Session V00: Poster Session III (1pm-4pm CST)

Determining finite contact size corrections in the transport characterization of topological insulators

We investigate the effect that finite size contacts have on the surface and bulk conductivity of topological insulators. Determining the contribution is non-trivial without first knowing the conductivity of the material, which requires knowing the effect the contacts have. In this work we systematically study Hall transport in an ideal topological insulator with contacts of varying sizes and using finite element analysis. We present how the correction factors associated with contacts evolve with the size of the contacts, the sample thickness, and the relative contributions of bulk and surface conduction in an ideal topological insulator.   

Topological phonons in two-dimensional materials

Phonons, the most fundamental emergent bosons in condensed matter systems, play an essential role in crystalline materials' thermal, mechanical, and electronic properties. Recently, the concept of topology has been introduced to phonon systems, and the nontrivial topological states also exist in phonons due to the constraint by the crystal symmetry of the space group. Although the classification of various topological phonons has been enriched theoretically, experimental studies were limited to several three-dimensional (3D) single crystals with inelastic X-ray or neutron scatterings. The experimental evidence of topological phonons in two-dimensional (2D) materials is absent. Using high-resolution electron energy loss spectroscopy following our theoretical predictions, we directly map out the phonon spectra of the atomically thin graphene in the entire 2D Brillouin zone and observe two nodal-ring phonons and four Dirac phonons. The closed loops of nodal-ring phonons and the conical structure of Dirac phonons in 2D momentum space are revealed by our measurements, in good agreement with our theoretical calculations.   

Control of Polarity in Kagome-NiAs Bismuthides

Kagome materials have attracted considerable attention due to the intrinsic properties such as flat bands, Dirac points and quantum magnetic frustration. Here, we report several new NiAs-type bismuthide materials in which the coupled formation of vacancies and interstitials generates kagome transition metal layers, which can further compositionally tuned towards breaking of inversion symmetry to give polar materials. A superstructure of the P6₃/mmc NiAs structure is reported in which kagome nets are stabilized in the octahedral transition metal layers of the compounds Ni_0.7Pd_0.2Bi, Ni_0.6Pt_0.4Bi, and Mn_0.99Pd_0.01Bi. The ordered vacancies that yield the true hexagonal kagome motif lead to filling of trigonal bipyramidal interstitial sites with the transition metal in this family of “kagome-NiAs” type materials. Further ordering of vacancies within these interstitial layers can be compositionally driven to simultaneously yield kagome-connected layers and a net polarization along the c axis in Ni_0.9Bi and Ni_0.79Pd_0.08Bi, which adopt polar Fmm2 symmetry. Interestingly, the polar Ni_0.9Bi exhibits an unconventional electronic transport, in contrast with the conventional metallic behavior of the non-polar members of this materials family, pointing to potentially extensive compositional control of structure and properties in a chemistry that affords the intensely studied kagome nets. This research demonstrates the ability to combine the important structural motifs and selectively break or maintain important crystallographic symmetries in the kagome-NiAs bismuthide family, which will stimulate extensive searches for new or polar kagome materials and associate studies of their physical properties, with ramifications for the control and understanding of electronic structure in the solid state.   

Constructing a device to study thermal transport properties of 2M-WS2 thin films and other topological materials.

Using electron beam lithography (EBL) and other nano-fabrication techniques, we construct a micro device to be used in conjunction with a Quantum Design Physical Property Measurement System (PPMS) to observe and measure thermal transport properties of monoclinic tungsten disulfide (2M-WS₂) thin films as well as other thin film topological materials.

 

Thermal transport property measurements, and more specifically thermal Hall measurements, are methods of determining topological superconductivity. In topological superconducting materials, it is theorized that Majorana particles exist on the surfaces or edges of the materials which could be observed through the thermal Hall measurements we obtain with this device.

 

This device was fabricated on a silicon substrate using EBL. It contains a resistive heater, a heat sink, and three platinum thermometers to measure both the longitudinal and transverse temperature gradients across the thin film samples. The resistive heater provides heat while the lithium fluoride (LiF) heat sink allows us to maintain a temperature gradient. From here, an out of plane magnetic field is applied to the system to generate the thermal Hall effect where potential Majorana particles will exhibit unique behavior in the thermal conductivity and thermal Hall measurements. We also plan to use this device to study Josephson Junction samples as well as other thin film topological materials.   

Bottom-up Synthesis of Magnetic Topological Insulator

The infusion of magnetic dopants into a topological insulator can induce magnetic order within the substance, disrupting the time-reversal symmetry inherent to its surface electronic states. The absence of this symmetry allows topological insulators to display a variety of unique quantum phenomena that are theoretically interesting and underexplored, such as the quantum anomalous Hall effect, chiral Majorana modes, and topological magnetoelectric effects. In this study, we employed atmospheric pressure chemical vapor deposition (CVD) to fabricate Mn-doped Bi₂Te₃, a magnetic topological insulator, from the bottom up. To characterize our samples, we utilized optical microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. By systematically modifying each CVD growth parameter individually, we were able to ascertain the optimal values and permissible ranges of several parameters. This research illustrates a straightforward, scalable, and cost-effective method for synthesizing Mn-doped Bi₂Te₃ crystals using CVD that can provide precise control over the stoichiometry of the grown material and facilitates the fabrication of high-quality, few-layer crystals. These attributes position CVD as a valuable and promising technique for future investigations into magnetic topological insulators.   

Theory of the spin-orbit coupling and topological flat band in the polyhedral π-conjugated molecules

The research on the spin-orbit coupling (SOC) has been one of the main themes in materials science since it induces the topological aspects in matter. Recently, the topological flat band has been focused on because of its new type of topological nature. In the present study, we focus on the triangular lattice of the organic molecules with C₃ symmetry. The band structure has been analyzed based on the fragment-molecular-orbital (fMO) picture, where the flat band appears and touches the dispersive band at the Γ point [1]. The dispersive and flat bands are constructed by the π-orbital, which is extended in the two-dimensional plane, and thus this system is an ideal two-dimensional system. Considering the Haldane model on this lattice in which the next-nearest spin-dependent hopping is introduced, it has been clarified that the infinitesimal spin dependent hopping induces a gap at the Γ point, and the topological flat band is realized with a nonzero Chern number [1]. In the present work, we derive the effective fMO model of the π-polyhedral molecule, including the effect of the intrinsic SOC. By treating the SOC term in the second-order perturbation theory, we relate the SOC to the next-nearest-neighbor hopping term in the polyhedral π-system. The structure of the effective Hamiltonian is different from that of graphene. We evaluate the order of magnitude of the spin-dependent hopping parameters and discuss their relevance to the material.

[1] Y. Shuku, R. Suizu, S. Nakano, M. Tsuchiizu, and K. Awaga, Phys Rev. B 107, 155123 (2023).   

Raman scattering from massive Dirac fermions in two dimensions

In recent years, Raman spectroscopy has been recognized as a valuable characterization tool for two dimensional (2D) materials. Although some of these materials are topological, the signatures of electronic band topology in Raman scattering have remained largely unexplored. In this talk, we will present a theoretical calculation on how the electronic Chern number influences the Raman tensor elements for ideal 2D massive Dirac fermions. We will conclude by extrapolating our findings to real 2D Dirac materials and commenting on the experimental challenges.   

Structural evolution in SrRuO3 thin film at low temperature

In the SrRuO₃ (SRO) system, the structural symmetry plays an important role in its physical properties. Structural investigation of SRO at high temperatures reveals the cubic symmetry above approximately 920 K. With reducing temperature, SRO undergoes a series of phase transitions, first from the cubic to tetragonal and then from the tetragonal to orthorhombic phases. At ambient temperature, SrRuO₃ exhibits the orthorhombic phase with the Pbnm space group, where mirror symmetry is present. The SRO thin film grown on SrTiO₃ (001) substrate shows a ferromagnetic phase transition with a Curie temperature (T_c) of about 150 K, below which a Weyl metal phase was recently discovered.  However, the possible structural evolution in the ferromagnetic SRO film is not experimentally studied in detail. In this study, we utilized high-precision synchrotron X-ray to investigate the structural properties of the SRO thin film down to around 10 K. The SRO thin film maintains the distorted orthorhombic phase with the Pbnm space group above T_c. Below T_c, the SRO (012) reflection appears and remains nearly unchanged with reducing temperature down to about 10 K, which can be associated with the P2₁/c symmetry with the absence of mirror planes. A systematic study on the temperature dependent structural evolution in SrRuO₃ /SrTiO₃ film will be presented and discussed.   

Study of Fermi liquid behaviour in Kagome-semimetal Ni3In2S2

The Kagome metals have been generating much interest due to their capacity to exhibit several intriguing properties, such as frustrated magnetism, topological state, superconductivity, charge order and correlated phenomena. Here, we report the synthesis and characterisation of the Kagome metal Ni₃In₂S₂ and study the transport properties of the Kagome metal. The T² dependency of resistivity, also known as the Baber law, is a property of a Fermi liquid. The majority of metals, however, only exhibit this behaviour at extremely low temperatures, often below 20 K. Our experimental study shows that this T² dependency holds over an extensive temperature range up to 86 K for the single-crystal Kagome semimetal Ni₃In₂S₂, showing that the electron-electron scattering mechanism dominates in this sample. Most importantly, with T² resistivity, Kadowaki-Woods scaling has been tested. The Debye temperature Ɵ_D=92.09K, obtained from the specific heat study near the cross-over temperature. A quantum linear magneto-resistance is seen in the magneto-transport experiment, reaching up to 286% at 2K and 6 T and showing no signs of saturation. This type of behaviour is due to open orbits on the Fermi surface.  However, our findings open a new door for exploring the exciting physics of electron-electron interaction and illuminate the possible reason for this unconventional behaviour.   

Broadband terahertz spectroscopy in reflection of three-dimensional Dirac semimetal Cd3As2

Three-dimensional Dirac semimetal Cd₃As₂ has gained attention recently for its extremely mobile carriers and terahertz nonlinear response. Charge carriers around the rotational symmetry protected inverted band crossings are suggested to host novel features in low-energy electrodynamics, including but not limited to terahertz nonlinear effects. In this work, we discuss new results from dynamical broadband terahertz experiments on an MBE grown film of Cd₃As₂, suggesting a complex many-body interaction of electrons with other collective modes of the system.   

Poster: Identification of Weyl Semimetals using Plasmonic Responses from Fourier Transform Infrared Spectroscopy

With increasing interests in exploring the extravagant capabilities and potential utilizations of Weyl Semimetals (WSM's) amongst other topological materials, there arises a need for a novel and rapid identification mechanism for realization of the characteristic chiral fermions present in these materials. The current experimental technique for determining WSM's is laser-based angle-resolved photoemission spectroscopy. It has been recently theorized that WSM's can be identified through the excitation of Surface Plasmon Polaritons (SPP's) and mapping out the resulting dispersion curves upon different orientations. In this work, we employ Fourier Transform Infrared (FTIR) spectroscopy along with the launching of SPP's through the Kretschmann geometrical configuration to extract an expeditious broadband dispersion curve from near infrared to far infrared regions. Along with that, we implement FTIR Spectrometry to obtain the dielectric function of materials through a pseudoellipsometric technique to then be used in current modeling software to computationally verify and further explore these WSM's.   

Pair Wavefunction Symmetry in UTe2 from Zero-Energy Surface State Visualization

Although nodal spin-triplet topological superconductivity appears probable in UTe₂, its superconductive order-parameter Δ_k has not yet been established. If spin-triplet, it should have odd parity so that Δ_-k = -Δ_k  and, in addition, may break time-reversal symmetry. A distinctive identifier of such nodal spin-triplet superconductors is the appearance of an Andreev bound state (ABS) on surfaces parallel to a nodal axis, due to the presence of a topological surface band (TSB). Moreover, theory shows that specific ABS characteristics observable in tunneling to an s-wave superconductor distinguish between chiral and non-chiral Δ_k. To search for such phenomena in UTe₂ we employ s-wave superconductive scan-tip imaging to discover a powerful zero-energy ABS signature at the (0 -1 1) crystal termination. Its imaging yields quasiparticle scattering interference signatures of two Δ_k nodes aligned with the crystal a-axis. Most critically, development of the zero-energy Andreev conductance peak into two finite-energy particle-hole symmetric conductance maxima as the tunnel barrier is reduced, signifies that UTe₂ superconductivity is non-chiral. Overall, this combination of a zero-energy ABS, internodal scattering along the a-axis, and splitting of Andreev conductance maximum due to s-wave proximity, categorizes the superconductive Δ_k of a D_2h-symmetry crystal as the odd-parity non-chiral B_3u state.   

Superconducting proximity effect in PbTe-Pb hybrid nanowires

A semiconductor nanowire coupled to a superconductor is an intriguing quantum system owing to the proximity effect. One example is the gate-tunable Josephson junction, which plays a key role in the gatemon superconducting qubit. Moreover, the interplay between strong spin-orbit coupling of the semiconductor and Zeeman energy may lead to topological phases hosting Majorana zero modes.
So far, most of those studies are based on III-V semiconductors such as InAs or InSb. The Majorana theory requires a clean semiconductor nanowire while the current experiments are disorder-limited. Recently, PbTe nanowires have been proposed to overcome the disorder issue owing to their large dielectric constant (~1350). Meanwhile, these nanowires were grown using selective area epitaxy and subjected to quantum transport characterizations on the field effect mobility, weak antilocalization, Aharonov-Bohm oscillations, and quantum dots. So far, the key question of whether superconducting proximity effect in PbTe nanowires exists is still pending. Here, we demonstrate the induced superconductivity in PbTe nanowires coupled to a superconductor Pb, whose gap is much larger than Al. Josephson junction devices based on this hybrid can reveal a gate-tunable supercurrent in the open regime and a hard-induced superconducting gap in the tunneling regime. We also have observed a hard superconducting gap with sharp coherence peaks in N(normal metal)-NW(nanowire)-S(superconductor) devices. The gap size is ∼ 1 meV. Our works show that the combination of PbTe and Pb could work as a new material platform for Majorana searches.   

Superconducting transition in a topological Dirac semimetal α-Sn thin film

Topological superconductivity (TSC) gathers much attention due to its potential application to topological quantum computing. One of the candidates for TSC is doped topological Dirac semimetal (TDS) [1]. In this study, we have discovered superconductivity in an elemental TDS α-Sn thin film (~ 5 nm) grown on an InSb (001) substrate. The as-grown α-Sn thin film showed semiconducting behavior, but it showed superconductivity (T_C = 4.2 K) 20 months later, probably caused by In diffusion into the α-Sn layer while the diamond crystal structure is maintained. Temperature dependence of the critical magnetic field is well explained by the 2D superconductivity model with GL coherence length of 20.5 nm. The in-plane critical magnetic field at 0 K estimated by the fitting reaches 11.8 T, which is far beyond the Pauli-limiting field H_P = 1.86T_C ~ 7.8 T. This indicates that paramagnetic pair-breaking effect is suppressed by the spin-orbit interaction of α-Sn. We observed Shubnikov-de Haas oscillations, indicating that the Fermi level has shifted by the aging, and a new linear band has appeared which may be the origin of superconductivity, considering calculated coherence length. Our result suggests that α-Sn is promising for TSC and quantum computing material.

[1] T. Hashimoto, et al., Phys. Rev. B 94, 014510 (2016).   

Reducing disorder in PbTe nanowires for Majorana research

Topological quantum computing is based on the braiding of Majorana zero modes encoding topological qubits. A promising candidate platform for Majorana zero modes is semiconductor-superconductor hybrid nanowires. The realization of topological qubits and braiding operations requires scalable and disorder-free nanowire networks. Although the scalability of in-plane InAs and InSb nanowires, combined with the shadow-wall growth of superconductors, has been demonstrated, the discernible lattice mismatch at the nanowire-substrate interface introduces disorder, posing a significant hurdle to progress. In this work, we address this challenge by combining selective area and shadow-wall growth to fabricate PbTe-Pb hybrid nanowires—a promising Majorana system—on a nearly perfectly lattice-matched ‘substrate’, Pb_1-xEu_xTe. Transmission electron microscopy reveals an atomically sharp surface of the Pb_1-xEu_xTe and clean PbTe-Pb interface, indicating potential for creating a clean nanowire system to explore Majorana zero modes and advance topological quantum computing.   

Quantized Zero-bias Conductance in Thin InAs-Al Nanowires

InAs-Al hybrid nanowires are one of the primary contenders in the search for Majorana zero modes (MZMs). One key prediction of MZM theory is a quantized zero bias peak (ZBP) at 2e² /h in tunneling conductance. Here, we report plateau regions for ZBPs within 5 percent of 2e² /h by sweeping gate voltages and magnetic field in a four-terminal thin InAs-Al device. In another device with a larger barrier transmission, we observe a continuous zero-bias peak-to-dip transition near 2e² /h driven by magnetic field. We further discuss their possible connections to MZMs or other alternative explanations, and also outline our future plans. Our results extensively explore the zero-bias conductance of thin InAs-Al nanowires, constituting a step forward towards establishing Majorana zero modes.   

Quantized anomalous Hall resistivity achieved in molecular beam epitaxy-grown MnBi2Te4 thin films

The intrinsic magnetic topological insulator MnBi2Te4 provides a feasible pathway to the high-temperature quantum anomalous Hall (QAH) effect as well as various novel topological quantum phases. Although quantized transport properties have been observed in exfoliated MnBi2Te4 thin flakes, it remains a big challenge to achieve molecular beam epitaxy (MBE)-grown MnBi2Te4 thin films even close to the quantized regime. Here, we report the realization of quantized anomalous Hall resistivity in MBE-grown MnBi2Te4 thin films with the chemical potential tuned by both controlled in situ oxygen exposure and top gating. We find that elongated post-annealing obviously elevates the temperature to achieve quantization of the Hall resistivity, but also increases the residual longitudinal resistivity, indicating a picture of high-quality QAH puddles weakly coupled by tunnel barriers. These results help to clarify the puzzles in previous experimental studies on MnBi2Te4 and to find a way out of the big difficulty in obtaining MnBi2Te4 samples showing quantized transport properties.   

Electron-phonon interactions in few-septuple-layers magnetic topological insulator MnBi2Te4

Intrinsic magnetic topological insulators exhibit chiral edge modes and quantum anomalous Hall effect (QAH) without any external magnetic field. Such an exotic quantum state opens a new pathway for lossless energy transport. MnBi₂Te₄ (MBT), an intrinsically magnetic compound, exhibits such an exotic quantum state depending on the thickness of the septuple layers (SL). For example, 1 SL MBT is a topologically trivial ferromagnetic (FM) insulator, while 3 SL MBT is a topologically nontrivial antiferromagnetic (AFM) QAH insulator. Although the magnetic ordering can exist beyond 100 K, the QAH state can only be observed below 1 K. Due to the rise of temperature, electron-phonon interactions (EPI) play a role in the QAH state. Based on the density functional perturbation theory (DFPT), we show that EPI in MBT strongly correlate with the magnetic ordering, septuple layers thickness and electronic band topology. In particular, the electron-phonon scattering is significantly lower in the QAH state (3 SL MBT) compared to 1 SL FM, 2 SL AFM, and 2 SL FM. We also observe a thickness-dependent phonon energy shift and dynamical stability of MBT with different thicknesses.   

Spin-orbit torque induce topological phase transition in magnetic topological insulator MnB2Te4

In modern spintronics, the efficient control of magnetic dynamics via electric field has been a central attraction for its potential applications. Spin-orbit torque in the magnetic system lacking inversion symmetry is the most promising mechanism for achieving such control. In the bulk antiferromagnetic system with local inversion symmetry breaking, an electric field can directly induce the symmetry-constrained staggered and uniform spin-orbit torque mediated by spin-orbit coupling, offering an efficient way to manipulate ultrafast magnetization dynamics in antiferromagnets.

Using first principle calculations, we have studied the intrinsic spin orbit torques in the magnetic topological insulator-MnBi₂Te₄ materials. We discover that the magnetization dynamics driven by the spin-orbit torque also drive the topological phase transitions. This provides a new avenue of manipulation and control of topological phases in magnetic materials.   

Emergence of charge density wave in ultrathin orthorhombic-CoSe2 film

Among transition metal chalcogenides (TMC), CoSex system has several kinds of stable phases, such as orthorhombic-CoSe2(o-CoSe2), hexagonal-CoSe2, 1T-CoSe2, tetragonal-CoSe. These stable phases of CoSex system have consecutive structural transitions across thermal annealing procedure.[1] We successfully fabricated ultrathin o-CoSe2 on epitaxial graphene substrates using MBE. We propose that o-CoSe2 possesses the Peierls-type charge density wave (CDW) in ultrathin thickness limit. In our DFT calculation results, Fermi surface of o-CoSe2 satisfies nesting condition with two parallel vectors unlike that of corresponding bulk. We experimentally confirmed the corresponding lattice distortion from scanning tunneling microscopy (STM) measurements and also analyzed related band structure maps from angle-resolved photoemission spectroscopy (ARPES) measurements. Our results suggest that the ultrathin CoSe2 film can serve as a interesting material platform for investigating two-dimensional electronic correlations.   

Curvature-induced quantum spin-Hall effect on a Möbius strip

The quantum Hall effect has been predicted and discovered in various condensed-matter systems. A promising quantum material for such topological effects is graphene. We report the numerical observation of a curvature-induced spin-Hall effect in a monolayer graphene Möbius strip. The solution of the Dirac equation on the nontrivial and non-Euclidean manifold reveals that despite the absence of a Hall current, a spin-Hall current is a natural consequence for such a topology, as predicted from symmetry arguments.   

Momentum Dependent Charge Density Wave Gap in an Antiferromagnetic Metal

Charge density wave (CDW) ordering has been an important topic of study for a long time owing

to its connection with other exotic phases such as superconductivity and magnetism. The RTe₃ (R = rare-earth elements) family of materials provides a fertile ground to study the dynamics of CDW in van der Waals layered materials, and the presence of magnetism in these materials allows to explore the interplay among CDW and long range magnetic ordering. Here, we have carried out a high-resolution angle-resolved photoemission spectroscopy (ARPES) study of a CDW material GdTe₃, which is antiferromagnetic below ∼ 12 K, along with thermodynamic, electrical transport, magnetic, and Raman measurements. Our ARPES data show a two-fold symmetric Fermi surface with both gapped and ungapped regions indicative of the partial nesting. The gap is momentum dependent, maximum along  Γ−Z  and gradually decreases going towards Γ−X. Our study provides a platform to study the dynamics of CDW and its interaction with other physical orders in two- and three-dimensions.   

Poster:Electric field-tunable Berry curvature in WTe2

    Berry curvature, as one of the central topics in topological physics, plays an important role in various quantum transport phenomena, such as various Hall effects and Circular dichroism [1]. Recently, Berry curvature dipole (BCD), i.e., the dipole moment of Berry curvature [2] is proposed to be able to induce second-order nonlinear Hall effect. The nonzero BCD can lead to high-frequency rectifiers, wireless charging, and energy harvesting through the nonlinear Hall effect, promising for nonlinear quantum devices. However, the maximum symmetry allowed for nonzero BCD is a single mirror symmetry, unfavorable for real applications.

    Here, via probing the nonlinear Hall effect, we demonstrate the electric field-manipulated Berry curvature and BCD [3]. A linear dependence between the BCD and the dc electric field is observed. The polarization direction of the Berry curvature is controlled by the relative orientation of the electric field and crystal axis, which can be further reversed by changing the polarity of the dc field. Our work provides a route to generate and control Berry curvature and BCD in broad material systems, and to facilitate the development of nonlinear quantum devices.

References

[1] D. Xiao, M.-C. Chang, and Q. Niu, Rev. Mod. Phys. 82, 1959 (2010).

[2] I. Sodemann and L. Fu, Phys. Rev. Lett. 115, 216806 (2015).

[3] X.-G. Ye, H. Liu, P.-F. Zhu, et al., Phys. Rev. Lett. 130, 016301 (2023).   

Cataloging topological phases of N stacked Su-Schrieffer-Heeger chains by a systematic breaking of symmetries

Two-dimensional (2D) model of a weak topological insulator with an N stacked Su-Schrieffer-Heeger chain is studied. This study starts with a basic model with all the fundamental symmetries (chiral, time reversal, and particle hole) preserved. Different topological phases are introduced in this model by systematically breaking the system's symmetries. The symmetries are broken by introducing different bonds (hopping terms) in the system. First, the chiral symmetry is broken by introducing hopping within each sublattice or intrasublattice hopping, where the hopping strengths of the sublattices are equal in magnitudes but opposite in sign. Then, following Haldane, the time-reversal (TR) symmetry is broken by replacing the real intrasublattice hopping strengths with imaginary numbers without changing the magnitudes. We find that breaking chiral and TR symmetries are essential for the weak topological insulator to be a Chern insulator. These models exhibit nontrivial topology with the Chern number C = ±1. The preservation of the particle-hole (PH) symmetry in the system facilitates an analytical calculation of C, which agrees with the numerically observed topological phase transition in the system. An interesting class of topologically nontrivial systems with C = 0 is also observed, where the nontriviality is identified by a quantized and fractional 2D Zak phase. Finally, the PH symmetry is broken in the system by introducing unequal amplitudes of intrasublattice hopping strengths, while the equal intrasublattice hopping strengths ensure the preservation of the inversion symmetry. We investigate the interplay of the PH and the inversion symmetries in the topological phase transition. A discussion on the possible experimental realizations of this model is also presented   

Chiral photoluminescence in topological insulators

In topological insulators, the strong SOC, in combination with the time-reversal symmetry, leads to the protected metallic surface states.  These states have opposite spins and propagate in opposite directions, forming a spin current as it occurs in the spin Hall effect. The surface states from this spin-dependent quantum Hall effect consist of counterpropagating states (called helical states) with opposite spins. The penetration depth of these states is of the order of the lattice constant if the surface states extend almost over the whole Brillouin zone (BZ). Theoretical and experimental studies show that there are two more surface bands near the Brillouin zone center (Γ-point) in Bi₂Se₃, a high-energy unoccupied Dirac cone (SS2) and a fully occupied Rashba-like surface state (RSS). Thus far, less is known about the properties of RSS and SS2.  We will use photoluminescence spectroscopy to investigate the properties of these states (SS2 and RSS) under different excitation photon energies and polarizations.   

Synthesizing a Magnetic Topological Insulator Using Chemical Vapor Deposition

Magnetic topological insulators have electronic and magnetic properties that are of interest to condensed matter physicists for potential applications in fields such as quantum computing and spintronics. In this work, we will synthesize Mn-doped Bi2Te3 using chemical vapor deposition (CVD) and characterize the resulting crystals using optical microscopy, Raman spectroscopy, and X-Ray Photoelectron Spectroscopy (XPS). We will study the effects of varying multiple parameters during the CVD process including temperature, presence of salts, types of carrier gases, substrate position, and more in an effort to produce high quality crystals in a scalable and reliable manner. Refining a CVD process to eventually produce crystalline MnBi2Te4 would mark a crucial step toward making this material a viable candidate to use in future projects that will help bring forth the next generation of quantum technologies and high-performance electronic devices.   

Spin-orbit coupling tuned crossover of gaped and gapless topological phases in the chalcopyrite HgSnX2(X=N/P):

The coupling between electron orbital momentum and spin momentum, known as spin-orbit coupling (SOC), is a fundamental origin of many fascinating physical phenomena. It holds paramount significance in topological materials. Our work has predicted the topological phase in Hg-based chalcopyrite compounds using the first principles density functional theory. The initial focus was on HgSnN$_2$, revealing it to be a nonmagnetic Weyl semimetal, while HgSnP$_2$ displayed characteristics of a strong topological insulator. What makes our work truly unique is that despite both compounds having the same SOC strength, they exhibit distinct topological phases due to the hybridization of bands. This finding prompted us to pursue another significant objective: understanding the correlation between topological phases and band hybridization within these intermetallic compounds. Our results indicate that we can tune the topological phase by manipulating SOC strength due to band hybridization between the dominant p orbitals of N or P and a minor contribution from Hg-$d$. This investigation stands as a remarkable illustration of the unique roles that hybridization plays in sculpting the topological properties of these compounds while simultaneously preserving their crystal symmetry and time reversal symmetry.   

Time Resolved Second Harmonic Generation in Topological Insulator Bi2Te3: Competing Contributions from Dirac Surface States, Surface Photovoltage and Band Bending

Nonlinear optical experiments, especially second harmonic generation (SHG) of topological insulators (TIs) such as Bi₂Te₃ and Bi₂Se₃ provide significant insights into the second order nonlinear susceptibility (χ⁽²⁾) of the topological surface states (TSS) and the DC electric field (E_dc), formed by the intrinsic band bending between the surface and bulk. Importantly, by selectively tuning the polarisation of the incident laser field from linear to circular, SHG in TIs enables us to extract these explicit contributions from the band bending and metallic surface states. In this work we report the relaxation dynamics of the differential change in SHG intensity (ΔI(τ_pp)) with respect to the pump-probe delay time (τ_pp) in Bi₂Te₃ crystal, mainly emphasizing on different pump-probe polarisation configurations to provide an understanding of the temporal evolution of ΔE_dc and Δχ⁽²⁾. Upon photoexcitation,  with linearly polarised pump-probe configuration, we observe two opposing contributions to ΔE_dc namely, the depletion electric field (DEF) and surface photovoltage (SPV) that arise from the spatial distribution of the bulk and surface carrier densities. However, on switching the probe polarisation from linear to circular, we show that the ΔI(τ_pp) has a predominant positive contribution from Δχ⁽²⁾ arising from the TSS. This is analytically understood by opening a band gap at the Dirac point due to the breaking of time-reversal symmetry using circularly polarised light.   

Nonequilibrium Kondo effect under finite thermal bias

The transport through strongly correlated nanostructures such as quantum dots, adatoms, nanowires or magnetic molecules have been under tremendous research interest. The electronic and thermoelectric properties of such nanostructures show fascinating behavior under the strong correlation regime. In particular, the equilibrium Kondo effect, a many-body screening effect shows a zero-bias conductance peak along with a sign-change in the Seebeck effect. The Numerical Renormalization Group method is a robust method that can describe the Kondo effect accurately in the linear response regime, but the nonlinear response regime requires sophisticated numerical techniques to describe the strong correlations accurately. In this work, we study the nonequilibrium Kondo effect in a quantum dot coupled to metallic leads with finite potential and temperature biases using the recently developed hybrid Numerical Renormalization Group-time-dependent Density Matrix Renormalization Group method (NRG-tDMRG). In particular, we describe the electronic conductance, the Seebeck coefficient, and the heat conductance of a Kondo-correlated quantum dot in the nonlinear transport regime.   

Topological phases on the real projective plane

We investigate two-dimensional spinless systems in which the fundamental domain in momentum space takes the form of a non-orientable closed manifold known as the real projective plane (RP2), in contrast to the usual case of a torus. We construct Wilson loops on RP2 to define a Z2 invariant that identifies topologically distinct phases. We find that the transition between the trivial and topological phases is mediated by an odd number of Weyl points within the fundamental domain, and that these Weyl points cannot all be annihilated. The topological phase is characterized by the presence of gapless bi-directional edge states, a feature attributed to the Fermi-arc connectivity of the Weyl points. Lastly, we demonstrate that these systems are examples of "momentum quadrupole insulators" that exhibit a linear response of momentum current to a translation gauge field.   

Non-Bloch band theory of sub-symmetry-protected topological phases

A generic feature of symmetry-protected topological (SPT) phases of matter is the bulkboundary correspondence (BBC) which connects the concept of bulk topology to the emergence of robust boundary states. In recent years, non-Hermitian systems have shown unconventional properties and phenomena such as exceptional points, non-Hermitian skin effect, and many more in different research fields without Hermitian analog. Therefore, the topological Bloch band theory with the notion of the Brillouin zone (BZ) has been extended to the non-Bloch band theory with the notion of the generalized Brillouin zone (GBZ) defined by generalized momenta which can take complex values. The non-Bloch band theory has successfully proven that non-Hermitian systems show two types of modified BBC: (i) complex eigenvalue topology of the bulk leads to non-Hermitian skin effect, where all bulk states localize at one boundary of the system, and (ii) the eigenstate topology in the GBZ leads to the conventional topological boundary modes.

In our recent work [1], we reported a new type of BBC in non-Hermitian systems, which originates from the intrinsic topology of the GBZ. Topologically non-trivial GBZ appears due to general boundary conditions that break the translation symmetry of the system locally. In our case, the topological phase transition is characterized by the generalized momentum touching of GBZ, which accompanies the emergence of exceptional points.

In this presentation, I will discuss a simple extension of our work to Hermitian systems. Firstly, we found that the intrinsic topology of GBZ successfully characterizes the SPT phases. In this case, the topological phase transition of GBZ is characterized by the generalized momenta touching of GBZ, which accompanies the emergence of band touching points. Moreover, the non-Bloch band theory provides a natural topological invariant characterizing the sub-symmetry-protected topological phases that a conventional Bloch topological invariant (winding number) fails to characterize.

Reference: [1] Sonu Verma and Moon Jip Park, arXiv:2305.08584.   

Interplay of Higher-Order van Hove Singularities and Band Topology in Kagome Systems

In 2D materials, van Hove Singularities (VHS) for the density of states usually correspond to saddle points on the energy bands in the momentum space. The Higher-order VHS (HOVHS) is a new classification of VHS that corresponds to a higher-order saddle point, where a power law describes the density of states. Here, we investigate the Kagome lattice using the tight-binding model and produced a set of phase diagrams demonstrating HOVHS emergence for the high-symmetry points in the Brillouin zone. We also discuss the onset of these HOVHS in topological Chern bands in the kagome lattice.   

Revealing higher-order topology through loop of spin-helical hinge states in Bi nanocrystal with proximity superconductivity

Higher-order topological insulators (HOTI) are newly proposed topological materials which host gapless boundary states in a codimension of order index. E.g., a 2nd order time-reversal invariant 3D TI will possess 1D boundary states residing in a series of hinges encircling the crystal. These hinge states are spin-helical which resembles the edge state of a quantum spin hall insulator. Experimentally, verifying a 3D HOTI would require systematical investigation on all the boundaries of a 3D crystal, which is challenging. Here we studied the element of Bismuth (Bi), a promising candidate of 2nd order TI, in the form of nanocrystals. The nanocrystals were fabricated on superconducting V3Si (111) substrate. Via scanning tunneling microscopy/spectroscopy (STM/S) measurement, we revealed dispersive 1D states on various hinges of the crystal, which are consistent with first-principle band calculation. After introducing ferromagnetic clusters, we found new scattering channels opened at certain hinges, which suggest they are spin-helical. Remarkably, these spin-helical states on different hinges formed a closed loop surrounding the nanocrystal, further supporting their topological origin. Thus our study provided direct evidence on the existence of HOTI state in nature. Moreover, we detected proximity-induced superconductivity in the hinge states, which enables HOTI as a novel platform for generating Majorana quasi-particles.   

A Machine Learning Approach for Understanding Chiral Materials

Machine learning has proven to be a very useful tool for analysis of materials science. In the search for qubits, we aim to use machine learning to find skyrmions and categorize quantum spin liquids by developing a machine learning model to predict a material's exchange parameters. Knowing the exchange parameters of a material J1,J2,J3 will allow us insight into the properties of different materials and accelerate further research into applications of skyrmions and quantum spin liquids.   

First-order topological phase transitions and disorder-induced Majorana modes in extended Kitaev chain

Using a combination of the mean-field Bogoliubov–de Gennes approach and the density matrix renormalization group method, we discover a first-order topological transition between topological superconducting and trivial insulating phases in a sawtooth lattice of intersite attractive fermions. The topological characterization of different phases is achieved in terms of winding numbers, Majorana edge modes, and entanglement spectra. By studying the effect of disorder on first-order topological phase transitions, we establish disorder-induced topological phase coexistence as a mechanism for generating a finite density of Majorana particles.   

Title: Stabilizing topological superconductivity in disordered spin-orbit coupled semiconductor-superconductor heterostructures

We theoretically consider the problem of one-dimensional semiconductor-superconductor (SM and SC) heterostructure with Rashba spin-orbit coupling and a parallel Zeeman field in the presence of short-ranged disorder from random charged impurities. With no disorder, this system was proposed as a model platform for realizing bulk topological superconductivity (TS) characterized by zero energy Majorana excitations localized at the wire ends. With disorder, however, it has been shown that disorder-induced trivial low-energy states can render the detection of topological superconductivity and zero-energy Majorana states experimentally very challenging, and, for strong disorder may even lead to the disappearance of the TS state from the experimentally accessible part of the phase diagram. Starting with the Hamiltonians of the SM and the SC and using the formalism of an effective SM Green's function by integrating out the SC, we show in this paper that for a strongly disordered SM, strong coupling to the SC is generically beneficial for stabilizing a robust TS state in the semiconductor. Furthermore, we find that, for the phase diagram defined by the chemical potential (μ) and Zeeman field (Γ), with increasing strength of disorder the robust topological regions move to the parts of the phase diagram defined by larger values of Γ. These results lead us to propose that (a) stronger SM and SC coupling by interface engineering, (b) SM systems with a larger gyromagnetic ratio, or (c) a stronger proximity effect allowing the application of larger values of the Zeeman field may help defeat the dominance of disorder-induced low-energy states in the semiconductor, revealing and expanding the underlying robust topological superconducting state in the phase diagram.   

Realizing attractive interacting topological surface fermions: A resonating TI- thin film hybrid platform

Here we put forward a practical way to realize topological surface Dirac fermions with tunable attractive interaction between them. The approach involves coating the surface of a topological insulator with a thin film metal and utilizing the strong-electron phonon coupling in the metal to induce interaction between the surface fermions. We found that for a given TI and thin film, the attractive interaction between the surface fermions can be maximally enhanced when the Dirac point of the TI surface resonates with one of the quasi-2D quantum-well bands of the thin film. This effect can be considered to be an example of 'quantum-well resonance'. We also demonstrate that the superconductivity of the resonating surface fermions can be further enhanced by choosing a strongly interacting thin film metal or by tuning the spin-orbit coupling of the TI. This TI-thin film hybrid configuration holds promise for applications in Majorana-based quantum computations and for the study of quantum critical physics of strongly attractively interacting surface topological matter with emergent supersymmetry.   

3+1d Boundary Topological Order of 4+1d Fermionic SPT state

We generalize the previous gapped boundary construction of 4+1d beyond-cohomology bosonic SPT phase to the fermionic SPT phase, which is now protected by a finite group symmetry including the fermion parity $Z_2^F$ symmetry. Via the crystalline correspondence, we related the classification of $Z_{2N}^F$ fermionic SPT phases with that of crystal rotational $C_N imes Z_2^F$ symmetry, where $C_N$ is the $N$-fold rotation. We construct TQFT boundaries for those SPT phases that do not forbid it and propose an exotic "$Z_K$ gauge theory" as a boundary TQFT, where the anomalous symmetry is implemented by codimension-1 invertible topological defects. We also give an explicit gapped boundary construction. In particular, for $N=2$, we relate the classification of $Z_4^{TF}$ with that of 3+1d topological superconductor classified by $Z_16$. For odd $ u in Z_{16}$, we give possible constructions of gapped phases but whose low energy theory is possibly beyond the conventional TQFT. For all other possible gapped phases, i.e. even $ u in Z_{16}$’s, we can further construct and identify the low energy TQFTs.   

Light-field dressing of transiently excited electronic states by time-resolved ARPES

Strong time-periodic light-field can be used to generate hybrid photon–electron states. Along this line, the interaction of light-field with Bloch states leads to Floquet–Bloch states, which can tailor properties of quantum materials. So far, most of our understanding on the Floquet engineering of topological insulators has been focused on the filled electronic states, however, little is known about the dressing of the electronic states above the Fermi level. For example, whether photo-excited electronic states above the Fermi energy can also be dressed by the light-field remains elusive. Revealing their dynamics of photo-excited carriers, light-field dressing and their interplay is an important question. Here by using time-and angle-resolved photoemission spectroscopy (Tr-ARPES) with mid-infrared (MIR) pumping, I will present new experimental results to address these interesting questions.   

Geometrically Disordered Network Models for the Integer Quantum Hall Transition via Loop Diagram Insertion

In [1], the transfer matrix method for the Chalker-Coddington (CC) Network Model of the Integer Quantum Hall transition was used on a lattice with random node removal to compute the localization critical exponent ν. We introduce a new method for including topological disorder in the lattice for the CC Network Model by randomly replacing nodes in the square lattice with diagrams including closed loops for which the transfer matrices can be explicitly computed. We then numerically compute ν for a variety of widths and random edge phases, and thus determine the characteristic localization length for electrons on the sample. By varying the node replacement probability, we also evaluate the effects of different levels of disorder on the value obtained for the localization exponent. 

[1] I. A. Gruzberg, A. Klu ̈mper, W. Nuding, and A. Sedrakyan. Geometrically disordered network models, quenched quantum gravity, and critical behavior at quantum hall plateau transitions. Phys. Rev. B, 95:125414, Mar 2017.   

High-order two-component fractional quantum Hall states in GaAs quantum wells

Two-component fractional quantum Hall (FQH) states have been known to occur in double and wide quantum wells for nearly three decades. The simplest of these states occurs at a total filling factor ν = 2/3 = (1/3,1/3), where 1/3 is the filling factor of each component (layer). Of special interest are so-called unbalanced states which indicate that it might be energetically favorable to have FQH states at different filling factors in each layer. To date, only a few such states have been observed, including ν = 11/15 = (1/3, 2/5), 19/15 = (2/3, 3/5), and 29/35 = (2/5, 3/7). Here, we report experimental evidence for many more higher-order fractions, up to ν = 109/99 = (5/9, 6/11) and 106/117 = (4/9, 6/13). These states emerge upon application of in-plane magnetic fields, but in contrast to earlier studies, we employ “normal,” single-subband quantum wells. The presence of many states allows us to investigate the role of imbalance. Surprisingly, we find that quite often unbalanced states are noticeably stronger than the neighboring balanced states.   

Vertex dominated superconductivity in intercalated FeSe

Bulk FeSe becomes superconducting below 9 K, but the critical temperature (Tc) is enhanced almost universally by a factor of ~4–5 when it is intercalated with alkali elements. How intercalation modifies the structure is known from in-situ X-ray and neutron scattering techniques, but why Tc changes so dramatically is not known. Here we show that there is one-to-one correspondence between the enhancement in magnetic instabilities at certain q vectors and superconducting pairing vertex, even while the nuclear spin relaxation rate 1/(T1T) may not reflect this enhancement. Intercalation modifies electronic screening both in the plane and also between layers. We disentangle quantitatively how superconducting pairing vertex gains from each such changes in electronic screening. Intercalated FeSe provides an archetypal example of superconductivity where information derived from the single-particle electronic structure appears to be insufficient to account for the origins of superconductivity, even when they are computed including correlation effects. We show that the five-fold enhancement in Tc on intercalation is not sensitive to the exact position of the dxy at Γ point, as long as it stays close to EF. Finally, we show that intercalation also significantly softens the collective charge excitations, suggesting the electron-phonon interaction could play some role in intercalated FeSe.   

AF-Spin fluctuations and its possible relation with the superconducting pairing mechanism and the superconducting energy gap symmetry of the stoichiometric iron-pnictide superconductor LiFeAs (Tc~18K).

In superconductivity (SC) discovery also understanding provide principal archetype, recently in 2008 discovery of iron-pnictide with several families, including spontaneous gauge symmetry breaking and Anderson-Higgs mechanism remain outstanding issue and opened new avenue of research in condensed matter physics. T_c raised in excess of 55ºK, bringing insights into bad-metal behavior, magnetism, and their striking interplay with unconventional SC.  Normal state AF-spin fluctuations and its possible relation with SC pairing mechanism and symmetry of SC energy gap function are important issue in iron-pnictide family of SC and are probed by lower critical field deduced from vortex penetration, specific heat measurements of SC single crystals and reversible magnetization of polycrystalline LiFeAs sample. LiFeAs seems to be almost isotropic and can be fit as a s-wave two-gap SC with the superfluid density strongly affected by the smaller gap. The specific heat of LiFeAs single crystals reveals s-multigap feature with a small gap of about 0.7 meV dominating low-temperature electronic quasiparticle excitations. A significant contribution from Einstein phonons is observed, as well as a noticeable residual linear term γ₀. Also, I will discuss magnetic properties, H_c1 anisotropy, anisotropic gaps, electronic correlations, and the role of orbital degrees of freedom. Developments and intense activity ensued commonalities and differences between high-T_c Fe-based, Cuprate, Heavy-fermion and Organic SC.   

Investigation of the Superconducting and Structural Properties of Trigonal PtBi2 Single Crystals

In this research, we explored the intriguing structural and superconducting properties of trigonal PtBi₂ single crystals. Layered material t-PtBi₂ exhibits a unique P31m structure. Our investigation focused on the complex interplay between their unique structural attributes and their emergent physical properties. Recent studies highlighted the novel topological states and transport behaviors of this material, notably the presence of triply degenerate point (TP) fermions and inherent topological semimetallic behaviors. Significant potential in its superconducting capabilities positions the trigonal PtBi₂ as a compelling subject in the realm of condensed matter physics.   

Superconductivity with high upper critical field in Ta-Hf alloys

Recently, there has been considerable interest in exploring superconducting alloys for potential applications in superconducting devices [1-3]. In this study, I will present our findings on superconductivity in Ta _x Hf _1-x alloys, utilizing magnetization, electrical resistivity, and specific heat measurements on polycrystalline samples [3].

The crystal structures of these alloys are composition dependent. Notably, when we substituted Hf (a Type-I superconductor with a critical temperature, T_C, of 0.12 K) with Ta (also a Type-I superconductor with a T_C of 4.4 K), a remarkable enhancement in T_C was observed, along with a transition to Type-II superconductivity featuring high upper-critical fields. This transition is of particular interest from the perspective of materials science. Our specific heat measurements provided evidence of strong coupling superconductivity, aligning with conventional principles of superconductivity. These metallic alloys exhibit both metallic properties and a high upper-critical field, making them highly promising for practical superconducting devices like SQUIDs and qubits.   

Absence of Superconductivity in Cu Doped Lead Apatite

We report on the structural, electrical and magnetic measurements in as-grown polycrystalline samples of Pb₁₀(PO₄)₆O and Cu doped Pb_10-xCu_x(PO₄)₆O.  The copper doped compound has been recently claimed to be a superconductor at room temperature. Structural characterizations (XRD, TEM, Raman) data are analyzed . This sample has 1.5% of Cu₂S as an impurity phase. No evidence of superconductivity is observed either in resistivity measurements or in magnetization measurements. A resistive transition with thermal hysteresis around 380 K is observed, which can be associated with possible structural transition of Cu₂S. Hall measurements provide evidence of hole doping through Cu substitution. The obtained value of band gap (E_g= 0.51eV) indicates that the obtained Cu doped sample is semiconducting.  In conclusion, we find no evidence for superconductivity in Cu doped lead apatite Pb₉Cu(PO4)₆O at ambient pressure.      

Evidence of s-wave pairing symmetry in 2H-TaSeS

In order to attain topological superconductivity, transition metal dichalcogenides are of great current significance. Potentially this can revolutionize the promised realization of scalable qubits. Here we report a thorough investigation into the synthesis and electromagnetic characterization of single crystals of 2H-TaSeS, which is thought to be a topological superconductor. At 4.15K, a superconducting transition is found to exist alongside a charge density wave ordering. The temperature dependence of radio frequency penetration depth using high resolution tunnel diode oscillator technique demonstrates s-wave properties in the weak coupling limit. This is in contrast to the predicted of p_x + ip_y type order parameter symmetry for topological superconductivity. Additionally, 2H-TaSeS is seen to be a typical superconductor based on the studies of critical fields. Upper critical fields exhibit moderate electronic anisotropy with a value of 1.52. DFT simulations further support the likelihood of superconducting behavior in TaSeS and indicate that the P6₃mc space group has the most stable structure. Phonon dispersion curves with negative values confirm the possibility of existence of CDW in 2H- TaSeS. Overall, all studies on single crystals of TaSeS shows conventional type-II superconductivity without any indication of topological characteristics.   

Ab initio quantum chemistry of high-temperature superconductors

We developed an ab initio quantum chemistry framework for faithful simulations of high-temperature superconductors (cuprates). The method allows for spin SU(2) and particle-number U(1) symmetry-breaking states such that the superconducting orders spontaneously emerge during the self-consistency. We directly computed the superconducting pairing order of several doped cuprate materials and structures. We found that we could correctly capture two well-known trends: the pressure effect, where the pairing order increases with intra-layer pressure, and the layer effect, where the pairing order varies with the number of copper-oxygen layers. From these calculations, we observed that the strength of superexchange and the covalency at optimal doping are the best descriptors of the maximal pairing order. Our microscopic analysis further identified short-range copper spin fluctuations, together with multi-orbital charge fluctuations, as central to the pairing trends. Our work demonstrates the possibility of a quantitative computational understanding of high-temperature superconducting materials.   

Magnetically mediated superconductivity in the pressurized Nickelate La3Ni2O7

With the discovery of high-temperature superconductivity in cuprates, understanding pairing mechanisms in strongly correlated phases of matter has prevailed as one of the most challenging problems in contemporary condensed matter physics. In particular, detailed microscopic insights into the relevant physics can open the way towards a targeted design of novel materials with high critical temperatures at ambient conditions. Very recently, the Ruddlesen-Popper bilayer perovskite nickelate La₃Ni₃O₇ (LNO) has joined the family of bulk superconductors above the boiling point of liquid nitrogen, whereby extraordinarily high critical temperatures (T_c) of 80 K at applied pressures above 14 GPa have been reported [1]. It has been argued that the low-energy physics of LNO can be described by the single band, strongly correlated mixed dimensional bilayer t-J model [2,3]. Our investigation of this bilayer system, utilizing density matrix renormalization group techniques, establishes an intricate understanding of the model and the magnetically induced pairing through comparison to the perturbative limit of dominating inter-layer spin couplings. In particular, this allows us to explain appearing finite-size effects as well as the role of all coupling parameters in the Hamiltonian, and we make predictions for binding energies and spin gaps in the two-dimensional limit. We estimate critical temperatures of the Berezinskii-Kosterlitz-Thouless transition in the perturbative regime, and discuss possible implications for the fate of T_c of LNO.

[1] Sun et al., Nature 621 (2023)

[2] Qu et al., arXiv:2307.16873 (2023)

[3] Oh et al., arXiv:2307.15706 (2023)   

Thermodynamics of superconducting transition in a metal with a purely repulsive interaction

Superconductivity may arise in a system with a purely repulsive frequency-dependent electron interaction as the self-consistency equation can be solved by a superconducting gap function that changes sign as a function of frequency. Recently, it was shown that this solution disappears as the constant repulsive part of the interaction is increased, which establishes a novel type of the superconducting quantum phase transition in the absence of disorder. In our work, we study thermodynamics of this system at small temperatures right above the critical point. We identify a logarithmically singular part of the interaction, which appears in the Cooper channel and study its impact on the specific heat of the system. The latter has a weak logarithmic singularity as a function of temperature above the quantum phase transition.   

Thermal activation energy for Cuprates from a Multilayer Boson-Fermion Model

In the mixed state, Type II superconductors, such as cuprates and pniptides, which present a cuasi-2D layered structure, permit a vortex flux flow penetration in the presence of an external magnetic field B, arranged in the form of an Abrikosov net.  However, between the lower critical field H_c1 and the upper critical field  H_c2 and near T_c , thermal fluctuations directly affect the vortex motion (TAFF) [1], so a thermal activation energy  U(T,B) must be overcome to allow flux motion and changing the  resistivity.

Here we report the thermal activation energy using the expression U(T,B)=Φ²₀H_c(T)/(2√2πλ(T)μ₀H), which is obtained combining the phenomenological relation from Yeshurun et al. with the G-L relation  H_c(T)=Φ₀/(2√2πλ(T)ξ(T)) [2], where we use the expressions for the coherence  ξ(T) and penetration λ(T)  lengths previously obtained applying the Multilayer Boson-Fermion model of superconductivity [3] for underdoped cuprates.

The comparison of our results with the experimental data show a better agreement than the ones reported in the literature using other models.

1. Hettinger, et al., PRL 62, 2044 (1989).

2. Tinkham, PRL 61, 1658 (1988).

3.  P. Salas et al. Physica C 524, 37 (2016).   

Ab initio Modeling of Superconducting Materials by Exploring Excited Electronic Configurations

High pressure hydrogen sulfide is a great candidate for industrial applications as a high temperature superconductor such as being used for frictionless railway systems and zero-resistance circuits at power plants as a way to efficiently transfer current. Hydrogen sulfide keeps its superconducting properties up to 200K when at extreme pressure. We mimic the electronic configuration features of this material relevant to superconductivity which includes constraints to spin projection, momentum by using spin polarized calculations and including momentum dispersion via k-points. K-points are included such that we can mimic a large segment of a material while explicitly simulating just a single unit cell.

This research shows that Cooper pairs are available to form in the sulfur trihydride material and can serve as the reason for superconductivity. Cooper pairs are formed when two electrons have opposite values of spin and momentum interacting via the distortion of the lattice. We computed the total energy for a range of relevant multi electronic configurations and compared it to the total energy of the reference neutral singlet configuration. We then introduce nuclear reorganization as part of our calculations.

We find that the energy of the certain excited states are more energetically favorable than the energy of the ground state. These more energetically favorable states are the states where Cooper pairs can form which is the primary reason for superconductivity.   

Control and manipulation of superconducting vortex lattices from nano to mesoscales

Single and few vortex manipulation opens a pathway to understand vortex properties and to tailor their dynamics to prospective applications in quantum computing. We will discuss two experiments using scanning tunneling microscopy (STM) which demonstrate direct control over vortex positions in FeSe superconductor. First, we observed that the twin boundary in the FeSe superconductor traps a relatively high density of vortices and acts as a barrier that aligns the vortices on the terrace parallel to the twin boundary. The alignment effect causes various phases of vortex lattice structures such as rectangular and one-dimensional vortex lattices – both with ordering qualitatively different from the commonly observed vortex glass [2]. Second, we found that the vortex shape can be controllably varied with the imaging conditions in STM, particularly at the extreme limit of very large local current density. We attribute the observations as direct evidence of vortex manipulation, by contrasting the behaviors of various types of vortices in proximity to twin boundaries on FeSe. Altogether we suggest that precise control over the high tunneling current, combined with specific structural topological defects, can translate into an effective strategy for vortex manipulation approach without destruction of the superconducting state, enabling STM to become a quantitative nanoscale probe of vortex dynamics and a platform to explore vortex manipulation in the context of topological quantum computing.   

Effect of Electron Irradiation on the Performance of Superconducting Devices

Low-energy electron irradiation can significantly impact the performance of superconducting devices, including short-term rise in quasiparticle population as well as long-term changes to the surface electronic structure and morphology. Here, we explore the influence of low-energy electron beams on the performance of microfabricated superconducting Nb and Ta microwires in both in-situ and ex-situ conditions. The irradiation is implemented in a custom-made ultrahigh vacuum reaction system at electron energies ranging from 500eV to 20 keV. The substrate temperature during the irradiation can be varied from 350 to 7 K using a closed-cycle GM cryocooler. Variations in microwires' resistance and critical current are monitored in-situ using a high-speed oscilloscope and lock-in amplifiers. This is complemented by ex-situ characterization of microwires' superconducting parameters and atomic-scale surface morphology. The information on the behavior of the superconducting microwires as a function of electron energy, fluence, and irradiation temperature can provide valuable insights into the application of superconducting devices in environments with medium to high probability of electron radiation.   

Two-tone spectroscopy of decoherence-causing defect in superconducting devices

Despite fast progress towards scalable superconducting quantum computing (QC) systems, one of the main detrimental factors hindering further advancement is the loss caused by so-called two-level system defects (TLS) which are believed to be hosted in amorphous native oxides on the surfaces of superconducting CQ devices.

Good phenomenological understanding of TLS is provided by standard tunneling model, and recent experiments with various superconducting materials and fabrication approaches led to improvements of QC devices. Yet, proper detailed characterization and understanding of microscopic nature of TLS is lacking.

We propose and demonstrate a new experimental technique to characterize spectroscopic features of TLS defects. To do that we use 3.9 GHz multi-cell 3D resonators and two-tone spectroscopy where we pump one of the pass-band modes of the 3D cavity with continuous wave signal, and probe internal quality factors of the other modes which are close in the frequency domain. Our measurements allow us to estimate the spectroscopic linewidth of TLS defects hosted in amorphous oxide layers of superconducting materials. This information allows to optimize fabrication procedures and choose optimal materials for high-coherence QC devices.   

Numerical implementation of the Eliashberg equations on the imaginary axis at strong coupling

In the presence of electron-phonon coupling, phonon dynamics within a superconductor give rise to an electronic retardation effect, which in turn results in a frequency-dependent order parameter. In Eliashberg theory, the frequency-dependent pairing and renormalization functions are determined self-consistently by solving the Eliashberg equations, which take into account the dynamical phonon-mediated interaction. We review and extend the numerical implementation of the finite-temperature Eliashberg equations on the imaginary axis, highlighting the technical difficulties in calculating the pairing function away from the weak-coupling regime. We similarly review the numerical calculation of physical quantities of interest, such as the critical temperature, and make comparisons with applicable results in the literature.   

Superconductivity and magnetism in the surface states of ABC-stacked multilayer graphene

ABC-stacked multilayer graphene (ABC-MLG) exhibits topological surface flat bands with a divergent density of states, leading to many-body instabilities at charge neutrality. Here, we explore electronic ordering within a mean-field approach with full generic treatment of all spin-isotropic, two-site charge density and spin interactions up to next-nearest neighbor (NNN) sites. We find that surface superconductivity and magnetism are significantly enhanced over bulk values. We find spin-singlet s wave and unconventional NNN bond spin-triplet f wave to be the dominant superconducting pairing symmetries, both with a full energy gap. By establishing the existence of ferromagnetic intra-sublattice interaction, (J2 < 0) we conclude that the f-wave state is favored in ABC-MLG. We trace this distinctive surface behavior to the strong sublattice polarization of the surface flat bands. We also find competing ferrimagnetic order, fully consistent with density functional theory (DFT) calculations. The magnetic order interpolates between sublattice ferromagnetism and antiferromagnetism, but only with the ratio of the sublattice magnetic moments (R) being insensitive to the DFT exchange correlation functional. By constraining the interactions to the DFT R-value we establish the phase diagram and find f-wave superconductivity being favored for all weak to moderately strong couplings J2 and as long as J2 is a sufficiently large part of the full interaction mix. Gating ABC-MLG away from charge neutrality further enhances the f-wave state over the ferrimagnetic state, establishing ABC-MLG as a strong candidate for f-wave superconductivity.   

Signatures of a charge-density wave quantum-critical point in superconducting 2H-TaS2-x induced by disorder

Many superconductors exist in close proximity to various forms of electronic order, particularly in unconventional superconductors and transition-metal dichalcogenides, where charge-density waves (CDWs) and superconductivity can coexist and compete. Anomalous electrical transport behaviour is often exhibited when such superconductors are tuned to a quantum critical point where superconductivity is optimized. Despite extensive research efforts, the origin of such strange-metal behaviour remains a mystery. Here we report the evolution of long-range CDW and superconductivity in 2H-TaS_2-x with various levels of disorder induced by sulfur vacancies. Measurements of complementary magnetization, electronic, and thermal transport properties show that the long-range CDW is continuously suppressed, leading to strange-metal behaviour with linear resistivity at the endpoint of the long-range CDW, which is accompanied by the emergence of a short-range CDW phase. The superconductivity shows at first a two-step-like behaviour but reaches a maximum at the endpoint of long-range CDW with a single homogeneous phase, suggesting an interplay between superconductivity and CDW order. Moreover, our results suggest that the strange-metal behaviour, which could arise from the short-range charge density fluctuations, is a signature of quantum criticality with Planckian dissipation.   

Absence of vestigial time-reversal symmetry breaking above $T_c$ in the strong-coupling limit of a twisted bilayer superconductor

Phase ordering in a 2D superconductor occurs via a BKT transition. However, in bilayer systems with vanishing first-order interlayer Josephson coupling, the superconducting phase can also exhibit spontaneous time-reversal symmetry breaking (TRSB) due to the locking of the phase difference $phi$ across the junction via a second-order Josephson coupling $J_2 cos (2phi)$ with a sign that favors $phi = pm pi/2$. In particular, in unconventional superconductors (SC), the first-order term $-J_1( heta) cos phi$ can be tuned to vanish via twisting the relative orientation (e.g., by  $ heta=pi/4$ in a $d$-wave SC), so that the second order coupling $J_2 cos (2phi)$ becomes the dominant interlayer interaction. However, it remains unsettled whether TRSB occurs above the BKT transition in the normal state ( extit{vestigial order}). Here we show that in the strong-coupling (large $J_2$) limit, 

the TRSB onsets precisely at the superconducting transition when $J_1= 0$, and at a lower critical temperature when $J_1>0$.   

Pair density wave state in the square lattice t-J model

Accumulating experimental evidence supports the existence of the pair density wave (PDW) state in cuprate superconductors. In such states the singlet pairing order parameter varies periodically with zero spatial average. The square lattice t-J model, which is believed to capture the essential physics of cuprates, is studied in this work using the fermionic tensor network approach, with focus on the small doping region ($delta$ < 0.10) and uniform states with bipartite structure. Using the simple update algorithm to perform imaginary time evolution on PEPS states, in addition to the usual $d_{x^2 - y^2}$-wave superconducting states, we also obtain PDW states with comparable energy.   

The search for ENZ and superconductivity near metal-insulator transition in Fe-B compounds

Recently, possible superconducting phase was reported in the thin film compositional spreads of FeB. In our study, thin films of two different FeB composition spreads were grown by co-sputtering to determine the conditions for appearance of superconductivity in these compounds. Previously, we have shown that metamaterial engineering can enhance superconductivity by creating materials with small dielectric constant (epsilon near zero, or ENZ). In a FeB compositional spread, we have found ENZ behavior in the vicinity of metal-insulator transition (MIT), when concentration of boron increases in the film. The results of our study of the transport, magnetic, and optical properties for these FeB compositional spreads will be shown. Correlation of MIT and ENZ behavior and their role in possibility of appearance of superconductivity will be discussed.   

Determining the Energy Gaps of Assymetrical All-MgB2 Thin Film Josephson Junctions

We have previously measured at very high resolution the momentum-dependent substructure within the two superconducting energy gaps (σ and π) of Magnesium diboride (MgB2 ) using tunneling spectroscopy of MgB2 heterojunctions [1,2]. In this presentation, we report tunneling spectroscopy measurements of all-MgB₂ thin film Josephson junctions. These were fabricated using two c-axis MgB2 films with very small contribution from the larger σ gap. The two MgB electrodes on either side of the junction show different critical temperatures and energy gap values. We attribute this to differences in growth conditions. Using a modified tunneling model where each electrode is represented as a weighted sum of two BCS densities of states, we analyze the differential conductance results we obtained. Our results show (1) a temperature-resolved transition from SIS to NIS behavior and (2) the presence of multiple quasiparticle peaks due to the sums and differences in pairwise combinations of π and σ gap values within each electrode.

¹“Energy Gap Substructures in Conductance Measurements of MgB₂-based Josephson Junctions” , S. Carabello, et al., Supercond. Sci. Technol. 28 (2015) 055015.

² "Momentum-dependent multiple gaps of MgB₂ probed by electron tunneling spectroscopy on MgB₂/native oxide/Pb junctions”, K. Chen, et. al, Nat. Commun. 3 : 619 doi: 10.1038/ncomms1626 (2012).   

Superconducting energy gap and Critical Temperature of Cuprates from the Multilayer Boson-Fermion Formalism

High temperature superconductors (HTSC) which present a cuasi-2D multilayer structure, such as cuprates, are not completely described by the BCS theory. Here we use the Boson-Fermion (BF) [1] formalism adjusted to incorporate multilayers [2] to calculate the properties of cuprates as a mixture composed by electrons as fermions and electron Cooper pairs (CP) as composite bosons. The multilayers are generated by applying a Dirac Comb Potential in the orthogonal direction with respect to the layers, thus the bosons and fermions satisfied the Kronig-Penney relation [3]. In X-Y planes, bosons have a linear energy-momentum relation [4] given by ε(K) = 2 E_F,2 - Δ₀ + c K_ρ, where E_F,2 is the Fermi energy in 2D, Δ₀ the energy gap and c a constant, while the electrons have the energy ε(k) = ħ²k²/2m_e, with m_e the electron mass. With the energy spectrum of the BF mixture we are able to calculate the energy gap, critical temperature and chemical potential, as functions of the plane impenetrability P and the separation a among them, by solving simultaneously the energy gap-like equation and the number equation. We compare our results with experimental cuprate data.

1. V.V. Tolmachev, Phys. Lett. A 266, 400 (2000); M. de Llano & V.V. Tolmachev, Physica A 347, 546

2. P. Salas et al. Physica C 524, 37 (2016)

3. R. de L. Kronig & W. G. Penney, Proc. Roy. Soc. (London) A 130, 499 (1931)

4. S.K. Adhikari, et al. Phys. Rev. B 62, 8671 (2000).   

Construction of a Cooper-Pair Heterostructure

Among unconventional superconductors, Sr₂RuO₄ (SRO) has attracted tremendous attention due to the potential application in quantum computing. Although discovered for 30 years, a lot of mysteries, including order parameters are still not resolved. One of the reasons is that SRO is notoriously sensitive to even the smallest amount of impurities and dislocations. For example, even a slight surface reconstruction of SRO is sufficient to suppress the superconducting state, imposing great challenges to surface sensitive studies such as STM and ARPES to reveal its order parameter. For the same reason, a two-dimensional (2D) state in SRO is not accessible so far. In this work, we present the construction of a Cooper-pair heterostructure Ba:SRO/SRO/Ba:SRO, which possess the same single electron state throughout. The minimal Ba doping almost does not affect crystal lattice or single-electronic structure of SRO above T_C, but locally breaks Cooper pairs below T_C, leaving behind a thin layer of superfluid in the middle layer of the heterostructure. We accessed a 2D superconducting state in these heterostructures and observed a resistive regime in the temperature-field phase diagram, due to the vortex liquid state, as a signature of the 2D superconductivity. In contrast to conventional 2D superconductors, the vortex liquid state in this case is very robust against magnetic field. We attribute such robustness to the very large vortex sizes due to the unconventionality of SRO.   

Superfluid weight of the superconducting condensate on a mirror asymmetric honeycomb lattice

The superfluid weight is the second derivative of superconducting free energy. A positive definite superfluid weight reflects a stable ground state. We calculate the superfluid weight of the BCS s-wave superconducting condensate as a function of the Rashba spin-orbit coupling in a mirror asymmetric honeycomb lattice. Mirror asymmetry introduces Rashba spin-orbit coupling, leading to the admixture of singlet and triplet order parameters. This model calculation provides insight into the possibility of finite momentum pairing states in mirror asymmetric honeycomb lattice-based 2D superconductors.   

Phonon state tomography probes of optically excited phonon-induced electron dynamics

Optical driving of quantum materials has opened a door for new modalities for engineering and controlling novel states of matter enabled by access to the excitation spectrum far away from equilibrium. One important class of experiments involves the optical excitation of specific vibrational modes (phonons) which, because of the dipole selection rules, can only couple nonlinearly to the electron density. A theory of these systems requires understanding the combined electron and phonon subsystems. In this poster, we propose a method, dubbed phonon state tomography (PST), in order to statistically unravel the electronic dynamics in terms of contributions from different phonon configurations. To demonstrate the effectiveness of this approach, we consider a model of a photo-pumped metal whose phonons are excited at an initial time by an optical pulse and show that i) spin and charge exhibit opposite trends as a function of the average phonon number, and ii) charge correlations increase with raising the width of the pump pulse. Thus, this technique may serve as a diagnostic tool for the phonon-induced electron dynamics in pump-probe experiments.   

Detecting Emergent Phases in Quantum Materials using High Harmonic Based Time and Angle Resolved Photoemission Spectroscopy

Time and angle resolved photoemission spectroscopy (tr-ARPES) is a powerful tool to capture the band structure in 1D, 2D and topological materials with energy, momentum and time resolution. By gently exciting a quantum material with an ultrafast laser pulse and probing it with extreme ultraviolet high harmonic pulses, we measure not only the response times, but also see precisely how the electronic band structure changes. In combination, this makes it possible to identify the many body interactions underlying the ground state of quantum materials and thereby the nature of the pseudogap.[1]   

Poster: Discovery of a single-band Mott insulator in a van der Waals flat-band compound

Many-body quantum phenomena can arise when the Fermi level lies within a flat band, because electron-electron interactions greatly exceed the quenched kinetic energy of electrons. For example, Mott insulator-like states and superconductivity at low temperatures were discovered in magic-angle twisted bilayer graphene with flat bands near the Fermi level. However, these low-energy bands are complex and exhibit multi-band features, making experimental verification and theoretical analysis challenging. Here, we report a single-band Mott insulator state in a van der Waals layered compound Nb₃Cl₈. In the high-temperature phase, this Mott insulator state can be ideally described by the typical single-band Hubbard model based on an isolated half filled flat band. The excellent agreement between experiment and theory unambiguously confirms that monolayer Nb₃Cl₈ is a single-band Mott insulator. Further, we revealed that the Hubbard bands form bonding/anti-bonding states in the low-temperature phase, leading to a non-magnetic spin-singlet state as its ground state, in which the Mott-Hubbard physics continues to dominate the insulating electronic state. The Mott insulator state in Nb₃Cl₈, which involves only one single low-energy band, provides an excellent platform for studying the intrinsic physics of the single-band Hubbard model. In the future, additional correlated states and exotic physical phenomena can be identified by tuning the Mott insulator state.   

Renormalization Group Equation of the Disordered Two-Dimensional Electron Gas with Particle-Hole Asymmetry

The non-linear sigma model (NLSM) is a powerful tool for studying the transport properties of disordered electron systems. However, the conventional NLSM cannot be used to study thermoelectric transport because it is particle-hole symmetric, while the thermoelectric transport coefficient is sensitive to particle-hole asymmetry. To overcome this limitation, a generalized NLSM with particle-hole asymmetry was recently developed. This generalized NLSM has already been used to calculate perturbatively the effects of interactions and disorder on the thermoelectric transport coefficient. A renormalization group (RG) analysis can provide further insights into the thermoelectric transport properties at low temperatures. In this project, we perform an RG analysis of the generalized NLSM to understand thermoelectric transport on the metallic side of the metal-insulator transition in the two-dimensional disordered electron liquid.   

Generalized multifractality at metal-insulator transitions and in metallic phases of two-dimensional disordered systems

We study generalized multifractality characterizing fluctuations and correlations of eigenstates in disordered systems of various symmetry classes. Both metallic phases and topological or nontopological Anderson-localization transitions are considered. By using the nonlinear sigma-model approach, we construct pure-scaling eigenfunction observables. The construction is verified by numerical simulations of appropriate microscopic models, which also yield numerical values of the corresponding exponents. In the metallic phases, the numerically obtained exponents satisfy Weyl symmetry relations as well as generalized parabolicity (proportionality to eigenvalues of the quadratic Casimir operator). At the same time, the generalized parabolicity is strongly violated at critical points of metal-insulator transitions, signaling violation of local conformal invariance. Moreover, in classes D and DIII, even the Weyl symmetry breaks down at critical points of metal-insulator transitions. This last feature is related to a peculiarity of the sigma-model manifolds in these symmetry classes: they consist of two disjoint components. Domain walls associated with these additional degrees of freedom are crucial for ensuring Anderson localization and, at the same time, lead to the violation of the Weyl symmetry.   

Metal-insulator transition of SrIrO3 thin film by SrTiO3 capping layer

When a SrIrO₃ thin film is deposited on a SrTiO₃ substrate using the PLD (Pulsed Laser Deposition) method, the temperature-dependent resistivity behavior is 'insulator-like', 'metal-like', or 'intermediate' depending on the deposition conditions. This is because the tilting angle of the Ir octahedron changes depending on the deposition conditions. When the SrIrO₃ thin film exceeds a certain thickness, its resistivity is small and its temperature-dependent resistivity behavior is insulator-like. However, if the thickness is small, the resistivity increases significantly and it becomes an insulator. Here we growth a SrTiO₃ capping layer on top of this SrIrO₃ thin film. When the thickness of the SrIrO₃ layer is large, there is no change due to the capping layer. However, when the thickness of the SrIrO₃ layer is small, the resistivity is greatly lowered due to the capping layer and shows metal-like resistance behavior. To investigate this more precisely, we systematically investigated the temperature-dependent resistivity behavior according to the thickness change of SrIrO₃. The thickness of the SrTiO₃ capping layer was fixed, and the resistivity was measured by varying the thickness of the SrIrO₃ thin film. When the thickness of SrIrO₃ is 9 to 20 unit cells, it showed insulator-like behavior. When it was 7 to 8 unit cells, it showed intermediate behavior. And when it was 1 to 6 unit cells, it showed metal-like behavior. Additionally, the “metal-like” SrIrO₃ thin film was not changed by the SrTiO₃ capping layer. We aim to determine the effect of SrTiO₃ capping layer on changing the resistivity of SrIrO3 thin films.   

Discovery of charge order in a cuprate Mott insulator

Cuprate superconductors exhibit multiple interacting electronic phases. Among them, charge/spin stripes and incommensurate charge density waves (CDWs) have been experimentally observed In underdoped cuprates with correlated metallic ground states. Here, we search for signatures of CDW order in very lightly hole-doped cuprates from the RBa₂Cu₃O_7 − δ family (RBCO; R = Y or rare earth) by using resonant X-ray scattering, electron transport, and muon spin rotation measurements to resolve its electronic and magnetic ground states. Specifically, we use Pr to substitute Y at the R-site to systematically underdope YBCO and access the insulating regime of the cuprate phase diagram without changing the oxygen stoichiometry of YBCO. Resonant X-ray scattering data on Pr-doped YBCO thin films reveal an in-plane CDW order with a peak onset temperature (T_CDW) above 300 K, similar to the T_CDW of the 3D charge order seen by Ruiz et al. in this system [1]. This in-plane CDW follows and extends the linear evolution of the wave vector versus hole concentration relationship present in oxygen-underdoped YBCO [2] all the way to the insulating and magnetically ordered Mott limit. Combined with the recent observation of the charge crystal phase on an insulating surface of Bi₂Sr₂CaCu₂O_8 + z [3], our results in RBCO suggest that this electronic symmetry breaking is universally present in very lightly doped CuO₂ planes. These findings bridge the gap between the Mott insulating and underdoped metallic states and underscore the prominent role that Coulomb-frustrated electronic phase separation plays among all cuprates.

[1] A. Ruiz et al. Nat. Comm. 13, article 6197 (2022)

[2] S. Blanco-Canosa et al. Phys. Rev. B 90, page 054513 (2014)

[3] H. Zhao et al. Nat. Mat. 18, page 103 (2019)   

Spin-polarized tunneling into a topological Kondo insulator

Motivated by a recent experiment [1], we propose a phenomenological model of spin-polarized tunneling of electrons into a heavy fermion Kondo lattice [2]. Specifically, we elaborate the crucial role of topological surface states that are naturally present at the interface between a topological Kondo insulator (TKI) and a normal metal, in driving a spin-selective tunneling current upon an external bias. Utilizing a self-consistent parton mean-field construction [3], we provide a qualitative description of the underlying mechanism, by focusing on the peak structure in the differential conductance. We discuss the application of this theory applied to various scanning tunneling microscope (STM) experiments where the STM tip is replaced by a TKI [4]. Unlike conventional spin-polarized STMs, the spin selectivity is voltage driven and does not depend on a magnetic tip.

[1] A. Aishwarya, et al., Science 377, 1218 (2022)

[2] M. Maltseva, M. Dzero, and P. Coleman, Phys. Rev. Lett. 103, 206402 (2009)

[3] Victor Alexandrov, Maxim Dzero, and Piers Coleman, Phys. Rev. Lett. 111, 226403 (2013)

[4] Maxim Dzero, Jing Xia, Victor Galitski, and Piers Coleman, Ann. Rev. Condens. Matter Phys. 7, 249 (2016)   

Investigation of carrier density in Ce1-xPrxOs4Sb12 (x = 0.1, 0.2)

The filled skutterudite CeOs₄Sb₁₂ is a heavy-fermion compensated semimetal that exhibits a spin-density-wave type antiferromagnetic phase below ~1 K. Theoretical predictions suggested that it may possess a topologically protected state where electron and hole Fermi surfaces coexist at low temperatures. Through previous studies of this compound have revealed a near spherical Fermi surface above 28 T, an enhancement of cyclotron mass below 35 T, and a valence transition with a striking reverse wedge-shape temperature (T) versus magnetic field (H) phase diagram [1,2,3]. Substituting the rare-earth element Pr for Ce introduces hole-doping, while applying pressure acts like electron-doping in CeOs₄Sb₁₂. In this report, we present the effects of Pr substitution on the electronic properties, evolution of T-H phase boundaries of the valence transition, and carrier density in Ce_1-xPr_xOs₄Sb₁₂ (x = 0.1 and 0.2) before superconductivity emerges.

Refs: [1] K. Götze et. al, New J. Phys. 24, 043044 (2022). [2] K. Götze et. al, PRB 101, 075102 (2020). [3] P.-C. Ho et. al, PRB 94, 205140 (2016).   

Origin of Ferromagnetism in the Copper(II) Triangle NMe4[Cu3(μ3-F)(TFA)6(Py)3]

Magnetic interactions in transition metal compounds are defined by the Goodenough-Kanamori rules. Experimental magnetic susceptibility and high-field electron paramagnetic resonance experiments on the triangular molecular magnet NMe₄[Cu₃(μ₃-F)(TFA)₆(Py)₃] [1] reveal dominant ferromagnetic spin-spin exchange as well as the importance of anisotropy in the system. Based on the Goodenough-Kanamori rules one would expect anti-ferromagnetic exchange interactions. Density functional theory and ab initio full multiplet ligand-field theory calculations confirm the conclusions drawn from the Goodenough-Kanamori rules, contradicting experimental observations. Only once correlations on the central ligand fluorine ion are considered within the theory we predict the magnetic interactions in agreement with experiment. Our results show the importance of correlations on the ligands for the understanding of magnetic interactions. These results should be generally valid and we discuss when correlations on the ligands change the magnetic interactions in transition metal compounds like Fe₃O₄ or doped H_Tc cuprate superconductors.

[1] Tereshchenko, D.S., Buzoverov, M.E., Glazunova, Russ. J. Inorg. Chem. (2023). https://doi.org/10.1134/S0036023623601666   

Revealing the unique domain boundary structure and topological defects of spin-density-wave state

Understanding the atomic-scale structure of magnetic domain walls is of fundamental importance for both the basics and applications of magnetism. Although the domain wall structures of local-moment magnets are well studied, little is known about those of itinerant magnetism, such as the spin-density-wave (SDW) state. Here by using scanning tunneling microscopy, we studied the domain wall structure of the SDW in Cr and its coexisting CDW state. On the Cr (001) surface, two single-Q (stripe-like) SDW/CDW domains with their wave vector Q perpendicular to each other are identified. On their domain boundaries, we observed grid-like CDW modulations. Detailed analysis shows that such modulations are not from the direct adding of two CDW stripes, but are the result of the coherent superposition of two single-Q SDW. This means a unique double-Q SDW state is formed at the domain wall. Moreover, the observed domain wall has a finite wall width, indicating an energy cost of forming a double-Q SDW state. Our simulation shows that the single-Q state exponentially decays when entering the double-Q region, which is a characteristic of the itinerant nature of SDW. Further, we observed topological defects of SDW state. Our work not only discovered a new type of magnetic domain wall which differs completely from local-moment magnetism, but also brings insights into the microscopic mechanisms of SDW state.   

Role of Spin-orbit Scattering in Complex Magnetoresistance behaviour in Mn intercalated NbS2 compound: Mn1/4NbS2

In this report, the structural, magnetic and transport properties of single crystals of Mn-intercalated NbS₂ compound i.e., Mn_1/4NbS₂ through X-ray diffraction, magnetization, resistivity and magnetoresistance are presented. The X-ray diffraction has confirmed the phase purity and single phase of Mn_1/4NbS₂ single crystals. Mn_1/4NbS₂ displays a ferromagnetic to paramagnetic transition around 104 K. The metallic behaviour of as-synthesized crystals is confirmed by four terminal longitudinal resistivity measurements. The transverse magnetoresistance under perpendicular applied magnetic field exhibit a negative magnetoresistance with the highest value obtained around -23% at 6 T and 120 K. Further, the interplay between negative and positive magnetoresistance with the increasing magnetic field is observed and explained in terms of competition between quantum-based time scales where the spin-orbit scattering comes into play. We observe a sharp cusp-like feature in the weak field region at low temperatures implying the localization effects in Mn_1/4NbS₂.   

Ground State Properties of an Interacting Bose gas Within an Imperfect 1D Crystal

For a weakly interacting boson gas within an imperfect one-dimensional crystal, we report the ground state properties as functions of the vacancy number in the crystal. To do this, we solve the corresponding Gross-Pitaevskii equation using the “Gradient Flow with Discrete Normalization”method, also known as the imaginary time method. We model the imperfect one-dimensional crystal as a Dirac Comb potential with several deltas removed at random.

We know that, in the case of a non-interacting Bose gas (IBG) within a 1D perfect crystal, the presence of a single vacancy [1] modifies the Bose gas energy spectrum by lowering only the bottom edge energy, which generates an energy gap between the ground state and the first excited state of the new energy spectrum. The gap magnitude is a function of the intensity of the removed delta. When we turn on the interactions between bosons within a 1D perfect crystal, the ground state energy (GSE) increases proportionally to the magnitude of the interaction. When we introduce the vacancies for a given interaction g, the GSE decreases almost linearly as a function of the vacancy number up to a value g [ ℏ²/2ma²] when we remove all the deltas. We also give the chemical potential as a function of the vacancy number and show some probability density functions for some vacancy number given an interaction g.

[1] E. I. Guerrero et al., March Meeting 2023 (G00.00382).   

Toward realization of the SYK model in graphene: Theory

Sachdev-Ye-Kitaev is a paradigmatic model relevant from both condensed matter and gravity perspective. We discuss detailed numerical modelling on interactions following the Sachdev-Ye-Kitaev (SYK) framework in disordered graphene flakes up to 300 000 atoms in size (∼100 nm in diameter) subjected to an out-of-plane magnetic field B of 5–20 Tesla within the tight-binding formalism. We investigate two sources of disorder: (i) irregularities at the system boundaries, and (ii) bulk vacancies—for a combination of which we find conditions that could be favorable for the formation of the phase with Sachdev-Ye-Kitaev features under realistic experimental conditions above the liquid helium temperature. This talk complements the experimental talk by L. E. Anderson and colleagues,   "Toward realization of the SYK model in graphene".   

Repulsive electron bound pairs in a two-leg ladder

Cooper pair is the basic ingredient for superconductivity and it is of great interest to understand the pairing mechanisms in different conditions. The experimental formation of bound two-atom pairs with a repulsive potential [1] was explained by analyzing the Hubbard Hamiltonian of two-boson states [2,3] in a 1D chain. In this work, we solve the two-electron repulsive bound states in a two-leg ladder within the Hubbard Hamiltonian. We found three bound electron pair bands, two bands have energies E≈U and properties very similar to the bound pairs found in [3] but a third band has surprisingly energies E≤0 for any value of U>0. The band structure and wavefunctions of the bound pairs are presented. We show the conditions of t, t' and U needed to find bound pairs with energies E<0 that are not the sum of two non-interacting electrons.

[1] K. Winkler, G. Thalhammer, F. Lang, R. Grimm, J. Hecker Denschlag, A. Daley, A. Kantian, H.P. Buchler, and P. Zoller, Repulsively bound atom pairs in an optical lattice, Nature, 441 (2006) 853.

[2] ] M. Valiente, and D. Petrosyan, Two-particles states in the Hubbard model, J. Phys. B, 41 (2008) 161002.

[3] M. Di Liberto, A. Recati, I. Carusotto, and C. Menotti, Two-body physics in the Su-Schrieffer-Heeger model, Phys. Rev. A, 94 (2016) 062704.   

Maximally Localized Wannier Functions, Interaction Models and Fractional Quantum Anomalous Hall Effect in Twisted Bilayer MoTe2

We investigate the moir'e band structures and the strong correlation effects in twisted bilayer MoTe$_2$ for a wide range of twist angles, employing a combination of various techniques. Using large-scale first principles calculations, we pinpoint realistic continuum modeling parameters, subsequently deriving the maximally localized Wannier functions for the top three moir'e bands. Simplifying our model with reasonable assumptions, we obtain a minimal two-band model, encompassing Coulomb repulsion, correlated hopping, and spin exchange. Our minimal interaction models pave the way for further exploration of the rich many-body physics in twisted MoTe$_2$. Furthermore, we explore the phase diagrams of the system through Hartree-Fock approximation and exact diagonalization. Our two-band exact diagonalization analysis underscores significant band-mixing effects in this system, which enlarge the optimal twist angle for fractional quantum anomalous Hall states.   

Enabling and suppressing quantum growth of Cu(111) nanostructures

Under certain conditions, the energy associated with quantum well states was found to play a critical role in determining the geometric structure of Cu(111) clusters grown on the surface of MoS₂ . This happens when depositions occur on a freshly prepared surface. The wave nature of electrons causes the total energy of the cluster to oscillate with thickness. Island stability increases as quantum states shift farther below the Fermi level, lowering the energy of the cluster. We believe the energy from the quantum states contributes significantly to the total energy due to the weak van der Waals forces at the Cu/MoS₂ interface, which reduces stress without the need for a wetting layer or significant lattice matching. The abrupt van der Waals gap at the interface also gives rise to strong electronic confinement, ideal for the formation of quantum well states. Subsequent depositions cause the Cu clusters to grow in a linear fashion with no evidence of any quantum growth. We believe this is due to the existence of previously deposited clusters hindering additional Cu atoms from reaching their lowest quantum energy state.   

Quantum magnetism meets cavity QED

Light-matter interactions are fundamental for both studying and

controlling quantum materials. It is of particular interest to consider

the case of nonclassical light, which can increase the precision of

measurements as well as lead to new phenomena. A particular example is

the quantum battery which couples multiple charge units coherently to a

cavity. With the cavity-enhanced light-matter interactions, the

electrons form polariton states with coherent photons.

A paradigmatic model for light-matter interaction is the Dicke model,

which exhibits a transition between the normal and superradiant phases.

A natural progression is to combine the Dicke model with interacting

spin models. We investigate a family of spin models with increasing

complexity from Ising to compass models. Leveraging the recently

developed variational non-Gaussian ED (NGSED) method, we unveil the

effects of spin interactions within the normal-superradiant phase

diagram. Moreover, we reverse the lens, discerning the influence of

quantum light on the phase diagram of the spin system.   

Dipolar quantum solids emerging in a Hubbard quantum simulator

Long-range interactions play an important role in nature; however, quantum simulations of lattice systems have largely not been able to realize such interactions. A wide range of efforts are underway to explore long-range interacting lattice systems using AMO and condensed matter platforms. We achieve novel quantum phases in a strongly correlated lattice system with long-range dipolar interactions using ultracold magnetic erbium atoms. As we tune the dipolar interaction to be the dominant energy scale in our system, we observe quantum phase transitions from a superfluid into dipolar quantum solids, which we directly detect using site-resolved quantum gas microscopy. Furthermore, we study quantum phase transitions in the context of $Z_2$ lattice gauge theory by mapping the hard-core Bose-Hubbard model to the mixed-dimensional spin model. In addition, we share progress toward studying extended Fermi-Hubbard physics with the fermionic isotope of erbium. This work demonstrates that novel strongly correlated quantum phases can be studied using dipolar interaction in optical lattices, opening the door to quantum simulations of a wide range of lattice models with long-range and anisotropic interactions.   

Interaction of in-plane Drude carrier with c-axis phonon in PdCoO2

We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO₂ thin films.

For the ab-plane, an optical peak near   Ω=750 cm^-1 drives  

the scattering rate γ^*(ω) and effective mass m^*(ω)  of the Drude carrier to increase and decrease respectively for ω > Ω. 

For the c-axis,

a longitudinal optical phonon (LO) is present at Ω. as evidenced by a peak in the loss function Im[-1/ε_c(ω)]. 

Further polarized measurements in different light propagation (q) and electric field (E) configurations indicate that 

the Peak at Ω results from an electron-phonon coupling of the ab-plane carrier with the c-LO phonon, which leads to the frequency-dependent γ^*(ω) and m^*(ω). 

This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared band and a c-LO. On the contrary, 

it is the Drude carrier that couples in PdCoO₂.  The coupling between the ab-plane Drude carrier and c-LO suggests that the c-LO phonon may play a significant role in the characteristic ab-plane electronic properties of PdCoO_2, including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.   

Gaussian Ensemble of varying randomness and Sparse Sachdev-Ye-Kitaev model

We study a system of N qubits, at large N, with a random Hamiltonian obtained by drawing coupling constants from Gaussian distributions in various ways. This results in a rich class of systems which include the GUE and the fixed q SYK theories. Starting with the GUE, we study the resulting behavior as the randomness is decreased. While in general the system goes from being chaotic to being more ordered as the randomness is decreased, the changes in various properties, including the density of states, the spectral form factor, the level statistics and out-of-time-ordered correlators; reveal interesting patterns. Subject to the limitations of our analysis which is mainly numerical, we find some evidence that the behavior changes in an abrupt manner when the number of non-zero independent terms in the Hamiltonian is exponentially large in N.   

Resonant ultrasound spectroscopy in the charge density wave and Weyl semimetal candidate compounds NbTe4 and TaTe4

Transition metal chalcogenides are an example of quantum materials, which show, topological states, charge density wave (CDW) formation, and ferroic orders. Within this family, the transition metal tetracalcogenides, NbTe4 and TaTe4, have captured the interest of the scientific community for being candidates to present axionic states connecting different Weyl points on the fermi surface through CDW formation. This represents a possibility to study the relationship between topological states of matter and correlated electronic states such as CDW. Diffraction, scanning tunneling spectroscopy, angle resolved photoemission spectroscopy and transport measurements have revealed important information about the CDW formation in these compounds. For instance, resistivity measurements as a function of temperature in these materials suggest possible phase transitions at certain temperatures, which, for the case of NbTe4, could be related to reconfigurations of the CDW order. However, bulk thermodynamic measurements that can give more insight into the nature of such transitions are still missing. Here we will present resonant ultrasound spectroscopy (RUS) measurements for NbTe4 and TaTe4 single crystals, in order to get thermodynamic information of the reported putative phase transitions at different temperatures, through the determination of the temperature dependence of the elastic constants. For NbTe4 we find that the resonance frequencies, directly related to the elastic constants, are highly hysteretical in the temperature range of 100K to 300K, which can be directly related to the presence and dynamics of the recently reported CDW domain walls (DWs)[1]. In contrast, TaTe4 do not show such hysteretical behavior. These results suggest that DWs in NbTe4 play a fundamental role in both, the electronic and elastic properties of NbTe4.

[1] Galvis, J. A. and Fang, A. and Jim'enez-Guerrero, D. and Rojas-Castillo, J. and Casas, J. and Herrera, O. and Garcia-Castro, A. C. and Bousquet, E. and Fisher, I. R. and Kapitulnik, A. and Giraldo-Gallo, P. Nanoscale phase-slip domain walls in the charge density wave state of the Weyl semimetal candidate NbTe4,American Physical Society 2023   

High-field magnetotransport and Fermi surface topology of the quasi-1D transition metal tetrachalcogenide TaTe4

Following the discovery of the quantum Hall effect, the exploration for new materials and subsequent identification of new phases, later appointed as ‘topological’, was initiated. This search has led to the discovery of exotic electronic phases with significant implications across various applications. The most notable examples include topological insulators, topological superconductors, Weyl semimetals and others. Furthermore, new families of materials have been postulated as potential candidates to host these new states, such as the transition metal tetrachalcogenides (TMTCs) . Due to their low dimensionality, these materials are prone to Peierls instabilities, leading to the ground state being described by a charge density wave (CDW) collective phase. Topological phases can coexist with a CDW and in this coexistence, the emergence of even more interesting new electronic states, such as axionic states, is predicted. In this work we focus on the TMTC compound TaTe₄ which hosts a CDW phase at room temperature and below, and it is also a candidate for being a Weyl semimetal, therefore being aperfect playground to study the coexistence between topological states and electronically correlated states. To study this, we performed high field (B < 35 T) magnetotransport measurements to reconstruct the Fermi surface of the material through the angle- and temperature dependence of Shubnikov de Haas (SdH) oscillations, as well as high-field current-voltaje curvesOur results show higher resolution than previous reports, as well as features that reveal the non-trivial topology of the band structure of this material.   

Evidence for saddle point driven charge density waves on the surface of heavily hole doped iron-based superconductor

When a band (a saddle point) is located just below the Fermi level (E_F) and is related to another saddle point via a wave vector q, Rice and Scott demonstrated that the wave vector dependent electron susceptibility χ(q) diverges logarithmically. A superlattice instability which does not have "nesting" in the usual sense can arise in the model. However, evidence for charge density wave (CDW) driven by saddle point has so far been elusive. Two-dimensional saddle points located near the Fermi level and unaffected by other energy bands have rarely been observed in previous materials. Here we use scanning tunneling microscope to demonstrate that 2×2 charge modulation along As-As direction with 2a_As period exists at the As-termination in heavily hole-doped 122 iron arsenide superconductors Ba_1-xK_xFe₂As₂ (x=0.77). An energy gap opens at the Fermi level and an intensity reversal exists in real space across the zero energy. A sharp peak in the tunneling spectra and theoretical calculation demonstrate the existence of saddle points in the middle of the principal axis of the first Brillouin zone. The vector of the 2×2 charge modulation connects the saddle points near the Fermi level, which gives strong evidence for the static charge ordering driven by saddle points. Our finding provides an intriguing platform for studying a new type of charge density wave on the arsenic (As) surface of heavily hole-doped 122 iron arsenide superconductors. Furthermore, it paves the way for exploring the intertwining between the unconventional superconducting pairing which breaks time reversal symmetry and the charge density wave.   

Simulation of Coulomb Crystallization of Molecules on Graphene

The KTHNY theory predicts continuous phase transitions of classical particles on 2D surfaces that are mediated by the evolution of defects. Previous experiments have utilized micron-sized colloidal particles to observe these transitions, but controlling these particles with high precision has been challenging. Through a molecularly decorated, gate-tunable graphene device, we can achieve more precise control over particle interactions and density, offering a more insightful approach to testing the predictions of the KTHNY theory. To determine the range of critical particle density for phase transitions, we conducted simulations of charged particles diffusing on a graphene surface using the Kinetic Monte Carlo method. We analyzed both the translational and rotational order parameters, as well as the structure factors of these particles in relation to particle density and interaction. Our simulations and analysis primarily agree with the hypotheses of the KTHNY theory, while also indicating directions for future experimental work.   

Potential prospects of 2D van der Waals quantum materials for new generation sensing and information science

Two dimensional (2D) van der Waals (vdW) materials refer to atomically thin materials that are stacked together through weak vdWs forces. These 2D materials exhibit unique properties that make them promising for quantum computing. The significance of exploring 2D vdW materials for quantum computing lies in several factors. Considering the scalability aspects, 2D materials can be easily fabricated into large-scale devices, making them compatible with the existing semiconductor industry. This scalability is crucial for the development of practical quantum technologies. Similarly, quantum computing aims to utilize quantum bits or qubits, which can exist in multiple states simultaneously. As a result, quantum information science in confined systems can enable secure communication protocols based on quantum entanglement. 2D vdW materials can be used to create quantum emitters, which generate single photons that can check nanoscale transport over long distances with high fidelity. Meanwhile, quantum sensors based on 2D vdW materials can achieve high sensitivity to external stimuli such as electric and magnetic fields. This can enable precise measurements and imaging techniques with applications in fields like medicine, materials science, and environmental monitoring. The current relevance of this topic stems from the ongoing research and development efforts in the field of quantum information science. Reports are available for investigating the properties of 2D vdW materials, exploring new ways to influence and control quantum states, and developing novel devices and architectures for quantum information processing. These materials offer scalability, potential for quantum computing and communication to unlock the full potential of 2D van der Waals materials and pave the way for practical quantum technologies in the near future.   

Moiré Exciton in Atomic-clean Assembled and Homogeneous MoSe2/WSe2 Heterobilayers

Fabricating moiré superlattices with sufficient uniformity and reproducibility, especially involving transition metal dichalcogenides (TMDs), has been a great challenge, severely hindering the progress of moiré system research. We present an assembly method that addresses this challenge well, and then report new finding on the properties of moiré excitons. By traditional dry transfer method of polycarbonate with specific assembling condition, including ultra-high vacuum and extremely low lamination speed, atomically clean interfaces can be achieved for Van der Waals heterostructures (vdWHs) such as TMD/TMD and TMD/hBN. Presentatively, we assemble atomic-clean monolayer MoSe₂/WSe₂ heterostructure with controlled angle encapsulated by hBN. Measuring by piezoresponse force microscopy, We find distortion of the moiré period is found to be only a few percentages across the sample area of about 5×10 μm². At low temperatures, we observe photoluminescence from the sample with high structural homogeneity. We observe homogeneous moiré exciton emissions across the whole heterostructure. Several samples with different twist angle shows reproductive spectra of moiré exciton. Without complex components enable us to further distribute the spectra of moiré exciton. We find twist-angle independent and dependent component of spectra, which could reveal the physical origin of moiré exciton.   

Integer and fractional moiré Chern insulators in van der Waals bilayers

Moiré van der Waals (vdW) materials have become established playground for exploring band topology and strong-correlations phenomena. Cryogenic nanopositioning and nanorotating setups of attocube systems are widely used in such nanoscale studies because they provide supreme stability and ultra-low-vibrations environment. These cryogenic scanning setups include confocal microscopes and various scanning probe microscopes, ranging from magnetic force microscope (MFM), SQUID microscope and nitrogen vacancy (NV) microscope to single-electron-transistor (SET) microscope and microwave impedance microscope (MIM).

We present here a selection of remarkable results achieved with attocube systems technology in labs of our customers with emphasis on integer and fractional moiré Chern insulators (MCIs) in vdW bilayers: Scanning magnetometry of an integer MCI MoTe₂/WSe₂ shows that its magnetization can be flipped with a very low current [1], which is appealing for utilization in energy-efficient magnetic memories. Magneto-optical study of the same heterostructure discovered a valley-nonpolarized quantum anomalous Hall state [2]. Magic angle twisted bilayer graphene (MATBG), identified as an integer MCI [3], reveals orbital magnetism induced by local Berry curvature as the function of the integer filling factor [4]. Moreover, MATBG can also exhibit fractional Chern insulating states (FCIS) even in low magnetic fields B<12T [5]. FCIS that survive in B=0 have been identified magneto-optically in twisted bilayer MoTe₂ using trions as the local probe for real-space imaging of spin polarization [6].

[1] C. L. Tschirhart et al., Nature Physics 19, 807 (2023).

[2] Z. Tao et al., preprint at https://arxiv.org/abs/2208.07452 (2022).

[3] J. Yu et al., Nature Physics 18, 825 (2022).

[4] S. Grover et al., Nature Physics 18, 885 (2022).

[5] Y. Xie et al., Nature 600, 439 (2021).

[6] J. Cai et al., Nature 622, 63 (2023)   

Micron-scale Characterization of Exfoliated Graphene by Raman Spectroscopy

Moiré quantum matter has emerged as a new tool for investigating the physics of strongly interacting systems. The most famous moiré system, twisted bilayer graphene (TBG), has been predicted and shown to exhibit both superconductivity and orbital ferromagnetism. The moiré patterns key to TBG devices are highly sensitive to both twist angle and strain, but the current fabrication processes are unable to control these parameters to the precision required, particularly when the 'magic angle' near 1.1 degrees is desired. Because the twist and strain parameters of most TBG devices cannot be tuned after fabrication, these devices are plagued by limited experimental repeatability.

TBG is commonly made by stacking exfoliated monolayer graphene (MLG). We describe a method to extract absolute values of uniaxial strain, biaxial strain, temperature, defect density, and charge carrier density with micron-level resolution over large areas of exfoliated MLG flakes using Raman spectroscopy. We characterize these parameters at successive stages of device fabrication, and in MLG exfoliated by differing techniques.   

Emulsifying properties of pristine low-dimensional carbon forms

Due to their extraordinary properties, low dimensional carbon forms hold great promise for applications ranging from optoelectronic, through environmental to biomedical technologies. However, the conflicting reports about their natures hinder potential applications. For many potential large scale processing routes to efficiently manufacture and commercialize graphene based devices, composites, coatings, membranes or inks it is essential to understand the fundamental colloidal properties of pristine graphene flakes (GF). Similarly, carbon nanotubes (CNTs) which have already demonstrated scientific and technological breakthroughs including scalable coatings, composites, tissue engineering, and biosensors, hold enormous, yet unfulfilled, potential in other macro-scale applications.

Our studies into colloidal properties of pristine GF and short and thin pristine CNT revealed why and how they can be used as emulsion stabilizers without using any additional surfactants. The rigorous theoretical and experimental studies explained the physico-chemical mechanism governing their amphiphatic nature. Pristine GF with the small surface areas and short thin pristine CNTs, due to high hydrophilic-to-hydrophobic surface regions ratio, exhibit sufficiently good emulsifying properties to form stable water-in-oil emulsions. Recently, we found out that also fullerenes can stabilize emulsions. I will compare the emulsifying properties of low dimensional carbon forms and explain how their amphipathic strength can be controlled.   

Magneto-Raman signature of chiral phonon in monolayer MoS2

Helicity-resolved Raman spectroscopy (HRRS) is a useful technique for discerning phonon symmetry in 2D materials. In our HRRS studies of monolayer MoS₂,  we observed a doubly degenerate chiral phonon at around 270 cm^-1. Our wavelength- and temperature-dependent measurements show that this chiral phonon is activated by the A exciton transition in MoS₂. Magneto-Raman spectroscopy measurements revealed phonon Zeeman splitting and a large magnetic moment of this chiral phonon mode. Theoretical calculation is developed to explain the chiral phonon behavior. Our work not only expands the understanding of monolayer MoS₂, but also provides a framework for extending these findings to other two-dimensional hexagonal lattices.

    

Nematicity from strong quantum spin fluctuations in a two-dimensional XY-type van der Waals magnet

We study XY-type magnet NiPS₃ to probe spin-induced nematicity in 2D system. We identified three Raman signatures for the static, long-range antiferromagnetic (AFM) order in bulk NiPS₃: suppressed spin fluctuations (revealed by ceased quasi-elastic scattering, QES), broken translational symmetry (revealed by the presence of a folded phonon at 30 cm^-1, P_BTS), and broken rotational symmetry (revealed by degeneracy-lifted phonons at around 180 cm^-1, P_BRS). We then track these three Raman features as the NiPS₃ thickness decreases and found their contrast behaviors in few-layer NiPS₃: the survival of QES, the disappearance of P_BTS, and the presence of P_BRS at 10 K, showing that the static, long-range AFM order in bulk evolves into a spin-induced nematic state in few-layer. We further observed three nematic domain states whose order parameters are rotated by 120^o from one another and are unchanged in different thermal cycles. Our results show the power of strong quantum fluctuations in creating new phases and offer an unprecedented venue for realizing novel 2D magnetic phases.   

Composite Fermions and Parton Wavefunctions in Graphene van der Waals Heterostructures

In a graphene-hexagonal Boron nitride (hBN) heterostructure, the presence of the hBN layer can lead to a gap and the breaking of the symmetry between carbon sublattices, with multicomponent fractional quantum Hall effect (FQHE) due to the electrons’ correlation. We report on the FQHE at filling factors ν = η/3 with ν > 1, and on the composite fermions (CFs) at in the ν < 1 lowest landau Level ν = 4/5, 5/7 and 2/3, which belong to the series of integral quantum Hall effect (IQHE) of CF electrons at ν = p/(2mp ± 1) and ν = 1- p/(2mp ± 1) for CF holes, where 2m is the number of magnetic flux quanta attached to the CF, or vorticity. Here we focus on the ν = 2/3 fraction and his description with a partons’ wave function; in this model the electron is broken down into sub-particles, the partons, each one residing in an IQHE state, fictitious particles that glued back together recover the physical electrons.   

Transport properties and electronic phases in potassium intercalated 2D MoS2 Devices

          Recently, ionic gating was expanded to intercalation of alkali metals into the vdW gaps of layered 2D materials, and a variety of gate-controlled quantum phases have been reported, as exemplified by the room temperature ferromagnetism in Li-intercalated Fe₃GeTe₂¹, BCS-BEC crossover in Li_xZrNCl², and FFLO state in LixMoS2³. Here we report the gate-controlled K intercalation of MoS₂ using Hall transport experimentation and describe a systematic evolution of low temperature phases.

          Resistivity is continuously reduced, showing a metallic behavior without superconductivity in the 2H phase. At V_G ~ 3 V, a dramatic resistivity drop occurs concurrently with a one order of magnitude carrier density decrease, indicating a transition from 2H to 1T’ structure.  Superconductivity of T_c = 5.5 K appears in the high carrier density 1T’ state. With further increased carrier density, the metallic state without superconductivity emerges.  During the deintercalation process, the phase apparently stays in the 1T’ structure and exhibits a metal-insulator transition. The present results indicate that K-intercalation exhibits a totally different phase diagram from that for the electrostatically doped MoS₂⁴.

 

¹Y. Deng et al. Nature 563, 94 (2018).

²Y. Nakagawa et al., Science 372, 190 (2021).

³D. Zhao et al., Nat. Phys. (2023) https://doi.org/10.1038/s41567-023-02202-4

⁴ J. T. Ye et al., Science 338, 1193 (2012).   

Automated and fast recognition of exfoliated two-dimensional materials .

Two-dimensional (2D) materials are a class of nanomaterials that consist of a single- or few-layers of atoms and possess exceptional physical and chemical properties. Such unique properties of 2D materials made them a focal point of research on novel nano-electronic devices. One of the easiest and widely used laboratory method of making 2D materials is the Mechanical Exfoliation also known as the Scotch Tape method, where flakes of various size and thickness are randomly split from bulk materials and transferred to a chosen substrate using adhesive tape. Then flakes need to be identified on the substrate and that process is usually done manually using optical microscopy as a starting tool and other lower-throughput methods for more reliable final identification. Manual search and identification of flakes is a time consuming and involved process that requires expertise to do it efficiently. There have been efforts to automate flake search and characterization using machine learning (ML). However, ML has some drawbacks: high volume of training data needed, variable samples/substrates, high requirement for computing power or time. Proposed solution to the efficient recognition and sorting of 2D material supported by the automation is to use a general non-ML algorithm that relies on some user input and general visual properties of 2D material flakes to identify and possibly characterize flakes with minimum computing power/time required.   

Investigation of van der Waals heterostructure interlayer coupling on large length scales through widefield pump-SHG probe microscopy

The strong light absorption, rich exciton diversity and non-epitaxial device construction make transition metal dichalcogenide (TMD) heterobilayers (HBLs) a popular candidate for next-generation optoelectronic devices. Strong interlayer coupling affords TMD HBL-based devices their novel capabilities which depends on a close spatial proximity between the constituent monolayers. One indicator of close interlayer contact is the increase in second harmonic generation (SHG) intensity in the presence of an optical pump [1]. The energetic offset of the band edges between the constituent monolayers drives the photoexcited electrons to reside in one layer and holes in the other, resulting in an SHG-active transient dipole at the interface. Here, we investigate on a large spatial scale (100s µm) the interlayer coupling of a WS₂-MoSe₂ HBL constructed from a popular gold-tape exfoliation technique [2]. A hyperspectral widefield microscope spatially monitors pump-induced changes of sub-band gap SHG from the HBL. We find there are abrupt differences in the pumped SHG intensity and therefore the layer coupling across the lateral extent of the HBL. These findings are not present in basic characterization methods such as photoluminescence and reflection contrast spectroscopy, which both exhibit strong interlayer coupling for the studied HBL. Our work suggests the widefield pump-SHG probe microscopy technique is supplementary to well-established characterization methods that describe interactions at the interface in TMD HBLs.   

The effects of light and electrostatic doping on the carrier dynamics of transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) are atomically thin semiconductors combined with van der Waals forces with unique properties that make them excellent for optoelectronic and photovoltaic applications. These materials, in both bulk and monolayer, have a variety of ultrafast photoinduced processes such as scattering, exciton formation, charge transfer and carrier recombination that can be studied with ultrafast transient absorption spectroscopy. Despite extensive previous research on the photophysical processes in these materials, the impact of a gate voltage via ionic-liquid gating on the carrier dynamics in these experiments remains limited. Ionic-liquid gating forms an electric double layer that acts as an electrostatic doping mechanism that creates even higher electric fields and charge densities than solid gate dielectrics and allows precise control over carrier concentration alongside photoexcitation. Our study uses ionic liquid gating to adjust the carrier concentration in TMDs such as MoS₂, altering photoinduced carrier dynamics as revealed by femtosecond transient absorption.   

Electrospun titania-zinc oxide nanofibers

Electrospinning of sol-gel solutions containing polymers and metal precursors can be utilized to fabricate metal-oxide nanofibers whose properties depend on experimental conditions and chemical compositions. Composite metal-oxide nanofibers have properties that can be tailored by varying ratio of metal precursors or calcination temperature. The titania-based composite fibers are attractive candidates for numerous applications, from photocatalytic to medical. We present morphological investigation of titania-zinc oxide nanofibers calcined at different temperatures, using SEM and BET methods. Extensive studies of structural properties by XRD and Raman scattering measurements reveal the amorphous nature of as-spun fibers, whereas in calcined samples crystalline titania phases (anatase and rutile) emerge, with their ratio dependent on the calcination temperature. The optical characteristics of nanofibers are examined using UV-visible spectroscopy, yielding consistent values for energy band gaps through the analysis of transmission and diffuse reflectance.   

Metavalent Bonding in 2D Chalcogenides: Structural Origin and Chemical Mechanisms

An unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which necessarily involves violation of the 8-N rule. Precise mechanisms of MVB and the relevance of lone pair of group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB and understanding its orbital mechanisms. Here, we present first-principles theoretical analysis of the nature of bonding in five distinct 2D lattices of group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out-of-plane expression of the lone pair versus in-plane bonding can be systematically explored.  While their honeycomb lattices respecting the 8-N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in-plane, with cationic lone pair activating the out-of-plane structural puckering that controls their relative stability. Anomalies in Born effective charges, dielectric constants, Grüneisen parameters occur only in their in-plane behaviour, confirming that MVB here is confined strictly to 2D and originates mostly from p-p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures exhibiting thermo- and ferro-electricity, and electronic topology important to quantum technologies.   

Gate-tunable spectral photoresponsivity of MoS2-WSe2 heterojunctions

Miniaturized computational spectrometers provide a path to integrating spectrometers into small scale, on-chip devices. One promising device architecture incorporates a van der Waals heterostructure as the light sensor. The heterostructure exhibits an electrically tunable spectral response which can be paired with a spectral reconstruction algorithm to determine optical spectra in the bandwidth 400 - 750 nm [1]. To optimize this reconstruction algorithm, a strong understanding of the heterojunction photoresponse is necessary. We are building heterojunctions of WSe₂ and MoS₂ and characterizing the electrically tunable spectral response via spectrally resolved photocurrent measurements. We are testing (a) the linearity of the photoresponse with respect to illumination power and (b) the linearity of photocurrent superposition when multiple narrow-band light sources illuminate the heterojunction. Our measurements will be used to improve spectral reconstruction algorithms and will probe the microscopic mechanisms governing the optoelectronic properties of van der Waals heterostructures. 

 

[1] H. H. Yoon et al., Miniaturized Spectrometers with a Tunable van Der Waals Junction, Science 378, 296 (2022).   

Studies of the electronic structure of VS2 and possible CDW transition

2D-transition metal dichalcogenides (2D-TMDs) host various phenomena of wide interest such as charge density wave (CDW), ferromagnetism, superconductivity, mott-insulator, and catalytic behavior. CDW ordering is common in 2D-TMDs and its mechanisms have been extensively studied in both bulk and monolayer specimens but remains a topic of debate. Here we report on the electronic structure of VS₂. The ARPES spectra reveal a Fermi surface largely dominated by V-3d bands which cross the Fermi level along A-L direction. Photon energy dependent ARPES measurements indicate minimal k_z dependence of the V-3d bands, consistent with its 2D nature. Most importantly, the Fermi surface map displays a pronounced decrease in the intensity of V-3d band away from the Brillouin zone (BZ) center. Such a feature has been attributed by some to a CDW gap opening in VSe₂ suggesting a similar phenomenon in VS₂. Furthermore, the energy distribution curves (EDCs) in the vicinity of the Fermi surface reveal the existence of dispersion-less flat bands. We also detect the presence of Dirac surface states, also observed in VTe₂, and VSe₂ suggesting its prevalence in VX₂ family of materials.    

From MAX to MXene: A Case Study of Sc2TlC MAX Phase, Sc2C Pristine MXene, and Surface-Functionalized Sc2CT2 (T=O, F) MXenes

Recent research has shown a growing interest in the exciting physical properties exhibited by MXenes-based two-dimensional (2D) materials. In this study, which utilizes first-principles approaches, we explore the Sc₂TlC MAX phase and its 2D derivatives named MXene. The primary focus is transforming Sc₂TlC into a 2D Sc₂C sheet and the subsequent surface functionalization with oxygen (O) and fluorine (F). The investigation of the electronic structures of these materials, both in their original states and following surface modifications, reveals unique and distinctive characteristics. The Sc₂TlC MAX phase has been confirmed to be metallic and nonmagnetic. However, when it transforms into the Sc₂C monolayer, it undergoes significant changes in its electronic structure while still maintaining its metallic properties. We also discuss the influence of surface functionalization on electronic properties, observing that Sc₂CT_x (T_x=O₂, F₂) MXenes exhibit distinct electronic states based on their surface terminations. Specifically, Sc₂CO₂ remains metallic, whereas Sc₂CF₂ displays semiconductor-like behavior, with energy gaps of approximately 1.325 eV for the up-spin state and 1.227 eV for the down-spin state. The maximum optical absorption of ultraviolet (UV) light has been determined for various materials, including Sc₂TlC MAX phase, Sc₂C, Sc₂CO₂, and Sc₂CF₂ MXenes, with values of 13.8×10⁵ cm^-1, 49.17 ×10⁴ cm^-1, 81.39 ×10⁴ cm^-1, and 69.0 ×10⁴ cm^-1, respectively. Examination of their optical characteristics shows that these materials exhibit substantial reflectance and absorption capabilities, especially when exposed to low-energy photons of light. This suggests their potential suitability for applications in optoelectronics and energy harvesting devices.   

Bose Condensation of Upper-Branch Exciton-Polaritons in a Transferable Microcavity

Exciton-polaritons as bosonic quasiparticles are ideal objects for realizing Bose-Einstein condensation in a solid environment due to the light-effective mass, which is the crucial step towards the next-generation optical device [1-2]. Simultaneously, the advent of transition-metal dichalcogenides (TMDs) monolayers has introduced a new realm of materials for polaritons [3-5]. Here, we report the first observation of upper polariton Bose-Einstein condensation in a transferrable WS₂ monolayer microcavity [6]. The evidence has been shown of a nonlinear intensity increasing, linewidth decreasing and energy blueshift in power-dependent measurement. The upper polaritons accumulate more in the ground state above the threshold. We also observe an increase in coherent time above the threshold. The dynamic rate equation shows a crucial conversion time from upper polariton to lower polariton, which should be larger than the upper polariton lifetime, leading to the possibility of condensation. Quantum Boltzmann equation simulation shows the upper polariton condensation would happen in a critical particle density and special detuning. Our work paves the way for further understanding condensation competition in open-dissipative quantum systems while linking to practical applications, shedding light on both fundamental aspects of quantum physics and practical laser technologies.   

Topological transitions of surface plasmon propagation in anisotropic 2D Dirac fermions

We explore topological transitions in the type of propagation of surface electromagnetic modes in massive anisotropic tilted two-dimensional (2D) Dirac systems. The presence of tilting and mass gives rise to an indirect band gap that strongly modifies the joint density of states compared to the gapless system. New Van Hove singularities appear, and the interplay between intra- and interband transitions leads to an anisotropic optical conductivity with imaginary parts acquiring opposite signs in orthogonal directions, opening the possibility of having hyperbolic propagation of plasmons. Isofrequency contours and low plasmon losses, as obtained from the dispersion relation, show that transitions between purely anisotropic quasielliptical and well-defined, highly directional, hyperbolic modes are attainable only when tilt and mass coexist via frequency and Fermi level variation. This behavior could be probed in massive tilted 2D Dirac materials like the organic-layered compound α−(BEDT-TTF)₂I₃ or WTe₂, in which hyperbolic plasmons were recently observed, through far-infrared absorption, optical nanoscopy, and similar current tools in graphene plasmonics.   

Probing the Properties of MPc/Multilayer-graphene/hBN heterostructures through quantum coherent transport

Multilayer-graphene/hBN heterostructures are often used to probe the electronic properties of other materials. For instance, magnetotransport measurements at low magnetic fields on multilayer graphene usually show a signature of weak localization, a consequence of the quantum interference of the electronic wave functions. The presence of metal-phthalocyanine (MPc) molecules in close proximity to the multilayer-graphene/hBNheterostructure can alter the quantum coherent transport, inducing for example weak antilocalization if the metal in the MPc molecule has an important spin-orbit coupling. In the same way, the back gate dependence of the resistance of the heterostructure before and after the deposition of the molecules, can reveal charge transfer between the molecules and the multilayer graphene. Here, we show data on a CuPc/multilayer-graphene/hBN heterostructure that puts in evidence charge transfer from graphene to the CuPc as well as consequences on weak localization measurements.   

Theory of Quasicrystals in Twisted Bilayer Graphene on Hexagonal Boron Nitride

We discuss the theoretical aspects of lattice relaxation in twisted bilayer graphene on hexagonal boron nitride (hBN) that are inspired by experimental investigations conducted by X. Lai et al. Despite the incommensurate nature of the graphene-hBN moiré interference pattern with respect to the twisted bilayer graphene (TBG) lattice, except for a few isolated commensurate twists, we discover a broad spectrum of relative alignments where lattice relaxation leads to the reestablishment of commensuration. Beyond a specific range of relative twists, where there is no separation between the moiré pattern and the emergent moiré-of-moiré scale, relaxation can no longer restore commensurability. In this regime, we observe the formation of a moiré quasicrystal characterized by the superposition of incommensurate periodicities. We also explore the implications for transport experiments involving these devices. Adjusting the alignment of TBG and hBN presents an intriguing means to manipulate and explore interactive physics within moiré patterns with new emergent periodicities.   

Diffraction in Twisted Bilayer Graphene

In twisted bilayer graphene (TBG), spatial variations in the twist angle produce variations in the local electron velocity, which produces diffraction of electrons that resembles the diffraction of light in an optical medium. We follow this analogy to investigate the creation of an "electron lens" using patterning of the twist angle in TBG. We show theoretically that electron diffraction in TBG is governed by a modified Snell's law, and we use theoretical analysis and numerical simulations to explore the implications of this modified Snell's law for designing an ideal lens and for focusing of electrons by a circular lens. Interestingly, a circular lens for electrons in TBG acts similar to the usual optical version, except that the optical axis and focal point are not horizontally symmetric, and are shifted away from the line of bilateral symmetry.   

Twisted-bilayer FeSe and the Fe-based superlattices

We derive BM-like continuum models for the bands of superlattice heterostructures formed out of Fe-chalcogenide monolayers: (1) a single monolayer experiencing an external periodic potential, and (2) twisted bilayers with long-range moire tunneling. A symmetry derivation for the inter-layer moire tunnelling is provided for both the $Gamma$ and $M$ high-symmetry points. In this paper, we focus on moire bands formed from hole-band maxima centered on $Gamma$, and show the possibility of moire bands with $C=0$ or $pm 1$ topological quantum numbers without breaking time-reversal symmetry. In the $C=0$ region for $theta-->0$ (and similarly in the limit of large superlattice period for (1) ), the system becomes a square lattice of 2D harmonic oscillators. We fit our model to FeSe and argue that it is a viable platform for the simulation of the square Hubbard model with tunable interaction strength.   

Flat-bands in two stacked Penrose lattices

Strongly correlated phases in Moiré materials with flat bands in twisted systems, play the main role to explain the superconductivity in Twisted Bilayer Graphene. Decagonal quasicrystals are a type of quasicrystals that has quasiperiodic layers stacked periodically. The Penrose tessellation is the two dimensional most used model to study quasicrystals due to presenting the 5-fold symmetry and quasiperiodicity, as in quasicrystals. In this work, flat bands are found in quasicrystals after a translation and rotation, using the Tight Binding model. The quasicrystalline phases and confined electronic states rise from rotating two Penrose lattices, which are analysed in detail to understand the emergence of this bands.   

Probing exotic phase transitions in bilayer graphene with NEMS resonator

The recent discovery of spin-polarized superconductivity in Bernal bilayer graphene has piqued interest in exploring exotic phases beyond twisted bilayer graphene.[1] By applying a proper carrier concentration and displacement field, it is possible to increase the density of state by a large amount in Bernal bilayer graphene, leading to observations of superconductivity, isospin magnetism, and quantum cascade of correlated phases.[2] In this study, we describe our efforts to investigate these exotic phase transitions using highly sensitive nano-electromechanical devices.[3]

[1]      H. Zhou et al., “Isospin magnetism and spin-polarized superconductivity in Bernal bilayer graphene,” Science (1979), vol. 375, no. 6582, pp. 774–778, Feb. 2022

[2]      A. M. Seiler et al., “Quantum cascade of correlated phases in trigonally warped bilayer graphene,” Nature, vol. 608, no. 7922, pp. 298–302, Aug. 2022

[3]      M. Šiškins et al., “Magnetic and electronic phase transitions probed by nanomechanical resonators,” Nat Commun, vol. 11, no. 1, p. 2698, Dec. 2020   

Shear-induced electronic confinement and one-dimensional bands in two-dimensional undulated bilayer semiconductors

 

Two-dimensional (2D) materials can significantly change their properties in response to external influences, such as stacking orientation or in-plane strain, forming materials with novel electronic structures. In this poster, I will show the emergence of an effective one-dimensional (1D) system (similar to quantum wells) using a periodic one-dimensional bending modulation of a 2D semiconductor bilayer without in-plane strain. The appearance of 1D electronic bands arising due to the confinement along the modulated direction is illustrated by a two-ladder one-dimensional tight binding (TB) model and also confirmed by density functional theory (DFT) calculations using undulated bilayer hexagonal boron nitride (hBN) as an example. The bending modulation of a 2D bilayer creates an interlayer shear gradient, which creates moiré patterns with different stackings along the modulated direction. The changing stacking pattern creates a long-range electrostatic confinement potential, resulting in 1D electronic flat bands. The obtained 1D states are very similar in property to the widely studied 1D quantum wells in bulk semiconductors and demonstrate that shear gradients generated by undulating 2D bilayers can be used as a novel way to create 1D quantum wells in 2D materials. Additionally, the 1D states of undulated 2D bilayers can significantly affect photogenerated carriers' lifetime and increase optoelectronic devices' performance.   

Quantum thermodynamics of the magnetic quantum phase-transition in P-doped graphene under biaxial strain

We explore the quantum-thermodynamic effects in a phosphorous (P)-doped graphene biaxial strain. We obtain thermodynamic quantities such as the electronic entropy and specific heat as a function of temperature and strain within a Fermi-Dirac statistical model. At zero temperature, a magnetic moment of 1.0 µ_B is induced by the P atom in the system, which can be tuned by the strain control parameter. From zero to about 5% of strain, the system exhibits a magnetic phase, transforming it into a paramagnetic phase for strain >5%. When the temperature is different from zero, we find entropy changes presenting sign changes within both a magnetic regime with magnetic moment of 1.0 µ_B (0-1.5% strain) and the paramagnetic regime, while in the intermediate regime with magnetic moment < 1.0 µ_B (~1.5-5% strain), the entropy changes are negative. Notably, the electronic specific heat for P-doped graphene increases in the magnetic phase up to 5% strain and abruptly drops in the paramagnetic regime at 5.5% strain, showing a magnetic quantum phase transition at low temperatures.   

Thermally Tunable Mid-Infrared Polarization Rotation and Ellipticity Based on Active Surface Phonon Polariton Nanocavity Arrays

Polarimetry is an invaluable tool for investigating material properties by observing how the material changes the polarization of light. Studies in the infrared are limited due to a lack of polarization-sensitive materials in far-infrared (IR). Here, we experimentally demonstrate an active polarization-control nanostructure, which avoids the limitations of methods that depend on bulk material effects. We measure the optical rotation and phase induced from gold subwavelength grating structures on top of vanadium dioxide (VO2) film on silicon carbide. The proposed structure has a resonance at 840 cm-1 due to the Fabry-Perot cavity array of coupled surface polaritons. Linearly polarized incident light can be decomposed into a statically reflected component and a partially absorbed temperature-dependent component. A novel polarimetric spectrometer was developed to measure the IR polarization spectra, and a simple two-polarizer method was used with laser light to quantify the polarization changes. The light near the resonance is optically rotated by 10^o, while the phase is shifted by 15^o as the temperature increases. Due to the device's small footprint, flexible operating wavelength, and potential to be used as a modulator, this work provides a solution for longwave IR polarimetry.   

Non-Perturbative High Harmonic Generation in Thin Film ITO

For decades, thin film materials made of metals or doped semiconductors have been a suitable candidate for supporting localized surface plasmon or phonon-polariton modes. More recently, the Epsilon Near Zero (ENZ) mode has been observed in subwavelength thin films. This ENZ mode is defined by the electric permittivity approaching zero, leading to an increased light-matter interaction. Thus, materials that can support the ENZ mode are highly desirable for sensitive optical measurements. Here, we show Indium Tin Oxide (ITO) as a suitable candidate for measuring high harmonic generation due to its tunability of the ENZ mode, allowing for the ENZ wavelength to be shifted over a wide range of wavelengths. These ITO thin film samples will be used to observe harmonic generation in the non-perturbative regime as well as transient harmonic generation measurements. The robust optical properties of ITO thin films suggest they can be utilized for future opto-electronic applications.   

Universal structural dependence of optical properties in 3D nanocolumnar metamaterial platforms

Due to the complexity of optical anisotropy possessed by highly porous nanostructure systems, performing the growth-time dependent critical dimension analysis and unraveling the correlation between the optical and structural parameters remain challenging.  With this study, we presented and discussed a route to extract a set of optical parameters, depolarization factors, that are extremely sensitive to the changes in critical dimensions of the nanostructure platforms. We pursue a comprehensive methodical series of studies to investigate the effect of column-aspect-ratio on the evolution of anisotropic homogenization parameters of slanted columnar thin films (SCTFs) from wide range of material choices. Hence, we fabricated sets of highly porous, spatially coherent SCTFs from zirconia (ultra-wide band gap), silicon (semiconductor), titanium (zero-band gap), and permalloy (metal alloy) on silicon substrates by using custom-built, ultra-high vacuum glancing-angle deposition technique. Subsequently, we used the anisotropic Bruggeman effective medium approximation to analyze spectroscopic ellipsometry data. We have successfully extracted the anisotropic optical characteristics, including the complex dielectric function, birefringence, and dichroism, for each metamaterial platform.   

Fano-resonance characteristics of nematic liquid crystal based all-dielectric metasurfaces

All-dielectric metasurface (ADMS) is transparent and has negligible losses due to lack of metal composite as in plasmonic metasurface. The ADMS can produce narrow Fano-resonance peak when incident with light. The light response can be dynamically control by regulating applied electric field across the nematic liquid crystal (NLC) integrated with ADMS. In this work, the Fano-resonance characteristics due to NLC integration with ADMS are studied. The NLC is integrated with ADMS as NLC-over-ADMS (scenario 1) and as ADMS-over-NLC (scenario 2). The resonance wavelength position 1568 nm of scenario 2 is red-shifted compared to 1552 nm of scenario 1 due to higher effective refractive index of scenario 2. Also, the field confinement among metasurface elements for scenario 2 is stronger than scenario 1, thus the produced Fano resonance linewidth (7 nm) is less and Q-factor (224) is high for scenario 2 compared to linewidth (12 nm) and Q-factor (129) for scenario 1. The modulation depth of Fano resonance is 87% for scenario 2 due to weak field coupling between ADMS and NLC compared to the modulation depth of Fano resonance is 99% for scenario 1 due to strong coupling between ADMS and NLC. These metasurface based devices find application as the bio-medical sensor, modulator, non-mechanical switching, tuning, colour displays etc., based on the desired values of performance parameters linewidth, Q-factor and modulation depth of Fano resonance. These metasurface structures can be fabricated by the standard lithography process.   

Multilayer surface waveguide cavity resonance based on coupled surface plasmon-phonon polaritons

Surface phonon polaritons have emerged as promising candidates for applications in the long-wave infrared spectrum. Recent advancements in resonant cavity design, based on coupled surface plasmon-phonon polaritons, have demonstrated near-perfect and controllable absorptions, making them a potential candidate for constructing building blocks in long-wave infrared metasurfaces. We have utilized phase-changing materials for active metasurfaces, particularly vanadium dioxide, which undergoes an insulator-to-metal transition near room temperature. However, vanadium dioxide exhibits a higher refractive index at long-wave infrared frequencies, leading to challenges such as higher optical power losses. We address these challenges by constructing a multilayer surface waveguide that can effectively transform into a cavity. The multilayer surface waveguide minimizes losses and accommodates larger device sizes for practical fabrication. We model a multilayer surface waveguide to investigate the dispersion of coupled surface plasmon-phonon polaritons and to identify the Fabry-Perot resonance condition that arises when the cavity is formed. Our study includes various types of polar dielectrics, namely SiC, Al2O3, and GaAs. The multilayer consists of passive SiO2 and active VO2 layers.   

P-doping Quantum materials by building heterostructures with low-workfunction 2D materials

The sheer size of conventional semiconductor components limits the ability of the computer, and the demand for stronger computing components is always increasing. One solution to meet this demand is by the use of 2D atomically thin materials to make smaller components(like the transistors), which are much thinner than the conventional materials used. In my project we plan on studying monolayer Molybdenum Ditelluride(MoTe2), a semiconductor with a band gap similar to silicon, by partially covering it with a low-work function 2D material to dope the semiconductor. We will be showing Raman, photoluminescence and transport data to characterise the level of doping and study the optoelectronic properties of this heterostack.    

Unconventional electromagnetic surface waves at the Ti-Si interface

It is typically assumed that electromagnetic waves do not penetrate lossy media beyond the skin depth.  However, recently it was theoretically predicted that in the presence of a gradual interface, the electromagnetic surface wave would experience low loss propagation along the diffuse boundary of two highly lossy media. Such a surface wave is predicted to propagate along diffuse boundaries over distances which are much larger than the skin depth. In this study, we have experimentally confirmed this prediction for the case of titanium/silicon diffused boundary.  Overlap of titanium and silicon thin films was created, and propagation of the visible light was detected along the boundary. This result may enable extension of plasmonics into the UV range and enable novel approaches to silicon photonics.   

Plasmon Assisted Random Lasing of Perovskite Materials

Random lasing occurs as the result of coherent optical feedback from random scattering centers. Plasmonic nanostructures, such as silver or gold nanoparticles, efficiently scatter light due to optical confinement and the formation of hot spots at the nanoscale. In this work, by using silver nanocubes as highly efficient light scattering centers, we demonstrated a plasmon assisted random lasing of halide perovskite materials. By embedding silver nanocubes in a crystalized MAPbI3 or/and MAPbBr3 matrix, we observed narrow bandwidth lasing modes having full-widths at half-maximum of approximately 1 nm. It is observed that the lasing thresholds of perovskites are different for glass and gold substrates, and for different nanocube concentrations. Results of time-resolved measurements indicate a significant shortening in the decay time of the emission at above the lasing threshold, implying a stimulated emission process.   

Superfluidity of indirect momentum space dark dipolar excitons in a double layer of 1T′-MoS2 with tilted Dirac bands<!-- notionvc: 2c6029ae-747a-44ba-b566-da8e15070f8a -->

We investigate the superfluidity of indirect momentum space dark dipolar excitons in a double layer of tilted 1T′-MoS₂ with Dirac bands. By considering the effects of an external vertical electric field and circularly polarized light, we analyze the exciton binding energies, wave functions, and critical temperature for superfluidity. Our calculations reveal that the anisotropic Bose gas of two-component excitons exhibits anisotropic critical velocity, collective excitations, and concentrations of the superfluid and normal component. We propose an experimental approach using phonon-assisted photoluminescence to observe the directional superfluidity of the indirect momentum space dark excitons in the double layer of 1T′-MoS₂.   

Thermal and electronic properties of twisted bilayer ZnO

We consider the bilayer structure of graphene-like ZnO. Bilayer ZnO is created by placing the Zn(O) atom on top of the O(Zn) atom which was predicted to be the most favorable stacked structure, with the cohesive energy slightly larger than the monolayer ZnO. We employ ab initio results to optimize the parameters of the Stillinger-Weber potential to describe the structural and vibrational properties of monolayer ZnO. Kolmogorov-Crespi potential is optimized to describe the interlayer interaction in experimentally observed bilayer ZnO. The optimized sets of interatomic potentials are used to investigate the twist angle dependent electronic properties ans thermal conductivity.   

Structural and Electronic Properties of Metals (Al, Ti, Mo, and Zr) Terminated Diamond (100) Surfaces

Diamond has exceptional structural, thermal, and electronic properties. Metals induce electronic properties such as negative electron affinity, thermal stability, and low contact resistance allowing charges to flow easily in between them. This is critical in the advancement of diamond- based devices, but no metal has previously been identified to make effective electrical contact with diamond. To begin addressing this issue surface reconstruction at the interface need to be explored. The goal of this study is to understand the surface chemistry of metals Aluminum (Al), Titanium (Ti), Zirconium (Zr), and Molybdenum (Mo) on diamond. In this presentation we will present structural and electronic information on monolayer films of metal, metal-oxide, and metal-carbides on diamond. The surface formation energies and electronic properties of diamond-metal surfaces Perder Burke Ernzerhof (PBE) exchange-correlation functional with a 6x6 monkhorst-Pack kpoint mesh and a plane wave cut off of 500eV. The optimized structure contained 42 atoms, allowing 14 layers of C to relax and reconstruct. In our study, we found that Mo (-0.652eV) and Zr (-0.857eV had a higher absorption potential energy compared to Al (0.180eV) and Ti (-0.220eV). The surface formation energy of four Zr atoms bound with four carbon atoms is -0.943eV. Additionally, for Zr when an oxygen atom is bridged across two metal atoms, in what is an ether configuration, it exhibits a highest formation energy for all metal- oxides. The surface formation energy indicates Zr and Mo has a greater strength attraction to a diamond (100) surface than Al and Ti.   

Interfacing Cubic Gallium Nitride with other Cubic Nitrides to Unlock Exotic Materials Properties

 

Group III-nitride materials such as GaN have become an integral part of the optoelectronics industry. GaN traditionally adopts a wurtzite structure (h-GaN), but it also has a metastable zinc blende allotrope (c-GaN). One of the potential advantages of this allotrope is its ability to interface much easier with the other transition metal nitrides, which are also cubic. If the growth of c-GaN could be stabilized epitaxial integration with these materials would open a world of new possibilities in band engineering, as well as the ability to create new device structures for areas such as nuclear fuels, quantum computing, and condensed matter physics.

We report successful synthesis of c-GaN, and its integration with known superconductor ZrN, via molecular beam epitaxy. The team stabilized c-GaN by carefully controlling the ratio of Ga and N atoms impinging on the substrate surface. The cubic structure of the c-GaN films was confirmed via in-situ RHEED and ex-situ XRD and photoluminescence. Our results suggest an atomic flux ratio closer to 1:1 helps to suppress the hexagonal phase. Superlattices of c-GaN/ZrN were also successfully deposited very recently. The structure of these films was confirmed via RHEED and XRD, and the interface quality is being investigated via TEM.

These results are informing future depositions, where the layer thicknesses will be carefully tuned to enhance superconductivity within ZrN. Integration of c-GaN with other nitrides such as Cr-, Mn-, and NiN is also planned.   

Observability of quantum pinch effect in the magnetized semiconducting quantum wires

We report on a two-component, cylindrical, quasi-one-dimensional quantum plasma subjected to

a radial confining harmonic potential and an applied magnetic field in the symmetric gauge. It is

demonstrated that such a system as can be realized in semiconducting quantum wires offers an

excellent medium for observing the quantum pinch effect at low temperatures. An exact analytical

solution of the problem allows us to make significant observations: surprisingly, in contrast to the

classical pinch effect, the particle density as well as the current density displays a determinable

maximum before attaining a minimum at the surface of the quantum wire. The effect will persist as

long as the equilibrium pair density is sustained. Therefore, the technological promise that emerges

is the route to the precise electronic devices that will control the particle beams at the nanoscale.   

Understanding Superlattices of Sb2Te3/Ge4Sb6Te7 for Low-Power and High-Speed Phase Change Memory

Conventional data storage technologies are approaching fundamental limits, and one of the most promising alternatives is based on phase-change materials, typically chalcogenides that can reversibly switch between low- and high-resistance states. The success of such phase change memory (PCM) [1] depends on achieving simultaneously low-power and high-speed operation at nanometer-scale dimensions. Here we introduce a novel phase-change superlattice (SL) material with Sb₂Te₃ and Ge₄Sb₆Te₇ layers (2/2 nm thin), which enable electron and phonon confinement and scattering, leading to material properties tunable with the SL layer thicknesses. In PCM devices, this SL enables record-low switching power density (5 MW/cm²) and fast switching speed (40 ns). Nanoscale device dimensions and strong phonon confinement within the SL enable the energy-efficient phase change in our devices [2]. Unlike traditional PCM materials, GST467 contains epitaxial SbTe nanoclusters within the Ge-Sb-Te matrix [3], which act as precursors for crystallization, boosting the switching speed of GST467. These results provide fundamental insights and practical applications of superlattice PCM for energy-efficient data storage.

Refs: [1] R. Ranica et al., IEDM (2020). [2] A. I. Khan, E. Pop et al., EDL (2021). [3] A. I. Khan, E. Pop et al., Advanced Materials (2023).   

Chemical motifs in natural superlattice design

Many natural bulk systems crystallize in patterns of reduced dimensionality. For example, transition metal dichalcogenides (TMDs) consist of layered, two-dimensional structures. One way to modulate the structure and electronic properties of such a system is through an external, periodic potential. To this end, 2D heterostructures have been used to artificially engineer exotic electronic phases. Here, we discuss the development of natural superlattices, in which such heterostructures may be realized in a natural crystal. We describe the chemical relations that contribute to the stability of these structures, and we discuss how these are realized in recent TMD superlattice materials. We discuss how this translates to geometric relations in physical structures, which can aid in materials discovery of new types of superlattices in natural crystals.   

Loss Induced Transmission Revival Based on Explicit Symmetry Violation

The introduction of lossy elements to a waveguide structure is conventionally associated with attenuation of its transmittances. In this research, we explore the elastodynamics of a one-dimensional, parity symmetric structure and reveal, both theoretically and experimentally, that upon appropriate excitation of the system, augmentation of a non-Hermitian, or damping, defect can trigger an unexpected revival in transmission. This phenomenon persists across the frequency spectrum and, specifically, occurs at eigenfrequencies corresponding to anti-symmetric modes of the underlying Hermitian system. Using a coupled mode theory framework, we demonstrate via perturbation theory and a Green's function approach, how a non-Hermitian perturbation can induce a disruption of the anti-symmetric mode nodal points and, subsequently, a revival in the transmission as the damping is increased. Our discoveries pave the way for innovative methods in full-spectrum detection of environmental variations.   

Abnormal resistance jump in Pb-Cu-P-O thin film

Pb_10-x Cu_x (PO₄)₆ O (LK-99), a potential room-temperature superconductor, has attracted the global attention since its paper was published on arXiv [1]. Many research teams are attempting to reproduce and verify LK-99, but its superconductivity remains unconfirmed. The chemical formula of LK-99 is Pb_10-x Cu_x (PO₄)₆ O, which is a hexagonal structure (P6₃/m, 176) with Cu²⁺ ions substituting for Pb²⁺ ions at the 2^nd position by approximately 1/4 in pure lead-phosphate (Pb₁₀(PO₄)₆O). For LK-99 thin film growth via molecular beam epitaxy (MBE), we selected Si (111) and Al₂O₃ substrates with mismatches of about 2%. The various growth temperatures were set after preheating at 600 ℃, and we individually adjusted the Pb and Cu ratio; x = 0.8 ~ 5. The growth process was monitored using reflection high-energy electron diffraction (RHEED). To determine the chemical composition of the sample, we analyzed it using an Energy Dispersive Spectrometer (EDS). We observed a resistance increase when it is exposed to air after growth. Interestingly, the resistance was decreased in PPMS system under vacuum (1 X 10^-2 Torr). Also we observed abnormal jump in temperature dependent resistance. In this talk, we will present on the structures, compositions, and unique transport properties of Pb-Cu-P-O compound in detail.

 

Reference

[1] S Lee, et.al., The Frist Room-Temperature Ambient-Pressure Superconductor. arXiv.2307.12008 (2023)   

Effect of temperature on the salt-assisted growth of monolayer tungsten disulfide via low-pressure chemical vapor deposition

Monolayer tungsten disulfide (WS₂) is a promising material for various device applications due to its unique electronic and optical properties. However, achieving an efficient and cost-effective method for synthesizing a large area, uniform WS₂ is still a challenge. In this work, we demonstrate the synthesis of single layer WS2 crystallites by salt-assisted low-pressure chemical vapor deposition and present a systematic study of the effect of temperature on the morphology, structure, thickness and optical properties of the as grown WS₂ films. An optimized amount of NaCl was mixed with WO₃ powder to use as the promoter. We observe transitions between triangular, hexagonal, fractal and dendritic structures as the temperature range is varied, as well as varied amounts of oxygen defects as evidenced by XPS and photoluminescence measurements. By varying the growth temperature between 700 °C to 900 °C, we further observed an increase in the size of the individual flakes up to 50 µm as well as an increase in the extent of growth. As grown WS₂ samples were further characterized under Raman spectroscopy, scanning electron microscopy,  and X-ray diffraction. The results of this work pave a way to optimize growth condition for obtaining large area, high quality uniform WS₂.   

Ion Energy Spectroscopy and its application on understanding the plasma kinetics during the Pulsed Laser Deposition of oxygen deficient single crystalline Strontium Titanate and Calcium Manganate thin films

Ion Energy Spectroscopy (IES) is a diagnostic tool that provides information about the kinetic energy distribution of ions in a plasma. Our study focuses on the plasma produced by ablation of perovskite transition metal-oxide targets during Pulsed Laser Deposition of thin films. The energy with which the ions, in the plasma impinges on the substrate, has a profound effect on the growth dynamics of thin films. Plasma plume kinetics depends mostly on target-substrate distance, laser fluence and the interaction of the plasma with the background gas. We will present data collected using IES, that maps the ion energy distribution with respect to energy, using a differential electrostatic retarding field that selects high energy cations after the removal of anions and electrons with the help of grids located in its sensor. The “spectrum” could be used to achieve films with reproducible structure-property relation for a particular target material for various combinations of growth conditions. Metal-insulator transitions of epitaxial Strontium Titanate thin films, grown under low background gas pressure (~ 10^-7 – 10^-6 Torr) are often difficult to reproduce due to stochastic distribution of defects including cation and anion off-stoichiometry. Conducting films with oxygen vacancies have shown to have excess Strontium when grown homoepitaxially. We will present data on the kinetic energy distribution of cations as function of growth parameters and discuss their correlation with the electrical properties. We will also present similar studies on the growth optimization of oxygen deficient ultra-thin films of Calcium Manganese Oxide.   

Thin Film Deposition of Nanoparticle Dispersions via Precision Inkjet Nano-Printing

This project deploys a Fujifilm Dimatix Materials Printer to print thin films of various nanoparticle dispersions such as titanium dioxide and organic thermochromic dyes. Immobilization of Titanium dioxide nanocoating will facilitate as the photocatalyst for breaking down the organic azo-dyes and polyfluoroalkyl compounds in wastewater. To successfully print a multi-layer coating on to the substrate, the size of the nanoparticle aggregations in the dispersion must be controlled using rotor-stator homogenization and subsequently with ultrasonic homogenization. The particle size is measured using dynamic light scattering, and the light absorption characteristics of the ink are measured using UV-Vis spectroscopy. The inkjet nano-printer enables precise control over the thickness of the thin films while allowing for a wide range of substrates to be used. The technology also allows for complex patterns, with resolutions of up to 30 microns, depending on the ink used. To analyze the thin films, the thickness is measured using an ellipsometer. In our preliminary research, we have optimized the homogenization processes to control the dispersed particle with size ranges below 200 nm for uniform coating and to avoid the nozzle clogging issues. Once the successful thin coating is validated, these immobilized films will be subjected to solar-photocatalysis experiment for wastewater treatment.   

Cation Interdiffusion in Amorphous Oxide Nanolaminates of TiO2 with GeO2 and with SiO2

Amorphous oxide nanolaminates consist of intercalated nanometer thick layers of two different oxides in which the optical and structural properties can be tailored by selecting the layer thicknesses. This study investigates cation interdiffusion in amorphous oxide nanolaminate designs of TiO₂ intercalated with GeO₂ and with SiO₂ after annealing at different temperatures. Nanolaminates are candidates for coatings of gravitational wave detectors to achieve low internal friction. Cation interdiffusion is assessed by X-ray Photoelectron Spectroscopy (XPS), while Grazing Incidence X-ray Diffraction (GIXRD) is used to determine the onset of crystallization. The results indicate cation interdiffusion between TiO₂ and GeO₂ occurs after annealing at 600°C, and similarly between TiO₂ and SiO₂ after annealing at 900°C and 1000°C. The onset of cation interdiffusion takes place at an average of 0.83 of the glass transition temperature (T_g) corresponding to the glass network formers, GeO₂ and SiO₂. The onset of crystallization of TiO₂ in anatase form is observed after annealing to 700°C and 600°C for the TiO₂/GeO₂ and TiO₂/SiO₂ nanolaminates, respectively. The differences in ionic radius and known modifications in the atomic range-order occurring close to the T_g could activate the interdiffusion, while thickness-dependent crystallization is observed for TiO₂.   

Effect of Etching Condition on Vibrational Characteristics of Two Dimensional MXenes

Two-dimensional materials based on transition metal carbides and nitrides called MXenes have been intensively studied due to their unique properties including metallic conductivity, hydrophilicity, and structural diversity and have shown great potential in several applications, for example, energy storage, sensing, and optoelectronics. In this work, we synthesized different batches of Ti₃C₂T_X MXenes by tuning temperature and time. The first batch of MXene was synthesized by adding Ti₃AlC₂ powders into the LiF/HCl solution and was etched for 7 days at room temperature whereas the second batch was etched for seven days at 70 °C. We observed several Raman bands including narrow peaks centered around 400 and 600 cm⁻¹ and broad peaks ~ 1365 and 1580 cm⁻¹. The presence of these lower wavenumber bands indicates the formation of Ti₃C₂T_X  whereas the broad peaks  centered  around  1350  and  1570  cm⁻¹  are related to  D and G bands of graphitic carbon. The relative intensity of Ti-C vibrational peaks is different between batch 1 and batch 2 of Ti₃C₂T_X  suggesting the change in surface structure.  The different intensity of D and G bands between two batches indicates that the extent of amorphous carbon can also be tuned by etching. These observations are further supported by X-ray diffraction and scanning electron microscopy images.     

Phase Transition in Copper Phthalocyanine Thin Films Observed in Surface Morphology

The surface morphology of organic semiconductor thin films determines the material performance for gas sensor and photovoltaic applications. Structural control of copper phthalocyanine thin films is achieved via post-annealing, altering the deposition temperature, and bi-thermal film deposition. The phase transition manifests in x-ray diffraction measurements through a peak shift from 6.8° to 7.0°. Simultaneously, the crystalline shape and alignments change in atomic force microscopy (AFM) images. At the α to β phase transition, the AFM images show local areas of both phases coexisting, while the x-ray diffraction data reveals only one phase. Thin films deposited sequentially at two deposition temperatures enable the production of crystals with predictable grain sizes, roughness, and surface morphologies.   

Interfacial Reactivity at the Interfaces of Titanium-Cobalt oxides

The chemical reactivity at the titanium-cobalt oxides has been investigated by the technique of x-ray photoelectron spectroscopy. Elemental cobalt was oxidized by two different processes: one in vacuum chamber and the other in a quartz tube furnace. These processes yielded the CoO and Co₃O₄ phases of the cobalt oxides. Thin films of titanium were deposited on these oxides kept at room temperature. Changes in the spectral features (such as the binding energy and the presence of the satellites) have been utilized to ascertain the nature of chemical reactivity at the Ti-cobalt oxide interfaces. The titanium overlayer has been observed to get oxidized resulting in the formation of TiO₂. The cobalt oxides were observed to get reduced. The CoO was observed to get reduced to elemental Co. In the case of Co₃O₄ substrate, the oxide first gets reduced to CoO for thin films of the titanium overlayer.  The reactivity has also been investigated for these interfaces as a function of substrate temperature.   

Investigating the catalytic properties of Co-doped ZnO for Fischer-Tropsch synthesis

We employ density functional theory (DFT) to examine the efficacy of Co-doped ZnO (Co:ZnO) as a Fischer-Tropsch synthesis (FTS) catalyst. This is done by investigating several molecules' adsorption and bond-breaking behavior on the 101̅0 surface of the Co:ZnO. Using Car-Parrinello molecular dynamics (CPMD), we obtain visualization of these adsorption and bond-breaking processes in CO, H2, and H2O. We also calculate the thermodynamic properties and analyze the energetics of the processes. Finally, we perform electron localization function (ELF) analysis to determine the nature of the bonds formed by the various molecules on the Co:ZnO surface. Our computational results show that Co:ZnO is a promising catalyst for the FTS process.   

Poster: An ab initio investigation of the cellulose-ice interface: towards controlled ice nucleation

Controlling the ice formation is critical to safe infrastructure design across the globe, as unwanted ice formation and its subsequent thawing can lead to catastrophic structural failures. Cellulosic hydrogels have been experimentally shown to modify the ice-nucleation kinetics by disrupting the water structure as it cools down to crystalize into ice, but the mechanistic origins of the process remains unknown. In the present work, our goal is to develop a multiscale understanding of these processes at the ice-cellulose interface from the atomic scale to the mesoscale. Here, we discuss the energetics of the cellulose-ice interface in detail by focusing on the hexagonal ice—cellulose Iβ using ab initio density functional theory (DFT) calculations by first determining the interfacial energies for various adsorption sites. Secondly, we use our findings to inform a coarse-grained model of water-polymer system with around 10,000 water molecules to examine the water-polymer interaction at room temperatures and varying pressures. Our results show that the resulting coarse-grained models based on the DFT description of the cellulose-ice interface suggest that polymer interaction can impede ice formation to varying degrees depending on the length of the polymer chains in our coarse-grained simulations. Finally, we discuss how this finding could be used to control ice-formation.   

Unexpected increase in droplet nanofriction on a lubricant-impregnated smooth surface.

Friction working between two shearing solids has been studied for a long time. Once the applied shearing force overcomes the maximum static friction force, the friction transitions to plateau dynamic mode. However, friction is not limited to solids. A recent study revealed that the droplets on a hydrophobic surface exhibit a similar friction to solids, which revealed that the effective parameters for droplet friction are the roughness of the surface, droplet width, and shearing speed.  

Recently, our group reported a new type of liquid-repellent surface, namely SPLASH(surface with π-interaction liquid adsorption, smoothness, and hydrophobicity). SPLASH is one of the lubricant-impregnated surfaces, whose base layer is smooth. Our motivation is to understand the droplet friction on the SPLASH; however, measuring the accurate droplet friction has been difficult for the classical method due to extremely low droplet friction. Here, we accurately measured the droplet friction on SPLASH using a sensitive capillary cantilever measurement that enables the detection of droplet friction with sub-µN accuracy. It seemed that droplet friction showed typical friction behavior. However, dynamical friction was increasing when droplet was reaching the edge of substrate. Investigations revealed that the friction increase is due to the long-range force from the edge of the substrate. This work experimentally study the origin of the increase in dynamic friction.    

Desorption kinetics from disordered surfaces

We present a computational study on the kinetics of desorption from disordered surfaces, both energetically homogeneous and energetically heterogeneous. When gas molecules adsorb onto struc- turally amorphous surfaces the extraction of Arrhenius parameters from experimental data becomes more difficult. By using a kinetic Monte Carlo algorithm we are able to keep track of the transient variations in the energy of activation and extract the overall preexponential factor. The presence of sites with multiple numbers of nearest neighbors result in various desorption rates, which has an effect on the preexponential factor, in addition to a reduction in configurational entropy arising from the fact that not all sites are equally accessible as they would be in an crystalline surface. Adding energetic heterogeneity to the disordered surface accentuates some of these observed effects.   

Investigation of the NiO surface reduction induced by Nd overlayers

The interface between the oxide and metal plays an important role in modern technologies like microelectronics, solar cells, magnetic storage media, and sensors. NiO is an important oxide material with potential application in several technologies. In this regard, the understanding of the interface between metals and NiO is an important research topic. Here we study the change in interface electronic properties of Nd-deposited NiO thin films utilizing X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS). We observed oxidation of deposited Nd by reacting with NiO lattice oxygen at the interface resulting in the reduction of a fraction of NiO to metallic Ni. The thickness of the interface metallic Ni layer increases linearly to 3.0 Å when the Nd overlayer thickness reaches 13.8 Å. An additional increase in Nd thickness has no further effect on interface Ni metal thickness indicating termination of the oxidation/reduction process. Such interface redox reactions could be used in special interface structures.   

Exquisite control of electronic and spintronic properties on highly porous 2D covalent organic frameworks (COFs)

Two-dimensional covalent organic frameworks (2D COFs) are layered crystalline organic porous materials. Their high surface area and excellent synthesis modularity make them attractive candidates for electronic, optoelectronic, and catalytic applications. However, their implementation is limited due to relatively poor π-delocalization. Practical applications require controlling and tuning their electronic structure. This work uses hybrid density functional theory to computationally explore a freestanding bilayer 2D COF architecture. We systematically study the intercalation of first-row transition metals in the bilayer as a method to enhance and fine-tune its electronic properties, we also explore all magnetic configurations compatible with the system's symmetry. This resulted in a total of one pristine bilayer, 64 intercalated bilayers, and one trilayer 2D COF. We find that the concentration and position of transition metals drastically change the 2D COFs' electronic and magnetic properties. Based on their spin-polarized electronic structure, we highlight potential applications in photocatalysis and optoelectronics. Finally, we discover that several of these compounds present spintronic features, including half-metal, half-semiconductor, and bipolar magnetic semiconductor behavior, which have not been widely studied for 2D COFs in the past and are difficult to find in the same family of materials.   

Effect of substrate and thickness on polycrystalline hexagonal Fe2MnSn films grown by magnetron sputtering method

Low-symmetry Heusler alloys are promising materials for next-generation spintronics devices with perpendicular anisotropy. In our prior work, we identified Fe₂MnSn as a promising material with a reasonably high magnetic anisotropy and high Curie temperature. In this work, we present the effect of substrate and thickness on polycrystalline Fe₂MnSn films grown by magnetron co-sputtering. Structure-property analysis uses pole figure x-ray diffraction, magnetoresistance, Hall effect, and Kerr effect measurements.  Films grown on c-Al₂O₃ substrate, which has a large lattice mismatch with Fe₂MnSn show a preference for the principle (201) and (200) peak. Films grown on STO(111), which has a good lattice match with the a-b plane of Fe₂MnSn, show a preference for both the (002) and (201) peaks. Pole figure scans on films grown on STO (111) show a six-fold dependence on the phi angle whereas films grown on c-Al₂O₃ and SiO₂ do not show this phi-dependent texturing. Longitudinal and transverse magnetoresistance (MR) appear qualitatively similar, ruling out anisotropic magnetoresistance as a dominant mechanism. Rather, we interpret the peaks in resistance at low fields as evidence of the suppression of spin-flip and grain boundary scattering, and the promotion of large magnetic domains and thus fewer domain walls. Longitudinal Kerr effect measurements show that films with partial (002) texture have higher in-plane magnetization 

compared to films with partial (200) texture, consistent with in-plane magnetic anisotropy in films with (002) orientation and out-of-plane anisotropy for (200) films.   

Universal Vortex Statistics and Stochastic Geometry of Bose-Einstein Condensation

The cooling of a Bose gas in finite time results in the formation of a Bose-Einstein condensate that is spontaneously proliferated with vortices. We propose that the vortex spatial statistics is described by a Poisson point process (PPP) with a density dictated by the Kibble-Zurek mechanism (KZM). We validate this model using numerical simulations of the two-dimensional stochastic Gross-Pitaevskii equation (SGPE) for both a homogeneous and a hard-wall trapped condensate. The KZM scaling of the average vortex number with the cooling rate is established along with the universal character of the vortex number distribution. The spatial statistics between vortices is brought out by analyzing the two-point defect-defect correlation function, the corresponding spacing distribution, and the random tessellation of the vortex pattern characterized by the Voronoi cell area statistics. Combining the PPP description with the KZM, we derive universal theoretical predictions for each of these quantities and find them in excellent agreement with the SGPE simulations. Our results establish the universal character of the spatial statistics of point-like topological defects generated during a continuous phase transition and the associated stochastic geometry.   

Multi-timescale ultrafast dynamics in multiferroic NiI2

Multiferroic materials have drawn a considerable amount of research attention in terms of their formation mechanisms and ferroic orderings characterization. In the context of applications that rely on dynamical control of ferroic ordering, research on non-equilibrium dynamics of the multiferroics is critical. Charge and magnetic ordering often arise from local perturbations which can respond to light on ultrafast timescales. Further, for a more comprehensive understanding, element-selective probing helps decode the intricate relationships and interplay among the material's constituents. Here, we study photoinduced ultrafast dynamics on NiI₂, a multiferroic material which has recently been reported to have multiferroic properties down to the monolayer limit, using table-top extreme-ultraviolet attosecond transient absorption spectroscopy. Leveraging the elemental sensitivity and few-femtosecond time resolution, ultrafast photoinduced electronic and structural dynamics are measured on iodine and nickel separately and simultaneously. In the electronic system of iodine, photoinduced dynamics are observed to begin abruptly and evolve on a timescale of tens of femtoseconds. Meanwhile, nickel seems to primarily respond through structural distortion after a few hundred femtoseconds. The contrast in the observed dynamics provides insights into the coupling between different ferroic orderings and paves the way for creating emerging functionalities on multiferroic materials.   

Propagation of avalanches in the disordered Heisenberg model: a computational study

The transition from ergodic to many-body localized phases is believed to be driven by an avalanche mechanism. In this process, thermalized regions form due to disorder fluctuations, promoting thermalization of their vicinity and consequent system delocalization, unless the disorder strength is sufficient to inhibit this process. We consider the XXZ model with uniform disorder in contact with a weakly disordered spin chain, comprising a finite thermal bath. By inspecting the time evolution of the two-body spin correlation functions with the bath, we are able to capture thermal avalanches spreading through the system, or a lack thereof, depending on the disorder strengths and system sizes. We also confirm that a weakly disordered bath Hamiltonian can be well approximated by a Gaussian Orthogonal Ensemble random matrix upon a proper energy rescaling. Finally, we comment on the recent result of Peacock and Sels (PRB 108, L020201 (2023)), noticing that a universal thermalizing behavior caused by a thermal inclusion may be an effect of the lack of energy conservation and our numerics suggests it does not occur for a time-independent Hamiltonian.   

Solidity of three quantum speed limits in few and many-body systems and their crossover

The quantum speed limit is a fundamental concept that governs the evolution of quantum systems, and there is an increasing demand for experimental certifications of these limits. In this study, we present a comprehensive set of experimental schemes encompassing both few-body systems (qubits and qutrits) and many-body systems (one-dimensional chains and two-dimensional square lattices) to examine how unitary time evolution is constrained by the Mandelstam and Tamm (MT), Margolus and Levitin (ML), and dual ML bounds. We rigorously demonstrate that the fidelity, denoted as ||, is strictly limited by a unified bound, which is the maximum among the MT, ML, and dual ML bounds. Moreover, our experimental setup allows us to observe crossovers between these three bounds. We provide a clear explanation for why the fidelity is bounded by the MT limit in short time intervals. For the qubit case, we analytically establish that it consistently satisfies Popoviciu's inequality, which is positioned on the semicircle of the phase diagram. By considering a unique initial pure state—a linear combination of two Fock states with maximal and minimal energy separately—we construct pathways within the phase diagram to illustrate how initially loosely bounded dynamics gradually become sufficiently constrained. Our research offers a comprehensive guide that outlines the expected features in experiments related to quantum speed limits, especially when applied to multi-qubit platforms that involve long-range couplings. This work contributes significantly to our understanding of the boundaries governing quantum evolution and provides valuable insights for experimental certification in quantum systems.   

Quantum turnstiles for robust measurement of full counting statistics

We present a scalable protocol for measuring full counting statistics (FCS) in experiments or tensor-network simulations. In this method, an ancilla in the middle of the system acts as a turnstile, with its phase keeping track of the time-integrated particle flux. Unlike quantum gas microscopy, the turnstile protocol faithfully captures FCS starting from number-indefinite initial states or in the presence of noisy dynamics. In addition, by mapping the FCS onto a single-body observable, it allows for stable numerical calculations of FCS using approximate tensor-network methods. We demonstrate the wide-ranging utility of this approach by computing the FCS of the transferred magnetization in a Floquet Heisenberg spin chain, as studied in a recent experiment with superconducting qubits, as well as the FCS of charge transfer in random circuits.   

Quantum phases in frustrated triangular and kagome optical lattices at negative absolute temperatures

We report on recent experiments investigating the phase diagram of bosons in the triangular and kagome optical lattices. We explore the frustrated physics of these lattices by creating negative absolute temperature states where the sign of the tunneling is effectively inverted. Notably, in this scheme the Hamiltonian remains static and there is no Floquet heating, leading to long lifetimes. In the triangular lattice, we observe that in the frustrated case the critical interaction strength for the bosonic superfluid to Mott insulator transition is strongly reduced, and also observe signatures of the chiral nature of the superfluid as well as indications for a chiral Mott insulator. For the kagome lattice, we investigate the many-body physics that comes into play when we melt a kagome Mott insulator into the flat band.   

Magnetic and structural properties of Cu1-xCoxFe2O4 nanoparticles prepared by a modified solgel method

A series of Cobalt substituted copper ferrite (Cu_1-xCo_xFe₂O₄) nanoparticles (x=0.0.1,0.2,0.4,0.6,0.8 and 1) are prepared by a modified sol-gel auto combustion method. The structural analysis carried out by the X-ray powder diffraction technique shows a structural transition from tetragonal (space group I41/amd) to cubic (space group Fd3m) phase is exhibited with the substitution of Cu²⁺ by Co²⁺. The crystallite size of samples varies from 18-32nm while an increase in the lattice parameter is observed with cobalt substitution. Magnetic hysteresis loop measurements have been performed using Vibrating Sample Magnetometer (VSM) at 300K and 5K over a field range of ±100kOe. Saturation magnetization shows an increasing pattern with cobalt concentration and attains a maximum value 76.13 emu/g and 82.528 emu/g for x=0.8 sample at 300K and 5K The variation of coercivity and magnetocrystalline anisotropy have been studied and the correlation between these parameters are investigated.   

Pressure and magnetic field control of the spin reorientation transition in the orthoferrite YbFeO3

YbFeO₃, a rare-earth orthoferrite, stands out due to its remarkably low spin-reorientation transition (SRT) temperature of approximately 8 K. This property makes it a compelling area of study to explore the interaction between noncollinear magnetism within the iron sublattice and the quasi-one-dimensional XXZ effective S = 1/2 chains of Yb³⁺ moments. We present the magnetic dynamics of YbFeO₃ using inelastic neutron scattering (INS), at temperatures below and above the SRT, under an applied hydrostatic pressure of 2 GPa, and in magnetic fields up to 4 T. At ambient pressure and temperatures below the SRT, the zero-field excitation spectrum is dominated by a gapped magnon mode ΔE = 0.84 meV, with dispersion exclusively along the [00L] direction. As the temperature rises above the SRT, a continuum emerges atop the magnon mode due to the increased thermal population of the magnon band. When subjected to a magnetic field, two distinct gapped modes become apparent in the INS spectra, regardless of whether the temperature is above or below the SRT. The SRT transition is clearly observable at lower magnetic fields (B < 1 T) but gradually diminishes at higher field strengths. The application of hydrostatic pressure 2 GPa effectively narrows the transition width (ΔT_SRT) and shifts the SRT to higher magnetic fields (B ≈ 3 T). We explore the impact of this applied pressure within the context of a modified mean-field theory, demonstrating that it influences the fourth-order anisotropy constant near the SRT, thus reducing ΔT_SRT.   

Improving MnAl tetragonal phase stability through doping elements

Permanent magnets are vital in countless commercial applications but the existing high performance magnets use  critical rare-earth (RE) elements making them vulnerable to supply chain disruption. This necessitates the exploration of alternative materials. Among the known intermetallic compounds, tetragonal MnAl (phase) is  one of the most studied alloys as a potential RE free magnet. However, the  phase-MnAl is metastable which can be stabilized using special synthesis  procedure. Here we have theoretically investigated the possibility of stabilizing the phase using elemental doping in empty sites. The density functional theory calculation indicates that the phase can be stabilized by doping MnAl with hydrogen, carbon, nitrogen, oxygen or fluorine. However, these increase only magnetization or magnetocrystalline anisotropy (MAE). While the hydrogen and carbon doping increase MAE but decrease magnetization, doping with fluorine increases magnetization but decreases MAE significantly. The doping with oxygen and nitrogen decrease both magnetization and MAE. However, only the carbon doped samples can be synthesized using traditional methods such as arc-melting. We have prepared carbon doped MnAl samples using arc-melting and annealing and the results are consistent with the theoretical prediction. We will discuss both  the theoretical and experimental results in this presentation.   

Magnetocaloric Effect and Mossbauer Spectroscopy Study for Defect Analysis in Gadolinium Garnets

This study discusses the effects of dopants, such as Praseodymium (Pr³⁺) and Calcium (Ca⁺²), on Gd₃Fe₅O₁₂ garnet. Samples are prepared using the auto-combustion method; all samples maintain cubic crystal structure. Praseodymium doping reduces the coordination of Fe³⁺ ions, forming penta-coordinated Fe³⁺ ions in the lattice. Similarly, maintaining charge balance, Ca²⁺ doping changes Fe³⁺ to Fe⁴⁺. The formation of Fe⁴⁺ at the down spin state (tetrahedral site) enhances the net magnetic moment of the compound. These different types of doping share similar mechanisms of lattice disorder resulting from the direct interaction of iron and oxygen tetrahedra. Thus, these defects alter the structural and magnetic order of the compound. The study explores the structural and magnetocaloric properties of the doped garnet. The magnetocaloric properties of doped garnet were assessed via isothermal magnetization curves in the field up to 5T. Gd_2.2Pr_0.8Fe₅O₁₂ compound exhibits a maximum entropy change of 2.33 J kg^-1K^-1 and a Relative Cooling power (RCP) value of 263 J kg^-1.  Meanwhile, the Gd_2.5Ca_0.5Fe₅O12 compound demonstrates even better magnetocaloric properties compared to the pure Gd₃Fe₅O₁₂ compound, with a maximum entropy change of 3.11 J kg^-1K^-1 and a notably high RCP value of 349 J kg^-1. Mossbauer studies are in progress to identify the effect of defects on iron coordination and hyperfine parameters. These findings suggest it is possible to fine-tune the magnetocaloric properties of Gd₃Fe₅O₁₂ via defects, thus offering a novel mechanism for fine-tuning the magnetocaloric properties of the compound.     

Activation energy studies for Fe-II spin crossover molecule [Fe(HB(tz)3)2] devices

Magnetic molecules continue to be competitive candidates for spintronic devices and promise ultrafast and low-power devices for data storage and magnetic sensing.  Spin crossover molecules undergo a spin-state change in the presence of an external stimulus such as light, temperature, and magnetic field, and this opens a path to nonvolatile memory devices. In particular,  [Fe(HB(tz)₃)₂] (tz = 1,2,4-triazol-1-yl) has a transition temperature above room temperature at T_c = 333 K and demonstrates a high cooperativity of 5.7 kJ mol^-1, making it suitable for spintronic device development. The power consumption of such a device directly depends on the activation energy required to change the spin state of the molecule. Previous results show that the measured activation energy of a spin crossover molecule as indicated by the transition temperature can vary with the characterization method.  For example, for a similar Fe-II spin crossover molecule, [Fe(H₂B(pz)₂)₂(bipy)] , the measured transition temperature decreased during XAS characterization as compared to magnetometry and transport measurements. Here, we characterize an [Fe(HB(tz)₃)₂] molecular device using temperature dependent Raman spectroscopy, UV-Vis spectroscopy, and transport measurements, and discuss feasibility for spintronic applications.    

Giant magnetostriction and magnetocapacitance effects in La2CoMnO6

In recent years, the B-site ordered double perovskite La₂BB’O₆ (B= Co, Ni and B’= Mn)   received much attention because of the magnetodielectric effect exhibited by them. The debate about the origin of this effect is still unsettled [1-2]. The magnetodielectric effect could also arise from the magnetostriction effect which refers to  the dimensional changes of a material under the action of an external magnetic field. However, no studies of magnetostriction available earlier, The first report of magnetostriction in La₂CoMnO₆ appeared only very recently[3]. Here, we try to understand correlation between the magnetostriction and magnetodielectric effects in La₂CoMnO₆. Polycrystalline insulating ferromagnetic double perovskite La₂CoMnO₆ possessing monoclinic structure and a high Curie temperature (T_C = 222 K) was rapidly synthesized (~30 min) by  the novel method of microwave synthesis, which involves irradiating stoichiometric mixture of oxides with microwaves of frequency 2.45 GHz. The sample exhibits negative magnetostriction (λ_par), i.e., contraction of length along the magnetic field direction in the ferromagnetic state. At 10 K, λ_par does not show saturation up to a magnetic field of 50 kOe where it reaches 610 x 10^-6 which is one of the highest values of magnetostriction found so far among perovskite oxides with 3d ions. The magnetodielecric effect of microwave synthesized shows the maximum value of 15% around 125 K for H = 5 T.  We compare the dc magnetoresistance, ac magnetoresistance, magnetic loss, and magnetostriction to address the origin of magnetodielectric effect in this compound.

References

[1] N.Ragodo et al.,  Adv. Mat. 17, 2225 (2005); P. Padhan et al., Appl. Phys. Lett.92, 022909 (2009).

[2] M. P. Singh et al., Appl. Phys. Lett. 91,042504 (2007); J. Krishnamurthy et al.,  J. Appl. Phys. 111, 024102 (2012); K. Manna et al., J. Appl. Phys. 119, 043906 (2016).

[3]  M. Manikandan, A. Ghosh and R. Mahendiran, Appl. Phys. Lett. 122, 022403 (2023).   

Tunable magnetic order in Fe-Mg co-doped montmoril-lonite nano-clay interfaced with amino acids.

The present study investigates the tunable magnetic order as well as the electrostatic and magnetic interactions due to the adsorption of the amino acids (AA) on the insulating montmorillonite (MMT) nano-clay in vacuum and in aqueous medium using the first principle density functional theory (DFT). A single layer MMT clay of thickness 0.68 nm has been co- doped with impurity atoms, Fe(II) and Mg(II), each of concentration 12.5 %. Our calculated values of interaction energies suggest that the water molecules enhances the binding affinity of AA molecules due to the formation of a strong hydrogen bonding with substantial charge transfer between AA molecules (charge donor) and nano-clay (charge acceptor). We also predicted the possible transition in magnetic orders (ferromagnetism, antiferromagnetism, and ferrimagnetism) due to adsorption of AA molecules while going from vacuum to aqueous medium which has not been reported yet. Such kind of study possess potential applications in tissue engineering, pharmacology, magnetic resonance imaging, and chemical engineering.   

Magnetic field-induced exotic ordering and peculiar ferroelectric polarization in L-type ferrimagnetic Fe2(MoO4)3

The prototype of L-type ferrimagnets (L-FiM) Fe₂(MoO₄)₃ was synthesized. It displayed a magnetic ordering at T_N1 ~12K, which is characterized by magnetic susceptibility (χ), specific heat capacity (C_p), and dielectric anomaly (ε') under the magnetic field 0≤H≤7T. Applying the magnetic field (H) induced an additional magnetic phase transition below T_N1, denoted as T_N2, along with a malleable ferroelectric polarization (P) appeared with controllable parameters of H and temperature (T) below T_N2. Consequently, a comprehensive phase diagram in the H-T space was established for spin-induced type-II multiferroics. Unlike traditional multiferroics, where a critical field-induced spin-flip P is observed, these multiferroic characteristics exhibit an exceptional nature. The unique behavior may arise from the intricate interplay of T- and H-dependent spin and lattice structures. Hence, a conceptual representation suggests H-induced potential variation in lattice symmetry and conical spin arrangement. These findings may introduce a novel mechanism for multiferroicity, necessitating theoretical and experimental investigations to understand this peculiar phenomenon comprehensively.   

Complex spin-ordering and magnetic field-induced spin-flip transition in Ni3(MoO4)2(TeO3)

High-quality single crystals of Ni₃(MoO₄)₂TeO₃ were synthesized via chemical vapor transport and verified using single-crystal X-ray diffraction (SXRD). Magnetization data revealed strong anisotropic magnetism, with two antiferromagnetic transitions at T_N1 (~17 K) and T_N2 (~14.5 K) along the H//b direction. Notably, T_N1 exhibited characteristics resembling ferromagnetism in the H⊥b direction. Neutron measurements disclosed a net magnetization along the b-axis for the Ni zig-zag chain, while adjacent chains along the a- and c-axes exhibited antiferromagnetic coupling with magnetic propagation vector of k = (1 0 0.5). The Ni(I) magnetic moment was aligned with the b-axis, whereas the Ni(II) site exhibited canting with components along ma, mb, and mc, contributing to the complex magnetic behavior of Ni₃(MoO₄)₂TeO_3.  A sharp magnetic-induced spin-flip transition occurred at H_C (critical magnetic field) of 2.5 T for H//b orientation. Furthermore, combined specific heat and magnetization measurements confirmed an unusual low-temperature shift of T_N2 with an external field up to H_C in the H//b direction and T_N2 transition disappeared for H > H_C.   

Magnetotransport characterization of spin structure and uncompensated magnetic moments in L10 MnPd (001) films

Antiferromagnets (AFs) have been attracting growing attention due to their advantages of multiple attractive properties, such as the absence of stray fields, immunity to external magnetic fields, and ultrafast terahertz dynamics. The spin structure and uncompensated moments on the AF surface play a critical role in manipulating the functional performance of AF spintronic devices. In this work, high-quality L10 MnPd(001) single crystal films were epitaxially grown on MgO(001) substrates. The antiferromagnetic attribute of L10 MnPd films is proven by the exchange bias observed in L10 MnPd/NiFe bilayers. The spin structure and the uncompensated magnetic moments in L10 MnPd(001) epitaxial films have been characterized directly by magnetoresistance measurements in various geometries and anomalous Hall effect, respectively. The Néel vector is identified to be perpendicular to the ab plane. Finally, exhibiting an anomaly at low temperatures, the sheet resistivity consists of the square and logarithmic temperature dependencies, due to magnon-electron scattering and an analogous orbital two-channel Kondo effect, respectively. Of most importance, the uncompensated magnetic moments are found to have a significant impact on the magnetotransport properties of L10 MnPd films.   

Experimental observation of the surface anomalous Hall effect in CoNi3 (001) epitaxial films

Although many attempts have been made to study the anomalous Hall effect (AHE), the contributions from the surface and the bulk have rarely been entangled. In this work, we have disentangled contributions from the surface and the bulk to the AHE in CoNi3 (001) epitaxial and CoNi3 polycrystalline films. The surface and the bulk anomalous Hall angles are found to have opposite signs. Due to the competition of the surface and the bulk effects, the anomalous Hall angle at a fixed temperature changes from negative to positive as the film thickness increases. Furthermore, when the temperature increases, the anomalous Hall angle decreases in thick films and increases in thin films. In particular, the scaling law of the bulk AHE is analyzed. The values of the scattering independent parameter in epitaxial and polycrystalline CoNi3 films are close to each other and the side-jump mechanism is negligible. Moreover, since the skew scattering parameters in polycrystalline films are much larger than those of epitaxial films and therefore strongly depend on the mechanism of the electron scattering. The experimental results are successfully replicated by first-principles calculations.   

Observation of orbital liquid in ultra-thin magnetic films

We experimentally demonstrate the existence of an orbital liquid, which similarly to quantum spin liquids is manifested by long-range correlations of orbital moments that lack ordering due to geometric frustration. Magnetoelectronic measurements of heterostructures based on ultrathin CoFeB films reveal two magnetic order parameters: one associated with spin ordering at the Curie point, and another "anomalous" order parameter with a critical point about 50K above the Curie point. Remarkably, magneto-optical Kerr effect measurements are not sensitive to the latter contribution, suggesting that its origin is qualitatively different from spin magnetism. X-ray magnetic circular dichroism measurements reveal that the "anomalous" order parameter is associated with orbital magnetism whose signatures vanish at low temperatures without the onset of orbital ordering. Our findings are consistent with ferromagnetic orbital correlations due to the Hund's interaction, which do not result in orbital ferromagnetism due to the geometric frustration. The results elucidate the role of orbital degrees of freedom in magnetism and suggest new possibilities for controlling orbital moments in magnetic materials for orbitronic applications.   

Current-induced Magnetization Switching in Ferromagnetic Heusler Alloy Co2MnAl-based Magnetic Trilayers

Spin-orbit torque (SOT) has emerged as a promising candidate for efficient magnetization manipulation of spintronic devices in the past decade [1]. In the request for high SOT efficiency, various materials and mechanisms have been explored. Recently, researchers have found ferromagnet (FM) as a high-efficiency spin source to achieve current-induced magnetization switching in the FM/Ti/FM trilayers [2,3]. In this regard, FM Heusler alloy Co2MnAl (CMA), a predicted Weyl semimetal [4] with strong anomalous Hall effect (AHE), anomalous Nernst effect and Spin Hall effect [5-7], shows great potential for SOT application. Here, current-induced magnetization switching is realized in a multilayer CMA/Ti/CoFeB(CFB)/MgO. The perpendicularly-magnetized CFB layer serves as a spin-current analyzer. The Ti spacer layer is deployed to decouple the ferromagnetic layers and to develop perpendicular magnetic anisotropy of CFB, but allows efficient spin current transmission [2,8]. The switching current density is 1.362 and 1.905 × 107 A/cm2 for ordered and disordered CMA, respectively. Furthermore, the spin Hall efficiency ξ has been determined to be -0.072 and -0.029 via the current-induced AHE hysteresis loop shift technique for the ordered and disordered CMA. The ξ of ordered CMA is larger than that of Ta, CFB and NiFe [2]. At last, we attribute the low switching current density and enhanced spin Hall efficiency to the increase of spin-orbit interaction strength with the ordered structure.

[1] A. Manchon, J. Zelezny, I. M. Miron, Reviews of Modern Physics, Vol. 91, p.035004 (2019).

[2] S. C. Baek, V. P. Amin, Y. W. Oh, Nat Mater, Vol. 17, p.509 (2018).

[3] J. Ryu, R. Thompson, J. Y. Park, Nature Electronics, Vol. 5, p.217 (2022).

[4] J. Kübler and C. Felser, EPL (Europhysics Letters), Vol. 114, p.47005 (2016).

[5] Y. M. Ji, W. X. Zhang, H. B. Zhang, New Journal of Physics, Vol. 24, p.053027 (2022).

[6] A. T. Breidenbach, H. Yu, T. A. Peterson, Physical Review B, Vol. 105, p.144405 (2022).

[7] P. Li, J. Koo, W. Ning, Nat Commun, Vol. 11, p.3476 (2020).

[8] Y. W. Oh, S. H. Chris Baek, Y. M. Kim, Nat Nanotechnol, Vol. 11, p.878 (2016).   

Room-temperature voltage-controlled spin-orbit torques in perpendicular 2DEG/Ti/CoFeB heterostructure

Two-dimensional electron gas (2DEG) emerged at the oxide interface has aroused great interest for its many fascinating properties, such as high mobility (μ), superconductivity, ferromagnetism, and strong Rashba spin-orbit coupling (SOC). The large Rashba-Edelstein effect and its inverse effect have been explored in 2DEG by using spin-torque ferromagnetic resonance (ST-FMR) and spin-pumping techniques. However, the 2DEG sandwiched between oxides requires state-of-art material engineering and limits its device design, in which direct contact of functional layer with 2DEG cannot be achieved. Here, we successfully created 2DEG at SrTiO₃ interface by utilizing the argon ions (Ar⁺) irradiation. Subsequently, the perpendicular magnetized Ti/CoFeB/MgO multilayers have been deposited on top of the 2DEG. The transmission electron microscopy (TEM) image shows the sharp interfaces and negligible inter-diffusion between layers in the 2DEG/Ti/CoFeB/MgO heterostructure. Gate voltage (V_g) dependence of longitudinal resistance (R_xx) and carrier density (n_H) at T = 300 K for the 2DEG/SiO₂ sample indicates that V_g can efficiently manipulate oxygen vacancies (OV). The magnetoresistance (ΔR_xx) at different angles (θ_H) for 2DEG/Ti/CoFeB/MgO heterostructure at T = 10 K exhibits the 2DEG characteristic. The sign change of ΔR_xx suggests that the magnetic field (H)-induced transition of interaction between the electrons and the Kondo-shielded magnetic impurities. The first (V_1f) and second (V_2f) harmonic Hall voltage at various V_g and the corresponding damping-like effective field (H_L), field-like effective field (H_T), and effective spin-torque efficiency (θ_‖,⊥) fitted from V_2f reveal that a large modulation of SOT about 550% has been achieved via V_g in 2DEG/Ti/CoFeB/MgO heterostructure. The voltage-controlled SOT in 2DEG opens the way for a new generation of spintronics devices.   

Micromagnetic Simulation Insights into the Coexistence of Neel and Bloch Skyrmions in [Pt/Co/Cu]N Multilayers

Magnetic skyrmions are one of the most promising candidates for next-generation information storage due to their small sizes, thermal stability, and high energy efficiency. Recently we have discovered magnetic skyrmions in epitaxial [Pt/Co/Cu]_N multilayers using magnetic force microscopy (MFM) and Lorentz transmission electron microscopy (LTEM) [1]. In our LTEM experiments on the devices patterned from [Pt/Co/Cu]₅ multilayers, we have found that the Bloch and Neel skyrmions can coexist upon applying current pulses. To understand this behavior, we have performed micromagnetic simulations in which [Pt/Co/Cu]_N multilayers are modeled as a stack of 3*N layers. With this micromagnetic model, we are able to reproduce several major experimental observations: (1). Out-of-plane single domain to multidomain transition between N = 4 to N = 5. (2). The diameter of skyrmions in [Pt/Co/Cu]₅ multilayers, which is ~120 nm. (3). The inherent coexistence of Neel and Bloch skyrmions that undergoes a transition with applied pulse current.   

Possible topological Hall effect in a binary Nd intermetallic compound

Skyrmions, which are particle-like spin textures with topological origin, are predicted in real space, and have been discovered in several compounds including MnSi, GdRu₂Si_2, and EuAl₄. So far, the magnetism in the materials hosting skyrmions has been attributed to either 3d transition metal ions or 4f⁷ rare earth ions with L = 0 (Gd³⁺ or Eu²⁺) [1]. Therefore, it is interesting to search for skyrmion material candidates beyond these magnetic ions, for instance, rare earth ions with L ≠ 0.

 

Here we identified an Nd intermetallic compound as such a candidate, where Nd³⁺ has L = 6. This system orders antiferromagnetically at T_N ~ 13.2 K, with multiple transitions below T_N. These transitions are also observed in zero-field neutron scattering measurements. Our compound is a superstructure of BaAl₄, with the former being structurally related to skyrmion material candidates [2,3].  We used magneto-transport data to map out a high-resolution field - temperature phase diagram. In addition, our preliminary magneto-transport measurements show a significant anomaly in the Hall effect in some of the magnetic phases, reaching values as high as 10⁵ Ohm^-1 cm^-1. This value far exceeds that expected from reciprocal space topology (10² – 10³ Ohm^-1 cm^-1), suggesting a possible origin from real space topology.

 

[1] Tokura, Y. and Kanazawa, N. Chem. Rev. 121, 2857-2897 (2021).

[2] Yasui, Y., et al. Nat. Commun. 11, 5925 (2020).

[3] Moya, J., et al. Phys. Rev. Mater. 6, 074201(2022).   

Spin Chirality in Mn3IrSi using Polarized Neutron Diffraction

Frustrated magnetic interactions within the materials create complex spin arrangements, providing an ideal platform to study spin states with nontrivial topology, such as magnetic skyrmions [1]. In this context, our study focuses on Mn₃IrSi. The crystal structure of Mn₃IrSi (P2₁3) is an ordered form of β-Mn (P4₁32/P4₃32) and exhibits a complex hyperkagome atomic arrangement [2].  Notably, the system exhibits antiferromagnetic ordering at ~ 230K. This transition is accompanied by a remarkable increase in electrical resistivity below T_N and a distinct λ-type anomaly in specific heat [3]. These observations provide compelling evidence for the possibility of a spin density wave or an antiferromagnetic gap at the Fermi surface in Mn₃IrSi, emphasizing the intricate interplay between its magnetic and electronic properties. As, the investigation of chirality in geometrically frustrated magnetic systems is of utmost importance due to its profound influence on emergent magnetic phenomena. Here, we utilized the polarized neutron scattering on the Mn₃IrSi crystal at zero field, revealing that this system exhibits mono-chiral with a left-handed proper screw-type magnetic modulation, which aligns with the non-centrosymmetric crystal structures.    

Fractional spin textures at the edges

Since the discovery of the magnetic skyrmion in MnSi, nano-scale non-collinear spin textures have attracted significant attention [1]. Of particular interest are Heusler compounds with D_2d symmetry that exhibit a new spin texture called an elliptical Bloch skyrmion, whose symmetry supports antiskyrmion [2,3,4,5]. It was found that dipole-dipole interactions and Dzyaloshinskii–Moriya interaction (DMI) contribute to the stabilization of distinct topological phases within a single compound. Recently, in the same compound, we have observed the formation and stability of fractional antiskyrmions and fractional Bloch skyrmions at the edges of thin specimens. The anisotropic DMI in Heusler compounds is a two-dimensional interaction that acts layer-wise to exhibit vertical tubular structures, which is responsible for their observation [6]. Further, we can continuously control their topological charges by tuning the field, as they either annihilate or convert into integer-charged objects. The D_2d Heusler compound family is unique for manipulating the real-space topology of spin textures since it contains both integer and fractionally charged spin textures in the same material.

 

[1] S. Mühlbauer et al., Science. 323, 915 (2009).

[2] A.K. Nayak et al., Nature. 548, 561 (2017).

[3] J. Jena et. al., Nano Lett. 20, 59 (2020).

[4] J. Jena et. al., Nat. Commun. 11, 1115 (2020).

[5] Sharma et. al., Adv. Mater. 33, 2101323 (2021).

[6] J. Jena et. al., Nat. Commun. 13, 2348 (2022).   

Generalizing Thiele equation

Recent systematic experimental results indicate a significant disparity between the skyrmion Hall angles with positive and negative topological charges. Conventional Thiele equation is not suitable for explaining the disparity. We propose a direct generalization of the Thiele equation with a transverse component of the collective coordinate of skyrmion center. The generalized Thiele equation successfully accommodate the disparity in the skyrmiin Hall angles.   

Controllable Skyrmionic Phase Transition between Néel Skyrmions and Bloch Skyrmionic Bubbles in van der Waals Ferromagnet Fe3-xGeTe2

The van der Waals (vdW) ferromagnet Fe3-xGeTe2 has attracted much research interest as a host for magnetic skyrmions. Despite numerous investigations, identifying the origin of the Dzyaloshinskii–Moriya interaction (DMI) and achieving a controllable acquisition of the skyrmion phase in Fe3-xGeTe2 remains challenging. In this study, we comprehensively investigated the crystal structure and magnetic properties of Fe3-xGeTe2 with varying Fe concentrations. Our results revealed a pronounced Fe atom displacement that increased with decreasing Fe content. Combined with first-principles calculations, we found that this atomic displacement caused the original centrosymmetric crystal structure to transform into non-centrosymmetric symmetry, resulting in a considerable DMI. Moreover, by tuning the Fe content and sample thickness, we achieved a controllable skyrmionic phase transition from Néel-type skyrmions to Bloch-type skyrmionic bubbles. Micromagnetic simulations revealed that this transition was governed by a delicate competition between dipole–dipole interaction and the DMI. Our findings may help resolve the protracted debate on the origin of the DMI and variable skyrmionic phases in Fe3-xGeTe2, which are of great importance for exploring vdW ferromagnet-based spintronic devices.   

Anisotropic Magnetoresistance in Co2MnGa Heusler Alloy Films in Near-zero Magnetic Field

We present precision magneto-transport measurements in 0.5 micron thick Co2MnGa films grown on sapphire substrates via magnetron sputtering. The sample is a Hall bar with dimensions 1 x 3 mm. At magnetic fields B between zero and approximately 500 Gauss, we observe a pronounced reduction in the sample resistance of order of 1-5 parts in 10,000, approximately quadratic in B, when the magnetic field is orthogonal to the film plane. This change is at least an order of magnitude larger than that with the magnetic field oriented along the direction of the sample current. We also present data for the Hall coefficient and discuss the implications of the findings for assessing magnetic order, including the formation of zero-dimensional topological textures (skyrmions), in these films.   

Spin Fluctuation Dominated Magneto-transport in Skyrmionic Co8Zn8Mn4

The β-Mn type Co-Zn-Mn alloys have attracted significant attention due to their ability to host skyrmions at room temperature. Here we discuss the synthesis and magneto-transport properties of high-quality single crystals of Co₈Zn₈Mn₄ with a Curie temperature of 275 K. A linear magnetoresistance (MR) is observed at low temperatures (Tc), the MR curve shows a nonlinear nature which corroborate to the dominance of spin fluctuations over magnons. Scaling of anomalous Hall resistivity with longitudinal resistivity reveals the dominance of skew scattering mechanism. The correlation between the skew scattering contribution and magnetoresistance reveals that the skew scattering contribution stems from the spin fluctuations. Our work provides new insight on the dominance of varied scattering mechanisms in chiral magnets with low conductivity.   

Witnessing entanglement of quantum spin liquids via covariance noise magnetometry with two diamond spin qubits

Dissipation in quantum systems, often considered as detrimental to the development of quantum computing platforms, has recently revealed promising applications in quantum metrology. In this context, noise can be harnessed to probe the quantum properties of exotic materials. Notably, NV (Nitrogen-Vacancy) sensors have emerged as a prominent platform for measuring local properties of magnetic materials. However, conventional measurement techniques are limited in accessing non-local properties. In this study, we propose a two NV sensor setup capable of detecting real-time detection of spin-spin correlations, even at extended distances. This innovative approach allows us to probe the dynamic structure factor, a property typically accessible only through neutron scattering experiments. We have evaluated the accuracy of this procedure using the Cramer-Rao bound and demonstrated its application to the Kitaev model. Our results reveal that quantum Fisher Information can be derived using this tabletop setup, thereby enabling us to characterize the entangled nature of such materials.   

Topological ferromagnetism in Rashba systems

We unravel a previously unexplored mechanism instigating magnetic instabilities both in three-dimensional and two-dimensional Rashba systems. These instabilities arise from the spin-dependent interactions of itinerant electrons, stemming from the Rashba spin-orbit coupling invoked by their mutual Coulomb fields. The ferromagnetic transition is associated with the rotation symmetry breaking not only in the spin, but also in the orbital spaces. The latter effect leads to deformation of the Fermi surface of the polarized itinerant electrons into the ellipsoidal or toroidal shape. It may undergo genus-changing topological Lifshitz transition between these shapes as a function of the spin-orbit coupling magnitude or interactions strength. The transition may have a number of observable manifestations in thermodynamic and and transport properties, as well as in spectra of Friedel and Shubnikov-de Haas oscillations.   

Nuclear spin induced scattering between quantum Hall edges with opposite spin polarization

We consider the nuclear spin induced scattering between two quantum Hall edge state channels with opposite spin polarization. Nonequilibrium spin polarization of nuclear spins is calculated, and dependence on temperature, voltage and magnetic field is analyzed. Nuclear spin relaxation due to spin diffusion caused by magnetic dipole-dipole interactions is considered. A renormalization group (RG) analysis of the hyperfine interaction is performed and a RG flow is obtained for the filling factor $ u=1/q$ case, $q$ being an odd integer. The RG flow appears to be different from that for edge states of a 2D topological insulator. Our results are relevant to recent experiments on nuclear spin related transport in the quantum Hall system and may provide a way to obtain dynamic nuclear polarization.   

Field-induced transformation of ellipsoidal cycloidal to conical spin order and possible magnetoelectric effect in a triangular antiferromagnet MnGeTeO6

The non-centrosymmetric layered honeycomb tellurate compound MnGeTeO₆ possesses a spin-frustrated triangular magnetic lattice, rendering it a compelling candidate for new multiferroic material. A high-quality MnTeGeO₆ polycrystalline sample was synthesized via solid-state synthesis. Magnetic and specific heat measurements confirmed an antiferromagnetic long-range ordered temperature (T_N) at 9.4 K, accompanying a metamagnetic phase transition observed in isothermal magnetization M (H) curves below T_N. Interestingly, a significant frequency-independent dielectric anomaly was observed near T_N in the temperature-dependent dielectric curves ε' (T). The scaling of magnetodielectric (MD) coupling with magnetization corresponds to the possible existence of higher-order magnetoelectric coupling. In this regard, the possible oblique conical type of spiral spin texture probably induces the observation of the higher-order magnetoelectric coupling, which underscores its substantial potential as a new magnetoelectric material.   

Feature extraction from Artificial Spin Ice using Restricted Boltzmann Machine

Magnetic Artificial Spin Ice are arrays of magnetic nanoparticles that can exhibit unusual phenomena due to geometric frustration and have been studied theoretically and experimentally for several years. Thermal properties are particularly interesting, and in recent years, various geometries have been found that show intriguing properties, such as kinetic topological ordering [1]. Many features can be difficult to identify from numerical or experimental data. We present the first results of a work in progress that is aimed towards developing tools that will be useful for identifying and categorizing novel features that can be observed in Artificial Spin Ice. We show how a Restricted Boltzmann Machine approach can identify fine structures in conditional probabilities that can be associated with observable quantities. We apply this to problems of parameter determination and defect mechanisms. Also, this approach can be used to generate data efficiently for sampling problem applications.

[1] R. F. Wang. Nature,439(7074):303–306, 2006   

Thermal Expansion Anomalies in Spinels: CdCr2O4 and Beyond

Although most materials tend to expand when heated, a select few, such as water ice, elastic bands, and invar alloy, display an anomalous phenomenon known as negative thermal expansion (NTE). Recently, it was discovered that CdCr2O4, a chromium spinel oxide, exhibits NTE within its ordered half-magnetization plateau phase. This phenomenon was also observed in an effective spin-lattice coupled bond-phonon model on the pyrochlore lattice [1].

In this study, we revisit CdCr2O4 using a more realistic site-phonon model that includes intra- and inter-tetrahedra interactions on the pyrochlore lattice [2]. We construct order parameters from the irreducible representations of the possible ground state spin configurations to map the temperature and magnetic field phase diagram through classical Monte Carlo simulations.

Our results demonstrate the presence of NTE in the half-magnetization plateau phase of the site-phonon model, thereby confirming the entropy-driven NTE mechanism presented in [1]. We further investigate the model's ability to explain other types of NTE observed in spinels, including at zero field.

[1] L. Rossi et. al., Physical Review LeAers 123, 027205 (2019).

[2] K. Aoyama et. al., Physical Review B 104, 184411 (2021).   

An Intuitive Understanding of the Spin Excitations of a 1D Antiferromagnet

Antiferromagnetic Heisenberg spin model is one of the basic models in quantum magnetism. Although in two- and three-dimensional world its ground state has a classical order, in one dimension (chains) there is no long-range order and the exact form of the ground state is provided by a rather complex Bethe Ansatz. Even more intricate are the low energy magnetic excitations of a spin chain — the notorious spinons, carrying fractional quantum numbers. In this poster we show an alternative approach to the ground state as well as the low-lying excitations of the one-dimensional Heisenberg antiferromagnet, which, although approximate, gives a more physically intuitive picture than the exact Bethe Ansatz solution.   

Title: From clusters to crystals: Magnetism of otherwise nonmagnetic elements in two-dimension

Clusters, due to their reduced size and uncommon geometry, exhibit properties unknown in their bulk phase. However, most of the clusters lose their identity when assembled into periodic structures. A recent combined experimental and theoretical paper showed that a cluster composed of one uranium and six gold atoms, (UAu₆), both of which are nonmagnetic in the bulk, possesses a magnetic moment of 3 localized at the uranium site and originating from its f-orbitals. A systematic theoretical investigation is conducted to analyse the physical and magnetic properties of UAu₆ and its cousin sisters UAg₆, UCu₆ clusters. We have found that the geometry coordination as well as the magnetic signature of the cluster remains intact when assembled into a two-dimensional structure UX₄ (X: Au, Ag, Cu). The ground state magnetic configurations (FM or AFM) have been identified for 2D monolayers and their magnetic phase transitions have been studied under external perturbations.   

Magnetic interactions in the single layer copper and nickel hydroxides synthetized in the confined space of layered silicate

Layered hydroxides, such as the copper hydroxide are quasi two dimensional magnetic materials whose structures consist of alternating stacking of the magnetic copper hydroxides and the carboxylate layers [1,2]. Its magnetic properties are fascinating, and it is the example of 2D Heisenberg spin ladder.

In our work, we prepared for the first time single-layer Cu(OH)₂ [3] and Ni(OH)₂, by synthesis inside the layered phyllosilicate sodium fluorohectorite clay (Fht) [4]. Fht is a structure composed of 1 nm thin sandwich of SiO₂ and metal oxides, and these sheets are negatively charged and kept together by interlayer cation.

We used several characterization techniques such as measurements of magnetic properties, X-ray diffraction and small angle neutron scattering. Instead of the single layer, hydroxide is in the form of app. 10 nm large 1 nm thin islands. For Cu(OH)₂, magnetic behaviour is very complex, exhibiting 3 types of magnetic interactions – in the Cu(OH)₂ itself, among the island inside the single interlayer and among islands across the Fht sheet.

So far, there is no method how to prepare single layers of Cu and Ni hydroxides, hence our method of synthesis in the confined space of layered silicate is very unique.

 

[1]. M. Suzuki, et al. Phys Rev B 64, 1–8 (2001) [2]. G. J. da Silva, et al. Phys Rev E 66, 1–8 (2002) [3] Wataru Fujita, et al. Appl Clay Sci 15, 281-303 (1999). [4]. H. Hemmen, et al. Langmuir 28,1678–1682 (2012)

    

Toroidal magnetic molecules stripped to their basics

Magnetic molecules are investigated with respect to their usability as units in future quantum devices. In view of quantum computing, a necessary prerequisite is a long coherence time of superpositions of low-lying levels. In this presentation, we investigate by means of numerical simulations whether a toroidal structure of single-ion easy anisotropy axes is advantageous as often conjectured. Our results demonstrate that there is no general advantage of toroidal magnetic molecules, but that arrangements of tilted anisotropy axes perform best in many cases.

[1] D. Pister, K. Irländer, D. Westerbeck, J. Schnack, Phys. Rev. Research 4, 033221 (2022)

[2] K. Irländer, J. Schnack, Phys. Rev. Research 5, 013192 (2023)   

Matsubara Green's function approach to study spin-1 lattice systems

The theoretical development of this work is focused on two main aspects. Firstly, it involves describing fundamental features of spin-1 systems, introducing spin-coherent and quadrupolar states that define the system's local physics. This includes the corresponding operators related to local order parameters. Secondly, it encompasses the introduction of theoretical techniques for studying local and non-local phases within a functional theory framework. This is achieved through constructing a path integral partition function using the Matsubara formalism, linking it to Green's functions, and computing observables using an effective theory developed through the Hubbard-Stratonovich transformation. at the end we get the characterization of the Ising-Ferromagnetic, Ising-Nematic, and XY-Ferrouadrupolar phases through local decoupling at low temperatures, along with their respective phase transitions. Furthermore, characterization of the Dimer phase and its properties at low temperatures is conducted with respect to the applied quadratic field through non-local analysis.   

The scaling relation of anomalous Hall effect in van der Waals ferromagnet Fe3GeTe2

According to the Mermin-Wagner theory, two-dimensional (2D) long-range magnetic order would be destroyed by thermal fluctuation at any nonzero temperature. Until 2017, magnetic 2D materials CrI3, Cr2Ge2Te6, and Fe3GeTe2 (FGT) were experimentally confirmed, which attributes to their strong magnetic anisotropy energy against the thermal fluctuation. Since then, the pursuit of high Curie temperature 2D magnets, such as FenGeTe2 (3 < n < 5, T_c=200∼400K) and Fe3GaTe2 (T_c~367 K), has emerged as a prominent research focus.
The vdWs ferromagnet FGT has recently been found to exhibit complex physics phenomena, including heavy fermions, Kondo lattice, and topological nodal lines. Here, we investigated the scaling relation of the anomalous Hall effect (AHE) in FGT nanoflakes by varying the flake thickness and the temperature. Our findings reveal that the scaling relation of the AHE in FGT falls within the cross-region between the intrinsic and dirty regimes, indicating both the intrinsic Berry curvature and hopping transport origin in the AHE of FGT. We observed that the scaling relation of AHE follows the relationship σ_AH ∝ σ_xx^γ, where γ ranges from 1.4 to 2 as temperature increases from 2 K to its Curie temperature ~200 K. Besides, the low-temperature anomaly of longitudinal (ρ_xx) and anomalous Hall resistivity (ρ_AH) are observed, which are interpreted as three-dimensional enhanced electron-electron interaction caused by Fe vacancies and its quantum correction to AHE, respectively.   

Spin Space Groups: Full Classification and Applications

In this work, we exhaust all the spin-space symmetries, which fully characterize collinear, non-collinear, commensurate, and incommensurate spiral magnetism, and investigate enriched features of electronic bands that respect these symmetries. We achieve this by systematically classifying the so-called spin space groups (SSGs) - joint symmetry groups of spatial and spin operations that leave the magnetic structure unchanged. Generally speaking, they are accurate (approximate) symmetries in systems where spin-orbit coupling (SOC) is negligible. We - for the first time - obtain the complete classifications of 1421, 9542, and 56512 distinct SSGs for collinear (N=1), coplanar (N=2), and non-coplanar (N=3) magnetism, respectively. SSG not only fully characterizes the symmetry of spin d.o.f., but also gives rise to exotic electronic states, which, in general, form projective representations of magnetic space groups (MSGs). Surprisingly, electronic bands in SSGs exhibit features never seen in MSGs, such as nonsymmorphic SSG Brillouin zone (BZ), where SSG operations behave as glide or screw when act on momentum and unconventional spin-momentum locking. To apply our theory, we identify the SSG for each of the 1604 published magnetic structures. Material examples exhibiting aforementioned novel features are discussed with emphasis. We also investigate new types of SSG-protected topological electronic states that are unprecedented in MSGs.   

Collective excitation of electron spin structure in titanium-based MXenes

Using the second quantized formalism, magnetism and the behavior of the quantized spin wave, called magnons, are analyzed in the titanium-based MXene.  Magnons are spin-1 bosons carrying energy and lattice momentum in crystals and depict the wave propagation due to a small fluctuation of electron’s moment around the natural direction in magnetic substances. The electronic and magnetic properties of these MXenes are evaluated using density functional calculations. Structural and termination group dependency of electronic and magnetic properties of MXenes make them very interesting. For example, pristine Ti₃N₂ has zero electronic band gap and shows ferromagnetic ordering with a total magnetization of about 0.8 Bohr magneton per formula unit in the hexagonal layered structure. Interestingly, ferromagnetism disappears in Ti₃N₂’s comparatively less symmetric Corundum trigonal structure. We semi-analytically investigate the collective excitation of electron spin structure in Ti₃N₂ and Ti₂N. The exchange energy parameter is evaluated using density functional theory, which is then used to set up the Heisenberg Hamiltonian and magnon dispersion relation. The Department of Energy BES-RENEW award number DE-SC0024611 has supported this research work.   

Structural and Physical properties of 2-dimensional Fe3GeTe2

Two-dimensional (2D) magnetic materials have received a considerable attention recently because they offer scientific and technological advancements due to low-dimensional quantization of electronic states. One of the most studied such systems is Fe₃GeTe₂. It crystalizes in a hexagonal structure and is an itinerant ferromagnetic system with a Curie temperature of 220 K of the bulk form and a strong magnetic anisotropy. In this study, the Fe₃GeTe₂ samples were synthesized using a chemical vapor transport method and characterized using TEM, EDAX, and Raman spectroscopy. To tune the electronic properties of 2D materials, strain engineering has been demonstrated as a powerful technique. Here, monolayers or few layers of Fe₃GeTe₂ were transferred on to polyethylene terephthalate (PET) and bent inward (compressive) or outward (tensile) to apply strain. In-situ Raman spectroscopy was utilized to study the effect of both compressive and tensile strain on the vibrational modes of the material. Since the magnetic domains of the magnetic materials are affected by the crystal orientation, polarized Raman spectroscopy was used to study the material. Magnetic phase transition (para to ferro) was explored using Electron paramagnetic Resonance Spectroscopy (EPR). Temperature and magnetic field dependent 4-probe resistance, R(T,B) and thermopower, S(T,B) were measured in the temperature range of 40-300 K and magnetic field -1 to +1 T.  R(T) showed a decrease of resistance with decreasing temperature confirming its metallic behavior, while S(T) showed a change of thermopower from its negative value at higher temperatures to a positive value below 180 K. The results of S(B), R(B) and magnetic field dependent Hall Voltage will also be presented.   

Temperature- and magnetic field-dependent Raman spectroscopy of layered, antiferromagnetic FePS3

J. A. BUCHMAN, J. R. SIMPSON, Towson University, Towson, MD 21252, T. T.MAI, K. F. GARRITY,  A. R. HIGHT WALKER, National Institute of Standards and Technology (NIST), Gaithersburg, MD20899, R. VALDE ́S AGUILAR, The Ohio State University, Columbus,OH 20899 —The recent discovery that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer stimulates a thorough Raman spectroscopic study of one such material, FePS₃, a large spin ($S = 2$) Mott insulator where the Fe atoms form a honeycomb lattice. Bulk FePS₃ was shown to be a quasi-2D Ising antiferromagnet, with additional features in the Raman spectra emerging below the Neel temperature (T_N ~ 120K). Previous magneto Raman measurements[1] demonstrated that one of these Raman-active modes below T_N is a magnon. Additional low-frequency, Raman-active modes demonstrate unusual temperature dependance, which we measure using a triple grating Raman spectrometer. In this process, we design and implement an entrance optical path, an alignment procedure, and an excitation light clean up scheme. Furthermore, we use group theory analysis of the polarization-dependent spectra to identify symmetry designations of Raman-active modes. Finally, we compare our spectra with predictions from density functional theory to discuss the origin and anomalous temperature dependence of additional low-frequency Raman-active modes. 

[1] A. McCreary et al., Phys. Rev. B 101, 064416 (2020).   

Chiral Inversion of Resonant Modes in GdxFe1-x Ferrimagnetic Alloy

Antiferromagnetic materials exhibit higher frequencies (up to THz) compared to ferromagnetic materials and demonstrate better magnetic field stability due to the absence of stray fields. They possess a twin sublattice structure, and their intrinsic magnetic excitation modes exhibit both left-handed and right-handed precession modes. However, controlling their magnetic order with an external magnetic field is challenging as their net magnetic moment is zero. In contrast, ferrimagnetic materials, with a twin sublattice structure and partial net magnetic moment, provide a suitable platform for studying antiferromagnetic spin dynamics. In ferrimagnetic alloys with specific composition ratios, compensation temperatures for magnetic moment and angular momentum exist due to thermal excitation. Moreover, as the composition ratio changes, points of magnetic moment compensation (PM) and angular momentum compensation (PA) are observed. In this study, we employed atomic-scale micromagnetic simulation techniques considering the lattice spatial structure to investigate the chirality reversal characteristics of GdxFe1-x ferrimagnetic alloys at different composition ratios and the variations of spin wave precession near the PA point under an external magnetic field. Our results indicate that the chirality of the resonant mode in GdxFe1-x alloys undergoes reversal with an increase in Gd ratio. Additionally, the energy degeneracy points of the two resonant modes deviate from zero magnetic field, distinguishing them from antiferromagnetic materials, owing to the different saturation magnetization strengths of the two sublattices in GdxFe1-x alloys. That is to say, the chirality of the spin resonance mode changes with variations in the external magnetic field under the same composition ratio. We also provide a theoretical explanation for this phenomenon.   

Magnon decays in magnets with long-range interactions

A recent experimental study realized long-range XY order in a ferromagnet at finite temperature using a 2D Rydberg atom array. To prepare this XY ordered state, a staggered magnetic field was applied; we aim to understand the effect of this staggered field on magnon decay rates and on spin correlations, and how the 1/r³ ("dipolar") interactions affect the kinematics of magnon decay. Using spin-wave theory to study the dipolar XY ferromagnet and antiferromagnet on the square lattice, we focus on (i) the kinematics of magnon decay and (ii) the magnon decay vertices. A field-independent region of kinematically allowed field-induced decays, dictated by the lattice symmetry, is observed. Cubic vertices are used to calculate magnon self-energies, which can then be used to calculate spin correlation functions.   

Magnon-Phonon Coupling in Twisted 2D van der Waals Magnets

Since the moiré superlattice was discovered in a graphene system, the new rise of the twistronics greatly expanding the application prospects of 2D materials. In heterostructures and homostructures formed by 2D atomic crystals, the stacking configuration of adjacent layers are strongly correlated to the physical properties of the van der Waals materials[1], bringing various novel phenomena such as unconventional superconductivity. Recently, it has been shown that the magnetic properties of 2D magnets can be controlled by the twist angle of magnetic moiré superlattices[2]. The effect of twist angel and local interlayer exchange coupling on the formation of domains has been reported in the literature[3]. According to some recent experimental works, we find that magnons may be strongly coupled to phonons at certain twist angles. So here, we propose a modified theoretical model based on the existing theories[4] to study the behavior of magnons in twisted 2D van der Waals magnets system. We introduce a term related to the action of phonons in the equation and improve the approximation method used for solving the equation. Our work is an enrichment of the magnetic dynamics in twisted 2D van der Waals magnets system.

Corresponding email: zhang@tongji.edu.cn   

Exploring Melting Phenomena in Self-Organized Magnetic Structures through Variational Autoencoder Analysis

The investigation of phase transition phenomena is a crucial aspect of various physics studies. However, defining order parameters in complex systems with self-organized structures presents challenges. This work introduces a method employing a variational autoencoder network for the definition of order parameters.

The effectiveness of our approach is demonstrated through the training of a deep learning network on a dataset containing spin configurations within a chiral magnetic system across diverse temperature conditions. By mitigating thermal fluctuations in the input data, the network preserves essential structural information, focusing on spin magnitudes. We utilize this information to establish an order parameter based on magnitude and validate our results against conventional analyses, revealing consistent outcomes.

Using the order parameter, we examine the thermal properties of the chiral magnetic system. Through systematic variations of physical parameters and data sizes, our investigation provides insights into the system's response to changing conditions, contributing to a nuanced understanding of its thermal behavior.   

Emergence of a high Tc magnetic phase in a time-elapsed vdW magnet

Long-range magnetic order in van der Waals (vdW) materials, exfoliated in few atomic layers, can be tuned via application of electric/magnetic field, interface engineering or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition causes degradation of 2D magnetic flakes which, in turn, affects the electronic/spintronic device performance in nanoscale. Counterintuitively, our current study reveals that exposure to the air at ambient atmosphere results in advent of a stable secondary ferromagnetic phase in the form of Cr₂Te₃ (T_C2 ~ 160 K) in the parent vdW magnetic semiconductor Cr₂Ge₂Te₆ (T_C1 ~ 69 K) [1]. As a result, the magnetic anisotropy energy (MAE) gets enhanced in the hybrid system by an order compared to the pristine Cr₂Ge₂Te₆, increasing the stability of the ferromagnetic ground state with time. The coexistence of the two ferromagnetic phases is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, Raman spectroscopy and magneto-transport measurements. In contrast to rather common poor environmental stability of the vdW magnets, our results open possibilities of finding air-stable novel materials having multiple magnetic phases [2].   

Toward Optically Induced Antiferromagnetic-to-Ferromagnetic Transitions in Few-Layer CrI3 and Fe3GeTe2

Materials with controllable antiferromagnetic-to-ferromagnetic (AFM-FM) transitions are high interest targets for data storage research. AFM-FM transitions in few-layer van der Waals magnets, such as CrI₃ and Fe₃GeTe₂, have been addressed with electrostatic fields, bearing promise for compact on-chip memories. Here, we explore the possibility of all-optical AFM-FM switching in bi-layer CrI₃ and Fe₃GeTe₂ systems in theory, using Density Functional Theory (DFT) and tight-binding models, and experiment. We attempt to extend the analysis to describe AC fields through numerical evaluation of model Hamiltonians and Time Dependent Density Functional Theory (TDDFT). The latter reveals nearly adiabatic evolution of CrI₃ under mid-IR excitation, suggesting that the observed AFM-FM switching seen with the application of static fields may also be realized optically. Our experimental focus is the investigation of the 2D material Fe₃GeTe₂. We report on successful fabrication of few-layer Fe₃GeTe₂ samples and present a method for realizing the switch via a pump-probe Magneto-Optical-Kerr-Effect (MOKE) setup. Our models and measurements will further our understanding of ultrafast magnetism at the 2D limit.   

Poster : Spectroscopic Study of the Electronic Structure of Two-dimensional Magnetic Materials

The recent discovery of ferromagnetism in 2D van der Waals chromium trihalides CrX₃ (X=Cl, Br, and I) down to the monolayer has gained research attraction because of their interesting electronic and magnetic properties. The magnetic properties of CrX₃ can be manipulated by applying perturbations such as external magnetic field, strain, and pressure. This makes CrX₃ prime candidates for spintronics and magneto-resistive memory applications. It also highlights the importance of determining the key energy scales properly to understand the physics of CrX₃ and build a more reliable base Hamiltonian. We have measured Cr L-edge soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) spectroscopy for all CrX₃ in order to understand their electronic structure. Through a systematic study, with the use of atomic multiplet simulations, we show that our approach has yielded a set of more reliably determined energy scale parameters. Ultimately, our goal is to achieve a detailed understanding of the electronic structure of CrX₃ and determine how it is related to magnetic order and excitations in these fascinating systems.   

Energy Efficient Manipulation of Stochastic Switching Behavior in Dual-biased Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) have been identified as excellent candidates for generating stochastic signal, making them a promising solution for implementing stochastic computing (SC) due to their inherent randomness [1,2]. By leveraging the competition between the spin transfer torque and the external magnetic field (dual biasing method) [3], it becomes possible to adjust the average switching rate and average output level independently, thereby addressing the issue of device variation arising from the fabrication process. However, a significant increase in energy consumption is anticipated when attempting to simultaneously bias all the MTJs in the array network due to the limited current spin efficiency in each device. This counteracts the advantage of stochastic computing. In this report, the voltage-controlled exchange coupling (VCEC) switching mechanism [4] is utilized in place of the spin transfer torque as one of the two biases. It is observed that the tunability of stochastic switching is maintained, with the estimated energy consumption being around 2 orders of lower than the typical STT-MTJs in previous dual-biased MTJs study [3]. The results demonstrate the feasibility of VCEC in dual biasing method, taking a significant step toward the application of MTJs in stochastic computing.

 

[1] Yang Lv, et al, “A single magnetic-tunnel-junction stochastic computing unit”, IEDM, DOI: 10.1109/IEDM.2017.8268504 (2017)

[2]    Brandon Zink, et al,. “Review of Magnetic Tunnel Junctions for Stochastic Computing.” IEEE Journal on Exploratory Solid-State Computational Devices and Circuits 8, no. 2 (December 2022): 173–84.

          [3]    Brandon Zink et al, “Influence of Intrinsic Thermal Stability on Switching Rate and Tunability of Dual-Biased Magnetic Tunnel Junctions for Probabilistic Bits.” IEEE Magnetics Letters 12 (2021): 1–5.

           [4] D. Zhang, et al., Nano Lett. 22, 622–629 (2022), DOI: 10.1021/acs.nanolett.1c03395..   

Pressure induced modification of electronic and magnetic properties of MnCrNbAl and MnCrTaAl

Spin-gapless semiconductor (SGS) is a new class of material that has been studied recently for potential applications in spintronic devices. These materials behave as an insulator for one spin channel, and as a gapless semiconductor for the opposite spin. In this work, we present results of a comprehensive computational study of two quaternary Heusler alloys, MnCrNbAl and MnCrTaAl that have been recently reported to exhibit spin-gapless semiconducting electronic structure. In particular, using density functional calculations we analyze the effect of external pressure on electronic and magnetic properties of these compounds. It is shown that while these two alloys indeed exhibit nearly SGS behavior at optimal lattice constants and at negative pressure (expansion), a closer inspection reveals that they are half-metals at equilibrium, and magnetic semiconductors at larger lattice constant. At the same time, reduction of the unit cell volume has a detrimental effect on electronic properties of these materials, by modifying the exchange splitting of their electronic structure and ultimately destroying their half-metallic / semiconducting behavior. Thus, our results indicate that both MnCrNbAl and MnCrTaAl may be attractive materials for practical device applications in spin-based electronics, but a potential compression of the unit cell volume (e.g. in thin-film applications) should be avoided.   

High magnon modulation with low current density in thin film lithium aluminum ferrite (Li0.5Al1.0Fe1.5O4)

Spin wave-based spintronics are an alternative to conventional electronics due to their potential for efficient energy consumption, improved processing speed and smaller device dimensions. The success of spin wave spintronics relies on the existence of low damping magnetic insulators that allow for efficient spin wave generation, manipulation, and detection. A promising candidate is the recently developed thin film Li_0.5Al_1.0Fe_1.5O₄ (LAFO) with low Gilbert damping parameter on the order of 10^-4 in 15 nm thick LAFO. In this talk, we present our results on non-local magnon transport in Li_0.5Al_1.0Fe_1.5O₄(LAFO), a magnetic insulator, grown on (100) oriented MgAl₂O₄(MAO). By studying the non-local voltage amplitude generated via the inverse spin Hall effect as a function of the separation of two overlaying Pt electrodes, we deduce a spin diffusion length in 15 nm thick LAFO of 1.9 um which is consistent with our previous results. By adding a third DC bias Pt electrode (modulator) between the two Pt electrodes, we can modulate the non-local voltage amplitude by a factor of 3.5 with current densities on the order of 10¹¹ A/cm², significantly lower than the current densities needed to achieve the same modulation in Y₃Fe₅O₁₂ (YIG). Additionally, from the relationship between the voltage amplitude and DC bias current, we can extract a critical current I_c, where the damping-like torque on the magnons underneath the Pt electrode is fully compensated by the spin-orbit torque from Pt. I_c is monotonically related to the width of the modulator which is in good agreement with theoretical predictions. These results show the promise of spin waves as a new platform for information processing.    

Optical measurement of temperature dependent antiferromagnetic resonance on CrSBr

NV centers in diamond are single-spin color centers whose photoluminescence (PL)depends on their spin-state populations. This spin population can be hyperpolarized into the m_s=0 state. Driven antiferromagnetic resonance (AFMR) modes decays into high frequency magnons, and multi-magnon scattering can generate electromagnetic radiation at frequencies matching the NV center magnetic resonance frequency thus enhancing spin relaxation that modifies the spin state populations and hence PL intensity. CrSBr is a van der Waals antiferromagnet with promising magnetic properties for memory devices. Below its Neel temperature (132K), CrSBr exhibits intralery ferromagnetic order; a weaker interlayer antiferromagnetic exchange coupling leads to AF order. The weakness of the AF coupling leads to AF resonant frequencies in the tens of GHz range. We present results on the optical measurement of this temperature dependent AFMR as measured with NV color centers.   

Effects of composition gradients on temperature dependent magnetization and coercivity in CoGd alloys

A great deal of attention has been given to ferrimagnetic alloys as potential building blocks of spintronic devices. Studies have recently shown the emergence of bulk Dzyaloshinskii-Moriya interactions and spin-orbit torques in vertically inhomogeneous single layer ferrimagnetic films. In this work, we utilize atomistic spin simulations to explore how the local magnetization varies in CoGd alloys, both due to the decreased coordination number at surfaces as well as due to vertical inhomogeneities, and how this in turn effects film behavior such as coercivities of selected films. We find that, while compositional modulation varies the local compensation point through the film thickness, it has no significant effect on the net compensation temperature of the alloy if the average composition stays the same, even with variations as large as 50% from top to bottom interfaces. However, even minor variations in composition can drastically reduce the out-of-plane coercivity of the films, or even preclude perpendicular anisotropy entirely. This provides new insights into limitations of ferrimagnetic materials with non-uniform properties for the incorporation into spintronic devices.   

Spin dynamics in van der Waals magnetic systems

The discovery of atomic monolayer magnetic materials has stimulated intense research activities in the two-dimensional (2D) van der Waals (vdW) materials community. The field is growing rapidly and there has been a large class of 2D vdW magnetic compounds with unique properties, which provides an ideal platform to study magnetism in the atomically thin limit. While the magnetic ground state has been extensively investigated, reliable characterization and control of spin dynamics play a crucial role in designing ultrafast spintronic devices. Ferromagnetic resonance (FMR) allows direct measurements of magnetic excitations, which provides insight into the key parameters of magnetic properties such as exchange interaction, magnetic anisotropy, gyromagnetic ratio, spin-orbit coupling, damping rate, and domain structure. In our review article, we present an overview of the essential progress in probing spin dynamics of 2D vdW magnets using FMR techniques including broadband FMR, optical FMR, and spin-torque FMR. Also, we discuss the recent advances in laboratory- and synchrotron-based FMR techniques and their opportunities to broaden the horizon of research pathways into atomically thin magnets.   

Search for ferromagnetism in Mn-doped lead halide perovskites

Lead halide perovskites are new key materials in various application areas such as high efficiency photovoltaics, lighting, and photodetectors. Doping with Mn, which is known to enhance the stability, has recently been reported to lead to ferromagnetism below 25 K in methylammonium lead iodide (MAPbI₃) mediated by superexchange. Two most recent reports confirm ferromagnetism up to room temperature but mediated by double exchange between Mn²⁺ and Mn³⁺ ions. Here we investigate a wide concentration range of MAMnxPb1−xI₃ and Mn-doped triple-cation thin films by soft X-ray absorption, X-ray magnetic circular dichroism, and quantum interference device magnetometry. The X-ray absorption lineshape shows clearly an almost pure Mn²⁺ configuration, confirmed by a sum-rule analysis of the dichroism spectra. A remanent magnetization is not observed down to 2 K. Curie-Weiss fits to the magnetization yield negative Curie temperatures. All data show consistently that significant double exchange and ferromagnetism do not occur. Our results show that Mn is not suitable for creating ferromagnetism in lead halide perovskites.   

Single Crystal Growth of FeRh from AuPb Flux

FeRh compound has been known for a long time as an itinerant magnet with a peculiar first-order antiferromagnetic (AFM) to ferromagnetic (FM) transition near room temperature. Although a lot of work has been done [1,2], the origin of the physical properties associated with the AFM to FM transition is still an ongoing debate and needs deeper investigation using a good single-crystal, single-phase material. Here, we report the single crystal growth of FeRh from the AuPb flux and confirm it by Laue diffraction, four-circle diffractometer measurements, and EPMA elemental analysis. These new single crystals will pave the way for a comprehensive understanding of the long-standing issues of the FeRh compound [3]. As expected, the preliminary measurement results, such as magnetization, are different from those previously reported.

[1] L. H. Lewis, C. H. Marrows, and S. Langridge, J. Phy. D 49, 323002 (2016).

[2] R.R Gimaev, A. A. Vaulin, A. F. Gubkin, and V. I. Zverev, Phys. Met. Mettalogr. 125, 823 (2020).

[3] A. Chirkova, K. P. Skokov, L. Schultz, N. V. Baranov, O. Gutfleisch, and T. G. Woodcock, Acta Mater. 106, 15 (2016).   

SpinView: General Interactive Visual Analysis Tool for Multiscale Computational Magnetism

Multiscale magnetic simulations, including micromagnetic and atomistic spin dynamics simulations, are widely used in the study of complex magnetic systems over a wide range of spatial and temporal scales. The advances in these simulation technologies have generated considerable amounts of data. However, a versatile and general tool for visualization, filtering, and denoising this data is largely lacking. To overcome these limitations, we have developed SpinView, a general interactive visual analysis tool for graphical exploration and data distillation. Combined with dynamic filters and a built-in database, it is possible to generate reproducible publication-quality images, videos, or portable interactive webpages within seconds. Since the basic input to SpinView is a vector field, it can be directly integrated with any spin dynamics simulation tool. With minimal effort on the part of the user, SpinView delivers a simplified workflow, speeds up analysis of complex datasets and trajectories, and enables new types of analysis and insight. SpinView is available from https://mxjk851.github.io/SpinView/.   

Charge-to-spin conversion by topological surface states of amorphous Gd-alloyed BixSe1-x

Topological insulators with large spin-orbit coupling are promising material candidates for efficient charge-spin interconversion [1,2]. Recently, large spin-orbit torque has been observed in amorphous materials [3] and topological states are predicted and demonstrated in the amorphous system [4,5]. Here we report the charge-to-spin conversion (SCC) in the amorphous Gd-alloyed Bi_xSe_1-x (BSG)/CoFeB bilayer [6]. The maximum SCC efficiency is 0.035 nm for the inverse Edelstein length, measured by spin pumping. Clear evidence of SCC is also observed by the THz time-domain spectroscopy. By studying the dependence of SCC efficiency on BSG thickness, we find the SCC could originate from the topological surface states of BSG. In addition, the angle-resolved photoemission spectroscopy on BSG shows dispersive two-dimensional surface states, which further supports the existence of amorphous topological states in the BSG. Our studies provide a new path toward the search of amorphous topological materials for spintronic applications.

 

References

[1] M. Dc et al. Nature materials 17.9 (2018).

[2] NHD Khang et al. Nature materials 17.9 (2018).

[3] T. Peterson, et al. Physical Review Materials 5.4 (2021).

[4] A. Agarwala et al. Physical Review Letters 118.23 (2017).

[5] P. Corbae et al. Nature Materials 22.2 (2023).

[6] P. Sahu et al. ACS Applied Materials & Interfaces 15.32 (2023).   

Searching for activated transitions in complex magnetic systems

The process of finding activated transitions in localized spin systems with continuous degrees of freedom is developed based on a magnetic variant of the Activation-Relaxation Technique (mART). In addition to the description of the method and the relevant local properties of the magnetic energy landscape, a criterion to efficiently recognize failed attempts and an expression for the step magnitude to control the convergence are proposed irrespective of the physical system under study. The implementation is exemplified on a 2D dipolar spin glass, revealing the characteristic energy distribution and the underlying physical processes of the activated dynamics.   

Phase-stability and magnetic properties of natural hydrogenated hematite – a potential water containing mineral on Mars

Hematite (α-Fe₂O₃) is the most prevalent iron oxide in sedimentary rocks and on the surface of Mars. Recent studies suggested that natural “hydrohematite” exists, wherein hematite accommodates H⁺ cations through compensation at the Fe³⁺-site, forming –OH bonds (i.e., structural water), with important implications for water in arid planetary environments. We present magnetic and Mössbauer characterizations to elucidate the magnetic response of H-doping on these samples, corroborating the results with first-principles calculations. The first-order reversal curves show a sharp -45° diagonal wing due to triaxial basal plane anisotropy. The Mössbauer spectra and field cooled/zero-field cooled M(T) both show the complete suppression of Morin transition (T_M) down to 5K, as opposed to T_M at ~260 K for hematite, indicating the presence of H⁺ as x-ray diffraction and electron microprobe analyses confirm the absence of the other elements. The defect calculations reveal the existence of stable bulk phase of hydrohematite, and the spin-anisotropies between the [0001] axis and the (0001) plane show an increasingly large energy difference with progressive H-doping in the structure – both interstitial and substitutional – verifying that hydrogenation of hematite would cause the T_M depression observed experimentally. These results provide a more comprehensive and fundamental understanding of natural hydrohematite and could serve as a blueprint for detecting this mineral on Mars via rover data or sample return.   

Achieving Complex Magnetic Anisotropy via 3D Printing to Incite Complex Motion in Soft Magnetic Elastomers.

Magnetic elastomers consist of magnetic particles suspended in a flexible elastomeric matrix. Such elastomers are applicable in areas of soft robotics where actuation without a direct line of sight is required. Magnetic elastomers with soft magnetic particulate generally display geometrically simple magnetically-actuated motion at large magnitudes, while samples with hard magnetic particulate display more complex motion, but at smaller magnitudes. The goal of this work is to study how the artificially created anisotropy from 3D-printed part shape and infill orientation can influence the complexity of motion in printed soft magnetic elastomer structures. Samples consist of 15% volume Iron particulate suspended in a thermoplastic polyurethane (TPU) matrix. Four sample beam sets printed with varying initial (-45deg, 0deg, 45deg) and twisting (0deg, 45deg, 90deg) angles are exposed to axial and transverse magnetic fields. Data is captured through images taken of the samples before and during exposure to the magnetic field. These images are then analyzed for degree of rotation and degree of deflection, combining to form complex motion. The complexity of the sample shape directly influences the complexity of the magneto active response of the sample. Samples with larger twist values show a larger rotational and deflection response when exposed to magnetic fields. This shows that complex motion is achievable using only soft magnetic elastomers through artificially structuring the underlying magnetic anisotropy via 3D printing.   

Magnetoactive Properties of Biocompatible Magnetic Hydrogel Composites: Effects of Magnetic Particulate Type, Loading content and Magnetic Poling

Polyvinyl alcohol (PVA) ferrogels were successfully fabricated by the freezing-thawing (FT) cycle technique, employing magnetite (Fe3O4) and strontium ferrite (SrFe12O19) as the materials for magnetic, micron-sized, additives. Ferrogels are magnetic hydrogel composites commonly utilized in tissue engineering, and drug delivery. The hydrogels’ similarity to biological tissue and use of smart additives leads them to be uniquely well suited for biomedical applications. The differing effects of magnetically soft and hard additives on the properties of PVA ferrogels were investigated by comparing the magnetic, magnetoactive, and mechanical properties of magnetically soft and hard PVA ferrogels, respectively. The effects of magnetic polling on ferrogels were investigated in this study and were compared to analogous, non-poled samples. Magnetic properties were determined via analysis of sample hysteresis loops, generated via vibrating sample magnetometer (VSM).  Magnetoactive properties are determined by quantifying the angle of sample deflection in an applied transverse magnetic field, via a custom digital image overlay program. Mechanical properties are determined via tensile testing. Magnetic additive distribution was investigated using scanning electron microscopy (SEM). Results yielded magnetically hard ferrogels that allow for more complex actuation and highly magnetoactive magnetically soft ferrogels with tunable responsiveness based upon magnetic particulate type, content, and effects of magnetic poling.   

Mapping the intrinsic photocurrent streamlines through micromagnetic heterostructure devices

Photocurrent in quantum materials is often collected at global contacts far away from the initial photoexcitation. This collection process is highly nonlocal. It involves an intricate spatial pattern of photocurrent flow (streamlines) away from its primary photoexcitation that depends sensitively on the configuration of current collecting contacts as well as the spatial nonuniformity and tensor structure of conductivity. Direct imaging to track photocurrent streamlines is challenging. In our recent work (doi.org/10.1073/pnas.2221815120), we demonstrate a microscopy method to image photocurrent streamlines through ultrathin heterostructure devices comprising platinum on yttrium iron garnet (YIG). We accomplish this by combining scanning photovoltage microscopy with a uniform rotating magnetic field. Here, local photocurrent is generated through a photo-Nernst type effect with its direction controlled by the external magnetic field. This enables the mapping of photocurrent streamlines in a variety of geometries that include conventional Hall bar-type devices, but also unconventional wing-shaped devices called electrofoils. In these, we find that photocurrent streamlines display contortion, compression, and expansion behavior depending on the shape and angle of attack of the electrofoil devices, much in the same way as tracers in a wind tunnel map the flow of air around an aerodynamic airfoil. This affords a powerful tool to visualize and characterize charge flow in optoelectronic devices.   

Algorithms of Image Processing and XRD Analyses for Characterization of Magnetic Nanostructures

Numerous physical and chemical properties including magnetism are influenced by morphological characteristics such as the shape, structure, and size of nanoparticles. The rapid progress in characterization techniques able to analyze the core-shell magnetic nanomaterials down to the same scale, such as high-resolution transmission electron microscopy (HR-TEM) and powdered X-ray diffraction (PXRD) analyses. Therefore, an innovative method capable of analyzing accurately the morphological properties of synthesized magnetic nanomaterials in a quantitative manner is in high demand. In a joint endeavor with the research group at Cal State LA, we introduce a pioneering algorithm tailored to interpret SEM images and effectively juxtapose these with PXRD elemental analyses. At the heart of our method lies a blend of conventional filtration techniques: from Fourier and inverse FT to node extraction and the novel reconnection algorithm empowered by 2D gaussian mapping. Our spotlight is on nanostructure classification by size, distinguishing between lengths for nanotubes/nanosheets and radii for nanospheres. But we don't stop there. Our exploration extends to understanding porosity, layer sizes, and their consequential magnetic attributes. We rounded it off with a dive into graphitization and amorphism. Our study encompasses a spectrum of synthesized core-shell nanoparticles such as metal (Fe, Ni, Co) and metal-free phthalocyanine and porphyrin, synthesized through intricate processes, all examined under the PPMS and EPR for their magnetic characteristics, to correlate the nanostructure layer design with its magnetic properties.   

Preliminary Faraday Rotation Results Associated with the Photoionized Gas of IC 1396

We present initial Faraday rotation measurements of extragalactic radio sources with lines of sight passing through or near to the nebula region of IC 1396. We measured the linear polarization of the sources with the Karl G. Jansky Very Large Array (VLA) at frequencies of ~5 GHz (6 cm). We estimate the background rotation measure (RM) in this region of the galaxy to be ~ -140 rad m^-2. We find the sources having lines of sight passing through IC 1396 have an excess rotation measure of |RM| ~ 62-465 rad m^-2 with respect to the background. We will discuss rotation measure values in the context of magnetized plasma of IC 1396. We compare our results to known models of rotation measure in our galaxy. We additionally develop a shell model to fit the structure of the nebula.   

A Physical Analysis of the Spinor Object as a Combination of Polar and Axial vectors: A Spinor as a Columnar Vortex

The Spinor is an abstract mathematical object found in Relativistic Quantum Mechanics that has defied simple physical description. However, it can be now understood through basic vector analysis as a combination of two parallel and equal vectors, one a polar vector, such as a velocity,  and the other and axial vector,  such as a vorticity. That is, as direction vector and rotation around that direction vector. Understood in this way, spinors are often seen in nature as tornados, dust devils, fire whirls and hurricanes.  The Spinor defined in this way is a local connected by inertial forces with the rest of the universe and allow persistent structure to appear in normally structureless media. Such objects can be shown to transform with the characteristic half-angle formula. This model can greatly assist in teaching of spinor physics to students in preparation to its appearance in Quantum Mechanics. .   

Tunneling time and Faraday/Kerr effects in PT symmetric systems

We review the generalization of tunneling time and anomalous behaviour of Faraday and Kerr rotation angles in parity and time systems. Similarities of two phenomena are discussed, both exhibit a phase transition-like anomalous behaviour in a certain range of model parameters. Anomalous behaviour of tunneling time and Faraday/Kerr angles in systems is caused by the motion of poles of scattering amplitudes in the energy/frequency complex plane.   

Amending General Relativity to account for dark energy and beyond

There is a spatial dimension missing from the Einstein equations. This dimension is as real as the other three spatial dimensions and can be visualized. Universe expansion in this dimension explains the illusion of dark energy. Math validation would show how the dimension expands with respect to decreasing mass density is equivalent to dark energy. When the Universe began, all matter and energy was confined to a tightly bound volume resulting in near infinite spacetime curvature. As the universe expanded mass density and curvature decreased. This resulted in Universe expansion in two directions – parallel and normal to the path of light. This is demonstrated by a tetherball: the curved path of light is represented by the ball moving around the pole. Movement away from the pole represents Universe expansion normal to the path of light. The rope keeps the ball from flying off from the pole. There is something yet unknown that keeps matter and energy from flying into the added dimension. Some matter and energy leaks into and out of the added dimension resulting in far reaching implications. One possibility is the unification of the space used for quantum mechanics and general relativity where a set of algorithms equally applies to both. This might be a steppingstone to the theory of everything. Low hanging fruit from such would be practical fusion reactors and quantum computers. Rather and unifying QM and GR the path would be to unify their definitions of space. Visualization comes from imagining how the new dimension expands.