Session W59: Electronic, Optical, and Transport Properties of Topological Materials

Fermiology of a topological, intercalated transition metal dichalcogenide compound

Transition metal dichalcogenides (TMDs) are known to have various phases such as charge density waves, superconductivity and Mott insulating phases. Here, we study the electronic structure of a transition metal dichalcogenide intercalated by triangular layers of Co atoms using angle-resolved photoemission spectroscopy. The measured Fermi surfaces after intercalation are different from pristine compounds and cannot be explained by a simple rigid band shift. Interestingly, recently a large anomalous hall effect (AHE) was observed in this family [1]. We found that AHE values can change signs with different intercalation levels. We utilize ARPES to track the Fermi level shifting with different dopings. Our study suggests the possibility of a tunable AHE system. Temperature dependence measurement also reveals how band structure changes across the magnetic transition. Finally, we conduct photon energy dependence to demonstrate the nontrivial band crossing. Our ARPES study indicates that this intercalated TMD compound hosts rich physics such as magnetism, tunable AHE and nontrivial band topology.     

Detection of time-reversal symmetry breaking via waveguide mode coupling

Time reversal symmetry is central to the characterization of a wide variety of quantum materials as it places fundamental constraints on electronic properties. Breaking such symmetry often reflects the topological features of the underlying ground state, points to the emergence of internal magnetic fields and gives rise to Hall currents. Experimental probes which are sensitive to this symmetry breaking without relying on contacts are relatively rare, and, as a result, it is often challenging to unequivocally establish the presence and microscopic origin of time-reversal symmetry breaking. Here, we present an alternative approach to detect time-reversal symmetry breaking based on coupling between electromagnetic modes in a cross-waveguide geometry. Specifically, we consider a small dielectric sample at the intersection of the waveguides and show how the breaking of time-reversal symmetry manifests in the scattered field. We conclude by discussing various experimental platforms where this technique may be applied.   

Effect of surface relaxation on the Dirac surface states of the topological crystalline insulator Bi

Recent theoretical studies revealed that bulk bismuth is a higher-order topological insulator and also a first-order crystalline topological insulator [1,2]. As a crystalline topological insulator, Bi exhibits two gapless Dirac states on (1-10)-type surfaces. Since the position of either of these Dirac cones in the surface Brillouin zone is not fixed by symmetry, the Dirac cone can be found at a general location within the zone, with the only constraint being that the two Dirac cones are related to each other by a twofold rotational symmetry. Here, using first-principles calculations, we study how surface relaxation changes the (1-10) topological surface states. We found that as the atoms move to their equilibrium positions, the locations of the Dirac cones shift significantly, and the surface band structure changes drastically. The possible physical reasons behind these phenomena are discussed.

1. F. Schindler et al., Nature Physics, 14, 918 (2018).

2. C-H. Hsu et al., PNAS, 116, 13255 (2019).   

Linearized band gap evolution in topologically gapped alloys: a case study of KZnSb1-xBix

Band gap is the most fundamental property underlying the device performance of a gapped crystalline material, either a semiconductor/insulator or a topological insulator. A well-established bowing curve characterizes the concentration dependence of trivial band gap in semiconductor alloys, while the concentration dependence of nontrivial gap in topological alloys remains largely unknown. This question is especially important because the topological alloys have usually a narrow gap which can be easily tuned by chemical composition where not only the size but also the “sign” of gap can be changed due to band inversion. We have systematically investigated the band gap (E_g) evolution of KZnSb_1-xBi_x alloy as a function of alloy concentration (x), using first-principles calculations within virtual crystal approximation. Interestingly, we found that the gap evolves linearly within each topological phase region (normal insulator, Dirac semimetal,  and topological crystalline insulator) separated by singular transition points, i.e., the band crossing and reopening points. This ideal linear topological gap function, in analogy to mixing enthalpy of ideal solution, is revealed to be driven by the interplay of chemical potential (on-site energy) and spin-orbit coupling of mixing, which differs from the parabolic bowing gap function of semiconductors, in analogy to regular solution, driven only by chemical potential of mixing.  We further show that the DSM phase in the range of x=0.23-0.45 displays non-trivial 1^st- and 2^nd-order band topology, with non-trivial surface states and higher order Fermi arc.   

ARPES and DFT study of the electronic structure of the electronically switchable antiferromagnet, CuMnAs

Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically reorientable Néel vector. Direct measurements of the electronic structure of single-crystalline thin films of tetragonal CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported, including Fermi surfaces (FS) and E-k dispersions.  After correcting for an unexpected chemical potential shift of ≈−390 meV (hole doping), there is excellent agreement of FS, orbital character of bands, and Fermi velocities between the experiment and Density Functional Theory calculations.  Additionally, 2x1 surface reconstructions are found in the low energy electron diffraction (LEED) and ARPES. This work underscores the need to control the chemical potential in tetragonal CuMnAs to enable the exploration and exploitation of the topological quantum switching of the Dirac point predicted to be above the chemical potential in the current samples.   

Dimensional crossover and band topology evolution in ultrathin semimetallic NiTe2films

Nickel ditelluride (NiTe₂), a recently identified Type-II Dirac semimetal possessing topological Dirac fermions close to the Fermi energy, is forecasted to show emergent two-gap superconductivity in the single-layer phase as well as pronounced dimensionality-mediated electronic tunability. Confirming these tantalizing phenomena necessitates fabricating ultrathin NiTe₂ films to unearth the hotly debated underlying physics. By undertaking photoemission band mappings of ultrathin NiTe₂ films grown via molecular beam epitaxy, we unveil spectroscopic proof for the strong thickness-mediated evolutions in the band structures of single-crystalline ultrathin NiTe₂ films. Specifically, when the film thickness is varied from one to five layers, the hybridization gap in the conical topological surface states closes within our experimental energy resolution. Furthermore, comparisons between experimental and first-principles data underline possible inherent difficulties in growing atomically smooth NiTe₂ films in the single-layer phase. Our comprehensive findings not only encourage further examinations of emergent physics in semimetallic NiTe₂ films but also underline the potential obstacles of integrating NiTe₂ into technological devices.   

Visualization of the spin-resolved band structure in the bulk Rashba material BiTeCl

The spin-orbit interaction has been studied intently, as it provides a mechanism for topologically non-trivial phenomena. Noncentrosymmetric materials with strong spin-orbit interactions can give rise to the bulk Rashba effect, such as the material family BiTeX (X=Cl, Br, I). Angle-resolved photoemission spectroscopy (ARPES) has shown that BiTeX possesses spin-split bands with some of the largest measured values of the Rashba parameter. Beyond conventional ARPES, spin-resolved ARPES is a powerful tool to directly probe their rich spin-orbital textures. In this talk, we will present our recent spin-resolved ARPES work on BiTeCl in which we visualize the detailed Rashba-split bands and quantitatively track the spin texture. We show that the momentum-dependent spin polarization cannot be fully described by the usual Rashba Hamiltonian. Thus, we apply a third-order Rashba model constrained by the crystal point group symmetry, which shows remarkable agreement with the measured spin structure. Furthermore, we use ab initio calculations to support our interpretations and give insights into the possible microscopic mechanisms that lead to such spin-orbital interactions.   

Driving Ultrafast Spin and Energy Modulation in Quantum Well Surface States via Photo-Induced Electric Fields

The future of modern optoelectronics and spintronic devices relies on our ability to control the spin and charge degrees of freedom at timescale that can compete with traditional silicon-based devices, operating at speeds >10 GHz.  Rashba spin-split quantum well states, 2D states that develop at the surface of strong spin-orbit coupling materials, are ideal given the easy tunability of their energy and spin states. So far however, most studies have only demonstrated such control in a static way. In this study, we demonstrate ultrafast control of the spin and energy degrees of freedom of surface quantum well states on Bi₂Se₃ at picosecond timescales. By means of a focused laser pulse, we modulate the band bending, producing picosecond time-varying electric fields at the material's surface, thereby reversibly modulating the quantum well spectrum and Rashba Effect. These results open a new pathway for light-driven spintronic devices with ultrafast switching of electronic phases, and offer the interesting prospect to extend this ultrafast photogating technique to a broader host of 2D materials.   

Shift-current response as a probe of quantum geometry and electron-electron interactions in twisted bilayer graphene.

Moiré materials, and in particular twisted bilayer graphene (TBG), exhibit a range of fascinating phenomena, that emerge from the interplay of band topology and interactions. We show that the non-linear second-order photoresponse is an appealing probe of this rich interplay. A dominant part of the photoresponse is the shift-current, which is determined by the geometry of the electronic wavefunctions and carrier properties, and thus becomes strongly modified by electron-electron interactions. We analyze its dependence on the twist angle and doping, and investigate the role of interactions. In the absence of interactions, the response of the system is dictated by two energy scales: the mean energy of direct transitions between the hole and electron flat bands, and the gap between flat and dispersive bands. Including electron-electron interactions, both enhance the response at the non-interacting characteristic frequencies as well as produce new resonances. We attribute these changes to the filling-dependent band renormalization in TBG. Our results highlight the connection between non-trivial geometric properties of TBG and its optical response, as well as demonstrate how optical probes can access the role of interactions in moiré materials.   

Plasmon Intrinsic Dipole Moment

Plasmons are usually described by macroscopic objects such as electric fields and currents. They are nevertheless quantum objects with internal structure. We show [1] this can include an intrinsic, static dipole moment that is tied to the quantum geometry of the Hilbert space in which the plasmon states reside.  This quantum geometric dipole offers a unique handle for controlling plasmon dynamics, via density modulation and electric fields. We demonstrate that a nonvanishing plasmon quantum geometric dipole gives rise to nonreciprocal skew scattering for plasmons from impurities in a valley dependent way.  This skew scattering can be directly observed in near field optical microscopy.   

Progress on photoemission studies of nodal plane in LaNiGa2

LaNiGa₂ is a time-reversal-symmetry-breaking superconductor below 2K. Recently, large platelet-style single crystal specimens have clarified the space group for the crystal structure which in turn predicts a novel electronic structure with Fermi surface degeneracies on one plane of the Brillouin zone. While the large platelet face of the crystals is most accessible to photoemission studies, the predicted degeneracies are on a plane orthogonal to the platelet face, making it difficult to probe with angle resolved photoemission spectroscopy (ARPES). In this talk we will present progress on accessing the predicted electronic degeneracies in LaNiGa2 directly with ARPES.   

Catalogue of Flat Band Stoichiometric Materials

Topological electronic flatten bands near or at the Fermi level are a promising avenue towards un-conventional superconductivity and correlated insulating states. In this work, we present a catalogueof all the three-dimensional stoichiometric materials with flat bands around the Fermi level that ex-ist in nature. We consider 55,206 materials from the Inorganic Crystal Structure Database (ICSD)catalogued using the Topological Quantum Chemistry website which provides their structural pa-rameters, space group (SG), band structure, density of states and topological characterization. Weidentify several signatures and properties of band flatness: bandwidth, peaks in the density of states,band topology, range of momenta over which the band is flat and the energy window around theFermi level where the flat bands is situated. Moreover, we perform a high-throughput analysis of allcrystal structures to identify those hosting line-graph or bipartite sublattices - either in two or threedimensions - that likely lead to flat bands. From this trove of information, we create the MaterialsFlatband Database website, a powerful search engine for future theoretical and experimental studies.We use it to manually extract a curated list of 2,379 materials potentially hosting flat bands whosecharge centers are not strongly localized on the atomic sites, out of which we present in minute de-tails the 345 most promising ones. In addition, we showcase five representative materials (KAg[CN]2in SG 163 (P -31c), Pb2Sb2O7in SG 227 (Fd -3m), Rb2CaH4in SG 139 (I4/mmm), Ca2NCl in SG166 (R -3m) and WO3in SG 221 (Pm -3m)) and provide a theoretical explanation for the origin of their flat bands that close to the Fermi energy using the S-matrix method.   

Theory of Difference Frequency Quantum Oscillations

Quantum oscillations (QO) describe the periodic variation of physical observables as a function of inverse magnetic field in metals. The Onsager relation connects the basic QO frequencies with the extremal areas of closed Fermi surface pockets, and the theory of magnetic breakdown explains the observation of sums of QO frequencies at high magnetic fields. Here we develop a quantitative theory of difference frequency QOs in metals with multiple Fermi pockets with parabolic or linearly dispersing excitations. We show that a non-linear interband coupling, e.g. in the form of interband impurity scattering, can give rise to otherwise forbidden QO frequencies which can persist to much higher temperatures compared to the basis frequencies. We discuss the experimental implications of our findings, for example, for topological materials with multifold fermion excitations.   

Enhanced Berry curvature dipole and persistent spin texture in Bi(110) monolayer

Non-vanishing Berry curvature dipole (BCD) and persistent spin texture (PST) are intriguing physical manifestations of electronic states in noncentrosymmetric 2D materials. The former induces a nonlinear Hall conductivity while the latter offers a coherent spin current. Based on density-functional-theory (DFT) calculations, we demonstrate the coexistence of both phenomena in a Bi(110) monolayer with a distorted phosphorene structure. Both effects are concurrently enhanced due to the strong spin-orbit coupling of Bi while the structural distortion creates internal in-plane ferroelectricity with inversion asymmetry. We further succeed in fabricating a Bi(110) monolayer in the desired phosphorene structure on NbSe₂ substrate. Detailed atomic and electronic structures of the Bi(110)/NbSe₂ heterostructure are characterized by scanning tunneling microscopy/spectroscopy and angle-resolved-photoemission spectroscopy. These results are consistent with DFT calculations which indicate the large BCD and PST retained. Our results suggest the Bi(110)/NbSe₂ heterostructure as a promising platform to exploit nonlinear Hall and coherent spin transport properties together.   

Network of topological nodal planes and point degeneracies in CoSi

We report the experimental identification of symmetry-enforced topological nodal planes in CoSi which together with multifold point degeneracies and Weyl points form a network of band crossings in total satisfying the Nielsen-Ninomiya no-go theorem. For this, we have combined measurements of Shubnikov-de Haas oscillations in CoSi with material-specific electronic structure calculations and a symmetry analysis of space group 198, in which CoSi crystallizes [1]. The observation of two nearly dispersionless Shubnikov-de Haas frequency branches is shown to provide clear evidence of four distinct Fermi surface sheets around the R point of the Brillouin zone (BZ) and of the symmetry-enforced orthogonality of the wave functions at the intersections with the nodal planes [2]. These results in combination with our symmetry analysis let us infer that the sum of topological charges of band crossings in the interior of the BZ is forced to be odd and therefore needs to be compensated by a topological charge of the nodal planes on the BZ boundary, as predicted in [3]. Therefore, a comprehensive analysis of all charges in the network needs to go beyond point degeneracies and take the topological nodal planes into account.

[1] Huber et al., arXiv:2107.02820 (2021)

[2] Wilde et al., Nature 594, 374 (2021)

[3] Yu et al., Phys. Rev. B 100, 041118 (2019)