Session S13: 2D Materials (General): Transport and Optical Phenomena -- Magnetism

Magneto-optical properties of bilayer transition metal dichalcogenides

In transition metal dichalcogenides (TMDCs) the spin-orbit interaction affects differently the conduction and valence band energies as functions of the wave vector k and the band gap is usually large except in few TMDCs that are metallic without band gaps. Consequently, when a perpendicular magnetic field B is applied, the conduction and valence band Landau levels are also different and this leads to a splitting of the interband optical absorption lines in both the absence and presence of an external electric field Ez. When B and Ez are present, the peaks in the imaginary part of the Hall conductivity give two distinct contributions of opposite sign to the interband spectrum. The real part of the right- and left-handed interband conductivity, however, retains its two-peak structure but the peaks are shifted in energy and amplitude with respect to each other in contrast with graphene.   

Magneto-Raman Spectroscopy of Layered Transition-Metal Dichalcogenides

Raman spectroscopy offers a non-contact, non-destructive, and high-throughput optical technique to probe the fundamental physics of two-dimensional (2D) layered materials. We discuss our unique magneto-Raman capability, which affords measurement of Raman spectra with simultaneous variation of temperature (4K to 400K), laser excitation wavelength (tunability from UV to near-IR), and magnetic field (up to 9 T). Coupling to a triple-grating spectrometer provides measurement of low-frequency phonons. Recent results on novel 2D materials will be presented to highlight instrumentation capabilities, including metallic TaSe₂, which exhibits transitions between commensurate and incommensurate charge-density wave (CDW) phases, and related, layered materials. The dependence of the observed Raman-active phonons in TaSe₂ on temperature and magnetic field will be presented and compared with earlier results on MoS₂ and calculations using density functional theory. Specifically, we observe the appearance of low-frequency, zone-folded modes in the CDW state. The phase, amplitude, and previously unanalyzed zone-folding modes are assigned to specific phonons observed in experimental spectra and DFT. Uniquely, the calculations show Ta-rich stripes emerge in the commensurate CDW phase.   

Magnetoplasmons in the pseudospin S=1 α-T3 lattice

We calculate the dynamical polarizability and the dispersion relation for magnetoplasmons for the α-T₃ model at zero temperature at integer filling. In the absence of magnetic field, the low energy spectrum described by a Dirac-Weyl Hamiltonian consists of a pair of Dirac cones and a dispersionless (flat) band in the K and K' valleys. We use the Peierls substitution to generate the Landau level spectrum for this structure with pseudospin S = 1 which is characterized by a parameter α for measuring the coupling strength between an additional atom at the center of the honeycomb graphene lattice and the A and B atoms of graphene. We present results for a doped layer for various α and magnetic fields in the random-phase approximation. These modes may be observed with the aid of inelastic light-scattering experiments.   

Negative Magnetoresistance from Berry Curvature and Orbital Magnetic Moment

Longitudinal negative magnetoresistance has attracted tremendous amount of attention since the emergence of three-dimensional topological materials, e.g. Weyl semimetals, due to the effect of chiral anomaly in these systems. In contrast to the extensively studied longitudinal negative magnetoresistance, transverse magnetoresistance in topological systems has been somewhat overlooked. We formulate transverse magnetoresistance for topological systems with non-zero Berry curvature based on semiclassical Boltzmann theory. Model calculation based on gapped two-dimensional valley systems shows explicitly that magnetoresistance is negative in large range of magnetic field, stemming from Berry curvature and orbital magnetic moment.   

Electrical and Magneto Transport Properties in Weyl Semimetal MoTe2

Molybdenum ditelluride MoTe2, a type II Weyl semimetal, has attracted intense interest due to novel properties such as large magnetoresistance, quantum spin Hall effect and massless carriers. Here we report on electrical and magneto transport studies of exfoliated multilayer MoTe2. Weak antilocalization is observed at low magnetic fields due to strong spin-orbit coupling. We also investigate layer dependent magnetoresistance at various temperatures. A back-gate voltage is applied to change the carrier density and Fermi level. It is shown such electronic properties can be effectively tuned by gate voltage.
  

Magnetotransport in monolayer 2H-MoTe2

Monolayer transtion metal dichalcogenides (TMD) have been investigated for a variety of interesting aspects: valley Hall effect, biexcitons, and 2-D superconductivity. In magnetotransport, monolayer TMDs break inversion symmetry and as a result has degenerate K/K' bands which are spin-split. In MoS2, this spin-splitting between the lower and upper K/K' bands in the conduction band is around 15 meV, 5x larger than expected, and similar results have been extracted from monolayer WSe2. In both cases, neither system can be investigated via electrical transport in both the n- and p-type carrier regime due to the large bandgap and issues with contacts. Monolayer, 2H-MoTe2 however, has a much smaller bandgap of 1.1 eV. Though there has been much work done on electrical characterization in MS2 and MSe2 (M = Mo or W), there has been almost no work done on 2H-MoTe2 as the monolayer is air sensitive and more difficult to contact than like 2-D air sensitive materials such as InSe or black phosphorus. In this study we report the magnetotranpsort of monolayer 2H-MoTe2 in the ultra-clean limit.   

Theory of magnetotransport for a cavity-embedded two-dimensional electron gas

We present a theory pointing out the crucial role of virtual polariton excitations in controlling the dc charge transport properties of cavity-embedded systems. Specifically, we consider the linear magnetotransport of a cavity-embedded two-dimensional electron gas (2DEG) in the regime where no real photons are injected or created in the resonator [1]. Our theory shows that, for a cavity photon mode with in-plane linear polarization, the dc bulk magnetoresistivity of the 2DEG is anisotropic. For high filling factors of the Landau levels, we predict a profound modification in the envelope of the Shubnikov-de Haas oscillations, with the resistivity being increased or reduced depending on the system parameters (an effect observed in recent experiments [2]). In the limit of low magnetic fields and in the ultrastrong light-matter coupling regime, the resistivity along the cavity-mode polarization direction is enhanced.

[1] N. Bartolo and C. Ciuti, preprint at arXiv:1805.02623, accepted on Phys. Rev. B.
[2] G.L. Paravicini-Bagliani, F. Appugliese, E. Richter, F. Valmorra, J. Keller, M. Beck, N. Bartolo, C. Rössler, T. Ihn, K. Ensslin, C. Ciuti, G. Scalari, and J. Faist, preprint at arXiv:1805.00846, accepted on Nature Physics.   

Coordinate-shift induced electron transport in strong magnetic field

In a strong magnetic field, electrons primarily execute cyclotron motion, with transverse transport in an electric field from the drifting of cyclotron orbits. We present a classical-Boltzmann theory of longitudinal transport in terms of coordinate shifts of such orbits upon scattering with impurities. During the electron-impurity scattering, the guiding centers suddenly shift their positions, which accumulatively contribute to a longitudinal current. Along with coordinate shifts, the cyclotron orbit radius are also changed, accounting for energy dissipation in the electric field. The resulting longitudinal conductivity is in agreement with the Drude phenomenological theory even beyond the weak scattering limit.   

Investigation on the Non-trivial Photoluminescence in 2D Antiferromagnetic Layers

Two-dimensional (2D) magnetic systems have attracted intense attention due to potentially applications in low-dimensional spintronic and magnetic devices. Recently, we observe a non-trivial photoluminescence in metal phosphorus chalcogenides, which are 2D antiferromagnetic materials. Multiple photoluminescence peaks are observed and some of them show a strong dependence on the excitation-wavelength and temperature in both the bulk and few-layer cases, which are different from the emission in conventional 2D semiconductors. A rational model based on the special electronic structures and transitions of the materials is proposed to explain this phenomenon. Our studies reveal the intrinsic electronic and magnetic properties in 2D crystals, and suggest the potential functionalities and applications of 2D magnetic layers.   

Spin filter and spin-polarized electric current in silicene nanoribbons induced by point defects

Half-metals, in which one spin channel is conductive but the other one is insulating or semiconducting, is the key ingredient to achieve spin-polarized currents. Hybrid structures of 2D materials and ferromagnetic insulators, like EuO, provide a route to induce half-metallicity and pave the way for spintronic applications. The ferromagnetic insulator induces a proximity exchange interaction between the spins in the material that results in a spin modulation of the structures. In this context, in this work we address the effects of a random distribution of adatoms on the electron transport properties of silicene nanoribbons. The tunnel coupling between adatoms and silicon atoms induces an electronic Fano effect that makes the conductance vanish when the Fermi level matches the resonant energy induced by the adatoms. The resonant energy is independent of the random distribution of adatoms, provided that they do not cluster. When the nanoribbon is in close proximity to a ferromagnetic insulator, the resonant energy depends on the electron spin and consequently the electric current can be highly spin polarized. Our results expand the base of available materials to designing a tunable source of polarized electrons for spintronics.   

Anisotropic spin relaxation in graphene/hBN heterostructures

Measurements show a large and tunable anisotropy in the spin relaxation of hBN encapsulated bilayer graphene^1,2, similar to what is observed in graphene/TMDC heterostructures³. Combining systematic first principles calculations for graphene/hBN heterostructures with a minimal tight-binding model, we extract spin-orbit coupling parameters of graphene in the µeV range. The extracted model parameters depend on (i) interlayer distances, (ii) stacking configurations, and (iii) an external electric field, resulting in a rich parameter space. Based on the Dyakonov-Perel formalism we calculate spin relaxation times for graphene, in the nanosecond range, in agreement with recent experimental measurements. A very important finding is that the spin relaxation anisotropy is maximum close to the charge neutrality point, decreasing with the doping level. In addition, we also show that the anisotropy can be tuned by means of an external electric field, via the precise control of the Rashba SOC.

¹ Xu et al., PRL 121, 127703 (2018)
² Leutenantsmeyer et al., PRL 121, 127702 (2018)
³ Cummings et al., PRL 119, 206601 (2017)   

Bilayer silicene in a magnetic field: spin localization and its effect on the integer and fractional Hall conductivity

The Hall conductivity of many condensed matter systems presents step structure when a uniform perpendicular magnetic field is applied. We report the emergence of fractions of the Hall conductivity due to the grouping in pairs of an electron with a bilayer silicene sublattice site due to strong spin-orbit coupling. The collective behavior of this pairing causes the conductivity to exhibit steps of e²/h or 4/5 e²/h high for the low-lying spectrum, depending on the dominating sublattices of Landau levels. This coexistence of integer and fractional Quantum Hall States arises from the interplay of lattice geometry, atomic interaction, spin-orbit coupling, and external magnetic field. We also report the significant effect of an external electric field on the evolution of integer and fractional quantum Hall effects.   

Fano Resonance in Two-Dimensional Magnetic Semiconductor CrPS4

The two-dimensional(2D) magnetic materials have attracted extensive research interests, not only because of their novel physical properties induced by quantum confinement, but also for their promise for spintronic devices. Chromium thiophosphate (CrPS₄) is a promising ternary antiferromagnetic semiconductor with a Neel temperature of 36 K. We synthesized CrPS₄ single crystals by chemical vapor transport method and performed spectroscopy study at different temperature. The spectra at low temperature showed an asymmetric Fano-resonance shape, which can be explained by coupling between a discrete state and continuum state. This is the first observation of Fano resonance in 2D magnetic materials in near infrared spectrum region. Moreover, we also studied Fano resonance of layered CrPS₄ on dependence of magnetic field, excitation power, laser polarization, etc. Our findings may provide insight of the electronic transition of CrPS₄, which is related to its intrinsic magnetic properties.   

Si-doped Defect in Monolayer Graphene: Magnetic Quantization

We explore the rich and unique magnetic quantization of Si-doped graphene defect systems for various concentrations and configurations using the generalized tight-binding model. This model takes into account simultaneously the non-uniform bond lengths, site energies and hopping integrals, as well as a uniform perpendicular magnetic field . The magnetic quantized Landau levels (LLs) could be classified into four different groups based on the probability distributions and oscillation modes. The magneto-optical selection rules, reflecting the main characteristics of LLs, cover Δn = |n^c-n^v| = 0 and 1. These rules for inter-LL excitations arise from the non-equivalence or equivalence of the and sublattices in a supercell. The spectral intensity can be controlled by oscillator strength using a canonical momentum as well as by density of states using concentration and distribution of doped Si atoms.