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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Spectroscopy of a tunable moiré system with a correlated and topological flat band

Abstract Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Giant resistance switch in twisted transition metal dichalcogenide tunnel junctions

Resistance switching in multilayer structures are typically based on materials possessing ferroic orders. Here, in this work, we predict an extremely large resistance switching based on the relative spin–orbit splitting in twisted transition metal dichalcogenide (TMD) monolayers tunnel junctions. Because of the valence band spin splitting which depends on the valley index in the Brillouin zone, the perpendicular electronic transport through the junction depends on the relative reciprocal space overlap of the spin-dependent Fermi surfaces of both layers, which can be tuned by twisting one layer. Our quantum transport calculations reveal a switching resistance larger than 10 6 % when the relative alignment of TMDs goes from 0° to 60° and when the angle is kept fixed at 60° and the Fermi level is varied. By creating vacancies, we evaluate how inter-valley scattering affects the efficiency and find that the resistance switching remains large (10 4 %) for typical values of vacancy concentration. Not only should this resistance switching be observed at room temperature due to the large spin splitting, but our results also show how twist angle engineering and control of van der Waals heterostructures could be used for next-generation memory and electronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Spin-valley locking and bulk quantum Hall effect in a noncentrosymmetric Dirac semimetal BaMnSb 2

Spin-valley locking in monolayer transition metal dichalcogenides has attracted enormous interest, since it offers potential for valleytronic and optoelectronic applications. Such an exotic electronic state has sparsely been seen in bulk materials. Here, we report spin-valley locking in a Dirac semimetal BaMnSb 2 . This is revealed by comprehensive studies using first principles calculations, tight-binding and effective model analyses, angle-resolved photoemission spectroscopy measurements. Moreover, this material also exhibits a stacked quantum Hall effect (QHE). The spin-valley degeneracy extracted from the QHE is close to 2. This result, together with the Landau level spin splitting, further confirms the spin-valley locking picture. In the extreme quantum limit, we also observed a plateau in the z-axis resistance, suggestive of a two-dimensional chiral surface state present in the quantum Hall state. These findings establish BaMnSb 2 as a rare platform for exploring coupled spin and valley physics in bulk single crystals and accessing 3D interacting topological states.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Photoinduced anomalous Hall effect in two-dimensional transition metal dichalcogenides

A circularly polarized ac pump field illuminated near resonance on two-dimensional transition metal dichalcogenides (TMDs) produces an anomalous Hall effect in response to a dc bias field. In this work, we develop a theory for this photoinduced anomalous Hall effect in undoped TMDs irradiated by a strong coherent laser field. The strong field renormalizes the equilibrium bands and opens up a dynamical energy gap where single-photon resonance occurs. The resulting photon dressed states, or Floquet states, are treated within the rotating-wave approximation. A quantum kinetic equation approach is developed to study the nonequilibrium density matrix and time-averaged transport currents under the simultaneous influence of the strong ac pump field and the weak dc probe field. Dissipative effects are taken into account in the kinetic equation that captures relaxation and dephasing. The photoinduced longitudinal and Hall conductivities display notable resonant signatures when the pump field frequency reaches the spin-split interband transition energies. Rather than valley polarization, we find that the anomalous Hall current is mainly driven by the intraband response of photon-dressed electron populations near the dynamical gap at both valleys, accompanied by a smaller contribution due to the interband response. In conclusion, these findings highlight the importance of photon-dressed bands and nonequilibrium distribution functions in achieving a proper understanding of the photoinduced anomalous Hall effect in a strong pump field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Gate Tunable Dark Trions in Monolayer WSe 2

Monolayer WSe 2 is an intriguing material to explore dark exciton physics. We have measured the photoluminescence from dark excitons and trions in ultraclean monolayer WSe 2 devices encapsulated by boron nitride. The dark trions can be tuned continuously between negative and positive trions with electrostatic gating. We reveal their spin-triplet configuration and distinct valley optical emission by their characteristic Zeeman splitting under a magnetic field. Here, the dark trion binding energies are 14–16 meV, slightly lower than the bright trion binding energies (21–35 meV). The dark trion lifetime (~1.3 ns) is two orders of magnitude longer than the bright trion lifetime (~10 ps) and can be tuned between 0.4 and 1.3 ns by gating. Such robust, optically detectable, and gate tunable dark trions may help us realize trion transport in two-dimensional materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

SU(4) Symmetry in Twisted Bilayer Graphene: An Itinerant Perspective

We study symmetry-broken phases in twisted bilayer graphene at small filling above charge neutrality and at van Hove filling. We argue that the Landau functionals for the particle-hole order parameters at these fillings both have an approximate SU(4) symmetry, but differ in the sign of quartic terms. We determine the order parameter manifold of the ground state and analyze its excitations. For small fillings, we find a strong first-order transition to an SU(3) Ⓧ U(1) manifold of orders that break spin-valley symmetry and induce a 3-1 splitting of fermionic excitations. For van Hove filling, we find a weak first-order transition to an SU(2) Ⓧ SU(2) Ⓧ U(1) manifold of orders that preserves the twofold band degeneracy. Here, we discuss the effect of particle-hole orders on superconductivity and compare with strong-coupling approaches.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electronic and thermoelectric properties of few-layer transition metal dichalcogenides

The electronic and thermoelectric properties of one to four monolayers of MoS{sub 2}, MoSe{sub 2}, WS{sub 2}, and WSe{sub 2} are calculated. For few layer thicknesses, the near degeneracies of the conduction band K and Σ valleys and the valence band Γ and K valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within k{sub B}T of a valley minimum changes with layer thickness. In all cases, the maximum ZT coincides with the greatest near-degeneracy within k{sub B}T of the band edge that results in the sharpest turn-on of the density of modes. The thickness at which this maximum occurs is, in general, not a monolayer. The transition from few layers to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Perfect spin-valley filter controlled by electric field in ferromagnetic silicene

The spin-valley currents in silicene-based normal/sublattice-dependent ferromagnetic/normal junction are investigated. Unlike that in graphene, the pseudo Dirac mass in silicene is generated by spin-orbit interaction and tunable by applying electric or exchange fields into it. This is due to silicon-based honeycomb lattice having buckled structure. As a result, it is found that the junction leads to currents perfectly split into four groups, spin up (down) in k- and k{sup ′}-valleys, when applying different values of the electric field, considered as a perfect spin-valley polarization (PSVP) for electronic application. The PSVP is due to the interplay of spin-valley-dependent Dirac mass and chemical potential in the barrier. The PSVP also occurs only for the energy comparable to the spin-orbit energy gap. This work reveals potential of silicene for spinvalleytronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Topological Insulators in Twisted Transition Metal Dichalcogenide Homobilayers

We show that moiré bands of twisted homobilayers can be topologically nontrivial, and illustrate the tendency by studying valence band states in ± K valleys of twisted bilayer transition metal dichalcogenides, in particular, bilayer MoTe 2 . Because of the large spin-orbit splitting at the monolayer valence band maxima, the low energy valence states of the twisted bilayer MoTe 2 at the + K (– K ) valley can be described using a two-band model with a layer-pseudospin magnetic field Δ(r) that has the moiré period. We show that Δ(r) has a topologically nontrivial skyrmion lattice texture in real space, and that the topmost moiré valence bands provide a realization of the Kane-Mele quantum spin-Hall model, i.e., the two-dimensional time-reversal-invariant topological insulator. As a result, the bands narrow at small twist angles, a rich set of broken symmetry insulating states can occur at integer numbers of electrons per moiré cell.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Valley-dependent exciton fine structure and Autler–Townes doublets from Berry phases in monolayer MoSe 2

The Berry phase of Bloch states can have profound effects on electron dynamics and lead to novel transport phenomena, such as the anomalous Hall effect and the valley Hall effect. Recently, it was predicted that the Berry phase effect can also modify the exciton states in transition metal dichalcogenide monolayers, and lift the energy degeneracy of exciton states with opposite angular momentum through an effective valley-orbital coupling. Here, we report the observation and control of the Berry phase-induced splitting of the 2p exciton states in monolayer molybdenum diselenide (MoSe 2 ) using the intraexciton optical Stark spectroscopy. We observe the time-reversal-symmetric analogue of the orbital Zeeman effect resulting from the valley-dependent Berry phase, which leads to energy difference of +14 (-14) meV between the 2p+ and 2p- exciton states in the K (K') valley, consistent with the ordering from our ab initio GW-Bethe–Salpeter equation results. In addition, we show that the light–matter coupling between intraexciton states is remarkably strong, leading to a prominent valley-dependent Autler–Townes doublet under resonant driving. Finally, our study opens up pathways to coherently manipulate the quantum states and excitonic excitation with infrared radiation in two-dimensional semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Spin-polarization anisotropy controlled by bending in tungsten diselenide nanoribbons and tunable excitonic states

A WSe 2 monolayer shows many interesting properties due to its spin-orbit coupling induced spin splitting in bands around the Fermi level and the spin-valley configuration. The orientation of the spin polarization in the relevant bands is crucial for the nature of exciton states and the optical valley selectivity. In this work, we studied the WSe 2 nanoribbons under different mechanical bending curvatures and electron/hole doping with density functional theory and their optical absorption and excitonic states with many-body perturbation GW and BSE (Bethe-Salpeter equation) methods. We found that the WSe 2 nanoribbons can exhibit an enhanced SOC effect and a spatially varying spin polarization in bands around the Fermi level under bending. The spin-polarization can show an anisotropy (or asymmetry) in those nearly degenerate bands, leading to a controllable magnetism via bending and electron/hole doping to the nanoribbons, suggesting a potential application in compact and controllable magnetic nanodevices and spintronics. The optical absorption spectrum of the nanoribbon presents a large tunability with bending within the near infrared region of about 0.4 to 1.5 eV, showing an enhanced absorption at a large bending condition. Finally, the exciton states generally show mixed or various spin configuration in the electron and hole pairs that are controlled by bending, potentially useful for applications in spin-based quantum information processes.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effective tight-binding model for M X 2 under electric and magnetic fields

We present a systematic method for developing a five band Hamiltonian for the metal d orbitals that can be used to study the effect of electric and magnetic fields on multilayer MX 2 (M=Mo,W and X=S,Se) systems. On a hexagonal lattice of d orbitals, the broken inversion symmetry of the monolayers is incorporated via fictitious s orbitals at the chalcogenide sites. A tight-binding Hamiltonian is constructed and then downfolded to get effective d orbital overlap parameters using quasidegenerate perturbation theory. The steps to incorporate the effects of multiple layers, external electric and magnetic fields are also detailed. We find that an electric field produces a linear-k Rashba splitting around the Γ point, while a magnetic field removes the valley pseudospin degeneracy at the ±K points. Lastly, our model provides a simple tool to understand the recent experiments on electric and magnetic control of valley pseudospin in monolayer dichalcogendies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS 2 and MoS 2 to 65 Tesla

In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality and so on). Here we report low-temperature polarized reflection spectroscopy of atomically thin WS 2 and MoS 2 in high magnetic fields to 65 T. Both the A and B excitons exhibit similar Zeeman splittings of approximately –230 μeV T–1 (g-factor ≃–4), thereby quantifying the valley Zeeman effect in monolayer transition-metal disulphides. Crucially, these large fields also allow observation of the small quadratic diamagnetic shifts of both A and B excitons in monolayer WS 2 , from which radii of ~1.53 and ~1.16 nm are calculated. Further, when analysed within a model of non-local dielectric screening, these diamagnetic shifts also constrain estimates of the A and B exciton binding energies (410 and 470 meV, respectively, using a reduced A exciton mass of 0.16 times the free electron mass). Lastly, these results highlight the utility of high magnetic fields for understanding new two-dimensional materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Detector-size upper bounds on dark hadron lifetime from cosmology

We show that in a confining hidden valley model where the lightest hidden particles are dark hadrons that have mass splittings larger than \( \mathcal{O}(0.1) \) GeV, if the lightest dark hadron is either stable or decays into Standard Model (SM) hadrons/charged leptons during the big-bang nucleosynthesis (BBN), at least one of the heavier dark hadrons needs to decay into SM particles within \( \mathcal{O}(10) \) nanosec. Once being produced at collider experiments, this heavier dark hadron is likely to decay within \( \mathcal{O}(10) \) meter distance, which strengthens the motivation of searching for long-lived particles with sub-meter scale decay lengths at colliders. To illustrate the idea, we study the lifetime constraint in scenarios where the lightest dark particle is a pseudo-scalar meson, and dark hadrons couple to SM particles either through kinetic mixing between the SM and dark photons or by mixing between the SM and dark Higgs. We study the annihilation and decay of dark hadrons in a thermal bath and calculate upper bounds on the lightest vector meson (scalar hadron) lifetime in the kinetic mixing (Higgs portal) scenario. We discuss the application of these lifetime constraints in long-lived particle searches that use the LHCb VELO or the AT-LAS/CMS inner detectors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Magnetic field mixing and splitting of bright and dark excitons in monolayer MoSe 2

Monolayers of semiconducting transition metal dichalcogenides (TMDCs) with unique spin-valley contrasting properties and remarkably strong excitonic effects continue to be a subject of intense research interests. These model 2D semiconductors feature two fundamental intravalley excitons species - optically accessible ' bright' excitons with anti-parallel spins and optically inactive 'dark' excitons with parallel spins. For applications exploiting radiative recombination of bright excitons or long lifetime dark excitons, it is essential to understand the radiative character of the exciton ground state and establish the energy separation between the lowest energy bright and dark excitons. Herein, we report a direct spectroscopic measure of dark excitons in monolayer MoSe 2 encapsulated in hexagonal boron nitride. By applying strong in-plane magnetic field, we induce mixing and splitting of bright and dark exciton branches, which enables an accurate spectroscopic determination of their energies. We confirm the bright character of the exciton ground state separated by a 1.5 meV gap from the higher energy dark exciton state, much smaller compared to the previous theoretical expectations. These results offer critical information for further improvement of the accurate theoretical description of TMDCs electronic structure.

molybdenum diselenide↗
DOE OSTI2018

Floquet Engineering of Gapped 2D Materials

It is demonstrated theoretically that the interaction of gapped 2D materials (gapped graphene and transition metal dichalchogenide monolayers) with a strong high-frequency electromagnetic field (dressing field) crucially changes the band structure of the materials. As a consequence, the renormalized band structure of the materials drastically depends on the field polarization. Particularly, a linearly polarized dressing field always decreases band gaps, whereas a circularly polarized field breaks the equivalence of band valleys in different points of the Brillouin zone and can both increase and decrease corresponding band gaps. It is shown also that a dressing field can turn both the band gaps and the spin splitting of the bands into zero. As a result, the dressing field can serve as an effective tool to control spin and valley properties of the materials in various optoelectronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Spatial Separation of Carrier Spin by the Valley Hall Effect in Monolayer WSe 2 Transistors

We investigate the valley Hall effect (VHE) in monolayer WSe 2 field-effect transistors using optical Kerr rotation measurements at 20 K. While studies of the VHE have so far focused on n-doped MoS 2 , we observe the VHE in WSe 2 in both the n- and p-doping regimes. Hole doping enables access to the large spin-splitting of the valence band of this material. The Kerr rotation measurements probe the spatial distribution of the valley carrier imbalance induced by the VHE. Under current flow, we observe distinct spin-valley polarization along the edges of the transistor channel. From analysis of the magnitude of the Kerr rotation, we infer a spin-valley density of 44 spins/μm, integrated over the edge region in the p-doped regime. As a result, assuming a spin diffusion length less than 0.1 μm, this corresponds to a spin-valley polarization of the holes exceeding 1%.

42 ENGINEERING↗
DOE OSTI2015

Polarized spin and valley transport across ferromagnetic silicene junctions

We study ballistic transport of Dirac fermions through silicene barriers, of width d, with an exchange field M and metallic gates above them that provide tunable potentials of height U. Away from the Dirac point (DP), the spin- and valley-resolved conductances, as functions of U, exhibit resonances and close to it a pronounced dip that becomes a transport gap when an appropriate electric field E{sub z} is applied. The charge conductance g{sub c} of such a junction changes from oscillatory to a monotonically decreasing function of d beyond a critical E{sub z}. This change of g{sub c} can be used to realize electric-field-controlled switching. The field M splits each resonance of g{sub c} in two spin-resolved peaks. The spin p{sub s} and valley p{sub v} polarizations of the current near the DP increase with E{sub z} or M and become nearly perfect above certain of their values. We also show that p{sub s} and p{sub v} can be inverted either by reversing the polarity of U or the direction of M. For two barriers, there is no splitting in g{sub c} when the fields M are in opposite directions. Most of these phenomena have no analogs in graphene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗