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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Strong Zeeman splitting in orbital-hybridized valleytronic interfaces

We report stacked heterointerfaces of two-dimensional (2D) materials are well-suited to obtain novel electronic functionality at small length scales, because the intrinsic properties of both materials can be blended by interlayer coupling with remarkable effects on the electronic band structure. In particular, band degeneracy at the K and K' Brillouin zone valleys of 2D materials can be broken by symmetry reductions by applying an external magnetic field, with possible application in valleytronic devices. Nevertheless, this usually requires impractically large external magnetic fields > 100 T. An alternative and more practical route is provided via stacking a non-centrosymmetric layered material on a magnetic substrate to induce valley splitting through the magnetic proximity effect; however, the splitting magnitude can vary sharply between similar interfaces. In this work, first-principle calculations are used to calculate the valley splitting of eight low-strain interfaces with transition metal dichalcogenides stacked on 2D magnetic substrates. It is shown that the interlayer band hybridization plays a major role when the two layers’ bands are closely aligned in energy, with the resulting band repulsion and the total valley splitting being strongly dependent on the Hubbard U correction , showing up to an order of magnitude change. The WSe 2 /CrGeTe 3 interface in particular shows strong interlayer interactions, which can potentially lead to a significantly high valley splitting of 26 meV; however further experimental investigations will be needed to resolve uncertainties in the predicted valley splitting.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Dressed photon-orbital states in a quantum dot: Intervalley spin resonance

Because of the symmetry in silicon quantum wells, silicon quantum dots have an extra degree of freedom leading to a small energy splitting called the valley splitting. This degree of freedom has been viewed alternately as a hazard, especially when the lowest valley-orbit splitting is small compared to the thermal energy, or as an asset, most prominently in proposals to use the valley degree of freedom itself as a qubit. Here we present experiments in which microwave electric field driving induces transitions between both valley-orbit and spin states. We show that this system is highly nonlinear and can be understood through the use of dressed photon-orbital states, enabling a unified understanding of six resonance lines we observe in these experiments.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Proximity-enhanced valley Zeeman splitting at the WS 2 /graphene interface

The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS 2 , systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS 2 /graphene stacks employing first-principles calculations and find that the lower conduction band of WS 2 at the <?CDATA $K/K^{^{\prime}}$?> K / K ′ valleys (the <:tex-math><?CDATA $\mathrm{CB}^-$?> C B − band) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at the Q point of WS 2 mediates the coupling between <?CDATA $\mathrm{CB}^-$?> C B − and graphene due to resonant energy conditions and strong coupling to the Dirac cone. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Resonant tunneling spectroscopy of valley eigenstates on a donor-quantum dot coupled system

We report on electronic transport measurements through a silicon double quantum dot consisting of a donor and a quantum dot. Transport spectra show resonant tunneling peaks involving different valley states, which illustrate the valley splitting in a quantum dot on a Si/SiO{sub 2} interface. The detailed gate bias dependence of double dot transport allows a first direct observation of the valley splitting in the quantum dot, which is controllable between 160 and 240 μeV with an electric field dependence 1.2 ± 0.2 meV/(MV/m). A large valley splitting is an essential requirement for implementing a physical electron spin qubit in a silicon quantum dot.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Out-of-plane carrier spin in transition-metal dichalcogenides under electric current

Significance Although out-of-plane current-induced spin polarization (CISP) is important for perpendicular-magnetization reorientation, and it has been realized by the crystalline symmetry breaking, its material examples are relatively fewer compared with in-plane CISP in Rashba systems. With the help of intrinsic spin–orbit coupling, an intriguing out-of-plane CISP is designed in transition-metal dichalcogenides by the symmetry breaking in the spin space, which provides opportunities for out-of-plane magnetization rotation and electric control of valley splitting. Moreover, the spin polarization is associated with valley-dependent responses to electric current and adds a dimension, valley degree of freedom, to the study of CISP. The symmetry argument and non-Rashba effective model also helps to illuminate these physics and broaden the scope of the CISP.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Magneto-optical spectra of transition metal dichalcogenides: A comparative study

Following our previous work [Ho et al., Phys. Rev. B 89, 155316 (2014)], we systematically calculate the magneto-optical properties of various transition metal dichalcogenide monolayers. The intrinsic spin-orbit coupling gives rise to the spin-split electronic states, while a perpendicular magnetic field further induces a valley splitting. In magneto-optical spectra with linearly polarized light, spectral features are spin and valley-polarized. Compounds are different from one another in terms of transition energies and appearance of twin peaks. Our numerical results can serve as a guide for future experimental identification.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Giant spin-valley polarization and multiple Hall effect in functionalized bismuth monolayers

Abstract Valleytronic materials, characterized by local extrema (valleys) in their bands, and topological insulators have separately attracted great interest recently. However, the interplay between valleytronic and topological properties in one single system, likely to enable important unexplored phenomena and applications, has been largely overlooked so far. Here, by combining a tight-binding model with first-principles calculations, we find the large-band-gap quantum spin Hall effects (QSHEs) and valley Hall effects appear simultaneously in the bismuth monolayers decorated with hydrogen/halogen elements, denoted as Bi 2 XY (X, Y = H, F, Cl, Br, or I). A staggered exchange field is introduced into the Bi 2 XY monolayers by transition-metal atom (Cr, Mo, or W) doping or LaFeO 3 magnetic substrates, which together with the strong spin-orbit coupling of bismuth atoms generates a time-reversal-symmetry-broken QSHE and a huge valley splitting (up to 513 meV) in the system. With gate control, QSHE and anomalous charge, spin, valley Hall effects can be observed in the single system. These predicted multiple and exotic Hall effects, associated with various degrees of freedom of electrons, could enable applications of the functionalized bismuth monolayers in electronics, spintronics, and valleytronics.

Zhou, Tong (ORCID:0000000345885263)↗
DOE OSTI2019

Valley-dependent Lorentz force and Aharonov-Bohm phase in strained graphene p - n junction

Veselago lens focusing in graphene p-n junction is promising for realizations of new generation electron optics devices. However, the effect of the strain-induced Aharonov-Bohm interference in a p-n junction has not been discussed before. We provide an experimentally feasible setup based on the Veselago lens in which the presence of strain can result in both the valley-dependent Lorentz force and Aharonov-Bohm interference. Inparticular, by employing the Green’s function and tight-binding methods, we study the strain induced by dislocations and line defects in a p-n junction and show how the resulting Aharonov-Bohm phase and interference can be detected. Furthermore, for a different strain configuration, e.g., corresponding to corrugated graphene, we find strong signatures of valley splitting induced by the fictitious magnetic field. Our proposal can be useful for mapping elastic deformations and defects, and for studying valley-dependent effects in graphene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Two-dimensional ferroelectric topological insulators in functionalized atomically thin bismuth layers

We introduce a class of two-dimensional (2D) materials that possess coexisting ferroelectric and topologically insulating orders. Such ferroelectric topological insulators (FETIs) occur in noncentrosymmetric atomic layer structures with strong spin-orbit coupling (SOC). Here, we showcase a prototype 2D FETI in an atomically thin bismuth layer functionalized by CH 2 OH, which exhibits a large ferroelectric polarization that is switchable by a ligand molecule rotation mechanism and a strong SOC that drives a band inversion leading to the topologically insulating state. An external electric field that switches the ferroelectric polarization also tunes the spin texture in the underlying atomic lattice. Moreover, the functionalized bismuth layer exhibits an additional quantum order driven by the valley splitting at the K and K' points in the Brillouin zone stemming from the symmetry breaking and strong SOC in the system, resulting in a remarkable state of matter with the simultaneous presence of the quantum spin Hall and quantum valley Hall effect. These phenomena are predicted to exist in other similarly constructed 2D FETIs, thereby offering a unique quantum material platform for discovering novel physics and exploring innovative applications.

2-dimensional systems↗
DOE OSTI2017

Van der Waals engineering of ferromagnetic semiconductor heterostructures for spin and valleytronics

The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage. We create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI 3 and a monolayer of WSe 2 . We observe unprecedented control of the spin and valley pseudospin in WSe 2 , where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe 2 valley splitting and polarization via flipping of the CrI 3 magnetization. The WSe2 photoluminescence intensity strongly depends on the relative alignment between photoexcited spins in WSe 2 and the CrI 3 magnetization, because of ultrafast spin-dependent charge hopping across the heterostructure interface. The photoluminescence detection of valley pseudospin provides a simple and sensitive method to probe the intriguing domain dynamics in the ultrathin magnet, as well as the rich spin interactions within the heterostructure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Multifunctional two-dimensional van der Waals Janus magnet Cr-based dichalcogenide halides

Two-dimensional van der Waals Janus materials and their heterostructures offer fertile platforms for designing fascinating functionalities. Here, by means of systematic first-principles studies on van der Waals Janus monolayer Cr-based dichalcogenide halides CrYX (Y = S, Se, Te; X = Cl, Br, I), we find that CrSX (X = Cl, Br, I) are the very desirable high T C ferromagnetic semiconductors with an out-of-plane magnetization. Excitingly, by the benefit of the large magnetic moments on ligand S 2- anions, the sought-after large-gap quantum anomalous Hall effect and sizable valley splitting can be achieved through the magnetic proximity effect in van der Waals heterostructures CrSBr/Bi 2 Se 3 /CrSBr and MoTe 2 /CrSBr, respectively. Additionally, we show that large Dzyaloshinskii–Moriya interactions give rise to skyrmion states in CrTeX (X = Cl, Br, I) under external magnetic fields. Our work reveals that two-dimensional Janus magnet Cr-based dichalcogenide halides have appealing multifunctionalities in the applications of topological electronic and valleytronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Modeling Si/SiGe quantum dot variability induced by interface disorder reconstructed from multiperspective microscopy

SiGe heteroepitaxial growth yields pristine host material for quantum dot qubits, but residual interface disorder can lead to qubit-to-qubit variability that might pose an obstacle to reliable SiGe-based quantum computing. By convolving data from scanning tunneling microscopy and high-angle annular dark field scanning transmission electron microscopy, we reconstruct 3D interfacial atomic structure and employ an atomistic multi-valley effective mass theory to quantify qubit spectral variability. The results indicate (1) appreciable valley splitting (VS) variability of ~50% owing to alloy disorder and (2) roughness-induced double-dot detuning bias energy variability of order 1–10 meV depending on well thickness. For measured intermixing, atomic steps have negligible influence on VS, and uncorrelated roughness causes spatially fluctuating energy biases in double-dot detunings potentially incorrectly attributed to charge disorder. Our approach yields atomic structure spanning orders of magnitude larger areas than post-growth microscopy or tomography alone, enabling more holistic predictions of disorder-induced qubit variability.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Correlated Hofstadter spectrum and flavour phase diagram in magic-angle twisted bilayer graphene

In magic-angle twisted bilayer graphene, the moiré superlattice potential gives rise to narrow electronic bands that support a multitude of many-body quantum phases. Further richness arises in the presence of a perpendicular magnetic field, where the interplay between moiré and magnetic length scales leads to fractal Hofstadter subbands. In this strongly correlated Hofstadter platform, multiple experiments have identified gapped topological and correlated states, but little is known about the phase transitions between them in the intervening compressible regimes. Here we simultaneously unveil sequences of broken-symmetry Chern insulators and resolve sharp phase transitions between competing states with different topological quantum numbers and different occupations of the spin-valley flavour. Our measurements determine the energy spectrum of interacting Hofstadter subbands in magic-angle twisted bilayer graphene and map out the phase diagram of flavour occupancy. In addition, we observe full lifting of the degeneracy of the zeroth Landau levels together with level crossings, indicating moiré valley splitting. We propose a unified flavour polarization mechanism to understand the intricate interplay of topology, interactions and symmetry breaking as a function of density and applied magnetic field in this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A direct measurement of the electronic structure of Si nanocrystals and its effect on optoelectronic properties

Since reports that silicon nanocrystals (Si-NCs) can exhibit direct transition emission, the silicon laser field is at a juncture where the importance of this discovery needs to be evaluated. Most theoretical models predicted a monotonic increase in the bandgap and experimental information currently available on the electronic structure at the Γ valley of these promising materials is circumstantial as it is obtained from emission measurements where competing non-radiative relaxation and recombination processes only provide an incomplete picture of the electronic structure of Si-NCs. Optical absorption, the most immediate probe of the electronic structure beyond the band-edges, showing the evolution of the Γ valley states with nanocrystal size has not been measured. Here, we show such measurements, performed with high dynamic range, allowing us to observe directly the effect of crystal size on the Γ valley splitting far above the band-edges. We show that the splitting is 100 s of meV more pronounced than predicted by pseudo potential calculations and Luttinger-Kohn model. We also show that ultrafast red-shifting emission can be observed in plasma enhanced chemical vapor deposition prepared Si-NCs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A reconfigurable gate architecture for Si/SiGe quantum dots

We demonstrate a reconfigurable quantum dot gate architecture that incorporates two interchangeable transport channels. One channel is used to form quantum dots, and the other is used for charge sensing. The quantum dot transport channel can support either a single or a double quantum dot. We demonstrate few-electron occupation in a single quantum dot and extract charging energies as large as 6.6 meV. Magnetospectroscopy is used to measure valley splittings in the range of 35–70 μeV. By energizing two additional gates, we form a few-electron double quantum dot and demonstrate tunable tunnel coupling at the (1,0) to (0,1) interdot charge transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Density dependence of the excitation gaps in an undoped Si/SiGe double-quantum-well heterostructure

In this work, we report low-temperature magneto-transport measurements of an undoped Si/SiGe asymmetric double quantum well heterostructure. The density in both layers is tuned independently utilizing top and bottom gates, allowing the investigation of quantum wells at both imbalanced and matched densities. Integer quantum Hall states at total filling factor v T =1 and v T =2 are observed in both density regimes, and the evolution of their excitation gaps is reported as a function of the density. The v T =1 gap evolution departs from the behavior generally observed for valley splitting in the single layer regime. Furthermore, by comparing the v T =2 gap to the single particle tunneling energy, Δ SAS , obtained from Schrödinger–Poisson (SP) simulations, evidence for the onset of spontaneous interlayer coherence is observed for a relative filling fraction imbalance smaller than ~50%.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗