Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Spin Valleytronics and Spin Orbit |
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Sponsoring Units: GMAG DMP FIAP Chair: Aubrey Hanbicki, Naval Research Laboratory Room: 320 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B18.00001: Valley polarization and intervalley scattering in monolayer MoS$_{2}$ G. Kioseoglou, A.T. Hanbicki, M. Currie, A.L. Friedman, D. Gunlycke, B.T. Jonker Single layer MoS$_{2}$ is a prime candidate material for implementing valleytronics because minima in the bandstructure at inequivalent K points of the Brillouin zone can be independently populated, thus making the valley index a potential state variable for information processing. Light of a particular helicity populates only one of the two K-valleys (either K or K') resulting in a strong emission at around 1.9 eV associated with a direct transition. We use energy and helicity dependent optical pumping to analyze the coupling of the valley and spin indices to the depolarization of emitted light. The circular polarization of the photoluminescence is very high for photo-excitation near the bandgap, and has a power-law decrease as the photo-excitation energy increases. We identify phonon-assisted intervalley scattering as the primary spin relaxation mechanism and present a model of depolarization that explains the wide variation in values for the optical polarization reported in the literature. Our results elucidate the basic processes that control the unique properties of this material and should help to realize future valleytronic applications. This work was supported by core programs at NRL and the NRL Nanoscience Institute. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B18.00002: Theoretical analysis of optical excitation and luminescence in MoS$_2$ monolayers Hanan Dery, Yang Song We analyze the absorption and circularly polarized luminescence spectra of MoS$_2$ monolayers. We show that indirect optical transitions can fully explain the observed decrease in circular polarization degree when increasing the lattice temperature or the exciting photon energy. This spin-conserving optical process is assisted by electron-phonon or electron-impurity interactions giving rise to intervalley transitions to intermediate virtual states. Spin-flip mechanisms, on the other hand, are shown to be insufficient in explaining the experimental results due to their relatively long timescales compared with the radiative timescales in monolayer dichalcogenides (tens of ps). [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B18.00003: Longitudinal and spin/valley Hall optical conductivity in single layer MoS$_2$ Zhou Li, Jules Carbotte A monolayer of MoS$_2$ has a non-centrosymmetric crystal structure, with spin polarized bands. It is a two valley semiconductor with direct gap falling in the visible range of the electromagnetic spectrum. Its optical properties are of particular interest in relation to valleytronic and possible device applications. Circular polarized light associated with each of the two valleys separately is considered and results are filtered according to spin polarization. Temperature can greatly change the spin mixture seen in the frequency window where they are not closely in balance.\\[4pt] [1] Zhou Li and J. P. Carbotte, submitted to Phys. Rev. B.\\[0pt] [2] D. Xiao et.al, Phys. Rev. Lett. 108,196802 (2012). [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B18.00004: Optical and Electrical Control of Valley Pseudospin in Atomically-Thin Semiconductors Invited Speaker: Xiaodong Xu Electronic valleys are energy extrema of Bloch bands in momentum space. In analogy to electrons with spin degrees of freedom, valley indexes can be considered as pseudospins for new modes of electronic and photonic device operation. In this talk, I will discuss the experimental progress on the investigation of these pseudospins using atomically-thin semiconductors, which are either single or bilayer group VI transition metal dichalcogenides. I will show that these new 2D semiconductors not only behave as remarkable excitonic systems, but also provide an ideal system for optical and electrical control of valley degrees of freedom. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B18.00005: Theoretical design of magnetic two-dimensional transition metal dichalcogenide semiconductors Wenguang Zhu, Di Xiao We explore the possibility of making single-layer dichalcogenide semiconductors magnetic by doping transition metal ions using density functional calculations. Optimal conditions of doping are suggested based on the study of the energetics and kinetics of magnetic ions in the host materials. The magnetic ordering and magnetic coupling mechanism between the magnetic dopants will also be discussed in this talk. This work may provide a new twist to form truly two-dimensional magnetic semiconductors for spintronic applications. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B18.00006: Group theory analysis of the intrinsic momentum and spin relaxation in monolayer dichalcogenides Yang Song, Hanan Dery Using group theory, we study the intrinsic momentum and spin relaxation of electrons and holes due to scattering with phonons in monolayer dichalcogenides. Double group symmetry representations of electron and hole states at high symmetry points ($K$, $K'$, and $\Gamma$ points as well as the $T$ axis) are identified with the help of results from absorption and photoluminescence experiments. We link the leading contributions to intravalley and intervalley scattering with symmetries of the nine phonon dispersion branches. Scattering matrix elements due to short-range interaction and the corresponding Elliott-Yafet spin-flip mechanism are expressed analytically, leading to explicit wavevector-dependence and scattering integrals. Long-range interaction are similarly analyzed. Due to the absence of inversion symmetry, valley-spin coupling is revealed to be a general feature in the spin-flip scattering. Using these results we estimate the temperature-dependent relaxation times. Intervalley scattering between valleys not connected by time-reversal is shown to be compound dependent. The $K$ to $T$ transition ($K$ to $\Gamma$ transition) in the conduction (valence) band is relevant in heavier (lighter) compounds such as WSe$_2$ (MoS$_2$). [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B18.00007: Spin-dependent phonon-assisted optical transition in Si and Ge under strain Pengke Li, Dhara Trivedi, Hanan Dery In indirect bandgap semiconductors like Si and Ge, the transfer of angular momentum between free carriers and photons is intricate since they involve both radiation-matter and electron-phonon interactions. Moreover, the multi-valley conduction band of Si and Ge leads to dependence on light propagation. By breaking the degeneracies of conduction valleys and of valence bands, strain could be used as an experimental tool to regulate and validate the relation between the measured circular polarization degree of photons and the spin polarization of charge carriers. Using symmetry arguments, we present a theoretical study of the spin-dependent selection rules for various phonon-assisted optical transitions. We show how these selection rules are changed under different configurations of strain. These selection rules are verified by rigorous numerical calculation of the spin-dependent luminescence spectra in strained Si and Ge, as well as in relaxed SiGe alloys. Lastly, we also provide results of the inverse process, namely optical orientation. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B18.00008: Spin communications under optimized external electric field in strained group IV semiconductors Lan Qing, Hanan Dery We investigate factors affecting an on-chip communication paradigm that is based on modulating spin polarization of a constant current in silicon or germanium wires. Strain that quenches certain intervalley scattering can prolong the spin lifetime considerably. Necessary external electric field can accelerate the transport by increasing drift velocity, yet it also enhances the spin relaxation by heating the electrons. We predict non-monotonic behaviors of the final spin signals versus the external electric fields. Simple approximate expressions are provided for the spin lifetimes of drifting electrons in strained silicon and germanium, which enable us to choose electric fields that maximize the signals in spin transport. Even at room temperature, we can expect no significant loss of the spin signal after transport across millimeter scale. Our theoretical results are supported by recent experimental breakthroughs. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B18.00009: Enhanced intervalley splitting and reduced spin relaxation in strained thin silicon films Dmitri Osintsev, Viktor Sverdlov, Siegfried Selberherr We investigate the influence of strain and spin-orbit interaction on the valley splitting, subband structure, subband wave functions, and spin relaxation matrix elements due to surface roughness scattering in thin silicon films. A ${\mathbf{k \cdot p}}$ approach suitable to describe the electron subband structure with spin [1] is generalized to include strain. The 4$\times$4 Hamiltonian is diagonalized with respect to the spin degree by a unitary transformation. The wave functions and eigenenergies are found analytically, when the thin film is approximated by an infinite square well potential. In relaxed films the unprimed subbands are degenerate. This degeneracy produces a large mixing between the spin-up and spin-down states, resulting in spin hot spots characterized by strong spin relaxation. These hot spots are contrasted with those appearing in the bulk system [1] due to the degeneracy of the opposite valleys along certain directions close to the X-point at the edge of the Brillouin zone. Shear strain efficiently lifts the degeneracy between the unprimed subbands. This removes the origin of the spin hot spots in a confined silicon system, which substantially improves the spin lifetime in silicon films. 1.P.Li and H.Dery, {\it Phys.Rev.Lett.} {\bf 107}, 107203 (2011). [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B18.00010: Unified theory of spin-dynamics in a two dimensional electron gase with arbitrary spin-orbit coping strength at finite temperature Xin Liu, Sinova Jairo We study the spin dynamics in the presence of impurity and electron-electron (e-e) scattering in a III-V semiconductor quantum well with arbitrary spin-orbit coupling (SOC) strength and symmetry at finite temperature. In the regime where the strength of the Rashba and linear Dresselhaus SOC match, known as the SU(2) symmetry point, experiments have observed the spin-helix mode with a large spin-lifetime whose unexplained nonmonotonic temperature dependence peaks at around 75 K. As a key test of our theory, we are able to naturally explain quantitatively this nonmonotonic dependence and show that it arises as a competition between the Dyakonov-Perel mechanism, suppressed at the SU(2) point, and the Elliott-Yafet mechanism. In the strong SOC regime, we show that our theory directly reproduces the previous known analytical result at the SU(2) symmetry point in the ballistic regime. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B18.00011: A critical phase induced by interplay of spin-orbit coupling and Coulomb interaction Eun-Gook Moon, Cenke Xu, Yong Baek Kim, Leon Balents We study long range Coulomb interaction effect on the Luttinger Hamiltonian in three spatial dimensions, which describes strong spin orbit coupling intrinsically. The Hamiltonian has energy spectrum of inverted band gap semiconductors as in well-known HgTe; only one quadratic band touching point exists at the gamma point in Brillouin zone protected by the cubic and time reversal symmetries. Using controlled renormalization group techniques, we find that long-range Coulomb interaction converts the quadratic band touching state into a non-Fermi liquid (NFL) state, in some ways analogous to the Luttinger liquid state in one dimension. Consequently, all physical quantities become scale invariant and show deviations from non-interacting electrons' properties. Temperature and field dependence of various thermodynamic functions are obtained. Moreover, our ground state can be viewed as a parent state of topological insulators, magnetic metals, and Weyl semi-metals by breaking either cubic symmetry or time-reversal symmetry. The strong Coulomb interaction changes phase boundaries qualitatively and phase diagrams with the Coulomb interaction are provided. Applications to iridium-oxides materials are also discussed. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B18.00012: 3D and 4D Topological Insulators based SU(2) Landau Levels Yi Li, Shou-Cheng Zhang, Congjun Wu Current studies of 3D topological insulators (TIs) based on the Bloch-wave band inversion have made great success in lattices. Independent of current routine, we propose a novel and simple mechanism achieving exactly flat topological spectra for electrons in the continuum at 3D and 4D without magnetic fields. By introducing spin-orbit couplings, helical Dirac modes or chiral Weyl modes with opposite helicities are spatially separated along an extra spatial dimension and robust at boundaries as protected by the time-reversal symmetry. Moreover, based on elegant analytic wavefunctions of high dimensional Landau levels, we construct the Laughlin type wavefunction at the fractional filling in 4d. Further, parallel to the 2D QHE, whose quantized Hall response demonstrates spatially separated (1+1)D chiral anomaly, the 4D SU(2) Landau levels explicitly show the quantized non-linear electromagnetic response, which exhibits spatially separated (3+1)D chiral anomaly with the same quantization in the unit of fundamental physical constants. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B18.00013: Spin polarization in the Hubbard model with Rashba spin-orbit coupling on a ladder Jose Riera The competition between on-site Coulomb repulsion and Rashba spin-orbit (RSO) coupling is studied on two-leg ladders by numerical techniques. Using DMRG it is found that the contribution to the current due to the RSO coupling for a fixed value of the Hubbard repulsion $U$ reaches a maximum at intermediate values of the RSO coupling-to-hopping ratio and eventually becomes negative. This point of maximum current is correlated with the maximum value of the spin polarization between the two legs of the ladder. The most important result is that for a fixed value of the RSO coupling, the spin polarization increases with $U$ and seems to saturate as $U\rightarrow \infty$. These behaviors are studied at various fillings in the metallic regime. Further support for these conclusions is provided by the study of persistent currents in Hubbard-Rashba models on ladder rings. The implications of this enhancement of the spin Hall effect with electron correlations for spintronic devices is discussed. [Preview Abstract] |
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