arXiv subjects

Luyi Yang

Publications and source records attributed to Luyi Yang.

28 records · Page 2Linked to original sources

arXiv2017

Gate controlled spin-valley locking of resident carriers in WSe2 monolayers

Using time-resolved Kerr rotation, we measure the spin/valley dynamics of resident electrons and holes in single charge-tunable monolayers of the archetypal transition-metal dichalcogenide (TMD) semiconductor WSe2. In the n-type regime, we observe long (70 ns) polarization relaxation of electrons that is sensitive to in-plane magnetic fields $B_y$, indicating spin relaxation. In marked contrast, extraordinarily long (2 microsecond) polarization relaxation of holes is revealed in the p-type regime, that is unaffected by $B_y$, directly confirming long-standing expectations of strong spin-valley locking of holes in the valence band of monolayer TMDs. Supported by continuous-wave Kerr spectroscopy and Hanle measurements, these studies provide a unified picture of carrier polarization dynamics in monolayer TMDs, which can guide design principles for future valleytronic devices.

cond-mat.mes-hall↗

arXiv2015

Spin coherence and dephasing of localized electrons in monolayer MoS$_2$

We report a systematic study of coherent spin precession and spin dephasing in electron-doped monolayer MoS$_2$. Using time-resolved Kerr rotation spectroscopy and applied in-plane magnetic fields, a nanosecond-timescale Larmor spin precession signal commensurate with $g$-factor $|g_0|\simeq 1.86$ is observed in several different MoS$_2$ samples grown by chemical vapor deposition. The dephasing rate of this oscillatory signal increases linearly with magnetic field, suggesting that the coherence arises from a sub-ensemble of localized electron spins having an inhomogeneously-broadened distribution of $g$-factors, $g_0 + Δg$. In contrast to $g_0$, $Δg$ is sample-dependent and ranges from 0.042 to 0.115.

cond-mat.mtrl-sci↗

arXiv2015

Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS_2 and WS_2

The recently-discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin-valley physics. Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments, PL timescales are necessarily constrained by short-lived (3-100ps) electron-hole recombination. Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole) doped TMDCs, which may persist long after recombination ceases, are at an early stage. Here we directly measure the coupled spin-valley dynamics in electron-doped MoS_2 and WS_2 monolayers using optical Kerr spectroscopy, and unambiguously reveal very long electron spin lifetimes exceeding 3ns at 5K (2-3 orders of magnitude longer than typical exciton recombination times). In contrast with conventional III-V or II-VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin-valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly-fluctuating internal spin-orbit field in TMDCs, driven by fast intervalley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide crucial insight into the physics underpinning spin and valley dynamics of resident electrons in atomically-thin TMDCs.

cond-mat.mtrl-sci↗

arXiv2014

Cross-correlation spin noise spectroscopy of heterogeneous interacting spin systems

We develop and apply a minimally invasive approach for characterization of inter-species spin interactions by detecting spin fluctuations alone. We consider a heterogeneous two-component spin ensemble in thermal equilibrium that interacts via binary exchange coupling, and we determine cross-correlations between the intrinsic spin fluctuations exhibited by the two species. Our theoretical predictions are experimentally confirmed using `two-color' optical spin noise spectroscopy on a mixture of interacting Rb and Cs alkali vapors. The results allow us to explore the rates of spin exchange and total spin relaxation under conditions of strict thermodynamic equilibrium.

cond-mat.mes-hall↗

arXiv2014

Two-color spin noise spectroscopy: Using spin fluctuation correlations to reveal homogeneous linewidths within quantum dot ensembles

"Spin noise spectroscopy" (SNS) is a powerful optical technique for probing electron and hole spin dynamics that is based on detecting their intrinsic and random fluctuations while in thermal equilibrium, an approach guaranteed by the fluctuation-dissipation theorem. Because SNS measures fluctuation properties rather than conventional response functions, we show that fluctuation \emph{correlations} can be exploited in multi-probe noise studies to reveal information that in general cannot be accessed by conventional linear optical spectroscopy, such as the underlying homogeneous linewidths of individual constituents within inhomogeneously-broadened systems. This is demonstrated in an ensemble of singly-charged (In,Ga)As quantum dots using two weak probe lasers: When the two lasers have the same wavelength, they are sensitive to the same QDs in the ensemble and their spin fluctuation signals are correlated. In contrast, two probe lasers that are widely detuned from each other measure different subsets of QDs, leading to uncorrelated fluctuations. Measuring the noise correlation versus laser detuning directly reveals the QD homogeneous linewidth even in the presence of a strong inhomogeneous broadening. Such noise-based correlation techniques are not limited to semiconductor spin systems, but can be widely applied to any system in which intrinsic fluctuations are measurable.

cond-mat.mes-hall↗

arXiv2014

Spin noise spectroscopy beyond thermal equilibrium and linear response

Per the fluctuation-dissipation theorem, the information obtained from spin fluctuation studies in thermal equilibrium is necessarily constrained by the system's linear response functions. However, by including weak radiofrequency magnetic fields, we demonstrate that intrinsic and random spin fluctuations even in strictly unpolarized ensembles \emph{can} reveal underlying patterns of correlation and coupling beyond linear response, and can be used to study non-equilibrium and even multiphoton coherent spin phenomena. We demonstrate this capability in a classical vapor of $^{41}$K alkali atoms, where spin fluctuations alone directly reveal Rabi splittings, the formation of Mollow triplets and Autler-Townes doublets, ac Zeeman shifts, and even nonlinear multiphoton coherences.

physics.atom-ph↗

arXiv2012

Coherent Propagation of Spin Helices in a Quantum-Well Confined Electron Gas

We use phase-resolved transient grating spectroscopy to measure the propagation of spin helices in a high mobility $n$-GaAs/AlGaAs quantum well with an applied in-plane electric field. At relatively low fields helical modes crossover from overdamped excitations where the spin-precession period exceeds the spin lifetime, to a regime of coherent propagation where several spin-precession periods can be observed. We demonstrate that the envelope of a spin polarization packet reaches a current-driven velocity of 10$^7$ cm s$^{-1}$ in an applied field of 70 V cm$^{-1}$.

cond-mat.mes-hall↗

arXiv2012

Doppler velocimetry of spin propagation in a two-dimensional electron gas

Controlling the flow of electrons by manipulation of their spin is a key to the development of spin-based electronics. While recent demonstrations of electrical-gate control in spin-transistor configurations show great promise, operation at room temperature remains elusive. Further progress requires a deeper understanding of the propagation of spin polarization, particularly in the high mobility semiconductors used for devices. Here we report the application of Doppler velocimetry to resolve the motion of spin-polarized electrons in GaAs quantum wells driven by a drifting Fermi sea. We find that the spin mobility tracks the high electron mobility precisely as a function of T. However, we also observe that the coherent precession of spins driven by spin-orbit interaction, which is essential for the operation of a broad class of spin logic devices, breaks down at temperatures above 150 K for reasons that are not understood theoretically.

cond-mat.mes-hall↗

arXiv2011

Measurement of electron-hole friction in an n-doped GaAs/AlGaAs quantum well using optical transient grating spectroscopy

We use phase-resolved transient grating spectroscopy to measure the drift and diffusion of electron-hole density waves in a semiconductor quantum well. The unique aspects of this optical probe allow us to determine the frictional force between a two-dimensional Fermi liquid of electrons and a dilute gas of holes. Knowledge of electron-hole friction enables prediction of ambipolar dynamics in high-mobility electron systems.

cond-mat.mes-hall↗

arXiv2011

Random walk approach to spin dynamics in a two-dimensional electron gas with spin-orbit coupling

We introduce and solve a semi-classical random walk (RW) model that describes the dynamics of spin polarization waves in zinc-blende semiconductor quantum wells. We derive the dispersion relations for these waves, including the Rashba, linear and cubic Dresselhaus spin-orbit interactions, as well as the effects of an electric field applied parallel to the spin polarization wavevector. In agreement with fully quantum mechanical calculations [Kleinert and Bryksin, Phys. Rev. B \textbf{76}, 205326 (2007)], the RW approach predicts that spin waves acquire a phase velocity in the presence of the field that crosses zero at a nonzero wavevector, $q_0$. In addition, we show that the spin-wave decay rate is independent of field at $q_0$ but increases as $(q-q_0)^2$ for $q\neq q_0$. These predictions can be tested experimentally by suitable transient spin grating experiments.

cond-mat.mes-hall↗