Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS_2 and WS_2 (1506.00996v1)

Abstract: 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.

• The paper demonstrates unusually long (>3 ns) electron spin lifetimes that far exceed typical exciton recombination times in TMDCs.
• It employs time-resolved Kerr rotation and Hanle measurements to uncover the influence of transverse magnetic fields and rapid intervalley scattering on spin dynamics.
• These findings suggest promising spintronic applications and provide a detailed model for spin-valley coupling in MoS₂ and WS₂.

Long-lived Nanosecond Spin Relaxation and Spin Coherence of Electrons in Monolayer MoS$_2$ and WS$_2$

The paper investigates the dynamics of electron spin relaxation and coherence in monolayer transition metal dichalcogenides (TMDCs), specifically in MoS$_2$ and WS$_2$. Using optical Kerr spectroscopy, the paper explores spin-valley coupling phenomena in n-type doped crystals at low temperatures and delineates a unique dephasing mechanism in these materials, facilitated by fast inter-valley scattering and spin-orbit coupling.

Major Findings

The authors present compelling evidence of unusually long-lived electron spin lifetimes in monolayer MoS$_2$ and WS$_2$, measured to be greater than 3 nanoseconds at 5K. These timescales greatly exceed the typical exciton recombination times, which are constrained to the range of 3-100 picoseconds. Such long relaxation spans underscore a fundamental difference in the spin dynamics of resident carriers in 2D TMDCs compared to their III-V and II-VI semiconductor counterparts.

Time-resolved Kerr rotation (KR) studies reveal that these extensive relaxation times are substantially reduced with small transverse magnetic fields $B_y$. This observation prompts a hypothesized novel spin relaxation mechanism that involves precession due to the combined effects of the fluctuating internal spin-orbit field and a rapidly oscillating magnetic environment, caused by fast intervalley electron scattering.

Moreover, these studies reveal sustained spin coherence at lower energies associated with localized states, which manifest pronounced oscillatory behavior with an electron g-factor near 1.8 in applied magnetic fields.

Theoretical Contributions

The authors support their experimental findings with a model of coupled spin-valley dynamics. This model illustrates the rapid dephasing induced by spin-conserving intervalley scattering and a significant internal spin-orbit field. It contrasts with conventional models by postulating that even small external fields can considerably accelerate spin relaxation, a feature uncommon in traditional semiconductors like GaAs or CdTe.

Methodological Insights

The crystals utilized were grown via chemical vapor deposition and characterized using low-temperature reflectance and photoluminescence (PL) spectroscopy. Hanle-Kerr measurements with continuous wave (cw) lasers and time-resolved KR with ultrafast laser pulses were performed to comprehensively characterize the spin/valley dynamics and electron spin relaxation properties as functions of applied magnetic fields and varying experimental conditions.

Implications and Future Directions

The identification of long spin relaxation times in MoS$_2$ and WS$_2$ at cryogenic temperatures identifies these materials as promising candidates for spintronic applications, leveraging their intrinsic long-lived electron spin polarization. The documented dependence on small magnetic fields and temperature suggests potential pathways for controlling and exploiting spin properties through external perturbations and environment modifications.

Future studies could explore the impact of substrate interactions on spin dynamics and further investigate the contribution of localized states to coherent spin phenomena. In addition, varying temperature conditions or different types of dielectric screenings may provide deeper insights into the electron-phonon interactions that underpin spin relaxation processes in these materials. The expansion of this research could enhance the development of low-dimensional semiconductor devices that utilize both charge and spin degrees of freedom, fostering advances in quantum technology and nanoelectronics.