부산대학교 도서관

Together with charge and spin degrees of freedom, many new 2D materials also permit information to be encoded in an electron's valley degree of freedom - that is, in particular momentum states in the material's Brillouin zone. With a view towards future generations of valley-based (opto)electronic technologies, the intrinsic timescales of scattering and relaxation between valleys therefore represent fundamental parameters of interest. Here we introduce and demonstrate an entirely passive, noise-based approach for exploring intrinsic valley dynamics in atomically-thin transition-metal dichalcogenide (TMD) semiconductors. Exploiting the valley-specific optical selection rules in monolayer TMDs, we use optical Faraday rotation to detect, under conditions of strict thermal equilibrium, the stochastic thermodynamic fluctuations of the valley polarization in a Fermi sea of resident carriers. Frequency spectra of this spontaneous "valley noise" reveal narrow Lorentzian lineshapes and therefore long exponentially-decaying intrinsic valley relaxation. Moreover, the valley noise signals are shown to validate both the relaxation times and the spectral dependence of conventional (perturbative) pump-probe measurements. These results provide a viable route toward quantitative measurements of intrinsic valley dynamics, free from any external perturbation, pumping, or excitation.
Comment: 6 pages, 5 figures