Ultrafast Strongly Anisotropic Valleytronics in SnSe

TL;DR

This work reveals ultrafast, strongly anisotropic valley dynamics in SnSe by combining time-resolved, polarization-controlled XUV photoemission with time-dependent Boltzmann equation simulations. The study shows one valley channel (X) can be nearly nonvolatile under ZZ excitation, while the orthogonal Y valley experiences ultrafast depolarization and polarization reversal under AC excitation due to intervalley scattering mediated by a particular in-plane optical phonon around 20 meV. The mechanism is rooted in mode-specific electron-phonon coupling and symmetry, highlighting a valleytronic platform with coexisting volatile and nonvolatile channels, distinct from conventional TMDC valleytronics and potentially enabling new valley-based quantum and optoelectronic functionalities.$f_{nk}(t)$ and $n_q$ are tracked to capture the coupled electron-phonon nonequilibrium dynamics, with a focus on how phonon-mediated scattering directs valley population flow toward the global CBM. Overall, the work broadens the landscape of valleytronics by identifying materials with intrinsically nondegenerate valleys that exhibit radically different ultrafast valley depolarization behavior, informing future device strategies for anisotropic valley control.

Abstract

Valleytronics aims to control electrons in a valley-specific manner for quantum information manipulation. Due to their strong in-plane anisotropy, which enables polarization-controlled optical transitions to distinct nondegenerate valleys, group-IV monochalcogenides have been recently proposed as promising candidates for next-generation valleytronic materials. However, ultrafast nonequilibrium dynamics following optical preparation of valley-polarized states remain completely unexplored in these systems. Combining time- and angle-resolved extreme-ultraviolet photoemission spectroscopy with time-dependent Boltzmann equation simulations, we investigate ultrafast valley polarization dynamics following polarization-controlled photoexcitation in SnSe. We show that selective excitation to valleys at global conduction minima yields nearly unity and time-independent valley polarization. In contrast, photoexcitation to the other valley channel leads to ultrafast decay and reversal of valley polarization on sub-picosecond timescales due to intervalley scattering mediated by strong electron-phonon coupling with an optical phonon mode. Our findings reveal strongly anisotropic and radically different nonequilibrium valley physics than in most common two-dimensional valleytronics materials.

Figures (7)

• Figure 1: Scheme of the experiment and ultrafast valleytronics in SnSe. (a) Schematic of the experimental setup, where s-polarized IR pump pulses (1.2 eV, 135 fs, 0.9 mJ/cm$^2$) and XUV probe pulses (21.6 eV) are sent onto a SnSe crystal along either the AC or ZZ crystallographic axis, with an angle of incidence of 65$^{\circ}$. Time-, energy-, and momentum-resolved photoemission intensities are measured using a time-of-flight momentum microscope. Constant energy contours are shown for energies corresponding to the valence band maximum (VBM: $\mathrm{E-E_{VBM} = 0.00 \pm 0.02~eV}$) and the conduction band (CB: $\mathrm{E-E_{VBM} = 1.15 \pm 0.18~eV}$) for the AC pump configuration. The lower subpanel depicts the real-space in-plane structure of SnSe with pump polarization along ZZ and AC directions. (b) Schematic of SnSe band structure along the AC ($\Gamma$-Y) and ZZ ($\Gamma$-X) high-symmetry directions. (c) Calculated contour plots near the CBM, showing both the X (red) and Y (blue) valleys. The color mapping of each contour progressively shifts toward lighter (whiter) shades with increasing energy. In (b)-(c), the red-blue arrows indicate the quasi-unidirectional intervalley scattering channel from Y to X valley, upon AC pumping.
• Figure 2: Ultrafast strongly anisotropic valley-resolved electron dynamics in SnSe. (a)-(d) Momentum distribution of excited electrons within conduction bands, in (a),(c) and (b),(d) following photoexcitation using IR polarization along ZZ and AC direction, respectively. (a),(b) and (c),(d) are measured at early (-125 fs to +125 fs) and late (+350 fs to +600 fs) time delays after polarization-controlled photoexcitation. (e),(f) Experimental and calculated valley-resolved electron dynamics following photoexcitation using IR polarization along ZZ and AC direction, respectively, and (g),(h) associated valley polarization dynamics extracted by taking the normalized difference between intensity within CB X and Y valleys shown in (e) and (f), respectively.
• Figure 3: Microscopic mechanism underlying ultrafast valley depolarization. (a)–(c) Phonon dispersion and momentum- and mode-resolved phonon temperatures along the $\Gamma$–M high-symmetry direction, at the end of the TDBE simulations (600 fs after the pump), following photoexcitation along the ZZ and AC directions, respectively. (c) Mode-resolved differential phonon temperature, obtained by subtracting the phonon temperature under AC (b) and ZZ (a) pumping. (d) Analysis of the phonon mode responsible for Y$\rightarrow$X intervalley scattering. The left and right panels show the electronic wave functions at the bottom of the conduction band at the Y and X valleys, respectively. The middle panel illustrates the atomic displacements (red arrows) associated with the strongly coupled optical phonons around 20 meV, which drive the Y$\rightarrow$X intervalley scattering.
• Figure S1: Valley- and polarization-resolved optical properties of SnSe. (a)-(b) Photon-energy- and valley-resolved (X-valley in pink and Y-valley in black) imaginary part of the electric susceptibility tensor for light polarization axis along ZZ ($\mathrm{Im}(\chi_{xx})$ in (a) and AC ($\mathrm{Im}(\chi_{yy})$ in (b)) directions. (c) Photon-energy-dependent valley polarization for light polarization axis along ZZ (in red) and AC (in blue) directions.
• Figure S2: Role of detuning in time-dependent Boltzmann equation simulations of polarization-dependent valley-resolved ultrafast electron dynamics in SnSe.(a)-(b) Valley-resolved electron dynamics following photoexcitation using IR polarization along ZZ and AC direction, respectively, as a function of pump photon energy detuning from the band edge ($\Delta$) and (c)-(d) associated valley polarization dynamics extracted by taking the normalized difference between population within CB X and Y valleys shown in (a) and (b), respectively .