SnSe Valleytronics Advances Ultrafast Information Control with Polarization-Selective Transitions

Valleytronics, the manipulation of electrons using valley-specific properties, promises new avenues for information technology, and recent attention has focused on group-IV monochalcogenides as promising materials for this field. Yiming Pan, from Christian-Albrechts-Universität zu Kiel, and Sotirios Fragkos, Dominique Descamps, and colleagues at Université de Bordeaux, now present a detailed investigation into the ultrafast dynamics of valley polarization in tin selenide (SnSe). Their research, combining advanced spectroscopic techniques with theoretical modelling, reveals a striking anisotropy in valley behaviour, demonstrating nearly complete and sustained polarization when exciting electrons into specific valleys. Conversely, excitation into the alternative valley channel triggers a remarkably rapid decay and reversal of polarization, occurring within picoseconds due to strong interactions between electrons and atomic vibrations. These findings establish SnSe as a uniquely anisotropic valleytronic material, offering radically different behaviour compared to commonly studied two-dimensional systems and potentially paving the way for novel optoelectronic devices.

These materials possess strong in-plane anisotropy, enabling polarization-controlled optical transitions to distinct, non-degenerate valleys, which are key to controlling electron behaviour. Scientists investigated the ultrafast dynamics of valley polarization following optical excitation in tin selenide, combining time- and angle-resolved extreme-ultraviolet photoemission spectroscopy with time-dependent Boltzmann equation simulations. The results demonstrate how valley polarization evolves following optical excitation, providing new insights into the potential of these materials for advanced technologies.

Valley Polarization in Two-Dimensional Materials

Valleytronics aims to exploit the valley degree of freedom in materials for information storage and processing. Valleys refer to specific points in the electronic band structure, and controlling their polarization could lead to novel electronic devices. This research focuses on two-dimensional materials, particularly tin selenide, which are attractive for valleytronics due to their unique electronic properties and strong spin-orbit coupling. The goal is to understand and control valley polarization, investigating the roles of material symmetry, external fields, electron-phonon interactions, and the chirality of valley excitons.

The research employs a combination of theoretical calculations and experimental techniques. Density Functional Theory calculates electronic structure and phonon spectra, while Wannier functions describe electronic states and facilitate electron-phonon coupling calculations. The EPW code is a key tool for calculating electron-phonon interactions, building upon initial calculations performed with Quantum ESPRESSO. Experimentally, momentum-resolved photoemission spectroscopy directly maps the electronic band structure, and time-resolved spectroscopy studies ultrafast electron and phonon dynamics. Data analysis utilizes an open-source workflow for processing photoemission data, and Monte Carlo simulations model electron and phonon behaviour. Specific research areas include investigating valley degeneracy splitting, exploring the possibility of a valley Hall effect, searching for ferrovalley materials, understanding the role of chirality in valley dynamics, and studying the ultrafast dynamics of electrons and phonons. The team also investigates how material symmetry influences valley polarization and how magnetic fields can be used to control it.

SnSe Exhibits Strong Valley Polarization Control

Scientists have demonstrated a new level of control over electrons in tin selenide, paving the way for advanced “valleytronics”. The research reveals radically different and anisotropic behaviour in how electrons respond to light, establishing tin selenide as a promising platform for next-generation electronic devices. Experiments combined time- and angle-resolved extreme-ultraviolet photoemission spectroscopy with sophisticated theoretical modeling. This allowed the team to investigate the ultrafast dynamics of valley polarization following optical excitation. When the material was excited with light polarized along a specific crystal axis, results showed nearly complete and time-independent valley polarization, meaning electrons remained confined to specific energy states without significant change over time.

Measurements confirmed that optical transitions occurred exclusively around these energy states, which represent the global conduction band minimum. Theoretical simulations accurately predicted the population of electrons in different valleys, demonstrating a rapid rise in photoemission intensity specifically from these states, while others exhibited no significant signal. In contrast, exciting the material with light polarized along a different direction yielded dramatically different results. Scientists observed ultrafast decay and even reversal of valley polarization on sub-picosecond timescales. This occurred due to intervalley scattering, where electrons moved between energy states, mediated by strong electron-phonon coupling with an optical phonon mode. The team identified this scattering as the primary driver of depolarization, establishing tin selenide as a unique material for valleytronics, where the population of electrons in different valleys can be precisely controlled by manipulating the polarization of incident light.

Valley Polarization Control and Ultrafast Dynamics

This research demonstrates precise control over valley polarization in tin selenide, a material with strong potential for next-generation valleytronic devices. Scientists investigated the ultrafast dynamics of valley polarization following optical excitation, revealing markedly different behaviours depending on the excitation direction. Specifically, selective excitation yields nearly complete and sustained valley polarization, while alternative excitation leads to rapid reversal of polarization on sub-picosecond timescales. This reversal is driven by intervalley scattering mediated by strong coupling to optical phonons.

The study further elucidates the underlying mechanisms by tracking the behaviour of photoholes, revealing a polarization-dependent response opposite to that of electrons. Through detailed analysis of phonon dynamics, the team identified specific phonon modes responsible for driving intervalley scattering, confirming the role of electron-phonon coupling in these processes. These findings establish a fundamental difference between the valley physics of group-IV monochalcogenides, like tin selenide, and more commonly studied two-dimensional materials, suggesting unique functionalities for future devices.