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Register a new userAnisotropic ultrafast spin/valley dynamics in WTe2 films
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WTe2 Weyl semimetal hosts the natural broken inversion symmetry and strong spin orbit coupling, making it promising for exotic spin/valley dynamics within a picosecond timescale. Here, we unveil an anisotropic ultrafast spin/valley dynamics in centimeter-scale, single-crystalline Td-WTe2 films using a femtosecond pump-probe technique at room temperature. We observe a transient (~0.8 ps) intra-valley transition and a subsequent polarization duration (~5 ps) during the whole spin/valley relaxation process. Furthermore, the relaxation exhibits the remarkable anisotropy of approximately six-fold and two-fold symmetries due to the intrinsic anisotropy along the crystalline orientation and the extrinsic matrix element effect, respectively. Our results offer a prospect for the ultrafast manipulation of spin/valleytronics in topological quantum materials for dissipationless high-speed spin/valleytronic devices.
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Valleytronics targets the exploitation of the additional degrees of freedom in materials where the energy of the carriers may assume several equal minimum values (valleys) at non-equivalent points of the reciprocal space. In single layers of transition metal dichalcogenides (TMDs) the lack of inversion symmetry, combined with a large spin-orbit interaction, leads to a conduction (valence) band with different spin-polarized minima (maxima) having equal energies. This offers the opportunity to manipulate information at the level of the charge (electrons or holes), spin (up or down) and crystal momentum (valley). Any implementation of these concepts, however, needs to consider the robustness of such degrees of freedom, which are deeply intertwined. Here we address the spin and valley relaxation dynamics of both electrons and holes with a combination of ultrafast optical spectroscopy techniques, and determine the individual characteristic relaxation times of charge, spin and valley in a MoS$_{2}$ monolayer. These results lay the foundations for understanding the mechanisms of spin and valley polarization loss in two-dimensional TMDs: spin/valley polarizations survive almost two-orders of magnitude longer for holes, where spin and valley dynamics are interlocked, than for electrons, where these degrees of freedom are decoupled. This may lead to novel approaches for the integration of materials with large spin-orbit in robust spintronic/valleytronic platforms.
Black phosphorus has recently emerged as a promising material for high performance electronic and optoelectronic device for its high mobility, tunable mid-infrared bandgap and anisotropic electronic properties. Dynamical evolution of photo excited carriers and its induced change of transient electronic properties are critical for materials high field performance, but remains to be explored for black phosphorus. In this work, we perform angle resolved transient reflection spectroscopy to study the dynamical evolution of anisotropic properties of black phosphorus under photo excitation. We find that the anisotropy of reflectivity is enhanced in the pump induced quasi-equilibrium state, suggesting an extraordinary enhancement of the anisotropy in dynamical conductivity in hot carrier dominated regime. These results raise enormous possibilities of creating high field, angle sensitive electronic, optoelectronic and remote sensing devices exploiting the dynamical electronic anisotropic with black phosphorus.
Ultrafast time-resolved differential reflectivity of Bi2Se3 crystals is studied using optical pump-probe spectroscopy. Three distinct relaxation processes are found to contribute to the initial transient reflectivity changes. The deduced relaxation timescale and the sign of the reflectivity change suggest that electron-phonon interactions and defect-induced charge trapping are the underlying mechanisms for the three processes. After the crystal is exposed to air, the relative strength of these processes is altered and becomes strongly dependent on the excitation photon energy.
Valleytronics is rapidly emerging as an exciting area of basic and applied research. In two dimensional systems, valley polarisation can dramatically modify physical properties through electron-electron interactions as demonstrated by such phenomena as the fractional quantum Hall effect and the metal-insulator transition. Here, we address the electrons spin alignment in a magnetic field in silicon-on-insulator quantum wells under valley polarisation. In stark contrast to expectations from a non-interacting model, we show experimentally that less magnetic field can be required to fully spin polarise a valley-polarised system than a valley-degenerate one. Furthermore, we show that these observations are quantitatively described by parameter free ab initio quantum Monte Carlo simulations. We interpret the results as a manifestation of the greater stability of the spin and valley degenerate system against ferromagnetic instability and Wigner crystalisation which in turn suggests the existence of a new strongly correlated electron liquid at low electron densities.
We investigate the ultrafast spin dynamics in an epitaxial hcp(1100) cobalt thin film. By performing pump-probe magneto-optical measurements with the magnetization along either the easy or hard magnetic axis, we determine the demagnetization and recovery times for the two axes. We observe a 35% slower dynamics along the easy magnetization axis, which we attribute to magneto-crystalline anisotropy of the electron-phonon coupling, supported by our ab initio calculations. This points towards an unambiguous and previously undisclosed role of anisotropic electron-lattice coupling in ultrafast magnetism.
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