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Register a new userEnhanced ultrafast relaxation rate in the Weyl semimetal phase of $mathbf{MoTe_2}$ measured by time-and angle-resolved photoelectron spectroscopy
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Added by
Alberto Crepaldi Dr.
Publication date
2017
fields
Physics
and research's language is
English
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$mathrm{MoTe_2}$ has recently been shown to realize in its low-temperature phase the type-II Weyl semimetal (WSM). We investigated by time- and angle- resolved photoelectron spectroscopy (tr-ARPES) the possible influence of the Weyl points in the electron dynamics above the Fermi level $mathrm{E_F}$, by comparing the ultrafast response of $mathrm{MoTe_2}$ in the trivial and topological phases. In the low-temperature WSM phase, we report an enhanced relaxation rate of electrons optically excited to the conduction band, which we interpret as a fingerprint of the local gap closure when Weyl points form. By contrast, we find that the electron dynamics of the related compound $mathrm{WTe_2}$ is slower and temperature-independent, consistent with a topologically trivial nature of this material. Our results shows that tr-ARPES is sensitive to the small modifications of the unoccupied band structure accompanying the structural and topological phase transition of $mathrm{MoTe_2}$.
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A Weyl semimetal is a new type of topological quantum phase with intriguing physics near the Weyl nodes. Although the equilibrium state of Weyl semimetals has been investigated, the ultrafast dynamics near the Weyl node in the nonequilibrium state is still missing. Here by performing time and angle resolved photoemission spectroscopy on type-II Weyl semimetal MoTe$_2$, we reveal the dispersion of the unoccupied states and identify the Weyl node at 70 meV above E$_F$. Moreover, by tracking the ultrafast relaxation dynamics near the Weyl node upon photo-excitation with energy, momentum and temporal resolution, two intrinsic recovery timescales are observed, a fast one of 430 fs and a slow one of 4.1 ps, which are associated with hot electron cooling by optical phonon cascade emission and anharmonic decay of hot optical phonons respectively. The electron population shows a metallic response, and the two temperature model fitting of the transient electronic temperature gives an electron-phonon coupling constant of $lambdalangleOmega^2ranglesimeq32$ $textrm{meV}^2$. Our work provides important dynamic information for understanding the relaxation mechanism of a Weyl semimetal and for exploiting potential applications using ultrafast optical control.
We utilized X-ray photoemission electron microscopy (XPEEM) and X-ray photoelectron spectroscopy (XPS) to investigate the crystal surface of Weyl semimetal NbAs. XPEEM images present white and black contrast in both the Nb 3d and As 3d core level spectra. Surface-sensitive XPS spectra indicate that the entire surface of the sample contains both surface states of Nb 3d and As 3d, in form of oxides, and bulk states of NbAs. Estimated atomic percentage values nNb/nAs suggest that the surface is Nb-rich and asymmetric for white and black areas.
In this work the complete valence-band structure of the molybdenum dichalcogenides MoS_2, MoSe_2, and alpha-MoTe_2 is presented and discussed in comparison. The valence bands have been studied using both angle-resolved photoelectron spectroscopy (ARPES) with synchrotron radiation, as well as, ab-initio band-structure calculations. The ARPES measurements have been carried out in the constant-final-state (CFS) mode. The results of the calculations show in general very good agreement with the experimentally determined valence-band structures allowing for a clear identification of the observed features. The dispersion of the valence bands as a function of the perpendicular component k_perp of the wave vector reveals a decreasing three-dimensional character from MoS_2 to alpha-MoTe_2 which is attributed to an increasing interlayer distance in the three compounds. The effect of this k_perp dispersion on the determination of the exact dispersion of the individual states as a function of k_parallel is discussed. By performing ARPES in the CFS mode the k_parallel-component for off-normal emission spectra can be determined. The corresponding k_perp-value is obtained from the symmetry of the spectra along the GammaA, KH, and ML line, respectively.
We report the pressure (p_max = 1.5 GPa) evolution of the crystal structure of the Weyl semimetal T_d-MoTe_2 by means of neutron diffraction experiments. We find that the fundamental non-centrosymmetric structure T_d is fully suppressed and transforms into a centrosymmertic 1T structure at a critical pressure of p_cr = 1.2 GPa. This is strong evidence for a pressure induced quantum phase transition (QPT) between topological to a trivial electronic state. Although the topological QPT has strong effect on magnetoresistance, it is interesting that the superconducting critical temperature T_c, the superfluid density, and the SC gap all change smoothly and continuously across p_cr and no sudden effects are seen concomitantly with the suppression of the T_d structure. This implies that the T_c, and thus the SC pairing strength, is unaffected by the topological QPT. However, the QPT requires the change in the SC gap symmetry from non-trivial s+- to a trivial s++ state, which we discuss in this work. Our systematic characterizations of the structure and superconducting properties associated with the topological QPT provide deep insight into the pressure induced phase diagram in this topological quantum material.
The relaxation dynamics of hot carriers in silicon (100) is studied via a novel holistic approach based on phase-resolved transient absorption spectroscopy with few-cycle optical pulses. After excitation by a sub-5 fs light pulse, strong electron-phonon coupling leads to an ultrafast momentum relaxation with time constant of 10 fs. The thermalization of the hot carriers occurs on a time constant of 150 fs, visible in the temporal evolution of the collision time as extracted from the Drude model. We find an increase of the collision time from 3 fs for the shortest timescales with a saturation at approximately 18 fs. Moreover, the optical effective mass of the hot carrier ensemble evolves on ultrafast timescales as well, with a bi-exponential decrease from 0.7 $m_e$ to about 0.125 $m_e$ and time constants of 4 fs and 58 fs. The presented information on the electron mass dynamics as well as the momentum-, energy-, and collision-scattering times with unprecedented time resolution is important for all hot carrier optoelectronic devices.
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