Ultrafast carrier dynamics in the large magnetoresistance material WTe2

Abstract

Ultrafast optical pump-probe spectroscopy is used to track carrier dynamics in the large magnetoresistance material WTe2. Our experiments reveal a fast relaxation process occurring on a sub-picosecond time scale that is caused by electron-phonon thermalization, allowing us to extract the electron-phonon coupling constant. An additional slower relaxation process, occurring on a time scale of ∼5-15 picoseconds, is attributed to phonon-assisted electron-hole recombination. As the temperature decreases from 300 K, the timescale governing this process increases due to the reduction of the phonon population. However, below ∼50 K, an unusual decrease of the recombination time sets in, most likely due to a change in the electronic structure that has been linked to the large magnetoresistance observed in this material. has aroused tremendous interest, due not only to its potential application in devices such as magnetic sensors and hard drives, but also to the enigmatic nature of this effect. The observed MR in Cd 3 As 2 has been attributed to the recovery of backscattering that is strongly suppressed in zero magnetic field [2]. In WTe 2 , the large non-saturating MR is believed to arise from perfect electron-hole (e-h) compensation [1], similar to bismuth (Bi) and graphite [4][5][6]. This is supported by angle-resolved photoemission spectroscopy (ARPES) [7] and quantum oscillation [8] experiments, which have found hole and electron pockets with the same size in WTe 2 at low temperatures. Further support for the e-h compensation scenario came from the application of pressure, which increases the difference between the sizes of the hole and electron pockets, dramatically suppressing the MR [9][10][11]. However, a recent high resolution ARPES study revealed a more complicated Fermi surface (FS) with nine pockets [12]. In addition, circular dichroism was observed in the photoemission spectra, signaling strong spin-orbit coupling, which may also play an important role in WTe 2 . Finally, a detailed study of the Shubnikov-de-Haas (SdH) effect, in combination with density functional theory (DFT) calculations, indicated that perfect e-h compensation breaks down under an external magnetic field in WTe 2 [13], challenging the e-h compensation scenario.More insight into the physics of WTe 2 can be obtained using ultrafast optical spectroscopy, which tracks the relaxation of photoexcited carriers in the time domain as they return to equilibrium. Carrier relaxation depends sensitively on the band structure and scattering mechanisms in a solid [14][15][16][17], and therefore an understanding of the ultrafast carrier dynamics in WTe 2 may shed new light on the mechanism of the anomalous MR. This also directly resolves the timescales that ultimately limit potential applications of WTe 2 in electronic devices, e.g., high speed solid-state drives. However, ultrafast optical studies have not yet been performed on this material.Here, we present a detailed ultrafast transient reflectivity (∆R(t)/R) study on WTe 2 while varying the temperatu...

Stable Weyl points, trivial surface states, and particle-hole compensation in WP2

“…These results are then consistent with the prediction that the XMR in WP 2 stems from electron-hole compensation. However, the turn-on behaviour cannot be explained as a result of changes to the band structure induced by temperature, as it has been proposed for WTe 2 [6,7,24]. In contrast to this, these results are more suggestive of theories based on strong T -dependence of the carrier mobility [25,26] and/or band structure modifications induced by the magnetic field itself [11].…”

Coherent optical phonon oscillation and possible electronic softening in WTe2 crystals

“…Following an abrupt change in reflectivity, the time-dependent signal is manifested as a multi-exponential decay curve entangled with certain periodic oscillations. The exponential decay components are related to the recovery dynamics of photo-excited carriers in WTe 2 8 . We subtract off the exponential decay parts to analyze the coherent vibrational dynamics in WTe 2 crystals.…”

“…The results suggest a high degree of lattice anharmonicity under intense excitation, implying possible involvement of electronic softening effect. From the dynamic point of view, the electronic softening effect is reasonable since the oscillation period is shorter than the lifetime of photo-excited carriers in WTe 2 8 . The fluence-dependent behavior of oscillatory signal is similar to the amplitude modes observed in some charge-density wave (CDW) systems 55 56 .…”

“…Moreover, intriguing features with strong spin-orbit interaction 5 and type-II Weyl points 6 at the Fermi surface have been observed, suggesting the existence of exotic electronic transport properties with promising application potentials for WTe 2 . While many efforts have been made towards understanding the physics relevant to electronic and spin states 2 5 7 8 9 10 11 12 13 14 15 , the temperature dependence 16 , three-dimensional anisotropy of MR effect 13 16 , and the drastic pressure effect on the MR effect 17 and superconductivity 18 19 show the important roles played by the lattice dynamics in this system, which, however, has been rarely investigated 20 21 22 .…”

Similar ultrafast dynamics of several dissimilar Dirac and Weyl semimetals

“…38 WTe 2 , a type-II Weyl semimetal, is similar: in samples with a momentum gap at E F , ∆R(t) decays in two parts, with τ B = 5 to 15 ps. 56,57 In contrast, the linear, Dirac-like dispersion in our materials speeds recombination because it enables low-momentum transitions between electron and hole states. The linear dispersion also ensures that Auger processes (both interband and intraband) automatically satisfy both energy and momentum conservation, provided they occur along a straight line in momentumspace.…”