We systematically measured the Hall effect in the extremely large magnetoresistance semimetal WTe 2 . By carefully fitting the Hall resistivity to a two-band model, the temperature dependencies of the carrier density and mobility for both electron-and hole-type carriers were determined. We observed a sudden increase of the hole density below ∼160 K, which is likely associated with the temperature-induced Lifshitz transition reported by a previous photoemission study. In addition, a more pronounced reduction in electron density occurs below 50 K, giving rise to comparable electron and hole densities at low temperature. Our observations indicate a possible electronic structure change below 50 K, which might be the direct driving force of the electronhole "compensation" and the extremely large magnetoresistance as well. Numerical simulations imply that this material is unlikely to be a perfectly compensated system.

We present a terahertz spectroscopic study of magnetic excitations in ferroelectric antiferromagnet BiFeO 3 . We interpret the observed spectrum of long-wavelength magnetic resonance modes in terms of the normal modes of the material's cycloidal antiferromagnetic structure. We find that the modulated Dzyaloshinski-Moriya interaction leads to a splitting of the out-of-plane resonance modes. We also assign one of the observed absorption lines to an electromagnon excitation that results from the magnetoelectric coupling between the ferroelectric polarization and the cycloidal magnetic structure of BiFeO 3 .

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PACS. 78.47.+p -Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter. PACS. 74.72.-h -High-Tc compounds. PACS. 74.76.Bz -High-Tc films.Abstract. -Using a femtosecond pump-probe optical technique, we measured the time-resolved change in reflectivity of three single-layer cuprate samples, including Bi2Sr2−xLaxCuO6+z, La2CuO4+y and La2−xSrxCuO4 grown on SrTiO3. We analyze the data in terms of a nonthermal electron relaxation model, and obtain quantitative agreement between the model and the data. The initial electron-electron thermalization process is much longer in the superconducting samples as opposed to the overdoped nonsuperconducting sample, suggesting a bottleneck in electron thermalization process due to the presence of the superconducting gap.c EDP Sciences

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...

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