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Register a new userUltrafast dynamics of the low frequency shear mode in $mathrm{1T-MoTe_2}$
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We report on the dynamics of coherent phonons in semimetal 1T-MoTe2 using femtosecond pump-probe spectroscopy. On an ultrafast sub-picosecond time scale at room temperature, a low frequency and long-lifetime shear phonon mode was observed at 0.39 THz, which was previously reported in the form of a characteristic phonon only in the low temperature Td-MoTe2 phase. Unlike the other optical phonon modes, the shear phonon mode was found to strongly couple with photoexcited carriers. Moreover, the amplitude of the shear mode surprisingly decreased with increasing excitation density, a phenomenon which can be attributed to be a consequence of the lattice temperature rising after excitation. These results provide useful physical information on ultrafast lattice symmetry switching between the normal semimetal 1T and the lattice inversion symmetry breaking Type-II Weyl semimetal Td phases.
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Quasi-two-dimensional transition metal dichalcogenides (TMDs) are a key platform for exploring emergent nanoscale phenomena arising from complex interactions. Access to the underlying degrees of freedom on their natural time scales motivates the use of advanced ultrafast probes that directly couple to the self-organised electronic and atomic structural order. Here, we report the first ultrafast investigation of TaTe2, which exhibits unique charge and lattice structural order characterised by a transition upon cooling from stripe-like trimer chains into a $(3 times 3)$ superstructure of trimer clusters. Utilising MeV-scale ultrafast electron diffraction, the TaTe2 structural dynamics is resolved following intense pulsed laser excitation at 1.2 eV. We observe a rapid $approx!1.4$ ps melting of the low-temperature ordered state, followed by recovery of the clusters via thermalisation into a hot superstructure persisting for extended times. Density-functional calculations indicate that the initial quench is triggered by Ta trimer bonding to nonbonding transitions that destabilises the clusters, unlike melting of charge density waves in other TaX2 compounds. Our work paves the way for further exploration and ultimately directed manipulation of the trimer superstructure for novel applications.
We investigate the ultrafast electron dynamics triggered by terahertz and optical pulses in thin platinum and gold films by probing their transient optical reflectivity. The response of the platinum film to an intense terahertz pulse is similar to the optically-induced dynamics of both films and can be described by a two-temperature model. Surprisingly, gold can exhibit a much smaller terahertz pulse-induced reflectivity change and with opposite sign. For platinum, we estimate a 20% larger electron-phonon coupling for the terahertz-driven dynamics compared to the optically-induced one, which we ascribe to an additional nonthermal electron-phonon coupling contribution. We explain the remarkable response of gold to terahertz radiation with the field emission of electrons due the Fowler-Nordheim tunneling process, in samples with thickness below the structural percolation threshold where near-field enhancement is possible. Our results provide a fundamental insight into the ultrafast processes relevant to modern electro- and magneto-optical applications.
We investigate the evolution of spin polarization, spontaneous Hall angle (SHA), saturation magnetization and Curie temperature of $B2$-ordered Fe$_{60}$Al$_{40}$ thin films under varying antisite disorder, induced by Ne$^{+}$-ion irradiation. The spin polarization increases monotonically as a function of ion fluence. A relatively high polarization of 46 % and the SHA of 3.1 % are achieved on 40 nm thick films irradiated with 2 $cdot$ 10$^{16}$ ions/cm$^2$ at 30 keV. An interesting divergence in the trends of the magnetization and SHA is observed for low disorder concentrations. The high spin polarization and its broad tunability range make ion-irradiated Fe$_{60}$Al$_{40}$ a promising material for application in spin electronic devices.
We use ultrafast x-ray pulses to characterize the lattice response of SrTiO3 when driven by strong terahertz (THz) fields. We observe transient changes in the diffraction intensity with a delayed onset with respect to the driving field. Fourier analysis reveals two frequency components corresponding to the two lowest energy zone-center optical modes in SrTiO3. The lower frequency mode exhibits clear softening as the temperature is decreased while the higher frequency mode shows slight temperature dependence.
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