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Register a new userStructural anomalies associated with the electronic and spin transitions in LnCoO3
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A powder X-ray diffraction study, combined with the magnetic susceptibility and electric transport measurements, was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100 - 1000 K. A non-standard temperature dependence of the observed thermal expansion was modelled as a sum of three contributions: (1) Weighted sum of lattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediate (IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population of excited (IS or HS) states of Co3+ ions at the course of the diamagnetic-paramagnetic transition. (3) An anomalous expansion due to excitations of Co3+ ions to another paramagnetic state accompanied by an insulator-metal transition. The anomalous expansion is governed by parameters that are found to vary linearly with the Ln ionic radius. In the case of the first magnetic transition it is the energy splitting E between the ground low spin state and the excited state, presumably the intermediate spin state. The energy splitting E, determined by a fit of magnetic susceptibility, decreases with temperature. The values of E determined for LaCoO3 and YCoO3 at T = 0 K as 164 K and 2875 K, respectively, fall to zero at T = 230 K for LaCoO3 and 860 K for YCoO3. The second anomalous expansion connected with a simultaneous magnetic and insulator-metal transition is characterized by its center at T = 535 K for LaCoO3 and 800 K for YCoO3. The change of the unit cell volume during each transition is independent on the Ln cation and is about 1% in both cases.
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The diamagnetic-paramagnetic and insulator-metal transitions in LnCoO3 perovskites (Ln = La, Y, rare earths) are reinterpreted and modeled as a two-level excitation process. In distinction to previous models, the present approach can be characterized as a LS-HS-IS (low-high-intermediate spin) scenario. The first level is the local excitation of HS Co3+ species in the LS ground state. The second excitation is based on the interatomic electron transfer between the LS/HS pairs, leading finally to a stabilization of the metallic phase based on IS Co3+. The model parameters have been quantified for Ln = La, Pr and Nd samples using the powder neutron diffraction on the thermal expansion of Co-O bonds, that is associated with the two successive spin transitions. The same model is applied to interpret the magnetic susceptibility of LaCoO3 and YCoO3.
Polarization- and temperature-dependent Raman data along with theoretical simulations are presented for the Kagome ferromagnet Fe_3Sn_2. Eight out of nine expected phonon modes were identified. The experimental energies compare well with those from the simulations. The analysis of the line widths indicates relatively strong phonon-phonon coupling in the range 0.1 to 1. The temperature-dependent frequencies of three A_{1g} modes show weak anomalies at approximately 100 K. In contrast, the linewidths of all phonon modes follow the conventional exponential broadening up to room temperature except for the softest A_{1g} mode, whose width exhibits a kink close to 100 K and becomes nearly constant for T > 100 K. These features are indicative of a spin reorientation taking place in the temperature range above 100 K which might arise from spin-phonon coupling. The low-energy part of the electronic continuum in E_g symmetry depends strongly on temperature. The possible reasons include particle-hole excitation tracking the resistivity, a spin-dependent gap or spin fluctuations.
The temperature dependence of the elastic properties of antiferroelectric PbHfO3 was investigated by Brillouin scattering. The two structural phase transitions of antiferroelectric-antiferroelectric-paraelectric phases were clearly identified by discontinuous changes in the acoustic mode frequencies and the hypersonic damping. The substantial softening of the mode frequency along with the remarkable increase in the acoustic damping observed in the paraelectric phase indicated the formation of precursor noncentrosymmetric (polar) clusters and their coupling to the acoustic waves. This was corroborated by the observation of quasi-elastic central peaks, the intensity of which grew upon cooling toward the Curie point. The obtained relaxation time exhibited a slowing-down behavior, suggesting that the dynamics of precursor clusters becomes more sluggish on approaching the phase transition temperature.
Manipulating crystal structure and the corresponding electronic properties in topological quantum materials provides opportunities for the exploration of exotic physics and practical applications. As prototypical topological materials, the bulk MoTe2 and WTe2 are identified to be Weyl semimetals and higher-order topological insulators. The non-centrosymmetric interlayer stacking in MoTe2 and WTe2 causes the Weyl semimetal phase while the intralayer Peierls distortion causes the higher-order topological insulator phase. Here, by ultrafast electron diffraction and TDDFT-MD simulations, we report the photoinduced concurrent intralayer and interlayer structural transitions in both the low temperature Td and room temperature 1T phase of MoTe2 and WTe2. The ultrafast reduction of the intralayer Peierls distortion within 0.3 ps is demonstrated to be driven by Mo-Mo (W-W) bond stretching and dissociation. Both the interlayer shear displacement and the suppression of the intralayer Peierls distortion are identified to be caused by photon excitation, which is a different mechanism compared to the THz field and optical field driven structural transitions reported previously. The revealed intralayer and interlayer structural transitions, provide an ultrafast switch from the rich topological nontrivial phases, such as the Weyl semimetal phase and (higher-order) topological insulator phase, to trivial phase in bulk MoTe2 and WTe2 and most monolayer TMDCs. Our work elucidates the pathway of the intralayer and interlayer structural transitions and the associated topological switches in atomic and femtosecond spatiotemporal scale, and sheds light on new opportunities for the ultrafast manipulation of topological and other exotic properties by photon excitation.
The effects of chemical disorder on the transport properties of the spin-filter material CrVTiAl are investigated experimentally and theoretically. Synchrotron X-ray diffraction experiments on bulk CrVTiAl and the associated Rietveld analysis indicate that the crystal structure consists primarily of a mixture of a partially ordered B2 phase, a fully disordered A2 phase and a small component of an ordered L2textsubscript{1} or Y phase. High temperature resistivity measurements confirm the existence of a band gap. First-principles, all-electron, self-consistent electronic structure computations show that the chemically disordered A2 and B2 phases are metallic, while the spin-filter properties of the ideal Y-type phase are preserved in the presence of L2textsubscript{1} disorder. The Hall coefficient is found to decrease with increasing temperature, similar to the measured increase in the conductivity, indicating the presence of thermally activated semiconductor-like carriers.
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