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Register a new userIonic Ordering in Thiospinel CuIr$_{2}$S$_{4}$
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Crystal structure of spinel compound CuIr$_{2}$S$_{4}$ was examined by powder X-ray diffraction for the insulating phase below the metal-insulator transition at $T_{MI}$ = 230 K. The superstructure spots are reproduced by considering the displacement of Ir atoms. A model for the ionic ordering of Ir$^{4+}$ and Ir$^{3+}$ with the same number is proposed for the insulating phase on the basis of the structural analysis. The model suggests that the structural change at $T_{MI}$ is driven by the formation Ir$^{4+}$ dimers. In addition, we found that the superstructure spots becomes significantly weak below 60 K, without any significant effects on electric and magnetic properties. Possible mechanism for the transition is discussed.
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Ultrafast dynamics across the photoinduced three-dimensional Peierls-like insulator-metal (IM) transition in CuIr$_{2}$S$_{4}$ was investigated by means of the all-optical ultrafast multi-pulse time-resolved spectroscopy. The structural coherence of the low-$T$ broken symmetry state is strongly suppressed on a sub-picosecond timescale above a threshold excitation fluence of $F_{mathrm{c}}approx3$ mJ/cm$^{2}$ (at 1.55-eV photon energy) resulting in a structurally inhomogeneous transient state which persists for several-tens of picoseconds before reverting to the original low-$T$ state. The electronic order shows a transient gap filling at a significantly lower fluence threshold of $sim0.6$~mJ/cm$^{2}$. The data suggest that the photoinduced-transition structural dynamics to the high-$T$ metallic phase is governed by first-order-transition nucleation kinetics that prevents the complete structural transition into the high-$T$ phase even at excitation fluences significantly larger than $F_{mathrm{c}}$. In contrast, the dynamically-decoupled electronic order is suppressed rather independently due to a photoinduced Mott transition.
The spinel-structure CuIr$_{2}$S$_{4}$ compound displays a rather unusual orbitally-driven three-dimensional Peierls-like insulator-metal transition. The low-T symmetry-broken insulating state is especially interesting due to the existence of a metastable irradiation-induced disordered weakly conducting state. Here we study intense femtosecond optical pulse irradiation effects by means of the all-optical ultrafast multi-pulse time-resolved spectroscopy. We show that the structural coherence of the low-T broken symmetry state is strongly suppressed on a sub-picosecond timescale above a threshold excitation fluence resulting in a structurally inhomogeneous transient state which persists for several-tens of picoseconds before reverting to the low-T disordered weakly conducting state. The electronic order shows a transient gap filling at a significantly lower fluence threshold. The data suggest that the photoinduced-transition dynamics to the high-T metallic phase is governed by first-order-transition nucleation kinetics that prevents the complete ultrafast structural transition even when the absorbed energy significantly exceeds the equilibrium enthalpy difference to the high-T metallic phase. In contrast, the dynamically-decoupled electronic order is transiently suppressed on a sub-picosecond timescale rather independently due to a photoinduced Mott transition.
We present measurements of the optical spectra on single crystals of spinel-type compound cis. This material undergoes a sharp metal-insulator transition at 230 K. Upon entering the insulating state, the optical conductivity shows an abrupt spectral weight transfer and an optical excitation gap opens. In the metallic phase, Drude components in low frequencies and an interband transition peak at $sim 2 eV$ are observed. In the insulating phase, a new peak emerges around $0.5 eV$. This peak is attributed to the transition of electrons from the occupied Ir$^{3+}$ $t_{2g}$ state to upper Ir$^{4+}$ $t_{2g}$ subband resulting from the dimerization of Ir$^{4+}$ ions in association with the simultaneous formations of Ir$^{3+}$ and Ir$^{4+}$ octamers as recently revealed by the x-ray diffraction experiment. Our experiments indicate that the band structure is reconstructed in the insulating phase due to the sudden structural transition.
We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS$_{4}$. Electrical resistivity increased with decreasing temperature according to the form proportional to $textrm{exp}(T_{0}/T)^{1/2} $, derived from variable range hopping with strong electron-electron interaction. Resistivity under magnetic fields was expressed by the same form with the characteristic temperature T0 decreasing with increasing magnetic field. Magnetoresistance ratio $rho (T,0)/rho(T,H)$ is 1.5 at 100 K for H=90 kOe and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. Possible mechanism of the large magnetoresistance is discussed.
The ordered hexagonal perovskite Ba2CuTeO6 hosts weakly coupled S=1/2 spin ladders produced by an orbital ordering of Cu2+. The magnetic susceptibility chi(T) of Ba2CuTeO6 is well described by that expected for isolated spin ladders with exchange coupling of J~86 K but shows a deviation from the expected thermally activated behavior at low temperatures below T*~25 K. An anomaly in chi(T), indicative of magnetic ordering, is observed at T_mag=16 K. No clear signature of long-range ordering, however, is captured in NMR, specific heat or neutron diffraction measurements at and below T_mag. The marginal magnetic transition, indicative of strong quantum fluctuations, is evidence that Ba2CuTeO6 is in very close proximity to a quantum critical point between a magnetically ordered phase and a gapped spin liquid controlled by inter-ladder couplings.
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