Nonmonotonic evolution of the charge gap in ZnV2O4 under pressure

A2+V2O4 spinels provide a unique opportunity for studying the evolution of the charge gap of Mott insulators that approach the itinerant electron limit under the application of external pressure. Here we report high-pressure resistivity and optical measurements in ZnV2O4 that provide unambiguous evidence of an unusual nonmonotonic behavior of the charge gap, , as a function of pressure P. These unexpected results suggest that ZnV2O4 undergoes a crossover from a Mott insulator with a charge gap dominated by the on-site Coulomb repulsion U, to a second type of insulator in the high pressure regime. Our Monte Carlo simulations of the three-band Hubbard model relevant for ZnV2O4 reproduce the nonmonotonic behavior of (P) and provide a partial understanding of this exotic phenomenonWe acknowledge financial support from the DFG (SFB 484 and SFB 608) and the Bayerische Forschungsstiftung. F.R. acknowledges support from Ministerio de Economa y Competitividad, Spain, through the project MAT2010-16157. Work at LANL was performed under the auspices of the US DOE Contract No. DE-AC52-06NA25396 through the LDRD programS

Pressure-induced quantum phase transition in Fe₁−ᵪCoᵪSi (x = 0.1,0.2)

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Study of the pressure effects in TiOCl by ab initio calculations

Electronic structure calculations on the low dimensional spin-1/2 compound TiOCl were performed at several pressures in the orthorhombic phase, finding that the structure is quasi-one-dimensional. The Ti3+ (d1) ions have one t2g orbital occupied (dyz) with a large hopping integral along the b direction of the crystal. The most important magnetic coupling is Ti-Ti along the b axis. The transition temperature (Tc) has a linear evolution with pressure, and at about 10 GPa this Tc is close to room temperature, leading to a room temperature spin-Peierls insulator-insulator transition, with an important reduction of the charge gap in agreement with the experiment. On the high-pressure monoclinic phase, TiOCl presents two possible dimerized structures, with a long or short dimerization. Long dimerized state occurs above 15 GPa, and below this pressure the short dimerized structure is the more stable phase.Comment: 3 pages, 3 embedded figures, 1 table. A. Pi\~neiro, et al.,J. Magn. Magn. Mater. (2009) (accepted

Interplay between crystal electric field and magnetic exchange anisotropies in the heavy-fermion antiferromagnet YbRhSb under pressure

We report the pressure effect on the magnetic ground state of the heavy-fermion (HF) canted antiferromagnet YbRhSb (orthorhombic ɛ-TiNiSi-type) by means of magnetization and resistivity measurements using a single crystal. At ambient pressure, this compound undergoes a transition at TM1=2.7 K into a canted antiferromagnetic (AF) state with a small spontaneous moment of 3×10-3 μB/Yb. With increasing pressure P above 1 GPa, another magnetic transition occurs at TM2 above TM1, and TM1(P) has a deep minimum of 2.5 K at 1.7 GPa. For P≥2 GPa, the canted AF structure changes to a ferromagnetic (FM) one, where a large moment 0.4 μB/Yb lies in the orthorhombic b-c plane and a metamagnetic transition occurs at B || a = 1.5 T. This unusual FM state below TM3≅4.3 K is ascribed to the balance between the single-ion crystalline electric field (CEF) anisotropy with easy direction || a and the intersite exchange interaction with easy b-c plane. Furthermore, we have investigated the pressure dependence of TM3 up to 20.4 GPa using electrical resistivity measurements. The structural stability under pressures up to 19 GPa was examined by x-ray diffraction. We find that TM3 above 2.5 GPa steeply increases up to about 7 K, showing a broad maximum and then slightly decreases with increasing pressure above 8 GPa, while the structure remains unchanged. We attribute the enhancement of TM3 above 2.5 GPa to an increase of the CEF anisotropy with respect to magnetic exchange anisotropy. Finally, we compare and discuss the volume dependence of magnetic phase diagram of YbRhSb with the isostructural HF ferromagnet YbNiSn

Nature of the electronic states involved in the chemical bonding and superconductivity at high pressure in SnO

We have investigated the electronic structure and the Fermi surface of SnO using density functional theory (DFT) calculations within recently proposed exchange-correlation potential (PBE+mBJ) at ambient conditions and high pressures up to 19.3 GPa where superconductivity was observed. It was found that the Sn valence states 5s, 5p, and 5d are strongly hybridized with the O 2p-states, and that our DFT-calculations are in good agreement with O K-edge X-ray spectroscopy measurements for both occupied and empty states. It was demonstrated that the metallic states appearing under pressure in the semiconducting gap stem due to the transformation of the weakly hybridized O 2p-Sn 5sp subband corresponding to the lowest valence state of Sn in SnO. We discuss the nature of the electronic states involved in chemical bonding and formation of the hole and electron pockets with nesting as a possible way to superconductivity.Comment: 5 pages, 6 figure

Multiphase transformations in undercooled melts

The current statements and novel results within the project MULTIPHAS (Non-equilibrium multi-phase transformations: eutectic solidification, spinodal decomposition and glass formation) are critically discussed in the present report. We describe and motivate experiments to be performed on board the International Space Station (ISS). Within the project, it is envisaged to measure the crystal growth velocity as a function of undercooling of intermetallic Cu50Zr50 and eutectic Cu56Zr44 alloy. Applying containerless levitation technique the intermetallic Cu50Zr50 alloy can be deeply undercooled (up to 310 K [1]), therefore, one may expect a serious influence of the temperature dependent diffusion coefficient and convective transport phenomena on the crystal growth kinetics. Also in the case of the eutectic Cu56Zr44 alloy, the growth kinetics is controlled by diffusion (at least at small and moderate undercoolings) and strongly dependent on convective transport occurring in the electromagnetic levitation facility. In addition to the problems in these binary alloys, the role of spinodal decomposition on the transition of the liquid to amorphous phase is investigated in Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 [2] and Pd40Cu30P20Ni10 alloys. Both alloys are belonging to bulk glass-forming alloys, which can be transferred to amorphous state by moderate cooling rates. The former shall be processed in the Electro-Magnetic Levitator (EML) currently under development by DLR/ESA within the COLUMBUS module of ISS while the latter one shall be studied using the Russian Multi Zone Electro Vacuum (MZEV) furnace currently under development by ROSKOSMOS on board the Russian module of the ISS. The transition “liquid state – amorphous state” especially is resolved by experimentation in microgravity and interpreted by theoretical modelling taking into consideration spinodal decomposition in the undercooled liquid prior to solidification. We acknowledge support from DFG (German Research Foundation) under the Project No. HE 160/19 and DLR Space Management under contract 50WM1140

Unusual pressure-induced metallic state in the correlated narrow band-gap semiconductor FeSi

Compressing FeSi induces a progressive semiconductor to metal transition, onset at P >= 15 GPa at temperatures below T-max determined by the degree of disorder in the sample. At high pressure preceding charge-gap closure, a broad maximum manifests in the rho(T) data at T-max and is a feature which persists into the metallic state. The extremum in rho(T) occurs at T-max similar to 40 K at similar to 11 GPa and shifts monotonically to similar to 240 K as pressure is increased to similar to 32 GPa, in the most detailed example of three series of measurements involving pressurized FeSi with different degrees of disorder. The transition to a metallic phase is an electronic change only, in that the B 20-type crystal structure is retained up to 30 GPa, with no evidence of a discontinuity in the volume-pressure equation of state data. Samples from the same ingot subjected to different quasihydrostatic conditions reveal different values of the critical pressure of the electronic transition, its width, and pressure dependences of T-max. This attests to sensitivity of the electronic transition to the degree of disorder in the investigated sample. The metallic state has neither Fermi-liquid nor non-Fermi-liquid behavior. Such an unusual pressure-induced correlated metallic state in FeSi is attributed to extended states within the 3d-3p hybridization gap originating from disorder and compression tuning of the mobility edge relative to the Fermi level. The metallic state has also been investigated in external magnetic fields up to 8 T at low temperatures (2 K <= T <= 20 K) at 15 and 19 GPa. This reveals a positive magnetoresistance, as observed in doped Fe1-xCoxSi samples at ambient pressure, suggesting that in the majority and minority spin bands there is a field-induced modification of the respective magnitudes of charge-carrier populations which have different mobilities