Anisotropic Hc2H_{c2} determined up to 92 T and the signature of multi-band superconductivity in Ca10_{10}(Pt4_{4}As8_{8})((Fe1−x_{1-x}Ptx_{x})2_{2}As2_{2})5_{5} superconductor

The upper critical fields, $H_{c2}$($T$), of single crystals of the superconductor Ca$_{10}$(Pt$_{4-\delta}$As$_{8}$)((Fe$_{0.97}$Pt$_{0.03}$)$_{2}$As$_{2}$)$_{5}$ ($\delta$ $\approx$ 0.246) are determined over a wide range of temperatures down to $T$ = 1.42 K and magnetic fields of up to $\mu_{0}H$ $\simeq$ 92 T. The measurements of anisotropic $H_{c2}$($T$) curves are performed in pulsed magnetic fields using radio-frequency contactless penetration depth measurements for magnetic field applied both parallel and perpendicular to the \textbf{ab}-plane. Whereas a clear upward curvature in $H_{c2}^{\parallel\textbf{c}}$($T$) along \textbf{H}$\parallel$\textbf{c} is observed with decreasing temperature, the $H_{c2}^{\parallel\textbf{ab}}$($T$) along \textbf{H}$\parallel$\textbf{ab} shows a flattening at low temperatures. The rapid increase of the $H_{c2}^{\parallel\textbf{c}}$($T$) at low temperatures suggests that the superconductivity can be described by two dominating bands. The anisotropy parameter, $\gamma_{H}$ $\equiv$ $H_{c2}^{\parallel\textbf{ab}}/H_{c2}^{\parallel\textbf{c}}$, is $\sim$7 close to $T_{c}$ and decreases considerably to $\sim$1 with decreasing temperature, showing rather weak anisotropy at low temperatures.Comment: 4pages, 3figures, accepted PRB Rapid Communicatio

Magnetic Field Induced Transition in Vanadium Spinels

We study vanadium spinels AV2O4 (A=Cd,Mg) in pulsed magnetic fields up to 65 T. A jump in magnetization at μ0H≈40 T is observed in the single-crystal MgV2O4, indicating a field induced quantum phase transition between two distinct magnetic orders. In the multiferroic CdV2O4, the field induced transition is accompanied by a suppression of the electric polarization. By modeling the magnetic properties in the presence of strong spin-orbit coupling characteristic of vanadium spinels, we show that both features of the field induced transition can be successfully explained by including the effects of the local trigonal crystal fieldWork at the LANL was performed under the auspices of the U.S. DOE Grant No. DE-AC52-06NA25396 through the LDRD program. The NHMFL facility is funded by the NSF through cooperative Grant No. DMR-1157490, by the DOE, and by the State of Florida. This material is based upon work supported in part by the NSF under Grant No. PHY-1066293 and the hospitality of the ACP. V.  P. acknowledges financial support from the Ramon y Cajal Program. F. R. acknowledges support from Xunta de Galicia 2012-CP071S

Low-temperature spin excitations in frustrated ZnCr2O4 probed by high-field thermal conductivity

The magnetoelastic excitations of spin frustrated ZnCr2O4 are studied by the magnetic field dependence of the thermal conductivity k down to 50 mK. Above the first-order magnetostructural transition at TN,S≈ 12.5 K, spin fluctuations are strongly coupled to acoustic phonons, leading to a glasslike dependence of k(T), up to Θ CW. In the symmetry broken phase below TN,S, k shows a dominant magnetic contribution even at the lowest temperatures probed in this work. Application of a magnetic field above 2.5 T destabilizes the spin-bond structure, leading to a sudden increase and a nonconventional temperature dependence of the thermal conductivity. The possibility of the coexistence of gapped and gapless excitations in this magnetic phase is discussedThis work was partially supported by MAT2010-16157 (Ministerio de Economía y Competitividad, Spain) and by the DFG via TRR 80 (Augsburg-Munich). Z.Y.Z. and X.F.S. acknowledge support from the National Natural Science Foundation of China, the National Basic Research Program of China (Grant Nos. 2009CB929502 and 2011CBA00111), and the Fundamental Research Funds for the Central Universities (Program No. WK2340000035). V.Z. acknowledges Los Alamos National Lab Directed Research Project 2010000043DRS