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Register a new userExperimental and theoretical analysis of the upper critical field in FSF trilayers
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The upper critical magnetic field H_{c2} in thin-film FSF trilayer spin-valve cores is studied experimentally and theoretically in geometries perpendicular and parallel to the heterostructure surface. The series of samples with variable thicknesses of the bottom and of the top Cu_{41}Ni_{59} F-layers are prepared in a single run, utilizing a wedge deposition technique. The critical field H_{c2} is measured in the temperature range $0.4-8$ K and for magnetic fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of the superconducting transition temperature versus F-layers thickness, induced by an external magnetic field, has been observed for the first time. In order to properly interpret the experimental data, we develop a quasiclassical theory, enabling one to evaluate the temperature dependence of the critical field and the superconducting transition temperature for an arbitrary set of the system parameters. A fairly good agreement between our experimental data and theoretical predictions is demonstrated for all samples, using a single set of fit parameters. This confirms adequacy of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual superconducting properties of the studied Cu_{41}Ni_{59}/Nb/Cu_{41}Ni_{59} spin-valve core trilayers.
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The upper critical field $mu_0H_{c2}(T_c)$ of LiFeAs single crystals has been determined by measuring the electrical resistivity using the facilities of pulsed magnetic field at Los Alamos. We found that $mu_0H_{c2}(T_c)$ of LiFeAs shows a moderate anisotropy among the layered iron-based superconductors; its anisotropic parameter $gamma$ monotonically decreases with decreasing temperature and approaches $gammasimeq 1.5$ as $Trightarrow 0$. The upper critical field reaches 15T ($Hparallel c$) and 24.2T ($Hparallel ab$) at $T=$1.4K, which value is much smaller than other iron-based high $T_c$ superconductors. The temperature dependence of $mu_0H_{c2}(T_c)$ can be described by the Werthamer-Helfand-Hohenberg (WHH) method, showing orbitally and (likely) spin-paramagnetically limited upper critical field for $Hparallel c$ and $Hparallel ab$, respectively.
We measure magnetotransport of F doped SmFeAsO samples up to 28T and we extract the upper critical fields, using different criteria. In order to circumvent the problem of criterion-dependence Hc2 values, we suggest a thermodynamic estimation of the upper critical field slope dHc2/dT based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as -12T/K is thus extracted for the optimally doped sample. We find evidence of a two-dimensional lowest Landau level (LLL) scaling for applied fields larger than mu_0H_LLL=8T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing Tc. In all cases, the coherence length values along the c axis are smaller than the interplanar distance, confirming the two-dimensional nature of superconductivity in this compound.
The superconducting parameters and upper critical field of the noncentrosymmetric superconductor BiPd have proven contentious. This material is of particular interest because it is a singular example of a $4f$-electron-free noncentrosymmetric superconductor of which crystals may be grown and cleaved, enabling surface-sensitive spectroscopies. Here, using bulk probes augmented by tunnelling data on defects, we establish that the lower of the previously reported upper critical fields corresponds to the bulk transition. The material behaves as a nearly-weak-coupled BCS s-wave superconductor, and we report its superconducting parameters as drawn from the bulk upper critical field. Possible reasons behind the order-of-magnitude discrepancy in the reported upper critical fields are discussed.
We present a detailed study of the electrical transport properties of YBa2Cu3O7-{delta} thin film. The irreversibility fields ({mu}_0 H_irr), upper critical fields ({mu}_0 H_C2), penetration depths ({lambda}) and coherence lengths ({xi} ) of the YBa2Cu3O7-{delta} materials are deduced from the resistivity curves. Itis observed that {mu}_0 H_irr, {mu}_0 H_C2 and {Delta}Tc of the film strongly depend on the direction and strength of the field. The coherence length {xi} (0) and penetration depth {lambda} (0) values at T = 0 K has been calculated from the irreversibility fields ({mu}_0 H_irr) and upper critical fields ({mu}_0 H_C2) respectively. Based on all the results, the change of the superconducting properties as a function of the magnetic field direction presents the anisotropy of the sample produced.
We present measurements of the superconducting critical temperature Tc and upper critical field Hc2 as a function of pressure in the transition metal dichalcogenide 2H-NbS2 up to 20 GPa. We observe that Tc increases smoothly from 6K at ambient pressure to about 8.9K at 20GPa. This range of increase is comparable to the one found previously in 2H-NbSe2. The temperature dependence of the upper critical field Hc2(T) of 2H-NbS2 varies considerably when increasing the pressure. At low pressures, Hc2(0) decreases, and at higher pressures both Tc and Hc2(0) increase simultaneously. This points out that there are pressure induced changes of the Fermi surface, which we analyze in terms of a simplified two band approach.
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