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Register a new userSuperconductivity with High Upper Critical Field in the Cubic Centrosymmetric $eta$-Carbide Nb$_4$Rh$_2$C$_{1-delta}$
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The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Copper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as $mu_0 H_{rm Pauli} approx 1.86{rm [T/K]} cdot T_{rm c}$ for a weak-coupling BCS superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy fermion and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic $eta$-carbide-type compound Nb$_4$Rh$_2$C$_{1-delta}$, with a normalized specific heat jump of $Delta C/gamma T_{rm c} =$ 1.64. We find that this material has a remarkably high upper critical field of $mu_0 H_{rm c2}{rm (0)}$ =~28.5~T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of $mu_0 H_{rm Pauli}$~=~18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.
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We report the effect of Nb doping on the upper critical field of the cubic Laves-phase superconductor HfV$_{2}$ studied in a series of HfV$_{2-x}$Nb$_{x}$ samples with 0 $leq$ $x$ $leq$ 0.3 under pulsed fields up to 30 T. The undoped HfV$_{2}$ undergoes a martensitic structural transition around 110 K, and becomes superconducting below $T_{rm c}$ = 9.4 K. Upon Nb doping, while the structural transition is suppressed for $x$ $geq$ 0.1, a maximum in $T_{rm c}$ of 10.1 K and zero-temperature upper critical field $B_{rm c2}$(0) of 22.4 T is found at $x$ = 0.2, which is ascribed to an increase of the density of states at the Fermi level. For all samples, the temperature dependence of $B_{rm c2}$ can be well described by the Werthamer-Helfand-Hohenberg (WHH) theory that takes into account both the spin paramagnetic effect and spin orbit scattering. In addition, a comparison is made between the $B_{rm c2}$ behavior of HfV$_{2-x}$Nb$_{x}$ and those of Nb-Ti and Nb$_{3}$Sn.
Recently, 12442 system of Fe-based superconductors has attracted considerable attention owing to its unique double-FeAs-layer structure. A steep increase in the in-plane upper critical field with cooling has been observed near the superconducting transition temperature, $T_c$, in KCa$_2$Fe$_4$As$_4$F$_2$ single crystals. Herein, we report a high-field investigation on upper critical field of this material over a wide temperature range, and both out-of-plane ($H|c$, $H_{c2}^{c}$) and in-plane ($H|ab$, $H_{c2}^{ab}$) directions have been measured. A sublinear temperature-dependent behavior is observed for the out-of-plane $H_{c2}^{c}$, whereas strong convex curvature with cooling is observed for the in-plane $H_{c2}^{ab}$. Such behaviors could not be described by the conventional Werthamer--Helfand--Hohenberg (WHH) model. The data analysis based on the WHH model by considering the spin aspects reveals a large Maki parameter $alpha=9$, indicating that the in-plane upper critical field is affected by a very strong Pauli paramagnetic effect.
We report the superconducting properties of new hexagonal Nb$_{10+2x}$Mo$_{35-x}$Ru$_{35-x}$Rh$_{10}$Pd$_{10}$ high-entropy alloys (HEAs) (0 $leq$ $x$ $leq$ 5). With increasing $x$, the superconducting transition temperature $T_{rm c}$ shows a maximum of 6.19 K at $x$ = 2.5, while the zero-temperature upper critical field $B_{rm c2}$(0) increases monotonically, reaching 8.3 T at $x$ = 5. For all $x$ values, the specific heat jump deviates from the Bardeen-Cooper-Schreiffer behavior. In addition, we show that $T_{rm c}$ of these HEAs is not determined mainly by the density of states at the Fermi level and would be enhanced by lowering the valence electron concentration.
We report synthesis, structural details and complete superconducting characterization of very recently discovered Nb2PdS5 new superconductor. The synthesized compound is crystallized in mono-clinic structure. Bulk superconductivity is seen in both magnetic susceptibility and electrical resistivity measurements with superconducting transition temperature (Tc) at 6K. The upper critical field (Hc2), being estimated from high field magneto-transport measure-ments is above 240kOe. The estimated Hc2(0) is clearly above the Pauli paramagnetic limit. Heat capacity measurements show clear transition with well defined peak at Tc, but with lower jump than as expected for a BCS type superconductor. The Sommerfield constant and Debye temperature as determined from low temperature fitting of heat capacity data are 32mJ/moleK2 and 263K respectively. Hall coefficients and resistivity in conjugation with electronic heat capacity indicates multiple gap superconductivity signatures in Nb2PdS5. We also studied the impact of hydrostatic pressure on superconductivity of Nb2PdS5 and found nearly no change in Tc for the given pressure range.
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