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Register a new userSuperconducting Order from Disorder in 2H-TaSe$_{2-x}$S$_{x}$ (0$leq$x$leq$2)
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We report on the emergence of robust superconducting order in single crystal alloys of 2H-TaSe$_{2-x}$S$_{x}$ (0$leq$x$leq$2) . The critical temperature of the alloy is surprisingly higher than that of the two end compounds TaSe$_{2}$ and TaS$_{2}$. The evolution of superconducting critical temperature T$_{c} (x)$ correlates with the full width at half maximum of the Bragg peaks and with the linear term of the high temperature resistivity. The conductivity of the crystals near the middle of the alloy series is higher or similar than that of either one of the end members 2H-TaSe$_{2}$ and/or 2H-TaS$_{2}$. It is known that in these materials superconductivity (SC) is in close competition with charge density wave (CDW) order. We interpret our experimental findings in a picture where disorder tilts this balance in favor of superconductivity by destroying the CDW order.
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We report the successful synthesis of FeSe$_{1-x}$S$_{x}$ single crystals with $x$ ranging from 0 to 1 via a hydrothermal method. A complete phase diagram of FeSe$_{1-x}$S$_{x}$ has been obtained based on resistivity and magnetization measurements. The nematicity is suppressed with increasing $x$, and a small superconducting dome appears within the nematic phase. Outside the nematic phase, the superconductivity is continuously suppressed and reaches a minimum $T_c$ at $x$ = 0.45; beyond this point, $T_c$ slowly increases until $x$ = 1. Intriguingly, an anomalous resistivity upturn with a characteristic temperature $T^*$ in the intermediate region of $0.31 leq x leq 0.71$ is observed. $T^{*}$ shows a dome-like behavior with a maximum value at $x$ = 0.45, which is opposite the evolution of $T_c$, indicating competition between $T^*$ and superconductivity. The origin of $T^*$ is discussed in detail. Furthermore, the normal state resistivity evolves from non-Fermi-liquid to Fermi-liquid behavior with S doping at low temperatures, accompanied by a reduction in electronic correlations. Our study addresses the lack of single crystals in the high-S doping region and provides a complete phase diagram, which will promote the study of relations among nematicity, superconductivity, and magnetism.
Remarkably, doping isovalent $d^{10}$ and $d^0$ cations onto the $B$ site in $A_2B$$B$O$_6$ double perovskites has the power to direct the magnetic interactions between magnetic $B$ cations. This is due to changes in orbital hybridization, which favors different superexchange pathways, and leads to the formation of alternative magnetic structures depending on whether $B$ is $d^{10}$ or $d^0$. Furthermore, the competition generated by introducing mixtures of $d^{10}$ and $d^0$ cations can drive the material into the realms of exotic quantum magnetism. Here, a W$^{6+}$ $d^0$ dopant was introduced to a $d^{10}$ hexagonal perovskite Ba$_2$CuTeO$_6$, which possesses a spin ladder geometry of Cu$^{2+}$ cations, creating a Ba$_2$CuTe$_{1-x}$W$_x$O$_6$ solid solution ($x$ = 0 - 0.3). Neutron and synchrotron X-ray diffraction show that W$^{6+}$ is almost exclusively substituted for Te$^{6+}$ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. This means the intra-ladder interactions are selectively tuned by the $d^0$ cations. Bulk magnetic measurements suggest this suppresses magnetic ordering in a similar manner to that observed for the spin-liquid like material Sr$_2$CuTe$_{1-x}$W$_x$O$_6$. This further demonstrates the utility of $d^{10}$ and $d^0$ dopants as a tool for tuning magnetic ground states in a wide range of perovskites and perovskite-derived structures.
We report $^{75}$As nuclear magnetic resonance (NMR) / nuclear quadrupole resonance (NQR) and transmission electron microscopy (TEM) studies on LaFeAsO$_{1-x}$F$_{x}$. There are two superconducting domes in this material. The first one appears at 0.03 $leq$ $x$ $leq$ 0.2 with $T_{rm c}$$^{max}$ = 27 K, and the second one at 0.25 $leq$ $x$ $leq$ 0.75 with $T_{rm c}$$^{max}$ = 30 K. By NMR and TEM, we demonstrate that a $C4$-to-$C2$ structural phase transition (SPT) takes place above both domes, with the transition temperature $T_{rm s}$ varying strongly with $x$. In the first dome, the SPT is followed by an antiferromagnetic (AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. In LaFeAsO$_{0.97}$F$_{0.03}$, we find that AF order and superconductivity coexist microscopically via $^{75}$As nuclear spin-lattice relaxation rate (1/$T_1$) measurements. In the coexisting region, 1/$T_1$ decreases at $T_{rm c}$ but becomes to be proportional to $T$ below 0.6$T_{rm c}$, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for $x leq$ 0.2 is quite similar to the doped BaFe$_{2}$As$_{2}$ system. The electrical resistivity in the second dome can be fitted by $rho = {{rho }_{0}}+A{{T}^{n}}$ with $n$ = 1 and a maximal coefficient $A$ at around $x_{opt}$ = 0.5$sim$0.55 where $T_{rm s}$ extrapolates to zero and $T_{rm c}$ is the maximal, which suggest the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO$_{1-x}$F$_{x}$, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.
Low energy polarized electronic Raman scattering of the electron doped superconductor Nd_1.85Ce_0.15CuO_4 (T_c=22 K) has revealed a nonmonotonic d_{x^2-y^2} superconducting order parameter. It has a maximum gap of 4.4 k_BT_c at Fermi surface intersections with antiferromagnetic Brillouin zone (the ``hot spots) and a smaller gap of 3.3 k_BT_c at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the ``hot spots emphasizes role of antiferromagnetic fluctuations and similarity in the origin of superconductivity for electron- and hole-doped cuprates.
We investigated the superconducting and transport properties in FeTe$_{1-x}$Se$_{x}$ (0.1 $leq$ $x$ $leq$ 0.4) single crystals prepared by O$_2$-annealing. Sharp superconducting transition width observed in magnetization measurement and the small residual resistivity prove the high quality of the crystals. All the crystals manifest large, homogeneous, and isotropic critical current density emph{J}$_c$ with self-field value over 10$^5$ A/cm$^2$ at 2 K. The large and field-robust critical current densities prove that the superconductivity in FeTe$_{1-x}$Se$_{x}$ (0.1 $leq$ $x$ $leq$ 0.4) is in bulk nature. The values of anisotropy parameter close to $T_c$ for crystals with different Se doping levels all reside in the range of 2 - 3. Hall coefficients $R_H$ keeps positive and almost constant value at high temperatures, followed by a sudden decreases before reaching $T$$_c$, which indicates that the electron-type charge carriers become dominant at low temperatures. Furthermore, the characteristic temperature for the sudden decrease in $R_H$ gradually increases with Se doping.
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