Pressure tuning of superconductivity independent of disorder in Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8+delta}$

Varying the superconducting transition temperature over a large scale of a cuprate superconductor is a necessary step for identifying the unsettled mechanism of superconductivity. Chemical doping or element substitution has been proven to be effective but also brings about lattice disorder. Such disorder can completely destroy superconductivity even at a fixed doping level. Pressure has been thought to be the most clean method for tuning superconductivity. However, pressure-induced increase of disorder was recognized from recent experiments. By choosing a disordered Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8+delta}$ at the optimal doping, we perform single-crystal x-ray diffraction and magnetic susceptibility measurements at high pressures. The obtained structural data provides evidence for the robust feature for the disorder of this material in the pressure range studied. This feature ensures the pressure effects on superconductivity distinguishable from the disorder. The derived parabolic-like behavior of the transition temperature with pressure up to near 30 GPa, having a maximum around 7 GPa, offers a platform for testing any realistic theoretical models in a nearly constant disorder environment. Such a behavior can be understood when considering the carrier concentration and the pairing interaction strength as two pressure intrinsic variables.

Superconducting properties of sulfur-doped iron selenide

The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of FeSe$_{1-x}$S$_{x}$ ($x$=0, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature $T_{c}$, anisotropy, upper critical field $H_{c2}$, and critical current density $J_{c}$. The upper critical field $H_{c2}(T)$ and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth $lambda _{ab}(T)$ and specific heat, we show clear evidence for strong coupling two-gap $s$-wave superconductivity. The temperature-dependence of $lambda _{ab}(T)$ calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with $s$-wave-like gaps. We find that a $d$-wave and single-gap BCS theory under the weak-coupling approach can not describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.

Superconductivity in Dense Rashba Semiconductor BiTeCl

Layered non-centrosymmetric bismuth tellurohalides are being examined as candidates for topological insulators. Pressure is believed to be essential for inducing and tuning topological order in these systems. Through electrical transport and Raman scattering measurements, we find superconductivity in two high-pressure phases of BiTeCl with the different normal state features, carrier characteristics, and upper critical field behaviors. Superconductivity emerges when the resistivity maximum or charge density wave is suppressed by the applied pressure and then persists till the highest pressure of 51 GPa measured. The huge enhancement of the resistivity with three magnitude of orders indicates the possible achievement of the topological order in the dense insulating phase. These findings not only enrich the superconducting family from topological insulators but also pave the road on the search of topological superconductivity in bismuth tellurohalides.

Superconductivity in efficient thermoelectric Cu$_3$Sb$_{0.98}$Al$_{0.02}$Se$_4$

Both superconductivity and thermoelectricity offer promising prospects for daily energy efficiency applications. The advancements of thermoelectric materials have led to the huge improvement of the thermoelectric figure of merit in the past decade. By applying pressure on a highly efficient thermoelectric material Cu$_{3}$Sb$_{0.98}$Al$_{0.02}$Se$_4$, we achieve dome-shape superconductivity developing at around 8.5 GPa but having a maximum critical temperature of 3.2 K at pressure of 12.7 GPa. The novel superconductor is realized through the first-order structural transformation from its initial phase to an orthorhombic one. The superconducting phase is determined in the ultimate formation of the Cu-Al-Sb-Se alloy.