铼基超导体因其非常规超导特性近期引起了广泛关注。本工作报道了新型超导Re$_{7}$X$_{3}$ (X = Nb, Ta, Ti, Zr, Hf)二元合金的合成与性质，该合金保持固定的铼成分，并根据X位元素结晶为中心对称至非中心对称晶体结构。利用粉末x射线衍射、AC输运、磁化和比热测量全面研究了结构与超导特性，并据此构建了超导相图。结果表明晶体结构与Re/X比例存在复杂的相互作用，这决定了自旋轨道耦合的强度并控制铼基超导体中的非常规超导行为。
英文摘要：
Rhenium-based superconductors have recently attracted significant interest due to their unconventional superconducting properties. In this work, we report the synthesis and properties of new superconducting Re$_{7}$X$_{3}$ (X = Nb, Ta, Ti, Zr, Hf) binary alloys which maintain a fixed composition of rhenium while crystallizing in centrosymmetric to non-centrosymmetric crystal structures, depending on the elements of the X site. Comprehensive structural and superconducting properties were investigated using powder x-ray diffraction, AC transport, magnetization, and specific heat measurements, and on the basis of these measurements, the superconducting phase diagram was constructed. The results suggest a complex interplay of crystal structure and the Re/X ratio, which governs the strength of spin-orbital coupling and controls the unconventional superconducting behavior in Re-based superconductors.
Strontium ruthenates have many similarities with copper oxide superconductors and are of particular interest for the investigation of the mechanisms and conditions which lead to high-temperature superconductivity. We report here on multiple experimental indications of superconductivity with onset at 40 K in strontium ruthenate doped by rhenium and selenium with chlorine used as the flux. The main experimental evidence arises from terahertz spectroscopy of this material followed by AC and DC magnetization, as well as measurements of its heat capacity and magnetoresistance. Structural and morpho…
Elemental rhenium exhibiting type-II superconductivity has been previously reported to break time-reversal symmetry in the superconducting state. We have investigated an arc-melted sample of rhenium exhibiting type-I superconductivity. Low temperature zero-field muon-spin relaxation measurements indicate that time-reversal symmetry is preserved in the superconducting state. Muon diffusion is observed, which is due to quantum mechanical tunneling between interstitial sites. The normal state behavior is characterized by the conduction electrons screening the muons and thermal broadening, and is …
We report a comprehensive study of the centrosymmetric Re$_3$B and noncentrosymmetric Re$_7$B$_3$ superconductors. At a macroscopic level, their bulk superconductivity (SC), with $T_c$ = 5.1 K (Re$_3$B) and 3.3 K (Re$_7$B$_3$), was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were investigated by means of muon-spin rotation/relaxation ($μ$SR). In both Re$_3$B and Re$_7$B$_3$ the low-$T$ zero-field electronic specific heat and the superfluid density (determined via tranverse-field $μ$SR) suggest a nod…
We show that electroplated Re films in multilayers with noble metals such as Cu, Au, and Pd have an enhanced superconducting critical temperature relative to previous methods of preparing Re. The dc resistance and magnetic susceptibility indicate a critical temperature of approximately 6 K. Magnetic response as a function of field at 1.8 K demonstrates type-II superconductivity, with an upper critical field on the order of 2.5 T. Critical current densities greater than 10^7 A/m^2 were measured above liquid-helium temperature. Low-loss at radio frequency was obtained below the critical temperat…
We report here the structural, electrical and magnetic properties of as-cast V$_{1-x}$Zr$_x$ alloys ($x$ =0 - 0.4) at low temperatures. We observe that all the alloys undergo successive peritectic and eutectic reactions during cooling from the melt which leads to the formation of five phases, namely, a body centred cubic $β$-V phase, two phases with slightly different compositions having face centred cubic ZrV$_2$ structure, a hexagonal closed packed $α$-Zr phase, and the $β$-Zr precipitates. The amount of each phase is found to be dependent on the concentration of zirconium in vanadium. The $…
While the pure elements tend to exhibit Type-I rather than Type-II superconductivity, nearly all compound superconductors are Type-II, with only a few known exceptions. We report single crystal growth and physical characterization of the rhenium aluminide Al$_6$Re, which we conclude is a Type-I superconductor based on magnetization, ac-susceptibility, and specific-heat measurements. This detection of superconductivity, despite the strong similarity of Al$_6$Re to a family of W and Mo aluminides that do not superconduct, suggests that these aluminides are an ideal testbed for identifying the re…
Here we report the observation of extraordinary superconductivity in a pressurized commercial niobium-titanium alloy. We find that its zero-resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record high pressure up to which a known superconducting state can continuously survives. Remarkably, at such an ultra-high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (TC) increases to ~19.1 K from ~9.6 K, and the critical magnetic field (HC2) at 1.8 K has be…
We present a detailed study of the superconducting properties in the beta-phase Mo$_{1-x}$Re$_x$ (x = 0.25 and 0.4) solid solution alloys pursued through magnetization and heat capacity measurements. The temperature dependence of the upper critical field H$_{C2}$(T) in these binary alloys shows a deviation from the prediction of the Werthamer-Helfand-Hohenberg (WHH) theory. The temperature dependence of superfluid density estimated from the variation of lower critical field H$_{C1}$ with temperature, cannot be explained within the framework of a single superconducting energy gap. The heat capa…
In a short review-article we first discuss the results, which are mainly devoted to the generalizations of the famous Kohn-Luttinger mechanism of superconductivity in purely repulsive fermion systems at low electron densities. In the context of repulsive-U Hubbard model and Shubin-Vonsovsky model we consider briefly the superconducting phase diagrams and the symmetries of the order parameter in novel strongly correlated electron systems including idealized monolayer and bilayer graphene. We stress that purely repulsive fermion systems are mainly the subject of unconventional low-temperature su…
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…
Herein, we report the synthesis and detailed superconducting properties of a new high entropy alloy superconductor with nominal composition Nb$_{20}$Re$_{20}$Zr$_{20}$Hf$_{20}$Ti$_{20}$ using powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), magnetization, transport, and thermodynamic measurements. The room temperature powder XRD confirms that the alloy is arranged on a simple body centered cubic crystal lattice with lattice parameter a = 3.38 (1) $\textÅ$. EDX measurement yields an average composition of Nb$_{21}$Re$_{16}$Zr$_{20}$Hf$_{23}$Ti$_{20}$ (in atomic $\%$).…
Niobium-zirconium (Nb-Zr) alloy is an old superconductor that is a promising new candidate for superconducting radio-frequency (SRF) cavity applications. Using density-functional and Eliashberg theories, we show that addition of Zr to a Nb surface in small concentrations increases the critical temperature $T_c$ and improves other superconducting properties. Furthermore, we calculate $T_c$ for Nb-Zr alloys across a broad range of Zr concentrations, showing good agreement with the literature for disordered alloys as well as the potential for significantly higher $T_c$ in ordered alloys near 75%N…
High Critical Temperature and Field Superconductivity in Nb$_{0.85}$X$_{0.15}$, (X = Ti, Zr, Hf) Alloys: Promising Candidates for Superconducting Devices
Indian Institute of Science Education and Research Bhopal（印度科学教育研究所博帕尔分校）
In the recent search for unconventional- and topological superconductivity, noncentrosymmetric superconductors (NCSCs) rank among the most promising candidate materials. Surprisingly, some of them -- especially those containing rhenium -- seem to exhibit also time-reversal symmetry (TRS) breaking in their superconducting state, while TRS is preserved in many other isostructural NCSCs. To date, a satisfactory explanation for such discrepant behavior, albeit crucial for understanding the unconventional superconductivity of these materials, is still missing. Here we review the most recent develop…
High-entropy alloys (HEA) form through the random arrangement of five or more chemical elements on a crystalline lattice. Despite the significant amount of resulting compositional disorder, a subset of HEAs enters a superconducting state below critical temperatures $T_c<10$ K. The superconducting properties of the known HEAs seem to suffice a Bardeen-Cooper-Schrieffer (BCS) description, but little is known about their superconducting order parameter and the microscopic role of disorder. We report on magnetic susceptibility measurements on films of the superconducting HEA (TaNb)$_{1-x}$(ZrHfTi)…
A polycrystalline sample of the high-entropy-alloy-type telluride AgInSnPbBiTe5 was synthesized using high-pressure synthesis. Superconductivity with a transition temperature (Tc) of 2.6 K was observed in AgInSnPbBiTe5. Elemental and structural analyses revealed that five metals are mixed in a metal site of an NaCl-type structure. Since AgInSnPbBiTe5 has a cation site (Ag, In, Sn, Pb, and Bi) and an anion site (Te), this is the first example of a high-entropy-alloy (HEA) pseudo-binary superconductor.
The superconducting high-entropy alloys (HEAs) recently attract considerable attention due to their exciting properties, such as the robustness of superconductivity against atomic disorder and extremely high-pressure. The well-studied crystal structure of superconducting HEAs is body-centered-cubic (bcc) containing Nb, Ti, and Zr atoms. The same elements are contained in Al5Nb24Ti40V5Zr26, which is a recently discovered bcc HEA and shows a gum-metal-like behavior after cold rolling. The gum metal is also an interesting system, exhibiting superelasticity and low Young's modulus. If gum metals s…
Spin-fluctuations in Ti$_{60}$V$_{40}$ alloy and its influence on the superconductivity
本文报道了Ti $_{0.6}$ V $_{0.4}$合金的电阻率和直流磁化率随温度和磁场的变化关系以及零场热容随温度的变化关系。该Ti $_{0.6}$ V $_{0.4}$合金的正常状态直流磁化率的温度依赖性显示T $^2$ lnT行为。电阻率的温度依赖性在20-50 K范围内遵循T $^2$行为。另一方面，在包含电子和晶格热容的表达式中需要项$T^3$ lnT，以解释在观察到电阻率的$T^2$依赖性的温度下热容的温度依赖性。直流磁化率、电阻率和热容的温度依赖性表明系统中存在自旋涨落。进一步的实验证据的自旋涨落的形式获得的T $^5$项的电阻率的温度依赖性的负值。详细讨论了自旋涨落对Ti $_{0.6}$ V $_{0.4}$超导性质的影响。通过对正常态和超导态的电阻率和磁化率的分析，我们发现Ti $_{0.6}$ V $_{0.4}$合金中存在的自旋涨落在本质上是巡游的。有一些证据表明，在远高于超导转变温度的温度范围内存在预制的库珀对。我们的研究表明，自旋涨落和超导性之间有趣的相关性实际上可能在超导材料中相当普遍，而不一定只限于某些类别的奇异化合物。
We report the observation of extraordinarily robust zero-resistance superconductivity in the pressurized (TaNb)0.67(HfZrTi)0.33 high entropy alloy - a new kind of material with a body-centered cubic crystal structure made from five randomly distributed transition metal elements. The transition to superconductivity (TC) increases from an initial temperature of 7.7 K at ambient pressure to 10 K at ~ 60 GPa, and then slowly decreases to 9 K by 190.6 GPa, a pressure that falls within that of the outer core of the earth. We infer that the continuous existence of the zero-resistance superconductivit…
We have found that body-centered cubic (bcc) HfMoNbTiZr is a type-II BCS high-entropy alloy (HEA) superconductor with a superconducting critical temperature of $T_\mathrm{c}$=4.1 K. By employing a Debye temperature $θ_\mathrm{D}$ of 263 K and $T_\mathrm{c}$, the electron-phonon coupling constant $λ_\mathrm{e-p}$ is calculated to be 0.63. The electronic structure calculation revealed band broadening with energy uncertainties due to atomic disorders. The superconducting properties are compared among equiatomic quinary bcc HEA superconductors. $T_\mathrm{c}$ decreases with decreasing $λ_\mathrm{e…
Hydrogen-based superconductors provide a route to the long-sought goal of room-temperature superconductivity, but the high pressures required to metallize these materials limit their immediate application. For example, carbonaceous sulfur hydride, the first room-temperature superconductor, can reach a critical temperature (Tc) of 288 K only at the extreme pressure of 267 GPa. The next recognized challenge is the realization of room-temperature superconductivity at significantly lower pressures. Here, we propose a strategy for the rational design of high-temperature superconductors at low press…
Superconducting high entropy alloy (HEA) are emerging as a new class of superconducting materials. It provides a unique opportunity to understand the complex interplay of disorder and superconductivity. We report the synthesis and detail bulk and microscopic characterization of Nb$_{60}$Re$_{10}$Zr$_{10}$Hf$_{10}$Ti$_{10}$ HEA alloy using transport, magnetization, specific heat, and muon spin rotation/relaxation ($μ$SR) measurements. Bulk superconductivity with transition temperature $T_{C}$ = 5.7 K confirmed by magnetization, resistivity, and heat capacity measurements. Zero-field $μ$SR measu…
Research on high-entropy-alloy (HEA) superconductors is a growing field in material science. In this study, we explored new HEA-type superconductors and discovered a CuAl2-type superconductor Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 with a HEA-type transition metal site. A superconducting transition was observed at 8.0 K after electrical resistivity, magnetization, and specific heat measurements. The bulk characteristics of the superconductivity were confirmed through the specific heat measurements. The discovery of superconductivity in HEA-type Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 will provide a novel pathway to…
High entropy alloys (HEAs) are the new class of materials with an attractive combination of tunable mechanical and physicochemical properties. They crystallize mainly in cubic structures, however, for practical applications, HEAs with hexagonal close-packed (hcp) structure are highly desirable in connection to their in general high hardness. Herein, we report the synthesis, structure and detailed superconducting properties of Re$_{0.56}$Nb$_{0.11}$Ti$_{0.11}$Zr$_{0.11}$Hf$_{0.11}$-the first hexagonal superconducting high entropy alloy (HEA) composed of five randomly distributed transition-meta…
High-entropy alloys (HEAs) are at the focus of current research for their diverse properties, including superconductivity and structural polymorphism. However, the polymorphic transition has been observed only in nonsuperconducting HEAs and mostly under high pressure. Here we report the discovery of superconductivity and temperature-driven polymorphism in the (V$_{0.5}$Nb$_{0.5}$)$_{3-x}$Mo$_{x}$Al$_{0.5}$Ga$_{0.5}$ (0.2 $\leq$ $x$ $\leq$ 1.4) HEAs. It is found that the as-cast HEA is of a single body-centered cubic (bcc) structure for $x$ = 0.2 and a mixture of the bcc and A15 structures for …
Based on the first-principles electronic structure calculations and the symmetry analysis, we predict that the topological superconductivity may occur on the surface of the LnPd$_{2}$Sn (Ln=Sc, Y, Lu) class of Heusler alloys. The calculated electronic band structure and topological invariant demonstrate that the LnPd$_{2}$Sn family is topologically nontrivial. The further slab calculations show that the nontrivial topological surface states of LnPd$_{2}$Sn exist within the bulk band gap and meanwhile they cross the Fermi level. Considering that the LnPd$_{2}$Sn class of compounds were all foun…
Effect of Deposition Pressure on the Superconductivity of Ti40V60 Alloy Thin Films
Free Electron Laser Utilization Laboratory, Raja Ramanna Center for Advanced Technology（自由电子激光利用实验室，拉贾·拉曼纳先进技术中心）；Homi Bhabha National Institute（霍米·巴巴国立研究所）；UGC-DAE Consortium for Scientific Research（UGC-DAE科学研究联盟）
NbSe$_{2}$ and NbS$_{2}$ are isostructural two-dimensional materials that exhibit contrasting superconducting properties when reduced to the single monolayer limit. Monolayer NbSe$_{2}$ is an Ising superconductor, while there have been no reports of superconductivity in monolayer NbS$_{2}$. NbS$_{x}$Se$_{2-x}$ alloys exhibit an intriguing non-monotonic dependence of the superconducting transition temperature with sulfur content, which has been interpreted as a manifestation of fractal superconductivity. However, several key questions about this result are not known: (1) Does the electronic str…
