By using mostly the muon-spin rotation/relaxation ($μ$SR) technique, we investigate the superconductivity (SC) of Nb$_5$Ir$_{3-x}$Pt$_x$O ($x = 0$ and 1.6) alloys, with $T_c = 10.5$ K and 9.1 K, respectively. At a macroscopic level, their superconductivity was studied by electrical resistivity, magnetization, and specific-heat measurements. In both compounds, the electronic specific heat and the low-temperature superfluid density data suggest a nodeless SC. The superconducting gap value and the specific heat discontinuity at $T_c$ are larger than that expected from the Bardeen-Cooper-Schrieffer theory in the weak-coupling regime, indicating strong-coupling superconductivity in the Nb$_5$Ir$_{3-x}$Pt$_x$O family. In Nb$_5$Ir$_3$O, multigap SC is evidenced by the field dependence of the electronic specific heat coefficient and the superconducting Gaussian relaxation rate, as well as by the temperature dependence of the upper critical field. Pt substitution suppresses one of the gaps, and Nb$_5$Ir$_{1.4}$Pt$_{1.6}$O becomes a single-gap superconductor. By combining our extensive experimental results, we provide evidence for a multiple- to single-gap SC crossover in the Nb$_5$Ir$_{3-x}$Pt$_x$O family.
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…
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…
Using density-functional-based method, we study the k-resolved sigma- and pi-band holes in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2} alloys. We find that the calculated profiles of the loss of sigma- and pi-band holes in these two systems as a function of impurity concentration are in qualitative agreement with experiments, as expected. We also describe its implications vis-a-vis superconductivity in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2}.
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…
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…
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 $…
We give a brief review of superconductivity at ambient pressure in elements, alloys, and simple three-dimensional compounds. Historically these were the first superconducting materials studied, and based on the experimental knowledge gained from them the BCS theory of superconductivity was developed in 1957. Extended to include the effect of phonon retardation, the theory is believed to describe the subset of superconducting materials known as `conventional superconductors', where superconductivity is caused by the electron-phonon interaction. These include the elements, alloys and simple comp…
We synthesized new REO0.5F0.5BiS2 (RE: rare earth) superconductors with the high-entropy-alloy-type (HEA-type) REO blocking layers. According to the RE concentration and the RE ionic radius, the lattice constant of a systematically changed in the HEA-type samples. A sharp superconducting transition was observed in the resistivity measurements for all the HEA-type samples, and the transition temperature of the HEA-type samples was higher than that of typical REO0.5F0.5BiS2. The sharp superconducting transition and the enhanced superconducting properties of the HEA-type samples may indicate the …
The Korringa-Kohn-Rostoker method with the coherent potental approximation (KKR-CPA) is applied to study the first superconducting high entropy alloy (HEA) Ta34Nb33Hf8Zr14Ti11 (discovered in 2014 with Tc = 7.3 K), focusing on estimations of the electron-phonon coupling constant λ. Electronic part of λhas been calculated using the rigid muffin-tin approximation (RMTA), while the phonon part has been approximated using average atomic mass and experimental Debye temperature. The estimated λ= 1.16 is close to the value determined from specific heat measurements, λ= 0.98, and suggests rather strong…
Using the Korringa-Kohn-Rostoker coherent-potential approximation in the atomic-sphere approximation (KKR-ASA CPA) we have studied the changes in the electronic structure and the superconducting transition temperature T_{c} in Nb_{1-x}B_{2} alloys as a function of x. We find that the variation in the electronic structure of Nb_{1-x}B_{2} alloys as a function of x is consistent with the rigid-band model. However, the variation of T_{c}, obtained using the Allen-Dynes equation within the Gaspari-Gyorffy formalism to estimate the electron-phonon matrix elements, does not follow the expected trend…
We present here, the effect of microstructure on the magnetic, electrical and thermal properties of (V$_{0.60}$Ti$_{0.40}$)-Gd alloys. The gadolinium is found to be immiscible and precipitates with a size $<$1.2~$μ$m in the (V$_{0.60}$Ti$_{0.40}$)-Gd alloys. These precipitates enhance the grain boundary density. The (V$_{0.60}$Ti$_{0.40}$)-Gd alloys become ferromagnetic below $T_{mc}$ = 295~K with an increase in the superconducting transition temperature ($T_{sc}$). Though the disorder increases with increasing Gd content, the electronic thermal conductivity ($κ_{e} (H = 0)$) reduces by at mos…
A recent experiment reported that robust superconductivity appears in NbTi alloys under ultrahigh pressures with an almost constant superconducting $T_c$ of ~19 K from 120 to 261.7 GPa [J. Guo et al., Adv. Mater. 31, 1807240 (2019)], which is very rare among the known superconductors. We investigate the origin of this novel superconducting behavior in NbTi alloys based on density functional theory and density functional perturbation theory calculations. Our results indicate that the pressure tends to transform NbTi alloys from a random phase to a uniformly ordered crystal phase, and the exotic…
The Pr-rich end of the alloy series Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been studied using muon spin rotation and relaxation. The end compound PrOs$_4$Sb$_{12}$ is an unconventional heavy-fermion superconductor, which exhibits a spontaneous magnetic field in the superconducting phase associated with broken time-reversal symmetry. No spontaneous field is observed in the Nd-doped alloys for x $>$ 0.05. The superfluid density is insensitive to Nd concentration, and no Nd$^{3+}$ static magnetism is found down to the lowest temperatures of measurement. Together with the slow suppression of the supe…
We report the synthesis of new equimolar high-entropy alloys (HEAs) formed from five or six 4d/5d transition metals that are each from a different Group of the Periodic Table. These include MoReRuRhPt and MoReRuIrPt, which have a valence electron count (VEC) of 8 and crystallise in the hexagonal close-packed (hcp) structure, making them isoelectronic and isostructural to the elements Ru and Os and the high-pressure phase of the well-known HEA CrMnFeCoNi. Analogous HEAs with VECs of 7, 6 and 5 crystallise with multiple phases, which we rationalise using the atomic size difference, $δ$, and the …
High-entropy-alloy-type (HEA-type) compound superconductors have been drawing much attention as a new class of exotic superconductors with local structural inhomogeneity. NaCl-type (Ag,In,Sn,Pb,Bi)Te is a typical HEA-type superconductor, but the carrier doping mechanism had been unclear. In this study, we synthesized (Ag,In,Sn,Pb,Bi)Te with various In concentration using high-pressure synthesis: the studied system is (AgSnPbBi)(1-x)/4InxTe (x = 0-0.4). Single-phase samples were obtained for x = 0-0.3. A semiconductor-like temperature dependence of resistivity was observed for x = 0, while supe…
Shear ruptures of Cu samples soldered with Sn96Ag4 and Sn60Pb40 alloys have been measured at 300 K and 77 K. An average degradation of about 37 % of the shear rupture strength has been observed at cold for samples soldered with the lead-free alloy. This effect can be attributed to the tin pest.
We synthesized a new high-entropy-alloy-type (HEA-type) superconductor (Fe,Co,Ni,Cu,Ga)Zr2 with a Tc of 2.9 K. The EDX analyses revealed that the actual composition of the transition-metal site (Tr-site) is Tr = Fe0.18Co0.18Ni0.16Cu0.25Ga0.23, which gives the configurational entropy of mixing ΔSmix = 1.60R for the Tr site. Neutron powder diffraction revealed that the sample has a tetragonal CuAl2-type (space group: #140). The lattice constant of a monotonically decreases with decreasing temperature, but the lattice constant of c does not exhibit a clear shrinkage. Isotropic displacement parame…
A microscopic mean-field theory of the phase coexistence between ferromagnetism and superconductivity in the weakly ferromagnetic itinerant electron system is constructed, while incorporating a realistic mechanism for superconducting pairing due to the exchange of critical spin fluctuations. The self-consistent solution of the resulting equations determines the superconducting transition temperature which is shown to depend strongly on the exchange splitting. The effect of phase crossover from isotropic (Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum phase transit…
We provide an analytic solution for the mean-field equations and for the relevant physical quantities at the Gaussian level, in terms of the complete elliptic integrals of the first and second kinds, for the crossover problem from BCS superconductivity to Bose-Einstein condensation of a three-dimensional system of free fermions interacting via an attractive contact potential at zero temperature. This analytic solution enables us to follow the evolution between the two limits in a particularly simple and transparent way, as well as to verify the absence of singularities during the evolution.
We outline a microscopic approach to the superconducting fluctuations and pairing correlations in the attractive Hubbard model above Tc, using the functional integral method. A crossover from BCS superconductivity to Bose condensation of preformed pairs is studied by constructing the appropriate Ginzburg-Landau functionals. The differences between the lattice and the continuum models are discussed. The case of quasi-two-dimensional superconductors as well as the model with non-local pairing interaction are also examined. The effects of gaussian fluctuations of the order parameter are analyzed …
We study LiFeP, LiFeAs and NaFeAs in their paramagnetic metallic phase including dynamical electronic correlations within a density functional theory + slave-spin mean-field framework. The three compounds are found to lie next to the crossover between a normal and a Hund's metal, where a region of enhanced electronic compressibility that may boost superconductivity is systematically present in this type of systems. We find that LiFeP lies in the normal metallic regime, LiFeAs at the crossover, and NaFeAs is in the Hund's metal regime, which possibly explains the different experimental trends f…
We have considered the BCS-BEC crossover at T=0 in the framework of a lattice fermion model (the extended Hubbard model). We have analyzed the case of on-site hard core repulsion ($U=\infty$) and intersite attraction ($W<0$) in a 3D system with extended s-wave pairing symmetry. The mechanisms of density-driven and attraction-strength-driven crossover have been analyzed. We have obtained the ground state phase diagram. The analyzed quantities are: chemical potential, order parameter, coherence length and fraction of condensed particles.
In paired Fermi systems, strong many-body effects exhibit in the crossover regime between the Bardeen-Cooper-Schrieffer (BCS) and the Bose-Einstein condensation (BEC) limits. The concept of the BCS-BEC crossover, which is studied intensively in the research field of cold atoms, has been extended to condensed matters. Here, by analyzing the typical superconductors within the BCS-BEC phase diagram, we find that FeSe-based superconductors are prone to shift their positions in the BCS-BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconductin…
Proximity effect facilitates the penetration of Cooper pairs that permits superconductivity in normal metal, offerring a promising approach to turn heterogeneous materials into superconducting and develop exceptional quantum phenomena. Here, we have systematically investigated proximity-induced anisotropic superconductivity in monolayer Ni-Pb binary alloy by combining scanning tunneling microscopy/ spectroscopy(STM/STS) with theoretical calculations. By means of high temperature growth, the(3root3by3root3)R30o Ni-Pb surface alloy has been fabricated on the Pb(111), where the appearance of doma…
The Kohn-Luttinger mechanism for superconductivity is investigated in a model for the electron doped cuprates. The symmetry of the order parameter of the superconducting phase is determined as a function of the geometry of the Fermi surface together with the structure of the electron-hole susceptibility. It is found to remain d_{x^2-y^2} wave within a large doping range. The shape of the gap anisotropy evolves with doping, with the maximum gap moving away from (pi ,0), in good agreement with recent experiments. As the shift of the maximum increases, a crossover to d_{xy}-symmetry is found.
The results of an extensive investigation of structure, surface morphology, composition and the superconducting-normal phase diagram of a new unconventional superconductor LaAg1-cMnc with nominal composition c = 0.0, 0.025, 0.05, 0.1, 0.2 and 0.3, reveal the following. The alloys with c = 0, 0.025 and 0.05 are essentially single phase alloys with the actual Mn concentration, x, same as the nominal one, i.e., c = x, whereas in the alloys with c = 0.1, 0.2 and 0.3, the actual Mn concentration of the majority phase (crystalline grains) is x = 0.050(1), 0.080(1) and 0.100(1), respectively. The ter…
A series of new Cr$_{5+x}$Mo$_{35-x}$W$_{12}$Re$_{35}$Ru$_{13}$C$_{20}$ high-entropy alloys (HEAs) have been synthesized and characterized by x-ray diffraction, scanning electron microscopy, electrical resistivity, magnetic susceptibility and specific heat measurements. It is found that the HEAs adopt a noncentrosymmetric cubic $β$-Mn type structure and exhibit bulk superconductivity for 0 $\leq$ $x$ $\leq$ 9. With increasing $x$, the cubic lattice parameter decreases from 6.7940(3) Å to 6.7516(3) Å. Meanwhile, the superconducting transition temperature $T_{\rm c}$ is suppressed from 5.49 K to…
Local distortions of the crystal structure and their influence on the electronic structure and superconductivity of the high-entropy alloy (TaNb)0.67(HfZrTi)0.33
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…
Based on the perturbative renormalization group (PRG) approach, we have studied dimensional crossovers in Hubbard ladders coupled via weak interladder one-particle hopping, $t_{\perp}$. We found that the one-particle crossover is strongly suppressed through growth of the intraladder scattering processes which lead the isolated Hubbard ladder system toward the spin gap metal (SGM) phase. Consequently when $t_{\perp}$ sets in, there exists, for any finite intraladder Hubbard repulsion, $U>0$, the region where the two-particle crossover dominates the one-particle crossover and consequently the d-…
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…
High-entropy alloys are made from random mixtures of principal elements on simple lattices, stabilized by a high mixing entropy. The recently discovered BCC Ta-Nb-Hf-Zr-Ti high entropy alloy superconductor appears to display properties of both simple crystalline intermetallics and amorphous materials, e.g. it has a well defined superconducting transition along with an exceptional robustness against disorder. Here we show that the valence-electron count dependence of the superconducting transition temperature in the high entropy alloy falls between those of analogous simple solid solutions and …
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…
Interactions between vortices in thin superconducting films are investigated in the crossover (intertype) regime between superconductivity types I and II. We consider two main factors responsible for this crossover: a) changes in the material characteristics of the film and b) variations of the film thickness controlling the effect of the stray magnetic fields outside superconducting sample. The analysis is done within the formalism that combines the perturbation expansion of the microscopic equations to one order beyond the Ginzburg-Landau theory with the leading contribution of the stray fie…
High-entropy-alloy (HEA) superconductors are a new class of disordered superconductors. In this study, we investigate the robustness of superconducting states in HEA-type metal telluride (MTe; M = Ag, In, Sn, Pb, Bi) under high pressure. PbTe exhibits a structural transition from a NaCl-type to an orthorhombic Pnma structure at low pressures, and further transitions to a CsCl-type structure at high pressures. When the superconductivity of the CsCl-type PbTe is observed, it is found that its superconducting transition temperature (Tc) decreases with pressure. However, in the HEA-type AgInSnPbBi…
Effects of pressure on the electronic structure, electron-phonon interaction, and superconductivity of the high entropy alloy (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ are studied in the pressure range 0 - 100 GPa. The electronic structure is calculated using the Korringa-Kohn-Rostoker method with the coherent potential approximation. Effects of pressure on the lattice dynamics are simulated using the Debye-Grüneisen model and the Grüneisen parameter at ambient conditions. In addition, the Debye temperature and Sommerfeld electronic heat capacity coefficient were experimentally determined. The electron…
The binary Re$_{1-x}$Mo$_x$ alloys, known to cover the full range of solid solutions, were successfully synthesized and their crystal structures and physical properties investigated via powder x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. By varying the Re/Mo ratio we explore the full Re$_{1-x}$Mo$_x$ binary phase diagram, in all its four different solid phases: hcp-Mg ($P6_3/mmc$), $α$-Mn ($I\overline{4}3m$), $β$-CrFe ($P4_2/mnm$), and bcc-W ($Im\overline{3}m$), of which the second is non-centrosymmetric with the rest being centrosymmetric. All Re$_{1-…
In general, conventional superconductivity and magnetism are competing phenomena. In some alloys this competition is a function of the concentration of the elements. Here we show that in the palladium alloys Pd$_{1-c}$(H/D/T)$_{c}$ (Pd-ides) the increase in the concentration $c$ of the ides: hydrogen, deuterium, tritium (H/D/T), lowers the predicted magnetism of amorphous palladium ($a$-Pd) gradually, allowing superconductivity to appear for $ c \approx 40\%$. This magnetism explains why superconductivity does not manifest for smaller values of $c$ ($c \leq 40\%$) in these Pd alloys. Also, the…
The most intriguing observation of cuprate experiments is most likely the metal-insulator-crossover (MIC), seen in the underdome region of the temperature-doping phase diagram of copper-oxides under a strong magnetic field, when the superconductivity is suppressed. This MIC, which results in such phenomena as heat conductivity downturn, anomalous Lorentz ratio, nonlinear entropy, insulating ground state, nematicity- and stripe-phases and Fermi pockets, reveals the nonconventional dielectric property of the pseudogap-normal phase. Since conventional superconductivity appears from a conducting n…
We derive thermodynamic Bethe ansatz equations for a model describing an $U\to\infty$ Anderson impurity embedded in a BCS superconductor. The equations are solved analytically in the zero-temperature limit, T=0. It is shown that the impurities depress superconductivity in the Kondo limit, however at T=0 the system remains in the superconducting state for any impurity concentration. In the mixed-valence regime, an impurity contribution to the density of states of the system near the Fermi level overcompensates a Cooper pairs weakening, and superconductivity is enhanced.
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…
In this paper we study an evolution of low-temperature thermodynamical quantities for an electron gas with a $ δ$-function attraction as the system crosses over from weak-coupling (BCS-type) to strong-coupling (Bose-type) superconductivity in three and two dimensions.
We carefully studied the nonsuperconducting sample of the magneto-superconducting RuSr2(Eu1-xCex)Cu2O10 series with composition RuSr2EuCeCu2O10. This compound seems to exhibit a complex magnetic state as revealed by host of techniques like resistivity, thermopower, magnetic susceptibility and MR measurements.
