Record‐High Superconductivity in Niobium–Titanium Alloy

The extraordinary superconductivity has been observed in a pressurized commercial niobium–titanium alloy. 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 survive. Remarkably, at such an ultra‐high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (T_C) increases to ≈19.1 K from ≈9.6 K, and the critical magnetic field (H_C2) at 1.8 K has been enhanced to 19 T from 15.4 T. These results set new records for both the T_C and the H_C2 among all the known alloy superconductors composed of only transition metal elements. The remarkable high‐pressure superconducting properties observed in the niobium–titanium alloy not only expand the knowledge on this important commercial superconductor but also are helpful for a better understanding on the superconducting mechanism.     

Superconductivity and Unusual Magnetic Features in Amorphous Carbon and in Other Unrelated Materials

Traces of superconductivity (SC) were observed in three different inhomogeneous sulfur-doped amorphous carbon (a-C) systems: (i) in old commercial powder, (ii) in fabricated (by pyrolytic decomposition of sucrose) powder, and (iii) in a-C thin films. (i) The commercial a-C powder contains 0.21 at% sulfur and reveals traces of two superconducting phases around T _C ?34 and T _C～65 K. (ii) The fabricated a-C powder is diamagnetic but not superconducting. However, mixtures of this powder with sulfur (a-CS) which were heated to 250 °C also show traces of SC up to T _C～42 K. (iii) Non-superconducting a-C thin films were grown by electron-beam induced deposition. SC at T _C～34 K emerged only after heat treatment with sulfur. It is proposed that the high SC states in a-CS (T _C～65 K at ambient pressure) and in H₃S materials (T _C = 203 K under >200 GPa) has the same origin. In both sulfur-containing systems, SC is induced by the interaction between electrons and the high frequencies of low mass atom (H or C) vibrations a-CS products obtained by heating commercial and fabricated a-C powders at 400 °C show unusual magnetic features: (a) pronounced irreversible peaks around 55–80 K which appear in the first zero-field-cooled (ZFC) runs only. This temperature range is close to the highest T _C observed. (b) These peaks are totally suppressed in the second ZFC sweeps measured a few minutes later. (c) The peaks reappear after 18 months. (d) Around the peak position, the field-cooled (FC) curves cross the ZFC plots (ZFC > FC). All these phenomena are intrinsic properties of the amorphous carbon materials and were not observed in crystalline graphite. The ZFC > FC state was also observed in chiral-based magnetic memory device and unexpectedly in the liver taken from a patient with mantle cell lymphoma. This peculiarity will be discussed.     

Decompression‐Driven Superconductivity Enhancement in In2Se3

An unexpected superconductivity enhancement is reported in decompressed In₂Se₃. The onset of superconductivity in In₂Se₃ occurs at 41.3 GPa with a critical temperature (T_c) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest T_c that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of T_c is driven by the pressure‐induced R‐3m to I‐43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression‐induced superconductivity enhancement implies that it is possible to maintain pressure‐induced superconductivity at lower or even ambient pressures with better superconducting performance.     

Coexistence of Superconductivity and Itinerant Ferromagnetism in Ferromagnetic Metals UCoGe and UIr

A microscopic coexistence of itinerant ferromagnetism (FM) and superconductivity (SC) is studied in a single band homogenous system, following an equation of motion method and Green’s function technique. Self-consistent equations for superconducting order parameter (Δ) and magnetization parameter (M) are derived. It is shown that there generally exists a coexistent (Δ≠0, M≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The expressions for electronic specific heat (C/T), density of states, free energy, transition probabilities, ultrasonic attenuation, and nuclear relaxation are also derived. The theory is applied to explain the observations in UCoGe and UIr. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, coexistence of FM and SC realized at a low enough temperature. The agreement between theory and experimental results for UCoGe and UIr is quite encouraging.     

Magnetic and Thermal Behavior of Ru<Subscript>0.9</Subscript>Sr<Subscript>2</Subscript>YCu<Subscript>2.1</Subscript>O<Subscript>7.9</Subscript> Magneto-Superconductor Synthesized by High-Pressure High-Temperature Technique

The structural, physical, and thermal property details of Ru_0.9Sr₂YCu_2.1O_7.9 (Y/Ru-1212) superconducting material synthesized through high pressure (6 GPa) and a high temperature (1400 °C) (HPHT) route are reported here. (Y/Ru-1212) crystallizes in P4/mmm tetragonal structure and is found free of any detectable impurities through X-ray diffractometry. Ru-spins are ordered magnetically above 145 K, with a sizeable ferromagnetic component at 5 K. Further clear diamagnetic transitions are seen in both zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility measurements and exhibiting superconductivity below 50 K. Both the thermoelectric power (S) and thermal conductivity (κ) measurements show superconductivity onset below 50 K with S=0 at 30 K and a broad hump in heat capacity C _p (T) below 30 K. Heat capacity (C _p ) measurements also exhibit the magnetic ordering temperature as a hump below 145 K. The appearance of a hump in C _p (T), instead of a clear transition, is indicative of short range magnetic correlations like spin glass (SG). Neither the high (145 K) nor the low (30 K) temperature humps of C _p (T) could be analyzed quantitatively because of short magnetic correlations in former and mixing of both superconductivity and FM components in a later case. The observed data is compared with various reported Ru-1212 systems synthesized under normal pressure conditions. It is concluded that HPHT synthesized Y/Ru-1212 is a bulk superconductor below 30 K with a substantial FM component.     

High-Temperature Superconductivity in Sulfur-Doped Amorphous Carbon Systems

Magnetization measurements performed on amorphous carbon (aC) powder that contained a small amount of sulfur revealed traces of inhomogeneous superconductivity (SC) at T _c=65 K. Thin films of granular aC-W composite obtained by Electron Beam Induced Deposition showed no sign of SC. However, SC at T _c≈40 K was induced upon treating this film with sulfur at 250 ^∘C for 24 hours. Although the superconducting volume fraction in both cases is very low, our results prove the necessity of sulfur for inducing SC in aC, and open new pathways to achieve high-temperature SC in the unique system of aC-based materials.     

Lattice Anomalies in the Oxygen Deficient NdFeAsO Superconductor

We compare the results of high-resolution synchrotron X-ray powder diffraction with low-temperature spectroscopic studies on the oxygen deficient NdFeAsO_0.85 superconductor. IR results indicate that a certain mode related to vibrations of the As ions softens at low temperatures. By employing a dense temperature sampling we have managed to reveal unnoticed structural modifications that start around ∼180 K, as the anomaly of the IR mode, and disappear at the transition temperature. The data show smaller structural changes in the Nd–O charge reservoir layer, while in the superconducting layer the FeAs₄ tetrahedron shows gradual modifications below ∼180 K, which peak around 140 K and then suddenly disappear at T _c . The observed lattice anomalies are not connected with a structural phase transition or an AF ordering; they are more evident in the superconducting Fe–As planes, and disappear across T _c , pointing to a connection to superconductivity.     

Superconductivity at 14 K in SmFe0.9Co0.1AsO

We report superconductivity in the SmFe_0.9Co_0.1AsO system being prepared by most easy and versatile single-step solid-state reaction route. The parent compound SmFeAsO is non-superconducting but shows the spin density wave (SDW) like antiferromagnetic ordering at around 140 K. To destroy the antiferromagnetic ordering and to induce the superconductivity in the parent system, the Fe²⁺ is partially substituted by Co³⁺. Superconductivity appears in SmFe_0.9Co_0.1AsO system at around 14 K. The Co doping suppresses the SDW anomaly in the parent compound and induces the superconductivity. Magnetization measurements show clearly the onset of superconductivity with T _c^dia at 14 K. The isothermal magnetization measurements exhibit the lower critical fields (H _c1) to be around 200 Oe at 2 K. The bulk superconductivity of the studied SmFe_0.9Co_0.1AsO sample is further established by open diamagnetic M(H) loops at 2 and 5 K. Normal-state (above T _c) linear isothermal magnetization M(H) plots excluded the presence of any ordered magnetic impurity in the studied compound.      

Theoretical Study of Specific Heat, Density of States and Free Energy of Itinerant Ferromagnetic Superconductor URhGe

Following the equation of motion method and Green’s function technique, the coexistence of itinerant ferromagnetism (FM) and superconductivity (SC) is investigated in a single band homogeneous system. Self-consistent equations for superconducting order parameter (Δ) and magnetic order parameter (Δ_FM) are derived. It is shown that there generally exists a coexistent (Δ≠0 and Δ_FM≠0) solutions to the coupled equations of the order parameter in the temperature range 0<T<min (T _C,T _FM) where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. Expressions for the specific heat, density of states and free energy are derived. The specific heat has a linear temperature dependence at low temperatures as opposed to the exponential decrease in the BCS theory. The density of states for a finite Δ_FM increases as opposed to that of a standard ferromagnetic metal. The free energy shows that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and therefore is realized at low enough temperature. The theory is applied to explain the observations of URhGe. The agreement between theory and experimental results is quite satisfactory.      

Interplay of Superconductivity and Ferromagnetism in UGe<Subscript>2</Subscript>

The emergence of pressure induced superconductivity (SC) under the background of ferromagnetic state in 5f-electron based itinerant ferromagnetic superconductor UGe₂ is studied in the single band model by using a mean-field approximation. The solutions to the coupled equations of superconducting gap (Δ) and magnetization (m) are obtained using Green’s function technique and equation of motion method. It is shown that there generally exists a coexistent (Δ≠0, m≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The study of electronic specific heat (C/T), density of states, free energy, etc. are also presented. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, realized at low enough temperature. The agreement between theory and experimental results for UGe₂ is quite satisfactory.      

Antiferromagnetism and superconductivity transitions with SO(5) symmetry

Using Monte Carlo simulation we have computed the phase diagram of a superspin model of the SO(5) symmetry between antiferromagnetism (AF) and d-wave superconductivity (SC). A bicritical point is observed at a finite temperature where the second-order AF and SC transition lines merge tangentially into a first-order line in the temperature vs. doping rate phase diagram. In the regime of the doping rate where the SC is achieved at low temperatures, maximal AF correlation length is observed at a temperature above the SC transition point T _c when AF coupling is much stronger than SC coupling. The relationship between this behavior and the spin-gap phenomenon is addressed.     

Evolution of Superconducting and Normal-State Properties in C60 “Polymer” Doped with K

Abstract

Abstract:

Bulk superconductivity was observed in a new iodine-treated aggregated form of solid C₆₀, believed to be a “polymer”, by doping with alkali metal. Evolution of superconducting (T_c = 17.2 K) and normal-state properties, has been studied as a function of doping with K. The normal-state dc susceptibility exhibited a complex behavior with the increasing K uptake, evolving from an enhanced paramagnetism at low doping level, to a weakly temperature-dependent diamagnetism at the optimum doping level for superconductivity, and ending with a strong temperature-independent diamagnetism in over-doped samples. The measured superconductivity parameters, such as T_c, London penetration depth and the Ginzburg-Landau coherence length, are compared with those of the monomer K₃C₆₀. and the main factors determining the differences between the two systems are discussed.     

Compressed H<Subscript>3</Subscript>S,Superfluid Density and the Quest for Room-Temperature Superconductivity

The discovery of superconductivity at 203 K in highly compressed sulphur hydride validates the ideas put forward by Ashcroft 50 years ago and galvanises the quest for room-temperature superconductivity. But at such temperatures, thermal fluctuations might be expected to break up Cooper pairs. For example, in the cuprates, fluctuations reduce T _c by 30% or more below the mean-field value. Similar effects are found in iron pnictides. Here, we ask: how does superconductivity survive in sulphur hydride at such high temperatures? To answer this, we examine the superfluid density which is the key parameter for quantifying fluctuations in both amplitude and phase. We show that dimensionality plays a key role in suppressing or enhancing thermal fluctuations to the benefit of hydrogen sulphide and the detriment of its more layered 2D competitors. We find that the temperature scale for phase fluctuations, T _φ , in superconducting H₃S exceeds 1200 K, and therefore, these are irrelevant at 200 K. But the amplitude fluctuation temperature scale, T _amp, at around 300 K is much lower and this has important implications for the ongoing quest for room temperature superconductivity. Appealing to the way in which superfluid density, T _φ , T _amp and T _c scale with each other it seems that room temperature superconductivity is nearly ruled out, but perhaps not quite. It will require 3D systems with a large Fermi velocity to achieve this goal.     

Effect of Y Doping on Superconductivity and Spin-Density-Wave States in Sm1?xYxFeAsO0.8F0.2 and Sm1?xYxFeAsO

Y doping effects on crystal structure, resistivity, superconductivity, and spin-density-wave states in the parent and F-doped SmFeAsO compounds have been studied. Y doping leads to shrinkage of the lattice parameters in both systems. The superconductivity of Sm_1−x Y _x FeAsO_0.8F_0.2 is suppressed by Y doping with onset T _c decreasing monotonically from 52.6 K at x=0 to 25 K at x=0.5. A weak resistivity anomalies around 120 K ascribed to the spin-density-wave instability is observed in the sample of x=0.5. In parent SmFeAsO compound, the resistivity anomaly temperature was detected to be around 150 K and shifted towards lower temperatures with the Y doping level increased. It is concluded that a negative pressure effect takes responsibilities for the decrease of T _c in Sm_1−x Y _x FeAsO_0.8F_0.2.     

The Superconducting Ferromagnet UCoGe

The correlated metal UCoGe is a weak itinerant ferromagnet with a Curie temperature T _C=3 K and a superconductor with a transition temperature T _s=0.6 K. We review its basic thermal, magnetic—on the macro and microscopic scale—and transport properties, as well as the response to high pressure. The data unambiguously show that superconductivity and ferromagnetism coexist below T _s=0.6 K and are carried by the same 5f electrons. We present evidence that UCoGe is a p-wave superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.     

Topological Quantum Phase Transition and Superconductivity Induced by Pressure in the Bismuth Tellurohalide BiTeI

A pressure‐induced topological quantum phase transition has been theoretically predicted for the semiconductor bismuth tellurohalide BiTeI with giant Rashba spin splitting. In this work, evolution of the electrical transport properties in BiTeI and BiTeBr is investigated under high pressure. The pressure‐dependent resistivity in a wide temperature range passes through a minimum at around 3 GPa, indicating the predicted topological quantum phase transition in BiTeI. Superconductivity is observed in both BiTeI and BiTeBr, while resistivity at higher temperatures still exhibits semiconducting behavior. Theoretical calculations suggest that superconductivity may develop from the multivalley semiconductor phase. The superconducting transition temperature, T_c, increases with applied pressure and reaches a maximum value of 5.2 K at 23.5 GPa for BiTeI (4.8 K at 31.7 GPa for BiTeBr), followed by a slow decrease. The results demonstrate that BiTeX (X = I, Br) compounds with nontrivial topology of electronic states display new ground states upon compression.     

Lattice instability in the normal state of La2?xSrxCuO4

Novel softening has been found in the transverse elastic constant (C ₁₁–C ₁₂)/2 below 50 K in single-crystalline La_1.86Sr_0.14CuO₄ (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT _c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT _c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T _c superconductivity and the lattice instability in LSCO.     

Superconductivity in alkali-doped fullerides with wood’s metal and heterofullerides with two different alkali metals A(1)A(2)MC60

Abstract

Here we report our experimental data on the synthesis and investigation of superconductivity in fullerides and heterofullerides synthesized by different methods and also doped by alloys with low melting temperature T_m: Wood’s metal (W – Sn,Pb,Bi,Cd; T_m?=?65°С), alloy Y (Y - Bi,Sn.Pb,In,Cd,Tl; Т_m?=?41.5°С) and Tl doped heterofullerides with two different alkali metals – ACsTlC₆₀ (A?=?K, Rb). We found superconductivity in heterofullerides K₂W₁C₆₀ (T_c?=?16 K), Rb₂W₁C₆₀ (T_c?=?8 K) and K₂Y₁C₆₀ (T_c?=?8 K). Also synthesized heterofullerides with Cesium KCsTlC₆₀ (Т_с?=?21.7 K) and RbCsTlC₆₀ (Т_с?=?26.4 K) are superconductors.     

Magnetic-Field Induced Superconductor–Antiferromagnet Transition in Lightly Doped RBa2Cu3O6+x (R = Lu, Y) Crystals

The remarkable sensitivity of the c-axis resistivity and magnetoresistance in cuprates to the spin ordering is used to clarify the doping-induced transformation from an antiferromagnetic (AF) insulator to a superconducting (SC) metal in RBa₂Cu₃O_6+x (R = Lu, Y) single crystals. The established phase diagram demonstrates that the AF and SC regions apparently overlap: The superconductivity in RBa₂Cu₃O_6+x , in contrast to La_2−x Sr _x CuO₄, sets in before the long-range AF order is completely destroyed by hole doping. Magnetoresistance measurements of superconducting crystals with low T _c ≤15–20 K give a clear view of the magnetic-field induced superconductivity suppression and recovery of the long-range AF state. What still remains to be understood is whether the AF order actually persists in the SC state or just revives when the superconductivity is suppressed, and in the former case, whether the antiferromagnetism and superconductivity reside in nanoscopically separated phases or coexist on an atomic scale.     

Effect of Oxygen Concentration on Superconducting Properties of Rubidium Tungsten Bronzes Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>WO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

Superconducting transition temperature (T _c) as a function of oxygen concentration for hexagonal rubidium tungsten bronzes Rb _x WO _y with 2.80 ≤ y ≤ 3.07 and x = 0.19, 0.23, and 0.27 has been systematically investigated. Three regions corresponding to T _c < 2 K (denoted as superconductivity suppressed region), T _c∼ 3 K (superconductivity uniform region) and T _c > 3 K (superconductivity enhanced region) were identified in T _c–y phase diagram for Rb_0.19WO _y and Rb_0.23WO _y . No superconductivity enhanced region was observed for Rb_0.27WO _y . The superconductivity suppressed region shifts toward higher oxygen content as rubidium concentration increases. The local ordering of the intercalated rubidium atoms might be responsible for the intriguing T _c–y phase diagram of Rb _x WO _y .