Sign Of Superconductivity Research Articles

A density-wave-like transition in the polycrystalline V3Sb2 sample with bilayer kagome lattice

Recently, transition-metal-based kagome metals have aroused much research interest as a novel platform to explore exotic topological quantum phenomena. Here we report on the synthesis, structure, and physical properties of a bilayer kagome lattice compound V3Sb2. The polycrystalline V3Sb2 samples were synthesized by conventional solid-state-reaction method in a sealed quartz tube at temperatures below 850 °C. Measurements of magnetic susceptibility and resistivity revealed consistently a density-wave-like transition at T dw ≈ 160 K with a large thermal hysteresis, even though some sample-dependent behaviors were observed presumably due to the different preparation conditions. Upon cooling through T dw, no strong anomaly in lattice parameters and no indication of symmetry lowering were detected in powder x-ray diffraction measurements. This transition can be suppressed completely by applying hydrostatic pressures of about 1.8 GPa, around which no sign of superconductivity was observed down to 1.5 K. Specific-heat measurements revealed a relatively large Sommerfeld coefficient γ = 18.5 mJ⋅mol–1⋅K–2, confirming the metallic ground state with moderate electronic correlations. Density functional theory calculations indicate that V3Sb2 shows a non-trivial topological crystalline property. Thus, our study makes V3Sb2 a new candidate of metallic kagome compound to study the interplay between density-wave-order, nontrivial band topology, and possible superconductivity.

Effect of Sr Doping on Structural and Transport Properties of Bi2Te3.

Search for doped superconducting topological insulators is of prime importance for new quantum technologies. We report on fabrication of Sr-doped BiTe single crystals. We found that Bridgman grown samples have p-type conductivity in the low 10 cm, high mobility of 4000 cmVs, crystal structure independent on nominal dopant content, and no signs of superconductivity. We also studied molecular beam epitaxy grown SrBiTe films on lattice matched (1 1 1) BaF polar surface. Contrary to the bulk crystals thin films have n-type conductivity. Carrier concentration, mobility and c-lattice constant demonstrate pronounced dependence on Sr concentration x. Variation of the parameters did not lead to superconductivity. We revealed, that transport and structural parameters are governed by Sr dopants incorporation in randomly inserted Bi bilayers into the parent matrix. Thus, our data shed light on the structural position of dopant in BiTe and should be helpful for further design of topological insulator-based superconductors.

Pressure-induced collapse of large-moment magnetic order and localized-to-itinerant electronic transition in the host-guest compound [Cs6Cl][Fe24Se26]

The magnetic properties of iron chalcogenides and pnictides have found much interest as magnetic fluctuations are suggested to drive the formation of Cooper pairs in iron-based superconductors. Here, we have studied the pressure dependence of the magnetic and electrical properties of the iron-selenide compound $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ by energy-domain synchrotron M\ossbauer spectroscopy up to 17 GPa and by resistivity measurements up to 45 GPa. Similar to the binary superconductor $\mathrm{F}{\mathrm{e}}_{1+x}\mathrm{Se}$, the host-guest-type crystal structure of $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ contains edge-sharing $\mathrm{FeS}{\mathrm{e}}_{4}$ units, but its ground state is antiferromagnetically ordered. A complex hyperfine pattern suggests a nontrivial spin structure like a spin spiral with large magnetic moments in the range 2 to $3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ at ambient pressure. High pressure drastically suppresses the N\'eel temperature from 220 K at ambient pressure to below 30 K at 12 GPa. Between 5 and 10 GPa the complex magnetic hyperfine pattern collapses and a low-moment magnetic ground state emerges at higher pressures. A concomitant decrease of the resistivity by several orders of magnitude indicates that the electronic system becomes more itinerant. Full metallization occurs near 25 GPa, but no sign of superconductivity down to 1.5 K was found at any pressure. It is suggested that the insulating ground state of $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ corresponds to an orbitally selective Mott phase, which due to an increased bandwidth, is transformed into a Hund's metal state under pressure.

Trilayer graphene shows signs of superconductivity

Trilayer graphene shows signs of superconductivity

From endohedral cluster superconductors to approximant phases: synthesis, crystal and electronic structure, and physical properties of Mo8Ga41-xZnx and Mo7Ga52-xZnx.

Using the crystal-growth joint flux technique based on the combination of two aliovalent low-melt metals, gallium and zinc, we adjust the gross valence electron count in the Mo-Ga-Zn system and produce the Mo8Ga41-xZnx and Mo7Ga52-xZnx intermetallic compounds. Gradual reduction in the valence electron count first leads to the Zn for Ga substitution in the Mo8Ga41 endohedral cluster superconductor, accompanied by the formation of Zn-containing clusters in the crystal structure and by the gradual suppression of superconductivity. Mo8Ga41-xZnx with x = 7.2(2) exhibits superconducting properties below TC = 4 K, whereas there is no superconducting transition at temperatures above 2 K for the limiting composition of x = 11.3(2). Further, the Mo7Ga52-xZnx phase is formed from the flux with a higher content of Zn. Mo7Ga52-xZnx crystallizes in the Mo7Sn12Zn40 structure type with a narrow homogeneity range and exhibits metallic behavior with no sign of superconductivity down to at least 1.8 K. Its experimental valence electron count of 2.9 e per atom is below that of endohedral gallium cluster superconductors. Electronic structures of Mo8Ga41-xZnx and Mo7Ga52-xZnx feature the opening of a pseudogap slightly below the Fermi level indicating the specific stability of these structure types at the valence electron count of 3.2 e per atom and 2.7 e per atom, respectively.

Magnetoresistance, Gating and Proximity Effects in Ultrathin NbN-Bi2Se3 Bilayers

Ultrathin Bi 2 Se 3 -NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with 10 nm thick Bi 2 Se 3 film which facilitates enhanced proximity coupling between them. Resistance versus temperature of the most resistive bilayers shows insulating behavior but with signs of superconductivity. We measured the magnetoresistance (MR) of these bilayers versus temperature with and without a magnetic field H normal to the wafer (MR = [R(H) − R(0)]/{[R(H) + R(0)]/2}), and under three electric gate-fields of 0 and ± 2 MV/cm. The MR results showed a complex set of gate sensitive peaks which extended up to about 30 K. The results are discussed in terms of vortex physics, and the origin of the different MR peaks is identified and attributed to flux-flow MR in the isolated NbN islands and the different proximity regions in the Bi 2 Se 3 cap-layer. The dominant MR peak was found to be consistent with enhanced proximity induced superconductivity in the topological edge currents regions. The high temperature MR data suggest a possible pseudogap phase or a highly extended fluctuation regime.

Synthesis, physical properties and band structure of non-magnetic Y3AlC

Y3AlC has been synthesized by arc melting and subsequent annealing. Rietveld analysis of the powder x-ray diffraction (XRD) data confirms cubic Pm-3m structure. Electrical resistivity (ρ) of Y3AlC exhibits metallic behaviour. No sign of superconductivity is observed down to the lowest measurement temperatures of 4.2K in ρ, and 2K in magnetic susceptibility (χ) and specific heat (Cp) measurements. The value of the electronic specific heat coefficient γ is 1.36mJ/K2mol from which the density of states (DOS) at the Fermi energy (EF) is obtained as 0.57 states/eV.unit cell. The value of Debye temperature θD is estimated to be 315K. Electronic band structure calculations of Y3AlC reveal a pseudo-gap in the DOS at EF leading to a small value of 0.5states/eVunit cell which matches quite well with that obtained from γ. Non-zero value of the DOS indicates metallic behaviour as confirmed by our ρ data. Covalent and ionic bonding seem to co-exist with metallic bonding in Y3AlC as indicated by van Arkel- Ketelaar triangle for Zintl-like systems.

Effect of Co and Ni substitution on the two magnetostructural phase transitions inFe1.12Te

Here we present the results of high-resolution x-ray diffraction experiments along with specific heat, resistivity, and magnetization measurements of chemically well-characterized ${\mathrm{Fe}}_{1.12\ensuremath{-}x}{M}_{x}\mathrm{Te}$ ($M=\mathrm{Co}$, Ni) samples. The motivation is to investigate how the two coupled magnetostructural phase transitions in the antiferromagnetic parent compound ${\mathrm{Fe}}_{1.12}\mathrm{Te}$ of chalcogenide superconductors can be tuned. While the two-step magnetostructural transition (tetragonal-to-orthorhombic followed by orthorhombic-to-monoclinic) persists in ${\mathrm{Fe}}_{1.10}{\mathrm{Co}}_{0.02}\mathrm{Te}$, only one, tetragonal-to-orthorhombic transition was observed in ${\mathrm{Fe}}_{1.10}{\mathrm{Ni}}_{0.02}\mathrm{Te}$. Upon increasing the Co and Ni substitution, the structural phase transitions and the long-range magnetic order are systematically suppressed without any sign of superconductivity. For high substitution levels $(x\ensuremath{\ge}0.05)$, a spin-glass-like behavior was observed and the low-temperature structure remains tetragonal. From our results, it can be inferred that the electron doping strongly suppresses the magnetostructural phase transitions.

Pressure-Induced Insulator to Metal Transition and Superconductivity of the Inert Gases

We present a comprehensive computational study of the transition from insulator to metal for the inert gases. The linearized augmented plane wave (LAPW) method, with the local density approximation (LDA) and generalized gradient approximation (GGA), was applied to study these transitions. Our calculations of the inert gases determined that the total energy of the fcc structure to be lower than the total energy in the bcc structure. This is in agreement with the experimentally verified results of the inert gases. We have confirmed that under high enough pressure, the inert gases, Ar, Kr, Xe, and Rn, transition from an insulator to a metal. Furthermore, using the Gaspari-Gyorffy-McMillan theory of superconductivity, we find significant signs of superconductivity for these materials under high pressure.

Gradual Localization of 5fStates in Orthorhombic UTX Ferromagnets:Polarized Neutron Diffraction Study of Ru Substituted UCoGe

We report on a microscopic study of the evolution of ferromagnetism in the Ru substituted ferromagnetic superconductor (FM SC) UCoGe crystallizing in the orthorhombic TiNiSi-type structure. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and characterized by magnetization, AC susceptibility, specific heat and electrical resistivity measurements. Both compounds have been found to order ferromagnetically below TC = 6.5 and 7.5 K, respectively, which is considerably higher than the TC = 3 K of the parent compound UCoGe. The higher values of TC are accompanied by enhanced values of the spontaneous moment μspont = 0.11 μB/f.u. and μspont = 0.21 μB/f.u., respectively in comparison to the tiny spontaneous moment of UCoGe (about 0.07 μB/f.u.). No sign of superconductivity was detected in either compound. The magnetic moments of the samples were investigated on the microscopic scale using polarized neutron diffraction (PND) and for UCo0.88Ru0.12Ge also ...

Low temperature resistivity, thermoelectricity, and power factor of Nb doped anatase TiO2

The resistivity of a very high quality anatase TiO2 doped with 6% of Nb was measured from 300 K down to 40 mK. No sign of superconductivity was detected. Instead, a minute quantity of cation vacancies resulted in a Kondo scattering. Measurements of thermo-electric power and resistivity were extended up to 600 K. The calculated power factor has a peak value of 14 μW/(K2cm) at 350 K, which is comparable to that of Bi2Te3 [Venkatasubramanian et al., Nature 413, 597 (2001)], the archetype thermolectrics. Taking the literature value for the thermal conductivity of Nb doped TiO2 thin films, the calculated figure of merit (ZT) is in the range of 0.1 above 300 K. This value is encouraging for further engineering of the material in order to reach ZT of 1 suitable for high temperature thermoelectrics.

Compositional disorder and transport peculiarities in the amorphous indium oxides

We present results of the disorder-induced metal-insulator transition (MIT) in three-dimensional amorphous indium-oxide films. The amorphous version studied here differs from the one reported by Shahar and Ovadyahu [Phys. Rev. B 46, 10917 (1992)] in that it has a much lower carrier concentration. As a measure of the static disorder we use the dimensionless parameter ${k}_{F}\ensuremath{\ell}$. Thermal annealing is employed as the experimental handle to tune the disorder. On the metallic side of the transition, the low temperature transport exhibits weak-localization and electron-electron correlation effects characteristic of disordered electronic systems. These include a fractional power-law conductivity versus temperature behavior anticipated to occur at the critical regime of the transition. The MIT occurs at a ${k}_{F}\ensuremath{\ell}\ensuremath{\approx}0.3$ for both versions of the amorphous material. However, in contrast with the results obtained on the electron-rich version of this system, no sign of superconductivity is seen down to $\ensuremath{\approx}$0.3 K even for the most metallic sample used in the current study. This demonstrates that using ${k}_{F}\ensuremath{\ell}$ as a disorder parameter for the superconductor-insulator transition (SIT) is an ill defined procedure. A microstructural study of the films, employing high resolution chemical analysis, gives evidence for spatial fluctuations of the stoichiometry. This brings to light that, while the films are amorphous and show excellent uniformity in transport measurements of macroscopic samples, they contain compositional fluctuations that extend over mesoscopic scales. These, in turn, reflect prominent variations of carrier concentrations thus introducing an unusual type of disorder. It is argued that this compositional disorder may be the reason for the apparent violation of the Ioffe-Regel criterion in the two versions of the amorphous indium oxide. However, more dramatic effects due to this disorder are expected when superconductivity sets in, which are in fact consistent with the prominent transport anomalies observed in the electron-rich version of indium oxide. The relevance of compositional disorder (or other agents that are effective in spatially modulating the BCS potential) to other systems near their SIT is discussed.

A Study of Ni-Substitution Effects on Heavy-Fermion CeCu2Si2–Similarities between Ni Substitution and High-Pressure Effects–

The effects of Ni substitution on Ce(Cu1-xNix)2Si2 have been studied by specific heat and electrical resistivity measurements. The specific heat measurement has revealed that the enhanced quantum fluctuations around an antiferromagnetic quantum critical point are markedly suppressed by Ni substitution, and that the Fermi liquid state recovers in the Nirich region (x > 0.12). The characteristic T-linear dependence of the resistivity has been observed at approximately x ~ 0.10 together with a sign of superconductivity. The variation of n in the form of rho - rho0 = aT^n against Tmax^1, at which the resistivity peaks, coincides with the case of high-pressure experiments on pure CeCu2Si2. The anomalous T-linear behavior appears to occur in the crossover region from the Kondo regime to the valence fluctuation regime rather than in the conventional antiferromagnetic quantum critical region.

Formation of superconductivity through interparticle interactions in ferrimagnetic-like Sn nanoparticle assemblies

We report on the observation of the development of superconductivity through interparticle interactions in 3, 5, 7, and 23 nm ferrimagnetic-like Sn nanoparticle assemblies. The Sn nanoparticles are fabricated using the gas condensation method. Each sample consists of a macroscopic amount of individual Sn nanoparticles without a capping molecule. Ferrimagnetism is found but no sign of superconductivity can be detected when the 3 nm particles are very loosely assembled. A reduction in the mean particle moment results when the packing fraction of the assembly is increased. Superconductivity occurs when a critical packing fraction is reached. Beyond this, the superconducting transition temperature T C continues to increase and noticeably exceeds that of the bulk T C. The enhancement of superconductivity by interparticle interactions has also been observed in 5, 7, and 23 nm particle assemblies, with the effect becoming less significant in larger particles. We attribute these observations to the transfer of electrons between the surface and the core regions of the nanoparticles triggered by finite size effects and interparticle interactions.

Electronic structure of superconducting Lu 2Ni 3Si 5 and its reference compound Y 2Ni 3Si 5 by ab initio calculations

Electronic structures of orthorhombic ternary nickel silicides: superconducting Lu 2Ni 3Si 5 and its non-superconducting counterpart, Y 2Ni 3Si 5, have been calculated employing the fully relativistic and full-potential local-orbital method within the density functional theory. Our investigations were focused particularly on the band structures and Fermi surfaces (FSs), being very similar for both ternaries. It appears that their FSs exist in four bands and contain electronlike and holelike three-dimensional sheets and small pockets, which suggests a presence of two- or even multi-band superconductivity (SC) in Lu 2Ni 3Si 5. The main difference between both systems is that only in Lu 2Ni 3Si 5 small electron FS pockets occur around the Γ point. It allows for arising BCS-like SC in this compound, as was deduced from previous heat-capacity measurements, while no sign of SC has been detected (at least down to 2 K) in Y 2Ni 3Si 5. In the latter system, a possible formation of a ferromagnetic ground state, which usually tends to destroy SC, has been excluded by our calculations.

Hole reduction and electron accumulation in YBa2Cu3Oythin films using an electrochemical technique: Evidence for ann-type metallic state

We demonstrated extreme hole reduction and electron accumulation in YBa${}_{2}$Cu${}_{3}$O${}_{y}$ films by an electrochemical technique. Following the transition from superconductor to insulator, we succeeded in doping electrons as evidenced by the decrease in resistivity and sign change of the Hall coefficient. Further doping resulted in metallic $n$-type YBa${}_{2}$Cu${}_{3}$O${}_{y}$ with a carrier density of \ensuremath{\sim}2.5 \ifmmode\times\else\texttimes\fi{} ${10}^{20}$ cm${}^{\ensuremath{-}3}$, but without any sign of superconductivity. The oxygen vacancies electrochemically induced in the CuO${}_{2}$ planes played a major role in the $p$-$n$ transition.

Single crystal growth of the metallic triangular-lattice antiferromagnet PdCrO 2

We report details of single crystal growth of the metallic triangular-lattice antiferromagnet PdCrO 2 consisting of layers of Pd triangles and Cr triangles stacking along the c-axis. We used the NaCl flux method and obtained the crystals with the size as large as 2×3.5×0.3 mm 3. We confirmed that single crystals have the delafossite structure with the R 3 ¯ m symmetry. The electrical resistivity along the c-axis and that in the ab plane exhibit metallic temperature dependence with the anisotropic ratio ρ c / ρ ab of over 300 at low temperatures. The residual resistivity of as small as ρ 0 , ab = 45 n Ω cm and the residual resistivity ratio of over 100 indicate high quality of the crystals investigated. Nevertheless, there is no sign of superconductivity down to 0.3 K. These crystals are useful for the investigation of anisotropic magnetic and transport properties including the unconventional anomalous Hall effect (AHE).

Superconductivity versus quantum criticality: what can we learn from heavyfermions?

Two quantum critical point (QCP) scenarios are being discussed for different classes ofantiferromagnetic (AF) heavy-fermion (HF) systems. In the itinerant one, where AF orderis of the spin-density wave (SDW) type, the heavy ‘composite’ charge carriers keep theirintegrity at the QCP. The second one implies a breakdown of the Kondo effectand a disintegration of the composite fermions at the AF QCP. We discuss twoisostructural compounds as exemplary materials for these two different scenarios:CeCu2Si2 exhibits a three-dimensional (3D) SDW QCP and superconductivity, presumably mediated bySDW fluctuations, as strongly suggested by recent inelastic neutron scattering experiments. InY bRh2Si2, the AF QCP is found to coincide with a Kondo-destroying one. However,in the latter compound these two QCPs can be detached by varying theaverage unit-cell volume, e.g. through the application of chemical pressure, asrealized by partial substitution of either Ir or Co for Rh. A comparison ofCeCu2Si2 andY bRh2Si2 indicates that the apparent differences in quantum critical behaviour go along with disparatebehaviour concerning the (non-) existence of superconductivity (SC). No sign of SC could be detectedin Y bRh2Si2 down to mK temperatures. A potential correlation between the specificnature of the QCP and the occurrence of SC, however, requires detailedstudies on further quantum critical HF superconductors, e.g. onβ-Y bAlB4,UBe13,CeCoIn5 andCeRhIn5.

From weak magnetism (spin density wave – SDW) to ferromagnetic state for SmFe1−xRuxAsO system with x = 0.0–0.50

The effect of ruthenium (Ru) doping in SmFe1−xRuxAsO (x=0.0–0.50) compound is studied here. The samples synthesized are mostly phase pure and not contaminated with various FeAs phases. It is found that with increasing Ru concentration x, the system turns from weak ferromagnetic (spin density wave (SDW)) to ferromagnetic state at x=0.20–0.30 without any sign of superconductivity.

Suppression of superconductivity in FeSe films under tensile strain

We have studied the effect of tensile strain on the superconductivity in FeSe films. 50, 100, and 200 nm FeSe films were grown on MgO, SrTiO3, and LaAlO3 substrates by using a pulsed laser deposition technique. X-ray diffraction analysis showed that the tetragonal phase is dominant in all of our FeSe films. The 50 nm FeSe films on MgO and SrTiO3 are under tensile strain, while the 50 nm FeSe film on LaAlO3 and the other thick FeSe films are unstrained. Superconducting transitions have been observed in unstrained FeSe films with Tonset≈8 K, which is close to the bulk value. However, no sign of superconductivity has been observed in FeSe films under tensile strain down to 5 K. This is evidence to show that tensile strain suppresses superconductivity in FeSe films.