Published Tunneling Results of Binnig et al Interpreted as Related to Surface Superconductivity in SrTiO<Subscript>3</Subscript>

In 1980, Binnig et al. reported tunneling measurements on Nb-doped SrTiO₃, and interpreted their results as indicating two-band superconductivity in the bulk of SrTiO₃. However, (1) effective masses determined from tunneling results in the normal state by Sroubek in 1969 and 1970 are much smaller than those determined by most other methods. The much smaller masses were attributed to properties of a surface layer by the present author in 1971. (2) The only other reports of two-band superconductivity in bulk SrTiO₃ can be used to infer much smaller values for the band separations than found by Binnig et al. In this paper, we give an alternative explanation of the results of Binnig et al. in terms of superconductivity in a surface layer. We obtain fair fits to the band gaps versus Fermi energy for the two bands in the three samples where two surface subbands are occupied and to the temperature dependence of the gaps in one crystal, using a model with three adjustable interaction parameters, an adjustable energy for the phonons which dominate the pairing, and an adjustable ratio of the mean-field T _c to the actual T _c . We show results for a combined fit to the low-temperature band gaps and to the T-dependence in one crystal. The phonon energy which gives the best fit is 21 meV. This is probably an appropriate average over the three longitudinal polar modes and acoustic modes in the material. A large value of about two is found for the ratio T _{c m f} /T _c , and we conjecture that this arises because a band with a small Fermi energy, not seen in the tunneling results, plays a part in increasing T _{c m f} /T _c .     

Superconductivity in Ca<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>FBiSe<Subscript>2</Subscript>

Through the measurement of resistivity, magnetic susceptibility, and Hall effect, we discovered a novel BiSe₂-based superconductor Ca_0.5La_0.5FBiSe₂ with T _c of 3.9 K. A strong diamagnetic signal below T _c in susceptibility χ(T) is observed indicating the bulk superconductivity. The negative Hall coefficient throughout the whole temperature regime implies the dominant electron-type carriers in the sample. Different to most of BiS₂-based compounds where superconductivity develops from a semiconducting-like normal state, its resistivity in the present compound exhibits a metallic behavior down to T _c . Together with the enhanced T _c , the metallic character of the normal state implies that the electronic structure of Ca_0.5La_0.5FBiSe₂ may be different to those in the other BiS₂-based compounds.     

Thermodynamic Properties of Superconductor with Competing Spin-Density Wave and Charge-Density Wave

The mean field Hamiltonian describing the coexistence of the spin-density wave (SDW) and charge-density wave (CDW) for high- T _c superconductor in the underdoped region before the onset of the superconductivity was studied. From self-consistent equations for SDW and CDW gaps derived from Green’s function method, the theromodynamic properties of the superconductor were derived. Our results show the competition between the SDW and CDW state on thermodynamic properties of superconductor that deviate gap-to- T _c ratio, critical temperature, and the specific heat jump from the BCS universal value.     

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.     

Structural and Superconductivity Properties of BaFe<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>2</Subscript>

Extraordinary magnetic behaviors, resistivity properties, and lattice parameters of the main sample BaFe₂As₂ and BaFe_2?x Pt _x As₂ in the variation of x from 0 to 0.4 with the step of 0.1 were investigated. The bulk materials have been prepared by the solid-state reaction method and sealed into a quartz tube. The crystal structure of all samples exhibited the ThCr₂Si₂-type crystal structure which is in harmony with earlier studies in the literature. The superconducting states with magnetization measurements have been detailed in the wide temperature range 5–170 K, up to a field of 20 Oe. Increasing Pt and decreasing Fe elements in the BaFe_2?x Pt _x As₂ compound deteriorated superconductivity. Using magnetization measurement data, we present the variation of superconducting critical temperature (T _c) correlating with a Pt dopant rate from x = 0 to x = 0.4. The dopant rate of x = 0.3 exhibited the limit rate for maximum T _c; deterioration of superconductivity was revealed with a dopant rate of more than x = 0.3. This should be explained by varying T _c related to a lattice shrinking and pressure effect (geometric factor).     

Anomalous Impact of Hydrostatic Pressure on Superconductivity of Polycrystalline LaO<Subscript>0.5</Subscript>F<Subscript>0.5</Subscript>BiSe<Subscript>2</Subscript>

We report bulk superconductivity at 2.5 K in LaO_0.5F_0.5BiSe₂ compound through the DC magnetic susceptibility and electrical resistivity measurements. The synthesized LaO_0.5F_0.5BiSe₂ compound is crystallized in tetragonal structure with space group P4/nmm and Reitveld refined lattice parameters are a = 4.15(1) Å and c = 14.02(2) Å. The lower critical field of H _c1 = 40 Oe, at temperature 2 K is estimated through the low field magnetization measurements. The LaO_0.5F_0.5BiSe₂ compound showed metallic normal state electrical resistivity with residual resistivity value of 1.35 m Ω cm. The compound is a type-II superconductor, and the estimated H _c2(0) value obtained by WHH formula is above 20 kOe for 90 % ρ _n criteria. The superconducting transition temperature decreases with applied pressure till around 1.68 GPa and with further higher pressures a high- T _c phase emerges with possible onset T _c of above 5 K for 2.5 GPa.     

Superconducting Properties of NdO<Subscript>0.5</Subscript><Emphasis Type="Bold">F</Emphasis><Subscript>0.5</Subscript>BiS<Subscript>2</Subscript> Single Crystals

We report on superconducting properties of high-quality single crystals of F-substituted NdOBiS₂ using low-temperature magnetization and transport measurements. Using the mixture of CsCl and KCl as the flux, we have synthesized our single crystals. This compound exhibits bulk superconductivity with a transition temperature of about T _c～4.6 K. The critical current density J _c as a function of temperature has been derived and decreases with the increasing temperature. We construct the phase diagram H _c2(T). The zero-temperature value for \(H_{\mathrm {c2}}^{B\parallel c}\) for value for \(T_{c}^{90~\%}\) and \(T_{c}^{0~\%}\) is estimated to be approximately 2.17 and 1.72 T respectively by using Werthamer-Helfand-Hohenberg model.     

The H-T and P-T Phase Diagram of the Superconducting Phase in Pd:Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

We study the magnetic field vs. temperature (H – T) and pressure vs. temperature (P – T) phase diagrams of the T _c ≈ 5.5 K superconducting phase in Pd _x Bi₂Te₃ (x ≈ 1) using electrical resistivity versus temperature measurements at various applied magnetic fields (H) and magnetic susceptibility versus temperature measurements at various applied magnetic fields (H) and pressure (P). The H – T phase diagram has an initial upward curvature as observed in some unconventional superconductors. The critical field extrapolated to T = 0 K is H _c (0) ≈ 6–10 kOe. The T _c is suppressed approximately linearly with pressure at a rate d T _c /d P ≈ ?0.28 K/GPa.     

X-ray Photoelectron Spectroscopy,Magnetotransport and Magnetisation Study of Nb<Subscript>2</Subscript>PdS<Subscript>5</Subscript> Superconductor

In the present report, we investigate various properties of the Nb₂PdS₅ superconductor. Scanning electron microscopy displayed slabs like laminar growth of Nb₂PdS₅ while X-ray photoelectron spectroscopy exhibited the hybridisation of sulphur (2p) with both palladium (3d) and niobium (3d). High-field (140 kOe) magneto-transport measurements revealed that superconductivity (\(T_{c}^{\text {onset}} =?7\) K and T cρ=?0 = 6.2 K) of the studied Nb₂PdS₅ material is quite robust against magnetic field with the upper critical field (H _c2) outside the Pauli paramagnetic limit. Thermally activated flux flow (TAFF) of the compound showed that resistivity curves follow Arrhenius behaviour. The activation energy for Nb₂PdS₅ is found to decrease from 15.15 meV at 10 kOe to 2.35 meV at 140 kOe. Seemingly, the single vortex pinning is dominant in low-field regions, while collective pinning is dominant in high-field region. The temperature dependence of AC susceptibility confirmed the T _c at 6 K, further varying amplitude and frequency, showed well-coupled granular nature of superconductivity. The lower critical field (H _c1) is extracted from DC magnetisation measurements at various T below T _c. It is found that H _c1(T) of Nb₂PdS₅ material seemingly follows the multiband nature of superconductivity.     

Comparative Experimental and Density Functional Theory (<Emphasis Type="Italic">DFT</Emphasis>) Study of the Physical Properties of MgB<Subscript>2</Subscript> and AlB<Subscript>2</Subscript>

In the present study, we report an intercomparison of various physical and electronic properties of MgB₂ and AlB₂. In particular, the results of phase formation, resistivity ρ(T), thermoelectric power S(T), magnetization M(T), heat capacity (C _P ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 Å, and c=3.524 Å of MgB₂ shrinks in c-direction for AlB₂ with a=3.006 Å, and c=3.254 Å. The resistivity ρ(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB₂. Superconductivity is not observed for AlB₂. Interestingly, the sign of S(T) is +ve for MgB₂ the same is ?ve for AlB₂. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB₂ to Debye–Einstein model and estimated the value of Debye temperature (Θ _D) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, the electronic density of states (DOS) at Fermi level N(E _F) is calculated. From the ratio of experimental N(E _F) and the one being calculated from DFT, we obtained value of λ to be 1.84, thus placing MgB₂ in the strong coupling BCS category. The electronic specific heat of MgB₂ is also fitted below T _c using α-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB₂ is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB₂, and hence it is not superconducting. DFT calculations demonstrated that for MgB₂, the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB₂ is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type, and hence obviously the AlB₂ is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB₂ and AlB₂.     

The influence of the diffusion cooling on the noise band of the superconductor NbN hot-electron bolometer operating in the terahertz range

Results of an experimental study of the noise temperature (T _n ) and noise bandwidth (NBW) of the superconductor NbN hot-electron bolometer (HEB) mixer as a function of its temperature (T _b ) are presented. It was determined that the NBW of the mixer is significantly wider at temperatures close to the critical ones (T _c ) than are values measured at 4.2 K. The NBW of the mixer measured at the heterodyne frequency of 2.5 THz at temperature T _b close to T _c was ~13 GHz, as compared with 6 GHz at Tb = 4.2 K. This experiment clearly demonstrates the limitation of the thermal flow from the NbN bridge at T _b ? T _c for mixers manufactured by the in situ technique. This limitation is close in its nature to the Andreev reflection on the superconductor/ metal boundary. In this case, the noise temperature of the studied mixer increased from 1100 to 3800 K.     

Superconducting Ti/TiN Thin Films for mm-Wave Absorption

Polarization-sensitive detectors at 120–500 GHz are required for the observation of the cosmic microwave background radiation. In this paper, superconducting thin films based on Ti/TiN bilayers are developed to be integrated as electromagnetic wave absorbers in suspended cooled silicon bolometers. The critical temperature (T_c) is tuned in the range of 600–800 mK through the superconductivity proximity effect between Ti and TiN to optimize the absorption of the incident power while minimizing the heat capacity of the system at low temperature. Ti/TiN bilayer samples are fabricated on silicon with two different thicknesses (100/5 and 300/5 nm). Electrical characterizations at low temperature have been performed and revealed the effect of thermal annealing (20–250 °C) on residual stress, T_c, critical magnetic field (H_c) and resistance above T_c. A physical characterization by X-ray photoelectron spectroscopy provides evidences of oxidized states which may explain these effects.     

Peculiarities of phase transitions in polytypes of monoclinic TlInS<Subscript>2</Subscript>

The existence of two polytypes at room temperatures, C-TlInS₂ and 2C-TlInS₂, with different monoclinic cell parameters, c and 2c, has been revealed. Significant differences in crystal lattice dynamics of these polytypes have been found. In particular, two phase transitions (PTs) have been detected for the polytype C-TlInS₂ as the temperature varies: a second-order PT from paraphase to incommensurate phase at T _i = 215 K and a first-order ferroelectric PT accompanied by a quadrupling of the parameter c at T _c = 197 K. No PT accompanied by an increase in unit cell parameter c has been found in the polytype 2C-TlInS₂, but a global temperature hysteresis characteristic of crystals with an incommensurately modulated structure has been detected at T = 180–230 K.     

In-field Conductivity Fluctuations in Ba<Subscript>0.72</Subscript>K<Subscript>0.28</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Fluctuations in the conductivity of Ba_0.72K_0.28Fe₂As₂ single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki?Thompson and Aslamakov?Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T _c. 3D fluctuations in superconductivity are realized near T _c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T ₀, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε ₀), as functions of magnetic field were used to determine the coherence length and the lifetime, τ _φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba_0.72K_0.28Fe₂As₂ single crystal gives a value of 3.76 × 10^??12 s for the τ _φ in the absence of magnetic field and it decreases to 2.4 × 10^??12 s in magnetic field of 13 T.     

High-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Hydrides: Interplay of Optical and Acoustic Modes and Comments Regarding the Upper Limit of <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript>

The phonon spectrum of many superconducting compounds and, especially, high-T_c hydrides, is broad and rather complicated, because of the presence of high-frequency optical modes. In order to analyze an interplay of optical and acoustic phonon branches, it is convenient to introduce two coupling constants, \(\lambda _{\text {opt.}}\) and \(\lambda _{\text {ac.}}\), along with characteristic frequencies. The correlation between the value of T_c and this interplay is demonstrated for the family of tantal hydrides (TaH₂/TaH₄/TaH₆). The problem of the upper limit of T_c is discussed. The phenomenon of room temperature superconductivity can be provided by the electron-phonon interaction and is described by the strong coupling theory.     

Appearance of Superconductivity at 15.9 K in Layered V<Subscript>2</Subscript>AlN

A bulk polycrystalline sample with the nominal compositions represented by V₂AlN_1?δ was synthesized by a two-step solid-state reaction. The structural characterization has been done via X-ray diffraction, followed by Rietveld refinements, which revealed that the layered V₂AlN is crystalized in cubic Fm3m space group with lattice parameters a = b = c = 6.127 Å. Both DC resistivity and magnetization measurements confirmed that V₂AlN is a bulk superconductor with superconducting transition temperature (T _c ) of 15.9 K.     

Coexistence of Superconductivity and Spin Density Wave (SDW) in Ferropnictide Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

This work focuses on the theoretical investigation of the coexistence of superconductivity and spin density wave (SDW) in ferropnictide Ba_1?x K _x Fe₂As₂. By developing a model Hamiltonian for the system and by using quantum field theory Green’s function formalism, we have obtained mathematical expressions for superconducting transition temperature (T _C), spin density wave transition temperature (T _sdw), superconductivity order parameter (Δ_Sc), and spin density wave order parameter (Δ_sdw). By employing the experimental and theoretical values of the parameters in the obtained expressions, phase diagrams of superconducting transition temperature (T _C) versus superconducting order parameter (Δ_Sc) and spin density wave transition temperature (T _sdw), versus spin density wave order parameter (Δ_sdw) have been plotted. By combining the two phase diagrams, we have demonstrated the possible coexistence of superconductivity and spin density wave (SDW) in ferropnictide Ba_1?x K _x Fe₂As₂.     

Stoichiometry Analysis and Normal-State Properties of SmBa2Cu3−x Ru x O7−δ Superconducting Phase

The electrical resistivity of normal and superconducting states for SmBa₂Cu_3?x Ru _x O_7?δ (Sm-123) phase with 0.00 ≤ x ≤ 0.50, prepared by the conventional solid-state reaction technique, was studied. X-ray powder diffraction (XRD), scanning electron microscope (SEM), particle-induced X-ray emission (PIXE), Rutherford backscattering spectroscopy (RBS) and electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution in Sm-123 phase. Both the phase formation and superconducting transition temperature T _c enhance up to x = 0.05. For x > 0.05, suppression of both the phase formation and T _c is observed and the superconductivity is completely destroyed around x = 0.50. The normal-state electrical resistivity was analyzed by the two- and three-dimensional variable range hopping (2D-VRH and 3D-VRH) and Coulomb gap CG. The dominant mechanism for Sm-123 phase is CG with x ≤ 0.20 while is 3D-VRH for x ≥ 0.30.