Pressure Dependence of T<Emphasis Type="Italic">c</Emphasis> in High Temperature Superconductors: Role of Interlayer Interactions

We have studied the role of interlayer interactions (W) in the pressure dependence of T _c of layered superconductors. The expressions for dT _c /dP are obtained by including the effects of layered structure within the framework of two different proposed models, namely the negative-U Hubbard model and the Hirsch model. We observe that the inclusion of interlayer interaction provides better explanation of pressure dependence of T _c . Our numerical results show that the systems having one CuO₂ layer per unit cell may be well described by small values of W while the larger values of W accounts for the systems having two or more superconducting layers in a unit cell. The calculated values of dT _c /dP vs. W are found to be in good agreement with those of experimental results obtained for various high T _c superconductors of cuprate family.     

Universal Correlations of T<Emphasis Type="Italic">c</Emphasis> Ion Mass Scaling with Distance

We present a short overview of the critical temperature correlations with different parameters for low and high T _c superconductors. The overview underlines the importance of crystalline structure in determining the critical temperature value. The structure of copper-based superconductors is studied in comparison with low-temperature superconductors and a new correlation of the transition temperature is found. Namely, the layered structure, more exactly a certain distribution of the mass of the ions along the c-axis is the key factor in determining the “intrinsic” critical temperature, parameter introduced by Kresin et al. [4].     

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.     

Skyrmion mechanism of hole pairing in the high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> cuprates

A topological mechanism of hole localization as two skyrmions in the CuO₂ layers of high-T _c superconductors is suggested on the basis of a nonlinear σ model.     

Methods of Extracting Current Density Dependence of the Effective Activation Energy <Emphasis Type="Italic">U</Emphasis><Subscript>eff</Subscript>(J) in High <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Superconductors

We have reviewed the methods of extracting current density dependence of the effective activation energy U_eff(J) from experimental data, including transport measurements and magnetic relaxations. Then we applied the method proposed by Maley etc. on our single-phase HgBaCaCuO-1223 sample to obtain the effective activation energy. The effective activation energy U_eff(J, H = 1~T) is extracted from the magnetization relaxation data. On the other hand, U_eff(J) can be theoretically estimated for the model of a sinusoidal washboard potential in superconductors. By comparing the two results we believe that the single curve obtained in the former way can be seen as real current density dependence of effective activation energy U_eff(J). In addition, we have analyzed the reasons why the magnetic decay data at various temperatures can be scaled onto a single curve. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J are thought as two important factors for this. In the temperature close to zero and near T_c, thermally assisted flux motion would no longer valid since other processes predominate.     

Sulfur Hydrides: Phase Diagram and the Transition into the Record-High <Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Bold">c</Emphasis></Subscript> State

Remarkable feature of the phase diagram of sulfur hydrides, the record-high T _c superconductors, is a sharp increase near P ≈ 150 GPa from T _c ≈ 120 to ≈ 200 K. This increase is a signature of the structural transition. The present study is concerned with the nature of this phase transition. One can demonstrate that the symmetry analysis along with an analysis of the impact of lattice deformations lead to the conclusion that we are dealing with the first-order transition. Such a transition is manifested in an abrupt appearance of small pockets on the Fermi surface and, correspondingly, the two-gap energy spectrum.     

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.     

Spin Dynamics in the Bi-Directional Charge Density Wave State of High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductors

In this paper, we investigate the dynamical spin susceptibility in the bi-directional charge density wave (BCDW) state by adopting a random-phase approximation. In the BCDW state, we find that no spin resonance exists and only a broad commensurate peak appears for the frequency dependence of the dynamical spin susceptibility at Q = (π,π), though a low-energy spin gap feature can also be found as in the superconducting state. While the “hourglass” type of the dispersion for the BCDW state bears some similarities with that in the superconducting state, the momentum distribution of Im χ ^+?(Q,ω) is just the opposite with the incommensurate peaks lying along the diagonal direction for the energy below ω _c and along the axial direction above ω _c . In the coexistence of SC and BCDW, the frequency dependence of the dynamical spin susceptibility at Q = (π,π) generally shows the two-peak structure, reflecting two energy scales of the spin excitations. These unique features may serve as signatures to verify whether or not the BCDW state is responsible for the formation of the Fermi arcs.     

Modeling of the Complex Doping Dependence of dT<Emphasis Type="Italic">c</Emphasis>/dP of YBa2Cu3O6+<Emphasis Type="Italic">x</Emphasis>

The pressure-induced intrinsic effects in the CuO₂ plane within the van Hove singularity (VHS) scenario is combined with the modified two-dimensional (2D) lattice gas phenomenology for the basal plane to model the complex structure of the hole-doping dependence of the pressure derivatives of T _c of YBa₂Cu₃O_6+x. The experimentally observed structure is found to be reproduced reasonably satisfactorily in the present formalism. This shows that the pressure-induced changes in the CuO₂ plane and the oxygen ordering in the basal plane both play important roles in explaining the doping dependent pressure derivative of T _c .     

The Boundary Effect on the Energy Gap of High <Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductors

Spatial dependence of the pairing potential across the thickness of the superconducting CuO₂ planes in high T _c superconductors (HTSC) copper oxides is found by using the Ginzburg-Landau-Gor’kov (GLG) theory. The potential turned out to be significantly suppressed due to an effect of non-superconducting layers, which separate the CuO₂ planes. The effect leads also to the reduction of the critical temperature of these superconductors. The temperature dependence of the effective energy gap was calculated in this work.     

Performance enhancement in high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> ceramics through melting by Joule heating,infrared lamps,and laser radiation

We have analyzed experimental data on the effect of short-term melting followed by recrystallization on the microstructure and critical current density of YBa₂Cu₃O_{7 ? x} , Bi₂Sr₂CaCu₂O_{8 ? x} , and Bi₂Sr₂Ca₂Cu₃O_{10 ? x} high-T _c ceramics. The ceramics were melted using different heat sources: infrared lamps, laser radiation, and electric current. A significant increase in the critical current density of Bi₂Sr₂Ca₂Cu₃O_{10 ? x} ceramics (by a factor of 40 at 20 K and by a factor of 8 at 77 K) was achieved using cw CO₂ laser irradiation. Melting TiC-doped (0.1%) Bi₂Sr₂Ca₂Cu₃O_{10 ? x} ceramics with a CO₂ laser, followed by annealing, insured an even larger increase in critical current density: by a factor of 35 at 77 K. We have calculated the thickness of the molten layer produced by laser heating of high-T _c ceramics.     

On the Role of Fermi Energy in Determining Properties of Superconductors: a Detailed Comparative Study of Two Elemental Superconductors (Sn and Pb), a Non-cuprate (MgB<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>) and Three Cuprates (YBCO,Bi-2212 and Tl-2212)

Fermi energies (E _Fs) of high- T _c superconductors (SCs) have of late been evincing considerable interest because they are believed to be the cause of their high T _cs and gap structures. Since Bardeen-Cooper-Schrieffer (BCS) equations for elemental and generalized-BCS equations for non-elemental SCs are derived under the blanket of the assumption E _F/ k θ > > 1 (k = Boltzmann constant, θ = Debye temperature), they cannot shed light on the E _Fs of these SCs. This fact leads us to address the gaps (Δ₀s) and T _cs of both types of SCs via recently derived equations which incorporate E _F as a variable. For the specification of the E _F of any SC, we now need another of its properties. Choosing j ₀, the critical current density of the SC at T = 0, and following an idea due to Pines, we present for both types of SCs new equations for j ₀ that depend solely on the following properties of the SC: E _F, θ, gram-atomic volume, electronic specific heat constant and a dimensionless construct \(y=k\theta \sqrt {2m\ast } \text {/}P_{\text {0}} \sqrt {E_{\mathrm {F}} } \text {,}\) where m* is the effective mass of superconducting electrons and P ₀ their critical momentum. Appeal to the experimental values of Δ₀, T _c and j ₀ of any SC then not only leads to values of E _F, m* and P ₀ but also provides plausible clues about how its j ₀—and therefore T _c—may be increased.     

The Anomalous Unloaded Quality Factor Behavior of High T<Emphasis Type="Italic">c</Emphasis> Superconducting Microstrip Resonator in Weak dc Magnetic Field

This paper reports the anomalous behavior of unloaded quality factor (Q _u ) of high T _c superconducting (HTSC) microstrip resonator in weak dc magnetic field (B_dc). The Q _u behavior increases with the elevated applied B_dc, but beyond the certain B_dc value, decreases linearly with B_dc. We speculate that this behavior may be caused by edge effect. The behavior is modeled both by the coupled-grain model accounting for a distribution of the grain dimension and Coffey-Clem model.     

Novel Solid-State Growth of <Emphasis Type="Italic">p</Emphasis>-Terphenyl: the Parent High-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Organic Superconductor (HTOS)

We report an easy and versatile route for the synthesis of the parent phase of the newest superconducting wonder material, i.e., p-terphenyl. Doped p-terphenyl has recently shown superconductivity with transition temperature as high as 120 K. For crystal growth, the commercially available p-terphenyl powder is pelletized, encapsulated in an evacuated (10^?4 Torr) quartz tube and subjected to high-temperature (260 ^°C) melt followed by slow cooling at 5 ^°C/h. A simple temperature-controlled heating furnace is used during the process. The obtained crystal is one piece, shiny, and plate like. Single crystal surface XRD (X-ray diffraction) showed unidirectional (00l) lines, indicating that the crystal is grown along the c-direction. Powder XRD of the specimen showed that as grown p-terphenyl is crystallized in monoclinic structure with space group P2 ₁/a space group, having lattice parameters a = 7.672 (2) Å, b = 5.772 (5) Å, and c = 13.526(3) Å and β = 91.484 (3)^°. Scanning electron microscopy (SEM) pictures of the crystal showed clear layered slab-like growth without any visible contamination from oxygen. Characteristic reported Raman active modes related to C–C–C bending, C–H bending, C–C stretching, and C–H stretching vibrations are seen clearly for the studied p-terphenyl crystal. The physical properties of the crystal are yet underway. The short letter reports an easy and versatile crystal growth method for obtaining quality p-terphenyl. The same growth method may probably be applied to doped p-terphenyl and to subsequently achieve superconductivity to the tune of as high 120 K for the newest superconductivity wonder, i.e., high- T _c organic superconductor (HTOS).     

Role of Interlayer and Intersite Interactions in Superconducting State in High-T<Emphasis Type="Italic">c</Emphasis> Layered Cuprates

We studied the role of interlayer and intersite interactions on transition temperature and specific heat of layered high-T _c cuprates. We used double-time Green's function technique in the spirit of mean field approximation in order to obtain the expressions for hole density, transition temperature, and specific heat. These expressions are found to be dependent on the carrier concentration and intersite and interlayer interactions. The numerical analysis shows that the effect of intersite interaction on transition temperature and specific heat is qualitatively similar to that of interlayer interactions and provides favorable conditions to establish long-range order in the superconducting state.     

The Temperature Evolution of the Inhomogeneous Magnetic Response of Cuprate Superconductors Above <Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>

Many different experiments and probes have displayed some form of anomalous behavior that may be related to charge inhomogeneity in different families of cuprate superconductors. In some materials, it appears to be associated with charge density waves and in others as local static domains of varying densities. The doping and temperature evolution of such charge instability is a matter of current intense research. We present here a model based on a phase separation transition to the temperature evolution of transverse field muon spin relaxation (TF- μSR) magnetic inhomogeneous response of cuprates above T _c recently measured.     

Progress in Critical Current Density (<Emphasis Type="Italic">J</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>) in Sintered MgB<Subscript>2</Subscript> Bulks

The present paper focuses on methods of further improving the flux pinning and critical current density of disk-shaped MgB₂ bulk superconductors by adding excess Mg metal in combination with an optimum silver content and optimized processing conditions. Bulk MgB₂ samples were produced by in situ solid-state reaction in Ar gas ambient using high purity commercial powders of Mg metal and 1.5 wt% carbon-coated amorphous B powders mixed in a fixed ratio of Mg/B = 1.1:2. Further, 4 wt% silver was added to improve flux pinning as well as mechanical performance of the bulk MgB₂ material. The magnetization measurements confirmed a sharp superconducting transition with T_c,onset at around 37 K, which is only by 1 K lower than in bulk MgB₂ material produced without carbon-coated amorphous boron. The critical current density (J_c) values significantly improved in the MgB₂ material with 4 wt% of silver and 1.5 wt% of carbon-coated amorphous boron, sintered at 775 ^°C for 3 h. At 20 K, this sample showed J_c at around 500 and 350 kA/cm² in the self-field and 1 T, respectively, which makes it suitable for several industrial applications.     

Evolution of Phononic Band Gaps in One-Dimensional Phononic Crystals that Incorporate High-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Superconductor and Magnetostrictive Materials

In this work, we calculate the reflectance of one-dimensional phononic crystals (1D PnCs) using the transfer matrix method. We present numerical results for two different PnC structures, the first one, PnCs1, contains high- T _c superconducting compound (Bi-2223) and the second, PnCs2, contains a giant magnetostrictive material (Terfenol-D). Magnetostriction is a property of ferromagnetic materials that causes them to change their shape/dimensions when subjected to external magnetic field. PnC studies that dealt with such materials are few. In this study, we focus on discussing the effects of the temperature and the magnetic field on the phononic gaps of these PnCs. For PnCs1, numerical results show that local resonant modes of elastic waves with brilliant sharpness can be realized. In addition, increasing the temperature leads to a decrease in the gap width which can be controlled by the magnetic field due to the effect of the magnetic field on the velocity of waves in the high- T _c superconducting compound, the magnetic field effectively can widen the gap. For PnCs2, numerical results show that the gap width increases by increasing the magnetic field because the magnetostrictive material directly expanded in the presence of the magnetic field.     

Noise and structural characteristics of high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductor films and the numerical simulation of bolometers based on such films

The structure, electrical properties, and noise characteristics of the epitaxial YBa₂Cu₃O_7?x high-T _c superconductor films grown by laser ablation and magnetron sputtering on CeO₂/Al₂O₃ and LaAlO₃ substrates have been studied. Experimental values of the film parameters (effective noise voltage V _n, bolometer resistance R _b, working temperature T _b, etc.) were used to calculate the main characteristics of bolometers based on such films, intended for the absolute radiometry of synchrotron radiation in the 150–3000 eV range. Numerical estimates of the equivalent noise power, NEP _Σ = 8 × 10^?11?1.3 × 10^?10 W/Hz^0.5, show that the proposed films can be used for the creation of a high-precision absolute radiometer capable of detecting soft x-ray (synchrotron) radiation in a broad frequency range with a power of about 1 μW at an error not exceeding 1%.     

Non-Resonant Microwave Absorption in SmFeAs<Emphasis Type="Bold">O</Emphasis><Subscript>0.80</Subscript><Emphasis Type="Bold">F</Emphasis><Subscript>0.20</Subscript>: Line Shape and Structure Evolution with Temperature

‘Non-resonant Microwave Absorption’ (NRMA) or the ‘Low field microwave absorption’ (LFMA) measurements on high-quality polycrystalline SmFeAsO_0.80F_0.20 superconducting sample were carried as functions of temperature and microwave power. The LFMA line shape is complex with two peaks namely; broad peak 1 and narrow peak 2 akin to one reported in SmFeAsO_0.88F_0.12 as reported by Onyancha et al (Supercond. Nov. Magn. 28, 2927–2934, 2015). This unquestionably illustrates that these peaks are a common feature in F-doped SmFeAsO. The LFMA signal as a function of temperature reveals that T _c ? T _? = 1K in SmFeAsO_0.80F_0.20 compared to 4 K in SmFeAsO_0.88F_0.12 (T _? is the characteristic temperature at which the narrow peak appears as we cool down the sample below T _c); hence inferring that the narrow peak is fluorine doping dependent. Furthermore, LFMA signal evolution with microwave power does not show phase reversal (anomalous absorption) at 2.227 mW which is a stark contrast to what was observed in SmFeAsO_0.88F_0.12 as reported by Onyancha et al (Physica C: Supercond. Appl. 533:49–52, 2017). The absence of phase reversal within measured microwave power indicates presence of hysteretic Josephson junction. These findings establish few non-superconducting inclusions in SmFeAsO_0.80F_0.20 system.