Model High-temperature Superconductor Research Articles

Theoretical Investigation of the Superconducting and Thermodynamic Properties of Two-Band Model High-Temperature Iron-Based Superconductor Ba1–xNaxFe2As2

This work presents the theoretical investigation of the superconducting and thermodynamic properties of the two-band model high-temperature iron-based superconductor Ba1-xNaxFe2As2. By developing a model Hamiltonian and by employing the well-knownvdouble-time temperature-dependent Green’s function formalism, we have computed the superconducting orderbparameters for the electron and hole intra- and inter-band transitions, superconducting transition temperature, densitiesbof states, and condensation energies. Furthermore, the electronic specific heat and the entropy for electron and hole intra-band transitions have been determined. By using appropriate experimental data and some credible approximations of the parameters in the computed expressions, we have found the phase diagrams of superconducting order parameters versus the temperature, superconducting critical temperature versus the inter-band interaction potential, temperature dependences of the electronic specific heat and entropy for electron and hole intra-band transitions, and densities of states for the electron and hole intra-band transitions as functions of the excitation energy at different values of the temperature. Finally, the phase diagrams of the condensation energy versus the temperature, inter-band pairing potential at T = 0 K versus the condensation energy, and condensation energy versus the superconducting transition temperature (TC) have been drawn. In some of the phase diagrams, the comparison between theoretical and experimental values has been made. The results are in a good agreement with previous findings.

Mechanism of the Generation of the Critical Current in High-Temperature Superconductors with Through Microdefects

The critical current of a model high-temperature superconductor (HTSC) with defects in the form of through holes (antidots) with characteristic sizes on the order of the magnetic-field penetration depth (or larger) is calculated. To this end, subprocesses equivalent to the magnetic-flux trapping by a hole and creation of a vortex near the hole edge are introduced into the model of a layered HTSC. It is shown that account for these subprocesses yields a physical mechanism, which makes it possible to describe correctly the nonmonotonic dependence of the critical current on the antidot characteristic size, which is similar to the experimental one. The calculations are carried out for a pure superconductor and a superconductor with nanoscale pinning centers. It is shown that the presence of nanoscale pinning centers along with antidots does not make qualitative changes in the relationship between the antidot radius and magnetic-flux pinning character and the critical-current behavior in an HTSC.

Механизм формирования критического тока высокотемпературных сверхпроводников, содержащих сквозные микроскопические дефекты

The critical current of a model high-temperature superconductor (HTSC) with defects in the form of through holes (antidots) with a characteristic size greater than or of the order of the penetration depth of the magnetic field is calculated. To do this, the subprocesses equivalent to the trapping of the magnetic flux by the hole and the creation of a vortex at the edge of the hole are introduced into the model of layered HTSC. It is shown that accounting for these subprocesses leads to the appearance of a physical mechanism that allows us to correctly describe the non-monotonic dependence of the critical current on the characteristic size of the antidot, similar to that observed in the experiment. Calculations were performed for a pure superconductor and a superconductor containing nanoscale pinning centers. It is shown that the presence of nanoscale pinning centers along with antipoints does not change the qualitative picture of the influence of the antidot radius on the pinning character of the magnetic flux and the behavior of the critical current in the HTSC.

Doping-Dependent Photon Scattering Resonance in the Model High-Temperature SuperconductorHgBa2CuO4+δRevealed by Raman Scattering and Optical Ellipsometry

We study the model high-temperature superconductor HgBa(2)CuO(4+δ) with electronic Raman scattering and optical ellipsometry over a wide doping range. The dependence of the resonant Raman cross section on the incident photon energy changes drastically as a function of doping, in a manner that corresponds to a rearrangement of the interband optical transitions seen with ellipsometry. This doping-dependent Raman resonance allows us to reconcile the apparent discrepancy between Raman and x-ray detection of magnetic fluctuations in superconducting cuprates. Intriguingly, the strongest variation occurs across the doping level where the antinodal superconducting gap appears to reach its maximum.

Magnetic order in the pseudogap phase of HgBa2CuO4+δstudied by spin-polarized neutron diffraction

Spin-polarized neutron diffraction experiments have revealed an unusual $q=0$ magnetic order in the model high-temperature superconductor HgBa${}_{2}$CuO${}_{4+\ensuremath{\delta}}$ (Hg1201) below the pseudogap temperature ${T}^{*}$ [Y. Li et al., Nature (London) 455, 372 (2008)]. Together with results for the structurally more complex compound YBa${}_{2}$Cu${}_{3}$O${}_{6+\ensuremath{\delta}}$ (YBCO) [B. Fauqu\'e et al., Phys. Rev. Lett. 96, 197001 (2006); H.A. Mook et al., Phys. Rev. B 78, 020506 (2008)], this establishes the universal existence of a genuine novel magnetic phase in underdoped cuprates with high maximal ${T}_{\mathrm{c}}$ (above 90 K at optimal doping). Here we report a systematic study of an underdoped Hg1201 sample (${T}_{\mathrm{c}}=75$ K), the result of which is consistent with the previously established doping dependence of the magnetic signal. We present an assumption-free analysis of all the data available for Hg1201. Depending on how the hole concentration is estimated, comparison with the results for YBCO leads to different scenarios for the competition between the $q=0$ magnetic order and the spin-density-wave order found in heavily underdoped YBCO.

Magnetic vortex lattice in HgBa2CuO4+δobserved by small-angle neutron scattering

We report the direct observation of the magnetic vortex lattice in the model high-temperature superconductor HgBa${}_{2}$CuO${}_{4+\ensuremath{\delta}}$. Using small-angle neutron scattering on high-quality crystals, we observe two equal domains of undistorted triangular vortex lattices well aligned with the tetragonal crystallographic axes. The signal decreases rapidly with increasing magnetic field and vanishes above $0.4$ T, which we attribute to a crossover from a three-dimensional to a two-dimensional vortex system, similar to previous results for the more anisotropic compound Bi${}_{2.15}$Sr${}_{1.95}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$. Our result indicates that a triangular vortex lattice (with or without distortion) at low magnetic fields is a generic property of cuprates with critical temperatures above $80$ K.

Demonstrating the model nature of the high-temperature superconductorHgBa2CuO4+δ

The compound ${\text{HgBa}}_{2}{\text{CuO}}_{4+\ensuremath{\delta}}$ (Hg1201) exhibits a simple tetragonal crystal structure and the highest superconducting transition temperature $({T}_{c})$ among all single Cu-O layer cuprates, with ${T}_{c}=97\text{ }\text{K}$ (onset) at optimal doping. Due to a lack of sizable single crystals, experimental work on this very attractive system has been significantly limited. Thanks to a recent breakthrough in crystal growth, such crystals have now become available. Here we demonstrate that it is possible to identify suitable heat treatment conditions to systematically and uniformly tune the hole concentration of Hg1201 crystals over a wide range, from very underdoped (${T}_{c}=47\text{ }\text{K}$, hole concentration $p\ensuremath{\approx}0.08$) to overdoped $({T}_{c}=64\text{ }\text{K},\text{ }p\ensuremath{\approx}0.22)$. We then present quantitative magnetic susceptibility and dc charge transport results that reveal the very high-quality nature of the studied crystals. Using x-ray photoemission spectroscopy on cleaved samples, we furthermore demonstrate that it is possible to obtain large surfaces of good quality. These characterization measurements demonstrate that Hg1201 should be viewed as a model high-temperature superconductor.

Crystal Growth and Characterization of the Model High‐Temperature Superconductor HgBa2CuO4+δ

Large, gram-sized single crystals (see figure) of the model high-temperature superconductor HgBa2CuO4+δ are obtained and characterized. The results demonstrate the high quality of the obtained crystals and the feasibility of inelastic neutron scattering measurements.

Flux pinning and phase transitions in model high-temperature superconductors with columnar defects.

We calculate the degree of flux pinning by defects in model high-temperature superconductors (HTSC's). The HTSC is modeled as a three-dimensional network of resistively shunted Josephson junctions in an external magnetic field, corresponding to a HTSC in the extreme type-II limit. Disorder is introduced either by randomizing the coupling between grains (model-A disorder) or by removing grains (model-B disorder). Three types of defects are considered: point disorder, random line disorder, and periodic line disorder; but the emphasis is on random line disorder. Static and dynamic properties of the models are determined by Monte Carlo simulations and by solution of the analogous coupled overdamped Josephson equations in the presence of thermal noise. Random line defects considerably raise the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$(B), and increase the apparent critical current density ${\mathit{J}}_{\mathit{c}}$(B,T), in comparison to the defect-free crystal. They are more effective in these respects than a comparable volume density of point defects, in agreement with the experiments of Civale et al. Periodic line defects commensurate with the flux lattice are found to raise ${\mathit{T}}_{\mathit{c}}$(B) even more than do random line defects. Random line defects are most effective when their density approximately equals the flux density. Near ${\mathit{T}}_{\mathit{c}}$(B), our static and dynamic results appear consistent with the anisotropic Bose-glass-scaling hypotheses of Nelson and Vinokur, but with possibly different critical indices.

Margins and yields of SFQ circuits in HTS materials

An analytical model has been developed to project the yield of superconductive integrated circuit chips as a function of circuit operating margins, fabrication process control, and component count. For Gaussian distributed deviations of critical component values from design specifications, chip yield was a highly nonlinear (threshold) function of the ratio of circuit margin to process standard deviation. Computer simulations of single-flux-quantum (SFQ) logic gates with model high-temperature superconductor (HTS) superconductor-normal-metal-superconductor (SNS) junctions operating at GHz clock rates showed at least 50-70% of the margins of similar Nb-Al/sub 2/O/sub 3/-Nb based circuits. Margins and maximum clock rate improved as I/sub c/R/sub n/ (critical-current-normal-resistance product) was increased from 200 to 500 mV.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Flux-flow resistivity in model high-temperature superconductors.

We calculate the resistivity of a model high-temperature superconductor'' consisting of a simple-cubic arrangement of superconducting grains'' coupled together by resistively shunted Josephson junctions. The effects of temperature are simulated by Langevin noise in each junction. We find a strong magnetoresistance for magnetic fields both parallel and perpendicular to the applied current, in agreement with the results of Kwok {ital et} {ital al}. (Phys. Rev. Lett. 64, 966 (1990)). When the magnetic field {bold B} and the current density {bold J} make an angle {phi}, the resistivity at sufficiently high temperatures roughly obeys the law {rho}({ital B},{ital T},{phi})={rho}{sub 0}({ital B},{ital T})+{Delta}{rho} sin{sup 2}({phi}) in agreement with experiment. The resistivity is strongly dependent on current density. At zero magnetic field it is found to satisfy the scaling relation {ital E}={xi}{sup {minus}1{minus}{ital z}F}{sub {plus minus}}({ital J}{xi}{sup {ital d}{minus}1}{Phi}{sub 0}/{ital ck}{sub {ital B}T}), where {ital E} is the electric field, {ital c} is the speed of light, {ital J} is the current density, {ital d} is the dimensionality, and {ital F}{sub {plus minus}} are scaling functions which apply above and below {ital T}{sub {ital c}}. The dynamical critical exponent is estimated for this model as 1.5{plus minus}0.5.

Disorder effect on non-ohmic resistive state in model high-temperature superconductor

Microscopic defects (twin boundaries, oxygen defects) are important for high-temperature superconductive properties even in single crystals. The current ( I) z. sbnd;voltage ( V) characteristics is calculated for two-dimensional (2D) intra-granular superconductor model with random defects of Josephson couplings. A non-ohmic power-law V ∝ I α is found where, notably, the exponent α seems logarithmically sensitive to defect fraction ( p): α ∝ ln( p o / p, as connected 2D vortex-antivortex excitation. This peculiar property qualitatively correlates with recent experiments on ion-irradiated Y-Ba-Cu-O.

Fluctuation specific heat and vortex configurations in model high-temperature superconductors

We calculate the fluctuation specific heat and diamagnetic susceptibility of a single layer of model high-temperature superconductor with parameters resembling YBa2Cu3O7−δ, as a function of perpendicular magnetic field, assuming an s-wave order parameter and fluctuations about a Ginzburg-Landau mean-field theory. Snapshots of the vortex configurations in a perpendicular field are also shown.

Specific heat of model high-temperature superconductors: Effects of magnetic fields.

We calculate, via Monte Carlo simulations, the fluctuation contribution to the specific heat of a Ginzburg-Landau-Wilson model for a single-crystal high-temperature superconductor. The result resembles that measured by Salamon et al., both in the sharply peaked structure that replaces the usual Bardeen-Cooper-Schrieffer step, and especially in the reduction of this peak by magnetic-field-induced frustration. The critical region of the model is probably broader than that measured.