Supercurrent Density Research Articles

Critical current measurements in superconductor-ferromagnet-superconductor junctions of YBa2Cu3Oy-SrRuO3-YBa2Cu3Oy: No evidence for dominant proximity-induced triplet superconductivity in the ferromagnetic barrier

Transport measurements in ramp-type junctions of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}{\ensuremath{-}\mathrm{SrRuO}}_{3}{\ensuremath{-}\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ with ${T}_{c}$ values of either 80--90 K or 60--70 K are reported. In both types of junctions but without a barrier (``shorts''), the supercurrent densities at 4.2 K reached 7.5 and 3.5 MA/cm${}^{2}$, respectively, indicating the high quality of the fabrication process. Plots of the critical current versus thickness of the ferromagnetic barrier at 4.2 K show exponential decays with decay lengths of 1.1 nm for the 90-K phase and 1.4 nm for the 60-K phase, which are much shorter than the relevant coherence lengths ${\ensuremath{\xi}}_{F}\ensuremath{\sim}5--6$ nm or ${\ensuremath{\xi}}_{N}\ensuremath{\sim}16$ nm of ${\mathrm{SrRuO}}_{3}$. We thus conclude that there is no dominant proximity induced triplet superconductivity in the ferromagnet in our junctions.

Gravity derivation of the Tisza-Landau model in AdS/CFT

We derive the fully backreacted bulk solution dual to a boundary superfluid with finite supercurrent density in AdS/CFT. The non-linear boundary hydrodynamical description of this solution is shown to be governed by a relativistic version of the Tisza-Landau two-fluid model to non-dissipative order. As previously noted, the phase transition can be both first order and second order, but in the strongly-backreacted regime at low charge q we find that the transition remains second order for all allowed fractions of superfluid density.

Superconducting nanowires: Interplay of discrete transverse modes with supercurrent

From a numerical solution of the Bogoliubov-de Gennes equations, we investigate an interplay of the transverse discrete modes with a longitudinal supercurrent in a metallic cylindrical superconducting nanowire. The superconductor-to-normal transition induced by a longitudinal superflow of electrons is found to occur as a cascade of jumps in the order parameter (supercurrent and superfluid density) as a function of the superfluid velocity for diameters $d<10--15\text{ }\text{nm}$ (for Al parameters) and sufficiently low temperatures $T<0.3--0.4{T}_{c}$, with ${T}_{c}$ the critical temperature. When approaching ${T}_{c}$, the jumps are smoothed into steplike but continuous drops. A similar picture occurs for $d>15--20\text{ }\text{nm}$. Only when the diameter exceeds 50--70 nm the quantum-size cascades are fully washed out, and we arrive at the mesoscopic regime. Below this regime the critical current density ${j}_{c}$ exhibits the quantum-size oscillations with pronounced resonant enhancements: the smaller the diameter, the more significant is the enhancement. Thickness fluctuations of real samples will smooth out such oscillations into an overall growth of ${j}_{c}$ with decreasing nanowire diameter.

Very High Critical Field and Superior Jc‐Field Performance in NdFeAsO0.82F0.18 with Tc of 51 K

been estimated to be higher than 55 or 63‐65 T in LaFeAsO0.9F0.1, [6,7] 70 T in PrFeAsO0.85F0.15, and over 100 T in SmFeAsO0.85F0.15. [7] The two gap superconductivity proposed for LaFeAsO0.9F0.1 suggests that the Hc2 could be further increased [6] to a significant extent. This is one of the unique features of the FeAs-based new superconductors. In this work, we show that Hc2(48K) ¼13 T, and that the Hc2(0) values can exceed 80‐230 T in a high-pressure (HP) fabricated NdO0.82F0.18FeAs bulk sample with Tc of 51K. We also demonstrate that the supercurrent density in fields from 1 up to 9 Tonly drops by a factor of 2‐6 for T <30K, significantly slower than for MgB2 and high-Tc cuprate superconductors. The very high Hc2 of the sample greatly surpasses those of MgB2 and classic low-temperature superconductors, and the superior Jc-field performance is promising for the use of the new NdFeAsO0.82F0.18 superconductors in highfield applications. The X-ray diffraction (XRD) and refinement results shown in Figure 1 indicate that the NdFeAsO0.82F0.18 sample is nearly single phase, with a tetragonal structure of the P4/nmm

Vortex phase diagram ofBa(Fe0.93Co0.07)2As2single crystals

Irreversible magnetic properties of $\text{Ba}{({\text{Fe}}_{0.93}{\text{Co}}_{0.07})}_{2}{\text{As}}_{2}$ single crystals were studied by using several techniques including transport, magnetization, and magneto-optics. The mixed-state response was found to be very similar to high-${T}_{c}$ cuprates, such as Y-Ba-Cu-O and Nd-Ce-Cu-O. In particular, we find a nonmonotonic ``fishtail'' shape of the magnetization loops $M(T=\text{const},H)$ and a corresponding feature in $M(T,H=\text{const})$ scans, as well as a very large magnetic relaxation rate. The supercurrent density, $j$, in the critical state was evaluated from magnetization and from direct transport measurements close to ${T}_{c}$. At 5 K we estimate a moderate $j\ensuremath{\approx}2.6\ifmmode\pm\else\textpm\fi{}0.2\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{A}/{\text{cm}}^{2}$. Analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning and creep (weak pinning and fast creep) model and suggests a crossover from the collective to the plastic creep regime in fields exceeding the field at which the fishtail magnetization is maximum. While the zero-field anisotropy of the magnetic properties in the superconducting state of $\text{Ba}{({\text{Fe}}_{0.93}{\text{Co}}_{0.07})}_{2}{\text{As}}_{2}$ is small, the vortex behavior changes for different orientations of magnetic field with respect to the crystal axes, implying field-dependent anisotropy of pinning properties.

Phase transition in the vortex structure of granular YBa2Cu3O7 − δ HTSCs in weak magnetic fields

This study aims at establishing the interrelation between the current-carrying capacity and peculiarities of magnetoresistance of granular YBa2Cu3O7 − δ HTSCs (T c = 92.5 K). The transverse magnetoresistance of several batches of YBa2Cu3O7 − δ HTSC samples with noticeably different values of critical supercurrent density j c is measured in magnetic fields H ext up to H ext max ≈ 500 Oe in a wide range of transport currents (5 mA ≤ I ≤ 1600 mA) at T = 77.4 K. Samples with relatively high values of j c (H ext = 0) ≥ 100 A/cm2 do not exhibit any anomalies in their field dependences. Magnetoresistance jumps δρBG-VG/ρ273K are observed for samples with low values of j c ≥ 20 A/cm2 in fields H BG-VG ≈ 200–260 Oe. The width ΔH BG-VG of the anomalous resistance region increases upon an increase in I. The magnetoresistance jumps decrease with increasing I in increasing field H ext(0 → H ext max ) and increase in decreasing field H ext(H ext max → 0). It is found that these peculiarities of the field dependences of magnetoresistance are associated with a first-order phase transition (in magnetic field) in the vortex structure of YBa2Cu3O7 − δ HTSCs of the Bragg glass-vortex glass type.

BCS Superconductivity of Dirac Electrons in Graphene Layers

Possible superconductivity of electrons with the Dirac spectrum is analyzed using the BCS model. We calculate the critical temperature, the superconducting energy gap, and the supercurrent as functions of the doping level and of the pairing interaction strength. Zero doping is characterized by the existence of a quantum critical point such that the critical temperature vanishes below some finite value of the interaction strength. However, the critical temperature remains finite for any nonzero electron or hole doping level when the Fermi energy is shifted away from the Dirac point. As distinct from usual superconductors, the supercurrent density is not proportional to the number of electrons but is strongly decreased due to the presence of the Dirac point.

Thermo-magnetic refrigeration as applied to hydrogen liquefaction I. Fundamentals and type-II superconductors

Gas liquefaction utilizing the entropy transport by flux lines (FLs) in the mixed state of type-II superconductors has been proposed. Refrigerating power and the Joulian heat generated by FL current are proportional to and to the square of the superconducting (super-) current, respectively. The optimum super-current, the corresponding field strength, and the net refrigerating power ( q n ) are expressed by the elemental parameters. Introducing the well-defined FL-flow viscosity coefficient, we show that the maximum q n appears at the field with 1 9 of the upper critical field under the corresponding super-current density, and we expect the optimum refrigeration power density of about 15.8 j 0 * and 4.49 j 0 * W / m 2 (where j 0 * is the critical super-current density) for Nb 3 Ge ( T 0 = 23.2 K , B C 2 = 14.5 T at T 0 ) and MgB 2 ( T 0 = 40.0 K , B C 2 = 16 T , at T 0 ), respectively.

Magneto-optics of spontaneous and field induced vortices in twinned YBa 2Cu 3O 7− δ/La 1− xSr xMnO 3 bilayers

The electromagnetic coupling between the magnetic and the superconducting layers in perovskite-heterostructures was investigated by means of the magneto-optical technique. The quantitative imaging of the magnetic field distribution allows the high resolution reconstruction of local supercurrent density. Two phenomena arising from the coupling between the manganite layer and the YBCO superconducting film deposited on top were investigated, i.e., the local effects of the electronic coupling between the compounds and the interaction of manganite magnetic moments on vortices in the superconductor. The first issue can be quantified in terms of the local superfluid density depression/enhancement in correspondence of the underlying magnetic structure, both domains and domain-walls (in dependence on the orientation and sign of the local magnetization) and of the spontaneous, macroscopic screening current loops generated by the manganite domain-walls. The interaction between the local manganite magnetization and the superconducting vortices is also influenced by structural defects because they modulate the magnetic pattern of the manganite. Different channeling phenomena in correspondence of natural twin-boundaries of the substrate, but locally magnetized by pinned domain-walls of the manganite on their locations, were recognized. In particular, it turns out that the vortex density along magnetized twin-boundaries can be controlled by the underlying magnetic moment orientation.

Temperature dependence of the persistent critical current and instabilities in MgB2 thin films

Temperature dependences of the persistent critical current density Jc(T) and its relaxation rate S=dlnJ∕dlnt were measured from 8.0K to Tc in MgB2 ring-shaped thin films. Jc(T) was observed to approach a Ginzburg-Landau-like temperature dependence with a decreasing Tc in magnesium deficient films. No correlation was found between the current density and the magnitude of the relaxation rate, suggesting percolative (filamentary) flow of the supercurrent. A suppression of the supercurrent density (instability) was observed between 8.0 and 10.5K in MgB2 thin films of Tc above 30K, but not in films of lower transition temperatures. Vacuum annealing shifts the instability to lower temperatures.

Assessment of the local supercurrent densities in long superconducting coated conductors

The authors report on measurements of the local supercurrent density in long Y1Ba2Cu3O7−δ based coated conductors by the magnetoscan technique. Significant inhomogeneities were found, which are well resolved by the resulting magnetic field map. A single central line scan along the length of the conductor reflects the inhomogeneities over the entire width of the sample, thus offering the possibility of very fast characterization. Modifying the applied field leads to different results highlighting either the overall critical current or details of the defect structure. In addition, numerical simulations of the current dynamics were carried out for a qualitative and quantitative interpretation of the results.

The intermodulation coefficient of an inhomogeneous superconductor

The high-Tc cuprate superconductors are now believed to be intrinsically inhomogeneous. We develop a theory to describe how this inhomogeneity affects the intermodulation coefficient of such a material. We show that the continuum equations describing intermodulation in a superconducting layer with spatially varying properties are formally equivalent to those describing an inhomogeneous dielectric with a nonzero cubic nonlinearity. Using this formal analogy, we calculate the effect of inhomogeneity on the intermodulation coefficient in a high-Tc material, using several assumptions about the topology of the layer and some simple analytical approximations to treat the nonlinearity. For some topologies, we find that the intermodulation critical supercurrent density JIMD is actually enhanced compared to a homogeneous medium, thereby possibly leading to more desirable material properties. We discuss this result in light of recent spatial mappings of the superconducting energy gap in BSCCO-2212.

Supercurrent density above 106A∕cm2 at 77K in a single-crystal film conductor of the cuprate high-Tc superconductor YBa2Cu3O7−δ—dream or reality?

Supercurrent transport phenomena in c-oriented epitaxial thin films of the the high-Tc superconducting (HTS) cuprate YBa2Cu3O7−δ (YBCO) with a high critical current density Jc(77K)⩾2×106A∕cm2 are investigated by four-probe transport measurements, low-frequency magnetic susceptibility studies, and SQUID magnetometry. The film samples are deposited on a single-crystal sapphire (r-cut) substrates with a CeO2 or LaAlO3(100) buffer layer by off-axis dc magnetron sputtering or pulsed laser ablation. A model of the mechanisms of Abrikosov vortex pinning and supercurrent limitation is developed and discussed by comparing its predictions with the results of measurements of the critical current and its dependence on applied magnetic fields of different strength and orientation and also with nanostructure data obtained by high-resolution transmission electron microscopy and electron diffraction in a backscattering geometry. It is shown that the low-angle subboundaries (LABs) formed between domains with a slight azimuthal misorientation during the epitaxial growth of the film play a key role in the phenomena observed in the transport of supercurrent. The tilt LABs form equidistant ordered rows of edge dislocations with nonsuperconducting cores about 3–4nm in diameter. The dislocation lines in the LABs are parallel to each other and perpendicular to the film plane. The average density of dislocations over the area of the film depends on the real statistics of the random system of LABs and can reach 1011cm−2. Since the diameter of the “normal” core of a dislocation is close to the diameter of the core of an Abrikosov vortex, the elementary pinning force of the vortex to the core of the dislocation is close to the maximum possible. The pinning on dislocation subboundaries has the following characteristics: 1) the achievement of high values Jc(77K)⩾2×106A∕cm2 and Jc(20K)&amp;gt;107A∕cm2 in epitaxial films and conductors; 2) the existence of a “plateau” on the Jc(H) curve, i.e., Jc(H)=const for H&amp;lt;Hm; 3) a logarithmic decline of Jc(H) for H&amp;gt;Hm, i.e., at the transition from the single-particle pinning regime to the collective pinning of the vortex lattice on the statistical ensemble of randomly distributed dislocation subboundaries; 4) the existence of a threshold field Hp that determines the limit up to which the vortices in a thin film (d⩽λ) remain rectilinear and perpendicular to the film even in a field inclined at a large angle; 5) the evolution of the angle dependence of Jc(θ) with a change of field strength is in complete agreement with the model of dominant pinning on “threading” edge dislocations. A new “peak effect”—an increase of Jc(H‖ab) with increasing longitudinal field—is observed for the first time for H&amp;gt;Hm, i.e, after the end of the “plateau” Jc(H‖ab)=const.

Basic properties of a vortex in a noncentrosymmetric superconductor

We numerically study the vortex core structure in a noncentrosymmetric superconductor such as CePt3Si without mirror symmetry about the xy plane. A single vortex along the z axis and a mixed singlet–triplet Cooper pairing model are considered. The spatial profiles of the pair potential, local density of states, supercurrent density, and radially-textured magnetic moment density around the vortex are obtained in the clean limit on the basis of the quasiclassical theory of superconductivity.

Classical and quantum percolation phenomena and controlling them in eutectic semiconductor-superconductor compositions

Experimental data on the conduction of heterogeneous systems have been traditionally interpreted in the context of the theory of percolation phenomena taking into account the relative threshold volume fraction ηC ≈ 0.16) of the high-conductivity phase. This work is concerned with the conduction of eutectic compositions semiconductor-normal metal at T > T c (the classical limit) and semiconductor-superconductor at T < T c (the quantum limit) obtained at various material growth rates; these materials contain metal particles as oriented whiskers in semiconducting matrices. The paper presents spatial and energy models of discrete, finite, and infinite clusters that well explain classical and quantum percolation conductivities. Depending on the growth rate of eutectic compositions, their classical and quantum conductivities can manifest themselves at arbitrary percolation thresholds ηp (0 < ηp ≤ ηc). It is shown that the density of whiskers, the distances between them, their diameters, and the critical supercurrent density per whisker can be controlled by varying the rate of composition growth.

Relaxation in bulkRBa2Cu3O7−δsuperconductors

The shape of the magnetic hysteresis curve, ``MHL,'' in bulk strongly doped $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ materials is analyzed and its correlation with relaxation characteristics is verified. It is shown that field and temperature dependencies of logarithmic relaxation rate, $Q=d\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}J∕d\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}dB∕dt$, can be obtained from $J(B)$ and $J(T)$ data ($J$ is the supercurrent density associated with total magnetic moment). In the case of the MHL with two well-distinguished peaks, the two additive contributions to $J$ are correlated with corresponding relaxation characteristics, which are given analytically. Plateau in $Q(T)$ dependence in such materials is shown to originate from scanning the specific shape of MHL at a fixed field. From $Q(J)$ plot pinning energy characteristics at low and intermediate fields are deduced.

On the origin of photogenerated terahertz radiation from current-biased superconducting YBa2Cu3O7−δ thin films

The origin of photogenerated terahertz radiation pulse emitted from current-biased superconducting YBa2Cu3O7−δ thin films excited by femtosecond optical laser pulses is delineated. By investigating the performance of the transient terahertz radiation generated under different operating parameters, pulse reshaping in the measured terahertz electric field caused by the kinetic inductance of the superconducting charge carriers is identified. After recovering the original wave forms of the emitted terahertz pulses, the transient supercurrent density directly correlated to the optically excited quasiparticle dynamics is obtained. A fast decreasing component of about 1.0 ps and a slower recovery process with a value of 2.5 ps are unambiguously delineated in the optically induced supercurrent modulation. The radiation mechanism of the transient terahertz pulse related to nonequilibrium superconductivity is discussed.

Temperature-induced crossover between0andπstates in S/F/S junctions

Ferromagnetic Josephson junctions can show at equilibrium a $\ensuremath{\pi}$ phase difference between the superconducting electrodes. We explain this $\ensuremath{\pi}$ state in an original way by a modified spectrum of Andreev bound states shifted by the exchange energy. A simplified expression for the spectral supercurrent density is calculated and the nonmonotonic temperature dependence of the critical current is discussed. This model accounts for the cancellation of the critical current with temperature observed in a small range of barrier thickness in our ${\mathrm{N}\mathrm{b}/\mathrm{C}\mathrm{u}}_{52}{\mathrm{Ni}}_{48}/\mathrm{Nb}$ junctions. This cancellation corresponds to an inversion of the supercurrent and to a ground-state crossover from a 0 state to a $\ensuremath{\pi}$ state. This transition is caused both by the thermal distribution of quasiparticles and by the temperature dependence of the exchange energy. The experimental curves are well reproduced by our theoretical expression except for the very small amplitude of the supercurrent attributed to a large spin-flip scattering.

Temperature dependence of the supercurrent density in bilayer cuprate superconductors

The present work deals with the study of the supercurrent density as a function of temperature and various microscopic interactions that exist in bilayer high temperature cuprate superconducting materials. For this purpose a tight binding bilayer Hubbard Hamiltonian has been considered that includes the in-plane (within CuO 2 plane) and out-of-plane interactions. The in-plane component of the model Hamiltonian is just in the BCS form with in-plane hopping energy and effective attractive interaction that comprises on-site and intersite interactions and is responsible to form pairing of the charge carriers within the plane. The out-of-plane part of the Hamiltonian includes single particle tunneling along with intrinsic Josephson like coupling between the planes within the unit cell which is responsible for transfer of single particle as well as Cooper pair from one layer to another and vice-versa, in the superconducting state. The situation for bilayered cuprates considered here is equivalent to a Josephson’s coupled SIS junction. To derive the expressions of the superconducting order parameter, carrier density and supercurrent density, we rely on the Green’s function equations of motion approach within the simple BCS formalism. The numerical analysis shows that in bilayer cuprates the supercurrent density depend on various in-plane and out-of-plane contributions as well as on temperature in an essential way. Finally, we have compared our theoretical results on supercurrent density with that of recent experimental results on temperature and pressure dependence of supercurrent density in bilayer superconducting cuprate materials and found to be in qualitative agreement.

Nonsinusoidal current-phase relation in SFS Josephson junctions

Various types of the current-phase relation I(phi) in superconductor-ferromagnet-superconductor (SFS) point contacts and planar double-barrier junctions are studied within the quasiclassical theory in the limit of thin diffusive ferromagnetic interlayers. The physical mechanisms leading to highly nontrivial I(phi) dependence are identified by studying the spectral supercurrent density. These mechanisms are also responsible for the 0-pi transition in SFS Josephson junctions.