Superconducting Glass State Research Articles

Скачкообразные квантовые фазовые переходы в вихревой системе и динамическая комплексная магнитная проницаемость двойниковых YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-x-=/SUB=- высокотемпературных сверхпроводников

A nonlocal high-frequency method for measuring the dynamic complex magnetic permeability with enhanced spatial resolution is developed to simultaneously determine the bulk and local character of stepwise penetration of a magnetic flux through twin boundaries (TBs) into a YBa2Cu3O7-x HTSC sample during its stepwise decomposition into twins. By the values of fields corresponding to the regions of steps, the thermodynamic first critical magnetic fields are determined: the penetration of a flux into a sample – a Josephson medium; the development of a critical state in the Josephson medium; the penetration of a flux into twins; and the values of the critical fields of phase transitions in the vortex system of the HTSC sample, such as the melting fields of a vortex crystal, the formation of a superconducting glass state in the sample, and the transition to the vortex glass and Bragg glass states.

Jump quantum phase transitions in a vortex sistem and the dinamic complex magnetic permeability of high-temperature binary YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-x-=/SUB=- superconductors

A nonlocal high-frequency method for measuring the dynamic complex magnetic permeability with enhanced spatial resolution is developed to simultaneously determine the bulk and local character of stepwise penetration of a magnetic flux through twin boundaries (TBs) into a YBa2Cu3O7-x HTSC sample during its stepwise decomposition into twins. By the values of fields corresponding to the regions of steps, the thermodynamic first critical magnetic fields are determined: the penetration of a flux into a sample --- a Josephson medium; the development of a critical state in the Josephson medium; the penetration of a flux into twins; and the values of the critical fields of phase transitions in the vortex system of the HTSC sample, such as the melting fields of a vortex crystal, the formation of a superconducting glass state in the sample, and the transition to the vortex glass and Bragg glass states. Keywords: high-frequency inductance, bulk and local dynamic complex magnetic permeability, magnetic flux trapping, effective demagnetization factor, twins (monodomains, crystallites, sub- and nanocrystallites), twin boundaries.

Observation of a superconducting glass state in granular superconducting diamond

The magnetic field dependence of the superconductivity in nanocrystalline boron doped diamond thin films is reported. Evidence of a superconducting glass state is presented, as demonstrated by the observation of a quasi de Almeida-Thouless line in the phase diagram and a logarithmic time dependence of the magnetism. The position of the phase boundary in the H-T plane is determined from electrical transport data by detailed fitting to quasi-zero-dimensional fluctuation conductivity theory. This allows determination of the boundary between resistive and non-resistive behaviour to be made with greater precision than the standard ad hoc onset/midpoint/offset criterion. We attribute the glassy superconductivity to the morphological granularity of the diamond films.

Vortex detection and quantum transport in mesoscopic graphene Josephson-junction arrays

We investigate mesoscopic Josephson junction arrays created by patterning superconducting disks on monolayer graphene, concentrating on the high-$T/T_c$ regime of these devices and the phenomena which contribute to the superconducting glass state in diffusive arrays. We observe features in the magnetoconductance at rational fractions of flux quanta per array unit cell, which we attribute to the formation of flux-quantized vortices. The applied fields at which the features occur are well described by Ginzburg-Landau simulations that take into account the number of unit cells in the array. We find that the mean conductance and universal conductance fluctuations are both enhanced below the critical temperature and field of the superconductor, with greater enhancement away from the graphene Dirac point.

Physical processes in high-temperature superconductors at the interface between the vortex-filled and meissner regions

A technique has been proposed for investigating the magnetic microstate of high-temperature superconductors with a simultaneous analysis of the crystalline microstate of the sample with the aim of elucidating the specific features of the interaction between the crystalline and magnetic microstructures of polycrystalline high-temperature superconductors. Qualitatively new results have been obtained for samples with different microstructures. In particular, it has been found that the magnetic field dependences of the trapped magnetic flux density Btr(H0) of polycrystalline and epitaxial films of high-temperature superconductors exhibit regular steps for both increasing and decreasing magnetic fields. The obtained results have demonstrated that, in strong magnetic fields, the studied epitaxial films, as well as bulk and thin-film polycrystalline high-temperature superconductors, “break down” into single domains, crystallites, and subcrystallites with different demagnetization factors. It has been revealed that the dependences Btr(H0) also exhibit steps due to the simultaneous penetration of vortices into crystallites of approximately the same sizes and into more regularly arranged subcrystallites. As the quality of the samples increases, these steps become more pronounced because of the increase in the short-range order. The absence of steps in the dependence Btr(H0) of the polycrystalline bulk samples clearly demonstrates the absence of long-range order in these samples. It is the vitreousness of the crystallographic microstructure of high-temperature superconductors which is responsible for the observed transformations in the vortex system. The similarity of the results obtained for samples with different microstructures indicates that the penetration (escape), distribution, and trapping of the magnetic flux in these samples occur through a universal mechanism. It has been found that the polycrystalline high-temperature superconductors are actually multi-step rather than two-step systems. It has been shown that the vitreousness of the microstructure of high-temperature superconductors and the presence of close-packed twin boundaries in samples lead to the penetration of a magnetic flux in the form of hypervortices into the sample and cause the formation of a superconducting glass state on a different physical basis as compared to the Ebner-Stroud model of a granulated glass.

A Method for Analyzing Physical Processes at the Vortex Front in High-Temperature Superconductors

A method of simultaneous analysis of the magnetic and crystalline microstates of superconductors is proposed to find out the specific features of interaction between the crystalline and magnetic microstructures of polycrystalline HTSCs. Qualitatively new results are obtained for samples with different microstructures. For example, regular steps are observed on the magnetic-field dependence of the trapped magnetic flux density Btr(H0) in polycrystalline and epitaxial YBCO films for both increasing and decreasing field. The results of the analysis imply that epitaxial films, as well as bulk and film polycrystalline HTSCs, are "decomposed" into monodomains, crystallites, and subcrystallites with different demagnetization factors. The simultaneous penetration of vortices into crystallites of about the same size and into more regularly arranged subcrystallites gives rise to the above-mentioned steps. As the quality of the samples increases, these steps become more prominent, which is attributed to the enhanced short-range ordering. The absence of steps on Btr(H0) in bulk polycrystalline samples clearly demonstrates the absence of long-range ordering in these samples. It is the vitreousness of the crystalline microstructure of HTSCs that is responsible for the transformations in the vortex system. The similarity of the results obtained in samples with different microstructures points to the universal mechanism of penetration, distribution, and trapping of magnetic flux in these samples. It is found that polycrystalline HTSCs are in fact multistep, rather than two-step, systems. It is shown that the vitreousness of the microstructure of HTSCs and the dense arrangement of twinning boundaries lead to the penetration of magnetic flux in the form of hypervortices into samples and are responsible for the formation of a superconducting glass state on physical principles different from those of the Ebner--Strode model of granulated glass.

Theory of proximity-induced superconductivity in graphene

We propose a way of making graphene superconductive by putting on it small superconductive islands which cover a tiny fraction of graphene area. We show that the critical temperature, T c , can reach several Kelvins at the experimentally accessible range of parameters. At low temperatures, T ≪ T c , and zero magnetic field, the density of states is characterized by a small gap E g ≤ T c resulting from the collective proximity effect. Transverse magnetic field H g ( T ) ∝ E g is expected to destroy the spectral gap driving graphene layer to a kind of a superconductive glass state. Melting of the glass state into a metal occurs at a higher field H g 2 ( T ) .

Proximity-induced superconductivity in graphene

We propose a way of making graphene superconductive by putting on it small superconductive islands which cover a tiny fraction of graphene area. We show that the critical temperature, T_c, can reach several Kelvins at the experimentally accessible range of parameters. At low temperatures, T<<T_c, and zero magnetic field, the density of states is characterized by a small gap E_g<T_c resulting from the collective proximity effect. Transverse magnetic field H_g(T) E_g is expected to destroy the spectral gap driving graphene layer to a kind of a superconductive glass state. Melting of the glass state into a metal occurs at a higher field H_{g2}(T).

Superconducting Glass State in Disordered Thin Films in a Parallel Magnetic Field

We develop a theory of mesoscopic fluctuations in disordered thin superconducting films in a parallel magnetic field. At zero temperature and sufficiently strong magnetic field the system undergoes a phase transition into a state characterized by a superfluid density which is random in sign. Consequently, in this regime, random supercurrents are spontaneously created in the ground state of the system, and it belongs to the same universality class as the two-dimensional $\mathrm{XY}$ spin glass with a random sign of the exchange interaction.

Angular dependence of the zero resistance superconducting glass state in YBCO thin films

The authors present current versus voltage and resistivity data as a function of angle, magnetic field, and temperature. The critical scaling exponents extracted from the data are consistent with the new theories of the vortex and Bose glass. However, the angle dependence of the glass transition temperature is inconsistent with the vortex glass model and is explainable in a model utilizing both the vortex and Bose glass theories. The samples, high quality epitaxial thin films of YBCO, were also studied using the atomic force microscope and X-ray diffraction techniques. The effects of the microstructure on angle-dependent current versus voltage measurements within the vortex glass framework are discussed. >

Scaling behaviour of relaxation dependences in metaloxide superconductors

Superconducting glass state has been investigated in different types of metaloxide ceramics, Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Ba-Pb-Bi-O, using the highly sensitive SQUID magnetometer. The analysis of long-time relaxation processes of thermoremanent magnetization m(sup trm) (+) = M(sub o) - Slnt displayed scaling dependence of the decay rate S = -dM/dlnt on quantity of trapped magnetic flux M(sub o): 1gs = 31g M(sub o) - observed universal dependence S is approximately M(sup 3) (sub o) seems to one of the features of superconducting glass state in metaloxide ceramics.

Resistive noise and heat effects in the granular oxide superconductor BaPb0.75Bi0.25O3

For weak applied magnetic fields the frequency spectrum of the voltage fluctuations was measured for Josephson ceramic samples of BaPb0.75Bi0.25O3 with Tc approximately=10 K. The measurements were carried out in the resistive state. It is shown that the spectral density Sv(f) in the frequency region 20 Hz<f<500 Hz is described by a function f- alpha , where alpha approximately=1 for small H and alpha approximately=2-2.5 for H>or approximately=10 Oe. For currents I>or approximately=0.8 mA through the sample, oscillations with a fundamental frequency f1<or=100 Hz and two harmonics f2 and f3 were observed. For larger currents these oscillations die out and instead several new oscillations appear. Flicker noise with alpha approximately=1 is due to the existence of the superconducting glass state. The transition to a alpha >or approximately=2 and the emergence of the oscillations apparently occur owing to heating effects.

Macroscopic quantum interference and superconducting glass state in YBa2(Cu1−xFex)3O7−δ

Macroscopic quantum phenomena and superconducting glass state have been observed simultaneously in a random system of linked Josephson junctions in lumps of suitable high temperature superconducting ceramics Y-Ba-Cu-O. This multiply connected Josephson medium was used for radio-frequency SQUID operation at temperatures up to 90 K. The effect of nonergodic glass state on SQUID behaviour readily was observed. The presence of a wide range of relaxation times in glass state leads to an appreciable noise of 1/f type in the SQUID output signal at f ≲ 103 Hz.

Flicker-noise in superconducting glass - ceramics BaPb 0.75Bi 0.25O 3

In ceramic superconductor BaPb 0.75 Bi 0.25 O 3 with current-voltage characteristics of the tunnel-type the spectrum of voltage fluctuations was measured for T = 4.2 K. The noise spectral density S V (f) in the frequency range 20 Hz ≤ f ≤ 500 Hz is described by the formula f -α (1 ≤ a ≤ 2.2), where α is substantially dependent on the external magnetic field H ≤ 16.6 Oe. The observed noise is apparently associated with the emergence of the superconducting glass state. The spectra contain also voltage oscillations of thermal origin with the fundamental frequency f o ≈ 100 Hz.

The bipolaronic superconducting glass state in the random infinite-range interaction Hubbard model

The phase diagram of the extended Hubbard model with on-site attraction and random inter-site Coulomb energies is studied for the half-filled band case. In the strong coupling limit the problem is mapped onto the system of hard-core bosons (bipolarons) on a lattice, described by the anisotropic pseudo-spin model with infinite-range random exchange interactions-in close analogy to the Sherrington-Kirkpatrick spin-glass approach. It is found that the disorder and frustration strongly affect properties of the system leading to the appearance of the bipolaronic superconducting phase, the bipolaronic superconducting glass state as well as the bipolaronic charge glass phase, depending on the temperature range and the degree of the disorder.

COMPLEX SUSCEPTIBILITY, MAGNETIZATION AND ELECTRICAL RESISTIVITY OF HIGH TEMPERATURE SUPERCONDUCTORS

ac complex susceptibility, χ′ and χ″, dc magnetization and electrical resistivity of sintered superconductors Bi–Pb–Sr–Ca–Cu–O and Y–Ba–Cu–O have been measured. The results show two non-linear 2/3 power behaviors: field dependence of χ″ peak temperature, a quasi de Almeida-Thouless line, and a irreversibility line. The former corresponds to the property of intergrain material measured at low field, it can be interpreted by superconductive glass state in high-T c superconductor and/or the weakly connected loop model. The latter corresponds to the property of intragrain material measured at higher field, it can be interpreted by the thermally activated flux creep model. Moreover, the field dependences of width and height of χ″ peak are also exhibited. Finally, we have measured the I – V characteristics at 103.8 K for Bi–Pb–Sr–Ca–Cu–O sample and suggested the relation between the χ″ peak temperature and the zero dissipation temperature.

Superconducting glass state in the random infinite-range interaction extended Hubbard model

The phase diagram of the extended Hubbard model with on-site attraction and random inter-site Coulomb energies is studied for the half-filled band case. In the strong coupling limit the problem is mapped onto the system of hard-core bosons (bipolarons) on a lattice, described by the anisotropic pseudo-spin model with infinite-range random exchange interactions.

Field induced microwave absorption in single crystal BI (2212): Evidence for a superconductive glass state

The field induced microwave absorption P(H) at 35 GHz in Bi 2212 micaceous single crystals exhibits remarkable variations as temperature is increased from 1.3 K to Tc. For T≲16 K, the hysteresis loops are highly reminiscent of those observed in spin glasses.At 30 ≲T ≲50 K, they resembel those of a type II superconductor. As suggested by the magnetization data, the micaceous nature causes a large enhancement in P(H) for 50<T<70 K, while above 70 K, the layers become effectively decoupled.

Nonlinear susceptibility of a polycrystalline YBa 2Cu 3O 7− y superconductor near T c

We have investigated nonlinear response to applied magnetic field in a YBa 2Cu 3O 7− y polycrystalline superconductor near T c , by using an a.c. mutual inductance bridge method. The third nonlinear susceptibility χ 2 shows a sharp peak with a negative sign and increases logarithmically near the peak temperature T g . And the weak magnetic field dependence of T g is found to fit the curve of the quasi-de Almeida-Thouless line. These experimental results are discussed in connection with the superconductive glass state in the high T c superconducting ceramics.

Diamagnetic correlations of the superconductive glass model

The dynamic structural factor for intragrain Josephson current induced magnetic correlations near the phase transition to the superconductive glass state is presented. In the mean field approximation critical exponents for scaling parameters are obtained. The paper predicts a strong magnetic field dependence of the intensity of the neutron scattering by intragrain diamagnetic correlations in high- T c superconductors.