Local Critical Temperature Research Articles

New mixed inhomogeneous phase in vortical gluon plasma: First-principle results from rotating SU(3) lattice gauge theory

Using first-principle numerical simulations, we find a new spatially inhomogeneous phase in rigidly rotating Nc=3 gluon plasma. This mixed phase simultaneously possesses both confining and deconfining phases in thermal equilibrium. Unexpectedly, the local critical temperature of the phase transition at the rotation axis does not depend on the angular frequency within a few percent accuracy. Even more surprisingly, an analytic continuation of our results to the domain of real angular frequencies indicates a profound breaking of the Tolman-Ehrenfest law in the vicinity of the phase transition, with the confining (deconfining) phase appearing far (near) the rotation axis.

Correlation between local structure variations and critical temperature of (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1-x(TiO2)x superconductor

TiO2 doping content affects the critical temperature (Tc) and the variations of local structure on Bi1.6Pb0.4Sr2Ca2Cu3O10+δ. The (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1-x(TiO2)x samples were fabricated with the solid-state reaction process, where x = 0, 0.002, 0.004, 0.006, 0.008, and 0.01. The Tc values of the samples were obtained by measuring resistivity versus temperature, indicating that Tc gradually decreased with increasing doping content (x). To explain the obtained degradation of Tc, carrier properties, role of interlayer coupling, and local structure were systematically investigated using Azlamozov–Larkin theory and its Lawrence–Doniach modification for strong anisotropic superconductors. The calculation of excess conductivity at the mean field region showed that the c-axis coherence length (ξc) and the effective inter-layering spacing (d) increased with increasing doping content. X-ray diffraction patterns also showed that the bond distances increased with increasing TiO2 content. The copper valence (V) and carrier concentration (p) of the samples were determined by analyzing the Cu L2,3-edge X-ray absorption near edge structure spectra. The values of V and p showed the same trend of decreasing with increasing x. A close correlation between the changes in local structure parameters and degradation of Tc of (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1-x(TiO2)x was then probably concluded.

Microstructure and superconducting properties of bulk EuBCO-Ag with and without holes

Bulk single-grain EuBCO-Ag superconductors with holes and without holes grown using a top-seeded melt-grow process are analysed. It turns out that the levitation force and the trapped magnetic field at 77 K are higher for the sample with holes (0,85 N and 1 T for the sample with a diameter of 28 mm and a height of 10 mm). Careful complex microstructural analyses undertaken by polarised light optical microscopy and scanning electron microscopy show that the reason for this behaviour is mainly the lower porosity of the sample with holes and the lower density of the of a/c-oxygenation cracks in non-porous areas along the holes. It also appears that other microstructural features, such as higher volume fraction of Eu2BaCuO5 particles in the a-growth sector than in the c-growth sector, uniform distribution of Ag particles in samples, presence of tetragonal EuBa2Cu3O7−x regions are similar on both types of samples and cannot significantly affect macroscopic superconducting properties. The influence of porosity on the local critical current density and transition temperature is also illustrated.

Analysis of magnetic vortex dissipation in Sn-segregated boundaries in Nb3Sn superconducting RF cavities

We study mechanisms of vortex nucleation in ${\mathrm{Nb}}_{3}\mathrm{Sn}$ superconducting RF (SRF) cavities using a combination of experimental, theoretical, and computational methods. Scanning transmission electron microscopy imaging and energy dispersive spectroscopy of some ${\mathrm{Nb}}_{3}\mathrm{Sn}$ cavities show Sn segregation at grain boundaries in ${\mathrm{Nb}}_{3}\mathrm{Sn}$ with Sn concentration as high as $\ensuremath{\sim}35$ at. % and widths $\ensuremath{\sim}3$ nm in chemical composition. Using ab initio calculations, we estimate the effect excess tin has on the local superconducting properties of the material. We model Sn segregation as a lowering of the local critical temperature. We then use time-dependent Ginzburg-Landau theory to understand the role of segregation on magnetic vortex nucleation. Our simulations indicate that the grain boundaries act as both nucleation sites for vortex penetration and pinning sites for vortices after nucleation. Depending on the magnitude of the applied field, vortices may remain pinned in the grain boundary or penetrate the grain itself. We estimate the superconducting losses due to vortices filling grain boundaries and compare with observed performance degradation with higher magnetic fields. We estimate that the quality factor may decrease by an order of magnitude $({10}^{10} \mathrm{to} {10}^{9})$ at typical operating fields if 0.03% of the grain boundaries actively nucleate vortices. We additionally estimate the volume that would need to be filled with vortices to match experimental observations of cavity heating.

Nano-patterning of cuprate superconductors by masked He+ ion irradiation: 3-dimensional profiles of the local critical temperature

Irradiation of cuprate high-Tc superconductors with light ions of moderate energy creates point defects that lead to a reduction or full suppression of the critical temperature. By shaping the ion flux with a stencil mask, nanostructures for emerging superconducting electronics can be fabricated. The 3-dimensional shape of such defect landscapes is examined, based on calculations of full collision cascades and atom displacements. A relation between the calculated defect density and experimental values of the critical temperature in thin YBa2Cu3O7−δ films is etablished that allows to determine the distribution of local Tc’s and its 3-dimensional visualization. The results confirm that, using 75 keV He+ ion irradiation and a stencil mask, well-defined patterns of non-superconducting material in the superconducting matrix can be produced with low blurring.

Mesoscale vortex pinning landscapes in a two component superconductor

We considered several mesoscale pinning/anti-pinning landscapes simulated with the parameter α(T) in the Ginzburg Landau free-energy density in a mesoscopic superconducting square of size L2. We analyzed the effects of the size, local critical temperature, position and number of the pinning/anti-pinning centers on the vortex configurations and magnetic response for several values of the Ginzburg-Landau parameter κ ([1 ≤ κ ≤ 8]). We shown that the fields depends strongly on the local critical temperature, configuration and number of the defects on the sample.

Ramp-Reversal Memory and Phase-Boundary Scarring in Transition Metal Oxides.

Transition metal oxides are complex electronic systems that exhibit a multitude of collective phenomena. Two archetypal examples are VO2 and NdNiO3 , which undergo a metal-insulator phase transition (MIT), the origin of which is still under debate. Here this study reports the discovery of a memory effect in both systems, manifested through an increase of resistance at a specific temperature, which is set by reversing the temperature ramp from heating to cooling during the MIT. The characteristics of this ramp-reversal memory effect do not coincide with any previously reported history or memory effects in manganites, electron-glass or magnetic systems. From a broad range of experimental features, supported by theoretical modelling, it is found that the main ingredients for the effect to arise are the spatial phase separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local transition temperature of the phase transition. We conclude that the emergent memory effect originates from phase boundaries at the reversal temperature leaving "scars" in the underlying lattice structure, giving rise to a local increase in the transition temperature. The universality and robustness of the effect shed new light on the MIT in complex oxides.

Superconductivity in highly disordered NbN nanowires

The topic of superconductivity in strongly disordered materials has attracted significant attention. These materials appear to be rather promising for fabrication of various nanoscale devices such as bolometers and transition edge sensors of electromagnetic radiation. The vividly debated subject of intrinsic spatial inhomogeneity responsible for the non-Bardeen–Cooper–Schrieffer relation between the superconducting gap and the pairing potential is crucial both for understanding the fundamental issues of superconductivity in highly disordered superconductors, and for the operation of corresponding nanoelectronic devices. Here we report an experimental study of the electron transport properties of narrow NbN nanowires with effective cross sections of the order of the debated inhomogeneity scales. The temperature dependence of the critical current follows the textbook Ginzburg–Landau prediction for the quasi-one-dimensional superconducting channel Ic ∼ (1-T/Tc)3/2. We find that conventional models based on the the phase slip mechanism provide reasonable fits for the shape of R(T) transitions. Better agreement with R(T) data can be achieved assuming the existence of short ‘weak links’ with slightly reduced local critical temperature Tc. Hence, one may conclude that an ‘exotic’ intrinsic electronic inhomogeneity either does not exist in our structures, or, if it does exist, it does not affect their resistive state properties, or does not provide any specific impact distinguishable from conventional weak links.

Analysis of the local superconducting properties in YBCO coated conductors with striations

In order to realize economical applications, it is important to reduce the ac loss of 2G high-temperature superconductor coated conductors. It seems to be reasonable that a multi-filamentary wire can decrease the magnetization loss. In this study, we prepared two samples of YBCO coated conductors with striations. We measured local superconducting properties of both samples by using Low Temperature Scanning Laser and Hall Probe Microscopy (LTSLHPM). The distribution of the local critical temperature of samples was analyzed from experimental results of Low Temperature Scanning Laser Microscopy (LTSLM) near the superconducting transition temperature. According to LTSLM results, spatial distributions of the local critical temperature of both samples are homogeneous. The local current density and the local magnetization in samples were explored from measuring stray fields by using Scanning Hall Probe Microscopy (SHPM). From SHPM results, the remanent field pattern of the one bridge sample in an external magnetic field confirms the Bean's critical state model and the three bridge sample has similar remanent field pattern of the one bridge sample. The local magnetization curve in the three bridge sample was measured from external fields from -500 Oe to 500 Oe. We visualized that the distribution of local hysteresis loss are related in the distribution of the remanent field of the three bridge sample. Although the field dependence of the critical current density must be taken into account, the relation of the local hysteresis loss and the remanent field from Bean's model was useful.

Pseudo-gap as a signature of inhomogeneous superconductivity in oxide interfaces

We present an explanation for recent tunneling experiments in LaAlO3/SrTiO3 interfaces which is based on the strongly inhomogeneous character of these interfaces. The measurements report signatures of superconductivity in the tunneling spectra while the global resistance of the sample is finite, i.e., a pseudo-gap state. In addition, even when the global resistance vanishes the zero-bias conductance remains finite. We show that these observations can be described by a model of superconducting (SC) islands embedded in a metallic background. The local critical temperatures of the SC island are randomly distributed, some of them necessarily exceeding the critical temperature for global percolation to the zero resistance state. Consequently, tunneling spectra display a suppression of the density of states and coherence peaks already well above the percolative transition. The temperature dependence of the spectra suggests that a sizable fraction of the metallic background becomes SC by proximity effect when the temperature is lowered.

Point-contact Andreev-reflection spectroscopy in Fe(Te,Se) films: multiband superconductivity and electron-boson coupling

We report on a study of the superconducting order parameter in thin films (with different Se contents: ) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude . The existence of this gap and its amplitude are confirmed by PCARS measurements in single crystals. The values of the two gaps and , once plotted as a function of the local critical temperature , turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.

Doping and critical-temperature dependence of the energy gaps in Ba(Fe1−xCox)2As2thin films

The dependence of the superconducting gaps in epitaxial Ba(Fe_{1-x}Co_{x})_2As_2 thin films on the nominal doping x (0.04 \leq x \leq 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the 2D Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps -- although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps \Delta_{S} and \Delta_{L} show similar monotonic trends as a function of the local critical temperature T_{c}^{A} (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e. to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with T_{c} according to the empirical law \Omega_{0}= 4.65*k_{B}*T_{c}, and with a total electron-boson coupling strength \lambda_{tot}= 2.22 for x \leq 0.10 (i.e. up to optimal doping) that slightly decreases to \lambda_{tot}= 1.82 in the overdoped samples (x = 0.15).

Metal–superconductor transition in low-dimensional superconducting clusters embedded in two-dimensional electron systems

Motivated by recent experimental data on thin film superconductors and oxide interfaces, we propose a random-resistor network apt to describe the occurrence of a metal–superconductor transition in a two-dimensional electron system with disorder on the mesoscopic scale. We consider low-dimensional (e.g. filamentary) structures of a superconducting cluster embedded in the two-dimensional network and we explore the separate effects and the interplay of the superconducting structure and of the statistical distribution of local critical temperatures. The thermal evolution of the resistivity is determined by a numerical calculation of the random-resistor network and, for comparison, a mean-field approach called effective medium theory (EMT). Our calculations reveal the relevance of the distribution of critical temperatures for clusters with low connectivity. In addition, we show that the presence of spatial correlations requires a modification of standard EMT to give qualitative agreement with the numerical results. Applying the present approach to an LaTiO3/SrTiO3 oxide interface, we find that the measured resistivity curves are compatible with a network of spatially dense but loosely connected superconducting islands.

Effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO

We studied the effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO by means of point-contact Andreev-reflection spectroscopy. The results show that the SmFe1−xRuxAsO0.85F0.15 system keeps a multigap character at least up to x = 0.50, and that the gap amplitudes Δ1 and Δ2 scale almost linearly with the local critical temperature . The gap ratios 2Δi/kBTc remain approximately constant only as long as Tc ≥ 30 K, and increase dramatically when Tc decreases further. This trend seems to be common to many Fe-based superconductors, irrespective of their family. Based on first-principle calculations of the bandstructure and of the density of states projected on the different bands, we show that this trend, as well as the Tc dependence of the gaps and the reduction of Tc upon Ru doping, can be explained within an effective three-band Eliashberg model as being due to a suppression of the superfluid density at finite temperature that, in turn, modifies the temperature dependence of the characteristic spin-fluctuation energy.

Temperature dependence of superconducting gaps in Mg 1− xAl xB 2 system investigated by SnS-Andreev spectroscopy

Detailed temperature dependence of both superconducting gaps was obtained directly by means of SnS-Andreev spectroscopy. The Δ σ , π ( T ) -curves were shown to be deviated from standard BCS-like behavior, due to k -space proximity effect between σ - and π -condensates, which could give a key to experimental determination of interband electron–phonon coupling constants. For the first time, an excellent qualitative agreement with theoretical predictions of Nicol and Carbotte, and Moskalenko and Suhl was shown. d I ( V ) / d V -spectra of SnS-Andreev contacts based on MgB 2 samples (with defects of crystal structure), and Mg 1− x Al x B 2 polycrystalline samples (with the local critical temperature T C variation 10 K ≤ T C ≤ 37 K ) were studied by means of the “break-junction” technique within the temperature range 4.2 K ≤ T ≤ T C .

Shift of the superconducting critical parameters due to correlated disorder

In a nonuniform superconductor with randomly distributed local critical temperature both the macroscopic critical temperature and the upper critical magnetic field strongly depend on the characteristic correlation length ρ 0 of correlated disorder. The shift of the macroscopic critical parameters from those for non-correlated disorder, which does not exist for white noise, is obtained for small ρ 0 in the framework of the Ginzburg–Landau theory.

Control of the vortex flow in microchannel arrays produced in YBCO films by heavy-ion lithography

High-energy heavy-ion lithography is a powerful tool for tuning both structural and electromagnetic properties of high temperature superconductors by inducing nanometer scale defects confined in micron scale patterns. We show how the vortex dynamics in YBCO thin films patterned by heavy-ion lithography can be controlled and potentially exploited for device applications. Both local critical temperature and local critical currents are effectively tailored by the imposed irradiation geometry. The direct visualization of the real-time dynamics of the magnetic pattern is achieved by the magneto-optical imaging technique, while confined vortex flow is revealed by the simultaneous measurement of the electrical resistance both along and perpendicular (Hall resistance) to the direction of the applied current. It is shown that, for microchannel arrays inclined with respect to the transport current flow, the direction of vortex motion is solely determined by the imposed irradiation pattern geometry, in a well-defined temperature range, for a given applied current.

Depth-dependent critical behavior inV2H

Using x-ray diffuse scattering, we investigate the critical behavior of an order-disorder phase transition in a defective ``skin layer'' of ${\text{V}}_{2}\text{H}$. In the skin layer, there exist walls of dislocation lines oriented normal to the surface. The density of dislocation lines within a wall decreases continuously with depth. We find that, because of this inhomogeneous distribution of defects, the transition effectively occurs at a depth-dependent local critical temperature. A depth-dependent scaling law is proposed to describe the corresponding critical ordering behavior.

Point-contact Andreev-reflection spectroscopy in segregation-freeMg1−xAlxB2 singlecrystals up to x = 0.32

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phaseMg1−xAlxB2 singlecrystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-bandBlonder–Tinkham–Klapwijk model allowed us to extract the gap amplitudesΔσ and Δπ. The gap values agree rather well with other PCAR results for Al-doped crystals and polycrystals upto x = 0.2 reported in the literature, and extend them to higher Al contents. In thelow-doping regime, however, we observed an increase in the small gapΔπ onincreasing x (or decreasing the local critical temperature of the junctions,TcA) which is not as clearly found for other samples. On further decreasingTcA below 30 K, both the gaps decrease and, up to the highest doping levelx = 0.32 anddown to TcA = 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashbergtheory shows that this gap trend can be explained as being mainly due to the band filling andto an increase in the interband scattering which is necessary to account for the increase inΔπ at low Alcontents (x<0.1). We suggest interpreting the following decrease ofΔπ for TcA<30 K as being governed by the onset of the inhomogeneity and disorder in the Al distributionthat partly mask the intrinsic effects of doping and are not taken into account in standardtheoretical approaches.

Excess conductivity in high-Tcsuperconducting thin films: Role of smooth doping disorder

While the time-dependent Ginzburg-Landau theory successfully describes a number of experimental results for homogeneous high-${T}_{c}$ superconductors, the proper value of the characteristic depairing electric field was recently found unreasonably small for Bi-Sr-Ca-Cu-O thin films. We present numerical simulations which show that these features can be the consequence of smooth spatial inhomogeneity of the local critical temperature. Because of the interplay of percolation and superconducting transition, even a small disorder affects strongly the nonlinear excess conductivity in the transition region, causing considerable rescaling of the characteristic electric field.