Breakdown Of Superconductivity Research Articles

The local rupture of the superconductivity and the curvature of the mesoscopic sample

In this article we revisit a well-known effect in superconductors, which is the penetration of the magnetic field by regions of greater radius of curvature (Tinkham, 2008; De Gennes, 1966). This is exposed in the literature but without a quantitative solution. Here we solve this problem by two distinct numerical simulations. We calculate the superconducting current density using numerical simulations via the London theory in the Meissner state and numerical simulations using the time-dependent Ginzburg–Landau (TDGL) theory for the vortex state. In both simulations, the results obtained are in agreement, as could not be otherwise. We show that in the Meissner state the current density increases much faster in the region where the radius of curvature is larger, thus reaching the critical value first in this location, and therefore local breakdown of superconductivity occurs. In the vortex state, our simulations, using TDGL, show that the vortices penetrate through exactly the same location pointed out by the previous simulation. For this study we worked with a superconducting needle with an ellipsoidal mesoscopic cross section.

Puddle formation and persistent gaps across the non-mean-field breakdown of superconductivity in overdoped (Pb,Bi)2Sr2CuO6+δ

The cuprate high-temperature superconductors exhibit many unexplained electronic phases, but the superconductivity at high doping is often believed to be governed by conventional mean-field Bardeen–Cooper–Schrieffer theory1. However, it was shown that the superfluid density vanishes when the transition temperature goes to zero2,3, in contradiction to expectations from Bardeen–Cooper–Schrieffer theory. Our scanning tunnelling spectroscopy measurements in the overdoped regime of the (Pb,Bi)2Sr2CuO6+δ high-temperature superconductor show that this is due to the emergence of nanoscale superconducting puddles in a metallic matrix4,5. Our measurements further reveal that this puddling is driven by gap filling instead of gap closing. The important implication is that it is not a diminishing pairing interaction that causes the breakdown of superconductivity. Unexpectedly, the measured gap-to-filling correlation also reveals that pair breaking by disorder does not play a dominant role and that the mechanism of superconductivity in overdoped cuprate superconductors is qualitatively different from conventional mean-field theory.

Magnetic field enhanced critical current and subharmonic structures in dissipative superconducting gold nanowires

Quantum breakdown of superconductivity in low-dimensional superconducting systems has attracted enormous attention due to the underlying fluctuation. Here we report exotic phenomena observed during the current-induced breakdown of superconductivity in gold nanowires contacted by superconducting electrodes. In a 1.0 μm-long nanowire, the resistance as a function of current exhibits a random resistance switching at 1.8 K and tail-like metallic state at higher temperatures, indicating phase slips in proximity-induced superconducting nanowire. Additionally, the upper critical current is enhanced under low magnetic field, which may be attributed to the formation of spin-triplet pairing in superconducting gold nanowire. In a 1.2 μm-long gold nanowire, unexpected subharmonic structures with characteristic energy smaller than superconducting gap are superimposed on tail-like structures. The signal might be related to the axions-triggered process in phase slip center Josephson junction and thus stimulate further studies in dissipative superconducting nanowires.

Detecting Induced p±ip Pairing at the Al-InAs Interface with a Quantum Microwave Circuit.

Superconductor-semiconductor hybrid devices are at the heart of several proposed approaches to quantum information processing, but their basic properties remain to be understood. We embed a two-dimensional Al-InAs hybrid system in a resonant microwave circuit, probing the breakdown of superconductivity due to an applied magnetic field. We find a fingerprint from the two-component nature of the hybrid system, and quantitatively compare with a theory that includes the contribution of intraband p±ip pairing in the InAs, as well as the emergence of Bogoliubov-Fermi surfaces due to magnetic field. Separately resolving the Al and InAs contributions allows us to determine the carrier density and mobility in the InAs.

Projected BCS theory for the unification of antiferromagnetism and strongly correlated superconductivity

A close connection between antiferromagnetism and superconductivity is at the core of high-temperature superconductivity. Here, we put forward the projected BCS theory for the unification of antiferromagnetism and strongly correlated superconductivity. Specifically, it is shown that, with the $d$-wave pairing symmetry, the projected BCS theory can provide excellent trial states at general doping for the exact ground states of the $t\text{\ensuremath{-}}J$ model in the square lattice, generating a unified theory of high-temperature superconductivity as a continuous function of hole concentration. A key to the success of the projected BCS theory is an accurate treatment of the strong correlation between Cooper pairs, which not only causes the breakdown of superconductivity itself at half filling but also defines the precise nature of strongly correlated superconductivity at moderate doping. Also, via the proper incorporation of particle number fluctuations, the projected BCS theory allows direct computation of the superconducting order parameter, shedding important light on the pseudogap phenomenon.

Grain boundary segregation and carbide precipitation in heat treated niobium superconducting radio frequency cavities

A fundamental understanding of superconducting radio frequency Nb cavity processing is necessary to achieve the desired improvement in their performance, which is needed for further upgrades of modern particle accelerators. To recognize the physical processes behind the losses in the accelerator modules, it is required to address not only the observed improvements but also the degradation occurring after different surface treatments. Here, we report on microscopic and spectroscopic studies of several cutouts from an extremely well performing cavity, which showed a systematic degradation after modified surface treatments and annealing conditions. Our results suggest that an abundance of low-angle grain boundaries surrounding the small sized grains can be related to the local superconductivity breakdown at high accelerating field gradients. Losses due to grain boundary segregated carbides are discussed to being most dominant and to leading to an anomalous Q-degradation of the whole cavity starting at low fields.

Supercurrent and Phase Slips in a Ballistic Carbon Nanotube Bundle Embedded into a van der Waals Heterostructure.

We demonstrate long-range superconducting correlations in a several-micrometers-long carbon nanotube bundle encapsulated in a van der Waals stack between hBN and NbSe2. We show that a substantial supercurrent flows through the nanotube section beneath the NbSe2 crystal as well as through the 2 μm long section not in contact with it. The large in-plane critical magnetic field of this supercurrent is an indication that even inside the carbon nanotube Cooper pairs enjoy a degree of paramagnetic protection typical of the parent Ising superconductor. As expected for superconductors of nanoscopic cross section, the current-induced breakdown of superconductivity is characterized by resistance steps due to the nucleation of phase slip centers. All elements of our hybrid device are active building blocks of several recently proposed setups for realization of Majorana fermions in carbon nanotubes.

Breakdown of superconductivity in a magnetic field with self-intersecting zero set

We prove that the lowest eigenvalue of the Laplace operator with a magnetic field having a self-intersecting zero set is a monotone function of the parameter defining the strength of the magnetic field, in a neighborhood of infinity. We apply this monotonicity result on the study of the transition from superconducting to normal states for the Ginzburg-Landau model, and prove that the transition occurs at a unique threshold value of the applied magnetic field.

Field-induced quantum breakdown of superconductivity in magnesium diboride

The quantum breakdown of superconductivity (QBS) is the reverse, comprehensive approach to the appearance of superconductivity. A quantum phase transition from superconducting to insulating states tuned by using nonthermal parameters is of fundamental importance to understanding the superconducting (SC) phase but also to practical applications of SC materials. However, the mechanism of the transition to a nonzero resistive state deep in the SC state is still under debate. Here, we report a systematic study of MgB2 bilayers with different thickness ratios for undamaged and damaged layers fabricated by low-energy iron-ion irradiation. The field-induced QBS is discovered at a critical field of 3.2 Tesla (=Hc), where the quantum percolation model best explains the scaling of the magnetoresistance near Hc. As the thickness of the undamaged layer is increased, strikingly, superconductivity is recovered from the insulating state associated with the QBS, showing that destruction of quantum phase coherence among Cooper electron pairs is the origin of the QBS.

Quantum breakdown of superconductivity in low-dimensional materials

In order to understand the emergence of superconductivity it is useful to study and identify the various pathways leading to the destruction of superconductivity. One way is to use the increase in Coulomb-repulsion due to the increase in disorder, which overpowers the attractive interaction responsible for Cooper-pair formation. A second pathway, applicable to uniformly disordered materials, is the competition between superconductivity and Anderson localization, which leads to electronic granularity in which phase and amplitude fluctuations of the superconducting order parameter play a role. Finally, a third pathway is an array of superconducting islands coupled by some form of proximity-effect, due to Andreev-reflections, and which leads from a superconducting state to a state with finite resistivity, which appears like a metallic groundstate. This review summarizes recent progress in understanding of these different pathways, including experiments in low dimensional materials and application in superconducting quantum devices.

The breakdown of superconductivity in the presence of magnetic steps

Many earlier works were devoted to the study of the breakdown of superconductivity in type-II superconducting bounded planar domains, submitted to smooth magnetic fields. In the present contribution, we consider a new situation where the applied magnetic field is piecewise-constant, and the discontinuity jump occurs along a smooth curve meeting the boundary transversely. To handle this situation, we perform a detailed spectral analysis of a new effective model. Consequently, we establish the monotonicity of the transition from a superconducting to a normal state. Moreover, we determine the location of superconductivity in the sample just before it disappears completely. Interestingly, the study shows similarities with the case of corner domains subjected to constant fields.

Persistence of superconductivity in thin shells beyond Hc1

In Ginzburg–Landau theory, a strong magnetic field is responsible for the breakdown of superconductivity. This work is concerned with the identification of the region where superconductivity persists, in a thin shell superconductor modeled by a compact surface [Formula: see text], as the intensity [Formula: see text] of the external magnetic field is raised above [Formula: see text]. Using a mean field reduction approach devised by Sandier and Serfaty as the Ginzburg–Landau parameter [Formula: see text] goes to infinity, we are led to studying a two-sided obstacle problem. We show that superconductivity survives in a neighborhood of size [Formula: see text] of the zero locus of the normal component [Formula: see text] of the field. We also describe intermediate regimes, focusing first on a symmetric model problem. In the general case, we prove that a striking phenomenon we call freezing of the boundary takes place: one component of the superconductivity region is insensitive to small changes in the field.

Enhancement of low-frequency fluctuations and superconductivity breakdown in Mn-dopedLa1−yYyFeAsO0.89F0.11superconductors

$^{19}\mathrm{F}$ NMR measurements in optimally electron-doped ${\mathrm{La}}_{1\text{\ensuremath{-}}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\text{\ensuremath{-}}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds $(y=0$ and $y=0.2)$ where the chemical pressure is varied by substituting La with Y. In the $y=0.2$ series superconductivity is suppressed for Mn contents an order of magnitude larger than for $y=0$. For both series a peak in the $^{19}\mathrm{F}$ NMR nuclear spin-lattice relaxation rate $1/{T}_{1}$ emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. $^{19}\mathrm{F}$ NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the $y=0$ series, at variance with what one would expect for $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn.

Characterization of superconducting radiofrequency breakdown by two-mode excitation

We show that thermal and magnetic contributions to the breakdown of superconductivity in radiofrequency (RF) fields can be separated by applying two RF modes simultaneously to a superconducting surface. We develop a simple model that illustrates how mode-mixing RF data can be related to properties of the superconductor. Within our model the data can be described by a single parameter, which can be derived either from RF or thermometry data. Our RF and thermometry data are in good agreement with the model. We propose to use mode-mixing technique to decouple thermal and magnetic effects on RF breakdown of superconductors.

NUMERICAL STUDY ON IRREVERSIBLE BEHAVIOR OF THz WAVE EMISSION FROM INTRINSIC JOSEPHSON JUNCTIONS

Recently novel current-driven resonant states characterized by the π-phase kinks were proposed in numerical and analytic studies on THz wave emission from intrinsic Josephson junctions based on the coupled sine-Gordon equation. In these states hysteresis behavior is observed with respect to the application process of current, and such behavior is due to nonlinearity in the Josephson coupling term. Varying the strength of the critical current, there exists a critical strength for the hysteresis behavior in the fundamental mode and at the critical strength the applied current at the emission peak coincides with the critical one, which means breakdown of superconductivity in actual systems. In higher-harmonic modes in the vicinity of the critical current, the strength of hysteresis becomes small and emission can be observed in the reverse process. Such "quasi-reversible" behavior may explain "reversible emission" reported in a recent experiment.

Effect of impurities on the superheating field of type-II superconductors

We consider the effect of nonmagnetic and magnetic impurities on the superheating field $H_s$ in a type-II superconductor. We solved the Eilenberger equations, which take into account the nonlinear pairbreaking of Meissner screening currents, and calculated $H_s(T)$ for arbitrary temperatures and impurity concentrations in a single-band s-wave superconductor with a large Ginzburg-Landau parameter. At low temperatures nonmagnetic impurities suppress a weak maximum in $H_s(T)$ which has been predicted for the clean limit, resulting instead in a maximum of $H_s$ as a function of impurity concentration in a moderately clean limit. It is shown that nonmagnetic impurities weakly affect $H_s$ even in the dirty limit, while magnetic impurities suppress both $H_s$ and the critical temperature $T_c$. The density of quasiparticles states $N(\epsilon)$ is strongly affected by an interplay of impurity scattering and current pairbreaking. We show that a clean superconductor at $H=H_s$ is in a gapless state, but a quasiparticle gap $\epsilon_g$ in $N(\epsilon)$ at $H=H_s$ appears as the concentration of nonmagnetic impurities increases. As the nonmagnetic scattering rate $\alpha$ increases above $\alpha_c=0.36$, the quasiparticle gap $\epsilon_g(\alpha)$ at $H=H_s$ increases, approaching $\epsilon_g\approx 0.32\Delta_0$ in the dirty limit $\alpha\gg 1$, where $\Delta_0$ is the superconducting gap parameter at zero field. The effects of impurities on $H_s$ can be essential for the nonlinear surface resistance and superconductivity breakdown by strong RF fields.

Analytical approach to the thermal instability of superconducting films under high current densities

Using the Green's function of the 3D heat equation, we develop an analytical account of the thermal behaviour of superconducting films subjected to electrical currents larger than their critical current in the absence of an applied magnetic field. Our model assumes homogeneity of films and current density, and besides thermal coefficients employs parameters obtained by fitting to experimental electrical field - current density characteristics at constant bath temperature. We derive both a tractable dynamic equation for the real temperature of the film up to the supercritical current density J^\ast (the lowest current density inducing transition to the normal state), and a thermal stability criterion that allows prediction of J^\ast . For two typical YBCO films, J^\ast predictions agree with observations to within 5%. These findings strongly support the hypothesis that a current-induced thermal instability is generally the origin of the breakdown of superconductivity under high electrical current densities, at least at temperatures not too far from Tc.

Current-induced in-plane superconducting transition in intrinsic Josephson junctions

In stacks of intrinsic Josephson junctions (IJJs) with lateral sizes of several microns,the current is non-uniform in many cases. In certain geometries a significantpart of the current flows along the superconducting planes and can reach thecritical value. The current-driven superconductivity breakdown within a singleCu2O4 plane can be seen as an extra branch structure of thec-axis current–voltage characteristics. This allows us to deduce the sheet critical current of a singleCu2O4 plane in different measurement configurations. The conditions for the observationof such a current-induced transition in different IJJ geometries are discussed.

Size Dependent Breakdown of Superconductivity in Ultranarrow Nanowires

Below a certain temperature T(c) (typically cryogenic), some materials lose their electric resistance R entering a superconducting state. Following the general trend toward a large scale integration of a greater number of electronic components, it is desirable to use superconducting elements in order to minimize heat dissipation. It is expected that the basic property of a superconductor, i.e., dissipationless electric current, will be preserved at reduced scales required by modern nanoelectronics. Unfortunately, there are indications that for a certain critical size limit of the order of approximately 10 nm, below which a "superconducting" nanowire is no longer a superconductor in a sense that it acquires a finite resistance even at temperatures close to absolute zero. In the present paper we report experimental evidence for a superconductivity breakdown in ultranarrow quasi-1D aluminum nanowires.

Inhomogeneously doped two-leg ladder systems

A chemical potential difference between the legs of a two-leg ladder is found to be harmful for Cooper pairing. The instability of superconductivity in such systems is analyzed by compairing results of various analytical and numerical methods. Within a strong coupling approach for the t-J model, supplemented by exact numerical diagonalization, hole binding is found unstable beyond a finite, critical chemical potential difference. The spinon-holon mean field theory for the t-J model shows a clear reduction of the the BCS gaps upon increasing the chemical potential difference leading to a breakdown of superconductivity. Based on a renormalization group approach and Abelian bosonization, the doping dependent phase diagram for the weakly interacting Hubbard model with different chemical potentials was determined.