Stability Of Superconducting State Research Articles

On the Kinetic Energy Driven Superconductivity in the Two-Dimensional Hubbard Model

We investigate the role of kinetic energy for the stability of superconducting state in the two-dimensional Hubbard model on the basis of an optimization variational Monte Carlo method. The wave function is optimized by multiplying by correlation operators of site off-diagonal type. This wave function is written in an exponential-type form given as ψλ=exp(−λK)ψG for the Gutzwiller wave function ψG and a kinetic operator K. The kinetic correlation operator exp(−λK) plays an important role in the emergence of superconductivity in large-U region of the two-dimensional Hubbard model, where U is the on-site Coulomb repulsive interaction. We show that the superconducting condensation energy mainly originates from the kinetic energy in the strongly correlated region. This may indicate a possibility of high-temperature superconductivity due to the kinetic energy effect in correlated electron systems.

Interplay of hopping conductivity and superconductivity in magnetic superconductor RuSr2(Eu1.5Ce0.5)Cu2O10−δ

In this work, we analyze the temperature dependencies of resistance and magnetoresistance for two RuSr2(Eu1.5Ce0.5)Cu2O10−δ ceramic samples, one of which was left in the as-prepared state while another one was oxygen saturated. The measurements on these samples were made soon after preparation and repeated after their long storage (10 years) in an ambient atmosphere when they lost most of their over-stoichiometric and part of stoichiometric oxygen. Having studied the widest possible range of oxygen concentrations, we are trying to clarify not only the questions of stability of superconducting state in ruthenocuprates, but also the interplay of various types of electronic hopping conductivity and superconductivity in granular magnetic material. Despite the significant progress made in understanding the properties of disordered conductors, the old question of mutual influence and competition between localization and superconductivity still not clear. Investigation of the properties of electron transport as it approaches the metal-insulator transition will be useful and important.

Superconducting state stability in second generation high-temperature superconducting wires with varying copper stabilizer thickness under AC overcurrent conditions

Superconducting state stability in high-temperature superconducting (HTS) wires is of great importance for the reliable operation of superconducting electric power equipment. To this end, a study was made of the steady state operation and transients under AC current overloading in second generation (2G) HTS tape wires with a copper stabilizing layer of varying thickness. The existence of a threshold stabilizer thickness has been found, such that determines the HTS tape I-V curve and transients when an overcritical current flows. If the current exceeds the critical value, primary instability takes place in a wire with no stabilizer layer as well as with a layer of any thickness. In a wire with the stabilizer thickness less than the threshold level, an irreversible superconducting-to-normal transition occurs. However, if the copper layer thickness exceeds the threshold thickness, the HTS wire stable overloaded state recovers after some transients that have an irregular oscillation shape. The recovery of the stable thermal state under overcurrent conditions in HTS wires with an over-threshold stabilizer thickness is applicable in a variety of superconducting electric power devices, such as transformers and current limiters with a high capability to withstand faults. The measurements also show that the loss of superconductivity under AC overcritical current is followed by the phase angle changing between voltage and current. This phenomenon could be used for fast response normal zone detection in current-carrying conductors of superconducting equipment.

Thermal instability of superconducting composites at magnetic flux creep

The conditions of the onset and development of thermal instabilities occurring under the action of thermal disturbance in superconducting composites with voltage-current characteristic described by a power equation are discussed. The investigation is done in the framework of the quasi-linear model, as in the Stekly approximation. The critical energies, which lead to the transition of the superconducting state to the normal one, and the velocities of irreversible propagation of the thermal instability along the composite are determined. It is shown that the largest current stably flowing in a superconducting composite can be either lower or higher than a priori defined critical current of a superconductor with a real (continuously increasing) voltage-current characteristic. The condition of the complete thermal stabilization for the superconducting composites with a power voltage-current characteristic is formulated. It is shown that the commonly accepted Stekly condition of the complete thermal stabilization, according to which the superconducting composite is completely stable to the thermal perturbations with any energy, does not allow to determine correctly the conditions providing the complete thermal stabilization of superconducting composites with real voltage-current characteristics. The results of the numerical simulations express that the existing theory of the thermal stabilization in the framework of which a jump transition from a superconducting state to a normal one is supposed leads to a limitation of the currents, at which a stable superconducting state exists.

Thermal Hall Conductivity in Superconducting Phase on Kagome Lattice

Motivated by a previous "$sd^2$-graphene" study, the pairing symmetry in the superconducting state and the thermal Hall conductivity are investigated by a self-consistent Bogoliubov--de Gennes approach on the kagome lattice with intrinsic spin-orbit coupling near van Hove fillings. While the topologically trivial state with broken time-reversal symmetry appears in the absence of spin-orbit coupling, the highest flat band becomes dispersive with a hexagonal symmetry due to spin-orbit coupling, which leads to a topological superconducting state. Since the thermal Hall conductivity in the low-temperature limit is associated with the topological property of time-reversal symmetry breaking superconductors, we study its temperature dependence near van Hove fillings. In particular, the pairing symmetry in the highest flat band is sensitive to the amplitudes of spin-orbit coupling and the attractive interaction, which is reflected remarkably in the thermal Hall conductivity. The obtained result may enable us to investigate the stable superconducting state on the kagome lattice.

Conditions for thermal stabilization of the superconductor’s critical state

Conditions for thermal stabilization of the electrodynamic states of a superconductor are studied. The macroscopic states are simulated in the nonisothermal approximation by numerically solving a set of the Fourier and Maxwell equations with the magnetic flux penetration boundary unknown. Stability criteria for the critical state described by the viscous flow model are formulated. The results are compared with those fol� lowing from the isothermal theory. It is shown that errors inherent in the isothermal approximation are sig� nificant for a thermally insulated superconductor. Therefore, the wellknow n adiabatic criterion of stability formulated in the isothermal approximation limits the domain of stable states, since a correct determination of conditions for the superconducting-normal state transition must take into account the thermal history of the stable superconducting state formation. On the whole, the error of loss calculation in the isothermal approximation increases when the heat transfer coefficient decreases or an external magnetic field sweep and the size of the superconductor's cross section increases. On the other hand, nonisothermal stability condi� tions expand the variety of allowable states, since they include conditions that links the currently developed theory of thermomagnetic instability, the theory of losses, and the theory of a superconductor's thermal sta�

Effect of the magnetic subsystem of the DyRh3.8Ru0.2B4 compound on its superconducting properties

A new DyRh3.8Ru0.2B4 magnetic superconductor with LuRu4B4-type structure is synthesized and studied. The magnetic subsystem of the compound is shown to substantially affect the superconducting sub-system. The field dependence of the magnetic moment M(B) of the complex DyRh3.8Ru0.2B4 rhodium boride differs markedly from that of the nonmagnetic classic YRh4B4 superconductor. The Bc2(T) dependence for the magnetic DyRh3.8Ru0.2B4 superconductor deviates from the classic parabolic function and exhibits an abrupt rise at 2.8 K, which is related to the transition of the magnetic subsystem of the compound from a ferrimagnetic (FM) to antiferromagnetic (AFM) state. The FM AFM transition of the magnetic subsystem is accompanied by the transition of the superconducting subsystem to a thermodynamically more stable superconducting state.

Model of an Exotic Chiral Superconducting Phase in a Graphene Bilayer

We theoretically demonstrate the formation of a new type of unconventional superconductivity in graphene materials, which exhibits a gapless property. The studied superconductivity is based on an interlayer pairing of chiral electrons in bilayer graphene, which results in an exotic s-wave spin-triplet condensate order with anomalous thermodynamic properties. These include the possibility of a temperature-induced condensation causing an increase of the pairing gap with increasing temperature and an entropy of the stable superconducting state which can be higher than its value in the normal state. Our study reveals the analogy of the interlayer superconductivity in graphene materials to the color superconductivity in dense quark matter and the gapless pairing states in nuclear matter and ultracold atomic gases.

Boron-doped diamond — Grained Mott's metal revealing superconductivity

Reconsidering the experimental data on superconducting transition in heavily boron-doped polycrystalline diamond films, we have suggested an original theoretical model based on the concept of disordered Mott's metal in which the transport above T C is controlled by weak localization of holes. It has been shown that in the Kelvin temperature range the spins of holes occupying the neighboring weakly localized orbits must be strongly correlated and that just this effect, which is fully analogous to the Cooper pairing known in classical superconductors, can account for the existence of superconductivity transition in this material. The possible influence of grain boundaries on the superconducting transition is discussed in the framework of a simple model. Assuming thermal noise as a source voltage difference between adjacent grains, a phase diagram determining the region of stability of superconducting state depending on grain size is derived.

Investigation of the Rb–W–O system in connexion with the superconducting properties of the hexagonal tungsten bronzes

The previous studies of the superconducting properties of the hexagonal tungsten bronzes have repeatedly come up against the lack of reproducibility of the data and, at the first place, of the data relating to the stability of superconducting state. We revisited this problem and identified the main causes of these contradictory data in Rb x WO 3, among which the major one is the ordering of the alkali atoms. Our study has emerged onto a determination of the x dependence of the order–disorder transition, of the lattice parameters and of the superconducting transition temperature T c. We have also clarified the crystal structure and examined the mechanisms involved in the oxidation and reduction of this compound. Finally, we have assembled our data to draw a plausible Rb–W–O phase diagram.

Joule heat release in a superconducting composite under a transport current charge

The physical peculiarities of temperature dependence of heat release in a composite superconductor induced by varying transport current are analysed. The proposed investigation was based on a model describing the simultaneous thermal and electromagnetic processes inside the composite superconductor with nonlinear current–voltage characteristic correctly. Numerical solutions of the set of Fourier and Maxwell's equations were used. The analytical expression of the heat release in the superconducting composite during the flux-creep regime is found for its stable superconducting state. The results are compared with the well-known formulae describing a current-sharing approximation of heat generation in the superconducting composite. It is shown that for a high rate of current charging and with a low heat transfer coefficient, the Joule heat generation in a composite increases both in the stage of the steady superconducting state and during the development of instabilities. In particular, the value of generated heat release may differ by several orders of magnitude from the amount of heat induced by slower rates of a current charge. In these cases a notable increase of the superconductor temperature is observed during the entire process of current penetration. Therefore, for a high rate of current charge and with low heat transfer coefficients, the value of the heat released in a superconducting composite is notably larger, through the entire charging process, than would be calculated from the existing thermal stability theory.

Heat release accompanying current injection into a composite superconductor in a stable superconducting state

An investigation is made of the Joule dissipation of energy in the superconducting phase of a composite superconductor as the current in it increases. It is shown that at high rates of current injection the dissipative processes are characterized by an appreciable release of heat, which substantially influences the critical energies for the loss of superconductivity and the velocity of the normal zone. An analytical expression is given for calculating the power of the thermal losses during current injection.

Study of the thermal equilibriums of a one-dimensional superconductor wire

The Maddock criterion of equal areas [B. J. Maddock, G. B. James, W. T. Norris, Superconductive composites, heat transfer and steady state stabilization, Cryogenics 9 (1969) 261] gives a necessary and sufficient condition in order that a one-dimensional superconductor of infinite length, submitted to fixed current density and magnetic field, could not evolve towards a resistive state after an accidental local overheating. In this paper, we study the temperature distribution in a portion of a one-dimensional superconductor located between two regions for which the temperature can be perfectly controlled. In consequence, the considered length L of the conductor is supposed finite. In this context we propose, as an extension of Bonzi [B. Bonzi, Etudes des équilibres thermiques d'un supraconducteur existence et stabilité, thèse de doctorat, INPL, France, 1991; B. Bonzi, H. Lanchon-Ducauquis, Equilibres et stabilité thermiques d'un supraconducteur, C. R. Acad. Sci. Paris, t317, II, 1993, pp. 899–903.] and El Khomssi [M. El Khomssi, Etude des équations paraboliques et elliptiques gérant l'état thermique d'un supraconducteur, Thèse de doctorat, INPL, France, 1994; B. Bonzi, M. El Khomssi, Practical criteria for the thermal stability of a unidimensional superconductor, J. Phys. III, France 4 (1994) 653.] works: 1. A necessary and sufficient criterion called optimal criterion which allows us to enlarge the superconductor thermal stability conditions; this criterion depends on the heat source term (competition between the heat power caused by the Joule effect and the one absorbed by the cryogenic bath), the length of the superconductor and its thermal conductivity. This criterion makes sure the global thermal stability of the conductor without any condition on the thermal disturbance. 2. A characterisation of the thermal stationary states which can exist when the optimal criterion is not realized. 3. Some numerical realistic applications from which we deduce several important parameters for the superconducting state stability.

Ramp rate limitation in AC multi-strand superconducting cable due to statistical scattering in inductive coupling between strands

Theoretical investigation results of the current-carrying capacity of a superconducting cable with insulated strands depending on the inductive coupling between the strands are presented. The superconducting state stability was analyzed by a non-isothermal model. On this model, permissible rise in temperature of each strand is bounded by the conditions of magnetic flux diffusion within this strand. It is shown that if the matrix of inductances between the cables strands has asymmetric elements then the degradation of maximum possible current (quench current) can be quite considerable over the whole range of current rates. The degradation becomes stronger with increase of both the number of strands and scattering in their mutual inductance.

Possible d-like symmetry pairing states in the extended Hubbard model

Abstract Superconducting phases of the extended Hubbard model has been investigated in the weak-coupling limit at different temperatures and doping levels in the presence of on-site, nearest-neighbour and next-nearest-neighbour couplings. The main result is the occurrence of stable pairing states of different symmetries depending on the tunable parameters of the model; attractive nearest-neighbour couplings give rise to a stable superconducting state having dxr y2 symmetry, while attractive next-nearest-neighbour interactions favour pairing states of dxy symmetry.

Flux creep and flux jumping.

We consider the flux jump instability of the Bean's critical state arising in the flux creep regime in type-II superconductors. We find the flux jump field, $B_j$, that determines the superconducting state stability criterion. We calculate the dependence of $B_j$ on the external magnetic field ramp rate, $\dot B_e$. We demonstrate that under the conditions typical for most of the magnetization experiments the slope of the current-voltage curve in the flux creep regime determines the stability of the Bean's critical state, {\it i.e.}, the value of $B_j$. We show that a flux jump can be preceded by the magneto-thermal oscillations and find the frequency of these oscillations as a function of $\dot B_e$.

Peculiarities of the stability of superheated superconducting grains near Tc

Abstract An original method of preparation and sorting out of micron-size tin grains has been elaborated, which makes it possible to prepare colloids with identical by size spherical grains. Just for such colloids the peculiarities of micron-size tin grains' superheated superconducting state stability near T c has been registered. The observed behavior of the superheated state's stability is apparently a sort of size effect and may be linked with the change of the nature of the phase transition at temperatures near T c , at which the penetration depth λ and the coherence length ξ become comparable with the grain size. A quantitative method for determination of the penetration depth at zero temperature λ 0 of grained superconducting systems has also been developed.

Superconducting state stability in current charging into composites

Numerical and analytical calculations of the current charging process into a round superconducting composite with properties homogenized over its cross-section are presented. A solution was sought which accounted for simultaneous thermal and electromagnetic processes taking place. A wire was considered with a current-voltage characteristic approximated by an exponential curve with the current density and temperature taken as parameters. The results obtained are compared with the pulsed thermal disturbances stability theory. It is shown that calculations of the superconducting state stability of the wire made at various rates of current charging, the I- V characteristics, the matrix materials, the heat transfer coefficients and other parameters agree qualitatively with the basic principles of the thermal stability theory. Accordingly, a limiting current exists below which the wire remains in the superconducting state and above which the wire is quenched when current charging is stopped; this current is somewhat less than the take-off current. The approximate analytical solution of the problem is defined. The non-isothermality of the wire, its heat capacity, and its thermal and electric conductivities are taken into account in the derived stability criterion.

Superconductivity of narrow-band metals in a model of multichannel scattering on electron density fluctuations

The authors consider the possibility that superconductivity (SC) may arise in narrow-band metals owing to electron-electron correlations. Consideration is given also to the superconductivity suppression mechanisms which is caused by the same correlations (spin fluctuations, or transition to dielectric phase). A new approach is proposed to describe high-Tc SC by introducing the Nambu field in the functional integral scheme. A stable superconducting state is realized in a rather small region of parameters: hole concentration and Hubbard repulsion. A method is proposed for approximating an interacting system by means of some simple multichannel models.

Heat transfer to liquid and supercritical helium in superconducting rotors

Cooling designs of superconducting generator rotors are important for maintaining a stable superconducting state of field windings, and it is essential to comprehend the heat transfer characteristics of helium in rotating fields. Experiments were carried out using a large-scale rotating cryostat with a cold rotor diameter of approximately 800 mm. Clear differences among the steady-state heat transfer characteristics of helium were confirmed between low centrifugal acceleration fields and high centrifugal acceleration fields, resulting in the collection of heat transfer data under the supercritical state necessary for cooling designs. The heat transfer characteristics of liquid and supercritical helium under conditions of gravitational and centrifugal acceleration fields are discussed. >