Isotropic Superconductors Research Articles

Nearly isotropic superconductivity in (Ba,K)Fe2As2

Superconductivity was recently observed in iron-arsenic-based compounds with a superconducting transition temperature (T(c)) as high as 56 K, naturally raising comparisons with the high-T(c) copper oxides. The copper oxides have layered crystal structures with quasi-two-dimensional electronic properties, which led to speculation that reduced dimensionality (that is, extreme anisotropy) is a necessary prerequisite for superconductivity at temperatures above 40 K (refs 8, 9). Early work on the iron-arsenic compounds seemed to support this view. Here we report measurements of the electrical resistivity in single crystals of (Ba,K)Fe(2)As(2) in a magnetic field up to 60 T. We find that the superconducting properties are in fact quite isotropic, being rather independent of the direction of the applied magnetic fields at low temperature. Such behaviour is strikingly different from all previously known layered superconductors, and indicates that reduced dimensionality in these compounds is not a prerequisite for 'high-temperature' superconductivity. We suggest that this situation arises because of the underlying electronic structure of the iron-arsenic compounds, which appears to be much more three dimensional than that of the copper oxides. Extrapolations of low-field single-crystal data incorrectly suggest a high anisotropy and a greatly exaggerated zero-temperature upper critical field.

Investigation of stimulated dynamics in strongly anisotropic high-temperature superconductors system Bi–Pb–Sr–Ca–Cu–O

It is used the mechanical method of Abrikosov vortex stimulated dynamics investigation in superconductors. With its help it was studied relaxation phenomena in vortex matter of high-temperature superconductors. It established that pulsed magnetic fields change the course of relaxation processes taking place in vortex matter. The study of the influence of magnetic pulses differing by their durations and amplitudes on vortex system of strongly anisotropic high-temperature superconductors system Bi 1.7Pb 0.3Sr 2Ca 2Cu 3O 10− δ showed the presence of threshold phenomena. The small duration pulses do not change the course of relaxation processes taking place in vortex matter. When the duration of pulses exceeds some critical value (threshold), then their influence changes the course of relaxation process which is revealed by stepwise change of relaxing mechanical moment τ rel . These investigations showed that the time for formatting of Abrikosov vortex lattice in Bi 1.7Pb 0.3Sr 2Ca 2Cu 3O 10− δ is of the order of 150 μs which on the order of value exceeds the time necessary for formation of a single vortex observed in isotropic high-temperature superconductor HoBa 2Cu 3O 7− δ and on two orders exceeds the creation time of a single vortex observed in classical type II superconductors.

Pseudogap and time reversal breaking in a holographic superconductor

Classical SU(2) Yang-Mills theory in 3+1 dimensional anti-de Sitter space is known to provide a holographic dual to a 2+1 system that undergoes a superconducting phase transition. We study the electrical conductivity and spectral density of an isotropic superconducting phase. We show that the theory exhibits a pseudogap at low temperatures and a nonzero Hall conductivity. The Hall conductivity is possible because of spontaneous breaking of time reversal symmetry.

S-Wave Order Parameter in the Presence of a Momentum-Dependent Impurity Scattering Potential

Momentum dependence of the impurity potential leads to the anisotropy of a local density of states in isotropic s-wave superconductors. We complete this result by discussing the influence of a momentum-dependent impurity potential on the s-wave order parameter. The scattering process is considered in the T-matrix approximation, and the model impurity potential consists of two separate terms representing isotropic (on-site) and anisotropic (momentum-dependent) parts of the potential. We show that the effect of the isotropic part on the order parameter is local whereas the influence of the momentum-dependent term is nonlocal.

Electrodynamics of Fulde-Ferrell-Larkin-Ovchinnikov superconducting state

The Ginzburg-Landau theory of the vortex lattice for clean isotropic three-dimensional superconductors is developed, including the space variation of currents and fields, in the limit when the critical field is mainly determined by the paramagnetic depairing effect and the orbital effect is of minor importance. Then, in addition to the Abrikosov vortex lattice, the formation of a Fulde-Ferrell-Larkin-Ovchinnikov state is favored. In this case, the diamagnetic superfluid currents mainly come from paramagnetic interaction of electron spins with the local magnetic field, and not from the kinetic energy response to the external field as usual. We find that the stable vortex lattice keeps its triangular structure as in the habitual Abrikosov mixed state, while the internal magnetic field acquires components perpendicular to the applied magnetic field. Experimental possibilities related to this prediction are discussed.

Macroscopic quantum tunneling and quasiparticle-tunneling blockade effect ins-wave/d-wave hybrid junctions

We have theoretically investigated macroscopic quantum tunneling (MQT) and the influence of nodal quasiparticles and zero energy bound states (ZES) on MQT in s-wave/ d-wave hybrid Josephson junctions. In contrast to d-wave/d-wave junctions, the low-energy quasiparticle dissipation resulting from nodal quasiparticles and ZES is suppressed due to a quasiparticle-tunneling blockade effect in an isotropic s-wave superconductor. Therefore, the inherent dissipation in these junctions is found to be very weak. We have also investigated MQT in a realistic s-wave/d-wave (Nb/Au/YBCO) junction in which Ohmic dissipation in a shunt resistance is stronger than the inherent dissipation and find that MQT is observable within the current experimental technology. This result suggests high potential of s-wave/d-wave hybrid junctions for applications in quantum information devices.

Investigation of relaxation phenomena in high-temperature superconductors HoBa2Cu3O7−δ at the action of pulsed magnetic fields

Abstract A mechanical method to investigate the stimulated Abrikosov vortex dynamics in superconductors has been used. We found out that pulsed magnetic fields change the path of relaxation processes taking place in vortex matter. The study of the influence of magnetic pulses differing in their durations and amplitudes on the vortex system of the isotropic high-temperature superconductor system HoBa 2 Cu 3 O 7− δ showed the presence of a threshold phenomena. Short duration pulses do not change the path of relaxation processes taking place in vortex matter. When the duration of pulses exceeds some critical value (threshold), then their influence changes the path of relaxation process which is revealed by a stepwise change of the relaxing mechanical moment τ rel . These investigations showed that the time for formatting of the Abrikosov vortex lattice in HoBa 2 Cu 3 O 7− δ is of the order of 20 μs which exceeds by one order of magnitude the time necessary for formation of a single vortex observed in type II superconductors.

Critical magnetic field of the vortex-free state in NbC thin films and prospects for its observation in MgB2

The critical magnetic fields H c‖ and H c2 are measured for thin films of the isotropic superconductor NbC. It is revealed that the critical fields exhibit strong anisotropy due to the vortex-free state of the film in a magnetic field aligned parallel to its surface. The H c‖/H c2 ratio at 2 K exceeds 6 and increases with increasing temperature. The dependence H c‖(T) agrees quantitatively with the concepts of microscopic theory on the vortex-free state of a thin film of a clean superconductor in the temperature range below T c . As the electron mean free path decreases under irradiation of the film with a low dose of He+ ions, the critical field H c‖ remains unchanged near T c but increases significantly at lower temperatures. The well-known theoretical models are used to estimate the electronic parameters and thicknesses of MgB2 films for which the specific features associated with the vortex-free state of the two-gap superconductor can manifest themselves in the temperature dependence of the critical magnetic field H c‖(T).

Multiband superconductivity in HoNi2B2C

HoNi2B2C single crystals prepared by the floating zone method were investigated by resistivity, magnetization, specific heat and point-contact measurements. The coexistence of superconductivity (SC) and antiferromagnetism (AFM) in HoNi2B2C is considered experimentally and theoretically for commensurate (C), incommensurate (IC) and metamagnetic magnetic structures within the multiband picture of Fermi surface sheets (FSS) obtained within full potential FPLO–LDA calculations. Special attention is paid to the admixture of Ho 5d states which mediate the pair breaking exchange between the Ho 4f derived local moments and the conduction electrons bearing the superconductivity. Within the paramagnetic and the uniform IC spiral phases, anisotropic multiband SC coexists with magnetism. Within the C-phase (below TN=5.3K) single band isotropic SC survives at a single FSS free of Ho 5d states in accord with isotropic standard WHH shaped upper critical fields Hc2(T). The superconducting energy gap obtained from point-contact data was found to disappear at a slightly higher temperature T∗=5.6K which is also observed in specific heat measurements. Using the experimental values for the energy gap Δ(0) and Hc2(0) at zero temperature, the characteristic phonon frequency, the electron–phonon coupling constant and the Fermi velocity of the residual superconducting phase were estimated from the standard isotropic single band model.

Meissner effect in a system of coupled one-dimensional superconducting wires: Monte Carlo simulations

Motivated by the observation of superconducting behavior in $4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ carbon nanotube-zeolite composites, we use Monte Carlo simulations to evaluate the Meissner effect in a system of coupled one-dimensional superconducting wires. We model the system by using an anisotropic Ginzburg-Landau free energy functional, with distinct transverse and longitudinal coupling constants. By varying the degree of anisotropy, we obtain behaviors ranging from independent wires with suppressed superconducting transition, to an isotropic superconducting system. We numerically evaluate the Meissner effect for finite-sized superconducting domains. Comparison with the experimental data for carbon nanotube-zeolite composite shows the size of the phase-coherent, superconducting domains to be small, i.e., on the order of one or a few transverse coherence lengths. We provide a consistent estimate of the material parameters for the nanotube-zeolite composite, ranging from the penetration length of transverse magnetic field to the longitudinal and transverse coherence lengths.

Interplay of paramagnetic, orbital, and impurity effects on the phase transition of a normal metal to the superconducting state

We derive the generalized Ginzburg-Landau free energy functional for conventional and unconventional singlet superconductors in the presence of paramagnetic, orbital and impurity effects. Within the mean field theory, we determine the criterion for appearence of the non uniform (Fulde-Ferrell-Larkin-Ovchinnikov) superconducting state, with vortex lattice structure and additional modulation along the magnetic field. We also discuss the possible change of the order of transition from normal to superconducting state. We find that the superconducting phase diagram is very sensitive to geometrical effects such as the nature of the order parameter and the shape of the Fermi surface. In particular, we obtain the qualitative phase diagrams for three-dimensional isotropic s-wave superconductors and in quasi two-dimensional d-wave superconductors under magnetic field perpendicular to the conducting layers. In addition, we determine the criterion for instability toward non uniform superconducting state in s-wave superconductors in the dirty limit.

Superconducting Gap of Filled Skutterudite Superconductor LaRu4P12 Studied by Sub-meV Resolution Photoemission Spectroscopy

We study the superconducting gap of the filled skutterudite superconductor LaRu 4 P 12 (transition temperature T c =7.2 K) using a newly developed sub-meV resolution photoemission spectrometer with a vacuum-UV laser as a light source. The result at 4.8 K shows the opening of a superconducting gap, whose spectral shape is well explained with an isotropic s-wave Dynes function using a gap size of 1.3 meV. This corresponds to the reduced gap 2Δ(0)/ k B T c of 4.2. The temperature dependence of the superconducting gap shows fairly good agreement with the conventional BCS theory. The present results are consistent with those of a moderately strong-coupling and isotropic s-wave superconductor.

Evidence for s-Wave Superconductivity with Antiferromagnetic Fluctuations in Filled Skutterudite LaFe4P12: 139La and 31P-NMR Studies

We have performed 139 La, 31 P-NMR studies on a filled skutterudite superconductor LaFe 4 P 12 with the critical temperature T c =4.1 K. In the normal state, the presence of antiferromagnetic (AFM) fluctuations at finite wave vectors is suggested from the relation between the nuclear spin-lattice relaxation rate 1/ T 1 and the Knight shift. In the superconducting (SC) state, the distinct coherence peak is observed just below T c , and is easily suppressed by applied fields. An exponential decrease of 1/ T 1 is observed, suggestive of the isotropic SC gap with 2Δ/( k B T c )=3.8. Besides, we observed the decrease of the Knight shift, indicative of the singlet-pair formation below T c . These results are clear evidence that LaFe 4 P 12 is a rare s -wave superconductor with significant AFM fluctuations.

Universal scaling for both isotropic and anisotropic irradiated superconductors

Abstract Current–voltage characteristics are investigated by transport measurements on heavy-ion irradiated Bi2Sr2CaCu2O8+δ (γ=380), Bi1.8Pb0.2Sr2CaCu2O8+δ (γ=60) and isotropic (K,Ba)BiO3 single crystals. In the range of magnetic fields from Bϕ/10 to Bϕ (where Bϕ is the matching field), the same scaling functions can be used to describe the transition in these systems. We obtain ν⊥=1.1±0.1, z=5.3±0.2 using α (≡ν∥/ν⊥)=2, in good agreement with numerical simulations for the case of strongly screened vortex line interactions. The most important finding is that the scaling laws of the I–V characteristics in these systems could be the same despite the difference in anisotropy.

Green functions for nearest- and next-nearest-neighbor hopping on the Bethe lattice

Dedicated to Bernhard Mühlschlegel on the occasion of his 80th birthday We investigate the local density of states and the order parameter structure inside an elliptic quantum corral on surfaces of isotropic and anisotropic superconductors. The Bogoliubov-de Gennes equations are solved in the presence of non-magnetic and magnetic impurities. We observe and discuss a variety of mirage and anti-mirage phenomena, which specifically reflect the nature of the superconducting pairing state.

Mirage phenomena in superconducting quantum corrals

AbstractDedicated to Bernhard Mühlschlegel on the occasion of his 80th birthdayWe investigate the local density of states and the order parameter structure inside an elliptic quantum corral on surfaces of isotropic and anisotropic superconductors. The Bogoliubov‐de Gennes equations are solved in the presence of non‐magnetic and magnetic impurities. We observe and discuss a variety of mirage and anti‐mirage phenomena, which specifically reflect the nature of the superconducting pairing state.

Mirage phenomena in superconducting quantum corrals

We investigate the local density of states and the order parameter structure inside an elliptic quantum corral on surfaces of isotropic and anisotropic superconductors. The Bogoliubov-de Gennes equations are solved in the presence of non-magnetic and magnetic impurities. We observe and discuss a variety of mirage and anti-mirage phenomena, which specifically reflect the nature of the superconducting pairing state.

Quasi-particle spectrum of nano-scale conventional and unconventional superconductors under magnetic field

We have developed a numerical method to solve the Bogoliubov-de Gennes equation for nano-scaled isotropic and d-wave superconductors. It is based on the finite element method, and therefore applicable to arbitrary geometries. We argue the difference of the local density of states between isotropic and a d-wave superconducting square plate. For d-wave case, it appears as intrinsic surface bound states.

Elasticity, fluctuations, and vortex pinning in ferromagnetic superconductors: A columnar elastic glass

We study the elasticity, fluctuations, and pinning of a putative spontaneous vortex solid in ferromagnetic superconductors. Using a rigorous thermodynamic argument, we show that in the idealized case of vanishing crystalline pinning anisotropy the long-wavelength tilt modulus of such a vortex solid vanishes identically, as guaranteed by the underlying rotational invariance. The vanishing of the tilt modulus means that, to lowest order, the associated tension elasticity is replaced by the softer, curvature elasticity. The effect of this is to make the spontaneous vortex solid qualitatively more susceptible to the disordering effects of thermal fluctuations and random pinning. We study these effects, taking into account the nonlinear elasticity, that, in three dimensions, is important at sufficiently long length scales, and showing that a ``columnar elastic glass'' phase of vortices results. This phase is controlled by a previously unstudied zero-temperature fixed point, and it is characterized by elastic moduli that have universal strong wave-vector dependence out to arbitrarily long length scales, leading to non-Hookean elasticity. We argue that, although translationally disordered for weak disorder, the columnar elastic glass is stable against the proliferation of dislocations and is, therefore, a topologically ordered elastic glass. As a result, the phenomenology of the spontaneous vortex state of isotropic magnetic superconductors differs qualitatively from a conventional, external-field-induced mixed state. For example, for weak external fields $H$, the magnetic induction scales universally like $B(H)\ensuremath{\sim}B(0)+c{H}^{\ensuremath{\alpha}}$, with $\ensuremath{\alpha}\ensuremath{\approx}0.72$.

Evidence fors-wave pairing from measurement of the lower critical field inMgCNi3

Magnetization measurements in the low field region have been carefully performed on a well-shaped cylindrical and an ellipsoidal sample of superconductor $MgCNi_3$. Data from both samples show almost the same results. The lower critical field $H_{c1}$ and the London penetration depth $\lambda$ are thus derived. It is found that the result of normalized superfluid density $\lambda^2(0)/\lambda^2(T)$ of $MgCNi_3$ can be well described by BCS prediction with the expectation for an isotropic s-wave superconductivity.