Pairing In Superconductors Research Articles

Bond-Length Fluctuations in the Copper-Oxide Superconductors

ABSTRACTThe phase diagram of La2-xSrxCuO4 is interpreted. From the virial theorem, it is argued that the cross-over from localized to itinerant electronic behavior in the range 0 < x < 0.3 is characterized by fluctuations between two equilibrium Cu-O bond lengths. Cooperative local fluctuations give rise to one-hole correlation bags of 5 to 6 copper centers on the underdoped side, to strong-correlation fluctuations in an itinerant-electron matrix on the overdoped side. Spinodal phase segregation between an antiferromagnetic, insulating parent phase and the superconductive phase occurs in the underdoped compositions, between the superconductive phase and the metallic overdoped phase on the other side of the phase diagram. Ordering of the fluctuations into a travelling bipolaronic charge-density/spin-density wave of composition x ≈?1/6 yields heavy fermions of symmetry (x2− y2) coexisting with light electrons; the high-temperature superconductive pairs are condensed out from the heavy fermions.

Tcfor non-s-wave pairing superconductors correlated with coherence length and effective mass

For unconventional heavy fermion superconductors, typified by ${\mathrm{UBe}}_{13},$ the superconducting transition temperatures ${T}_{c}$ are shown to correlate with a characteristic energy ${\ensuremath{\Elzxh}}^{2}{/(m}^{*}{\ensuremath{\xi}}^{2}{),m}^{*}$ being the effective mass, and $\ensuremath{\xi}$ the coherence length. For four of the six materials for which ${T}_{c},$ ${m}^{*},$ and $\ensuremath{\xi}$ are available, ${k}_{B}{T}_{c}\ensuremath{\sim}20{\ensuremath{\Elzxh}}^{2}{/(m}^{*}{\ensuremath{\xi}}^{2}).$ One heavy fermion material, ${\mathrm{UPd}}_{2}{\mathrm{Al}}_{3},$ reveals a tendency for the above linear behavior to saturate at substantially larger ${\ensuremath{\Elzxh}}^{2}{/(m}^{*}{\ensuremath{\xi}}^{2})$ than for ${\mathrm{UBe}}_{13}.$ The sixth material considered, ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2},$ falls between ${\mathrm{UBe}}_{13}$ and ${\mathrm{UPd}}_{2}{\mathrm{Al}}_{3}.$ To embrace d-wave pairing in cuprates, a $\mathrm{log}\ensuremath{-}\mathrm{log}$ plot reveals that ${k}_{\mathrm{B}}{T}_{c}\ensuremath{\sim}{\ensuremath{\Elzxh}}^{2}{/(m}^{*}{\ensuremath{\xi}}^{2}),$ but more materials for which ${m}^{*}$ and $\ensuremath{\xi}$ are measured will be required to substantiate the correlation in these high-${T}_{c}$ substances.

Remnant Fermi surfaces in cuprates

We investigate a new methodology proposed by Ronning et al. for analyzing the angle resolved photoemission data on high Tc cuprates - the remnant Fermi surface (rFs). This is the locus of the points in k-space where the momentum distribution n(k) is one half, and can be determined even in strongly correlated electron systems that lack the Fermi surface. There are two fundamentally different classes of remnant Fermi surfaces depending on whether the interaction leads to pairing or to a generalized nesting (magnetic or charge) instability. For pairing the remnant Fermi surface follows faithfully the Fermi surface, whereas for magnetic or charge instabilities the rFs maps the corresponding superlattice Brillouin zone. We exemplify these concepts by determining the rFs for a particular model presenting two phases: flux phase (nesting) and d-wave superconductivity (pairing).

Andreev scattering in quasi-two-dimensional superconducting/semiconducting heterojunctions: Subharmonic gap-structure and exchange-correlation effects

The differential resistance of a quasi-two-dimensional electron gas of a semiconducting layer with proximity-induced superconducting pair potentials is computed from the solutions of the (mean field) Time-dependent Bogoliubov-de Gennes Equations. Exchange-correlation effects are taken into account in the Time-dependent Density-Functional Bogoliubov-de Gennes Equations. We discuss their effects on Andreev scattering in junctions involving semiconductors and high temperature superconductors.

Normal-state properties of BEDT-TTF compounds and the superconductivity pairing mechanism

Normal-state properties of BEDT-TTF compounds and the superconductivity pairing mechanism

Ferromagnetism Stabilized by Lattice Distortion at the Surface of the p -Wave Superconductor Sr 2 RuO 4

Ferromagnetic (FM) spin fluctuations are believed to mediate the spin-triplet pairing for the p-wave superconductivity in Sr(2)RuO(4). Our experiments show that, at the surface, a bulk soft-phonon mode freezes into a static lattice distortion associated with an in-plane rotation of the RuO(6) octahedron. First-principle calculations confirm this structure and predict a FM ground state at the surface. This coupling between structure and magnetism in the environment of broken symmetry at the surface allows a reconsideration of the coupling mechanism in the bulk.

Resonance as a measure of pairing correlations in the high-Tc superconductor YBa2Cu3O6.6

One of the most striking properties of the high-transition-temperature (high-Tc) superconductors is that they are all derived from insulating antiferromagnetic parent compounds. The intimate relationship between magnetism and superconductivity in these copper oxide materials has intrigued researchers from the outset, because it does not exist in conventional superconductors. Evidence for this link comes from neutron-scattering experiments that show the unambiguous presence of short-range antiferromagnetic correlations (excitations) in the high-Tc superconductors. Even so, the role of such excitations in the pairing mechanism for superconductivity is still a subject of controversy. For YBa2Cu3O(6+x), where x controls the hole-doping level, the most prominent feature in the magnetic excitation spectrum is a sharp resonance (refs 6-11). Here we show that for underdoped YBa2Cu3O6.6, where x and Tc are below their optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular to the CuO2 planes than for parallel fields. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in high-Tc superconductivity. The persistence of a field effect above Tc favours mechanisms in which the superconducting electron pairs are pre-formed in the normal state of underdoped copper oxide superconductors, awaiting transition to the superconducting state.

High- [formula omitted] microwave ring oscillators

YBa 2Cu 3O 7− δ (YBCO) superconducting ring resonators with YBCO ground plane were fabricated. The resonators exhibit a high quality ( Q>10 4) at 16 K, indicating high potential for applications. In addition, from the analyses of the surface resistance R s( T) and reactance X s( T), it was found that the derived London penetration depth λ(T)−λ(5 K) displays a linear behavior over a wide range of temperatures. The results suggest that a distinct line node feature in the gap function of a d-wave pairing superconductor may extend to temperatures higher than previously reported.

Evidence for linking an oxygen pairing process and superconductivity in YBa 2Cu 3O 7− δ between 86 and 130 K

Ultrasonic studies at 85 kHz about the T c region of melt-textured YBa 2Cu 3O 7− δ (Y123) yielded details of strain amplitude dependent (SAD) and time dependent phenomena. This has provided strong evidence for an oxygen pairing process associated with a structural change, which was evident for temperatures below T c up to 130 K. This pairing process has been linked, through a variety of experiments, to superconductivity and has provided evidence for the pseudogap phenomenon as well as a possible mechanism for channeling the superconducting carriers. The experiments included an enhanced oxygen related ultrasonic breakaway process in the T c region, lowering the T c of Y123 ultrasonically at high strain amplitudes, enhancing superconductivity at low ultrasonic strain amplitudes in the T c transition, oxygen pairing up to 130 K.

Phase Fluctuations and BCS-LP Crossover in 2D Short Coherence Length Superconductors

We study the effects of phase fluctuations in 2D short-coherence length superconductors on the basis of the extended Hubbard model with intersite attraction. In the Kosterlitz-Thouless scenario the critical temperatures for anisotropic pairings were determined and compared with the ones from BCS Hartree-Fock approximation. The Uemura-type plots, i.e. the critical tem- perature vs. zero temperature phase stiffness, were obtained for extended s-wave and d x 2- y 2-wave pairings. We also discuss the crossover from BCS to local pair superconductivity for d-wave pairing.

Superconductor/ferromagnet proximity effect in Fe/Pb/Fe trilayers

We report on measurements of structural, superconducting, and magnetic properties of trilayer and bilayer systems combined of superconducting Pb and ferromagnetic Fe. The Pb/Fe layers can be grown on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ with reasonably flat interfaces, there is no alloying of the components at the interface and Fe is found to be ferromagnetic down to the monolayer range. This is a favorable situation for an $S/F$ proximity system, since it corresponds closely to the situation treated in theoretical models. We find an oscillation of the superconducting transition temperature when plotted versus the thickness of the ferromagnetic layer, which we regard as a clear indication of an unconventional, propagating superconducting pair wave function in the Pb/Fe system. We fit our results using recent theoretical model calculations and find evidence for a strongly reduced transparency of the Pb/Fe interface. We regard this as an essential feature of the proximity effect in Pb/Fe and discuss its physical origin.

Pseudogaps and charge band in the Parisi solution of insulating and superconducting electronic spin glasses at arbitrary fillings

We report progress in understanding the fermionic Ising spin glass with arbitrary filling. A crossover from a magnetically disordered single band phase via two intermediate bands just below the freezing temperature to a 3-band structure at still lower temperatures - beyond an almost random field instability - is shown to emerge in the magnetic phase. An attempt is made to explain the exact solution in terms of a quantum Parisi phase. A central nonmagnetic band is found and seen to become sharply separated at T = 0 by gaps from upper and lower magnetic bands. The gap sizes tend towards zero as the number of replica symmetry breaking steps increases towards infinity. In an extended model, the competition between local pairing superconductivity and spin glass order is discussed.

Ion-acoustic wakefields and molecular ions in dense plasmas

It is shown, both analytically and through one-dimensional numerical computations, that highly charged ions propagating through a plasma with thermal or faster than thermal velocities, drive significant ion-acoustic wakefields. By a mechanism similar to electron pairing in superconductors, such wakefields may contribute to the formation of transient molecular ions. Comparison is made with the results of experiments in which dense high temperature plasmas are created by laser irradiated colliding foils.

Random magnetic interactions and spin glass order competing with superconductivity: Interference of the quantum Parisi phase

We analyse the competition between spin glass (SG) order and local pairing superconductivity (SC) in the fermionic Ising spin glass with frustrated fermionic spin interaction and nonrandom attractive interaction. The phase diagram is presented for all temperatures T and chemical potentials \mu. SC-SG transitions are derived for the relevant ratios between attractive and frustrated-magnetic interaction. Characteristic features of pairbreaking caused by random magnetic interaction and/or by spin glass proximity are found. The existence of low-energy excitations, arising from replica permutation symmetry breaking (RPSB) in the Quantum Parisi Phase, is shown to be relevant for the SC-SG phase boundary. Complete 1-step RPSB-calculations for the SG-phase are presented together with a few results for infinity-step breaking. Suppression of reentrant SG - SC - SG transitions due to RPSB is found and discussed in context of ferromagnet - SG boundaries. The relative positioning of the SC and SG phases presents a theoretical landmark for comparison with experiments in heavy fermion systems and high T_c superconductors. We find a crossover line traversing the SG-phase with (\mu=0,T=0) as its quantum critical (end)point in complete RPSB, and scaling is proposed for its vicinity. We argue that this line indicates a random field instability and suggest Dotsenko-Mezard vector replica symmetry breaking to occur at low temperatures beyond.

Dual order parameter for the nodal liquid

The guiding conception of vortex-condensation-driven Mott insulating behavior is central to the theory of the nodal liquid. We amplify our earlier description of this idea and show how vortex condensation in 2D electronic systems is a natural extension of 1D Mott insulating and 2D bosonic Mott insulating behavior. For vortices in an underlying superconducting pair field, there is an important distinction between the condensation of flux hc/2e and flux hc/e vortices. The former case leads to spin-charge confinement, exemplified by the band insulator and the charge-density-wave. In the latter case, spin and charge are liberated leading directly to a 2D Mott insulator exhibiting *spin-charge separation*. Possible upshots include not only the nodal liquid, but also a novel undoped antiferromagnetic insulating phase with gapped excitations exhibiting spin-charge separation.

Correlation effects in ET compounds

The ET salt family is described by a two parameter 2D Fermi surface model, in which bandwidth and departure from perfect nesting can be varied. We have studied the effect of spin fluctuations on the normal state properties in a RPA approach. The calculated NMR relaxation rate exhibits a peak in 1/T 1T, which strongly decreases when the departure from perfect nesting of the Fermi surface and the bandwidth increase. These results are in perfect agreement with NMR experiments done in (BEDT-TTF) 2 X (X= Cu[N(CN) 2]Br, Cu(NCS) 2), at least qualitatively. We investigate the superconductivity pairing due to spin fluctuations and a model of quasi-2D electrons close to SDW instability, described by a two-parameter Fermi surface model, in which bandwidth and departure from perfect nesting can be varied. The coupling constants of various channels are calculated. They strongly depend on the nesting properties of the Fermi surface and on the bandwidth.

Pseudogap and Unconventional Pairing in the Hubbard Model

The attractive (U < 0) and repulsive (U > 0) Hubbard models have been studied using the Fermi liquid perturbation approach. The attractive Hubbard model (U < 0) is an adequate model for 3 He , an incompressible and strongly paramagnetic liquid [Formula: see text], [Formula: see text] for |U|N F = 0.9) with a pseudogap in the charge response. A pairing instability and superfluidity for U < 0 exists in the spin channel only: spin-triplet with l = 0, or spin-singlet with l = 1 (p-wave orbital state j = s + l = 1), where l is the orbital momentum of a pair. The repulsive Hubbard model (U > 0) represents a highly compressible and nearly antiferromagnetic liquid [Formula: see text], [Formula: see text] for UN F = 0.9) with a pseudogap in the spin response. However, for U > 0 a pairing instability and superconductivity exist in the charge channel only: spin-singlet with l = 0 (s-wave), or with l = 2 (d-wave) in the case of an anisotropic Fermi surface.

Ground State and Thermodynamic Properties of an Induced-Pairing Model

We study the phase diagrams and thermodynamic properties of a system of coexisting wide-band current carriers (c-particles) and narrow-band electrons (d-electrons) which can form local pairs. There are two distinct mechanisms of superconductivity in the model considered: (i) the intersubsystem charge exchange, which leads to the superconducting state involving both types of electrons and (ii) the pair hopping interaction of d-electrons, leading to the superconducting state of d-particles only. In contrast to previous works on the subject, we assume an arbitrary value of on-site density interaction of d-electrons U, which allows us to study the effects of reduced d-pair binding energy. Within the approach in which the U term is exactly treated we determine the evolution of superconducting properties as a function of interactions and relative position of the bands. In a definite range of parameters the system shows features which are intermediate between those of a local pair superconductor and those of a classical BCS model. The increasing on-site Coulomb repulsion U competing with the intersubsystem charge exchange and the pair hopping interaction reduces the superconducting critical temperature. Moreover, it can induce a change of the superconducting transition into the first order one. Above a critical value of U, dependent on concentration of electrons and other interactions, the superconducting state cannot be stable at any temperature.

Hole pairing and flux quantization in modelCu-O clusters

In the three-band Hubbard model, the fully symmetric Cu-O clusters with a central Cu and appropriate hole numbers have many-body ground states in which holes are paired. Here we investigate the mechanism of pairing, focusing on clusters with ≤ 21 atoms and 4 holes. We show that spin-flip diagrams yield the effective interaction. The analysis agrees with the results of exact diagonalization and shows that singlet-triplet splitting measures the strength of the pairing interaction. Moreover, the quantization at half-integer values of a magnetic text flux is consistent with supercon-ducting pairing.

HTSCs with apical oxygen replaced by halogens

The fundamental structural unity of familiar oxide HTSCs and their oxyhalide counterparts, with apical oxygen ions replaced by halogen ions, is demonstrated and discussed. As a consequence of this unity, major physical phenomena are expected to be common for both classes, such as doping effects, concentration-driven insulator–metal transitions, the unusual pair superconductivity, and the temperature dependence anomalies in the critical magnetic field, linear expansion coefficient, and heat capacity jumps. New experimental results are discussed, accounted for by the exciton mechanism of superconductivity with the local nature of excitations taken into account.