Flux flow of Abrikosov-Josephson vortices along grain boundaries in high-temperature superconductors

Low-angle grain boundaries (GBs) in superconductors exhibit intermediate Abrikosov vortices with Josephson cores, whose length l along GB is smaller than the London penetration depth, but larger than the coherence length. We found an exact solution for a periodic vortex structure moving along GBs in a magnetic field H and calculated the flux flow resistivity R(F)(H), and the nonlinear voltage-current characteristics. The predicted R(F)(H) dependence describes well our experimental data on 7 unirradiated and irradiated YBa(2)Cu(3)O(7) bicrystals, from which the core size l(T), and the intrinsic depairing density J(b)(T) on nanoscales of a few GB dislocations were measured for the first time. The observed J(b)(T) = J(b0)(1-T/T(c))(2) indicates a significant order parameter suppression on GB.     

Smectic vortex phase in optimally doped YBa2Cu3O7 thin films

Angular dependent resistivity measurements of optimally doped YBa2Cu3O7 films in fields H pulsed to 50 T are presented. Up to the highest H, the vortex melting field Hm increases and vortex motion is reduced for H aligned with the correlated pinning centers along the main crystalline axes, otherwise 3D anisotropic scaling describes the vortex dynamics. For H parallel ab, the rapid increase in Hm at low temperatures and a critical exponent analysis near Hm confirm the presence of the liquid-crystalline smectic phase predicted for layered superconductors.     

New Josephson plasma modes in underdoped YBa2Cu3O6.6 induced by a parallel magnetic field

The c-axis reflectivity spectrum of underdoped YBa2Cu3O6.6 (YBCO) is measured below T(c)=59 K in parallel magnetic fields H parallel CuO2 up to 7 T. Upon application of a parallel field, a new peak appears at finite frequency in the optical conductivity at the expense of suppression of c-axis condensate weight. We conclude that the dramatic change originates from different Josephson coupling strengths between bilayers with and without Josephson vortices. We find that the 400 cm(-1) broad conductivity peak in YBCO gains the spectral weight under parallel magnetic field; this indicates that the condensate weight at omega=0 is distributed to this peak as well as to the new optical Josephson mode.     

Origin of the magnetic field induced changes of the transverse plasma mode in the c-axis infrared response of underdoped YBa2Cu3O(7-δ)

We report on results of our theoretical study of magnetic field induced changes of the c-axis infrared response of bilayer cuprate superconductors using the phenomenological multilayer model involving the conductivity of the spacing layers and that of the bilayer units. For H perpendicular to the planes, the local conductivities have been expressed in terms of a two-fluid approximation--as weighted averages of the superconducting state ones and the normal state ones representing contributions of the vortex cores, the weight of the latter increasing linearly with the field. This allows us to reproduce and interpret the fast decrease with increasing H of the well known 400 cm(-1) peak (transverse plasma mode) in the c-axis conductivity, observed by LaForge and co-workers. For the local conductivities of underdoped YBa(2)Cu(3)O(7-δ) with T(c)=58 K reported by Dubroka and co-workers and the fraction of the normal state (T ≈ T(c)) component given by (μ(0)H/25 T), the computed field induced changes of the reflectivity are in quantitative agreement with the data. This suggests that the response at H=0 and T ≈ T(c) is close to that at H=25 T < H(c2) and T ? T(c), in accord with theories attributing the above T(c) state to that of a superconductor lacking long-range phase coherence. Also discussed are changes of the response induced by H parallel to the CuO(2) planes.     

Inhomogeneous magnetic-field response of YBa2Cu3Oy and La2-xSrxCuO4 persisting above the bulk superconducting transition temperature

We report that in YBa2Cu3Oy and La2-xSrxCuO4 there is a spatially inhomogeneous response to the magnetic field for temperatures T extending well above the bulk-superconducting transition temperature Tc. An inhomogeneous magnetic response is observed above Tc even in ortho-II YBa2Cu3O6.50, which has highly ordered doping. The degree of the field inhomogeneity above Tc tracks the hole-doping dependences of both Tc and the density of the superconducting carriers below Tc, and therefore is apparently coupled to superconductivity.     

Quantum oscillations in the underdoped cuprate YBa2Cu4O8

We report the observation of quantum oscillations in the underdoped cuprate superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed magnetic fields up to 85 T. There is a clear signal, periodic in inverse field, with frequency 660+/-15 T and possible evidence for the presence of two components of slightly different frequency. The quasiparticle mass is m(*)=3.0+/-0.3m(e). In conjunction with the results of Doiron-Leyraud et al. for YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a general feature of underdoped copper oxide planes and provide information about the doping dependence of the Fermi surface.     

Crossover between channeling and pinning at twin boundaries in YBa2Cu3O7 thin films

The critical current (Jc) of highly twinned YBa2Cu3O7 films has been measured as a function of temperature, magnetic field, and angle. For much of the parameter space we observe a strong suppression of Jc for fields in the twin boundary (TB) directions; this is quantitatively modeled as flux-cutting-mediated vortex channeling. For certain temperatures and fields a crossover occurs to a regime in which channeling is blocked and the TBs act as planar pinning centers so that TB pinning enhances the overall Jc. In this regime, intrinsic pinning along the TBs is comparable to that between the twins.     

Scanning tunneling spectroscopy in the superconducting state and vortex cores of the beta-pyrochlore KOs2O6

We performed the first scanning tunneling spectroscopy measurements on the pyrochlore superconductor KOs2O6 (T(c)=9.6 K) in both zero magnetic field and the vortex state at several temperatures above 1.95 K. This material presents atomically flat surfaces, yielding spatially homogeneous spectra which reveal fully gapped superconductivity with a gap anisotropy of 30%. Measurements performed at fields of 2 and 6 T display a hexagonal Abrikosov flux line lattice. From the shape of the vortex cores, we extract a coherence length of 31-40 A, in agreement with the value derived from the upper critical field H(c2). We observe a reduction in size of the vortex cores (and hence the coherence length) with increasing field which is consistent with the unexpectedly high and unsaturated upper critical field reported.     

Do columnar defects produce bulk pinning?

From magneto-optical imaging performed on heavy-ion-irradiated YBa(2)Cu(3)O(7-delta) single crystals, it is found that at fields and temperatures where strong single vortex pinning by individual irradiation-induced amorphous columnar defects is to be expected, vortex motion is limited by the nucleation of vortex kinks at the specimen surface. In the material bulk, vortex motion occurs through (easy) kink sliding. Depinning in the bulk determines the screening current only at fields comparable to or larger than the matching field, at which the majority of vortices is not trapped by an ion track.     

Absence of orbital currents in superconducting YBa2Cu4O8 using a zeeman-perturbed nuclear-quadrupole-resonance technique

Zeeman perturbed nuclear quadrupole resonance was applied to evaluate weak magnetic fields in the context of orbital currents in cuprate superconductors. The magnetic environment of the barium atom in c-axis oriented powder samples of YBa(2)Cu(4)O(8) was investigated in the pseudogap phase at 90 K. No evidence for orbital currents was found: any static and dynamic field must be less than 0.07 and 0.7 mT, respectively.     

Experimental evidence for the vortex glass phase in untwinned, proton irradiated YBa2Cu3O7-delta

We report on magnetoresistance measurements of the effects of 9 MeV proton irradiation on a clean, untwinned single crystal of YBa2Cu3O7-delta. For the first time, evidence for a vortex glass transition is detected in an untwinned single crystal of YBa2Cu3O7-delta with induced pointlike disorder, in which the first order vortex melting transition is completely suppressed after proton irradiation. Our results suggest that a sufficiently high pinning disorder is required in order for the vortex glass phase to be observed.     

Charge and spin structure in of YBa(2)Cu(3)O(6.35)

Neutron scattering has been used to measure the charge and spin structure in the highly underdoped superconductor YBa(2)Cu(3)O(6.35). Incommensurate static charge ordering is found that remains at high temperatures. The magnetic pattern is complex with a resonance and incommensurate structure observed at low temperatures. The results clarify the role of striped phases in YBa(2)Cu(3)O(6+x) superconductors.     

Dynamics of the local moment induced by nonmagnetic defects in cuprates

We present a study of the spin dynamics of magnetic defects induced by Li substitution of the plane Cu in the normal state of YBa2Cu3O6+x. The fluctuations of the coupled Cu magnetic moments in the vicinity of Li are probed by near-neighbor 89Y and 7Li NMR spin lattice relaxation. The data indicate that the magnetic perturbation fluctuates as a single entity with a correlation time tau which scales with the local static susceptibility. This behavior is reminiscent of the low T Kondo state of magnetic impurities in conventional metals. Surprisingly it extends well above the "Kondo" temperature for the underdoped pseudogapped case.     

Delocalized quasiparticles in the vortex state of an overdoped high-T(c) superconductor probed by 63Cu NMR

We report the spin Knight shift (K(s)) and the nuclear spin-lattice relaxation rate (1/T1) in the vortex state as a function of magnetic field (H) up to 28 T in the high-Tc superconductor TlSr2CaCu2O6.8 (Tc = 68 K). At low temperatures well below Tc, both K(s) and 1/T1 measured around the middle point between the two nearest vortices (saddle point) increase substantially with increasing field, which indicate that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of states (DOS) around the saddle point is found to be kappaN(0)square root[H/H(c2)], with kappa = 0.5-0.7 and N0 being the normal-state DOS.     

Triangular to square flux lattice phase transition in YBa2Cu3O7

We have used the technique of small-angle neutron scattering to observe magnetic flux lines directly in a YBa2Cu3O7 single crystal at fields higher than previously reported. For field directions close to perpendicular to the CuO2 planes, we find that the flux lattice structure changes smoothly from a distorted triangular coordination to nearly perfectly square as the magnetic induction approaches 11 T. The orientation of the square flux lattice is as expected from recent d-wave theories but is 45 degrees from that recently observed in La(1.83)Sr(0.17)CuO(4+delta).     

d-Wave-induced Josephson current counterflow in YBa2Cu3O7/Nb zigzag junctions

Well-defined zigzag-shaped ramp-type Josephson junctions between YBa2Cu3O7 and Nb have been studied. The magnetic field dependencies of the critical currents provide evidence for d-wave--induced alternations in the direction of the Josephson current between neighboring sides of the zigzag structure. The arrays present controllable model systems to study the influences of pi facets in high-angle high- T(c) grain boundaries. From the characteristics, we estimate a possible imaginary s-wave admixture to the order parameter of the YBa2Cu3O7 to be below 1%.     

Oscillatory dynamics and organization of the vortex solid in YBa2Cu3o7 single crystals

We report on the degree of order of the vortex solid in YBa2Cu3O7 single crystals observed in ac susceptibility measurements. We show that when vortices are "shaken" by a temporarily symmetric ac field they are driven into an easy-to-move, ordered structure but, on the contrary, when the ac field is temporarily asymmetric, they are driven into a more pinned disordered state. This is characteristic of tearing of the vortex lattice and shows that ordering due to symmetric ac fields is essentially different from an equilibration process or a dynamical crystallization that is expected to occur at high driving currents.     

Anisotropic vortex plasticity in the liquid state of YBa2Cu3O7: evidence for quenched c-axis vortex correlation length

A generalized methodology, based on anisotropic angular dependent magnetoresistance measurements, is presented to deconvolute the contributions to the vortex activation energy (i.e., plastic and pinning energies) in the vortex liquid state of high temperature superconductors. Experimental evidence is given for the appearance of a partially entangled liquid vortex state in YBa 2Cu 3O (7) when random quenched and correlated disorders compete, as in twinned melt textured YBa 2Cu 3O (7) quasisingle crystals with Y 2BaCuO (5) precipitates. The hallmark of this new phase is a quench of the c-axis vortex correla-tion length.     

de Haas-van Alphen oscillations in the underdoped high-temperature superconductor YBa2Cu3O6.5

The de Haas-van Alphen effect was observed in the underdoped cuprate YBa2Cu3O6.5 via a torque technique in pulsed magnetic fields up to 59 T. Above a field of approximately 30 T the magnetization exhibits clear quantum oscillations with a single frequency of 540 T and a cyclotron mass of 1.76 times the free electron mass, in excellent agreement with previously observed Shubnikov-de Haas oscillations. The oscillations obey the standard Lifshitz-Kosevich formula of Fermi-liquid theory. This thermodynamic observation of quantum oscillations confirms the existence of a well-defined, closed, and coherent, Fermi surface in the pseudogap phase of cuprates.     

Flux-flow resistivity anisotropy in the instability regime of the a-b plane of epitaxial superconducting YBa2Cu3O7-delta thin films

Measurements of the nonlinear flux-flow resistivity rho and the critical vortex velocity vphi* at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov (Z. Eksp. Teor. Fiz 68, 1915 (1975) [Sov. Phys. JETP 41, 960 (1976)]) are reported along the node and antinode directions of the d-wave order parameter in the a-b plane of epitaxial YBa2Cu3O7-delta films. In this pinning-free regime, rho and vphi* are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of rho is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of YBa2Cu3O7-delta.