Vortex lattice symmetry and electronic structure in YBa2Cu 3O7

Evidence for a Square-Square Vortex Lattice Transition in a High-T_{c} Cuprate Superconductor

Using sound velocity and attenuation measurements in high magnetic fields, we identify a new transition in the vortex lattice state of La_{2-x}Sr_{x}CuO_{4}. The transition, observed in magnetic fields exceeding 35 T and temperatures far below zero field T_{c}, is detected in the compression modulus of the vortex lattice, at a doping level of x=p=0.17. Our theoretical analysis based on Eilenberger's theory of the vortex lattice shows that the transition corresponds to the long-sought 45° rotation of the square vortex lattice, predicted to occur in d-wave superconductors near a van Hove singularity.

Anomalous magnetic field dependence of Y Ba(2)Cu(3)O(7 - δ): effect of orthorhombic distortion

In this work an attempt has been made to ascertain the origin of the mixed symmetry state of the order parameters in the high-T(c) superconductor Y Ba(2)Cu(3)O(7 - δ), with the contesting candidates being the orthorhombic distortion associated with the CuO(2) planes and an s-wave order parameter component originating from the CuO chains present in the system. Due to its applicability over the entire range of applied magnetic field H(c1) < H < H(c2) the present theoretical model has been employed to study various unusual properties which are signatures of the unconventional pairing state symmetry of this material. The results presented in this paper and their agreement with the relevant experimental data suggest that the origin of the mixed symmetry state of pairing and the various experimentally observed unusual properties of Y Ba(2)Cu(3)O(7 - δ) including the anomalous magnetic field dependence of the vortex lattice structure, unusual upward curvature of the H(c2)(T) plot, etc, can be attributed to the orthorhombic distortion of the CuO(2) planes present in this material. However, the effect discussed in the present work pertains to the orthorhombic distortion arising only through a second-order coupling; the distortion arising from other possible sources, namely anisotropic mixed gradient coupling, has not been taken into account here. Hence this discussion is restricted to a qualitative explanation of the effect due to orthorhombic distortion arising from the second-order coupling only.

The effect of in-plane mass anisotropy on the properties of high temperature superconductors in states of mixed symmetry

Experiments have suggested that the high- T(c) cuprate YBCO shows marked anisotropy in penetration depth and coherence length measurements. In order to take into account the presence of this anisotropy in the system and its corresponding effect on the various properties of the high- T(c) superconducting materials, we have developed an anisotropic two-order parameter Ginzburg-Landau (GL) theory involving a mixed symmetry state of the order parameter components. For this we have generalized a two-order parameter GL theory, recently developed for the isotropic high- T(c) superconductors involving a mixed symmetry state of the order parameter components (Karmakar and Dey 2008 J. Phys. Condens. Matter 20 255218), in which the effect of the presence of in-plane anisotropy has been taken into account by an effective mass approximation, with the anisotropy being characterized by the parameter γ = m(x)/m(y). The work goes beyond the limitations of the earlier studies in this field as it enables us to carry out a detailed study of the various properties of the system over the entire range of applied magnetic field and wide range of temperature for arbitrary values of the GL parameter κ(y) and vortex lattice symmetry. The model successfully explains not only the observed oblique vortex lattice structure in the presence of in-plane anisotropy but also the experimentally observed angle between the primitive axes of the vortex lattice. The generation of two-fold symmetry of the vortices in the presence of in-plane anisotropy, for a very low applied magnetic field can also be analyzed by our model. We have also compared our theoretical results with various other experiments on high- T(c) cuprate YBCO.

Fermi surface and order parameter driven vortex lattice structure transitions in twin-free YBa2Cu3O7

We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.

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).

Hexagonal and square flux line lattices in CeCoIn5

Using small-angle neutron scattering, we have imaged the magnetic flux line lattice (FLL) in the d-wave heavy-fermion superconductor CeCoIn5. At low fields we find a hexagonal FLL. Around 0.6 T this undergoes what is most likely a first-order transition to square symmetry, with the nearest neighbors oriented along the gap node directions. This orientation of the square FLL is consistent with theoretical predictions based on the d-wave order parameter symmetry.

Direct evidence for an intrinsic square vortex lattice in the overdoped high- T(c) superconductor La(1.83)Sr(0.17)CuO(4+delta)

We report here the first direct observations of a well ordered vortex lattice in the bulk of a La(2-x)Sr(x)CuO(4+delta) single crystal (slightly overdoped, x = 0.17). Our small angle neutron scattering investigation of the mixed phase reveals a crossover from triangular to square coordination with increasing magnetic field. The existence of an intrinsic square vortex lattice has never been observed in high-temperature superconductors and is indicative of the coupling of the vortex lattice to a source of anisotropy, such as those provided by a d-wave order parameter or the presence of stripes.

Staggered currents in the mixed state

The current pattern in the mixed state of high-T(c) superconductors is studied in the U(1) mean-field theory of the t-J model. Assuming the absence of spin antiferromagnetism we find (1) at moderate doping staggered-current pattern exists in vortex cores and eventually percolates with increasing magnetic field. (2) The polarity of the staggered current is pinned by the magnetic field. (3) Vortex cores with (without) staggered current are slightly negatively (positively) charged. (4) Gutzwiller projection does not wash out the staggered current. We discuss the signature of staggered current in neutron scattering and scanning tunneling microscopy.

Superheating and supercooling of vortex matter in a Nb single crystal: direct evidence for a phase transition at the peak effect from neutron diffraction

We report the first observation of a striking history dependence of the structure function of vortex matter in the peak effect regime in a Nb single crystal by using small angle neutron scattering combined with in situ magnetic susceptibility measurements. Metastable phases of vortex matter, supercooled vortex liquid and superheated vortex solid, have been identified. We interpret our results as direct structural evidence for a first-order vortex solid-liquid transition at the peak effect.