R. Gilardi

Direct Evidence for an Intrinsic Square Vortex Lattice in the Overdoped High- Tc Superconductor La1.83Sr0.17CuO4+δ

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.

Direct Observation of the Flux-Line Vortex Glass Phase in a Type II Superconductor

The order of the vortex state in La1.9Sr0.1CuO4 is probed using muon-spin rotation and small-angle neutron scattering. A transition from a Bragg glass to a vortex glass is observed, where the latter is composed of disordered vortex lines. In the vicinity of the transition the microscopic behavior reflects a delicate interplay of thermally induced and pinning-induced disorder.

Magnetic-Field-Induced Spin Excitations and Renormalized Spin Gap of the Underdoped La1:895Sr0:105CuO4 Superconductor

High-resolution neutron inelastic scattering experiments in applied magnetic fields have been performed on La1.895Sr0.105CuO4 (LSCO). In zero field, the temperature dependence of the low-energy peak intensity at the incommensurate momentum transfer QIC=(0.5,0.5+/-delta,0),(0.5+/-delta,0.5,0) exhibits an anomaly at the superconducting Tc which broadens and shifts to lower temperature upon the application of a magnetic field along the c axis. A field-induced enhancement of the spectral weight is observed, but only at finite energy transfers and in an intermediate temperature range. These observations establish the opening of a strongly downward renormalized spin gap in the underdoped regime of LSCO. This behavior contrasts with the observed doping dependence of most electronic energy features.

Unusual interplay between copper-spin and vortex dynamics in slightly overdoped La1.83Sr0.17CuO4

Our inelastic neutron scattering experiments of the spin excitations in the slightly overdoped La1.83Sr0.17CuO4 compound show that, under the application of a magnetic field of 5 tesla, the low-temperature susceptibility undergoes a weight redistribution centered at the spin gap energy. Furthermore, by comparing the temperature dependence of the neutron data with ac susceptibility and magnetization measurements, we conclude that the filling-in of the spin gap tracks the irreversibility/melting temperature rather than Tc2, which indicates an unusual interplay between the magnetic vortices and the spin excitations even in the slightly overdoped regime of high-temperature superconductors.

Influence of higher d-wave gap harmonics on the dynamical magnetic susceptibility of high-temperature superconductors

Using a fermiology approach to the computation of the magnetic susceptibility measured by neutron scattering in hole-doped high-T-c superconductors, we estimate the effects on the incommensurate peaks caused by higher d-wave harmonics of the superconducting order parameter induced by underdoping. The input parameters for the Fermi surface and d-wave gap are taken directly from angle-resolved photoemission experiments on Bi2Sr2CaCu2O8+x (Bi2212). We find that higher d-wave harmonics lower the momentum-dependent spin gap at the incommensurate peaks as measured by the lowest spectral edge of the imaginary part in the frequency dependence of the magnetic susceptibility of Bi2212. This effect is robust whenever the fermiology approach captures the physics of high-T-c superconductors. At energies above the resonance we observe diagonal incommensurate peaks. We show that the crossover from parallel incommensuration below the resonance energy to diagonal incommensuration above it is connected to the values and the degeneracies of the minima of the two-particle energy continuum.

A Small Angle Neutron Scattering Study of the Vortex Matter in La2-xSrxCuO4 (x=0.17)

The magnetic phase diagram of slightly overdoped La2-xSrxCuO4 (x=0.17) is characterised by a field-induced hexagonal to square transition of the vortex lattice at low fields (~ 0.4 Tesla) [R. Gilardi et al., Phys. Rev. Lett.88, 217003 (2002)]. Here we report on a small angle neutron scattering study of the vortex lattice at higher fields, that reveals no further change of the coordination of the square vortex lattice up to 10.5 Tesla applied perpendicular to the CuO2 planes. Moreover, it is found that the diffraction signal disappears at temperatures well below Tc, due to the melting of the vortex lattice.

Glassy low-energy spin fluctuations and anisotropy gap in La1.88Sr0.12CuO4

We present high-resolution triple-axis neutron scattering studies of the high-temperature superconductor La1.88Sr0.12CuO4 (Tc=27 K). The temperature dependence of the low-energy incommensurate magnetic fluctuations reveals distinctly glassy features. The glassiness is confirmed by the difference between the ordering temperature TN≃Tc inferred from elastic neutron scattering and the freezing temperature Tf≃11 K obtained from muon spin rotation studies. The magnetic field independence of the observed excitation spectrum as well as the observation of a partial suppression of magnetic spectral weight below 0.75 meV for temperatures smaller than Tf, indicate that the stripe frozen state is capable of supporting a spin anisotropy gap, of a magnitude similar to that observed in the spin and charge stripe-ordered ground state of La1.875Ba0.125CuO4. The difference between TN and Tf implies that the significant enhancement in a magnetic field of nominally elastic incommensurate scattering is caused by strictly inelastic scattering—at least in the temperature range between Tf and Tc—which is not resolved in the present experiment. Combining the results obtained from our study of La1.88Sr0.12CuO4 with a critical reappraisal of published neutron scattering work on samples with chemical composition close to p=0.12, where local probes indicate a sharp maximum in Tf(p), we arrive at the view that the low-energy fluctuations are strongly dependent on composition in this regime, with anisotropy gaps dominating only sufficiently close to p=0.12 and superconducting spin gaps dominating elsewhere.

Field-induced hexagonal to square transition of the vortex lattice in overdoped La1.8Sr0.2CuO4

Abstract We report on a small angle neutron scattering study of the vortex lattice in overdoped La 2− x Sr x CuO 4 ( x =0.2) up to high magnetic fields (9.5 T) applied perpendicular to the CuO 2 planes. At low magnetic fields we observe a crossover from hexagonal to square coordination of the vortex lattice. This field-induced transition confirms the results obtained in slightly overdoped La 2− x Sr x CuO 4 ( x =0.17).

Muons as Local Probes of Three-Body Correlations in the Mixed State of Type-II Superconductors

The vortex glass state formed by magnetic flux lines in a type-II superconductor is shown to possess nontrivial three-body correlations. While such correlations are usually difficult to measure in glassy systems, the magnetic fields associated with the flux vortices allow us to probe these via muon-spin rotation measurements of the local field distribution. We show via numerical simulations and analytic calculations that these observations provide detailed microscopic insight into the local order of the vortex glass and more generally validate a theoretical framework for correlations in glassy systems.

Combined neutron scattering and muon-spin rotation investigations of the Abrikosov state of high-temperature superconductors

The magnetic phase diagram of high-temperature superconductors can contain many exotic vortex phases not observed in conventional superconducting materials. For example, the familiar vortex lattice may melt at high temperatures into a vortex liquid. The influence of defects, which pin the vortices, is of particular interest from both a theoretical and an experimental point of view. We have used a combination of small angle neutron scattering (SANS) and muon-spin rotation to probe the order of the vortex system on a microscopic scale and have succeeded, for the first time, to measure a well-ordered vortex lattice (VL) structure at all doping regimes of LSCO. In the optimally to overdoped regime a field-induced transition from hexagonal to square coordination is reported. The possible connections of our neutron results to photoemission data, as well as the implications for various competing theoretical models will be discussed. In the underdoped regime we observe, as a function of applied magnetic field, a transition from an ordered vortex state to a vortex glass phase that results from the presence of random pinning. Finally, recent measurements of the VL on electron doped high-temperature superconductors are presented.