Reentrant vortex lattice transformation in fourfold symmetric superconductors

Flux jumps, cluster distribution model and vortex phase diagram of oxygenated YBa2Cu3-xAlxO6+δ single crystals for H|| ab

This work reports magnetic field direction dependent second magnetisation peak (SMP) anomaly in single crystals of oxygenated [Formula: see text] for [Formula: see text] ab. Detailed investigations on crystal A revealed the direction dependence of SMP anomaly at temperatures below 25 K, above which the direction dependence vanishes. The state of spatial order of the vortex lattice was found to be correlated to the vortex lattice symmetry that underwent a change at certain fields and was captured via single flux jumps observed in the third and fifth quadrant of magnetisation hysteresis loops. Bean's critical state profiles drawn to explain the flux jumps required the presence of clusters of [Formula: see text] in the basal plane of [Formula: see text] with closely lying binding energies. Large full width at half maximum observed in Cu, O, Y and Ba photoelectron spectra confirmed the validity of the proposed cluster model. A vortex phase diagram incorporating all features in the magnetisation hysteresis loops has been made for oxygenated [Formula: see text] for [Formula: see text] || ab depicting various phases.

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.

Controlled Generation of Quantized Vortex-Antivortex Pairs in a Superconducting Condensate

Quantized vortices, as topological defects, play an important role in both physics and technological applications of superconductors. Normally, the nucleation of vortices requires the presence of a high magnetic field or current density, which allow the vortices to enter from the sample boundaries. At the same time, the controllable generation of individual vortices inside a superconductor is still challenging. Here, we report the controllable creation of single quantum vortices and antivortices at any desirable position inside a superconductor. We exploit the local heating effect of a scanning tunneling microscope (STM) tip: superconductivity is locally suppressed by the tip and vortex-antivortex pairs are generated when supercurrent flows around the hot spot. The experimental results are well-explained by theoretical simulations within the Ginzburg-Landau approach.

High-Temperature Superconductivity in Single-Unit-Cell FeSe Films on Anatase TiO_{2}(001)

We report on the observation of high-temperature (T_{c}) superconductivity and magnetic vortices in single-unit-cell FeSe films on anatase TiO_{2}(001) substrate by using scanning tunneling microscopy. A systematic study and engineering of interfacial properties has clarified the essential roles of substrate in realizing the high-T_{c} superconductivity, probably via interface-induced electron-phonon coupling enhancement and charge transfer. By visualizing and tuning the oxygen vacancies at the interface, we find their very limited effect on the superconductivity, which excludes interfacial oxygen vacancies as the primary source for charge transfer between the substrate and FeSe films. Our findings have placed severe constraints on any microscopic model for the high-T_{c} superconductivity in FeSe-related heterostructures.

Temperature dependence of clusters with attracting vortices in superconducting niobium studied by neutron scattering

We investigated the intermediate mixed state of a superconducting niobium sample using very small angle neutron scattering. We show that this state is stabilized through a sequence where a regular vortex lattice appears, which then coexists with vortex clusters before vanishing at low temperature. Vortices in clusters have a constant periodicity regardless of the applied field and exhibit a temperature dependence close to the one of the penetration depth. The clusters disappear in the high temperature limit. All the results agree with an explanation in terms of vortex attraction due to non-local effects and indicate a negligible role for pinning. Phase coexistence between the Abrikosov vortex lattice and vortex clusters is reported, showing the first-order nature of the boundary line.

First-order reorientation transition of the flux-line lattice in CaAlSi

The flux-line lattice in CaAlSi has been studied by small-angle neutron scattering. A well-defined hexagonal flux-line lattice is seen just above H(c1) in an applied field of only 54 Oe. A 30° reorientation of this vortex lattice has been observed in a very low field of 200 Oe. This reorientation transition appears to be first-order and could be explained by nonlocal effects. The magnetic field dependence of the form factor is well-described by a single penetration depth of λ=1496(1) Å and a single coherence length of ξ=307(1) Å at 2 K. At 1.5 K, the penetration depth anisotropy is γ(λ)=2.7(1), with the field applied perpendicular to the c axis, and agrees with the coherence length anisotropy determined from critical field measurements.

Morphology of the superconducting vortex lattice in ultrapure niobium

The morphology of the superconducting flux line lattice (FLL) of Nb comprises gradual variations with various lock-in transitions and symmetry breaking rotations. We report a comprehensive small-angle neutron scattering study of the FLL in an ultrapure single crystal of Nb as a function of the orientation of the applied magnetic field. We attribute the general morphology of the FLL and its orientation to three dominant mechanisms; first, nonlocal contributions, second, the transition between open and closed Fermi surface sheets and, third, the intermediate mixed state between the Meissner and the Shubnikov phase.

Instability of square vortex lattice in d-wave superconductors is due to paramagnetic depairing

The effects of paramagnetic depairing on structural transitions between vortex lattices of a quasi-two-dimensional d-wave superconductor are examined. We find that, when the Maki parameter alphaM is of order unity, a square lattice induced by a d-wave pairing is destabilized with increasing fields, and that a reentrant rhombic lattice occurs in higher fields. Further, a weak Fermi surface anisotropy competitive with the pairing symmetry induces another structural transition near Hc2. These results are consistent with the structure changes of the vortex lattice in CeCoIn5 in H parallel c determined from recent neutron scattering data.

Pauli paramagnetic effects on vortices in superconducting TmNi2B2C

The magnetic field distribution around the vortices in TmNi2B2C in the paramagnetic phase was studied experimentally as well as theoretically. The vortex form factor, measured by small-angle neutron scattering, is found to be field independent up to 0.6Hc2 followed by a sharp decrease at higher fields. The data are fitted well by solutions to the Eilenberger equations when paramagnetic effects due to the exchange interaction with the localized 4f Tm moments are included. The induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor.

Spontaneous symmetry-breaking vortex lattice transitions in pure niobium

We report an extensive investigation of magnetic vortex lattice (VL) structures in single crystals of pure niobium with the magnetic field applied parallel to a fourfold symmetry axis, so as to induce frustration between the cubic crystal symmetry and hexagonal VL coordination expected in an isotropic situation. We observe new VL structures and phase transitions; all the VL phases observed (including those with an exactly square unit cell) spontaneously break some crystal symmetry. One phase even has the lowest possible symmetry of a two-dimensional Bravais lattice. This is quite unlike the situation in high-Tc or borocarbide superconductors, where VL structures orient along particular directions of high crystal symmetry. The causes of this behavior are discussed.

Anisotropic diamagnetic response in type-II superconductors with gap and Fermi-surface anisotropies

The effects of anisotropic gap structures on a diamagnetic response are investigated in order to demonstrate that the field-angle-resolved magnetization [ML(chi)] measurement can be used as a spectroscopic method to detect gap structures. Our microscopic calculation based on the quasiclassical Eilenberger formalism reveals that ML(chi) in a superconductor with a fourfold gap displays a fourfold oscillation reflecting the gap and Fermi-surface anisotropies, and the sign of this oscillation changes at a field between Hc1 and Hc2. As a prototype of unconventional superconductors, magnetization data for borocarbides are also discussed.

Square vortex lattice at anomalously low magnetic fields in electron-doped Nd1.85Ce0.15CuO4

We report here on the first direct observations of the vortex lattice in the bulk of electron-doped Nd1.85Ce0.15CuO4 single crystals. Using small-angle neutron scattering, we have observed a square vortex lattice with the nearest neighbors oriented at 45 degrees from the Cu-O bond direction, which is consistent with theories based on the d-wave superconducting gap. However, the square symmetry persists down to unusually low magnetic fields. Moreover, the diffracted intensity from the vortex lattice is found to decrease rapidly with increasing magnetic field.

Evidence for the coexistence of an anisotropic superconducting gap and nonlocal effects in the nonmagnetic superconductor LuNi2B2C

A study of the dependence of the heat capacity C(p)(alpha) on the field angle in LuNi2B2C reveals an anomalous disorder effect. For pure samples, C(p)(alpha) exhibits a fourfold variation as the field H<H(c2) is rotated in the [001] plane, with minima along <100> (alpha=0). A slightly disordered sample, however, develops anomalous secondary minima along <110> for mu(0)H>1 T, leading to an eightfold pattern at 2 K and 1.5 T. The anomalous pattern is discussed in terms of coexisting superconducting gap anisotropy and nonlocal effects.

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).

Vortex lattice structural transitions: a Ginzburg-Landau model approach

We analyze the rhombic to square vortex lattice phase transition in anisotropic superconductors using a variant of Ginzburg-Landau theory. The mean-field phase diagram is determined to second order in the anisotropy parameter, and shows a reorientation transition of the square vortex lattice with respect to the crystal lattice. We then derive the long-wavelength elastic moduli of the lattices, and use them to show that thermal fluctuations produce a reentrant rhombic to square lattice transition line, similar to recent studies which used a nonlocal London model.