Vu Hung Dao

Type-1.5 Superconductivity

We demonstrate the existence of a novel superconducting state in high quality two-component MgB2 single crystalline superconductors where a unique combination of both type-1 (lambda{1}/xi{1}<1/sqrt[2]) and type-2 (lambda{2}/xi{2}>1/sqrt[2]) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.

Scanning SQUID microscopy of vortex clusters in multiband superconductors

In type-1.5 superconductors, vortices emerge in clusters, which grow in size with increasing magnetic field. These novel vortex clusters and their field dependence are directly visualized by scanning SQUID microscopy at very low vortex densities in MgB2 single crystals. Our observations are elucidated by simulations based on a two-gap Ginzburg-Landau theory in the type-1.5 regime.

Stable fractional flux vortices in mesoscopic superconductors

Conventional superconductors have vortices carrying integer multiples of magnetic flux quantum while unconventional ones, with p- or d-wave order parameter, allow half-integer fluxes. Here, we show that mesoscopic size effects stabilize fractional flux vortices in the thermodynamical ground state of s-wave two-gap superconductors. The value of these fluxes can be an arbitrary fraction of flux quantum and can be measured directly from distributions of magnetic fields on the samples.

Thermodynamically stable noncomposite vortices in mesoscopic two-gap superconductors

In mesoscopic two-gap superconductors with sizes of the order of the coherence length noncomposite vortices are found to be thermodynamically stable in a large domain of the T-H phase diagram. In these phases the vortex cores of one condensate are spatially separated from the other condensate ones, and their respective distributions can adopt distinct symmetries. The appearance of these vortex phases is caused by a non-negligible effect of the boundary of the sample on the superconducting order parameter and represents therefore a genuine mesoscopic effect. For low values of interband Josephson coupling vortex patterns with L1 ≠ L2 can arise in addition to the phases with L1 = L2, where L1 and L2 are total vorticities in the two condensates. The calculations show that noncomposite vortices could be observed in thin mesoscopic samples of MgB2.

Giant vortices, rings of vortices, and reentrant behavior in type-1.5 superconductors

We predict that in a bulk type-1.5 superconductor, the competing magnetic responses of the two components of the order parameter can result in group-stabilized giant vortices and individual rings of vortices in the absence of any extrinsic pinning or confinement mechanism. We also determine within the Ginzburg-Landau theory a condition for the robustness of type 1.5 in the vicinity of the critical temperature, and we find a rich phase diagram with successions of behaviors such as type 1{yields} type 1.5{yields} type 2{yields} type 1.5 when temperature decreases.

Size of stripe domains in a superconducting ferromagnet

In a superconducting ferromagnet, the superconducting state appears in the ferromagnetic phase where usually a domain structure has already developed. We study the influence of the superconducting screening currents on a stripe structure with out-of-plane magnetization, in a film of arbitrary thickness. We find that superconductivity always induces a shrinkage of the domains, and there is a critical value of penetration depth below which a mono-domain structure is more stable than the periodic one. Furthermore we investigate the possible different effects of singlet and triplet superconductivity on the domain width, as well as the conditions for the existence of vortices in the domains. The obtained results are then discussed in light of the experimental data of superconducting ferromagnets URhGe, UGe2, and UCoGe.

Role of crystal anisotropy on the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase

We demonstrate that the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase may be very different depending on the field orientation relative to the crystalline axes. We numerically calculate the upper critical field near the tricritical point taking into account the modulation of the order parameter along the magnetic field as well as the higher Landau levels. For s-wave superconductors with the anisotropy described by an elliptical Fermi surface, we propose a general scheme for the analysis of the angular dependence of the upper critical field at all temperatures on the basis of an exact solution for the order parameter. Our results show that the transitions (with tilting magnetic field) between different types of mixed states may be a salient feature of the FFLO phase.

Collective modes in the paramagnetic phase of the Hubbard model

The charge dynamical response function of the Hubbard model is investigated on the square lattice in the thermodynamical limit. The obtained charge excitation spectra consist of a continuum, a gapless collective mode with anisotropic zero-sound velocity, and a correlation induced high-frequency mode at

Intrinsic granularity in nanocrystalline boron-doped diamond films measured by scanning tunneling microscopy

\omega\approx U

Inhomogeneity of initial flux penetration in MgB2 single crystals

. The correlation function is calculated from Gaussian fluctuations around the paramagnetic saddle-point within the Kotliar and Ruckenstein slave-boson representation. Its dependence on the on-site Coulomb repulsion