Local mapping of dissipative vortex motion

in Physical Review. B : Condensed Matter (2012), 86

We explore, with unprecedented single vortex resolution, the dissipation and motion of vortices in a superconducting ribbon under the influence of an external alternating magnetic field. This is achieved ... [more ▼]

We explore, with unprecedented single vortex resolution, the dissipation and motion of vortices in a superconducting ribbon under the influence of an external alternating magnetic field. This is achieved by combining the phase sensitive character of ac susceptibility, allowing us to distinguish between the inductive and dissipative responses, with the local power of scanning Hall probe microscopy. Whereas the induced reversible screening currents contribute only inductively, the vortices do leave a fingerprint in the out-of-phase component. The observed large phase-lag demonstrates the dissipation of vortices at time scales comparable to the period of the driving force (i.e., 13 ms). These results indicate the presence of slow microscopic loss mechanisms mediated by thermally activated hopping transport of vortices between metastable states. [less ▲]

Thermodynamic, thermoelectric, and magnetic properties of FeSb2: A combined first-principles and experimental study

in Physical Review. B : Condensed Matter (2011), 84

Microscopic picture of the critical state in a superconductor with a periodic array of antidots

in Physical Review. B : Condensed Matter (2011), 83(2),

By using scanning Hall microscopy we visualize the progressive formation of the critical state with individual vortex resolution in a Pb thin film with a periodic pinning array. Slightly above the first ... [more ▼]

By using scanning Hall microscopy we visualize the progressive formation of the critical state with individual vortex resolution in a Pb thin film with a periodic pinning array. Slightly above the first penetration field, we directly observe a terraced critical state as proposed theoretically by Cooley and Grishin [Phys. Rev. Lett. 74, 2788 (1995)]. However, at higher fields, the flux front tends to disorder and the classical Bean profile is restored. This study allows us to establish a clear link between the widely used integrated response measurements in the superconducting state and the nanoscale landscape defined by individual vortices. [less ▲]

Spin correlations in the extended kagome system YBaCo3FeO7

in Physical Review. B : Condensed Matter (2011), 84(22),

The transition metal-based oxide YBaCo3FeO7 is structurally related to the mineral Swedenborgite SbNaBe4O7, a polar noncentrosymmetric crystal system. The magnetic Co3Fe sublattice consists of a ... [more ▼]

The transition metal-based oxide YBaCo3FeO7 is structurally related to the mineral Swedenborgite SbNaBe4O7, a polar noncentrosymmetric crystal system. The magnetic Co3Fe sublattice consists of a tetrahedral network containing kagome-like layers with trigonal interlayer sites. This geometry causes frustration effects for magnetic ordering, which were investigated by magnetization measurements, Mossbauer spectroscopy polarized neutron diffraction, and neutron spectroscopy. Magnetization measurement and neutron diffraction do not show long range ordering even at low temperature (1 K), although a strong antiferromagnetic coupling (similar to 2000 K) is deduced from the magnetic susceptibility. Below 590 K we observe two features, a spontaneous weak anisotropic magnetization hysteresis along the polar crystallographic axis and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the interlayer sites remain idle. Below similar to 50 K the onset of a hyperfine field shows the development of moments static on the Mossbauer time scale also for the Fe interlayer sites. Simultaneously, an increase of spin correlations is found by polarized neutron diffraction. The relaxation part of the dynamic response has been further investigated by high-resolution neutron spectroscopy, which reveals that the spin correlations start to freeze below similar to 50 K. Monte Carlo simulations show that the neutron scattering results at lower temperatures are compatible with a recent proposal that the particular geometric frustration in the Swedenborgite structure promotes quasi-one-dimensional partial order. [less ▲]

Direct visualization of the Campbell regime in superconducting stripes

in Physical Review. B : Condensed Matter (2010), 81(14),

A combination of scanning Hall microscopy and scanning ac-susceptibility measurements in superconducting stripes (ribbons) of width w < 10 mu m was used to observe the dimensional phase transitions of the ... [more ▼]

A combination of scanning Hall microscopy and scanning ac-susceptibility measurements in superconducting stripes (ribbons) of width w < 10 mu m was used to observe the dimensional phase transitions of the vortex lattice and its stability under alternating fields. At low dc magnetic fields applied perpendicularly to the plane of the stripes, vortices form a one-dimensional chain at the center of the stripes. Above a certain field H*(w), the vortex chain splits in two parallel rows displaced laterally in such a way that a zigzag vortex pattern is observed. By shaking the vortices with an external magnetic ac field and detecting their in-phase motion locally, we can identify the degree of mobility of each individual vortex. This technique allows us (i) to directly visualize the transition from intravalley (Campbell regime) to intervalley vortex motion as the amplitude of the ac modulation is increased and (ii) to accurately determine the temperature at which the vortex lattice freezes in a field-cooling experiment. [less ▲]

Current-induced giant vortex and asymmetric vortex confinement in microstructured superconductors

in Physical Review. B : Condensed Matter (2009), 80(5),

Based on the time-dependent Ginzburg-Landau equations, we study numerically current-driven vortices in a micrometer size square type-II superconductor. We demonstrate that the applied current ... [more ▼]

Based on the time-dependent Ginzburg-Landau equations, we study numerically current-driven vortices in a micrometer size square type-II superconductor. We demonstrate that the applied current significantly influences the dynamics of the vortices entering the sample. Strikingly, we find that a giant vortex can be created by the current-assisted collision of two singly quantized vortices. [less ▲]

Direct visualization of magnetic vortex pinning in superconductors

in Physical Review. B : Condensed Matter (2009), 79(14),

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high-resolution imaging techniques: Bitter decoration and scanning Hall ... [more ▼]

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high-resolution imaging techniques: Bitter decoration and scanning Hall-probe microscopy (SHPM). The periodicity and strength of the magnetic pinning potential generated by the square microrings are controlled by the magnetic history of the template. When the square rings are in the magnetized dipolar state, known as the onion state, the strong stray field generated at the domain walls prevents the decoration of vortices. SHPM images show that the stray field generated by the dipoles is much stronger than the vortex field, in agreement with the results of simulations. Real-space vortex imaging has revealed that in the onion state, the corners of the square rings act as effective pinning centers for vortices. [less ▲]

Simplifying strong electronic correlations in uranium: Localized uranium heavy-fermion UM(2)Zn(20) (M = Co, Rh) compounds

in Physical Review. B : Condensed Matter (2008), 78(11),

The physical properties including magnetic susceptibility, magnetization, specific heat, and dynamic susceptibility chi ''(E) are reported for single crystals of the cubic UM(2)Zn(20) (M = Co, Rh ... [more ▼]

The physical properties including magnetic susceptibility, magnetization, specific heat, and dynamic susceptibility chi ''(E) are reported for single crystals of the cubic UM(2)Zn(20) (M = Co, Rh) materials. Maxima in the thermodynamic data at T(max) similar to 10 K for both compounds and a broad peak in chi ''(E) at 5 K in UCo(2)Zn(20) of width Gamma=5 meV indicate a heavy-fermion state characterized by a Kondo temperature T(K) similar to 20-30 K arising from weak hybridization of f- and conduction-electron states. Anderson impurity model fits to the data in the Kondo limit including crystalline electric-field effects corroborate an ionic-like uranium electronic configuration in UM(2)Zn(20). [less ▲]

Different regimes of nucleation of superconductivity in mesoscopic superconductor/ferromagnet hybrids

in Physical Review. B : Condensed Matter (2008), 77(21),

The competition between two regimes of the nucleation of superconductivity is investigated experimentally and theoretically in a mesoscopic disk-shaped superconductor/ferromagnet hybrid. By changing the ... [more ▼]

The competition between two regimes of the nucleation of superconductivity is investigated experimentally and theoretically in a mesoscopic disk-shaped superconductor/ferromagnet hybrid. By changing the magnetic state of a multilayered Co/Pt disk one can reversibly affect the magnetic-field dependence of the critical temperature T(c)(H) of an Al layer. We demonstrate that an enhancement of the magnetic field near the edge of the out-of-plane magnetized disk either stimulates the nucleation of superconductivity at the disk perimeter due to the field compensation effect or prevents it due to edge magnetic barrier (for relatively low parallel to H parallel to values). As a consequence, the presence of such magnetic-field pattern makes it possible to eliminate boundary effects for mesoscopic superconducting samples. Switching from one nucleation regime to another while sweeping H leads to an abrupt change of the slope of the T(c)(H) envelope. [less ▲]

Effects of short-range interactions on transport through quantum point contacts: A numerical approach

in Physical Review. B : Condensed Matter (2007), 75(4),

We study electronic transport through a quantum point contact, where the interaction between the electrons is approximated by a contact potential. Our numerical approach is based on the nonequilibrium ... [more ▼]

We study electronic transport through a quantum point contact, where the interaction between the electrons is approximated by a contact potential. Our numerical approach is based on the nonequilibrium Green-function technique which is evaluated at the Hartree-Fock level. We show that this approach allows us to reproduce relevant features of the so-called "0.7 anomaly" observed in the conductance at low temperatures, including the characteristic features in recent shot-noise measurements. This is consistent with a spin-splitting interpretation of the process, and indicates that the 0.7 anomaly should also be observable in transport experiments with ultracold fermionic atoms. [less ▲]