Superconducting switch permits measurement of small voltages at cryogenic temperatures

Dual-coil, superconducting, on-off switch measures small, thermoelectrically generated voltages produced by thermocouples in a liquid helium bath. Placed in a shunt configuration between the thermocouple and the measuring device, the measuring device sees the sum of the voltage to be measured and the spurious thermoelectric voltages

Measurements of thermoelectric power in annealed and quenched gold-platinum alloys

Report gives measurements of absolute thermoelectric powers of dilute gold-platinum alloys and influence of quenched-in lattice vacancies on their thermoelectric powers. It investigates phonon-drag component of thermoelectric power as a function of platinum concentration, and change in phonon-drag thermoelectric power by lattice vacancies

Simultaneous Micro-Characterization of the Superconducting and Structural Properties of High-Tc Superconducting Films

Important information on the local values of the critical temperature and the critical current density in high-Tc superconducting films is obtained by low-temperature scanning electron microscopy (LTSEM). The imaging principle of LTSEM is based on the local beam-induced heating effect and the detection of a voltage response signal in the current-biased specimen film. During the scanning process the sample is mounted on a low-temperature stage the temperature of which is electronically stabilized at some specific value in the range of interest. The local superconducting quantities measured by LTSEM can be correlated with the microstructure of the specimen film investigated by standard techniques. Recently we have studied polycrystalline and epitaxial YBaCuO films on various substrates, and the results are summarized. The spatial resolution of LTSEM has been found to approach 1 μm

Experimental search for anisotropic flux flow resistivity in the a-b plane of optimally doped epitaxial thin films of YBCO

Transport measurements along the node and anti-node directions in the a-b plane of optimally doped and epitaxial thin films of YBCO are reported. Low bias magnetoresistance measurements near and below T_c show that the flux flow resistivity along the node and anti-node directions versus magnetic field are indistinguishable. This result suggests that within the experimental error of our measurements, no correspondence is found between the flux pinning properties in YBCO and the d-wave nature of the order parameter.Comment: 5 figure

The Ballistic Phonon Signal in Low Temperature Scanning Electron Microscopy

By scanning the surface of a specimen cooled to liquid-helium temperature with the electron beam, ballistic phonons are generated which can be used for acoustic imaging. The anisotropy of the ballistic phonon propagation caused by the phonon focusing effect has been observed in recent experiments. The simultaneous operation of two or more small-area phonon detectors during the scanning process appears promising for three-dimensional acoustic imaging of structural inhomogeneities even far from the specimen surface. The principles of this scheme for acoustic imaging are discussed and the spatial resolution limits are estimated

Dynamic Formation of Metastable Intermediate State Patterns in Type-I Superconductors

Structure of the intermediate state in type-I superconducting lead (Pb) is shown to be very sensitive to the ramp rate of an applied magnetic field. The configurations of resulting static patterns depend sensitively on the shape of the specimen. In particular, geometric barrier, present in the samples with rectangular cross-section, plays an important role in determining the sharp boundary between the phases of different topology. We propose that seemingly laminar (stripe) pattern obtained as a result of the fast field ramp is simply an imprint left behind by the fast-moving flux tubes. Our results confirm that flux tube phase is topologically favorable.Comment: to be presented at LT-25 (Amsterdam, 2008

Nernst effect, quasiparticles, and d-density waves in cuprates

We examine the possibility that the large Nernst signal observed in the pseudogap regime of hole-doped cuprates originates from quasiparticle transport in a state with d-density wave (DDW) order, proposed by S. Chakravarty et al. [Phys. Rev. B 63, 094503 (2001)]. We find that the Nernst coefficient can be moderately enhanced in magnitude by DDW order, and is generally of negative sign. Thus, the quasiparticles of the DDW state cannot account for the large and positive Nernst signal observed in the pseudogap phase of the cuprates. However, the general considerations outlined in this paper may be of broader relevance, in particular to the recent measurements of Bel et al. in NbSe_2 and CeCoIn_5 [Phys. Rev. Lett. 91, 066602 (2003); ibid. 92, 217002 (2004)].Comment: 9 pages, 3 figures; published versio

Impeded Growth of Magnetic Flux Bubbles in the Intermediate State Pattern of Type I Superconductors

Normal state bubble patterns in Type I superconducting Indium and Lead slabs are studied by the high resolution magneto-optical imaging technique. The size of bubbles is found to be almost independent of the long-range interaction between the normal state domains. Under bubble diameter and slab thickness proper scaling, the results gather onto a single master curve. On this basis, in the framework of the "current-loop" model [R.E. Goldstein, D.P. Jackson and A.T. Dorsey, Phys. Rev. Lett. 76, 3818 (1996)], we calculate the equilibrium diameter of an isolated bubble resulting from the competition between the Biot-and-Savart interaction of the Meissner current encircling the bubble and the superconductor-normal interface energy. A good quantitative agreement with the master curve is found over two decades of the magnetic Bond number. The isolation of each bubble in the superconducting matrix and the existence of a positive interface energy are shown to preclude any continuous size variation of the bubbles after their formation, contrary to the prediction of mean-field models.Comment: \'{e}quipe Nanostructures Quantique

Dynamics of stripe patterns in type-I superconductors subject to a rotating field

The evolution of stripe patterns in type-I superconductors subject to a rotating in-plane magnetic field is investigated magneto-optically. The experimental results reveal a very rich and interesting behavior of the patterns. For small rotation angles, a small parallel displacement of the main part of the stripes and a co-rotation of their very ends is observed. For larger angles, small sideward protrusions develop, which then generate a zigzag instability, ultimately leading to a breaking of stripes into smaller segments. The short segments then start to co-rotate with the applied field although they lag behind by approximately $10^\circ$. Very interestingly, if the rotation is continued, also reconnection of segments into longer stripes takes place. These observations demonstrate the importance of pinning in type-I superconductors.Comment: To appear in Phys. Rev.

Numerical simulations of two dimensional magnetic domain patterns

I show that a model for the interaction of magnetic domains that includes a short range ferromagnetic and a long range dipolar anti-ferromagnetic interaction reproduces very well many characteristic features of two-dimensional magnetic domain patterns. In particular bubble and stripe phases are obtained, along with polygonal and labyrinthine morphologies. In addition, two puzzling phenomena, namely the so called `memory effect' and the `topological melting' observed experimentally are also qualitatively described. Very similar phenomenology is found in the case in which the model is changed to be represented by the Swift-Hohenberg equation driven by an external orienting field.Comment: 8 pages, 8 figures. Version to appear in Phys. Rev.