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We present the results of a systematic study of thin-films of amorphous indium-oxide near the superconductor-insulator transition. We show that the film's resistivity follows a simple, well-defined, power-law dependence on the perpendicular magnetic field. This dependence holds well into the insulating state. Our results indicate that vortices play a central role in the transport of our films in the superconducting as well as insulating phases., Comment: Final accepted version; 11 pages with 5 embedded figures

We fabricated cantilevers for magnetic force microscopy with carbon nanotube tips coated with magnetic material. Images of a custom hard drive demonstrated 20 nm lateral resolution, with prospects for further improvements., Comment: Accepted to be published in Applied Physics Letters

There is a long-standing debate about whether spin–charge separation is the root cause of the peculiar normal-state properties and high superconducting transition temperatures of the high-Tc materials. In the proposed1 state of matter, the elementary excitations are not electron-like, as in conventional metals, but rather the electron ‘fractionalizes’ to give excitations that are chargeless spin-1/2 fermions (spinons) and charge +e bosons (chargons). Although spin–charge separation has been well established in one dimension, the theoretical situation for two dimensions is controversial and experimental evidence for it in the high-Tc materials is indirect. A model2 with sharp experimental tests for a particular type of separation in two dimensions has recently been proposed. Here we report the results of those experimental tests, placing a conservative upper limit of 190 K on the energy of the proposed topological defects known as visons. There is still debate3 about the extent to which this experiment can settle the issue of spin–charge separation in the high-Tc copper oxides, because some forms of the separation are able to avoid the need for visons. But at least one class4,5,6 of theories that all predict a vortex-memory effect now are unlikely models for the copper oxides.

Magnetic flux in superconductors is usually quantized in units of $h/2e$. Here we report scanning SQUID and scanning Hall probe studies of single fluxoids in high purity ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.35}$ crystals ( ${T}_{c}\ensuremath{\lesssim}13\mathrm{K}$), extending flux quantization studies to a region of the cuprate phase diagram where the superfluid density is sufficiently low that novel behavior has been predicted. Some scenarios in which superconductivity results from spin-charge separation predict $h/e$ fluxoids in materials with low superfluid density. Our observations of only $h/2e$ fluxoids set limits on these theories.

Extensive mapping of local electronic structure in copper oxide superconductors reveals fluctuating stripe-like electron patterns that appear as a high-temperature precursor to superconductivity. See Letter p.677 A long-standing question in the superconductor field is the nature of the interplay between the pseudogap, which is generic to all hole-doped copper oxide superconductors, and stripes, whose static form occurs in only one family of copper oxides over a narrow range of the phase diagram. Parker et al. report observations of the spatial reorganization of electronic states with the onset of the pseudogap state at T* in the high-temperature superconductor Bi2Sr2CaCu2O8+x using scanning tunnelling microscopy. They find that the onset of the pseudogap phase coincides with the appearance of electronic patterns that have the predicted characteristics of fluctuating stripes. The experiments indicate that stripes are a consequence of pseudogap behaviour rather than its cause.

Phase sensitive experiments in tricrystal grain boundary samples[1, 2, 3, 4, 5, 6] have demonstrated that a number of the high-Tc cuprate superconductors have predominantly d-wave symmetry. The reproducibility of these experiments is remarkable, when one considers the roughness of the grain boundary interfaces. Here we briefly describe how this roughness affects the magnetic properties of grain boundaries in high-Tc superconductors. We then describe experiments designed and performed to make conclusive symmetry tests despite this roughness.

We have magnetically imaged interlayer Josephson vortices emerging parallel to the planes of single crystals of the organic superconductor κ-(BEDT-TTF)2 Cu(NCS)2, and the single layer cuprate high-Tc superconductors Tl2Ba2CuO6+δ (Tl-2201) and (Hg,Cu)Ba2CuO4+δ) (Hg-1201), using a scanning Superconducting Quantum Interference Device (SQUID) microscope. These images provide a direct measurement of the interlayer penetration depth, which is approximately 63μm for κ-(BEDT-TTF)2 Cu(NCS)2, 18μm for Tl-2201 and 8μm for Hg-1201. The lengths for the cuprates are about a factor of 10 larger than originally predicted by the interlayer tunneling model for the mechanism of superconductivity in layered compounds, indicating that this mechanism alone cannot account for the high critical temperatures in these materials.

We present the solution to London's equations for the magnetic fields of a vortex oriented parallel to the plane, and normal to a crystal face, of a layered superconductor. These expressions account for flux spreading at the superconducting surface, which can change the apparent size of the vortex along the planes by as much as 30%. We compare these expressions with experimental results., 13 pages, 5 figures

We have magnetically imaged interlayer Josephson vortices emerging from an ac face of single crystals of the single layer cuprate high-Tc superconductor (Hg,Cu)Ba2CuO4+d. These images provide a direct measurement of the c-axis penetration depth of about 10 microns. This length is a factor of 10 longer than predicted by the interlayer tunneling model for the mechanism of superconductivity in layered compounds, indicating that the condensation energy available through this mechanism is 100 times smaller than is required for superconductivity., 10 pages, 4 figures

The spontaneous flux generation and magnetic field modulation of the critical current in a 0-pi Josephson junction are calculated for different ratios of the junction length to the Josephson penetration depth, and different ratios of the 0-junction length to the pi-junction length. These calculations apply to a Pb-YBCO c-axis oriented junction with one YBCO twin boundary, as well as other experimental systems. Measurements of such a junction can provide information on the nature of the c-axis Josephson coupling and the symmetry of the order parameter in YBCO. We find spontaneous flux even for very short symmetric 0-pi junctions, but asymmetric junctions have qualitatively different behavior., 13 pages, TEX,+ 7 figures, postscript

Direct observations of the half-integer flux quantum effect in rings, disks, and blanket films of the high-Tc cuprate superconductorsY Ba 2 Cu 3O 7−δ (YBCO),GdBa 2 Cu 3O 7−δ (GdBCO),Tl 2 Ba 2 CuO 6+δ (Tl2201), andBi 2 Sr 2 CaCu 2O 8+δ (BSCCO) with three tricrystal substrate geometries show that their gaps have sign changes consistent with d-wave symmetry. The blanket geometry gives the Josephson penetration depth directly, and provides a means for symmetry tests without patterning. Tl2201 is a tetragonal, single-layer, cuprate without chains. When combined with photoemission results, our BSCCO results imply that the gap followsd x 2−y2 symmetry closely. Asymmetric 45 degree bicrystal grain boundary samples show spontaneous magnetization, as expected for d-wave superconductors with interface roughness.

The anisotropy of the magnetic field dependence of the specific heat of YBa$_2$Cu$_3$O$_{7 - {\rm \delta}}$ can be used to identify different low-energy excitations, which include phonons, spin-$ \frac{1}{2}$ particles, and electronic contributions. With a magnetic field H applied perpendicular to the copper oxide planes, we find that the specific heat includes a linear-T term proportional to $\sqrt{H}$. The nonlinear field dependence of the density of states at the Fermi level suggests that there are quasiparticle excitations throughout the entire vortex, not just in the vortex core. The $\sqrt{H} T$ term agrees quantitatively with G. Volovik's prediction for a superconductor with lines of nodes in the gap. A similar, but much smaller, effect is predicted for fields parallel to the planes, and sensitive measurements of the in-plane anisotropic magnetic field dependence of the specific heat could be used to map out the nodes., Comment: 14 pages (LaTeX2e) plus 6 uuencoded .eps figures

We have designed and built a scanning SQUID microscope in a dilution refrigerator, capable of magnetic imaging at temperatures down to 20 mK . As sensors we use susceptometer SQUIDs with two pickup loops and on-chip field coils to allow measurement of both the magnetic susceptibility of the sample and the magnetic field at the sample surface on a mesoscopic length scale. The instrument is useful for studying superconductivity and magnetic effects in novel materials and electronic coherence effects (such as persistent currents) in mesoscopic systems.

Locally applied magnetic fields can be used to create vortex–antivortex pairs in superconducting films and thin crystals. These pairs typically annihilate on some timescale which depends on temperature. We use a 21 μm diameter field coil integrated onto a scanning superconducting quantum interference device to create and observe vortex–antivortex pairs. We present measurements of the distribution of annihilation times as a function of temperature, which should allow us to determine the pinning forces for vortices in these highly underdoped samples of YBa 2 Cu 3 O 6.354 .

This paper analyzes the use of Raman transitions to select a narrow velocity distribution of atoms. We determine the evolution of the atomic wave function comprised of both the internal state and the external momentum of the atom in the presence of two counterpropagating laser beams. The effects of a single \ensuremath{\pi} pulse, two separated Ramsey \ensuremath{\pi}/2 pulses, and a sequence of four \ensuremath{\pi}/2 pulses are analyzed.

Stimulated Raman transitions between the ground-level hyperfine states of atoms have been used to manipulate slowly moving atoms in an atomic fountain. An ensemble of sodium atoms with an inferred velocity spread along one dimension of 270 \ensuremath{\mu}m/sec has been prepared by this technique. We also show that this velocity-selection method is effective in measuring ultracold temperatures of laser-cooled atoms in a regime where traditional ballistic methods fail.

Recent work done at Stanford in the manipulation of atoms and particles is summarized. Techniques to further increase our control of neutral particles such as atomic fountains, funnels, and trampolines have been demonstrated. These techniques are now being combined with a new type of velocity selection in order to study atom/surface interactions and to improve the limit on the charge neutrality of atoms. Trapping techniques have also allowed us to manipulate single molecules of DNA in aqueous solution while observing the molecules in fluorescence.

A new scheme has been developed that produces a slow atomic beam of high brightness with a narrow velocity distribution. The technique can provide long measurement times and high counting rates for future precision experiments. In our initial work, densities of 10{sup 8} Na atoms/cm{sup 3} at a velocity of 270 cm/sec and at a temperature of {approx}200 {mu}K were obtained.

Grain boundaries (GBs) have been shown to limit critical current density, Jc, in YBa2Cu3O7 (YBCO) coated conductors. Here we use transport measurements and scanning Hall probe microscopy coupled with current reconstruction to demonstrate that GB geometry, such as the in-plane meandering observed in films grown by metalorganic deposition (MOD) on rolling assisted biaxially textured substrate (RABiTS), can lead to higher GB Jc. We observe current-induced flux entry into such a coated conductor, then model its behavior by imaging films with single, straight GBs tilted at various angles to the applied current.

The authors have fabricated and characterized GaAs/Al{sub x}Ga{sub 1-x}As two-dimensional electron gas scanning Hall probes for imaging perpendicular magnetic fields at surfaces. The Hall crosses range from 85 x 85 to 1000 x 1000 nm{sup 2}. They study low-frequency noise in these probes, especially random telegraph noise, and show that low-frequency noise can be significantly reduced by optimizing the voltage on a gate over the Hall cross. The authors demonstrate a 100 nm Hall probe with a sensitivity of 0.5 G/{radical}Hz (flux sensitivity of 0.25m {Phi}{sub 0}/{radical}Hz; spin sensitivity of 1.2 x 10{sup 4} {mu}{sub B}/{radical}Hz) at 3 Hz and 9 K.

We demonstrate the ability to straighten and align metal-coated carbon nanotubes with a focused ion beam. The metal-coated nanotubes align toward the source of the ion beam allowing their orientation to be changed at precise angles. By this technique, metal-coated nanotube tips that are several micrometers in length are prepared for scanning probe microscopy. We image high-aspect-ratio structures on the surface of a cell using these tips.

We have constructed a scanning Hall-probe microscope that combines a 1×4cm scan range with 200 nm positioning resolution by coupling stepper motors to high-resolution drivers and reducing gears. The instrument is uniquely suited for efficient magnetic imaging of mesoscopic devices, media, and materials, operating from 4 K to room temperature with fast turn-around time. Its potential for studying dissipation in coated conductors—high-Tc superconducting tapes—is demonstrated via model systems. We image an entire sample of YBa2Cu3O7−δ, then zoom in to individual fluxons. Flux penetration into a single artificial grain boundary is imaged with 4×10−3G∕Hz field resolution and 25μs time resolution by averaging over cycles of ac driving current. Using the resulting magnetic movie, we map out ac power losses.

Our recent experiments on BSCCO (2:2:1:2) have detected a significant anisotropic magnetic-field dependence of the specific heat, with an excess contribution for a perpendicular magnetic field (along the c axis), relative to that for zero or parallel field. One possible explanation for these observations is the thermal excitation of undamped vibrational modes of the lattice of pancake vortices. In the London model, the resulting heat capacity Cν has a Debye-like form appropriate for two-dimensional phonons, with a characteristic Debye temperature Θν = hwc/2kB (for B = 10T and free electrons, Θν ≈ 6.7 K).

We have determined the magnetic field dependence of the electron density of states at the Fermi level, NF(H), by specific heat measurements on single-crystal YBCO. The total specific heat includes contributions from phonons and impurities, as well as the electronic specific heat. The electronic specific heat is best described by a model which includes two characteristics of d-wave superconductors: a T2 term in zero field, and a linear term that corresponds to N F ( H ) ∝ ( H/H c2 ) 1 2 , as predicted by Volovik for superconductors with lines of nodes in the gap.

A number of experimental applications of laser cooling of neutral atoms are described. In an experiment on atomic fountains, laser-cooled atoms pushed up on a vertical trajectory by radiation pressure were observed to turn around due to gravity. A compact cesium fountain that would give three to four orders of magnitude increase in the counting rate over the first experiment can be designed, and the trapping and cooling can be done with diode lasers, suggesting a practical clock. In order to increase the time-averaged flux of such a fountain and also create a continuous fountain, an atomic funnel that can produce an atomic beam of very low velocity ( approximately 270 cm/s) at densities of 10/sup 8/ atoms/cm/sup 2/ has been devised. >