Search

We have designed and characterized a micro-SQUID with dispersive readout for use in low temperature scanning probe microscopy systems. The design features a capacitively shunted RF SQUID with a tunable resonance frequency from 5 to 12 GHz, micrometer spatial resolution, and integrated superconducting coils for local application of magnetic fields. The SQUID is operated as a nonlinear oscillator with a flux- and power-dependent resonance frequency. Measurements for device characterization and noise benchmarking were carried out at 4 K. The measured flux noise above 10 kHz at 4 K is 80 nΦ0 Hz−1∕2 at a bandwidth of 200 MHz. Estimations suggest that one can benefit from parametric gain based on inherent nonlinearity of the Josephson junction and reduce the flux noise to 30 nΦ0Hz–1∕2 at 100 mK, which corresponds to 10.6 μBHz–1∕2 for a magnetic moment located at the center of the pickup loop. [ABSTRACT FROM AUTHOR]

We present depth resolved X-ray micro-Laue diffraction experiments on the low temperature domain structure of SrTiO3. At 80K, monochromatic X-ray diffraction shows an elongated out-of-plane unit cell axis within a matrix of in-plane oriented tetragonal unit cells. Full deviatoric strain mappings from white beam diffraction show a dominance of two tetragonal domain orientations (x- and z-axes) over a large area of sample surface. This information sets an upper bound on domain wall widths and offers a method for studying 3D domain structure at low temperatures. [ABSTRACT FROM AUTHOR]

We employed a scanning Hall probe microscope to detect the hopping of individual vortices between pinning sites along grain boundaries in YBa2Cu3O6+δ thin films in the presence of an applied current. Detecting the motion of individual vortices allowed us to probe the current-voltage (I-V) characteristics of the grain boundary with voltage sensitivity below a femtovolt. We find a very sharp onset of dissipation with V∝In with an unprecedented high exponent of n≈290 that shows essentially no dependence on temperature or grain boundary angle. Our data have no straightforward explanation within the existing grain boundary transport models. [ABSTRACT FROM AUTHOR]

The authors have fabricated and characterized GaAs/AlxGa1-xAs two-dimensional electron gas scanning Hall probes for imaging perpendicular magnetic fields at surfaces. The Hall crosses range from 85×85 to 1000×1000 nm2. 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/[Square_Root]Hz (flux sensitivity of 0.25m[uppercase_phi_synonym]0/[Square_Root]Hz; spin sensitivity of 1.2×104μB/[Square_Root]Hz) at 3 Hz and 9 K. [ABSTRACT FROM AUTHOR]

Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth λ on a nonsuperconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a number of limits. Using our results, we fit experimental susceptibility data as a function of the sample-sensor spacing for three samples: (1) δ-doped SrTiO3, which has a predominantly diamagnetic response, (2) a thin film of LaNiO3, which has a predominantly paramagnetic response, and (3) the two-dimensional electron layer at a SrTiO3/LaAlO3 interface, which exhibits both types of response. These formulas will allow the determination of the concentrations of paramagnetic spins and superconducting carriers from fits to scanning SQUID susceptibility measurements. [ABSTRACT FROM AUTHOR]

We use scanning superconducting quantum interference device (SQUID) microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: They tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in twinned samples in pnictides and other superconductors. [ABSTRACT FROM AUTHOR]

The extent to which business coalitions and their employer members are catalysts for improving quality of care is of interest to policymakers, who need to know where and under what circumstances the marketplace succeeds on its own in assuring quality. Using data from the 1998 National Business Coalition on Health annual survey, this paper indicates that most coalitions have an infrastructure in place that could be tapped to advance quality goals. Although the survey data cannot tell us the extent to which coalitions are exercising their enhanced market influence specifically to improve quality, interviews with coalition leaders provide insights about how quality considerations can factor into coalition strategies. [ABSTRACT FROM AUTHOR]

Magnetic images of high density vertically recorded media using metal-coated carbon nanotube tips exhibit a doubling of the spatial frequency under some conditions (Deng et al 2004 Appl. Phys. Lett. 85 6263). Here we demonstrate that this spatial frequency doubling is due to the switching of the moment direction of the nanotube tip. This results in a signal which is proportional to the absolute value of the signal normally observed in MFM. Our modeling indicates that a significant fraction of the tip volume is involved in the observed switching, and that it should be possible to image high bit densities with nanotube magnetic force sensors.

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 YBa2Cu3O6.35 crystals (T(c) less, similar 13 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.