25 research outputs found
Local Measurement of Microwave Response with Local Tunneling Spectra Using Near Field Microwave Microscopy
We have designed and built a near-field scanning microwave microscope, which
has been used to measure the local microwave response and the local
density-of-states (LDOS) in the area including the boundary between the gold
deposited and the non-deposited region on highly-orientated pyrolytic graphite
at a frequency of about 7.3 GHz. We have succeeded in measuring the spatial
variation of both the LDOS and the surface resistance. It can be observed that
the surface resistance in gold deposited region with the metallic tunneling
spectra is smaller than that in the non-deposited region with the U-shaped
tunneling spectra.Comment: 3 pages, 3 figures
Vortex matching effect in engineered thin films of NbN
We report robust vortex matching effects in antidot arrays fabricated on thin
films of NbN. The near absence of hysteresis between field sweep directions
indicates a negligible residual pinning in the host thin film. Owing to the
very small coherence length of NbN thin films (), the observations
suggests the possibility of probing physics of vortices at true nanometer
length scales in suitably fabricated structures.Comment: Submitted to Appl. Phys. Let
Evidence for field-induced excitations in low-temperature thermal conductivity of Bi_2Sr_2CaCu_2O_8
The thermal conductivity ,, of Bi_2Sr_2CaCu_2O_8 was studied as a
function of magnetic field. Above 5 K, after an initial decrease,
presents a kink followed by a plateau, as recently reported by Krishana et al..
By contrast, below 1K, the thermal conductivity was found to \emph{increase}
with increasing field. This behavior is indicative of a finite density of
states and is not compatible with the existence of a field-induced fully gapped
state which was recently proposed to describe the
plateau regime. Our low-temperature results are in agreement with recent works
predicting a field-induced enhancement of thermal conductivity by Doppler shift
of quasi-particle spectrum.Comment: 4 pages including 4 eps figures, submitted to Phys. Rev. Let
Where does the transport current flow in Bi2Sr2CaCu2O8 crystals?
A new measurement technique for investigation of vortex dynamics is
introduced. The distribution of the transport current across a crystal is
derived by a sensitive measurement of the self-induced magnetic field of the
transport current. We are able to clearly mark where the flow of the transport
current is characterized by bulk pinning, surface barrier, or a uniform current
distribution. One of the novel results is that in BSCCO crystals most of the
vortex liquid phase is affected by surface barriers resulting in a thermally
activated apparent resistivity. As a result the standard transport measurements
in BSCCO do not probe the dynamics of vortices in the bulk, but rather measure
surface barrier properties.Comment: 11 pages, 4 figures, accepted for publication in Natur
Vortex states of Bi2Sr2CaCu2O8+y with antidot array probed by c-axis transport measurements
AbstractTo study the vortex states in Bi2Sr2CaCu2O8+y (Bi2212) high-Tc superconductor with periodic array of antidots, we have measured the c-axis critical current in a stack of the intrinsic Josephson junctions with a square lattice of antidots, which gives us information on the c-axis correlation of the pancake vortices. Without the antidot array, the c-axis critical current can probe the first-order vortex lattice melting transition. Additionally, a suppression of the critical current in the vicinity of zero magnetic field is found, which may be related to the instability of vortex lattice under the parallel current flow. With the square lattice of antidots, enhancements of the critical current due to the matching effect are observed in both the integer and fractional matching fields. By mapping the critical current on H-T diagram, the influence of the matching effect on the phase boundary of vortex solid and liquid is shown