42 research outputs found
Driven weak to strong pinning crossover in partially nanopatterned 2H-NbSe2 single crystal
Investigations into the heterogeneous pinning properties of the vortex state
created by partially nano-patterning single crystals of 2H-NbSe2 reveal an
atypical magnetization response which is significantly drive dependent.
Analysis of the magnetization response shows non-monotonic behavior of the
magnetization relaxation rate with varying magnetic field sweep rate. With all
the patterned pinning centers saturated with vortices, we find that the pinning
force experienced by the vortices continues to increase with increasing drive.
Our studies reveal an unconventional dynamic weak to strong pinning crossover
where the flow of the vortex state appears to be hindered or jammed as it is
driven harder through the interstitial voids in the patterned pinning lattice.Comment: 15 pages with 5 figure
Anomalous local magnetic field distribution and strong pinning in CaFe1.94Co0.06As2 single crystals
Magneto-optical imaging of a single crystal of CaFe1.94Co0.06As2, shows
anomalous remnant magnetization within Meissner like regions of the
superconductor. The unconventional shape of the local magnetization hysteresis
loop suggests admixture of superconducting and magnetic fractions governing the
response. Near the superconducting transition temperature, local magnetic field
exceeds the applied field resulting in a diamagnetic to positive magnetization
transformation. The observed anomalies in the local magnetic field distribution
are accompanied with enhanced bulk pinning in the CaFe1.94Co0.06As2 single
crystals. We propose our results suggest a coexistence of superconductivity and
magnetic correlations.Comment: 6 pages, 4 figures. arXiv admin note: substantial text overlap with
arXiv:1201.369
MicroSQUID Force microscopy in a dilution refrigerator
We present a new generation of a scanning MicroSQUID microscope operating in
an inverted dilution refrigerator. The MicroSQUIDs have a size of 1.21$ \
\mum\textsuperscript{2} and a magnetic flux sensitivity of 120 \mu\Phi_{0} /
\sqrt{\textrm{Hz}}550^{-6} \ \Phi_{0} /
\sqrt{\textrm{Hz}} \ \mu \textrm{G}/ \sqrt{\textrm{Hz}}\mu$m and a coarse displacement of 5 mm in x
and y direction has been implemented. The MicroSQUID-to-sample distance is
regulated using a tuning fork based force detection. A MicroSQUID-to-sample
distance of 420 nm has been obtained. The reliable knowledge of this distance
is necessary to obtain a trustworthy estimate of the absolute value of the
superconducting penetration depth. An outlook will be given on the ongoing
direction of development
Metastable inhomogeneous vortex configuration with non-uniform filling fraction inside a blind hole array patterned in a BSCCO single crystal and concentrating magnetic flux inside it
Using magneto-optical imaging technique, we map local magnetic field
distribution inside a hexagonally ordered array of blind holes patterned in
BSCCO single crystals. The nature of the spatial distribution of local magnetic
field and shielding currents across the array reveals the presence of a
non-uniform vortex configuration partially matched with the blind holes at
sub-matching fields. We observe that the filling fraction is different in two
different regions of the array. The mean vortex configuration within the array
is described as a patchy vortex configuration with the patches having different
mean filling fraction. The patchy nature of the vortex configuration is more
pronounced at partial filling of the array at low fields while the
configuration becomes more uniform with a unique filling fraction at higher
fields. The metastable nature of this patchy vortex configuration is revealed
by the application of magnetic field pulses of fixed height or individual
pulses of varying height to the array. The metastability of the vortex
configuration allows for a relatively easy way of producing flux reorganization
and flux focusing effects within the blind hole array. Effect of the magnetic
field pulses modifies the vortex configuration within the array and produces a
uniform enhancement in the shielding current around the patterned array edges.
The enhanced shielding current concentrates magnetic flux within the array by
driving vortices away from the edges and towards the center of the array. The
enhanced shielding current also prevents the uninhibited entry of vortices into
the array. We propose that the metastable patchy vortex configuration within
the blind hole array is due to a non-uniform pinning landscape leading to
non-uniform filling of individual blind holes.Comment: 15 pages, 6 figure
Generating strong magnetic flux shielding regions in a single crystal of Bi2Sr2CaCu2O8 using a blind hole array
Magneto-optical imaging studies in a single crystal of Bi2Sr2CaCu2O8
partially patterned with a hexagonal array of pinning centers (blind holes)
reveals local features in the patterned region which are distinct compared to
the pristine unpatterned regions in the sample. The patterned area exhibits a
strongly diamagnetic local magnetization response and is characterized by a
local penetration field enhanced by a factor of three. We show that strong
shielding currents around the periphery of the nanopatterned region create a
barrier which prevents vortex entry into the patterned region thus sustaining
an effectively flux-free state upto the enhanced penetration field.Comment: 11 pages, 4 figure
Critical behavior at de-pinning of a driven disordered vortex matter in 2H-NbS2
We report unusual jamming in driven ordered vortex flow in 2H-NbS2.
Reinitiating movement in these jammed vortices with a higher driving force, and
halting it thereafter once again with a reduction in drive, unfolds a critical
behavior centered around the de-pinning threshold via divergences in the
lifetimes of transient states, validating the predictions of a recent
simulation study, which also pointed out a correspondence between plastic
de-pinning in vortex matter and the notion of random organization proposed in
the context of sheared colloids undergoing diffusive motion.Comment: Phys. Rev. B (in press, 2012). The paper has 14 pages of Text+ Refs.
with 4 figures. (Note as some of the figure files are large in size, to
enable faster download, the file size has been kept small and the figure
resolution are low. The online version of the paper to appear in PRB will
contain the higher resolution figures
A Scanning Hall Probe Microscope for high resolution, large area, variable height Magnetic Field Imaging
International audienceWe present a Scanning Hall Probe Microscope operating in ambient conditions. One of the unique features of this microscope is the use of the same stepper motors for both sample positioning as well as scanning, which makes it possible to have a large scan range (few mm) in x and y directions, with a scan resolution of 0.1 µm. Protocols have been implemented to enable scanning at different heights from the sample surface. The z range is 35 mm. Microstructured Hall probes of size 1-5 µm have been developed. A minimum probe-sample distance < 2 µm has been obtained by the combination of new Hall probes and probe-sample distance regulation using a tuning fork based force detection technique. The system is also capable of recording local B(z) profiles. We discuss the application of the microscope for the study of micro-magnet arrays being developed for applications in micro-system
Visualizing a dilute vortex liquid to solid phase transition in a Bi2Sr2CaCu2O8 single crystal
Using high sensitivity magneto-optical imaging we find evidence for a jump in
local vortex density associated with a vortex liquid to solid phase transition
just above the lower critical field in a single crystal of Bi2Sr2CaCu2O8. We
find the regions of the sample where the jump in vortex density occurs are
associated with low screening currents. In the field - temperature vortex phase
diagram we identify phase boundaries demarcating a dilute vortex liquid phase
and the vortex solid phase. The phase diagram also identifies a coexistence
regime of the dilute vortex liquid and solid phases and shows the effect of
pinning on the vortex liquid to solid phase transition line. We find the phase
boundary lines can be fitted to the theoretically predicted expression for the
low-field portion of the phase boundary delineating a dilute vortex solid from
a vortex liquid phase. We show that the same theoretical fit can be used to
describe the pinning dependence of the low-field phase boundary lines provided
a dependence of the Lindemann number on pinning strength is considered.Comment: 16 pages and 6 figures (Published
From Romantic Gothic to Victorian Medievalism: 1817 and 1877
"The Cambridge History of the Gothic was conceived in 2015, when Linda Bree, then Editorial Director at Cambridge University Press, first suggested the idea to us