436 research outputs found
Vortex configurations and dynamics in elliptical pinning sites for high matching fields
Using numerical simulations we study the configurations, dynamics, and
melting properties of vortex lattices interacting with elliptical pinning sites
at integer matching fields with as many as 27 vortices per pin. Our pinning
model is based on a recently produced experimental system [G. Karapetrov et
al., Phys. Rev. Lett. 95, 167002 (2005)], and the vortex configurations we
obtain match well with experimental vortex images from the same system. We find
that the strong pinning sites capture more than one vortex each, and that the
saturation number of vortices residing in a pin increases with applied field
due to the pressure from the surrounding vortices. At high matching fields, the
vortices in the intestitial regions form a disordered triangular lattice. We
measure the depinning thresholds for both the x and y directions, and find
distinctive dynamical responses along with highly anisotropic thresholds. For
melting of the vortex configurations under zero applied current, we find
multi-step melting transitions in which the interstitial vortices melt at a
much lower temperature than the pinned vortices. We associate this with
signatures in the specific heat.Comment: 11 pages, 13 postscript figure
Graphite based Schottky diodes formed on Si, GaAs and 4H-SiC substrates
We demonstrate the formation of semimetal graphite/semiconductor Schottky
barriers where the semiconductor is either silicon (Si), gallium arsenide
(GaAs) or 4H-silicon carbide (4H-SiC). Near room temperature, the forward-bias
diode characteristics are well described by thermionic emission, and the
extracted barrier heights, which are confirmed by capacitance voltage
measurements, roughly follow the Schottky-Mott relation. Since the outermost
layer of the graphite electrode is a single graphene sheet, we expect that
graphene/semiconductor barriers will manifest similar behavior.Comment: 5 pages, 3 figures, 1 tabl
Tunneling magnetoresistance in (La,Pr,Ca)MnO3 nanobridges
The manganite (La,Pr,Ca)MnO3 is well known for its micrometer scale phase
separation into coexisting ferromagnetic metallic and antiferromagnetic
insulating (AFI) regions. Fabricating bridges with widths smaller than the
phase separation length scale has allowed us to probe the magnetic properties
of individual phase separated regions. We observe tunneling magnetoresistance
across naturally occurring AFI tunnel barriers separating adjacent
ferromagnetic regions spanning the width of the bridges. Further, near the
Curie temperature, a magnetic field induced metal-to-insulator transition among
a discrete number of regions within the narrow bridges gives rise to abrupt and
colossal low-field magnetoresistance steps at well defined switching fields.Comment: 13 pages, 3 figures, submitted to Applied Physics Letter
Ordered low-temperature structure in K4C60 detected by infrared spectroscopy
Infrared spectra of a K4C60 single-phase thin film have been measured between
room temperature and 20 K. At low temperatures, the two high-frequency T1u
modes appear as triplets, indicating a static D2h crystal-field stabilized
Jahn-Teller distortion of the (C60)4- anions. The T1u(4) mode changes into the
known doublet above 250 K, a pattern which could have three origins: a dynamic
Jahn-Teller effect, static disorder between "staggered" anions, or a phase
transition from an orientationally-ordered phase to one where molecular motion
is significant.Comment: 4 pages, 2 figures submitted to Phys. Rev.
Unusual Shubnikov-de Haas oscillations in BiTeCl
We report measurements of Shubnikov-de Haas (SdH) oscillations in single
crystals of BiTeCl at magnetic fields up to 31 T and at temperatures as low as
0.4 K. Two oscillation frequencies were resolved at the lowest temperatures,
Tesla and Tesla. We also measured the
infrared optical reflectance and Hall effect; we
propose that the two frequencies correspond respectively to the inner and outer
Fermi sheets of the Rashba spin-split bulk conduction band. The bulk carrier
concentration was cm and the effective
masses for the inner and for the
outer sheet. Surprisingly, despite its low effective mass, we found that the
amplitude of is very rapidly suppressed with increasing temperature,
being almost undetectable above K
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