2,956 research outputs found
Low Velocity Granular Drag in Reduced Gravity
We probe the dependence of the low velocity drag force in granular materials
on the effective gravitational acceleration (geff) through studies of spherical
granular materials saturated within fluids of varying density. We vary geff by
a factor of 20, and we find that the granular drag is proportional to geff,
i.e., that the granular drag follows the expected relation Fprobe = {\eta}
{\rho}grain geff dprobe hprobe^2 for the drag force, Fprobe on a vertical
cylinder with depth of insertion, hprobe, diameter dprobe, moving through
grains of density {\rho}grain, and where {\eta} is a dimensionless constant.
This dimensionless constant shows no systematic variation over four orders of
magnitude in effective grain weight, demonstrating that the relation holds over
that entire range to within the precision of our data
Quantum-Classical Reentrant Relaxation Crossover in Dy2Ti2O7 Spin-Ice
We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through
measurements of the a.c. magnetic susceptibility. While the characteristic spin
relaxation time is thermally activated at high temperatures, it becomes almost
temperature independent below Tcross ~ 13 K, suggesting that quantum tunneling
dominates the relaxation process below that temperature. As the low-entropy
spin ice state develops below Tice ~ 4 K, the spin relaxation time increases
sharply with decreasing temperature, suggesting the emergence of a collective
degree of freedom for which thermal relaxation processes again become important
as the spins become highly correlated
Structural, orbital, and magnetic order in vanadium spinels
Vanadium spinels (ZnV_2O_4, MgV_2O_4, and CdV_2O_4) exhibit a sequence of
structural and magnetic phase transitions, reflecting the interplay of lattice,
orbital, and spin degrees of freedom. We offer a theoretical model taking into
account the relativistic spin-orbit interaction, collective Jahn-Teller effect,
and spin frustration. Below the structural transition, vanadium ions exhibit
ferroorbital order and the magnet is best viewed as two sets of
antiferromagnetic chains with a single-ion Ising anisotropy. Magnetic order,
parametrized by two Ising variables, appears at a tetracritical point.Comment: v3: streamlined introductio
Low Temperature Spin Freezing in Dy2Ti2O7 Spin Ice
We report a study of the low temperature bulk magnetic properties of the spin
ice compound Dy2Ti2O7 with particular attention to the (T < 4 K) spin freezing
transition. While this transition is superficially similar to that in a spin
glass, there are important qualitative differences from spin glass behavior:
the freezing temperature increases slightly with applied magnetic field, and
the distribution of spin relaxation times remains extremely narrow down to the
lowest temperatures. Furthermore, the characteristic spin relaxation time
increases faster than exponentially down to the lowest temperatures studied.
These results indicate that spin-freezing in spin ice materials represents a
novel form of magnetic glassiness associated with the unusual nature of
geometrical frustration in these materials.Comment: 24 pages, 8 figure
Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures
We demonstrate the manipulation of the Curie temperature of buried layers of
the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the
effect of annealing. Patterning the GaAs-capped ferromagnetic layers into
nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As
layers and thus allows the removal of Mn interstitials using annealing. This
leads to an enhanced Curie temperature and reduced resistivity compared to
unpatterned samples. For a fixed annealing time, the enhancement of the Curie
temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections
Evidence for a Self-Bound Liquid State and the Commensurate-Incommensurate Coexistence in 2D He on Graphite
We made heat-capacity measurements of two dimensional (2D) He adsorbed on
graphite preplated with monolayer He in a wide temperature range (0.1 80 mK) at densities higher than that for the 4/7 phase (= 6.8
nm). In the density range of 6.8 8.1 nm, the 4/7
phase is stable against additional He atoms up to 20% and they are promoted
into the third layer. We found evidence that such promoted atoms form a
self-bound 2D Fermi liquid with an approximate density of 1 nm from the
measured density dependence of the -coefficient of heat capacity. We
also show evidence for the first-order transition between the commensurate 4/7
phase and the ferromagnetic incommensurate phase in the second layer in the
density range of 8.1 9.5 nm.Comment: 6 pages, 4 figure
Thermodynamic Study of Excitations in a 3D Spin Liquid
In order to characterize thermal excitations in a frustrated spin liquid, we
have examined the magnetothermodynamics of a model geometrically frustrated
magnet. Our data demonstrate a crossover in the nature of the spin excitations
between the spin liquid phase and the high-temperature paramagnetic state. The
temperature dependence of both the specific heat and magnetization in the spin
liquid phase can be fit within a simple model which assumes that the spin
excitations have a gapped quadratic dispersion relation.Comment: 5 figure
Experimental determination of superconducting parameters for the intermetallic perovskite superconductor ${\text {MgCNi}}_3
We have measured upper-critical-field , specific heat C, and
tunneling spectra of the intermetallic perovskite superconductor MgCNi
with a superconducting transition temperature K. Based
on these measurements and relevant theoretical relations, we have evaluated
various superconducting parameters for this material, including the
thermodynamic critical field (0), coherence length (0),
penetration depth (0), lower-critical-field (0), and
Ginsberg-Landau parameter (0). From the specific heat, we obtain the
Debye temperature 280 K. We find a jump of
=2.3 at (where is the
normal state electronic specific coefficient), which is much larger than the
weak coupling BCS value of 1.43. Our tunneling measurements revealed a gap
feature in the tunneling spectra at with 4.6, again larger than the weak-coupling value
of 3.53. Both findings indicate that MgCNi is a strong-coupling
superconductor. In addition, we observed a pronounced zero-bias conductance
peak (ZBCP) in the tunneling spectra.
We discuss the possible physical origins of the observed ZBCP, especially in
the context of the pairing symmetry of the material.Comment: 5 pages, 4 figure
Performance of a GridPix detector based on the Timepix3 chip
A GridPix readout for a TPC based on the Timepix3 chip is developed for
future applications at a linear collider. The GridPix detector consists of a
gaseous drift volume read out by a single Timepix3 chip with an integrated
amplification grid. Its performance is studied in a test beam with 2.5 GeV
electrons. The GridPix detector detects single ionization electrons with high
efficiency. The Timepix3 chip allowed for high sample rates and time walk
corrections. Diffusion is found to be the dominating error on the track
position measurement both in the pixel plane and in the drift direction, and
systematic distortions in the pixel plane are below 10 m. Using a
truncated sum, an energy loss (dE/dx) resolution of 4.1% is found for an
effective track length of 1 m.Comment: To be published in Nuclear Instruments and Methods in Physics
Research Section
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