915 research outputs found
Pressure Induced Change in the Magnetic Modulation of CeRhIn5
We report the results of a high pressure neutron diffraction study of the
heavy fermion compound CeRhIn5 down to 1.8 K. CeRhIn5 is known to order
magnetically below 3.8 K with an incommensurate structure. The application of
hydrostatic pressure up to 8.6 kbar produces no change in the magnetic wave
vector qm. At 10 kbar of pressure however, a sudden change in the magnetic
structure occurs. Although the magnetic transition temperature remains the
same, qm increases from (0.5, 0.5, 0.298) to (0.5, 0.5, 0.396). This change in
the magnetic modulation may be the outcome of a change in the electronic
character of this material at 10 kbar.Comment: 4 pages, 3 figures include
The bulk correlation length and the range of thermodynamic Casimir forces at Bose-Einstein condensation
The relation between the bulk correlation length and the decay length of
thermodynamic Casimir forces is investigated microscopically in two
three-dimensional systems undergoing Bose-Einstein condensation: the perfect
Bose gas and the imperfect mean-field Bose gas. For each of these systems, both
lengths diverge upon approaching the corresponding condensation point from the
one-phase side, and are proportional to each other. We determine the
proportionality factors and discuss their dependence on the boundary
conditions. The values of the corresponding critical exponents for the decay
length and the correlation length are the same, equal to 1/2 for the perfect
gas, and 1 for the imperfect gas
Heat capacity studies of Ce and Rh site substitution in the heavy fermion antiferromagnet CeRhIn_5;: Short-range magnetic interactions and non-Fermi-liquid behavior
In heavy fermion materials superconductivity tends to appear when long range
magnetic order is suppressed by chemical doping or applying pressure. Here we
report heat capacity measurements on diluted alloyes of the heavy fermion
superconductor CeRhIn_5;. Heat capacity measurements have been performed on
CeRh_{1-y}Ir_{y}In_5; (y <= 0.10) and Ce_{1-x}La_{x}Rh_{1-y}Ir_{y}In_5; (x <=
0.50) in applied fields up to 90 kOe to study the affect of doping and magnetic
field on the magnetic ground state. The magnetic phase diagram of
CeRh_{0.9}Ir_{0.1}In_5; is consistent with the magnetic structure of CeRhIn_5;
being unchanged by Ir doping. Doping of Ir in small concentrations is shown to
slightly increase the antiferromagnetic transition temperature T_{N} (T_{N}=3.8
K in the undoped sample). La doping which causes disorder on the Ce sublattice
is shown to lower T_{N} with no long range order observed above 0.34 K for
Ce_{0.50}La_{0.50}RhIn_5;. Measurements on Ce_{0.50}La_{0.50}RhIn_5; show a
coexistence of short range magnetic order and non-Fermi-liquid behavior. This
dual nature of the Ce 4f-electrons is very similar to the observed results on
CeRhIn_5; when long range magnetic order is suppressed at high pressure.Comment: 8 pages, 9 figure
Enhanced stability of the square lattice of a classical bilayer Wigner crystal
The stability and melting transition of a single layer and a bilayer crystal
consisting of charged particles interacting through a Coulomb or a screened
Coulomb potential is studied using the Monte-Carlo technique. A new melting
criterion is formulated which we show to be universal for bilayer as well as
for single layer crystals in the case of (screened) Coulomb, Lennard--Jones and
1/r^{12} repulsive inter-particle interactions. The melting temperature for the
five different lattice structures of the bilayer Wigner crystal is obtained,
and a phase diagram is constructed as a function of the interlayer distance. We
found the surprising result that the square lattice has a substantial larger
melting temperature as compared to the other lattice structures. This is a
consequence of the specific topology of the defects which are created with
increasing temperature and which have a larger energy as compared to the
defects in e.g. a hexagonal lattice.Comment: Accepted for publication in Physical Review
Numerical convergence of the block-maxima approach to the Generalized Extreme Value distribution
In this paper we perform an analytical and numerical study of Extreme Value
distributions in discrete dynamical systems. In this setting, recent works have
shown how to get a statistics of extremes in agreement with the classical
Extreme Value Theory. We pursue these investigations by giving analytical
expressions of Extreme Value distribution parameters for maps that have an
absolutely continuous invariant measure. We compare these analytical results
with numerical experiments in which we study the convergence to limiting
distributions using the so called block-maxima approach, pointing out in which
cases we obtain robust estimation of parameters. In regular maps for which
mixing properties do not hold, we show that the fitting procedure to the
classical Extreme Value Distribution fails, as expected. However, we obtain an
empirical distribution that can be explained starting from a different
observable function for which Nicolis et al. [2006] have found analytical
results.Comment: 34 pages, 7 figures; Journal of Statistical Physics 201
Magnetic structure of CeRhIn_5 as a function of pressure and temperature
We report magnetic neutron-diffraction and electrical resistivity studies on
single crystals of the heavy-fermion antiferromagnet CeRhIn at pressures
up to 2.3 GPa. These experiments show that the staggered moment of Ce and the
incommensurate magnetic structure change weakly with applied pressure up to
1.63 GPa, where resistivity, specific heat and NQR measurements confirm the
presence of bulk superconductivity. This work places new constraints on an
interpretation of the relationship between antiferromagnetism and
unconventional superconductivity in CeRhIn.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
Normal and Lateral Casimir Forces between Deformed Plates
The Casimir force between macroscopic bodies depends strongly on their shape
and orientation. To study this geometry dependence in the case of two deformed
metal plates, we use a path integral quantization of the electromagnetic field
which properly treats the many-body nature of the interaction, going beyond the
commonly used pairwise summation (PWS) of van der Waals forces. For arbitrary
deformations we provide an analytical result for the deformation induced change
in Casimir energy, which is exact to second order in the deformation amplitude.
For the specific case of sinusoidally corrugated plates, we calculate both the
normal and the lateral Casimir forces. The deformation induced change in the
Casimir interaction of a flat and a corrugated plate shows an interesting
crossover as a function of the ratio of the mean platedistance H to the
corrugation length \lambda: For \lambda \ll H we find a slower decay \sim
H^{-4}, compared to the H^{-5} behavior predicted by PWS which we show to be
valid only for \lambda \gg H. The amplitude of the lateral force between two
corrugated plates which are out of registry is shown to have a maximum at an
optimal wavelength of \lambda \approx 2.5 H. With increasing H/\lambda \gtrsim
0.3 the PWS approach becomes a progressively worse description of the lateral
force due to many-body effects. These results may be of relevance for the
design and operation of novel microelectromechanical systems (MEMS) and other
nanoscale devices.Comment: 20 pages, 5 figure
Using the Wigner-Ibach Surmise to Analyze Terrace-Width Distributions: History, User's Guide, and Advances
A history is given of the applications of the simple expression generalized
from the surmise by Wigner and also by Ibach to extract the strength of the
interaction between steps on a vicinal surface, via the terrace width
distribution (TWD). A concise guide for use with experiments and a summary of
some recent extensions are provided.Comment: 11 pages, 4 figures, reformatted (with revtex) version of refereed
paper for special issue of Applied Physics A entitled "From Surface Science
to Device Physics", in honor of the retirements of Prof. H. Ibach and Prof.
H. L\"ut
Local fluctuations in quantum critical metals
We show that spatially local, yet low-energy, fluctuations can play an
essential role in the physics of strongly correlated electron systems tuned to
a quantum critical point. A detailed microscopic analysis of the Kondo lattice
model is carried out within an extended dynamical mean-field approach. The
correlation functions for the lattice model are calculated through a
self-consistent Bose-Fermi Kondo problem, in which a local moment is coupled
both to a fermionic bath and to a bosonic bath (a fluctuating magnetic field).
A renormalization-group treatment of this impurity problem--perturbative in
, where is an exponent characterizing the spectrum
of the bosonic bath--shows that competition between the two couplings can drive
the local-moment fluctuations critical. As a result, two distinct types of
quantum critical point emerge in the Kondo lattice, one being of the usual
spin-density-wave type, the other ``locally critical.'' Near the locally
critical point, the dynamical spin susceptibility exhibits scaling
with a fractional exponent. While the spin-density-wave critical point is
Gaussian, the locally critical point is an interacting fixed point at which
long-wavelength and spatially local critical modes coexist. A Ginzburg-Landau
description for the locally critical point is discussed. It is argued that
these results are robust, that local criticality provides a natural description
of the quantum critical behavior seen in a number of heavy-fermion metals, and
that this picture may also be relevant to other strongly correlated metals.Comment: 20 pages, 12 figures; typos in figure 3 and in the main text
corrected, version as publishe
Dust Devil Tracks
Dust devils that leave dark- or light-toned tracks are common on Mars and they can also be found on the Earth’s surface. Dust devil tracks (hereinafter DDTs) are ephemeral surface features with mostly sub-annual lifetimes. Regarding their size, DDT widths can range between ∼1 m and ∼1 km, depending on the diameter of dust devil that created the track, and DDT lengths range from a few tens of meters to several kilometers, limited by the duration and horizontal ground speed of dust devils. DDTs can be classified into three main types based on their morphology and albedo in contrast to their surroundings; all are found on both planets: (a) dark continuous DDTs, (b) dark cycloidal DDTs, and (c) bright DDTs. Dark continuous DDTs are the most common type on Mars. They are characterized by their relatively homogenous and continuous low albedo surface tracks. Based on terrestrial and martian in situ studies, these DDTs most likely form when surficial dust layers are removed to expose larger-grained substrate material (coarse sands of ≥500 μm in diameter). The exposure of larger-grained materials changes the photometric properties of the surface; hence leading to lower albedo tracks because grain size is photometrically inversely proportional to the surface reflectance. However, although not observed so far, compositional differences (i.e., color differences) might also lead to albedo contrasts when dust is removed to expose substrate materials with mineralogical differences. For dark continuous DDTs, albedo drop measurements are around 2.5 % in the wavelength range of 550–850 nm on Mars and around 0.5 % in the wavelength range from 300–1100 nm on Earth. The removal of an equivalent layer thickness around 1 μm is sufficient for the formation of visible dark continuous DDTs on Mars and Earth. The next type of DDTs, dark cycloidal DDTs, are characterized by their low albedo pattern of overlapping scallops. Terrestrial in situ studies imply that they are formed when sand-sized material that is eroded from the outer vortex area of a dust devil is redeposited in annular patterns in the central vortex region. This type of DDT can also be found in on Mars in orbital image data, and although in situ studies are lacking, terrestrial analog studies, laboratory work, and numerical modeling suggest they have the same formation mechanism as those on Earth. Finally, bright DDTs are characterized by their continuous track pattern and high albedo compared to their undisturbed surroundings. They are found on both planets, but to date they have only been analyzed in situ on Earth. Here, the destruction of aggregates of dust, silt and sand by dust devils leads to smooth surfaces in contrast to the undisturbed rough surfaces surrounding the track. The resulting change in photometric properties occurs because the smoother surfaces have a higher reflectance compared to the surrounding rough surface, leading to bright DDTs. On Mars, the destruction of surficial dust-aggregates may also lead to bright DDTs. However, higher reflective surfaces may be produced by other formation mechanisms, such as dust compaction by passing dust devils, as this may also cause changes in photometric properties. On Mars, DDTs in general are found at all elevations and on a global scale, except on the permanent polar caps. DDT maximum areal densities occur during spring and summer in both hemispheres produced by an increase in dust devil activity caused by maximum insolation. Regionally, dust devil densities vary spatially likely controlled by changes in dust cover thicknesses and substrate materials. This variability makes it difficult to infer dust devil activity from DDT frequencies. Furthermore, only a fraction of dust devils leave tracks. However, DDTs can be used as proxies for dust devil lifetimes and wind directions and speeds, and they can also be used to predict lander or rover solar panel clearing events. Overall, the high DDT frequency in many areas on Mars leads to drastic albedo changes that affect large-scale weather patterns
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