7,213 research outputs found
Macroscopic Degeneracy and order in the 3d plaquette Ising model
The purely plaquette 3d Ising Hamiltonian with the spins living at the
vertices of a cubic lattice displays several interesting features. The
symmetries of the model lead to a macroscopic degeneracy of the low-temperature
phase and prevent the definition of a standard magnetic order parameter.
Consideration of the strongly anisotropic limit of the model suggests that a
layered, "fuki-nuke" order still exists and we confirm this with multicanonical
simulations. The macroscopic degeneracy of the low-temperature phase also
changes the finite-size scaling corrections at the first-order transition in
the model and we see this must be taken into account when analysing our
measurements.Comment: arXiv admin note: text overlap with arXiv:1412.442
Multicanonical analysis of the plaquette-only gonihedric Ising model and its dual
The three-dimensional purely plaquette gonihedric Ising model and its dual
are investigated to resolve inconsistencies in the literature for the values of
the inverse transition temperature of the very strong temperature-driven
first-order phase transition that is apparent in the system. Multicanonical
simulations of this model allow us to measure system configurations that are
suppressed by more than 60 orders of magnitude compared to probable states.
With the resulting high-precision data, we find excellent agreement with our
recently proposed nonstandard finite-size scaling laws for models with a
macroscopic degeneracy of the low-temperature phase by challenging the
prefactors numerically. We find an overall consistent inverse transition
temperature of 0.551334(8) from the simulations of the original model both with
periodic and fixed boundary conditions, and the dual model with periodic
boundary conditions. For the original model with periodic boundary conditions,
we obtain the first reliable estimate of the interface tension, 0.12037(18),
using the statistics of suppressed configurations
Exact solutions to plaquette Ising models with free and periodic boundaries
An anisotropic limit of the 3d plaquette Ising model, in which the plaquette
couplings in one direction were set to zero, was solved for free boundary
conditions by Suzuki (Phys. Rev. Lett. 28 (1972) 507), who later dubbed it the
fuki-nuke, or "no-ceiling", model. Defining new spin variables as the product
of nearest-neighbour spins transforms the Hamiltonian into that of a stack of
(standard) 2d Ising models and reveals the planar nature of the magnetic order,
which is also present in the fully isotropic 3d plaquette model. More recently,
the solution of the fuki-nuke model was discussed for periodic boundary
conditions, which require a different approach to defining the product spin
transformation, by Castelnovo et al. (Phys. Rev. B 81 (2010) 184303).
We clarify the exact relation between partition functions with free and
periodic boundary conditions expressed in terms of original and product spin
variables for the 2d plaquette and 3d fuki-nuke models, noting that the
differences are already present in the 1d Ising model. In addition, we solve
the 2d plaquette Ising model with helical boundary conditions. The various
exactly solved examples illustrate how correlations can be induced in finite
systems as a consequence of the choice of boundary conditions.Comment: v5 - The title is changed to better reflect the contents and the
exposition is streamlined. Version accepted for publicatio
Photovoltaic effect in an electrically tunable van der Waals heterojunction
Semiconductor heterostructures form the cornerstone of many electronic and
optoelectronic devices and are traditionally fabricated using epitaxial growth
techniques. More recently, heterostructures have also been obtained by vertical
stacking of two-dimensional crystals, such as graphene and related two-
dimensional materials. These layered designer materials are held together by
van der Waals forces and contain atomically sharp interfaces. Here, we report
on a type- II van der Waals heterojunction made of molybdenum disulfide and
tungsten diselenide monolayers. The junction is electrically tunable and under
appropriate gate bias, an atomically thin diode is realized. Upon optical
illumination, charge transfer occurs across the planar interface and the device
exhibits a photovoltaic effect. Advances in large-scale production of
two-dimensional crystals could thus lead to a new photovoltaic solar
technology.Comment: 26 pages, 14 figures, Nano Letters 201
Optical imaging of strain in two-dimensional crystals
Strain engineering is widely used in material science to tune the
(opto-)electronic properties of materials and enhance the performance of
devices. Two-dimensional atomic crystals are a versatile playground to study
the influence of strain, as they can sustain very large deformations without
breaking. Various optical techniques have been employed to probe strain in
two-dimensional materials, including micro-Raman and photoluminescence
spectroscopy. Here we demonstrate that optical second harmonic generation
constitutes an even more powerful technique, as it allows to extract the full
strain tensor with a spatial resolution below the optical diffraction limit.
Our method is based on the strain-induced modification of the nonlinear
susceptibility tensor due to a photoelastic effect. Using a two-point bending
technique, we determine the photoelastic tensor elements of molybdenum
disulfide. Once identified, these parameters allow us to spatially image the
two-dimensional strain field in an inhomogeneously strained sample.Comment: 13 pages, 4 figure
Physical characterization and origin of binary near-Earth asteroid (175706) 1996 FG3
The near-Earth asteroid (NEA) (175706) 1996 FG3 is a particularly interesting
spacecraft target: a binary asteroid with a low-DeltaV heliocentric orbit. The
orbit of its satellite has provided valuable information about its mass density
while its albedo and colors suggest it is primitive or part of the C-complex
taxonomic grouping. We extend the physical characterization of this object with
new observations of its emission at mid-Infrared (IR) wavelengths and with
near-IR reflection spectroscopy. We derive an area-equivalent system diameter
of 1.90 \pm 0.28 km (corresponding to approximate component diameters of 1.83
km and 0.51 km, respectively) and a geometric albedo of 0.039 \pm 0.012.
1996 FG3 was previously classified as a C-type asteroid, though the combined
0.4--2.5 micron spectrum with thermal correction indicates classification as
B-type; both are consistent with the low measured albedo. Dynamical studies
show that 1996 FG3 has most probably originated in the inner main asteroid
belt. Recent work has suggested the inner Main Belt (142) Polana family as the
possible origin of another low-DeltaV B-type NEA, (101955) 1999 RQ36. A similar
origin for 1996 FG3 would require delivery by the overlapping Jupiter 7:2 and
Mars 5:9 mean motion resonances rather than the nu-6 resonance, and we find
this to be a low probability, but possible, origin.Comment: Published in Ap
Physical Characterization of Warm Spitzer-observed Near-Earth Objects
Near-infrared spectroscopy of Near-Earth Objects (NEOs) connects diagnostic
spectral features to specific surface mineralogies. The combination of
spectroscopy with albedos and diameters derived from thermal infrared
observations can increase the scientific return beyond that of the individual
datasets. To that end, we have completed a spectroscopic observing campaign to
complement the ExploreNEOs Warm Spitzer program that obtained albedos and
diameters of nearly 600 NEOs (Trilling et al. 2010). Here we present the
results of observations using the low-resolution prism mode (~0.7-2.5 microns)
of the SpeX instrument on the NASA Infrared Telescope Facility (IRTF). We also
include near-infrared observations of ExploreNEOs targets from the MIT-UH-IRTF
Joint Campaign for Spectral Reconnaissance. Our dataset includes near-infrared
spectra of 187 ExploreNEOs targets (125 observations of 92 objects from our
survey and 213 observations of 154 objects from the MIT survey). We identify a
taxonomic class for each spectrum and use band parameter analysis to
investigate the mineralogies for the S-, Q-, and V-complex objects. Our
analysis suggests that for spectra that contain near-infrared data but lack the
visible wavelength region, the Bus-DeMeo system misidentifies some S-types as
Q-types. We find no correlation between spectral band parameters and
ExploreNEOs albedos and diameters. We find slightly negative Band Area Ratio
(BAR) correlations with phase angle for Eros and Ivar, but a positive BAR
correlation with phase angle for Ganymed. We find evidence for spectral phase
reddening for Eros, Ganymed, and Ivar. We identify the likely ordinary
chondrite type analog for a subset of our sample. Our resulting proportions of
H, L, and LL ordinary chondrites differ from those calculated for meteorite
falls and in previous studies of ordinary chondrite-like NEOs.Comment: 6 Tables, 9 Figure
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