4,360 research outputs found
Casimir Energy and Entropy between perfect metal Spheres
We calculate the Casimir energy and entropy for two perfect metal spheres in
the large and short separation limit. We obtain nonmonotonic behavior of the
Helmholtz free energy with separation and temperature, leading to parameter
ranges with negative entropy, and also nonmonotonic behavior of the entropy
with temperature and with the separation between the spheres. The appearance of
this anomalous behavior of the entropy is discussed as well as its
thermodynamic consequences.Comment: 10 pages and 8 figures. Accepted for publication in the Proceedings
of the tenth conference on Quantum Field Theory under the influence of
external conditions - QFEXT'1
Low-temperature behavior of the statistics of the overlap distribution in Ising spin-glass models
Using Monte Carlo simulations, we study in detail the overlap distribution
for individual samples for several spin-glass models including the
infinite-range Sherrington-Kirkpatrick model, short-range Edwards-Anderson
models in three and four space dimensions, and one-dimensional long-range
models with diluted power-law interactions. We study three long-range models
with different powers as follows: the first is approximately equivalent to a
short-range model in three dimensions, the second to a short-range model in
four dimensions, and the third to a short-range model in the mean-field regime.
We study an observable proposed earlier by some of us which aims to distinguish
the "replica symmetry breaking" picture of the spin-glass phase from the
"droplet picture," finding that larger system sizes would be needed to
unambiguously determine which of these pictures describes the low-temperature
state of spin glasses best, except for the Sherrington-Kirkpatrick model which
is unambiguously described by replica symmetry breaking. Finally, we also study
the median integrated overlap probability distribution and a typical overlap
distribution, finding that these observables are not particularly helpful in
distinguishing the replica symmetry breaking and the droplet pictures.Comment: 11 pages, 6 figure
The role of sign in students' modeling of scalar equations
We describe students revising the mathematical form of physics equations to
match the physical situation they are describing, even though their revision
violates physical laws. In an unfamiliar air resistance problem, a majority of
students in a sophomore level mechanics class at some point wrote Newton's
Second Law as F = -ma; they were using this form to ensure that the sign of the
force pointed in a direction consistent with the chosen coordinate system while
assuming that some variables have only positive value. We use one student's
detailed explanation to suggest that students' issues with variables are
context-dependent, and that much of their reasoning is useful for productive
instruction.Comment: 5 pages, 1 figure, to be published in The Physics Teache
Understanding and Affecting Student Reasoning About Sound Waves
Student learning of sound waves can be helped through the creation of
group-learning classroom materials whose development and design rely on
explicit investigations into student understanding. We describe reasoning in
terms of sets of resources, i.e. grouped building blocks of thinking that are
commonly used in many different settings. Students in our university physics
classes often used sets of resources that were different from the ones we wish
them to use. By designing curriculum materials that ask students to think about
the physics from a different view, we bring about improvement in student
understanding of sound waves. Our curriculum modifications are specific to our
own classes, but our description of student learning is more generally useful
for teachers. We describe how students can use multiple sets of resources in
their thinking, and raise questions that should be considered by both
instructors and researchers.Comment: 23 pages, 4 figures, 3 tables, 28 references, 7 notes. Accepted for
publication in the International Journal of Science Educatio
Continuous transition between decagonal quasicrystal and approximant by formation and ordering of out-of-phase domains
The transformation between a quasicrystal and an orthorhombic approximant is studied at the nominal composition Al72.7Ni8.3Co19 by electron diffraction and high-resolution transmission electron microscopy. A series of transition states indicating a continuous transformation is monitored. First, the material transforms to a single-oriented one-dimensional quasicrystal. In the course of this process out-of-phase domains are formed. The approximant results from ordering of these domains to a periodic structure
Energy levels and decoherence properties of single electron and nuclear spins in a defect center in diamond
The coherent behavior of the single electron and single nuclear spins of a
defect center in diamond and a 13C nucleus in its vicinity, respectively, are
investigated. The energy levels associated with the hyperfine coupling of the
electron spin of the defect center to the 13C nuclear spin are analyzed.
Methods of magnetic resonance together with optical readout of single defect
centers have been applied in order to observe the coherent dynamics of the
electron and nuclear spins. Long coherence times, in the order of microseconds
for electron spins and tens of microseconds for nuclear spins, recommend the
studied system as a good experimental approach for implementing a 2-qubit gate.Comment: 4 pages, 4 figure
Solving the Einstein-Podolsky-Rosen puzzle: the origin of non-locality in Aspect-type experiments
So far no mechanism is known, which could connect the two measurements in an
Aspect-type experiment. Here, we suggest such a mechanism, based on the phase
of a photon's field during propagation. We show that two polarization
measurements are correlated, even if no signal passes from one point of
measurement to the other. The non-local connection of a photon pair is the
result of its origin at a common source, where the two fields acquire a well
defined phase difference. Therefore, it is not actually a non-local effect in
any conventional sense. We expect that the model and the detailed analysis it
allows will have a major impact on quantum cryptography and quantum
computation.Comment: 5 pages 1 figure. Added an analysis of quantum steering. The result
is that under certain conditions the experimental result at B can be
predicted if the polarization angle and the result at A are known. The paper
has been accepted for publication in Frontiers of Physics. arXiv admin note:
substantial text overlap with arXiv:1108.435
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