17,638 research outputs found
Magnetic quantum phase transitions of the antiferromagnetic J_{1}-J_{2} Heisenberg model
We obtain the complete phase diagram of the antiferromagnetic -
model, , within the framework of the
nonlinear sigma model. We find two magnetically ordered phases, one with N\'
eel order, for , and another with collinear order, for
, separated by a nonmagnetic region, for , where a gapped spin liquid is found. The transition at is of
the second order while the one at is of the first order and the
spin gaps cross at . Our results are exact at
and agree with numerical results from different methods.Comment: 4 pages, 5 figure
Particle Creation by a Moving Boundary with Robin Boundary Condition
We consider a massless scalar field in 1+1 dimensions satisfying a Robin
boundary condition (BC) at a non-relativistic moving boundary. We derive a
Bogoliubov transformation between input and output bosonic field operators,
which allows us to calculate the spectral distribution of created particles.
The cases of Dirichlet and Neumann BC may be obtained from our result as
limiting cases. These two limits yield the same spectrum, which turns out to be
an upper bound for the spectra derived for Robin BC. We show that the particle
emission effect can be considerably reduced (with respect to the
Dirichlet/Neumann case) by selecting a particular value for the oscillation
frequency of the boundary position
Dynamical Casimir effect with cylindrical waveguides
I consider the quantum electromagnetic field in a coaxial cylindrical
waveguide, such that the outer cylindrical surface has a time-dependent radius.
The field propagates parallel to the axis, inside the annular region between
the two cylindrical surfaces. When the mechanical frequency and the thickness
of the annular region are small enough, only Transverse Electromagnetic (TEM)
photons may be generated by the dynamical Casimir effect. The photon emission
rate is calculated in this regime, and compared with the case of parallel
plates in the limit of very short distances between the two cylindrical
surfaces. The proximity force approximation holds for the transition matrix
elements in this limit, but the emission rate scales quadratically with the
mechanical frequency, as opposed to the cubic dependence for parallel plates.Comment: 6 page
Generation of Superposition States and Charge-Qubit Relaxation Probing in a Circuit
We demonstrate how a superposition of coherent states can be generated for a
microwave field inside a coplanar transmission line coupled to a single
superconducting charge qubit, with the addition of a single classical magnetic
pulse for chirping of the qubit transition frequency. We show how the qubit
dephasing induces decoherence on the field superposition state, and how it can
be probed by the qubit charge detection. The character of the charge qubit
relaxation process itself is imprinted in the field state decoherence profile.Comment: 6 pages, 4 figure
The Hamilton-Jacobi Approach to Teleparallelism
We intend to analyse the constraint structure of Teleparallelism employing
the Hamilton-Jacobi formalism for singular systems. This study is conducted
without using an ADM 3+1 decomposition and without fixing time gauge condition.
It can be verified that the field equations constitute an integrable system.Comment: 12 pages, no figur
Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)X
High-resolution thermal expansion measurements have been performed for
exploring the mysterious "structureless transition" in (TMTTF)X (X =
PF and AsF), where charge ordering at coincides with the
onset of ferroelectric order. Particularly distinct lattice effects are found
at in the uniaxial expansivity along the interstack
-direction. We propose a scheme involving a charge
modulation along the TMTTF stacks and its coupling to displacements of the
counteranions X. These anion shifts, which lift the inversion symmetry
enabling ferroelectric order to develop, determine the 3D charge pattern
without ambiguity. Evidence is found for another anomaly for both materials at
0.6 indicative of a phase transition
related to the charge ordering
New forms of BRST symmetry in rigid rotor
We derive the different forms of BRST symmetry by using the
Batalin-Fradkin-Vilkovisky formalism in a rigid rotor. The so called
"dual-BRST" symmetry is obtained from usual BRST symmetry by making a canonical
transformation in the ghost sector. On the other hand, a canonical
transformation in the sector involving Lagrange multiplier and its
corresponding momentum leads to a new form of BRST as well as dual-BRST
symmetry.Comment: 10 Pages, revtex, No Fig
Inertial forces in the Casimir effect with two moving plates
We combine linear response theory and dimensional regularization in order to
derive the dynamical Casimir force in the low frequency regime. We consider two
parallel plates moving along the normal direction in dimensional space. We
assume the free-space values for the mass of each plate to be known, and obtain
finite, separation-dependent mass corrections resulting from the combined
effect of the two plates. The global mass correction is proportional to the
static Casimir energy, in agreement with Einstein's law of equivalence between
mass and energy for stressed rigid bodies.Comment: 9 pages, 1 figure; title and abstract changed; to appear in Physical
Review
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