55,821 research outputs found
Quantum Tunneling of Spin Particles in Periodic Potentials with Asymmetric Twin Barriers
The tunneling effect of a periodic potential with an asymmetric twin barrier
per period is calculated using the instanton method. The model is derived from
the Hamiltonian of a small ferromagnetic particle in an external magnetic field
using the spin-coherent-state path integral. The instantons in two neighbouring
barriers differ and lead to different level shifts . We derive with Bloch theory the energy spectrum which has
formally the structure of an energy band. The spectrum depends on both level
shifts. The removal of Kramer's degeneracy by an external magnetic field is
discussed. In addition we find a new kind of quenching of macroscopic quantum
coherence which is irrelevant to Kramer's degeneracy.Comment: 18 pages, LaTex, one figur
Calculation of Spin Tunneling Effects in the Presence of an Applied Magnetic Field
The tunneling splitting of the energy levels of a ferromagnetic particle in
the presence of an applied magnetic field - previously derived only for the
ground state with the path integral method - is obtained in a simple way from
Schroedinger theory. The origin of the factors entering the result is clearly
understood, in particular the effect of the asymmetry of the barriers of the
potential. The method should appeal particularly to experimentalists searching
for evidence of macroscopic spin tunneling.Comment: 10 pages, LaTex, 4 figures included using graphic
Arithmetic purity of strong approximation for homogeneous spaces
We prove that any open subset of a semi-simple simply connected quasi-split linear algebraic group with over a number field satisfies strong approximation by establishing a fibration of over a toric variety. We also prove a similar result of strong approximation with Brauer-Manin obstruction for a partial equivariant smooth compactification of a homogeneous space where all invertible functions are constant and the semi-simple part of the linear algebraic group is quasi-split. Some semi-abelian varieties of any given dimension where the complements of a rational point do not satisfy strong approximation with Brauer-Manin obstruction are given
Interacting Dirac fermions under spatially alternating pseudo-magnetic field: Realization of spontaneous quantum Hall effect
Both topological crystalline insulators surfaces and graphene host
multi-valley massless Dirac fermions which are not pinned to a high-symmetry
point of the Brillouin zone. Strain couples to the low-energy electrons as a
time-reversal invariant gauge field, leading to the formation of pseudo-Landau
levels (PLL). Here we study periodic pseudo-magnetic fields originating from
strain superlattices. We study the low-energy Dirac PLL spectrum induced by the
strain superlattice and analyze the effect of various polarized states. Through
self-consistent Hartree-Fock calculations we establish that, due to the strain
superlattice and PLL electronic structure, a valley-ordered state spontaneously
breaking time-reversal and realizing a quantum Hall phase is favored, while
others are suppressed.Comment: 13 pages + 2 appendices, 9 figure
Zeeman-Induced Gapless Superconductivity with Partial Fermi Surface
We show that an in-plane magnetic field can drive two-dimensional
spin-orbit-coupled systems under superconducting proximity effect into a
gapless phase where parts of the normal state Fermi surface are gapped, and the
ungapped parts are reconstructed into a small Fermi surface of Bogoliubov
quasiparticles at zero energy. Charge distribution, spin texture, and density
of states of such "partial Fermi surface" are discussed. Material platforms for
its physical realization are proposed.Comment: 5 pages, 2 figure
Macroscopic Quantum Coherence in Small Antiferromagnetic Particle and the Quantum Interference Effects
Starting from the Hamiltonian operator of the noncompensated two-sublattice
model of a small antiferromagnetic particle, we derive the effective Lagrangian
of a biaxial antiferromagnetic particle in an external magnetic field with the
help of spin-coherent-state path integrals. Two unequal level-shifts induced by
tunneling through two types of barriers are obtained using the instanton
method. The energy spectrum is found from Bloch theory regarding the periodic
potential as a superlattice. The external magnetic field indeed removes
Kramers' degeneracy, however a new quenching of the energy splitting depending
on the applied magnetic field is observed for both integer and half-integer
spins due to the quantum interference between transitions through two types of
barriers.Comment: 9 pages, Latex, 4 Postscript figure
NeIII/OII as an oxygen abundance indicator in the HII regions and HII galaxies
To calibrate the relationship between Ne3O2 (Ne3O2 =
log(\neiii/\oii)) and oxygen abundances, we present a
sample of 3000 \hii galaxies from the Sloan Digital Sky Survey (SDSS)
data release four. They are associated with a sample from the literature
intended to enlarge the oxygen abundance region. We calculated the electron
temperatures () of 210 galaxies in the SDSS sample with the direct method,
and of the other 2960 galaxies in SDSS sample calculated with an
empirical method. Then, we use a linear least-square fitting to calibrate the
Ne3O2 oxygen abundance indicator. It is found that the Ne3O2 estimator follows
a linear relation with \zoh\ that holds for the whole abundance range covered
by the sample, from approximately 7.0 to 9.0. The best linear relationship
between the Ne3O2 and the oxygen abundance is calibrated. The dispersion
between oxygen abundance and Ne3O2 index in the metal rich galaxies may come
partly from the moderate depletion of oxygen onto grains. The method
has the virtue of being single-valued and not affected by internal reddening.
As a result, the method can be a good metallicity indicator in the \hii
regions and \hii galaxies, especially in high-redshift galaxies.Comment: 7 pages, 6 figures. A&A accepte
BRST invariant approach to quantum mechanical tunneling
A new approach with BRST invariance is suggested to cure the degeneracy
problem of ill defined path integrals in the path-integral calculationof
quantum mechanical tunneling effects in which the problem arises due to the
occurrence of zero modes. The Faddeev-Popov procedure is avoided and the
integral over the zero mode is transformed in a systematic way into a well
defined integral over instanton positions. No special procedure has to be
adopted as in the Faddeev-Popov method in calculating the Jacobian of the
transformation. The quantum mechanical tunneling for the Sine-Gordon potential
is used as a test of the method and the width of the lowest energy band is
obtained in exact agreement with that of WKB calculations.Comment: 9 pages, LaTe
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