506 research outputs found
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
On the Observability of Meso- and Macro-scopic Quantum Coherence of Domain Walls in Magnetic Insulators
Results are presented of a numerical calculation of the tunneling gap for a
domain wall moving in the double well potential of a pair of voids in a
magnetic insulator. Both symmetric and asymmetric double well potentials are
considered. It is found that, even in the absence of dissipation, the prospects
for observing quantum coherence on a meso- or macro-scopic scale appears
unlikely.Comment: 15 pages, Plain LaTex, UBC TP-93-1
Sugarbeet Production Under Reduced Tillage Prospects And Problems
A study was initiated in the fall of 1977 to obtain base line data on the applicability of reduced tillage sugarbeet production in the Red River Valley. Three reduced tillage systems were compared to a conventional system which consisted of fall plow plus secondary tillage. Results indicated warmer early spring soil temperatures, better seedling emergence, lower ground-level wind speed and no significant yield loss under reduced tillage as compared to the conventional system
Path integrals for spinning particles, stationary phase and the Duistermaat-Heckman theorem
We examine the problem of the evaluation of both the propagator and of the
partition function of a spinning particle in an external field at the classical
as well as the quantum level, in connection with the asserted exactness of the
saddle point approximation (SPA) for this problem. At the classical level we
argue that exactness of the SPA stems from the fact that the dynamics (on the
two--sphere ) of a classical spinning particle in a magnetic field is the
reduction from \br^4 to of a linear dynamical system on \br^4. At the
quantum level, however, and within the path integral approach, the restriction,
inherent to the use of the SPA, to regular paths clashes with the fact that no
regulators are present in the action that enters the path integral. This is
shown to lead to a prefactor for the path integral that is strictly divergent
except in the classical limit. A critical comparison is made with the various
approaches to the same problem that have been presented in the literature. The
validity of a formula given in literature for the spin propagator is extended
to the case of motion in an arbitrary magnetic field.Comment: 29 pages, Late
Unusual condensates in quark and atomic systems
In these lectures we discuss condensates which are formed in quark matter
when it is squeezed and in a gas of fermionic atoms when it is cooled. The
behavior of these two seemingly very different systems reveals striking
similarities. In particular, in both systems the Bose-Einstein condensate to
Bardeen--Cooper-Schrieffer (BEC-BCS) crossover takes place.Comment: Lectures delivered at 8th Moscow school of Physics (33rd ITEP Winter
School of Physics
Quantum Phase Interference for Quantum Tunneling in Spin Systems
The point-particle-like Hamiltonian of a biaxial spin particle with external
magnetic field along the hard axis is obtained in terms of the potential field
description of spin systems with exact spin-coordinate correspondence. The
Zeeman energy term turns out to be an effective gauge potential which leads to
a nonintegrable pha se of the Euclidean Feynman propagator.
The phase interference between clockwise and anticlockwise under barrier
propagations is recognized explicitly as the Aharonov-Bohm effect. An
additional phase which is significant for quantum phase interference is
discovered with the quantum theory of spin systems besides the known phase
obtained with the semiclassical treatment of spin. We also show the energ y
dependence of the effect and obtain the tunneling splitting at excited states
with the help of periodic instantons.Comment: 19 pages, no figure, to appear in PR
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
Model Exact Low-Lying States and Spin Dynamics in Ferric Wheels; Fe to Fe
Using an efficient numerical scheme that exploits spatial symmetries and
spin-parity, we have obtained the exact low-lying eigenstates of exchange
Hamiltonians for ferric wheels up to Fe. The largest calculation
involves the Fe ring which spans a Hilbert space dimension of about 145
million for M=0 subspace. Our calculated gaps from the singlet ground state
to the excited triplet state agrees well with the experimentally measured
values. Study of the static structure factor shows that the ground state is
spontaneously dimerized for ferric wheels. Spin states of ferric wheels can be
viewed as quantized states of a rigid rotor with the gap between the ground and
the first excited state defining the inverse of moment of inertia. We have
studied the quantum dynamics of Fe as a representative of ferric wheels.
We use the low-lying states of Fe to solve exactly the time-dependent
Schr\"odinger equation and find the magnetization of the molecule in the
presence of an alternating magnetic field at zero temperature. We observe a
nontrivial oscillation of magnetization which is dependent on the amplitude of
the {\it ac} field. We have also studied the torque response of Fe as a
function of magnetic field, which clearly shows spin-state crossover.Comment: Revtex, 24 pages, 8 eps figure
Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles
A formula suitable for a quantitative evaluation of the tunneling effect in a
ferromagnetic particle is derived with the help of the instanton method. The
tunneling between n-th degenerate states of neighboring wells is dominated by a
periodic pseudoparticle configuration. The low-lying level-splitting previously
obtained with the LSZ method in field theory in which the tunneling is viewed
as the transition of n bosons induced by the usual (vacuum) instanton is
recovered. The observation made with our new result is that the tunneling
effect increases at excited states. The results should be useful in analyzing
results of experimental tests of macroscopic quantum coherence in ferromagnetic
particles.Comment: 18 pages, LaTex, 1 figur
Dicke-Type Energy Level Crossings in Cavity-Induced Atom Cooling: Another Superradiant Cooling
This paper is devoted to energy-spectral analysis for the system of a
two-level atom coupled with photons in a cavity. It is shown that the
Dicke-type energy level crossings take place when the atom-cavity interaction
of the system undergoes changes between the weak coupling regime and the strong
one. Using the phenomenon of the crossings we develop the idea of
cavity-induced atom cooling proposed by the group of Ritsch, and we lay
mathematical foundations of a possible mechanism for another superradiant
cooling in addition to that proposed by Domokos and Ritsch. The process of our
superradiant cooling can function well by cavity decay and by control of the
position of the atom, at least in (mathematical) theory, even if there is
neither atomic absorption nor atomic emission of photons.Comment: 15 pages; 8 figure
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