658 research outputs found
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
Phase transition between quantum and classical regimes for the escape rate of a biaxial spin system
Employing the method of mapping the spin problem onto a particle one, we have
derived the particle Hamiltonian for a biaxial spin system with a transverse or
longitudinal magnetic field. Using the Hamiltonian and introducing the
parameter where (U_{min})
corresponds to the top (bottom) of the potential and is the energy of the
particle, we have studied the first- or second-order transition around the
crossover temperature between thermal and quantum regimes for the escape rate,
depending on the anisotropy constant and the external magnetic field. It is
shown that the phase boundary separating the first- and second-order transition
and its crossover temperature are greatly influenced by the transverse
anisotropy constant as well as the transverse or longitudinal magnetic field.Comment: 5 pages + 3 figures, to be published in Phys. Rev.
Nonadiabatic Landau Zener tunneling in Fe_8 molecular nanomagnets
The Landau Zener method allows to measure very small tunnel splittings \Delta
in molecular clusters Fe_8. The observed oscillations of \Delta as a function
of the magnetic field applied along the hard anisotropy axis are explained in
terms of topological quantum interference of two tunnel paths of opposite
windings. Studies of the temperature dependence of the Landau Zener transition
rate P gives access to the topological quantum interference between exited spin
levels. The influence of nuclear spins is demonstrated by comparing P of the
standard Fe_8 sample with two isotopically substituted samples. The need of a
generalized Landau Zener transition rate theory is shown.Comment: 5 pages, 6 figure
Instanton picture of the spin tunneling in the Lipkin model
A consistent theory of the ground state energy and its splitting due to the
process of tunneling for the Lipkin model is presented. For the functional
integral in terms of the spin coherent states for the partition function of the
model we accurately calculate the trivial and the instanton saddle point
contributions. We show that such calculation has to be perfomed very accurately
taking into account the discrete nature of the functional integral. Such
accurate consideration leads to finite corrections to a naive continous
consideration. We present comparison with numerical calculation of the ground
state energy and the tunneling splitting and with the results obtained by the
quasiclassical method and get excellent agreement.Comment: REVTEX, 32 pages, 3 figure
The Relationship Between Minority Business Enterprises and Corporate Purchasing Personnel: Perceptions from Both Sides of the Table
This paper addresses the nature of the difficulties MBEs face when conducting business with large companies through MBE purchasing programs. Data collected from MBEs and purchasing personnel were analyzed with logistic regression to demonstrate that MBEs and their corporate purchasing counterparts have different perceptions across human, environmental, and organizational dimensions of transaction cost economics. These differences help to explain the problems: (1) that MBEs have in selling to large companies and the problems that MBEs and purchasing personnel have in implementing MBE purchasing programs; (2) of reaching agreement in the marketplace; and, (3) of collectively pursuing the economic development of the minority business community. We offer recommendations for improving the relationship between these parties
Low-Temperature Quantum Relaxation in a System of Magnetic Nanomolecules
We argue that to explain recent resonant tunneling experiments on crystals of
Mn and Fe, particularly in the low-T limit, one must invoke dynamic
nuclear spin and dipolar interactions. We show the low-, short-time
relaxation will then have a form, where depends on the
nuclear , on the tunneling matrix element between the two
lowest levels, and on the initial distribution of internal fields in the
sample, which depends very strongly on sample shape. The results are directly
applicable to the system. We also give some results for the long-time
relaxation.Comment: 4 pages, 3 PostScript figures, LaTe
Magnon Exchange Mechanism of Ferromagnetic Superconductivity
The magnon exchange mechanism of ferromagnetic superconductivity
(FM-superconductivity) was developed to explain in a natural way the fact that
the superconductivity in , and is confined to the
ferromagnetic phase.The order parameter is a spin anti-parallel component of a
spin-1 triplet with zero spin projection. The transverse spin fluctuations are
pair forming and the longitudinal ones are pair breaking. In the present paper,
a superconducting solution, based on the magnon exchange mechanism, is obtained
which closely matches the experiments with and . The onset of
superconductivity leads to the appearance of complicated Fermi surfaces in the
spin up and spin down momentum distribution functions. Each of them consist of
two pieces, but they are simple-connected and can be made very small by varying
the microscopic parameters. As a result, it is obtained that the specific heat
depends on the temperature linearly, at low temperature, and the coefficient
is smaller in the superconducting phase than in the
ferromagnetic one. The absence of a quantum transition from ferromagnetism to
ferromagnetic superconductivity in a weak ferromagnets and is
explained accounting for the contribution of magnon self-interaction to the
spin fluctuations' parameters. It is shown that in the presence of an external
magnetic field the system undergoes a first order quantum phase transition.Comment: 9 pages, 7 figures, accepted for publication in Phys.Rev.
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
Crossover from thermal hopping to quantum tunneling in Mn_{12}Ac
The crossover from thermal hopping to quantum tunneling is studied. We show
that the decay rate with dissipation can accurately be determined near
the crossover temperature. Besides considering the Wentzel-Kramers-Brillouin
(WKB) exponent, we also calculate contribution of the fluctuation modes around
the saddle point and give an extended account of a previous study of crossover
region. We deal with two dangerous fluctuation modes whose contribution can't
be calculated by the steepest descent method and show that higher order
couplings between the two dangerous modes need to be taken into considerations.
At last the crossover from thermal hopping to quantum tunneling in the
molecular magnet Mn_{12}Ac is studied.Comment: 10 pages, 3 figure
Ward identity and optical-conductivity sum rule in the d-density wave state
We consider the role of the Ward identity in dealing with the transport
properties of an interacting system forming a d-wave modulated charge-density
wave or staggered flux phase. In particular, we address this issue from the
point of view of the restricted optical-conductivity sum rule. Our aim is to
provide a controlled approximation for the current-current correlation function
which allows us also to determine analytically the corresponding sum rule. By
analyzing the role of the vertex functions in both the microscopic interacting
model and in the effective mean-field Hamiltonian, we propose a non-standard
low-energy sum-rule for this system. We also discuss the possible applicability
of these results for the description of cuprate superconductors in the
pseudogap regime.Comment: Revised version, accepted for publication in Phys. Rev.
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