232 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
Quantum phase interference and spin parity in Mn12 single-molecule magnets
Magnetization measurements of Mn12 molecular nanomagnets with spin ground
states of S = 10 and S = 19/2 showresonance tunneling at avoided energy level
crossings. The observed oscillations of the tunnel probability as a function of
the magnetic field applied along the hard anisotropy axis are due to
topological quantum phase interference of two tunnel paths of opposite
windings. Spin-parity dependent tunneling is established by comparing the
quantum phase interference of integer and half-integer spin systems.Comment: 5 pages, 5 figure
Film properties and in-situ optical analysis of TiO2 layers synthesized by remote plasma ALD
TiO2 is a widely studied material due to its optical and photocatalytic properties and its hydrophilic nature after prolonged UV exposure. When synthesized by atomic layer deposition (ALD) the TiO2 can be deposited with ultimate growth control with a high conformality on demanding topologies and even at room temperature when e.g. using a plasma based process. We report on the deposition of TiO2 films using remote plasma ALD with titanium (IV) isopropoxide as precursor and O2 plasma as oxidant. Stochiometric TiO2 films with carbon and hydrogen levels below the detection limit of Rutherford backscattering/elastic recoil detection (<2 at.%) have been deposited within the temperature range of 25°C to 300°C. Depending on the ALD conditions and film thickness amorphous films turn anatase for temperatures higher than 200°C as revealed by X-ray diffraction. It is demonstrated that this change in crystal phase can also be observed by spectroscopic ellipsometry revealing an increase in growth rate per cycle (from typically 0.45 Å/cycle to 0.7 Å/cycle) and change in bandgap (from 3.4 eV to 3.7 eV) when the TiO2 becomes anatase. An accompanying change in surface topology is clearly observed by atomic force microscopy. The hydrophilicity of low temperature TiO2 films is studied by contact angle measurements for adhesion purposes revealing that the amorphous films are super-hydrophilic after UV exposure
Suppression of tunneling by interference in half-integer--spin particles
Within a wide class of ferromagnetic and antiferromagnetic systems, quantum
tunneling of magnetization direction is spin-parity dependent: it vanishes for
magnetic particles with half-integer spin, but is allowed for integer spin. A
coherent-state path integral calculation shows that this topological effect
results from interference between tunneling paths.Comment: 14 pages (RevTeX), 2 postscript figures available upon reques
Secure exchange of information by synchronization of neural networks
A connection between the theory of neural networks and cryptography is
presented. A new phenomenon, namely synchronization of neural networks is
leading to a new method of exchange of secret messages. Numerical simulations
show that two artificial networks being trained by Hebbian learning rule on
their mutual outputs develop an antiparallel state of their synaptic weights.
The synchronized weights are used to construct an ephemeral key exchange
protocol for a secure transmission of secret data. It is shown that an opponent
who knows the protocol and all details of any transmission of the data has no
chance to decrypt the secret message, since tracking the weights is a hard
problem compared to synchronization. The complexity of the generation of the
secure channel is linear with the size of the network.Comment: 11 pages, 5 figure
Quantum Tunneling of Magnetization in Single Molecular Magnets Coupled to Ferromagnetic Reservoirs
The role of spin polarized reservoirs in quantum tunneling of magnetization
and relaxation processes in a single molecular magnet (SMM) is investigated
theoretically. The SMM is exchange-coupled to the reservoirs and also subjected
to a magnetic field varying in time, which enables the quantum tunneling of
magnetization (QTM). The spin relaxation times are calculated from the Fermi
golden rule. The exchange interaction with tunneling electrons is shown to
affect the spin reversal due to QTM. Furthermore, it is shown that the
switching is associated with transfer of a certain charge between the leads.Comment: 5 pages, 3 EPS figures, final version as publishe
Large Deviation Property of Free Energy in p-Body Sherrington-Kirkpatrick Model
Cumulant generating function phi(n) and rate function Sigma(f) of the free
energy is evaluated in p-body Sherrington-Kirkpatrick model by using the
replica method with the replica number n finite. From a perturbational
argument, we show that the cumulant generating function is constant in the
vicinity of n = 0. On the other hand, with the help of two analytic properties
of phi(n), the behavior of phi(n) is derived again. However this is also shown
to be broken at a finite value of n, which gives a characteristic value in the
rate function near the thermodynamic value of the free energy. Through the
continuation of phi(n) as a function of n, we find out a way to derive the 1RSB
solution at least in this model, which is to fix the RS solution to be a
monotone increasing function.Comment: 7 pages, 5 figures. accepted for publication in J.Phs.Soc.Jp
Quenched Spin Tunneling and Diabolical Points in Magnetic Molecules: II. Asymmetric Configurations
The perfect quenching of spin tunneling first predicted for a model with
biaxial symmetry, and recently observed in the magnetic molecule Fe_8, is
further studied using the discrete phase integral (or
Wentzel-Kramers-Brillouin) method. The analysis of the previous paper is
extended to the case where the magnetic field has both hard and easy
components, so that the Hamiltonian has no obvious symmetry. Herring's formula
is now inapplicable, so the problem is solved by finding the wavefunction and
using connection formulas at every turning point. A general formula for the
energy surface in the vicinity of the diabolo is obtained in this way. This
formula gives the tunneling apmplitude between two wells unrelated by symmetry
in terms of a small number of action integrals, and appears to be generally
valid, even for problems where the recursion contains more than five terms.
Explicit results are obtained for the diabolical points in the model for Fe_8.
These results exactly parallel the experimental observations. It is found that
the leading semiclassical results for the diabolical points appear to be exact,
and the points themselves lie on a perfect centered rectangular lattice in the
magnetic field space. A variety of evidence in favor of this perfect lattice
hypothesis is presented.Comment: Revtex; 4 ps figures; follow up to cond-mat/000311
Derivation of Hebb's rule
On the basis of the general form for the energy needed to adapt the
connection strengths of a network in which learning takes place, a local
learning rule is found for the changes of the weights. This biologically
realizable learning rule turns out to comply with Hebb's neuro-physiological
postulate, but is not of the form of any of the learning rules proposed in the
literature.
It is shown that, if a finite set of the same patterns is presented over and
over again to the network, the weights of the synapses converge to finite
values.
Furthermore, it is proved that the final values found in this biologically
realizable limit are the same as those found via a mathematical approach to the
problem of finding the weights of a partially connected neural network that can
store a collection of patterns. The mathematical solution is obtained via a
modified version of the so-called method of the pseudo-inverse, and has the
inverse of a reduced correlation matrix, rather than the usual correlation
matrix, as its basic ingredient. Thus, a biological network might realize the
final results of the mathematician by the energetically economic rule for the
adaption of the synapses found in this article.Comment: 29 pages, LaTeX, 3 figure
Quantum-Classical Phase Transition of Escape rate in Biaxial Spin Particles
The escape rates of the biaxial single domain spin particles with and without
an applied magnetic field are investigated. Using the strict potential field
description of spin systems developed by Ulyanov and Zaslavskii we obtain new
effective Hamiltonians which are considered to be in exact spin-coordinate
correspondence unlike the well studied effective Hamiltonians with the
approximate correspondence. The sharp first-order transition is found in both
cases. The phase diagram of the transitions depending on the anisotropy
constant and the external field is also given.Comment: 15 pages, 8 figure
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