4,697 research outputs found
The Metallacyclopentane-Olefin Interchange Reaction
Tris(triphenylphosphine)tetramethylenenickel(II) and biscyclopentadienyltetramethylenetitanium, prepared from the reaction of a 1,4-dilithiobutane and the transition metal dihalides react with olefins to produce substituted metallacyclopentanes; the stereochemistry and substitution pattern of the metallocycles formed from propene, octa-1,7-diene, and norbornadiene has been determined
Deceptive Apparent Nonadiabatic Magnetization Process
We discuss the effect of the thermal environment on the low-temperature
response of the magnetization of uniaxial magnets to a time-dependent applied
magnetic field. At sufficiently low temperatures the staircase magnetization
curves observed in molecular magnets such as Mn_{12} and Fe_8 display little
temperature dependence. However the changes of the magnetization at each step
do not seem to be directly related to the probability for a quantum mechanical
nonadiabatic transition. In order to explain this deceptive apparent
nonadiabatic behavior, we study the quantum dynamics of the system in a thermal
environment and propose a relation between the observed magnetization steps and
the quantum mechanical transition probability due to the nonadiabatic
transition.Comment: 4 pages, 7 eps figure
Quantum Annealing in the Transverse Ising Model
We introduce quantum fluctuations into the simulated annealing process of
optimization problems, aiming at faster convergence to the optimal state.
Quantum fluctuations cause transitions between states and thus play the same
role as thermal fluctuations in the conventional approach. The idea is tested
by the transverse Ising model, in which the transverse field is a function of
time similar to the temperature in the conventional method. The goal is to find
the ground state of the diagonal part of the Hamiltonian with high accuracy as
quickly as possible. We have solved the time-dependent Schr\"odinger equation
numerically for small size systems with various exchange interactions.
Comparison with the results of the corresponding classical (thermal) method
reveals that the quantum annealing leads to the ground state with much larger
probability in almost all cases if we use the same annealing schedule.Comment: 15 pages, RevTeX, 8 figure
Quantum Spin Dynamics and Quantum Computation
We describe a simulation method for a quantum spin model of a generic,
general purpose quantum computer. The use of this quantum computer simulator is
illustrated through several implementations of Grover's database search
algorithm. Some preliminary results on the stability of quantum algorithms are
presented.Comment: 6 pages, 4 figures ; Minor errors corrected and figures update
Half-magnetization plateau stabilized by structural distortion in the antiferromagnetic Heisenberg model on a pyrochlore lattice
Magnetization plateaus, visible as anomalies in magnetic susceptibility at
low temperatures, are one of the hallmarks of frustrated magnetism. We show how
an extremely robust half-magnetization plateau can arise from coupling between
spin and lattice degrees of freedom in a pyrochlore antiferromagnet, and
develop a detailed symmetry of analysis of the simplest possible scenario for
such a plateau state. The application of this theory to the spinel oxides
CdCr2O4 and HgCr2O4, where a robust half magnetization plateau has been
observed, is discussed.Comment: 4 pages, 4 figure
Quantum Fluctuation-Induced Phase Transition in S=1/2 XY-like Heisenberg Antiferromagnets on the Triangular Lattice
The selection of the ground state among nearly degenerate states due to
quantum fluctuations is studied for the S=1/2 XY-like Heisenberg
antiferromagnets on the triangular lattice in the magnetic field applied along
the hard axis, which was first pointed out by Nikuni and Shiba. We find that
the selected ground state sensitively depends on the degree of the anisotropy
and the magnitude of the magnetic field. This dependence is similar to that in
the corresponding classical model at finite temperatures where various types of
field induced phases appear due to the entropy effect. It is also found that
the similarity of the selected states in the classical and quantum models are
not the case in a two-leg ladder lattice, although the lattice consists of
triangles locally and the ground state of this lattice in the classical case is
the same as that of the triangular lattice.Comment: 15 pages, 35 figure
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