4,136 research outputs found
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
Feedback Effect on Landau-Zener-Stueckelberg Transitions in Magnetic Systems
We examine the effect of the dynamics of the internal magnetic field on the
staircase magnetization curves observed in large-spin molecular magnets. We
show that the size of the magnetization steps depends sensitively on the
intermolecular interactions, even if these are very small compared to the
intra-molecular couplings.Comment: 4 pages, 3 Postscript figures; paper reorganized, conclusions
modifie
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
Magnetization Process of Nanoscale Iron Cluster
Low-temperature magnetization process of the nanoscale iron cluster in
linearly sweeped fields is investigated by a numerical analysis of
time-dependent Schrdinger equation and the quantum master
equation. We introduce an effective basis method extracting important states,
by which we can obtain the magnetization process effectively. We investigate
the structure of the field derivative of the magnetization. We find out that
the antisymmetric interaction determined from the lattice structure reproduces
well the experimental results of the iron magnets and that this interaction
plays an important role in the iron cluster. Deviations from the adiabatic
process are also studied. In the fast sweeping case, our calculations indicate
that the nonadiabatic transition dominantly occurs at the level crossing for
the lowest field. In slow sweeping case, due to the influence of the thermal
environment to the spin system, the field derivative of the magnetization shows
an asymmetric behavior, the magnetic Fhn effect, which explains
the substructure of the experimental results in the pulsed field.Comment: 5 pages of text and 2 pages of 6 figures. To appear in J. Phys. Soc.
Jp
Nonexponential Relaxation of Magnetization at the Resonant Tunneling Point under a Fluctuating Random Noise
Nonexponential relaxation of magnetization at resonant tunneling points of
nanoscale molecular magnets is interpreted to be an effect of fluctuating
random field around the applied field. We demonstrate such relaxation in
Langevin equation analysis and clarify how the initial relaxation (square-root
time) changes to the exponential decay. The scaling properties of the
relaxation are also discussed.Comment: 4 pages, 4 fgiure
Adiabatic Landau-Zener-St\"uckelberg transition with or without dissipation in low spin molecular system V15
The spin one half molecular system V15 shows no barrier against spin
reversal. This makes possible direct phonon activation between the two levels.
By tuning the field sweeping rate and the thermal coupling between sample and
thermal reservoir we have control over the phonon-bottleneck phenomena
previously reported in this system. We demonstrate adiabatic motion of molecule
spins in time dependent magnetic fields and with different thermal coupling to
the cryostat bath. We also discuss the origin of the zero-field tunneling
splitting for a half-integer spin.Comment: to appear in Phys. Rev. B - Rapid Communication
Growth Dynamics of Photoinduced Domains in Two-Dimensional Charge-Ordered Conductors Depending on Stabilization Mechanisms
Photoinduced melting of horizontal-stripe charge orders in
quasi-two-dimensional organic conductors
\theta-(BEDT-TTF)2RbZn(SCN)4[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene]
and
\alpha-(BEDT-TTF)2I3 is investigated theoretically. By numerically solving
the time-dependent Schr\"odinger equation, we study the photoinduced dynamics
in extended Peierls-Hubbard models on anisotropic triangular lattices within
the
Hartree-Fock approximation. The melting of the charge order needs more energy
for \theta-(BEDT-TTF)2RbZn(SCN)4 than for \alpha-(BEDT-TTF)2I3, which is a
consequence of the larger stabilization energy in \theta-(BEDT-TTF)2RbZn(SCN)4.
After local photoexcitation in the charge ordered states, the growth of a
photoinduced domain shows anisotropy. In \theta-(BEDT-TTF)2RbZn(SCN)4, the
domain hardly expands to the direction perpendicular to the horizontal-stripes.
This is because all the molecules on the hole-rich stripe are rotated in one
direction and those on the hole-poor stripe in the other direction. They
modulate horizontally connected transfer integrals homogeneously, stabilizing
the charge order stripe by stripe. In \alpha-(BEDT-TTF)2I3, lattice distortions
locally stabilize the charge order so that it is easily weakened by local
photoexcitation. The photoinduced domain indeed expands in the plane. These
results are consistent with recent observation by femtosecond reflection
spectroscopy.Comment: 9 pages, 8 figures, to appear in J. Phys. Soc. Jpn. Vol. 79 (2010)
No.
Quantum Phase Transition of Randomly-Diluted Heisenberg Antiferromagnet on a Square Lattice
Ground-state magnetic properties of the diluted Heisenberg antiferromagnet on
a square lattice are investigated by means of the quantum Monte Carlo method
with the continuous-time loop algorithm. It is found that the critical
concentration of magnetic sites is independent of the spin size S, and equal to
the two-dimensional percolation threshold. However, the existence of quantum
fluctuations makes the critical exponents deviate from those of the classical
percolation transition. Furthermore, we found that the transition is not
universal, i.e., the critical exponents significantly depend on S.Comment: RevTeX, 4 pages including 5 EPS figure
Proton Spin Relaxation Induced by Quantum Tunneling in Fe8 Molecular Nanomagnet
The spin-lattice relaxation rate and NMR spectra of H in
single crystal molecular magnets of Fe8 have been measured down to 15 mK. The
relaxation rate shows a strong temperature dependence down to 400
mK. The relaxation is well explained in terms of the thermal transition of the
iron state between the discreet energy levels of the total spin S=10. The
relaxation time becomes temperature independent below 300 mK and is
longer than 100 s. In this temperature region stepwise recovery of the
H-NMR signal after saturation was observed depending on the return field of
the sweep field. This phenomenon is attributed to resonant quantum tunneling at
the fields where levels cross and is discussed in terms of the Landau-Zener
transition.Comment: 13 pages, 5 figure
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