1,069 research outputs found
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
Periodic Bounce for Nucleation Rate at Finite Temperature in Minisuperspace Models
The periodic bounce configurations responsible for quantum tunneling are
obtained explicitly and are extended to the finite energy case for
minisuperspace models of the Universe. As a common feature of the tunneling
models at finite energy considered here we observe that the period of the
bounce increases with energy monotonically. The periodic bounces do not have
bifurcations and make no contribution to the nucleation rate except the one
with zero energy. The sharp first order phase transition from quantum tunneling
to thermal activation is verified with the general criterions.Comment: 17 pages, 5 postscript figures include
Coexistence of Itinerant Electrons and Local Moments in Iron-Based Superconductors
In view of the recent experimental facts in the iron-pnictides, we make a
proposal that the itinerant electrons and local moments are simultaneously
present in such multiband materials. We study a minimal model composed of
coupled itinerant electrons and local moments to illustrate how a consistent
explanation of the experimental measurements can be obtained in the leading
order approximation. In this mean-field approach, the spin-density-wave (SDW)
order and superconducting pairing of the itinerant electrons are not directly
driven by the Fermi surface nesting, but are mainly induced by their coupling
to the local moments. The presence of the local moments as independent degrees
of freedom naturally provides strong pairing strength for superconductivity and
also explains the normal-state linear-temperature magnetic susceptibility above
the SDW transition temperature. We show that this simple model is supported by
various anomalous magnetic properties and isotope effect which are in
quantitative agreement with experiments.Comment: 7 pages, 4 figures; an expanded versio
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
Macroscopic quantum coherence in mesoscopic ferromagnetic systems
In this paper we study the Macroscopic Quantum Oscillation (MQO) effect in
ferromagnetic single domain magnets with a magnetic field applied along the
hard anistropy axis. The level splitting for the ground state, derived with the
conventional instanton method, oscillates with the external field and is
quenched at some field values. A formula for quantum tunneling at excited
levels is also obtained. The existence of topological phase accounts for this
kind of oscillation and the corresponding thermodynamical quantities exhibit
similar interference effects which resembles to some extent the electron
quantum phase interference induced by gauge potential in the Aharonov-Bohm
effect and the -vacuum in Yang-Mills field theory..Comment: 12 pages, 4 figures, to appear in Phys. Rev.
Electronic structure, magnetic and optical properties of intermetallic compounds R2Fe17 (R=Pr,Gd)
In this paper we report comprehensive experimental and theoretical
investigation of magnetic and electronic properties of the intermetallic
compounds Pr2Fe17 and Gd2Fe17. For the first time electronic structure of these
two systems was probed by optical measurements in the spectral range of 0.22-15
micrometers. On top of that charge carriers parameters (plasma frequency and
relaxation frequency) and optical conductivity s(w) were determined.
Self-consistent spin-resolved bandstructure calculations within the
conventional LSDA+U method were performed. Theoretical interpetation of the
experimental s(w) dispersions indicates transitions between 3d and 4p states of
Fe ions to be the biggest ones. Qualitatively the line shape of the theoretical
optical conductivity coincides well with our experimental data. Calculated by
LSDA+U method magnetic moments per formula unit are found to be in good
agreement with observed experimental values of saturation magnetization.Comment: 16 pages, 5 figures, 1 tabl
Spin Tunneling in Magnetic Molecules: Quasisingular Perturbations and Discontinuous SU(2) Instantons
Spin coherent state path integrals with discontinuous semiclassical paths are
investigated with special reference to a realistic model for the magnetic
degrees of freedom in the Fe8 molecular solid. It is shown that such paths are
essential to a proper understanding of the phenomenon of quenched spin
tunneling in these molecules. In the Fe8 problem, such paths are shown to arise
as soon as a fourth order anisotropy term in the energy is turned on, making
this term a singular perturbation from the semiclassical point of view. The
instanton approximation is shown to quantitatively explain the magnetic field
dependence of the tunnel splitting, as well as agree with general rules for the
number of quenching points allowed for a given value of spin. An accurate
approximate formula for the spacing between quenching points is derived
Mean first-passage times of non-Markovian random walkers in confinement
The first-passage time (FPT), defined as the time a random walker takes to
reach a target point in a confining domain, is a key quantity in the theory of
stochastic processes. Its importance comes from its crucial role to quantify
the efficiency of processes as varied as diffusion-limited reactions, target
search processes or spreading of diseases. Most methods to determine the FPT
properties in confined domains have been limited to Markovian (memoryless)
processes. However, as soon as the random walker interacts with its
environment, memory effects can not be neglected. Examples of non Markovian
dynamics include single-file diffusion in narrow channels or the motion of a
tracer particle either attached to a polymeric chain or diffusing in simple or
complex fluids such as nematics \cite{turiv2013effect}, dense soft colloids or
viscoelastic solution. Here, we introduce an analytical approach to calculate,
in the limit of a large confining volume, the mean FPT of a Gaussian
non-Markovian random walker to a target point. The non-Markovian features of
the dynamics are encompassed by determining the statistical properties of the
trajectory of the random walker in the future of the first-passage event, which
are shown to govern the FPT kinetics.This analysis is applicable to a broad
range of stochastic processes, possibly correlated at long-times. Our
theoretical predictions are confirmed by numerical simulations for several
examples of non-Markovian processes including the emblematic case of the
Fractional Brownian Motion in one or higher dimensions. These results show, on
the basis of Gaussian processes, the importance of memory effects in
first-passage statistics of non-Markovian random walkers in confinement.Comment: Submitted version. Supplementary Information can be found on the
Nature website :
http://www.nature.com/nature/journal/v534/n7607/full/nature18272.htm
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
Two-dimensional amine and hydroxy functionalized fused aromatic covalent organic framework
Ordered two-dimensional covalent organic frameworks (COFs) have generally been synthesized using reversible reactions. It has been difficult to synthesize a similar degree of ordered COFs using irreversible reactions. Developing COFs with a fused aromatic ring system via an irreversible reaction is highly desirable but has remained a significant challenge. Here we demonstrate a COF that can be synthesized from organic building blocks via irreversible condensation (aromatization). The as-synthesized robust fused aromatic COF (F-COF) exhibits high crystallinity. Its lattice structure is characterized by scanning tunneling microscopy and X-ray diffraction pattern. Because of its fused aromatic ring system, the F-COF structure possesses high physiochemical stability, due to the absence of hydrolysable weak covalent bonds
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