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Quantum Fluctuations (quantum + fluctuation)

Distribution by Scientific Domains

Physics and Astronomy	92%

Selected Abstracts

Four-sublattice ferrimagnetic systems: I. Quantum fluctuations of spins at zero temperature

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2004
Rong-ke Qiu
Abstract Within the framework of the linear spin wave approximation, the quantum fluctuations of spins at zero temperature in four-sublattice ferrimagnetic systems are studied by employing retard Green's functions. The effects of exchange constants on the quantum fluctuations of spins are discussed for three different spin-configurations. The magnetic properties of these spin configurations are related to their magnetically structural symmetry. When the parameters of the exchange couplings are adjusted, the crossover of the spin configurations results in the strong quantum fluctuations, owing to the behaviors of the non-three-dimensional magnetically system. When two of the four exchange-constants in the present four-sublattice bulk systems are set to be zero, the system behaves as a non-three-dimensionally magnetic system, although the structure of the system is still three-dimensional. All the exchange couplings involve in the quantum competition of the systems, but the effects of antiferromagnetic and ferromagnetic exchange couplings are different evidently. The antiferromagnetic exchange couplings play an important role in a balance of the quantum competition. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Magnetic-field-induced polarization and magnetocapacitance in quantum paraelectric EuTiO3

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 13 2004
S. J. Gong
Abstract EuTiO3 is a quantum paraelectric as well as an antiferromagnet. The most attractive property for EuTiO3 is the magnetoelectric coupling effect. In the present work, we investigate the influence of the external fields on EuTiO3, including the electric field E and magnetic field h. The application of the electric field counteracts the quantum fluctuation and forces the system into an electrically ordered state. The applied magnetic field directly induces the fluctuation of the spin-pair correlation and indirectly influences the dielectric behaviors through the magnetoelectric coupling. In addition, we notice that the contribution of the magnetic field depends closely on the temperature condition and the electric field background. The (p, h, E, T) and (,, h, T) diagrams are presented showing the temperature, magnetic and electric field dependence of the polarization and the dielectric capacitance. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Spin-wave spectra and magnetization of ferro,ferrimagnetic double layers

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2008
Wei Jiang
Abstract The spin-wave spectra and magnetization of the ferro,ferrimagnetic double layers are studied by using a linear spin-wave approximation and retarded Green's-function method. We obtain the four branches of the spin-wave spectra. Two energy gaps are found to exist in the energy band. The effects of the interlayer exchange coupling, the intralayer exchange coupling and the spin quantum numbers on the spin-wave spectra and the energy gaps are discussed. The minimum (maximum) value point on the spin-wave spectra and energy gaps correspond to a system that has a high symmetrical magnetic structure and the balance of quantum competitions among the exchange couplings and the spin quantum numbers of the system. There is a crossover between sublattice magnetizations in ferromagnetic layer that is affected by quantum fluctuations, thermal fluctuations and frustration of spins. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Four-sublattice ferrimagnetic systems: I. Quantum fluctuations of spins at zero temperature

Four-sublattice ferrimagnetic systems: II.

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2004
Effects of the spin quantum number
Abstract The effects of the spin quantum number of each sublattice on the quantum fluctuations are discussed for different spin configurations in four-sublattice ferrimagnetic systems. In multi-sublattice ferrimagnets, although the individual sublattice magnetization vectors do not offset each other, but their deviations vectors can cancel out. Namely, the sum of the deviations of magnetization of sites with same initiate spin direction, equals to that of sites with opposite initiate spin direction ,i , = ,j ,, i and j denote respectively the spins along the up and down initiate spin directions). The role of the spin quantum number of each site on magnetic properties of the system is correlative with properties of the exchange couplings surrounding the site. The results show that the proportion of ferromagnetic and antiferromagnetic exchange couplings, the spin quantum number of each sublattice and the magnetically structural symmetry of the system all play important roles on the quantum fluctuations of the systems. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Full counting statistics for electron number in quantum dots

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2008
Yasuhiro Utsumi
Abstract Measurements of the average current and its fluctuations (noise) have been powerful tools to study the quantumtransport in mesoscopic systems. Recently it became possible to measure the probability distribution of current, ,full counting statistics' (FCS), by using quantum point-contact charge-detectors. Motivated by recent experiments, we developed the FCS theory for the joint probability distribution of the current and the electron number inside quantum dots (QDs). We show that a non-Gaussian exponential distribution appears when there is no dot state close to the lead chemical potentials. We show that the measurement of the joint probability distribution of current and electron number would reveal nontrivial correlations, which reflect the asymmetry of tunnel barriers. We also show that for increasing strength of tunneling, the quantum fluctuations of charge qualitatively change the probability distribution of the electron number. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnets

ANNALEN DER PHYSIK, Issue 10-11 2009
B. Vogt
Abstract The physical properties of quantum systems, which are described by the anisotropic Heisenberg model, are influenced by thermal as well as by quantum fluctuations. Such a quantum Heisenberg system can be profoundly changed towards a classical system by tuning two parameters, namely the total spin and the anisotropy field: Large easy-axis anisotropy fields, which drive the system towards the classical Ising model, as well as large spin quantum numbers suppress the quantum fluctuations and lead to a classical limit. We elucidate the incipience of this reduction of quantum fluctuations. In order to illustrate the resulting effects we determine the critical temperatures for ferro- and antiferromagnets and the ground state sublattice magnetization for antiferromagnets. The outcome depends on the dimension, the spin quantum number and the anisotropy field and is studied for a widespread range of these parameters. We compare the results obtained by: Classical Mean Field, Quantum Mean Field, Linear Spin Wave and Random Phase Approximation. Our findings are confirmed and quantitatively improved by numerical Quantum Monte Carlo simulations. The differences between the ferromagnet and antiferromagnet are investigated. We finally find a comprehensive picture of the classical trends and elucidate the suppression of quantum fluctuations in anisotropic spin systems. In particular, we find that the quantum fluctuations are extraordinarily sensitive to the presence of small anisotropy fields. This sensitivity can be quantified by introducing an "anisotropy susceptibility". [source]

Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnets

On the pre-metric foundations of wave mechanics I: massless waves

ANNALEN DER PHYSIK, Issue 4 2009
D.H. Delphenich
Abstract The mechanics of wave motion in a medium are founded in conservation laws for the physical quantities that the waves carry, combined with the constitutive laws of the medium, and define Lorentzian structures only in degenerate cases of the dispersion laws that follow from the field equations. It is suggested that the transition from wave motion to point motion is best factored into an intermediate step of extended matter motion, which then makes the dimension-codimension duality of waves and trajectories a natural consequence of the bicharacteristic (geodesic) foliation associated with the dispersion law. This process is illustrated in the conventional case of quadratic dispersion laws, as well as quartic ones, which include the Heisenberg,Euler dispersion law. It is suggested that the contributions to geodesic motion from the non-quadratic nature of a dispersion law might represent another source of quantum fluctuations about classical extremals, in addition to the diffraction effects that are left out by the geometrical optics approximation. [source]

Semiclassical quantum gravity: statistics of combinatorial Riemannian geometries

ANNALEN DER PHYSIK, Issue 8 2005
L. Bombelli
Abstract This paper is a contribution to the development of a framework, to be used in the context of semiclassical canonical quantum gravity, in which to frame questions about the correspondence between discrete spacetime structures at "quantum scales" and continuum, classical geometries at large scales. Such a correspondence can be meaningfully established when one has a "semiclassical" state in the underlying quantum gravity theory, and the uncertainties in the correspondence arise both from quantum fluctuations in this state and from the kinematical procedure of matching a smooth geometry to a discrete one. We focus on the latter type of uncertainty, and suggest the use of statistical geometry as a way to quantify it. With a cell complex as an example of discrete structure, we discuss how to construct quantities that define a smooth geometry, and how to estimate the associated uncertainties. We also comment briefly on how to combine our results with uncertainties in the underlying quantum state, and on their use when considering phenomenological aspects of quantum gravity. [source]

Entangling Light in its Spatial Degrees of Freedom with Four-Wave Mixing in an Atomic Vapor

CHEMPHYSCHEM, Issue 5 2009
Vincent Boyer Dr.
Abstract Nonlinearities in atomic vapors allow the production of "entangled images",beams of light whose transverse light distributions exhibit localized correlations in their unavoidable quantum fluctuations (see picture). These spatially entangled beams may prove useful to reduce the noise in absorption imaging and beam positioning below the quantum noise level, as well as for quantum information applications. The entanglement properties of two beams of light can reside in subtle correlations that exist in the unavoidable quantum fluctuations of their amplitudes and phases. Recent advances in the generation of nonclassical light with four-wave mixing in an atomic vapor have permitted the production and the observation of entanglement that is localized in almost arbitrary transverse regions of a pair of beams. These multi-spatial-mode entangled beams may prove useful for an array of applications ranging from noise-free imaging and improved position sensing to quantum information processing. [source]