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Fri, 16 Feb 2001 00:00:00 +0100Fri, 16 Feb 2001 00:00:00 +0100Instanton-Sphaleron transition in the d = 2 Abelian-Higgs model on a Circle
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1176
Abstract: The transition from the instanton-dominated quantum regime to the sphaleron-dominated classical regime is studied in the d = 2 abelian-Higgs model when the spatial coordinate is compactified to S1. Contrary to the noncompactified case, this model allows both sharp first-order and smooth second-order transitions depending on the size of the circle. This finding may make the model a useful toy model for the analysis of baryon number violating processes.D.K. Park; H. J. W. Müller-Kirsten; J.-Q. Liang; A. V. Shurgaiapreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1176Fri, 16 Feb 2001 00:00:00 +0100Macroscopic Quantum Phase Interference in Antiferromagnetic Particles
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1178
Abstact. The tunnel splitting in biaxial antiferromagnetic particles is studied with a magnetic field applied along the hard anisotropy axis. We observe the oscillation of tunnel splitting as a function of the magnetic field due to the quantum phase interference of two tunneling paths of opposite windings. The oscillation is similar to the recent experimental result with Fe8 molecular clusters.Yi-Hang Nie; Yan-Hong Jin; J.-Q. Liang; H.J.W. Müller-Kirsten; D.K. Park; F.-C. Pupreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1178Fri, 16 Feb 2001 00:00:00 +0100Quantum Phase Interference for Quantum Tunneling in Spin Systems
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1179
Abstract: 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 phase 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 energy dependence of the effect and obtain the tunneling splitting at excited states with the help of periodic instantons.J.-Q. Liang; H. J. W. Müller-Kirsten; D. K. Park; F.-C. Pupreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1179Fri, 16 Feb 2001 00:00:00 +0100Winding number transitions at finite temperature in the d = 2 Abelian-Higgs model
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1180
Abstract: Following our earlier investigations we examine the quantum-classical winding number transition in the Abelian-Higgs system. It is demonstrated that the winding number transition in this system is of the smooth second order type in the full range of parameter space. Comparison of the action of classical vortices with that of the sphaleron supports our finding.D.K. Park; H.J.W. Müller-Kirsten; J.-Q. Liangpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1180Fri, 16 Feb 2001 00:00:00 +0100Winding Number Transitions in the Mottola-Wipf Model on a Circle
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1181
Abstract: Winding number transitions from quantum to classical behavior are studied in the case of the 1+1 dimensional Mottola-Wipf model with the space coordinate on a circle for exploring the possibility of obtaining transitions of second order. The model is also studied as a prototype theory which demonstrates the procedure of such investigations. In the model at hand we find that even on a circle the transitions remain those of first order.D.K. Park; H.J.W. Müller-Kirsten; J.-Q. Liangpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1181Fri, 16 Feb 2001 00:00:00 +0100Periodic Bounce for Nucleation Rate at Finite Temperature in Minisuperspace Models
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1182
Abstract: 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.J.-Q. Liang; H. J. W. Müller-Kirsten; Y.-B. Zhang; A.V. Shurgaia; S.-P Kou; D.K. Parkpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1182Fri, 16 Feb 2001 00:00:00 +0100Absorption Cross Section of Scalar Field in Supergravity Background
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1155
Abstract: It has recently been shown that the equation of motion of a massless scalar field in the background of some specific p branes can be reduced to a modified Mathieu equation. In the following the absorption rate of the scalar by a D3 brane in ten dimensions is calculated in terms of modified Mathieu functions of the first kind, using standard Mathieu coefficients. The relation of the latter to Dougall coefficients (used by others) is investigated. The S-matrix obtained in terms of modified Mathieu functions of the first kind is easily evaluated if known rapidly convergent low energy expansions of these in terms of products of Bessel functions are used. Leading order terms, including the interesting logarithmic contributions, can be obtained analytically.R. Manvelyan; H. J. W. Müller-Kirsten; J.-Q. Liang; Yunbo Zhangpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1155Fri, 09 Feb 2001 00:00:00 +0100Macroscopic Quantum Coherence in Small Antiferromagnetic Particle and the Quantum Interference Effects
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1158
Starting from the Hamiltonian operator of the noncompensated two-sublattice model of a small antiferromagnetic particle, we derive the e effective Lagrangian of a biaxial antiferromagnetic particle in an external magnetic field with the help of spin-coherent-state path integrals. Two unequal level-shifts induced by tunneling through two types of barriers are obtained using the instanton method. The energy spectrum is found from Bloch theory regarding the periodic potential as a superlattice. The external magnetic field indeed removes Kramers' degeneracy, however a new quenching of the energy splitting depending on the applied magnetic field is observed for both integer and half-integer spins due to the quantum interference between transitions through two types of barriers.Yi-Hang Nie; Y.-B. Zhang; J.-Q. Liang; H.J.W. Müller-Kirsten; F.-C. Pupreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1158Fri, 09 Feb 2001 00:00:00 +0100Finite Temperature Tunneling and Phase Transitions in SU(2)-Gauge Theory
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1151
A pure Yang-Mills theory extended by addition of a quartic term is considered in order to study the transition from the quantum tunneling regime to that of classical, i.e. thermal, behaviour. The periodic field confiurations are found, which interpolate between the vacuum and sphaleron field configurations. It is shown by explicit calculation that only smooth second order transitions occur for all permissible values of the parameter A introduced with the quartic term. The theory is one of the rare cases which canbe handled analytically.A.V. Shurgaia; H. J. W. Müller-Kirsten; J.-Q. Liang; D.K. Parkpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1151Mon, 22 Jan 2001 00:00:00 +0100Nucleation at Finite Temperature Beyond the Superminispace Model
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1152
Abstract: The transition from the quantum to the classical regime of the nucleation of the closed Robertson-Walker Universe with spacially homogeneous matter fields is investigated with a perturbation expansion around the sphaleron configuration. A criterion is derived for the occurrence of a first-order type transition, and the related phase diagram for scalar and vector fields is obtained. For scalar fields both the first and second order transitions can occur depending on the shape of the potential barrier. For a vector field, here that of an O (3) nonlinear o-model, the transition is seen to be only of the first order. PACS numbers: 11.15.Kc, 03.65Sq, 05.70.Fh, 98.80.CqJ.-Q. Liang; H. J. W. Müller-Kirsten; D.K. Park; A. V. Shurgaiapreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1152Mon, 22 Jan 2001 00:00:00 +0100Quantisation of 2D-gravity with Weyl and area-preserving diffeomorphism invariances
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/887
The constraint structure of the induced 2D-gravity with the Weyl and area-preserving diffeomorphism invariances is analysed in the ADM formulation. It is found that when the area-preserving diffeomorphism constraints are kept, the usual conformal gauge does not exist, whereas there is the possibility to choose the so-called "quasi-light-cone" gauge, in which besides the area-preserving diffeomorphism invariance, the reduced Lagrangian also possesses the SL(2,R) residual symmetry. This observation indicates that the claimed correspondence between the SL(2,R) residual symmetry and the area-preserving diffeomorphism invariance in both regularisation approaches does not hold. The string-like approach is then applied to quantise this model, but a fictitious non-zero central charge in the Virasoro algebra appears. When a set of gauge-independent SL(2,R) current-like fields is introduced instead of the string-like variables, a consistent quantum theory is obtained, which means that the area-preserving diffeomorphism invariance can be maintained at the quantum level.J.-Q. Liang; H. J. W. Müller-Kirsten; Zhenjiu Zhang; J.-G. Zhou; Y.-G. Miaopreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/887Mon, 03 Apr 2000 00:00:00 +0200Periodic instanton and phase transition in quantum tunneling of spin systems
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1068
Y. Zhang; Y. Nie; S. Kou; J.-Q. Liang; H.J.W. Müller-Kirsten; F.-C. Pupreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1068Fri, 10 Mar 2000 00:00:00 +0100Quantum-classical phase transition of escape rate in biaxial spin particles
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1069
Y.-B. Zhang; J.-Q. Liang; H.J.W. Müller-Kirsten; F.-C. Pupreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1069Fri, 10 Mar 2000 00:00:00 +0100The Wess-Zumino term and quantum tunneling
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1071
J.-Q. Liang; L. Maharana; H. J. W. Müller-Kirstenpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1071Fri, 10 Mar 2000 00:00:00 +0100