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1.
The infinite rangeXY spin glass model in a transverse field is investigated by means of the Trotter-Suzuki approach. The exact phase diagram is obtained showing that a spin glass transition takes place for non-zero values of the transverse field up to a critical value c =1.440.01. The present numerically exact calculations are in good agreement with our previous approximate results and they clear remaining discrepancies from previous work.  相似文献   

2.
Magnetic properties of the cubic nanowire, which consists of a ferromagnetic core of spin-1 and a ferromagnetic shell of spin-3/2 with ferrimagnetic interface coupling, are investigated by the use of the effective-field theory with correlations (EFT). Two compensation points have been found for certain values of the system parameters in the nanowire. In particular, the effects of the transverse field and the crystal field on the magnetization and phase transition are described in detail.  相似文献   

3.
The Hamiltonian limit of the ANNNI model in (1+1) dimensions is studied by using the Quantum Statistical Monte Carlo method. Even if recent results suggest that Monte Carlo calculations may prove unreliable in the study of this system, the phase diagram of the quantum version of the model was successfully obtained. In particular, the clusive transitions between the disordered, the floating incommensurate and the degenerate 2, 2 are determined by analysing the correlation length behaviour in finite lattices.Partially supported by CONICET Argentina  相似文献   

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The Ground state phase diagram of the 1D spin-1/2 XXZ model in a transverse magnetic field is studied by the numerical Lanczos method. Also by computing the energy gap and different spin-spin structure factors, quantum phase transitions in whole range of the anisotropy parameter are determined.  相似文献   

6.
We study the phase diagram of the two-dimensional anisotropic next-nearest neighbor Ising (ANNNI) model by comparing the time evolution of two distinct spin configurations submitted to the same thermal noise. We clearly see several dynamical transitions between ferromagnetic, paramagnetic, antiphase, and floating phases. These dynamical transitions seem to occur rather close to the transition lines determined previously in the literature.  相似文献   

7.
We consider a quantum spin-1/2 Ising chain with competing nearest and next-nearest neighbor interactions in a transverse magnetic field, which is known to be equivalent to the classical two-dimensional ANNNI model. Within a perturbation theory for small transverse field (corresponding to low temperatures in the classical ANNNI model) we derive two effective Hamiltonians: the free model describing free fermions on a fictitious lattice that excludes particular heavy excitations of the original system; and the complete model, which incorporates creation and annihilation of these fermions. Whereas the former possesses only three phases (ferromagnetic, floating and anti phase), the latter contains the full physics of the 2d ANNNI model, including a paramagnetic phase between the ferromagnetic and floating phase and a Kosterlitz-Thouless transition. New analytic results are derived for the free model, e.g. the excitation spectrum that turns out to be non-trivial. Our effective Hamiltonians are defined on a restricted Hilbert space so that exact diagonalization calculations can be done for much larger system sizes. Results from extensive Lanczos calculations for system sizes up to L = 32 are presented confirming the original predictions from Villain and Bak.  相似文献   

8.
The self-consistent mean field approximation of the two-flavor NJL model,with a free parameter a to reflect the competition between the "direct" channel and the "exchange" channel,is employed to study the QCD phase structure at finite iso spin chemical potential μ_I,finite bary on chemical potential μ_B and finite temperature T,and especially to study the location of the QCD critical point.Our results show that in order to match the corresponding lattice results of iso spin density and energy density,the contributions of the "exchange" channel need to be considered in the framework of the NJL model,and a weighting factor α=0.5 should be taken.It is also found that for fixed isospin chemical potentials,the lower temperature of the phase transition is obtained with increasing a in the T-μ_I plane,and the largest difference of the phase transition temperature with different a's appears at μ_I~1.5 mπ.At μ_I=0 the temperature of the QCD critical end point(CEP) decreases with increasing a,while the critical baryon chemical potential increases.At high isospin chemical potential(μ_I=500 MeV),the temperature of the QCD tricritical point(TCP) increases with increasing a,and in the low temperature regions the system will transition from the pion superfluidity phase to the normal phase as μ_B increases.At low density,the critical temperature of the QCD phase transition with different a's rapidly increases with μ_I at the beginning,and then increases smoothly around μ_I 300 MeV.In the high baryon density region,the increase of the iso spin chemical potential will raise the critical baryon chemical potential of the phase transition.  相似文献   

9.
The stationary states of the random-field Ising model are determined through the master equation approach, where the contact with the heat bath is simulated by the Glauber stochastic dynamics. The phase diagram of the model is constructed from the stationary values of the magnetization as a function of temperature and field amplitude. The continuous phase transitions coincide with the equilibrium ones, while the first-order transitions occur at fields larger than the corresponding values at equilibrium. The difference between the fields at the limit of stability of the ordered phase and that of the equilibrium is maximum at zero temperature and vanishes at the tricritical point. We also find the mean field time auto-correlation function at the stationary states of the model. Received: 4 June 1997 / Revised: 5 August 1997 / Accepted: 10 November 1997  相似文献   

10.
The mean field phase diagram of the discrete Frenkel-Kontorova model (harmonic chain of particles exposed to a sinusoidal potential) is studied using a fixed irrational value for the ratio of atoms and available sites as a boundary condition. Independent variables are the temperature and the height of the sinusoidal potential. The phase diagram contains two different solid phases, namely an intrinsically pinned incommensurate and an unpinned incommensurate phase. The position of the phase boundary and the nature of the phases are determined. We also investigate the influence of an electric field and the resulting pinning-unpinning transition for fields smaller than the threshold field.  相似文献   

11.
The mean field equations of the simple cubic or tetragonal ANNNI model are studied on finite lattices. Structure combination branching processes are found which allow us to considerably refine previous mean field calculations on the model.  相似文献   

12.
Wetting phenomena at various interface types in the ordered phases of the two-dimensional axial next-nearest neighbour Ising (or ANNNI) model in a field are studied using a transfer matrix approach and Monte Carlo techniques. Different types of wetting at distinct wetting lines or at the bulk phase boundary are identified, which may be tested experimentally in adsorbate systems with a (p×1) ordering, forp equal to two, three or four.  相似文献   

13.
Ghenuche P  Quidant R  Badenes G 《Optics letters》2005,30(14):1882-1884
The dramatic field enhancement at the extremity of finite chains of strongly coupled gold nanoparticles illuminated under total internal reflection is investigated numerically. We demonstrate that high enhancement factors can be achieved by exploiting the in-plane forward scattering of the particles, with geometries achievable by state-of-the-art lithographic techniques.  相似文献   

14.
The behavior of the axial next-nearest-neighbor Ising (ANNNI) model in an external magnetic field is investigated using a low-temperature expansion of the free energy. Unusual cascades of phase transitions and complete devil's staircases, unexpected for the ANNNI model, are found.  相似文献   

15.
正At extremely high temperature T~200 Me V and low chemical potentialμ,the quarks and gluons get released from the nucleon and form the quark-gluon plasma(QGP)state,which is accessible in the heavy ion collisions(HIC)globally[1,2].Theoretically,ab initio lattice QCD simulation found the transition from hadrons to QGP at lowμis continuous crossover instead of phase transition.In this connection,RHIC is running the beam energy scan program to look into the territory with higherμ[3].One important goal is to determine if there is the first order phase transition as expected by many model studies and locate the critical end point(CEP)if it exists.  相似文献   

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17.
The phase diagram of the model of spinless fermions with repulsive nearest neighbour interaction is calculated analytically on a hypercubic lattice in infinite dimensions (d → ∞). In spite of its simplicity the model displays a rich phase diagram depending on the doping δ, the interaction U and the temperature T. The system can be in the homogeneous phase (HOM), the nonsegregated AB charge density wave (AB-CDW), the AB phase separation region (PS-AB/HOM; coexistence of AB-CDW and HOM), the incommensurate phase (IP) or the IP phase separation region (PS-AB/IP; coexistence of AB-CDW and IP). We identify three important values of the interaction UIPL = 0.572 < UIPH = 1.914 < UIP/PS = 4.212 which distinguish four intervals of U. These imply four different types of phase diagrams. In all the three phase diagrams with U below UIP/PS the IP appears. We propose a new general ansatz for the order parameter of this phase. A competition between the IP, the PS-AB/IP and the PS-AB/HOM is found. The relevance of our findings for the phase scenario of the Hubbard model is shown.  相似文献   

18.
The partition function zeros of the anisotropic XY chain in a complex transverse field are studied analytically and numerically. It is found that the partition function zeros of the periodic and quasiperiodic quantum Ising chain lie on the circle at zero temperature and the radius equal to the values of the critical field. For the periodic and quasiperiodic anisotropic XY chains, the closed curves are split to two or three closed curves as the anisotropic parameter gamma decreases at a given ratio of two kinds of exchange interactions. For the isotropic XX case, the partition function zeros lie on the straight segments which are parallel to the real axis and the segments move towards the real axis as the temperature goes to zero.  相似文献   

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20.
郭灿  王志军  王锦程  郭耀麟  唐赛 《物理学报》2013,62(10):108104-108104
研究了直接相关函数峰宽及 k=0处峰对双模晶体相场模型相图的影响. 结果表明, 随着峰宽的增加, 有序相的相区范围不断增大. 而增大峰宽比可使体心立方(bcc)相区范围缩小, 面心立方(fcc)相区范围大幅增加.直接相关函数 k=0峰的引入则使相图被压缩, 两相共存区变窄, 液相稳定区域增加. 关键词: 晶体相场模型 双模近似 相图 直接相关函数  相似文献   

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