共查询到20条相似文献,搜索用时 218 毫秒
1.
The large-n expansion is applied to the calculation of thermal critical exponents describing the critical behavior of spatially anisotropic d-dimensional systems at m -axial Lifshitz points. We derive the leading non-trivial 1/n correction for the perpendicular correlation-length exponent νL2 and hence several related thermal exponents to order O(1/n). The results are consistent with known large-n expansions for d -dimensional critical points and isotropic Lifshitz points, as well as with the second-order epsilon expansion about the upper critical dimension d?=4+m/2 for generic m∈[0,d]. Analytical results are given for the special case d=4, m=1. For uniaxial Lifshitz points in three dimensions, 1/n coefficients are calculated numerically. The estimates of critical exponents at d=3, m=1 and n=3 are discussed. 相似文献
3.
The grand partition functions Z(T,B) of the Ising model on L×L triangular lattices with fully periodic boundary conditions, as a function of temperature T and magnetic field B , are evaluated exactly for L<12 (using microcanonical transfer matrix) and approximately for L?12 (using Wang–Landau Monte Carlo algorithm). From Z(T,B), the distributions of the partition function zeros of the triangular-lattice Ising model in the complex temperature plane for real B≠0 are obtained and discussed for the first time. The critical points aN(x) and the thermal scaling exponents yt(x) of the triangular-lattice Ising antiferromagnet, for various values of x=e−2βB, are estimated using the partition function zeros. 相似文献
4.
We consider a free quantum scalar field satisfying modified dispersion relations in curved spacetimes, within the framework of Einstein–Aether theory. Using a power counting analysis, we study the divergences in the adiabatic expansion of 〈?2〉 and 〈Tμν〉, working in the weak field approximation. We show that for dispersion relations containing up to 2s powers of the spatial momentum, the subtraction necessary to renormalize these two quantities on general backgrounds depends on s in a qualitatively different way: while 〈?2〉 becomes convergent for a sufficiently large value of s , the number of divergent terms in the adiabatic expansion of 〈Tμν〉 increases with s. This property was not apparent in previous results for spatially homogeneous backgrounds. 相似文献
5.
We investigate a new method to probe the helicity of the photon emitted in the b→sγ transition. The method relies on the observation of interference effects between two resonance contributions, B→K∗(Kγ)γ and B→ηc(γγ)K or B→χc0(γγ)K to the same final state Kγγ . Decays of the type B→Kres(Kγ)γ dominate the B→Kγγ yield throughout most of the phase space, and may be accessible at current B meson facilities already. 相似文献
6.
To complement existing knowledge of the density matrix γF(x,y) of independent fermions for N particles in one dimension under harmonic confinement, the corresponding matrix γIB(x,y) for impenetrable bosons is given for N=2 and 3 (with the N=4 form available also). For fermions the momentum density is then obtained and illustrated numerically for N=10. The boson momentum density is studied analytically at high momentum p , the coefficients of the p−4 and p−6 terms being tabulated for N=2–5 inclusive. Their dependence on powers of N is exhibited numerically. Finally, the functional relationship between γIB(x,y) and γF(x,y) is formally set out and illustrated. 相似文献
7.
Beisert et al. have identified an integrable SU(2,2) quantum spin chain which gives the one-loop anomalous dimensions of certain operators in large Nc QCD. We derive a set of nonlinear integral equations (NLIEs) for this model, and compute the scattering matrix of the various (in particular, magnon) excitations. 相似文献
8.
We argue that relatively compact charmonium states, J/ψ, ψ(2S), χc, can very likely be bound inside light hadronic matter, in particular inside higher resonances made from light quarks and/or gluons. The charmonium state in such binding essentially retains its properties, so that the bound system decays into light mesons and the particular charmonium resonance. Thus such bound states of a new type, which we call hadro-charmonium, may explain the properties of some of the recently observed resonant peaks, in particular of Y(4.26), Y(4.32–4.36), Y(4.66), and Z(4.43). We discuss further possible implications of the suggested picture for the observed states and existence of other states of hadro-charmonium and hadro-bottomonium. 相似文献
9.
Amovilli and March (2006) [8] used diffusion quantum Monte Carlo techniques to calculate the non-relativistic ionization potential I(Z) in He-like atomic ions for the range of (fractional) nuclear charges Z lying between the known critical value Zc=0.911 at which I(Z) tends to zero and Z=2. They showed that it is possible to fit I(Z) to a simple quadratic expression. Following that idea, we present here a semiempirical fine-tuning of Hartree–Fock ionization potentials for the isoelectronic series of He, Be, Ne, Mg and Ar-like atomic ions that leads to excellent estimations of Zc for these series. The empirical information involved is experimental ionization and electron affinity data. It is clearly demonstrated that Hartree–Fock theory provides an excellent starting point for determining I(Z) for these series. 相似文献
10.
12.
It is shown that four-dimensional N=1 supersymmetric QCD with massive flavors in the fundamental representation of the gauge group can be realized in the hidden sector of E8×E8 heterotic string vacua. The number of flavors can be chosen to lie in the range of validity of the free-magnetic dual, using which one can demonstrate the existence of long-lived meta-stable non-supersymmetric vacua. This is shown explicitly for the gauge group Spin(10), but the methods are applicable to Spin(Nc), SU(Nc) and Sp(Nc) for a wide range of color index Nc. Hidden sectors of this type can potentially be used as a mechanism to break supersymmetry within the context of heterotic M-theory. 相似文献
13.
Motivated by experiments in nanoscopic systems, we study a generalized Anderson, which consist of two spin degenerate doublets hybridized to a singlet by the promotion of an electron to two conduction bands, as a function of the energy separation δ between both doublets. For δ=0 or very large, the model is equivalent to a one-level SU(N) Anderson model, with N=4 and 2 respectively. We study the evolution of the spectral density for both doublets (ρ1σ(ω) and ρ2σ(ω)) and their width in the Kondo limit as δ is varied, using the non-crossing approximation (NCA). As δ increases, the peak at the Fermi energy in the spectral density (Kondo peak) splits and the density of the doublet of higher energy ρ2σ(ω) shifts above the Ferrmi energy. The Kondo temperature TK (determined by the half-width at half maximum of the Kondo peak in density of the doublet of lower energy ρ1σ(ω)) decreases dramatically. The variation of TK with δ is reproduced by a simple variational calculation. 相似文献
14.
Motivated by the necessity of discrete ZN symmetries in the MSSM to insure baryon stability, we study the origin of discrete gauge symmetries from open string sector U(1)?s in orientifolds based on rational conformal field theory. By means of an explicit construction, we find an integral basis for the couplings of axions and U(1) factors for all simple current MIPFs and orientifolds of all 168 Gepner models, a total of 32 990 distinct cases. We discuss how the presence of discrete symmetries surviving as a subgroup of broken U(1)?s can be derived using this basis. We apply this procedure to models with MSSM chiral spectrum, concretely to all known U(3)×U(2)×U(1)×U(1) and U(3)×Sp(2)×U(1)×U(1) configurations with chiral bi-fundamentals, but no chiral tensors, as well as some SU(5) GUT models. We find examples of models with Z2 (R-parity) and Z3 symmetries that forbid certain B and/or L violating MSSM couplings. Their presence is however relatively rare, at the level of a few percent of all cases. 相似文献
15.
The physical properties of arbitrary half-integer spins F=N−1/2 fermionic cold atoms loaded into a one-dimensional optical lattice are investigated by means of a conformal field theory approach. We show that for attractive interactions two different superfluid phases emerge for F?3/2: A BCS pairing phase, and a molecular superfluid phase which is formed from bound-states made of 2N fermions. In the low-energy approach, the competition between these instabilities and charge-density waves is described in terms of ZN parafermionic degrees of freedom. The quantum phase transition for F=3/2,5/2 is universal and shown to belong to the Ising and three-state Potts universality classes respectively. In contrast, for F?7/2, the transition is non-universal. For a filling of one atom per site, a Mott transition occurs and the nature of the possible Mott-insulating phases are determined. 相似文献
16.
The particle in a symmetrical squared tangent potential well is studied by examining its Shannon information entropy and standard deviations. The position and momentum information entropy densities ρs(x), ρs(p) and probability densities ρ(x), ρ(p) are illustrated with different potential range L and potential depth U . We present analytical position information entropies Sx for the lowest two states. We observe that the sum of position and momentum entropies Sx and Sp expressed by Bialynicki-Birula–Mycielski (BBM) inequality is satisfied. Some eigenstates exhibit entropy squeezing in the position. The entropy squeezing in position will be compensated by an increase in momentum entropy. We also note that the Sx increases with the potential range L, while decreases with the potential depth U . The variation of Sp is contrary to that of Sx. 相似文献
17.
Minos A. Neto J. Roberto Viana J. Ricardo de Sousa 《Journal of magnetism and magnetic materials》2012
In this work we study the critical behavior of the quantum spin-1/2 anisotropic Heisenberg antiferromagnet in the presence of a longitudinal field on a body centered cubic (bcc) lattice as a function of temperature, anisotropy parameter (Δ) and magnetic field (H ), where Δ=0 and 1 correspond the isotropic Heisenberg and Ising models, respectively. We use the framework of the differential operator technique in the effective-field theory with finite cluster of N =4 spins (EFT-4). The staggered ms=(mA−mB)/2 and total m=(mA+mB)/2 magnetizations are numerically calculated, where in the limit of ms→0 the critical line TN(H,Δ) is obtained. The phase diagram in the T−H plane is discussed as a function of the parameter Δ for all values of H∈[0,Hc(Δ)], where Hc(Δ) correspond the critical field (TN=0). Special focus is given in the low temperature region, where a reentrant behavior is observed around of H=Hc(Δ)≥Hc(Δ=1)=8J in the Ising limit, results in accordance with Monte Carlo simulation, and also was observed for all values of Δ∈[0,1]. This reentrant behavior increases with increase of the anisotropy parameter Δ. In the limit of low field, our results for the Heisenberg limit are compared with series expansion values. 相似文献
18.
G. Narsinga Rao Saibal Roy Chung-Yuan Mou J.W. Chen 《Journal of magnetism and magnetic materials》2006
We have investigated the magnetotransport and magnetic properties on polycrystalline samples of Sr2−xLaxFeMoO6 (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0). The magnitude of intergrain tunneling magnetoresistance with low magnetic field of 0.88 T for x=0.2 and 0.4 samples are as large as 5% and 7% at room temperature and 13% and 10% at 10 K, respectively. The increase of coercivity (Hc), ratio of remanent magnetization with respect to saturation magnetization (Mr/Ms), high saturation fields, and reduction of the saturation magnetization indicate that random disorder of spin orientation is mainly responsible for enhancement of the low-field magnetoresistance for samples with x?0.4. Whereas rapid drop of Hc, Mr/Ms, Mr, and saturation fields for samples with x>0.4 signifies the growth of antiphase boundary, which gives rise to lower values of low-field MR. 相似文献
19.
20.
Current experimental data indicate that two unitarity triangles of the CKM quark mixing matrix V are almost the right triangles with α≈90°. We highlight a very suggestive parametrization of V and show that its CP-violating phase ? is nearly equal to α (i.e., ?−α≈1.1°). Both ? and α are stable against the renormalizaton-group evolution from the electroweak scale MZ to a superhigh energy scale MX or vice versa, and thus it is impossible to obtain α=90° at MZ from ?=90° at MX. We conjecture that there might also exist a maximal CP-violating phase φ≈90° in the MNS lepton mixing matrix U. The approximate quark–lepton complementarity relations, which hold in the standard parametrizations of V and U, can also hold in our particular parametrizations of V and U simply due to the smallness of |Vub| and |Ve3|. 相似文献