18 research outputs found
Ferromagnetic phase transition and Bose-Einstein condensation in spinor Bose gases
Phase transitions in spinor Bose gases with ferromagnetic (FM) couplings are
studied via mean-field theory. We show that an infinitesimal value of the
coupling can induce a FM phase transition at a finite temperature always above
the critical temperature of Bose-Einstein condensation. This contrasts sharply
with the case of Fermi gases, in which the Stoner coupling can not lead
to a FM phase transition unless it is larger than a threshold value . The
FM coupling also increases the critical temperatures of both the ferromagnetic
transition and the Bose-Einstein condensation.Comment: 4 pages, 4 figure
Neel probability and spin correlations in some nonmagnetic and nondegenerate states of hexanuclear antiferromagnetic ring Fe6: Application of algebraic combinatorics to finite Heisenberg spin systems
The spin correlations \omega^z_r, r=1,2,3, and the probability p_N$ of
finding a system in the Neel state for the antiferromagnetic ring Fe(III)6 (the
so-called `small ferric wheel') are calculated. States with magnetization M=0,
total spin 0<=S<=15 and labeled by two (out of four) one-dimensional
irreducible representations (irreps) of the point symmetry group D_6 are taken
into account. This choice follows from importance of these irreps in analyzing
low-lying states in each S-multiplet. Taking into account the Clebsch--Gordan
coefficients for coupling total spins of sublattices (SA=SB=15/2) the global
Neel probability p*_N can be determined. Dependencies of these quantities on
state energy (per bond and in the units of exchange integral J) and the total
spin S are analyzed. Providing we have determined p_N(S) etc. for other
antiferromagnetic rings (Fe10, for instance) we could try to approximate
results for the largest synthesized ferric wheel Fe18. Since thermodynamic
properties of Fe6 have been investigated recently, in the present
considerations they are not discussed, but only used to verify obtained values
of eigenenergies. Numerical results re calculated with high precision using two
main tools: (i) thorough analysis of symmetry properties including methods of
algebraic combinatorics and (ii) multiple precision arithmetic library GMP. The
system considered yields more than 45 thousands basic states (the so-called
Ising configurations), but application of the method proposed reduces this
problem to 20-dimensional eigenproblem for the ground state (S=0). The largest
eigenproblem has to be solved for S=4; its dimension is 60. These two facts
(high precision and small resultant eigenproblems) confirm efficiency and
usefulness of such an approach, so it is briefly discussed here.Comment: 13 pages, 7 figs, 5 tabs, revtex
Magnetic-field-induced quantum criticality in a spin-
The effects of single-ion anisotropy on quantum criticality in a
d-dimensional spin-S planar ferromagnet is explored by
means of the two-time Green’s function method. We work at the Tyablikov decoupling level
for exchange interactions and the Anderson-Callen decoupling level for single-ion
anisotropy. In our analysis a longitudinal external magnetic field is used as the
non-thermal control parameter and the phase diagram and the quantum critical properties
are established for suitable values of the single-ion anisotropy
parameter D. We find that the single-ion anisotropy has sensible
effects on the structure of the phase diagram close to the quantum critical point.
However, for values of the uniaxial crystal-field parameter below a positive threshold,
the conventional magnetic-field-induced quantum critical scenario remains unchanged
Scaling functions for classical to quantum crossover in the transverse Ising model via an effective Wilsonian renormalization group approach in 4-\epsilon dimensions;
The classical to quantum crossover, which occurs in d-dimensional transverse field Ising model-like systems decreasing the temperature to zero in the influence domain of the quantum critical point (QCP), is described by employing an effective Wilsonian renormalization group approach in 4 - ε dimensions. The basic ingredient of the treatment is the static action arising from a preliminary one-loop averaging over non-zero frequency modes, which enter the original quantum one. The crossover scaling functions for susceptibility and related thermodynamic quantities are obtained to first order in ε as explicit functions of the temperature and the applied magnetic field. In our static framework, which can be easily extended to other quantum systems exhibiting a critical line which terminates in a QCP, the suitable procedure for observing this type of crossover through genuine thermodynamic measurements is clarified consistently with available experiments. Remarkably, our basic idea and results may be usefully employed to explore also the dimensional crossover which takes place in classical Ising-like systems with slab or film geometry and, possibly, in other finite-size classical systems