548 research outputs found
Critical behaviour of a spin-tube model in a magnetic field
We show that the low-energy physics of the spin-tube model in presence of a
critical magnetic field can be described by a broken SU(3) spin chain. Using
the Lieb-Schultz-Mattis Theorem we characterize the possible magnetization
plateaus and study the critical behavior in the region of transition between
the plateaus m=1/2 and m=3/2 by means of renormalization group calculations
performed on the bosonized effective continuum field theory. We show that in
certain regions of the parameter space of the effective theory the system
remains gapless, and we compute the spin-spin correlation functions in these
regions. We also discuss the possibility of a plateau at m=1, and show that
although there exists in the continuum theory a term that might cause the
appearance of a plateau there, such term is unlikely to be relevant. This
conjecture is proved by DMRG techniques. The modifications of the three-leg
ladder Hamiltonian that might show plateaus at m =1,5/6,7/6 are discussed, and
we give the expected form of correlation functions on the m=1 plateau.Comment: RevTeX, 43 pages, 5 EPS figure
Gauge theory for mixed -spin glasses
Physical quantities in the mixed -spin glasses are evaluated with
Nishimori's gauge theory and several variance inequalities. The -symmetry breaking and the replica-symmetry breaking are studied in finite
and infinite dimensions. Obtained bounds on the expectation of the square of
the magnetization and spontaneous magnetization enable us to clarify properties
of paramagnetic and spin glass phases. It is proven that variances of
ferromagnetic and spin glass order parameters vanish on the Nishimori line in
the infinite volume limit. These results imply the self-averaging of these
order parameters on the Nishimori line. The self-averaging of the spin glass
order parameter rigorously justifies already argued absence of replica-symmetry
breaking on the Nishimori line.Comment: 13 page
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