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J. M. Roman

Publications and source records attributed to J. M. Roman.

13 recordsLinked to original sources
arXiv2004

Charge and Spin Quantum Fluids Generated by Many-Electron Interactions

In this paper we describe the electrons of the 1D Hubbard model by a fluid of unpaired rotated electrons and a fluid of zero-spin rotated-electron pairs. The rotated electrons are related to the original electrons by a mere unitary transformation. For all finite values of energy and for the whole parameter space of the model this two-fluid picture leads to a description of the energy eigenstates in terms of occupancy configurations of $η$-spin 1/2 holons, spin 1/2 spinons, and $c$ pseudoparticles only. The electronic degrees of freedom couple to external charge (and spin) probes through the holons and $c$ pseudoparticles (and spinons). Our results refer to very large values of the number of lattice sites $N_a$. The holon (and spinon) charge (and spin transport is made by $2ν$-holon (and $2ν$-spinon) composite pseudoparticles such that $ν=1,2,...$.

cond-mat.str-el↗
arXiv2003

Log-periodic behavior of finite size effects in field theories with RG limit cycles

We compute the finite size effects in the ground state energy, equivalently the effective central charge c_{eff}, based on S-matrix theories recently conjectured to describe a cyclic regime of the Kosterlitz-Thouless renormalization group flows. The effective central charge has periodic properties consistent with renormalization group predictions. Whereas c_{eff} for the massive case has a singularity in the very deep ultra-violet, we argue that the massless version is non-singular and periodic on all length scales.

hep-th↗
arXiv2003

Russian Doll Renormalization Group, Kosterlitz-Thouless Flows, and the Cyclic sine-Gordon model

We investigate the previously proposed cyclic regime of the Kosterlitz-Thouless renormalization group (RG) flows. The period of one cycle is computed in terms of the RG invariant. Using bosonization, we show that the theory has $U_q (\hat{sl(2)})$ quantum affine symmetry, with $q$ {\it real}. Based on this symmetry, we study two possible S-matrices for the theory, differing only by overall scalar factors. We argue that one S-matrix corresponds to a continuum limit of the XXZ spin chain in the anti-ferromagnetic domain $Δ< -1$. The latter S-matrix has a periodicity in energy consistent with the cyclicity of the RG. We conjecture that this S-matrix describes the cyclic regime of the Kosterlitz-Thouless flows. The other S-matrix we investigate is an analytic continuation of the usual sine-Gordon one. It has an infinite number of resonances with masses that have a Russian doll scaling behavior that is also consistent with the period of the RG cycles computed from the beta-function. Closure of the bootstrap for this S-matrix leads to an infinite number of particles of higher spin with a mass formula suggestive of a string theory.

hep-th↗
arXiv2003

The elementary excitations of the BCS model in the canonical ensemble

We summarize previous works on the exact ground state and the elementary excitations of the exactly solvable BCS model in the canonical ensemble. The BCS model is solved by Richardson equations, and, in the large coupling limit, by Gaudin equations. The relationship between this two kinds of solutions are used to classiffy the excitations.

cond-mat.supr-con↗
arXiv2003

What are the elementary excitations of the BCS model in the canonical ensemble?

We have found the elementary excitations of the exactly solvable BCS model for a fixed number of particles. These turn out to have a peculiar dispersion relation, some of them with no counterpart in the Bogoliubov picture, and unusual counting properties related to an old conjecture made by Gaudin. We give an algorithm to count the number of excitations for each excited state and a graphical interpretation in terms of paths and Young diagrams. For large systems the excitations are described by an effective Gaudin model, which accounts for the finite size corrections to BCS.

cond-mat.supr-con↗
arXiv2002

Large N limit of the exactly solvable BCS model: analytics versus numerics

We have studied the numerical solutions of Richardson equations of the BCS model in the limit of large number of energy levels at half-filling, and compare them with the analytic results derived by Gaudin and Richardson, which in turn leads to the standard BCS solution. We focus on the location and density of the roots, the eigenvalues of the conserved quantities, and the scaling properties of the total energy for the equally spaced and the two-level models.

cond-mat.supr-con↗
arXiv2000

Spin Waves in Canted Phases: An Application to Doped Manganites

We present the effective lagrangian for low energy and momentum spin waves in canted phases at next to leading order in the derivative expansion. The symmetry breaking pattern SU(2) --> 1 of the internal spin group and that of the crystallographic space group imply that there is one ferromagnetic and one antiferromagnetic spin wave. The interaction of the spin waves with the charge carriers is also discussed for canted, ferromagnetic and antiferromagnetic phases. All this together allows us to write the doping dependence of the dispersion relation parameters for doped manganites. We point out that the spin waves posses distinctive characteristics which may allow us to experimentally differentiate canted phases from phase separation regions in doped manganites.

cond-mat.str-el↗
arXiv1999

Spin wave mediated non-reciprocal effects in antiferromagnets

By using an effective field theory for the electromagnetic interaction of spin waves, we show that, in certain antiferromagnets, the latter induce non-reciprocal effects in the microwave region, which should be observable in the second harmonic generation and produce gyrotropic birefringency. We calculate the various (non-linear) susceptibilities in terms of a few parameters the order of magnitude of which is under control.

cond-mat↗
arXiv1999

Continuum Double Exchange Model

We present a continuum model for doped manganites which consist of two species of quantum spin 1/2 fermions interacting with classical spin fields. The phase structure at zero temperature turns out to be considerably rich: antiferromagnetic insulator, antiferromagnetic two band conducting, canted two band conducting, canted one band conducting and ferromagnetic one band conducting phases are identified, all of them being stable against phase separation. There are also regions in the phase diagram where phase separation occurs.

cond-mat.str-el↗
arXiv1999

Effective Field Theory Approach to Ferromagnets and Antiferromagnets in Crystaline Solids

We present a systematic construction of effective lagrangians for the low energy and momentum region of ferromagnetic and antiferromagnetic spin waves in crystalline solids. We fully exploit the spontaneous symmetry breaking pattern $SU(2)\to U(1)$, the fact that spin waves are its associated Goldstone modes, the crystallographic space group and time reversal symmetries. We show how to include explicit SU(2) breaking terms due to spin-orbit and magnetic dipole interactions. The coupling to electromagnetic fields is also discussed in detail. For definiteness we work with the space group $R\bar 3 c$ and present our results to next to leading order.

cond-mat↗
arXiv1998

The Matrix Product Approach to Quantum Spin Ladders

We present a manifestly rotational invariant formulation of the matrix product method valid for spin chains and ladders. We apply it to 2 legged spin ladders with spins 1/2, 1 and 3/2 and different magnetic structures labelled by the exchange coupling constants, which can be ferromagnetic or antiferromagnetic along the legs and the rungs of the ladder We compute ground state energy densities, correlation lengths and string order parameters. We present numerical evidence of the duality properties of the 3 different non ferromagnetic spin 1/2 ladders. We show that the long range topological order characteristic of isolated spin 1 chains is broken by the interchain coupling. The string order correlation function decays exponentially with a finite correlation length that we compute. A physical picture of the spin 1 ladder is given in terms of a collection of resonating spin 1 chains. Finally for ladders with spin equal or greater than 3/2 we define a class of AKLT states whose matrix product coefficients are given by 9-j symbols.

cond-mat.str-el↗
arXiv1995

The regulated four parameter one dimensional point interaction

The general four parameter point interaction in one dimensional quantum mechanics is regulated. It allows the exact solution, but not the perturbative one. We conjecture that this is due to the interaction not being asymptotically free. We then propose a different breakup of unperturbed theory and interaction, which now is asymptotically free but leads to the same physics. The corresponding regulated potential can be solved both exactly and perturbatively, in agreement with the conjecture.

hep-th↗