33 research outputs found
Inhomogeneous superconducting phases in the frustrated Kondo-Heisenberg chain
We use bosonization and renormalization group methods to determine the ground
state phase diagram of a one-dimensional frustrated Kondo-Heisenberg system
consisting of a one-dimensional spin-1/2 Luttinger liquid coupled by a Kondo
exchange interaction to a frustrated quantum antiferromagnetic Heisenberg
chain, with a nearest-neighbor exchange coupling and a
next-nearest-neighbor (frustrating) exchange interaction . We analyze the
interplay of quantum frustration in the antiferromagnetic chain with the Kondo
exchange coupling with the Luttinger liquid. We discuss the structure of
the phase diagram of this system as a function of the ratios ,
and of the parameters of the Luttinger liquid. In particular we
discuss in detail the regimes in which a pair-density-wave state may be
realized and its relation with the spin correlations in the frustrated
antiferromagnetic chain.Comment: 16 pages, 1 figure, 39 references; v2 with a new paragraph. Published
versio
Relevance of nonadiabatic effects in TiOCl
We analyze the effect of the phonon dynamics on a recently proposed model for
the uniform-incommensurate transition seen in TiOX compounds. The study is
based on a recently developed formalism for nonadiabatic spin-Peierls systems
based on bosonization and a mean field RPA approximation for the interchain
coupling. To reproduce the measured low temperature spin gap, a spin-phonon
coupling quite bigger than the one predicted from an adiabatic approach is
required. This high value is compatible with the renormalization of the phonons
in the high temperature phase seen in inelastic x-ray experiments. Our theory
accounts for the temperature of the incommensurate transition and the value of
the incommensurate wave vector at the transition point.Comment: 4 pages, 2 figure
Generation of chiral solitons in antiferromagnetic chains by a quantum quench
We analyze the time evolution of a magnetic excitation in a spin-1/2
antiferromagnetic Heisenberg chain after a quantum quench. By a proper
modulation of the magnetic exchange coupling, we prepare a static soliton of
total spin 1/2 as an initial spin state. Using bosonization and a numerical
time dependent density matrix renormalization group algorithm, we show that the
initial excitation evolves to a state composed of two counter-propagating
chiral states, which interfere to yield = 1/4 for each mode. We find that
these dynamically generated states remain considerably stable as time evolution
is carried out. We propose spin-Peierls materials and ultracold-atom systems as
suitable experimental scenarios in which to conduct and observe this mechanism.Comment: Published version. Title changed due to reinterpretation of results.
5 pages, 4 figure
Magnetic end-states in a strongly-interacting one-dimensional topological Kondo insulator
Topological Kondo insulators are strongly correlated materials, where
itinerant electrons hybridize with localized spins giving rise to a
topologically non-trivial band structure. Here we use non-perturbative
bosonization and renormalization group techniques to study theoretically a
one-dimensional topological Kondo insulator. It is described as a
Kondo-Heisenberg model where the Heisenberg spin-1/2 chain is coupled to a
Hubbard chain through a Kondo exchange interaction in the p-wave channel - a
strongly correlated version of the prototypical Tamm-Shockley model. We derive
and solve renormalization group equations at two-loop order in the Kondo
parameter, and find that, at half-filling, the charge degrees of freedom in the
Hubbard chain acquire a Mott gap, even in the case of a non-interacting
conduction band (Hubbard parameter ). Furthermore, at low enough
temperatures, the system maps onto a spin-1/2 ladder with local ferromagnetic
interactions along the rungs, effectively locking the spin degrees of freedom
into a spin- chain with frozen charge degrees of freedom. This structure
behaves as a spin-1 Haldane chain, a prototypical interacting topological spin
model, and features two magnetic spin- end states for chains with open
boundary conditions. Our analysis allows to derive an insightful connection
between topological Kondo insulators in one spatial dimension and the
well-known physics of the Haldane chain, showing that the ground state of the
former is qualitatively different from the predictions of the naive mean-field
theory.Comment: 13 pages, 2 figures, 1 appendix. New version with typos correcte
Haldane phase in one-dimensional topological Kondo insulators
We investigate the groundstate properties of a recently proposed model for a
topological Kondo insulator in one dimension (i.e., the -wave
Kondo-Heisenberg lattice model) by means of the Density Matrix Renormalization
Group method. The non-standard Kondo interaction in this model is different
from the usual (i.e., local) Kondo interaction in that the localized spins
couple to the "-wave" spin density of conduction electrons, inducing a
topologically non-trivial insulating groundstate. Based on the analysis of the
charge- and spin-excitation gaps, the string order parameter, and the spin
profile in the groundstate, we show that, at half-filling and low energies, the
system is in the Haldane phase and hosts topologically protected spin-1/2
end-states. Beyond its intrinsic interest as a useful "toy-model" to understand
the effects of strong correlations on topological insulators, we show that the
-wave Kondo-Heisenberg model can be implemented in band optical lattices
loaded with ultra-cold Fermi gases.Comment: 8 pages, 4 figures, 1 appendi
Bending mode fluctuations and structural stability of graphene nanoribbons
We analyze the thermal fluctuations of a narrow graphene nanoribbon. Using a continuum membranelike model in the harmonic approximation, we study the height-height correlation functions and the destabilization modes corresponding to two different boundary conditions: fixed and free edges. For the first case, the thermal spectrum has a gap and the correlations along the ribbon decay exponentially. Thermal fluctuations produce only local perturbations of the flat situation. However, the long range crystalline order is not distorted. For free edges the situation changes as thermal excitations are gapless. The low energy spectrum decouples into a bulk and an edge excitation. The bulk excitation tends to destabilize the crystalline order producing a homogeneous rippling. Furthermore, we can relate the edge mode to a precluding perturbation leading to scrolled edges, as seen in suspended graphene samples. We also analyze the implications of our results in the thermal conductivity of graphene nanoribbons.Fil: Scuracchio, Pablo Martín. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); ArgentinaFil: Dobry, Ariel Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentin
From spinons to magnons in explicit and spontaneously dimerized antiferromagnetic chains
We reconsider the excitation spectra of a dimerized and frustrated
antiferromagnetic Heisenberg chain. This model is taken as the simpler example
of compiting spontaneous and explicit dimerization relevant for Spin-Peierls
compounds. The bosonized theory is a two frequency Sine-Gordon field theory. We
analize the excitation spectrum by semiclassical methods. The elementary
triplet excitation corresponds to an extended magnon whose radius diverge for
vanishing dimerization. The internal oscilations of the magnon give rise to a
series of excited state until another magnon is emited and a two magnon
continuum is reached. We discuss, for weak dimerization, in which way the
magnon forms as a result of a spinon-spinon interaction potential.Comment: 5 pages, latex, 3 figures embedded in the tex
Microscopic model for magnetoelectric coupling through lattice distortions
We propose a microscopic magnetoelectric model in which the coupling between spins and electric dipoles is mediated by lattice distortions. The magnetic sector is described by a spin S=1/2 Heisenberg model coupled directly to the lattice via a standard spin-Peierls term and indirectly to the electric dipole variables via the distortion of the surrounding electronic clouds. Electric dipoles are described by Ising variables for simplicity. We show that the effective magnetoelectric coupling which arises due to the interconnecting lattice deformations is quite efficient in one-dimensional arrays. More precisely, we show using bosonization and extensive density matrix renormalization group numerical simulations that increasing the magnetic field above the spin-Peierls gap, a massive polarization switch-off occurs due to the proliferation of soliton pairs. We also analyze the effect of an external electric field when the magnetic system is in a gapped (plateau) phase and show that the magnetization can be electrically switched between clearly distinct values. More general quasi-one-dimensional models and two-dimensional systems are also discussed.Fil: Cabra, Daniel Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física de Líquidos y Sistemas Biológicos. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física de Líquidos y Sistemas Biológicos; ArgentinaFil: Dobry, Ariel Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; ArgentinaFil: Gazza, Claudio Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; ArgentinaFil: Rossini, Gerardo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin
Microscopic theory for the incommensurate transition in TiOCl
We propose a microscopic mechanism for the incommensurate phase in TiOX
compounds. The model includes the antiferromagnetic chains of Ti ions immersed
in the phonon bath of the bilayer structure. Making use of the Cross-Fisher
theory, we show that the geometrically frustrated character of the lattice is
responsible for the structural instability which leads the chains to an
incommensurate phase without an applied magnetic field. In the case of TiOCl,
we show that our model is consistent with the measured phonon frequencies at
and the value of the incommensuration vector at the transition
temperature. Moreover, we find that the dynamical structure factor shows a
progressive softening of an incommensurate phonon near the zone boundary as the
temperature decreases. This softening is accompanied by a broadening of the
peak which gets asymmetrical as well when going towards the transition
temperature. These features are in agreement with the experimental inelastic
X-ray measurements.Comment: 6 pages, 5 figures. Published versio
On the Path Integral Representation for Spin Systems
We propose a classical constrained Hamiltonian theory for the spin. After the
Dirac treatment we show that due to the existence of second class constraints
the Dirac brackets of the proposed theory represent the commutation relations
for the spin. We show that the corresponding partition function, obtained via
the Fadeev-Senjanovic procedure, coincides with the one obtained using coherent
states. We also evaluate this partition function for the case of a single spin
in a magnetic field.Comment: To be published in J.Phys. A: Math. and Gen. Latex file, 12 page