arXiv subjects

A. Dobry

Publications and source records attributed to A. Dobry.

At least 19 recordsLinked to original sources

arXiv2014

Tuning the polarized quantum phonon transmission in graphene nanoribbons

We propose systems that allow a tuning of the phonon transmission function T($ω$) in graphene nanoribbons by using C$^{13}$ isotope barriers, antidot structures, and distinct boundary conditions. Phonon modes are obtained by an interatomic fifth-nearest neighbor force-constant model (5NNFCM) and T($ω$) is calculated using the non-equilibrium Green's function formalism. We show that by imposing partial fixed boundary conditions it is possible to restrict contributions of the in-plane phonon modes to T($ω$) at low energy. On the contrary, the transmission functions of out-of-plane phonon modes can be diminished by proper antidot or isotope arrangements. In particular, we show that a periodic array of them leads to sharp dips in the transmission function at certain frequencies $ω_ν$ which can be pre-defined as desired by controlling their relative distance and size. With this, we demonstrated that by adequate engineering it is possible to govern the magnitude of the ballistic transmission functions T$(ω)$ in graphene nanoribbons. We discuss the implications of these results in the design of controlled thermal transport at the nanoscale as well as in the enhancement of thermo-electric features of graphene-based materials.

cond-mat.mes-hall↗

arXiv2014

The role of atomic vacancies and boundary conditions on ballistic thermal transport in graphene nanoribbons

Quantum thermal transport in armchair and zig-zag graphene nanoribbons are investigated in the presence of single atomic vacancies and subject to different boundary conditions. We start with a full comparison of the phonon polarizations and energy dispersions as given by a fifth-nearest-neighbor force-constant model (5NNFCM) and by elasticity theory of continuum membranes (ETCM). For free-edges ribbons we discuss the behavior of an additional acoustic edge-localized flexural mode, known as fourth acoustic branch (4ZA), which has a small gap when it is obtained by the 5NNFCM. Then, we show that ribbons with supported-edges have a sample-size dependent energy gap in the phonon spectrum which is particularly large for in-plane modes. Irrespective to the calculation method and the boundary condition, the dependence of the energy gap for the low-energy optical phonon modes against the ribbon width W is found to be proportional to 1/W for in-plane, and 1/W$^2$ for out-of-plane phonon modes. Using the 5NNFCM, the ballistic thermal conductance and its contributions from every single phonon mode are then obtained by the non equilibrium Green's function technique. We found that, while edge and central localized single atomic vacancies do not affect the low-energy transmission function of in-plane phonon modes, they reduce considerably the contributions of the flexural modes. On the other hand, in-plane modes contributions are strongly dependent on the boundary conditions and at low temperatures can be highly reduced in supported-edges samples. These findings could open a route to engineer graphene based devices where it is possible to discriminate the relative contribution of polarized phonons and to tune the thermal transport on the nanoscale.

cond-mat.mes-hall↗

arXiv2012

Bending mode fluctuations and structural stability of graphene nanoribbons

We analyze the thermal fluctuations of a narrow graphene nanoribbon. Using a continuum, membrane-like model we study the height-height correlation functions and the destabilization modes corresponding to two different boundaries conditions: ribbons which are fixed or free on the edges. For the first situation, 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 an homogeneous rippling. Furthermore, we associate the edge mode with a precluding perturbation leading to scrolled edges, as seen in suspended graphene samples.

cond-mat.mes-hall↗

arXiv2011

From the 2D graphene honeycomb lattice to 1D nanoribbons: dimensional crossover signals in the structural thermal fluctuations

We study the dimensional crossover from 2D to 1D type behavior, which takes place in the thermal excited rippling of a graphene honeycomb lattice, when one of the dimensions of the layer is reduced. Through a joint study, by Monte Carlo (MC) atomistic simulations using a quasi-harmonic potential and analytical calculations, we find that the normal-normal correlation function does not change its power law behavior in the long wavelength limit. However the system size dependency of the square of out of plane displacement $ $ changes its scaling behavior when going from a layer to a nanoribbon. We show that a new scaling law appears which corresponds to a truly 1D behavior and we estimate the ratio of the sample dimensions where the crossover takes place as $R_{2D \leftrightarrow 1D}\approx 1.609$. Having explored a wide number of realistic systems sizes, we conclude that narrow ribbons present stronger corrugations than the square graphene sheets and we discuss the implications for the electronic properties of freestanding graphene systems.

cond-mat.mes-hall↗

arXiv2011

Incommensurate spin Luttinger liquid phase in a model for the spin-Peierls materials TiOBr and TiOCl

In the present work we aim to characterize the lattice configurations and the magnetic behavior in the incommensurate phase of spin-Peierls systems. This phase emerges when the magnetic exchange interaction is coupled to the distortions of an underlying triangular lattice and has its experimental realization in the quasi-one dimensional compound family TiOX (X = Cl, Br). With a simple model of spin-1/2 chains inserted in a planar triangular geometry which couples them elastically, we are able to obtain the uniform-incommensurate and incommensurate-dimerized phase transitions seen in these compounds. Moreover, we follow the evolution of the wave-vector of the distortions with temperature inside the incommensurate phase. Finally, we predict gapless spin excitations for the intermediate phase of TiOX compounds along with incommensurate spin-spin correlations. This exotic Luttinger liquid-like behavior could be observed in future experiments.

cond-mat.str-el↗

arXiv2010

Spectral gap induced by structural corrugation in armchair graphene nanoribbons

We study the effects of the structural corrugation or rippling on the electronic properties of undoped armchair graphene nanoribbons (AGNR). First, reanalyzing the single corrugated graphene layer we find that the two inequivalent Dirac points (DP), move away one from the other. Otherwise, the Fermi velocity decrease by increasing rippling. Regarding the AGNRs, whose metallic behavior depends on their width, we analyze in particular the case of the zero gap band-structure AGNRs. By solving the Dirac equation with the adequate boundary condition we show that due to the shifting of the DP a gap opens in the spectra. This gap scale with the square of the rate between the high and the wavelength of the deformation. We confirm this prediction by exact numerical solution of the finite width rippled AGNR. Moreover, we find that the quantum conductance, calculated by the non equilibrium Green's function technique vanish when the gap open. The main conclusion of our results is that a conductance gap should appear for all undoped corrugated AGNR independent of their width.

cond-mat.mes-hall↗

arXiv2008

Dimerization process and elementary excitations in spin-Peierls chains coupled by frustrated interactions

We consider the ground state and the elementary excitations of an array of spin-Peierls chains coupled by elastic and magnetic interactions. It is expected that the effect of the magnetic interchain coupling will be to reduce the dimerization amplitude and that of the elastic coupling will be to confine the spin one-half solitons corresponding to each isolated chain. We show that this is the case when these interactions are not frustrated. On the other hand, in the frustrated case we show that the amplitude of dimerization in the ground state is independent of the strength of the interchain magnetic interaction in a broad range of values of this parameter. We also show that free solitons could be the elementary excitations when only nearest neighbor interactions are considered. The case of an elastic interchain coupling is analyzed on a general energetic consideration. To study the effect of the magnetic interchain interaction the problem is simplified to a two-leg ladder which is solved using DMRG calculations. We show that the deconfinement mechanism is effective even with a significantly strong antiferromagnetic interchain coupling.

cond-mat.str-el↗

arXiv2005

Magnetization plateaux induced by a coupling to the lattice

We investigate a simple model of a frustrated spin-1/2 Heisenberg chain coupled to adiabatic phonons under an external magnetic field. Using field theoretic methods complemented by extensive Density Matrix Renormalisation Group techniques generalized to include self-consistent lattice distortions, we show that magnetization plateaux at non-trivial rational values of the magnetization can be stabilized by the lattice coupling. We suggest that such a scenario could be relevant for some low dimensional frustrated spin-Peierls compounds.

cond-mat.str-el↗

arXiv2005

Pressure dependence of the melting mechanism at the limit of overheating in Lennard-Jones crystals

We study the pressure dependence of the melting mechanism of a surface free Lennard-Jones crystal by constant pressure Monte Carlo simulation. The difference between the overheating temperature($T_{OH}$) and the thermodynamical melting point($T_M$) increase for increasing pressure. When particles move into the repulsive part of the potential the properties at $T_{OH}$ change. There is a crossover pressure where the volume jump becomes pressure-independent. The overheating limit is pre-announced by thermal excitation of big clusters of defects. The temperature zone where the system is dominated by these big clusters of defects increases with increasing pressure. Beyond the crossover pressure we find that excitation of defects and clusters of them start at the same temperature scale related with $T_{OH}$.

cond-mat.mtrl-sci↗

arXiv2003

Dislocation lines as the precursor of the melting of crystalline solids observed in Monte Carlo simulations

The microscopic mechanism of the melting of a crystal is analyzed by the constant pressure Monte Carlo simulation of a Lennard-Jones fcc system. Beyond a temperature of the order of 0.8 of the melting temperature, we found that the relevant excitations are lines of defects. Each of these lines has the structure of a random walk of various lengths on an fcc defect lattice. We identify these lines with the dislocation ones proposed in recent phenomenological theories of melting. Near melting we find the appearance of long lines that cross the whole system. We suggest that these long lines are the precursor of the melting process.

cond-mat.mtrl-sci↗

arXiv2001

Manifestation of marginal Fermi liquid and phonon excitations in photoemision experiments of cuprate superconductors

Recent ARPES experiments in cuprates superconductors show a kink in the electron dispersion near the Fermi energy. This kink coexists with a linear frequency dependence of the imaginary part of the electron self-energy. In this paper we show that both features could be accounted for if an electron-phonon interaction is included in a model where the electrons are described by a marginal Fermi liquid theory. Phonons provide the energy scale seen in the experiments but the quasiparticle weight at the Fermi level is zero. At high binding energy, in agreement with the experiment, the electron dispersion does not go to the one-electron band. We analyze the compatibility between the electron scattering rate extracted from ARPES experiment and the one extracted from transport properties. We conclude that the electron-phonon interaction relevant for transport properties is strongly screened respect to the one extracted from ARPES. This is in agreement with recent studies in the context of 1/N expansion on t-J model.

cond-mat.str-el↗

arXiv2001

Non-perturbative effective field theory for two-leg antiferromagnetic spin ladders

We study the long wavelength limit of a spin 1/2 Heisenberg antiferromagnetic two-leg ladder, treating the interchain coupling in a non-perturbative way. We perform a mean field analysis and then include exactly the fluctuations. This allows for a discussion of the phase diagram of the system and provides an effective field theory for the low energy excitations. The coset fermionic Lagrangian obtained corresponds to a perturbed SU(4)_1/U(1) Conformal Field Theory (CFT). This effective theory is naturally embedded in a SU(2)_2 x Z_2 CFT, where perturbations are easily identified in terms of conformal operators in the two sectors. Crossed and zig-zag ladders are also discussed using the same approach.

cond-mat.str-el↗

arXiv1999

Antiferromagnetism in doped anisotropic two-dimensional spin-Peierls systems

We study the formation of antiferromagnetic correlations induced by impurity doping in anisotropic two-dimensional spin-Peierls systems. Using a mean-field approximation to deal with the inter-chain magnetic coupling, the intra-chain correlations are treated exactly by numerical techniques. The magnetic coupling between impurities is computed for both adiabatic and dynamical lattices and is shown to have an alternating sign as a function of the impurity-impurity distance, hence suppressing magnetic frustration. An effective model based on our numerical results supports the coexistence of antiferromagnetism and dimerization in this system.

cond-mat.str-el↗

arXiv1997

Numerical Study of the Incommensurate Phase in Spin-Peierls Systems

We analyze several properties of the lattice solitons in the incommensurate phase of spin-Peierls systems using exact diagonalization and quantum Monte Carlo. These systems are modelled by an antiferromagnetic Heisenberg chain with nearest and next-nearest neighbor interactions coupled to the lattice in the adiabatic approximation. Several relations among features of the solitons and magnetic properties of the system have been determined and compared with analytical predictions. We have studied in particular the relation between the soliton width and the spin-Peierls gap. Although this relation has the form predicted by bosonized field theories, we have found some important quantitative differences which could be relevant to describe experimental studies of spin-Peierls materials.

cond-mat↗

arXiv1997

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.

cond-mat↗

arXiv1996

Stability of correlated electronic systems under the influence of the electron-phonon interaction

We have used an exact diagonalization technique to study the stability of the $t-J$-Holstein and Hubbard-Holstein models under the influence of the electron-phonon interaction. Previous results have been obtained using frozen-phonon technique or introducing only a few dynamical phonon modes due to the large Hilbert space. To check these results we have done exact diagonalization in a small cluster (four sites) including all the phonon modes allowed by symmetry. We compare our results with those obtained by using the adiabatic approximation.

cond-mat↗

arXiv1995

Exact diagonalization study of the two-dimensional t-J-Holstein model

We study by exact diagonalization the two-dimensional t-J-Holstein model near quarter filling by retaining only few phonon modes in momentum space. This truncation allows us to incorporate the full dynamics of the retained phonon modes. The behavior of the kinetic energy, the charge structure factor and other physical quantities, show the presence of a transition from a delocalized phase to a localized phase at a finite value of the electron-phonon coupling. We have also given some indications that the e-ph coupling leads in general to a suppression of the pairing susceptibility at quarter filling.

cond-mat↗

arXiv1994

Polaron Formation in the Three-Band Peierls-Hubbard Model for Cuprate Superconductors

Exact diagonalization calculations show a continuous transition from delocalized to small polaron behavior as a function of intersite electron-lattice coupling. A transition, found previously at Hartree-Fock level [Yonemitsu et al., Phys. Rev. Lett. {\bf 69}, 965 (1992)], between a magnetic and a non magnetic state does not subsist when fluctuations are included. Local phonon modes become softer close to the polaron and by comparison with optical measurements of doped cuprates we conclude that they are close to the transition region between polaronic and non-polaronic behavior. The barrier to adiabatically move a hole vanishes in that region suggesting large mobilities.

cond-mat↗