arXiv subjects

A. Langari

Publications and source records attributed to A. Langari.

71 records · Page 4Linked to original sources

arXiv2005

The effect of Aharanov-Bohm phase on the magnetic-field dependence of two-pulse echos in glasses at low temperatures

The anomalous response of glasses in the echo amplitude experiment is explained in the presence of a magnetic field. We have considered the low energy excitations in terms of an effective two level system. The effective model is constructed on the flip-flop configuration of two interacting two level systems. The magnetic field affects the tunneling amplitude through the Aharanov-Bohm effect. The effective model has a lower scale of energy in addition to the new distribution of tunneling parameters which depend on the interaction. We are able to explain some features of echo amplitude versus a magnetic field, namely, the dephasing effect at low magnetic fields, dependence on the strength of the electric field, pulse separation effect and the influence of temperature. However this model fails to explain the isotope effects which essentially can be explained by the nuclear quadrupole moment. We will finally discuss the features of our results.

cond-mat.dis-nn↗

arXiv2005

Ferromagnetism in a hard-core boson model

The problem of ferromagnetism -- associated with a ground state with maximal total spin -- is discussed in the framework of a hard-core model, which forbids the occupancy at each site with more than one particle. It is shown that the emergence of ferromagnetism on finite square lattices crucially depends on the statistics of the particles. Fermions (electrons) lead to the well-known instabilities for finite hole densities, whereas for bosons (with spin) ferromagnetism appears to be stable for all hole densities.

cond-mat.str-el↗

arXiv2004

Formation of clusters in the ground state of the $t-J$ model on a two leg ladder

We investigate the ground state properties of the $t-J$ model on a two leg ladder with anisotropic couplings ($t,α=J/t$) along rungs and ($t',α'=J'/t'$) along legs. We have implemented a cluster approach based on 4-site plaqettes. In the strong asymmetric cases $α/α'\ll 1$ and $α'/α\ll 1$ the ground state energy is well described by plaquette clusters with charges $Q=2,4$. The interaction between the clusters favours the condensation of plaquettes with maximal charge -- a signal for phase separation. The dominance of Q=2 plaquettes explains the emergence of tightly bound hole pairs. We have presented the numerical results of exact diagonalization to support our cluster approach.

cond-mat.str-el↗

arXiv2004

Cumulant expansion for ferrimagnetic spin (S_1, s_2) systems

We have generalized the application of cumulant expansion to ferrimagnetic systems of large spins. We have derived the effective Hamiltonian in terms of classical variables for a quantum ferrimagnet of large spins. A noninteracting gas of ferrimagnetic molecules is studied systematically by cumulant expansion to second order of ($Js/T$) where $J$ is the exchange coupling in each molecule, $s$ is the smaller spin ($S_1, s_2$) and $T$ is temperature. We have observed fairly good results in the convergent regime of the expansion, i.e $T > Js$. We then extend our approach to a system of interacting ferrimagnetic molecules. For one dimensional nearest neighbor interaction we have observed that the correlation of more than two neighboring sites is negligible at moderate and high temperature behavior. Thus the results of a single molecule can be applied to the chain of interacting molecules for temperatures greater than classical energy scale, i.e $T>JS_1s_2$. Finally we will discuss the effect of spin inhomogeneity on the accuracy of this method.

cond-mat.str-el↗

arXiv2004

Quantum renormalization group of XYZ model in a transverse magnetic field

We have studied the zero temperature phase diagram of XYZ model in the presence of transverse magnetic field. We show that small anisotropy (0 =< Delta <1) is not relevant to change the universality class. The phase diagram consists of two antiferromagnetic ordering and a paramagnetic phases. We have obtained the critical exponents, fixed points and running of coupling constants by implementing the standard quantum renormalization group. The continuous phase transition from antiferromagnetic (spin-flop) phase to a paramagnetic one is in the universality class of Ising model in transverse field. Numerical exact diagonalization has been done to justify our results. We have also addressed on the application of our findings to the recent experiments on Cs_2CoCl_4.

cond-mat.str-el↗

arXiv2003

Charge density plateaux and insulating phases in the $t-J$ model with ladder geometry

We discuss the occurrence and the stability of charge density plateaux in ladder-like $t-J$ systems (at zero magnetization M=0) for the cases of 2- and 3-leg ladders. Starting from isolated rungs at zero leg coupling, we study the behaviour of plateaux-related phase transitions by means of first order perturbation theory and compare our results with Lanczos diagonalizations for $t-J$ ladders ($N=2\times 8$) with increasing leg couplings. Furthermore we discuss the regimes of rung and leg couplings that should be favoured for the appearance of the charge density plateaux.

cond-mat↗

arXiv2002

The anisotropic Heisenberg model in a transverse magnetic field

One dimensional spin-1/2 $XXZ$ model in a transverse magnetic field is studied. It is shown that the field induces the gap in the spectrum of the model with easy-plain anisotropy. Using conformal invariance the field dependence of the gap at small fields is found. The ground state phase diagram is obtained. It contains four phases with different types of the long range order (LRO) and a disordered one. These phases are separated by critical lines, where the gap and the long range order vanish. Using scaling estimations and a mean-field approach as well as numerical calculations in the vicinity of all critical lines we found the critical exponents of the gap and the LRO. It is shown that transition line between the ordered and disordered phases belongs to the universality class of the transverse Ising model.

cond-mat.str-el↗

arXiv2001

Coherent motion in the interaction model of cold glasses

We have studied the collective phenomena of multicomponent glasses at ultra low temperatures [Strehlow, et. al, Phys. Rev. Lett 80, 5361 (1998)] by taking into account the proper interaction between tunneling centers. We have considered both double and triple well potentials with different types of interactions. We show that a phase with coherent motion appears for a range of parameters when the path of tunneling is coursed by an interaction of the XY type, while the usual Ising like interaction does not lead to the expected collective phenomena. In the phase of coherent motion, the dipole moment and the low-energy levels oscillate with a frequency proportional to the number of tunneling centers in the system. Simultaneous level crossing occurs between the ground and first excited states. The effects of long-range interactions and also of random couplings have been also studied for a one- and two-dimensional array of tunneling centers. We find that long-range interactions do not affect the coherent motion, while a wide distribution of random couplings destroys the collective effects.

cond-mat.dis-nn↗

arXiv2001

Exact diagonalisation study of charge order in the quarter-filled two-leg ladder system NaV2O5

The charge ordering transition in the layer compound NaV2O5 is studied by means of exact diagonalization methods for finite systems. The 2-leg ladders of the V-Trellis lattice are associated with one spin variable of the vanadium 3d-electron in the rung and a pseudospin variable that describes its positional degree of freedom. The charge ordering (CO) due to intersite Coulomb interactions is described by an effective Ising-like Hamiltonian for the pseudo-spins that are coupled to the spin fluctuations along the ladder. We employ a Lanczos algortihm on 2D lattice to compute charge (pseudo-spin) and spin-correlation functions and the energies of the low lying excited states. A CO-phase diagram is constructed and the effect of intra-ladder exchange on the CO transition is studied. It is shown that a phase with no-longe range order (no-LRO) exists between the in-line and zig-zag ordered structures. We provide a finite-size scaling analysis for the spin excitation gap and also discuss the type of excitations. In addition we studied the effect of bond-alternation of spin exchange and derived a scaling form for the spin gap in terms of the dimerization parameter.

cond-mat.str-el↗

arXiv2001

Superfluid-spiral state of quantum ferrimagnets in magnetic field

We study the phase diagram of one-dimensional quantum ferrimagnets by using a numerical exact diagonalization of a finite size system along with a field-theoretical non-linear $σ$ model of the quantum ferrimagnets at zero temperature and its effective description in the presence of the external magnetic field in terms of the quantum XY-model. The low- and the high-field phases correspond respectively to the classical Néel and the fully polarized ferromagnetic states where in the intermediate magnetic field ($h_{c1} < h < h_{c2}$), it is an XXZ+h model with easy plane anisotropy, which possess the spiral (superfluid) states that carry the dissipationless spin-supercurrent. We derive the critical exponents, and then will study the stability of the XY spiral state against these spin-supercurrents and the hard axis fluctuations. We will show a first order phase transition from the easy plane spiral state to a saturated ferromagnetic state occurs at $h=h_{c2}$ if the spin-supercurrent reaches to its critical value.

cond-mat↗

arXiv2000

Alternating-Spin Ladders in a Magnetic Field: New Magnetization Plateaux

We study numerically the formation of magnetization plateaux with the Lanczos method in 2-leg ladders with mixed spins of magnitudes $(S_1,S_2)=(1,1/2)$ located at alternating positions along the ladder and with dimerization $γ$. For interchain coupling $J'>0$ and $γ=0$, we find normalized plateaux at $m=1/3$ starting at zero field and $m=1$ (saturation), while when $γ\ne 0$ is columnar, another extra plateau at $m=2/3$ shows up. For $J'<0$, when $γ<γ_c(J')$ we find no plateau while for $γ>γ_c(J')$ we find four plateaux at $m=0,1/3,2/3,1$. We also apply several approximate analytical methods (Spin Wave Theory, Low-Energy Effective Hamiltonians and Bosonization) to understand these findings and to conjeture the behaviour of ferrimagnetic ladders with a bigger number of legs.

cond-mat.str-el↗

arXiv2000

Low Energy Properties of Ferrimagnetic 2-leg Ladders: a Lanczos study

We apply the Lanczos method to a 2-leg ladder with mixed spins of magnitudes $(S_1,S_2)=(1,1/2)$ located at alternating positions along the ladder. The effect of dimerization $γ$ is also considered according to two different patterns. A Spin Wave Theory (SWT) is applied to this model predicting one gapless branch with ferromagnetic properties and another gapful branch with antiferromagnetic nature as low energy excitations of the model. We compute the ground state energies, Ferro- and AF-excitation gaps, magnetizations and correlation functions as a function of $J'$ and $γ$ which results into a fine estimate of the phase diagram. The Lanczos results are compared with the SWT analysis and a qualitative agreement is found but with numerical discrepancies. We also study numerically the Spin-Peierls instability and find that it is conditional for any value of $ J' \in (0, 2)$ and both dimerization patterns.

cond-mat.str-el↗

arXiv1999

Phase diagram of ferrimagnetic ladders with bond-alternation

We study the phase diagram of a 2-leg bond-alternation spin-(1/2, 1) ladder for two different configurations using a quantum renormalization group approach. Although d-dimensional ferrimagnets show gapless behavior, we will explicitly show that the effect of the spin mixing and the bond-alternation can open the possibility for observing an energy gap. We show that the gapless phases of such systems can be equivalent to the 1-dimensional half-integer antiferroamgnets, besides the gapless ferrimagnetic phases. We therefore propose a phase transition between these two gapless phases that can be seen in the parameter space.

cond-mat↗

arXiv1999

Quantum Ferrimagnets

We study quantum ferrimagnets in one, two, and three dimensions by using a variety of methods and approximations. These include: (i) a treatment based on the spin coherent state path-integral formulation of quantum ferrimagnets by taking into account the leading order quantum and thermal fluctuations (ii) a field-theoretical (non-linear $σ$-model type) formulation of the special case of one-dimensional quantum ferrimagnets at zero temperature (iii) an effective description in terms of dimers and quantum rotors, and (iv) a quantum renormalization group study of ferrimagnetic Heisenberg chains. Some of the formalism discussed here can be used for a unified treatment of both ferromagnets and antiferromagnets in the semiclassical limit. We show that the low (high) energy effective Hamiltonian of a (S_1, S_2) Heisenberg ferrimagnet is a ferromagnetic (antiferromagnetic) Heisenberg model. We also study the phase diagram of quantum ferrimagnets in the presence of an external magnetic field h ($h_{c1} < h < h_{c2}$) and show that the low- and the high-field phases correspond respectively to the classical Néel and the fully polarized ferromagnetic states. We also calculate the transition temperature for the Berezinskii-Kosterlitz-Thouless phase transition in the special case of two-dimensional quantum ferrimagnets.

cond-mat↗

arXiv1998

Phase diagram of the anti-ferromagnetic xxz model in the presence of an external magnetic field

The anisotropic s=1/2 anti-ferromagnetic Heisenberg chain in the presence of an external magnetic field is studied by using the standard quantum renormalization group. We obtain the critical line of the transition from partially magnetized (PM) phase to the saturated ferromagnetic (SFM) phase. The crossover exponent between the PM phase and anti-ferromagnetic Ising (AFI) phase is evaluated. Our results show that the anisotropy($\d$) term is relevant and causes crossover. These results indicate that the standard RG approach yields fairly good values for the critical points and their exponents. The magnetization curve, correlation functions and the ground state energy per site are obtained and compared with the known exact results.

cond-mat.str-el↗

arXiv1998

Quantum Renormalization Group for 1 Dimensional Fermion Systems

Inspired by the superblock method of White, we introduce a simple modification of the standard Renormalization Group (RG) technique for the study of quantum lattice systems. Our method which takes into account the effect of Boundary Conditions(BC), may be regarded as a simple way for obtaining first estimates of many properties of quantum lattice systems. By applying this method to the 1-dimensional free and interacting fermion system, we obtain the ground state energy with much higher accuracy than the standard RG. We also calculate the density-density correlation function in the free-fermion case which shows good agreement with the exact result.

cond-mat.stat-mech↗

arXiv1998

A Modified Quantum Renormalization Group for xxz Spin Chain

A simple modification of the standard Renormalization Group (RG) technique for the study of quantum spin systems is introduced. Our method which takes into account the effect of boundary conditions by employing the concept of superblock, may be regarded as a simple way for obtaining first estimates of many properties of spin systems. By applying this method to the xxz spin 1/2 Heisenberg chain, we obtain the ground state energy with much higher accuracy than the standard RG. We have also obtained the staggered magnetization and the z-component of spin-spin correlation function which confirms the absence of long-range order in the massless region of the 1D xxz model.

cond-mat.stat-mech↗