Umesh K. Yadav

An extended Falicov-Kimball model on a triangular lattice

The combined effect of frustration and correlation in electrons is a matter of considerable interest lately. In this context a Falicov-Kimball model on a triangular lattice with two localized states, relevant for certain correlated systems, is considered. Making use of the local symmetries of the model, our numerical study reveals a number of orbital ordered ground states, tuned by the small changes in parameters while quantum fluctuations between the localized and extended states produce homogeneous mixed valence. The inversion symmetry of the Hamiltonian is broken by most of these ordered states leading to orbitally driven ferroelectricity. We demonstrate that there is no spontaneous symmetry breaking when the ground state is inhomogeneous. The study could be relevant for frustrated systems like GdI2, NaTiO2 (in its low-temperature C2/m phase) where two Mott localized states couple to a conduction band.

A ground state phase diagram of a spinless, extended Falicov–Kimball model on the triangular lattice

Correlated systems with hexagonal layered structures have come to the fore with renewed interest in cobaltates, transition metal dichalcogenides and GdI(2). While superconductivity, unusual metal and possible exotic states (prevented from long-range order by strong local fluctuations) appear to come from frustration and correlation working in tandem in such systems, they freeze at a lower temperature to crystalline states. The underlying effective Hamiltonian in some of these systems is believed to be the Falicov-Kimball model and therefore, a thorough study of the ground state of this model and its extended version on a non-bipartite lattice is important. Using a Monte Carlo search algorithm, we identify a large number of different possible ground states with charge order as well as valence and metal-insulator transitions. Such competing states, close in energy, give rise to complex charge order and other broken symmetry structures as well as the phase segregations observed in the ground state of these systems.

Phase transitions in a spinless, extended Falicov–Kimball model on the triangular lattice

Abstract A numerical diagonalization technique with canonical Monte-Carlo simulation algorithm is used to study the phase transitions from low temperature (ordered) phase to high temperature (disordered) phase of spinless Falicov–Kimball model on a triangular lattice with correlated hopping ( t ′ ). It is observed that the low temperature ordered phases (i.e. regular, bounded and segregated) persist up to a finite critical temperature ( T c ). In addition, we observe that the critical temperature decreases with increasing the correlated hopping in regular and bounded phases whereas it increases in the segregated phase. Single and multi peak patterns seen in the temperature dependence of specific heat ( C v ) and charge susceptibility ( χ ) for different values of parameters like on-site Coulomb correlation strength ( U ), correlated hopping ( t ′ ) and filling of localized electrons ( n f ) are also discussed.

Thermodynamic studies of the two dimensional Falicov-Kimball model on a triangular lattice

Abstract Thermodynamic properties of the spinless Falicov-Kimball model are studied on a triangular lattice using numerical diagonalization technique with Monte-Carlo simulation algorithm. Discontinuous metal-insulator transition is observed at finite temperature. Unlike the case of square lattice, here we observe that the finite temperature effect is not able to smear out the discontinuous metal-insulator transition seen in the ground state. Calculation of specific heat (Cv) shows single and double peak structures for different values of parameters like on-site correlation strength (U), f-electron energy (Ef) and temperature.

Study of ground state phases for spin-1/2 Falicov-Kimball model on a triangular lattice

The spin-dependent Falicov–Kimball model (FKM) is studied on a triangular lattice using numerical diagonalization technique and Monte-Carlo simulation algorithm. Magnetic properties have been explored for different values of parameters: on-site Coulomb correlation U, exchange interaction J and filling of electrons. We have found that the ground state configurations exhibit long range Neel order, ferromagnetism or a mixture of both as J is varied. The magnetic moments of itinerant (d) and localized (f) electrons are also studied. For the one-fourth filling case we found no magnetic moment from d- and f-electrons for U less than a critical value.

The role of intra- and inter-site exchange correlations in the extended Falicov–Kimball model on a triangular lattice

Abstract Ground state magnetic properties of the spin-dependent Falicov–Kimball model (FKM) are studied by incorporating the intrasite exchange correlation J (between itinerant d - and localized f -electrons) and intersite (superexchange) correlation J se (between localized f -electrons) on a triangular lattice for two different fillings. Numerical diagonalization and Monte-Carlo techniques are used to determine the ground state magnetic properties. Transitions from antiferromagnetic to ferromagnetic and again to re-entrant antiferromagnetic phase is observed in a wide range of parameter space. The magnetic moments of d - and f -electrons are observed to depend strongly on the value of J , J se and also on the total number of d -electrons ( N d ).

Thermodynamic studies of Spin-1/2 Falicov-Kimball Model (FKM) on a Triangular Lattice

Thermodynamic properties of the spin-dependent Falicov-Kimball model are studied on a triangular lattice for one-fourth filled case. Numerical diagonalization and Monte-Carlo simulation are used to study the thermodynamic properties. Continuous phase transitions are observed at finite temperature. We have observed that critical temperature (Tc) increases with the increase in on-site Coulomb correlation U. The second order nature of the transition is also revealed from the temperature dependence of specific heat.

Effect of super-exchange interaction on ground state magnetic properties of spin-dependent Falicov-Kimball model on a triangular lattice

Ground state magnetic properties are studied by incorporating the super-exchange interaction (Jse) in the spin-dependent Falicov-Kimball model (FKM) between localized (f-) electrons on a triangular lattice for half filled case. Numerical diagonalization and Monte-Carlo simulation are used to study the ground state magnetic properties. We have found that the magnetic moment of (d-) and (f-) electrons strongly depend on the value of Hund’s exchange (J), super-exchange interaction (Jse) and also depends on the number of (d-) electrons (Nd). The ground state changes from antiferromagnetic (AFM) to ferromagnetic (FM) state as we decrease (Nd). Also the density of d electrons at each site depends on the value of J and Jse.

Study of magnetic properties of spin-dependent Falicov-Kimball model on a triangular lattice

Numerical diagonalization technique and Monte-Carlo simulation algorithm is used to study the ground state properties of spin-dependent Falicov-Kimball model (FKM) on a triangular lattice for 1/3 filling of itinerant (d) and localized (f) electrons. We have found that the ground state configurations are of long range Neel ordered antiferromagnetic, ferromagnetic or mixture of anti-ferromagnetic and ferromagnetic type for different values of exchange correlation (J). The magnetization of d and f-electrons increases with increasing the exchange correlation (J) between d and f-electrons and then decreases with further increasing the value of J.

Ground state study of spin-1/2 Falicov-Kimball model on a triangular lattice

The ground state magnetic properties of two dimensional spin-1/2 Falicov-Kimball model on a triangular lattice are studied using numerical diagonalization with Monte Carlo simulation techniques. In the ground state we observe large range magnetic ordered phases like anti-ferromagnetic and ferromagnetic depending on different values of parameters of onsite Coulomb repulsion U and exchange correlation J. The variation of the magnetization of d- and f-electron with J is also studied.