arXiv subjects

S. S. Kancharla

Publications and source records attributed to S. S. Kancharla.

10 recordsLinked to original sources

arXiv2007

Anomalous superconductivity and its competition with antiferromagnetism in doped Mott insulators

Proximity to a Mott insulating phase is likely to be an important physical ingredient of a theory that aims to describe high-temperature superconductivity in the cuprates. Quantum cluster methods are well suited to describe the Mott phase. Hence, as a step towards a quantitative theory of the competition between antiferromagnetism (AFM) and d-wave superconductivity (SC) in the cuprates, we use Cellular Dynamical Mean Field Theory to compute zero temperature properties of the two-dimensional square lattice Hubbard model. The d-wave order parameter is found to scale like the superexchange coupling J for on-site interaction U comparable to or larger than the bandwidth. The order parameter also assumes a dome shape as a function of doping while, by contrast, the gap in the single-particle density of states decreases monotonically with increasing doping. In the presence of a finite second-neighbor hopping t', the zero temperature phase diagram displays the electron-hole asymmetric competition between antiferromagnetism and superconductivity that is observed experimentally in the cuprates. Adding realistic third-neighbor hopping t'' improves the overall agreement with the experimental phase diagram. Since band parameters can vary depending on the specific cuprate considered, the sensitivity of the theoretical phase diagram to band parameters challenges the commonly held assumption that the doping vs T_{c}/T_{c}^{max} phase diagram of the cuprates is universal. The calculated ARPES spectrum displays the observed electron-hole asymmetry. Our calculations reproduce important features of d-wave superconductivity in the cuprates that would otherwise be considered anomalous from the point of view of the standard BCS approach.

cond-mat.str-el↗

arXiv2007

Electron Doping of Cuprates via Interfaces with Manganites

The electron doping of undoped high-$T_c$ cuprates via the transfer of charge from manganites (or other oxides) using heterostructure geometries is here theoretically discussed. This possibility is mainly addressed via a detailed analysis of photoemission and diffusion voltage experiments, which locate the Fermi level of manganites above the bottom of the upper Hubbard band of some cuprate parent compounds. A diagram with the relative location of Fermi levels and gaps for several oxides is presented. The procedure discussed here is generic, allowing for the qualitative prediction of the charge flow direction at several oxide interfaces. The addition of electrons to antiferromagnetic Cu oxides may lead to a superconducting state at the interface with minimal quenched disorder. Model calculations using static and dynamical mean-field theory, supplemented by a Poisson equation formalism to address charge redistribution at the interface, support this view. The magnetic state of the manganites could be antiferromagnetic or ferromagnetic. The former is better to induce superconductivity than the latter, since the spin-polarized charge transfer will be detrimental to singlet superconductivity. It is concluded that in spite of the robust Hubbard gaps, the electron doping of undoped cuprates at interfaces appears possible, and its realization may open an exciting area of research in oxide heterostructures.

cond-mat.str-el↗

arXiv2007

Band insulator to Mott insulator transition in a bilayer Hubbard model

The ground state phase diagram of the half-filled repulsive Hubbard model in a bilayer is investigated using cluster dynamical mean field theory. For weak to intermediate values of Coulomb repulsion $U$, the system undergoes a transition from a Mott insulating phase to a metallic phase and then onto a band insulating phase as the interlayer hopping is increased. In the strong coupling case, the model exhibits a direct crossover from a Mott insulating phase to a band insulating phase. These results are robust with respect to the presence or absence of magnetic order.

cond-mat.str-el↗

arXiv2007

Novel Phase Between Band and Mott Insulators in Two Dimensions

We investigate the ground state phase diagram of the half-filled repulsive Hubbard model in two dimensions in the presence of a staggered potential $Δ$, the so-called ionic Hubbard model, using cluster dynamical mean field theory. We find that for large Coulomb repulsion, $U\gg Δ$, the system is a Mott insulator (MI). For weak to intermediate values of $Δ$, on decreasing $U$, the Mott gap closes at a critical value $U_{c1}(Δ)$ beyond which a correlated insulating phase with possible bond order (BO) is found. Further, this phase undergoes a first-order transition to a band insulator (BI) at $U_{c2}(Δ)$ with a finite charge gap at the transition. For large $Δ$, there is a direct first-order transition from a MI to a BI with a single metallic point at the phase boundary.

cond-mat.str-el↗

arXiv2006

Metallic interface at the boundary between band and Mott insulators

Motivated by experiments on atomically smooth layers of LaTiO$_3$, a Mott insulator, sandwiched between layers of SrTiO$_3$, a band insulator, a simple model for such heterostructures is studied using quasi one-dimensional lattices and the Lanczos method. Taking both the local and long-range Coulomb interactions into account, and computing the layer dependent local density of states, a metallic state was found at the interface whose extent strongly depends on the dielectric constant of the material. We also observed that the antiferromagnetic correlations in the bulk Mott phase persist into the metallic region. Our conclusions are in excellent agreement with recently reported results for this model in the opposite limit of infinite dimensions, thus providing an alternative tool to study electronic reconstruction effects in heterostructures.

cond-mat.str-el↗

arXiv2006

Pseudogap induced by short-range spin correlations in a doped Mott insulator

We study the evolution of a Mott-Hubbard insulator into a correlated metal upon doping in the two-dimensional Hubbard model using the Cellular Dynamical Mean Field Theory. Short-range spin correlations create two additional bands apart from the familiar Hubbard bands in the spectral function. Even a tiny doping into this insulator causes a jump of the Fermi energy to one of these additional bands and an immediate momentum dependent suppression of the spectral weight at this Fermi energy. The pseudogap is closely tied to the existence of these bands. This suggests a strong-coupling mechanism that arises from short-range spin correlations and large scattering rates for the pseudogap phenomenon seen in several cuprates.

cond-mat.str-el↗

arXiv2004

Dynamical Breakup of the Fermi Surface in a doped Mott Insulator

The evolution from an anomalous metallic phase to a Mott insulator within the two-dimensional Hubbard model is investigated by means of the Cellular Dynamical Mean-Field Theory. We show that the density-driven Mott metal-insulator transition is approached in a non-uniform way in different regions of the momentum space. This gives rise to a breakup of the Fermi surface and to the formation of hot and cold regions, whose position depends on the hole or electron like nature of the carriers in the system.

cond-mat.str-el↗

arXiv2004

Cluster Dynamical Mean-Field Theory of the density-driven Mott transition in the one-dimensional Hubbard model

The one-dimensional Hubbard model is investigated by means of two different cluster schemes suited to introduce short-range spatial correlations beyond the single-site Dynamical Mean-Field Theory, namely the Cluster-Dynamical Mean-Field Theory and its periodized version. It is shown that both cluster schemes are able to describe with extreme accuracy the evolution of the density as a function of the chemical potential from the Mott insulator to the metallic state. Using exact diagonalization to solve the cluster impurity model, we discuss the role of the truncation of the Hilbert space of the bath, and propose an algorithm that gives higher weights to the low frequency hybridization matrix elements and improves the speed of the convergence of the algorithm.

cond-mat.str-el↗

arXiv2002

Cellular Dynamical Mean Field Theory for the 1D Extended Hubbard Model

We explore the use of exact diagonalization methods for solving the self consistent equations of the cellular dynamical mean field theory (CDMFT) for the one dimensional regular and extended Hubbard models. We investigate the nature of the Mott transition and convergence of the method as a function of cluster size as well as the optimal allocation of computational resources between bath and `cluster-impurity' sites, with a view to develop a renormalization group method in higher dimensions. We assess the performance of the method by comparing results for the Green's functions in both the spin density wave (SDW) and charge density wave (CDW) phases with accurate density matrix renormalization group (DMRG) calculations.

cond-mat.str-el↗

arXiv2000

Optical response in one dimensional Mott Insulators

We study the optical response of a Mott Hubbard system in the framework of the half--filled Extended Hubbard Model using the Density Matrix Renormalization Group (DMRG) method. We discuss the appearance of excitonic features inside the spectral gap as the system goes from the Spin Density Wave (SDW) to the Charge Density Wave (CDW) phase.

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