I will present recent results on the evolution from an anomalous metallic phase to a Mott insulator within the two dimensional Hubbard model, using a cluster extension of dynamical mean field theory. In particular, 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.

http://absimage.aps.org/image/MAR05/MWS_MAR05-2004-002024.pdf

Cluster Dynamical Mean Field analysis of the Mott transition

We report the numerical investigation of strain induced superconductor-insulator quantum phase transition on a Lieb lattice. Based on a non perturbative Monte Carlo technique, we show that in two dimensions an s-wave superconductor undergoes transition to a highly correlated bosonic insulator under the influence of strain, applied as staggered hopping amplitudes. We further demonstrate a strain induced BCS-BEC like crossover in the superconducting state, such that the superconductor-insulator transition takes place between a bosonic superconductor and a bosonic insulator. Our results suggest that it is the contribution of the dispersive bands towards the superconducting order, which dictates this crossover. To the best of our knowledge, this is the first work to report theoretical investigation of "disorder free" superconductor-insulator phase transition in systems with Lieb lattice structure. With the recent experimental realization of the Lieb lattice in ultracold atomic gases, photonic lattices as well as in solid state systems, we believe that the results presented in this paper would be of importance to initiate experimental investigation of such novel quantum phase transitions. We further discuss the fate of such systems at finite temperature, highlighting the effect of fluctuations on the superconducting pair formations, thermal scales and quasiparticle behavior. Our non perturbative numerical approach to the problem enables us to capture the thermal scales of the system accurately and provides us with mean field estimates of the ground state properties. The high temperature quasiparticle signatures discussed in this paper are expected to serve as benchmarks for experiments such as radio frequency and momentum resolved radio frequency spectroscopy measurements carried out on systems such as ultracold atomic gases.

Strain induced superconductor-insulator transition on Lieb lattice

The authors propose that the superconducting phase fluctuations influences the strain induced superconductor-insulator phase transition on a Lieb lattice systems. The paper establishes the distinctly different roles played by the flat and the dispersive bands in dictating the quantum phases in such designer lattices

Strain-induced superconductor-insulator transition on a Lieb lattice

We investigate the bandwidth-controlled Mott transition in the Hubbard model on the checkerboard lattice at half filling using the path-integral renormalization group (PIRG) method. It is demonstrated that the system undergoes a first-order phase transition to the plaquette-singlet insulating phase at a finite Hubbard interaction. This conclusion is drawn via a detailed analysis of the spin and charge correlations around the phase transition point by means of the PIRG method aided with a new iteration scheme introduced in this paper.

Mott Transition in the Hubbard Model on Checkerboard Lattice(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

We develop a real-space theory of the voltage bias driven transition from a Mott insulator to a correlated metal. Within our Keldysh mean-field approach the problem reduces to a self-consistency scheme for the charge and spin profiles in this open system. We solve this problem for a two dimensional antiferromagnetic Mott insulator at zero temperature. The charge and spin magnitude is uniform over the system at zero bias, but a bias $V$ leads to spatial modulation over a length scale $\ensuremath{\xi}(V)$ near the edges. $\ensuremath{\xi}(V)$ grows rapidly and becomes comparable to system size as $V$ increases towards a threshold scale ${V}_{c}$. The linear response conductance of the insulator is zero with the current being exponentially small for $V\ensuremath{\ll}{V}_{c}$. The current increases rapidly as $V\ensuremath{\rightarrow}{V}_{c}$. Beyond ${V}_{c}$, we observe an inhomogeneous low moment antiferromagnetic metal, and at even larger bias, a current saturated paramagnetic metal. We suggest an approximate scheme for the spectral features of this nonequilibrium system.

Spatial behavior in a Mott insulator near the voltage driven resistive transition

The checkerboard lattice, with alternating 'crossed' plaquettes, serves as the two dimensional analog of the pyrochlore lattice. The corner sharing plaquette structure leads to a hugely degenerate ground state, and no magnetic order, for classical spins with short range antiferromagnetic interaction. For the half-filled Hubbard model on this structure, however, we find that the Mott insulating phase involves virtual electronic processes that generate longer range and multispin couplings. These couplings lift the degeneracy, selecting a 'flux like' state in the Mott insulator. Increasing temperature leads, strangely, to a sharp crossover from this state to a '120 degree' correlated state and then a paramagnet. Decrease in the Hubbard repulsion drives the system towards an insulator-metal transition - the moments reduce, and a spin disordered state wins over the flux state. Near the insulator-metal transition the electron system displays a pseudogap extending over a large temperature window.

Magnetic Order and Mott Transition on the Checkerboard Lattice

The rare-earth based pyrochlore molybdates involve orbitally degenerate electrons Hund's coupled to local moments. The large Hund's coupling promotes ferromagnetism, the superexchange between the local moments prefers antiferromagnetism, and Hubbard repulsion tries to open a Mott gap. The phase competition is tuned by the rare-earth ionic radius, decreasing which leads to change from a ferromagnetic metal to a spin disordered highly resistive ground state, and ultimately an “Anderson-Mott” insulator. We attempt a quantitative theory of the molybdates by studying their minimal model on a pyrochlore geometry, using a static auxiliary field based Monte Carlo. We establish a thermal phase diagram that closely corresponds to the experiments, predict the hitherto unexplored orbital correlations, quantify and explain the origin of the anomalous resistivity, and present dynamical properties across the metal-insulator transition.

Mott transition and anomalous resistive state in the pyrochlore molybdates

The checkerboard lattice, with alternating 'crossed' plaquettes, serves as the two dimensional analog of the pyrochlore lattice. The corner sharing plaquette structure leads to a hugely degenerate ground state, and no magnetic order, for classical spins with short range antiferromagnetic interaction. For the half-filled Hubbard model on this structure, however, we find that the Mott insulating phase involves virtual electronic processes that generate longer range and multispin couplings. These couplings lift the degeneracy, selecting a 'flux like' state in the Mott insulator. Increasing temperature leads, strangely, to a sharp crossover from this state to a '120 degree' correlated state and then a paramagnet. Decrease in the Hubbard repulsion drives the system towards an insulator-metal transition-the moments reduce, and a spin disordered state wins over the flux state. Near the insulator-metal transition the electron system displays a pseudogap extending over a large temperature window.

Nyayabanta Swain

Papers

Magnetic Order and Mott Transition on the Checkerboard Lattice

Mott transition and anomalous resistive state in the pyrochlore molybdates

Magnetic order and Mott transition on the checkerboard lattice