Pinaki Majumdar

Harish-Chandra Research Institute | HRI

An electron-phonon system at commensurate filling often displays charge order (CO) in the ground state. Such a system subject to a laser pulse shows a wide variety of behaviour. A weak pulse sets up low amplitude oscillations in the order parameter, with slow decay to a slightly suppressed value. A strong pulse leads to the destruction of the charg...

We study the equilibrium dynamics of magnetic moments in the Mott insulating phase of the Hubbard model on the square and triangular lattice. We rewrite the Hubbard interaction in terms of an auxiliary vector field and use a recently developed Langevin scheme to study its dynamics. A thermal noise, derivable approximately from the Keldysh formalism...

We use a Langevin dynamics approach to map out the thermal phases of an antiferromagnetic Mott insulator pushed out of equilibrium by a large voltage bias. The Mott insulator is realised in the half-filled Hubbard model in a three dimensional bar geometry with leads at voltage $\pm V/2$ connected at the two ends. We decouple the strong Hubbard inte...

We study the equilibrium dynamics of magnetic moments in the Mott insulating phase of the Hubbard model on the square and triangular lattice. We rewrite the Hubbard interaction in terms of an auxiliary vector field and use a recently developed Langevin scheme to study its dynamics. A thermal `noise', derivable approximately from the Keldysh formali...

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 temperatu...

We solve for the finite temperature collective mode dynamics in the Holstein-double-exchange problem, using coupled Langevin equations for the phonon and spin variables. We present results in a strongly anharmonic regime, close to a polaronic instability. For our parameter choice, the system transits from an undistorted ferromagnetic metal at low t...

We present a method that generalises the standard mean field theory of correlated lattice bosons to include amplitude and phase fluctuations of the U(1) field that induces onsite particle number mixing. We solve the resulting problem, initially, by using a classical approximation for the particle number mixing field and a Monte Carlo treatment of t...

We solve for the finite temperature collective mode dynamics in the Holstein-double exchange problem, using coupled Langevin equations for the phonon and spin variables. We present results in a strongly anharmonic regime, close to a polaronic instability. For our parameter choice the system transits from an `undistorted' ferromagnetic metal at low...

Optical lattice experiments which probe the effect of disorder on superfluidity often use a speckle pattern for generating the disorder. Such speckle disorder is spatially correlated. While fermionic superfluidity in the presence of uncorrelated disorder is well studied, the impact of correlated disorder, particularly on thermal properties of the s...

We investigate the thermal physics of a Bose-Hubbard model with Rashba spin-orbit coupling starting from a strong coupling mean-field ground state. The essential role of the spin-orbit coupling γ is to promote condensation of the bosons at a finite wave vector k0. We find that the bosons display either homogeneous or phase-twisted or orbital ordere...

We use a Langevin approach to treat the finite temperature dynamics of displacement variables in the half-filled spinless Holstein model. Working in the adiabatic regime we exploit the smallness of the adiabatic parameter to simplify the memory effects and estimate displacement costs from an “instantaneous” electronic Hamiltonian. We use a phenomen...

We use a Langevin approach to treat the finite temperature dynamics of displacement variables in the half-filled spinless Holstein model. Working in the adiabatic regime we exploit the smallness of the adiabatic parameter to simplify the memory effects and estimate displacement costs from an "instantaneous" electronic Hamiltonian. We use a phenomen...

We use a Langevin approach to treat the finite temperature dynamics of displacement variables in the half-filled spinless Holstein model. Working in the adiabatic regime we exploit the smallness of the adiabatic parameter to simplify the memory effects and estimate displacement costs from an "instantaneous" electronic Hamiltonian. We use a phenomen...

We investigate the thermal physics of a Bose-Hubbard model with Rashba spin-orbit coupling starting from a strong coupling mean-field ground state. The essential role of the spin-orbit coupling $\left(\gamma\right)$ is to promote condensation of the bosons at a finite wavevector ${k}_{0}$. We find that the bosons display either homogeneous or phase...

We explore the anisotropy in low frequency conductivity as a function of hole doping in multiorbital models of pnictides by analyzing the Drude weight in the x and y directions of a (π, 0) spin-density wave state. A reduction in the conductivity anisotropy with increased hole doping, and subsequent sign reversal, both observed in experiments, is fo...

Impurity scattering is found to lead to quasi-one-dimensional nanoscale modulation of the local density of states in iron pnictides and chalcogenides. This “quasiparticle interference” feature is remarkably similar across a wide variety of pnictide and chalcogenide phases, suggesting a common origin. We show that a unified understanding of the expe...

We explore the anisotropy in low frequency conductivity as a function of hole doping in multiorbital models of pnictides by analyzing the Drude weight in the $x$ and $y$ directions of a ($\pi$, 0) spin-density wave state. A reduction in the conductivity anisotropy with increased hole doping, and subsequent sign reversal, both observed in experiment...

Optical lattice experiments which probe the effect of disorder on superfluidity often use a speckle pattern for generating the disorder. Such speckle disorder is spatially correlated. While fermionic superfluidity in the presence of uncorrelated disorder is well studied, the impact of correlated disorder, particularly on thermal properties of the s...

An electron system with pre-existing local moments and an effective electron-electron attraction can exhibit simultaneous magnetic and superconducting order. Increasing the magnetic coupling weakens pairing and the ground state loses superconductivity at a critical coupling. In the vicinity of the critical coupling magnetic order dramatically modif...

We investigate the dynamical structure factor associated with lattice fluctuations in a model that approximates the manganites. It involves electrons strongly coupled to core spins, and to lattice distortions, in a weakly disordered background. This model is solved in the adiabatic limit in two dimensions via Monte Carlo, retaining all the thermal...

Impurity scattering is found to lead to quasi-one dimensional nanoscale modulation of the local density of states in the iron pnictides and chalcogenides. This `quasiparticle interference' feature is remarkably similar across a wide variety of pnictide and chalcogenide phases, suggesting a common origin. We show that an unified understanding of the...

We compute the single particle spectral function of a Bose liquid on a lattice, at integer filling, close to the superfluid-Mott transition. We use a `static path approximation' that retains all the classical thermal fluctuations in the problem, and a real space implementation of the random phase approximation (RPA) for the Green's functions on the...

We study the Holstein model on a square lattice across the polaronic crossover by using an adiabatic approximation to compute the lattice distorted backgrounds and a Gaussian expansion in dynamical modes to compute phonon spectral properties. We capture phonon dynamics from the perturbatively renormalised weak coupling end to the `polaron insulator...

We present a method that generalises the standard mean field theory of correlated lattice bosons to include amplitude and phase fluctuations of the $U(1)$ field that induces onsite particle number mixing. This arises formally from an auxiliary field decomposition of the kinetic term in a Bose Hubbard model. We solve the resulting problem, initially...

We investigate the impurity scattering induced quasiparticle interference in the ($\pi, 0$) spin-density wave phase of the iron pnictides. We use a five orbital tight binding model and our mean field theory in the clean limit captures key features of the Fermi surface observed in angle-resolved photoemission. We use a t-matrix formalism to incorpor...

We study s-wave superconductivity in the two dimensional attractive Hubbard model in an applied magnetic field, assume the extreme Pauli limit, and examine the role of spatial fluctuations in the coupling regime corresponding to BCS-BEC crossover. We establish the full thermal phase diagram of this strong coupling problem, revealing $T_c$ scales an...

We study the behavior of magnetic superconductors which involve a local attractive interaction between electrons, and a coupling between local moments and the electrons. We solve this `Hubbard-Kondo' model through a variational minimization at zero temperature and validate the results via a Monte Carlo based on static auxiliary field decomposition...

We study the effect of high population imbalance in the two dimensional attractive Hubbard model, in the coupling regime corresponding to BCS-BEC crossover, and focus on thermal effects on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Using a method that retains all the classical thermal fluctuations on the FFLO state we estimate a very low Tc...

We study s-wave superconductivity in the two dimensional attractive Hubbard model in an applied magnetic field, assume the extreme Pauli limit, and examine the role of spatial fluctuations in the coupling regime corresponding to BCS-BEC crossover. We establish the full thermal phase diagram of this strong coupling problem, revealing $T_c$ scales an...

In a Fermi superfluid increasing population imbalance leads initially to reduction of the transition temperature, then the appearance of modulated Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, and finally the suppression of pairing itself. For interaction strength such that the `balanced' system has a normal state pseudogap, increasing imbalance...

We study s-wave superconductivity in the attractive Hubbard model in an applied magnetic field and assume the extreme Pauli limit where the orbital critical field is much greater than the Zeeman critical field. We work at a coupling corresponding roughly to the peak $T_c$ in the BCS to BEC crossover window and retain the crucial amplitude and phase...