arXiv subjects

Pinaki Majumdar

Publications and source records attributed to Pinaki Majumdar.

At least 37 records · Page 2Linked to original sources

arXiv2019

The impact of speckle disorder on a superfluid Fermi system

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 superfluid, is poorly understood. We provide a detailed study of the impact of speckle disorder, for varying speckle size and disorder magnitude, on the ground state and thermal properties of a Fermi superfluid. We work in the strong coupling regime of BCS-BEC crossover in a two dimensional lattice. For a fixed disorder strength, an increase in speckle size leads to smoothening of the self-consistent background potential, increase in the critical disorder needed for a superfluid-insulator transition, and an increase in superfluid $T_c$ . Along with these hints at decrease in effective disorder, speckle correlations also suppress the superfluid gap and the gap formation temperature - effects normally associated with increasing disorder. We correlate these effects with the effective potential and the single particle localisation effects in the ground state

cond-mat.str-el↗

arXiv2019

A Langevin approach to lattice dynamics in a charge ordered polaronic system

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 phenomenological damping rate, and uncorrelated thermal noise. The low temperature state has checkerboard charge order (CO) and the Langevin scheme generates equilibrium thermodynamic properties that accurately match Monte Carlo results. It additionally yields the dynamical structure factor, $D({\bf q}, ω)$, from the displacement field $x({\bf r}, t)$. We observe four regimes with increasing temperature, $T$, classified in relation to the charge ordering temperature, $T_c$, and the `polaron formation' temperature $T_P$, with $ T_c \ll T_P$. For $T \ll T_c$ the oscillations are harmonic, leading to dispersive phonons, with increasing $T$ bringing in anharmonic, momentum dependent, corrections. For $T \sim T_c$, thermal tunneling events of the $x({\bf r})$ field occur, with a propagating `domain' pattern at wavevector ${\bf q} \sim (π, π)$ and low energy weight in $D({\bf q}, ω)$. When $T_c < T < T_P$, the disordered polaron regime, domain structures vanish, the dispersion narrows, and low energy weight is lost. For $T \gtrsim T_P$ we essentially have uncorrelated local oscillations. We propose simple models to analyse this rich dynamics.

cond-mat.str-el↗

arXiv2019

Thermal transitions of the modulated superfluid for spin-orbit coupled correlated bosons in an optical lattice

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 $\left(γ\right)$ is to promote condensation of the bosons at a finite wavevector ${k}_{0}$. We find that the bosons display either homogeneous or phase-twisted or orbital ordered superfluid phases, depending on $γ$ and the inter-species interaction strength $λ$. We show that an increase of $γ$ leads to suppression of the critical interaction $U_c$ for the superfluid to Mott insulator transition in the ground state, and a reduction of the $T_c$ for superfluid to Bose-liquid transition at a fixed interaction. We capture the thermal broadening in the momentum distribution function, and the real space profiles of the thermally disordered magnetic textures, including their homogenization for $T \gtrsim T_{c}$. We provide a Landau theory based description of the ground state phase boundaries and thermal transition scales, and discuss experiments which can test our theory.

cond-mat.str-el↗

arXiv2018

Drude weight anisotropy in the doped iron pnictides: the primary role of orbital weight redistribution along the reconstructed Fermi surfaces

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 found to be a common trend in these models. This behavior arises due to the interplay of low energy orbitally-resolved density of states and the geometrical features of the Fermi surface. An understanding of anisotropy in the electron doped regime, however, would require additional ingredients.

cond-mat.str-el↗

arXiv2018

Quasi-one Dimensional Nanostructures as Sign of Nematicity in Iron Pnictides and Chalcogenides

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 a unified understanding of the experiments can be obtained by simply invoking a four-fold symmetry breaking $d_{xz}-d_{yz}$ orbital splitting, of a magnitude already suggested by the experiments. This can explain the one-dimensional characteristics in the local density of states observed in the orthorhombic nematic, tetragonal paramagnetic, as well as the spin-density wave and superconducting states in these materials.

cond-mat.supr-con↗

arXiv2018

Thermally induced gaplessness and Fermi arcs in a "s-wave" magnetic superconductor

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 modifies the quasiparticle dispersion in the superconductor, creating low energy spectral weight and significant gap anisotropy in the notional 's-wave' state. Using a Monte Carlo approach to the Hubbard-Kondo lattice problem we establish a thermal phase diagram, for varying magnetic coupling, that corresponds qualitatively to the borocarbide superconductors. In addition to the superconducting and magnetic transition temperatures, we identify two new thermal scales in this nominal s-wave system. These are associated, respectively, with crossover from gapped to gapless superconductivity, and from an anisotropic (nodal) 'Fermi surface' at low temperature, through a Fermi arc regime, to an isotropic Fermi surface at high temperature. Some of the spectral effects are already visible in the Ho and Er based borocarbides, others can be readily tested.

cond-mat.str-el↗

arXiv2018

Thermal phases of correlated lattice boson: a classical fluctuation theory

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, by using a classical approximation for the particle number mixing field and a Monte Carlo treatment of the resulting bosonic model. In two dimensions we obtain $T_c$ scales that dramatically improve on mean field theory and are within about 20% of full quantum Monte Carlo estimates. The `classical approximation' ground state, however, is still mean field, with an overestimate of the critical interaction, $U_c$, for the superfluid to Mott transition. By further including low order quantum fluctuations in the free energy functional we improve significantly on the $U_c$, and the overall thermal phase diagram. The classical approximation based method has a computational cost linear in system size. The methods readily generalise to multispecies bosons and the presence of traps.

cond-mat.str-el↗

arXiv2018

Modeling dynamical phonon fluctuations across the magnetically driven polaron crossover in the manganites

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 fluctuations. In the metallic phase near the polaronic crossover this approach captures the effect of thermally induced polaron formation, and their short range correlation, on the electronic spectral functions. The dynamical fluctuations of the optical and acoustic phonon modes are computed at a `one loop' level by calculating the electronic polarisability in the the thermally fluctuating backgrounds, and solving the phonon Dyson equations in real space. We present phonon lineshapes across the ferromagnet to paramagnet thermal transition and correlate them with changing electronic properties. We compare our results with inelastic neutron scattering data on the metallic manganites, and also predict what one may find in the more insulating phases.

cond-mat.str-el↗

arXiv2017

Thermal decoherence in a strongly correlated Bose liquid

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 thermally fluctuating backgrounds. This leads to the standard RPA answers in the ground state but captures the progressive damping of the excitations with increasing temperature. We focus on the momentum resolved lineshape across the superfluid to Bose liquid thermal transition. In the superfluid regime we observe a gapped `amplitude' mode, and gapless `phase' modes of positive and negative energy. The dispersion and weight of these modes changes with interaction but are almost temperature independent, even into the normal state, except near critical coupling. The damping of the modes varies roughly as $T^α f_{\bf k}$, where $T$ is the temperature and ${\bf k}$ the momentum, with $α\sim 0.5$ and $f_{\bf k}$ having non trivial momentum dependence. The Mott phase has gapped dispersive spectra. Near critical coupling the thermal Bose `liquid' is gapped, with progressive widening of the gap with increasing temperature, a feature that it shares with the Mott insulator.

cond-mat.str-el↗

arXiv2017

Mott transition and anomalous resistive state in the pyrochlore molybdates

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.

cond-mat.str-el↗

arXiv2017

Highly anisotropic quasiparticle interference patterns in the spin-density wave state of the iron pnictides

We investigate the impurity scattering induced quasiparticle interference in the ($π, 0$) 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 incorporate the effect of doping induced impurities on this state. The impurities lead to a spatial modulation of the local density of states about the impurity site, with a periodicity of $\sim 8a_{{\rm Fe}-{\rm Fe}}$ along the antiferromagnetic direction. The associated momentum space quasiparticle interference pattern is anisotropic, with major peaks located at $\sim (\pm π/4,0)$, consistent with spectroscopic imaging scanning tunneling microscopy. We trace the origin of this pattern to an elliptical contour of constant energy around momentum (0,0), with major axis oriented along the (0,1) direction, in the mean field electronic structure.

cond-mat.str-el↗

arXiv2017

Giant magnetoelectric effect in pure manganite-manganite heterostructures

Obtaining strong magnetoelectric couplings in bulk materials and heterostructures is an ongoing challenge. We demonstrate that manganite heterostructures of the form ${\rm (Insulator)/(LaMnO_3)_n/(CaMnO_3)_n/(Insulator)}$ show strong multiferroicity in magnetic manganites where ferroelectric polarization is realized by charges leaking from ${\rm LaMnO_3}$ to ${\rm CaMnO_3}$ due to repulsion. Here, an effective nearest-neighbor electron-electron (electron-hole) repulsion (attraction) is generated by cooperative electron-phonon interaction. Double exchange, when a particle virtually hops to its unoccupied neighboring site and back, produces magnetic polarons that polarize antiferromagnetic regions. Thus a striking giant magnetoelectric effect ensues when an external electrical field enhances the electron leakage across the interface.

cond-mat.str-el↗

arXiv2016

Giant magnetoelectric effect in pure manganite-manganite heterostructures

cond-mat.str-el↗

arXiv2016

Magnetic Order and Mott Transition on the Checkerboard Lattice

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.

cond-mat.str-el↗

arXiv2016

Mott-Hubbard transition and spin-liquid state on the pyrochlore lattice

The pyrochlore lattice involves corner sharing tetrahedra and the resulting geometric frustration is believed to suppress any antiferromagnetic order for Mott insulators on this structure. There are nevertheless short-range correlations which could be vital near the Mott-Hubbard insulator-metal transition. We use a static auxiliary-field-based Monte Carlo to study this problem in real space on reasonably large lattices. The method reduces to unrestricted Hartree-Fock at zero temperature but captures the key magnetic fluctuations at finite temperature. Our results reveal that increasing interaction drives the non magnetic (semi) metal to a 'spin disordered' metal with small local moments, at some critical coupling, and then, through a small pseudogap window, to a large moment, gapped, Mott insulating phase at a larger oupling. The spin disordered metal has a finite residual resistivity which grows with interaction strength, diverging at the upper coupling. We present the resistivity, optical conductivity, and density of states across the metal-insulator transition and for varying temperature. These results set the stage for the more complex cases of Mott transition in the pyrochlore iridates and molybdates.

cond-mat.str-el↗

arXiv2016

Non-collinear order and gapless superconductivity in s-wave magnetic superconductors

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 of the Hubbard interaction. Over a magnetic coupling window that widens with increasing attractive interaction the ground state supports simultaneous magnetic and superconducting order. The pairing amplitude remains s-wave like, without significant spatial modulation, while the magnetic phase evolves from a ferromagnet, through non-collinear `spiral' states, to a Neel state with increasing density and magnetic coupling. We find that at intermediate magnetic coupling the antiferromagnetic-superconducting state is gapless, except for the regime of Neel order. We map out the phase diagram in terms of density, magnetic coupling and attractive interaction, establish the electron dispersion and effective `Fermi surface' in the ground state, provide an estimate of the magnetic and superconducting temperature scales via Monte Carlo, and compare our results to available data on the borocarbides.

cond-mat.supr-con↗

arXiv2016

Anomalous pseudogap in population imbalanced Fermi superfluids

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 reveals anomalous spectral behavior. At a fixed weak imbalance (small polarization) the stable homogeneous superfluid occurs only above a certain temperature. The density of states has a minimum at the Fermi level, then a weak peak {\it within the gap}, and then the large, gap edge, coherence features. On heating, this non monotonic energy dependence changes to a more conventional fluctuation driven pseudogap, with a monotonic energy dependence. At large imbalance the ground state is FFLO and `pseudogapped' due to the modulated order. It changes to a gapless normal state on heating, and then shows a pseudogap again at a higher temperature. These weak imbalance and strong imbalance features both involve effects well beyond mean field theory. We establish them by using a Monte Carlo technique on large lattices, motivate the results in terms of the pairing field distribution, and compare them to spectroscopic results in the imbalanced unitary Fermi gas.

cond-mat.quant-gas↗

arXiv2015

Strong coupling s-wave superconductors in the extreme Pauli limit: I.The breached pair and metastable FFLO phases

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 fluctuations. At low field, as expected, the superconductor undergoes a second order thermal transition to the normal state, and is only weakly magnetized near $T_c$. At intermediate fields the thermal transition is still second order, but the magnetization is significantly larger, characteristic of a `breached pair' state. At strong field, the thermal transition is first order and our Monte Carlo reveals the presence of a metastable Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. At even higher fields we observe the true FFLO ground state. We present the full `field-temperature' phase diagram of this strong coupling problem, revealing $T_c$ scales an order of magnitude below the mean field estimate, compute the superconducting and magnetic order parameters, and map out the directly measurable magnetic structure factor. We compare these trends to results on the Pauli limited heavy fermion CeCoIn$_5$, and the cold atomic Fermi gas at unitarity. This paper focuses primarily on the homogeneous superconducting state, another deals with thermal effects in the FFLO regime.

cond-mat.supr-con↗