arXiv subjects

Pinaki Majumdar

Publications and source records attributed to Pinaki Majumdar.

At least 55 records · Page 3Linked to original sources

arXiv2014

A Real Space Description of Magnetic Field Induced Melting in the Charge Ordered Manganites: I. The Clean Limit

We study the melting of charge order in the half doped manganites using a model that incorporates double exchange, antiferromagnetic superexchange, and Jahn-Teller coupling between electrons and phonons. We primarily use a real space Monte Carlo technique to study the phase diagram in terms of applied field $(h)$ and temperature $(T)$, exploring the melting of charge order with increasing $h$ and its recovery on decreasing $h$. We observe hysteresis in this response, and discover that the `field melted' high conductance state can be spatially inhomogeneous even without extrinsic disorder. The hysteretic response plays out in the background of field driven equilibrium phase separation. Our results, exploring $h$, $T$, and the electronic parameter space, are backed up by analysis of simpler limiting cases and a Landau framework for the field response. This paper focuses on our results in the `clean' systems, a companion paper studies the effect of cation disorder on the melting phenomena.

cond-mat.str-el↗

arXiv2014

A Real Space Description of Field Induced Melting in the Charge Ordered Manganites: II. the Disordered Case

We study the effect of A site disorder on magnetic field induced melting of charge order (CO) in half doped manganites using a Monte-Carlo technique. Strong A-site disorder destroys CO even without an applied field. At moderate disorder, the zero field CO state survives but has several intriguing features in its field response. Our spatially resolved results track the broadening of the field melting transition due to disorder and explain the unusual dependence of the melting scales on bandwidth and disorder. In combination with our companion paper on field melting of charge order in clean systems we provide an unified understanding of CO melting across all half doped manganites.

cond-mat.str-el↗

arXiv2014

Tunneling Spectroscopy Across the Superconductor-Insulator Thermal Transition

Advances in scanning tunneling spectroscopy reveal the presence of superconducting nanoregions well past the bulk thermal transition in strongly disordered superconductors. We use a Monte Carlo tool to capture the spatially differentiated amplitude and phase fluctuations in such a material and establish spatial maps of the coherence peak as the superconductor is driven through the thermal transition. Analysis of the local density of states reveals that superconducting regions shrink and fragment with increasing temperature, but survive in small clusters to a temperature $T_{clust} >> T_{c}$ . The gap (or pseudogap) in the spectrum survives in general to another independent scale, $T_{g}$, depending on the strength of interaction. This multiple scale description is consistent with recent measurements and defines the framework for analysing strongly disordered superconductors.

cond-mat.str-el↗

arXiv2014

Charge Dynamics Across the Disorder Driven Superconductor-Insulator Transition

While insensitive to weak non magnetic disorder, an s-wave superconductor can be driven insulating by strong disorder. Using a scheme that captures the correct ground state, and fully retains thermal amplitude and phase fluctuations, we describe the disorder driven superconductor-insulator transition at finite temperature. Our results on the resistivity suggest that beyond moderate disorder the low temperature superconducting state can arise out of an insulating normal state. We also find that the low frequency weight in the density of states and optical conductivity are non monotonic in disorder, with a maximum near critical disorder, and their high temperature value correlate with the superconducting fraction in the disordered ground state.

cond-mat.supr-con↗

arXiv2014

A real space auxiliary field approach to the BCS-BEC crossover

The BCS to BEC crossover in attractive Fermi systems is a prototype of weak to strong coupling evolution in many body physics. While extensive numerical results are available, and several approximate methods have been developed, most of these schemes are unsuccessful in the presence of spatial inhomogeneity. Such situations call for a real space approach that can handle large spatial scales and retain the crucial thermal fluctuations. With this in mind, we present comprehensive results of a real space auxiliary field approach to the BCS to BEC crossover in the attractive Hubbard model in two dimensions. The scheme reproduces the Hartree-Fock-Bogoliubov ground state, and leads to a $T_c$ scale that agrees with quantum Monte Carlo estimates to within a few percent. We provide results on the $T_c$, amplitude and phase fluctuations, density of states, and the momentum resolved spectral function over the entire interaction and temperature window. We suggest how the method generalises successfully to the presence of disorder, trapping, and population imbalance.

cond-mat.str-el↗

arXiv2013

Radio frequency spectroscopy of the attractive Hubbard model in a trap

Attractive interaction between fermions can lead to pairing and superfluidity in an optical lattice. In contrast to the `continuum', on a lattice the trap induced density variation can generate a non monotonic profile of the pairing amplitude, and completely modify the spectral signatures of any possible pseudogap phase. Using a tool that fully captures the inhomogeneity and strong thermal fluctuations, we demonstrate how the crucial radio frequency signatures of pairing are `inverted' in a trapped attractive fermion lattice compared to the traditional continuum case. These features would be central in interpreting any spectroscopic hint of fermion pairing and superfluidity.

cond-mat.str-el↗

arXiv2013

On soft linear spaces and soft normed linear spaces

In this paper an idea of soft linear spaces and soft norm on soft linear spaces are given and some of their properties are studied. Soft vectors in soft linear spaces are introduced and their properties are studied. Completeness of soft normed linear space is also studied and equivalent soft norms and convex soft sets are studied in soft normed linear space settings.

math.GM↗

arXiv2013

Mott Transition and Glassiness in the Face Centered Cubic Lattice

We study the half-filled Hubbard model on the geometrically frustrated face centered cubic (FCC) lattice, using an auxiliary field based real space technique. The low temperature state is a paramagnetic metal at weak interaction, an antiferromagnetic insulator (AFI) with flux like order at intermediate interaction, and an AFI with `C type' order at very strong interaction. Remarkably, there is a narrow window between the paramagnetic metal and the AFI where the system exhibits spin glass behaviour arising from the presence of disordered but `frozen' local moments. The spin glass state is metallic at weaker interaction but shows crossover to pseudogap behaviour and an insulating resistivity with growing interaction. We compare our results to available experiments on FCC and pyrochlore based materials and suggest that several of these features are typical of three dimensional correlated systems with geometric frustration.

cond-mat.str-el↗

arXiv2013

The Crossover from a Bad Metal to a Frustrated Mott Insulator

We use a novel Monte Carlo method to study the Mott transition in an anisotropic triangular lattice. The real space approach, retaining extended spatial correlations, allows an accurate treatment of non trivial magnetic fluctuations in this frustrated structure. Choosing the degree of anisotropy to mimic the situation in the quasi-two dimensional organics, $κ-$(BEDT-TTF)$_2$Cu[N(CN)$_2$]-X, we detect a wide pseudogap phase, with anomalous spectral and transport properties, between the `ungapped' metal and the `hard gap' Mott insulator. The magnetic fluctuations also lead to pronounced momentum dependence of quasiparticle damping and pseudogap formation on the Fermi surface as the Mott transition is approached. Our predictions about the `bad metal' state have a direct bearing on the organics where they can be tested via tunneling, angle resolved photoemission, and magnetic structure factor measurement.

cond-mat.str-el↗

arXiv2012

The Magnon Spectrum in the Domain Ferromagnetic State of Antisite Disordered Double Perovskites

In their ideal structure, double perovskites like Sr_2FeMoO_6 have alternating Fe and Mo along each cubic axes, and a homogeneous ferromagnetic metallic ground state. Imperfect annealing leads to the formation of structural domains. The moments on mislocated Fe atoms that adjoin each other across the domain boundary have an antiferromagnetic coupling between them. This leads to a peculiar magnetic state, with ferromagnetic domains coupled antiferromagnetically. At short distance the system exhibits ferromagnetic correlation while at large lengthscales the net moment is strongly suppressed due to inter-domain cancellation. We provide a detailed description of the spin wave excitations of this complex magnetic state, obtained within a 1/S expansion, for progressively higher degree of mislocation, i.e., antisite disorder. At a given wavevector the magnons propagate at multiple energies, related, crudely, to `domain confined' modes with which they have large overlap. We provide a qualitative understanding of the trend observed with growing antisite disorder, and contrast these results to the much broader spectrum that one obtains for uncorrelated antisites.

cond-mat.str-el↗

arXiv2011

Huge Positive Magnetoresistance in Antiferromagnetic Double Perovskite Metals

Metals with large positive magnetoresistance are rare. We demonstrate that antiferromagnetic metallic states, as have been predicted for the double perovskites, are excellent candidates for huge positive magnetoresistance. An applied field suppresses long range antiferromagnetic order leading to a state with short range antiferromagnetic correlations that generate strong electronic scattering. The field induced resistance ratio can be more than tenfold, at moderate field, in a structurally ordered system, and continues to be almost twofold even in systems with upto 25 % antisite disorder. Although our explicit demonstration is in the context of a two dimensional spin-fermion model of the double perovskites, the mechanism we uncover is far more general, complementary to the colossal negative magnetoresistance process, and would operate in other local moment metals that show a field driven suppression of non-ferromagnetic order.

cond-mat.str-el↗

arXiv2011

Pairing Fluctuations and Anomalous Transport Above the BCS-BEC Crossover in the Two Dimensional Attractive Hubbard Model

A Fermi liquid with weak attractive interaction undergoes a BCS transition to a superconductor with reducing temperature. With increasing interaction strength, the thermal transition is progressively modified as the high temperature `metallic' phase develops a pseudogap due to pairing fluctuations and the resistivity above T_c shows insulating behaviour. The crossover to insulating character occurs much before the system can be considered to be in the BEC regime of preformed fermion pairs. We use a new Monte Carlo tool to map out the BCS-BEC crossover in the attractive Hubbard model on large two dimensional lattices and explicitly compute the resistivity to demonstrate how the metal to superconductor (MS) thermal transition at weak coupling crosses over to an insulator to superconductor (IS) transition at intermediate coupling. Our high resolution access to the single particle and optical spectrum at finite temperature allows us to completely describe the transport crossover in this longstanding problem.

cond-mat.str-el↗

arXiv2011

Non-collinear Magnetic Order in the Double Perovskites: Double Exchange on a Geometrically Frustrated Lattice

Double perovskites of the form A_2BB'O_6 usually involve a transition metal ion, B, with a large magnetic moment, and a non magnetic ion B'. While many double perovskites are ferromagnetic, studies on the underlying model reveal the possibility of antiferromagnetic phases as well driven by electron delocalisation. In this paper we present a comprehensive study of the magnetic ground state and T_c scales of the minimal double perovskite model in three dimensions using a combination of spin-fermion Monte Carlo and variational calculations. In contrast to two dimensions, where the effective magnetic lattice is bipartite, three dimensions involves a geometrically frustrated face centered cubic (FCC) lattice. This promotes non-collinear spiral states and `flux' like phases in addition to collinear anti-ferromagnetic order. We map out the possible magnetic phases for varying electron density, `level separation' epsilon_B - epsilon_B', and the crucial B'-B' (next neighbour) hopping t'.

cond-mat.str-el↗

arXiv2011

Thermally Fluctuating Inhomogeneous Superfluid State of Strongly Interacting Fermions in an Optical Lattice

The presence of attractive interaction between fermions can lead to pairing and superfluidity in an optical lattice. The temperature needed to observe superfluidity is about a tenth of the tunneling energy in the optical lattice, and currently beyond experimental reach. However, at strong coupling the precursors to global superfluidity should be visible at achievable temperatures, in terms of fluctuating domains with strong pairing correlations. We explore this regime of the attractive two dimensional fermion Hubbard model, in the presence of a confining potential, using a new Monte Carlo technique. We capture the low temperature inhomogeneous superfluid state with its unusual spectral signatures but mainly focus on the experimentally accessible intermediate temperature state. In this regime, and for the trap center density we consider, there is a large pairing amplitude at the center, spatially correlated into domains extending over several lattice spacings. We map out the thermal evolution of the local density, the double occupancy, the pairing correlations, and the momentum distribution function across this phase fluctuation window.

cond-mat.quant-gas↗

arXiv2010

Antiferromagnetic Order and Phase Coexistence in Antisite Disordered Double Perovskites

In addition to the well known ferromagnetism, double perovskites are also expected to exhibit antiferromagnetic (AF) order driven by electron delocalisation. This has been seen in model Hamiltonian studies and confirmed via ab initio calculations. The AF phases should occur, for example, on sufficient electron doping of materials like Sr_2FeMoO_6 (SFMO) via La substitution for Sr. Clear experimental indication of such AF order is limited, possibly because of increase in antisite disorder with La doping on SFMO, although intriguing signatures of non ferromagnetic behaviour are seen. We study the survival of electronically driven antiferromagnetism in the presence of spatially correlated antisite disorder and extract the signals in magnetism and transport. We discover that A and G type AF order, that is predicted in the clean limit, is actually suppressed less strongly than ferromagnetism by antisite disorder. The AF phases are metallic, and, remarkably, more conducting that the ferromagnet for similar antisite disorder. We also highlight the phase coexistence window that connects the ferromagnetic regime to the A type antiferromagnetic phase.

cond-mat.str-el↗

arXiv2010

Antisite Domains in Double Perovskite Ferromagnets: Impact on Magnetotransport and Half-metallicity

Several double perovskite materials of the form A_2BB'O_6 exhibit high ferromagnetic T_c, and significant low field magnetoresistance. They are also a candidate source of spin polarized electrons. The potential usefulness of these materials is, however, frustrated by mislocation of the B and B' ions, which do not organise themselves in the ideal alternating structure. The result is a strong dependence of physical properties on preparative conditions, reducing the magnetization and destroying the half-metallicity. We provide the first results on the impact of spatially correlated antisite disorder, as observed experimentally, on the ferromagnetic double perovskites. The antisite domains suppress magnetism and half-metallicity, as expected, but lead to a dramatic enhancement of the low field magnetoresistance.

cond-mat.str-el↗

arXiv2009

Conductance Switching and Inhomogeneous Field Melting in the Charge Ordered Manganites

The field induced switching of conductance in the charge ordered half-doped manganites is controlled by the combination of metastability, an inhomogeneous high field state, and cation disorder. We study this non-equilibrium problem via real space Monte Carlo on a disordered strong coupling model appropriate to the manganites. We reproduce the variation of the switching fields with the mean ionic radius r_A and cation disorder σ_A, and demonstrate how the experimental features arise from the proximity of several phases in the Landau free energy landscape. Our prediction for the field melted state is consistent with a growing body of experimental evidence.

cond-mat.str-el↗

arXiv2009

Magnetic Order Beyond RKKY in the Classical Kondo Lattice

We study the Kondo lattice model of band electrons coupled to classical spins, in three dimensions, using a combination of variational calculation and Monte Carlo. We use the weak coupling `RKKY' window and the strong coupling regime as benchmarks, but focus on the physically relevant intermediate coupling regime. Even for modest electron-spin coupling the phase boundaries move away from the RKKY results, the non interacting Fermi surface no longer dictates magnetic order, and weak coupling `spiral' phases give way to collinear order. We use these results to revisit the classic problem of 4f magnetism and demonstrate how both electronic structure and coupling effects beyond RKKY control the magnetism in these materials.

cond-mat.str-el↗