arXiv subjects

Pinaki Majumdar

Publications and source records attributed to Pinaki Majumdar.

At least 73 records · Page 4Linked to original sources

arXiv2008

A Magnetic Model for the Ordered Double Perovskites

We construct an effective spin model from the coupled spin-fermion problem appropriate to double perovskites of the form A_2BB'O_6. The magnetic model that emerges is reminiscent of double exchange and we illustrate this `reduction' in detail for the case of perfect B-B' structural order, i.e, no antisite disorder. We estimate the effective exchange between the magnetic B ions in terms of the electronic parameters, study the `classical' magnetic model using Monte Carlo techniques, and compare this approach to a full numerical solution of the spin-fermion problem. The agreement is reasonable, and promises a quick estimate of magnetic properties when coupled with ab initio electronic structure. The scheme generalises to the presence of antisite disorder.

cond-mat.str-el↗

arXiv2008

Exploiting B Site Disorder for Phase Control in the Manganites

Disorder on the active d element site is usually very disruptive for conduction and long range order in perovskite transition metal oxides. However, in the background of phase competition such `B site' dopants also act to promote one ordered phase at the expense of another. This occurs either through valence change of the transition metal or via creation of `defects' in the parent magnetic state. We provide a framework for understanding the complex variety of phenomena observed in B site doped manganites and identify the key parameters that control the physics. Using a spatially resolved analysis of B ions in various manganite phases we explain the existing data and predict new situations where highly polarisable phase separated states can be created.

cond-mat.str-el↗

arXiv2008

Structural Ordering and Antisite Defect Formation in Double Perovskites

We formulate an effective model for B-B' site ordering in double perovskite materials A_2BB'O_6. Even within the simple framework of lattice-gas type models, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B' species. We also study ordering in the `ternary' compounds A_2BB'_{1-y}B''_yO_6 Although our emphasis is on the double perovskites, our results are easily generalizable to a wide variety of binary and ternary alloys.

cond-mat.mtrl-sci↗

arXiv2007

The Distinct Effects of Homogeneous Weak Disorder and Dilute Strong Scatterers on Phase Competetion in the Manganites

We study the two orbital double-exchange model in two dimensions in the presence of antiferromagnetic (AF) superexchange, strong Jahn-Teller coupling, and substitutional disorder. At hole doping x=0.5 we explore the `bicritical' regime where the energy of a ferromagnetic metal and a charge and orbital ordered (CO-OO) CE state are closely balanced, and compare the impact of weak homogeneous disorder to that of a low density of strong scatterers. Even moderate homogeneous disorder suppresses the CE-CO-OO phase and leads to a glass with nanoscale correlations. Dilute strong scatterers of comparable strength, however, convert the CE-CO-OO phase to a phase separated state with ferromagnetic and AF-CO-OO clusters. We provide the first spatial description of these phenomena and compare our results in detail to experiments on the half-doped manganites.

cond-mat.str-el↗

arXiv2006

The Effect of Disorder in an Orbitally Ordered Jahn-Teller Insulator

We study a two dimensional, two-band double-exchange model for $e_g$ electrons coupled to Jahn-Teller distortions in the presence of quenched disorder using a recently developed Monte-Carlo technique. In the absence of disorder the half-filled system at low temperatures is an orbitally ordered ferromagnetic insulator with a staggered pattern of Jahn-Teller distortions. We examine the finite temperature transition to the orbitally disordered phase and uncover a qualitative difference between the intermediate and strongly coupled systems, including a thermally driven insulator to metal crossover in the former case. Long range orbital order is suppressed in the presence of disorder and the system displays a tendency towards metastable states consisting of orbitally disordered stripe-like structures enclosing orbitally ordered domains.

cond-mat.str-el↗

arXiv2006

Domain Formation and Orbital Ordering Transition in a Doped Jahn-Teller Insulator

The ground state of a double-exchange model for orbitally degenerate $e_g$ electrons with Jahn-Teller lattice coupling and weak disorder is found to be spatially inhomogeneous near half filling. Using a real space Monte-Carlo method we show that doping the half-filled orbitally ordered insulator leads to the appearance of hole-rich disordered regions in an orbitally ordered environment. The doping driven orbital order to disorder transition is accompanied by the emergence of metallic behavior. We present results on transport and optical properties along with spatial patterns for lattice distortions and charge densities, providing a basis for an overall understanding of the low doping phase diagram of La$_{1-x}$Ca$_x$MnO$_3$.

cond-mat.dis-nn↗

arXiv2006

Insulator-Metal Phase Diagram of the Optimally Doped Manganites from the Disordered Holstein-Double Exchange Model

We study the Holstein-Double Exchange model in three dimensions in the presence of substitutional disorder. Using a new Monte Carlo technique we establish the phase diagram of the clean model and then focus on the effect of varying electron-phonon coupling and disorder at fixed electron density. We demonstrate how extrinsic disorder controls the interplay of lattice polaron effects and spin fluctuations and leads to widely varying regimes in transport. Our results on the disorder dependence of the ferromagnetic T_c and metal-insulator transitions bear direct comparison to data on the `optimally doped', x=0.3-0.4, manganites. We highlight disorder induced polaron formation as a key effect in these materials, organise a wide variety of data into a simple `global phase diagram', and make several experimental predictions.

cond-mat.str-el↗

arXiv2005

Giant Tunneling Magnetoresistance, Glassiness, and the Energy Landscape at Nanoscale Cluster Coexistence

We present microscopic results on the giant tunneling magnetoresistance that arises from the nanoscale coexistence of ferromagnetic metallic (FMM) and antiferromagnetic insulating (AFI) clusters in a disordered two dimensional electron system with competing double exchange and superexchange interactions. Our Monte Carlo study allows us to map out the different field regimes in magnetotransport and correlate it with the evolution of spatial structures. At coexistence, the isotropic O(3) model shows signs of slow relaxation, and has a high density of low energy metastable states, but no genuine glassiness. However, in the presence of weak magnetic anisotropy, and below a field dependent irreversibility temperature $T_{irr}$, the response on field cooling (FC) differs distinctly from that on zero field cooling (ZFC). We map out the phase diagram of this `phase coexistence glass', highlight how its response differs from that of a standard spin glass, and compare our results with data on the manganites.

cond-mat.dis-nn↗

arXiv2005

Transport and Localisation in the Presence of Strong Structural and Spin Disorder

We study a tight binding model including both on site disorder and coupling of the electrons to randomly oriented magnetic moments. The transport properties are calculated via the Kubo-Greenwood scheme, using the exact eigenstates of the disordered system and large system size extrapolation of the low frequency optical conductivity. We first benchmark our method in the model with only structural disorder and then use it to map out the transport regimes and metal- insulator transitions in problems involving (i) scattering from random magnetic moments, and (ii) the combined effect of structural disorder and magnetic scattering. We completely map out the dependence of the d.c conductivity on electron density (n) the structural disorder (Δ) and the magnetic coupling (J'), and locate the insulator-metal phase boundary in the space of n-Δ-J'. These results serve as a reference for understanding transport in systems ranging from magnetic semiconductors to double exchange `colossal magnetoresistance' systems. A brief version of this study appears in our earlier paper Europhys. Lett. vol 65, 75 (2004).

cond-mat.dis-nn↗

arXiv2005

Double Exchange Models: Self Consistent Renormalisation

We propose a scheme for constructing classical spin Hamiltonians from Hunds coupled spin-fermion models in the limit J_H/t \to \infinity. The strong coupling between fermions and the core spins requires self-consistent calculation of the effective exchange in the model, either in the presence of inhomogeneities or with changing temperature. In this paper we establish the formalism and discuss results mainly on the ``clean'' double exchange model, with self consistently renormalised couplings, and compare our results with exact simulations. Our method allows access to system sizes much beyond the reach of exact simulations, and we can study transport and optical properties of the model without artificial broadening. The method discussed here forms the foundation of our papers Phys. Rev. Lett. 91, 246602 (2003), and Phys. Rev. Lett. 92, 126602 (2004).

cond-mat.str-el↗

arXiv2005

Singular Effect of Disorder on Electronic Transport in Strong Coupling Electron-Phonon Systems

We solve the disordered Holstein model in three dimensions considering the phonon variables to be classical. After mapping out the phases of the `clean' strong coupling problem, we focus on the effect of disorder at strong electron-phonon (EP) coupling. The presence of even weak disorder (i) enormously enhances the resistivity (ρ) at T=0, simultaneously suppressing the density of states at the Fermi level, (ii) suppresses the temperature dependent increase of ρ, and (iii) leads to a regime with dρ/dT <0. We locate the origin of these anomalies in the disorder induced tendency towards polaron formation, and the associated suppression in effective carrier density and mobility. These results, explicitly at `metallic' density, are of direct relevance to disordered EP materials like covalent semiconductors, the manganites, and to anomalous transport in the A-15 compounds.

cond-mat.str-el↗

arXiv2005

The Many Electron Ground State of the Adiabatic Holstein Model in Two and Three Dimensions

We present the complete ground state phase diagram of the Holstein model in two and three dimension considering the phonon variables to be classical. We first establish the overall structure of the phase diagram by using exact diagonalisation based Monte Carlo (ED-MC) on small lattices and then use a new ``travelling cluster'' approximation (TCA) for annealing the phonon degrees of freedom on large lattices. The phases that emerge include a Fermi liquid (FL), with no lattice distortions, an insulating polaron liquid (PL) at strong coupling, and a charge ordered insulating (COI) phase around half- filling. The COI phase is separated from the Fermi liquid by a regime of phase coexistence whose width grows with increasing electron-phonon coupling. We provide results on the electronic density of states, the COI order parameter, and the spatial organisation of polaronic states, for arbitrary density and electron-phonon coupling. The results highlight the crucial role of spatial correlations in this strong coupling problem.

cond-mat.str-el↗

arXiv2005

Structural Disorder Induced Polaron Formation and Magnetic Scattering in the Disordered Holstein-Double Exchange Model

In this paper we present results on the disordered Holstein-Double Exchange model, explicitly in three dimension and `metallic' densities, obtained by using a recently developed Monte Carlo approach. Following up on our earlier paper, cond-mat/0406085, here we provide a detailed microscopic picture of the thermally driven metal-insulator transition (MIT) that arises close to the ferromagnet to paramagnet transition in this problem. This paper is focused mainly on the `diagnostics', clarifying the origin of the effective disorder that drives the MIT in this system. To that effect, we provide results on the thermal evolution of the distributions of (i) lattice distortions, (ii) the net `structural disorder' and (iii) the `hopping disorder' arising from spin randomness feeding back through the Hunds coupling. We suggest a phenomenology for the thermally driven MIT, viewing it as an `Anderson-Holstein' transition.

cond-mat.str-el↗

arXiv2004

The Travelling Cluster Approximation for Strong Correlation Models of Lattice Fermions Coupled to Classical Fields

We suggest and implement a new Monte Carlo strategy for correlated models involving fermions strongly coupled to classical degrees of freedom, with accurate handling of quenched disorder as well. Current methods iteratively diagonalise the full Hamiltonian for a system of N sites with computation time tau_N proportional to N^4. This limits achievable sizes to N \sim 100. In our method the energy cost of a Monte Carlo update is computed from the Hamiltonian of a cluster, of size N_c, constructed around the reference site, and embedded in the larger system. As MC steps sweep over the system, the cluster Hamiltonian also moves, being reconstructed at each site where an update is attempted. In this method tau_{N,N_c} is proportional to NN_c^3. Our results are obviously exact when N_c=N, and converge quickly to this asymptote with increasing N_c. The accuracy improves in systems where the effective disorder seen by the fermions is large. We provide results of preliminary calculations on the Holstein model and the Double Exchange model. The `locality' of the energy cost, as evidenced by our results, suggests that several important but inaccessible problems can now be handled with control.

cond-mat.str-el↗

arXiv2004

Nanoscale Phase Coexistence and Percolative Quantum Transport

We study the nanoscale phase coexistence of ferromagnetic metallic (FMM) and antiferromagnetic insulating (AFI) regions by including the effect of AF superexchange and weak disorder in the double exchange model. We use a new Monte Carlo technique, mapping on the disordered spin-fermion problem to an effective short range spin model, with self-consistently computed exchange constants. We recover `cluster coexistence' as seen earlier in exact simulation of small systems. The much larger sizes, $\sim 32 \times 32$, accessible with our technique, allows us to study the cluster distribution for varying electron density, disorder, and temperature. We track the magnetic structure, obtain the density of states, with its `pseudogap' features, and, for the first time, provide a fully microscopic estimate of the resistivity in a phase coexistence regime, comparing it with the `percolation' scenario.

cond-mat.str-el↗

arXiv2003

Anti-Localisation to Strong Localisation: The Interplay of Magnetic Scattering and Structural Disorder

We study the effect of magnetic scattering on transport in a system with strong structural disorder, using exact finite size calculation of the low frequency optical conductivity. At weak electron-spin coupling spin disorder leads to a decrease in resistivity by weakening the quantum interference precursors to Anderson localisation. However, at strong electron-spin coupling, the double exchange limit, magnetic scattering increases the effective disorder, sharply increasing the resistivity. We illustrate the several unusual transport regimes in this strong disorder problem, identify a re-entrant insulator-metal-insulator transition, and map out the phase diagram at a generic electron density.

cond-mat.dis-nn↗

arXiv2003

Inhomogeneous Ferromagnetism and Unconventional Charge Dynamics in Disordered Double Exchange Magnets

We solve the double exchange model in the presence of arbitrary substitutional disorder by using a self consistently generated effective Hamiltonian for the spin degrees of freedom. The magnetic properties are studied through classical Monte Carlo while the effective exchange, $D_{ij}$, are calculated by solving the disordered fermion problem, and renormalised self-consistently with increasing temperature. We present exact results on the conductivity, magnetoresistance, optical response and `real space' structure of the inhomogeneous ferromagnetic state, and compare our results with charge dynamics in disordered La_{1-x}Sr_xMnO_3. The large sizes, ${\cal O} (10^3)$, accessible within our method allows a complete, controlled calculation on the disordered strongly interacting problem.

cond-mat.str-el↗

arXiv2003

Anderson-Mott Transition Driven by Spin Disorder: Spin Glass Transition and Magnetotransport in Amorphous GdSi

A zero temperature Anderson-Mott transition driven by spin disorder can be `tuned' by an applied magnetic field to achieve colossal magnetoconductance. Usually this is not possible since spin disorder by itself cannot localise a high density electron system. However, the presence of strong structural disorder can realise this situation, self consistently generating a disordered magnetic ground state. We explore such a model, constructed to understand amorphous GdSi, and highlight the emergence of a spin glass phase, Anderson-Mott signatures in transport and tunneling spectra, and unusual magneto-optical conductivity. We solve a disordered strong coupling fermion-spin-lattice problem essentially exactly on finite systems, and account for all the qualitative features observed in magnetism, transport, and the optical spectra in this system.

cond-mat.str-el↗