Pinaki Majumdar

Dependence of magnetoresistivity on charge-carrier density in metallic ferromagnets and doped magnetic semiconductors

Magnetoresistance—the field-dependent change in the electrical resistance of a ferromagnetic material—finds applications in technologies such as magnetic recording. Near and above the Curie point, T c, corresponding to the onset of magnetic order, scattering of charge carriers by magnetic fluctuations can substantially increase the electrical resistance,. These fluctuations can be suppressed by a magnetic field, leading to a negative magnetoresistance. Magnetic scattering might also have a role in the ‘colossal’ magnetoresistance observed in some perovskite manganese oxides, but is it not yet clear how to reconcile this behaviour with that of the conventional ferromagnetic materials. Here we show that, in generic models of magnetic scattering, the bulk low-field magnetoresistance (near and above T c) is determined by a single parameter: the charge-carrier density. In agreement with experiment,,, the low-field magnetoresistance scales with the square of the ratio of the field-induced magnetization to the saturation magnetization. The scaling factor is C ≈ x −2/3, where x is the number of charge carriers per magnetic unit cell. Data from very different ferromagnetic metals and doped semiconductors are in broad quantitative agreement with this relationship, with the notable exception of the perovskite manganese oxides (in which dynamic lattice distortions complicate and enhance, the effects of pure magnetic scattering). Our results might facilitate searches for new materials with large bulk magnetoresistive properties.

Quantum Many Particle Physics

This set of lectures focuses on some problems in itinerant Bose and Fermi systems. The first lecture will be an introduction to many-particle physics. In the second we will discuss phase transitions and symmetry breaking and the mean field approach to condensation in Bose systems. In the third we will look at strong interactions in ‘normal’ Fermi systems. The final lecture will be on strong coupling superconductivity which involves both strong interactions and symmetry breaking.

Distinct effects of homogeneous weak disorder and dilute strong scatterers on phase competition in manganites.

We study the two orbital double-exchange model in two dimensions including antiferromagnetic (AFM) superexchange, Jahn-Teller coupling, and substitutional disorder. At hole doping x = 0.5 we focus on phase competition between the ferromagnetic metal (FMM) and the charge-ordered (CO) and orbital-ordered (OO) CE state 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, while dilute strong scatterers of comparable strength convert the CE-CO-OO phase to a phase separated state with ferromagnetic metal and AFM-CO-OO clusters.

Exchange interactions and magnetic phases of transition metal oxides: benchmarking advanced ab initio methods.

The magnetic properties of the transition metal monoxides MnO and NiO are investigated at equilibrium and under pressure via several advanced first-principles methods coupled with Heisenberg Hamiltonian MonteCarlo. The comparative first-principles analysis involves two promising beyond-local density functionals approaches, namely the hybrid density functional theory and the recently developed variational pseudo-self-interaction correction method, implemented with both plane-wave and atomic-orbital basis sets. The advanced functionals deliver a very satisfying rendition, curing the main drawbacks of the local functionals and improving over many other previous theoretical predictions. Furthermore, and most importantly, they convincingly demonstrate a degree of internal consistency, despite differences emerging due to methodological details (e.g. plane waves vs. atomic orbitals)

Structural ordering and antisite defect formation in double perovskites

We formulate an effective model for B-B′ site ordering in double perovskite materials A2BB′O6. 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 A2BB′1−yB″yO6. Although our emphasis is on the double perovskites, our results are easily generalizable to a wide variety of binary and ternary alloys.

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 rA and cation disorder σA, 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.

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 La1 - xCaxMnO3.

Novel sound phenomena in superfluid helium in aerogel and other impure superfluids

During the last decade new techniques for producing impure superfluids with unique properties have been developed. This new class of systems includes superfluid helium confined to aerogel, HeII with different impurities (D2, N2, Ne, Kr), superfluids in Vycor glasses, and watergel. These systems exhibit very unusual properties including unexpected acoustic features. We discuss the sound properties of these systems and show that sound phenomena in impure superfluids are modified from those in pure superfluids. We calculate the coupling between temperature and pressure oscillations for impure superfluids and for superfluid He in aerogel. We show that the coupling between these two sound modes is governed either by c∂ρ/∂c or σρaρs (for aerogel) rather than thermal expansion coefficient ∂ρ/∂T, which is enormously small in pure superfluids. This replacement plays a fundamental role in all sound phenomena in impure superfluids. It enhances the coupling between the two sound modes that leads to the existence of such phenomena as the slow mode and heat pulse propagation with the velocity of first sound observed in superfluids in aerogel. This means that it is possible to observe in impure superfluids such unusual sound phenomena as slow pressure (density) waves and fast temperature (entropy) waves. The enhancement of the coupling between the two sound modes decreases the threshold values for nonlinear processes as compared to pure superfluids. Sound conversion, which has been observed in pure superfluids only by shock waves should be observed at moderate sound amplitude in impure superfluids. Cerenkov emission of second sound by first sound (which never been observed in pure superfluids) could be observed in impure superfluids.

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 Tc scale that agrees with quantum Monte Carlo estimates to within a few percent. We provide results on the Tc, 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.

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 and 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 ∼ 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.