G. Venketeswara Pai

Theory of insulator metal transition and colossal magnetoresistance in doped manganites.

The persistent proximity of insulating and metallic phases, a puzzling characteristic of manganites, is argued to arise from the self-organization of the twofold degenerate e(g) orbitals of Mn into localized Jahn-Teller (JT) polaronic levels and broad band states due to the large electron-JT phonon coupling present in them. We describe a new two band model with strong correlations and a dynamical mean-field theory calculation of equilibrium and transport properties. These explain the insulator metal transition and colossal magnetoresistance quantitatively, as well as other consequences of two state coexistence.

Raman scattering in charge-ordered Pr0.63Ca0.37MnO3: Anomalous temperature dependence of linewidth

We report on the evolution of the Raman-active Ag phonon modes in the charge-ordered manganite Pr0.63Ca0.37MnO3 as a function of temperature from 300 K to 25 K. Our studies reveal that the linewidths of the Ag(2) and Ag(4) phonons increase as temperature decreases. This anomalous temperature dependence of phonon lineshapes, seen for the first time in charge-ordered manganites, can be quantitatively understood in terms of a strong spin-phonon coupling involving t2g spins and Ag phonons.

Magnetic phases of electron-doped manganites

We study the anisotropic magnetic structures exhibited by electron-doped manganites using a model which incorporates the double exchange between orbitally degenerate eg electrons and the superexchange between t2g electrons with realistic values of the Hund’s coupling (JH), the superexchange coupling (JAF), and the bandwidth (W). We look at the relative stabilities of the G-, C-, and A-type antiferromagnetic phases. In particular we find that the G phase is stable for low electron doping as seen in experiments. We find good agreement with the experimentally observed magnetic phase diagrams of electron-doped manganites (x.0.5) such as Nd12xSrxMnO3, Pr1 2xSrxMnO3, and Sm12xCaxMnO3. We can also explain the experimentally observed orbital structures of the C and A phases. We also extend our calculation for electron-doped bilayer manganites of the form R222xA112xMn2O7 and predict that the C phase will be absent in these systems due to their reduced dimensionality.

Nonlinear current of strongly irradiated quantum Hall gas

Two dimensional electrons in weakly disordered high Landau levels are considered. The current-field response in the presence of a strong microwave field, is computed. The disordered Floquet evolution operator allows us to treat the short range disorder perturbatively, at any strength of electric fields. A simplifying Random Matrix Approximation reproduces the broadened Landau levels density of states and structure factor. We derive the magnitude of the Microwave Induced Resistivity Oscillations. The disorder short wavelength cut-off determines the non-linear electric fields of the Zero Resistance State and the Hall Induced Resistivity Oscillations. We discuss wider implications of our results on experiments and other theories.

Zero-temperature insulator-metal transition in doped manganites

We study the transition at T = 0 from a ferromagnetic insulating to a ferromagnetic metallic phase in manganites as a function of hole doping using an effective low-energy model Hamiltonian proposed by us recently. The model incorporates the quantum nature of the dynamic Jahn-Teller (JT) phonons strongly coupled to orbitally degenerate electrons as well as strong Coulomb correlation effects, and leads naturally to the coexistence of localized (JT polaronic) and band-like electronic states. We study the insulator-metal transition as a function of doping as well as of the correlation strength U and JT gain in energy EJT, and find, for realistic values of parameters, a ground-state phase diagram in agreement with experiments. We also discuss how several other features of manganites as well as differences in behaviour among manganites can be understood in terms of our model.

Theory of Manganites Exhibiting Colossal Magnetoresistance

The electronic properties of many transition metal oxide systems require new ideas concerning the behaviour of electrons in solids for their explanation. A recent example, subsequent to that of cuprate superconductors, is of rare earth manganites doped with alkaline earths, namely Re 1-x A x MnO 3 , which exhibit colossal magnetoresistance, metal insulator transition and many other poorly understood phenomena. Here we show that the strong Jahn Teller coupling between the twofold degenerate (d x 2 -y 2 and d 3z 2 -r 2 )e g orbitals of Mn and lattice modes of vibration (of the oxygen octahedra surrounding the Mn ions) dynamically reorganizes the former into a set of states (which we label l) which are localized with large local lattice distortion and exponentially small intersite overlap, and another set (labelled b) which form a broad band. This hitherto unsuspected but microscopically inevitable coexistence of radically different l and b states, and their relative energies and occupation as influenced by doping x, temperature T, local Coulomb repulsion U etc., underlies the unique effects seen in manganites. We present results from strong correlation calculations using the dynamical mean-field theory which accord with a variety of observations in the orbital liquid regime (say, for 0.2 ≲ x ≲ 0.5).We outline extensions to include intersite l coherence and spatial correlations/long range order.

Colossal magnetoresistance manganites: A new approach ¶

Manganites of the LA1−xCaxMnO3 family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach recently proposed by us, where we argue that due to strong Jahn-Teller (JT) coupling with phonons the twofold degenerateeg states at the Mn sites dynamically reorganize themselves into localised, JT polaronsl with exponentially small inter-site hopping, and band-like, nonpolaronic statesb, leading to anew 2-band model for manganites which includes strong Coulomb and Hund’s couplings. We also discuss some results from a dynamical mean-field theory treatment of the model which explains quantitatively a wide variety of experimental results, including insulator-metal transitions and CMR, in terms of the influence of physical conditions on the relative energies and occupation of thel andb states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and spatial correlation is the key to manganite physics.

Small Polarons in Dense Lattice Systems

There is considerable evidence for the persistence of small polaron like entities in colossal magnetoresistance oxides, which are dense electronic systems with electron density n≲1 per site. This has brought up again the question of whether and how small (narrow band) polaronic states survive in a dense electronic system. We investigate this question in a simple one band Holstein polaron model, in which spinless electrons on a tight binding lattice cause an on-site lattice distortion x0. In the small polaron limit, each electron is localized, and the electron hopping tij is neglected. We develop a systematic approach in powers of tij, identify classical t0, quantum mean field t1, and quantum fluctuation t2 terms, and show that the last two terms are relatively small, even for dense systems, so long as the narrowed polaron bandwidth t*=t exp(−u) is much smaller than the Einstein phonon energy ħω0. (Here u=(x20/2x2zp) with xzp being the zero point phonon displacement.) The relevance of these results for CMR oxides is briefly discussed.