27 research outputs found
Colossal magnetoresistance manganites: a new approach
Manganites of the LA1-x Ca xMnO3 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 degenerate eg 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
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 E_{JT}, 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.Comment: To be published in Europhysics Letter
Density wave and supersolid phases of correlated bosons in an optical lattice
Motivated by the recent experiment on the Bose-Einstein condensation of
Cr atoms with long-range dipolar interactions (Werner J. et al., Phys.
Rev. Lett., 94 (2005) 183201), we consider a system of bosons with repulsive
nearest and next-nearest neighbor interactions in an optical lattice. The
ground state phase diagram, calculated using the Gutzwiller ansatz, shows,
apart from the superfluid (SF) and the Mott insulator (MI), two modulated
phases, \textit{i.e.}, the charge density wave (CDW) and the supersolid (SS).
Excitation spectra are also calculated which show a gap in the insulators,
gapless, phonon mode in the superfluid and the supersolid, and a mode softening
of superfluid excitations in the vicinity of the modulated phases. We discuss
the possibility of observing these phases in cold dipolar atoms and propose
experiments to detect them
Theory of Insulator Metal Transition and Colossal Magnetoresistance in Doped Manganites
The persistent proximity of insulating and metallic phases, a puzzling
characterestic 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
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 orbital
ly degenerate electrons and the super-exchange between
electrons with realistic values of the Hund's coupling(), the
super-exchange coupling(), and the bandwidth(). We look at the
relative stabilities of the G, C and A type antiferromagnetic ph ases. 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
() such as NdSrMnO, PrSrMnO,
and SmCaMnO. We can also explain the experimentally
observed orbital structures of the C a nd A phases.
We also extend our calculation for electron-doped bilayer manganites of the
form RAMnO and predict that the C-phase will be
absent in t hese systems due to their reduced dimensionality.Comment: 7 .ps files included. To appear in Phys. Rev. B (Feb 2001
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 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 . In the small polaron limit, each electron is localized, and the electron hopping t_i_j is neglected. We develop a systematic approach in powers of t_i_j, identify classical , quantum mean field , and quantum fluctuation 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 . (Here u= with x_z_p being the zero point phonon displacement.) The relevance of these results for CMR oxides is briefly discussed