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 $^{52}$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

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 $e_{g}$ electrons and the super-exchange between $t_{2g}$ electrons with realistic values of the Hund's coupling($J_H$), the super-exchange coupling($J_{AF}$), and the bandwidth($W$). 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 ($x > 0.5$) such as Nd$_{1-x}$Sr$_{x}$MnO$_{3}$, Pr$_{1-x}$Sr$_{x}$MnO$_{3}$, and Sm$_{1-x}$Ca$_{x}$MnO$_{3}$. 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 R$_{2-2x}$A$_{1+2x}$Mn$_2$O$_7$ 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 $n^{<}_{\approx}1$ 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 $x_0$. 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 $t^0$, quantum mean field $t^1$, and quantum fluctuation $t^2$ 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 $\hbar \omega _0$. (Here u=$(x^2{} _0 /2x ^2_{}z_p)$ with x_z_p being the zero point phonon displacement.) The relevance of these results for CMR oxides is briefly discussed

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 0 . (Here u=(x20 /2x2zp) with xzp being the zero point phonon displacement.) The relevance of these results for CMR oxides is briefly discussed

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 Pr0.63Ca0.37MnO3Pr_{0.63} Ca_{0.37}MnO_3: anomalous temperature dependence of linewidth

We report on the evolution of the Raman-active $A_g$ phonon modes in the charge-ordered manganite $Pr_{0.63} Ca_{0.37}MnO_3$as a function of temperature from 300 K to 25 K. Our studies reveal that the linewidths of the $A_g(2)$ and $A_g(4)$ 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 t_2_g spins and $A_g$ phonons