Session U20: Focus Session: Metal-Insulator Transition and Electron Phonon Coupling in Perovskites

'CMR' Manganites: Strongly or Weakly Correlated?

A newly developed ``semiclassical impurity solver'' [1] is used to perform dynamical mean field calculations of the kinetic energy, optical conductivity and magnetic transition temperature of the two-orbital double-exchange model for colossal magnetoresistance manganites, including the full Kanamori ($U-U'-J$) interactions for the $e_g$ multiplet as well as the $e_g-t_{2g}$ Hunds coupling $J_H$. The effective correlation strength is shown to be weak in the ferromagnetic ground state, while in the high temperature paramagnetic state the multiplet interactions block many of the possible final states, leading to an effectively strongly correlated situation characterized by a large effective $J_H$.. \newline [1] S. Okamoto et. al., Phys. Rev. {\bf B71} 235113 (2005).   

Resistivity peak in the one-band double-exchange model with cooperative phonons

We present the results of Monte Carlo simulations of a single-band double-exchange model with cooperative phonons, both with and without quenched disorder, at a carrier density $n$=0.3. Both in two and three dimensions, the simulations reveal a peak in the resistivity at $T_{\rm Curie}$, in agreement with previous studies by other groups. The peak gets destroyed upon the application of an external field, resulting in a large magnetoresistance. The results are in good agreement with experiments involving CMR manganites. Studies at other densities are also presented, and an intuitive picture for the presence of the resistivity peak is discussed.   

Exchange Striction and Heat Conduction in Ca$_{1-y}$Sr$_y$MnO$_3$ ($0\leq y\leq 0.5$)

CaMnO$_3$, a G-type antiferromagnet with orthorhombic structure, exhibits a substantial enhancement of its thermal conductivity$^{a,b}$ ($\kappa$) for $T [Preview Abstract]

Strong current dependence of resistivity in CaMnO$_3$

The perovskite manganite CaMnO$_3$ (CMO) has a G-type antiferromagnetic ground state with N\'eel temperature $T_N$=125K. Prior transport measurements in the magnetic and paramagnetic phases$^{a}$ establish that CMO is a $n$-type semiconductor with $n\sim 10^{18}-10^{19} {\rm cm}^{-3}$ (from native defects like oxygen vacancies) and modestly heavy (large- polaron) mass, $m^{\ast}\sim 10m_0$. Here we report transport measurements on single crystal and polycrystalline CMO which reveal a strong current dependence of the resistivity ($\rho$) at low temperatures where $\rho >10^6\ \Omega\ {\rm cm}$ and impurity-band conduction predominates. For example, at 30~K, $\rho$ decreases by an order of magnitude for small current densities ($J<100\ \mu$A/cm$^2$), indicating that the effect is not associated with Joule heating. The possible role of spin-polarized hopping in this phenomenon will be discussed.\hfill \vskip .05in \noindent $^a$ J. L. Cohn, C. Chiorescu, and J. J. Neumeier, Phys. Rev. B {\bf 72}, 024422 (2005).   

On the nature of the pressure-induced insulator-to-metal transition in LaMnO$_3$

Since the discovery of colossal magnetoresistance, manganites have been intensively studied. We focus on the pressure induced insulator-to-metal (IM) transition which was found experimentally~[1] in the undoped parent compound LaMnO$_3$ with configuration $t^3_{2g}e^1_g$. This transition occurs at room temperature (above $T_N$=140 K) and at a hydrostatic pressure of 32 GPa where the Jahn-Teller distortion appears to be completely suppressed~[1]. The IM transition thus seems to be a bandwidth-driven Mott-Hubbard transition of the $e_g$ electrons. We employ the local density approximation combined with static and dynamical mean-field theories (LDA+$U$ and LDA+DMFT) and conclude that the IM transition observed at 32 GPa in paramagnetic LaMnO$_3$ at room temperature is {\it not} a Mott transition, but is caused by the overlap of the majority-spin $e_g$ bands, orbitally polarized by the Coulomb repulsion. Crucial are also the bandwidth reduction of $\sim0.6$ and $2/3$ arising from, respectively, the gap generated by the crystal-field splitting and the random, spatially uncorrelated direction of the $t_{2g}$ spin at room temperature. [1] I. Loa {\it et al}., Phys. Rev. Lett. {\bf 87}, 125501 (2001).   

Double-exchange driven metal-insulating transition in Mn-doped CuO

Doping antiferromagnetic CuO with Mn causes a uncommon metal-insulating transition where the low-temperature ($T < T_c$=80 K) phase is ferromagnetic, with a large but metallic-like resistivity, while the high-temperature phase is paramagnetic and insulating, but with a resistivity typical of Mott insulators in the hopping conducting regime. Applying a first-principles, self-interaction corrected local spin density approach, we are able to understand and rationalize this puzzling behavior: each doping Mn in CuO acts as a single donor, inducing a double-exchange driven metallic regime and a Mn-Mn ferromagnetic allignement. Nicely, here double-exchange can also work at rather low Mn concentrations since carriers can freely flow within the CuO (x,y) planes and only need the Mn assistance to move through the c axis. In the antiferromagnetic phase the system is insulating, but a polaron hopping conductivity through a few meV-wide Coulomb gap is envisaged. This scenario depicts the intriguing possibility of designing double-exchange driven ferromagnetic cuprates.   

Computational Studies of Magnetoresistance in Double-Exchange-Based Models for Manganites

Double-exchange-based models for manganites are studied using Monte Carlo techniques, with both exact-diagonalization and polynomial-expansion of the fermionic sector. One and two- orbital models are investigated, with and without phonons. We focus our analysis on the study of the resistance (R) vs. temperature (T) using the Landauer formalism. Highlights of investigations by our group and others are: (1) R vs. T curves, parametric with magnetic fields, that closely resemble experimental data for the case of one orbital and considering cooperative phonons; and (2) an insulator to bad metal transition induced by quenched disorder for the two-orbital model. In spite of these positive results, we remark that this is just the beginning of a frontal attack to the manganite problem using realistic models and efficient algorithms that scale linearly with the lattice volume. Future directions and open problems will also be discussed.   

Neutron Scattering Study of Electron- Phonon Coupling in La$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ Perovskite

La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta }$ compounds are reported to have an unusual magneto-structural transition at low temperatures. Below $\sim $210$K$, it is proposed that charge disproportionation occurs according to 2Fe$^{4+}=>$Fe$^{3+}$+Fe$^{5+}$, thereby creating different valence on the iron sites. The different iron valences order along the body diagonal [111]$_{c, }$resulting in a change in crystal structure from rhombohedral to orthorhombic and antiferromagnetic ordering. Inelastic neutron scattering was used to determine the effect of simultaneous charge and magnetic ordering on the phonon and spin wave excitations. We find that the high frequency oxygen phonons ($\sim $80 meV) soften above the transition by several meV. Spin wave excitations appear below the transition with a characteristic energy of 50 meV at the Brillouin zone boundary. The result and relationship between the charge ordering and the electron-phonon interaction are discussed.   

Magnetic polarons in a one-dimensional antiferromagnetic chain

We present several results concerning magnetic polarons in a double-exchange system. We use a simplified model consisting on an antiferromagnetic (AF) chain doped with donor impurities. First, we study the ``back-effect'' in the AF background of a conduction electron bound to its donor impurity. We show that a region with extended spin distortions appear in the AF structure, similar to a domain wall and stabilizing the magnetic polaron. Second, we discuss the effect of these distortions in a system doped with a finite density of donor impurities. We show that from a critical density a non-trivial energy dependence on conduction electrons density appears, which can be interpreted as an attractive interaction between magnetic polarons. Third, we analyze temperature evolution of such a system of magnetic polarons. We show that they remain stable up to rather high temperatures, larger than the N\'eel temperature of the undoped chain. Our results may be relevant to understand the physics of low-doped manganites.   

Zener Polarons Ordering Variants Induced by A-Site Ordering in Half-Doped Manganites

Zener Polaron (ZP) ordering [1] provides a still polemic [2] and elusive interpretation of the charge ordering (CO) phenomenon in A site disordered half doped (A$_{1/2}$Ca$_{1/2}$) MnO$_3$, which is classically pictured by the Goodenough model (GM) of Mn$^{3+}$ and Mn$^{4+}$ CO [3,4]. ZP ordering considers instead the ordering of pre-formed ferromagnetic Mn pairs sharing an charge and keeping Mn in a Mn$^{+3.5}$ valence state. The recently synthesized A site cation ordered ABaMn$_2 $O$_6$ were shown to not present the generic magnetic CE state found of (A$_{1/2}$Ca$_{1/2}$)MnO$_3$ [5]. We present our magnetic structure determination of YBaMn$_2$O$_6$: the non- collinear magnetic order obtained unexpectedly reveals ferromagnetic plaquettes of four Mn attributable to larger 4-Mn ZPs, whose presence additionally fits very well the effective paramagnetic moments inferred from susceptibility measurements. The results unambiguously reveal the possible existence of ZP ordering variant in charge ordered manganites. [1] A. Daoud-Aladine et al., Phys. Rev. Lett. 89, 097205 (2002) [2] S. Grenier et al., Phys. Rev. B 69, 134419 (2004) [3] J. B. Goodenough, Phys. Rev. 100, 564 (1955) [4] P.G. Radaelli et al., Phys. Rev. B, 55, 3015 (1997) [5] T. Arima et al., Phys. Rev. B 66, 140408 (2002)   

Evolution of the local Jahn-Teller distortion across the phase diagram of La$\bf _{1-x}$Ca$\bf _x$MnO$\bf _3$ ($0 \le x \le 0.5$)

We report on the most comprehensive study to date of the {\it local} Jahn-Teller (JT) distortion across the phase diagram of the colossal magnetoresistive (CMR) La$\bf _{1-x}$Ca$\bf _x$MnO$\bf _3$ ($0 \le x \le 0.5$). The local structure has been measured, using the neutron powder diffraction based atomic pair distribution function (PDF) approach, over the wide temperature and Ca-doping range. These results are compared to the conventional crystallographic results obtained by Rietveld analysis. The results will be compared with both homogeneous and inhomogeneous models of the electronic structure. The magnitude of the {\it local} JT distortion is quantified over the entire phase diagram. In agreement with earlier work, we see the local JT distortion disappear in the metallic phase. However, in contrast with some earlier studies, we show that in the insulating phases the magnitude of the JT distortion decreases with increasing doping, becoming constant at higher doping. This new result should be incorporated in theoretical models of CMR manganites.   

Structural and Transport Study of La$_{0.9}$MnO$_{3}$ Under Pressure.

The detailed transport and structural properties of the self-doped system La$_{0.9}$MnO$_{3}$ have been studied under hydrostatic pressures extending to 6GPa and 11GPa, respectively.~ Like the doped manganite systems previously studied by our group, in the La deficiency (x=0.9) system a maximum shift of the peak resistivity is obtained at around 3.4GPa.~~ A crossover from metallic to insulating behavior is observed above this optimum pressure.~ The x-ray diffraction study results show that the self-doped system is compatible with a single phase of monoclinic space group up to 11 GPa. The detailed changes in the lattice parameters, volume, bond distances and bond angles have been obtained.~Comparisons with pressure dependent measurements on the doped manganites will be made.~ This work was supported by NSF DMR-0209243 and DMR-0512196.   

A quantum phase transition in the monoxides of the first transition series

The monoxides of the 3d metals (MO; M= Ti to Ni) provide an isostructural series (cubic, Fm $\bar{3}$m) in which to study the transition from metallic paramagnetism (TiO) to insulating antiferromagnetism (MnO, FeO, CoO, NiO). But the transition is not smooth, and while CrO has eluded synthesis over the years, the intrinsic properties of VO are under discussion. Here we present strong experimental and computational evidence that VO is a strongly correlated metal with Non-Fermi Liquid low temperature thermodynamics, a pseudogap in the density of states and an unusually strong spin-lattice coupling. All these properties are interpreted as signatures of the proximity to a magnetic quantum phase transition. Interestingly, TiOx displays a superconducting transition with a dome-shape dependence of the superconducting critical temperature with doping in x (0.8$<$x$<$1.2). The analogies with the phase diagram of the High Tc cuprates and their structural and electronic simplicity makes 3d monoxides ideal candidates to make progress in the understanding of correlated electron systems.