Pressure-Induced Metal-Insulator Transition inLaMnO3Is Not of Mott-Hubbard Type

Abstract

Calculations employing the local density approximation combined with static and dynamical mean-field theories (LDA+U and LDA+DMFT) indicate that the metal-insulator transition observed at 32 GPa in paramagnetic LaMnO3 at room temperature is not a Mott-Hubbard transition, but is caused by orbital splitting of the majority-spin eg bands. For LaMnO3 to be insulating at pressures below 32 GPa, both on-site Coulomb repulsion and Jahn-Teller distortion are needed.PACS numbers: 71.30.+h 71.20.-b 71.27.+a Since the discovery of colossal magnetoresistance (CMR) [1], manganites have been intensively studied. The key to understand CMR is the high-temperature paramagnetic insulating-like phase, which is characterized not only by an increase of resistivity with decreasing temperature, but also by unusual dynamical properties, such as low spectral weight at the Fermi level for a wide range of doping [2,3,4]. Theoretical understanding of this hole-doped paramagnetic phase remains incomplete, and CMR transition temperatures are lower than technologically desirable.In this Letter, we shall focus on the pressure-induced insulator-metal (IM) transition in the undoped parent compound LaMnO 3 with configuration t 3 2g e g . This transition occurs at room temperature, well above the magnetic ordering temperature (T N =140 K) , well below the cooperative Jahn-Teller (JT) temperature (T oo =740 K at 0 GPa), and at a hydrostatic pressure of 32 GPa where the JT distortion appears to be completely suppressed [5]. The IM transition thus seems to be a bandwidthdriven Mott-Hubbard transition of the e g electrons and points to the dominating importance of the Coulomb repulsion between two e g electrons on the same site. This is supported by recent self-interaction-corrected local density approximation (LDA) calculations, performed, however, for the cubic structure and magnetically ordered states at low temperature [6]. Structural distortions at 0 K as functions of pressure were recently calculated with the LDA+U method [7]. On the theoretical side, it has been an issue of long debate whether the JT distortion or the Coulomb repulsion is responsible for the insulating behavior of LaMnO 3 at normal pressure. The highpressure experiment [5] seems to favor the latter.Here, we shall study the room-temperature electronic structure of LaMnO 3 at normal pressure and the pressure-induced IM transition by means of LDA + U [8] and LDA + dynamical mean field theory (DMFT) [9] calculations. Upon going from the insulating to the metallic, high-pressure phase, we shall find that the orbital polarization and the concomitant splitting of the two majority-spin e g bands are gradually reduced. The IM transition takes place when the bands start to overlap. Since this occurs within the (orbitally) symmetry-broken phase, this IM transition is not a Mott-Hubbard transition. The Coulomb interaction, as well as the JT distortion are needed for a proper description of this transition and the insulating nature of LaMnO 3 .The orthorhombic crystal structure of LaMnO ...

Pressure-induced phase transitions and metallization inVO2

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Computation of correlation-induced atomic displacements and structural transformations in paramagneticKCuF3andLaMnO3

“…The overall e g band width is about 2.8-3.0 eV, which is remarkably much smaller than the estimates of the Coulomb interaction parameter U found in the literature. 59,61 In Fig. 8 we display our results for the GGA total energy as a function of the JT distortion δ JT .…”

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Bandwidth-driven nature of the pressure-induced metal state of LaMnO 3

“…As far as theoretical approaches are concerned, ab initio calculations using soft pseudo-potentials support the conservation of a local distortion up to IM transition pressure 6,7,16,38 . Trimarchi and coworkers predicted the occurrence of a structural phase transition around 15 GPa leading to a situation with a mixture of poly-types of antiferromagnetic order 16 .…”