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A Band Theory Perspective on Molecular Orbitals in Complex Oxides  [PDF]
Kateryna Foyevtsova, George A. Sawatzky
Journal of Modern Physics (JMP) , 2019, DOI: 10.4236/jmp.2019.108062
Abstract: In view of the growing interest in molecular orbitals (MOs) encountered in certain complex oxides, we review some of their properties from the band theory perspective and provide detailed examples based on real materials. Our discussion includes some technical aspects of identifying MOs in electronic structure calculations and considers cases when MOs can be both orthogonal and non-orthogonal. We also describe orthonormalization of MOs, a procedure converting them into Wannier functions, and discuss the problem of Wannier functions possibly being rather spatially extended and how using MO, rather than atomic orbital, based effective Hamiltonians might be a better choice in describing certain strongly correlated systems as well as systems with strong electron-phonon coupling. Furthermore, we address the problem of strongly correlated MOs and how it can be treated in band theory calculations.
Molecular beam epitaxy growth of SrO buffer layers on graphite and graphene for the integration of complex oxides  [PDF]
Adam Ahmed,Hua Wen,Taisuke Ohta,Igor Pinchuk,Tiancong Zhu,Thomas Beechem,Roland Kawakami
Physics , 2015,
Abstract: We report the successful growth of high-quality SrO films on highly-ordered pyrolytic graphite (HOPG) and single-layer graphene by molecular beam epitaxy. The SrO layers have (001) orientation as confirmed by x-ray diffraction (XRD) while atomic force microscopy measurements show continuous pinhole-free films having rms surface roughness of <1.5 {\AA}. Transport measurements of exfoliated graphene after SrO deposition show a strong dependence between the Dirac point and Sr oxidation. Subsequently, the SrO is leveraged as a buffer layer for more complex oxide integration via the demonstration of (001) oriented SrTiO3 grown atop a SrO/HOPG stack.
Composite Polarons in Ferromagnetic Narrow-band Metallic Manganese Oxides  [PDF]
Liang-Jian Zou,H. Q. Lin,Qing-Qi Zheng
Physics , 1998, DOI: 10.1088/0953-8984/9/41/009
Abstract: A new mechanism is proposed to explain the colossal magnetoresistance and related phenomena. Moving electrons accompanied by Jahn-Teller phonon and spin-wave clouds may form composite polarons in ferromagnetic narrow-band manganites. The ground-state and finite-temperature properties of such composite polarons are studied in the present paper. By using a variational method, it is shown that the energy of the system at zero temperature decreases with the formation of composite polaron; the energy spectrum and effective mass of the composite polaron at finite temperature is found to be strongly renormalized by the temperature and the magnetic field. It is suggested that the composite polaron contribute significantly to the transport and the thermodynamic properties in ferromagnetic narrow-band metallic manganese oxides.
Polarons, free charge localisation and effective dielectric permittivity in oxides  [PDF]
Mario Maglione
Physics , 2010,
Abstract: This review will deal with several types of free charge localisation in oxides and their consequences on the effective dielectric spectra of such materials. The first one is the polaronic localisation at the unit cell scale on residual impurities in ferroelectric networks. The second one is the collective localisation of free charge at macroscopic interfaces like surfaces, electrodes and grain boundaries in ceramics. Polarons have been observed in many oxide perovskites mostly when cations having several stable electronic configurations are present. In manganites, the density of such polarons is so high as to drive a net lattice of interacting polarons. On the other hand, in ferroelectric materials like BaTiO3 and LiNbO3, the density of polarons is usually very small but they can influence strongly the macroscopic conductivity. The contribution of such polarons to the dielectric spectra of ferroelectric materials is described. Even residual impurities as for example Iron can induce well defined anomalies at very low temperatures. This is mostly resulting from the interaction between localised polarons and the highly polarisable ferroelectric network in which they are embedded. The case of such residual polarons in SrTiO3 will be described in more details, emphasizing the quantum polaron state at liquid helium temperatures. Recently, several non-ferroelectric oxides have been shown to display giant effective dielectric permittivity. It is first shown that the frequency/temperature behaviour of such parameters is very similar in very different compounds (donor doped BaTiO3, CaCu3Ti4O12, LuFe2O4,Li doped NiO,...). This similarity calls for a common origin of the giant dielectric permittivity in these compounds. A space charge localisation at macroscopic interfaces can be the key for such extremely high dielectric permittivity.
Impurity Conduction and Magnetic Polarons in Antiferromagnetic Oxides  [PDF]
C. Chiorescu,J. L. Cohn,J. J. Neumeier
Physics , 2007, DOI: 10.1103/PhysRevB.76.020404
Abstract: Low-temperature transport and magnetization measurements for the antiferromagnets SrMnO(3) and CaMnO(3) identify an impurity band of mobile states separated by energy E from electrons bound in Coulombic potentials. Very weak electric fields are sufficient to excite bound electrons to the impurity band, increasing the mobile carrier concentration by more than three orders of magnitude. The data argue against the formation of self-trapped magnetic polarons (MPs) predicted by theory, and rather imply that bound MPs become stable only for kT<
Spin, Orbital, and Spin-Orbital Polarons in Transition Metal Oxides  [PDF]
Krzysztof Wohlfeld
Physics , 2009, DOI: 10.1063/1.3225485
Abstract: I give a brief overview of a polaron formation in three distinct transition metal oxides: (i) spin polaron when a hole is added to the antiferromagnetic (AF) ordered plane in La$_2$CuO$_4$, (ii) orbital polaron when a hole is added to the alternating orbital (AO) ordered plane in LaMnO$_3$, and (iii) spin-orbital polaron when a hole is added to the AF and AO ordered plane in LaVO$_3$. Comparison of the distinct features of the above polarons can shed some light on the basic differences between the experimental phase diagrams of the lightly doped transition metal oxides La$_{2-x}$Sr$_x$CuO$_4$, La$_{1-x}$Sr$_x$MnO$_3$, and La$_{1-x}$Sr$_{x}$VO$_3$.
Nonlinear Insulator in Complex Oxides  [PDF]
Z. Q. Liu,D. P. Leusink,W. M. Lü,X. Wang,X. P. Yang,K. Gopinadhan,A. Annadi,S. Dhar,Y. P. Feng,H. B. Su,G. Xiong,T. Venkatesan,Ariando
Physics , 2010, DOI: 10.1103/PhysRevB.84.165106
Abstract: The insulating state is one of the most basic electronic phases in condensed matter. This state is characterised by an energy gap for electronic excitations that makes an insulator electrically inert at low energy. However, for complex oxides, the very concept of an insulator must be re-examined. Complex oxides behave differently from conventional insulators such as SiO2, on which the entire semiconductor industry is based, because of the presence of multiple defect levels within their band gap. As the semiconductor industry is moving to such oxides for high-dielectric (high-k) materials, we need to truly understand the insulating properties of these oxides under various electric field excitations. Here we report a new class of material called nonlinear insulators that exhibits a reversible electric-field-induced metal-insulator transition. We demonstrate this behaviour for an insulating LaAlO3 thin film in a metal/LaAlO3/Nb-SrTiO3 heterostructure. Reproducible transitions were observed between a low-resistance metallic state and a high-resistance non-metallic state when applying suitable voltages. Our experimental results exclude the possibility that diffusion of the metal electrodes or oxygen vacancies into the LaAlO3 layer is occurring. Instead, the phenomenon is attributed to the formation of a quasi-conduction band (QCB) in the defect states of LaAlO3 that forms a continuum state with the conduction band of the Nb-SrTiO3. Once this continuum (metallic) state is formed, the state remains stable even when the voltage bias is turned off. An opposing voltage is required to deplete the charges from the defect states. Our ability to manipulate and control these defect states and, thus, the nonlinear insulating properties of complex oxides will open up a new path to develop novel devices.

Chen Nianyi,Wen Yuankai,

金属学报 , 1979,
Abstract: Factors concerning with the stability of binary complex oxides has been stu-died. For complex oxides with ionic bonds,the main factors determining the heatof formation of such oxides appear to be the relevant ionic radius and the va-lency of the component elements. For oxides with partial covalency, however, thescreening parameter proposed in this paper may also be important in addition tothe factors mentioned above.
Adiabatic large polarons in anisotropic molecular crystals
Zoran Ivi , eljko Pr ulj
Journal of Research in Physics , 2011, DOI: 10.2478/v10242-012-0002-2
Abstract: We study the large polaron whose motion is confined to a single chain in a system composed of the collection of parallel molecular chains embedded in threedimensional lattice. It is found that the interchain coupling has a significant impact on the large polaron characteristics. In particular, its radius is quite larger while its effective mass is considerably lighter than that estimated within the one-dimensional models. We believe that our findings should be taken into account for the proper understanding of the possible role of large polarons in the charge and energy transfer in quasi-one-dimensional substances.
Phonon Overlaps: Polyacetylene, Polarons, and Molecular Size  [PDF]
Connie Te-ching Chang,James P. Sethna
Physics , 2006,
Abstract: We provide a theory for the effects of polarons and phonons in mediating and suppressing the quantum tunneling of electrons into single molecules of conducting polymers, motivated by experiments on molecular quantum dots. The effects of both phonons and excitations of the polaron particle-in-a-box excitations are calculated. Using both the Su-Schrieffer-Heeger (SSH) model of polyacetylene and direct density-functional theory (DFT) calculations, we calculate the suppression of ground--state to ground--state transitions and the position and strength of the side-bands.
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