1,721,055 research outputs found
Double-exchange driven ferromagnetic metal-paramagnetic insulator transition in Mn-doped CuO
No full textEmploying ab initio self-interaction-corrected local-spin-density calculations, we explain the nature of the ferromagnetic, metallic phase of Mn-doped CuO (an antiferromagnetic insulator when undoped), and of its concurrent transitions to a paramagnetic, insulating phase. Mn-induced donor levels enable conduction through ferromagnetically aligned Mn centers and ferromagnetic CuO planes via double exchange. In the paramagnetic insulating phase, a polaron hopping mechanism consistent with the experiments is envisaged. Our results suggest the intriguing possibility of designing double-exchange driven ferromagnetic cuprates
Magnetic ordering under strain and spin-peierls dimerization in GeCuO3
No full textStudying from first principles the competition between ferromagnetic (FM) and antiferromagnetic (AF) interactions in the charge-transfer-insulator GeCuO3, we predict that a small external pressure should switch the uniform AF ground state to FM, and estimate (using exchange parameters computed as a function of strain) the competing AF couplings and the transition temperature to the dimerized spin-Peierls state. Although idealized as a one-dimensional Heisenberg antiferromagnet, GeCuO3 is found to be influenced by nonideal geometry and side groups
Magnetic ordering in CuO from first principles: A cuprate antiferromagnet with fully three-dimensional exchange interactions
No full textWe investigate the interplay of bonding and magnetism in CuO by a first-principles self-interaction-free density-functional approach. Our analysis reveals that, at variance with typical low-dimensional cuprates, a fully three-dimensional view of the exchange interactions is needed to describe accurately the magnetic ground state and low-energy excitations in CuO. The apparent one-dimensional behavior of antiferromagnetic order is due to the presence of a single spin-polarized hole of d(z)(2) character. This induces a strongly anisotropic magnetic ordering built up by ferromagnetic (x, y) layers, and antiferromagnetic chains along z, with exchange interactions of similar magnitude
Metal-insulator transitions and singlet polarons in one-dimensional Ca(2+x)Y(2-x)Cu(5)O(10)
No full textOne-dimensional dopable Ca(2+x)Y(2-x)Cu(5)O(10) is characterized by the presence of Zhang-Rice singlets (ZRSs) in the electronic ground state and a series of magnetic and metal-insulating phase transitions. With the use of unconventional first-principles calculations suited to the study of strongly correlated materials, we describe the change in the electronic and magnetic properties through the whole doping range (up to one hole per CuO(2) unit). Singlet polarons, whose space and energy distributions are described in detail, are found by our calculations to be stable above a concentration of similar to 0.3 holes per Cu. We give a sound interpretation of the observed insulating behavior and loss of antiferromagnetic ordering at low doping and predict unusual properties in the doping region, where experimental data are not yet available. The system is antiferromagnetic and hopping conducting for x/n up to about 0.25, gapped around 0.3-0.4, metallic from 0.4 to 1, and gapped and ZRS-saturated at x/n=1
Fully Spin-Polarized, Two-Dimensional Hole and Electron Gases at the Highly Polar (111) EuTiO3/SrTiO3 Interface
Full text linkThe first experimental observation of a two-dimensional hole gas in (001) SrTiO3/LaAlO3/SrTiO3 heterostructures with carrier mobilities of similar to 10(3) cm(2)/(V s) at low temperature has opened innovative avenues for future oxide electronics and demonstrated that the elimination of point defects is instrumental to achieving spontaneously confined hole carriers. Stimulated by these findings, we analyze the highly polar (111) orientation of the SrTiO3 and EuTiO3/SrTiO3 structures. Using first-principles calculations, we show that tightly confined and fully spin-polarized hole and electron gases can be realized at the p-doped and n-doped terminations of these structures, respectively. The high polarity is key to achieving strong, spontaneous carrier attraction toward the surface, provided that the macroscopic fields generated by the polarity in the interior of the material are efficiently screened out. Our findings present the (111) EuTiO3/SrTiO3 interface as a credible material for innovative spintronic applications, even capable of outperforming its (001) counterpart
Faceting and stress of missing-row reconstructed transition-metal (110) surfaces
No full textWe present ab initio total energy and stress calculations for the unreconstructed and (2x1)-missing-row reconstructed Ir (110) and Rh (110) surfaces. We then use those results to set up a model rationalizing the (2x1) reconstruction as a faceting transition to a long-wavelength-corrugated (111)-like surface. Next, we discuss the qualitative extension of such model to a general nx1 reconstruction, using ab initio results, elasticity theory, and classical dynamics simulations for Al (110). Remarkably, despite the severe inherent limitations of the model, the 3x1 structure is found to be the most stable for Ir. Finally, we use the stress density to analyze the stress increase upon reconstruction, and find it to be due to a changed balance of tensile and compressive contributions in the near-surface region, which closely matches previous interpretations of the reconstruction mechanism. We conclude that, as for the (100) surface, the reconstruction basically originates from the strong relativistic contraction effects on the electronic structure of end-of-series 5d metals. [S0163-1829(99)04643-3]
Theory and applications of the stress density
No full textDrawing on the theory of quantum-mechanical stress, we introduce the stress density in density-functional theory, and give specific prescriptions for its practical and efficient implementation in the plane-wave ultrasoft pseudopotential method within the local-density approximation. In analogy with the Chetty-Martin energy density, the stress density provides a spatial resolution of the contributions to the integrated macroscopic stress tensor. While this resolution is inherently nonunique (gauge dependent), there exist gauge-independent ways of using it in practice. Here we adopt the following ones: (a) calculating integrated macroscopic stresses over appropriately defined parts of a system; (b) analyzing macroscopic averages of the stress density; (c) analyzing changes in the stress density in response to external perturbation. The abilities of the stress density are demonstrated for a set of representative test cases from surface and interface physics: in perspective, the stress density emerges as vastly more powerful and predictive than the integrated macroscopic stress
Self-interaction-free density-functional band theory for magnetic cuprates
No full textWe overview the capability of the pseudo-SIC -a self-interaction-free density-functional approach- to explore the electronic and magnetic properties of cuprates. We consider the relevant test cases of CuO, GeCuO3, and YBa2Cu3O6, and give evidence that this approach provides a description in qualitatively improved agreement with the experiments. (c) 2006 Elsevier B.V. All rights reserved
Coexistence of ionic and metallic bonding in noble-metal oxides
No full textWe investigate the nature of bonding in noble-metal oxides Cu2O, Ag2O, and Au2O using a combination of first-principles density-functional-based methodologies including self-interaction correction, polarization theory, and the electron localization function. A long-standing question concerns the type of bonding operating in these compounds. They are prevailingly ionic, but their observed cubic structure is stabilized by weak cation-cation interactions whose nature is not fully understood. A currently credited picture is that electron promotion out of the filled cation d shells produces nonspherical charge distributions on the cations, which interact in a van der Waals fashion according to some or form covalent cation-cation bonds according to others. In this work we give a detailed and consistent picture of bonding in Cu2O (and related compounds) at some variance with that hitherto assumed. First, we show that no trace of interstitial covalency is to be found, which negates the covalent bonding hypothesis. Second, we show the cation on-site s-d hybridization causes only a marginal deviation from the exact sphericity of the electron charge and contributes negligibly to the stabilization of the cubic phase. Third, we show that the cubic phase is stable because of incomplete ionic charge transfer, leaving on the cation an excess of delocalized charge which establishes weakly metallic cation-cation bonding
Hybrid perovskites for photovoltaics: Insights from first principles
No full textThe methylammonium lead iodide perovskites at the core of recently proposed solar cells with exceptionally large quantum conversion efficiency are studied by first-principles methods. Large absorption coefficients (0.03-0.04 nm-1 for wavelength ∼500 nm) and small effective masses suited for both n-type and p-type transport are obtained as a consequence of their peculiar structural and electronic characteristics. In particular, the presence of a direct gap between highly dispersed Pb(6s)-I(5p) valence bands and Pb(6p) conduction bands is the key ingredient at the basis of their excellent performance in photovoltaic applications. © 2014 American Physical Society
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