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Sep 18, 2013
09/13
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Alexander Yu. Dobin; Wenhui Duan; Renata M. Wentzcovitch
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We have successfully calculated the electronic and structural properties of chromia (Cr_2O_3) in the Local Spin Density Approximation (LSDA). We predict a transformation from the corundum to the Rh_2O_3(II) structure around 15 GPa in the anti-ferromagnetic (AFM) phase as well as in the paramagnetic (PM) insulating state which occurs above the Neel temperature (T_N). This transition is relevant to interpreting the optical anomalies observed in the absorption spectrum of ruby under pressure. We...
Source: http://arxiv.org/abs/cond-mat/0006242v1
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Jul 20, 2013
07/13
by
Burak Himmetoglu; Renata M. Wentzcovitch; Matteo Cococcioni
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We investigate the electronic and structural properties of CuO, which shows significant deviations from the trends obeyed by other transition-metal monoxides. Using an extended Hubbard corrective functional, we uncover an orbitally ordered insulating ground state for the cubic phase of this material, which was expected but never found before. This insulating state results from a fine balance between the tendency of Cu to complete its d-shell and Hund's rule magnetism. Starting from the ground...
Source: http://arxiv.org/abs/1107.4399v1
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Sep 22, 2013
09/13
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Han Hsu; Peter Blaha; Matteo Cococcioni; Renata M. Wentzcovitch
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Using density functional theory plus Hubbard $U$ calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O$_3$ perovskite, a major mineral phase in the Earth's lower mantle, has high-spin ferric iron ($S=5/2$) at both the dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site iron undergoes a spin-state crossover to the low-spin state ($S=1/2$), while the A-site iron remains in the high-spin state. Our calculation shows that the B-site spin-state crossover in the...
Source: http://arxiv.org/abs/1101.3819v2
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44
Jul 20, 2013
07/13
by
Han Hsu; Peter Blaha; Renata M. Wentzcovitch
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With local density approximation + Hubbard $U$ (LDA+$U$) calculations, we show that the ferromagnetic (FM) insulating state observed in tensile-strained LaCoO$_3$ epitaxial thin films is most likely a mixture of low-spin (LS) and high-spin (HS) Co, namely, a HS/LS mixture state. Compared with other FM states, including the intermediate-spin (IS) state (\textit{metallic} within LDA+$U$), which consists of IS Co only, and the insulating IS/LS mixture state, the HS/LS state is the most favorable...
Source: http://arxiv.org/abs/1203.1861v1
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Sep 21, 2013
09/13
by
Min Ji; Koichiro Umemoto; Cai-Zhuang Wang; Kai-Ming Ho; Renata M. Wentzcovitch
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We propose three new phases of H2O under ultrahigh pressure. Our structural search was performed using an adaptive genetic algorithm which allows an extensive exploration of crystal structure. The new sequence of pressure-induced transitions beyond ice X at 0 K should be ice X - Pbcm - Pbca - Pmc21 - P21 - P21/c phases. Across the Pmc21 - P21 transition, the coordination number of oxygen increases from 4 to 5 with a significant increase of density. All stable crystalline phases have nonmetallic...
Source: http://arxiv.org/abs/1108.4164v1
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Sep 21, 2013
09/13
by
Pierre Carrier; João F. Justo; Renata M. Wentzcovitch
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The quasiharmonic approximation (QHA), in its simplest form also called the statically constrained (SC) QHA, has been shown to be a straightforward method to compute thermoelastic properties of crystals. Recently we showed that for non-cubic solids SC-QHA calculations develop deviatoric thermal stresses at high temperatures. Relaxation of these stresses leads to a series of corrections to the free energy that may be taken to any desired order, up to self-consistency. Here we show how to correct...
Source: http://arxiv.org/abs/0808.3373v1
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Sep 23, 2013
09/13
by
Tao Sun; Koichiro Umemoto; Zhongqing Wu; Jin-Cheng Zheng; Renata M. wentzcovitch
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Platinum is widely used as a pressure calibration standard. However, the established thermal EOS has uncertainties, especially in the high $P$-$T$ range. We use density functional theory to calculate the thermal equation of state of platinum, up to 550 GPa and 5000 K. The static lattice energy is computed by using the LAPW method, with LDA, PBE, and the recently proposed WC functional. The electronic thermal free energy is evaluated using the Mermin functional. The vibrational part is computed...
Source: http://arxiv.org/abs/0801.3448v2
