Orbital occupation, local spin and exchange interactions in V2O3

We present the results of an LDA and LDA+U band structure study of the monoclinic and the corundum phases of V2O3 and argue that the most prominent (spin 1/2) models used to describe the semiconductor metal transition are not valid. Contrary to the generally accepted assumptions we find that the large on site Coulomb and exchange interactions result in a total local spin of 1 rather than 1/2 and especially an orbital occupation which removes the orbital degeneracies and the freedom for orbital ordering. The calculated exchange interaction parameters lead to a magnetic structure consistent with experiment again without the need of orbital ordering. While the low-temperature monoclinic distortion of the corundum crystal structure produces a very small effect on electronic structure of v2o3, the change of magnetic order leads to drastic differences in band widths and band gaps. The low temperature monoclinic phase clearly favors the experimentally observed magnetic structure, but calculations for corundum crystal structure gave two consistent sets of exchange interaction parameters with nearly degenerate total energies suggesting a kind of frustration in the paramagnetic phase. These results strongly suggest that the phase transitions in V2O3 which is so often quoted as the example of a S=1/2 Mott Hubbard system have a different origin. So back to the drawing board

The influence of the rare earth ions radii on the Low Spin to Intermediate Spin state transition in lanthanide cobaltite perovskites: LaCoO3 vs. HoCoO3

We present first principles LDA+U calculations of electronic structure and magnetic state for LaCoO3 and HoCoO3. Low Spin to Intermediate Spin state transition was found in our calculations using experimental crystallographic data for both materials with a much higher transition temperature for HoCoO3, which agrees well with the experimental estimations. Low Spin state t6e0 (non-magnetic) to Intermediate Spin state t5e1 (magnetic) transition of Co(3+) ions happens due to the competition between crystal field t_2g-e_g splitting and effective exchange interaction between 3$d$ spin-orbitals. We show that the difference in crystal structure parameters for HoCoO3 and LaCoO3 due to the smaller ionic radius of Ho ion comparing with La ion results in stronger crystal field splitting for HoCoO3 (0.09 eV ~ 1000 K larger than for LaCoO3) and hence tip the balance between the Low Spin and Intermediate Spin states to the non-magnetic solution in HoCoO3.Comment: 13 pages, 6 figure

Time-dependent scattering theory for Schr\"odinger operators on scattering manifolds

We construct a time-dependent scattering theory for Schr\"odinger operators on a manifold $M$ with asymptotically conic structure. We use the two-space scattering theory formalism, and a reference operator on a space of the form $R\times \partial M$, where $\partial M$ is the boundary of $M$ at infinity. We prove the existence and the completeness of the wave operators, and show that our scattering matrix is equivalent to the absolute scattering matrix, which is defined in terms of the asymptotic expansion of generalized eigenfunctions. Our method is functional analytic, and we use no microlocal analysis in this paper.Comment: 24 page

Two Aspects of the Mott-Hubbard Transition in Cr-doped V_2O_3

The combination of bandstructure theory in the local density approximation with dynamical mean field theory was recently successfully applied to V$_2$O$_3$ -- a material which undergoes the f amous Mott-Hubbard metal-insulator transition upon Cr doping. The aim of this sh ort paper is to emphasize two aspects of our recent results: (i) the filling of the Mott-Hubbard gap with increasing temperature, and (ii) the peculiarities of the Mott-Hubbard transition in this system which is not characterized by a diver gence of the effective mass for the $a_{1g}$-orbital.Comment: 2 pages, 3 figures, SCES'04 conference proceeding

The study of the canonical Watson-Crick DNA base pairs by Moller-Plesset perturbation method: the nature of their stability

Gas-phase gradient optimization was carried out on the canonical Watson-Crick DNA base pairs using the second-order Moller-Plesset (MP2) perturbation method at the 6-31G* and 6-31G*(0.25) basis sets. It is detected that full geometry optimization at the MP2 level leads to an intrinsically nonplanar propeller-twisted and buckled geometry of G-C and A-T base pairs. Morokuma-Kitaura (MK) and reduced variational space (RVS) methods of the decomposition for molecular Hartree-Fock interaction energies were used for the investigation of the hydrogen bonding in the Watson-Crick base pairs in question. It is shown that the stability of the hydrogen-bonded DNA base pairs originates mainly from electrostatic interactions. At the same time the polarization, charge transfer and dispersion interactions also make considerable contribution to the attraction energy of bases.Використовуючи теорію збурень Моллера-Плессета у другому порядку, проведено градієнтну оптимізацію канонічних уотсон-криківських пар основ у базисах 6-31G* і 6-31G*(0,25). Знайдено, що повна оптимізація геометрії на рівні МР2 спричинює внутрішньо неплоску пропелер-повернуту і вигнуту геометрію пар основ G-C і А-Т. Для дослідження водневих зв'язків в уотсон-криківських парах використано методи Мо­року ми-Кітаури та зменшеного варіаційного простору для розкладу хартрі-фоківської енергії молекулярної взаємодії. По­казано, що стабільність водневих зв'язків пар основ переважно з'являється за рахунок електростатичної взаємодії. В той же час поляризаційна і исперсійна компоненти та взаємодія з перенесенням заряду також роблять значний внесок в енергію притяжіння основ.Используя теорию возмущений Моллера-Плессета во втором порядке, проведена градиентная оптимизация канонических уотсон-криковских пар оснований в базисах 6-31G* и 6-31G* (0,25). Обнаружено, что полная оптимизация геометрии на уровне МР2 приводит к внутренне неплоской пропеллер-повернутой и изогнутой геометрии пар оснований G-C и А-Т. Для исследования водородных связей в уотсон-криковских парах оснований использованы методы Морокумы-Китауры и умень­шенного вариационного пространства для разложения хартри-фоковской энергии молекулярного взаимодействия. Показано, что стабильность водородно-связанных пар оснований возни­кает, как правило, благодаря электростатическим взаимодей­ствиям. В то же время поляризационная и дисперсионная компоненты, а также взаимодействие с переносом заряда в свою очередь вносят значительный вклад в энергию притяже­ния оснований

Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory

We use first principles density functional theory to calculate electronic and magnetic properties of TiF3 using the full potential linearized augmented plane wave method. The LDA approximation predicts a fully saturated ferromagnetic metal and finds degenerate energy minima for high and low symmetry structures. The experimentally observed Jahn-Teller phase transition at Tc=370K can not be driven by the electron-phonon interaction alone, which is usually described accurately by LDA. Electron correlations beyond LDA are essential to lift the degeneracy of the singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are important, the direction of the t2g-level splitting is determined by the dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic insulator with an orbitally ordered ground state. The input parameters U=8.1 eV and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on the TiF$_6^{2-}$ ion using the molecular NRLMOL code. We estimate the Heisenberg exchange constant for spin-1/2 on a cubic lattice to be approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.

Orbital Ordering in Paramagnetic LaMnO3 and KCuF3

{\it Ab-initio} studies of the stability of orbital ordering, its coupling to magnetic structure and its possible origins (electron-phonon and/or electron-electron interactions) are reported for two perovskite systems, LaMnO$_3$ and KCuF$_3$. We present a new Average Spin State (ASS) calculational scheme that allowed us to treat a paramagnetic state. Using this scheme, we succesfully described the experimental magnetic/orbital phase diagram of both LaMnO$_3$ and KCuF$_3$ in crystal structures when the Jahn-Teller distortions are neglected. Hence, we conclude that the orbital ordering in both compounds is purely electronic in origin.Comment: 10 pages, 5 figure

On-site Coulomb interaction and the magnetism of (GaMn)N and (GaMn)As

We use the local density approximation (LDA) and LDA+U schemes to study the magnetism of (GaMn)As and (GaMn)N for a number of Mn concentrations and varying number of holes. We show that for both systems and both calculational schemes the presence of holes is crucial for establishing ferromagnetism. For both systems, the introduction of $U$ increases delocalization of the holes and, simultaneously, decreases the p-d interaction. Since these two trends exert opposite influences on the Mn-Mn exchange interaction the character of the variation of the Curie temperature (T$_C$) cannot be predicted without direct calculation. We show that the variation of T$_C$ is different for two systems. For low Mn concentrations we obtain the tendency to increasing T$_C$ in the case of (GaMn)N whereas an opposite tendency to decreasing T$_C$ is obtained for (GaMn)As. We reveal the origin of this difference by inspecting the properties of the densities of states and holes for both systems. The main body of calculations is performed within a supercell approach. The Curie temperatures calculated within the coherent potential approximation to atomic disorder are reported for comparison. Both approaches give similar qualitative behavior. The results of calculations are related to the experimental data.Comment: to appear in Physical Review

Protecting the past for the public good: archaeology and Australian heritage law

Archaeological remains have long been recognised as fragile evidence of the past, which require protection. Legal protection for archaeological heritage has existed in Australia for more than thirty years but there has been little analysis of the aims and effectiveness of that legislation by the archaeological profession. Much Australian heritage legislation was developed in a period where the dominant paradigm in archaeological theory and practice held that archaeology was an objective science. Australian legislative frameworks continue to strongly reflect this scientific paradigm and contemporary archaeological heritage management practice is in turn driven by these legislative requirements. This thesis examines whether archaeological heritage legislation is fulfilling its original intent. Analysis of legislative development in this thesis reveals that legislators viewed archaeological heritage as having a wide societal value, not solely or principally for the archaeological community. Archaeological heritage protection is considered within the broader philosophy of environmental conservation. As an environmental issue, it is suggested that a ‘public good’ conservation paradigm is closer to the original intent of archaeological heritage legislation, rather than the “scientific” paradigm which underlies much Australian legislation. Through investigation of the developmental history of Australian heritage legislation it is possible to observe how current practice has diverged from the original intent of the legislation, with New South Wales and Victoria serving as case studies. Further analysis is undertaken of the limited number of Australian court cases which have involved substantial archaeological issues to determine the court’s attitude to archaeological heritage protection. Situating archaeological heritage protective legislation within the field of environmental law allows the examination of alternate modes of protecting archaeological heritage and creates opportunities for ‘public good’ conservation outcomes. This shift of focus to ‘public good’ conservation as an alternative to narrowly-conceived scientific outcomes better aligns with current public policy directions including the sustainability principles, as they have developed in Australia, as well as indigenous rights of self-determination. The thesis suggests areas for legal reforms which direct future archaeological heritage management practice to consider the ‘public good’ values for archaeological heritage protection

Large Orbital Magnetic Moment and Coulomb Correlation effects in FeBr2

We have performed an all-electron fully relativistic density functional calculation to study the magnetic properties of FeBr2. We show for the first time that the correlation effect enhances the contribution from orbital degrees of freedom of $d$ electrons to the total magnetic moment on Fe$^{2+}$ as opposed to common notion of nearly total quenching of the orbital moment on Fe$^{2+}$ site. The insulating nature of the system is correctly predicted when the Hubbard parameter U is included. Energy bands around the gap are very narrow in width and originate from the localized Fe-3$d$ orbitals, which indicates that FeBr2 is a typical example of the Mott insulator.Comment: 4 pages, 3 figures, revtex4, PRB accepte