Obtaining correct orbital ground states in ff electron systems using a nonspherical self-interaction corrected LDA+UU method

The electronic structure of lanthanide and actinide compounds is often characterized by orbital ordering of localized $f$-electrons. Density-functional theory (DFT) studies of such systems using the currently available LDA+$U$ method are plagued by significant orbital-dependent self-interaction, leading to erroneous orbital ground states. An alternative scheme that modifies the exchange, not Hartree, energy is proposed as a remedy. We show that our LDA+$U$ approach reproduces the expected degeneracy of $f^1$ and $f^2$ states in free ions and the correct ground states in solid PrO$_2$. We expect our method to be useful in studying compounds of $f$- and heavy-$d$ elements.Comment: 11 pages, 4 figure

Magnetic defects promote ferromagnetism in Zn1-xCoxO

Experimental studies of Zn1-xCoxO as thin films or nanocrystals have found ferromagnetism and Curie temperatures above room temperature and that p- or n-type doping of Zn1-xCoxO can change its magnetic state. Bulk Zn1-xCoxO with a low defect density and x in the range used in experimental thin film studies exhibits ferromagnetism only at very low temperatures. Therefore defects in thin film samples or nanocrystals may play an important role in promoting magnetic interactions between Co ions in Zn1-xCoxO. The electronic structures of Co substituted for Zn in ZnO, Zn and O vacancies, substituted N and interstitial Zn in ZnO were calculated using the B3LYP hybrid density functional in a supercell. The B3LYP functional predicts a band gap of 3.34 eV for bulk ZnO, close to the experimental value of 3.47 eV. Occupied minority spin Co 3d levels are at the top of the valence band and unoccupied levels lie above the conduction band minimum. Majority spin Co 3d levels hybridize strongly with bulk ZnO states. The neutral O vacancy and interstitial Zn are deep and shallow donors, respectively. The Zn vacancy is a deep acceptor and the acceptor level for substituted N is at mid gap. The possibility that p- or n-type dopants promote exchange coupling of Co ions was investigated by computing total energies of magnetic states of ZnO supercells containing two Co ions and an oxygen vacancy, substituted N or interstitial Zn in various charge states. The neutral N defect and the singly-positively charged O vacancy are the only defects which strongly promote ferromagnetic exchange coupling of Co ions at intermediate range.Comment: 9 pages, 11 figure

Don’t Go Protectionist! Trade and Investment Relations between the EU and China. Asia Policy Brief 2012/1, June 2012

The European Parliament has proposed the creation of a body to monitor foreign – in particular Chinese – investment in the EU. The initiative, driven by fears of unfair competition and a hidden political agenda behind Chinese investments, should be rejected. There are better ways to promote openness and transparency in Sino-European economic relations

MicroRNA-mediated regulatory circuits: outlook and perspectives

MicroRNAs have been found to be necessary for regulating genes implicated in almost all signaling pathways, and consequently their dysfunction influences many diseases, including cancer. Understanding of the complexity of the microRNA-mediated regulatory network has grown in terms of size, connectivity and dynamics with the development of computational and, more recently, experimental high-throughput approaches for microRNA target identification. Newly developed studies on recurrent microRNA-mediated circuits in regulatory networks, also known as network motifs, have substantially contributed to addressing this complexity, and therefore to helping understand the ways by which microRNAs achieve their regulatory role. This review provides a summarizing view of the state-of-the-art, and perspectives of research efforts on microRNA-mediated regulatory motifs. In this review, we discuss the topological properties characterizing different types of circuits, and the regulatory features theoretically enabled by such properties, with a special emphasis on examples of circuits typifying their biological significance in experimentally validated contexts. Finally, we will consider possible future developments, in particular regarding microRNA-mediated circuits involving long non-coding RNAs and epigenetic regulators

Asian Middle Classes - Drivers of Political Change? Asia Policy Brief 2014/06, November 2014

Asia watchers have been kept exceptionally busy by recent political developments in the region. An unprecedented landslide victory in India’s general elections, pro-democracy protests in Hong Kong, close elections in Indonesia, a coup in Thailand – the list goes on. As unrelated as these events appear, analysts may find a missing link among a social group that is currently exploding in numbers: Asia’s middle classes. Often discussed simply in terms of its economic potential, Asia’s middle-income population is also flexing its political muscle. A closer look at its influence throughout the region in recent months seems to confirm for the field of politics what economists have known for some time: The rise of the Asian middle classes constitutes one of the most fundamental transformations of our time. The consequences remain to be seen

Low-Frequency Noise Phenomena in Switched MOSFETs

In small-area MOSFETs widely used in analog and RF circuit design, low-frequency (LF) noise behavior is increasingly dominated by single-electron effects. In this paper, the authors review the limitations of current compact noise models which do not model such single-electron effects. The authors present measurement results that illustrate typical LF noise behavior in small-area MOSFETs, and a model based on Shockley-Read-Hall statistics to explain the behavior. Finally, the authors treat practical examples that illustrate the relevance of these effects to analog circuit design. To the analog circuit designer, awareness of these single-electron noise phenomena is crucial if optimal circuits are to be designed, especially since the effects can aid in low-noise circuit design if used properly, while they may be detrimental to performance if inadvertently applie

Prediction of higher thermoelectric performance in BiCuSeO by weakening electron-polar optical phonon scattering

BiCuSeO is a promising thermoelectric material, but its applications are hindered by low carrier mobility. We use first-principles calculations to analyse electron–phonon scattering mechanisms and evaluate their contributions to the thermoelectric figure of merit ZT. The combined scattering of carriers by polar optical (PO) and longitudinal acoustic (LA) phonons yields an intrinsic hole mobility of 32 cm^{2} V^{−1} s^{−1} at room temperature and a temperature power law of T^{−1.5} between 100–875 K, which agree well with experiments. We demonstrate that electron–phonon scattering in the Cu–Se layer dominates at low T (< 500 K), while contributions from the Bi–O layer become increasingly significant at higher T. At room temperature, ZT is calculated to be 0.48 and can be improved by 30% through weakening PO phonon scattering in the Cu–Se layer. This finding agrees with the experimental observation that weakening the electron–phonon interaction by Te substitution in the Cu–Se layer improves mobility and ZT. At high T, the figure of merit is improved by weakening the electron–PO phonon scattering in the Bi–O layer instead. The theoretical ZT limit of BiCuSeO is calculated to be 2.5 at 875 K

Effects of Octahedral Tilting on Band Structure and Thermoelectric Power Factor of Titanate Perovskites: A First-Principles Study on SrTiO₃

Doped SrTiO_{3} and other perovskite structured titanates are attracting interest as n-type thermoelectric materials due to their relatively high thermoelectric power factor, low toxicity, and modest cost. Taking SrTiO_{3} as an example, the effects of octahedral tilting on the electronic band structure and thermoelectric power factor of titanate perovskites have been studied from first-principles calculations. By utilizing Glazer’s notation, six representative tilt systems, including three out-of-phase (a^{0}a^{0}c^{–}, a^{0}b^{-}b^{–}, and a^{–}a^{–}a^{–}) and three in-phase tilt systems (a^{0}a^{0}c^{+}, a^{0}b^{+}b^{+}, and a^{+}a^{+}a^{+}), were investigated. It is found that out-of-phase tilting improves the optimum power factor as compared to the cubic aristotype, while in-phase tilting marginally lowers the optimum power factor. The largest increase in power factor (∼100%) is obtained in the one-tilt system a^{0}a^{0}c^{–} at a tilt angle of 15°, which can be achieved with an energy cost of only 44 kJ mol^{–1} per formula unit. These findings agree with the experimental evidence that increased power factors are found in a^{0}a^{0}c^{–} and a^{–}a^{–}a^{–} tilt systems of titanate perovskites. The predicted increase of Seebeck coefficient as a function of tilt angle in the a^{–}a^{–}a^{–} tilt system of SrTiO_{3} is also consistent with the experimental increase of Seebeck coefficient in a^{–}a^{–}a^{–} titanates of La_{0.55}K_{0.45}TiO_{3} and La_{0.5}Na_{0.5}Ti_{0.9}Nb_{0.1}O_{3}. Our simulations provide valuable insights into tuning the thermoelectric power factor of titanate perovskites by controlling octahedral tilting

Dynamical friction and the evolution of satellites in virialized halos: the theory of linear response

The evolution of a small satellite inside a more massive truncated isothermal spherical halo is studied using both the Theory of Linear Response for dynamical friction and N-Body simulations. The analytical approach includes the effects of the gravitational wake, of the tidal deformation and the shift of the barycenter of the primary, so unifying the local versus global interpretation of dynamical friction. Sizes, masses, orbital energies and eccentricities are chosen as expected in hierarchical clustering models. We find that in general the drag force in self-gravitating backgrounds is weaker than in uniform media and that the orbital decay is not accompanied by a significant circularization. We also show that the dynamical friction time scale is weakly dependent on the initial circularity. We provide a fitting formula for the decay time that includes the effect of mass and angular momentum loss. Live satellites with dense cores can survive disruption up to an Hubble time within the primary, notwithstanding the initial choice of orbital parameters. Dwarf spheroidal satellites of the Milky Way, like Sagittarius A and Fornax, have already suffered mass stripping and, with their present masses, the sinking times exceed 10 Gyr even if they are on very eccentric orbits.Comment: 27 pages including 9 figures. Accepted for publication in the Astrophysical Journal. Part 2, issue November 10 1999, Volume 52