Christian Joas

Random-phase approximation and its applications in computational chemistry and materials science

The random-phase approximation (RPA) as an approach for computing the electronic correlation energy is reviewed. After a brief account of its basic concept and historical development, the paper is devoted to the theoretical formulations of RPA, and its applications to realistic systems. With several illustrating applications, we discuss the implications of RPA for computational chemistry and materials science. The computational cost of RPA is also addressed which is critical for its widespread use in future applications. In addition, current correction schemes going beyond RPA and directions of further development will be discussed.

Microwave photoconductivity of a modulated two-dimensional electron gas due to intra-Landau-level transitions

Motivated by a recent experiment, we study the microwave-induced photoconductivity of a two-dimensional electron gas arising from intra-Landau-level transitions within a model where the electrons are subject to a unidirectional periodic potential in addition to a weaker impurity potential. With appropriate identifications, our results can be compared to experiment and allow us to explain the sign of the photocurrent, its dependence on magnetic field and microwave frequency, as well as the microwave-induced suppression of the Shubnikov-deHaas oscillations.

Electronic theory for superconductivity in Sr2RuO4: Triplet pairing due to spin-fluctuation exchange

Using a Hubbard Hamiltonian for the three electronic bands crossing the Fermi level in Sr2RuO4, we calculate the band structure and spin susceptibility χ(q,ω) in quantitative agreement with nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) experiments. The susceptibility has two peaks at Qi = (2π/3a,2π/3a,0) due to the nesting Fermi surface properties and at qi = (0.2π/a,0,0) due to the tendency towards ferromagnetism. Applying spin-fluctuation exchange theory as in layered cuprates we determine from χ(q,ω), electronic dispersions, and Fermi surface topology that superconductivity in Sr2RuO4 consists of triplet pairing. Using χ(q,ω) we can exclude s- and d-wave symmetry for the superconducting order parameter. Furthermore, within our analysis and approximations we find that the order parameter will have a node between neighboring RuO2-planes and that in the RuO2-plane fx2 − y2-wave and p-wave symmetry are close in energy.

Parametric resonance of a two-dimensional electron gas under bichromatic irradiation

In an ultrahigh mobility 2D electron gas, even a weak nonparabolicity of the electron dispersion, by violating Kohn’s theorem, can have a drastic effect on dc magnetotransport under ac drive. In this paper, we study theoretically the manifestation of this effect in the dc response to the combined action of two driving ac fields (bichromatic irradiation). Compared to the case of monochromatic irradiation, which is currently intensively studied both experimentally and theoretically, the presence of a second microwave source provides additional insight into the properties of an ac-driven 2D electron gas in weak magnetic field. In particular, we find that nonparabolicity, being the simplest cause for a violation of Kohn’s theorem, gives rise to new qualitative effects specific to bichromatic irradiation. Namely, when the frequencies v1 and v2 are well away from the cyclotron frequency, vc, our simple classical considerations demonstrate that the system becomes unstable with respect to fluctuations with frequency 1 2 sv1 +v2d. The most favorable condition for this parametric instability is 1 sv1 +v2 d. vc. The saturation level of this instability is also determined by the nonparabolicity. We also demonstrate that, as an additional effect of nonparabolicity, this parametric instability can manifest itself in the dc properties of the system. This happens when v1, v2, and vc are related as 3:1:2, respectively. Even for weak detuning between v1 and v2, the effect of the bichromatic irradiation on the dc response in the presence of nonparabolicity can differ dramatically from the monochromatic case. In particular, we demonstrate that, beyond a critical intensity of the two fields, the equations of motion acquire multistable solutions. As a result, the diagonal dc conductivity can assume several stable negative values at the same magnetic field.

Theory for phonon-induced superconductivity inMgB2

We analyze superonductivity in MgB

Geometric characterization of nodal domains: the area-to-perimeter ratio

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Superconductivity—A Challenge to Modern Physics

observed below

The Physics of Cold in the Cold War—“On-Line Computing” Between the ICBM Program and Superconductivity

T_c=39

Supraleitung und Interkontinentalraketen : 'online computing' zwischen Militär, Industrie und Wissenschaft

K resulting from electron-phonon coupling involving a mode at

THEORY FOR INELASTIC NEUTRON SCATTERING IN HIGH-Tc SUPERCONDUCTORS: DOPING AND TEMPERATURE DEPENDENCE OF TWO CHARACTERISTIC FREQUENCIES

\hbar \omega_1 = 24