Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
10 results
Search Results
Item Open Access Electrons and holes in InSb under crossed magnetic and stress fields. 1, Theory(1988) Trebin, Hans-Rainer; Wolfstädter, Bernd; Pascher, Harald; Häfele, Hans-GeorgIn a series of four papers magnetooptical transitions are presented for InSb crystals, which are subjected to uniaxial stress perpendicular to the magnetic field. Here, in the first paper, we establish an 8×8 k⋅p Hamiltonian matrix for stress Tǁ[100] and field Bǁ[001] and diagonalize it exactly. The dependence of valence and conduction states on stress and longitudinal momentum is discussed and compared with the geometry of parallel fields TǁBǁ[001]. Characteristic features are extracted for inter- and intraband transitions. Under crossed fields, the levels are separated much stronger with stress, yielding more insight than in the parallel configuration.Item Open Access Stability of monatomic and diatomic quasicrystals and the influence of noise(1990) Roth, Johannes; Schilling, Rolf; Trebin, Hans-RainerThe stability of quasicrystals endowed with atomic Lennard-Jones–like pair potentials was investigated with use of the method of steepest descent. Starting from two- and three-dimensional Penrose patterns, the basic units were decorated in various fashions with one or two sorts of atoms. In accord with previous studies, all monatomic two-dimensional quasicrystals decay to a hexagonal periodic crystal with defects; diatomic systems remain stable when the relative size of the atoms is suitably chosen. In three dimensions, the monatomic quasicrystalline unit-sphere packing was proven stable as well as the structure of truncated icosahedra, even if in the initial configuration the atoms were displaced statistically up to 7% and 25%, respectively, of the edge length (noise). A series of decorations (among them one involving Mackay icosahedra) relaxed to the amorphous state. In these transitions the atoms arrange in families of Fibonacci planes whose separations scale down to atomic distances in a self-similar fashion.Item Open Access Metallic glasses and chiral nematic liquid crystals(1987) Trebin, Hans-Rainer; Longa, Lech; Salzgeber, BennoGeometrical frustration exists in both metallic glasses and chiral nematic liquid crystals. Applying results from the Landau-Ginzburg theory of liquid crystals and from polytope models for the amorphous state, a frustrated free-energy density for metallic glasses is derived. It is expressed in terms of an order parameter that characterizes the bond orientation of neighbouring atoms.Item Open Access Structure of the elastic free energy for chiral nematic liquid crystals(1989) Longa, Lech; Trebin, Hans-RainerIn Landau–de Gennes theory, the free energy f of liquid crystals is expanded into powers of a symmetric, traceless tensor order parameter Q αβ and its derivatives Q αβ,γ. The expansion is subject to the condition that f is a scalar, i.e., invariant under all rotations of the group SO(3). Using the method of integrity basis, we have established the most general SO(3)-invariant free-energy density up to all powers in Q αβ and up to second order in Q αβ,γ. It turns out that this free-energy density is composed of 39 invariants, which are multiplied by arbitrary polynomials in TrQ 2 and TrQ 3. On the other hand, these 39 invariants can be expressed as polynomials of 33 so-called irreducible invariants. Interestingly, among the irreducible invariants there are only three chiral terms (i.e., linear in Q αβ,γ). They locally give rise to three independent helix modes in chiral, biaxial liquid crystals. This conclusion generalizes results of Trebin [J. Phys. (Paris) 42, 1573 (1981)] and Govers and Vertogen [Phys. Rev. A 31, 1957 (1985); 34, 2520 (1986)] and contradicts a statement of Pleiner and Brand [Phys. Rev. A 24, 2777 (1981); 34, 2528 (1986)], according to which only one twist term is supposed to exist.Item Open Access Integrity basis approach to the elastic free energy functional of liquid crystals. 1, Classification of basic elastic modes(1989) Longa, Lech; Trebin, Hans-RainerUsing the method integrity basis, the most general SO(3)-invariant free energy density up to all powers in xβ and up to second order in Q xβ,y is established. The method provides all analytically independent elastic modes for nematics and cholesterics in the form of 33 so-called, irreducible invariants. Interestingly, among the irreducible invariants there are only three chiral terms (i.e. linear in Q δ,β,y ). They give rise locally to three independent helix modes in chiral, biaxial liquid crystals. This conclusion generalizes results of Trebin and Govers and Vertogen and contradicts a statement of Pleiner and Brandt, according to which only one twist term is supposed to exist. The most general free energy expansion can be written as sum of 39 additive invariants, which are multiplied by arbitrary polynomials in TrQ 2 and TrQ 3.Item Open Access Electrons and holes in InSb under crossed magnetic and stress fields. 4, Stimulated Raman scattering in the valence bands(1988) Wolfstädter, Bernd; Trebin, Hans-Rainer; Pascher, Harald; Häfele, Hans-GeorgIn a series of four papers magnetooptical transitions are presented for InSb crystals, which are subjected to uniaxial stress perpendicular to the magnetic field. Here, in the first paper, we establish an 8×8 k⋅p Hamiltonian matrix for stress Tǁ[100] and field Bǁ[001] and diagonalize it exactly. The dependence of valence and conduction states on stress and longitudinal momentum is discussed and compared with the geometry of parallel fields TǁBǁ[001]. Characteristic features are extracted for inter- and intraband transitions. Under crossed fields, the levels are separated much stronger with stress, yielding more insight than in the parallel configuration.Item Open Access Localization and diffusion of positive muons in metals(1979) Orth, Helmut; Döring, Klaus-Peter; Gladisch, Michael; Herlach, Dierk; Maysenhölder, Waldemar; Metz, Harald; Putlitz, Gisbert zu; Seeger, Alfred; Vetter, J.; Wahl, W.; Wigand, M.; Yagi, EiichiDie Grundeigenschaften positiver Müonen (positive Elementarladung, Spin 1/2, Massenverhältnis Müon/Proton ≈ 1/9, großes magnetisches Moment, paritätsverletzender Zerfall in ein Positron und zwei Neutrinos) und ihre Verfügbarkeit in Form intensiver spinpolarisierter Strahlen in "Mesonen-Fabriken" gestatten es, mit Hilfe der μ+SR ("Müon-Spin-Rotation")-Methode die Wechselwirkung der Müonen mit Magnetfeldern in Metallen zu studieren. In dieser Arbeit geben wir einen kurzen Überblick über die μ+SR-Methode und ihre Anwendung auf das Studium der Lokalisierung und Diffusion von Müonen in Metallen für den Fall der dipolaren Wechselwirkung mit den magnetischen Momenten der Kerne. Die Zusammenhänge zwischen den gemessenen Relaxationsraten und der Müonenbeweglichkeit und dem Einfang und Entweichen an Verunreinigungen werden dargestellt und anhand von Messungen an hochreinen Be-, Nb- und Ta-Einkristallen veranschaulicht.Item Open Access Electrons and holes in InSb under crossed magnetic and stress fields. 3, Magnetoreflection(1988) Wolfstädter, Bernd; Trebin, Hans-Rainer; Pascher, Harald; Häfele, Hans-GeorgIn a series of four papers magnetooptical transitions are presented for InSb crystals, which are subjected to uniaxial stress perpendicular to the magnetic field. Here, in the first paper, we establish an 8×8 k⋅p Hamiltonian matrix for stress Tǁ[100] and field Bǁ[001] and diagonalize it exactly. The dependence of valence and conduction states on stress and longitudinal momentum is discussed and compared with the geometry of parallel fields TǁBǁ[001]. Characteristic features are extracted for inter- and intraband transitions. Under crossed fields, the levels are separated much stronger with stress, yielding more insight than in the parallel configuration. We have investigated the electronic interband transitions close to the band gap of InSb by observing magnetoreflectivity under uniaxial stress. The sensitivity of the experiment was increased by application of a magnetic-field-modulation technique. By this method we can resolve transitions from the conduction band to various valence-band Landau levels. The measurements were carried out with light polarized linearly along and perpendicular to the magnetic induction B (EǁB and E⊥B). In addition to well-known electric dipole resonances we observed new transitions induced by stress. These additional lines are connected with transition matrix elements which arise from lowering the symmetry. The observed transition energies are compared with band-structure calculations.Item Open Access Phase diagrams of cholesteric liquid crystals obtained with a generalized Landau-de Gennes theory(1989) Longa, Lech; Monselesan, Didier; Trebin, Hans-RainerPhase diagrams of chiral nematic liquid crystals are studied within the framework of a generalized Landau-Ginzburg-de Gennes theory. Using the parametrization of Grebel, Hornreich, and Shtrikman for the tensor order parameter Q, all relevant elastic terms are included for the helicoidal phase and the blue phases of chiral nematic liquid crystals up to fourth order in Q and its gradient ∂Q. The influence of the additional elastic terms on the phase diagrams of the chiral nematic phases is then investigated. The theory correctly describes the variation of the pitch with temperature and the induced biaxiality of the cholesteric phase. The results resolve the discrepancies encountered by Hornreich and Shtrikman in the comparison of experiment and theory. New features in the topology of the phase diagrams of blue phases, like re-entrant phase transitions, are predicted.Item Open Access Electrons and holes in InSb under crossed magnetic and stress fields. 2, Recombination radiation(1988) Wolfstädter, Bernd; Trebin, Hans-Rainer; Pascher, Harald; Häfele, Hans-GeorgStimulated recombination radiation and spin-flip Raman scattering have been observed in InSb with uniaxial stress Tapplied perpendicular to a magnetic field B Tǁ[100] Bǁ[001. A k·p Hamiltonian was established within an 8 x 8 Kane model and diagonalized exactly for this particular geometry. Energy levels, transition energies, and corresponding oscillator strengths were calculated and compared to the experimental data.