## Fachbereich Physik

### Refine

#### Year of publication

- 1999 (78) (remove)

#### Document Type

- Preprint (53)
- Article (16)
- Course Material (6)
- Doctoral Thesis (1)
- Master's Thesis (1)
- Periodical Part (1)

#### Keywords

- Praktikum (6)
- Brillouin light scattering spectroscopy (2)
- Wannier-Stark systems (2)
- entropy (2)
- localization (2)
- quantum mechanics (2)
- resonances (2)
- spin wave quantization (2)
- 90° orientation (1)
- Brillouin light scattering (1)

- Wall energy and wall thickness of exchange-coupled rare-earth transition-metal triple layer stacks (1999)
- The room-temperature wall energy sw 54.0310 23 J/m 2 of an exchange-coupled Tb 19.6 Fe 74.7 Co 5.7 /Dy 28.5 Fe 43.2 Co 28.3 double layer stack can be reduced by introducing a soft magnetic intermediate layer in between both layers exhibiting a significantly smaller anisotropy compared to Tb+- FeCo and Dy+- FeCo. sw will decrease linearly with increasing intermediate layer thickness, d IL , until the wall is completely located within the intermediate layer for d IL d w , where d w denotes the wall thickness. Thus, d w can be obtained from the plot sw versus d IL .We determined sw and d w on Gd+- FeCo intermediate layers with different anisotropy behavior ~perpendicular and in-plane easy axis! and compared the results with data obtained from Brillouin light-scattering measurements, where exchange stiffness, A, and uniaxial anisotropy, K u , could be determined. With the knowledge of A and K u , wall energy and thickness were calculated and showed an excellent agreement with the magnetic measurements. A ten times smaller perpendicular anisotropy of Gd 28.1 Fe 71.9 in comparison to Tb+- FeCo and Dy+- FeCo resulted in a much smaller sw 51.1310 23 J/m 2 and d w 524 nm at 300 K. A Gd 34.1 Fe 61.4 Co 4.5 with in-plane anisotropy at room temperature showed a further reduced sw 50.3310 23 J/m 2 and d w 517 nm. The smaller wall energy was a result of a different wall structure compared to perpendicular layers.

- Universalität im Spektrum des Dirac-Operators (1999)
- Diese Dissertation ist ein Beitrag zur Untersuchung der Anwendbarkeit der Random-Matrix-Theorie (RMT) in der Quantenchromodynamik (QCD). Untersucht werden die Fluktuationen der kleinen Eigenwerte des Dirac-Operators mit Kogut-Susskind-Fermionen und SUc(2)-Eichfeldern. Diese werden mit Hilfe eines Hybrid-Monte-Carlo-Algorithmus erzeugt. Die Universalität der Fluktuationen kleiner Eigenwerte, das heisst die Übereinstimmung der numerisch berechneten Spektren mit den Vorhersagen des chiralen Random-Matrix-Modells wird in dieser Arbeit nachgewiesen. Die Bedeutung dieses Resultats liegt in der Allgemeinheit des Ansatzes, die QCD-Zustandssumme für ein endliches Volumen durch ein Random-Matrix-Modell zu approximieren.

- The role of infrared divergence for decoherence (1999)
- Continuous and discrete superselection rules induced by the interaction with the environment are investigated for a class of exactly soluble Hamiltonian models. The environment is given by a Boson field. Stable superselection sectors can only emerge if the low frequences dominate and the ground state of the Boson field disappears due to infrared divergence. The models allow uniform estimates of all transition matrix elements between different superselection sectors.

- The construction of trigonometric invariants for Weyl groups and the derivation of corresponding exactly solvable Sutherland models (1999)
- Trigonometric invariants are defined for each Weyl group orbit on the root lattice. They are real and periodic on the coroot lattice. Their polynomial algebra is spanned by a basis which is calculated by means of an algorithm. The invariants of the basis can be used as coordinates in any cell of the coroot space and lead to an exactly solvable model of Sutherland type. We apply this construction to the \(F_4\) case.

- Switching behavior of a Stoner particle beyond the relaxation time limit (1999)
- We report results of the switching properties of Stoner-like magnetic particles subject to short magnetic field pulses, obtained by numerical investigations. We discuss the switching properties as a function of the external field pulse strength and direction, the pulse length and the pulse shape. For field pulses long compared to the ferromagnetic resonance precession time the switching behavior is governed by the magnetic damping term, whereas in the limit of short field pulses the switching properties are dominated by the details of the precession of the magnetic moment. In the latter case, by choosing the right field pulse parameters, the magnetic damping term is of minor importance and ultrafast switching can be achieved. Switching can be obtained in an enlarged angular range of the direction of the applied field compared to the case of long pulses.

- Stark Resonances in dc fields from short time propagation of the field-free Hamiltonian (1999)
- A new method for calculating Stark resonances is presented and applied for illustration to the simple case of a one-particle, one-dimensional model Hamiltonian. The method is applicable for weak and strong dc fields. The only need, also for the case of many particles in multi-dimensional space, are either the short time evolution matrix elements or the eigenvalues and Fourier components of the eigenfunctions of the field-free Hamiltonian.

- Spontaneous emission and level shifts in absorbing disordered dielectrics and dense atomic gases: A Green's function approach (1999)
- Abstract: Spontaneous emission and Lamb shift of atoms in absorbing dielectrics are discussed. A Green's-function approach is used based on the multipolar interaction Hamiltonian of a collection of atomic dipoles with the quantised radiation field. The rate of decay and level shifts are determined by the retarded Green's-function of the interacting electric displacement field, which is calculated from a Dyson equation describing multiple scattering. The positions of the atomic dipoles forming the dielectrics are assumed to be uncorrelated and a continuum approximation is used. The associated unphysical interactions between different atoms at the same location is eliminated by removing the point-interaction term from the free-space Green's-function (local field correction). For the case of an atom in a purely dispersive medium the spontaneous emission rate is altered by the well-known Lorentz local-field factor. In the presence of absorption a result different from previously suggested expressions is found and nearest-neighbour interactions are shown to be important.