Equilibrium and nonequilibrium dynamics of the sub-Ohmic spin-boson model

Employing the nonperturbative numerical renormalization group method, we study the dynamics of the spin-boson model, which describes a two-level system coupled to a bosonic bath with a spectral density J(omega) proportional to omega(s). We show that, in contrast with the case of Ohmic damping, the delocalized phase of the sub-Ohmic model cannot be characterized by a single energy scale only, due to the presence of a nontrivial quantum phase transition. In the strongly sub-Ohmic regime, s<1, weakly damped coherent oscillations on short time scales are possible even in the localized phase--this is of crucial relevance, e.g., for qubits subject to electromagnetic noise.     

An endohedral single-molecule magnet with long relaxation times: DySc2N@C80

The magnetism of DySc(2)N@C(80) endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in Dy(III) ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc(2)N@C(80) samples with C(60) increases the zero-field 4f electron relaxation time at 2 K to several hours.     

Steady-state currents through nanodevices: a scattering-states numerical renormalization-group approach to open quantum systems

We propose a numerical renormalization group (NRG) approach to steady-state currents through nanodevices. A discretization of the scattering-states continuum ensures the correct boundary condition for an open quantum system. We introduce two degenerate Wilson chains for current carrying left- and right-moving electrons reflecting time-reversal symmetry in the absence of a finite bias V. We employ the time-dependent NRG to evolve the known steady-state density operator for a noninteracting junction into the density operator of the fully interacting nanodevice at finite bias. We calculate the differential conductance as function of V, T, and the external magnetic field.     

Messung der Lebensdauer von Elektronen im 3P- und 4D-Zustand des Natrium

Lifetimes of electrons in the 3P- and 4D-levels of sodium are measured. The sodium-atoms are excited by short pulses of electrons with energies between 2 and 8 eV. The time dependence of the emitted light is determined by direct oscillographing via photomultiplier as well as by counting single quantas. The counting technique corresponds to the delayed coincidence method and employs a sampling oscilloscope. Lifetimes of the 3P- and 4D-levels were found to be 14.0 and 52.1 nsec respectively. From these values the absorption oscillator strengths 1.11 and 0.095 were calculated for the 3S-3P- and 3P-4D-transitions.     

Studying nanomagnets and magnetic heterostructures with X-ray PEEM at the Swiss Light Source

Polarization dependent X-ray absorption spectroscopy and microscopy enables the element selective investigation of magnetic systems at the nanoscale. At the Swiss Light Source a photoemission electron microscope is used for the study of a broad variety of systems. Here, a review of recent activities is presented with a focus on instrumental and analytical developments. A new procedure for the 3 dimensional determination of the magnetization vector has been developed, and is demonstrated for GdFeCo microstructures displaying in-plane and out-of-plane domains, and sub-20 nm Fe nanoparticles. The recent progress for measurements in applied magnetic fields is presented and a new set-up for time-resolved measurements employing femtosecond laser pulses is described.     

Ultrafast, stretched-pulse thulium-doped fiber laser with a fiber-based dispersion management

An ultrafast thulium-doped fiber laser with stretched-pulse operation has been realized and investigated. The passively mode-locked oscillator emitted 119 fs pulses at a peak wavelength of 1912 nm. A normal-dispersion fiber with a high numerical aperture and small core was used for intracavity dispersion management and external compression. Numerical simulations were performed and are in good agreement with the experimental results.