[en] The following topics are dealt with: Artificial atoms and molecules, tailored from solids, fractional flux quanta, molecular magnets, controlled interaction in quantum gases, the theory of quantum correlations in mott matter, cold gases, and mesoscopic systems, Bose-Einstein condensates on the chip, on the route to the quantum computer, a quantum computer in diamond. (HSI)

[en] In lanthanides, such as holmium, ordering of the local magnetic moments strongly affects the scattering rate of the conduction electrons (Kondo lattice). We performed infrared measurements on single crystalline holmium in a range from 1.2 meV to 1.2 eV at temperatures between 5 and 300 K. In the ordered state a reduced scattering rate in the Drude regime reflects the decrease of phonon as well as magnetic scattering. We observe an exchange splitting of 372 meV at the Fermi level and a low-frequency feature at about 37 meV

[en] The blue bronzes A_0.3MoO₃ (A=Rb,K,Tl) are well known quasi-one-dimensional materials. They exhibit a Peierls-type metal-insulator phase transition around 180 K which is connected to the formation of a charge density wave (CDW) in the conducting direction. While temperature dependent dc-resistivity indicates the CDW in all three directions, so far most measurements concentrated on the conducting b- and a*-direction and no thorough investigations of the physical properties of the insulating c*-direction have been carried out. We report first optical conductivity measurements on the potassium blue bronze K_0.3MoO₃ along the b- and the c*-direction in order to evaluate the influence of the long range order of the CDW on the perpendicular direction. Measurements were performed with polarized light in a wide frequency range from 30 cm^-1 up to 11000 cm^-1 for several selected temperatures between 300 K and 50 K. At high frequencies we have carefully analyzed the changes in the strong interband transition related to the gap opening. In addition two phonon lines are observed around 900 cm^-1 and attributed to stretching vibrations of terminal O- with Mo-atoms. Degenerated at high temperatures, those lines split and shift in frequency once the CDW develops.

[en] We present a broadband microwave spectrometer covering the range from 45 MHz up to 20 GHz (in some cases up to 40 GHz) which employs the Corbino geometry, meaning that the flat sample terminates the end of a coaxial transmission line. This setup is optimized for low-temperature performance (temperature range 1.7-300 K) and for the study of highly conductive samples. The actual sensitivity in reflection coefficient can be as low as 0.001, leading to a resolution of 10% in absolute values of the impedance or complex conductivity. For optimum accuracy a full low-temperature calibration is necessary; therefore up to three calibration measurements (open, short, and load) are performed at the same temperature as the sample measurement. This procedure requires excellent reproducibility of the cryogenic conditions. We compare further calibration schemes based on just a single low-temperature calibration measurement or employing a superconducting sample as a calibration standard for its normal state, and we document the capability of the instrument with test measurements on metallic thin films. Finally we apply the spectrometer to thin films of a heavy-fermion compound as an example for a strongly correlated electron system

[en] Organic metals exhibit unusual electronic properties in their charge and spin degrees of freedom that have puzzled physicists for decades. By now this behaviour is established as intrinsic and related to electronic interactions. Like other correlated electron systems, such as heavy fermions or transition-metal oxides, organic conductors are located next to some ordered phase in the spin or charge sectors. Theory predicts quantum fluctuations to become important at low temperatures and quantum critical behaviour present in most physical properties. Here we survey the experimental evidence of quantum criticality in well-established organic model compounds and look for indications of non-Fermi-liquid behaviour. (topical review)

[en] A broadband microwave spectrometer has been constructed to determine the complex conductivity of thin metal films at frequencies from 45 MHz to 20 GHz working in the temperature range from 0.45 K to 2 K (in a ³He cryostat). The setup follows the Corbino approach: a vector network analyzer measures the complex reflection coefficient of a microwave signal hitting the sample as termination of a coaxial transmission line. As the calibration of the setup limits the achievable resolution, we discuss the sources of error hampering different types of calibration. Test measurements of the complex conductivity of a heavy-fermion material demonstrate the applicability of the calibration procedures.

[en] Planar superconducting resonators have multiple applications, e.g. in quantum information processing or spectroscopy on unconventional solids. Here, Pb can be used as a basis of microwave resonators with a critical temperature of 7.2 K and a critical magnetic field of 80 mT. It shows interesting behavior with residual normal conducting areas after entering the superconducting phase with applied external magnetic field. We present a study of superconducting Pb microwave stripline resonators in variable external magnetic field up to 140 mT and a temperature range of 1.6 K to 6.5 K. We are able to determine the complex conductivity of Pb in variable magnetic field and we find striking similarities to measurements in variable temperature, like a coherence peak. The quality factor of the resonances shows strong hysteresis effects after exceeding the critical magnetic field, indicating residual normal conducting areas in the resonator persisting in the superconducting state. We are able to determine the critical magnetic field for a set of given temperatures both for the Pb resonator as well as a Sn sample placed on top of the underlying Pb resonator. These results demonstrate that Pb resonators can be applied for different spectroscopy studies in zero as well as finite magnetic field.

[en] The transport properties of [2,2]-paracyclophane thin films prepared by molecular beam epitaxy were studied by space-charge-limited current measurements. This organic semiconductor, which has a three-dimensional π-system comparable to that of C₆₀, is an interesting material for molecular electronic applications due to its remarkably high mobility and its insensitivity against photo-oxidation. The current-voltage characteristics were recorded using a coplanar electrode geometry which has the advantage, that after deposition no further processing of the organic semiconducting layer is necessary. In polycrystalline films hole mobilities up to 10^-2 cm² V^-1 s^-1 were observed at room temperature

[en] Microwave spectroscopy is a powerful experimental tool to study the electrodynamics of metals and superconductors at comparably low frequencies. To overcome some disadvantages of the established cryogenic microwave techniques, we use stripline resonators as a probe where the bulk sample under study represents one of the ground planes of the transmission line. We discuss striplines made of copper as well as superconducting lead, and we show the applicability of this technique with measurements at the superconducting transition of a bulk sample of tantalum.

[en] A novel method to study high-frequency electron spin resonance is introduced, which sweeps the frequency at fixed magnetic fields. We describe the main features of our frequency-domain spectrometer, which works in the spectral range from 30 GHz to 1.5 THz and at magnetic fields up to 8 T; the temperature can be as low as 0.5 K. Applications include the investigations of paramagnetic ions, in general, and molecular magnets, in particular