Dephasing of a superconducting qubit induced by photon noise

We have studied the dephasing of a superconducting flux qubit coupled to a dc-SQUID based oscillator. By varying the bias conditions of both circuits we were able to tune their effective coupling strength. This allowed us to measure the effect of such a controllable and well-characterized environment on the qubit coherence. We can quantitatively account for our data with a simple model in which thermal fluctuations of the photon number in the oscillator are the limiting factor. In particular, we observe a strong reduction of the dephasing rate whenever the coupling is tuned to zero. At the optimal point we find a large spin-echo decay time of .     

Quantum ratchet effect for vortices

We have measured a quantum ratchet effect for vortices moving in a quasi-one-dimensional Josephson junction array. In this solid-state device the shape of the vortex potential energy, and consequently the band structure, can be accurately designed. This band structure determines the presence or absence of the quantum ratchet effect. In particular, asymmetric structures possessing only one band below the barrier do not exhibit current rectification at low temperatures and bias currents. The quantum nature of transport is also revealed in a universal/nonuniversal power-law dependence of the measured voltage-current characteristics for samples without/with rectification.     

A study of the magnetic,molecular and electronic structure of some alkali biphenyl ion pairs in the solid state

A magnetic resonance and susceptibility study of the molecular and magnetic structure of some alkali biphenyl (Bp) polyglyme single crystals and powders has been performed. For RbBp · 2Ttg (Ttg=tetraglyme), space group C2/c, and NaBp · 2Tg (Tg=triglyme), space group P2₁, the average g tensors have been measured. From the average g tensor and the known crystal structure of RbBp · 2Ttg the molecular g tensor of the Bp anion could be derived. The molecular g tensor was calculated with Stone's theory. Excellent agreement was found with the experimental values. With the aid of the molecular g tensor of the Bp anion and the average g tensor of NaBp · 2Tg, for which the crystal structure is unknown, the orientation of the symmetry axes of Bp with respect to the crystal axes could be calculated. Susceptibility measurements reveal ferromagnetic coupling in both KBp · 2Ttg and NaBp · 2Tg, whereas antiferromagnetic coupling exists in RbBp · 2Ttg. The results of alkali N.M.R. experiments are related to the molecular structure. A lower limit is found for the electrical conductivity of NaBp · 2Tg.     

Tc enhancement!

The equilibrium critical temperature of a superconducting aluminum strip has been determined with three different methods. The results agree to within 1 mK. By applying microwave radiation superconductivity was maintained up to 57 mK above this critical temperature. These data clearly establish the existense of T_c enhancement.     

Magneto-optical and magnetic properties of some solid solutions of C1b-Heusler compounds

The large Kerr effect in PtMnSb has been ascribed to details of its unusual band structure. Substitution of atoms will modify the band structure and consequently alter the Kerr effect. In the present investigation we have determined the magneto-optical properties at room temperature in the energy range 0.5–5 eV of several solid solutions of C1_b−Heusler compounds, comprising the solid solution series Pt_1−xNi_xMnSb, Pt_1−xRh_xMnSb, Ni_1−xCr_xMnSb, Ni_1−xFe_xMnSb and PtMnSb_0.9Sn_0.1. The magnetic and crystallographic properties were determined for the solid solution series Pt_1−xNi_xMnSb, Pt_1−xRh_xMnSb and Ni_1−xCr_xMnSb. Neither of the substitutions leads to a larger Kerr effect than that present in the pure compound. Moreover, it is found that the shape of the spectra does not change significantly. For one case, Ni substituted in PtMnSb, band structure calculations were made for hypothetically ordered compounds. The results agree with the experimental findings that near 1.7 eV magneto-optically active transitions persist on substituting Ni in PtMnSb.