arXiv subjects
Publications and source records attributed to R. Held.
A new concept for nonvolatile superconducting memories is proposed. The devices combine ferromagnetic dots for the storage of the data and Josephson junctions for their readout. Good scalability is expected for large scale integration. First memory cells have been implemented using 3 $μ$m-Nb-technology and permalloy dots. Nonvolatile data storage at 300 K was demonstrated.
The current-induced dissipation in YBCO grain boundary tunnel junctions has been measured between 4.2 K and 300 K. It is found that the resistance of 45 degree (100)/(110) junctions decreases linearly by a factor of four when their temperature is increased from 100 K to 300 K. At the superconducting transition temperature Tc the grain boundary resistance of the normal state and of the superconducting state extrapolate to the same value.
The problems of hazardous radiation and collisions with matter on a warp driven ship pose considerable obstacles to this possibility for interstellar travel. A solution to these problems lies in the Broeck metric. It is demonstrated that both threats to the ship will be greatly reduced. It is also shown that the horizon problem no longer exists.
It will be shown that while horizons do not exist for warp drive spacetimes traveling at subluminal velocities horizons begin to develop when a warp drive spacetime reaches luminal velocities. However it will be shown that the control region of a warp drive ship lie within the portion of the warped region that is still causally connected to the ship even at superluminal velocities, therefore allowing a ship to slow to subluminal velocities. Further it is shown that the warped regions which are causally disconnected from a warp ship have no correlation to the ship velocity.
The temperature dependence of the conductance of a quantum point contact has been measured. The conductance as a function of the Fermi energy shows temperature-independent fixed points, located at roughly multiple integers of $e^{2}/h$. Around the first fixed point at e$^{2}$/h, the experimental data for different temperatures can been scaled onto a single curve. For pure thermal smearing of the conductance steps, a scaling parameter of one is expected. The measured scaling parameter, however, is significantly larger than 1. The deviations are interpreted as a signature of the potential landscape of the quantum point contact, and of the source-drain bias voltage. We relate our results phenomenologically to the metal-insulator transition in two dimensions.
A single-electron transistor has been realized in a Ga[Al]As heterostructure by oxidizing lines in the GaAs cap layer with an atomic force microscope. The oxide lines define the boundaries of the quantum dot, the in-plane gate electrodes, and the contacts of the dot to source and drain. Both the number of electrons in the dot as well as its coupling to the leads can be tuned with an additional, homogeneous top gate electrode. Pronounced Coulomb blockade oscillations are observed as a function of voltages applied to different gates. We find that, for positive top-gate voltages, the lithographic pattern is transferred with high accuracy to the electron gas. Furthermore, the dot shape does not change significantly when in-plane voltages are tuned.
A quantum wire is spatially displaced by suitable electric fields with respect to the scatterers inside a semiconductor crystal. As a function of the wire position, the low-temperature resistance shows reproducible fluctuations. Their characteristic temperature scale is a few hundred millikelvin, indicating a phase-coherent effect. Each fluctuation corresponds to a single scatterer entering or leaving the wire. This way, scattering centers can be counted one by one.