arXiv subjects
Publications and source records attributed to S. Heinze.
We show that the ground state of a Bose-Einstein condensate of atoms with hyperfine spin f = 2 can be either spin aligned, condensed into pairs of atoms coupled to F = 0, or condensed into triplets of atoms coupled to F = 0. The complete phase diagram is constructed for f = 2 and the generic properties of the phase diagram are obtained for f > 2.
Degeneracies near the real axis in a complex-extended parameter space of a hermitian Hamiltonian are studied. We present a method to measure distributions of such degeneracies on the Riemann sheet of a selected level and apply it in classification of quantum phase transitions. The degeneracies are shown to behave similarly as complex zeros of a partition function.
With decreasing device dimensions, the performance of carbon nanotube field-effect transistors (CNFETs) is limited by high Off currents except at low drain voltages. We show that an asymmetric design improves the performance, reducing Off currents and extending the usable range of drain voltage. The improvement is most dramatic for ambipolar Schottky-barrier CNFETs. Moreover, this approach allows a single device to exhibit equally good performance as an n- or p-type transistor, by changing only the sign of the drain voltage. Even for CNFETs having ohmic contacts, an asymmetric design can greatly improve the performance for small-bandgap nanotubes.
While decreasing the oxide thickness in carbon nanotube field-effect transistors (CNFETs) improves the turn-on behavior, we demonstrate that this also requires scaling the range of the drain voltage. This scaling is needed to avoid an exponential increase in Off-current with drain voltage, due to modulation of the Schottky barriers at both the source and drain contact. We illustrate this with results for bottom-gated ambipolar CNFETs with oxides of 2 and 5 nm, and give an explicit scaling rule for the drain voltage. Above the drain voltage limit, the Off-current becomes large and has equal electron and hole contributions. This allows the recently reported light emission from appropriately biased CNFETs.
We show that carbon nanotube transistors exhibit scaling that is qualitatively different than conventional transistors. The performance depends in an unexpected way on both the thickness and the dielectric constant of the gate oxide. Experimental measurements and theoretical calculations provide a consistent understanding of the scaling, which reflects the very different device physics of a Schottky barrier transistor with a quasi-one-dimensional channel contacting a sharp edge. A simple analytic model gives explicit scaling expressions for key device parameters such as subthreshold slope, turn-on voltage, and transconductance.
We show that carbon nanotube transistors operate as unconventional "Schottky barrier transistors", in which transistor action occurs primarily by varying the contact resistance rather than the channel conductance. Transistor characteristics are calculated for both idealized and realistic geometries, and scaling behavior is demonstrated. Our results explain a variety of experimental observations, including the quite different effects of doping and adsorbed gases. The electrode geometry is shown to be crucial for good device performance.