Ehrenfest time dependent suppression of weak localization

The Ehrenfest time dependence of the suppression of the weak localization correction to the conductance of a {\em clean} chaotic cavity is calculated. Unlike in earlier work, no impurity scattering is invoked to imitate diffraction effects. The calculation extends the semiclassical theory of K. Richter and M. Sieber [Phys. Rev. Lett. {\bf 89}, 206801 (2002)] to include the effect of a finite Ehrenfest time.Comment: 3 Pages, 1 Figure, RevTe

Quantum-to-classical crossover of mesoscopic conductance fluctuations

We calculate the system-size-over-wave-length ($M$) dependence of sample-to-sample conductance fluctuations, using the open kicked rotator to model chaotic scattering in a ballistic quantum dot coupled by two $N$-mode point contacts to electron reservoirs. Both a fully quantum mechanical and a semiclassical calculation are presented, and found to be in good agreement. The mean squared conductance fluctuations reach the universal quantum limit of random-matrix-theory for small systems. For large systems they increase $\propto M^2$ at fixed mean dwell time $\tau_D \propto M/N$. The universal quantum fluctuations dominate over the nonuniversal classical fluctuations if $N < \sqrt{M}$. When expressed as a ratio of time scales, the quantum-to-classical crossover is governed by the ratio of Ehrenfest time and ergodic time.Comment: 5 pages, 5 figures: one figure added, references update

Ab initio electronic-structure calculations on the Nb/Ta multilayer system

Ab initio electronic-structure calculations are performed for Nb/Ta multilayers with small modulation wavelengths in the [100], [110] and [111] BCC modulation directions. The [110] modulated Nb/Ta multilayer with the smallest modulation wavelength was also calculated with an 8% increased modulation wavelength on the one hand, with an undisturbed structure but containing 50% interstitial hydrogen on the other hand, and with a combination of both. The ease with which Nb/Ta multilayers can be grown in different modulation directions is understood from total energy calculations which show energy differences of less than 1 mRyd between the various directions. Further, it is found that interstitial hydrogen lowers the density of states at the Fermi level and that, but to a lesser extent, the lattice expansion on hydrogenation raises the density of states at the Fermi level. The net effect of a lowering of the density of states at the Fermi level is in agreement with the measured reduction of the superconducting critical temperature on hydrogenation.

Ab initio electronic-structure calculations on the Au/Ag multilayer system and Au epitaxy on the Ag(110) surface

Ab initio electronic-structure calculations are performed for Au/Ag multilayers with small modulation wavelengths in the [100], [110], and [111] fcc modulation directions and for Ag [110] slabs covered with none, one, two, or three Au(110) layers. The total energies of the multilayers can be ordered according to the number of Au-Ag nearest-neighbor pairs in these systems, from which the bonding at the interface can be estimated. The multilayer and slab results taken together allow for a decomposition of the energetics of Au adlayer behavior on the Ag(110) surface. It is found that a bare Ag(110) surface is energetically more favorable than an Ag(110) surface covered by Au monolayers. This is in agreement with the difference in surface energy of Au and Ag surfaces, and it implies that the bonding energy gained at the Au-Ag interface is insufficient to overcome this difference. Therefore Au will tend to grow in islands on an Ag(110) surface rather than wetting it. Our results, based on Au-Ag bonding and the open geometry of the Ag(110) surface, support the bilayer-growth model that has recently been reported in the literature, but this support is only for the first two layers in the islands. Whereas the density-of-states (DOS) curves of the multilayers show hardly any variation for the different systems, the layer-resolved densities of states for the slabs show pronounced changes on going from the surface layers inward. For all slabs, with or without Au atop, an interface state can be observed, localized between the first and second surface layer. Furthermore, a shift in the d-state density to higher energies at the interface and to lower energies for the layer below the interface occurs as compared with the elemental DOS curves.

Ab initio electronic-structure calculations on the Nb/Cu multilayer system

Ab initio electronic-structure calculations are reported for coherent and incoherent Nb/Cu multilayem. An incoherent unit cell describing three Nb BCC (110) layers and three Cu FCC (111) layers is constructed for the layers in the Nishiyama- Wasserman orientation and with relaxed atomic positions at the interface. It is found that the total density of states is a combination of the broadened DOS curves of the parent metals and that at the interface Nb has a decreased, and Cu has an increased, density of states at the Fermi energy. These results are in agreement with experimetital results and for Nb can be explained by a broadening of the density of states. Possible coherency for small modulation wavelengths is investigated by calculating the total energies for overall BCC [110] and overall FCC [111] Nb/Cu multilayers consisting of one layer of each metal and comparing these with the total energy results of the incoherent structure. The positive interface energy found for the Nb/Cu system favours the incoherent multilayer over the coherent multilayers, where a positive structural energy is also involved.

Ab initio electronic-structure calculations on the Nb/Zr multilayer system

Ab initio electronic-structure calculations are performed for the Nb/Zr metallic multilayer system in the coherent bcc structure and in the incoherent bcc/hcp structure, observed for small and larger modulation wavelengths, respectively. A new calculational scheme, the localized-spherical-wave method, has been used. This method is optimized for handling large unit cells since it avoids laborious lattice summations. For the coherent structure a range of 1 to 6 monolayers for each metal is considered. The results for the incoherent structure are restricted to 5 monolayers per constituent metal. A model for the Nb/Zr incoherent bcc/hcp unit cell is given, including a relaxed structure at the interface, which leads to 55 atoms inside the unit cell. The measured modulation-wavelength dependence of the electronic specific-heat coefficient γ and the superconducting transition temperature Tc are explained in terms of the calculated results for the density of states. Furthermore, the coherent to incoherent structural phase transition is in agreement with the modulation-wavelength dependence of the total energies. The effects of multilayering both on the density of states and the band structure are traced by comparing the results with those for the constituent pure metallic systems.