Constant-temperature molecular-dynamics algorithms for mixed hard-core/continuous potentials

We present a set of second-order, time-reversible algorithms for the isothermal (NVT) molecular-dynamics (MD) simulation of systems with mixed hard-core/continuous potentials. The methods are generated by combining real-time Nose' thermostats with our previously developed Collision Verlet algorithm [Mol. Phys. 98, 309 (1999)] for constant energy MD simulation of such systems. In all we present 5 methods, one based on the Nose'-Hoover [Phys. Rev. A 31, 1695 (1985)] equations of motion and four based on the Nose'-Poincare' [J.Comp.Phys., 151 114 (1999)] real-time formulation of Nose' dynamics. The methods are tested using a system of hard spheres with attractive tails and all correctly reproduce a canonical distribution of instantaneous temperature. The Nose'-Hoover based method and two of the Nose'-Poincare' methods are shown to have good energy conservation in long simulations.Comment: 9 pages, 5 figure

A High-Precision Pulse-Width Modulator Source

A novel high-resolution pulse-width modulator (PWM) is being developed for a new digital regulator for the Advanced Photon Source power converters. The circuit features 82-ps setability over an 80-{micro}s range. Our application requires a 50-{micro}s fill-scale range; therefore the 82-ps setability is equivalent to better than 19 bits. The circuit is presently implemented as a VME module and is an integral part of the digital regulator prototype. The design concept and performance results will be presented

Intense Flowing Hollow Cathode Lamp

An inexpensive, simple, and versatile hollow cathode lamp has been developed which produces intense ion resonance radiation of about 1 mW into 4π/25 sr. The lamp employs flowing helium gas to sustain the discharge. The construction permits a variety of cathode seed materials to be easily interchanged. The same lamp has been used as a source of ion resonance radiation for Ca, Ba, Zn, Mg, Sr, Yb, and Eu

Excited-ion Lifetimes Using Penning-ionizing Collisions

We show that in the Penning-ionizing collision He(23S1)+X→He(11S0)+(X+)*+e the precession of the Hem spin at its Larmor frequency forces the J polarization of the Penning ion to rotate at the same frequency but shifted in phase. The tangent of the phase shift is proportional to the lifetime of the ionic state and presents a unique method for measuring excited-ion lifetimes. © 1972 The American Physical Society

Radiative Lifetimes And Alignment Depolarization Cross Sections For YbI And II By The Hanle Effect In A Flowing Helium System

The radiative lifetimes of the 17992-,25068-,28857-, 37414-, 40564-, and 44017-cm-1 neutral levels and the 30392-cm-1 ion level of Yb have been measured by the Hanle method in a fast-flowing He system. The lifetimes (in units of 10-9 sec) were found to be 820(20), 5.12(0.12), 14.4(0.4), 77.4(6.0), 9.32(0.6), 39.1(3.5), and 5.8(0.6), respectively. In a flowing system with He as a buffer gas the alignment depolarization cross sections with Yb were obtained and are reported here for the first time. They are (in units of 10-15 cm2) 5.0(1.0), 5.9(1.0), 5.9(1.2), 17.9(2.0), 28.2(3.0), 5.16(4.0), and 7.2(2.5), respectively. © 1976 The American Physical Society

On the weak-coupling limit and complete positivity

We consider two non-interacting systems embedded in a heat bath. If they remain dynamically independent, physical inconsistencies are avoided only if the single-system reduced dynamics is completely positive also beyond the weak-coupling limit.Comment: 11 pages, plain-Te

Quantum transport in carbon nanotubes

Carbon nanotubes are a versatile material in which many aspects of condensed matter physics come together. Recent discoveries, enabled by sophisticated fabrication, have uncovered new phenomena that completely change our understanding of transport in these devices, especially the role of the spin and valley degrees of freedom. This review describes the modern understanding of transport through nanotube devices. Unlike conventional semiconductors, electrons in nanotubes have two angular momentum quantum numbers, arising from spin and from valley freedom. We focus on the interplay between the two. In single quantum dots defined in short lengths of nanotube, the energy levels associated with each degree of freedom, and the spin-orbit coupling between them, are revealed by Coulomb blockade spectroscopy. In double quantum dots, the combination of quantum numbers modifies the selection rules of Pauli blockade. This can be exploited to read out spin and valley qubits, and to measure the decay of these states through coupling to nuclear spins and phonons. A second unique property of carbon nanotubes is that the combination of valley freedom and electron-electron interactions in one dimension strongly modifies their transport behaviour. Interaction between electrons inside and outside a quantum dot is manifested in SU(4) Kondo behavior and level renormalization. Interaction within a dot leads to Wigner molecules and more complex correlated states. This review takes an experimental perspective informed by recent advances in theory. As well as the well-understood overall picture, we also state clearly open questions for the field. These advances position nanotubes as a leading system for the study of spin and valley physics in one dimension where electronic disorder and hyperfine interaction can both be reduced to a very low level.Comment: In press at Reviews of Modern Physics. 68 pages, 55 figure

Institutional Need-based and Non-need Grants: Trends and Differences Among College and University Sectors

This study uses data from the National Postsecondary Student Aid Survey to analyze the use of need-based versus non-need financial aid awarded by colleges and universities in 1989 and 1995. Descriptive and trend analyses were used to examine differences in the use of these types of aid among varying institutional types, and to examine how financial aid awards have changed for full-time undergraduate students from different income backgrounds

Inheritance of Black Hair Patterns in Cattle Lacking the Extension Factor for Black (E.). III, A Multiple Allelic Hypothesis to Explain the Inheritance of Blackish and Blackish Pattern

Author Institution: Department of Dairy Science and the Institute of Genetics, The Ohio State University, Columbus and Department of Dairy Science, The Ohio Agricultural Experiment Station, Wooste

Symplectic algorithm for constant-pressure molecular dynamics using a Nose-Poincare thermostat

We present a new algorithm for isothermal-isobaric molecular-dynamics simulation. The method uses an extended Hamiltonian with an Andersen piston combined with the Nos'e-Poincar'e thermostat, recently developed by Bond, Leimkuhler and Laird [J. Comp. Phys., 151, (1999)]. This Nos'e-Poincar'e-Andersen (NPA) formulation has advantages over the Nos'e-Hoover-Andersen approach in that the NPA is Hamiltonian and can take advantage of symplectic integration schemes, which lead to enhanced stability for long-time simulations. The equations of motion are integrated using a Generalized Leapfrog Algorithm and the method is easy to implement, symplectic, explicit and time reversible. To demonstrate the stability of the method we show results for test simulations using a model for aluminum.Comment: 7 page