Effective time-reversal symmetry breaking in the spin relaxation in a graphene quantum dot

We study the relaxation of a single electron spin in a circular gate-tunbable quantum dot in gapped graphene. Direct coupling of the electron spin to out-of-plane phonons via the intrinsic spin-orbit coupling leads to a relaxation time T_1 which is independent of the B-field at low fields. We also find that Rashba spin-orbit induced admixture of opposite spin states in combination with the emission of in-plane phonons provides various further relaxation channels via deformation potential and bond-length change. In the absence of valley mixing, spin relaxation takes place within each valley separately and thus time-reversal symmetry is effectively broken, thus inhibiting the van Vleck cancellation at B=0 known from GaAs quantum dots. Both the absence of the van Vleck cancellation as well as the out-of-plane phonons lead to a behavior of the spin relaxation rate at low magnetic fields which is markedly different from the known results for GaAs. For low B-fields, we find that the rate is constant in B and then crosses over to ~B^2 or ~B^4 at higher fields.Comment: 5 pages, 2 figures, 1 tabl

Cloud fluid models of gas dynamics and star formation in galaxies

The large dynamic range of star formation in galaxies, and the apparently complex environmental influences involved in triggering or suppressing star formation, challenges the understanding. The key to this understanding may be the detailed study of simple physical models for the dominant nonlinear interactions in interstellar cloud systems. One such model is described, a generalized Oort model cloud fluid, and two simple applications of it are explored. The first of these is the relaxation of an isolated volume of cloud fluid following a disturbance. Though very idealized, this closed box study suggests a physical mechanism for starbursts, which is based on the approximate commensurability of massive cloud lifetimes and cloud collisional growth times. The second application is to the modeling of colliding ring galaxies. In this case, the driving processes operating on a dynamical timescale interact with the local cloud processes operating on the above timescale. The results is a variety of interesting nonequilibrium behaviors, including spatial variations of star formation that do not depend monotonically on gas density

Spin-orbit-induced strong coupling of a single spin to a nanomechanical resonator

We theoretically investigate the deflection-induced coupling of an electron spin to vibrational motion due to spin-orbit coupling in suspended carbon nanotube quantum dots. Our estimates indicate that, with current capabilities, a quantum dot with an odd number of electrons can serve as a realization of the Jaynes-Cummings model of quantum electrodynamics in the strong-coupling regime. A quantized flexural mode of the suspended tube plays the role of the optical mode and we identify two distinct two-level subspaces, at small and large magnetic field, which can be used as qubits in this setup. The strong intrinsic spin-mechanical coupling allows for detection, as well as manipulation of the spin qubit, and may yield enhanced performance of nanotubes in sensing applications.Comment: 5 pages, 3 figures + appendix; published versio

Spin exchange interaction with tunable range between graphene quantum dots

We study the spin exchange between two electrons localized in separate quantum dots in graphene. The electronic states in the conduction band are coupled indirectly by tunneling to a common continuum of delocalized states in the valence band. As a model, we use a two-impurity Anderson Hamiltonian which we subsequently transform into an effective spin Hamiltonian by way of a two-stage Schrieffer-Wolff transformation. We then compare our result to that from a Coqblin-Schrieffer approach as well as to fourth order perturbation theory.Comment: 8 pages, 3 figure

On incorporating uncertainty analysis in abstract building performance simulation tools

Building Performance Simulation (BPS) is a powerful measure to educate the building design process. However, its use in practice is too large extends limited to the detailed design stage dedicated to the code compliance analysis of worked out design proposals. BPS is not much used to support the conceptual design stage (CDS). To date BPS – tools are regarded as pure analysis tools, which do not provide design information. It is hypothesized that, when integrating uncertainty analysis techniques to existing BPS – tools (incremental research approach) valuable design information can be provided. The paper gives an update on the process to integrate an uncertainty assessment capability to a tool specifically developed to support the conceptual design stage. The resulting prototype should be capable of providing information about the deviation of specific design parameters (simulation output) based on the uncertainties of the building concept specification (simulation input). In addition to the total uncertainties, the results also indicate sensitivities of the design parameters as a result of the concept specification variation. This paper describes an approach to add extra-capabilities to legacy simulation tools and presents a comparison of uncertainties and total sensitivities calculated with one detailed design analysis (DDA) tool, IES, and one conceptual design analysis (CDA) tool , LEA. The main focus was herby the representation of material properties and its impact on the model uncertainty

On incorporating uncertainty analysis in abstract building performance simulation tools

Building Performance Simulation (BPS) is a powerful measure to educate the building design process. However, its use in practice is too large extends limited to the detailed design stage dedicated to the code compliance analysis of worked out design proposals. BPS is not much used to support the conceptual design stage (CDS). To date BPS – tools are regarded as pure analysis tools, which do not provide design information. It is hypothesized that, when integrating uncertainty analysis techniques to existing BPS – tools (incremental research approach) valuable design information can be provided. The paper gives an update on the process to integrate an uncertainty assessment capability to a tool specifically developed to support the conceptual design stage. The resulting prototype should be capable of providing information about the deviation of specific design parameters (simulation output) based on the uncertainties of the building concept specification (simulation input). In addition to the total uncertainties, the results also indicate sensitivities of the design parameters as a result of the concept specification variation. This paper describes an approach to add extra-capabilities to legacy simulation tools and presents a comparison of uncertainties and total sensitivities calculated with one detailed design analysis (DDA) tool, IES, and one conceptual design analysis (CDA) tool , LEA. The main focus was herby the representation of material properties and its impact on the model uncertainty

Tunable gauge potential for neutral and spinless particles in driven lattices

We present a universal method to create a tunable, artificial vector gauge potential for neutral particles trapped in an optical lattice. The necessary Peierls phase of the hopping parameters between neighboring lattice sites is generated by applying a suitable periodic inertial force such that the method does not rely on any internal structure of the particles. We experimentally demonstrate the realization of such artificial potentials, which generate ground state superfluids at arbitrary non-zero quasi-momentum. We furthermore investigate possible implementations of this scheme to create tuneable magnetic fluxes, going towards model systems for strong-field physics