158 research outputs found
Spectroscopic studies in open quantum systems
The spectroscopic properties of an open quantum system are determined by the
eigenvalues and eigenfunctions of an effective Hamiltonian H consisting of the
Hamiltonian H_0 of the corresponding closed system and a non-Hermitian
correction term W arising from the interaction via the continuum of decay
channels. The eigenvalues E_R of H are complex. They are the poles of the
S-matrix and provide both the energies and widths of the states. We illustrate
the interplay between Re(H) and Im(H) by means of the different interference
phenomena between two neighboured resonance states. Level repulsion along the
real axis appears if the interaction is caused mainly by Re(H) while a
bifurcation of the widths appears if the interaction occurs mainly due to
Im(H). We then calculate the poles of the S-matrix and the corresponding
wavefunctions for a rectangular microwave resonator with a scatter as a
function of the area of the resonator as well as of the degree of opening to a
guide. The calculations are performed by using the method of exterior complex
scaling. Re(W) and Im(W) cause changes in the structure of the wavefunctions
which are permanent, as a rule. At full opening to the lead, short-lived
collective states are formed together with long-lived trapped states. The
wavefunctions of the short-lived states at full opening to the lead are very
different from those at small opening. The resonance picture obtained from the
microwave resonator shows all the characteristic features known from the study
of many-body systems in spite of the absence of two-body forces. The poles of
the S-matrix determine the conductance of the resonator. Effects arising from
the interplay between resonance trapping and level repulsion along the real
axis are not involved in the statistical theory.Comment: The six jpg files are not included in the tex-fil
Spin-orbit effects in armchair carbon nanotubes: analytical results
Energy spectra and transport properties of armchair nanotubes with curvature
induced spin-orbit interaction are investigated thoroughly. The spin-orbit
interaction consists of two terms: the first one preserves the spin symmetry in
rotating frame, while the second one breaks it. It is found that the both terms
are equally important: i)at scattering on the potential step which mimics a
long-range potential in the nanotubes; ii)at transport via nanotube quantum
dots. It is shown that an armchair nanotube with the first spin-orbit term
works as an ideal spin-filter, while the second term produces a parasitic
inductance.Comment: 11 pages, 10 figure
Spin control in semiconductor quantum wires
We show that spin-flip rotation in a semiconductor quantum wire, caused by
the Rashba and the Dresselhaus interactions (both of arbitrary strengths), can
be suppressed by dint of an in-plane magnetic field. We found a new type of
symmetry, which arises at a particular set of intensity and orientation of the
magnetic field and explains this suppression. Based on our findings, we propose
a transport experiment to measure the strengths of the Rashba and the
Dresselhaus interactions.Comment: 4 pages, 4 figure
Resonant tuning of Langevin transducers for ultrasonically assisted machining applications
This paper provides a fundamental study into the trade-offs between the location of piezoceramic elements, resonant frequency, and achievable ultrasonic vibration amplitude at the working end of the Bolted Langevin-style Transducers (BLT) for Ultrasonically Assisted Machining (UAM) applications. Analytical models and Finite Element (FE) models are established for theoretical study, which are then validated by experiments on four real electro-mechanical transducers. Results suggest that resonant frequency and oscillation amplitude of the BLTs depend essentially on the dimensions of the system and the location of piezoceramic elements. The highest resonant frequency and the maximal vibration are achieved when the piezoceramic elements are at the longitudinal displacement node, where the highest effective electro-mechanical coupling coefficient value is exhibited. However, the minimal resonant frequency and the lowest vibration, which is almost equal to zero, are observed when the piezoceramic elements are located at the displacement anti-node. In addition, the longitudinal displacement node locations are dependent on the resonant frequency of the devices rather than the locations of the piezoceramic elements
Mechanistic insights into allosteric regulation of the A2A adenosine G protein-coupled receptor by physiological cations.
Cations play key roles in regulating G-protein-coupled receptors (GPCRs), although their mechanisms are poorly understood. Here, 19F NMR is used to delineate the effects of cations on functional states of the adenosine A2A GPCR. While Na+ reinforces an inactive ensemble and a partial-agonist stabilized state, Ca2+ and Mg2+ shift the equilibrium toward active states. Positive allosteric effects of divalent cations are more pronounced with agonist and a G-protein-derived peptide. In cell membranes, divalent cations enhance both the affinity and fraction of the high affinity agonist-bound state. Molecular dynamics simulations suggest high concentrations of divalent cations bridge specific extracellular acidic residues, bringing TM5 and TM6 together at the extracellular surface and allosterically driving open the G-protein-binding cleft as shown by rigidity-transmission allostery theory. An understanding of cation allostery should enable the design of allosteric agents and enhance our understanding of GPCR regulation in the cellular milieu
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