463 research outputs found
Semiclassical Scattering in a Circular Semiconductor Microstructure
The conductance of a microscopic junction shows fluctuations caused by quantum interference of waves that follow different paths between the leads. We give a semiclassical formula for these fluctuations. The theory utilizes trajectories which travel between the centers of the lead apertures; it also incorporates diffraction at these apertures. We extend the theory to include ‘‘ghost paths,’’ which scatter diffractively off the lead mouths. Semiclassical S-matrix elements are computed for a circular junction over a range of Fermi wave numbers, and the large-scale structure of these matrix elements shows good agreement with quantum results. Finally, we propose a hypothesis about the effect of the quantum coherence length on the S matrix and on the semiclassical sum. © 1996 The American Physical Society
Excitation of weakly bound Rydberg electrons by half-cycle pulses
The interaction of a weakly bound Rydberg electron with an electromagnetic
half-cycle pulse (HCP) is described with the help of a multidimensional
semiclassical treatment. This approach relates the quantum evolution of the
electron to its underlying classical dynamics. The method is nonperturbative
and is valid for arbitrary spatial and temporal shapes of the applied HCP. On
the basis of this approach angle- and energy-resolved spectra resulting from
the ionization of Rydberg atoms by HCPs are analyzed. The different types of
spectra obtainable in the sudden-impact approximation are characterized in
terms of the appearing semiclassical scattering phenomena. Typical
modifications of the spectra originating from finite pulse effects are
discussed.Comment: Submitted to Phys. Rev.
Negative length orbits in normal-superconductor billiard systems
The Path-Length Spectra of mesoscopic systems including diffractive
scatterers and connected to superconductor is studied theoretically. We show
that the spectra differs fundamentally from that of normal systems due to the
presence of Andreev reflection. It is shown that negative path-lengths should
arise in the spectra as opposed to normal system. To highlight this effect we
carried out both quantum mechanical and semiclassical calculations for the
simplest possible diffractive scatterer. The most pronounced peaks in the
Path-Length Spectra of the reflection amplitude are identified by the routes
that the electron and/or hole travels.Comment: 4 pages, 4 figures include
Quantum control and the Strocchi map
Identifying the real and imaginary parts of wave functions with coordinates
and momenta, quantum evolution may be mapped onto a classical Hamiltonian
system. In addition to the symplectic form, quantum mechanics also has a
positive-definite real inner product which provides a geometrical
interpretation of the measurement process. Together they endow the quantum
Hilbert space with the structure of a K\"{a}ller manifold. Quantum control is
discussed in this setting. Quantum time-evolution corresponds to smooth
Hamiltonian dynamics and measurements to jumps in the phase space. This adds
additional power to quantum control, non unitarily controllable systems
becoming controllable by ``measurement plus evolution''. A picture of quantum
evolution as Hamiltonian dynamics in a classical-like phase-space is the
appropriate setting to carry over techniques from classical to quantum control.
This is illustrated by a discussion of optimal control and sliding mode
techniques.Comment: 16 pages Late
Structure calculation, refinement and validation using CcpNmr Analysis
CcpNmr Analysis provides a streamlined pipeline for both NMR chemical shift assignment and structure determination of biological macromolecules. In addition, it encompasses tools to analyse the many additional experiments that make NMR such a pivotal technique for research into complex biological questions. This report describes how CcpNmr Analysis can seamlessly link together all of the tasks in the NMR structure-determination process. It details each of the stages from generating NMR restraints [distance, dihedral,hydrogen bonds and residual dipolar couplings (RDCs)],exporting these to and subsequently re-importing them from structure-calculation software (such as the programs CYANA or ARIA) and analysing and validating the results obtained from the structure calculation to, ultimately, the streamlined deposition of the completed assignments and the refined ensemble of structures into the PDBe repository. Until recently, such solution-structure determination by NMR has been quite a laborious task, requiring multiple stages and programs. However, with the new enhancements to CcpNmr Analysis described here, this process is now much more intuitive and efficient and less error-prone
Diffraction and boundary conditions in semi-classical open billiards
The conductance through open quantum dots or quantum billiards shows
fluctuations, that can be explained as interference between waves following
different paths between the leads of the billiard. We examine such systems by
the use of a semi-classical Green's functions. In this paper we examine how the
choice of boundary conditions at the lead mouths affect the diffraction. We
derive a new formula for the S-matrix element. Finally we compare
semi-classical simulations to quantum mechanical ones, and show that this new
formula yield superior results.Comment: 7 pages, 4 figure
Estimation of interdomain flexibility of N-terminus of factor H using residual dipolar couplings
Characterization of segmental flexibility is needed to understand the biological mechanisms of the very large category of functionally diverse proteins, exemplified by the regulators of complement activation, that consist of numerous compact modules or domains linked by short, potentially flexible, sequences of amino acid residues. The use of NMR-derived residual dipolar couplings (RDCs), in magnetically aligned media, to evaluate interdomain motion is established but only for two-domain proteins. We focused on the three N-terminal domains (called CCPs or SCRs) of the important complement regulator, human factor H (i.e. FH1-3). These domains cooperate to facilitate cleavage of the key complement activation-specific protein fragment, C3b, forming iC3b that no longer participates in the complement cascade. We refined a three-dimensional solution structure of recombinant FH1-3 based on nuclear Overhauser effects and RDCs. We then employed a rudimentary series of RDC datasets, collected in media containing magnetically aligned bicelles (disk-like particles formed from phospholipids) under three different conditions, to estimate interdomain motions. This circumvents a requirement of previous approaches for technically difficult collection of five independent RDC datasets. More than 80% of conformers of this predominantly extended three-domain molecule exhibit flexions of < 40 °. Such segmental flexibility (together with the local dynamics of the hypervariable loop within domain 3), could facilitate recognition of C3b via initial anchoring and eventual reorganization of modules to the conformation captured in the previously solved crystal structure of a C3b:FH1-4 complex
Mapping the encounter state of a transient protein complex by PRE NMR spectroscopy
Many biomolecular interactions proceed via a short-lived encounter state, consisting of multiple, lowly-populated species invisible to most experimental techniques. Recent development of paramagnetic relaxation enhancement (PRE) nuclear magnetic resonance (NMR) spectroscopy has allowed to directly visualize such transient intermediates in a number of protein-protein and protein-DNA complexes. Here we present an analysis of the recently published PRE NMR data for a protein complex of yeast cytochrome c (Cc) and cytochrome c peroxidase (CcP). First, we describe a simple, general method to map out the spatial and temporal distributions of binding geometries constituting the Cc-CcP encounter state. We show that the spatiotemporal mapping provides a reliable estimate of the experimental coverage and, at higher coverage levels, allows to delineate the conformational space sampled by the minor species. To further refine the encounter state, we performed PRE-based ensemble simulations. The generated solutions reproduce well the experimental data and lie within the allowed regions of the encounter maps, confirming the validity of the mapping approach. The refined encounter ensembles are distributed predominantly in a region encompassing the dominant form of the complex, providing experimental proof for the results of classical theoretical simulations
Geometry-dependent scattering through quantum billiards: Experiment and theory
We present experimental studies of the geometry-specific quantum scattering
in microwave billiards of a given shape. We perform full quantum mechanical
scattering calculations and find an excellent agreement with the experimental
results. We also carry out the semiclassical calculations where the conductance
is given as a sum of all classical trajectories between the leads, each of them
carrying the quantum-mechanical phase. We unambiguously demonstrate that the
characteristic frequencies of the oscillations in the transmission and
reflection amplitudes are related to the length distribution of the classical
trajectories between the leads, whereas the frequencies of the probabilities
can be understood in terms of the length difference distribution in the pairs
of classical trajectories. We also discuss the effect of non-classical "ghost"
trajectories that include classically forbidden reflection off the lead mouths.Comment: 4 pages, 4 figure
Optical creation of vibrational intrinsic localized modes in anharmonic lattices with realistic interatomic potentials
Using an efficient optimal control scheme to determine the exciting fields,
we theoretically demonstrate the optical creation of vibrational intrinsic
localized modes (ILMs) in anharmonic perfect lattices with realistic
interatomic potentials. For systems with finite size, we show that ILMs can be
excited directly by applying a sequence of femtosecond visible laser pulses at
THz repetition rates. For periodic lattices, ILMs can be created indirectly via
decay of an unstable extended lattice mode which is excited optically either by
a sequence of pulses as described above or by a single picosecond far-infrared
laser pulse with linearly chirped frequency. In light of recent advances in
experimental laser pulse shaping capabilities, the approach is experimentally
promising.Comment: 20 pages, 7 eps figures. Accepted, Phys. Rev.
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