7,308 research outputs found
Nonadiabatic Effects on Peptide Vibrational Dynamics Induced by Conformational Changes
Quantum dynamical simulations of vibrational spectroscopy have been carried out for glycine dipeptide (CH3-CO-NH-CH2-CO-NH-CH3). Conformational structure and dynamics are modeled in terms of the two Ramachandran dihedral angles of the molecular backbone. Potential energy surfaces and harmonic frequencies are obtained from electronic structure calculations at the density functional theory (B3LYP/6-31+G(d)) level. The ordering of the energetically most stable isomers (C7 and C5) is reversed upon inclusion of the quantum mechanical zero point vibrational energy. Vibrational spectra of various isomers show distinct differences, mainly in the region of the amide modes, thereby relating conformational structures and vibrational spectra. Conformational dynamics is modeled by propagation of quantum mechanical wave packets. Assuming a directed energy transfer to the torsional degrees of freedom, transitions between the C7 and C5 minimum energy structures occur on a sub-picosecond timescale (700 ... 800 fs). Vibrationally non-adiabatic effects are investigated for the case of the coupled, fundamentally excited amide I states. Using a two state-two mode model, the resulting wave packet dynamics is found to be strongly non-adiabatic due to the presence of a seam of the two potential energy surfaces. Initially prepared adiabatic vibrational states decay upon conformational change on a timescale of 200 ... 500 fs with population transfer of more than 50 % between the coupled amide I states. Also the vibrational energy transport between localized (excitonic) amide I vibrational states is strongly influenced by torsional dynamics of the molecular backbone where both enhanced and reduced decay rates are found. All these observations should allow the detection of conformational changes by means of time-dependent vibrational spectroscopy
Archeops, mapping the CMB sky from large to small angular scales
Archeops is a balloon-borne experiment designed to measure the temperature
fluctuations of the CMB on a large region of the sky () with a high
angular resolution (10 arcminutes) and a high sensitivity ( per
pixel). Archeops will perform a measurement of the CMB anisotropies power
spectrum from large angular scales () to small angular scales
(). Archeops flew for the first time for a test flight in July
1999 from Sicily to Spain and the first scientific flight took place from
Sweden to Russia in January 2001. The data analysis is on its way and I present
here preliminary results, realistic simulations showing the expected accuracy
on the measurement of the power spectrum and perspectives for the incoming
flights (Winter 2001/2003).Comment: 6 pages, 6 figures, proceedings to TAUP2001 conference, LNGS, Italy,
Sept. 200
QCD at non-zero chemical potential and temperature from the lattice
A study of QCD at non-zero chemical potential, mu, and temperature, T, is
performed using the lattice technique. The transition temperature (between the
confined and deconfined phases) is determined as a function of mu and is found
to be in agreement with other work. In addition the variation of the pressure
and energy density with mu is obtained for small positive mu. These results are
of particular relevance for heavy-ion collision experiments.Comment: Invited paper presented at the Joint Workshop on Physics at the
Japanese Hadron Facility, March 2002, Adelaide. 10 pages, uses
ws-procs9x6.cls style file (provided
Motional sidebands and direct measurement of the cooling rate in the resonance fluorescence of a single trapped ion
Resonance fluorescence of a single trapped ion is spectrally analyzed using a
heterodyne technique. Motional sidebands due to the oscillation of the ion in
the harmonic trap potential are observed in the fluorescence spectrum. From the
width of the sidebands the cooling rate is obtained and found to be in
agreement with the theoretical prediction.Comment: 4 pages, 4 figures. Final version after minor changes, 1 figure
replaced; to be published in PRL, July 10, 200
On Hausdorff dimension of the set of closed orbits for a cylindrical transformation
We deal with Besicovitch's problem of existence of discrete orbits for
transitive cylindrical transformations
where is an
irrational rotation on the circle \T and \varphi:\T\to\R is continuous,
i.e.\ we try to estimate how big can be the set
D(\alpha,\varphi):=\{x\in\T:|\varphi^{(n)}(x)|\to+\infty\text{as}|n|\to+\infty\}.
We show that for almost every there exists such that the
Hausdorff dimension of is at least . We also provide a
Diophantine condition on that guarantees the existence of
such that the dimension of is positive. Finally, for some
multidimensional rotations on \T^d, , we construct smooth
so that the Hausdorff dimension of is positive.Comment: 32 pages, 1 figur
Exciton-magnon transitions in the frustrated chromium antiferromagnets CuCrO2, alpha-CaCr2O4, CdCr2O4, and ZnCr2O4
We report on optical transmission spectroscopy of the Cr-based frustrated
triangular antiferromagnets CuCrO2 and alpha-CaCr2O4, and the spinels CdCr2O4
and ZnCr2O4 in the near-infrared to visible-light frequency range. We explore
the possibility to search for spin correlations far above the magnetic ordering
temperature and for anomalies in the magnon lifetime in the magnetically
ordered state by probing exciton-magnon sidebands of the spin-forbidden
crystal-field transitions of the Cr3+ ions (spin S = 3/2). In CuCrO2 and
alpha-CaCr2O4 the appearance of fine structures below T_N is assigned to magnon
sidebands by comparison with neutron scattering results. The temperature
dependence of the line width of the most intense sidebands in both compounds
can be described by an Arrhenius law. For CuCrO2 the sideband associated with
the 4A2 -> 2T2 transition can be observed even above T_N. Its line width does
not show a kink at the magnetic ordering temperature and can alternatively be
described by a Z2 vortex scenario proposed previously for similar materials.
The exciton-magnon features in alpha-CaCr2O4 are more complex due to the
orthorhombic distortion. While for CdCr2O4 magnon sidebands are identified
below T_N and one sideband excitation is found to persist across the magnetic
ordering transition, only a weak fine structure related to magnetic ordering
has been observed in ZnCr2O4.Comment: 14 pages, 10 figures, all comments are welcome and appreciated,
accepted for publication in PR
Global monitoring of tropospheric water vapor with GPS radio occultation aboard CHAMP
The paper deals with application of GPS radio occultation (RO) measurements
aboard CHAMP for the retrieval of tropospheric water vapor profiles. The GPS RO
technique provides a powerful tool for atmospheric sounding which requires no
calibration, is not affected by clouds, aerosols or precipitation, and provides
an almost uniform global coverage. We briefly overview data processing and
retrieval of vertical refractivity, temperature and water vapor profiles from
GPS RO observations. CHAMP RO data are available since 2001 with up to 200 high
resolution atmospheric profiles per day. Global validation of CHAMP water vapor
profiles with radiosonde data reveals a bias of about 0.2 g/kg and a standard
deviation of less than 1 g/kg specific humidity in the lower troposphere. We
demonstrate potentials of CHAMP RO retrievals for monitoring the mean
tropospheric water vapor distribution on a global scale.Comment: 7 pages, 4 figure
Cold Trapped Ions as Quantum Information Processors
In this tutorial we review physical implementation of quantum computing using
a system of cold trapped ions. We discuss systematically all the aspects for
making the implementation possible. Firstly, we go through the loading and
confining of atomic ions in the linear Paul trap, then we describe the
collective vibrational motion of trapped ions. Further, we discuss interactions
of the ions with a laser beam. We treat the interactions in the travelling-wave
and standing-wave configuration for dipole and quadrupole transitions. We
review different types of laser cooling techniques associated with trapped
ions. We address Doppler cooling, sideband cooling in and beyond the Lamb-Dicke
limit, sympathetic cooling and laser cooling using electromagnetically induced
transparency. After that we discuss the problem of state detection using the
electron shelving method. Then quantum gates are described. We introduce
single-qubit rotations, two-qubit controlled-NOT and multi-qubit controlled-NOT
gates. We also comment on more advanced multi-qubit logic gates. We describe
how quantum logic networks may be used for the synthesis of arbitrary pure
quantum states. Finally, we discuss the speed of quantum gates and we also give
some numerical estimations for them. A discussion of dynamics on off-resonant
transitions associated with a qualitative estimation of the weak coupling
regime and of the Lamb-Dicke regime is included in Appendix.Comment: 44 revtex pages, 23 figures, to appear in Journal of Modern Optic
Sympathetic ground state cooling and coherent manipulation with two-ion-crystals
We have cooled a two-ion-crystal to the ground state of its collective modes
of motion. Laser cooling, more specific resolved sideband cooling is performed
sympathetically by illuminating only one of the two Ca ions in the
crystal. The heating rates of the motional modes of the crystal in our linear
trap have been measured, and we found them considerably smaller than those
previously reported by Q. Turchette {\em et. al.} Phys. Rev. A 61, 063418
(2000) in the case of trapped Be ions. After the ground state is
prepared, coherent quantum state manipulation of the atomic population can be
performed. Within the coherence time, up to 12 Rabi oscillations are observed,
showing that many coherent manipulations can be achieved. Coherent excitation
of each ion individually and ground state cooling are important tools for the
realization of quantum information processing in ion traps
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