88 research outputs found
Non-Markovian Dynamics in Ultracold Rydberg Aggregates
We propose a setup of an open quantum system in which the environment can be
tuned such that either Markovian or non-Markovian system dynamics can be
achieved. The implementation uses ultracold Rydberg atoms, relying on their
strong long-range interactions. Our suggestion extends the features available
for quantum simulators of molecular systems employing Rydberg aggregates and
presents a new test bench for fundamental studies of the classification of
system-environment interactions and the resulting system dynamics in open
quantum systems.Comment: 13 pages, 4 figure
Quantum simulation of energy transport with embedded Rydberg aggregates
We show that an array of ultracold Rydberg atoms embedded in a laser driven
background gas can serve as an aggregate for simulating exciton dynamics and
energy transport with a controlled environment. Spatial disorder and
decoherence introduced by the interaction with the background gas atoms can be
controlled by the laser parameters. This allows for an almost ideal realization
of a Haken-Reineker-Strobl type model for energy transport. Physics can be
monitored using the same mechanism that provides control over the environment.
The degree of decoherence is traced back to information gained on the
excitation location through the monitoring, turning the setup into an
experimentally accessible model system for studying the effects of quantum
measurements on the dynamics of a many-body quantum system.Comment: 5 pages, 4 figures, 3 pages supp. in
Optomechanical interactions in non-Hermitian photonic molecules
We study optomechanical interactions in non-Hermitian photonic molecules that support two photonic states and one acoustic mode. The nonlinear steady-state solutions and their linear stability landscapes are investigated as a function of the system\u27s parameters and excitation power levels. We also examine the temporal evolution of the system and uncover different regimes of nonlinear dynamics. Our analysis reveals several important results: (1) parity-time () symmetry is not necessarily the optimum choice for maximum optomechanical interaction. (2) Stable steady-state solutions are not always reached under continuous wave optical excitations. (3) Accounting for gain saturation effects can regulate the behavior of the otherwise unbounded oscillation amplitudes. Our study provides a deeper insight into the interplay between optical non-Hermiticity and optomechanical coupling and can thus pave the way for new device applications
Pressure-induced metallization and structural phase transition of the Mott-Hubbard insulator TiOBr
We investigated the pressure-dependent optical response of the
low-dimensional Mott-Hubbard insulator TiOBr by transmittance and reflectance
measurements in the infrared and visible frequency range. A suppression of the
transmittance above a critical pressure and a concomitant increase of the
reflectance are observed, suggesting a pressure-induced metallization of TiOBr.
The metallic phase of TiOBr at high pressure is confirmed by the presence of
additional excitations extending down to the far-infrared range. The
pressure-induced metallization coincides with a structural phase transition,
according to the results of x-ray powder diffraction experiments under
pressure.Comment: 4 pages, 3 figure
Two pressure-induced structural phase transitions in TiOCl
We studied the crystal structure of TiOCl up to pressures of =25~GPa at
room temperature by x-ray powder diffraction measurements. Two pressure-induced
structural phase transitions are observed: At 15~GPa emerges
an 22 superstructure with -axis unique monoclinic
symmetry (space group P2/). At 22~GPa all lattice
parameters of the monoclinic phase show a pronounced anomaly. A fraction of the
sample persists in the ambient orthorhombic phase (space group ) over the
whole pressure range.Comment: 5 pages, 5 figures; accepted for publication in Phys. Rev.
Full counting statistics of laser excited Rydberg aggregates in a one-dimensional geometry
We experimentally study the full counting statistics of few-body Rydberg
aggregates excited from a quasi-one-dimensional Rydberg gas. We measure
asymmetric excitation spectra and increased second and third order statistical
moments of the Rydberg number distribution, from which we determine the average
aggregate size. Direct comparisons with numerical simulations reveal the
presence of liquid-like spatial correlations, and indicate sequential growth of
the aggregates around an initial grain. These findings demonstrate the
importance of dissipative effects in strongly correlated Rydberg gases and
introduce a way to study spatio-temporal correlations in strongly-interacting
many-body quantum systems without imaging.Comment: 6 pages plus supplemen
The (3+1)-dimensional superspace description of the commensurately modulated structure of p-chlorobenzamide (alpha-form) and its relation to the gamma-form
The room temperature structure (a-form) of the organic compound p-chlorobenzamide, C7H6ClNO, can be described as a classical three-dimensional periodic superstructure (alpha(s)) and also as a commensurately modulated structure (alpha(m)) in (3 + 1)-dimensional superspace. The diffraction pattern is characterized by a clear difference in intensity between main and satellite reflections. All reflections can be indexed with four indices hklm in a triclinic unit cell and one modulation vector in the superspace group P (1) over bar(alphabetagamma) with the modulation vector (q) over right arrow = 1/3 . (d) over right arrow*. The structure undergoes a phase transition at higher temperature. In this phase transition the modulation vanishes as indicated by the disappearance of the satellite reflections. The resulting high temperature phase (gamma-form) contains one molecule in the asymmetric unit, the space group is P (1) over bar. The high temperature structure (gamma) is very close to the reference structure of the modulated model at room temperature. The phase transition can therefore be understood as a loss of the modulation at high temperature
Axial-vector contributions in two-photon reactions: Pion transition form factor and deeply-virtual Compton scattering at NNLO in QCD
Using the approach based on conformal symmetry we calculate the two-loop coefficient function for the axial-vector contributions to two-photon processes in the ¯¯¯¯¯¯MS scheme. This is the last missing element for the complete next-to-next-to-leading order (NNLO) calculation of the pion transition form factor γ∗γ→π in perturbative QCD. The corresponding high-statistics measurement is planned by the Belle II collaboration and will allow one to put strong constraints on the pion light-cone distribution amplitude. The calculated NNLO corrections prove to be rather large and have to be taken into account. The same coefficient function determines the contribution of the axial-vector generalized parton distributions to deeply virtual Compton scattering (DVCS) which is investigated at the JLAB 12 GeV accelerator, by COMPASS at CERN, and in the future will be studied at the Electron Ion Collider EIC
Search for the Hypothetical pi -> mu x Decay
The KARMEN collaboration has reported the possible observation of a hitherto
unknown neutral and weakly interacting particle x, which is produced in the
decay pi -> mu + x with a mass m(x) = 33.9 MeV. We have searched for this
hypothetical decay branch by studying muons from pion decay in flight with the
LEPS spectrometer at the piE3 channel at PSI and find branching ratios BR(pi-
to mu- anti-x) < 4e-7 and BR(pi+ to mu+ x) < 7e-8 (95\% C.L.). Together with
the limit BR > 2e-8 derived in a recent theoretical paper our result would
leave only a narrow region for the existence of x if it is a heavy neutrino.Comment: 10 pages, TeX (uses epsf), 3 Postscript figures uu-encode
Mott-Hubbard gap closure and structural phase transition in the oxyhalides TiOBr and TiOCl under pressure
Pressure-dependent transmittance and reflectance spectra of TiOBr and TiOCl
single crystals at room temperature suggest the closure of the Mott-Hubbard
gap, i.e., the gap is filled with additional electronic states extending down
to the far-infrared range. According to pressure-dependent x-ray powder
diffraction data the gap closure coincides with a structural phase transition.
The transition in TiOBr occurs at slightly lower pressure (=14 GPa) compared
to TiOCl (=16 GPa) under hydrostatic conditions, which is discussed in terms
of the chemical pressure effect. The results of pressure-dependent
transmittance measurements on TiOBr at low temperatures reveal similar effects
at 23 K, where the compound is in the spin-Peierls phase at ambient pressure.Comment: 11 pages, 12 figures; to appear in Phys. Rev.
- …