4,914 research outputs found
A General Effective Theory for Dense Quark Matter
A general effective action for quark matter at nonzero temperature and/or
nonzero density is derived. Irrelevant quark modes are distinguished from
relevant quark modes, and hard from soft gluon modes, by introducing two
separate cut-offs in momentum space, one for quarks, , and one for
gluons, . Irrelevant quark modes and hard gluon modes are then
exactly integrated out in the functional integral representation of the QCD
partition function. Depending on the specific choice for and
, the resulting effective action contains well-known effective
actions for hot and/or dense quark matter, for instance the ``Hard Thermal
Loop'' (HTL) or the ``Hard Dense Loop'' (HDL) action, as well as the
high-density effective theory proposed by Hong and others.Comment: 10 pages, 6 figures, contribution to proceedings of SEWM 200
Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?
During the exothermic adsorption of molecules at solid surfaces dissipation
of the released energy occurs via the excitation of electronic and phononic
degrees of freedom. For metallic substrates the role of the nonadiabatic
electronic excitation channel has been controversially discussed, as the
absence of a band gap could favour an easy coupling to a manifold of
electronhole pairs of arbitrarily low energies. We analyse this situation for
the highly exothermic showcase system of molecular oxygen dissociating at
Pd(100), using time-dependent perturbation theory applied to first-principles
electronic-structure calculations. For a range of different trajectories of
impinging O2 molecules we compute largely varying electron-hole pair spectra,
which underlines the necessity to consider the high-dimensionality of the
surface dynamical process when assessing the total energy loss into this
dissipation channel. Despite the high Pd density of states at the Fermi level,
the concomitant non-adiabatic energy losses nevertheless never exceed about 5%
of the available chemisorption energy. While this supports an electronically
adiabatic description of the predominant heat dissipation into the phononic
system, we critically discuss the non-adiabatic excitations in the context of
the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed
V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin
asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)
Waveform sampling using an adiabatically driven electron ratchet in a two-dimensional electron system
We utilize a time-periodic ratchet-like potential modulation imposed onto a
two-dimensional electron system inside a GaAs/AlGaAs
heterostructure to evoke a net dc pumping current. The modulation is induced by
two sets of interdigitated gates, interlacing off center, which can be
independently addressed. When the transducers are driven by two identical but
phase-shifted ac signals, a lateral dc pumping current results, which
strongly depends on both, the phase shift and the waveform of the
imposed gate voltages. We find that for different periodic signals, the phase
dependence closely resembles . A simple linear model of
adiabatic pumping in two-dimensional electron systems is presented, which
reproduces well our experimental findings.Comment: 3 figure
Magneto-capacitance probing of the many-particle states in InAs dots
We use frequency-dependent capacitance-voltage spectroscopy to measure the
tunneling probability into self-assembled InAs quantum dots. Using an in-plane
magnetic field of variable strength and orientation, we are able to obtain
information on the quasi-particle wave functions in momentum space for 1 to 6
electrons per dot. For the lowest two energy states, we find a good agreement
with Gaussian functions for a harmonic potential. The high energy orbitals
exhibit signatures of anisotropic confinement and correlation effects.Comment: 3 pages, 3 figure
First-principles embedded-cluster calculations of the neutral and charged oxygen vacancy at the rutile TiO<sub>2</sub>(110) surface
We perform full-potential screened-hybrid density-functional theory calculations to compare the thermodynamic stability of neutral and charged states of the surface oxygen vacancy at the rutile TiO2(110) surface. Solid-state (QM/MM) embedded-cluster calculations are employed to account for the strong TiO2 polarization response to the charged defect states. Similarly to the situation for the bulk O vacancy, the +2 charge state VO2+ is found to be energetically by far the most stable. Only for Fermi-level positions very close to the conduction band, small polarons may at best be trapped by the charged vacancy. The large decrease in the VO2+ formation energy with decreasing Fermi-level position indicates strongly enhanced surface O vacancy concentrations for p-doped samples
Renormalization Group Flow of Quantum Gravity in the Einstein-Hilbert Truncation
The exact renormalization group equation for pure quantum gravity is used to
derive the non-perturbative \Fbeta-functions for the dimensionless Newton
constant and cosmological constant on the theory space spanned by the
Einstein-Hilbert truncation. The resulting coupled differential equations are
evaluated for a sharp cutoff function. The features of these flow equations are
compared to those found when using a smooth cutoff. The system of equations
with sharp cutoff is then solved numerically, deriving the complete
renormalization group flow of the Einstein-Hilbert truncation in . The
resulting renormalization group trajectories are classified and their physical
relevance is discussed. The non-trivial fixed point which, if present in the
exact theory, might render Quantum Einstein Gravity nonperturbatively
renormalizable is investigated for various spacetime dimensionalities.Comment: 58 pages, latex, 24 figure
Split-gate quantum point contacts with tunable channel length
We report on developing split-gate quantum point contacts (QPCs) that have a
tunable length for the transport channel. The QPCs were realized in a
GaAs/AlGaAs heterostructure with a two- dimensional electron gas (2DEG) below
its surface. The conventional design uses 2 gate fingers on the wafer surface
which deplete the 2DEG underneath when a negative gate voltage is applied, and
this allows for tuning the width of the QPC channel. Our design has 6 gate
fingers and this provides additional control over the form of the electrostatic
potential that defines the channel. Our study is based on electrostatic
simulations and experiments and the results show that we developed QPCs where
the effective channel length can be tuned from about 200 nm to 600 nm.
Length-tunable QPCs are important for studies of electron many-body effects
because these phenomena show a nanoscale dependence on the dimensions of the
QPC channel
Stabilizing nuclear spins around semiconductor electrons via the interplay of optical coherent population trapping and dynamic nuclear polarization
We experimentally demonstrate how coherent population trapping (CPT) for
donor-bound electron spins in GaAs results in autonomous feedback that prepares
stabilized states for the spin polarization of nuclei around the electrons. CPT
was realized by excitation with two lasers to a bound-exciton state.
Transmission studies of the spectral CPT feature on an ensemble of electrons
directly reveal the statistical distribution of prepared nuclear spin states.
Tuning the laser driving from blue to red detuned drives a transition from one
to two stable states. Our results have importance for ongoing research on
schemes for dynamic nuclear spin polarization, the central spin problem and
control of spin coherence.Comment: 5 pages, 4 figure
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