6,690 research outputs found
Nanomechanics of a Hydrogen Molecule Suspended between Two Equally Charged Tips
Geometric configuration and energy of a hydrogen molecule centered between
two point-shaped tips of equal charge are calculated with the variational
quantum Monte-Carlo (QMC) method without the restriction of the
Born-Oppenheimer (BO) approximation. Ground state nuclear distribution,
stability, and low vibrational excitation are investigated. Ground state
results predict significant deviations from the BO treatment that is based on a
potential energy surface (PES) obtained with the same QMC accuracy. The quantum
mechanical distribution of molecular axis direction and bond length at a
sub-nanometer level is fundamental for understanding nanomechanical dynamics
with embedded hydrogen. Because of the tips' arrangement, cylindrical symmetry
yields a uniform azimuthal distribution of the molecular axis vector relative
to the tip-tip axis. With approaching tips towards each other, the QMC sampling
shows an increasing loss of spherical symmetry with the molecular axis still
uniformly distributed over the azimuthal angle but peaked at the tip-tip
direction for negative tip charge while peaked at the equatorial plane for
positive charge. This directional behavior can be switched between both stable
configurations by changing the sign of the tip charge and by controlling the
tip-tip distance. This suggests an application in the field of molecular
machines.Comment: 20 pages, 10 figure
Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots
The time evolution of optically excited carriers in semiconductor quantum
wells and quantum dots is analyzed for their interaction with LO-phonons. Both
the full two-time Green's function formalism and the one-time approximation
provided by the generalized Kadanoff-Baym ansatz are considered, in order to
compare their description of relaxation processes. It is shown that the
two-time quantum kinetics leads to thermalization in all the examined cases,
which is not the case for the one-time approach in the intermediate-coupling
regime, even though it provides convergence to a steady state. The
thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.
A classification of ideals in Steinberg and Leavitt path algebras over arbitrary rings
We give a one-to-one correspondence between ideals in the Steinberg algebra
of a Hausdorff ample groupoid , and certain families of ideals in the group
algebras of isotropy groups in . This generalises a known ideal
correspondence theorem for Steinberg algebras of strongly effective groupoids.
We use this to give a complete graph-theoretic description of the ideal lattice
of Leavitt path algebras over arbitrary commutative rings, generalising the
classification of ideals in Leavitt path algebras over fields
Identifying predictors for energy poverty in Europe using machine learning
In this paper we identify drivers for energy poverty in Europe using machine learning. The establishment of predictors for energy poverty valid across countries is a call made by many experts, since it could provide a basis to effectively target energy-poor households with adequate policy measures. We apply a “low income, high expenditure” framework that classifies households as being at risk of energy poverty to a dataset from a survey conducted at the household-level in 11 European countries with vastly different economies, cultures, and climates. A gradient boosting classifier is successfully trained on a set of socio-economic features hypothesized as predictors for energy poverty in this diverse set of countries. The classifier's internal model is analyzed, providing novel insights into the intricacies that underlie energy poverty. We find that besides the main driver - income - floor area and household size can be confirmed as predictors. Our results suggest the presence of universal predictors that are valid across Europe, and contextual ones that are governed by local characteristics. To facilitate advanced research into energy poverty in Europe, we recommend to increase and streamline household data collection efforts, both at the country- and EU-level
Effective calculation of LEED intensities using symmetry-adapted functions
The calculation of LEED intensities in a spherical-wave representation can be substantially simplified by symmetry relations. The wave field around each atom is expanded in symmetry-adapted functions where the local point symmetry of the atomic site applies. For overlayer systems with more than one atom per unit cell symmetry-adapted functions can be used when the division of the crystal into monoatomic subplanes is replaced by division into subplanes containing all symmetrically equivalent atomic positions
A novel procedure for fast surface structural analysis based on LEED intensity data
By evaluating LEED intensities from different diffraction beams taken only at discrete energy intervals (which may be as large as 15–20 eV) the same degree of reliability in surface structure determination can be reached as with the conventional techniques based on analysis of continuous I/V-spectra. The minimum of the corresponding R-factor can be found by a least-squares fit method, as will be exemplified with a system in which 8 structural parameters were subject to simultaneous refinement
Self-induced decoherence approach: Strong limitations on its validity in a simple spin bath model and on its general physical relevance
The "self-induced decoherence" (SID) approach suggests that (1) the
expectation value of any observable becomes diagonal in the eigenstates of the
total Hamiltonian for systems endowed with a continuous energy spectrum, and
(2), that this process can be interpreted as decoherence. We evaluate the first
claim in the context of a simple spin bath model. We find that even for large
environments, corresponding to an approximately continuous energy spectrum,
diagonalization of the expectation value of random observables does in general
not occur. We explain this result and conjecture that SID is likely to fail
also in other systems composed of discrete subsystems. Regarding the second
claim, we emphasize that SID does not describe a physically meaningful
decoherence process for individual measurements, but only involves destructive
interference that occurs collectively within an ensemble of presupposed
"values" of measurements. This leads us to question the relevance of SID for
treating observed decoherence effects.Comment: 11 pages, 4 figures. Final published versio
Potentially Diagnostic Electron Paramagnetic Resonance Spectra Elucidate the Underlying Mechanism of Mitochondrial Dysfunction in the Deoxyguanosine Kinase Deficient Rat Model of a Genetic Mitochondrial DNA Depletion Syndrome
A novel rat model for a well-characterized human mitochondrial disease, mitochondrial DNA depletion syndrome with associated deoxyguanosine kinase (DGUOK) deficiency, is described. The rat model recapitulates the pathologic and biochemical signatures of the human disease. The application of electron paramagnetic (spin) resonance (EPR) spectroscopy to the identification and characterization of respiratory chain abnormalities in the mitochondria from freshly frozen tissue of the mitochondrial disease model rat is introduced. EPR is shown to be a sensitive technique for detecting mitochondrial functional abnormalities in situ and, here, is particularly useful in characterizing the redox state changes and oxidative stress that can result from depressed expression and/or diminished specific activity of the distinct respiratory chain complexes. As EPR requires no sample preparation or non-physiological reagents, it provides information on the status of the mitochondrion as it was in the functioning state. On its own, this information is of use in identifying respiratory chain dysfunction; in conjunction with other techniques, the information from EPR shows how the respiratory chain is affected at the molecular level by the dysfunction. It is proposed that EPR has a role in mechanistic pathophysiological studies of mitochondrial disease and could be used to study the impact of new treatment modalities or as an additional diagnostic tool
Raman, infrared and optical spectra of the spin-Peierls compound NaV_2O_5
We have measured polarized spectra of Raman scattering, infrared and optical
transmission of NaV_2O_5 single crystals above the temperature of the
spin-Peierls transition Tsp=35 K. Some of the far-infrared (FIR) phonon lines
are strongly asymmetric, due to the spin-phonon interaction. In addition to the
phonon lines, a broad band was observed in the c(aa)c Raman spectrum and in the
E||a FIR transmission spectrum. A possible origin of these bands is discussed.
The absorption band at 10000 cm-1 1.25 eV is attributed to vanadium d-d
electronic transitions while the absorption edge above 3 eV is supposed to
correspond to the onset of charge-transfer transitions.Comment: 7 figures, 8 page
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