arXiv subjects

M. Schmidt

Publications and source records attributed to M. Schmidt.

At least 163 records · Page 9Linked to original sources

arXiv2008

A gauge model for quantum mechanics on a stratified space

In the Hamiltonian approach on a single spatial plaquette, we construct a quantum (lattice) gauge theory which incorporates the classical singularities. The reduced phase space is a stratified Kähler space, and we make explicit the requisite singular holomorphic quantization procedure on this space. On the quantum level, this procedure furnishes a costratified Hilbert space, that is, a Hilbert space together with a system which consists of the subspaces associated with the strata of the reduced phase space and of the corresponding orthoprojectors. The costratified Hilbert space structure reflects the stratification of the reduced phase space. For the special case where the structure group is $\mathrm{SU}(2)$, we discuss the tunneling probabilities between the strata, determine the energy eigenstates and study the corresponding expectation values of the orthoprojectors onto the subspaces associated with the strata in the strong and weak coupling approximations.

hep-th↗

arXiv2008

Mobility in Graphene Double Gate Field Effect Transistors

In this work, double-gated field effect transistors manufactured from monolayer graphene are investigated. Conventional top-down CMOS-compatible processes are applied except for graphene deposition by manual exfoliation. Carrier mobilities in single- and double gated graphene field effect transistors are compared. Even in double-gated graphene FETs, the carrier mobility exceeds the universal mobility of silicon over nearly the entire measured range. At comparable dimensions, reported mobilities for ultra thin body silicon-on-insulator MOSFETs can not compete with graphene FET values.

cond-mat.mtrl-sci↗

arXiv2008

Curvature effect on the interaction between folded graphitic surface and silver clusters

Evidence of curvature effects on the interaction and binding of silver clusters on folded graphitic surfaces has been shown from both experiment and theory. Density Functional Theory (DFT) calculations within the local density and generalized gradient approximations have been performed for the structural relaxation of both Ag and Ag$_2$ on curved surfaces, showing a cross-over from quantum to classical behaviour. Using Lennard-Jones potential to model the interaction between a single cluster and the graphene surface, evidence is found for the curvature effect on the binding of silver nano-particles to folding graphitic surfaces. The theoretical results are compared to SEM and AFM images of samples obtained from pre-formed silver cluster deposition on carboneous substrates exhibiting anisotropic pleat structures.

cond-mat.mtrl-sci↗

arXiv2008

On the Topology of the Reduced Classical Configuration Space of Lattice QCD

We study the topological structure of the quotient of $SU(3)\times SU(3)$ by diagonal conjugation. This is the simplest nontrivial example for the classical reduced configuration space of chromodynamics on a spatial lattice in the Hamiltonian approach. We construct a cell complex structure of the quotient in such a way that the closures of strata are subcomplexes and we compute the homology and cohomology groups of the strata and their closures.

hep-th↗

arXiv2007

Structural investigations on $ε$-FeGe at high pressure and low temperature

The structural parameters of $ε$-FeGe have been determined at ambient conditions using single crystal refinement. Powder diffraction have been carried out to determine structural properties and compressibility for pressures up to 30 GPa and temperatures as low as 82 K. The discontinuous change in the pressure dependence of the shortest Fe-Ge interatomic distance might be interpreted as a symmetry-conserving transition and seems to be related to a magnetic phase boundary line.

cond-mat.str-el↗

arXiv2007

Optical evidence for heavy charge carriers in FeGe

The optical spectrum of the cubic helimagnetic metal FeGe has been investigated in the frequency range from 0.01 - 3.1 eV for different temperatures from 30 K to 296 K. The optical conductivity shows the evolution of a low energy (0.22 eV) interband transition and the development of a narrow free carrier response with a strong energy and temperature dependence. The frequency dependent effective mass and scattering rate derived from the optical data indicate the formation of dressed quasi-particles with a mass renormalization factor of 12. Similar to FeSi the spectral weight in FeGe is not recovered over a broad frequency range, an effect usually attributed to the influence of the on-site Coulomb interaction.

cond-mat.str-el↗

arXiv2007

INTEGRAL observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth

We study the spectrum of the cosmic X-ray background (CXB) in energy range $\sim$5-100 keV. Early in 2006 the INTEGRAL observatory performed a series of four 30ksec observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background(CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. The spectrum of the cosmic X-ray background in the energy band 5-100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalization is $\sim$10% higher. This difference in normalization can (at least partly) be traced to the different assumptions on the absolute flux from the Crab Nebulae. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20-50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CXB origin.

astro-ph↗

arXiv2006

Understanding the insulating phase in CMR manganites: Shortening of the Jahn-Teller long-bond across the phase diagram of La_{1-x} Ca_x Mn O_3

The detailed evolution of the magnitude of the local Jahn-Teller (JT) distortion in La_{1-x} Ca_x Mn O_3 is obtained across the phase diagram for 0 < x < 0.5 from high quality neutron diffraction data using the atomic pair distribution function (PDF) method. A local JT distortion is observed in the insulating phase for all Ca concentrations studied. However, in contrast with earlier local structure studies its magnitude is not constant, but decreases continuously with increasing Ca content. This observation is at odds with a simple small-polaron picture for the insulating state.

cond-mat.str-el↗

arXiv2006

Metallic State in Cubic FeGe beyond its Quantum Phase Transition

We report on results of electrical resistivity and structural investigations on the cubic modification of FeGe under high pressure. The long-wavelength helical order ($T_C=280$ K) is suppressed at a critical pressure $p_c\approx 19$ GPa. An anomaly in the resistivity data at $T_X(p)$ and strong deviations from a Fermi-liquid behavior in a wide pressure range above $p_c$ suggest that the suppression of $T_C$ disagrees with the standard notion of a quantum critical phase transition. The metallic ground state persisting at high pressure can be described by band-structure calculations if structural disorder due to zero-point motion is included. Discontinuous changes in the pressure dependence of the shortest Fe-Ge interatomic distance occurring close to the $T_C(p)$ phase line could be interpreted as a symmetry-conserving transition of first order.

cond-mat.str-el↗

arXiv2006

A Lattice Gauge Model of Singular Marsden-Weinstein Reduction. Part I. Kinematics

The simplest nontrivial toy model of a classical SU(3) lattice gauge theory is studied in the Hamiltonian approach. By means of singular symplectic reduction, the reduced phase space is constructed. Two equivalent descriptions of this space in terms of a symplectic covering as well as in terms of invariants are derived.

hep-th↗

arXiv2006

Post-AGB stars as testbeds of nucleosynthesis in AGB stars

We construct a data base of 125 post-AGB objects (including R CrB and extreme helium stars) with published photospheric parameters (effective temperature and gravity) and chemical composition. We estimate the masses of the post-AGB stars by comparing their position in the (log Teff, log g) plane with theoretical evolutionary tracks of different masses. We construct various diagrams, with the aim of finding clues to AGB nucleosynthesis. This is the first time that a large sample of post-AGB stars has been used in a systematic way for such a purpose and we argue that, in several respects, post-AGB stars should be more powerful than planetary nebulae to test AGB nucleosynthesis. Our main findings are that: the vast majority of objects which do not show evidence of N production from primary C have a low stellar mass (M* < 0.56 Msun); there is no evidence that objects which did not experience 3rd dredge-up have a different stellar mass distribution than objects that did; there is clear evidence that 3rd dredge-up is more efficient at low metallicity. The sample of known post-AGB stars is likely to increase significantly in the near future thanks to the ASTRO-F and follow-up observations, making these objects even more promising as testbeds for AGB nucleosynthesis.

astro-ph↗

arXiv2005

Soft core fluid in a quenched matrix of soft core particles: A mobile mixture in a model gel

We present a density-functional study of a binary phase-separating mixture of soft core particles immersed in a random matrix of quenched soft core particles of larger size. This is a model for a binary polymer mixture immersed in a crosslinked rigid polymer network. Using the replica `trick' for quenched-annealed mixtures we derive an explicit density functional theory that treats the quenched species on the level of its one-body density distribution. The relation to a set of effective external potentials acting on the annealed components is discussed. We relate matrix-induced condensation in bulk to the behaviour of the mixture around a single large particle. The interfacial properties of the binary mixture at a surface of the quenched matrix display a rich interplay between capillary condensation inside the bulk matrix and wetting phenomena at the matrix surface.

cond-mat.soft↗

arXiv2005

Luminosity-dependent evolution of soft X-ray selected AGN: New Chandra and XMM-Newton surveys

We present new results on the cosmological evolution of unabsorbed (type-1) active galactic nuclei (AGN) selected in the soft (0.5-2 keV) X-ray band. From a variety of ROSAT, XMM-Newton and Chandra surveys we selected a total of ~1000 AGN with an unprecedented spectroscopic and photometric optical/NIR identification completeness. For the first time we are able to derive reliable space densities for low-luminosity (Seyfert-type) X-ray sources at cosmological redshifts. The evolutionary behaviour of AGN shows a strong dependence on X-ray luminosity: while the space density of high-luminosity AGN reaches a peak around z~2, similar to that of optically selected QSO, the space density of low-luminosity AGNs peaks at redshifts below z=1. This confirms previous ROSAT findings of a luminosity-dependent density evolution. Using a rigorous treatment of the optical identification completeness we are able to show that the space density of AGN with X-ray luminosities L_x < 10^45 erg s^-1 declines significantly towards high redshifts.

astro-ph↗

arXiv2005

Crystallographic disorder and electron scattering on structural two-level systems in ZrAs1.4Se0.5

Single crystals of ZrAs1.4Se0.5 (PbFCl type structure) were grown by chemical vapour transport. While their thermodynamic and transport properties are typical for ordinary metals, the electrical resistivity exhibits a shallow minimum at low temperatures. Application of strong magnetic fields does not influence this anomaly. The minimum of the resistivity in ZrAs1.4Se0.5 apparently originates from interaction between the conduction electrons and structural two-level systems. Significant disorder in the As-Se substructure is inferred from X-ray diffraction and electron microprobe studies.

cond-mat.str-el↗

arXiv2005

The Galaxy Populations of Double Cluster RX J1053.7+5735 at z=1.13

We present a study of the galaxy population in the cluster RX J1053.7+5735, one of the most distant X-ray selected clusters of galaxies, which also shows an unusual double-lobed X-ray morphology, indicative of a possible equal-mass cluster merger. Using Keck-DEIMOS spectroscopic observations of galaxies in the 2x1.5 arcmin region surrounding RX J1053.7+5735, we secured redshifts for six galaxies in the range 1.129 < z < 1.139, with a mean redshift =1.134. The mean redshift agrees well with the cluster X-ray redshift previously estimated from the cluster X-ray Fe-K line, confirming the presence of a cluster at z~1.135. Galaxies with concordant redshifts are located in both eastern and western sub-clusters of the double cluster structure, indicating that both sub-clusters are at similar redshifts. This result is also consistent with a previous claim that both eastern and western X-ray lobes have similar X-ray redshifts. Based on their separation of ~ 250 kpc/h, these results support the interpretation that RX J1053.7+5735 is an equal-mass cluster merger taken place at z ~ 1, although further direct evidence for dynamical state of the cluster is needed for a more definitive statement about the cluster merging state. The six galaxies have a line-of-sight velocity dispersion = 650 km/s. All six galaxies show clear absorption features of CaII H & K, and several Balmer lines, typical of early galaxies at the present epoch, in agreement with their I-K colors. A color-magnitude diagram, constructed from deep optical/NIR observations of the RX J1053.7+5735 field, shows a clear red color sequence. There is an indication that the red sequence in RX J1053.7+5735 lies ~ 0.3 to the blue of the Coma line, qualitatively consistent with previous studies investigating other clusters at z ~ 1.

astro-ph↗

arXiv2005

Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers

An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and non-adsorbing polymers, that takes polymer non-ideality into account through a repulsive stepfunction pair potential between polymers, is studied with grand canonical Monte Carlo simulations and density functional theory. Simulation results validate previous theoretical findings for the shift of the bulk fluid demixing binodal upon increasing strength of polymer-polymer repulsion, promoting the tendency to mix. For increasing strength of the polymer-polymer repulsion, simulation and theory consistently predict the interfacial tension of the free colloidal liquid-gas interface to decrease significantly for fixed colloid density difference in the coexisting phases, and to increase for fixed polymer reservoir packing fraction.

cond-mat.soft↗

arXiv2004

On the Stratified Classical Configuration Space of Lattice QCD

The stratified structure of the configuration space $\mb G^N = G \times ... \times G$ reduced with respect to the action of $G$ by inner automorphisms is investigated for $G = SU(3) .$ This is a finite dimensional model coming from lattice QCD. First, the stratification is characterized algebraically, for arbitrary $N$. Next, the full algebra of invariants is discussed for the cases $N = 1$ and $N =2 .$ Finally, for $N = 1$ and $N =2 ,$ the stratified structure is investigated in some detail, both in terms of invariants and relations and in more geometric terms. Moreover, the strata are characterized explicitly using local cross sections.

hep-th↗

arXiv2003

INTEGRAL timing and localization performance

In this letter we report on the accuracy of the attitude, misalignment, orbit and time correlation which are used to perform scientific analyses of the INTEGRAL data. The boresight attitude during science pointings has an accuracy of 3 arcsec. At the center of the field, the misalignments have been calibrated leading to a location accuracy of 4 to 40 arcsec for the different instruments. The spacecraft position is known within 10 meters. The relative timing between instruments could be reconstructed within 10 microsec and the absolute timing within 40 microsec.

astro-ph↗