arXiv subjects

R. Schmidt

Publications and source records attributed to R. Schmidt.

At least 73 records · Page 4Linked to original sources

arXiv2011

Zooming into the broad line region of the gravitationally lensed quasar Q2237+0305 = the Einstein Cross: III. Determination of the size and structure of the CIV and CIII] emitting regions using microlensing

We aim to use microlensing taking place in the lensed quasar Q2237+0305 to study the structure of the broad line region and measure the size of the region emitting the CIV and CIII] lines. Methods: Based on 39 spectrophotometric monitoring data points obtained between Oct. 2004 and Dec. 2007, we derived lightcurves for the CIV and CIII] emission lines. We used three different techniques to analyse the microlensing signal. Different components of the lines (narrow, broad and very broad) are identified and studied. We built a library of simulated microlensing lightcurves that reproduce the signal observed in the continuum and in the lines provided only the source size is changed. A Bayesian analysis scheme is then developed to derive the size of the various components of the BLR. Results: 1. The half-light radius of the region emitting the CIV line is found to be R_CIV ~ 66^{+110}_{-46} lt-days = 0.06$^{+0.09}_{-0.04}$ pc = 1.7$^{+2.8}_{-1.1}$\,10$^{17}$ cm (at 68.3% CI). Similar values are obtained for CIII]. Relative sizes of the carbon-line and V-band continuum emitting-regions are also derived with median values of R(line)/R(cont) in the range 4 to 29, depending of the FWHM of the line component. 2. The size of the CIV emitting region agrees with the Radius-Luminosity relationship derived from reverberation mapping. Using the virial theorem we derive the mass of the black hole in Q2237+0305 to be M_BH ~ 10^{8.3+/-0.3} M_sun. 3. We find that the CIV and CIII] lines are produced in at least 2 spatially distinct regions, the most compact one giving rise to the broadest component of the line. The broad and narrow line profiles are slightly different for CIV and CIII]. 4. Our analysis suggests a different structure for the CIV and FeII+III emitting regions, with the latter produced in the inner part of the BLR or in a less extended emitting region than CIV.

astro-ph.CO↗

arXiv2010

Optical interface created by laser-cooled atoms trapped in the evanescent field surrounding an optical nanofiber

Trapping and optically interfacing laser-cooled neutral atoms is an essential requirement for their use in advanced quantum technologies. Here we simultaneously realize both of these tasks with cesium atoms interacting with a multi-color evanescent field surrounding an optical nanofiber. The atoms are localized in a one-dimensional optical lattice about 200 nm above the nanofiber surface and can be efficiently interrogated with a resonant light field sent through the nanofiber. Our technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes. Moreover, it is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices.

quant-ph↗

arXiv2009

Efimov effect from functional renormalization

We apply a field-theoretic functional renormalization group technique to the few-body (vacuum) physics of non-relativistic atoms near a Feshbach resonance. Three systems are considered: one-component bosons with U(1) symmetry, two-component fermions with U(1)\times SU(2) symmetry and three-component fermions with U(1) \times SU(3) symmetry. We focus on the scale invariant unitarity limit for infinite scattering length. The exact solution for the two-body sector is consistent with the unitary fixed point behavior for all considered systems. Nevertheless, the numerical three-body solution in the s-wave sector develops a limit cycle scaling in case of U(1) bosons and SU(3) fermions. The Efimov parameter for the one-component bosons and the three-component fermions is found to be approximately s=1.006, consistent with the result of Efimov.

cond-mat.stat-mech↗

arXiv2009

The Evolution of Galaxy Clusters Across Cosmic Time

The large scale structure of the present Universe is determined by the growth of dark matter density fluctuations and by the dynamical action of dark energy and dark matter. While much progress has been made in recent years in constraining the cosmological parameters, and in reconstructing the evolution in the large--scale structure of the dark matter distribution, we still lack an understanding of the evolution of the baryonic component of the Universe. Located at nodes of the cosmic web, clusters of galaxies are the largest collapsed structures in the Universe with total masses up to 10$^{15}$ M$_{\sun}$. Over 80% of their mass resides in the form of dark matter. The remaining mass is composed of baryons, most of which (about 85%) is a diffuse, hot plasma that radiates primarily in X-rays. X-ray observations of the evolving cluster population provide a unique opportunity to address such open and fundamental questions as: How do hot diffuse baryons dynamically evolve in dark matter potentials? How and when was the excess energy which we observe in the intergalactic medium generated? What is the cosmic history of heavy-element production and circulation? Our current knowledge comes primarily from detailed studies of clusters in the relatively nearby Universe (z$<$0.5). Major advances will come from high throughput, high spectral and spatial resolution X-ray observations that measure the thermodynamical properties and metal content of the first low mass clusters emerging at z $\sim$ 2 and directly trace their evolution into today's massive clusters.

astro-ph.CO↗

arXiv2009

Functional renormalization for trion formation in ultracold fermion gases

The energy spectrum for three species of identical fermionic atoms close to a Feshbach resonance is computed by use of a nonperturbative flow equation. Already a simple truncation shows that for large scattering length $|a|$ the lowest energy state is a "trion" (or trimer) bound state of three atoms. At the location of the resonance, for $|a|\to\infty$, we find an infinite set of trimer bound states, with exponentially decreasing binding energy. This feature was pointed out by Efimov. It arises from limit cycle scaling, which also leads to a periodic dependence of the three body scattering coupling on $\ln |a|$. Extending our findings by continuity to nonzero density and temperature we find that a "trion phase" separates a BEC and a BCS phase, with interesting quantum phase transitions for T=0.

cond-mat.supr-con↗

arXiv2008

Three-body loss in lithium from functional renormalization

We use functional integral methods for an estimate of the three-body loss in a three-component $^6$Li ultracold atom gas. We advocate a simple picture where the loss proceeds by the formation of a three-atom bound state, the trion. In turn, the effective amplitude for the trion formation from three atoms is estimated from a simple effective boson exchange process. The energy gap of the trion and other key quantities for the loss coefficient are computed in a functional renormalization group framework.

cond-mat.other↗

arXiv2007

Enhanced magnetocaloric effect in frustrated magnetic molecules with icosahedral symmetry

We investigate the magnetocaloric properties of certain antiferromagnetic spin systems that have already been or very likely can be synthesized as magnetic molecules. It turns out that the special geometric frustration which is present in antiferromagnets that consist of corner-sharing triangles leads to an enhanced magnetocaloric effect with high cooling rates in the vicinity of the saturation field. These findings are compared with the behavior of a simple unfrustrated spin ring as well as with the properties of the icosahedron. To our surprise, also for the icosahedron large cooling rates can be achieved but due to a different kind of geometric frustration.

cond-mat.str-el↗

arXiv2007

Neutron density distributions from antiprotonic 208Pb and 209Bi atoms

The X-ray cascade from antiprotonic atoms was studied for 208Pb and 209Bi. Widths and shifts of the levels due to the strong interaction were determined. Using modern antiproton-nucleus optical potentials the neutron densities in the nuclear periphery were deduced. Assuming two parameter Fermi distributions (2pF) describing the proton and neutron densities the neutron rms radii were deduced for both nuclei. The difference of neutron and proton rms radii /\r_np equal to 0.16 +-(0.02)_{stat} +- (0.04)_{syst} fm for 208Pb and 0.14 +- (0.04)_{stat} +- (0.04)_{syst} fm for 209Bi were determined and the assigned systematic errors are discussed. The /\r_np values and the deduced shapes of the neutron distributions are compared with mean field model calculations.

nucl-ex↗

arXiv2005

Frustration effects in magnetic molecules

By means of exact diagonalization we study the ground-state and the low-temperature physics of the Heisenberg antiferromagnet on the cuboctahedron and the icosidodecahedron. Both are frustrated magnetic polytopes and correspond to the arrangement of magnetic atoms in the magnetic molecules Cu12La8 and Mo72Fe30. The interplay of strong quantum fluctuations and frustration influences the ground state spin correlations drastically and leads to an interesting magnetization process at low temperatures. Furthermore the frustration yields low-lying non-magnetic excitations resulting in an extra low-temperature peak in the specific heat.

cond-mat.str-el↗

arXiv2003

Conductance of a molecular junction mediated by unconventional metal-induced gap states

The conductance of a molecular junction is commonly determined by either charge-transfer-doping, where alignment of the Fermi energy to the molecular levels is achieved, or tunnelling through the tails of molecular resonances within the highest-occupied and lowest-unoccupied molecular-orbital gap. Here, we present an alternative mechanism where electron transport is dominated by electrode surface states. They give rise to metallization of the molecular bridge and additional, pronounced conductance resonances allowing for substantial tailoring of its electronic properties via, e.g. a gate voltage. This is demonstrated in a field-effect geometry of a fullerene-bridge between two metallic carbon nanotubes.

cond-mat.mes-hall↗

arXiv2003

Spin-1/2 J1-J2 model on the body-centered cubic lattice

Using exact diagonalization (ED) and linear spin wave theory (LSWT) we study the influence of frustration and quantum fluctuations on the magnetic ordering in the ground state of the spin-1/2 J1-J2 Heisenberg antiferromagnet (J1-J2 model) on the body-centered cubic (bcc) lattice. Contrary to the J1-J2 model on the square lattice, we find for the bcc lattice that frustration and quantum fluctuations do not lead to a quantum disordered phase for strong frustration. The results of both approaches (ED, LSWT) suggest a first order transition at J2/J1 $\approx$ 0.7 from the two-sublattice Neel phase at low J2 to a collinear phase at large J2.

cond-mat.str-el↗

arXiv2002

Interference Blockade in the Conductance of Organic Molecules

The conductance of {\em cis/trans} isomers of stilbene molecules connected to armchair single wall carbon nanotubes is studied in the Landauer formalism combined with a density-functional based approach. For a given arrangement of the electrodes, dramatic differences in the transmission between both isomers are found. For a given isomer, the conductance can be varied by orders of magnitude by changing the molecule-electrode relative orientation. Both effects can be explained by a simple, physically transparent interference rule, which suggests a straightforward conductance control in such molecular systems by different switching mechanisms.

cond-mat.mes-hall↗

arXiv2002

Nucleon density in the nuclear periphery determined with antiprotonic x-rays: cadmium and tin isotopes

The x-ray cascade from antiprotonic atoms was studied for 106Cd, 116Cd, 112Sn, 116Sn, 120Sn, and 124Sn. Widths and shifts of the levels due to strong interaction were deduced. Isotopic effects in the Cd and Sn isotopes are clearly seen. The results are used to investigate the nucleon density in the nuclear periphery. The deduced neutron distributions are compared with the results of the previously introduced radiochemical method and with HFB calculations.

nucl-ex↗

arXiv2002

Precise Measurement of Muon Capture on the Proton

The aim of the MuCap experiment is a 1% measurement of the singlet capture rate Lambda_S for the basic electro-weak reaction mu + p -> n + nu_mu. This observable is sensitive to the weak form-factors of the nucleon, in particular to the induced pseudoscalar coupling constant g_P. It will provide a rigorous test of theoretical predictions based on the Standard Model and effective theories of QCD. The present method is based on high precision lifetime measurements of mu^- in hydrogen gas and the comparison with the free mu^+ lifetime. The mu^- experiment will be performed in ultra-clean, deuterium-depleted H_2 gas at 10 bar. Low density compared to liquid H_2 is chosen to avoid uncertainties due to ppmu formation. A time projection chamber acts as a pure hydrogen active target. It defines the muon stop position in 3-D and detects rare background reactions. Decay electrons are tracked in cylindrical wire-chambers and a scintillator array covering 75% of 4 pi.

nucl-ex↗

arXiv2001

Fullerene based devices for molecular electronics

We have investigated the electronic properties of a C_60 molecule in between carbon nanotube leads. This problem has been tackled within a quantum chemical treatment utilizing a density functional theory-based LCAO approach combined with the Landauer formalism. Owing to low-dimensionality, electron transport is very sensitive to the strength and geometry of interfacial bonds. Molecular contact between interfacial atoms and electrodes gives rise to a complex conductance dependence on the electron energy exhibiting spectral features of both the molecule and electrodes. These are attributed to the electronic structure of the C_60 molecule and to the local density of states of the leads, respectively.

cond-mat.mes-hall↗

arXiv2001

An `all-carbon' molecular device

We have performed parameter-free calculations of electron transport across a carbon molecular junction consisting of a C$_{60}$ molecule sandwiched between two semi-infinite metallic carbon nanotubes. It is shown that the Landauer conductance of this carbon hybrid system can be tuned within orders of magnitude not only by varying the tube-C$_{60}$ distance, but more importantly at fixed distances by i) changing the orientation of the Buckminsterfullerene or ii) rotating one of the tubes around its cylinder axis. Furthermore, it is explicitely shown that structural relaxation determines qualitatively the transmission spectrum of such devices.

cond-mat.mes-hall↗

arXiv2001

Information on antiprotonic atoms and the nuclear periphery from the PS209 experiment

In the PS209 experiments at CERN two kinds of measurements were performed: the in-beam measurement of X-rays from antiprotonic atoms and the radiochemical, off-line determination of the yield of annihilation products with mass number A_t -1 (less by 1 than the target mass). Both methods give observables which allows to study the peripheral matter density composition and distribution.

nucl-ex↗

arXiv1997

Excitation and relaxation in atom-cluster collisions

Electronic and vibrational degrees of freedom in atom-cluster collisions are treated simultaneously and self-consistently by combining time-dependent density functional theory with classical molecular dynamics. The gradual change of the excitation mechanisms (electronic and vibrational) as well as the related relaxation phenomena (phase transitions and fragmentation) are studied in a common framework as a function of the impact energy (eV...MeV). Cluster "transparency" characterized by practically undisturbed atom-cluster penetration is predicted to be an important reaction mechanism within a particular window of impact energies.

physics.atm-clus↗