arXiv subjects

S. Fritz

Publications and source records attributed to S. Fritz.

12 recordsLinked to original sources

arXiv2019

Structural and Nanochemical Properties of AlOx Layers in $Al/AlO_x/Al$-Layer Systems for Josephson Junctions

The structural and nanochemical properties of thin $AlO_x$ layers are decisive for the performance of advanced electronic devices. For example, they are frequently used as tunnel barriers in Josephson junction-based superconducting devices. However, systematic studies of the influence of oxidation parameters on structural and nanochemical properties are rare up to now, as most studies focus on the electrical properties of $AlO_x$ layers. This study aims to close this gap by applying transmission electron microscopy in combination with electron energy loss spectroscopy to analyze the structural and nanochemical properties of differently fabricated $AlO_x$ layers and correlate them with fabrication parameters. With respect to the application of $AlO_x$ as tunnel barrier in superconducting Josephson junctions, $Al/AlO_x/Al$-layer systems were deposited on Si substrates. We will show that the oxygen content and structure of amorphous $AlO_x$ layers is strongly dependent on the fabrication process and oxidation parameters. Dynamic and static oxidation of Al yields oxygen-deficient amorphous $AlO_x$ layers, where the oxygen content ranges from x = 0.5 to x = 1.3 depending on oxygen pressure and substrate temperature. Thicker layers of stoichiometric crystalline $γ-Al_2O_3$ layers were grown by electron-beam evaporation of $Al_2O_3$ and reactive sputter deposition.

cond-mat.mtrl-sci↗

arXiv2019

Optimization of $Al/AlO_x/Al$-Layer Systems for Josephson Junctions from a Microstructure Point of View

$Al/AlO_x/Al$-layer systems are frequently used for Josephson junction-based superconducting devices. Although much work has been devoted to the optimization of the superconducting properties of these devices, systematic studies on influence of deposition conditions combined with structural analyses on the nanoscale are rare up to now. We have focused on the optimization of the structural properties of $Al/AlO_x/Al$-layer systems deposited on Si(111) substrates with a particular focus on the thickness homogeneity of the $AlO_x$-tunnel barrier. A standard high-vacuum electron-beam deposition system was used and the effect of substrate pretreatment, different Al-deposition temperatures and Al-deposition rates was studied. Transmission electron microscopy was applied to analyze the structural properties of the $Al/AlO_x/Al$-layer systems to determine the thickness homogeneity of the $AlO_x$ layer, grain size distribution in the Al layers, Al-grain boundary types and the morphology of the $Al/AlO_x$ interface. We show that the structural properties of the lower Al layer are decisive for the structural quality of the whole $Al/AlO_x/Al$-layer system. Optimum conditions yield an epitaxial Al(111) layer on a Si(111) substrate with an Al-layer thickness variation of only 1.6 nm over more than 10 $μm$ and large lateral grain sizes up to 1 $μm$. Thickness fluctuations of the $AlO_x$-tunnel barrier are minimized on such an Al layer which is essential for the homogeneity of the tunnel current. Systematic variation of the Al-deposition rate and deposition temperature allows to develop an understanding of the growth mechanisms.

cond-mat.mtrl-sci↗

arXiv2017

Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits

This work is concerned with Al/Al-oxide(AlO$_{x}$)/Al-layer systems which are important for Josephson-junction-based superconducting devices such as quantum bits. The device performance is limited by noise, which has been to a large degree assigned to the presence and properties of two-level tunneling systems in the amorphous AlO$_{x}$ tunnel barrier. The study is focused on the correlation of the fabrication conditions, nanostructural and nanochemical properties and the occurrence of two-level tunneling systems with particular emphasis on the AlO$_{x}$-layer. Electron-beam evaporation with two different processes and sputter deposition were used for structure fabrication, and the effect of illumination by ultraviolet light during Al-oxide formation is elucidated. Characterization was performed by analytical transmission electron microscopy and low-temperature dielectric measurements. We show that the fabrication conditions have a strong impact on the nanostructural and nanochemical properties of the layer systems and the properties of two-level tunneling systems. Based on the understanding of the observed structural characteristics, routes are derived towards the fabrication of Al/AlO$_{x}$/Al-layers systems with improved properties.

quant-ph↗

arXiv2011

The broad iron Kalpha line of Cygnus X-1 as seen by XMM-Newton in the EPIC-pn modified timing mode

We present the analysis of the broadened, flourescent iron Kalpha line in simultaneous XMM-Newton and RXTE data from the black hole Cygnus X-1. The XMM-Newton data were taken in a modified version of the timing mode of the EPIC-pn camera. In this mode the lower energy threshold of the instrument is increased to 2.8 keV to avoid telemetry drop outs due to the brightness of the source, while at the same time preserving the signal-to-noise ratio in the Fe Kalpha band. We find that the best-fit spectrum consists of the sum of an exponentially cut off power-law and relativistically smeared, ionized reflection. The shape of the broadened Fe Kalpha feature is due to strong Compton broadening combined with relativistic broadening. Assuming a standard, thin accretion disk, the black hole is close to rotating maximally.

astro-ph.HE↗

arXiv2007

Thermal and Chemical Freeze-out in Spectator Fragmentation

Isotope temperatures from double ratios of hydrogen, helium, lithium, beryllium, and carbon isotopic yields, and excited-state temperatures from yield ratios of particle-unstable resonances in 4He, 5Li, and 8Be, were determined for spectator fragmentation, following collisions of 197Au with targets ranging from C to Au at incident energies of 600 and 1000 MeV per nucleon. A deviation of the isotopic from the excited-state temperatures is observed which coincides with the transition from residue formation to multi-fragment production, suggesting a chemical freeze-out prior to thermal freeze-out in bulk disintegrations.

nucl-ex↗

arXiv2007

The broadband spectrum of Cygnus X-1

The Black Hole (BH) binary Cygnus X-1 has been observed simultaneously by INTEGRAL, RXTE, and XMM-Newton for four times in November and December 2004, when Cyg X-1 became first observable with XMM-Newton. During these observations the source was found in one of its transitional states between the hard state and the soft state. We obtained a high signal to noise spectrum of Cyg X-1 from 3 keV to 1 MeV which allows us to put constraints on the nature of the Comptonizing plasma by modeling the continuum with Comptonization models as eqpair (Coppi 1992). Using XMM-Newton we were also able to confirm the presence of a relativistically broadened Fe K-alpha line.

astro-ph↗

arXiv2006

A torque reversal of 4U 1907+09

We present an analysis of the accreting X-ray pulsar system 4U 1907+09 based on INTEGRAL data. The main focus of this analysis is a study of the timing behavior of this source. In addition we also show an analysis of the 5-90 keV spectrum. The data were extracted using the official INTEGRAL software OSA 5.1. Timing analysis was performed using epoch folding and pulsar pulse phasing. We have measured 12 individual pulse periods for the years 2003 to 2005. We confirm earlier RXTE results that during 2003 the spin down became slower and show furthermore that after this phase 4U 1907+09 started to spin up with dP/dt = -0.158 s/yr in 2004. The similarity of the pulse period histories of 4U 1907+09 and 4U 1626-26 suggests that accretion onto an oblique rotator, as recently proposed by Perna et al., is a possible explanation for this change.

astro-ph↗

arXiv2005

Long Term Variability of Cygnus X-1. IV. Spectral Evolution 1999-2004

(abbreviated abstract) We present the long term spectral evolution of the Galactic black hole candidate Cygnus X-1 in the X-rays and at 15GHz using ~200 pointed observations taken between early 1999 and late 2004 with RXTE and the Ryle radio telescope. The X-ray spectra are remarkably well described by a simple broken power law spectrum with an exponential cutoff. Physically motivated Comptonization models, e.g., compTT or eqpair, can reproduce this simplicity. Broken power law models reveal a significant linear correlation between the photon index of the lower energy power law and the hardening of the power law at approximately 10keV. Comptonization models show that the bolometric flux of a soft excess (e.g., disk component) is strongly correlated with the compactness ratio of the Comptonizing medium. We also find that the fraction of the time spent in low radio emission/soft X-ray spectral states has increased from ~10% in 1996-2000 to ~34% since early 2000. Radio flares typically occur during state transitions and failed state transitions. There is also a strong correlation between the 10-50keV X-ray flux and the radio luminosity of the source. We demonstrate that rather than there being distinctly separated states, in contrast to the timing properties the spectrum of Cyg X-1 shows variations between extremes of properties, with clear cut examples of spectra at every intermediate point in the observed spectral correlations.

astro-ph↗

arXiv2004

GX 301-2 as seen by Integral

We present observations of the High Mass X-ray Binary GX 301-2 taken during the Galactic Plane Scan (GPS) with Integral following our RXTE observations (Kreykenbohm et al. 2004). The optical companion of GX 301-2 is the B1Ia+ hypergiant Wray 977 with a luminosity of 1.3 x 10^6 Lsun and a mass of ~48 Msun making the system one of the most massive X-ray binaries known. The system was observed 24 times during the GPS thus covering many orbital phases including the pre-periastron flare. The source is clearly detected in all Integral instruments in most pointings. The derived X-ray light curves show strong variability on all timescales. Depending on the orbital phase of the pointings, the luminosity changes by factor of more than 10 during the pre-periastron flare compared to other pointings. Furthermore, broad band spectra using all instruments available on Integral are compared with data taken by the RXTE instruments PCA and HEXTE (using the same data as in Kreykenbohm et al. 2004).

astro-ph↗

arXiv2002

Generalized Isoscaling of Isotopic Distributions

Generalized isoscaling relationships are proposed that may permit one to relate the isotopic distributions of systems that may not be at the same temperature. The proposed relationships are applied to multifragmentation excitation functions for central Kr+Nb and Ar+Sc collisions.

nucl-ex↗

arXiv1999

Breakup Density in Spectator Fragmentation

Proton-proton correlations and correlations of protons, deuterons and tritons with alpha particles from spectator decays following 197Au + 197Au collisions at 1000 MeV per nucleon have been measured with two highly efficient detector hodoscopes. The constructed correlation functions, interpreted within the approximation of a simultaneous volume decay, indicate a moderate expansion and low breakup densities, similar to assumptions made in statistical multifragmentation models. PACS numbers: 25.70.Pq, 21.65.+f, 25.70.Mn, 25.75.Gz

nucl-ex↗

arXiv1997

Emission temperatures and freeze out densities from light particle correlation functions in Au+Au collisions at 1 A GeV

A study of emission temperatures extracted from excited state populations and of freeze out radii from light particle intensity interferometry is presented. Three high resolution dE-E-Hodoscopes with a total of 216 detectors are combined with the ALADiN setup in order to study Au+Au collisions at 1 A GeV. In contrast to measurements with the isotope thermometer the extracted apparent temperatures do not vary with impact parameter thus with excitation energy. From the extracted radii a freeze out density was determined which decreases from $0.2ρ_0$ for the most peripheral to less than $0.1ρ_0$ for the most central collisions. A density--dependent feeding correction is applied to the different temperature measurements.

nucl-ex↗