Andreas Nowack

We have measured the current-voltage characteristics of point contacts between U${\mathrm{Be}}_{13}$ or U${\mathrm{Pt}}_{3}$ and the normal metals W or Pt (metallic point contacts) or GaAs (Schottky-barrier tunneling contact) in the temperature range between 50 mK and 1 K. In the metal-point-contact characteristics ($\frac{\mathrm{dV}}{\mathrm{dI}}$ vs $V$) there appear zero-bias minima of width $2\ensuremath{\Delta}$ below ${T}_{c}$. The ratio $\frac{2\ensuremath{\Delta}}{{k}_{B}{T}_{c}}$ is close to the BCS value. The tunneling spectra of U${\mathrm{Pt}}_{3}$ exhibit weak additional structure below ${T}_{c}$. A value $2\ensuremath{\Delta}$ has been estimated, which is a factor of 2 larger than that for the metal point contacts.

We performed point-contact and tunneling measurements on the organic superconductors β-(BEDT-TTF)2I3 and β-(BEDT-TTF)2IAuI, in the normal and superconducting states. The point-contact measurement in the normal state provides the Eliashberg function α2(ω)F (ω). This function has maxima at 4 and 15 mV, as well as a sharp peak at 1 mV which seems to indicate a very soft phonon strongly coupled to the electrons. The measurements in the superconducting state provide the superconducting gap, which for the tunneling measurements has a value 2Δ/kB Tc≅4, just slightly above the BCS value.

The d2U/dI2-characteristics of point-contacts between the organic superconductor β-(BEDT-TTF)2I3 in the normal metallic state and a normal metal (copper) are measured and found to be quite similar to those of point-contacts between normal metals which are well known to be correlated with the electron phonon coupling and the phonon density of states.

We report point-contact spectroscopy I–V data on superconducting αt-(BEDT-TTF)2I3. The I–V curves show structure in 3 regions: For energies of order 0.25 meV, of about 4.5 meV and for energies of about 10 meV. No structure is observed near the BCS gap (1.3–1.4 meV). We attribute the structure at 4.5 meV to interaction with the phonon possessing this frequency which was observed independently by Raman spectroscopy, and the structure near 0.25 meV to an excitation energy considerably smaller than the BCS gap.

The self-aligned in-line technique has been applied to the preparation of ultrasmall low-capacitance metallic tunnel junctions. By using e-beam lithography the area of Al/AlOx/Al contacts has so far been reduced to less than 0.005 μm2. At low temperatures high-ohmic double junctions with a small metallic island between them show the Coulomb blockade effect. The current through such a device could be modulated by a voltage applied to a gate electrode capacitively coupled to the island (single-electron transistor). Both single-charge phenomena have been observed at temperatures up to 1 K.

The zero-bias anomaly in point-contact characteristics of the organic superconductor αt-(BEDT-TTF)2I3 is investigated as a function of temperature and magnetic field. It is found that the zero-bias anomaly is insensitive to magnetic fields up to 5 T. In contrast, a structure at 5 meV, conventionally designated as the superconducting gap–but which is 4 times larger than the expected BCS gap–is strongly affected by magnetic fields above 1 T.

Point contacts between the heavy-fermion superconductor ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ and Nb are studied. A finite dc Josephson current is found in contacts aligned parallel to the $a$- $b$ directions of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$, whereas it is absent in contacts aligned along the $c$ direction. We attribute this extreme anisotropy of the Josephson current to an unconventional superconducting order parameter in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$, with a symmetry leading to destructive interference for Josephson currents along the $c$ direction.

The CMS Silicon Strip Tracker will be equipped with 16000 silicon microstrip detector modules covering a surface of approximately 200m2. The APV Readout Controller system was developed at RWTH Aachen, III. Physikalisches Institut in order to perform full readout tests of hybrids and modules at each production step. From the experience derived from initial module production, an automated fault finding algorithm has been developed which uses the full correlations between different electrical tests. The results of a recent production of over 250 Tracker Outer Barrel and 25 Tracker End Cap modules at UCSB demonstrate that the testing protocols are sufficient to find all known faults and that electrical module components produced have a high quality. The results are typical of all CMS tracker assembly and bonding sites.

Abstract The CMS tracker will be equipped with 16,000 silicon microstrip detector modules covering a surface of approximately 220 m 2 . For quality control, a compact and inexpensive DAQ system is needed to monitor the mass production in industry and in the CMS production centres. To meet these requirements a set-up called APV Readout Controller (ARC) system was developed and distributed among all collaborating institutes to perform full readout tests of hybrids and modules at each production step. The system consists of all necessary hardware components, C++ based readout software using LabVIEW 1 as graphical user interface and provides full database connection to track every single module component during the production phase. Two preseries of Tracker End Cap (TEC) silicon detector modules have been produced by the TEC community and tested with the ARC system at Aachen. The results of the second series are presented.

The dV/dI(V) characteristics of point contacts on single crystals of UPd2Al3 and UNi2Al3 were measured in the temperature range 1.6–30 K. The dV/dI of UPd2Al3 contacts reveal distinct peculiarities related to the antiferromagnetic ordering, contrary to UNi2Al3 point contacts. Spectral features manifesting a partially gapped Fermi surface in the antiferromagnetic state were not observed in the dV/dI curves.

Abstract A system of detector modules consisting of a large size Gas Electron Multiplier (GEM), coupled to Micro Strip Gas Counters (MSGC), has been exposed to a pion beam at the Paul Scherrer Institute Cyclotron facility. As part of a CMS tracker milestone, the aim of this test was to investigate the robustness of such detectors when exposed to experimental conditions close to what is expected at the Large Hadron Collider (LHC) of CERN. Eighteen detector modules have been operated at voltage settings corresponding to 98% detection efficiency for Minimum Ionizing Particles during a period of 5 weeks. Sparking rates and strip losses have been monitored throughout the exposure. An operation margin of at least a factor of three with respect to the required gas gain has been demonstrated.

This paper presents a web-based Job Monitoring framework for individual Grid sites that allows users to follow in detail their jobs in quasi-real time. The framework consists of several independent components : (a) a set of sensors that run on the site CE and worker nodes and update a database, (b) a simple yet extensible web services framework and (c) an Ajax powered web interface having a look-and-feel and control similar to a desktop application. The monitoring framework supports LSF, Condor and PBS-like batch systems. This is one of the first monitoring systems where an X.509 authenticated web interface can be seamlessly accessed by both end-users and site administrators. While a site administrator has access to all the possible information, a user can only view the jobs for the Virtual Organizations (VO) he/she is a part of. The monitoring framework design supports several possible deployment scenarios. For a site running a supported batch system, the system may be deployed as a whole, or existing site sensors can be adapted and reused with the web services components. A site may even prefer to build the web server independently and choose to use only the Ajax powered web interface. Finally, the system is being used to monitor a glideinWMS instance. This broadens the scope significantly, allowing it to monitor jobs over multiple sites.

The differential resistance dV/dI versus applied voltage for URu2Si2 homocontacts at helium temperatures shows a distinct maximum at zero bias for current flow in the ab plane which vanishes as the temperature reaches the Neel temperature TN⋍ 17.5 K. For current flow parallel to the c direction this structure was not so pronounced. We connect the observed maximum with the spin-density wave gap opening below TN on part of the Fermi surface in the ab plane. The value of the gap estimated from the width of the zero-bias maxima in the symmetric part of the dV/dI is of order 10meV. The measured temperature dependence shows a rapidly decreasing intensity of this maximum, while its width is practically constant up to temperatures close to TN.

The assembly and test of 18 MSGC+GEM modules each equipped with four wedge-shaped substrates is described. Measurements of gain and current were performed to determine the fields for maximum transparency of the GEM. These detectors were operated for five weeks under a high rate environment at the Paul Scherrer Institute (Switzerland) to study their stability and uniformity. The large scale MSGC+GEM system was operated for 376 h at the nominal working point of signal to noise around 37 with a negligible sparking rate. A safety margin of a factor of two to three in signal to noise guarantees a safe operation under LHC like conditions.

We have measured the differential resistance of point-contacts between Pt and the MeBe13-compounds with Me = La,Ce,Yb and U as function of bias U. The characteristics of Ce- and YbBe13 show an asymmetry which is typical for an f-instability, and so does UBe13.

In order to check the system aspects of the forward–backward MSGC tracker designed for the future CMS experiment at LHC, 38 trapezoidal MSGC counters assembled in six multi-substrates detector modules were built and exposed to a muon beam at the CERN SPS. Results on the gain uniformity along the wedge-shaped strip pattern and across the detector modules are shown together with measurements of the detection efficiency and the spatial resolution.

A small series production of detector modules made of MicroStrip Gas Counters (MSGC) and a Gas Electron Multiplier (GEM) foil has been exposed to a high-intensity hadron beam. We report about the reproductibility and stability of the detector responses and about the occurrence and consequences of discharges in the detector. The interdependence of the four voltage differences used in the detector has been studied by simulation and with X-ray measurements. Rate dependence of the signal amplitude is observed. The behaviour of the MSGC+GEM is compared to that of a state-of-the-art MSGC. Influence of various parameters on the detector response is investigated.

The Compact Muon Solenoid (CMS) is one of two general purpose detectors which are foreseen to operate at the Large Hadron Collider (LHC), which is presently being built at the European laboratory for particle physics (CERN) in Switzerland. The Central Tracker of CMS consists of a Pixel System, which is located close to the interaction point and a Silicon Strip Tracker (SST) which instruments the intermediate and outer region. The SST is composed of 15148 Silicon Microstrip Detector Modules which contain the readout electronics (hybrids) and sensors. These modules will be assembled into substructures with control electronics and optics for transmitting data. The substructures will be integrated into the subsystems of the SST. The SST will be operated for up to ten years in the harsh radiation environment of the LHC. The lifetime of the SST will be extended by operating the detector at lowered temperature. The sensors, which are very delicate with respect to radiation damage, will be operated at a maximum temperature of -10/spl deg/C. Since the assembly of the modules as well as the mounting on substructures is done at room temperature, tests in a CMS-like environment are necessary to prove the mechanical and electrical stability.

Among rare-earth and actinide ${\mathrm{Be}}_{13}$ intermetallic compounds, ${\mathrm{YbBe}}_{13}$ is anomalous, with evidence for mixed-valence and heavy-fermion behavior. The resistivity is comparatively low and the resulting large mean free path enables point-contact spectroscopy in the ballistic regime. In the ${d}^{2}{V/dI}^{2}$ characteristics of point contacts between ${\mathrm{YbBe}}_{13}$ and normal metals, measured at temperatures between 0.2 and 40 K, we find structures due to crystal-field and Kondo scattering. The characteristics show a pronounced asymmetry, which we explain by the enhanced effective electronic mass in ${\mathrm{YbBe}}_{13}$ and by a shift of the chemical potential in the contact region due to the nonthermal occupation of excited crystal-field levels.

The self-aligned in-line (SAIL) technique has been applied to the preparation of ultrasmall low-capacitance metallic tunnel junctions. By using e-beam lithography the area of Al/AIOx/Al contacts has so far been reduced to less than 0.005 μm2. At low temperatures high-ohmic double junctions with a small metallic island between them show the Coulomb blockade effect. The current through such a device could be modulated by a voltage applied to a gate electrode capacitively coupled to the island (single-electron transistor). Both single-charge phenomena have been observed at temperatures up to 1 K.

The SAIL (self-aligned in-line) technique has been applied to the preparation of ultrasmall metallic tunnel junctions. By using e-beam lithography and sputter methods the area of both Al/AIO x /Al and Nb/AIO x /Nb contacts has so far been reduced to less than 0.005pm 2 . At low temperatures high-ohmic double junctions with a small metallic island in between show the Coulomb blockade effect. The current through such a device can be modulated by a voltage applied to a gate electrode capacitively coupled to the island (single-electron transistor). Both these single-charge phenomena have been observed at temperatures of a few hundred mK.

The efficient exploitation of worldwide distributed storage and computing resources available in the grids require a robust, transparent and fast deployment of experiment specific software. The approach followed by the CMS experiment at CERN in order to enable Monte-Carlo simulations, data analysis and software development in an international collaboration is presented. The current status and future improvement plans are described.

The efficient exploitation of worldwide distributed storage and computing resources available in the grids require a robust, transparent and fast deployment of experiment specific software. The approach followed by the CMS experiment at CERN in order to enable Monte-Carlo simulations, data analysis and software development in an international collaboration is presented. The current status and future improvement plans are described.

We report the results of systematic investigations on operating properties of MSGC+GEM detectors exposed to different sources of X-rays.The influence of the drift field on the detector response has been studied experimentally and by simulation.Charging-up effects depending on the drift field will be discussed.

At about the same time when Little proposed room-temperature superconductivity in organic polymers, it vas suggested that the high-Tc of the more conventional A-15’s is associated with their one-dimensional electronic hand structure1. When TTF-TCNQ was discovered in 1973, it was suggested that the electron-phonon coupling in this 1-D organic molecular crystal is responsible for the metal-to-insulator (Peierls) transition at 52 K2, and reducing λ will cause a crossover to a superconducting state3. Since then, the electronic structure, the conduction mechanism, and the superconducting mechanism were subject to controversy. Therefore, it is of some importance to establish whether the electronic band structure, and conduction mechanism, are similar to those in more conventional metals, and whether the superconductivity mechanism is the normal BCS phonon-mediated interaction

We have performed point-contact measurements on the organic superconductor αT-(BEDT—TTF)2I3 (Tc ≈ 8 K) with a Pt counterelectrode. The dV/dI versus V characteristics show gap-like structures which can be well described by the BTK theory. The width of the gap is in agreement with the BCS value (2Δ/kBTc = 3.52) in contrast to previous measurements on homocontacts of αT-(BEDT—TTF)2I3.

We report on point contact measurements on the organic superconductor αt-(BEDT-TTF)2I3 (Tc~8K). The dV/dI vs V characteristics of homocontacts of this material show strong nonlinearities which, interpreted as a gap-structure, lead to extremly high values of Δ/κTc. Among other possibilities, we discuss the anomaly in terms of a strongly enhanced McMillan-Rowell structure Δ(e) at e = Δ BCS + α ph. The enhancement is caused by deviations from Eliashberg equations due to strong coupling (λ = 1-2) together with an Einstein phonon spectrum.

Differential resistance dV/dl(V) of point contacts based on the heavy-fermion compound (HFC) URu2Si2 is measured in the temperature range 1.5-20 К along two principal crystallographic directions. A distinguishing feature of dVldl(V) measured below the Neel temperature TN — \1.5 К for URu2Si2 point contacts is a peak located asymmetrically with respect to zero voltage (V=0). On the symmetric part of the dependence dV/dI(V) for homojunctions, such a peak is observed for currents directed at right angles to the c-axis, while the peak is suppressed or missing along the c-axis. The existence of such a peak is associated with anisotropic energy gap in the ab-plane, which is attributed to the spin-density waves (SDW) and appears on a part of the Fermi surface at temperatures below 7jV. The magnitude of the gap, estimated from the width of the peak in the symmetric part of dV/dl(V) for V — 0, was found to be equal to 10±1 mV. The reproducible asymmetry of the differential resistance of heterojunctio ns between URu2Si2 and simple metals is studied and attributed to the effect of the peak of the high-density electron states, located asymmetrically with respect to the Fermi level in the HFC. © 1995 American Institute of Physics.

We have measured the differential resistance of point-contacts between Pt and the MeBe13-compounds with Me = La,Ce,Yb and U as function of bias U. The characteristics of Ce- and YbBe13 show an asymmetry which is typical for an f-instability, and so does UBe13.