The silicon pixel tracking system for the ATLAS experiment at the Large Hadron Collider is described and the performance requirements are summarized. Detailed descriptions of the pixel detector electronics and the silicon sensors are given. The design, fabrication, assembly and performance of the pixel detector modules are presented. Data obtained from test beams as well as studies using cosmic rays are also discussed.

The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems. But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAlO3-SrTiO3 interface as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field. We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen-Cooper-Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity.

The ATLAS IBL CollaborationDuring the shutdown of the CERN Large Hadron Collider in 2013-2014, an additional pixel layer was installed between the existing Pixel detector of the ATLAS experiment and a new, smaller radius beam pipe. The motivation for this new pixel layer, the Insertable B-Layer (IBL), was to maintain or improve the robustness and performance of the ATLAS tracking system, given the higher instantaneous and integrated luminosities realised following the shutdown. Because of the extreme radiation and collision rate environment, several new radiation-tolerant sensor and electronic technologies were utilised for this layer. This paper reports on the IBL construction and integration prior to its operation in the ATLAS detector.The ATLAS [1] general purpose detector is used for the study of proton-proton (pp) and heavy-ion collisions at the CERN Large Hadron Collider (LHC) [2]. It successfully collected data at pp collision energies of 7 and 8 TeV in the period of 2010-2012, known as Run 1. Following an LHC shutdown in 2013-2014 (LS1), it has collected data since 2015 at a pp collision energy of 13 TeV (the so-called Run 2).The ATLAS inner tracking detector (ID) [1, 3] provides charged particle tracking with high efficiency in the pseudorapidity 1 range of |η| < 2.5. With increasing radial distance from the interaction region, it consists of silicon pixel and micro-strip detectors, followed by a transition radiation tracker (TRT) detector, all surrounded by a superconducting solenoid providing a 2 T magnetic field.The original ATLAS pixel detector [4,5], referred to in this paper as the Pixel detector, was the innermost part of the ID during Run 1. It consists of three barrel layers (named the B-Layer, Layer 1 and Layer 2 with increasing radius) and three disks on each side of the interaction region, to guarantee at least three space points over the full tracking |η| range. It was designed to operate for the Phase-I period of the LHC, that is with a peak luminosity of 1 × 10 34 cm −2 s −1 and an integrated luminosity of approximately 340 fb −1 corresponding to a TID of up to 50 MRad 2 and a fluence of up to 1 × 10 15 n eq /cm 2 NIEL. However, for luminosities exceeding 2 × 10 34 cm −2 s −1 , which are now expected during the Phase-I operation, the read-out efficiency of the Pixel layers will deteriorate. This paper describes the construction and surface integration of an additional pixel layer, the Insertable B-Layer (IBL) [6], installed during the LS1 shutdown between the B-Layer and a new smaller radius beam pipe. The main motivations of the IBL were to maintain the full ID tracking performance and robustness during Phase-I operation, despite read-out bandwidth limitations of the Pixel layers (in particular the B-Layer) at the expected Phase-I peak luminosity, and accumulated radiation damage to the silicon sensors and front-end electronics. The IBL is designed to operate until the end of Phase-I, when a full tracker upgrade is planned [7] for high luminosity LHC (HL-LHC) operation from approximately ...

We observe the total filling factor nuT=1 quantum Hall state in a bilayer two-dimensional electron system with virtually no tunneling. We find thermally activated transport in the balanced system with a monotonic increase of the activation energy with decreasing d/lB below 1.65. In the imbalanced system we find activated transport in each of the layers separately, yet the activation energies show a striking asymmetry around the balance point, implying a different excitation spectrum for the separate layers forming the condensed state.

The magneto resistance of a narrow single quantum well is spectacularly different from the usual behavior. At filling factors 2 3 and 3 5 we observe large and sharp maxima in the longitudinal resistance instead of the expected minima. The peak value of the resistance exceeds those of the surrounding magnetic field regions by a factor of up to three. The formation of the maxima takes place on very large time scales which suggests a close relation with nuclear spins. We discuss the properties of the observed maxima due to a formation of domains of different electronic states.