The FIR-absorption of short period quantum wires and the transition from one to two dimensions

We investigate the FIR-absorption of short period parallel quantum wires in a perpendicular quantizing magnetic field. The external time-dependent electric field is linearly polarized along the wire modulation. The mutual Coulomb interaction of the electrons is treated self-consistently in the ground state and in the absorption calculation within the Hartree approximation. We consider the effects of a metal gate grating coupler, with the same or with a different period as the wire modulation, on the absorption. The evolution of the magnetoplasmon in the nonlocal region where it is split into several Bernstein modes is discussed in the transition from: narrow to broad wires, and isolated to overlapping wires. We show that in the case of narrow and not strongly modulated wires the absorption can be directly correlated with the underlying electronic bandstructure.Comment: 15 pages, 9 figures, Revtex, to appear in Phys. Rev.

Stabilizing isolated skyrmions at low magnetic fields exploiting vanishing magnetic anisotropy

Skyrmions are topologically protected non-collinear magnetic structures. Their stability and dynamics, arising from their topological character, have made them ideal information carriers e.g. in racetrack memories. The success of such a memory critically depends on the ability to stabilize and manipulate skyrmions at low magnetic fields. The driving force for skyrmion formation is the non-collinear Dzyaloshinskii-Moriya exchange interaction (DMI) originating from spin-orbit coupling (SOC). It competes with both the nearest neighbour Heisenberg exchange interaction and the magnetic anisotropy, which favour collinear states. While skyrmion lattices might evolve at vanishing magnetic fields, the formation of isolated skyrmions in ultra-thin films so far required the application of an external field which can be as high as several T. Here, we show that isolated skyrmions in a monolayer (ML) of Co epitaxially grown on a Ru(0001) substrate can be stabilized at magnetic fields as low as 100 mT. Even though SOC is weak in the 4d element Ru, a homochiral spin spiral ground state and isolated skyrmions could be detected and laterally resolved using a combination of tunneling and anisotropic tunneling magnetoresistance effect in spin-sensitive scanning tunneling microscopy (STM). Density functional theory (DFT) calculations confirm these chiral magnetic textures, even though the stabilizing DMI interaction is weak. We find that the key factor is the absence of magnetocristalline anisotropy in this system which enables non-collinear states to evolve in spite of weak SOC, opening up a wide choice of materials beyond 5d elements

Qualification Tests of the R11410-21 Photomultiplier Tubes for the XENON1T Detector

The Hamamatsu R11410-21 photomultiplier tube is the photodetector of choice for the XENON1T dual-phase time projection chamber. The device has been optimized for a very low intrinsic radioactivity, a high quantum efficiency and a high sensitivity to single photon detection. A total of 248 tubes are currently operated in XENON1T, selected out of 321 tested units. In this article the procedures implemented to evaluate the large number of tubes prior to their installation in XENON1T are described. The parameter distributions for all tested tubes are shown, with an emphasis on those selected for XENON1T, of which the impact on the detector performance is discussed. All photomultipliers have been tested in a nitrogen atmosphere at cryogenic temperatures, with a subset of the tubes being tested in gaseous and liquid xenon, simulating their operating conditions in the dark matter detector. The performance and evaluation of the tubes in the different environments is reported and the criteria for rejection of PMTs are outlined and quantified.Comment: 24 pages, 16 figure

Climatic and geologic controls on suspended sediment flux in the Sutlej River Valley, western Himalaya

The sediment flux through Himalayan rivers directly impacts water quality and is important for sustaining agriculture as well as maintaining drinking-water and hydropower generation. Despite the recent increase in demand for these resources, little is known about the triggers and sources of extreme sediment flux events, which lower water quality and account for extensive hydropower reservoir filling and turbine abrasion. Here, we present a comprehensive analysis of the spatiotemporal trends in suspended sediment flux based on daily data during the past decade (2001–2009) from four sites along the Sutlej River and from four of its main tributaries. In conjunction with satellite data depicting rainfall and snow cover, air temperature and earthquake records, and field observations, we infer climatic and geologic controls of peak suspended sediment concentration (SSC) events. Our study identifies three key findings: First, peak SSC events (≥ 99th SSC percentile) coincide frequently (57–80%) with heavy rainstorms and account for about 30% of the suspended sediment flux in the semi-arid to arid interior of the orogen. Second, we observe an increase of suspended sediment flux from the Tibetan Plateau to the Himalayan Front at mean annual timescales. This sediment-flux gradient suggests that averaged, modern erosion in the western Himalaya is most pronounced at frontal regions, which are characterized by high monsoonal rainfall and thick soil cover. Third, in seven of eight catchments, we find an anticlockwise hysteresis loop of annual sediment flux variations with respect to river discharge, which appears to be related to enhanced glacial sediment evacuation during late summer. Our analysis emphasizes the importance of unconsolidated sediments in the high-elevation sector that can easily be mobilized by hydrometeorological events and higher glacial-meltwater contributions. In future climate change scenarios, including continuous glacial retreat and more frequent monsoonal rainstorms across the Himalaya, we expect an increase in peak SSC events, which will decrease the water quality and impact hydropower generation

Stationary Entangled Radiation from Micromechanical Motion

Mechanical systems facilitate the development of a new generation of hybrid quantum technology comprising electrical, optical, atomic and acoustic degrees of freedom. Entanglement is the essential resource that defines this new paradigm of quantum enabled devices. Continuous variable (CV) entangled fields, known as Einstein-Podolsky-Rosen (EPR) states, are spatially separated two-mode squeezed states that can be used to implement quantum teleportation and quantum communication. In the optical domain, EPR states are typically generated using nondegenerate optical amplifiers and at microwave frequencies Josephson circuits can serve as a nonlinear medium. It is an outstanding goal to deterministically generate and distribute entangled states with a mechanical oscillator. Here we observe stationary emission of path-entangled microwave radiation from a parametrically driven 30 micrometer long silicon nanostring oscillator, squeezing the joint field operators of two thermal modes by 3.40(37) dB below the vacuum level. This mechanical system correlates up to 50 photons/s/Hz giving rise to a quantum discord that is robust with respect to microwave noise. Such generalized quantum correlations of separable states are important for quantum enhanced detection and provide direct evidence for the non-classical nature of the mechanical oscillator without directly measuring its state. This noninvasive measurement scheme allows to infer information about otherwise inaccessible objects with potential implications in sensing, open system dynamics and fundamental tests of quantum gravity. In the near future, similar on-chip devices can be used to entangle subsystems on vastly different energy scales such as microwave and optical photons.Comment: 13 pages, 5 figure