Negative ions in liquid helium

Structure of negative ions in liquid ⁴He is analyzed. The possibility of cluster or bubble formation around impurity ions of both signs is discussed. It is demonstrated that in superfluid helium, around negative alkalineearth metal ions, bubbles are formed and, around halogen ions, clusters are formed. The nature of “fast” and “exotic” negative ions is also discussed. It is assumed that the “fast” ions are negative ions of helium excimer molecules localized inside bubbles. The “exotic” ions are stable negative impurity ions, which are always present in small amounts in gas discharge plasma. Around such ions, bubbles or clusters are created with radius smaller the radius of electron bubbles

Performance of a cryogenic system prototype for the XENON1T Detector

We have developed an efficient cryogenic system with heat exchange and associated gas purification system, as a prototype for the XENON1T experiment. The XENON1T detector will use about 3 ton of liquid xenon (LXe) at a temperature of 175K as target and detection medium for a dark matter search. In this paper we report results on the cryogenic system performance focusing on the dynamics of the gas circulation-purification through a heated getter, at flow rates above 50 Standard Liter per Minute (SLPM). A maximum flow of 114 SLPM has been achieved, and using two heat exchangers in parallel, a heat exchange efficiency better than 96% has been measured

Neutron Scattering Study of Crystal Field Energy Levels and Field Dependence of the Magnetic Order in Superconducting HoNi2B2C

Elastic and inelastic neutron scattering measurements have been carried out to investigate the magnetic properties of superconducting (Tc~8K) HoNi2B2C. The inelastic measurements reveal that the lowest two crystal field transitions out of the ground state occurat 11.28(3) and 16.00(2) meV, while the transition of 4.70(9) meV between these two levels is observed at elevated temperatures. The temperature dependence of the intensities of these transitions is consistent with both the ground state and these higher levels being magnetic doublets. The system becomes magnetically long range ordered below 8K, and since this ordering energy kTN ~ 0.69meV << 11.28meV the magnetic properties in the ordered phase are dominated by the ground-state spin dynamics only. The low temperature structure, which coexists with superconductivity, consists of ferromagnetic sheets of Ho{3+ moments in the a-b plane, with the sheets coupled antiferromagnetically along the c-axis. The magnetic state that initially forms on cooling, however, is dominated by an incommensurate spiral antiferromagnetic state along the c-axis, with wave vector qc ~0.054 A-1, in which these ferromagnetic sheets are canted from their low temperature antiparallel configuration by ~17 deg. The intensity for this spiral state reaches a maximum near the reentrant superconducting transition at ~5K; the spiral state then collapses at lower temperature in favor of the commensurate antiferromagnetic state. We have investigated the field dependence of the magnetic order at and above this reentrant superconducting transition. Initially the field rotates the powder particles to align the a-b plane along the field direction, demonstrating that the moments strongly prefer to lie within this plane due to the crystal field anisotropy. Upon subsequently increasing the field atComment: RevTex, 7 pages, 11 figures (available upon request); Physica

Spin-polarized Tunneling in Hybrid Metal-Semiconductor Magnetic Tunnel Junctions

We demonstrate efficient spin-polarized tunneling between a ferromagnetic metal and a ferromagnetic semiconductor with highly mismatched conductivities. This is indicated by a large tunneling magnetoresistance (up to 30%) at low temperatures in epitaxial magnetic tunnel junctions composed of a ferromagnetic metal (MnAs) and a ferromagnetic semiconductor (GaMnAs) separated by a nonmagnetic semiconductor (AlAs). Analysis of the current-voltage characteristics yields detailed information about the asymmetric tunnel barrier. The low temperature conductance-voltage characteristics show a zero bias anomaly and a V^1/2 dependence of the conductance, indicating a correlation gap in the density of states of GaMnAs. These experiments suggest that MnAs/AlAs heterostructures offer well characterized tunnel junctions for high efficiency spin injection into GaAs.Comment: 14 pages, submitted to Phys. Rev.

Charged lepton electric dipole moments with the localized leptons and the new Higgs doublet in the two Higgs doublet model

We study the lepton electric dipole moments in the split fermion scenario, in the two Higgs doublet model, where the new Higgs scalars are localized around the origin in the extra dimension, with the help of the localizer field. We observe that the numerical value of the electron (muon, tau) electric dipole moment is at the order of the magnitude of 10^{-31} (10^{-24}, 10^{-22}) (e-cm) and this quantity is sensitive the new Higgs localization in the extra dimension.Comment: 20 pages, 7 figure

Mesoscopic Stern-Gerlach device to polarize spin currents

Spin preparation and spin detection are fundamental problems in spintronics and in several solid state proposals for quantum information processing. Here we propose the mesoscopic equivalent of an optical polarizing beam splitter (PBS). This interferometric device uses non-dispersive phases (Aharonov-Bohm and Rashba) in order to separate spin up and spin down carriers into distinct outputs and thus it is analogous to a Stern-Gerlach apparatus. It can be used both as a spin preparation device and as a spin measuring device by converting spin into charge (orbital) degrees of freedom. An important feature of the proposed spin polarizer is that no ferromagnetic contacts are used.Comment: Updated to the published versio

A Sawtooth-like Timeline for the First Billion Year of Lunar Bombardment

We revisit the early evolution of the Moon's bombardment. Our work combines modeling (based on plausible projectile sources and their dynamical decay rates) with constraints from the lunar crater record, radiometric ages of the youngest lunar basins, and the abundance of highly siderophile elements in the lunar crust and mantle. We deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history, but also there was no prominent and "narrow" impact spike some 3.9 Gy ago, unlike that typically envisioned in the lunar cataclysm scenario. Instead, we show the timeline of the lunar bombardment has a sawtooth-like profile, with an uptick in the impact flux near 4.1 Gy ago. The impact flux at the beginning of this weaker cataclysm was 5-10 times higher than the immediately preceding period. The Nectaris basin should have been one of the first basins formed at the sawtooth. We predict the bombardment rate since about 4.1Gy ago declined slowly and adhered relatively close to classic crater chronology models (Neukum and Ivanov (1994)). Overall we expect that the sawtooth event accounted for about 1/4 of the total bombardment suffered by the Moon since its formation. Consequently, considering that about 12-14 basins formed during the sawtooth event, we expect that the net number of basins formed on the Moon was about 45-50. From our expected bombardment timeline, we derived a new and improved lunar chronology suitable for use on Pre-Nectarian surface units. According to this chronology, a significant portion of the oldest lunar cratered terrains has an age of 4.38-4.42 Gyr. Moreover, the largest lunar basin, South Pole Aitken, is older than 4.3Gy, and therefore was not produced during the lunar cataclysm.Comment: In press in EPS

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Magnetocaloric effect in spin-1/2 XXXX chains with three-spin interactions

We consider the exactly solvable spin-1/2 $XX$ chain with the three-spin interactions of the $XZX+YZY$ and $XZY-YZX$ types in an external (transverse) magnetic field. We calculate the entropy and examine the magnetocaloric effect for the quantum spin system. We discuss a relation between the cooling/heating efficiency and the ground-state phase diagram of the quantum spin model. We also compare ability to cool/heat in the vicinity of the quantum critical and triple points. Moreover, we examine the magnetocaloric effect for the spin-1/2 $XX$ chain with three-spin interactions in a random (Lorentzian) transverse magnetic field.Comment: 10 pages, 8 figure