Search

Gas Electron Multiplier (GEM) technology is being considered for the forward muon upgrade of the CMS experiment in Phase 2 of the CERN LHC. Its first implementation is planned for the GE1/1 system in the $1.5 < \mid\eta\mid < 2.2$ region of the muon endcap mainly to control muon level-1 trigger rates after the second long LHC shutdown. A GE1/1 triple-GEM detector is read out by 3,072 radial strips with 455 $\mu$rad pitch arranged in eight $\eta$-sectors. We assembled a full-size GE1/1 prototype of 1m length at Florida Tech and tested it in 20-120 GeV hadron beams at Fermilab using Ar/CO$_{2}$ 70:30 and the RD51 scalable readout system. Four small GEM detectors with 2-D readout and an average measured azimuthal resolution of 36 $\mu$rad provided precise reference tracks. Construction of this largest GEM detector built to-date is described. Strip cluster parameters, detection efficiency, and spatial resolution are studied with position and high voltage scans. The plateau detection efficiency is [97.1 $\pm$ 0.2 (stat)]\%. The azimuthal resolution is found to be [123.5 $\pm$ 1.6 (stat)] $\mu$rad when operating in the center of the efficiency plateau and using full pulse height information. The resolution can be slightly improved by $\sim$ 10 $\mu$rad when correcting for the bias due to discrete readout strips. The CMS upgrade design calls for readout electronics with binary hit output. When strip clusters are formed correspondingly without charge-weighting and with fixed hit thresholds, a position resolution of [136.8 $\pm$ 2.5 stat] $\mu$rad is measured, consistent with the expected resolution of strip-pitch/$\sqrt{12}$ = 131.3 $\mu$rad. Other $\eta$-sectors of the detector show similar response and performance., Comment: 8 pages, 32 figures, submitted to Proc. 2014 IEEE Nucl. Sci. Symposium, Seattle, WA, reference added

The crystal structure of the thermoelectric material tin selenide has been investigated with angle-dispersive synchrotron x-ray powder diffraction under hydrostatic pressure up to 27 GPa. With increasing pressure, a continuous evolution of the crystal structure from the GeS type to the higher-symmetry TlI type was observed, with a critical pressure of 10.5(3) GPa. The orthorhombic high-pressure modification, beta'-SnSe, is closely related to the pseudo-tetragonal high-temperature modification at ambient pressure. The similarity between the changes of the crystal structure at elevated temperatures and at high pressures suggests the possibility that strained thin films of SnSe may provide a route to overcoming the problem of the limited thermal stability of beta-SnSe at high temperatures., Comment: 11 pages, 7 figures

The introduction of A-site vacancies in SrTiO3 results in a glass-like thermal conductivity while Nb substituted samples maintains good electrical conductivity. This unexpected result brings SrTiO3 one step closer to being a high-performing phonon-glass electron-crystal thermoelectric material.

Inelastic neutron scattering experiments were performed to investigate the crystalline electric field (CEF) excitations of Nd3+ (J = 9/2) in the iron pnictide NdFeAsO. The crystal field level structures for both the high-temperature paramagnetic phase and the low-temperature antiferromagnetic phase of NdFeAsO are constructed. The variation of CEF excitations of Nd3+ reflects not only the change of local symmetry but also the change of magnetic ordered state of the Fe sublattice. By analyzing the crystal field interaction with a crystal field Hamiltonian, the crystal field parameters are obtained. It was found that the sign of the fourth and sixth-order crystal field parameters change upon the magnetic phase transition at 140 K, which may be due to the variation of exchange interactions between the 4f and conduction electrons., Comment: 5 pages, 4 figures

We carried out AC magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK. Below ~1 K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the AC susceptibility revealed a dilute-spin-glass like transition below ~ 1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons in triply degenerate t2g orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously been interpreted in terms of a freezing to a heterogeneous state with non-magnetic sites where 4d^1 electrons have paired in spin-singlets dimers, and residual unpaired Mo5+ 4d1 electrons. Based on the magnetic heat capacity data it has been suggested that this heterogeneity is the result of kinetic constraints intrinsic to the physics of the pure system (possibly due to topological overprotection), leading to a self-induced glass of valence bonds between neighbouring 4d1 electrons. The muSR relaxation unambiguously points to a static heterogeneous state with a static arrangement of unpaired electrons isolated by spin-singlet (valence bond) dimers between the majority of Mo5+ 4d electrons. The AC susceptibility data indicate that the residual magnetic moments freeze into a dilute-spin-glass-like state. This is in apparent contradiction with the muon-spin decoupling at 50 mK in fields up to 200 mT, which indicates that, remarkably, the time scale of the field fluctuations from the residual moments is ~ 5 ns. Comparable behaviour has been observed in other geometrically frustrated magnets with spin-liquid-like behaviour and the implications of our observations on Ba2YMoO6 are discussed in this context., Comment: 11 pages, 3 Figures. Published in New Journal of Physics

Thermoelectric figures of merit, ZT > 0.5, have been obtained in arc-melted TiNiSn-based ingots. This promising conversion efficiency is due to a low lattice thermal conductivity, which is attributed to excess nickel in the half-Heusler structure.

The CMS GEM collaboration is considering Gas Electron Multipliers (GEMs) for upgrading the CMS forward muon system in the 1.5<|eta|<2.4 endcap region. GEM detectors can provide precision tracking and fast trigger information. They would improve the CMS muon trigger and muon momentum resolution and provide missing redundancy in the high-eta region. Employing a new faster construction and assembly technique, we built four full-scale Triple-GEM muon detectors for the inner ring of the first muon endcap station. We plan to install these or further improved versions in CMS during the first long LHC shutdown in 2013/14 for continued testing. These detectors are designed for the stringent rate and resolution requirements in the increasingly hostile environments expected at CMS after the second long LHC shutdown in 2018/19. The new prototypes were studied in muon/pion beams at the CERN SPS. We discuss our experience with constructing the new full-scale production prototypes and present preliminary performance results from the beam test. We also tested smaller Triple-GEM prototypes with zigzag readout strips with 2 mm pitch in these beams and measured a spatial resolution of 73 microns. This readout offers a potential reduction of channel count and consequently electronics cost for this system while maintaining high spatial resolution., Comment: 5 pages, 9 figures, submitted to Proc. 2012 IEEE Nucl. Sci. Symposium, Anaheim, CA

The low-temperature magnetic structure of NdFeAsO has been revisited using neutron powder diffraction and symmetry analysis using the Sarah representational analysis program. Four magnetic models with one magnetic variable for each of the Nd and Fe sublattices were tested. The best fit was obtained using a model with Fe moments pointing along the c-direction, and Nd moments along the a-direction. This signals a significant interplay between rare-earth and transition metal magnetism, which results in a spin-reorientation of the Fe sublattice upon ordering of the Nd moments. All models that fit the data well, including collinear models with more than one magnetic variable per sublattice, were found to have an Fe moment of 0.5 BM and a Nd moment of 0.9 BM, demonstrating that the low-temperature Fe moment is not substantially enhanced compared to the spin-density wave (SDW) state., Comment: accepted to J. Phys.: Cond. Mat

The phase stability of the (Bi2)m(Bi2Te3)n natural superlattices has been investigated through the low temperature solid state synthesis of a number of new binary BixTe1-x compositions. Powder X-ray diffraction revealed that an infinitely adaptive series forms for 0.44 < x < 0.70, while an unusual 2-phase region with continuously changing compositions is observed for 0.41 < x < 0.43. For x > 0.70, mixtures of elemental Bi and an almost constant composition (Bi2)m(Bi2Te3)n phase are observed. Rietveld analysis of synchrotron X-ray powder diffraction data collected on Bi2Te (m = 2, n = 1) revealed substantial interchange of Bi and Te between the Bi2 and Bi2Te3 blocks, demonstrating that the block compositions are variable. All investigated phase pure compositions are degenerate semiconductors with low residual resistivity ratios and moderate positive magnetoresistances (R/R0 = 1.05 in 9 T). The maximum Seebeck coefficient is +80 muV K-1 for x = 0.63, leading to an estimated thermoelectric figure of merit, zT = 0.2 at 250 K., Comment: accepted to a special issue on thermoelectric materials in the Journal of Solid State Chemistry

At present, part of the forward RPC muon system of the CMS detector at the CERN LHC remains uninstrumented in the high-\eta region. An international collaboration is investigating the possibility of covering the 1.6 < |\eta| < 2.4 region of the muon endcaps with large-area triple-GEM detectors. Given their good spatial resolution, high rate capability, and radiation hardness, these micro-pattern gas detectors are an appealing option for simultaneously enhancing muon tracking and triggering capabilities in a future upgrade of the CMS detector. A general overview of this feasibility study will be presented. The design and construction of small (10\times10 cm2) and full-size trapezoidal (1\times0.5 m2) triple-GEM prototypes will be described. During detector assembly, different techniques for stretching the GEM foils were tested. Results from measurements with x-rays and from test beam campaigns at the CERN SPS will be shown for the small and large prototypes. Preliminary simulation studies on the expected muon reconstruction and trigger performances of this proposed upgraded muon system will be reported., Comment: 7 pages, 25 figures, submitted for publication in conference record of the 2011 IEEE Nuclear Science Symposium, Valencia, Spain

Gas Electron Multipliers (GEM) are an interesting technology under consideration for the future upgrade of the forward region of the CMS muon system, specifically in the $1.6<| \eta |<2.4$ endcap region. With a sufficiently fine segmentation GEMs can provide precision tracking as well as fast trigger information. The main objective is to contribute to the improvement of the CMS muon trigger. The construction of large-area GEM detectors is challenging both from the technological and production aspects. In view of the CMS upgrade we have designed and built the largest full-size Triple-GEM muon detector, which is able to meet the stringent requirements given the hostile environment at the high-luminosity LHC. Measurements were performed during several test beam campaigns at the CERN SPS in 2010 and 2011. The main issues under study are efficiency, spatial resolution and timing performance with different inter-electrode gap configurations and gas mixtures. In this paper results of the performance of the prototypes at the beam tests will be discussed.

We investigated the low energy excitations in the parent compound NdFeAsO of the Fe-pnictide superconductor in the $\mu$eV range by a back scattering neutron spectrometer. The energy scans on a powder NdFeAsO sample revealed inelastic peaks at E = 1.600 $ \pm 0.003 \mu$eV at T = 0.055 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at about 6 K. We interpret the inelastic peaks to be due to the transition between hyperfine-split nuclear level of the $^{143}$Nd and $^{145}$Nd isotopes with spin $I = 7/2$. The hyperfine field is produced by the ordering of the electronic magnetic moment of Nd at low temperature and thus the present investigation gives direct evidence of the ordering of the Nd magnetic sublattice of NdFeAsO at low temperature.

A combination of synchrotron X-ray, neutron powder diffraction, magnetization, heat capacity and electrical resistivity measurements reveals that NdMnAsO is an antiferromagnetic semiconductor with large Neel temperature (TN = 359(2) K). At room temperature the magnetic propagation vector k = 0 and the Mn moments are directed along the crystallographic c-axis (mMn = 2.41(6) BM). Upon cooling a spin reorientation (SR) transition of the Mn moments into the ab-plane occurs (TSR = 23 K). This coincides with the long range ordering of the Nd moments, which are restricted to the basal plane. The magnetic propagation vector remains k = 0. At base temperature (1.6 K) the fitted moments are mab,Mn = 3.72(1) BM and mab,Nd = 1.94(1) BM. The electrical resistivity is characterized by a broad maximum at 250 K, below which it has a metallic temperature dependence but semiconducting magnitude (rho250K = 50 Ohm cm, residual resistivity ratio = 2), and a slight upturn at the SR transition.

We report on the unconventional magnetism in the cubic B-site ordered double perovskite Ba2YMoO6, using AC and DC magnetic susceptibility, heat capacity and muSR. No magnetic order is observed down to 2 K while the Weiss temperature is ~ -160 K. This is ascribed to the geometric frustration in the lattice of edge-sharing tetrahedra with orbitally degenerate Mo5+ s=1/2 spins. Our experimental results point to a gradual freezing of the spins into a disordered pattern of spin-singlets, quenching the orbital degeneracy while leaving the global cubic symmetry unaffected, and providing a rare example of a valence bond glass., Comment: 4 pages, 5 figures. Published in Phys. Rev. Lett.

The structural, electronic, and magnetic properties of the superconducting NdFeAsO$_{1-x}$F$_{x}$ phases ($T_C=43 {\rm K}$ for $x=0.15$) have been investigated experimentally by high-resolution synchrotron x-ray powder diffraction, magnetization and $^{75}$As NMR measurements. Density-functional calculations were performed to calculate and analyze the electric field gradient and the density of states. Compared to LaFeAsO family, the NdFeAsO family shows a contraction of the lattice parameters with shorter rare-earth (RE)-As bond distances, an increased thickness of the As-Fe$_2$-As layer, and less distorted Fe-As$_4$ tetrahedra. The $^{75}$As quadrupole frequencies are enhanced with respect to the La analogs. This is due to a more prolate As $4p$ electron distribution mainly caused by the reduced lattice parameters and not by the presence of Nd $4f$ electrons. A non-negligible hyperfine coupling between the $^{75}$As nuclei and the Nd $4f$ states indicates a weak coupling between the REO and FeAs layer and possibly opens the channel for a Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction between localized Nd $4f$ moments mediated by itinerant Fe 3d and/or Nd $5d$ states., Comment: 7 pages, 6 figures, replaces 0806.1450

A suppression of superconductivity in the late rare earth RFeAsO1-xFx materials is reported. The maximum critical temperature (Tc) decreases from 51 K for R = Tb to 36 K for HoFeAsO0.9F0.1, which has been synthesised under 10 GPa pressure. This suppression is driven by a decrease in the Fe-As-Fe angle below an optimimum value of 110.6 degrees, as the angle decreases linearly with unit cell volume (V) across the RFeAsO1-xFx series. A crossover in electronic structure around this optimum geometry is evidenced by a change in sign of the compositional dTc/dV, from negative values for previously reported large R materials to positive for HoFeAsO0.9F0.1., Comment: accepted to PRB

We report measurements of the phonon density-of-states in iron oxypnictide superconductors by inelastic x-ray scattering. A good agreement with ab-initio calculations that do not take into account strong electronic correlations is found, and an unpredicted softening of phonon branches under F doping of these compounds is observed. Raman scattering experiments lead us to conclude that this softening is not related to zone center phonons, and consequently imply an important softening of the relevant phonon branches at finite momentum transfer Q., Comment: 4 pages, 4 figures. Originally submitted 07 Jul 08, accepted for publication in Phys. Rev. B (R)

The magnetic properties and structure of LixCoO2 for x between 0.5 and 1.0 are reported. Co4+ is found to be high-spin in LixCoO2 for x between 0.94 and 1.0 and low-spin for x between 0.50 and 0.78. Weak antiferromagnetic coupling is observed, increasing in strength as more Co4+ is introduced. At an x value of about 0.65, the temperature-independent contribution to the magnetic susceptibility and the electronic contribution to the specific heat are largest. Neutron diffraction analysis reveals that the lithium oxide layer expands perpendicular to the basal plane and the Li ions displace from their ideal octahedral sites with decreasing x. A comparison of the structures of the NaxCoO2 and LixCoO2 systems reveals that the CoO2 layer changes substantially with alkali content in the former but is relatively rigid in the latter. Further, the CoO6 octahedra in LixCoO2 are less distorted than those in NaxCoO2. We postulate that these structural differences strongly influence the physical properties in the two systems.

The structures and thermoelectric properties of the (Bi2)m(Bi2Te3)n homologous series, derived from stacking hexagonal Bi2 and Bi2Te3 blocks, are reported. The end-members of this series are metallic Bi and semiconducting Bi2Te3; nine members of the series have been studied. The structures form an infinitely adaptive series and a unified structural description based on a modulated structure approach is presented. The as-synthesized samples have thermopowers (S) that vary from n-type for Bi2Te3 to p-type for phases rich in Bi2 blocks but with some Bi2Te3 blocks present, to n-type again for Bi metal. The thermoelectric power factor (S2/rho) is highest for Bi metal (43 muW/K2 cm at 130 K), followed by Bi2Te3 (20 muW/K2 cm at 270 K), while Bi2Te (m:n = 5:2) and Bi7Te3 (m:n = 15:6) have 9 muW/K2 cm (at 240 K) and 11 muW/K2 (at 270 K), respectively. The results of doping studies with Sb and Se into Bi2Te are reported., Comment: accepted for publication in PRB

Ferromagnetism is observed below 10 K in [Bi0.75Te0.125Mn0.125]Te. This material has the BiTe structure, which is made from the stacking of two Te-Bi-Te-Bi-Te blocks and one Bi-Bi block per unit cell. Crystal structure analysis shows that Mn is localized in the Bi2 blocks, and is accompanied by an equal amount of TeBi anti-site occupancy in the Bi2Te3 blocks. These TeBi anti-site defects greatly enhance the Mn solubility. This is demonstrated by comparison of the [Bi1-xMnx]Te and [Bi1-2xTexMnx]Te series; in the former, the solubility is limited to x = 0.067, while the latter has xmax = 0.125. The magnetism in [Bi1-xMnx]Te changes little with x, while that for [Bi1-2xTexMnx]Te shows a clear variation, leading to ferromagnetism for x > 0.067. Magnetic hysteresis and the anomalous Hall Effect are observed for the ferromagnetic samples., Comment: Accepted for publication in Phys. Rev. B

Charge density waves (CDWs) are periodic modulations of the conduction electron density in solids. They are collective states that arise from intrinsic instabilities often present in low dimensional electronic systems. The layered dichalcogenides are the most well-studied examples, with TiSe_2 one of the first CDW-bearing materials known. The competition between CDW and superconducting collective electronic states at low temperatures has long been held and explored, and yet no chemical system has been previously reported where finely controlled chemical tuning allows this competition to be studied in detail. Here we report how, upon controlled intercalation of TiSe_2 with Cu to yield Cu_xTiSe_2, the CDW transition is continuously suppressed, and a new superconducting state emerges near x = 0.04, with a maximum T_c of 4.15 K found at x = 0.08. Cu_xTiSe_2 thus provides the first opportunity to study the CDW to Superconductivity transition in detail through an easily-controllable chemical parameter, and will provide new insights into the behavior of correlated electron systems., Comment: Accepted to Nature Physics

The crystal structures of the NaxCoO2 phases based on three-layer NaCoO2, with x=0.32, x=0.51, x=0.60, x=0.75 and x=0.92, determined by powder neutron diffraction, are reported. The structures have triangular CoO2 layers interleaved by sodium ions, and evolve with variation in Na content in a more complex way than has been observed in the two-layer NaxCoO2 system. The highest and lowest Na containing phases studied (x=0.92 and x=0.32) are trigonal, with three CoO2 layers per cell and octahedral Na ion coordination. The intermediate compositions have monoclinic structures. The x=0.75 compound has one CoO2 layer per cell, with Na in octahedral coordination and an incommensurate superlattice. The x=0.6 and x=0.5 phases are also single-layer, but the Na is found in trigonal prismatic coordination. The magnetic behavior of the phases is similar to that observed in the two-layer system. Both the susceptibility and the electronic contribution to the specific heat are largest for x=0.6., Comment: 24 pages, 14 figures, submitted to Phys.Rev. B