IR-spectroscopy and AFM-microscopy of the surface of gamma-irradiated GaS and GaS:Yb layered single crystals

For the first time, information on the surface relief of the layered GaS and doped GaS:Yb single crystals subjected to gamma-irradiation was obtained using atomic force microscopy (AFM) and Fourier-transform infrared spectroscopy (FTIR). It was found that GaS is characterized by a non-uniform distribution of irregularities with different heights and periodicities, and when doping crystals with Yb atoms, the distribution of irregularities becomes more orderly, the height and periodicity of irregularities decreases. In the FTIR spectra, changes in the reflection coefficients of the surface of GaS and GaS:Yb single crystals are observed as a function of the gammairradiation dose (Фγ = 30…200 krad), and on the basis of spectroscopic and microscopic changes, it was found that doped single crystals are the most radiation-resistant.Вперше методами атомно-силової мікроскопії (АСМ) і ІК-фур'є-спектроскопії отримано інформацію про рельєф поверхні нелегованих GaS і легованих монокристалів GaS:Yb, підданих гамма-опроміненню. Встановлено, що для монокристалів GaS характерний нерівномірний розподіл нерівностей з різною висотою і періодичністю, а при легуванні кристалів атомами Yb розподіл нерівностей упорядкується, їх висота і періодичність зменшаться. В ІК-фур’є-спектрах спостерігаються зміни коефіцієнтів відбиття поверхні монокристалів GaS і GaS:Yb в залежності від дози гамма-опромінення (Фγ = 30...200 крад), і на основі цих змін встановлено, що леговані монокристали є більш радіаційно стійкими.Впервые методами атомно-силовой микроскопии (АСМ) и ИК-фурье-спектроскопии получена информация о рельефе поверхности нелегированных GaS и легированных монокристаллов GaS:Yb, подвергнутых гамма-облучению. Установлено, что для монокристаллов GaS характерно неравномерное распределение неровностей с различной высотой и периодичностью, а при легировании кристаллов атомами Yb распределение неровностей упорядочится, их высота и периодичность уменьшатся. В ИК-фурьеспектрах наблюдаются изменения коэффициентов отражения поверхности монокристаллов GaS и GaS:Yb в зависимости от дозы гамма-облучения (Фγ = 30…200 крад), и на основе этих изменений установлено, что легированные монокристаллы являются более радиационно стойкими

Anisotropy of Upper Critical Field near Tc and Magnetic Gap of Superconducting URu2Si2 Single Crystal.

Studying thermal conductivity and STM spectra in the normal and magnetic phase of superconducting URu2Si2 single crystal, we found, that the magnetic gap, partially opened on the Fermi surface below Neel temperature TN = 17.5 K, is strongly anisotropic: gapped states mainly correspond to tetragonal ab plane

Low-frequency noise and tunnelling magnetoresistance in Fe(110)/MgO(111)/Fe(110) epitaxial magnetic tunnel junctions

We report on tunnelling magnetoresistance (TMR), current-voltage (IV) characteristics and low frequency noise in epitaxially grown Fe(110)/MgO(111)/Fe(110) magnetic tunnel junctions (MTJs) with dimensions from 2x2 to 20x20 um2. The evaluated MgO energy barrier (0.50+/-0.08 eV), the barrier width (13.1+/-0.5 angstrom) as well as the resistance times area product (7+/-1 Mohmsum2) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. The noise power, though exhibiting large variation, was observed to be roughly anticorrelated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behaviour could be related to an interplay between the magnetic state and the local barrier defects structure of the epitaxial MTJsComment: 9 pages and 3 figure

Anomalous heavy-fermion and ordered states in the filled skutterudite PrFe4P12

Specific heat and magnetization measurements have been performed on high-quality single crystals of filled-skutterudite PrFe_4P_{12} in order to study the high-field heavy-fermion state (HFS) and low-field ordered state (ODS). From a broad hump observed in C/T vs T in HFS for magnetic fields applied along the direction, the Kondo temperature of ~ 9 K and the existence of ferromagnetic Pr-Pr interactions are deduced. The {141}-Pr nuclear Schottky contribution, which works as a highly-sensitive on-site probe for the Pr magnetic moment, sets an upper bound for the ordered moment as ~ 0.03 \mu_B/Pr-ion. This fact strongly indicates that the primary order parameter in the ODS is nonmagnetic and most probably of quadrupolar origin, combined with other experimental facts. Significantly suppressed heavy-fermion behavior in the ODS suggests a possibility that the quadrupolar degrees of freedom is essential for the heavy quasiparticle band formation in the HFS. Possible crystalline-electric-field level schemes estimated from the anisotropy in the magnetization are consistent with this conjecture.Comment: 7 pages and 7 figures. Accepted for publication in Phys. Rev.

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results

Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC

Measurements of inclusive jet suppression in heavy ion collisions at the LHC provide direct sensitivity to the physics of jet quenching. In a sample of lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the anti-kt algorithm with values for the distance parameter that determines the nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp. Jet production is found to be suppressed by approximately a factor of two in the 10% most central collisions relative to peripheral collisions. Rcp varies smoothly with centrality as characterized by the number of participating nucleons. The observed suppression is only weakly dependent on jet radius and transverse momentum. These results provide the first direct measurement of inclusive jet suppression in heavy ion collisions and complement previous measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables, submitted to Physics Letters B. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02

Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment

This paper describes an analysis of the angular distribution of W->enu and W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and the missing transverse energy, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw > 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour, are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables, revised author list, matches European Journal of Physics C versio

Observation of a new chi_b state in radiative transitions to Upsilon(1S) and Upsilon(2S) at ATLAS

The chi_b(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider (LHC) at sqrt(s) = 7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4 fb^-1, these states are reconstructed through their radiative decays to Upsilon(1S,2S) with Upsilon->mu+mu-. In addition to the mass peaks corresponding to the decay modes chi_b(1P,2P)->Upsilon(1S)gamma, a new structure centered at a mass of 10.530+/-0.005 (stat.)+/-0.009 (syst.) GeV is also observed, in both the Upsilon(1S)gamma and Upsilon(2S)gamma decay modes. This is interpreted as the chi_b(3P) system.Comment: 5 pages plus author list (18 pages total), 2 figures, 1 table, corrected author list, matches final version in Physical Review Letter

Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS

We present the results of a search for new, heavy particles that decay at a significant distance from their production point into a final state containing charged hadrons in association with a high-momentum muon. The search is conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS detector operating at the Large Hadron Collider. Production of such particles is expected in various scenarios of physics beyond the standard model. We observe no signal and place limits on the production cross-section of supersymmetric particles in an R-parity-violating scenario as a function of the neutralino lifetime. Limits are presented for different squark and neutralino masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final version to appear in Physics Letters

Measurement of the inclusive isolated prompt photon cross-section in pp collisions at sqrt(s)= 7 TeV using 35 pb-1 of ATLAS data

A measurement of the differential cross-section for the inclusive production of isolated prompt photons in pp collisions at a center-of-mass energy sqrt(s) = 7 TeV is presented. The measurement covers the pseudorapidity ranges |eta|<1.37 and 1.52<=|eta|<2.37 in the transverse energy range 45<=E_T<400GeV. The results are based on an integrated luminosity of 35 pb-1, collected with the ATLAS detector at the LHC. The yields of the signal photons are measured using a data-driven technique, based on the observed distribution of the hadronic energy in a narrow cone around the photon candidate and the photon selection criteria. The results are compared with next-to-leading order perturbative QCD calculations and found to be in good agreement over four orders of magnitude in cross-section.Comment: 7 pages plus author list (18 pages total), 2 figures, 4 tables, final version published in Physics Letters