[{"type":"speech","title":"Effect of dc magnetic field on the microwave losses in MgB\u2082 thin films","issued":{"date-parts":[["2007"]]},"author":[{"family":"Zaitsev","given":"A."},{"family":"Schneider","given":"R."},{"family":"Hott","given":"R."},{"family":"Schwarz","given":"T."},{"family":"Geerk","given":"J."}],"note":"71.Jahrestagung der Deutschen Physikalischen Gesellschaft und DPG Fr\u00fchjahrstagung des Arbeitskreises Festk\u00f6rperphysik, Fachverband Tiefe Temperaturen, Regensburg, 26.-30.M\u00e4rz 2007 Verhandlungen der Deutschen Physikalischen Gesellschaft, R.6, B.42(2007) TT 34.3","abstract":"Section Low Temperature Physics (TT)\nTT 33.8\nFri 12:15\nFriday\nH18\nNonmagnetic impurities in fractionalized spin liquids \u2014\n\u2022Alexei Kolezhuk1 and Subir Sachdev2 \u2014 1 Institut f\u00a8 r Theou\nretische Physik, Universit\u00a8t Hannover, 30167 Hannover, Germany \u2014\n2 Physics Department, Harvard University, Cambridge MA 02138, USA\nSpin correlations in the vicinity of a nonmagnetic impurity are analyzed for two models of gapless U(1) spin liquids in two dimensions:\n(1) decon\ufb01ned critical point between the Neel and valence-bond-solid\n(VBS) phase; (2) staggered \ufb02ux spin liquid. The impurity susceptibility in both cases exhibits a 1\/T temperature dependence with an\nanomalous Curie constant. It is shown that an external magnetic \ufb01eld\ninduces both uniform and staggered magnetization around the impurity in the Neel-VBS case, while in the staggered \ufb02ux phase only the\nuniform component is present.\nTT 33.9\nFri 12:30\nH18\nCon\ufb01nement of spinons in the spiral staircase model \u2014\n\u00a8\n\u2022Christian Brunger and Fakher Assaad \u2014 Universit\u00a8t W\u00a8 rzburg,\nu\nGermany\nWe consider two antiferromagnetic spin 1\/2 chains with coupling constants J and J cos2 (\u03b8\/2) coupled ferromagnetically, J\u22a5 . In the limit\nJ\u22a5 \u2192 \u221e the model maps onto the Haldane spin 1 chain irrespective\nof te choice of the angle \u03b8. The question we address here is the con\ufb01nement of spinons in the weak coupling region. As expected from\nbosonization [1] at \u03b8 = 0 the spin gap varies linearly with J\u22a5 . On\nthe other hand in the limit \u03b8 = \u03c0 our results suggest that this spin\ngap opens as (J\u22a5 )\u03b1 with \u03b1\n2. This statement follows from mean\ufb01eld, exact diagonalization as well as large scale quantum Monte Carlo\nsimulations.\n[1] D. G. Shelton, A. A. Nersesyan and A.M. Tsvelik, Phys. Rev. B\n53, 8521 (1996)\nTT 33.10\nFri 12:45\nH18\nExperimental Consequences of O(3) Decon\ufb01ned Criticality\nin 2+1 D \u2014 \u2022David Santiago1 and Zaira Nazario2 \u2014 1 InsituutLorentz, Leiden University, P.O. Box 9506, NL-2300 RA Leiden, The\nNetherlands \u2014 2 Max Planck Institute for the Physics of Complex Systems, N\u00a8thnitzer Str. 38, 01187 Dresden, Germany\nThe paramagnetic phase of 2+1 D antiferromagnets can be described\nin terms of electrodynamics of charged, massive bosonic spinons interacting through an emergent compact U(1) gauge \ufb01eld. Spinons in\nthe paramagnet are con\ufb01ned due to the presence of nontrivial tunneling e\ufb00ects, instantons which provide a long range interaction between\nthe gauge \ufb01elds and the charges that gaps the gauge \ufb01elds and provides a linear potential for the charges. The instantons responsible\nfor spinon con\ufb01nement in the paramagnetic phase vanish at the quantum critical point. Therefore, spinons are decon\ufb01ned at criticality. We\nhave recently obtained the e\ufb00ective theory that describes the universal\nphysics of these decon\ufb01ned critical points. From the decon\ufb01ned critical\ntheory, we calculate the critical Neel \ufb01eld propagator and \ufb01nd a critical exponent eta=1. We also obtain measurable e\ufb00ects and quantities\nthat follow from the prediction eta=1 and serve as characterization of\nO(3) decon\ufb01ned criticality. Those are the inelastic and elastic neutron\nscattering response, Nuclear Magnetic Resonance (NMR) response,\nmagnetic \ufb01eld response and the speci\ufb01c heat. All of these response\nfunctions serve to de\ufb01ne the O(3) decon\ufb01ned universality class.\nTime: Friday 10:15\u201313:00\nLocation: H19\nTT 34.1\nFri 10:15\nH19\nCommensurability e\ufb00ects in Nb thin \ufb01lms with (quasi)periodic pinning arrays \u2014 \u2022Matthias Kemmler1 , Daniel\nu\nD-72076T\u00a8bingen \u2014 2 IMS, Universit\u00a8t Karlsruhe,Hertzstr. 16, Du\nWe study experimentally the critical depinning current Ic versus applied magnetic \ufb01eld B in Nb thin \ufb01lms which contain 2D arrays of\ncircular antidots placed on the nodes of a \ufb01vefold Penrose lattice.\nFor measurements of electric transport we use a highly sensitive liquid Helium-cooled dc SQUID ampli\ufb01er. The sample temperature is\ncontrolled and stabilized close to the Nb transition temperature Tc via\nan optical, very low noise heating system.\nOur experiments con\ufb01rm essential features in the Ic (B) patterns as\npredicted by Misko et al.[1] , close to the transition temperature Tc of\nthe Nb \ufb01lms.\nIn order to \ufb01nd the arrangement of optimal pinning we compare the\nperformance of Nb \ufb01lms containing quasiperiodic pinning arrays, triangular pinning arrays, randomly distributed antidots, or no antidots.\nSome of the results are published in [2].\n[1] V.R. Misko, S.Savel\u2019ev, F.Nori, Phys. Rev. Lett. 95 (2005)\n177007\n[2] M. Kemmler et al., Phys. Rev. Lett. 97 (2006) 147003\nTT 34.2\nFri 10:30\nFour fold VL patterns, breaking the crystal symmetry have been identi\ufb01ed in Niobium, which can be explained by non-local corrections in\nthe Eilenberger model. Furthermore, recent polarised neutron studies\nof the VL in Niobium will be presented.\nTT 34.3\nFri 10:45\nH19\nE\ufb00ect of dc magnetic \ufb01eld on the microwave losses in MgB2\nthin \ufb01lms \u2014 \u2022Alexander Zaitsev, Rudolf Schneider, Roland\nu\nThe microwave surface impedance (Zs = Rs + iXs ) of in situ MgB2\nthin \ufb01lms was measured as a function of temperature and parallel dc\nmagnetic \ufb01eld at several frequencies between 5.7 GHz and 18.5 GHz using a dielectric resonator technique. The results are consistent with the\nexpectations for a classical type-II superconductor and, consequently,\nquite di\ufb00erent from those of the high-Tc cuprates. The \ufb01lms cooled\nin zero-\ufb01eld revealed a clear indication of the lower critical \ufb01eld, Bc1 ,\nwith a small hysteresis around B \u2264 Bc1 . In higher \ufb01elds (B > Bc1 ) the\nlosses followed the Co\ufb00ey-Clem and Brandt model, including the frequency dependences, whereas high-Tc Y-Ba-Cu-O \ufb01lms did not show a\nreasonable agreement with this model. Both the relatively high values\nof \u2206Xs \/\u2206Rs ratio and their frequency dependence indicate a weak\ne\ufb00ect of the \ufb02ux creep on the measured microwave loss in MgB2 \ufb01lms.\nThe temperature dependence of \u2206Xs \/\u2206Rs ratio can be described by\na microscopic pinning model for BCS-superconductors.\nH19\nVortex Structures in Tantalum, Vandium and Niobium \u2014\n\u00a8\n\u00a8\nRobert Georgii2 , Edward Forgan3 , Charlotte Bowell3 , and\nu\nof Physics and Astronomy, Birmingham (UK)\nSmall angle neutron scattering directly maps the fourier transform of\nthe vortex lattice (VL) of type II superconductors and gives valuable\ninformations on both the underlying fermi surface and the mechanism\nof the superconducting pairing. But the symmetry of the VL is also\nmainly in\ufb02uenced by pinning and impurity e\ufb00ects. Recent studies of\nthe VL in the classical superconductors Tantalum, Vanadium and Niobium with \ufb01eld applied along the four-fold (100) axis will be presented.\nTT 34.4\nFri 11:00\nH19\nSuperconducting Transition Broadening in MgB2 \u2014\n\u2022Anatolie Sidorenko1 , Vladimir Zdravkov1 , Andrej Sudru1 ,\nDimitriu Ghitsu1 , Thomas Koch2 , and Thomas Schimmel2,3 \u2014\n1 Institute of Electronic Engineering and Industrial Technologies,\n3 Institute of Applied Physics, University of Karlsruhe D-76128 Karlsruhe, Germany\nSuperconducting properties of high-quality \ufb01lms of the \ufb01rst multiband superconductor, magnesium diboride, have been investigated.\nTwo mechanisms with intrinsic origin of the superconducting transition broadening for MgB2 were found. The dominating role of twodimensional \ufb02uctuations in the vicinity of the critical temper","kit-publication-id":"230067506"}]