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| Condensed Matter : Observation of the Little-Parks Oscillations in a System of Asymmetric Superconducting Rings |
| arXiv:0805.1223 (May 2008) | | Observation of the Little-Parks Oscillations in a System of Asymmetric Superconducting Rings | | A. A. Burlakov, V. L. Gurtovoi, S. V. Dubonos, A. V. Nikulov and V. A. Tulin | Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences, 142432 Chernogolovka, Moscow District, RUSSIA.
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. Little-Parks oscillations are observed in a system of 110 series-connected aluminum rings 2000 nm in diameter with the use of measuring currents from 10 nA to 1000 nA. The measurements show that the amplitude and character of the oscillations are independent of the relation between the measuring current and the amplitude of the persistent current. By using asymmetric rings, it is demonstrated that the persistent current has clockwise or contra-clockwise direction. This means that the total current in one of the semi-rings may be directed against the electric field at measurement of the Little-Parks oscillations. The measurements at zero and low measuring current have revealed that the persistent current, like the conventional circulating current, causes a potential difference on the semi-rings with different cross sections in spite of the absence of the Faraday's voltage. | | Categories. cond-mat.supr-con cond-mat.stat-mech | | Subject. Superconductivity; Statistical Mechanics | | Comment. 5 pages, 6 figures | | Journal-ref. JETP Letters vol. 86, pp. 517-521 (2007) License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Condensed Matter : Dislocations in cubic crystals described by discrete models |
| arXiv:0805.1221 (May 2008) | | Dislocations in cubic crystals described by discrete models | | L. L. Bonilla, A. Carpio and I. Plans | Modelización, Simulación Numérica y Matemática Industrial, Universidad Carlos III de Madrid, Avenida de la Universidad 30, 28911 Leganés, Spain
Departamento de Matemática Aplicada, Universidad Complutense de Madrid, 28040 Madrid, Spain
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. Discrete models of dislocations in cubic crystal lattices having one or two atoms per unit cell are proposed. These models have the standard linear anisotropic elasticity as their continuum limit and their main ingredients are the elastic stiffness constants of the material and a dimensionless periodic function that restores the translation invariance of the crystal and influences the dislocation size. For these models, conservative and damped equations of motion are proposed. In the latter case, the entropy production and thermodynamic forces are calculated and fluctuation terms obeying the fluctuation-dissipation theorem are added. Numerical simulations illustrate static perfect screw and 60$^circ$ dislocations for GaAs and Si. | | Categories. cond-mat.mtrl-sci | | Subject. Materials Science | | Comment. 26 pages, 4 figures | | Journal-ref. Physica A 376, 361-377 (2007) License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Condensed Matter : Periodized discrete elasticity models for defects in graphene |
| arXiv:0805.1220 (May 2008) | | Periodized discrete elasticity models for defects in graphene | | A. Carpio and L. L. Bonilla | Departamento de Matemática Aplicada, Universidad Complutense de Madrid, 28040 Madrid, Spain
G. Millán Institute for Fluid Dynamics, Nanoscience and Industrial Mathematics, Universidad Carlos III de Madrid, 28911 Leganés, Spain
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. The cores of edge dislocations, edge dislocation dipoles and edge dislocation loops in planar graphene have been studied by means of periodized discrete elasticity models. To build these models, we have found a way to discretize linear elasticity on a planar hexagonal lattice using combinations of difference operators that do not involve symmetrically all the neighbors of an atom. At zero temperature, dynamically stable cores of edge dislocations may be heptagon-pentagon pairs (glide dislocations) or octagons (shuffle dislocations) depending on the choice of initial configuration. Possible cores of edge dislocation dipoles are vacancies, pentagon-octagon-pentagon divacancies, Stone-Wales defects and 7-5-5-7 defects. While symmetric vacancies, divacancies and 7-5-5-7 defects are dynamically stable, asymmetric vacancies and 5-7-7-5 Stone-Wales defects seem to be unstable. | | Categories. cond-mat.mtrl-sci | | Subject. Materials Science | | Comment. 21 pages, 7 figures License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Condensed Matter : In search of multipolar order on the Penrose tiling |
| arXiv:0805.1216 (May 2008) | | In search of multipolar order on the Penrose tiling | | E. Y. Vedmedenko, Shahar Even-Dar Mandel and Ron Lifshitz | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. Based on Monte Carlo calculations, multipolar ordering on the Penrose tiling, relevant for two-dimensional molecular adsorbates on quasicrystalline surfaces and for nanomagnetic arrays, has been analyzed. These initial investigations are restricted to multipolar rotors of rank one through four - described by spherical harmonics Ylm with l=1...4 and restricted to m=0 - positioned on the vertices of the rhombic Penrose tiling. At first sight, the ground states of odd-parity multipoles seem to exhibit long-range multipolar order, indicated by the appearance of a superstructure in the form of the decagonal Hexagon-Boat-Star tiling, in agreement with previous investigations of dipolar systems. Yet careful analysis establishes that long-range multipolar order is absent in all cases investigated here, and only short-range order exists. This result should be taken as a warning for any future analysis of order in either real or simulated arrangements of multipoles on quasiperiodic templates. | | Categories. cond-mat.mtrl-sci License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ | | Subject. Materials Science; |
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| Condensed Matter : Scanning magnetic field microscope with a diamond single-spin sensor |
| arXiv:0805.1215 (May 2008) | | Scanning magnetic field microscope with a diamond single-spin sensor | | C. L. Degen | IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose CA, 95120, USA.
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. We describe a scanning device where a single spin is used as an ultra-sensitive, nanoscale magnetic field sensor. As this "probe spin" we consider a single nitrogen-vacancy defect center in a diamond nanocrystal, attached to the tip of the scanning device. Changes in the local field seen by the probe spin are then detected by optically monitoring its electron paramagnetic resonance (EPR) transition. The scanning device may be useful for performing nanoscale magnetic resonance imaging and spectroscopy, and for the characterization of magnetic nanostructures down to the single atom level. It may also serve as a quantum read-out device for other single spins. | | Categories. cond-mat.mes-hall | | Subject. Mesoscopic Systems and Quantum Hall Effect | | Comment. 4 pages, 1 figure, 1 table License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Condensed Matter : Instabilities in Attractor Networks with Fast Synaptic Fluctuations and Partial Updating of the Neurons Activity |
| arXiv:0805.1210 (May 2008) | | Instabilities in Attractor Networks with Fast Synaptic Fluctuations and Partial Updating of the Neurons Activity | | J. J. Torres, J. Marro, J. M. Cortes and B. Wemmenhove | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. We present and study a probabilistic neural automaton in which the fraction of simultaneously-updated neurons is a parameter, rho (0, 1) . For small rho, there is relaxation towards one of the attractors and a great sensibility to external stimuli and, for rho >= rho_c, itinerancy among attractors. Tuning rho in this regime, oscillations may abruptly change from regular to chaotic and vice versa, which allows one to control the efficiency of the searching process. We argue on the similarity of the model behavior with recent observations and on the possible role of chaos in neurobiology. | | Categories. cond-mat.dis-nn | | Subject. Disordered Systems and Neural Networks | | Comment. 19 pages, 5 figures License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Condensed Matter : A 57Fe Moessbauer Spectroscopy Study of the 7 K Superconductor LaFePO |
| arXiv:0805.1208 (May 2008) | | A 57Fe Moessbauer Spectroscopy Study of the 7 K Superconductor LaFePO | | Marcus Tegel, Inga Schellenberg, Rainer Pöttgen and Dirk Johrendt | | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. A polycrystalline sample of superconducting LaFePO was prepared in a tin flux at 1123 K. The structure was determined from single crystal data (ZrCuSiAs-type, P4/nmm, a = 3.9610(1), c = 8.5158(2) A, Z = 2) and the phase analysis was performed by the Rietveld method. LaFePO is Pauli-paramagnetic and becomes superconducting at 7 K after removing the ferromagnetic impurity phase Fe2P from the sample. 57Fe Moessbauer spectroscopy measurements at 298, 77, 4.2 and 4 K show single signals at isomer shifts around 0.35 mm/s, subject to weak quadrupole splitting. At 4 K, a symmetric line broadening appears, resulting from a small transferred magnetic hyperfine field of 1.15(1) T and accompanied by an angle of 54.7(5) between Bhf and Vzz, the main component of the electric field gradient tensor. | | Categories. cond-mat.supr-con cond-mat.str-el | | Subject. Superconductivity; Strongly Correlated Electrons | | Comment. submitted to Z. Naturforsch. B License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Condensed Matter : Interplay between coarsening and nucleation in an Ising model with dipolar interactions |
| arXiv:0805.1207 (May 2008) | | Interplay between coarsening and nucleation in an Ising model with dipolar interactions | | S. A. Cannas, M. F. Michelon, D. A. Stariolo and F. A. Tamarit | Instituto de FÃsica de la Facultad de Matemática, AstronomÃa y FÃsica (IFFAMAF-CONICET), Universidad Nacional de Córdoba Ciudad Universitaria, 5000 Córdoba, Argentina
Departamento de FÃsica, Universidade Federal do Rio Grande do Sul CP 15051, 91501-970 Porto Alegre, RS, Brazil
Institituo de FÃsica Gleb Wataghin, Universidade Estadual de Campinas CP 6165, 13083-970 Campinas, SP, Brazil
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. We study the dynamical behavior of a square lattice Ising model with exchange and dipolar interactions by means of Monte Carlo simulations. After a sudden quench to low temperatures we find that the system may undergo a coarsening process where stripe phases with different orientations compete or alternatively it can relax initially to a metastable nematic phase and then decay to the equilibrium stripe phase through nucleation. We measure the distribution of equilibration times for both processes and compute their relative probability of occurrence as a function of temperature and system size. This peculiar relaxation mechanism is due to the strong metastability of the nematic phase, which goes deep in the low temperature stripe phase. We also measure quasi-equilibrium autocorrelations in a wide range of temperatures. They show a distinct decay to a plateau that we identify as due to a finite fraction of frozen spins in the nematic phase. We find indications that the plateau is a finite size effect. Relaxation times as a function of temperature in the metastable region show super-Arrhenius behavior, suggesting a possible glassy behavior of the system at low temperatures. | | Categories. cond-mat.stat-mech cond-mat.soft License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ | | Subject. Statistical Mechanics; Soft Condensed Matter; |
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| Condensed Matter : Percolative properties of hard oblate ellipsoids of revolution with a soft shell |
| arXiv:0805.1181 (May 2008) | | Percolative properties of hard oblate ellipsoids of revolution with a soft shell | | Gianluca Ambrosetti, Niklaus Johner, Claudio Grimaldi, Andrea Danani and Peter Ryser | LPM, Ecole Polytechnique Fédérale de Lausanne, Station 17, CH-1015 Lausanne, Switzerland
ICIMSI, University of Applied Sciences of Southern Switzerland, CH-6928 Manno, Switzerland
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. We present an in-depth analysis of the geometrical percolation behavior in the continuum of random assemblies of hard oblate ellipsoids of revolution. Simulations where carried out by considering a broad range of aspect-ratios, from spheres up to aspect-ratio 100 plate-like objects, and with various limiting two particle interaction distances, from 0.05 times the major axis up to 4.0 times the major axis. We confirm the widely reported trend of a consistent lowering of the hard particle critical volume fraction with the increase of the aspect-ratio. Moreover, assimilating the limiting interaction distance to a shell of constant thickness surrounding the ellipsoids, we propose a simple relation based on the total excluded volume of these objects which allows to estimate the critical concentration from a quantity which is quasi-invariant over a large spectrum of limiting interaction distances. Excluded volume and volume quantities are derived explicitly. | | Categories. cond-mat.stat-mech | | Subject. Statistical Mechanics | | Comment. 11 pages, 8 figures License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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| Condensed Matter : Path integral formulation of fractional Brownian motion for general Hurst exponent |
| arXiv:0805.1170 (May 2008) | | Path integral formulation of fractional Brownian motion for general Hurst exponent | | Ivan Calvo and Raul Sanchez | Laboratorio Nacional de Fusión, Asociación EURATOM-CIEMAT, 28040 Madrid, Spain
Fusion Energy Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
| | Received. 08 May 2008 Last updated. 08 May 2008 | | Abstract. In J. Phys. A: Math. Gen. 28, 4305 (1995), K. L. Sebastian gave a path integral computation of the propagator of subdiffusive fractional Brownian motion (fBm), i.e. fBm with a Hurst or self-similarity exponent $Hin(0,1/2)$. The extension of Sebastian's calculation to superdiffusion, $Hin(1/2,1]$, becomes however quite involved due to the appearance of additional boundary conditions on fractional derivatives of the path. In this paper, we address the construction of the path integral representation in a different fashion, which allows to treat both subdiffusion and superdiffusion on an equal footing. The derivation of the propagator of fBm for general Hurst exponent is then performed in a neat and unified way. | | Categories. cond-mat.stat-mech | | Subject. Statistical Mechanics | | Comment. 6 pages License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/ |
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