463 research outputs found
Quantum statistics of interacting dimer spin systems
The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Wurtz [Phys. Rev. B 50, 13 515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important
Exploring the fragile antiferromagnetic superconducting phase in CeCoIn5
CeCoIn5 is a heavy fermion Type-II superconductor which exhibits clear
indications of Pauli-limited superconductivity. A variety of measurements give
evidence for a transition at high magnetic fields inside the superconducting
state, when the field is applied either parallel to or perpendicular to the c
axis. When the field is perpendicular to the c axis, antiferromagnetic order is
observed on the high-field side of the transition, with a magnetic wavevector
of (q q 0.5), where q = 0.44 reciprocal lattice units. We show that this order
remains as the magnetic field is rotated out of the basal plane, but the
associated moment eventually disappears above 17 degrees, indicating that the
anomalies seen with the field parallel to the c axis are not related to this
magnetic order. We discuss the implications of this finding.Comment: Accepted Physical Review Letters, September 2010. 4 pages, 4 figure
Fragmentation is crucial for the steady-state dynamics of actin filaments
Despite the recognition that actin filaments are important for numerous cellular processes, and decades of investigation, the dynamics of in vitro actin filaments are still not completely understood. Here, we follow the time evolution of the length distribution of labeled actin reporter filaments in an unlabeled F-actin solution via fluorescence microscopy. Whereas treadmilling and diffusive length fluctuations cannot account for the observed dynamics, our results suggest that at low salt conditions, spontaneous fragmentation is crucial
Fractal Behaviour in the O(3) Model
We study domain formation in the two-dimensional O(3) model near criticality.
The fractal dimension of these domains is determined with good statistical
accuracy.Comment: 6 pages + 3 figures (concatenated PS files, uuencoded gz-compressed
Competing superconducting and magnetic order parameters and field-induced magnetism in electron doped Ba(FeCo)As
We have studied the magnetic and superconducting properties of
Ba(FeCo)As as a function of temperature and
external magnetic field using neutron scattering and muon spin rotation. Below
the superconducting transition temperature the magnetic and superconducting
order parameters coexist and compete. A magnetic field can significantly
enhance the magnetic scattering in the superconducting state, roughly doubling
the Bragg intensity at 13.5 T. We perform a microscopic modelling of the data
by use of a five-band Hamiltonian relevant to iron pnictides. In the
superconducting state, vortices can slow down and freeze spin fluctuations
locally. When such regions couple they result in a long-range ordered
antiferromagnetic phase producing the enhanced magnetic elastic scattering in
agreement with experiments.Comment: 9 pages, 6 figure
Flux pinning and phase separation in oxygen rich La2-xSrxCuO4+y system
We have studied the magnetic characteristics of a series of super-oxygenated
La2-xSrxCuO4+y samples. As shown in previous work, these samples spontaneously
phase separate into an oxygen rich superconducting phase with a TC near 40 K
and an oxygen poor magnetic phase that also orders near 40 K. All samples
studied are highly magnetically reversible even to low temperatures. Although
the internal magnetic regions of these samples might be expected to act as
pinning sites, our present study shows that they do not favor flux pinning.
Flux pinning requires a matching condition between the defect and the
superconducting coherence length. Thus, our results imply that the magnetic
regions are too large to act as pinning centers. This also implies that the
much greater flux pinning in typical La2-xSrxCuO4 materials is the result of
nanoscale inhomogeneities that grow to become the large magnetic regions in the
super-oxygenated materials. The superconducting regions of the phase separated
materials are in that sense cleaner and more homogenous than in the typical
cuprate superconductor.Comment: 4 figures 8 pages Submitted to PR
Quantitative comparison of filtering methods in lattice QCD
We systematically compare filtering methods used to extract topological
excitations (like instantons, calorons, monopoles and vortices) from lattice
gauge configurations, namely APE-smearing and spectral decompositions based on
lattice Dirac and Laplace operators. Each of these techniques introduces
ambiguities, which can invalidate the interpretation of the results. We show,
however, that all these methods, when handled with care, reveal very similar
topological structures. Hence, these common structures are free of ambiguities
and faithfully represent infrared degrees of freedom in the QCD vacuum. As an
application we discuss an interesting power-law for the clusters of filtered
topological charge.Comment: 6 pages, 18 plots in 5 figures; final version as published in EPJ A;
section 4 was adde
Multiferroicity in the generic easy-plane triangular lattice antiferromagnet RbFe(MoO4)2
RbFe(MoO4)2 is a quasi-two-dimensional (quasi-2D) triangular lattice
antiferromagnet (TLA) that displays a zero-field magnetically-driven
multiferroic phase with a chiral spin structure. By inelastic neutron
scattering, we determine quantitatively the spin Hamiltonian. We show that the
easy-plane anisotropy is nearly 1/3 of the dominant spin exchange, making
RbFe(MoO4)2 an excellent system for studying the physics of the model 2D
easy-plane TLA. Our measurements demonstrate magnetic-field induced
fluctuations in this material to stabilize the generic finite-field phases of
the 2D XY TLA. We further explain how Dzyaloshinskii-Moriya interactions can
generate ferroelectricity only in the zero field phase. Our conclusion is that
multiferroicity in RbFe(MoO4)2, and its absence at high fields, results from
the generic properties of the 2D XY TLA.Comment: 5 pages, 5 figures, accepted in PRB as a Rapid Communicatio
Coexistence of ferromagnetism and superconductivity in the hybrid ruthenate-cuprate compound RuSr_2GdCu_2O_8 studied by muon spin rotation (\mu SR) and DC-magnetization
We have investigated the magnetic and the superconducting properties of the
hybrid ruthenate-cuprate compound RuSr_{2}GdCu_{2}O_{8} by means of zero-field
muon spin rotation- (ZF-SR) and DC magnetization measurements. The
DC-magnetisation data establish that this material exhibits ferromagnetic order
of the Ru-moments () below T_{Curie} = 133 K and
becomes superconducting at a much lower temperature T_c = 16 K. The ZF-SR
experiments indicate that the ferromagnetic phase is homogeneous on a
microscopic scale and accounts for most of the sample volume. They also suggest
that the magnetic order is not significantly modified at the onset of
superconductivity.Comment: improved version submitted to Phys. Rev.
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