394 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
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
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
Bose-Einstein Condensation of S = 1 Ni spin degrees of freedom in NiCl2-4SC(NH2)2
It has recently been suggested that the organic compound
NiCl-4SC(NH) (DTN) exhibits Bose-Einstein Condensation (BEC) of the
Ni spin degrees of freedom for fields applied along the tetragonal c-axis. The
Ni spins exhibit 3D XY-type antiferromagnetic order above a field-induced
quantum critical point at T. The Ni spin fluid can be
characterized as a system of effective bosons with a hard-core repulsive
interaction in which the antiferromagnetic state corresponds to a Bose-Einstein
condensate (BEC) of the phase coherent Ni spin system. We have
investigated the the high-field phase diagram and the occurrence of BEC in DTN
by means of specific heat and magnetocaloric effect measurements to dilution
refrigerator temperatures. Our results indicate that a key prediction of BEC is
satisfied; the magnetic field-temperature quantum phase transition line
approaches a power-law at low temperatures,
with an exponent at the quantum critical point,
consistent with the BEC theory prediction of .Comment: 4 pages, 4 figure
A Prismatic Analyser concept for Neutron Spectrometers
A development in modern neutron spectroscopy is to avoid the need of large
samples. We demonstrate how small samples together with the right choice of
analyser and detector components makes distance collimation an important
concept in crystal analyser spectrometers. We further show that this opens new
possibilities where neutrons with different energies are reflected by the same
analyser but counted in different detectors, thus improving both energy
resolution and total count rate compared to conventional spectrometers. The
technique can be combined with advanced focusing geometries and with
multiplexing instrument designs. We present a combination of simulations and
data with 3 energies from one analyser. The data was taken on a prototype
installed at PSI, Switzerland, and shows excellent agreement with the
predictions. Typical improvements will be 2 times finer resolution and a factor
1.9 in flux gain compared to a Rowland geometry or 3 times finer resolution and
a factor 3.2 in flux gain compared to a single flat analyser slab
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