5,954 research outputs found
The fraction of Bose-Einstein condensed triplons in TlCuCl3 from magnetization M(T,H)-data
In this study we calculate the fraction of condensed magnetic quasiparticles
of TlCuCl3 from magnetization M(T,H)-data. It is independent of the direction
of the magnetic field and slightly decreases with increasing magnetic field.Comment: 8 pages, 10 figure
Defect propagation in one-, two-, and three-dimensional compounds doped by magnetic atoms
Inelastic neutron scattering experiments were performed to study
manganese(II) dimer excitations in the diluted one-, two-, and
three-dimensional compounds CsMn(x)Mg(1-x)Br(3), K(2)Mn(x)Zn(1-x)F(4), and
KMn(x)Zn(1-x)F(3) (x<0.10), respectively. The transitions from the ground-state
singlet to the excited triplet, split into a doublet and a singlet due to the
single-ion anisotropy, exhibit remarkable fine structures. These unusual
features are attributed to local structural inhomogeneities induced by the
dopant Mn atoms which act like lattice defects. Statistical models support the
theoretically predicted decay of atomic displacements according to 1/r**2, 1/r,
and constant (for three-, two-, and one-dimensional compounds, respectively)
where r denotes the distance of the displaced atoms from the defect. The
observed fine structures allow a direct determination of the local exchange
interactions J, and the local intradimer distances R can be derived through the
linear law dJ/dR.Comment: 22 pages, 5 figures, 2 table
Cross Sections for Charm Production in Collisions: Massive versus Massless Scheme
The next--to--leading order inclusive cross section for large-
photoproduction of charm quarks at HERA is calculated in two different
approaches. In the first approach the charm quarks are treated as massive
objects which are strictly external to the proton and the photon while in the
second approach the charm mass is neglected and the quark is assumed to be
one of the active flavours in the proton and photon structure functions. We
present single-inclusive distributions in transverse momentum and rapidity
including direct and resolved photons. The cross section in the massless
approach is found to be significantly larger than in the massive scheme. The
deviation originates from several contributions which are disentangled. We
argue that large- photoproduction of charm quarks at HERA will be
sensitive to the charm content of the photon structure function.Comment: 11 pages, Latex, epsfig, 6 figures appended as uuencoded file
(hardcopy can be obtained upon request from [email protected]
Precision Calculations for Associated WH and ZH Production at Hadron Colliders
Recently the next-to-next-to-leading order QCD corrections and the
electroweak O(alpha) corrections to the Higgs-strahlung processes ppbar/pp ->
WH/ZH + X have been calculated. Both types of corrections are of the order of
5-10%. In this article the various corrections are briefly discussed and
combined into state-of-the-art predictions for the cross sections. The
theoretical uncertainties from renormalization/factorization scales and from
the parton distribution functions are discussed.Comment: 8 pages, Contributed to 3rd Les Houches Workshop: Physics at TeV
Collider
Quantum Gravitational Contributions to the CMB Anisotropy Spectrum
We derive the primordial power spectrum of density fluctuations in the
framework of quantum cosmology. For this purpose we perform a Born-Oppenheimer
approximation to the Wheeler-DeWitt equation for an inflationary universe with
a scalar field. In this way we first recover the scale-invariant power spectrum
that is found as an approximation in the simplest inflationary models. We then
obtain quantum gravitational corrections to this spectrum and discuss whether
they lead to measurable signatures in the CMB anisotropy spectrum. The
non-observation so far of such corrections translates into an upper bound on
the energy scale of inflation.Comment: 4 pages, v3: sign error in Eq. (5) and its consequences correcte
Spin Configuration in the 1/3 Magnetization Plateau of Azurite Determined by NMR
High magnetic field Cu NMR spectra were used to determine the local
spin polarization in the 1/3 magnetization plateau of azurite,
Cu(CO)(OH), which is a model system for the distorted diamond
antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the
Cu2 (dimer) sites is found to be field independent, negative and strongly
anisotropic, corresponding to 10 % of fully polarized spin in a
-orbital. This is close to the expected configuration of the "quantum"
plateau, where a singlet state is stabilized on the dimer. However, the
observed non-zero spin polarization points to some triplet admixture, induced
by strong asymmetry of the diamond bonds and .Comment: Phys. Rev. Lett. 102, in press (2009
Magnetotransport of lanthanum doped RuSr2GdCu2O8 - the role of gadolinium
Strongly underdoped RuSr_1.9La_0.1GdCu_2O_8 has been comprehensively studied
by dc magnetization, microwave measurements, magnetoresistivity and Hall
resistivity in fields up to 9 T and temperatures down to 1.75 K. Electron
doping by La reduces the hole concentration in the CuO2 planes and completely
suppresses superconductivity. Microwave absorption, dc resistivity and ordinary
Hall effect data indicate that the carrier concentration is reduced and a
semiconductor-like temperature dependence is observed. Two magnetic ordering
transitions are observed. The ruthenium sublattice orders antiferromagnetically
at 155 K for low applied magnetic field and the gadolinium sublattice
antiferromagnetically orders at 2.8 K. The magnetoresistivity exhibits a
complicated temperature dependence due to the combination of the two magnetic
orderings and spin fluctuations. It is shown that the ruthenium magnetism
influences the conductivity in the RuO2 layers while the gadolinium magnetism
influences the conductivity in the CuO2 layers. The magnetoresistivity is
isotropic above 4 K, but it becomes anisotropic when gadolinium orders
antiferromagnetically.Comment: 7 pages, 9 figures, submitted to European Physical Journal
Propagation of sound in a Bose Einstein condensate in an optical lattice
We study the propagation of sound waves in a Bose-Einstein condensate trapped
in a one-dimensional optical lattice. We find that the velocity of propagation
of sound wavepackets decreases with increasing optical lattice depth, as
predicted by the Bogoliubov theory. The strong interplay between nonlinearities
and the periodicity of the external potential raise new phenomena which are not
present in the uniform case. Shock waves, for instance, can propagate slower
than sound waves, due to the negative curvature of the dispersion relation.
Moreover, nonlinear corrections to the Bogoliubov theory appear to be important
even with very small density perturbations, inducing a saturation on the
amplitude of the sound signal
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