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 epep Collisions: Massive versus Massless Scheme

The next--to--leading order inclusive cross section for large-$p_\perp$ 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 $c$ 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-$p_\perp$ 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 $^{63,65}$Cu NMR spectra were used to determine the local spin polarization in the 1/3 magnetization plateau of azurite, Cu$_3$(CO$_3$)$_2$(OH)$_2$, 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 $\approx$10 % of fully polarized spin in a $d$-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 $J_1$ and $J_3$.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