Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2

Low energy Raman scattering of the s=1/2 spin tetrahedra system Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated temperatures this mode shows a soft mode-like decrease in energy pointing to an instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2 with a presumably larger inter-tetrahedra coupling does not show such a low energy mode. Instead its excitation spectrum and thermodynamic properties are compatible with long range Neel-ordering. We discuss the observed effects in the context of quantum fluctuations and competing ground states.Comment: 5 pages, 2 figures, ISSP-Kashiwa 2001, Conference on Correlated Electron

Momentum distribution of confined bosons: temperature dependence

The momentum distribution function of a parabolically confined gas of bosons with harmonic interparticle interactions is derived. In the Bose-Einstein condensation region, this momentum distribution substantially deviates from a Maxwell-Boltzmann distribution. It is argued that the determination of the temperature of the boson gas from the Bose-Einstein momentum distribution function is more appropriate than the currently used fitting to the high momentum tail of the Maxwell-Boltzmann distribution.Comment: 5 REVTEX pages + 2 postscript figures. Accepted in Phys. Rev.

Thermodynamics of Coupled Identical Oscillators within the Path Integral Formalism

A generalization of symmetrized density matrices in combination with the technique of generating functions allows to calculate the partition function of identical particles in a parabolic confining well. Harmonic two-body interactions (repulsive or attractive) are taken into account. Also the influence of a homogeneous magnetic field, introducing anisotropy in the model, is examined. Although the theory is developed for fermions and bosons, special attention is payed to the thermodynamic properties of bosons and their condensation.Comment: 13 REVTEX pages + 9 postscript figure

Enhanced quasiparticle dynamics of quantum well states: the giant Rashba system BiTeI and topological insulators

In the giant Rashba semiconductor BiTeI electronic surface scattering with Lorentzian linewidth is observed that shows a strong dependence on surface termination and surface potential shifts. A comparison with the topological insulator Bi2Se3 evidences that surface confined quantum well states are the origin of these processes. We notice an enhanced quasiparticle dynamics of these states with scattering rates that are comparable to polaronic systems in the collision dominated regime. The Eg symmetry of the Lorentzian scattering contribution is different from the chiral (RL) symmetry of the corresponding signal in the topological insulator although both systems have spin-split surface states.Comment: 6 pages, 5 figure

TiOCl, an orbital-ordered system?

We present first principles density functional calculations and downfolding studies of the electronic and magnetic properties of the layered quantum spin system TiOCl. We discuss explicitely the nature of the exchange pathes and attempt to clarify the concept of orbital ordering in this material. An analysis of the electronic structure of slightly distorted structures according to the phononic modes allowed in this material suggests that this system is subject to large orbital fluctuations driven by the electron-phonon coupling. Based on these results, we propose a microscopic explanation of the behavior of TiOCl near the phase transition to a spin-gapped system.Comment: Some figures are compressed, for higher quality please contact the author

Xenogeneic, extracorporeal liver perfusion in primates improves the ratio of branched-chain amino acids to aromatic amino acids (Fischer's ratio)

In fulminant hepatic failure (FHF), the development of hepatic encephalopathy is associated with grossly abnormal concentrations of plasma amino acids (PAA). Normalization of the ratio of branched-chain amino acids to aromatic amino acids (Fischer's ratio) correlates with clinical improvement. This study evaluated changes in PAA metabolism during 4 h of isolated, normothermic extracorporeal liver perfusion using a newly designed system containing human blood and a rhesus monkey liver. Bile and urea production were within the physiological range. Release of the transaminases AST, ALT and LDH were minimal. The ratio of branched (valine, leucine, isoleucine) to aromatic (tyrosine, phenylalanine) amino acids increased significantly. These results indicate that a xenogeneic extracorporeal liver perfusion system is capable of significantly increasing Fischer's ratio and may play a role in treating and bridging patients in FHF in the future

Longitudinal magnon in the tetrahedral spin system Cu2Te2O5Br2 near quantum criticality

We present a comprehensive study of the coupled tetrahedra-compound Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report the observation of a longitudinal magnon in Raman scattering in the ordered state close to quantum criticality. We show that the excited tetrahedral-singlet sets the energy scale for the magnetic ordering temperature T_N. This energy is determined experimentally. The ordering temperature T_N has an inverse-log dependence on the coupling parameters near quantum criticality

A Non-Sequential Representation of Sequential Data for Churn Prediction

We investigate the length of event sequence giving best predictions when using a continuous HMM approach to churn prediction from sequential data. Motivated by observations that predictions based on only the few most recent events seem to be the most accurate, a non-sequential dataset is constructed from customer event histories by averaging features of the last few events. A simple K-nearest neighbor algorithm on this dataset is found to give significantly improved performance. It is quite intuitive to think that most people will react only to events in the fairly recent past. Events related to telecommunications occurring months or years ago are unlikely to have a large impact on a customer’s future behaviour, and these results bear this out. Methods that deal with sequential data also tend to be much more complex than those dealing with simple nontemporal data, giving an added benefit to expressing the recent information in a non-sequential manner

Density and Pair Correlation Function of Confined Identical Particles: the Bose-Einstein Case

Two basic correlation functions are calculated for a model of $N$ harmonically interacting identical particles in a parabolic potential well. The density and the pair correlation function of the model are investigated for the boson case. The dependence of these static response properties on the complete range of the temperature and of the number of particles is obtained. The calculation technique is based on the path integral approach of symmetrized density matrices for identical particles in a parabolic confining well.Comment: 8 pages (REVTEX) + 6 figures (postscript

Bulk antiferromagnetism in Na0.82CoO2\bf Na_{0.82}CoO_2 single crystals

Susceptibility, specific heat, and muon spin rotation measurements on high-quality single crystals of $\rm Na_{0.82}CoO_2$ have revealed bulk antiferromagnetism with N\'{e}el temperature $\rm T_N = 19.8 \pm 0.1$ K and an ordered moment perpendicular to the $\rm CoO_2$ layers. The magnetic order encompasses nearly 100% of the crystal volume. The susceptibility exhibits a broad peak around 30 K, characteristic of two-dimensional antiferromagnetic fluctuations. The in-plane resistivity is metallic at high temperatures and exhibits a minimum at $\rm T_N$.Comment: published versio