Dynamical Properties of an Antiferromagnet near the Quantum Critical Point: Application to LaCuO_2.5

For a system of two-chain spin ladders, the ground state for weak interladder coupling is the spin-liquid state of the isolated ladder, but is an ordered antiferromagnet (AF) for sufficiently large interactions. We generalize the bond-operator mean-field theory to describe both regimes, and to focus on the transition between them. In the AF phase near the quantum critical point (QCP) we find both spin waves and a low-lying but massive amplitude mode which is absent in a conventional AF. The static susceptibility has the form $\chi(T) = \chi_0 + a T^2$, with $\chi_0$ small for a system near criticality. We consider the dynamical properties to examine novel features due to the presence of the amplitude mode, and compute the dynamic structure factor. LaCuO$_{2.5}$ is thought to be such an unconventional AF, whose ordered phase is located very close to the QCP of the transition to the spin liquid. From the N\'eel temperature we deduce the interladder coupling, the small ordered moment and the gap in the amplitude mode. The dynamical properties unique to near-critical AFs are expected to be observable in LaCuO$_{2.5}$.Comment: 29 pages in RevTeX preprint format, 10 figures included. Figure 11 (930kB compressed) available on the WWW at http://itp.ethz.ch/Preprints/Norman

Identification of relaxation and diffusion mechanisms in amorphous silicon

The dynamics of amorphous silicon at low temperatures can be characterized by a sequence of discrete activated events, through which the topological network is locally reorganized. Using the activation-relaxation technique, we create more than 8000 events, providing an extensive database of relaxation and diffusion mechanisms. The generic properties of these events - size, number of atoms involved, activation energy, etc. - are discussed and found to be compatible with experimental data. We introduce a complete and unique classification of defects based on their topological properties and apply it to study of events involving only four-fold coordinated atoms. For these events, we identify and present in detail three dominant mechanisms.Comment: 4 pages, three figures, submitted to PR

Magnon Dispersion in the Field-Induced Magnetically Ordered Phase of TlCuCl3

The magnetic properties of the interacting dimer system TlCuCl3 are investigated within a bond-operator formulation. The observed field-induced staggered magnetic order perpendicular to the field is described as a Bose condensation of magnons which are linear combinations of dimer singlet and triplet modes. This technique accounts for the magnetization curve and for the field dependence of the magnon dispersion curves observed by high-field neutron scattering measurements.Comment: 4 pages, 4 figures, REVTeX

Statistical and Clinical Aspects of Hospital Outcomes Profiling

Hospital profiling involves a comparison of a health care provider's structure, processes of care, or outcomes to a standard, often in the form of a report card. Given the ubiquity of report cards and similar consumer ratings in contemporary American culture, it is notable that these are a relatively recent phenomenon in health care. Prior to the 1986 release of Medicare hospital outcome data, little such information was publicly available. We review the historical evolution of hospital profiling with special emphasis on outcomes; present a detailed history of cardiac surgery report cards, the paradigm for modern provider profiling; discuss the potential unintended negative consequences of public report cards; and describe various statistical methodologies for quantifying the relative performance of cardiac surgery programs. Outstanding statistical issues are also described.Comment: Published in at http://dx.doi.org/10.1214/088342307000000096 the Statistical Science (http://www.imstat.org/sts/) by the Institute of Mathematical Statistics (http://www.imstat.org

Interplay between shear loading and structural aging in a physical gel

We show that the aging of the mechanical relaxation of a gelatin gel exhibits the same scaling phenomenology as polymer and colloidal glasses. Besides, gelatin is known to exhibit logarithmic structural aging (stiffening). We find that stress accelerates this process. However, this effect is definitely irreducible to a mere age shift with respect to natural aging. We suggest that it is interpretable in terms of elastically-aided elementary (coil$\to$helix) local events whose dynamics gradually slows down as aging increases geometric frustration

Suppression of static stripe formation by next-neighbor hopping

We show from real-space Hartree-Fock calculations within the extended Hubbard model that next-nearest neighbor (t') hopping processes act to suppress the formation of static charge stripes. This result is confirmed by investigating the evolution of charge-inhomogeneous corral and stripe phases with increasing t' of both signs. We propose that large t' values in YBCO prevent static stripe formation, while anomalously small t' in LSCO provides an additional reason for the appearance of static stripes only in these systems.Comment: 4 pages, 5 figure

Electronic and Magnetic Structure of LaCuO2.5_{2.5}

The recently-discovered ``ladder'' compound LaCuO$_{2.5}$ has been found to admit hole doping without altering its structure of coupled copper oxide ladders. While susceptibility measurements on the parent compound suggest a spin gap and a spin-liquid state, NMR results indicate magnetic order at low temperatures. These seemingly contradictory results may be reconciled if in fact the magnetic state is near the crossover from spin liquid to antiferromagnet, and we investigate this possibility. From a tight-binding fit to the valence LDA bandstructure, we deduce that the strength of the interladder hopping term is approximately half that of intraladder hopping, showing that the material is three-dimensional in character. A mean-field treatment of the insulating magnetic state gives a spin-liquid phase whose spin gap decreases with increasing interladder coupling, vanishing (signalling a transition to the ordered phase) at a value somewhat below that obtained for LaCuO$_{2.5}$. The introduction of an on-site repulsion term, $U$, to the band scheme causes a transition to an antiferromagnetic insulator for rather small but finite values of $U$, reflecting the predominance of (one-dimensional) ladder behavior, and an absence of any special nesting features.Comment: 8 pages + 5 figure