Diagnosing order by disorder in quantum spin systems

In this paper we study the frustrated J1-J2 quantum Heisenberg model on the square lattice for J2 > 2J1, in a magnetic field. In this regime the classical system is known to have a degenerate manifold of lowest energy configurations, where standard thermal order by disorder occurs. In order to study its quantum version we use a path integral formulation in terms of coherent states. We show that the classical degeneracy in the plane transverse to the magnetic field is lifted by quantum fluctuations. Collinear states are then selected, in a similar pattern to that set by thermal order by disorder, leaving a Z2 degeneracy. A careful analysis reveals a purely quantum mechanical effect given by the tunneling between the two minima selected by fluctuations. The effective description contains two planar (XY -like) fields conjugate to the total magnetization and the difference of the two sublattice magnetizations. Disorder in either or both of these fields produces the locking of their conjugate observables. Furthermore, within this scenario we argue that the quantum state is close to a product state.Comment: 8 pages, 3 figure

Structural studies of mesoporous ZrO2_{2}-CeO2_{2} and ZrO2_{2}-CeO2_{2}/SiO2_{2} mixed oxides for catalytical applications

In this work the synthesis of ZrO$_{2}$-CeO$_{2}$ and ZrO$_{2}$-CeO$_{2}$/SiO$_{2}$ were developed, based on the process to form ordered mesoporous materials such as SBA-15 silica. The triblock copolymer Pluronic P-123 was used as template, aiming to obtain crystalline single phase walls and larger specific surface area, for future applications in catalysis. SAXS and XRD results showed a relationship between ordered pores and the material crystallization. 90% of CeO$_{2}$ leaded to single phase homogeneous ceria-zirconia solid solution of cubic fluorite structure (Fm$\bar{3}$m). The SiO$_{2}$ addition improved structural and textural properties as well as the reduction behavior at lower temperatures, investigated by XANES measurements under H$_{2}$ atmosphere

Comparison between disordered quantum spin 1/2 chains

We study the magnetic properties of two types of one dimensional XX spin 1/2 chains. The first type has only nearest neighbor interactions which can be either antiferromagnetic or ferromagnetic and the second type which has both nearest neighbor and next nearest neighbor interactions, but only antiferromagnetic in character. We study these systems in the presence of low transverse magnetic fields both analytically and numerically. Comparison of results show a close relation between the two systems, which is in agreement with results previously found in Heisenberg chains by means of a numerical real space renormalization group procedure.Comment: 7 page

CFD Analysis of Biologically-Inspired Marine Propulsors

In the present work, the design and testing of two propulsion mechanisms which emulate fish swimming are presented. The motivation comes from the high effi ciency and maneuverality that fi sh demonstrate over conventional rotary propellers. In order to know the fl uid fl ow pattern in detail, a 3D CFD model was developed. Details of the velocity and pressure fi elds were obtained, as well as the hydrodynamic forces, cruising velocity, power and effi ciency. The results obtained using this CFD model were validated with the experimental prototypes, obtaining a reasonable agreement. Once validated, the CFD model was used to analyze several confi gurations of the propelling fi n, obtaining that thunniform swimming mode is the most effi cient

Reentrant behaviour in Landau Fermi liquids with spin-split Pomeranchuk instabilities

We study the effects of spin-antisymmetric interactions on the stability of a Landau-Fermi liquid on the square lattice, using the generalized Pomeranchuk method for two-dimensional lattice systems. In particular, we analyze interactions that could induce instabilities of the so called spin-split type, that is when spin-up and spin-down Fermi surfaces are displaced with respect to each other. The phase space is studied as a function of the strength of the interaction $V$, the electron chemical potential $\mu$ and an external magnetic field $h$. We find that such interactions produce in general an enhancement of the instability region of the Landau-Fermi liquid. More interestingly, in certain regions of the $V$-$\mu$ phase space, we find a reentrant behaviour as a function of the magnetic field $h$, similar to that found in recent experiments, e.g. in URu$_2$Si$_2$ and Sr$_3$Ru$_2$O$_7$.Comment: 5 pages, 3 figure

Using step width to compare locomotor biomechanics between extinct, non-avian theropod dinosaurs and modern obligate bipeds

How extinct, non-avian theropod dinosaurs locomoted is a subject of considerable interest, as is the manner in which it evolved on the line leading to birds. Fossil footprints provide the most direct evidence for answering these questions. In this study, step width—the mediolateral (transverse) distance between successive footfalls—was investigated with respect to speed (stride length) in non-avian theropod trackways of Late Triassic age. Comparable kinematic data were also collected for humans and 11 species of ground-dwelling birds. Permutation tests of the slope on a plot of step width against stride length showed that step width decreased continuously with increasing speed in the extinct theropods (p < 0.001), as well as the five tallest bird species studied (p < 0.01). Humans, by contrast, showed an abrupt decrease in step width at the walk–run transition. In the modern bipeds, these patterns reflect the use of either a discontinuous locomotor repertoire, characterized by distinct gaits (humans), or a continuous locomotor repertoire, where walking smoothly transitions into running (birds). The non-avian theropods are consequently inferred to have had a continuous locomotor repertoire, possibly including grounded running. Thus, features that characterize avian terrestrial locomotion had begun to evolve early in theropod history

On the Phase Covariant Quantum Cloning

It is known that in phase covariant quantum cloning the equatorial states on the Bloch sphere can be cloned with a fidelity higher than the optimal bound established for universal quantum cloning. We generalize this concept to include other states on the Bloch sphere with a definite $z$ component of spin. It is shown that once we know the $z$ component, we can always clone a state with a fidelity higher than the universal value and that of equatorial states. We also make a detailed study of the entanglement properties of the output copies and show that the equatorial states are the only states which give rise to separable density matrix for the outputs.Comment: Revtex4, 6 pages, 5 eps figure

Incidence of postoperative acute kidney injury in patients with chronic kidney disease undergoing minimally invasive valve surgery

BackgroundWe hypothesize that minimally invasive valve surgery in patients with chronic kidney disease (CKD) is superior to a conventional median sternotomy.MethodsWe retrospectively analyzed 1945 consecutive patients who underwent isolated valve surgery. Included were patients with CKD stages 2 to 5. In-hospital mortality, composite complication rates, and intensive care unit and total hospital lengths of stay of those who underwent a minimally invasive approach were compared with those who underwent a standard median sternotomy. Resource use was approximated based on intensive care unit and total hospital lengths of stay.ResultsThere were 688 patients identified; 510 (74%) underwent minimally invasive surgery, and 178 (26%) underwent a median sternotomy. There was no significant difference in mortality. Minimally invasive surgery was associated with fewer composite complications (33.1% vs 49.4%; odds ratio, 0.5; P ≤ .001), shorter intensive care unit (48 [interquartile range {IQR}, 33-74] hours vs 71 [IQR, 42-96] hours; P < .01), and hospital (8 [IQR, 6-9] days vs 10 [IQR, 8-15] days; P < .001) lengths of stay, and a lower incidence of acute kidney injury (8% vs 14.7%; odds ratio, 0.5; P = .01), compared with median sternotomy. In a multivariable analysis, minimally invasive surgery was associated with a 60% reduction in the risk of development of postoperative acute kidney injury.ConclusionsIn patients with CKD undergoing isolated valve surgery, minimally invasive valve surgery is associated with reduced postoperative complications and lower resource use