Universal Magnetic Properties of La2−δSrδCuO4La_{2-\delta} Sr_{\delta} Cu O_4 at Intermediate Temperatures

We present the theory of two-dimensional, clean quantum antiferromagnets with a small, positive, zero temperature ($T$) stiffness $\rho_s$, but with the ratio $k_B T / \rho_s$ arbitrary. Universal scaling forms for the uniform susceptibility ($\chi_u$), correlation length($\xi$), and NMR relaxation rate ($1/T_1$) are proposed and computed in a $1/N$ expansion and by Mont\'{e}-Carlo simulations. For large $k_B T/\rho_s$, $\chi_u (T)/T$ and $T\xi(T)$ asymptote to universal values, while $1/T_{1}(T)$ is nearly $T$-independent. We find good quantitative agreement with experiments and some numerical studies on $La_{2-\delta} Sr_{\delta} Cu O_4$.Comment: 14 pages, REVTEX, 1 postscript figure appende

Recursion and Path-Integral Approaches to the Analytic Study of the Electronic Properties of C60C_{60}

The recursion and path-integral methods are applied to analytically study the electronic structure of a neutral $C_{60}$ molecule. We employ a tight-binding Hamiltonian which considers both the $s$ and $p$ valence electrons of carbon. From the recursion method, we obtain closed-form {\it analytic} expressions for the $\pi$ and $\sigma$ eigenvalues and eigenfunctions, including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states, and the Green's functions. We also present the local densities of states around several ring clusters, which can be probed experimentally by using, for instance, a scanning tunneling microscope. {}From a path-integral method, identical results for the energy spectrum are also derived. In addition, the local density of states on one carbon atom is obtained; from this we can derive the degree of degeneracy of the energy levels.Comment: 19 pages, RevTex, 6 figures upon reques

Oscillons and oscillating kinks in the Abelian-Higgs model

We study the classical dynamics of the Abelian Higgs model employing an asymptotic multiscale expansion method, which uses the ratio of the Higgs to the gauge field amplitudes as a small parameter. We derive an effective nonlinear Schr\"{o}dinger equation for the gauge field, and a linear equation for the scalar field containing the gauge field as a nonlinear source. This equation is used to predict the existence of oscillons and oscillating kinks for certain regimes of the ratio of the Higgs to the gauge field masses. Results of numerical simulations are found to be in very good agreement with the analytical findings, and show that the oscillons are robust, while kinks are unstable. It is also demonstrated that oscillons emerge spontaneously as a result of the onset of the modulational instability of plane wave solutions of the model. Connections of the obtained solutions with the phenomenology of superconductors is discussed.Comment: arXiv admin note: substantial text overlap with arXiv:1306.386

Invariant spin coherent states and the theory of quantum antiferromagnet in a paramagnetic phase

The consistent theory of the Heisenberg quantum antiferromagnet in the disordered phase with short range antiferromagnetic order was developed on the basis of the path integral for the spin coherent states. We have presented the Lagrangian of the theory in a form which is explicitly invariant under rotations and have found natural variables in the term of which one can construct a natural perturbation theory. The short wave spin fluctuations are similar to the spin wave theory ones, and the long wave spin fluctuations are governed by the nonlinear sigma model. We have also demonstrated that the short wave spin fluctuations have to be considered accurately in the framework of the discrete version in time of the path integral. In the framework of our approach we have obtained the response function for the spin fluctuations for the whole region of the frequency $\omega$ and the wave vector ${\bf k}$ and have calculated the free energy of the system.Comment: 7 pages, LATEX2

On the temperature dependence of correlation functions in the space like direction in hot QCD

We study the temperature dependence of quark antiquark correlations in the space like direction. In particular, we predict the temperature dependence of space like Bethe-Salpeter amplitudes using recent Lattice gauge data for the space like string potential. We also investigate the effect of the space like string potential on the screening mass and discuss possible corrections which may arise when working with point sources.Comment: 15 pages 8 figures (not included, will be sent on request), (SUNY-NTG-94-3

Spin mapping, phase diagram, and collective modes in double layer quantum Hall systems at ν=2\nu=2

An exact spin mapping is identified to simplify the recently proposed hard-core boson description (Demler and Das Sarma, Phys. Rev. Lett., to be published) of the bilayer quantum Hall system at filling factor 2. The effective spin model describes an easy-plane ferromagnet subject to an external Zeeman field. The phase diagram of this effective model is determined exactly and found to agree with the approximate calculation of Demler and Das Sarma, while the Goldstone-mode spectrum, order parameter stiffness and Kosterlitz-Thouless temperature in the canted antiferromagnetic phase are computed approximately.Comment: 4 pages with 2 figures include

Multi-wavelength observations of IGR J17544-2619 from quiescence to outburst

In this paper we report on a long multi-wavelength observational campaign of the supergiant fast X-ray transient prototype IGR J17544-2619. A 150 ks-long observation was carried out simultaneously with XMM-Newton and NuSTAR, catching the source in an initial faint X-ray state and then undergoing a bright X-ray outburst lasting about 7 ks. We studied the spectral variability during outburst and quiescence by using a thermal and bulk Comptonization model that is typically adopted to describe the X-ray spectral energy distribution of young pulsars in high mass X-ray binaries. Although the statistics of the collected X-ray data were relatively high we could neither confirm the presence of a cyclotron line in the broad-band spectrum of the source (0.5-40 keV), nor detect any of the previously reported tentative detection of the source spin period. The monitoring carried out with Swift/XRT during the same orbit of the system observed by XMM-Newton and NuSTAR revealed that the source remained in a low emission state for most of the time, in agreement with the known property of all supergiant fast X-ray transients being significantly sub-luminous compared to other supergiant X-ray binaries. Optical and infrared observations were carried out for a total of a few thousands of seconds during the quiescence state of the source detected by XMM-Newton and NuSTAR. The measured optical and infrared magnitudes were slightly lower than previous values reported in the literature, but compatible with the known micro-variability of supergiant stars. UV observations obtained with the UVOT telescope on-board Swift did not reveal significant changes in the magnitude of the source in this energy domain compared to previously reported values.Comment: Accepted for publication on A&A. V2: few typos correcte

Physics of π\pi-Meson Condensation and High Temperature Cuprate Superconductors

The idea of condensation of the Goldstone $\pi$-meson field in nuclear matter had been put forward a long time ago. However, it was established that the normal nuclear density is too low, it is not sufficient to condensate $\pi$-mesons. This is why the $\pi$-condensation has never been observed. Recent experimental and theoretical studies of high temperature cuprate superconductors have revealed condensation of Goldstone magnons, the effect fully analogous to the $\pi$-condensation. The magnon condensation has been observed. It is clear now that quantum fluctuations play a crucial role in the condensation, in particular they drive a quantum phase transition that destroys the condensate at some density of fermions