Charge correlations and optical conductivity in weakly doped antiferromagnets

We investigate the dynamical charge-charge correlation function and the optical conductivity in weakly doped antiferromagnets using Mori-Zwanzig projection technique. The system is described by the two-dimensional t-J model. The arising matrix elements are evaluated within a cumulant formalism which was recently applied to investigate magnetic properties of weakly doped antiferromagnets. Within the present approach the ground state consists of non-interacting hole quasiparticles. Our spectra agree well with numerical results calculated via exact diagonalization techniques. The method we employ enables us to explain the features present in the correlation functions. We conclude that the charge dynamics at weak doping is governed by transitions between excited states of spin-bag quasiparticles.Comment: 5 pages, 2 figures, to appear in Europhys. Letter

A multiwavlength study of PSR B0628-28: The first overluminous rotation-powered pulsar?

The ROSAT source RX J0630.8-2834 was suggested by positional coincidence to be the X-ray counterpart of the old field pulsar PSR B0628-28. This association, however, was regarded to be unlikely based on the computed energetics of the putative X-ray counterpart. In this paper we report on multiwavelength observations of PSR B0628-28 made with the ESO/NTT observatory in La Silla, the Jodrell Bank radio observatory and XMM-Newton. Although the optical observations do not detect any counterpart of RX J0630.8-2834 down to a limiting magnitude of V=26.1 mag and B=26.3 mag, XMM-Newton observations finally confirmed it to be the pulsar's X-ray counterpart by detecting X-ray pulses with the radio pulsar's spin-period. The X-ray pulse profile is characterized by a single broad peak with a second smaller peak leading the main pulse component by ~144 degree. The fraction of pulsed photons is (38 +- 7)% with no strong energy dependence in the XMM-Newton bandpass. The pulsar's X-ray spectrum is well described by a single component power law with photon index 2.63^{+0.23}_{-0.15}, indicating that the pulsar's X radiation is dominated by non-thermal emission processes. A low level contribution of thermal emission from residual cooling or from heated polar caps, cannot be excluded. The pulsar's spin-down to X-ray energy conversion efficiency is obtained to be ~16% for the radio dispersion measure inferred pulsar distance. If confirmed, PSR B0628-28 would be the first X-ray overluminous rotation-powered pulsar identified among all ~1400 radio pulsars known today.Comment: Accepted for publication in ApJ. Find a paper copy with higher resolution images at ftp://ftp.xray.mpe.mpg.de/people/web/astro-ph-0505488_rev2.pd

The 2-10 keV emission properties of PSR B1937+21

We present the results of a BeppoSAX observation of the fastest pulsar known: PSR B1937+21. The ~ 200 ks observation (78.5 (34) ks MECS (LECS) exposure times) allowed us to investigate with high statistical significance both the spectral properties and the pulse profile shape. The absorbed power law spectral model gave a photon index of ~ 1.7 and N_H ~ 2.3 x 10^22 cm^-2. These values explain both a) the ROSAT non-detection and b) the deviant estimate of a photon index of ~ 0.8 obtained by ASCA. The pulse profile appears, for the first time, clearly double peaked with the main component much stronger than the other. The statistical significance is 10 sigma (main peak) and 5 sigma (secondary peak). The 1.6-10 keV pulsed fraction is consistent with 100%; only in the 1.6-4 keV band there is a ~ 2 sigma indication for a DC component. The secondary peak is detected significantly only for energies above 3 / 4 keV. The unabsorbed (2-10 keV) flux is F_2-10 = 3.7 x 10^-13 erg cm^-2 s^-1, implying a luminosity of L_X = 4.6 x 10^31 Theta (d/3.6 kpc)^2 erg s^-1 and an X-ray efficiency of eta = 4 x 10^-5 Theta, where Theta is the solid angle spanned by the emission beam. These results are in agreement with those obtained by ASCA.Comment: 4 pages, 4 figures, 2 tables. To appear in the Proceedings of the 270. WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan. 21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper. Proceedings are available as MPE-Report 27

How cold is cold dark matter? Small scales constraints from the flux power spectrum of the high-redshift Lyman-alpha forest

We present constraints on the mass of warm dark matter (WDM) particles derived from the Lyman-alpha flux power spectrum of 55 high- resolution HIRES spectra at 2.0 < z < 6.4. From the HIRES spectra, we obtain a lower limit of mwdm > 1.2 keV 2 sigma if the WDM consists of early decoupled thermal relics and mwdm > 5.6 keV (2 sigma) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman-alpha flux power spectrum, we get mwdm > 4 keV and mwdm > 28 keV (2 sigma) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor two.Comment: Some issues clarified (especially resolution related). Conclusions unchanged. Accepted version by PR

Optical measurement methods in thermogasdynamics

A review is presented of a number of optical methods of flow measurements. Consideration is given to such spectroscopic methods as emission and absorption techniques, electron beam-stimulated fluorescence, and light scattering - Rayleigh, Raman and Mie - methods. The following visualization methods are also discussed: shadow photography, schlieren photography, interferometry, holographic interferometry, laser anemometry, particle holography, and electron-excitation imaging. A large bibliography is presented and the work is copiously illustrated with figures and photographs

ROSAT HRI Observations of the Crab Pulsar: An Improved Temperature upper limit for PSR 0531+21

ROSAT HRI observations have been used to determine an upper limit of the Crab pulsar surface temperature from the off-pulse count rate. For a neutron star mass of 1.4 \Mo and a radius of 10 km as well as the standard distance and interstellar column density, the redshifted temperature upper limit is\/ $T_s^\infty \le 1.55\times 10^6$ K $(3\sigma)$. This is the lowest temperature upper limit obtained for the Crab pulsar so far. Slightly different values for $T_s^\infty$ are computed for the various neutron star models available in the literature, reflecting the difference in the equation of state.Comment: 5 pages, uuencoded postscript, to be published in the Proceedings of the NATO Advanced Study Insitute on "Lives of the Neutron Stars", ed. A. Alpar, U. Kiziloglu and J. van Paradijs ( Kluwer, Dordrecht, 1995 )

Tropical Pacific spatial trend patterns in observed sea level: internal variability and/or anthropogenic signature?

In this study we focus on the sea level trend pattern observed by satellite altimetry in the tropical Pacific over the 1993–2009 time span (i.e. 17 yr). Our objective is to investigate whether this 17-yr-long trend pattern was different before the altimetry era, what was its spatio-temporal variability and what have been its main drivers. We try to discriminate the respective roles of the internal variability of the climate system and of external forcing factors, in particular anthropogenic emissions (greenhouse gases and aerosols). On the basis of a 2-D past sea level reconstruction over 1950–2009 (based on a combination of observations and ocean modelling) and multi-century control runs (i.e. with constant, preindustrial external forcing) from eight coupled climate models, we have investigated how the observed 17-yr sea level trend pattern evolved during the last decades and centuries, and try to estimate the characteristic time scales of its variability. For that purpose, we have computed sea level trend patterns over successive 17-yr windows (i.e. the length of the altimetry record), both for the 60-yr long reconstructed sea level and the model runs. We find that the 2-D sea level reconstruction shows spatial trend patterns similar to the one observed during the altimetry era. The pattern appears to have fluctuated with time with a characteristic time scale of the order of 25–30 yr. The same behaviour is found in multi-centennial control runs of the coupled climate models. A similar analysis is performed with 20th century coupled climate model runs with complete external forcing (i.e. solar plus volcanic variability and changes in anthropogenic forcing). Results suggest that in the tropical Pacific, sea level trend fluctuations are dominated by the internal variability of the ocean–atmosphere coupled system. While our analysis cannot rule out any influence of anthropogenic forcing, it concludes that the latter effect in that particular region is stillhardly detectable

Resonant enhancements of high-order harmonic generation

Solving the one-dimensional time-dependent Schr\"odinger equation for simple model potentials, we investigate resonance-enhanced high-order harmonic generation, with emphasis on the physical mechanism of the enhancement. By truncating a long-range potential, we investigate the significance of the long-range tail, the Rydberg series, and the existence of highly excited states for the enhancements in question. We conclude that the channel closings typical of a short-range or zero-range potential are capable of generating essentially the same effects.Comment: 7 pages revtex, 4 figures (ps files

Strong-field approximation for intense-laser atom processes: the choice of gauge

The strong-field approximation can be and has been applied in both length gauge and velocity gauge with quantitatively conflicting answers. For ionization of negative ions with a ground state of odd parity, the predictions of the two gauges differ qualitatively: in the envelope of the angular-resolved energy spectrum, dips in one gauge correspond to humps in the other. We show that the length-gauge SFA matches the exact numerical solution of the time-dependent Schr\"odinger equation.Comment: 5 pages, 3 figures, revtex