η\eta collective mode as A1g_{1g} Raman resonance in cuprate superconductors

We discuss the possible existence a spin singlet excitation with charge $\pm2$ ($\eta$-mode) originating the $A_{1g}$ Raman resonance in cuprate superconductors. This $\eta$-mode relates the $d$-wave superconducting singlet pairing channel to a $d$-wave charge channel. We show that the $\eta$ boson forms a particle-particle bound state below the $2\Delta$ threshold of the particle-hole continuum where $\Delta$ is the maximum $d$-wave gap. Within a generalized random phase approximation and Bethe-Salpether approximation study, we find that this mode has energies similar to the resonance observed by Inelastic Neutron Scattering (INS) below the superconducting (SC) coherent peak at $2\Delta$ in various SC cuprates compounds. We show that it is a very good candidate for the resonance observed in Raman scattering below the $2\Delta$ peak in the $A_{1g}$ symmetry. Since the $\eta$-mode sits in the $S=0$ channel, it may be observable via Raman, X -ray or Electron Energy Loss Spectroscopy probes

Catching the fish - Constraining stellar parameters for TX Psc using spectro-interferometric observations

Stellar parameter determination is a challenging task when dealing with galactic giant stars. The combination of different investigation techniques has proven to be a promising approach. We analyse archive spectra obtained with the Short-Wavelength-Spectrometer (SWS) onboard of ISO, and new interferometric observations from the Very Large Telescope MID-infrared Interferometric instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The aim of this work is to determine stellar parameters using spectroscopy and interferometry. The observations are used to constrain the model atmosphere, and eventually the stellar evolutionary model in the region where the tracks map the beginning of the carbon star sequence. Two different approaches are used to determine stellar parameters: (i) the 'classic' interferometric approach where the effective temperature is fixed by using the angular diameter in the N-band (from interferometry) and the apparent bolometric magnitude; (ii) parameters are obtained by fitting a grid of state-of-the-art hydrostatic models to spectroscopic and interferometric observations. We find a good agreement between the parameters of the two methods. The effective temperature and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star just recently, which means that the star is still in a 'quiet' phase compared to the subsequent strong-wind regime. This is in agreement with the C/O ratio being only slightly larger than 1.Comment: 11 pages, 9 figures, 5 table

Unconventional high-energy-state contribution to the Cooper pairing in under-doped copper-oxide superconductor HgBa2_2Ca2_2Cu3_3O8+δ_{8+\delta}

We study the temperature-dependent electronic B1g Raman response of a slightly under-doped single crystal HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ with a superconducting critical temperature Tc=122 K. Our main finding is that the superconducting pair-breaking peak is associated with a dip on its higher-energy side, disappearing together at Tc. This result hints at an unconventional pairing mechanism, whereas spectral weight lost in the dip is transferred to the pair-breaking peak at lower energies. This conclusion is supported by cellular dynamical mean-field theory on the Hubbard model, which is able to reproduce all the main features of the B1g Raman response and explain the peak-dip behavior in terms of a nontrivial relationship between the superconducting and the pseudo gaps.Comment: 7 pages 4 figure

Geometrical model fitting for interferometric data: GEM-FIND

We developed the tool GEM-FIND that allows to constrain the morphology and brightness distribution of objects. The software fits geometrical models to spectrally dispersed interferometric visibility measurements in the N-band using the Levenberg-Marquardt minimization method. Each geometrical model describes the brightness distribution of the object in the Fourier space using a set of wavelength-independent and/or wavelength-dependent parameters. In this contribution we numerically analyze the stability of our nonlinear fitting approach by applying it to sets of synthetic visibilities with statistically applied errors, answering the following questions: How stable is the parameter determination with respect to (i) the number of uv-points, (ii) the distribution of points in the uv-plane, (iii) the noise level of the observations?Comment: 14 pages, 12 figures; to appear in SPIE Astronomical Telescopes and Instrumentation, SPIE Conference Series 844

Suppressed antinodal coherence with a single d-wave superconducting gap leads to two energy scales in underdoped cuprates

Conventional superconductors are characterized by a single energy scale, the superconducting gap, which is proportional to the critical temperature Tc . In hole-doped high-Tc copper oxide superconductors, previous experiments have established the existence of two distinct energy scales for doping levels below the optimal one. The origin and significance of these two scales are largely unexplained, although they have often been viewed as evidence for two gaps, possibly of distinct physical origins. By measuring the temperature dependence of the electronic Raman response of Bi2Sr2CaCu2O8+d (Bi-2212) and HgBa2CuO4+d (Hg-1201) crystals with different doping levels, we establish that these two scales are associated with coherent excitations of the superconducting state which disappears at Tc. Using a simple model, we show that these two scales do not require the existence of two gaps. Rather, a single d-wave superconducting gap with a loss of Bogoliubov quasiparticle spectral weight in the antinodal region is shown to reconcile spectroscopic and transport measurements.Comment: 3 figure

Intrinsic Low Temperature Paramagnetism in B-DNA

We present experimental study of magnetization in $\lambda$-DNA in conjunction with structural measurements. The results show the surprising interplay between the molecular structures and their magnetic property. In the B-DNA state, $\lambda$-DNA exhibits paramagnetic behaviour below 20 K that is non-linear in applied magnetic field whereas in the A-DNA state, remains diamagnetic down to 2 K. We propose orbital paramagnetism as the origin of the observed phenomena and discuss its relation to the existence of long range coherent transport in B-DNA at low temperature.Comment: 5 pages, 4 figures, submitted to Physical Review Letters October 200

Fundamental parameters of 16 late-type stars derived from their angular diameter measured with VLTI/AMBER

Thanks to their large angular dimension and brightness, red giants and supergiants are privileged targets for optical long-baseline interferometers. Sixteen red giants and supergiants have been observed with the VLTI/AMBER facility over a two-years period, at medium spectral resolution (R=1500) in the K band. The limb-darkened angular diameters are derived from fits of stellar atmospheric models on the visibility and the triple product data. The angular diameters do not show any significant temporal variation, except for one target: TX Psc, which shows a variation of 4% using visibility data. For the eight targets previously measured by Long-Baseline Interferometry (LBI) in the same spectral range, the difference between our diameters and the literature values is less than 5%, except for TX Psc, which shows a difference of 11%. For the 8 other targets, the present angular diameters are the first measured from LBI. Angular diameters are then used to determine several fundamental stellar parameters, and to locate these targets in the Hertzsprung-Russell Diagram (HRD). Except for the enigmatic Tc-poor low-mass carbon star W Ori, the location of Tc-rich stars in the HRD matches remarkably well the thermally-pulsating AGB, as it is predicted by the stellar-evolution models. For pulsating stars with periods available, we compute the pulsation constant and locate the stars along the various sequences in the Period -- Luminosity diagram. We confirm the increase in mass along the pulsation sequences, as predicted by the theory, except for W Ori which, despite being less massive, appears to have a longer period than T Cet along the first-overtone sequence.Comment: 15 pages, 9 figures, 6 table