Comment on: Nonmonotonic dx2−y2d_{x^{2}-y^{2}} Superconducting Order Parameter in Nd2−x_{2-x}Cex_xCuO4_4

In a recent letter Blumberg and collaborators claim that a non-monotonic $d_{x^2-y^2}$ form for the superconducting order parameter is required to explain their Raman scattering measurements in Nd$_{2-x}$Ce$_x$CuO$_4$ . In this comment we show with a simple model calculation that the basis for this conclusion is insufficient. The proposed functional dependence of the gap is neither consistent with their measured spectra nor compatible with other experimental results. Therefore the issue of the superconduing gap in electron-doped systems cannot be considered solved by now.Comment: Comment to the paper by Blumberg et al., Phys. Rev. Lett., 88, 107002 (2002

Raman scattering from a superconductivity-induced bound state in MgB2MgB_2

It is shown that the sharp peak in the $E_{2g}$ Raman spectrum of superconducting $MgB_2$ is due to a bound state caused by the electron-phonon coupling. Our theory explains why this peak appears only in the spectra with $E_{2g}$ symmetry and only in the $\sigma$ but not $\pi$ bands. The properties of the bound state and the Raman spectrum are investigated, also in the presence of impurity scattering.Comment: 4 pages, 4 figures, will appear in PR

Nernst effect in the electron-doped cuprates

We calculate the normal state Nernst signal in the cuprates resulting from a reconstruction of the Fermi surface due to spin density wave order. An order parameter consistent with the reconstruction of the Fermi surface detected in electron-doped materials is shown to sharply enhance the Nernst signal close to optimal doping. Within a semiclassical treatment, the obtained magnitude and position of the enhanced Nernst signal agrees with Nernst measurements in electron-doped cuprates.Comment: 9 pages, 5 figures, revised version as accepted by Phys. Rev. B, changed several citations and reference

Minimal energy cost of entanglement extraction

We compute the minimal energy cost for extracting entanglement from the ground state of a bosonic or fermionic quadratic system. Specifically, we find the minimal energy increase $\Delta E_{\mathrm{min}}$ in the system resulting from replacing an entangled pair of modes, sharing entanglement entropy $\Delta S$, by a product state, and we show how to construct modes achieving this minimal energy cost. Thus, we obtain a protocol independent lower bound on the extraction of pure state entanglement from quadratic systems. Due to their generality, our results apply to a large range of physical systems, as we discuss with examples.Comment: 30+13 pages, 9 figure

Superconducting Gap and Pseudogap in Bi-2212

We present results of Raman scattering experiments in differently doped Bi-2212 single crystals. Below Tc the spectra show pair-breaking features in the whole doping range. The low frequency power laws confirm the existence of a $d_{x^2-y^2}$-wave order parameter. In the normal state between Tc and T* = 200K we find evidence for a pseudogap in B2g symmetry. Upon doping its effect on the spectra decreases while its energy scale appears to be unchanged.Comment: 2 pages, 1 EPS figure; LT22 Proceedings to appear in Physica

Band and momentum dependent electron dynamics in superconducting Ba(Fe1−xCox)2As2{\rm Ba(Fe_{1-x}Co_{x})_2As_2} as seen via electronic Raman scattering

We present details of carrier properties in high quality ${\rm Ba(Fe_{1-x}Co_{x})_2As_2}$ single crystals obtained from electronic Raman scattering. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition highlighting the importance of complex band-dependent interactions. The presence of low energy spectral weight deep in the superconducting state suggests a gap with accidental nodes which may be lifted by doping and/or impurity scattering. When combined with other measurements, our observation of band and momentum dependent carrier dynamics indicate that the iron arsenides may have several competing superconducting ground states.Comment: 5 pages, 4 figure