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Infrared spectroscopic study of phonons coupled to charge excitations in FeSi

From an investigation of the optical conductivity of FeSi single crystals using FTIR spectroscopy in the frequency range from 30 to 20000 wavenumbers we conclude that the transverse effective charge of the Fe and Si ions is approximately 4e. Of the five optical phonons which are allowed by symmetry we observe only four, three of which have a Fano line shape presumably resulting from an interaction of these modes with the electronic continuum. We show that the large oscillator strength of the phonons results from a relatively weak coupling (lambda of the order of 0.1) of the lattice degrees of freedom to an electronic resonance above the semiconductor gap, which is also responsible for the large electronic polarizability of the medium.Comment: Revtex, 10 pages, 2 postscript pictures, to be published in Phys. Rev. B Rapid Comm

Magnetism in heavy-fermion U(Pt,Pd)3 studied by mSR

We report mSR experiments carried out on a series of heavy-electron pseudobinary compounds U(Pt1-xPdx)3 (x<=0.05). For x<=0.005 the zero-field muon depolarisation is described by the Kubo-Toyabe function. However the temperature variation of the Kubo-Toyabe relaxation rate does not show any sign of the small-moment antiferromagnetic phase with TN~6 K (signalled by neutron diffraction), in contrast to previous reports. The failure to detect the small ordered moment suggests it has a fluctuating (> 10 MHz) nature, which is consistent with the interpretation of NMR data. For 0.01<=x<=0.05 the muon depolarisation in the ordered state is described by two terms of equal amplitude: an exponentially damped spontaneous oscillation and a Lorentzian Kubo-Toyabe function. These terms are associated with antiferromagnetic order with substantial moments. The Knight-shift measured in a magnetic field of 0.6 T on single-crystalline U(Pt0.95Pd0.05)3 in the paramagnetic state shows two signals for B perpendicular to c, while only one signal is observed for B||c. The observation of two signals for B perpendicular to c, while there is only one muon localisation site (0,0,0), points to the presence of two spatially distinct regions of different magnetic response.Comment: 25 pages including 12 figures (PS), J. Phys.: Condens. Matter, in prin

Strict limit on in-plane ordered magnetic dipole moment in URu2Si2

Neutron diffraction is used to examine the polarization of weak static antiferromagnetism in high quality single crystalline URu2Si2. As previously documented, elastic Bragg-like diffraction develops for temperature T<T_{HO}= 17.5 K at q=(100) but not at wave vector transfer q=(001). The peak width indicates correlation lengths \xi_c=230(12) \AA \ and \xi_a=240(15) \AA. The integrated intensity of the T-dependent peaks corresponds to a sample averaged c-oriented staggered moment of \mu_{c}=0.022(1) \mu_B at T=1.7 K. The absence of T-dependent diffraction at q=(001) places a limit \mu_{\perp}<0.0011 \mu_B on an f- or d-orbital based in-plane staggered magnetic dipole moment, which is associated with multipolar orders proposed for URu_2Si_2.Comment: 9 pages, 7 figure

Muon localization site in U(Pt,Pd)3

The angular and temperature (10-250 K) variation of the Knight shift of single-crystalline U(Pt0.95Pd0.05)3 has been measured in transverse field (B=0.6 T) mSR experiments. By analysing the temperature variation of the Knight shift with a modified Curie-Weiss expression the muon localization site in this hexagonal material is determined at (0,0,0).Comment: 12 pages (including 4 figures); postscript file; Proc. 8th Int. Conf. on Muon Spin Rotation, Relaxation and Resonance (Aug.30-Sept.3, Les Diablerets); 2nd version with minor correction

Damping mechanism for the strongly renormalized cc-axis charge transport in high-TcT_c cuprate superconductors

We analyze the $c$-axis infrared reflectivity of La$_{1.85}$Sr$_{0.15}$CuO$_4$ single crystals. The plasma edge near 6 meV, observed below $T_c$, is due to Cooper-pair tunneling. This low value of the plasma edge is consistent with the $c$-axis plasma frequency ($\nu_p$) obtained from LDA calculations ($>0.1$ eV) if we take into account that the single-particle charge transport along the $c$ axis is strongly incoherent both above and below $T_c$. We find no evidence for a reduction of the $c$-axis scattering rate ($\gamma$) below $T_c$. Our investigation suggests $h\gamma>h\nu_{p}\gg 3.5k_BT_c$, which is exactly opposite to the clean limit. VSGD.94.6.1Comment: 4 pages, figures on request. Revtex, version 2, Materials Science Center Internal Report Number VSGD.94.6.