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D. van der Marel

Publications and source records attributed to D. van der Marel.

138 records · Page 8Linked to original sources
arXiv1997

Experimental Test of the Inter-Layer Pairing Models for High-Tc Superconductivity Using Grazing Incidence Infrared Reflectometry

From measurements of the far-infrared reflectivity at grazing angles of incidence with p-polarized light we determined the c-axis Josephson plasma frequencies of the single layer high T_c cuprates Tl_2Ba_2CuO_6 and La_{2-x}Sr_xCuO_4. We detected a strong plasma resonance at 50 cm^{-1} for La_{2-x}Sr_xCuO_4 in excellent agreement with previously published results. For Tl_2Ba_2CuO_6 we were able to determine an upper limit of the unscreened c-axis Josephson plasma frequency 100 cm^{-1} or a c-axis penetration depth > 15 μm. The small value of $ω_J$ stands in contrast to recent a prediction based on the inter-layer tunneling mechanism of superconductivity.

cond-mat.str-el↗
arXiv1996

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.

cond-mat.str-el↗
arXiv1996

Transverse optical plasmons in layered superconductors

We discuss the possible existance of transverse optical plasma modes in superlattices consisting of Josephson coupled superconducting layers. These modes appear as resonances in the current-current correlation function, as opposed to the usual plasmons which are poles in the density-density channel. We consider both bilayer superlattices, and single layer lattices with a spread of interlayer Josephson couplings. We show that our model is in quantitative agreement with the recent experimental observation by a number of groups of a peak at the Josephson plasma frequency in the optical conductivity of La$_{1.85}$Sr$_{0.15}$CuO$_4$

cond-mat↗
arXiv1996

Electrodynamical Properties of High Tc Superconductors Studied with Polarized Angle Resolved Infrared Spectroscopy

Using infrared spectroscopy at grazing angle of incidence we study the electrodynamical properties of high temperature superconductors. We review some of our experiments where transverse polarized light is absorbed by longitudinal optical modes with their mode of oscillation perpendicular to the plane. This is particularly useful for the study of the plasmons and phonons perpendicular to the plane, and allows us to study in detail the c-axis dynamical properties of flux grown single crystals for which usually no samples with large dimensions in the c-direction exist.

cond-mat↗
arXiv1996

Grazing Incidence Infrared Reflectivity of La$_{1.85}$Sr$_{0.15}$CuO$_4$ and NbN

Infrared reflectivity measurements, using p-polarized light at a grazing angle of incidence, show an increased sensitivity to the optical conductivity of highly reflecting superconducting materials. We demonstrate that when this measurement technique is applied to the conventional s-wave superconductor NbN, the results are in perfect agreement with BCS theory. For the in-plane response of a La$_{1.85}$Sr$_{0.15}$CuO$_4$ single crystal, in the superconducting state, we find a reduction of the optical conductivity in the frequency range below 20 meV. The observed frequency dependence excludes an isotropic s-wave gap, but agrees well with model calculations assuming a d-wave order parameter.

cond-mat↗
arXiv1994

Doping Dependence of the Chemical Potential in cuprate high T$_c$ superconductors II: (Bi,Pb)$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$% O$_{10+\mbox{\small\protect\boldmath $δ$}}$

Using X-ray photoelectron spectroscopy, a systematic study is performed of the doping dependence of the chemical potential $μ$ in two systems of (Bi,Pb)$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$. The doping is varied by changing the O content of the sample. The measured shifts of the chemical potential are compared with present models for the doping behaviour of $μ$ in high-$T_{\rm c}$ materials. VSGD.94.6.3

cond-mat↗
arXiv1994

Observation of the Holstein shift in high $T_c$ superconductors with thermal modulation reflectometry

We use the experimental technique of thermal modulation reflectometry to study the relatively small temperature dependence of the optical conductivity of superconductors. Due to a large cancellation of systematic errors, this technique is shown to a be very sensitive probe of small changes in reflectivity. We analyze thermal modulation reflection spectra of single crystals and epitaxially grown thin films of YBa$_2$Cu$_3$O$_{7-δ}$ and obtain the ${α_tr}^2F(ω)$ function in the normal state, as well as the superconductivity induced changes in reflectivity. We present detailed model calculations, based on the Eliashberg-Migdal extension of the BCS model, which show good qualitative and quantitative agreement with the experimental spectra. VSGD.93.12.th

cond-mat↗
arXiv1994

Collective modes of spin, density, phase and amplitude in exotic superconductors

The equations of motion of pair-like excitations in the superconducting state are studied for various types of pairing using the random phase approximation. The collective modes are computed of a layered electron gas described by a $t-t'$ tight-binding band, where the electrons experience besides the long-range Coulomb repulsion an on-site Hubbard U repulsion and a nearest-neighbour attractive interaction. {}From numerical calculations we see, that the collective mode spectrum now becomes particularly rich. Several branches can occur below the continuum of quasi-particle excitations, corresponding to order-parameter fluctuations of various symmetries of pairing, and collective spin-density fluctuations. {}From the collective mode softening near the nesting vectors it is concluded, that in the d-wave paired state an instability occurs toward the formation of a spin-density wave.

cond-mat↗
arXiv1994

Long range Coulomb forces and the behaviour of the chemical potential of electrons in metals at a second order phase transition

We give a general thermodynamic analyzis of the behaviour of the chemical potential of electrons in metals at a second order phase transition, including in our analysis the effect of long range Coulomb forces. It is shown, that this chemical potential can have a kink at T$_c$, both for fixed sample volume and fixed external pressure. The Coulomb term transfers the changes in chemical potential of the electrons into an experimentally observable shift of the surface potential if the sample is electrically connected to a ground potential. VSGD.93.9.th1

cond-mat↗
arXiv1994

Damping mechanism for the strongly renormalized $c$-axis charge transport in high-$T_c$ 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 ($ν_p$) 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 ($γ$) below $T_c$. Our investigation suggests $hγ>hν_{p}\gg 3.5k_BT_c$, which is exactly opposite to the clean limit. VSGD.94.6.1

cond-mat↗
arXiv1993

Weak coupling model for s*- and d-wave superconductivity

The phase diagram of the unconstrained $t-J$ model is calculated using the random phase approximation. It is found that the extended $s$ and the $d_{x^2-y^2}$-channels are {\em not} degenerate near half filling. Extended $s$-pairing with a low $T_c$ occurs only for a band containing less then 0.4 electrons or holes per unit cell, whereas in a large region around half-filling $d$-wave pairing is the only stable superconducting solution. At half filling superconductivity is suppressed due to the formation of the anti-ferromagnetic Mott-Hubbard insulating state. By extending the analysis to the unconstrained $t-t'-J$ model, it is proven that, if a Fermi surface is assumed similar to the one that is known to exist in cuprous oxide superconductors, the highest superconducting $T_c$ is reached for about $0.7$ electron per site, whereas the anti-ferromagnetic solution still occurs for $1$ electron per site. It is shown, that the maximum $d$-wave superconducting mean field transition temperature is half the maximum value that the Neèl temperature can have in the Mott-insulating state.

cond-mat↗