Research Unit FOR538
Doping Dependence of Phase Transitions and Ordering Phenomena in Copper-Oxygen Superconductors

P7 Infrared Ellipsometry Studies of Cuprate High-Tc Superconductors

This project is concerned with precise broad-band, 0.005 - 5.5 eV (FIR-UV), ellipsometric measurements on cuprate high temperature superconductors (HTSC). Systematic studies are planned for several HTSC compounds in order to reveal the evolution of their dielectric properties as a function of charge carrier concentration and impurity substitution. The ultimate goal is to obtain information about the interactions of the charge carriers that are at the heart of superconductivity as well as of the so-called pseudogap phenomenon that determines the unusual normal state properties of underdoped samples.


The technique of ellipsometry is especially suited for this purpose since it enables one to perform direct and very precise measurements of the real and imaginary parts of the dielectric function. It does not require reference measurements or a Kramers-Kronig transformation analysis that involves the extrapolation of the data to zero and infinite frequencies.


The following projects are envisaged:

(i) Anomalous superconductivity induced spectral weight redistribution:

By measuring the temperature dependent changes of the dielectric function over a very wide frequency range we will search for a superconductivity induced redistribution of spectral weight that involves an unusually large energy scale (well beyond the gap energy). In particular, we will address the question whether such a spectral weight redistribution can be associated with a change in the kinetic energy of the charge carriers which could be the signature of an exotic pairing mechanism.

(ii) Pseudogap and spectral weight redistribution in the normal state:

The pseudogap phenomenon is most readily seen in the c-axis response perpendicular to the conducting CuO2 planes, where it gives rise to a gap-like suppression of the far-infrared conductivity already in the normal state. We want to identify the presently unknown energy scale of the related spectral weight redistribution. Of particular interest is the comparison between the gap features in the normal and the superconducting states. Ultimatively, we want to learn whether the pseudogap and the superconducting gap have a common origin or rather are distinct (possibly even competitive) phenomena.

Especially important is the comparison of different hole and electron-doped HTSC compounds as well as the influence of structural defects like magnetic Ni and non-magnetic Zn impurities that allow one to suppress superconductivity.

(iii) Low-energy collective electronic modes:

We want to study the evolution of the low-energy IR-active phonons and collective electronic excitations that may be the signature of dynamic charge density modulations. The universality of these modes will be tested by studying the in-plane dielectric response of several HTSC compounds including under- to overdoped YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+δ. This investigation aims at resolving the question whether the electronic ground state exhibits aspects of heterogeneity.

A very important aspect is the close collaboration with the other groups within the Forschergruppe concerning the results of their spectroscopic techniques (Raman scattering, tunneling, angle resolved photo-emission and neutron scattering) that will provide complementary information on the charge and spin excitations. The interaction with the theory group will help in understanding and interpretation of our results as well as in the development of new ideas for improved experiments.

(iv) Exploring in detail the insulator to metal (and superconductor) transition at very low hole and electron doping:

We want to learn how the Drude-peak due to the free carriers and the so-called MIR band due to weakly bound charges emerge from the Mott insulator state.

(v) Clarify whether and how spin fluctuations, phonons, charge fluctuations, or multiband effect due to the bilayer coupling give rise to the so-called "dip feature" in the optical conductivity around 500-800 cm-1 that is frequently viewed as a fingerprint of the mode that yields the superconducting pairing interaction.

Selected Papers:

Spectroscopic distinction between the normal state pseudogap and the superconducting gap of
cuprate high Tc superconductors
Li Yu, D. Munzar, A.V. Boris, P. Yordanov, J. Chaloupka, Th. Wolf, C.T. Lin, B. Keimer & C. Bernhard
Phase separation in superoxygenated La2-xSrxCuO4+y
H.E. Mohottala, B.O. Wells, J.I. Budnick, W.A. Hines, C. Niedermayer, L. Udby, C. Bernhard, A.R. Moodenbaugh & Fang-Cheng Chou
Nature Materials
Isotope effect on the optical phonons of YBa2Cu4O8 studied by far-infrared ellipsometry and Raman scattering
A. Trajnerowicz, A. Golnik, C. Bernhard, L. Machtoub, C. Ulrich, J.L. Tallon & M. Cardona
Phys. Rev. B
Interpretation of in-plane infrared response of high-Tc cuprate superconductors involving spin fluctuations using quasiparticle spectral functions
P. Casek, C. Bernhard, J. Humlicek & D. Munzar
Phys. Rev. B
Nickel Impurity-Induced Enhancement of the Pseudogap of Cuprate High-Tc Superconductors
A.V. Pimenov, A.V. Boris, Li Yu, V. Hinkov, Th. Wolf, J.L. Tallon, B. Keimer & C. Bernhard
Phys. Rev. Lett.
In-Plane Spectral Weight Shift of Charge Carriers in YBa2Cu3O6.9
A.V. Boris, N.N. Kovaleva, O.V. Dolgov, T. Holden, C.T. Lin, B. Keimer & C. Bernhard