SRC Users' Meeting - Synchrotron Radiation Center - University of ...

Epitaxial strain in La 2-x Sr x CuO 4 : Fermi Surface change and T c enhancement.
M. Abrecht 1* , D. Ariosa 1 , D. Cloëtta 1 , G. Margaritondo 1 , M. Onellion 2 , and D. Pavuna 1 .
1 IPMC, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
2 Department of Physics, University of Wisconsin-Madison, Madison, WI-53706, USA
Extended abstract
Using our dedicated Pulsed Laser Deposition (PLD) system and in-situ Angle Resolved
Photoemission Spectroscopy (ARPES) as opposed to the traditional cleaving or scraping of
samples in UHV, we measured the band dispersion of epitaxially strained La 2-x Sr x CuO 4
(0.1x0.2) thin films to investigate the links between strain induced increase in
superconducting transition temperature (T c ) and low energy electronic features. Such
measurements are the first direct probing of the electronic structure of any high T c
superconductor under epitaxial strain, and were made possible only thanks to our new approach
of sample growth at the Synchrotron Radiation Center.
We emphasize the importance of such measurements since strain has been known to increase
substantially the superconducting properties of cuprates: the critical temperature of La 2-x Sr x CuO 4
(LSCO) under epitaxial strain for example can reach 50K whereas for a non strained sample, the
T c value does not exceed 39K whatever the Sr (doping) content [Locquet].
Our APRES results on in-plane compressed films show a band dispersing along the k x reciprocal
space direction and crossing the Fermi level before the Brillouin zone boundary (figure 1), in
sharp contrast to a flat band staying well below the Fermi level for unstrained samples with equal
doping [Ino]. Our results were confirmed on various sampes with different doping values x
[Abrecht] using the Scienta analyzers of the SRC. Very surprisingly, the critical temperatures of
the strained samples are enhanced by more than 6 degrees despite a tremendous reduction of the
density of states due to the disappearance of the saddle point near the Fermi level (figure 2).
Based on a simple 2D tight binding approximation, we were able to predict the Fermi Surface
evolution with doping and strain (figure 3), a simulation that proved accurate for both Ino’s bulk
data [Ino] and our strained and unstrained film samples. Implications of our findings on possible
superconducting mechanisms in the cuprates include that the van-Hove singularity is not
essential to obtain high critical temperatures, and that T c =T c (x, ), where is a parameter related
to strain that should be included in the Superconducting phase diagram.
* Present address: National Institute of Standards and Technology, Magnetic Technology Division, 325 Broadway,
Boulder CO-80305.