Gaussian fluctuations in a cuprate superconductor

It is still unclear why the transition temperature Tc of cuprate superconductors falls with underdoping. The doping dependence of the critical magnetic field Hc2 is directly relevant to this question, but different estimates of Hc2 are in sharp contradiction. We resolve this contradiction by tracking the characteristic field scale of superconducting fluctuations as a function of doping, via measurements of the Nernst effect in La1.8−xEu0.2SrxCuO4. Hc2 is found to fall with underdoping, with a minimum where stripe order is strong. The same non-monotonic behaviour is observed in the archetypal cuprate superconductor YBa2Cu3Oy. We conclude that competing states such as stripe order weaken superconductivity and cause both Hc2 and Tc to fall as cuprates become underdoped.

Broken rotational symmetry in the pseudogap phase of a high-Tc superconductor

Johan Juul Chang

The nature of the pseudogap phase is a central problem in the effort to understand the high-transition-temperature (high-Tc) copper oxide superconductors. A fundamental question is what symmetries are broken when the pseudogap phase sets in, which occurs when the temperature decreases below a value T*. There is evidence from measurements of both polarized neutron diffraction and the polar Kerr effect that time-reversal symmetry is broken, but at temperatures that differ significantly from one another. Broken rotational symmetry was detected from both resistivity measurements and inelastic neutron scattering at low doping, and from scanning tunnelling spectroscopy at low temperature, but showed no clear relation to T*. Here we report the observation of a large in-plane anisotropy of the Nernst effect in YBa 2Cu3 Oy that sets in precisely at T* throughout the doping phase diagram. We show that the CuO chains of the orthorhombic lattice are not responsible for this anisotropy, which is therefore an intrinsic property of the CuO2 planes. We conclude that the pseudogap phase is an electronic state that strongly breaks four-fold rotational symmetry. This narrows the range of possible states considerably, pointing to stripe or nematic order. © 2010 Macmillan Publishers Limited. All rights reserved.

2010

Photoemission studies of thin films grown by pulsed laser deposition

Mike Abrecht

In this thesis I report on the unique novel possibilities offered by combining pulsed laser deposition (PLD) film growth of high temperature superconductors with angle integrated and angle resolved photoemission spectroscopy (PES and ARPES). The originality of the procedure rests on the in situ preparation and transfer of samples, hence by-passing the usual cumbersome cleaving or scraping practices to obtain high quality surfaces suitable for photoemission studies. Photoemission spectroscopy is one of the major experimental techniques to analyze the electronic structure that holds the key to understanding the properties of the high Tc cuprates (copper oxides) for which a complete microscopic theory still needs to materialize. In chapter 1, I introduce these complex copper oxide materials. The vast majority of ARPES measurements on the cuprate superconductors have so far been limited to the easily cleavable Bi2Sr2CaCu2O8+x (BSCCO-2212) single crystals. In chapter 2, I present the principle of an ARPES measurement, and show as an example (obtained in the "traditional way" of cleaving single crystals) a crossover in the electronic response at a special temperature T+>Tc in overdoped samples. In chapter 3, our unique laser ablation set-up, built on site at the Wisconsin Synchrotron Radiation Center (SRC), is introduced. This is followed by reports on film characterization such as transport, crystal and surface properties which enables the optimization of growth parameters. From here on to the end of the thesis, we show that instead of being a competing technique to cleaving samples, in situ growth of films appears to be a complementary one. In particular, our results show that preparing cuprates which contain oxygen chains, like Re-Ba2Cu3O6+x (RBCO-123; R=Y, Nd), for the surface sensitive ARPES measurements is very difficult due to a material intrinsic loss of oxygen near the surface. This, in turn, curbs the photoemission signal near the chemical potential due to the loss of superficial conductivity (chapter 3). Among the cuprates studied, La2-xSrxCuO4 (LSCO) and Bi2Sr2CaCu2O8+x films, however, both exhibit a sharp Fermi step in angle integrated photoemission spectra, indicating a conducting behavior at the surface. Moreover, thanks to using film samples, we were able to study ultra thin cuprates, down to about 1 unit cell (UC), and we were able to induce epitaxial strain in the samples which can remarkably enhance superconductivity. In chapter 4, combining core level, angle integrated valence band photoemission, and x-ray diffraction, we demonstrate that a structural phase transition occurs in the early growth of BSCCO-2212 films on SrTiO3 (STO) substrates, transforming the precursor Bi6O7 natural reactive buffer layer into the BSCCO-2212 structure. This is qualitatively different from the initial growth of YBCO-123 characterized by nucleation of well separated islands. Chapter 5 then presents the main success of our method: the first ever ARPES data on the low energy electronic structure of any high Tc superconductor under strain. In particular, we confirm that the Fermi surface (FS) of LSCO evolves with doping, but changes even more significantly with strain. More specifically, the FS becomes electron-like for in-plane compressed samples (for all doping values x studied, 0.1

EPFL2003

Normal state transport properties of novel superconductors

Balazs Sipos

The discovery of high temperature superconductivity in the cuprates in 1986 has boosted the research in strongly correlated materials. One strong motivation was and stays the understanding the high-Tc phenomenon with the hope that one can ultimately engineer new materials with even higher Tc. Besides the in-depth investigation of cuprates, there is a strong tendency in the solid state community to find new superconductors, which by themselves are interesting for applications, or by their properties they can contribute to the understanding of the high-Tc phenomenon. The program of my doctoral thesis was three-fold: i) to address one important issue in the cuprate superconductors, that of the role of homogeneity in the underdoped part of the phase diagram; ii) what is the effect of disorder in MgB2 superconductor, which has high potentials for applications; iii) to discover new superconductors in the family of transition metal dichalcogenides. All these materials are in some sense unconventional superconductors. The cuprates by their high Tc and the symmetry of the order parameter, MgB2 by its two-band superconductivity and Tc of 39 K, and the dichalcogenides by the appearance of superconductivity on the background of competing interactions. Measurements of transport properties, such as resistivity and thermoelectric power, were used to get insight in the behavior of these materials. Besides temperature as variable, I applied high pressure, extreme magnetic fields and controlled disorder introduced by fast electron irradiation. In the first part I present the pressure dependent study of two members of the transition metal dichalcogenides having 1T structure, 1T-TiSe2 and 1T-TaS2, where superconductivity was never observed in a pristine sample. 1T-TiSe2 has a CDW phase below 220 K which origin, weather it is driven by an excitonic mechanism or by a Jahn-Teller distortion, is an ongoing question. By applying pressure I showed that the pristine sample is superconducting in the pressure range of 2.0–4.0 GPa. This range remarkably coincides with the short range fluctuating CDW before its disappearance at the upper pressure value. If CDW is due to excitonic interactions than our observations suggest that it can be at the origin of superconductivity, as well. The second dichalcogenide is the 1T-TaS2, where a Mott-insulator phase appears on the top of a commensurate CDW. By applying pressure I was able to melt that Mott-phase, and reveal that the material is superconducting above 2.5 GPa with Tc of 5.9 K. Unexpectedly, superconductivity is born from a nonmetallic phase, and stays remarkably stable up to the highest applied pressure of 24 GPa. In the second part I tried to give my contribution to the field of high-Tc superconductors. I carefully selected few high quality underdoped Bi2Sr2PrxCa1-xCu2O8-δ sample, to address the nature of the low temperature ground state by applying high magnetic field. Although former measurements by other groups showed that at high underdoping, the ground state is an insulator, I found that a sample with as low Tc as 15 K exhibits metallic behavior up to 60 T. Furthermore, I showed that a inhomogeneous distribution of the doping atoms can completely mask the intrinsic normal state of a high-Tc superconductor. In the last part of my thesis I focused on the two-band superconductor MgB2 by studying the scattering between the bands by the means of the Matthiessen's rule. I made a systematic study of the influence of defects created by fast electron irradiation, and found that the the Matthiessen's rule is satisfied for the defect concentration range I induced. I further compare the influence of defects on the critical temperature and the residual resistivity in MgB2 with superconductors with various order parameters, and found that the decrease-rate of Tc in our system is within the range of a response of a superconductor with an s-wave order parameter.