Strain-engineering Mott-insulating La2CuO4

The transition temperature T-c of unconventional superconductivity is often tunable. For a monolayer of FeSe, for example, the sweet spot is uniquely bound to titanium-oxide substrates. By contrast for La2-xSrxCuO4 thin films, such substrates are sub-optimal and the highest T-c is instead obtained using LaSrAlO4. An outstanding challenge is thus to understand the optimal conditions for superconductivity in thin films: which microscopic parameters drive the change in T-c and how can we tune them? Here we demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of La2CuO4 thin films can be enhanced by compressive strain. Our experiments and theoretical calculations establish that the substrate producing the largest T-c under doping also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest optimising the parent Mott state as a strategy for enhancing the superconducting transition temperature in cuprates.

Spectroscopic Studies of the Electronic Structure in Layered Cuprates and Ruthenates

Claudia Giuseppina Fatuzzo

In materials where electrons interact strongly, a number of exotic and exciting phenomena arise. The mechanisms at the base of many of these phenomena remain debated, as strongly correlated electron physics represents one of the biggest challenges for modern condensed matter physics. Transition metal oxides are a class of strongly correlated systems which exhibit a multitude of physical properties, among which unconventional superconductivity, incommensurate charge and spin ordering, partial anisotropic gapping, strangemetallicity, colossal magnetoresistance, multiferroicity, antiferromagneticMott insulation, etc.. These properties are often observed in close vicinity, when pressure, temperature or chemical composition are varied. As a consequence, it is challenging to disentangle not only their origin, but also their effects on the elementary excitations of the system. In this thesis, high quality single crystals of copper and ruthenium based transition metal oxides (cuprate and ruthenate compounds) were investigated with synchrotron-based spectroscopic techniques. Our angle resolved photoemission spectroscopy (ARPES) studies focused on the normal state of cuprates, at hole content varying from under- to over-doping. First, the low energy single particle excitations in overdoped uprates were verified to fulfill the mathematical conditions for Landau Fermi quasiparticles. In a second study, the evolution of the spectral gap was followed as function of doping and temperature, across the charge order, pseudogap and strangcuprates, ruthenates, strong correlations, RIXS, ARPES, XAS, pseudogap, strange metal, spin orbit coupling, crystal field.e metal phases of a cuprate compound, La1.6¡xNd0.4SrxCuO4. This systematic study allowed the identification of an optimal doping regime for the investigation of the pseudogap physics in cuprates. The orbital structure of single layer ruthenates was explored combining X-ray absorption and resonant inelastic X-ray spectroscopies (XAS and RIXS). Since spectroscopies at the L absorption edge of 4d materials are challenged by insufficient energy resolution, we performed our studies at the oxygen K edge, in the soft X-ray energy range. By exploiting the strong orbital hybridization between the oxygen 2p and the ruthenium 4d states, we obtained high resolution experimental access to the 4d electron properties. The results were interpreted through a simple model Hamiltonian, with analytic solutions that provide a consistent description of the low energy features observed.

EPFL2017

Kinks in the angle resolved photoemission and inelastic x-ray spectra of high temperature superconductors

Jeff Graf

Though the high superconducting transition temperature (Tc) is the most interesting technological aspect of high temperature superconductors, the complex way in which the spin, lattice and electronic degrees of freedom interplay, makes them of the highest scientific interest. This is illustrated by their rich phase diagram characterized by a variety of exotic groundstate including; Mott insulating, pseudogap and high temperature superconductivity. One of the most serious barriers that has prevented our understanding of the mechanism of high temperature superconductors thus far is the lack of information on how low energy Landau quasiparticles result from an organization of real particles. This organization, often colloquially referred to as "dressing", is a fundamental general concept in physics that explains a variety of physical phenomena, such as exotic particle formations and phase transitions. The role of the lattice in this dressing is particularly controversial. Phonons are quanta of lattice vibration energy, and play a crucial role in conventional superconductivity. They provide an attractive interaction allowing the electrons to condensate in superconducting Cooper pairs. However, high temperature superconductivity in the cuprates in achieved through hole doping in an antiferromagnetic Mott insulator. In this case, the antiferromagnetic background, the strong coulomb repulsion and the anisotropic superconducting gap all suggest a marginal role of the phonons. In order to assess experimentally the exact role of the phonon, angle resolved photoemission spectroscopy (ARPES) is the weapon of choice given its unique ability to probe the electronic structure of solids in an energy- and momentum-resolved manner. In this thesis, I will use ARPES to characterize the various form of dressing of the quasiparticles in the high temperature superconductor from the Fermi level all the way to the valence band complex. I'll show that when combined with isotope substitution and inelastic x-rays scattering, the strong electron-phonon interaction can be probed in vivid details. This thesis presents three fundamental results. 1) Using the isotope effect, the important and unusual role of the phonon is demonstrated, as well as the strong interplay between the magnetic and phonic degrees of freedom. 2) Using inelastic scattering, the intriguing interplay between the Fermi surface and the bond stretching phonon is exposed. And 3) By exploring the ARPES data up to high energy, two new energy scales at high binding energy as well as the spectral waterfalls phenomenon are revealed. When all these new elements are considered together, they clearly show the need to consider simultaneously the spin, lattice, Fermi surface topology and electronic degrees of freedom. The spectacular progress in ARPES techniques over the past two decades was critical for these results, and I will present in this thesis our current effort in pushing the techniques even further. In particular I'll present my efforts in building a laser based ARPES setup with a hemispherical analyzer and a spin resolved time of flight analyzer.

EPFL2008

Atomic-Layer Engineering of Oxide Superconductors

Ivan Bozovic, Guy Joseph Günter Dubuis, Davor Pavuna

Molecular beam epitaxy technique has enabled synthesis of atomically smooth thin films, multilayers, and superlattices of cuprates and other complex oxides. Such heterostructures show high temperature superconductivity and enable novel experiments that probe the basic physics of this phenomenon. For example, it was established that high temperature superconductivity and anti-ferromagnetic phases separate on Ångström scale, while the pseudo-gap state apparently mixes with high temperature superconductivity over an anomalously large length scale (the “Giant Proximity Effect”). We review some recent experiments on such films and superlattices, including X-ray diffraction, atomic force microscopy, angle-resolved time of flight ion scattering and recoil spectroscopy, transport measurements, high- resolution transmission electron microscopy, resonant X-ray scattering, low-energy muon spin resonance, and ultrafast photo-induced reflection high energy electron diffraction. The results include an unambiguous demonstration of strong coupling of in-plane charge excitations to out-of-plane lattice vibrations, a discovery of interface high temperature superconductivity that occurs in a single CuO2 plane, evidence for local pairs, and establishing tight limits on the temperature range of superconducting fluctuations.