Magnetic, orbital and charge fluctuations in layered cuprates studied by resonant soft X-ray scattering

This thesis presents the results achieved on cuprates with resonant soft x-ray scattering (RXS) during my activity in the group of Prof. G. Ghiringhelli and Prof. L. Braicovich of the Physics Department of Politecnico di Milano (Italy). The group has a well-established experience in synchrotron-based spectroscopies for the study of magnetic and electronic properties of transition-elements and rare earth compounds. In the last years they focused their activity especially on resonant inelastic x-ray scattering (RIXS), contributing substantially to the development of the technique, both from the point of view of science and instrumentation. They designed and built two high resolution spectrometers dedicated to RIXS: AXES (Advanced X-ray Emission Spectrometer) and SAXES (Super-AXES). AXES is working since 1995 at the beamline ID08 of the European Synchrotron Radiation Facility in Grenoble, France. SAXES, which is the evolution of AXES, has been installed in 2006 at the ADRESS beamline at the Swiss Light Source in Villigen, Switzerland. It is working since July 2007 and by now it holds the world record of resolving power with a combined resolution of 130 meV at Cu L3 edge (930 eV). In this thesis I present some of the results obtained with these spectrometers on insulating and superconducting layered cuprates using Cu L3 edge RIXS. This technique is shown to be the optimal probe to study magnetic, orbital and charge fluctuations in the CuO2 planes of these compounds, allowing energy and momentum-resolved measurements and adding crucial pieces to the puzzle of high temperature superconducitivity. CuO2 planes are the common feature of all layered cuprates and the set where high-Tc superconductivity emerges: these planes consist of Cu2+ ions alternated to O2− ions and they are separated each other by “blocking layers”. Although it has one hole per Cu site, each CuO2 plane is originally insulating, due to the large electron correlation typical of transition-element oxides. The Cu2+ ions (3d9 configuration) have one unpaired spin-1/2 per site and they are coupled via superexchange interaction J, i.e. the exchange mediated by oxygen, so to produce a bidimensional antiferromagnetic (AF) lattice. The hybridization with the oxygen ions is so strong that the superexchange is exceptionally high in cuprates (J > 100 meV), allowing the study of the associated magnetic excitations without the need of a few meV resolution. When the insulating parent compounds are doped, the additional degrees of freedom from dopant charges further complicate the electronic situation. A critical doping is required to destroy the various long range orders and superconductivity emerges when charges coming from the blocking layers dope the CuO2 sheets in a number that alters the situation and triggers the transition. Despite more than 25 years of studies, the origin of the superconducting state in cuprates is still unclear and remains the subject of intense scrutiny. In particular one of the central unanswered questions concerns the nature of the normal-state spin fluctuations that may be responsible for the pairing. Because of technical limitations, the experimental investigation of doped cuprates has been until now largely focused on low-energy excitations in a small range of momentum space. In this thesis we used high resolution RIXS to show that a large family of high-Tc superconductors (HTS), i.e. (Y,Nd)Ba2Cu3O6+x (RBCO), exhibits high-energy damped spin excitations (paramagnons) over a wide range of doping, with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. The comprehensive experimental description that comes out from our systematic data acquisition enables quantitative tests of magnetic Cooper pairing models and supports the paramagnons as strong candidates to cover the role of glue for the Cooper’s pairs. Subsequently we have exploited the capability of RIXS to work very well on thin films, in order to study both insulating and superconducting cuprate-based heterostructures. Recently the technical progress in epitaxial growth has lead to the discovery of a panoply of exceptional magnetic and transport properties in artificial heterostructures of 3d transition metal oxides in general. Electronic, lattice and orbital reconstruction occurring at the interfaces can in fact influence the charge transfer between the oxides, while the modified dimensionality can affect the magnetic properties of the oxides. Among these heterostructures cuprate-based superlattices (SLs) are particularly interesting since they can be considered as new, artificial HTS, offering the opportunity of freely choosing the two building blocks i.e., the superconducting CuO2 planes and the charge reservoir blocking layers. We have carried out Cu L3 RIXS measurements on both insulating and superconducting (CaCuO2)m/(SrTiO3)n SLs and compared the results with those on a 14 nm thick CaCuO2 film, in order to understand what happens to magnons when the CuO2 planes are at the interfaces of a SL and if a (para)magnon-mediated superconductivity could still be possible. In all insulating samples spin excitations are in the form of dispersing magnons and in the SLs magnons have similar spectral intensity but reduced dynamics with respect to pure CaCuO2. This is the demonstration that the AF order is preserved in the insulating SLs, down to very small cuprate layer thickness and despite the chemical and structural alterations at the interfaces. On the other hand the superconducting SLs exhibit dispersing paramagnons, similarly to the case of superconducting RBCO. Moreover the orbital excitations, visible in RIXS spectra together with magnons and due to the ligand field felt by Cu2+ ions, have revealed a pyramidal coordination of copper atoms at the CaCuO2/SrTiO3 interfaces. These findings open the way to the production of new, artificial HTS based on cuprate/noncuprate SLs where the charge reservoir layer is constituted by the interface itself. Any successful theory for HTS should require a detailed understanding not only of the spin but also of the charge correlations in the normal state from which superconductivity emerges. Therefore we have studied charge fluctuations in the CuO2 planes by means of RXS. Despite intense efforts, to the present date only two clear ordering phenomena have been reported for correlations in the copper oxide sheets of cuprates: the above cited uniform AF in undoped cuprates and a uniaxially modulated AF, combined with charge order, in the so-called “214” family [with chemical composition La2−x−y(Sr,Ba)x(Nd,Eu)yCuO4]. The latter is known as “stripe order”, with a commensurate charge modulation with a period 4 lattice units, which greatly reduces the superconducting transition temperature of 214 materials at a doping level p ≈ 1/8 per planar Cu atom. Incommensurate spin and charge fluctuations in 214 materials with p ≠ 1/8 have been interpreted as evidence of fluctuating stripes. These findings have generated a long-standing debate around the questions of whether stripe order is a generic feature of the copper oxides and if stripe fluctuations are essential for superconductivity. We have used RXS to assess this issue and identify two-dimensional charge fluctuations with an incommensurate periodicity of 3.2 lattice units in the CuO2 planes of the superconductors RBCO, with hole concentrations p of 0.09 to 0.13 per planar Cu ion. The intensity and correlation length of the signal increase strongly upon cooling towards Tc, while further cooling below Tc abruptly reverses the divergence of the charge correlations. In combination with earlier observations, these data indicate an incipient charge density wave (CDW) instability that competes with superconductivity and, for the first time, we have the evidence that the anomalously low Tc found in underdoped cuprates is due to CDW, and not other phenomena.

Questa tesi è basata sui miei 3 anni di attività nel gruppo dei Prof. L. Braicovich e G. Ghiringhelli durante i quali ho partecipato attivamente a numerosi esperimenti di Resonant Inelastic X-ray Scattering (RIXS) e X-ray Absorption Spectroscopy (XAS), presso la beamline ID08 dell’European Synchrotron Radiation Facility (ESRF) di Grenoble (FR), dove mi sono anche trasferito stabilmente per un anno, e la beamline ADRESS dello Swiss Light Source (SLS) di Villigen (CH), per un totale di oltre 30 settimane di esperimenti. Dopo una breve introduzione sulla tecnica spettroscopica e la strumentazione utilizzata, la tesi presenta i principali risultati ottenuti con il RIXS, lavorando prevalentemente allo spigolo L3 del Rame. Abbiamo studiato una vasta gamma di sistemi cuprati con diversi drogaggi, sia in forma di film sottili che di cristalli, in particolare Nd1.2Ba1.8Cu3O6+x (NBCO) e YBa2Cu3O6+x (YBCO). In questi sistemi abbiamo dimostrato la permanenza nei piani CuO2 di un’eccitazione disperdente, figlia del magnone visibile nei parent compounds isolanti, anche nel caso di campioni superconduttori fortemente drogati. Tale eccitazione “paramagnonica” costituisce una fortissima prova a sostegno dell’interpretazione della superconduttività ad alta temperatura che assegna all’ordine magnetico un ruolo dominante e propone i magnoni come collanti delle coppie di Cooper nei cuprati. Seguendo lo stesso percorso logico abbiamo misurato le fluttuazioni magnetiche in alcuni superreticoli (SR) artificiali a base di SrTiO3 e CaCuO2, un cuprato dalla struttura cristallina molto semplice che lo rende un sistema modello per lo studio dei piani CuO2. Questi SR, benché composti di materiali isolanti, si sono recentemente dimostrati superconduttori se cresciuti in alta pressione di ossigeno. Anche in questo caso abbiamo riscontrato la presenza di eccitazioni magnetiche di energia tale da poter sostenere la superconduttività mentre le informazioni sul campo cristallino fornite dalle eccitazioni dd ci hanno permesso di comprendere come varia la struttura elettronica del rame alle interfacce di questi SR e di fare alcune interessanti ipotesi sulla struttura di quest’ultime e sul loro ruolo nel meccanismo di drogaggio. Infine abbiamo studiato le fluttuazioni di carica nei piani CuO2 di cuprati drogati, raccogliendo una ricca casistica relativa alla presenza di una risonanza quasi-elastica al variare del momento trasferito negli spettri RIXS. Tale risonanza è connessa con un ordinamento dei piani CuO2 in cui strisce di carica si alternano a strisce in cui è preservato l’ordine antiferromagnetico. Abbiamo documentato questo effetto nei sistemi (Y,N)BCO ottenendo un quadro completo della fenomenologia: in particolare al diminuire della temperatura la risonanza cresce, è massima alla temperatura critica e quindi decresce, confermando la presenza di un ordine di carica di tipo charge density wave che compete con la superconduttività e mai osservato prima in questi materiali.