Angle-Resolved Photoemission Spectroscopy of Tetragonal CuO: Evidence for Intralayer Coupling Between Cupratelike Sublattices

We investigate by angle-resolved photoemission the electronic structure of in situ grown tetragonal CuO, a synthetic quasi-two-dimensional edge-sharing cuprate. We show that, in spite of the very different nature of the copper oxide layers, with twice as many Cu in the CuO layers of tetragonal CuO as compared to the CuO2 layers of the high-T-c cuprates, the low-energy electronic excitations are surprisingly similar, with a Zhang-Rice singlet dispersing on weakly coupled cupratelike sublattices. This system should thus be considered as a member of the high-T-c cuprate family, with, however, interesting differences due to the intralayer coupling between the cupratelike sublattices.

ARPES Studies of Cuprate and Pnictide Superconductors

Elia Razzoli

Since the discovery of cuprate superconductors in 1986, the subject of high temperature superconductivity has triggered a lot of scientific activity. Despite enormous efforts, the origin of high-temperature superconductivity is still the subject of intense debate, revived recently by the discovery of Fe-base superconducting pnictides. Angle-resolved photoemission spectroscopy (ARPES) has been a fundamental tool in the study of the electronic band structure of cuprate and Fe-based superconductors and of the connection between the electronic degrees of freedom and the origin of unconventional superconductivity in these systems. This thesis presents an experimental study of pnictide LaRu2P2 and of cuprate La2−xSrxCuO4 (LSCO) by means of ARPES carried out at Surface/Interface spectroscopy (SIS) beamline and at ADRESS soft x-ray beamline of Swiss Light Source (SLS). In the first part of the thesis I present soft x-ray (SX) ARPES measurments of superconducting LaRu2P2 (Tc = 4K) which is isostructural with “122” Fe-pnictides superconductors. The results show that both the Fermi surface topology and the bandwidth renormalization of LaRu2P2 are very different from their high-temperature superconducting Fe-pnictide counterparts. We also demonstrate that, compared to ultraviolet (UV) ARPES, the increased photoelectron mean free path of SX-ARPES is essential in determining of the bulk electronic structure of LaRu2 P2 . In the second part of the thesis I present a comprehensive ARPES study of the electronic structure of La2−xSrxCuO4 in a wide doping range (x), from non-superconducting samples (x = 0.02) to highly doped superconducting ones (x = 0.22). The measurements show that, while the main Fermi surface changes monotonically with adding more charge carriers to the system, two folded bands that resemble a copy of the main band but shifted by (π, 0) or (π, π) have an unusual dependence on doping. Third part of the thesis deals with the ARPES study of the momentum and temperature dependence of the energy gap in the electronic excitation spectra of highly underdoped superconducting (x = 0.08) and non-superconducting (x = 0.02 − 0.03) LSCO. This study reveals that in this compound the low-energy electronic excitation spectra are gapped along the entire underlying Fermi surface at low temperatures and that the gap structure evolves to a dx2−y2 form as increasing temperature or doping, consistent with the vast majority of ARPES studies of cuprates.

EPFL2012

Role of a higher-dimensional interaction in stabilizing charge density waves in quasi-one-dimensional NbSe3 revealed by angle-resolved photoemission spectroscopy

Helmuth Berger, Christophe Berthod

We revisit charge density wave (CDW) behavior in the archetypal quasi-one-dimensional (quasi-1D) material NbSe3 by high-resolution angle-resolved photoemission spectroscopy measurements utilizing a microfocused laser with a photon energy of 6.3 eV. We present a detailed view of the electronic structure of this complex multiband system and unambiguously resolve CDW gaps at the Fermi level (E-F). By employing a tight-binding model, we argue that these gaps are the result of interband coupling between electronic states that reside predominantly on distinct 1D chains within the material. Two such localized states are found to couple to an electronic state that extends across multiple 1D chains, highlighting the importance of a higher-dimensional interaction in stabilizing the CDW ordering in this material. In addition, the temperature evolution of intrachain gaps caused by the CDW periodicities far below E-F deviate from the behavior expected for a Peierls-type mechanism driven by nesting; the upper and lower bands of the renormalized CDW dispersions maintain a fixed peak-to-peak distance while the gaps are gradually removed at higher temperatures. This points toward a gradual loss of long-range phase coherence as the dominant effect in reducing the CDW order parameter, which may correspond to the loss of coherence between the coupled chains. Furthermore, one of the gaps is observed above the bulk and surface CDW transition temperatures, implying the persistence of short-range incoherent CDW order. The influence of such higher-dimensional interactions likely plays an important role in a range of low-dimensional systems.

2020

Three-dimensional dispersion induced by extreme tensile strain in La_(2−x)Sr_xCuO_4 films

Mike Abrecht, Claudia Cancellieri, Dominique Cloëtta, Slobodan Mitrovic, Davor Pavuna

The electronic band structure probed by angle-resolved photoemission spectroscopy on thin epitaxial La_(2−x)Sr_(x)CuO_(4) films under extreme tensile strain shows anomalous features compatible with c-axis dispersion. This result is in striking contrast with the usual quasi-two-dimensional (2D) dispersion observed up to now in most superconducting cuprates, including relaxed and compressively strained La_(2−x)Sr_(x)CuO_(4) films grown under the same conditions. The data were analyzed using a 3D tight-binding dispersion for a body-centeredtetragonal lattice. We relate the enhancement of the c-axis dispersion to the significant displacement of the apical oxygen induced by epitaxial strain.

2006

ARPES Studies of Cuprate and Pnictide Superconductors

Elia Razzoli

EPFL2012