ARPES Studies of Cuprate and Pnictide Superconductors

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.

The Interplay Between Structural and Electronic Degrees of Freedom in Superconductors

Mathieu Julien Gino Cottet

More than one hundred years after the discovery of superconductivity in Leiden, the intriguing physics of several unconventional classes of superconductors continue to fascinate and challenge scientists from all over the world. The majority of these compounds belongs to a major class of materials known as strongly correlated systems that exhibit interesting and diverse physical properties, like the high-temperature superconductivity, the Mott-insulator transition, the colossal magnetoresistance or the structurally-driven electronic phase transitions. Rigorous and deep investigations of the electronic properties, as well as their tunability, should lead to new attractive commercial applications. With the use of time-resolved experiments or bulk sensitive techniques, which represent the main subject of this thesis, the electronic and magnetic properties of strongly correlated type-II superconductors and high-temperature copper-oxide superconductors are accessible. These techniques take advantages of the photon tunability in the case of optical pump-probe by allowing one to record the transient response of the material over the whole visible spectrum, or in the case of X-ray diffraction, the tunability of the synchrotron radiation thanks to the slicing operating at the SLS (Swiss Light Source) in Villigen, Switzerland. Cryo-LTEM allows one to work across the (H ,T ) phase diagram of type-II superconductors, thus enabling the study of the coexistence between normal and superconducting state by careful regulation of the applied magnetic field near the specimen. The Chapter 1 (1) is a general introduction dedicated to the historical evolution of our understanding of superconductivity, according to the theories and general concepts used and developed is this thesis. Chapter 2 (3) is dedicated to the experimental techniques. A careful description of the different setups, as well as a theoretical development needed for the calculations is presented. In Chapter 3 (4), the investigation of magnesium diboride superconductor by means of cryo-LTEM is presented. By observing the field-dependent vortex lattices in this system and performing calculations using the theoretical frameworks developed by Ginzburg-Landau and Abrikosov, I found that this compound belongs to the type-II family [1], and not to the novel type 1.5 class of superconductor. In Chapter 4 (5), I present the investigation related the charge density waves compound Lu5Ir4Si10. The use of ultrafast time-resolved optical pump-probe experiments, coupled with ab-anitio theoretical calculations of the electron-phonon coupling and the electronic structure, permits to shine new light on the nature of this transition which we identified as a structurally-driven Peierls transition [2]. The Chapter 5 (6) is dedicated to the investigation of the high-temperature copper-oxide superconductor, La2−x Srx CuO4, for different doping levels; the experiment was carrying out at the SLS on the Micro-XAS beamline, By using a transient temperature model and comparing our experimental results with temperature-dependent density of states calculations, we report on the electronic temperature dependence of the electron-phonon coupling constant (submitted to Phys. Rev. B).

EPFL2013

Spectroscopic Studies on Semiconducting Interfaces with Giant Spin Splitting

Emmanouil Frantzeskakis

The application of an external magnetic field can lift the spin degeneracy of electronic states through its interaction with the electronic magnetic moment. A closely-related phenomenon is the Rashba-Bychkov (RB) effect where symmetry breaking at surfaces or interfaces gives rise to an electric field which is in turn seen as an effective magnetic field in the electrons' rest frame. The resulting k-dependent energy splitting of spin-polarized electronic states has been observed on various metal surfaces but the effect is much larger in artificially-grown surface alloys; such as the BiAg2 grown at the surface of Ag(111). The spin splitting magnitude observed in these systems might be very useful in spintronics applications since it could decrease the spin precession time in a spin transistor and distinguish between the extrinsic and intrinsic spin Hall effects. Nevertheless, their metallic character poses serious obstacles in the exploitation of the RB effect due to the presence of spin-degenerate electronic states at the Fermi level which would dominate transport experiments. We have used angle-resolved photoelectron spectroscopy (ARPES) to explore the RB effects on various artificially grown structures, formed on semiconducting substrates. The interplay of quantum confinement and giant RB splitting on a trilayer Si(111)-Ag-BiAg2 system reveals the formation of a complex spin-dependent structure, which can be externally tuned by varying the Ag layer thickness. This provides a means to tailor the electronic structure and spin polarization near the Fermi level, with potential applications on Si-compatible spintronic devices. Moreover, we have discovered a giant spin splitting in a true semiconducting system, namely the Si(111)-Bi trimer phase. The size of the RB parameters is comparable to those of metallic surface alloys. Using theoretical models we have identified the peculiar band topology as the origin of the giant spin splitting on the Bi/Si(111) system. All our findings are supported by relativistic first-principles calculations. Finally, a chapter of this thesis manuscript is devoted to the description of phenomeno-logical theoretical simulation, which can capture the experimental results related to the RB effect on low-dimensional systems. A parallel experimental project is discussed in a separate chapter. It has been focused on the band topology of the novel p(2 × 2) reconstruction of the Pt(111)-Ag-Bi trilayer. We investigated the symmetry properties of the interface states by varying the amount of Ag. ARPES results present the electronic signature of a strain-related structural transition.

EPFL2010

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.