The growth by laser ablation and electronic properties of thin cuprate films

In my thesis work I have concentrated on the growth and the in-depth analysis of high temperature superconducting thin films with the central aim to elucidate their electronic properties, predominantly by in-situ angle resolved photoemission spectroscopy (ARPES). I have used two somewhat complementary approaches and two laser ablation set-ups. The first one, developed previously in Wisconsin, was used mainly for studies of strained La2-xSrxCuO4 (LSCO) with a transfer to the Scienta analyzer via an appropriate suitcase. The second one, at the EPFL, where I have built a new pulsed laser deposition (PLD) system, was used to optimize the growth of Bi2Sr2-xLaxCuO4 (Bi-2201) and study in-situ ARPES. In-situ ARPES is the most direct tool to probe the electronic structure. We performed it at the Synchrotron Radiation Center (SRC, University of Wisconsin), where we used the aforementioned experimental set-up consisting in a dedicated PLD system coupled with the SCIENTA beamline. The sample transfer procedure assures that the surface quality is preserved on the way to the SCIENTA analyzer. There we studied in detail the effect of strain in LSCO thin films. In a previous work the in-plane compressive strain was studied and the main result was that the Fermi surface (FS) topology changed from hole-like to electron-like. The tensile strained films showed completely different results. ARPES analysis show evidence for a 3-dimensional (3D) electronic dispersion relation in contrast to the strictly 2-dimensional (2D) dispersion observed in all other studied LSCO films. In this thesis this result has been confirmed mapping the FS at different photon energies. We found that the strain related to the thickness of the films, is playing an important role in inducing a 3D dispersion. Furthermore, the 3D parameters, evolve according to the level of strain. Moreover, we observe a staircase structure for different photon energies, revealing both the 3D nature of the electronic dispersion and the quantization of the electron wave vector along the direction normal to the film surface. Taking advantage of the wavevector quantization we were able to determine directly the band parameters and map the FS without using the nearly-free-electron approximation (NFEA). Moreover, introducing an effective anisotropic photoelectron effective mass, related to the local structure of the excited band, improves the use of the NFEA for single photon energy measurements. In parallel, I have built an improved PLD system at the EPFL which can be connected to the SCIENTA analyzer, and which enables us to perform in-situ ARPES measurements at any time rather than only during allocated beamtimes at the synchrotron. We also produced our own targets for the laser ablation and all the films were fully characterized at the EPFL performing X-ray diffraction (XRD), resistivity and magnetic measurements. I analyzed in detail the growth mechanism of Bi-2201 and I investigated the presence of random intergrowths. We developed a model to explain the presence of these polytypes and studied their presence as a function of the deposition parameters and the annealing treatment. The model predicts a very particular spatial distribution of defects: a Markovian-like sequence of displacements along the grow direction, as well as a two-component in-plane correlation function, characteristic of self-organized intercalates. We varied the growth conditions in order to study the presence of intergrowths and to produce single-phase samples. Subsequently, we performed in-situ photoemission experiments on thin films of Bi-2201 films free from intergrowths and we analyzed their FS. This method is successful and can be extended to other related oxide films.

Properties of La2-xSrxCuO4 under epitaxial strain

Dominique Cloëtta

The subject of this thesis is the growth and analysis of high temperature superconductor (HTSC) films and the study of their electronic structure and properties. In particular, the effect of epitaxial strain is investigated, predominantly by means of in-situ angle resolved photoemission spectroscopy (ARPES), as well as X-ray diffraction, resistivity and susceptibility measurements. In order to achieve that goal we have developed a unique experimental set-up at the Synchrotron Radiation Center (University of Wisconsin), consisting in a dedicated pulsed laser deposition system, which enables us to grow thin films with excellent surface quality. Our sample transfer procedures assure that this surface quality is not affected on the way to the ARPES analyzer, which is connected to a beamline. We have built a similar film growth system at the EPFL with the aim to connect it to the SCIENTA analyzer at the IPN and to an analyzer on a beamline at the SLS in Villigen. For the first time we were able to perform ARPES measurements on in-situ grown films of HTSC. Previously to our work all the ARPES measurements were carried out on cleaved or scraped samples, predominantly single crystals of Bi2Sr2CaCu2O8 compounds. Thin films offer the possibility to study the effect of epitaxial strain induced through lattice mismatch between the film and its substrate. Compressive strain in the CuO2 plane has been known to enhance the critical temperature (TC) up to 50%, therefore we expected to see a signature of strain in the electronic dispersion. The Fermi surface of unstrained La2-xSrxCuO4 evolves with doping as reported for scraped single crystals, but also changes strongly with strain. Our studies show, that the in-plane compressive strain changes the Fermi surface topology from hole-like to electron-like. It enhances band dispersion and the Fermi level is crossed before the Brillouin zone boundary, in sharp contrast to the "usual" saddle point remaining ~30 meV below the Fermi level measured along the direction of the Cu-O bonds on unstrained samples. The associated reduction of the density of states near the Fermi level does not diminish the superconductivity; TC is enhanced in all our compressively strained samples. This result is rather surprising since such a reduction of the density of states, according to many mean field models, does not favor the increase of TC measured in our films. By comparing the ARPES measurements on our films with measurements on bulk crystals, we could also show that the results from our relaxed films are equivalent to those on bulk crystals, therefore excluding an explanation through finite size effects other than strain. Our latest results on films under huge tensile strain (1% change in c-axis) are significantly different: ARPES shows evidence for a 3-dimensional dispersion, in contrast with the strictly 2-dimensional dispersion observed on compressively strained films. Already the conduction band of relaxed La2-xSrxCuO4 is atypical: It has considerable apical-oxygen pz and Cu3dz2-r2 out-of-plane character, while for the rest of the cuprate HTSC, those orbitals hybridize far less with the conduction band. We relate the observed z-axis dispersion with the significant displacement of the apical-oxygen towards the CuO2 plane, induced by the epitaxial strain. Resistivity measurements show an insulating behavior of films under extreme tensile strain and no TC. Films with weaker tensile strain still exhibit superconductivity, but diminished as compared to the relaxed films. In summary, while the in-plane compressive strain tends to push the apical oxygen far away from the CuO2 plane, enhances the 2-dimensional character of the dispersion and increases TC, the tensile strain seems to act exactly in the opposite direction and the resulting dispersion is 3-dimensional. We have established the shape of the Fermi surface for both cases, yet further experiments are required to clarify fine details.

EPFL2005

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

A Theoretical Investigation of Topological Insulator Nanostructures

Naunidh Singh Virk

The topology of the electron wavefunctions in certain band insulators can give rise to novel topological phases. Materials harbouring such topological phases are termed topological insulators (TI). A gapped bulk electronic spectrum, described by a topological invariant, and gapless boundary modes, tend to characterize the non-trivial topology. This work describes a theoretical investigation of the Z2 topological insulator phase in Bi2Se3 and Bi2Te3, and the topological crystalline insulator (TCI) phase in SnTe, subject to nanoscale confinement. Specifically, it details the electronic structure, and properties of low-dimensional nanostructures derived from the bulk topological phase. For the bismuth chalcogenides, a first principles methodology is applied to compute the energetics of high-index surfaces, followed by an analysis of the electronic properties of corresponding topological surface state charge carriers. Our calculations find several stable terminations of high-index surfaces, which can be realized at different values of the chemical potential of one of constituent elements. For the uniquely defined stoichiometric termination, the Dirac fermion surface states exhibit a strong anisotropy, with a clear dependence of Fermi velocities and spin polarization on the surface orientation. Non-stoichiometric surfaces undergo self-doping effects, which results in the presence of topologically trivial mid-gap states. These findings guide the construction of Bi2Se3 nanostructures of a nanowire (NW) and nanoribbon (NR) morphology. A tight-binding formalism is utilised to study, firstly, the impact of finite-size effects on the electronic spectrum of each nanostructure. Secondly, the effects of confinement on the topological properties of two-dimensional (2D) Dirac fermion surface states. Quantum confinement around each nanostructure perimeter entails the formation of a series of discrete one-dimensional (1D) sub-bands in the bulk gap. An analysis of how the band gap varies as a function of nanostructure dimensions finds that the dependence is highly sensitive to nanostructure morphology. We reveal a clear correspondence between the spin helicity of the 2D surface Dirac cone and the spin properties of the 1D sub-bands. This is exemplified in the real space spin textures of each nanostructure. For the NW morphology, this correspondence gives rise to an energy dependent spin polarization density. Whereas for the NR morphology the presence of two separate surface types results in a more complex relationship. Finally, via a similar tight-binding formalism, we establish how the crystal-symmetry-dependent topological phases of SnTe (001) thin films are exhibited in lower dimensional nanowires. SnTe (001) thin films, defined by either mirror or glide symmetry, realise distinct 2D TCI phases. As the band dispersion of NWs are characterised by which of these symmetry classes they belong to, we subsequently connect the distinctive NW surface states to the respective parent 2D TCI phase. Lastly, we show that the robust topological protection offered by the mirror symmetry protected 2D TCI phase is manifested in robust surface states of NWs of equivalent symmetry.

EPFL2016