Andrea Damascelli


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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Spin-orbit coupling in iridates (2020)

Transition-metal oxides (TMOs) are a widely studied class of materials with fascinating electronic properties and a great potential for applications. Sr₂IrO₄ is such a TMO, with a partially filled 5d t₂g shell. Given the reduced Coulomb interactions in these extended 5d orbitals, the insulating state in Sr₂IrO₄ is quite unexpected. To explain this state, it has been proposed that SOC entangles the t₂g states into a filled jeff = 3/2 state and a half-filled jeff = 1/2 state, in which a smaller Coulomb interaction can open a gap. This new scheme extends filling and bandwidth, the canonical control parameters for metal-insulator transitions, to the relativistic domain. Naturally the question arises whether in this case, SOC can in fact drive such a transition. In order to address this question, we have studied the behaviour of Sr₂IrO₄ when substituting Ir for Ru or Rh. Both of these elements change the electronic structure and drive the system into a metallic state. A careful analysis of filling, bandwidth, and SOC, demonstrates that only SOC can satisfactorily explain the transition. This establishes the importance of SOC in the description of metal-insulator transitions and stabilizing the insulating state in Sr₂IrO₄.It has furthermore been proposed that the jeff = 1/2 model in Sr₂IrO₄ is an analogue to the superconducting cuprates, realizing a two-dimensional pseudo-spin 1/2 model. We test this directly by measuring the spin-orbital entanglement using circularly polarized spin-ARPES. Our results indicate that there is a drastic change in the spin-orbital entanglement throughout the Brillouin zone, implying that Sr₂IrO₄ can not simply be described as a pseudo-spin 1/2 insulator, casting doubt on direct comparisons to the cuprate superconductors. We thus find that the insulating ground state in Sr₂IrO₄ is mediated by SOC, however, SOC is not strong enough to fully disentangle the jeff = 1/2 state, requiring that Sr₂IrO₄ is described as a multi-orbital relativistic Mott insulator.

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Designing quantum phases in monolayer graphene (2019)

The physics of quantum materials is at the heart of current condensed matter research. The interactions in these materials between electrons themselves, with other excitations, or external fields can lead to a number of macroscopic quantum phases like superconductivity, the quantum Hall effect, or density wave orders. But the experimental study of these materials is often hindered by complicated structural and chemical properties as well as by the involvement of toxic elements.Graphene, on the other hand, is a purely two-dimensional material consisting of a simple honeycomb lattice of carbon atoms. Since it was discovered experimentally, graphene has become one of the most widely studied materials in a range of research fields and remains one of the most active areas of research today. However, even though graphene has proven to be a promising platform to study a plethora of phenomena, the material itself does not exhibit the effects of correlated electron physics.In this thesis, we show two examples of how epitaxially grown large-scale graphene can be exploited as a platform to design quantum phases through interaction with a substrate and intercalation of atoms. Graphene under particular strain patterns exhibits pseudomagnetic fields. This means the Dirac electrons in the material behave as if they were under the influence of a magnetic field, even though no external field is applied. We are able to create large homogeneous pseudomagnetic fields using shallow nanoprisms in the substrate, which allows us to study the strain-induced quantum Hall effect in a momentum-resolved fashion using angle-resolved photoemission spectroscopy (ARPES).In the second part, we show how the intercalation of gadolinium can be used to couple flat bands in graphene to ordering phenomena in gadolinium. Flat bands near the Fermi level are theorised to enhance electronic correlations, and in combination with novel ordering phenomena, play a key role in many quantum material families. Our ARPES and resonant energy-integrated X-ray scattering (REXS) measurements reveal a complex interplay between different quantum phases in the material, including pseudogaps and evidence for a density wave order.

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Magnetic field study for a new generation high resolution mass separator (2018)

The work presented in this thesis is part of the design of the high resolution mass separator for the ARIEL facility under construction at TRIUMF, located in the UBC campus. This new facility, together with the existing ISAC facility, will produce rare isotope beams for nuclear physics experiments and nuclear medicine. The delivery of such beams requires a stage of separation after production to select the isotope of interest. The required separation is expressed in terms of resolving power defined as the inverse of the relative mass difference between two isotopes that need to be separated. The higher the mass the greater the resolving power required. The challenge is the separation of two isobars rather than two isotopes that by definition require a much lower resolving power. A resolving power of twenty thousand is the minimum required to achieve isobaric separation up to the uranium mass. The state of the art for existing heavy ion mass separators is a resolving power in the order of ten thousand for a transmitted emittance of less than three micrometers. The more typical long term operational value is well below ten thousand for larger emittances. The main goal of this project is to develop a mass separator that maintains an operational resolving power of twenty thousand. Different aspects influence the performance of the mass separator; the two main ones are the optics design and the field quality of the magnetic dipole(s) that provides the core functionality of the mass separator. In this thesis we worked from the hypothesis that minimizing the magnetic field integral variation with respect to the design mass resolution is equivalent to minimizing the aberration of the optical system. During this work we investigated how certain geometric parameters influence the field quality, as for example the dependency of the field flatness on the magnet pole gap. We also developed a new technique to control the mesh in the finite element analysis to facilitate particle tracking calculations. Beyond demonstrating our hypothesis, we ultimately produced a final magnet design where the field integral variation is less than one part in one hundred thousand.

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Electron-phonon mediated superconductivity probed by ARPES : from MgB2 to lithium-decorated graphene (2015)

This thesis traces a path from conventional superconductivity in a bulk material to the introduction of superconductivity and other novel phenomena in graphene. Magnesium diboride is a conventional superconductor, where the pairing is mediated by the electron-phonon coupling. ARPES (angle resolved photoemission spectroscopy) is shown to be an excellent probe to quantitatively study the momentum dependence of the electron-phonon coupling, demonstrating the origin of the distribution of superconducting gap sizes previously observed with other experimental techniques.Next, we exploit our understanding of the electron-phonon coupling to study how it can be tuned in a low dimensional system. It is shown that the electron-phonon coupling in graphene can be strongly enhanced by the deposition of alkali adatoms. High resolution, low temperature ARPES measurements provide the first experimental evidence of superconductivity in this two-dimensional system, showing a temperature dependent pairing gap, and an estimated Tc of ~ 6K.Finally, we present a study of another graphene-adatom system expected to show novel physics. Thallium on graphene has been predicted to enhance the spin-orbit coupling, leading to a robust topological insulator state. From ARPES measurements characterizing this system, we disentangle the long-range and short-range scattering contributions and show that thallium atoms act as surprisingly strong short-range scatterers. Our results are consistent with theoretical predictions for this system, indicating it is a good place to search for a two-dimensional topological insulator.

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Charge localization phenomena in correlated oxides (2014)

Charge segregation is very common in correlated oxides, spanning from the extreme limit of the Mott insulator, characterized by strong charge localization and suppressed charge dynamics, towards more mildly correlated states of matter, characterized by partial charge reorganization (charge-density-wave).In this thesis work I have investigated how the spatial organization of valence charge evolves with electrostatics (carrier doping) and chemistry (going down in the periodic table). For this reason, the focus is on the experimental study of the strongly-correlated 3d-oxides and the spin-orbit coupled 5d-oxides.These investigations have been performed with a bundle of state-of-the-art spectroscopic techniques in the field of quantum materials, namely: angle-resolved photoemission (ARPES), low-energy electron diffraction (LEED), resonant elastic X-ray scattering (REXS), X-ray diffraction (XRD), scanning tunnelling microscopy (STM) and optical spectroscopy. To support experimental data, we have used theoretical tools such as conventional density functional theory (DFT) for the 5d-oxides and developed ad-hoc approaches for the more complex 3d-materials.The all-around study of underdoped high-Tc Bi-based cuprates allowed us to shed new light on the universality and origin of charge-ordering instabilities in these materials and understand their interplay with superconductivity and pseudogap. These phenomena have been investigated in detail in underdoped samples of Bi2201, using a various experimental techniques (ARPES, REXS, XRD, LEED, STM) and various tailored theoretical approaches. The results of these studies are presented in Chapters 2 and 3.In the 5d-based iridates (in particular, Na₂IrO₃) we have revealed and characterized a novel form of Mott-Hubbard physics. This has been possible thanks to the combination of ARPES and optics for probing the electronic structure near the Fermi energy, and DFT for providing the theoretical framework to understand the electronic ground state in these materials. Ultimately, this approach helped demonstrate the crucial role of spin-orbit interaction in driving a novel Mott phase in materials where the Mott criterion might be violated (Chapter 4).Altogether, the resulting phenomena discovered in copper- and iridium based oxides have revealed novel unconventional aspects of the physics of correlated materials, thus paving the way for future explorations of the complex but fascinating jungle of transition metal oxides.

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Implementation of a Coherent Lyman-� Source for Laser Cooling and Spectroscopy of Antihydrogen (2014)

No abstract available.

Spin- and angle-resolved photoemission spectroscopy on unconventional superconductor strontium ruthenate (2013)

This thesis represents two bodies of work: a detailed look at what angle- resolved photoemission spectroscopy (ARPES) measures, as well as ARPES and circularly polarized photon spin- and angle-resolved photoemission spectroscopy (CPS-ARPES) measurements on the unconventional superconductor Sr₂RuO₄.In the first part I present a study of both established methods of ARPES analysis and some new variations on model spectral functions. This modelling was done in a realistic regime, yet far from the limits often assumed. Away from these limits I show that any "effective coupling" inferred from quasiparticle renormalizations differs drastically and unpredictably from the true coupling. Conversely, I show that perturbation theory retains good predictive power where expected, that the momentum dependence of the self-energy can be revealed via the relationship between velocity renormalization and quasiparticle strength, and that it is often possible to infer the self-energy and bare electronic structure through lineshape analysis.In the second part I present experimental ARPES and CPS-ARPES data on Sr₂RuO₄. Newly discovered and unexplained ARPES features are characterized and compared with a variety of different possible structural distortions through bulk and slab local-density approximation (LDA) band structure calculations. I thereby rule out phases driven by electronic interaction, such as Dirac- or Rashba-type surface states, and instead find that there exists a progressive structural modulation whereby both the surface and (at a minimum) sub-surface layers exhibit a (√2 x √2)R45° reconstruction.Through CPS-ARPES on Sr₂RuO₄ I also directly demonstrate that the effects of spin-orbit (SO) coupling are not limited to a modification of the band structure, but fundamentally entangle the spin and spatial parts of the wave-function. This must drive the superconducting state in Sr₂RuO₄ to be even more unconventional than is generally assumed, with mixing between singlet and triplet states that varies around the Fermi surface, and thereby offers a possible resolution to a number of experiments that clash with the categorization of Sr₂RuO₄ as a hallmark spin-triplet chiral p-wave superconductor.

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Unconventional Peierl's physics in underdoped Bi2Sr2CuO6+d (2013)

No abstract available.

Angle-resoloved photemission spectroscopy studies of Tl2Ba2CuO6+d and YBA2Cu3O7-d: analysis of recent results and the construction of a new system (2012)

No abstract available.

Metal-Insulation Transition, Orbital Symmetries and Gaps in Correlated Oxides: An Impurity Control Approach (2010)

No abstract available.

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