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Instruments / Technical Area	Publication Details	Published
I05-ARPES	Direct comparison of ARPES, STM, and quantum oscillation data for band structure determination in Sr2RhO4 I. Battisti , W. O. Tromp , S. Ricco , R. S. Perry , A. P. Mackenzie , A. Tamai , F. Baumberger , M. P. Allan Npj Quantum Materials 5 DOI: 10.1038/s41535-020-00292-4 Diamond Proposal Number(s): [13398, 5282] Open Access Show Abstract ▼ Abstract: Discrepancies in the low-energy quasiparticle dispersion extracted from angle-resolved photoemission, scanning tunneling spectroscopy, and quantum oscillation data are common and have long haunted the field of quantum matter physics. Here, we directly test the consistency of results from these three techniques by comparing data from the correlated metal Sr2RhO4. Using established schemes for the interpretation of the experimental data, we find good agreement for the Fermi surface topography and carrier effective masses. Hence, the apparent absence of such an agreement in other quantum materials, including the cuprates, suggests that the electronic states in these materials are of different, non-Fermi liquid-like nature. Finally, we discuss the potential and challenges in extracting carrier lifetimes from photoemission and quasiparticle interference data.	Dec 2020
I05-ARPES	Surface and bulk electronic structure of aluminium diboride V. Sunko , D. Milosavljević , F. Mazzola , O. J. Clark , U. Burkhardt , T. K. Kim , H. Rosner , Yu. Grin , A. P. Mackenzie , P. D. C. King Physical Review B 102 DOI: 10.1103/PhysRevB.102.035143 Diamond Proposal Number(s): [18705] Open Access Show Abstract ▼ Abstract: We report a combined experimental and theoretical study of the surface and bulk electronic structure of aluminium diboride, a nonsuperconducting sister compound of the superconductor MgB 2 . We perform angle-resolved photoemission measurements with variable photon energy, and compare them to density functional theory calculations to disentangle the surface and bulk contributions to the measured spectra. Aluminium diboride is known to be aluminium deficient, Al 1 − δ B 2 , which would be expected to lead to a hole doping as compared to the nominally stoichimoetric compound. Nonetheless, we find that the bulk σ states, which mediate superconductivity in MgB 2 , remain more than 600 meV below the Fermi level. However, we also observe σ states originating from the boron terminated surface, with an order of magnitude smaller binding energy of 70 meV , and demonstrate how surface hole-doping can bring these across the Fermi level.	Jul 2020
I05-ARPES	Electronically driven spin-reorientation transition of the correlated polar metal Ca3Ru2O7 Igor Markovic , Matthew D. Watson , Oliver J. Clark , Federico Mazzola , Edgar Abarca Morales , Chris A. Hooley , Helge Rosner , Craig M. Polley , Thiagarajan Balasubramanian , Saumya Mukherjee , Naoki Kikugawa , Dmitry A. Sokolov , Andrew P. Mackenzie , Phil D. C. King Proceedings Of The National Academy Of Sciences vol. 105 DOI: 10.1073/pnas.2003671117 Diamond Proposal Number(s): [21986, 25040] Show Abstract ▼ Abstract: The interplay between spin–orbit coupling and structural inversion symmetry breaking in solids has generated much interest due to the nontrivial spin and magnetic textures which can result. Such studies are typically focused on systems where large atomic number elements lead to strong spin–orbit coupling, in turn rendering electronic correlations weak. In contrast, here we investigate the temperature-dependent electronic structure of Ca3Ru2O7 , a 4d oxide metal for which both correlations and spin–orbit coupling are pronounced and in which octahedral tilts and rotations combine to mediate both global and local inversion symmetry-breaking polar distortions. Our angle-resolved photoemission measurements reveal the destruction of a large hole-like Fermi surface upon cooling through a coupled structural and spin-reorientation transition at 48 K, accompanied by a sudden onset of quasiparticle coherence. We demonstrate how these result from band hybridization mediated by a hidden Rashba-type spin–orbit coupling. This is enabled by the bulk structural distortions and unlocked when the spin reorients perpendicular to the local symmetry-breaking potential at the Ru sites. We argue that the electronic energy gain associated with the band hybridization is actually the key driver for the phase transition, reflecting a delicate interplay between spin–orbit coupling and strong electronic correlations and revealing a route to control magnetic ordering in solids.	Jun 2020
I05-ARPES I09-Surface and Interface Structural Analysis	Probing spin correlations using angle-resolved photoemission in a coupled metallic/Mott insulator system Veronika Sunko , F. Mazzola , S. Kitamura , S. Khim , P. Kushwaha , O. J. Clark , M. D. Watson , I. Markovic , D. Biswas , L. Pourovskii , T. K. Kim , T.-l. Lee , P. K. Thakur , H. Rosner , A. Georges , R. Moessner , T. Oka , A. P. Mackenzie , P. D. C. King Science Advances 6 DOI: 10.1126/sciadv.aaz0611 Diamond Proposal Number(s): [19479, 17699] Show Abstract ▼ Abstract: A nearly free electron metal and a Mott insulating state can be thought of as opposite ends of the spectrum of possibilities for the motion of electrons in a solid. Understanding their interaction lies at the heart of the correlated electron problem. In the magnetic oxide metal PdCrO2, nearly free and Mott-localized electrons exist in alternating layers, forming natural heterostructures. Using angle-resolved photoemission spectroscopy, quantitatively supported by a strong coupling analysis, we show that the coupling between these layers leads to an “intertwined” excitation that is a convolution of the charge spectrum of the metallic layer and the spin susceptibility of the Mott layer. Our findings establish PdCrO2 as a model system in which to probe Kondo lattice physics and also open new routes to use the a priori nonmagnetic probe of photoemission to gain insights into the spin susceptibility of correlated electron materials.	Feb 2020
I05-ARPES	Direct observation of a uniaxial stress-driven Lifshitz transition in Sr2RuO4 Veronika Sunko , Edgar Abarca Morales , Igor Markovic , Mark E. Barber , Dijana Milosavljević , Federico Mazzola , Dmitry A. Sokolov , Naoki Kikugawa , Cephise Cacho , Pavel Dudin , Helge Rosner , Clifford Hicks , Philip D. C. King , Andrew P. Mackenzie Npj Quantum Materials 4 DOI: 10.1038/s41535-019-0185-9 Diamond Proposal Number(s): [20427] Open Access Show Abstract ▼ Abstract: Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems, and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr2RuO4, leading to a pronounced peak in Tc versus strain whose origin is still under active debate. Here we develop a simple and compact method to passively apply large uniaxial pressures in restricted sample environments, and utilise this to study the evolution of the electronic structure of Sr2RuO4 using angle-resolved photoemission. We directly visualise how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in Tc. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr2RuO4. More generally, our experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.	Dec 2019
I05-ARPES	Itinerant ferromagnetism of the Pd-terminated polar surface of PdCoO 2 Federico Mazzola , Veronika Sunko , Seunghyun Khim , Helge Rosner , Pallavi Kushwaha , Oliver J. Clark , Lewis Bawden , Igor Markovic , Timur K. Kim , Moritz Hoesch , Andrew P. Mackenzie , Phil D. C. King Proceedings Of The National Academy Of Sciences 15 DOI: 10.1073/pnas.1811873115 Diamond Proposal Number(s): [12469, 14927, 16262] Show Abstract ▼ Abstract: The ability to modulate the collective properties of correlated electron systems at their interfaces and surfaces underpins the burgeoning field of “designer” quantum materials. Here, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability to itinerant ferromagnetism at the Pd-terminated surface of the nonmagnetic delafossite oxide metal PdCoO2. Combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we show how this leads to a rich multiband surface electronic structure. We find similar surface state dispersions in PdCrO2, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.	Dec 2018
I05-ARPES	Maximal Rashba-like spin splitting via kinetic-energy-coupled inversion-symmetry breaking Veronika Sunko , H. Rosner , P. Kushwaha , S. Khim , F. Mazzola , L. Bawden , O. J. Clark , J. M. Riley , D. Kasinathan , M. W. Haverkort , T. K. Kim , M. Hoesch , J. Fujii , I. Vobornik , A. P. Mackenzie , Philip King Nature 549, 492-496 DOI: 10.1038/nature23898 Diamond Proposal Number(s): [12469, 14927, 18267] Show Abstract ▼ Abstract: Engineering and enhancing the breaking of inversion symmetry in solids—that is, allowing electrons to differentiate between ‘up’ and ‘down’—is a key goal in condensed-matter physics and materials science because it can be used to stabilize states that are of fundamental interest and also have potential practical applications. Examples include improved ferroelectrics for memory devices and materials that host Majorana zero modes for quantum computing1, 2. Although inversion symmetry is naturally broken in several crystalline environments, such as at surfaces and interfaces, maximizing the influence of this effect on the electronic states of interest remains a challenge. Here we present a mechanism for realizing a much larger coupling of inversion-symmetry breaking to itinerant surface electrons than is typically achieved. The key element is a pronounced asymmetry of surface hopping energies—that is, a kinetic-energy-coupled inversion-symmetry breaking, the energy scale of which is a substantial fraction of the bandwidth. Using spin- and angle-resolved photoemission spectroscopy, we demonstrate that such a strong inversion-symmetry breaking, when combined with spin–orbit interactions, can mediate Rashba-like3, 4 spin splittings that are much larger than would typically be expected. The energy scale of the inversion-symmetry breaking that we achieve is so large that the spin splitting in the CoO2- and RhO2-derived surface states of delafossite oxides becomes controlled by the full atomic spin–orbit coupling of the 3d and 4d transition metals, resulting in some of the largest known Rashba-like3, 4 spin splittings. The core structural building blocks that facilitate the bandwidth-scaled inversion-symmetry breaking are common to numerous materials. Our findings therefore provide opportunities for creating spin-textured states and suggest routes to interfacial control of inversion-symmetry breaking in designer heterostructures of oxides and other material classes.	Sep 2017
I05-ARPES	Quasi-two-dimensional Fermi surface topography of the delafossite PdRhO 2 F. Arnold , M. Naumann , S. Khim , H. Rosner , V. Sunko , Federico Mazzola , P. D. C. King , A. P. Mackenzie , E. Hassinger Physical Review B 96 DOI: 10.1103/PhysRevB.96.075163 Diamond Proposal Number(s): [14927] Show Abstract ▼ Abstract: We report on a combined study of the de Haas-van Alphen effect and angle-resolved photoemission spectroscopy on single crystals of the metallic delafossite PdRhO2 rounded off by ab initio band structure calculations. A high-sensitivity torque magnetometry setup with superconducting quantum interference device readout and synchrotron-based photoemission with a light spot size of 50μm enabled high-resolution data to be obtained from samples as small as 150×100×20(μm)3. The Fermi surface shape is nearly cylindrical with a rounded hexagonal cross section enclosing a Luttinger volume of 1.00(1) electrons per formula unit.	Aug 2017
I05-ARPES	Nearly free electrons in a 5d delafossite oxide metal P. Kushwaha , V. Sunko , P. J. W. Moll , L. Bawden , J. M. Riley , N. Nandi , H. Rosner , M. P. Schmidt , F. Arnold , E. Hassinger , T. K. Kim , M. Hoesch , A. P. Mackenzie , P. D. C King Science Advances 1 (9), e1500692 DOI: 10.1126/sciadv.1500692 PMID: 26601308 Diamond Proposal Number(s): [10040] Open Access Show Abstract ▼ Abstract: Understanding the role of electron correlations in strong spin-orbit transition-metal oxides is key to the realization of numerous exotic phases including spin-orbit–assisted Mott insulators, correlated topological solids, and prospective new high-temperature superconductors. To date, most attention has been focused on the 5d iridium-based oxides. We instead consider the Pt-based delafossite oxide PtCoO2. Our transport measurements, performed on single-crystal samples etched to well-defined geometries using focused ion beam techniques, yield a room temperature resistivity of only 2.1 microhm·cm (μΩ-cm), establishing PtCoO2 as the most conductive oxide known. From angle-resolved photoemission and density functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along kz. Despite being predominantly composed of d-orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14me. Moreover, the sharp spectral features observed in photoemission remain well defined with little additional broadening for more than 500 meV below EF, pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO2 as a model nearly-free–electron system in a 5d delafossite transition-metal oxide.	Oct 2015

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