Anna Tamai

Creation and control of a two-dimensional electron liquid at the bare SrTiO 3 surface

Many-body interactions in transition-metal oxides give rise to a wide range of functional properties, such as high-temperature superconductivity, colossal magnetoresistance or multiferroicity . The seminal recent discovery of a two-dimensional electron gas (2DEG) at the interface of the insulating oxides LaAlO(3) and SrTiO(3) (ref. 4) represents an important milestone towards exploiting such properties in all-oxide devices. This conducting interface shows a number of appealing properties, including a high electron mobility, superconductivity and large magnetoresistance, and can be patterned on the few-nanometre length scale. However, the microscopic origin of the interface 2DEG is poorly understood. Here, we show that a similar 2DEG, with an electron density as large as 8×10(13)  cm(-2), can be formed at the bare SrTiO(3) surface. Furthermore, we find that the 2DEG density can be controlled through exposure of the surface to intense ultraviolet light. Subsequent angle-resolved photoemission spectroscopy measurements reveal an unusual coexistence of a light quasiparticle mass and signatures of strong many-body interactions.

Strong Electron Correlations in the Normal State of the Iron-Based FeSe0.42Te0.58 Superconductor Observed by Angle-Resolved Photoemission Spectroscopy

A. Tamai, A.Y. Ganin, E. Rozbicki, J. Bacsa, W. Meevasana, P.D.C. King, M. Caffio, R. Schaub, S. Margadonna, K. Prassides, M.J. Rosseinsky, and F. Baumberger School of Physics and Astronomy, University of St. Andrews, St. Andrews, Fife KY16 9SS, United Kingdom Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, United Kingdom School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, United Kingdom School of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, United Kingdom Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom (Dated: February 10, 2010)

Fermi Surface and van Hove Singularities in the Itinerant Metamagnet Sr3Ru2O7

The low-energy electronic structure of the itinerant metamagnet Sr3Ru2O7 is investigated by angle-resolved photoemission and density-functional calculations. We find well-defined quasiparticle bands with resolution-limited linewidths and Fermi velocities up to an order of magnitude lower than in single layer Sr2RuO4. The complete topography, the cyclotron masses, and the orbital character of the Fermi surface are determined, in agreement with bulk sensitive de Haas-van Alphen measurements. An analysis of the dxy band dispersion reveals a complex density of states with van Hove singularities near the Fermi level, a situation which is favorable for magnetic instabilities.

Quasiparticle dynamics and spin–orbital texture of the SrTiO3 two-dimensional electron gas

Two-dimensional electron gases (2DEGs) in SrTiO3 have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the d-electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orbital and lattice degrees of freedom in this system. We reveal how they lead to pronounced orbital ordering, mediate an orbitally enhanced Rashba splitting with complex subband-dependent spin-orbital textures and markedly change the character of electron-phonon coupling, co-operatively shaping the low-energy electronic structure of the 2DEG. Our results allow for a unified understanding of spectroscopic and transport measurements across different classes of SrTiO3-based 2DEGs, and yield new microscopic insights on their functional properties.

Spin-orbit splitting of the Shockley surface state on Cu(111)

We present angle-resolved photoemission data from Cu(111). Using a focused 6 eV continuous-wave laser for photoexcitation, we achieve a high effective momentum resolution, enabling detection of the Rashba spin splitting in the Shockley surface state on Cu(111). The magnitude of the spin splitting of Δk∼0.006 A−1 is surprisingly large and exceeds values predicted for the analogous surface state on Ag(111), but is reproduced by first-principles calculations. We further resolve a kink in the dispersion, which we attribute to electron-phonon coupling.

A facility for the analysis of the electronic structures of solids and their surfaces by synchrotron radiation photoelectron spectroscopy

A synchrotron radiation beamline in the photon energy range of 18-240 eV and an electron spectroscopy end station have been constructed at the 3 GeV Diamond Light Source storage ring. The instrument features a variable polarisation undulator, a high resolution monochromator, a re-focussing system to form a beam spot of 50 × 50 μm2, and an end station for angle-resolved photoelectron spectroscopy (ARPES) including a 6-degrees-of-freedom cryogenic sample manipulator. The beamline design and its performance allow for a highly productive and precise use of the ARPES technique at an energy resolution of 10-15 meV for fast k-space mapping studies with a photon flux up to 2 ⋅ 1013 ph/s and well below 3 meV for high resolution spectra.

Spectroscopic Indications of Polaronic Behavior of the Strong Spin-Orbit Insulator Sr3Ir2O7

We investigate the bilayer Ruddlesden-Popper iridate Sr3Ir2O7 by temperature-dependent angle-resolved photoemission. At low temperatures, we find a fully gapped correlated insulator, characterized by a small charge gap and narrow bandwidths. The low-energy spectral features show a pronounced temperature-dependent broadening and non-quasiparticle-like Gaussian line shapes. Together, these spectral features provide experimental evidence for a polaronic ground state. We observe similar behavior for the single-layer cousin Sr2IrO4, indicating that strong electron-boson coupling dominates the low-energy excitations of this exotic family of 5d compounds.

Structural Origin of Apparent Fermi Surface Pockets in Angle-Resolved Photoemission of Bi 2 Sr 2-x La x CuO 6+δ

We observe apparent hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission. From detailed low-energy electron diffraction measurements and an analysis of the angle-resolved photoemission polarization dependence, we show that these pockets are not intrinsic but arise from multiple overlapping superstructure replicas of the main and shadow bands. We further demonstrate that the hole pockets reported recently from angle-resolved photoemission [Meng et al., Nature (London) 462, 335 (2009)] have a similar structural origin and are inconsistent with an intrinsic hole pocket associated with the electronic structure of a doped CuO₂ plane.

Control of a two-dimensional electron gas on SrTiO3 (111) by atomic oxygen

We report on the formation of a two-dimensional electron gas (2DEG) at the bare surface of (111) oriented SrTiO3. Angle resolved photoemission experiments reveal highly itinerant carriers with a sixfold symmetric Fermi surface and strongly anisotropic effective masses. The electronic structure of the 2DEG is in good agreement with self-consistent tight-binding supercell calculations that incorporate a confinement potential due to surface band bending. We further demonstrate that alternate exposure of the surface to ultraviolet light and atomic oxygen allows tuning of the carrier density and the complete suppression of the 2DEG.

The electronic structure of a surfactant layer: Pb/Cu(1 1 1)

The electronic structure of an incommensurate Pb layer on Cu(1 1 1) is investigated by means of angle scanned ultraviolet photoemission. Despite of the rather complex atomic structure with three competing periodicities, we observe a well defined band structure, reflecting essentially the fundamental (1 1)-periodicity of the adlayer. Comparison with band structure calculations for a free standing Pb-layer shows that all observed bands can be fully attributed to the overlayer. 2003 Elsevier Science B.V. All rights reserved.