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KTaO3(001) Preparation Methods in Vacuum: Effects on Surface Stoichiometry, Crystallography, and in-gap States
Authors:
Andrea M. Lucero Manzano,
Esteban D. Cantero,
Emanuel A. Martínez,
F. Y. Bruno,
Esteban A. Sánchez,
Oscar Grizzi
Abstract:
KTaO3 single crystals with different orientations are used as substrates for the epitaxial growth of thin films and/or as hosts for two-dimensional electron gases. Due to the polar nature of the KTaO3(001) surface, one can expect difficulties and challenges to arise in its preparation. Maintaining good insulating characteristics without adding undesirable in-gap electronic states, obtaining good c…
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KTaO3 single crystals with different orientations are used as substrates for the epitaxial growth of thin films and/or as hosts for two-dimensional electron gases. Due to the polar nature of the KTaO3(001) surface, one can expect difficulties and challenges to arise in its preparation. Maintaining good insulating characteristics without adding undesirable in-gap electronic states, obtaining good crystalline order up to the top surface layer, a sufficiently flat surface, and complete cleanliness of the surface (without water, C or OH contaminants), are in general difficult conditions to accomplish simultaneously. Cleaving in vacuum is likely the best option for obtaining a clean surface. However, since KTaO3 is cubic and lacks a well-defined cleavage plane, this method is notsuitable for sample growth or reproducible device fabrication. Here, we systematically evaluate the effect of typical preparation methods applied on the surfaces of KTaO3(001) single crystals. In particular, we used annealing in vacuum at different temperatures, light sputtering with Ar+ ions at low energy (500 eV) followed by annealing, heavy Ar+ ion bombardment and annealing, and grazing Ar+ ion bombardment under continuous azimuthal rotation combined with both annealing in vacuum and in O2 atmosphere. Possible side effects after each treatment are evaluated by a combination of techniques, including low-energy ion scattering at forward angles, Auger electron spectroscopy, low-energy electron energy loss, X-ray photoelectron spectroscopy, low-energy electron diffraction, and time of flightsecondary ion mass spectrometry. Advantages and shortcomings of each preparation method are discussed in detail.
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Submitted 14 May, 2025; v1 submitted 2 April, 2025;
originally announced April 2025.
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Fermi surface and pseudogap in highly doped Sr$_{2}$IrO$_{4}$
Authors:
Y. Alexanian,
A. de la Torre,
S. McKweon Walker,
M. Straub,
G. Gatti,
A. Hunter,
S. Mandloi,
E. Cappelli,
S. Riccò,
F. Y. Bruno,
M. Radovic,
N. C. Plumb,
M. Shi,
J. Osiecki,
C. Polley,
T. K. Kim,
P. Dudin,
M. Hoesch,
R. S. Perry,
A. Tamai,
F. Baumberger
Abstract:
The fate of the Fermi surface in bulk electron-doped Sr$_{2}$IrO$_{4}$ remains elusive, as does the origin and extension of its pseudogap phase. Here, we use high-resolution angle-resolved photoelectron spectroscopy (ARPES) to investigate the electronic structure of Sr$_{2-x}$La$_{x}$IrO$_{4}$ up to $x=0.2$, a factor of two higher than in previous work. We find that the antinodal pseudogap persist…
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The fate of the Fermi surface in bulk electron-doped Sr$_{2}$IrO$_{4}$ remains elusive, as does the origin and extension of its pseudogap phase. Here, we use high-resolution angle-resolved photoelectron spectroscopy (ARPES) to investigate the electronic structure of Sr$_{2-x}$La$_{x}$IrO$_{4}$ up to $x=0.2$, a factor of two higher than in previous work. We find that the antinodal pseudogap persists up to the highest doping level, and thus beyond the sharp increase in Hall carrier density to $\simeq 1+x$ recently observed above $x^{*}\simeq 0.16$ [Y.-T. Hsu et al., Nature Physics 20, 1593 (2024)]. This suggests that doped iridates host a unique phase of matter in which a large Hall density coexists with an anisotropic pseudogap, breaking up the Fermi surface into disconnected arcs. The temperature boundary of the pseudogap is $T^{*}\simeq 200$ K for $x=0.2$, comparable to cuprates and to the energy scale of short range antiferromagnetic correlations in cuprates and iridates.
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Submitted 4 September, 2025; v1 submitted 27 November, 2024;
originally announced November 2024.
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High stability 2D electron gases formed in Si3N4/Al//KTaO3 heterostructures: synthesis and in-depth interfacial characterization
Authors:
E. A. Martínez,
A. M. Lucero,
E. D. Cantero,
N. Biškup,
A. Orte,
E. A. Sánchez,
M. Romera,
N. M. Nemes,
J. L. Martínez,
M. Varela,
O. Grizzi,
F. Y. Bruno
Abstract:
The two-dimensional electron gas (2DEG) found in KTaO3-based interfaces has garnered attention due to its remarkable electronic properties. In this study, we investigated the conducting system embedded at the Si3N4/Al//KTO(110) heterostructure. We demonstrate that the Al/KTO interface supports a conducting system, with the Si3N4 passivation layer acting as a barrier to oxygen diffusion, enabling e…
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The two-dimensional electron gas (2DEG) found in KTaO3-based interfaces has garnered attention due to its remarkable electronic properties. In this study, we investigated the conducting system embedded at the Si3N4/Al//KTO(110) heterostructure. We demonstrate that the Al/KTO interface supports a conducting system, with the Si3N4 passivation layer acting as a barrier to oxygen diffusion, enabling ex-situ characterization. Our findings reveal that the mobility and carrier density of the system can be tuned by varying the Al layer thickness. Using scanning transmission electron microscopy, electron energy-loss spectroscopy, X-ray photoemission spectroscopy, and time-of-flight secondary ion mass spectrometry, we characterized the structural and chemical composition of the interface. We found that the Al layer fully oxidizes into AlOx, drawing oxygen from the KTaO3 substrate. The oxygen depletion zone extends 3-5 nm into the substrate and correlates to the Al thickness. Heterostructures with thicker Al layers exhibit higher carrier densities but lower mobilities, likely due to interactions with the oxygen vacancies that act as scattering centers. These findings highlight the importance of considering the effect and extent of the oxygen depletion zone when designing and modeling two-dimensional electron systems in complex oxides.
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Submitted 18 September, 2024;
originally announced September 2024.
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Anisotropic Electronic Structure of the Two-Dimensional Electron Gas at the AlOx/KTaO3(110) interface
Authors:
E. A. Martínez,
J. Dai,
M. Tallarida,
N. M. Nemes,
F. Y. Bruno
Abstract:
Oxide-based two-dimensional electron gases (2DEGs) have generated significant interest due to their potential for discovering novel physical properties. Among these, 2DEGs formed in KTaO3 stand out due to the recently discovered crystal face-dependent superconductivity and large Rashba splitting, both of which hold potential for future oxide electronics devices. In this work, angle-resolved photoe…
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Oxide-based two-dimensional electron gases (2DEGs) have generated significant interest due to their potential for discovering novel physical properties. Among these, 2DEGs formed in KTaO3 stand out due to the recently discovered crystal face-dependent superconductivity and large Rashba splitting, both of which hold potential for future oxide electronics devices. In this work, angle-resolved photoemission spectroscopy is used to study the electronic structure of the 2DEG formed at the (110) surface of KTaO3 after deposition of a thin Al layer. Our experiments revealed a remarkable anisotropy in the orbital character of the electron-like dispersive bands, which form a Fermi surface consisting of two elliptical contours with their major axes perpendicular to each other. The measured electronic structure is used to constrain the modeling parameters of self-consistent tight-binding slab calculations of the band structure. In these calculations, an anisotropic Rashba splitting is found with a value as large as 4 meV at the Fermi level along the [-110] crystallographic direction. This large unconventional and anisotropic Rashba splitting is rationalized based on the orbital angular momentum formulation. These findings provide insights into the interpretation of spin-orbitronics experiments and help to constrain models for superconductivity in the KTO(110)-2DEG system.
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Submitted 28 April, 2023;
originally announced April 2023.
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BinPo: An open-source code to compute the band structure of two-dimensional electron systems
Authors:
Emanuel A. Martínez,
Juan I. Beltrán Fínez,
Flavio Y. Bruno
Abstract:
We introduce BinPo, an open-source Python code to compute electronic properties of two-dimensional electron systems. Its usage is focused on the ABO$_3$ perovskite structure based systems, such as SrTiO$_3$ and KTaO$_3$, because of their increasing impact in materials community and possible applications in spintronic devices. BinPo has a Schrödinger-Poisson solver to obtain the self-consistent pot…
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We introduce BinPo, an open-source Python code to compute electronic properties of two-dimensional electron systems. Its usage is focused on the ABO$_3$ perovskite structure based systems, such as SrTiO$_3$ and KTaO$_3$, because of their increasing impact in materials community and possible applications in spintronic devices. BinPo has a Schrödinger-Poisson solver to obtain the self-consistent potential energy in a slab system. The tight binding slab Hamiltonian of the system is created from the transfer integrals in the maximally localized Wannier functions basis, thus reaching a higher accuracy than conventional tight binding methods. The band structure, energy slices, and other properties, along with different projections and orientations can be computed. High resolution and publishable figures of the simulations can be generated. In BinPo, priority has been given to ease-of-use, efficiency, readability and modularity, therefore becoming suitable to produce reliable electronic structures simulations at low computational cost. Along with the code itself, we provide files from first-principles calculations, instructions of use and detailed examples of its wide range of capabilities. We detail the approaches used in the code, so that it can be further exploited and adapted to other problems, such as adding new materials and functionalities which can strength the initial code scopes.
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Submitted 21 March, 2022;
originally announced March 2022.
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Electronic structure of the highly conductive perovskite oxide SrMoO$_3$
Authors:
E. Cappelli,
A. Hampel,
A. Chikina,
E. Bonini Guedes,
G. Gatti,
A. Hunter,
J. Issing,
N. Biskup,
M. Varela,
Cyrus E. Dreyer,
A. Tamai,
A. Georges,
F. Y. Bruno,
M. Radovic,
F. Baumberger
Abstract:
We use angle-resolved photoemission to map the Fermi surface and quasiparticle dispersion of bulk-like thin films of SrMoO$_3$ grown by pulsed laser deposition. The electronic self-energy deduced from our data reveals weak to moderate correlations in SrMoO$_3$, consistent with our observation of well-defined electronic states over the entire occupied band width. We further introduce spectral funct…
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We use angle-resolved photoemission to map the Fermi surface and quasiparticle dispersion of bulk-like thin films of SrMoO$_3$ grown by pulsed laser deposition. The electronic self-energy deduced from our data reveals weak to moderate correlations in SrMoO$_3$, consistent with our observation of well-defined electronic states over the entire occupied band width. We further introduce spectral function calculations that combine dynamical mean-field theory with an unfolding procedure of density functional calculations and demonstrate good agreement of this approach with our experiments.
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Submitted 11 March, 2022;
originally announced March 2022.
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A Laser-ARPES View of the 2D Electron Systems at LaAlO3/SrTiO3 and Al/SrTiO3 Interfaces
Authors:
Siobhan McKeown Walker,
Margherita Boselli,
Emanuel A. Martínez,
Stefano Gariglio,
Flavio Y. Bruno,
Felix Baumberger
Abstract:
We have measured the electronic structure of the two-dimensional electron system (2DES) found at the Al/SrTiO3 (Al/STO) and LaAlO3/SrTiO3 (LAO/STO) interfaces by means of laser angle resolved photoemission spectroscopy, taking advantage of the large photoelectron escape depth at low photon energy to probe these buried interfaces. We demonstrate the possibility of tuning the electronic density in A…
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We have measured the electronic structure of the two-dimensional electron system (2DES) found at the Al/SrTiO3 (Al/STO) and LaAlO3/SrTiO3 (LAO/STO) interfaces by means of laser angle resolved photoemission spectroscopy, taking advantage of the large photoelectron escape depth at low photon energy to probe these buried interfaces. We demonstrate the possibility of tuning the electronic density in Al/STO by varying the Al layer thickness and show that the electronic structure evolution is well described by self-consistent tight binding supercell calculations, but differs qualitatively from a rigid band shift model. We show that both 2DES are strongly coupled to longitudinal optical phonons, in agreement with previous reports of a polaronic ground state in similar STO based 2DESs. Tuning the electronic density in Al/STO to match that of LAO/STO and comparing both systems, we estimate that the intrinsic LAO/STO 2DES has a bare band width of ~ 60 meV and a carrier density of ~ 6 10^13 cm-2.
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Submitted 11 February, 2022;
originally announced February 2022.
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A laser-ARPES study of LaNiO3 thin films grown by sputter deposition
Authors:
Edoardo Cappelli,
Willem Tromp,
Siobhan McKeown Walker,
Anna Tamai,
Marta Gibert,
Felix Baumberger,
Flavio Y. Bruno
Abstract:
Thin films of the correlated transition-metal oxide LaNiO$_3$ undergo a metal-insulator transition when their thickness is reduced to a few unit cells. Here, we use angle-resolved photoemission spectroscopy to study the evolution of the electronic structure across this transition in a series of epitaxial LaNiO$_3$ films of thicknesses ranging from 19 to 2 u.c. grown in situ by RF magnetron sputter…
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Thin films of the correlated transition-metal oxide LaNiO$_3$ undergo a metal-insulator transition when their thickness is reduced to a few unit cells. Here, we use angle-resolved photoemission spectroscopy to study the evolution of the electronic structure across this transition in a series of epitaxial LaNiO$_3$ films of thicknesses ranging from 19 to 2 u.c. grown in situ by RF magnetron sputtering. Our data show a strong reduction of the electronic mean free path as the thickness is reduced below 5 u.c. This prevents the system from becoming electronically two-dimensional, as confirmed by the largely unchanged Fermi surface seen in our experiments. In the insulating state we observe a strong suppression of the coherent quasiparticle peak but no clear gap. These features resemble previous observations of the insulating state of NdNiO$_3$.
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Submitted 6 February, 2020;
originally announced February 2020.
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Bulk and surface electronic structure of the dual-topology semimetal Pt2HgSe3
Authors:
I. Cucchi,
A. Marrazzo,
E. Cappelli,
S. Ricco,
F. Y. Bruno,
S. Lisi,
M. Hoesch,
T. K. Kim,
C. Cacho,
C. Besnard,
E. Giannini,
N. Marzari,
M. Gibertini,
F. Baumberger,
A. Tamai
Abstract:
We report high-resolution angle resolved photoemission measurements on single crystals of Pt2HgSe3 grown by high-pressure synthesis. Our data reveal a gapped Dirac nodal line whose (001)-projection separates the surface Brillouin zone in topological and trivial areas. In the non-trivial $k$-space range we find surface states with multiple saddle-points in the dispersion resulting in two van Hove s…
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We report high-resolution angle resolved photoemission measurements on single crystals of Pt2HgSe3 grown by high-pressure synthesis. Our data reveal a gapped Dirac nodal line whose (001)-projection separates the surface Brillouin zone in topological and trivial areas. In the non-trivial $k$-space range we find surface states with multiple saddle-points in the dispersion resulting in two van Hove singularities in the surface density of states. Based on density functional theory calculations, we identify these surface states as signatures of a topological crystalline state which coexists with a weak topological phase.
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Submitted 11 September, 2019;
originally announced September 2019.
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Band Structure and Spin-Orbital Texture of the (111)-KTaO3 Two-Dimensional Electron Gas
Authors:
F. Y. Bruno,
S. McKeown Walker,
S. Riccò,
A. de la Torre,
Z. Wang,
A. Tamai,
T. K. Kim,
M. Hoesch,
M. S. Bahramy,
F. Baumberger
Abstract:
Two-dimensional electron gases (2DEGs) in oxides show great potential for discovering new physical phenomena and at the same time hold promise for electronic applications. In this work we use angle resolved photoemission to determine the electronic structure of a 2DEG stabilized in the (111)-oriented surface of the strong spin orbit coupling material KTaO3. Our measurements reveal multiple sub-ban…
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Two-dimensional electron gases (2DEGs) in oxides show great potential for discovering new physical phenomena and at the same time hold promise for electronic applications. In this work we use angle resolved photoemission to determine the electronic structure of a 2DEG stabilized in the (111)-oriented surface of the strong spin orbit coupling material KTaO3. Our measurements reveal multiple sub-bands that emerge as a consequence of quantum confinement and form a six-fold symmetric Fermi surface. This electronic structure is well reproduced by self-consistent tight-binding supercell calculations. Based on these calculations we determine the spin and orbital texture of the 2DEG. We show that the 2DEG Fermi surface is derived from bulk J = 3/2 states and exhibits an unconventional anisotropic Rashba-like lifting of the spin-degeneracy. Spin-momentum locking holds only for high symmetry directions and a strong out-of-plane spin component renders the spin-texture three-fold symmetric. We find that the average spin-splitting on the Fermi surface is an order of magnitude larger than in SrTiO3, which should translate into an enhancement in the spin-orbitronic response of (111)-KTaO3 2DEG based devices.
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Submitted 14 March, 2019;
originally announced March 2019.
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High-resolution photoemission on Sr2RuO4 reveals correlation-enhanced effective spin-orbit coupling and dominantly local self-energies
Authors:
A. Tamai,
M. Zingl,
E. Rozbicki,
E. Cappelli,
S. Ricco,
A. de la Torre,
S. McKeown Walker,
F. Y. Bruno,
P. D. C. King,
W. Meevasana,
M. Shi,
M. Radovic,
N. C. Plumb,
A. S. Gibbs,
A. P. Mackenzie,
C. Berthod,
H. Strand,
M. Kim,
A. Georges,
F. Baumberger
Abstract:
We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about two, due to electronic correlations. The se…
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We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about two, due to electronic correlations. The self-energies for the $β$ and $γ$ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects, but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies, and provides strong evidence for an electronic origin of the observed non-linear frequency dependence of the self-energies, leading to `kinks' in the quasiparticle dispersion of Sr2RuO4.
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Submitted 7 May, 2019; v1 submitted 16 December, 2018;
originally announced December 2018.
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Microfocus laser-ARPES on encapsulated mono-, bi-, and few-layer 1T'-WTe$_2$
Authors:
Irène Cucchi,
Ignacio Gutiérrez-Lezama,
Edoardo Cappelli,
Siobhan McKeown Walker,
Flavio Y. Bruno,
Giulia Tenasini,
Lin Wang,
Nicolas Ubrig,
Céline Barreteau,
Enrico Giannini,
Marco Gibertini,
Anna Tamai,
Alberto F. Morpurgo,
Felix Baumberger
Abstract:
Two-dimensional crystals of semimetallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few layer 1T'-WTe$_2$ and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiment about the momentum space electr…
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Two-dimensional crystals of semimetallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few layer 1T'-WTe$_2$ and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiment about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T'-WTe$_2$ by laser-based micro-focus angle resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe$_2$ comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T'-WTe$_2$ and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.
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Submitted 12 November, 2018;
originally announced November 2018.
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Observation of out-of-plane spin texture in a SrTiO3 (111) two-dimensional electron gas
Authors:
Pan He,
S. McKeown Walker,
Steven S. -L. Zhang,
F. Y. Bruno,
M. S. Bahramy,
Jongmin Lee,
Rajagopalan Ramaswamy,
Kaiming Cai,
Olle Heinonen,
Giovanni Vignale,
F. Baumberger,
Hyunsoo Yang
Abstract:
We explore the second order bilinear magnetoelectric resistance (BMER) effect in the d-electron-based two-dimensional electron gas (2DEG) at the SrTiO3 (111) surface. We find an evidence of a spin-split band structure with the archetypal spin-momentum locking of the Rashba effect for the in-plane component. Under an out-of-plane magnetic field, we find a BMER signal that breaks the six-fold symmet…
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We explore the second order bilinear magnetoelectric resistance (BMER) effect in the d-electron-based two-dimensional electron gas (2DEG) at the SrTiO3 (111) surface. We find an evidence of a spin-split band structure with the archetypal spin-momentum locking of the Rashba effect for the in-plane component. Under an out-of-plane magnetic field, we find a BMER signal that breaks the six-fold symmetry of the electronic dispersion, which is a fingerprint for the presence of a momentum dependent out-of-plane spin component. Relativistic electronic structure calculations reproduce this spin-texture and indicate that the out-of-plane component is a ubiquitous property of oxide 2DEGs arising from strong crystal field effects. We further show that the BMER response of the SrTiO3 (111) 2DEG is tunable and unexpectedly large.
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Submitted 5 June, 2018;
originally announced June 2018.
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In-situ strain-tuning of the metal-insulator-transition of Ca$_{2}$RuO$_{4}$ in angle-resolved photoemission experiments
Authors:
S. Riccò,
M. Kim,
A. Tamai,
S. McKeown Walker,
F. Y. Bruno,
I. Cucchi,
E. Cappelli,
C. Besnard,
T. K. Kim,
P. Dudin,
M. Hoesch,
M. Gutmann,
A. Georges,
R. S. Perry,
F. Baumberger
Abstract:
We report the evolution of the $k$-space electronic structure of lightly doped bulk Ca$_{2}$RuO$_{4}$ with uniaxial strain. Using ultrathin plate-like crystals, we achieve strain levels up to $-4.1\%$, sufficient to suppress the Mott phase and access the previously unexplored metallic state at low temperature. Angle-resolved photoemission experiments performed while tuning the uniaxial strain reve…
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We report the evolution of the $k$-space electronic structure of lightly doped bulk Ca$_{2}$RuO$_{4}$ with uniaxial strain. Using ultrathin plate-like crystals, we achieve strain levels up to $-4.1\%$, sufficient to suppress the Mott phase and access the previously unexplored metallic state at low temperature. Angle-resolved photoemission experiments performed while tuning the uniaxial strain reveal that metallicity emerges from a marked redistribution of charge within the Ru $t_{2g}$ shell, accompanied by a sudden collapse of the spectral weight in the lower Hubbard band and the emergence of a well defined Fermi surface which is devoid of pseudogaps. Our results highlight the profound roles of lattice energetics and of the multiorbital nature of Ca$_{2}$RuO$_{4}$ in this archetypal Mott transition and open new perspectives for spectroscopic measurements.
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Submitted 24 October, 2018; v1 submitted 1 March, 2018;
originally announced March 2018.
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Structural, magnetic, and electronic properties of GdTiO3 Mott insulator thin films grown by pulsed laser deposition
Authors:
M. N. Grisolia,
F. Y. Bruno,
D. Sando,
H. J. Zhao,
E. Jacquet,
X. M. Chen,
L. Bellaiche,
A. Barthelemy,
M. Bibes
Abstract:
We report on the optimization process to synthesize epitaxial thin films of GdTiO3 on SrLaGaO4 substrates by pulsed laser deposition. Optimized films are free of impurity phases and are fully strained. They possess a magnetic Curie temperature TC = 31.8 K with a saturation magnetization of 4.2 muB per formula unit at 10 K. Transport measurements reveal an insulating response, as expected. Optical…
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We report on the optimization process to synthesize epitaxial thin films of GdTiO3 on SrLaGaO4 substrates by pulsed laser deposition. Optimized films are free of impurity phases and are fully strained. They possess a magnetic Curie temperature TC = 31.8 K with a saturation magnetization of 4.2 muB per formula unit at 10 K. Transport measurements reveal an insulating response, as expected. Optical spectroscopy indicates a band gap of 0.7 eV, comparable to the bulk value. Our work adds ferrimagnetic orthotitanates to the palette of perovskite materials for the design of emergent strongly correlated states at oxide interfaces using a versatile growth technique such as pulsed laser deposition.
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Submitted 29 August, 2017;
originally announced August 2017.
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Atomically precise lateral modulation of a two-dimensional electron liquid in anatase TiO2 thin films
Authors:
Z. Wang,
Z. Zhong,
S. McKeown Walker,
Z. Ristic,
J. -Z. Ma,
F. Y. Bruno,
S. Ricco,
G. Sangiovanni,
G. Eres,
N. C. Plumb,
L. Patthey,
M. Shi,
J. Mesot,
F. Baumberger,
M. Radovic
Abstract:
Engineering the electronic band structure of two-dimensional electron liquids (2DELs) confined at the surface or interface of transition metal oxides is key to unlocking their full potential. Here we describe a new approach to tailoring the electronic structure of an oxide surface 2DEL demonstrating the lateral modulation of electronic states with atomic scale precision on an unprecedented length…
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Engineering the electronic band structure of two-dimensional electron liquids (2DELs) confined at the surface or interface of transition metal oxides is key to unlocking their full potential. Here we describe a new approach to tailoring the electronic structure of an oxide surface 2DEL demonstrating the lateral modulation of electronic states with atomic scale precision on an unprecedented length scale comparable to the Fermi wavelength. To this end, we use pulsed laser deposition to grow anatase TiO2 films terminated by a (1 x 4) in-plane surface reconstruction. Employing photo-stimulated chemical surface doping we induce 2DELs with tunable carrier densities that are confined within a few TiO2 layers below the surface. Subsequent in-situ angle resolved photoemission experiments demonstrate that the (1 x 4) surface reconstruction provides a periodic lateral perturbation of the electron liquid. This causes strong backfolding of the electronic bands, opening of unidirectional gaps and a saddle point singularity in the density of states near the chemical potential.
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Submitted 1 March, 2017;
originally announced March 2017.
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ARPES Studies of Two-Dimensional Electron Gases at Transition Metal Oxide Surfaces
Authors:
S. McKeown Walker,
F. Y. Bruno,
F. Baumberger
Abstract:
High mobility two-dimensional electron gases (2DEGs) underpin today's silicon based devices and are of fundamental importance for the emerging field of oxide electronics. Such 2DEGs are usually created by engineering band offsets and charge transfer at heterointerfaces. However, in 2011 it was shown that highly itinerant 2DEGs can also be induced at bare surfaces of different transition metal oxid…
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High mobility two-dimensional electron gases (2DEGs) underpin today's silicon based devices and are of fundamental importance for the emerging field of oxide electronics. Such 2DEGs are usually created by engineering band offsets and charge transfer at heterointerfaces. However, in 2011 it was shown that highly itinerant 2DEGs can also be induced at bare surfaces of different transition metal oxides where they are far more accessible to high resolution angle resolved photoemission (ARPES) experiments. Here we review work from this nascent field which has led to a systematic understanding of the subband structure arising from quantum confinement of highly anisotropic transition metal d-states along different crystallographic directions. We further discuss the role of different surface preparations and the origin of surface 2DEGs, the understanding of which has permitted control over 2DEG carrier densities. Finally, we discuss signatures of strong many-body interactions and how spectroscopic data from surface 2DEGs may be related to the transport properties of interface 2DEGs in the same host materials.
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Submitted 12 December, 2016;
originally announced December 2016.
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Electronic structure of buried LaNiO3 layers in (111)-oriented LaNiO3/LaMnO3 superlattices probed by soft x-ray ARPES
Authors:
F. Y. Bruno,
M. Gibert,
S. McKeown Walker,
O. E. Peil,
A. de la Torre,
S. Riccò,
Z. Wang,
S. Catalano,
A. Tamai,
F. Bisti,
V. N. Strocov,
J-M Triscone,
F. Baumberger
Abstract:
Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy we report electronic structure measurements of (111)-oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial films. For thin films we observe a 3D Fermi surface with an electron pocket at the Brillouin zone center and hole pockets at the zone vertices. Superlattices with thick nickelate…
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Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy we report electronic structure measurements of (111)-oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial films. For thin films we observe a 3D Fermi surface with an electron pocket at the Brillouin zone center and hole pockets at the zone vertices. Superlattices with thick nickelate layers present a similar electronic structure. However, as the thickness of the LaNiO3 is reduced the superlattices become insulating. These heterostructures do not show a marked redistribution of spectral weight in momentum space but exhibit a pseudogap of 50 meV.
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Submitted 25 November, 2016;
originally announced November 2016.
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Fermi arcs and their topological character in the candidate type-II Weyl semimetal MoTe2
Authors:
A. Tamai,
Q. S. Wu,
I. Cucchi,
F. Y. Bruno,
S. Ricco,
T. K. Kim,
M. Hoesch,
C. Barreteau,
E. Giannini,
C. Bernard,
A. A. Soluyanov,
F. Baumberger
Abstract:
We report a combined experimental and theoretical study of the candidate type-II Weyl semimetal MoTe2. Using laser-based angle-resolved photoemission we resolve multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces. All surface states observed experimentally are reproduced by an electronic structure calculation for the experimental crystal structure that predicts a topolog…
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We report a combined experimental and theoretical study of the candidate type-II Weyl semimetal MoTe2. Using laser-based angle-resolved photoemission we resolve multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces. All surface states observed experimentally are reproduced by an electronic structure calculation for the experimental crystal structure that predicts a topological Weyl semimetal state with 8 type-II Weyl points. We further use systematic electronic structure calculations simulating different Weyl point arrangements to discuss the robustness of the identified Weyl semimetal state and the topological character of Fermi arcs in MoTe2.
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Submitted 21 July, 2016; v1 submitted 27 April, 2016;
originally announced April 2016.
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Observation of Large Topologically Trivial Fermi-Arcs in the Candidate Type-II Weyl Semimetal WTe2
Authors:
F. Y. Bruno,
A. Tamai,
Q. S. Wu,
I. Cucchi,
C. Barreteau,
A. de la Torre,
S. McKeown Walker,
S. Riccò,
Z. Wang,
T. K. Kim,
M. Hoesch,
M. Shi,
N. C. Plumb,
E. Giannini,
A. A. Soluyanov,
F. Baumberger
Abstract:
We report angle-resolved photoemission experiments resolving the distinct electronic structure of the inequivalent top and bottom (001) surfaces of WTe2. On both surfaces, we identify a surface state that forms a large Fermi-arc emerging out of the bulk electron pocket. Using surface electronic structure calculations, we show that these Fermi arcs are topologically trivial and that their existence…
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We report angle-resolved photoemission experiments resolving the distinct electronic structure of the inequivalent top and bottom (001) surfaces of WTe2. On both surfaces, we identify a surface state that forms a large Fermi-arc emerging out of the bulk electron pocket. Using surface electronic structure calculations, we show that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. This implies that the observation of surface Fermi arcs alone does not allow the identification of WTe2 as a topological Weyl semimetal. We further use the identification of the two different surfaces to clarify the number of Fermi surface sheets in WTe2.
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Submitted 12 September, 2016; v1 submitted 8 April, 2016;
originally announced April 2016.
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Absence of Giant Spin Splitting in the Two-Dimensional Electron Liquid at the Surface of SrTiO$_3$ (001)
Authors:
S. McKeown Walker,
S Riccò,
F. Y. Bruno,
A. de la Torre,
A. Tamai,
E. Golias,
A. Varykhalov,
D. Marchenko,
M. Hoesch,
M. S. Bahramy,
P. D. C. King,
J. Sánchez-Barriga,
F. Baumberger
Abstract:
We reinvestigate the putative giant spin splitting at the surface of SrTiO$_3$ reported by Santander-Syro $et~al.$ [Nature Mat. 13, 1085 (2014)]. Our spin- and angle-resolved photoemission experiments on (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocurrent. We demonstrate that this result excludes a…
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We reinvestigate the putative giant spin splitting at the surface of SrTiO$_3$ reported by Santander-Syro $et~al.$ [Nature Mat. 13, 1085 (2014)]. Our spin- and angle-resolved photoemission experiments on (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocurrent. We demonstrate that this result excludes a giant spin splitting while it is fully consistent with the unconventional Rashba-like splitting seen in band structure calculations that reproduce the experimentally observed ladder of quantum confined subbands.
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Submitted 1 March, 2016;
originally announced March 2016.
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Centimeter-scale synthesis of ultrathin layered MoO3 by van der Waals epitaxy
Authors:
Aday J. Molina-Mendoza,
Jose Luis Lado,
Joshua Island,
Miguel Angel Niño,
Lucía Aballe,
Michael Foerster,
Flavio Y. Bruno,
Alejandro López-Moreno,
Luis Vaquero-Garzon,
Herre S. J. van der Zant,
Gabino Rubio-Bollinger,
Nicolas Agraït,
Emilio Perez,
Joaquin Fernandez-Rossier,
Andres Castellanos-Gomez
Abstract:
We report on the large-scale synthesis of highly oriented ultrathin MoO3 layers using a simple and low-cost atmospheric pressure by van der Waals epitaxy growth on muscovite mica substrates. By this method we are able to synthetize high quality centimeter-scale MoO3 crystals with thicknesses ranging from 1.4 nm (two layers) up to a few nanometers. The crystals can be easily transferred to an arbit…
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We report on the large-scale synthesis of highly oriented ultrathin MoO3 layers using a simple and low-cost atmospheric pressure by van der Waals epitaxy growth on muscovite mica substrates. By this method we are able to synthetize high quality centimeter-scale MoO3 crystals with thicknesses ranging from 1.4 nm (two layers) up to a few nanometers. The crystals can be easily transferred to an arbitrary substrate (such as SiO2) by a deterministic transfer method and extensively characterized to demonstrate the high quality of the resulting crystal. We also study the electronic band structure of the material by density functional theory calculations. Interestingly, the calculations demonstrate that bulk MoO3 has a rather weak electronic interlayer interaction and thus it presents a monolayer-like band structure. Finally, we demonstrate the potential of this synthesis method for optoelectronic applications by fabricating large-area field-effect devices (10 micrometers by 110 micrometers in lateral dimensions), finding responsivities of 30 mA/W for a laser power density of 13 mW/cm2 in the UV region of the spectrum and also as an electron acceptor in a MoS2-based field-effect transistor.
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Submitted 20 June, 2016; v1 submitted 14 December, 2015;
originally announced December 2015.
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Tailoring the nature and strength of electron-phonon interactions in the SrTiO$_3$(001) two-dimensional electron liquid
Authors:
Z. Wang,
S. McKeown Walker,
A. Tamai,
Y. Wang,
Z. Ristic,
F. Y. Bruno,
A. de la Torre,
S. Riccò,
N. C. Plumb,
M. Shi,
P. Hlawenka,
J. Sánchez-Barriga,
A. Varykhalov,
T. K. Kim,
M. Hoesch,
P. D. C. King,
W. Meevasana,
U. Diebold,
J. Mesot,
B. Moritz,
T. P. Devereaux,
M. Radovic,
F. Baumberger
Abstract:
Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO$_3$(001) surface to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density…
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Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO$_3$(001) surface to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO$_3$/SrTiO$_3$ interface, our angle-resolved photoemission data show replica bands separated by 100\,meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies strong long-range coupling to a single longitudinal optical phonon mode. In the overdoped regime the preferential coupling to this mode decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow for a unified understanding of the transport properties in SrTiO$_3$-based 2DELs.
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Submitted 19 February, 2016; v1 submitted 3 June, 2015;
originally announced June 2015.
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Collapse of the Mott gap and emergence of a nodal liquid in lightly doped Sr$_2$IrO$_4$
Authors:
A. de la Torre,
S. McKeown Walker,
F. Y. Bruno,
S. Ricco,
Z. Wang,
I. Gutierrez Lezama,
G. Scheerer,
G. Giriat,
D. Jaccard,
C. Berthod,
T. K. Kim,
M. Hoesch,
E. C. Hunter,
R. S. Perry,
A. Tamai,
F. Baumberger
Abstract:
Superconductivity in underdoped cuprates emerges from an unusual electronic state characterised by nodal quasiparticles and an antinodal pseudogap. The relation between this state and superconductivity is intensely studied but remains controversial. The discrimination between competing theoretical models is hindered by a lack of electronic structure data from related doped Mott insulators. Here we…
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Superconductivity in underdoped cuprates emerges from an unusual electronic state characterised by nodal quasiparticles and an antinodal pseudogap. The relation between this state and superconductivity is intensely studied but remains controversial. The discrimination between competing theoretical models is hindered by a lack of electronic structure data from related doped Mott insulators. Here we report the doping evolution of the Heisenberg antiferromagnet Sr$_2$IrO$_4$, a close analogue to underdoped cuprates. We demonstrate that metallicity emerges from a rapid collapse of the Mott gap with doping, resulting in lens-like Fermi contours rather than disconnected Fermi arcs as observed in cuprates. Intriguingly though, the emerging electron liquid shows nodal quasiparticles with an antinodal pseudogap and thus bares strong similarities with underdoped cuprates. We conclude that anisotropic pseudogaps are a generic property of two-dimensional doped Mott insulators rather than a unique hallmark of cuprate high-temperature superconductivity.
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Submitted 1 June, 2015;
originally announced June 2015.
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Control of a two-dimensional electron gas on SrTiO3(111) by atomic oxygen
Authors:
S. McKeown Walker,
A. de la Torre,
F. Y. Bruno,
A. Tamai,
T. K. Kim,
M. Hoesch,
M. Shi,
M. S. Bahramy,
P. D. C. King,
F. Baumberger
Abstract:
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 6-fold 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 incorporat…
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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 6-fold 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.
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Submitted 13 May, 2014;
originally announced May 2014.
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Upper limit to magnetism in LaAlO3/SrTiO3 heterostructures
Authors:
M. R. Fitzsimmons,
N. Hengartner,
S. Singh,
M. Zhernenkov,
F. Y. Bruno,
J. Santamaria,
A. Brinkman,
M. Huijben,
H. Molegraaf,
J. de la Venta,
Ivan K. Schuller
Abstract:
Using polarized neutron reflectometry (PNR) we measured the neutron spin dependent reflectivity from four LaAlO3/SrTiO3 superlattices. This experiment implies that the upper limit for the magnetization induced by an 11 T magnetic field at 1.7 K is 2 emu/cm3. SQUID magnetometry of the superlattices sporadically finds an enhanced moment, possibly due to experimental artifacts. These observations set…
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Using polarized neutron reflectometry (PNR) we measured the neutron spin dependent reflectivity from four LaAlO3/SrTiO3 superlattices. This experiment implies that the upper limit for the magnetization induced by an 11 T magnetic field at 1.7 K is 2 emu/cm3. SQUID magnetometry of the superlattices sporadically finds an enhanced moment, possibly due to experimental artifacts. These observations set important restrictions on theories which imply a strongly enhanced magnetism at the interface between LaAlO3 and SrTiO3.
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Submitted 6 September, 2011;
originally announced September 2011.