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Direct Fabrication of a Superconducting Two-Dimensional Electron Gas on KTaO3(111) via Mg-Induced Surface Reduction
Authors:
Chun Sum Brian Pang,
Bruce A. Davidson,
Fengmiao Li,
Mohamed Oudah,
Peter C. Moen,
Steef Smit,
Cissy T. Suen,
Simon Godin,
Sergey A. Gorovikov,
Marta Zonno,
Pinder Dosanjh,
Sergey Zhdanovich,
Giorgio Levy,
Matteo Michiardi,
Alannah M. Hallas,
George A. Sawatzky,
Robert J. Green,
Andrea Damascelli,
Ke Zou
Abstract:
Two-dimensional electron gases (2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong spin-orbit coupling, Rashba physics, and low-carrier-density superconductivity. Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both generate carriers and can obscure the native electronic structure, making spectrosco…
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Two-dimensional electron gases (2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong spin-orbit coupling, Rashba physics, and low-carrier-density superconductivity. Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both generate carriers and can obscure the native electronic structure, making spectroscopic access to the underlying 2DEG challenging. Here, we demonstrate a simple and direct method to generate a superconducting 2DEG on KTaO3(111) using Mg-induced surface reduction in molecular-beam epitaxy (MBE). Mg has an extremely low sticking coefficient at elevated temperatures, enabling the formation of an ultrathin (less than 1-2 monolayers) MgO layer that is transparent to soft x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES). This allows direct measurement of the surface chemistry and low-energy electronic structure of the pristine reduced surface without the need for a several-nanometer-thick capping layer. XPS shows clear reduction of Ta5+ to lower oxidation states, while ARPES reveals a parabolic Ta 5d conduction band with an approximately 150 meV bandwidth and additional subband features arising from quantum confinement. Transport measurements confirm a superconducting transition below 0.7 K. Together, these results demonstrate a chemically straightforward and controllable pathway for fabricating spectroscopically accessible superconducting 2DEGs on KTaO3(111), and provide a powerful new platform for investigating the mechanisms underlying orientation-dependent superconductivity in KTaO3-based oxide interfaces.
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Submitted 20 April, 2026; v1 submitted 21 December, 2025;
originally announced December 2025.
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Magnetic Ni-N-Ni Centers in N-substituted NiO
Authors:
Simon Godin,
Ilya S. Elfimov,
Fengmiao Li,
Bruce A. Davidson,
Ronny Sutarto,
Jonathan D. Denlinger,
Liu Hao Tjeng,
George A. Sawatzky,
Ke Zou
Abstract:
To explore how anion substitution modifies the existing magnetism in strongly correlated oxides, we investigate local electronic states and magnetic ordering in nickel oxide (NiO) induced by substituting oxygen (O) with nitrogen (N). Each N introduces an additional N 2p hole and modifies the magnetic moment of a neighboring nickel (Ni) cation site, as the exchange interaction between this hole and…
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To explore how anion substitution modifies the existing magnetism in strongly correlated oxides, we investigate local electronic states and magnetic ordering in nickel oxide (NiO) induced by substituting oxygen (O) with nitrogen (N). Each N introduces an additional N 2p hole and modifies the magnetic moment of a neighboring nickel (Ni) cation site, as the exchange interaction between this hole and the Ni eg electrons exceeds the Ni-O-Ni superexchange interaction. This leads to the formation of Ni-N-Ni centers consisting of five spins, without perturbing the antiferromagnetic NiO lattice. These centers are studied using density functional theory and confirmed through high-resolution spectroscopy on N-substituted NiO thin films grown by molecular beam epitaxy. This type of magnetic design may enable future advances in quantum technologies based on strongly correlated materials, such as quantum sensors and spin-based qubits.
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Submitted 25 July, 2025; v1 submitted 26 December, 2024;
originally announced December 2024.
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Electronically-driven switching of topology in LaSbTe
Authors:
J. Bannies,
M. Michiardi,
H. -H. Kung,
S. Godin,
J. W. Simonson,
M. Oudah,
M. Zonno,
S. Gorovikov,
S. Zhdanovich,
I. S. Elfimov,
A. Damascelli,
M. C. Aronson
Abstract:
In the past two decades, various classes of topological materials have been discovered, spanning topological insulators, semimetals, and metals. While the observation and understanding of the topology of a material has been a primary focus so far, the precise and easy control of topology in a single material remains largely unexplored. Here, we demonstrate full experimental control over the topolo…
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In the past two decades, various classes of topological materials have been discovered, spanning topological insulators, semimetals, and metals. While the observation and understanding of the topology of a material has been a primary focus so far, the precise and easy control of topology in a single material remains largely unexplored. Here, we demonstrate full experimental control over the topological Dirac nodal loop in the square-net material LaSb$_\mathrm{x}$Te$_\mathrm{2-x}$ by chemical substitution and electron doping. Using angle-resolved photoemission spectroscopy (ARPES), we show that changing the antimony concentration x from 0.9 to 1.0 in the bulk opens a gap as large as 400 meV in the nodal loop. Our symmetry analysis based on single-crystal X-ray diffraction and a minimal tight binding model establishes that the breaking of \textit{n} glide symmetry in the square-net layer is responsible for the opening of the gap. Remarkably, we can also realize this topological phase transition \textit{in situ} on the surface of LaSb$_\mathrm{x}$Te$_\mathrm{2-x}$ by chemical gating using potassium deposition, which enables the reversible switching of the topology from gapped to gapless nodal loop. The underlying control parameter for the structural and topological transition in the bulk and on the surface is the electron concentration. It opens a pathway towards applications in devices based on switching topology by electrostatic gating.
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Submitted 11 July, 2024;
originally announced July 2024.
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Method of Mechanical Exfoliation of Bismuth with Micro-Trench Structures
Authors:
Oulin Yu,
Raphaela Allgayer,
Simon Godin,
Jacob Lalande,
Paolo Fossati,
Chunwei Hsu,
Thomas Szkopek,
Guillaume Gervais
Abstract:
The discovery of graphene led to a burst in search for 2D materials originating from layered atomic crystals coupled by van der Waals force. While bulk bismuth crystals share this layered crystal structure, unlike other group V members of the periodic table, its interlayer bonds are stronger such that traditional mechanical cleavage and exfoliation techniques have shown to be inefficient. In this…
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The discovery of graphene led to a burst in search for 2D materials originating from layered atomic crystals coupled by van der Waals force. While bulk bismuth crystals share this layered crystal structure, unlike other group V members of the periodic table, its interlayer bonds are stronger such that traditional mechanical cleavage and exfoliation techniques have shown to be inefficient. In this work, we present a novel mechanical cleavage method for exfoliating bismuth by utilizing the stress concentration effect induced by micro-trench SiO2 structures. As a result, the exfoliated bismuth flakes can achieve thicknesses down to the sub-10 nm range which are analyzed by AFM and Raman spectroscopy.
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Submitted 5 June, 2024; v1 submitted 2 November, 2023;
originally announced November 2023.
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Critical Role of Disorder for Superconductivity in the Series of Epitaxial Ti(O,N) Films
Authors:
Fengmiao Li,
Oliver Dicks,
Myung-Geun Han,
Solveig Aamlid,
Giorgio Levy,
Ronny Sutarto,
Chong Liu,
Hsiang-Hsi Kung,
Oleksandr Foyevstov,
Simon Godin,
Bruce A. Davidson,
Andrea Damascelli,
Yimei Zhu,
Christoph Heil,
Ilya Elfimov,
George A. Sawatzky,
Ke Zou
Abstract:
Realizing experimental control of superconductivity is of paramount importance to advancing both basic research and technological applications. Disorder, generally existing in most superconductors, intricately interacts with Cooper pairs and also impacts the performance of quantum devices. In this paper, we report the study of a series of Ti(O,N) crystalline films prepared via molecular beam epita…
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Realizing experimental control of superconductivity is of paramount importance to advancing both basic research and technological applications. Disorder, generally existing in most superconductors, intricately interacts with Cooper pairs and also impacts the performance of quantum devices. In this paper, we report the study of a series of Ti(O,N) crystalline films prepared via molecular beam epitaxy (MBE). We discover that substituting nitrogen (N) for oxygen (O) in TiO, namely TiO(N), considerably increases the normal-state conductivity and the superconducting transition temperature Tc. The Tc of TiO(N) falling between those of TiO (about 0.5 K) and TiN (about 6 K) is contrary to their comparable Tc predicted by the Migdal Eliasberg theory. It is found that their resistivity vs temperature obeys the Mooij rule, known as the characteristic of metallic glasses. Density functional theory (DFT) calculations demonstrate that strong disorder severely scatters the Bloch electron waves at nonzero momenta, which consequently weakens electron-phonon coupling in TiO(N).
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Submitted 24 November, 2024; v1 submitted 4 October, 2023;
originally announced October 2023.