Periodic Reporting for period 1 - SURFER (Superconducting and ferroelectric two-dimensional electron gases (2DEGs) at oxide interfaces.)
Okres sprawozdawczy: 2023-06-01 do 2025-05-31
Complex oxide materials represent one of the most promising frontiers for realizing multifunctional devices that address pressing global challenges in energy efficiency, sustainable electronics, and emerging quantum technologies. These materials uniquely combine diverse functional properties – such as superconductivity (SC), ferroelectricity (FE), and ferromagnetism (FM) – within a single crystalline framework. Traditionally, many of these properties were thought to be mutually exclusive due to their antagonistic physical origins. However, recent breakthroughs in materials synthesis, nanoscale interface engineering, and strain control have demonstrated that such properties can not only coexist but also couple in ways that generate entirely new functionalities. In particular, the interplay between SC and FE has garnered significant attention in the condensed matter and quantum materials community. While bulk SrTiO3 (STO) has served as a model system for exploring this coexistence, practical device applications remain limited because conventional bulk doping renders the entire crystal conducting, eliminating the possibility of electric-field control over the electronic phases. This constraint is a major barrier to realizing tunable, low-power quantum devices based on the SC–FE coupling. Recently, it has been shown that two-dimensional electron gases (2DEGs) formed at carefully designed oxide interfaces can exhibit ferroelectric ordering while remaining highly conductive, offering an unprecedented platform to electrically modulate superconducting properties. However, only a handful of systems – most notably LaAlO3/Ca-doped STO – have shown experimental signatures of both FE and SC coexisting at the interface, and even these do not display the anticipated enhancement of the superconducting transition temperature. This underlines the urgent scientific need to discover, engineer, and understand new oxide 2DEG systems that can unlock the full potential of SC–FE coupling.
The SURFER project directly addresses this knowledge and technology gap by moving beyond bulk-doped oxides to design and fabricate oxide heterostructures that host robust, switchable 2DEGs with coupled superconducting and ferroelectric properties. By leveraging state-of-the-art thin-film deposition techniques, atomic-scale interface engineering, and epitaxial strain tuning, SURFER will pioneer new pathways for manipulating quantum states in low-dimensional systems. Specifically, the project will systematically investigate three underexplored but promising oxide platforms: (1) strained SrTiO3 (STO), where epitaxial strain can stabilize ferroelectricity; (2) BaTiO3 (BTO), a well-known robust ferroelectric whose single-domain structure can be precisely controlled through strain; and (3) Nb-doped KTaO3 (KTN), which combines emergent superconductivity with strong spin-orbit coupling, offering an additional handle for quantum functionality. To create high-mobility 2DEGs in these systems, SURFER will implement two complementary approaches: (a) deposition of reactive metals to induce redox-driven carrier formation at the oxide surface, and (b) growth of polar ABO3 oxides to trigger electronic reconstruction at the interface. The overarching objectives of SURFER are to discover new material systems that support tunable SC – FE coupling, unravel the underlying physical mechanisms that link these phases, identify routes to enhance superconducting transition temperatures through ferroelectric ordering, and develop strategies for electric-field control of superconductivity in 2DEGs. The insights gained will advance fundamental understanding at the intersection of quantum materials, oxide electronics, and interface physics.
By doing so, SURFER aligns with the European Union’s strategic goals under the Green Deal and Quantum Technologies Flagship, paving the way for future generations of energy-efficient, multifunctional devices such as ultra-low-power switches, quantum sensors, and reconfigurable superconducting circuits. In the longer term, these advances could have far-reaching impacts on information processing, secure communication, and sustainable digital infrastructures. In addition to its scientific and technological ambitions, SURFER integrates social sciences perspectives to assess the societal readiness and potential ethical implications of these emerging quantum technologies, ensuring that the breakthroughs are developed responsibly and with public trust.
In summary, SURFER sets the scene for a new class of oxide-based quantum materials with controllable multifunctional properties, strengthening Europe’s leadership in advanced functional oxides and positioning European research and industry at the forefront of the next technological revolution.
The SURFER project directly addresses this knowledge and technology gap by moving beyond bulk-doped oxides to design and fabricate oxide heterostructures that host robust, switchable 2DEGs with coupled superconducting and ferroelectric properties. By leveraging state-of-the-art thin-film deposition techniques, atomic-scale interface engineering, and epitaxial strain tuning, SURFER will pioneer new pathways for manipulating quantum states in low-dimensional systems. Specifically, the project will systematically investigate three underexplored but promising oxide platforms: (1) strained SrTiO3 (STO), where epitaxial strain can stabilize ferroelectricity; (2) BaTiO3 (BTO), a well-known robust ferroelectric whose single-domain structure can be precisely controlled through strain; and (3) Nb-doped KTaO3 (KTN), which combines emergent superconductivity with strong spin-orbit coupling, offering an additional handle for quantum functionality. To create high-mobility 2DEGs in these systems, SURFER will implement two complementary approaches: (a) deposition of reactive metals to induce redox-driven carrier formation at the oxide surface, and (b) growth of polar ABO3 oxides to trigger electronic reconstruction at the interface. The overarching objectives of SURFER are to discover new material systems that support tunable SC – FE coupling, unravel the underlying physical mechanisms that link these phases, identify routes to enhance superconducting transition temperatures through ferroelectric ordering, and develop strategies for electric-field control of superconductivity in 2DEGs. The insights gained will advance fundamental understanding at the intersection of quantum materials, oxide electronics, and interface physics.
By doing so, SURFER aligns with the European Union’s strategic goals under the Green Deal and Quantum Technologies Flagship, paving the way for future generations of energy-efficient, multifunctional devices such as ultra-low-power switches, quantum sensors, and reconfigurable superconducting circuits. In the longer term, these advances could have far-reaching impacts on information processing, secure communication, and sustainable digital infrastructures. In addition to its scientific and technological ambitions, SURFER integrates social sciences perspectives to assess the societal readiness and potential ethical implications of these emerging quantum technologies, ensuring that the breakthroughs are developed responsibly and with public trust.
In summary, SURFER sets the scene for a new class of oxide-based quantum materials with controllable multifunctional properties, strengthening Europe’s leadership in advanced functional oxides and positioning European research and industry at the forefront of the next technological revolution.
During this project, the researcher systematically investigated two ferroelectric oxide systems: (1) strain-engineered ferroelectric SrTiO3 (STO) and (2) BaTiO3 (BTO) thin films.
1. For the STO system, high-quality STO films were grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates using hybrid molecular beam epitaxy (hMBE) in collaboration with a partner laboratory. The lattice mismatch between STO (3.905 Å) and LSAT (3.868 Å) results in a coherent compressive strain of -0.95%, confirmed by X-ray diffraction (XRD) 2θ-ω scans and reciprocal space mapping (RSM). The films showed excellent crystallinity with an out-of-plane lattice parameter elongated to 3.932 ± 0.001 Å, consistent with stoichiometric STO under compressive strain and a Poisson ratio of 0.25. Raman spectroscopy confirmed the ferroelectric nature of the strained STO films through the clear observation of TO2, TO₄, and LO₄ phonon modes characteristic of the tetragonal polar phase. The ferroelectric transition temperature (Tc) was determined to be ~165 ± 10 K, significantly higher than that of bulk Ca-substituted STO single crystals (~30-50 K).
These high-quality STO films were further used to generate two-dimensional electron gases (2DEGs) by depositing thin Al layers via DC magnetron sputtering at room temperature. X-ray photoelectron spectroscopy (XPS) verified the formation of the 2DEG via redox reactions, evidenced by the presence of Ti3+ and Ti2+ states alongside Ti4+. Monochromated scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) further resolved Ti3+ near the interface at the Ti-L2,3 edge. The dimensionality of the electron gas is also confirmed by performing the in-plane angle-dependent magneto-transport measurements on our 2DEG samples, by varying the angle between applied magnetic field and the sample plane. We observe a negative MR of approximately 4% change when the field is applied perpendicular to the current, whereas MR nearly vanishes when the field was applied parallel. This is consistent with weak localization in a 2D transport system.
One of our major outcomes is that these 2DEGs in strained STO films showed significantly higher low-temperature electron mobilities compared to similar STO-based 2DEGs fabricated by pulsed laser deposition (PLD), highlighting the superior crystal quality achieved by hMBE. Another important outcome is that in our 2DEG samples (Al/STO//LSAT), ferroelectric ordering persists up to ~ 130 K, revealed by transport and Raman measurements, notably higher than that reported for Ca-doped STO-based 2DEGs (~ 30 K). To investigate interfacial structural evolution, STEM-geometrical phase analysis was conducted on pristine strained STO films and on the Al//STO heterostructures at low temperatures. An additional c-axis elongation of ~ 4.3% near the interface was observed on top of the ~ 1.7% strain from epitaxy, indicating a c-axis gradient possibly linked to the ferroelectric transition.
Overall, this work demonstrates the fabrication of high-quality strained ferroelectric STO films and the realization of robust ferroelectric 2DEGs with significantly enhanced transition temperatures and mobilities, offering a promising platform for the development of ferroelectric 2DEGs functional at higher temperatures.
2. For BTO system, the researcher investigated BTO films grown on (110) GdScO3 (GSO) substrates using two deposition techniques: hMBE and PLD. High-quality BTO films grown by hMBE were provided by a collaborating laboratory while the researcher optimized the PLD growth conditions to achieve single-domain BTO films directly on GSO and on SrRuO3 bottom electrodes. Films grown by both techniques demonstrated excellent structural quality and coherent strain, as confirmed by XRD and RSM. Temperature-dependent XRD measurements indicated a ferroelectric transition temperature (Tc) of approximately 700 K. The BTO films exhibited robust ferroelectric switching behavior and remarkable endurance, maintaining ferroelectric performance without fatigue even after more than 10¹¹ polarization switching cycles, underscoring their potential for device applications. Similar to the STO system, a 2DEG was generated at the surface of the BTO films through a redox process by depositing an Al layer. XPS confirmed the formation of the 2DEG, and transport measurements showed metallic conductivity, with properties tunable by adjusting the Al thickness. Polarization maps derived from GPA provided direct evidence for the coexistence of ferroelectricity and interfacial conductivity in the BTO 2DEG region.
In addition, the researcher demonstrated spin-to-charge conversion at room temperature in these BTO-based 2DEGs via inverse Edelstein effect (IEE), enabled by interfacing the BTO 2DEG with a ferromagnetic NiFe layer for spin current injection. Our results demonstrate the feasibility of integrating high-Tc ferroelectric 2DEGs in spintornics and present a critical step toward the development of energy-efficient, ferroelectric spin-orbit (FESO) for next-generation spintronic applications.
1. For the STO system, high-quality STO films were grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates using hybrid molecular beam epitaxy (hMBE) in collaboration with a partner laboratory. The lattice mismatch between STO (3.905 Å) and LSAT (3.868 Å) results in a coherent compressive strain of -0.95%, confirmed by X-ray diffraction (XRD) 2θ-ω scans and reciprocal space mapping (RSM). The films showed excellent crystallinity with an out-of-plane lattice parameter elongated to 3.932 ± 0.001 Å, consistent with stoichiometric STO under compressive strain and a Poisson ratio of 0.25. Raman spectroscopy confirmed the ferroelectric nature of the strained STO films through the clear observation of TO2, TO₄, and LO₄ phonon modes characteristic of the tetragonal polar phase. The ferroelectric transition temperature (Tc) was determined to be ~165 ± 10 K, significantly higher than that of bulk Ca-substituted STO single crystals (~30-50 K).
These high-quality STO films were further used to generate two-dimensional electron gases (2DEGs) by depositing thin Al layers via DC magnetron sputtering at room temperature. X-ray photoelectron spectroscopy (XPS) verified the formation of the 2DEG via redox reactions, evidenced by the presence of Ti3+ and Ti2+ states alongside Ti4+. Monochromated scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) further resolved Ti3+ near the interface at the Ti-L2,3 edge. The dimensionality of the electron gas is also confirmed by performing the in-plane angle-dependent magneto-transport measurements on our 2DEG samples, by varying the angle between applied magnetic field and the sample plane. We observe a negative MR of approximately 4% change when the field is applied perpendicular to the current, whereas MR nearly vanishes when the field was applied parallel. This is consistent with weak localization in a 2D transport system.
One of our major outcomes is that these 2DEGs in strained STO films showed significantly higher low-temperature electron mobilities compared to similar STO-based 2DEGs fabricated by pulsed laser deposition (PLD), highlighting the superior crystal quality achieved by hMBE. Another important outcome is that in our 2DEG samples (Al/STO//LSAT), ferroelectric ordering persists up to ~ 130 K, revealed by transport and Raman measurements, notably higher than that reported for Ca-doped STO-based 2DEGs (~ 30 K). To investigate interfacial structural evolution, STEM-geometrical phase analysis was conducted on pristine strained STO films and on the Al//STO heterostructures at low temperatures. An additional c-axis elongation of ~ 4.3% near the interface was observed on top of the ~ 1.7% strain from epitaxy, indicating a c-axis gradient possibly linked to the ferroelectric transition.
Overall, this work demonstrates the fabrication of high-quality strained ferroelectric STO films and the realization of robust ferroelectric 2DEGs with significantly enhanced transition temperatures and mobilities, offering a promising platform for the development of ferroelectric 2DEGs functional at higher temperatures.
2. For BTO system, the researcher investigated BTO films grown on (110) GdScO3 (GSO) substrates using two deposition techniques: hMBE and PLD. High-quality BTO films grown by hMBE were provided by a collaborating laboratory while the researcher optimized the PLD growth conditions to achieve single-domain BTO films directly on GSO and on SrRuO3 bottom electrodes. Films grown by both techniques demonstrated excellent structural quality and coherent strain, as confirmed by XRD and RSM. Temperature-dependent XRD measurements indicated a ferroelectric transition temperature (Tc) of approximately 700 K. The BTO films exhibited robust ferroelectric switching behavior and remarkable endurance, maintaining ferroelectric performance without fatigue even after more than 10¹¹ polarization switching cycles, underscoring their potential for device applications. Similar to the STO system, a 2DEG was generated at the surface of the BTO films through a redox process by depositing an Al layer. XPS confirmed the formation of the 2DEG, and transport measurements showed metallic conductivity, with properties tunable by adjusting the Al thickness. Polarization maps derived from GPA provided direct evidence for the coexistence of ferroelectricity and interfacial conductivity in the BTO 2DEG region.
In addition, the researcher demonstrated spin-to-charge conversion at room temperature in these BTO-based 2DEGs via inverse Edelstein effect (IEE), enabled by interfacing the BTO 2DEG with a ferromagnetic NiFe layer for spin current injection. Our results demonstrate the feasibility of integrating high-Tc ferroelectric 2DEGs in spintornics and present a critical step toward the development of energy-efficient, ferroelectric spin-orbit (FESO) for next-generation spintronic applications.
This project has delivered results that significantly advance the state of the art in ferroelectric oxide thin films and ferroelectric 2DEGs. For the strain-engineered STO system, the successful growth of coherently strained, high-crystallinity STO films by hybrid MBE has enabled the realization of ferroelectricity with a transition temperature (~165 K) far exceeding that of bulk Ca-substituted STO crystals (~30–50 K). The generation of high-mobility 2DEGs demonstrates that these strained STO film-based 2DEGs show ferroelectric ordering up to ~130 K, setting a new benchmark for ferroelectric 2DEGs compared to conventional Ca-STO systems.
For the BTO system, the project achieved single-domain, high-endurance BTO thin films with exceptional ferroelectric switching stability (no fatigue after 10¹¹ cycles) and a high Tc (~710 K). Importantly, the coexistence of ferroelectricity and metallic 2DEGs at the BTO surface was directly confirmed through GPA-derived polarization maps and XPS. We obtained efficient room-temperature spin-to-charge conversion at BTO 2DEGs and larger Rashba compared to SrTiO₃-based 2DEGs.
For the BTO system, the project achieved single-domain, high-endurance BTO thin films with exceptional ferroelectric switching stability (no fatigue after 10¹¹ cycles) and a high Tc (~710 K). Importantly, the coexistence of ferroelectricity and metallic 2DEGs at the BTO surface was directly confirmed through GPA-derived polarization maps and XPS. We obtained efficient room-temperature spin-to-charge conversion at BTO 2DEGs and larger Rashba compared to SrTiO₃-based 2DEGs.