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In situ Al$_2$O$_3$ passivation of epitaxial tantalum and aluminum films enables long-term stability in superconducting microwave resonators
Authors:
Yi-Ting Cheng,
Hsien-Wen Wan,
Wei-Jie Yan,
Lawrence Boyu Young,
Yen-Hsun Glen Lin,
Kuan-Hui Lai,
Wan-Sin Chen,
Chao-Kai Cheng,
Ko-Hsuan Mandy Chen,
Tun-Wen Pi,
Yen-Hsiang Lin,
Jueinai Kwo,
Minghwei Hong
Abstract:
Long-term stability of superconducting microwave resonators is essential for scalable quantum technologies; however, surface and interface degradation continue to limit device stability. Here, we demonstrate exceptional stability in microstrip resonators fabricated from epitaxial tantalum and aluminum films, protected by in situ deposited Al$_2$O$_3$ under ultra-high vacuum. These resonators initi…
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Long-term stability of superconducting microwave resonators is essential for scalable quantum technologies; however, surface and interface degradation continue to limit device stability. Here, we demonstrate exceptional stability in microstrip resonators fabricated from epitaxial tantalum and aluminum films, protected by in situ deposited Al$_2$O$_3$ under ultra-high vacuum. These resonators initially exhibit internal quality factors (Qi) exceeding one million and maintain high performance with minimal degradation after up to fourteen months of air exposure. In contrast, devices relying on native surface oxides show substantial declines in Qi over time, indicating increased microwave losses. X-ray photoelectron spectroscopy reveals that the in situ Al$_2$O$_3$ effectively suppresses interfacial oxidation and preserves the chemical integrity of the underlying superconducting films, whereas native oxides permit progressive oxidation, leading to device degradation. These findings establish a robust, scalable passivation strategy that addresses a longstanding materials challenge in the development of superconducting quantum circuits.
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Submitted 2 August, 2025;
originally announced August 2025.
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1/ f noise and two-level systems in MBE-grown Al thin films
Authors:
Shouray Kumar Sahu,
Yen-Hsun Glen Lin,
Kuan-Hui Lai,
Chao-Kai Cheng,
Chun-Wei Wu,
Elica Anne Heredia,
Ray-Tai Wang,
Yen-Hsiang Lin,
Juainai Kwo,
Minghwei Hong,
Juhn-Jong Lin,
Sheng-Shiuan Yeh
Abstract:
Aluminum thin films are essential to the functionalities of electronic and quantum devices, where two-level systems (TLS) can degrade device performance. MBE-grown Al films may appeal to these applications due to their low TLS densities. We studied the energy distributions of TLS densities, g(E), in 10-nm-thick MBE-grown and electron-beam evaporated Al films through 1/f noise measurements between…
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Aluminum thin films are essential to the functionalities of electronic and quantum devices, where two-level systems (TLS) can degrade device performance. MBE-grown Al films may appeal to these applications due to their low TLS densities. We studied the energy distributions of TLS densities, g(E), in 10-nm-thick MBE-grown and electron-beam evaporated Al films through 1/f noise measurements between 80 and 360 K. At 300 K, the noise magnitudes in MBE-grown films are about three times lower than in the electron-beam evaporated films, corresponding to the g(E) values about ten times lower in the former than in the latter. Compared with previously established observations, we identified that the 1/f noise was generated by thermally activated TLS at grain boundaries.
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Submitted 2 July, 2025;
originally announced July 2025.
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Electrically Sign-Reversible Topological Hall Effect in a Top-Gated Topological Insulator (Bi,Sb)2Te3 on a Ferrimagnetic Insulator Europium Iron Garnet
Authors:
Jyun-Fong Wong,
Ko-Hsuan Mandy Chen,
Jui-Min Chia,
Zih-Ping Huang,
Sheng-Xin Wang,
Pei-Tze Chen,
Lawrence Boyu Young,
Yen-Hsun Glen Lin,
Shang-Fan Lee,
Chung-Yu Mou,
Minghwei Hong,
Jueinai Kwo
Abstract:
Topological Hall effect (THE), an electrical transport signature of systems with chiral spin textures like skyrmions, has been observed recently in topological insulator (TI)-based magnetic heterostructures. However, the intriguing interplay between the topological surface state and THE is yet to be fully understood. In this work, we report a large THE of ~10 ohm (~4 micro-ohm*cm) at 2 K with an e…
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Topological Hall effect (THE), an electrical transport signature of systems with chiral spin textures like skyrmions, has been observed recently in topological insulator (TI)-based magnetic heterostructures. However, the intriguing interplay between the topological surface state and THE is yet to be fully understood. In this work, we report a large THE of ~10 ohm (~4 micro-ohm*cm) at 2 K with an electrically reversible sign in a top-gated 4 nm TI (Bi0.3Sb0.7)2Te3 (BST) grown on a ferrimagnetic insulator (FI) europium iron garnet (EuIG). Temperature, external magnetic field angle, and top gate bias dependences of magnetotransport properties were investigated and consistent with a skyrmion-driven THE. Most importantly, a sign change in THE was discovered as the Fermi level was tuned from the upper to the lower parts of the gapped Dirac cone and vice versa. This discovery is anticipated to impact technological applications in ultralow power skyrmion-based spintronics.
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Submitted 13 April, 2023; v1 submitted 31 December, 2022;
originally announced January 2023.
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Single-crystal epitaxial europium iron garnet films with strain-induced perpendicular magnetic anisotropy: structural, strain, magnetic, and spin transport properties
Authors:
M. X. Guo,
C. K. Cheng,
Y. C. Liu,
C. N. Wu,
W. N. Chen,
T. Y Chen,
C. T. Wu,
C. H. Hsu,
S. Q. Zhou,
C. F. Chang,
L. H. Tjeng,
S. F. Lee,
C. F. Pai,
M. Hong,
J. Kwo
Abstract:
Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, part…
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Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with roughness as low as 0.1 nm as observed using atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed in-plane epitaxial film growth, very smooth film/substrate interface, excellent film crystallinity with a small full width at half maximum of 0.012$^{\circ}$ in the rocking curve scans, and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The measured squarish out-of-plane magnetization-field hysteresis loops by vibrating sample magnetometry in conjunction with the measurements from angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. We have tailored the magnetic properties of the EuIG thin films, including saturation magnetization ranging from 71.91 to 124.51 emu/c.c. (increase with the (Eu/Fe) ratios), coercive field from 27 to 157.64 Oe, and the strength of PMA field ($H_\bot$) increasing from 4.21 to 18.87 kOe with the in-plane compressive strain from -0.774 to -1.044%. We have also investigated spin transport in Pt/EuIG bi-layer structure and evaluated the real part of spin mixing conductance to be $3.48\times10^{14} Ω^{-1}m^{-2}$. We demonstrated the current-induced magnetization switching with a low critical switching current density of $3.5\times10^6 A/cm^2$, showing excellent potential for low-dissipation spintronic devices.
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Submitted 11 January, 2022;
originally announced January 2022.
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Enormous Berry-Curvature-Driven Anomalous Hall Effect in Topological Insulator (Bi,Sb)2Te3 on Ferrimagnetic Europium Iron Garnet beyond 400 K
Authors:
Wei-Jhih Zou,
Meng-Xin Guo,
Jyun-Fong Wong,
Zih-Ping Huang,
Jui-Min Chia,
Wei-Nien Chen,
Sheng-Xin Wang,
Keng-Yung Lin,
Lawrence Boyu Young,
Yen-Hsun Glen Lin,
Mohammad Yahyavi,
Chien-Ting Wu,
Horng-Tay Jeng,
Shang-Fan Lee,
Tay-Rong Chang,
Minghwei Hong,
Jueinai Kwo
Abstract:
To realize the quantum anomalous Hall effect (QAHE) at elevated temperatures, the approach of magnetic proximity effect (MPE) was adopted to break the time-reversal symmetry in the topological insulator (Bi0.3Sb0.7)2Te3 (BST) based heterostructures with a ferrimagnetic insulator europium iron garnet (EuIG) of perpendicular magnetic anisotropy. Here we demonstrate phenomenally large anomalous Hall…
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To realize the quantum anomalous Hall effect (QAHE) at elevated temperatures, the approach of magnetic proximity effect (MPE) was adopted to break the time-reversal symmetry in the topological insulator (Bi0.3Sb0.7)2Te3 (BST) based heterostructures with a ferrimagnetic insulator europium iron garnet (EuIG) of perpendicular magnetic anisotropy. Here we demonstrate phenomenally large anomalous Hall resistance (RAHE) exceeding 8 Ω (\r{ho}AHE of 3.2 μΩ*cm) at 300 K and sustaining to 400 K in 35 BST/EuIG samples, surpassing the past record of 0.28 Ω (\r{ho}AHE of 0.14 μΩ*cm) at 300 K. The remarkably large RAHE as attributed to an atomically abrupt, Fe-rich interface between BST and EuIG. Importantly, the gate dependence of the AHE loops shows no sign change with varying chemical potential. This observation is supported by our first-principles calculations via applying a gradient Zeeman field plus a contact potential on BST. Our calculations further demonstrate that the AHE in this heterostructure is attributed to the intrinsic Berry curvature. Furthermore, for gate-biased 4 nm BST on EuIG, a pronounced topological Hall effect (THE) coexisting with AHE is observed at the negative top-gate voltage up to 15 K. Interface tuning with theoretical calculations has opened up new opportunities to realize topologically distinct phenomena in tailored magnetic TI-based heterostructures.
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Submitted 30 September, 2021; v1 submitted 30 March, 2021;
originally announced March 2021.
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Topological insulator Bi2Se3 films on rare earth iron garnets and their high-quality interfaces
Authors:
C. C. Chen,
K. H. M. Chen,
Y. T. Fanchiang,
C. C. Tseng,
S. R. Yang,
C. N. Wu,
M. X. Guo,
C. K. Cheng,
C. T. Wu,
M. Hong,
J. Kwo
Abstract:
The integration of quantum materials like topological insulators (TIs) with magnetic insulators (MIs) has important technological implications for spintronics and quantum computing. Here we report excellent crystallinity of c-axis oriented epitaxial TI films Bi2Se3 grown on MI films, a rare earth iron garnet (ReIG), such as thulium iron garnet (Tm3Fe5O12, TmIG) by molecular beam epitaxy (MBE) with…
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The integration of quantum materials like topological insulators (TIs) with magnetic insulators (MIs) has important technological implications for spintronics and quantum computing. Here we report excellent crystallinity of c-axis oriented epitaxial TI films Bi2Se3 grown on MI films, a rare earth iron garnet (ReIG), such as thulium iron garnet (Tm3Fe5O12, TmIG) by molecular beam epitaxy (MBE) with a Se-buffered low-temperature (SBLT) growth technique. We demonstrated a streaky reflection high-energy electron diffraction pattern starting from the very first quintuple layer of Bi2Se3, indicating the high-quality interface between TmIG and Bi2Se3, a prerequisite for studying interfacial exchange coupling effects. The strong interfacial exchange interaction was manifested by observations of anomalous Hall effect in the Bi2Se3/TmIG bilayer and a shift of ferromagnetic resonance field of TmIG induced by Bi2Se3. We have reproducibly grown high-quality Bi2Se3/ReIG and interfaces using this new TI growth method, which may be applied to grow other types of van der Waals (vdW) hetero-structures.
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Submitted 12 September, 2018;
originally announced September 2018.