Researchers at The University of Tokyo and New York University are exploring a new path toward advanced superconducting devices by demonstrating the coexistence of quantum superconductivity and ferromagnetism within a novel junction. The work centers on iron-doped, narrow-gap III-V ferromagnetic semiconductors, materials expected to form highly conductive interfaces with superconductors and allow tuning of both carrier concentration and magnetic exchange energy. This approach offers a potential workaround to the typical suppression of superconducting coherence length seen with non-diluted ferromagnets, potentially enabling the creation of “Josephson π-junctions, spin-triplet superconductivity, and superconducting diode effect.”
Fe-Doped III-V Semiconductors for Proximity Superconductivity
Epitaxially grown heterostructures with atomically clean interfaces are proving valuable for applications including gatemons and topological qubits, and a new study highlights the potential of iron-doped, narrow-gap III-V ferromagnetic semiconductors as hosts for proximity-induced superconductivity. Researchers focused on these materials because they are expected to form Ohmic and transparent interfaces with superconductors, reducing energy barriers as demonstrated in previous work with aluminum and indium arsenide.
The ability to create such interfaces is critical for enabling the flow of supercurrents between dissimilar materials. Both the carrier concentration and ferromagnetic exchange energy within these iron-doped semiconductors can be adjusted by applying an electrical gate voltage, offering a degree of control previously difficult to achieve. These experimental results align with theoretical predictions regarding proximity-induced superconductivity in ferromagnets, and suggest a pathway toward manipulating superconducting properties with external stimuli.
The team’s work also points to a potential role for this system in exploring Majorana physics, a field concerned with exotic particles with potential applications in quantum computing. To date, proximity-induced superconductivity in ferromagnetic semiconductors has been demonstrated in niobium and iron-doped indium arsenide heterostructures, where long-range effects and hysteretic behavior indicated the presence of spin-triplet supercurrents. However, previous samples were grown ex-situ, meaning the materials were exposed to air between growth stages, potentially degrading interface quality.
The current study addresses this limitation by using in-situ growth within a single ultrahigh-vacuum system, ensuring atomically clean interfaces between the aluminum, indium arsenide and iron-doped gallium antimonide layers. “Owing to the semiconductor nature of InAs, both the proximitised superconductivity and the magnetic proximity effect are controlled by applying a gate voltage,” the researchers write. These gate-controllable ferromagnetic Josephson junctions offer a promising platform for studying the coexistence of superconductivity and ferromagnetism, and for developing new superconducting quantum devices.
InAs/(Ga,Fe)Sb Heterostructure Growth via Low-Temperature MBE
Growth of aluminum on the indium arsenide/(gallium, iron) antimonide heterostructure establishes a platform for investigating proximity-induced superconductivity and ferromagnetism under electrostatic control, a process refined through careful manipulation of growth parameters. After establishing AlSb/AlAs buffer layers on a gallium arsenide substrate, a 5 nanometer (In,Ga)As layer, followed by 15 nanometers of InAs and finally 20 nanometers of (Ga,Fe)Sb was deposited at 250℃ to minimize iron segregation within the (Ga,Fe)Sb layer. Following buffer layer deposition, the substrate temperature was reduced to 250℃, and low-temperature molecular beam epitaxy was used to deposit the layered structure.
High-Mobility InAs Layer Characterization via Magnetotransport
Magnetotransport measurements reveal electron mobility in the indium arsenide channel reaching 3.2×10³ cm²V⁻¹s⁻¹ along the [1̅10] crystal direction, a key characteristic for high-performance devices. This value, determined through Shubnikov-de Haas oscillations observed at 3.7 K under perpendicular magnetic fields, indicates a highly conductive layer despite the adjacent ferromagnetic material. The electron carrier density within the InAs layer was also quantified at 3.5×10¹² cm⁻², providing further insight into the material’s electrical properties and potential for manipulation.
Transmission electron microscopy confirms an atomically sharp and clean interface between the aluminum and (In,Ga)As layers, a critical factor for efficient electron transport. Resistance measurements along the [1̅10] crystal axis, alongside Hall resistance, provide a comprehensive picture of charge carrier behavior within the heterostructure. These findings are essential for understanding the interplay between superconductivity and magnetism in the fabricated Josephson junctions.
Magnetic circular dichroism (MCD) and SQUID magnetometry confirmed the ferromagnetic properties of the (Ga,Fe)Sb layer. The consistency between MCD and SQUID results at 5 Kelvin strengthens the reliability of the magnetic characterization.
Al/(In,Ga)As Interface Quality Verified by TEM Analysis
Earlier work relied on depositing the superconducting niobium onto iron-doped indium arsenide after air exposure, potentially compromising the connection between materials. This new approach aims to circumvent those issues by building the layers sequentially within a single vacuum environment. The resulting interface quality directly impacts the potential for long-range proximity effects and the observation of spintriplet supercurrents, phenomena central to the development of advanced superconducting devices.
This gating capability is particularly important for practical applications, offering a means to control and optimize supercurrent flow within the heterostructure. Quantitative evaluation of the Josephson junction interface transparency yielded parameters α = 0.92 and α′ = 0.59, corresponding to 29% and 44% of the ballistic theoretical limits for α, and 22% and 40% for α′, respectively. These values indicate a significant degree of transparency, allowing for efficient transfer of superconducting pairing across the interface.
Analysis of the supercurrent density distribution, obtained through Fast Fourier Transform (FFT) analysis of the junction data, revealed the spatial characteristics of current flow and provided insights into the junction’s behavior under varying magnetic fields. The critical current itself is expressed as Ic(Γβ), where β is dependent on the applied magnetic field and layer thicknesses, and is treated as a complex function for FFT analysis.
Tunable Spin Splitting in InAs via Magnetic Proximity Effect
Spin splitting as large as 18 meV has been observed within the indium arsenide channel, a result of a strong magnetic proximity effect induced by the adjacent ferromagnetic semiconductor (Ga,Fe)Sb layer, and this splitting is tunable via gate voltage modulation of the electron wavefunction and concentration. This control over spin splitting stems from the unique type-III band alignment, allowing for significant penetration of the electron wavefunction into the (Ga,Fe)Sb layer, effectively linking the magnetic properties of the semiconductor to the electronic behavior of the indium arsenide.
The ability to modulate this interaction represents an advancement in manipulating spin-based phenomena within the heterostructure. Transmission electron microscopy confirms the preservation of the zinc-blende crystal structure throughout the molecular beam epitaxy growth process, revealing that the aluminum film grows along the face-centered cubic crystal direction, consistent with X-ray diffraction data.
A 10 nanometer-thick aluminum film was grown in situ at temperatures below -40°C, forming the superconducting component of the junctions, and a thin layer of indium gallium arsenide, containing 19% indium, was inserted between the aluminum and indium arsenide layers to improve interface quality. The estimated superconducting coherent length, approximately 1.7 micrometers, is shorter than the 5-micrometer junction width, opening a discussion regarding the potential for these edge channels to contribute to long-range proximity effects.
Superconductor/Ferromagnetic Semiconductor Junction Design
The interface transparency of these aluminum/ferromagnetic semiconductor heterostructures reaches 44% of the theoretical diffusive limit, a value comparable to epitaxially grown junctions but exceeding those created via sputtering. Achieving this level of transparency is important for efficient superconducting current flow and validates the quality of the fabricated interfaces, particularly given the challenges in growing superconducting films on ferromagnetic materials. This success stems from the careful material selection and in situ growth process, which preserves the zinc-blende crystal structure of the semiconductor layers as confirmed by transmission electron microscopy.
Further analysis of the Josephson junctions reveals a quantifiable modulation of the critical current through electrostatic gating, a feature essential for practical device applications. This specific composition facilitates a strong magnetic proximity effect within the adjacent InAs layer, potentially enabling the exploration of exotic superconducting states.
Fulde-Ferrell-LarkinOvchinnikov (FFLO) State Potential in Heterostructures
The potential for realizing the Fulde-Ferrell-LarkinOvchinnikov (FFLO) state, a superconducting phase characterized by unconventional Cooper pairing, is enhanced in heterostructures using semiconductors with a large g-factor, materials where induced Zeeman splitting can drive non-zero pairing momentum. The ability to manipulate this state through external control is important for scalable quantum architectures, and the current work focuses on achieving this via electrostatic gating.
The team fabricated aluminum/indium arsenide/(gallium, iron) antimonide heterostructures using low-temperature molecular beam epitaxy, growing a 5 nanometer thick (indium, gallium) arsenide layer atop a 15 nanometer thick indium arsenide layer, followed by a 20 nanometer (gallium, iron) antimonide layer on a GaAs substrate.
Critical current measurements revealed a value of 3.00 μA, alongside an excess current of 1.91 μA and a normal resistance of 52.5 Ω, characteristics extracted from the observed zero-resistance state at low bias currents and linear behavior at higher currents. Analysis of the resulting dI/dV spectrum showed multiple peaks, indicative of multiple Andreev reflections, a phenomenon where Cooper pairs are reflected at the interface between the superconductor and the semiconductor.
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