arXiv subjects

Keita Ishihara

Publications and source records attributed to Keita Ishihara.

4 recordsLinked to original sources

arXiv2026

Orientation selection and superconducting properties of epitaxial Al on ferromagnetic semiconductor (In,Fe)As

Superconductor/ferromagnet heterostructures provide a versatile platform for exploring spin-dependent superconducting phenomena arising from interfacial proximity effects. In this article, we investigate the structural and superconducting properties of Al thin films grown in situ by molecular beam epitaxy on strained Fe-doped ferromagnetic semiconductor (FMS) (In,Fe)As layers. X-ray diffraction and transmission electron microscopy reveal the epitaxial growth of single-crystalline Al layers, with the growth orientation changing from (110) to (111) as the in-plane lattice constant of (In,Fe)As increases. The superconducting critical temperature of Al varies systematically with the film surface morphology and grain size. In addition, the critical magnetic field of Al exhibits an anomalous decrease below 0.5 K, possibly reflecting magnetic coupling to the underlying FMS (In,Fe)As layer. These findings provide a guideline for material design of epitaxial Al/(In,Fe)As heterostructures, which may serve as a promising platform for investigating proximity-induced superconducting and magnetic phenomena in semiconductor-based hybrid quantum devices.

cond-mat.supr-con↗

arXiv2026

Interplay of superconductivity and ferromagnetism in ferromagnetic semiconductor-based Josephson junctions

The interplay between superconductivity and ferromagnetism has long been pursued as a route to unconventional Josephson effects, yet suitable material platforms remain limited. Here we report Josephson junctions based on epitaxial Al/InAs/(Ga,Fe)Sb heterostructures grown by low-temperature molecular beam epitaxy, achieving atomically abrupt superconductor/semiconductor/ferromagnetic interfaces. The devices exhibit clear proximity-induced superconductivity, including multiple Andreev reflections and gate-tunable supercurrents, confirming transparent coupling across the hybrid structure. Under perpendicular magnetic fields, the junctions reveal highly unconventional Fraunhofer interference patterns with hysteresis, flux jumps, asymmetric lobe evolution, and clear nonreciprocity, providing strong evidence of induced ferromagnetism and broken time-reversal symmetry in the superconducting channel. Gate control further modulates the critical current, highlighting the semiconducting nature of the system. Our results demonstrate that ferromagnetic semiconductor heterostructures can serve as a highly tunable platform for exploring proximity-induced superconductivity and superconducting diode effects, and for advancing device concepts at the intersection of magnetism and quantum electronics.

cond-mat.supr-con↗

arXiv2023

Allotropic transition of Dirac semimetal α-Sn to superconductor β-Sn induced by irradiation of focused ion beam

Diamond-type structure allotrope α-Sn is attracting much attention as a topological Dirac semimetal (TDS). In this study, we demonstrate that α-Sn undergoes a phase transition to another allotrope β-Sn with superconductivity at low temperature by irradiating with a focused Ga ion beam (FIB). To clarify the transition mechanism, we performed X-ray photoemission spectroscopy (XPS) measurements on an α-Sn thin film irradiated with FIB and an as-grown α-Sn thin film. The XPS results suggest that the local annealing, which is one of the side effects of FIB, causes the transformation from α-Sn into β-Sn. Furthermore, the difference in the chemical states between α-Sn and β-Sn can be quantitatively explained by the crystal structures rather than the degree of metallicity reflecting the conductivity. These results propose a new way of fabricating TDS/superconductor in-plane heterostructures based on α-Sn and β-Sn.

cond-mat.mtrl-sci↗

arXiv2023

Giant superconducting diode effect in ion-beam patterned Sn-based superconductor nanowire / topological Dirac semimetal planar heterostructures

Superconductor/topological material heterostructures are intensively studied as a platform for topological superconductivity and Majorana physics. However, the high cost of nanofabrication and the difficulty of preparing high-quality interfaces between the two dissimilar materials are common obstacles that hinder the observation of intrinsic physics and the realisation of scalable topological devices and circuits. Here, we demonstrate an innovative method to directly draw nanoscale superconducting beta-tin (beta-Sn) patterns of any shape in the plane of a topological Dirac semimetal (TDS) alpha-tin (alpha-Sn) thin film by irradiating a focused ion beam (FIB). We utilise the property that alpha-Sn undergoes a phase transition to superconducting beta-Sn upon heating by FIB. In beta-Sn nanowires embedded in a TDS alpha-Sn thin film, we observe giant non-reciprocal superconducting transport, where the critical current changes by 69% upon reversing the current direction. The superconducting diode rectification ratio reaches a maximum when the magnetic field is applied parallel to the current, distinguishing itself from all the previous reports. Moreover, it oscillates between alternate signs with increasing magnetic field strength. The angular dependence of the rectification ratio on the magnetic field and current directions is similar to that of the chiral anomaly effect in TDS alpha-Sn, suggesting that the non-reciprocal superconducting transport may occur at the beta-Sn/alpha-Sn interfaces. The ion-beam patterned Sn-based superconductor/TDS planar structures thus show promise as a universal platform for investigating novel quantum physics and devices based on topological superconducting circuits of any shape.

cond-mat.supr-con↗