arXiv subjects

Yoichi Ando

Publications and source records attributed to Yoichi Ando.

At least 19 recordsLinked to original sources

arXiv2026

Flux-tunable global and local superconductivity in a topological insulator nano-SQUID

Topological systems are defined by global properties that enforce the existence of local boundary modes. Three-dimensional topological insulators (TIs) were among the earliest proposed systems for hosting topological superconductivity, but experimental focus subsequently shifted to other platforms. Here, we revisit bulk-insulating TIs using a columnar nano-superconducting quantum interference device (nano-SQUID) architecture. This geometry optimises the proximity effect on the TI surface and enables simultaneous probing of global superconducting properties - via the critical current through the nano-SQUID - alongside the local states at the ends of the nano-SQUID via tunnel junctions. We observe several global superconducting features that appear to show a flux-driven global phase transition consistent with entering the topological regime, including periodic critical current oscillations and a sign reversal in the superconducting diode effect. Simultaneously, tunnelling spectroscopy reveals spectral jumps in local and nonlocal conductance that align with these global features. However, zero-bias peaks (ZBPs) in local conductance are present both within the predicted topological range of magnetic fields and in theoretically trivial regimes, including at zero magnetic field. Ultimately, the lack of correlation between local ZBP signatures and global signatures emphasises that conclusively identifying Majorana bound states will necessitate a combined approach, integrating the establishment of global topological properties with the use of local and other, more advanced, probes.

cond-mat.mes-hall↗

arXiv2026

Fabrication of high-quality topological insulator nanodevices from bulk-insulating air-sensitive Sb-Bi$_2$Se$_3$

High-quality topological insulator (TI) materials are essential for the realization and detection of Majorana bound states (MBSs) in TI-superconductor hybrid platforms. Widely used compensated TIs exhibit substantial disorder and charge inhomogeneity, which may be detrimental for Majorana devices. In this regard, Sb-substituted Bi$_2$Se$_3$ (SBS) is promising, because it is non-compensated and yet achieves very low bulk carrier density. We systematically investigate the impact of thermal processing during microfabrication on the transport properties of SBS. We developed a room-temperature fabrication protocol that preserves the low carrier density of exfoliated SBS upon fabrication of Hall bar and nanowire devices as evidenced from the observation of quantum interference oscillations in nanowires, a large gate tunability, and clear signatures of weak antilocalization (WAL).

cond-mat.mes-hall↗

arXiv2026

Imaging geometry- and phase-controlled spectra in a surface-state Andreev cavity

Andreev cavities provide a setting in which superconducting proximity spectra are shaped by phase-coherent electron-hole motion along extended trajectories. While such Andreev physics is well established in transport, local spectra in two-dimensional cavities remain largely unexplored in real space. Here we use scanning tunnelling spectroscopy to study confined Cu(111) surface states coupled to superconducting Nb(110). The in-plane magnetic-field scale for the collapse of the resolved low-energy spectrum is controlled by the transverse extent available to Andreev trajectories, while the zero-field excitation energy evolves with the characteristic trajectory length. These trends, together with spatial variations within individual islands and the response to vortex phase textures, are captured by a minimal semiclassical phase-accumulation picture. Our results identify geometry-defined Andreev trajectories as a design principle for phase-coherent superconducting cavities accessible by local spectroscopy.

cond-mat.supr-con↗

arXiv2026

Topological insulator single-electron transistors for charge sensing applications

We present topological insulator (TI)-based single-electron transistors (SETs) as magnetic-field-compatible charge sensing devices that are easily integrable with TI-superconductor hybrid platforms. We observe well-resolved Coulomb diamonds in the charge-stability diagrams of our devices confirming the charge quantization and single-electron transport. In some devices, the Coulomb resonances show persistent shifts corresponding up to $\sim$ e/2 charge. An axial magnetic field further displaces these shifts to higher or lower gate voltages. We find that the axial magnetic-field dependence of the shifts is consistent with the Zeeman shift of a trap state coupled to the SET, and we reproduce the observations using numerical simulations. The resonance shifts are therefore identified as a consequence of the sensitivity of our TI-SET devices to charges in proximity. Establishing this charge sensing capability is a first step toward integrating TI-SETs as charge sensors in more complex TI-based hybrid devices, with the overarching goal of detecting and braiding Majorana zero modes.

cond-mat.mes-hall↗

arXiv2026

Microscopic origin of an exceptionally large phonon thermal Hall effect from charge puddles in a topological insulator

We present the experimental observation of a drastically enhanced thermal Hall effect in the topological insulator material TlBi$_{0.15}$Sb$_{0.85}$Te$_2$. Although heat transport is dominated by phonons, moderate magnetic fields generate a thermal Hall ratio ($\kappa_{xy}/\kappa_{xx}$) above 2\%, an unprecedented value for a nonmagnetic material. The transverse thermal conductivity $\kappa_{xy}$ exhibits a pronounced maximum in fields of a few Tesla. This characteristic field dependence allows us to identify the microscopic origin of the thermal Hall effect in this system. Small densities of charged impurities induce locally conducting regions, so-called charge puddles, within the bulk insulating matrix. Via electron-phonon coupling, these charge puddles imprint a large thermal Hall effect onto the phonons accounting for both the magnitude and the magnetic-field dependence of the observed effect.

cond-mat.str-el↗

arXiv2025

Magnetic-Field and Temperature Limits of a Kinetic-Inductance Traveling-Wave Parametric Amplifier

Kinetic-inductance traveling-wave parametric amplifiers (KI-TWPAs) offer broadband near-quantum-limited amplification with high saturation power. Due to the high critical magnetic fields of high-kinetic-inductance materials, KI-TWPAs should be resilient to magnetic fields. In this work, we study how magnetic field and temperature affect the performance of a KI-TWPA based on a thin-NbTiN inverse microstrip with a Nb ground plane. This KI-TWPA can provide substantial signal-to-noise ratio improvement ($\Delta SNR$) up to in-plane magnetic fields of 0.35T and out-of-plane fields of 50mT, considerably higher than what has been demonstrated with TWPAs based on Josephson junctions. The field compatibility can be further improved by incorporating vortex traps and by using materials with higher critical fields. We also find that the gain does not degrade when the temperature is raised to 3K (limited by the Nb ground plane) while $\Delta SNR$ decreases with temperature consistently with expectation. This demonstrates that KI-TWPAs can be used in experiments that need to be performed at relatively high temperatures. The operability of KI-TWPAs in high magnetic field opens the door to a wide range of applications in spin qubits, spin ensembles, topological qubits, low-power NMR, and the search for axion dark matter.

quant-ph↗

arXiv2025

Reply to the Comment by Tikhonov and Khrapai on "Long-range crossed Andreev reflection in a topological insulator nanowire proximitized by a superconductor"

The comment (arXiv:2505.23490) fails to identify any scientific errors and its central arguments actually support the main conclusions of our publication [Nat. Phys. 21, 708 (2025)]. Firstly, the whole argument of the comment to try to explain our data explicitly relies on the existence of a large crossed Andreev reflection (CAR) effect. The presence of a sizable CAR transmission probability over a surprisingly long distance is the first conclusion of our publication. Secondly, the comment discusses the complex interplay of CAR and elastic co-tunneling, especially in the presence of local effects. This complex interplay is precisely the second conclusion of our publication. In essence, the comment amounts to merely pointing out that there is a broader sense in the notion of "dominant CAR" when nonlinear effects become relevant.

cond-mat.mes-hall↗

arXiv2025

Dynamics of current-induced switching in the quantum anomalous Hall effect

Ferromagnetic topological insulators in the quantum anomalous Hall (QAH) regime host chiral, dissipationless edge states whose propagation direction is determined by the internal magnetization. Under suitable conditions, a strong electrical bias can induce magnetization reversal, and thus flip the propagation direction. In this work, we perform time-resolved measurements to investigate the switching dynamics. Our results reveal characteristics consistent with a disordered magnetic landscape and demonstrate that the reversal process is thermally activated, driven by Joule heating during the current pulse. The understanding of the magnetization dynamics in QAH systems opens pathways for local, controlled manipulation of chiral edge states via thermal effects.

cond-mat.mes-hall↗

arXiv2025

Ex Situ Fabrication of Superconducting Nanostructures for Low-Temperature STM

Nanofabrication of topological insulator (TI) devices is essential for accessing edge and interface states, but conventional lithography and etching compromise the atomically clean surfaces required for scanning tunneling microscopy and spectroscopy (STM/STS). We demonstrate a fabrication strategy that combines ex situ lithographic patterning with in situ ultrahigh-vacuum cleaving and flipping, yielding atomically clean, nanopatterned TI-superconductor heterostructures suitable for STM/STS. In Design I, nanoribbons were defined by etching trenches into a TI film and capping with Nb. This enabled spectroscopy on large areas, although edge quality was limited by etch debris. In Design II, local thinning defined buried nanoribbons within a continuous TI film, producing pristine planar surfaces. STM/STS revealed well-developed superconducting gaps in the surrounding film, with suppressed gaps on the nanoribbons, consistent with vertical proximity coupling. This approach establishes a reproducible pathway for high-resolution STM/STS studies of proximitized nanostructures, providing a scalable platform for exploring topological superconductivity.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic-field-induced nonlocal transport in the topological semimetal ZrTe$_5$

Nonlocal transport, which goes beyond the Ohm's law, can be a key in understanding systems with topological order or edge states. Here we report an unusual nonlocal charge transport in the nodal-line semimetal ZrTe$_5$ that occurs in the ultra-quantum limit driven by the magnetic field applied along the $a$-axis. Surprisingly, the observed decay length of the nonlocality exceeds 100 $\mu$m and it increases linearly with the sample width. This nonlocal transport is detected not only in the longitudinal configuration, but also in the transverse one as an unusual nonlocal Hall effect. Our findings demonstrate that the nonlocal response can offer unprecedented insights into topological quantum materials.

cond-mat.mes-hall↗

arXiv2025

Generic Chiral Anomaly and Planar Hall Effect in a Non-Weyl System

The condensed-matter version of the chiral anomaly describes how electrons are pumped from a Weyl node with negative chirality to a Weyl node with positive chirality using parallel electric and magnetic fields. Key experimental signatures are a negative longitudinal magnetoresistance (LMR) and the planar Hall effect (PHE), both of which have been experimentally observed. Here, we show that the chiral anomaly explains key features of magnetotransport in the nodal-line semimetal ZrTe$_5$ despite the absence of Weyl points. The anomaly physics applies generically to materials in the quantum limit, when electron transport becomes quasi-one-dimensional, provided that Fermi velocities remain sufficiently large. This explains not only the negative LMR but also the PHE with a gigantic Hall angle and a highly unusual magnetic-field-angle dependence in ZrTe$_5$.

cond-mat.mes-hall↗

arXiv2025

Breakdown of the quantum anomalous Hall effect under microwave drives

Quantum anomalous Hall (QAH) insulators exhibit chiral dissipationless edge states without an external magnetic field, making them a promising material for quantum metrology and microwave applications. However, the breakdown of the zero-resistance state at low currents hinders progress. We investigate and characterize this breakdown under microwave fields (1-25 GHz) by measuring the increase of longitudinal resistance in RF Hall bars and RF Corbino devices made from V-doped (Bi,Sb)$_2$Te$_3$ films. Our results point to the role of heating of electron-hole puddles under microwave irradiation, thereby fostering hopping transport. Our work offers insights critical for GHz-range QAH applications.

cond-mat.mes-hall↗

arXiv2025

Quasiparticle interference on the surface of Bi$_{\mathbf{2}}$Se$_{\mathbf{3}}$ terminated (PbSe)$_{\mathbf 5}$(Bi$_{\mathbf 2}$Se$_{\mathbf 3}$)$_{\mathbf 6}$

Among the family of topological superconductors derived from {\BiSe}, $\mathrm{Cu}_x(\mathrm{PbSe})_{5}(\mathrm{Bi}_{2}\mathrm{Se}_{3})_{6}$ is unique in its surface termination of a single quintuple layer (QL) of the topological insulator (TI) \BiSe{} on an ordinary insulator PbSe. Here, we report a combined scanning tunneling microscopy (STM) and density functional theory (DFT) characterization of the cleaved surface of the parent compound $(\mathrm{PbSe})_{5}(\mathrm{Bi}_{2}\mathrm{Se}_{3})_{6}$ (PSBS). Interestingly, the potential disorder due to the random distribution of native defects is only $\Gamma \sim 4~\mathrm{meV}$, comparable to the smallest reported for TIs. Performing high-resolution quasiparticle interference imaging (QPI) near the Fermi energy ($E-E_\mathrm{F} = -1~\mathrm{eV}~\mathrm{to}~0.6~\mathrm{eV}$) we reconstruct the dispersion relation of the dominant spectral feature and our ab initio calculations show that this surface feature originates from two bands with Rashba-like splitting due to strong spin-orbit coupling and inversion symmetry breaking. Moreover, only a small hexagonal distortion of the calculated Fermi surface is seen in the full momentum space distribution of the measured scattering data. Nevertheless, the scattering pattern at lower energies transforms into a flower-like shape with suppressed intensity along the $\overline{\Gamma \mathrm{K}}$ direction. We show that this effect is not due to the forbidden backscattering in the spin-momentum locked surface state in Bi$_2$Se$_3$ but reflects the threefold symmetry of the scattering potential.

cond-mat.mes-hall↗

arXiv2024

Tunable superconducting diode effect in a topological nano-SQUID

A Josephson diode passes current with zero resistance in one direction but is resistive in the other direction. While such an effect has been observed in several platforms, a large and tunable Josephson diode effect has been rare. Here we report that a simple device consisting of a topological-insulator (TI) nanowire side-contacted by superconductors to form a lateral Josephson junction presents a large diode effect with the efficiency $\eta$ reaching 0.3 when a parallel magnetic field $B_{||}$ is applied. Interestingly, the sign and the magnitude of $\eta$ is tunable not only by $B_{||}$ but also by the back-gate voltage. This diode effect can be understood by modeling the system as a nano-SQUID, in which the top and bottom surfaces of the TI nanowire each form a line junction and $B_{||}$ creates a magnetic flux to thread the SQUID loop. This model further shows that the observed diode effect marks the emergence of topological superconductivity in TI-nanowire-based Josephson junction.

cond-mat.supr-con↗

arXiv2024

Topological Insulator nano-SQUID: Flux-tunable platform for topological superconductivity

Many efforts have been made in the past decade to realize topological superconductivity using superconducting proximity effect, but an ideal platform is still lacking. A 3D topological insulator (TI) is promising for this purpose due to the spin-momentum-locked surface state. Here we propose a novel yet simple TI platform which gives rise to a topological phase that is robust against disorder. It consists of a bulk-insulating rectangular TI nanowire laterally sandwiched by two superconductors. In this structure, the top and bottom surfaces individually work as SNS line junctions, forming a nanometer-scale columnar SQUID in which the nanowire cross-section defines the threading magnetic flux $\Phi$ in axial magnetic fields. We theoretically show that, when the two junctions are asymmetric, a robust topological phase occurs periodically for a wide range of $\Phi$, independently of the chemical potential. Our experiment found that a TI device of this structure indeed behaves as a columnar nano-SQUID where the supercurrent flows only through the top and bottom surfaces with vanishing bulk contribution. Furthermore, the top/bottom asymmetry can be tuned by a back gate, a key ingredient for the topological phase.

cond-mat.mes-hall↗

arXiv2024

Non-Majorana-origin of the half-integer conductance quantization elucidated by multi-terminal superconductor-quantum anomalous Hall insulator heterostructure

Chiral one-dimensional transport can be realized in thin films of a surface-insulating ferromagnetic topological insulator called quantum anomalous Hall insulator (QAHI). When superconducting (SC) pairing correlations are induced in the surface of such a material by putting an $s$-wave superconductor on the top, the resulting topological superconductivity gives rise to chiral Majorana edge-modes. A quantized two-terminal conductance of $\frac{1}{2}(e^2/h)$ was proposed as a smoking-gun evidence for the topological SC phase associated with a single chiral Majorana edge-mode. There have been experiments to address this proposal, but the conclusion remains unclear. Here, we formulate the edge transport in a multi-terminal superconductor-QAHI heterostructure using the Landauer-B\"uttiker formalism. Compared to the original proposal for the $\frac{1}{2}(e^2/h)$-quantization based on a simple two-terminal model, our formalism allows for deeper understanding of the origin of the quantization. The analysis of our experiments on multi-terminal devices unambiguously shows that the half-integer conductance quantization arises from the equilibration of the potentials of the incoming edge states at the SC electrode, and hence it is not of Majorana origin.

cond-mat.mes-hall↗

arXiv2024

Characterizing the chemical potential disorder in the topological insulator (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ thin films

We use scanning tunneling microscopy and spectroscopy under ultra-high vacuum and down to 1.7 K to study the local variations of the chemical potential on the surface of the topological insulator (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ thin films (thickness 7 - 30 nm) with varying Sb-concentration $x$, to gain insight into the charge puddles formed in thin films of a compensated topological insulator. We found that the amplitude of the potential fluctuations, $\Gamma$, is between 5 to 14 meV for quasi-bulk conducting films and about 30 - 40 meV for bulk-insulating films. The length scale of the fluctuations, $\lambda$, was found to span the range of 13 - 54 nm, with no clear correlation with $\Gamma$. Applying a magnetic field normal to the surface, we observe the condensation of the two-dimensional topological surface state into Landau levels and find a weak but positive correlation between $\Gamma$ and the spectral width of the Landau-level peaks, which suggests that quantum smearing from drift motion is the source of the Landau level broadening. Our systematic measurements give useful guidelines for realizing $(\mathrm{Bi}_{1-x}\mathrm{Sb}_x)_2\mathrm{Te}_3$ thin films with an acceptable level of potential fluctuations. In particular, we found that $x\approx 0.65$ realizes the situation where $\Gamma$ shows a comparatively small value of 14 meV and the Dirac point lies within $\sim$10 meV of the Fermi energy.

cond-mat.mes-hall↗

arXiv2024

Parallel-Field Hall effect in ZrTe$_5$

Parallel-field Hall effect is the appearance of a Hall voltage $V_{\rm H}$ that is transverse to the current $I$ when the magnetic field $B$ is applied parallel to $I$ (i.e. $B \parallel I \perp V_{\rm H}$). Such an effect is symmetry forbidden in most cases and hence is very unusual. Interestingly, the existence of a finite parallel-field Hall effect was reported for the layered topological semimetal ZrTe$_5$ and was proposed to be due to Berry curvature. However, it is forbidden for the known symmetry of ZrTe$_5$ and the possible existence of a misaligned out-of-plane magnetic field was not completely ruled out. Here, we elucidate the existence of the parallel-field Hall effect in ZrTe$_5$ with careful magnetic-field alignment. We interpret this result to originate from symmetry breaking and quantitatively explain the observed parallel-field Hall signal by considering a tilting of the Fermi surface allowed by broken symmetry.

cond-mat.mtrl-sci↗