arXiv subjects

Takashi Uchihashi

Publications and source records attributed to Takashi Uchihashi.

At least 19 recordsLinked to original sources

arXiv2026

Direct Observation of the Zigzag Edge States of a Supramolecular Diatomic Kagome Lattice

Lattice geometry plays a fundamental role in the behavior of Bloch electrons in a crystal. The diatomic Kagome lattice, an extension of the honeycomb and Kagome lattices, is predicted to give rise to emergent and topological phenomena, but its experimental investigation has been limited thus far. Here, we fabricate a diatomic Kagome lattice through self-assembly of a triptycene derivative with phenazine moieties (Trip-Phz)---a $\mathrm{C_3}$-symmetric, non-planar $π$-conjugated molecule. Our scanning tunneling microscopy (STM) observations show that Trip-Phz forms a highly ordered diatomic Kagome lattice terminated by zigzag-type edges on the Pb(111) surface. Combined STM measurements and tight-binding calculations provide direct evidence for the existence of the edge states that correspond to those of graphene. These states are topological edge states dictated by the quantization of the Zak phase and the bulk-edge correspondence.This work reveals an ideal platform for exploring quantum materials with unique lattice geometries using supramolecular technology.

cond-mat.mes-hall↗

arXiv2026

Anisotropic transport of Josephson vortices in atomic-layer superconductors on vicinal surfaces

Atomic steps have strong influences on surface two-dimensional superconductors. Josephson vortices formed at the atomic steps under magnetic fields may dominate transport phenomena at low temperatures, but its experimental verification is still lacking. Here, we report the vortex transport properties of atomic-layer superconductor Si(111)-$(\sqrt{7}\times\sqrt{3})$-In with vicinal surfaces, for which Josephson vortices are directly observed by scanning tunneling microscopy. A sharp drop in resistance with decreasing temperature $T$, detected under out-of-plane magnetic field $B$, reveals a distinctive anisotropy with respect to the atomic step direction. The anisotropy of sheet resistance, proportional to that of vortex mobility, amounts to the order of $10^3$ at intermediate magnetic fields. In the high-$T$ and low-$B$ region, Josephson vortices exhibit thermally excited creep motions with anisotropic activation energy $U_\mathrm{act}$. A further increase in $B$ suppresses $U_\mathrm{act}$ toward zero anisotropically, resulting in one-dimensional pinning-free vortex flow at $0.10 \lesssim B \lesssim 0.20$ T. At the lowest temperatures, the vortex motion is governed by quantum tunneling. A $B$-$T$ phase diagram constructed based on these measurements reveals multiple regions characterized by directionally dependent vortex-transport mechanisms.

cond-mat.supr-con↗

arXiv2024

Moiré superlattices of antimonene on a Bi(111) substrate with van Hove singularity and Rashba-type spin polarization

Moiré superlattices consisting of two-dimensional materials have attracted immense attention because of emergent phenomena such as flat band-induced Mott insulating states and unconventional superconductivity. However, the effects of spin-orbit coupling on these materials have not yet been fully explored. Here, we show that single- and double-bilayer antimony honeycomb lattices, referred to as antimonene, form moiré superlattices on a Bi(111) substrate due to lattice mismatch. Scanning tunnelling microscopy (STM) measurements reveal the presence of spectral peaks near the Fermi level, which are spatially modulated with the moiré period. Angle-resolved photoemission spectroscopy (ARPES) combined with density functional theory calculations clarify the surface band structure with saddle points near the Fermi level, which allows us to attribute the observed STM spectral peaks to the van Hove singularity. Moreover, spin-resolved ARPES measurements reveal that the observed surface states are Rashba-type spin-polarized. The present work has significant implications in that Fermi surface instability and symmetry breaking may emerge at low temperatures, where the spin degree of freedom and electron correlation also play important roles.

cond-mat.mes-hall↗

arXiv2023

Field resilient superconductivity in atomic layer crystalline materials

A recent study [S. Yoshizawa {\it et al}., Nature Communications {\bf 12}, 1462 (2021)] reported the occurrence of field-resilient superconductivity, that is, enhancement of the in-plane critical magnetic field $H^{||}_{\rm c2}$ beyond the paramagnetic limiting field, in atomic-layer crystalline ($\sqrt{7}\times\sqrt{3}$)-In on a Si(111) substrate. The present article elucidates the origin of the observed field-resilient noncentrosymmetric superconductivity in this highly crystalline two-dimensional material. We develop the quasiclassical theory of superconductivity by incorporating the Fermi surface anisotropy together with an anisotropic spin splitting and texture specific to atomic-layer crystalline systems. In Si(111)-($\sqrt{7}\times\sqrt{3}$)-In, a typical material with a large antisymmetric spin-orbit coupling (ASOC), we show an example where the combination of the ASOC and disorder effect suppresses the paramagnetic depairing and can lead to an enhancement of $H^{||}_{\rm c2}$ compared to an isotropic system only when a magnetic field is applied in a particular direction due to an anisotropic spin texture. We also study the parity-mixing effect to demonstrate that the enhancement of $H^{||}_{\rm c2}$ is limited in the moderately clean regime because of the fragile $s$+$p$-wave pairing against nonmagnetic scattering in the case of the dominant odd-parity component of a pair wavefunction. Furthermore, from analysis of the transition line, we identify the field-resilience factor taking account of the scattering and suppression of paramagnetic effects and discuss the origin of the field-resilient superconductivity. Through fitting of the $H^{||}_{\rm c2}$ data, the normal-state electron scattering is discussed with a prime focus on the role of atomic steps on a Si(111) surface.

cond-mat.supr-con↗

arXiv2022

Non-charge-transfer origin of Tc Enhancement in a Surface Superconductor Si(111)-(root7xroot3)-In with Adsorbed Organic Molecules

The effects of adsorption of Zn-phthalocyanine (ZnPc) molecules on the superconductivity of the Si(111)-(root7xroot3)-In surface are studied through transport measurements under ultrahigh vacuum environment. The ZnPc molecules are found to increase the transition temperature Tc by 11% at maximum, which is about 2.7 times the Tc increase previously reported using CuPc. By contrast, angle-resolved photoemission spectroscopy measurements and ab initio calculations show that charge transfer from the In atomic layers to ZnPc is substantially smaller than that to CuPc. This clearly shows that charge transfer should be excluded as the origin of the increase in Tc. The push-back effect induced by physical adsorption of molecules is discussed as a possible mechanism for the Tc enhancement.

cond-mat.supr-con↗

arXiv2021

High-Mobility p-Channel Wide Bandgap Transistors Based on h-BN/Diamond Heterostructures

Field-effect transistors made of wide-bandgap semiconductors can operate at high voltages, temperatures and frequencies with low energy losses, and have been of increasing importance in power and high-frequency electronics. However, the poor performance of p-channel transistors compared with that of n-channel transistors has constrained the production of energy-efficient complimentary circuits with integrated n- and p-channel transistors. The p-type surface conductivity of hydrogen-terminated diamond offers great potential for solving this problem, but surface transfer doping, which is commonly believed to be essential for generating the conductivity, limits the performance of transistors made of hydrogen-terminated diamond because it requires the presence of ionized surface acceptors, which cause hole scattering. Here, we report on fabrication of a p-channel wide-bandgap heterojunction field-effect transistor consisting of a hydrogen-terminated diamond channel and hexagonal boron nitride ($h$-BN) gate insulator, without relying on surface transfer doping. Despite its reduced density of surface acceptors, the transistor has the lowest sheet resistance ($1.4$ k$Ω$) and largest on-current ($1600$ $μ$m mA mm$^{-1}$) among p-channel wide-bandgap transistors, owing to the highest hole mobility (room-temperature Hall mobility: $680$ cm$^2$V$^{-1}$s$^{-1}$). Importantly, the transistor also shows normally-off behavior, with a high on/off ratio exceeding $10^8$. These characteristics are suited for low-loss switching and can be explained on the basis of standard transport and transistor models. This new approach to making diamond transistors paves the way to future wide-bandgap semiconductor electronics.

cond-mat.mtrl-sci↗

arXiv2021

Atomic-layer Rashba-type superconductor protected by dynamic spin-momentum locking

Spin-momentum locking is essential to the spin-split Fermi surfaces of inversion-symmetry broken materials, which are caused by either Rashba-type or Zeeman-type spin-orbit coupling (SOC). While the effect of Zeeman-type SOC on superconductivity has experimentally been shown recently, that of Rashba-type SOC remains elusive. Here we report on convincing evidence for the critical role of the spin-momentum locking on crystalline atomic-layer superconductors on surfaces, for which the presence of the Rashba-type SOC is demonstrated. In-situ electron transport measurements reveal that in-plane upper critical magnetic field is anomalously enhanced, reaching approximately three times the Pauli limit at $T = 0$. Our quantitative analysis clarifies that dynamic spin-momentum locking, a mechanism where spin is forced to flip at every elastic electron scattering, suppresses the Cooper pair-breaking parameter by orders of magnitude and thereby protects superconductivity. The present result provides a new insight into how superconductivity can survive the detrimental effects of strong magnetic fields and exchange interactions.

cond-mat.supr-con↗

arXiv2020

Charge-carrier mobility in hydrogen-terminated diamond field-effect transistors

Diamond field-effect transistors (FETs) have potential applications in power electronics and high-output high-frequency amplifications. In such applications, high charge-carrier mobility is desirable for a reduced loss and high-speed operation. We have recently fabricated diamond FETs with a hexagonal-boron-nitride gate dielectric and observed a high mobility above 300 cm$^{2}$V$^{-1}$s$^{-1}$. In this study, we examine which scattering mechanism limits the mobility of our FETs through theoretical calculations. Our calculations reveal that the dominant carrier scattering is caused by surface charged impurities with the density of $\approx$1$\times10^{12}$ cm$^{-2}$, and suggest a possible increase in mobility over 1000 cm$^{2}$V$^{-1}$s$^{-1}$ by reducing the impurities.

cond-mat.mes-hall↗

arXiv2019

Quantum oscillations in diamond field effect transistors with a h-BN gate dielectric

Diamond has attracted attention as a next-generation semiconductor because of its various exceptional properties such as a wide bandgap and high breakdown electric field. Diamond field effect transistors, for example, have been extensively investigated for high-power and high-frequency electronic applications. The quality of their charge transport (i.e., mobility), however, has been limited due to charged impurities near the diamond surface. Here, we fabricate diamond field effect transistors by using a monocrystalline hexagonal boron nitride as a gate dielectric. The resulting high mobility of charge carriers allows us to observe quantum oscillations in both the longitudinal and Hall resistivities. The oscillations provide important information on the fundamental properties of the charge carriers, such as effective mass, lifetime, and dimensionality. Our results indicate the presence of a high-quality two-dimensional hole gas at the diamond surface and thus pave the way for studies of quantum transport in diamond and the development of low-loss and high-speed devices.

cond-mat.mes-hall↗

arXiv2016

Two-dimensional Superconductors with Atomic-scale Thicknesses

Recent progress in two-dimensional superconductors with atomic-scale thicknesses is reviewed mainly from the experimental point of view. The superconducting systems treated here involve a variety of materials and forms: elemental-metal ultrathin films and atomic layers on semiconductor surfaces; interfaces and superlattices of heterostructures made of cuprates, perovskite oxides, and rare-earth metal heavy-fermion compounds; interfaces of electric-double-layer transistors; graphene and atomic sheets of transition-metal dichalcogenide; iron selenide and organic conductors on oxide and metal surfaces, respectively. Unique phenomena arising from the ultimate two-dimensionality of the system and the physics behind them are discussed.

cond-mat.supr-con↗

arXiv2016

Two-dimensional atomic layer-molecule hybrid superconductors with controllable exchange coupling

The coexistence and competition of superconductivity and magnetism can lead to a variety of rich physics and technological applications. Recent discovery of atomic-layer superconductors and self-assembly of magnetic molecules on solid surfaces should allow one to create a new two-dimensional (2D) hybrid superconducting system, but its possibility has never been fully investigated so far. Here we report the fabrication of highly ordered 2D hybrid superconductors based on indium atomic layers on silicon surfaces and magnetic metal-phthalocyanines (MPc) and clarify their detailed structural, superconducting and magnetic properties. Our primary findings include a substantial controllability of the superconducting transition temperatures (Tc) through substitution of central metal ions (M = Cu, Fe, Mn) of the molecules. This is attributed to charge transfers between the magnetic molecules and the superconducting layers and to different degrees of exchange coupling between them, which originates from anisotropic distributions of the relevant d-orbitals. The present study opens a route for designing and creating exotic 2D superconductors with an atomic-scale precision.

cond-mat.supr-con↗

arXiv2015

Engineering topological superconductors using surface atomic-layer/molecule hybrid materials

Surface atomic-layer (SAL) superconductors consisting of epitaxially grown metal adatoms on a clean semiconductor surface have been recently established. Compared to conventional metal thin films, they have two important features: i) space-inversion symmetry breaking throughout the system and ii) high sensitivity to surface adsorption of foreign species. These potentially lead to manifestation of the Rashba effect and a Zeeman field exerted by adsorbed magnetic organic molecules. After introduction of archetypical SAL superconductor Si(111)-(root7xroot3)-In, we describe how these features are utilized to engineer topological superconductor with Majorana fermions, and discuss its promises and expected challenges.

cond-mat.mes-hall↗

arXiv2015

Disorder-induced suppression of superconductivity in the Si(111)-($\sqrt{7}\times\sqrt{3}$)-In surface: Scanning tunneling microscopy study

The critical effect of disorder on the two-dimensional (2D) surface superconductor Si(111)-($\sqrt{7}\times\sqrt{3}$)-In is clarified by comparing two regions with different degrees of disorder. Low-temperature scanning tunneling microscopy measurements reveal that superconductivity is retained in the less disordered region, judging from the characteristic differential conductance ($dI/dV$) spectra and from the formation of vortices under magnetic fields. In striking contrast, the absence of those features in the highly disordered region shows that superconductivity is strongly suppressed there. Analysis of observed zero-bias anomalies in $dI/dV$ spectra allows us to estimate the reduction in the transition temperature $T_{\rm c}$, which explains the fate of superconductivity in each region.

cond-mat.mes-hall↗

arXiv2015

Locality and lateral modulations of quantum well states in Ag(100) thin films studied using a scanning tunneling microscope

We investigate Ag(100) thin films epitaxially grown on a Fe(100) substrate using a low-temperature scanning tunneling microscope. Fabrication of a wedge structure by evaporating Ag through a shadow mask allows us to observe systematic evolution of quantum well (QW) states for layer thicknesses varying from 3 to 16 monolayers (ML). Close inspection of differential conductance spectra and images reveal significant modulations of QW states in the lateral directions, presumably due to the local defects at the Ag/Fe interface. The area where QW states are modulated extends over ~ 5 nm. In clear contrast, near a surface atomic step, QW states exhibit negligible changes at least up to 1 nm away from the step, leaving unmixed the two sets of neighboring QW states belonging to different thicknesses. The results are discussed in terms of a simple electron wave diffraction model.

cond-mat.mes-hall↗

arXiv2014

Imaging Josephson Vortices on the Surface Superconductor Si(111)-(root7xroot3)-In using a Scanning Tunneling Microscope

We have studied the superconducting Si(111)-(root7xroot3)-In surface using a 3He-based low-temperature scanning tunneling microscope (STM). Zero-bias conductance (ZBC) images taken over a large surface area reveal that vortices are trapped at atomic steps after magnetic fields are applied. The crossover behavior from Pearl to Josephson vortices is clearly identified from their elongated shapes along the steps and significant recovery of superconductivity within the cores. Our numerical calculations combined with experiments clarify that these characteristic features are determined by the relative strength of the interterrace Josephson coupling at the atomic step.

cond-mat.mes-hall↗

arXiv2011

Macroscopic Superconducting Current through a Silicon Surface Reconstruction with Indium Adatoms: Si(111)-(R7$\times$R3)-In

Macroscopic and robust supercurrents are observed by direct electron transport measurements on a silicon surface reconstruction with In adatoms (Si(111)-(R7xR3)-In). The superconducting transition manifests itself as an emergence of the zero resistance state below 2.8 K. $I-V$ characteristics exhibit sharp and hysteretic switching between superconducting and normal states with well-defined critical and retrapping currents. The two-dimensional (2D) critical current density $J_\mathrm{2D,c}$ is estimated to be as high as $1.8 \ \mathrm{A/m}$ at 1.8 K. The temperature dependence of $J_\mathrm{2D,c}$ indicates that the surface atomic steps play the role of strongly coupled Josephson junctions.

cond-mat.mes-hall↗

arXiv2011

One-dimensional surface states on a striped Ag thin film with stacking fault arrays

One-dimensional (1D) stripe structures with a periodicity of 1.3 nm are formed by introduction of stacking fault arrays into a Ag thin film. The surface states of such striped Ag thin films are studied using a low temperature scanning tunneling microscope. Standing waves running in the longitudinal direction and characteristic spectral peaks are observed by differential conductance (dI/dV) measurements, revealing the presence of 1D states on the surface stripes. Their formation can be attributed to quantum confinement of Ag(111) surface states into a stripe by stacking faults. To quantify the degree of confinement, the effective potential barrier at the stacking fault for Ag(111) surface states is estimated from independent measurements. A single quantum well model with the effective potential barrier can reproduce the main features of dI/dV spectra on stripes, while a Kronig-Penney model fails to do so. Thus the present system should be viewed as decoupled 1D states on individual stripes rather than as anisotropic 2D Bloch states extending over a stripe array.

cond-mat.mes-hall↗

arXiv2010

Strong Electron Confinement By Stacking-fault Induced Fractional Steps on Ag(111) Surfaces

The electron reflection amplitude $R$ at stacking-fault (SF) induced fractional steps is determined for Ag(111) surface states using a low temperature scanning tunneling microscope. Unexpectedly, $R$ remains as high as $0.6 \sim 0.8$ as energy increases from 0 to 0.5 eV, which is in clear contrast to its rapidly decreasing behavior for monatomic (MA) steps [L. B{ü}rgi et al., Phys. Rev. Lett. \textbf{81}, 5370 (1998)]. Tight-binding calculations based on {\em ab-initio} derived band structures confirm the experimental finding. Furthermore, the phase shifts at descending SF steps are found to be systematically larger than counterparts for ascending steps by $\approx 0.4 π$. These results indicate that the subsurface SF plane significantly contributes to the reflection of surface states.

cond-mat.mes-hall↗