arXiv subjects

Yukio Hasegawa

Publications and source records attributed to Yukio Hasegawa.

17 recordsLinked to original sources

arXiv2026

Electrothermal control of spin-reorientation transition in Co/Fe_3GaTe_2 heterostructures

Electrical control of magnetic anisotropy in van der Waals (vdWs) magnets is a key step toward reconfigurable two-dimensional spintronics, yet how a conventional metallic ferromagnet competes with a van der Waals magnet across a direct interface has remained largely unexplored. Here we demonstrate reversible thermal and electrothermal control of a spin-reorientation transition in Co/Fe_3GaTe_2 (FGaT) heterostructures. As Joule heating weakens the FGaT anisotropy, the heterostructure switches from an out-of-plane- to an in-plane-dominated state at a reorientation temperature of approximately 311 K, well below the Curie temperature, consistent with an exchange-mediated anisotropy competition between the Co overlayer and FGaT. An electrically driven device shows a closely matching loop evolution within an 80-100 mW power window, reversibly over five measurement cycles, consistent with an electrothermal origin. In a Co-free FGaT device, Kerr microscopy traces the switching to a power-tunable domain nucleation barrier and demonstrates power-thresholded, field-assisted magnetization reversal at a threshold near 15 mW. These results demonstrate electrothermal anisotropy competition as a route to heat-assisted and device-level control of vdWs magnetism.

cond-mat.mes-hall↗

arXiv2025

Vortex Pinning in Niobium covered by a thin polycrystalline Gold

Owing to its superconducting properties, Niobium (Nb) is an excellent candidate material for superconducting electronics and applications in quantum technology. Here we perform scanning tunneling microscopy and spectroscopy experiments on Nb films covered by a thin gold (Au) film. We investigate the minigap structure of the proximitized region and provide evidence for a highly transparent interface between Nb and Au, beneficial for device applications. Imaging of Abrikosov vortices in presence of a perpendicular magnetic field is reported. The data show vortex pinning by the granular structure of the polycrystalline Au film. Our results show robust and homogeneous superconducting properties of thin Nb film in the presence of a gold capping layer. The Au film not only protects the Nb from surface oxidation but also preserves its excellent superconducting properties.

cond-mat.supr-con↗

arXiv2025

In-situ nanoscale transport measurements on monoatomic metal films by low-temperature scanning tunneling potentiometry

Investigation of transport properties is fundamental for characterizing electronic properties and phase transitions. However, most of the transport measurements on conductive layers have been performed at macroscopic scales, and thus the development of microscopic methods to measure transport is important. Scanning tunneling potentiometry (STP) is a powerful tool for investigating surface conductivity at nano-scale spatial resolutions. However, it is still challenging to conduct STP studies at low temperatures and most of the low-temperature studies were performed on samples that were prepared ex-situ. In this study, we developed a low-temperature STP and demonstrated its performance on monoatomic metal films formed on Si(111) substrates that were prepared in-situ. Stable operation at low temperatures enables us to extract the electrochemical potential originating from the surface transport by canceling out the potential due to thermal differences and artifacts arising from the nonlinearity of the density of states (DOS). We also formulated the nonlinear-DOS artifact and confirmed it by comparing with the nonlinearity obtained by scanning tunneling spectroscopy.

cond-mat.mes-hall↗

arXiv2025

Quantum spin excitations in a dual-core magnetic molecule

Magnetic excitations are important quantum phenomena in magnetic systems and have been widely studied in individual magnetic atoms and molecules as well as their assembled structures over the past few decades. Using scanning tunneling microscopy/spectroscopy (STM/S) combined with density functional theory (DFT) and the state-of-the-art ab initio wavefunction calculations, we investigated the properties of a novel dual-core Cr2Br6 molecule, which consists of two Cr ions coupled via superexchange through a single near-90° Cr-Br-Cr scissors bond. Under zero magnetic field, we observed a Fano peak with multi-steps through STS. When an external magnetic field is applied, some steps exhibit additional splitting, while others change little. We find that the Cr2Br6, exhibits a spin-degenerate ground state, and the complex peak splitting arises from the coexistence of vibrational and magnetic excitations in the molecule. Our results reveal rich quantum spin behavior in a well-defined two-core magnetic trihalide complex at the atomic scale, offering not only a minimal model for superexchange-coupled multi-spin quantum excitations but also a possible foundational unit for future molecule-based quantum functionalities.

cond-mat.mtrl-sci↗

arXiv2022

Multi-Band Superconductivity in Strongly Hybridized 1T'-WTe$_2$/NbSe$_2$ Heterostructures

The interplay of topology and superconductivity has become a subject of intense research in condensed matter physics for the pursuit of topologically non-trivial forms of superconducting pairing. An intrinsically normal-conducting material can inherit superconductivity via electrical contact to a parent superconductor via the proximity effect, usually understood as Andreev reflection at the interface between the distinct electronic structures of two separate conductors. However, at high interface transparency, strong coupling inevitably leads to changes in the band structure, locally, owing to hybridization of electronic states. Here, we investigate such strongly proximity-coupled heterostructures of monolayer 1T'-WTe$_2$, grown on NbSe$_2$ by van-der-Waals epitaxy. The superconducting local density of states (LDOS), resolved in scanning tunneling spectroscopy down to 500~mK, reflects a hybrid electronic structure, well-described by a multi-band framework based on the McMillan equations which captures the multi-band superconductivity inherent to the NbSe$_2$ substrate and that induced by proximity in WTe$_2$, self-consistently. Our material-specific tight-binding model captures the hybridized heterostructure quantitatively, and confirms that strong inter-layer hopping gives rise to a semi-metallic density of states in the 2D WTe$_2$ bulk, even for nominally band-insulating crystals. The model further accurately predicts the measured order parameter $Δ\simeq 0.6$~meV induced in the WTe$_2$ monolayer bulk, stable beyond a 2~T magnetic field. We believe that our detailed multi-band analysis of the hybrid electronic structure provides a useful tool for sensitive spatial mapping of induced order parameters in proximitized atomically thin topological materials.

cond-mat.supr-con↗

arXiv2017

Atomic-scale visualization of surface-assisted orbital order

Orbital-related physics attracts growing interest in condensed matter research, but direct real-space access of the orbital degree of freedom is challenging. Here we report a first, real-space, imaging of a surface- assisted orbital ordered structure on a cobalt-terminated surface of the well-studied heavy fermion compound CeCoIn5. Within small tip-sample distances, the cobalt atoms on a cleaved (001) surface take on dumbbell shapes alternatingly aligned in the [100] and [010] directions in scanning tunneling microscopy topographies. First- principles calculations reveal that this structure is a consequence of the staggered dxz-dyz orbital order triggered by enhanced on-site Coulomb interaction at the surface. This so-far-overlooked surface-assisted orbital ordering may prevail in transition metal oxides, heavy fermion superconductors and other materials.

cond-mat.str-el↗

arXiv2017

Dirac fermions in borophene

Honeycomb structures of group IV elements can host massless Dirac fermions with non-trivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials especially in monolayer structures. We present a detailed investigation of the β12 boron sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that the lattice of the β12-sheet could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calculations. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices.

cond-mat.mtrl-sci↗

arXiv2016

Compressed sensing in scanning tunneling microscopy/spectroscopy for observation of quasi-particle interference

We applied a method of compressed sensing to the observation of quasi-particle interference (QPI) by scanning tunneling microscopy/spectroscopy to improve efficiency and save measurement time. To solve an ill-posed problem owing to the scarcity of data, the compressed sensing utilizes the sparseness of QPI patterns in momentum space. We examined the performance of a sparsity-inducing algorithm called least absolute shrinkage and selection operator (LASSO), and demonstrated that LASSO enables us to recover a double-circle QPI pattern of the Ag(111) surface from a dataset whose size is less than that necessary for the conventional Fourier transformation method. In addition, the smallest number of data required for the recovery is discussed on the basis of cross validation.

cond-mat.mes-hall↗

arXiv2016

Experimental verification of the rotational sense and type of chiral spin spiral structure by spin-polarized scanning tunneling microscopy

We report on experimental verification of the rotational sense and type of homogeneous chiral spin spiral order in a Mn monolayer on a W(110) substrate using spin-polarized scanning tunneling microscopy. We found that the magnetic contrast due to the spin spiral order almost vanishes with a magnetic tip magnetized normal to the (001) plane, indicating that the spin spiral rotates in the plane. From a shift in the most-contrasted sites by changing the tip magnetization direction within the rotating plane, we reveal that the rotational sense is left-handed, consistent with the previous results predicted by first-principle calculations. By comparing the current system with a chiral magnetic domain wall in Fe double layers on the same substrate, we found that the polarity of the Dzyaloshinskii--Moriya interaction, the driving force of those chiral magnets, is dominantly determined by the choice of the substrate rather than the overlayer.

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

Site-dependent conduction channel transmission in atomic-scale superconducting junctions

Using scanning tunneling microscopy, we reproducibly obtained electrical conductance spectra of superconductor- superconductor atomic point contacts formed on various atomic sites of the substrate. From the analysis of the subharmonic features due to the multiple Andreev reflection, spatial distributions of the number of the conduction channels and their transmission probabilities were obtained. It is found that the number of channels and the transmission probabilities strongly depend on an atomic site where the contact is formed. We also revealed how the conduction channels and their transmission probability evolve from the tunneling to contact regimes. The Josephson current also exhibits the atomic site dependence, which is explained by the site-dependent transmission probabilities. Our results demonstrate a crucial role of the atomic geometry in the conduction channels of the ultimately confined conductor for the first time experimentally.

cond-mat.mes-hall↗

arXiv2015

Electronic and magnetic influences of a stacking fault in cobalt nanoscale islands on the Ag(111) surface

Utilizing spin-polarized scanning tunneling microscopy and spectroscopy, we found coexistence of perpendicularly and in-plane magnetized cobalt nanoscale islands on the Ag(111) surface, and the relationship between the moire corrugation amplitude and the magnetization direction of the islands; the islands with the stronger moire corrugation show the perpendicular magnetization, and the ones with the weaker moire corrugation do the in-plane. Density functional theory calculations reproduce the relationship and explain the differences between the two types of the islands with an fcc stacking fault in the intrinsic hcp stacking of cobalt.

cond-mat.mes-hall↗

arXiv2015

Site-dependent evolution of electrical conductance from tunneling to atomic point contact

Using scanning tunneling microscopy (STM), we investigated the evolution of electrical conductance between a Pb tip and Pb(111) surface from tunneling to atomic point contact at a site that was defined with atomic precision. We found that the conductance evolution depended on the contact site, for instance, on-top, bridge, or hollow (hcp and fcc) sites in the Pb lattice. In the transition from tunneling to contact regimes, the conductance measured at the on-top site was enhanced. In the point contact regime, the hollow sites had conductances larger than those of the other sites, and between the hollow sites, the hcp site had a conductance larger than that of the fcc site. We also observed the enhancement and reversal of the apparent height in atomically resolved high-current STM images, consistent with the results of the conductance traces. Our results indicate the importance of atomic configuration in the conductance of atomic junctions and suggest that attractive chemical interactions have a significant role in electron transport between contacting atoms.

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↗

arXiv2014

Enhancement and termination of the superconducting proximity effect due to atomic-scale defects visualized by scanning tunneling microscopy

Using low-temperature scanning tunneling microscopy and spectroscopy, we have studied the proximity effect at the interfaces between superconducting Pb island structures and metallic Pb-induced striped-incommensurate phase formed on a Si(111) substrate. Our real-space observation revealed that the step structures on the two-dimensional metallic layer exhibit significant roles on the propagation of the superconducting pair correlation; the proximity effect is terminated by the steps, and in the confined area by the interface and the steps the effect is enhanced. The observed results are explained quantitatively with an elastic reflection of electrons at the step edges based on calculations with the quasi-classical Green's function formulation using Usadel equation.

cond-mat.mes-hall↗

arXiv2012

Thermally Assisted Penetration and Exclusion of Single Vortex in Mesoscopic Superconductors

A single vortex overcoming the surface barrier in a mesoscopic superconductor with lateral dimensions of several coherence lengths and thickness of several nanometers provides an ideal platform to study thermal activation of a single vortex. In the presence of thermal fluctuations, there is non-zero probability for vortex penetration into or exclusion from the superconductor even when the surface barrier does not vanish. We consider the thermal activation of a single vortex in a mesoscopic superconducting disk of circular shape. To obtain statistics for the penetration and exclusion magnetic fields, slow and periodic magnetic fields are applied to the superconductor. We calculate the distribution of the penetration and exclusion fields from the thermal activation rate. This distribution can also be measured experimentally, which allows for a quantitative comparison.

cond-mat.supr-con↗

arXiv2011

A comparsion of force sensors for atomic force microscopy based on quartz tuning forks and length extensional resonators

The force sensor is key to the performance of atomic force microscopy (AFM). Nowadays, most AFMs use micro-machined force sensors made from silicon, but piezoelectric quartz sensors are applied at an increasing rate, mainly in vacuum. These self sensing force sensors allow a relatively easy upgrade of a scanning tunneling microscope to a combined scanning tunneling/atomic force microscope. Two fundamentally different types of quartz sensors have achieved atomic resolution: the 'needle sensor' that is based on a length extensional resonator and the 'qPlus sensor' that is based on a tuning fork. Here, we calculate and measure the noise characteristics of these sensors. We find four noise sources: deflection detector noise, thermal noise, oscillator noise and thermal drift noise. We calculate the effect of these noise sources as a factor of sensor stiffness, bandwidth and oscillation amplitude. We find that for self sensing quartz sensors, the deflection detector noise is independent of sensor stiffness, while the remaining three noise sources increase strongly with sensor stiffness. Deflection detector noise increases with bandwidth to the power of 1.5, while thermal noise and oscillator noise are proportional to the square root of the bandwidth. Thermal drift noise, however, is inversely proportional to bandwidth. The first three noise sources are inversely proportional to amplitude while thermal drift noise is independent of the amplitude. Thus, we show that the earlier finding that quoted optimal signal-to-noise ratio for oscillation amplitudes similar to the range of the forces is still correct when considering all four frequency noise contributions. Finally, we suggest how the signal-to-noise ratio of the sensors can be further improved and briefly discuss the challenges of mounting tips.

cond-mat.mes-hall↗