arXiv subjects

Y. Soh

Publications and source records attributed to Y. Soh.

17 recordsLinked to original sources

arXiv2025

Creating and Deleting a Single Dipolar Skyrmion by Surface Spin Twists

We report deterministic operations on single dipolar skyrmions confined in nanostructured cuboids using in-plane currents. We achieve highly reversible writing and deleting of skyrmions in the simple cuboid without any artificial defects or pinning sites. The current-induced creation of skyrmions is well-understood through the spin-transfer torque acting on surface spin twists of the spontaneous 3D ferromagnetic state, caused by the magnetic dipole-dipole interaction of the uniaxial Fe3Sn2 magnet with a low-quality factor. Current-induced deletions of skyrmions result from the combined effects of magnetic hysteresis and Joule thermal heating. Our results are replicated consistently through 3D micromagnetic simulations. Our approach offers a viable method for achieving reliable single-bit operations in skyrmionic devices for applications such as random-access memories.

cond-mat.mes-hall↗

arXiv2025

Strong electron-electron interactions in a dilute weakly-localized metal near a metal-to-insulator transition

Because it is easily switched from insulator to metal either via chemical doping or electrical gating, silicon is at the core of modern information technology and remains a candidate platform for quantum computing. The metal-to-insulator transition in this material has therefore been one of the most studied phenomena in condensed matter physics, and has been revisited with considerable profit each time a new fabrication technology has been introduced. Here we take advantage of recent advances in creating ultra-thin layers of Bohr-atom-like dopants to realize the two-dimensional disordered Hubbard model at half-filling and its metal-to-insulator transition (MIT) as a function of mean distance between atoms. We use gas-phase dosing of dopant precursor molecules on silicon to create arsenic and phosphorus $\delta$-layers as thin as 0.4~nm and as dilute as 10$^{13}$~cm$^{-2}$. On approaching the insulating state, the conventional weak localization effects, prevalent at high dopant densities and due to orbital motion of the electrons in the plane, become dominated by electron-electron interaction contributions which obey a paramagnetic Zeeman scaling law. The latter make a negative contribution to the conductance, and thus cannot be interpreted in terms of an emergent Kondo regime near the MIT.

quant-ph↗

arXiv2024

In-plane Antiferromagnetism in Ferromagnetic Kagome Semimetal Co3Sn2S2

Co3Sn2S2 has been reported to be a Weyl semimetal with broken time-reversal symmetry with c axis ferromagnetism (FM) below a Curie temperature of 177 K. Despite the large interest in Co3Sn2S2, the magnetic structure is still under debate and recent studies have challenged our understanding of the magnetic phase diagram of Co3Sn2S2 by reporting unusual magnetic phases including the presence of exchange bias. Understanding the magnetism of Co3Sn2S2 is important since its electronic band structure including the much-celebrated flat bands and Weyl nodes depend on the magnetic phase. In this work, using X-ray Magnetic Circular Dichroism (XMCD), we establish that the magnetic moment in Co arises from the spin, with negligible orbital moment. In addition, we detect an in-plane AFM minority phase in the sea of a FM phase using spatially-resolved angle-resolved photoemission spectroscopy ({\mu}-ARPES) combined with density functional theory (DFT) calculation. Separately, we detect a sharp flat band precisely at the Fermi level (EF) at some regions in the sample, which we attribute to a surface state. The AFM phase survives even to the low temperature of 6 K. This example of entirely different magnetic ground states in a stoichiometric intermetallic invites further efforts to explore the observed AFM phase and understand the origin and nature of the magnetic and electronic inhomogeneity on the mesoscale and the interface between the AFM and FM phases.

cond-mat.mtrl-sci↗

arXiv2023

Robust room temperature ferromagnetism in an itinerant van der Waals antiferromagnet

The coexistence of antiferromagnetic and ferromagnetic order at room temperature in single-phase van der Waals materials, particularly within the two-dimensional limit, has attracted significant research interest. Nonetheless, such materials are rare. In this work, we introduce an itinerant van der Waals antiferromagnet (Fe0.56Co0.44)5GeTe2, where the ferromagnetic order of its exfoliated flakes remains discernible up to room temperature, extending down to the monolayer limit. A notable phenomenon observed is the evident odd-even layer-number effect at high temperature (e.g., T = 150 K). Such behaviour can be expounded by a linear-chain model. Of particular interest is the robust ferromagnetic order observed in even-layer flakes at low temperature (e.g., T = 2 K), which could potentially be attributed to spin-polarized defects. The intricate interplay among magnetic field strength, layer number, and temperature gives rise to a diverse array of phenomena, holding promise not only for new physics but also for practical applications.

cond-mat.str-el↗

arXiv2023

Charge fluctuations in the intermediate-valence ground state of SmCoIn$_5$

The microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn$_5$ and other Ce-based heavy fermion materials, depends strongly on the efficiency with which $f$ electrons are delocalized from the rare earth sites and participate in a Kondo lattice. Replacing Ce$^{3+}$ ($4f^1$, $J=5/2$) with Sm$^{3+}$ ($4f^5$, $J=5/2$), we show that a combination of crystal field and on-site Coulomb repulsion causes SmCoIn$_5$ to exhibit a $\Gamma_7$ ground state similar to CeCoIn$_5$ with multiple $f$ electrons. Remarkably, we also find that with this ground state, SmCoIn$_5$ exhibits a temperature-induced valence crossover consistent with a Kondo scenario, leading to increased delocalization of $f$ holes below a temperature scale set by the crystal field, $T_v$ $\approx$ 60 K. Our result provides evidence that in the case of many $f$ electrons, the crystal field remains the most important tuning knob in controlling the efficiency of delocalization near a heavy fermion quantum critical point, and additionally clarifies that charge fluctuations play a general role in the ground state of "115" materials.

cond-mat.str-el↗

arXiv2023

Spin waves and orbital contribution to ferromagnetism in a topological metal

Special arrangements of atoms with more than one atom per unit cell, including honeycomb or kagome (woven bamboo mat) lattices, can host propagating excitations with non-trivial topology as defined by their evolution along closed paths in momentum space. Excitations on such lattices can also be momentum-independent, meaning that they are localized notwithstanding strong hopping of the underlying disturbances between neighbouring sites. The associated flat bands are interesting because the interactions between the heavy quasiparticles inhabiting them will become much more important than for strong dispersion, resulting in novel quantum solid and liquid states. Different stackings of two-dimensional lattices, for example twisted graphene bilayers, provide routes to further engineer topology and many-body effects. Here, we report the discovery, using circularly polarized x-rays for the unambiguous isolation of magnetic signals, of a nearly flat spin wave band and large (compared to elemental iron) orbital moment for the metallic ferromagnet Fe3Sn2, built from compact AB-stacked kagome bilayers and which has a topologically non-trivial electronic band structure controllable by modest external magnetic fields. As a function of out-of-plane momentum, the nearly flat optical mode and the global rotation symmetry-restoring acoustic mode are out of phase, consistent with a bilayer exchange coupling that is larger than the already large in-plane couplings. The defining units of this topological metal are therefore a triangular lattice of octahedral iron clusters rather than weakly coupled kagome planes. The spin waves are strongly damped when compared to elemental iron, opening the topic of interactions of topological bosons (spin waves) and fermions (electrons) with the very specific target of explaining boson lifetimes.

cond-mat.str-el↗

arXiv2022

Combined Magnetic Imaging and Anisotropic Magnetoresistance Detection of Dipolar Skyrmions

Magnetic skyrmions are localized particle-like nontrivial swirls that are promising in building high-performance topological spintronic devices. The read-out functions in skyrmionic devices require the translation of magnetic skyrmions to electrical signals. Here, we report combined real-space magnetic imaging and anisotropic magnetoresistance studies on dipolar skyrmions. A single skyrmion chain and single skyrmion are observed using Lorentz transmission electron microscopy imaging. Meanwhile, the field, helicity, and skyrmion count dependence of anisotropic magnetoresistance of the Fe3Sn2 nanostructures are obtained simultaneously. Our results demonstrate that the anisotropic magnetoresistance of skyrmions is independent of the helicity and proportional to the skyrmion count. Our work could promote read-out operations in skyrmion-based spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Transfer Learning Application of Self-supervised Learning in ARPES

Recent development in angle-resolved photoemission spectroscopy (ARPES) technique involves spatially resolving samples while maintaining the high-resolution feature of momentum space. This development easily expands the data size and its complexity for data analysis, where one of it is to label similar dispersion cuts and map them spatially. In this work, we demonstrate that the recent development in representational learning (self-supervised learning) model combined with k-means clustering can help automate that part of data analysis and save precious time, albeit with low performance. Finally, we introduce a few-shot learning (k-nearest neighbour or kNN) in representational space where we selectively choose one (k=1) image reference for each known label and subsequently label the rest of the data with respect to the nearest reference image. This last approach demonstrates the strength of the self-supervised learning to automate the image analysis in ARPES in particular and can be generalized into any science data analysis that heavily involves image data.

physics.ins-det↗

arXiv2022

Non-destructive X-ray imaging of patterned delta-layer devices in silicon

The progress of miniaturisation in integrated electronics has led to atomic and nanometre-sized dopant devices in silicon. Such structures can be fabricated routinely by hydrogen resist lithography, using various dopants such as phosphorous and arsenic. However, the ability to non-destructively obtain atomic-species-specific images of the final structure, which would be an indispensable tool for building more complex nano-scale devices, such as quantum co-processors, remains an unresolved challenge. Here we exploit X-ray fluorescence to create an element-specific image of As dopants in silicon, with dopant densities in absolute units and a resolution limited by the beam focal size (here $\sim1~\mu$m), without affecting the device's low temperature electronic properties. The As densities provided by the X-ray data are compared to those derived from Hall effect measurements as well as the standard non-repeatable, scanning tunnelling microscopy and secondary ion mass spectroscopy, techniques. Before and after the X-ray experiments, we also measured the magneto-conductance, dominated by weak localisation, a quantum interference effect extremely sensitive to sample dimensions and disorder. Notwithstanding the $1.5\times10^{10}$ Sv ($1.5\times10^{16}$ Rad/cm$^{-2}$) exposure of the device to X-rays, all transport data were unchanged to within experimental errors, corresponding to upper bounds of 0.2 Angstroms for the radiation-induced motion of the typical As atom and 3$\%$ for the loss of activated, carrier-contributing dopants. With next generation synchrotron radiation sources and more advanced optics, we foresee that it will be possible to obtain X-ray images of single dopant atoms within resolved radii of 5 nm.

quant-ph↗

arXiv2022

Anomalous quasiparticles in the zone center electron pocket of the kagom\'e ferromagnet Fe3Sn2

One material containing kagome bilayers and featuring both exceptional magnetism and electron transport is the ferromagnetic metal Fe3Sn2. Notwithstanding the widespread interest in Fe3Sn2, crystal twinning, difficulties in distinguishing surface from bulk states, and a large unit cell have until now prevented the synchrotron-based spectroscopic observation of sharply resolved quasiparticles near the Fermi surface which could be responsible for the anomalous properties appearing at low temperatures for the material. Here we report microfocused laser-based angle-resolved photoemission spectroscopy (micro-ARPES), which offers the first look at such quasiparticles. The high spatial resolution allows individual crystal twin domains to be examined in isolation, resulting in the discovery of three-fold symmetric electron pockets at the Brillouin zone (BZ) center, not predicted by early tight-binding descriptions but in agreement with density functional theory (DFT) calculations, which also feature Weyl nodes. The quasiparticles in these pockets have remarkably long mean free paths, and their Fermi surface area is consistent with reported quantum oscillations. At the same time, though, the best-defined Fermi surface is reduced at low temperature, and the quasiparticles generally are marginal in the sense that their wavelength uncertainty is of order the deviation of the quasiparticle wavelength from the Fermi vector. We attribute these manifestations of strong electron-electron interactions to a flat band predicted by our DFT to lie just above the dispersive bands seen in this experiment. Thus, beyond demonstrating the impact of twin averaging for ARPES measurements of band structures, our experiments reveal many-body physics unaccounted for by current theories of metallic kagome ferromagnets.

cond-mat.str-el↗

arXiv2022

Current-Controlled Topological Magnetic Transformations in a Nanostructured Kagome Magnet

Topological magnetic charge Q is a fundamental parameter that describes the magnetic domains and determines their intriguing electromagnetic properties. The ability to switch Q in a controlled way by electrical methods allows for flexible manipulation of electromagnetic behavior in future spintronic devices. Here we report the room-temperature current-controlled topological magnetic transformations between Q = -1 skyrmions and Q = 0 stripes or type-II bubbles in a kagome crystal Fe$_3$Sn$_2$. We show that the reproducible and reversible skyrmion-bubble and skyrmion-stripe transformations can be achieved by tuning the density of nanosecond pulsed current of the order of ~10$^{10}$ A$^{-2}$. Further numerical simulations suggest that spin-transfer torque combined with Joule thermal heating effects determine the current-induced topological magnetic transformations.

cond-mat.mes-hall↗

arXiv2022

Two-dimensional characterization of three-dimensional magnetic bubbles in Fe$_3$Sn$_2$ nanostructures

We report differential phase contrast scanning transmission electron microscopy (TEM) of nanoscale magnetic objects in Kagome ferromagnet Fe$_3$Sn$_2$ nanostructures. This technique can directly detect the deflection angle of a focused electron beam, thus allowing clear identification of the real magnetic structures of two magnetic objects including three-ring and complex arch-shaped vortices in Fe$_3$Sn$_2$ by Lorentz transmission electron microscopy imaging. Numerical calculations based on real material-specific parameters well reproduced the experimental results, showing that the magnetic objects can be attributed to integral magnetizations of two types of complex three-dimensional (3D) magnetic skyrmion bubbles with depth-modulated spin twisting. Magnetic configurations obtained using the high-resolution TEM are generally considered as two-dimensional (2D) magnetic objects previously. Our results imply the importance of the integral magnetizations of underestimated 3D magnetic structures in 2D TEM magnetic characterizations.

cond-mat.mtrl-sci↗

arXiv2021

Tuning the electronic band structure in a kagome ferromagnetic metal via magnetization

Materials with zero energy band gap display intriguing properties including high sensitivity of the electronic band structure to external stimulus such as pressure or magnetic field. An interesting candidate for zero energy band gap are Weyl nodes at the Fermi level EF. A prerequisite for the existence of Weyl nodes is to either have inversion or time reversal symmetry broken. Weyl nodes in systems with broken time reversal symmetry are ideal to realize the tunability of the electronic band structure by magnetic field. Theoretically, it has been shown that in ferromagnetic Weyl materials, the band structure is dependent upon the magnetization direction and thus the electronic bands can be tuned by controlling the magnetization direction. Here, we demonstrate tuning of the band structure in a kagome Weyl ferromagnetic metal Fe3Sn2 with magnetization and magnetic field. Owing to spin-orbit coupling, we observe changes in the band structure depending on the magnetization direction that amount to a decrease in the carrier density by a factor of four when the magnetization lies in the kagome plane as compared to when the magnetization is along the c axis. Our discovery opens a way for tuning the carrier density in ferromagnetic materials.

cond-mat.str-el↗

arXiv2020

Anomalous planar Hall effect in a kagome ferromagnet

The macroscopic signature for Weyl nodes so far has been the negative longitudinal magnetoresistance arising from the chiral anomaly. However, negative longitudinal magnetoresistance is not unique to chiral anomaly and can arise due to completely different mechanisms such as current jetting or in ferromagnetic systems due to the suppression of scattering with magnons. Therefore, a macroscopic effect that can be uniquely attributed to the presence of Weyl nodes is desirable. Here we show that the planar Hall effect could be a hallmark for Weyl nodes. We investigated the anisotropic magnetoresistance and planar Hall effect in Fe$_3$Sn$_2$, which has a kagome lattice and has been predicted to be a type II Weyl metal. We discover that the planar Hall effect contains a field antisymmetric contribution in addition to the ordinary field symmetric contribution. The field antisymmetric planar Hall effect has a 3-fold rotational symmetry, distinctively different from the symmetric planar Hall effect, but consistent with the 3-fold rotational degeneracy of the magnetization in Fe$_3$Sn$_2$. The temperature and field dependence of the antisymmetric planar Hall effect rules out an interpretation based on contribution from the anomalous Hall effect and is different from the symmetric planar Hall effect, pointing to a different origin. We attribute the antisymmetric planar Hall effect to the topological nature of Fe$_3$Sn$_2$ due to the presence of Weyl II nodes. Our finding offers a promising route for macroscopically probing Weyl systems.

cond-mat.mes-hall↗

arXiv2019

Magnetotransport as diagnostic of spin reorientation: kagome ferromagnet as a case study

While in most ferro or antiferromagnetic materials there is a unique crystallographic direction, including crystallographically equivalent directions, in which the moments like to point due to spin-orbit coupling, in some, the direction of the spin reorients as a function of a certain physical parameter such as temperature, pressure etc. Fe3Sn2 is a kagome ferromagnet with an onset of ferromagnetism below 650 K, and undergoes a spin reorientation near 150 K. While it is known that the moments in Fe3Sn2 point perpendicular to the kagome plane at high temperatures and parallel to the kagome plane at low temperatures, how the distribution of the magnetic domains in the two different spin orientations evolve throughout the spin reorientation is not well known. Furthermore, while there have been various reports on the magnetotransport properties in the Hall configuration, the angular dependence of magnetoresistance has not been studied so far. In this paper, we have examined the spin reorientation by using anisotropic magnetoresistivity in detail, exploiting the dependence of the resistivity on the direction between magnetization and applied current. We are able to determine the distribution of the magnetic domains as a function of temperature between 360 K to 2 K and the reorientation transition to peak at 120 K. We discover that both out of plane and in plane phases coexist at temperatures around the spin reorientation, indicative of a first order transition. Although the volume of the magnetic domains in the different phases sharply changes at the spin reorientation transition, the electronic structure for a specific magnetization is not influenced by the spin reorientation. In contrast, we observe an electronic transition around 40 K, hitherto unreported, and reflected in both the zero-field resistivity and anisotropic resistivity.

cond-mat.mtrl-sci↗

arXiv2019

Bulk single crystal growth of the theoretically predicted magnetic Weyl semimetals $R$AlGe ($R$ = Pr, Ce)

We explore two methods for single crystal growth of the theoretically proposed magnetic Weyl semimetals $R$AlGe ($R$ = Pr,Ce), which prove that a floating zone technique, being both crucible- and flux-free, is crucial to obtain perfectly stoichiometric $R$AlGe crystals. In contrast, the crystals grown by a flux growth technique tend to be Al-rich. We further present both structural and elemental analysis, along with bulk magnetization and electrical resistivity data on the crystals prepared by the floating zone technique. Both systems with the intended 1:1:1 stoichiometry crystallize in the anticipated polar I4$_{1}$md (No. 109) space group, although neither displays the theoretically expected ferromagnetic ground state. Instead PrAlGe displays a spin-glass-like transition below 16 K with an easy-c-axis and CeAlGe has an easy-ab-plane antiferromagnetic order below 5 K. The grown crystals provide an ideal platform for microscopic studies of the magnetic field-tunable correlation physics involving magnetism and topological Weyl nodes.

cond-mat.mtrl-sci↗

arXiv2018

Switchable Weyl nodes in topological Kagome ferromagnet Fe3Sn2

The control of topological quantum materials is the prerequisite for novel devices exploiting these materials. Here we propose that the room temperature ferromagnet Fe3Sn2, whose fundamental building blocks are Kagome bilayers of iron, hosts Weyl nodes at the Fermi level which can move in momentum space depending on the direction of the magnetization, itself readily controlled either by modest external fields or temperature. The proposal is derived from density functional calculations, including a mean field treatment of Hubbard repulsion U, which have been validated by comparison with angle-resolved photoemission data. Ferromagnetism with magnetization along certain directions is shown to lift the Weyl degeneracies, while at the same time inducing texture in the quasiparticle spin polarizations mapped in reciprocal space. In particular, the polarization is attenuated and then rotated from parallel to perpendicular to antiparallel to the magnetization as Weyl points derived from crossing of majority and minority spin bands are traversed.

cond-mat.str-el↗