arXiv subjects

Seung-Hwan Do

Publications and source records attributed to Seung-Hwan Do.

16 recordsLinked to original sources

arXiv2025

Diverse Magnetic Phase Diagram and Anomalous Hall Effect in Antiferromagetic LuMn$_6$Sn$_6$

The interactions between conduction electrons and magnetism can significantly enhance the Hall signal, a phenomenon known as the anomalous Hall effect (AHE). While the AHE is generally not expected in antiferromagnets, a large AHE is observed in certain antiferromagnets with noncollinear spin textures and nonvanishing Berry curvature. In this work, we present a rich temperature and magnetic phase diagram with eight distinct magnetic phases for the antiferromagnetic kagome compound LuMn$_6$Sn$_6$. The Hall effect analysis in LuMn$_6$Sn$_6$ reveals both intriguing physical phenomena and methodological challenges. In the coplanar canted antiferromagnetic phase, we observe an AHE, which likely originates from the intrinsic effects. At low temperatures, upon entering the ferromagnetic phase, the AHE sharply increases and exceeds the conventional limits expected from intrinsic mechanisms. We also demonstrate the limitations of standard experimental methods in extracting the topological contribution to the Hall effect data. We show the importance of considering magnetoresistance anisotropy when estimating the anomalous and topological Hall effects. These shortcomings in current approaches in partitioning the Hall response necessitate new tools to interpret transport results in complex magnetic materials such as LuMn$_6$Sn$_6$.

cond-mat.str-el↗

arXiv2023

Topological Nernst and topological thermal Hall effect in rare-earth kagome ScMn$_6$Sn$_6$

Thermal and thermoelectric measurements are known as powerful tools to uncover the physical properties of quantum materials due to their sensitivity towards the scattering and chirality of heat carriers. We use these techniques to confirm the presence of momentum and real-space topology in ScMn$_6$Sn$_6$. There is an unconventional dramatic increase in the Seebeck coefficient on entering the transverse conical spiral (TCS) below $T$ = 200 K suggesting an unusual scattering of heat carriers. In addition, the observed anomalous thermal Hall effect and the anomalous Nernst effect indicates non-zero Berry curvature in $k$-space. Furthermore, we identify a significant topological contribution to the thermal Hall and Nernst signals in the TCS phase revealing the impacts of real-space Berry curvature. We discuss the presence of topological thermal Hall effect and topological Nernst effect for the first time in the diverse HfFe$_6$Ge$_6$ family. This study illustrates the importance of transverse thermal and thermoelectric measurements to investigate the origin of topological transport in the non-coplanar magnetic phases in this family of kagome metals.

cond-mat.str-el↗

arXiv2023

Field-induced spin level crossings within a quasi-XY antiferromagnetic state in Ba$_{2}$FeSi$_{2}$O$_{7}$

We present a high-field study of the strongly anisotropic easy-plane square lattice $S$ = 2 quantum magnet Ba$_{2}$FeSi$_{2}$O$_{7}$. This compound is a rare high-spin antiferromagnetic system with very strong easy-plane anisotropy, such that the interplay between spin level crossings and antiferromagnetic order can be studied. We observe a magnetic field-induced spin level crossing occurring within an ordered state. This spin level crossing appears to preserve the magnetic symmetry while producing a non-monotonic dependence the order parameter magnitude. The resulting temperature-magnetic field phase diagram exhibits two dome-shaped regions of magnetic order overlapping around 30 T. The ground state of the lower-field dome is predominantly a linear combination of $| S^{z} = 0 \rangle$ and $ |S^{z} = 1 \rangle$ states, while the ground state of the higher-field dome can be approximated by a linear combination of $| S^{z} = 1 \rangle $ and $ | S^{z} = 2\rangle$ states. At 30 T, where the spin levels cross, the magnetization exhibits a slanted plateau, {\color {black}the magnetocaloric effect shows a broad hump, and the electric polarization shows a weak slope change}. We determined the detailed magnetic phase boundaries and the spin level crossings using measurements of magnetization, electric polarization, and the magnetocaloric effect in pulsed magnetic fields to 60 T. We calculate these properties using a mean field theory based on direct products of SU(5) coherent states and find good agreement. Finally, we measure and calculate the magnetically-induced electric polarization that reflects magnetic ordering and spin level crossings. This multiferroic behavior provides another avenue for detecting phase boundaries and symmetry changes.

cond-mat.str-el↗

arXiv2023

Cubic Double Perovskites Host Noncoplanar Spin Textures

Magnetic materials with noncoplanar magnetic structures can show unusual physical properties driven by nontrivial topology. Topologically-active states are often multi-q structures, which are challenging to stabilize in models and to identify in materials. Here, we use inelastic neutron-scattering experiments to show that the insulating double perovskites Ba2YRuO6 and Ba2LuRuO6 host a noncoplanar 3-q structure on the face-centered cubic lattice. Quantitative analysis of our neutron-scattering data reveals that these 3-q states are stabilized by biquadratic interactions. Our study identifies double perovskites as a highly promising class of materials to realize topological magnetism, elucidates the stabilization mechanism of the 3-q state in these materials, and establishes neutron spectroscopy on powder samples as a valuable technique to distinguish multi-q from single-q states, facilitating the discovery of topologically-nontrivial magnetic materials.

cond-mat.str-el↗

arXiv2022

Understanding temperature-dependent SU($3$) spin dynamics in the $S=1$ antiferromagnet Ba$_2$FeSi$_2$O$_7$

Quantum magnets admit more than one classical limit and $N$-level systems with strong single-ion anisotropy are expected to be described by a classical approximation based on SU($N$) coherent states. Here we test this hypothesis by modeling finite temperature inelastic neutron scattering (INS) data of the effective spin-one antiferromagnet \bfso{}. The measured dynamic structure factor is calculated with a generalized Landau-Lifshitz dynamics for SU($3$) spins. Unlike the traditional classical limit based on SU($2$) coherent states, the results obtained with classical SU($3$) spins are in good agreement with the measured temperature-dependent spectrum. The SU($3$) approach developed here provides a general framework to understand the broad class of materials comprising weakly coupled antiferromagnetic dimers, trimers, or tetramers, and magnets with strong single-ion anisotropy.

cond-mat.str-el↗

arXiv2022

Gaps in Topological Magnon Spectra: Intrinsic vs. Extrinsic Effects

For topological magnon spectra, determining and explaining the presence of a gap at a magnon crossing point is a key to characterize the topological properties of the system. An inelastic neutron scattering study of a single crystal is a powerful experimental technique that is widely employed to probe the magnetic excitation spectra of topological materials. Here, we show that when the scattering intensity rapidly disperses in the vicinity of a crossing point, such as a Dirac point, the apparent topological gap size is extremely sensitive to experimental conditions including sample mosaic, resolution, and momentum integration range. We demonstrate these effects using comprehensive neutron-scattering measurements of CrCl$_3$. Our measurements confirm the gapless nature of the Dirac magnon in CrCl$_3$, but also reveal an artificial, i.e. extrinsic, magnon gap unless the momentum integration range is carefully controlled. Our study provides an explanation of the apparent discrepancies between spectroscopic and first-principles estimates of Dirac magnon gap sizes, and provides guidelines for accurate spectroscopic measurement of topological magnon gaps.

cond-mat.mes-hall↗

arXiv2022

Weak-coupling to strong-coupling quantum criticality crossover in a Kitaev quantum spin liquid $α$-RuCl$_3$

We report an unprecedented quantum criticality crossover representing two different universal scaling behaviors in a Kitaev quantum magnetic material $α$-RuCl$_3$. $α$-RuCl$_3$ presents both a symmetry breaking antiferromagnetic order and a long-range entangled topological order of a quantum spin liquid, and thus could be a candidate system for a new universality class involving deconfined fractionalized excitations of the local Z$_2$ fluxes and itinerant Majorana fermions. Theoretical analyses on the inelastic neutron scattering and specific heat results demonstrate that Wilson-Fisher-Yukawa-type 'conventional' weak-coupling quantum criticality in high energy scales crosses over to heavy-fermion-type 'local' strong-coupling one in low energy scales. Our findings provide deep insight on how the quantum criticality evolves in fermion-boson coupled topological systems with different types of deconfined fermions.

cond-mat.str-el↗

arXiv2021

Decay and renormalization of a longitudinal mode in a quasi-two-dimensional antiferromagnet

An ongoing challenge in the study of quantum materials, is to reveal and explain collective quantum effects in spin systems where interactions between different modes types are important. Here we approach this problem through a combined experimental and theoretical study of interacting transverse and longitudinal modes in an easy-plane quantum magnet near a continuous quantum phase transition. Our inelastic neutron scattering measurements of Ba$_{2}$FeSi$_{2}O$_{7}$ reveal the emergence, decay, and renormalization of a longitudinal mode throughout the Brillouin zone. The decay of the longitudinal mode is particularly pronounced at the zone center. To account for the many-body effects of the interacting low-energy modes in anisotropic magnets, we generalize the standard spin-wave theory. The measured mode decay and renormalization is reproduced by including all one-loop corrections. The theoretical framework developed here is broadly applicable to quantum magnets with more than one type of low energy mode.

cond-mat.str-el↗

arXiv2021

Physical properties of a quasi-two-dimensional square lattice antiferromagnet Ba$_2$FeSi$_2$O$_7$

We report the magnetization ($χ$, $M$), specific heat ($C_{\text{P}}$), and neutron powder diffraction results on a quasi-two-dimensional $S$ = 2 square lattice antiferromagnet Ba$_2$FeSi$_2$O$_7$ consisting of FeO$_4$ tetragons with a large compressive distortion (27%). Despite of the quasi-two-dimensional lattice structure, both $χ$ and $C_{\text{P}}$ present three dimensional magnetic long-range order below the Néel temperature $T_{\text{N}}$ = 5.2 K. Neutron diffraction data shows a collinear $Q_{m}$ = (1,0,0.5) antiferromagnetic (AFM) structure with the in-plane ordered magnetic moment suppressed by 26% below $T_{\text{N}}$. Both the AFM structure and the suppressed moments are well explained by the Monte Carlo simulation with a large single-ion ab-plane anisotropy $D$ = 1.4 meV and a rather small in-plane Heisenberg exchange $J_{\text{intra}}$ = 0.15 meV. The characteristic two dimensional spin fluctuations can be recognized in the magnetic entropy release and diffuse scattering above $T_{\text{N}}$. This new quasi-2D magnetic system also displays unusual non-monotonic dependence of the $T_{\text{N}}$ as a function of magnetic field $H$.

cond-mat.str-el↗

arXiv2021

Damped Dirac magnon in a metallic kagome antiferromagnet FeSn

The kagome lattice is a fertile platform to explore topological excitations with both Fermi-Dirac and Bose-Einstein statistics. While relativistic Dirac Fermions and flat-bands have been discovered in the electronic structure of kagome metals, the spin excitations have received less attention. Here we report inelastic neutron scattering studies of the prototypical kagome magnetic metal FeSn. The spectra display well-defined spin waves extending up to 120 meV. Above this energy, the spin waves become progressively broadened, reflecting interactions with the Stoner continuum. Using linear spin wave theory, we determine an effective spin Hamiltonian that reproduces the measured dispersion. This analysis indicates that the Dirac magnon at the K-point remarkably occurs on the brink of a region where well-defined spin waves become unobservable. Our results emphasize the influential role of itinerant carriers on the topological spin excitations of metallic kagome magnets.

cond-mat.str-el↗

arXiv2020

Complex magnetic phases in polar tetragonal intermetallic NdCoGe$_3$

Polar materials can host a variety of topologically significant magnetic phases, which often emerge from a modulated magnetic ground state. Relatively few noncentrosymmetric tetragonal materials have been shown to host topological spin textures and new candidate materials are necessary to expand the current theoretical models. This manuscript reports on the anisotropic magnetism in the polar, tetragonal material NdCoGe$_3$ via thermodynamic and neutron diffraction measurements. The previously reported $H$-$T$ phase diagram is updated to include several additional phases, which exist for both $H$ = 0 and with an applied field H$\perp$ c. Neutron diffraction data reveal that the magnetic structures below $T_{N1}$ = 3.70 K and $T_{N2}$ = 3.50 K are incommensurate, with a ground state magnetic order that is incommensurate in all directions with the propagation vector $\vec{k}$ = (0.494, 0.0044, 0.385) at 1.8 K. A unique magnetic structure solution is not achievable, but the possible single and multi-$\vec{k}$ spin models are discussed. These results demonstrate that NdCoGe3 hosts complicated magnetic order derived from modulated magnetic moments.

cond-mat.str-el↗

arXiv2020

Observation of a random singlet state in a diluted Kitaev honeycomb material

We report a $^{35}$Cl nuclear magnetic resonance (NMR) study of the diluted Kitaev material $α$-Ru$_{1-x}$Ir$_x$Cl$_3$ ($x=0.1$ and $0.2$) where non-magnetic Ir$^{3+}$ dopants substitute Ru$^{3+}$ ions. Upon dilution, the $^{35}$Cl spectra exhibit unusual large magnetic inhomogeneity, which sets in at temperatures below the Kitaev exchange energy scale. At the same time, the $^{35}$Cl spin-lattice relaxation rate $T_1^{-1}$ as a function of dilution and magnetic field unravels a critical doping of $x_c\approx 0.22$, towards which both the field-induced spin gap and the zero-field magnetic ordering are simultaneously suppressed, while novel gapless low-energy spin excitations dominate the relaxation process. These NMR findings point to the stabilization of a random singlet phase in $α$-Ru$_{1-x}$Ir$_x$Cl$_3$, arising from the interplay of dilution and exchange frustration in the quantum limit.

cond-mat.str-el↗

arXiv2019

Magnon bound states vs. anyonic Majorana excitations in the Kitaev honeycomb magnet $α$-RuCl$_3$

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as $α$-RuCl$_3$. Currently, two competing scenarios exist for the intermediate field phase of this compound ($B=7-10$ T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.

cond-mat.str-el↗

arXiv2018

Sub-gap optical response across the structural phase transition in van der Waals layered α-RuCl$_3$

We report magnetic, thermodynamic, thermal expansion, and on detailed optical experiments on the layered compound $α$-RuCl$_3$ focusing on the THz and sub-gap optical response across the structural phase transition from the monoclinic high-temperature to the rhombohedral low-temperature structure, where the stacking sequence of the molecular layers is changed. This type of phase transition is characteristic for a variety of tri-halides crystallizing in a layered honeycomb-type structure and so far is unique, as the low-temperature phase exhibits the higher symmetry. One motivation is to unravel the microscopic nature of spin-orbital excitations via a study of temperature and symmetry-induced changes. We document a number of highly unusual findings: A characteristic two-step hysteresis of the structural phase transition, accompanied by a dramatic change of the reflectivity. An electronic excitation, which appears in a narrow temperature range just across the structural phase transition, and a complex dielectric loss spectrum in the THz regime, which could indicate remnants of Kitaev physics. Despite significant symmetry changes across the monoclinic to rhombohedral phase transition, phonon eigenfrequencies and the majority of spin-orbital excitations are not strongly influenced. Obviously, the symmetry of the single molecular layers determine the eigenfrequencies of most of these excitations. Finally, from this combined terahertz, far- and mid-infrared study we try to shed some light on the so far unsolved low energy (< 1eV) electronic structure of the ruthenium $4d^5$ electrons in $α$-RuCl$_3$.

cond-mat.str-el↗

arXiv2017

Electronic and phonon excitations in α-RuCl$_3$

We report on THz, infrared reflectivity and transmission experiments for wave numbers from 10 to 8000 cm$^{-1}$ ($\sim$ 1 meV - 1 eV) and for temperatures from 5 to 295 K on the Kitaev candidate material α-RuCl$_3$. As reported earlier, the compound under investigation passes through a first-order structural phase transition, from a monoclinic high-temperature to a rhombohedral low-temperature phase. The phase transition shows an extreme and unusual hysteretic behavior, which extends from 60 to 166 K. In passing this phase transition, in the complete frequency range investigated we found a significant reflectance change, which amounts almost a factor of two. We provide a broadband spectrum of dielectric constant, dielectric loss and optical conductivity from the THz to the mid infrared regime and study in detail the phonon response and the low-lying electronic density of states. We provide evidence for the onset of an optical energy gap, which is of order 200 meV, in good agreement with the gap derived from measurements of the DC electrical resistivity. Remarkably, the onset of the gap exhibits a strong blue shift on increasing temperatures.

cond-mat.str-el↗

arXiv2017

Incarnation of Majorana Fermions in Kitaev Quantum Spin Lattice

Kitaev quantum spin liquid is a topological magnetic quantum state characterized by Majorana fermions of fractionalized spin excitations, which are identical to their own antiparticles. Here, we demonstrate emergence of Majorana fermions thermally fractionalized in the Kitaev honeycomb spin lattice α-RuCl3. The specific heat data unveil the characteristic two-stage release of magnetic entropy involving localized and itinerant Majorana fermions. The inelastic neutron scattering results further corroborate these two distinct fermions by exhibiting quasielastic excitations at low energies around the Brillouin zone center and Y-shaped magnetic continuum at high energies, which are evident for the ferromagnetic Kitaev model. Our results provide an opportunity to build a unified conceptual framework of fractionalized excitations, applicable also for the quantum Hall states, superconductors, and frustrated magnets.

cond-mat.str-el↗