arXiv subjects

Minhyea Lee

Publications and source records attributed to Minhyea Lee.

At least 19 recordsLinked to original sources

arXiv2025

Field-induced magnetic phase transition driven by a ground state level-crossing in CsErSe$_2$

We report a comprehensive study of the low-temperature magnetic properties of the insulating rare-earth triangular magnet CsErSe$_2$. We uncover a field-induced level crossing at the crystal electric field (CEF) ground state, which gives rise to a first order phase transition as well as to distinctive magnetic properties. This crossing is identified by the accurate determination of the single-ion Hamiltonian, the reliability of which is substantially enhanced by field-dependent optical spectroscopy that directly tracks the Zeeman splittings of Kramers doublets. We also observe spontaneous antiferromagnetic ordering in CsErSe$_2$ below $T_N \approx 110$ mK, and resolve the corresponding magnetic structure using elastic neutron scattering. We discuss how the rich magnetic behavior of CsErSe$_2$ arises from the interplay of non-trivial field-dependent single-ion physics and spontaneous ordering, and highlight the implications of these results for understanding magnetic phenomena across a wide range of insulating rare-earth magnets.

cond-mat.str-el↗

arXiv2025

Evolution of magnetic bubble domains in the uniaxial ferromagnet CeRu$_2$Ga$_2$B inferred from the Hall effect and ac magnetic susceptibility

We study the Hall effect, AC magnetic susceptibility ($\chi_{\rm ac}$), and magnetic force microscopy of the uniaxial ferromagnet CeRu$_2$Ga$_2$B with a centrosymmetric crystal structure. We observe a finite topological Hall effect (THE) within the ordered phase before the magnetization is polarized by applied field. By comparing the field dependences of the area fraction of the magnetic bubbles, the derivative of $\chi_{\rm ac}$, and the THE signal, we deduce that the magnetic bubbles of CeRu$_2$Ga$_2$B evolve from the trivial to topological spin texture with field. Our findings will be utilized to expand the search for magnetic materials hosting the topological spin textures to ones with uniaxial anisotropy, and open a new possibility to tailor the topological spin texture.

cond-mat.str-el↗

arXiv2024

Refining the Two-Band Model for Highly Compensated Semimetals Using Thermoelectric Coefficients

In studying compensated semimetals, the two-band model has proven extremely useful in capturing electrical conductivity under magnetic field, as a function of density and mobility of electron-like and hole-like carriers. However, it rarely offers practical insight into magneto-thermoelectric properties. Here, we report the field dependence of thermoelectric (TE) coefficients in a highly compensated semimetal NbSb$_2$, where we find the Seebeck and Nernst coefficients increase quadratically and linearly with applied magnetic field, respectively. Such field dependence was predicted in previous work that studied a system of two parabolic bands, within semiclassical Boltzmann transport theory when the following two conditions are simultaneously met:$\omega_c\tau \gg 1$ and $\tan\theta_H \ll 1$. Under these conditions, we find the field dependence of the TE coefficients directly provides a relation between the electron-like ($n_e$) and hole-like ($n_h$) carrier densities, which in turn can be used to refine two-band model fitting. With this, we find the compensation factor ($\frac{|\Delta n|}{n_e}$) of NbSb$_2$ is two orders of magnitude smaller than what was found in unrestricted fitting, resulting in a larger saturation field scale for magnetoresistance. Within the same framework of the semiclassical theory, we also deduce that the thermoelectric Hall angle $\tan\theta_{\gamma} = \frac{S_{xy}}{S_{xx}}$ can be expressed as $\big(\frac{|\Delta n|}{n_e} \times \omega_c\tau\big)^{-1}$, which serves as a parameter to predict the degree of compensation. Our findings offer crucial insights into identifying empirical conditions for field-induced enhancement of TE performance and into engineering efficient thermoelectric devices based on semimetallic materials.

cond-mat.str-el↗

arXiv2024

Heat conduction in magnetic insulators via hybridization of acoustic phonons and spin-flip excitations

We present a comprehensive study on the longitudinal magneto-thermal transport in a paramagnetic effective spin-1/2 magnetic insulator CsYbSe$_2$, by introducing a minimal model requiring only Zeeman splitting and magnetoelastic coupling. We use it to argue that hybridized excitations -- formed from acoustic phonons and localized spin-flip-excitations across the Zeeman gap of the crystal electric field ground doublet -- are responsible for a non-monotonic field dependence of longitudinal thermal conductivity. Beyond highlighting a starring role for phonons, our results raise the prospect of universal magneto-thermal transport phenomena in paramagnetic insulators that originate from simple features shared across many systems.

cond-mat.str-el↗

arXiv2024

Complex Antiferromagnetic Order in the Metallic Triangular Lattice Compound SmAuAl$_4$Ge$_2$

The compounds $Ln$AuAl$_4$Ge$_2$ ($Ln$ $=$ lanthanide) form in a structure that features two-dimensional triangular lattices of $Ln$ ions that are stacked along the crystalline $c$ axis. Together with crystal electric field effects, magnetic anisotropy, and electron-mediated spin exchange interactions, this sets the stage for the emergence of strongly correlated spin and electron phenomena. Here we investigate SmAuAl$_4$Ge$_2$, which exhibits weak paramagnetism that strongly deviates from conventional Curie-Weiss behavior. Complex antiferromagnetic ordering emerges at $T_{\rm{N1}}$ $=$ 13.2 K and $T_{\rm{N2}}$ $=$ 7.4 K, where heat capacity measurements show that these transitions are first and second order, respectively. These measurements also reveal that the Sommerfeld coefficient is not enhanced compared to the nonmagnetic analog YAuAl$_4$Ge$_2$, consistent with the charge carrier quasiparticles exhibiting typical Fermi liquid behavior. The temperature-dependent electrical resistivity follows standard metallic behavior, but linear magnetoresistance unexpectedly appears within the ordered state. We compare these results to other $Ln$AuAl$_4$Ge$_2$ materials, which have already been established as localized $f$-electron magnets that are hosts for interesting magnetic and electronic phases. From this, SmAuAl$_4$Ge$_2$ emerges as a complex quantum spin metal, inviting further investigations into its properties and the broader family of related materials.

cond-mat.str-el↗

arXiv2022

Field-Induced Magnetic States in the Metallic Rare-Earth Layered Triangular Antiferromagnet TbAuAl$_4$Ge$_2$

Magnetic frustration in metallic rare earth lanthanides ($Ln$) with $4f$-electrons is crucial for producing interesting magnetic phases with high magnetic anisotropy where intertwined charge and spin degrees of freedom lead to novel phenomena. Here we report on the magnetic, thermodynamic, and electrical transport properties of TbAuAl$_4$Ge$_2$. Tb ions form 2-dimensional triangular lattice layers which stack along the crystalline $c$-axis. The magnetic phase diagram reveals multiple nearly degenerate ordered states upon applying field along the magnetically easy $ab$-plane before saturation. The magnetoresistance in this configuration exhibits intricate field dependence that closely follows that of the magnetization while the specific heat reveals a region of highly enhanced entropy, suggesting the possibility of a non-trivial spin textured phase. For fields applied along the $c$-axis (hard axis), we find linear magnetoresistance over a wide range of fields. We compare the magnetic properties and magnetoresistance with an isostructral GdAuAl$_4$Ge$_2$ single crystals. These results identify TbAuAl$_4$Ge$_2$ as an environment for complex quantum spin states and pave the way for further investigations of the broader $Ln$AuAl$_4$Ge$_2$ family of materials.

cond-mat.str-el↗

arXiv2022

Direct link between disorder, mobility and magnetoresistance in topological semimetals

The extent to which disorder influences the properties of topological semimetals remains an open question and is relevant to both the understanding of topological states and the use of topological materials in practical applications. Here, we achieve unmatched and systematic control of point defect concentrations in the prototypical Dirac semimetal Cd$_3$As$_2$ to gain important insight into the role of disorder on electron transport behavior. We find that arsenic vacancies introduce localized states near the Fermi level and strongly influence the electron mobility. Reducing arsenic vacancies by changing the As/Cd flux ratio used during deposition results in an increase in the magnetoresistance from 200%-1000% and an increase in mobility from 5000-18,000 cm$^2$/Vs. However, the degree of linear magnetoresistance, which has previously been linked to disorder, is found here to correlate inversely with measures of disorder, including disorder potential and disorder correlation lengths. This finding yields important new information in the quest to identify the origin of linear magnetoresistance in a wider range of materials.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic Ordering in GdAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$: layered compounds with triangular lanthanide nets

We report the synthesis of the entire $Ln$AuAl$_4$Ge$_2$ ($Ln$ = Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) series and focus on the magnetic properties of GdAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$. Temperature and magnetic field dependent magnetization, heat capacity, and electrical resistivity measurements reveal that both compounds exhibit several magnetically ordered states at low temperatures, with evidence for magnetic fluctuations extending into the paramagnetic temperature region. For magnetic fields applied in the $ab$-plane there are several ordered state regions that are associated with metamagnetic phase transitions, consistent with there being multiple nearly degenerate ground states. Despite Gd being an isotropic $S$-state ion and Tb having an anisotropic $J$-state, there are similarities in the phase diagrams for the two compounds, suggesting that factors such as the symmetry of the crystalline lattice, which features well separated triangular planes of lanthanide ions, or the Ruderman-Kittel-Kasuya-Yosida interaction as defined by the Fermi surface topography control the magnetism. We also point out similarities to other centrosymmetric compounds that host skyrmion lattices such as Gd$_2$PdSi$_3$, and propose that the $Ln$AuAl$_4$Ge$_2$ family of compounds are of interest as reservoirs for complex magnetism and electronic behaviors such as the topological Hall effect.

cond-mat.str-el↗

arXiv2022

Systematic extraction of crystal electric-field effects and quantum magnetic model parameters in triangular rare-earth magnets

A primary goal at the interface of theoretical and experimental quantum magnetism is the investigation of exotic spin states, most notably quantum spin liquids (QSLs). Magnetic rare-earth ions go beyond the straightforward paradigm of geometrical frustration in Heisenberg antiferromagnets by introducing competing energy scales, and in particular their strong spin-orbit coupling creates multiple split crystal electric-field (CEF) levels, leading to anisotropic effective spin models with intrinsic frustration. While rare-earth delafossites have a triangular-lattice geometry, and thus have gained recent attention as candidates for hosting spin-1/2 QSL physics, the reliable extraction of effective spin models from the initial many-parameter CEF spectrum is a hard problem. Using the example of CsYbSe2, we demonstrate the unambiguous extraction of the Stevens-operators coefficients dictating the full CEF spectrum of Yb$^{3+}$ by translating these into parameters with a direct physical interpretation. Specifically, we combine low-field susceptibility measurements with resonant torsion magnetometry experiments in fields up to 60 T to determine a sufficiently large number of physical parameters -- effective Zeeman splittings, anisotropic van Vleck coefficients, and magnetotropic coefficients -- that the set of Stevens-operator coefficients is unique. Our crucial identification of the strong corrections to the Zeeman splitting of Kramers doublets as van Vleck coefficients has direct consequences for the interpretation of all anisotropic magnetic susceptibility measurements. Our results allow us to determine the nature and validity of an effective spin-1/2 model for CsYbSe2, to provide input for theoretical studies of such models on the triangular lattice, and to provide additional materials insight into routes for achieving magnetic frustration and candidate QSL systems in rare-earth compounds.

cond-mat.str-el↗

arXiv2021

The ground state in a proximity to a possible Kitaev spin liquid: An undistorted honeycomb iridate NaxIrO3 (0.60 < x < 0.80)

We report results of our study of a newly synthesized honeycomb iridate NaxIrO3 (0.60 < x < 0.80). Single-crystal NaxIrO3 adopts a honeycomb lattice noticeably without distortions and stacking disorder inherently existent in its sister compound Na2IrO3. The oxidation state of the Ir ion is a mixed valence state resulting from a majority Ir5+(5d4) ion and a minority Ir6+(5d3) ion. NaxIrO3 is a Mott insulator likely with a predominant pseudospin = 1 state. It exhibits an effective moment of 1.1 Bohr Magneton/Ir and a Curie-Weiss temperature of -19 K but with no discernable long-range order above 1 K. The physical behavior below 1 K features two prominent anomalies at Th = 0.9 K and Tl = 0.12 K in both the heat capacity and AC magnetic susceptibility. Intermediate between Th and Tl lies a pronounced temperature linearity of the heat capacity with a large slope of 77 mJ/mole K2, a feature expected for highly correlated metals but not at all for insulators. These results along with comparison drawn with the honeycomb lattices Na2IrO3 and (Na0.2Li0.8)2IrO3 point to an exotic ground state in a proximity to a possible Kitaev spin liquid.

cond-mat.str-el↗

arXiv2020

Quantum liquid from strange frustration in the trimer magnet Ba4Ir3O10

Quantum spin systems such as magnetic insulators usually show classical magnetic order, but such classical states can give way to quantum liquids with exotic entanglement through two known mechanisms of frustration: geometric frustration in lattices with triangle motifs, and spin-orbit-coupling frustration in the exactly solvable quantum liquid of Kitaev's honeycomb lattice. Here we present the experimental observation of a new kind of frustrated quantum liquid arising in an unlikely place: the magnetic insulator Ba4Ir3O10 where Ir3O12 trimers form an unfrustrated square lattice. Experimentally we find a quantum liquid state persisting down to 0.2 K that is stabilized by strong antiferromagnetic interaction with Curie-Weiss temperature - 766 K. The astonishing frustration parameter of 3800 is beyond any known iridate thus far. Heat capacity and thermal conductivity are both linear at low temperatures, a familiar feature in metals but here in an insulator pointing to an exotic quantum liquid state. A mere 2% Sr substitution for Ba produces long-range order at 130 K and destroys the linear-T features. Although the Ir4+(5d5) ions in Ba4Ir3O10 appear to form Ir3O12 trimers of face-sharing IrO6 octahedra, we propose that intra-trimer exchange is reduced and the lattice recombines into an array of coupled 1D chains with additional spins. An extreme limit of decoupled 1D chains can explain most but not all of the striking experimental observations, indicating that the inter-chain coupling plays an important role in the novel frustration mechanism leading to this quantum liquid.

cond-mat.str-el↗

arXiv2020

Spinon Excitations in the Quasi-1D S = 1/2 Chain Cs4CuSb2Cl12

The spin-1/2 Heisenberg antiferromagnetic chain is ideal for realizing one of the simplest gapless quantum spin-liquids (QSLs), supporting a many-body ground state whose elementary excitations are fractional fermionic excitations called spinons. Here we report the discovery of such a 1D QSL in Cs4CuSb2Cl12. Compared to previously reported S = 1/2 1D chains, this material possesses a wider temperature range over which the QSL state is stabilized. We identify spinon excitations extending at T > 0.8 K, with a large T-linear contribution to the specific heat, gamma = 31.5(2) mJ mol-1 K-2 which contribute itinerantly to thermal transport up to temperatures as high as T = 35 K. At T = 0.7 K, we find a second-order phase transition, suggesting a weak spin-Peierls transition that is unchanged by a 5 T magnetic field. Cs4CuSb2Cl12 reveals new phenomenology deep in the 1D QSL regime, supporting a gapped QSL phase over a wide temperature range compared to many other experimental realizations.

cond-mat.str-el↗

arXiv2019

Giant thermal magnetoconductivity in CrCl$_3$ and a general model for spin-phonon scattering

Insulating quantum magnets lie at the forefront both of fundamental research into quantum matter and of technological exploitation in the increasingly applied field of spintronics. In this context, the magnetic thermal transport is a particularly sensitive probe of the elementary spin and exotic topological excitations in unconventional magnetic insulators. However, magnetic contributions to heat conduction are invariably intertwined with lattice contributions, and thus the issue of spin-phonon coupling in determining the spin and thermal transport properties becomes more important with emergent topological magnetic system. Here we report the observation of an anomalously strong enhancement of the thermal conductivity, occurring at all relevant temperatures, in the layered honeycomb material CrCl$_3$ in the presence of an applied magnetic field. Away from the magnetically ordered phase at low temperatures and small fields, there is no coherent spin contribution to the heat conduction, and hence the effect must be caused by a strong suppression of the phonon thermal conductivity due to magnetic fluctuations, which are in turn suppressed by the field. We build an empirical model for the thermal conductivity of CrCl$_3$ within a formalism assuming an independently determined number of spin-flip processes and an efficiency of the phonon scattering events they mediate. By extracting the intrinsic phonon thermal conductivity we obtain a quantitative description at all fields and temperatures and demonstrate that the scattering efficiency is entirely independent of the field. In this way we use CrCl$_3$ as a model system to understand the interactions between spin and phonon excitations in the context of thermal transport. We anticipate that the completely general framework we introduce will have broad implications for the interpretation of transport phenomena in magnetic quantum materials.

cond-mat.str-el↗

arXiv2018

Non-saturating large magnetoresistance in semimetals

The rapidly expanding class of quantum materials known as {\emph{topological semimetals}} (TSM) display unique transport properties, including a striking dependence of resistivity on applied magnetic field, that are of great interest for both scientific and technological reasons. However, experimental signatures that can identify or discern the dominant mechanism and connect to available theories are scarce. Here we present the magnetic susceptibility ($χ$), the tangent of the Hall angle ($\tanθ_H$) along with magnetoresistance in four different non-magnetic semimetals with high mobilities, NbP, TaP, NbSb$_2$ and TaSb$_2$, all of which exhibit non-saturating large MR. We find that the distinctly different temperature dependences, $χ(T)$ and the values of $\tanθ_H$ in phosphides and antimonates serve as empirical criteria to sort the MR from different origins: NbP and TaP being uncompensated semimetals with linear dispersion, in which the non-saturating magnetoresistance arises due to guiding center motion, while NbSb$_2$ and TaSb$_2$ being {\it compensated} semimetals, with a magnetoresistance emerging from nearly perfect charge compensation of two quadratic bands. Our results illustrate how a combination of magnetotransport and susceptibility measurements may be used to categorize the increasingly ubiquitous non-saturating large magnetoresistance in TSMs.

cond-mat.str-el↗

arXiv2017

Multiple magnetic states within the A-phase determined by field-orientation dependence of Mn0.9Fe0.1Si

We report three distinct regions within the A-phase in Fe-doped MnSi, based on the evolution of magnetoresistance and the Hall effect as a function of orientation of applied field. Fe impurities as pinning centers and crystalline anisotropy are found non-negligible only at the boundary of the A-phase. Electrical transport characteristics unique to the A-phase not only remain robust, but also indicate a freely rotating skyrmion lattice, decoupled from underlying crystal structure or impurity pinning.

cond-mat.str-el↗

arXiv2017

Anomalous thermal conductivity and magnetic torque response in the honeycomb magnet $α$-RuCl$_3$

We report on the unusual behavior of the in-plane thermal conductivity ($κ$) and torque ($τ$) response in the Kitaev-Heisenberg material $α$-RuCl$_3$. $κ$ shows a striking enhancement with linear growth beyond H = 7 T, where magnetic order disappears, while $τ$ for both of the in-plane symmetry directions shows an anomaly at the same field. The temperature- and field-dependence of $κ$ are far more complex than conventional phonon and magnon contributions, and require us to invoke the presence of unconventional spin excitations whose properties are characteristic of a field-induced spin-liquid phase related to the enigmatic physics of the Kitaev model in an applied magnetic field

cond-mat.str-el↗

arXiv2015

Out-of-Plane Spin-Orientation Dependent Magnetotransport Properties in the Anisotropic Helimagnet Cr$_{1/3}$NbS$_2$

Understanding the role of spin-orbit coupling (SOC) has been crucial to controlling magnetic anisotropy in magnetic multilayer films. It has been shown that electronic structure can be altered via interface SOC by varying the superlattice structure, resulting in spontaneous magnetization perpendicular or parallel to the plane. In lieu of magnetic thin films, we study the similarly anisotropic helimagnet Cr$_{1/3}$NbS$_2$, where the spin polarization direction, controlled by the applied magnetic field, can modify the electronic structure. As a result, the direction of spin polarization can modulate the density of states, and in turn affect the in-plane electrical conductivity. In Cr$_{1/3}$NbS$_2$, we found an enhancement of in-plane conductivity when the spin polarization is out-of-plane, as compared to in-plane spin polarization. This is consistent with the increase of density of states near the Fermi energy at the same spin configuration, found from first principles calculations. We also observe unusual field dependence of the Hall signal in the same temperature range. This is unlikely to originate from the non-collinear spin texture, but rather further indicates strong dependence of electronic structure on spin orientation relative to the plane.

cond-mat.str-el↗

arXiv2014

Spin Structure of the Anisotropic Helimagnet Cr$_{1/3}$NbS$_2$ in a Magnetic Field

In this letter we describe the ground-state magnetic structure of the highly anisotropic helimagnet Cr$_{1/3}$NbS$_2$ in a magnetic field. A Heisenberg spin model with Dyzaloshinkii-Moriya interactions and magne- tocrystalline anisotropy allows the ground state spin structure to be calculated for magnetic fields of arbitrary strength and direction. Comparison with magnetization measurements shows excellent agreement with the predicted spin structure.

cond-mat.str-el↗