arXiv subjects

Seongsu Lee

Publications and source records attributed to Seongsu Lee.

17 recordsLinked to original sources

arXiv2024

Geometric Remove-and-Retrain (GOAR): Coordinate-Invariant eXplainable AI Assessment

Identifying the relevant input features that have a critical influence on the output results is indispensable for the development of explainable artificial intelligence (XAI). Remove-and-Retrain (ROAR) is a widely accepted approach for assessing the importance of individual pixels by measuring changes in accuracy following their removal and subsequent retraining of the modified dataset. However, we uncover notable limitations in pixel-perturbation strategies. When viewed from a geometric perspective, we discover that these metrics fail to discriminate between differences among feature attribution methods, thereby compromising the reliability of the evaluation. To address this challenge, we introduce an alternative feature-perturbation approach named Geometric Remove-and-Retrain (GOAR). Through a series of experiments with both synthetic and real datasets, we substantiate that GOAR transcends the limitations of pixel-centric metrics.

cs.LG↗

arXiv2023

Structural Investigation of BaIrO$_3$ by Neutron Diffraction

We report a temperature-dependent neutron diffraction (ND) study on polycrystalline monoclinic BaIrO$_3$ which is famous for charge density wave (CDW) and weak ferromagnetic phase transitions at T$_C$$\sim$180 K simultaneously. A Rietveld analysis on the ND patterns reveals that even though there is no symmetry breaking in crystal structure, a noticeable change in the four kinds of IrO$_{6}$ octahedra is isolated as the temperature approaches to T$_C$. Based on the structure analysis results, we calculated the $d$-orbital energy level splittings by crystal electric field for each type of the IrO$_6$ octahedra. By taking into account the strong spin-orbit coupling in Ir 5$d$ orbitals and the lattice distortions obtained from the ND analysis, we propose an electronic configuration model to understand the phase transition of the system, where an effective $J_{\rm eff, 1/2}$ Mott insulating phase and a charge gap phase induced by bonding states between the $J_{\rm eff,1/2}$ states compete each other.

cond-mat.str-el↗

arXiv2022

Magnetic phase diagram of a 2-dimensional triangular lattice antiferromagnet Na$_2$BaMn(PO$_4$)$_2$

We report the magnetic phase transitions of a spin-5/2, 2-dimensional triangular lattice antiferromagnet (AFM) Na$_2$BaMn(PO$_4$)$_2$. From specific heat measurements, we observe two magnetic transitions at temperatures 1.15 K and 1.30 K at zero magnetic field. Detailed AC magnetic susceptibility measurements reveal multiple phases including the $\uparrow$$\uparrow$$\downarrow$ (up-up-down)-phase between 1.9 T and 2.9 T at 47 mK when magnetic field is applied along the $c$ axis, implying that Na$_2$BaMn(PO$_4$)$_2$ is a classical 2$d$ TL Heisenberg AFM with easy-axis anisotropy. However, it deviates from an ideal model as evidenced by a hump region with hysteresis between the $\uparrow$$\uparrow$$\downarrow$ and $V$-phases and weak phase transitions. Our work provides another experimental example to study frustrated magnetism in 2$d$ TL AFM which also serves as a reference to understand the possible quantum spin liquid behavior and anomalous phase diagrams observed in sibling systems Na$_2$Ba$M$(PO$_4$)$_2$ ($M$ = Co, Ni).

cond-mat.str-el↗

arXiv2021

Experimental determination of the magnetic interactions of frustrated Cairo pentagon lattice materials

We present inelastic neutron scattering measurements of the Cairo pentagon lattice magnets Bi$_2$Fe$_4$O$_9$ and Bi$_4$Fe$_5$O$_{13}$F, supported by high field magnetisation measurements of Bi$_2$Fe$_4$O$_9$. Using linear spin wave theory and mean field analyses we determine the spin exchange interactions and single-ion anisotropy in these materials. The Cairo lattice is geometrically frustrated and consists of two inequivalent magnetic sites, both occupied by Fe$^{3+}$ ions and connected by two competing nearest neighbour interactions. We found that one of these interactions, coupling nearest neighbour spins on the three-fold symmetric sites, is extremely strong and antiferromagnetic. These strongly coupled dimers are then weakly coupled to a framework formed from spins occupying the other inequivalent site. In addition we found that the Fe$^{3+}$ $S=5/2$ spins have a non-negligible single-ion anisotropy, which manifests as a spin anisotropy gap in the neutron spectrum and a spin-flop transition in high field magnetisation measurements.

cond-mat.str-el↗

arXiv2017

Spin glass behavior in frustrated quantum spin system CuAl2O4 with a possible orbital liquid state

CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu2+. It is a rather unique feature that the Cu2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu2+ is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f = Theta_CW/Tm ~67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4 should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.

cond-mat.str-el↗

arXiv2017

Magnetic Transitions in the Chiral Armchair-Kagome System Mn$_2$Sb$_2$O$_7$

The competition between interactions in frustrated magnets allows a wide variety of new ground states, often exhibiting emergent physics and unique excitations. Expanding the suite of lattices available for study enhances our chances of finding exotic physics. Mn$_2$Sb$_2$O$_7$ forms in a chiral, kagome-based structure in which a fourth member is added to the kagome-plane triangles to form an armchair unit and link adjacent kagome planes. This structural motif may be viewed as intermediate between the triangles of the kagome network and the tetrahedra in the pyrochlore lattice. Mn$_2$Sb$_2$O$_7$ exhibits two distinct magnetic phase transitions, at 11.1 and 14.2K, at least one of which has a weak ferromagnetic component. The magnetic propagation vector does not change through the lower transition, suggesting a metamagnetic transition or a transition involving a multi-component order parameter. Although previously reported in the $P3_121$ space group, Mn$_2$Sb$_2$O$_7$ actually crystallizes in $P2$, which allows ferroelectricity, and we show clear evidence of magnetoelectric coupling indicative of multiferroic order. The quasi-two-dimensional `armchair-kagome' lattice presents a promising platform for probing chiral magnetism and the effect of dimensionality in highly frustrated systems.

cond-mat.str-el↗

arXiv2016

Spontaneous structural distortion of metallic Shastry-Sutherland system DyB4 by quadrupole-spin-lattice coupling

DyB4 has a two-dimensional Shastry-Sutherland (Sh-S) lattice with strong Ising character of the Dy ions. Despite the intrinsic frustrations, surprisingly, it undergoes two successive transitions: a magnetic ordering at TN = 20K, and a quadrupole ordering at TQ=12.5 K. From high-resolution neutron and synchrotron X-ray powder diffraction studies, we have obtained full structural information on this material in all phases, and demonstrate that structural modifications occurring at quadrupolar transition lead to the lifting of frustrations inherent in the Sh-S model. Our study thus provides a complete experimental picture of how the intrinsic frustration of the Sh-S lattice can be lifted by the coupling to quadrupole moments. We show that two other factors, i.e. strong spin-orbit coupling and long-range RKKY interaction in metallic DyB4, play an important role in this behavior.

cond-mat.str-el↗

arXiv2015

High-resolution structure studies and magnetoelectric coupling of relaxor multiferroic Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$

Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$ (PFN), one of the few relaxor multiferroic systems, has a $G$-type antiferromagnetic transition at $T_N$ = 143 K and a ferroelectric transition at $T_C$ = 385 K. By using high-resolution neutron-diffraction experiments and a total scattering technique, we paint a comprehensive picture of the long- and short-range structures of PFN: (i) a clear sign of short-range structural correlation above $T_C$, (ii) no sign of the negative thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric coupling below $T_N$. We conclude that at the heart of the unusual relaxor multiferroic behavior lies the disorder between Fe$^{3+}$ and Nb$^{5+}$ atoms. We argue that this disorder gives rise to short-range structural correlations arising from O disorder in addition to Pb displacement.

cond-mat.mtrl-sci↗

arXiv2014

Temperature-Dependent Interplay of Dzyaloshinskii-Moriya Interaction and Single-Ion Anisotropy in Multiferroic BiFeO$_3$

Low-energy magnon excitations in multiferroic BiFeO$_3$ were measured in detail as a function of temperature around several Brillouin zone centers by inelastic neutron scattering experiments on single crystals. Unique features around 1 meV are directly associated with the interplay of the Dzyaloshinskii-Moriya interaction and a small single-ion anisotropy. The temperature dependence of these and the exchange interactions were determined by fitting the measured magnon dispersion with spin-wave calculations. The spectra best fits an easy-axis type magnetic anisotropy and the deduced exchange and anisotropy parameters enable us to determine the anharmonicity of the magnetic cycloid. We then draw a direct connection between the changes in the parameters of spin Hamiltonian with temperature and the physical properties and structural deformations of BiFeO$_3$.

cond-mat.mtrl-sci↗

arXiv2013

Enhanced Upper Critical Fields in a New Quasi-one-dimensional Superconductor Nb2PdxSe5

We report a discovery of superconductivity with Tc = 5.5 K in Nb2PdxSe5, in which one-dimensional (1D) Nb-Se chains existing along the b-direction hybridize each other to form the conducting b-c* plane. The magnetic susceptibility and specific heat data in both single- and poly-crystals show evidence of bulk superconductivity. The resistivity, Hall coefficient, and magneto-resistance data all indicate the presence of an energy scale T* = ~ 50 K, which becomes systematically lowered under hydrostatic pressure and competes with the stabilization of superconductivity. Combined with the band calculation results showing the Fermi surfaces with 1D character, we postulate that the energy scale T* is related to a formation of a density wave or a stabilization of low dimensional electronic structure. The zero temperature upper critical field, Hc2(0), of the single crystal is found to be 10.5, 35 and 22 T in the a', b and c*-directions, respectively. While the linearly increasing Hc2(T) for H // c* indicates the multi-band effect, Hc2(0) for H // b and c* are found to be much bigger than the BCS Pauli limiting field, 1.84 Tc ~ 9 T. The suppressed Pauli paramagnetic effect points to a possibility of the enhanced spin-orbit scattering related to the low dimensional electronic structure or the presence of heavy elements such as Pd.

cond-mat.supr-con↗

arXiv2013

Negative magnetostrictive magnetoelectric coupling of BiFeO3

How the magnetoelectric coupling actually occurs on a microscopic level in multiferroic BiFeO3 is not well known. By using the high-resolution single crystal neutron diffraction techniques, we have determined the electric polarization of each individual elements of BiFeO3, and concluded that the magnetostrictive coupling suppresses the electric polarization at the Fe site below TN. This negative magnetoelectric coupling appears to outweigh the spin current contributions arising from the cycloid spin structure, which should produce a positive magnetoelectric coupling.

cond-mat.str-el↗

arXiv2012

Spin wave measurements over the full Brillouin zone of multiferroic BiFeO3

Using inelastic neutron scattering technique, we measured the spin wave dispersion over the entire Brillouin zone of room temperature multiferroic BiFeO3 single crystals with magnetic excitations extending to as high as 72.5 meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian with a nearest neighbor exchange interaction (J=4.38 meV), a next nearest neighbor exchange interaction (J'=0.15 meV), and a Dzyaloshinskii-Moriya-like term (D=0.107 meV). This simple Hamiltonian determined, for the first time, for BiFeO3 provides a fundamental ingredient for understanding of the novel magnetic properties of BiFeO3.

cond-mat.mtrl-sci↗

arXiv2011

Temperature-dependent properties of the magnetic order in single-crystal BiFeO3

We report neutron diffraction and magnetization studies of the magnetic order in multiferroic BiFeO3. In ferroelectric monodomain single crystals, there are three magnetic cycloidal domains with propagation vectors equivalent by crystallographic symmetry. The cycloid period slowly grows with increasing temperature. The magnetic domain populations do not change with temperature except in the close vicinity of the N¶eel temperature, at which, in addition, a small jump in magneti- zation is observed. No evidence for the spin-reorientation transitions proposed in previous Raman and dielectric studies is found. The magnetic cycloid is slightly anharmonic for T=5 K. The an- harmonicity is much smaller than previously reported in NMR studies. At room temperature, a circular cycloid is observed, within errors. We argue that the observed anharmonicity provides important clues for understanding electromagnons in BiFeO3.

cond-mat.str-el↗

arXiv2011

Long wavelength magnetic and magnetoelectric excitations in the ferroelectric antiferromagnet BiFeO3

We present a terahertz spectroscopic study of magnetic excitations in ferroelectric antiferromagnet BiFeO3. We interpret the observed spectrum of long-wavelength magnetic resonance modes in terms of the normal modes of the material's cycloidal antiferromagnetic structure. We find that the modulated Dzyaloshinski-Moriya interaction leads to a splitting of the out-of-plane resonance modes. We also assign one of the observed absorption lines to an electromagnon excitation that results from the magnetoelectric coupling between the ferroelectric polarization and the cycloidal magnetic structure of BiFeO3.

cond-mat.mtrl-sci↗

arXiv2008

3:1 magnetization plateau and suppression of ferroelectric polarization in an Ising chain multiferroic

Ferroelectric Ising chain magnet Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ ($x\simeq$0.96) was studied in magnetic fields up to 33 T. Magnetization and neutron scattering measurements reveal successive metamagnetic transitions from the zero-field $\uparrow \uparrow \downarrow \downarrow$ spin configuration to the $\uparrow \uparrow \uparrow \downarrow$ state with a broad magnetization plateau, and then to the $\uparrow \uparrow \uparrow \uparrow$ state. The absence of hysteresis in these plateaus reveals an intriguing coupling between the intra-chain state and the three-dimensional geometrically frustrated magnetic system. Inversion symmetry, broken in the $\uparrow \uparrow \downarrow \downarrow$ state, is restored in the $\uparrow \uparrow \uparrow \downarrow$ state, leading to the complete suppression of the electric polarization driven by symmetric superexchange.

cond-mat.str-el↗

arXiv2008

Terahertz wave generation via optical rectification from multiferroic BiFeO3

We detected broadband coherent terahertz (THz) emission from multiferroic BiFeO3 after illuminating a high-quality bulk single ferroelectric domain crystal with a ~100 fs optical pulse. The dependence of the emitted THz waveform on the energy and polarization of the optical pulse is consistent with the optical rectification mechanism of THz emission. The THz emission provides a sensitive probe of the electric polarization state of BiFeO3, enabling applications in ferroelectric memories and ferroelectric domain imaging. We also report room-temperature THz optical constants of BiFeO3.

cond-mat.mtrl-sci↗

arXiv2008

Singe ferroelectric and chiral magnetic domain of single-crystalline BiFeO$_3$ in an electric field

We report polarized neutron scattering and piezoresponse force microscopy studies of millimeter-sized single crystals of multiferroic BiFeO$_3$. The crystals, grown below the Curie temperature, consist of a single ferroelectric domain. Two unique electric polarization directions, as well as the populations of equivalent spiral magnetic domains, can be switched reversibly by an electric field. A ferroelectric monodomain with a single-$q$ single-helicity spin spiral can be obtained. This level of control, so far unachievable in thin films, makes single-crystal BiFeO$_3$ a promising object for multiferroics research.

cond-mat.mtrl-sci↗