arXiv subjects

Myung-Hwan Whangbo

Publications and source records attributed to Myung-Hwan Whangbo.

At least 19 recordsLinked to original sources

arXiv2026

Chemical Perspectives on Cuprate Superconductivity: Hole-Protected Spin Dimers as d Wave Cooper Pairs

The observation that the coherence lengths in cuprate superconductors are extremely small motivated us to search for its real-space structure by examining the consequence of hole-doping in CuO4 units in cuprates. We analyzed aggregates of hole-doped CuO4 units that phase-separate from hole-doped CuO2 layers in terms of their segregation-pressure indices defined in this work. Our search suggests that each d-wave Cooper pair is an antiferromagnetically coupled spin dimer protected in an antiferromagnetic hole dimer (AHD), and that the antiferromagnetic chains of AHDs provide channels through which such spin dimers can move collectively without perturbation. Based on these suggestions and their implications, we examined the causes for several seemingly puzzling observations on cuprate superconductors to find that they are naturally explained by the suggestions.

cond-mat.supr-con↗

arXiv2026

Field-Induced Local Excitations Causing Zero-Magnetization Plateaus in Antiferromagnets of Antiferromagnetic Spin Dimers Under Magnetic Field

Certain antiferromagnets composed of antiferromagnetic spin dimers exhibit a zero-magnetization plateau despite that the single-ion anisotropy of their magnetic ions is negligible. The cause for this observation was investigated by analyzing how a magnetic field affects the energy spectrum of an antiferromagnetic chain composed of antiferromagnetic spin dimers made up of two spin-half ions and by carrying out specific heat measurements for potassium copper chloride as a function magnetic field at 2 K.

cond-mat.str-el↗

arXiv2026

Reduction of Ordered Spin Moments in Antiferromagnets of S = 5/2 Ions (Fe3+, Mn2+) Driven by Local Magnetic Excitation

For antiferromagnets composed of spin five-half ions, the moments of these ions in the ordered antiferromagnetic state can be significantly smaller than 5 BM (i.e., 1.56 - 4.48 BM) if these ions form quantum fluctuating entities (QFEs), for example, quasi one-dimensional uniform antiferromagnetic chains or quasi zero-spin antiferromagnetic spin dimers. It is reasonable to suppose that the stronger the quantum fluctuation in such an antiferromagnet, the greater the reduction in its ordered moment would become, but this supposition has not yet been confirmed because quantifying the strength of quantum fluctuation is not a straightforward matter. Here we show that the local magnetic excitations involving the QFEs can be used to quantify the strength of quantum fluctuation by analyzing six antiferromagnets showing significant reduction in their ordered spin moments.

cond-mat.str-el↗

arXiv2025

Le Chatelier principle and field-induced change in magnetic entropy leading to spin lattice partitioning and magnetization plateau

For a certain antiferromagnet, the magnetization does not increase gradually with increasing magnetic field but exhibits field region(s) typically at an integer fraction of its saturation magnetization. This phenomenon is understood by the supposition that such an antiferromagnet undergoes field-induced partitioning of its spin lattice into ferrimagnetic fragments. We searched for a theoretical basis for this supposition by investigating how external magnetic fields affect the magnetic entropy of such an antiferromagnet, to find that the field region of the magnetization plateau has a single magnetic phase, but a nonzero slope region of the magnetization curve has two magnetic phases of different magnetic entropy, and that the magnetic entropy of a single-phase region does not depend on magnetic field but that of a two-phase region does. We tested these predictions by carrying out magnetization and specific heat measurements for g-Mn3(PO4)2. It was found that the magnetic entropy of the two-phase region increases with field, indicating that field-induced breaking of magnetic bonds and hence field-induced partitioning of an antiferromagnetic spin lattice are time-averaged results of all allowed spin arrangements that occur repeatedly during static magnetization measurements. The temperature-dependent magnetic specific heats of g-Mn3(PO4)2 between 2 - 6 K shows a larger excitation gap when measured at 9 T than at 0 T, suggesting that these energy gaps reflect the two successive local excitations of linear Mn2+-Mn2+-Mn2+ ferrimagnetic trimers embedded in the antiferromagnetic spin lattice of g-Mn3(PO4)2 and arise from the Boltzmann factor associated with these excitations

cond-mat.str-el↗

arXiv2024

Magnetization Plateaus by the Field-Induced Partitioning of Spin Lattices

To search for a conceptual picture describing the magnetization plateau phenomenon, we surveyed the crystal structures and the spin lattices of those magnets exhibiting plateaus in their magnetization vs. magnetic field curves by probing the three questions: (a) why only certain magnets exhibit magnetization plateaus, (b) why there occur several different types of magnetization plateaus, and (c) what controls the widths of magnetization plateaus. We show that the answers to these questions lie in how the magnets under field absorb Zeeman energy hence changing their magnetic structures. The magnetic structure of a magnet insulator is commonly described in terms of its spin lattice, which requires the determination of the spin exchanges nonnegligible strengths between the magnetic ions. Our work strongly suggests that a magnet under magnetic field partitions its spin lattice into antiferromagnetic (AFM) or ferrimagnetic fragments by breaking its weak magnetic bonds. Our supposition of the field-induced partitioning of spin lattice into magnetic fragments is supported by the anisotropic magnetization plateaus of Ising magnets and by the highly anisotropic width of the 1/3-magnetization plateau in azurite. The answers to the three questions (a) - (c) emerge naturally by analyzing how these fragments are formed under magnetic field.

cond-mat.mtrl-sci↗

arXiv2022

High-spin orbital interactions across van der Waals gaps controlling the interlayer ferromagnetism in van der Waals ferromagnets

We examined what interactions control the sign and strength of the interlayer coupling in van der Waals ferromagnets such as Fe3-xGeTe2, Cr2Ge2Te6, CrI3 and VI3, to find that high-spin orbital interaction across the van der Waals gaps are a key to understanding their ferromagnetism. Interlayer ferromagnetic coupling in Fe3-xGeTe2 Cr2Ge2Te6, and CrI3 is governed by the high-spin two-orbital two-electron destabilization, but that in VI3 by the high-spin four-orbital two-electron stabilization. These interactions explain a number of seemingly puzzling observations in van der Waals fer-romagnets.

cond-mat.str-el↗

arXiv2020

Peculiarities of magnetic ordering in the S = 5/2 two-dimensional square-lattice antimonate NaMnSbO4

An orthorhombic compound, NaMnSbO4, represents a square net of magnetic Mn2+ ions residing in vertex-shared oxygen octahedra. Its static and dynamic magnetic properties were studied using magnetic susceptibility, specific heat, magnetization, electron spin resonance (ESR), nuclear magnetic resonance (NMR) and density functional calculations. Thermodynamic data indicate an establishment of the long-range magnetic order with TN about 44 K, which is preceded by a short-range one at about 55 K. In addition, a non-trivial wasp-waisted hysteresis loop of the magnetization was observed, indicating that the ground state is most probably canted antiferromagnetic. Temperature dependence of the magnetic susceptibility is described reasonably well in the framework of 2D square lattice model with the main exchange parameter J = -5.3 K, which is in good agreement with density functional analysis, NMR and ESR data.

cond-mat.mtrl-sci↗

arXiv2018

Interband electron pairing for superconductivity from the breakdown of the Born-Oppenheimer approximation

The origin of interband electron pairing responsible for enhancing superconductivity and the factors controlling its strength were examined. We show that the interband electron pairing is a natural consequence of breaking down the Born-Oppenheimer approximation during the electron-phonon interactions. Its strength is determined by the pair-state excitations around the Fermi surfaces that take place to form a superconducting state. Fermi surfaces favorable for the pairing were found and its implications were discussed.

cond-mat.supr-con↗

arXiv2018

Probable cause for the superconductor-like properties of alkane-wetted graphite and single-layer graphene above room temperature under ambient pressure

Recently Kawashima has reported that, when wetted with alkanes, several forms of graphite and single-layer graphene exhibit superconductor-like properties above room temperature under ambient pressure [AIP Adv. 2013, 3, 052132; arXiv:1612.05294; arXiv:1801.09376]. Under the assumption that these seemingly unlikely properties arise from the presence of paired electrons brought about by the alkane-wetting, we explored their implications to arrive at a probable mechanism for strong electron-pairing driven by Fermi surface nesting and acoustic phonon. This mechanism explains why alkane-wetting is essential for the graphene systems to become superconductor-like above room temperature and why the Tc of alkane-wetted pitch-based graphite fibers increases almost linearly from 363 to 504 K with increasing the molecular weight of alkane from heptane to hexadecane. It also provides a number of experimentally-verifiable predictions, the confirmation of which will provide a strong support for the superconductivity driven by Fermi surface nesting and acoustic phonon.

cond-mat.supr-con↗

arXiv2018

Spin density wave as a superposition of two magnetic states of opposite chirality and its implications

A magnetic solid with weak spin frustration tends to adopt a noncollinear magnetic structure such as cycloidal structure below a certain temperature and a spin density wave (SDW) slightly above this temperature. The causes for these observa-tions were explored by studying the magnetic structures of BaYFeO4, which undergoes an SDW and a cycloidal phase transi-tion below 48 and 36 K, respectively, in terms of density func-tional theory calculations. We show that an SDW structure aris-es from a superposition of two magnetic states of opposite chi-rality, an SDW state precedes a chiral magnetic state due to the lattice relaxation, and whether an SDW is transversal or longitudinal is governed by the magnetic anisotropy of magnetic ions.

cond-mat.str-el↗

arXiv2018

Large second harmonic generation of LiCs2PO4 caused by the metal-cation-centered groups

We evaluated the individual atom contributions to the second harmonic generation (SHG) coefficients of LiCs2PO4 (LCPO) by introducing the partial response functionals on the basis of first principles calculations. The SHG response of LCPO is dominated by the metal-cation-centered groups CsO6 and LiO4, not by the nonmetal-cation-centered groups PO4 one expects from the existing models and theories. The SHG coefficients of LCPO are determined mainly by the occupied orbitals O-2p and Cs-5p as well as by the unoccupied orbitals Cs-5d and Li-2p. For the SHG response of a material, the polarizable atomic orbitals of the occupied and the unoccupied states are both important.

cond-mat.mtrl-sci↗

arXiv2017

The road map toward room temperature superconductivity: manipulating different pairing channels in systems composed of multiple electronic components

While it is known that the amplification of the superconducting critical temperature Tc is possible in a system of multiple electronic components in comparison with a single component system, many different road maps for room temperature superconductivity have been proposed for a variety of multicomponent scenarios. Here we focus on the scenario where the first electronic component is assumed to have a vanishing Fermi velocity corresponding to a case of the intermediate polaronic regime, and the second electronic component is in the weak coupling regime with standard high Fermi velocity using a mean field theory for multiband superconductivity. This roadmap is motivated by compelling experimental evidence for one component in the proximity of a Lifshitz transition in cuprates, diborides and iron based superconductors. By keeping a constant and small exchange interaction between the two electron fluids, we search for the optimum coupling strength in the electronic polaronic component which gives the largest amplification of the superconducting critical temperature in comparison with the case of a single electronic component.

cond-mat.supr-con↗

arXiv2017

Single-domain ferromagnet of noncentrosymmetric uniaxial magnetic ions and magnetoelectric interaction

The feasibility of a single-domain ferromagnet based on uniaxial magnetic ions was examined. For a noncentrosymmetric uniaxial magnetic ion of magnetic moment at a site of local electric dipole moment p, it is unknown to date whether the magnetic moment prefers to be parallel or antiparallel to the electric dipole moment. The nature of this magnetoelectric interaction was probed in terms of analogical reasoning based on the Rashba effect and density functional calculations. We show that the magnetic and electric dipole moments prefer an antiparallel arrangement, predict that Fe-doped CaZnOS is a single-domain ferromagnet like a bar magnet, and find the probable cause for the ferromagnetism and weak magnetization hysteresis in Fe-doped hexagonal ZnO and ZnS at very low dopant concentrations.

cond-mat.str-el↗

arXiv2016

Condensed-matter equation of states covering a wide region of pressure studied experimentally

The relationships among the pressure P, volume V, and temperature T of solid-state materials are described by their equations of state (EOSs), which are often derived from the consideration of the finite-strain energy or the interatomic potential.1-3 These EOSs consist of typically three parameters to determine from experimental P-V-T data by fitting analyses. In the empirical approach to EOSs, one either refines such fitting parameters or improves the mathematical functions3-5 to better simulate the experimental data. Despite over seven decades of studies on EOSs, none has been found to be accurate for all types of solids over the whole temperature and pressure ranges studied experimentally.3,6,7 Here we show that the simple empirical EOS, P = α1(PV) + α2(PV)2 + α3(PV)3, in which the pressure P is indirectly related to the volume V through a cubic polynomial of the energy term PV with three fitting parameters α1 - α3, provides accurate descriptions for the P-vs-V data of condensed matter in a wide region of pressure studied experimentally even in the presence of phase transitions

cond-mat.mtrl-sci↗

arXiv2016

Possibility of Combining Ferroelectricity and Rashba-like spin splitting in Monolayers of 1T-type Transition-Metal Dichalcogenides MX2

First-principles calculations were carried out to explore the possible coupling between spin-polarized electronic states and ferroelectric polarization in monolayers of transition-metal dichalcogenides MX 2 (M = Mo, W; X = S, Se, Te) with distorted octahedrally coordinated 1T structures.For d^2 metal ions, two competing metal clustering effects can take place, where metal ions are arranged in trimers or zigzag chains. Among these, the former structural distortion comes along with an improper ferroelectric phase which persists in the monolayer limit. Switchable Rashba-like spin-polarization features are predicted in the trimerized polytype, which can be permanently tuned by acting on its ferroelectric properties. The polar trimerized structure is found to be stable for 1T-MoS 2 only, while the nonpolar polytype with zigzag metal clustering is predicted to stabilize for other transition-metal dichalcogenides with d 2 metal ions.

cond-mat.str-el↗

arXiv2016

Magnetic Properties from the Viewpoints of Electronic Hamiltonian: Spin Exchange Parameters, Spin Orientation and Spin-Half Misconception

In this chapter we review the quantitative and qualitative aspects of describing the properties of magnetic solids on the basis of electronic Hamiltonian. We show that a spin Hamiltonian approach becomes consistent with an electronic Hamiltonian approach if the spin lattice and its associated spin exchange parameters, to be used for the spin Hamiltonian, are determined by the energy-mapping analysis based on DFT calculations. The preferred spin orientation (i.e., the magnetic anisotropy) of a magnetic ion is not predicted by a spin Hamiltonian because it does not include the orbital degree of freedom explicitly. In contrast, the magnetic anisotropy is readily predicted by electronic structure theories employing both orbital and spin degrees of freedom, if one takes into consideration the spin-orbit coupling (SOC). It was shown that the preferred spin orientation of a magnetic ion can be predicted and understood in terms of the HOMO-LUMO interactions of the magnetic ion by taking SOC as perturbation. A spin Hamiltonian gives rise to the spin-half misconception, namely, the blind belief that spin-half magnetic ions do not possess magnetic anisotropy that arise from SOC. This misconception is a direct consequence from the limitedness of a spin Hamiltonian that it lacks the orbital degree of freedom. We show that the magnetic properties of 5d ion oxides are better explained by the LS-coupling than by the jj-coupling scheme of SOC, that the spin-orbital entanglement of 5d ions is not as strong as has been assumed.

cond-mat.str-el↗

arXiv2016

Structure and Composition of the 200 K-Superconducting Phase of H2S under Ultrahigh Pressure: The Perovskite (SH-)(H3S+)

H2S is converted under ultrahigh pressure (> 110 GPa) to a metallic phase that becomes superconducting with a record Tc of 200 K. It has been proposed that the superconducting phase is body-centered cubic H3S ( Im3m , a = 3.089 Å) resulting from a decomposition reaction 3H2S --> 2H3S + S. The analogy of H2S and H2O leads us to a very different conclusion. The well-known dissociation of water into H3O+ and OH- increases by orders of magnitude under pressure. An equivalent behavior of H2S is anticipated under pressure with the dissociation, 2H2S --> H3S+ + SH- forming a perovskite structure (SH-)(H3S+), which consists of corner-sharing SH6 octahedra with SH- at each A-site (i.e., the center of each S8 cube). Our DFT calculations show that the perovskite (SH-)(H3S+) is thermodynamically more stable than the Im3m structure of H3S, and suggest that the A-site H atoms are most likely fluxional even at Tc.

cond-mat.supr-con↗

arXiv2016

Spin orientations of the spin-half Ir4+ ions in Sr3NiIrO6, Sr2IrO4 and Na2IrO3: Density functional, perturbation theory and Madelung potential analyses

The spins of the low-spin Ir4+ (S = 1/2, d5) ions at the octahedral sites of the oxides Sr3NiIrO6, Sr2IrO4 and Na2IrO3 exhibit preferred orientations with respect to their IrO6 octahedra. We evaluated the magnetic anisotropies of these S = 1/2 ions on the basis of DFT calculations including spin-orbit coupling (SOC), and probed their origin by performing perturbation theory analyses with SOC as perturbation within the LS coupling scheme. The observed spin orientations of Sr3NiIrO6 and Sr2IrO4 are correctly predicted by DFT calculations, and are accounted for by the perturbation theory analysis. As for the spin orientation of Na2IrO3, both experimental studies and DFT calculations have not been unequivocal. Our analysis reveals that the Ir4+ spin orientation of Na2IrO3 should have nonzero components along the c- and a-axes directions. The spin orientations determined by DFT calculations are sensitive to the accuracy of the crystal structures employed, which is explained by perturbation theory analyses when interactions between adjacent Ir4+ ions are taken into consideration. There are indications implying that the 5d electrons of Na2IrO3 are less strongly localized compared with those of Sr3NiIrO6 and Sr2IrO4. This implication was confirmed by showing that the Madelung potentials of the Ir4+ ions are less negative in Na2IrO3 than in Sr3NiIrO6, Sr2IrO4. Most transition-metal S = 1/2 ions do have magnetic anisotropies because the SOC induces interactions among their crystal-field split d-states, and the associated mixing of the states modifies only the orbital parts of the states. This finding cannot be mimicked by a spin Hamiltonian because this model Hamiltonian lacks the orbital degree of freedom, thereby leading to the spin-half syndrome. The spin-orbital entanglement for the 5d spin-half ions Ir4+ is not as strong as has been assumed lately.

cond-mat.str-el↗