arXiv subjects

B. I. Min

Publications and source records attributed to B. I. Min.

At least 19 recordsLinked to original sources

arXiv2025

Isotope Effects and the Negative Thermal Expansion Phenomena in Ice and Water

H2O is a unique substance with exceptional thermal properties arising from the subtle interplay between its electronic, phononic, and structural degrees of freedom. Of particular interest in H2O are the negative thermal expansion (NTE) phenomena, observed in its solid phase (ice) at low temperature, and in its liquid phase (water) near the freezing temperature. Furthermore, ice and water exhibit the abnormal volume isotope effect (VIE), where volume expansions occur when replacing H with its heavier isotope, deuterium (D). In order to capture more conceptual and intuitive understanding of intriguing NTE and VIE phenomena in ice and water, we have explored isotope effects in their NTE and melting properties by employing a type of Born-Oppenheimer-approximation approach and the Lindemann criterion. Our findings demonstrate that unusual isotope effects in these phenomena stem from competition between zero-point-energy phonons, thermal phonons, and the hydrogen bonding in H2O. All these components originate from nuclear quantum mechanical (QM) processes, revealing that QM physics plays a crucial role in the seemingly classical ice/water systems.

physics.chem-ph↗

arXiv2024

Weyl Fermion with various chiralities in a f-electron ferromagnetic system: PrB4

Rare-earth tetraborides ($R$B$_{4}$) have attracted a lot of recent attention due to their intriguing electronic, magnetic, and topological properties. We have theoretically investigated topological properties of PrB$_{4}$, which is unique among $R$B$_{4}$ family due to its ferromagnetic ground state. We have discovered that PrB$_{4}$ is an intrinsic magnetic Weyl system possessing multiple topological band crossings with various chiral charges. Density-functional-theory band calculations combined with tight-binding band analysis reveal large Fermi-arc surface states, which are characteristic fingerprints of Weyl fermions. Anomalous Hall conductivity is estimated to be very large, ranging from 500 to 1000 ($Ω\cdot$cm)$^{-1}$ near the Fermi level, which also demonstrates the topological Weyl character of ferromagnetic PrB$_{4}$. These findings suggest that PrB$_{4}$, being a potential candidate of magnetic Weyl system, would be a promising rare-earth topological system for applications to next-generation spintronic and photonic devices.

cond-mat.str-el↗

arXiv2019

Magnetic field-induced type-II Weylsemimetallic state in geometrically frustrated Shastry-Sutherland lattice GdB4

Weyl semimetal is a topologically non-trivial phase of matter with pairs of Weyl nodes in the k-space, which act as monopole and anti-monopole pairs of Berry curvature. Two hallmarks of the Weyl metallic state are the topological surface state called the Fermi arc and the chiral anomaly. It is known that the chiral anomaly yields anomalous magneto-transport phenomena. In this study, we report the emergence of the type-II Weyl semimetallic state in the geometrically frustrated non-collinear antiferromagnetic Shastry-Sutherland lattice (SSL) GdB4 crystal. When we apply magnetic fields perpendicular to the noncollinear moments in SSL plane, Weyl nodes are created above and below the Fermi energy along the M-A line (tau-band) because the spin tilting breaks the time-reversal symmetry and lifts band degeneracy while preserving C4z or C2z symmetry. The unique electronic structure of GdB4 under magnetic fields applied perpendicular to the SSL gives rise to a non-trivial Berry phase, detected in de Haas-van Alphen experiments and chiral-anomaly-induced negative magnetoresistance. The emergence of the magnetic field-induced Weyl state in SSL presents a new guiding principle to develop novel types of Weyl semimetals in frustrated spin systems.

cond-mat.mtrl-sci↗

arXiv2019

Multiple topological Dirac cones in a mixed-valent Kondo semimetal: g-SmS

We demonstrate theoretically that the golden phase of SmS ($g$-SmS), a correlated mixed-valent system, exhibits nontrivial surface states with diverse topology. It turns out that this material is an ideal playground to investigate different band topologies in different surface terminations. We have explored surface states on three different (001), (111), and (110) surface terminations. Topological signature in the (001) surface is not apparent due to a hidden Dirac cone inside the bulk-projected bands. In contrast, the (111) surface shows a clear gapless Dirac cone in the gap region, demonstrating the unambiguous topological Kondo nature of $g$-SmS. Most interestingly, the (110) surface exhibits both topological-insulator-type and topological-crystalline-insulator (TCI)-type surface states simultaneously. Two different types of double Dirac cones, Rashba-type and TCI-type, realized on the (001) and (110) surfaces, respectively, are analyzed with the mirror eigenvalues and mirror Chern numbers obtained from the model-independent \emph{ab initio} band calculations.

cond-mat.str-el↗

arXiv2019

Topological Phase Transition in an Archetypal f-electron Correlated System: Ce

A typical f-electron Kondo lattice system Ce exhibits the well-known isostructural transition, the so-called gamma-alpha transition, accompanied by an enormous volume collapse. Most interestingly, we have discovered that a topological-phase transition also takes place in elemental Ce, concurrently with the gamma-alpha transition. Based on the dynamical mean-field theory approach combined with density functional theory, we have unravelled that the non-trivial topology in alpha-Ce is driven by the f-d band inversion, which arises from the formation of coherent 4f band around the Fermi level. We captured the formation of the 4f quasi-particle band that is responsible for the Lifshitz transition and the non-trivial Z2 topology establishment across the phase boundary. This discovery provides a concept of 'topology switch' for topological Kondo systems. The 'on' and 'off' switching knob in Ce is versatile in a sense that it is controlled by available pressure (around 1 GPa) at room temperature.

cond-mat.str-el↗

arXiv2017

Optical Signatures of Spin-Orbit Exciton in Bandwidth Controlled Sr$_2$IrO$_4$ Epitaxial Films via High-Concentration Ca and Ba Doping

We have investigated the electronic and optical properties of (Sr$_{1-x}$Ca$_{x}$)$_2$IrO$_4$ (x= 0 - 0.375) and (Sr$_{1-y}$Ba$_y$)$_2$IrO$_4$ (y= 0 - 0.375) epitaxial thin-films, in which the bandwidth is systematically tuned via chemical substitutions of Sr ions by Ca and Ba. Transport measurements indicate that the thin-film series exhibits insulating behavior, similar to the J$_{eff}$= 1/2 spin-orbit Mott insulator Sr$_2$IrO$_4$. As the average A-site ionic radius increases from (Sr$_{1-x}$Ca$_{x}$)$_2$IrO$_4$ to (Sr$_{1-y}$Ba$_y$)$_2$IrO$_4$, optical conductivity spectra in the near-infrared region shift to lower energies, which cannot be explained by the simple picture of well-separated J$_{eff}$= 1/2 and J$_{eff}$= 3/2 bands. We suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the J$_{eff}$= 1/2 band. Our experimental results are consistent with this interpretation as implemented by a multi-orbital Hubbard model calculation: namely, incorporating a strong Fano-like coupling between the spin-orbit exciton and inter-site d-d transitions within the J$_{eff}$= 1/2 band.

cond-mat.str-el↗

arXiv2017

Experimental Realization of Type-II Dirac Fermions in PdTe$_2$ Superconductor

A Dirac fermion in a topological Dirac semimetal is a quadruple-degenerate quasi-particle state with a relativistic linear dispersion. Breaking either time-reversal or inversion symmetry turns this system into a Weyl semimetal that hosts double-degenerate Weyl fermion states with opposite chiralities. These two kinds of quasi-particles, although described by a relativistic Dirac equation, do not necessarily obey Lorentz invariance, allowing the existence of so-called type-II fermions. Recent theoretical discovery of type-II Weyl fermions evokes the prediction of type-II Dirac fermions in PtSe$_2$-type transition metal dichalcogenides, expecting an experimental confirmation. Here, we report an experimental realization of type-II Dirac fermions in PdTe$_2$ by angle-resolved photoemission spectroscopy combined with {\it ab-initio} band calculations. Our experimental finding makes the first example that has both superconductivity and type-II Dirac fermions, which turns the topological material research into a new phase.

cond-mat.mtrl-sci↗

arXiv2016

Edwards polaron formation : From one to three dimension

Employing a self-consistent (optimized) variational diagonalization scheme, we investigate the formation of polaronic quasiparticles in a spinless fermion-boson transport model that couples the movement of charge carriers to fluctuations and correlations of a background medium. The background is parameterized by bosonic degrees of freedom. The variational fermion-boson Hilbert space is constructed to achieve high accuracy in one to three spatial dimensions with modest computational requirements. To characterize the ground-state properties of the Edwards model in the single-particle sector, we present exact numerical results for the polaron band dispersion, quasiparticle weight, Drude weight, mass enhancement, and the particle-boson correlations in a wide parameter regime. In the Edwards model, transport will be quasifree, diffusive or boson-assisted in the weakly fermion-boson coupled, fluctuation-dominated or strongly correlated regimes, respectively. Thereby correlated transport is not only assisted but also limited by the bosonic excitations. As a result, the Drude weight remains finite even in the limit of very small boson frequencies. For a strongly correlated background, closed loops are important, in any dimension, to generate a finite effective particle mass even when the free fermion has an infinite mass.

cond-mat.str-el↗

arXiv2016

Magnetic effects in sulfur-decorated graphene

The interaction between two different materials can present novel phenomena that are quite different from the physical properties observed when each material stands alone. Strong electronic correlations, such as magnetism and superconductivity, can be produced as the result of enhanced Coulomb interactions between electrons. Two-dimensional materials are powerful candidates to search for the novel phenomena because of the easiness of arranging them and modifying their properties accordingly. In this work, we report magnetic effects of graphene, a prototypical non-magnetic two-dimensional semi-metal, in the proximity with sulfur, a diamagnetic insulator. In contrast to the well-defined metallic behaviour of clean graphene, an energy gap develops at the Fermi energy for the graphene/sulfur compound with decreasing temperature. This is accompanied by a steep increase of the resistance, a sign change of the slope in the magneto-resistance between high and low fields, and magnetic hysteresis. A possible origin of the observed electronic and magnetic responses is discussed in terms of the onset of low-temperature magnetic ordering. These results provide intriguing insights on the search for novel quantum phases in graphene-based compounds.

cond-mat.mtrl-sci↗

arXiv2016

Electronic structure of YbB$_{6}$: Is it a Topological Insulator or not?

To resolve the controversial issue of the topological nature of the electronic structure of YbB$_{6}$, we have made a combined study using density functional theory (DFT) and angle resolved photoemission spectroscopy (ARPES). Accurate determination of the low energy band topology in DFT requires the use of modified Becke-Johnson exchange potential incorporating the spin-orbit coupling and the on-site Coulomb interaction $U$ of Yb $4f$ electrons as large as 7 eV. We have double-checked the DFT result with the more precise GW band calculation. ARPES is done with the non-polar (110) surface termination to avoid band bending and quantum well confinement that have confused ARPES spectra taken on the polar (001) surface termination. Thereby we show definitively that YbB$_{6}$ has a topologically trivial B 2$p$-Yb 5$d$ semiconductor band gap, and hence is a non-Kondo non-topological insulator (TI). In agreement with theory, ARPES shows pure divalency for Yb and a $p$-$d$ band gap of 0.3 eV, which clearly rules out both of the previous scenarios of $f$-$d$ band inversion Kondo TI and $p$-$d$ band inversion non-Kondo TI. We have also examined the pressure-dependent electronic structure of YbB$_{6}$, and found that the high pressure phase is not a Kondo TI but a \emph{p}-\emph{d} overlap semimetal.

cond-mat.str-el↗

arXiv2015

Substrate-tuning of correlated spin-orbit oxides

We have systematically investigated substrate-strain effects on the electronic structures of two representative Sr-iridates, a correlated-insulator Sr$_2$IrO$_4$ and a metal SrIrO$_3$. Optical conductivities obtained by the \emph{ab initio} electronic structure calculations reveal that the tensile strain shifts the optical peak positions to higher energy side with altered intensities, suggesting the enhancement of the electronic correlation and spin-orbit coupling (SOC) strength in Sr-iridates. The response of the electronic structure upon tensile strain is found to be highly correlated with the direction of magnetic moment, the octahedral connectivity, and the SOC strength, which cooperatively determine the robustness of $J_{eff}$=1/2 ground states. Optical responses are analyzed also with microscopic model calculation and compared with corresponding experiments. In the case of SrIrO$_3$, the evolution of the electronic structure near the Fermi level shows high tunability of hole bands, as suggested by previous experiments.

cond-mat.mtrl-sci↗

arXiv2015

The Insulating Nature of Na2IrO3: Mott-type or Slater-type?

We have investigated temperature-dependent electronic structures of Na2IrO3 to unravel its insulating nature. Employing the combined scheme of the density-functional theory (DFT) and the dynamical mean-field theory (DMFT), we have shown that the insulating state persists even above the Neel temperature (T_{N}), which reveals that Na2IrO3 is classified into a Mott-type insulator. The measured photoemission spectrum in the paramagnetic (PM) state is well described by the electronic structure obtained from the DFT+DMFT for the insulating state above T_{N}. The analysis of optical conductivity, however, suggests that the non-local correlation effect is also important in Na2IrO3. Therefore, Na2IrO3 is not to be a standard Mott insulator in that the extended nature and the non-local correlation effect of Ir 5d electrons are important as well in describing its electronic and magnetic properties.

cond-mat.str-el↗

arXiv2015

The nature of Itinerant Ferromagnetism of SrRuO3 : A DFT+DMFT Study

We have investigated the temperature (T)-dependent evolution of electronic structures and magnetic properties of an itinerant ferromagnet SrRuO3, employing the combined scheme of the density functional theory and the dynamical mean-field theory (DFT+DMFT). The inclusion of finite dynamical correlation effects beyond the DFT well describes not only the incoherent hump structure observed in the photoemission experiment but also the T-dependent magnetic properties in accordance with experiments. We have shown that the magnetization of SrRuO3 evolves with the Stoner behavior below the Curie temperature (Tc), reflecting the weak itinerant ferromagnetic behavior, but the local residual magnetic moment persists even above Tc, indicating the local magnetic moment behavior. We suggest that the ferromagnetism of SrRuO3 has dual nature of both weak and local moment limits, even though the magnetism of SrRuO3 is more itinerant than that of Fe.

cond-mat.str-el↗

arXiv2014

Topological properties and the dynamical crossover from mixed-valence to Kondo-lattice behavior in golden phase of SmS

We have investigated temperature-dependent behaviors of electronic structure and resistivity in a mixed-valent golden phase of SmS, based on the dynamical mean-field theory band structure calculations. Upon cooling, the coherent Sm 4$f$ bands are formed to produce the hybridization induced pseudogap near the Fermi level, and accordingly the topology of Fermi surface is changed to exhibit a Lifshitz-like transition. The surface states emerging in the bulk gap region are found to be not topologically protected states but just typical Rashba spin-polarized states, indicating that SmS is not a topological Kondo semimetal. From the analysis of anomalous resistivity behavior in SmS, we have identified universal energy scales, which characterize the Kondo/mixed-valent semimetallic systems.

cond-mat.str-el↗

arXiv2014

Dimerization-Induced Fermi-Surface Reconstruction in IrTe2

We report a de Haas-van Alphen (dHvA) oscillation study on IrTe2 single crystals showing complex dimer formations. By comparing the angle dependence of dHvA oscillations with band structure calculations, we show distinct Fermi surface reconstruction induced by a 1/5-type and a 1/8-type dimerizations. This verifies that an intriguing quasi-two-dimensional conducting plane across the layers is induced by dimerization in both cases. A phase transition to the 1/8 phase with higher dimer density reveals that local instabilities associated with intra- and interdimer couplings are the main driving force for complex dimer formations in IrTe2.

cond-mat.str-el↗

arXiv2014

The coexistence of charge density wave and superconductivity in Pt-based layered superconductors : SrPt$_2$As$_2$ and LaPt$_2$Si$_2$

The intriguing coexistence of the charge density wave (CDW) and superconductivity in SrPt$_2$As$_2$ and LaPt$_2$Si$_2$ has been investigated based on the {\it ab initio} density functional theory band structure and phonon calculations. We have found that the local split distortions Pt atoms in the [As-Pt-As] layers play an essential role in driving the five-fold supercell CDW instability as well as the phonon softening instability in SrPt$_2$As$_2$. By contrast, the CDW and phonon softening instabilities in LaPt$_2$Si$_2$ occur without split distortions of Pt atoms, indicating that the driving mechanisms of the CDW in SrPt$_2$As$_2$ and LaPt$_2$Si$_2$ are different. We have found that the CDW instability for the former arises from the Fermi surface nesting, while, for the latter, from the saddle point scattering. The phonon calculations, however, suggest that the CDW and the superconductivity coexist in [{\it X}-Pt-{\it X}] layers ({\it X} = As or Si) for both cases.

cond-mat.supr-con↗

arXiv2014

Structural instability and the Mott-Peierls transition in a half-metallic hollandite : K$_{2}$Cr$_{8}$O$_{16}$

In order to explore the driving mechanism of the concomitant metal-insulator and structural transitions in quasi-one-dimensional hollandite K$_{2}$Cr$_{8}$O$_{16}$, electronic structures and phonon properties are investigated by employing the {\it ab initio} density functional theory (DFT) calculations. We have found that the imaginary phonon frequency reflecting the structural instability appears only in the DFT+$U$ ($U$: Coulomb correlation) calculation, which indicates that the Coulomb correlation plays an essential role in the structural transition. The lattice displacements of the softened phonon at X explain the observed lattice distortions in K$_{2}$Cr$_{8}$O$_{16}$ perfectly well, suggesting the Peierls distortion vector {\bf Q} of X (0, 0, 1/2). The combined study of electronic and phonon properties reveals that half-metallic K$_{2}$Cr$_{8}$O$_{16}$, upon cooling, undergoes the correlation-assisted Peierls transition to become a Mott-Peierls ferromagnetic insulator at low temperature.

cond-mat.str-el↗

arXiv2014

Termination-dependent Surface In-gap States in a Mixed-valent Topological Insulator: SmB$_6$

We have investigated the surface states of a potential mixed-valent topological insulator SmB$_6$ based on the first principles density functional theory slab band structure analysis. We have found that metallic surface states are formed in the bulk band gap region, providing evidence for the topological insulating nature of SmB$_6$. The obtained surface in-gap states are quite different from those in existing reports in that they are formed differently depending on the Sm or B$_6$ surface termination, and are composed of mainly Sm $4f$ state indicating the essentiality of including $f$ electrons in describing the surface states. We have obtained the spin chiral structures of the Fermi surfaces, which are also in accordance with the topological insulating nature of SmB$_6$.

cond-mat.str-el↗