arXiv subjects

B. I. Min

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

At least 37 records · Page 2Linked to original sources

arXiv2014

Universal metastability of the low-spin state in Co$^{2+}$ systems: non-Mott type pressure-induced spin-state transition in CoCl$_{2}$

We have investigated the pressure-induced spin-state transition in Co$^{2+}$ systems in terms of a competition between the Hund's exchange energy ($J$) and the crystal-field splitting ($Δ_{CF}$). First, we show the universal metastability of the low-spin state in octahedrally coordinated Co$^{2+}$ systems. Then we present the strategy to search for a Co$^{2+}$ system, for which the mechanism of spin-state and metal-insulator transitions is governed not by the Mott physics but by $J$ vs. $Δ_{CF}$ physics. Using CoCl$_{2}$ as a prototypical Co$^{2+}$ system, we have demonstrated the pressure-induced spin-state transition from high-spin to low-spin, which is accompanied with insulator-to-metal and antiferromagnetic to half-metallic ferromagnetic transitions. Combined with metastable character of Co$^{2+}$ and the high compressibility nature of CoCl$_{2}$, the transition pressure as low as 27 GPa can be identified on the basis of $J$ vs. $Δ_{CF}$ physics.

cond-mat.str-el↗

arXiv2014

Temperature Dependence of Linked Gap and Surface State Evolution in the Mixed Valent Topological Insulator SmB6

Taken together and viewed holistically, recent theory, low temperature (T) transport, photoelectron spectroscopy and quantum oscillation experiments have built a very strong case that the paradigmatic mixed valence insulator SmB6 is currently unique as a three-dimensional strongly correlated topological insulator (TI). As such, its many-body T-dependent bulk gap brings an extra richness to the physics beyond that of the weakly correlated TI materials. How will the robust, symmetry-protected TI surface states evolve as the gap closes with increasing T? For SmB6 exploiting this opportunity first requires resolution of other important gap-related issues, its origin, its magnitude, its T-dependence and its role in bulk transport. In this paper we report detailed T-dependent angle resolved photoemission spectroscopy (ARPES) measurements that answer all these questions in a unified way.

cond-mat.str-el↗

arXiv2014

Termination-dependent Electronic and Magnetic Properties of Ultrathin SrRuO$_{3}$ (111) Film on SrTiO$_{3}$

We have investigated electronic and magnetic properties of ultrathin SrRuO$_{3}$ (SRO) film grown on (111) SrTiO$_{3}$ substrate using the {\it ab initio} electronic structure calculations. Ru-terminated SRO (111) film suffers from strong surface atomic relaxations, while SrO$_{3}$-terminated one preserves the surface structure of ideal perovskites. Both Ru- and SrO$_{3}$-terminated SRO (111) film show unexpected interlayer antiferromagnetic (AFM) structure at the surface, but with different characters and mechanisms. The AFM structure for the former results from the large surface atomic relaxation, whereas that for the latter results from the truncated film effect. Interestingly, for the SrO$_{3}$-termination case, the half-metallic nature emerges despite the interlayer AFM structure. Upon reducing the thickness, the collapsing behavior of magnetic anisotropy from out-of-plane to in-plane easy axis is found to occur for the Ru-termination case, which, however, does not pertain to SrO$_{3}$-termination case.

cond-mat.mtrl-sci↗

arXiv2014

Electronic excitations in the edge-shared relativistic Mott insulator: Na$_2$IrO$_3$

We have investigated the excitation spectra of $j_{eff}$=$\frac{1}{2}$ Mott insulator Na$_2$IrO$_3$. Taking into account a relativistic multiplet structure of Ir ions, we have calculated the optical conductivity $σ(ω)$ and resonant inelastic x-ray scattering (RIXS) spectra, which manifest different features from those of a canonical $j_{eff}$=$\frac{1}{2}$ system Sr$_2$IrO$_4$.Distinctly from the two-peak structure in Sr$_2$IrO$_4$, $σ(ω)$ in Na$_2$IrO$_3$ has a broad single peak dominated by interband transitions from $j_{eff}$=$\frac{3}{2}$ to $\frac{1}{2}$. RIXS spectra exhibit the spin-orbit (SO) exciton that has a two-peak structure arising from the crystal-field effect, and the magnon peak at energies much lower than in Sr$_2$IrO$_4$. In addition, a small peak near the optical absorption edge is found in RIXS spectra, originating from the coupling between the electron-hole ($e$-$h$) excitation and the SO exciton. Our findings corroborate the validity of the relativistic electronic structure and importance of both itinerant and local features in Na$_2$IrO$_3$.

cond-mat.str-el↗

arXiv2014

Interacting Holstein and extended-Holstein bipolarons

Employing the recently developed self-consistent variational basis generation scheme, we have investigated the bipolaron-bipolaron interaction within the purview of Holstein-Hubbard and the extended-Holstein-Hubbard (F2H) model on a discrete one-dimensional lattice. The density-matrix renormalization group (DMRG) method has also been used for the Holstein-Hubbard model. We have shown that there exists no bipolaron-bipolaron attraction in the Holstein-Hubbard model. In contrast, we have obtained clear-cut bipolaron-bipolaron attraction in the F2H model. Composite bipolarons are formed above a critical electron-phonon coupling strength, which can survive the finite Hubbard $U$ effect. We have constructed the phase diagram of F2H polarons and bipolarons, and discussed the phase separation in terms of the formation of composite bipolarons.

cond-mat.str-el↗

arXiv2013

Band symmetries of mixed-valence topological insulator: SmB6

We have investigated the band structure and the band symmetry of mixed-valence insulator SmB$_{6}$ systematically within the density functional theory (DFT). We have analyzed the symmetries and characters of Sm $4f$ and $5d$ bands near the Fermi level ($E_F$) in terms of the non-relativistic real cubic bases as well as the relativistic complex bases incorporating the spin-orbit coupling and the cubic crystal field. Further, we have found that the semi-core band located at $\sim 15$ eV below $E_F$ has the mixed parity and thereby affects the parity eigenvalues of the special ${\bf k}$-points considerably. We have discussed the possible surface states and topological class of SmB$_{6}$, based on the bulk parity tables.

cond-mat.str-el↗

arXiv2013

Kink during the formation of the Kondo resonance band in the heavy fermion system

We have investigated the formation of the Kondo resonance (KR) band and the hybridization gap (HG) in the heavy fermion compound CeCoGe2, using a combined approach of the density functional theory (DFT) and the dynamical mean field theory (DMFT). Low temperature (T) spec-tral functions show dispersive KR states, similarly to the recent experimental observation. During the evolution from the nonf conduction band state at high T to the dispersive KR band state at low T, which have topologically different band shapes, we have found the existence of kinks in the spectral function near EF . The observation of kink is clearly in correspondence with the multiple temperature scale of the formation of the KR band. We suggest that the detailed analysis on the abrupt change of electron velocity and the scattering rate near the Fermi level will provide crucial information for the heavy fermion system.

cond-mat.str-el↗

arXiv2013

Correlated Electronic Structures and the Phase Diagram of Hydrocarbon-based Superconductors

We have investigated correlated electronic structures and the phase diagram of electron-doped hydrocarbon molecular solids, based on the dynamical mean-field theory. We have found that the ground state of hydrocarbon-based superconductors such as electron-doped picene and coronene is a multi-band Fermi liquid, while that of non-superconducting electron-doped pentacene is a single-band Fermi liquid in the proximity of the metal-insulator transition. The size of the molecular orbital energy level splitting plays a key role in producing the superconductivity of electron-doped hydrocarbon solids. The multi-band nature of hydrocarbon solids would boost the superconductivity through the enhanced density of states at the Fermi level.

cond-mat.str-el↗

arXiv2013

A new self-consistent basis generation scheme for polaron and bipolaron systems

We have developed a new self-consistent scheme of generating variational basis based on the exactdiagonalization, which can be applied efficiently to various types of electron-phonon systems. This scheme is quite general and brings down the size of the variational space by an order of magnitude or even more in some cases to reproduce the most precise ground state energies and correlation functions available in the literature. This method has enormous potential for application to systems with more electrons or in higher dimensions, which are still beyond the reach of exact-diagonalization just because of the sheer size of their variational space needed to get reasonably convergent results.

cond-mat.str-el↗

arXiv2013

Electronic Structures and Phonon Spectra in Boronitride Superconductors LaMBN (M= Ni, Pt)

We have investigated electronic structures and phonon spectra of newly discovered isostructural superconductors LaNiBN (T_c = 4.1 K) and LaPtBN (T_c = 6.7 K). We have found that their electronic structures are substantially three-dimensional, leading to metallicity both in NiB (PtB) and the intervening LaN layers. Our ab initio phonon calculations show that almost all phonon modes contribute to the electron-phonon coupling (EPC) mechanism, reflecting that both layers are involved in the superconductivity. For LaNiBN, we obtain an EPC strength of λ= 0.52 and a logarithmically averaged characteristic phonon frequency of ω_{log} = 376 K, leading to T_c = 3.9 K. Compared with the Ni B_{1g} mode in LaNiBN, the Pt B_{1g} mode in LaPtBN is reduced by ~70%, leading to a slightly enhanced λ= 0.56 and an ~20 % reduced ω_{log}. The estimated T_c is 5.4 K for LaPtBN, in good agreement with the experiment. We do not find any indication of magnetic instability for either LaNiBN or LaPtBN, which implies that both systems are EPC mediated superconductors. Further, we have found an interesting trend of monotonic increase of T_c with respect to the boron height in the NiB (PtB) layer of both borocarbide and boronitride superconductors, which suggests a possible way to enhance T_c in these systems.

cond-mat.supr-con↗

arXiv2013

Quantum Oscillations of the Metallic Triangular-lattice Antiferromagnet PdCrO2

We report the de Haas-van Alphen (dHvA) oscillations and first-principle calculations for triangular antiferromagnet PdCrO2 showing unconventional anomalous Hall effect (AHE). The dHvA oscillations in PdCrO2 reveal presence of several 2 dimensional Fermi surfaces of smaller size than found in nonmagnetic PdCoO2. This evidences Fermi surface reconstruction due to the non-collinear 120 antiferromagnetic ordering of the localized Cr, consistent with the first principle calculations. The temperature dependence of dHvA oscillations shows no signature of additional modification of Cr spin structure below TN. Considering that the 120 helical ordering of Cr spins has a zero scalar spin chirality, our results suggest that PdCrO2 is a rare example of the metallic triangular antiferromagnets whose unconventional AHE can not be understood in terms of the spin chirality mechanism.

cond-mat.str-el↗

arXiv2012

Magnetic couplings, optical spectra, and spin-orbit exciton in 5d electron Mott insulator Sr2IrO4

Based on the microscopic model including strong spin-orbit coupling, on-site Coulomb and Hund's interactions, as well as crystal field effects, we have investigated magnetic and optical properties of Sr2IrO4. Taking into account all possible intermediate state multiplets generated by virtual hoppings of electrons, we calculated the isotropic Heisenberg, pseudodipolar, and Dzyaloshinsky-Moriya coupling constants, which describe the experiment quite well. We have also evaluated the optical conductivity σ(ω) by employing the exact diagonalization method on small clusters, and obtained two peaks at \sim 0.5 and \sim 1.0 eV in agreement with experiment. The two peak structure of σ(ω) arises from the unusual Fano-type overlap between electron-hole continuum of the Jeff = 1/2 band and the intrasite spin-orbit exciton observed recently in Sr2IrO4 by resonant x-ray scattering.

cond-mat.str-el↗

arXiv2012

Effects of the Spin-Orbit Coupling and the Superconductivity in simple-cubic alpha-Polonium

We have investigated the mechanism of stabilizing the simple-cubic (SC) structure in polonium (alpha- Po), based on the phonon dispersion calculations using the first-principles all-electron band method. We have demonstrated that the stable SC structure results from the suppression of the Peierls instability due to the strong spin-orbit coupling (SOC) in alpha-Po. Further, we have explored the possible superconductivity in alpha-Po, and predicted that it becomes a superconductor with Tc ~ 4 K. The transverse soft phonon mode at q ~ 2/3 R, which is greatly influenced by the SOC, plays an important role both in the structural stability and the superconductivity in alpha-Po. We have discussed effects of the SOC and the volume variation on the phonon dispersions and superconducting properties of alpha-Po.

cond-mat.supr-con↗

arXiv2012

Electron and phonon band-structure calculations for the antipolar SrPt$_{3}$P antiperovskite superconductor: Evidence of low-energy two-dimensional phonons

SrPt3P has recently been reported to exhibit superconductivity with Tc = 8.4 K. To explore its superconducting mechanism, we have performed electron and phonon band calculations based on the density functional theory, and found that the superconductivity in SrPt3P is well described by the strong coupling phonon-mediated mechanism. We have demonstrated that superconducting charge carriers come from pdπ-hybridized bands between Pt and P ions, which couple to low energy (~ 5 meV) phonon modes confined on the ab in-plane. These in-plane phonon modes, which do not break antipolar nature of SrPt3P, enhance both the electron-phonon coupling constant λand the critical temperature Tc. There is no hint of a specific phonon softening feature in the phonon dispersion, and the effect of the spin-orbit coupling on the superconductivity is found to be negligible.

cond-mat.supr-con↗

arXiv2012

Correlation Assisted Phonon Softenings and the Mott-Peierls Transition in VO$_{2}$

To explore the driving mechanisms of the metal-insulator transition (MIT) and the structural transition in VO2, we have investigated phonon dispersions of rutile VO2 (R-VO2) in the DFT and the DFT+U (U : Coulomb correlation) band calculations. We have found that the phonon softening instabilities occur in both cases, but the softened phonon mode only in the DFT+U describes properly both the MIT and the structural transition from R-VO2 to monoclinic VO2 (M1-VO2). This feature demonstrates that the Coulomb correlation effect plays an essential role of assisting the Peierls transition in R-VO2. We have also found from the phonon dispersion of M1-VO2 that M1 structure becomes unstable under high pressure. We have predicted a new phase of VO2 at high pressure that has a monoclinic CaCl2-type structure with metallic nature.

cond-mat.str-el↗

arXiv2011

Pressure-induced Phonon Softenings and the Structural and Magnetic Transitions in CrO$_{2}$

To investigate the pressure-induced structural transitions of chromium dioxide (CrO$_{2}$), phonon dispersions and total energy band structures are calculated as a function of pressure. The first structural transition has been confirmed at P$\approx$ 10 GPa from the ground state tetragonal CrO$_{2}$ (t-CrO$_{2}$) of rutile type to orthorhombic CrO$_{2}$ (o-CrO$_{2}$) of CaCl$_{2}$ type. The half-metallic property is found to be preserved in o-CrO$_{2}$. The softening of Raman-active B$_{1g}$ phonon mode, which is responsible for this structural transition, is demonstrated. The second structural transition is found to occur for P$\geq$ 61.1 GPa from ferromagnetic (FM) o-CrO$_{2}$ to nonmagnetic (NM) monoclinic CrO$_{2}$ (m-CrO$_{2}$) of MoO$_{2}$ type, which is related to the softening mode at {\bf q} = R(1/2,0,1/2). The third structural transition has been newly identified at P= 88.8 GPa from m-CrO$_{2}$ to cubic CrO$_{2}$ of CaF$_{2}$ type that is a FM insulator.

cond-mat.mtrl-sci↗

arXiv2011

Fermi Surface Reconstruction in CeTe2 Induced by Charge Density Wave: ARPES Study

Electronic structures of a charge-density-wave CDW system CeTe_2-xSb_x (x=0, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES). The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe_2 both in shapes and sizes. The metallic states crossing the Fermi level(E_F) are observed in ARPES. The carriers near E_F have mainly the Te(1) 5p character, with the negligible contribution from Ce 4f states to the CDW formation. The supercell (shadow) bands and the corresponding very weak FS's are found to arise from band-folding due to the interaction with Ce-Te(2) layers. This work shows that the origin of the CDW formation in CeTe_2 is the FS nesting and that the CDW modulation vector is along Gamma-X (Q_CDW ~ X)

cond-mat.str-el↗

arXiv2011

Density functional calculations of the electronic structure and magnetic properties of the hydrocarbon K3picene superconductor near the metal-insulator transition

We have investigated the electronic structures and magnetic properties of of K3picene, which is a first hydrocarbon superconductor with high transition temperature T_c=18K. We have shown that the metal-insulator transition (MIT) is driven in K3picene by 5% volume enhancement with a formation of local magnetic moment. Active bands for superconductivity near the Fermi level E_F are found to have hybridized character of LUMO and LUMO+1 picene molecular orbitals. Fermi surfaces of K3picene manifest neither prominent nesting feature nor marked two-dimensional behavior. By estimating the ratio of the Coulomb interaction U and the band width W of the active bands near E_F, U/W, we have demonstrated that K3picene is located in the vicinity of the Mott transition.

cond-mat.supr-con↗