arXiv subjects

B. I. Min

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

71 records · Page 4Linked to original sources

arXiv2002

Electronic structures of doped anatase $\rm TiO_{2}$: $\rm Ti_{1-x}M_{x}O_{2}$ (M=Co, Mn, Fe, Ni)

We have investigated electronic structures of a room temperature diluted magnetic semiconductor : Co-doped anatase $\rm TiO_{2}$. We have obtained the half-metallic ground state in the local-spin-density approximation(LSDA) but the insulating ground state in the LSDA+$U$+SO incorporating the spin-orbit interaction. In the stoichiometric case, the low spin state of Co is realized with the substantially large orbital moment. However, in the presence of oxygen vacancies near Co, the spin state of Co becomes intermediate. The ferromagnetisms in the metallic and insulating phases are accounted for by the double-exchange-like and the superexchange mechanism, respectively. Further, the magnetic ground states are obtained for Mn and Fe doped $\rm TiO_{2}$, while the paramagnetic ground state for Ni-doped $\rm TiO_{2}$.

cond-mat.mtrl-sci↗

arXiv2002

Electronic structure of metallic antiperovskite compound GaCMn$_3$

We have investigated electronic structures of antiperovskite GaCMn$_3$ and related Mn compounds SnCMn$_3$, ZnCMn$_3$, and ZnNMn$_3$. In the paramagnetic state of GaCMn$_3$, the Fermi surface nesting feature along the $Γ{\rm R}$ direction is observed, which induces the antiferromagnetic (AFM) spin ordering with the nesting vector {\bf Q} $\sim Γ{\rm R}$. Calculated susceptibilities confirm the nesting scenario for GaCMn$_3$ and also explain various magnetic structures of other antiperovskite compounds. Through the band folding effect, the AFM phase of GaCMn$_3$ is stabilized. Nearly equal densities of states at the Fermi level in the ferromagnetic and AFM phases of GaCMn$_3$ indicate that two phases are competing in the ground state.

cond-mat.mtrl-sci↗

arXiv2001

Half-metallic antiferromagnets in double perovskites: LaAVRuO$_6$ (A=Ca, Sr, and Ba)

Based on the theoretical exploration of electronic structures, we propose that the ordered double perovskites LaAVRuO$_6$ and LaVO$_3$/ARuO$_3$ (001) superlattice (A = Ca, Sr and Ba) are strong candidates for half-metallic (HM) antiferromagnets (AFMs). %LaAVRuO$_6$ and LaVO$_3$/ARuO$_3$ have the %100% spin polarizations at the Fermi level but with zero %total magnetic moments. We have shown that the HM-AFM nature in LaAVRuO$_6$ is very robust regardless of (i) divalent ion replacement at A-sites, (ii) oxygen site relaxation, (iii) the inclusion of the Coulomb correlation, and (iv) cation disorder. A type of the double exchange interaction is expected to be responsible for the half-metallicity and the antiferromagnetism in these systems.

cond-mat.str-el↗

arXiv2001

Electronic Structures of Antiperovskite Superconductor MgCNi$_3$ and Related Compounds

Electronic structure of a newly discovered antiperovskite superconductor MgCNi$_3$ is investigated by using the LMTO band method. The main contribution to the density of states (DOS) at the Fermi energy $E_{\rm F}$ comes from Ni 3$d$ states which are hybridized with C 2$p$ states. The DOS at $E_{\rm F}$ is varied substantially by the hole or electron doping due to the very high and narrow DOS peak located just below $E_{\rm F}$. We have also explored electronic structures of C-site and Mg-site doped MgCNi$_3$ systems, and described the superconductivity in terms of the conventional phonon mechanism.

cond-mat.supr-con↗

arXiv2001

New High Temperature Diboride Superconductors: AgB2 and AuB2

Based on electronic structure calculations, we have found that noble metal diborides, AgB2 and AuB2, are potential candidates for conventional BCS-type high temperature superconductors. B 2p density of states dominates at the Fermi level in comparison with Ag 4d and Au 5d states. Furthermore, the electron-phonon coupling constant λis much larger in AgB2 and AuB2 than in MgB2. Estimated transition temperatures for AgB2 and AuB2 are T_c = 59 K and 72 K, respectively. These are about 2 ~ 3 times higher than the estimated T_c = 27 K in MgB2 and almost comparable to those in cuprate superconductors.

cond-mat.supr-con↗

arXiv2001

Half-metallic antiferromagnets in thiospinels

We have theoretically designed the half-metallic (HM) antiferromagnets (AFMs) in thiospinel systems, $\rm Mn(CrV)S_{4}$ and $\rm Fe_{0.5}Cu_{0.5}(V_{0.5}Ti_{1.5})S_{4}$, based on the electronic structure studies in the local-spin-density approximation (LSDA). We have also explored electronic and magnetic properties of parent spinel compounds of the above systems; $\rm CuV_{2}S_{4}$ and $\rm CuTi_{2}S_{4}$ are found to be HM ferromagnets in their cubic spinel structures, while $\rm MnCr_{2}S_{4}$ is a ferrimagnetic insulator. We have discussed the feasibility of material synthesis of HM-AFM thiospinel systems.

cond-mat.mtrl-sci↗

arXiv2001

Electronic structures of antiperovskite superconductors: MgXNi$_3$ (X=B,C,N)

We have investigated electronic structures of a newly discovered antiperovskite superconductor MgCNi$_3$ and related compounds MgBNi$_3$ and MgNNi$_3$. In MgCNi$_3$, a peak of very narrow and high density of states is located just below $\rm E_F$, which corresponds to the $π^*$ antibonding state of Ni-3d and C-$2p$ but with the predominant Ni-3d character. The prominent nesting feature is observed in the $Γ$-centered electron Fermi surface of an octahedron-cage-like shape that originates from the 19th band. The estimated superconducting parameters based on the simple rigid-ion approximation are in reasonable agreement with experiment, suggesting that the superconductivity in MgCNi$_3$ is described well by the conventional phonon mechanism.

cond-mat.supr-con↗

arXiv2000

Charge and Orbital Ordering and Spin State Transition Driven by Structural Distortion in YBaCo_2O_5

We have investigated electronic structures of antiferromagnetic YBaCo_2O_5 using the local spin-density approximation (LSDA) + U method. The charge and orbital ordered insulating ground state is correctly obtained with the strong on-site Coulomb interaction. Co^{2+} and Co^{3+} ions are found to be in the high spin (HS) and intermediate spin (IS) state, respectively. It is considered that the tetragonal to orthorhombic structural transition is responsible for the ordering phenomena and the spin states of Co ions. The large contribution of the orbital moment to the total magnetic moment indicates that the spin-orbit coupling is also important in YBaCo_2O_5.

cond-mat.str-el↗

arXiv2000

Unquenched large orbital magnetic moment in NiO

Magnetic properties of NiO are investigated by incorporating the spin-orbit interaction in the LSDA+U scheme. It is found that the large part of orbital moment remains unquenched in NiO. The orbital moment contributes about mu_L = 0.29 mu_B to the total magnetic moment of M = 1.93 mu_B, as leads to the orbital-to-spin angular momentum ratio of L/S = 0.36. The theoretical values are in good agreement with recent magnetic X-ray scattering measurements.

cond-mat.str-el↗

arXiv2000

Itinerant ferromagnetism in half-metallic CoS_2

We have investigated electronic and magnetic properties of the pyrite-type CoS_2 using the linearized muffin-tin orbital (LMTO) band method. We have obtained the ferromagnetic ground state with nearly half-metallic nature. The half-metallic stability is studied by using the fixed spin moment method. The non-negligible orbital magnetic moment of Co 3d electrons is obtained as $μ_L = 0.06 μ_B$ in the local spin density approximation (LSDA). The calculated ratio of the orbital to spin angular momenta /< S_z > = 0.15 is consistent with experiment. The effect of the Coulomb correlation between Co 3d electrons is also explored with the LSDA + U method. The Coulomb correlation at Co sites is not so large, $U \lesssim 1$ eV, and so CoS_2 is possibly categorized as an itinerant ferromagnet. It is found that the observed electronic and magnetic behaviors of CoS_2 can be described better by the LSDA than by the LSDA + U.

cond-mat.str-el↗

arXiv1999

Origin of the giant magnetic moments of Fe impurities on and in Cs films

To explore the origin of the observed giant magnetic moments ($\sim 7 μ_B$) of Fe impurities on the surface and in the bulk of Cs films, we have performed the relativistic LSDA + U calculations using the linearized muffin-tin orbital (LMTO) band method. We have found that Fe impurities in Cs behave differently from those in noble metals or in Pd. Whereas the induced spin polarization of Cs atoms is negligible, the Fe ion itself is found to be the source of the giant magnetic moment. The 3d electrons of Fe in Cs are localized as the 4f electrons in rare-earth ions so that the orbital magnetic moment becomes as large as the spin magnetic moment. The calculated total magnetic moment of $M = 6.43 μ_B$, which comes mainly from Fe ion, is close to the experimentally observed value.

cond-mat.str-el↗

arXiv1999

The small polaron crossover transition in colossal magnetoresistance (CMR) manganites

Based on the combined model of the double exchange and the polaron, we have studied the small-to-large polaron crossover transition and explored its effects on the magnetic and transport properties in colossal magnetoresistance (CMR) manganites. We have used the variational Lang-Firsov canonical transformation, and shown that the magnetic and transport properties of both high and low $T_C$ manganites are well described in terms of a single formalism. We have reproduced the rapid resistivity drop below $T_C$, a realistic CMR ratio, and the {\it first-order-like} sharp magnetic phase transition, which are observed in low $T_C$ manganites.

cond-mat.str-el↗

arXiv1999

Isotope effects and the charge gap formation in the charge ordered phase of colossal magnetoresistance manganites

Giant oxygen isotope effects observed in colossal magnetoresistance manganites are investigated by employing the combined model of the double exchange and interacting lattice polaron mechanism. We have shown that the isotope effects on $ T_C$ in the metallic phase and $ T_{CO}$ in the charge ordered phase of manganites can be explained well in terms of the double exchange and polaron narrowing factors with reasonable physical parameters.

cond-mat.str-el↗

arXiv1997

Polaron transport and lattice dynamics in colossal magnetoresistance manganites

Based on the model combining the spin double exchange and the lattice polaron, we have studied the colossal magnetoresistance phenomena observed in perovskite manganites R_{1-x}A_xMnO_3. First, effects of both the double exchange and the electron-phonon interaction on the transport property are investigated. We have evaluated the temperature dependent resistance and the magnetoresistance using the Kubo formula, and examined the crossover from tunneling to hopping regime of small polarons. Second, effects of the double exchange interaction on the lattice degree of freedom are explored. It is found that both the hardening of the phonon frequency and the reduction of the phonon damping take place with decreasing the temperature.

cond-mat↗

arXiv1997

Charge ordering in doped manganese oxides: lattice dynamics and magnetic structure

Based on the Hamiltonian of small polarons with the strong nearest neighbor repulsion, we have investigated the charge ordering phenomena observed in half-doped manganites R_{1/2}A_{1/2}MnO_3. We have explored possible consequences of the charge ordering phase in the half-doped manganites. First, we have studied the renormalization of the sound velocity around $T_{CO}$, considering the acoustic phonons coupled to the electrons participating in the charge ordering. Second, we have found a new antiferromagnetic phase induced by the charge ordering, and discussed its role in connection with the specific CE-type antiferromagnetic structure observed in half-doped manganites.

cond-mat↗

arXiv1997

Effect of Quantum Confinement on Electron Tunneling through a Quantum Dot

Employing the Anderson impurity model, we study tunneling properties through an ideal quantum dot near the conductance minima. Considering the Coulomb blockade and the quantum confinement on an equal footing, we have obtained current contributions from various types of tunneling processes; inelastic cotunneling, elastic cotunneling, and resonant tunneling of thermally activated electrons. We have found that the inelastic cotunneling is suppressed in the quantum confinement limit, and thus the conductance near its minima is determined by the elastic cotunneling at low temperature ($k_BT \ll Γ$, $Γ$: dot-reservoir coupling constant), or by the resonant tunneling of single electrons at high temperature ($k_BT \gg Γ$).

cond-mat.mes-hall↗

arXiv1996

The Finite-$U$ Impurity Anderson Model in the presence of an external magnetic field

We have investigated effects of an external magnetic field in the impurity Anderson model with a finite on-site Coulomb repulsion $U$. Large $N_f$ expansion is employed in the slave boson representation, by taking into account $f^0$, $f^1$, and $f^2$ subspaces. To evaluate the vertex function for the ``empty state boson" self-energy, we have devised two approximations which reduce much computational efforts without losing general features of the model. It is found that the Kondo temperature is reduced by the presence of a magnetic field, and that at low field and at low temperature, the field dependence of both the Kondo temperature and the impurity magnetization exhibits a scaling behavior with high accuracy. Further, some interesting features are found in the field dependence of the impurity magnetization at finite temperature, the physical implications of which are discussed in terms of the renormalized Kondo temperature.

cond-mat↗