Jaejun Yu

Novel Jeff=1/2 Mott state induced by relativistic spin-orbit coupling in Sr2IrO4.

We investigated the electronic structure of 5d transition-metal oxide Sr2IrO4 using angle-resolved photoemission, optical conductivity, x-ray absorption measurements, and first-principles band calculations. The system was found to be well described by novel effective total angular momentum Jeff states, in which the relativistic spin-orbit coupling is fully taken into account under a large crystal field. Despite delocalized Ir 5d states, the Jeff states form such narrow bands that even a small correlation energy leads to the Jeff=1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of Jeff quantum spin driven correlated-electron phenomena.

Electronic structure and properties of YBa2Cu3O7-δ, a low dimensional, low density of states superconductor

Abstract The electronic structure of the high T c superconductor, YBa 2 Cu 3 O 7-δ , determined from highly precise all-electron local density calculations yields a relatively simple highly 2D electronic band structure consisting of two 2D Cu2-O and two 1D Cu1-O bands (one almost empty and one almost full at δ =0, becoming full at δ ⩾0.1) near E F . Detailed features (multi-peaks) of the density of states (DOS) are correlated with the band structure of the 36 Cu-O band complex. Surprising features include: (i) the low DOS at E F , especially for δ ⩾0.1 which is much longer than that in La 2− x Sr x CuO 4 - in agreement with experiment - and (ii) a relatively large magnetic Stoner factor.

Electronic structure and properties of Bi2Sr2CaCu2O8, the third high-Tc superconductor

Abstract Results of a highly precise local density determination of the electronic structure (energy bands, densities of states, Fermi surface and charge densities) of the new high-Tc superconductor Bi2Sr2CaCu2O8 are presented. The calculations employed the full-potential linearized augmented plane wave (FLAPW) method and the subcell structure parameters given by the work of Hazen et al. and Sunshine et al. As in the case of the other Cu-O superconductors, we find a relatively simple band structure at EF and strongly anisotropic highly 2D properties. The Sr and Ca atoms are highly ionic, with the Ca2+ ions serving to insulate the Cu-O planes. The Bi-O planes contribute substantially to N(EF), the density of states at EF, and to the transport properties whereas neither the Sr or Ca do. The N(EF) contribution from the Cu-O planes (∼1.0 states/eV-Cu atom) is lower (per Cu atom) than that found previously for the La-Sr-Cu-O and Y-Ba-Cu-O superconductors. The highly 2D Fermi surface shows regions of strong Cu-O and Bi-O hybridization from which highly anisotropic superconducting energy gaps are predicted. Here, too, an electron-phonon calculation of Tc (using the crude rigid ion approximation) is found to be inadequate. As in the case of YBa2Cu3O7-δ, strongly localized states at EF (arising from Bi-O and Cu-O) may be responsible for charge transfer excitations (excitons) which cause the high Tc.

Bonds, bands, charge transfer excitations and superconductivity of YBa2Cu3O7-δ

Abstract The different energy dispersion and Fermi surfaces arising from the bands which dominate the electronic band structure near EF in YBa2Cu3O7-δ are presented and related to their orbital charge density distributions for the Cu2 plane and Cu1 chain arrangements thereby relating their physical and chemical descriptions. Magnetic isolation of the Y ions is found to explain the existence of high Tc superconductivity of the RBa2Cu3O7-δ compounds (where R = magnetic heavy lanthanides). A conventional phonon mechanism is found to be inadequate for obtaining high Tc. Charge transfer excitations (“excitons”) of occupied Cu1-O dpπ bonding orbitals into their empty Cu1-O dpσ antibonding orbital partners result in poorly screened Cu3+-Cu4+-like charge fluctuations which induce attractive interactions (-U centers) both in the chains and to the 2D (Cu2) bands-thereby promoting the high Tc via exchange of these “excitons”.

Dimensionality-controlled insulator-metal transition and correlated metallic state in 5d transition metal oxides Sr n+1Ir nO3n+1 (n=1, 2, and infinity).

We investigated the electronic structures of the 5d Ruddlesden-Popper series Sr n+1Ir nO3n+1 (n=1, 2, and infinity) using optical spectroscopy and first-principles calculations. As 5d orbitals are spatially more extended than 3d or 4d orbitals, it has been widely accepted that correlation effects are minimal in 5d compounds. However, we observed a Mott insulator-metal transition with a change of bandwidth as we increased n. In addition, the artificially synthesized perovskite SrIrO3 showed a very large mass enhancement of about 6, indicating that it was in a correlated metallic state.

Physical properties of transparent perovskite oxides (Ba,La)SnO3with high electrical mobility at room temperature

Transparent electronic materials are increasingly in demand for a variety of optoelectronic applications. BaSnO3 is a semiconducting oxide with a large band gap of more than 3.1 eV. Recently, we discovered that La doped BaSnO3 exhibits unusually high electrical mobility of 320 cm^2(Vs)^-1 at room temperature and superior thermal stability at high temperatures [H. J. Kim et al. Appl. Phys. Express. 5, 061102 (2012)]. Following that work, we report various physical properties of (Ba,La)SnO3 single crystals and films including temperature-dependent transport and phonon properties, optical properties and first-principles calculations. We find that almost doping-independent mobility of 200-300 cm^2(Vs)^-1 is realized in the single crystals in a broad doping range from 1.0x10^19 to 4.0x10^20 cm^-3. Moreover, the conductivity of ~10^4 ohm^-1cm^-1 reached at the latter carrier density is comparable to the highest value. We attribute the high mobility to several physical properties of (Ba,La)SnO3: a small effective mass coming from the ideal Sn-O-Sn bonding, small disorder effects due to the doping away from the SnO2 conduction channel, and reduced carrier scattering due to the high dielectric constant. The observation of a reduced mobility of ~70 cm^2(Vs)^-1 in the film is mainly attributed to additional carrier-scatterings which are presumably created by the lattice mismatch between the substrate SrTiO3 and (Ba,La)SnO3. The main optical gap of (Ba,La)SnO3 single crystals remained at about 3.33 eV and the in-gap states only slightly increased, thus maintaining optical transparency in the visible region. Based on these, we suggest that the doped BaSnO3 system holds great potential for realizing all perovskite-based, transparent high-frequency high-power functional devices as well as highly mobile two-dimensional electron gas via interface control of heterostructured films.

Electronic structure of Nd-Ce-Cu-O, a Fermi liquid superconductor

Abstract Results of highly precise local density energy band studies of the new electron doped superconductor, Nd 2− x Ce x CuO 4 , are presented which demonstrate the close similarity to the other hole doped superconductors (the important role of the strongly two-dimensional Cu-O antibonding dpσ band, a somewhat lower density of states at the Fermi energy and a highly 2D Fermi surface). This adds additional evidence to support the Fermi liquid description of the normal metallic state of the high T c superconductors.

Electronic structure and properties of the high-Tc superconductors: Tl2Ba2CaCu2O8 and Tl2Ba2Ca2Cu3O10

Abstract We present results of highly precise local density calculations of the electronic structure of the two high- T c superconductors Tl 2 Ba 2 CaCuO 8 ( T c ≅112 K) and Tl 2 Ba 2 Ca 2 Cu 3 O 10 ( T c ≅125 K), as obtained with the full potential linearized augmented plane wave (FLAPW) method. A relatively simple band structure is found near E F and strong 2D properties are predicted. Again as in the case of the other high- T c materials, Ba and Ca are highly ionic, with the Ca 2+ ions insulating the Cu-O planes. The Tl-O complexes, which are decoupled from the Cu-O planes, produce small electron pockets at E F with predominant O character that contribute to the transport properties. Each Cu-O plane is found to contribute ≃ 1 state/(eV-Cu atom) to the density of states (DOS) at E F , similar to the Bi 2 Sr 2 CaCu 2 O 8 case. Remarkable strong fermi surface nesting is found along the (100) and (010) directions for the 2D Cu-O dpσ bands. Significantly, we show that the usual simple nearest-neighbors only tight-binding model cannot properly describe these states. Crude rigid ion calculations show that a purely electron-phonon mechanism is inadequate to explain the observed high T c . As in the case of the other high- T c superconductors, a charge transfer excitation (excitonic) mechanism appears likely. Interestingly, the partial DOS structure shows that both systems can be viewed as a metal-semiconductor (or metal-semimetal) superlattice indicating a possible relation to the Allender, Bray, and Bardeen model of excitonic superconductivity.

Dominant role of the 2D Van Hove singularity on the Fermi surface and generalized susceptibility of the quasi-2D superconductor La2−xMxCuO4 (M = Sr, Ba, …)☆

The dominant role of the 2D van Hove saddle point singularity on the Fermi surface (FS) and generalized susceptibility, χ(q), in the quasi-2D superconductor La2-xMxCuO4 is shown for varying compositions x of divalent additions M=Sr, Ba …. Dramatic topological changes in the FS with x indicate that composition can play the role of pressure in the classic work of Lifshitz. Very close correlations of large peaks in χ(q) with Fermi surface nesting features are found, along the Γ-X and Γ-N directions in the Brillouin zone, for varying x. The peak at X and N for x=0 which apparently drives, via a soft phonon mode, the transition from the bct to the orthorhombic phase is shifted to smaller q values. These results are consistent with the semiconducting behavior of La2CuO4 and superconductivity when stabilized with x>0 additions. Finally, orbital frequencies (areas) and masses are given which show the range required for de Haas-van Alphen or other experiments.

Fundamental Thickness Limit of Itinerant Ferromagnetic SrRuO3 Thin Films

We report on a fundamental thickness limit of the itinerant ferromagnetic oxide SrRuO(3) that might arise from the orbital-selective quantum confinement effects. Experimentally, SrRuO(3) films remain metallic even for a thickness of 2 unit cells (uc), but the Curie temperature T(C) starts to decrease at 4 uc and becomes zero at 2 uc. Using the Stoner model, we attributed the T(C) decrease to a decrease in the density of states (N(o)). Namely, in the thin film geometry, the hybridized Ru d(yz,zx) orbitals are terminated by top and bottom interfaces, resulting in quantum confinement and reduction of N(o).