arXiv subjects

H. Ishida

Publications and source records attributed to H. Ishida.

26 records · Page 2Linked to original sources

arXiv2008

Multisite versus multiorbital Coulomb correlations studied within finite-temperature exact diagonalization dynamical mean-field theory

The influence of short-range Coulomb correlations on the Mott transition in the single-band Hubbard model at half-filling is studied within cellular dynamical mean field theory for square and triangular lattices. Finite-temperature exact diagonalization is used to investigate correlations within two-, three-, and four-site clusters. Transforming the non-local self-energy from a site basis to a molecular orbital basis, we focus on the inter-orbital charge transfer between these cluster molecular orbitals in the vicinity of the Mott transition. In all cases studied, the charge transfer is found to be small, indicating weak Coulomb induced orbital polarization despite sizable level splitting between orbitals. These results demonstrate that all cluster molecular orbitals take part in the Mott transition and that the insulating gap opens simultaneously across the entire Fermi surface. Thus, at half-filling we do not find orbital-selective Mott transitions, nor a combination of band filling and Mott transition in different orbitals. Nevertheless, the approach towards the transition differs greatly between cluster orbitals, giving rise to a pronounced momentum variation along the Fermi surface, in agreement with previous works. The near absence of Coulomb induced orbital polarization in these clusters differs qualitatively from single-site multi-orbital studies of several transition metal oxides, where the Mott phase exhibits nearly complete orbital polarization as a result of a correlation driven enhancement of the crystal field splitting. The strong single-particle coupling among cluster orbitals in the single-band case is identified as the source of this difference.

cond-mat.str-el↗

arXiv2007

The Mott insulator LaTiO_3 in heterostructures with SrTiO_3 is metallic

It is shown that LaTiO_3 in superlattices with SrTiO_3 is not a Mott insulator but a strongly correlated metal. The tetragonal lattice geometry imposed by the SrTiO_3 substrate leads to an increase of the Ti 3d t2g band width and a reversal of the t2g crystal field relative to the orthorhombic bulk geometry. Using dynamical mean field theory based on finite-temperature multi-band exact diagonalization we show that, as a result of these effects, local Coulomb interactions are not strong enough to induce a Mott transition in tetragonal LaTiO_3. The metalicity of these heterostructures is therefore not an interface property but stems from all LaTiO_3 planes.

cond-mat.str-el↗

arXiv2007

Coulomb correlations do not fill the e'_g hole pockets in Na_{0.3}CoO_2

There exists presently considerable debate over the question whether local Coulomb interactions can explain the absence of the small e'_g Fermi surface hole pockets in photoemission studies of Na_{0.3}CoO_2. By comparing dynamical mean field results for different single particle Hamiltonians and exact diagonalization as well as quantum Monte Carlo treatments, we show that, for realistic values of the Coulomb energy U and Hund exchange J, the e'_g pockets can be slightly enhanced or reduced compared to band structure predictions, but they do not disappear.

cond-mat.str-el↗

arXiv2006

Subband filling and Mott transition in Ca_{2-x}Sr_xRuO_4

A new concept is proposed for the paramagnetic metal insulator transition in the layer perovskite Ca_{2-x}Sr_xRuO_4. Whereas the pure Sr compound is metallic up to very large Coulomb energies due to strong orbital fluctuations, structural changes induced by doping with Ca give rise to a interorbital charge transfer which makes the material extremely sensitive to local correlations. Using dynamical mean field theory based on finite temperature multi-band exact diagonalization it is shown that the combination of crystal field splitting and onsite Coulomb interactions leads to complete filling of the d_xy band and to a Mott transition in the half-filled d_xz,yz bands.

cond-mat.str-el↗

arXiv2006

Exact Diagonalization Dynamical Mean Field Theory for Multi-Band Materials: Effect of Coulomb correlations on the Fermi surface of Na_0.3CoO_2

Dynamical mean field theory combined with finite-temperature exact diagonalization is shown to be a suitable method to study local Coulomb correlations in realistic multi-band materials. By making use of the sparseness of the impurity Hamiltonian, exact eigenstates can be evaluated for significantly larger clusters than in schemes based on full diagonalization. Since finite-size effects are greatly reduced this approach allows the study of three-band systems down to very low temperatures, for strong local Coulomb interactions and full Hund exchange. It is also shown that exact diagonalization yields smooth subband quasi-particle spectra and self-energies at real frequencies. As a first application the correlation induced charge transfer between t2g bands in Na_0.3CoO_2 is investigated. For both Hund and Ising exchange the small eg' Fermi surface hole pockets are found to be slightly enlarged compared to the non-interacting limit, in agreement with previous Quantum Monte Carlo dynamical mean field calculations for Ising exchange, but in conflict with photoemission data.

cond-mat.str-el↗

arXiv2005

Effect of Dynamical Coulomb Correlations on the Fermi Surface of Na_0.3CoO_2

The t2g quasi-particle spectra of Na_0.3CoO_2 are calculated within the dynamical mean field theory. It is shown that as a result of dynamical Coulomb correlations charge is transfered from the nearly filled e_g' subbands to the a_1g band, thereby reducing orbital polarization among Co t2g states. Dynamical correlations therefore stabilize the small e_g' Fermi surface pockets, in contrast to angle-resolved photoemission data, which do not reveal these pockets.

cond-mat.str-el↗

arXiv2004

Coulomb Correlations and Orbital Polarization in the Metal Insulator Transition of VO_2

The quasi-particle spectra in the metallic rutile and insulating monoclinic phases of VO$_2$ are shown to be dominated by local Coulomb interactions. In the rutile phase the small orbital polarization among V 3d t_2g states leads to weak static but strong dynamical correlations. In the monoclinic phase the large 3d orbital polarization caused by the V--V Peierls distortion gives rise to strong static correlations which are shown to be the primary cause of the insulating behavior.

cond-mat.str-el↗

arXiv2004

Spin Fluctuation and Crystal Field Excitation of a Heavy Fermion Compound YbAgGe studied by Inelastic Neutron Scattering

Inelastic neutron scattering experiment has been performed on a new heavy fermion compound YbAgGe for a polycrystalline sample at zero magnetic field. A quasielastic scattering and a crystal field excitation at 12 meV was observed. Both of them are broadened with an intrinsic width of 0.9 meV at 1.5 K which increases to about 3 meV at 300 K. The temperature dependence follows Gamma_0+AT^(1/2) typically observed in Kondo systems. Crystal field parameters were also deduced. To explain the result that only one excitation isobserved at 12 meV, it is concluded that the B22 term should be the main component.

cond-mat.str-el↗