arXiv subjects

S. Suga

Publications and source records attributed to S. Suga.

At least 37 records · Page 2Linked to original sources

arXiv2009

ARPES spectral functions and Fermi surface for La(1.86)Sr(0.14)CuO(4) compared with LDA+DMFT+Sk calculations

Slightly underdoped high-Tc system La(1.86)Sr(0.14)CuO(4) (LSCO) is studied by means of high energy high resolution angular resolved photoemission spectroscopy (ARPES) and combined computational scheme LDA+DMFT+Sk. Corresponding one band Hubbard model is solved via dynamical mean-field theory (DMFT), while model parameters needed are obtained from first principles within local density approximation (LDA). An "external" k-dependent self-energy Sk describes interaction of correlated electrons with antiferromagnetic (AFM) pseudogap fluctuations. Experimental and theoretical data clearly show "destruction" of the LSCO Fermi surface in the vicinity of the (pi,0) point and formation of "Fermi arcs" in the nodal directions. ARPES energy distribution curves (EDC) as well as momentum distribution curves (MDC) demonstrate deviation of the quasiparticle band from the Fermi level around (pi,0) point. The same behavior of spectral functions follows from theoretical calculations suggesting AFM origin of the pseudogap state.

cond-mat.str-el↗

arXiv2009

Magnetic Anisotropy and Magnetization Dynamics of Individual Atoms and Clusters of Fe and Co on Pt(111)

The recently discovered giant magnetic anisotropy of single magnetic Co atoms raises the hope of magnetic storage in small clusters. We present a joint experimental and theoretical study of the magnetic anisotropy and the spin dynamics of Fe and Co atoms, dimers, and trimers on Pt(111). Giant anisotropies of individual atoms and clusters as well as lifetimes of the excited states were determined with inelastic scanning tunneling spectroscopy. The short lifetimes due to hybridization-induced electron-electron scattering oppose the magnetic stability provided by the magnetic anisotropies.

cond-mat.mtrl-sci↗

arXiv2008

Zero-temperature Phase Diagram of Two Dimensional Hubbard Model

We investigate the two-dimensional Hubbard model on the triangular lattice with anisotropic hopping integrals at half filling. By means of a self-energy functional approach, we discuss how stable the non-magnetic state is against magnetically ordered states in the system. We present the zero-temperature phase diagram, where the normal metallic state competes with magnetically ordered states with $(π, π)$ and $(2π/3, 2π/3)$ structures. It is shown that a non-magnetic Mott insulating state is not realized as the ground state, in the present framework, but as a meta-stable state near the magnetically ordered phase with $(2π/3, 2π/3)$ structure.

cond-mat.str-el↗

arXiv2008

Origin of "hot-spots" in the pseudogap regime of Nd(1.85)Ce(0.15)CuO(4): LDA+DMFT+Sigma_k study

Material specific electronic band structure of the electron-doped high-Tc cuprate Nd(1.85)Ce(0.15)CuO(4) (NCCO) is calculated within the pseudo gap regime, using the recently developed generalized LDA+DMFT+Sigma_k scheme. LDA/DFT (density functional theory within local density approximation) provides model parameters (hopping integral values, local Coulomb interaction strength) for the one-band Hubbard model, which is solved by DMFT (dynamical mean-field theory). To take into account pseudogap fluctuations LDA+DMFT is supplied with "external" k-dependent self-energy Sigma_k, which describes interaction of correlated conducting electrons with non-local Heisenberg-like antiferromagnetic (AFM) spin fluctuations responsible for pseudo gap formation. Within this LDA+DMFT+Sigma_k approach we demonstrate the formation of pronounced "hot-spots" on the Fermi surface (FS) map in NCCO, opposite to our recent calculations for Bi(2)Sr(2)CaCu(2)O(8-d) (Bi2212), which have produced rather extended region of FS "destruction". There are several physical reasons for this fact: (i) the "hot-spots" in NCCO are located closer to Brillouin zone center; (ii) correlation length of AFM fluctuations ξis larger for NCCO; (iii) pseudogap potential Δis stronger, than in Bi2212. Comparison of our theoretical data with recent bulk sensitive high-energy angle-resolved photoemission (ARPES) data for NCCO provides good semiquantitative agreement. Based on that comparison alternative explanation of the van-Hove singularity at -0.3 eV is proposed. Optical conductivity both for Bi2212 and NCCO is also calculated within LDA+DMFT+Sigma_k and compared with experimental results, demonstrating satisfactory agreement.

cond-mat.str-el↗

arXiv2007

Electronic structures of CeRu$_2X_2$ ($X$ = Si, Ge) in the paramagnetic phase studied by soft X-ray ARPES and hard X-ray photoelectron spectroscopy

Soft and hard X-ray photoelectron spectroscopy (PES) has been performed for one of the heavy fermion system CeRu$_2$Si$_2$ and a $4f$-localized ferromagnet CeRu$_2$Ge$_2$ in the paramagnetic phase. The three-dimensional band structures and Fermi surface (FS) shapes of CeRu$_2$Si$_2$ have been determined by soft X-ray $hν$-dependent angle resolved photoelectron spectroscopy (ARPES). The differences in the Fermi surface topology and the non-$4f$ electronic structures between CeRu$_2$Si$_2$ and CeRu$_2$Ge$_2$ are qualitatively explained by the band-structure calculation for both $4f$ itinerant and localized models, respectively. The Ce valences in CeRu$_2X_2$ ($X$ = Si, Ge) at 20 K are quantitatively estimated by the single impurity Anderson model calculation, where the Ce 3d hard X-ray core-level PES and Ce 3d X-ray absorption spectra have shown stronger hybridization and signature for the partial $4f$ contribution to the conduction electrons in CeRu$_2$Si$_2$.

cond-mat.str-el↗

arXiv2007

Three-Dimensional Bulk Electronic Structures of Ca1.5Sr0.5RuO4 Studied by Soft X-ray Angle-Resolved Photoemission

We report on experimental data of the three-dimensional bulk Fermi surfaces of the layered strongly correlated Ca1.5Sr0.5RuO4 system. The measurements have been performed by means of hn-depndent bulk-sensitive soft x-ray angle-resolved photoemission technique. Our experimental data evinces the bulk Fermi surface topology at kz~0 to be qualitatively different from the one observed by surface-sensitive low-energy ARPES. Furthermore, stronger kz dispersion of the circle-like gamma Fermi surface sheet is observed compared with Sr2RuO4. Thus in the paramagnetic metal phase, Ca1.5Sr0.5RuO4 compound is found to have rather three-dimensional electronic structure.

cond-mat.str-el↗

arXiv2006

The case for the bulk nature of the spectroscopic Luttinger liquid signatures observed in angle resolved photoemission of Li0.9Mo6O17

Angle resolved photoemission spectroscopy (ARPES) has been performed on quasi-one dimensional Li0.9Mo6O17 using photon energy 500 eV. Measured band dispersions are in agreement with those from both low photon energy measurements and band structure calculations. The momentum integrated ARPES spectrum is well fit by the finite temperature Luttinger liquid (LL)spectral function, with an anomalous exponent 0.6 that is the same within experimental uncertainty as the value found with photon energy 30 eV. These identical findings at both low and high photon energies are entirely consistent with reasoning based on the crystal structure, that the quasi-one dimensional chains lie two layers below the cleavage plane so that the observed spectroscopic LL behavior of Li0.9Mo6O17 is a bulk property.

cond-mat.str-el↗

arXiv2006

Photoemission study of (V$_{1-x}$M$_x$)$_2$O$_3$ (M=Cr, Ti)

We present high-resolution bulk-sensitive photoemission spectra of (V$_{1-x}$M$_x$)$_2$O$_3$ (M=Cr, Ti). The measurements were made for the paramagnetic metal (PM), paramagnetic insulator (PI), and antiferromagnetic insulator (AFI) phases of (V$_{1-x}$M$_x$)$_2$O$_3$ with the samples of $x$ = 0, 0.012, and 0.028 for Cr-doping and $x$ = 0.01 for Ti-doping. In the PM phase, we observe a prominent quasiparticle peak in general agreement with theory, which combines dynamical mean-field theory with the local density approximation (LDA+DMFT). The quasiparticle peak shows a significantly larger peak width and weight than in the theory. For both the PI and AFI phases, the vanadium 3d parts of the valence spectra are not simple one peak structures. For the PI phase, there is not yet a good theoretical understanding of these structures. The size of the electron removal gap increases, and spectral weight accumulates in the energy range closer to the chemical potential, when the PI to AFI transition occurs. Spectra taken in the same phases with different compositions show interesting monotonic changes as the dopant concentration increases, regardless of the dopant species. With increased Cr-doping, the AFI phase gap decreases and the PI phase gap increases.

cond-mat.str-el↗

arXiv2006

Static vs. dynamical mean field theory of Mott antiferromagnets

Studying the antiferromagnetic phase of the Hubbard model by dynamical mean field theory, we observe striking differences with static (Hartree-Fock) mean field: The Slater band is strongly renormalized and spectral weight is transferred to spin-polaron side bands. Already for intermediate values of the interaction $U$ the overall bandwidth is larger than in Hartree-Fock, and the gap is considerably smaller. Such differences survive any renormalization of $U$. Our photoemission experiments for Cr-doped V$_2$O$_3$ show spectra qualitatively well described by dynamical mean field theory.

cond-mat.str-el↗

arXiv2006

Observation of three-dimensional bulk Fermi surfaces for a strongly correlated material by soft x-ray $hν$-dependent (700-860 eV) ARPES

Three-dimensional Fermi surfaces at a high temperature have been clarified for a strongly correlated Ce compound, ferromagnet CeRu$_2$Ge$_2$ in the paramagnetic phase, by virtue of a soft x-ray $hν$-dependent (700-860 eV) ARPES. Although the observed Fermi surfaces as well as quasiparticle dispersions are partly explained by a band-structure calculation based on a localized $4f$ model, qualitative discrepancy in experiments, between our APRES in the paramagnetic phase and de Haas-van Alphen measurement in the ferromagnetic phase, is revealed. This suggests a fundamental change in the $4f$ contribution to the Fermi surfaces across the magnetic phase transition widely seen for Ce compounds.

cond-mat.str-el↗

arXiv2006

Evidence for strong electronic correlations in the spectra of Sr$_2$RuO$_4$

The importance of electronic correlation effects in the layered perovskite Sr$_2$RuO$_4$ is evidenced. To this end we use state-of-the-art LDA+DMFT (Local Density Approximation + Dynamical Mean-Field Theory) in the basis of Wannier functions to compute spectral functions and the quasiparticle dispersion of Sr$_2$RuO$_4$. The spectra are found to be in good agreement with various spectroscopic experiments. We also calculate the $\textbf{k}$-dependence of the quasiparticle bands and compare the results with new angle resolved photoemission (ARPES) data. Two typical manifestations of strong Coulomb correlations are revealed: (i) the calculated quasiparticle mass enhancement of $m^*/m \approx2.5$ agrees with various experimental results, and (ii) the satellite structure at about 3 eV binding energy observed in photoemission experiments is shown to be the lower Hubbard band. For these reasons Sr$_2$RuO$_4$ is identified as a strongly correlated 4$d$ electron material.

cond-mat.str-el↗

arXiv2005

Tomonaga-Luttinger Liquid in a Quasi-One-Dimensional S=1 Antiferromagnet Observed by the Specific Heat

Specific heat experiments on single crystals of the S=1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C_9H_24N_4)(NO_2)ClO_4, alias NTENP, have been performed in magnetic fields applied both parallel and perpendicular to the spin chains. We have found for the parallel field configuration that the magnetic specific heat (C_mag) is proportional to temperature (T) above a critical field H_c, at which the energy gap vanishes, in a temperature region above that of the long-range ordered state. The ratio C_mag/T increases as the magnetic field approaches H_c from above. The data are in good quantitative agreement with the prediction of the c=1 conformal field theory in conjunction with the velocity of the excitations calculated by a numerical diagonalization, providing a conclusive evidence for a Tomonaga-Luttinger liquid.

cond-mat.str-el↗

arXiv2005

High-energy photoemission on Fe3O4: Small polaron physics and the Verwey transition

We have studied the electronic structure and charge ordering (Verwey) transition of magnetite (Fe3O4) by soft x-ray photoemission. Due to the enhanced probing depth and the use of different surface preparations we are able to distinguish surface and volume effects in the spectra. The pseudogap behavior of the intrinsic spectra and its temperature dependence give evidence for the existence of strongly bound small polarons consistent with both dc and optical conductivity. Together with other recent structural and theoretical results our findings support a picture in which the Verwey transition contains elements of a cooperative Jahn-Teller effect, stabilized by local Coulomb interaction.

cond-mat.str-el↗

arXiv2005

Charge dynamics in strongly correlated one-dimensional Cu-O chain systems revealed by inelastic X-ray scattering

We report on the Cu 1s resonant inelastic X-ray scattering (RIXS) of Cu-O one-dimensional (1D) strongly correlated insulator systems with contrasting atomic arrangements, namely edge-sharing CuGeO3 and corner-sharing Sr2CuO3. Owing to good statistics of the high-resolution RIXS data, so far unresolved fine structures are revealed. Detailed photon-energy and momentum dependence of the RIXS spectra in comparison with theoretical calculations has clarified the natures of the low-energy charge excitations and hybridization of the electronic states.

cond-mat.str-el↗

arXiv2005

Spin excitations under fields in an anisotropic bond-alternating quantum S=1 chain: contrast with Haldane spin chains

Inelastic neutron scattering experiments on the S=1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4 have been performed under magnetic fields below and above a critical field Hc at which the energy gap closes. Normal field dependece of Zeeman splitting of the excited triplet modes below Hc has been observed, but the highest mode is unusually small and smears out with increasing field. This can be explained by an interaction with a low-lying two magnon continuum at q=pi that is present in dimerized chains but absent in uniform ones. Above Hc, we find only one excited mode, in stark contrast with three massive excitations previously observed in the structurally similar Haldane-gap material NDMAP [A. Zheludev et al., Phys. Rev. B 68, 134438 (2003)].

cond-mat.str-el↗

arXiv2004

High-resolution soft x-ray photoemission study of a Kondo semiconductor and related compounds

We have performed the bulk-sensitive high-resolution soft x-ray photoemission study of a Kondo semiconductor CeRhAs and related compounds CeNiSn and CePdSn. The comparison of the spectra of polycrystalline CePdSn on the fractured and scraped surfaces shows that the fracturing of the samples is much better than the scraping in order to obtain intrinsic photoemission spectra. The Ce 4d core-level spectra show clear differences in the electronic states among the materials.

cond-mat.str-el↗

arXiv2004

Technique for bulk Fermiology by photoemission applied to layered ruthenates

We report the Fermi surfaces of the superconductor Sr2RuO4 and the non-superconductor Sr1.8Ca0.2RuO4 probed by bulk-sensitive high-energy angle-resolved photoemission. It is found that there is one square-shaped hole-like, one square-shaped electron-like and one circle-shaped electron-like Fermi surface in both compounds. These results provide direct evidence for nesting instability giving rise to magnetic fluctuations. Our study clarifies that the electron correlation effects are changed with composition depending on the individual band.

cond-mat.str-el↗

arXiv2004

Mutual Experimental and Theoretical Validation of Bulk Photoemission Spectra of Sr1-xCaxVO3

We report high-resolution high-energy photoemission spectra together with parameter-free LDA+DMFT (local density approximation + dynamical mean-field theory) results for Sr1-xCaxVO3, a prototype 3d1 system. In contrast to earlier investigations the bulk spectra are found to be insensitive to x. The good agreement between experiment and theory confirms the bulk-sensitivity of the high-energy photoemission spectra.

cond-mat.str-el↗