Ultrafast nematic-orbital excitation in FeSe

The electronic nematic phase is an unconventional state of matter that spontaneously breaks the rotational symmetry of electrons. In iron-pnictides/chalcogenides and cuprates, the nematic ordering and fluctuations have been suggested to have as-yet-unconfirmed roles in superconductivity. However, most studies have been conducted in thermal equilibrium, where the dynamical property and excitation can be masked by the coupling with the lattice. Here we use femtosecond optical pulse to perturb the electronic nematic order in FeSe. Through time-, energy-, momentum- and orbital-resolved photo-emission spectroscopy, we detect the ultrafast dynamics of electronic nematicity. In the strong-excitation regime, through the observation of Fermi surface anisotropy, we find a quick disappearance of the nematicity followed by a heavily-damped oscillation. This short-life nematicity oscillation is seemingly related to the imbalance of Fe 3dxz and dyz orbitals. These phenomena show critical behavior as a function of pump fluence. Our real-time observations reveal the nature of the electronic nematic excitation instantly decoupled from the underlying lattice

Doping-dependence of nodal quasiparticle properties in high-TcT_{\rm c} cuprates studied by laser-excited angle-resolved photoemission spectroscopy

We investigate the doping dependent low energy, low temperature ($T$ = 5 K) properties of nodal quasiparticles in the d-wave superconductor Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+\delta}$ (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near $E_{F}$, mean free paths and scattering rates ($\Gamma$) of quasiparticles. For optimally and overdoped, we obtain very sharp quasiparticle peaks of 8 meV and 6 meV full-width at half-maximum, respectively, in accord with terahertz conductivity. For all doping levels, we find the energy-dependence of $\Gamma \sim |\omega |$, while $\Gamma$($\omega =0$) shows a monotonic increase from overdoping to underdoping. The doping dependence suggests the role of electronic inhomogeneity on the nodal quasiparticle scattering at low temperature (5 K \lsim 0.07T_{\rm c}), pronounced in the underdoped region

Orbital-dependent modifications of electronic structure across magneto-structural transition in BaFe2As2

Laser angle-resolved photoemission spectroscopy (ARPES) is employed to investigate the temperature (T) dependence of the electronic structure in BaFe2As2 across the magneto-structural transition at TN ~ 140 K. A drastic transformation in Fermi surface (FS) shape across TN is observed, as expected by first-principles band calculations. Polarization-dependent ARPES and band calculations consistently indicate that the observed FSs at kz ~ pi in the low-T antiferromagnetic (AF) state are dominated by the Fe3dzx orbital, leading to the two-fold electronic structure. These results indicate that magneto-structural transition in BaFe2As2 accompanies orbital-dependent modifications in the electronic structure.Comment: 13 pages, 4 figures. accepted by Physical Review Letter

Bulk and surface-sensitive high-resolution photoemission study of Mott-Hubbard systems SrVO3_3 and CaVO3_3

We study the electronic structure of Mott-Hubbard systems SrVO$_{3}$ and CaVO$_3$ with bulk and surface-sensitive high-resolution photoemission spectroscopy (PES), using a VUV laser, synchrotron radiation and a discharge lamp ($h\nu$ = 7 - 21 eV). A systematic suppression of the density of states (DOS) within $\sim$ 0.2 eV of the Fermi level ($E_F$) is found on decreasing photon energy i.e. on increasing bulk sensitivity. The coherent band in SrVO$_{3}$ and CaVO$_3$ is shown to consist of surface and bulk derived features, separated in energy. The stronger distortion on surface of CaVO$_{3}$ compared to SrVO$_{3}$ leads to higher surface metallicity in the coherent DOS at $E_F$, consistent with recent theory.Comment: 4 pages 5 figures (including 2 auxiliary figures); A complete analysis of the spectra based on the surface and bulk analysis shows in auxiliary figures Fig. A1 and A

Co-NMR Knight Shift of NaxCoO2 \dot yH2O Studied in Both Superconducting Regions of the Tc-nuQ3 Phase Diagram Divided by the Nonsuperconducting Phase

In the temperature (T)-nuQ3 phase diagram of NaxCoO2 \dot yH2O, there exist two superconducting regions of nuQ3 separated by the nonsuperconducting region, where nuQ3 is usually estimated from the peak position of the 59Co-NQR spectra of the 5/2-7/2 transition and well-approximated here as nuQ3~3nuQ,nuQ being the interaction energy between the nuclear quadrupole moment and the electric field gradient. We have carried out measurements of the 59Co-NMR Knight shift (K) for a single crystal in the higher-nuQ3 superconducting phase and found that K begins to decrease with decreasing T at Tc for both magnetic field directions parallel and perpendicular to CoO2-planes. The result indicates together with the previous ones that the superconducting pairs are in the spin-singlet state in both phases, excluding the possibility of the spin-triplet superconductivity in this phase diagram. The superconductivity of this system spreads over the wide nuQ3 regions, but is suppressed in the narrow region located at the middle point of the region possibly due to charge instability.Comment: 8 pages, 5 figures, submitted to J. Phys. Soc. Jp

Three-dimensional bulk band dispersion in polar BiTeI with giant Rashba-type spin splitting

In layered polar semiconductor BiTeI, giant Rashba-type spin-split band dispersions show up due to the crystal structure asymmetry and the strong spin-orbit interaction. Here we investigate the 3-dimensional (3D) bulk band structures of BiTeI using the bulk-sensitive $h\nu$-dependent soft x-ray angle resolved photoemission spectroscopy (SX-ARPES). The obtained band structure is shown to be well reproducible by the first-principles calculations, with huge spin splittings of ${\sim}300$ meV at the conduction-band-minimum and valence-band-maximum located in the $k_z=\pi/c$ plane. It provides the first direct experimental evidence of the 3D Rashba-type spin splitting in a bulk compound.Comment: 9 pages, 4 figure

A Possible Phase Transition in beta-pyrochlore Compounds

We investigate a lattice of interacting anharmonic oscillators by using a mean field theory and exact diagonalization. We construct an effective five-state hopping model with intersite repulsions as a model for beta-pyrochlore AOs_2O_6(A=K, Rb or Cs). We obtain the first order phase transition line from large to small oscillation amplitude phases as temperature decreases. We also discuss the possibility of a phase with local electric polarizations. Our theory can explain the origin of the mysterious first order transition in KOs_2O_6.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp