We investigated electronic structure of hexagonal multiferroic YMnO3 using the polarization dependent x-ray absorption spectroscopy (XAS) at O K and Mn L(2,3) edges. The spectra exhibit strong polarization dependence at both edges, reflecting anisotropic Mn 3d orbital occupation. Moreover, the O K edge spectra show that Y 4d states are strongly hybridized with O 2p ones, resulting in large anomalies in Born effective charges on off-centering Y and O ions. These results manifest that the Y d(0)-ness with rehybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites.

We report a quantum ring-like toroidal cavity naturally formed in a vertical-cavity-like active microdisk plane due to Rayleigh's band of whispering gallery modes. The √ T -dependent redshift and a square-law property of microampere-range threshold currents down to 2 µA are consistent with a photonic quantum wire view, due to whispering gallery mode-induced dimensional reduction.PACS number: 42.55. Sa, 42.60.Da For the last several years, there have been intensive developments of microdisk semiconductor lasers of whispering gallery(WG) modes for low-power and high-density photonic array applications. These efforts of earlier photo-pumped thumbtack-type WG lasers [1] have been evolving to photonic-wire lasers [2] and electro-pumped thumbtack-type WG lasers [3,4] of submilliampere threshold currents.In this letter, we demonstrate a cylindrical vertical-cavity surface-emitting laser(VCSEL)-like diode that exhibits WG modes with unusual photonic quantum ring(PQR) characteristics such as µA-range threshold currents and spectral √ Tdependence, in addition to the usual VCSEL mode. We will thus illustrate and analyze the two-threshold behavior of successive lasings, first the PQR and then the VC-SEL as well. Indeed, the quantum wire behavior of the PQR is vividly demonstrated in the √ T -dependent spectral peak shift data. We note that the quantum-ring-like WG modes are naturally formed in the circumferential Rayleigh band region [5] of the active microdisk of a regular multi-quantum-well VCSEL-like structure but without any intentional and real quantum ring patterning.The PQR device differs from the previous WG lasers [ [1][2][3][4]6] in that the vertical confinement is improved by the top and bottom DBR layers, and that stripe or segmented top electrodes are used for vertical output coupling. The metal-organic vapor-phase epitaxy-grown device employs a one-λ thick active layer with four 80 A Al 0.11 Ga 0.89 As quantum wells(QWs), whose details have been described elsewhere [8,9,12]. In the active disk plane, the PQR region is defined by Rayleigh's bandwidth, W Rayleigh = (φ/2)(1 − n ef f /n), where the WG mode occurs, which is well described in Ref. [7]: Rayleigh's annular band is defined by the active disk's radius R(= φ/2) for outer boundary and the inner reflection point r in = Rn M /n, where n is the refractive index of the active medium and n M is the effective refractive An exploded segment shows the effective toroidal structure of the PQR, where R is the index-guided outer limit and rin is the inner reflection limit which may also be reinforced by gain guiding. The radial evanescent field results from forward and backward helical traveling waves, and its intensity peaks at a distance t ≤ 1 µm from the outer boundary.The calculated profile of the annular WG emission based on Rayleigh's Bessel function analysis is shown in Fig. 1(a) for our device with R = 7.5 µm. The azimuthal mode number is rather large here, M = 2πRn ef f /λ = 188, and hence the 376 azimuthal peaks are not well distinguishable, unlike the i...

Quasiparticle tunneling spectra of the electron-doped (n-type) infinite-layer cuprate Sr 0.9 La 0.1 CuO 2 reveal characteristics that counter a number of common phenomena in the hole-doped (p-type) cuprates. The optimally doped Sr 0.9 La 0.1 CuO 2 with T c 43 K exhibits a momentum-independent superconducting gap D 13.0 6 1.0 meV that substantially exceeds the BCS value, and the spectral characteristics indicate insignificant quasiparticle damping by spin fluctuations and the absence of pseudogap. The response to quantum impurities in the Cu sites also differs fundamentally from that of the p-type cuprates with d x 2 2y 2 -wave pairing symmetry. 74.50.+r, 74.62.Dh The predominantly d x 2 2y 2 pairing symmetry [1,2], the existence of spin fluctuations in the CuO 2 planes [3,4], and the pseudogap phenomena [3][4][5] [7,8], and it has been suggested that the pairing symmetry in the one-layer n-type cuprates may change from d x 2 2y 2 to s, depending on the electron doping level [9]. The nonuniversal pairing symmetries in cuprate superconductors imply that the symmetry is likely the result of competing orders rather than a sufficient condition for pairing. Nonetheless, an important consequence of either d x 2 2y 2 or ͑d x 2 2y 2 1 s͒-wave pairing is that the resulting nodal quasiparticles can interact strongly with the quantum impurities in the CuO 2 planes [10,11], such that a small concentration of impurities can give rise to strong suppression of superconductivity and modification of the collective Cu 21 spin excitations [6,[12][13][14][15][16][17]. In addition, Kondo effects could be induced by nonmagnetic impurities through breaking the nearestneighbor antiferromagnetic Cu 21 -Cu 21 interaction [18]. Such strong response to nonmagnetic impurities is in sharp contrast to conventional s-wave superconductivity [19,20]. Despite significant progress in the studies of cuprate superconductivity, the research on the simplest form of cuprates, the infinite-layer system Sr 12x L x CuO 2 (L La, Gd, Sm), has been limited [21][22][23] superconducting volume and a sharp superconducting transition temperature at T c 43 K, thus enabling reliable spectroscopic studies of the pairing symmetry and the effects of quantum impurities. These single-phased infinitelayer cuprates are n-type with P4͞mmm symmetry, which differ significantly from other cuprates in that no excess charge reservoir block exists between consecutive CuO 2 planes except a single layer of Sr(La), as illustrated in Fig. 1(a), suggesting stronger CuO 2 interplanar coupling. Furthermore, the c-axis superconducting coherence length ͑j c 0.53 nm͒ is found to be longer than the c-axis lattice constant ͑c 0 0.347 nm͒ [25], in stark contrast to other cuprate superconductors with j c ø c 0 . Hence, the superconducting properties of the infinite-layer system are expected to be more three-dimensional, as opposed to the quasi-two-dimensional nature of all other cuprates. In this Letter, we report experimental findings based on the scanning tunneling spectroscopy studies...

A c-axis oriented thin film and a high density sintered pellet of MgB 2 have been studied by x-ray photoemission spectroscopy, and compared to measurements from MgO and MgF 2 single crystals. The as-grown surface has a layer which is Mg-rich and oxidized, which is effectively removed by a nonaqueous etchant. The subsurface region of the pellet is Mgdeficient. This nonideal near-surface region may explain varied scanning tunneling spectroscopy results. The MgB 2 core level and Auger signals are similar to measurements from metallic Mg and transition metal diborides, and the measured valence band is consistent with the calculated density of states.

We investigated the magnetic nature of Fe(1/4)TaS2 using x-ray absorption spectroscopy, photoemission spectroscopy, and first principles band calculations. The results show a large unquenched orbital magnetic moment (∼1.0  μ(B)/Fe) at intercalated Fe sites, resulting in a gigantic magnetic anisotropy (H(A)≃60  T). The magnetic coupling is well understood in terms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, suggesting a novel RKKY ferromagnet with Ising-type spin states. We also found that this indirect exchange coupling between the neighboring Fe spins is ferromagnetic and maximized at the Fe-Fe distance of 2×2 superstructure.