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Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of f-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the experimental determination of the limiting cases for which MV emerges. Here we address this question for SmB6, a prototypical MV system characterized by two nearly-degenerate Sm2+ and Sm3+ configurations. By combining angle-resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS), we track the evolution of the mean Sm valence, vSm, in the SmxLa1−xB6 series. Upon substitution of Sm ions with trivalent La, we observe a linear decrease of valence fluctuations to an almost complete suppression at x = 0.2, with vSm ~ 2; surprisingly, by further reducing x, a re-entrant increase of vSm develops, approaching the value of vimp ~ 2.35 in the dilute-impurity limit. Such behaviour departs from a monotonic evolution of vSm across the whole series, as well as from the expectation of its convergence to an integer value for x → 0. Our ARPES and XAS results, complemented by a phenomenological model, demonstrate an unconventional evolution of the MV character in the SmxLa1−xB6 series, paving the way to further theoretical and experimental considerations on the concept of MV itself, and its influence on the macroscopic properties of rare-earth compounds in the dilute-to-intermediate impurity regime. This study reveals a non-monotonic evolution of the mixed-valence character in the SmxLa1−xB6 series, with near-complete suppression of valence fluctuations in the intermediate substitution regime, followed by a re-emergent mixed-valence behavior in the dilute-impurity limit. [ABSTRACT FROM AUTHOR]

Charge-ordered ground states permeate the phenomenology of 3d-based transition metal oxides, and more generally represent a distinctive hallmark of strongly correlated states of matter. The recent discovery of charge order in various cuprate families has fuelled new interest into the role played by this incipient broken symmetry within the complex phase diagram of high-Tc superconductors. Here, we use resonant X-ray scattering to resolve the main characteristics of the charge-modulated state in two cuprate families: Bi2Sr2−xLaxCuO6+δ (Bi2201) and YBa2Cu3O6+y (YBCO). We detect no signatures of spatial modulations along the nodal direction in Bi2201, thus clarifying the inter-unit-cell momentum structure of charge order. We also resolve the intra-unit-cell symmetry of the charge-ordered state, which is revealed to be best represented by a bond order with modulated charges on the O-2p orbitals and a prominent d-wave character. These results provide insights into the origin and microscopic description of charge order in cuprates, and its interplay with superconductivity. [ABSTRACT FROM AUTHOR]

Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive tool for the study of subtle charge, orbital, and spin superlattice orders driven by the valence electrons, which therefore escape detection in conventional x-ray diffraction (XRD). Although the power of REXS has been demonstrated by numerous studies of complex oxides performed in the soft x-ray regime, the cross section and photon wavelength of the material-specific elemental absorption edges ultimately set the limit to the smallest superlattice amplitude and periodicity one can probe. Here we show - with simulations and REXS on Mn-substituted Sr3Ru2O7 - that these limitations can be overcome by performing resonant scattering experiments at the absorption edge of a suitably-chosen, dilute impurity. This establishes that - in analogy with impurity-based methods used in electron-spin-resonance, nuclear-magnetic resonance, and Mössbauer spectroscopy - randomly distributed impurities can serve as a non-invasive, but now momentum-dependent probe, greatly extending the applicability of resonant x-ray scattering techniques. [ABSTRACT FROM AUTHOR]

Central to the understanding of high-temperature superconductivity is the evolution of the electronic structure as doping alters the density of charge carriers in the CuO2 planes. Superconductivity emerges along the path from a normal metal on the overdoped side to an antiferromagnetic insulator on the underdoped side. This path also exhibits a severe disruption of the overdoped normal metal’s Fermi surface. Angle-resolved photoemission spectroscopy (ARPES) on the surfaces of easily cleaved materials such as Bi2Sr2CaCu2O8+δ shows that in zero magnetic field the Fermi surface breaks up into disconnected arcs. However, in high magnetic field, quantum oscillations at low temperatures in YBa2Cu3O6.5 indicate the existence of small Fermi surface pockets. Reconciling these two phenomena through ARPES studies of YBa2Cu3O7−δ (YBCO) has been hampered by the surface sensitivity of the technique. Here, we show that this difficulty stems from the polarity and resulting self-doping of the YBCO surface. Through in situ deposition of potassium atoms on cleaved YBCO, we can continuously control the surface doping and follow the evolution of the Fermi surface from the overdoped to the underdoped regime. The present approach opens the door to systematic studies of high-temperature superconductors, such as creating new electron-doped superconductors from insulating parent compounds. [ABSTRACT FROM AUTHOR]

Competition between magnetism and the kinetic energy of mobile carriers (typically holes) in doped antiferromagnets may lead to ‘stripe’ phases, which are charged rivers separating regions of oppositely phased antiferromagnetism. In copper oxides the main experimental evidence for such coexisting static spin and charge order comes from neutron scattering in La1.48Nd0.4Sr0.12CuO4 (LNSCO; ref. 5) and La1.875Ba0.125CuO4 (LBCO; ref. 6). However, as a neutron is neutral, it does not detect charge but rather its associated lattice distortion, so it is not known whether the stripes involve ordering of the doped holes. Here we present a study of the charge order in LBCO with resonant soft X-ray scattering (RSXS). We observe giant resonances near the Fermi level as well as near the correlated gap, demonstrating significant modulation in both the doped-hole density and the ‘Mottness’, or the degree to which the system resembles a Mott insulator. The peak-to-trough amplitude of the valence modulation is estimated to be 0.063 holes, which suggests an integrated area of 0.59 holes under a single stripe, close to the expected 0.5 for half-filled stripes. [ABSTRACT FROM AUTHOR]

Isotopic shift (IS) and hyperfine structure (hfs) measurements were performed in 15 lines to the ground configurations (5 d+6 s) of iridium. In a parametric description according to Bauche and Champeau accurate IS parameters were determined, and together with the IS parameters of 5 other 5 d-elements, systematic trends could be noticed. The hfs analysis (with the help of the effective operator method of Sandars and Beck) yielded reliable radial integrals which permit a comparison with data from various self consistent methods. From the IS parameter s and the hfs parameter a an accurate value for the nuclear parameter λ=0.044(4) fm could be derived. [ABSTRACT FROM AUTHOR]

We discuss the separation problem of bulk vs interface contributions in optical Second-Harmonic Generation (SHG). A new method is presented, which - in special cases - provides a full separation of all interface and bulk tensor components. The method is based on measurements in a thin-film geometry, and the relation between two bulk tensor components obtained from group-theoretical arguments. The separation problem in its strict sense is shown to be absent in Magnetization-induced SHG (MSHG). Though, in practice, also in the MSHG case interface and bulk contributions can be closely resemblant. The approach is illustrated by second-harmonic experiments on C thin films. SHG in this system is found to be completely dominated by bulk processes. [ABSTRACT FROM AUTHOR]

Isotope shift (IS) and hyperfine structure (hfs) measurements have been performed on seven lines of the platinum I spectrum with interference as well as laser fluorescence spectroscopy. In the latter case a frequency-doubled single-mode cw dye laser was applied. The IS ofPt with a natural abundance of only 0.01% was determined to be δν=−38.65(8) mK in the λ306.47 nm transition. The IS parameters and the effective hfs integrals in the configuration (5 d+6 s) were determined in intermediate coupling by a least squares fit of the IS and hfs data with eigenvectors obtained from the platinum fine structure (fs). The results are compared with theoretical values and yield improved δ〈 r〉 data. [ABSTRACT FROM AUTHOR]

Presents research which investigated magnetization reversal in response to a change in temperature in YVO3. Factors in the lowest total energy of a magnet in a magnetic field; Reversal of magnetization in ferrimagnetic materials; Cause of the net magnetic moment in YVO3; Abrupt switching at 77K; Gradual reversal at approximately 95K; Possible change in orbital ordering in transition metal oxides.

We present a combined resonant soft X-ray reflectivity and electric transport study of LaAlO 3 / SrTiO 3 field effect devices. The depth profiles with atomic layer resolution that are obtained from the resonant reflectivity reveal a pronounced temperature dependence of the two-dimensional electron liquid at the LaAlO 3 / SrTiO 3 interface. At room temperature the corresponding electrons are located close to the interface, extending down to 4 unit cells into the SrTiO 3 substrate. Upon cooling, however, these interface electrons assume a bimodal depth distribution: They spread out deeper into the SrTiO 3 and split into two distinct parts, namely one close to the interface with a thickness of about 4 unit cells and another centered around 9 unit cells from the interface. The results are consistent with theoretical predictions based on oxygen vacancies at the surface of the LaAlO 3 film and support the notion of a complex interplay between structural and electronic degrees of freedom. [ABSTRACT FROM AUTHOR]

Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive tool for the study of subtle charge, orbital, and spin superlattice orders driven by the valence electrons, which therefore escape detection in conventional x-ray diffraction (XRD). Although the power of REXS has been demonstrated by numerous studies of complex oxides performed in the soft x-ray regime, the cross section and photon wavelength of the material-specific elemental absorption edges ultimately set the limit to the smallest superlattice amplitude and periodicity one can probe. Here we show - with simulations and REXS on Mn-substituted Sr3Ru2O7 - that these limitations can be overcome by performing resonant scattering experiments at the absorption edge of a suitably-chosen, dilute impurity. This establishes that - in analogy with impurity-based methods used in electron-spin-resonance, nuclear-magnetic resonance, and Mössbauer spectroscopy - randomly distributed impurities can serve as a non-invasive, but now momentum-dependent probe, greatly extending the applicability of resonant x-ray scattering techniques. [ABSTRACT FROM AUTHOR]

Competition between magnetism and the kinetic energy of mobile carriers (typically holes) in doped antiferromagnets may lead to ‘stripe’ phases, which are charged rivers separating regions of oppositely phased antiferromagnetism. In copper oxides the main experimental evidence for such coexisting static spin and charge order comes from neutron scattering in La1.48Nd0.4Sr0.12CuO4 (LNSCO; ref. 5) and La1.875Ba0.125CuO4 (LBCO; ref. 6). However, as a neutron is neutral, it does not detect charge but rather its associated lattice distortion, so it is not known whether the stripes involve ordering of the doped holes. Here we present a study of the charge order in LBCO with resonant soft X-ray scattering (RSXS). We observe giant resonances near the Fermi level as well as near the correlated gap, demonstrating significant modulation in both the doped-hole density and the ‘Mottness’, or the degree to which the system resembles a Mott insulator. The peak-to-trough amplitude of the valence modulation is estimated to be 0.063 holes, which suggests an integrated area of 0.59 holes under a single stripe, close to the expected 0.5 for half-filled stripes. [ABSTRACT FROM AUTHOR]