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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]

Determining the nature of the electronic phases that compete with superconductivity in high-transition-temperature (high- Tc) superconductors is one of the deepest problems in condensed matter physics. One candidate is the `stripe' phase, in which the charge carriers (holes) condense into rivers of charge that separate regions of antiferro magnetism. A related but lesser known system is the `spin ladder', which consists of two coupled chains of magnetic ions forming an array of rungs. A doped ladder can be thought of as a high-Tc material with lower dimensionality, and has been predicted to exhibit both superconductivity and an insulating `hole crystal' phase in which the carriers are localized through many-body interactions. The competition between the two resembles that believed to operate between stripes and superconductivity in high-Tc materials. Here we report the existence of a hole crystal in the doped spin ladder of Sr14Cu24O41 using a resonant X-ray scattering technique. This phase exists without a detectable distortion in the structural lattice, indicating that it arises from many-body electronic effects. Our measurements confirm theoretical predictions, and support the picture that proximity to charge ordered states is a general property of superconductivity in copper oxides. [ABSTRACT FROM AUTHOR]

Multiferroicity in LiCu2O2 single crystals is studied using resonant soft x-ray magnetic scattering, hard x-ray diffraction, heat capacity, magnetic susceptibility, and electrical polarization. Two magnetic transitions are found at 24.6 K (T1) and 23.2 K (T2). Our data are consistent with a sinusoidal spin structure at T21 and with a helicoidal spin structure at T2, giving rise to ferroelectricity. Surprisingly, above T2, the correlation lengths of the spin structures increase as the temperature increases with dramatic changes of ∼42% occurring along the c axis [ABSTRACT FROM AUTHOR]

We report a study of the pressure-induced antiferrodistortive cubic-to-tetragonal phase transition in strontium titanate (SrTiO3) at ambient temperature. High-resolution inelastic x-ray scattering measurements reveal the softening of a phonon mode at the zone boundary; a static distortion sets in at a critical pressure of 9.5 GPa, which corresponds to a critical volume reduction of 5.3%. Prior studies have shown that similar phonon softening and ensuing lattice distortion can be induced under ambient pressure by lowering the sample temperature through a critical temperature of 105 K. The relationship between the two phase transitions is clarified by comparing the power laws of the pressure and temperature dependences of the softening behavior. The results are explained in terms of the analytic properties of the soft mode. [ABSTRACT FROM AUTHOR]

We report temperature-dependent transport and x-ray diffraction measurements of the influence of Ti hole doping on the charge density wave (CDW) in 1T-Ta1-xTixS2. Confirming past studies, we find that even trace impurities eliminate the low-temperature commensurate (C) phase in this system. Surprisingly, the magnitude of the in-plane component of the CDW wave vector in the nearly commensurate (NC) phase does not change significantly with Ti concentration, as might be expected from a changing Fermi surface volume. Instead, the angle of the CDW in the basal plane rotates, from 11.9° at x = 0 to 16.4° at x = 0.12. Ti substitution also leads to an extended region of coexistence between incommensurate (IC) and NC phases, indicating heterogeneous nucleation near the transition. Finally, we explain a resistive anomaly originally observed by Di Salvo [F. J. Di Salvo et al., Phys. Rev. B 12, 2220 (1975)] as arising from pinning of the CDW on the crystal lattice. Our study highlights the importance of commensuration effects in the NC phase, particularly at x ~ 0.08. [ABSTRACT FROM AUTHOR]

Superconductivity in so-called unconventional superconductors is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between superconductivity and fluctuations in some other order parameter. To better understand this connection, we used high-pressure X-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe2, which was previously shown to exhibit superconductivity when the CDW is suppressed by pressure or intercalation of Cu atoms. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at Pc = 5.1 ± 0.2 GPa, which is more than 1 GPa beyond the end of the superconducting region. Unexpectedly, at P = 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that superconductivity in TiSe2 may not be connected to the QCP itself, but to the formation of CDW domain walls. [ABSTRACT FROM AUTHOR]

We have used x-ray diffraction to study the structure of strained, epitaxial BiFeO3 (BFO) films, which exhibitordered arrays of stripelike ferroelectric domains, in which the polarization vector P alternates by either 109° or 71°. Diffraction maps exhibit an intricate satellite structure that arises from coherent, gratinglike diffraction fromthe domain structure. In the case of the 109° arrays, the domain structure was found to exert a strain modulationon the DyScO3 substrate, with the same periodicity, indicating that domains in BFO can have an influence onthe substrate structure. In the case of the 71° arrays, in which there is no contrast between neighboring domainsand coherent scattering is not expected, weak scattering is nonetheless observed, which we interpret as evidencefor previously unobserved, internal strains in these domain walls. To understand the x-ray data, we introducea simple, single-scattering model that incorporates Gaussian disorder and fits the diffraction maps, providingdomain periods and surface "puckering" angles that are in good agreement with atomic force microscopy andpiezoresponse force microscopy measurements. Our study demonstrates a simple, computationally inexpensivetechnique for semiquantitatively interpreting coherent domain scattering in ferroelectric films, and suggests thattuning domain structures is a potential route to engineering the near-surface properties of perovskite oxides. [ABSTRACT FROM AUTHOR]

The optical design of the Intermediate Energy X-ray (IEX) beamline at the APS has been optimized to study emergent phenomena via angle-resolved photoemission spectroscopy (ARPES) and resonant soft X-ray scattering (RSXS). Driven by the scientific requirements of these two techniques the beamline consists of two separate branchlines sharing the same source, heat absorbing optics and monochromator, but each having its own refocusing optics. The ARPES branch is optimized for a very high “resolving power” and a small spot at the sample; whereas, the RSXS branch is designed for high flux and a low beam divergence at the sample. The very high “resolving power” in the ARPES branch is achieved by using a monochromator which consists of a plane mirror and a variable line spacing grating that focuses the beam at all photon energies. A flux higher than photons/s at a “resolving power” better than 5×104 will be available at the sample position of the ARPES beamline on a spot size (FWHM) smaller than (hor.×ver.) for photon energies between 250 and 1660eV. A second grating will increase the flux by more than a factor of 10 at a “resolving power” of 104. The flux expected at the RSXS branch for energies between 250eV and 1900eV is higher than 4×1012 photons/s at “resolving power” around 2.5×103. The expected spot in this branch is sub-millimeter and the beam divergence is less than 0.3mrad (FWHM). [ABSTRACT FROM AUTHOR]

The ferrimagnetic spinels MnB2O2 (S = Mn,V) exhibit a similar sedes of closely spaced magnetic and structural phase transitions at low temperatures, reflecting both magnetic frustration and a strong coupling between the spin and lattice degrees of freedom. Careful studies of excitations in MnB2O2 (B = Mn,V), and the evolution of these excitations with temperature, are important for obtaining a microscopic description of the role that magnetic excitations and spin-lattice coupling play in the low-temperature phase transitions of these materials. We report an inelastic light (Raman) scattering study of the temperature and magnetic-field dependences of oneand two-magnon excitations in MnV2O4 and Mn3O4. We observe a pair of q = 0 one-magnon modes at 74 and 81 cm -1 in MnV2O4, which is in contrast with the single 80-cm-1 q = 0 magnon that has been reported for MnV2O2 based on previous neutron-scattering measurements and spin-wave calculations. Additionally, we find that the two-magnon energy of MnV2O4 decreases ("softens") with decreasing temperature below TN, which we attribute to strong coupling between magnetic and vibrational excitations near the zone boundary. [ABSTRACT FROM AUTHOR]