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Synchrotron-based X-ray spectroscopic and microscopic techniques are used to identify the origin of enhancement of the photoelectrochemical (PEC) properties of BiVO4 (BVO) that is coated on ZnO nanodendrites (hereafter referred to as BVO/ZnO). The atomic and electronic structures of core-shell BVO/ZnO nanodendrites have been well-characterized, and the heterojunction has been determined to favor the migration of charge carriers under the PEC condition. The variation of charge density between ZnO and BVO in core-shell BVO/ZnO nanodendrites with many unpaired O 2p-derived states at the interface forms interfacial oxygen defects and yields a band gap of approximately 2.6 eV in BVO/ZnO nanocomposites. Atomic structural distortions at the interface of BVO/ZnO nanodendrites, which support the fact that there are many interfacial oxygen defects, affect the O 2p-V 3d hybridization and reduce the crystal field energy 10Dq ∼2.1 eV. Such an interfacial atomic/electronic structure and band gap modulation increase the efficiency of absorption of solar light and electron-hole separation. This study provides evidence that the interfacial oxygen defects act as a trapping center and are critical for the charge transfer, retarding electron-hole recombination, and high absorption of visible light, which can result in favorable PEC properties of a nanostructured core-shell BVO/ZnO heterojunction. Insights into the local atomic and electronic structures of the BVO/ZnO heterojunction support the fabrication of semiconductor heterojunctions with optimal compositions and an optimal interface, which are sought to maximize solar light utilization and the transportation of charge carriers for PEC water splitting and related applications.

CuB2O4 has attracted considerable attention because of its unique chemical and physical properties and potential practical applications. In this paper, we investigated the optical properties of CuB...

Copper metaborate had a unique crystal structure and exhibited noteworthy magnetic phase transitions at 21 and 10 K. The electronic structure and lattice dynamics of copper metaborate Cu11B2O4 single crystals were investigated and compared with the optical properties of CuB2O4, to assess the boron isotope effect. The optical absorption spectrum at room temperature revealed two charge-transfer bands at approximately 4.30 and 5.21 eV with an extrapolated direct optical band gap of 3.16 ± 0.07 eV. Compared with the data on CuB2O4, the electronic transitions were shifted to lower energies upon the replacement of a heavier boron isotope. The band gap was also determined to be lower in Cu11B2O4. Anomalies in the temperature dependence of the optical band gap were observed below 21 K. Furthermore, 38 Raman-active phonon modes were identified in the room-temperature Raman scattering spectrum of Cu11B2O4, which were also observed in CuB2O4 with a shift to lower frequencies. No broadening caused by isotopic changes was observed. As the temperature decreased, phonon frequencies shifted to higher wavenumbers and the linewidth decreased. Anomalous softening in the Raman peaks below 21 K was also revealed.

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO5 (YBCFO). An onset of increase of resistivity at ~180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab-plane and is also observed with strong anisotropy between the ab-plane and the c-axis. The XLD spectra at the Fe L3,2-edge revealed a change in Fe 3d eg holes from the preferential $${\bf{3}}{{\boldsymbol{d}}}_{{{\bf{x}}}^{{\bf{2}}}{\boldsymbol{-}}{{\bf{y}}}^{{\bf{2}}}}$$3dx2−y2 orbital at high temperature (300–150 K) to the $${\bf{3}}{{\boldsymbol{d}}}_{{{\bf{3}}{\bf{z}}}^{{\bf{2}}}{\boldsymbol{-}}{{\bf{r}}}^{{\bf{2}}}}$$3d3z2−r2 orbital at/below 125 K. The analysis of the Fe K-edge EXAFS data of YBCFO further revealed an unusual increase in the Debye-Waller factor of the nearest-neighbor Fe-O bond length at/below 125 K, suggesting phonon-softening behavior, resulting in the breaking of lattice symmetry, particularly in the ab-plane of Fe-related square pyramids. These findings demonstrate a close correlation between electrical resistivity and coupling of the preferred Fe 3d orbital with lattice distortion of a single crystal of YBCFO.

Effects of electronic and atomic structures of V-doped 2D layered SnS

Synchrotron-based X-ray spectroscopic and microscopic techniques are used to identify the origin of enhancement of the photoelectrochemical (PEC) properties of BiVO

The potential applications of perovskite manganite R1-xAxMnO3 (R = rare earth element; A = Sr, Ca) thin films have been continuously explored due to their multi-functional properties. In particular, the optimally hole-doped La0.67Ca0.33MnO3 thin film demonstrates a colossal magneto-resistance that is beneficial to the performance of spintronic devices. To understand the effect of R and A ions on the material properties, we systematically measure the resistivity, magnetization, and electronic energy states for three optimally hole-doped R0.67A0.33MnO3 thin films with R = La, Sm and A = Sr, Ca. Various energy parameters are derived based on the X-ray absorption and X-ray photoelectron spectra, including the band gap, the charge frustration energy and the magnetic exchange energy. It is interesting to find that the replacement of La with Sm is more effective than that of Sr with Ca in terms of tuning the electrical property, the Curie temperature, and the band gap. The strain-induced reduction of the O 2p- Mn 3d hybridization and the interplay of R/A site disorder and strain effect are discussed. The results of this study provide useful information for the band design of perovskite oxide films.

This study probes the temperature-dependent strain that is strongly correlated with the orbital and magnetic structures of epitaxial films of Nd0.35Sr0.65MnO3 (NSMO) that are fabricated by pulsed laser deposition with two thicknesses, 17 (NS17) and 103 nm (NS103) on SrTiO3 (STO) substrate. This investigation is probed using X-ray diffraction (XRD) and absorption-based techniques, X-ray linear dichroism (XLD) and the X-ray magnetic circular dichroism (XMCD). XRD indicates a significant shift in the (004) peak position that is associated with larger strain in NS17 relative to that of NS103 at both 30 and 300 K. Experimental and atomic multiplet simulated temperature-dependent Mn L3,2-edge XLD results reveal that the stronger strain in a thinner NS17 film causes less splitting of Mn 3d eg state at low temperature, indicating an enhancement of orbital fluctuations in the band above the Fermi level. This greater Mn 3d orbital fluctuation can be the cause of both the enhanced ferromagnetism (FM) as a result of spin moments and the reduced Néel temperature of C-type antiferromagnetism (AFM) in NS17, leading to the FM coupling of the canted-antiferromagnetism (FM-cAFM) state in NSMO/STO epitaxial films at low temperature (T = 30 K). These findings are also confirmed by Mn L3,2-edge XMCD measurements.

We combined spectroscopic ellipsometry, Raman scattering spectroscopy, and first-principles calculations to explore the optical properties of YBaCuFeO5 single crystals. Measuring the optical absorption spectrum of YBaCuFeO5 at room temperature revealed a direct optical band gap at approximately 1.41 eV and five bands near 1.69, 2.47, 3.16, 4.26, and 5.54 eV. Based on first-principles calculations, the observed optical excitations were appropriately assigned. Analysis of the temperature dependence of the band gap indicated anomalies in antiferromagnetic phase transition at 455 and 175 K. Additionally, a hardening in the frequency of the Eg phonon mode was observed at 175 K. The value of the spin–phonon coupling constant was 15.7 mRy/Å2. These results suggest a complex nature of spin–charge–lattice interactions in YBaCuFeO5.

Charge orders (COs) and entangled topology denote the fundamentals of correlated electron systems at low temperatures, and their profound revelation at atomic scale awaits experimental advances. Here, we report such a spatially resolved investigation using cryogenic atomic-scale electron microscopy and spectroscopy at 100 K, with the CO insulator of $\mathrm{Hg}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ below 240 K as an exemplification. By the cryogenic technique, we can resolve the charge and lattice characteristics of the insulating CO phase at atomic resolution and identify the order parameters (OPs) at play as charge-density-modulated (\ensuremath{\sim}31.4 \AA{}, modulation length) and antiphase (\ensuremath{\sim}10.5 \AA{}) OPs. Both OPs are largely unaddressed forms of COs in correlated oxides and display the emergent topology of intertwining mesoscopic stripes of 10--40 nm in width and \ensuremath{\ge}500 nm in length, distinctly different from the atomically narrow CO stripes conventional to correlated systems and depicting the unique electronic contour of the phase. This preliminary demonstration of the cryogenic unveiling of the OPs and topological landscape paves the way to disentangling the multifarious COs and hidden topology in condensed matters and, meanwhile, calls for further developments in the atomic-scale cryogenic spectral probing at an enhanced speed.

Copper metaborate had a unique crystal structure and exhibited noteworthy magnetic phase transitions at 21 and 10 K. The electronic structure and lattice dynamics of copper metaborate Cu

This investigation reports on anisotropy in the magnetic interaction, lattice-orbital coupling and degree of phonon softening in single crystal Ni3TeO6 (NTO) using temperature- and polarization-dependent X-ray absorption spectroscopic techniques. The magnetic field-cooled and zero-field-cooled measurements and temperature-dependent Ni L3,2-edge X-ray magnetic circular dichroism spectra of NTO reveal a weak Ni-Ni ferromagnetic interaction close to ~60 K (TSO: temperature of the onset of spin ordering) with a net alignment of Ni spins (the uncompensated components of the Ni moments) along the crystallographic c-axis, which is absent from the ab-plane. Below the Néel temperature, TN~ 52 K, NTO is stable in the antiferromagnetic state with its spin axis parallel to the c-axis. The Ni L3,2-edge X-ray linear dichroism results indicate that above TSO, the Ni 3d eg electrons preferentially occupy the out-of-plane 3d3z2−r2 orbitals and switch to the in-plane 3dx2−y2 orbitals below TSO. The inherent distortion of the NiO6 octahedra and anisotropic nearest-neighbor Ni-O bond lengths between the c-axis and the ab-plane of NTO, followed by anomalous Debye-Waller factors and orbital-lattice in conjunction with spin-phonon couplings, stabilize the occupied out-of-plane (3d3z2−r2) and in-plane (3dx2−y2) Ni eg orbitals above and below TSO, respectively.

Effects of electronic and atomic structures of V-doped 2D layered SnS2 are studied using X-ray spectroscopy for the development of photocatalytic/photovoltaic applications. Extended X-ray absorption fine structure measurements at V K-edge reveal the presence of VO and VS bonds which form the intercalation of tetrahedral OVS sites in the van der Waals (vdW) gap of SnS2 layers. X-ray absorption near-edge structure (XANES) reveals not only valence state of V dopant in SnS2 is ≈4+ but also the charge transfer (CT) from V to ligands, supported by V Lα,β resonant inelastic X-ray scattering. These results suggest V doping produces extra interlayer covalent interactions and additional conducting channels, which increase the electronic conductivity and CT. This gives rapid transport of photo-excited electrons and effective carrier separation in layered SnS2 . Additionally, valence-band photoemission spectra and S K-edge XANES indicate that the density of states near/at valence-band maximum is shifted to lower binding energy in V-doped SnS2 compare to pristine SnS2 and exhibits band gap shrinkage. These findings support first-principles density functional theory calculations of the interstitially tetrahedral OVS site intercalated in the vdW gap, highlighting the CT from V to ligands in V-doped SnS2 .

Through the detailed studies utilizing x-ray and neutron powder diffraction and magnetometry measurements, we report the magnetic phase transitions in the B-site disorder perovskite oxide PrFe0.5Cr0.5O3. The Fe3+/Cr3+ sublattice was observed to form a long-range magnetic order with the GxFz configuration below TN = 270 K from neutron powder diffraction. It then undergoes the second-order phase transition to FxGz spin configuration below 210 K and the transition ends at ~170 K. On the other hand, long range magnetic ordering with FxCy configuration at Pr3+ sublattice can be identified at base temperature. The magnetic phase diagram is derived with spontaneous spin ordering and reorientation at A- and B-sublattices.

The open question of crystal symmetry for YBaCuFeO5, still under debate, is resolved by using diffraction anomalous fine structure and resonant multi-beam X-ray diffraction at the Fe and Cu K edges. The different asymmetric intensity distributions observed at 7, 7.1305 and 8.9945 keV give direct evidence that the space group of YBaCuFeO5 should be P4/mmm with the distance between Cu and apical O atoms longer than that between Fe and O atoms. This approach provides a direct way to resolve detailed structural symmetry which is indeterminate in conventional Bragg diffraction.

A modified traveling solvent floating zone (TSFZ) technique was used to successfully grow a large size and high quality single crystal of multiferroic material YBaCuFeO 5 . This modified TSFZ growth uses a stoichiometric feed rod and pure copper oxide as the initial flux without prior knowledge of the complex phase diagram involving four elements, and the optimal flux for the growth of incongruently melt crystal is self-adjusted after a prolonged stable pulling. The wetting of the feed rod edge that often perturbs the molten zone stability was avoided by adding 2 wt% B 2 O 3 . The optimal flux concentration for the YBaCuFeO 5 growth can be extracted to be near YBaCuFeO 5 :CuO=13:87 in molar ratio. The crystal quality was confirmed by the satisfactory refinement of crystal structure of space group P 4 mm and the two consecutive anisotropic antiferromagnetic phase transitions near 455 K and 170 K.

The local electronic and atomic structures of the high-quality single crystal of SrFeO3-δ (δ~0.19) were studied using temperature-dependent x-ray absorption and valence-band photoemission spectroscopy (VB-PES) to investigate the origin of anisotropic resistivity in the ab-plane and along the c-axis close to the region of thermal hysteresis (near temperature for susceptibility maximum, Tm~78 K). All experiments herein were conducted during warming and cooling processes. The Fe L3,2-edge X-ray linear dichroism results show that during cooling from room temperature to below the transition temperature, the unoccupied Fe 3d eg states remain in persistently out-of-plane 3d3z2-r2 orbitals. In contrast, in the warming process below the transition temperature, they change from 3d3z2-r2 to in-plane 3dx2-y2 orbitals. The nearest-neighbor (NN) Fe-O bond lengths also exhibit anisotropic behavior in the ab-plane and along the c-axis below Tm. The anisotropic NN Fe-O bond lengths and Debye-Waller factors stabilize the in-plane Fe 3dx2-y2 and out-of-plane 3d3z2-r2 orbitals during warming and cooling, respectively. Additionally, a VB-PES study further confirms that a relative band gap opens at low temperature in both the ab-plane and along the c-axis, providing the clear evidence of the charge-density-wave nature of SrFeO3-δ (δ~0.19) single crystal.

Using magnetization, dielectric constant, and neutron diffraction measurements on a high quality single crystal of YBaCuFeO

Elastic and inelastic neutron scattering (ENS and INS) experiments were performed on a single crystal of $(\mathrm{N}{\mathrm{i}}_{0.4}\mathrm{M}{\mathrm{n}}_{0.6})\mathrm{Ti}{\mathrm{O}}_{3}$ (NMTO) to study the spatial correlations and dynamics of spins in the XY-like spin-glass (SG) state. Magnetization measurements reveal signatures of SG behavior in NMTO with a freezing temperature of ${T}_{\mathrm{SG}}\ensuremath{\sim}9.1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The ENS experiments indicate that the intensity of magnetic diffuse scattering starts to increase around 12 K, which is close to ${T}_{\mathrm{SG}}$. Also, spin-spin correlation lengths $(\ensuremath{\xi})$ at 1.5 K are approximately $(21\ifmmode\pm\else\textpm\fi{}1)$ and $(73\ifmmode\pm\else\textpm\fi{}2)\phantom{\rule{0.16em}{0ex}}\AA{}$ in the interlayer and the in-plane directions, respectively, demonstrating that magnetic correlations in NMTO exhibit quasi-two-dimensional antiferromagnetic order. In addition, critical exponent $(\ensuremath{\beta})$ is determined to be $0.37\ifmmode\pm\else\textpm\fi{}0.02$ from the intensity of magnetic diffuse scattering confirms the XY-like SG state of NMTO. INS results show quasielastic neutron scattering (QENS) profiles below ${T}_{\mathrm{SG}}$. The lifetime of dynamic correlations $\ensuremath{\tau}\ensuremath{\sim}\ensuremath{\hbar}/{\mathrm{\ensuremath{\Gamma}}}_{L}$, obtained from the half-width at half-maximum of the Lorentzian $({\mathrm{\ensuremath{\Gamma}}}_{L})$ QENS profiles, are approximately $(16\ifmmode\pm\else\textpm\fi{}1)$ and $(16\ifmmode\pm\else\textpm\fi{}2)$ ps at 10 K for two positions (0.00, 0.00, 1.52) and (0.01, 0.01, 1.50), respectively. Therefore, our experimental findings demonstrate that short-range-ordered antiferromagnetic clusters with short-lived spin correlations are present in the XY-like SG state of NMTO.

X-ray scattering (XRS), x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopic techniques were used to study the electronic and atomic structures of the high-quality Sr3Ir4Sn13 (SIS) single crystal below and above the transition temperature (T* ≈ 147 K). The evolution of a series of modulated satellite peaks below the transition temperature in the XRS experiment indicated the formation of a possible charge density wave (CDW) in the (110) plane. The EXAFS phase derivative analysis supports the CDW-like formation by revealing different bond distances [Sn1(2)-Sn2] below and above T* in the (110) plane. XANES spectra at the Ir L3-edge and Sn K-edge demonstrated an increase (decrease) in the unoccupied (occupied) density of Ir 5d-derived states and a nearly constant density of Sn 5p-derived states at temperatures T * in the (110) plane. These observations clearly suggest that the Ir 5d-derived states are closely related to the anomalous resistivity transition. Accordingly, a close relationship exists between local electronic and atomic structures and the CDW-like phase in the SIS single crystal.

Physical review / B 95(6), 064424(2017). doi:10.1103/PhysRevB.95.064424, We present the results of resonant x-ray scattering measurements and electronic structure calculations on the monoarsenide FeAs. We elucidate details of the magnetic structure, showing the ratio of ellipticity of the spin helix is larger than previously thought, at 2.58(3), and reveal both a right-handed chirality and an out-of-plane component of the magnetic moments in the spin helix. We find that electronic structure calculations and analysis of the spin-orbit interaction are able to qualitatively account for this canting., Published by APS, Woodbury, NY

It is known that the nanoscale morphological organization of donors or acceptors in bulk heterojunction (BHJ) solar cells is critical to device performance and strongly affects carrier generation, transporting, and collection. This work demonstrates the dependence of nanocrystal dimensionality and organization on the polymer nanomorphology in P3HT:TiO2 hybrid bulk heterojunctions, which were revealed using grazing-incidence X-ray diffraction (GIXRD) using a synchrotron X-ray beam and electron tomography. We further performed a multiscale molecular dynamic simulation to understand the morphological orientation of a polymer blended with TiO2 nanoparticles (NPs) or nanorods (NRs). The correlation between polymer nanoscale morphology and the dimensionality and anisotropy of nanocrystals in P3HT:TiO2 hybrids clearly explains the observation of different optical absorption and carrier transport behaviors in directions perpendicular or parallel to the film substrate. Our results provide crucial information towar...

Full polarisation analysis of resonant X-ray magnetic scattering (RXMS) is shown to advance the determination of magnetic moment directions of complex magnetic structures within single crystals. Key features of this novel method are the variation of the incident beam polarisation through the use of an X-ray phase plate, and the measurement of the scattered beam polarisation in terms of Poincare–Stokes parameters. Contrary to the established method, no azimuthal rotation is required. Thus, the major sources of systematic error are eliminated, and the method is compatible with exotic and complex sample environments. The technique is demonstrated with the example of TbMn 2 O 5 . The RXMS theory briefly outlined in this paper was fitted to the data, and the local and delocalized, band-specific moment directions associated with the magnetic order of the resonating species was refined with a high degree of accuracy.

C-axis oriented InN films with wurtzite structure were grown on sapphire (0001) substrate by MOCVD method. Superconductivity with transition onset temperature T c,onset around 3.5 K has been characterized by magnetotransport measurements in fields up to 9 Tesla for films with carrier concentration in the range of 1×10 19 cm -3 to 7×10 20 cm -3 . Among them, the film with a nitridation buffer layer has the highest zero-resistance temperature T c0 of 2 K. The normal-state magnetoresistance follows Kohler's rule ΔR/Rα (H / R) 2 , indicating that there is a single species of charge carrier with single scattering time at all points on the Fermi surface. The extrapolated value of zero-temperature upper critical field H c2 ab (0) and H c2 c (0) is estimated to be 5900 G and 2800 G, respectively, giving rise to the anisotropy parameter γ about 2.1. The angular dependence of the upper critical field is in good agreement with the behavior predicted by Lawrence-Doniach model in the two-dimensional (2D) limit strongly suggesting that the InN film is a quasi-2D superconductor.

We report an unusual transition behavior of charge stripes in ${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_{4}$ using x-ray scattering. The segregated holes in ${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_{4}$ are observed to form anisotropic stripes in the $a\ifmmode\times\else\texttimes\fi{}b$ plane of the crystal space below the transition temperature $T\ensuremath{\simeq}238$ K, and at the same time, display an unusual inverse order-disorder transition along the $c$ axis. Using a phenomenological Landau theory, we show that this inverse transition is due to the interlayer coupling between the charge and spin orders. This discovery points to the importance of the interlayer correlations in the strongly correlated electrons system.

Using magnetization, dielectric constant, and neutron diffraction measurements on a high quality single crystal of YBaCuFeO5 (YBCFO), we demonstrate that the crystal shows two antiferromagnetic transitions at [Formula: see text] K and [Formula: see text] K, and displays a giant dielectric constant with a characteristic of the dielectric relaxation at T N2. It does not show the evidence of the electric polarization for the crystal used for this study. The transition at T N1 corresponds with a paramagnetic to antiferromagnetic transition with a magnetic propagation vector doubling the unit cell along three crystallographic axes. Upon cooling, at T N2, the commensurate spin ordering transforms to a spiral magnetic structure with a propagation vector of ([Formula: see text] [Formula: see text] [Formula: see text]), where [Formula: see text], [Formula: see text], and [Formula: see text] are odd, and the incommensurability δ is temperature dependent. Around the transition boundary at T N2, both commensurate and incommensurate spin ordering coexist.

Inverse melting refers to the rare thermodynamic phenomenon in which a solid melts into a liquid upon cooling, a transition that can occur only when the ordered (solid) phase has more entropy than the disordered (liquid) phase, and that has so far only been observed in a handful of systems. Here we report the first experimental observation for the inverse melting of an electronic liquid crystalline order in strontium-doped lanthanum nickelate, a compound isostructural with the superconducting cuprates, with a hole doping concentration of 1/3. Using x-ray scattering, we demonstrate that the isotropic charge modulation is driven to nematic order by fluctuating spins and shows an inverse melting transition. Using a phenomenological Landau theory, we show that this inverse melting transition is due to the interlayer coupling between the charge and spin orders. This discovery points to the importance of the interlayer correlations in the system, and provides a new perspective to study the intricate nature of the electronic liquid crystal phases in strongly correlated electronic systems, including possibly the Cu- and Fe-based high-Tc superconductors., 7 pages, 7 figures

Using x-ray scattering we have measured the response of the incommensurate charge-density wave (CDW) modulation in the quasi-low-dimensional material 2H-NbSe2 to applied magnetic fields at low temperatures. The application of a magnetic field, either perpendicular or parallel to the layers of a single crystal of 2H-NbSe2, caused no significant change to either the correlation length or the intensity of the CDW satellites for magnetic fields up to 10 T. These results suggest that the enhancement of the resistance observed in low-dimensional CDW materials exposed to the applied magnetic field does not result from an appreciable conversion of carriers from the normal state to the CDW state. In addition 2H-NbSe2 is a superconductor at low temperatures (Tc = 7.2 K), whilst still within the incommensurate CDW state. This material therefore affords an opportunity to study any interaction between the CDW and superconducting condensates. High magnetic fields can suppress the superconducting state yet no change in the incommensurate CDW satellite correlation length or intensity was observed at high applied magnetic fields. These results conflict with spectroscopic measurements, which suggest a coupling between the superconducting gap excitations and the CDW.

Using high-resolution x-ray scattering, we have demonstrated the existence of quenched disordered charge stripes in a single crystal of La 53Sr13NiO4 at low temperatures. Above the second-order transition critical scattering was observed due to fluctuations into the charge stripe phase. The charge stripes are shown to be two dimensional in nature both by measurements of their correlation lengths ja 185 A, jb 400 A, and jc 25 A) and by the critical exponents of the charge strip transition. The charge stripe ordering did not develop long-range order even at low temperatures, indicating that the charge stripes are disordered and that the length scale of the disorder is quenched.

A single crystal of ${(\mathrm{B}\mathrm{i}}_{0.24}{\mathrm{Ca}}_{0.76}{)\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ has been extensively studied using both laboratory and synchrotron radiation x-ray scattering. Using x-ray techniques we can directly observe the formation of charge stripes associated with the low-temperature structural phase transition. Weak satellite peaks with a modulation wave vector $\mathbf{q}\ensuremath{\approx}(0.24,0.24,0)$ were observed below the transition temperature. These are associated with the charge ordering of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ ions into charge (and spin) stripes within the perovskite structure. Measurements of the temperature dependence of the intensity and width of the charge-ordering satellites over the temperature range 10\char21{}240 K were also completed. These demonstrate the first-order nature of the structural phase transition that accompanies the formation of charge and spin ordering. Measurements of the width of the charge-order satellites demonstrate that the correlation length of the charge order is long range at all temperatures, and in all directions, below the structural phase transition.

The polycrystalline Li1+xTi2O4 has intricate physical behaviors in normal and superconducting state. High-resolution X-ray diffraction shows that it is the pure spinel phase and there is no structural phase transition down to 20 K. In addition to the field-induced broadening of R(T) near the superconducting regime, an anomalous upturn in R(T) appears near the transition temperature when magnetic field is above 3 T. This anomalous resistivity upturn regime diminishes with the adding of Li, which increases the conductivity of sample. The superconductivity is suppressed at field above 14 T and temperature above 4.2 K, meanwhile the low temperature resistance behaves semiconductor-like. It resembles to the metal-insulator transition observed in La214 under high magnetic field. Furthermore, the sample exhibits other interesting features: a significant positive magnetoresistance below 40 K, e.g., ΔR(H)/R(H=0)∼3% at T=14 K and H=16 T; ZFC and FC curves of magnetization measured below 1000 G split around 170 K; and Hall measurements also show that charge carriers change sign from negative to positive around 170 K. These experimental facts suggest that magnetic property and electronic structure strongly correlate with each other.

Using magnetization, conductivity and x-ray scattering measurements, we demonstrate that the giant magnetoresistance of the oxygen-deficient ferrite SrFeO2.875±0.02 is a consequence of the coupling between the charge and spin order parameters and the tetragonal to monoclinic structural distortion. Upon cooling the sample at T sime 120 K we find a shoulder in both field-cool and zero field cool magnetization data and the simultaneous appearance of incommensurate structural satellites observed using x-ray diffraction. These satellites are shown to be due to incommensurate charge ordering with the high temperature delocalized Fe${}^{3.5+}$ ions becoming localized with a charge disproportion forming an incommensurate charge-ordered phase. Strong resonant enhancement of these satellites at the Fe L III absorption edge confirms that this charge ordering is occurring at the Fe(2) sites. Further cooling increases the charge order correlation until T sime 62 K where there is a full structural transition from the tetragonal phase to a mononclinic phase. This causes a jump in the charge order wavevector from an incommensurate value of 0.610 to a commensurate ground state position of 5/8. This first-order structural transition displays considerable hysteresis as well as dramatic reductions in the magnetization, resistivity and magnetoresistance. The transition also causes an antiferromagnetic spin-ordering with a doubled unit cell along the c-axis. Well as observing new commensurate magnetic reflections at the FeIII edge we also observed resonant enhancement at the oxygen K-edge showing considerable hybridization between the Fe 3d and oxygen 2p states at low temperatures. Our results show that the formation of a magnetic long-rage ordered ground state drives the charge ordering from an incommensurate ordering to a commensurate ground state. This is evidence of a strong coupling between the magnetic and charge order parameters which is the basis for the unusual magnetoresistive effects observed at the transition.

We report the observation of a modulated structure and a ferromagnetic insulating state in a high quality single crystal of a nine-layer BaRuO3. Using x-ray scattering, the modulated satellites were observed to double the unit cell along the c-axis at low temperature. The ferromagnetic insulating state is confirmed by magnetic and resistivity measurements. Analyzing the peak profiles from the modulation and host structure respectively, showed a lattice distortion at T ∼ 55 K. These findings elucidate the intimate relationship between ferromagnetism and lattice distortion in a nine-layer BaRuO3. (Some figures in this article are in colour only in the electronic version)

In multiferroic TbMn${}_{2}$O${}_{5}$, the behavior of the terbium ions forms a crucial part of the magnetoelectric coupling. The result is a magnetically induced reversal of the electric polarization at 2 T. In this article we present the first direct measurement of the terbium magnetic structure under applied magnetic fields. Contrary to the current interpretation of the magnetic properties of TbMn${}_{2}$O${}_{5}$, we show that upon the electric polarization reversal the terbium sublattice adopts a canted antiferromagnetic structure with a large component of magnetic moment parallel to the $a$ axis. Furthermore, we provide evidence for a coupling between the manganese $3d$ magnetic structure and the terbium $4f$ magnetism, which is of great significance in the elusive magnetoelectric mechanisms at play.

We have employed resonant x-ray magnetic scattering to specifically probe the magnetic order of the rare-earth ions in multiferroic ${\text{TbMn}}_{2}{\text{O}}_{5}$. Two energy resonances were observed, one originated from the E1-E1 dipolar transition and the other from the E2-E2 quadrupolar transition. These resonances directly probe the valence $5d$ band and the partially occupied $4f$ band, respectively. First, full polarization analysis, which is a measurement of the scattered polarization as a function of incident polarization, confirmed a spin polarization of the terbium valence states (probed by the E1-E1 transition) by the ${\text{Mn}}^{4+}$ spin density in the commensurate phase. Second, full polarization analysis data were collected in the low-temperature incommensurate and commensurate phases when tuned to the E2-E2 resonance. By employing a least-squares fitting procedure, the spin orientations of the terbium ion sublattice were refined.

We report a sequent phase transition using high-resolution synchrotron x-ray scattering in a single crystal Nd1/2Sr1/2MnO3. By measuring the peak profile of Bragg reflections, upon cooling, we observed an increase in the width of the Bragg reflections around the Curie temperature (252 K) corresponding to the transition from a paramagnetic to a ferromagnetic state. Below approximately 200 K, dramatic changes in the width and integrated intensity were observed. Changes continued until the formation of charge ordering with q = (½,0,0) at TCO = 152 K. This charge ordering was observed to be the first order transition with a large hysteresis width of 10 K. This sequent transition is understood by the formation of different magnetic domains at different temperature ranges as that observed by neutron powder diffraction.

The first observation of the X-ray multiple-wave interaction in an incommensurate charge-density-wave (CDW) modulated structure at low temperatures is reported for an example of a quasi-two-dimensional material, NbSe(2)-2H. Via the coherent interaction between the X-ray waves propagating in the CDW-modulated structure and the host structure, the phase-dependent intensity variations of a CDW reflection were detected. In accord with a centrosymmetric structure, the phases of the structure-factor triplets of two CDW reflections and a Bragg reflection of the host structure were determined to be either 0 or 180 degrees, and not to vary with temperature. Relative phase differences of the two CDW reflections are also deduced.

The electronic and magnetic properties of tetravalent-ion-doped La0.85Zr0.15MnO3 (LZMO) thin films that were epitaxially grown on SrTiO3 (STO) and MgO substrates were studied using temperature-dependent x-ray diffraction (XRD), x-ray absorption near-edge structure, x-ray linear dichroism, and x-ray magnetic circular dichroism at the Mn L3,2- and K-edge. XRD studies reveal that the LZMO thin films have compressive and tensile strains (along the c-axis) on the STO and MgO substrates, respectively. As the temperature is reduced from room temperature to below magnetic transition temperature, the preferentially occupied Mn majority-spin eg orbital changes from the in-plane dx2-y2 to the out-of-plane d3z2-r2 orbital for LZMO/STO, and vice versa for LZMO/MgO. Experimental results suggest that the new hopping path that is mediated by the Mn2+ ions triggers a stronger d3z2-r2 orbital ordering of Mn3+ ions and enhances the ferromagnetic coupling between the Mn spin moments of t2g electrons in LZMO/STO, whereas the ...

Single crystals of both (Bi0 24Ca0 76)MnO3 and (La1 67Sr0 33)NiO4 have been studied extensively using both laboratory and synchrotron radiation x-ray scattering. Using x-ray techniques, we have detected directly the formation of charge stripes. Weak satellite peaks with a modulation wave-vector q≈(0.25, 0.25, 0) were observed below the transition temperature in the perovskite (Bi0 24Ca0 76)MnO3. Measurements of the temperature dependence of the intensity and width of the charge-ordering satellites over the temperature range 10–240 K demonstrate that the correlation length of the charge-order stripes is long range at all temperatures and in all directions. A first-order structural phase transition accompanying the charge ordering was also studied. We also undertook SR x-ray measurements on (La1 67Sr0 33)NiO4, and found quasi-2D-like charge stripes with a modulation wave-vector q≈(2/3, 2/3, 1). Detailed critical scattering measurements of the melting of the charge stripes gave critical exponents compatible with those expected of a 2D-Ising system.