H. Zheng

Competing magnetic interactions in the extended Kagome system YBaCo{sub 4}O{sub 7}

YBaCo{sub 4}O{sub 7} belongs to a new class of geometrically frustrated magnets like the pyrochlores, in which Co spins occupy corners of tetrahedra. The structure can be viewed as an alternating stacking of Kagome and triangular layers. Exactly half of the triangular units of the Kagome plane are capped by Co ions to form columns running perpendicular to the Kagome sheets. Neutron powder diffraction reveals a broad temperature range of diffuse magnetic scattering, followed by long-range magnetic ordering below 110 K. A unique low-temperature magnetic structure simultaneously satisfies an S=0 arrangement in the uncapped triangular units and antiferromagnetic coupling along the columns. A spin reorientation above 30 K tracks the relative strengths of the in-plane and out-of-plane interactions.

Magnetotransport of single crystalline YSb

We report transport measurement in zero and applied magnetic field on a single crystal of NbAs. Transverse and longitudinal magnetoresistance in the plane of this tetragonal structure does not saturate up to 9 T. In the transverse configuration (H ∥ c, I ⊥ c) it is 230,000% at 2 K. The Hall coefficient changes sign from hole-like at room temperature to electron-like below ∼150 K. The electron carrier density and mobility calculated at 2 K based on a single band approximation are 1.8 × 10(19) cm(-3) and 3.5 × 10(5) cm(2) Vs(-1), respectively. These values are similar to reported values for TaAs and NbP, and further emphasize that this class of noncentrosymmetric, transition-metal monopnictides is a promising family to explore the properties of Weyl semimetals and the consequences of their novel electronic structure.

Magnetic correlations in the extended kagome YBaCo4O7 probed by single-crystal neutron scattering.

We have studied the frustrated system YBaCo4O7.0 generally described as an alternating stacking of kagome and triangular layers of magnetic ions on a trigonal lattice, by single-crystal neutron diffraction experiments above the Néel ordering transition. Experimental data reveal pronounced magnetic diffuse scattering, which is successfully modeled by direct Monte Carlo simulations. Long-range magnetic correlations are found along the c axis, due to the presence of corner-sharing bipyramids, creating quasi-one-dimensional order at finite temperature. In contrast, in the kagome layers (ab plane), the spin-spin correlation function, displaying a short-range 120 degrees configuration, decays rapidly as typically found in spin liquids. YBaCo4O7 experimentally realizes a new class of two-dimensional frustrated systems where the strong out-of-plane coupling does not lift the in-plane degeneracy, but instead acts as an external field.

Local Structure of La1 xSrxCoO3 Determined from EXAFS and Neutron Pair Distribution Function Studies

The combined local structure techniques, extended x-ray absorption fine structure and neutron pair distribution function analysis, have been used for temperatures 4< or =T< or =330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co sites (x< or =0.35), indicating few, if any, JT-active, singly occupied e_{g} Co sites exist.The combined local structure techniques, extended x-ray absorption fine structure and neutron pair distribution function analysis, have been used for temperatures 4 {le} T {le} 330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La{sub 1-x}Sr{sub x}CoO{sub 3} for a significant fraction of Co sites (x {le} 0.35), indicating few, if any, JT-active, singly occupied e{sub g} Co sites exist.

Spin-ordering and magnetoelastic coupling in the extended kagome system YBaCo4O7

Low-temperature magnetic and structural behavior of the extended Kagome system YBaCo{sub 4}O{sub 7} has been studied by single-crystal neutron diffraction and high-resolution powder x-ray diffraction. Long-range magnetic ordering associated with a structural transition from orthorhombic Pbn2{sub 1} to monoclinic P2{sub 1} symmetry has been found at T{sub 1} {approx} 100 K. The interplay between the structural and magnetic degrees of freedom indicates that the degeneracy of the magnetic ground state, present in the orthorhombic phase, is lifted through a strong magnetoelastic coupling, as observed in other frustrated systems. At T{sub 2} {approx} 60 K, an additional magnetic transition is observed, though isosymmetric. Models for the magnetic structures below T1 and T2 are presented, based on refinements using a large number of independent reflections. The results obtained are compared with previous single-crystal and powder-diffraction studies on this and related compositions.

Surface effects on the orbital order in the single-layered manganite La0.5Sr1.5MnO4.

The question of how bulk electronic order is terminated at a surface is an intriguing one, and one with possible practical implications--for example in nanoscaled systems that may be characterized by their surface behaviour. One example of such order is orbital order, and in principle it should be possible to probe the termination of this order with surface X-ray scattering. Here, we report the first observation of the scattering arising from the termination of bulk orbital order at the surface of a crystal--so-called orbital truncation rods. The measurements, carried out on a cleaved perovskite, La(0.5)Sr(1.5)MnO(4), reveal that whereas the crystallographic surface is atomically smooth, the orbital surface, which is observed through the atomic displacements caused by the orbital order, is much rougher, with a typical scale of the surface roughness of approximately 7 degrees A. Interestingly, the temperature dependence of this scattering shows evidence of a surface-induced second-order transition.

Signature of checkerboard fluctuations in the phonon spectra of a possible polaronic metal La1.2Sr1.8Mn2O7

Charge carriers in low-doped semiconductors may distort the atomic lattice around them and through this interaction form so-called small polarons. High carrier concentrations on the other hand can lead to short-range ordered polarons (large polarons) and even to a long-range charge and orbital order. These ordered systems should be insulating with a large electrical resistivity. However, recently a polaronic pseudogap was found in a metallic phase of La(2-2x)Sr(1+2x)Mn(2)O(7) (ref. 7). This layered manganite is famous for colossal magnetoresistance associated with a phase transition from this low-temperature metallic phase to a high-temperature insulating phase. Broad charge-order peaks due to large polarons in the insulating phase disappear when La(2-2x)Sr(1+2x)Mn(2)O(7) becomes metallic. Investigating how polaronic features survive in the metallic phase, here we report the results of inelastic neutron scattering measurements showing that inside the metallic phase polarons remain as fluctuations that strongly broaden and soften certain phonons near the wavevectors where the charge-order peaks appeared in the insulating phase. Our findings imply that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase. Our findings are highly relevant to cuprate superconductors with both a pseudogap and a similar phonon effect associated with a competing stripe order.

d-d excitations in bilayer manganites probed by resonant inelastic x-ray scattering

We report a high-resolution resonant inelastic x-ray scattering investigation of the bilayer manganites La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} with x=0.36 and 0.5. The momentum dependence along the crystallographic (110) direction for energy losses 1 eV {le} {Delta}E {le} 15 eV has been measured in detail with the data analysis focusing on the energy-loss region 1 eV {le} {Delta}E {le} 5 eV, which includes a strong peak located at {Delta}E {approx} 2 eV. We observe a clear dispersion of up to 0.5 eV in the measured q range, which is direct evidence of the nonlocal character of this excitation. Further, we found that the intensity in this low-energy region strongly depends on both the reduced wave vector q=(h,h,0), h=0.1-0.5, and temperature, i.e., different ordered phases. Results can be explained via an intersite d-d charge-transfer excitation, proposed for pseudocubic manganites, where the hopping rate is strongly increased (decreased) by ferromagnetic (antiferromagnetic) alignment of neighboring in-plane Mn ion core spins.

Response of acoustic phonons to charge and orbital order in the 50% doped bilayer manganite LaSr2Mn2O7.

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite LaSr(2)Mn(2)O(7). For excitation energies less than 15 meV, we observe an abrupt increase (decrease) of the phonon energies (linewidths) of a transverse acoustic phonon branch at q = (h, h, 0), h ≤ 0.3, upon entering the low temperature charge and orbital ordered state (T(COO) = 225 K). This indicates a reduced electron-phonon coupling due to a decrease of electronic states at the Fermi level leading to a partial removal of the Fermi surface below T(COO) and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.

Observation of Electronic Inhomogeneity and Charge Density Waves in a Bilayer La 2 − 2 x Sr 1 + 2 x Mn 2 O 7 Single Crystal

We employed a scanning tunneling microscope to image the (001) surface topography and local density of states (LDOS) in La(2-2x)Sr(1+2x)Mn(2)O(7) (x=0.32, LSMO) single crystals below the Curie temperature (T(C)≈120 K). The LDOS maps revealed a stripelike modulation propagating along the tetragonal a axis with a wavelength of about 16 Å, which is indicative of a charge density wave (CDW). The observed CDW in the x=0.32 sample is far from the Fermi surface nesting instability as compared with the data of angle resolved photoemission spectroscopy in an x=0.40 sample. The stripe model developed previously for cuprates can explain the observed CDW in our LSMO sample, indicating that competing interactions between localized and itinerant phases are the origin of the spatial modulations present intrinsically in cuprates and manganites.