123 research outputs found
Anisotropic optical response of the mixed-valent Mott-Hubbard insulator NaCu2O2
We report the results of a comprehensive spectroscopic ellipsometry study of
NaCu2O2, a compound composed of chains of edge-sharing Cu2+O4 plaquettes and
planes of Cu1+ ions in a O-Cu1+-O dumbbell configuration, in the spectral range
0.75-6.5 eV at temperatures 7 -300 K. The spectra of the dielectric function
for light polarized parallel to the Cu1+ planes reveal a strong in-plane
anisotropy of the interband excitations. Strong and sharp absorption bands
peaked at 3.45 eV (3.7 eV) dominate the spectra for polarization along
(perpendicular) to the Cu2+O2 chains. They are superimposed on flat and
featureless plateaux above the absorption edges at 2.25 eV (2.5 eV). Based on
density-functional calculations, the anomalous absorption peaks can be assigned
to transitions between bands formed by Cu1+ 3dxz(dyz) and Cu2+ 3dxy orbitals,
strongly hybridized with O pstates. The major contribution to the background
response comes from transitions between Cu1+ 3dz2 and 4px(py) bands. This
assignment accounts for the measured in-plane anisotropy. The dielectric
response along the Cu2+O2 chains develops a weak two-peak structure centered at
2.1 and 2.65 eV upon cooling below 100 K, along with the appearance of spin
correlations along the Cu2+O2 chains. These features bear a striking
resemblance to those observed in the single-valent Cu2+O2 chain compound
LiCuVO4, which were identified as an exciton doublet associated with
transitions to the upper Hubbard band that emerges as a consequence of the
long-range Coulomb interaction between electrons on neighboring Cu2+ sites
along the chains. An analysis of the spectral weights of these features yields
the parameters characterizing the on-site and long-range Coulomb interactions.Comment: 12 pages, 12 figure
Magnetic structure of the edge-sharing copper oxide chain compound NaCu2O2
Single-crystal neutron diffraction has been used to determine the
incommensurate magnetic structure of NaCu2O2, a compound built up of chains of
edge-sharing CuO4 plaquettes. Magnetic structures compatible with the lattice
symmetry were identified by a group-theoretical analysis, and their magnetic
structure factors were compared to the experimentally observed Bragg
intensities. In conjunction with other experimental data, this analysis yields
an elliptical helix structure in which both the helicity and the polarization
plane alternate among copper-oxide chains. This magnetic ground state is
discussed in the context of the recently reported multiferroic properties of
other copper-oxide chain compounds
Magnetic phase diagram of
Magnetometry, electrical transport, and neutron scattering measurements were
performed on single crystals of the Fe^{4+}-containing perovskite-related phase
Sr_3Fe_2O_7-x as a function of oxygen content. Although both the crystal
structure and electron configuration of this compound are closely similar to
those of well-studied ruthenates and manganates, it exhibits very different
physical properties. The fully-oxygenated compound (x=0) exhibits a
charge-disproportionation transition at T_D = 340 K, and an antiferromagnetic
transition at T_N = 115 K. For temperatures T \leq T_D, the material is a
small-gap insulator; the antiferromagnetic order is incommensurate, which
implies competing exchange interactions between the Fe^{4+} moments. The
fully-deoxygenated compound (x=1) is highly insulating, and its Fe^{3+} moments
exhibit commensurate antiferromagnetic order below T_N ~ 600 K. Compounds with
intermediate x exhibit different order with lower T_N, likely as a consequence
of frustrated exchange interactions between Fe^{3+} and Fe^{4+} sublattices. A
previous proposal that the magnetic transition temperature reaches zero is not
supported.Comment: 8 pages, 6 figure
Magnetic excitations and phonons in the spin-chain compound NaCu2O2
We report an inelastic light scattering study of single-crystalline
NaCuO, a spin-chain compound known to exhibit a phase with helical
magnetic order at low temperatures. Phonon excitations were studied as a
function of temperature and light polarization, and the phonon frequencies are
compared to the results of ab-initio lattice dynamical calculations, which are
also reported here. The good agreement between the observed and calculated
modes allows an assignment of the phonon eigenvectors. Two distinct high-energy
two-magnon features as well as a sharp low-energy one-magnon peak were also
observed. These features are discussed in terms of the magnon modes expected in
a helically ordered state. Their polarization dependence provides evidence of
substantial exchange interactions between two closely spaced spin chains within
a unit cell. At high temperatures, the spectral features attributable to
magnetic excitations are replaced by a broad, quasielastic mode due to
overdamped spin excitations
Spin liquid in a single crystal of the frustrated diamond lattice antiferromagnet CoAl2O4
We study spin liquid in the frustrated diamond lattice antiferromagnet
CoAl2O4 by means of single crystal neutron scattering in zero and applied
magnetic field. The magnetically ordered phase appearing below TN=8 K remains
nonconventional down to 1.5 K. The magnetic Bragg peaks at the q=0 positions
remain broad and their profiles have strong Lorentzian contribution.
Additionally, they are connected by weak diffuse streaks along the
directions. These observations are explained within the spiral spin liquid
model as short-range magnetic correlations of spirals populated at these finite
temperatures, as the energy minimum around q=0 is flat and the energy of
excited states with q=(111) is low. The agreement is only qualitative, leading
us to suspect that microstructure effects are also important. Magnetic field
significantly perturbs spin correlations. The 1.5 K static magnetic moment
increases from 1.58 mB/Co at zero field to 2.08 mB/Co at 10 T, while the
magnetic peaks, being still broad, acquire almost Gaussian profile. Spin
excitations are rather conventional spin waves at zero field, resulting in the
exchange parameters J1=0.92(1) meV, J2=0.101(2) meV and the anisotropy term
D=-0.0089(2) meV for CoAl2O4. The application of a magnetic field leads to a
pronounced broadening of the excitations at the zone center, which at 10 T
appear gapless and nearly featureless
Magnetoresistance Effects in SrFeO(3-x): Dependence on Phase Composition and Relation to Magnetic and Charge Order
Single crystals of iron(IV) rich oxides SrFeO(3-x) with controlled oxygen
content have been studied by Moessbauer spectroscopy, magnetometry,
magnetotransport measurements, Raman spectroscopy, and infrared ellipsometry in
order to relate the large magnetoresistance (MR) effects in this system to
phase composition, magnetic and charge order. It is shown that three different
types of MR effects occur. In cubic SrFeO3 (x = 0) a large negative MR of 25%
at 9 T is associated with a hitherto unknown 60 K magnetic transition and a
subsequent drop in resistivity. The 60 K transition appears in addition to the
onset of helical ordering at ~130 K. In crystals with vacancy-ordered
tetragonal SrFeO(3-x) as majority phase (x ~0.15) a coincident
charge/antiferromagnetic ordering transition near 70 K gives rise to a negative
giant MR effect of 90% at 9 T. A positive MR effect is observed in tetragonal
and orthorhombic materials with increased oxygen deficiency (x = 0.19, 0.23)
which are insulating at low temperatures. Phase mixtures can result in a
complex superposition of these different MR phenomena. The MR effects in
SrFeO(3-x) differ from those in manganites as no ferromagnetic states are
involved
Disparity of superconducting and pseudogap scales in low-Tc Bi-2201 cuprates
We experimentally study transport and intrinsic tunneling characteristics of
a single-layer cuprate Bi(2+x)Sr(2-y)CuO(6+delta) with a low superconducting
critical temperature Tc < 4 K. It is observed that the superconducting energy,
critical field and fluctuation temperature range are scaling down with Tc,
while the corresponding pseudogap characteristics have the same order of
magnitude as for high-Tc cuprates with 20 to 30 times higher Tc. The observed
disparity of the superconducting and pseudogap scales clearly reveals their
different origins.Comment: 5 page
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