248 research outputs found
Commensurate Fluctuations in the Pseudogap and Incommensurate spin-Peierls Phases of TiOCl
X-ray scattering measurements on single crystals of TiOCl reveal the presence
of commensurate dimerization peaks within both the incommensurate spin-Peierls
phase and the so-called pseudogap phase above T_c2. This scattering is
relatively narrow in Q-space indicating long correlation lengths exceeding ~
100 A below T* ~ 130 K. It is also slightly shifted in Q relative to that of
the commensurate long range ordered state at the lowest temperatures, and it
coexists with the incommensurate Bragg peaks below T_c2. The integrated
scattering over both commensurate and incommensurate positions evolves
continuously with decreasing temperature for all temperatures below T* ~ 130 K.Comment: To appear in Physical Review B: Rapid Communications. 5 page
Suppression of the commensurate spin-Peierls state in Sc-doped TiOCl
We have performed x-ray scattering measurements on single crystals of the
doped spin-Peierls compound Ti(1-x)Sc(x)OCl (x = 0, 0.01, 0.03). These
measurements reveal that the presence of non-magnetic dopants has a profound
effect on the unconventional spin-Peierls behavior of this system, even at
concentrations as low as 1%. Sc-doping suppresses commensurate fluctuations in
the pseudogap and incommensurate spin-Peierls phases of TiOCl, and prevents the
formation of a long-range ordered spin-Peierls state. Broad incommensurate
scattering develops in the doped compounds near Tc2 ~ 93 K, and persists down
to base temperature (~ 7 K) with no evidence of a lock-in transition. The width
of the incommensurate dimerization peaks indicates short correlation lengths on
the order of ~ 12 angstroms below Tc2. The intensity of the incommensurate
scattering is significantly reduced at higher Sc concentrations, indicating
that the size of the associated lattice displacement decreases rapidly as a
function of doping.Comment: 7 pages, 5 figure
Hour-glass magnetic excitations induced by nanoscopic phase separation in cobalt oxides LaSrCoO
The magnetic excitations in the cuprate superconductors might be essential
for an understanding of high-temperature superconductivity. In these cuprate
superconductors the magnetic excitation spectrum resembles an hour-glass and
certain resonant magnetic excitations within are believed to be connected to
the pairing mechanism which is corroborated by the observation of a universal
linear scaling of superconducting gap and magnetic resonance energy. So far,
charge stripes are widely believed to be involved in the physics of hour-glass
spectra. Here we study an isostructural cobaltate that also exhibits an
hour-glass magnetic spectrum. Instead of the expected charge stripe order we
observe nano phase separation and unravel a microscopically split origin of
hour-glass spectra on the nano scale pointing to a connection between the
magnetic resonance peak and the spin gap originating in islands of the
antiferromagnetic parent insulator. Our findings open new ways to theories of
magnetic excitations and superconductivity in cuprate superconductors.Comment: Nature Communications 5, 5731 (2014
Electronic and magnetic nano phase separation in cobaltates LaSrCoO
The single-layer perovskite cobaltates have attracted enormous attention due
to the recent observation of hour-glass shaped magnetic excitation spectra
which resemble the ones of the famous high-temperature superconducting
cuprates. Here, we present an overview of our most recent studies of the spin
and charge correlations in floating-zone grown cobaltate single crystals. We
find that frustration and a novel kind of electronic and magnetic nano phase
separation are intimately connected to the appearance of the hour-glass shaped
spin excitation spectra. We also point out the difference between nano phase
separation and conventional phase separation.Comment: * plenary talk SUPERSTRIPES conference 201
Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)
Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and
dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15).
Static dielectric constants are determined from the low-T limiting behavior of
the permittivity. The estimated polarizability for bound holes ~ 10^{-22}
cm^{-3} implies a radius comparable to the interatomic spacing, consistent with
the small polaron picture established from prior transport studies near room
temperature and above on nearby compositions. Relaxation peaks in the
dielectric loss associated with charge-carrier hopping yield activation
energies in good agreement with low-T hopping energies determined from
variable-range hopping fits of the dc resistivity. The doping dependence of
these energies suggests that the orthorhombic, canted antiferromagnetic ground
state tends toward an insulator-metal transition that is not realized due to
the formation of the ferromagnetic insulating state near Mn(4+) concentration ~
0.13.Comment: PRB in press, 5 pages, 6 figure
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