546 research outputs found
Spin dynamics near the critical doping in weakly-superconducting underdoped YBa2Cu3O6.35 (Tc=18K)
Using neutron scattering we have determined the magnetic structure and
fluctuations in the YBa2Cu3O6.35 superconductor (Tc=18 K). The long-range
ordered collinear spins of the insulating antiferromagnet are replaced by a
commensurate central mode arising from slow, isotropically polarized,
short-range spin correlations. The inelastic spectrum up to 30 meV is broad in
wave vector and commensurate. In contrast to the the resonance peak of higher
Tc superconductors, the spins exhibit a single overdamped spectrum whose rate
of relaxation decreases on cooling and saturates at 5 meV below 50 K. As the
relaxation rate saturates the quasi-static spin correlations grow and become
resolution limited in energy. The spin susceptibility at high temperatures
illustrates that the dominant energy scale is set by the temperature. At low
temperatures, the scale length is geometric and not linked by velocity to
dynamic widths. There is no observable suppression of the spin fluctuations or
central mode upon the onset of superconductivity. The spins respond not to
coherent charge pairs but to hole doping allowing coexistence of glassy short
range spin order with superconductivity. Since the physics of the weakly
superconducting system YBCO6.35 must connect continuously with that in more
strongly superconducting YBCO6.5, we find that neither incommensurate
stripe-like spin modulations nor a well-defined neutron spin resonance are
essential for the onset with doping of pairing in a high temperature cuprate
superconductor.Comment: 22 pages, 19 figures, accepted for publication in Phys. Rev.
Anisotropy of the incommensurate fluctuations in Sr2RuO4: a study with polarized neutrons
The anisotropy of the magnetic incommensurate fluctuations in Sr2RuO4 has
been studied by inelastic neutron scattering with polarized neutrons. We find a
sizeable enhancement of the out of plane component by a factor of two for
intermediate energy transfer which appears to decrease for higher energies. Our
results qualitatively confirm calculations of the spin-orbit coupling, but the
experimental anisotropy and its energy dependence are weaker than predicted.Comment: 4 pages, 4 figure
Intrinsic Localized Modes Observed in the High Temperature Vibrational Spectrum of NaI
Inelastic neutron measurements of the high-temperature lattice excitations in
NaI show that in thermal equilibrium at 555 K an intrinsic mode, localized in
three dimensions, occurs at a single frequency near the center of the spectral
phonon gap, polarized along [111]. At higher temperatures the intrinsic
localized mode gains intensity. Higher energy inelastic neutron and x-ray
scattering measurements on a room-temperature NaI crystal indicate that the
creation energy of the ground state of the intrinsic localized mode is 299 meV.Comment: 17 pages, 5 figures Revised version; final versio
Soft Phonon Anomalies in the Relaxor Ferroelectric Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3
Neutron inelastic scattering measurements of the polar TO phonon mode
dispersion in the cubic relaxor Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3 at 500K reveal
anomalous behavior in which the optic branch appears to drop precipitously into
the acoustic branch at a finite value of the momentum transfer q=0.2 inverse
Angstroms, measured from the zone center. We speculate this behavior is the
result of nanometer-sized polar regions in the crystal.Comment: 4 pages, 4 figure
A Universal Phase Diagram for PMN-xPT and PZN-xPT
The phase diagram of the Pb(Mg1/3Nb2/3)O3 and PbTiO3 solid solution (PMN-xPT)
indicates a rhombohedral ground state for x < 0.32. X-ray powder measurements
by Dkhil et al. show a rhombohedrally split (222) Bragg peak for PMN-10%PT at
80 K. Remarkably, neutron data taken on a single crystal of the same compound
with comparable q-resolution reveal a single resolution-limited (111) peak down
to 50 K, and thus no rhombohedral distortion. Our results suggest that the
structure of the outer layer of these relaxors differs from that of the bulk,
which is nearly cubic, as observed in PZN by Xu et al.Comment: Replaced Fig. 3 with better versio
Helical spin-waves, magnetic order, and fluctuations in the langasite compound Ba3NbFe3Si2O14
We have investigated the spin fluctuations in the langasite compound
Ba3NbFe3Si2O14 in both the ordered state and as a function of temperature. The
low temperature magnetic structure is defined by a spiral phase characterized
by magnetic Bragg peaks at q=(0,0,tau ~ 1/7) onset at TN=27 K as previously
reported by Marty et al. The nature of the fluctuations and temperature
dependence of the order parameter is consistent with a classical second order
phase transition for a two dimensional triangular antiferromagnet. We will show
that the physical properties and energy scales including the ordering
wavevector, Curie-Weiss temperature, and the spin-waves can be explained
through the use of only symmetric exchange constants without the need for the
Dzyaloshinskii-Moriya interaction. This is accomplished through a set of
``helical" exchange pathways along the c direction imposed by the chiral
crystal structure and naturally explains the magnetic diffuse scattering which
displays a strong vector chirality up to high temperatures well above the
ordering temperature. This illustrates a strong coupling between magnetic and
crystalline chirality in this compound.Comment: 16 pages, 16 figures, submitted to Physical Review
Strong Influence of the diffuse component on the lattice dynamics in Pb(MgNb)O
The temperature and zone dependence of the lattice dynamics in
Pb(MgNb)O is characterized using neutron inelastic
scattering. A strong correlation between the diffuse and phonon scattering is
measured. The lattice dynamics in Brillouin zones where the diffuse scattering
is strong is observed to display qualitatively different behavior than those
zones where the diffuse scattering is weak. In the (220) and (200) zones, where
there is a weak diffuse component, the dynamics are well described by coupled
harmonic oscillators. Compared with SrTiO, the coupling is weak and
isotropic, resulting in only a small transfer of spectral weight from one mode
to another. A comparison of the scattering in these zones to the (110) zone,
where a strong diffuse component is present, reveals a strong coupling of the
diffuse (or central) component to the acoustic mode. We speculate that the
coupling to the central peak is the reason for several recent conflicting
interpretations of the lattice dynamics based on data from zones with a strong
diffuse component.Comment: 7 pages, 7 figure
Magnetism and Structural Distortion in the La0.7Sr0.3MnO3 Metallic Ferromagnet
Neutron scattering studies on a single crystal of the highly-correlated
electron system, La1-xSrxMnO3 with x~0.3, have been carried out elucidating
both the spin and lattice dynamics of this metallic ferromagnet. We report a
large measured value of the spin wave stiffness constant, which directly shows
that the electron transfer energy of the d band is large. The spin dynamics,
including magnetic critical scattering, demonstrate that this material behaves
similar to other typical metallic ferromagnets such as Fe or Ni. The crystal
structure is rhombohedral, as previously reported, for all temperatures studied
(below ~425K). We have observed new superlattice peaks which show that the
primary rhombohedral lattice distortion arises from oxygen octahedra rotations
resulting in an R-3c structure. The superlattice reflection intensities which
are very sensitive to structural changes are independent of temperature
demonstrating that there is no primary lattice distortion anomaly at the
magnetic transition temperature, Tc = 378.1 K, however there is a lattice
contraction.Comment: Submitted to Phys. Rev. B. (03Aug95) Uuencoded gz-compressed .tar
file of Postscript text (12 pages) and 6 figures. Also available by WWW from
http://insti.physics.sunysb.edu/~mmartin/ under my list of publications or by
e-mail reques
High Temperature Emissivity, Reflectivity, and X-ray absorption of BiFeO3
We report on the lattice evolution of BiFeO3 as function of temperature using
far infrared emissivity, reflectivity, and X-ray absorption local structure. A
power law fit to the lowest frequency soft phonon in the magnetic ordered phase
yields an exponent {\beta}=0.25 as for a tricritical point. At about 200 K
below TN~640 K it ceases softening as consequence of BiFeO3 metastability. We
identified this temperature as corresponding to a crossover transition to an
order-disorder regime. Above ~700 K strong band overlapping, merging, and
smearing of modes are consequence of thermal fluctuations and chemical
disorder. Vibrational modes show band splits in the ferroelectric phase as
emerging from triple degenerated species as from a paraelectric cubic phase
above TC~1090 K. Temperature dependent X-ray absorption near edge structure
(XANES) at the Fe K-edge shows that lower temperature Fe3+ turns into Fe2+.
While this matches the FeO w\"ustite XANES profile, the Bi LIII-edge downshift
suggests a high temperature very complex bond configuration at the distorted A
perovskite site. Overall, our local structural measurements reveal high
temperature defect-induced irreversible lattice changes, below, and above the
ferroelectric transition, in an environment lacking of long-range coherence. We
did not find an insulator to metal transition prior to melting.Comment: Accepted for publicatio
Neutron Diffuse Scattering from Polar Nanoregions in the Relaxor Pb(Mg1/3Nb2/3)O3
We have studied the neutron diffuse scattering in the relaxor PMN. The
diffuse scattering appears around the Burns temperature (~620K), indicating its
origin from the polar nanoregions (PNR). While the relative diffuse intensities
are consistent with previous reports, they are entirely different from those of
the lowest-energy TO phonon. Because of that, it has been considered that this
TO mode could not be the ferroelectric soft mode. Recently, a neutron
scattering study has unambiguously shown that the TO mode does soften on
cooling. If the diffuse scattering in PMN originates from the soft mode
condensation, then the atomic displacements must satisfy the center of mass
condition. But, the atomic displacements determined from diffuse scattering
intensities do not fulfill this condition. To resolve this contradiction, we
propose a simple model in which the total atomic displacement consists of two
components: is created by the soft mode condensation, satisfying
the center of mass condition, and, represents a uniform
displacement of the PNR along their polar direction relative to the surrounding
(unpolarized) cubic matrix. Within this framework, we can successfully describe
the neutron diffuse scattering intensities observed in PMN.Comment: 7 pages, 7 figures (Revised: 11-16-2001
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