216 research outputs found
Effect of localized holes on the long-range order in bilayer antiferromagnets
The effect of localized holes on the long-range antiferromagnetic order in
bilayer cuprates is studied, by applying the renormalization group to the
appropriate non-linear sigma model. The theory accounts quantitatively for the
magnetic phase diagram of Ca doepd YBa_2Cu_3O_6.Comment: 2 pages, to appear in Physica
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
Flux pinning and phase separation in oxygen rich La2-xSrxCuO4+y system
We have studied the magnetic characteristics of a series of super-oxygenated
La2-xSrxCuO4+y samples. As shown in previous work, these samples spontaneously
phase separate into an oxygen rich superconducting phase with a TC near 40 K
and an oxygen poor magnetic phase that also orders near 40 K. All samples
studied are highly magnetically reversible even to low temperatures. Although
the internal magnetic regions of these samples might be expected to act as
pinning sites, our present study shows that they do not favor flux pinning.
Flux pinning requires a matching condition between the defect and the
superconducting coherence length. Thus, our results imply that the magnetic
regions are too large to act as pinning centers. This also implies that the
much greater flux pinning in typical La2-xSrxCuO4 materials is the result of
nanoscale inhomogeneities that grow to become the large magnetic regions in the
super-oxygenated materials. The superconducting regions of the phase separated
materials are in that sense cleaner and more homogenous than in the typical
cuprate superconductor.Comment: 4 figures 8 pages Submitted to PR
The Accelerated expansion of the Universe as a crossover phenomenon
We show that the accelerated expansion of the Universe can be viewed as a
crossover phenomenon where the Newton constant and the Cosmological constant
are actually scaling operators, dynamically evolving in the attraction basin of
a non-Gaussian infrared fixed point, whose existence has been recently
discussed. By linearization of the renormalized flow it is possible to evaluate
the critical exponents, and it turns out that the approach to the fixed point
is ruled by a marginal and a relevant direction. A smooth transition between
the standard Friedmann--Lemaitre--Robertson--Walker (FLRW) cosmology and the
observed accelerated expansion is then obtained, so that at late times.Comment: 12 pages, latex, use bibtex. In the final version, the presentation
has been improved, and new references have been adde
Magnetic ground state and magnon-phonon interaction in multiferroic h-YMnO
Inelastic neutron scattering has been used to study the magneto-elastic
excitations in the multiferroic manganite hexagonal YMnO. An avoided
crossing is found between magnon and phonon modes close to the Brillouin zone
boundary in the -plane. Neutron polarization analysis reveals that this
mode has mixed magnon-phonon character. An external magnetic field along the
-axis is observed to cause a linear field-induced splitting of one of the
spin wave branches. A theoretical description is performed, using a Heisenberg
model of localized spins, acoustic phonon modes and a magneto-elastic coupling
via the single-ion magnetostriction. The model quantitatively reproduces the
dispersion and intensities of all modes in the full Brillouin zone, describes
the observed magnon-phonon hybridized modes, and quantifies the magneto-elastic
coupling. The combined information, including the field-induced magnon
splitting, allows us to exclude several of the earlier proposed models and
point to the correct magnetic ground state symmetry, and provides an effective
dynamic model relevant for the multiferroic hexagonal manganites.Comment: 12 pages, 10 figure
Magnetic-field-induced spin excitations and renormalized spin gap of the underdoped superconductor LaSrCuO
High-resolution neutron inelastic scattering experiments in applied magnetic
fields have been performed on LaSrCuO (LSCO). In zero
field, the temperature dependence of the low-energy peak intensity at the
incommensurate momentum-transfer $\mathbf{Q}^{\
}_{\mathrm{IC}}=(0.5,0.5\pm\delta,0),(0.5\pm\delta,0.5,0)T^{\}_{c}$ which broadens and shifts to lower
temperature upon the application of a magnetic field along the c-axis. A
field-induced enhancement of the spectral weight is observed, but only at
finite energy transfers and in an intermediate temperature range. These
observations establish the opening of a strongly downward renormalized spin gap
in the underdoped regime of LSCO. This behavior contrasts with the observed
doping dependence of most electronic energy features.Comment: accepted for publication in Phys. Rev. Let
Suppression of the structural phase transition and lattice softening in slightly underdoped Ba(1-x)K(x)Fe2As2 with electronic phase separation
We present x-ray powder diffraction (XRPD) and neutron diffraction
measurements on the slightly underdoped iron pnictide superconductor
Ba(1-x)K(x)Fe2As2, Tc = 32K. Below the magnetic transition temperature Tm =
70K, both techniques show an additional broadening of the nuclear Bragg peaks,
suggesting a weak structural phase transition. However, macroscopically the
system does not break its tetragonal symmetry down to 15 K. Instead, XRPD
patterns at low temperature reveal an increase of the anisotropic microstrain
proportionally in all directions. We associate this effect with the electronic
phase separation, previously observed in the same material, and with the effect
of lattice softening below the magnetic phase transition. We employ density
functional theory to evaluate the distribution of atomic positions in the
presence of dopant atoms both in the normal and magnetic states, and to
quantify the lattice softening, showing that it can account for a major part of
the observed increase of the microstrain.Comment: 7 pages, 4 figure
Similarity of slow stripe fluctations between Sr-doped cuprates and oxygen-doped nickelates
Stripe fluctuations in La2NiO4.17 have been studied by 139La NMR using the
field and temperature dependence of the linewidth and relaxation rates. In the
formation process of the stripes the NMR line intensity is maximal below 230K,
starts to diminish around 140K, disappears around 50K and recovers at 4K. These
results are shown to be consistent with, but completely complementary to
neutron measurements, and to be generic for oxygen doped nickelates and
underdoped cuprates.Comment: 4 pages including 4 figure
Specific heat of MgB in a one- and a two-band model from first-principles calculations
The heat capacity anomaly at the transition to superconductivity of the
layered superconductor MgB is compared to first-principles calculations
with the Coulomb repulsion, , as the only parameter which is fixed to
give the measured . We solve the Eliashberg equations for both an
isotropic one-band and a two-band model with different superconducting gaps on
the and Fermi surfaces. The agreement with experiments is
considerably better for the two-band model than for the one-band model.Comment: final published versio
Coupling of magnetic and ferroelectric hysteresis by a multi-component magnetic structure in Mn2GeO4
The olivine compound Mn2GeO4 is shown to feature both a ferroelectric
polarization and a ferromagnetic magnetization that are directly coupled and
point along the same direction. We show that a spin spiral generates
ferroelectricity (FE), and a canted commensurate order leads to weak
ferromagnetism (FM). Symmetry suggests that the direct coupling between the FM
and FE is mediated by Dzyaloshinskii-Moriya interactions that exist only in the
ferroelectric phase, controlling both the sense of the spiral rotation and the
canting of the commensurate structure. Our study demonstrates how
multi-component magnetic structures found in magnetically-frustrated materials
like Mn2GeO4 provide a new route towards functional materials that exhibit
coupled FM and FE.Comment: Supplementary material available on request, or at publisher websit
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