208 research outputs found

### Magnetic Order and Dynamics in Stripe-Ordered La2-xSrxNiO4

We have studied magnetic correlations in several compositions of
stripe-ordered La2-xSrxNiO4. In this paper we show how polarized-neutron
scattering has helped uncover important features of the magnetic ordering and
spin dynamics. In particular, polarization analysis has enabled us (1) to
characterize a spin reorientation transition, (2) to identify anisotropy gaps
in the spin excitation spectrum, and (3) to investigate an anomalous dip in the
spin-wave intensity suggestive of coupling between collective spin and charge
excitations.Comment: 4 pages, 3 figs. Presented at PNSXM, Venice. To appear in Physica

### Origin of the "Waterfall" Effect in Phonon Dispersion of Relaxor Perovskites

Inelastic neutron scattering study of the perovskite relaxor ferroelectric
PZN:8%PT elucidates the origin of the previously reported unusual kink on the
low frequency transverse phonon dispersion curve (known as "waterfall" effect).
We show that its position depends on the choice of the Brillouin zone and that
the relation of its position to the size of the polar nanoregions is highly
improbable. The observation is explained in the framework of a simple model of
coupled damped harmonic oscillators representing the acoustic and optic phonon
branches.Comment: 4 pages, 6 figures, LaTe

### Anharmonic Self-Energy of Phonons: Ab Initio Calculations and Neutron Spin Echo Measurements

We have calculated (ab initio) and measured (by spin-echo techniques) the
anharmonic self-energy of phonons at the X-point of the Brillouin zone for
isotopically pure germanium. The real part agrees with former, less accurate,
high temperature data obtained by inelastic neutron scattering on natural
germanium. For the imaginary part our results provide evidence that transverse
acoustic phonons at the X-point are very long lived at low temperatures, i.e.
their probability of decay approaches zero, as a consequence of an unusual
decay mechanism allowed by energy conservation.Comment: 8 pages, 2 figures, pdf fil

### Magnetic excitations in the metallic single-layer Ruthenates Ca(2-x)Sr(x)RuO(4) studied by inelastic neutron scattering

By inelastic neutron scattering, we have analyzed the magnetic correlations
in the paramagnetic metallic region of the series Ca(2-x)Sr(x)RuO(4),
0.2<=x<=0.62. We find different contributions that correspond to 2D
ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors
(0.11,0,0), (0.26,0,0) and (0.3,0.3,0). These components constitute the
measured response as function of the Sr-concentration x, of the magnetic field
and of the temperature. A generic model is applicable to metallic
Ca(2-x)Sr(x)RuO(4) close to the Mott transition, in spite of their strongly
varying physical properties. The amplitude, characteristic energy and width of
the incommensurate components vary only little as function of x, but the
ferromagnetic component depends sensitively on concentration, temperature and
magnetic field. While ferromagnetic fluctuations are very strong in
Ca1.38Sr0.62RuO4 with a low characteristic energy of 0.2 meV at T=1.5 K, they
are strongly suppressed in Ca1.8Sr0.2RuO4, but reappear upon the application of
a magnetic field and form a magnon mode above the metamagnetic transition. The
inelastic neutron scattering results document how the competition between
ferromagnetic and incommensurate antiferromagnetic instabilities governs the
physics of this system

### Spin-wave spectrum of copper metaborate in the commensurate phase 10K<T<21K

We have investigated the spin-wave spectrum of copper metaborate,
CuB$_2$O$_4$, by means of inelastic neutron scattering in the commensurate
magnetic phase. We have found two branches of spin-wave excitations associated
with the two magnetic sublattices Cu(A) and Cu(B), respectively. In the
temperature regime $10K \le T \le 21K$, where only the Cu(A) magnetic moments
are ordered, the interaction between the two sublattices is found to be
negligible. With this approximation we have determined the `easy plane'
exchange parameters of the Cu(A) subsystem within standard spin-wave theory.Comment: 4 figure

### No Evidence for Orbital Loop Currents in Charge Ordered YBa$_2$Cu$_3$O$_{6+x}$ from Polarized Neutron Diffraction

It has been proposed that the pseudogap state of underdoped cuprate
superconductors may be due to a transition to a phase which has circulating
currents within each unit cell. Here, we use polarized neutron diffraction to
search for the corresponding orbital moments in two samples of underdoped
YBa$_2$Cu$_3$O$_{6+x}$ with doping levels $p=0.104$ and 0.123. In contrast to
some other reports using polarized neutrons, but in agreement with nuclear
magnetic resonance and muon spin rotation measurements, we find no evidence for
the appearance of magnetic order below 300 K. Thus, our experiment suggests
that such order is not an intrinsic property of high-quality cuprate
superconductor single crystals. Our results provide an upper bound for a
possible orbital loop moment which depends on the pattern of currents within
the unit cell. For example, for the CC-$\theta_{II}$ pattern proposed by Varma,
we find that the ordered moment per current loop is less than 0.013 $\mu_B$ for
$p=0.104$.Comment: Comments in arXiv:1710.08173v1 fully addresse

### Dynamic displacement disorder of cubic BaTiO3

The three-dimensional distribution of the x-ray diffuse scattering intensity of BaTiO3 has been recorded in a synchrotron experiment and simultaneously computed using molecular dynamics simulations of a shell model. Together, these have allowed the details of the disorder in paraelectric BaTiO3 to be clarified. The narrow sheets of diffuse scattering, related to the famous anisotropic longitudinal correlations of Ti ions, are shown to be caused by the overdamped anharmonic soft phonon branch. This finding demonstrates that the occurrence of narrow sheets of diffuse scattering agrees with a displacive picture of the cubic phase of this textbook ferroelectric material. The presented methodology allows one to go beyond the harmonic approximation in the analysis of phonons and phonon-related scattering

### Spin correlations among the charge carriers in an ordered stripe phase

We have observed a diffuse component to the low-energy magnetic excitation
spectrum of stripe-ordered La5/3Sr1/3NiO4 probed by neutron inelastic
scattering. The diffuse scattering forms a square pattern with sides parallel
and perpendicular to the stripe directions. The signal is dispersive, with a
maximum energy of ~10 meV. Probed at 2 meV the scattering decreases in strength
with increasing temperature, and is barely visible at 100 K. We argue that the
signal originates from dynamic, quasi- one-dimensional, antiferromagnetic
correlations among the stripe electrons.Comment: 4 pages, 4 figures. To be published in Physical Review Letter

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