53 research outputs found
Zn and Ni doping effects on the low-energy spin excitations in LaSrCuO
Impurity effects of Zn and Ni on the low-energy spin excitations were
systematically studied in optimally doped La1.85Sr0.15Cu1-yAyO4 (A=Zn, Ni) by
neutron scattering. Impurity-free La1.85Sr0.15CuO4 shows a spin gap of 4meV
below Tc in the antiferromagnetic(AF) incommensurate spin excitation. In
Zn:y=0.004, the spin excitation shows a spin gap of 3meV below Tc. In
Zn:y=0.008 and Zn:y=0.011, however, the magnetic signals at 3meV decrease below
Tc and increase again at lower temperature, indicating an in-gap state. In
Zn:y=0.017, the low-energy spin state remains unchanged with decreasing
temperature, and elastic magnetic peaks appear below 20K then exponentially
increase. As for Ni:y=0.009 and Ni:y=0.018, the low-energy excitations below
3meV and 2meV disappear below Tc. The temperature dependence at 3meV, however,
shows no upturn in constrast with Zn:y=0.008 and Zn:y=0.011, indicating the
absence of in-gap state. In Ni:y=0.029, the magnetic signals were observed also
at 0meV. Thus the spin gap closes with increasing Ni. Furthermore, as omega
increases, the magnetic peak width broadens and the peak position, i.e.
incommensurability, shifts toward the magnetic zone center (pi pi). We
interpret the impurity effects as follows: Zn locally makes a
non-superconducting island exhibiting the in-gap state in the superconducting
sea with the spin gap. Zn reduces the superconducting volume fraction, thus
suppressing Tc. On the other hand, Ni primarily affects the superconducting
sea, and the spin excitations become more dispersive and broaden with
increasing energy, which is recognized as a consequence of the reduction of
energy scale of spin excitations. We believe that the reduction of energy scale
is relevant to the suppression of Tc.Comment: 13pages, 14figures; submitted to Phys. Rev.
Static magnetic correlations near the insulating-superconducting phase boundary in LaSrCuO
An elastic neutron scattering study has been performed on several single
crystals of LaSrCuO for {\it x} near the lower critical
concentration {\it x} for superconductivity. % In the insulating
spin-glass phase ({\it x} = 0.04 and 0.053), the previously reported
one-dimensional spin modulation along the orthorhombic {\it b}-axis is
confirmed. % Just inside the superconducting phase ({\it x} = 0.06), however,
two pairs of incommensurate magnetic peaks are additionally observed
corresponding to the spin modulation parallel to the tetragonal axes. % These
two types of spin modulations with similar incommensurabilities coexist near
the boundary. % The peak-width along spin-modulation direction
exhibits an anomalous maximum in the superconducting phase near {\it x},
where the incommensurability monotonically increases upon doping
across the phase boundary. % These results are discussed in connection with the
doping-induced superconducting phase transition.Comment: 9pages, 9figure
Characterization of low-energy magnetic excitations in chromium
The low-energy excitations of Cr, i.e. the Fincher-Burke (FB) modes, have
been investigated in the transversely polarized spin-density-wave phase by
inelastic neutron scattering using a single-(Q+-) crystal with a propagation
vector (Q+-) parallel to [0,0,1]. The constant-momentum-transfer scans show
that the energy spectra consist of two components, namely dispersive FB modes
and an almost energy-independent cross section. Most remarkably, we find that
the spectrum of the FB modes exhibits one peak at 140 K near Q = (0,0,0.98) and
two peaks near Q = (0,0,1.02), respectively. This is surprising because Cr
crystallizes in a centro-symmetric bcc structure. The asymmetry of those energy
spectra decreases with increasing temperature. In addition, the observed
magnetic peak intensity is independent of Q suggesting a transfer of
spectral-weight between the upper and lower FB modes. The energy-independent
cross section is localized only between the incommensurate peaks and develops
rapidly with increasing temperature.Comment: 6 pages, 8 figure
Dual structures in PZN-xPT ferroelectric relaxors
We performed x-ray diffraction studies on a series of
(1-)Pb(ZnNb)O-PbTiO (PZN-PT) single crystals
with different incident photon energies, and therefore different penetration
depths. Our results show that outer-layers of to 50 m thick are
present in all samples. The structure of those outer-layers is different from
that of the inside of the crystals, by having much greater (rhombohedral)
distortions. With increasing , rhombohedral-type lattice distortions
develop, both in the outer-layer and the inside.Comment: submitted to Appl. Phys. Let
Magnetization under High Pressure in MnSi
The magnetization M(H) has been measured in the weakly helimagnetic itinerant
compound MnSi under high pressure up to 10.2 kbar and high magnetic field up to
9 Tesla. We interpret the simultaneous decrease under pressure of the saturated
magnetization, , and the Curie temperature, in the frame of the
self-consistent renormalization theory (SCR) of spin fluctuations. From the
analysis of the so-called Arrot-plot ( versus ) and
the respective volume dependence of and , we estimate the evolution
of the characteristic spin fluctuation temperatures, and when the
system approaches its critical pressure, =15 kbar, corresponding to the
disappearance of the long range magnetic order at T=0.Comment: 12 pages, 5 figures. Submitted to Phys. Rev.
Incommensurate spin correlations induced by magnetic Fe ions substituted into overdoped Bi1.75Pb0.35Sr1.90CuO6+z
Spin correlations in the overdoped region of Bi1.75Pb0.35Sr1.90CuO6+z have
been explored with Fe-doped single crystals characterized by neutron
scattering, muon-spin-rotation (muSR) spectroscopy, and magnetic susceptibility
measurements. Static incommensurate spin correlations induced by the Fe spins
are revealed by elastic neutron scattering. The resultant incommensurability
delta is unexpectedly large (~0.2 r.l.u.), as compared with delta ~ 1/8 in
overdoped superconductor La2-xSrxCuO4. Intriguingly, the large delta in this
overdoped region is close to the hole concentration p. This result is
reminiscent of the delta ~ p trend observed in underdoped La2-xSrxCuO4;
however, it is inconsistent with the saturation of delta in the latter compound
in the overdoped regime. While our findings in Fe-doped
Bi1.75Pb0.35Sr1.90CuO6+z support the commonality of incommensurate spin
correlations in high-Tc cuprate superconductors, they also suggest that the
magnetic response might be dominated by a distinct mechanism in the overdoped
region.Comment: 4 pages, 5 figures. Revision in introduction, discussion, and
conclusion
Renormalization of Commensurate Magnetic Peak in Ni-doped LaSrCuO
We have studied the magnetic excitations in impurity doped
LaSrCuAO (A=Ni or Zn) by neutron
scattering. The dispersion for Zn: is similar to that for the impurity
free sample: incommensurate peaks with the incommensurability
(rlu) do not change their positions up to 21 meV. On the
other hand, for Ni:, two incommensurate peaks observed at low energies
suddenly change into a broad commensurate peak at meV.
Compared to the impurity free sample with a similar Sr-concentration ,
[B. Vignolle {\it et al.} Nature Physics {\bf 3} (2007) 163],
for Ni: is decreased by nearly the same factor for
the reduction in . This is very similar to the shift of the resonance
energy () in Ni-doped YBaCuO.[Y. Sidis {\it
et al.}: Phys. Rev. Lett. {\bf 84} (2000) 5900]. These common impurity effects
on the shift of and suggest the same
magnetic origin for the resonance peak in YBaCuO and
that for a crossing point of upward and downward dispersions in the
LaSrCuO. We propose that the sudden change in the
dispersion is better described by a crossover from incommensurate spin
fluctuations to a gapped spin wave rather than a hourglass-like dispersion.Comment: 11 pages, 5 figure
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