255 research outputs found
Tunable Charge and Spin Order in PrNiO Thin Films and Superlattices
We have used polarized Raman scattering to probe lattice vibrations and
charge ordering in 12 nm thick, epitaxially strained PrNiO films, and in
superlattices of PrNiO with the band-insulator PrAlO. A carefully
adjusted confocal geometry was used to eliminate the substrate contribution to
the Raman spectra. In films and superlattices under tensile strain, which
undergo a metal-insulator transition upon cooling, the Raman spectra reveal
phonon modes characteristic of charge ordering. These anomalous phonons do not
appear in compressively strained films, which remain metallic at all
temperatures. For superlattices under compressive strain, the Raman spectra
show no evidence of anomalous phonons indicative of charge ordering, while
complementary resonant x-ray scattering experiments reveal antiferromagnetic
order associated with a modest increase in resistivity upon cooling. This
confirms theoretical predictions of a spin density wave phase driven by spatial
confinement of the conduction electrons.Comment: PRL, in pres
Momentum-dependent charge correlations in YBaCuO superconductors probed by resonant x-ray scattering: Evidence for three competing phases
We have used resonant x-ray scattering to determine the momentum dependent
charge correlations in YBaCuO samples with highly ordered
chain arrays of oxygen acceptors (ortho-II structure). The results reveal
nearly critical, biaxial charge density wave (CDW) correlations at in-plane
wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity
exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length
are enhanced as superconductivity is weakened by an external magnetic field.
Analogous experiments were carried out on a YBaCuO crystal with
a dilute concentration of spinless (Zn) impurities, which had earlier been
shown to nucleate incommensurate magnetic order. Compared to pristine crystals
with the same doping level, the CDW amplitude and correlation length were found
to be strongly reduced. These results indicate a three-phase competition
between spin-modulated, charge-modulated, and superconducting states in
underdoped YBaCuO.Comment: 6 pages, 3 figures revised version, to appear in Phys. Rev. Let
Long-range charge density wave proximity effect at cuprate-manganate interfaces
The interplay between charge density waves (CDWs) and high-temperature
superconductivity is currently under intense investigation. Experimental
research on this issue is difficult because CDW formation in bulk copper-oxides
is strongly influenced by random disorder, and a long-range-ordered CDW state
in high magnetic fields is difficult to access with spectroscopic and
diffraction probes. Here we use resonant x-ray scattering in zero magnetic
field to show that interfaces with the metallic ferromagnet
LaCaMnO greatly enhance CDW formation in the optimally
doped high-temperature superconductor YBaCuO (), and that this effect persists over several tens of nm. The wavevector
of the incommensurate CDW serves as an internal calibration standard of the
charge carrier concentration, which allows us to rule out any significant
influence of oxygen non-stoichiometry, and to attribute the observed phenomenon
to a genuine electronic proximity effect. Long-range proximity effects induced
by heterointerfaces thus offer a powerful method to stabilize the charge
density wave state in the cuprates, and more generally, to manipulate the
interplay between different collective phenomena in metal oxides.Comment: modified version published in Nature Material
Transfer of Magnetic Order and Anisotropy through Epitaxial Integration of 3d and 4f Spin Systems
Resonant x ray scattering at the Dy M 5 and Ni L 3 absorption edges was used to probe the temperature and magnetic field dependence of magnetic order in epitaxial LaNiO3 DyScO3 superlattices. For superlattices with 2 unit cell thick LaNiO3 layers, a commensurate spiral state develops in the Ni spin system below 100 K. Upon cooling below T ind 18 K, Dy Ni exchange interactions across the LaNiO3 DyScO3 interfaces induce collinear magnetic order of interfacial Dy moments as well as a reorientation of the Ni spins to a direction dictated by the strong magnetocrystalline anisotropy of Dy. This transition is reversible by an external magnetic field of 3 T. Tailored exchange interactions between rare earth and transition metal ions thus open up new perspectives for the manipulation of spin structures in metal oxide heterostructures and device
Site-selective Probe of Magnetic Excitations in Rare-earth Nickelates using Resonant Inelastic X-ray Scattering
We have used high-resolution resonant inelastic x-ray scattering (RIXS) to
study a thin film of NdNiO, a compound whose unusual spin- and bond-ordered
electronic ground state has been of long-standing interest. Below the magnetic
ordering temperature, we observe well-defined collective magnon excitations
along different high-symmetry directions in momentum space. The magnetic
spectra depend strongly on the incident photon energy, which we attribute to
RIXS coupling to different local electronic configurations of the expanded and
compressed NiO octahedra in the bond-ordered state. Both the noncollinear
magnetic ground state and the observed site-dependent magnon excitations are
well described by a model that assumes strong competition between the
antiferromagnetic superexchange and ferromagnetic double-exchange interactions.
Our study provides direct insight into the magnetic dynamics and exchange
interactions of the rare-earth nickelates, and demonstrates that RIXS can serve
as a site-selective probe of magnetism in these and other materials.Comment: Phys. Rev. X, in pres
Dispersion, damping, and intensity of spin excitations in the single-layer (Bi,Pb)(Sr,La)CuO cuprate superconductor family
Using Cu- edge resonant inelastic x-ray scattering (RIXS) we measured
the dispersion and damping of spin excitations (magnons and paramagnons) in the
high- superconductor (Bi,Pb)(Sr,La)CuO
(Bi2201), for a large doping range across the phase diagram (). Selected measurements with full polarization analysis
unambiguously demonstrate the spin-flip character of these excitations, even in
the overdoped sample. We find that the undamped frequencies increase slightly
with doping for all accessible momenta, while the damping grows rapidly, faster
in the (0,0)(0.5,0.5) nodal direction than in the
(0,0)(0.5,0) antinodal direction. We compare the experimental
results to numerically exact determinant quantum Monte Carlo (DQMC)
calculations that provide the spin dynamical structure factor
of the three-band Hubbard model. The theory reproduces
well the momentum and doping dependence of the dispersions and spectral weights
of magnetic excitations. These results provide compelling evidence that
paramagnons, although increasingly damped, persist across the superconducting
dome of the cuprate phase diagram; this implies that long range
antiferromagnetic correlations are quickly washed away, while short range
magnetic interactions are little affected by doping.Comment: 11 pages, 9 figure
Doping dependent charge order correlations in electron-doped cuprates
Understanding the interplay between charge order (CO) and other phenomena
(e.g. pseudogap, antiferromagnetism, and superconductivity) is one of the
central questions in the cuprate high-temperature superconductors. The
discovery that similar forms of CO exist in both hole- and electron-doped
cuprates opened a path to determine what subset of the CO phenomenology is
universal to all the cuprates. Here, we use resonant x-ray scattering to
measure the charge order correlations in electron-doped cuprates (La2-xCexCuO4
and Nd2-xCexCuO4) and their relationship to antiferromagnetism, pseudogap, and
superconductivity. Detailed measurements of Nd2-xCexCuO4 show that CO is
present in the x = 0.059 to 0.166 range, and that its doping dependent
wavevector is consistent with the separation between straight segments of the
Fermi surface. The CO onset temperature is highest between x = 0.106 and 0.166,
but decreases at lower doping levels, indicating that it is not tied to the
appearance of antiferromagnetic correlations or the pseudogap. Near optimal
doping, where the CO wavevector is also consistent with a previously observed
phonon anomaly, measurements of the CO below and above the superconducting
transition temperature, or in a magnetic field, show that the CO is insensitive
to superconductivity. Overall these findings indicate that, while verified in
the electron-doped cuprates, material-dependent details determine whether the
CO correlations acquire sufficient strength to compete for the ground state of
the cuprates.Comment: Supplementary information available upon reques
Polarization resolved Cu -edge resonant inelastic x-ray scattering of orbital and spin excitations in NdBaCuO
High resolution resonant inelastic x-ray scattering (RIXS) has proven
particularly effective in the determination of crystal field and spin
excitations in cuprates. Its strength lies in the large Cu resonance
and in the fact that the scattering cross section follows quite closely the
single-ion model predictions, both in the insulating parent compounds and in
the superconducting doped materials. However, the spectra become increasingly
broader with (hole) doping, hence resolving and assigning spectral features has
proven challenging even with the highest energy resolution experimentally
achievable. Here we have overcome this limitation by measuring the complete
polarization dependence of the RIXS spectra as function of momentum transfer
and doping in thin films of NdBaCuO. Besides
confirming the previous assignment of and spin excitations (magnon,
bimagnon) in the antiferromagnetic insulating parent compound, we unequivocally
single out the actual spin-flip contribution at all dopings. We also
demonstrate that the softening of excitations is mainly attributed to the
shift of the peak to lower energy loss. These results provide a definitive
assessment of the RIXS spectra of cuprates and demonstrate that RIXS
measurements with full polarization control are practically feasible and highly
informative.Comment: 14 pages, 10 figure
Long-range incommensurate charge fluctuations in (Y,Nd)Ba2Cu3O(6+x)
There are increasing indications that superconductivity competes with other
orders in cuprate superconductors, but obtaining direct evidence with
bulk-sensitive probes is challenging. We have used resonant soft x-ray
scattering to identify two-dimensional charge fluctuations with an
incommensurate periodicity of lattice units in the copper-oxide
planes of the superconductors (Y,Nd)BaCuO with hole
concentrations per planar Cu ion. The intensity and
correlation length of the fluctuation signal increase strongly upon cooling
down to the superconducting transition temperature, ; further cooling
below abruptly reverses the divergence of the charge correlations. In
combination with prior observations of a large gap in the spin excitation
spectrum, these data indicate an incipient charge-density-wave instability that
competes with superconductivity.Comment: to appear in Scienc
Connection between charge-density-wave order and charge transport in the cuprate superconductors
Charge-density-wave (CDW) correlations within the quintessential CuO
planes have been argued to either cause [1] or compete with [2] the
superconductivity in the cuprates, and they might furthermore drive the
Fermi-surface reconstruction in high magnetic fields implied by quantum
oscillation (QO) experiments for YBaCuO (YBCO) [3] and
HgBaCuO (Hg1201) [4]. Consequently, the observation of bulk
CDW order in YBCO was a significant development [5,6,7]. Hg1201 features
particularly high structural symmetry and recently has been demonstrated to
exhibit Fermi-liquid charge transport in the relevant temperature-doping range
of the phase diagram, whereas for YBCO and other cuprates this underlying
property of the CuO planes is partially or fully masked [8-10]. It
therefore is imperative to establish if the pristine transport behavior of
Hg1201 is compatible with CDW order. Here we investigate Hg1201 ( = 72 K)
via bulk Cu L-edge resonant X-ray scattering. We indeed observe CDW
correlations in the absence of a magnetic field, although the correlations and
competition with superconductivity are weaker than in YBCO. Interestingly, at
the measured hole-doping level, both the short-range CDW and Fermi-liquid
transport appear below the same temperature of about 200 K. Our result points
to a unifying picture in which the CDW formation is preceded at the higher
pseudogap temperature by = 0 magnetic order [11,12] and the build-up of
significant dynamic antiferromagnetic correlations [13]. Furthermore, the
smaller CDW modulation wave vector observed for Hg1201 is consistent with the
larger electron pocket implied by both QO [4] and Hall-effect [14]
measurements, which suggests that CDW correlations are indeed responsible for
the low-temperature QO phenomenon
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