303 research outputs found
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
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
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
Abrupt Onset of Second Energy Gap at Superconducting Transition of Underdoped Bi2212
The superconducting gap - an energy scale tied to the superconducting
phenomena-opens on the Fermi surface at the superconducting transition
temperature (TC) in conventional BCS superconductors. Quite differently, in
underdoped high-TC superconducting cuprates, a pseudogap, whose relation to the
superconducting gap remains a mystery, develops well above TC. Whether the
pseudogap is a distinct phenomenon or the incoherent continuation of the
superconducting gap above TC is one of the central questions in high-TC
research. While some experimental evidence suggests they are distinct, this
issue is still under intense debate. A crucial piece of evidence to firmly
establish this two-gap picture is still missing: a direct and unambiguous
observation of a single-particle gap tied to the superconducting transition as
function of temperature. Here we report the discovery of such an energy gap in
underdoped Bi2212 in the momentum space region overlooked in previous
measurements. Near the diagonal of Cu-O bond direction (nodal direction), we
found a gap which opens at TC and exhibits a canonical (BCS-like) temperature
dependence accompanied by the appearance of the so-called Bogoliubov
quasiparticles, a classical signature of superconductivity. This is in sharp
contrast to the pseudogap near the Cu-O bond direction (antinodal region)
measured in earlier experiments. The emerging two-gap phenomenon points to a
picture of richer quantum configurations in high temperature superconductors.Comment: 16 pages, 4 figures, authors' version Corrected typos in the abstrac
Two Energy Scales and two Quasiparticle Dynamics in the Superconducting State of Underdoped Cuprates
The superconducting state of underdoped cuprates is often described in terms
of a single energy-scale, associated with the maximum of the (d-wave) gap.
Here, we report on electronic Raman scattering results, which show that the gap
function in the underdoped regime is characterized by two energy scales,
depending on doping in opposite manners. Their ratios to the maximum critical
temperature are found to be universal in cuprates. Our experimental results
also reveal two different quasiparticle dynamics in the underdoped
superconducting state, associated with two regions of momentum space: nodal
regions near the zeros of the superconducting gap and antinodal regions. While
antinodal quasiparticles quickly loose coherence as doping is reduced, coherent
nodal quasiparticles persist down to low doping levels. A theoretical analysis
using a new sum-rule allows us to relate the low-frequency-dependence of the
Raman response to the temperature-dependence of the superfluid density, both
controlled by nodal excitations.Comment: 16 pages, 5 figure
Imaging the Two Gaps of the High-TC Superconductor Pb-Bi2Sr2CuO6+x
The nature of the pseudogap state, observed above the superconducting
transition temperature TC in many high temperature superconductors, is the
center of much debate. Recently, this discussion has focused on the number of
energy gaps in these materials. Some experiments indicate a single energy gap,
implying that the pseudogap is a precursor state. Others indicate two,
suggesting that it is a competing or coexisting phase. Here we report on
temperature dependent scanning tunneling spectroscopy of Pb-Bi2Sr2CuO6+x. We
have found a new, narrow, homogeneous gap that vanishes near TC, superimposed
on the typically observed, inhomogeneous, broad gap, which is only weakly
temperature dependent. These results not only support the two gap picture, but
also explain previously troubling differences between scanning tunneling
microscopy and other experimental measurements.Comment: 6 page
Energy and symmetry of excitations in undoped layered cuprates measured by Cu resonant inelastic x-ray scattering
We measured high resolution Cu edge resonant inelastic x-ray scattering
(RIXS) of the undoped cuprates LaCuO, SrCuOCl, CaCuO
and NdBaCuO. The dominant spectral features were assigned to
excitations and we extensively studied their polarization and scattering
geometry dependence. In a pure ionic picture, we calculated the theoretical
cross sections for those excitations and used them to fit the experimental data
with excellent agreement. By doing so, we were able to determine the energy and
symmetry of Cu-3 states for the four systems with unprecedented accuracy and
confidence. The values of the effective parameters could be obtained for the
single ion crystal field model but not for a simple two-dimensional cluster
model. The firm experimental assessment of excitation energies carries
important consequences for the physics of high superconductors. On one
hand, having found that the minimum energy of orbital excitation is always
eV, i.e., well above the mid-infrared spectral range, leaves to
magnetic excitations (up to 300 meV) a major role in Cooper pairing in
cuprates. On the other hand, it has become possible to study quantitatively the
effective influence of excitations on the superconducting gap in cuprates.Comment: 22 pages, 11 figures, 1 tabl
Dispersive charge density wave excitations and temperature dependent commensuration in Bi2Sr2CaCu2O8+{\delta}
Experimental evidence on high-Tc cuprates reveals ubiquitous charge density
wave (CDW) modulations, which coexist with superconductivity. Although the CDW
had been predicted by theory, important questions remain about the extent to
which the CDW influences lattice and charge degrees of freedom and its
characteristics as functions of doping and temperature. These questions are
intimately connected to the origin of the CDW and its relation to the
mysterious cuprate pseudogap. Here, we use ultrahigh resolution resonant
inelastic x-ray scattering (RIXS) to reveal new CDW character in underdoped
Bi2Sr2CaCu2O8+{\delta} (Bi2212). At low temperature, we observe dispersive
excitations from an incommensurate CDW that induces anomalously enhanced phonon
intensity, unseen using other techniques. Near the pseudogap temperature T*,
the CDW persists, but the associated excitations significantly weaken and the
CDW wavevector shifts, becoming nearly commensurate with a periodicity of four
lattice constants. The dispersive CDW excitations, phonon anomaly, and
temperature dependent commensuration provide a comprehensive momentum space
picture of complex CDW behavior and point to a closer relationship with the
pseudogap state
Dispersive spin excitations in highly overdoped cuprates revealed by resonant inelastic x-ray scattering
Using resonant inelastic x-ray scattering (RIXS) at the Cu -absorption
edge, we have observed intense, dispersive spin excitations in highly overdoped
TlBaCuO (superconducting K), a compound whose
normal-state charge transport and thermodynamic properties have been studied
extensively and shown to exhibit canonical Fermi-liquid behavior. Complementary
RIXS experiments on slightly overdoped TlBaCuO (
K) and on YCaBaCuO compounds spanning a wide
range of doping levels indicate that these excitations exhibit energies and
energy-integrated spectral weights closely similar to those of
antiferromagnetic magnons in undoped cuprates. The surprising coexistence of
Fermi-liquid-like charge excitations and high-energy spin excitations
reminiscent of antiferromagnetic insulators in highly overdoped compounds poses
a challenge to current theoretical models of the cuprates
Influence of apical oxygen on the extent of in-plane exchange interaction in cuprate superconductors
In high Tc superconductors the magnetic and electronic properties are
determined by the probability that valence electrons virtually jump from site
to site in the CuO2 planes, a mechanism opposed by on-site Coulomb repulsion
and favored by hopping integrals. The spatial extent of the latter is related
to transport properties, including superconductivity, and to the dispersion
relation of spin excitations (magnons). Here, for three antiferromagnetic
parent compounds (single-layer Bi2Sr0.99La1.1CuO6+delta, double-layer
Nd1.2Ba1.8Cu3O6 and infinite-layer CaCuO2) differing by the number of apical
atoms, we compare the magnetic spectra measured by resonant inelastic x-ray
scattering over a significant portion of the reciprocal space and with
unprecedented accuracy. We observe that the absence of apical oxygens increases
the in-plane hopping range and, in CaCuO2, it leads to a genuine 3D
exchange-bond network. These results establish a corresponding relation between
the exchange interactions and the crystal structure, and provide fresh insight
into the materials dependence of the superconducting transition temperature.Comment: 9 pages, 4 figures, 1 Table, 42 reference
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