413 research outputs found
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
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
Charge order driven by Fermi-arc instability in Bi2201
The understanding of the origin of superconductivity in cuprates has been
hindered by the apparent diversity of intertwining electronic orders in these
materials. We combined resonant x-ray scattering (REXS), scanning-tunneling
microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) to
observe a charge order that appears consistently in surface and bulk, and in
momentum and real space within one cuprate family, Bi2201. The observed wave
vectors rule out simple antinodal nesting in the single-particle limit but
match well with a phenomenological model of a many-body instability of the
Fermi arcs. Combined with earlier observations of electronic order in other
cuprate families, these findings suggest the existence of a generic
charge-ordered state in underdoped cuprates and uncover its intimate connection
to the pseudogap regime.Comment: A high resolution version can be found at
http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/Bi2201_CDW_REXS_STM.pdf
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
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
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
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
Tracing magnetism and pairing in FeTe-based systems
In order to examine the interplay between magnetism and superconductivity, we
monitor the non- superconducting chalcogenide FeTe and follow its transitions
under insertion of oxygen, doping with Se and vacancies of Fe using
spin-polarized band structure methods (LSDA with GGA) starting from the
collinear and bicollinear magnetic arrangements. We use a supercell of Fe8Te8
as our starting point so that it can capture local changes in magnetic moments.
The calculated values of magnetic moments agree well with available
experimental data while oxygen insertions lead to significant changes in the
bicollinear or collinear magnetic moments. The total energies of these systems
indicate that the collinear-derived structure is the more favorable one prior
to a possible superconducting transition. Using a 8-site Betts-cluster-based
lattice and the Hubbard model, we show why this structure favors electron or
hole pairing and provides clues to a common understanding of charge and spin
pairing in the cuprates, pnictides and chalcogenides
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