45 research outputs found
Cu nuclear magnetic resonance study of charge and spin stripe order in LaBaCuO
We present a Cu nuclear magnetic/quadrupole resonance study of the charge
stripe ordered phase of LBCO, with detection of previously unobserved
('wiped-out') signal. We show that spin-spin and spin-lattice relaxation rates
are strongly enhanced in the charge ordered phase, explaining the apparent
signal decrease in earlier investigations. The enhancement is caused by
magnetic, rather than charge fluctuations, conclusively confirming the
long-suspected assumption that spin fluctuations are responsible for the
wipeout effect. Observation of the full Cu signal enables insight into the spin
and charge dynamics of the stripe-ordered phase, and measurements in external
magnetic fields provide information on the nature and suppression of spin
fluctuations associated with charge order. We find glassy spin dynamics, in
agreement with previous work, and incommensurate static charge order with
charge modulation amplitude similar to other cuprate compounds, suggesting that
the amplitude of charge stripes is universal in the cuprates.Comment: 7 pages, 5 figure
Probing optically silent superfluid stripes in cuprates
Unconventional superconductivity in the cuprates emerges from, or coexists
with, other types of electronic order. However, these orders are sometimes
invisible because of their symmetry. For example, the possible existence of
superfluid charge stripes in the normal state of single layer cuprates cannot
be validated with infrared optics, because interlayer tunneling fluctuations
vanish on average. Similarly, it is not easy to establish if charge orders are
responsible for dynamical decoupling of the superconducting layers over broad
ranges of doping and temperatures. Here, we show that TeraHertz pulses can
excite nonlinear tunneling currents between linearly de-coupled charge-ordered
planes. A giant TeraHertz third harmonic signal is observed in
La1.885Ba0.115CuO4 far above Tc=13 K and up to the charge ordering temperature
TCO = 55 K. We model these results by considering large order-parameter-phase
oscillations in a pair density wave condensate, and show how nonlinear mixing
of optically silent tunneling modes can drive large dipole-carrying
super-current oscillations. Our results provide compelling experimental support
for the presence of hidden superfluid order in the normal state of cuprates.
These experiments also underscore the power of nonlinear TeraHertz optics as a
sensitive probe of frustrated excitations in quantum solids.Comment: 9 pages main text, 5 figures, 12 page supplementar
Measurement of the dynamic charge response of materials using low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS)
One of the most fundamental properties of an interacting electron system is
its frequency- and wave-vector-dependent density response function, . The imaginary part, , defines the
fundamental bosonic charge excitations of the system, exhibiting peaks wherever
collective modes are present. quantifies the electronic compressibility
of a material, its response to external fields, its ability to screen charge,
and its tendency to form charge density waves. Unfortunately, there has never
been a fully momentum-resolved means to measure at the
meV energy scale relevant to modern elecronic materials. Here, we demonstrate a
way to measure with quantitative momentum resolution by applying
alignment techniques from x-ray and neutron scattering to surface
high-resolution electron energy-loss spectroscopy (HR-EELS). This approach,
which we refer to here as "M-EELS," allows direct measurement of with meV resolution while controlling the momentum with an accuracy
better than a percent of a typical Brillouin zone. We apply this technique to
finite-q excitations in the optimally-doped high temperature superconductor,
BiSrCaCuO (Bi2212), which exhibits several phonons
potentially relevant to dispersion anomalies observed in ARPES and STM
experiments. Our study defines a path to studying the long-sought collective
charge modes in quantum materials at the meV scale and with full momentum
control.Comment: 26 pages, 10 sections, 7 figures, and an appendi
Ultrafast Melting of Superconductivity in an Iron-Based Superconductor
Intense debate has recently arisen regarding the photoinduced changes to the
iron-chalcogenide superconductors, including the enhancement of
superconductivity and a metastable state. Here, by employing high energy
resolution, we directly observe the melting of superconductivity on ultrafast
timescales. We demonstrate a distinctly nonequilibrium response on short
timescales, where the gap fills in prior to the destruction of the
superconducting peak, followed by a metastable response. We propose that the
former is due to pair phase decoherence and speculate that the latter is due to
the increase in double stripe correlations that are known to compete with
superconductivity. Our results add to exciting new developments on the
iron-based superconductors, indicating that the photoinduced metastable state
possibly competes with superconductivity
Magnetic order tuned by Cu substitution in Fe1.1-zCuzTe
We study the effects of Cu substitution in Fe1.1Te, the non-superconducting
parent compound of the iron-based superconductor, Fe1+yTe1-xSex, utilizing
neutron scattering techniques. It is found that the structural and magnetic
transitions, which occur at \sim 60 K without Cu, are monotonically depressed
with increasing Cu content. By 10% Cu for Fe, the structural transition is
hardly detectable, and the system becomes a spin glass below 22 K, with a
slightly incommensurate ordering wave vector of (0.5-d, 0, 0.5) with d being
the incommensurability of 0.02, and correlation length of 12 angstrom along the
a axis and 9 angstrom along the c axis. With 4% Cu, both transition
temperatures are at 41 K, though short-range incommensurate order at (0.42, 0,
0.5) is present at 60 K. With further cooling, the incommensurability decreases
linearly with temperature down to 37 K, below which there is a first order
transition to a long-range almost-commensurate antiferromagnetic structure. A
spin anisotropy gap of 4.5 meV is also observed in this compound. Our results
show that the weakly magnetic Cu has large effects on the magnetic
correlations; it is suggested that this is caused by the frustration of the
exchange interactions between the coupled Fe spins.Comment: 7 pages, 7 figures, version as appeared on PR
Enhanced low-energy magnetic excitations via suppression of the itinerancy in Fe0.98-zCuzTe0.5Se0.5
We have performed resistivity and inelastic neutron scattering measurements
on three samples of Fe0.98-zCuzTe0.5Se0.5 with z = 0, 0.02, and 0.1. It is
found that with increasing Cu doping the sample's resistivity deviates
progressively from that of a metal. However, in contrast to expectations that
replacing Fe with Cu would suppress the magnetic correlations, the low-energy
(no larger than 12 meV) magnetic scattering is enhanced in strength, with
greater spectral weight and longer dynamical spin-spin correlation lengths.
Such enhancements can be a consequence of either enlarged local moments or a
slowing down of the spin fluctuations. In either case, the localization of the
conduction states induced by the Cu doping should play a critical role. Our
results are not applicable to models that treat 3d transition metal dopants
simply as effective electron donors.Comment: 5 pages, 5 figures. To appear in PR