41 research outputs found
Electron-phonon interaction in n-doped cuprates: an Inelastic X-ray Scattering study
Inelastic x-ray scattering (IXS) with very high (meV) energy resolution has
become a valuable spectroscopic tool, complementing the well established
coherent inelastic neutron scattering (INS) technique for phonon dispersion
investigations. In the study of crystalline systems IXS is a viable alternative
to INS, especially in cases where only small samples are available. Using IXS,
we have measured the phonon dispersion of Nd_{1.86}Ce_{0.14}CuO_{4+\delta}
along the [x,0,0] and [x,x,0] in-plane directions. Compared to the undoped
parent compound, the two highest longitudinal optical (LO) phonon branches are
shifted to lower energies because of Coulomb-screening effects brought about by
the doped charge carriers. An additional anomalous softening of the highest
branch is observed around q=(0.2,0,0). This anomalous softening, akin to what
has been observed in other compounds, provides evidence for a strong
electron-phonon coupling in the electron-doped high-temperature
superconductors.Comment: Proceedings of the SATT11 conference, Vietri sul Mare - Italy (March
2002); accepted for publication on Int. J. Mod. Phys.
Quasiparticles dynamics in high-temperature superconductors far from equilibrium: an indication of pairing amplitude without phase coherence
We perform time resolved photoelectron spectroscopy measurements of optimally
doped \tn{Bi}_2\tn{Sr}_2\tn{CaCu}_2\tn{O}_{8+\delta} (Bi-2212) and
\tn{Bi}_2\tn{Sr}_{2-x}\tn{La}_{x}\tn{Cu}\tn{O}_{6+\delta} (Bi-2201). The
electrons dynamics show that inelastic scattering by nodal quasiparticles
decreases when the temperature is lowered below the critical value of the
superconducting phase transition. This drop of electronic dissipation is
astonishingly robust and survives to photoexcitation densities much larger than
the value sustained by long-range superconductivity. The unconventional
behaviour of quasiparticle scattering is ascribed to superconducting
correlations extending on a length scale comparable to the inelastic path. Our
measurements indicate that strongly driven superconductors enter in a regime
without phase coherence but finite pairing amplitude. The latter vanishes near
to the critical temperature and has no evident link with the pseudogap observed
by Angle Resolved Photoelectron Spectroscopy (ARPES).Comment: 7 pages, 5 Figure
Bond stretching phonon softening and angle-resolved photoemission kinks in optimally doped Bi2Sr1.6La0.4Cu2O6 superconductors
We report the first measurement of the optical phonon dispersion in optimally
doped single layer Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering.
We found a strong softening of the Cu-O bond stretching phonon at about
q=(0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates.
A direct comparison with angle-resolved photoemission spectroscopy measurements
taken on the same sample, revealed an excellent agreement in terms of energy
and momentum between the ARPES nodal kink and the soft part of the bond
stretching phonon. Indeed, we find that the momentum space where a 63 meV kink
is observed can be connected with a vector q=(xi,0,0) with xi~0.22, which
corresponds exactly to the soft part of the bond stretching phonon mode. This
result supports an interpretation of the ARPES kink in terms of electron-phonon
coupling.Comment: submited to PR
Dynamics of the magnetic and structural a -> e phase transition in Iron
We have studied the high-pressure iron bcc to hcp phase transition by
simultaneous X-ray Magnetic Circular Dichroism (XMCD) and X-ray Absorption
Spectroscopy (XAS) with an X-ray dispersive spectrometer. The combination of
the two techniques allows us to obtain simultaneously information on both the
structure and the magnetic state of Iron under pressure. The magnetic and
structural transitions simultaneously observed are sharp. Both are of first
order in agreement with theoretical prediction. The pressure domain of the
transition observed (2.4 0.2 GPa) is narrower than that usually cited in
the literature (8 GPa). Our data indicate that the magnetic transition slightly
precedes the structural one, suggesting that the origin of the instability of
the bcc phase in iron with increasing pressure is to be attributed to the
effect of pressure on magnetism as predicted by spin-polarized full potential
total energy calculations
Interplay between charge-lattice interaction and strong electron correlations in cuprates: phonon anomaly and spectral kinks
We investigate the interplay between strong electron correlations and
charge-lattice interaction in cuprates. The coupling between half breathing
bond stretching phonons and doped holes in the t-t'-J model is studied by
limited phonon basis exact diagonalization method. Nonadiabatic electron-phonon
interaction leads to the splitting of the phonon spectral function at half-way
to the zone boundary at and
to low energy kink feature in the electron dispersion, in agreement with
experimental observations. Another kink due to strong electron correlation
effects is observed at higher energy, depending on the strength of the
charge-lattice coupling.Comment: 4 pages, 3 figure
Correlation between band structure and magneto-transport properties in far-infrared detector modulated nanostructures superlattice
We report here carrier’s magneto-transport properties and the band structure results for II-IV semiconductors. HgTe is a zero gap semiconductor when it is sandwiched between CdTe layers to yield to a small gap HgTe/CdTe superlattice which is the key of an infrared detector. Our sample, grown by MBE, had a period d (100 layers) of 18 nm (HgTe) / 4.4 nm (CdTe). Calculations of the spectra of energy E(kz) and E(kp), respectively, in the direction of growth and in the plane of the superlattice were performed in the envelope function formalism. The angular dependence of the transverse magnetoresistance follows the two-dimensional (2D) behavior with Shubnikov-de Haas oscillations. At low temperature, the sample exhibits p type conductivity with a hole mobility of 900 cm²/V.s. A reversal the sign of the weak-field Hall coefficient occurs at 25 K with an electron mobility of 3 104 cm2/Vs. In intrinsic regime, the measured Eg ≈ 38 meV agrees with calculated Eg(Γ,300 K) = 34 meV which coincide with the Fermi level energy. The formalism used here predicts that this narrow gap sample is semi metallic, quasi-two-dimensional and far-infrared detector.We report here carrier’s magneto-transport properties and the band structure results for II-IV semiconductors. HgTe is a zero gap semiconductor when it is sandwiched between CdTe layers to yield to a small gap HgTe/CdTe superlattice which is the key of an infrared detector. Our sample, grown by MBE, had a period d (100 layers) of 18 nm (HgTe) / 4.4 nm (CdTe). Calculations of the spectra of energy E(kz) and E(kp), respectively, in the direction of growth and in the plane of the superlattice were performed in the envelope function formalism. The angular dependence of the transverse magnetoresistance follows the two-dimensional (2D) behavior with Shubnikov-de Haas oscillations. At low temperature, the sample exhibits p type conductivity with a hole mobility of 900 cm²/V.s. A reversal the sign of the weak-field Hall coefficient occurs at 25 K with an electron mobility of 3 104 cm2/Vs. In intrinsic regime, the measured Eg ≈ 38 meV agrees with calculated Eg(Γ,300 K) = 34 meV which coincide with the Fermi level energy. The formalism used here predicts that this narrow gap sample is semi metallic, quasi-two-dimensional and far-infrared detector
Bulk charge density wave and electron-phonon coupling in superconducting copper oxychlorides
Bulk charge density waves (CDWs) are now reported in nearly all
high-temperature superconducting (HTS) cuprates, with the noticeable exception
of one particular family: the copper oxychlorides. Here, we used resonant
inelastic X-ray scattering (RIXS) to reveal a bulk CDW in these materials.
Combining RIXS with non-resonant IXS, we investigate the interplay between the
lattice excitations and the CDW, and evidence bond-stretching (BS) phonon
anomalies at the CDW wave-vector. We propose that such electron-phonon
anomalies occur in the presence of dispersive charge excitations emanating from
the CDW and interacting with the BS phonon. Our observations in a structurally
simple cuprate promises to better connect bulk and surface properties and
bridge the gap between theory and experiment
Magnetic and structural properties of the iron silicide superconductor LaFeSiH
The magnetic and structural properties of the recently discovered
pnictogen/chalcogen-free superconductor LaFeSiH (~K) have been
investigated by Fe synchrotron M{\"o}ssbauer source (SMS) spectroscopy,
x-ray and neutron powder diffraction and Si nuclear magnetic resonance
spectroscopy (NMR). No sign of long range magnetic order or local moments has
been detected in any of the measurements and LaFeSiH remains tetragonal down to
2 K. The activated temperature dependence of both the NMR Knight shift and the
relaxation rate is analogous to that observed in strongly overdoped
Fe-based superconductors. These results, together with the
temperature-independent NMR linewidth, show that LaFeSiH is an homogeneous
metal, far from any magnetic or nematic instability, and with similar Fermi
surface properties as strongly overdoped iron pnictides. This raises the
prospect of enhancing the of LaFeSiH by reducing its carrier
concentration through appropriate chemical substitutions. Additional SMS
spectroscopy measurements under hydrostatic pressure up to 18.8~GPa found no
measurable hyperfine field
Paramagnon dispersion and damping in doped NaCaCuOCl
Using Resonant Inelastic X-ray Scattering, we measure the paramagnon
dispersion and damping of undoped, antiferromagnetic CaCuOCl as
well as doped, superconducting NaCaCuOCl. Our estimation
of the spin-exchange parameter and width of the paramagnon peak at the zone
boundary confirms that no simple relation can be drawn between
these parameters and the critical temperature . Consistently with
other cuprate compounds, we show that upon doping there is a slight softening
at , but not at the zone boundary . In combination with these
measurements we perform calculations of the dynamical spin structure factor of
the one-band Hubbard model using cluster dynamical mean-field theory. The
calculations are in excellent agreement with the experiment in the undoped
case, both in terms of energy position and width. While the increase in width
is also captured upon doping, the dynamical spin structure factor shows a
sizable softening at , which provides insightful information on the
length-scale of the spin fluctuations in doped cuprates.Comment: 11 pages, 5 figures, 2 tables, V2 typo corrected in title and
reference