15,191 research outputs found
Raman spectroscopy study of the interface structure in (CaCuO2)n/(SrTiO3)m superlattices
Raman spectra of CaCuO2/SrTiO3 superlattices show clear spectroscopic marker
of two structures formed in CaCuO2 at the interface with SrTiO3. For
non-superconducting superlattices, grown in low oxidizing atmosphere, the 425
cm-1 frequency of oxygen vibration in CuO2 planes is the same as for CCO films
with infinite layer structure (planar Cu-O coordination). For superconducting
superlattices grown in highly oxidizing atmosphere, a 60 cm-1 frequency shift
to lower energy occurs. This is ascribed to a change from planar to pyramidal
Cu-O coordination because of oxygen incorporation at the interface. Raman
spectroscopy proves to be a powerful tool for interface structure
investigation
Correlation between the transition temperature and the superfluid density in BCS superconductor NbB_2+x
The results of the muon-spin rotation experiments on BCS superconductors
NbB_2+x (x = 0.2, 0.34) are reported. Both samples, studied in the present
work, exhibit rather broad transitions to the superconducting state, suggesting
a distribution of the volume fractions with different transition temperatures
(T_c)'s. By taking these distributions into account, the dependence of the
inverse squared zero-temperature magnetic penetration depth (\lambda_0^{-2}) on
T_c was reconstructed for temperatures in the range 1.5K<T_c<8.0K.
\lambda_0^{-2} was found to obey the power law dependence \lambda_0^{-2}\propto
T_c^{3.1(1)} which appears to be common for some families of BCS
superconductors as, {\it e.g.}, Al doped MgB_2 and high-temperature cuprate
superconductors as underdoped YBa_2Cu_3O_{7-\delta}.Comment: 9 pages, 7 figures. Accepted for publication in Phys. Rev.
Charge-fluctuation contribution to the Raman response in superconducting cuprates
We calculate the Raman response contribution due to collective modes, finding
a strong dependence on the photon polarizations and on the characteristic
wavevectors of the modes. We compare our results with recent Raman spectroscopy
experiments in underdoped cuprates, and
, where anomalous low-energy peaks are
observed, which soften upon lowering the temperature. We show that the specific
dependence on doping and on photon polarizations of these peaks is only
compatible with charge collective excitations at finite wavelength.Comment: 5 pages, 3 figure
Hidden Ferronematic Order in Underdoped Cuprates
We study a model for low doped cuprates where holes aggregate into oriented
stripe segments which have a vortex and an antivortex fixed to the extremes. We
argue that due to the interaction between segments a state with macroscopic
polarization is stabilized, which we call a ferronematic. This state can be
characterized as a charge nematic which, due to the net polarization, breaks
inversion symmetry and also exhibits an incommensurate spin modulation. Our
calculation can reproduce the doping dependent spin structure factor of
lanthanum cuprates in excellent agreement with experiment and allows to
rationalize experiments in which the incommensurability has an order
parameter-like temperature dependence.Comment: 5 pages, 4 figure
Tuning topological disorder in MgB
We carried out Raman measurements on neutron-irradiated and Al-doped MgB
samples. The irradiation-induced topological disorder causes an unexpected
appearance of high frequency spectral structures, similar to those observed in
lightly Al-doped samples. Our results show that disorder-induced violations of
the selection rules are responsible for the modification of the Raman spectrum
in both irradiated and Al-doped samples. Theoretical calculations of the phonon
density of states support this hypothesis, and demonstrate that the high
frequency structures arise mostly from contributions at of the
E phonon mode.Comment: 4 pages, 4 figure
Effect of Al doping on the optical phonon spectrum in Mg(1-x)Al(x)B(2)
Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected
for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the
peak frequency, the width and the intensity of the observed Raman lines has
been carefully analized. A peculiar x-dependence of the optical modes is
pointed out for two different Al doping ranges. In particular the onset of the
high-doping structural phase previously observed in diffraction measurements is
marked by the appearence of new spectral components at high frequencies. A
connection between the whole of our results and the observed suppression of
superconductivity in the high doping region is established
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