84 research outputs found
Spectral weight redistribution in (LaNiO3)n/(LaMnO3)2 superlattices from optical spectroscopy
We have studied the optical properties of four
(LaNiO)/(LaMnO) superlattices (SL) (=2, 3, 4, 5) on
SrTiO substrates. We have measured the reflectivity at temperatures from 20
K to 400 K, and extracted the optical conductivity through a fitting procedure
based on a Kramers-Kronig consistent Lorentz-Drude model. With increasing
LaNiO thickness, the SLs undergo an insulator-to-metal transition (IMT)
that is accompanied by the transfer of spectral weight from high to low
frequency. The presence of a broad mid-infrared band, however, shows that the
optical conductivity of the (LaNiO)/(LaMnO) SLs is not a linear
combination of the LaMnO and LaNiO conductivities. Our observations
suggest that interfacial charge transfer leads to an IMT due to a change in
valence at the Mn and Ni sites.Comment: Accepted for publication in Phys. Rev. Lett. 5 pages, 5 figure
Metallic ground state and glassy transport in single crystalline URhGe: Enhancement of disorder effects in a strongly correlated electron system
We present a detailed study of the electronic transport properties on a
single crystalline specimen of the moderately disordered heavy fermion system
URhGe. For this material, we find glassy electronic transport in a
single crystalline compound. We derive the temperature dependence of the
electrical conductivity and establish metallicity by means of optical
conductivity and Hall effect measurements. The overall behavior of the
electronic transport properties closely resembles that of metallic glasses,
with at low temperatures an additional minor spin disorder contribution. We
argue that this glassy electronic behavior in a crystalline compound reflects
the enhancement of disorder effects as consequence of strong electronic
correlations.Comment: 5 pages, 4 figures, accepted for publication in PR
Evidence of a pressure-induced metallization process in monoclinic VO
Raman and combined trasmission and reflectivity mid infrared measurements
have been carried out on monoclinic VO at room temperature over the 0-19
GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence
obtained for both lattice dynamics and optical gap shows a remarkable stability
of the system up to P*10 GPa. Evidence of subtle modifications of V ion
arrangements within the monoclinic lattice together with the onset of a
metallization process via band gap filling are observed for PP*. Differently
from ambient pressure, where the VO metal phase is found only in
conjunction with the rutile structure above 340 K, a new room temperature
metallic phase coupled to a monoclinic structure appears accessible in the high
pressure regime, thus opening to new important queries on the physics of
VO.Comment: 5 pages, 3 figure
Electrodynamics of superconducting pnictide superlattices
It has been recently reported (S. Lee et al., Nature Materials 12, 392, 2013)
that superlattices where layers of the 8% Co-doped BaFe2As2 superconducting
pnictide are intercalated with non superconducting ultrathin layers of either
SrTiO3 or of oxygen-rich BaFe2As2, can be used to control flux pinning, thereby
increasing critical fields and currents, without significantly affecting the
critical temperature of the pristine superconducting material. However, little
is known about the electron properties of these systems. Here we investigate
the electrodynamics of these superconducting pnictide superlattices in the
normal and superconducting state by using infrared reflectivity, from THz to
visible range. We find that multi-gap structure of these superlattices is
preserved, whereas some significant changes are observed in their electronic
structure with respect to those of the original pnictide. Our results suggest
that possible attempts to further increase the flux pinning may lead to a
breakdown of the pnictide superconducting properties.Comment: 4 pages, two figure
Pressure dependence of the single particle excitation in the charge-density-wave CeTe system
We present new data on the pressure dependence at 300 K of the optical
reflectivity of CeTe, which undergoes a charge-density-wave (CDW) phase
transition well above room temperature. The collected data cover an
unprecedented broad spectral range from the infrared up to the ultraviolet,
which allows a robust determination of the gap as well as of the fraction of
the Fermi surface affected by the formation of the CDW condensate. Upon
compressing the lattice there is a progressive closing of the gap inducing a
transfer of spectral weight from the gap feature into the Drude component. At
frequencies above the CDW gap we also identify a power-law behavior, consistent
with findings along the Te series (i.e., chemical pressure) and
suggestive of a Tomonaga-Luttinger liquid scenario at high energy scales. This
newest set of data is placed in the context of our previous investigations of
this class of materials and allows us to revisit important concepts for the
physics of CDW state in layered-like two-dimensional systems
Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2
Neutron scattering studies on powder and single crystals have provided new
evidences for unconventional magnetism in Cu2Te2O5Cl2. The compound is built
from tetrahedral clusters of S=1/2 Cu2+ spins located on a tetragonal lattice.
Magnetic ordering, emerging at TN=18.2 K, leads to a very complex multi-domain,
most likely degenerate, ground state, which is characterized by an
incommensurate (ICM) wave vector k ~ [0.15, 0.42,1/2]. The Cu2+ ions carry a
magnetic moment of 0.67(1) mB/ Cu2+ at 1.5 K and form a four helices spin
arrangement with two canted pairs within the tetrahedra. A domain
redistribution is observed when a magnetic field is applied in the tetragonal
plane (Hc≈0.5 T), but not for H||c up to 4 T. The excitation spectrum is
characterized by two well-defined modes, one completely dispersionless at 6.0
meV, the other strongly dispersing to a gap of 2 meV. The reason for such
complex ground state and spin excitations may be geometrical frustration of the
Cu2+ spins within the tetrahedra, intra- and inter-tetrahedral couplings having
similar strengths and strong Dzyaloshinski-Moriya anisotropy. Candidates for
the dominant intra- and inter-tetrahedral interactions are proposed
Optical investigation of the charge-density-wave phase transitions in
We have measured the optical reflectivity of the quasi
one-dimensional conductor from the far infrared up to the
ultraviolet between 10 and 300 using light polarized along and normal to
the chain axis. We find a depletion of the optical conductivity with decreasing
temperature for both polarizations in the mid to far-infrared region. This
leads to a redistribution of spectral weight from low to high energies due to
partial gapping of the Fermi surface below the charge-density-wave transitions
at 145 K and 59 K. We deduce the bulk magnitudes of the CDW gaps and discuss
the scattering of ungapped free charge carriers and the role of fluctuations
effects
Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies
We measured the THz reflectance properties of a high quality epitaxial thin
film of the Fe-based superconductor BaFeCoAs with
T=22.5 K. The film was grown by pulsed laser deposition on a DyScO
substrate with an epitaxial SrTiO intermediate layer. The measured
spectrum, i.e. the reflectivity ratio between the superconducting and
normal state reflectance, provides clear evidence of a superconducting gap
close to 15 cm. A detailed data analysis shows that a
two-band, two-gap model is absolutely necessary to obtain a good description of
the measured spectrum. The low-energy gap results to be
well determined (=15.50.5 cm), while the value of the
high-energy gap is more uncertain (=557 cm).
Our results provide evidence of a nodeless isotropic double-gap scenario, with
the presence of two optical gaps corresponding to 2 values close
to 2 and 7.Comment: Published Versio
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