338 research outputs found
Signatures of Electronic Correlations in Optical Properties of LaFeAsOF
Spectroscopic ellipsometry is used to determine the dielectric function of
the superconducting LaFeAsOF ( = 27 K) and undoped LaFeAsO
polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 350 K. The free charge carrier response in both samples is heavily
damped with the effective carrier density as low as 0.0400.005 electrons
per unit cell. The spectral weight transfer in the undoped LaFeAsO associated
with opening of the pseudogap at about 0.65 eV is restricted at energies below
2 eV. The spectra of superconducting LaFeAsOF reveal a
significant transfer of the spectral weight to a broad optical band above 4 eV
with increasing temperature. Our data may imply that the electronic states near
the Fermi surface are strongly renormalized due to electron-phonon and/or
electron-electron interactions.Comment: 4 pages, 4 figures, units in Fig.2 adde
Effect of annealing on the specific heat of optimally doped Ba(FeCo)As
We report the temperature dependence of the low-temperature specific heat
down to 400 mK of the electron-doped Ba(FeCo)As
superconductors. We have measured two samples extracted from the same batch:
first sample has been measured just after preparation with no additional heat
treatment. The sample shows =20 K, residual specific heat
=3.6 mJ/mol K and a Schottky-like contribution at low
temperatures. A second sample has been annealed at 800 for two weeks
and shows = 25 K and =1.4 mJ/mol~K. By subtracting
the lattice specific heat, from pure BaFeAs, the temperature
dependence of the electronic specific heat has been obtained and studied. For
both samples the temperature dependence of clearly indicate the
presence of low-energy excitations in the system. Their specific heat data
cannot be described by single clean s- or d-wave models and the data requires
an anisotropic gap scenario which may or may not have nodesComment: SCES 2010, 5 pages, 2 figure
Ferromagnetism and Lattice Distortions in the Perovskite YTiO
The thermodynamic properties of the ferromagnetic perovskite YTiO are
investigated by thermal expansion, magnetostriction, specific heat, and
magnetization measurements. The low-temperature spin-wave contribution to the
specific heat, as well as an Arrott plot of the magnetization in the vicinity
of the Curie temperature K, are consistent with a
three-dimensional Heisenberg model of ferromagnetism. However, a magnetic
contribution to the thermal expansion persists well above , which
contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a
comparison with the corresponding model system EuS. The pressure dependences of
and of the spontaneous moment are extracted using thermodynamic
relationships. They indicate that ferromagnetism is strengthened by uniaxial
pressures and is weakened by uniaxial
pressures and hydrostatic pressure.
Our results show that the distortion along the - and -axes is further
increased by the magnetic transition, confirming that ferromagnetism is favored
by a large GdFeO-type distortion. The c-axis results however do not fit
into this simple picture, which may be explained by an additional
magnetoelastic effect, possibly related to a Jahn-Teller distortion.Comment: 12 pages, 13 figure
Dipole-active optical phonons in YTiO_3: ellipsometry study and lattice-dynamics calculations
The anisotropic complex dielectric response was accurately extracted from
spectroscopic ellipsometry measurements at phonon frequencies for the three
principal crystallographic directions of an orthorhombic (Pbnm) YTiO_3 single
crystal. We identify all twenty five infrared-active phonon modes allowed by
symmetry, 7B_1u, 9B_2u, and 9B_3u, polarized along the c-, b-, and a-axis,
respectively. From a classical dispersion analysis of the complex dielectric
functions \tilde\epsilon(\omega) and their inverses -1/\tilde\epsilon(\omega)
we define the resonant frequencies, widths, and oscillator strengths of the
transverse (TO) and longitudinal (LO) phonon modes. We calculate
eigenfrequencies and eigenvectors of B_1u, B_2u, and B_3u normal modes and
suggest assignments of the TO phonon modes observed in our ellipsometry spectra
by comparing their frequencies and oscillator strengths with those resulting
from the present lattice-dynamics study. Based on these assignments, we
estimate dynamical effective charges of the atoms in the YTiO_3 lattice. We
find that, in general, the dynamical effective charges in YTiO_3 lattice are
typical for a family of perovskite oxides. By contrast to a ferroelectric
BaTiO_3, the dynamical effective charge of oxygen related to a displacement
along the c-axis does not show the anomalously large value. At the same time,
the dynamical effective charges of Y and ab-plane oxygen exhibit anisotropy,
indicating strong hybridization along the a-axis.Comment: 8 pages, 7 figure
Large single crystal growth of BaFe1.87Co0.13As2 using a nucleation pole
Co-doped iron arsenic single crystal of BaFe1.87Co0.13As2 with dimension up
to 20 x 10 x 2 mm3 were grown by a nucleation pole: an alumina stick served as
nucleation center during growth. The high quality of crystalline was
illustrated by the measurements of neutron rocking curve and X-ray diffraction
pattern. A very sharp superconducting transition temperature Tc~25 K was
revealed by both resistivity and susceptibility measurements. A nearly 100%
shielding fraction and bulk nature of the superconductivity for the single
crystal were confirmed using magnetic susceptibility data.Comment: 4 pages, 5 figure
Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices
The competition between collective quantum phases in materials with strongly
correlated electrons depends sensitively on the dimensionality of the electron
system, which is difficult to control by standard solid-state chemistry. We
have fabricated superlattices of the paramagnetic metal LaNiO3 and the wide-gap
insulator LaAlO3 with atomically precise layer sequences. Using optical
ellipsometry and low-energy muon spin rotation, superlattices with LaNiO3 as
thin as two unit cells are shown to undergo a sequence of collective
metalinsulator and antiferromagnetic transitions as a function of decreasing
temperature, whereas samples with thicker LaNiO3 layers remain metallic and
paramagnetic at all temperatures. Metal-oxide superlattices thus allow control
of the dimensionality and collective phase behavior of correlated-electron
systems
Specific-heat study of superconducting and normal states in FeSe1-xTex (0.6<=x<=1) single crystals: Strong-coupling superconductivity, strong electron-correlation, and inhomogeneity
The electronic specific heat of as-grown and annealed single-crystals of
FeSe1-xTex (0.6<=x<=1) has been investigated. It has been found that annealed
single-crystals with x=0.6-0.9 exhibit bulk superconductivity with a clear
specific-heat jump at the superconducting (SC) transition temperature, Tc. Both
2Delta_0/kBTc [Delta_0: the SC gap at 0 K estimated using the single-band BCS
s-wave model] and Delta C/(gamma_n-gamma_0)Tc [Delta C$: the specific-heat jump
at Tc, gamma_n: the electronic specific-heat coefficient in the normal state,
gamma_0: the residual electronic specific-heat coefficient at 0 K in the SC
state] are largest in the well-annealed single-crystal with x=0.7, i.e., 4.29
and 2.76, respectively, indicating that the superconductivity is of the strong
coupling. The thermodynamic critical field has also been estimated. gamma_n has
been found to be one order of magnitude larger than those estimated from the
band calculations and increases with increasing x at x=0.6-0.9, which is
surmised to be due to the increase in the electronic effective mass, namely,
the enhancement of the electron correlation. It has been found that there
remains a finite value of gamma_0 in the SC state even in the well-annealed
single-crystals with x=0.8-0.9, suggesting an inhomogeneous electronic state in
real space and/or momentum space.Comment: 22 pages, 1 table, 6 figures, Version 2 has been accepted for
publication in J. Phys. Soc. Jp
Upper critical magnetic field in Ba_0.68K_0.32Fe_2As_2 and Ba(Fe_0.93Co_0.07)_2As_2
We report measurements of the temperature dependence of the radio-frequency
magnetic penetration depth in Ba_0.68K_0.32Fe_2As_2 and
Ba(Fe_0.93Co_0.07)_2As_2 single crystals in pulsed magnetic fields up to 60 T.
From our data, we construct an H-T phase diagram for the inter-plane (H || c)
and in-plane (H || ab) directions for both compounds. For both field
orientations in Ba_0.68K_0.32Fe_2As_2, we find a concave curvature of the
Hc2(T) lines with decreasing anisotropy and saturation towards lower
temperature. Taking into account Pauli spin paramagnetism we can describe
Hc2(T) and its anisotropy. In contrast, we find that Pauli paramagnetic pair
breaking is not essential for Ba(Fe_0.93Co_0.07)_2As_2. For this electron-doped
compound, the data support a Hc2(T) dependence that can be described by the
Werthamer Helfand Hohenberg model for H || ab and a two-gap behavior for H ||
c.Comment: 7 pages, 8 figure
Iron pnictides: Single crystal growth and effect of doping on structural, transport and magnetic properties
We demonstrate the preparation of large, free standing iron pnictide single
crystals with a size up to 20 x 10 x 1 mm3 using solvents in zirconia crucibles
under argon atmosphere. Transport and magnetic properties are investigated to
study the effect of potassium doping on the structural and superconducting
property of the compounds. The spin density wave (SDW) anomaly at Ts ~138 K in
BaFe2As2 single crystals from self-flux shifts to Ts ~85 K due to Sn solvent
growth. We show direct evidence for an incorporation of Sn on the Fe site. The
electrical resistivity data show a sharp superconducting transition temperature
Tc~38.5 K for the single crystal of Ba0.68K0.32Fe2As2. A nearly 100% shielding
fraction and bulk nature of the superconductivity for the single crystal were
confirmed by magnetic susceptibility data. A sharp transition Tc~25 K occurred
for the single crystal of Sr0.85K0.15Fe2As2. There is direct evidence for a
coexistence of the SDW and superconductivity in the low doping regime of
Sr1-xKxFe2As2 single crystals. Structural implications of the doping effects as
well as the coexistence of the two order parameters are discussed.Comment: 22 pages, 9 figure
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