311 research outputs found
Ultrafast quasiparticle dynamics in superconducting iron pnictide CaFe1.89Co0.11As2
Nonequilibrium quasiparticle relaxation dynamics is reported in
superconducting CaFe1.89Co0.11As2 single crystal using femtosecond
time-resolved pump-probe spectroscopy. The carrier dynamics reflects a
three-channel decay of laser deposited energy with characteristic time scales
varying from few hundreds of femtoseconds to order of few nanoseconds where the
amplitudes and time-constants of the individual electronic relaxation
components show significant changes in the vicinity of the spin density wave
(T_SDW ~ 85 K) and superconducting (T_SC ~ 20 K) phase transition temperatures.
The quasiparticles dynamics in the superconducting state reveals a charge gap
with reduced gap value of 2_0/k_BT_SC ~ 1.8. We have determined the
electron-phonon coupling constant \lemda to be ~ 0.14 from the temperature
dependent relaxation time in the normal state, a value close to those reported
for other types of pnictides. From the peculiar temperature-dependence of the
carrier dynamics in the intermediate temperature region between the
superconducting and spin density wave phase transitions, we infer a temperature
scale where the charge gap associated with the spin ordered phase is maximum
and closes on either side while approaching the two phase transition
temperatures.Comment: 6 pages, 4 figures (revised manuscript);
http://dx.doi.org/10.1016/j.ssc.2013.02.00
Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of superconducting iron pnictide Ca(Fe_0.944Co_0.056)_2As_2
We report temperature evolution of coherently excited acoustic and optical
phonon dynamics in superconducting iron pnictide single crystal
Ca(Fe_0.944Co_0.056)_2As_2 across the spin density wave transition at T_SDW ~
85 K and superconducting transition at T_SC ~20 K. Strain pulse propagation
model applied to the generation of the acoustic phonons yields the temperature
dependence of the optical constants, and longitudinal and transverse sound
velocities in the temperature range of 3.1 K to 300 K. The frequency and
dephasing times of the phonons show anomalous temperature dependence below T_SC
indicating a coupling of these low energy excitations with the Cooper-pair
quasiparticles. A maximum in the amplitude of the acoustic modes at T ~ 170 is
seen, attributed to spin fluctuations and strong spin-lattice coupling before
T_SDW.Comment: 6 pages, 4 figures (revised manuscript
Structural, magnetic, and transport properties of CoFeSi Heusler films
We report the deposition of thin CoFeSi films by RF magnetron sputtering.
Epitaxial (100)-oriented and L2 ordered growth is observed for films grown
on MgO(100) substrates. (110)-oriented films on AlO(110) show several
epitaxial domains in the film plane. Investigation of the magnetic properties
reveals a saturation magnetization of 5.0 at low temperatures. The
temperature dependence of the resistivity exhibits a crossover
from a T^3.5 law at T<50K to a T^1.65 behaviour at elevated temperatures.
shows a small anisotropic magnetoresistive effect. A weak
dependence of the normal Hall effect on the external magnetic field indicates
the compensation of electron and hole like contributions at the Fermi surface.Comment: 10 pages, 9 figures to be published in J. Phys. D: Appl. Phy
Spin reorientation in Na-doped BaFeAs studied by neutron diffraction
We have studied the magnetic ordering in Na doped BaFeAs by
unpolarized and polarized neutron diffraction using single crystals. Unlike
previously studied FeAs-based compounds that magnetically order,
BaNaFeAs exhibits two successive magnetic transitions: For
x=0.35 upon cooling magnetic order occurs at 70\ K with in-plane magnetic
moments being arranged as in pure or Ni, Co and K-doped BaFeAs samples.
At a temperature of 46\ K a second phase transition occurs, which the
single-crystal neutron diffraction experiments can unambiguously identify as a
spin reorientation. At low temperatures, the ordered magnetic moments in
BaNaFeAs point along the direction. Magnetic
correlations in these materials cannot be considered as Ising like, and
spin-orbit coupling must be included in a quantitative theory.Comment: 5 pages, 4 figure
Phonon Anomalies, Orbital-Ordering and Electronic Raman Scattering in iron-pnictide Ca(Fe0.97Co0.03)2As2: Temperature-dependent Raman Study
We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a
wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal
to orthorhombic structural transition as well as magnetic transition at Tsm ~
160K. The mode frequencies of two first-order Raman modes B1g and Eg, both
involving displacement of Fe atoms, show sharp increase below Tsm.
Concomitantly, the linewidths of all the first-order Raman modes show anomalous
broadening below Tsm, attributed to strong spin-phonon coupling. The high
frequency modes observed between 400-1200 cm-1 are attributed to the electronic
Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The
splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which
increases as temperature decreases below Tsm. A broad Raman band observed at ~
3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d
antiferromagnet.Comment: Accepted for Publication in JPC
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