162 research outputs found
Superconductivity and charge carrier localization in ultrathin bilayers
/ (LSCO15/LCO) bilayers
with a precisely controlled thickness of N unit cells (UCs) of the former and M
UCs of the latter ([LSCO15\_N/LCO\_M]) were grown on (001)-oriented {\slao}
(SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and
reciprocal space map (RSM) studies confirmed the epitaxial growth of the
bilayers and showed that a [LSCO15\_2/LCO\_2] bilayer is fully strained,
whereas a [LSCO15\_2/LCO\_7] bilayer is already partially relaxed. The
\textit{in situ} monitoring of the growth with reflection high energy electron
diffraction (RHEED) revealed that the gas environment during deposition has a
surprisingly strong effect on the growth mode and thus on the amount of
disorder in the first UC of LSCO15 (or the first two monolayers of LSCO15
containing one plane each). For samples grown in pure
gas (growth type-B), the first LSCO15 UC next to the SLAO
substrate is strongly disordered. This disorder is strongly reduced if the
growth is performed in a mixture of and gas
(growth type-A). Electric transport measurements confirmed that the first UC of
LSCO15 next to the SLAO substrate is highly resistive and shows no sign of
superconductivity for growth type-B, whereas it is superconducting for growth
type-A. Furthermore, we found, rather surprisingly, that the conductivity of
the LSCO15 UC next to the LCO capping layer strongly depends on the thickness
of the latter. A LCO capping layer with 7~UCs leads to a strong localization of
the charge carriers in the adjacent LSCO15 UC and suppresses superconductivity.
The magneto-transport data suggest a similarity with the case of weakly hole
doped LSCO single crystals that are in a so-called {"{cluster-spin-glass
state}"
Terahertz ellipsometry study of the soft mode behavior in ultrathin SrTiO<sub></sub> films
We present a combined study with time-domain terahertz and conventional far-infrared ellipsometry of the temperature dependent optical response of SrTiO₃thin films (82 and 8.5 nm) that are grown by pulsed-laser deposition on (La0.3Sr0.7) (Al0.65Ta0.35)O₃ (LSAT) substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical response of these thin films, in particular, to the soft mode of SrTiO₃. We show that for the 82 nm film the eigenfrequency of the soft mode is strongly reduced by annealing at 1200 °C, whereas for the 8.5 nm film it is hardly affected. For the latter, after annealing the mode remains at 125 cm⁻¹ at 300 K and exhibits only a weak softening to about 90 cm⁻¹ at 10 K. This suggests that this ultrathin film undergoes hardly any relaxation of the compressive strain due to the LSAT substrate
Scaling of the Fano effect of the in-plane Fe-As phonon and the superconducting critical temperature in BaKFeAs
By means of infrared spectroscopy we determine the temperature-doping phase
diagram of the Fano effect for the in-plane Fe-As stretching mode in
BaKFeAs. The Fano parameter , which is a
measure of the phonon coupling to the electronic particle-hole continuum, shows
a remarkable sensitivity to the magnetic/structural orderings at low
temperatures. More strikingly, at elevated temperatures in the
paramagnetic/tetragonal state we find a linear correlation between and
the superconducting critical temperature . Based on theoretical
calculations and symmetry considerations, we identify the relevant interband
transitions that are coupled to the Fe-As mode. In particular, we show that a
sizable orbital component at the Fermi level is fundamental for the Fano
effect and possibly also for the superconducting pairing.Comment: Supplemental materials are available upon reques
Infrared spectroscopy study of the in-plane response of YBa2Cu3O6.6 in magnetic fields up to 30 Tesla
With Terahertz and Infrared spectroscopy we studied the in-plane response of
an underdoped YBa2Cu3O6.6 single crystal with Tc=58(1) K in high magnetic
fields up to B=30 Tesla applied along the c-axis. Our goal was to investigate
the field-induced suppression of superconductivity and to observe the
signatures of the three dimensional (3d) incommensurate copper charge density
wave (Cu-CDW) which was previously shown to develop at such high magnetic
fields. Our study confirms that a B-field in excess of 20 Tesla gives rise to a
full suppression of the macroscopic response of the superconducting condensate.
However, it reveals surprisingly weak signatures of the 3d Cu-CDW at high
magnetic fields. At 30 Tesla there is only a weak reduction of the spectral
weight of the Drude-response (by about 3%) that is accompanied by an
enhancement of two narrow electronic modes around 90 and 240 cm-1, that are
interpreted in terms of pinned phase modes of the CDW along the a- and
b-direction, respectively, and of the so-called mid-infrared (MIR) band. The
pinned phased modes and the MIR band are strong features already without
magnetic field which suggests that prominent but short-ranged and slowly
fluctuating (compared to the picosecond IR-time scale) CDW correlations exist
all along, i.e., even at zero magnetic field.Comment: 12 pages, 3 figure
Coexistence and competition of magnetism and superconductivity on the nanometer scale in underdoped BaFe1.89Co0.11As2
We report muon spin rotation (muSR) and infrared (IR) spectroscopy
experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and
superconductivity (SC) coexist and compete on the nanometer length scale. Our
combined data reveal a bulk magnetic order, likely due to an incommensurate
spin density wave (SDW), which develops below Tmag \approx 32 K and becomes
reduced in magnitude (but not in volume) below Tc = 21.7 K. A slowly
fluctuating precursor of the SDW seems to develop alrady below the structural
transition at Ts \approx 50 K. The bulk nature of SC is established by the muSR
data which show a bulk SC vortex lattice and the IR data which reveal that the
majority of low-energy states is gapped and participates in the condensate at T
<< Tc
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