15 research outputs found
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
Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF
We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at Ts=121 K and a magnetic one to a spin density wave (SDW) state at TN=110 K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300cm−1 that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the BaFe2As2 system [Z.-G. Chen et al., Phys. Rev. Lett. 119, 096401 (2017)]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250cm−1) develops a strongly asymmetric line shape in the SDW state and note that this behavior can be explained in terms of a strong coupling with the Dirac fermions
Phase transitions in the spinless Falicov-Kimball model with correlated hopping
The canonical Monte-Carlo is used to study the phase transitions from the
low-temperature ordered phase to the high-temperature disordered phase in the
two-dimensional Falicov-Kimball model with correlated hopping. As the
low-temperature ordered phase we consider the chessboard phase, the axial
striped phase and the segregated phase. It is shown that all three phases
persist also at finite temperatures (up to the critical temperature )
and that the phase transition at the critical point is of the first order for
the chessboard and axial striped phase and of the second order for the
segregated phase. In addition, it is found that the critical temperature is
reduced with the increasing amplitude of correlated hopping in the
chessboard phase and it is strongly enhanced by in the axial striped and
segregated phase.Comment: 17 pages, 6 figure
Granular superconductivity and charge/orbital order in YBa 2 Cu 3 O 7 /manganite trilayers
We studied how the electronic, superconducting, and magnetic properties of YBa2Cu3O7/Nd1−x(Ca1−ySry )xMnO3 multilayers depend on the tolerance factor and the hole doping of the manganite. In particular, we investigated the granular superconducting state and the related magnetic-field-driven insulator-to- superconductor transition that was previously discovered in corresponding multilayers with Pr0.5La0.2Ca0.3MnO3 [B. P. P. Mallett et al., Phys. Rev. B 94, 180503(R) (2016)]. We found that this granular uperconducting state occurs only when the manganite layer is in a charge/orbital ordered and CE-type antiferromagnetic state (Mn-CO/OO). The coupling mechanism underlying this intriguing proximity effect seems to involve the domain boundaries of the Mn-CO/OO and/or the charge disordered regions of the manganite layer that become more numerous as the hole doping is reduced below x = 0.5
Transform-limited photons from a coherent tin-vacancy spin in diamond
Solid-state quantum emitters that couple coherent optical transitions to
long-lived spin qubits are essential for quantum networks. Here we report on
the spin and optical properties of individual tin-vacancy (SnV) centers in
diamond nanostructures. Through cryogenic magneto-optical and spin
spectroscopy, we verify the inversion-symmetric electronic structure of the
SnV, identify spin-conserving and spin-flipping transitions, characterize
transition linewidths, measure electron spin lifetimes and evaluate the spin
dephasing time. We find that the optical transitions are consistent with the
radiative lifetime limit even in nanofabricated structures. The spin lifetime
is phononlimited with an exponential temperature scaling leading to
10 ms, and the coherence time, reaches the nuclear spin-bath limit upon
cooling to 2.9 K. These spin properties exceed those of other
inversion-symmetric color centers for which similar values require millikelvin
temperatures. With a combination of coherent optical transitions and long spin
coherence without dilution refrigeration, the SnV is a promising candidate for
feasable and scalable quantum networking applications
Scaling of the Fano effect of the in-plane Fe-As phonon and the superconducting critical temperature in {\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}
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 Ba1−xKxFe2As2. The Fano parameter 1=q2, which is a measure of the phonon coupling to the electronic particle-hole continuum, shows a remarkable sensitivity to the magnetic and structural orderings at low temperatures. Most strikingly, at elevated temperatures in the paramagnetic tetragonal state we observe a linear correlation between 1=q2 and the superconducting critical temperature Tc. 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 xy orbital component at the Fermi level is fundamental for the Fano effect and, thus, possibly also for the superconducting pairing
MICADO: The data import engine of the CERN engineering and equipment data management system
The CERN Engineering and Equipment Data Management System (EDMS) is one of the largest and most complex data management systems of its kind. For the last 10 years, it has received huge quantities of data generated in the different LHC project phases. Capturing all this information would not have been possible without a robust, failsafe, yet flexible and user-friendly data import engine. For this purpose the Micado toolkit was developed at CERN using XML standards, providing multi-level data verifications, an advanced queuing mechanism and batch processing of large amounts of import requests. Whereas Micado originally was developed to facilitate the capturing of LHC manufacturing data, its modular architecture has allowed a cost-effective extension to also cater for the LHC installation and hardware commissioning data import processes. Recently Micado has broadened its scope even further, including also imports of safety inspections, and at the same time the next generation of the tool providing Web Services is already being developed
Kesterite Inorganic-Organic Heterojunction for Solution Processable Solar Cells
New synthesis of solution processable kesterite and kesterite-phenoxazine nanopowders were presented. The direct band-gap semiconductor Cu2ZnSnS4 has attracted the attention of many due to its large absorption coefficient (alpha > 10(4) cm(-1)) and (optical) band-gap energy close to the optimal value for solar light conversion (1.4-1.6 eV). The presence of a kesterite nanocrystal structure has been investigated and confirmed by (HR) TEM, X-ray powder diffraction, EDX and EXAFS measurements. Low-temperature photoluminescence (PL) measurements indicate the absence of PL in the Cu2ZnSnS4 nanocrystals. Electrochemical studies helped to prove that an inorganic-organic heterojunction of nanokesterite-phenoxazine was obtained. Device studies showed a two fold improvement in efficiency upon addition of a kesterite or phenoxazines-kesterite layer. (C) 2016 Elsevier Ltd. All rights reserved