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N. Xu,
P. Richard, X. -P. Wang,
X. Shi,
A. van Roekeghem,
T. Qian,
E. Ieki,
K. Nakayama,
T. Sato,
E. Rienks,
S. Thirupathaiah,
J. Xing,
H. -H. Wen,
M. Shi,
T. Takahashi,
H. Ding
[show abstract]
[hide abstract]
ABSTRACT: We used high-energy resolution angle-resolved photoemission spectroscopy to
extract the momentum dependence of the superconducting gap of Ru-substituted
Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ ($T_c = 15$ K). Despite a strong
out-of-plane warping of the Fermi surface, the magnitude of the superconducting
gap observed experimentally is nearly isotropic and independent of the
out-of-plane momentum. More precisely, we respectively observed 5.7 meV and 4.5
meV superconducting gaps on the inner and outer $\Gamma$-centered hole Fermi
surface pockets, whereas a 4.8 meV gap is recorded on the M-centered electron
Fermi surface pockets. Our results are consistent with the $J_1-J_2$ model with
a dominant antiferromagnetic exchange interaction between the next-nearest Fe
neighbors.
Physical Review B 03/2013; 87:094513. · 3.69 Impact Factor
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Y. -B. Huang,
P. Richard,
J. -H. Wang, X. -P. Wang,
X. Shi,
N. Xu,
Z. Wu,
A. Li,
J. -X. Yin,
T. Qian,
B. Lv,
C. W. Chu,
S. H. Pan,
M. Shi,
H. Ding
[show abstract]
[hide abstract]
ABSTRACT: We performed a combined angle-resolved photoemission spectroscopy and
scanning tunneling microscopy study of the electronic structure of
electron-doped Ca$_{0.83}$La$_{0.17}$Fe$_2$As$_2$. A surface reconstruction
associated with the dimerization of As atoms is observed directly in the real
space, as well as the consequent band folding in the momentum space. Besides
this band folding effect, the Fermi surface topology of this material is
similar to that reported previously for BaFe$_{1.85}$Co$_{0.15}$As$_2$, with
$\Gamma$-centred hole pockets quasi-nested to M-centred electron pockets by the
antiferromagnetic wave vector. Although no superconducting gap is observed by
ARPES possibly due to low superconducting volume fraction, a gap-like density
of states depression of $7.7\pm 2.9$ meV is determined by scanning tunneling
microscopy.
10/2012;
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X. -P. Wang,
P. Richard,
A. van Roekeghem,
Y. -B. Huang,
E. Razzoli,
T. Qian,
E. Rienks,
S. Thirupathaiah,
H. -D. Wang,
C. -H. Dong,
M. -H. Fang,
M. Shi,
H. Ding
[show abstract]
[hide abstract]
ABSTRACT: We performed a high-resolution angle-resolved photoemission spectroscopy
study on superconducting (SC) Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$ ($T_c=29$
K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic $\sim$
8.2 meV 2-dimensional (2D) SC gap ($2\Delta/k_BT_c\sim7$) on quasi-2D electron
Fermi surfaces (FSs) located around M$(\pi,0,0)$-A$(\pi,0,\pi)$, we observe a
$\sim 6.2$ meV isotropic SC gap ($2\Delta/k_BT_c\sim5$) on the Z-centered
electron FS that rules out any d-wave pairing symmetry and rather favors an
s-wave symmetry. All isotropic SC gap amplitudes can be fit by a single gap
function derived from a local strong coupling approach suggesting an
enhancement of the next-next neighbor exchange interaction in the
ferrochalcogenide superconductors.
EPL (Europhysics Letters) 08/2012; 99:67001. · 2.17 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The superconducting gap is the fundamental parameter that characterizes the
superconducting state, and its symmetry is a direct consequence of the
mechanism responsible for Cooper pairing. Here we discuss about angle-resolved
photoemission spectroscopy measurements of the superconducting gap in the
Fe-based high-temperature superconductors. We show that the superconducting gap
is Fermi surface dependent and nodeless with small anisotropy, or more
precisely, a function of momentum. We show that while this observation is
inconsistent with weak coupling approaches for superconductivity in these
materials, it is well supported by strong coupling models and global
superconducting gaps. We also suggest that the strong anisotropies measured by
other probes sensitive to the residual density of states are not related to the
pairing interaction itself, but rather emerge naturally from the smaller
lifetime of the superconducting Cooper pairs that is a direct consequence of
the momentum dependent interband scattering inherent to these materials.
08/2012;
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N Xu,
T Qian,
P Richard,
Y B Shi, X P Wang,
P Zhang,
Y B Huang,
Y-M Xu,
H Miao,
G Xu,
G F Xuan,
W H Jiao,
Z A Xu,
G H Cao,
H Ding
[show abstract]
[hide abstract]
ABSTRACT: We report a systematic angle-resolved photoemission spectroscopy study on Ba(Fe$_{1−x}$Ru$_x$)$_2$As$_2$ for a wide range of Ru concentrations (0.15 $\leq$ x $\leq$ 0.74). We observed a crossover from two dimensions to three dimensions for some of the holelike Fermi surfaces with Ru substitution and a large reduction in the mass renormalization close to optimal doping. These results suggest that isovalent Ru substitution has remarkable effects on the low-energy electron excitations, which are important for the evolution of superconductivity and antiferromagnetism in this system.
Physical Review B 08/2012; 86(6):064505. · 3.69 Impact Factor
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X.-P. Wang,
P. Richard,
Y.-B. Huang,
H. Miao,
L. Cevey,
N. Xu,
Y.-J. Sun,
T. Qian,
Y.-M. Xu,
M. Shi,
J.-P. Hu,
X. Dai,
H. Ding
[show abstract]
[hide abstract]
ABSTRACT: In order to determine the orbital characters on the various Fermi surface pockets of the Fe-based superconductors Ba0.6K0.4Fe2As2 and FeSe0.45Te0.55, we introduce a method to calculate photoemission matrix elements. We compare our simulations to experimental data obtained with various experimental configurations of beam orientation and light polarization. We show that the photoemission intensity patterns revealed from angle-resolved photoemission spectroscopy measurements of Fermi surface mappings and energy-momentum plots along high-symmetry lines exhibit asymmetries carrying precious information on the nature of the states probed, information that is destroyed after the data symmetrization process often performed in the analysis of angle-resolved photoemission spectroscopy data. Our simulations are consistent with Fermi surfaces originating mainly from the dxy, dxz, and dyz orbitals in these materials.
Physical Review B 06/2012; 85(21):214518. · 3.69 Impact Factor
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N. Xu,
T. Qian,
P. Richard,
Y. -B. Shi, X. -P. Wang,
P. Zhang,
Y. -B. Huang,
Y. -M. Xu,
H. Miao,
G. Xu,
G. -F. Xuan,
W. -H. Jiao,
Z. -A. Xu,
G. -H. Cao,
H. Ding
[show abstract]
[hide abstract]
ABSTRACT: We report a systematic angle-resolved photoemission spectroscopy study on
Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ for a wide range of Ru concentrations (0.15
$\leq$ \emph{x} $\leq$ 0.74). We observed a crossover from two-dimension to
three-dimension for some of the hole-like Fermi surfaces with Ru substitution
and a large reduction in the mass renormalization close to optimal doping.
These results suggest that isovalent Ru substitution has remarkable effects on
the low-energy electron excitations, which are important for the evolution of
superconductivity and antiferromagnetism in this system.
03/2012;
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H. Miao,
P. Richard,
Y. Tanaka,
K. Nakayama,
T. Qian,
K. Umezawa,
T. Sato,
Y.-M. Xu,
Y.-B. Shi,
N. Xu, X.-P. Wang,
P. Zhang,
H.-B. Yang,
Z.-J. Xu,
J. S. Wen,
G.-D. Gu,
X. Dai,
J.-P. Hu,
T. Takahashi,
H. Ding
[show abstract]
[hide abstract]
ABSTRACT: We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe1−xSex, which has the simplest structure of all Fe-based superconductors. We found isotropic superconducting gaps on all Fermi surfaces whose sizes can be fitted by a single gap function derived from a strong coupling approach, promoting local antiferromagnetic exchange interactions as a serious candidate for the pairing origin.
Physical Review B 03/2012; 85(9):094506. · 3.69 Impact Factor
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T Qian, X-P Wang,
W-C Jin,
P Zhang,
P Richard,
G Xu,
X Dai,
Z Fang,
J-G Guo,
X-L Chen,
H Ding
[show abstract]
[hide abstract]
ABSTRACT: We have performed an angle-resolved photoemission spectroscopy study of the new iron-based superconductor K(0.8)Fe(1.7)Se(2) (T(c)∼30 K). Clear band dispersion is observed with the overall bandwidth renormalized by a factor of 2.5 compared to our local density approximation calculations, indicating relatively strong correlation effects. Only an electronlike band crosses the Fermi energy, forming a nearly circular Fermi surface (FS) at M (π, 0). The holelike band at Γ sinks ∼90 meV below the Fermi energy, with an indirect band gap of 30 meV, to the bottom of the electronlike band. The observed FS topology in this superconductor favors (π, π) inter-FS scattering between the electronlike FSs at the M points, in sharp contrast to other iron-based superconductors which favor (π, 0) inter-FS scattering between holelike and electronlike FSs.
Physical Review Letters 05/2011; 106(18):187001. · 7.37 Impact Factor
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T. Qian,
N. Xu,
Y.-B. Shi,
K. Nakayama,
P. Richard,
T. Kawahara,
T. Sato,
T. Takahashi,
M. Neupane,
Y.-M. Xu, X.-P. Wang,
G. Xu,
X. Dai,
Z. Fang,
P. Cheng,
H.-H. Wen,
H. Ding
[show abstract]
[hide abstract]
ABSTRACT: We have performed an angle-resolved photoemission spectroscopy study of the iron-based superconductor (SC) Sr2VFeAsO3. While V 3d orbitals are found to be in a Mott-insulating state and show an incoherent peak at ~1 eV below the Fermi level, the dispersive Fe 3d bands form several hole- and electronlike Fermi surfaces (FSs), some of which are quasinested by the (π,0) wave vector. This differs from the local-density approximation (LDA) calculations, which predict non-nested FSs for this material. However, LDA + U with a large effective Hubbard energy U on V 3d electrons can reproduce the experimental observation reasonably well. The observed fermiology in SC Sr2VFeAsO3 strongly supports that (π,0) interband scattering between quasinested FSs may play an important role for SC in pnictides.
Physical Review B 04/2011; 83(14). · 3.69 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We performed a high-resolution angle-resolved photoemission spectroscopy
study of the Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$ superconductor ($T_c=29$
K). We show the existence of two electronlike bands at the M$(\pi, 0)$ point
which cross the Fermi level at similar Fermi wave vectors to form nearly
circular electronlike Fermi surface pockets. We observe a nearly isotropic
$\sim$ 8.5 meV superconducting gap ($\Delta/k_BT_c\sim 7$) on these Fermi
surfaces. Our analysis of the band structure around the Brillouin zone centre
reveals two additional electronlike Fermi surfaces: a very small one and a
larger one with $k_F$ comparable to the FS pockets at M. Interestingly, a SC
gap with a magnitude of $\sim$ 8 meV also develops along the latter FS. Our
observations are consistent with the s-wave strong coupling scenario.
01/2011;
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[show abstract]
[hide abstract]
ABSTRACT: In this letter, we study the influence of the Pt top-electrode thickness and of the chamber atmosphere during cell operation on the resistive switching of TiN HfO <sub>2</sub> Pt cells. The oxygen permeability of the Pt electrode directly in contact with the atmosphere significantly affects the resistive switching and the resistance states of the cell. The results provide strong experimental indications that the electroforming operation leads to oxygen-vacancy formation and that the subsequent reset operation relies on the available oxygen species in the filament neighborhood. Significant implications with respect to endurance and retention assessment of resistive-switching memory devices are discussed.
Applied Physics Letters 01/2011; · 3.84 Impact Factor
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T. Qian, X. -P. Wang,
W. -C. Jin,
P Zhang,
P. Richard,
G Xu,
X Dai,
Z Fang,
J. G. Guo,
X-L Chen,
H Ding
[show abstract]
[hide abstract]
ABSTRACT: We have performed an angle-resolved photoemission spectroscopy study of the
new iron-based superconductor K0.8Fe1.7Se2 (Tc ~ 30 K). Clear band dispersion
is observed with the overall bandwidth renormalized by a factor of 2.5 compared
to our local density approximation calculations, indicating relatively strong
correlation effects. Only an electronlike band crosses the Fermi energy,
forming a nearly circular Fermi surface (FS) at M ({\pi}, 0). The holelike band
at {\Gamma} sinks ~ 90 meV below the Fermi energy, with an indirect band gap of
30 meV to the bottom of the electronlike band. The observed FS topology in this
superconductor favors ({\pi}, {\pi}) inter-FS scattering between the
electronlike FSs at the M points, in sharp contrast with other iron-based
superconductors which favor ({\pi}, 0) inter-FS scattering between holelike and
electronlike FSs.
12/2010;
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T. Qian,
N Xu,
Y. -B. Shi,
K Nakayama,
P. Richard,
T Kawahara,
T Sato,
T Takahashi,
M. Neupane,
Y. -M. Xu, X. -P. Wang,
G Xu,
X Dai,
Z Fang,
P Cheng,
H. H. Wen,
H Ding
[show abstract]
[hide abstract]
ABSTRACT: We have performed an angle-resolved photoemission spectroscopy study of a new
iron-based superconductor Sr4V2O6Fe2As2. While V 3d orbitals are found to be in
a Mott insulator state and show an incoherent peak at ~ 1 eV below the Fermi
level, the dispersive Fe 3d bands form several hole- and electron-like Fermi
surfaces (FSs), some of which are quasi-nested by the (pi, 0) wave vector. This
differs from the local density approximation (LDA) calculations, which predict
non-nested FSs for this material. However, LDA+U with a large effective Hubbard
energy U on V 3d electrons can reproduce the experimental observation
reasonably well. The observed fermiology in superconducting Sr4V2O6Fe2As2
strongly supports that (pi, 0) interband scattering between quasi-nested FSs is
indispensable to superconductivity in pnictides.
08/2010;
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[show abstract]
[hide abstract]
ABSTRACT: This paper deals with the bipolar switching properties of Ni\NiO\Ni cells for which the NiO layer is made by thermal oxidation
of Ni layers. Physical characterization of the NiO layer revealed a strong deficiency in oxygen and a defective crystal structure.
This microstructure allowed reproducible bipolar switching, which we attributed to the drift of oxygen anionic species and
redox reactions at the anode. Conductive atomic force microscopy (C-AFM) experiments on Ni\NiO surfaces showed the scaling
potential of the bipolar switching. C-AFM and macroscopic electrical measurements showed similarities and consistent results
related to oxygen movement effects at the nanometer scale through relaxation processes and switching events. A discussion
is conducted on the origin of the change in resistance, observed either at a microscopic or at a macroscopic scale. A distinction
is made between electrochemical redox reactions taking place along conductive channels near the Ni\NiO\Ni top electrode and
local changes of oxygen-vacancy densities.
Journal of The Electrochemical Society. 07/2010; 157(8):G187-G192.
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N. Menou, X.P. Wang,
B. Kaczer,
W. Polspoel,
M. Popovici,
K. Opsomer,
M.A. Pawlak,
W. Knaepen,
C. Detavernier,
T. Blomberg, [......],
J. Swerts,
J.W. Maes,
P. Favia,
H. Bender,
B. Brijs,
W. Vandervorst,
S. Van Elshocht,
D.J. Wouters,
S. Biesemans,
J.A. Kittl
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate for the first time record low Leakage-EOT (3.5 times 10<sup>-7</sup> A/cm<sup>2</sup> at 1V, EOT=0.49 nm) MIM capacitors fabricated using a low temperature (250degC) ALD SrTiO<sub>3</sub> (STO) deposition process on ALD TiN bottom electrode. While most previous work on STO used deposition techniques not compatible with high aspect ratio DRAM applications, recent work on ALD STO showed promise on noble-like metal electrodes (Ru, Pt) [1,2]. In this work, a low temperature ALD process with alternative precursor set and carefully optimized deposition and processing conditions enables the use of low-cost, manufacturable-friendly TiN electrode MIMcaps for future DRAM nodes. Composition (Sr-rich) and process optimization allowed minimization of interfacial EOT penalties and leakage reduction by decreasing the density of leakier STO grains.
Electron Devices Meeting, 2008. IEDM 2008. IEEE International; 01/2009
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S Van Elshocht,
C Adelmann,
S Clima,
G Pourtois,
T Conard,
A Delabie,
A Franquet,
P Lehnen,
J Meersschaut,
N Menou, [......],
A Hardy,
D Dewulf,
M K Van Bael,
T Blomberg,
D Pierreux,
J Swerts,
J W Maes,
D J Wouters,
S De Gendt,
J A Kittl
Journal of Vacuum Science & Technology B. 01/2009; 27(1):209-213.
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[show abstract]
[hide abstract]
ABSTRACT: Surface soil moisture content exhibits a high degree of spatial and temporal variability. The purpose of this study was (a) to characterize the spatial and temporal variations in soil moisture contents in the depths of 0–15 and 0–30 cm soil layer in an artificially re-vegetated desert area in Shapotou, China; and (b) to make inferences regarding the environmental factors that influence such variability. Over a period of 7-month experiment, soil moisture contents were measured with Time Domain Reflectometry (TDR) at an interval of every half a month at a grid space with 10 m intervals horizontally covering 4500 m2 area, which include a topography from dune crest to dune hollow, and again to dune crest. Results indicated that the distribution of soil moisture at the grid scale was in a pattern of normal distribution with a significant temporal–spatial variability; the variability of soil moisture decreased with decreased mean soil moisture content, and it was higher in the depth of 0–15 cm than that in the depth of 0–30 cm. Relative elevation was a main influence factor of soil moisture variability especially after rainfall events and its influence was more tremendous in the depth of 0–30 cm than that in the depth of 0–15 cm. There was a consistent time-series pattern between the vegetation cover and the soil moisture content and the relative elevation, which increased under the rainstorm events. Soil texture (particle size distribution) had an opposite time-series pattern with relative elevation and vegetation cover, and the correlation was higher in the depth of 0–15 cm than that in the depth of 0–30 cm. We may conclude that local topography and vegetation cover were the main influencing factors of soil moisture variations immediately after the rainstorm events, whereas the soil texture was an important influencing factor as the soil dried up in the study area.
Journal of Arid Environments.
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[show abstract]
[hide abstract]
ABSTRACT: Cadmium-metallothionein (Cd-MT) may have a role in the pathogenesis and irreversibility of Cd nephrotoxicity. In the present study, rats were injected with 0.3 mg Cd/kg body wt per week as Cd-MT for 5 consecutive weeks and a group of rats (n = 3) was killed 24 hr after each injection. A group of three rats was kept for an additional week after the S weeks of Cd-MT injection for recovery. After the first injection, urinary Cd and protein levels and kidney/body wt ratio were increased. The electrophoretic pattern of urinary protein showed increased excretion of low-molecular-weight proteins, especially after the first injection of Cd-MT. Tubular cell necrosis occurred after the first week with renal Cd levels of only 10 μg/g and gradually progressed to severe necrosis with inflammation in 3 weeks and then to interstitial fibrosis in 5 weeks. The levels of Cd and MT in kidney increased with repeated injection of Cd-MT, but renal Cd was about 40 μg/g after 5 weeks of injection. Urinary Cd and MT levels progressively increased during the Cd exposure period, but returned to pretreatment levels during the sixth week (recovery period). Renal cell necrosis and inflammation were absent at the sixth week, but interstitial fibrosis persisted. This study indicates that nephrotoxicity of Cd in this model is related to urinary excretion of Cd-MT and that renal cell injury may be independent of Cd in the renal cortex. Nephrotoxicity occurs at levels much lower than the proposed critical concentration for Cd (200 μg Cd/g) following long-term exposure to CdCl2. However, in the absence of continued Cd exposure from liver or circulation, the Cd-MT-induced renal damage is reversible.
Toxicology and Applied Pharmacology.
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J.A. Kittl,
K. Opsomer,
M. Popovici,
N. Menou,
B. Kaczer, X.P. Wang,
C. Adelmann,
M.A. Pawlak,
K. Tomida,
A. Rothschild, [......],
W. Vandervorst,
T. Conard,
A. Franquet,
P. Favia,
H. Bender,
B. Brijs,
S. Van Elshocht,
M. Jurczak,
J. Van Houdt,
D.J. Wouters
[show abstract]
[hide abstract]
ABSTRACT: The requirements and development of high-k dielectric films for application in storage cells of future generation flash and Dynamic Random Access Memory (DRAM) devices are reviewed. Dielectrics with k-value in the 9–30 range are studied as insulators between charge storage layers and control gates in flash devices. For this application, large band gaps (>6 eV) and band offsets are required, as well as low trap densities. Materials studied include aluminates and scandates. For DRAM metal–insulator–metal (MIM) capacitors, aggressive scaling of the equivalent oxide thickness (with targets down to 0.3 nm) drives the research towards dielectrics with k-values >50. Due to the high aspect ratio of MIMCap structures, highly conformal deposition techniques are needed, triggering a substantial effort to develop Atomic Layer Deposition (ALD) processes for the deposition of metal gates and high-k dielectrics. Materials studied include Sr- and Ba-based perovskites, with SrTiO3 as one of the most promising candidates, as well as tantalates, titanates and niobates.
Microelectronic Engineering.
