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H. Xiao,
T. Hu,
S. K. He, B. Shen,
W. J. Zhang,
B. Xu,
K. F. He,
J. Han,
Y. P. Singh,
H. H. Wen,
X. G. Qiu,
C. Panagopoulos,
C. C. Almasan
[show abstract]
[hide abstract]
ABSTRACT: We show magnetotransport results on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.0 \leq
x \leq 0.13$) single crystals. We identify the low temperature resistance step
at 23 K in the parent compound with the onset of filamentary superconductivity
(FLSC), which is suppressed by an applied magnetic field in a similar manner to
the suppression of bulk superconductivity (SC) in doped samples. FLSC is found
to persist across the phase diagram until the long range antiferromagnetic
order is completely suppressed. A significant suppression of FLSC occurs for
$0.02<x<0.04$, the doping concentration where bulk SC emerges. Based on these
results and the recent report of an electronic anisotropy maximum for 0.02
$\leq x \leq$ 0.04 [Science 329, 824 (2010)], we speculate that, besides spin
fluctuations, orbital fluctuations may also play an important role in the
emergence of SC in iron-based superconductors.
08/2012;
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J. -Ph. Reid,
A. Juneau-Fecteau,
R. T. Gordon,
S. Rene de Cotret,
N. Doiron-Leyraud,
X. G. Luo,
H. Shakeripour,
J. Chang,
M. A. Tanatar,
H. Kim, [......],
T. Saito,
H. Fukazawa,
Y. Kohori,
K. Kihou,
C. H. Lee,
A. Iyo,
H. Eisaki, B. Shen,
H. -H. Wen,
Louis Taillefer
[show abstract]
[hide abstract]
ABSTRACT: The nature of the pairing state in iron-based superconductors is the subject
of much debate. Here we argue that in one material, the stoichiometric iron
pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state,
characterized by symmetry-imposed vertical line nodes in the superconducting
gap. This evidence is reviewed, with a focus on thermal conductivity and the
strong impact of impurity scattering on the critical temperature Tc. We then
compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the
pairing symmetry is s-wave and the Tc is ten times higher. The transition from
d-wave to s-wave within the same crystal structure provides a rare opportunity
to investigate the connection between band structure and pairing mechanism. We
also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which
the pairing symmetry is probably not d-wave, but more likely s-wave with
accidental line nodes.
07/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Approximately 20-30% of patients with ulcerative colitis would eventually require surgery despite recent advances in medical therapy. Ileal pouch-anal anastomosis has become the surgical treatment of choice after total proctocolectomy. A subset of patients who had a preoperative diagnosis of ulcerative colitis may develop Crohn's disease or a Crohn's disease-like condition of the ileal pouch after surgery. Diagnosis, differential diagnosis, and management of Crohn's disease of the ileal pouch have been challenging. A combined approach with the assessment of clinical history, endoscopy, histology, abdominal/pelvic imaging, and examination under anesthesia is necessary for an accurate diagnosis, disease classification, management and improvement in outcome. A multidisciplinary approach with gastroenterologists, colorectal surgeons, gastrointestinal pathologists and radiologists for proper medical, endoscopic, and surgical treatment is advocated.
Minerva gastroenterologica e dietologica 06/2012; 58(2):123-35.
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E. Hassinger,
G. Gredat,
F. Valade,
S. Rene de Cotret,
A. Juneau-Fecteau,
J. -Ph. Reid,
H. Kim,
M. A. Tanatar,
R. Prozorov, B. Shen,
H. -H. Wen,
N. Doiron-Leyraud,
Louis Taillefer
[show abstract]
[hide abstract]
ABSTRACT: The electrical resistivity rho of the iron-arsenide superconductor
Ba1-xKxFe2As2 was measured in applied pressures up to 2.6 GPa for four
underdoped samples, with x = 0.16, 0.18, 0.19 and 0.21. The antiferromagnetic
ordering temperature T_N, detected as a sharp anomaly in rho(T), decreases
linearly with pressure. At pressures above around 1.0 GPa, a second sharp
anomaly is detected at a lower temperature T_0, which rises with pressure. We
attribute this second anomaly to the onset of a phase that causes a
reconstruction of the Fermi surface. This new phase expands with increasing x
and it competes with superconductivity. We discuss the possibility that a
second spin-density wave orders at T_0, with a Q vector distinct from that of
the spin-density wave that sets in at T_N.
05/2012;
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[show abstract]
[hide abstract]
ABSTRACT: BaZr0.2Ti0.8O3 thin films on Pt/Ti/SiO2/Si substrates have been fabricated under low temperature conditions by a sol–gel-hydrothermal technique. The dielectric constant
is 247–83 in the frequency range of 1kHz–1MHz. The corresponding dielectric loss is ~10−2. The capacitance–voltage curve shows strong non-linear dielectric behavior leading to a high tunability, up to ~30% at 1kHz.
The remanent polarization and coercive field at room temperature are measured to be ~1.5μC/cm2 and ~90kV/cm. The infrared optical properties of the thin films are investigated using an infrared spectroscopic ellipsometry
in the wave number range of 800–4,000cm−1. Optical constants of the thin films are simultaneously obtained.
KeywordsThin films-Low temperature technique-Dielectric properties
Journal of Sol-Gel Science and Technology 05/2012; 55(3):343-347. · 1.63 Impact Factor
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D. V. Evtushinsky,
V. B. Zabolotnyy,
T. K. Kim,
A. A. Kordyuk,
A. N. Yaresko,
J. Maletz,
S. Aswartham,
S. Wurmehl,
A. V. Boris,
D. L. Sun,
C. T. Lin, B. Shen,
H. H. Wen,
A. Varykhalov,
R. Follath,
B. Büchner,
S. V. Borisenko
[show abstract]
[hide abstract]
ABSTRACT: Among numerous hypotheses, recently proposed to explain superconductivity in
iron-based superconductors [1-9], many consider Fermi surface (FS) nesting [2,
4, 8, 10] and dimensionality [4, 9] as important contributors. Precise
determination of the electronic spectrum and its modification by
superconductivity, crucial for further theoretical advance, were hindered by a
rich structure of the FS [11-17]. Here, using the angle-resolved photoemission
spectroscopy (ARPES) with resolution of all three components of electron
momentum and electronic states symmetry, we disentangle the electronic
structure of hole-doped BaFe2As2, and show that nesting and dimensionality of
FS sheets have no immediate relation to the superconducting pairing.
Alternatively a clear correlation between the orbital character of the
electronic states and their propensity to superconductivity is observed: the
magnitude of the superconducting gap maximizes at 10.5 meV exclusively for iron
3dxz;yz orbitals, while for others drops to 3.5 meV. Presented results reveal
similarities of electronic response to superconducting and magneto-structural
transitions [18, 19], implying that relation between these two phases is more
intimate than just competition for FS, and demonstrate importance of orbital
physics in iron superconductors.
04/2012;
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B Shen,
E S H Chu,
G Zhao,
K Man,
C-W Wu,
J T Y Cheng,
G Li,
Y Nie,
C M Lo,
N Teoh,
G C Farrell,
J J Y Sung,
J Yu
[show abstract]
[hide abstract]
ABSTRACT: We have previously demonstrated that peroxisome proliferator-activated receptor (PPARγ) activation inhibits hepatocarcinogenesis. We aim to investigate the effect of PPARγ on hepatocellular carcinoma (HCC) metastatic potential and explore its underlying mechanisms.
Human HCC cells (MHCC97L, BEL-7404) were infected with adenovirus-expressing PPARγ (Ad-PPARγ) or Ad-lacZ and treated with or without PPARγ agonist (rosiglitazone). The effects of PPARγ on cell migration and invasive activity were determined by wound healing assay and Matrigel invasive model in vitro, and in an orthotopic liver tumour metastatic model in mice.
Pronounced expression of PPARγ was demonstrated in HCC cells (MHCC97L, BEL-7404) treated with Ad-PPARγ, rosiglitazone or Ad-PPARγ plus rosiglitazone, compared with control (Ad-LacZ). Such induction markedly suppressed HCC cell migration. Moreover, the invasiveness of MHCC97L and BEL-7404 cells infected with Ad-PPARγ, or treated with rosiglitazone was significantly diminished up to 60%. Combination of Ad-PPARγ and rosiglitazone showed an additive effect. Activation of PPARγ by rosiglitazone significantly reduced the incidence and severity of lung metastasis in an orthotopic HCC mouse model. Key mechanisms underlying the effect of PPARγ in HCC include upregulation of cell adhesion genes, E-cadherin and SYK (spleen tyrosine kinase), extracellular matrix regulator tissue inhibitors of metalloproteinase (TIMP) 3, tumour suppressor gene retinoblastoma 1, and downregulation of pro-metastatic genes MMP9 (matrix metallopeptidase 9), MMP13, HPSE (heparanase), and Hepatocyte growth factor (HGF). Direct transcriptional regulation of TIMP3, MMP9, MMP13, and HPSE by PPARγ was shown by ChIP-PCR.
Peroxisome proliferator-activated receptor-gamma exerts an inhibitory effect on the invasive and metastatic potential of HCC in vitro and in vivo, and is thus, a target for the prevention and treatment of HCC metastases.
British Journal of Cancer 04/2012; 106(9):1486-94. · 5.04 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report the observation of a pseudogap in the \emph{ab}-plane optical
conductivity of underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($x = 0.2$ and
0.12) single crystals. Both samples show prominent gaps opened by a spin
density wave (SDW) order and superconductivity at the transition temperatures
$T_{\it SDW}$ and $T_c$, respectively. In addition, we observe an evident
pseudogap below $T^{\ast} \sim$ 75 K, a temperature much lower than $T_{\it
SDW}$ but much higher than $T_{c}$. A spectral weight analysis shows that the
pseudogap is closely connected to the superconducting gap, indicating the
possibility of its being a precursor of superconductivity. The doping
dependence of the gaps is also supportive of such a scenario.
02/2012;
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E. Hassinger,
G. Gredat,
F. Valade,
S. René de Cotret,
A. Juneau-Fecteau,
J. Ph Reid,
H. Kim,
M. A. Tanatar,
R. Prozorov, B. Shen,
H. H. Wen,
N. Doiron-Leyraud,
Louis Taillefer
Physical Review B. 01/2012; 86(14):140502.
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Physical Review B. 01/2012; 86(10).
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X. Y. Peng,
Q. Zhang, B. Shen,
J. R. Shi,
C. M. Yin,
X. W. He,
F. J. Xu,
X. Q. Wang,
N. Tang,
C. Y. Jiang,
Y. H. Chen,
K. Chang
[show abstract]
[hide abstract]
ABSTRACT: An anomalous linear photogalvanic effect (LPGE) in AlxGa1−xN/GaN heterostructures under infrared irradiation has been observed. This effect exhibits a similar dependence as the ordinary LPGE on the laser polarization state but is closely related to the inhomogeneity of the laser intensity. This effect is believed to originate from the optical momentum alignment effect and a microscopic model has been developed based on that. This anomalous LPGE provides a sensitive way for the detection of photoinduced momentum anisotropy in semiconductors.
Physical Review B 08/2011; 84(7):075341. · 3.69 Impact Factor
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Q. X. Guo,
H. Senda,
K. Saito,
T. Tanaka,
M. Nishio,
J. Ding,
T. X. Fan,
D. Zhang,
X. Q. Wang,
S. T. Liu, B. Shen,
R. Ohtani
[show abstract]
[hide abstract]
ABSTRACT: We have investigated the electronic structure of GaInN semiconductors by performing x-ray absorption near-edge fine structure (XANES) spectroscopy at Ga K -edge and self-consistent-field real-space multiple-scattering theory calculations. It was demonstrated that the nondestructive Ga K -edge XANES spectra can be used as the fingerprints of structure and composition for GaInN. The theoretical calculations gave a reasonable reproduction of the experimental spectral features. The results revealed that the combination of the experimental XANES and the theoretical calculations is a powerful tool for studying the electronic structure of GaInN semiconductors.
Applied Physics Letters 06/2011; · 3.84 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Hole mobility in wurtzite InN at low electric fields is studied by an ensemble Monte Carlo calculation. Scatterings of holes by polar optical phonons, nonpolar optical phonons, acoustic phonons, ionized and neutral impurities, and threading dislocations are taken into account. Mobility of holes is ∼ 220 cm2/V s at 300 K in the InN, where holes are only scattered by the lattice. It decreases to 20–70 cm2/V s when the present quality of InN with threading dislocation density of ∼ 1010 cm−2 and residual donor concentration of over 1017 cm−3 is considered. The calculated mobility coincides well with the recent experimental observation.
Applied Physics Letters 05/2011; 98(19):192114-192114-3. · 3.84 Impact Factor
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C. C. Huang,
F. J. Xu,
X. D. Yan,
J. Song,
Z. Y. Xu,
L. B. Cen,
Y. Wang,
J. H. Pan,
X. Q. Wang,
Z. J. Yang, B. Shen,
B. S. Zhang,
X. S. Chen,
W. Lu
[show abstract]
[hide abstract]
ABSTRACT: Defects and strain control in AlxGa1−xN/GaN multiple quantum wells (MQWs) for intersubband transitions (ISBTs) at atmospheric window grown on GaN/sapphire templates by metal-organic chemical vapor deposition have been investigated adopting strain modulation technique using AlN/GaN superlattices (SLs) interlayer. It is found that cracking in the MQWs can be effectively avoided adopting AlN/GaN SLs interlayer. It is demonstrated that AlN/GaN SLs interlayer acts as a flexible layer and relieves most of the tensile strain through buried microcracks in AlN/GaN SLs interlayer. The intersubband absorptions at 3.6–4.1 μm wavelength region have been observed on the crack-free AlxGa1−xN/GaN MQWs. Our results open up prospects to realize crack-free and high quality AlxGa1−xN/GaN MQWs on GaN/sapphire templates for ISBTs devices at 3–5 μm atmospheric window.
Applied Physics Letters 03/2011; 98(13):132105-132105-3. · 3.84 Impact Factor
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C. M. Yin,
N. Tang,
S. Zhang,
J. X. Duan,
F. J. Xu,
J. Song,
F. H. Mei,
X. Q. Wang, B. Shen,
Y. H. Chen,
J. L. Yu,
H. Ma
[show abstract]
[hide abstract]
ABSTRACT: The photocurrent has been measured in Al <sub>0.25</sub> Ga <sub>0.75</sub> N / GaN heterostructures at room temperature, and the photoinduced anomalous Hall effect (AHE) was observed. The AHE current changes linearly with the varied longitudinal electric fields. Due to the strong Rashba spin–orbit coupling of the two-dimensional electron gas in Al <sub>0.25</sub> Ga <sub>0.75</sub> N / GaN heterostructures, the intrinsic anomalous Hall mechanism is supposed to contribute to the photoinduced AHE. The photoinduced AHE measurement proposed in this study could be used to other spin related measurements at room temperature.
Applied Physics Letters 03/2011; 98(12):122104. · 3.84 Impact Factor
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Z. L. Miao,
N. Tang,
F. J. Xu,
L. B. Cen,
K. Han,
J. Song,
C. C. Huang,
T. J. Yu,
Z. J. Yang,
X. Q. Wang,
G. Y. Zhang, B. Shen,
K. Wei,
J. Huang,
X. Y. Liu
[show abstract]
[hide abstract]
ABSTRACT: Magnetotransport properties of the two-dimensional electron gas (2DEG) in lattice-matched In0.18Al0.82N/AlN/GaN heterostructures have been studied at low temperatures and high magnetic fields. The double subband occupancy of the 2DEG in the triangular quantum well at the heterointerface is observed. The 2DEG density is determined to be 2.09×1013 cm−2 and the energy separation between the first and the second subbands is 191 meV. Both of them are significantly higher than those in AlxGa1−xN/AlN/GaN heterostructures owing to the stronger spontaneous polarization effect. The evident difference of the quantum scattering times in the two subbands of the 2DEG indicates that the interface roughness scattering plays an important role in the transport properties of the 2DEG in InxAl1−xN/AlN/GaN heterostructures.
Journal of Applied Physics 01/2011; 109(1):016102-016102-3. · 2.17 Impact Factor
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J. Song,
F. J. Xu,
X. D. Yan,
F. Lin,
C. C. Huang,
L. P. You,
T. J. Yu,
X. Q. Wang, B. Shen,
K. Wei,
X. Y. Liu
[show abstract]
[hide abstract]
ABSTRACT: A correlation between microstructures and high gate leakage current density of Schottky contacts on lattice-matched InxAl1−xN/GaN heterostructures has been investigated by means of current-voltage measurements, conductive atom force microscopy (C-AFM), and transmission electron microscopy (TEM) investigations. It is shown that the reverse-bias gate leakage current density of Ni/Au Schottky contacts on InxAl1−xN/GaN heterostructures is more than two orders of magnitude larger than that on AlxGa1−xN/GaN ones. C-AFM and TEM observations indicate that screw- and mixed-type threading dislocations (S/M-TDs) are efficient leakage current channels in InxAl1−xN barrier and In segregation is formed around S/M-TDs. It is believed that In segregation around S/M-TDs reduces local Schottky barrier height to form conductive channels and leads to high leakage current density of Schottky contacts on InxAl1−xN/GaN heterostructures.
Applied Physics Letters 12/2010; 97(23):232106-232106-3. · 3.84 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The influence of polarization-induced electric fields on the coherent electron tunneling probability in AlN/GaN coupled double quantum wells (CDQWs) has been performed by solving Schrödinger and Poisson equations self-consistently. It is found that when the first excited state (E2) and the second excited state (E3) resonate in AlN/GaN CDQWs, the coherent electron tunneling probability is 16 times higher than that in AlN/GaN single quantum well, which is attributed to the Fabry–Perot quantum interference mechanism in AlN/GaN CDQWs. However, the coherent electron tunneling probability decreases rapidly with the polarization-induced electric fields increasing, which is attributed to the resonance between the E2 and E3 subbands weakening with the polarization-induced electric fields increasing in AlN/GaN CDQWs.
Journal of Applied Physics 12/2010; 108(11):113107-113107-4. · 2.17 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The interband impact ionization transition rate (IIR) of electrons and holes in wurtzite GaN and AlN is calculated using full local density approximation (LDA) band structures under the scissor approximation. The IIR of AlN is first calculated and the results show a large difference between the IIR of electrons and holes. The IIR of electrons grows from zero to 1014 at energies of ~6.6–10 eV, while the IIR of holes is zero below 12.2 eV. The difference is interpreted by the energy gap between the sixth and the seventh valence bands. Analysis shows that it is hard for holes in AlN to tunnel through the large energy gap between the sixth and the seventh valence bands; as a result, holes can hardly get enough energy for impact ionization in the electrical field of several MV cm−1. This indicates that AlN is a good material for avalanche photodetectors of controllable gain, low noise and wide bandwidth with a cutoff wavelength of 210 nm. To further facilitate Monte Carlo simulation, the AlN and GaN IIR are fitted to the Keldysh formula.
Semiconductor Science and Technology 10/2010; 25(11):115010. · 1.72 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Owing to the extremely high-time resolution of impulse radio ultra-wideband (IR-UWB), time of arrival (TOA) estimation has been an important and tempting issue of this technology ever since its emergence. Conventional TOA estimation (TOAE) schemes require prohibitively high sampling rate and a priori knowledge of the received signal, and hence render a practical implementation rigorous or even infeasible. To tackle these drawbacks, this paper proposes a low-complexity energy detection-based non-coherent TOAE scheme, which is composed of two processing stages: initial signal acquisition (ISA) and fine timing estimation (FTE). In the ISA stage, a linear quadrature optimisation (LQO)-based weighting scheme is proposed to coarsely capture the arrival of the IR-UWB signals. Capitalising on the acquisition of the IR-UWB signal in a relatively short time range, the authors then develop in the FTE stage, a double-threshold test (DTT) tailored for locating the leading edge of the IR-UWB signal. Simulations illustrate that the LQO algorithm yields a considerably increased probability of seizing the arrival of the IR-UWB signals in a blind manner, and the DTT strategy significantly ameliorates the TOAE accuracy in terms of mean absolute error, compared with the conventional energy detection-based TOAE methods.
IET Communications 09/2010; · 0.83 Impact Factor
