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J D Moore,
L. F. Cohen,
Y. Yeshurun,
A. D. Caplin,
K Morrison,
K. A. Yates,
C.M. McGilvery,
J. M. Perkins,
D.W. McComb,
C. Trautmann,
Z. A. Ren, J Yang,
W Lu,
X. L. Dong,
Z. X. Zhao
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ABSTRACT: Oxypnictide superconductor NdFeAsO0.85 sample was irradiated with 2 GeV Ta ions at a fluence of 5x10^10 ions/cm2. High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetisation in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is enhanced at fields below the matching field (~1 Tesla) but only marginally. The pinning enhancement is anisotropic and maximum along the defect direction at high temperatures but the pinning then becomes more isotropic at low temperatures. The creep rate is suppressed at high temperatures and at fields below the matching field, indicating the columnar defects are efficient pinning sites at these H and T conditions. Comment: 10 pages, 4 figures
07/2009;
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W. L. Yang,
P. O. Velasco,
J. D. Denlinger,
A. P. Sorini,
C C Chen,
B. Moritz,
W. S. Lee,
F. Vernay,
B. Delley,
J. -H. Chu,
J. G. Analytis,
I. R. Fisher,
Z. A. Ren, J Yang,
W Lu,
Z. X. Zhao,
J. van den Brink,
Z. Hussain,
Z. -X. Shen,
T. P. Devereaux
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ABSTRACT: Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe $L$ edges is investigated in Fe pnictide materials, and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters $U\lesssim 2$eV and $J\approx 0.8$eV. Comment: 11 pages, 12 figures
05/2009;
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ABSTRACT: We have measured the thermoelectric power S of the iron-based superconductor SmFeAsO0.85 as a function of temperature at various pressures up to 1.1 GPa. The magnitude of thermoelectric power increases with decreasing temperature and exhibits a maximum at a characteristic temperature T* (~110 K at ambient pressure), whereas the temperature dependence of the resistance shows metallic behavior. The superconducting transition temperature Tc and T* decrease monotonically with pressure. We discuss our results in terms of the effects of Fermi-surface nesting and orbit degeneracy in the new iron-oxypnictides.
New Journal of Physics 02/2009; 11(2):025006. · 4.18 Impact Factor
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ABSTRACT: Here the authors report that by a high pressure synthesis method, superconductivity with an onset transition temperature at 52 K was discovered in a fluorine-doped quaternary iron-arsenide compound Pr[O1-xFx]FeAs, with the zero resistivity and Meissner transition appeared at 44 K and 50 K respectively. As the first non-cuprate compound that superconducts above 50 K, this discovery places these iron arsenide compounds to the second high temperature superconducting family explicitly.
Material Research Innovations 08/2008; 12(3):105-106. · 0.41 Impact Factor
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A Yamamoto,
A A Polyanskii,
J Jiang,
F Kametani,
C Tarantini,
F Hunte,
J Jaroszynski,
E E Hellstrom,
P J Lee,
A Gurevich,
D C Larbalestier,
Z A Ren, J Yang,
X L Dong,
W Lu,
Z X Zhao
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ABSTRACT: Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either widely spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto-optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the intergranular and intragranular current densities have distinctively different temperature dependences and differ in magnitude by about 1000. If the highest current density loops lie only within grains, their magnitude is ~5 × 106 A cm−2 at 5 K and self-field. Whole sample current densities, though two orders of magnitude lower at 1000–10 000 A cm−2, are some two orders of magnitude higher than in random polycrystalline cuprates. We cannot yet be certain whether this large difference in global and intragrain current density is intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because the samples contain a significant second phase, some of which wets the grain boundaries and may cause a superconducting–normal–superconducting proximity effect in the whole sample critical current.
Superconductor Science and Technology 07/2008; 21(9):095008. · 2.66 Impact Factor
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ABSTRACT: Diamagnetic susceptibility measurements under high hydrostatic pressure (up to 1.03 GPa) were carried out on a newly discovered Fe-based superconductor LaO1-xFxFeAs (x=0.11). The transition temperature TC, defined as a point at the maximum slope of superconducting transition, was enhanced almost linearly by hydrostatic pressure, yielding a dTC/dP of about 1.2 K GPa−1. Differential diamagnetic susceptibility curves indicate that the underlying superconducting state is complicated. It is suggested that pressure plays an important role on pushing the low TC superconducting phase toward the main (optimal) superconducting phase.
New Journal of Physics 06/2008; 10(6):063026. · 4.18 Impact Factor
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J. Jaroszynski,
S. C. Riggs,
F. Hunte,
A. Gurevich,
D. C. Larbalestier,
G. S. Beobinger,
F. F. Balakirev,
A. Migliori,
Z. A. Ren,
W. Lu, J. Yang,
X. L. Shen,
X. L. Dong,
Z. X. Zhao,
R. Jin,
A. S. Sefat,
M. A. McGuire,
B. C. Sales,
D. K. Christen,
D. Mandrus
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ABSTRACT: The recent discovery of a new class of superconducting oxypnictides with high
transition temperatures may have profound implications for understanding
unconventional high-temperature superconductivity. Like the cuprates, the
oxypnictides seem to manifest an interleaving of charge donor and
superconducting layers emerging upon doping of an antiferromagnetic parent
semi-metal. Here we report magneto-transport measurements of three rare earth
(Re = La, Nd, Sm) oxypnicide compounds with the transition temperatures near
the maximum reported to date, in very high DC and pulsed magnetic fields up to
45 and 54 T, respectively. Our resistivity, Hall coefficient and critical
magnetic fields data suggest that these oxypnictide superconductors bridge the
gap between MgB$_2$ and YBaCu$_3$O$_{7-x}$ as far as electromagnetic and vortex
properties are concerned.
06/2008;
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J D Moore,
K Morrison,
K. A. Yates,
A. D. Caplin,
Y. Yeshurun,
L. F. Cohen,
J. M. Perkins,
C.M. McGilvery,
D.W. McComb,
Z. A. Ren, J Yang,
W Lu,
X. L. Dong,
Z. X. Zhao
[show abstract]
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ABSTRACT: Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 103 A/cm2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 104 A/cm2. We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry. Comment: 11 pages, 4 figures
06/2008;
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ABSTRACT: Diamagnetic susceptibility measurements under high hydrostatic pressure (up to 1.03 GPa) were carried out on the newly discovered Fe-based superconductor LaO_{1-x}F_{x}FeAs(x=0.11). The transition temperature T_c, defined as the point at the maximum slope of superconducting transition, was enhanced almost linearly by hydrostatic pressure, yielding a dT_c/dP of about 1.2 K/GPa. Differential diamagnetic susceptibility curves indicate that the underlying superconducting state is complicated. It is suggested that pressure plays an important role on pushing low T_c superconducting phase toward the main (optimal) superconducting phase. Comment: 7 pages, 4 figures
03/2008;
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J. Jaroszynski,
Scott C. Riggs,
F. Hunte,
A. Gurevich,
D.C. Larbalestier,
G.S. Boebinger,
F.F. Balakirev,
Albert Migliori,
Z.A. Ren,
W. Lu, J. Yang,
X.L. Shen,
X.L. Dong,
Z.X. Zhao,
R. Jin,
A.S. Sefat,
M.A. McGuire,
B.C. Sales,
D.K. Christen,
D. Mandrus
Physical review. B, Condensed matter 01/2008; 78:064511.
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Advanced Materials 04/2004; 16(8):713 - 716. · 13.88 Impact Factor
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J. Jaroszynski,
Scott C. Riggs,
F. Hunte,
A. Gurevich,
D. C. Larbalestier,
G. S. Boebinger,
F. F. Balakirev,
Albert Migliori,
Z. A. Ren,
W. Lu, J. Yang,
X. L. Shen,
X. L. Dong,
Z. X. Zhao,
R. Jin,
A. S. Sefat,
M. A. McGuire,
B. C. Sales,
D. K. Christen,
D. Mandrus
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ABSTRACT: We compare magnetotransport of the three iron-arsenide-based compounds ReFeAsO (Re=La, Sm, Nd) in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Each sample studied exhibits a superconducting transition temperature near the maximum reported to date for that particular compound. While high magnetic fields do not suppress the superconducting state appreciably, the resistivity, Hall coefficient, and critical magnetic fields, taken together, suggest that the phenomenology and superconducting parameters of the oxypnictide superconductors bridges the gap between MgB2 and YBCO.
Phys. Rev. B. 78(6).
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ABSTRACT: The temperature dependent optical spectra are investigated in a well-characterized titanate system, Nd1−xTiO3, between 50 and 40 000 cm−1 at three different doping levels, x=0.019, 0.046, and 0.095, corresponding to a Mott-Hubbard insulator, a semiconductor and a correlated metal, respectively. Mid-gap states develop inside the Hubbard gap with hole doping. Based on the room-temperature spectra of the optical conductivity, the evolution rate of the excitations below 1.2 eV with doping is dependent on the electron correlation strength (U∕W) of the parent insulator, which has been observed in other titanates as well. In the metallic sample (x=0.095), an anomalous enhancement of spectral weight below 1 eV develops with decreasing temperature. The partial spectral weight shows a quadratic dependence on temperatures up to the plasma frequency. Meanwhile, the metallic sample displays a Fermi-liquid behavior at low frequencies.
Phys. Rev. B. 73(19).
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ABSTRACT: In the present paper, starting from elemental Bi, Sb and Te granules with 5 N purity and 1–5 mm diameter, p-type single phase (Bi2Te3)x(Sb2Te3)1−x (x=0.20, 0.225, 0.25, 0.275 and 0.30) thermoelectric materials with high densification (>99% theoretical density) were prepared via bulk mechanical alloying (BMA) and hot pressing (HP). Their thermoelectric properties, are less composition sensitive in the composition range of x≤0.25 and are comparable to that of those materials prepared by travelling heater method (THM); the figures of merit of (Bi2Te3)x(Sb2Te3)1−x are about 3×10−3/K. When x>0.25, the figures of merit of (Bi2Te3)x(Sb2Te3)1−x decreased rapidly with increasing x.
Journal of Alloys and Compounds.
