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ABSTRACT: The high pressure synthesis, structure and magnetic properties of
Ca_25Co_22O_56(OH)_28 are reported. The compound has a misfit structure,
consisting of double, square calcium oxide hydroxide rock-salt-like layers
between hexagonal CoO_2 layers. The misfit compound crystallizes in the
monoclinic space group C2/m, and can be characterized by the coexistence of two
subsystems with common a=4.893(5)A, c=8.825(9)A and b=95.745(8) parameters, and
different b parameters: b_RS=4.894(5)A, and b_HEX=2.809(3)A, for the rock-salt
and hexagonal type planes respectively. The compound shows Curie-Weiss
paramagnetism with an antiferromagnetic Weiss temperature of -43K and a reduced
Co moment. Substantial deviations from Curie-Weiss behavior are seen below 50K
with no indication of magnetic ordering. No superconductivity was observed down
to a temperature of 2K.
10/2005;
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ABSTRACT: Single phase samples of the monoclinic and orthorhombic polymorphs of Nb12O29 have been identified and isolated. These polymorphs have different arrangements of nanometer dimension structural columns of ReO3-type niobium oxygen arrays: They differ in structure after a 3-nm translation in one of three dimensions while being the same in the other two. Magnetic susceptibility measurements show that one polymorph displays an antiferromagnetic transition at 12 K, along with short-range order fluctuations up to 25 K, while the other shows strict Curie-Weiss behavior down to 2 K.
Phys. Rev. B. 07/2005; 72(3).
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ABSTRACT: In a ferromagnet, an anomalous Hall heat current, given by the off-diagonal Peltier term alpha(xy), accompanies the anomalous Hall current. By combining Nernst, thermopower, and Hall experiments, we have measured how alpha(xy) varies with hole density and lifetime tau in CuCr2Se4-xBrx. At low temperatures T, we find that alpha(xy) is independent of tau, consistent with anomalous-velocity theories. Its magnitude is fixed by a microscopic geometric area A approximately 34 A(2). Our results are incompatible with some models of the Nernst effect in ferromagnets.
Physical Review Letters 12/2004; 93(22):226601. · 7.37 Impact Factor
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ABSTRACT: In a ferromagnet, an anomalous-Hall heat current, given by the off-diagonal Peltier term $\alpha_{xy}$, accompanies the anomalous Hall current. By combining Nernst, thermopower and Hall experiments, we have measured how $\alpha_{xy}$ varies with hole density and lifetime $\tau$ in $\rm CuCr_2Se_{4-x}Br_x$. At low temperatures $T$, we find that $\alpha_{xy}$ is independent of $\tau$, consistent with anomalous-velocity theories. Its magnitude is fixed by a microscopic geometric area ${\cal A}\sim 34 \mathrm{\AA}^2$. Our results are incompatible with some models of the Nernst effect in ferromagnets. Comment: 4 pages, 5 figures, revision with publication info
07/2004;
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ABSTRACT: In a ferromagnet, an applied electric field $\bf E$ invariably produces an anomalous Hall current ${\bf J}_H$ that flows perpendicular to the plane defined by $\bf E$ and $\bf M$ (the magnetization). For decades, the question whether ${\bf J}_H$ is dissipationless (independent of the scattering rate), has been keenly debated without experimental resolution. In the ferromagnetic spinel CuCr$_2$Se$_{4-x}$Br$_x$, the resistivity $\rho$ (at low temperature) may be increased 1000 fold by varying $x$(Br), without degrading the $\bf M$. We show that ${\bf J}_H/E$ (normalized per carrier, at 5 K) remains unchanged throughout. In addition to resolving the controversy experimentally, our finding has strong bearing on the generation and study of spin-Hall currents in bulk samples. Comment: 7 pages, 6 figures
05/2004;
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ABSTRACT: In a ferromagnet, an applied electric field E invariably produces an anomalous Hall current JH that flows perpendicular to the plane defined by E and M (the magnetization). For decades, the question of whether JH is dissipationless (independent of the scattering rate) has been debated without experimental resolution. In the ferromagnetic spinel CuCr2Se4-xBrx, the resistivity rho (at low temperature) may be increased by several decades by varying x (Br) without degrading M. We show that JH/E (normalized per carrier, at 5 kelvin) remains unchanged throughout. In addition to confirming the dissipationless nature of JH, our finding has implications for the generation and study of spin-Hall currents in bulk samples.
Science 04/2004; 303(5664):1647-9. · 31.20 Impact Factor
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ABSTRACT: Neutron scattering has been used to investigate the crystal structure and lattice dynamics of superconducting Na0.3CoO2⋅1.4(H/D)2O, and the “parent” Na0.3CoO2 material. The structure of Na0.3CoO2 consists of alternate layers of CoO2 and Na and is the same as the structure at higher Na concentrations. For the superconductor, the water forms two additional layers between the Na and CoO2, increasing the c-axis lattice parameter of the hexagonal P63/mmc space group from 11.16 Å to 19.5 Å. The Na ions are found to occupy a different configuration from the parent compound, while the water forms a structure that replicates the structure of ice. Both types of sites are only partially occupied. The CoO2 layer in these structures is robust, on the other hand, and we find a strong inverse correlation between the CoO2 layer thickness and the superconducting transition temperature (TC increases with decreasing thickness). The phonon density of states for Na0.3CoO2 exhibits distinct acoustic and optic bands, with a high-energy cutoff of ∼100meV. The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes that are quite similar to ice, supporting the structural model that the water intercalates and forms ice-like layers in the superconductor.
Phys. Rev. B. 12/2003; 68(21).
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ABSTRACT: We report results of specific heat measurements on polycrystalline samples of the layered superconductor, Na(0.3)CoO(2).1.3H(2)O. The electronic contribution to the specific heat, gamma, is found to be 12.2 mJ/mol-K2. The feature at the superconducting transition is rather sharp, becoming broad and strongly suppressed in an applied magnetic field. The data indicate a residual normal state electronic specific heat at low temperatures, implying that there is a sizable population of normal state electrons in the samples even below Tc. Inhomogeneity in the Na content, to which the superconducting state is exquisitely sensitive, appears to be the most likely explanation for these results. These results further indicate that special sample handling is required for an accurate characterization of the superconducting state in this material.
08/2003;
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ABSTRACT: Neutron scattering has been used to investigate the crystal structure and lattice dynamics of superconducting Na0.3CoO2 1.4(H/D)2O, and the parent Na0.3CoO2 material. The structure of Na0.3CoO2 consists of alternate layers of CoO2 and Na and is the same as the structure at higher Na concentrations. For the superconductor, the water forms two additional layers between the Na and CoO2, increasing the c-axis lattice parameter of the hexagonal P63/mmc space group from 11.16 A to 19.5 A. The Na ions are found to occupy a different configuration from the parent compound, while the water forms a structure that replicates the structure of ice. Both types of sites are only partially occupied. The CoO2 layer in these structures is robust, on the other hand, and we find a strong inverse correlation between the CoO2 layer thickness and the superconducting transition temperature (TC increases with decreasing thickness). The phonon density-of-states for Na0.3CoO2 exhibits distinct acoustic and optic bands, with a high-energy cutoff of ~100 meV. The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes that are quite similar to ice, supporting the structural model that the water intercalates and forms ice-like layers in the superconductor. Comment: 14 pages, 7 figures, Phys. Rev. B (in press). Minor changes + two figures removed as requested by referee
07/2003;
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M. L. Foo,
R. E. Schaak, V. L. Miller,
T. Klimczuk,
N. S. Rogado,
Yayu Wang,
G. C. Lau,
C. Craley,
H. W. Zandbergen,
N. P. Ong,
R. J. Cava
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ABSTRACT: The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2*1.4H2O is shown through X-ray diffraction and thermogravimetric studies to be one of a series of hydrated phases of Na0.3CoO2. Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with c=13.8 angstroms implies that the superconductivity turns on in this system between CoO2 layer spacings of 6.9 and 9.9 angstroms at nominally constant chemical doping. Comment: 10 pages and 4 figures
04/2003;
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ABSTRACT: We report results of specific heat measurements on polycrystalline samples of the layered superconductor, Na0.3CoO2 · 1.3H2O. The electronic contribution to the specific heat, γ, is found to be 12.2 mJ/mol K2. The feature at the superconducting transition is rather sharp, becoming broad and strongly suppressed in an applied magnetic field. The data indicate a residual normal state electronic specific heat at low temperatures, implying that there is a sizable population of normal state electrons in the samples even below Tc. Inhomogeneity in the Na content, to which the superconducting state is exquisitely sensitive, appears to be the most likely explanation for these results. These results further indicate that special sample handling is required for an accurate characterization of the superconducting state in this material.
Physica C: Superconductivity.
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Physical Review B, 72 (3).
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M.L. Foo,
R.E. Schaak, V.L. Miller,
T. Klimczuk,
N.S. Rogado,
Y. Wang,
G.C. Lau,
C. Craley,
H.W. Zandbergen,
N.P. Ong,
R.J. Cava
[show abstract]
[hide abstract]
ABSTRACT: The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2·1.4H2O is shown through X-ray diffraction and thermogravimetric studies to be one of the series of hydrated phases of Na0.3CoO2. Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with implies that the superconductivity turns on in this system between CoO2 layer spacings of 6.9 and 9.9 Å at nominally constant chemical doping.
Solid State Communications.
