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ABSTRACT: We present a detailed in-situ Raman analysis of stage-1 KC8, CaC6, and LiC6
graphite intercalation compounds (GIC) to unravel their intrinsic finger print.
Four main components were found between 1200 cm-1 and 1700 cm-1, and each of
them were assigned to a corresponding vibrational mode. From a detailed line
shape analysis of the intrinsic Fano-lines of the G- and D-line response we
precisely determine the position ({\omega}ph), line width ({\Gamma}ph) and
asymmetry (q) from each component. The comparison to the theoretical calculated
line width and position of each component allow us to extract the
electron-phonon coupling constant of these compounds. A coupling constant
{\lambda}ph < 0.06 was obtained. This highlights that Raman active modes alone
are not sufficient to explain the superconductivity within the electron-phonon
coupling mechanism in CaC6 and KC8.
04/2012;
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Y. J. Um,
A. Subedi,
P. Toulemonde,
A. Y. Ganin,
L. Boeri,
M. Rahlenbeck,
Y. Liu,
C. T. Lin,
S. J. E. Carlsson,
A. Sulpice, M. J. Rosseinsky,
B. Keimer,
M. Le Tacon
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ABSTRACT: We report on an investigation of the lattice dynamical properties in a range of Fe1+yTe1−xSex compounds, with special emphasis on the c-axis polarized vibration of Fe with B1g symmetry, a Raman active mode common to all families of Fe-based superconductors. We have carried out a systematic study of the temperature dependence of this phonon mode as a function of Se x and excess Fe y concentrations. In parent compound Fe1+yTe, we observe an unconventional broadening of the phonon between room temperature and magnetic ordering temperature TN. The situation smoothly evolves toward a regular anharmonic behavior as Te is substituted for Se and long-range magnetic order is replaced by superconductivity. Irrespective to Se contents, excess Fe is shown to provide an additional damping channel for the B1g phonon at low temperatures. We performed density functional theory ab initio calculations within the local density approximation to calculate the phonon frequencies, including magnetic polarization and Fe nonstoichiometry in the virtual crystal approximation. We obtained a good agreement with the measured phonon frequencies in the Fe-deficient samples, while the effects of Fe excess are poorly reproduced. This may be due to excess Fe-induced local magnetism and low-energy magnetic fluctuations that cannot be treated accurately within these approaches. As recently revealed by neutron scattering and muon spin rotation studies, these phenomena occur in the temperature range where anomalous decay of the B1g phonon is observed and suggests a peculiar coupling of this mode with local moments and spin fluctuations in Fe1+yTe1−xSex.
Phys. Rev. B. 02/2012; 85(6).
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ABSTRACT: Bi(Fe2/8Mg3/8Ti3/8)O3 (BFTM) is a member of a small class of pure Bi3+ A site perovskites which are stable without recourse to high pressure synthesis. BFTM is polar in the R3c space group, but ferroelec. switching of the polarisation is not attainable in bulk ceramics. Formation of solid solns. with BaTiO3 produces enhanced functional behavior. The compn. 0.75Bi(Fe2/8Mg3/8Ti3/8)O3-0.25BaTiO3 displays ferroelec. hysteresis loops and piezoelec. response (high field d33 of 85 pC N-1 at 0.1 Hz and low field d33 of 16 pC N-1). This change in functional behavior is assocd. with significant changes in the av. structure, where the rhombohedral distortion is reduced and transformed to a pseudo-cubic R3m space group, as substitution of the larger Ba2+ cation suppresses tilting of the BO6 octahedra. Polar Bi displacements are refined solely along the pseudocubic <111>p direction of the perovskite subcell, with the off-axis displacements characteristic of BFTM being suppressed. The local structure deviates
Chemical science (Royal Society of Chemistry) 01/2012; 3(5):1426-1435. · 7.53 Impact Factor
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Microporous and Mesoporous Materials 01/2012; 157:24-32. · 3.29 Impact Factor
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ABSTRACT: Ordering of the tetrahedral site vacancies in two crystals of refined
compositions K0.93(1)Fe1.52(1)Se2 and K0.862(3)Fe1.563(4)Se2 produces a
fivefold expansion of the parent ThCr2Si2 unit cell in the ab plane which can
accommodate 20% vacancies on a single site within the square FeSe layer. The
iron charge state is maintained close to +2 by coupling of the level of alkali
metal and iron vacancies, producing a potential doping mechanism which can
operate at both average and local structure levels.
02/2011;
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Z.-R. Tang,
S. A. Kondrat,
C. Dickinson,
J. K. Bartley,
A. F. Carley,
S. H. Taylor,
T. E. Davies,
M. Allix, M. J. Rosseinsky,
J. B. Claridge,
Z. Xu,
S. Romani,
M. J. Crudace,
G. J. Hutchings
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ABSTRACT: A series of high surface area nanocryst. copper manganese oxide catalysts have been prepd. by supercrit. anti-solvent (SAS) pptn. using CO2 and tested for the ambient temp. oxidn. of CO. The catalysts were prepd. by pptn. from an ethanol/metal acetate soln. and the addn. of small quantities of water was found to result in a mixed acetate precursor with surface areas >200 m2 g-1, considerably higher than those prepd. by conventional pptn. methods. The surface area of the final calcined mixed oxide was found to be dependent upon the initial water concn. XRD and FT-IR anal. indicated that the addn. of water promoted the formation of carbonate species in the amorphous acetate precursor, with high resoln. TEM and STEM showing the material to consist of spherical agglomerations of fibrous strings of ca. 30 nm length. This is in contrast to the material prepd. in the absence of water, using the same SAS methodol., which typically yields quasi-spherical particles of 100 nm size. [on SciFinder(R)]
Catalysis Science & Technology 01/2011; 1(5):64-70.
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ABSTRACT: An 8-layer B-site deficient twinned hexagonal perovskite Ba(8)Ga(4-x)Ta(4+0.6x)O(24) has been synthesized and its structure and microwave dielectric properties characterized. This hexagonal perovskite consists of eight close-packed BaO(3) layers stacked by a sequence of (ccch)(2), where c and h refer to cubic and hexagonal BaO(3) layers, respectively. The Ba(8)Ga(4-x)Ta(4+0.6x)O(24) ceramic materials exhibit composition-independent dielectric permittivity e;4 29, improved Qf value with the B-site vacancy content increase, and tunable temperature coefficient of resonant frequency tau(f) from negative to positive. An optimum microwave dielectric performance was achieved for Ba(8)Ga(0.8)Ta(5.92)O(24): Qf approximate to 29 000 GHz and tau(f) approximate to 11 ppm/degrees C. The factors controlling the microwave dielectric properties are discussed in comparison with 8-layer twinned analogues and related 10-layer twinned hexagonal perovskites based on their structural and property data.
Chemistry of Materials. 01/2011; 23(22):5058-5067.
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ABSTRACT: We report X-band electron paramagnetic resonance (EPR) and 125Te and 77Se NMR measurements on single-crystalline superconducting FeSe0.42Te0.58 [Tc=11.5(1) K]. The data provide indications for the coexistence of intrinsic localized and itinerant electronic states. In the normal state, localized moments couple to itinerant electrons in the Fe(Se,Te) layers and affect the local spin susceptibility and spin fluctuations. Below Tc, spin fluctuations become rapidly suppressed and an unconventional superconducting state emerges in which 1/T1 is reduced at a much faster rate than expected for conventional s- or s±-wave symmetry. We suggest that the localized states arise from the strong electronic correlations within one of the Fe-derived bands. The multiband electronic structure together with the electronic correlations thus determine the normal and superconducting states of the FeSe1−xTex family, which appears much closer to other high-Tc superconductors than previously anticipated.
Physical Review B. 10/2010; 82(14):140508.
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J Rabone,
Y-F Yue,
S Y Chong,
K C Stylianou,
J Bacsa,
D Bradshaw,
G R Darling,
N G Berry,
Y Z Khimyak,
A Y Ganin,
P Wiper,
J B Claridge, M J Rosseinsky
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ABSTRACT: Porous materials find widespread application in storage, separation, and catalytic technologies. We report a crystalline porous solid with adaptable porosity, in which a simple dipeptide linker is arranged in a regular array by coordination to metal centers. Experiments reinforced by molecular dynamics simulations showed that low-energy torsions and displacements of the peptides enabled the available pore volume to evolve smoothly from zero as the guest loading increased. The observed cooperative feedback in sorption isotherms resembled the response of proteins undergoing conformational selection, suggesting an energy landscape similar to that required for protein folding. The flexible peptide linker was shown to play the pivotal role in changing the pore conformation.
Science 08/2010; 329(5995):1053-7. · 31.20 Impact Factor
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ABSTRACT: We present data for the temperature dependence of the magnetic penetration depth lambda(T), heat capacity C(T), resistivity R(T) and magnetic torque ?tau for highly homogeneous single crystal samples of Fe1:0Se0:44(4)Te0:56(4). lambda(T) was measured down to 200mK in zero field. We find lambda(T) follows a power law lambda~T^n with n = 2.2 +/- 0.1. This is similar to some 122 iron-arsenides and likely results from a sign-changing pairing state combined with strong scattering. Magnetic fields of up to B =55T or 14T were used for the ? tau(B) and C(T)/R(T) measurements respectively. The specific heat, resistivity and torque measurements were used to map out the (H,T) phase diagram in this material. All three measurements were conducted on exactly the same single crystal sample so that the different information revealed by these probes is clearly distinguished. Heat capacity data strongly resemble those found for the high Tc cuprates, where strong fluctuation effects wipe-out the phase transition at Hc2. Unusually, here we find the fluctuation effects appear to be strongly anisotropic. Comment: 10 pages, 9 figures, submitted to PRB
07/2010;
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ABSTRACT: We report X-band EPR and $^{125}$Te and $^{77}$Se NMR measurements on
single-crystalline superconducting FeSe$_{0.42}$Te$_{0.58}$ ($T_c$ = 11.5(1)
K). The data provide evidence for the coexistence of intrinsic localized and
itinerant electronic states. In the normal state, localized moments couple to
itinerant electrons in the Fe(Se,Te) layers and affect the local spin
susceptibility and spin fluctuations. Below $T_c$, spin fluctuations become
rapidly suppressed and an unconventional superconducting state emerges in which
$1/T_1$ is reduced at a much faster rate than expected for conventional $s$- or
$s_\pm$-wave symmetry. We suggest that the localized states arise from the
strong electronic correlations within one of the Fe-derived bands. The
multiband electronic structure together with the electronic correlations thus
determine the normal and superconducting states of the FeSe$_{1-x}$Te$_x$
family, which appears much closer to other high-$T_c$ superconductors than
previously anticipated.
06/2010;
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A Tamai,
A Y Ganin,
E Rozbicki,
J Bacsa,
W Meevasana,
P D C King,
M Caffio,
R Schaub,
S Margadonna,
K Prassides, M J Rosseinsky,
F Baumberger
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ABSTRACT: We investigate the normal state of the "11" iron-based superconductor FeSe0.42Te0.58 by angle-resolved photoemission. Our data reveal a highly renormalized quasiparticle dispersion characteristic of a strongly correlated metal. We find sheet dependent effective carrier masses between approximately 3 and 16m{e} corresponding to a mass enhancement over band structure values of m{*}/m{band} approximately 6-20. This is nearly an order of magnitude higher than the renormalization reported previously for iron-arsenide superconductors of the "1111" and "122" families but fully consistent with the bulk specific heat.
Physical Review Letters 03/2010; 104(9):097002. · 7.37 Impact Factor
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A Tamai,
A. Y. Ganin,
E. Rozbicki,
J. Bacsa,
W. Meevasana,
P D C King,
M. Caffio,
R. Schaub,
S. Margadonna,
K Prassides, M J Rosseinsky,
F Baumberger
[show abstract]
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ABSTRACT: We investigate the normal state of the '11' iron-based superconductor FeSe0.42Te0.58 by angle resolved photoemission. Our data reveal a highly renormalized quasiparticle dispersion characteristic of a strongly correlated metal. We find sheet dependent effective carrier masses between ~ 3 - 16 m_e corresponding to a mass enhancement over band structure values of m*/m_band ~ 6 - 20. This is nearly an order of magnitude higher than the renormalization reported previously for iron-arsenide superconductors of the '1111' and '122' families but fully consistent with the bulk specific heat. Comment: 5 pages, 4 figures, to appear in Phys. Rev. Lett
12/2009;
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ABSTRACT: We report on the ambient pressure 13C and 133Cs NMR study of disorder-free A15-structured Cs3C60, which shows non-BCS superconductivity at 38 K under pressure. Temperature-independent 13C and 133Cs spin-lattice relaxation rates prove an insulating ground state with a small effective magnetic moment 1.3–1.6μB for T>100 K. Below 46 K, A15 Cs3C60 orders into an antiferromagnetic structure best described by a magnetic wave vector, q=(1/2,1/2,1/2) with a staggered magnetic moment ∼0.9μB per C60. The large (U/W) ratio (∼2.2), the opening of a gap in the electronic excitation spectrum and the small C603− magnetic moment all underline the importance of electronic correlations in A15 Cs3C60.
Phys. Rev. B. 11/2009; 80(19).
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ABSTRACT: Magnetic, structural and reduction data on the K2NiF4 structure CrIII/NiIII oxides, LaSrCrxNi1-xO4+δ (0.1 ≤ x ≤ 0.9, where δ may be negative) are reported; samples with x > 0.2 adopt the tetragonal I4/mmm space group, while for x ≤ 0.2, the samples are multiphasic due to competition between a novel vacancy-ordered Immm structure and the undistorted K2NiF4 phase. Magnetic studies in the temperature range 5 ≤ T/K ≤ 300 show spin glass behavior for x < 0.9, with freezing temperatures of between 12 and 16 K: at higher temperature the susceptibilities display Curie−Weiss behavior. The magnetism of the x = 0.9 sample resembles that reported for LaSrCrO4, displaying an antiferromagnetic transition at 150 K and weak ferromagnetism below 130 K. Reduction of the title compounds in 1 atm of flowing H2 at 500 °C produces stable, oxygen-deficient compounds, notably La2Sr2CrNiO7, which contains NiI with a thermal stability unprecedented in an oxide.
12/2008;
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ABSTRACT: Superconductivity emerges for the A15 polymorph of the fulleride Cs3C60 upon compression to a pressure of approximately 4 kbar. Using density functional theory we study the bonding in the A15 phase as a function of unit cell volume comparing it to that in the fcc polymorph. We find that, despite its smaller packing density, the bcc-derived A15 phase has both a substantially wider bandwidth for the partially occupied t1u band and a higher density of states at the Fermi level. This result can be traced to the striking differences in the nature of the interanion Tc--the two sphere packings (body centered versus face centered) observed experimentally produce two electronically distinct classes of fulleride superconductors.
Physical Review Letters 10/2008; 101(13):136404. · 7.37 Impact Factor
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ABSTRACT: The introduction of mixed valency into extended main-group solids is discussed using the example of hole-doped LiBC, where a combination of experimental measurements and density functional theory calculations is used to understand the observed electronic properties in terms of deviation from the expected rigid-band electronic structure behaviour.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 02/2008; 366(1862):55-62. · 2.77 Impact Factor
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ABSTRACT: A2B2O7 complex oxides with the pyrochlore structure feature a wide variety of chem. compns. and corresponding diverse elec. and phys. properties. A new pyrochlore, Yb0.96Ca1.04TiNbO6.98, is reported and characterized in terms of structural stability and elec. properties. The material is stable to ∼1450° in 5% H2/N2 and has a high electronic cond. in a reducing atm., e.g., 9 S/cm at 800° in 5% H2/N2 after high-temp. redn. Other cation substitutions enhance the low-temp. kinetics of cond. response to changes in oxygen partial pressure. [on SciFinder(R)]
Chemistry of Materials 01/2008; 20(22):6911-6916. · 7.29 Impact Factor
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ABSTRACT: The first examples of anionic-neutral mixed ligand frameworks prepared from 1,3, 5-benzenetrisbenzoate (BTB) and 4,4 '-bipyridyl (bipy) are reported. Framework 1, of composition [Ni(HBTB)(bipy)]center dot(guests), is a triply interpenetrated (3,5)-connected gra binodal network that has a 1-D channel structure exhibiting permanent porosity. Framework 2, which was prepared under identical conditions but in the presence of toluene, has the formula [Ni-3(BTB)(2)(EG)(3)(bipy)(3)]center dot(toluene) (where EG ethylene glycol). 2 is a dense phase of triply interpenetrated rare (3,4)-connected tfz binodal networks that contains intercalated toluene in isolated voids. The primary influence for the formation of 2 are numerous hydrophobic interactions between the internal framework surface and the included toluene guest molecule.
CrystEngComm 01/2008; 10(11):1687-1692. · 3.84 Impact Factor
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ABSTRACT: In this work, we present a detailed study of the ammoniated fullerides (NH3)KRb2C60 and (NH3)Rb3C60 using the μ+ spin relaxation/rotation (μ+SR) technique in its zero-field (ZF), longitudinal-field (LF) and transverse-field (TF) variants as a function of temperature and cooling rate. The ZF and TF μ+SR spectra of the investigated ammoniated alkali fullerides clearly reveal the onset of long-range antiferromagnetic (AF) order at low temperatures. In the AF state, the local field exhibits large spatial inhomogeneities, while its quasi-static nature is confirmed by the complete recovery of the asymmetry in an LF of 200–500 Oe. A large fraction of paramagnetic domains survives well into the AF state. In addition, the cooling rates of the samples affect sensitively the coherent ordering of the magnetic moments, while the Néel temperature TN remains essentially unchanged. Finally, the anomalous evolution of the Néel temperatures with x across the (NH3)K3−xRbxC60 (0≤x≤3) series is ascribed to the different low-temperature orientationally ordered superstructures appearing between x = 2 and 3.
Journal of Physics Condensed Matter 09/2007; 19(38):386235. · 2.55 Impact Factor
