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ABSTRACT: We report the lattice parameter variation and the band gap tuning of CdO with Ca by preparing thermodynamically stable Cd1−xCaxO solid solution in the entire composition range 0 ≤ x ≤ 1. The functional dependence of the lattice parameter on Ca concentration is found to deviate from Vegard’s law. The principal band gap is found to vary nonmonotonically over a wide range, from 1.4 to 3.9 eV for 0 ≤ x ≤ 0.8. First principles density functional theory calculations, using full potential linearized augmented plane wave methods also predict a nonlinear variation for the lattice parameter and the optical band gap with Ca concentration. From these calculations, contributions from volume deformation, electron transfer, and structural relaxation are estimated and the results are compared with experiments.
Journal of Applied Physics 01/2011; 109(1):013510-013510-7. · 2.17 Impact Factor
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ABSTRACT: The occurrence of bulk superconductivity at ~22 K is reported in polycrystalline samples of BaFe2-xRuxAs2 for nominal Ru content in the range of x=0.75 to 1.125. A systematic suppression of the spin density wave transition temperature (TSDW) precedes the appearance of superconductivity in the system. A phase diagram is proposed based on the measured TSDW and superconducting transition temperature (TC) variations as a function of Ru composition. Band structure calculations, indicate introduction of electron carriers in the system upon Ru substitutiom. The calculated magnetic moment on Fe shows a minimum at x=1.0, suggesting that the suppression of the magnetic moment is associated with the emergence of superconductivity. Results of low temperature and high field Mossbauer measurements are presented. These indicate weakening of magnetic interaction with Ru substitution Comment: 20 pages 10 figures
02/2009;
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S. Dhara,
P. Magudapathy,
R. Kesavamoorthy,
S. Kalavathi, V. S. Sastry,
K. G. M. Nair,
G. M. Hsu,
L C Chen,
K H Chen,
K. Santhakumar,
T. Soga
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ABSTRACT: InN phase is grown in crystalline InP(100) substrates by 50 keV N+ implantation at an elevated temperature of 400 deg C followed by annealing at 525 deg C in N2 ambient. Crystallographic structural and Raman scattering studies are performed for the characterization of grown phases. Temperature- and power-dependent photoluminescence studies show direct band-to-band transition peak ~1.06 eV at temperatures <=150K. Implantations at an elevated temperature with a low ion beam current and subsequent low temperature annealing step are found responsible for the growth of high-quality InN phase.
09/2007;
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ABSTRACT: Structural and magnetic studies have been carried out on SrRu <sub>1-x</sub>M<sub>x</sub>O<sub>3</sub> ( M= Ga :x=0.1 ; Cu and Zn: x=0.1 and 0.2) in order to understand the dependence of the ferromagnetic transition temperature of SrRuO <sub>3</sub> on heterovalent magnetic and nonmagnetic cation substitution at the Ru site. Powder x-ray diffraction has been carried out to analyze the crystal structure using the Rietveld refinement procedure. T<sub>c</sub> decreases more rapidly for divalent cation dopants as compared to the trivalent cations. The refinement results show that there are distortions of the RuO <sub>6</sub> octahedra in the doped compounds, with little change in the lattice parameters. However, the direction and magnitudes of the tilt of the octahedra for all the compounds do not show correlation with the T<sub>c</sub> . Therefore, the change of T<sub>c</sub> is attributed to the charge and spin disorders introduced by the heterovalent dopant cations. The filled e<sub>g</sub> orbitals in Ga, Cu, and Zn ions are expected to have weak hybridization with the oxygen p orbitals as compared to the pristine compound, leading to a weakening of the overall ferromagnetic interactions between the Ru ions.
Journal of Applied Physics 08/2007; · 2.17 Impact Factor
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ABSTRACT: The effect of charge state modification at the Mn site on the physical properties of CMR manganites is reported. With a view to avoiding additional complexity of local spin coupling effects, Mn site substitution of La0.67Ca0.33MnO3 is carried out with appropriate diamagnetic ions—Zn2+, Zr4+, Ta5+ and W6+—of different valence states. The substitution results in size changes of the unit cell and enhanced local structural distortions, which increase in the order Zn, Zr, Ta and W. The ground state is ferromagnetic–metallic below a certain critical concentration xc of the substituents, beyond which the magnetic ground state shows a glassy behaviour. The phase transition temperatures (TMI and Tc) decrease with substitution, but to different extents. The observed suppression rates of the Curie temperature, Tc, of ~39 K/at.% and ~45 K/at.% respectively for Ta5+ and W6+ substituted compounds are the highest reported in the Mn site substituted CMR manganites. Besides the modification of majority carrier concentration due to the increased (decreased) Mn3+ concentration and enhanced local structural effects, the local electrostatic potential of the substituents seems to contribute to the unusually strong reduction in the itinerant ferromagnetism and the observed glassy states.
Journal of Physics Condensed Matter 05/2007; 19(23):236207. · 2.55 Impact Factor
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ABSTRACT: We discuss the modified magnetic state of single-phase La0.67Ca0.33Mn1−xTaxO3 (x≤0.10) colossal magnetoresistive compounds with orthorhombic Pnma symmetry. Pentavalent Ta substitution results in an extraordinarily strong reduction of the magnetic phase transition temperatures (~39 K/at.%) in the ferromagnetic–metallic regime (x≤0.03) and suppresses the metal–insulator transition for x≥0.05. Many experimental features show evidence for the cluster-glass behaviour of the compounds with x≥0.05. These features are large thermomagnetic irreversibility just below the Curie temperature (Tc), monotonic increase of the field cooled magnetization down to 4.2 K, non-saturation of magnetization, and the observation of two distinct magnetic transitions in ac susceptibility in an appropriate dc field—one close to Tc and the other at a lower temperature (the spin-freezing temperature (Tg)). These results find additional support from non-equilibrium spin dynamics with a maximum in magnetic viscosity at Tg and a linear low-temperature magnetic specific heat with a characteristic broad maximum close to Tg.
Journal of Physics Condensed Matter 04/2007; 19(21):216218. · 2.55 Impact Factor
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ABSTRACT: In this letter we report observation of room temperature ferromagnetism in bulk Ga$_2$O$_3$ . We also observe large (10-60%) increase/decrease in photoluminescence. In the red(700 nm wavelength)/blue(500 nm), with the application of small magnetic field(0.4 Tesla). We argue, that ferromagnetism occurs entirely due to chains of oxygen(O(3) sites, Fig. 5) vacancies. We propose a simple model to explain, origin and location of moment, formation of ferromagnetic dislocation needles, and strong increase/decrease ofred/blue photoluminescence intensity with magnetic field.
02/2007;
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ABSTRACT: We report the charge state modification effects at the Mn site on the ground
state properties of colossal magnetoresistive manganites. Ta5+
substitution results in an appreciable increase in the lattice parameters
and unit cell volume due to increased Mn3+ concentration. The
ferromagnetic-metallic ground state modifies to a cluster glass insulator
for
x \geqslant 0.05x \geqslant 0.05
. The reduction in the transition temperatures with
increasing x is ∼39K/at.%. Besides the modification of majority carrier concentration due to increased Mn3+ concentration and enhanced local structural effects, the local
electrostatic potential of the substituent seems to contribute to the
unusually strong reduction of the transition temperatures of the compounds.
Thermo magnetic irreversibility just below Curie temperature (Tc),
non-saturation of magnetization, two distinct magnetic transitions in ac
susceptibility in an appropriate static field: close to Tc and other at
low temperature (the spin freezing temperature (Tg)) and non-stationary
dynamics with a characteristic maximum in the magnetic viscosity close to
Tg confirm a cluster glass state for
x \geqslant 0.05x \geqslant 0.05
. These results
find additional support from a linear low temperature magnetic specific heat
of x = 0.10 with a characteristic broad maximum close to Tg.
Physics of Condensed Matter 01/2007; 58(2):115-121. · 1.53 Impact Factor
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ABSTRACT: In this paper, we examine the possible influence of extrinsic factors on the electrical and magnetotransport of La(0.67)Ca(0.33)Mn(1-x)Ru(x)O(3) (x≤0.10). Ru substitution results in double metal-insulator transitions (MITs) at T(MI1) and T(MI2), both exhibiting magnetoresistance (MR). No additional magnetic signal corresponding to a second low-temperature maximum (LTM) at T(MI2) could be observed, either in ac susceptibility (χ(')) or in specific heat (C(p)). Typical grain sizes of ∼18 000-20 000 nm, as estimated from the scanning electron microscope (SEM) micrographs, are not so small as to warrant an LTM. The absence of additional peaks in the high statistics powder x-ray diffraction (XRD), a linear systematic increase of the unit cell parameters, close matching of the transition temperatures in resistivity, χ(') and C(p) and their linear systematic decrease with x, and an homogeneous distribution of Mn, Ru and O at arbitrarily selected regions within and across the grains exclude chemical inhomogeneity in the samples. The insensitivity of grain boundary MR at 5 K to Ru composition indicates that the grain boundary is not altered to result in an LTM. Oxygen stoichiometry of all the compounds is close to the nominal value of 3. These results not only exclude the extrinsic factors, but also establish that double MITs, both exhibiting MR, are intrinsic to Ru substituted La(0.67)Ca(0.33)MnO(3).
Journal of Physics Condensed Matter 05/2006; 18(17):4427-42. · 2.55 Impact Factor
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ABSTRACT: In this paper, we discuss the most deleterious effect of charge state modification at the Mn site on the ground state of the CMR manganites.
01/2006;
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ABSTRACT: We discuss the effects of local structure on the electrical transport and magnetic properties of La(0.67)Ca(0.33)Mn(1-x)Ti(x)O(3) system.Based on the intercomparison of the structure, transport and magnetic properties of the Mn site substituted La(0.67)Ca(0.33)MnO(3) with isovalent diamagnetic and paramagnetic ions, we argue that local structural effects have a decisive role to play, compared to the local spin coupling effects, in the ferromagnetic-metallic ground state of the CMR manganites
01/2006;
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ABSTRACT: In this paper, we examine the possible influence of extrinsic factors on the electrical and magnetotransport of La(0.67)Ca(0.33)Mn(1-x)Ru(x)O(3) (x < 0.10). These results not only exclude the extrinsic factors, but establishes the fact that the metal transitions both exhibiting MR is intrinsic to Ru substituted La(0.67)Ca(0.33)MnO(3) and the system. These results substantiate our hypothesis that Ru substituted system undergoes a magnetic phase separation involving the co-existence of two ferromagnetic-metallic phases in its ground state. Comment: 12 pages: Figures are included in the text. (To communicate to J. Appl. Phys)
12/2005;
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ABSTRACT: We report on the effect of Ni substitution at the Co site on the physical properties of NaxCoO2 system by investigating the series NaxCo1-yNiyO2 (x=0.75, 0<y<0.15). An upturn in the resistivity is observed in all Ni substituted samples as the temperature is lowered, suggestive of the occurrence of a Metal-Insulator Transition (MIT). The temperature at which this transition occurs increases with Ni content. The temperature dependence of the resistivity in the metallic region in the Ni substituted samples shows a T2 dependence, which is qualitatively different from that observed in the pristine sample. The evolution of the Fano asymmetry parameter, extracted by analyzing the lineshape of the IR active in-plane Co-O mode, both as a function of Ni concentration and temperature corroborates the occurrence of the MIT. It is argued that the progressive substitution of the Co4+ ions with Ni increases the probability of double occupancy and therefore the on-site Coulomb interaction energy leading to a shift in the thermodynamically driven MIT to higher temperatures. Comment: 17 pages 9 figures; submitted to Journal Of Phys. Cond. Mat
07/2005;
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S Dhara,
Sharat Chandra,
P Magudapathy,
S Kalavathi,
B K Panigrahi,
K G M Nair, V S Sastry,
C W Hsu,
C T Wu,
K H Chen,
L C Chen
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ABSTRACT: Photoluminescence study using the 325 nm He-Cd excitation is reported for the Au nanoclusters embedded in SiO(2) matrix. Au clusters are grown by ion beam mixing with 100 KeV Ar(+) irradiation on Au [40 nm]/SiO(2) at various fluences and subsequent annealing at high temperature. The blue bands above approximately 3 eV match closely with reported values for colloidal Au nanoclusters and supported Au nanoislands. Radiative recombination of sp electrons above Fermi level to occupied d-band holes are assigned for observed luminescence peaks. Peaks at 3.1 and 3.4 eV are correlated to energy gaps at the X- and L-symmetry points, respectively, with possible involvement of relaxation mechanism. The blueshift of peak positions at 3.4 eV with decreasing cluster size is reported to be due to the compressive strain in small clusters. A first principle calculation based on density functional theory using the full potential linear augmented plane wave plus local orbitals formalism with generalized gradient approximation for the exchange correlation energy is used to estimate the band gaps at the X- and L-symmetry points by calculating the band structures and joint density of states for different strain values in order to explain the blueshift of approximately 0.1 eV with decreasing cluster size around L-symmetry point.
The Journal of Chemical Physics 01/2005; 121(24):12595-9. · 3.33 Impact Factor
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ABSTRACT: For the first time,we report the effect of pentavalent subtitution at the Mn site of La(0.67)Ca(0.33)MnO(3). The ferromagnetic -metallic ground state modifies to a cluster-glass insulator beyond x=0.03. The observed suppression in the transition temperatures (39K/at.%) is the largest reported for the Mn site subtituted manganites. We argue that charge state(valence) of the substituent has a dominant role in affecting the ferromagnetic - metallic ground state of CMR manganites.
11/2004;
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ABSTRACT: Carbon has been substituted into the MgB2 lattice, forming the series Mg(B1−xCx)2, by heat treatments in sealed Ta tubes starting with elemental constituents. The superconducting transition temperature, TC, was measured by diamagnetic susceptibility. The superconducting transitions are sharp and the x-ray diffraction on the samples show only trace amounts of impurity phases. The C fraction that substitutes the B atoms into the MgB2 lattice, as determined from the now well established correlation between the 'a' lattice parameter and x in Mg(B1−xCx)2 is found to be very close to the targeted values. The temperature dependence of the normalized resistance is similar to that seen in C doped MgB2 single crystals along the ab plane. Rn/R(300 K), where Rn is the resistance just above TC, increases with x.
Superconductor Science and Technology 10/2004; 17(12):1401. · 2.66 Impact Factor
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ABSTRACT: Surface acoustic phonon modes in pelletized polycrystalline Pb(Mg1∕3Nb2∕3)O3 are investigated in the frequency range from 5 to 40 cm−1 at various temperatures from 25 to 550 °C. These spectra support the hypothesis of the core-shell nanoclusters with 1:1 ordered, negatively charged nanocore surrounded by the charge compensating positively charged disordered nanoshell. These surface acoustic modes exist even up to 550 °C revealing the existence of the charged core-shell nanoclusters which is well beyond the dissolution temperature of the polar clusters in the material.
Applied Physics Letters 08/2004; 85(5):798-800. · 3.84 Impact Factor
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ABSTRACT: Ion-beam mixing of the thermally immiscible Fe/Ag system is studied by 90 keV Ar <sup>+</sup> -ion irradiation of Fe/Ag multilayer at 325 K and the evolution of microstructure is studied by x-ray diffraction (XRD), x-ray reflectivity, and transmission electron microscopy (TEM). XRD measurements did not show any evidence of a additional phase formation or increase in solubility due to mixing of Fe in Ag. X-ray reflectivity analysis shows an initial sharpening at the interface for a fluence of 1×10<sup>16</sup> ions/cm <sup>2</sup> and the destruction of multilayer periodicity is observed for higher fluences. The microstructural studies by TEM show a reduction in Ag particle size with fluence. The size of Ag nanoclusters was optimum at a fluence of 7×10<sup>16</sup> ions/cm <sup>2</sup> and was found to be in the range of 10–20 nm. The particle density decreases at low fluence and on further irradiation, the particle density increases at higher ion fluences. This indicates the nucleation of Ag particles presumably due to long-range transport of Ag atoms. These Ag atom clusters give rise to a granular microstructure. © 2004 American Institute of Physics.
Journal of Applied Physics 06/2004; · 2.17 Impact Factor
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ABSTRACT: In order to explore the possibility of tuning superconducting and ferromagnetic transitions by Cu doping (for Ru) in GdSr2RuCu2O8, we have carried out synthesis and characterization of GdSr2Ru{1-x}Cu{2+x)O8 (x = 0, 0.05, 0.1, 0.2) and studied their physical properties. Coexistence of superconductivity and ferromagnetism is observed in all the Cu doped samples studied here. The zero field susceptibility data suggests formation of a spontaneous vortex phase. Cu doping decreases the ferromagnetic Curie temperature, whereas the superconducting transition temperature increases until an optimal concentration x~0.1. This reflects an increase in hole transfer to the CuO2 planes and reduction of ferromagnetic order within the ruthenate layers.
04/2003;
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ABSTRACT: Results of our study on the superconducting and normal state properties of the recently discovered superconductor MgCNi_3, the effect of Fe and Co substitution at the Ni site and the effect of pressure are reported. It is shown that a two band model provides a consistent interpretation of the temperature dependence of the normal state resistance and the Hall constant. Whereas band structure calculations suggest an increase in T_c upon partial substitution of Ni with Fe and Co, Co substitution quenches superconductivity and Fe substitution leads to an increase followed by a decrease in T_c. The observed variation of T_c may be explained in terms of a competition between increase in T_c due to increase in density of states and a decrease due to spin fluctuations. Based on these results, it is suggested that the spin fluctuations are weaker in Fe doped samples as compared to the Co doped ones. An initial decrease in T_c (and the normal state resistance) followed by an increase is observed on application of pressure. The decrease in T_c for small applied pressures can be understood in terms of the decrease in the density of states at the Fermi level. The subsequent increase in T_c with pressure is due to a lattice softening or a structural phase transition, consistent with the band structure calculations. It is conjectured that suppression of spin fluctuations by pressure may also be responsible for the observed increase in T_c at higher pressures.
03/2002;
