S. Karmakar

Magnetic behavior of iron-filled multiwalled carbon nanotubes

Using vibrating-sample magnetometry, magnetic properties of iron-filled multiwalled carbon nanotubes have been investigated. The field dependence of dc magnetization at high magnetic fields suggests that these tubes behave as a one-dimensional exchange-coupled ferromagnetic system. At 5K, the saturation magnetization (MS) of the nanowires is found to be 85emu∕g, which is much less than the expected bulk value ∼210emu∕g. The observed exchange bias, in spite of the small fraction of γ‐Fe in our samples, implies that γ‐Fe may not be the only antiferromagnetic component responsible for the exchange bias in these Fe-filled carbon nanotubes. Quantitative study on the temperature dependence of saturation magnetization, remanent magnetization and coercivity has been carried out.

Pressure Effects on Single Wall Carbon Nanotube Bundles

We report high pressure Raman studies on single wall carbon nanotube bundles under hydrostatic conditions using two different pressure transmitting media, alcohol mixture and pure water. The radial and tangential modes show a blue shift when SWNT bundle is immersed in the liquids at ambient pressures. The pressure dependence of the radial modes is the same in both liquids. However, the pressure derivatives dw/dP of the tangential modes are slightly higher for the water medium. Raman results are compared with studies under non-hydrostatic conditions and with recent high-pressure X-ray studies. It is seen that the mode frequencies of the recovered sample after pressure cycling from 26 GPa are downshifted by

Structural changes in single-walled carbon nanotubes under non-hydrostatic pressures: x-ray and Raman studies

~7-10 cm^{-1}

Low-temperature and high-pressure Raman and X-ray studies of pyrochlore Tb 2 Ti 2 O 7 : phonon anomalies and possible phase transition

as compared to the starting sample.

Pressure induced structural, electronic topological, and semiconductor to metal transition in AgBiSe2

Using in situ x-ray diffraction and Raman scattering techniques, we have investigated the behaviour of single-walled carbon nanotubes bundles under non-hydrostatic pressures. It is seen that the diffraction line corresponding to the two-dimensional triangular lattice in the bundles is not reversible for pressures beyond 5 GPa, in sharp contrast to earlier results under hydrostatic pressure conditions. Most interestingly, radial breathing and tangential Raman modes of the pressure-cycled samples from 21 and 30 GPa match very well with those of the starting sample. Raman and x-ray results put together clearly suggest that the ordering of tubes in the bundles is only marginally regained with a very short coherence length on decompression.

High pressure structural and vibrational properties of the spin-gap system Cu2PO4(OH)

We have carried out temperature- and pressure-dependent Raman and x-ray measurements on single crystals of Tb2Ti2O7. We attribute the observed anomalous temperature dependence of phonons to phonon-phonon anharmonic interactions. The quasiharmonic and anharmonic contributions to the temperature-dependent changes in phonon frequencies are estimated quantitatively using mode Gruneisen parameters derived from pressure-dependent Raman experiments and bulk modulus from high-pressure x-ray measurements. Further, our Raman and x-ray data suggest a subtle structural deformation of the pyrochlore lattice at ~9 GPa. We discuss possible implications of our results on the spin-liquid behavior of Tb2Ti2O7.

Structural and optical investigations of Fe1.03Se0.5Te0.5 under high pressure

We report the effect of strong spin orbit coupling inducing electronic topological and semiconductor to metal transitions on the thermoelectric material AgBiSe2 at high pressures. The synchrotron X-ray diffraction and the Raman scattering measurement provide evidence for a pressure induced structural transition from hexagonal (α-AgBiSe2) to rhombohedral (β-AgBiSe2) at a relatively very low pressure of around 0.7 GPa. The sudden drop in the electrical resistivity and clear anomalous changes in the Raman line width of the A1g and Eg(1) modes around 2.8 GPa was observed suggesting a pressure induced electronic topological transition. On further increasing the pressure, anomalous pressure dependence of phonon (A1g and Eg(1)) frequencies and line widths along with the observed temperature dependent electrical resistivity show a pressure induced semiconductor to metal transition above 7.0 GPa in β-AgBiSe2. First principles theoretical calculations reveal that the metallic character of β-AgBiSe2 is induced mainl...

High pressure structural investigations of copper metaborate (CuB2O4)

The structural and vibrational properties of the spin-gapped system Cu(2)PO(4)(OH) have been investigated at room temperature under high pressure up to ~20 GPa by Raman scattering and synchrotron-based x-ray diffraction and infrared (IR) spectroscopic measurements. The orthorhombic phase (space group Pnnm, z = 4) remains stable up to at least 7 GPa where it undergoes a weakly first order structural transition (with negligible volume drop) to a monoclinic phase (space group P2(1)/n, z = 4) with an abrupt monoclinic distortion. Refinement of atomic positions has been performed for the low pressure phase. The conspicuous changes in the vibrational spectra (Raman as well as far-IR) confirm this phase transition. At further higher pressures the monoclinic angle increases rapidly and the system transforms irreversibly into a disordered phase. Detailed vibrational analyses have been performed in the orthorhombic phase and pressure-induced structural evolution has been correlated with the vibrational modes corresponding to the Cu-O bonds. A strong negative pressure dependence of hydroxyl mode frequencies (as observed from the mid-IR absorption spectra) supports the pressure-induced structural disordering at higher pressures.

High pressure structural and superconducting properties of Ca1-xEuxFeAs2

Optimally doped iron-chalcogenide superconductor Fe1.03Se0.5Te0.5 has been investigated under high pressures using synchrotron-based x-ray diffraction and mid-infrared reflectance measurements at room temperature. The superconducting transition temperature (Tc) of the same sample has been determined by temperature-dependent resistance measurements up to 10 GPa. The tetragonal phase (P4/nmm) is found to exist in phase-separated states where both the phases have remarkably high compressibility. A first-order structural transition to the orthorhombic phase (Pbnm) is reported above 10 GPa. For the tetragonal phase, a strong correlation is observed between the Fe(Se,Te)4 tetrahedral deformation and the sharp rise of Tc up to ∼ 4 GPa, above which Tc shows marginal pressure dependence at least up to 10 GPa. The evolution with pressure of the optical conductivity shows that with increasing pressure the tetragonal phase approaches towards a conventional metallic state. Above ∼ 6 GPa, the Drude term reduces drastically, indicating poor metallic character of the high-pressure orthorhombic phase.

High pressure structural and vibrational properties of Cu3VS4

High pressure in situ x-ray diffraction and Raman measurements have been performed on tetragonal CuB2O4 up to a pressure of 25 GPa. The ambient tetragonal structure remains stable up to the highest pressure of this study. Pressure induced mode softening of some of the lattice modes indicate a possible structural transition at higher pressures.