Physics Department

We report phonon renormalization in bilayer graphene as a function of doping. The Raman G peak stiffens and sharpens for both electron and hole doping as a result of the nonadiabatic Kohn anomaly at the ⌫ point. The bilayer has two conduction and valence subbands, with splitting dependent on the interlayer coupling. This gives a change of slope in the variation of G peak position with doping which allows a direct measurement of the interlayer coupling strength.

We show simultaneous p-and n-type carrier injection in a bilayer graphene channel by varying the longitudinal bias across the channel and the top-gate voltage. The top gate is applied electrochemically using solid polymer electrolyte and the gate capacitance is measured to be 1.5 μF cm −2 , a value about 125 times higher than the conventional SiO 2 back-gate capacitance. Unlike the single-layer graphene, the drain-source current does not saturate on varying the drain-source bias voltage. The energy gap opened between the valence and conduction bands using top-and back-gate geometry is estimated.

A strong electron-phonon interaction which limits the electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in situ Raman scattering from a single-layer MoS 2 electrochemically top-gated field-effect transistor (FET), we show softening and broadening of the A 1g phonon with electron doping, whereas the other Raman-active E 1 2g mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why the A 1g mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single-layer MoS 2 -based FETs, which have a high on-off ratio and are of technological significance.

Spin-phonon coupling originated from spin-lattice correlation depends upon different exchange interactions in transition metal oxides containing 3d magnetic ions. Spin-lattice coupling can influence the coupling mechanism in magnetoelectric material. To understand the spin-lattice correlation in inverse trirutile Fe 2 TeO 6 (FTO), magnetic properties and phonon spectra are studied. Signature of short-range magnetic correlation induced by 5/2-5/2 dimeric interaction and magnetic anomaly at 150 K is observed apart from the familiar sharp transition (T N~2 10K) corresponding to long-range order by magnetization and heat capacity measurements. The magnetic transitions and the spin dynamics are further locally probed by muon spin resonance (μSR) measurement in both zero fields (ZF) and longitudinal field (LF) mode. Three dynamically distinct temperature regimes; (i) T >T N , (ii) T N > T >150 K, and (iii) T<150 K, are observed. A change in the spin dynamics is realized at 150 K by μSR, though previous studies suggest long-range antiferromagnetic order. The renormalization of phonon frequencies observed in Raman spectra below 210 K suggests the existence of spin-phonon coupling in the material. The coupling strength is quantified as in the range 0.1-1.2 cm-1 following the mean-field and two-spin cluster approximations. We propose that the spin-phonon coupling mediated by the Fe

The electronic properties of graphene sheets decorated with nanodiamond (ND) particles have been investigated. The chemical fusion of ND to the graphene lattice creates pockets of local defects with robust interfacial bonding. At the ND-bonded regions, the atoms of graphene lattice follow sp 3-like bonding, and such regions play the role of conduction bottlenecks for the percolating sp 2 graphene network. The low-temperature charge transport reveals an insulating behavior for the disordered system associated with Anderson localization for the charge carriers in graphene. A large negative magnetoresistance is observed in this insulating regime, and its origin is discussed in the context of magnetic correlations of the localized charge carriers with local magnetic domains and extrinsic metal impurities associated with the ND.

High pressure Raman experiments on Boron Nitride multiwalled nanotubes show that the intensity of the vibrational mode at ~ 1367 cm-1 vanishes at ~ 12 GPa and it does not recover under decompression. In comparison, the high pressure Raman experiments on hexagonal Boron Nitride show a clear signature of a phase transition from hexagonal to wurtzite at ~ 13 GPa which is reversible on decompression. These results are contrasted with the pressure behavior of carbon nanotubes and graphite.

Theoretical studies have shown that electron-electron (e-e) and electron-hole (e-h) interactions play important roles in many observed quantum properties of graphene making this an ideal system to study many body effects. In this report we show that spectroscopic ellipsometry can enable us to measure this interactions quantitatively. We present spectroscopic data in two extreme systems of graphene on quartz (GOQ), an insulator, and graphene on copper (GOC), a metal which show that for GOQ, both e-e and e-h interactions dominate while for GOC e-h interactions are screened. The data further enables the estimation of the strength of the many body interaction through the effective fine structure constant, α * g. The α * g for GOQ indicates a strong correlation with an almost energy independent value of about 1.37. In contrast, α * g value of GOC is photon energy dependent, is almost two orders of magnitude lower at low energies indicating very weak correlation.

We have used Raman spectroscopy to study the behavior of multi-walled boron nitride nanotubes and hexagonal boron nitride crystals under high pressure. While boron nitride nanotubes show an irreversible transformation at about 12 GPa, hexagonal boron nitride exhibits a reversible phase transition at 13 GPa. We also present molecular dynamics simulations which suggest that the irreversibility of the pressure-induced transformation in boron nitride nanotubes is due to the polar nature of the bonds between boron and nitrogen.

We have used Raman spectroscopy to study the behavior of double-walled carbon nanotubes (DWNT) under hydrostatic pressure. We find that the rate of change of the tangential mode frequency with pressure is higher for the sample with traces of polymer compared to the pristine sample. We have performed classical molecular dynamics simulations to study the collapse of single (SWNT) and double-walled carbon nanotube bundles under hydrostatic pressure. The collapse pressure p c was found to vary as 1/R 3 , where R is the SWNT radius or the DWNT effective radius. The bundles showed ∼30% hysteresis and the hexagonally close packed lattice was completely restored on decompression. The p c of a DWNT bundle was found to be close to the sum of its values for the inner and the outer tubes considered separately as SWNT bundles, demonstrating that the inner tube supports the outer tube and that the effective bending stiffness of DWNT, D DWNT ∼ 2D SWNT .

Temperature-dependent Raman spectra of TbMnO 3 from 5 K to 300 K in the spectral range of 200 to 1525 cm-1 show five first-order Raman allowed modes and two high frequency modes. The intensity ratio of the high frequency Raman band to the corresponding first order Raman mode is nearly constant and high (~ 0.6) at all temperatures, suggesting a orbiton-phonon mixed nature of the high frequency mode. One of the first order phonon modes shows anomalous softening below T N (~ 46 K), suggesting a strong spin-phonon coupling.

We report temperature-dependent Raman spectra of CeFeAsO 0.9 F 0.1 from 4 to 300 K in the spectral range of 60-1800 cm −1 and interpret them using estimates of phonon frequencies obtained from first-principles density functional calculations. We find evidence for strong coupling between the phonons and crystal field excitations; in particular the Ce 3+ crystal field excitation at 432 cm −1 couples strongly with the E g oxygen vibration at 389 cm −1. Below the superconducting transition temperature, the phonon mode near 280 cm −1 shows softening, signaling its coupling with the superconducting gap. The ratio of the superconducting gap to T c , thus estimated to be ∼10, suggests CeFeAsO 0.9 F 0.1 to be a strong coupling superconductor. In addition, two high frequency modes observed at 1342 and 1600 cm −1 are attributed to electronic Raman scattering from the (x 2-y 2) to xz /yz d-orbitals of Fe.

Anomalous temperature dependence of Raman phonon wavenumbers attributed to phonon-phonon anharmonic interactions has been studied in two different families of pyrochlore titanates. We bring out the role of the ionic size of titanium and the inherent vacancies of pyrochlore in these anomalies by studying the effect of replacement of Ti 4 + by Zr 4 + in Sm 2 Ti 2 O 7 and by stuffing Ho 3 + in place of Ti 4 + in Ho 2 Ti 2 O 7 with appropriate oxygen stoichiometry. Our results show that an increase in the concentration of the larger ion, i.e. Zr 4 + or Ho 3 + , reduces the phonon anomalies, thus implying a decrease in the phonon-phonon anharmonic interactions. In addition, we find signatures of coupling between a phonon and crystal field transition in Sm 2 Ti 2 O 7 , manifested as an unusual increase in the phonon intensity with increasing temperature.

We revisit the assignment of Raman phonons of rare-earth titanates by performing Raman measurements on single crystals of O 18 isotope-rich spin ice Dy 2 Ti 2 O 18 7 and nonmagnetic Lu 2 Ti 2 O 18 7 pyrochlores and compare the results with their O 16 counterparts. We show that the low-wavenumber Raman modes below 250 cm À1 are not due to oxygen vibrations. A mode near 200 cm À1 , commonly assigned as F 2g phonon, which shows highly anomalous temperature dependence, is now assigned to a disorderinduced Raman active mode involving Ti 4+ vibrations. Moreover, we address here the origin of the 'new' Raman mode, observed below T C~1 10 K in Dy 2 Ti 2 O 7 , through a simultaneous pressure-dependent and temperature-dependent Raman study. Our study confirms the 'new' mode to be a phonon mode. We find that dT C /dP = + 5.9 K/GPa. Temperature dependence of other phonons has also been studied at various pressures up to~8 GPa. We find that pressure suppresses the anomalous temperature dependence. The role of the inherent vacant sites present in the pyrochlore structure in the anomalous temperature dependence is also discussed.

A simple procedure was developed for the fabrication of electrochemical glucose biosensors using glucose oxidase (GOx), with graphene or multi-walled carbon nanotubes (MWCNTs). Graphene and MWCNTs were dispersed in 0.25% 3-aminopropyltriethoxysilane (APTES) and drop cast on 1% KOH-pre-treated glassy carbon electrodes (GCEs). The EDC (1-ethyl-(3-dimethylaminopropyl) carbodiimide)-activated GOx was then bound covalently on the graphene-or MWCNT-modified GCE. Both the graphene-and MWCNT-based biosensors detected the entire pathophysiological range of blood glucose in humans, 1.4-27.9 mM. However, the direct electron transfer (DET) between GOx and the modified GCE's surface was only observed for the MWCNT-based biosensor. The MWCNT-based glucose biosensor also provided over a four-fold higher

We have carried out temperature and pressure-dependent Raman and x-ray measurements on single crystals of Tb 2 Ti 2 O 7. 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 Grüneisen 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 behaviour of Tb 2 Ti 2 O 7 .

We present here temperature-dependent Raman, x-ray diffraction and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound Dy 2 T i 2 O 7 and its non-magnetic analogue Lu 2 T i 2 O 7. Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in Dy 2 T i 2 O 7 , appearing below a temperature of T c =110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low temperature Raman experiments on O 18-isotope substituted Dy 2 T i 2 O 7 confirm the phonon origin of the "new" mode. These findings, absent in Lu 2 T i 2 O 7 , suggest that the room temperature cubic lattice of the pyrochlore Dy 2 T i 2 O 7 undergoes a "subtle" structural transformation near T c. We find anomalous red-shift of some of the phonon modes in both the Dy 2 T i 2 O 7 and the Lu 2 T i 2 O 7 as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.