Achintya Bera

Symmetry-dependent phonon renormalization in monolayer MoS2 transistor

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 MoS2 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-2g(1) 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 MoS2-based FETs, which have a high on-off ratio and are of technological significance.

Sharp Raman anomalies and broken adiabaticity at a pressure induced transition from band to topological insulator in Sb2Se3.

The nontrivial electronic topology of a topological insulator is thus far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E(g)(2) phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT) in Sb(2)Se(3) crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how nonadiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator.

Raman evidence for the superconducting gap and spin–phonon coupling in the superconductor Ca(Fe0.95Co0.05)2As2

Inelastic light scattering studies on a single crystal of electron-doped Ca(Fe0.95Co0.05)2As2 superconductor, covering the tetragonal-to-orthorhombic structural transition as well as the magnetic transition at TSM ~ 140 K and the superconducting transition temperature Tc ~ 23 K, reveal evidence for superconductivity-induced phonon renormalization. In particular, the phonon mode near 260 cm − 1 shows hardening below Tc, signaling its coupling with the superconducting gap. All three Raman active phonon modes show anomalous temperature dependence between room temperature and Tc, i.e. the phonon frequency decreases with lowering temperature. Further, the frequency of one of the modes shows a sudden change in temperature dependence at TSM. Using first-principles density functional theory based calculations, we show that the low temperature phase (Tc < T < TSM) exhibits short-ranged stripe antiferromagnetic ordering, and estimate the spin–phonon couplings that are responsible for these phonon anomalies.

Insights into Vibrational and Electronic Properties of MoS2 Using Raman, Photoluminescence, and Transport Studies

We review the relevant vibrational and electronic properties of a single and a few layer MoS2 to understand their resonant and nonresonant Raman scattering results. In particular, the optical modes and low frequency shear and layer breathing modes show significant dependence on the number of MoS2 layers. Further, the electron doping of the MoS2 single layer achieved using top-gating in a field effect transistor renormalizes the two optical modes A 1g and \( E_{2g}^{1} \) differently due to symmetry-dependent electron–phonon coupling. The issues related to carrier mobility, the Schottky barrier at the MoS2–metal contact pads and the modifications of the dielectric environment are addressed. The direct optical transitions for single-layer MoS2 involve two excitons at K-point in the Brillouin zone and their stability with temperature and pressure is reviewed. Finally, the Fermi level dependence of spectral shift for a quasiparticle, called trion, is discussed.

Enhanced Raman and photoluminescence response in monolayer MoS2 due to laser healing of defects

Bound quasiparticles, negatively charged trions and neutral excitons are associated with the direct optical transitions at the K-points of the Brillouin zone for monolayer MoS2. The change in the carrier concentration, surrounding dielectric constant, and defect concentration can modulate the photoluminescence and Raman spectra. Here, we show that exposing the monolayer MoS2 in air to a modest laser intensity for a brief period of time enhances simultaneously the photoluminescence intensity associated with both trions and excitons, together with similar to 3 to 5 times increase of the Raman intensity of first-order and second-order modes. The simultaneous increase of photoluminescence from trions and excitons cannot be understood based only on known scenario of depletion of electron concentration in MoS2 by adsorption of O-2 and H2O molecules. This is explained by laser-induced healing of defect states resulting in reduction of nonradiative Auger processes. This laser healing is corroborated by an observed increase of intensity of both the first-order and second-order longitudinal acoustic Raman modes at the M-point of Brillouin zone by a factor of similar to 3 to 5. The A(1g) mode hardens by similar to 1.4 cm(-1), whereas the E-2g(1) mode softens by similar to 1 cm(-1). The second-order longitudinal acoustic Raman mode at the M-point of Brillouin zone at similar to 440 cm(-1) shows an increase in wavenumber by similar to 8 cm(-1) with laser exposure. These changes are a combined effect of change in electron concentrations and oxygen-induced lattice displacements. Copyright (c) 2017 John Wiley & Sons, Ltd.

Pressure-induced phase transition in Bi2Se3 at 3 GPa: electronic topological transition or not?

In recent years, a low pressure transition around P3 GPa exhibited by the A2B3-type 3D topological insulators is attributed to an electronic topological transition (ETT) for which there is no direct evidence either from theory or experiments. We address this phase transition and other transitions at higher pressure in bismuth selenide (Bi2Se3) using Raman spectroscopy at pressure up to 26.2 GPa. We see clear Raman signatures of an isostructural phase transition at P2.4 GPa followed by structural transitions at ∼ 10 GPa and 16 GPa. First-principles calculations reveal anomalously sharp changes in the structural parameters like the internal angle of the rhombohedral unit cell with a minimum in the c/a ratio near P3 GPa. While our calculations reveal the associated anomalies in vibrational frequencies and electronic bandgap, the calculated Z2 invariant and Dirac conical surface electronic structure remain unchanged, showing that there is no change in the electronic topology at the lowest pressure transition.

Pressure-dependent semiconductor to semimetal and Lifshitz transitions in 2H-MoTe2: Raman and first-principles studies

High pressure Raman spectroscopy of bulk 2H-MoTe2 up to  ∼29 GPa is shown to reveal two phase transitions (at  ∼6 and 16.5 GPa), which are analyzed using first-principles density functional theoretical calculations. The transition at 6 GPa is marked by changes in the pressure coefficients of A 1g and [Formula: see text] Raman mode frequencies as well as in their relative intensity. Our calculations show that this is an isostructural semiconductor to a semimetal transition. The transition at  ∼16.5 GPa is identified with the changes in linewidths of the Raman modes as well as in the pressure coefficients of their frequencies. Our theoretical analysis clearly shows that the structure remains the same up to 30 GPa. However, the topology of the Fermi-surface evolves as a function of pressure, and abrupt appearance of electron and hole pockets at [Formula: see text] GPa marks a Lifshitz transition.

Superconducting fluctuations and anomalous phonon renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of 5K to 300 K, covering the superconducting transition temperature T-c = 28.3 K, reveal that the Raman mode at similar to 230 cm(-1) shows a sharp jump in frequency by similar to 2% and linewidth increases by similar to 175% at T-o similar to 60 K. Below T-o, anomalous softening of the mode frequency and a large decrease by similar to 10 cm(-1) in the linewidth are observed. These precursor effects at T-0 (similar to 2T(c)) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weak coupling between the well separated (similar to 15 angstrom) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K and 60 K (similar to 7%) indicates strong spin-phonon coupling in this superconductor