Achintya Bera - Academia.edu (original) (raw)
Papers by Achintya Bera
Physical Review B
Isostructural transitions in layered MX 2 compounds are governed by competing van der Waals (vdW)... more Isostructural transitions in layered MX 2 compounds are governed by competing van der Waals (vdW) and Coulomb interactions. While an isostructural transition (at P ∼ 20 GPa) has been observed before metallization in MoS 2 when subjected to pressure, it is surprisingly missing in layered MoSe 2 and MoTe 2. Using synchrotron x-ray diffraction and Raman spectroscopic measurements of structural and vibrational properties of layered MoSSe crystals subjected to pressures up to 30 GPa and first-principles density functional theoretical analysis, we demonstrate a layer sliding isostructural transition from its 2H c structure (space group P6 3 mc) to a mixedphase of 2H a + 2H c at P ∼ 10.8 GPa, marked by discontinuity in lattice parameters, pressure coefficients of Raman modes, and accompanying changes in electronic structure. The origin of the unusually lower transition pressure of MoSSe compared with MoS 2 is shown to be linked to chemical ordering of S and Se atoms on the anionic sublattice, possible because of moderate lattice mismatch between the parent compounds MoS 2 and MoSe 2 and large interlayer space in the vdW-bonded structure. Notably, we also report a lower-pressure transition observed at P ∼ 3 GPa and not reported earlier in the isostructural Mo-based chalcogenides, marked by a discontinuity in the pressure coefficient of the c/a ratio and indirect band gap. The transition observed at P ∼ 10.8 GPa appears due to the change in the sign of the pressure coefficient of the direct band gap originating from inversion of the lowest-energy conduction bands. Our theoretical analysis shows that the phase transition at P ∼ 18 GPa marked by sharp changes in pressure coefficients of A 1 Raman modes is associated with the metallization of the 2H a phase.
Journal of Physics: Condensed Matter
Journal of Applied Physics
Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of s... more Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. However, as yet no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steadystate phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state parameters. To answer the latter question, we revisited pulsed excitation, and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We performed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed TR-MC and TR-PL measurements as function of excitation intensity on the same samples of different MAPbI 3 thin films, and find good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of phototransport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity.
Physical chemistry chemical physics : PCCP, Jan 7, 2018
In-gap states in solar cell absorbers that are recombination centers determine the cell's pho... more In-gap states in solar cell absorbers that are recombination centers determine the cell's photovoltaic performance. Using scanning tunneling spectroscopy (STS), temperature-dependent photoconductivity and steady-state photocarrier-grating measurements we probed, directly and indirectly, the energies of such states, both at the surface and in the bulk of two similar, but different halide perovskites, the single cation MAPbI (here MAPI) and the mixed cation halide perovskite, FAMACsPb(IBr) (here MCHP). We found a correlation between the energy distribution of the in-gap states, as determined by STS measurements, and their manifestation in the photo-transport parameters of the MCHP absorbers. In particular, our results suggest that the in-gap recombination centers in the MCHP are shallower than those of MAPI. This can be one explanation for the better photovoltaic efficiency of the former.
Advanced Materials
TOC text and figure Halide perovskites are fascinating crystalline materials for optoelectronic d... more TOC text and figure Halide perovskites are fascinating crystalline materials for optoelectronic devices and can yield efficient solar cells. We provide a critical review of their structural and optoelectronic characteristics and find that they exhibit a unique combination of properties. Based on this notion, the Essay addresses open questions about these materials.
Journal of Raman Spectroscopy
Journal of Physics: Condensed Matter
In recent years, a low pressure transition around P3 GPa exhibited by the A2B3-type 3D topologica... more 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.
Journal of physics. Condensed matter : an Institute of Physics journal, Jan 15, 2017
High pressure Raman spectroscopy of bulk 2H-MoTe2 up to ∼29 GPa is shown to reveal two phase tra... more 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.
Lecture Notes in Nanoscale Science and Technology, 2013
We review vibrational and electronic properties of single and a few layer MoS 2 relevant to under... more We review vibrational and electronic properties of single and a few layer MoS 2 relevant to understand their resonant and non-resonant Raman scattering results. In particular, the optical modes and low frequency shear and layer breathing modes show significant dependence on the number of MoS 2 layers. Further, the electron doping of the MoS 2 single layer achieved using top-gating in a field effect transistor renormalizes the two optical modes A 1g and E 1 2g differently due to symmetry-dependent electron-phonon coupling. The issues related to carrier mobility, the Schottky barrier at the MoS 2-metal contact pads and the modifications of the dielectric environment are addressed. The direct optical transitions for single layer-MoS 2 involve two excitons at K-point in the Brillouin zone and their stability with temperature and pressure has been reviewed. Finally, the Fermi-level dependence of spectral shift for a quasiparticle, called trion, has been discussed.
Solid State Communications, 2012
We report Raman signatures of electronic topological transition (ETT) at 3.6 GPa and rhombohedral... more We report Raman signatures of electronic topological transition (ETT) at 3.6 GPa and rhombohedral (α-Bi 2 Te 3) to monoclinic (β-Bi 2 Te 3) structural transition at ~ 8 GPa. At the onset of ETT, a new Raman mode appears near 107 cm-1 which is dispersionless with pressure. The structural transition at ~ 8 GPa is marked by a change in pressure derivative of A 1g and E g mode frequencies as well as by appearance of new modes near 115 cm-1 and 135 cm-1. The mode Grüneisen parameters are determined in both the α and β-phases.
Physical Review Letters, 2013
The non-trivial electronic topology of a topological insulator is so far known to display signatu... more The non-trivial electronic topology of a topological insulator is so 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 2 g 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 non-adiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator.
Physical Review B, 2012
Strong electron-phonon interaction which limits electronic mobility of semiconductors can also ha... more Strong electron-phonon interaction which limits 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 single layer MoS2 electrochemically top-gated field effect transistor (FET), we show softening and broadening of A1g 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 A1g 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 enormous technological significance.
Journal of Physics: Condensed Matter, 2011
Inelastic light scattering studies on single crystal of electron-doped Ca(Fe 0.95 Co 0.05) 2 As 2... more Inelastic light scattering studies on single crystal of electron-doped Ca(Fe 0.95 Co 0.05) 2 As 2 superconductor, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at T SM ~ 140 K and superconducting transition temperature T c ~ 23 K, reveal evidence for superconductivity-induced phonon renormalization; in particular the phonon mode near 260 cm-1 shows hardening below T c , signaling its coupling with the superconducting gap. All the three Raman active phonon modes show anomalous temperature dependence between room temperature and T c i.e phonon frequency decreases with lowering temperature. Further, frequency of one of the modes shows a sudden change in temperature dependence at T SM. Using first-principles density functional theory-based calculations, we show that the low temperature phase (T c < T < T SM) exhibits short-ranged stripe anti-ferromagnetic ordering, and estimate the spinphonon couplings that are responsible for these phonon anomalies.
Journal of Photochemistry and Photobiology A: Chemistry, 2013
ABSTRACT This paper deals with a study of the photophysical property of poly(ether imine) (PETIM)... more ABSTRACT This paper deals with a study of the photophysical property of poly(ether imine) (PETIM) dendritic macromolecule in the presence of aromatic compounds. The inherent photoluminescence property of the dendrimer undergoes quenching in the presence of guest aromatic nitro-compounds. From life-time measurements study, it is inferred that the lifetimes of luminescent species of the dendrimer are not affected with nitrophenols as guest molecules, whereas nitrobenzenes show a marginal change in the lifetimes of the species. Raman spectral characteristic of the macromolecular host–guest complex is conducted in order to identify conformational change of the dendrimer and a significant change in the stretching frequencies of methylene moieties of the dendrimer is observed for the complex with 1,3,5-trinitrobenzene, when compared to other complexes, free host and guest molecules. The photophysical behavior of electron-rich, aliphatic, neutral dendritic macromolecule in the presence of electron-deficient aromatic molecules is illustrated in the present study.
Applied Physics Letters, 2012
Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 ... more Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature T c ~ 28.3 K, reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at T o ~ 60 K. Below T o, anomalous softening of the mode frequency and a large decrease by ~ 10 cm-1 in the linewidth is observed. These precursor effects at T 0 (~ 2T c) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K (~ 7 %) indicates strong spin-phonon coupling in this superconductor.
Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 ... more Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature T c ~ 28.3 K, reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at T o ~ 60 K. Below T o, anomalous softening of the mode frequency and a large decrease by ~ 10 cm-1 in the linewidth is observed. These precursor effects at T 0 (~ 2T c) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K (~ 7 %) indicates strong spin-phonon coupling in this superconductor.
Physical Review B
Isostructural transitions in layered MX 2 compounds are governed by competing van der Waals (vdW)... more Isostructural transitions in layered MX 2 compounds are governed by competing van der Waals (vdW) and Coulomb interactions. While an isostructural transition (at P ∼ 20 GPa) has been observed before metallization in MoS 2 when subjected to pressure, it is surprisingly missing in layered MoSe 2 and MoTe 2. Using synchrotron x-ray diffraction and Raman spectroscopic measurements of structural and vibrational properties of layered MoSSe crystals subjected to pressures up to 30 GPa and first-principles density functional theoretical analysis, we demonstrate a layer sliding isostructural transition from its 2H c structure (space group P6 3 mc) to a mixedphase of 2H a + 2H c at P ∼ 10.8 GPa, marked by discontinuity in lattice parameters, pressure coefficients of Raman modes, and accompanying changes in electronic structure. The origin of the unusually lower transition pressure of MoSSe compared with MoS 2 is shown to be linked to chemical ordering of S and Se atoms on the anionic sublattice, possible because of moderate lattice mismatch between the parent compounds MoS 2 and MoSe 2 and large interlayer space in the vdW-bonded structure. Notably, we also report a lower-pressure transition observed at P ∼ 3 GPa and not reported earlier in the isostructural Mo-based chalcogenides, marked by a discontinuity in the pressure coefficient of the c/a ratio and indirect band gap. The transition observed at P ∼ 10.8 GPa appears due to the change in the sign of the pressure coefficient of the direct band gap originating from inversion of the lowest-energy conduction bands. Our theoretical analysis shows that the phase transition at P ∼ 18 GPa marked by sharp changes in pressure coefficients of A 1 Raman modes is associated with the metallization of the 2H a phase.
Journal of Physics: Condensed Matter
Journal of Applied Physics
Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of s... more Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. However, as yet no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steadystate phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state parameters. To answer the latter question, we revisited pulsed excitation, and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We performed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed TR-MC and TR-PL measurements as function of excitation intensity on the same samples of different MAPbI 3 thin films, and find good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of phototransport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity.
Physical chemistry chemical physics : PCCP, Jan 7, 2018
In-gap states in solar cell absorbers that are recombination centers determine the cell's pho... more In-gap states in solar cell absorbers that are recombination centers determine the cell's photovoltaic performance. Using scanning tunneling spectroscopy (STS), temperature-dependent photoconductivity and steady-state photocarrier-grating measurements we probed, directly and indirectly, the energies of such states, both at the surface and in the bulk of two similar, but different halide perovskites, the single cation MAPbI (here MAPI) and the mixed cation halide perovskite, FAMACsPb(IBr) (here MCHP). We found a correlation between the energy distribution of the in-gap states, as determined by STS measurements, and their manifestation in the photo-transport parameters of the MCHP absorbers. In particular, our results suggest that the in-gap recombination centers in the MCHP are shallower than those of MAPI. This can be one explanation for the better photovoltaic efficiency of the former.
Advanced Materials
TOC text and figure Halide perovskites are fascinating crystalline materials for optoelectronic d... more TOC text and figure Halide perovskites are fascinating crystalline materials for optoelectronic devices and can yield efficient solar cells. We provide a critical review of their structural and optoelectronic characteristics and find that they exhibit a unique combination of properties. Based on this notion, the Essay addresses open questions about these materials.
Journal of Raman Spectroscopy
Journal of Physics: Condensed Matter
In recent years, a low pressure transition around P3 GPa exhibited by the A2B3-type 3D topologica... more 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.
Journal of physics. Condensed matter : an Institute of Physics journal, Jan 15, 2017
High pressure Raman spectroscopy of bulk 2H-MoTe2 up to ∼29 GPa is shown to reveal two phase tra... more 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.
Lecture Notes in Nanoscale Science and Technology, 2013
We review vibrational and electronic properties of single and a few layer MoS 2 relevant to under... more We review vibrational and electronic properties of single and a few layer MoS 2 relevant to understand their resonant and non-resonant Raman scattering results. In particular, the optical modes and low frequency shear and layer breathing modes show significant dependence on the number of MoS 2 layers. Further, the electron doping of the MoS 2 single layer achieved using top-gating in a field effect transistor renormalizes the two optical modes A 1g and E 1 2g differently due to symmetry-dependent electron-phonon coupling. The issues related to carrier mobility, the Schottky barrier at the MoS 2-metal contact pads and the modifications of the dielectric environment are addressed. The direct optical transitions for single layer-MoS 2 involve two excitons at K-point in the Brillouin zone and their stability with temperature and pressure has been reviewed. Finally, the Fermi-level dependence of spectral shift for a quasiparticle, called trion, has been discussed.
Solid State Communications, 2012
We report Raman signatures of electronic topological transition (ETT) at 3.6 GPa and rhombohedral... more We report Raman signatures of electronic topological transition (ETT) at 3.6 GPa and rhombohedral (α-Bi 2 Te 3) to monoclinic (β-Bi 2 Te 3) structural transition at ~ 8 GPa. At the onset of ETT, a new Raman mode appears near 107 cm-1 which is dispersionless with pressure. The structural transition at ~ 8 GPa is marked by a change in pressure derivative of A 1g and E g mode frequencies as well as by appearance of new modes near 115 cm-1 and 135 cm-1. The mode Grüneisen parameters are determined in both the α and β-phases.
Physical Review Letters, 2013
The non-trivial electronic topology of a topological insulator is so far known to display signatu... more The non-trivial electronic topology of a topological insulator is so 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 2 g 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 non-adiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator.
Physical Review B, 2012
Strong electron-phonon interaction which limits electronic mobility of semiconductors can also ha... more Strong electron-phonon interaction which limits 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 single layer MoS2 electrochemically top-gated field effect transistor (FET), we show softening and broadening of A1g 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 A1g 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 enormous technological significance.
Journal of Physics: Condensed Matter, 2011
Inelastic light scattering studies on single crystal of electron-doped Ca(Fe 0.95 Co 0.05) 2 As 2... more Inelastic light scattering studies on single crystal of electron-doped Ca(Fe 0.95 Co 0.05) 2 As 2 superconductor, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at T SM ~ 140 K and superconducting transition temperature T c ~ 23 K, reveal evidence for superconductivity-induced phonon renormalization; in particular the phonon mode near 260 cm-1 shows hardening below T c , signaling its coupling with the superconducting gap. All the three Raman active phonon modes show anomalous temperature dependence between room temperature and T c i.e phonon frequency decreases with lowering temperature. Further, frequency of one of the modes shows a sudden change in temperature dependence at T SM. Using first-principles density functional theory-based calculations, we show that the low temperature phase (T c < T < T SM) exhibits short-ranged stripe anti-ferromagnetic ordering, and estimate the spinphonon couplings that are responsible for these phonon anomalies.
Journal of Photochemistry and Photobiology A: Chemistry, 2013
ABSTRACT This paper deals with a study of the photophysical property of poly(ether imine) (PETIM)... more ABSTRACT This paper deals with a study of the photophysical property of poly(ether imine) (PETIM) dendritic macromolecule in the presence of aromatic compounds. The inherent photoluminescence property of the dendrimer undergoes quenching in the presence of guest aromatic nitro-compounds. From life-time measurements study, it is inferred that the lifetimes of luminescent species of the dendrimer are not affected with nitrophenols as guest molecules, whereas nitrobenzenes show a marginal change in the lifetimes of the species. Raman spectral characteristic of the macromolecular host–guest complex is conducted in order to identify conformational change of the dendrimer and a significant change in the stretching frequencies of methylene moieties of the dendrimer is observed for the complex with 1,3,5-trinitrobenzene, when compared to other complexes, free host and guest molecules. The photophysical behavior of electron-rich, aliphatic, neutral dendritic macromolecule in the presence of electron-deficient aromatic molecules is illustrated in the present study.
Applied Physics Letters, 2012
Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 ... more Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature T c ~ 28.3 K, reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at T o ~ 60 K. Below T o, anomalous softening of the mode frequency and a large decrease by ~ 10 cm-1 in the linewidth is observed. These precursor effects at T 0 (~ 2T c) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K (~ 7 %) indicates strong spin-phonon coupling in this superconductor.
Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 ... more Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature T c ~ 28.3 K, reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at T o ~ 60 K. Below T o, anomalous softening of the mode frequency and a large decrease by ~ 10 cm-1 in the linewidth is observed. These precursor effects at T 0 (~ 2T c) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K (~ 7 %) indicates strong spin-phonon coupling in this superconductor.