B1g Symmetry Research Articles

Electron-hole asymmetry in the electron-phonon coupling in top-gated phosphorene transistor

Using in situ Raman scattering from phosphorene channel in an electrochemically top-gated field effect transistor, we show that phonons with Ag symmetry depend much more strongly on concentration of electrons than that of holes, wheras phonons with Bg symmetry are insensitive to doping. With first-principles theoretical analysis, we show that the observed electon–hole asymmetry arises from the radically different constitution of its conduction and valence bands involving π and σ bonding states respectively, whose symmetry permits coupling with only the phonons that preserve the lattice symmetry. Thus, Raman spectroscopy is a non-invasive tool for measuring electron concentration in phosphorene-based nanoelectronic devices.

Two-Magnon Scattering in Spin–Orbital Mott Insulator Ba2IrO4

A spin–orbit induced Mott insulator Ba2IrO4 with the pseudo-spin Jeff = 1/2, showing an antiferromagnetic order (TN = 240 K), has been investigated by Raman spectroscopy. A broad peak with the B1g symmetry is found in a wide temperature region up to 400 K, which is ascribed to the two-magnon scattering. From the peak position and width, the exchange coupling and the antiferromagnetic correlation length are estimated to be 590 cm−1 and 45 A at 90 K, respectively. The results are compared with the antiferromagnet La2CuO4 with the spin S = 1/2. We conclude that there is no significant difference in the short wavelength spin-excitation between the S = 1/2 and Jeff = 1/2 systems.

Theoretical Study on Resonance Raman Spectra of Tetraoxaporphyrin Dication by TDDFT Calculation

The B state excited resonance Raman scattering of tetraoxaporphyrin dication (TOP2+) was theoretically studied with DFT/TDDFT calculations and the sum-over-states approach of polarizability including both the A and B terms contributions. The resonance Raman spectra calculated with PBE1PBE, B3LYP, Cam-B3LYP, and B3LYP-D3 functionals are similar to each other in general, with PBE1PBE and B3LYP being better in reproducing resonance Raman intensities in comparison with the experiment. The calculated relative intensities of the totally symmetric modes are excellently consistent with the experiment. The TDDFT calculations manifested a considerable deformation of the B state along the ν2, ν6, ν7, and ν8 modes, which is responsible for the strong resonance Raman intensities of these modes. The resonance Raman intensities of non-totally symmetric modes were calculated to be weaker than the totally symmetric modes by one or two order of magnitude, which qualitatively agrees with the experiment. However, the resonance Raman intensity of the ν10 mode (CβCβ stretch, B1g symmetry) predicted by TDDFT calculations is unexpectedly small whereas that of the ν11 mode (symmetric CαCm stretch, B1g symmetry) is too large, which is assumed to be caused by the Jahn-Teller instability for the B state of TOP2+.

Symmetry-dependent carrier relaxation dynamics and charge–density–wave transition in DyTe3 probed by polarized femtosecond spectroscopy

Photo-induced quasi-particle (QP) relaxation dynamics with different symmetries have been investigated for the multiple charge–density–wave (CDW) compound DyTe3 by using ultrafast polarized pump-probe spectroscopy. By performing symmetry analysis, the QP dynamics with isotropic A1g and anisotropic B2g symmetry were found to show unique anomalies at the first and second CDW transitions. Both the temperature dependence and pump fluence dependence indicate that the B2g response is very sensitive to the underlying lattice deformation, which provides critical insight into the multiple CDW formations.

Raman scattering as a probe of nematic correlations

We use the symmetry constrained low energy effective Hamiltonian of iron based superconductors to study the Raman scattering in the normal state of underdoped iron-based superconductors. The incoming and scattered Raman photons couple directly to orbital fluctuations and indirectly to the spin fluctuations. We computed both couplings within the same low energy model. The symmetry constrained Hamiltonian yields the coupling between the orbital and spin fluctuations of only the same symmetry type. Attraction in B2g symmetry channel was assumed for the system to develop the subleading instability towards the discrete in-plane rotational symmetry breaking, referred to as Ising nematic transition. We find that upon approaching this instability, the Raman spectral function develops a quasi-elastic peak as a function of energy transferred by photons to the crystal. We attribute this low-energy B2g scattering to the critical slow-down associated with the build up of nematic correlations.

Electronic and Chemical Properties of Germanene: The Crucial Role of Buckling

The heavier analogues of graphene, namely silicene and germanene, are known to be buckled. Such buckling leads to interesting properties like direct band gap in hydrogenated germanene, known as germanane. This article shows that the sequential replacement of C by Ge in benzene leads to increasing buckling with the maximal buckling distance (d = 0.61 Å) in Ge6H6. The origin of such buckling induced lowering of symmetry (D6h → D3d) is traced to pseudo Jahn–Teller (PJT) distortion along the b2g normal mode arising out of mixing of the nondegenerate (A1g) ground state with low lying (Δ0 = 4.36 eV) excited state of B2g symmetry. Buckling also leads to enhanced chemical reactivity of germanene toward hydrogen to form germanane. The large affinity of germanene toward hydrogenation explains the experimental synthesis of exfoliated layers of germanane by Goldberger and co-workers [ACS Nano 2013, 7, 4414−4421]. Germanene → germanane formation leads to the opening up of a large band gap making hydrogenation a chemical route to control the electronic properties in these new 2D materials. The presence of buckling in germanene leads to higher hole reorganization energies than polyaromatic hydrocarbons (PAH) of the same nuclearity.

Multipole interactions in a LiTmF4 single crystal

We have considered magnetic and magnetoelastic characteristics of a van Vleck paramagnet LiTmF4 taking into account the interaction between thulium ions via the phonon field. We have calculated parameters of the multipole interaction that is caused by the interaction of Tm3+ ions with dynamic lattice deformations of the Bg symmetry. We have presented a self-consistent description of previously published results of measurements of temperature dependences of elastic constants and the nonlinear Zeeman effect in the optical spectrum of the LiTmF4 single crystal, as well as dependences of the magnetostriction on temperature, magnitude, and direction of the external magnetic field.

Assignments of the Raman modes of monoclinic erbium oxide

As a heavy rare earth oxide, erbium oxide (Er2O3) has many attractive properties. Monoclinic Er2O3 has useful properties not found in stable cubic Er2O3, such as unique optical properties and high radiation damage tolerance. In this study, cubic Er2O3 coating and Er2O3 coating with mixed phases were prepared. The Raman scattering spectra of these coatings were investigated by using a confocal micro-Raman spectrometer equipped with 325, 473, 514, 532, 633, and 784 nm lasers. A total of 17 first-order Raman modes of monoclinic Er2O3 were identified and assigned. The modes at 83, 112, 152, 170, 278, 290, 409, 446, 478, 521, 603, and 622 cm−1 are of Ag symmetry, whereas modes at 71, 98, 333, 409, 446, and 468 cm−1 are of Bg symmetry. This research provides basic data necessary for the characterization of monoclinic Er2O3 by Raman spectroscopy.

The temperature dependence of the Raman spectra of chromium-doped titanite (CaTiOSiO4 )

Summary The temperature dependence of the Raman bands of Cr4+ modes which show enhanced intensities due to pre-resonance effects is reported from 293 to 673 K in chromium-doped titanite (CaTiOSiO4). Some aspects of the temperature dependence of Raman bands in pure, synthetic titanite which have not been previously published are also included in this study. Two Raman-active components of Ag and Bg symmetry, respectively, of the symmetric Si–O mode in titanite are predicted under P21/a symmetry and also been identified in this phase for the first time. The one component of Bg symmetry disappears just above the antiferroelectric–paraelectric transition at ~500 K in accordance with the predictions under A2/a symmetry for the high-temperature phase. Two resonance-enhanced components of the Cr4+–O stretch are also evident in the P21/a phase and only one could be identified in the A2/a phase, again in accordance with group-theoretical predictions. These observations can be used to characterize the P21/a and A2/a phases of pure synthetic and chromium-doped titanite. The temperature dependence of the Cr4+–O modes can be approximated by two-dimensional Ising behavior with the critical exponent β ≈ 1/8 below 450 K. Between 450 and 498 K, anomalous behavior is observed and this could be due to the appearance of mobile anti-phase boundaries (APBs). Anomalous behavior also persists to temperatures above 500 K. The half-width of the Ti–O stretching mode reflects the influence of the order parameter (Ti–O displacements) as well as mobile anti-phase boundaries. No evidence could be found of the existence of other ions such as Cr4+-ions in Ti-sites and/or Cr3+-ions also in Ti-positions in Cr-doped titanite in the Raman spectra using different laser lines to excite the spectra. Copyright © 2013 John Wiley & Sons, Ltd.

Electronic Raman scattering from orbital nematic fluctuations

We compute Raman scattering intensities via the lowest-order coupling to the bosonic propagator associated with orbital nematic fluctuations in a minimal model for iron pnictides. The model consists of two bands on a square lattice exhibiting four Fermi pockets and a transition from the normal to a nematic state. It is shown that the orbital fluctuations produce in the B1g channel strong quasi-elastic light scattering around the nematic critical temperature Tn, both above and below Tn. This holds for the A1g symmetry only below Tn whereas no low-energy scattering from orbital fluctuations is found in the B2g symmetry. Due to the nematic distortion the electron pocket at the X-point may disappear at low temperatures. Such a Lifshitz transition causes in the B2g spectrum a large upward shift of spectral weight in the high energy region whereas no effect is seen in the other symmetries.

Ab initio and shell model studies of structural, thermoelastic and vibrational properties of SnO2 under pressure

The pressure dependences of the structural, thermoelastic and vibrational properties of SnO2 in its rutile phase are studied, as well as the pressure-induced transition to a CaCl2-type phase. These studies have been performed by means of ab initio (AI) density functional theory calculations using the localized basis code SIESTA. The results are employed to develop a shell model (SM) for application in future studies of nanostructured SnO2. A good agreement of the SM results for the pressure dependences of the above properties with the ones obtained from present and previous AI calculations as well as from experiments is achieved. The transition is characterized by a rotation of the Sn-centered oxygen octahedra around the tetragonal axis through the Sn. This rotation breaks the tetragonal symmetry of the lattice and an orthorhombic distortion appears above the critical pressure Pc. A zone-center phonon of B1g symmetry in the rutile phase involves such rotation and softens on approaching Pc. It becomes an Ag mode which stabilizes with increasing pressure in the CaCl2 phase. This behavior, together with the softening of the shear modulus (C11−C12)/2 related to the orthorhombic distortion, allows a precise determination of a value for Pc. An additional determination is provided by the splitting of the basal plane lattice parameters. Both the AI and the experimentally observed softening of the B1g mode are incomplete, indicating a small discontinuity at the transition. However, all results show continuous changes in volume and lattice parameters, indicating a second-order transition. All these results indicate that there should be sufficient confidence for the future employment of the shell model.

Electronic Raman response in electron-doped cuprate superconductors

The electronic Raman response in the electron-doped cuprate superconductors is studied based on the t-t′-J model. It is shown that although the domelike shape of the doping dependent peak energy in the B2g symmetry is a common feature for both electron-doped and hole-doped cuprate superconductors, there are pronounced deviations from a cubic response in the B2g channel and a linear response in the B2g channel for the electron-doped case in the low energy limit. It is also shown that these pronounced deviations are mainly caused by a nonmonotonic d-wave gap in the electron-doped cuprate superconductors.

A DFT study of reversed isotope shifts in H/D substitution of free-base porphyrin and related free-base tetrapyrroles

DFT/B3LYP calculations are used to analyse the occurrence of reverse isotope shift ratios (ISR) in H/D substitution of the free-base tetrapyrroles, in situations where the frequency ratio νH/νD is less than 1. The reverse ISR effect is found to be most evident in the out-of-plane bending modes (b2g and b3u symmetry) involving some N–H motion for the four molecules studied, viz., porphine (H2P), tetraaza-porphine (H2TAP), tetrabenzo-porphine (H2TBP), and phthalocyanine (H2Pc). It was analysed by following the evolution of the normal mode frequencies with incremental variation of the H atom masses from 1 to 2 amu. This method allows direct, unambiguous mode correlations to be established between the light and the heavy isotopologues. When the NH(D) motion is predominant, the H to D frequency evolution decreases in a continuous manner for a particular normal mode. In the case of two modes of the same symmetry and whose frequencies are similar, their frequency evolutions could cross, depending on the extent of NH(D) motion involved in them. The evolution diagrams may show avoided crossings of various extents, which thereby reflects the degree of the NH(D) motion in the modes. The reverse ISR effect is directly correlated to these avoided crossings. Because the isotope shifts are quite small (<10 cm–1) and occur in the congested 1500–500 cm–1 spectral region, high-resolution methods yielding narrow line transitions are required for experimental analysis. The matrix isolation technique is particularly well suited for this work and is proposed for use in a search for this effect.

Electronic Raman response in electron-doped cuprate superconductors

The electronic Raman response in the electron-doped cuprate superconductors is studied based on the t–t′–J model. It is shown that although the domelike shape of the doping dependent peak energy in the B2g symmetry is a common feature for both electron-doped and hole-doped cuprate superconductors, there are pronounced deviations from a cubic response in the B1g channel and a linear response in the B2g channel for the electron-doped case in the low energies. It is also shown that these pronounced deviations are mainly caused by a nonmonotonic d-wave gap in the electron-doped cuprate superconductors.

Temperature dependence of coherent phonons in TbVO4 crystal probed by ultrafast optical spectroscopy

Coherent optical phonons in terbium vanadate (TbVO4) are investigated by using femtosecond time-resolved pump-probe spectroscopy at temperatures from 20 to 300 K. Combined with the Raman spectrum, the coherent phonon mode is attributed to an optical phonon mode of B1g symmetry. The main generation mechanism of the coherent optical phonons is revealed to be the impulsive stimulated Raman scattering. The temperature dependence of the dephasing time reveals that the main mechanism of the coherent phonon population decay is anharmonic phonon–phonon coupling, which causes a redshift of the coherent phonon frequency with increasing temperature.

Investigation of the Vibration Spectra of a SbSBrxI1-x Crystal in Harmonic and Quasiharmonic Approximation

The vibration spectra of SbSBrxI1-x (x = 0 ÷ 0.75) crystal have been determined in quasiharmonic approximation by diagonalization of dynamical matrix. Applying the model atom A with its own parameters (mass and force constant) we were able to get exact normal modes’ vibration frequencies. The obtained normal modes’ frequencies were superposed with experimental data. It is proved that in Brillouin zone's point k = 0 the lower frequency IR (infrared) mode of B1u symmetry and lower frequency R (Raman) mode of B1g symmetry are soft-alike modes whereas in Brillouin zone's point k = 0.5 an acoustic and Raman mode become active, but lower frequency IR mode becomes inactive. The interaction between phonons creates the anharmonicity of soft-alike IR, R and A vibration modes.

Raman study of zircon-structured RPO4 (R = Y, Tb, Er, Tm) phosphates at high pressures

The Raman spectra of tetragonal RPO4 (R = Y, Tb, Er, Tm) phosphates belonging to the zircon-structure class (space group D4h19) have been measured at high pressures up to 7.9 GPa (YPO4, ErPO4) and 15.5 GPa (TbPO4, TmPO4). Most of the 12 Raman active modes of this crystal class have been observed at high pressures using a Diamond Anvil Cell. A characteristic feature in the spectra of all crystals studied is the anomalous pressure dependence of the B2g symmetry internal bending mode which softens substantially with increasing pressure to the extent that a level crossing occurs between this mode and one normal Eg rotational mode. These observations imply that an incipient structural phase transition of the crystals may occur at a higher pressure. Such a phase transition has been observed for TbPO4 at a pressure of ∼9.5 GPa. The abrupt spectral variations observed at this pressure, indicate that TbPO4 undergoes a first-order phase transition from tetragonal zircon to a lower symmetry structure which is quenchable upon pressure release.

Polarized Raman scattering and lattice eigenmodes of antiferromagnetic NdFeO3

AbstractThe first‐ and second‐order Raman‐active phonons in the orthorhombic Pbnm NdFeO3 single crystals were studied by means of polarized Raman scattering and lattice dynamics computations (LDC). The zone‐center phonons of Ag symmetry were distinguished from the B1g eigenmodes by performing polarized Raman scattering experiments using two parallel polarization configurations, X′(ZZ)X′ and Z(X′X′)Z. With the help of LDC, we were able to assign most of the observed Raman‐active modes, including phonons of B2g and B3g symmetry. The LDC results indicated that among the 16 force constants employed, the one corresponding to the stretching vibration between the central Fe cation and the axial oxygen atom in a FeO6 octahedron unit had the largest value. This suggests that the B‐site Fe cation is more tightly bound to the axial O1 ion than the other two equatorial O2 ions. It was further shown that at higher wavenumbers, the displacement of oxygen atoms contributed dominantly to the zone‐center vibrations. Copyright © 2008 John Wiley & Sons, Ltd.

Inelastic light scattering studies of overdoped Y1–xCax Ba2Cu3O7–δ thin film

AbstractThe vibrational and magnetic Raman excitation spectra were examined in overdoped Y1–xCaxBa2Cu3O7–δ thin films. Below Tc, certain phonons in undoped YBa2Cu3O7–δ (YBCO) show strong self‐energy effects, which gradually vanish with increasing Ca concentration into the overdoped regime. The observed B1g symmetry two‐magnon excitation peak near 2900 cm–1 in YBCO is significantly broadened, weakened, and shifts to the lower frequency with increasing Ca content, indicating the effective value of magnetic superexchange energy decreases and that the life time of the magnons becomes shorter with increasing hole concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Asymmetric band profile of the Soret band of deoxymyoglobin is caused by electronic and vibronic perturbations of the heme group rather than by a doming deformation

We measured the Soret band of deoxymyoglobin (deoxyMb), myoglobin cyanide (MbCN), and aquo-metmyoglobin (all from horse heart) with absorption and circular dichroism (CD) spectroscopies. A clear non-coincidence was observed between the absorption and CD profiles of deoxyMb and MbCN, with the CD profiles red- and blueshifted with respect to the absorption band position, respectively. On the contrary, the CD and absorption profiles of aquametMb were nearly identical. The observed noncoincidence indicates a splitting of the excited B state due to heme-protein interactions. CD and absorption profiles of deoxyMb and MbCN were self-consistently analyzed by employing a perturbation approach for weak vibronic coupling as well as the relative intensities and depolarization ratios of seven bands in the respective resonance Raman spectra measured with B-band excitation. The respective B(y) component was found to dominate the observed Cotton effect of both myoglobin derivatives. The different signs of the noncoincidences between CD and absorption bands observed for deoxyMb and MbCN are due to different signs of the respective matrix elements of A(1g) electronic interstate coupling, which reflects an imbalance of Gouterman's 50:50 states. The splitting of the B band reflects contributions from electronic and vibronic perturbations of B(1g) symmetry. The results of our analysis suggest that the broad and asymmetric absorption band of deoxyMb results from this band splitting rather than from its dependence on heme doming. Thus, we are able to explain recent findings that the temperature dependences of CO rebinding to myoglobin and the Soret band profile are uncorrelated[Ormos et al., Proc. Natl. Acad. Sci U.S.A. 95, 6762 (1998)].