B1g Phonon Research Articles

Probing the effect of lightly doped Iron in Bi2S3 nanostructures on Physiochemical properties and efficacy in anti-microbial activity

In this study, Bi2S3 nanostructures doped with Iron (Fe) at various concentrations i.e., FexBi2-xS3 (x = 0.0, 0.4, 0.6, 0.8 wt%) were synthesized using the reverse micelle method. EDAX (Energy Dispersive Analysis of x-rays) has shown that the prepared samples are in stoichiometry without any kind of impurities. Rietveld refinement XRD (x-ray diffraction) pattern confirmed the orthorhombic crystal structure and showed good crystallinity of all the samples with increase in Fe content. The unit cell volume is found to be varied from 12.34 nm to 19.39 nm. HRTEM (High Resolution Transmission Electron Microscopy) has shown that the prepared nanostructures are nanorods and nanocylinders with high crystallinity and corroborates with our XRD results. Diffuse reflectance spectroscopy analysis indicated that the band gap has increased from 1.550 eV for pure Bi2S3 to 1.592 eV for Fe0.08Bi1.92S3 nanostructures reflecting the blue shift compared to bulk sample. The photoluminescence spectra (PL) recorded with 250 nm excitation wavelength for powder samples has shown that with increase in Iron concentration the intensity of 440 nm peak increases whereas the peaks at 470 nm and 510 nm decreases. The PL spectra is also recorded for nanostructures dispersed in liquid media and has shown that the peaks at 501 nm is observed while rest of the two peaks are quenched. Raman spectra dependent on temperature is obtained for FexBi2-xS3 (x = 0.0, 0.4, 0.6, 0.8 wt%) samples in pellet form in the range of 80 K to 280 K. All samples have shown B1g and Ag phonon modes with higher intensity. The Gruneisen parameter determined for B1g mode varies from 1.21 to 14.13 whereas for Ag mode it varies from 0.60 to 7.91 with the exception of a negative value of −3.10 for Fe0.06Bi1.94S3 sample. VSM (vibrating sample magnetometer) showed the diamagnetic behavior of Bi2S3 and ferromagnetic behaviour of FexBi2-xS3 (x = 0.0, 0.4, 0.6, 0.8 wt%) samples. The saturation magnetization is found to be reaching to a value of 127.5 emu gm−1 for 0.6 wt% of Fe doping in Bi2S3 and then decreases drastically to 40.34 emu gm−1 for 0.8 wt% Fe doping. The antibacterial efficacy showed that as Fe concentration increases, the MIC (minimal inhibitory concentration) fluctuates between 60 to 70 μg ml−1 and is found to be maximum for Fe0.08Bi1.92S3 sample. It is also found that Fe0.04Bi1.96S3 nanostructures show the lowest MIC value for Gram +ve and Gram –ve bacteria in comparison to Bi2S3 nanostructures.

Lattice thermal conductivity calculation of phosphorene using molecular dynamics and spectral energy density

The anisotropic structure of black phosphorene makes it potentially a unique material with special anisotropic properties. Its applications and special properties call for a study of phonon dispersion and thermal transport properties. In this case, we calculate the spectral energy density from molecular dynamics to extract the phonon relaxation times and phonon mean free paths. The phosphorene's thermal conductivity was calculated at 300 K and resulted as 13.45 W/mK and 28.97 W/mK in the armchair and zigzag directions respectively which show a clear anisotropy in different directions because of the anisotropic structure of phosphorene. ZA mode phonons have a lower role in conductivity in comparison with graphene; LA phonons in the zigzag direction have a very important role and in the armchair direction both LA and TA mode phonons have higher values. In optical branches, the B1g phonons have an important role in both zigzag and armchair directions.

Emergent charge order from correlated electron-phonon physics in cuprates

Charge-density wave order is now understood to be a widespread feature of underdoped cuprate high-temperature superconductors, although its origins remain unclear. While experiments suggest that the charge-ordering wavevector is determined by Fermi-surface nesting, the relevant sections of the Fermi surface are featureless and provide no clue as to the underlying mechanism. Here, focusing on underdoped YBa2Cu3O6+x, we propose that charge-density waves form from the incipient softening of a bond-buckling phonon. The momentum dependence of its coupling to itinerant electrons favourably selects the wavevector found in experiments. But, it requires quasiparticle renormalization by strong electronic correlations to enable a unique enhancement of the charge susceptibility near the B1g-phonon selected wavevector. The B1g phonon frequency softens by a few percent, and finite-range charge-density wave correlations will form locally, if nucleated by defects or dopant disorder. These results suggest that underdoped cuprates cannot be understood in the context of strong electronic correlations alone.

Role of Bi/Tm substitution in Bi-2212 system on crystal structure quality, pair wave function and polaronic states

This comprehensive study finds strongly out the crucial variations in the dc electrical resistivity, superconducting, crystal structural and flux pinning mechanisms with the partial replacement of homovalent Tm+3 inclusions by Bi+3 impurity in the active layers of Bi-2212 superconducting material. Materials of type Bi2.1-xTmxSr2.0Ca1.1Cu2.0Oy with molar ratio changes of 0.00 ≤ x ≤ 0.30 are prepared by conventional solid-state reaction route in atmospheric pressure and the characterizations are exerted by the bulk density, dc electrical resistivity (ρ-T), powder X-ray diffraction (XRD), critical current density (Jc), scanning electron microscopy (SEM) and electron dispersive X-ray (EDX) experimental measurements. The combination of experimental results evaluated from the bulk density, dc electrical resistivity, XRD and EDX measurements points out that the Tm foreign impurities mostly incorporate successfully into the Bi-2212 crystal lattice. In fact, the EDX investigations verify that the thulium impurities may mostly be substituted for the bismuth sites in the crystal structure. All the experimental results declare that the characteristic features improve regularly with the increment in the Tm impurity level up to x = 0.07 beyond which the properties degrade dramatically. In this respect, the sample with x = 0.07 exhibits highest electrical conductive/metallic characteristics as a consequence of the refinement in the crystal structure quality and connectivity between the superconducting grains, being favored by bulk density and related relative degrees of granularity surveys. Likewise, the material presents the maximum offset-onset critical transition temperature values of 85.61 K–85.85 K due to the increment in the formation of effective and strong electron-phonon coupling probabilities and optimization of mobile hole carrier concentrations in the Cu-O2 consecutively stacked layers. In other words, the optimum content level leading to transit inherently over-doped nature into optimally doped state strengthens the amplitude of pair wave function for the Bi-2212 material. In more sophisticated interpretations, the presence of optimum dopant in the crystal structure changes the vibrational mode intensities of O (1)CuA1g, B1g phonons and O (2)SrA1g phonon so that the formation possibility of bipolaron out of two polarons increases strongly in a polarizable lattice (polaronic effect). Additionally, the highest self-field Jc of 95 A/cm2 confirms the fact that the optimum dopant augments the effective nucleation centers along the intragrain and inter-grain boundaries in the crystal system. Similarly, the material prepared with x = 0.07 presents the smoothest, densest, largest average crystalline distribution, lowest porosity and most uniform surface appearance with the finest connection between the superconducting grains. The XRD results (the increased high phase, c-axis length and average grain size but decreased a-axis length) also show the optimum dopant level of x = 0.07 for Bi-2212 crystal system. All in all, the paper developing a strong methodology about why the characteristic properties improve with the presence of Tm impurity in the Bi-2212 system may be a pioneering research to construct newly, novel and feasible market areas for the Bi-2212 superconducting ceramics in the universe economy.

Phonon spectrum of single-crystalline FeSe probed by high-resolution electron energy-loss spectroscopy

Utilizing high-resolution electron energy-loss spectroscopy (HREELS) we measure the phonon frequencies of β–FeSe(001), cleaved under ultra-high vacuum conditions. At the zone center (Γ¯–point) three prominent loss features are observed at loss energies of about ≃ 20.5 and 25.6 and 40 meV. Based on the scattering selection rules we assign the observed loss features to the A1g, B1g, and A2u phonon modes of β–FeSe(001). The experimentally measured phonon frequencies do not agree with the results of density functional based calculations in which a nonmagnetic, a checkerboard or a strip antiferromagnetic order is assumed for β–FeSe(001). Our measurements suggest that, similar to the other Fe-based materials, magnetism has a profound impact on the lattice dynamics of β–FeSe(001).

Phonon anomalies in FeS

We present results from light scattering experiments on tetragonal FeS with the focus placed on lattice dynamics. We identify the Raman active A1g and B1g phonon modes, a second order scattering process involving two acoustic phonons, and contributions from potentially defect-induced scattering. The temperature dependence between 300 and 20K of all observed phonon energies is governed by the lattice contraction. Below 20K the phonon energies increase by 0.5-1 cm$^{-1}$ thus indicating putative short range magnetic order. Along with the experiments we performed lattice-dynamical simulations and a symmetry analysis for the phonons and potential overtones and find good agreement with the experiments. In particular, we argue that the two-phonon excitation observed in a gap between the optical branches becomes observable due to significant electron-phonon interaction.

Polarized Raman scattering on single crystals of rare earth orthochromite RCrO3 (R = La, Pr, Nd, and Sm)

AbstractWe report on polarized Raman scattering on orthorhombic RCrO3 (R = La, Pr, Nd, and Sm) single crystals at room temperature. The analysis of polarized Raman spectra as a function of polarizer and analyzer angles with respect to crystallographic axes allowed to observe in these flux grown crystals phonon modes that were not detected yet. On the basis of both Raman selection rules and lattice dynamics calculations within density functional perturbations theory framework, the observed Ag, B1g, B2g, and B3g phonon modes were unambiguously assigned. Phonon mode wave number evolutions as a function of orthorhombic distortion, related to rare earth ionic radius, were investigated and highlighted a complex behavior for several low wave number modes allowed crossing. For the modes of same symmetry, a mode mixing behavior is observed, and numerical calculations based on two or three coupling systems were performed to fit experimental data and to extract the coupling constants.

Raman and X-ray diffraction studies of superconducting FeTe1−xSex compounds

Iron-based superconducting layered compounds have the second highest transition temperature after cuprate superconductors. Their discovery is a milestone in the history of high-temperature superconductivity and will have profound implications for high-temperature superconducting mechanism as well as industrial applications. We have measured X-ray diffraction and Raman spectra of poly-crystalline FeTe1−xSex samples with x = 0.0, 0.5 and 1 at room temperature. X-ray profile of FeTe and FeTe0.5Se0.5 confirmed the primitive tetragonal unit cell. FeSe is found to be in double phase with tetragonal phase (about 66.62%) and orthorhombic (33.38%). A1g and B1g phonon modes were observed in Raman spectra. At room temperature, these lines are located at 95 and 130 cm−1 for FeTe, 93 and 129 cm-1 for FeTe0.5Se0.5 and 93 and 129 cm-1 for FeSe samples.

Determination of optimum diffusion annealing temperature for Au surface-layered Bi-2212 ceramics and dependence of transition temperatures on disorders

This study aims not only to declare experimentally the effect of diffusion annealing temperature (600 °C-850 °C) on the crystal structure, electrical and superconducting properties of Au surface-layered Bi-2212 ceramics by dc resistivity and powder X-ray diffraction measurements, but also to advance in-depth understanding of empirical relationship between the disorders and transition temperatures. It is found that the diffusion annealing temperature of 800 °C is found to be optimum for the improvement of crystal structure due to the retrogression of disorders and local structural-distortions in the layers, increasing metastability, σ-gap anisotropy and softening of Ag and B1g phonons. Accordingly, the Au nanoparticles transit from inherently over-doped nature of the compound to optimally-doped state. Additionally, the empirical relations between critical temperatures and Δρ parameters (related to disorders) show that it is possible to achieve maximum onset and offset critical temperature values of 90.467 K and 90.359 K for the Bi-2212 ceramics.

Electron–phonon coupling in BaFe2As2: A possible origin of nematic phase

Iron-based superconductors have the second-highest superconducting transition temperature next to cuprate superconductors. There are many electronic phases such as superconducting, antiferromagnetic and nematic phases, coexisting in the family of iron pnictides. The charge/spin orders are the key factors for understanding the mechanism of high-temperature superconductivity. In this paper we have performed a Raman scattering study of parent compound BaFe2As2. A relatively strong electron–phonon coupling is observed for B1g phonon mode and is weakened after the structural transition at ∼140 K. Through a careful symmetry analysis, we revealed the possibility that B1g mode can be effectively coupled with dxz and dyz orbits. The coupling can remove the degeneracy of the two orbits and further cause the fluctuations of electronic nematic phases. The study suggests that electron–phonon coupling can be a possible origin of nematic phase.

Phonon Anharmonicity of PdO Studied by Raman Spectrometry

Raman spectra were measured for palladium oxide (PdO) at temperatures from 298 to 973 K. The first-order scattering B1g and Eg phonon modes show prominent frequency and line width dependencies on temperature. The phonon behaviors are described well by a model considering contributions from thermal expansion related quasiharmonicity and explicit lattice anharmonicity from both cubic and quartic phonon decay processes. Compared to the quasiharmonicity for the line shift, the explicit anharmonicity is found to be relatively smaller for the B1g mode with the cubic anharmonicity leading the quartic term. The Eg mode has a smaller line shift which mostly follows a quartic phonon decay mechanism while the cubic contribution is canceled by thermal expansion effects. The cubic anharmonicity dominates mostly the line broadening with more prominent quartic effects responsible for the larger broadening of the Eg mode. Multiple potential asymmetric cubic and quartic anharmonic decay channels including down-conversion ...

Raman Spectroscopy of Ba(Fe1−x Mn x )2As2

Raman scattering measurements on iron–pnictide Mn-doped BaFe2As2 single crystals are reported. Single crystals were grown out of a FeAs self-flux using conventional high-temperature solution growth and characterized by X-ray diffraction, atomic force microscopy, and Raman. Raman spectra were obtained at room temperature and 77 K on ab- and a(b)c-planes. Two of four phonon modes allowed by symmetry were found and identified. It was observed that the scattering intensity of A1g mode and the frequencies of the A1g and B1g phonons are dependent upon doping of Mn. The dependence of scattering intensity and frequency of A1g mode on Mn doping might indicate that the Mn ion also occupies the As site.

Signatures of the spin-phonon coupling in [formula omitted] alloys

Raman scattering spectra of Fe1+yTe1−xSex (x=0, y=0.07; x=0.1, y=0.05 and x=0.4, y=0.02) alloys are measured in a temperature range between 20K and 300K. The A1g and B1g Raman active modes have been experimentally observed at energies 156 and 198cm−1, which is in rather good agreement with the lattice dynamics calculation. The antiferromagnetic spin ordering below 70K in Fe1.07Te leaves a fingerprint only in the B1g phonon mode linewidth and energy, whose temperature dependence follows the normalized magnetic susceptibility, indicating the presence of the spin-phonon coupling. The frequency and the linewidth of the A1g mode assume a conventional anharmonic temperature dependence in all measured samples, which is also the case for the B1g mode in the Se doped samples. The linewidth (energy) of the A1g mode decreases (increases) with doping, whereas the opposite is seen for the B1g mode.

Raman scattering study of the lattice dynamic of URu2Si2 and sample’s preparation

We report Raman scattering measurements on URu2Si2 single crystals as a function of temperature down to 2 K. We probe all the Raman active symmetries. Only when the sample is prepared with a surface perpendicular to a-axis, we observe an extrinsic hardening and broadening of the A1g and B1g phonons after polishing, which disappears after annealing. Moreover, a parasitic phase with Si-excess compared to URu2Si2 composition appears on the a-axis surface when annealing is at 1075 °C. No parasitic phase is induced when annealing is done at 950 °C. The temperature dependance of the A1g, and two Eg phonons shows a hardening with decreasing temperature. The B1g phonon mode’s behavior is more unusual, its energy stays stable down to ∼ 30 K before softens at lower temperature. An electron-phonon coupling is certainly at play here.

Phononic and magnetic excitations in the quasi-one-dimensional Heisenberg antiferromagnet KCuF3

The Raman-active phonons and magnetic excitations in the orbitally ordered, quasi one-dimensional Heisenberg antiferromagnet KCuF3 are studied as a function of temperature in the range between 3 and 290 K in different scattering configurations. The low-energy Eg and B1g phonon modes show an anomalous softening (∼25% and ∼13%, respectively) between room temperature and the characteristic temperature TS = 50 K. In this temperature range, a freezing-in of F− ion dynamic displacements is proposed to occur. In addition, the Eg mode at about 260 cm−1 clearly splits below TS. The width of the phonon lines above TS follows an activated behavior with an activation energy of about 50 K. Our observations clearly evidence reduction of the lattice symmetry below TS and indicate strong coupling of lattice, spin, and orbital fluctuations for T > TS. A strongly polarization dependent quasielastic scattering is observed at temperatures above TN = 39 K due to magnetic-energy fluctuations. For temperatures below TN a rich spectrum of additional modes is observed, with different lineshape, polarization and temperature dependence. We attribute them to longitudinal and transverse magnetic modes as well as to a continuum of magnetic excitations.

Cooperative effects of Coulomb and electron-phonon interactions in the two-dimensional 16-bandd−pmodel for iron-based superconductors

We study the electronic states and the superconductivity in the two-dimensional 16-band d-p model coupled with A1g, B1g and Eg local phonons and obtain the rich phase diagram including the magnetic, charge and orbital ordered phases on the parameter plane of the Coulomb and electron-phonon interactions. When the electron-phonon interaction is dominant, the charge fluctuations induce the s++ wave superconductivity, while when the Coulomb interaction is dominant, the magnetic fluctuations induce the s+- wave superconductivity. Remarkably, the orbital fluctuations are enhanced due to the cooperative effects of the Coulomb and electron-phonon interactions and induce the s++ wave and the nodal s+- wave superconductivities.

Raman Line Shapes of Optical Phonons of Different Symmetries in Anatase TiO2 Nanocrystals

Raman spectroscopic investigations of phonons of different symmetries in anatase TiO2 nanocrystals synthesized by the sol−gel method are carried out. Out of six Raman active phonons, the line shapes of two Eg modes and one B1g mode have been analyzed quantitatively to distinguish between the contributions of laser-induced local heating, phonon confinement effects, and defects to the line broadening. The line shape asymmetry arising from confinement of optical phonons in the 397 cm−1 B1g mode is found to be of opposite nature than those in the 144 and 639 cm−1 Eg modes. This arises due to the negative dispersion of the 397 cm−1 B1g phonon dispersion curve. The measured spectra show larger broadening than those predicted by a phonon confinement model. The excess broadening, attributed to intrinsic defects, is found to be least for the 144 cm−1 Eg mode and largest for the 397 cm−1 B1g mode. In addition, finite laser power of the excitation wavelength is found to raise the temperature of the nanocrystals, with heating being maximum for the smallest size nanocrystals.

Anisotropy of gap and kink energies in the trilayer high-Tc cuprate superconductor Bi2Sr2Ca2Cu3O10+δ

We have investigated the electronic structure of the optimally doped trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+δ by angle-resolved photoemission spectroscopy. We have found that the d-wave like superconducting gap opens below Tc and the pseudogap in the antinodal region persists at slightly above Tc, consistent with the previous report. The energy position of the kink in the quasi-particle dispersion was nearly isotropic below Tc, but became momentum dependent above Tc, consistent with the scenario that the antinodal states are coupled to the B1g phonon mode and the nodal states to the half-breathing mode.

Structure and superconductivity in the ternary silicide CaAlSi

Using the linear response-linearized Muffin-tin orbital (LR-LMTO) method, we study the electronic band structure, phonon spectra, electron-phonon coupling and superconductivity for c-axis ferromagnetic-like (F-like) and antiferromagnetic-like (AF-like) structures in ternary silicide CaAlSi. The following conclusions are drawn from our calculations. If Al and Si atoms are assumed to arrange along the c axis in an F-like long-range ordering (-Al-Al-Al-and-Si-Si-Si-), one could obtain the ultrasoft B1g phonon mode and thus very strong electron-phonon coupling in CaAlSi. However, the appearance of imaginary frequency phonon modes indicates the instability of such a structure. For Al and Si atoms arranging along the c axis in an AF-like long-range ordering (-Al-Si-Al-), the calculated electron-phonon coupling constant is equal to 0.8 and the logarithmically averaged frequency is 146.8 K. This calculated result can correctly yield the superconducting transition temperature of CaAlSi by the standard BCS theory in the moderate electron-phonon coupling strength. We propose that an AF-like superlattice model for Al (or Si) atoms along the c direction may mediate the inconsistency estimated from theory and experiment, and explain the anomalous superconductivity in CaAlSi.

Synthesis and Room‐Temperature Ultraviolet Photoluminescence Properties of Zirconia Nanowires

AbstractThis paper reports the synthesis of tetragonal zirconia nanowires using template method. An as‐prepared sample was characterized by scanning and transmission electron microscopy. It was found that the as‐prepared materials were tetragonal zirconia nanowires with average diameters of ca. 80 nm and length of over 10 μm. The Raman spectrum showed peaks at 120, 461, and 629 cm–1, which are attributed to the Eg, Eg, and B1g phonon modes of the tetragonal zirconia structure, respectively. The UV‐vis absorption spectrum showed an absorption peak at 232.5 nm (5.33 eV in photon energy). Photoluminescence (PL) spectra of zirconia nanowires showed a strong emission peak at ca. 388 nm at room temperature, which is attributed to the ionized oxygen vacancy in the zirconia nanowires system.