A1g Phonon Research Articles

Anisotropy of coherent phonon in Bismuth crystal

In this article, we investigate the coherent A1g phonon mode in bismuth crystal using transient reflectivity measurements, focusing on two distinct crystallographic orientations: one with the principal axis ((111)-direction in the trigonal cell representation) perpendicular to the sample surface, and the other with the principal axis parallel to the surface. Our results demonstrate significant variations in the amplitude, frequency, and lifetime of the coherent phonon mode between these two orientations, even when identical pumping and probing conditions are applied. We attribute these differences to the anisotropy of the electron effective mass, which influences electron mobility and, in turn, affects the phonon dynamics in bismuth.

Controlling Ultrafast Magnetization Dynamics via Coherent Phonon Excitation in a Ferromagnet Monolayer.

Exploring ultrafast magnetization control in 2D magnets via laser pulses is established, yet the interplay between spin dynamics and the lattice remains underexplored. Utilizing real-time time-dependent density functional theory (rt-TDDFT) coupled with Ehrenfest dynamics and nonadiabatic molecular dynamics (NAMD) simulations, we systematically investigate the laser-induced spin-nuclei dynamics with pre-excited A1g and E2g coherent phonons in the 2D ferromagnet Fe3GeTe2 (FGT) monolayer. Selective pre-excitation of coherent phonons under ultrafast laser irradiation significantly alters the local spin moment of FGT, consequently inducing additional spin loss attributed to the nuclear motion-induced asymmetric interatomic charge transfer. Excited spin-resolved charge undergoes a bidirectional spin-flip between spin-down and spin-up states, characterized by a subtle change in the spin moment within approximately 100 fs, followed by unidirectional spin-flip, which will further contribute to the spin moment loss of FGT within tens of picoseconds. Our results shed light on the coupling of coherent phonons with magnetization dynamics in 2D limit.

Interlayer Exciton-Phonon Coupling in MoSe2/WSe2 Heterostructures.

Transition metal dichalcogenide heterostructures have garnered strong interest for their robust excitonic properties, mixed light-matter states such as exciton-polaritons, and tailored properties, vital for advanced device engineering. Two-dimensional heterostructures inherit their physics from monolayers with the addition of interlayer processes that have been particularly emphasized for their electronic and optical properties. Here, we demonstrate the interlayer coupling of the MoSe2 phonons to WSe2 excitons in a WSe2/MoSe2 heterostructure using resonant Raman scattering. The WSe2 monolayer induces an interlayer resonance in the Raman cross-section of the MoSe2 A1g phonons. Frozen-phonon calculations within density functional theory reveal a strong deformation-potential coupling between the A1g MoSe2 phonon and the electronic states of the close-by WSe2 layer approaching 20% of the intralayer coupling to the MoSe2 electrons. Understanding the vibrational properties of van der Waals heterostructures requires going beyond the sum of their constituents and considering cross-material coupling.

Investigation of pressure-driven superconductivity in TlInTe2: an ab initio study

The Zintl compound TlInTe2 is an intriguing material because of its outstanding thermoelectric properties at ambient pressure. Interestingly, it has recently been found that TlInTe2 exhibits a V-shape dependence of the superconducting critical temperature (T c) under increasing pressure, which has been linked to the reversed behavior of the Raman active Ag phonon mode and anharmonic effects. In this study, we have performed first-principles calculations of the electron-phonon interactions and the superconducting properties of TlInTe2 in order to understand this unusual pressure-induced response. In contrast to experiment, we find a dome-shaped pressure-induced dependence of T c with a maximum value of 0.23 K at 18 GPa, significantly lower than the experimental results. Electron doping has the potential to adjust the T c to fall within the experimental range, but it necessitates considerably high levels of doping. Furthermore, our analysis of the phonon spectra and phonon lifetimes, including anharmonic effects, show that anharmonicity is unlikely to influence the superconducting properties of TlInTe2. It remains an open question whether there is indeed an unusual V-shape T c dependence with pressure or whether the phonon-mediated theory of superconductivity used here breaks down in this system.

Spin-charge-lattice couplings and Co doping effect in Ba(Fe 1−x Co x )2As2: an ab initio molecular dynamics study

As one of the typical iron-based superconductors, Ba(Fe 1−x Co x )2As2 exhibits a dome-like evolution of superconducting transition temperature T c with the Co doping (x), which provides an ideal platform to explore the mechanism of unconventional superconductivity. Through ab initio molecular dynamics (AIMD) studies, we find that the spin, charge, and lattice degrees of freedom in Ba(Fe 1−x Co x )2As2 are intimately coupled with a characteristic frequency around 5.5 THz, which originates from the A1g phonon mode of the As atoms. Compared with the undoped BaFe2As2, the spin dynamics in Ba(Fe 1−x Co x )2As2 (x = 0.0625, 0.1250, 0.1875) shows enhanced spatial inhomogeneities, though the features of the charge dynamics are rarely modified. Along with the increasing Co doping, the spatial inhomogeneities of Fe spin dynamics display nonmonotonic changes, meanwhile the lattice vibrational spectra first show peak broadening and then restore the main peak around 5.5 THz, being both consistent with the nonmonotonic behavior of superconducting T c. Our study provides a molecular dynamics approach to investigate the couplings of spin, charge, and lattice in doped BaFe2As2, which could also be applied to explore the dome-like evolution of superconductivity in other Fe-based superconductors.

Tip-enhanced Raman scattering and near-field optical imaging of semiconducting monolayer and few-layer MoTe2

Transition metal dichalcogenides (TMDs) offer exceptional platforms to study unique phenomena in two-dimensional (2D) materials. The understanding of phonon interactions in atomically thin TMDs is crucial for the development of novel applications. However, advancing our knowledge on phonon dynamics in TMDs under optical, thermal, electric, and strain perturbations requires comprehensive and minimally invasive chemical imaging techniques with nanoscale spatial resolution. Tip-enhanced Raman scattering (TERS) provides new promising avenues towards this goal while also enabling a detailed analysis of structural heterogeneity. Here, we demonstrate gap-mode TERS imaging of mechanically exfoliated monolayer and few-layer 2H–MoTe2. At 633 nm, 671 nm, and 785 nm laser excitations, we report an overwhelmingly selective TERS enhancement of the out-of-plane A1g phonon mode relative to in-plane E12g phonon mode for mono-to few-layer MoTe2. The pure near-field spectral line shapes are clearly distinct from the well-characterized far-field counterparts. We demonstrate that near-field interactions of selective phonon modes are extremely sensitive to the excitation wavelength, sample thickness, and structural defects. Our results therefore provide fundamental information for the nanoscale characterization of semiconducting MoTe2-based devices.

Three-stage ultrafast demagnetization dynamics in a monolayer ferromagnet

Intense laser pulses can be used to demagnetize a magnetic material on an extremely short timescale. While this ultrafast demagnetization offers the potential for new magneto-optical devices, it poses challenges in capturing coupled spin-electron and spin-lattice dynamics. In this article, we study the photoinduced ultrafast demagnetization of a prototype monolayer ferromagnet Fe3GeTe2 and resolve the three-stage demagnetization process characterized by an ultrafast and substantial demagnetization on a timescale of 100 fs, followed by light-induced coherent A1g phonon dynamics which is strongly coupled to the spin dynamics in the next 200–800 fs. In the third stage, chiral lattice vibrations driven by nonlinear phonon couplings, both in-plane and out-of-plane are produced, resulting in significant spin precession. Nonadiabatic effects are found to introduce considerable phonon hardening and suppress the spin-lattice couplings during demagnetization. Our results advance our understanding of dynamic charge-spin-lattice couplings in the ultrafast demagnetization and evidence angular momentum transfer between the phonon and spin degrees of freedom.

Anharmonic strong-coupling effects at the origin of the charge density wave in CsV3Sb5

The formation of charge density waves is a long-standing open problem, particularly in dimensions higher than one. Various observations in the vanadium antimonides discovered recently further underpin this notion. Here, we study the Kagome metal CsV3Sb5 using polarized inelastic light scattering and density functional theory calculations. We observe a significant gap anisotropy with 2Δmax/kBTCDW≈20\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2{\\Delta }_{\\max }/{k}_{{{{{{{{\\rm{B}}}}}}}}}{T}_{{{{{{{{\\rm{CDW}}}}}}}}}\\, \\approx \\, 20$$\\end{document}, far beyond the prediction of mean-field theory. The analysis of the A1g and E2g phonons, including those emerging below TCDW, indicates strong phonon-phonon coupling, presumably mediated by a strong electron-phonon interaction. Similarly, the asymmetric Fano-type lineshape of the A1g amplitude mode suggests strong electron-phonon coupling below TCDW. The large electronic gap, the enhanced anharmonic phonon-phonon coupling, and the Fano shape of the amplitude mode combined are more supportive of a strong-coupling phonon-driven charge density wave transition than of a Fermi surface instability or an exotic mechanism in CsV3Sb5.

Low-Frequency Raman Study of Large-Area Twisted Bilayers of WS2 Stacked by an Etchant-Free Transfer Method.

Monolayer transition metal dichalcogenides have strong intracovalent bonding. When stacked in multilayers, however, weak van der Waals interactions dominate interlayer mechanical coupling and, thus, influence their lattice vibrations. This study presents the frequency evolution of interlayer phonons in twisted WS2 bilayers, highly subject to the twist angle. The twist angle between the layers is controlled to modulate the spacing between the layers, which, in turn, affects the interlayer coupling that is probed by Raman spectroscopy. The shifts of high-frequency E2g1 (Γ) and A1g (Γ) phonon modes and their frequency separations are dependent on the twist angle, reflecting the correlation between the interlayer mechanical coupling and twist angle. In this work, we fabricated large-area, twisted bilayer WS2 with a clean interface with controlled twist angles. Polarized Raman spectroscopy identified new interlayer modes, which were not previously reported, depending on the twist angle. The appearance of breathing modes in Raman phonon spectra provides evidence of strong interlayer coupling in bilayer structures. We confirm that the twist angle can alter the exciton and trion dynamics of bilayers as indicated by the photoluminescence peak shift. These large-area controlled twist angle samples have practical applications in optoelectronic device fabrication and twistronics.

Multiple coherent amplitude modes and exciton-phonon coupling in quasi-one-dimensional excitonic insulator Ta2NiSe5.

An excitonic insulator (EI) is an intriguing correlated electronic phase of condensed excitons. Ta2NiSe5 is a model material for investigating condensed excitonic states. Herein, femtosecond pump-probe spectroscopy is used to study the coherent phonon dynamics and associated exciton-phonon coupling in single-crystal Ta2NiSe5. The reflectivity time series consists of exponential decay due to hot carriers and damped oscillations due to the Ag phonon vibration. Given the in-plane anisotropic thermal conductivity of Ta2NiSe5, coherent phonon oscillations are stronger with perpendicular polarization to its quasi-one-dimensional chains. The 1-, 2-, and 4-THz vibration modes show coherent amplitude responses in the EI phase of Ta2NiSe5 with increasing temperature, totally different from those of normal coherent phonons (the 3- and 3.7-THz modes). The amplitude modes at higher frequencies decouple with the EI order parameter at lower temperatures, as supported by theoretical analysis with a model Hamiltonian of the exciton-phonon coupling system. Our work provides valuable insights into the character of the EI order parameter and its coupling to multiple coherent amplitude modes.

Multimodal Microscopy Characterisation of 2D Materials

Spatial characterisation of transition metal dichalcogenides (TMDCs) is crucial for understanding their optoelectronic properties and optimising film growth conditions. In this work, we present a multimodal microscopy platform combining conventional widefield, Raman, photoluminescence and second harmonic generation imaging for the characterisation of 2D materials.The platform was used to characterise CVD grown WSe2 crystals. Raman imaging identified the presence of both monolayer and multilayer WSe2 through a change in the intensity and position of the E12g / A1g phonon modes. Photoluminescence imaging confirmed the presence of monolayer WSe2 with emission at 780 nm and identified two distinct multilayer regions through changes in the photoluminescence wavelength and intensity. Lastly, second harmonic generation imaging under femtosecond laser excitation was used to obtain the relative growth orientation of the three identified domains in the crystal.Figure 1 highlights Raman imaging of WSe2. (a) Intensity of the E12g / A1g (250 cm-1) Raman band, (b) Peak position of the E12g / A1g (250 cm-1) Raman band, (c) least squares spectral matching revealing three distinct Raman spectral areas, (d) Averaged Raman spectra from areas A, B and C. Figure 1

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.

Strong Coupling of Coherent Phonons to Excitons in Semiconducting Monolayer MoTe2.

The coupling of the electron system to lattice vibrations and their time-dependent control and detection provide unique insight into the nonequilibrium physics of semiconductors. Here, we investigate the ultrafast transient response of semiconducting monolayer 2H-MoTe2 encapsulated with hBN using broadband optical pump-probe microscopy. The sub-40 fs pump pulse triggers extremely intense and long-lived coherent oscillations in the spectral region of the A' and B' exciton resonances, up to ∼20% of the maximum transient signal, due to the displacive excitation of the out-of-plane A1g phonon. Ab initio calculations reveal a dramatic rearrangement of the optical absorption of monolayer MoTe2 induced by an out-of-plane stretching and compression of the crystal lattice, consistent with an A1g -type oscillation. Our results highlight the extreme sensitivity of the optical properties of monolayer TMDs to small structural modifications and their manipulation with light.

Photoinduced Ultrafast Phase Transition in Bilayer CrI3.

In two-dimensional magnets, the ultrafast photoexcited method represents a low-power and high-speed method of switching magnetic states. Bilayer CrI3 (BLC) is an ideal platform for studying ultrafast photoinduced magnetic phase transitions due to its stacking-dependent magnetic properties. Here, by using time-dependent density functional theory, we explore the photoexcitation phase transition in BLC from the R- to M-stacked phase. This process is found to be induced by electron-phonon interactions. The activated Ag and Bg phonon modes in the xy direction drive the horizontal relative displacements between the layers. The activated Ag mode in the z direction leads to a transition potential reduction. Furthermore, this phase transition can invert the sign of the interlayer spin interaction, indicating a photoinduced transition from ferromagnet to antiferromagnet. This investigation has profound implications for magnetic phase engineering strategies.

Temperature-dependent Raman spectroscopy analysis of single spiral [formula omitted] nanosheets with screw dislocations

This study presents Raman spectroscopy measurements of Screw Dislocation Single Spiral Pattern SDD-SSP-WS2 samples supported on SiO2/Si at various locations, including the edge, middle, and center across temperatures spanning from 80 K to 560 K. A physical model that encompasses both the temperature effect and volume effect has been employed to comprehensively comprehend the nonlinearity temperature dependence of the A1g mode in a quantitative manner. It is shown that the four-phonon scattering and thermal expansion effect are the major causes of the Raman mode’s non-linear dependency. The thermal expansion effect and four phonons positively correlate with thicknesses, while the three-phonon scattering effect has an inverse relationship. The investigation of the full width at half maximum (FWHM) of the A1g mode has been conducted through experimental and theoretical means, utilizing various expressions within the identical temperature range. The observed broadening of FWHM A1g mode can be attributed to the decay of the A1g phonon. This study aims to enhance comprehension of the anharmonic characteristics and behaviors of phonons in SDD-SSP-WS2 flakes of varying thicknesses.

The coupling between the interlayer magnetic order and Davydov splitting modes in few-layer CrI3

The magnetic order in 2D material CrI3 has a significant impact on the Raman polarization selection rules of phonons, as evidenced by magneto-optical Raman spectroscopy. In a monolayer, the forbidden Raman scattering of A1g mode can be detected in the XY channel at low temperatures. However, in the bilayer, the Ag mode splits into two modes, and the selection rules of the splitting modes are determined by the magnetic order and symmetry. In contrast, the inversion symmetry of the spin structure is maintained in both antiferromagnetic (AFM) and ferromagnetic (FM) states in the trilayer (3L) CrI3, and the evolution of its Raman scattering with magnetic field is not yet clear. In this work, we use magneto-optical polarized Raman spectroscopy and reflective magnetic circular dichroism to investigate the Davydov splitting of the Ag phonon mode and its coupling to layered magnetism in 2–5L CrI3. Our results show that the Raman scattering of the Ag mode is strongly coupled to the layered magnetic order, indicating strong spin–phonon coupling in CrI3. Our study might shed light on the research on interaction between magnetic and vibrational properties of 2D magnetic materials and provide important implications for developing novel 2D spintronic devices.

Ultrafast melting of charge-density wave fluctuations at room temperature in 1T-TiSe2 monitored under non-equilibrium conditions

We investigate the ultrafast lattice dynamics in 1T-TiSe2 using femtosecond reflection pump–probe and pump–pump–probe techniques at room temperature. The time-domain signals and Fourier-transformed spectra show the A1g phonon mode at 5.9 THz. Moreover, we observe an additional mode at ≈ 3 THz, corresponding to the charge-density wave (CDW) amplitude mode (AM), which is generally visible below Tc≈200 K. We argue that the emergence of the CDW amplitude mode at room temperature can be a consequence of fluctuations of order parameters based on the additional experiment using the pump–pump–probe technique, which exhibited suppression of the AM signal within the ultrafast timescale of ∼0.5 ps.

Anharmonic and glass-like phonon transport in the Tetrahedrite-manner Ag6Si6Sn4P12

In this study, the thermal conductivity properties of the tetrahedrite-structures compound Ag6Si6Sn4P12, whose phonon properties have not been reported so far, are investigated in depth using self-consistent phonon calculations that consider up to fourth-order anharmonic terms. Compared to the previously studied Ag6Ge10P12, a tetrahedrite-manner phosphide, Ag6Si6Sn4P12 without lone pairs, has a unique feature in that it exhibits softening of the Ag and Sn phonon modes at 300 K and has a short phonon lifetime owing to its relatively high anharmonicity. This is believed to be the cause of the extremely low lattice thermal conductivity of Ag6Si6Sn4P12. Additionally, we also observed a nonnegligible contribution of the coherent component to the thermal conductivity of Ag6Si6Sn4P12 because of its wide phonon linewidth, suggesting that Ag6Si6Sn4P12 exhibits glass-like phonon transport properties.

Transient gap generation in BaFe2As2 driven by coherent lattice vibrations.

Iron-based superconductors provide a rich platform to investigate the interplay between unconventional superconductivity, nematicity, and magnetism. The electronic structure and the magnetic properties of iron-based superconductors are highly sensitive to the pnictogen height. Coherent excitation of the phonon by femtosecond laser directly modulates the pnictogen height, which has been used to control the physical properties of iron-based superconductors. Previous studies show that the driven phonon resulted in a transient increase of the pnictogen height in BaFeAs, favoring an enhanced Fe magnetic moment. However, there are no direct observations on either the enhanced Fe magnetic moments or the enhanced spin-density wave (SDW) gap. Here, we use time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFeAs in the phonon-driven state. Below the SDW transition temperature, we observe a transient gap generation at early-time delays. A similar transient feature is observed in the normal state up to room temperature.

Raman Spectroscopy of the Trapezoidal Bi2O2Se

AbstractThe 2D Bi2O2Se has recently attracted significant interest with large Hall mobility at low temperature due to the depressed electron–phonon scattering. However, the phonon properties of Bi2O2Se are not well explored, and some Raman modes are under debate. Here, substrate engineering is used to grow an unexplored trapezoidal Bi2O2Se, which facilitates the Raman measurements in different orientation planes. Two additional Raman modes at 55 and 429 cm−1 are detected on the slope of the trapezoid besides the 159 and 362 cm−1 modes found on the top plane of the trapezoidal Bi2O2Se. Based on the polarized Raman spectroscopy, density functional theory calculations, and group theory analysis, the Raman modes of 55, 159, 362, and 429 cm−1 are assigned to the Eg, A1g, B1g, and Eg modes, respectively. There are two distinct types of angle‐resolved polarized Raman spectroscopy for the A1g mode on the top and edge planes of Bi2O2Se, which can be used to determine the crystal orientation unambiguously. Furthermore, strong thermal anisotropy is found in the top and edge planes of trapezoidal Bi2O2Se, as reflected from the first‐order temperature coefficient of A1g phonon mode. The novel growth strategy and interesting anisotropic phonon properties may open up potential applications of Bi2O2Se in nanoelectronics.