Fano Asymmetry Research Articles

Probing electron–phonon coupling in magnetic van der Waals material NiPS3: A non-magnetic site-dilution study

NiPS3 is a van der Waals antiferromagnet which has been shown to exhibit spin-phonon and spin-charge coupling in the antiferromagnetically ordered state below TN = 155 K. It is also a rare Ni-based negative charge-transfer-type insulator with Ni valence in a linear superposition state ψ= αd8+ βd9L−+ γ d10L−2 , where L− is the ligand hole. Here, we study high-quality single-crystals of Ni1−x Zn x PS3 (0 < x < 0.2) using temperature-dependent specific heat and Raman spectroscopy probes. We show that in pristine NiPS3, the phonon mode at 176 cm−1 (P2), associated with the vibrations of Ni ions, exhibits a distinct Fano asymmetry. The Fano resonance is particularly pronounced in the paramagnetic phase above TN, which was further confirmed by temperature dependent Raman data on the Zn-doped crystals. In the Zn-doped crystals, while the magnetism weakens following the mean-field prediction for site-dilution in a honeycomb lattice, the Fano coupling |1/q| strengthens, increasing monotonically with increasing Zn-doping. The x-ray photoemission spectra suggest an increase in the weight of the d9 and d10 components in the Zn-doped crystals. These observations indicate the presence of strong electron–phonon coupling in Ni1−x Zn x PS3, in addition to the spin-phonon, and spin-charge coupling previously reported.

The Evident of Coupled Spin and Electron-phonon Interactions in Cd2Os2O7 Pyrochlore

We present the results of the temperature dependence of the Raman spectra of pyrochlore Cd2Os2O7 single crystal in a temperature range of 100-270 K. We found that, among the six observed Raman active modes, the 762 cm-1 phonon has a significant peak shift and asymmetric Fano lineshape. The temperature-dependent of the frequency and full width at half maximum (FWHM) of the phonon shows the kinks near the Néel temperature. The complex temperature-dependent asymmetry Fano parameter and the anomalies of the FWHM below Néel temperature indicate coupling of the 762 cm-1 phonon to both electron and spin degrees of freedom. In addition, we found the Fano asymmetry rapidly drops below the Néel temperature towards its minimum value of ~ 200 K. Thus, at temperature ranging from 200 K to TN, the pyrochlore is suggested to be in an all-in -all-out ordered semimetallic state. We also confirm that Raman scattering temperature dependence provides a simple and powerful method for investigating temperature transitions and coupling in pyrochlore materials.&#x0D; &#x0D; &#x0D;

Unconventional room-temperature carriers in the triangular-lattice Mott insulator TbInO3

The strong correlations between electrons in Mott insulator materials may produce highly entangled many-body states with unconventional emergent excitations. The signatures of such excitations, if any, are commonly believed to be observable only at low temperatures. Here, we challenge this common belief and show using terahertz time-domain spectroscopy that exotic carriers exist even at room temperature in TbInO3, a candidate material for realizing a low-temperature quantum spin-liquid phase. In particular, over the entire temperature range of 1.5–300 K, we observe a quadratic frequency dependence in the real part of the in-plane optical conductivity as well as Fano asymmetry of an optical phonon mode strongly interacting with the excitation continuum. These features are robust even under external magnetic fields of up to 7 T. Our data confirm the presence of emergent charge carriers within the Mott charge gap of TbInO3, suggesting that it is possible to probe and manipulate highly entangled quantum many-body states at room temperature.

Weak-localization effect in Fano asymmetry of C implanted rutile TiO2 nanostructure

Fano resonance is the degree of asymmetry that describes the interference of continuum states with discrete states and causes asymmetric line shape in the optical response. The hydrothermally synthesized TiO 2 nanostructures in this work are doped with 1.5 MeV carbon (C) ion beams to explore the Fano effect via the asymmetry in the Raman spectra. It is observed that the E g and A 1 g modes are asymmetric toward the lower wavenumber. The Fano effect is responsible for the peak asymmetry, which rises with C ion fluence. The Breit–Wigner–Fano line shape fits well the E g and A 1 g modes. The calculated asymmetry parameter (1/q) for implanted TiO 2 nanostructures represents weakly localized asymmetry with temperature. The electron–phonon coupling strength grows with the C ion fluence. The effects of temperature and defect level on the asymmetry parameter are also discussed.

Fano asymmetry in zero-detuned exciton-plasmon systems.

Plasmonic resonances in metallic nanostructures can strongly enhance the emission from quantum emitters, as commonly used in surface-enhanced spectroscopy techniques. The extinction and scattering spectrum of these quantum emitter-metallic nanoantenna hybrid systems are often characterized by a sharp Fano resonance, which is usually expected to be symmetric when a plasmonic mode is resonant with an exciton of the quantum emitter. Here, motivated by recent experimental work showing an asymmetric Fano lineshape under resonant conditions, we study the Fano resonance found in a system composed of a single quantum emitter interacting resonantly with a single spherical silver nanoantenna or with a dimer nanoantenna composed of two gold spherical nanoparticles. To analyze in detail the origin of the resulting Fano asymmetry we develop numerical simulations, an analytical expression that relates the asymmetry of the Fano lineshape to the field enhancement and to the enhanced losses of the quantum emitter (Purcell effect), and a set of simple models. In this manner we identify the contributions to the asymmetry of different physical phenomena, such as retardation and the direct excitation and emission from the quantum emitter.

Laser-induced Fano asymmetry, electron-phonon coupling, and phase transition in lanthanide sesquioxide (Ln2O3; Ln = Eu, Gd, Dy) nanoparticles: A Raman spectroscopic investigation

Laser power-dependent Raman spectroscopy is deployed to probe Fano interference in asymmetrically broadened Tg modes and the associated line shift in three technologically sound, meticulously characterized rare-earth sesquioxide systems. Group theoretical analysis is accompanied to introspect the Raman-active optic modes in cubic, monoclinic, and trigonal phases and identify the laser heating-induced local phase transitions. With increasing laser intensity, a regular redshift and larger negative asymmetry in the Raman peaks are detected, which is attributed to moderations in Fano scattering by enhanced electron–phonon coupling amid the focussed photoexcited electron plasma and is illustrated using a Feynman diagram. A quantitative study is thereby performed to unveil the intrinsic nature of discrete-continuum Fano resonance in the nanoparticles of interest emphasizing the high sensitivity of Raman spectra to the excitation strength that perturbs the generic vibrational features at the Brillouin zone center by influencing the interference conditions, force constant, and length of the associated bonds compelled by tensile stress. A rising trend of the charge–phonon coupling constant (λ) with laser power validates a stronger particle–quasiparticle coupling, whereas a shorter anharmonic phonon lifetime (τanh) indicates faster interactions. Using Allen's formalism, the charge density of states [N(εF)] at the Fermi level per spin and molecule is calculated, which pertains to a negative regression dependence in the λN(εF)−τanh dynamics.

Raman fingerprints of fractionalized Majorana excitations in the honeycomb iridate Ag3LiIr2O6

We report low-temperature (down to $\sim$5 K) Raman signatures of the recently discovered intercalated honeycomb magnet Ag$_3$LiIr$_2$O$_6$, a putative Kitaev quantum spin liquid (QSL) candidate. The Kitaev QSL is predicted to host Majorana fermions as its emergent elementary excitations through a thermal fractionalization of entangled spins $S = 1/2$. We observe evidence of this fractionalization in the low-energy magnetic continuum whose temperature evolution harbours signatures of the predicted Fermi statistics obeyed by the itinerant Majorana quasiparticles. The magnetic Raman susceptibility evinces a crossover from the conventional to a Kitaev paramagnetic state below the temperature of $\sim$80 K. Additionally, the development of the Fano asymmetry in the low frequency phonon mode and the enhancement of integrated Raman susceptibilities below the crossover temperature signifies prominent coupling between the vibrational and Majorana fermionic excitations.

Plasmon-induced coherence, exciton-induced transparency, and Fano interference for hybrid plasmonic systems in strong coupling regime.

We present an analytical model describing the transition to a strong coupling regime for an ensemble of emitters resonantly coupled to a localized surface plasmon in a metal-dielectric structure. The response of a hybrid system to an external field is determined by two distinct mechanisms involving collective states of emitters interacting with the plasmon mode. The first mechanism is the near-field coupling between the bright collective state and the plasmon mode, which underpins the energy exchange between the system components and gives rise to exciton-induced transparency minimum in scattering spectra in the weak coupling regime and to emergence of polaritonic bands as the system transitions to the strong coupling regime. The second mechanism is the Fano interference between the plasmon dipole moment and the plasmon-induced dipole moment of the bright collective state as the hybrid system interacts with the radiation field. The latter mechanism is greatly facilitated by plasmon-induced coherence in a system with the characteristic size below the diffraction limit as the individual emitters comprising the collective state are driven by the same alternating plasmon near field and, therefore, all oscillate in phase. This cooperative effect leads to scaling of the Fano asymmetry parameter and of the Fano function amplitude with the ensemble size, and therefore, it strongly affects the shape of scattering spectra for large ensembles. Specifically, with increasing emitter numbers, the Fano interference leads to a spectral weight shift toward the lower energy polaritonic band.

Vibrational and electronic structures of tin selenide nanowires confined inside carbon nanotubes

We study vibrational and electronic properties of tin selenide (SnSe) nanowires encapsulated in single walled carbon nanotubes (SWCNT) by combining experimental Raman spectroscopy and density functional theory (DFT) calculations at the Heyd-Scuseria-Ernzerhof (HSE) level. The theoretically investigated standalone SnSe nanowires are Sn4Se4 with square (2 × 2) atomic arrangement and Sn6Se6 with a repeating hexagonal Mo6S6-like structure. Raman data support the theoretical prediction that the square (2 × 2) nanowires possess specific modes at 151 and 185 cm−1, whereas the hexagonal Sn6Se6 structure is characterized by a mode appearing at ~ 235 cm−1. Calculations predict that the (2 × 2) nanowire has an electronic gap of 1.5 eV and the Sn6Se6 nanowire presents a semi-metallic character. Raman spectra of composite SnSe@SWCNT samples show that the radial breathing mode of the nanotubes is strongly suppressed indicating interaction between SWCNT and the encapsulated SnSe nanowire while the Fano asymmetry parameter of the G band is increased.

Fano resonance of optical phonons in a multilayer graphene stack

We studied Fano interference between the Raman spectrum of G-band phonons and electron continuum in a multilayer graphene stack. The thickness and power dependencies of the Fano interference coefficient ∣1/q∣ in the G-mode, where q is the Fano asymmetry parameter, were spatially visualized and analyzed using the Gaussian-convoluted Breit–Wigner–Fano function. The estimated ∣1/q∣ decreases with an increase in the layer number and laser power in the low-power region at least for monolayer, bilayer, and trilayer graphene. In the higher-power region, ∣1/q∣ increases with power only for monolayer graphene. The observed behaviors of ∣1/q∣ reflect the phase difference of Raman signals from the electron continuum and G-band and possibly originate from changes in the electronic relaxation time and the Fermi level of graphene owing to the laser heating of the sample.

Micro-Raman investigation of p-type B doped Si(1 0 0) revisited

The doping concentration of B doped single-crystal Czochralski Si(1 0 0) wafers (6 × 1014-5 × 1019 cm−3) has been monitored via micro-Raman spectroscopy using visible (633 and 532 nm) and near-UV (355 nm) laser excitations at low power (5 mW). Data have been analysed with unprecedented accuracy via a convoluted Fano-Gaussian model of the first-order Raman Stokes mode of Silicon. This allowed the determination of the fitting spectral parameters (peak position and width) with an accuracy of 0.01 cm−1, which enables a reliable probing of the concentration. We observed, independently on the excitation wavelength used, a widening (up to 6.5 cm−1), a frequency-softening (up to 1.5 cm−1) and an intensity reduction (down to 90%) of the Si peak with the doping concentration. The widening and frequency-softening follow a strictly linear dependence with doping concentration, allowing a calibration. A linear dependence of the reciprocal Fano asymmetry parameter (q-1) with excitation energy is verified, with the slope showing a linear behavior with the doping concentration and providing a direct estimate on the hole-phonon interaction strength. Results are reproduced with surface-sensitive near-UV Raman spectroscopy on BF2+ ion implanted and laser thermal annealed (LTA) Si, demonstrating the full portability of the Raman technique to state-of-the-art nanoelectronics.

Influence of the surface roughness on electron-phonon interaction in an intrinsic CdTe single crystal

In this work, we found that the electron-phonon interaction (EPI) is dependent on the surface roughness. Simulations of Resonance Raman Spectroscopy (RRS) spectra, show that the Fano asymmetry parameter q, decreases as the Mean Surface Roughness (MSR) increases. They also make it clear, that the magnitude of the EPI strength increases with increasing roughness. Our results also show that the EPI along with the surface morphology in an undoped CdTe sample allow the manifestation of other phonons in the RRS spectra. This work contributes to understand and potentially control the EPI in semiconductors.

Fano effect in resonant Raman spectrum of CdTe

In this work, we show a line-shape analysis of the resonant Raman scattering spectrum of un-doped CdTe single crystal. We found that the main peak correspondent to the longitudinal-optic (LO) phonon mode is asymmetrically broadened toward the high-frequency side. We attributed this asymmetry to an electron-phonon coupling effect, which was demonstrated using a Fano-type function to describe the spectrum line-shape. In our fitting we have additionally considered the weak contribution (in the low-frequency region) of two added phonons, whose presence is attributed to the rupture of the symmetry due to the surface. Results show an excellent agreement between the experimental data and the fitting model, where Fano asymmetry parameter was found to be q=4.5. The results obtained, are a tool that contributes to the understanding of the origin of LO phonon line-shape asymmetry of the Raman spectrum; and its use will be helpful to investigate the strength of electron-phonon coupling in semiconductors.

Raman scattering study of two-dimensional magnetic van der Waals compound VI3**Project supported by the Science Fund from the Ministry of Science and Technology of China (Grant Nos. 2017YFA0302904 and 2016YFA0300504), the National Natural Science Foundation of China (Grant Nos. 11774419, U1932215, 11774423, and 11822412), the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University

The layered magnetic van der Waals materials have generated tremendous interest due to their potential applications and importance in fundamental research. Previous x-ray diffraction (XRD) studies on the magnetic van der Waals compound VI3, revealed a structural transition above the magnetic transition but output controversial analysis on symmetry. In this paper we carried out polarized Raman scattering measurements on VI3 from 10 K to 300 K, with focus on the two Ag phonon modes at ∼ 71.1 cm−1 and 128.4 cm−1. Our careful symmetry analysis based on the angle-dependent spectra demonstrates that the crystal symmetry can be well described by C2h rather than D3d both above and below structural phase transition. We further performed temperature-dependent Raman experiments to study the magnetism in VI3. Fano asymmetry and anomalous linewidth drop of two Ag phonon modes at low temperatures, point to a significant spin–phonon coupling. This is also supported by the softening of 71.1-cm−1 mode above the magnetic transition. The study provides the fundamental information on lattice dynamics and clarifies the symmetry in VI3. And spin–phonon coupling existing in a wide temperature range revealed here may be meaningful in applications.

Terahertz Fano resonances induced by combining metamaterial modes of the same symmetry.

Fano resonances are observed in a composite metamaterial that consists of an electric split ring resonator eSRR and an I-shaped resonator ISR. By adjusting the length of the ISR the degree of asymmetry in the line shape of the composite metamaterial can be controlled and even made to be symmetric. In contrast to other methods to create Fano resonances, the individual modes of the eSRR and ISR have the same symmetry and are not evanescently coupled to each other. The transmission is simulated using the finite difference time domain method and a coupled oscillator model is used to obtain nominal values of the Fano asymmetry factor q. Composite metamaterials and individual eSRR and ISR metamaterials are fabricated, and their transmission is measured with terahertz time-domain spectroscopy.

Uncovering electron-phonon scattering and phonon dynamics in type-I Weyl semimetals

Weyl semimetals are 3D phases of matter with topologically protected states that have remarkable macroscopic transport behaviors. As phonon dynamics and electron-phonon scattering play a critical role in the electrical and thermal transport, we pursue a fundamental understanding of the origin of these effects in type-I Weyl semimetals NbAs and TaAs. In the temperature-dependent Raman spectra of NbAs, we reveal a previously unreported Fano lineshape, a signature stemming from the electron-phonon interaction. Additionally, the temperature dependence of the $A_{1}$ phonon linewidths in both NbAs and TaAs strongly deviate from the standard model of anharmonic decay. To capture the mechanisms responsible for the observed Fano asymmetry and the atypical phonon linewidth, we present first principles calculations of the phonon self-energy correction due to the electron-phonon interaction. Finally, we investigate the relationship between Fano lineshape, electron-phonon coupling, and locations of the Weyl points in these materials. Through this study of the phonon dynamics and electron-phonon interaction in these Weyl semimetals, we consider specific microscopic pathways which contribute to the nature of their macroscopic transport.

Phonon anomalies in pyrochlore iridates studied by Raman spectroscopy

We report polarization-resolved Raman scattering measurements on single crystals of the pyrochlore compound ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ which exhibits a ground state with noncollinear magnetic order, as well as its paramagnetic counterpart ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. The spectra reveal the six Raman-active optical phonons expected for the pyrochlore lattice symmetry. Combined with density functional calculations, polarization analysis of the Raman intensity allows us to assign all observed peaks to specific vibration patterns. Whereas most phonon profiles are weakly temperature dependent, an Ir-O-Ir bond-bending vibration in ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ exhibits a pronounced Fano asymmetry in the paramagnetic state, and marked softening and line-shape anomalies at the magnetic phase transition. These observations indicate strong electron-phonon interactions, which must be considered in models of the electronic properties and phase behavior of the pyrochlore iridates. In addition, a prominent feature appears at 210 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the Raman spectrum of ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ below the magnetic transition temperature, possibly due to a magnetic excitation.

Correlated paramagnetism and interplay of magnetic and phononic degrees of freedom in 3d-5d coupled La2CuIrO6

Conventional paramagnetism–a state with finite magnetic moment per ion sans long range magnetic ordering, but with lowering temperature the moment each ion picks up a particular direction, breaking spin rotational symmetry, and results into long-range magnetic ordering. However, in systems with competing multiple degrees of freedom this conventional notion may easily break and results into short range correlation much above the global magnetic transition temperature. La2CuIrO6 with complex interplay of spins (s = 1/2) on Cu site and pseudo-spin (j = 1/2) on Ir site owing to strong spin–orbit coupling provides fertile ground to observe such correlated phenomena. By a comprehensive temperature dependent Raman study, we have shown the presence of such a correlated paramagnetic state in La2CuIrO6 much above the long-range magnetic ordering temperature (TN). Our observation of strong interactions of phonons, associated with Cu/Ir octahedra, with underlying magnetic degrees of freedom mirrored in the observed Fano asymmetry, which remarkably persists as high as ~3.5TN clearly signals the existence of correlated paramagnetism hence broken spin rotational symmetry. Our detailed analysis also reveals anomalous changes in the self-energy parameters of the phonon modes, i.e. mode frequencies and linewidth, below TN, providing a useful gauge for monitoring the strong coupling between phonons and magnetic degrees of freedom.

Nanoscale n++-p junction formation in GeOI probed by tip-enhanced Raman spectroscopy and conductive atomic force microscopy

Ge-on-Si and Ge-on-insulator (GeOI) are the most promising materials for the next-generation nanoelectronics that can be fully integrated with silicon technology. To this day, the fabrication of Ge-based transistors with a n-type channel doping above 5 × 1019 cm−3 remains challenging. Here, we report on n-type doping of Ge beyond the equilibrium solubility limit (ne ≈ 6 × 1020 cm−3) together with a nanoscale technique to inspect the dopant distribution in n++-p junctions in GeOI. The n++ layer in Ge is realized by P+ ion implantation followed by millisecond-flashlamp annealing. The electron concentration is found to be three times higher than the equilibrium solid solubility limit of P in Ge determined at 800 °C. The millisecond-flashlamp annealing process is used for the electrical activation of the implanted P dopant and to fully suppress its diffusion. The study of the P activation and distribution in implanted GeOI relies on the combination of Raman spectroscopy, conductive atomic force microscopy, and secondary ion mass spectrometry. The linear dependence between the Fano asymmetry parameter q and the active carrier concentration makes Raman spectroscopy a powerful tool to study the electrical properties of semiconductors. We also demonstrate the high electrical activation efficiency together with the formation of ohmic contacts through Ni germanidation via a single-step flashlamp annealing process.

Alkali Metal Doped Mono- and Bilayer Graphene: Band Structure, Raman Spectrum and Electronic Transport

We use ultra-high vacuum (UHV) Raman spectroscopy in tandem with angle-resolved photoemission (ARPES) to investigate the doping-dependent Raman spectrum of epitaxially grown graphene. The evolution of the Raman spectra from pristine to heavily Cs doped graphene up to a carrier concentration of 4.4x10^14 cm-2 is investigated [1]. Renormalization effects reduce the electronic bandwidth at this doping level when graphene is at the onset of the Lifshitz transition. Ultraviolet (UV) light allows to probe the optical transition at the saddle point in the Brillouin zone achieving resonance Raman conditions in close vicinity to the van Hove singularity in the joint density of states. The position of the Raman G band of fully doped graphene/Ir(111) shifts down by 60 cm-1 and the G band asymmetry of Cs doped epitaxial graphene on Ir(111) assumes an unusual strong Fano asymmetry opposite to that of the G band of doped graphene on insulators. We compare these results on Cs doped monolayer graphene to studies of Cs doped bilayer graphene. Growth of bilayer graphene is realized by intercalation of carbon atoms under monolayer graphene using molecular beam epitaxy. We probe each step of the intercalation process using UHV Raman which reveals distinct changes in the linshape. Finally, the graphene is transferred from the metal substrate onto Si wafers and probed by UHV electronic transport during alkali metal deposition. Our work demonstrates high sensitivity towards sensing of alkali atoms and a giant decrease of the four-point resistance by three orders of magnitude upon Cs doping. [1] M.G.Hell at al. Nano Letters 2018 18 (9), 6045-6056 Figure 1