arXiv subjects

Omer Yaffe

Publications and source records attributed to Omer Yaffe.

At least 19 recordsLinked to original sources

arXiv2026

Resolving Transient Electron-Phonon Coupling with Time-Resolved Spontaneous Raman Spectroscopy

Understanding the interaction of charge carriers with lattice vibrations in the quasi-equilibrium regime is crucial for semiconductor functionality. However, the structural signatures of these interactions are often too subtle for conventional ultrafast techniques to detect. We developed a time-resolved spontaneous Raman technique based on time-correlated single-photon counting to track the spectral response following photoexcitation, providing sub-wavenumber spectral resolution and a few-hundred-picosecond temporal resolution. Unlike traditional pump-probe schemes, our method utilizes a modulated continuous-wave probe to maintain high spectral resolution, enabling detection of low-frequency Raman shifts down to 10 cm$^{-1}$. Applied to lightly boron-doped silicon, we resolve intra-valence band and inter-valence band electronic transitions. A coupled-mode analysis of transient phonon asymmetry, resulting from interference with the inter-valence band transitions, reveals electron-phonon coupling parameters that directly relate to carrier recombination. By capturing these subtle dynamical shifts, we demonstrate that this platform offers a powerful probe for investigating electron-phonon interactions in long-lived excited states.

cond-mat.mtrl-sci↗

arXiv2024

Ab initio theory of the non-resonant Raman effect in crystals at finite temperature in comparison to experiment: The examples of GaN and BaZrS3

We present an ab initio theory of the non-resonant Raman scattering process in crystals at finite temperature in direct comparison with experiments. The theory incorporates the scattering geometry and polarization dependence of the Raman process and the small but finite wave vectors of the phonons for correctly describing the scattering with longitudinal optical (LO) modes in optically anisotropic solids. We implement the theory for first-order Raman scattering and showcase the approach for wurtzite Gallium Nitride and the complex chalcogenide perovskite BaZrS3 in comparison to experiment. We subsequently discuss several common estimates for second-order Raman scattering in complex materials, and highlight similarities and differences to established theoretical approaches and simulation protocols both from phonon theory and molecular dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Noncollinear electric dipoles in a polar, chiral phase of CsSnBr$_3$ perovskite

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two unstudied phases of the 3D perovskite, CsSnBr$_3$, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K<$T$<85 K, space group $P2_1/m$) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I ($T$<77 K, space group $P2_1$) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and a large, intrinsic polarization$-$electrostriction coefficient along the polar axis ($Q_{22}\approx$1.1 m$^4$ C$^{-2}$), resulting in acute negative thermal expansion ($\alpha_V=-9\times10^{-5}$ K$^{-1}$) through the onset of spontaneous polarization. The unprecedented structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and $^{81}$Br nuclear quadrupole resonance spectroscopies. Relativistic electronic structure scenarios compatible with reported photoluminescence measurements are discussed. Together, the polar symmetry, small bandgap, large spin-orbit splitting of Sn 5$p$ orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr$_3$ or its neighboring solid solutions as strong candidates for bulk Rashba effects.

cond-mat.mtrl-sci↗

arXiv2023

Disentangling the Effects of Structure and Lone-Pair Electrons in the Lattice Dynamics of Halide Perovskites

Metal halide perovskites have shown great performance as solar energy materials, but their outstanding optoelectronic properties are paired with unusually strong anharmonic effects. It has been proposed that this intriguing combination of properties derives from the "lone pair" 6$s^2$ electron configuration of the Pb$^{2+}$ cations, and associated weak pseudo-Jahn-Teller effect, but the precise impact of this chemical feature remains unclear. Here we show that in fact an $ns^2$ electron configuration is not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in this class of materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and first-principles molecular dynamics calculations to directly contrast the lattice dynamics of CsSrBr$_3$ with those of CsPbBr$_3$, two compounds which bear close structural similarity but with the former lacking the propensity to form lone pairs on the 5$s^0$ octahedral cation. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint to detect anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This work highlights the key role of structure in perovskite lattice dynamics, providing important design rules for the emerging class of soft perovskite semiconductors for optoelectronic and light-harvesting devices.

cond-mat.mtrl-sci↗

arXiv2023

Anharmonic Fluctuations Govern the Band Gap of Halide Perovskites

We determine the impact of anharmonic thermal vibrations on the fundamental band gap of CsPbBr$_3$, a prototypical model system for the broader class of halide perovskite semiconductors. Through first-principles molecular dynamics and stochastic calculations, we find that anharmonic fluctuations are a key effect in the electronic structure of these materials. We present experimental and theoretical evidence that important characteristics, such as a mildly changing band-gap value across a temperature range that includes phase-transitions, cannot be explained by harmonic phonons thermally perturbing an average crystal structure and symmetry. Instead, the thermal characteristics of the electronic structure are microscopically connected to anharmonic vibrational contributions to the band gap that reach a fairly large magnitude of 450 meV at 425 K.

cond-mat.mtrl-sci↗

arXiv2022

Static and Dynamic Disorder in Formamidinium Lead Bromide Single Crystals

We show that formamidinium lead bromide is unique among the halide perovskite crystals because its inorganic sub-lattice exhibits intrinsic local static disorder that co-exists with a well-defined average crystal structure. Our study combines THz-range Raman-scattering with single-crystal X-ray diffraction and first-principles calculations to probe the inorganic sub-lattice dynamics evolution with temperature in the range of 10-300 K. The temperature evolution of the Raman spectra shows that low-temperature, local static disorder strongly affects the crystal's structural dynamics and phase transitions at higher temperatures.

cond-mat.mtrl-sci↗

arXiv2022

The Disorder Origin of Raman Scattering In Perovskites Single Crystals

The anharmonic lattice dynamics of oxide and halide perovskites play a crucial role in their mechanical and optical properties. Raman spectroscopy is one of the key methods used to study these structural dynamics. However, despite decades of research, existing interpretations cannot explain the temperature dependence of the observed Raman spectra. We demonstrate the non-monotonic evolution with temperature of the scattering intensity and present a model for 2nd-order Raman scattering that accounts for this unique trend. By invoking a low-frequency anharmonic feature, we are able to reproduce the Raman spectral line-shapes and integrated intensity temperature dependence. Numerical simulations support our interpretation of this low-frequency mode as a transition between two minima of a double-well potential surface. The model can be applied to other dynamically disordered crystal phases, providing a better understanding of the structural dynamics, leading to favorable electronic, optical, and mechanical properties in functional materials.

cond-mat.mtrl-sci↗

arXiv2022

Dynamic disorder in Bi sub-lattice of $\delta$-Bi$_2$O$_3$

$\delta$-Bi$_2$O$_3$ is one of the fastest known solid oxide ion conductors owing to its intrinsically defective fluorite-like structure with 25\% vacant sites in the O sub-lattice. Numerous diffraction measurements and molecular dynamics simulations indicate that the Bi ions construct a cubic, fcc lattice, and the O ions are %distributing and migrating through it. Nonetheless, in this study we present Raman scattering measurements that clearly show that the Bi sub-lattice preserves the monoclinic symmetry of the low temperature phase ($\alpha$-Bi$_2$O$_3$) up to the melting temperature of the crystal. The apparent contradiction between our observations and previous findings suggests that Bi ions oscillate between local minima of the effective potential energy surface. These minima represent the monoclinic phase while the time-averaged structure is the cubic phase. We discuss the implication of these low-frequency oscillations on ion conduction.

cond-mat.mtrl-sci↗

arXiv2022

Phonon-phonon interactions in the polarizarion dependence of Raman scattering

We have found that the polarization dependence of Raman scattering in organic crystals at finite temperatures can only be described by a fourth-rank formalism. This generalization of the second-rank Raman tensor $\mathcal{R}$ stems from the effect of off-diagonal components in the crystal self-energy on the light scattering mechanism. We thus establish a novel manifestation of phonon-phonon interaction in inelastic light scattering, markedly separate from the better-known phonon lifetime.

cond-mat.mtrl-sci↗

arXiv2022

Anharmonic Lattice Dynamics in Sodium Ion Conductors

We employ THz-range temperature-dependent Raman spectroscopy and first-principles lattice-dynamical calculations to show that the undoped sodium ion conductors Na$_3$PS$_4$ and isostructural Na$_3$PSe$_4$ both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice -- Na$^+$ ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na$_3$PSe$_4$ shows almost purely displacive character with the soft modes disappearing in the cubic phase as the change of symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na$_3$PS$_4$ instead shows order-disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining active in Raman in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice -- mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity in these Na$^+$ ion conductors.

cond-mat.mtrl-sci↗

arXiv2021

The dielectric response of rock-salt crystals at finite temperatures from first principles

We combine ab initio simulations and Raman scattering measurements to demonstrate explicit anharmonic effects in the temperature dependent dielectric response of a NaCl single crystal. We measure the temperature evolution of its Raman spectrum and compare it to both a quasi-harmonic and anharmonic model. Results demonstrate the necessity of including anharmonic lattice dynamics to explain the dielectric response of NaCl, as it is manifested in Raman scattering. Our model fully captures the linear dielectric response of a crystal at finite temperatures and may therefore be used to calculate the temperature dependence of other material properties governed by it.

cond-mat.mtrl-sci↗

arXiv2021

Nanofibers coated with Rare-Earth complexes

Crystals and fibers doped with Rare Earth (RE) ions provide the basis to most of today's solid-state optical systems, from lasers and telecom devices to emerging potential quantum applications such as quantum memories and optical to microwave conversion. The two platforms, doped crystals and doped fibers, seem mutually exclusive, each having its own strengths and limitations- the former providing high homogeneity and coherence, and the latter offering the advantages of robust optical waveguides. Here we present a hybrid platform that does not rely on doping but rather on coating the waveguide - a tapered silica optical fiber - with a monolayer of complexes, each containing a single RE ion. The complexes offer an identical, tailored environment to each ion, thus minimizing inhomogeneity and allowing tuning of their properties to the desired application. Specifically, we use highly luminescent Yb$^{+3}$[Zn(II)$_{MC}$(QXA)] complexes, which isolate the RE ion from the environment and suppress non-radiative decay channels. We demonstrate that the beneficial optical transitions of the Yb$^{+3}$ are retained after deposition on the tapered fiber, and observe an excited-state lifetime of over 0.9 ms, on par with state-of-the-art Yb doped inorganic crystals.

physics.optics↗

arXiv2020

Fast and anomalous exciton diffusion in two-dimensional hybrid perovskites

Two-dimensional hybrid perovskites are currently in the spotlight of condensed matter and nanotechnology research due to their intriguing optoelectronic and vibrational properties with emerging potential for light-harvesting and -emitting applications. While it is known that these natural quantum wells host tightly bound excitons, the mobilities of these fundamental optical excitations at the heart of the optoelectronic applications are still largely unexplored. Here, we directly monitor the diffusion of excitons through ultrafast emission microscopy from liquid helium to room temperature in hBN-encapsulated two-dimensional hybrid perovskites. We find very fast diffusion with characteristic hallmarks of free exciton propagation for all temperatures above 50 K. In the cryogenic regime we observe nonlinear, anomalous behavior with an exceptionally rapid expansion of the exciton cloud followed by a very slow and even negative effective diffusion. We discuss our findings in view of efficient exciton-phonon coupling, highlighting two-dimensional hybrids as promising platforms for many-body physics research and optoelectronic applications on the nanoscale.

cond-mat.mes-hall↗

arXiv2020

The tetragonal phase of CH$_{3}$NH$_{3}$PbI$_{3}$ is strongly anharmonic

Halide perovskite (HP) semiconductors exhibit unique strong coupling between the electronic and structural dynamics. The high-temperature cubic phase of HPs is known to be entropically stabilized, with imaginary frequencies in the calculated phonon dispersion relation. Similar calculations, based on the static average crystal structure, predict a stable tetragonal phase with no imaginary modes. This work shows that in contrast to standard theory predictions, the room-temperature tetragonal phase of CH$_{3} $NH$_{3} $PbI$_{3}$ is strongly anharmonic. We use Raman polarization-orientation (PO) measurements and \textit{ab initio} molecular dynamics (AIMD) to investigate the origin and temperature evolution of the strong structural anharmonicity throughout the tetragonal phase. Raman PO measurements reveal a new spectral feature that resembles a soft mode. This mode shows an unusual continuous increase in damping with temperature which is indicative of an anharmonic potential surface. The analysis of AIMD trajectories identifies two major sources of anharmonicity: the orientational unlocking of the [CH$_{3} $NH$_{3}$]$^+$ ions and large amplitude octahedral tilting that continuously increases with temperature. Our work suggests that the standard phonon picture cannot describe the structural dynamics of tetragonal CH$_{3} $NH$_{3} $PbI$_{3}$.

cond-mat.mtrl-sci↗

arXiv2020

Anharmonic Host Lattice Dynamics Enable Fast Ion Conduction in Superionic AgI

Basic understanding of the driving forces of ion conduction in solids is critical to the development of new solid-state ion conductors. Physical understanding of ion conduction is limited due to strong deviations from harmonic vibrational dynamics in these systems that are difficult to characterize experimentally and theoretically. We overcome this challenge in superionic AgI by combining THz-frequency Raman polarization-orientation measurements and ab-initio molecular dynamics computations. Our findings demonstrate clear signatures of strong coupling between the mobile ions and host lattice that are of importance to the diffusion process. We first derive a dynamic structural model from the Raman measurements that captures the simultaneous crystal-like and fluid-like properties of this fast-ion conductor. Then we show and discuss the importance of anharmonic relaxational motion that arises from the iodine host lattice by demonstrating its strong impact on ion conduction in superionic AgI.

cond-mat.mtrl-sci↗

arXiv2020

Anharmonic Lattice Vibrations in Small-Molecule Organic Semiconductors

The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, we use polarization-orientation (PO) Raman measurements to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, we show that at 10 K the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows a subtle phase transition between 80-150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. Our findings lay the groundwork for accurate predictions and new design rules for high-mobility organic semiconductors at room temperature.

cond-mat.mtrl-sci↗

arXiv2019

Halide perovskites under polarized light: Vibrational symmetry analysis using polarized Raman

In the last decade, hybrid organic-inorganic halide perovskites have emerged as a new type of semiconductor for photovoltaics and other optoelectronic applications. Unlike standard, tetrahedrally bonded semiconductors (e.g. Si and GaAs), the ionic thermal fluctuations in the halide perovskites (i.e. structural dynamics) are strongly coupled to the electronic dynamics. Therefore, it is crucial to obtain accurate and detailed knowledge about the nature of atomic motions within the crystal. This has proved to be challenging due to low thermal stability and the complex, temperature dependent structural phase sequence of the halide perovskites. Here, these challenges are overcome and a detailed analysis of the mode symmetries is provided in the low-temperature orthorhombic phase of methylammonium-lead iodide. Raman measurements using linearly- and circularly- polarized light at 1.16 eV excitation are combined with density functional perturbation theory (DFPT). By performing an iterative analysis of Raman polarization-orientation dependence and DFPT mode analysis, the crystal orientation is determined. Subsequently, accounting for birefringence effects detected using circularly polarized light excitation, the symmetries of all the observed Raman-active modes at 10 K are assigned.

cond-mat.mtrl-sci↗

arXiv2018

Liquid-like Free Carrier Solvation and Band Edge Luminescence in Lead-Halide Perovskites

We report a strongly temperature dependent luminescence Stokes shift in the electronic spectra of both hybrid and inorganic lead-bromide perovskite single crystals. This behavior stands in stark contrast to that exhibited by more conventional crystalline semiconductors. We correlate the electronic spectra with the anti-Stokes and Stokes Raman vibrational spectra. Dielectric solvation theories, originally developed for excited molecules dissolved in polar liquids, reproduce our experimental observations. Our approach, which invokes a classical Debye-like relaxation process, captures the dielectric response originating from an anharmonic LO phonon at about 20 meV (160 cm-1) in the lead-bromide framework. We reconcile the liquid-like picture with more standard solid-state theories of the Stokes shift in crystalline semiconductors.

cond-mat.mtrl-sci↗