Electron-lattice Interaction Research Articles

Spin and Orbital Symmetry Breakings Central to the Laser-Induced Ultrafast Demagnetization of Transition Metals

The role of spin and orbital rotational symmetry on the laser-induced magnetization dynamics of itinerant-electron ferromagnets was theoretically investigated. The ultrafast demagnetization of transition metals is shown to be the direct consequence of the fundamental breaking of these conservation laws in the electronic system, an effect that is inherent to the nature of spin-orbit and electron-lattice interactions. A comprehensive symmetry analysis is complemented by exact numerical calculations of the time evolution of optically excited ferromagnetic ground states in the framework of a many-body electronic Hamiltonian. Thus, quantitative relations are established between the strength of the interactions that break the rotational symmetries and the time scales that are relevant for the magnetization dynamics.

Opposite pressure impact on electron–phonon coupling in Eu2+ and Ce3+ doped AlN

This paper analyzes the influence of pressure on electron–lattice interactions in the 5d excited states of Ce3+ and Eu2+ in the AlN host based on pressure-dependent photoluminescence and photoluminescence excitation spectra.

Interface Scattering as a Nonlocal Transport Phenomenon in Semiconductors

Macroscopically non-local effects are common in electron transport in semiconductor devices, occurring whenever the mean free path and/or the deBroglie wavelength are not small compared to geometry/flow length scales. When such effects are important, standard diffusion-drift (DD) theory becomes inaccurate and in need of revision, with the best known example being density-gradient (DG) theory wherein a gradient term is added to the electron gas equation of state to approximate the effect of quantum non-locality. Here we consider a similarly motivated gradient correction to the electron-lattice interaction that accounts for non-locality in the transport physics. Versions of DD and DG theory with this correction are discussed, and then are applied to the analysis of long-channel field-effect transistors where they are found to provide a physics-based approach for modeling and understanding interface scattering.

Rheological and optical response of hydroxypropyl methylcellulose under variable temperatures for optical switching based on thermo‐optical effect

AbstractThis work is devoted to investigate novel aspects of the rheological and optical response of hydroxypropyl methylcellulose (HPMC) as a result of temperature variations from 308.15 to 338.15 K. First, new insights on correlation between the solution flow behavior and the film thickness uniformity, under distinct temperatures, are established. Then, the HPMC film is examined from the point of view of some basic surface parameters describing the surface morphology, such as root mean square roughness, surface area ratio, texture direction index, kurtosis, skewness and surface bearing index. The refraction properties of HPMC are reported for the first time as a function of wavelength and temperature. The changes in the optical dispersion parameters as a function of temperature increase reveal the reduction of strength of interband optical transitions from 15.147 to 14.086 eV and a smaller band gap from 6.475 to 6.137 eV. The latter is caused by temperature‐induced dilatation of the polymer lattice and its effect on the electron–lattice interactions. The thermo‐optic coefficient of HPMC is found to be larger than that of other polymers, while its dependence on the wavelength is small, varying from −2.03 × 10−4 K−1 at 489 nm to −2.31 × 10−4 K−1. The resulted properties are corresponding to the demands imposed for materials used in optical switching based on thermo‐optical effect.

Non-local transport effects in semiconductors under low-field conditions

Macroscopically non-local effects arise in semiconductor devices whenever the mean free path and/or the deBroglie wavelength are not small compared to geometry/flow length scales. In such cases standard diffusion-drift (DD) theory becomes inaccurate and in need of revision, with the best known example being density-gradient (DG) theory. Here we consider a similarly motivated gradient correction to the electron-lattice interaction that accounts for non-locality in the transport physics. Versions of DD and DG theory with this correction are discussed and the DD-based approach is illustrated by applying it to the analysis of long-channel field-effect transistors where it provides a new physics-based approach for treating surface scattering.

Combination of ultrafast time-resolved spectroscopy techniques for the analysis of electron dynamics of heliumlike impurity centers in silicon

Helium-like interstitial donors in silicon have larger binding energies compared to their hydrogen-like counterparts. Long lifetimes of the excited states are expected since one-phonon transitions are not available. Here, the authors study their relaxation dynamics at two different free electron laser facilities using photon echo, transient grating, and pump-probe spectroscopy. In contrast to the theoretical predictions and previous experimental studies, it is found that the relaxation rates are extremely high. These results shed new light on the electron-lattice interaction and open the possibility for further studies of similar materials.

Enhancement of superconductivity and its relation to lattice expansion in InxTe (0.84≤x≤1)

The quest to govern the driving forces behind superconductivity and gain control over the superconducting transition temperature $T_{c}$ is as old as the phenomenon itself. Microscopically, this requires a proper understanding of the evolution of electron-lattice interactions in their parameter space. We report such a controlled study on $T_{c}$ in In$_{x}$Te via fine-tuning the In stoichiometry $x$. We find that increasing $x$ from 0.84 to 1 results in an enhancement of $T_{c}$ from 1.3 K to 3.5 K accompanied by an increase of the electron-phonon coupling constant from 0.45 to 0.63. Employing first-principles calculations, we show that this behavior is driven by two factors, each taking the dominant role depending on $x$. For $x\lesssim 0.92$, the major role is played by the density of electronic states at the Fermi level. Above $x\sim 0.92$, the change in the density of states flattens while the enhancement of $T_{c}$ continues. We attribute this to a systematic softening of lattice vibrations, amplifying the electron-phonon coupling, and hence, $T_{c}$.

Ferroelectricity modulates polaronic coupling at multiferroic interfaces

Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework based on screening of the polarization field and depolarizing charges. Additional effects still unexplored are the band dependence of the interfacial charge modulation and the associated changes of the electron-phonon interaction, coupling the charge and lattice degrees of freedom. Here, multiferroic heterostructures of the colossal-magnetoresistance manganite La1-xSrxMnO3 buried under ferroelectric BaTiO3 and PbZrxTi1-xO3 are investigated using soft-X-ray angle-resolved photoemission. The experimental band dispersions from the buried La1-xSrxMnO3 identify coexisting two-dimensional hole and three-dimensional electron charge carriers. The ferroelectric polarization modulates their charge density, affecting the coupling of the 2D holes and 3D electrons with the lattice which forms large Fröhlich polarons inherently reducing mobility of the charge carriers. Our k-resolved results on the orbital occupancy, band filling and electron-lattice interaction in multiferroic oxide heterostructures modulated by the ferroelectric polarization disclose most fundamental physics of these systems needed for further progress of beyond-CMOS ferro-functional electronics.

Band versus Polaron: Charge Transport in Antimony Chalcogenides.

Antimony sulfide (Sb2S3) and selenide (Sb2Se3) are emerging earth-abundant absorbers for photovoltaic applications. Solar cell performance depends strongly on charge-carrier transport properties, but these remain poorly understood in Sb2X3 (X = S, Se). Here we report band-like transport in Sb2X3, determined by investigating the electron–lattice interaction and theoretical limits of carrier mobility using first-principles density functional theory and Boltzmann transport calculations. We demonstrate that transport in Sb2X3 is governed by large polarons with moderate Fröhlich coupling constants (α ≈ 2), large polaron radii (extending over several unit cells), and high carrier mobility (an isotropic average of >10 cm2 V–1 s–1 for both electrons and holes). The room-temperature mobility is intrinsically limited by scattering from polar phonon modes and is further reduced in highly defective samples. Our study confirms that the performance of Sb2X3 solar cells is not limited by intrinsic self-trapping.

Competing electronic states emerging on polar surfaces

Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO2 termination of KTaO3(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.

Dynamic short-range correlation in photoinduced disorder phase transitions

Ultrafast photoexcitation can induce a nonequilibrium dynamic with electron-lattice interaction, offering an effective way to study photoinduced phase transitions (PIPTs) in solids. The issue that atomic displacements after photoexcitation belong to coherent change or disordered process, has become a controversy in the PIPT community. Using real-time time-dependent density functional theory (rt-TDDFT) simulations, we obtain both the coherent and the disordered PIPTs (dimer dissociation) in IrTe2 with the different electronic occupations. More importantly, we found that in the disordered phase transition, there exists a local correlation between different dimers regarding their dissociation status. One can define vertical groups across the layers. The dimers in the same group will dissociate in a correlated fashion: they either all dissociate, or all not dissociate. On the other hand, the dimers in neighboring groups will have an anti-correlation: if the dimers in one group dissociate, the dimers in the neighboring group tend not to be dissociated, and vice versus.

Interplay of the Jahn-Teller effect and spin-orbit coupling: The case of trigonal vibrations

We study an interplay between the orbital degeneracy and spin-orbit coupling giving rise to spin-orbital entangled states. As a specific example, we analyze the interaction of electrons occupying triply degenerate single-ion $t_{2g}$ levels with trigonal vibronic modes (the $t\otimes T$ problem). A more general problem of the electron-lattice interaction involving both tetragonal and trigonal vibrations is also considered. It is shown that the result of such interaction crucially depends on the occupation of $t_{2g}$ levels leading to either the suppression or enhancement of the Jahn-Teller effect by the spin-orbit coupling.

Mechanism of length-induced magnetism in polyacene molecules

Within the celebrated Su-Schrieffer-Heeger model including electron-lattice and electron-electron interactions, the electronic ground state of polyacene ($n$-acene) is studied as a function of molecular length $n$. The results demonstrate that the ground state exhibits a phase transition from a nonmagnetic state to an antiferromagnetic state at a critical length of $n=7$. The magnetism is explained by calculating the lattice distortion and the related average hopping rate of the \ensuremath{\pi} electrons. It is revealed that in polyacenes, there exist two competing mechanisms that minimize the molecular energy, namely the lattice dimerization and the formation of an antiferromagnetic spin density. Since the dimerization is restricted to the regions near the ends of the molecules the former mechanism is dominant for short molecules, which therefore assume a nonmagnetic state. When the length is increased beyond the critical value, the lattice dimerization in the middle of the molecule is reduced and a rapid drop of the average hopping rate occurs. This makes the second mechanism dominant and the associated antiferromagnetic state is stabilized. The effect of different interaction strengths and values of hopping integrals on the critical length is discussed. The results are confirmed by first-principles calculations. Suggestions for the design of organic antiferromagnets with a similar ladder structure are also given.

Spin Engineering of VO2 Phase Transitions and Removal of Structural Transition.

Vanadium dioxide undergoes a metal-to-insulator transition, where the energy of electron-electron, electron-lattice, spin-spin, and spin-lattice interactions are of the same order of magnitude. This leads to the coexistence of electronic and structural transitions in VO2 that limit the lifetime and speed of VO2-based devices. However, the closeness of interaction energy of lattice-electron-spin can be turned into an opportunity to induce some transitions while pinning others via external stimuli. That is, the contribution of spin, charge, orbital, and lattice degrees of freedom can be manipulated. In this study, spin engineering has been exploited to affect the spin-related interactions in VO2 by introducing a ferromagnetic Ni layer. The coercivity in the Ni layer is engineered by controlling the shape anisotropy via kinetics of growth. Using spin engineering, the structural pinning of the monoclinic M2 phase of VO2 is successfully achieved, while the electronic and magnetic transitions take place.

Charge and Lattice Dynamics in Excitonic Insulator Ta2NiSe5 Investigated Using Ultrafast Reflection Spectroscopy

An excitonic insulator, in which an optical gap is formed by exciton condensation, is expected to exhibit large photoresponses. Here, we report the results of femtosecond reflection spectroscopy covering a wide infrared region on a typical excitonic insulator, Ta2NiSe5, which gives direct information not only about a change of excitonic gap but also a metallization. With a weak excitation, the excitonic gap decreases due to the weakening of excitonic effect by photocarriers. With a strong excitation, a Drude-like high-reflection band appears, showing an excitonic-insulator-to-metal transition. Analyses of spectral and temporal changes of reflectivity revealed that the metallization is accompanied by large structural changes. This demonstrates an important role of electron–lattice interaction on the stabilization of excitonic phase.

Model of charge triplets for high-[formula omitted] cuprates

Starting with a minimal model for the CuO2 planes with the on-site Hilbert space reduced to a charge triplet of the three effective valence centers [CuO4]7−,6−,5− (nominally Cu1+,2+,3+) with different conventional spin, different orbital symmetry, and different local lattice configuration, we develop a unified non-BCS spin–pseudospin model to describe the main phase states of doped cuprates. We argue that antiferromagnetic insulating, charge ordered, superconducting, and Fermi-liquid phases are possible phase states of a model parent cuprate, while typical phase state of a doped cuprate, in particular mysterious pseudogap phase, is a result of a phase separation. Superconductivity of cuprates is not a consequence of pairing of doped holes, but the result of quantum transport of on-site composite hole bosons, whereas main peculiarities of normal state can be related to an electron–hole interplay for unusual Fermi-liquid phase and features of the phase separation. Puzzlingly, but it is the electron–lattice interaction, which in the BCS model determines s-wave pairing, in the model of local composite bosons gives dx2−y2-symmetry of the superconducting order parameter, thus showing once again a substantial involvement of the lattice in the cuprate’s HTSC.

Migration dynamics of excitons/biexcitons induced by a funnel-like nonuniform compression strain over organic polymers

As a promising strategy, strain engineering has been employed on different semiconductor materials, aiming to modulate their photoelectric properties, thus innovate device applications. An interesting experimental result is the directional exciton migration induced by creating some local nonuniform strains in materials, such as “exciton funneling.” In this Letter, to clarify the dynamical mechanism and the impacting factors, we theoretically investigate the migration dynamics of excitons/biexcitons along organic polymers induced by a funnel-like nonuniform compression strain. First, the migration dynamics of an exciton/biexciton are separately demonstrated right after the strain created. It is found that both of them will migrate toward the strain center with the speeds up to 7–10 nm/ps, comparable with the experimental observations in a bending ZnO microwire, where the difference between the exciton and biexciton migration dynamics is also emphasized. We attribute the present exciton/biexciton migration mechanism to the strain-induced gradient of the exciton/biexciton creation energy along polymers. Furthermore, some typical factors impacting the exciton migration dynamics are considered, such as the strain gradient, the initial ratio of the exciton located in the strain region, the electron-lattice interaction, etc. Finally, based on these findings, we briefly discuss the possible results about the strain-modulated luminescence of organic polymers.

Cryogenic Electron Microscopy on Strongly Correlated Quantum Materials.

ConspectusQuantum materials refers to a class of materials with exotic properties that arise from the quantum mechanical nature of their constituent electrons, exhibiting, for example, high-temperature superconductivity, colossal magnetoresistivity, multiferroicity, and topological behavior. Quantum materials often have incompletely filled d- or f-electron shells with narrow energy bands, and the conduct of their electrons is strongly correlated. One distinct characteristic of the materials is that their electronic states are often spatially inhomogeneous and thus well suited for study using a spatially resolved electron beam with its great scattering power and sensitivity to atomic ionicity. Furthermore, most of these exotic properties only manifest at very low temperatures, posing a challenge to modern electron microscopy. It requires extraordinarily instrument stabilities at cryogenic temperatures with critical spatial, temporal, and energy resolutions in both static and dynamic manner to probe these materials. On the other hand, the ability to directly visualize the atomic, electronic and spin structures and inhomogeneities of quantum materials and correlate them to their functionalities creates enormous opportunities. At the most elementary levels of condensed matter physics, understanding the competing order of electron, spin, orbital, and lattice and their degrees of freedom, the impacts of defects and interfaces, and the site-specific quantum phenomena and phase transitions that give rise to the emergent behaviors allows us to discover and control novel materials for quantum information science and technologies.In this Account, several of our research examples are selected to highlight the use of cryogenic electron microscopy (cryo-EM) to study strongly correlated quantum materials. We focus on the critical roles of heterogeneity, interfaces, defects, and disorder in crystal structure, magnetic structure, and electronic structure to understand the physical properties of the materials that cryo-EM enables. We show how electron crystallography coupled with Bragg diffraction and diffuse scattering analysis empowers us to reveal the nature of structural modulations, lattice distortion, and phonons and how quantitative electron diffraction can be used to map the distributions of the valence electrons that bond atoms together. We exploit transformative advances in imaging capabilities including the use of femtosecond laser and ultrafast electron diffraction to probe electron-lattice interactions and photoinduced transitions beyond equilibrium of matter. We review our Lorentz phase microscopy studies to illustrate the intriguing transformations among various topological chiral spin states under applied magnetic field at various cryogenic temperatures. Finally, we show that atomically resolved imaging and electron energy-loss spectroscopy at 10 K can be used to understand interface-enhanced superconductivity. The wide range of research and progress on quantum materials at low temperature reported here may inspire and attract more researchers in this ever-expanding field of cryo-EM.

Electron-insulator interaction and secondary electron yield at any Kelvin temperature

The universal formulae for λ(χreal, Eg, T) and λ(χreal, Eg, T)p of insulator having less impurities are experimentally proved, where λ(χreal, Eg, T) is the mean escape depth at Kelvin temperature T of secondary electrons emitted from insulators with band gap Eg and efficient electron affinity χreal, λ(χreal, Eg, T)p is the λ(χreal, Eg, T) due to electron-lattice interaction. Based on the mechanism of electron-insulator interaction, experimental data, existing formulae, ESTAR program and formulae for λ(χreal, Eg, T) and λ(χreal, Eg, T)p of insulator having less impurities, the methods of obtaining λ(χreal, Eg, T)e-e, λ(χreal, Eg, T)e-defect and λ(χreal, Eg, T)i are presented, where λ(χreal, Eg, T)e-e is the λ(χreal, Eg, T) due to electron–electron interaction, λ(χreal, Eg, T)e-defect is the λ(χreal, Eg, T) due to electron–defect interaction, λ(χreal, Eg, T)i is the λ(χreal, Eg, T) due to electron-impurity interaction. The λ(χreal, Eg, T)p, λ(χreal, Eg, T)e-e, λ(χreal, Eg, T)e-defect and λ(χreal, Eg, T)i can provide corresponding information regarding electron-insulator interaction. Thus, it can be concluded that the method of investigating electron-insulator interaction by secondary electron emission SEE is successfully presented in this work. According to experimental data, formula for λ(χreal, Eg, T), ESTAR program, calculated parameters of primary range and existing formulae for SEE, the method of calculating the ẟ at different T and in the range of 1.0 keV ≤ Epo ≤ 100.0 keV from insulator having less impurities is successfully presented, where ẟ is secondary electron yield, Epo is incident energy of primary electron.

Anisotropy in cubic UO2 caused by electron-lattice interactions

Despite many years of research, the full complexity of the electron-lattice interactions in ${\mathrm{UO}}_{2}$ is not fully understood. We present x-ray inelastic scattering at low temperature showing that the interaction between electronic degrees of freedom and transverse acoustic phonons is strong only along the reciprocal space direction [100]. The anisotropy is reflected in the phonon-linewidth broadening, which persists also well above the N\'eel temperature. This intrinsic effect infers an anisotropy in the thermal conductivity, which has been observed, but which is formally forbidden in a cubic material. We have no model capable of connecting our experimental observations with the low thermal conductivity of ${\mathrm{UO}}_{2}$ below room temperature.