Electron-phonon Interaction Research Articles

Self-cooling in reverse biased p-InAsSbP/n-InAs0.9Sb0.1 heterostructures

2D thermal radiation distribution together with I–V and L–I characteristics has been measured and analyzed in forward and reverse biased p–n heterostructures based on InAs0.9Sb0.1 and grown onto n-InAs substrates. The measurements revealed a sufficient difference in the temperature distribution onto the sample surface at forward and reverse bias, which is explained by an impact of heat pump operation initiated by an electron–phonon interaction at the p–n junction and diode contacts at U < 0.

Structural Transition Analysis of Bilayer Black Phosphorus under High Pressure via Ultralow-Frequency Raman Spectroscopy.

The unique anisotropic electron-photon and electron-phonon interactions of black phosphorus (BP) set it apart from other isotropic 2D materials. These anisotropic properties can be adjusted by varying the stacking thickness and sequence as well as by applying external pressure and strain. In contrast to multilayer or bulk BP, the effects of pressure on bilayer BP are still not fully elucidated. This study presents experimental evidence that explores the high-pressure response (0-20 GPa) of bilayer BP within the extreme low Raman shift region (5-150 cm-1), aiming at verifying structural phase transitions and stacking sequence variations. Notably, phase transitions were observed at ∼7 GPa for the orthorhombic to rhombohedral transition and at ∼14-16 GPa for a further transition to a simple cubic structure. The effects of pressure on bilayer BP with three distinct stacking sequences (AA, AB, and AC) were analyzed using angular-resolved polarized Raman spectroscopy (ARPR). The recovery of all characteristic Raman modes after the application of pressure substantiates the reversibility of the structural phase transitions in bilayer BP across all three stacking sequences. However, significant alterations in the stacking sequences were observed before and after the application of hydrostatic pressure. Specifically, AA and AC stacking sequences showed a tendency to relax into an optimized interlayer AB stacking structure upon the release of pressure from 20 to 0 GPa. This observed change in the stacking sequences of bilayer BP under pressure is detected for the first time in this work. The pressure-induced modifications in the stacking sequences, and consequently in the anisotropic properties of bilayer BP, highlight its potential as a promising material for pressure- (or strain-) sensitive optoelectronic nanodevices. Additionally, this study offers novel insights into the analysis of anisotropic interlayer interactions in other van der Waals-stacked 2D materials.

Discovery of superconductivity and electron-phonon drag in the non-centrosymmetric Weyl semimetal LaRhGe3

We present an exploration of the effect of electron-phonon coupling and broken inversion symmetry on the electronic and thermal properties of the semimetal LaRhGe3. Our transport measurements reveal evidence for electron-hole compensation at low temperatures, resulting in a large magnetoresistance of 3000% at 1.8 K and 14 T. The carrier concentration is on the order of 1021/cm3 with high carrier mobilities of 2000 cm2/Vs. When coupled to our theoretical demonstration of symmetry-protected almost movable Weyl nodal lines, we conclude that LaRhGe3 supports a Weyl semimetallic state. We discover superconductivity in this compound with a Tc of 0.39(1) K and Bc(0) of 2.2(1) mT, with evidence from specific heat and transverse-field muon spin relaxation. We find an exponential dependence in the normal state electrical resistivity below ~50 K, while Seebeck coefficient and thermal conductivity measurements each reveal a prominent peak at low temperatures, indicative of strong electron-phonon interactions. To this end, we examine the temperature-dependent Raman spectra of LaRhGe3 and find that the lifetime of the lowest energy A1 phonon is dominated by phonon-electron scattering instead of anharmonic decay. We conclude that LaRhGe3 has strong electron-phonon coupling in the normal state, while the superconductivity emerges from weak electron-phonon coupling. These results open up the investigation of electron-phonon interactions in the normal state of superconducting non-centrosymmetric Weyl semimetals.

Dissipation in quantum tunnel junctions

Based on experimental data, we propose a model to evaluate the energy dissipated during quantum tunneling processes in solid-state junctions. This model incorporates a nonlinear friction force expressed in the general form f(x)=γv(x)α, where γ is the frictional coefficient, which is fitted to data. We study this by applying voltages just below the barrier height up to near breakdown voltages. Furthermore, by lowering the temperature and adjusting the applied voltage to the junction, the effect on dissipation caused by the variation in barrier height is examined. We underline that the crucial dependency of dissipation on the fraction of particle energy lost is modulated by two primary mechanisms: the application of voltage and the variation of temperature. The fraction of energy dissipated decreases, in general, for increasing energies of the tunneling particles at a given temperature. However, for a given energy of the tunneling particle, the present work demonstrates a turning point at a temperature of 137 K, after which the dissipated energy starts increasing for higher temperatures. The latter can possibly be due to the increase of electron–phonon interactions, which become predominant over barrier height reduction at higher temperatures, and hence, we identify T = 137 K as a critical temperature for a change in the dissipative characteristics of the solid-state junction under consideration. Notably, the study also identifies significant changes in dissipation parameters, γ and α, above 137 K, exhibiting a linear decline and underscoring the importance of further research at higher temperatures.

Mitigating Band Tailing in Kesterite Solar Absorbers: Ab Initio Quantum Dynamics.

Open-circuit voltage deficits are limiting factors in kesterite solar cells. Addressing this issue by suppressing band tailing and nonradiative charge recombination is essential for enhancing the performance. We employ ab initio nonadiabatic molecular dynamics to elucidate the origin of band tailing and charge losses and propose a mitigation strategy. The simulations show that Cu-Zn disorder, associated with antisite defect clusters [CuZn+ZnCu], is a significant source of band tailing in kesterites, as evidenced by the much larger Urbach energy in disordered than ordered kesterites. Cu-Zn disorder gives rise to new sulfur-centered coordination polyhedra, increases structural inhomogeneity, changes electrostatic potential at sulfur centers, and shifts the S(3p) orbital energy. Differences in the S(3p)/Cu(3d) and S(3p)/Sn(5s) hybridization strengths and the S(3p) orbital energy shift reduce the band gap by 0.37 eV. Furthermore, Cu-Zn disorder enhances vibrational motion of sulfur anions and surrounding cations, increasing band gap fluctuations by 15 meV. The stronger electron-phonon interactions reduce charge carrier lifetimes and limit the kesterite solar cell efficiency. Partial substitution of Zn with Cd facilitates structural ordering and significantly suppresses band tailing, particularly in disordered systems. The improvement can be attributed to the larger atomic radius and mass of Cd, which weakens bonding around the anion, suppresses S-related vibrations within the covalent tetrahedra, and reduces nonadiabatic coupling, thereby increasing charge carrier lifetimes. The reported results establish the key influence of cation disorder on band tailing and reduced charge carrier lifetimes in kesterites and highlight cation disorder engineering as a strategy to achieve high-efficiency kesterite solar cells.

Respective Roles of Electron-Phonon and Electron-Electron Interactions in the Transport and Quasiparticle Properties of SrVO_{3}.

The spectral and transport properties of strongly correlated metals, such as SrVO_{3} (SVO), are widely attributed to electron-electron (e-e) interactions, with lattice vibrations (phonons) playing a secondary role. Here, using first-principles electron-phonon (e-ph) and dynamical mean field theory calculations, we show that e-ph interactions play an essential role in SVO: they govern the electron scattering and resistivity in a wide temperature range down to 30K, and induce an experimentally observed kink in the spectral function. In contrast, the e-e interactions control quasiparticle renormalization and low temperature transport, and enhance the e-ph coupling. We clarify the origin of the near T^{2} temperature dependence of the resistivity by analyzing the e-e and e-ph limited transport regimes. Our work disentangles the electronic and lattice degrees of freedom in a prototypical correlated metal, revealing the dominant role of e-ph interactions in SVO.

Possible Superconductivity for Layered Metal Boride Carbide Compounds MB2C2 (M = Alkali, Alkaline-Earth, or Rare-Earth Metals).

The possible emergence of superconductivity in layered metal boride carbide compounds MB2C2 (M = Sc, Y, Be, Ca) was investigated using density functional theory calculations upon the topology of a boron-carbon network and the nature of the metal. ScB2C2 and YB2C2 show metallic and superconductive properties with low critical temperatures (Tcs). The semiconducting BeB2C2 compound may show superconductivity upon carrier doping with a high Tc of 47.8 K by hole doping─comparable to the structurally related MgB2 superconductor─but with a low Tc by electron doping. In contrast, the semiconducting CaB2C2 compound is predicted to be a superconductor by hole and electron doping but with low Tcs. These differences arise from the spatial distribution of electrons at the Fermi level. For compounds with low Tcs, electrons at the Fermi level are localized primarily on B and C π states perpendicular to the BC layers, experiencing minimal influence from atomic oscillations and resulting in weak electron-phonon interactions. Conversely, for a high Tc, electrons are found in σ-bonding states, leading to strong electron-phonon interactions. Electrons at the Fermi level in boron-carbon σ-bonding states seem to be a prerequisite to expect high Tc superconductivity in this kind of compound.

Phonon Drag Contribution to Thermopower for a Heated Metal Nanoisland on a Semiconductor Substrate.

The possible contribution of phonon drag effect to the thermoelectrically sustained potential of a heated nanoisland on a semiconductor surface was estimated in a first principal consideration. We regarded electrons and phonons as interacting particles, and the interaction cross-section was derived from the basic theory of semiconductors. The solution of the equation of motion for average electrons under the simultaneous action of phonon drag and electric field gave the distributions of phonon flux, density of charge carriers and electric potential. Dimensional suppression of thermal conductance and electron-phonon interaction were accounted for but found to be less effective than expected. The developed model predicts the formation of a layer with a high density of charge carriers that is practically independent of the concentration of dopant ions. This layer can effectively intercept the phonon flow propagating from the heated nanoisland. The resulting thermoEMF can have sufficient magnitudes to explain the low-voltage electron emission capability of nanoisland films of metals and sp2-bonded carbon, previously studied by our group. The phenomenon predicted by the model can be used in thermoelectric converters with untypical parameters or in systems for local cooling.

Enhancing Extraction and Suppressing Cooling of Hot Electrons in Lead Halide Perovskites by Dipolar Surface Passivation.

Slowing hot carrier (HC) cooling and improving HC extraction are considered two pivotal factors for enhancing power conversion efficiency in emerging HC photovoltaic applications of perovskites and other materials. Employing ab initio quantum dynamics simulations, we demonstrate the simultaneous slow cooling and efficient extraction of hot electrons at the C60/CsPbI3 interface through dipolar surface passivation with phenethylammonium and 4-fluorophenethylammonium ligands. The passivation effectively suppresses I-Pb lattice vibrations, weakens the hot electron-phonon interaction in CsPbI3, and thus slows down the HC cooling. At the same time, the dipolar surface passivation elevates the LUMO + 1 state in C60 and reduces the energy gap for HC extraction. Concurrently, higher-frequency vibrations of the dipolar layer enhance the coupling between C60 and CsPbI3, promoting efficient HC extraction further. These phenomena are intensified with increased polarity of the dipolar layer. Furthermore, we find that dipolar passivation has the opposite influence on cold electron collection at the band edge, underscoring the fact that the observed improvement in photovoltaic performance stems preferentially from the effective utilization of HCs rather than cold electrons. The work provides a new strategy for achieving high-performance HC perovskite solar cells.

Influence of Dimensional Quantization Effects on the Effective Mass of Major Charge Carriers in LED Heterostructures with InxGa1−xN/GaN Multiple Quantum Wells

By numerically solving the self-consistent system of the Schr¨odinger equations and Poisson electroneutrality, zone diagrams of LED heterostructures with InxGa1−xN/GaN multiple quantum wells have been calculated. The effect of electron–phonon interaction, nonparabolicity of the dispersion relation, and hybridization of the wave function on the values of the effective mass of major charge carriers in the InxGa1−xN/GaN quantum wells has been studied. The long-wave shift of 2D-plasmon resonances is associated with the temperature renormalization of the effective mass of two-dimensional carriers. To describe the temperature dependence of the effective mass, the function for the displacement of the 2D-plasmon resonance frequency is introduced.

Machine learning electron-phonon interactions in two-dimensional semiconducting materials: The case of zero-point renormalization

Machine learning electron-phonon interactions in two-dimensional semiconducting materials: The case of zero-point renormalization

Nanometer-Scale Acoustic Wave Packets Generated by Stochastic Core-Level Photoionization Events

We demonstrate that the absorption of femtosecond hard x-ray pulses excites quasispherical, high-amplitude, and high-wave-vector coherent acoustic phonon wave packets using an all hard-x-ray pump-probe scattering experiment. The time- and momentum-resolved diffuse scattering signal is consistent with an ensemble of 3D strain wave packets induced by the rapid electron cascade dynamics following photoionization at uncorrelated excitation centers. We quantify key parameters of this process, including the localization size of the stress field and the photon energy conversion efficiency into elastic energy. The parameters are determined by the photoelectron and Auger electron cascade dynamics, as well as the electron-phonon interaction. In particular, we obtain the localization size of the observed strain wave packet to be 1.5 and 2.5 nm for bulk SrTiO3 and KTaO3 single crystals, respectively. The results provide crucial information on the mechanism of x-ray energy deposition into matter and shed light on the shortest collective length scales accessible to coherent acoustic phonon generation using x-ray excitation. Published by the American Physical Society 2024

Excitonic properties of deep-blue emitting fragmented CsCl:Eu2+ nanocrystals

In this work, polycrystalline CsCl:Eu2+ nanocrystals were grown by the hot injection method. The CsCl:Eu2+ nanocrystals were fragmented into particles of a few nanometers by dispersion in organic solvents, and they exhibited strong deep-blue emission under ultraviolet light. Excitonic emission at room temperature was observed near 410 nm, with a full width at half-maximum of 37.8 nm and a photoluminescence quantum yield of 4.1%. Additionally, the average exciton lifetime was determined to be 6.45 ns. In the temperature range of 10 K to 300 K, the excitonic emission intensity gradually decreased while the energy of the peak position slightly increased, corresponding to an increase in the temperature. From the result, the exciton binding energy and average longitudinal phonon energy were calculated to be about 29 and 13 meV, respectively, comparable to the energy calculations of similar Cs-series perovskites. Thermal expansion and electron-phonon interaction also increased steadily with the temperature, and these two phenomena gave rise to a monotonic blue-shift.

Electron trapping in HfO2 layer deposited over a HF last treated silicon substrate

Electron trapping in HfO2-based MOS structures was studied through pulsed capacitance-voltage (C-V) technique. 10 nm HfO2 layer was deposited by atomic layer deposition over a HF last treated Si substrate. The C-V curves were observed to shift to positive voltages driven by the positive applied voltage along the pulses, consistent with electron trapping due to tunneling transitions between the substrate and pre-existing defects within the oxide and the subsequent lattice relaxation through electron-phonon interaction. The dependences of the voltage shift for a given capacitance value (ΔVC) with stress bias and time, allowed to distinguish two mechanisms. An initial trapping process occurs for times shorter than the microsecond, probably associated with a thin non-stoichiometric SiOx interfacial layer, which is followed by a trapping process that starts after tens of μs and progressively slowed down, associated with traps within the HfO2 layer. Numerical simulations yield for the HfO2 traps an energy of 1.3 eV below the conduction band edge, decreasing exponentially with the distance from the Si interface with a characteristic length of 1.7 nm; and phonon and relaxation energies of 50 meV and 1 eV, respectively. These physical parameters are consistent with previous reports of electron trapping in HfO2 layers deposited on a controlled interfacial layer, suggesting that trapping properties of defects inside the HfO2 layer are insensitive to the treatment of the Si surface before HfO2 deposition. On the other hand, the observed large initial trapping suggests that the non-controlled SiOx interfacial region is more defective than a controlled one.

Phonon-Coupled High-Harmonic Generation for Exploring Nonadiabatic Electron-Phonon Interactions.

High harmonic generation (HHG) have received significant attention for the exploration of material properties and ultrafast dynamics. However, the lack of consideration for couplings between HHG and other quasiparticles, such as phonons, has been impeding the understanding of many-body interactions in HHG. Here, we reveal the many-body electron-phonon mechanism in the quasiparticle-coupled strong-field dynamics by investigating the nonadiabatic (NA) coherent-phonon-coupled HHG. Coherent phonons are revealed to effectively affect HHG via the adiabatic band modulation induced by phonon deformation effects and the NA and nonequilibrium distribution of photocarriers in multiple valleys. The adiabatic and NA mechanisms leave their fingerprint via influencing the phonon period and phase delay in the oscillation of HHG intensity, both of which are experimentally measurable. Investigation of these quantities enables the direct probing of the electron-phonon interaction in materials.

Hydrogen-Doped c-BN as a Promising Path to High-Temperature Superconductivity Above 120 K at Ambient Pressure.

Finding high-temperature superconductivity in light-weight element containing compounds at atmosphere pressure is currently a research hotspot but has not been reached yet. Here it is proposed that hard or superhard materials can be promising candidates to possess the desirable high-temperature superconductivity. By studying the electronic structures and superconducting properties of H and Li doped c-BN within the framework of the first-principles, it is demonstrated that the doped c-BN are indeed good superconductors at ambient pressure after undergoing the phase transition from the insulating to metallic behavior, though holding different nature of metallization. Li doped c-BN is predicted to exhibit the superconducting transition temperature of ≈58 K, while H doped c-BN has stronger electron-phonon interaction and possesses a higher transition temperature of 122 K. These results and findings thus point out a new direction for exploring the ambient-pressure higher-temperature superconductivity in hard or superhardmaterials.

Synthesis of CuO/Fe3O4 Nanocomposite for enhanced solar thermal desalination

Nanoparticle (NPs) have attracted the attention of scientists and researchers in water remediation due to the improvement in its processability, and its cost effectiveness. The present work analyzed the synergistic effect of CuO (NPs) with different ratios (0, 0.5, 0.6, 0.7, 0.8, 0.9 and 1 wt %) on the structural, and optical properties of Fe3O4 (NPs), that can be used in water desalination process. The general methodology for preparation and structural properties of CuO (NPs), Fe3O4 (NPs) and CuO/Fe3O4 (NCs) were analyzed by (XRD) and (TEM) measurements. The morphological combination of (SEM) with (EDX), were used for determining the elemental identification of CuO/Fe3O4 (NCs). The characterization topography was found to be strongly affected by the change of the CuO concentration. Furthermore, the Urbach energy (EU), steepness parameter (σ), electron-phonon interaction (Ee-p), optical band gap (Eg), refractive index (n) and optical conductivity (σopt) were calculated. The optical absorption studies in the UV–Visible region, revealed that the CuO/Fe3O4 (NCs) was found to be direct allowed transition, and the energy bandgap value decreased from 2.52 eV to 2.15 eV based on the percentage change of CuO (NPs) in CuO/Fe3O4 (NCs). Moreover, the value of thermal conductivity (k), and thermal efficiency (η), showed an increase with the increase of the ratio of CuO (NPs), which may be an indication for opening an innovation way that can satisfy the need of seawater desalination technology.

High-throughput screening of 2D materials identifies p-type monolayer WS2 as potential ultra-high mobility semiconductor

Abstract2D semiconductors offer a promising pathway to replace silicon in next-generation electronics. Among their many advantages, 2D materials possess atomically-sharp surfaces and enable scaling the channel thickness down to the monolayer limit. However, these materials exhibit comparatively lower charge carrier mobility and higher contact resistance than 3D semiconductors, making it challenging to realize high-performance devices at scale. In this work, we search for high-mobility 2D materials by combining a high-throughput screening strategy with state-of-the-art calculations based on the ab initio Boltzmann transport equation. Our analysis singles out a known transition metal dichalcogenide, monolayer WS2, as the most promising 2D semiconductor, with the potential to reach ultra-high room-temperature hole mobilities in excess of 1300 cm2/Vs should Ohmic contacts and low defect densities be achieved. Our work also highlights the importance of performing full-blown ab initio transport calculations to achieve predictive accuracy, including spin–orbital couplings, quasiparticle corrections, dipole and quadrupole long-range electron–phonon interactions, as well as scattering by point defects and extended defects.

Probing carrier transport of sphalerite Cadmium Chalcogenide (CdX, X=S, Se) from first-principles

Cadmium chalcogenide (CdX, X=S, Se) traps photons in the visible and near-infrared regions, and it has been widely used in optoelectronic devices such as solar cells and light-emitting diodes. The carrier concentration and intrinsic mobility are prerequisites for the optimized design of optoelectronic devices, while the underlying physics that determines the carrier mobility of sphalerite CdX remains elusive. Herein, we explore the temperature-dependent carrier transport properties and scattering mechanisms of sphalerite CdX from first-principles. The calculation results indicate that the isotropic electron mobility for CdS and CdSe are 180.2 cm2/Vs and 656.8 cm2/Vs at room temperature, respectively. An in-depth analysis reveals that polar optical phonon (POP) scattering is the main scattering mechanism limiting the mobility of sphalerite CdX, and electrons are mainly affected by long-range longitudinal optical phonon scattering. The lower electron mobility of CdS compared to CdSe is attributed to the larger Fröhlich coupling constant, -larger Pauling ionicity, and stronger electron-phonon interactions. As the temperature rises, the electron mobility of CdX consistently decreases due to the enhancement of POP scattering and lattice vibrations. The electron mobility of CdX at high doping concentrations decreases dramatically, attributing to the enhancement of ionized impurity scattering i.e. collisional scattering of electrons and ionized impurity ions. This work provides a systematic study of the electron transport properties of sphalerite cadmium chalcogenide from first-principles, which provides a theoretical guideline for the functional regulation and improvement of optoelectronic devices.

Structure Evolution and Optical Tuning of One-Dimensional Post-perovskite (TDMP)PbBr4 under High Pressure.

Optimizing the structure and tuning the optical properties in low-dimensional organic-inorganic halide perovskites are crucial to practical applications for stable solid-state lighting. Herein, we performed high-pressure investigations on one-dimensional (1D) postperovskite (TDMP)PbBr4 (TDMP = trans-2,5-dimethylpiperaziniium), and structure and optical properties under pressure are studied. (TDMP)PbBr4 exhibits color tunable emission from cool white light to yellow orange as the pressure increases from atmospheric pressure to 20.0 GPa. It was found that high pressure would facilitate trapping the free exciton (free exciton) to form a self-trapped exciton (STE) state due to increased electron-phonon interaction, thus enhancing STE emission in the pressure range of 4.0-7.0 GPa. At above 7.0 GPa, the STE emission is quenched, which is due to the phonon-assisted nonradiative relaxation. Meanwhile, (TDMP)PbBr4 displays reversible piezochromism from colorless to yellow under pressure as a result of the compound undergoing a reversible structural transformation. This work provides an insightful perspective on revealing the relationship between structure and optical properties of 1D postperovskites under high pressure.