Electron Phonon 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.

Effective phonon dispersion and low field transport in AlxGa1−xN alloys using supercells: An ab initio approach

To investigate the transport properties in random alloys, it is important to model the alloy disorder using supercells. Although computationally expensive, the local disorder in the system is accurately captured as translational symmetry that is imposed on the system over larger length scales. Additionally, in supercells, the error introduced by self-image interaction between the impurities is reduced. In this work, we have investigated the Effective Phonon Dispersion (EPD) and transport properties, from first principle calculations using supercells in AlxGa1−xN alloy systems. Using an in-house developed code for phonon-band unfolding, the EPD of AlGaN is obtained and the individual phonon modes are identified with good agreement with experimental values. Moreover, we report an in-house developed method to calculate low-field transport properties directly from supercells without phonon band unfolding. First, to validate our methods, we have solved the Boltzmann transport equation using Rode’s method to compare the phonon limited mobility in the 4 atom GaN primitive cell and 12 atom GaN supercell. Using the same technique, we have investigated the low field transport in random AlxGa1−xN alloy systems. The quadrupole interaction is included for transport properties of GaN and AlN to accurately capture the physics in these materials. Our calculations show that along with alloy scattering, electron–phonon scattering may also play an important role at room temperature and high-temperature device operation. This technique opens up the path for calculating phonon-limited transport properties in random alloy systems.

All-optical manipulation of bandgap dynamics via coherent phonons

The ability to actively and dynamically control electronic states at ultrafast timescales opens up a wide range of potential applications across optoelectronics, quantum computing and sensing, energy conversion and storage, etc. Yet, achieving dynamic electronic manipulation via coherent phonons has posed a considerable challenge. Here, employing time-resolved high-harmonic generation (tr-HHG) spectroscopy, we demonstrate the manipulation of bandgap dynamics in a BaF2 crystal by coherent phonons. The tr-HHG spectrum perturbed by a triply degenerate phonon mode T2g exhibits simultaneously a remarkable two-dimensional (2D) sensitivity, i.e., in both intensity and energy domains. The dynamic compression and enhancement of the harmonics in the intensity domain showed a π/2 phase shift compared to the manifestation of shifts of the harmonics in the energy domain, an astounding example of a physical phenomenon being observed simultaneously in two different perspectives. We employed a quantum model incorporating the electron–phonon coupling to complement our experimental observations, successfully reproducing the results. In addition, we demonstrated complete control over the strength and initial phase of the coherent phonon oscillations by varying the incident electric field polarizations across different crystal orientations. Our findings lay a foundation for engineering the electronic structure through coherent phonons within the terahertz frequency and picosecond to nanosecond time regimes.

High-throughput structural screening and property study of noncentrosymmetric superconductors in Th-based ZrNiAl-like compounds

Noncentrosymmetric superconductors (NCSs) are emerging as potential materials for the investigation of nontraditional phenomena, including both the presence of a spin-singlet pairing configuration and the occurrence of a spin-triplet pairing configuration, as well as the manifestation of topological superconductivity. In this work, we constructed a high-throughput theoretical design and performed a computational screening of Th-based NCSs with ZrNiAl-like structures based on the first principles method. Through systematic stability evaluations, we expanded the range of Th-based ZrNiAl-like compounds with superior stability to 45 materials. In this group of substances, we have pinpointed 17 prospective entities exhibiting notable band separation in the vicinity of the Fermi level. Electron–phonon coupling calculations indicated that 14 of these compounds are superconducting. By performing an analysis of electronic properties, we proposed that ThAlRu and ThAlOs are two promising NCSs in this family. This work may aid in the exploration of the design and synthesis of Th-based ZrNiAl-like materials, leading to the discovery of more NCSs with unique superconducting properties.

Discovery of PdHx compounds using genetic algorithms and first-principles calculations

Abstract In the present work, the variable-composition crystal structure prediction algorithm USPEX was combined with first-principles calculations to systematically explore stable PdH x compounds in the pressure range from 0 to 100 GPa. The predicted structures are PdH, Pd4H5, Pd2H, and PdH4, of which PdH has previously been synthesized. The electronic structures show that these compounds can be metallic, with Pd states dominating at the Fermi level. However, the hydrogen atoms do not play a main role in the electronic properties; their vibrations do contribute to the electron–phonon coupling. The predicted Pd4H5 has a superconducting transition temperature of ∼13 K, which is higher than that of PdH (∼8 K in the experiment). Additionally, PdH, Pd4H5, Pd2H, and PdH4 can be classified as phonon-mediated superconductors, falling into the low-coupling regime. Our results demonstrate that as the hydrogen concentration is increased in PdH x compounds, the superconducting transition temperature rises due to hydrogen vibrations, which is consistent with the experiment. Hence, the present study paves the way for discovering new PdH x compounds with promising superconducting transition temperatures.

Extremely weak sub-Kelvin electron–phonon coupling in InAs on Insulator

We are proposing a hybrid superconductor–semiconductor platform using indium arsenide (InAs) grown on an insulating layer of indium aluminum arsenide heterostructure (InAsOI) as an ideal candidate for coherent caloritronic devices. These devices aim to heat or cool electrons out of equilibrium with respect to the phonon degree of freedom. However, their performances are usually limited by the strength of the electron–phonon (e-ph) coupling and the associated power loss. Our work discusses the advantages of the InAsOI platform, which are based on the significantly low e-ph coupling measured compared to all-metallic state-of-the-art caloritronic devices. Our structure demonstrates values of the e-ph coupling constant up to two orders of magnitude smaller than typical values in metallic structures.

Ultrafast thermal modulation dynamics during ripple formation induced by a femtosecond laser multi-pulse vortex beam

Abstract This work theoretically investigated the ultrafast thermal modulation dynamics during early formation of ripples on an Au film induced by femtosecond laser multi-pulse vortex beam irradiation. An extended two-temperature dynamics model that comprehensively considers optical interference modulation for the formation of seed ripples, transient reflectivity and non-equilibrium thermal transfer was self-consistently built to predict high-contrast ripple formation. The two-dimensional evolution of electron and phonon temperature modulations during ripple formation in a high non-equilibrium state of Au film were obtained via femtosecond laser multi-pulse vortex beam irradiation. It was revealed that ripple contrast can be significantly amplified by shortening the laser wavelength, increasing the pulse number, or enlarging the laser fluence of the vortex beam. Moreover, the electron–phonon coupling time during ripple formation is fully explored in detail. This study provides valuable insights into optimizing laser parameters for controlled high-contrast ripple formation on Au films.

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.

Achieving Luminous Efficiency Enhancement and Near‐Infrared Emission in Moisture‐Stable 0D Zinc‐Based Halides

AbstractZero‐dimensional (0D) metal halides are attractive due to their structure‐dependent and tunable photoluminescence properties. Herein, a new 0D organic–inorganic hybrid Zn‐based halide, (C4H12N2)ZnBr4, featuring a long‐term stable crystal structure and moisture‐stable PL emission under various extreme conditions is reported. A strong electron–phonon coupling effect enables the Zn‐based halide to display highly efficient blue light at 472 nm with a large Stokes shift of 7385 cm−1. Intriguingly, heterovalent substitution of Cu+‐Zn2+ further enhances the photoluminescence quantum efficiency to 60% as the introduction of Cu+ effectively suppresses the nonradiative recombination process. Besides, the formation of twisted [SbBr4]− tetrahedra via Sb3+‐Zn2+ substitution help to achieve a broadband near‐infrared (NIR) emission (760 nm) with full width at half maxima (FWHM) of 203 nm, enabling the potential applications in night‐vision and nondestructive fruit damage inspection. Detailed structural and optical analyses are used to investigate the photophysical processes of different self‐trapped exciton (STE) emission for pristine and Cu+/Sb3+‐doped 0D metal halides. These findings advance the understanding of spectral regulation mechanism via heterovalent substitution and initiate more exploitation of luminescent metal halides for emerging applications.

Superconductivity in Ternary Mg4Pd7As6

AbstractThe synthesis and characterization of a new compound Mg4Pd7As6, which is found to be a superconductor with Tc = 5.45 K is reported. Powder X‐ray diffraction confirms the U4Re7Si6 structure (space group Im‐3m, no. 229) with the lattice parameter a = 8.2572(1) Å. Magnetization, specific heat, and electrical resistivity measurements indicate that it is a moderate‐coupling (λ = 0.72) type‐II superconductor. The electronic and phonon structures are calculated, highlighting the importance of antibonding Pd–As interactions in determining the properties of this material. The calculated electron–phonon coupling parameter λ = 0.76 agrees very well with the experimental finding, which confirms the conventional pairing mechanism in Mg4Pd7As6.

Enhancement of thermoelectric transport in surface halogenated Ti2O MOenes via electron–phonon drag effect

Designing efficient and environmentally friendly thermoelectric materials near room temperature is critical, and the electron–phonon drag effect on thermoelectric transport in monolayers remains largely unexplored. This manuscript systematically investigates the electron–phonon drag effect in surface halogenated Ti2O MOenes (Ti2OX2, X = F, Cl) by solving fully coupled electron–phonon Boltzmann transport equations. We find that the phonon drag effect significantly enhances the total Seebeck coefficient and various electronic transport coefficients, including the thermal response of electrons to an electric field, electrical conductivity, and electronic thermal conductivity at zero field, while the electron drag effect notably increases the lattice thermal conductivity, especially at high carrier concentrations and low temperatures. Furthermore, the electron–phonon drag effect significantly increases the thermoelectric figure of merit (zT) across 100–––900 K, with the greatest enhancement at low temperatures. At room temperature, zT increases by 13.73 times for Ti2OF2 and 2.82 times for Ti2OCl2, achieving maximum values of 0.92 and 0.84, respectively. Our work underscores the superior thermoelectric performance of surface halogenated Ti2O MOenes near room temperature and the potential of leveraging electron–phonon drag effects to enhance the electrical, thermal and thermoelectric transport in monolayers.

Ultrafast laser-enabled optoacoustic characterization of Three-dimensional, nanoscopic interior features of microchips

The recent advances in micromanufacturing have been pushing boundaries of the new generation of semiconductor devices, which, in the meantime, brings new challenges in the material and structural characterization – a key step to ensure the device quality through the micromanufacturing process. An ultrafast laser-enable optoacoustic characterization methodology is developed, targeting in situ calibration and delineation of the three-dimensional (3-D), nanoscopic interior features of opaque semiconductor chips. With the guidance of ultrafast electron–phonon coupling effect and velocity-perturbated optical interference, a femtosecond-laser pump–probe set-up based on Sagnac interferometer is configured to generate and acquire picosecond ultrasonic bulk waves (P-UBWs) traversing the microchips. The interior features of the microchips shift the phase of acquired P-UBW signals, reflected in the perturbed probe laser beam. The phase shifts are calibrated to compute signal correlation of P-UBW signals between different acquiring positions, whereby to delineate the interior features in an intuitive manner. The approach is experimentally validated by characterizing nanoscopic, invisible interior aurum(Au)-gratings with periodically varied depths in typical microchips. Results highlight that the 3-D nanoscopic features of the microchips can be revealed with a microscopic and a nanoscopic spatial resolution, respectively along the transverse and depth directions of the chip, where the Au-gratings become “visible” with a depth variance of a few tens of nanometers only. This proposed approach has provided a fast, nondestructive approach to “see” through an opaque microchip with a nanoscopic resolution.

Emergence of superconductivity and superionicity in the electride [formula omitted]La under extreme conditions

Superconducting electrides, a unique type of conventional superconductors, have recently attracted considerable attention. Inspired by the discovery of high critical temperature (Tc) in lanthanum hydrides and the feasibility of forming electrides in lithium-based compounds, we have conducted a systematic investigation the phase diagram and superconductivity of Li-La alloys under high pressure. Using the crystal structure prediction method and first-principles calculations, we uncovered three pressure-stabilized compounds with stoichiometries LiLa3, LiLa2 and Li6La. All three compounds exhibit metallic behavior with electrons transferred from Li to La atoms. Notably, Li6La emerges as a prototypical superconducting electride with Tc of 11.6 K at 350 GPa, which increases to 16.3 K as pressure decreases to 200 GPa. Electron–phonon analysis reveal that the enhancement of superconductivity mainly originates from the coupling between the increased interstitial quasiatoms (ISQs) and low-frequencies phonons. Additionally, akin to lanthanum hydrides, Li6La is predicted to enter a superionic phase characterized by high lithium diffusion coefficients under high pressure and temperature. Our findings indicate the coexistence of superconductivity and superionicity in the electride Li6La, which may prompt further experimental investigations.

Electronic structure, lattice dynamics and superconductivity in Li2CuX and LiCu2X (X = Si, Ge and Sn) Heusler compounds

First-principles computational methods have been employed to explore the electronic, lattice dynamics, and superconducting properties of the intermetallic Heusler compounds Li2CuX and LiCu2X (X = Si, Ge, or Sn). The electronic structure calculations suggest that all of these compounds exhibit metallic properties. The dynamic stability of the compounds has been examined by computing the phonon dispersion spectra and phonon density of states. Six out of twelve phases are found to be dynamically stable. Additionally, the superconductivity of stable and metastable compounds has been investigated using the electron–phonon-mediated mechanism. In calculations, it turns out that, regular cubic full Heusler Fm-3m LiCu2Ge exhibited the highest superconducting critical temperature (Tc) of 2.0 K, with a calculated electron–phonon coupling constant (λ) of 0.58.

Band order reversal and valley engineering driving to dual high electron and hole mobility in strained monolayer BS

Strain engineering is an effective method to tune the physical properties of materials. In this work, we found that applying 5% biaxial compression to monolayer boron sulfur can simultaneously induce a band order reversal of valence bands and valley engineering of conduction bands. This change significantly enhances the room temperature intrinsic mobility, boosting it from 2 to 260 cm2 V−1 s−1 for holes and 63 to 543 cm2 V−1 s−1 for electrons. The high hole mobility surpasses that of bulk GaN and silicon. We further demonstrate that the valence band order reversal not only lifts the px/y state above the pz states, giving to a more dispersive highest valence band, and thus smaller electron scattering channels and hole effective mass, but also shifts the stronger σz bonding to weaker π-bonding characteristics, resulting in smaller electron–phonon coupling strength. Those combined effects results in a substantial increase in hole mobility.

Strong Quantized Electron–Phonon Coupling Induced by the Unique LA Phonon Mode in 2D Kramers Semimetal InTe

Strong Quantized Electron–Phonon Coupling Induced by the Unique LA Phonon Mode in 2D Kramers Semimetal InTe

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.

Measurements of fluctuation-induced in-plane magneto-conductivity of granular aluminum film

The phenomenon of Berezinskii–Kosterlitz–Thouless (BKT) phase fluctuations and the superconducting fluctuations is investigated in a 40 nm thick granular aluminum film using magneto-transport measurements. The transport measurements suggest the possibility of strong electron–phonon (el–ph) interactions in contrast to a Bardeen–Cooper–Schrieffer superconductor. It shows a BKT transition of 2.304 K and a superconducting mean-field transition at 2.32 K. The presence of the resistive tail even before the BKT transition reflects the abundance of thermally activated free vortices. By analyzing the excess conductivity, Gaussian–Ginzburg–Landau superconducting fluctuations are observed above the superconducting transition, which causes rounding of the transition region even before the superconducting transition. The temperature dependence of the fluctuation conductivity in zero magnetic field exhibits distinct signatures of the two-dimensional direct Aslamazov–Larkin theory, with a significant contribution from the Maki–Thompson (MT) model. Furthermore, the anomalous behavior of the fluctuation conductivity at higher temperatures and perpendicular magnetic fields (up to 700 mT) is explained in terms of the total-energy cutoff (=0.72) in the low-wavelength region of the superconducting fluctuations and a pair-breaking parameter (∼0.031). Further studies on the pair-breaking parameter indicate the presence of the el–ph scattering, which diminishes the MT contribution. Our study carries important bearings on how the BKT phase fluctuations and superconducting amplitude fluctuations control the conductivity of granular superconductor near and above the transition region as non-equilibrium properties of weakly disordered granular superconductors. This research is of significance, offering insights into the fundamental properties of granular superconductivity and aiding in the comprehension of nano-structured thin film devices.

Atomic-Scale Mechanism of Enhanced Electron-Phonon Coupling at the Interface of MgB2 Thin Films.

In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, electron-phonon coupling is the fundamental mechanism of superconductivity. For instance, the superconductivity of magnesium diboride (MgB2) comes from the coupling between E2g modes (in-plane boron-boron bond vibrations) and self-doped charge carriers. In thin films and ceramics of BCS superconductors, interfaces with discontinuous chemical bonds may alter the local electron-phonon coupling. However, such effects remain largely unexplored. Here, we investigate the heterointerface of the MgB2 film on the SiC substrate at the atomic scale using electron microscopy and spectroscopy. We detect the presence of a thin MgO layer with a thickness of ∼1 nm between MgB2 and SiC. Atomic-level electron energy loss spectra (EELS) show MgB2-E2g mode splitting and softening near the MgB2/MgO interface, which enhances electron-phonon coupling at the interface. Our findings highlight the potential of interface engineering to enhance superconductivity via modulating local phonon states and/or electron states.

Cumulated Effects of Magnetic Field, Electron–Phonon and Spin–Orbit Interaction on Thermodynamics Properties of Mono Layer Graphene

Cumulated Effects of Magnetic Field, Electron–Phonon and Spin–Orbit Interaction on Thermodynamics Properties of Mono Layer Graphene