Phonon Coupling Research Articles

Mechanical Gain Enhanced Propagation of Light in a Hybrid Spin‐Mechanical System

AbstractThe spin‐mechanical hybrid systems provide a potential platform to reach quantum science and technology, but practical applications with such hybrid quantum systems need the integration of more physical units. In the article, a multi‐mode spin‐mechanical hybrid quantum system is proposed with two‐mode coupling nanomechanical carbon nanotube (CNT) resonators, in which one CNT resonator is loss and the other CNT resonator can be loss, neutral, or gain. The two carbon nanotube (CNT) resonators can interact through a phase‐dependent phonon‐exchange mechanism, which couples them to the same nitrogen vacancy centers in diamond via magnetomechanical effects. By modulating the phase of phonon–phonon coupling and leveraging the Fano‐like resonance phenomenon, double electromagnetically induced transparency can be achieved in this system. This is accompanied by rapid dispersion, resulting in subtle advancements or delays in light propagation. The group index can be manipulated and periodically switched by tuning the modulation phase, with fast‐ and slow‐light effects being particularly pronounced when one CNT resonator is in an active (gain) state compared to lossy or neutral states. This study establishes a foundation for the application of phonon‐mediated optical information storage and processing.

Abnormal Blueshift of Mn d-d Emission Unlocked by Decreasing Phonon Coupling under High Pressure.

Mn d-d emissionhas been a star transition-metal activated phosphors that were extensivelyused in optoelectronic devices. However, due to the complex and elusive luminescence mechanism, the full potential of this material remains largely untapped. Here, we designed a core-shell structure of MnS@CdS quantum dots to investigate the effect of the internal strain on the Mn emission. Upon pressure processing, an unexpected blueshift of Mn emission was achieved. When the pressure reached a pressure of 2.1 GPa, the conventional redshift of Mn emission reappeared. Remarkable color transition from red to orange and then back to red was observed during pressure treatment. Further analysis revealed that the application of external pressure facilitated the diffusion of Mn atoms into the CdS shell, strengthened host-dopant coupling and mitigated the lattice mismatch rate within the MnS@CdS QDs. These factors resulted in a reduction in phonon coupling strength and an increase in energy transfer efficiency from the exciton of the CdS host lattice to the Mn dopants, thus ultimately accounting for the abnormal blueshift of Mn d-d emission. Our findings represent a significant breakthrough in the quest for precise control and regulation of Mn-related emission, offeringinsights into the underlyingluminescence mechanism of Mn emission.

Abnormal Blueshift of Mn d–d Emission Unlocked by Decreasing Phonon Coupling under High Pressure

Mn d–d emission has been a star transition‐metal activated phosphors that were extensively used in optoelectronic devices. However, due to the complex and elusive luminescence mechanism, the full potential of this material remains largely untapped. Here, we designed a core‐shell structure of MnS@CdS quantum dots to investigate the effect of the internal strain on the Mn emission. Upon pressure processing, an unexpected blueshift of Mn emission was achieved. When the pressure reached a pressure of 2.1 GPa, the conventional redshift of Mn emission reappeared. Remarkable color transition from red to orange and then back to red was observed during pressure treatment. Further analysis revealed that the application of external pressure facilitated the diffusion of Mn atoms into the CdS shell, strengthened host‐dopant coupling and mitigated the lattice mismatch rate within the MnS@CdS QDs. These factors resulted in a reduction in phonon coupling strength and an increase in energy transfer efficiency from the exciton of the CdS host lattice to the Mn dopants, thus ultimately accounting for the abnormal blueshift of Mn d–d emission. Our findings represent a significant breakthrough in the quest for precise control and regulation of Mn‐related emission, offering insights into the underlying luminescence mechanism of Mn emission.

Effect of electron–phonon coupling on thermal transport in metals: A Monte Carlo approach for solving the coupled electron–phonon Boltzmann transport equation

In this work, the effect of electron–phonon (e–ph) coupling on both electron and phonon transport of metals is investigated. A Monte-Carlo (MC) approach for solving the coupled electron–phonon Boltzmann transport equation is developed to study the thermal conductivity of α-U and Ag. In this approach, the anisotropic electron band structure, phonon dispersion in the full Brillouin zone, and mode-dependent thermal relaxation time of electrons and phonons are calculated from first principles. Using this approach, MC simulations of coupled e–ph thermal transport at different temperatures in α-U and Ag are performed. Results indicate the e–ph relaxation time is orders of magnitude smaller than the phonon relaxation time. In phonon thermal transport, the impact of ph–e scattering is almost negligible and the ph–ph scattering dominates phonon transport. At high temperatures, the electrons dominate thermal transport in both α-U and Ag. However, at low temperatures, the phonon contribution to the total thermal conductivity of α-U is significant. Moreover, the Lorenz ratio deviates from the Sommerfeld value at low to intermediate temperatures, where the Wiedemann–Franz law is not applicable. Finally, we show that the Ag electron thermal conductivity shows a stronger size effect than the phonon thermal conductivity.

Electron–phonon coupling and coherent energy superposition induce spin-sensitive orbital degeneracy for enhanced acidic water oxidation

The development of acid-stable water oxidation electrocatalysts is crucial for high-performance energy conversion devices. Different from traditional nanostructuring, here we employ an innovative microwave-mediated electron–phonon coupling technique to assemble specific Ru atomic patterns (instead of random Ru-particle depositions) on Mn0.99Cr0.01O2 surfaces (RuMW-Mn1-xCrxO2) in RuCl3 solution because hydrated Ru-ion complexes can be uniformly activated to replace some Mn sites at nearby Cr-dopants through microwave-triggered energy coherent superposition with molecular rotations and collisions. This selective rearrangement in RuMW-Mn1-xCrxO2 with particular spin-differentiated polarizations can induce localized spin domain inversion from reversed to parallel direction, which makes RuMW-Mn1-xCrxO2 demonstrate a high current density of 1.0 A cm−2 at 1.88 V and over 300 h of stability in a proton exchange membrane water electrolyzer. The cost per gallon of gasoline equivalent of the hydrogen produced is only 43% of the 2026 target set by the U.S. Department of Energy, underscoring the economic significance of this nanotechnology.

Defect-mediated electron–phonon coupling in halide double perovskite

Optically active defects often play a crucial role in governing the light emission as well as the electronic properties of materials. Moreover, defect-mediated states in the midgap region can trap electrons, thus opening a path for the recombination of electrons and holes in lower energy states that may require phonons in the process. Considering this, we have probed electron–phonon interaction in halide perovskite systems with the introduction of defects and investigated the thermal effect on this interaction. Here, we report Raman spectroscopic study of the thermal evolution of electron–phonon coupling, which is tunable with the crystal growth conditions, in the halide perovskite systems Cs2AgInCl6 and Cs2NaInCl6. The signature of electron–phonon coupling is observed as a Fano anomaly in the lowest frequency phonon mode (51 cm−1), which evolves with temperature. In addition, we observe a broad band in the photoluminescence (PL) measurements for the defect-mediated systems, which is otherwise absent in defect-free halide perovskite. The simultaneous observation of the Fano anomaly in the Raman spectrum and the emergence of the PL band suggests the defect-mediated midgap states and the consequent existence of electron–phonon coupling in the double perovskite.

Tailored large-particle quantum dots with high color purity and excellent electroluminescent efficiency.

High-quality quantum dots (QDs) possess superior electroluminescent efficiencies and ultra-narrow emission linewidths are essential for realizing ultra-high definition QD light-emitting diodes (QLEDs). However, the synthesis of such QDs remains challenging. In this study, we present a facile high-temperature successive ion layer adsorption and reaction (HT-SILAR) strategy for the growth of precisely tailored Zn1-xCdxSe/ZnSe shells, and the consequent production of high-quality, large-particle, alloyed red CdZnSe/Zn1-xCdxSe/ZnSe/ZnS/CdZnS QDs. The transitional Zn1-xCdxSe/ZnSe shells serve to effectively suppress heavy hole energy level splitting and weaken the exciton-longitudinal optical phonon coupling of QDs, thus facilitating the formation of highly luminescent QDs with a near-unity photoluminescence quantum yield of 97.8% and narrow emission with a full width at half maximum of 17.1nm. In addition, the introduction of transitional shells can extend the particle size of QDs to 19.0nm, which is beneficial for efficient carrier recombination and reduced Joule heating in QD-based LEDs. As a result, the fabricated QLEDs can achieve a record external quantum efficiency of 38.2%, luminance over 120,000cdm-2, and exceptional operational stability T95 (tested at 1,000cdm-2) of 24,100h. These findings provide new avenues for synthesizing high-quality QDs with high color purity.

Angle-resolved Raman scattering study of anisotropic two-dimensional tellurium nanoflakes

As an elemental crystal, anisotropic two-dimensional (2D) tellurium (Te) flakes have recently garnered significant attention due to their exceptional chemical stability, tunable bandgap, low thermal conductivity, and high carrier mobility. To further investigate the anisotropic properties of Te nanoflakes, a rapid and effective method of determining their crystal axes is essential. In this study, it is demonstrated that the intensity of the Raman-active mode in Te nanoflakes exhibits a laser-polarization-dependence, varying periodically with the polarization angle. The crystal axis in two-dimensional Te nanoflakes can be identified using angle-resolved polarized Raman spectroscopy. Specifically, the Raman intensity of the A1 mode is the highest when the incident light is polarized along the [12¯10] direction and the lowest when polarized along the [0001] direction. This identification of the crystal axis via Raman spectroscopy is further verified by transmission electron microscopy measurements. In addition, theoretical simulations reveal that anisotropic Raman scattering is closely associated with the interference effect in a multilayer stacking system, as well as anisotropic absorption and anisotropic electron–phonon coupling in Te nanoflakes. This discovery not only provides a rapid method for locating the crystal axes in Te nanoflakes but also offers new insights into the scattering phenomenon in anisotropic materials beyond Te nanoflakes.

Exploring superconductivity in dynamically stable carbon-boron clathrates trapping molecular hydrogen

Abstract The recent discovery of type-VII boron–carbon clathrates with calculated superconducting transition temperatures approaching ~100 K has sparked interest in exploring new conventional superconductors that may be stabilized at ambient pressure. The electronic structure of the clathrate is highly tunable based on the ability to substitute different metal atoms within the cages, which may also be large enough to host small molecules. Here we introduce molecular hydrogen (H2) within the clathrate cages and investigate its impact on electron–phonon coupling interactions and the superconducting transition temperature (Tc). Our approach involves combining molecular hydrogen with the new diamond-like covalent framework, resulting in a hydrogen-encapsulated clathrate, (H2)B3C3. A notable characteristic of (H2)B3C3 is the dynamic behavior of the H2 molecules, which exhibit nearly free rotations within the B–C cages, resulting in a dynamic structure that remains cubic on average. The static structure of (H2)B3C3 (a snapshot in its dynamic trajectory) is calculated to be dynamically stable at ambient and low pressures. Topological analysis of the electron density reveals weak van der Waals interactions between molecular hydrogen and the B–C cages, marginally influencing the electronic structure of the material. The electron count and electronic structure calculations indicate that (H2)B3C3 is a hole conductor, in which H2 molecules donate a portion of their valence electron density to the metallic cage framework. Electron–phonon coupling calculation using the Migdal–Eliashberg theory predicts that (H2)B3C3 possesses a Tc of 46 K under ambient pressure. These results indicate potential for additional light-element substitutions within the type-VII clathrate framework and suggest the possibility of molecular hydrogen as a new approach to optimizing the electronic structures of this new class of superconducting materials.

How Does a Ceramic Melt Under Laser? Tunnel Ionization Dominant Femtosecond Ultrafast Melting in Magnesium Oxide

Laser-induced melting plays a crucial role in advanced manufacturing technology and ultrafast science; however, its atomic processes and microscopic mechanisms, especially in a wide-gap ceramic, remain elusive due to complex interplays between many degrees of freedom within a timescale of ~100 fs. We report here that laser melting is greatly accelerated by intense laser-induced tunnel ionization, instead of a priori multiphoton absorption, in the archetypal ceramic magnesium oxide (MgO). The tunneling processes generate a large number of photocarriers and results in intense energy absorption, instantaneously altering the potential energy surface of lattice configuration. The strong electron–phonon couplings and fast carrier relaxation enable efficient energy transfer between electrons and the lattice. These results account well for the latest ultrafast melting experiments and provide atomistic details and nonequilibrium mechanism of photoinduced ultrafast phase transitions in wide-gap materials. The laser modulation of melting thresholds and phase boundary demonstrate the possibility of manipulating phase transition on demand. A shock wave curve is also obtained at moderate conditions ( P = 2 GPa), extending Hugoniot curve to new regimes.

Ultrafast carrier and coherent phonon dynamics in van der Waals ferromagnet CrI3

We investigate the dynamics of carriers and coherent phonons in CrI3. The relaxation of phonon A11g undergoes a sudden change at 60 and 220 K, which is attributed to the weakening of spin–phonon coupling and increased vacancy concentrations.

Quantitative Measurement of Temperature-Dependent Quasiparticle Scattering of the Topological Surface States in Bi2Se3

Abstract The quasiparticle scattering processes of the topological surface state (TSS) in three-dimensional topological insulators (TIs) have a vital effect on the many-body interactions and potential applications of topological materials. In this study, we performed high-resolution temperature-dependent angle-resolved photoemission spectroscopy analysis of the 3D strong TI Bi2Se3. Using an ab initio simulation, we analyzed the temperature dependence of the electronic structure and lifetime broadening of the TSS, which are closely associated with the quasiparticle scattering process, i.e., electron–phonon coupling and spin-dependent scattering. We show that, at a low temperature (7 K), the spin-dependent electron scattering facilitates the anisotropic scattering rate of the TSS. Conversely, at room temperature (300 K), the electron–phonon coupling dominates the contribution to the scattering rate. The scattering rate increases with temperature and becomes uniform in momentum space owing to the temperature dependence of quasiparticle scattering. The quantitative study of temperature-dependent scattering rates in TSS is crucial to understanding the topological property and transport mobility of Dirac fermions for fundamental studies and potential applications.

Phonon Inverse Faraday Effect from Electron-Phonon Coupling.

The phonon inverse Faraday effect describes the emergence of a dc magnetization due to circularly polarized phonons. In this work we present a microscopic formalism for the phonon inverse Faraday effect. The formalism is based on time-dependent second order perturbation theory and electron phonon coupling. While our final equation is general and material independent, we provide estimates for the effective magnetic field expected for the ferroelectric soft mode in the oxide perovskite SrTiO_{3}. Our estimates are consistent with recent experiments showing a huge magnetization after a coherent excitation of circularly polarized phonons with THz laser light. Hence, the theoretical approach presented here is promising for shedding light into the microscopic mechanism of angular momentum transfer between ionic and electronic angular momentum, which is expected to play a central role in the phononic manipulation of magnetism.

Prediction of p-block-based ternary superconductors XC2H8 at low pressure

Achieving room-temperature superconductivity under ambient conditions is one of the most important goals in solid-state physics and material science. Recent discoveries of high-Tc superconductivity in binary hydrides H3S and LaH10 at high pressures have focused the search for room-temperature superconductors on dense hydrides with conventional phonon-mediated pairing mechanisms. In this study, we predict a novel family of superconducting ternary hydrides under moderate compression, XC2H8 (X = Ga, In, Tl, Sn, Pb, Sb, Bi, Te, Po). Unlike H3S and LaH10, these new materials are stable at just around 20 GPa. Among the analyzed compounds, SbC2H8 exhibits the highest critical temperature of 73 K at a pressure of 100 GPa, which is attributed to its energetically favorable high-symmetry crystal structure (Fm3¯m), high density of states at the Fermi level (1.27 states/eV) and strong electron–phonon coupling constant (1.02). We expect that our findings provide crucial insights into achieving high-temperature superconductivity at moderate pressures and accelerate the progress of experimental research.

Experimental and Theoretical Evidence of Weak Spin–Lattice Coupling in the Double Perovskite Sr2YRuO6

ABSTRACTSr2YRuO6 is a material that provides an ideal platform for studying magnetic frustration in three‐dimensional geometries. Herein, we combine Raman spectroscopy and density functional theory calculations to establish connections between the lattice dynamics and magnetic states in Sr2YRuO6 single crystals. The x‐ray diffraction profiles reveal that Sr2YRuO6 possesses an ordered double‐perovskite structure with distorted monoclinic symmetry. Three magnetic phase transitions are observed and linked to the presence of weak ferromagnetism at 135 K and short‐ and long‐range antiferromagnetic orderings at 32 and 26 K. The oxygen‐octahedron antistretching and stretching modes, observed at 570 and 766 cm−1, exhibit anomalies near the magnetic phase transition temperatures, indicating an intriguing interplay between the lattice and spin degrees of freedom. Their spin–phonon coupling constants of 0.7 cm−1 reflect the weak spin–lattice interactions in Sr2YRuO6.

Simultaneous study of acoustic and optic phonon scattering of electrons and holes in undoped GaAs/AlxGa1−xAs heterostructures

The study of phonon coupling in doped semiconductors via electrical transport measurements is challenging due to unwanted temperature-induced effects such as dopant ionization and parallel conduction. Here, we study phonon scattering in 2D electrons and holes in the 1.6–92.5 K range without the use of extrinsic doping, where both acoustic and longitudinal optic (LO) phonons come into effect. We use undoped GaAs/AlxGa1−xAs heterostructures and examine the temperature dependence of the sample resistivity, extracting phonon coupling constants and the LO activation energy. Our results are consistent with results obtained through approaches other than transport measurements and highlight the benefit of this approach for studying electron–phonon and hole–phonon coupling.

Incorporation and Compensatory Doping Processes of Cu into ZnO Nanowires Investigated at the Local Scale

The Cu compensatory doping of ZnO nanowires is of great interest to face the challenge arising from the detrimental screening of the piezoelectric potential generated under mechanical solicitations. However, the incorporation processes of Cu into ZnO nanowires are largely unknown. Here, they are investigated locally by combining mass spectrometry and optical spectroscopy with X‐Ray linear dichroism using synchrotron radiation. By varying the Cu(NO3)2/Zn(NO3)2 concentration ratio from 0 to 10% in a chemical bath kept at high pH, it is shown that the amount of Cu incorporated into ZnO nanowires varies from around 4.5 × 1016 to 3.6 × 1018 at cm−3. However, only 15% of the incorporated Cu forms CuZn‐related defects, while the remaining Cu lies on the surfaces of ZnO nanowires. Importantly, thermal annealing under O2 atmosphere is found to electrically activate the incorporated Cu, resulting in the formation of CuZn‐related defect complexes involving nearby VZn, the structured green emission band with a strong phonon coupling, and the increase in the electrical resistivity. These findings shed light on the local environment of Cu incorporated into ZnO nanowires and the required conditions for electrically activating the compensatory doping, as an important outcome for enhanced piezoelectric nanogenerators and stress/strain sensors.

Strong electron–phonon coupling in magic-angle twisted bilayer graphene

The unusual properties of superconductivity in magic-angle twisted bilayer graphene (MATBG) have sparked considerable research interest1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12–13. However, despite the dedication of intensive experimental efforts and the proposal of several possible pairing mechanisms14, 15, 16, 17, 18, 19, 20, 21, 22, 23–24, the origin of its superconductivity remains elusive. Here, by utilizing angle-resolved photoemission spectroscopy with micrometre spatial resolution, we reveal flat-band replicas in superconducting MATBG, where MATBG is unaligned with its hexagonal boron nitride substrate11. These replicas show uniform energy spacing, approximately 150 ± 15 meV apart, indicative of strong electron–boson coupling. Strikingly, these replicas are absent in non-superconducting twisted bilayer graphene (TBG) systems, either when MATBG is aligned to hexagonal boron nitride or when TBG deviates from the magic angle. Calculations suggest that the formation of these flat-band replicas in superconducting MATBG are attributed to the strong coupling between flat-band electrons and an optical phonon mode at the graphene K point, facilitated by intervalley scattering. These findings, although they do not necessarily put electron–phonon coupling as the main driving force for the superconductivity in MATBG, unravel the electronic structure inherent in superconducting MATBG, thereby providing crucial information for understanding the unusual electronic landscape from which its superconductivity is derived.

Crystal and Electronic Structure of β‐Nb2N Thin Films Grown by Molecular Beam Epitaxy

AbstractNiobium nitrides have garnered significant research interest since their discovery due to their exceptional properties and broad applications. In this study, NbNx thin films are successfully grown on 4H(6H)‐SiC(001) substrates using nitrogen‐assisted molecular beam epitaxy. Through scanning transmission electron microscopy and X‐ray diffraction, it is confirmed that the crystal structure of the thin films corresponds to the β‐Nb2N. Different from the previously reported structure of the βA phase (P63/mmc) of β‐Nb2N, the results clearly match with the βB phase (). Resistivity measurements reveal that β‐Nb2N exhibits superconductivity at ≈10 K with an upper critical field of ≈5 T. Its 3D electronic structure is further elucidated using angle‐resolved photoemission spectroscopy combined with theoretical calculations. The observed superconductivity in β‐Nb2N is attributed to its relatively high electron–phonon coupling strength and density of states at Fermi level. Interestingly, it is found that the sample is close to a Lifshitz transition, suggesting potential for tunable physical properties. The results provide a comprehensive understanding of the crystal and electronic structures of β‐Nb2N, facilitating its future applications.

Room temperature polarization-resolved Raman and photoluminescence in uniaxially strained layered MoS2

Transition metal dichalcogenides (TMDCs) are one of the material systems of choice toward achieving room temperature quantum coherence. Externally applied strain is used as a more common control mechanism to tune electro-optical properties in TMDCs like molybdenum disulfide (MoS2). However, room temperature electron–phonon interactions in the presence of strain in transition metal dichalcogenides are still not fully explored. In this work, we employ uniaxial strain dependent Raman and photoluminescence (PL) studies on monolayer and bilayer MoS2 to explore electron–phonon physics. Helicity-resolved Raman in MoS2 obeys robust selection rules. Our studies reveal clear modification in these helicity-based selection rules in the presence of moderate uniaxial strain (ϵ = 0.4%–1.2%). The selection rules are restored upon clear symmetry breaking of the in-plane vibrational mode (ϵ > 1.2%). We assign these changes to the onset of Fröhlich interaction in this moderate strain regime. The changes in Raman scattering are accompanied by changes in valley selective relaxation observed through non-resonant photoluminescence (PL). The moderate strain regime also exhibits the onset of PL polarization for indirect excitonic emission under non-resonant excitation. Our experimental observations point toward electron–phonon coupling mechanisms affecting both valley-selective electron relaxation during PL emission as well as polarization-selective Raman scattering of two-dimensional semiconductors at room temperature.