Role Of Phonons Research Articles

Photophysics of a single quantum emitter based on vanadium phthalocyanine molecules

Single quantum emitters play a fundamental role in the development of quantum technologies such as quantum repeaters, and quantum information processing. Isolating individual molecules with stable optical emission is an essential step for these applications, especially for those molecules that present large coherence times at room temperature. Among them, vanadium-oxide phthalocyanine (VOPc) molecules stand out as promising candidates due to the large coherence times of their ground state electronic spin, which are on the order of microseconds when measured in the ensemble. However, the optical properties of such VOPc molecules at the single emitter level have not yet been reported. Here we show that single VOPc molecules with stable optical properties at room temperature can be isolated. We find that the optical response of the molecule under laser illumination of different polarizations agrees well with a system having pyramidal C4v symmetry. Furthermore, we provide theoretical calculations that support our experimental findings and provide insight into the role of phonons and the internal electronic structure of the molecule. These results demonstrate that this single paramagnetic molecule can function as a single quantum emitter while displaying optical stability under ambient conditions to have their intrinsic properties investigated.

Comparative Studies Between Lattice Dynamical Study and Normal Coordinate Analysis of HTSC EuBa2Cu3O6

It is generally believed, that is needed to sign post the way to the development of materials with high critical temperature, even to room temperature and above, is the formulation of a proper theoretical understanding of superconductivity in these materials. Many experimental studies suggest an intimate relationship between phonons, and the superconducting transition in the high Tc oxides.For example, Raman frequencies and line widths, elastic constants and thermal conductivity all show changes around Tc. Nevertheless, there is not a well- developed theory to explain how the phonons are related to Tc.. In conventional superconductors, the role of phonons is well established. However, in high Tc superconductors, the role played by phonons is controversial. Even if they do not mediate the pairing mechanism, they are likely to play an important subsidiary role. Hence there is much interest in the phonon structure of these materials. The normal state and superconducting properties are believed to arise from the strongly correlated motion of the electronic charge carries (electrons or holes) in the CuO2 planes that essentially define the layered cuprate HTSC. The other cations and oxygen atom in the structure provides structural stability and control the number of charge carries in the CuO2 planes. The Raman spectroscopy of HTSC explains that the role played by the phonons in the mechanism of superconductivity in the perovskite family of superconductors is not fully understood although a large series of experimental results has shown considerable coupling of some phonons to electronic excitation in these systems. Optical spectroscopy and especially Raman and Infrared spectroscopy revealed strong direct influences of the changes in the electronic states to the phonon at the center of the Brillouin.

Unveiling the potential of phonons and photons in quantum computing and communication

This article explores the cutting-edge advancements in quantum technologies, focusing on the innovative use of atomic interactions and the role of phonons and photons in quantum systems. We delve into the recent discovery by researchers at the University of Washington, who have demonstrated the potential of atomic vibrations for tracking and transmitting quantum information, laying the groundwork for new communication systems, high-precision sensors, and powerful computational architectures. The discussion extends to the domain of optomechanics, where light and mechanical vibrations are intricately linked, offering new quantum phenomena for controlling single photons through integrated optical circuits. The study of excitons and their interaction with photons highlights the crucial role these quasi-particles play in the absorption and emission of light in semiconductors, and their potential for encoding and transmitting quantum information. Furthermore, the article addresses the challenges and solutions in leveraging phonons within quantum technologies, emphasizing their application in quantum computing, communication, and sensing. The inherent challenges of simulating quantum systems on classical computers are also discussed, alongside the pivotal role of software simulations in quantum algorithm development, education, and research. In conclusion, the article underscores the ongoing journey of quantum technology development, marked by significant challenges but driven by the immense potential to revolutionize computing, communication, and sensing. The integration of quantum principles into practical applications continues to push the boundaries of what is technologically feasible, heralding a new era of innovation and discovery in the quantum realm.

Anharmonic Exciton‐Phonon Coupling in Metal‐Organic Chalcogenides Hybrid Quantum Wells

AbstractIn contrast to inorganic quantum wells, hybrid quantum wells (HQWs) based on metal‐organic semiconductors are characterized by relatively soft lattices, in which excitonic states can strongly couple to lattice phonons. Therefore, understanding the lattice's impact on exciton dynamics is essential for harnessing the optoelectronic potential of HQWs. Beyond 2D metal halide perovskites, layered metal‐organic chalcogenides (MOCs), which are an air‐stable, underexplored material class hosting room‐temperature excitons, can be exploited as photodetectors, light emitting devices, and ultrafast photoswitches. Here, the role of phonons in the optical transitions of the prototypical MOC [AgSePh]∞ is elucidated. Impulsive stimulated Raman scattering (ISRS) allows the detection of coherent exciton oscillations driven by Fröhlich interaction with low‐energy optical phonons. Steady state absorption and Raman spectroscopies reveal a strong exciton‐phonon coupling (Huang‐Rhys parameter ≈1.7) and its anharmonicity, manifested as a nontrivial temperature‐dependent Stokes shift. The ab initio calculations support these observations, hinting at an anharmonic behavior of the low‐energy phonons <200 cm−1. These results untangle complex exciton‐phonon interactions in MOCs, establishing an ideal testbed for room‐temperature many‐body phenomena.

The role of phonons in switchable MOFs: a model material perspective

The role of phonons in switchable DUT-8(M) MOFs involving Ni, Co, Zn, or Cu as metal (M) was studied by Raman spectroscopy, inelastic neutron scattering (INS), and phonon acoustic spectroscopy (PAS) and density functional theory (DFT) calculations.

Intervalley scattering induced significant reduction in lattice thermal conductivities for phosphorene.

The thermal transport properties of buckled phosphorene (β-P) and antimonene (β-Sb) are investigated using first-principles methods. The large acoustic-optical phonon gaps of 3.8 THz and 2.2 THz enable the four-phonon interaction to play an important role in phonon scattering for both β-P and β-Sb. Considering the electron-phonon coupling, the lattice thermal conductivity can further undergo 84% decrease to 4.9 W mK-1 for p-type β-P at n = 5 × 1013 cm-2. By quantitatively describing the scattering probability of electrons in different paths combined with electron-phonon coupling matrix element analysis, it is found that multi-valley features of electronic band structure and strong electron-phonon coupling strength make electrons have strong intervalley scattering behavior in β-P. The former plays an important role in the energy conservation condition of the scattering process, and the latter determines the selection rule. Our work elucidates the contribution of higher-order phonon interactions as well as electron-phonon coupling effects to lattice thermal conductivity, and provides a new idea for finding materials with low lattice thermal conductivity induced by intervalley scattering.

Correlated Mott insulators in strong electric fields: Role of phonons in heat dissipation

We study the spectral and transport properties of a Mott insulator driven by a static electric field into a non-equilibrium steady state. For the dissipation, we consider two mechanisms: Wide-band fermion reservoirs and phonons included within the Migdal approximation. The electron correlations are treated via non-equilibrium dynamical mean field theory with an impurity solver suitable for strong correlations. We find that dissipation via phonons is limited to restricted ranges of field values around Wannier-Stark resonances. To cover the full range of field strengths, we allow for a small coupling with fermionic baths, which stabilizes the solution. When considering both dissipation mechanisms, we find that phonons enhance the current for field strengths close to half of the gap while lowering it at the gap resonance as compared to the purely electronic dissipation used by Murakami and Werner [arXiv:1804.08257]. Once phonons are the only dissipation mechanism, the current in the metallic phase is almost one order of magnitude smaller than the typical values obtained by coupling to a fermionic bath. In this case, the transport regime is characterized by an accumulation of charge in the upper Hubbard band described by two effective chemical potentials.

Achieving High Thermoelectric Performance in Severely Distorted YbCd2Sb2

AbstractAlloying scattering of phonons is particularly effective in reducing the thermal conductivity. The alloying model considers the mass and strain fluctuations but the substitutional atoms are assumed to be positioned at the ideal lattice point, i.e., without lattice distortion. In the real case, the existence of the lattice distortion in the alloy is inevitable, and it should have an additional contribution to the phonon scattering. Such an effect, however, is usually ignored and can be partially ascribed to the difficulty of experimentally identifying and quantifying the lattice distortion. In this work, significant distortion of the crystal lattice is directly observed by the scanning transmission electron microscopy in YbCd2Sb2 with multiple elements alloying. These results show that the plane distance of adjacent Ytterbium atoms along the direction b fluctuates in the range between 0.34 and 0.46 nm, a distortion from ≈−11.7% to ≈16.0%. The lattice distortion plays a remarkable role in phonon scattering and substantially reduces the lattice thermal conductivity to ≈0.45 Wm–1 K–1 at 700 K. As a result, a peak zT of ≈1.4 is achieved in (Yb0.9Mg0.1)Cd1.2Mg0.4Zn0.4Sb2. These results indicate that tuning the lattice distortion can be a promising strategy for enhancing thermoelectric performance.

Phonon-assisted phase separation in strongly correlated systems

We relate the phase separation observed in many crystals with pronounced electron correlations to the regions of negative electron compressibility (NEC). They were found in several models describing strong electron correlations. At low temperatures, these regions arise near chemical potentials corresponding to the change of the ground state in the site Hamiltonian. The NEC leads to the separation of the system into electron-rich and electron-poor domains. The energy released in the course of this separation is absorbed by phonons. Another role of phonons is to give a definite form — stripes or checkerboards — to lattice distortions and domains of different electron concentrations. The shape, direction and periodicity of such textures are determined by wavevectors of lattice distortions, which most strongly scatter electrons.

The Role of Phonons and Oxygen Vacancies in Non-Cubic SrVO3.

Combining neutron diffraction with pair distribution function analysis, we have uncovered hidden reduced symmetry in the correlated metallic d1 perovskite, SrVO3. Specifically, we show that both the local and global structures are better described using a GdFeO3 distorted (orthorhombic) model as opposed to the ideal cubic ABO3 perovskite type. Recent reports of imaginary phonon frequencies in the density functional theory (DFT)-calculated phonon dispersion for cubic SrVO3 suggest a possible origin of this observed non-cubicity. Namely, the imaginary frequencies computed could indicate that the cubic crystal structure is unstable at T = 0 K. However, our DFT calculations provide compelling evidence that point defects in the form of oxygen vacancies, and not an observable symmetry breaking associated with calculated imaginary frequencies, primarily result in the observed non-cubicity of SrVO3. These experimental and computational results are broadly impactful because they reach into the thin-film and theoretical communities who have shown that SrVO3 is a technologically viable transparent conducting oxide material and have used SrVO3 to develop theoretical methods, respectively.

Pairing in magic-angle twisted bilayer graphene: Role of phonon and plasmon umklapp

Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties are unique and likely play an important role in some of the phenomenological complexity. Some of the salient features include an electronic bandwidth smaller than the characteristic phonon bandwidth and a non-trivial structure of the underlying Bloch wavefunctions. We perform a theoretical study of the cooperative effects due to phonons and plasmons on pairing in order to disentangle the distinct role played by these modes on superconductivity. We consider a variant of MATBG with an enlarged number of fermion flavors, $N \gg 1$, where the study of pairing instabilities reduces to the conventional (weak-coupling) Eliashberg framework. In particular, we show that certain umklapp processes involving mini-optical phonon modes, which arise physically as a result of the folding of the original acoustic branch of graphene due to the moir\'e superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings, and study the effect of external screening due to a metallic gate on superconductivity. At strong coupling the dynamical pairing interaction leaves a spectral mark in the single particle tunneling density of states. We thus predict such features will appear at specific frequencies of the umklapp phonons corresponding to the sound velocity of graphene times an integer multiple of the Brillouin zone size.

The spin selectivity effect in chiral materials

We overview experiments performed on the chiral induced spin selectivity (CISS) effect using various materials and experimental configurations. Through this survey of different material systems that manifest the CISS effect, we identify several attributes that are common to all the systems. Among these are the ability to observe spin selectivity for two point contact configurations, when one of the electrodes is magnetic, and the correlation between the optical activity of the chiral systems and a material’s spin filtering properties. In addition, recent experiments show that spin selectivity does not require pure coherent charge transport and the electron spin polarization persists over hundreds of nanometers in an ordered medium. Finally, we point to several issues that still have to be explored regarding the CISS mechanism. Among them is the role of phonons and electron–electron interactions.

The Role of Phonons in a Topological Material

Unusual interactions occur between phonons and electrons in the topological semimetal tungsten diphosphide, a finding that could explain some of the material’s strange properties.

Intense ionizing irradiation-induced atomic movement toward recrystallization in 4H-SiC

An ultrafast thermal spike within a time interval of a few pico-seconds generated by intense ionizing energy deposited using 100 MeV Ag ions is utilized to study the atomistic details of damage recovery in 4H-SiC. Sequential single ion irradiations were performed using 300 keV Ar and 100 MeV Ag in ⟨0001⟩ 4H-SiC to invoke swift heavy ion (SHI) beam induced epitaxial recrystallization in samples with different degrees of pre-damaged conditions. SHI irradiation was carried out at both room temperature and a low temperature of ∼80K. Low-temperature irradiation was carried out to arrest thermal diffusion of defects and to isolate ionization-induced defect migration in 4H-SiC. Insights into the thermal spike generated by ionizing events in crystalline and amorphous regions at both the temperatures predict a SiC response to SHI. The results emphasize the role of different degrees of pre-damage induced physico-chemical conditions and irradiation temperatures against SHI-induced recrystallization as evaluated by Rutherford backscattering/channeling, Raman spectroscopy, and hard x-ray photoelectron spectroscopy. Understanding the dependence of ion-beam damage accumulation and their recovery on the inelastic to elastic energy loss ratio is important for the performance prediction of SiC intended for extreme environments such as space, defense, and nuclear radiation. We report substantial damage recovery even at a near liquid nitrogen temperature of ∼80K. The recovery gets impeded mainly by the formation of complex defects having homonuclear bonds. The results are explained in the framework of the inelastic thermal spike model, and the role of phonon in the damage recovery process is emphasized.

Diffusion and trapping of hydrogen in carbon steel at different temperatures

The presence of interstitial hydrogen in steel produces embrittlement, which poses a severe risk to the integrity of structural components. Although real steel, as a multi-phase material with crystal defects at several scales is too complex a system to be modelled utilizing ab-initio calculations, mechanisms of hydrogen (H) diffusion in metals have attracted interest and have been widely described in the literature. Here, we study from first-principles (DFT code CASTEP) the role of phonons on the diffusion of hydrogen at different temperatures in a bcc iron lattice (Fe16H) via a calculation of Helmholtz’s free energy, which has been fed into COMSOL for finite element calculations. The diffusion coefficient of hydrogen between 250 K and 700 K was obtained and, the increment in the diffusion barrier at the higher temperatures, traditionally attributed to a transition regime, is now explained by the contribution of phonons. The effect of different traps sites on hydrogen diffusion is studied by using the FEM model.

Elastic Softening in Zirconia During Stage III of Flash Experiments

We report in-situ measurements of the elastic modulus of dense polycrystalline zirconia held in a constant state of flash, i.e. during Stage III of flash experiments. The modulus, measured with and without flash, is lowered by 10 - 35 %, in the temperature range of 1100-1400 oC. The specimen temperature, measured with a pyrometer, was consistent with the estimate from a black body radiation model. These measurements echo recent publications on the role of phonons in the onset of flash, namely the Debye temperature as a lower bound for flash, and the formation of Frenkel pairs in molecular dynamics calculations by the proliferation of phonons above the Debye temperature. However, several questions remain, for example, the rise in electrical conductivity, and electroluminescence that are nearly universal signatures of the flash phenomenon.

Plasmonic Hot Carrier Induced Photosensitization of CdSe Quantum Dots: Role of Phonons

Generation of hot carriers from nonradiative surface-plasmon decay and subsequent injection into the semiconductor is the start of a new paradigm in the field of optoelectronics and photocatalysis....

(Invited) Polaron Mediated Cooling Dynamics in Perovskite Materials: Effect of Temperature

During last decade huge number of research investigations are going on different kind of halide perovskite materials due to their exciting optoelectronic properties which includes ability to provide very efficient solar devices. Charge carrier cooling process is one of the key process for design and development of any higher efficient optoelectronic devices. Ultrafast hot carrier cooling is the key loss channel which limits achievable solar conversion efficiency. Delaying the carrier cooling high efficient hot-carrier solar cell can be realized. Thus, delayed hot carrier cooling in the cell absorber layer can make hot carrier extraction a relatively easier task, assisting in the realization of hot carrier solar cells. There have been plentitude of reports concerning the slow carrier cooling in perovskite materials, which has eventually triggered interest in the radical understanding of the native photophysics driving the device design. In our recent investigation, we have demonstrated dramatic dip in the cooling rate in CsPbBr3 NC after growing Cs4PbBr6 shell over it, in contrast to the bare CsPbBr3 core NCs. By employing ultrafast transient absorption spectroscopy, we have investigated the disparity in the hot carrier thermalization pathways in the CsPbBr3 and CsPbBr3@Cs4PbBr6 core-shell NCs under same laser fluence, which can be validated as a corollary of polaron formation in the later NCs. Role of phonon in the cooling process has also been investigated by carrying out the ultrafast transient studies in the temperature range of 5K-300K.

Thermophysical Properties of Solid Yttrium–Holmium Solutions in the Temperature Range from Room Temperature to 1400 K

The thermophysical properties (thermal diffusivity, specific heat, and thermal conductivity) and resistance of solid solutions (alloys) of rare-earth metals (yttrium and holmium) have been experimentally studied. It is established that, on the whole, these properties obey the same laws as pure rare-earth metals. Heat transfer in Y–Ho alloys at the considered temperatures is mainly performed by electrons as well. The existing quantitative differences between the properties are due to the different influences of the different mechanisms of carrier (electron) scattering. A procedure to separate the contributions to electron scattering based on the Mott model is considered. The roles of phonon-, magnetic-, and impurity-scattering mechanisms are determined. It is established that the magnetic-scattering intensity monotonically decreases upon heating and with a decrease in the holmium concentration. Nordheim’s rule is valid for impurity scattering, which indicates stability of the energy spectrum structure of collective alloy electrons.

Anomalously large lattice thermal conductivity in metallic tungsten carbide and its origin in the electronic structure

Usually, the thermal conductivity is predominantly contributed by electrons in metals. In this work, by using first-principles calculations we find that in tungsten carbide (WC) the phonon-contributed thermal conductivities (κph) are 131 and 158 Wm−1K−1 along the a and c axes, respectively, three times as much as the electronic contribution (κe). In isotopically pure samples, κph can be further increased to 204 and 249 Wm−1K−1 along the a and c axes, respectively, which is comparable to the κe of Al. The anomalously large κph is attributed to the weak phonon-phonon and electron-phonon scattering, both of which have their origin in the electronic structure of the group-VI carbides. The Fermi energy falls within the pseudogap between the bonding and antibonding states, suggesting stronger interatomic bonding and weaker electron-phonon scattering than in group-IV and V carbides. The unique combination of strong interatomic bonding and large atomic mass of W results in a large acoustic-optical gap in the phonon dispersion, suppressing phonon-phonon scattering. In contrast, in another group-VI carbide, MoC, also with strong interatomic bonding, the smaller atomic mass of Mo increases the acoustic phonon frequencies and reduces the acoustic-optical gap. Furthermore, electron-phonon scattering, though not very strong in absolute magnitude, also plays an important role in phonon scattering, leading to a weak temperature dependence of κph in WC. The large thermal conductivity, persisting at high temperatures, facilitates the use of this material in applications such as cutting tools.