Strong Electron-phonon Interaction Research Articles

Investigating the normal state and superconducting state properties of orthorhombic and hexagonal ZrRuP: A first-principles study

We have executed ab initio pseudopotential calculations on the structural, electronic, elastic, mechanical, vibrational, and electron-phonon interaction properties of hexagonal ZrRuP (h-ZrRuP) and orthorhombic ZrRuP (o-ZrRuP). The electronic states of both phases near the Fermi energy are found to be dominated by the $d$ electrons of transition metal atoms, suggesting that they play a more active role in the generation of superconducting state for both phases of ZrRuP. A critical assessment of their elastic and mechanical properties reveals that the lattice of h-ZrRuP is softer than that of o-ZrRuP. A comparison of phonon dispersion curves for both phases indicates that the lower transverse acoustic branch of h-ZrRuP is much softer than that of o-ZrRuP. The soft character of this phonon branch gives rise to strong electron-phonon interaction in h-ZrRuP. Therefore the electron-phonon coupling parameter for h-ZrRuP equals to 1.25 which is considerably larger than the corresponding value of 0.57 for o-ZrRuP. As a consequence, phonon and electron-phonon interaction properties are crucial in making superconducting transition temperature much higher for h-ZrRuP than o-ZrRuP. At the end, the value of this temperature is found to be 12.49 K for h-ZrRuP and 3.89 K for o-ZrRuP which coincide with their experimental values of 12.93 and 3.82 K.

Effects of Material Dimensionality on the Optical Properties of CsPbBr3 Nanomaterials

Inorganic perovskites such as CsPbBr3 have attracted intensive research attention due to their excellent thermal stability and high photoluminescence quantum efficiency. With the decrease of the material’s dimension, quantum confinement effects will take place, which will induce many interesting properties. Here, quasi two-dimensional perovskite nanoplatelets (NPLs) and three-dimensional perovskite nanocrystals (NCs) have been grown, and their optical properties have been compared and discussed. It is interesting to find that the photon energy of CsPbBr3 NPLs hardly changes with temperature due to strong electron–phonon interaction. At the same time, the strong quantum confinement effect results in a smaller Stokes shift and a faster fluorescence lifetime. In addition, although the NPLs have many surface defects, they exhibit a relatively strong negative thermal quenching effect of emission. These results demonstrate the unique optical properties of perovskite NPLs that can pave the way for future nanosca...

Phonon anomalies in pyrochlore iridates studied by Raman spectroscopy

We report polarization-resolved Raman scattering measurements on single crystals of the pyrochlore compound ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ which exhibits a ground state with noncollinear magnetic order, as well as its paramagnetic counterpart ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. The spectra reveal the six Raman-active optical phonons expected for the pyrochlore lattice symmetry. Combined with density functional calculations, polarization analysis of the Raman intensity allows us to assign all observed peaks to specific vibration patterns. Whereas most phonon profiles are weakly temperature dependent, an Ir-O-Ir bond-bending vibration in ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ exhibits a pronounced Fano asymmetry in the paramagnetic state, and marked softening and line-shape anomalies at the magnetic phase transition. These observations indicate strong electron-phonon interactions, which must be considered in models of the electronic properties and phase behavior of the pyrochlore iridates. In addition, a prominent feature appears at 210 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the Raman spectrum of ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ below the magnetic transition temperature, possibly due to a magnetic excitation.

Effect of electron-phonon interaction on lattice thermal conductivity of SiGe alloys

While it is well-known that electron-phonon scattering often determines the electron mobility, its impact on lattice thermal conductivity is less clear. Dominant phonon scattering mechanisms that determine the lattice thermal conductivity have been attributed to phonon-phonon and phonon-defect interactions. However, recent studies in silicon have shown that strong electron-phonon interaction can also lead to significant phonon scatterings at high carrier concentrations. Here, we use first-principles simulations to study thermal transport in SiGe alloys and show that the effect of electron-phonon interaction on thermal transport is even more significant than that in Si because mass disorder scattering leaves long mean free path phonons behind, which are more strongly scattered by electrons. At the carrier concentration of 1 × 1020 cm−3, the room temperature lattice thermal conductivity of the Si0.9Ge0.1 alloy including electron-phonon interaction is only 40% of the value without this interaction. The results show that thermal transport in alloys at a high doping level can be significantly impacted by the free carriers, providing important insights into heat conduction mechanisms in thermoelectric materials which are mostly based on heavily doped alloys.

Fabrication of Fe3O4/ZnS nanocomposites towards ultrasensitive resonant Raman scattering-based immunoassays

Magnetic-optical nanocomposites, which consist of iron oxide nanoparticles and quantum dots, have aroused considerable research interest owing to their unique magnetic and luminescent properties. However, the luminescence of quantum dots tends to be quenched by the magnetic component. Resonant Raman scattering (RRS) of semiconductor nanocrystals, which originates from a strong electron-phonon interaction, provides a new candidate property to be exploited as an analytical tool. Herein, Fe3O4/ZnS nanocomposites were prepared with the seed-mediated growth approach. The nanocomposites exhibited superparamagnetism and characteristic multi-order phonon resonant Raman lines. By using biofunctionalized Fe3O4/ZnS nanocomposites as RRS probes, a femtomolar immunoassay of a model protein, human IgG, was realized with the assistance of an external magnetic field. The results provide guidance for the broad applications of magnetic-optical nanocomposites.

Nonlinear resonant tunneling as mechanism of cool electrons filtration

The paper is devoted to the explanation of the effect of suppressing the temperature distribution of the current carriers in the tunneling device where a nanoparticle embedded in a dielectric matrix served as a quantum dot (QD). Due to a spatial confinement the quantization of energy takes place in the QD. Each level assumed to be four-fold degenerated. We suppose the existence of the strong electron–phonon interaction in the QD leading to the attraction of electrons that exceeds the Coulomb repulsion. This interaction results in breaking the degeneracy. The discrete levels can have lower energies than the Fermi energy of the electrode. The tunneling current through these levels will not be affected by the temperature. In the proposed mechanism of the cool electrons filtration, fitting parameters are absent and nevertheless a good agreement between the theory and experiment has been achieved.

Preferred states of open electronic systems

System-environment interaction may introduce dynamic destruction of quantum coherence, resulting in a special representation named as pointer states. In this work, pointer states of an open electronic system are studied. The decoherence effect is taken into account through two different ways which are Büttiker's virtual probe model and strong electron-phonon interaction in the polaron picture. The pointer states of the system with different coupling strength are investigated. The pointer states are identified by tracking the eigenstates of the density matrix in real-time propagation. It is found that the pointer states can emerge for arbitrary coupling strength. And the pointer states deform to the eigenstates of the system in the strong coupling limit, which indicates the vanish of quantumness in the strong coupling limit.

Defect-induced electronic structures on SnSe surfaces

Using a low-temperature scanning tunneling microscope (STM), we investigated the role of atomic-scale defects of a cleaved SnSe(100) surface on the electronic structure around them. We found in empty-state STM images that Sn vacancies induce dark contrast around them, proving their role as a p-type dopant. The vacancies are also accompanied with an X-shaped dark pattern reflecting the structural asymmetry of the terminating layer. In addition, in empty-state STM images, a 1 × 3 superstructure was observed around a bright-contrasted defect, which is presumed to be a defect-induced charge density wave driven by strong electron-phonon interaction.

Urbach Rule in MnGa2Se4 Single Crystals Upon Optical Absorption

The Urbach rule in MnGa2Se4 single crystals is studied. It is found that the long-wavelength absorption tail obeys the Urbach rule at photon energies in the range 2.1–2.2 eV. It is shown that a strong electron–phonon interaction exists in MnGa2Se4 single crystals.

Photoluminescence and nonlinear transmission of Cu-doped CdSe quantum dots

In this paper we investigate the photoluminescence (PL) properties and nonlinear transmission of Cu-doped CdSe colloidal quantum dots (QDs) under a nanosecond laser pulse excitation. We find a strong difference of the pump intensity dependent behavior between the basic exciton transition and Cu dopants associated PL. We also study the saturation of absorption in Cu-doped CdSe QDs. The role of a strong electron-phonon interaction in the PL properties and nonlinear transmission of colloidal Cu-doped CdSe QDs is explained. The presence of biexciton PL exciton-phonon interaction and its strong influence on the basic exciton PL and non-linear properties of Cu-doped colloidal CdSe QDs are proved by the simultaneous linear growth of basic excitons PL, the growth of the Stokes shift and significant decrease of absorption at the basic exciton transition wavelength with the increase of pump intensity.

Nonrenewal statistics in quantum transport from the perspective of first-passage and waiting time distributions

The waiting time distribution has, in recent years, proven to be a useful statistical tool for characterising transport in nanoscale quantum transport. In particular, as opposed to moments of the distribution of transferred charge, which have historically been calculated in the long-time limit, waiting times are able to detect non-renewal behaviour in mesoscopic systems. They have failed, however, to correctly incorporate backtunneling events. Recently, a method has been developed that can describe unidirectional and bidirectional transport on an equal footing: the distribution of first-passage times. Rather than the time between successive electron tunnelings, the first-passage refers to the first time the number of extra electrons in the drain reaches $+1$. Here, we demonstrate the differences between first-passage time statistics and waiting time statistics in transport scenarios where the waiting time either cannot correctly reproduce the higher order current cumulants or cannot be calculated at all. To this end, we examine electron transport through a molecule coupled to two macroscopic metal electrodes. We model the molecule with strong electron-electron and electron-phonon interactions in three regimes: (i) sequential tunneling and cotunneling for a finite bias voltage through the Anderson model, (ii) sequential tunneling with no temperature gradient and a bias voltage through the Holstein model, and (iii) sequential tunneling at zero bias voltage and a temperature gradient through the Holstein model. We show that, for each transport scenario, backtunneling events play a significant role; consequently, the waiting time statistics do not correctly predict the renewal and non-renewal behaviour, whereas the first-passage time distribution does.

Evaluation of NSAIDs antioxidant activity on lipid peroxidation in splenocyte membranes

Here we summarized the nanoscale layered transition metal chalcogenide superconductors, mainly based on the experimental results. In transition-metal chalcogenides the interplay between strong electron-electron and electron-phonon interactions produces a wide variety of instability ranging from charge density wave to superconducting state. The majority of bulk transition-metal dichalcogenides’ Tcs are normally between 2 and 4 K. At present, superconducting transition-metal chalcogenides generally show low transition temperature (Tc < 10 K). Fe based transition-metal chalcogenides have certain higher Tc (in the range of 10 ~ 50 K). As have been reported, nano-transition-metal chalcogenides may be one of the routes to improve the superconducting transition temperature. In this review, we would like to give a brief introduction of superconductor development and crystal structure of transition-metal chalcogenides. Furthermore, we will describe major synthesis and physical properties of nano-transition-metal chalcogenides. Finally, recent status and outlook of superconductor based on nano-transition-metal chalcogenides are discussed.

Электрон-фононное взаимодействие в квантовых ямах на основе одноосных материалов

AbstractThe interaction of charged particles with interface optical phonons in quantum wells composed of uniaxially symmetric materials is studied theoretically. It is shown that the character of this interaction depends considerably on the degree of anisotropy of the phonon spectrum in the materials forming the quantum well and the barriers. In the case of strong anisotropy, the interaction turns out to be significantly weaker than it is in similar structures made of materials with cubic symmetry. In the case of weak phonon anisotropy, the interaction of charged particles with optical phonons can under certain conditions be described by a Fröhlich-type coupling constant, and both weak and strong electron–phonon interaction can take place in different structures. The results obtained extend the possibilities of the optical diagnostics of quantum nanostructures.

Effects of material structure on device efficiency of III-nitride intersubband photodetectors

&lt;sec&gt; The photodetection potential applications of III-nitride semiconductor are not only restricted in the ultraviolet range, but also extended to the infrared and even terahertz wave range through intersubband transition (ISBT) process between quantum-confined electronic states. The large conduction band offset (~1.75 eV for GaN/AlN heterostructures) and the strong electron-phonon interaction in these materials enable the III-nitride ISBT photodetectors to operate with ultrahigh speed for near-infrared telecommunication applications. Furthermore, due to the large energy of longitudinal optical phonons of nitride materials (~92 meV for GaN), the corresponding ISBT photodetectors are demonstrated as a promising candidate for operating in a specific terahertz (THz) range (5－12 THz) at high temperature, which is fundamentally in accessible to As-based devices. One of the major difficulties in realizing high performance III-nitride ISBT photodetectors is the existence of undesired polarization field inherited from conventional c-axis nitrides, which greatly makes the design complicated. In this paper, we theoretically study the influences of critical material structure parameters on the band structure, electron distribution, polarization field intensity, and tunneling effect based on near-infrared photovoltaic photodetectors and terahertz double-step photodetectors. &lt;/sec&gt; &lt;sec&gt; III-nitride ISBT photodetectors operating in near-infrared are designed to be able to suppress the internal electric field in the active region, so that as many quantum wells as possible can be populated by electrons to enhance the absorption efficiency. It is found that with the increase of well periods and well doping concentration, a weaker average polarization field is observed, and as a result more wells will be populated with electrons. In addition, keeping the lattice constant of the contact layer material the same as the average lattice constant of the active region is beneficial to making more quantum wells effectively populated with electrons. On the other hand, the devices operating in a terahertz range require a more accurate design of energy band than the near-infrared ones due to its rather small energy interval. The design difficulties lie in how to ensure both the efficient absorption of photons and the following transportation of photo-excited electrons in the presence of polarization-induced field. The simulation results indicate that the polarization field in an active region is sensitive to the aluminum mole composition of barrier layer, the thickness of step well and step barrier layers, while even if the thickness of well and barrier layers change, the polarization field in barrier layer will stay the same. Moreover, reducing the aluminum mole composition of step barrier layer or appropriately increasing its thickness will be helpful in enhancing the photo-excited electrons tunneling. The above knowledge is beneficial to the optimal design of III-nitride ISBT photodetectors with high efficiency. &lt;/sec&gt;

Правила Урбаха в монокристаллах MnGa-=SUB=-2-=/SUB=-Sе-=SUB=-4-=/SUB=- при оптических поглощениях

AbstractThe Urbach rule in MnGa_2Se_4 single crystals is studied. It is found that the long-wavelength absorption tail obeys the Urbach rule at photon energies in the range 2 . 1–2 . 2 eV. It is shown that a strong electron–phonon interaction exists in MnGa_2Se_4 single crystals.

Relationship of alcohol and/or cannabis use in the last year and the severity of the Internet gaming disorder symptoms

In this paper we investigate the photoluminescence (PL) properties and nonlinear transmission of Cu-doped CdSe colloidal quantum dots (QDs) under a nanosecond laser pulse excitation. We find a strong difference of the pump intensity dependent behavior between the basic exciton transition and Cu dopants associated PL. We also study the saturation of absorption in Cu-doped CdSe QDs. The role of a strong electron-phonon interaction in the PL properties and nonlinear transmission of colloidal Cu-doped CdSe QDs is explained. The presence of biexciton PL exciton-phonon interaction and its strong influence on the basic exciton PL and non-linear properties of Cu-doped colloidal CdSe QDs are proved by the simultaneous linear growth of basic excitons PL, the growth of the Stokes shift and significant decrease of absorption at the basic exciton transition wavelength with the increase of pump intensity.

Spontaneous mass generation due to phonons in a two-dimensional Dirac fermion system

Fermions with one and two Dirac nodes are coupled to in-plane phonons to study a spontaneous transition into the Hall insulating phase. At sufficiently strong electron–phonon interaction a gap appears in the spectrum of fermions, signaling a transition into a phase with spontaneously broken parity and time-reversal symmetry. The structure of elementary excitations above the gap in the corresponding phase reveals the presence of scale invariant parity breaking terms which resemble Chern–Simons excitations. Evaluating the Kubo formula for both models we find quantized Hall plateaux in each case, with conductance of binodal model exactly twice as large as of the mononodal model.

Intermolecular coupling and superconductivity in PbMo6S8 and other Chevrel phase compounds

To understand superconductivity in Chevrel phase compounds and guide the search for interesting properties in materials created with Chevrel phase molecules as building blocks, we use ab-initio methods to study the properties of single Mo$_6$X$_8$ molecules with $X=S$, $Se$, $Te$ as well as the bulk solid PbMo$_6$S$_8$. In bulk PbMo$_6$S$_8$, the different energy scales from strong to weak are: the band kinetic energy, the intra-molecular Coulomb interaction, the on-molecule Jahn-Teller energy and the Hund's exchange coupling. The metallic state is stable with respect to Mott and polaronic insulating states. The bulk compound is characterized by a strong electron-phonon interaction with the largest coupling involving phonon modes with energies in the range from 11 meV to 17 meV and with a strong inter-molecule (Peierls) character. A two-band Eliashberg equation analysis shows that the superconductivity is strong-coupling, with different gaps on the two Fermi surface sheets. A Bergman-Rainer analysis of the functioanl derivative of the transition temperature with respect to the electron-phonon coupling reveals that the Peierls modes provide the most important contribution to the superconductivity. This work illustrates the importance of inter-molecular coupling for collective phenomena in molecular solids.

Influence of electron-phonon coupling on the low-temperature phases of metallic single-wall carbon nanotubes

We investigate the effect of electron-phonon coupling on low temperature phases in metallic single-wall carbon nanotubes. We obtain low-temperature phase diagrams of armchair and zigzag type nanotubes with screened interactions with a weak-coupling renormalization group approach. In the absence of electron-phonon coupling, two types of nanotubes have similar phase diagrams. A $D$-Mott phase or $d$-wave superconductivity appears when the on-site interaction is dominant, while a charge-density wave or an excitonic insulator phase emerges when the nearest neighbor interaction becomes comparable to the on-site interaction. The electron-phonon coupling, treated by a two-cutoff scaling scheme, leads to different behavior in two types of nanotubes. For strong electron-phonon interactions, phonon softening is induced and a Peierls insulator phase appears in armchair nanotubes. We find that this softening of phonons may occur for any intraband scattering phonon mode. On the other hand, the effect of electron-phonon coupling is negligible for zigzag nanotubes. The distinct behavior of armchair and zigzag nanotubes against lattice distortion is explained by analysis of the renormalization group equations.

Anomalous thermoelectric power and polaronic transport in the vicinity of topological phase transition of Pb1-xSnxTe

Pb1−xSnxTe compounds exhibits the topological phase transition from trivial semiconductor (PbTe) to topological crystalline insulator (SnTe). In the vicinity of topological phase transition in the region of Pb1−xSnxTe (x= 0.2, 0.3, and 0.4), the band touching of conduction L6− and valence L6+ bands gives rise to topological Dirac semimetal. Here we investigated thermoelectric properties in the region of topological Dirac semimetal in Pb1−xSnxTe compounds. From the temperature-dependent Hall RH(T) and Seebeck coefficients S(T), we found the abnormal sign difference between Hall (positive RH) and Seebeck (negative S) coefficients in the compounds of x= 0.3 and x = 0.4 at intermediate temperature range. We analyzed the abnormal sign difference between the Hall and Seebeck coefficients in terms of the Aharanov-Bohm effect of non-adiabatic 3-site polaron hopping. The polaronic hopping in the compounds can be justified by strong electron-phonon coupling. The temperature-dependent behavior of Hall mobility μH(T) and carrier density nH(T) indicates the strong electron-phonon scattering. From the temperature-dependent thermal conductivity κ(T) measurement, the broad peaks at low temperature, driven by freezing-out of Umklapp scattering, are systematically suppressed by increasing Sn concentration, which consistent with the strong electron-phonon interaction in the system.