Weak Electron-phonon Interaction Research Articles

Exciton photoluminescence of CsPbBr3@SiO2 quantum dots and its application as a phosphor material in light-emitting devices

In this report, we mainly investigate the optical property differences between CsPbBr3@SiO2 quantum dots (QDs) and CsPbBr3 QDs. The photoluminescence demonstrates that CsPbBr3@SiO2 QDs and CsPbBr3 QDs have similar exciton binding energy. Both CsPbBr3 and CsPbBr3@SiO2 QDs present optical bandgaps and photoluminescence (PL) linewidth broadening as the temperature increases from 10 K to room temperature, which is attributed to the thermal expansion and electron-phonon coupling. The fitting results show that CsPbBr3 and CsPbBr3@SiO2 QDs have the similar bandgap thermal expansion coefficient, but the CsPbBr3@SiO2 QDs have weaker electron-phonon interaction. Temperature-dependent time-resolved photoluminescence (TRPL) demonstrates that the PL lifetime increases with the temperature and CsPbBr3@SiO2 QDs have longer PL lifetime than CsPbBr3 QDs after 110 K. In addition, the CsPbBr3@SiO2 QDs integrated on the blue light-emitting diode chip as green phosphor material show better thermal stability in ambient air.

Modification of electron-phonon coupling by micromachining and suspension

Weak electron-phonon interaction in metals at low temperatures forms the basis of operation for cryogenic hot-electron bolometers and calorimeters. Here, we develop a thermometry scheme based on proximity supercurrent to study the thermal response of a thin gold film on a SiO2 platform at temperatures below 100 mK. We find that the exponent of the power law describing electron-phonon coupling in the film drops by approximately 1/2 as the platform is micromachined and released from its substrate. This contrasts the conventional theory for bulklike geometries that predicts integer-valued exponents. We attribute the fractional change to a modified phonon spectrum described by recent theoretical developments.

Comparisons of electrical/magneto-transport properties of degenerate semiconductors BiCuXO (X = S, Se and Te) and their electron-phonon-interaction evolution

Layered BiCuXO (X = S, Se, and Te) compounds attract much attention recently because of their superior thermoelectric properties. However, the major shortcoming of these compounds is the relatively large electrical resistance. Exploring the mechanism of electrical scattering of BiCuXO can give some hints on how to optimize their electrical conductivity. Here, we compared electrical and magnetotransport properties of degenerate semiconductor BiCuXO. These compounds show metallic electrical properties, and the in-plane resistivity is linearly dependent on the temperature at the high-temperature regime (>150 K). The magnetoresistance of these compounds is approximately quadratically dependent on the square of magnetic field B, and magnetoresistance decreases from 10%–16% to 0.5% when the temperature is increased from 2 to 300 K. The Hall measurements substantiate that the electrical carriers of BiCuXO are holes, which perhaps come from Bi vacancies. The electron-optical-phonon interaction is the major mechanism of electron scattering when the temperature is higher than 80 K. Analyzing the linear coefficient ρxx-T of BiCuXO (X = S, Se, and Te) suggests that the averaged electron-phonon interaction of BiCuSO is nearly six times larger than that of BiCuSeO/BiCuTeO. Based on the Thomas-Fermi model, compared with BiCuSO, the weak electron-phonon interaction in BiCuSeO/BiCuTeO could be attributed to a highly screened Coulomb interaction between electrons and ions. This work may be useful to optimize the electrical properties of BiCuXO to satisfy the requirement of thermoelectric materials.

Raman scattering study of NaFe0.53Cu0.47As

We use polarization-resolved Raman scattering to study lattice dynamics in NaFe$_{0.53}$Cu$_{0.47}$As single crystals. We identify 4 $A_{1g}$ phonon modes at 125, 172, 183 and 197 cm$^{-1}$, and 4 $B_{3g}$ phonon modes at 101, 138, 173, 226 cm$^{-1}$. The phonon spectra are consistent with the $Ibam$ group, which confirms that the Cu and Fe atoms form a stripe order. The temperature dependence of the phonon spectra suggests weak electron-phonon and magneto-elastic interactions.

Potentially high-temperature superconductivity in K1−xB6: A first-principles prediction

Electronic structure, lattice dynamics, and superconducting properties for KB6 are obtained by first-principles calculations. We show that the stoichiometric KB6 have a weak electron-phonon interaction. The obtained electron-phonon coupling strength λ and the superconducting transition temperature TC for KB6 is only 0.22 and 0.14 K (μ*=0.1), respectively. However, hole doping can greatly enhance the electron-phonon coupling strength for KB6. For K1-xB6, the e-ph coupling strength λ and TC can reach as high as 1.13 and 65.3 K, respectively. This indicates that K1−xB6 is potentially high-temperature superconducting.

Vacuum Referred Binding Energy Scheme, Electron–Vibrational Interaction, and Energy Transfer Dynamics in BaMg2Si2O7:Ln (Ce3+, Eu2+) Phosphors

The host structure and the synchrotron radiation VUV–UV luminescence properties of samples BaMg2Si2O7 (BMSO):Ln (Ce3+, Eu2+) at different doping levels and different temperatures were investigated in detail. Three important aspects are studied to elucidate the luminescence properties of samples: (1) the vacuum referred binding energy (VRBE) scheme is constructed with the electron binding in the BMSO host bands and in the Ce3+ and Eu2+ impurity levels with the aim to explain the different thermal stabilities of Ce3+ and Eu2+ emissions; (2) the electron–vibrational interaction analysis on the narrow Eu2+ emission indicates a weak electron–phonon interaction in the current case; (3) by using three models (Inokuti–Hirayama, Yokota–Tanimoto, and Burshteĭn models) at different conditions, the energy transfer dynamics between Ce3+ and Eu2+ was analyzed. It reveals that the energy transfer from Ce3+ to Eu2+ via electric dipole–dipole (EDD) interaction is dominant while energy migration between Ce3+ is negligible....

Optical transmittance and reflectance studies and evidence of weak electron–phonon interaction in Type-I Ge clathrate Ba8Ga16Ge30

The optical properties of ternary type-I Ge clathrate Ba8Ga16Ge30 are investigated by optical reflectance and transmittance measurements. The refractive index and extinction coefficient are calculated from the reflectance spectrum via the modified Kramers–Kronig analysis method between 0.5 and 3.2 eV. The photon energy dependence of the optical absorption coefficient reveals that Ba8Ga16Ge30 is an indirect band gap semiconductor with a gap energy Eg of 0.69 eV at 5.7 K and 0.66 eV at 285 K. The temperature dependence of Eg can be satisfactorily described by an equation based on the Bose–Einstein statistics model. When compared with those of common elemental, III–V, and II–VI semiconductors, the small temperature coefficient dEg/dT of the Ba8Ga16Ge30 can be considered to represent the weak electron–phonon interaction in the Ba8Ga16Ge30 clathrate.

Chemical Trend of Superconducting Critical Temperatures in Hole-Doped CuBO2, CuAlO2, CuGaO2, and CuInO2

We calculated the superconducting transition temperature $T_{\rm c}$ of hole-doped CuBO$_2$, CuAlO$_2$, CuGaO$_2$ and CuInO$_2$ using first-principles. The calculated $T_{\rm c}$ are about 50 K for CuAlO$_2$, 10 K for CuBO$_2$ and CuGaO$_2$ and 1 K for CuInO$_2$ at maximum in the optimum hole-doping concentration. The low $T_{\rm c}$ of CuInO$_2$ is attributed to the weak electron-phonon interaction caused by the low covalency and heavy atomic mass.

High luminescent brightness and thermal stability of red emitting Li3Ba2Y3(WO4)8:Eu3+ phosphor

A series of Li3Ba2Y3−x(WO4)8:xEu3+ (x=0.1, 1, 1.5, 2 and 2.8) phosphors were synthesized by a high temperature solid-state reaction method. Under the excitation of near ultraviolet (NUV) light, the as-prepared phosphor exhibits intense red luminescence originating from the characteristic transitions of Eu3+ ions, which is 1.8 times as strong as the commercial Y2O2S:Eu3+ phosphor. The optimal doping concentration of Eu3+ ions here is confirmed as x=1.5. The electric dipole-quadrupole (D-Q) interaction is deduced to be responsible for concentration quenching of Eu3+ ions in the Li3Ba2Y3(WO4)8 phosphor. The analysis of optical transition and Huang-Rhys factor reveals a weak electron-phonon coupling interaction. The temperature-dependent emission spectra also indicate that the as-prepared Li3Ba2Y3(WO4)8:Eu3+ phosphor has better thermal stability than that of the commercial Y2O2S:Eu3+ phosphor. Therefore, our results show that the as-prepared Li3Ba2Y3(WO4)8:Eu3+ phosphor is a promising candidate as red emitting component for white light emitting diodes (LEDs).

Hot carrier dynamics in HfN and ZrN measured by transient absorption spectroscopy

The hot carrier solar cell is a promising concept for high efficiency photovoltaics. Obtaining the dynamic temperature information for the hot carriers in an absorber is of crucial importance to this concept. In this paper, we present a dynamical hot carrier theory that extracts the carrier temperature and the instantaneous carrier occupation from transient absorption spectra. We have applied this model to two transition metal nitride materials, HfN and ZrN. The analysis showed that for HfN the initial carrier temperature rose to 360K immediately after a femtosecond pulse excitation and then dropped rapidly to 330K within 0.13ps. After this initial process, the carrier temperature still remained relatively elevated at above 320K for as long as 2ns. The initial fast process was attributed to the strong carrier–carrier scattering in the material while the slow decay of temperature might be due to unusually weak electron–phonon interactions. A shorter cooling time and overall lower hot carrier temperatures were observed in ZrN.

Photon and phonon-assisted shot noise in a molecular quantum-dot-ferromagnetic system in the Coulomb blockade regime

The dynamic shot noise in a molecular quantum dot connected to two non-collinear ferromagnetic terminals under the perturbation of ac fields has been investigated by the nonequilibrium Green’s function approach. The formulas of current, current correlation, and shot noise have been presented in the weak electron-phonon (el-ph) interaction regime to address the cooperated behaviors generated by the photon and phonon perturbation. The photon and phonon perturbations induce quite different current correlations, and the noise suppression occurs unambiguously by increasing the phonon energy in the valley regime. The suppression of shot noise comes from drawing the unbalanced current correlation towards the balanced one, where coherent current correlation takes major role. The suppression of photon-assisted shot noise can be completed mainly by rotating polarization angle of the terminals, and increasing the phonon energy ħω 0. The Fano factor is enhanced considerably by the el-ph interaction, and it is also enhanced by increasing the polarization angle from θ = 0 to θ = π.

Hot-electron effect in spin relaxation of electrically injected electrons in intrinsic Germanium

The hot-electron effect in the spin relaxation of electrically injected electrons in intrinsic germanium is investigated by the kinetic spin Bloch equations both analytically and numerically. It is shown that in the weak-electric-field regime with E ≲ 0.5 kV cm−1, our calculations have reasonable agreement with the recent transport experiment in the hot-electron spin-injection configuration (2013 Phys. Rev. Lett. 111 257204). We reveal that the spin relaxation is significantly enhanced at low temperature in the presence of weak electric field E ≲ 50 V cm−1, which originates from the obvious center-of-mass drift effect due to the weak electron–phonon interaction, whereas the hot-electron effect is demonstrated to be less important. This can explain the discrepancy between the experimental observation and the previous theoretical calculation (2012 Phys. Rev. B 86 085202), which deviates from the experimental results by about two orders of magnitude at low temperature. It is further shown that in the strong-electric-field regime with 0.5 ≲ E ≲ 2 kV cm−1, the spin relaxation is enhanced due to the hot-electron effect, whereas the drift effect is demonstrated to be marginal. Finally, we find that when 1.4 ≲ E ≲ 2 kV cm−1 which lies in the strong-electric-field regime, a small fraction of electrons (≲5%) can be driven from the L to Γ valley, and the spin relaxation rates are the same for the Γ and L valleys in the intrinsic sample without impurity. With the negligible influence of the spin dynamics in the Γ valley to the whole system, the spin dynamics in the L valley can be measured from the Γ valley by the standard direct optical transition method.

Conditions for formation of electron pairs in a metal

In an isotropic model of the electron system of metal that is presented by the Fröhlich’s initial Hamiltonian, in the approximation of a weak electron–phonon interaction at T=0, first time, we show that the ground state of the system is the state with pairing correlations of electrons (the pair correlations of occupied electron states). In contrast to the BCS approach, the initial point in our approach is not electron pairing but is the maximum reduction of the energy of the considered system due to virtual processes of the electron–phonon interaction and to the exchange effect for the indirect electron–electron interaction (which is induced by certain phonon modes separately from others). In contrast to the BCS approach, we take into account the portion of the energy of the electron system that is connected with the above exchange effect. In the BCS approach, the corresponding portion is missed, and its role is prescribed to the portion that does not relate to the electron pairing.We show that expectation values of the above Hamiltonian for different wave functions for two interacting electrons above the Fermi sea of the non-interacting system (with interaction between the electrons that is induced by different phonon modes separately from others) are minimum for a certain structure of these functions and simultaneously for phonon modes that can induce the transitions of the interacting electrons between electron states in which they are (without violation of the Pauli exclusion principle and at everything else being equal). On the basis of the above results, the mechanism of the maximum reduction of the energy of the considered electron system is considered. In this mechanism each electron interact with the very different phonons, but in such way that give the maximum-possible negative contribution to the energy of the considered electron system.The theme of the article is conditions for the formation of electron pairs in a metal. This requires our understanding for the mechanism of the formation of electron pairs in a metal. The absence of this understanding is the main drawback of the BCS theory. The considered mechanism gives the solution. If this mechanism is feasible for a metal at T=0, the electron system of this metal can be described by the Hamiltonian that is similar to the BCS reduced Hamiltonian and the ground-state wave function is similar to the BCS ground-state wave function.The considered mechanism combines the simplicity and universality of the BCS model with giving wide opportunities to study conditions for the formation of the state of the electron system in a metal with the pair correlation of conduction electrons near the Fermi surface and with a gap in the spectrum of electronic excitations of this system and to study the dependence of these conditions on crystalline structure and structure of the conduction band of metals. It is so, because the considered mechanism has the universal nature but the above dependence is largely determined by the nature of virtual pairs in a metal.

Nonequilibrium optical conductivity in materials with localized electronic states

A wide range of disordered materials contain electronic states that are spatially well localized. In this work, we investigated the electrical response of such systems in non-equilibrium conditions to external electromagnetic field. We obtained the expression for optical conductivity valid for any non-equilibrium state of electronic subsystem. In the case of incoherent non-equilibrium state, this expression contains only the positions of localized electronic states, Fermi's golden rule transition probabilities between the states and the populations of electronic states. The same form of expression is valid both in the case of weak electron-phonon interaction and weak electron-impurity interaction that act as perturbations of electronic Hamiltonian. The derivation was performed by expanding the general expression for AC conductivity in powers of small electron-phonon interaction or electron-impurity interaction parameter. Applications of the expression to two model systems, a simple one- dimensional Gaussian disorder model and the model of a realistic three-dimensional organic polymer material, were presented, as well.

Weak Electron-Phonon Coupling and Unusual Electron Scattering of Topological Surface States in Sb(111) by Laser-Based Angle-Resolved Photoemission Spectroscopy

High-resolution laser-based angle-resolved photoemission (ARPES) measurements are carried out on Sb(111) single crystal. Two kinds of Fermi surface sheets are observed, which are derived from the topological surface states: one small hexagonal electron-like Fermi pocket around Γ point and the other six elongated lobes of hole-like Fermi pockets around the electron pocket. Clear Rashba-type band splitting due to the strong spin-orbit coupling is observed to be anisotropic in the momentum space. Our super-high-resolution ARPES measurements reveal no obvious kink in the surface band dispersions, indicating a weak electron-phonon interaction in the surface states. In particular, the electron scattering rate for these topological surface states is nearly a constant over a large energy window near the Fermi level that is unusual in terms of the conventional picture.

Highly sensitive hot electron bolometer based on disordered graphene.

A bolometer is a device that makes an electrical resistive response to the electromagnetic radiation resulted from a raise of temperature due to heating. The combination of the extremely weak electron-phonon interactions along with its small electron heat capacity makes graphene an ideal material for applications in ultra-fast and sensitive hot electron bolometer. However, a major issue is that the resistance of pristine graphene weakly depends on the electronic temperature. We propose using disordered graphene to obtain a strongly temperature dependent resistance. The measured electrical responsivity of the disordered graphene bolometer reaches 6 × 106 V/W at 1.5 K, corresponding to an optical responsivity of 1.6 × 105 V/W. The deduced electrical noise equivalent power is 1.2 , corresponding to the optical noise equivalent power of 44 . The minimal device structure and no requirement for high mobility graphene make a step forward towards the applications of graphene hot electron bolometers.

Dissipative time-dependent quantum transport theory

A dissipative time-dependent quantum transport theory is developed to treat the transient current through molecular or nanoscopic devices in presence of electron-phonon interaction. The dissipation via phonon is taken into account by introducing a self-energy for the electron-phonon coupling in addition to the self-energy caused by the electrodes. Based on this, a numerical method is proposed. For practical implementation, the lowest order expansion is employed for the weak electron-phonon coupling case and the wide-band limit approximation is adopted for device and electrodes coupling. The corresponding hierarchical equation of motion is derived, which leads to an efficient and accurate time-dependent treatment of inelastic effect on transport for the weak electron-phonon interaction. The resulting method is applied to a one-level model system and a gold wire described by tight-binding model to demonstrate its validity and the importance of electron-phonon interaction for the quantum transport. As it is based on the effective single-electron model, the method can be readily extended to time-dependent density functional theory.

Impact Excitation and Electron–Hole Multiplication in Graphene and Carbon Nanotubes

In semiconductor photovoltaics, photoconversion efficiency is governed by a simple competition: the incident photon energy is either transferred to the crystal lattice (heat) or transferred to electrons. In conventional materials, energy loss to the lattice is more efficient than energy transferred to electrons, thus limiting the power conversion efficiency. Quantum electronic systems, such as quantum dots, nanowires, and two-dimensional electronic membranes, promise to tip the balance in this competition by simultaneously limiting energy transfer to the lattice and enhancing energy transfer to electrons. By exploring the optical, thermal, and electronic properties of quantum materials, we may perhaps find an ideal optoelectronic material that provides low cost fabrication, facile systems integration, and a means to surpass the standard limit for photoconversion efficiency. Nanoscale carbon materials, such as graphene and carbon nanotubes, provide ideal experimental quantum systems in which to explore optoelectronic behavior for applications in solar energy harvesting. Within essentially the same material, researchers can achieve a broad spectrum of energetic configurations, from a gapless semimetal to a large band-gap semiconducting nanowire. Owing to their nanoscale dimensions, graphene and carbon nanotubes exhibit electronic and optical properties that reflect strong electron-electron interactions. Such strong interactions may lead to exotic low-energy electron transport behavior and high-energy electron scattering processes such as impact excitation and the inverse process of Auger recombination. High-energy processes, which become very important under photoexcitation, may be particularly efficient in nanoscale carbon materials due to the relativistic-like, charged particle band structure and sensitivity to the dielectric environment. In addition, due to the covalently bonded carbon framework that makes up these materials, electron-phonon coupling is very weak. In carbon nanomaterials, strong electron-electron interactions combined with weak electron-phonon interactions results in excellent optical, thermal and electronic properties, the exploration of which promises to reveal fundamentally new physical processes and deliver advanced nanotechnologies. In this Account, we review the results of novel optoelectronic experiments that explore the intrinsic photoresponse of carbon nanomaterials integrated into nanoscale devices. By fabricating gate voltage-controlled photodetectors composed of atomically thin sheets of graphene and individual carbon nanotubes, we are able to fully explore electron transport in these systems under optical illumination. We find that strong electron-electron interactions play a key role in the intrinsic photoresponse of both materials, as evidenced by hot carrier transport in graphene and highly efficient multiple electron-hole pair generation in nanotubes. In both of these quantum systems, photoexcitation leads to high-energy electron-hole pairs that relax energy predominantly into the electronic system, rather than heating the lattice. Due to highly efficient energy transfer from photons into electrons, graphene and carbon nanotubes may be ideal materials for solar energy harvesting devices with efficiencies that could exceed the Shockley-Queisser limit.

Temperature-Dependent Fluorescence in Carbon Dots

Carbon dots are cost-effective, environmental friendly, and biocompatible nanoparticles with many potential applications in optoelectronics and biophotonics. Their dual fluorescence bands were observed and could be attributed to core and surface state emission. We also conduct temperature-dependent fluorescence measurements from cryogenic to room temperatures. The dual emission bands exhibit similar temperature dependence. The strong electron–electron interactions and weak electron–phonon interactions could account for the very broad photoluminescence (PL) band even at 77 K. Our experimental results also suggest that carbon dots exhibit similar temperature behavior as metallic quantum dots (nanoclusters) but are different from inorganic semiconductor quantum dots. Here, for the first time, we present the temperature-dependent spectroscopic results to shed some light on the presently unclear fluorescence mechanism.

Chemical trend of superconducting transition temperature in hole-doped delafossite of CuAlO2, AgAlO2 and AuAlO2

We have performed the first-principles calculations about the superconducting transition temperature Tc of hole-doped delafossite CuAlO2, AgAlO2 and AuAlO2. Calculated Tc are about 50K (CuAlO2), 40K (AgAlO2) and 3K(AuAlO2) at maximum in the optimum hole-doping concentration. The low Tc of AuAlO2 is attributed to the weak electron–phonon interaction caused by the low covalency and heavy atomic mass.