Electron-phonon Coupling Interaction Research Articles

Tuning Electron–Phonon Coupling Interaction for the Efficient Wide Blue Emission of Pb2+‐Doped Cs2InCl5·H2O

AbstractLead‐free perovskites exhibit unique crystal structures and optical properties due to their low‐dimensional structure. However, the 0D structure of Cs2InCl5·H2O usually demonstrates poor optical properties at room temperature due to their extremely strong electron–phonon coupling interaction. In this study, a doping strategy is employed by introducing Pb2+ into Cs2InCl5·H2O, resulting in the achievement of an efficient broadband blue emission, with a photoluminescence quantum yield (PLQY) of 58%. The temperature‐dependent PL spectra reveal that the efficient emission is attributed to the weakening of the electron–phonon coupling strength in the lattice and the inhibition of the phonon‐assisted non‐radiative recombination process through Pb2+ doping. The validity of this deduction is further corroborated by the results of the Raman spectra. The results of the first‐principle calculation demonstrate that the broadband emission originates from multiple emission paths in In3+ and Pb2+ polyhedrons. In addition, Pb2+‐doped Cs2InCl5·H2O can realize the transformation from sky‐blue emission to deep‐blue emission by absorbing water in the air. Remarkably, this change is reversible during the heating–cooling cycles, demonstrating excellent optical stability with repeatable recyclability.

Possible charge ordering and anomalous transport in graphene/graphene quantum dot heterostructure

Observations of superconductivity and charge density waves (CDW) in graphene have been elusive thus far due to weak electron–phonon coupling (EPC) interactions. Here, we report a unique observation of anomalous transport and multiple charge ordering phases at high temperatures ( T1∼213K , T2∼325K ) in a 0D−2D van der Waals (vdW) heterostructure comprising of single layer graphene (SLG) and functionalized (amine) graphene quantum dots (GQD). The presence of functionalized GQD contributed to charge transfer with shifting of the Dirac point ∼ 0.05 eV above the Fermi level (ab initio simulations) and carrier density n∼−0.3×1012 cm−2 confirming p-doping in SLG and two-fold increase in EPC interaction was achieved. Moreover, we elucidate the interplay between electron–electron and electron–phonon interactions to substantiate high temperature EPC driven charge ordering in the heterostructure through analyses of magnetotransport and weak anti-localization (WAL) framework. Our results provide impetus to investigate strongly correlated phenomena such as CDW and superconducting phase transitions in novel graphene based heterostructures.

Origin of charge density wave in topological semimetals SrAl4 and EuAl4

Topological semimetals in BaAl4-type structure show many interesting behaviors, such as charge density wave (CDW) in SrAl4 and EuAl4, but not the isostructural and isovalent BaAl4, SrGa4, and BaGa4. Here using Wannier functions based on density functional theory, we calculate the susceptibility functions with millions of k-points to reach the small q-vector and study the origin and driving force behind the CDW. Our comparative study reveals that the origin of the CDW in SrAl4 and EuAl4 is the strong electron-phonon coupling interaction for the transverse acoustic mode at small q-vector along the Γ-Z direction besides the maximum of the real part of the susceptibility function from the nested Fermi surfaces of the Dirac-like bands, which explains well the absence of CDW in the other closely related compounds in a good agreement with experiment. We also connect the different CDW behaviors in the Al compounds to the macroscopic elastic properties.

Electron Spin Decoherence Dynamics in Magnetic Manganese Hybrid Organic-Inorganic Crystals: The Effect of Lattice Dimensionality.

Organic-inorganic metal hybrids with their tailorable lattice dimensionality and intrinsic spin-splitting properties are interesting material platforms for spintronic applications. While the spin decoherence process is extensively studied in lead- and tin-based hybrids, these systems generally show short spin decoherence lifetimes, and their correlation with the lattice framework is still not well-understood. Herein, we synthesized magnetic manganese hybrid single crystals of (4-fluorobenzylamine)2MnCl4, ((R)-3-fluoropyrrolidinium)MnCl3, and (pyrrolidinium)2MnCl4, which represent a change in lattice dimensionality from 2D and 1D to 0D, and studied their spin decoherence processes using continuous-wave electron spin resonance spectroscopy. All manganese hybrids exhibit nanosecond-scale spin decoherence time τ2 dominated by the symmetry-directed spin exchange interaction strengths of Mn2+-Mn2+ pairs, which is much longer than lead- and tin-based metal hybrids. In contrast to the similar temperature variation laws of τ2 in 2D and 0D structures, which first increase and gradually drop afterward, the 1D structure presents a monotonous rise of τ2 with the temperatures, indicating the strong correlation of spin decoherence with the lattice rigidity of the inorganic framework. This is also rationalized on the basis that the spin decoherence is governed by the competitive contributions from motional narrowing (prolonging the τ2) and electron-phonon coupling interaction (shortening the τ2), both of which are thermally activated, with the difference that the former is more pronounced in rigid crystalline lattices.

Polaron level spectrum in double-potential quantum wells

The description of macroscopic systems of quasi-particles is a bright spot in quantum physics, providing an accurate description of the physics development. The current study presents a theoretical study of the energy level spectrum of weakly coupled polarons in polar crystals (GaAs, PbSe, PbTe) in double potential quantum wells (parabolic and asymmetric Gaussian confinement potentials) using the linear combination operator and two simple unitary transformations. Rigorous electron-phonon coupling interaction results are obtained under the weak coupling limitation. The results indicate that the polaron energy is relatively lower than that of the electron and the coupling strength affects the energy difference between polaron and electron. The influence of quantum wells’ confinement potential on energy is also discussed. These findings provide the possibility for potential application of polaron.

Deep investigation of phonon induced photoluminescence evolution of YAG:Cr transparent ceramics at a wide temperature range

• In this study, we focused mainly on the vibronic coupling induced luminescence behavior of Cr 3+ doped YAG deep-red transparent ceramics, and two highlights have been listed as below. • Room-temperature PL excitation spectrum was experimentally recorded, which was further theoretically simulated to reveal the underlying electron-phonon coupling interaction with giving different Huang-Rhys factor values. • The dominant phonon energy involving in the vibronic coupling process was determined to be 57 meV which was consistent with that of the fitting work on temperature-dependent PL integrated intensity.

Improving Tc in sodalite-like boron-nitrogen compound M2(BN)6

To explore the improvement of the critical temperature (Tc) of boron-nitrogen materials, the doping effect of other elements into the sodalite-like clathrate compounds M2(BN)6 is studied, based on the first-principles calculations. The stability, electronic structures, dynamical properties, and electron-phonon coupling interactions of these materials upon doping suggest that they are potential high-temperature superconductors. The result shows that the highest Tc of boron-nitrogen clathrate compound at ambient pressure reaches 72 K by doping Al. Boron-nitrogen clathrates doped by H, In, Tl, Si, and Ge also possess the high Tc values of 45, 54, 67, 58, and 41 K at ambient pressure, respectively. By increasing the doping concentration, the Tc value of boron-nitrogen clathrate can be obviously improved and nearly doubled. The predicted Tc of 72 K is the highest value for all boron-nitrogen materials at ambient pressure. It is much higher than those of magnesium diboride and alkali fullerides.

Pressure-induced structural transition, metallization, and topological superconductivity in PdSSe

Pressure not only provides a powerful way to tune the crystal structure of transition metal dichalcogenides (TMDCs) but also promotes the discovery of exotic electronic states and intriguing phenomena. Structural transitions from the quasi-two-dimensional layered orthorhombic phase to three-dimensional cubic pyrite phase, metallization, and superconductivity under high pressure have been observed experimentally in TMDCs materials PdS2 and PdSe2. Here, we report a theoretical prediction of the pressure-induced evolutions of crystal structure and electronic structure of PdSSe, an isomorphous intermediate material of the orthorhombic PdS2 and PdSe2. A series of pressure-induced structural phase transitions from the layered orthorhombic structure into an intermediate phase, then to a cubic phase are revealed. The intermediate phase features the same structure symmetry as the ambient orthorhombic phase, except for drastic collapsed interlayer distances and striking changes of the coordination polyhedron. Furthermore, the structural phase transitions are accompanied by electronic structure variations from semiconductor to semimetal, which are attributed to bandwidth broaden and orbital-selective mechanisms. Especially, the cubic phase PdSSe is distinct from the cubic PdS2 and PdSe2 materials by breaking inversion and mirror-plane symmetries, but showing similar superconductivity under high pressure, which is originated from strong electron-phonon coupling interactions concomitant with topologically nontrivial Weyl and high-fold Fermions. The intricate interplay between lattice, charge, and orbital degrees of freedom as well as the topologically nontrivial states in these compounds will further stimulate wide interest to explore the exotic physics of the TMDCs materials.

Anion-Centered Polyhedron Strategy for Strengthening Photon Emission Induced by Electron-Phonon Coupling.

Electron-phonon coupling emerges as a growing frontier in the heart of condensed matter from physical symmetry to the electronic quantum state, but its quantitative strength dependence on the chemical structure has not been assessed. Here, we originally proposed the anion-centered polyhedron (ACP) strategy for elaborating the electron-phonon coupling interaction in rare-earth (RE) materials comprising three chemical factors, RE-O bond length, the effective charge of the coordinated atom, and structural dimensionality. Using Gd3+ cation with 4f7 configuration as a fluorescence probe, we found that the "free-O"-centered polyhedron is the most crucial motif in strengthening the phonon-assisted energy transfer and photon emission. The temperature-dependent Huang-Rhys S factors were calculated to identify the electron-phonon coupling intensity based on the fluorescence spectrum quantitatively. Finally, beyond conventional wisdom, a series of structural criteria were presented, serving as useful guidelines for discovering strongly coupled rare-earth optical materials. Our study breaks the long-time "blind"-searching diagram and provides reliable principles for many functional materials associated with electron-phonon coupling, such as superconductors, multiferroics, and phosphors.

Predicted structures and superconductivity of LiYHn (n = 5-10) under high pressure.

The structures of LiYHn (n = 5-10) compounds in the pressure range of 0-300 GPa have been extensively explored using the CALYPSO structure prediction method based on the particle swarm optimization algorithm and first-principles calculation. Four stable structures (P21/m LiYH6, C2/c LiYH8, P1̄ LiYH9, R3̄m LiYH10) and three metastable phases (Pnma LiYH6, P1̄ LiYH8, Immm LiYH9) were predicted. They all exhibit metallic and superconducting behavior in their respective stable pressure ranges, and the predicted superconducting transition temperature Tc is within 22-109 K when the pressure is greater than 100 GPa. It was found that after doping Li into YHn (n = 6, 9, 10), the H2 units in the system increased, the electron-phonon coupling interaction weakened, and Tc decreased when the structural characteristics, electronic density of states distribution, and superconductivity of LiYHn and YHn (n = 6, 8, 9, 10) were compared. Systems that have a high density of H_s states and a low number of Y_d states at the Fermi level have stronger electron-phonon coupling (EPC) interactions and higher Tc.

Superconductivity in metal intercalated graphite-like boron-carbon-nitrogen

Ternary B-C-N materials have been widely investigated during recent decades. Additionally, previous investigations have indicated that intercalation of alkali metal could induce superconductivity emerged in layer structure. Motivated by this strategy, we attempt to establish the bulk structure of K-BC2N and explore its related properties by using density functional theory calculations. Two stable phases of K-BC2N were identified with anticipated metallic feature. Therefore, we calculated electron-phonon coupling interaction of K-BC2N within Allen-Dynes modified McMillan equation. The results confirm that bulk K-BC2N is a phonon-mediated superconductor with estimated critical temperature of 3∼6 K, providing a feasible approach for design and synthesis new superconductor at atmospheric pressure.

Second group of high-pressure high-temperature lanthanide polyhydride superconductors

Rare-earth polyhydrides formed under pressure are promising conventional superconductors, with the critical temperature ${T}_{c}$ in compressed ${\mathrm{LaH}}_{10}$ almost reaching room temperature. Here, we report a systematic computational investigation of the structural and superconducting properties of rare-earth (RE) polyhydrides formed under pressure across the whole lanthanide series. Analyses of the electronic and dynamical properties and electron-phonon coupling interaction for the most hydrogen-rich hydrides ${\mathrm{REH}}_{n}$ ($n=8,9,10$) that can be stabilized below 400 GPa show that enhanced ${T}_{c}$ correlates with a high density of H $s$ states and low number of RE $f$ states at the Fermi level. In addition to previously predicted and measured ${\mathrm{LaH}}_{10}$ and ${\mathrm{CeH}}_{9}$, we suggest ${\mathrm{YbH}}_{10}$ and ${\mathrm{LuH}}_{8}$ as additional potential high-${T}_{c}$ superconducters. They form a ``second island'' of high-${T}_{c}$ superconductivity amongst the late lanthanide polyhydrides, with an estimated ${T}_{c}$ of 102 K for ${\mathrm{YbH}}_{10}$ at 250 GPa.

Charge density wave instability and pressure-induced superconductivity in bulk 1T−NbS2

Charge-density-wave (CDW) instability and pressure-induced superconductivity in bulk 1T-NbS2 are predicted theoretically by first-principles calculations. We reveal a CDW instability towards the formation of a stable commensurate CDW order, resulting in a sqart(13)*sqart(13) structural reconstruction featured with star-of-David clusters. The CDW phase exhibits one-dimensional metallic behavior with in-plane flat-band characteristics, and coexists with an orbital-density-wave order predominantly contributed by 4d_(z^2-r^2 ) orbital from the inner Nb atoms of the star-of-David cluster. By doubling the cell of the CCDW phase along the layer stacking direction, a metal-insulator transition may be realized in the CDW phase in case the interlayer antiferromagnetic ordering and Coulomb correlation effect have been considered simultaneously. Bare electron susceptibility, phonon linewidth and electron-phonon coupling calculations suggest that the CDW instability is driven by softened phonon modes due to the strong electron-phonon coupling interactions. CDW order can be suppressed by pressure, concomitant with appearance of superconductivity. Our theoretical predictions call for experimental investigations to further clarify the transport and magnetic properties of 1T-NbS2. Furthermore, it would also be very interesting to explore the possibility to realize the CDW order coexisting with the superconductivity in bulk 1T-NbS2.

Accurate Model for Temperature Dependence of Solar Cell Performance According to Phonon Energy Correction

Abstract In many experiments, it has been reported that the performance of solar cells decreases with increasing temperature. This effect arises due to an increase in the intrinsic carrier concentration of material that directly affects the reverse saturation currents (J 0). As a result, the open circuit voltage which is inversely proportional to J 0 will decrease quite rapidly with increasing temperature. The intrinsic carrier concentration is determined by the bandgap energy of a material and its temperature. The Varshni relationship is a relation for the variation of the bandgap energy with temperature in semiconductors that has been used extensively in the model of a solar cell performance. But the problem is the Varshni relation just calculates the contribution of the vibrational part at the temperature, which is much greater than the Debye temperature. These works proposed a model of temperature dependence of solar cell performance that involves phonon energy correction and electron-phonon coupling interaction. This correction is applied because the electron-phonon coupling interaction is an intrinsic interaction of semiconductors. The existence of interaction cannot be avoided either experimentally or theoretically. The proposed model is compared with experimental data, which have fairly high accuracy.

Effect of electrons scattered by optical phonons on superconductivity in MH3 ( M=S , Ti, V, Se)

For exploring the superconductivity mechanisms and seeking potential high-temperature superconductors in hydrogen-rich compounds, we perform a systematical investigation of the phase diagram, crystal structures, electronic properties, and electron-phonon coupling (EPC) of titanium hydrides under high pressures. Strikingly, the low ${T}_{\mathrm{c}}$ in $\mathrm{Ti}{\mathrm{H}}_{3}$ (\ensuremath{\sim}4 K) contrast sharply with the high ${T}_{\mathrm{c}}$ (above 200 K) in $\mathrm{S}{\mathrm{H}}_{3}$, though they both possess the same stoichiometry and both crystallize in high-symmetrical cubic crystal. The large difference of ${T}_{\mathrm{c}}$ motivates us to discover the superconductive mechanism by probing the electron-phonon coupling interaction in the cubic crystal of $\mathrm{S}{\mathrm{H}}_{3}, \mathrm{Ti}{\mathrm{H}}_{3}, \mathrm{V}{\mathrm{H}}_{3}$, and $\mathrm{Se}{\mathrm{H}}_{3}$. The analyses of phonon linewidths and Fermi surface nesting functions reveal that the contrasting ${T}_{\mathrm{c}}$ is mainly attributed to the disparate intensity of electrons interacting with optic phonons, rather than the contributions from global electronic structures. Furthermore, it is found that the strong dependency of optic phonon frequencies on the wave vector is an essential ingredient for strong EPC \ensuremath{\lambda}.

Hot electron-hole plasma dynamics and amplified spontaneous emission in ZnTe nanowires.

Key to optimizing and tailoring the optoelectronic properties of semiconductor nanostructures for practical applications is a clear understanding of their carrier interactions and recombination dynamics. Herein, the electron-hole (e-h) plasma dynamics and the electron-phonon coupling interactions in zincblende ZnTe nanowires (NWs) were systematically investigated by time-resolved photoluminescence (TRPL) spectroscopy over a wide range of lattice temperatures (4-300 K) and pump densities. Following intense, non-resonant femtosecond (fs) laser pulse excitation, the excited carriers thermalize to quasi-equilibrium distribution through carrier-carrier and carrier-phonon scattering within a few picoseconds. The peak temperature of the hot electron gas (Te0) is much higher than the lattice temperature and increases sub-linearly with the pump fluence. The hot electron gas thermalizes in two characteristic carrier density-dependent regimes - i.e., within 35 ps under high carrier densities (e-h plasma) while persisting to 360 ps under low carrier densities (exciton). Temperature-dependent studies of the ZnTe NWs revealed that the acoustic phonons play a significant role in the cooling of the hot e-h plasma in these NWs and the emission band broadening arises from the interplay of the contributions from crystal imperfections, LA and LO phonon scattering and most importantly, from the hot carrier thermalization. For demonstration, e-h plasma-amplified spontaneous emission in ZnTe NWs at room temperature by one- and two-photon excitation was realized. The results provide new insights into carrier interactions and recombination dynamics of ZnTe NWs and highlight their potential for high-efficiency e-h plasma light emitters, sensors and in plasma photochemotherapy.

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).

Fabrication of cuprate superconducting La1.85Sr0.15CuO4 nanofibers by electrospinning and subsequent calcination in oxygen

The synthesis of high-temperature cuprate superconducting La1.85Sr0.15CuO4 (LSCO) flute-like nanofibers (FNFs) viaelectrospinning and subsequent calcination in oxygen is reported. A reduced superconducting critical temperature was found to be around 19.2 K, a considerable decrease in comparison to its bulk counterpart (∼ 37 K). The weakened superconductivity in the LSCO FNFs is relevant to the following observations that (i) the presence of disorder or the amount of structural defects in the FNFs, (ii) the ab-plane dilatation (∼ 0.41%) and c-axis-compression (∼ 0.16%) distortions, leads to the reduced in-plane hybridization, and (iii) the suppression of A1g Raman-active phonon mode ∼ 430 cm−1 and enhancement of B1g mode ∼ 370 cm−1, is associated with the weakened electron–phonon coupling interaction. Our work demonstrates a “clean” high-yield nanofabrication of high-temperature cuprate superconducting materials.

Induced effects by the substitution of Mg in MgCNi3

The electronic structures and magnetic properties induced by the substitution of Mg in MgCNi3 were investigated by performing the first-principles density functional calculations. It is confirmed that AlCNi3, GaCNi3, and InCNi3 are paramagnetic nonsuperconductors resulting from total energy, density of states at Fermi level, and electron-phonon coupling constant. A large peak of density of states lies on the Fermi level, resulting in ferromagnetism rather than superconductivity in NaCNi3. The substitution of dual elements for Mg, such as Al0.5Na0.5CNi3 and Al0.5Li0.5CNi3, was inspected too. Evaluating the electron-phonon coupling interaction, the possible superconductivity exists in near ferromagnetic Al0.5Na0.5CNi3.

High-pressure phase diagram in the manganites: a two-site model study

The pressure dependence of the Curie temperature TC in manganites, recently studied over a wide pressure range, is not qualitatively accounted for by the quenching of Jahn–Teller distortions, and suggests the occurrence of a new pressure-activated localizing process. On the basis of several high-pressure experiments, the antiferromagnetic (AF) interaction between the core spins has been suggested as a good candidate for such a localizing mechanism. We present a theoretical calculation of TC based on a two-site double-exchange model with electron–phonon coupling interaction and direct superexchange (SE) between the t2g core spins. We calculate the pressure dependence of TC and compare it with the experimental phase diagram. Our results describe the experimental behaviour quite well only if a pressure-activated enhancement of the AF–SE interaction is assumed.