Strong Electron-phonon Interaction Research Articles

Ab initio investigation on the experimental observation of metallic hydrogen

The optical spectra of hydrogen at $\sim$500 GPa were studied theoretically using a combination of \emph{ab initio} methods. Among the four most competitive structures, i.e. C2/c-24, Cmca-12, Cmca-4, and I41/amd, only the atomic phase I41/amd can provide satisfactory interpretations of the recent experimental observation, and the electron-phonon interactions (EPIs) play a crucial role. Anharmonic effects (AHEs) due to lattice vibration are non-negligible but not sufficient to account for the experimentally observed temperature dependence of the reflectance. The drop of the reflectance at 2 eV is not caused by diamond's band gap reducing or interband plasmon, but very likely by defects absorptions in diamond. These results provide theoretical support for the recent experimental realization of metallic hydrogen. The strong EPIs and the non-negligible AHEs also emphasize the necessity for quantum treatments of both the electrons and the nuclei in future studies.

Temperature dependent photoluminescence of anatase and rutile TiO2 single crystals: Polaron and self-trapped exciton formation

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO2, the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Franck-Condon model. Both phases show thus a very strong interaction between the photogenerated carriers and the lattice.

Anomalous pressure effect on the Nèel temperature and volume of DyB6

The pressure effect on the Nèel temperature TN of DyB6 has been investigated by measuring the temperature dependence of electrical resistivity (ρ) at various pressures up to 8 GPa. It was found that TN increases almost linearly with pressure in the range of 0-2 GPa and then tends to saturate above 2 GPa. The pressure derivative of TN below 2 GPa was estimated to be 2.6 K/GPa. The pressure effect on the volume of DyB6 has also been measured at high pressure up to 14 GPa at room temperature using X-ray diffraction and diamond anvil cell. The pressure dependence of volume of LaB6 was also measured for the comparison. It is revealed that DyB6 is more compressible than LaB6. The origin for this observation is suggested to be due to the difference of the ionic size of the rare earth elements, or the strong electron-phonon interaction in DyB6.

Unveiling the Superconducting Mechanism of Ba0.51K0.49BiO3

The mechanism of high superconducting transition temperatures (T_{c}) in bismuthates remains under debate despite more than 30years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba_{0.51}K_{0.49}BiO_{3} reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions-the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics.Furthermore, we observe an isotropic superconducting gap with 2Δ_{0}/k_{B}T_{c}=3.51±0.05, and strong electron-phonon interactions with a coupling constant λ∼1.3±0.2. These findings solve a long-standing mystery-Ba_{0.51}K_{0.49}BiO_{3} is an extraordinary Bardeen-Cooper-Schrieffer superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high T_{c}. Such effects will also be critical for finding new superconductors.

Study of supersolidity in the two-dimensional Hubbard–Holstein model

We derive an effective Hamiltonian for the two-dimensional Hubbard-Holstein model in the regimes of strong electron-electron and strong electron-phonon interactions by using a nonperturbative approach. In the parameter region where the system manifests the existence of a correlated singlet phase, the effective Hamiltonian transforms to a $t_1-V_1-V_2-V_3$ Hamiltonian for hard-core-bosons on a checkerboard lattice. We employ quantum Monte Carlo simulations, involving stochastic-series-expansion technique, to obtain the ground state phase diagram. At filling $1/8$, as the strength of off-site repulsion increases, the system undergoes a first-order transition from a superfluid to a diagonal striped solid with ordering wavevector $\vec{Q}=(\pi/4,3\pi/4)$ or $(\pi/4,5\pi/4)$. Unlike the one-dimensional situation, our results in the two-dimensional case reveal a supersolid phase (corresponding to the diagonal striped solid) around filling $1/8$ and at large off-site repulsions. Furthermore, for small off-site repulsions, we witness a valence bond solid at one-fourth filling and tiny phase-separated regions at slightly higher fillings.

Strain-Mediated Converse Magnetoelectric Coupling in La0.7Sr0.3MnO3/Pb(Mg1/3Nb2/3)O3–PbTiO3 Multiferroic Heterostructures

The converse magnetoelectric (ME) coupling of a La0.7Sr0.3MnO3 (LSMO) thin film, deposited on piezoelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) single crystal substrate by the pulsed laser deposition (PLD) technique, was investigated by magneto-optical Kerr effect (MOKE) system. Because of the piezoelectric nature of the PMN–PT, the strain status of the LSMO thin film is tunable by the electric field applied on the substrate, as it is well-known that the Manganite thin film has very strong electron–phonon interaction; thus, the magnetic property change as a function of external electric field can be realized. The change of the Kerr signal of LSMO film upon applying direct current (DC) voltage to the PMN–PT single crystal was recorded. In particular, the voltage-induced change of the Kerr signal in the LSMO by applying the alternative current (AC) actuation voltage bias to the PMN–PT single crystal can be in situ recorded without external magnetic fields. The gained Kerr signal versus voltage loop basically t...

Nonadiabatic coupling effects in MgB2 reexamined

The unusual Raman spectrum of MgB$_2$ and its formidable temperature dependence are successfully reproduced by means of a parameter-free \emph{ab initio} nonadiabatic theory that accounts for the electron-hole pair scattering mechanisms with the system phonons. This example turns out to be a prototypical case where a strong nonadiabatic renormalization of the phonon frequency is partially washed out by the aforementioned scattering events, bringing along a characteristic temperature dependence. Both electron-hole pair lifetime and energy renormalization effects due to dynamical electron-phonon coupling turn out to play a crucial role. This theory could aid in comprehending other Raman spectra characterized with unconventionally strong electron-phonon interaction.

Temperature Dependent Photoinduced Reversible Phase Separation in Mixed-Halide Perovskite

Even though tandem solar cells comprised of mixed-halide perovskites CH3NH3Pb(I1–xBrx)3 were expected to have much higher efficiency, the observation that they undergo photoinduced phase separation/demixing put forth a limitation to their possible utility. Herein, using temperature dependent photoluminescence studies, we show that the stated photoinduced phase separation occurs only in a narrow temperature range and above a particular bromine concentration. Our observation of the disappearance of phase separation at elevated temperatures suggests the possibility that these tandem solar cells may indeed work better at elevated temperatures. Further, we provide the first experimental proof for the demixing transition temperature as predicted by Bischak et al. and also observe that demixing and remixing temperatures are pinned to crystallographic phase transition temperatures. Longer carrier lifetime of iodide-rich clusters is observed confirming the strong electron–phonon interaction (polaronic effect) whic...

Electronic structure, elastic and phonon properties of perovskite-type hydrides MgXH3 (X = Fe, Co) for hydrogen storage

Metal–hydrogen systems have been continuously investigated due to the ability of metallic atoms to absorb large amounts of hydrogen. According to this viewpoint, the electronic structure, elastic and phonon properties of perovskite hydrides MgXH3 (X = Fe, Co) are searched using ab initio calculations based on density-functional theory (DFT) in this study. The lattice constants, bulk modulus, second order elastic modulus, anisotropy factors, Poisson's ratio, Young's and Shear moduli are calculated. From the elastic constants, both hydrides were found to be stable mechanically. According to obtained results, MgFeH3 and MgCoH3 can be classified as brittle and ductile material, respectively. Both hydrides show a metallic character due to very strong electron phonon interaction. The electronic band structures, phonon dispersion curves, the corresponding total and partial density of states (DOS), as well as the specific heat capacity, entropy and phonon free energy in terms of temperature are also researched for the first time in this study.

Thermal rectification in a double quantum dots system with a polaron effect

We investigate the rectification of heat current carried by electrons through a double quantum dot (DQD) system under a temperature bias. The DQD can be realized by molecules such as suspended carbon nanotube and be described by the Anderson-Holstein model in presence of electron-phonon interaction. Strong electron-phonon interaction can lead to formation of polaronic states in which electronic states are dressed by phonon cloud. Dressed tunneling approximation (DTA), which is nonperturbative in dealing with strong electron-phonon interaction, is employed to obtain the heat current expression. In DTA, self-energies are dressed by phonon cloud operator and are temperature dependent. The temperature dependency of imaginary part of dressed retarded self-energy gives rise to the asymmetry of the system and is the necessary condition of thermal rectification. On top of this, one can either tune DQD effective energy levels such that $|\bar{\epsilon}_1|\neq |\bar{\epsilon}_2|$ or have asymmetric dot-lead couplings to achieve thermal rectification. We numerically find that increasing electron-phonon coupling and reducing inter dot coupling can both improve thermal rectification effect, while the electronic heat current is reduced.

Polymeric Copper Oxide: Preparation and Investigation of Its Structure and Optical Properties

A complex of polyvinyl alcohol (PVA) with copper hydroxide was used as a precursor to obtain polymeric copper oxide through thermal decomposition. The absence of Cu(OH)2 crystalline phase was observed for the component ratio up to 1 Cu(OH)2 molecular unit to 3 PVA residuals. The formation of crystalline copper oxide was not observed after the dehydration of this material. UV–VIS and IR spectroscopy, and computational modeling were used to study the structure and properties of the obtained materials. A comparison with other similar materials was drawn. It was found that experimental data are in general accordance with the computations based on the polymeric model for copper hydroxide/oxide as a component of hybrid interpolymeric complex with PVA. A distinctive feature observed for polymeric copper oxide is strong broadening of the optical absorption band at 400 nm. It is suggested that this effect is caused by strong electron–phonon interaction, which is also responsible for superconductivity of copper oxide based systems.

Electron-phonon interaction in efficient perovskite blue emitters.

Low-dimensional perovskites have-in view of their high radiative recombination rates-shown great promise in achieving high luminescence brightness and colour saturation. Here we investigate the effect of electron-phonon interactions on the luminescence of single crystals of two-dimensional perovskites, showing that reducing these interactions can lead to bright blue emission in two-dimensional perovskites. Resonance Raman spectra and deformation potential analysis show that strong electron-phonon interactions result in fast non-radiative decay, and that this lowers the photoluminescence quantum yield (PLQY). Neutron scattering, solid-state NMR measurements of spin-lattice relaxation, density functional theory simulations and experimental atomic displacement measurements reveal that molecular motion is slowest, and rigidity greatest, in the brightest emitter. By varying the molecular configuration of the ligands, we show that a PLQY up to 79% and linewidth of 20 nm can be reached by controlling crystal rigidity and electron-phonon interactions. Designing crystal structures with electron-phonon interactions in mind offers a previously underexplored avenue to improve optoelectronic materials' performance.

Influence of the Diagonal and Off-Diagonal Electron–Phonon Interactions on the Formation of Local Polarons and Their Band Structure in Materials with Strong Electron Correlations

For systems with strong electron correlations and strong electron–phonon interaction, we analyze the electron–phonon interaction in local variables. The effects of the mutual influence of electron–electron and electron–phonon interactions that determine the structure of local Hubbard polarons are described. Using a system containing copper–oxygen layers as an example, we consider the competition between the diagonal and off-diagonal interactions of electrons with the breathing mode as the polaron band structure is formed within a corrected formulation of the polaron version of the generalized tight-binding method. The band structure of Hubbard polarons is shown to depend strongly on the temperature due to the excitation of Franck–Condon resonances. For an undoped La2CuO4 compound we have described the evolution of the band structure and the spectral function from the hole dispersion in an antiferromagnetic insulator at low temperatures with the valence band maximum at point (π/2, π/2) to the spectrum with the maximum at point (π, π) typical for the paramagnetic phase. The polaron line width at the valence band top and its temperature dependence agree qualitatively with angle-resolved photoemission spectroscopy for undoped cuprates.

Charge Transfer Effects in Naturally Occurring van der Waals Heterostructures (PbSe)_{1.16}(TiSe_{2})_{m} (m=1, 2).

van der Waals heterostructures (VDWHs) exhibit rich properties and thus has potential for applications, and charge transfer between different layers in a heterostructure often dominates its properties and device performance. It is thus critical to reveal and understand the charge transfer effects in VDWHs, for which electronic structure measurements have proven to be effective. Using angle-resolved photoemission spectroscopy, we studied the electronic structures of (PbSe)_{1.16}(TiSe_{2})_{m} (m=1, 2), which are naturally occurring VDWHs, and discovered several striking charge transfer effects. When the thickness of the TiSe_{2} layers is halved from m=2 to m=1, the amount of charge transferred increases unexpectedly by more than 250%. This is accompanied by a dramatic drop in the electron-phonon interaction strength far beyond the prediction by first-principles calculations and, consequently, superconductivity only exists in the m=2 compound with strong electron-phonon interaction, albeit with lower carrier density. Furthermore, we found that the amount of charge transferred in both compounds is nearly halved when warmed from below 10K to room temperature, due to the different thermal expansion coefficients of the constituent layers of these misfit compounds. These unprecedentedly large charge transfer effects might widely exist in VDWHs composed of metal-semiconductor contacts; thus, our results provide important insights for further understanding and applications of VDWHs.

Strong Electron–Phonon Interaction Establishes Correlation between Superconducting and Charge Density Wave States in NbSe<sub>2</sub>

Strong Electron–Phonon Interaction Establishes Correlation between Superconducting and Charge Density Wave States in NbSe<sub>2</sub>

Strong electron–phonon interaction and colossal magnetoresistance in EuTiO3

At low temperatures, EuTiO$_3$ system has very large resistivities and exhibits colossal magnetoresistance. Based on a first principle calculation and the dynamical mean-field theory for small polaron we have calculated the transport properties of EuTiO$_3$. It is found that due to electron-phonon interaction the conduction band may form a tiny subband which is close to the Fermi level. The tiny subband is responsible for the large resistivity. Besides, EuTiO$_3$ is a weak antiferromagnetic material and its magnetization would slightly shift the subband via exchange interaction between conduction electrons and magnetic atoms. Since the subband is close to the Fermi level, a slight shift of its position gives colossal magnetoresistance.

Origin and Dynamics of Highly Efficient Broadband Photoluminescence of Aqueous Glutathione-Capped Size-Selected Ag–In–S Quantum Dots

The 2–3 nm size-selected glutathione-capped Ag–In–S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by precipitation/redissolution from an aqueous colloidal ensemble. The QDs reveal broadband photoluminescence (PL) with a quantum yield of up to 60% for the most populated fraction of the core/shell AIS/ZnS QDs. The PL band shape can be described by a self-trapped exciton model implying the PL band being a sequence of phonon replica of a zero-phonon line resulting from strong electron–phonon interaction and a partial conversion of the electron excitation energy into lattice vibrations. It can be concluded that the position and shape of the PL bands of AIS QDs originate not from energy factors (depth and distribution of trap states) but rather from the dynamics of the electron–phonon interaction and the vibrational relaxation in the QDs. The rate of vibrational relaxation of the electron excitation energy in AIS QDs is found to be size-dependent, increasing almost twice from the largest to the ...

Strong electron-phonon interaction retarding phonon transport in superconducting hydrogen sulfide at high pressures.

Realistic representation of quantum interactions between electrons and phonons is crucial to uncovering the fundamental origin of superconductivity and predicting superconductors with a high transition temperature (Tc). We study the influence of the intrinsic electron-phonon interaction (EPI) on the phonon transport of superconducting hydrogen sulfide at high pressures, and its variation under an external magnetic field using all-electron first-principles calculations. High pressure greatly shortens interatomic distances, modifies the Fermi surface with sharp variation around sulfide atoms and leads to a strong EPI. Such a strong EPI substantially broadens the linewidths of major heat-carrying phonons and induces discontinuity, the so-called Kohn anomalies, in the phonon dispersions. In the superconducting state, the strong EPI competes with the weakened phonon-phonon interactions and induces a local peak in lattice thermal conductivity well below Tc. Under an external magnetic field, the reduced electron density around the Fermi surface leads to a slightly decreased EPI and Tc, which is consistent with previous experimental observation. The results gained from this work help us to understand the quantum behaviors of electrons and phonons and the influence of their interaction on the phonon transport of high-Tc superconductors.

Numerical modelling of doped graphane superconducting and normal properties

The normal properties of graphane with various degrees of doping are calculated with the help of the generalized Eliashberg theory. Within the theory of strong electron-phonon interaction, a superconducting order parameter of the doped graphane has been found. The critical temperature of superconductivity of graphane has been calculated as a function of the doping degree taking into account the renormalization of the electron mass, the chemical potential, and the density of electronic states.

Competition among Superconducting, Antiferromagnetic, and Charge Orders with Intervention by Phase Separation in the 2D Holstein-Hubbard Model.

Using a variational MonteCarlo method, we study the competition of strong electron-electron and electron-phonon interactions in the ground state of the Holstein-Hubbard model on a square lattice. At half filling, an extended intermediate metallic or weakly superconducting (SC) phase emerges, sandwiched between antiferromagnetic and charge order (CO) insulating phases. By carrier doping into the CO insulator, the SC order dramatically increases for strong electron-phonon couplings, but is largely hampered by wide phase separation (PS) regions. Superconductivity is optimized at the border to the PS.