Electron-phonon Coupling Mechanism Research Articles

Universal inverse deuterium isotope effect on the Tc of BEDT-TTF-based molecular superconductors

Deuterium substitution of the ethylene end groups of the ET [ET: BEDT-TTF or bis(ethylenedithio)tetrathiafulvalene] electron-donor molecule results in a 0.25 K increase in the superconducting transition temperature of three molecular superconductors derived from this molecule. Simplistically speaking, this change in T c is in contradiction to that predicted by the electron–phonon coupling mechanism of the BCS theory. We suggest that the slight shortening of the C–D bond relative to the C–H bond, coupled with the recent findings of a large, negative uniaxial pressure derivative of T c in κ-(ET) 2Cu(SCN) 2 and β ′′-(ET) 2SF 5CH 2CF 2SO 3, can explain this unique effect. Herein we report the first study of the effect of deuterium substitution on the superconducting transition temperature in a molecular-based superconductor in which the electron-donor molecules are packed in a β ′′ motif, viz., β ′′-(ET) 2SF 5CH 2CF 2SO 3. This compound is ideally suited for this study because it contains discrete (non-polymeric) anions, has a completely ordered structure, is indefinitely stable in air at room temperature, and is free from possible magnetic impurities. Substitution of the eight hydrogen atoms of the ET molecule by deuterium causes the T c of β ′′-(ET) 2SF 5CH 2CF 2SO 3 to increase from 4.34±0.05 to 4.61±0.03 K. These values were determined by measuring several representative crystals from various parallel electrocrystallization experiments containing h 8- or d 8-ET that was prepared in parallel syntheses. This is the first example which demonstrates that the inverse (positive) isotope effect previously observed in κ-phase salts is also present in a β ′′-phase superconductor.

Possibilities of molecular magnetic metals and high Tc superconductors in field effect transistor configurations

AbstractAb initio molecular orbital (MO) and density functional theory (DFT) calculations of neutral and anion radical states of anthracene are performed to elucidate their vibrational frequencies. They are used to calculate the transition temperatures of superconductivity of these species in the field effect transistor (FET) configuration on the basis of the electron–phonon coupling mechanism. Anderson and Kondo lattice models are also examined in relation to possible electronic phases of segregated columns of donors or acceptors with radical groups, and conducting polymers with radical groups in the FET configuration. The effective exchange interactions within conduction electrons and localized spins are calculated by theoretical methods. The electron or hole doping into molecular crystals and/or thin films of these systems in the FET configuration are studied in relation to possible magnetic states, Kondo and dense Kondo states, and superconducting states via spin fluctuations. The cooperative mechanism between the electron–phonon and spin fluctuation is also discussed to search the high Tc superconductor. Finally, the magnetic modifications of helical conducting polymers and DNA chains with chiral spin sources are discussed in relation to possibility of molecular solenoid, which may be generated in the FET configuration. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Picosecond thermal pulses in thin metal films

A simplified approach is introduced to describe the thermal behavior of a thin metal film exposed to picoseconds thermal pulses. The approach is based on the assumption that the metal film thermal behavior occurs in two separate stages. In the first stage, electron gas transmits its energy to the solid lattice through electron-phonon coupling and other mechanisms of energy transport are negligible. In the second stage, electron gas and solid lattice are in thermal equilibrium, the energy transfer through electron-phonon coupling is negligible, and thermal diffusion dominates. The proposed approach eliminates the coupling between the energy equations and the reduced differential equations are easier to handle. The proposed approach applies to metal films whenever GL2/Ke≫1.

Electron-phonon coupling effects on Yb3+ spectra inseveral laser crystals

The complex absorption spectra of Yb3+ in YAG, LiNbO3 and YLF, three important laser crystals, are analysed in terms of electron-phonon coupling effects. The local symmetry for Yb3+ in these crystals is noncentrosymmetric allowing the coexistence of several electron-phonon coupling mechanisms in the formation of vibronic spectra. The shape of several Yb3+ absorption lines is interpreted in terms of the near resonant electron-phonon coupling theory. The computer modelling, by taking into account more phonons in the resonance region enables the assignment of crystal-field levels of the Yb3+ 2F5/2 multiplet and an estimation of electron-phonon coupling strengths. The phonons involved in the near resonance processes were observed in Raman spectra too. For YAG the modelling with a phonon density with four peaks in the near resonant region is made and it gives a good description of the complex experimental pattern. In the case of Yb3+ in LiNbO3 and YLF the use of polarization data evidences also the symmetry characteristics of the electron-phonon coupling.

Two-beam photoacoustic phase measurement of the thermal diffusivity of a Gd-doped bulk YBCO superconductor

The two-beam photoacoustic phase measurement was applied to measure quantitatively the thermal diffusivity (αs) of a ceramic bulk high-Tc superconductor. Neglecting the effects of thermal dilation, and thermoelastic bending was proved valid in accordance with our composite piston model for the chosen experimental conditions. It was found that αs shows different features at the onset and offset temperatures corresponding to the normal–superconducting (NS) transition. A dip was seen at the resistivity transition onset temperature and a cusp at the offset temperature where the electrical resistance disappears. The presence of the cusp at the offset temperature is proposed to be related to weak coupling between superconducting grains. Our studies indicate that the two-beam phase measurement is a very sensitive method for superconductor characterization and NS transition detection. The experimental results also confirm the presence of a large energy gap and strong electron–phonon coupling mechanism in the YBCO superconductor.

Secondary Retarded Effect in Layered High-Temperature Superconductors

Assuming a weak electron-phonon coupling mechanism in the high-temperature superconductivity in copper oxides, the possibility of effective electron-electron interaction mediated by the secondary retarded effect associated with inter- CuO2 layer coupling has been examined. It is pointed out that under the appropriate phonon phase there is an attraction even for |εn,k±q-εn,k|>\\hbarωq, circumventing the conventional BCS restriction.

Electron-phonon interactions and excitonic dephasing in semiconductor nanocrystals.

The size dependence of the contribution to the excitonic dephasing rate in semiconductor nanocrystals is clarified for various electron-phonon coupling mechanisms. On the basis of these dependencies, the commonly observed linearly temperature-dependent term of the excitonic dephasing rate and the proportionality of its magnitude to the inverse square of the nanocrystal size are attributed to pure dephasing due to deformation-potential coupling. The calculated coefficients of the linearly temperature-dependent term are quantitatively in good agreement with the experimental results on CdSe and CuCl nanocrystals.

Infrared and Resonance Raman Spectroscopic Studies of AgTCNQ

Abstract AgTCNQ has been prepared, and studied by infrared and Raman spectroscopies. The various preparation techniques were found to lead to the same Ag+TCNQ− composition. The Raman spectra show strong resonance enhancement of the totally symmetric modes. Differences in the degree of coupling of these modes with the electronic transitions were observed. The infrared spectra also show the presence of the totally symmetric modes, which become infrared active through the electron-phonon coupling mechanism.

Thermal diffusivity of superconducting YBa2Cu3O7−x

Thermal diffusivity results for superconducting YBa2Cu3O7−x systems are reported. Two laser excitation techniques were used: photoacoustic phase-lag and the flash method. We observed an anomalous jump in the thermal diffusivity at the critical temperature and related it to the electronic specific heat anomaly. This correlation yielded a characteristic phonon frequency of 320 cm−1 in the range of an active Raman mode already observed in a single crystal of the same compound. This result reinforces the current electron-phonon coupling mechanism as responsible for superconductivity.

X-ray-absorption studies ofYBa2Cu3O7−δandGdBa2Cu3O7−δsuperconductors

X-ray-absorption measurements of the high-${T}_{c}$ superconductors $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ show that their near-edge and extended fine structures (EXAFS) are essentially identical over the temperature range of 4.2 to 688 K. The results indicate that not only are $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ very similar to each other in local bonding and electronic structure, but also remain virtually unchanged, electronically and structurally, over this wide range in temperature, from far below to well above ${T}_{c}$. The positions of the Cu near-edge spectra for these superconductors suggest a Cu valence between that of Cu in CuO, which has a formal valence of II, and that in KCu${\mathrm{O}}_{2}$, with a formal valence of III. Our EXAFS structural results agree well with those from neutron diffraction data. Recently proposed alternate structures are shown to be inconsistent with our data. The characteristic vibration frequencies of the near-neighbor atoms show that the two-dimensional Cu---O planes and the one-dimensional Cu---O chains form the most rigid parts of the structure, and these layers are bound more weakly along the $c$ axis via the Ba and Y planes. These oscillator frequencies are identified with specific near-neighbor pairs, and the values are consistent with those for some of the modes seen by inelastic neutron scattering as well as by infrared and Raman spectroscopy. No evidence of phonon softening is apparent in any of the phonon modes studied, providing evidence against a strong electron-phonon coupling mechanism.

Hidden superconductivity

A new vibronically induced interelectron attractive force is found. This is not brought about by the conventional mechanism of electron-phonon coupling in condensed matter physics. The sign of the vibronic interaction with respect to the Coulombic repulsion is “−” and is not “+” in the conventional electron-phonon coupling model. This predicts a “hidden” vibronic mechanism of superconductivity.

Complete vibronic Hamiltonian and "hidden" superconductivity.

A field theory of non-Born-Oppenheimer vibronic interaction in a molecular system is formulated using a differential geometrical study of quantum molecular dynamics [A. Tachibana and T. Iwai, Phys. Rev. A 33, 2262 (1986)]. In contrast to the conventional approach of many-body formalism, the present approach stresses the finite-system aspects of the theory. The formalism is, of course, general and exact and applies to infinitely large systems. Remarkably, a new vibronically induced interelectron attraction is revealed, which is not brought about by the conventional mechanism of electron-phonon coupling in condensed-matter physics. It is shown that the ``sign'' of the vibronic interaction with respect to the Coulombic repulsion is - and is not + in the conventional electron-phonon coupling model. The physical origin of the newly found interelectron attraction is clarified in terms of the ``instantaneous'' nature of vibronic interaction and the primordial ``spin-flip'' mechanism of non-Born-Oppenheimer electron scattering processes [A. Tachibana, K. Hori, and T. Yamabe, Chem. Phys. Lett. 112, 279 (1984)]. This predicts a ``hidden'' vibronic mechanism of superconductivity.

Electron-longitudinal-phonon coupling in a transition metal: Ta

Recent tunneling studies of tantalum using a proximity technique reveal stronger coupling to the longitudinal modes relative to the transverse modes than observed previously for any transition metal. This result has implications for modeling the electron-phonon coupling mechanism and for future tunneling investigations. We obtain parameter values for Ta of λ = 0.73, μ ph ∗ = 0.10 ,in agreement with optimized conventional results.

Raman scattering and far-infrared studies of the vanadium dihalides with layered structure

The vanadium dihalides with layered structure, V${\mathrm{Cl}}_{2}$, V${\mathrm{Br}}_{2}$, and V${\mathrm{I}}_{2}$, have been investigated by Raman and infrared spectroscopy. A compilation of the measured optical frequencies is given. The new lines appearing in the Raman spectrum of V${\mathrm{I}}_{2}$ below its N\'eel temperature ${T}_{N}$ are attributed to zone-boundary-phonon Raman scattering induced by Bragg scattering from the spin superstructure. We discuss the symmetry properties and selection rules for the proposed spin-dependent electron-phonon coupling mechanism due to a phonon modulation of the exchange interaction. The measured two-phonon absorption spectra of the three materials show a shift to lower frequencies with respect to the calculated two-phonon density of states. Some possible reasons for this unusually large shift are discussed. The Raman spectra of V${\mathrm{Cl}}_{2}$ and V${\mathrm{Br}}_{2}$ show additional features at low temperature, different from the sharp lines observed in V${\mathrm{I}}_{2}$ below ${T}_{N}$. This additional scattering is also shifted to lower frequencies compared to the calculated one-phonon density of states. We explain the appearance of the additional Raman scattering by a spin-phonon coupling via the exchange modulation mechanism, whereas the shift in the Raman spectra should be due to a self-energy renormalization by the coupling mechanism.

Non-radiative recombination in chalcogenide glasses

Non-radiative recombination of electron-hole pairs in chalcogenide glasses is described using a mechanism of a strong electron-phonon coupling. Recombination proceeds through a transient bonding rearrangement that creates a defect similar to those giving localized electronic states. Incomplete recombination which leaves the defect in a metastable state explains the reversible photostructural change.

Polarized Fluorescence and Absorption Spectra of Tetracene in p-Terphenyl

Polarized absorption and fluorescence spectra of the 5000 Å B12u⇌1A1g transitions of tetracene (naphthacene) dissolved in p-terphenyl have been measured at 4.2°K. The vibronic band-widths are sufficiently sharp to permit resolution of a 3 cm−1 site splitting and to enable assignments to be made of the 12 ground and excited state a1g and several b3g fundamentals between 300 and 1600 cm−1. The phonon structures in absorption and fluorescence are characteristic of strong and weak electron—phonon coupling, respectively. A dependence of the structure in the tetracene fluorescence spectrum on the wavelength of the exciting light has been observed. It is shown that this wavelength dependence is the result of unusual crystal site relaxation processes which occur through electron—phonon coupling mechanisms.