Coupled Electron-phonon Systems Research Articles

Zigzag HgTe Nanowires Modify the Electron-Phonon Interaction in Chirality-Refined Single-Walled Carbon Nanotubes.

Atomically thin nanowires (NWs) can be synthesized inside single-walled carbon nanotubes (SWCNTs) and feature unique crystal structures. Here we show that HgTe nanowires formed inside small-diameter (<1 nm) SWCNTs can advantageously alter the optical and electronic properties of the SWCNTs. Metallic purification of the filled SWCNTs was achieved by a gel column chromatography method, leading to an efficient extraction of the semiconducting and metallic portions with known chiralities. Electron microscopic imaging revealed that zigzag HgTe chains were the dominant NW geometry in both the semiconducting and metallic species. Equilibrium-state and ultrafast spectroscopy demonstrated that the coupled electron–phonon system was modified by the encapsulated HgTe NWs, in a way that varied with the chirality. For semiconducting SWCNTs with HgTe NWs, Auger relaxation processes were suppressed, leading to enhanced photoluminescence emission. In contrast, HgTe NWs enhanced the Auger relaxation rate of metallic SWCNTs and created faster phonon relaxation, providing experimental evidence that encapsulated atomic chains can suppress hot carrier effects and therefore boost electronic transport.

Quantum dynamics of electric-dipole coupled defect centers in solids

We investigate the quantum dynamics of two defect centers in solids, which are coupled by vacuum-induced dipole–dipole interactions. When the interaction between defects and phonons is taken into account, the two coupled electron–phonon systems make up two equivalent multilevel atoms. By making Born–Markov and rotating wave approximations, we derive a master equation describing the dynamics of the coupled multilevel atoms. The results indicate the concepts of subradiant and superradiant states can be applied to these systems and the population transfer process presents different behaviors from those of the two dipolar-coupled two-level atoms due to the participation of phonons.

Coupled electron-impurity and electron-phonon systems as trivial non-Fermi liquids

We consider an electron gas, both in two (2D) and three (3D) dimensions, interacting with quenched impurities and phonons within leading order finite-temperature many body perturbation theories, calculating the electron self-energies, spectral functions, and momentum distribution functions at finite temperatures. The resultant spectral function is in general highly non-Lorentzian, indicating that the system is not a Fermi liquid in the usual sense. The calculated momentum distribution function cannot be approximated by a Fermi function at any temperature, providing a rather simple example of a non-Fermi liquid with well-understood properties.

The (eg \u2297 eu) \u2297 Eg product Jahn\u2013Teller effect in the neutral group-IV vacancy quantum bits in diamond

The product Jahn–Teller effect may occur for such coupled electron–phonon systems in solids where single electrons occupy double degenerate orbitals. We propose that the excited state of the neutral XV split-vacancy complex in diamond, where X and V labels a group-IV impurity atom of X = Si, Ge, Sn, Pb and the vacancy, respectively, is such a system with eg and eu double degenerate orbitals and Eg quasi-localized phonons. We develop and apply ab initio theory to quantify the strength of electron–phonon coupling for neutral XV complexes in diamond, and find a significant impact on the corresponding optical properties of these centers. Our results show good agreement with recent experimental data on the prospective SiV(0) quantum bit, and reveals the complex nature of the excited states of neutral XV color centers in diamond.

Properties of ground state and anomalous quantum fluctuations in one-dimensional polaron—soliton systems—the effects of electron-two-phonon interaction and non-adiabatic quantum correlations

Based on the Holstein model Hamiltonian of one-dimensional molecular crystals, by making use of the expansion approach of the correlated squeezed-coherent states of phonon instead of the two-phonon coherent state expansion scheme, the properties of the ground state and the anomalous quantum fluctuations are investigated in a strongly coupled electron—phonon system with special consideration of the electron-two-phonon interaction. The effective renormalization (α̃i) of the displacement of the squeezed phonons with the effect of the squeezed-coherent states of phonon and both the electron-displaced phonon and the polaron-squeezed phonon correlations have been combined to obtain the anomalous quantum fluctuations for the corrections of the coherent state. Due to these non-adiabatic correlations, the effective displacement parameter α̃i is larger than the ordinary parameter αi(0). In comparison with the electron-one-phonon interaction (g) corrected as α̃ig, we have found the electron-two-phonon interaction (g1) corrected as α̃i2g1 is enhanced significantly. For this reason, the ground state energy (E0(2)) contributed by the electron-two-phonon interaction is more negative than the single-phonon case (E0(1)) and the soliton solution is more stable. At the same time, the effects of the electron-two-phonon interaction greatly increase the polaron energy and the quantum fluctuations. Furthermore, in a deeper level, we have considered the effect of the polaron-squeezed phonon correlation (f-correlation). Since this correlation parameter f > 1, this effect will strengthen the electron-one and two-phonon interactions by fα̃ig and f2α̃i2g1, respectively. The final results show that the ground state energy and the polaron energy will appear more negative further and the quantum fluctuations will gain further improvement.

Properties of nonadiabatic quantum fluctuations of the strongly coupled electron-phonon systems

A new variational-ansatz of states of electrons and phonons was proposed on the basis of the Holstein model in strongly coupled electron-phonon systems for studying the influence of nonadiabatic phonon fluctuation, arising from the motion and density fluctuation of electrons, on the properties of ground state, uncertainty relation, stability of polarons, charge density wave (CDW) and phonon staggered ordering. The new ansatz represents the correlation among the displacement and squeezing states of phonons and polaron’s state of electrons as well as the squeezing-antisqueezing effect. The correlation and squeezing-antisqueezing effect result in the decrease of ground state energy, enhancement of stability of the systems, increase of binding energy of the polarons, weakening of the growing speed of polaron narrowing of the electron band, increase of the charge density wave order and suppression of the increased tendency of anomalous quantum fluctuation of the phonons in such a case, when compared with the uncorrelated case in the systems. The results obtained show that the ground state determined by the new state ansatz is most stable, thus the new ansatz describing the properties of the coupled electron phonon systems is very relevant and available, especially in strongly coupled and largely squeezed cases.

Roles of spin fluctuations and rattling in magnetic and thermoelectric properties of AT 4Sb 12 (A=Ca, Sr, Ba, La; T=Fe, Ru, Os)

Magnetic and thermoelectric properties of the skutterudites AT 4Sb 12 filled with non-magnetic atoms, A=Ca, Sr, Ba and La, are found to depend strongly on the transition-metal elements, T=Fe, Ru and Os. The strong spin fluctuations in the Fe-based compounds manifest themselves as an enhanced specific-heat coefficient γ=100 mJ/K 2 mol. The maximum of the magnetic susceptibility at 50 K observed for A=Ca, Sr and Ba is a characteristic of a nearly ferromagnetic system. The increase of the paramagnetic Curie temperature from 3 to 54 K on going from A=La, Ba, Sr to Ca indicates the enhancement of ferromagnetic interactions in the Fe 3d narrow band. Except for A=La, a marked shoulder appears in the thermopower at 50 K which is totally suppressed by the application of a magnetic field of 14 T. The Ru compounds are metallic diamagnets with γ=10 mJ/K 2 mol, while the Os compounds are strongly enhanced Pauli paramagnets with γ=45 mJ/K 2 mol and show behaviors of a strongly coupled electron–phonon system. The lattice thermal conductivity for either A=La or T=Os does not show a peak near 50 K and its value is much smaller than other compounds. The strong phonon scattering can be caused by rattling of the relatively small La 3+ ion in the T 4Sb 12 cage or tunneling of Sr 2+ and Ba 2+ ions in the Os 4Sb 12 cage.

Phonon-assisted tunnelling in interacting suspended single-wall carbon nanotubes

Transport in suspended metallic single-wall carbon nanotubes in the presence of strong electron–electron interaction is investigated. We consider a tube of finite length and discuss the effects of the coupling of the electrons to the deformation potential associated to the acoustic stretching and breathing modes. Treating the interacting electrons within the framework of the Luttinger liquid model, the low-energy spectrum of the coupled electron–phonon system is evaluated. The discreteness of the spectrum is reflected in the differential conductance which, as a function of the applied bias voltage, exhibits three distinct families of peaks. The height of the phonon-assisted peaks is very sensitive to the parameters. The phonon peaks are best observed when the system is close to the Wentzel–Bardeen singularity.

Singular Effect of Disorder on Electronic Transport in Strongly Coupled Electron-Phonon Systems

We solve the disordered Holstein model in three dimensions considering the phonon variables to be classical. After mapping out the phases of the "clean" strong coupling problem, we focus on the effect of disorder at strong electron-phonon (EP) coupling. The presence of even weak disorder (i) enormously enhances the resistivity (rho) at T=0, simultaneously suppressing the density of states at the Fermi level, (ii) suppresses the temperature dependent increase of rho, and (iii) leads to a regime with drho/dT<0. We locate the origin of these anomalies in the disorder induced tendency towards polaron formation, and the associated suppression in effective carrier density and mobility. These results, explicitly at "metallic" density, are of direct relevance to disordered EP materials such as covalent semiconductors, the manganites, and to anomalous transport in the A-15 compounds.

Phonon hardening due to the small-polaron effect

The influence of the small-polaron effect on the vibrational properties of the strongly coupled electron–phonon system is investigated. It was found that polaron–phonon interaction may cause a noticeable renormalization of the phonon spectra. These changes may be observed by the measurements of the speed of sound for which a very specific dependence on temperature, values of coupling constant and adiabatic parameter is expected. We also found that the rise of the small-polaron effective mass as a function of the coupling constant should be considerably moderated than that anticipated without taking into account the modification of the vibrational spectra.

Electronic transport in molecular devices: the role of coherent and incoherent electron–phonon scattering

The Green function for the coupled electron–phonon system is derived from a quantized version of the SSH Hamiltonian and is solved exactly for relevant modes of vibration. This Green function is used to compute the elastic and inelastic current components thought a di-thio-phenylene molecule in between Au contacts.

CHANGES IN THE PHYSICAL PROPERTIES OF NONADIABATICALLY COUPLED ELECTRON–PHONON SYSTEMS ARISING FROM SQUEEZING–ANTISQUEEZING EFFECT

Changes in the physical properties such as the ground state properties, charge density wave ordering, binding energy and energy bandwidth of polaron and quantum fluctuation, and minimum uncertainty relation of phonons and nonadiabatically coupled electron–phonon systems with spin-1/2 have been investigated by our new state ansatz which can account for correlation among the phononic displacement, squeezing and polaron effects using variational method in one-dimensional Holstein model. The investigation here shows that the squeezing–antisqueezing effect (correlated) results in a decrease of the ground state energy, an increase of the binding energy of polarons, the reduction of the uncertainty and quantum fluctuation of the phonons, a decrease of polaron narrowing of electron bandwidth, an increase of tunneling effect of the polarons and an increase of CDW ordering and phonon staggered ordering when compared with the uncorrelated case. Therefore, this shows that the ground state determined by the new state ansatz is the most stable. The new ansatz which include the squeezing–antisqueezing (correlated) effect is very relevant for the coupled electron–phonon systems, especially in strongly coupled and highly squeezed cases.

Electron gas with polaronic effects: beyond the mean‐field theory

AbstractThe occurrence of polaronic behaviour in real solids is often accompanied by strong exchange and correlation effects in the electron gas besides being strongly entangled to the phononic degrees of freedom. Such effects can be relevant for the instability towards Wigner crystallization or charge density waves, or yet for the polaron role in high‐Tc superconductors and in manganites. The inclusion of exchange and correlation in the coupled electron–phonon system is thus highly desirable. In this work, we exploit the density functional theory and its time‐dependent extension to construct an appropriate effective potential useful for studying the properties of interacting polarons. The validity and the possible developments of this approach are discussed, and future applications are proposed.

THE PROPERTIES OF NONADIABATIC FLUCTUATIONS IN STRONGLY COUPLED ELECTRON–PHONON SYSTEMS WITH CORRELATED DISPLACEMENT AND SQUEEZING

A new variational ansatz for the coherent phonons with correlated displacement and squeezing, including interaction with electrons are used to study the properties of (i) the nonadiabatic phonon fluctuation on the ground state, (ii) the uncertainty principle, (iii) the phonon-staggered ordering, and (iiv) the charge-density wave order, of the systems due to fluctuations of the electron density in a coupled electron–phonon systems. The results obtained show that the correlation effect can lower the energy of the ground state of the systems, enhance the stabilization of the polaron and suppress the quantum increase of the classical polaron narrowing of electron band. Moreover, it add to the effects of nonadiabaticity on the minimum quantum uncertainty for the phonon coordinate by a strong coupling with the electron density for a large squeezing as compared with the adiabatic case and the uncorrelated case. Therefore, the new ansatz which can represent the correlation effects of displacement and squeezing is especially relevant for system with sufficiently large squeezing and strong coupling.

The properties of quantum fluctuations in strongly coupled electron–phonon systems with correlated displacement and squeezing

Influences of the non-adiabatic phonon fluctuations on the ground state, and on the minimum uncertainty principle of phonons were investigated by introducing a new state-ansatz including correlation between the displacement and squeezing of the phonons in the strongly coupled electron–phonon systems. For a large squeezing, the correlation lowers, obviously, the energy of the ground state, and increases the binding energy of the polaron when compared with the uncorrelated case. Thus the stabilization of the polarons and the stability of the systems are all enhanced. The correlation was also found to suppress the quantum reduction of the classical narrowing of the polaron band, and to weaken the tunnelling effect of the polarons, and to enhance the quantum uncertainty for the phonon coordinate due to the coupling with the electron density fluctuation. Generally, the correlated effects and new ansatz introduced here were found to be relevant for a sufficiently large squeezing in the strongly and intermediately coupled electron–phonon systems.

The properties of the ground state of strongly coupled electron–phonon systems with squeezing-antisqueezed effect

Influences of the squeezing-antisqueezed effect of phonons arising from polaron motion on the ground state in strongly coupled electron–phonon systems with high and low electron-densities were studied by a new state ansatz including the correlated effects among displacement and squeezing of phonons and polarons in the Holstein model. The squeezing-antisqueezed effect caused significant decrease of the ground state energy of the systems and increase of the binding energy of the polarons appeared in the systems. Therefore the stabilities of the polaron and of the systems are enhanced simultaneously. The results obtained show that the new state ansatz is very relevant and accurate for the nonadiabatically coupled electron–phonon systems, especially in strongly coupling and large squeezing cases.

The Stabilities of the Ground State and the Polaron Arising from the Squeezing ? Antisqueezing Effect in Strongly Coupled Electron ? Phonon Systems

The influences of the squeezing–antisqueezing effect arising from variations of the electron density and motion of polarons and nonadiabatic phonon fluctuations on the properties of the ground state and the binding energy of the polaron in the Holstein model of coupled one-dimensional electron–phonon systems with spin 1/2 in the cases of high and low electron densities have been studied by using the Bogolyubov transformation and a new ansatz for states that includes correlation effects among the one-phonon coherent and two-phonon squeezed and polaronic states. The results obtained show that the squeezing–antisqueezing (correlation) effect results in that the ground state energy of the systems is significantly lowered, and the binding energy of the polaron is considerably increased. Thus, the stability of the systems and of the polaron are obviously enhanced in such a case when compared with the uncorrelated case. This shows that the ground state determined by the new state ansatz is most stable, and the new ansatz introduced here is very relevant for the coupled electron–phonon systems, especially in strongly coupled and largely squeezed cases.

The properties of the ground state of the nonadiabatically coupled electron-phonon systems with correlated displacement and squeezing

Effects of the nonadiabatic phonon fluctuations on the ground state of coupled one-dimensional electron-phonon systems with high and low electron-densities were investigated by a new ansatz including correlated interaction of the displacement and squeezing. The correlated effect resulted in noticeable reduction of ground state energy of the systems, and obvious increases of binding energy of the polarons occurred. Thus, the stabilities of the polaron and of the systems were significantly enhanced, exactly steady ground state of the Holstein model, then, was obtained by the new ansatz for the coupled electron-phonon systems. In general, the new ansatz were found to be very relevant for the strong coupling and large squeezing cases in nonadiabatically coupled electron-photon systems.

Effects of squeezing-antisqueezing in strongly coupled electron–phonon systems

The effects of squeezing-antisqueezing resulting from the motion and density fluctuation of the electrons on the properties of both electrons and phonons have been studied by using a new variational ansatz with correlated displacement and squeezing in strongly coupled electron–phonon systems. The effects results in (1) reduction of the ground state energy, and enhancement of stability of the systems, (2) increase of the binding energy of the polaron occurred and weakening of growing speed of polaron narrowing of electron band, (3) increase of the charge density wave order and (4) suppression of increased tendency of anomalous quantum fluctuation of the phonons in the systems. The antisqueezed effect plays an important role in determining the properties of the electrons and phonons in the strongly coupled electron–phonon systems.

Variational Approach to One-Dimensional Electrons Non-Adiabatically Coupled to Phonons

The effects of the nonadiabatic phonon behaviours on the ground state of the system and the quantum modifications of the polaron characteristics and the modification form of the uncertainty relation between the phonon coordinate and momentum due to fluctuations of the electron density in the coupled electron-phonon systems have been studied by using a new variational ansatz for coherent phonons with correlated displacement and squeezing including internations with electrons. The correlated effect results in the following effect: (1) the energy of the ground state of the system is found to be lower than that for the phonons with uncorrelated displacement and squeezing; (2) the binding energy of the polaron was found to be increased; (3) the nonadiabatic coupling with electrons enhances the quantum uncertainty for the phonon coordinate and momentum when compared with the adiabatic and uncorrelated case.