Strong Electron-phonon Interaction Research Articles

Graphene-based terahertz photodetector by noise thermometry technique

We report the characteristics of graphene-based terahertz (THz) photodetector based on noise thermometry technique by measuring its noise power at frequency from 4 to 6 GHz. Hot electron system in graphene microbridge is generated after THz photon pumping and creates extra noise power. The equivalent noise temperature and electron temperature increase rapidly in low THz pumping regime and saturate gradually in high THz power regime which is attributed to a faster energy relaxation process involved by stronger electron-phonon interaction. Based on this detector, a conversion efficiency around 0.15 from THz power to noise power in 4–6 GHz span has been achieved.

Temperature-independent pseudogap and thermally activated c-axis hopping conductivity in layered cuprate superconductors

The carrier localization (confinement) and c-axis charge transport in high-Tc layered cuprates have been investigated in the temperature-independent pseudogap state over a wide doping range from the underdoped to the overdoped regime, focusing on the nanoscale phase separation into two different domains (i.e. carrier-rich CuO2 layers and carrier-poor regions between the CuO2 layers) caused by inherent nanoscale electronic inhomogeneities and on the real-space pairing of polaronic carriers caused by strong electron–phonon interactions in carrier-poor regions. A simple microscopic model for layered cuprate superconductors is proposed to describe the carrier confinement and formation of localized bipolarons in the low-doping regions between the CuO2 layers and to simulate the c-axis hopping transport at the thermal dissociation of such bipolarons. This model is found to describe properly the c-axis charge transport, which is governed by thermally activated hopping of polarons residing in the localized state between the CuO2 layers due to the existence of the temperature-independent bipolaronic pseudogap, and the insulating behavior of the c-axis resistivity ρc(T) observed above Tc in various cuprate superconductors, from the underdoped to the overdoped regime. It is found that the insulating behavior of ρc(T) is changed to metallic behavior with disappearing of bipolaronic pseudogap.

Thermoelectric Transport Across Nanoscale Polymer–Semiconductor–Polymer Junctions

There is an increasing interest in the thermoelectric (TE) properties of hybrid organic–inorganic structures, such as molecular junctions, organic–inorganic multilayers, and nanocomposites, owing to the recent success in nanostructuring of inorganic materials for high-efficiency thermoelectrics and the ability to synthesize hybrid materials in close analog to inorganic counterparts with much lower cost and greater flexibility. Compared to inorganic counterparts, the development of hybrid inorganic-polymer structures for TE applications are in a nascent stage, where theoretical understanding is very much needed and many potential nanoscale structures are yet to be explored. In this work, we study quasi-one-dimensional TE transport in a nanoscale polymer–semiconductor–polymer (PSP) junction, where a semiconductor quantum dot (QD) is trapped between two bulk polymers with aligned polymer chains. The Holstein small polaron model, which can be used for strong electron–phonon interaction in polymers beyond the ...

Optical conductivity spectra of rattling phonons and charge carriers in the type-VIII clathrate Ba8Ga16Sn30

We have investigated optical conductivity spectra of $n$- and $p$-type Ba$_8$Ga $_{16}$Sn$_{30}$ ($\alpha$-BGS) with type-VIII clathrate structure, at temperatures from 296\,K down to 6\,K with a terahertz time-domain spectrometer (0.2\,-\,2.5\,THz). The continuous spectra contributed from charge carriers are dispersive in this frequency range and also temperature- and carrier type-dependent. The Drude-Smith model taking multiple-scatterings of charge carriers into account well reproduces those data. The relaxation rate of the $n$-type carriers decreases more sharply than that in the $p$-type material, suggesting that a stronger electron-phonon interaction may exist in the $n$-type than in the $p$-type. On the other hand, the localized infrared-active modes observed at 1.3\,THz and 1.7\,THz, identified as the rattling phonons of the Ba$^{2+}$ ion's quasi-on-center vibrations, become soft and broad significantly with decreasing temperature as well as observed in type-I BGS and BGG (Ba$_8$Ga$_{16}$Ge$_{30}$) clathrates. The softening in the $n$-type is smaller by about 30% than in the $p$-type, whereas the linewidth brodening is almost the same independently on the carrier type. The difference in the softening is discussed with a scenario where the interaction of rattling phonons with carriers can modify the anharmonic potential of the guest ions. The anomalous broadening at low temepratures is also discussed by the impurity-scattering model presented for a rattling-phonon system strongly hybridized with acoustic cage phonons.

Electrical and Optical Properties of Sb-Doped BaSnO3

The electronic and optical properties of the cubic perovskite, BaSnO3, are compared with the well-known transparent conducting oxides (TCOs), SnO2 and In2O3. The optical band gaps of the undoped compounds, as measured by diffuse reflectance spectroscopy on powdered samples, are 3.1 eV for BaSnO3 and 3.8 eV for SnO2. Electronic structure calculations show that both compounds possess a large conduction band dispersion, which suggests that BaSnO3, like SnO2, should be a good TCO if it can be n-doped. To explore this possibility the properties of substitutionally doped, BaSn1–xSbxO3 samples were investigated. The electrical conductivity increases drastically for BaSn1–xSbxO3 samples with x ≤ 0.05, showing a transition from an insulating to a metallic state. For higher doping levels, 0.05 < x < 0.15, the conductivity saturates at 4 S cm–1. This is accompanied by saturation in the expansion of the cubic lattice parameter. The color of the pellets changed from white (x = 0) to bluish black (x = 0.15). This darkening originates from the formation of an intense and broad optical absorption band centered at 1200–1300 nm for the more highly doped samples. This absorption band spans both the visible and the near-infrared regions, resulting in a loss of transparency. Various spectroscopic techniques were used to elucidate the observed behavior. Mössbauer spectroscopy reveals the presence of mixed valent antimony, Sb3+(5s2)/Sb5+(5s0), as x increases. ESR spectra collected on a BaSn0.99Sb0.01O3 sample indicate that only ∼5% of unpaired electrons are present as delocalized carriers. The other 95% are localized to form a Sn3+ trapped electron center. To explain the tendency for carrier localization in doped BaSnO3 a mechanism based on strong electron–phonon interactions in perovskites containing cations with (n – 1)d10ns0 electronic configuration is proposed. This mechanism explains why BaSn1–xSbxO3 possesses relatively poor TCO characteristics, in sharp contrast to widely used TCO materials Sn1–xSbxO2 and In1–xSnxO1.5.

Phonon influence on the measurement of spin states in double quantum dots using the quantum point contact

We study the influence of phonon scattering on the noise characteristics of a quantum point contact coupled to a two-electron system in a double quantum dot, as proposed for a singlet-triplet measurement scheme in a double-dot system. We point out that at low temperatures phonon-induced relaxation to the ground state suppresses transitions to doubly occupied singlet states which are the source of detectable current fluctuations in this measurement scheme. Thus, for a relatively strong electron-phonon interaction present in the system, the two configurations display the same noise characteristics. In this way, coupling to phonons reduces the distinguishability between the singlet and triplet configurations. Under such conditions, the proposed measurement scheme is no longer valid even though the times of the measurement-induced decoherence of an initial singlet-triplet superposition and of the localization into the singlet or triplet subspace remain essentially unchanged.

Many-body effects and possible superconductivity in the two-dimensional metallic surface states of three-dimensional topological insulators

We theoretically consider temperature and density-dependent electron-phonon interaction induced many-body effects in the two-dimensional (2D) metallic carriers confined on the surface of the 3D topological insulator (e.g. Bi$_2$Se$_3$). We calculate the temperature and the carrier density dependence of the real and imaginary parts of the electronic self-energy, the interacting spectral function, and the phonon-induced velocity renormalization, enabling us to obtain a simple density and temperature dependent effective dimensionless electron-phonon coupling constant parameter, which increases (decreases) strongly with increasing density (temperature). Our theoretical results can be directly and quantitatively compared with experimental ARPES or STS studies of the 2D spectral function of topological insulator surface carriers. In particular, we predict the possible existence of surface superconductivity in Bi$_2$Se$_3$ induced by strong electron-phonon interaction.

Effect of A-site cation radius on thermodynamic properties of ARuO3 (A = Ca, Sr, and Ba)

The thermodynamic properties of alkaline earth ruthenate ARuO3 (A = Ca, Sr, and Ba) perovskites have been investigated for the first time by means of a modified rigid ion model at temperature 1 K ≤ T ≤ 300 K. As strong electron–phonon interactions are present in these compounds, the lattice contribution to the specific heat deserves proper attention. The values of specific heat calculated by us have shown remarkably good agreement with corresponding experimental data. We have found that in ARuO3 (A = Ca, Sr, and Ba) ruthenate family, Debye temperature increases inversely with the ionic radius of the alkaline earth A cations. In addition, the results on the temperature dependence of thermal expansion coefficient (α), cohesive energy (ϕ), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θD), and Gruneisen parameter (γ) are also reported.

Effect of screening of the electron-phonon interaction on the temperature of Bose-Einstein condensation of intersite bipolarons

Here, we consider an interacting electron-phonon system within the framework of extended Holstein-Hubbard model at strong enough electron-phonon interaction limit in which (bi)polarons are the essential quasiparticles of the system. It is assumed that the electron-phonon interaction is screened and its potential has Yukawa-type analytical form. An effect of screening of the electron-phonon interaction on the temperature of Bose-Einstein condensation of the intersite bipolarons is studied for the first time. It is revealed that the temperature of Bose-Einstein condensation of intersite bipolarons is higher in the system with the more screened electron-phonon interaction.

Microscopic theoretical study of Raman spectra in charge and spin ordered cuprate systems

Raman scattering is one of the most powerful methods to investigate the electron as well as the phonon excitations in the systems. In this communication, we present a theoretical study of Raman scattering in the normal state of the high-TC systems in the under-doped region displaying the interplay of the spin-density-wave (SDW) and charge-density-wave (CDW) interactions. The SDW order arises from the repulsive Coulomb interaction of electrons, while the CDW order arises due to strong electron–phonon interaction giving rise to Fermi surface instability. We calculate phonon response function in order to examine the possibility of observing the SDW excitation mode in presence of the CDW interaction present in the same conduction band. The Raman scattering intensity is calculated from the imaginary part of the phonon Green’s function assigning an arbitrary spectral width. The spectral density function displays two mixed modes of excitation peaks at energies 2(Δc±Δs). The evolution of excitation peaks are investigated by varying CDW coupling, SDW coupling and the phonon momentum transfer energy.

On the important role of the anti-Jahn-Teller effect in underdoped cuprate superconductors

In this paper it is shown that the "anti-Jahn-Teller effect" plays an essential role in giving rise to a small Fermi surface of Fermi pockets above Tc and d-wave superconductivity below Tc in underdoped cuprates. In the first part of the present paper, we review the latest developments of the model proposed by Kamimura and Suwa, which bears important characteristics born from the interplay of Jahn-Teller Physics and Mott Physics. It is shown that the feature of Fermi surfaces in underdoped LSCO is the Fermi pockets in the nodal region constructed by doped holes under the coexistence of a metallic state and of the local antiferromagnetic order. In the antinodal region in the momentum space, there are no Fermi surfaces. Then it is discussed that the phonon-involved mechanism based on the Kamimura-Suwa model leads to the d-wave superconductivity. In particular, it is shown that the origin of strong electron-phonon interactions in cuprates is due to the anti-Jahn-Teller effect. In the second part a recent theoretical result on the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below Tc is discussed. It is shown that the feature of ARPES profiles of underdoped cuprates consists of a coherent peak in the nodal region and the real transitions of photoexcited electrons from occupied states below the Fermi level to a free-electron state above the vacuum level in the antinodal region, where the latter transitions form a broad hump. From this feature, the origin of the two distinct gaps observed by ARPES is elucidated without introducing the concept of the pseudogap. Finally, a remark is made on the phase diagram of underdoped cuprates.

Influence of surface optical phonons on exciton binding energies of a quasi-one-dimensional wurtzite GaN-based nanowire: Quantum size effect

Based on the surface optical phonon states and their electron-phonon coupling functions obtained recently, the surface polaronic exciton states in a quasi-one-dimensional (Q1D) wurtzite nanowire (NW) are investigated by using the variational method and Lee-Low-Pines transform. In order to reflect the different confined features of Q1D wurtzite NWs in axial and radial directions and anisotropy of wurtzite nitride crystals, a two-parameter variational approach is proposed and applied to describe the polaronic exciton states in the NWs. Numerical calculations are performed for GaN NWs. The quantum size effects on the binding energies of polaronic exciton and the contributions of surface phonon modes are analyzed in detail. Our results show that the binding energy of polaronic exciton and surface phonon contribution in the wurtzite GaN NWs reaches 190 and 95 meV, respectively, which are one or two orders of magnitude larger than those in cubic GaAs-based quantum wells and NWs with the same radius. This is mainly ascribed to the strong electron-phonon interaction, the large effective masses of carriers and relatively small dielectric constants in GaN material. The numerical results also show that the two-parameter variational approach is reasonable and necessary for the description of polaronic exciton states in Q1D wurtzite GaN NWs. Moreover, the behaviors of the two variational parameters for the polaronic exciton states are quite different from those for the bound polaron states in Q1D wurtzite NWs.

Crossover from adiabatic to antiadiabatic phonon-assisted tunneling in single-molecule transistors

The crossover between two customary limits of phonon-assisted tunneling, the adiabatic and antiadiabatic regimes, is studied systematically in the framework of a minimal model for molecular devices: a resonant level coupled by displacement to a localized vibrational mode. Conventionally associated with the limits where the phonon frequency is either sufficiently small or sufficiently large as compared to the bare electronic hopping rate, we show that the crossover between the two regimes is governed for strong electron-phonon interactions primarily by the polaronic shift rather than the phonon frequency. In particular, the perturbative adiabatic limit is approached only as the bare hopping rate \Gamma exceeds the polaronic shift, leaving an extended window of couplings where \Gamma well exceeds the phonon frequency and yet the physics is basically that of the antiadiabatic regime. We term this intermediate regime the extended antiadiabatic regime. The effective low-energy Hamiltonian in the traditional and the extended antiadiabatic regime is shown to be the (purely fermionic) interacting resonant-level model, with parameters that we extract from numerical renormalization-group calculations. The extended antiadiabatic regime is followed in turn by a true crossover region where the polaron gets progressively undressed. The renormalized tunneling rate, which serves as the low-energy scale in the problem and thus sets the width of the tunneling resonance, is found to follow an approximate scaling form on going from the adiabatic to the antiadiabatic regime. Charging properties are governed by two distinct mechanisms at the extended antiadiabatic and into the crossover region, giving rise to distinctive shoulders in the low-temperature conductance as a function of gate voltage.

Effects of electron-phonon coupling in the Kondo regime of a two-orbital molecule

We study the interplay between strong electron-electron and electron-phonon interactions within a two-orbital molecule coupled to metallic leads, taking into account Holstein-like coupling of a local phonon mode to the molecular charge as well as phonon-mediated interorbital tunneling. By combining canonical transformations with numerical renormalization-group calculations to address the interactions nonperturbatively and on equal footing, we obtain a comprehensive description of the system's many-body physics in the anti-adiabatic regime where the phonons adjust rapidly to changes in the orbital occupancies, and are thereby able to strongly affect the Kondo physics. The electron-phonon interactions strongly modify the bare orbital energies and the Coulomb repulsion between electrons in the molecule, and tend to inhibit tunneling of electrons between the molecule and the leads. The consequences of these effects are considerably more pronounced when both molecular orbitals lie near the Fermi energy of the leads than when only one orbital is active. In situations where a local moment forms on the molecule, there is a crossover with increasing electron-phonon coupling from a regime of collective Kondo screening of the moment to a limit of local phonon quenching. At low temperatures, this crossover is associated with a rapid increase in the electronic occupancy of the molecule as well as a marked drop in the linear electrical conductance through the single-molecule junction.

Phonon-assisted thermoelectric effects in a two-level molecule

Thermoelectric properties of a two-level molecule attached to the metallic electrodes are analyzed using the equation of motion technique within the Green function formalism. Results show that the electrical conductance is strongly dependent on the electron and phonon temperatures and the electron–phonon coupling strength. In addition, it is observed that the thermal conductance peaks in the electron–hole symmetry points are vanished in the presence of the strong electron–phonon interaction. It is also found that the figure of merit is strongly suppressed in the strong electron–phonon interaction. The violation of the Wiedemann–Franz law is also observed coming from the Coulomb interactions.

Surface and subsurface phonons of Bi(111) measured with helium atom scattering

The surface phonon dispersion curves of Bi(111) have been measured by inelastic helium atom scattering (HAS) along the two symmetry directions. The complex set of observed dispersion curves, including several branches in the acoustic region, plus a localized and a resonant branch in the optical region, is interpreted by means of calculations based on density functional perturbation theory (DFPT). It is recognized that the upper phonon branches in the acoustic region starting at about $5.3$ meV and $4.3$ meV at zero wave vector correspond to shear-vertical and longitudinal modes localized on the third surface layer (second bilayer), respectively. The HAS ability of detecting subsurface phonons previously observed for Pb(111) multilayers is attributed to the comparatively strong electron-phonon interaction, and confirmed through a DFPT calculation of the phonon-induced surface charge-density oscillations. A comparison of the integrated HAS intensities with those of Pb(111) multilayers measured under similar kinematic conditions allows for an estimation of the electron-phonon mass-enhancement parameter for the Bi(111) surface. An anomaly at the $\overline{\mathrm{M}}$ point, quite sharp at 123 K but very broad at room temperature, is associated with recombination processes of bulk $\overline{\mathrm{M}}$-point pocket electrons with bulk pocket holes at either $\overline{\ensuremath{\Gamma}}$ or equivalent $\overline{\mathrm{M}}$ points.

Electron and Phonon Transport in n- and p-type Skutterudites

ABSTRACTFilled skutterudites are one of the most promising materials for thermoelectric (TE) power generation applications at intermediate temperatures due to their superior TE and thermomechanical performance as compared to other materials. In the past, we have demonstrated that n-type skutterudites can be optimized so that their maximum TE figure of merit reaches 1.7 at 850 K. TE performance of the p-type, however, is lagging behind, which hinders the optimization of skutterudites-based TE module development. In this paper we reveal that the underlying reasons for inferior TE properties of the p-type root in their electronic band structures, which result in higher thermal conductivity at elevated temperatures due to bipolar lattice thermal conduction and lower power factor because of heavy valance bands induced strong electron-phonon interactions. We also identify means of improving the power factor and reducing bipolar effect.

First and second differentials of the ultrasonic parameter as an effective tool to identify phase transitions in R 1-x A x MnO3 perovskites

Abstract In order to examine the existence of different phase transitions, the ultrasonic longitudinal modulus (L) and its first (ΔL/ΔT) and second (Δ2 L/Δ2 T) differentials have been established for two different perovskite manganites over a wide range of temperatures, La0.67Sr0.33MnO3 from 460 to 300 K and Nd0.67Sr0.33MnO3 300 to 150 K. The anomalies in L and its differentials at T C (≈378 K) in La0.67Sr0.33MnO3 are due to the ferromagnetic metal transition; the changes at T C (≈234 K), T CO (≈194 K) in Nd0.67Sr0.33MnO3 represent the ferromagnetic transition and charge ordering transition respectively. The existing phase transitions are due to a strong electron-phonon interaction arising from the Jahn-Teller effect and spin-phonon interaction from the linear magnetostriction effect. The first (ΔL/ΔT) and second (Δ2 L/Δ2 T) differentials of L are identified as an effective approach for the clear identification/detection of different phase transitions in the above systems over a wide range of temperatures.

Thermophysical properties of Pr1−xCaxCoO3

In the present paper, the thermophysical properties of PrCoO3 cobaltates is mapped here with reference to the changed local environment at A-site due to calcium doping for the first time using Rigid Ion Model (RIM). The specific heat and there by thermal expansion for temperature (1K≤T≤1000K) of Pr1−xCaxCoO3 (x=0.0–0.5) are presented. The trends of variation of our computed results on specific heat with temperature are in more or less similar with corresponding experimental data for almost all the compositions (x) of Pr1−xCaxCoO3. Strong electron–phonon interactions are present in these compounds which causes the variation of the lattice specific heat (Cv(lattice)) with cation doping of varying size and valence. In addition, we have computed the thermal expansion (α), bulk modulus (B), cohesive energy (ϕ), molecular force constant (ƒ), and Restsrahalen frequency (υ) whose results are discussed in detail.

High-temperature superconductivity from realistic Coulomb and Fröhlich interactions

In recent years ample experimental evidence has shown that charge carriers in high-temperature superconductors are strongly correlated but also coupled with lattice vibrations (phonons), signalling that the true origin of high-Tc superconductivity can only be found in a proper combination of Coulomb and electron-phonon interactions. On this basis, we propose and study a model for high-Tc superconductivity which accounts for realistic Coulomb repulsion, strong electron-phonon (Fröhlich) interaction and residual on-site (Hubbard-) correlations without any ad hoc assumptions on their relative strength and interaction range. In the framework of this model, which exhibits a phase transition to a superconducting state with a critical temperature Tc well in excess of 100 K, we emphasize the role of as the driving parameter for a BEC/BCS crossover. Our model lays a microscopic foundation for the polaron-bipolaron theory of superconductivity. We argue that the high-Tc phenomenon originates in competing Coulomb and Fröhlich interactions beyond the conventional BCS description.