Electron Mass Enhancement Research Articles

Nonlinear phenomena in charge transport properties of a hole-doped organic spin-liquid compound

The nonlinear electric conductivity of κ-(BEDT-TTF)4Hg2.89Br8, which is known as a hole-doped spin liquid, has been observed by single crystal transport measurements using dc and ac excitations. This compound is known as a charge transfer complex with strong magnetic fluctuations accompanied by electron mass enhancement at ambient pressure. We discuss the nonlinear dc conductivity drastically emerging below 100 K. We also performed ac impedance measurements of this compound and compared the results with those of a typical dimer-Mott compound of deuterated κ-(d8-BEDT-TTF)2Cu[N(CN)2]Br, located just near the Mott boundary. The analyses of the Nyquist plots of κ-(d8-BEDT-TTF)2Cu[N(CN)2]Br and κ-(BEDT-TTF)4Hg2.89Br8 reveal qualitatively different features. The former shows a behavior pertinent to the inhomogeneous distribution of domains due to a mixing of metal and insulating phases, taking into account the influence of the proximity effect, while the doped spin liquid has a distinct semicircle-type frequency dependence in its Nyquist plot in the whole temperature range studied. We conclude that the nonlinear conductivity is intrinsically peculiar to the doped dimer Mott system, where the charge degrees of freedom dominate the itinerancy. We attribute the anomalous features, such as non-Fermi liquid behavior, heat capacity enhancement, and strong antiferromagnetic fluctuations in κ-(BEDT-TTF)4Hg2.89Br8, to a kind of charge confinement effect retained in the hole-doped spin-liquid state.

Theoretical analysis of perturbative high harmonic wave mixing from plasma surfaces

High harmonic generation modulated by a weakly perturbing laser field enables new wave mixing frequency components, thus allowing in-situ spatiotemporal measurements and wavefront control of attosecond optical pulses. However, perturbative high harmonic wave mixing from plasma surfaces has not been investigated extensively. In this study, we theoretically analyze the plasma high harmonic generation process in the relativistic regime modulated by a perturbing laser field with an arbitrary frequency. New wave mixing frequency components satisfying the conservation laws of photon energy and momentum are observed. The wave mixing component intensities adhere to a power law for the perturbating laser photon number as the perturbing laser intensity increases, thereby revealing perturbative behaviors in the nonperturbative, extremely nonlinear optical process of high harmonic generation. Detailed studies reveal the polarization selection rule and physical mechanism of high harmonic wave mixing. The modulation of the relativistic factor or mass enhancement of electrons on the plasma surface by the perturbing laser field is believed to result in high harmonic wave mixing in the relativistic regime.

Phonon-Mediated Quasiparticle Lifetime Renormalizations in Few-Layer Hexagonal Boron Nitride.

Understanding the collective behavior of the quasiparticles in solid-state systems underpins the field of nonvolatile electronics, including the opportunity to control many-body effects for well-desired physical phenomena and their applications. Hexagonal boron nitride (hBN) is a wide-energy-bandgap semiconductor, showing immense potential as a platform for low-dimensional device heterostructures. It is an inert dielectric used for gated devices, having a negligible orbital hybridization when placed in contact with other systems. Despite its inertness, we discover a large electron mass enhancement in few-layer hBN affecting the lifetime of the π-band states. We show that the renormalization is phonon-mediated and consistent with both single- and multiple-phonon scattering events. Our findings thus unveil a so-far unknown many-body state in a wide-bandgap insulator, having important implications for devices using hBN as one of their building blocks.

Giant electron-phonon interaction for a prototypical semiconductor interface: Sn/Ge(111)−(3×3)

We report an experimental and theoretical study of the electron-phonon coupling for $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}/\mathrm{Ge}(111)$, a prototypical triangular lattice surface, closely related to $\mathrm{Sn}/\mathrm{Si}(111)\text{\ensuremath{-}}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$, where recent experimental evidence has found superconductivity [X. Wu et al., Phys. Rev. Lett. 125, 117001 (2020)]. We concentrate our study on the ($3\ifmmode\times\else\texttimes\fi{}3$) phase of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}/\mathrm{Ge}(111)$ that appears between 150 and 120 K and has a well-known geometry with a half-filled electronic band around the Fermi energy. We show that this surface presents a giant electron-phonon interaction that can be considered at least partially responsible for the different phases that this system shows at very low temperature. Our theoretical results indicate that indeed the electron-phonon interaction in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}/\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}3)$ is unusually large, since we find that $\ensuremath{\lambda}$, the electron mass enhancement for the half-filled band, is $\ensuremath{\lambda}=1.3$. This result is in good agreement with the experimental value obtained from high-resolution angle-resolved photoemission spectroscopy measurements, which yield $\ensuremath{\lambda}=1.45\ifmmode\pm\else\textpm\fi{}0.1$.

Hybridization and correlation between f - and d -orbital electrons in a valence fluctuating compound EuNi2P2

The interaction between localized f and itinerant conduction electrons is crucial in the electronic properties of heavy fermion and valence fluctuating compounds. Using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structure of the archetypical valence fluctuating compound EuNi2P2 that hosts multiple f electrons. At low temperatures, we reveal the hybridization between Eu 4f and Ni 3d states, which contributes to the electron mass enhancement, consistent with the periodic Anderson model. With increasing temperature, interestingly, we observe opposite temperature evolution of electron spectral function above and below the Kondo coherence temperature near 110 K, which is in contrast to the monotonic valence change and beyond the expectation of the periodic Anderson model. We argue that both f-d hybridization and correlation are imperative in the electronic properties of EuNi2P2. Our results shed light on the understanding of novel properties, such as heavy fermion behaviors and valence fluctuation, of rare-earth transition-metal intermetallic compounds with multiple f electrons.

Many-body Green's function approaches to the doped Fröhlich solid: Exact solutions and anomalous mass enhancement

In polar semiconductors and insulators, the Fr\"ohlich interaction between electrons and long-wavelength longitudinal optical phonons induces a many-body renormalization of the carrier effective masses and the appearence of characteristic phonon sidebands in the spectral function, commonly dubbed 'polaron satellites'. The simplest model that captures these effects is the Fr\"ohlich model, whereby electrons in a parabolic band interact with a dispersionless longitudinal optical phonon. The Fr\"ohlich model has been employed in a number of seminal papers, from early perturbation-theory approaches to modern diagrammatic Monte Carlo calculations. One limitation of this model is that it focuses on undoped systems, thus ignoring carrier screening and Pauli blocking effects that are present in real experiments on doped samples. To overcome this limitation, we here extend the Fr\"ohlich model to the case of doped systems, and we provide exact solutions for the electron spectral function, mass enhancement, and polaron satellites. We perform the analysis using two approaches, namely Dyson's equation with the Fan-Migdal self-energy, and the second-order cumulant expansion. We find that these two approaches provide qualitatively different results. In particular, the Dyson's approach yields better quasiparticle masses and worse satellites, while the cumulant approach provides better satellite structures, at the price of worse quasiparticle masses. Both approaches yield an anomalous enhancement of the electron effective mass at finite doping levels, which in turn leads to a breakdown of the quasiparticle picture in a significant portion of the phase diagram.

The sensitive tunability of superconducting critical temperature in high-buckled plumbene by shifting Fermi level

As the newly member added to the graphene family, the high-buckled (HB) plumbene is a metallic two-dimensional(2D) material with the electronic structure characterized by the existence of some band edges in close proximity to the Fermi level. Hence in such a 2D material the electronic density of states (DOS) presents sharp peaks around the Fermi level. In this work, we perform first-principles calculations of the conventional superconducting critical temperature (Tc) of HB plumbene. We find that the electron-phonon (e-ph) coupling strength which measures the electron mass enhancement is proportional to the DOS at the Fermi level. As a result, Tc of HB plumbene has a sizable increase (from 5.29 K to 17.21 K) when the Fermi level is shifted to the nearby DOS peaks from its intrinsic position. For 2D materials, the adjustment of Fermi level can be readily realized by applying a gate voltage in a device structure of electronic transport, instead of a chemical carrier doping. Therefore, our study proposes a feasible way to tune Tc of those 2D metallic materials such as HB plumbene which have some band edges or flat band around Fermi level.

Mass enhancement in 3d and s−p perovskites from symmetry breaking

In some d-electron oxides the measured effective mass m(exptl) has long been known to be significantly larger than the model effective mass m(model) deduced from mean-field band theory, i.e., m(exptl) = beta * m(model), where beta > 1 is the "mass enhancement", or "mass renormalization" factor. Previous applications of density functional theory (DFT), based on the smallest number of possible magnetic, orbital, and structural degrees of freedom, missed such mass enhancement, a fact that was taken as evidence of strong electronic correlation being the exclusive enabling physics. The current paper reports that known modalities of energy-lowering symmetry-broken spin and structural effects included in mean-field DFT show mass enhancement for both electrons and holes in a range of d-electron perovskites SrVO3, SrTiO3, BaTiO3, and LaMnO3 as well as p-electron perovskites CsPbI3 and SrBiO3, including metals (SrVO3) and insulators (the rest). This is revealed only when enlarged unit cells of the same parent global symmetry, which are large enough to allow for symmetry breaking distortions and concomitant variations in spin order, are explored for their ability to lower the total energy. The paper analyzes the contributions of different symmetry-broken modalities to mass enhancement, finds common effects in the range of d- as well as p-electron perovskites. Thus, symmetry breaking in mean-field theory could describe effects that were previously attributed exclusively to complex correlated treatments. Indeed, broken symmetries are often experimentally observed, e.g., octahedral tilting, Jahn-Teller distortions, or bond disproportionation, and often provide intuitive explanations in terms of degeneracy removal due to lowering of structural symmetry, or as shifting of band energies in explainable directions.

Crystal electric field effects and heavy-fermion behavior in cubic PrCu2Au3 compound

We report the crystal structure as well as the magnetic, electronic and transport properties of PrCu2Au3 which crystallizes in the MgCu4Sn-type structure with the cubic space group F4¯3m (No. 216). In this structure, the Pr3+ ion sits on a lattice site with the cubic Td symmetry which dictates the crystal electric field (CEF) splitting of the Hund’s 3H4 ground multiplet into four levels. The sample has been investigated by measurements of temperature dependences of specific heat, Cp(T), electrical resistivity, ρ(T), magnetic susceptibility, χ(T), isothermal magnetization, M(B,T), thermal conductivity, κT(T) and thermoelectric power, S(T). χ(T) follows a Curie-Weiss behavior above 100 K with an effective magnetic moment, μeff = 3.3 μB/Pr which is comparable to a calculated value of 3.58 μB/Pr for a free Pr3+ ion and a Weiss temperature, θCW = 15.70 K indicating a dominant ferromagnetic interaction. From the analysis of M(B,T) and the 4f-electron derived specific heat (C4f(T)) results, the CEF ground state is a Γ5 magnetic triplet and the CEF energy scheme is presented. The Sommerfeld coefficient, γ estimated from the low-temperature C4f(T) is 304 mJ/(mol K2) indicating a mass enhancement of the conduction electrons at low temperatures. Further analysis of the heavy fermion properties based on the Sommerfeld-Wilson ratio supports the formation of a heavy fermion state in PrCu2Au3. However, the absence of an enhancement in S/T at low temperatures possibly suggests a weak correlation between the conduction electrons and the 4f-electrons at the Fermi energy. The thermal transport properties suggest that PrCu2Au3 is likely a compensated metal.

Electronic correlations in uranium hydride UH5 under pressure

We report results of calculations based on density functional theory and dynamical mean-field theory for the electronic structure of uranium hydride UH5 under pressure, a compound of the uranium-based hydride family some members of which have been predicted to be superconducting. The effective electronic mass enhancement m*/m ∼ 1.4 indicates that the Coulomb correlations have a moderate strength. However, the topology of the Fermi surface changes strongly at the influence of the correlation effects: one hourglass-like pocket running along the Γ–A direction splits into two elliptical pockets centered at the A point. This result shows the possibility of an unconventional pairing mechanism for uranium hydrides in addition to the electron–phonon pairing that was studied in previous investigations.

Metal-insulator crossover and massive Dirac fermions in electron-doped FeTe

We explore the role played by electron doping in the electronic structure and transport properties of tetragonal FeTe using the local density approximation plus dynamical mean-field theory treatment. Semiconducting and metal-insulator crossover behavior observed in a paramagnetic and strained FeTe superconductor are shown to be driven by the interplay between multiorbital electron interactions and doping-induced normal-state massive Dirac fermions. The doping-dependent self-energy pole structure we derive is promising in the sense that it leads to results that explain why moderate electron band filling can generate orbital-selective Dirac valleys with strong electron mass enhancement consistent with experiments.

Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates

We present a detailed investigation of the specific heat of Ca3(Ru1-xMx)2O7 (M = Ti, Fe, Mn) single crystals. Depending on the dopant and doping level, three distinct regions are present: a quasitwo-dimensional metallic state with antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-b), a Mott insulating state with G-type AFM order (G-AFM), and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn doping. We observed not only an anomalous large mass enhancement, but also an additional term in the specific heat, i.e., C ∝ T2, in the localized region. The C ∝ T2 term is most likely due to long-wavelength excitations with both FM and AFM components. A decrease in the Debye temperature is observed in the G-type AFM region, indicating lattice softening associated with the Mott transition.

Nonlocal approach to energy bands in periodic lattices and emergence of electron mass enhancement

In this study, we analyze energy bands in periodic lattices based on the nonlocal-in-time kinetic energy approach introduced recently by Suykens. We consider the electron dynamics in the periodic potential and construct a nonlocal approach to the weak binding case in three dimensions. A number of features are revealed. In particular, we demonstrate an effective mass enhancement for electrons of around m∗≈167α−2Gev/c2 for typical solids, where α is a real free parameter.

Electronic structure and correlation in β−Ti3O5 and λ−Ti3O5 studied by hard x-ray photoelectron spectroscopy

We have conducted hard x-ray photoelectron spectroscopy investigations of the electronic structure changes and electron correlation phenomena which take place upon the photoinduced reversible phase transition between \ensuremath{\beta}- and $\ensuremath{\lambda}\text{\ensuremath{-}}T{\mathrm{i}}_{3}\mathrm{O}$. From valence band spectra of \ensuremath{\beta}- and $\ensuremath{\lambda}\text{\ensuremath{-}}T{\mathrm{i}}_{3}{\mathrm{O}}_{5}$, we have identified the bipolaron caused by the \ensuremath{\sigma}-type bonding of ${d}_{xy}$ orbitals in $\ensuremath{\beta}\text{\ensuremath{-}}T{\mathrm{i}}_{3}{\mathrm{O}}_{5}$ and the \ensuremath{\pi} stacking between the ${d}_{xy}$ orbitals between different Ti sites in $\ensuremath{\lambda}\text{\ensuremath{-}}T{\mathrm{i}}_{3}{\mathrm{O}}_{5}$, previously predicted by ab initio calculations. This indicates that the single electron band picture is valid for the description of photoinduced phase transitions. On the other hand, the Ti $2p$ and Ti $1s$ core level spectra exhibit nonlocal screening satellite features, which are typical spectroscopic signs of strong electron correlation in the coherent $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}{t}_{2g}$ states. The most striking result we obtain is that correlation in the valence band also manifests to reduce the plasmon energy, which results in an enhancement of the valence electron mass by a factor of 2.7.

On-site electron correlations in Bi2Se3 topological insulator

We explore the role of local, multi-orbital electron correlations in Bi2Se3 topological insulator. Based on local-density-approximation plus dynamical-mean-field-theory (LDA + DMFT) we derive a Kondo-Mott electronic transition, predicting maximally non-metallicity in three-dimensional Bi-chalcogenide topological insulators. Our results within the spin-orbit Kondo state show good agreement with scanning tunneling spectroscopy taken on pure and Cu-doped Bi2Se3 samples and effective mass enhancement of Dirac electrons for Cu x Bi2Se3, confirming the hypothesis of electron correlations in three-dimensional p-band topological insulators.

Relation between electron mass enhancement and potential shape: Numerical analysis of two-site anharmonic Holstein-Hubbard model

In order to promote our understanding on the emergence of magnetically robust heavy-electron state due to the interaction between conduction electrons and local anharmonic oscillations of guest atom in cage-structure materials, we investigate how the electron mass enhancement factor depends on the potential shape of the guest atom by analyzing numerically the two-site anharmonic Holstein-Hubbard model. It is found that the electron mass enhancement factor becomes maximum when the bottom of the potential becomes wide and flat. Such a potential is called a rattling type, which appears in the transition region between single-well and double-well types. Thus, the anharmonic oscillation in the rattling potential is considered to play a key role for the formation of the heavy-electron state in cage-structure compounds. We briefly discuss the reason why the electron mass enhancement factor becomes maximum in the rattling type potential region.

Orbital-dependent electronic masses in Ce heavy-fermion materials studied via Gutzwiller density-functional theory

A series of cerium-based heavy fermion materials is studied using a combination of local density functional theory and the many-body Gutzwiller approximation. Computed orbital-dependent electronic mass enhancement parameters are compared with available experimental data extracted from measured values of the Sommerfeld coefficient. Gutzwiller density functional theory is shown to remarkably follow the trends across a variety of Ce compounds and to give important insights into the phenomenon of orbital-selective mass renormalization which in turn allows for a better understanding of a wide spread of data.

Nonlinear resonance absorption of laser in an inhomogeneous plasma

A formalism of nonlinear resonance absorption of p-polarized laser in a plasma with density gradient is developed. The nonlinearity is taken to arise through the relativistic mass effect. As the laser incident at an angle to the density gradient approaches the turning point, dielectric swelling leads to field enhancement, causing strong electron mass enhancement and reduction in local plasma frequency. When the critical layer is within a laser wavelength away from the turning point, the laser field is greatly enhanced at the critical layer and absorption of the laser is strongly affected by the nonlinear effect.

Evidence for persistent spin fluctuations in uranium sesquicarbide

The low-temperature magnetic susceptibility, heat capacity, and electrical resistivity of uranium sesquicarbide have been determined down to 2 K by combining measurements carried out on samples of U${}_{2}$C${}_{3}$, containing UC as minority phase, and on pure UC specimens. The presence of spin fluctuations with characteristic temperature ${T}_{SF}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}7$ K is revealed by a nonanalytic contribution to the specific heat behaving (well below ${T}_{SF}$) as ${T}^{3}\text{ln}(T/{T}_{SF})$, and confirmed by a ${T}^{2}$ increase of the electrical resistivity and a $1\ensuremath{-}\ensuremath{\kappa}{T}^{2}$ decrease of the low-temperature magnetic susceptibility. The analysis of the specific-heat data above ${T}_{SF}$ provides a Debye temperature ${\ensuremath{\Theta}}_{D}$ $\ensuremath{\approx}$ 256 K and a moderate high value of the Sommerfeld coefficient, $\ensuremath{\gamma}$ $\ensuremath{\approx}$ 42 mJ mol${}_{U}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}2}$. The zero-temperature many-body enhancement of the electron mass is found to be ${m}^{*}/m$ $\ensuremath{\approx}$ 2.7. Our data rule out the occurrence of magnetic ordering in U${}_{2}$C${}_{3}$. First-principles electronic structure calculations support the absence of a magnetically ordered ground state but suggest that dynamical spin fluctuations are responsible for the electron mass enhancement.

On the idea of low-energy nuclear reactions in metallic lattices by producing neutrons from protons capturing “heavy” electrons

The present article is a critical comment on Widom and Larsens speculations concerning low-energy nuclear reactions (LENR) based on spontaneous collective motion of protons in a room temperature metallic hydride lattice producing oscillating electric fields that renormalize the electron self-energy, adding significantly to the effective electron mass and enabling production of low-energy neutrons. The frequency and mean proton displacement estimated on the basis of neutron scattering from protons in palladium and applied to the Widom and Larsens model of the proton oscillations yield an electron mass enhancement less than one percent, far below the threshold for the proposed neutron production and even farther below the mass enhancement obtained by Widom and Larsen assuming a high charge density. Neutrons are not stopped by the Coulomb barrier, but the energy required for the neutron production is not low.