Electron Self Research Articles

Two loop low temperature corrections to electron self energy

We recalculate the two loop corrections in the background heat bath using real time formalism. The procedure of the integrations of loop momenta with dependence on finite temperature before the momenta without it has been followed. We determine the mass and wavefunction renormalization constants in the low temperature limit of QED, for the first time with this preferred order of integrations. The correction to electron mass and spinors in this limit is important in the early universe at the time of primordial nucleosynthesis as well as in astrophysics.

Full quantum treatment of spin-dependent beam-beam processes at linear colliders

Depolarisation processes at future linear colliders need to be understood as precisely as possible. To that end a theoretical consideration of the spin flip process and its radiative corrections is presented here. The spin flip process contains a divergence and it is useful to repeat the calculation of its transition rate using a coordinate system which makes the physical nature of the divergence apparent. It is argued that the radiative corrections to the spin flip process should be considered within the Furry Picture. The Electron Self Energy in the external field is being explicitly re-examined in order to establish the presence of UV divergences and the procedure required to remove them. A calculation of the Vertex Correction in an external field is being performed and results obtained so far for special kinematics are consistent with known results.

QED in Krein Space Quantization

In this paper we consider the QED in Krein space quantization. We show that the theory is automatically regularized. The three primitive divergences integrals in usual QED are considered in Krein QED. The photon self energy, electron self energy and vertex function are calculated in this formalism. We show that these quantities are finite. The infrared and ultraviolet divergencies do not appear. We discuss that Krein space quantization is similar to Pauli-Villars regularization, so we have called it the “Krein regularization”.

Nonequilibrium transport via spin-induced subgap states in superconductor/quantum dot/normal metal cotunnel junctions

We study low-temperature transport through a Coulomb blockaded quantum dot (QD) contacted by a normal (N), and a superconducting (S) electrode. Within an effective cotunneling model the conduction electron self energy is calculated to leading order in the cotunneling amplitudes and subsequently resummed to obtain the nonequilibrium T-matrix, from which we obtain the nonlinear cotunneling conductance. For even occupied dots the system can be conceived as an effective S/N-cotunnel junction with subgap transport mediated by Andreev reflections. The net spin of an odd occupied dot, however, leads to the formation of sub-gap resonances inside the superconducting gap which gives rise to a characteristic peak-dip structure in the differential conductance, as observed in recent experiments.

Momentum-space anisotropy and pseudogaps: A comparative cluster dynamical mean-field analysis of the doping-driven metal-insulator transition in the two-dimensional Hubbard model

Cluster dynamical mean field calculations based on 2, 4, 8 and 16 site clusters are used to analyze the doping-driven metal-insulator transition in the two dimensional Hubbard model. Comparison of results obtained on different clusters enables a determination of those aspects of the physics that are common to all clusters and permits identification of artifacts associated with particular cluster geometries. A modest particle-hole asymmetry in the underlying band structure is shown to lead to qualitatively different behavior on the hole doped side than on the electron doped side. For particle-hole asymmetry of the sign and magnitude appropriate to high-$T_c$ cuprates, the approach to the insulator from the hole-doping side is found to proceed in two stages from a high-doping region where the properties are those of a Fermi liquid with moderately renormalized parameters and very weak momentum dependence. As doping is reduced the system first enters an intermediate doping regime where the Fermi liquid renormalizations are larger and the electron self energy varies significantly around the Fermi surface and then passes to a small doping regime characterized by a gap in some regions of the Fermi surface but gapless behavior in other regions. On the electron doped side the partially gapped regime does not occur, and the momentum dependence of the electron self energy is less pronounced. Implications for the high-$T_c$ cuprates and for the use of cluster dynamical mean field methods in wider classes of problems are discussed.

Effect of electron-phonon interaction and temperature dependence of resistivity in some heavy fermion systems

Here we have considered the electron-phonon interaction in the Periodic Anderson Model (PAM) to describe the temperature dependence of resistivity in some heavy fermion (HF) systems. Since the resistivity is related to the imaginary part of the electron self energy, the expression of the same is evaluated from electron Green function by the double time temperature dependant Green function technique of the Zubarev type. In order to understand the effect of electron-phonon interaction in these systems, we couple the phonons to both the f-electrons as well as to the electrons of the hybridization band of both conduction and f-electrons. The influence of various model parameters, namely, the position of 4f level E 0, the electron-phonon coupling strengths f 1(q) and f 2(q), the effecting coupling strength g = N(0)γ 0 2 /ω 0 have been studied on the temperature dependence of resistivity in HF systems. The numerical analysis is performed for q = 0 and for finite temperature in the static limit. The analysis of the results gives satisfactorily in comparison to the experimental observations.

The Refinement of Self-Trapped Excitons Structure in CaF$_{2}$ and SrF$_{2}$ Crystals: An Ab Initio Study

We present the results of ab initio calculations of self-trapped excitons (STE) in CaF2 and SrF2 crystals performed with BHHLYP density functional in embedded cluster approach. As a motivation for this theoretical study, we also report some previously unpublished experimental results on STE luminescence. They concern new luminescence bands of STE approximately 1 eV higher then the main band. The new bands have much lower intensities in pure crystals, however they remain almost unaffected by impurity doping while the main band is quenched by impurities. As possible candidates for excitonic configuration responsible for these high-energy bands, four possible configurations of the off-center STE are considered and rejected. For the on-center STE, i.e., VK + e, stable configuration has been found. The calculated luminescence energy of on-center STE is 6.57 eV in CaF2 and 6.31 eV in SrF2 crystal. On the basis of these energies (much higher than energies of new excitonic luminescence bands) and low stability of on-center STE, it is also rejected as a candidate for new bands. Finally, the hypothesis of electron self-trapping is considered. We have found electron self trapping in CaF2 and SrF2 crystals to be energetically favorable, however the value of energy gain lies within the possible calculation error. It was also found that there is a barrier about 0.3 eV for electron self-trapping. Self-trapped electron is a center with trigonal symmetry and can be considered as perturbed F-center. Calculated isotropic hyperfine couplings of self-trapped electron are given. On the basis of these results, we tentatively suggest that self-trapped electrons are the precursors of yet unknown excitonic configuration responsible for the new luminescence bands.

Prompt optical emission and synchrotron self-absorption constraints on emission site of GRBs

We constrain the distance of the Gamma-Ray Burst (GRB) prompt emission site from the explosion centre, R, by determining the location of the electron's self absorption frequency in the GRB prompt optical-to-X/gamma-ray spectral energy distribution, assuming that the optical and the gamma-ray emissions are among the same synchrotron radiation continuum of a group of hot electrons. All possible spectral regimes are considered in our analysis. The method has only two assumed parameters, namely, the bulk Lorentz factor of the emitting source Gamma, and the magnetic field strength B in the emission region (with a weak dependence). We identify a small sample of 4 bursts that satisfy the following three criteria: (1) they all have simultaneous optical and gamma-ray detections in multiple observational time intervals; (2) they all show temporal correlations between the optical and gamma-ray light curves; and (3) the optical emission is consistent with belonging to the same spectral component as the gamma-ray emission. For all the time intervals of these 4 bursts, it is inferred that R \geq 10^{14} (Gamma/300)^{3/4} (B/10^5 Gauss)^{1/4} cm. For a small fraction of the sample, the constraint can be pinned down to R \approx 10^{14} - 10^{15} cm for Gamma ~ 300. For a second sample of bursts with prompt optical non-detections, only upper limits on R can be obtained. We find no inconsistency between the R-constraints for this non-detection sample and those for the detection sample.

GWquasiparticle corrections to theLDA+U∕GGA+Uelectronic structure of bcc hydrogen

In this paper, we study the quasiparticle electronic structure of atomic hydrogen in the body-centered cubic structure for various densities. We employ the $GW$ approach to compute the electron self energy. For this model system, we use the local density approximation $(\mathrm{LDA})+U$/generalized gradient approximation $(\mathrm{GGA})+U$ method as the mean-field solution starting point, which is known to work better than LDA/GGA for systems with strongly correlated electrons. In the low-density insulating phase, we find that the calculated $GW$ quasiparticle gap is quite insensitive to the value of the on-site repulsive $U$ employed over a wide range of physically reasonable values. Moreover, our result for the electronic gap agrees with the measured difference between ionization energy and electron affinity in the atomic limit.

Theory of bulk and surface quasiparticle spectra for Fe, Co, and Ni

The correlated electronic structure of iron, cobalt and nickel is investigated within the dynamical mean-field theory formalism, using the newly developed full-potential LMTO-based LDA+DMFT code. Detailed analysis of the calculated electron self-energy, density of states and the spectral density are presented for these metals. It has been found that all these elements show strong correlation effects for majority spin electrons, such as strong damping of quasiparticles and formation of a density of states satellite at about -7 eV below the Fermi level. The LDA+DMFT data for fcc nickel and cobalt (111) surfaces and bcc iron (001) surface is also presented. The electron self energy is found to depend strongly on the number of nearest neighbors, and it practically reaches the bulk value already in the second layer from the surface. The dependence of correlation effects on the dimensionality of the problem is also discussed.

Properties of halo nuclei from atomic isotope shifts

This paper reviews progress that has been made in obtaining essentially exact solutions to the nonrelativistic three-body problem for helium by a combination of variational and asymptotic expansion methods. The calculation of relativistic and quantum electrodynamic corrections by perturbation theory is discussed, and in particular, methods for the accurate calculation of the Bethe logarithm part of the electron self energy are presented. As an example, the results are applied to the calculation of isotope shifts for the short-lived 'halo' nucleus 6 He relative to 4 He in order to determine the nuclear charge radius of 6 He from high precision spectroscopic measurements carried out at the Argonne National Laboratory. The results demonstrate that the high precision that is now available from atomic theory is creating new opportunities to create novel measurement tools, and helium, along with hydrogen, can be regarded as a fundamental atomic system whose spectrum is well understood for all practical purposes.

Theory of the Luttinger surface in doped Mott insulators

We prove that the Mott insulating state is characterized by a divergence of the electron self energy at well-defined values of momenta in the first Brillouin zone. When particle-hole symmetry is present, the divergence obtains at the momenta of the Fermi surface for the corresponding non-interacting system. Such a divergence gives rise to a surface of zeros (the Luttinger surface) of the single-particle Green function and offers a single unifying principle of Mottness from which pseudogap phenomena, spectral weight transfer, and broad spectral features emerge in doped Mott insulators. We also show that only when particle-hole symmetry is present does the volume of the zero surface equal the particle density. We identify that the general breakdown of Luttinger's theorem in a Mott insulator arises from the breakdown of a perturbative expansion for the self energy in the single-particle Green function around the non-interacting limit. A modified version of Luttinger's theorem is derived for special cases.

Femtosecond evolution of electronic interactions at the Ni(111) surface

We used time-resolved photoemission spectroscopy to measure the changes in electron self energy for the occupied surface resonance near the surface Brillouin zone center of Ni/W(110) single crystalline films following optical excitations with a fs laser pulse. Binding energy and lifetime display pronounced variations on a 100 fs time scale. We provide evidence that this is directly linked to the laser induced population of electronic levels.

Perturbative Formulation of Pure Space-Like Axial Gauge QED with Infrared Divergences Regularized by Residual Gauge Fields

We construct a new perturbative formulation of pure space-like axial gauge QED in which the inherent infrared divergences are regularized by residual gauge fields. For that purpose we perform our calculations in coordinates $x^{\mu}=(x^+,x^-,x^1,x^2)$, where $x^+=x^0\sin{\theta}+x^3\cos {\theta}$ and $x^-=x^0\cos{\theta}-x^3\sin{\theta}$. $A_-=A^0\cos{\theta}+A^3 \sin{\theta}=n{\cdot}A=0$ is taken as the gauge fixing condition. We show in detail that, in perturbation theory, infrared divergences resulting from the residual gauge fields cancel infrared divergences resulting from the physical parts of the gauge field. As a result we obtain the gauge field propagator prescribed by Mandelstam and Leibbrandt. By taking the limit $\theta {\to} \frac{\pi}{4}$ we can construct the light-cone formulation which is free from infrared difficulty. With that analysis complete, we perform a successful calculation of the one loop electron self energy, something not previously done in light-cone quantization and light-cone gauge.

Self-quenching of a photo-excited ruthenium complex in component-controllable mixed Langmuir–Blodgett films

The miscibility and phase behavior of the mixed monolayer of Ru(dpphen) 3 2+ (dpphen=4,7-diphenyl-1,10-phenanthroline) (abbreviated as Ru(II)), octadecyltrichlorosilane (OTS) and stearic acid (SA) at air/water interface had been investigated in detail. Two-dimensional density of Ru(II) could be easily adjusted by changing the mixing ratio of Ru(II)/SA/OTS. The mixed monolayer of Ru(II)/SA/OTS with different molar proportions had been deposited and characterized by low angle X-ray diffraction, UV–visible absorption spectra and photo-induced emission spectra. The possible structure of its resulting LB film was proposed based on the results of the π – A isotherms and low angle X-ray diffraction. Photo-induced electron transfer and self quenching of Ru(II) in mixed LB films had been investigated in detail.

Self-quenching of a photo-excited ruthenium complex in component-controllable mixed Langmuir–Blodgett films

The miscibility and phase behavior of the mixed monolayer of Ru(dpphen) 3 2+ (dpphen=4,7-diphenyl-1,10-phenanthroline) (abbreviated as Ru(II)), octadecyltrichlorosilane (OTS) and stearic acid (SA) at air/water interface had been investigated in detail. Two-dimensional density of Ru(II) could be easily adjusted by changing the mixing ratio of Ru(II)/SA/OTS. The mixed monolayer of Ru(II)/SA/OTS with different molar proportions had been deposited and characterized by low angle X-ray diffraction, UV–visible absorption spectra and photo-induced emission spectra. The possible structure of its resulting LB film was proposed based on the results of the π– A isotherms and low angle X-ray diffraction. Photo-induced electron transfer and self quenching of Ru(II) in mixed LB films had been investigated in detail.

Angle-resolved photoemission studies of the cuprate superconductors

This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature in this field. The low energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and d-wave-like dispersion, evidence of electronic inhomogeneity and nano-scale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides a brief overview of the scientific issues relevant to the investigation of the low energy electronic structure by ARPES. The rest of the paper is devoted to the review of experimental results from the cuprates and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self energy and collective modes. Within each topic, ARPES data from the various copper oxides are presented.

Self‐consistent model of solar wind electrons

We model the transformation of the steady state electron velocity distribution function in the collisional transition region of the solar wind by solving the Fokker‐Planck equation. Alongside the proton and electron Coulomb collisions, effects of gravitational and electric and magnetic forces are also considered. The Coulomb collision term is calculated for any background velocity distribution function using a spectral method that we have developed and applied previously [Pierrard et al., 1999], Consistent treatment of electron self collisions is obtained using an iterative numerical method to match the velocity distribution functions of “test” and “background” electron distributions. Electron collisions with background solar wind protons are also taken into account. We find that Coulomb collisions have important effects on angular scattering of the electrons [i.e., on the pitch angle distribution] without changing their average density or mean radial temperature distribution. Finally, the importance of boundary conditions on the solution of the Fokker‐Planck equation is discussed and emphasized.

Thermoelectric behavior near the magnetic quantum critical point

We use the coupled 2d-spin-3d-fermion model proposed by Rosch {\sl et. al.} (Phys. Rev. Lett. {\bf 79}, 159 (1997)) to study the thermoelectric behaviour of a heavy fermion compound when it is close to an antiferromagnetic quantum critical point. When the low energy spin fluctuations are quasi two dimensional, as has been observed in ${\rm YbRh}_2{\rm Si}_2$ and $ {\rm CeCu}_{6-x}{\rm Au}_x $, with a typical 2d ordering wavevector and 3d Fermi surface, the ``hot'' regions on the Fermi surface have a finite area. Due to enhanced scattering with the nearly critical spin fluctuations, the electrons in the hot region are strongly renormalized. We argue that there is an intermediate energy scale where the qualitative aspects of the renormalized hot electrons are captured by a weak-coupling perturbative calculation. Our examination of the electron self energy shows that the entropy carried by the hot electrons is larger than usual. This accounts for the anomalous logarithmic temperature dependence of specific heat observed in these materials. We show that the same mechanism produces logarithmic temperature dependence in thermopower. This has been observed in $ {\rm CeCu}_{6-x}{\rm Au}_x $. We expect to see the same behaviour from future experiments on ${\rm YbRh}_2{\rm Si}_2$.

Nonquasiparticle structure in the photoemission spectra from the Be(0001) surface and determination of the electron self energy

Complex structure in photoemission spectra of Be(0001) surface states near the Fermi energy is observed and explained as the effect of strong electron-phonon coupling. The weak momentum dependence of the electron-phonon contribution to the electron self energy \ensuremath{\Sigma} is exploited to determine \ensuremath{\Sigma} by direct inversion of the spectra.