Finite Effective Mass Research Articles

Momentum average approximation for models with boson-modulated hopping: Role of closed loops in the dynamical generation of a finite quasiparticle mass

We generalize the momentum average approximation to study the properties of single polarons in models with boson affected hopping, where the fermion-boson scattering depends explicitly on both the fermion's and the boson's momentum. As a specific example, we investigate the Edwards fermion-boson model in both one and two dimensions. In one dimension, this allows us to compare our results with exact diagonalization results, to validate the accuracy of our approximation. The generalization to two-dimensional lattices allows us to calculate the polaron's quasiparticle weight and dispersion throughout the Brillouin zone and to demonstrate the importance of Trugman loops in generating a finite effective mass even when the free fermion has an infinite mass.

Current-Voltage Characteristics of Superconductive Heterostructure Arrays

A systematic approach is presented to calculate the I-V characteristic of a general superconductive multilayer heterostructure, consisting of superconductors(S), normal metals(N), semiconductors(Sm) and insulators(I), with finite transparency and effective mass mismatch at the junctions. The transfer matrix method is used to obtain the scattering probability amplitudes of quasiparticles for the Andreev and ordinary reflection as well as the transmission with and without branch-crossing based on the solutions of the Bogoliubov-de Gennes equation. The method is then applied to study a niobium NSNSN structure with variable thickness of layers.

First direct observation of Dirac fermions in graphite

Originating from relativistic quantum field theory, Dirac fermions have been invoked recently to explain various peculiar phenomena in condensed-matter physics, including the novel quantum Hall effect in graphene1,2, the magnetic-field-driven metal–insulator-like transition in graphite3,4, superfluidity in 3He (ref. 5) and the exotic pseudogap phase of high-temperature superconductors6,7. Despite their proposed key role in those systems, direct experimental evidence of Dirac fermions has been limited. Here, we report the first direct observation of relativistic Dirac fermions with linear dispersion near the Brillouin zone (BZ) corner H, which coexist with quasiparticles that have a parabolic dispersion near another BZ corner K. In addition, we also report a large electron pocket that we attribute to defect-induced localized states. Thus, graphite presents a system in which massless Dirac fermions, quasiparticles with finite effective mass and defect states all contribute to the low-energy electronic dynamics.

Low energy excitations in graphite: The role of dimensionality and lattice defects

In this paper, we present a high resolution angle resolved photoemission spectroscopy (ARPES) study of the electronic properties of graphite. We found that the nature of the low energy excitations in graphite is particularly sensitive to interlayer coupling as well as lattice disorder. As a consequence of the interlayer coupling, we observed for the first time the splitting of the π bands by ≈0.7 eV near the Brillouin zone corner K. At low binding energy, we observed signatures of massless Dirac fermions with linear dispersion (as in the case of graphene), coexisting with quasiparticles characterized by parabolic dispersion and finite effective mass. We also report the first ARPES signatures of electron–phonon interaction in graphite: a kink in the dispersion and a sudden increase in the scattering rate. Moreover, the lattice disorder strongly affects the low energy excitations, giving rise to new localized states near the Fermi level. These results provide new insights on the unusual nature of the electronic and transport properties of graphite.

Production and evolution of perturbations of sterile neutrino dark matter

Sterile neutrinos, fermions with no standard model couplings [SU(2) singlets], are predicted by most extensions of the standard model, and may be the dark matter. I describe the nonthermal production and linear perturbation evolution in the early universe of this dark matter candidate. I calculate production of sterile neutrino dark matter including effects of Friedmann dynamics dictated by the quark-hadron transition and particle population, the alteration of finite temperature effective mass of active neutrinos due to the presence of thermal leptons, and heating of the coupled species due to the disappearance of degrees of freedom in the plasma. These effects leave the sterile neutrinos with a nontrivial momentum distribution. I also calculate the evolution of sterile neutrino density perturbations in the early universe through the linear regime and provide a fitting function form for the transfer function describing the suppression of small-scale fluctuations for this warm dark matter candidate. The results presented here differ quantitatively from previous work due to the inclusion of the relevant physical effects during the production epoch.

Threshold Energy Reduction in Photodisintegration and PhotoParticle Interaction Processes in Plasmas

The consequences of the finite effective mass of transverse photons in plasmas due to Coulomb effects on the value of threshold energies of inelastic photon-induced cosmic interaction processes such as photodisintegration and photopair and photomeson production are investigated. In all cases, a reduction of the value of the threshold energy as compared to the vacuum case occurs. Because the effective photon mass scales with the plasma density, the strongest reduction effect is found for stellar plasmas; in particular, the photon threshold energy for photopair production in stellar cores is 46% smaller than in the vacuum case. For the dilute intergalactic space plasma, we demonstrate that the threshold energy reductions for intergalactic photopair absorption and for the Greisen-Zatsepin-Kuzmin (GZK) effect on ultra-high-energy cosmic radiation are negligibly small.

Thermodynamics of photons in relativistic e+e- gamma plasmas.

Thermodynamic and spectral properties of a photon gas in e(+)e(-)gamma plasmas are studied. The effect of a finite effective mass of a photon, associated with the plasma frequency cutoff, is self-consistently included. In the ultrarelativistic plasma, the photon spectrum turns out to be universal with the temperature normalized plasma frequency cutoff as a fundamental constant independent of plasma parameters. Such a universality does not hold in the nonrelativistic plasma.

Effective mass of composite fermions and fermionic Chern-Simons theory in the temporal gauge

The definitions of the effective mass for the composite fermion are discussed in half filled lowest-Landau-level systems. In a recent work, Shankar and Murthy show a finite effective mass for the composite fermion using a canonical transformation, while the perturbative calculation gives the logarithmic divergent one at the Fermi surface. We emphasize that the definition of the effective mass depends on how strong the low-frequency Chern-Simons fluctuations are in the physical process. We work with the standard Halperin-Lee-Read model in the temporal gauge. The advantage of this gauge is that the finite effective mass may be calculated in the Hartree-Fock approximation. Furthermore, it is precisely equal to the result obtained by Shankar and Murthy, which fits very well the numerical calculation from the ground-state energy analysis as well as the semiclassical estimation. However, if we consider further the random-phase approximation, we find that the divergence of the effective mass at the Fermi surface emerges again whether we work with the temporal or Coulomb gauge. On the other hand, the behavior of the response functions in the small band mass limit is discussed systematically in the lowest-order calculation if there exists a self-interaction among the magnetoplasmons.

Nuclear matter with a Bose condensate of dibaryons in a relativistic Hartree approximation

The Green's functions are constructed and a one-loop calculation is given for the scalar and vector densities and for the equation of state of nuclear matter with a Bose condensate of dibaryons. This is the lowest approximation in the loop expansion of quantum hadrodynamics, sufficient to account for the presence of dibaryons not in the condensate in the heterophase nucleon-dibaryon matter. It leads to a finite effective nucleon mass and remains consistent with increasing the density.

Magnetic Field Induced Triplet Exciton Polariton in CuCl

AbstractThe integrated optical absorption I (B) due to the Z3‐triplet exciton in CuCl is measured at 4.2 K as a function of an external magnetic field B (O ≦ B ≦ 0.2 T) in Voigt configuration. I (B) shows the characteristic B‐dependence whose behaviour is ranging between quadratic and linear on B. It is found that this B‐dependence of I is caused by the correlation between the B‐dependent singlet‐triplet mixing and the sample‐dependent damping of the Z3‐triplet exciton. The effects of spatial dispersion due to the finite effective mass of the triplet exciton are taken into account assuming Pekar's ABC. The conceptional transition from a classical Lorentz model to the polariton model is demonstrated experimentally. An application to the characterization of the sample is also discussed.

Time-resolved reflection of light from InP crystals

The reflection dynamics of picosecond light pulses is investigated under the Brewster angle of incidence near the lowest exciton resonance in InP. The reflected pulses acquire a nonexponential tail which follows the resonant polarization decay in the reflecting surface layer. Both finite effective mass of excitons and dephasing must be included in the theoretical model to yield a numerical fit of the experimental data. The observed intensity dependence of the reflection dynamics can be understood by an enhanced dephasing process due to exciton-exciton collisions.

Photon propagation in nuclear matter and the correction to the theoretical nuclear charge density

The polarization function is derived for a virtual photon propagating in uncorrelated nuclear matter. The modification of the photon propagator and the induced charge distribution in the presence of the external field source are expressed in terms of a finite effective photon mass which is dependent on the Fermi momentum. The application is made to a finite nucleus by subtracting the local field due to the surrounding infinite matter distribution. It is found that the correction to the potential due to the scattering of exchanged virtual photons by the nucleus results in a short range repulsive force on the orbiting muon in muonic atoms. The estimated energy shifts are -560 eV for the $2{p}_{\frac{1}{2}}\ensuremath{\rightarrow}1{s}_{\frac{1}{2}}$ transition, and -291 eV for the $3{d}_{\frac{5}{2}}\ensuremath{\rightarrow}2{p}_{\frac{3}{2}}$ transition in $^{208}\mathrm{Pb}$; and -58 eV and -2.5 eV, respectively, in $^{90}\mathrm{Zr}$. The effects are negligible for higher orbits.NUCLEAR STRUCTURE $^{90}\mathrm{Zr}$, $^{208}\mathrm{Pb}$; calculated induced charge density in muonic atoms. Nucleon Compton scattering effect on photon propagator.

Electromechanical Behavior of the Bender-Activated Tonpilz

This paper relates to transducers using one or several bender elements that are clamped in a rigid structure of finite effective mass. Transducers of this type were first conceived by E. E. Turner of Raytheon (patent pending). The analysis of dynamic behavior of clamped-clamped benders is extended to the case where the damping structure partakes in the motion. The ratio of the effective mass of the clamps to the effective mass of the bender determines the location of the nodes, in close analogy to the case of the classical tonpilz. An equivalent circuit is found from which the Q and resonance frequency may be determined. From the stress distribution of the particular bending mode used optimal conditions may be specified for the attainment of a high electromechanical coupling coefficient.