Bare Electron Mass Research Articles

Properties of the superconducting state in molecular metallic hydrogen under pressure at 347 GPa

The thermodynamic properties of the superconducting state induced in metallic molecular hydrogen under the influence of pressure 347 GPa were determined. In particular, it has been shown that the critical temperature ( T C ) changes in the range from 120 to 90 K for μ ⁎ ∈ 〈 0.08 , 0.15 〉 , where μ ⁎ is the value of the Coulomb pseudopotential. Next, the energy gap near the temperature of zero Kelvin ( 2 Δ ( 0 ) ) was calculated. It has been stated, that the dimensionless ratio 2 Δ ( 0 ) / k B T C slightly decreases with the increase of μ ⁎ from 3.98 to 3.84. In the last step, the ratio of effective electron mass ( m ⁎ e ) to the bare electron mass ( m e )) was determined. It has been proved that m ⁎ e / m e takes its highest value equal to 1.96 for T= T C .

Tuning the Dimensionality of the Heavy Fermion Compound CeIn 3

Condensed-matter systems that are both low-dimensional and strongly interacting often exhibit unusual electronic properties. Strongly correlated electrons with greatly enhanced effective mass are present in heavy fermion compounds, whose electronic structure is essentially three-dimensional. We realized experimentally a two-dimensional heavy fermion system, adjusting the dimensionality in a controllable fashion. Artificial superlattices of the antiferromagnetic heavy fermion compound CeIn3 and the conventional metal LaIn3 were grown epitaxially. By reducing the thickness of the CeIn3 layers, the magnetic order was suppressed and the effective electron mass was further enhanced. Heavy fermions confined to two dimensions display striking deviations from the standard Fermi liquid low-temperature electronic properties, and these are associated with the dimensional tuning of quantum criticality.

Light-mass Bragg cavity polaritons in planar quantum dot lattices

The exciton-polariton modes of a quantum dot lattice embedded in a planar optical cavity are theoretically investigated. Umklapp terms, in which an exciton interacts with many cavity modes differing by reciprocal lattice vectors, appear in the Hamiltonian due to the periodicity of the dot lattice. We focus on Bragg polariton modes obtained by tuning the exciton and the cavity modes into resonance at high symmetry points of the Brillouin Zone. Depending on the microcavity design these polaritons modes at finite in-plane momentum can be guided and can have long lifetimes. Moreover, their effective mass can be extremely small, of the order of $10^{-8} m_0$ ($m_0$ is the bare electron mass), and they constitute the lightest exciton-like quasi-particles in solids.

Theory of Spin Hall Effect: Extension of the Drude Model

An extension of the Drude model is proposed that accounts for the spin and spin-orbit interaction of charge carriers. Spin currents appear due to the combined action of the external electric field, crystal field, and scattering of charge carriers. The expression for the spin Hall conductivity is derived for metals and semiconductors that is independent of the scattering mechanism. In cubic metals, the spin Hall conductivity sigma s and charge conductivity sigma c are related through sigma s=[2pi variant /(3mc2)]sigma2c with m being the bare electron mass. The theoretically computed value is in agreement with experiment.

Free-electron-like dispersion in an organic monolayer film on a metal substrate

Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications. The electronic properties of these materials and their interfaces are therefore worth investigating intensively, particularly the degree of electron delocalization that can be achieved. If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion. But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available, and the mechanism(s) of electron delocalization in molecular materials are also not well understood. Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47m(e), where m(e) is the bare electron mass. This dispersion is far stronger than expected for the organic film alone, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer.

Composite bosons in bilayerν=1system: An application of the Murthy-Shankar formalism

We calculate the dispersion of the out-of-phase mode characteristic for the bilayer nu = 1 quantum Hall system applying the version of Chern-Simons theory of Murthy and Shankar that cures the unwanted bare electron mass dependence in the low-energy description of quantum Hall systems. The obtained value for the mode when d, distance between the layers, is zero is in a good agreement with the existing pseudospin picture of the system. For d nonzero but small we find that the mode is linearly dispersing and its velocity to a good approximation depends linearly on d. This is in agreement with the Hartree-Fock calculations of the pseudospin picture that predicts a linear dependance on d, and contrary to the naive Hartree predictions with dependence on the square-root of d. We set up a formalism that enables one to consider fluctuations around the found stationary point values. In addition we address the case of imbalanced layers in the Murthy-Shankar formalism.

Inertial mass of the Abrikosov vortex.

We show that a large contribution to the inertial mass of the Abrikosov vortex comes from transversal displacements of the crystal lattice. The corresponding part of the mass per unit length of the vortex line is M(l)=(m(2)(e)c(2)/64 pi alpha(2)mu lambda(4)(L))ln((lambda(L)/xi), where m(e) is the bare electron mass, c is the speed of light, alpha=e(2)/Planck's over 2 pi c approximately 1/137 is the fine structure constant, mu is the shear modulus of the solid, lambda(L) is the London penetration length, and xi is the coherence length. In conventional superconductors, this mass can be comparable to or even greater than the vortex core mass computed by Suhl [Phys. Rev. Lett. 14, 226 (1965)]].

Electron Effective Mass and Resonant Polaron Effect in CdTe/CdMgTe Quantum Wells

Cyclotron resonance in CdTe/CdMgTe quantum wells (QWs) was studied. Due to the polaron effect the zero-field effective mass is strongly influenced by the QW width. The experimental data have been described theoretically by taking into account electron–phonon coupling and the non-parabolicity of the conduction band. The subband structure was calculated self-consistently. The best fit was obtained for an electron–phonon coupling constant α = 0.3 and bare electron mass of mb = 0.092m0.

Resistance Spikes and Domain Wall Loops in Ising Quantum Hall Ferromagnets

We explain the recent observation of resistance spikes and hysteretic transport properties in Ising quantum Hall ferromagnets in terms of the unique physics of their domain walls. Self-consistent RPA/Hartree-Fock theory is applied to microscopically determine properties of the ground state and domain wall excitations. In these systems domain wall loops support one-dimensional electron systems with an effective mass comparable to the bare electron mass and may carry charge. Our theory is able to account quantitatively for the experimental Ising critical temperature and to explain qualitative characteristics of the resistive hysteresis loops.

Strong coupling between local moments and superconducting 'heavy' electrons in UPd2Al3.

The electronic structure of heavy-fermion compounds arises from the interaction of nearly localized 4f- or 5f-shell electrons (with atomic magnetic moments) with the free-electron-like itinerant conduction-band electrons. In actinide or rare-earth heavy-fermion materials, this interaction yields itinerant electrons having an effective mass about 100 times (or more) the bare electron mass. Moreover, the itinerant electrons in UPd2Al3 are found to be superconducting well below the magnetic ordering temperature of this compound, whereas magnetism generally suppresses superconductivity in conventional metals. Here we report the detection of a dispersive excitation of the ordered f-electron moments, which shows a strong interaction with the heavy superconducting electrons. This 'magnetic exciton' is a localized excitation which moves through the lattice as a result of exchange forces between the magnetic moments. By combining this observation with previous tunnelling measurements on this material, we argue that these magnetic excitons may produce effective interactions between the itinerant electrons, and so be responsible for superconductivity in a manner analogous to the role played by phonons in conventional superconductors.

Normal state, superconductivity and quasiparticle Fermi surface of the strongly correlated oxide Sr 2RuO 4

The oxide superconductor Sr 2RuO 4 is emerging as an archetypal strongly correlated electron system. We review the experimental situation, focusing on Fermi surface specific probes. Quantum oscillation measurements have revealed a Fermi surface consisting of three slightly warped cylinders populated by quasiparticles with masses ranging from 3.4 to 14.6 times the bare electron mass. There is strong evidence that Sr 2RuO 4 is a spin-triplet superconductor, and the simplicity of the Fermi surface combined with detailed knowlege of the quasiparticle properties from quantum oscillation and orbital specific NMR measurements means that Sr 2RuO 4 may be a key system for understanding unconventional superconductivity. We compare Sr 2RuO 4 with three-dimensional nearly ferromagnetic 3d-metals, and suggest that quasi-two-dimensional spin-fluctuations may be responsible for the fact that Sr 2RuO 4 is the first nearly ferromagnetic metal to show superconductivity.

Unified treatment of some Casimir energies and Lamb shifts: A dielectric between two ideal conductors

Lamb shifts and Casimir energies, often thought of as long-range and short-range effects, respectively, and studied separately, are each a manifestation of quantum electrodynamics and can be studied together. We do so for a dielectric medium between two parallel ideal conducting plates. We extract the usual Casimir energy and a bulk Lamb shift by studying the quantum fluctuations of the radiation field in the dielectric. We derive finite expressions for the Casimir energy and the bulk Lamb shift valid for any permittivity satisfying the Kramers-Kr\"onig relation; some of the Casimir shifts obtained are simpler in form than any in the literature. We separate the divergent and finite contributions to the bulk Lamb shift. For the dilute nonrelativistic gas we show that the divergent contribution to the bulk Lamb shift defines the bare electron mass in terms of the physically observed free-electron mass. Although we lack a physical interpretation for the necessary subtraction in the case of an arbitrary dielectric, it is natural to interpret the finite part of the bulk energy as a ``Lamb shift'' in this case too. We show that the derived finite bulk Lamb shift and Casimir energies in the limit of a dilute homogeneous gas are consistent with earlier results for a single atom between two ideal conductors, and for an atom near one of the walls. As an application, the radiative contribution to the interaction energy of a single electron with uniform probability density between two ideal walls is obtained.

INDUCED MASSES AND SPIN SELF-INTERACTION FROM FIVE-DIMENSIONAL EINSTEIN–CARTAN–DIRAC THEORY

The Einstein–Cartan theory with a dilaton field coupling arising from five-dimensional principal fiber bundle structure of the metric is investigated. A Dirac type spinor field is coupled to the metric and to translational gauge fields. After the dimensional reduction, the theory introduces a gravitational spin–spin contact interaction coming from the translational degrees of freedom, which is repulsive between Dirac particles whose spins are aligned and attractive when spins are opposed. A classically stable ground state for the dilaton field exists, which is infinitely degenerate with respect to a rescaling of the metric. The induced mass term in the Dirac equation can be interpreted, e.g. as the bare electron mass, as long as we regard this theory as an effective "medium" energy model coming from finite string field theories.

Composite-fermion Hall conductance at ν=

We show that in the limit of vanishing bare electron mass, and in the presence of particle-hole symmetric disorder (which can be of vanishing strength), the composite-fermion Hall conductance is constrained to be -${\mathrm{e}}^{2}$/h. We discuss the implications of this result for the existence and nature of a composite Fermi liquid in the lowest Landau level.

EFFECTIVE DIPOLE-RADIATION-FIELD THEORY II: ALL ORDERS BEYOND STANDARD COUPLING

The novel treatment of the interaction between a charged particle and the electromagnetic field, as presented in paper I [H. Dekker, Int. J. Mod. Phys.B8, 1–19 (1994)], is generalized to all orders beyond the standard dipole model. The resulting nonlinear problem is then again statistically linearized and the ensuing dynamics is solved exactly for a harmonically bound nonrelativistic electron. The earlier noted ultraviolet divergence in the system’s quantum mechanics is found to be absent, unless the bare electron mass were exactly zero. Inter alia it is also found that the electron’s generic quantum distribution is Gaussian.

Effects of the finite band width in a two-dimensional electron gas: The renormalization constant and the effective mass.

We have studied effects of a finite band width on the renormalization constant ${\mathit{Z}}_{\mathit{F}}$ and the effective mass ${\mathit{m}}^{\mathrm{*}}$ in a two-dimensional interacting electron gas. We first determine the polarization function (Lindhard function), taking into account a finite band width. Using the modified polarization function, we have evaluated ${\mathit{Z}}_{\mathit{F}}$ and ${\mathit{m}}^{\mathrm{*}}$ with respect to the fraction of the occupied band in the GW random phase approximation. The effect of the finite band width is found to be larger than or comparable to that of the local field correction. In particular, the finite band effects are much pronounced in the low-density regime. Further, it is found that, in an almost filled band system, ${\mathit{m}}^{\mathrm{*}}$ can be smaller than the bare electron mass due to the pure many-body interaction effects. It is also shown that results of ${\mathit{m}}^{\mathrm{*}}$ considering the finite band width describe well the experimental results in the high-density regime (${\mathit{r}}_{\mathit{s}}$=0.5--2.0). \textcopyright{} 1996 The American Physical Society.

Transient-estimate Monte Carlo in the two-dimensional electron gas.

Energies of the ground state and low-lying excited states of the two-dimensional electron gas have been calculated by a transient-estimate Monte Carlo method. This is an exact fermion quantum Monte Carlo method that systematically improves upon the results of a variational energy without imposing nodal constraints. We focus upon the density r s51, where our previous variational Monte Carlo calculation found qualitative differences in the effective mass from other theoretical approaches. Starting from a wave function with backflow and two-body correlations, the best trial function in our previous variational study, we find a ground-state energy only very slightly lower than the previously reported backflow fixed-node energy, reinforcing the conclusion that backflow wave functions are quite accurate. The effective mass derived from excitation energies does not differ significantly from the variational Monte Carlo results, giving a value of m*/m50.9360.01, so we conclude that the effective mass is indeed less than bare electron mass for a range of densities around r s51.

Dynamics of an Acoustic Polaron in One-Dimensional Electron-Lattice System

The dynamical behavior of an acoustic polaron in typical non-degenerate conjugated polymer, polydiacetylene, is numerically studied by using Su-Schrieffer-Heeger's model for the one dimensional electron-lattice system. It is confirmed that the velocity of a polaron accelerated by a constant electric field shows a saturation to a velocity close to the sound velocity of the system, and that the width of a moving polaron decreases as a monotonic function of the velocity tending to zero at the saturation velocity. The effective mass of a polaron is estimated to be about one hundred times as heavy as the bare electron mass. Furthermore the linear mode analysis in the presence of a polaron is carried out, leading to the conclusion that there is only one localized mode, i.e. the translational mode. This is confirmed also from the phase shift of extended modes. There is no localized mode corresponding to the amplitude mode in the case of the soliton in polyacetylene. Nevertheless the width of a moving polaron shows small oscillations in time. This is found to be related to the lowest odd symmetry extended mode and to be due to the finite size effect.

Magnetic oscillations in the heavy-fermion superconductor UBe13.

We present measurements of the magnetic torque acting upon a single crystal of ${\mathrm{UBe}}_{13}$, measurements made with a capacitive magnetometer. We observe pronounced magnetic oscillations in the torque resulting from the de Haas--van Alphen effect. A conventional analysis reveals that these oscillations result from ``light fermions'' with an effective mass ${\mathit{m}}^{\mathrm{*}}$=0.017${\mathit{m}}_{\mathit{e}}$ (${\mathit{m}}_{\mathit{e}}$ is the bare electron mass). The Fermi surface associated with these light quasiparticles is anisotropic. We present arguments that these oscillations represent an intrinsic phenomenon and compare our results with those of other groups.

Observation of heavy fermion quasiparticles in UPt 3 above the metamagnetic transition

Quantum oscillations of the transverse magnetoresistance have been observed in the heavy fermion metal UPt 3 in fields up to 24.5 T applied in the basal plane, providing the first observation of the quasiparticle Fermi surface above the metamagnetic transition. The observed quasiparticle masses remain strongly renormalised in the high field state, ranging from 30 to approximately 100 times the bare electron mass.