Light Effective Mass Research Articles

Theory of static and dynamic antiferromagnetic vortices in LSCO superconductors

A key prediction of the SO(5) theory is the antiferromagnetic vortex state. Recent neutron scattering experiment on LSCO superconductors revealed enhanced antiferromagnetic order in the vortex state. Here we review theoretical progress since the original proposal and present a theory of static and dynamic antiferromanetic vortices in LSCO superconductors. It is shown that the antiferromagnetic region induced by the vortices can be greater than the coherence length, due to the light effective mass of the dynamic antiferromagnetic fluctuations at optimal doping, and close proximity to the antiferromagentic state in the underdoped regime. Systematic experiments are proposed to unambiguously determine that the field induced magnetic scattering originates from the vortices and not from the bulk.

Erratum: “Neutrinoless double beta decay” at a neutrino factory [Phys. Rev. D63, 054031 (2001)

We examine the prospects of detecting an analogous process of neutrinoless double beta decay at a neutrino factory from a high energy muon storage ring. Limits from LEP experiments, neutrinoless double beta decay as well as from global fits have to be incorporated and severely restrict the results. We investigate what limits on light and heavy effective Majorana neutrino masses can be obtained and compare them with existing ones. Discussed are also contributions from right-handed neutrinos and purely right-handed interactions. Other ``new physics'' contributions to the same final state might produce large event numbers.

“Neutrinoless double beta decay” at a neutrino factory

We examine the prospects of detecting an analogous process of neutrinoless double beta decay at a neutrino factory from a high energy muon storage ring. Limits from LEP experiments, neutrinoless double beta decay as well as from global fits have to be incorporated and severely restrict the results. We investigate what limits on light and heavy effective Majorana neutrino masses can be obtained and compare them with existing ones. Discussed are also contributions from right-handed neutrinos and purely right-handed interactions. Other ``new physics'' contributions to the same final state might produce large event numbers.

Large mixing angle MSW and atmospheric neutrinos from single right-handed neutrino dominance and U(1) family symmetry

Single right-handed neutrino dominance (SRHND) in the 23 sector of the light effective neutrino mass matrix has been proposed as a natural explanation for the concurrent large 23 mixing angle and large 23 mass hierarchy. In this paper we show how large 12 mixing angles, suitable for the large mixing angle (LMA) MSW solution to the solar neutrino problem, may arise from SRHND. In order to understand the conditions for SRHND and LMA MSW we first consider the case of one and two right-handed neutrinos, and obtain simple analytic conditions which are then extended to the case of three right-handed neutrinos. We then introduce a single U(1) family symmetry and show how these analytic conditions may be translated into U(1) charge assignments and perform a systematic search for the simplest examples.

In-plane propagation of excitonic cavity polaritons

By exciting resonantly a microcavity with a pulsed laser beam, we have observed the in-plane propagation of excitonic cavity polaritons over distances much greater than the wavelength. The initial in-plane wave vector is conserved during the propagation, showing that it is a good quantum number for cavity polaritons. The polariton group velocities were measured as a function of both the in-plane wave vector and detuning. Propagation is faster for negative detunings where the polaritons have a light effective mass. The observations are in good agreement with a simple two-oscillator model.

Propagation of Excitonic Polaritons in a Microcavity

By exciting resonantly a microcavity with a pulsed laser beam, we have observed the in-plane propagation of excitonic cavity polaritons on distances much greater than the wavelength. The initial in-plane wave vector is conserved during several tens of picoseconds showing it is a good quantum number for cavity polaritons. The polariton group velocities were measured both as a function of the in-plane wave vector and detuning. Propagation is larger at negative detunings where polaritons have a light effective mass. The observations are in good agreement with a simple two-oscillator model.

Resonant tunneling between transverseXstates in GaAs/AlAs double-barrier structures under elevated hydrostatic pressure

We present a model to describe the mechanisms involved in tunneling between the quasiconfined $X$ subbands of the AlAs layers in GaAs/AlAs double-barrier structures at pressures up to the type-II transition. The model involves self-consistent Schr\"odinger-Poisson calculation of the potential profiles within the device for a given relative alignment between the two $X$-like quantum wells, and thus allows prediction of the bias positions at which certain resonant tunneling processes will occur. By systematic variation of the parameters involved, in particular the charge distribution between the two AlAs layers, these predictions have been fitted to the measured vertical transport characteristics of a series of samples of different AlAs layer width. In the cases of those samples with $X$ ground states that are transverse in nature, very good agreement has been obtained. The level of insight afforded by the model opens up alternative methods for the determination of important band-structure parameters, such as the light effective mass of the $X$ minima in AlAs, and the ${\ensuremath{\Gamma}}_{\mathrm{GaAs}}{\ensuremath{-}X}_{\mathrm{AlAs}}$ conduction-band offset. It also proves to be an extremely sensitive probe of the degree of symmetry between the AlAs layer widths of near-symmetric devices, and we are thus able to measure an asymmetry of around one monolayer in each of our devices. This explains entirely the asymmetry exhibited by the low bias ${X}_{t}(1)\ensuremath{\rightarrow}{X}_{t}(1)$ resonances of some nominally symmetric double-barrier structures, which has been a source of some debate in recent years.

Dissipative Zener Tunneling in Silicon Semiconductors

In the present work studying the dissipative Zener tunneling, we assume the existence of an electric field (acting for finite time intervals) perpendicular to the surface of a parabolic quantum well, and the potential energy of a valence band electron in the well to be that of the usual harmonic oscillator coupled to a phonon bath (ignoring the Brownian motion). As it has been proved for the case of silicon (Si) semiconductor the current density increases with an increase of the electric field strength, and the amount of the increase depends on the effective valence band electron mass as well as on the dissipation parameter γ. Actually, heavy effective valence band electron mass gives lower current density in comparison with light effective valence band electron mass. Furthermore, an increase of the dissipation parameter γ increases the localization of the electron and the current density.

Cyclotron resonance in ultra-low-hole-density narrow p-type GaAs/(Al,Ga)As quantum wells

The cyclotron-resonance behavior of a series of p-type, low ${\mathrm{p}}_{\mathrm{s}}$ , asymmetrically doped (311)A quantum wells has been measured over two orders of magnitude in energy. Landau levels for the structures studied have been calculated for comparison with experiment. The calculations use a rotated Luttinger Hamiltonian and invoke the axial approximation. In the quantum limit and at certain far-infrared energies we observe two resonances widely separated in field. We find at the lowest fields that the samples exhibit a light effective mass. For the narrowest quantum well the cyclotron resonance (CR) broadens and shifts to a slightly lighter mass into the quantum limit, above about 2 T. The CR energy is modeled by inter-Landau-level transitions from the highest level, and a discontinuous evolution to a higher effective mass, observed as the second resonance above 8 T, is explained by crossing of the highest two Landau levels, as modeled in the calculation. The discontinuous evolution to a higher mass is found to shift to lower fields as the well width increases, as suggested from the modeling. For the wider quantum wells the influence of additional anticrossings at intermediate fields, due to the proximity of the second subband, is observed. CR energy for these samples is adequately modeled at energies above and below the region of subband anticrossing. The limitations of the modeling procedure have been discussed and a number of features of the experimental data are explained qualitatively in terms of the expected modifications to the hole Landau-level structure when full Landau-level mixing is incorporated into the modeling. The influence of hole-hole interactions and factors influencing the CR scattering time are also discussed briefly.

Complex Airy analysis of photoreflectance spectra for III-V semiconductors.

We present a detailed analysis of photoreflectance (PR) spectra of semiconductors using complex Airy functions and their derivatives. We demonstrate that photoreflectance spectra can be treated in terms of a single complex Airy function with an energy-dependent broadening parameter. We show analytically and numerically that this complex Airy PR treatment is functionally equivalent within field conditions appropriate for PR to the model of R. N. Bhattacharya, H. Shen, P. Parayanthal, F. H. Pollak, T. Coutts, and A. Aharoni [Phys. Rev. B 37, 4044 (1988)], where the effects of gradient electric field and non-flat-band modulation are treated explicitly. The equivalence occurs because the field gradient and non-flat-band modulation effects are included in our model in the energy dependence of the phenomenological broadening parameter ${\mathrm{\ensuremath{\Gamma}}}^{\mathrm{*}}$=(${\mathrm{\ensuremath{\Gamma}}}_{0}$/\ensuremath{\Elzxh}\ensuremath{\theta})exp[\ensuremath{\delta}(\ensuremath{\Elzxh}\ensuremath{\omega}-${\mathit{E}}_{\mathit{g}}$)], where \ensuremath{\Elzxh}\ensuremath{\omega} is the photon energy, ${\mathit{E}}_{\mathit{g}}$ is the band-gap energy, ${\mathrm{\ensuremath{\Gamma}}}_{0}$ is the nominal broadening at the band-gap energy, and \ensuremath{\delta} is a parameter directly proportional to the electric-field gradient and modulation between two finite fields. The major utility of our model is that a single effective layer can be treated instead of a more computationally intensive laminar model.We apply our complex Airy model to bulk semiconductors such as GaAs, InP, and ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As. In the photoreflectance spectra of these semiconductors, our model considers three distinct but convolved features at ${\mathit{E}}_{0}$ which represent the light- and heavy-hole valence bands and an exciton-impurity feature below ${\mathit{E}}_{0}$. At ${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$ our model considers two features which are related to the spin-orbit-split valence band and a second state just below this critical point. For GaAs, we determined from our PR modeling that the band-gap energy for these films was 1.422\ifmmode\pm\else\textpm\fi{}0.003 eV, which agreed, within experimental error, with the band-gap energy measured by room-temperature photoluminescence. A feature was found below the ${\mathit{E}}_{0}$ gap in the GaAs samples with energies ranging from 3 to 4 meV below the band gap, which is similar to excitonic binding energies in this material system.A below-critical-point feature was evident in one GaAs sample at 11 meV below the ${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$ transition. Also, in GaAs we determined the ratio of light and heavy interband effective masses, ${\mathrm{\ensuremath{\mu}}}_{\mathrm{LH}}$/${\mathrm{\ensuremath{\mu}}}_{\mathrm{HH}}$, to be 0.6865, which is in good agreement with values determined in previous studies. For bulk InP the band-gap energy was determined to be 1.340\ifmmode\pm\else\textpm\fi{}0.003 eV, which agreed, within experimental error, with the band-gap energy determined from PL. Below-critical-point features were also found for InP with energies of 4.0 and 3.5 meV below the ${\mathit{E}}_{0}$ and ${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$ transitions, respectively. For both GaAs and InP, the surface electric fields determined from the ${\mathit{E}}_{0}$ and ${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$ critical points were in agreement within experimental error. For ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As on InP films near lattice-matched conditions, the complex Airy line shape was applied to both intermediate electric field (Franz-Keldysh oscillations) and low-field-like PR spectra illustrating the utility of the complex Airy functional analysis in fitting both simple and complicated PR responses. We found that the band-gap energy from the PR spectral fits to those determined from photoluminescence measurements agreed within experimental error for these ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As films.

Boron delta doping in Si and SiGe and its application toward field-effect transistor devices

The mobility behavior of boron delta (δ)-doped Si, Si1−xGex(0≤x≤1) is investigated, which includes the first mobility measurements reported for B δ-doped Si1−xGex and Ge. The expected mobility enhancement from δ doping is not realized in Si:B due to the heavy effective mass of holes. However, some enhancement may be possible at lower doping levels for narrower wells. Indications of mobility enhancement have been realized for δ-doped Ge:B because of the relatively light effective mass. We also present the implementation of boron δ-doped layers in the fabrication of (i) the first SiGe:B δ-FET and (ii) the first coupled δ-layer Si:B δ-FET.

Dependence of valence-subband structures on the substrate orientation in ZnSxSe1−x/ZnyMg1−ySzSe1−z quantum wells

We present a theoretical investigation of the growth orientation dependence of valence-subband structures in ZnSxSe1−x/ZnyMg1−ySzSe1−z quantum wells grown in the [001], [115], [113], [112], and [111] directions. The results indicate that the in-plane effective mass of the heavy-hole subband in the [111]-oriented structure is substantially smaller than that in the [001] quantum wells. For applications to quantum-well lasers, the lighter effective mass will lead to a smaller threshold current density, and therefore a better laser performance. Our investigations should provide useful guidelines for the design of II-VI quantum-well blue lasers.

Simple evaluation of linewidth-enhancement factor in quantum well laser with strain

A simple formula for linewidth-enhancement factor α in quantum well laser with strain was derived and compared with Westbrook and Adams theory [IEE Proc. 135. Pt.J, 223 (1988)]. Strain is incorporated through light and heavy hole effective masses. It has been found that tensile strain results in better improvement of ‖αmat‖ than compressive strain.

Electron wave interference effects in CMT and quantum-size devices

The authors report on electron interference effects in n-Hg0.8Cd0.2Te (CMT). This narrow-gap semiconductor has a very light effective electron mass and a comparatively long electron wave coherence length. Due to these properties a non-classical conductivity was observed in bulk samples at elevated temperatures up to 35 K. Dimensional crossovers were discovered in samples as large as 1*2.5* 35 mu m3: with decreasing temperature size-quantization from three- via two- to one-dimensional behaviour takes place. The evidence of pronounced electron wave interference effects favours CMT as a candidate for novel quantum size devices, as cutoff wavelength transistors or quantum-valve infrared detectors.

High field cyclotron resonance and the electron effective masses in AlAs

We have measured far-infrared cyclotron resonance in n-type AlAs in pulsed high magnetic fields up to 150T. At a wavelength of 119μm, a prominent resonance peak was observed at 42T corresponding to the heavy effective mass of m ∗ tm ∗ l =0.47±0.01m 0 on the ellipsoidal equinergy surface at the X-point. Also observed a shoulder-like structure centered at 22T. Assigning it to the light effective mass m ∗ t , the effective masses were determined as m ∗ t=0.25±0.01m 0 and m ∗ l=0.88±0.06m 0 .

Electronic structure of ultrathin (GaAs)n(AlAs)n [001] superlattices and the Ga0.5Al0.5As alloy

Using self-consistent electronic structure calculations we contrast the energy levels of the ultrathin (GaAs)n(AlAs)n [001] superlattices (n=1,2) with those of the disordered Ga0.5Al0.5As alloy and a long period (n→∞) superlattice. Conventional Kronig–Penney and effective mass models suggest that, because of the relatively light electron effective masses and small barrier heights, only delocalized superlattice conduction states would exist in the n=1 limit. We find a number of such conventional ‘‘averaging states’’ (delocalized on both sublattices). In addition, we also find states localized on a single sublattice. For small n’s, the latter are divided into two classes: (i) ‘‘repelling states’’ (distinct alloy states which fold in the superlattice into states of identical symmetry, which, in turn, repel each other and tend to localize), and (ii) ‘‘segregating states’’ (a pair of localized states Ψα and Ψβ, where symmetry compels Ψα to have a vanishing angular momentum component l on a subset α of unit cell atoms, whereas the complementary state Ψβ is localized on the other atoms β. These states are split by the potential difference Vβl −Vαl). We analyze new luminescence, reflectance, and Raman data in light of our theoretical model. Studies of the II-VI superlattices (CdTe)1(HgTe)1 shows similar behavior.

Band Structures and Optical Properties of Semiconducting Layer Compounds GaS and GaSe

The band structures of GaS and GaSe near the Fermi levels are derived in a semiempirical way. In view of the anisotropic structures, the π band structures are mainly studied. The valence band consists of a π band with the heavy effective masses. The conduction band at the center of the Brillouin zone has the light effective masses for the both directions parallel and perpendicular to the layers, while its minimum at the zone edge has a two-dimensional character. The characteristics of the optical properties of these compounds are the existence of the sharp peaks associated with the nearly two-dimensional pair bands at the saddle points and further the limited spatial extension of an exciton along the c -axis. Most of the structures in reflectivities in GaS and GaSe and the binding energy of an exciton in GaSe are explained fairly well.