Decrease Of Effective Mass Research Articles

Extended versus standard Holstein model: Results in two and three dimensions

We present numerically exact solutions to the problem of a single electron interacting through an extended interaction with optical phonons in two and three dimensions. Comparisons are made with results for the standard Holstein model and with perturbative approaches from both the weak coupling and strong coupling sides. We find, in agreement with earlier work, that the polaron effective mass increases (decreases) in the weak (strong) coupling regime, respectively. However, in two dimensions, the decrease in effective mass still results in too large an effective mass to be relevant in realistic models of normal metals. In three dimensions the decrease can be more relevant but exists only over a very limited range of coupling strengths.

Temperature Dependent Diquark and Baryon Masses

Temperature dependence of diquark mass has been investigated in the frame work of the quasi particle diquark model. The effective mass of the diquark has been suggested to have a temperature dependence which shows a power law behavior. The variation of the diquark mass with temperature has been studied. A decrease in effective mass at temperature T < Tc, where Tc is the critical temperature has been observed. Some features of the phase transition have been discussed. The phase transition is found to be of second order. Temperature variation of baryon masses has also been studied. The results are compared and discussed with available works.

Modeling the transport properties of epitaxially grown thermoelectric oxide thin films using spectroscopic ellipsometry

The influence of oxygen vacancies on the transport properties of epitaxial thermoelectric (Sr,La)TiO3 thin films is determined using electrical and spectroscopic ellipsometry (SE) measurements. Oxygen vacancy concentration was varied by ex-situ annealing in Ar and Ar/H2. All films exhibited degenerate semiconducting behavior, and electrical conductivity decreased (258–133 S cm−1) with increasing oxygen content. Similar decrease in the Seebeck coefficient is observed and attributed to a decrease in effective mass (7.8–3.2 me), as determined by SE. Excellent agreement between transport properties deduced from SE and direct electrical measurements suggests that SE is an effective tool for studying oxide thin film thermoelectrics.

Influence of the band structure of BiSb alloy on the magneto-Seebeck coefficient

The influence of the band structure of BiSb alloys on the magneto-Seebeck coefficient was estimated by the Boltzmann equation using an energy-dependent relaxation time approximation at 100 K. The magneto-Seebeck coefficient along the trigonal direction was calculated under a magnetic field along the bisectrix direction. The scattering factor was supposed as being an acoustic deformation potential scattering. In order to estimate the influence of band structure on the magneto-Seebeck coefficient, both two band and three band models were utilized. The two band model consists of an electron surface at the L-point and a hole surface at the T-point. The influence of the change in band overlap and the effective electron mass was investigated using the two band model. The three band model consists of an electron surface at the L-point and a hole surface at the T-point. The influence of change in the Sb concentration of BiSb alloy was also investigated using the three band model. As a result, the Seebeck coefficient was observed to increase as the band overlap decreased. The band overlap has little influence on the improvement of the magneto-Seebeck coefficient. The Seebeck coefficient decreases as the effective mass decreases; however, improvement in the magneto-Seebeck coefficient increases as the effective mass decreases. The magneto-Seebeck coefficient of Bi95Sb5 was effectively improved under a low magnetic field.

Polaron effects due to interface optical‐phonons in wurtzite GaN/AlN quantum wells

AbstractConsidering the effects of the built‐in electric field (BEF) induced by the spontaneous and piezoelectric polarizations of wurtzite GaN/AlN quantum wells (QW's), the polaron energy shift and the effective mass due to the electron interactions with the interface optical‐phonons are investigated theoretically by means of Lee‐Low‐Pines variational approach. We find that the BEF has a remarkable influence on the polaron effects especially for a QW with well width d &gt; 6 nm. The polaron energy shift increases slowly and its effective mass approaches to a constant if d is further increased. On the contrary, both the polaron energy shift and the effective mass decrease slowly with the increasing of d if the BEF is ignored. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Unification of the Band Anticrossing and Cluster-State Models of Dilute Nitride Semiconductor Alloys

We show that a quantitative description of the conduction band in Ga(In)NAs is obtained by combining the experimentally motivated band anticrossing model with detailed calculations of nitrogen cluster states. The unexpectedly large electron effective mass values observed in many GaNAs samples are due to hybridization between the conduction band edge E- and nitrogen cluster states close to the band edge. Similar effects explain the difficulty in observing the higher-lying E+ level at low N composition. We predict a decrease of effective mass with hydrostatic pressure in many GaNAs samples.

Spin splitting effect on the effective mass of GaAs/AlxGa1−xAs quantum wells

The enhancement of the electronic effective mass in GaAs/AlxGa1−xAs quantum wells is investigated within the framework of a 14-band k.p theory. In this theory, the bulk conduction band dispersion is expanded to fourth order in the wave vector, k, and the anisotropy and spin splitting effects are also included. The analytical expressions for the non-confinement and confinement-direction masses are reformulated to take into account the spin splitting effect. The dependence of the effective mass with the quantum well width is also discussed for different compositions of the barrier material (AlxGa1−xAs). It results that the spin splitting effect on the effective mass decrease with increasing widths of the GaAs/AlxGa1−xAs quantum wells. For values of the well width greater than 10 nm, a value close to the bulk GaAs is obtained. This behaviour is similar to that reported in the literature for GaAs/AlxGa1−xAs quantum wires.

Modification of valence-band symmetry and Auger threshold energy in biaxially compressed InAs1-xSbx.

Strained-layer superlattices (SLS's) with biaxially compressed ${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ were characterized using magnetophotoluminescence and compared with unstrained ${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ alloys. Holes in the SLS exhibited a decrease in effective mass, approaching that of the electrons. In the two-dimensional limit, a large increase in the Auger threshold energy accompanies this strain-induced change in SLS valence-band symmetry. Correspondingly, the activation energy for nonradiative recombination in the SLS's displayed a marked increase compared with that of the unstrained alloys. Strained-layer superlattices and alloy activation energies are in agreement with estimated Auger threshold energies.

Magnetophotoluminescence of biaxially compressed InAsSb quantum wells

Heterostructures with biaxially compressed InAsSb are being considered as active regions for midwave infrared diode lasers. Quantum wells of biaxially compressed InAs1−xSbx (x≊0.09) in unstrained InAs were characterized using magnetophotoluminescence. The quantum size shift of the photoluminescence for a series of quantum wells produced an estimate of the conduction band offset. In magnetic field studies, the holes in the strained quantum wells exhibited a decrease in effective mass, approaching that of the electrons. A type I band offset was observed for these InAsSb/InAs heterostructures. Throughout this study, magnetoexcitonic behavior is observed; our analysis indicates that the exciton binding energy increases with quantum confinement.

Using the quartz microbalance principle for sensing mass changes and damping properties

A brief summary of the most important development steps of a special crystal sensing technique is presented. Then, a lumped-element equivalent circuit model is introduced that describes the near-resonance electrical characteristics of a quartz-crystal microbalance (QCM) simultaneously loaded by a surface mass layer and contacting analyte. This model predicts that surface mass accumulation causes a shift of the resonance peak while increasing the density—viscosity product of the sensitive layer. These two effects cause both a translation and a decrease of the resonance peak. Experimental results are given which demonstrate interactions between sensitive materials (adsorbates) coated on the quartz crystal and various solvent vapours (analytes). Different polymers, modified ethanolamines and compounds with long alkyl-chains are used as coatings. Two of these coated sensors show an anomalous effect: the exposure to solvent vapours results in an increase of frequency because of the effective mass decrease. The apparent discrepancy implies that the physical properties of the film must change, requiring a more detailed characterization of the coated quartz.

Effective mass decrease due to electron-phonon interaction in heavy fermion systems

It is argued that in heavy fermion systems the electron-phonon interaction results in a decrease of the quasiparticle mass. This is in contrast to ordinary metals where this interaction is known to increase the effective mass. A simple expression is derived which relates the effective mass change to the electronic Gruneisen parameter, the Kondo temperature and the bulk modulus. For CeRu2Si2 the effective-mass decrease is estimated to be 25%.

Hole superconductivity and the high-Tc oxides.

We discuss predictions of the hole-pairing mechanism of superconductivity for various experimentally measurable properties of the high-${\mathit{T}}_{\mathit{c}}$ oxides. The model considered describes conduction by holes through the oxygen anion network in the high-${\mathit{T}}_{\mathit{c}}$ oxides, and pairing originates in a term in the Hamiltonian describing an enhanced hopping amplitude for a hole in the presence of other holes. The model includes on-site and nearest-neighbor Coulomb repulsions and different hopping amplitudes within and between planes. We use information from experiments to determine suitable ranges of parameters in the model and examine various properties of the superconducting state within this model using BCS theory. Among the most notable predictions of the model for the high-${\mathit{T}}_{\mathit{c}}$ oxides are: (1) the superconducting state is nearly isotropic despite the anisotropic band structure; (2) the pressure dependence of ${\mathit{T}}_{\mathit{c}}$ is very different for pressure applied perpendicular and parallel to the planes; and (3) the upper critical field and effective mass decrease rapidly and monotonically with hole doping, as a crossover occurs between strong- and weak-coupling regimes.

Vacuum polarization effects on meson propagators

The stability of uniform nuclear matter in the relativistic Hartree approximation to the Walecka model is investigated by calculating the meson propagators in the one-loop approximation. In this renormalizable model, Dirac nucleons interact with neutral scalar and vector mesons. At the one-loop level, nuclear matter is found to be unstable to short-wavelength modes because of vacuum polarization effects. This occurs at momenta greater than twice the free-nucleon mass. The onset of instability is pushed to larger momenta with increasing baryon density as the nucleon effective mass decreases. The implications of short-range correlations and vertex corrections are discussed.