Showing papers on "Effective mass (solid-state physics) published in 2011"
TL;DR: In this paper, an elastic metamaterial which exhibits simultaneously negative effective mass density and bulk modulus is presented with a single unit structure made of solid materials, which is achieved through a chiral microstructure that is capable of producing simultaneous translational and rotational resonances.
Abstract: In this letter, an elasticmetamaterial which exhibits simultaneously negative effective mass density and bulk modulus is presented with a single unit structure made of solid materials. The double-negative properties are achieved through a chiralmicrostructure that is capable of producing simultaneous translational and rotational resonances. The negative effective mass density and effective bulk modulus are numerically determined and confirmed by the analysis of wave propagation. The left-handed wave propagation property of this metamaterial is demonstrated by the negative refraction of acoustic waves.
366 citations
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TL;DR: In this article, the authors demonstrate the beneficial effect of light effective mass leading to high power factor in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass.
Abstract: High Seebeck coefficient by creating large density of state (DOS) around the Fermi level through either electronic structure modification or manipulating nanostructures, is commonly considered as a route to advanced thermoelectrics. However, large density of state due to flat bands leads to large effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of high effective mass is a lower thermoelectric figure of merit when the carriers are predominantly scattered by acoustic phonons according to the deformation potential theory of Bardeen-Shockley. We demonstrate the beneficial effect of light effective mass leading to high power factor in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction.
357 citations
TL;DR: A single-crystal sample is prepared with a large shielding fraction and the specific-heat anomaly associated with the superconductivity is measured, which suggests a fully gapped, strong-coupling superconducting state but the BCS theory is not in full agreement with the data, which hints at a possible unconventional pairing.
Abstract: The superconductivity recently found in the doped topological insulator Cu(x)Bi₂Se₃ offers a great opportunity to search for a topological superconductor. We have successfully prepared a single-crystal sample with a large shielding fraction and measured the specific-heat anomaly associated with the superconductivity. The temperature dependence of the specific heat suggests a fully gapped, strong-coupling superconducting state, but the BCS theory is not in full agreement with the data, which hints at a possible unconventional pairing in Cu(x)Bi₂Se₃. Also, the evaluated effective mass of 2.6m(e) (m(e) is the free electron mass) points to a large mass enhancement in this material.
279 citations
TL;DR: In this paper, the band gap and electron effective mass (EEM) of C/BN HBLs can be modulated effectively by tuning the interlayer spacing and stacking arrangement.
Abstract: Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of graphene/boron nitride heterobilayers (C/BN HBLs) can be modulated effectively by tuning the interlayer spacing and stacking arrangement. The HBLs have smaller EEM than that of graphene bilayers (GBLs), and thus higher carrier mobility. For specific stacking patterns, the nearly linear band dispersion relation of graphene monolayer can be preserved in the HBLs accompanied by a small band-gap opening. The tunable band gap and high carrier mobility of these C/BN HBLs are promising for building high-performance nanodevices.
220 citations
TL;DR: In this paper, the authors calculated the localization lengths of the electrons and holes in InGaN/GaN quantum wells using numerical solutions of the effective mass Schrodinger equation.
Abstract: Localization lengths of the electrons and holes in InGaN/GaN quantum wells have been calculated using numerical solutions of the effective mass Schrodinger equation. We have treated the distribution of indium atoms as random and found that the resultant fluctuations in alloy concentration can localize the carriers. By using a locally varying indium concentration function we have calculated the contribution to the potential energy of the carriers from band gap fluctuations, the deformation potential, and the spontaneous and piezoelectric fields. We have considered the effect of well width fluctuations and found that these contribute to electron localization, but not to hole localization. We also simulate low temperature photoluminescence spectra and find good agreement with experiment.
171 citations
TL;DR: In this article, the authors present a comprehensive study of the properties of the deeply bound excitons with particular focus on the ${Y}_{0}$ transition at 3.33 and 3.35 eV.
Abstract: ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons ($Y$ lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the ${Y}_{0}$ transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers ($I$ lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all $Y$ lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II--VI and III--V semiconductors.
166 citations
TL;DR: In this paper, the temperature dependence of the bandgap of perovskite semiconductor compound CsSnI3 was determined by measuring excitonic emission at low photoexcitation in a temperature range from 9 to 300 K.
Abstract: The temperature dependence of the bandgap of perovskite semiconductor compound CsSnI3 is determined by measuring excitonic emission at low photoexcitation in a temperature range from 9 to 300 K. The bandgap increases linearly as the lattice temperature increases with a linear coefficient of 0.35 meV K−1. This behavior is distinctly different than that in most of tetrahedral semiconductors. First-principles simulation is employed to predict the bandgap change with the rigid change of lattice parameters under a quasi-harmonic approximation. It is justified that the thermal contribution dominates to the bandgap variation with temperature, while the direct contribution of electron-phonon interaction is conjectured to be negligible likely due to the unusual large electron effective mass for this material.
165 citations
TL;DR: In this paper, a new high-precision density functional DD-MEδ is presented which includes four mesons, σ, ω, δ, and ρ, with density-dependent meson-nucleon couplings.
Abstract: Although ab initio calculations of relativistic Brueckner theory lead to large scalar isovector fields in nuclear matter, at present, successful versions of covariant density functional theory neglect the interactions in this channel. A new high-precision density functional DD-MEδ is presented which includes four mesons, σ , ω, δ, and ρ, with density-dependent meson-nucleon couplings. It is based to a large extent on microscopic ab initio calculations in nuclear matter. Only four of its parameters are determined by adjusting to binding energies and charge radii of finite nuclei. The other parameters, in particular the density dependence of the meson-nucleon vertices, are adjusted to nonrelativistic and relativistic Brueckner calculations of symmetric and asymmetric nuclear matter. The isovector effective mass m ∗ − m ∗ derived from relativistic Brueckner theory is used to determine the coupling strength of the δ meson and its density dependence.
143 citations
TL;DR: It is demonstrated that resonant Raman scattering is an important tool to probe the electronic structure of novel materials and elucidate the band structure of wurtzite GaAs at the Γ point.
Abstract: In semiconductor nanowires, the coexistence of wurtzite and zinc-blende phases enables the engineering of the electronic structure within a single material. This presupposes an exact knowledge of the band structure in the wurtzite phase. We demonstrate that resonant Raman scattering is a important tool to probe the electronic structure of novel materials. Exemplarily, we use this technique to elucidate the band structure of wurtzite GaAs at the Γ point. Within the experimental uncertainty we find that the free excitons at the edge of the wurtzite and the zinc-blende band gap exhibit equal energies. For the first time we show that the conduction band minimum in wurtzite GaAs is of Γ7 symmetry, meaning a small effective mass. We further find evidence for a light-hole–heavy-hole splitting of 103 meV at 10 K.
131 citations
TL;DR: In this paper, the intersubband electron-related nonlinear optical absorption and optical rectification in GaAs-Ga1-xAlxAs asymmetric double quantum wells are studied, under the influence of combined or independent applied electric and magnetic fields as well as hydrostatic pressure.
117 citations
TL;DR: In this article, band-to-band tunneling is given a rigorous quantum mechanical treatment to incorporate confinement effects, multiple electron and hole valleys, and interactions with phonons, and the model reveals that the strong band bending near the gate dielectric results in quantization of the energy bands.
Abstract: Being the working principle of a tunnel field-effect transistor, band-to-band tunneling is given a rigorous quantum mechanical treatment to incorporate confinement effects, multiple electron and hole valleys, and interactions with phonons. The model reveals that the strong band bending near the gate dielectric, required to create short tunnel paths, results in quantization of the energy bands. Comparison with semiclassical models reveals a big shift in the onset of tunneling. The effective mass difference of the distinct valleys is found to reduce the subthreshold swing steepness.
TL;DR: In this article, the process of neutrinoless double electron (0νECEC) capture is revisited for those cases where the two participating atoms are nearly degenerate in mass.
TL;DR: In this article, the authors investigated the accuracy of different transfer matrix approaches, widely used to solve the stationary effective mass Schrodinger equation for arbitrary one-dimensional potentials, and showed that a symmetrized transfer matrix approach yields a similar accuracy as the Airy function method at a significantly reduced numerical cost.
Abstract: The accuracy of different transfer matrix approaches, widely used to solve the stationary effective mass Schrodinger equation for arbitrary one-dimensional potentials, is investigated analytically and numerically. Both the case of a constant and a position dependent effective mass are considered. Comparisons with a finite difference method are also performed. Based on analytical model potentials as well as self-consistent Schrodinger-Poisson simulations of a heterostructure device, it is shown that a symmetrized transfer matrix approach yields a similar accuracy as the Airy function method at a significantly reduced numerical cost, moreover avoiding the numerical problems associated with Airy functions.
TL;DR: In this paper, the authors analyzed the properties of a single impurity immersed in a Fermi sea and calculated its energy, quasiparticle residual energy, and effective mass.
Abstract: We analyze the properties of a single impurity immersed in a Fermi sea. At positive
energy and scattering lengths, we show that the system possesses a well-defined but
metastable excitation, the repulsive polaron, and we calculate its energy, quasiparticle
residue and effective mass. From a thermodynamic argument we obtain the number of
particles in the dressing cloud, illustrating the repulsive character of the polaron.
Identifying the important 2- and 3-body decay channels, we furthermore calculate the
lifetime of the repulsive polaron. The stability conditions for the formation of fully
spin polarized (ferromagnetic) domains are then examined for a binary mixture of atoms
with a general mass ratio. Our results indicate that mass imbalance lowers the critical
interaction strength for phase-separation, but that very short quasiparticle decay times
will complicate the experimental observation of itinerant ferromagnetism. Finally, we
present the spectral function of the impurity for various coupling strengths and momenta.
TL;DR: In this article, a broadband acoustic metamaterial with strongly anisotropic effective mass density is presented, which is composed of arrays of solid inclusions in an air background, and the anisotropy is controlled by the rotational asymmetry of these inclusions.
Abstract: We present the experimental realization and characterization of a broadband acoustic metamaterial with strongly anisotropic effective mass density. The metamaterial is composed of arrays of solid inclusions in an air background, and the anisotropy is controlled by the rotational asymmetry of these inclusions. Transmission and reflection measurements inside a one-dimensional waveguide are used to extract the relevant components of the effective density tensor together with the effective bulk modulus of the metamaterial. Its broadband anisotropy is demonstrated by measurements that span 500–3000 Hz. Excellent agreement between these measurements and numerical simulations confirms the accuracy of the design approach.
TL;DR: In this article, a quantitative analysis of the absorption and luminescence of colloidal PbSe/CdSe core/shell quantum dots (QDs) is presented.
Abstract: We present a quantitative analysis of the absorption and luminescence of colloidal PbSe/CdSe core/shell quantum dots (QDs). In absorption, both the energy and the oscillator strength of the first exciton transition coincide with that of plain PbSe QDs. In contrast, luminescence lifetime measurements indicate that the oscillator strength of the emitting transition is reduced by at least a factor of 4 compared to PbSe core QDs. Moreover, the addition of an electron scavenger quenches the PbSe/CdSe emission, while a hole scavenger does not. This implies that the electron wave function reaches the QD surface, while the hole is confined to the PbSe core. These observations are consistent with calculations based on the effective mass model, which show that PbSe/CdSe QDs are at the boundary between the type-I and quasi-type-II regime, where the electron spreads over the entire nanoparticle and the hole remains confined in the PbSe core. However, as this only leads to a minor reduction of the oscillator strength, it follows that the drastic reduction of the oscillator strength in emission cannot be explained in terms of electron delocalization. In combination with the increased Stokes shift for PbSe/CdSe QDs, this indicates that the emission results from lower energy states that are fundamentally different from the absorbing states.
TL;DR: In this paper, the authors explain the two important reasons for the introduction of strain into the active region of a quantum-well laser, which greatly enhance almost all characteristics of semiconductor lasers and make possible a wide range of applications.
Abstract: This tutorial article explains the two important reasons for the introduction of strain into the active region of a quantum-well laser. First, it reduces the density of states at the top of the valence band, which allows population inversion to be obtained at a lower carrier density. Second, it distorts the 3-D symmetry of the crystal lattice and matches it more closely to the 1-D symmetry of the laser beam. Together these effects greatly enhance almost all characteristics of semiconductor lasers and make possible a wide range of applications. Combinations of compressive and tensile strain can also be used, for example, to produce nonabsorbing mirrors and polarization-insensitive semiconductor optical amplifiers.
TL;DR: Using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) as discussed by the authors, the effects of strain on the energetic ordering and effective mass of the lowest conduction-band states in SrTiO${}_{3}$.
Abstract: Using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) we explore the effects of strain on the energetic ordering and effective mass of the lowest conduction-band states in SrTiO${}_{3}$. We predict that biaxial stress in the (001) or (110) planes results in the lowest-energy conduction-band state having significantly smaller electron mass in the in-plane directions compared to the unstrained SrTiO${}_{3}$, thus suggesting that pseudomorphic growth is a promising route to increasing the electron mobility in epitaxial films. We propose possible substrates that may lead to SrTiO${}_{3}$ films with enhanced electron mobilities, and report deformation potentials that allow accurate prediction of conduction-band splittings for arbitrary strain configurations.
TL;DR: In this paper, it was shown that high-field superconductivity in heavy fermion metals is sustained only when the effective mass of its conduction electrons diverge and not by a vanishing of the Fermi velocity to zero.
Abstract: It is widely believed that high-field superconductivity in heavy fermion metals is sustained only when the effective mass of its conduction electrons diverge. Measurements of magnetically driven changes in the electronic topology of URhGe suggest it is not divergence of the effective mass to infinity but a vanishing of the Fermi velocity to zero that supports this behaviour.
TL;DR: It is shown that the Fermi surface in the antiferromagnetic phase of BaFe( 2)As(2) is composed of one hole and two electron pockets, all of which are three dimensional and closed, in sharp contrast to the FS observed by angle-resolved photoemission spectroscopy.
Abstract: We show that the Fermi surface (FS) in the antiferromagnetic phase of BaFe(2)As(2) is composed of one hole and two electron pockets, all of which are three dimensional and closed, in sharp contrast to the FS observed by angle-resolved photoemission spectroscopy. Considerations on the carrier compensation and Sommerfeld coefficient rule out existence of unobserved FS pockets of significant sizes. A standard band structure calculation reasonably accounts for the observed FS, despite the overestimated ordered moment. The mass enhancement, the ratio of the effective mass to the band mass, is 2-3.
TL;DR: In this paper, the full plane-wave expansions of the Bloch functions at the conduction band minima have been investigated and compared with different approximations and formalisms adopted in the literature.
Abstract: Orbital degeneracy of the electronic conduction band edge in silicon is a potential roadblock to the storage and manipulation of quantum information involving the electronic spin degree of freedom in this host material. This difficulty may be mitigated near an interface between Si and a barrier material, where intervalley scattering may couple states in the conduction ground state, leading to nondegenerate orbital ground and first excited states. The level splitting is experimentally found to have a strong sample dependence, varying by orders of magnitude for different interfaces and samples. The basic physical mechanisms leading to such coupling in different systems are addressed. We expand our recent study based on an effective mass approach, incorporating the full plane-wave expansions of the Bloch functions at the conduction band minima. Physical insights emerge naturally from a simple Si/barrier model. In particular, we present a clear comparison between ours and different approximations and formalisms adopted in the literature and establish the applicability of these approximations in different physical scenarios.
TL;DR: In amorphous lattices the bands comprise localized states, but it is found that defect states residing in the gap are more localized than the localization length of states within the band.
Abstract: We study, experimentally and numerically, amorphous photonic lattices and the existence of band gaps therein. Our amorphous system comprises 2D waveguides distributed randomly according to a liquidlike model responsible for the absence of Bragg peaks, as opposed to ordered lattices with disorder which always exhibit Bragg peaks. In amorphous lattices the bands comprise localized states, but we find that defect states residing in the gap are more localized than the localization length of states within the band. Finally, we show how the concept of effective mass carries over to amorphous photonic lattices.
TL;DR: In this paper, the authors examine band-structure engineering in extremely tetragonal ferroelectric perovskites to make these materials suitable for photovoltaic applications.
Abstract: We examine band-structure engineering in extremely tetragonal ferroelectric perovskites ($\mathit{AB}$O${}_{3}$) to make these materials suitable for photovoltaic applications. Using first-principles calculations, we study how $B$-site ordering, lattice strain, cation identity, and oxygen octahedral cage tilts affect the energies and the compositions of the valence and conduction bands. We find that extreme tetragonality makes the band gap highly sensitive to the $B$-cation ordering, with a layered $B$-site arrangement exhibiting a small band gap. It also leads to a strong sensitivity of the band gap to the oxygen octahedral tilting. These effects only occur for cations with filled $d$ states located near the valence-band maximum or empty $d$ states at the conduction-band minimum; this criterion is explained by crystal field theory. In addition to a smaller band gap, the layered $B$-site ordering has a strong impact on the carrier mobility. We find that the excited electron effective mass is similar to that found in Si and other classic semiconductors. Moreover, the hole effective mass is strongly anisotropic, indicating a two-dimensional hole gas in the layered $B$-cation ordering.
TL;DR: In this article, the subband structure and optical properties of a cylindrical quantum well wire under intense non-resonant laser field are investigated by taking into account the correct dressing effect for the confinement potential.
TL;DR: In this article, the position-dependent effective mass in a GaAs/AlxGa1−xAs cubic quantum dot is studied and an analytic relation for studying the position dependent effective mass is obtained for the intersubband optical absorption coefficient and refractive index change in the quantum dot.
Abstract: In this paper, we first obtain an analytic relation for studying the position-dependent effective mass in a GaAs/AlxGa1−xAs cubic quantum dot. Then, the effect of position-dependent effective mass on the intersubband optical absorption coefficient and the refractive index change in the quantum dot are studied. Our numerical calculations are performed using both a constant effective mass and the position-dependent effective mass. We calculate the linear, nonlinear and total intersubband absorption coefficient and refractive index change as a function of the incident optical intensity and structural parameters such as dot length. The results obtained from the present work show that spatially varying electron effective mass plays an important role in the intersubband optical absorption coefficient and refractive index change in a cubic quantum dot.
TL;DR: In this article, the optical and structural properties of wurtzite GaN nanowires containing zinc-blende GaN inclusions of different thicknesses are investigated, and the quantum confinement in these type-II crystal phase heterostructures was simulated in the framework of a one-dimensional effective mass model.
Abstract: The optical and structural properties of wurtzite GaN nanowires containing zinc-blende GaN inclusions of different thicknesses are investigated. Micro-photoluminescence spectra of single nanowires exhibit a series of narrow emission peaks with linewidth as low as 0.8 meV in the interval 3.1–3.42 eV. The peak energy blue-shifts with increasing excitation power following a ∼I1/3 law due to the progressive band filling and to the screening of the internal field. The quantum confinement in these type-II crystal phase heterostructures was simulated in the framework of a one-dimensional effective mass model, accounting for the internal electrical polarization of the wurtzite GaN. The predicted transition energies are in good agreement with the energy statistics realized on more than 30 single nanowire emission spectra.
TL;DR: In this article, self-energies of a minimally coupled scalar field with quartic and trilinear interactions are calculated in a de Sitter background, using a position space propagator.
Abstract: Self-energies of a minimally coupled scalar field with quartic and trilinear interactions are calculated in a de Sitter background, using a position space propagator. For quartic interactions, we recover earlier results for the seagull diagram, namely, that it contributes an effective mass for the scalar field at leading order in the infrared enhancement in a steady-state de Sitter background. We further show that the sunset diagram also contributes to this effective mass and argue that these two contributions are sufficient in order to determine a self-consistent dynamical mass. In addition, trilinear interactions also induce a dynamical mass for the scalar field which we calculate. Since an interacting scalar field in de Sitter acquires a dynamical mass through these loop corrections, the infrared divergences of the two-point correlator are naturally self-regulated.
TL;DR: In this article, the edge configuration and quantum confinement effects on electron transport in armchair-edged graphene nanoribbons (A-GNRs) were investigated by using a computational approach.
Abstract: We investigated edge configuration and quantum confinement effects on electron transport in armchair-edged graphene nanoribbons (A-GNRs) by using a computational approach. We found that the edge bond relaxation has a significant influence not only on the bandgap energy but also on the electron effective mass. We also found that A-GNRs with N = 3m family (N is the number of atoms in its transverse direction, and m is a positive integer) exhibits smaller effective mass by comparing it at the same bandgap energy. As a result, A-GNRs with N = 3m family are found to be favorable for use in channels of field-effect transistors.
TL;DR: In this article, the authors consider the discrete eigenvalues of the Schrodinger operator He = − Δ+ V (x )+ e2Q(ex), where V(x) is periodic and Q(y) is localized on R d, d ≥ 1.
Abstract: We consider the discrete eigenvalues of the operator He = − Δ+ V (x )+ e2Q(ex), where V (x) is periodic and Q(y) is localized on R d , d ≥ 1. For e> 0 and sufficiently small, discrete eigenvalues may bifurcate (emerge) from spectral band edges of the periodic Schrodinger operator, H0 = −Δx + V (x), into spectral gaps. The nature of the bifurcation depends on the homogenized Schrodinger operator LA,Q = −∇y · A∇y + Q(y). Here, A denotes the inverse effective mass matrix, associated with the spectral band edge, which is the site of the bifurcation.
University of Basel1, University of Bonn2, Petersburg Nuclear Physics Institute3, University of Groningen4, Florida State University5, University of South Carolina6, University of Mainz7, Bhabha Atomic Research Centre8, Indian Institutes of Technology9, Forschungszentrum Jülich10, Savitribai Phule Pune University11
TL;DR: In this article, the quasi-free photoproduction of η-mesons off the deuteron has been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies up to 2.5GeV.
Abstract: Precise data for quasi-free photoproduction of η-mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies up to 2.5GeV. The η-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to η-production off the free proton. No significant effects beyond the folding of the free cross-section with the momentum distribution of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of the η-nucleon pairs reconstructed from the final-state four-momenta of the particles. The total cross-section for quasi-free η-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant-mass spectrum of the η-neutron pairs. Position and width of the peak in the invariant-mass spectrum are W ≈ 1665 MeV and FWHM Γ ≈ 25 MeV. The data are compared to the results of different models.