Showing papers on "Effective mass (solid-state physics) published in 1999"

Estimation of the thermal band gap of a semiconductor from Seebeck measurements

Abstract: It is shown that the magnitude of the Seebeck coefficient of a semiconductor has a maximum value that is close to one-half the energy gap divided by eT. An expression for the position of the Fermi level at which the Seebeck coefficient has a maximum or minimum value is derived, with account taken of the mobility and effective mass ratios. It is concluded that measurement of the Seebeck coefficient as a function of temperature on any novel semiconductor is one of the simplest ways of estimating its band gap.

The `Friction' of Vacuum, and other Fluctuation-Induced Forces

Abstract: The static Casimir effect describes an attractive force between two conducting plates, due to quantum fluctuations of the electromagnetic (EM) field in the intervening space. Thermal fluctuations of correlated fluids (such as critical mixtures, super-fluids, liquid crystals, or electrolytes) are also modified by the boundaries, resulting in finite-size corrections at criticality, and additional forces that affect wetting and layering phenomena. Modified fluctuations of the EM field can also account for the ``van der Waals'' interaction between conducting spheres, and have analogs in the fluctuation-induced interactions between inclusions on a membrane. We employ a path integral formalism to study these phenomena for boundaries of arbitrary shape. This allows us to examine the many unexpected phenomena of the dynamic Casimir effect due to moving boundaries. With the inclusion of quantum fluctuations, the EM vacuum behaves essentially as a complex fluid, and modifies the motion of objects through it. In particular, from the mechanical response function of the EM vacuum, we extract a plethora of interesting results, the most notable being: (i) The effective mass of a plate depends on its shape, and becomes anisotropic. (ii) There is dissipation and damping of the motion, again dependent upon shape and direction of motion, due to emission of photons. (iii) There is a continuous spectrum of resonant cavity modes that can be excited by the motion of the (neutral) boundaries.

Many-Body Effects in Angle-Resolved Photoemission: Quasiparticle Energy and Lifetime of a Mo(110) Surface State

Abstract: Recent investigations of strongly correlated electron systems have questioned the validity of one of the most fundamental paradigms in solid state physics— Fermi liquid theory. The latter picture is based on the existence of “quasiparticles,” or single-particle-like low energy excitations which obey the exclusion principle and have lifetimes long enough to be considered as particles. Strictly speaking, the quasiparticle concept is restricted to zero temperature and a narrow region around the Fermi level [1], but its usefulness often continues to finite temperatures, and energies away from the Fermi level [2]. Indications for possible non-Fermi-liquid behavior have been found in some organic one-dimensional conductors [3] and in the normal state of high temperature superconductors [4]. A whole variety of experimental techniques have been employed in the search for such behavior, including resistivity measurements [5], infrared spectroscopy [6], scanning tunneling spectroscopy [7], and time-resolved two-photon photoemission [8]. Angle-resolved photoemission spectroscopy (ARPES) has an advantage, in that the energy and lifetime of the photohole are directly observable in the experiment. ARPES in principle measures the quasiparticle spectral function [9]:

Supersymmetric approach to quantum systems with position-dependent effective mass

Abstract: We consider the application of the supersymmetric quantum-mechanical formalism to the Schr\"odinger equation describing a particle characterized by a position-dependent effective mass $m(x).$ We show that any one-dimensional quantum system with effective mass has a supersymmetric partner system characterized by the same position dependence of the mass, but with a new potential function. The form of this supersymmetric partner potential ${V}_{2}(x)$ depends on both the form of the original potential ${V}_{1}(x)$ and the form of the mass x dependence. We also generalize the concept of shape invariance to the nonconstant mass scenario. As illustrative examples we provide, for a given form $m(x)$ of the effective mass, shape-invariant potentials exhibiting (a) harmonic-oscillator-like spectra and (b) Morse-like spectra. In both cases the energy eigenvalues and eigenfunctions can be obtained in algebraic fashion.

Neutron matter model

Abstract: The Bertsch, nonparametric model of neutron matter is analyzed and strong indications are found that, in the infinite system limit, the ground state is a Fermi liquid with an effective mass, except for a set of measure zero.

Effects of doping on the transport properties of CoSb3

Abstract: Effects of doping on the transport properties of CoSb3 have been systematically investigated using Ni, Pd, and Pt as donor impurities. It is shown that the Hall mobility, the Seebeck coefficient, and the electrical conductivity depend strongly not only on the carrier concentration but also on these donor impurities. Our theoretical analysis suggests that the electron effective mass and the conduction band deformation potential are significantly affected by both the doping levels and the donor impurities. These doping effects in CoSb3 can be attributed to (1) the changes in the electronic structure with doping and (2) the specific nature of the conduction band structure, in particular, the nonparabolicity of the band which can be explained in terms of a two-band Kane model. The observed changes in the electronic properties with doping are also consistent with the predictions of a recent band structure calculation of CoSb3. On the other hand, the lattice thermal conductivity decreases markedly with increasi...

Mobile small polaron

Abstract: small radius of the wave function but a large size of the lattice distortion. We calculate the energy dispersion and the effective mass of the polaron with the 1/� perturbation theory and with the exact Monte Carlo method in the nonadiabatic and adiabatic regimes, respectively. The "small" Frohlich polaron is found to be lighter than the small Holstein polaron by one or more orders of magnitude. A free electron interacting with the dielectric polaris- able continuum was studied by Pekar (1) and Frohlich (2) in the strong and weak coupling limit, respectively. This is the case of carriers interacting with optical phonons in ionic crystals under the condition that the size of the self-trapped state is large compared to the lattice con- stant so the lattice discreteness is irrelevant (3). The most sophisticated treatment of this "large" or "contin- uum" polaron is due to Feynman and co-workers (4) with the path-integral method, substantially extended in the past decade (5). This treatment leads to a mass enhance- ment, but not to a hopping conduction or to a narrow polaron band. When the electron-phonon coupling constantis large, all the states in the Brillouin zone are involved in the for- mation of the polaron wave function, so the polaron ra- dius becomes comparable with the lattice constant a and the continuum approximation is no longer valid. Basic features of the small polaron were well recognised a long time ago by Tjablikov (6), Yamashita and Kurosawa (7), Sewell (8), Holstein (9), Lang and Firsov (10) and oth- ers, and are described in several review papers and text- books (11-15). So far, analytical and numerical studies have been mainly confined to the Holstein model with a short-range electron-phonon interaction. Exact diag- onalization of several vibrating molecules coupled with one electron (16,17), variational (18,19) and Monte Carlo calculations (20) revealed an excellent agreement with an- alytical results of Holstein (9) and Lang and Firsov (10) for the energy of the ground state and first excited states at large �. Polaron mass is very large in the Holstein model, unless phonon frequencies are extremely high. The size of the region, where the small Holstein polaron is localised, is about the same as the size of the lattice distortion, each of the order of the lattice constant. Both sizes are almost identical also for the large Frohlich polaron, but much larger. In this Letter we study a problem of the lattice po- laron with a long-range Frohlich interaction (21). This quasiparticle has a small (atomic) size of the electron lo- calization region but a large size of the lattice distortion. While the large Frohlich polaron is heavier than the large Holstein polaron, the small Frohlich polaron (SFP) turns out to be much lighter than the small Holstein polaron (SHP) with the same binding energy. We argue that SFPs are relevant quasiparticles in the cuprates. A quite general electron-phonon lattice Hamiltonian with one electron and the "density-displacement" type of interaction is given by (9,12,15) H = − X nn' tnnc †'cn + X q� ¯q�(d †�dq� + 1/2)

Large Dispersive Effects near the Band Edges of Photonic Crystals

Abstract: We have used phase-sensitive ultrashort-pulse interferometry to study the modification of optical pulse propagation near the photonic band edges in colloidal crystals consisting of polystyrene spheres in water. A strong suppression of the group velocity is found at frequencies near the $L$ gap of the fcc lattice. The group velocity dispersion diverges at the band edges and shows branches of both normal dispersion and anomalous dispersion, which can be interpreted as large changes in the effective mass, both positive and negative. We obtain excellent agreement with the dynamical diffraction theory.

The rate of radiative recombination in the nitride semiconductors and alloys

Abstract: The radiative recombination rates of free carriers and lifetimes of free excitons have been calculated in the wide band gap semiconductors GaN, InN, and AlN of the hexagonal wurtzite structure, and in their solid solutions GaxAl1−xN, InxAl1−xN and GaxIn1−xN on the base of existing data on the energy band structure and optical absorption in these materials. We determined the interband matrix elements for the direct optical transitions between the conduction and valence bands, using the experimental photon energy dependence of absorption coefficient near the band edge. In our calculations we assumed that the material parameters of the solid solutions (the interband matrix element, carrier effective masses, and so on) could be obtained by a linear interpolation between their values in the alloy components. The temperature dependence of the energy gap was taken in the form proposed by Varshni [Physica 34, 149 (1967)]. The calculations of the radiative recombination rates were performed in a wide range of temp...

Low-energy electrodynamics of SmB 6

Abstract: We have performed direct measurements of the low-temperature dynamical conductivity and dielectric permittivity of single crystalline ${\mathrm{SmB}}_{6}$ in the spectral range from 0.6 to 4.5 meV, i.e., below the hybridization gap. The obtained results together with the data of Hall-effect and infrared reflection measurements give evidence for a 19-meV energy gap in the density of states and an additional narrow donor-type band lying only 3 meV below the bottom of the upper conduction band. It is shown that at temperatures $5\mathrm{K}lTl20\mathrm{K}$ the electrodynamic response and the dc conductivity of ${\mathrm{SmB}}_{6}$ are determined by quasifree carriers thermally excited in the conduction band. We evaluate the microscopic parameters of these carriers: the spectral weight, the concentration, the effective mass, the scattering rate, and the mobility. Below 8 K the concentration of carriers in the conduction band freezes out exponentially and finally the electronic properties of ${\mathrm{SmB}}_{6}$ are determined by the localized carriers in the narrow band with the typical signature of hopping conductivity.

Effective mass in quantum effects of radiation pressure

Abstract: We study the quantum effects of radiation pressure in a high-finesse cavity with a mirror coated on a mechanical resonator. We show that the optomechanical coupling can be described by an effective susceptibility which takes into account every acoustic modes of the resonator and their coupling to the light. At low frequency this effective response is similar to a harmonic response with an effective mass smaller than the total mass of the mirror. For a plano-convex resonator the effective mass is related to the light spot size and becomes very small for small optical waists, thus enhancing the quantum effects of optomechanical coupling.

Effective mass in quantum effects of radiation pressure

Abstract: We study the quantum effects of radiation pressure in a high-finesse cavity with a mirror coated on a mechanical resonator. We show that the optomechanical coupling can be described by an effective susceptibility which takes into account every acoustic modes of the resonator and their coupling to the light. At low frequency this effective response is similar to a harmonic response with an effective mass smaller than the total mass of the mirror. For a plano-convex resonator the effective mass is related to the light spot size and becomes very small for small optical waists, thus enhancing the quantum effects of optomechanical coupling.

Excitonic exchange splitting in bulk semiconductors

Abstract: We present an approach to calculate the excitonic fine-structure splittings due to electron-hole short-range exchange interactions using the local-density approximation pseudopotential method, and apply it to bulk semiconductors CdSe, InP, GaAs, and InAs. Comparing with previous theoretical results, the current calculated splittings agree well with experiments. Furthermore, we provide an approximate relationship between the short-range exchange splitting and the exciton Bohr radius, which can be used to estimate the exchange splitting for other materials. The current calculation indicates that a commonly used formula for exchange splitting in quantum dot is not valid. Finally, we find a very large pressure dependence of the exchange splitting: a factor of 4.5 increase as the lattice constant changes by 3.5%. This increase is mainly due to the decrease of the Bohr radius via the change of electron effective mass.

Electron effective mass in hexagonal GaN

Abstract: Far-infrared magneto-optical investigations of shallow donors in epitaxial GaN layers on sapphire were carried out by means of Fourier transform spectrometry up to 23 T. From the splitting of the donor p states in a magnetic field, the cyclotron effective mass for conduction electrons was found to be m*=0.222 m0. A precise determination of the mass was made possible by the high quality of the spectra and by taking into account high magnetic field data above 12 T.

Electronic States of Boron and Phosphorus in Diamond

Abstract: The electronic states of boron and phosphorus in diamond have been studied by infrared absorption and photo-thermal ionisation spectroscopies. High quality boron doped synthetic diamond (p-type conductive) and phosphorus-doped CVD diamond film (n-type conductive) were used for this study. In the case of boron-doped diamond, the four main excited states of the bound hole follow a Rydberg series, suggesting that boron has a hydrogen-like behaviour, with a weak splitting of the excited states. The consistent values of the optical ionisation energy (E0 = 382 meV), of an “average” effective mass (m* = 0.74m0) and of the Bohr radius of the ground state (a* = 4.1 A) deduced from the Rydberg series support this suggestion. The comparison with the effective mass approximation, applied for acceptor states in diamond, suggests that the top of the valence band of diamond is different from that of silicon and germanium. In the case of phosphorus-doped diamond, two excited states of the bound electron have been observed for the first time, at 523 and 562 meV from the ground level. The good agreement with the effective mass approximation suggests that phosphorus is a shallow donor, and allows us to propose a first value of the optical ionisation energy of phosphorus in diamond of about 600 meV, consistent with Hall effect measurements.

Light quarks in the instanton vacuum at finite baryon density

Abstract: We consider the finite density, zero-temperature behavior of quark matter in the instanton picture. Since the instanton-induced interactions are attractive in both $\overline{q}q$ and $\mathrm{qq}$ channels, a competition ensues between phases of matter with condensation in either or both. It results in chiral symmetry restoration due to the onset of diquark condensation, a ``color superconductor,'' at finite density. Also possible is a state with both manners of condensation; however, such a phase is at best metastable for any chemical potential. The properties of quark matter in each phase are discussed, with emphasis on the microscopic effects of the effective mass and superconducting energy gap.

Ultraviolet-blue emission and electron-hole states in ZnSe quantum dots

Abstract: We observed the quantum-confined band edge emission from ZnSe quantum dots and the size dependence of the energy states, spin-orbit interaction, and Stokes shift. The band edge emission occurs in the ultraviolet blue. The energy gap=Eg+C/dn where d is the diameter and n is 1.19±0.13 and 1.21±0.13 for the first and second electron-hole transitions, respectively. The separation between these transitions approaches the bulk spin-orbit splitting, while the Stokes shift decreases with particle size. Effective mass theories cannot explain these results. Trap emission is observed in some samples in the green and red, resulting from Se-related traps.

Leakage current of sol-gel derived Pb(Zr, Ti)O3 thin films having Pt electrodes

Abstract: The electrical conduction behaviors of sol-gel derived Pb(Zr, Ti)O3 (PZT) thin films on Pt electrodes were analyzed based on a fully depleted film, thermionic field emission, and space charge limited conduction model in the low and high electric field regions, respectively. For films having thicknesses ranging from 150 to 250 nm, no thickness-dependent variation in the dielectric constant was observed due to the relatively large thicknesses. The rather small film-thickness-dependent leakage current characteristics in the low-field region elucidates that the positive space charge density in the film is about 1018 cm−3, which is a smaller value than that of the sputter deposited (Ba, Sr)TiO3 thin films by an order of magnitude. The calculated interfacial potential barrier height and effective mass of electrons were 0.93 eV and 0.09m0, respectively. The slope larger than 2 from the log J vs log V plot in the high-field region implies that the energy level of electron traps are continuously distributed in the energy band gap.

Polaron effective mass, band distortion, and self-trapping in the Holstein molecular-crystal model

Abstract: We present polaron effective masses and selected polaron band structures of the Holstein molecular crystal model in one dimension as computed by the Global-Local variational method over a wide range of parameters. These results are augmented and supported by leading orders of both weak- and strong-coupling perturbation theory. The description of the polaron effective mass and polaron band distortion that emerges from this work is comprehensive, spanning weak, intermediate, and strong electron-phonon coupling, and nonadiabatic, weakly adiabatic, and strongly adiabatic regimes. Using the effective mass as the primary criterion, the self-trapping transition is precisely defined and located. Using related band-shape criteria at the Brillouin-zone edge, the onset of band narrowing is also precisely defined and located. These two lines divide the polaron parameter space into three regimes of distinct polaron structure, essentially constituting a polaron phase diagram. Though the self-trapping transition is thus shown to be a broad and smooth phenomenon at finite parameter values, consistency with the notion of self-trapping as a critical phenomenon in the adiabatic limit is demonstrated. Generalizations to higher dimensions are considered, and resolutions of apparent conflicts with well-known expectations of adiabatic theory are suggested. {copyright} {ital 1999} {ital The American Physical Society}

Investigations on the Optical Properties of Undoped, Fluorine Doped and Antimony Doped Tin Oxide Films

Abstract: Undoped, fluorine doped and antimony doped tin oxide films are prepared on quartz plates by Spray pyrolysis technique. The films grown at the optimum substrate temperature with different doping levels have been chosen for this study. Optical properties of these films are investigated in the entire UV-Visible -IR region (0.2 - 10 mikrom). The observed absorption edge lies at 3.65 eV for undoped tin oxide and on doping it shifts towards higher energies, which is due to the Moss - Burstein effect. For fluorine doping depending upon the fluorine concentration, the absorption edge lies in the range 3.9 - 4.14 eV and for antimony doping it lies in the range 3.82 - 4.1 eV. In the undoped tin oxide films the direct allowed transition occurs at 4.02 eV and indirect allowed transition occurs at 2.43 eV, whereas for fluorine doped tin oxide and antimony doped tin oxide films, the direct allowed transitions occur in the range 4.18 - 4.28 and 4.13 - 4.22 eV respectively and the indirect allowed transitions occur in the range 2.63 - 2.73 and 2.54 - 2.65 eV respectively. Optical properties near the plasma edge have been analyzed using Drude's theory. The dependence of effective mass on carrier concentration has been explained on the basis of nonparabolicity of the conduction band. The shift in the fermi energy, calculated on the basis of energy dependent effective mass, is consistent with the measured shift in the absorption edge.

Role of the intrinsic surface state in the decay of image states at a metal surface

Abstract: The role of the intrinsic surface state $(n=0)$ in the decay of the first image state $(n=1)$ at the (111) surface of copper is investigated. Inelastic linewidths are evaluated from the knowledge of the imaginary part of the electron self energy, which we compute, within the GW approximation of the many-body theory, by going beyond a free-electron description of the metal surface. Single-particle wave functions are obtained by solving the Schr\"odinger equation with a realistic one-dimensional model potential, and departure of the motion along the surface from free-electron behavior is considered through the introduction of the effective mass. The decay of the first image state of Cu(111) into the intrinsic surface state is found to result in a linewidth that represents 40% of the total linewidth. The dependence of linewidths on the momentum of the image state parallel to the surface is also investigated.

Effect of Nitrogen‐Induced Modification of the Conduction Band Structure on Electron Transport in GaAsN Alloys

Abstract: We report pressure studies of conductivity, photoconductivity and Hall effect in Si doped GaAs0.986N0.014. We observed that pressure causes an unusually large reduction of the electron mobility in this compound (factor of 2.5 in the pressure range 0 to 2 GPa). We also measured the pressure coefficient of the energy gap and we found it, in agreement with previous reports, to be reduced by almost 40% compared to that of GaAs. These results can be successfully explained by a recently proposed, phenomenological model based on the concept of an anticrossing interaction between localized N-related states (resonant with the conduction band) and conduction band states. This model explains well the reduction of the pressure coefficient of the bandgap and predicts an enhancement of the effective mass of GaAs1—xNx. The decrease of electron mobility observed in the present study is in agreement with this latter prediction.

Fermi surface properties and de Haas–van Alphen oscillation in both the normal and superconducting mixed states of URu2Si2

Abstract: We have succeeded in growing a high-quality single crystal of URu2Si2 with a residual resistivity ratio of 255 and performed magnetoresistance and de Haas–van Alphen (dHvA) experiments. The dHvA oscillations were observed clearly in both the normal and superconducting mixed states. From the magnetoresistance experiments, it is concluded that URu2Si2 is a compensated metal with equal carrier numbers of electrons and holes. In the dHvA experiments we observed three kinds of Fermi surfaces. We studied precisely the Fermi surface properties in both the normal and mixed states. The dHvA frequency does not change in magnitude between the normal and mixed states, while the cyclotron effective mass is reduced, and the corresponding Dingle temperature or scattering rate of the conduction electron increases in the mixed state.

Electron mobility in extremely thin single-gate silicon-on-insulator inversion layers

Abstract: Inversion-layer mobility has been investigated in extremely thin silicon-on-insulator metal–oxide–semiconductor field-effect transistors with a silicon film thickness as low as 5 nm. The Poisson and Schrœdinger equations have been self-consistently solved to take into account inversion layer quantization. To evaluate the electron mobility, the Boltzmann transport equation has been solved by the Monte Carlo method, simultaneously taking into account phonon, surface-roughness, and Coulomb scattering. We show that the reduction of the silicon layer has several effects on the electron mobility: (i) a greater confinement of the electrons in the thin silicon film, which implies an increase in the phonon-scattering rate and therefore a mobility decrease; (ii) a reduction in the conduction effective mass and the intervalley-scattering rate due to the redistribution of carriers in the two subband ladders as a consequence of size quantization resulting in a mobility increase; and (iii) an increase in Coulomb scattering because of a greater number of interface traps in the buried Si–SiO2 and to a closer approach of these charged centers to the mobile carriers. The dependence of these effects on the silicon-layer thickness and on the inversion-charge concentration causes the mobility to be a nontrivial function of these variables. A detailed explanation of the mobility behavior is provided. Mobility for samples with silicon thickness below 10 nm is shown to increase in an electric field range that depends on the charged center concentration, while for silicon layers over 10 nm mobility always decreases as the silicon-layer thickness is reduced.

Specific heat of high temperature superconductors in high fields at Tc: from BCS to the Bose–Einstein condensation

Abstract: We consider the experimental trends in a database of specific heat measurements near T c in high magnetic fields for type-II superconductors with a large value of κ = λ/ξ, including mostly high-temperature superconductors. Whereas the BCS limiting case is well established in low-T c superconductors, the exact 3D-XY behavior illustrated by the λ-transition of 4 He applies only in the particular case of optimally doped YBa 2 Cu 3 O x . Otherwise, transitions are intermediate either between the BCS and the 3D-XY models (e.g., YBa 2 Cu 3 O 7.00 ), or between the 3D-XY model and the Bose-Einstein condensation (BEC) (e.g., Bi 2 Sr 2 CaCu 2 O 8 ). The key parameter in ordering this sequence appears to be the product k F ξ of the Fermi wave number by the coherence length, as evaluated from tunneling spectra in the vortex cores. Such a trend, which is consistent with theoretical descriptions of the strong coupling limit, is visible in the thermodynamics of the phase transition. Implications on the effective mass, the density of pairs just above T c , the pseudo-gap behavior, etc., are discussed.

Evidence for localized Si-donor state and its metastable properties in AlGaN

Abstract: Transport studies of AlxGa1−xN (0.5

Fermi Surface and de Haas-van Alphen Oscillation in both the Normal and Superconducting Mixed States of UPd2Al3.

Abstract: The dHvA oscillation was observed clearly in both the normal and superconducting mixed states for UPd 2 Al 3 with the hexagonal structure. Eight kinds of dHvA branches were detected, which are well explained by the results of band calculations based on a fully-relativistic spin-polarized LAPW method. An inner orbit of a band 41-hole Fermi surface, named ζ, has the largest dHvA frequency of 9.45×10 7 Oe and cyclotron mass of 65 m 0 , reflecting a large electronic specific heat coefficient of 145 mJ/K 2 ·mol. Branch α of a band 40-hole Fermi surface was observed in both the normal and superconducting mixed states, where the dHvA frequency does not change in magnitude between the normal and mixed states. The cyclotron effective mass of branch α is, however, found to be reduced and the corresponding Dingle temperature or scattering rate of the conduction electron increases with decreasing the field in the mixed state. An anisotropic energy gap with a line node for UPd 2 Al 3 is discussed from the angular depe...