Anisotropic Effective Mass Research Articles

Low angle deviation from the anisotropic effective mass model in epitaxial YBa2(Cu1−xZnx)3O7−δ thin films

The magnetic field effect on the electrical in-plane resistivity ϱab has been investigated for high quality c-axis oriented YBa2(Cu1−xZnx)3O7−δ thin films (x=0.02 and 0.04) by magnetoresistance measurements up to 20 Teslas for different orientations of the magnetic field with respect to the a-b plane, at different temperatures below Tc. Deviation in the scaling behaviour expected from the anisotropic effective mass model is observed for small misalignement between the magnetic field direction and the superconducting CuO2 planes, which is attributed to the intrinsic pinning effect. The Zn doping influence on the anisotropic ratio γ = ξab/ξc is studied from the high angle scaling properties of the film.

Magnetic anisotropy in single crystals of Bi2.1Sr1.9Ca1Cu2Ox

The anisotropic magnetisation of large and high quality single crystals of Bi2.1Sr1.9Ca1Cu2Ox has been studied using a 7 Tesla SQUID magnetometerequipped with a sample rotation system having an angle adjustment accuracy 0.1 degree when the magnetic field was applied at different angles from 0°–90° with respect to the crystallographic a-axis of the crystals. Two large single crystals of 5*5.5*1.2 mm3 and 5*3.6*0.45 mm3 were used in the experiments. The high quality of the crystals was confirmed by neutron diffraction analysis. The results showed the effective mass anisotropy parameter γ is much larger than 250. The irreversibility line Hirr(T) for a magnetic field applied along the a-axis of the crystals is 700 times higher than the for Hirr(T) a field applied along the c-axis of the crystals.

Diamagnetism of quasicrystals due to Bragg's reflexions

We point out that in quasicrystals, one has to take into account a large but finite number of effective Bragg's reflexions out of the dense set. This leads to small gaps on the Fermi surface (some tenths of meV), much narrower than the usual Hume-Rothery one's (0.2-0.5 eV) invoked for explaining their stability. Small pockets of electrons and holes with very anisotropic effective masses are thus existing. The very low effective masses in some directions for electron pockets are responsible for the diamagnetism of quasicrystals. As these gaps are very sensitive to impurities, phasons, so is the diamagnetism.

Quantum size effects of charge-transfer excitonsin nonpolar molecular organic thin films

We use a quantum-mechanical model to analyze charge-transfer (CT) excitons in closely packed, nonpolar organic molecular crystal thin films grown by the ultrahigh-vacuum process of organic molecular beam deposition. The exciton Hamiltonian includes both polarization effects and the periodic pseudopotential of the crystal. This model takes into account the very large anisotropy characteristic of many organic materials such as the archetype molecular crystal, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and PTCDA-based multilayers. Using a single-exciton Hamiltonian, we quantitatively model experimental electroabsorption data and the absorption spectral shifts observed in ultrathin organic multilayers or ``multiple quantum wells.'' The data analyzed from several such experiments give independent and consistent estimations of the anisotropic effective mass tensor and exciton radii for PTCDA along different crystal axes. This treatment is general, and is found to extend to other CT and Wannier exciton systems found in many interesting, nonpolar organic molecular and inorganic semiconductor crystals such GaAs, suggesting similar physical origins for Wannier and CT excitons in a wide range of materials.

Magnetoluminescence study on the effective mass anisotropy of CuPtB-ordered GaInP2 alloys

Photoluminescence measurements in a magnetic field between 0 and 13.6 T were carried out on CuPtB-ordered GaInP at liquid-helium temperature. Four samples of different degrees of ordering (η, varying from 0 to 0.54) were studied. Experimental results show that the ordering not only induces a band-gap reduction and valence-band splitting, it also causes changes in effective masses and an effective mass anisotropy. By measuring and analyzing the magnetoluminescence with the magnetic field aligned along both ordering (the [1̄11]) and growth (the [001]) directions, we demonstrate that, for the band-edge excitonic state, the reduced mass in the plane perpendicular to the ordering direction is smaller than that in the ordering direction and also smaller than that of a disordered alloy. The exciton binding energy is found nearly independent of the degree of ordering, in agreement with theoretical predictions.

Schottky barriers on anisotropic semiconductor GaTe

Abstract Schottky barriers were studied on p-GaTe single crystals both normal and parallel to the layer planes. From the C-V characteristics the barrier heights qφB normal to the layer planes were found to be 0.75, 0.63 and 0.485eV for Al, Ag and Au respectively dependent on the metal work functions. From the temperature dependence of the I-V characteristics of Al Schottky barriers, the effective Richardson constants A* were determined to be 120 and 56.4 A cm-2 K-2 for current flow normal and parallel respectively to the layer planes, in excellent agreement with values obtained from the anisotropic hole effective masses. The corresponding ideality factors were 1.44 and 2.5 respectively at 300 K. The carrier concentration n increased while φB decreased with decrease in temperature owing to increased tunnelling and recombination currents. From the variation in φBP with φM the interface index S was found to be 0.30 as predicted by Kurtin, McGill and Mead in 1971 from electronegativity considerations.

Electronic structure and thermoelectric properties of bismuth telluride and bismuth selenide

The electronic structures of the two thermoelectric materials and are studied using density-functional theory with the spin - orbit interaction included. The electron states in the gap region and the chemical bonding can be described in terms of interaction between the atomic p orbitals within the `quintuple' layer. For , we find both the valence-band maximum as well as the conduction-band minimum, each with a nearly isotropic effective mass, to occur at the zone centre in agreement with experimental results. For , we find that the six valleys for the valence-band maximum are located in the mirror planes of the Brillouin zone and they have a highly anisotropic effective mass, leading to an agreement between the de Haas - van Alphen data for the p-doped samples and the calculated Fermi surface. The calculated conduction band, however, has only two minima, instead of the six minima indicated from earlier experiments. The calculated Seebeck coefficients for both p-type and n-type materials are in agreement with the experiments.

Crystal growth of alkali metal tungsten brozes M xWO 3 (M  K, Rb, Cs), and their optical properties

Attempts have been made to grow crystalsof alkali metal hexagonal (HTB) and tetragonal (TTB) tungsten bronzes (M x WO 3) by chemical transport in a temperature gradient. Different transport agents were used (HgCl 2, HgBr 2, HgI 2, Cl 2, PrCl 3). Needle-shaped crystals of up to 6 mm in length could be grown using only HgCl 2 and HgBr 2. It was observed that the size of HTB crystals and the transport rate drop with increasing content of talkali metal. TTB crystals larger than 0.1 mm in dimensions could not be prepared. The electrical properties can be explained by using the Drude free carrier model. According to this there are plasma 20 000 cm −1. The spectral properties can be explained by using the Drude free carrier model. According to this there are plasma frequencies which are shifted systematically towards larger values x 1 and anisotropic effective masses m ∗(E⊥)≈3m ∗(E/ic) with m ∗(E⊥c) between 0.5 and 1.3 which also increase systematically with respect to x. An exclusion from Drude behavior is observed for K-HTB for the E⊢ c polarization. For these cases the line profile can better be described using a Lorentz-oscillator model.

Momentum dependence of the dimensionality of the electronic states in heterostructures

Bound states of electrons (holes) in quantum wells and wires with asymmetric barriers can exist in bounded regions of two-and one-dimensional momentum space, respectively. As the corresponding momentum increases, both the disappearance (increase of dimensionality) and appearance (decrease of dimensionality) of bound states as well as the existence of a sequence of several such transformations of dimensionality are possible. In the case of anisotropic effective masses in the quantum wells and barriers, the forms of the lines of disappearance and appearance of bound states are different from the forms of the isoenergy lines. Therefore there is a finite energy interval (i.e., electron density interval) where bound states exist on only a part of an isoenergy line. The dimensionality of the states can be controlled with an electric field; this should be observable in a number of the experiments discussed.

Scaling in Angle Resolved Electrical Transport Measurements on $\mathsf{\kappa}$-(BEDT-TTF)$\mathsf{_2}$Cu(NCS)$\mathsf{_2}$

We report on single crystal electrical transport measurements on k-(BEDT-TTF)2Cu(NCS)2 in an external magnetic field applied at different angles with respect to the superconducting layers. By measuring the in-plane resistivity versus temperature we can reveal a thermally activated behavior in the vortex liquid phase and are able to extract the angular dependence of the activation energy. Voltage versus current measurements lead to angle dependent values for the critical current J c . Our data can be fitted succesfully within the framework of the anisotropic effective mass model resulting in an estimate for the anisotropy parameter γ = ζ ab /ζc 40 - 110.

Optimization of the confinement energy of quantum-wire states in T-shaped GaAs/AlxGa1-xAs structures.

We report on an optimization of the wire confinement energies of the confined electronic states at the T-shaped intersection of GaAs and ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells. These structures can be produced by the cleaved edge overgrowth technique. We present an analytical model for the confinement to give insight into the basic mechanism. The optimization of the confinement energy is done by calculations in a six-band k\ensuremath{\cdot}p approximation for the valence band and in an isotropic effective-mass approximation for the conduction band. The confined valence-band states are only weakly bound at the T-shaped intersection due to the large and anisotropic hole effective masses. Employing optimized sample parameters, confinement energies for the free-electron-hole pairs are nearly doubled compared to symmetric structures, and 34 meV are predicted for a 3-nm overgrown GaAs well. This is expected to be further enhanced by the Coulomb interaction, that is neglected in the numerical model. The experimental structures grown using the optimized geometry show wire confinement energies of up to 54 meV, which is significantly larger than KT at room temperature and larger than previously reported. \textcopyright{} 1996 The American Physical Society.

Surface superconductivity in the heavy-fermion superconductor UPt3.

We present a study of the surface critical field ${\mathit{H}}_{\mathit{c}3}$(\ensuremath{\Phi},\ensuremath{\Theta},T) measured for two needlelike whiskers of ${\mathrm{UPt}}_{3}$. Dominant surface effects were observed in the angular dependence of the critical field by means of ac-resistivity measurements. These surface superconductivity effects show a surprisingly nonlinear thermal variation of ${\mathit{H}}_{\mathit{c}3}$ contrary to behavior expected from conventional theory, where ${\mathit{H}}_{\mathit{c}3}$/${\mathit{H}}_{\mathit{c}2}$ = 1.69 is predicted. The ratio ${\mathit{H}}_{\mathit{c}3}$/${\mathit{H}}_{\mathit{c}2}$ is strongly depressed from its initial value 1.7 when going from the A to the C phase as the temperature is decreased. It seems to remain constant in the C phase for even lower T. Nevertheless, for temperatures close to ${\mathit{T}}_{\mathit{c}+}$ it is possible to describe the angular behavior of ${\mathit{H}}_{\mathit{c}3}$(\ensuremath{\Theta},\ensuremath{\Phi}) with a standard model by introducing an effective-mass anisotropy of the heavy quasiparticles. These results are compared to recent ${\mathit{H}}_{\mathit{c}3}$ calculations for different representations of the order parameter and seem to provide a direct evidence for the suppression of one component of the order parameter at the surface. The restrictions imposed by these measurements on the choice of the representations of the unconventional order parameter will be discussed by also taking into account the limitations imposed due to the temperature dependence of the basal plane ${\mathit{H}}_{\mathit{c}2}$ modulation. \textcopyright{} 1996 The American Physical Society.

Anisotropic Magnetic Response in the Superconducting Mixed State of UPt3.

Equilibrium magnetization ${M}_{\mathrm{eq}}$ of the superconducting mixed state of UP${\mathrm{t}}_{3}$ has been measured as a function of field. At low temperatures, the discontinuity of ${\mathrm{dM}}_{\mathrm{eq}}/dH$ at the upper critical field ${H}_{c2}$ exhibits marked anisotropy between the two principal directions of the hexagonal crystal, being indiscernibly small for $H\ensuremath{\perp}c$, where the normal state paramagnetic susceptibility is largest. The results are not simply explained by the effective mass anisotropy nor by the ordinary paramagnetic effect of a spin-singlet pairing; they are rather in favor of an odd-parity pairing with an appreciable anisotropy in the pair-spin correlation.

Plasmapolaron selfenergy and effective mass in uniaxial polar crystals

AbstractWe study the polaron effects in a uniaxial polar semiconductor with electrons in the conduction band. We describe the system as an external charge interacting with three boson fields: two optical longitudinal phonons and a plasmon. The combined effects of these three fields lead to a quasiparticle which we call plasmapolaron. The selfenergy and the anisotropic effective mass of the plasmapolaron as a function of the anisotropy parameters and the electron density are calculated in the intermediate regime for the electron‐phonon and electron‐plasmon coupling. We find that, when the crystal has high polarizability along the c‐axis and a large polaron radius in the a‐b plane, the screening effects due to the plasmons on the electron‐phonon interaction are small, so that the latter is present even at relatively high electron density. In these conditions the induced charges are localized mainly on the a‐b plane. We also calculate the selfenergy of the plasmapolaron in the strong coupling limit and compare it with that calculated in the intermediate regime. We find that, increasing the electron density, the selfenergy in the intermediate regime becomes always lower than that in the strong limit. A possible connection of our results with the high‐Tc superconductors is discussed.

Guided modes in an arbitrarily oriented Si-quantum wire and their control

The eigenmodes of electron wave propagating through a Si-quantum wire are aanalyzed self-consistently by using a finite element method. The anisotropic effective mass tensor of the conduction band valleys in Si and the continuity of the probability current densities at the SiSiO 2 interface are duly taken into account. As a result, the eigenenergies of electron wave, the electron density distribution and their control by the gate voltage are presented.

Electronic transmission coefficient for solid‐state structures with spatially variable anisotropic electronic effective mass

AbstractThe method for calculating the electronic transmission coefficient for elastic electronic transport through a layered structure with spatially variable anisotropic effective mass and one‐electron potential energy is proposed. In this method, the transmission coefficient is evaluated by means of a certain matrix quantity assigned to any region of the sample. Thismatrix is determined by functional shapes of the reciprocal effective mass tensor and a one‐particle potential energy in a given region as wellas by the energy and group velocity of an incident electron. As an example, the transmission coefficients are calculated for a triple‐barrier structure as well as for a structure obtained by replacing the inner potential barrier by an anisotropic conductive layer.

Two‐photon absorption and magnetoabsorption of P excitons in β‐ZnP2

AbstractElectronic transitions in biaxial β‐ZnP2 are studied by two‐photon absorption. The anisotropy of the crystal leads to a large energy splitting of the P excitons. The energy shift of the P excitons and Landau transitions are observed in a magnetic field up to 6T. We present a theory which describes the anisotropic P excitons in a magnetic field. The analysis of the experimental data allows the determination of the anisotropic dielectric constants and effective masses of the valence and conduction bands.

Temperature dependence of upper critical field and anisotropy of YBa 2Cu 3O 7

Like the conventional superconductors, the temperature dependence of upper critical field ( B c2) and anisotropy of effective mass (γ) of YBa 2Cu 3O 7 can be determined from the peak position of the specific heat in magnetic fields. It is argued that the upper critical field at T = 0 K ( B c20) cannot be extrapolted from the measurement in the critical region. The procedure to obtain B c2 and a possible way to extrapolate B c20 from specific-heat measurements is presented.

Effective mass anisotropy and many-body effects in 2D GaAs/(Ga,Al)As hole gases observed in very high magnetic fields: comparison of theory and experiment

The effective masses of two-dimensional GaAs/(Ga,Al)As hole systems grown with (100), (311)B, (211)B, (111)B and (110) substrate orientations have been measured in magnetic fields of up to 80 T. The measured anisotropy is discussed in terms of bulk effects using a newly developed theory. Low energy (millimetre wave) cyclotron resonance absorptions show a 15% shift to higher fields in the temperature range 1.4 to 4.2 K over two orders of magnitude of energy. Many-body effects are used to discuss this observation.

Superconducting gap symmetries in YBa2Cu3O7- delta compatible with electronic Raman efficiencies and the local-density-approximation band structure.

Superconductivity induced absolute electronic Raman efficiencies are presented for single-domain ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ in a-b plane polarization geometries and compared with spectra calculated from gauge invariant theory using realistic Raman vertices derived from the local-density-approximation band structure. Effective mass fluctuations, screening, gap size, and anisotropy on the multisheeted Fermi surface are systematically investigated. It is argued that the observed anisotropy of the ${\mathit{A}}_{1\mathit{g}}$, ${\mathit{B}}_{2\mathit{g}}$, and ${\mathit{B}}_{1\mathit{g}}$ spectra requires two different gaps on the ${\mathrm{CuO}}_{2}$ plane Fermi surfaces of even and odd parity. Respective time reversal invariant gaps of ${\mathit{B}}_{1\mathit{g}}$ and ${\mathit{A}}_{1\mathit{g}}$ symmetry (e.g., ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{even}}$=${\mathit{d}}_{{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}}$, ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{odd}}$=0.5is) or of ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{A}}_{2\mathit{g}}$ symmetry (e.g., ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{even}}$=${\mathit{s}}_{\mathrm{|}{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}\mathrm{|}}$, ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{odd}}$=${\mathit{s}}_{\mathit{xy}({\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2})}$) are found compatible with the Raman data, band-structure calculations, and with corner-superconducting quantum interference device and c-axis tunneling results. \textcopyright{} 1996 The American Physical Society.