Anisotropic Effective Mass Research Articles

Electron-Hole Bilayer TFET: Experiments and Comments

We investigate Si/Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.85</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> fully depleted-SOI tunnel FET (TFET) devices operated in the electron-hole bilayer (EHB) mode. The application of negative bias on front gate and positive bias on back gate results in confined hole and electron layers that are expected to enable vertical band-to-band tunneling (BTBT). The idea of the EHB-TFET device is to enhance the tunneling current by expanding the BTBT generation area from the narrow lateral source/channel junction to the entire channel region. Our systematic measurements on a variety of TFETs with variable geometry and channel materials do not offer support to this attractive concept. Self-consistent simulations confirm that the vertical BTBT transitions do not produce an appreciable current in our devices, due to size-and bias-induced quantization, effective mass anisotropy, and incomplete formation of the bilayer. We examine the conditions for efficient vertical BTBT to occur and show that they cannot be met simultaneously, at least in Si or Si/SiGe devices.

Low anisotropy of the upper critical field in a strongly anisotropic layered cuprateBi2.15Sr1.9CuO6+δ:Evidence for a paramagnetically limited superconductivity

We study angular-dependent magnetoresistance in a low-${T}_{c}$ layered cuprate ${\mathrm{Bi}}_{2.15}{\mathrm{Sr}}_{1.9}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$. The low ${T}_{c}\ensuremath{\sim}4$ K allows complete suppression of superconductivity by modest magnetic fields and facilitates accurate analysis of the upper critical field ${H}_{c2}$. We observe a universal exponential decay of fluctuation conductivity in a broad range of temperatures above ${T}_{c}$ and propose a method for extraction of ${H}_{c2}(T)$ from the scaling analysis of the fluctuation conductivity at $T&gt;{T}_{c}$. Our main result is observation of a surprisingly low ${H}_{c2}$ anisotropy $\ensuremath{\sim}2$, which is much smaller than the effective mass anisotropy of the material $\ensuremath{\sim}300$. We show that the anisotropy is decreasing with increasing field and saturates at a small value when the field reaches the paramagnetic limit. We argue that the dramatic discrepancy of high-field and low-field anisotropies is clear evidence for paramagnetically limited superconductivity.

Exchange coupling between silicon donors: The crucial role of the central cell and mass anisotropy

Donors in silicon are now demonstrated as one of the leading candidates for implementing qubits and quantum information processing. Single qubit operations, measurements and long coherence times are firmly established, but progress on controlling two qubit interactions has been slower. One reason for this is that the inter donor exchange coupling has been predicted to oscillate with separation, making it hard to estimate in device designs. We present a multivalley effective mass theory of a donor pair in silicon, including both a central cell potential and the effective mass anisotropy intrinsic in the Si conduction band. We are able to accurately describe the single donor properties of valley-orbit coupling and the spatial extent of donor wave functions, highlighting the importance of fitting measured values of hyperfine coupling and the orbital energy of the $1s$ levels. Ours is a simple framework that can be applied flexibly to a range of experimental scenarios, but it is nonetheless able to provide fast and reliable predictions. We use it to estimate the exchange coupling between two donor electrons and we find a smoothing of its expected oscillations, and predict a monotonic dependence on separation if two donors are spaced precisely along the [100] direction.

Angular dependence of the peak effect in [formula omitted

Evolution of the peak effect with the angle between the c-axis and the external magnetic field, using pure and carbon substituted MgB2 single crystals field is presented. For given angle Θ(c,H) the peak effect appears only for H>H∗(Θ), where H∗(Θ) increases with the angle for the crystals x=0 and x=0.04, while practically remains constant for x=0.1 The peak critical current diminishes with angle and disappears when the magnetic field is parallel to the c-axis. Both the irreversibility and the second peak lines, for each temperature, can be reproduced by the equation H(Θ)=H(0)/cos2Θ+(1/γ)2sin2Θ, for at least Θ<80°, with the same effective mass anisotropy parameter, γ. This type of scaling implies that the peak effect and the irreversibility lines are driven by thermal fluctuations. The relation of the experimental results with the type of pinning centers producing the peak effect will be discussed.

Broken symmetries in URu2Si2

To resolve the nature of the hidden order below 17.5 K in the heavy-fermion compound URuSi, identifying which symmetries are broken below the hidden-order transition is one of the most important steps. Several recent experiments on the electronic structure have shown that the Fermi surface in the hidden-order phase is quite close to the result of band-structure calculations within the framework of itinerant electron picture assuming the antiferromagnetism. This provides strong evidence for the band folding along the axis with the ordering vector of , corresponding to broken translational symmetry. In addition to this, there is growing evidence for fourfold rotational symmetry breaking in the hidden-order phase from measurements of the in-plane magnetic anisotropy and the effective mass anisotropy in the electronic structure, as well as the orthorhombic lattice distortion. This broken fourfold symmetry gives a stringent constraint that the symmetry of the hidden-order parameter should belong to the degenerate -type irreducible representation. We also discuss a possibility that time reversal symmetry is also broken, which further narrows down the order parameter that characterizes the hidden order.

Interstitial channels that control band gaps and effective masses in tetrahedrally bonded semiconductors.

We find that electron states at the bottom of the conduction bands of covalent semiconductors are distributed mainly in the interstitial channels and that this floating nature leads to the band-gap variation and the anisotropic effective masses in various polytypes of SiC. We find that the channel length, rather than the hexagonality prevailed in the past, is the decisive factor for the band-gap variation in the polytypes. We also find that the floating nature causes two-dimensional electron and hole systems at the interface of different SiC polytypes and even one-dimensional channels near the inclined SiC surface.

Calculation of the electron mobility for the Δ1 model of the conduction band of germanium single crystals

The concentration dependences of the charge-carrier mobility are obtained for the Δ1 model of the conduction band of n-Ge crystals on the basis of anisotropic scattering at 77 K. It is shown that the absolute-minimum inversion of the L 1-Δ1 type caused by single-axis pressure on n-Ge crystals along the [100] crystallographic direction substantially decreases the charge-carrier mobility. This is explained by a decrease in the relaxation time because the effective electron masses differ only slightly in terms of different minima. For the other two cases of inversion of the L 1-Δ1 absolute minimum under hydrostatic and single-axis pressure along the [110] crystallographic direction, a decrease in the electron mobility is caused mainly by an increase in the effective mass. It is shown also that it is the degree of effective-mass anisotropy that substantially affects the efficiency of charge-carrier scattering in anisotropic semiconductors in this case.

Electronic states and wavefunctions of diatomic donor molecular ions in silicon: multi-valley envelope function theory

Using the Burt–Foreman envelope function theory and effective mass approximation, we develop a theoretical model for an arbitrary number of interacting donor atoms embedded in silicon which reproduces the electronic energy spectrum with high computational efficiency, taking into account the effective mass anisotropy and the valley–orbit coupling. We show that the variation of the relative magnitudes of the electronic coupling between the donor atoms with respect to the valley–orbit coupling as a function of the internuclear distance leads to different kinds of spatial interference patterns of the wavefunction. We also report on the impact of the orientation of the diatomic phosphorus donor molecular ion in the crystal lattice on the ionization energy and on the energy separation between the ground state and the lowest excited state.

Simulation Analysis of Anisotropy of Electron Transport in Silicon

Electron transport in silicon is analyzed using Ensemble Monte Carlo simulation based on analytic band model. For <001>, larger valley occupation in X valleys on the kz-axis with smaller drift velocity leads to stronger anisotropy of total electron drift velocity. Generalizing from this result, larger valley occupation in X valleys with smaller drift velocity leads to stronger anisotropy of total electron drift velocity. This phenomenon is due to the anisotropy of electron effective masses in X valley.

Anisotropy of effective electron masses in highly doped nonpolar GaN

The anisotropic effective electron masses in wurtzite GaN are determined by generalized infrared spectroscopic ellipsometry. Nonpolar (112¯0) oriented thin films allow accessing both effective masses, m⊥* and m∥*, by determining the screened plasma frequencies. A n-type doping range up to 1.7 × 1020 cm−3 is investigated. The effective mass ratio m⊥*/m∥* is obtained with highest accuracy and is found to be 1.11 independent on electron concentration up to 1.2 × 1020 cm−3. For higher electron concentrations, the conduction band non-parabolicity is mirrored in changes. Absolute values for effective electron masses depend on additional input of carrier concentrations determined by Hall effect measurements. We obtain m⊥*=(0.239±0.004)m0 and m∥*=(0.216±0.003)m0 for the parabolic range of the GaN conduction band. Our data are indication of a parabolic GaN conduction band up to an energy of approximately 400 meV above the conduction band minimum.

Anisotropy of the electron effective mass in rutile SnO2 determined by infrared ellipsometry

AbstractGeneralized infrared spectroscopic ellipsometry (GIRSE) was applied to Sb‐doped rutile SnO2(101) films grown by plasma‐assisted molecular beam epitaxy (PAMBE). Coupled longitudinal‐optical (LO) phonon–plasmon (LPP) modes for the two principal polarization directions of the optically anisotropic SnO2 were identified and their frequencies were determined as a function of electron concentrations obtained by Hall effect measurements. The analysis of these modes yielded very accurate values for the plasma frequencies and finally the anisotropy of the electron effective masses as a function of carrier density. Comparison to Hall effect electron concentrations yielded a non‐parabolicity of the conduction band.

First-principles electronic functionalization of silicene and germanene by adatom chemisorption

This study presents first-principles results on the electronic functionalization of silicene and germanene monolayers by means of chemisorption of adatom species H, Li, F, Sc, Ti, V. Three general adatom-monolayer configurations are considered, each having its distinct effect on the electronic structure, yielding metallic or semiconducting dispersions depending on the adatom species and configuration. The induced bandgap is a (in)direct Γ gap ranging from 0.2 to 2.3eV for both silicene and germanene. In general the alternating configuration was found to be the most energetically stable. The boatlike and chairlike conformers are degenerate with the former having anisotropic effective carrier masses. The top configuration leads to the planar monolayer and predominately to a gapped dispersion. The hollow configuration with V adatoms retains the Dirac cone, but with strong orbital planar hybridization at the Fermi level. We also observe a planar surface state the Fermi level for the latter systems.

Angular dependence of the resistive upper critical field of the iron-based superconductor Fe1+δ (Te,Se) in high magnetic fields

We have investigated the angular dependence of the upper critical fields (µ0Hc2) of a single crystal of Fe1+δ(Te,Se) in pulsed high magnetic fields up to 52 T. At T = 10 K, the µ0Hc2 increases with increasing θ, which is the angle between the c-axis and the direction of the applied magnetic field. Using the effective-mass model based on the three-dimensional anisotropic Ginzburg-Landau (G-L) theory, we estimate the effective-mass anisotropy ratio γ = (mc* /mab*)1/2 to be 1.6 at 10 K. This result is consistent with that of our previous study on the anisotropy coefficient of the resistive upper critical field, Γ ≡ Hc2ab/Hc2c.

Multicomponent Skyrmion Lattices and Their Excitations

We study quantum Hall ferromagnets with a finite density of topologically charged spin textures in the presence of internal degrees of freedom such as spin, valley, or layer indices, so that the system is parametrized by a d-component spinor field. In the absence of anisotropies we find a hexagonal Skyrmion lattice that completely breaks the underlying SU(d) symmetry with the low-lying excitation spectrum separating into d(2) - 1 gapless acoustic magnetic modes and a magnetophonon. The ground state charge density modulations, which inevitably exist in these lattices, vanish exponentially in d. We discuss the role of effective mass anisotropy for SU(3)-valley Skyrmions relevant to experiments with AlAs quantum wells. Here we find a transition which breaks a sixfold rotational symmetry of the triangular lattice, followed by the formation of a square lattice at large values of anisotropy strength.

Electron mass anisotropy of BaZrO3 doped YBCO thin films in pulsed magnetic fields up to 30 T

The high anisotropy of the critical current density in the high-temperature superconductor YBa2Cu3O6+x can be compensated by changing the pinning landscape, for example, through doping with BaZrO3. We measure the change due to BaZrO3 doping in the effective electron mass anisotropy, γ, by resistive measurements of the upper critical field in pulsed high magnetic fields. It is found that the angular dependence of the upper critical field follows the Blatter scaling up to 30 T but the irreversibility field does not. We also report a significant reduction in the effective electron mass anisotropy from 6.0 to 3.4 by BaZrO3 doping.

Anisotropy of effective masses in CuInSe2

Anisotropy of the valence band is experimentally demonstrated in CuInSe2, a key component of the absorber layer in one of the leading thin-film solar cell technology. By changing the orientation of applied magnetic fields with respect to the crystal lattice, we measure considerable differences in the diamagnetic shifts and effective g-factors for the A and B free excitons. The resulting free exciton reduced masses are combined with a perturbation model for non-degenerate independent excitons and theoretical dielectric constants to provide the anisotropic effective hole masses, revealing anisotropies of 5.5 (4.2) for the A (B) valence bands.

Anisotropic Magnetic Fluctuations and Superconductivity in Ferromagnetic Superconductor UCoGe

We have performed 59Co NQR / NMR measurements on the single-crystalline UCoGe, in order to investigate the relationship between ferromagnetism and superconductivity. The measurements of Knight-shift and nuclear spin-lattice relaxation rate provide clear evidence that both static and dynamic susceptibilities are ferromagnetic with Ising anisotropy. In addition, Hc2 also shows extremely large anisotropy which can not be explained by the GL model with anisotropy of effective mass. These data suggest intimate relationship between Ising magnetization and anisotropic superconductivity in UCoGe.

Occurrence of a gap in the spectrum of magnetoplasma excitations of a two-dimensional electron disk subjected to a strong in-plane magnetic field

The effect of a parallel magnetic field on the dispersion of bulk and edge magnetoplasmons in a disk-shaped two-dimensional electron system is investigated. It is found that the anisotropy of the electron effective mass that appears in a parallel magnetic field lifts the degeneracy of plasma oscillations in the disk. This is accompanied by the occurrence of a gap in the spectrum of magnetoplasmons, and the magnetic dispersion of these excitations changes from a linear to a parabolic one. The width of the gap is determined by the difference between the frequencies of plasma oscillations along the field and transverse to the field and grows quadratically with the in-plane field strength.

Microstructural design and experimental validation of elastic metamaterial plates with anisotropic mass density

A microstructure design of anisotropic resonant inclusions is investigated for the elastic metamaterial plate with the aid of the numerically based effective medium model. Experimental validation is then conducted in the anisotropic metamaterial plate through both harmonic and transient wave testing, from which the anisotropic effective dynamic mass density, group, and phase velocities are determined as functions of frequency. The strongly anisotropic mass density along two principal orientations is observed experimentally and the prediction from the experimental measurements agrees well with that from the numerical simulation. Finally, based on the numerically obtained effective dynamic properties, a continuum theory is developed to simulate different guided wave modes in the elastic metamaterial plate. Particularly, high-order guided wave coupling and repulsion as well as the preferential energy flow in the anisotropic elastic metamaterial plate are discussed.

Angular-dependences of giant in-plane and interlayer magnetoresistances in Bi2Te3 bulk single crystals

Angular-dependences of in-plane and interlayer magnetotransport properties in n-type Bi2Te3 bulk single crystals have been investigated over a broad range of temperatures and magnetic fields. Giant in-plane magnetoresistances (MR) of up to 500% and interlayer MR of up to 200% were observed, respectively. The observed MR exhibits quadratic field dependences in low fields and linear field dependences in high fields. The angular dependences of the MR represent strong anisotropy and twofold oscillations. The observed angle-dependent, giant MR might result from the strong coulomb scattering of electrons as well as impurity scattering in the bulk conduction bands of n-type Bi2Te3. The strong anisotropy of the MR may be attributable to the anisotropy of electron mobility, effective mass, and relaxation time in the Fermi surface. The observed giant anisotropic MR in n-type Bi2Te3 bulk single crystals paves the way for Bi2Te3 single crystals to be useful for practical applications in magnetoelectronic devices such as disk reading heads, anisotropic magnetic sensors, and other multifunctional electromagnetic applications.