Thermoelectric power in superlattices of nonparabolic semiconductors with graded interfaces under magnetic quantization

Summary We study the thermoelectric power of the electrons under magnetic quantization in III–V, II–VI, PbTe/PbSnTe and strained layer superlattices with graded interfaces and compare the same with the corresponding bulk specimens of the constituent materials by formulating the respective expressions incorporating the broadening. It is found, by taking GaAs/Ga_1−x Al _x As, CdS/CdTe, PbTe/PbSnTe and InAs/GaSb superlattices with graded interfaces as examples, that the thermoelectric power exhibits oscillatory dependence with the inverse quantizing magnetic field due to Shubnikov-de Hass effect and increases with decreasing electron concentration in an oscillatory manner in all the aforementioned cases. The thermopower in graded superlattices is greater than that of constituent bulk materials together with the fact that the oscillations in superlattices show up much more significantly as compared to the respective constituent materials. In addition, the well-known expressions for bulk specimens of wide-gap semiconductors have also been obtained as special cases from our generalized expressions under certain limiting conditions.     

The nature of the vectorial photoelectric effect in the threshold energy region

Summary An immediate correlation has been proved to exist between the vectorial photoelectric effect and electronic surface structure modified due to adsorption. Experimental studies were performed for Cs overlayers on W(110) and Si(111) faces using visible exciting photons with energies below those of both bulk and surface plasmons. Considerable (more than 20 fold) growth of the effect was observed forp-polarized light in the vicinity of threshold as the light frequency approached the position of the maximum of local density in the surface-state band. A special case of the vectorial photoelectric effect forp-polarized light has been found in the absence of bulk photoemission excited bys-polarized light. Our results prove that the anomalous vectorial photoelectric effect is caused by the normal electric vector component of thep-polarized light under appropriate conditions for the particular photoemission enhancement on the surface. The effect can be related to optical absorption by the surface-state band and to subsequent electron emission into the vacuum, the normal electric vector component of thep-polarized light being essential for both processes.     

Composition dependence of photoconductivity in amorphous thin films of Se70Te30−xSbx

Summary The present paper reports the composition dependence of steady-state and transient photoconductivity in amorphous thin films of Se₇₀Te_30-xSb_x, wherex is varied from 0 to 10. The results indicate that photoconductivity is highly composition dependent in this system and a discontinuity in various electrical parameters is observed at 4 at. % of Sb. This is explained in terms of increased disorder up to 4 at. % of Sb. However, at higher concentration of Sb, an ordered structure may be established due to the formation of microcrystalline phases as observed in X-ray diffraction patterns.     

Electron transport in thin metal films having ionized impurities and unequal surface specularity parameters

Summary In this paper analytical expressions for the electrical conductivity and thermoelectric power of thin metal films have been calculated for films containing ionized impuritiesand having unequal surface specularity parameters. The effects of alteration in the energy dependence of the electron relaxation time as well as unequal probabilities of scattering from the two surfaces are thus completely taken into account.     

The electrical transport properties of thin metal films containing neutral impurities

Summary In this paper we have calculated the expressions for the electrical conductivity and the thermoelectric power of thin metal films containing unionized or neutral impurities and having equal surface specularity parameters. The calculation is based on the solutions of the Boltzmann transport equation in the presence of a weak electric field and a small temperature gradient. The results indicate that the electrical conductivity decreases whereas the thermoelectric power increases.     

On the thermoelectric power in quantum well of A 3 II B 2 V semiconductors in the presence of a classically large magnetic field

Summary An attempt is made to study the thermoelectric power of the carriers in quantum well of A ₃ ^II B ₂ ^V semiconductors, taking Cd₃As₂ quantum well as an example. It is found, on the basis of newly derived 2DE−k _s dispersion relation by including various types of anisotropies in the energy spectrum that the thermoelectric power decreases with increasing electron concentration and decreasing film thickness respectively. In addition, the corresponding well-known results for bulk specimens of isotropic parabolic energy bands are also obtained from the expressions derived.     

Comparative study of magnetic and electric field induced insulator-metal-transitions in Pr1-xCaxMnO3 films

The insulator-metal (IM)-transition in Pr_1-xCa_xMnO₃ (PCMO) is of particular interest because it can be induced by a variety of external forces, such as magnetic and electric fields, photon exposure and hydrostatic pressure. In this paper, we present a comparative study of the IM-transition in magnetic and electric fields for epitaxial thin films prepared by pulsed laser deposition. The transport data as a function of applied field or temperature give strong evidence for the presence of electronic phase separation. However, the observed different IM-transitions in magnetic and electric fields indicate that two different areas of spatially inhomogeneous electronic ground states in the phase diagram of PCMO are involved.     

Effect of the substrate ferroelastic transition on epitaxial La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films grown on LaAlO<Subscript>3</Subscript>

Thin films of La_0.7Sr_0.3MnO₃ were grown by molecular beam epitaxy on (001)LaAlO₃ crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the ferroelastic transition at T_F = 813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic irreversibility which is ascribed to the quenched disorder related to twinning and discussed in terms of coexisting ferromagnetic and spin disordered regions connected with the undeformed domain cores and strained domain walls respectively.     

Column buckling of doubly parallel slender nanowires carrying electric current acted upon by a magnetic field

Axial buckling of current-carrying double-nanowire-systems immersed in a longitudinal magnetic field is aimed to be explored. Each nanowire is affected by the magnetic forces resulted from the externally exerted magnetic field plus the magnetic field resulted from the passage of electric current through the adjacent nanowire. To study the problem, these forces are appropriately evaluated in terms of transverse displacements. Subsequently, the governing equations of the nanosystem are constructed using Euler–Bernoulli beam theory in conjunction with the surface elasticity theory of Gurtin and Murdoch. Using a meshless technique and assumed mode method, the critical compressive buckling load of the nanosystem is determined. In a special case, the obtained results by these two numerical methods are successfully checked. The roles of the slenderness ratio, electric current, magnetic field strength, and interwire distance on the axial buckling load and stability behavior of the nanosystem are displayed and discussed in some detail.     

On the relation between the fermi energies in the presence and absence of magnetic quantization in degenerate semiconductors having non-parabolic energy bands

An investigation is made of the influence of band non-parabolicity on the relation between the Fermi energies in the presence and absence of magnetic quantization in degenerate semi-conductors through derivation of a simplified form of the relation.On leave of absence from theDept. of Physics, B. N. College, Patna University, Patna 800004.     

Wannier–Stark resonances in Zener tunneling diodes with GaAs/AlAs superlattices

We present DC transport measurements of the valence to conduction band (Zener) tunneling current in ap–i–ndiode with an ultrathin intrinsic layer containing a (GaAs)₅/(AlAs)₂multi-quantum well structure. According to recent theoretical predictions, the DC current should show maxima as a function of the reverse bias voltage that reflect the formation of Wannier–Stark resonances. So far, Wannier–Stark resonances have only been observed optically and never in a regime of strong Zener tunneling. Experimentally, we find the second derivative of the current-voltage characteristics to show a weak oscillatory structure indeed, indicating the existence of Wannier–Stark resonances in Zener tunneling.     

Ground state energies of bound polarons in semiconductors in a strong magnetic field

Effect of the electron-lattice coupling on the ground state energies of shallow donors in a strong magnetic field is investigated for a few polar semiconductors. Extending the theory of Bajaj for bound polarons in the weak coupling limit, and using the ground state wave function of Yafet et al. for a shallow donor in a magnetic field, it is found that the coupling enhances the ground state energies. While this effect is negligible for most of the semiconductors, the results indicate softening of the coupling in a magnetic field.     

Effects of magnetic field on photon-induced quantum transport in a single dot-cavity system

In this study,we show how a static magnetic field can control photon-induced electron transport through a quantum dot system coupled to a photon cavity.The quantum dot system is connected to two electron reservoirs and exposed to an external perpendicular static magnetic field.The propagation of electrons through the system is thus influenced by the static magnetic and the dynamic photon fields.It is observed that the photon cavity forms photon replica states controlling electron transport in the system.If the photon field has more energy than the cyclotron energy,then the photon field is dominant in the electron transport.Consequently,the electron transport is enhanced due to activation of photon replica states.By contrast,the electron transport is suppressed in the system when the photon energy is smaller than the cyclotron energy.     

Simple theoretical analysis of the thermoelectric power in quantum dot superlattices of non-parabolic heavily doped semiconductors with graded interfaces under strong magnetic field

We study theoretically the thermoelectric power in the presence of a large magnetic field (TPM) in heavily doped III–V, II–VI, PbTe/PbSnTe, strained layer and HgTe/CdTe quantum dot superlattices (QDSLs) with graded structures on the basis of newly formulated electron energy spectra and compare the same with that of the constituent materials. It has been found, taking heavily doped GaAs/Ga_1−xAl_xAs, CdS/CdTe, PbTe/PbSnTe, InAs/GaSb and HgTe/CdTe QDSLs as examples, that the TPM increases with increasing inverse electron concentration and film thickness, respectively, in different oscillatory manners and the nature of oscillations is totally band structure dependent. We have also suggested the experimental methods of determining the Einstein relation for the diffusivity–mobility ratio, the Debye screening length and the electronic contribution to the elastic constants for materials having arbitrary dispersion laws.     

The thermoelectric power in superlattices of III–V semiconductors with graded structures under strong magnetic quantization

Summary An attempt is made to formulate the thermoelectric power under strong magnetic quantization (TPM) in superlattices (SLS) of III–V semiconductors with graded structures and to compare the same with that of the forming materials. It is found, taking Ga_0.8In_0.14P_0.78Sb_0.22/GaAs SL as an example, that the TPM increases with increasing quantizing magnetic field and decreases with increasing electron concentration respectively in an oscillatory manner. The TPM in SL with graded structures is greater than that of the constituent bulk materials for III–V SL.     

Hard axis magnetic field dependence on current-induced magnetization switching in MgO-based magnetic tunnel junctions

We conducted a detailed study of hard axis magnetic field (H_hard) dependence on current-induced magnetization switching (CIMS) in MgO-based magnetic tunnel junctions (MTJs) with various junction sizes and various uniaxial anisotropy fields. The decreases in critical current density (J_c) and the intrinsic critical current density (J_c0) estimated from the pulse duration dependence on J_c in CIMS are observed when applying H_hard for all MTJs. The decrease in energy barrier of CIMS is also observed except for the largest sample. These results indicate that the reduction of J_c is attributable to both the increase of spin-transfer efficiency and the decrease in energy barrier in the case of applying H_hard. The J_c0 decreases with increase in the mutual angle between the direction of magnetization and the easy axis (θ_f), which is consistent with the theoretical prediction proposed by Slonczewski. The degree of the reduction of J_c0 for the same value of H_hard decreases with decreasing size of MTJs. This behavior is considered to be related to not only decrease in θ_f due to the increase in anisotropy field in MTJs, but also to the increase in the variance of the initial angle of magnetization due to the thermally activated magnon excitation. The stable switching endurance related to CIMS was observed in a wide range of MTJ sizes when applying H_hard. Moreover, we proposed a new architecture and a new switching method considering write disturbance. These results would be useful for application to spin memory and other spin-electronic devices.     

Lattice Boltzmann simulation of the two-dimensional flow of a magnetic suspension between two parallel walls under a non-uniform magnetic field

We have developed a lattice Boltzmann method based on fluctuation hydrodynamics that is applicable to the flow problem of a particle suspension. In this method, we have introduced the viscosity-modifying method, rather than the velocity-scaling method, in which a modified viscosity is used for generating random forces in lattice Boltzmann simulations. The viscosity-modifying method is found to be applicable to the simulation of a magnetic particle suspension. We have applied this method to the two-dimensional Poiseuille flow of a magnetic suspension between two parallel walls in order to investigate the behavior of magnetic particles in a non-uniform applied magnetic field. From the results of the snapshots, the pair correlation function between the magnetic pole and the magnetic particles and the averaged local particle velocity and magnetization distributions, it was observed that the behavior of the magnetic particles changes significantly depending upon which factor dominates the phenomenon in the balance between the magnetic particle–particle interaction, the non-uniform applied magnetic field and the translational and rotational Brownian motion.     

Theoretical analysis of the gate capacitance inn-channel inversion layers on ternary chalcopyrite semiconductors in the presence of a quantizing magnetic field

An attempt is made to study theoretically, the dependence of the gate capacitance inn-channel inversion larges on ternary chalcopyrite semiconductors on a quantizing magnetic field, takingn-channel inversion layers on CdGeAs₂ as an example. It is found, on the basis of a newly derived electron energy spectrum of the above class of semiconductors, that the gate capacitance exhibits spiky oscillations with changing magnetic field and the oscillatory behaviour is in qualitative agreement with the experimental observation reported in the literature for the MOS structure of Hg_1–x Cd _x Te.     

Transport through a double barrier in Large Radius Carbon Nanotubes with a transverse magnetic field

We discuss the Luttinger Liquid behaviour of Large Radius Carbon Nanotube e.g. the Multi Wall ones (MWNT), under the action of a transverse magnetic field B. Our results imply a reduction with B in the value of the bulk critical exponent, α_bulk, for the tunneling density of states, which is in agreement with that observed in transport experiments. Then, the problem of transport through a Quantum Dot formed by two intramolecular tunneling barriers along the MWNT, weakly coupled to Tomonaga-Luttinger liquids is studied, including the action of a strong transverse magnetic field B. We predict the presence of some peaks in the conductance G versus B, related to the magnetic flux quantization in the ballistic regime, at a very low temperature T, and also at higher values of T, where the Luttinger behaviour dominates. The temperature dependence of the maximum G_max of the conductance peak according to the Sequential Tunneling follows a power law, G ∝T^γe-1 with γ_e linearly dependent on the critical exponent, α_end, strongly reduced by B.