Impurity-induced tuning of quantum-well States in spin-dependent resonant tunneling

We report exact model calculations of the spin-dependent tunneling in double magnetic tunnel junctions in the presence of impurities in the well. We show that the impurity can tune selectively the spin channels giving rise to a wide variety of interesting and novel transport phenomena. The tunneling magnetoresistance, the spin polarization, and the local current can be dramatically enhanced or suppressed by impurities. The underlying mechanism is the impurity-induced shift of the quantum well states (QWSs), which depends on the impurity potential, impurity position, and the symmetry of the QWS.     

Hydrogen tunneling modes in α-Mn suppressed by elastic stresses

Behavior of the tunneling mode of hydrogen in MnH_0.04 and MnH_0.07 under high pressures in sapphire anvils was studied by the method of incoherent inelastic neutron scattering (INS). It is established that the INS peak corresponding to the hydrogen tunneling in a double-well potential disappears at a pressure of 0.8 GPa in a quasi-hydrostatic regime, while being retained without visible changes under pure hydrostatic conditions. An analogous, albeit weaker, suppression of the tunneling mode takes place upon grinding of a freshly prepared sample. The effect of suppression of the hydrogen tunneling modes by applied inhomogeneous elastic stresses is explained by a shift of the energy levels in the adjacent wells caused by the static displacements.     

Atomic tunneling states in low-temperature dielectric and elastic losses of diamagnetic glassy semiconductors

A mechanism of low-temperature dielectric and elastic losses in some diamagnetic amorphous structures (e.g. glassy semiconductors) is presented. The mechanism is associated with atomic tunneling states which (as earlier stated) cause also the low-temperature anomalies of thermal properties of such systems. Some related questions are also discussed.     

Spin-polarized tunneling spectroscopy of co(0001) surface States

Spin-polarized tunneling was studied on Co surfaces of exchange coupled Co/Cu/Co samples, using an Fe tip. A spin-polarized surface state of Co(0001) was found to exist at -0.43 eV relative to E(F), with FWHM of 0.23 eV in the spectra. The state exhibits negative magnetoresistance with an effective spin polarization of less than -23%, suggesting negatively high spin polarization of the surface state. Our first-principles calculations have supported the existence of the surface state. From the calculation, the state is identified as a minority spin Gamma-centered d(2)(z)-like surface state.     

A new approach to the elastic tunneling current in metal-insulator-metal diodes

A new formulation of the elastic coherent tunneling current has been developed. Electron transport is described by quasifree electrons in the metals and by an effective one-particle barrier in the insulator. This phenomenological theory needs three parameters to characterize the barrier (barrier height φ, tunneling length s and an asymmetric parameter V_b). Four independent quantities containing these three parameters have been experimentally determined to check the consistency of this theory and an excellent agreement has been found. Finally, the theory is extended to a trapezoidal barrier explaining the experimental results obtained on AlIAl diodes.     

Resonant tunneling and a nonlinear response in RF fields

The problem of resonant tunneling through a double-barrier nanostructure in a strong alternating electric field is solved completely. To this end, a perturbation method is proposed. Electron wavefunctions and a nonlinear response are obtained in analytical form over wide ranges of field frequencies and amplitudes, using the perturbation method and the semiclassical approximation. The semiclassical expression for the current allows for contributions of all orders with respect to the field, i.e., electron transitions with the emission and absorption of any number of photons. This enables one to find the limits of resonant current and output power. The case of ?ω?Γ is considered, where ? is the rationalized Planck constant, ω is the field frequency, and Γ is the resonance level width. It is established that the maximum resonant current is approximately as high as half the resonant constant current. For the quantum regime of oscillation, the output power can be 10⁶–10⁷ W/cm² at ω=10¹³ s^?1 and the output power rises with ω, in contrast to the well-known classical regime, where the power decreases rapidly.     

Inhomogeneous elastic response of silica glass

Using large scale molecular dynamics simulations we investigate the properties of the nonaffine displacement field induced by macroscopic uniaxial deformation of amorphous silica, a strong glass according to Angell's classification. We demonstrate the existence of a length scale xi characterizing the correlations of this field (corresponding to a volume of about 1000 atoms), and compare its structure to the one observed in a standard fragile model glass. The "boson-peak" anomaly of the density of states can be traced back in both cases to elastic inhomogeneities on wavelengths smaller than xi where classical continuum elasticity becomes simply unapplicable.     

A modular scanning tunneling microscope with an interchangeable elastic closed cell and external actuators

We introduce a novel modular cell based scanning tunneling microscope with external piezoelectric actuators. A tip and a sample are contained in a closed interchangeable cell, consisting of a stiff top plate and a bottom part, fastened together by an elastic material. The bottom part, containing a scanning tip, is fastened to a base unit while the top plate, containing a sample, is capable of scanning motion by external piezoelectric actuators mounted in the same base unit. The actuators are pre-loaded by the deformation of the elastic material of the cell, giving an increased stability. This design is expected to simplify the scanning tunneling microscope (STM) operation in difficult environments greatly by enclosing only the tip and sample in a small cell-module, which is pluggable to a scanning mechanism and other supportive functionalities. A frequency characterization and an image scan showing atomic resolution of highly oriented graphite in air, at room temperature, is presented.     

High-pressure properties of group IV clathrates

We review the investigations on clathrate materials having C, Si, Ge or Sn framework atoms in which the pressure parameter plays an important role. In this article, we studied the synthesis of clathrates, superconducting properties and those studies relative to the structural cohesivity and phase diagrams. We have tried to extensively review these subjects. Key references on other important properties of the group IV clathrates are provided in the introductory sections. As a main result of this review, we note that pressure appears as a key parameter for the elaboration of present and future clathrates in particular those exhibiting superconducting properties. We show how high-pressure research has also played an important role in the understanding of the parameters governing clathrate superconductivity. In contrast, the study of the structural evolution of group IV clathrates (bulk modulus and stability) gives abundant clues for tailoring new materials with improved mechanical properties. Finally, practically all investigations converge to point out that today hypothetical carbon clathrates are candidates for extraordinary superconducting and mechanical properties.     

Single-dot spectroscopy via elastic single-electron tunneling through a pair of coupled quantum dots

We study the electronic structure of a single self-assembled InAs quantum dot by probing elastic single-electron tunneling through a single pair of weakly coupled dots. In the region below pinch-off voltage, the nonlinear threshold voltage behavior provides electronic addition energies exactly as the linear, Coulomb blockade oscillation does. By analyzing it, we identify the s and the p shell addition spectrum for up to six electrons in the single InAs dot, i.e., one of the coupled dots. The evolution of the shell addition spectrum with magnetic field provides Fock-Darwin spectra of the s and p shells.     

Thermodynamic properties of clathrates: I. The heat capacity and entropy of argon in the argon quinol clathrates

Measurements have been made of the heat capacity C_p from ～ 13°k to ～ 273°k of five clathrates of argon and β-quinol. The argon content ranged from ～ 20 per cent to ～ 80 per cent of the maximum possible amount. Over much of the temperature range studied, C_p proved to be a linear function of the argon content, but from 13°k to 20°k, and from 50°k to 100°k the relation between C_p and argon content is obscure, and may, in fact, be non-linear. Estimates have been made of the contribution to C_p made by a mole of argon in the temperature region where C_p is a linear function of composition, and these experimental values have been compared with those calculated according to the theory of J. H. van der Waals, which is based on the cell model of Lennard-Jones and Devonshire. The agreement between theory and experiment is satisfactory.     

Exceptional ideal strength of carbon clathrates

We study by means of ab initio calculations the ideal tensile and shear strengths of the C-46 clathrate phase. While its bulk modulus and elastic constants are smaller than in diamond, its strength is found to be in all directions larger than the critical stresses associated with the diamond [111] planes of easy slip. This can be related to the frustration by the clathrate cage structure of the diamond to graphite instability under nonhydrostatic stress conditions [corrected] The criteria for designing strong materials are discussed.     

Theory of inelastic tunneling spectroscopy of a single molecule - Competition between elastic and inelastic current

A theory of inelastic electron tunneling spectroscopy of a single molecule with scanning tunneling microscope is presented using the Keldysh Green’s function method for an adsorbate-induced resonance coupled to the molecular vibration. It is found that the correction to the tunneling current is expressed in terms of the transmission probability; the correction is negative for the elastic part of the current and positive for the inelastic one. The differential conductance (dI/dV) exhibits an increase or decrease at the threshold corresponding to the opening of inelastic channel depending on the sign of the correction, and the size of this conductance jump is scaled with the vibrational damping due to electron-hole pair excitation. The lineshape of d²I/dV²-spectra calculated using a renormalized adsorbate Green’s function evolves from an antisymmetric dip to a peak through the derivative-like one as the position of the adsorbate resonance recedes from the Fermi level of the substrate.     

Pressure induced phase transformation of semiconductor clathrates

Pressure induced phase transformation and amorphization for Ge-based type-I clathrates have been investigated by means of synchrotron XRD and Raman experiments under high pressure. The XRD results of Sr₈Ga₁₆Ge₃₀, Ba₈Ga₁₆Ge₃₀, and I₈Sb₈Ge₃₈ demonstrated volume collapse phase transitions at 18, 33, and 42 GPa, respectively. Reitveld analyses performed for I₈Sb₈Ge₃₈ reveal a deformation of six-member rings of 14-hedron cages with increasing pressure.     

Thermoelectric properties of tin clathrates under hydrostatic pressure

We report the temperature dependence of electrical resistance (R) and thermopower (S) of clathrate Cs₈Sn₄₄ under high pressure up to 1.2 GPa. We observe a reversible gap widening, prominent relaxation effect of R, irreversible increase of |S| under high pressure. We also find that the power factor S²σ (σ: electrical conductivity) reaches a maximum at pressure of 0.3 GPa. Comparison of the experimental results with band structure calculations suggests that the intrinsic vacancy in the clathrate structure of Cs₈Sn₄₄ plays an important role in transport properties under high pressure. Measurements on Cs₈Zn₄Sn₄₂, a clathrate which has defects other than vacancies, are compared with Cs₈Sn₄₄. The results indicate that replacing Sn by Zn has similar effect as the intrinsic vacancy on S.     

Thin-film formation of Si clathrates on Si wafers

In this study, we prepared Si clathrate films (Na₈Si₄₆ and Na_xSi₁₃₆) using a single-crystalline Si substrate. Highly oriented film growth of Zintl-phase sodium silicide, which is a precursor of Si clathrate, was achieved by exposing Na vapour to Si substrates under an Ar atmosphere. Subsequent heat treatment of the NaSi film at 400 °C (3 h) under vacuum (<10⁻² Pa) resulted in a film of Si clathrates having a thickness of several micrometres. Furthermore, this technique enabled the selective growth of Na₈Si₄₆ and Na_xSi₁₃₆ using the appropriate crystalline orientation of Si substrates.     

Structural principles and amorphouslike thermal conductivity of Na-doped Si clathrates

The postulated low thermal conductivity and the possibility of altering the electronic conductivity of metal-doped clathrates with semiconducting host elements have stimulated great interest in exploring these compounds as promising thermoelectric materials. The electronic and thermal properties of the prototypical Na xSi (46) system are studied in detail here. It is shown that, despite the fact that the Na/Si clathrate is metallic, its thermal conductivity resembles that of an amorphous solid. A theoretical model is developed to rationalize the structural stability of the peculiar structural topology, and a general scheme for rational design of high efficiency thermoelectric materials is presented.