Virtual Gap Research Articles

Variation of band bending at the surface of Mg-doped InGaN: Evidence ofp-type conductivity across the composition range

The variation of band bending as a function of composition at oxidized (0001) surfaces of Mg-doped ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ is investigated using x-ray photoelectron spectroscopy. Distinctly different trends in barrier height are seen for the Mg-doped compared to undoped alloys, which is explained in terms of Fermi-level pinning at the surface and virtual gap states. Solutions of Poisson's equation within the modified Thomas-Fermi approximation are used to model the band bending and corresponding variation of carrier concentration with depth below the surface. A transition from a surface inversion layer for In-rich alloys to a surface hole depletion layer for Ga-rich alloys occurs at $x\ensuremath{\approx}0.49$. The trend in barrier height, calculated space-charge profiles, and difference of barrier height for undoped and Mg-doped InN indicate that Mg doping induces bulk $p$-type conductivity across the entire composition range.

Dynamic clamp: a powerful tool in cardiac electrophysiology

Dynamic clamp is a collection of closely related techniques that have been employed in cardiac electrophysiology to provide direct answers to numerous research questions regarding basic cellular mechanisms of action potential formation, action potential transfer and action potential synchronization in health and disease. Building on traditional current clamp, dynamic clamp was initially used to create virtual gap junctions between isolated myocytes. More recent applications include the embedding of a real pacemaking myocyte in a simulated network of atrial or ventricular cells and the insertion of virtual ion channels, either simulated in real time or simultaneously recorded from an expression system, into the membrane of an isolated myocyte. These applications have proven that dynamic clamp, which is characterized by the real-time evaluation and injection of simulated membrane current, is a powerful tool in cardiac electrophysiology. Here, each of the three different experimental configurations used in cardiac electrophysiology is reviewed. Also, directions are given for the implementation of dynamic clamp in the cardiac electrophysiology laboratory. With the growing interest in the application of dynamic clamp in cardiac electrophysiology, it is anticipated that dynamic clamp will also prove to be a powerful tool in basic research on biological pacemakers and in identification of specific ion channels as targets for drug development.

Review of debris bed cooling in the TMI-2 accident

Still today after decades of severe accident research it is not well understood why the molten corium did not attack or even penetrate the lower head vessel wall in the TMI 2 accident. The findings can only be explained by additional assumptions which have been proposed by various authors. This paper is also looking for an explanation by examining the role of the debris cooling in the TMI lower head. The present knowledge of debris cooling is based on small-scale experiments with simulant debris. It is argued that the experiments have been stopped to early and therefore do not reveal the potential of debris bed coolability in case of a corium debris bed. It is also argued that in TMI the debris bed was in a state of turbulence and fluidization such that the coolabilty was much higher than in the small-scale experiments. Basically this paper hypothesizes that the unknown phenomenon in the TMI 2 accident is the strong interaction of the debris particles of a wide range of grain sizes and the strong turbulent motion of the cooling fluid and vapor mixture in conjunction with a “virtual gap” at the vessel wall.

Pseudogap, superconducting energy scale, and Fermi arcs of underdoped cuprate superconductors

Through the measurements of magnetic field dependence of specific heat in $La_{2-x}Sr_xCuO_4$ in zero temperature limit, we determined the nodal slope $v_\Delta$ of the quasiparticle gap. It is found that $v_\Delta$ has a very similar doping dependence of the pseudogap temperature $T^*$ or value $\Delta_p$. Meanwhile the virtual maximum gap at ($\pi,0$) derived from $v_\Delta$ is found to follow the simple relation $\Delta_q=0.46k_BT^*$ upon changing the doping concentration. This strongly suggests a close relationship between the pseudogap and superconductivity. It is further found that the superconducting transition temperature is determined by both the residual density of states of the pseudogap phase and the nodal gap slope in the zero temperature limit, namely, $T_c \approx \beta v_\Delta \gamma_n(0)$, where $\gamma_n(0)$ is the extracted zero temperature value of the normal state specific heat coefficient which is proportional to the size of the residual Fermi arc $k_{arc}$. This manifests that the superconductivity may be formed by forming a new gap on the Fermi arcs near nodes below $T_c$. These observations mimic the key predictions of the SU(2) slave boson theory based on the general resonating-valence-bond (RVB) picture.

Behavior of transformer cores with multistep-lap joints

Recently, no-load losses P and noise n of transformer cores have been reduced by replacing the former single-step-lap (SSL) joints by multistep-lap (MSL) joints. On the other hand, the application of MSL in model cores was found not only to decrease P and n, but in some cases they increase. The aim of this paper is the investigation of the difference between SSL and MSL with respect to local magnetic conditions at joint regions and their relevance for global conditions in the core. The results show that the joint is the key region of the transformer core, affecting its dynamical behavior in a distinct way. To minimize the iron losses and the noise of the core, the step lap number N should be chosen carefully to avoid the appearance of "virtual temporary air gaps".

シミュレーションによるキーパー媒体・薄膜ヘッド系の記録再生特性

Two-dimensional read/write simulation based on the Maxwell equations was performed by a finite element method on a system consisting of a keepered medium (KM) with a thin soft magnetic layer (keeper layer) over a recording layer and a thin-film head. In the write process, the KM is given a weaker head field than the UKM. But the KM’s magnetization is larger than that of the UKM because of an image effect in the write and after-write processes. In the read process, the keeper layer reduces the practical spacing by means of the image effect, and improves the read resolution by introducing a virtual gap (VG). Then, the keeper layer with the optimum parameters (e.g. μ = 100, t = 15 nm, Bs = 1.5 T) decreases the demagnetizing field in the recording layer (e.g. Mr = 560 kA/m, t = 36 nm, Hc = 167 kA/m) by 1/2, and increases the recorded magnetization by 1/4. As a result, the thermal stability of the recording magnetization can be doubled. Therefore, the KM will be an attractive medium with better thermal stability if the optimum parameters are chosen.

An empirical model for predicting the gap light index in an even-aged oak stand

An empirical model for predicting light levels under a canopy gap in an even-aged oak stand is presented. The effects of gap size, distance from the gap, and directional position (north–south) within the gap on gap light index (GLI) [sensu Canham, C.D. 1988. An index for understory light levels in and around canopy gaps. Ecology 69, 1634–1638] are quantified by multiple regression analysis. Data were obtained by the `erasing method' (Yanagisawa et al., in preparation 1 In order to estimate the light levels after the cutting, hemispherical photographs taken before the actual cutting were computerized and all the crowns to be cut (identified by field observations) were digitally erased (named `erasing method'). Light levels predicted by using the erasing method were highly correlated with that calculated by the ordinary photograph-analyzing method after the actual cutting ( R 2=0.911, P<0.001). 1 ) which predicts the GLI from computerized images of hemispherical photographs. Forty-five `virtual canopy gaps' were assumed, and at 945 points, light levels were calculated using HEMIPHOT [ter Steege, H. 1993. HEMIPHOT, A Programme to Analyze Vegetation Indices, Light and Light Quality from Hemispherical Photographs. The Tropenbos Foundation, Wageningen, The Netherlands]. The model presented in this study has the advantage that the indices needed for the prediction can be measured easily in field survey. The applicability of the model is discussed.

Elementary calculation of the branch-point energy in the continuum of interface-induced gap states

The band alignment at ideal semiconductor interfaces may be described by the continuum of interface-induced gap states. These intrinsic interface states derive from the virtual gap states (ViGS) of the complex semiconductor band structure. The energy at which their character changes from predominantly donorlike to acceptor-like is defined as their branch point. This branch point is located slightly below the middle of its band gap at the mean-value point in the Brillouin zone. By comparison with GW band-structure data it is demonstrated that, first, the width of the band gap at the mean-value point equals the dielectric band gap and that, second, the empirical tight-binding approximation reproduces the dispersion of the GW valence bands from the middle to the mean-value point of the Brillouin zone. Branch points computed by Tersoff for 15 semiconductors are found slightly below the middle of the respective dielectric band gaps at the mean-value point of the Brillouin zone. These results make possible simple calculations of branch points for other zincblende-structure semiconductors.

Empirical tight-binding calculation of the branch-point energy of the continuum of interface-induced gap states

The band lineup at metal–semiconductor contacts as well as at semiconductor heterostructures may be described by one and the same physical concept, the continuum of interface-induced gap states. These intrinsic interface states derive from the virtual gap states (ViGS) of the complex semiconductor band structure and their character varies from predominantly donorlike closer to the valence band to mostly acceptorlike nearer to the conduction band. Calculations are presented of the respective branch points for elemental and binary as well as ternary compound semiconductors which make use of Baldereschi’s concept of mean-value points in the Brillouin zone [Phys. Rev. B 7, 5212 (1973)], Penn’s idea of dielectric band gaps [Phys. Rev. 128, 2093 (1962)], and the empirical tight-binding approximation (ETB). The results are as follows. First, at the mean-value point the band gaps calculated in the GW approximation have the same widths as the dielectric band gaps. Second, the ETB approximation reproduces the GW valence-band energies at the mean-value point. Third, the branch points of the ViGS are slightly below midgap at the mean-value point. The ETB branch-point energies excellently reproduce the barrier heights of gold Schottky contacts on 19 semiconductors and the valence-band offsets of Al1−xGaxAs/GaAs heterostructures.

Adapting Adolescents to Market Relations

Today, the usual differences between parents and children are turning into a virtual generation gap, because a clear change of values is taking place—collectivism replaced by individualism, romanticism by pragmatism, chaste love by sex. The youngsters assimilate the new values with ease, which is often cause for alarm among adults concerned about the social legacy. Will the younger generation want to work, or know how to? Will they retain a sense of the value of human life, or will they be nurtured to turn into killers from an early age? Is the new generation going to be lost in the sense of a break in the continuity of the process of cultural development? Is normal language going to be supplanted by deviant language (there has been talk on television about legitimizing obscenity)? In some sense, the older generation is also worried about itself—tomorrow, you see, they will be in charge of us.

Historical Data on Graduate-Level Participation of Women in a Large Geoscience Department

Participation by women in the graduate program in geological sciences at the University of Texas at Austin has varied dramatically at different times since the founding of the department in 1897. Two periods of relatively high participation rates are documented, based on percentages of women among graduates awarded MA and PhD degrees. A peak of participation from the late 1920s through the early 1940s ended with World War II. Participation of women in the graduate program did not again reach comparable levels until after the mid-1970s oil-price rise. Causes of this virtual 30-year gap in participation by women remain obscure. During the periods of higher participation, a very small number of individuals among the total faculty fostered a disproportionate number of graduate degree completions by women.

Fluctuation growth of the transverse resistivity of layered superconductors at the edge of transition

Superconducting fluctuations in strongly anisotropic planar superconductors substantially affect the resistivity above the transition temperature. In a wide temperature range above T c, the main effect of these fluctuations is to create a virtual gap in the electronic spectrum, causing an increase of transverse resistivity. This effect becomes especially competitive due to the suppression of paraconductivity in c-axis direction because of the hopping character of electron motion. The qualitative aspects of the phenomenon are discussed in detail.

The peculiarities of c-axis transport in the normal phase of high-T c materials

The c-axis transport of impure layered metals is discussed in the cases of clean and dirty limits. The crossover from the Drude regime to the hopping regime in transverse conductivity is treated qualitatively as the change from anisotropic diffusion to Zener oscillations of electrons. It is shown that the superconducting fluctuations in strongly anisotropic layered superconductors substantially affect the transport coefficients above the transition temperature. In a wide temperature range above Tc the main effect of these fluctuations is to create a virtual gap in the electron spectrum, causing an increase of transverse electric and heat resistivities and decrease of thermoelectric power. This effect turns out to be specially competitive due to the suppression of paraconductivity along the c-axis direction due to the hopping character of electron motion.

Chemical interactions and Schottky barrier determinations at the Mg/Si(100) interface studied using X-ray photoelectron spectroscopy

Low work function metal-on-semiconductor systems have technological importance as efficient photocathodes or as thermionic energy converters. Mg-on-Si provides one such system that we have studied here. When the metal is deposited onto Si(100) at room temperature, it reacts to form a thin (approximately two monolayers) layer of Mg 2Si. Upon further deposition, the silicide behaves as a reaction barrier preventing the reactants coming into contact. Mg metal therefore grows on top of the silicide. Furthermore, it adopts a layer-by-layer growth mode. The Schottky barrier formed at this interface is very close in value to that quoted by Mönch [Phys. Rev. Lett. 58 (1987) 1260], i.e. approximately 0.5 eV. This close agreement suggests that the final pinning position may be a consequence of metal-induced virtual gap states.

The influence of wave dispersion characteristics on the determination of mechanical relaxation spectra

A new rheometrical technique has been developed to determine the mechanical relaxation spectra of viscoelastic materials by measurement of shear wave group and phase velocities in a virtual gap rheometer. The technique’s sensitivity and accuracy in providing relaxation spectra directly from measurements arises from exploitation of marked changes in wave dispersion characteristics in the vicinity of stress relaxation mechanisms. The new technique is compared with two conventional, wave‐based rheometrical techniques (based on pulse‐propagation and resonance measurements), which are adversely affected by wave dispersion. The superior accuracy and sensitivity of the new technique over conventional rheometry is demonstrated with reference to the Maxwell model and published relaxation spectra. Experimental results obtained on a concentrated colloidal dispersion serve to demonstrate the measurement principles.

Metal-semiconductor contacts: electronic properties

Rectification in metal-semiconductor contacts was first described by Braun in 1874. We owe the explanation of this observation to Schottky. He demonstrated that depletion layers exist on the semiconductor side of such interfaces. The current transport across such contacts is determined by their barrier heights, i.e., the respective energy difference between the Fermi level and the edge of the majority-carrier band. Since Schottky had published his pioneering work in 1938 the mechanisms, which determine the barrier heights of metal-semiconductor contacts, have remained under discussion. In 1947, Bardeen attributed the failure of the early Schottky-Mott rule to the neglect of electronic interface states. The foundations for a microscopic description of interface states in ideal Schottky contacts was laid by Heine in 1965. He demonstrated that a continuum of metal-induced gap states (MIGS), as they were called later, derives from the virtual gap states of the complex semiconductor band-structure. Neither this MIGS model nor any of the many other monocausal approaches, the most prominent is Spicer's Unified Defect Model, can explain the experimental data. In 1987, Mönch concluded that the continuum of MIG states represents the primary mechanism, which determines the barrier heights in ideal, i.e., intimate, abrupt, and homogeneous metal-semiconductor contacts. He attributed deviations from what is predicted by the MIGS model to other and then secondary mechanisms. In this respect, interface defects, structure-related interface dipoles. interface strain, interface compound formation, and interface intermixing, to name a few examples, were considered.

Rheological spectroscopy of concentrated colloidal dispersions

A new instrument, the virtual gap rheometer (VGR) for the determination of dynamic shear moduli (and mechanical relaxation spectra therefrom) at frequencies above 100 Hz is described. Several features of the VGR which make it particularly suited to the study of concentrated viscoelastic colloidal dispersions are discussed. The underlying theory is presented and the instrument's operation illustrated with results obtained on aqueous dispersions of a model clay colloid.

Effect of superconducting fluctuations on the transverse resistance of high-Tc superconductors.

Superconducting fluctuations in strongly anisotropic planar superconductors substantially affect the resistivity above the transition temperature. In a wide temperature range above T(c), the main effect of these fluctuations is to create a virtual gap in the electronic spectrum, causing an increase in resistivity. As a result the resistivity shows a maximum close to the transition temperature. Closer to the transition temperature the correction to the resistivity becomes negative and grows.as 1/(T - T(c))2, WhiCh is followed by the usual (T - T(c))1/2 law still closer to the transition temperature. This observation may help to understand the unusual behavior of the resistivity in the c direction in high-T(c) cuprates.

Atomic nitrogen on InAs(110) surfaces at room temperature

The interaction of atomic nitrogen with cleaved InAs(110) surfaces at room temperature was investigated by using a Kelvin probe and photoemission spectroscopy excited with Mg(Kα), Zr(Mζ) as well as monochromatic He I and He II radiation. The increase of the N(1s) signal by a simultaneous decrease of the As(3s) intensity accompanied by a constant In(3d 5/2) signal indicates an anion exchange. An N-induced build-up of inversion layers was found on samples doped p-type and of accumulation layers on those doped n-type. For larger exposures the Fermi level becomes pinned at 0.58 eV above the top of the valence band at the surface irrespective of the type of doping. This energy position of the Fermi level agrees very well with the charge-neutrality level of the virtual gap states. With increasing nitrogen exposure two new components to the In(4d) core-level spectra were observed which are chemically shifted by 0.23 and 0.64 eV to larger binding energies with respect to the bulk component. With the As(3d) level a component shifted by 2.1 eV to larger binding energies was detected. All shifts are caused by bonds with nitrogen. The ionization energy initially decreases but increases for larger exposures. The minimum occurs at an N-coverage of one monolayer. It is explained by the removal of the relaxation and a build-up of two additional surface dipoles.

The determination of dynamic moduli at high frequencies

A new technique for the measurement of dynamic moduli at high frequencies is presented, based upon the properties of a virtual gap formed by two unequal shear wave path lengths. Measuring procedures and underlying theory are presented which overcome many of the difficulties inherent in the high-frequency rheometry of concentrated dispersions. Use of the technique is illustrated with results obtained on dispersions of a commercially available model clay colloid.