Anomalous Dip Research Articles

Small-signal substrate resistance effect in RF CMOS identified through device simulations

An anomalous dip in the measured s/sub 22/ characteristic, as well as a decrease in the output resistance, of MOS devices for rf applications was found to be a pure ac effect caused by the small-signal substrate transconductance. The study of the ac characteristics of multifinger transistors in rf applications with a high-resistivity substrate also puts a question mark on the possibility of achieving fully scalable models, considering the observed ac substrate effect.

Anomalous Sharp Dip of Large Field-Induced Refractive Index Change in GaAs/AlGaAs Five-Layer Asymmetric Coupled Quantum Well

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well for ultrafast and low-voltage optical modulators and switches. Almost linear and large electrorefractive index change can be obtained in the transparency wavelength regions. In the GaAs/AlGaAs FACQW, an abrupt change in refractive index change Δn due to an applied electric field F occurs at a certain electric field range, which results in an anomalous sharp dip of Δn versus F. The physical origin and the elimination of the dip are discussed in detail. The abrupt change of refractive index is caused by significant changes of the wavefunction overlap integrals (and exciton binding energies) of transitions between the ground states for an electron (e1) and a heavy hole (hh1), and transitions between e1 and the first excited state for a heavy hole (hh2). The overlap changes are mainly due to shifts of the wavefunction distribution of hh1 and hh2, respectively. The dip can be eliminated by changing the position or Al content of the AlGaAs barrier layer in the FACQW. In addition, the larger negative index change in a modified FACQW structure is demonstrated.

Effect of hydrogen etching on 6H SiC surface morphology studied by reflection high-energy positron diffraction and atomic force microscopy

Hydrogen-etched 6H SiC (0001) surfaces have been studied by reflection high-energy positron diffraction and atomic force microscopy. It was found that residual damage on the surfaces were effectively removed by the hydrogen etching as compared to the HF etching after the oxidation. The hydrogen-etched surfaces were atomically flat. After the oxidation following the hydrogen etching, the surface roughness was found to increase and an anomalous dip structure appeared in the rocking curve of the reflection high-energy positron diffraction.

Effects of non-uniform solvation on thermal response in poly(N-isopropylacrylamide) gels

The quantitative analyses on the dehydration of poly(N-isopropylacrylamide) (NIPA) hydrogels were conducted by means of differential scanning calorimetry (DSC) as a function of polymer volume fraction, φ. It was found that the thermal properties of NIPA gels were strongly dependent on the history of the gel treatment before DSC measurements. In particular, a disagreement in the hydrophobic dissociation temperature at which an endothermic peak appears, Thd, and in the enthalpy of dehydration, ΔH, was clearly detected between gels having different histories: one prepared by adding a given amount of water to a dried gel (non-equilibrated gels) and the other made by gradual shrinking along the isobar line by heating (equilibrated gels). The phase diagram, i.e. the plot of Thd vs. φ, for the equilibrated gels has a convex function of φ, whereas that of the non-equilibrated gel shows an anomalous dip for φ>φst≈0.4 (i.e. above the stoichiometric concentration of NIPA-water hydration). The values of ΔH for the non-equilibrated gels are less than those for the equilibrated gels in the concentration regime of φ>φst. This implies that the non-equilibrated gels have a higher degree of non-uniform solvation than the equilibrated gels.

110]TA 1 phonon dispersion relation of the β 1-phases in Ni–Co–Al alloys

The present study is mainly concerned with the relation between electron concentration and anomalous dip appearing on the [110]TA 1 phonon branch of the β 1-phase (B2-structure) in Ni–Al alloys. By substituting Co for Ni, the dip-position shifts toward the Brillouin zone center. In addition, we prove that the dip appears also in the equilibrium β 1-phase existing above T b-temperature, below which Ni 3Al precipitates. The obtained results indicate clearly that the dip is not any precursor of the martensitic transition but a reflection of the electronic property in the β 1-phase.

Spectroscopic study of self-organized quantum dot like structures in Ga–In–P superlattices on (311) GaAs

We report temperature dependent photoluminescence, contactless electroreflectance and photoluminescence excitation study of (GaP)2(InP)2.5 strained short period superlattices sandwiched between GaxIn1−xP alloy layers grown on GaAs (311)A substrates. Transmission electron microscope pictures of these samples reveal the presence of self-organized In rich globular structures with Ga rich surroundings in the superlattice planes. The variation of the peak position of the photoluminescence band with decreasing temperature has an anomalous dip. We show that this is not due to an anomalous change in the band gap with temperature but is due to the interplay between two luminescence pathways associated with two phases, one which has the original (GaP)2(InP)2.5 superlattice and the other being the self-organized composition modulated In rich regions within the superlattice layers. We also present spectroscopic results which indicate quantum dot like nature of the self-organized In rich structures in these samples.

RXTE highlights of the 34.85-day cycle of Her X-1

An analysis of the publically available RXTE archive on Her X-1 including data on 23 34.85-day cycles is performed. The turn-on times for these cycles are determined. The number of cycles with a duration of 20.5 orbits has been found exceedingly larger than of shorter (20 orbits) or longer (21 orbits). A correlation between the duration of a cycle and its mean X-ray flux is noted. The mean X-ray light curve shows a very distinct short-on state. The anomalous X-ray absorption dip is found during the first orbit after the turn-on in the main-on state for the cycles starting near the binary phase 0.25, while is present during two successive orbits in the low-on state. The post-eclipse recovery feature have not been found in the main-on state but appears at least for two orbits during the low-on state. The pre-eclipse dips are present both in main-on and low-on state and demonstrate the behaviour as in early observations. The comparison of durations of the main-on and short-on states enabled us to constrain the accretion disk semi-thickness and its inclination to the orbital plane.

RXTE highlights of the 34.85-day cycle of Her X-1

An analysis of the publically available RXTE archive on Her X-1 including data on 23 34.85-day cycles is performed. The turn-on times for these cycles are determined. The number of cycles with a duration of 20.5 orbits has been found exceedingly larger than of shorter (20 orbits) or longer (21 orbits). A correlation between the duration of a cycle and its mean X-ray flux is noted. The mean X-ray light curve shows a very distinct short-on state. The anomalous X-ray absorption dip is found during the first orbit after the turn-on in the main-on state for the cycles starting near the binary phase 0.25, while is present during two successive orbits in the low-on state. The post-eclipse recovery feature have not been found in the main-on state but appears at least for two orbits during the low-on state. The pre-eclipse dips are present both in main-on and low-on state and demonstrate the behaviour as in early observations. The comparison of durations of the main-on and short-on states enabled us to constrain the accretion disk semi-thickness and its inclination to the orbital plane.

Phonon-mediated unconventional superconductivity in strongly correlated systems

We propose an adiabatic approach for Hubbard models in the Fermi-liquid regime coupled to phonons. The Hubbard parameters are associated with the bandwidth $W$ via an interpolation formula between the trivial for $W$ strong- and weak-coupling limits of Hubbard models. Phonons are introduced via an adiabatic random-phase approximation scheme. We obtain simple conditions for phonon-driven instabilities in a Fermi liquid with short-ranged interactions. We report phonon-driven instabilities without nesting and describe the elimination of the Peierls instability in a nested system by Coulomb correlations. We also report the possibility for a phonon-driven phase separation (PS) instability as well as the strong enhancement of forward processes in the effective electron-phonon scattering near the phase separation instability. We show that the proximity to PS induces momentum decoupling (MD) in superconductivity which implies a tendency for decorrelation between the physics in the different regions of the Fermi surface. MD could induce anisotropic superconductivity with unconventional gap symmetry such as $d$ wave. Whether anisotropy in the high-${T}_{c}$ oxides is driven by MD or by anisotropic scattering (for example with spin fluctuations) becomes a crucial question. We discuss some qualitative implications of MD that explain puzzling qualitative aspects of superconductivity in the oxides and could advocate that MD is at the origin of anisotropies. Such effects are the marginality of the superconducting gap symmetry for the condensation free energy and the resulting possibility of gap symmetry transitions with the doping, the temperature dependence of the shape of the anisotropy, and the behavior of the anomalous dip above the gap in the density of states. We also show that in the MD regime the orthorhombic distortion of the ${\mathrm{CuO}}_{2}$ planes in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ could be sufficient to explain the mixing of $s$-gap components in the dominantly $d$-wave gap. On the other hand, if spin fluctuations mediate the pairing in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ at least 25% of the condensate must be located in the chains. Our analysis could rehabilitate phonons as potential mediators of the pairing in all ``unconventional'' superconductors including heavy-fermion and organic compounds.

Magnetic studies on the field-driven transition from decoupled to coupled pancake vortex phase inBi2Sr2CaCu2O8+δwith columnar defects

The reversible ${(M}_{\mathrm{rev}})$ and the irreversible magnetization of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals with columnar defects were measured as a function of field and temperature. In contrast to the conventional $\mathrm{ln}B$ dependence of ${M}_{\mathrm{rev}}$ observed in the unirradiated sample, the field dependence of ${M}_{\mathrm{rev}}$ for the irradiated samples exhibits an anomalous dip, corresponding to the expulsion of vortices from the sample. We show that the coupling transition, which, to our knowledge, has not been considered previously in analyzing the magnetization data, is very important to understand the anomalous behavior of ${M}_{\mathrm{rev}}.$ From the measurement, we obtain the almost temperature-independent transition line ${B}_{\mathrm{cp}}(T)\ensuremath{\sim}{B}_{\ensuremath{\Phi}}/3$, where ${B}_{\ensuremath{\Phi}}$ is a matching field. This is in good accordance with the recent computer simulation. The anomalous peak effect is observed at the same field, $B\ensuremath{\sim}{B}_{\ensuremath{\Phi}}/3$, in Bose-glass (BG) regime, which accompanies the reentrant behavior of BG melting line.

Ab initiostudy of the anomalies in the He-atom-scattering spectra of H/Mo(110) and H/W(110)

Helium-atom-scattering (HAS) studies of the H-covered Mo(110) and W(110) surfaces reveal a twofold anomaly in the respective dispersion curves. In order to explain this unusual behavior we performed density-functional theory calculations of the atomic and electronic structure, the vibrational properties, and the spectrum of electron-hole excitations of those surfaces. Our work provides evidence for hydrogen-adsorption induced Fermi-surface nesting. The respective nesting vectors are in excellent agreement with the HAS data and recent angle resolved photoemission experiments of the H-covered alloy system ${\mathrm{Mo}}_{0.95}{\mathrm{Re}}_{0.05}$(110). Also, we investigated the electron-phonon coupling and discovered that the Rayleigh phonon frequency is lowered for those critical wave vectors. Moreover, the smaller indentation in the HAS spectra can be clearly identified as a Kohn anomaly. Based on our results for the susceptibility and the recently improved understanding of the He-scattering mechanism we argue that the larger anomalous dip is due to a direct interaction of the He atoms with electron-hole excitations at the Fermi level.

Anomalies in He atom scattering spectra of the H-covered Mo(110) and W(110) surfaces

Helium atom scattering (HAS) studies of the H-covered Mo(110) and W(110) surfaces reveal a two-fold anomaly in the respective dispersion curves. In order to explain this unusual behavior we performed density-functional theory calculations of the atomic and electronic structure, the vibrational properties, and the spectrum of electron-hole excitations of those surfaces. Our work provides evidence for hydrogen adsorption induced Fermi-surface nesting. The respective nesting vectors are in excellent agreement with the HAS data and recent angle resolved photoemission experiments of the H-covered alloy system Mo 0.95Re 0.05 (110). Also, we investigated the electron-phonon coupling and discovered that the Rayleigh phonon frequency is lowered for those critical wave vectors compared to the clean surfaces. Moreover, the smaller indentation in the HAS spectra can be clearly identified as a Kohn anomaly. Based on our results and the recently improved understanding of the He scattering mechanism we argue that the larger anomalous dip is due to electron-hole excitations by the He scattering.

Sharp Anomaly in Magnetoresistance of the Spin-Density-Wave State of(TMTSF)2PF6

Magnetoresistance of an organic superconductor, (TMTSF) 2 PF 6 , where TMTSF is tetrameth- yltetraselenafulvalene, was measured in the spin-density-wave state for the electrical current perpendicular to the conducting a b -plane and the magnetic field within the plane. The magnetic field dependence of the magnetoresistance shows an anomalous dip at 1.2 T ( B d ). The value of B d is independent of temperature and field orientation within the a b -plane below 4.2 K and the dip anomaly is more distinct at lower temperature. Clear negative magnetoresistance appears below 4.2 K. We attempted to interpret the angular-independent dip anomaly in terms of the closed orbital motion of the carriers on the side of the Fermi tube and its disappearance at a kind of quantum limit.

Frequency dependent magnetic susceptibility of Ni(OH) 2 monolayer nanoclusters

The a.c. magnetic susceptibility χ ac was measured for Ni(OH) 2 monolayer nanoclusters (Ni-MNC) in the temperature region 4.2–20 K. The frequency dependence of χ ac was investigated between 98 Hz and 8 kHz with an a.c. field amplitude of 1.8 Oe. The magnetic freezing temperature T′ f was measured above 10 K using the real and imaginary parts of χ ac, ( χ′ 0, χ′' 0). The value of dynamic critical exponent zv was determined by applying a dynamic scaling law to the present results. It is considered that the finite two-dimensional ferromagnetic system of Ni-MNCs has magnetic order at 9.5 K, at which the non-linear a.c. magnetic susceptibility χ′ 2 has an anomalous dip.

Decoupling between the antiferromagnetic ordering and the structural phases transformation in Zn-doped La 2− xBa xCuO 4 around [formula omitted

The high- T c oxide La 2− x Ba x CuO 4 shows a local minimum in the midst of the curve of T c versus x around x ≊ 0.115, where the antiferromagnetic ordering and the structural phase transformation form the orthogonal phase to the tetragonal are reported to take place. In order to investigate the relation between these two phase transitions and the anomalous dip in T c , Zn-concentration dependence of their transition temperatures has been studied by 139La-NMR and the ultrasonic attenuation. By Zn-substitution up to 6%, the Neel temperature showed significant reduction while the structural phase transformation temperature stayed almost constant, suggesting that these two are independent of each other.

63/65Cu/ 139La-NMR study on the antiferromagnetic ordering in La-based high- Tc cuprate La 2− xSr xCuO 4 around [formula omitted

In order to investigate the origin of the anomalous dip in T c at x ≊ 0.115 for La 2x Sr x CuO 4 , the temperature dependence of 63/65Cu/ 139La-NMR spectra has been studied on samples prepared by the spray-dry method. Spectra of 63/65Cu-NMR for the sample x ≊ 0.115 showed a huge broadening at 4.2 K, demonstrating the existence of the antiferromagnetic ordering of 3 d-spins. The Néel temperature determined by 139La-NMR, showed maximum of T N ≊ 32 K at x=0.115 , which just coincided with the location of the dip in T c , suggesting the crucial interplay between the antiferromagnetism and the suppression in T c .

A Study on the Unusual Spectroscopic Behaviour of the Rhizobium Capsular Polysaccharide System

When changes in molecular organization of the capsular polysaccharide of Rhizobium trifolii (strain TA-I) are monitored by the standard technique of optical rotation (OR), a particularly strange phenomenon is observed. The OR change, during heating (to melt the gel network), continues initially in the same direction as the change observed on cooling (negative contribution), and is then followed by the expected reversal of sign (positive contribution), giving rise to an anomalous dip in the OR-temperature profile. This unusual feature is absent in the temperature dependence of optical rotation on heating at concentrations below about 5.5 mg/mL and reaches its maximum depth at the highest concentration used (16 mg/mL). As a result, the change in specific rotation decreases by about a factor of 4 between 0.1 and 16 mg/mL but the accompanying enthalpy change shows no evidence of a reduction in the extent of conformational ordering at higher concentrations. However, rapid decrease of temperature for the Rhizobium samples produces a large positive shift in OR, suggesting an antagonism between network formation (negative contribution), which is favoured by slow cooling rates, and chiral structures of competing optical activity (positive contribution) which are more effective on rapid cooling. The chiral aggregates, entrapped within the network, ate also responsible for positive differential scattering as detected by circular dichroism.

Resistivity and magnetoresistance of amorphous FexGe1-x alloys near the metal-insulator transition

Electrical transport mechanisms near the metal-insulator transition are investigated in the amorphous FexGe1-x system (0.05<or=x<or=0.30) by measuring the resistivity between 100 mK and 273 mK and the magnetoresistance between 100 mK and 6 K in magnetic fields up to 3.86 T. In the metallic samples, an anomalous dip in the resistivity is observed on cooling below 50 K, followed by an increase in the resistivity below about 10 K. The results of the measurements are compared to the predictions of current theories of variable-range hopping with interactions on the insulating side of the transition, and electron-electron interactions and weak localization on the metallic side of the transition.

Displacive-replacive phase transformation in a Ni62.5Al37.5 β2-phase studied by high-resolution electron microscopy and microdiffraction

Abstract The structure of the metastable Ni2Al phase, which has long been a matter of controversy, has been carefully re-examined by means of high-resolution trans-mission electron microscopy (HREM) and electron microdiffraction. First, it is concluded that the as-quenched Nix Al100−x (60 ⩽ x ⩽ 65) material already exhibits a partial ω-type collapse in a one-dimensional fashion which and is consistent with the anomalous dip in the phonon dispersion curve. Ni2 Al precipitates are formed on annealing by thermal decomposition of the high-temperature NixAl100−x B2 phase and still retain the smali ω-type shuffle. The amount of displacement in the well developed Ni2Al phase was estimated to be between 20 and 50% of the ideal ω collapse; this was determined by means of a combined technique of HREM and microdiffraction together with dynamical calculations of HREM images and diffraction intensities.

Low‐frequency magnetic field measurements near the epicenter of the Ms 7.1 Loma Prieta Earthquake

We report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the MS 7.1 Loma Prieta earthquake of 17 October 1989. Our measurements cover 25 narrow frequency bands in the more than six‐decade frequency range 0.01 Hz–32 kHz, with a time resolution varying from a half hour in the ULF range (0.01–10 Hz) to one second in the ELF/VLF range (10 Hz–32 kHz). The ULF system is located near Corralitos, about 7 km from the epicenter. The ELF/VLF system is located on the Stanford campus, about 52 km from the epicenter. Analysis of the ELF/VLF data has revealed no precursor activity that we can identify at this time. However, the ULF data have some distinctive and anomalous features. First, a narrow‐band signal appeared in the range 0.05–0.2. Hz around September 12 and persisted until the appearance of the second anomalous feature, which consisted of a substantial increase in the noise background starting on 5 October and covering almost the entire frequency range of the ULF system. Third, there was an anomalous dip in the noise background in the range 0.2–5 Hz, starting one day ahead of the earthquake. Finally, and perhaps most compelling, there was an increase to an exceptionally high level of activity in the range 0.01–0.5 Hz starting approximately three hours before the earthquake. There do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase. Further, while our systems are sensitive to motion, seismic measurements indicate that there were no significant shocks preceding the quake. Thus, the various anomalous features in our data, and in particular the large‐amplitude increase in activity starting three hours before the quake, may have been magnetic precursors.