Combined DC Research Articles

Combined capillary gas chromatography/ion trap mass spectrometry quantitative methods using labeled or unlabeled internal standards

Three methods of operating an ion trap mass spectrometer (ITMS) were investigated for the determination of quantitative information by combined capillary GC/MS and GC/MS/MS. Separate regression lines for a hexynone drug were examined by using both unlabeled and stable-isotope-labeled internal standards. The ITMS modes of operation examined were (1) the full-scan, rf-voltage-only mode, useful on ion trap detectors as well as on the ITMS, (2) a scan using combined rf and dc voltages (rf/dc) for mass-selective storage analyses, and (3) rf/dc followed by collision-induced dissociation. Results of over 200 analyses in the 0.1–10 ng on-column range, with 5 ng of internal standard, showed that the unlabeled internal standard, separated by retention time on the capillary GC column, gave the best relative standard deviations (less than 5% over the range) and linear correlations ( r 2 typically > 0.9992).

Combined DC and ELF magnetic fields can alter cell proliferation

Effects of extremely-low-frequency (ELF) electromagnetic fields on proliferation of rabbit-ligament fibroblasts were examined. Markedly different effects, ranging from inhibition to stimulation of proliferation, were obtained, depending on the signal parameters of amplitude, frequency, and DC magnetic field. These results demonstrate that simple, single-frequency signals can have dramatic effects on ligament cells, which may be useful for bioelectrical stimulation of growth and repair. Also, the range of frequencies examined in this study covered those commonly used in transmission of electrical energy (50 or 60 Hz), which indicates that further study of possible effects of exposure to fields generated by power transmission equipment is warranted.

Evaluation of the effects of erythro-9(2-hydroxy-3-nonyl) adenine (EHNA) on HIV-1 production [formula omitted

Effects of erythro-9(2-hydroxy-3-nonyl) adenine (EHNA), an inhibitor of the common Adenosine deaminase (adenosine aminohydrolase, EC 3.5.4.4), on HIV-1 production was evaluated in vitro . Reverse transcriptase (RT) activity in the supernatant was inhibited by nearly 50% in EHNA-treated HIV-1 infected H9 cells, when compared with untreated but infected H9 cells. There was also a significant decrease in cell viability, but this was reversed following the addition of deoxycytidine (dC) to these cultures. The combined treatment was also effective in suppressing HIV-1 release from HIV-1-infected U937 cells. This combined EHNA plus dC treatment had no effect on RT activity in the cell lysates, suggesting that the inhibition of HIV-1 production may be due to the disturbance of virus release from infected cells.

Noise and ac-dc interference phenomena in the charge-density-wave conductor K0.3MoO

We demonstrate that extremely thin (l1 \ensuremath{\mu}m thick) samples of the charge-density-wave (CDW) conductor ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ display high-quality narrow-band noise spectra when driven by a dc electric field. In the presence of combined dc and large-amplitude ac fields, the CDW becomes mode locked to the external signal, giving rise to Shapiro steps in the dc I-V characteristics. In the presence of combined dc and small-amplitude ac fields, distinct resonance features in the complex ac conductivity \ensuremath{\sigma}(\ensuremath{\omega}) are observed whenever the internal and external frequencies coincide. Analysis of the Shapiro-step interference suggests that the ac-dc interference arises from a direct coupling between the driving fields and the low-frequency dielectric relaxation mode in ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$. Resonances in the ac conductivity are shown to be analogous to those present in a phase-shifted resonant harmonic oscillator. Both noise and interference experiments yield a ratio between the narrow-band noise frequency and the CDW current-density of ${f}_{\mathrm{NBN}/{J}_{\mathrm{CDW}}}$=12\ifmmode\pm\else\textpm\fi{}3 kHz ${\mathrm{cm}}^{2}$/A, suggesting that the intrinsic pinning potential in ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ has a periodicity equal to the CDW wavelength.

The dynamics of charge-density waves

In many materials with a highly anisotropic band structure, electron-phonon interactions lead to a novel type of ground state called the charge-density wave. The condensate is pinned to the underlying lattice by impurities and by boundary effects, but can, even for small electric fields, carry current in a fashion originally envisioned by Fr\"ohlich. This review discusses some of the underlying theories and the main experimental observations on this new collective transport phenomenon. The frequency- and electric-field-dependent conductivity, current oscillations, electric-field-dependent transport coefficients and elastic properties, together with nuclear-magnetic-resonance experiments, provide clear evidence for a translational motion of the condensate. Various theories, involving classical and quantum-mechanical concepts, are able to account for a broad variety of experimental findings, which were also made in the presence of combined dc and ac fields.

Experiment versus the classical model of deformable charge-density waves: Interference phenomena and mode locking.

The differential resistance of ${\mathrm{NbSe}}_{3}$ measured in the presence of combined ac and dc electric fields exhibits interference features. Our experimental results for the dependence of these features on ac amplitude and frequency are in complete and dramatic disagreement with predictions of the classical model of deformable charge-density waves, and prove that pinning by impurities does not vanish at large fields and frequencies.

Measurement Scheme for the Macroscopic Quantum Coherence Experiment

An experimental strategy to test for macroscopic quantum coherence in a rf SQUID is described. The back action of broadband noise sources in a conventional dc SQUID magnetometer measurement system destroy the coherent state. A novel measurement scheme is suggested in which the rf SQUID is weakly coupled to an intermediate bistable system. This system consists of a pair of dc SQUIDs biased below threshold. The combined rf and dc threshold SQUID system is described by a single potential. The form of this potential is varied by changes in the external bias to the dc SQUIDs at the moment in which the measurement is to occur. This causes a transition in the dc SQUID system which serves to freeze the coherent oscillations. Amplification by a conventional SQUID linear amplifier chain then follows.

Phase locking in charge-density-wave transport.

Steps of constant current are observed in the dc I-V characteristic of charge-density-wave (CDW) systems subjected to combined ac and dc drives. The step widths are calculated by using a proposed form for the pinning potential, and by assuming phase locking between the applied ac drive and the CDW. Preliminary results indicate good qualitative and quantitative agreement with experiment for both harmonic and subharmonic steps in ${\mathrm{NbSe}}_{3}$.

Elastic properties of charge-density-wave conductors: ac-dc electric field coupling.

We have measured Young's modulus $Y$ and internal friction $\ensuremath{\delta}$ of the charge-density-wave (CDW) conductors Ta${\mathrm{S}}_{3}$ and Nb${\mathrm{Se}}_{3}$, in the presence of combined ac and dc electric fields. In the pinned CDW state, the elastic properties are sensitive to even small ac-field amplitudes. In the sliding-CDW state, sharp harmonic and subharmonic anomalies are observed in $Y$ and $\ensuremath{\delta}$; these coincide with Shapiro-type interference in the electronic response. Our experiments demonstrate that internal degrees of freedom (incoherence) of the CDW condensate are intimately tied to the dc- and ac-field-induced elastic anomalies.

Shapiro-step spectrum and phase-velocity coherence in NbSe3 in a uniform temperature gradient.

We have measured the Shapiro-step spectrum of the charge-density-wave conductor ${\mathrm{NbSe}}_{3}$, under conditions of combined ac and dc electric fields and an applied uniform temperature gradient. Increasing the temperature gradient from zero results in a well-defined breakup of the Shapiro-step spectrum. Under fixed temperature gradient, the isothermal Shapiro-step spectrum can be effectively recovered by application of a sufficiently-large-amplitude ac driving field. We identify different interference peaks of the broken spectrum with different macroscopically phase-velocity-coherent regions of the crystal, and we explicitly determine the volumes of these dynamically coherent domains.

Microwave absorption by hydrogen atoms in high Rydberg states.

The behavior of Rydberg atoms in a recent experiment involving selective excitation in combined ac and dc electric fields is shown to be consistent with the predictions of a one-dimensional model incorporating only the extreme level of each Stark manifold. For low microwave powers, the excitation is resonant and is described qualitatively by multiphoton perturbation theory. For higher powers, an alternative stochastic theory predicts rapid ionization at all frequencies, as observed experimentally.

Harmonic and subharmonic shapiro steps in orthorhombic Tas 3

Observation of harmonic and subharmonic Shapiro steps in orthorhombic TaS 3 in the presence of combined rf and dc drive is reported, demonstrating that both the harmonic and subharmonic steps are a general characteristic of charge-density-wave response. Compared to NbSe 3, the noise spectrum is typically much broader in TaS 3, leading to poorly defined steps at small rf voltages. The steps sharpen with larger amplitudes of rf. Increased rf amplitude is also accompanied by an increase in the CDW current density at a given step, but this increase approaches a limiting value for large rf levels. The results are interpreted as evidence for increased coherence of the CDW response in the presence of large rf driving fields.

Surface flashover of conical insulators in vacuum

The surface flashover of a Macor glass-ceramic conical insulator in ultrahigh vacuum is investigated using direct current (dc), alternating current (ac at 60 Hz), standard lightning impulse (1.2/50 μs), and combinations of dc+1.2/50 μs and dc+ac. The dc and the 1.2/50-μs flashover voltages are found to be at about the same level when the cone angle θ, which the surface makes with the applied field, is varied from −55° to +55°. At a fixed positive value of θ and a fixed thickness of the insulator, the lightning impulse, and the dc flashover voltages are higher than the ac. The level and the polarity of the dc prestress influence the combined dc+impulse flashover voltage in conical insulators. The ac+dc flashover depends on the dc prestress level and increases with increasing the prestress until a saturation is reached which is at a slightly higher voltage than the dc flashover. The electric field and the potential distributions at the interface of the cone insulator are calculated for different cone angles in the range 5° to 55°.

Improved Performance of Cylindrical Solid Insulators with Concave Curved Edges in Vacuum

The withstand voltage capability of a cylindrical solid insulator can be improved considerably by incorporating a concave geometry. in the insulator ends at the contact region with the conductors. It is found that the higher the value of the angle γ between the curved edgèof the insulator and the ambient field the lower is the value of the tangential field at the triple junction. The influence of the relative dielectric constant of the material and the length of the insulator on the field at the triple junction is found to be negligibly small. Measurements of the. flashover voltage in. ultrahigh vacuum (< 10-8 Torr) are conducted for cylindrical rods made of glass-ceramic with concave curvature at both-ends. γis varied in the range 0-65°. The DC, AC and 1.2/50 Ps lightning impulse flashover voltages are between 20 to 30% higher than the respective voltages of a straight cylindrical insulator of the same thickness. The DC pre-stress has negligible influence (< 10%) on the subsequent combined DC + impulse and DC + AC flashover voltages, of a concave insulator made of Macor glass-ceramic.

The mechanism of the Cd(II) reduction in 1 M fluoride, perchlorate and chloride solutions at the DME, a combined DC, AC admittance and demodulation study

On the basis of a recently published theoretical approach further proof is given for the CEE mechanism proposed earlier for the Cd(II) reduction in fluoride solution. It is shown that in perchlorate base electrolyte solution the same mechanism applies and that the rate constant of the chemical step is quite close to the value in fluoride solution. The theories available at present are found not to apply to the Cd(II) reduction in chloride solution because of the invalidity of the Randles equivalent circuit, contrary to experimental evidence published so far. The outstanding potentialities of the demodulation method for the study of charge-transfer mechanisms is clearly demonstrated by means of the systems mentioned.

Subharmonic Shapiro Steps and Devil's-Staircase Behavior in Driven Charge-Density-Wave Systems

Subharmonic steps in the dc $I\ensuremath{-}V$ characteristics are reported in the model compound Nb${\mathrm{Se}}_{3}$, when subjected to combined dc and ac drives. The steps occur at currents which correspond to frequencies ${\ensuremath{\omega}}_{\mathrm{int}}=(\frac{p}{q}){\ensuremath{\omega}}_{\mathrm{ext}}$, with $p$ and $q$ integers. These steps cannot be explained in terms of the familiar classical equation of motion in a single degree of freedom, because the inertial term is too small. The observed behavior is compared with the devil's staircase, and a fractal dimension of 0.91 \ifmmode\pm\else\textpm\fi{} 0.03 is extracted.

Chaotic response of NbSe3: Evidence for a new charge-density-wave phase

We have measured the charge-density-wave response characteristics of Nb${\mathrm{Se}}_{3}$ for combined ac and dc driving fields. In the switching regime a clear period-doubling route to chaos is observed, where the bifurcation cascade is periodic in applied dc bias and ac amplitude. These observations provide strong evidence for a new charge-density-wave phase which, in terms of classical dynamics, behaves underdamped.

Single-mode operation of 1.3 µm InGaAsP/InP buried crescent lasers using a short external optical cavity

Highly single longitudinal mode operation of 1.3 μm InGaAsP/InP buried crescent lasers has been obtained using a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60-600 \mu</tex> m long external cavity defined by a spherical mirror. The intensity of the neighboring modes was suppressed by a factor of 1200 for dc excitation and by a factor of 800 for combined dc and 250 MHz current modulation.

Effect of DC Pre-Stress on AC and DC Surface Flashover of Solid Insulators in Vacuum

The dc, ac (60 Hz) and ac superimposed on dc surface flashover of Teflon® (εr = 2.2), quartz-silica (εr = 3.8) and glass-ceramic (εr = 3.8) is investigated in vacuum prior to and following the application of a dc pre-stress. The surface flashover is measured at ~ 300 K, at a residual gas pressure of <10-8 Torr (< 10-6 Pa) after the solid insulators have been thoroughly degassed at elevated temperatures (~ 200° C). The ac surface flashover voltage is unaffected by the dc pre-stress. The combined dc and ac flashover voltage exceeds the withstand voltage of the ac by itself. Depending on the level of the applied dc pre-stress, the value of the combined dc and ac flashover is between 1.5 to 1.8 of the ac flashover voltage. The dc flashover voltage measured subsequent to applying a pre-stress with an opposite polarity is found to be considerably lower than the dc flashover voltage without a pre-stress. Applying a dc pre-stress with the same polarity improves only slightly the subsequent dc flashover strength. The observed phenomena are attributed to the changes in the electric fields at both electrode junctions due to the creation of charges on the surface of the solid insulator upon the application of the voltage stress.

Generation of short optical pulses in semiconductor lasers by combined DC and microwave current injection

We report picosecond pulse generation in low threshold buried optical guide lasers using combined direct and microwave current excitation. The pulse widths were obtained as a function of direct current for several levels of RF excitation using lasers 125 and 380 μm long. The pulses have a Gaussian shape with full widths at half maximum intensity ranging from 19 to 57 ps. The pulse widths were obtained from the second harmonic autocorrelation. The experimental results are in reasonable agreement with the theory of short pulse formation in sinusoidally modulated lasers. The pulse width decreases with increasing dc and microwave current, and decreasing laser length. The shortest pulses were obtained with a 125μm long laser using 0.25 W RF at 1 GHz and 35 mA de bias. Multiple pulses are emitted at high dc excitation levels. Using the above laser at the indicated current levels the emission consists of a burst of ∼10 pulses which are separated by 30 ps, and has an approximately exponentially decaying amplitude. Saturable absorption was introduced in the lasers by degradation and results in shortening the pulses. A comparison is made of the pulse widths obtained for sinusoidal microwave current modulation and for pulsed excitation where the excitation is obtained from a step recovery diode. Reduced pulse widths are obtained for short current pulse excitation.