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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Neoclassical effects on the stabilization of tearing mode by current modulation

The neoclassical effects on the stabilization of tearing modes by current modulation have been investigated. Neoclassical effects enhance the resistivity and reduce the resistive diffusion time of the modulation current. Therefore, the oscillating current can penetrate deeper into the plasma. With an oscillating loop voltage, the plasma oscillates radially at the Ware-pinch velocity. These neoclassical effects improve the efficiency of tearing mode stabilization by the current modulation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Modulation of effective damping constant using spin Hall effect

We have investigated modulation of the effective damping constant α{sub eff} via spin currents through the spin Hall effect for Permalloy/Pt bilayer films with various thicknesses. The observed linear and sinusoidal dependences of current density and field direction on α{sub eff} are in agreement with the analytical model. By comparing the thickness dependence of spin Hall angle obtained from the damping modulation with that previously obtained by spin-torque-induced ferromagnetic resonance, we show that there is no clear extrinsic contribution in the present method. We also show the large modulation of the effective damping constant (down to ∼20%) in the high-current-density region.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Modulation of drift-wave envelopes in a nonuniform quantum magnetoplasma

We study the amplitude modulation of low-frequency, long-wavelength electrostatic drift-wave envelopes in a nonuniform quantum magnetoplasma consisting of cold ions and degenerate electrons. The effects of tunneling associated with the quantum Bohm potential and the Fermi pressure for nonrelativistic degenerate electrons, as well as the equilibrium density and magnetic field inhomogeneities are taken into account. Starting from a set of quantum magnetohydrodynamic equations, we derive a nonlinear Schrödinger equation (NLSE) that governs the dynamics of the modulated quantum drift-wave packets. The NLSE is used to study the modulational instability (MI) of a Stoke's wave train to a small plane wave perturbation. It is shown that the quantum tunneling effect as well as the scale length of inhomogeneity plays crucial roles for the MI of the drift-wave packets. Thus, the latter can propagate in the form of bright and dark envelope solitons or as drift-wave rogons in degenerate dense magnetoplasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Modulation of over 10{sup 14} cm{sup −2} electrons in SrTiO{sub 3}/GdTiO{sub 3} heterostructures

We demonstrate charge modulation of over 10{sup 14} cm{sup −2} electrons in a two-dimensional electron gas formed in SrTiO{sub 3}/GdTiO{sub 3} inverted heterostructure field-effect transistors. Increased charge modulation was achieved by reducing the effect of interfacial region capacitances through thick SrTiO{sub 3} cap layers. Transport and device characteristics of the heterostructure field-effect transistors were found to match a long channel field effect transistor model. SrTiO{sub 3} impurity doped metal–semiconductor field effect transistors were also demonstrated with excellent pinch-off and current density exceeding prior reports. The work reported here provides a path towards oxide-based electronics with extreme charge modulation exceeding 10{sup 14} cm{sup −2}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

An ultrafast carbon nanotube terahertz polarisation modulator

We demonstrate ultrafast modulation of terahertz radiation by unaligned optically pumped single-walled carbon nanotubes. Photoexcitation by an ultrafast optical pump pulse induces transient terahertz absorption in nanowires aligned parallel to the optical pump. By controlling the polarisation of the optical pump, we show that terahertz polarisation and modulation can be tuned, allowing sub-picosecond modulation of terahertz radiation. Such speeds suggest potential for semiconductor nanowire devices in terahertz communication technologies.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Au-gated SrTiO{sub 3} field-effect transistors with large electron concentration and current modulation

We report the fabrication of low-leakage rectifying Pt and Au Schottky diodes and Au-gated metal-semiconductor field effect transistors (MESFETs) on n-type SrTiO{sub 3} thin films grown by hybrid molecular beam epitaxy. In agreement with previous studies, we find that compared to Pt, Au provides a higher Schottky barrier height with SrTiO{sub 3}. As a result of the large dielectric constant of SrTiO{sub 3} and the large Schottky barrier height of Au, the Au-gated MESFETs are able to modulate ∼1.6 × 10{sup 14 }cm{sup −2} electron density, the highest modulation yet achieved using metal gates in any material system. These MESFETs modulate current densities up to ∼68 mA/mm, ∼20× times larger than the best demonstrated SrTiO{sub 3} MESFETs. We also discuss the roles of the interfacial layer, and the field-dependent dielectric constant of SrTiO{sub 3} in increasing the pinch off voltage of the MESFET.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Study of spatial spin-modulated structures by Mössbauer spectroscopy using SpectrRelax

SpectrRelax is an application for analysis and fitting of absorption and emission Mössbauer spectra. It includes a large selection of static and relaxation spectrum models, and allows fitting and searching for optimal model parameters. Recently, we have added new models for Mössbauer spectra of nuclides in spatial spin modulated structures. In these structures, spin density or direction changes in a periodic way along a single direction, and this wave is incommensurate with the underlying lattice. The models include Spin/Charge density wave, where the shape of this wave is represented as a sum of odd harmonics, Anharmonic spin modulation where the spin direction has a cycloidal type modulation, and a Spiral-like spin structure, in which magnetic moments rotate in a plane perpendicular to the wave propagation vector, forming a spiral.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Modulation instability and dissipative rogue waves in ion-beam plasma: Roles of ionization, recombination, and electron attachment

The amplitude modulation of ion-acoustic waves is investigated in an unmagnetized plasma containing positive ions, negative ions, and electrons obeying a kappa-type distribution that is penetrated by a positive ion beam. By considering dissipative mechanisms, including ionization, negative-positive ion recombination, and electron attachment, we introduce a comprehensive model for the plasma with the effects of sources and sinks. Via reductive perturbation theory, the modified nonlinear Schrödinger equation with a dissipative term is derived to govern the dynamics of the modulated waves. The effect of the plasma parameters on the modulation instability criterion for the modified nonlinear Schrödinger equation is numerically investigated in detail. Within the unstable region, first- and second-order dissipative ion-acoustic rogue waves are present. The effect of the plasma parameters on the characteristics of the dissipative rogue waves is also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Constructive polarization modulation for coherent population trapping clock

We propose a constructive polarization modulation scheme for atomic clocks based on coherent population trapping (CPT). In this scheme, the polarization of a bichromatic laser beam is modulated between two opposite circular polarizations to avoid trapping the atomic populations in the extreme Zeeman sublevels. We show that if an appropriate phase modulation between the two optical components of the bichromatic laser is applied synchronously, the two CPT dark states which are produced successively by the alternate polarizations add constructively. Measured CPT resonance contrasts up to 20% in one-pulse CPT and 12% in two-pulse Ramsey-CPT experiments are reported, demonstrating the potential of this scheme for applications to high performance atomic clocks.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Thickness modulated MoS{sub 2} grown by chemical vapor deposition for transparent and flexible electronic devices

Two-dimensional (2D) materials have been a great interest as high-performance transparent and flexible electronics due to their high crystallinity in atomic thickness and their potential for variety applications in electronics and optoelectronics. The present study explored the wafer scale production of MoS{sub 2} nanosheets with layer thickness modulation from single to multi-layer by using two-step method of metal deposition and CVD process. The formation of high-quality and layer thickness-modulated MoS{sub 2} film was confirmed by Raman spectroscopy, AFM, HRTEM, and photoluminescence analysis. The layer thickness was identified by employing a simple method of optical contrast value. The image contrast in green (G) channel shows the best fit as contrast increases with layer thickness, which can be utilized in identifying the layer thickness of MoS{sub 2}. The presence of critical thickness of Mo for complete sulphurization, which is due to the diffusion limit of MoS{sub 2} transformation, changes the linearity of structural, electrical, and optical properties of MoS{sub 2}. High optical transparency of >90%, electrical mobility of ∼12.24 cm{sup 2} V{sup −1} s{sup −1}, and I{sub on/off} of ∼10{sup 6} characterized within the critical thickness make the MoS{sub 2} film suitable for transparent and flexible electronics as compared to conventional amorphous silicon (a-Si) or organic films. The layer thickness modulated large scale MoS{sub 2} growth method in conjunction with the layer thickness identification by the nondestructive optical contrast will definitely trigger development of scalable 2D MoS{sub 2} films for transparent and flexible electronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Modulating action of low frequency oscillations on high frequency instabilities in Hall thrusters

It is found that the low frequency oscillations have modulating action on high frequency instabilities in Hall thrusters. The physical mechanism of this modulation is discussed and verified by numerical simulations. Theoretical analyses indicate that the wide-range fluctuations of plasma density and electric field associated with the low frequency oscillations affect the electron drift velocity and anomalous electron transport across the magnetic field. The amplitude and frequency of high frequency oscillations are modulated by low frequency oscillations, which show the periodic variation in the time scale of low frequency oscillations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Parasitic modulation of electromagnetic signals caused by time-varying plasma

An experiment on the propagation of electromagnetic (EM) signals in continuous time-varying plasma is described. The time-varying characteristics of plasma are considered to cause a parasitic modulation in both amplitude and phase, and the strength of this modulation, which carries the information of the electron density profile, is closely related to the plasma frequency and the incident wave frequency. Through theoretical analysis, we give an explanation and mechanism of the interaction between the continuous time-varying plasma and EM waves, which is verified by a comparative analysis with experiments performed under the same conditions. The effects of this modulation on the EM signals in the plasma sheath cannot be ignored.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Enhancing the pH sensitivity by laterally synergic modulation in dual-gate electric-double-layer transistors

The sensitivity of a standard ion-sensitive field-effect transistor is limited to be 59.2 mV/pH (Nernst limit) at room temperature. Here, a concept based on laterally synergic electric-double-layer (EDL) modulation is proposed in order to overcome the Nernst limit. Indium-zinc-oxide EDL transistors with two laterally coupled gates are fabricated, and the synergic modulation behaviors of the two asymmetric gates are investigated. A high sensitivity of ∼168 mV/pH is realized in the dual-gate operation mode. Laterally synergic modulation in oxide-based EDL transistors is interesting for high-performance bio-chemical sensors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

The electrochemical impact on electrostatic modulation of the metal-insulator transition in nickelates

For physical studies of correlated electron systems and for realizing novel device concepts, electrostatic modulation of metal-insulator transitions (MITs) is desired. The inherently high charge densities needed to modulate MITs make this difficult to achieve. The high capacitance of ionic liquids are attractive but, voltages are needed that can be in excess of the electrochemical stability of the system. Here, we show temperature/resistivity data that suggest electrostatic modulation of the MIT temperature of NdNiO{sub 3} in a wide regime. However, additional voltammetric and x-ray photoelectron spectroscopy measurements demonstrate the electrochemical impact of the electrostatic doping approach with ionic liquids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Photon-counting 1.0 GHz-phase-modulation fluorometer

We have constructed an improved version of a photon-counting phase-modulation fluorometer (PC-PMF) with a maximum modulation frequency of 1.0 GHz, where a phase domain measurement is conducted with a time-correlated single-photon-counting electronics. While the basic concept of the PC-PMF has been reported previously by one of the authors, little attention has been paid to its significance, other than its weak fluorescence measurement capability. Recently, we have recognized the importance of the PC-PMF and its potential for fluorescence lifetime measurements. One important aspect of the PC-PMF is that it enables us to perform high-speed measurements that exceed the frequency bandwidths of the photomultiplier tubes that are commonly used as fluorescence detectors. We describe the advantages of the PC-PMF and demonstrate its usefulness based on fundamental performance tests. In our new version of the PC-PMF, we have used a laser diode (LD) as an excitation light source rather than the light-emitting diode that was used in the primary version. We have also designed a simple and stable LD driver to modulate the device. Additionally, we have obtained a sinusoidal histogram waveform that has multiple cycles within a time span to be measured, which is indispensable for precise phase measurements. With focus on the fluorescence intensity and the resolution time, we have compared the performance of the PC-PMF with that of a conventional PMF using the analogue light detection method.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Modulation of superconducting critical temperature in niobium film by using all-solid-state electric-double-layer transistor

An all-solid-state electric-double-layer transistor (EDLT) was fabricated for electrical modulation of the superconducting critical temperature (T{sub c}) of Nb film epitaxially grown on α-Al{sub 2}O{sub 3} (0001) single crystal. In an experiment, T{sub c} was modulated from 8.33 to 8.39 K while the gate voltage (V{sub G}) was varied from 2.5 to −2.5 V. The specific difference of T{sub c} for the applied V{sub G} was 12 mK/V, which is larger than that of an EDLT composed of ionic liquid. A T{sub c} enhancement of 300 mK was found at the Li{sub 4}SiO{sub 4}/Nb film interface and is attributed to an increase in density of states near the Fermi level due to lattice constant modulation. This solid electrolyte gating method should enable development of practical superconducting devices highly compatible with other electronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Pattern manipulation via on-chip phase modulation between orbital angular momentum beams

An integrated approach to thermal modulation of relative phase between two optical vortices with opposite chirality has been demonstrated on a silicon-on-insulator substrate. The device consists of a silicon-integrated optical vortex emitter and a phase controlled 3 dB coupler. The relative phase between two optical vortices can be actively modulated on chip by applying a voltage on the integrated heater. The phase shift is shown to be linearly proportional to applied electrical power, and the rotation angle of the interference pattern is observed to be inversely proportional to topological charge. This scheme can be used in lab-on-chip, communications and sensing applications. It can be intentionally implemented with other modulation elements to achieve more complicated applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Mapping power-law rheology of living cells using multi-frequency force modulation atomic force microscopy

We present multi-frequency force modulation atomic force microscopy (AFM) for mapping the complex shear modulus G* of living cells as a function of frequency over the range of 50–500 Hz in the same measurement time as the single-frequency force modulation measurement. The AFM technique enables us to reconstruct image maps of rheological parameters, which exhibit a frequency-dependent power-law behavior with respect to G{sup *}. These quantitative rheological measurements reveal a large spatial variation in G* in this frequency range for single cells. Moreover, we find that the reconstructed images of the power-law rheological parameters are much different from those obtained in force-curve or single-frequency force modulation measurements. This indicates that the former provide information about intracellular mechanical structures of the cells that are usually not resolved with the conventional force measurement methods.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗