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Faculty Publications

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Articles 1081 - 1110 of 1132

Full-Text Articles in Physics

Origin Of The Peaked Structure In The Conductance Of One-Dimensional Silicon Accumulation Layers, Richard A. Webb, A. Hartstein, J. J. Wainer, A. B. Fowler Apr 1985

Origin Of The Peaked Structure In The Conductance Of One-Dimensional Silicon Accumulation Layers, Richard A. Webb, A. Hartstein, J. J. Wainer, A. B. Fowler

Faculty Publications

We have made extensive studies of the temperature, gate voltage, and electric field dependences of the conductance peaks in small silicon inversion layers in order to distinguish between resonant-tunneling models and a hopping model. We find that many of the peaks are consistent only with a hopping model, whereas some could be consistent with an early resonant-tunneling model. None of our structure is consistent with resonant tunneling if the recent formulation of Stone and Lee is correct.

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Example Of A Group Action Determined Phase Transition, Jeffrey W. Felix, Dorian M. Hatch Feb 1985

Example Of A Group Action Determined Phase Transition, Jeffrey W. Felix, Dorian M. Hatch

Faculty Publications

The principles of the group action approach to structural phase transitions are outlined. It is assumed that all properties of the transition are determined by the action of a single physically irreducible represention of the space group of the more symmetric phase. We determine the isotropy groups using the image space of the representation. The free energy minima are determined to fourth order and to all orders using the results of Gufan and then compared. This theory is applied to Calcite (Roverline3c) to determine all possible continuous commensurate phase transitions.

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New Shubnikov-De Haas Effects In A Two-Dimensional Electron-Hole System, S. Washburn, Richard A. Webb, E. E. Mendez, L. L. Chang, L. Esaki Jan 1985

New Shubnikov-De Haas Effects In A Two-Dimensional Electron-Hole System, S. Washburn, Richard A. Webb, E. E. Mendez, L. L. Chang, L. Esaki

Faculty Publications

We report the temperature dependence of the Shubnikov–de Haas oscillations from a two-dimensional electron-hole system in GaSb-InAs-GaSb quantum wells at very low temperatures. The samples are double heterostructures containing separate electron and hole layers. The oscillations arising from the electron layer behave regularly with temperature. Additional oscillations, characterized by strong temperature dependence, and relatively large peak widths are believed to arise from the presence of hole layers.

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Quantum Electrodynamics Based On Self-Energy: Lamb Shift And Spontaneous Emission Without Field Quantization, A. O. Barut, Jean F. Van Huele Jan 1985

Quantum Electrodynamics Based On Self-Energy: Lamb Shift And Spontaneous Emission Without Field Quantization, A. O. Barut, Jean F. Van Huele

Faculty Publications

The theory of radiative processes in quantum theory is formulated on the basis of self-energy, in analogy to classical radiation theory, and is explicitly carried out for the calculation of the Lamb shift and spontaneous emission.

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Absence Of Minimum Metallic Conductivity In Gd(3-X)Vxs4 At Very Low Temperature And Evidence For A Coulomb Gap, S. Washburn, Richard A. Webb, S. Von Molnar, F. Holtzberg Nov 1984

Absence Of Minimum Metallic Conductivity In Gd(3-X)Vxs4 At Very Low Temperature And Evidence For A Coulomb Gap, S. Washburn, Richard A. Webb, S. Von Molnar, F. Holtzberg

Faculty Publications

Gd(3-x)vxS4 provides a convenient analog of a compensated semiconductor in which, for x≃0.3, the mobility edge can be tuned smoothly through the Fermi energy by the application of a magnetic field. The results of a search for a minimum metallic conductivity demonstrate that, down to T=6 mK, the metal-insulator transition is smooth. In the insulating regime, the temperature dependence of the conductivity was more consistent with the theory of mutual interactions than with the theory of pure localization.

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Magnetoresistance Of Small, Quasi-One-Dimensional, Normal-Metal Rings And Lines, C. P. Umbach, S. Washburn, R. B. Laibowitz, Richard A. Webb Oct 1984

Magnetoresistance Of Small, Quasi-One-Dimensional, Normal-Metal Rings And Lines, C. P. Umbach, S. Washburn, R. B. Laibowitz, Richard A. Webb

Faculty Publications

The magnetoresistance of sub-0.4-μm-diam Au and Au60Pd40 rings was measured in a perpendicular magnetic field at temperatures as low as 5 mK in search of simple, periodic resistance oscillations that would be evidence of flux quantization in normal-metal rings. However, instead of simple oscillations, a very complex structure developed in the magnetoresistance at low temperatures. Fourier analysis of all the data did not reveal convincing evidence for flux quantization in the rings. Complex structure similar to that observed in the rings was also found in the magnetoresistance of short, narrow, Au and Au60Pd …

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Isotropy Group Description Of A Phase Transition In Nano3, W. Scott Stornetta, Dorian M. Hatch Jun 1984

Isotropy Group Description Of A Phase Transition In Nano3, W. Scott Stornetta, Dorian M. Hatch

Faculty Publications

Predictions of the possible post-transition symmetries of an R 3c zone center continuous phase transition are made using recent extensions of the Landau theory. The Landau theory is summarized and direct group theoretical methods are applied to the R 3c phase. General techniques for solving systems of nonlinear polynomial equations are reviewed and these techniques are applied to the minimization of the free energy F for the aforementioned possible symmetries. The results of the direct methods and the minimization procedures are shown to be compatible. It is shown that the transition to R 3c in NaNO3 is consistent with the …

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Weak Localization Of Two-Dimensional Conduction Holes, S. Washburn, Richard A. Webb, E. E. Mendez, L. L. Chang, L. Esaki Mar 1984

Weak Localization Of Two-Dimensional Conduction Holes, S. Washburn, Richard A. Webb, E. E. Mendez, L. L. Chang, L. Esaki

Faculty Publications

We report transport measurements which we interpret as weak localization of two-dimensional conduction holes in a GaSb-InAs-GaSb quantum-well structure. This system is unique in that it has parallel conduction channels containing both holes and electrons. The longitudinal resistance of the sample was measured for temperatures between 0.006 and 25 K; the magnetoresistance was measured in a perpendicular magnetic field. Weak localization of the holes was indicated by negative magnetoresistance and by a large logarithmic correction to the conductivity.

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Velocity Measurements Of Humans By Computers, J. B. Rafert, R. C. Nicklin Jan 1984

Velocity Measurements Of Humans By Computers, J. B. Rafert, R. C. Nicklin

Faculty Publications

Distance and time, two fundamental quantities, are discussed early in most introductory physics courses. By dividing a change in distance by time to get velocity, and by dividing a change in velocity by time to get acceleration, two more important quantities result. With these the real world of automobiles and jet planes and applications of Newton's second law is available for interpretation from an analytical point of view. In order to get students thinking reliably about these important ideas it seems important to have direct measurements of distance and time over short enough intervals to talk sensibly about "instantaneous" velocities.

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Magnetic Field Behavior Of A Josephson-Junction Array: Two-Dimensional Flux Transport On A Periodic Substrate, Richard A. Webb, Richard F. Voss, G. Grinstein, P. M. Horn Aug 1983

Magnetic Field Behavior Of A Josephson-Junction Array: Two-Dimensional Flux Transport On A Periodic Substrate, Richard A. Webb, Richard F. Voss, G. Grinstein, P. M. Horn

Faculty Publications

This Letter reports measurements down to 3 mK of the magnetic field dependence of the resistance and critical current of a weakly coupled periodic array of Josephson junctions. Below 700 mK the resistance is an oscillatory function of field. The temperature dependence of the resistance for integral numbers of flux quanta per cell suggests the existence of a Kosterlitz-Thouless transition involving the unbinding of defects in the flux lattice.

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Unification Of Ernst-Equation Backlund Transformations Using A Modified Wahlquist-Estabrook Technique; Wahlquist-Estabrook, B. Kent Harrison Aug 1983

Unification Of Ernst-Equation Backlund Transformations Using A Modified Wahlquist-Estabrook Technique; Wahlquist-Estabrook, B. Kent Harrison

Faculty Publications

The three known Backlund transformations for the Ernst equation are derived using a modification of the Wahlquist-Estabrook prolongation procedure. The modification requires that the equation to be studied be cast into a set of differential forms and their exterior derivatives, such that all coefficients are constant (a "CC ideal''). Analysis of the resulting equations produces 16 solutions composed of the three basic transformations combined with identity and other essentially trivial transformations. The group structure of the transformations is discussed. A Backlund transformation (already known) for the Ernst-Maxwell equations can be found by the same method. Promising generalizations are mentioned.

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Conductance In Restricted-Dimensionality Accumulation Layers, A. B. Fowler, A. Hartstein, Richard A. Webb Jan 1982

Conductance In Restricted-Dimensionality Accumulation Layers, A. B. Fowler, A. Hartstein, Richard A. Webb

Faculty Publications

Conductance has been studied in metal-oxide-silicon field-effect transistor accumulation-layer samples in which it is possible to constrict the channel to small dimensions both perpendicular to the surface and perpendicular to the channel. A temperature-dependent conductance σ=σ0exp[-(T0/T)n] is observed, where n=1/2 for small channel widths and n=1/3 for larger channel widths. It is believed that this behavior arises from a transition from one-dimensional to two-dimensional variable-range hopping in the sample.

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Classroom Demonstration Of Sunspots, Thomas O. Callaway, Harry D. Downing, Glen T. Clayton Jan 1982

Classroom Demonstration Of Sunspots, Thomas O. Callaway, Harry D. Downing, Glen T. Clayton

Faculty Publications

No abstract provided.

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Physical Science Workshop Course For Elementary Teachers, Glen T. Clayton, Harry D. Downing, Thomas O. Callaway Jan 1982

Physical Science Workshop Course For Elementary Teachers, Glen T. Clayton, Harry D. Downing, Thomas O. Callaway

Faculty Publications

Science for elementary school grades K-6 varies somewhat in content from one textbook series to another, but it is fairly common that 40-60% of the material covered is directly related to physics. These texts suggest that certain learning activities such as experiments, demonstrations, and observations be carried out by the teacher and the class. Often the apparatus for a learning activity is to be constructed from simple, readily available materials. The typical elementary school teacher has had little experience in performing experiments and is often frustrated in his/her attempts at carrying out the suggested learning activity. To meet this need …

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Macroscopic Quantum Tunneling In 1-Μm Nb Josephson Junctions, Richard F. Voss, Richard A. Webb Jul 1981

Macroscopic Quantum Tunneling In 1-Μm Nb Josephson Junctions, Richard F. Voss, Richard A. Webb

Faculty Publications

The probability distributions for switching out of the superconducting state of low-cur-rent-density 1-μm Nb Josephson junctions with capacitance ≈ 0.1 pF have been measured as a function of temperature T down to 3 mK. Below 100 mK the distribution widths become independent of T. The results are in excellent agreement with predictions for the quantum tunneling of the (macroscopic) junction phase that include the reduction of tunneling rates due to dissipation.

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Landau Free Energy Form At The F Point Of The Roverline3 Structure, Dorian M. Hatch, Daniel L. Decker Feb 1981

Landau Free Energy Form At The F Point Of The Roverline3 Structure, Dorian M. Hatch, Daniel L. Decker

Faculty Publications

A free energy form compatible with the F point of the trigonal D63d(Roverline3c) symmetry is constructed. It is a function of the order parameter, elastic strain, and the stress. The resulting free energy exhibits a close formalistic analogy to the stressed cubic perovskite structures and their associated multicriticality.

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Wave Demonstration Device, Glen T. Clayton, Harry D. Downing, Thomas O. Callaway Jan 1981

Wave Demonstration Device, Glen T. Clayton, Harry D. Downing, Thomas O. Callaway

Faculty Publications

Students often have difficulty gaining an understanding of wave motion without effective demonstrations being performed in the lecture. Many excellent approaches utilizing ripple tanks, waves on a spring, chain, rope, etc., are customarily employed. Perhaps the most well-known device for demonstrating many of the effects of wave motion is the shive or bell wave motion machine.1 The purpose of the present note is to describe a simple, inexpensive device which can be easily constructed by a teacher or student and can be used to demonstrate many of the properties of wave motion.

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Spin-Glass-Like Behavior In Very Dilute Pdfe At Very Low Temperatures, Richard A. Webb, G. W. Crabtree, J. J. Vuillemin Sep 1979

Spin-Glass-Like Behavior In Very Dilute Pdfe At Very Low Temperatures, Richard A. Webb, G. W. Crabtree, J. J. Vuillemin

Faculty Publications

Measurements of the electrical resistivity of palladium, residual resistivity ratio ∼22 500, containing 1.7 ppm of Fe have been made from 5.5 K to 1.8 mK. 15 Hz ac susceptibility measurements from 1 K to 1.8 mK together with the resistivity data suggest that a transition into a spin-glass state occurs near 7 mK.

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Chemical Vapor Deposited Amorphous Silicon For Use In Photothermal Conversion, David D. Allred, D. C. Booth, M. Janai, G. Weiser, B. O. Seraphin Nov 1978

Chemical Vapor Deposited Amorphous Silicon For Use In Photothermal Conversion, David D. Allred, D. C. Booth, M. Janai, G. Weiser, B. O. Seraphin

Faculty Publications

Efficient photothermal conversion requires surfaces of high solar absorptance and low thermal emittance. This can be accomplished by the tandem action of a good infrared reflector overlaid by a film of sufficient solar absorptance that is transparent in the infrared. Crystalline silicon is a suitable candidate for the absorber layer. Its indirect band gap, however, results in a shallow absorption edge that extends to far into the visible. In contrast, the absorption edge of amorphous silicon is steeper and located farther into the infrared, resulting in a larger solar absorptance. We report on the fabrication of amorphous silicon absorbers by …

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Chemical Vapor Deposited Molybdenum Films For Use In Photothermal Conversion, David D. Allred, G. E. Carver, B. O. Seraphin Nov 1978

Chemical Vapor Deposited Molybdenum Films For Use In Photothermal Conversion, David D. Allred, G. E. Carver, B. O. Seraphin

Faculty Publications

High infrared reflectance, coupled with higher solar absorptance, is required for efficient photothermal conversion. Converters can be fabricated by depositing an absorber on a highly reflecting metal. The absorber functions in the visible, yet becomes transparent in the near infrared, allowing the metal to suppress the thermal emittance. Economic considerations demand the use of thin films, rather than bulk materials. The thin film reflector must be capable of withstanding high temperatures of operation. Compatibility of the reflector with the substrate below, and the absorber above, is required for long-time service. Highly reflective silver films suffer reflectance losses by agglomeration, and …

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Tip-Angle-Dependent Magnetic Relaxation In Superfluid 3he, Richard A. Webb Mar 1978

Tip-Angle-Dependent Magnetic Relaxation In Superfluid 3he, Richard A. Webb

Faculty Publications

Using a pulsed superconducting-quantum-interference-device (SQUID) NMR technique, relaxation of the longitudinal magnetization in superfluid 3He has been measured in magnetic fields of 31, 102, 180, and 306 Oe in the pressure range 16-26 bar. It is observed that in both superfluid phases there is a temperature-dependent, well-defined spin-tipping angle, θc, for which the relaxation of the magnetization changes from an exponential to a nonexponential behavior.

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The Stephen F. Austin Traveling Science Show, James C. Dennis Jan 1978

The Stephen F. Austin Traveling Science Show, James C. Dennis

Faculty Publications

No abstract provided.

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Ferromagneticlike Resonance Behavior In Superfluid 3he-B, Richard A. Webb Oct 1977

Ferromagneticlike Resonance Behavior In Superfluid 3he-B, Richard A. Webb

Faculty Publications

Nonlinear cw NMR behavior, similar to that observed on ferromagnets at high rf powers, has been observed in superfluid 3He-B using a superconducting quantum interference device (SQUID) magnetometer in fields of 31, 102, 180, and 308 Oe and at a pressure of 21 bar. It is observed that beyond some threshold value of the transverse H1 field the z component of magnetization anomalously decreases and is followed by a rapid increase back to the normal resonance curve as the frequency of the H1 field is swept through resonance.

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The Use Of Nuclear Reactions And Sims For Quantitative Depth Profiling Of Hydrogen In Amorphous Silicon, David D. Allred, G. J. Clark, C. W. White, B. R. Appleton, C. W. Magee, D. E. Carlson Aug 1977

The Use Of Nuclear Reactions And Sims For Quantitative Depth Profiling Of Hydrogen In Amorphous Silicon, David D. Allred, G. J. Clark, C. W. White, B. R. Appleton, C. W. Magee, D. E. Carlson

Faculty Publications

Depth profiles for hydrogen in amorphous silicon have been determined by the use of resonant nuclear reactions [1H(15N,αγ)12C and 1H(19F,αγ)16O] and by secondary ion mass spectroscopy (SIMS). Independent calibration procedures were used for the two techniques. Measurements were made on the same amorphous silicon film to provide a direct comparison of the two hydrogen analysis techniques. The hydrogen concentration in the bulk of the film was determined to be about 9 at % H. The SIMS results agree with the resonant nuclear reaction results to within 10%, which demonstrates that quantitative hydrogen depth profiles can be obtained by SIMS analysis …

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Measurement Of The Difference Between The Dynamic Nmr And Static Susceptibilites Of Superfluid 3he-B Using An Rf-Biased Superconducting Quantum-Interference Device, Richard A. Webb May 1977

Measurement Of The Difference Between The Dynamic Nmr And Static Susceptibilites Of Superfluid 3he-B Using An Rf-Biased Superconducting Quantum-Interference Device, Richard A. Webb

Faculty Publications

The temperature-dependent susceptiblity of superfluid 3He-B has been measured both statically and via a pulse technique in a field of 309 G using an rf-biased superconducting quantum-interference device (SQUID). In the pressure range 26.5 to 18 bar, the dynamic NMR susceptiblity agrees qualitatively with the theoretical weak-coupling predictions for the Balian-Werthamer state. However, the static susceptiblity, measured using the same rf-biased SQUID and detection system, is significantly smaller thatn the dynamic susceptibility.

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Computer Model Of A Fast Toroidal Plasma Compression, With Application To The Topolotron, H. Mark Nelson, Keith H. Brown, Charles A. Hart Nov 1976

Computer Model Of A Fast Toroidal Plasma Compression, With Application To The Topolotron, H. Mark Nelson, Keith H. Brown, Charles A. Hart

Faculty Publications

The method is developed for a comuputer sequence which models a fast toroidal theta or screw pinch for a highly conducting axially symmetric plasma. The computer sequence takes into account the self-inductances of the plasma and the external conductors which drive the compression as well as the mutual inductance which electromagnetically couples the two. The computer sequence is divided into three phases: a snowplow compression phase, an adiabatic compression phase, and a crowbarred circuit phase. The computer sequence is applied to a topolotron and an example is given of a magnetohydrodynamic equilibrium geometry for which the plasma surface possesses an …

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Behavior Of A Manganin Coil At Elevated Temperatures And Pressures, Jonathan D. Weiss, Daniel L. Decker, H. B. Vanfleet Sep 1976

Behavior Of A Manganin Coil At Elevated Temperatures And Pressures, Jonathan D. Weiss, Daniel L. Decker, H. B. Vanfleet

Faculty Publications

We have determined the temperature dependence in the first-order pressure coefficient of a manganin coil to be dBetaB/dT= (4.2±0.8) ×10^-7 (kbar degrees C)^-1 in the equation R (T,P) =R (T,0)(1+BetaP+GammaP2), where R (T,P) is the coil resistance at temperature T and pressure P. The measurements were performed by simultaneously measuring the coil resistance and temperature at the transitions of mercury and bismuth, which were observed by differential thermal analysis.

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Relaxation Of The Wall-Pinned Magnetization Ringing Mode In Superfluid 3he-B, Richard A. Webb, R. E. Sager, J. C. Wheatley Oct 1975

Relaxation Of The Wall-Pinned Magnetization Ringing Mode In Superfluid 3he-B, Richard A. Webb, R. E. Sager, J. C. Wheatley

Faculty Publications

Observations of the wall-pinned mode in 3He-B allow new magnetic relaxation phenomena to be studied. Excepting the quantitative value of the zero-time ringing frequency, comparison of experiment with theory is satisfactory, including a linear dependence of the square of the ringing period on time and a square-root singularity near Tc in the relaxation parameter.

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Nonlinear Parallel Ringing Of Magnetization In Superfluid 3he, Richard A. Webb, R. E. Sager, J. C. Wheatley Oct 1975

Nonlinear Parallel Ringing Of Magnetization In Superfluid 3he, Richard A. Webb, R. E. Sager, J. C. Wheatley

Faculty Publications

Experiments based on an analogy to the ac Josephson effect have shown in both 3He-A and 3He-B that a pairing theory of the superfluidity of 3He is essentially correct. Additional observations of parallel ringing are not in agreement with the simple "pendulum models" used to describe nonlinear dynamic magnetic effects.

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Relationship Between The Linear Ringing Frequencies In 3he-A And 3he-B Near The Polycritical Point, Richard A. Webb, R. E. Sager, J. C. Wheatley Sep 1975

Relationship Between The Linear Ringing Frequencies In 3he-A And 3he-B Near The Polycritical Point, Richard A. Webb, R. E. Sager, J. C. Wheatley

Faculty Publications

New measurements of parallel ringing in a quasi-ideal geometry for 3He-B near the temperature and pressure of the polycritical point suggest fB2/fA2∼5/2, where fB and fA are the linear parallel-ringing frequencies at a given temperature near Tc. This result approaches the prediction of theory using the Anderson-Brinkman-Morel and Balian-Werthamer states to describe 3He-A and 3He-B, respectively, and hence the results of Osheroff at melting pressure, but disagrees with earlier observations at pressures near the polycritical point.

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