Author
A. M. Portis
Bio: A. M. Portis is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Resonance & Faraday rotator. The author has an hindex of 6, co-authored 6 publications receiving 498 citations.
Papers
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TL;DR: In this paper, the suppression by electric fields of longitudinal resistivity anomalies at 145 and 59 K in the compound Nb${\mathrm{Se}}_{3} was reported, and attributed the observed suppression to Zener breakdown across extremely small gaps introduced by the presence of charge density waves.
Abstract: We report the suppression by electric fields of longitudinal resistivity anomalies at 145 and 59 K in the compound Nb${\mathrm{Se}}_{3}$. Sample resistance was determined by conventional four-probe dc measurement as well as with short current pulses. We attribute the observed suppression to Zener breakdown across extremely small gaps introduced by the presence of charge density waves.
342 citations
TL;DR: In this article, the limits of electron spin resonance detection sensitivity were investigated for absorption and induction spectrometers employing superheterodyne detection, and a balanced homodyne detector was used in place of the conventional second detector.
Abstract: The limits of electron spin resonance detection sensitivity are investigated for absorption and induction spectrometers employing superheterodyne detection. The sensitivity of the conventional bridge absorption spectrometer is found to decrease rapidly below the theoretical limit set by the noise figure of the receiver when klystron power is increased above about 10 mw. This decrease is shown to arise from the extreme frequency dependence of the bridge balance. The bimodal cavity induction spectrometer, which has a highly stable frequency independent balance, shows no departure from the theoretical limit. Flicker noise is found to be produced by the i.f. current in the mixer diodes, but it may be greatly reduced by the use of high i.f. gain. The use of a balanced homodyne detector in place of the conventional second detector is found to reduce the over‐all noise figure of the receiver from 13 to 9 db by suppression of carrier noise. The performance of the bridge spectrometer is greatly improved by the homodyne detector which discriminates against frequency noise that enters when the bridge drifts away from balance.
63 citations
TL;DR: In this article, the design and analysis of a bimodal cavity for the observation of microwave Faraday rotation is presented and the coupling between degenerate cavity modes is expressed in terms of the elements of the susceptibility tensor of the material producing the rotation.
Abstract: The design and analysis of a bimodal cavity for the observation of microwave Faraday rotation is presented. An equivalent circuit of lumped elements is developed and the coupling between degenerate cavity modes is expressed in terms of the elements of the susceptibility tensor of the material producing the rotation. The theory is checked against experimental results with a paramagnetic salt and substantial agreement is obtained. A cavity of this type when used in conjunction with superheterodyne detection appears to provide a high sensitivity spectrometer for the observation of magnetic resonance.
50 citations
TL;DR: In this article, the nuclear spin temperature as a function of the power absorbed by the electronic antiferromagnetic resonance was observed and an apparent cooling of the nuclei via their interactions with the electron system was discovered.
Abstract: The Mn/sup 55/ nuclear resonance in KMnF/sub 3/ was observed by a double resonance technique. Also, by observing the nuclear spin temperature as a function of power absorbed by the electronic antiferromagnetic resonance, an apparent cooling of the nuclei via their interactions with the electron system was discovered. (L.N.N.)
29 citations
TL;DR: In this paper, it was found that with increasing rf power a second antiferromagnetic resonance grows at a shifted field position, and that the power level at which substantial growth takes place is in good agreement with theory.
Abstract: The Mn55 nuclear resonance absorption in KMnF3 has been monitored through the observation of antiferromagnetic resonance at 4.2°K and below. It is found that with increasing rf power a second antiferromagnetic resonance grows at a shifted field position. At 4.2°K a shifted resonance can be observed for driving frequencies between 580 and 680 Mc. As the driving frequency is reduced the position of the shifted resonance moves toward that of the unshifted line. These results are interpreted in terms of the nucleation of regions of the sample with nuclear resonance at the driving frequency. Such regions are present in the absence of excitation as the result of electronic pinning. With rf excitation these regions are expected to grow. The power level at which substantial growth takes place is in good agreement with theory.
15 citations
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TL;DR: 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 as mentioned in this paper, which can, even for small electric fields, carry current in a fashion originally envisioned by Frohlich.
Abstract: 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.
1,308 citations
TL;DR: In this article, the authors survey recent theories on the transition between commensurate (C) and incommensurate phases, and on properties of the I phase, and the devil's staircase concept for the I phases is described.
Abstract: Surveys recent theories on the transition between commensurate (C) and incommensurate (I) phases, and on properties of the I phase. The devil's staircase concept for the I phase is described. Differences between theories in two and three dimensions are discussed, together with those on chaotic structures.
1,042 citations
TL;DR: In this article, the authors present the most salient developments of research on organic conductors and superconductors during the past 10 years, including lattice instability in TTF-TCNQ and related compounds, superconducting or antiferromagnetic instabilities in the (TMTSF) 2 X series.
Abstract: This review attempts to present the most salient developments of research on organic conductors and superconductors during the past 10 years. A theoretical introduction treats instabilities of quasi-one-dimensional electron systems and associated precursor effects which are relevant to the experimental results on organic conductors. We then describe the characterization of quasi-one-dimensional organic conductors by their transport, optical and magnetic properties. Finally, two sections are devoted to the experimental investigation of the low temperature instabilities: lattice instability in TTF-TCNQ and related compounds, superconducting or antiferromagnetic instabilities in the (TMTSF) 2 X series. The importance of one-dimensional fluctuations is emphasized in both lattice and superconducting instabilities.
804 citations
TL;DR: In this paper, the static and dynamic properties of spontaneous superstructures formed by electrons are reviewed, and a special attention is paid to the collective effects in pinning and sliding of these structures, and the glassy properties at low temperature.
Abstract: This article reviews the static and dynamic properties of spontaneous superstructures formed by electrons. Representations of such electronic crystals are charge density waves (CDW) and spin density waves in inorganic as well as organic low-dimensional materials. A special attention is paid to the collective effects in pinning and sliding of these superstructures, and the glassy properties at low temperature. Charge order and charge disproportionation which occur in organic materials resulting from correlation effects are analysed. Experiments under magnetic field, and more specifically field-induced CDWs are discussed. Properties of meso-and nanostructures of CDWs are also reviewed.
397 citations
238 citations