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Recent measurements of the low-field, induced torque in sodium and potassium by Elliott and Datars show that the resistivity anisotropy increases with increasing magnetic field. The zero-field resistivity anisotropy, unexpected for cubic symmetry, is explained by the charge-density-wave (CDW) structure. Due to the wave-function mixing caused by the CDW potential, the momentum transfer (by isotropic impurities) is much larger for electrons near the CDW energy gap. This is modeled by an anisotropic relaxation time in k\ensuremath{\rightarrow} space. The Boltzmann transport equation in a magnetic field can then be solved exactly. The computed resistivity anisotropy is higher for ${\ensuremath{\omega}}_{c}$\ensuremath{\tau}g1 compared with its zero-field value. The effect of the magnetic field is to ``stir'' the electron distribution f(k\ensuremath{\rightarrow}); this feeds electrons into the region of rapid relaxation and thereby increases the resistivity anisotropy.

Oscillations reported by Dunifer et al. in microwave transmission through thin K layers are found to be periodic in 1/H. The oscillations arise from conduction-electron Landau levels passing through a small cylindrical sheet of the Fermi surface. This cylinder had been envisioned theoretically after incorporating both charge-density-wave and crystalline potentials in Schr\"odinger's equation. The cylinder's cross-sectional area is found to be \ensuremath{\pi}${\mathit{k}}_{\mathit{F}}^{2}$/69, in agreement with the area inferred from the perpendicular-field cyclotron resonance, discovered by Grimes in the surface impedance.

Neutron diffraction from a stress-free crystal of potassium shows no increase in its (\ensuremath{\sim}0.03\ifmmode^\circ\else\textdegree\fi{}) mosaic width after repeated cycling between 4 and 300 K. Furthermore, no temperature-dependent, Huang-type diffuse scattering near [011] and [022] was found. Properties such as these, indicative of thermally-induced plastic deformation, were reported for the potassium crystals studied by Pintschovius et al. and by Blaschko et al. Their data also exhibited a sample-caused background as much as 10 times larger than our stress-free crystal. Using 2.35-\AA{}-wavelength neutrons and tight collimation, we find that the satellite structure near [011] retains the same spacing near [022]. The structure cannot be attributed to a double-scattering artifact, which requires a spacing proportional to tan${\mathrm{\ensuremath{\theta}}}_{\mathit{B}}$. The data are consistent with a charge-density-wave interpretation.

The sections in this article are 1 Introduction 2 Cross Relaxation 3 Nuclear Polarization by ESR Saturation 4 Verification 5 ESR Frequency Shift 6 Biogrpahical Sketch

Induced-torque anisotropies observed in single-crystal spheres of potassium prove that the Fermi surface is multiply connected. Cyclotron orbits which intersect heterodyne gaps created by the charge-density-wave broken symmetry lead to an anisotropic Hall effect having longitudinal components. Thereby the theoretical induced torque (in a 360\ifmmode^\circ\else\textdegree\fi{} magnet rotation) has four evenly spaced minima and four maxima with a staggered spacing. The maxima grow almost $\ensuremath{\sim}{H}^{2}$ and can be 30 times higher than the minima. All such features have been observed. (Details depend on crystal growth and orientation.) All are impossible in a spherical crystal with a simply connected Fermi surface.

Phonon spectra of fct indium and hcp magnesium are calculated using the dynamic pseudopotential theory developed by Wang and Overhauser. Good agreement with experimental data is achieved with two adjustable pseudopotential parameters.

The microwave surface resistance of potassium in a perpendicular magnetic field, measured by Baraff, Grimes, and Platzman in 1969, has never been completely explained until now. The sharp cyclotron resonance peak (at a magnetic field ${\mathit{H}}_{\mathit{c}}$) is caused by the small cylindrical section of Fermi surface created by the charge-density-wave (CDW) minigaps, having periodicities K${\ensuremath{\rightarrow}}_{\mathit{n}}$=(n+1)Q\ensuremath{\rightarrow}-${\mathit{nG}\ensuremath{\rightarrow}}_{110}$. The shape of the observed resonance requires a tilt of the CDW vector Q\ensuremath{\rightarrow} away from [110], predicted by Giuliani and Overhauser in 1979. An abrupt drop of the surface resistance for |H|\ensuremath{\gtrsim}|${\mathit{H}}_{\mathit{c}}$| is caused by the heterodyne gaps, which have periodicities K${\ensuremath{\rightarrow}}_{\mathit{n}}$=n(G${\ensuremath{\rightarrow}}_{110}$-Q\ensuremath{\rightarrow}). These very small gaps, which begin to undergo magnetic breakdown for fields H\ensuremath{\gtrsim}1 T, interrupt the cyclotron motion of equatorial orbits. The abrupt drop in surface resistance for |H|\ensuremath{\gtrsim}|${\mathit{H}}_{\mathit{c}}$| is caused by the resulting partial loss of carrier effectiveness for electrons having velocities nearly parallel to the surface. \textcopyright{} 1996 The American Physical Society.

In recent experiments it was found that the Kapitza conductance between diamond and Pb at room temperature is as much as 100 times larger than predicted by the acoustic mismatch model. Because the diamond and Pb are in intimate contact, the phonon modes near the interface are joint vibrational modes, extending a phonon mean free path on each side of the interface. Since the phonon mean-free path at room temperature is of the order of 4000 \AA{} in diamond but only of the order of 10 \AA{} in Pb, the energy of a joint mode resides primarily in the diamond. Consequently, the thermal population of a joint mode is determined by the temperature of the diamond. It is shown here that the direct energy transfer between metallic electrons at the Pb temperature and joint vibrational modes at the diamond temperature explains the observed heat conduction across the interface.

Behavior of the isotope effect when the superconducting [ital T][sub [ital c]] becomes large on account of an attractive, short-range interaction is reviewed. We explain the difference between results obtained using the two-square-well model and those found by using McMillan's equation.

Five signals which emerge after transmission through potassium slabs (\ensuremath{\sim}0.1 mm thick) have been studied by Dunifer, Sambles, and Mace at 1.3 K with 79-GHz microwaves and large magnetic fields, Hz^, perpendicular to the slabs. They are (i) conduction-electron-spin resonance (together with spin-wave sidebands); (ii) Gantmakher-Kaner (GK) oscillations; (iii) cyclotron resonance; (iv) cyclotron-resonance subharmonics; and (v) high-frequency oscillations. Spin-resonance transmission has been understood for many years, but not the other four. The isotropic-Fermi-surface model is used here to calculate the transmitted power versus H by solving Maxwell's equations self-consistently with the Boltzmann transport equation. GK oscillations do emerge, but the (theoretical) transmitted power is too large by a factor \ensuremath{\sim}10 000. In contrast, the remaining three signals should not even exist. It has been shown that a charge-density-wave broken symmetry creates a family of (higher-order) minigaps which cuts through the Fermi surface near its extremities along the slab normal. These gaps diminish the effectiveness of conduction electrons having ${\mathit{v}}_{\mathit{z}}$ near the Fermi velocity. Calculation confirms that this ineffectiveness dramatically reduces the predicted amplitude of the GK oscillation, and thereby provides an interpretation of the huge discrepancy. The minigaps also give rise to a small Fermi-surface cylinder which (as recently shown) leads to Landau-level oscillations that quantitatively explain signal (v). Signals (iii) and (iv) can also be attributed to the small Fermi-surface cylinder, but only because its axis is tilted \ensuremath{\sim}45\ifmmode^\circ\else\textdegree\fi{} relative to z^. A nonlocal theory for such a scenario is yet to be formulated.

The influence of ordinary (nonmagnetic) impurities on the transition temperature of a traditional singlet-pairing superconductor is revisited. The theory of Anderson is juxtaposed with that due to Abrikosov and Gor'kov (AG). Whereas Anderson's treatment leads to no substantial decrease of ${\mathit{T}}_{\mathit{c}}$ with impurity substitution, the Abrikosov-Gor'kov theory predicts a large decrease of ${\mathit{T}}_{\mathit{c}}$, linear in the impurity concentration. This latter result is not well known because an unallowed mathematical manipulation canceled a large ${\mathit{T}}_{\mathit{c}}$ decrease required by a correct evaluation of the AG theory. The controversy that arises is herein settled in favor of Anderson. The Green's functions employed in the AG method are not appropriate for an impure Bardeen-Cooper-Schrieffer superconductor. It is shown that if the required projected Green's functions (not derivable from a Dyson equation) are employed, Anderson's theorem is recovered.

A hypothetical transition-metal superconductor having a narrow {ital d} band and a sublattice of filled-shell negative ions is studied. The transverse collective vibrations of the last-filled negative-ion shells (e.g., {ital L} shells for O{sup 2{minus}} or {ital M} shells for S{sup 2{minus}}) are invoked to mediate an interaction between the Bloch states of a nearly localized {ital d} band. The coupling constant {lambda}{sub {ital c}} that determines the critical temperature is surprisingly large. {Tc}{similar to}{ital w} exp({minus}1/({lambda}{sub {ital c}}{minus}{mu})), where {ital w} is the {ital d}-band width and {mu} is the unreduced Coulomb parameter (i.e., not {mu}{sup *}). The explicit expression for {lambda}{sub {ital c}} provides guidelines for selection of potential superconducting compounds.

The dynamic pseudopotential model for phonons in metals involves division of each ion into an inner-ion core and a last-filled electron shell. Vibrations of the cores and shells have different amplitudes. Only Coulomb interactions, screened by the conduction-electron sea, enter the theory. The core and shell pseudopotentials involve just one parameter each, and these are determined phenomenologically for Na, K, Rb, Cu, Ag, Au, Ca, Sr, Ba, Yb, Al, and Pb. Distortion of the ions is found to be very important in the heavy metals. A recently discovered anomaly in the phonon dispersion of Ba can be attributed to exchange interactions between the (last-filled) shell and the conduction-electron sea. The (so far unmeasured) spectra for fcc Sr and bcc Yb are predicted.

Superconductivity in a metal with a spin-density-wave (SDW) broken symmetry is investigated within the framework of the Bardeen-Cooper-Schrieffer (BCS) theory. The gap equation is solved analytically, leading to a gap that falls to zero at the SDW gap for a spiral SDW, and to a small, but nonzero value, for a linear SDW. The electronic specific heat in the superconducting state acquires a low-temperature tail, far in excess of a BCS exponential falloff. The ultrasonic attenuation exhibits a similar deviation from the usual BCS result.

We have investigated the cyclotron resonance for two degenerate electron systems interacting with one another through the electron-electron collision time ${\ensuremath{\tau}}_{e}(T)$ defined as the relaxation time of the relative momentum. The coupled kinetic equations for the total and the relative momentum are solved in the presence of a static magnetic field and a frequency-dependent electric field. The solution for the power absorption is discussed in terms of the concentration ratio of the electrons, $\frac{{n}_{1}}{{n}_{2}}$, and the parameters $\frac{{\ensuremath{\tau}}_{1}}{{\ensuremath{\tau}}_{e}}$, $\frac{{\ensuremath{\tau}}_{2}}{{\ensuremath{\tau}}_{e}}$, where ${\ensuremath{\tau}}_{1,2}$ are ordinary scattering times. We find that our results are consistent with the observed temperature dependence of the cyclotron resonance of inversion layer electrons in Si, for $T\ensuremath{\gtrsim}25\ifmmode^\circ\else\textdegree\fi{}$K. At lower temperatures also, the frequency dependence of ${\ensuremath{\tau}}_{e}$ becomes important and affects the cyclotron resonance in a two-electron system, even as $T\ensuremath{\rightarrow}0$.

A criterion for the preferred direction of the wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}$ of a charge-density wave (CDW) is obtained by means of a simple theory. Screening of the electric field caused by the CDW is provided by a sinusoidal distortion of the positive-ion lattice. The optimum $\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}$ direction is that which minimizes the elastic energy of distortion. For potassium the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}$ direction is found to be tilted about 4\ifmmode^\circ\else\textdegree\fi{} away from a [110] direction. The exact value of the tilt depends on the magnitude of $\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}$.

We have sought to determine whether an inhomogeneous residual resistivity could cause the low-temperature induced-torque anisotropy observed in single-crystal spheres of potassium, especially the nonsaturating, four-peak patterns observed between 5 and 25 kOe. On the basis of our previous results and the analysis presented herein, we conclude that the residual-resistivity gradient cannot explain induced-torque anomalies.

A theoretical study is made of the generation of ultrasound as a function of an applied, static magnetic field for propagation along a [110] direction in a potassium crystal. For certain orientations of the incident radio-frequency field the variation of acoustic amplitude with field presents a cusp. The position of this minimum can be used to determine the departure of the Hall coefficient from its free-electron value.

Anomalies observed in electromagnetic generation of ultrasound in aluminum are explained on the basis of known Fermi-surface geometry. At large values of ${\ensuremath{\omega}}_{c} \ensuremath{\tau}$ the topology of the second-zone hole sheet permits only holelike cyclotron orbits, while for small values of ${\ensuremath{\omega}}_{c} \ensuremath{\tau}$ the local curvature of the hole sheet is mostly electronlike. If follows that the effective number of electronlike and holelike carriers must be allowed to vary with magnetic field. These features, together with the Bragg-reflection force (caused by accelerating holes), are incorporated into the theory of electromagnetic generation. The observed anomalies are explained without using any adjustable parameter.