Hans Mueller

Bio: Hans Mueller is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Light scattering & Birefringence. The author has an hindex of 10, co-authored 14 publications receiving 396 citations.

Properties of Rochelle Salt

Abstract: In a qualitative way the anomalous properties of Rochelle salt can be explained on the basis of four different theories: ($a$) on the dipole theory, ($b$) by assuming polymorphic transitions at the Curie points, ($c$) by postulating an anomalous piezoelectric effect, and ($d$) with an inter-action theory which assumes that the structure and the fundamental properties of the crystal have no unusual characteristics and which ascribes the observed anomalies to an accidental degeneration of the piezoelectric interaction between the elastic deformation and the electric polarization. The existence of a polymorphic transition at the upper Curie point is demonstrated in a new experiment and it is shown that this transition is a spontaneous elastic deformation. It can be explained on the basis of either one of the four theories. A quantitative analysis of the experimental data, which leads to a new interpretation of the elastic and piezoelectric measurements, shows that the theories ($b$) and ($c$) are not valid for Rochelle salt. The experimental data verify the conclusions of the dipole theory insofar as they lead to the result that Rochelle salt would show an abnormal temperature dependence of the dielectric constant even if the crystal were not piezo-electric or if the piezoelectric deformations could be suppressed. However, it is doubtful whether the clamped crystal has a Curie point. The observed Curie points of the free crystal result from the lattice-dipole coupling as described by the interaction theory.

The Intensity and Polarization of the Light Diffracted by Supersonic Waves in Solids

Abstract: The intensity and polarization of the light diffracted by supersonic waves in solids are calculated by considering the photoelastic effect due to the strains created by the elastic waves. For natural incident light the diffracted light is partially polarized. In glasses and cubic crystals the diffracted light is partially depolarized if the incident light is polarized. The intensities are evaluated with the help of the theory of Raman and Nath and it is shown that this theory explains the measurements better than Brillouin's approximation. The calculated intensity distributions for natural and polarized incident light agree with the observations of Schaefer and Bergmann on glasses, quartz and calcite. This agreement justifies Fues' and Ludloff's assumption that all forced vibrations of the same frequency oscillate with the same amplitude.

Why Are There so Few Magnetic Ferroelectrics

Abstract: Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.

XCVI. Theory of barium titanate

Abstract: Summary The theory of the dielectric and crystallographic properties of barium titanate is considered. By expanding the free energy as a function of polarization and strain and making reasonable assumptions about the coefficients, it is found possible to account for the various crystal transitions. Calculations are made of the dielectric constants, crystal stains, internal energy, and self polarization as functions of temperature. Finally relations are obtained between the coefficients in the free energy and the ionic force constants. These are used to estimate some of the coefficients which are not completely determined by experimental data.

Quadrupole Effects in Nuclear Magnetic Resonance Studies of Solids

Abstract: Publisher Summary This chapter discusses quadrupole effects in nuclear magnetic resonance studies of solids. The first evidence that many nuclei possess magnetic moments came from the study of the hyperfine structure of atomic spectra in the visible region. The interaction of the nuclear magnetic moment with the magnetic field produced by the atomic electrons gives rise to a hyperfine spectrum that is relatively simple, being characterized by the well known “interval rule.” Marked departures from this interval rule do occur in a few cases, however, and some of the departures can definitely be attributed to the presence of a nuclear electric quadrupole interaction. The methods of radio-frequency spectroscopy are very well adapted for the investigation of the very small interaction energies to which nuclear moments give rise. They have led not only to much more precise determinations of nuclear magnetic moments, but also to a vastly increased knowledge of nuclear electric quadrupole effects. The first outstanding success along this line was the discovery, by the molecular beam resonance method, of the quadrupole moment of the deuteron. The field of electric quadrupole interactions in nuclear magnetic resonance can be divided roughly into two areas according to the relative magnitude of the nuclear quadrupole interactions.

Theory of the Transition in KH2PO4

Abstract: Potassium dihydrogen phosphate contains phosphate groups connected by hydrogen bonds. Different possible arrangements of the hydrogens result effectively in different orientations of the (H2PO4)— dipoles. Since these have the lowest energy when pointing along the axis of the crystal, there is a tendency toward spontaneous polarization along this axis, resulting in the well‐known transition, similar to Rochelle salt, with polarization below the Curie point. The theory of this transition is worked out, using statistical methods to count the number of arrangements of hydrogens consistent with each total polarization of the crystal, and deriving the free energy. It is found that the theory predicts a phase change of the first order, with sudden transition from the polarized state at low temperature to the unpolarized state at high temperature, rather than the lambda‐point transition or phase change of the second order which is observed. However, the observed transition is confined to a very narrow temperature range compared to that predicted, for instance, by the Weiss theory, so that it seems as if it might be merely a broadened transition of the first order. It is suggested that the broadening may result from the irregular shifts of transition temperatures of individual domains in the crystal on account of stresses resulting from the large piezoelectric effect and the resulting deformation of the crystal below the transition point. The susceptibility above the Curie point comes out by the theory to be 4.33 times as great as it should according to the Weiss theory, a result which seems to be in general agreement with experiment. The entropy change in the transition is given by the theory as 0.69 unit, somewhat smaller than the observed value of about 0.8 unit. No explanation is suggested for this discrepancy.