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## Full-Text Articles in Physics

Symmetry, Spin-Orbit Interactions, And Spin Anisotropies, Taner Yildirim, A. Brooks Harris, Ora Entin-Wohlman, Amnon Aharony

#### Symmetry, Spin-Orbit Interactions, And Spin Anisotropies, Taner Yildirim, A. Brooks Harris, Ora Entin-Wohlman, Amnon Aharony

*Department of Physics Papers*

The origins of anisotropy in the effective spin Hamiltonian, describing the ground manifold of Hubbard models with spin-orbit interactions, are critically discussed. For tetragonal symmetry, we show exactly that spin anisotropy can arise only if one includes *both* spin-orbit and Coulomb exchange interactions. For lower symmetries, additional anisotropies arise from terms which were hitherto neglected. Our analytic results are supported by numerical solutions for single bond clusters. These results can explain the easy plane anisotropy in the antiferromagnetic cuprates.

Mean-Field Theory For Alkali-Metal-Doped Polyacetylene, A. Brooks Harris

#### Mean-Field Theory For Alkali-Metal-Doped Polyacetylene, A. Brooks Harris

*Department of Physics Papers*

The Landau theory used by Choi and Mele (CM) to treat their rotor model on a triangular lattice for the orientational ordering of polyacetylene chains in alkali-metal-doped polyacetylene is studied. A reanalysis of the higher-order terms in the Landau expansion indicates that cosine ordering can support a nonzero cubic term in the Landau expansion whereas the sine-ordered phase has no such term. To construct a phase diagram requires a numerical solution of the self-consistent equations of mean-field theory. Although this analysis does not convincingly treat the incommensurate phases found by CM, it does identify an unusually rich variety of thermodynamically ...

Influence Of Boundary Reflection And Refraction On Diffusive Photon Transport, Douglas J. Durian

#### Influence Of Boundary Reflection And Refraction On Diffusive Photon Transport, Douglas J. Durian

*Department of Physics Papers*

We report computer simulations which test the accuracy of the diffusion theories used in the analysis of multiple light scattering data. Explicitly including scattering anisotropy and boundary reflections, we find that the predicted probability for transmission through a slab is accurate to 1% if the slab thickness is greater than about 5 transport mean free paths. For strictly isotropic scattering and no boundary reflections, the exact diffusion theory prediction is accurate to this level for all thicknesses. In addition, we predict how the angular distribution of transmitted photons is affected by boundary reflectivity, both with and without refraction. Simulations show ...

Symmetry Analysis Of The 2a Phase Of C_{60}, A. Brooks Harris, Ravi Sachidanandam, Taner Yildirim

#### Symmetry Analysis Of The 2a Phase Of C60, A. Brooks Harris, Ravi Sachidanandam, Taner Yildirim

*Department of Physics Papers*

A symmetry analysis of the 2*a* phase recently observed in some samples of C_{60} is presented. This phase is described by a unit cell with eight molecules in inequivalent orientations. We first show that if this structure is assumed to be exactly cubic, there are only three allowed space groups, none of which corresponds to the *Pa*3^{¯} arrangement of threefold axes previously established for C_{60} by several groups. Our calculated powder diffraction spectra for these space groups are not consistent with existing experimental data. Second, if the symmetry of the *Pa*3^{¯} structure is lowered by ...

Spin Structures Of Tetragonal Lamellar Copper Oxides, Taner Yildirim, A. Brooks Harris, Ora Entin-Wohlman, Amnon Aharony

#### Spin Structures Of Tetragonal Lamellar Copper Oxides, Taner Yildirim, A. Brooks Harris, Ora Entin-Wohlman, Amnon Aharony

*Department of Physics Papers*

The spin Hamiltonian of tetragonal lamellar antiferromagnets is shown to contain several novel anisotropies. Symmetry allows *bond-dependent* anisotropic exchange interactions, which lead to (a) interplane mean-field coupling and (b) an in-plane anisotropy which vanishes classically but arises from quantum zero point energy (QZPE). A similar QZPE involving the interplane isotropic interaction prefers collinear spins. Adding also diploar anisotropy, the competition between all these effects explains for the first time the spin structures of many cuprates.

Viscous And Elastic Fingering Instabilities In Foam, S. S. Park, Douglas J. Durian

#### Viscous And Elastic Fingering Instabilities In Foam, S. S. Park, Douglas J. Durian

*Department of Physics Papers*

We investigate pattern formation in the immiscible displacement of foam from a confining geometry. The dominant length scales with the driving pressure gradient as for a Newtonian fluid, but depends surprisingly on the ratio of plate separation to bubble diameter. The pattern morphology exhibits a transition from a jagged, presumably elastic, character to a smooth, viscous character as the shear strain rate rises far above the natural internal relaxation time scale of the foam. These features may result from the two-dimensional nature of the flows and the unusual rheology of foams.

Phase Diagram Of The Dilute Ising Spin Glass In General Spatial Dimension, Shye Shapira, Lior Klein, Joan Adler, Amnon Aharony, A. Brooks Harris

#### Phase Diagram Of The Dilute Ising Spin Glass In General Spatial Dimension, Shye Shapira, Lior Klein, Joan Adler, Amnon Aharony, A. Brooks Harris

*Department of Physics Papers*

We use high-temperature and low-concentration series to treat the dilute spin glass within a model with nearest-neighbor interactions which randomly assume the values +*J*, 0, -*J* with probabilities *p*/2, 1-*p*, *p*/2, respectively. Using the Harris no-free-end diagrams scheme in general spatial dimension, we obtained 15th-order series for χ^{EA} as a function of temperature for arbitrary dilution, 14th-order series for χ^{EA} as a function of dilution for selected temperatures, and 11th-order series for two higher derivatives of χ^{EA} with respect to the ordering field, where χ^{EA} is the Edwards-Anderson spin-glass susceptibility. Analysis of these ...

Solid H_{2} And D_{2}: Remarkable Differences In Some Nmr Properties, A. Brooks Harris, Horst Meyer, X. Qin

#### Solid H2 And D2: Remarkable Differences In Some Nmr Properties, A. Brooks Harris, Horst Meyer, X. Qin

*Department of Physics Papers*

The differences in the observed properties of solid H_{2} and D_{2} are reviewed, and in particular those encountered in NMR experiments. The failure to detect a sharp NMR (*I*=1) impurity ‘‘isolated pair’’ spectrum in *p*-D2 is discussed in terms of a larger crystalline field than in H_{2}, where an intense and sharp pair spectrum has been observed. Furthermore, we discuss the dramatic (*I*=1) solid echo signal loss with decreasing temperature which is observed in solid D_{2}, but not for solid H_{2}. A theory of the solid echo damping through orientational fluctuations is ...

Mean-Field Theory For C_{70}, Ravi Sachidanandam, A. Brooks Harris

#### Mean-Field Theory For C70, Ravi Sachidanandam, A. Brooks Harris

*Department of Physics Papers*

Orientational ordering transitions in C_{70} are studied by constructing a Landau free energy in terms of order parameters describing long-range orientational order. This theory predicts that the transition from the orientationally disordered state into a partially ordered state, where the long axes of the molecules are parallel to one another, is discontinuous with an elastic distortion. Order parameters describing the lower temperature transition, where spinning about the long axis becomes hindered, are also discussed.

Measure Factors, Tension, And Correlations Of Fluid Membranes, W Cai, T C. Lubensky, Philip C. Nelson, Thomas Powers

#### Measure Factors, Tension, And Correlations Of Fluid Membranes, W Cai, T C. Lubensky, Philip C. Nelson, Thomas Powers

*Department of Physics Papers*

We study two geometrical factors needed for the correct construction of statistical ensembles of surfaces. Such ensembles appear in the study of fluid bilayer membranes, though our results are more generally applicable. The naive functional measure over height fluctuations must be corrected by these factors in order to give correct, self-consistent formulas for the free energy and correlation functions of the height. While one of these corrections the Faddeev, Popov determinant has been studied extensively, our derivation proceeds from very simple geometrical ideas, which we hope removes some of its mystery. The other factor is similar to the Liouville correction ...