Electric Polarization of Heteropolar Nanotubes as a Geometric Phase

The three-fold symmetry of planar boron nitride, the III-V analog to graphene, prohibits an electric polarization in its ground state, but this symmetry is broken when the sheet is wrapped to form a BN nanotube. We show that this leads to an electric polarization along the nanotube axis which is controlled by the quantum mechanical boundary conditions on its electronic states around the tube circumference. Thus the macroscopic dipole moment has an {\it intrinsically nonlocal quantum} mechanical origin from the wrapped dimension. We formulate this novel phenomenon using the Berry's phase approach and discuss its experimental consequences.Comment: 4 pages with 3 eps figures, updated with correction to Eqn (9

Antiferromagnetic Domains and Superconductivity in UPt3

We explore the response of an unconventional superconductor to spatially inhomogeneous antiferromagnetism (SIAFM). Symmetry allows the superconducting order parameter in the E-representation models for UPt3 to couple directly to the AFM order parameter. The Ginzburg-Landau equations for coupled superconductivity and SIAFM are solved numerically for two possible SIAFM configurations: (I) abutting antiferromagnetic domains of uniform size, and (II) quenched random disorder of `nanodomains' in a uniform AFM background. We discuss the contributions to the free energy, specific heat, and order parameter for these models. Neither model provides a satisfactory account of experiment, but results from the two models differ significantly. Our results demonstrate that the response of an E_{2u} superconductor to SIAFM is strongly dependent on the spatial dependence of AFM order; no conclusion can be drawn regarding the compatibility of E_{2u} superconductivity with UPt3 that is independent of assumptions on the spatial dependence of AFMComment: 12 pages, 13 figures, to appear in Phys. Rev.

Local Defect in Metallic Quantum Critical Systems

We present a theory of a single point, line or plane defect coupling to the square of the order parameter in a metallic system near a quantum critical point at or above its upper critical dimension. At criticality, a spin droplet is nucleated around the defect with droplet core size determined by the strength of the defect potential. Outside the core a universal slowly decaying tail of the droplet is found, leading to many dissipative channels coupling to the droplet and to a complete suppression of quantum tunneling. We propose an NMR experiment to measure the impurity-induced changes in the local spin susceptibility.Comment: 2 figures; 5 page

Stacking faults in UPt3

Atomic resolution transmission electron microscopy and X-ray diffraction measurements have been combined to investigate the microstructure of superconducting UPt3. Regions of a second double hexagonal phase with a typical dimension of 25-30 A are found to occupy approximately 3% of the total sample volume.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30799/1/0000453.pd

Domains and domain nucleation in magnetron-sputtered CoCr thin films

CoCr thin films have been of interest for a number of years due to their strong perpendicular anisotropy, favoring magnetization normal to the film plane. The microstructure and magnetic properties of CoCr films prepared by both rf and magnetron sputtering have been examined in detail. By comparison, however, relatively few systematic studies of the magnetic domain structure and its relation to the observed film microstructure have been reported. In addition, questions still remain as to the operative magnetization reversal mechanism in different film thickness regimes. In this work, the magnetic domain structure in magnetron sputtered Co-22 at.%Cr thin films of known microstructure were examined by Lorentz transmission electron microscopy. Additionally, domain nucleation studies were undertaken via in-situ heating experiments.It was found that the 50 nm thick films, which are comprised of columnar grains, display a “dot” type domain configuration (Figure 1d), characteristic of a perpendicular magnetization. The domain size was found to be on the order of a few structural columns in diameter.</jats:p

In situ transmission electron microscopy study of the paraelectric to ferroelectric (cubic to tetragonal) phase transformation in lanthanum-modified lead titanate ceramics

Lead titanate (PbTiO3) forms the ferroelectric end of the ferroelectric-antiferroelectric solid solution series PbTiO3-PbZrO3 (PZT), which is of considerable interest in various piezoelectric applications. Above the Curie point (TC), the material is cubic (paraelectric), point group m3m. Below this temperature transformation occurs to the tetragonal (ferroelectric) phase, point group 4mm. The strain generated in this transformation is relieved via the formation of twin-related structural (ferroelectric) domains. Lanthanum is added to polycrystalline lead titanate to aid in the poling operation (alignment of electric dipoles in individual grains via the application of a large electric field), through a reduction of the c/a ratio in the tetragonal phase. This transformation has been observed in this material in-situ via transmission electron microscopy (TEM).</jats:p

High Resolution Electron Microscopy of thin Co-Cr films

Co-Cr thin films have been studied extensively as leading candidates for perpendicular recording media. The development of microstructure in this system has been reported by a number of investigators. These studies have revealed a general growth scenario in which a fine grained, randomly oriented "transition layer" forms first, followed by the development of columnar grains having their hcp c axes textured normal to the film plane. However, it has been found that such columns are not present in very thin (∽10 nm) layers. Also, it has been inferred from electron microdiffraction studies that both c and a axis texturing occurs in these same films. However, direct evidence for this texturing is still needed. In this work, high resolution electron microscopy (HREM) has been employed to examine very thin (5-10 nm) Co-Cr layers.</jats:p