81 research outputs found
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
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
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.
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
Materials Science and Engineering A334 (2002) 173 -- 178
We have conducted pulling and bending tests on individual carbon nanotubes in-situ in a transition electron microscope. Based on our observation of the force required to break the tube, a tensile strength of 0.15 TPa was computed. From corresponding bending studies on such nanotubes, the Young's modulus was estimated to be 0.9 TPa (0.8 TPa after `sub continuum' corrections). These results suggest a strength that is a large fraction of the elastic modulus, although previous measurements of their elastic stiffness have yielded higher modulus values, by as much as a factor of 2. The result does indicate that individual nanotubes can fail as essentially defect-free materials. Furthermore, we observed no obvious reduction in cross-sectional area prior to the failure. In addition, the bending experiments revealed a remarkable flexibility in these tubes. These unique properties support the potential of nanotubes as reinforcement fibers in structural materials. 2002 Elsevier Science B.V. All rights reserved
Magnetic force imaging of a chain of biogenic magnetite and Monte Carlo analysis of tip–particle interaction
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