Scaling in the Fan of an Unconventional Quantum Critical Point

We present results of extensive finite-temperature Quantum Monte Carlo simulations on a SU(2) symmetric S=1/2 quantum antiferromagnet with a four-spin interaction [Sandvik, Phys. Rev. Lett. 98, 227202 (2007)]. Our simulations, which are free of the sign-problem and carried out on lattices containing in excess of 1.6 X 10^4 spins, indicate that the four-spin interaction destroys the N\'eel order at an unconventional z=1 quantum critical point, producing a valence-bond solid paramagnet. Our results are consistent with the `deconfined quantum criticality' scenario.Comment: published version, minor change

Tailoring the ground state of the ferrimagnet La2Ni(Ni1/3Sb2/3)O6

We report on the magnetic and structural properties of La2Ni(Ni1/3Sb2/3)O6 in polycrystal, single crystal and thin film samples. We found that this material is a ferrimagnet (Tc ~ 100 K) which possesses a very distinctive and uncommon feature in its virgin curve of the hysteresis loops. We observe that bellow 20 K it lies outside the hysteresis cycle, and this feature was found to be an indication of a microscopically irreversible process possibly involving the interplay of competing antiferromagnetic interactions that hinder the initial movement of domain walls. This initial magnetic state is overcome by applying a temperature dependent characteristic field. Above this field, an isothermal magnetic demagnetization of the samples yield a ground state different from the initial thermally demagnetized one.Comment: 21 pages, 8 figures, submitted to JMM

Black Hole Entropy from a Highly Excited Elementary String

Suggested correspondence between a black hole and a highly excited elementary string is explored. Black hole entropy is calculated by computing the density of states for an open excited string. We identify the square root of oscillator number of the excited string with Rindler energy of black hole to obtain an entropy formula which, not only agrees at the leading order with the Bekenstein-Hawking entropy, but also reproduces the logarithmic correction obtained for black hole entropy in the quantum geometry framework. This provides an additional supporting evidence for correspondence between black holes and strings.Comment: revtex, 4 page

Large-N estimates of universal amplitudes of the CP^{N-1} theory and comparison with the JQ model

We present computations of certain finite-size scaling functions and universal amplitude ratios in the large-N limit of the CP^{N-1} field theory. We pay particular attention to the uniform susceptibility, the spin stiffness and the specific heat. Field theoretic arguments have shown that the long-wavelength description of the phase transition between the Neel and valence bond solid states in square lattice S=1/2 anti-ferromagnets is expected to be the non-compact CP^1 field theory. We provide a detailed comparison between our field theoretic calculations and quantum Monte Carlo data close to the Neel -VBS transition on a S=1/2 square-lattice model with competing four-spin interactions (the JQ model).Comment: 15 page

In situ characterization of vertically oriented carbon nanofibers for three-dimensional nano-electro-mechanical device applications

We have performed mechanical and electrical characterization of individual as-grown, vertically oriented carbon nanofibers (CNFs) using in situ techniques, where such high-aspect-ratio, nanoscale structures are of interest for three-dimensional (3D) electronics, in particular 3D nano-electro-mechanical-systems (NEMS). Nanoindentation and uniaxial compression tests conducted in an in situ nanomechanical instrument, SEMentor, suggest that the CNFs undergo severe bending prior to fracture, which always occurs close to the bottom rather than at the substrate–tube interface, suggesting that the CNFs are well adhered to the substrate. This is also consistent with bending tests on individual tubes which indicated that bending angles as large as ~70° could be accommodated elastically. In situ electrical transport measurements revealed that the CNFs grown on refractory metallic nitride buffer layers were conducting via the sidewalls, whereas those synthesized directly on Si were electrically unsuitable for low-voltage dc NEMS applications. Electrostatic actuation was also demonstrated with a nanoprobe in close proximity to a single CNF and suggests that such structures are attractive for nonvolatile memory applications. Since the magnitude of the actuation voltage is intimately dictated by the physical characteristics of the CNFs, such as diameter and length, we also addressed the ability to tune these parameters, to some extent, by adjusting the plasma-enhanced chemical vapor deposition growth parameters with this bottom-up synthesis approach

Imaging bond order near non-magnetic impurities in square lattice antiferromagnets

We study the textures of generalized "charge densities" (scalar objects invariant under time reversal), in the vicinity of non-magnetic impurities in square-lattice quantum anti-ferromagnets, by order parameter field theories. Our central finding is the structure of the "vortex" in the generalized density wave order parameter centered at the non-magnetic impurity. Using exact numerical data from quantum Monte Carlo simulations on an antiferromagnetic spin model, we are able to verify the results of our field theoretic study. We extend our phenomenological approach to the period-4 bond-centered density wave found in the underdoped cuprates.Comment: 4 pages, 4 figure

Fabrication of wide-IF 200–300 GHz superconductor–insulator–superconductor mixers with suspended metal beam leads formed on silicon-on-insulator

We report on a fabrication process that uses SOI substrates and micromachining techniques to form wide-IF SIS mixer devices that have suspended metal beam leads for rf grounding. The mixers are formed on thin 25 µm membranes of Si, and are designed to operate in the 200–300 GHz band. Potential applications are in tropospheric chemistry, where increased sensitivity detectors and wide-IF bandwidth receivers are desired. They will also be useful in astrophysics to monitor absorption lines for CO at 230 GHz to study distant, highly redshifted galaxies by reducing scan times. Aside from a description of the fabrication process, electrical measurements of these Nb/Al–AlNx/Nb trilayer devices will also be presented. Since device quality is sensitive to thermal excursions, the new beam lead process appears to be compatible with conventional SIS device fabrication technology