Magnetic Structure and Spin Waves in the Kagom\'{e} Jarosite compound KFe3(SO4)2(OH)6{\bf KFe_3(SO_4)_2(OH)_6}

We present a detailed study of the magnetic structure and spin waves in the Fe jarosite compound ${\rm KFe_3(SO_4)_2(OH)_6}$ for the most general Hamiltonian involving one- and two-spin interactions which are allowed by symmetry. We compare the calculated spin-wave spectrum with the recent neutron scattering data of Matan {\it et al.} for various model Hamiltonians which include, in addition to isotropic Heisenberg exchange interactions between nearest ($J_1$) and next-nearest ($J_2$) neighbors, single ion anisotropy and Dzyaloshinskii-Moriya (DM) interactions. We concluded that DM interactions are the dominant anisotropic interaction, which not only fits all the splittings in the spin-wave spectrum but also reproduces the small canting of the spins out of the Kagom\'e plane. A brief discussion of how representation theory restricts the allowed magnetic structure is also given.Comment: 23 pages, 17 figures, submitted to Phys. Rev. B (March 2006

Smelling in Chemically Complex Environments: An Optofluidic Bragg Fiber Array for Differentiation of Methanol Adulterated Beverages

Cataloged from PDF version of article.A novel optoelectronic nose for analysis of alcohols (ethanol and methanol) in chemically complex environments is reported. The cross-responsive sensing unit of the optoelectronic nose is an array of three distinct hollow-core infrared transmitting photonic band gap fibers, which transmit a specific band of IR light depending on their Bragg mirror structures. The presence of alcohol molecules in the optofluidic core quenches the fiber transmissions if there is an absorption band of the analyte overlapping with the transmission band of the fiber; otherwise they remain unchanged. The cumulative response data of the fiber array enables rapid, reversible, and accurate discrimination of alcohols in chemically complex backgrounds such as beer and fruit juice. In addition, we observed that humidity of the environment has no effect on the response matrix of the optoelectronic nose, which is rarely achieved in gas sensing applications Consequently, it can be reliably used in virtually any environment without precalibration for humidity or drying the analytes. Besides the discussed application in counterfeit alcoholic beverages, with its superior sensor parameters, this novel concept proves to be a promising contender for many other applications including food quality control, environmental monitoring, and breath analysis for disease diagnostics

Frustration and Quantum Fluctuations in Heisenberg fcc Antiferromagnets

We consider the quantum Heisenberg antiferromagnet on a face-centered-cubic lattice in which J, the second-neighbor (intrasublattice) exchange constant, dominates J′, the first-neighbor (intersublattice) exchange constant. It is shown that the continuous degeneracy of the classical ground state with four decoupled (in a mean-field sense) simple cubic antiferromagnetic sublattices is removed so that at second order in J′/J the spins are collinear. Here we study the degeneracy between the two inequivalent collinear structures by analyzing the contribution to the spin-wave zero-point energy which is of the form Heff/J=C0+C4σ1σ2σ3σ4(J′/J)4+O(J′/J)5, where σi specifies the phase of the ith collinear sublattice, C0 depends on J′/J but not on the σ’s, and C4 is a positive constant. Thus the ground state is one in which the product of the σ’s is −1. This state, known as the second kind of type A, is stable in the range |J′|\u3c2|J| for large S. Using interacting spin-wave theory, it is shown that the main effect of the zero-point fluctuations is at small wave vector and can be well modeled by an effective biquadratic interaction of the form ΔEQeff=−1/2Q∑i,j[S(i)⋅S(j)]2/S3. This interaction opens a spin gap by causing the extra classical zero-energy modes to have a nonzero energy of order J′√S. We also study the dependence of the zero-point spin reduction on J′/J and the sublattice magnetization on temperature. The resulting experimental consequences are discussed

Three-Dimensional Ordering in bct Antiferromagnets Due to Quantum Disorder

Quantum effects on magnetic ordering in body-centered-tetragonal antiferromagnets with only nearest-neighbor interactions are studied in detail using interacting spin-wave theory. The model consists of M noninteracting (in a mean-field sense) antiferromagnetic planes which together form a body-centered-tetragonal structure. We obtain the leading quantum correction of order 1/S from the zero-point energy for a system of M planes whose staggered moments have arbitrary orientations. The infinite degeneracy of the ground-state manifold of this system is partially removed by collinear ordering in view of effects previously calculated by Shender at relative order J2⊥/(J2S), where J, the antiferromagnetic in-plane exchange interaction, is assumed to dominate J⊥, the out-of-plane interaction which can be of either sign. We study the complete removal of the remaining degeneracy of the collinear spin structures by assigning an arbitrary sign σi (i=1,2,...M) to the staggered moment of the planes. Our result for the zero-point energy (for M\u3e2) up to the sixth order in j=J⊥/J is E({σi}) =E1+CEG(j6/S)[-2σ1σ3-2σM−2σM+2∑i =1M-2σiσi+2-3∑i=1M-3σiσi+1σi+2σi+3], where C\u3e0 and E1 are constants independent of the σ’s, and EG is the classical ground-state energy. (Here sums from i to j when j\u3ci are interpreted to be zero.) Surprisingly, there is no σ-dependent contribution at order j4/S. This result shows that for M\u3e4 second-neighboring planes are antiferromagnetically coupled in the ground state and thus the three-dimensional spin structure cannot be described by a single wave vector, as is often assumed. At order j4, σ-dependent terms first appear at order 1/S3 and these also favor antiferromagnetic coupling of alternate planes

Photonic bandgap narrowing in conical hollow core Bragg fibers

Cataloged from PDF version of article.We report the photonic bandgap engineering of Bragg fibers by controlling the thickness profile of the fiber during the thermal drawing. Conical hollow core Bragg fibers were produced by thermal drawing under a rapidly alternating load, which was applied by introducing steep changes to the fiber drawing speed. In conventional cylindrical Bragg fibers, light is guided by omnidirectional reflections from interior dielectric mirrors with a single quarter wave stack period. In conical fibers, the diameter reduction introduced a gradient of the quarter wave stack period along the length of the fiber. Therefore, the light guided within the fiber encountered slightly smaller dielectric layer thicknesses at each reflection, resulting in a progressive blueshift of the reflectance spectrum. As the reflectance spectrum shifts, longer wavelengths of the initial bandgap cease to be omnidirectionally reflected and exit through the cladding, which narrows the photonic bandgap. A narrow transmission bandwidth is particularly desirable in hollow waveguide mid-infrared sensing schemes, where broadband light is coupled to the fiber and the analyte vapor is introduced into the hollow core to measure infrared absorption. We carried out sensing simulations using the absorption spectrum of isopropyl alcohol vapor to demonstrate the importance of narrow bandgap fibers in chemical sensing applications. © 2014 AIP Publishing LLC

Robust Cassie State of Wetting in Transparent Superhydrophobic Coatings

Cataloged from PDF version of article.This paper investigates the stability of the Cassie state of wetting in transparent superhydrophobic coatings by comparing a single-layer microporous coating with a double-layer micro/nanoporous coating. Increasing pressure resistance of superhydrophobic coatings is of interest for practical use because high external pressures may be exerted on surfaces during operation. The Cassie state stability against the external pressure of coatings was investigated by squeezing droplets sitting on surfaces with a hydrophobic plate. Droplets on the single-layer coating transformed to the Wenzel state and pinned to the surface after squeezing, whereas droplets on the double-layer micro/nanoporous coating preserved the Cassie state and rolled off the surface easily. In addition, the contact angle and contact-line diameter of water droplets during evaporation from surfaces were in situ investigated to further understand the stability of coatings against Wenzel transition. A droplet on a microporous coating gradually transformed to the Wenzel state and lost its spherical shape as the droplet volume decreased (i.e., the internal pressure of the droplet increased). The contact line of the droplet during evaporation remained almost unchanged. In contrast, a water droplet on a double-layer surface preserved its spherical shape even at the last stages of the evaporation process, where pressure differences as high as a few thousand pascals were generated. For this case, the droplet contact line retracted during evaporation and the droplet recovered the initial water contact angle. The demonstrated method for the preparation of robust transparent superhydrophobic coatings is promising for outdoor applications such as self-cleaning cover glasses for solar cells and nonwetting windows

Surface Textured Polymer Fibers for Microfluidics

Cataloged from PDF version of article.This article introduces surface textured polymer fibers as a new platform for the fabrication of affordable microfluidic devices. Fibers are produced tens of meters-long at a time and comprise 20 continuous and ordered channels (equilateral triangle grooves with side lengths as small as 30 micrometers) on their surfaces. Extreme anisotropic spreading behavior due to capillary action along the grooves of fibers is observed after surface modification with polydopamine (PDA). These flexible fibers can be fixed on any surface - independent of its material and shape - to form three-dimensional arrays, which spontaneously spread liquid on predefined paths without the need for external pumps or actuators. Surface textured fibers offer high-throughput fabrication of complex open microfluidic channel geometries, which is challenging to achieve using current photolithography-based techniques. Several microfluidic systems are designed and prepared on either planar or 3D surfaces to demonstrate outstanding capability of the fiber arrays in control of fluid flow in both vertical and lateral directions. Surface textured fibers are well suited to the fabrication of flexible, robust, lightweight, and affordable microfluidic devices, which expand the role of microfluidics in a scope of fields including drug discovery, medical diagnostics, and monitoring food and water quality. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Hidden Symmetries and their Consequences in t2gt_{2g} Cubic Perovskites

The five-band Hubbard model for a $d$ band with one electron per site is a model which has very interesting properties when the relevant ions are located at sites with high (e. g. cubic) symmetry. In that case, if the crystal field splitting is large one may consider excitations confined to the lowest threefold degenerate $t_{2g}$ orbital states. When the electron hopping matrix element ($t$) is much smaller than the on-site Coulomb interaction energy ($U$), the Hubbard model can be mapped onto the well-known effective Hamiltonian (at order $t^{2}/U$) derived by Kugel and Khomskii (KK). Recently we have shown that the KK Hamiltonian does not support long range spin order at any nonzero temperature due to several novel hidden symmetries that it possesses. Here we extend our theory to show that these symmetries also apply to the underlying three-band Hubbard model. Using these symmetries we develop a rigorous Mermin-Wagner construction, which shows that the three-band Hubbard model does not support spontaneous long-range spin order at any nonzero temperature and at any order in $t/U$ -- despite the three-dimensional lattice structure. Introduction of spin-orbit coupling does allow spin ordering, but even then the excitation spectrum is gapless due to a subtle continuous symmetry. Finally we showed that these hidden symmetries dramatically simplify the numerical exact diagonalization studies of finite clusters.Comment: 26 pages, 3 figures, 520 KB, submitted Phys. Rev.