Showing papers in "Canadian Journal of Physics in 2001"

Magnets with strong geometric frustration

Abstract: A nontechnical introduction to the theory of magnets with strong geometric frustration is given, concentrating on magnets on corner-sharing (Kagome, pyrochlore, SCGO, and GGG) lattices. Their rich ...

Thermal radiation and buoyancy effects on MHD free convective heat generating flow over an accelerating permeable surface with temperature-dependent viscosity

Abstract: The paper presents a study of the flow of a viscous incompressible fluid over an accelerating permeable surface with temperature-dependent viscosity, taking into account the effect of thermal radiation and thermal buoyancy in the presence of a magnetic field. The fluid viscosity is assumed to vary as an inverse linear function of temperature. The governing equations for laminar free convection of fluid are changed to dimensionless ordinary differential equations by similarity transformation. They are solved by a shooting method. The effects of various parameters on the velocity and temperature profiles as well as the skin friction coefficient and wall heat transfer are presented graphically and in tabulated form. PACS Nos.: 47.65ta, 52.30–q

Breit-Pauli energy levels and transition rates for the carbonlike sequence

Abstract: Breit–Pauli results for energy levels, lifetimes, and some transition data are reported for all levels up to 2s22p3d 1PJo of the C-like spectrum for 7 Z 15. For all but the lowest members of the sequence, these include all the states of the 2s22p2, 2s2p3, 2p4, 2s22p3s, 2s22p3p, and 2s22p3d configurations. For carbon, only the states for the 2s22p2, 2s2p3, and 2s22p3s configurations are included. A simultaneous optimization scheme was applied so that a radial basis could be determined for a set of terms that mix in the Breit–Pauli approximation. All results have been posted at a Web site. In this paper, lifetimes are reported and, in addition, results for N II, O III, and Si IX are critically evaluated. Energy levels are compared with experiment and adjusted transition rates, along with uncertainty estimates are presented. Selected results are compared with other theory and experiment. PACS Nos.: 31.25Jf, 32.30Jc, 32.70Cs

Origin of spin-ice behavior in Ising pyrochlore magnets with long-range dipole interactions: an insight from mean-field theory

Abstract: Recent experiments suggest that the Ising pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 display qualitative properties of the ferromagnetic nearest-neighbor spin-ice model proposed by Harris et al. Phys...

Laser-assisted elastic electron scattering from helium

Abstract: We report measurements of the differential cross section for scattering of electrons, from helium, with energies in the range 1–20 eV through scattering angles of 14°–140° in the presence of a high-intensity carbon dioxide laser. These results were obtained with beam geometries for which the Kroll–Watson approximation, normally applied to these types of measurements, predicts zero cross sections. The cross sections reported are not zero but confirm similar experiments with argon as the scattering centre. PACS No.: 34.80Qb

Soft-X-ray spectra of highly charged Au ions in an electron-beam ion trap

Abstract: Systematic variation of the electron-beam energy in an electron-beam ion trap has been employed to produce soft-X-ray spectra (20 to 60 A) of Au with well-defined maximum charge states ranging from...

Effective Hamiltonians and dilution effects in Kagome and related anti-ferromagnets

Abstract: What is the zero-temperature ordering pattern of a Heisenberg anti-ferromagnet with large spin length S (and possibly small dilution), on the Kagome lattice, or others built from corner-sharing tri...

A realistic quasi-physical model of the 100 m dash

Abstract: A quasi-physical model (having both physical and mathematical roots) of sprint performances is presented, accounting for the influence of drag modification via wind and altitude variations.The race...

Discrete quantum gravity and causal sets

Abstract: This paper provides a thorough introduction to the physical and conceptual need for a theory of quantum gravity; some knowledge of general relativity and nonrelativistic quantum mechanics is assume...

Frustrated order by disorder: The pyrochlore anti-ferromagnet with bond disorder

Abstract: The classical Heisenberg anti-ferromagnet on the pyrochlore lattice is macroscopically and continuously degenerate and the system remains disordered at all temperatures, even in the presence of weak dilution with nonmagnetic ions. We show that, in contrast, weak-bond disorder lifts the ground-state degeneracy in favour of locally collinear spin configurations. We present a proof that for a single tetrahedron the ground state is perfectly collinear but identify two mechanisms that preclude the establishment of a globally collinear state; one due to frustration and the other due to higher order effects. We thus obtain a rugged energy landscape, which is necessary to account for the glassy phenomena found in real systems such as the pyrochlore Y2Mo2O7 recently reported by Booth et al. (Phys. Rev. B: Condens. Matter Mater. Phys. 62, R755 (2000).) to contain a substantial degree of bond disorder. PACS Nos.: 75.10.Hk, 75.40.Mg, 75.40.Gb

Spin-liquid phase in the pyrochlore anti-ferromagnet

Abstract: Correlation functions (CFs) of the classical Heisenberg anti-ferromagnet on the pyrochlore lattice are studied by solving exactly the infinite-component spin-vector model. As in many fully frustrated lattices, the constraint due to the minimization of the energy and the particular structure based on corner-sharing tetrahedra both contribute to the creation of local degrees of freedom. The resulting degeneracy destroys any magnetic order at all temperatures and we obtain no sign of criticality, even at T = 0. Calculated neutron-scattering cross sections have their maxima beyond the first Brillouin zone and reproduce experimental results obtained on Y(Sc)Mn2 and CsCrNiF6 as well as theoretical predictions previously obtained by classical Monte Carlo simulations. Evidences for thermal and spatial decoupling of the magnetic modes are found so that the magnetic fluctuations in this system can be approximated by S(q,T) f(q) h(T). PACS Nos.: 75.10Hk, 75.50Ee, 75.40Cx, 75.40-s

Conventional and unconventional orderings in the jarosites

Abstract: The jarosites make up the most studied family of Kagome anti-ferromagnets. The flexibility of the structure to substitution of the A and B ions allows a wide range of compositions to be synthesized with the general formula AB3(SO4)2(OH)6 (A = Na+, K+, Ag+, Rb+, H3O+, NH4+, 1/2Ba2+, and 1/2Pb2+; B = Fe3+, Cr3+, and V3+). Additional chemical tuning of the exchange between layers is also possible by substitution of the (SO4)2– groups by (SeO4)2– or (CrO4)2–. Thus, a variety of S = 5/2, 3/2, and 1 systems can be engineered to allow study of the effects of frustration in both the classical and more quantum limits. Within this family both conventional long-ranged magnetic order and more exotic unconventional orderings have been found. This article reviews the different types of magnetic orderings that occur and examines some of the parameters that are their cause. PACS Nos.: 75.25+z, 75.30Et, 75.30Gw, 75.50Ee, 75.50Lk

Some analytic results on the Uehling correction to the g-factor of a bound electron (muon)

Abstract: We calculate vacuum-polarization corrections to the g-factor of a bound electron in the ground state of a hydrogenlike atom. The result is found in a closed analytic form for an arbitrary value of ...

Structural and magnetic characterization of Ho3SbO7 and Dy3SbO7

Abstract: We present an experimental investigation of the structural and magnetic properties of Ho3SbO7 and Dy3SbO7. These compounds adopt the Y3TaO7 structure, space group C2221. The magnetic rare-earth ion...

Modeling the spectral line shapes with speed-dependent broadening and Dicke narrowing

Abstract: Shapes of pressure- and Doppler-broadened spectral lines are obtained by solving exactly a three-dimension transport and (or) relaxation equation. The speed dependence of collisional broadening and shifting caused by dephasing collisions and Dicke narrowing caused by velocity-changing collisions are taken into account within the impact approximation. The Rautian–Sobelman and Keilson–Storer models are used to describe velocity-changing collisions. We show in the high density or hydrodynamic limit that both models lead to a profile that is the weighted sum of Lorentz profiles if the collisional broadening is much greater than the frequency of velocity-changing collisions and to the ordinary Lorentz profile in the opposite case. This shows that the relative size of the optical and kinetic cross section is important in determining the shape of absorption curves. The analytical solutions may be useful in modeling absorption profiles for remote sensing of the atmosphere. PACS No.: 32.70Jz

Magnetic susceptibility study of the heavy rare-earth stannate pyrochlores

Abstract: The series of magnetic rare-earth pyrochlore stannates R2Sn2O7 (R = rare earth, except Ce and Pm) have been investigated by powder susceptibility measurements down to T = 1.8 K. The results are compared with results for the analogous titanate series, which are well known frustrated magnets. Unlike the titanates, the whole series can be formed in the cubic pyrochlore structure. Possible experimental advantages of studying the stannates are discussed. PACS Nos.: 75.10Nr, 75.50Ee, 75.50Lk

Multichannel quantum defect theory treatment of triplet gerade and ungerade d-symmetry levelsof H2 and its isotopomers

Abstract: This work presents a systematic multichannel quantum defect theory (MQDT) analysis of the triplet d-symmetry levels of the hydrogen molecule. First, a new compilation of the best available experimental term values for these levels was prepared. Second, R-dependent quantum defect matrices for the 3Πu, 3Πg, and 3Δg states of H2 were obtained from ab initio potential-energy curves and used in an ab initio MQDT calculation of all known triplet d-symmetry rovibronic levels of H2, HD, and D2. For a few of these levels previous ab initio calculations have been reported. The agreement currently obtained is generally significantly better than that in previous work. Finally, the quantum defect matrices are used to calculate the electronic transition moments 3Πu 3Πg, 3Δg as functions of energy and internuclear distance, R, for application in a companion article. PACS Nos: 31.15Ar, 33.20Wr, 34.10+x, 34.80Kw

Spin- and chirality-orderings of frustrated magnets - stacked-triangular anti-ferromagnets and spin glasses

Abstract: "Chirality" is a multispin quantity representing the sense or the handedness of the noncollinear spin structures induced by spin frustration. Recent studies have revealed that the chirality often p...

Three- and two-photon absorption spectroscopy: REMPI of HCl and HBr

Abstract: Approximation expressions for absorption line strengths due to Ω' = 0,1,2,3 ¬ Ω" = 0 (Σ, Π, Δ, and Φ ¬ Σ) transitions for three-, two-, and one-photon absorptions are summarized. Those for three- a...

Heavy-fermion behavior of itinerant frustrated systems: β-Mn, Y(Sc)Mn2, and LiV2O4

Abstract: These three metallic systems do not exhibit any magnetic ordering despite experiments showing the existence of localized moments with large anti-ferromagnetic exchange: we propose that this is a direct consequence of the strong geometric frustration since Y(Sc)Mn2 and LiV2O4 have the pyrochlore structure, while β-Mn has a more complicated frustrated structure. Another similarity between these compounds is their very large specific-heat coefficient γ = C/T (420 mJ mol–1 K–2 for LiV2O4). Several explanations have been proposed for this "3d heavy-fermion behavior", including a 3d-Kondo effect. However, the similarities between the three compounds indicate that frustration plays a big role. We describe a new model in which we take into account the existence of two types of 3d-electrons (localized and itinerant) and a frustrated anti-ferromagnetic exchange between the localized 3d electrons. PACS Nos.: 75.10Hk, 75.50Ee, 75.40Cx, 75.40-s

On the dissociation energy of molecular hydrogen

Abstract: The status of our knowledge of the dissociation energy of molecular hydrogen (H2, HD, D2) from the earliest determinations in 1926–1927 is briefly reviewed. Present values obtained by high-resoluti...

Cavity ring-down spectroscopy of transient O 2 -O 2 dimers

Abstract: Cavity ring-down spectroscopy has been used to obtain the spectra of transient (O2)2 at 577 and 629 nm, at room temperature and pressures ranging from about 0.25 to 10.0 atm. A selection of these spectra are displayed showing the overlapping monomer and dimer features. Pressure-dependent cross sections have been obtained and Rayleigh extinction has been observed. The derived band parameters are compared to recent results from others. PACS Nos.: 33.20kf, 33.70Fd

κ(BEDTTTF)2X organic crystals: Superconducting versus anti-ferromagnetic instabilities in the Hubbard model on an anisotropic triangular lattice

Abstract: A Hubbard model at half-filling on an anisotropic triangular lattice has been proposed as the minimal model to describe conducting layers of κ–(BEDT–TTF)2X organic materials. The model interpolates between the square lattice and decoupled chains. The κ–(BEDT–TTF)2X materials have many similarities with cuprates, such as the presence of unconventional metallic properties and the close proximity of superconducting and anti-ferromagnetic phases. As in cuprates, spin fluctuations are expected to play a crucial role in the onset of superconductivity. We perform a weak-coupling renormalization-group analysis to show that a superconducting instability occurs. Frustration in the anti-ferromagnetic couplings, which arises from the underlying geometrical arrangement of the lattice, breaks the perfect nesting of the square lattice at half-filling. The spin-wave instability is suppressed and a superconducting instability predominates. For the isotropic triangular lattice, there are again signs of long-range magnetic ...

Lanczos study of the S = 1/2 frustrated square-lattice anti-ferromagnet in a magnetic field

Abstract: We study the zero-temperature phase diagram of the frustrated square-lattice S = 1/2 anti-ferromagnet in an external magnetic field numerically with the Lanczos algorithm. For strong frustration, w...

Magnetic ordering and fluctuation in Kagome lattice anti-ferromagnets Fe and Cr jarosites

Abstract: Jarosite family compounds, KFe3(OH)6(SO4)2 (abbreviated to Fe jarosite) and KCr3(OH)6(SO4)2 (Cr jarosite) are typical examples of Heisenberg anti-ferromagnets on the Kagome lattice and have been investigated by means of magnetization and NMR experiments. The susceptibility of Cr jarosite deviates from the Curie–Weiss law due to the short-range spin correlation below about 150 K and shows the magnetic transition at 4.2 K, while Fe jarosite has the transition at 65 K. The measured susceptibility fits well with the calculated one on the high-temperature expansion for the Heisenberg anti-ferromagnet on the Kagome lattice. The values of the exchange interactions of Cr jarosite and Fe jarosite are derived to be JCr = 4.9 K and JFe = 23 K, respectively. The 1H-NMR spectra of Fe jarosite suggest that the ordered spin structure is the q = 0 type 120° configuration with +1 chirality. The transition is considered to be caused by a weak single-ion type anisotropy. The spin-lattice relaxation rate, 1/T1, of Fe jarosit...

Exact diagonalization studies of two-dimensional frustrated anti-ferromagnet models

Abstract: We describe the four kinds of behavior found in two-dimensional isotropic quantum anti-ferromagnets. Two of them display long-range order at T = 0: the Neel state and the valence bond crystal. The last two are spin liquids. Properties of these different states are briefly described and open questions are underlined. PACS Nos.: 75.10Jm, 75.50Ee, 75.40-s

Convective heat transfer past a continuously moving plate embedded in a non-Darcian porous medium in the presence of a magnetic field

Abstract: In the present study, free-convection and heat-transfer behavior of an electrically conducting fluid is investigated near a stretching sheet embedded in a non-Darcian medium. The temperature of the stretching sheet is varied. The sheet is stretched linearly with variable velocity and temperature. Boundary-layer equations are derived. The resulting approximate nonlinear ordinary differential equations are solved numerically. Velocity and temperature profiles as well as the local Nusselt number and skin-friction coefficient are computed for various values of the magnetic field, the Prandtl number, the free convective parameter, and the inertia parameter. PACS No.: 44.30+v

l-Type doubling: Herzberg versus Nielsen

Abstract: In 1942 Gerhard Herzberg demonstrated the existence of l-type doubling in a number of bending fundamental levels of linear molecules. He attributed the doubling partly to asymmetric-top effects and partly to Coriolis interactions with the stretching modes, but Harald Nielsen and Wave Shaffer claimed that it is a purely Coriolis effect. The dispute, which was due mainly to errors in theoretical calculations, lasted for several years. This paper presents a summary of the controversy, based on published papers and letters exchanged between the main participants. PACS No.: 33.10

Convective heat transfer, by the presence of radiation, in an electrically conducting fluid at a stretching surface

Abstract: The problem of free-convection heat transfer due to the simultaneous action of buoyancy, radiation, and transverse magnetic field is investigated near an isothermal sheet. The sheet is linearly stretched in the presence of heat generation or absorption. The resulting coupled nonlinear differential equations are solved numerically. A parametric study is performed to illustrate the influence of the radiation parameter, magnetic parameter, heat generation or absorption parameter, and Prandtl number on the profiles of the velocity and temperature functions. Numerical data for the skin friction and the heat flux functions have been tabulated for various parametric conditions. PACS No.: 44.25+f

Spectroscopy of the C2 molecule: Valence and Rydberg states in the 710 eV region. An ab initio study

Abstract: The potential curves of selected valence and Rydberg states of C2 lying in the 7–10 eV region are reported for the first time. The states studied, using MRCI wave funtions, include (2–5)3,1Πu, (1–3)3Σ+g, (2–5)3Σ–g, (1–3)3Δg, and relevant quintet states. The f 3Σ–g, g3Δg, and F1Πu states observed in absorption from 8.88 to 9.25 eV by Herzberg et al. have originally been assumed to be πu ® 3s Rydberg states, with respect to a 3Πu(13Πu) for the triplets and to X1Σg+ for the singlet. Our calculations partially support such assignments: f 3 Σg– corresponding to 33Σg– has mixed valence σuσgπu 3πg and Rydberg σu 2σgπu 23s character, g 3Δg corresponding to 23 Δg has a valence σuσgπu 3πg structure, while F 1 Πu(21Πu) is confirmed to be a σu 2πu 33s state. The calculated f00-values for these bands are 0.027 for f¬ a, 0.051 for g ¬ a, and 0.098 for F ¬ X. The singlet–singlet transition F ¬ X constitutes the strongest absorption band reported so far for C2. Our theoretical value agrees with f00(F¬X) = 0.10 ± 0.01 der...