Unusual Ground State Research Articles

Composite fermion description of rotating Bose-Einstein condensates

We study the properties of rotating Bose-Einstein condensates in parabolic traps, with coherence length large compared to the system size. In this limit, it has been shown that unusual ground states form which cannot be understood within a conventional many-vortex picture. Using comparisons with exact numerical results, we show that these ground states can be well-described by a model of noninteracting ``composite fermions.'' Our work emphasizes the similarities between the unusual states that appear in rotating Bose-Einstein condensates and incompressible fractional quantum Hall states.

Metal-insulator transition in a degenerate Hubbard model

We have studied the metal-insulator transition at integer fillings in a triply degenerate Hubbard model using the Lanczos method. The critical Coulomb interaction strength U-c, is found to depend strongly on the band filling, with U-c similar to root 3 W (W is the bandwidth) at half filling for this case with threefold degeneracy. We discuss the implications of our results on metal-insulator transitions in strongly correlated systems in general, and on the unusual electronic ground state of the alkali-metal-doped fullerenes, in particular. [S0163-1829(99)11003-8].

The mechanism of antiferromagnetism in chromium

It is shown that two special properties of Cr are needed to explain its antiferromagnetism. One special property is the well known sensitivity to antiferromagnetic spin-density waves due to nesting of the Fermi surface. A second new special property comes from first-principles total-energy calculations on bcc Cr, which show that, although the lowest energy state is nonmagnetic, a small expansion of the lattice brings a second-order transition into a type-I antiferromagnetic phase with rapidly rising local moments. The combined properties provide a mechanism for stabilization of the unusual antiferromagnetic ground state, since a spin-density wave which modulates the moments of the antiferromagnetic phase can be used to compensate the strain energy of the lattice expansion. This combined mechanism also explains various properties of Cr, such as the great sensitivity of the antiferromagnetism to pressure, that are otherwise puzzling.

An unusual ground-state stabilization effect and origins of the alpha-effect in aminolyses of Y-substituted phenyl X-substituted benzoates

Second-order rate constants have been measured spectrophotometrically for the reactions of X-C6H4CO2C6H4-Y with a series of primary amines in H2O containing 20 mol% DMSO at 25.0 ± 0.1°C. The reactivity increases as the substituent (X and Y) becomes a stronger electron-withdrawing group. The sigma + constants give better Hammett correlation than sigma constants for the reactions of 4-nitrophenyl X-substituted benzoates with glycylglycine (glygly) and hydrazine (NH2NH2), indicating that the ground-state stabilization effect is unusually significant on the reaction rates. The reactions of X-C6H4CO2C6H4-Y with glygly and NH2NH2 appear to proceed through the same mechanism, but the degree of leaving-group departure and the negative charge developed in the acyl moiety at the rate-determining TS is considered to be more significant for the glygly system than the NH2NH2 system based on ßlg and rho X values. The magnitude of the alpha -effect is observed to be not always dependent on the ßnuc value but dependent on the electronic nature of the substituent X and Y, i.e., an electron-donating substituent increases the alpha -effect, while an electron-withdrawing one decreases the alpha -effect. The present study has led to the conclusion that the ground-state effect is important for the reaction rates but it is not solely responsible for the alpha -effect, and the intramolecular H-bonding interactions (4) are proposed for the cause of the increasing or decreasing alpha -effect trends observed in the present system.Key words: alpha -effect, intramolecular H-bonding interaction, ground-state stabilization effect.

An unusual ground-state stabilization effect and origins of the alpha-effect in aminolyses of Y-substituted phenyl X-substituted benzoates

An unusual ground-state stabilization effect and origins of the alpha-effect in aminolyses of Y-substituted phenyl X-substituted benzoates

NMR studies of the electronic structure of low-spin iron(III) phosphonite complexes: unusual (d xz , d yz )4(d xy )1 configuration

Complexation of dimethylphenylphosphonite to (5,10,15,20-tetraphenylporphyrinato)iron(III) yields a low-spin derivative [Fe(tpp){P(OMe)2Ph}2]ClO4, showing NMR and EPR properties consistent with an iron(III) ion in the unusual ground-state configuration(dxy,dyz)4(dxy)1.

Unusual ground-state properties of UPd2Al3: Implications for the coexistence of heavy-fermion superconductivity and local-moment antiferromagnetism.

We report measurements of the resistivity and the specific heat, performed as a function of temperature, magnetic field, and hydrostatic pressure on the antiferromagnetic (${\mathit{T}}_{\mathit{N}}$=14 K) heavy-fermion superconductor (${\mathit{T}}_{\mathit{c}}$=2 K) ${\mathrm{UPd}}_{2}$${\mathrm{Al}}_{3}$. Our results suggest a coexistence of two subsystems with more localized 5f states responsible for the magnetic properties and less localized states responsible for the superconducting properties. The latter are compatible with an order parameter of octagonal d-wave symmetry.

Paramagnetic Pd 2+ ions in trigonal sites of CsMgCl 3 crystals

Crystals of CsMgCl 3 grown from melts containing small amounts of PdCl 2 exhibit a noticeable coloration indicating the incorporation of Pd 2+ ions into the lattice. The EPR spectra of the crystals contain the features characteristic of a S=1 ion in a trigonal site with spin Hamiltonian parameters of: g ∥ = 2.18, g ‖ = 2.16, and |D| = 0.25 cm −1. This paper describes the spectra and discusses the unusual paramagnetic ground state for Pd 2+.

Torelons and unusual ground states

States of non-zero electric flux encircling one or more periodic boundary conditions (torelons) are studied by lattice gauge theory simulation for pure gauge interactions with SU(2) colour. Both small and large spatial sizes are used and interesting results are obtained. For a range of intermediate volumes, the ground state of the (110) torelon sector is found to be antisymmetric under x− y interchange. This unusual behaviour of a ground state which is not in the most symmetric representation is discussed. It has a bearing on 't Hooft's conjecture concerning the energy at large volumes of torelon states and their string-like nature.

Interband pairs and reduced Josephson current: a comment on tunnelling experiments in two-band superconductors

Tunnelling experiments in superconductors involving interband pair states are considered. We discuss qualitatively the results obtained in heavy-fermion systems which could be understood as an evidence of this unusual ground state.

Inflationary universe without GUTs

The existence of a primordial inflationary era is unavoidable due to the puzzling nature of semiclassical gravitation, regulated by Einstein's equations and the laws of quantum mechanics. This interaction appears to be controlled by a mass-dependent effective gravitational coupling constant. The latter undergoes an unexpected transition from a classical gravitational attractive to an antigravitational repulsive regime when the corresponding mass of a quantum matter field passes through a definite threshold. This induces in turn a gravitational, spontaneously broken symmetry phenomenon responsible for the presence of an unusual non-Minkowskian ground state: the inflationary de Sitter space-time. This then acquires the status of the primordial cosmological vacuum, the generic configuration of our cosmological history.

Prediction of a low-spin ground state in the GaAs:V2+ impurity system.

All 3d impurities observed to date in tetrahedral semiconductors have a high-spin ground state, in agreement with Hund's rule. Using first-principles self-consistent Green's-function calculations for substitutional GaAs:V within the local-spin-density formalism, we predict that the as yet unobserved ground state of GaAs:${\mathrm{V}}^{2+}$ is of the low-spin type. The origin of this unusual ground state is explained.

Physico-chemical study of copper(II) and cobalt(II) chelates of tetra-2,3-pyridinoporphyrazine

The electronic and e.s.r. spectral data of the copper(II) and cobalt(II) chelates of tetra-2,3-pyridinoporphyrazine show that the electronic environment provided by the ligand is similar to that of the phthalocyanines. Potentiometric pH, spectrophotometric, and e.s.r. spectral measurements show that the water soluble NN′N″N‴-tetramethyl quaternized tetracationic forms of the copper(II) and cobalt(II) tetra-2,3-pyridinoporphyrazinates are free of solute-solute interactions, stable in a wide range of acidic solutions, but give rise to some decomposition in slightly alkaline solution. The cobalt(II) chelate forms five- and six-co-ordinate complexes with certain heterocyclic nitrogen ligands. The e.s.r. spectrum due to the imidazole complex of cobalt(II)NN′N″N‴-tetramethyltetra-2,3-pyridinoporphyrazinate indicates an unusual ground state of cobalt(II) in these circumstances. Spectrophotometric and e.s.r. spectral data indicate that the cobalt(II) chelate in aqueous solution undergoes a one-electron reduction of the metal centre as a result of reaction with ascorbic acid. The d.c. polarographic behaviour of the cobalt(II), nickel(II), and copper(II) chelates consists of waves with E½ in the region of –0.55 and –1.20 V versus the saturated calomel electrode. The waves are polarographically irreversible but it has been shown that the first wave is due to a one-electron reduction of the chelates at the dropping mercury electrode.

ESR and NMR of Fe(CN) 5 NH [formula omitted

The ESR of Fe(CN) 5NH 3 2− in single crystals of Na 2Fe(CN) 5NO· 2H 20 has been studied at 4.2°K. The principal g values were determined to be g z = 2.995, g x = 0.845, and g γ = 2.177. In addition, the hyperfine interaction for the ammine nitrogen was observed and found to be A z = +8.1 × 010 −4 cm −1, A x⪕ +2.2 × 10 −4 cm −1, A y⪖ -3.7 × 10 −4 cm −1. The g values are shown to be consistent only with a ground state in which the unpaired electron is primarily in a d xz orbital rather than the expected d x 2−y 2 orbital. It is shown that a 20 % transfer of electron spin to the ammine nitrogen is required to explain the large anisotropy in the hyperfine interaction. The unusual ground state and covalency of the FeN bond suggest a much stronger ligand field strength for NH 3 in this complex than is found in other complex ions. The data obtained in this study is used to explain the large paramagnetic shift observed for the 14N NMR resonance of NH 3 in this complex.

Theoretical study of cyclopropene and its C3H4 isomers

Ab initio SCF MO and CI calculations are reported for the three C3H4 isomers cyclopropene, methylacetylene and allene. The closed shell nature of each of these molecules allows for a good theoretical comparison of their stabilities and ionization potentials by the SCF method. It is pointed out that cyclopropene differs from most 22 electron systems with a triatomic skeleton in that it possesses a non-linear equilibrium structure; this fact is related to its unusual ground state electronic configuration, which corresponds to a highly excited state in simple triatomics such as CO2 and N 3 − . The electronic spectrum and the methylene rotational barrier of cyclopropene are also investigated.

Ground-state conformation of dimethylcarbodi-imide

The unusual ground-state geometry of dimethyl-carbodi-imide has been calculated by the INDO molecular orbital method.