40 research outputs found
The Magnetization of Cu_2(C_5H_{12}N_2)_2Cl_4 : A Heisenberg Spin Ladder System
We study the magnetization of a Heisenberg spin ladder using exact
diagonalization techniques, finding three distinct magnetic phases. We consider
the results in relation to the experimental behaviour of the new copper
compound Cu_2(C_5H_{12}N_2)_2Cl_4 and deduce that the compound is well
described by such a model with a ratio of `chain' to `rung' bond strengths
(J/J^\prime) of the order of 0.2, consistent with results from the magnetic
susceptibility. The effects of temperature, spin impurities and additional
diagonal bonds are presented and we give evidence that these diagonal bonds are
indeed of a ferromagnetic nature.Comment: Latex file (4 pages), related figures (encapsulated postscript)
appende
Non-local space-time supersymmetry on the lattice
We show that several well-known one-dimensional quantum systems possess a
hidden nonlocal supersymmetry. The simplest example is the open XXZ spin chain
with \Delta=-1/2. We use the supersymmetry to place lower bounds on the ground
state energy with various boundary conditions. For an odd number of sites in
the periodic chain, and with a particular boundary magnetic field in the open
chain, we can derive the ground state energy exactly. The supersymmetry thus
explains why it is possible to solve the Bethe equations for the ground state
in these cases. We also show that a similar space-time supersymmetry holds for
the t-J model at its integrable ferromagnetic point, where the space-time
supersymmetry and the Hamiltonian it yields coexist with a global u(1|2) graded
Lie algebra symmetry. Possible generalizations to other algebras are discussed.Comment: 12 page
Frustration-Induced Two Dimensional Quantum Disordered Phase in Piperazinium Hexachlorodicuprate
Piperazinium Hexachlorodicuprate (PHCC) is shown to be a frustrated
quasi-two-dimensional quantum Heisenberg antiferromagnet with a gapped
spectrum. Zero-field inelastic neutron scattering and susceptibility and
specific heat measurements as a function of applied magnetic field are
presented. At T = 1.5 K, the magnetic excitation spectrum is dominated by a
single propagating mode with a gap, Delta = 1 meV, and bandwidth of
approximately 1.8 meV in the (h0l) plane. The mode has no dispersion along the
b* direction indicating that neighboring a-c planes of the triclinic structure
are magnetically decoupled. The heat capacity shows a reduction of the gap as a
function of applied magnetic field in agreement with a singlet-triplet
excitation spectrum. A field-induced ordered phase is observed in heat capacity
and magnetic susceptibility measurements for magnetic fields greater than H_c1
approximately equal to 7.5 Tesla. Analysis of the neutron scattering data
reveals the important exchange interactions and indicates that some of these
are highly frustrated.Comment: 13 pages with 14 figures, 7 pages of text, 6 pages of figures.
Submitted to Phys. Rev. B 4/7/2001. email comments to [email protected] or
[email protected]
Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl3
Magnetization and neutron elastic scattering measurements under a hydrostatic
pressure were performed on KCuCl3, which is a three-dimensionally coupled spin
dimer system with a gapped ground state. It was found that an intradimer
interaction decreases with increasing pressure, while the sum of interdimer
interactions increases. This leads to the shrinkage of spin gap. A quantum
phase transition from a gapped state to an antiferromagnetic state occurs at Pc
? 8.2 kbar. For P > P c, magnetic Bragg reflections were observed at reciprocal
lattice points equivalent to those for the lowest magnetic excitation at zero
pressure. This confirms that the spin gap decreases and closes under applied
pressure.Comment: 7 pages, 10 figures, submitted to J. Phys. Soc. Jp
Field-Induced Two-Step Phase Transitions in the Singlet Ground State Triangular Antiferromagnet CsFeBr
The ground state of the stacked triangular antiferromagnet CsFeBr is a
spin singlet due to the large single ion anisotropy . The
field-induced magnetic ordering in this compound was investigated by the
magnetic susceptibility, the magnetization process and specific heat
measurements for an external field parallel to the -axis. Unexpectedly, two
phase transitions were observed in the magnetic field higher than 3 T. The
phase diagram for temperature versus magnetic field was obtained. The mechanism
leading to the successive phase transitions is discussed.Comment: 8 pages, 9 figures, 10 eps files, jpsj styl
Critical Properties of Condensation of Field-Induced Triplet Quasiparticles
A review on the field-induced magnetic ordering is given, together with some
results of a quantum Monte Carlo simulation focused on the critical behevior
near the quantum critical point.Comment: Proceedings of SPQS, Sendai, 200
High-field specific-heat and susceptibility measurements: Relevance to the spin-Peierls phase diagram and the validity of a soliton picture
Géodynamique andine : résumés étendus = Andean geodynamics : extended abstracts = Geodinamica andina : resumenes ampliados
pH dependent Ni(II) binding and aggregation of Escherichia coli and Helicobacter pylori NikR.
International audienceNikR proteins are bacterial metallo-regulatory transcription factors that control the expression of the nickel uptake system and/or nickel containing enzymes such as urease, and are involved in the acid stress response. Here, a comparative study is reported on NikR from Helicobacter pylori (HpNikR) and Escherichia coli (EcNikR), as well as the Q2E mutant of EcNikR. Most attention was focused on the Ni(II) binding properties of these proteins, as a function of pH. The influence of the pH on the Ni(II) binding and aggregation properties was studied using gel filtration analysis and UV-visible absorption spectroscopy in the presence of an increasing concentration of nickel. Q2E and wt EcNikR are identical in Ni(II) binding but the Q2E mutant is impaired to some extent in DNA-binding. For EcNikR it is shown that between pH 6 and 8, addition of Ni(II) above 1 equiv. induces mass aggregation and precipitation, concomitant with binding of Ni(II) up to a maximum of 5-8 Ni(II) ions per monomer. The Ni(II) site with highest affinity is the well-described square planar site with three histidines and one cysteine ligands. Aggregation is complete in the presence of less than 1 extra equiv. of Ni(II) and aggregation is fully reversible and precipitates are rapidly solubilized by addition of EDTA. The sensitivity of EcNikR to aggregation decreases with decreasing pH, concurrent with histidines being the main ligands of the site responsible for aggregation. HpNikR does not display aggregation except at alkaline pH, where 3 Ni(II) equiv. are needed. The participation of a cluster consisting of surface-exposed histidines present in EcNikR but not in HpNikR, is proposed to be involved in aggregation. Our results on HpNikR are compatible with the crystallographic data and with the ability of this protein to bind more than one nickel