256,324 research outputs found
Monte Carlo Study of the S=1/2 and S=1 Heisenberg Antiferromagnet on a Spatially Anisotropic Square Lattice
We present a quantum Monte Carlo study of a Heisenberg antiferromagnet on a
spatially anisotropic square lattice, where the coupling strength in the
x-direction () is different from that in the y-direction (). By
varying the anisotropy from 0 to 1, we interpolate between the
one-dimensional chain and the two-dimensional isotropic square lattice. Both
and S=1 systems are considered separately in order to facilitate
comparison. The temperature dependence of the uniform susceptibility and the
spin-spin correlation length are computed down to very low temperatures for
various values of . For S=1, the existence of a quantum critical point
at as well as the scaling of the spin gap is
confirmed. Universal quantities predicted from the nonlinear
model agree with our results at without any adjustable
parameters. On the other hand, the results are consistent with
, as discussed by a number of previous theoretical studies.
Experimental implications for compounds such as SrCuO are also
discussed.Comment: 8 pages, 7 figures, to be published in Phys. Rev.
A direct proof of Kim's identities
As a by-product of a finite-size Bethe Ansatz calculation in statistical
mechanics, Doochul Kim has established, by an indirect route, three
mathematical identities rather similar to the conjugate modulus relations
satisfied by the elliptic theta constants. However, they contain factors like
and , instead of . We show here that
there is a fourth relation that naturally completes the set, in much the same
way that there are four relations for the four elliptic theta functions. We
derive all of them directly by proving and using a specialization of
Weierstrass' factorization theorem in complex variable theory.Comment: Latex, 6 pages, accepted by J. Physics
Validation of a model of regulation in the tryptophan operon against multiple experiment data using global optimisation
This paper is concerned with validating a mathematical model of regulation in the tryptophan operon using global optimization. Although a number of models for this biochemical network are proposed, in many cases only qualitative agreement between the model output and experimental data was demonstrated, since very little information is currently available to guide the selection of parameter values for the models. This paper presents a model validating method using both multiple experimental data and global optimization
MgB2 tunnel junctions with native or thermal oxide barriers
MgB2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native
oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers
therefore survive the deposition of the second electrode at 300oC, even over
junction areas of ~1 mm2. Studies of such junctions, and those of the type
MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers
grown on MgB2 exhibit a wide range of barrier heights and widths.Comment: 9 pages, 3 figure
Electron-hole asymmetry in Co- and Mn-doped SrFe2As2
Phase diagram of electron and hole-doped SrFe2As2 single crystals is
investigated using Co and Mn substitution at the Fe-sites. We found that the
spin-density-wave state is suppressed by both dopants, but the superconducting
phase appears only for Co (electron)-doping, not for Mn (hole)-doping. Absence
of the superconductivity by Mn-doping is in sharp contrast to the hole-doped
system with K-substitution at the Sr sites. Distinct structural change, in
particular the increase of the Fe-As distance by Mn-doping is important to have
a magnetic and semiconducting ground state as confirmed by first principles
calculations. The absence of electron-hole symmetry in the Fe-site-doped
SrFe2As2 suggests that the occurrence of high-Tc superconductivity is sensitive
to the structural modification rather than the charge doping.Comment: 7 pages, 6 figure
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