96 research outputs found
Drivers of long-term growth? Assessing the impacts of emerging countries on Sub-Saharan african economies
After the âlost decadesâ Sub-Saharan African economies have exhibited positive growth rates due
to emerging countries. The paper shows the complexity of causalities, which depend on: i)
channels (trade, investment); ii) the emerging country (especially China); and iii) African
countriesâ market structures. On the one hand, this growth relies on structural asymmetries and
is generated by distorted export structures that are based on commodities. It falls if prices
decline and increases the specialisation of African economies in commodities. On the other
hand, emerging countries have positive impacts via their investments in infrastructure, which
fosters industrialisation. Finally, developed countriesâ aid, due to the detrimental effects of
conditionality, induces far greater asymmetries than does the aid of emerging countries
Interaction between static holes in a quantum dimer model on the kagome lattice
A quantum dimer model (QDM) on the kagome lattice with an extensive
ground-state entropy was recently introduced [Phys. Rev. B 67, 214413 (2003)].
The ground-state energy of this QDM in presence of one and two static holes is
investigated by means of exact diagonalizations on lattices containing up to
144 kagome sites. The interaction energy between the holes (at distances up to
7 lattice spacings) is evaluated and the results show no indication of
confinement at large hole separations.Comment: 6 pages, 3 figures. IOP style files included. To appear in J. Phys.:
Condens. Matter, Proceedings of the HFM2003 conference, Grenobl
Spin-1/2 frustrated antiferromagnet on a spatially anisotopic square lattice: contribution of exact diagonalizations
The phase diagram of a spin-1/2 model is investigated by means of
exact diagonalizations on finite samples. This model is a generalization of the
model on the square lattice with two different nearest-neighbor
couplings and may be also viewed as an array of coupled Heisenberg
chains. The results suggest that the resonnating valence bond state predicted
by Nersesyan and Tsvelik [Phys. Rev. B {\bf 67}, 024422 (2003)] for is realized and extends beyond the limit of small interchain coupling
along a curve nearly coincident with the line where the energy per spin is
maximum. This line is likely bordered on both side by a columnar dimer long
range order. This columnar order could extends for which correspond
to the model.Comment: 14 pages, 21 figures, final versio
Valence-bond crystals in the kagome spin-1/2 Heisenberg antiferromagnet: a symmetry classification and projected wave function study
In this paper, we do a complete classification of valence-bond crystals
(VBCs) on the kagome lattice based on general arguments of symmetry only and
thus identify many new VBCs for different unit cell sizes. For the spin-1/2
Heisenberg antiferromagnet, we study the relative energetics of competing
gapless spin liquids (SLs) and VBC phases within the class of
Gutzwiller-projected fermionic wave functions using variational Monte Carlo
techniques, hence implementing exactly the constraint of one fermion per site.
By using a state-of-the-art optimization method, we conclusively show that the
U(1) Dirac SL is remarkably stable towards dimerizing into all 6-, 12- and
36-site unit cell VBCs. This stability is also preserved on addition of a
next-nearest-neighbor super-exchange coupling of both antiferromagnetic and
ferromagnetic (FM) type. However, we find that a 36-site unit cell VBC is
stabilized on addition of a very small next-nearest-neighbor FM super-exchange
coupling, i.e. |J2|~0.045, and this VBC is the same in terms of space-group
symmetry as that obtained in an effective quantum dimer model study. It breaks
reflection symmetry, has a nontrivial flux pattern and is a strong dimerization
of the uniform RVB SL.Comment: 16 pages, 25 figures. Invited paper for Focus issue on "Quantum Spin
Liquids" of the New Journal of Physic
Low-lying excitations and magnetization process of coupled tetrahedral systems
We investigate low-lying singlet and triplet excitations and the
magnetization process of quasi-1D spin systems composed of tetrahedral spin
clusters. For a class of such models, we found various exact low-lying
excitations; some of them are responsible for the first-order transition
between two different ground states formed by local singlets. Moreover, we find
that there are two different kinds of magnetization plateaus which are
separated by a first-order transition.Comment: To appear in Phys.Rev.B (Issue 01 August 2002). A short comment is
adde
Magnetization curve of the kagome-strip-lattice antiferromagnet
We study the magnetization curve of the Heisenberg model on the
quasi-one-dimensional kagome-strip lattice that shares the same lattice
structure in the inner part with the two-dimensional kagome lattice. Our
numerical calculations based on the density matrix renormalization group method
reveal that the system shows several magnetization plateaus between zero
magnetization and the saturated one; we find the presence of the magnetic
plateaus with the n=7 height of the saturation for n =1,2,3,4,5 and 6 in the S
=1/2 case, whereas we detect only the magnetic plateaus of n =1,3,5 and 6 in
the S =1 case. In the cases of n =2,4 and 6 for the S=1/2 system, the
Oshikawa-Yamanaka-Affleck condition suggests the occurrence of the
translational symmetry breaking (TSB). We numerically confirm this non-trivial
TSB in our results of local magnetizations. We have also found that the
macroscopic jump appears near the saturation field irrespective of the spin
amplitude as well as the two-dimensional kagome model.Comment: 6pages, 3figures, accepted for publication in Journal of Low
Temperature Physic
Magnetization Process of Kagome-Lattice Heisenberg Antiferromagnet
The magnetization process of the isotropic Heisenberg antiferromagnet on the
kagome lattice is studied. Data obtained from the numerical-diagonalization
method are reexamined from the viewpoint of the derivative of the magnetization
with respect to the magnetic field. We find that the behavior of the derivative
at approximately one-third of the height of the magnetization saturation is
markedly different from that for the cases of typical magnetization plateaux.
The magnetization process of the kagome-lattice antiferromagnet reveals a new
phenomenon, which we call the "magnetization ramp".Comment: 4 pages, 5figures, accepted in J. Phys. Soc. Jpn
Magneto-thermodynamics of the spin-1/2 Kagome antiferromagnet
In this paper, we use a new hybrid method to compute the thermodynamic
behavior of the spin-1/2 Kagome antiferromagnet under the influence of a large
external magnetic field. We find a T^2 low-temperature behavior and a very low
sensitivity of the specific heat to a strong external magnetic field. We
display clear evidence that this low temperature magneto-thermal effect is
associated to the existence of low-lying fluctuating singlets, but also that
the whole picture (T^2 behavior of Cv and thermally activated spin
susceptibility) implies contribution of both non magnetic and magnetic
excitations. Comparison with experiments is made.Comment: 4 pages, LaTeX 2.09 and RevTeX with 3 figures embedded in the text.
Version to appear in Phys. Rev. Let
Ga NMR study of the local susceptibility in SrCr8Ga4O19: pseudogap and paramagnetic defects
We present the first Ga(4f) NMR study of the Cr susceptibility in the
archetype of Kagome based frustrated antiferromagnets,
SrCrGaO. Our major finding is that the susceptibility of the
frustrated lattice goes through a maximum around 50 K. Our data also supports
the existence of paramagnetic ``clusters'' of spins, responsible for the Curie
behavior observed in the macroscopic susceptibility at low T. These results set
novel features for the constantly debated physics of geometrically frustrated
magnets.Comment: 4 pages, 5 figures Submitted to Phys. Rev. Let
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