131 research outputs found
Ladders in a magnetic field: a strong coupling approach
We show that non-frustrated and frustrated ladders in a magnetic field can be
systematically mapped onto an XXZ Heisenberg model in a longitudinal magnetic
field in the limit where the rung coupling is the dominant one. This mapping is
valid in the critical region where the magnetization goes from zero to
saturation. It allows one to relate the properties of the critical phase
(, , the critical exponents) to the exchange integrals and
provide quantitative estimates of the frustration needed to create a plateau at
half the saturation value for different models of frustration.Comment: One mistake corrected, one reference adde
Exact Ground States of Frustrated Spin-1 Ising-Heisenberg and Heisenberg Ladders in a Magnetic Field
Ground states of the frustrated spin-1 Ising-Heisenberg two-leg ladder with
Heisenberg intra-rung coupling and only Ising interaction along legs and
diagonals are rigorously found by taking advantage of local conservation of the
total spin on each rung. The constructed ground-state phase diagram of the
frustrated spin-1 Ising-Heisenberg ladder is then compared with the analogous
phase diagram of the fully quantum spin-1 Heisenberg two-leg ladder obtained by
density matrix renormalization group (DMRG) calculations. It is demonstrated
that both investigated spin models exhibit quite similar magnetization
scenarios, which involve intermediate plateaux at one-quarter, one-half and
three-quarters of the saturation magnetization.Comment: 3 pages, 1 figure, contribution to proceedings of CSMAG'13 conferenc
Exploring the spin-orbital ground state of Ba3CuSb2O9
Motivated by the absence of both spin freezing and a cooperative Jahn-Teller
effect at the lowest measured temperatures, we study the ground state of
Ba3CuSb2O9. We solve a general spin-orbital model on both the honeycomb and the
decorated honeycomb lattice, revealing rich phase diagrams. The spin-orbital
model on the honeycomb lattice contains an SU(4) point, where previous studies
have shown the existence of a spin-orbital liquid with algebraically decaying
correlations. For realistic parameters on the decorated honeycomb lattice, we
find a phase that consists of clusters of nearest-neighbour spin singlets,
which can be understood in terms of dimer coverings of an emergent square
lattice. While the experimental situation is complicated by structural
disorder, we show qualitative agreement between our theory and a range of
experiments.Comment: 12 pages, 9 figure
First order transition between magnetic order and valence-bond order in a 2D frustrated Heisenberg model
We study the competition between magnetic order and valence bond order in a
two dimensional (2D) frustrated Heisenberg model introduced some time ago by
Shastry and Sutherland ({\sc B. Sriram Shastry} and {\sc Bill Sutherland}, {\em
Physica} 108{\bf B},1069 (1981) ) for which a configuration of dimers is known
to be the ground state in a certain range of parameters. Using exact
diagonalisation of small clusters, linear spin wave theory and Schwinger boson
mean field theory, we show that the transition between the two types of order
is first-order, and that it takes place inside the domain where magnetic
long-range order is stable with respect to quantum fluctuations.Comment: Revtex, 8 pages, 1 latex figure and 2 uuencoded postscript figure
Iterative Deconvolution of Quadrupole Split NMR Spectra
We propose a simple method to deconvolute NMR spectra of quadrupolar nuclei
in order to separate the distribution of local magnetic hyperfine field from
the quadrupole splitting. It is based on an iterative procedure which allows to
express the intensity of a single NMR line directly as a linear combination of
the intensities of the total experimental spectrum at a few related
frequencies. This procedure is argued to be an interesting complement to
Fourier transformation since it can lead to a significant noise reduction in
some frequency ranges. This is demonstrated in the case of the 11B-NMR spectrum
in SrCu2(BO3)2 at a field of 31.7 T, where a magnetization plateau at 1/6 of
the saturation has been observed.Comment: 4 pages, 2 figure
Quantum and thermal transitions out of the supersolid phase of a 2D quantum antiferromagnet
We investigate the thermodynamic properties of a field-induced supersolid
phase in a 2D quantum antiferromagnet model. Using quantum Monte Carlo
simulations, a very rich phase diagram is mapped out in the temperature -
magnetic field plane, with an extended supersolid region where a diagonal
(solid) order coexists with a finite XY spin stiffness (superfluid). The
various quantum and thermal transitions out of the supersolid state are
characterized. Experimental consequences in the context of field-induced
magnetization plateau materials are briefly discussed.Comment: To appear in Phys. Rev. Let
Theory of the field-induced BEC in the frustrated spin-1/2 dimer compound BaCuSi2O6
Building on recent neutron and NMR experiments, we investigate the
field-induced exotic criticality observed in the frustrated spin-1/2 dimer
compound BaCuSi2O6 using a frustrated model with two types of bilayers. A
semiclassical treatment of the effective hard-core boson model shows that
perfect inter-layer frustration leads to a 2D-like critical exponent phi=1
without logarithmic corrections and to a 3D low temperature phase with
different but non vanishing triplet populations in both types of bilayers.
These results further suggest a simple phenomenology in terms of a
field-dependent transverse coupling in the context of which we reproduce the
entire field-temperature phase diagram with Quantum Monte Carlo simulations
Spectral Function and Self-Energy of the One-Dimensional Hubbard Model in the U --> infinity Limit
The interpretation of the k dependent spectral functions of the
one-dimensional, infinite U Hubbard model obtained by using the factorized
wave-function of Ogata and Shiba is revisited. The well defined feature which
appears in addition to low energy features typical of Luttinger liquids, and
which, close to the Fermi energy, can be interpreted as the shadow band
resulting from spin fluctuations, is further investigated. A calculation
of the self-energy shows that, not too close to the Fermi energy, this feature
corresponds to a band, i.e. to a solution of the Dyson equation .Comment: Latex, 3 pages, 2 figures, to appear in Z. Phys. B, Proceedings of
the Euroconfernce on "Correlations in Unconventional Quantum Liquids", Evora,
Portugal, October 7 to 11, 199
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