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 ($H_c^1$, $H_c^2$, 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

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

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

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 $2k_F$ 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 $\omega - \epsilon(k) - Re \Sigma (k,\omega) =0$.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