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Register a new userExcitations with fractional spin less than 1/2 in frustrated magnetoelastic chains
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Added by
Claudio Javier Gazza
Publication date
2006
fields
Physics
and research's language is
English
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We study the magnetic excitations on top of the plateaux states recently discovered in spin-Peierls systems in a magnetic field. We show by means of extensive density matrix renormalization group (DMRG) computations and an analytic approach that one single spin-flip on top of $M=1-frac2N$ ($N=3,4,...$) plateau decays into $N$ elementary excitations each carrying a fraction $frac1N$ of the spin. This fractionalization goes beyond the well-known decay of one magnon into two spinons taking place on top of the M=0 plateau. Concentrating on the $frac13$ plateau (N=3) we unravel the microscopic structure of the domain walls which carry fractional spin-$frac13$, both from theory and numerics. These excitations are shown to be noninteracting and should be observable in x-ray and nuclear magnetic resonance experiments.
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We study the finite-size behavior of the low-lying excitations of spin-1/2 Heisenberg chains with dimerization and next-to-nearest neighbors interaction, J_2. The numerical analysis, performed using density-matrix renormalization group, confirms previous exact diagonalization results, and shows that, for different values of the dimerization parameter delta, the elementary triplet and singlet excitations present a clear scaling behavior in a wide range of ell=L/xi (where L is the length of the chain and xi is the correlation length). At J_2=J_2c, where no logarithmic corrections are present, we compare the numerical results with finite-size predictions for the sine-Gordon model obtained using Luschers theory. For small delta we find a very good agreement for ell > 4 or 7 depending on the excitation considered.
The static structure factor S(q) of frustrated spin-1/2 chains with isotropic exchange and a singlet ground state (GS) diverges at wave vector q_m when the GS has quasi-long-range order (QLRO) with periodicity 2pi/q_m but S(q_m) is finite in bond-order-wave (BOW) phases with finite-range spin correlations. Exact diagonalization and density matrix renormalization group (DMRG) calculations of S(q) indicate a decoupled phase with QLRO and q_m = pi/2 in chains with large antiferromagnetic exchange between second neighbors. S(q_m) identifies quantum phase transitions based on GS spin correlations.
We investigated the magnetoelastic properties of the quasi-one-dimensional spin-1/2 frustrated magnet LiCuVO$_4$. Longitudinal-magnetostriction experiments were performed at 1.5 K in high magnetic fields of up to 60 T applied along the $b$ axis, i.e., the spin-chain direction. The magnetostriction data qualitatively resemble the magnetization results, and saturate at $H_{text{sat}} approx 54$ T, with a relative change in sample length of $Delta L/L approx 1.8times10^{-4}$. Remarkably, both the magnetostriction and the magnetization evolve gradually between $H_{text{c3}} approx 48$ T and $H_{text{sat}}$, indicating that the two quantities consistently detect the spin-nematic phase just below the saturation. Numerical analyses for a weakly coupled spin-chain model reveal that the observed magnetostriction can overall be understood within an exchange-striction mechanism. Small deviations found may indicate nontrivial changes in local correlations associated with the field-induced phase transitions.
Using Lanczos exact diagonalization, stochastic analytic continuation of quantum Monte Carlo data, and perturbation theory, we investigate the dynamic spin structure factor $mathcal{S}(q,omega)$ of the $S=1/2$ antiferromagnetic Heisenberg trimer chain. We systematically study the evolution of the spectrum by varying the ratio $g=J_2/J_1$ of the intertrimer and intratrimer coupling strengths and interpret the observed features using analytical and numerical calculations with the trimer eigenstates. The doublet ground states of the trimers form effective interacting $S=1/2$ degrees of freedom described by a Heisenberg chain with coupling $J_{rm eff}=(4/9)J_2$. Therefore, the conventional two-spinon continuum of width $propto J_1$ when $g=1$ evolves into to a similar continuum of width $propto J_2$ in the reduced Brillouin zone when $gto 0$. The high-energy modes (at $omega propto J_1$) for $g alt 0.5$ can be understood as weakly dispersing propagating internal trimer excitations (which we term doublons and quartons), and these fractionalize with increasing $g$ to form the conventional spinon continuum when $g to 1$. The coexistence of two kinds of emergent spinon branches for intermediate values of $g$ give rise to interesting spectral signatures, especially at $g approx 0.7$ where the gap between the low-energy spinon branch and the high energy band of mixed doublons, quartons, and spinons closes. These features should be observable in inelastic neutron scattering experiments if a quasi-one-dimensional quantum magnet with the linear trimer structure and $J_2 < J_1$ can be identified. We suggest that finding such materials would be useful, enabling detailed studies of coexisting exotic excitations and their interplay within a relatively simple theoretical framework.
We study the frustrated ferromagnetic spin-1 chains, where the ferromagnetic nearest-neighbor coupling competes with the antiferromagnetic next-nearest-neighbor coupling. We use the density matrix renormalization group to obtain the ground states. Through the analysis of spin-spin correlations we identify the double Haldane phase as well as the ferromagnetic phase. It is shown that the ferromagnetic coupling leads to incommensurate correlations in the double Haldane phase. Such short-range correlations transform continuously into the ferromagnetic instability at the transition to the ferromagnetic phase. We also compare the results with the spin-1/2 and classical spin systems, and discuss the string orders in the system.
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