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Register a new userFrom deconfined spinons to coherent magnons in an antiferromagnetic Heisenberg chain with long range interactions
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We study the spectrum and the nature of the excitations of an antiferromagnetic (AFM) Heisenberg chain with staggered long range interactions, both numerically using the time-dependent density matrix renormalization group (tDMRG) method and by means of a multi-spinon approximation that qualitatively explains its main features. The unfrustrated long-range nature of the exchange effectively increases the dimensionality of the system and the chain is able to undergo true symmetry breaking and develop long range order, transitioning from a gapless spin liquid to a gapless ordered AFM phase. We calculated the momentum resolved spin dynamical structure factor and found that for weakly decaying interactions the emergence of Neel order can be associated to the formation of bound states of spinons that become coherent magnons. The quasiparticle band leaks out from the two-spinon continuum that is pushed up to higher energies. Our physical picture is also supported by an analysis of the behavior of the excitations in real-time.
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Anderson localization is a general phenomenon of wave physics, which stems from the interference between multiple scattering paths1,2. It was originally proposed for electrons in a crystal, but later was also observed for light3-5, microwaves6, ultrasound7,8, and ultracold atoms9-12. Actually, in a crystal, besides electrons there may exist other quasiparticles such as magnons and spinons. However the search for Anderson localization of these magnetic excitations is rare so far. Here we report the first observation of spinon localization in copper benzoate, an ideal compound of spin-1/2 antiferromagnetic Heisenberg chain, by ultra-low-temperature specific heat and thermal conductivity measurements. We find that while the spinon specific heat Cs displays linear temperature dependence down to 50 mK, the spinons thermal conductivity ks only manifests the linear temperature dependence down to 300 mK. Below 300 mK, ks/T decreases rapidly and vanishes at about 100 mK, which is a clear evidence for Anderson localization. Our finding opens a new window for studying such a fundamental phenomenon in condensed matter physics.
Inelastic neutron scattering was used to measure the magnetic field dependence of spin excitations in the antiferromagnetic S=1/2 chain CuCl_2 2(dimethylsulfoxide) (CDC) in the presence of uniform and staggered fields. Dispersive bound states emerge from a zero-field two-spinon continuum with different finite energy minima at wave numbers q=pi and q_i approx pi (1-2<S_z>). The ratios of the field dependent excitation energies are in excellent agreement with predictions for breather and soliton solutions to the quantum sine-Gordon model, the proposed low-energy theory for S=1/2 chains in a staggered field. The data are also consistent with the predicted soliton and n=1,2 breather polarizations and scattering cross sections.
We study a generalized quantum spin ladder with staggered long range interactions that decay as a power-law with exponent $alpha$. Using the density matrix renormalization group (DMRG) method and exact diagonalization, we show that this model undergoes a transition from a rung-dimer phase characterized by a non-local string order parameter, to a symmetry broken Neel phase at $alpha_csim 2.1$. We find evidence that the transition is second order with a dynamic critical exponent $z=1$ and $ uapprox 1.2$. In the magnetically ordered phase, the spectrum exhibits gapless modes, while excitations in the gapped phase are well described in terms of triplons -- bound states of spinons across the legs. We obtained the momentum resolved spin dynamic structure factor numerically and found that the triplon band is well defined at high energies and adiabatically connected to the magnon dispersion. However, at low energies it emerges as the lower edge of continuum of excitations that shifts to high energies across the transition. We further discuss the possibility of deconfined criticality in this model.
In conventional quasi-one-dimensional antiferromagnets with quantum spins, magnetic excitations are carried by either magnons or spinons in different energy regimes: they do not coexist independently, nor could they interact with each other. In this Letter, by combining inelastic neutron scattering, quantum Monte Carlo simulations and Random Phase Approximation calculations, we report the discovery and discuss the physics of the coexistence of magnons and spinons and their interactions in Botallackite-Cu2(OH)3Br. This is a unique quantum antiferromagnet consisting of alternating ferromagnetic and antiferromagnetic Spin-1/2 chains with weak inter-chain couplings. Our study presents a new paradigm where one can study the interaction between two different types of magnetic quasiparticles, magnons and spinons.
The phase transition in the compound LiVGe2O6 has been proposed as a unique example of a spin-Peierls transition in an S=1 antiferromagnetic chain. We report neutron and x-ray diffraction measurements of LiVGe2O6 above and below the phase transition at T=24 K. No evidence is seen for any structural distortion associated with the transition. The neutron results indicate that the low temperature state is antiferromagnetic, driven by ferromagnetic interchain couplings.
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