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Register a new userInteraction-Induced Weakening of Localization in Few-Particle Disordered Heisenberg Chains
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We investigate real-space localization in the few-particle regime of the XXZ spin-$1/2$ chain with a random magnetic field. Our investigation focuses on the time evolution of the spatial variance of non-equilibrium densities, as resulting for a specific class of initial states, namely, pure product states of densely packed particles. Varying the strength of both particle-particle interactions and disorder, we numerically calculate the long-time evolution of the spatial variance $sigma(t)$. For the two-particle case, the saturation of this variance yields an increased but finite localization length, with a parameter scaling different to known results for bosons. We find that this interaction-induced increase is the stronger the more particles are taken into account in the initial condition. We further find that our non-equilibrium dynamics are clearly inconsistent with normal diffusion and instead point to subdiffusive dynamics with $sigma(t) propto t^{1/4}$.
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We uncover a novel mechanism for inducing a gapful phase in interacting many-body quantum chains. The mechanism is nonperturbative, being triggered only in the presence of both strong interactions and strong aperiodic (disordered) modulation. In the context of the critical antiferromagnetic spin-1/2 XXZ chain, we identify an emerging dimerization which removes the system from criticality and stabilizes the novel phase. This mechanism is shown to be quite general in strongly interacting quantum chains in the presence of strongly modulated quasiperiodic disorder which is, surprisingly, perturbatively irrelevant. Finally, we also characterize the associated quantum phase transition via the corresponding critical exponents and thermodynamic properties.
Using numerical diagonalization techniques, we explore the effect of local and bond disorder on the finite temperature spin and thermal conductivities of the one dimensional anisotropic spin-1/2 Heisenberg model. High-temperature results for local disorder show that the dc conductivties are finite, apart from the uncorrelated - XY case - where dc transport vanishes. Moreover, at strong disorder, we find finite dc conductivities at all temperatures $T$, except T=0. The low frequency conductivities are characterized by a nonanalytic cusp shape. Similar behavior is found for bond disorder.
The ground state spin-wave excitations and thermodynamic properties of two types of ferrimagnetic chains are investigated: the alternating spin-1/2 spin-5/2 chain and a similar chain with a spin-1/2 pendant attached to the spin-5/2 site. Results for magnetic susceptibility, magnetization and specific heat are obtained through the finite-temperature Lanczos method with the aim in describing available experimental data, as well as comparison with theoretical results from the semiclassical approximation and the low-temperature susceptibility expansion derived from Takahashis modified spin-wave theory. In particular, we study in detail the temperature vs. magnetic field phase diagram of the spin-1/2 spin-5/2 chain, in which several low-temperature quantum phases are identified: the Luttinger Liquid phase, the ferrimagnetic plateau and the fully polarized one, and the respective quantum critical points and crossover lines.
Field-induced excitation gaps in quantum spin chains are an interesting phenomenon related to confinements of topological excitations. In this paper, I present a novel type of this phenomenon. I show that an effective magnetic field with a fourfold screw symmetry induces the excitation gap accompanied by dimer orders. The gap and dimer orders induced so exhibit characteristic power-law dependence on the fourfold screw-symmetric field. Moreover, the field-induced dimer order and the field-induced Neel order coexist when the external uniform magnetic field, the fourfold screw-symmetric field, and the twofold staggered field are applied. This situation is in close connection with a compound [Cu(pym)(H$_2$O)$_4$]SiF$_6$ [J. Liu et al., Phys. Rev. Lett. 122, 057207 (2019)]. In this paper, I discuss a mechanism of field-induced dimer orders by using a density-matrix renormalization group method, a perturbation theory, and quantum field theories.
In conformal field theory, key properties of spin-1/2 chains, such as the ground state energy per site and the excitation gap scale with dimerization delta as delta^alpha with known exponents alpha and logarithmic corrections. The logarithmic corrections vanish in a spin chain with nearest (J=1) and next nearest neighbor interactions (J_2), for J_2c=0.2411. DMRG analysis of a frustrated spin chain with no logarithmic corrections yields the field theoretic values of alpha, and the scaling relation is valid up to the physically realized range, delta ~ 0.1. However, chains with logarithmic corrections (J_2<0.2411 J) are more accurately fit by simple power laws with different exponents for physically realized dimerizations. We show the exponents decreasing from approximately 3/4 to 2/3 for the spin gap and from approximately 3/2 to 4/3 for the energy per site and error bars in the exponent also decrease as J_2 approaches to J_2c.
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