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Frustrated mixed-spin ladders: An intermediate phase between rung-singlet and Haldane phases

Nader Ahmadi, Jahanfar Abouie, Reza Haghshenas

Published Jun 15, 2021• 1 citation
OpenAlex

Abstract

In frustrated spin ladders composed of antiferromagnetically coupled chains, homogeneous or inhomogeneous, the interplay of frustration and correlations causes the emergence of two phases, Haldane (H) phase and rung singlet (RS) phase, in which the transition between these two phases had been under debate. In this paper we investigate the ground state phase diagram of frustrated mixed-spin-(1, 1/2) ladders, using the notions from quantum information theory such as entanglement entropy, Schmidt gap and entanglement levels degeneracies, and also defining various local and nonlocal order parameters. Employing two numerical techniques, the infinite time-evolving block decimation (iTEBD) and density matrix renormalization group (DMRG) algorithms, we obtain the ground state phase diagram of the ladder, and demonstrate that there is an intermediate phase between RS and H phases, where the ground state is disordered and the entanglement spectrum follow no particular pattern.

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In frustrated spin ladders composed of antiferromagnetically coupled chains, homogeneous or inhomogeneous, the interplay of frustration and correlations causes the emergence of two phases, Haldane (H) phase and rung singlet (RS) phase, in which the transition between these two phases had been under debate. In this paper we investigate the ground state phase diagram of frustrated mixed-spin-(1, 1/2) ladders, using the notions from quantum information theory such as entanglement entropy, Schmidt gap and entanglement levels degeneracies, and also defining various local and nonlocal order parameters. Employing two numerical techniques, the infinite time-evolving block decimation (iTEBD) and density matrix renormalization group (DMRG) algorithms, we obtain the ground state phase diagram of the ladder, and demonstrate that there is an intermediate phase between RS and H phases, where the ground state is disordered and the entanglement spectrum follow no particular pattern.
In frustrated spin ladders, the interplay of frustration and correlations leads to the familiar Haldane (H) and rung-singlet (RS) phases. The nature of the transition between these two phases is still under debate. In this paper we tackle this issue using tools of quantum information theory. We consider frustrated mixed-spin-(1, 1/2) ladders with antiferromagnetic leg, rung and diagonal couplings, and calculate various quantities, such as the entanglement entropy (EE), the Schmidt gap, and the level degeneracy of the entanglement spectrum (ES). We use two numerical techniques, the infinite time-evolving block decimation (iTEBD) and the density matrix renormalization group (DMRG). We demonstrate that there exists an intermediate phase in which the ES levels do not exhibit the characteristic degeneracies of the H and RS phases. To understand the underlying physics in this phase, we investigate short-range spin correlations along legs, rungs and diagonals and show that in this intermediate phase long-wavelength modulations occur, akin to bond-order waves.
We study the ground-state phase diagram of a spin-12\frac{1}{2}21​ frustrated XXZ ladder, in which two antiferromagnetic chains are coupled by competing rung and diagonal interactions, ${J}_{\ensuremath{\perp}}$ and ${J}_{\ifmmode\times\else\texttimes\fi{}}$. Previous studies on the isotropic model have revealed that a fluctuation-induced effective dimer attraction between the legs stabilizes the columnar dimer (CD) phase in the highly frustrated regime ${J}_{\ensuremath{\perp}}\ensuremath{\approx}2{J}_{\ifmmode\times\else\texttimes\fi{}}$, especially for ferromagnetic ${J}_{\ensuremath{\perp},\ifmmode\times\else\texttimes\fi{}}<0$. By means of effective field theory and numerical analyses, we extend this analysis to the XXZ model, and we obtain a rich phase diagram. The diagram includes four gapped featureless phases with no symmetry breaking: the rung singlet (RS) and Haldane phases as well as their twisted variants, the RS* and Haldane* phases, which are all distinct in the presence of certain symmetries. Significantly, the Haldane-CD transition point in the isotropic model turns out to be a crossing point of two transition lines in the XXZ model, and the stripe N\'eel and RS* phases appear between these lines. This indicates a nontrivial interplay between the effective dimer attraction and the exchange anisotropy. In the easy-plane regime, the four featureless phases and two critical phases are found to compete in a complex manner depending on the signs of ${J}_{\ensuremath{\perp},\ifmmode\times\else\texttimes\fi{}}$.
We investigate a spin-1/21/21/2 two-leg honeycomb ladder with frustrating next-nearest-neighbor (NNN) coupling along the legs, which is equivalent to two ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ spin chains coupled with ${J}_{\ensuremath{\perp}}$ at odd rungs. The full parameter region of the model is systematically studied using conventional and infinite density-matrix renormalization group as well as bosonization. The rich phase diagram consists of five distinct phases: a Haldane phase, a NNN-Haldane phase, and a staggered dimer phase when ${J}_{\ensuremath{\perp}}<0$; a rung singlet phase and a columnar dimer phase when ${J}_{\ensuremath{\perp}}>0$. An interesting reentrant behavior from the dimerized phase into the Haldane phase is found as the frustration J2{J}_{2}J2​ increases. The universalities of the critical phase transitions are fully analyzed. Phase transitions between dimerized and disordered phases belong to the two-dimensional Ising class with a central charge c=1/2c=1/2c=1/2. The transition from the Haldane phase to NNN-Haldane phase is of a weak topological first order, while the continuous transition between the Haldane phase and rung singlet phase has a central charge c=2c=2c=2.
In the present paper, by using the density-matrix renormalization-group technique, we investigate the existence of dimerized phases in a frustrated spin-1/2 ladder model. By calculating the two-rung localizable entanglement and, in particular, its first-order derivative in the ground state, we show that an intermediate columnar dimerized phase exists indeed in a narrow parameter region between the rung-singlet and the Haldane phases. Furthermore, by introducing an additional interchain next-nearest-neighbor exchange interaction, the stability of this phase can be greatly enhanced and a staggering dimerized phase also appears in the phase diagram. Our results confirm the previous conclusions derived by the bosonization methods.
We study the impact of the diagonal frustrating couplings on the quantum phase diagram of a two-leg ladder composed of alternating spin-1 and spin-12\frac{1}{2}21​ rungs. As the coupling strength is increased the system successively exhibits two gapped paramagnetic phases (a rung-singlet and a Haldane-type nondegenerate states) and two ferrimagnetic phases with different ferromagnetic moments per rung. The first two states are similar to the phases studied in the frustrated spin-12\frac{1}{2}21​ ladder, whereas the magnetic phases appear as a result of the mixed-spin structure of the model. A detailed characterization of these phases is presented using density-matrix renormalization-group calculations, exact diagonalizations of periodic clusters, and an effective Hamiltonian approach inspired by the analysis of numerical data. The present theoretical study was motivated by the recent synthesis of the quasi-one-dimensional ferrimagnetic material FeIIFeIII{\text{Fe}}^{\text{II}}{\text{Fe}}^{\text{III}}FeIIFeIII (trans-1,4-cyclohexanedicarboxylate) exhibiting a similar ladder structure.
Low-Lying Energy Properties of a Frustrated Antiferromagnetic Spin-12\frac 1 221​ Ladder
Using the Density Matrix Renormalization Group method, we determined the phase diagram of a frustrated antiferromagnetic spin-12\frac 1 221​ ladder at zero temperature. Two spin-gapped phases, the Haldane phase and the singlet phase, are identified. A phase transition between the two phases occurs at any non-zero value of frustration coupling J×J_{\times}J×​. On the phase boundary, the spin gap vanishes for sufficiently small J×J_{\times}J×​. Crossing this non-gapped line, the transition is of second order, while of first order for larger J×J_{\times}J×​. Striking frustration effects are predicted for ladder materials.