We study the ground state of three-leg $S=1/2$ Heisenberg tube using the density-matrix renormalization group method. The dimerization order-parameter and spin-excitation gap are calculated in a wide range of leg exchange interactions. We confirm that a gapped state with the dimerization order is realized even when the leg exchange interactions are ferromagnetic. Furthermore, the topological configuration of spin-singlet pairs in the ordered state is determined from the results of the Berry phase of each spin coupling as well as the structure factor of singlet-singlet correlation functions. We find that there exist three kinds of configurations of the valence-bond states depending on the ratio of leg and rung exchange interactions.
Soliton binding and low-lying singlets in frustrated odd-legged S=1/2 spin tubes
受钒自旋管Na 2 V3 O 7的有趣性质的启发，我们表明，一个有效的自旋手征模型类似于标准海森堡奇腿S=1/2自旋管可以推导出阻挫环间耦合，但与自旋手征耦合常数α，可以是任意小。利用密度矩阵重整化群和分析参数，我们表明，而自发二聚总是存在的，孤子成为绑定到低躺单作为α减少。强挫管的实验影响进行了讨论。
We investigate the spin-1/2 twisted three-leg antiferromagnetic quantum spin tube in the presence of the easy-plane anisotropy, using the numerical diagonalization of finite-size clusters. And we observe cases of the existence and the absence of magnetization plateau at 1/3 height of saturation magnetization when the anisotropy is tuned. The phenomenological-renormalization analysis indicates a quantum phase transition between the 1/3 magnetization plateau phase and the plateauless one. The phase diagram is also presented.
We study the ground state phase diagram of the twisted three-leg spin tube in magnetic fields by the density matrix renormalization group (DMRG) method. The twisted spin tube is composed of triangular unit cells and possesses strong quantum fluctuations under geometrical frustration. We apply the sine square deformation method to remove the strong boundary effects and obtain smooth magnetization curves without steps of finite systems. With the analysis of the magnetization curves and correlation functions we determine the ground state phase diagram consisting of (a) a Tomonaga-Luttinger (TL) l…
We study the ground state phase diagram of a frustrated spin-1/2 four-leg spin tube in an external magnetic field. We explore the parameter space of this model in the regime of all-antiferromagnetic exchange couplings by means of three different approaches: analysis of low-energy effective Hamiltonian (LEH), a Hartree variational approach (HVA) and density matrix renormalization group (DMRG) for finite clusters. We find that in the limit of weakly interacting plaquettes, low-energy singlet, triplet and quintuplet states play an important role in the formation of fractional magnetization platea…
We derive an effective electronic Hamiltonian for square lattice Hubbard-Holstein model (HHM) in the strong electron-electron (e-e) and electron-phonon (e-ph) coupling regime and under non-adiabatic conditions ($t/ω_0 \leq 1$), $t$ and $ω_0$ being the electron hopping and phonon frequency respectively. Using Density Matrix Renormalization Group method, we simulate this effective electronic model on $4-$Leg cylinder system at quarter-filling and present a phase diagram in $g-U$ plane where $g$ and $U$ are being the e-ph coupling constant and Hubbard on-site interaction respectively. For larger …
N-leg integer-spin ladders and tubes in commensurate external fields -Nonlinear sigma model approach-
本文研究了具有反铁磁链内耦合的N臂整数自旋梯和管的低能性质。在奇腿管，自动对焦横档耦合造成挫折。为了系统地处理所有梯和管，我们应用基于非线性西格玛模型的Sénéchal方法[Phys. Rev. B 52，15319（1995）]。这种策略在弱链间（梯级）耦合机制中是有效的。我们发现所有的阻挫管都具有六重简并自旋1的磁振子带，作为最低激发态，而其他的阶梯和管具有标准的三重简并带。我们还考虑了四种塞曼项的影响：均匀的，交错只沿梯级，只沿链，或两个方向。上述无场情形的预测意味着，由于双简并最低磁振子在阻挫管中的凝聚，足够强的均匀场产生双组分Tomonaga-Luttinger液体（TLL）。相反，在所有其他系统中，场诱导标准的单分量TLL。这一点得到了基于金兹伯格-朗道理论的对称性和玻色子化论点的支持。玻色化还表明，当均匀场变大，次低磁振子带接触零能线时，双分量TLL消失，出现单分量TLL。在磁化过程中，这种转变可以被观察到为尖点奇异性。对具有交错Zeeman项的一维AF自旋系统的分析表明，双简并横磁振子和单纵磁振子的出现对于具有弱交错场的一维AF自旋系统是普遍的.
The $S=1/2$ twisted three-leg spin tube with the lattice distortion from the regular triangle to the isosceles one is investigated using the numerical diagonalization of finite-size clusters and the phenomenological renormalization group analysis. It is found that the quantum phase transition occurs from the spin gap phase to another one with respect to this lattice distortion.
We study the ground-state magnetic phase diagram of a spin S=1/2 antiferromagnetic two-leg ladder in the presence of period two lattice units modulated, Dzyaloshinskii-Moriya (DM) interaction along the legs. We consider the case of collinear DM vectors and strong rung exchange and magnetic field. In this limit we map the initial ladder model onto the effective spin $σ=1/2$ XXZ chain and study the latter using the continuum-limit bosonization approach. We identified four quantum phase transitions and corresponding critical magnetic fields, which mark transitions from the spin gapped regimes int…
The spin-1/2 Ising-Heisenberg three-leg tube composed of the Heisenberg spin triangles mutually coupled through the Ising inter-triangle interaction is exactly solved in a zero magnetic field. By making use of the local conservation for the total spin on each Heisenberg spin triangle the model can be rigorously mapped onto a classical composite spin-chain model, which is subsequently exactly treated through the transfer-matrix method. The ground-state phase diagram, correlation functions, concurrence, Bell function, entropy and specific heat are examined in detail. It is shown that the spin fr…
We study the antiferromagnetic spin-half Heisenberg ladder in the presence of an additional frustrating rung spin which is motivated and relevant also for the description of real two-dimensional materials such as the two-dimensional trimer magnet Ba$_4$Ir$_3$O$_{10}$. We study the zero-temperature phase diagram, where we combine numerical and analytical methods into an overall consistent description. All numerical simulations are also accompanied by studies of the dynamical spin structure factor obtained via the density matrix renormalization group. Overall, we find in the regime of strong run…
Correlation functions and low-energy excitations are investigated in the asymmetric two-leg ladder consisting of a Hubbard chain and a noninteracting tight-binding (Fermi) chain using the density matrix renormalization group method. The behavior of charge, spin and pairing correlations is discussed for the four phases found at half filling, namely, Luttinger liquid, Kondo-Mott insulator, spin-gapped Mott insulator and correlated band insulator. Quasi-long-range antiferromagnetic spin correlations are found in the Hubbard leg in the Luttinger liquid phase only. Pair-density-wave correlations ar…
Low-energy behavior of the spin-tube and spin-orbital models
在强链间耦合的极限下，用双二次交换作用耦合的XXZ链表示了具有周期性边界条件的三条耦合自旋链（自旋管）的低能有效哈密顿量.提出了类似的一类模型来描述诸如NaV 2 O 5的材料中自旋与轨道自由度的耦合。我们利用玻色化和重整化群技术研究了这些模型，发现一般相图包括一个无隙区和由自旋液相和各种反铁磁相组成的有隙区.我们讨论了自旋液相的性质，特别是基态和基元激发的性质，然后研究了磁场的作用，得出结论：足够强的磁场可以抑制二聚化相，导致双组分Luttinger液体。在过渡gapful-gapless的临界指数的计算和显示是非普遍的自旋管的情况下，以及在一般的自旋轨道问题。
Whether a doped hole propagates as a Bloch wave or not is an important issue of doped Mott physics. Here we examine this problem based on the quasiparticle spectral weight $Z$ distribution, calculated by density matrix renormalization group (DMRG). By tuning the anisotropy of a two-leg $t$-$J$ ladder without closing the background spin gap, the $Z$ distribution unambiguously reveals a transition of the single hole state from a Bloch wave to a novel one with spontaneous translational symmetry breaking. We further establish a direct connection of such a transition with a nonlocal phase string en…
The existence of a gapped chiral spin liquid has been recently suggested in the vicinity of the metal-insulator transition of the Hubbard model on the triangular lattice, by intensive density-matrix renormalization group (DMRG) simulations [A. Szasz, J. Motruk, M.P. Zaletel, and J.E. Moore, Phys. Rev. X {\bf 10}, 021042 (2020)]. Here, we report the results obtained within the variational Monte Carlo technique based upon Jastrow-Slater wave functions, implemented with backflow correlations. As in DMRG calculations, we consider $N$-leg cylinders. For $N=4$ and in the presence of a next-nearest n…
Thermodynamic properties of the twisted three-leg spin tube under magnetic field are studied by the finite-$T$ density-matrix renormalization group method. The specific heat, spin, and chiral susceptibilities of the infinite system are calculated for both the original and its low-energy effective models. The obtained results show that the presence of the chirality is observed as a clear peak in the specific heat at low temperature and the contribution of the chirality dominates the low-temperature part of the specific heat as the exchange coupling along the spin tube decreases. The peak struct…
This article reviews low-temperature heat transport studies of spin-gapped quantum magnets in the last few decades. Quantum magnets with small spins and low dimensionality exhibit a variety of novel phenomena. Among them, some systems are char-acteristic of having quantum-mechanism spin gap in their magnetic excitation spectra, including spin-Peierls systems, S = 1 Haldane chains, S = 1/2 spin ladders, and spin dimmers. In some particular spin-gapped systems, the XY-type antiferromagnetic state induced by magnetic field that closes the spin gap can be described as a magnon Bose-Einstein conden…
Magnetic excitations in the strong-leg quantum spin ladder compound (C$_7$H$_{10}$N)$_2$CuBr$_4$ (known as DIMPY) in the field-induced Tomonaga-Luttinger spin liquid phase are studied by means of high-field electron spin resonance (ESR) spectroscopy. The presence of a gapped ESR mode with unusual non-linear frequency-field dependence is revealed experimentally. Using a combination of analytic and exact diagonalization methods, we compute the dynamical structure factor and identify this mode with longitudinal excitations in the antisymmetric channel. We argue that these excitations constitute a…
An overview on the phase diagram of the frustrated two-leg ladder model
利用密度矩阵重整化群，我们研究了具有Heisenberg相互作用的两足阶梯的一般相图。我们确认，所有的反铁磁带隙态属于相同的普适类的Halfoxin相。在三维相图中，我们确定了一个连续的表面与奇异的字符串-序参量或其一阶导数，对应于两个Haldron相位之间的过渡具有不同的拓扑顺序。这个过渡曲面的某些部分是临界的，带隙为零，弦序参量为零。在互补部分，跃迁是一阶的，具有有限的间隙和弦序。这个表面与铁磁区域的边界是临界端线，当表面是临界的时，并且在其他任何地方是三线。该边界的一部分与D. V. Dmitriev、V. Ya Krivnov和A.A. Ovchinnikov [Phys.Rev.B56，5985（1997）]。
Some magnets due to particular geometry of the exchange bonds do not undergo transition to the conventional magnetically ordered state despite of the presence of significant exchange couplings. Instead, a collective paramagnetic state is formed. The later state can remain stable down to $T=0$ if the ground state of this magnet turns out to be nonmagnetic singlet separated from the excited triplet states by an energy gap. Low-temperature spin dynamics of the collective paramagnets with gapped excitations spectrum (or spin-gap magnets) can be described in terms of a dilute gas of the triplet exc…
We consider the spin-1/2 two-leg ladders with ferromagnetic (FM) interactions along legs and rungs. Using the stochastic series expansion QMC method, we study the low-temperature magnetic behavior of the system. An isolated spin-1/2 FM two-leg ladder is in the gapped saturated FM phase at zero temperature. As soon as the spin-1/2 FM two-leg ladders are connected with antiferromagnetic (AFM) inter-ladder interaction, a first-order commensurate-incommensurate quantum phase transition occurs in the ground state magnetic phase diagram. In fact a jump in the magnetization curve is observed. We foun…
