We explore the ground-state phase diagram of the S = 1/2 two-leg ladder. The isotropic leg interactions J 1, a and J 1, b between nearest neighbor spins in the legs a and b , respectively, are different from each other. The xy and z components of the uniform rung interactions are denoted by J r and Δ J r , respectively, where Δ is the XXZ anisotropy parameter. This system has a frustration when J 1, a J 1, b < 0 irrespective of the sign of J r . The phase diagrams on the Δ (0≤Δ<1) versus J 1, b plane in the cases of J 1, a = − 0.2 and J 1, a = 0.2 with J r = −1 are determined numerically. We employ the physical consideration, the level spectroscopy analysis of the results obtained by the exact diagonalization method and also the density-matrix renormalization-group method. It is found that the non-collinear ferrimagnetic (NCFR) state appears as the ground state in the frustrated region of the parameters. Furthermore, the direct-product triplet-dimer (TD) state in which all rungs form the TD pair is the exact ground state , when J 1, a + J 1, b = 0 and 0≤ Δ ≲ 0.83. The obtained phase diagrams consist of the TD, XY and Haldane phases as well as the NCFR phase.
We explore the ground-state phase diagram of the
S=1/2 two-leg ladder with different leg interactions. The
xy and
z components of the leg interactions between nearest-neighbor spins in the
a (
b) leg are respectively denoted by
Jl,a and
ΔlJl,a (
Jl,b and
ΔlJl,b). On the other hand, the
xy and
z components of the uniform rung interactions are respectively denoted by
ΓrJr and
Jr. In the above,
Δl and
Γr are the
XXZ-type anisotropy parameters for the leg and rung interactions, respectively. This system has a frustration when
Jl,aJl,b<0 irrespective of the sign of
Jr. The phase diagram on the
Δl (
∣Δl∣≤1.0) versus
Jl,b (
−2.0≤Jl,b≤3.0) plane in the case where
Jl,a=0.2,
Jr=−1.0, and
Γr=0.5 is determined numerically. We employ the physical consideration, and the level spectroscopy and phenomenological renormalization-group analyses of the numerical date obtained by the exact diagonalization method. The resultant phase diagram contains the ferromagnetic, Haldane, N{é}el, nematic Tomonaga-Luttinger liquid (TLL), partial ferrimagnetic, and
XY1 phases. Interestingly enough, the nematic TLL phase appears in the strong-rung unfrustrated region as well as in the strong-rung frustrated one. We perform the first-order perturbational calculations from the strong rung coupling limit to elucidate the characteristic features of the phase diagram. Furthermore, we make the density-matrix renormalization-group calculations for some physical quantities such as the energy gaps, the local magnetization, and the spin correlation functions to supplement the reliability of the phase diagram.