The ground-state phase diagram of frustrated S = 1 XXZ spin chains with the competing nearest-and next-nearest-neighbor antiferromagnetic couplings is studied using the infinitesystem density-matrix renormalization-group method. We find six different phases, namely, the Haldane, gapped chiral, gapless chiral, double Haldane, Néel, and double Néel (uudd) phases. The gapped and gapless chiral phases are characterized by the spontaneous breaking of parity, in which the long-range order parameter is a chirality, κ l =S , whereas the spin correlation decays either exponentially or algebraically. These chiral ordered phases appear in a broad region in the phase diagram for ∆ < 0.95, where ∆ is an exchange-anisotropy parameter. The critical properties of phase transitions are also studied.KEYWORDS: frustration, spin-1 chain, ground-state phase diagram, density-matrix renormalization-group method, chiral ordering §1. Introduction Frustrated antiferromagnetic quantum spin systems have attracted considerable attention over the decades since they exhibit a wealth of fascinating phenomena in their ground states and lowlying excitations. In general, frustration supresses antiferromagnetic correlations and the tendency towards the Néel order. Classical systems, for example, often show a helical ordered state in their ground states in the presence of strong frustration. In quantum systems, the interplay of frustration and quantum fluctuations plays an important role which causes exotic phenomena, e.g., a spin-liquid state and a novel type of spontaneous symmetry breaking.In this paper, we study a one-dimensional anisotropic spin system with the antiferromagnetic nearest-neighbor coupling J 1 and the frustrating next-nearest-neighbor coupling J 2 . The model is * E-mail address: hikihara@phys03.phys.sci.kobe-u.ac. where S l is a spin-S spin operator at site l and ∆ ≥ 0 represents an exchange anisotropy. Hereafter, we put j ≡ J 2 /J 1 (j ≥ 0).In the classical limit, S → ∞, the system exhibits a magnetic long-range order (LRO) characterized by a wavenumber q. The order parameter is defined bywhere L is the total number of spins. While the magnetic LRO is of the Néel-type (q = π) when j is smaller than a critical value, j ≤ 1/4, it becomes of helical-type for j > 1/4 with q = cos −1 (−1/4j).It should be noticed that both the time-reversal and parity symmetries are broken in this helical ordered state. When the system has an XY -like anisotropy (∆ < 1), the helical ordered state possesses a two-fold discrete degeneracy according as the helix is either right-or left-handed, in addition to a continuous degeneracy associated with the original U (1) symmetry of the XY spin.This discrete degeneracy is characterized by mutually opposite signs of the total chirality definedNote that this chirality is distinct from the scalar chirality of the Heisenberg spin often discussed in the literature 2) defined by χ l = S l−1 · S l × S l+1 : The chirality O κ changes its sign under the parity operation but is invariant under the time-reversal op...