We develop a microscopic theory of finite-temperature spin-nematic orderings in three-dimensional spatially anisotropic magnets consisting of weakly coupled frustrated spin-1/2 chains with nearest-neighbor and next-nearest-neighbor couplings in a magnetic field. Combining a field theoretical technique with density-matrix renormalization group results, we complete finite-temperature phase diagrams in a wide magnetic-field range that possess spin-bond-nematic and incommensurate spin-density-wave ordered phases. The effects of a four-spin interaction are also studied. The relevance of our results to quasi-one-dimensional edge-shared cuprate magnets such as LiCuVO(4) is discussed.

The ground-state phase diagram of a spin-1 2 XXZ chain with competing ferromagnetic nearestneighbor (J1 < 0) and antiferromagnetic second-neighbor (J2 > 0) exchange couplings is studied by means of the infinite time evolving block decimation algorithm and effective field theories. For the SU(2)-symmetric (Heisenberg) case, we show that the nonmagnetic phase in the range −4 < J1/J2 < 0 has a small but finite ferromagnetic dimer order. We argue that this spontaneous dimer order is associated with effective spin-1 degrees of freedom on dimerized bonds, which collectively form a valence bond solid state as in the spin-1 antiferromagnetic Heisenberg chain (the Haldane spin chain). We thus call this phase the Haldane dimer phase. With easy-plane anisotropy, the model exhibits a variety of phases including the vector chiral phase with gapless excitations and the even-parity dimer and Néel phases with gapped excitations, in addition to the Haldane dimer phase. Furthermore, we show the existence of gapped phases with coexisting orders in narrow regions that intervene between the gapless chiral phase and any one of Haldane dimer, even-parity dimer, and Néel phases. Possible implications for quasi-one-dimensional edge-sharing cuprates are discussed.

We show by unbiased numerical calculations that the ferromagnetic nearest-neighbor exchange interaction stabilizes a vector spin chiral order against the quantum fluctuation in a frustrated spin-1/2 chain relevant to multiferroic cuprates, LiCu2O2 and LiCuVO4. Our realistic semiclassical analyses for LiCu2O2 resolve controversies on the helical magnetic structure and unveil the pseudo-Nambu-Goldstone modes as the origin of experimentally observed electromagnons.

1Spin liquid is a state of electron spins where quantum fluctuation breaks magnetic ordering with keeping spin correlation [1]. It has been one of central topics of magnetism because of its relevance to fascinating phenomena such as high-T c superconductivity [2, 3] and topological states [4]. In spite of the profound physics, on the other hand, spin liquid itself has been quite difficult to utilize.Typical spin liquid states are realized in one-dimensional spin systems, called quantum spin chains [5, 6]. Here we show that a spin liquid in a spin-1/2 quantum chain generates and carries spin current via its long-range spin fluctuation. This is demonstrated by observing an anisotropic negative spin Seebeck effect [7][8][9][10][11][12] along the spin chains in Sr 2 CuO 3 . The result shows that spin current can flow even in an atomic channel owing to the spin liquid state, which can be used for atomic spin-current wiring.A flow of electrons spin angular momentum is called spin current [13]. In condensed matter science, transport properties of spin current have attracted considerable interest since the discovery of various spin-current phenomena [14, 15]. In spintronics [16], on the other hand, it is of critical importance to find materials which can carry spin angular momentum efficiently in integrated microscopic devices.Two types of spin current have experimentally been explored so far. The first one is conduction-electron spin current, which is mediated by an electron motion in metals and semiconductors. Its velocity and propagation length are thus limited by electron diffusion [17]. The other type is spin-wave spin current [18,19], where spin waves, wavelike propagation of spin motions in magnets, carry spin angular momentum. Its excitation gap is equal to a spin-wave gap, proportional to magnetic anisotropy. Importantly, spin-wave spin current can exist even in insulators in which spin relaxation via conduction electrons is absent, an advantage which may realize fast and long-range spin current transmission, opening a new field of insulator-based spintronics. However, spin-wave spin current in classical magnets may not be suitable for microscopic devices, since handling spin waves becomes difficult when devices are miniaturized toward atomic scale; in ferromagnets, spontaneous magnetization brings about significant stray fields, causing crosstalk. In an antiferromagnetic system, on the other hand, spin ordering patterns should be broken or interfered when a device is in atomic scale; in both cases, spin waves become vulnerable. Therefore, to realize spin-current transport in microscopic devices, spin ordering is expected to vanish with 2 keeping strong interaction among spins.Here, we would like to make a new type of spin current debut: spinon spin current, which may provide a channel for atomic spin transmission to satisfy the requirements. A spinon generally refers to magnetic elementary excitation in quantum spin liquid states [1]. When system size of a magnet is reduced to atomic scale, quantum spin fluct...