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We have studied the longitudinal spin Seebeck effect in a polar antiferromagnet α-Cu2V2O7 in contact with a Pt film. Below the antiferromagnetic transition temperature of α-Cu2V2O7, spin Seebeck voltages whose magnetic field dependence is similar to that reported in antiferromagnetic MnF2|Pt bilayers are observed. Though a small weak-ferromagnetic moment appears owing to the Dzyaloshinskii-Moriya interaction in α-Cu2V2O7, the magnetic field dependence of spin Seebeck voltages is found to be irrelevant to the weak ferromagnetic moments. The dependences of the spin Seebeck voltages on magnetic fields and temperature are analyzed by a magnon spin current theory. The numerical calculation of spin Seebeck voltages using magnetic parameters of α-Cu2V2O7 determined by previous neutron scattering studies reveals that the magnetic-field and temperature dependences of the spin Seebeck voltages for α-Cu2V2O7|Pt are governed by the changes in magnon lifetimes with magnetic fields and temperature.Low-dimensional quantum magnets have attracted much attention in condensed matter physics for many decades [1,2]. It is known that in a one-dimensional antiferromagnetic system, long-range ordering is absent even at zero temperature [3], leading to exotic magnetic ground states, e.g. quantum spin liquid states [4]. Spin excitations in spin liquid states are spinons, which are regarded as spin-1/2 excitations as opposed to spin-1 excitations of magnons. Very recently, spin currents carried by spinons were demonstrated experimentally using the spin Seebeck effect (SSE) in Sr 2 CuO 3 |Pt systems [5]. Unusual magnetic properties of low-dimensional quantum magnets are intriguing for the search of new spin current effects.Among various compounds, a copper divanadates α-Cu 2 V 2 O 7 is a low-dimensional antiferromagnetic spin-1/2 system with fascinating magnetic properties. The oxide compound Cu 2 V 2 O 7 crystallizes in dichromate structure with three different polymorphs, i.e. α, β, and γ phases [6]. The structures of the β and γ phases are centrosymmetric, while the α phase possesses a noncentrosymmetric crystal structure with a polar point group (mm2) [6,7]. The space group of α-Cu 2 V 2 O 7 is F dd2 with lattice constants of a = 20.645Å, b = 8.383Å, and c = 6.442Å [8]. As illustrated in Fig. 1(a), all Cu 2+ ions form two sets of almost perpendicular zigzag chains [6][7][8][9].Magnetic properties of α-Cu 2 V 2 O 7 are governed by Cu 2+ S = 1/2 spins, since V 5+ ions are nonmagnetic. In the proposed spin Hamiltonian [10], there are three important terms to explain the magnetic properties of α-Cu 2 V 2 O 7 . The first term is isotropic exchange interactions. In the zigzag spin chains, Cu 2+ spins interact with their nearest neighbors. Since the magnetic interactions between nearest (J 1 ), second-nearest (J 2 ), and third-nearest (J 3 ) neighbors are all antiferromagnetic [6-10], α-Cu 2 V 2 O 7 exhibits antiferromagnetism. The second term is an anisotropic exchange interaction, which arises from the multiorbital correlation effect [10]. ...

14High-field magnetization of the spin-1/2 antiferromagnet α-Cu2V2O7 was measured in pulsed magnetic fields of up to 56 T in order to study its magnetic phase diagram. When the field was applied along the easy axis (the a-axis), two distinct transitions were observed at Hc1 = 6.5 T and Hc2 = 18.0 T. The former is a spin-flop transition typical for a collinear antiferromagnet and the latter is believed to be a spin-flip transition of canted moments. The canted moments, which are induced by the Dzyaloshinskii-Moriya interactions, anti-align for Hc1 < H < Hc2 due to the anisotropic exchange interaction that favors the antiferromagnetic arrangement along the a-axis. Above Hc2, the Zeeman energy of the applied field overcomes the antiferromagnetic anisotropic interaction and the canted moments are aligned along the field direction. Density functional theory was employed to compute the exchange interactions, which were used as inputs for quantum Monte Carlo calculations and then further refined by fitting to the magnetic susceptibility data. Contrary to our previous report in Phys. Rev. B 92, 024423, the dominant exchange interaction is between the third nearest-neighbor spins, which form zigzag spin-chains that are coupled with one another through an intertwining network of the nonnegligible nearest and second nearest-neighbor interactions. In addition, elastic neutron scattering under the applied magnetic fields of up to 10 T reveals the incommensurate helical spin structure in the spin-flop state.