Putative spin-nematic phase in BaCdVO(PO4)2

Abstract

We report neutron scattering and AC magnetic susceptibility measurements of the 2D spin-1/2 frustrated magnet BaCdVO(PO4)2. At temperatures well below T N ≈ 1K, we show that only 34% of the spin moment orders in an up-up-down-down strip structure. Dominant magnetic diffuse scattering and comparison to published µsr measurements indicates that the remaining 66% is fluctuating. This demonstrates the presence of strong frustration, associated with competing ferromagnetic and antiferromagnetic interactions, and points to a subtle ordering mechanism driven by magnon interactions. On applying magnetic field, we find that at T = 0.1 K the magnetic order vanishes at 3.8 T, whereas magnetic saturation is reached only above 4.5 T. We argue that the putative high-field phase is a realization of the long-sought bond-spin-nematic state.In the search for new states of matter, the realization of a spin-nematic state -a quantum version of a liquid crystal -has proved an enduring but elusive goal [1]. Of particular interest is the bond spin nematic (BSN), believed to exist in spin-1/2 materials with competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions [1,2]. This state is remarkable in that it combines the long-range entanglement characteristic of a quantum spin liquid [3][4][5][6], with a nematic order that breaks spin-rotational symmetry, while preserving both translational-and time-reversal symmetry [1,2]. As a consequence, the BSN state does not produce a static internal magnetic field, making it difficult to observe in experiment [5,[7][8][9]. Nonetheless, there is now a wellestablished scenario for BSN order ocurring through the condensation of bound pairs of magnons in high magnetic field [2, 10-21], cf. Fig. 1, and a number of promising candidate materials where this may occur [22][23][24][25][26].Of particular note is BaCdVO(PO 4 ) 2 , one of a family of square-lattice, spin-1/2 vanadates [27][28][29][30][31][32]. Early measurements of the heat capacity identified a phase transi-tion with T N ≈ 1 K, for fields H ≤ 3.5 T, while the magnetization was found to saturate for a field H > ∼ 4 T [31]. These results were interpreted in terms of a model with 1 st -neighbor exchange J 1 ≈ −3.6 K and 2 nd -neighbor exchange J 2 ≈ 3.2 K [31], for which the low-field ordered state would be a canted AFM with propagation vector q sq = (1/2, 0) [33][34][35]. Subsequent thermodynamic measurements have extended the magnetic phase diagram of BaCdVO(PO 4 ) 2 , filling in the gaps at high field, and identifying a new low-temperature phase bordering the saturated state [36], precisely where one might expect a BSN to occur [2,10,[14][15][16][18][19][20][21]. This is an exciting development, since the range of fields involved, 4 < ∼ H < ∼ 5 T [36], is much lower than in other candidate systems [22][23][24][25], making it accessible to a wider range of experimental techniques. Despite this progress, the nature of the low-field phase in BaCdVO(PO 4 ) 2 , the form of its magnetic interactions, and the possibility of a BSN in high f...

Presaturation phase with no dipolar order in a quantum ferro-antiferromagnet

“…That no magnetic structure with a different propagation vector exists in higher fields is clear from powder diffraction experiments of Ref. [12]. We thus identify the region H > H c2 as fully saturated and the field range H c1 < H < H c2 as corresponding to the potential spin-nematic phase.…”

“…The upper boundary assigned to the nematic phase in Ref. [12] does not correspond to any thermodynamic anomalies in single crystals. Counter-intuitively it moves out to higher fields with increased temperature.…”

“…Long range magnetic order in zero applied field is of a peculiar "up-up-down-down" type, propagation vector (0, 1/2, 0), and appears below T N = 1.05 K [12]. It is incompatible with the simple J 1 -J 2 model and was attributed to additional 3rd-nearest neighbor (3nn) interactions in the V 4+ layers [12,16]. There is, however, a more natural explanation.…”

Possible observation of quantum spin-nematic phase in a frustrated magnet

“…We note that while this article was under review, two studies have also reported the possibility of SN order in the square-lattice frustrated ferromagnet BaCdVO(PO 4 ) 2 (29, 30).…”

Realizing square and diamond lattice S=1/2 Heisenberg antiferromagnet models in the α and β phases of the coordination framework, Abdeldaim1, Li2, Farrar3 et al. 2020Phys. Rev. Materials

“…Indeed, in a recent theoretical and experimental investigation of the solid-solution, Sr 2 Cu(Te 1-x W x )O 6 where x = 0 and x = 1 represent Néel and columnar ordered systems, respectively, QSL signatures have been observed for the x = 0.5 compound [17]. Interestingly, the behavior exhibited by complex materials derived from the FSL, such as BaCdVO(PO 4 ) 2 [6], where signatures of a spinnematic ground state have been observed in an applied field [18,19], has further enriched the phase diagram of FSL materials, leading to new theoretical predictions [20]. By extending the J 1 −J 2 model to incorporate the effects of interplanar coupling, J 3 , magnetic ground states inaccessible to the pure FSL model can also be realized [21].…”