A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between Néel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor interaction dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use d10–d0 cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr2Cu(Te0.5W0.5)O6, where the isostructural end phases Sr2CuTeO6 and Sr2CuWO6 are Néel and columnar type antiferromagnets, respectively. We show that magnetism in Sr2Cu(Te0.5W0.5)O6 is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr2Cu(Te0.5W0.5)O6—similar to several spin liquid candidates—a plateau in muon spin relaxation rate and a strong T-linear dependence in specific heat. Our observations for Sr2Cu(Te0.5W0.5)O6 highlight the role of disorder in addition to magnetic frustration in spin liquid physics.

Sr 2 CuWO 6 is a double perovskite proposed to be at the border between two-and three-dimensional magnetism, with a square lattice of S = 1 2 Cu 2+ ions. We have used inelastic neutron scattering to investigate the spin wave excitations of the system, to find out how they evolve as a function of temperature, as well as to obtain information about the magnetic exchange interactions. We observed well defined dispersive spin wave modes at 6 K, which partially survive above the magnetic ordering temperature T N = 24 K. Linear spin wave theory is used to determine the exchange interactions revealing them to be highly two dimensional in nature. Density functional theory calculations are presented supporting this experimental finding, which is in contrast to a previous ab initio study of the magnetic interactions. Our analysis confirms that not the nearest neighbor, but the next nearest neighbor interactions in the tetragonal ab plane are the strongest. Low incident energy measurements reveal the opening of a 0.6(1) meV gap below T N , which suggests the presence of a very weak single ion anisotropy term in the form of an easy axis alongâ.

The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum disordered ground state when magnetic frustration is maximized by competing nearest-neighbor J1 and next-nearest-neighbor J2 interactions (J2/J1 ≈ 0.5). The double perovskites Sr2CuTeO6 and Sr2CuWO6 are isostructural spin-1/2 square-lattice antiferromagnets with Néel (J1 dominates) and columnar (J2 dominates) magnetic order, respectively. Here we characterize the full isostructural solid solution series Sr2Cu(Te1-xWx)O6 (0 ≤ x ≤ 1) tunable from Néel order to quantum disorder to columnar order. A spin-liquid-like ground state was previously observed for the x = 0.5 phase, but we show that the magnetic order is suppressed below 1.5 K in a much wider region of x ≈ 0.1-0.6. This coincides with significant T-linear terms in the lowtemperature specific heat. However, density functional theory calculations predict most of the materials are not in the highly frustrated J2/J1 ≈ 0.5 region square-lattice Heisenberg model.Thus, a combination of both magnetic frustration and quenched disorder is the likely origin of the spin-liquid-like state in x = 0.5.

Establishing the physical mechanism governing exchange interactions is fundamental for exploring exotic phases such as quantum spin liquids (QSLs) in real materials. In this work, we address exchange interactions in Sr2CuTexW1−xO6, a series of double perovskites that realize a spin-1/2 square lattice and are suggested to harbor a QSL ground state arising from the random distribution of non-magnetic ions. Our ab initio multi-reference configuration interaction calculations show that replacing Te atoms with W atoms changes the dominant couplings from nearest to nextnearest neighbor due to the crucial role of unoccupied states of the non-magnetic ions in the supersuperexchange mechanism. Combined with spin-wave theory simulations, our calculated exchange couplings provide an excellent description of the inelastic neutron scattering spectra of the parent compounds, as well as explaining that the magnetic excitations in Sr2CuTe0.5W0.5O6 emerge from bond-disordered exchange couplings. Our results demonstrate the crucial role of the non-magnetic cations in exchange interactions paving the way to further explore QSL phases in bond-disordered materials.In 3d transition metal (TM) oxides, the on-site Coulomb repulsive interactions between the electrons are strong enough to confine them to the TM sites, leading to the formation of localized spin or spin-orbital moments [1]. The manner in which these moments couple to each other is primarily governed by the underlying exchange interactions, which may be direct and/or mediated by the intermediate anions or ligands (L), the latter is also referred to as the superexchange. There are many possible ways these interactions can manifest, resulting in a plethora of magnetically ordered states such as ferromagnetic and different types of antiferromagnetic (AFM) order, magnetic spirals or more exotic topologically protected magnetic textures such as Skyrmions [1][2][3][4].Even more fascinating ground states that stem from exchange interactions are those which do not undergo any magnetic ordering even at absolute zero temperature, e.g. spin-liquid states in low-dimensional magnetic systems [5]. Broken-symmetry valence-bond solids and QSLs where symmetry is conserved are examples of such phases [5][6][7]. In these quantum paramagnetic phases, the long-range magnetic order is typically destroyed by frustrated exchange interactions and quantum fluctuations [8]. In the simplistic and prototypical two-dimensional spin-1/2 Heisenberg square lattice (HSL) model, the ratio of nearest-neighbor (NN) J 1 and AF next-nearest neighbor (NNN) J 2 exchange interac-tions of ∼0.5 results in magnetic frustration and a QSL ground state [7].The exchange mechanisms in TM compounds, principally the superexchange, are reasonably well understood in the form of the Goodenough-Kanamori-Anderson (GKA) rules [1]. The highly successful GKA rules correctly predict the sign of magnetic coupling for the 180 • and 90 • TM-L-TM bond angles. In double perovskite compounds like Sr 2 CuTeO 6 and Sr 2 CuWO 6 the magnetic Cu...