Magnetic Excitations and Electronic Interactions inSr2CuTeO6: A Spin-1/<

Abstract

Sr 2 CuTeO 6 presents an opportunity for exploring low-dimensional magnetism on a square lattice of S ¼ 1=2 Cu 2þ ions. We employ ab initio multireference configuration interaction calculations to unravel the Cu 2þ electronic structure and to evaluate exchange interactions in Sr 2 CuTeO 6 . The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr 2 CuTeO 6 is an almost ideal realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV. DOI: 10.1103/PhysRevLett.117.237203 Mott insulators are a subject of intense interest due to the observation of many different quantum phenomena [1,2]. In low-dimensional systems, frustration and quantum fluctuations can destroy long-range magnetic order giving rise to quantum paramagnetic phases such as valence-bond solids with broken lattice symmetry or spin liquids, where symmetry is conserved but with possible new collective behaviors involving emergent gauge fields and fractional excitations [3][4][5]. The spin-1=2 frustrated square lattice with nearest-neighbor (NN) J 1 and nextnearest neighbor J 2 exchange interactions is one of the simplest models for valence-bond solids and spin liquids [4,6]. Yet, despite the many theoretical efforts, experimental realizations of the J 1 -J 2 model have been rather scarce. The double perovskite oxides are particularly interesting as magnetic interactions can be tuned by changing structure, stoichiometry, and cation order [7,8].In the search for a quantum magnet with weak exchange energies, Sr 2 CuTeO 6 has been proposed [9,10].The tetragonal crystal structure of the double perovskite Sr 2 CuTeO 6 [11] consists of corner sharing CuO 6 and TeO 6 octahedra that are rotated in a staggered fashion about the c axis; see Figs. 1(a) and 1(b). The CuO 6 octahedra are elongated along the c axis, effectively resulting in the ground state of a Cu 2þ (3d 9 ) ion having a hole in the inplane d x 2 −y 2 orbital, where z is along the c axis. This could eventually result in quasi-2D magnetism in Sr 2 CuTeO 6 with dominant intraplane exchange interactions. In the basal ab plane, the exchange that couples the Cu 2þ ions is the super-superexchange interaction mediated through the bridging TeO 6 octahedra as shown in Fig. 1(b), which is expected to reduce the coupling strength in Sr 2 CuTeO 6 .Magnetic susceptibility and heat capacity measurements on Sr 2 CuTeO 6 indicate a quasi-2D magnetic behavior, suggesting that it is a realization of the square-lattice J 1 -J 2 model [10]. More recently, neutron diffraction measurements on Sr 2 CuTeO 6 have shown it to order in a Néel antiferromagnetic (AFM) structure below T N ≃ 29 K with moments in the ab plane [12]; see Fig. 1(a). The ordered moment at 1.5 K was found to be reduced to 0.69ð6Þμ B , from the classical value of 1 μ B [12], indicating a renormalization by q...

Spin wave excitations in the tetragonal double perovskiteSr2CuWO6

“…Now we compare the exchange parameters estimated for Sr 2 CuWO 6 with those of Sr 2 CuTeO 6 [33,34]. Based on magnetic susceptibility measurements, it is estimated that in isostructural Sr 2 CuTeO 6 the ratio J 2 /J 1 < 0.07 [33], compared to ∼8 in Sr 2 CuWO 6 from the INS results, while recent inelastic neutron scattering measurements have reduced the Sr 2 CuTeO 6 value even further to 0.03 [34].…”

“…Based on magnetic susceptibility measurements, it is estimated that in isostructural Sr 2 CuTeO 6 the ratio J 2 /J 1 < 0.07 [33], compared to ∼8 in Sr 2 CuWO 6 from the INS results, while recent inelastic neutron scattering measurements have reduced the Sr 2 CuTeO 6 value even further to 0.03 [34]. Koga et al propose that this reversal in relative strengths of nearest and next nearest neighbor interactions is due to the outermost filled orbital in Te 6+ being d x 2 −y 2 , such that the two hole spins must be antiparallel, giving antiferromagnetic exchange J 1 and J 2 ; whereas in W 6+ the p x and p y orbitals are orthogonal so that the two hole spins are parallel, which would give a ferromagnetic exchange for J 1 and an antiferromagnetic exchange for J 2 .…”

Magnetic excitations from the two-dimensional interpenetrating Cu framework in Ba2Cu3O4Cl2

“…The magnetic zone-boundary in S = 1/2 square-lattice AFMs displays a number of intriguing quantum effects [9,13,16,17,42,[44][45][46][47]. This is reflected on the spectrum in the following ways: (i) the zone-boundary at (π, 0) is around 8% lower than at (π/2, π/2); (ii) half of the single-magnon intensity at (π/2, π/2) is missing; and (iii) a continuum of intensity is found at (π/2, π/2).…”

Assessing the performance of the random phase approximation for exchange and superexchange coupling constants in magnetic crystalline solids

“…However, in Ref. [63] it was demonstrated that agreement with experiments could be obtained with sophisticated quantum chemistry methods.…”

“…Another magnetic material that has recently been scrutinized theoretically [63] is the double perovskite Sr 2 CuTeO 6 . Here the magnetic Cu 2+ ions comprise two-dimensional (2D) square planes where the superexchange interactions are mediated by intermediate TeO 6 octahedra, which make the interactions rather weak compared to oxygen-mediated superexchange.…”