Spin dynamics of coupled spin ladders near quantum criticality in Ba2CuTeO6

Abstract

We report inelastic neutron scattering measurements of the magnetic excitations in Ba2CuTeO6, proposed by ab initio calculations to magnetically realize weakly coupled antiferromagnetic two-leg spin-1 2 ladders. Isolated ladders are expected to have a singlet ground state protected by a spin gap. Ba2CuTeO6 orders magnetically, but with a small Néel temperature relative to the exchange strength, suggesting that the interladder couplings are relatively small and only just able to stabilize magnetic order, placing Ba2CuTeO6 close in parameter space to the critical point separating the gapped phase and Néel order. Through comparison of the observed spin dynamics with linear spin wave theory and quantum Monte Carlo calculations, we propose values for all relevant intra-and interladder exchange parameters, which place the system on the ordered side of the phase diagram in proximity to the critical point. We also compare high field magnetization data with quantum Monte Carlo predictions for the proposed model of coupled ladders. arXiv:1806.04052v2 [cond-mat.str-el] 7 Dec 2018 FIG. 1. (Color online) Schematic phase diagram of two-leg spin-1 2 AFM ladders as a function of the in-plane interladder exchange coupling J [see Fig. 2(b)]. Below a critical coupling J c , the ground state is an overall singlet with a spin gap ∆. The J = 0 state is schematically illustrated in the inset in the strong rung limit (Jrung J leg ), showing the Cu 2+ ions (blue circles) and spin singlet bonds (black ovals). For J > J c , AFM Néel order is expected below a finite temperature TN for nonzero interplane coupling J3D. Ba2CuTeO6 has been proposed to be located close to the quantum critical point, on the ordered side of the phase diagram. 21,22

Site-Selective d10/d0 Substitution in an S = 1/2 Spin Ladder Ba2CuTe1–xWxO6 (0 ≤ x ≤ 0.3)

“…41 The transition (Ttrans), arising from further axial elongation of the Jahn-Teller active Cu-O bonds, is weak meaning the structural integrity of the spin ladder geometry is retained and there is little impact on the magnetic interactions. 38 The transition was marked by peak splitting, as can be observed by comparing the neutron diffraction patterns of Ba2CuTe0.7W0.3O6 at (a) 1.44 K and (b) 300 K in Figure 4a. As x in Ba2CuTe1-xWxO6 increased, peak splitting was suppressed to lower temperatures.…”

“…36 Ba2CuTeO6 is an excellent system to test this due to its hexagonal perovskite structure that results in a spin ladder magnetic geometry. [37][38][39][40] Within the spin ladder, Cu 2+ cations are linked via three key Cu-O-Te-O-Cu exchange interactions illustrated in Figure 1c. These are the intraladder Jleg and Jrung interactions via the corner-sharing Te(1)O6 units and the inter-ladder interaction via the face-sharing Te(2)O6 units within the Cu-Te(2)-Cu trimers.…”

“…37 However, the low dimensional magnetic behavior means the system does not present a classical AFM ordering cusp. Instead, magnetic order has been detected using muon and neutron techniques placing the magnetic transition at TN = 14 K. 38,40 The W 6+ doped samples are similar in that they all display the same broad Tmax feature, but the position of Tmax shifts to lower temperatures as x increases as shown in Table 2, suggesting weakening of the short-range ladder interactions. More dramatic differences are observed at low temperatures, where the upturn in the susceptibility data gradually becomes stronger with x and this 'Curie-tail' like feature is most pronounced in the x = 0.3 sample.…”

“…Ba2CuTeO6 is known to lie on the ordered side of the S = 1/2 spin ladder phase diagram, but is very close to the quantum critical point between the Néel ordered and spin-singlet states. 38,40,[62][63][64][65] The introduction of W 6+ d 0 to the B''(c) site could modify the ratio between the Jleg and…”

“…42 The intraladder interactions are antiferromagnetic and equally strong with J rung / J leg ∼ 1, while the interladder interaction is weaker. 38 , 39 In principle, W 6+ could be doped onto either of the Te 6+ B″ sites. This offers the possibility of tuning the J leg and J rung interactions independently of the J inter , forming a more complex phase space than cubic perovskites.…”