Field-induced effects in the spin liquid candidate PbCuTe2_{2}O6_{6}

PbCuTe$_2$O$_6$ is considered as one of the rare candidate materials for a three-dimensional quantum spin liquid (QSL). This assessment was based on the results of various magnetic experiments, performed mainly on polycrystalline material. More recent measurements on single crystals revealed an even more exotic behavior, yielding ferroelectric order below $T_{\text{FE}}\approx 1\,\text{K}$, accompanied by distinct lattice distortions, and a somewhat modified magnetic response which is still consistent with a QSL. Here we report on low-temperature measurements of various thermodynamic, magnetic and dielectric properties of single crystalline PbCuTe$_2$O$_6$ in magnetic fields $B\leq 14.5\,\text{T}$. The combination of these various probes allows us to construct a detailed $B$-$T$ phase diagram including a ferroelectric phase for $B \leq$ $8\,\text{T}$ and a $B$-induced magnetic phase at $B \geq$ $11\,\text{T}$. These phases are preceded by or coincide with a structural transition from a cubic high-temperature phase into a distorted non-cubic low-temperature state. The phase diagram discloses two quantum critical points (QCPs) in the accessible field range, a ferroelectric QCP at $B_{c1}$ = $7.9\,\text{T}$ and a magnetic QCP at $B_{c2}$ = $11\,\text{T}$. Field-induced lattice distortions, observed in the state at $T>$ $1\,\text{K}$ and which are assigned to the effect of spin-orbit interaction of the Cu$^{2+}$-ions, are considered as the key mechanism by which the magnetic field couples to the dielectric degrees of freedom in this material

Spin liquid and ferroelectricity close to a quantum critical point in PbCuTe2O6

Geometrical frustration among interacting spins combined with strong quantum fluctuations destabilize long-range magnetic order in favour of more exotic states such as spin liquids. By following this guiding principle, a number of spin liquid candidate systems were identified in quasi-two-dimensional (quasi-2D) systems. For 3D, however, the situation is less favourable as quantum fluctuations are reduced and competing states become more relevant. Here we report a comprehensive study of thermodynamic, magnetic and dielectric properties on single crystalline and pressed-powder samples of PbCuTe$_2$O$_6$, a candidate material for a 3D frustrated quantum spin liquid featuring a hyperkagome lattice. Whereas the low-temperature properties of the powder samples are consistent with the recently proposed quantum spin liquid state, an even more exotic behaviour is revealed for the single crystals. These crystals show ferroelectric order at $T_{\text{FE}} \approx 1\,\text{K}$, accompanied by strong lattice distortions, and a modified magnetic response -- still consistent with a quantum spin liquid -- but with clear indications for quantum critical behaviour.Comment: 59 pages, 15 figures, This version of the article has been accepted for publication, after peer review but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available onlin