2017 Volume 27 Issue 4 Pages 140-143
A hidden order that emerges in the frustrated pyrochlore Tb2+xTi2-xO7+y with Tc = 0.53 K is studied using neutron scattering, specific heat, magnetization measurements on a high-quality single crystal. Semiquantitative analyses based on a pseudospin-1/2 Hamiltonian for ionic non-Kramers magnetic doublets demonstrate that it is a long-range order of electric quadrupole moments intrinsic to the Tb3+ ions. This is the first example of electric quadrupole order in frustrated magnetic systems, and opens new research directions for "frustrated quadrupole systems". It is also found that the elusive spin liquid state of the nominal Tb2Ti2O7, studied for more than a decade, is most likely a U(1) quantum spin-liquid state.