  Quantum phase transitions occur at zero temperature upon variation of some
nonthermal control parameters. The Ising chain in a transverse field is
probably the most-studied model undergoing such a transition, from
ferromagnetic to paramagnetic state1,2. This model can be exactly solved by
using a Jordan-Wigner transformation, which transforms the spins into
noninteracting spinless fermions1. At the quantum critical point, the magnetic
excitations can carry arbitrarily low energy and dominate the low temperature
properties. Here we report the unveiling of such quantum critical point in
quasi-one-dimensional Ising ferromagnet CoNb2O6 by ultra-low-temperature
thermal conductivity measurements. We find that in the paramagnetic state,
phonons are scattered by the magnetic excitations above certain temperature Ts,
which corresponds to a gap. As predicted by the theoretical model1, this gap
linearly goes to zero with decreasing the transverse field, thus determining
the quantum critical point of the Ising chain.
