Quantum Criticality of the Ising-like Screw Chain Antiferromagnet SrCo 2 V 2 O 8 in a Transverse Magnetic Field

The quantum criticality of an Ising-like screw chain antiferromagnet SrCo_{2}V_{2}O_{8}, with a transverse magnetic field applied along the crystalline a axis, is investigated by ultralow temperature NMR measurements. The Néel temperature is rapidly and continuously suppressed by the field, giving rise to a quantum critical point (QCP) at H_{C_{1}}≈7.03 T. Surprisingly, a second QCP at H_{C_{2}}≈7.7 T featured with gapless excitations is resolved from both the double-peak structure of the field-dependent spin-lattice relaxation rate 1/^{51}T_{1} at low temperatures and the weakly temperature-dependent 1/^{51}T_{1} at this field. Our data, combined with numerical calculations, suggest that the induced effective staggered transverse field significantly lowers the critical fields, and leads to an exposed QCP at H_{C_{2}}, which belongs to the one-dimensional transverse-field Ising universality.