Quantum Criticality of the Ising-like Screw Chain Antiferromagnet SrCo2V2O8 in a Transverse Magnetic Field

The quantum criticality of an Ising-like screw chain antiferromagnet SrCo2V2O8, 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 HC1≈7.03 T. Surprisingly, a second QCP at HC2≈7.7 T featured with gapless excitations is resolved from both the double-peak structure of the field-dependent spin-lattice relaxation rate 1/T511 at low temperatures and the weakly temperature-dependent 1/T511 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 HC2, which belongs to the one-dimensional transverse-field Ising universality.