In a recent study, researchers at High Magnetic Field Laboratory, Chinese Academy of Sciences (CHMFL) discovered field-driven quantum criticality in the spinel magnet ZnCr₂Se₄ in ultra-low temperature and high magnetic field.

The chalcogenide spinels ACr₂X₄ (A = Zn, Cd, Hg; X = O, S, Se), as a model frustrated magnetic material, exhibits some novel magnetic ground states and many fascinating physical effects, such as field-induced fractional magnetization plateaus, zero energy mode and so on.

With increasing field, the ACr₂X₄ system goes from a helix spin state to a spiral spin state to an unidentified regime and then a fully polarized state at the measured temperatures.

Two possibilities have been proposed for this unidentified regime: an umbrella state or a spin nematic state, which both break the spin rotational symmetry.

To address this unidentified regime, researchers completed the magnetic phase diagram of ZnCr₂Se₄, through a joint study of dc/ac susceptibility, heat capacity, and thermal conductivity measurements.

They found that the new unidentified phase locates between the critical fields H_C2 and H_C3. Upon cooling, H_C2 shifts to higher fields, while H_C3 moves to lower fields. And these two critical fields merge at a quantum critical point, indicating a quantum phase transition between spiral spin state and fully polarized state at zero temperature.

As the system approaches the quantum criticality, strong quantum fluctuations of the spins could be observed with behaviors such as an unconventional T²-dependent specific heat and temperature-independent mean free path for the thermal transport.

This quantum critical regime has an unconventional critical mode, which cannot be simply described by Ising or Gaussian criticality.

This finding of the unconventional quantum criticality calls for future works and is likely to provide a unique example of frustrated quantum criticality for further studies.

This study was jointly conducted by researchers in High Magnetic Field Laboratory, University of Science and Technology of China, Fudan University, University of Tennessee & National High Magnetic Field Laboratory (NHMFL) in Tallahassee.

H-T phase diagram of ZnCr₂Se₄. (Image by GU Chuanchuan)