Researchers at High Magnetic Field Laboratory, Chinese Academy of Sciences (CHMFL) discovered new topological quantum phase transition induced by high magnetic field in Dirac Semimetal ZrTe₅.

Dirac semimetals can be transformed into Weyl semimetals by breaking the time reversal symmetry under magnetic field. The obtained Weyl semimetal phase is robust since Weyl points can only disappear if pairs of opposite Weyl points meet and annihilate each other.

Generally, the annihilation of Weyl points by magnetic field is hard because most Dirac/Weyl semimetals have a small Zeeman g factor and large band width. Thus for a finite magnetic field, it is unlikely to separate and annihilate the Weyl points at, for example, Brillouin zone boundary.

The Dirac semimetal ZrTe₅ is a layered material with a large Zeeman g factor and small band width along interlayer direction, which provides an ideal platform to study the annihilation of Weyl points under high magnetic field.

The group led by TIAN Mingliang at CHMFL presented a kind of Weyl point annihilation in ZrTe₅ single crystals induced by the Zeeman splitting under high magnetic field.

In this work, they found that when a magnetic field of about 8 T is applied along the interlayer direction of ZrTe₅ bulk crystals, the Zeeman energy will exceed the band width in this direction.

Thus, two separated Weyl points will meet and annihilate each other (with a gap opened) at Brillouin zone boundary. Correspondingly, a two-dimensional (2D) massive Dirac metal is formed above 8 T.

Experimentally, a transformation of Fermi surface from three-dimension to quasi two-dimension near 8 T has been identified by the angular-dependent magnetoresistance (MR) quantum oscillations, hinting a quasi 2D motion of electrons at high field.

Furthermore, an orbital splitting can be trigged by magnetic field in such a 2D massive Dirac metal, when the Fermi level is driven into the lowest Landau level (n=1) above 8 T, leading to a shift of zeroth Landau level and a sharp increase of the carrier density. And the extra carriers induced by the orbital splitting will largely enhance the conductivity of the system, leading to a sharp drop of MR near 8 T.

This experiment reveals a probable topological quantum phase transition of field induced Weyl points annihilation in Dirac semimetal ZrTe₅ and gives an alternative explanation for the drop of MR at high field.

This work has been published on Phys. Rev. B, with the title "Field-induced topological phase transition from a three-dimensional Weyl semimetal to a two-dimensional massive Dirac metal in ZrTe5."

The sharp drop of magnetoresistivity near 8 T in ZrTe5 single crystals.

The Zeeman effect and orbit splitting under high magnetic field in ZrTe5.