PSILOGO

Laboratory for Particle Physics (LTP)


LTP Colloquium

Weyl Fermions fluctuating from Dirac Sea - a condensed Matter Version

Thursday, November 21, 2019, 16:00
OFLG/402

Junzhang Ma, PSI

Abstract:
Weyl semimetals (WSMs) host low-energy quasiparticles governed by the Weyl equation. These are massless fermionic excitations near a crossing point of two bands, which, in contrast to Dirac points, are spin non-degenerate. The spin splitting results from the breaking of either time-reversal (T) or inversion (P) symmetry. Long-range magnetic order can be an effective way to break the T symmetry. Nevertheless, despite considerable efforts, experimental evidence for magnetically induced WSMs is scarce. Here, using angle-resolved photoemission spectroscopy (ARPES), we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd2As2 at temperatures up to ~ 100 K, which is ten times larger than the antiferromagnetic ordering temperature of EuCd2As2. We attribute this effect to the itinerant electrons experiencing quasi-static and quasi-long-range ferromagnetic fluctuations below ~ 100 K. Combining ARPES results with first-principles calculations, we reveal that the spin-nondegenerate band structure harbors a pair of Weyl nodes close to the Fermi level. Hence, we have shown that the spontaneous breaking of T symmetry is not necessary for the existence of WSM states because this electronic phase can also be driven by magnetic fluctuations and that fluctuated WSM states can emerge in a wider range of condensed-matter systems than previously thought.