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.