Topological phase transition in a magnetic Weyl semimetal

Abstract

Topological Weyl semimetals (TWSs) are exotic crystals possessing emergent relativistic Weyl fermions connected by unique surface Fermi-arcs (SFAs) in their electronic structures. To realize the TWS state, certain symmetry (such as the inversion or time reversal symmetry) must be broken, leading to a topological phase transition (TPT). Despite the great importance in understanding the formation of TWSs and their unusual properties, direct observation of such a TPT has been challenging. Here, using a recently discovered magnetic TWS Co3Sn2S2, we were able to systematically study its TPT with detailed temperature dependence of the electronic structures by angle-resolved photoemission spectroscopy. The TPT with drastic band structures evolution was clearly observed across the Curie temperature (TC = 177 K), including the disappearance of the characteristic SFAs and the recombination of the spin-split bands that leads to the annihilation of Weyl points with opposite chirality. These results not only reveal important insights on the interplay between the magnetism and band topology in TWSs, but also provide a new method to control their exotic physical properties.

Correlation driven near-flat band Stoner excitations in a Kagome magnet

“…2 a, b we present the spin-resolved d -orbital electronic bands in the FM state calculated with U = 5.0 eV and Hund’s exchange J H = 0.9 eV, showing the presence of dispersing as well as flat bands in Co 3 Sn 2 S 2 . The overall low-energy band dispersions and energies are in good agreement with reported ARPES data on Co 3 Sn 2 S 2 (see “Methods”) 22 , 32 . Along with the spin-waves, electron–hole pair excitations with electrons and holes in the bands of opposite spins are also elementary to itinerant ferromagnets, and are called the Stoner excitations.…”

“…4 a, is expected because the bands become partially non-spin-polarised and shift in energy above T C (see Supplementary information Fig. S8) 32 . This is also reflected in the concomitant shift towards higher energy in the calculated S ( q , ω ) in the PM state (Fig.…”

Transport, refraction and interface arcs in junctions of Weyl semimetals

“…They host surface states in the form of Fermi arcs, which are connected to nontrivial Hall response, magnetoconductance and thermal transport phenomena [12][13][14]. Compounds in the pyrochlore iridates family and the ferromagnet HgCr 2 Se 4 were the first candidate Weyl semimetals [15,16], followed by various layered materials [17] , such as Co 3 Sn 2 S 2 [18][19][20] and the antiferromagnets Mn 3 X (X=Sn, Ge) [21], in which the Weyl nodes were identified. Magnetic Heusler alloys also provide excellent candidates for time-reversal-broken Weyl semimetals [9,17,22], with various experimental confirmations, including Ti 2 MnAl [23][24][25], Co 2 TiX (X=Si, Ge, Sn), GdSI [26] and Co 2 MnGa [27].…”

Light-field-driven non-Ohmic current and Keldysh crossover in a Weyl semimetal

“…However, when we use Weyl semimetals [7][8][9][10][11] , the electron scattering becomes relatively long (several hundred femtosecondsseveral picoseconds), owing to the suppression of the back-scattering process 12 . This study presents the light-eld-driven dynamics by the THz pulse (~1 ps) to Weyl semimetal Co 3 Sn 2 S 2 [13][14][15][16][17][18][19][20] , where the intense THz pulse of a monocycle electric eld nonlinearly generates direct current (DC) via coherent acceleration without scattering 5,6 and non-adiabatic excitation 1,5,21,. In other words, the non-Ohmic current appears in the Weyl semimetal with a combination of the long relaxation time and an intense THz pulse.…”

“…This study shows the light-eld-driven electrodynamics of the Weyl semimetal Co 3 Sn 2 S 2 (CSS) [13][14][15][16][17][18][19][20] in the THz frequency (~ 1ps). CSS has a relatively long electron scattering time of 0.1-10 ps, a considerably slower timescale than the previously reported graphene 5 and organic material 6 .…”