Weyl semimetal made ideal with a crystal of Raman light and atoms

• 作者: Xiaopeng Li W.Vincent Liu
• 来源期刊: 科学通报（英文版）
• 年，卷（期）: 2021,（13）
• 所属期刊栏目: NEWS & VIEWS
• 页码范围: 1253-1255
• 页数: 3页
• 语种: 英文

摘要

Chiral symmetry played a spectacular role in the qualitative understanding of the low energy effective theory of quantum chro-modynamics(QCD)and in the formulation of the standard model of electroweak and strong interactions in high energy physics.For the former,it is through the chiral symmetry breaking phe-nomena such as the emergence of pions.For the latter,it provides constraints by requiring all axial-vector currents-generated by chiral symmetry transformation-be anomaly free and conserved when coupled to any gauge field(see,for instance,Ref.[1]).This is the symmetry that left-and right-handed particles enjoy two independent sets of transformation while leaving all dynamics and physical observables invariant.Such particles were first intro-duced by Hermann Weyl in 1929 as a chirality-definite solution to Dirac equation at the massless limit.In the family of elementary particles,neutrinos which seemed to have vanishing mass in the beta decay experiment were initially expected to be Weyl fer-mions.This was later invalidated with mounting experimental evi-dence of non-zero neutrino masses.In condensed matter physics,Weyl fermions have been found to emerge as quasi-particle excita-tions of an exotic quantum many-body state of Weyl semimetals[2],with the vanishing effective mass protected by topological Berry phase effects in momentum space(Fig.1),which has attracted tremendous research efforts in the last decade.Various exotic prop-erties as mediated by Weyl fermions have been predicted,including fascinating Fermi-arc surface states[2],chiral anomalous quantum transport[3],and topological phase transitions[4].In the first pro-posal of material realization with strongly correlated iridium pyr-ochlores R2Ir2O7,twenty-four Weyl nodes were predicted with first principle calculations[2].A conceptually much simpler and cleaner proposal was provided based on stacking topological insu-lator thin films separated by ordinary-insulator spacer layers,lead-ing to a most fundamental Weyl semimetal,which is an ideal Weyl semimetal having only two Weyl nodes,the smallest possible number allowed in lattice models as proved by topological homo-topy theory[5].