Changjiang Yi

Experimental evidence of hourglass fermion in the candidate nonsymmorphic topological insulator KHgSb

Topological insulators (TIs) host novel states of quantum matter, distinguished from trivial insulators by the presence of nontrivial conducting boundary states connecting the valence and conduction bulk bands. Up to date, all the TIs discovered experimentally rely on the presence of either time reversal or symmorphic mirror symmetry to protect massless Dirac-like boundary states. Very recently, it has been theoretically proposed that several materials are a new type of TIs protected by nonsymmorphic symmetry, where glide-mirror can protect novel exotic surface fermions with hourglass-shaped dispersion. However, an experimental confirmation of such new nonsymmorphic TI (NSTI) is still missing. Using angle-resolved photoemission spectroscopy, we reveal that such hourglass topology exists on the (010) surface of crystalline KHgSb while the (001) surface has no boundary state, which is fully consistent with first-principles calculations. We thus experimentally demonstrate that KHgSb is a NSTI hosting hourglass fermions. By expanding the classification of topological insulators, this discovery opens a new direction in the research of nonsymmorphic topological properties of materials.Photoemission established KHgSb as a nonsymmorphic topological insulator, which hosts hourglass-shaped surface states. Topological insulators (TIs) host novel states of quantum matter characterized by nontrivial conducting boundary states connecting valence and conduction bulk bands. All TIs discovered experimentally so far rely on either time-reversal or mirror crystal symmorphic symmetry to protect massless Dirac-like boundary states. Several materials were recently proposed to be TIs with nonsymmorphic symmetry, where a glide mirror protects exotic surface fermions with hourglass-shaped dispersion. However, an experimental confirmation of this new fermion is missing. Using angle-resolved photoemission spectroscopy, we provide experimental evidence of hourglass fermions on the (010) surface of crystalline KHgSb, whereas the (001) surface has no boundary state, in agreement with first-principles calculations. Our study will stimulate further research activities of topological properties of nonsymmorphic materials.

Interplay of Dirac electrons and magnetism in CaMnBi2 and SrMnBi2

Dirac materials exhibit intriguing low-energy carrier dynamics that offer a fertile ground for novel physics discovery. Of particular interest is the interplay of Dirac carriers with other quantum phenomena such as magnetism. Here we report on a two-magnon Raman scattering study of AMnBi2 (A=Ca, Sr), a prototypical magnetic Dirac system comprising alternating Dirac carrier and magnetic layers. We present the first accurate determination of the exchange energies in these compounds and, by comparison with the reference compound BaMn2Bi2, we show that the Dirac carrier layers in AMnBi2 significantly enhance the exchange coupling between the magnetic layers, which in turn drives a charge-gap opening along the Dirac locus. Our findings break new grounds in unveiling the fundamental physics of magnetic Dirac materials, which offer a novel platform for probing a distinct type of spin–Fermion interaction. The results also hold great promise for applications in magnetic Dirac devices.

Heavy fermion behavior in the quasi-one-dimensional Kondo lattice CeCo2Ga8

Dimensionality plays an essential role in determining the anomalous non-Fermi liquid properties in heavy fermion systems. So far most heavy fermion compounds are quasi-two-dimensional or three-dimensional. Here we report the synthesis and systematic investigations of the single crystals of the quasi-one-dimensional Kondo lattice CeCo2Ga8. Resistivity measurements at ambient pressure reveal the onset of coherence at T * ≈ 20 K and non-Fermi liquid behavior with linear temperature dependence over a decade in temperature from 2 to 0.1 K. The specific heat increases logarithmically with lowering temperature between 10 and 2 K and reaches 800 mJ/mol K2 at 1 K, suggesting that CeCo2Ga8 is a heavy fermion compound in the close vicinity of a quantum critical point. Resistivity measurements under pressure further confirm the non-Fermi liquid behavior in a large temperature–pressure range. The magnetic susceptibility is found to follow the typical behavior for a one-dimensional spin chain from 300 K down to T *, and first-principles calculations predict flat Fermi surfaces for the itinerant f-electron bands. These suggest that CeCo2Ga8 is a rare example of the quasi-one-dimensional Kondo lattice, but its non-Fermi liquid behaviors resemble those of the quasi-two-dimensional YbRh2Si2 family. The study of the quasi-one-dimensional CeCo2Ga8 family may therefore help us to understand the role of dimensionality on heavy fermion physics and quantum criticality.Heavy fermion materials: evidence for a quasi-one-dimensional Kondo latticeEvidence is provided for a heavy fermion material that is an unusual example of a quasi-one-dimensional Kondo lattice system. Heavy fermion materials are compounds with localised magnetic moments that interact strongly with the surrounding conduction electrons, causing a rich variety of exotic behaviour to arise. And when these moments form a periodic array, a Kondo lattice can be formed —where scattering off the moments becomes coherent. An international team of researchers led by Yi-feng Yang and Youguo Shi from the Institute of Physics, Chinese Academy of Sciences now present an unusual example of a Kondo lattice system that is quasi-one-dimensional in nature. Using a combination of experiments and first-principles calculations they show that the cerium atoms in single crystals of CeCo2Ga8 form chains, leading to quasi-one-dimensional Kondo lattice behaviour and unusual quantum critical scalings that are not expected in the conventional quantum critical theory.

Raman scattering study of large magnetoresistance semimetals TaAs 2 and NbAs 2

We have performed polarized and temperature-dependent Raman scattering measurements on extremely large magnetoresistance compounds

Robustness of topological states with respect to lattice instability in the nonsymmorphic topological insulator KHgSb

{\mathrm{TaAs}}_{2}

Coherent helix vacancy phonon and its ultrafast dynamics waning in topological Dirac semimetal Cd3As2

and

Large negative magnetoresistance of a nearly Dirac material: Layered antimonide EuMnSb2

{\mathrm{NbAs}}_{2}

Effect of hydrostatic pressure on the superconducting properties of quasi-1D superconductor K2Cr3As3

. In both crystals, all the Raman active modes, including six

Quasiparticle interference of Fermi arc states in the type-II Weyl semimetal candidate WTe2

{A}_{g}

Observation of bulk nodal lines in topological semimetal ZrSiS

modes and three