arXiv subjects

Yige Chen

Publications and source records attributed to Yige Chen.

26 records · Page 2Linked to original sources

arXiv2017

Helical Majorana fermions at the interface of Weyl semimetal and d-wave superconductor: Application to Iridates and high-Tc Cuprates

Majorana bound states exist inside a core of half-quantum vortex in spin-triplet superconductors. Despite intense efforts, they have not been discovered in any spin-triplet superconductor candidate materials. After the success of topological insulator, another route to achieve Majorana fermions has been suggested: heterostructures of s-wave superconductors and topological insulator or semimetal with strong spin-orbit coupling provide an effective spinless $p+ip$ pairing which supports Majorana bound states in a single vortex. This theoretical observation has stimulated both experimental and theoretical communities to search for Majorana fermions, and recently a localized Majorana state at the end of one-dimensional wire has been reported. Here we study the two-dimensional interface of Weyl semimetal and d-wave superconductor which promotes a pair of helical Majorana fermions propagating along the edge of the interface. We suggest that Iridium oxide layer IrO$_2$ classified as two-dimensional Weyl semimetal in close proximity to high temperature Cuprates would be a best example to explore these helical Majorana modes.

cond-mat.mes-hall↗

arXiv2017

Elemental topological Dirac semimetal: α-Sn on InSb(111)

Three-dimensional (3D) topological Dirac semimetals (TDSs) are rare but important as a versatile platform for exploring exotic electronic properties and topological phase transitions. A quintessential feature of TDSs is 3D Dirac fermions associated with bulk electronic states near the Fermi level. Using angle-resolved photoemission spectroscopy (ARPES), we have observed such bulk Dirac cones in epitaxially-grown α-Sn films on InSb(111), the first such TDS system realized in an elemental form. First-principles calculations confirm that epitaxial strain is key to the formation of the TDS phase. A phase diagram is established that connects the 3D TDS phase through a singular point of a zero-gap semimetal phase to a topological insulator (TI) phase. The nature of the Dirac cone crosses over from 3D to 2D as the film thickness is reduced.

cond-mat.mtrl-sci↗

arXiv2016

3D $Z_2$ Topological Nodes in Nonsymmorphic Photonic Crystals: Ultrastrong Coupling and Anomalous Refraction

We propose to simulate 3D Dirac points and line-nodes with nontrivial $Z_2$ topology in nonsymmorphic all-dielectric photonic-crystals with space-time reversal symmetry, which can be realized at infrared and microwave frequencies. Double degeneracy of all Bloch states in high symmetry planes is achieved via nonsymmorphic screw symmetries despite the fundamental obstacle of no Kramers degeneracy in photonic crystals. Two orthogonal screw axes lead to 3D $Z_2$ Dirac points on high symmetry Brillouin zone boundary lines. On the other hand, $Z_2$ line-nodes emerge as protected twofold degeneracy of Bloch bands with opposite mirror parities on the $k_z=0$ plane. The lowest frequency line-node is deterministic because of a degenerate-partner switching mechanism guaranteed by the fundamental properties of Maxwell equations and the nonsymmorphic screw symmetry. A pair of Fermi arcs with opposite chirality due to $Z_2$ topological Dirac points emerge below the light-line on (100) and (010) photonic-crystal-air interfaces. These robust surface states offer an unique opportunity to realize an "open cavity" with strong interaction between quantum emitters and engineered vacuum with nontrivial Berry phases --- an important step toward topological states of strongly interacting bosons. Realistic calculation for resonant coupling between cavity-photons and phonons in boron nitride thin film yields ultrastrong coupling with vacuum Rabi splitting reaching to $23\%$ of photon frequency. We also show that type-II Dirac cones have anomalous valley selective refraction: birefringence with both positive and negative refractions for one valley, while no refraction for the other.

physics.optics↗

arXiv2016

Topological crystalline semimetals in non-symmorphic lattices

Numerous efforts have been devoted to reveal exotic semimetallic phases with topologically non-trivial bulk and/or surface states in materials with strong spin-orbit coupling. In particular, semimetals with nodal line Fermi surface (FS) exhibit novel properties, and searching for candidate materials becomes an interesting research direction. Here we provide a generic condition for a four-fold degenerate nodal line FS in non-symmorphic crystals with inversion and time-reversal symmetry (TRS). When there are two glide planes or screw axes perpendicular to each other, a pair of Bloch bands related by non-symmorphic symmetry become degenerate on a Brillouin Zone (BZ) boundary. There are two pairs of such bands, and they disperse in a way that the partners of two pairs are exchanged on other BZ boundaries. This enforces a nodal line FS on a BZ boundary plane protected by non-symmorphic symmetries. When TRS is broken, four-fold degenerate Dirac points or Weyl ring FS could occur depending on a direction of the magnetic field. On a certain surface double helical surface states exist, which become double Ferm arcs as TRS is broken.

cond-mat.str-el↗

arXiv2015

Topological nodal line semimetals with and without spin-orbital coupling

We theoretically study three-dimensional topological semimetals (TSMs) with nodal lines protected by crystalline symmetries. Compared with TSMs with point nodes, e.g., Weyl semimetals and Dirac semimetals, where the conduction and the valence bands touch at discrete points, in these new TSMs the two bands cross at closed lines in the Brillouin zone. We propose two new classes of symmetry protected nodal lines in the absence and in the presence of spin-orbital coupling (SOC), respectively. In the former, we discuss nodal lines that are protected by the combination of inversion symmetry and time-reversal symmetry; yet unlike any previously studied nodal lines in the same symmetry class, each nodal line has a $Z_2$ monopole charge and can only be created (annihilated) in pairs. In the second class, with SOC, we show that a nonsymmorphic symmetry (screw axis) protects a four-band crossing nodal line in systems having both inversion and time-reversal symmetries.

cond-mat.mes-hall↗

arXiv2015

Surface States of Perovskite Iridates AIrO$_3$; Signatures of Topological Crystalline Metal with Nontrivial $\mathbb{Z}_2$ Index

There have been increasing efforts in realizing topological metallic phases with nontrivial surface states. It was suggested that orthorhombic perovskite iridates are classified as a topological crystalline metal (TCM) with flat surface states protected by lattice symmetries. Here we perform first-principles electronic structure calculations for epitaxially stabilized orthorhombic perovskite iridates. Remarkably, two different types of topological surface states are found depending on surface directions. On side surfaces, flat surface states protected by lattice symmetries emerge, manifesting the topological crystalline character. On the top surface, on the other hand, an unexpected Dirac cone appears, indicating surface states protected by a time-reversal symmetry, which is confirmed by the presence of a nontrivial topological $\mathbb{Z}_2$ index. These results suggest that the orthorhombic iridates are unique systems exhibiting both lattice- and global-symmetry-protected topological phases and surface states. Transitions to weak and strong topological insulators and implications of surface states in light of angle resolved photoemission spectroscopy are also discussed.

cond-mat.mes-hall↗

arXiv2015

Topological Crystalline Metal in Orthorhombic Perovskite Iridates

Since topological insulators were theoretically predicted and experimentally observed in semiconductors with strong spin-orbit coupling, more and more attention has been drawn to topological materials which host exotic surface states. These surface excitations are stable against perturbations since they are protected by global or spatial/lattice symmetries. Succeeded in achieving various topological insulators, a tempting challenge now is to search for metallic materials with novel topological properties. Here we predict that orthorhombic perovskite iridates realize a new class of metals dubbed topological crystalline metals, which support zero-energy surface states protected by certain lattice symmetry. These surface states can be probed by photoemission and tunnelling experiments. Furthermore, we show that by applying magnetic fields, the topological crystalline metal can be driven into other topological metallic phases, with different topological properties and surface states.

cond-mat.mes-hall↗

arXiv2014

Topological phases in Iridium oxide superlattices: quantized anomalous charge or valley Hall insulators

We study topological phases in Iridium (Ir) oxide superlattices of orthorhombic perovskite-type grown along the [001] crystallographic axis. Due to strong spin-orbit coupling of Ir 5d-orbitals and electronic correlation effects, Ir oxide bilayer superlattices display topological magnetic insulators exhibiting quantized anomalous Hall effects. Depending on stacking of two layers, we also found a valley Hall insulator with counter-propagating edge currents from two different valleys and a topological crystalline insulator with edge states protected by the crystal lattice symmetry. In a single layer Ir oxide superlattice, a topological insulator can be achieved, when a strain field is applied to break the symmetry of a glide plane protecting the Dirac points. In the presence of a magnetic ordering or in-plane magnetic field, it turns into a topological magnetic insulator. We discuss essential ingredients for these topological phases and experimental signatures to test our theoretical proposals.

cond-mat.mes-hall↗