Concrete

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Dirac Hamiltonians for bosonic spectra. (arXiv:2001.02694v1 [cond-mat.mes-hall])

2020-01-10T04:30:19+00:00January 10th, 2020|Categories: Publications|Tags: , |

Dirac materials are of great interest as condensed matter realizations of the Dirac and Weyl equations. In particular, they serve as a starting point for the study of topological phases. This physics has been extensively studied in electronic systems such as graphene, Weyl- and Dirac semi-metals. In contrast, recent studies have highlighted several examples of Dirac-like cones in collective excitation spectra, viz. in phonon, magnon and triplon bands. These cannot be directly related to the Dirac or Weyl equations as they are bosonic in nature with pseudo-unitary band bases. In this article, we show that any Dirac-like equation can be smoothly deformed into a form that is applicable to bosonic bands. The resulting bosonic spectra bear a two-to-one relation to that of the parent Dirac system. Their dispersions inherit several interesting properties including conical band touching points and a gap-opening-role for `mass’ terms. The relationship also extends to the band eigenvectors with the bosonic states carrying the same Berry connections as the parent fermionic states. The bosonic bands thus inherit topological character as well. If the parent fermionic system has non-trivial topology that leads to mid-gap surface states, the bosonic analogue also hosts surface states that lie within the corresponding band gap. The proposed bosonic Dirac structure appears in several known models. In materials, it is realized in Ba$_2$CuSi$_2$O$_6$Cl$_2$ and possibly in CoTiO$_3$ as well as in paramagnetic honeycomb ruthenates. Our results allow for a rigorous understanding of Dirac phononic and magnonic systems and enable concrete predictions, e.g., of surface

Published : "arXiv Mesoscale and Nanoscale Physics".

Tunable Layer Circular Photogalvanic Effect in Twisted Bilayers. (arXiv:1911.04049v1 [cond-mat.mes-hall])

2019-11-12T08:31:23+00:00November 12th, 2019|Categories: Publications|Tags: , |

We develop a general theory of the layer circular photogalvanic effect (LCPGE) in quasi two-dimensional chiral bilayers, which refers to the appearance of a polarization-dependent, out-of-plane dipole moment induced by circularly polarized light. We elucidate the geometric origin of the LCPGE as two types of interlayer coordinate shift weighted by the quantum metric tensor and the Berry curvature, respectively. As a concrete example, we calculate the LCPGE in twisted bilayer graphene, and find that it exhibits a resonance peak whose frequency can be tuned from visible to infrared as the twisting angle varies. The LCPGE thus provides a promising route towards frequency-sensitive, circularly-polarized light detection, particularly in the infrared range.

Published : "arXiv Mesoscale and Nanoscale Physics".

Two-dimensional antiferromagnetic Dirac fermions in monolayer ${mathrm{TaCoTe}}_{2}$

2019-11-04T14:34:49+00:00November 4th, 2019|Categories: Publications|Tags: |

Author(s): Si Li, Ying Liu, Zhi-Ming Yu, Yalong Jiao, Shan Guan, Xian-Lei Sheng, Yugui Yao, and Shengyuan A. YangDirac points in two-dimensional (2D) materials have been a fascinating subject of research. Recently, it has been theoretically predicted that Dirac points may also be stabilized in 2D magnetic systems. However, it remains a challenge to identify concrete 2D materials which host such magnetic Dirac …[Phys. Rev. B 100, 205102] Published Mon Nov 04, 2019

Published in: "Physical Review B".

Topological charge pumping in twisted bilayer graphene. (arXiv:1910.09001v1 [cond-mat.mes-hall])

2019-10-22T04:30:38+00:00October 22nd, 2019|Categories: Publications|Tags: , , |

We show that a sliding motion between the two layers of a moire superlattice induces an electric current and realizes a two-dimensional version of the topological Thouless pump when the Fermi energy lies in one of the minigaps. Interestingly, a chiral charge pump, namely, a transverse current induced by the sliding motion, is possible in twisted homobilayers. This result is confirmed by a concrete calculation of the adiabatic current in twisted bilayer graphene. Our work reveals an interesting link between mechanical motion and electricity unique to moire superlattices, and may find applications in nanogenerators and nanomotors.

Published : "arXiv Mesoscale and Nanoscale Physics".

Two-dimensional antiferromagnetic Dirac fermions in monolayer TaCoTe$_2$. (arXiv:1910.07716v1 [cond-mat.mtrl-sci])

2019-10-18T02:29:37+00:00October 18th, 2019|Categories: Publications|Tags: |

Dirac point in two-dimensional (2D) materials has been a fascinating subject of research. Recently, it has been theoretically predicted that Dirac point may also be stabilized in 2D magnetic systems. However, it remains a challenge to identify concrete 2D materials which host such magnetic Dirac point. Here, based on first-principles calculations and theoretical analysis, we propose a stable 2D material, the monolayers TaCoTe$_2$, as an antiferromagnetic (AFM) 2D Dirac material. We show that it has an AFM ground state with an out-of-plane N'{e}el vector. It hosts a pair of 2D AFM Dirac points on the Fermi level in the absence of spin-orbit coupling (SOC). When the SOC is considered, a small gap is opened at the original Dirac points. Meanwhile, another pair of Dirac points appear on the Brillouin zone boundary below the Fermi level, which are robust under SOC and have a type-II dispersion. Such a type-II AFM Dirac point has not been observed before. We further show that the location of this Dirac point as well as its dispersion type can be controlled by tuning the N'{e}el vector orientation.

Published in: "arXiv Material Science".