Topological phases in pyrochlore thallium niobate Tl2Nb2O6+x

Abstract

The discovery of new topological electronic materials brings a chance to uncover novel physics and plays a key role in observing and controlling various intriguing phenomena. Up to now, many materials have been theoretically proposed and experimentally proved to host different kinds of topological states. Unfortunately, there is little convincing experimental evidence for the existence of topological oxides. The reason is that oxidation of oxygen leads to ionic crystal in general and makes band inversion unlikely. In addition, the realization of different topological states in a single material is quite difficult, but strongly needed for exploring topological phase transitions. In this work, using first-principles calculations and symmetry analysis, we propose that the experimentally tunable continuous solid solution of oxygen in pyrochlore Tl2Nb2O6+x (0 ≤ x ≤ 1.0) leads to various topological states. Topological insulator, Dirac semimetal and triply degenerate nodal point semimetal can be realized in it via changing the oxygen content and/or tuning the crystalline symmetries. When x = 1, it is a semimetal with quadratic band touching point at Fermi level. It transits into a Dirac semimetal or a topological insulator depending on the in-plane strain. When x = 0.5, the inversion symmetry is spontaneously broken in Tl2Nb2O6.5, leading to triply degenerate nodal points. When x = 0, Tl2Nb2O6 becomes a trivial insulator with a narrow band gap. These topological phase transitions driven by solid solution of oxygen are unique and physically plausible due to the variation of valence state of Tl 1+ and Tl 3+ . This topological oxide will be promising for studying correlation induced topological states and potential applications.

Possible Three-Dimensional Topological Insulator in Pyrochlore Oxides

“…First, we show the band structure of Pb2Ta2O7 in Figure 2a. Similar to other quasi-FB oxides [14][15][16][17][18], this compound also shows a characteristic quasi-FB just below EF. The origin of this quasi-FB is explained as follows.…”

“…In those papers, we mainly focused on the possible ferromagnetism induced by the quasi-FB. We have already shown [14] (0 ≤ x ≤ 1) leads to various topological phases accompanied with lattice distortion [18].…”

“…Lattice distortion can greatly affect the topological properties. Zhang et al have shown that Tl 2 Nb 2 O 7 can transform from a semi-metal to a TI by in-plane strain [18]. Since the topological properties are not dependent on the detail of the band structure, it is highly possible that the TI is also induced by in-plane strain in Tl 2 Ta 2 O 7 , In 2 Nb 2 O 7 and In 2 Ta 2 O 7 , which have similar band structures.…”

A Review on Piezoelectric, Magnetostrictive, and Magnetoelectric Materials and Device Technologies for Energy Harvesting Applications

“…The tetragonal phase, as an intermediate, provided a low-energy pathway for the rotation of the rhombohedral phase domains from other directions to the direction of the external field, which explained the enhanced magnetostrictive response around the MPB. Zhang et al [154] proposed a real-space phase field model solved by the finite element method for Fe-Ga alloys. The domain evolution of nanorings with different geometries was studied to investigate the control of vortex chirality and the magnetic switching mechanism.…”

Twist-induced magnon Landau levels in honeycomb magnets

“…In particular, twisting one layer of bilayer graphene with respect to the other by certain "magic" angles [6] results in spatial modulation of electron tunneling between the layers and produces flat "Moiré bands" responsible for the correlated insulating phase [7] and the unconventional superconductivity [8]. Properly tuned strain can close or open band gaps in topological quantum matter and induce phase transitions between distinct topological phases [9][10][11][12][13][14].…”