Abstract
Discovering new topological phases of matter is a major theme in fundamental physics and materials science. Dirac semimetal provides an exceptional platform for exploring topological phase transitions under symmetry breaking. Recent theoretical studies have revealed that a three-dimensional Dirac semimetal can harbor fascinating hinge states, a higher-order topological manifestation not known before. However, its realization in experiment is yet to be achieved. In this Letter, we propose a minimum model to construct a spinless higher-order Dirac semimetal protected by C_{6v} symmetry. By breaking different symmetries, this parent phase transitions into a variety of novel topological phases including higher-order topological insulator, higher-order Weyl semimetal, and higher-order nodal-ring semimetal. Furthermore, for the first time, we experimentally realize this unprecedented higher-order topological phase in a sonic crystal and present an unambiguous observation of the desired hinge states via momentum-space spectroscopy and real-space visualization. Our findings may offer new opportunities to manipulate classical waves such as sound and light.
Highlights
1, the hallmark of HO topological phases is the existence of gapless modes at boundaries of co-dimension
For a HO Dirac semimetal, its gapped 2D slices may be classified into two HO topologically distinct insulators, for which the transition occurs exactly at the Dirac points[4,5]
Similar topological transitions emerge in HO Weyl semimetals[12,13,14,15]
Summary
1, the hallmark of HO topological phases is the existence of gapless modes at boundaries of co-dimension. It features zero-energy hinge modes (red lines) connecting the projected Dirac points.
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