## Purely rotational symmetry-protected topological crystalline insulator $$\alpha$$ -Bi _{4} Br _{4}

## Abstract

Rotational-symmetry-protected topological crystalline insulators (TCIs) are expected to host unique boundary modes, in that the surface normal to the rotational axis can feature surface states with ‘unpinned’ Dirac points, which are not constrained to lie on high symmetry points or lines, but can lie at any general k point in the Brillouin zone. Also, as a higher order bulk boundary correspondence is involved here, a three-dimensional (3D) TCI can support one-dimensional (1D) helical edge states. Using first-principles band structure calculations, we identify the van der Waals material $$\alpha$$ -Bi _{4} Br _{4} as a purely rotation symmetry protected TCI. We show that the (0 1 0) surface of Bi _{4} Br _{4} exhibits a pair of unpinned topological Dirac fermions which are related to the presence of a two-fold rotation axis. These unpinned Dirac fermions possess an exotic spin texture which will be highly favorable for spin transport, and a band structure that consists of van Hove singularities due to a Lifshitz transition. We also identify 1D topological hinge states along the edges of an $$\alpha$$ -Bi _{4} Br _{4} rod. We comment on how the predicted topological features in $$\alpha$$ -Bi _{4} Br _{4} could be accessed experimentally.

- Authors:

- National Univ. of Singapore (Singapore)
- National Cheng Kung Univ., Tainan City (Taiwan)
- Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
- Northeastern Univ., Boston, MA (United States)
- Academia Sinica, Taipei (Taiwan)
- National Cheng Kung Univ., Tainan City (Taiwan); Center for quantum frontiers of research & technology (QFort), Tainan (Taiwan)

- Publication Date:

- Research Org.:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)

- Sponsoring Org.:
- USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division; USDOE

- OSTI Identifier:
- 1515095

- Alternate Identifier(s):
- OSTI ID: 1526942

- Grant/Contract Number:
- AC02-07CH11358; AC02-05CH11231

- Resource Type:
- Published Article

- Journal Name:
- 2D Materials

- Additional Journal Information:
- Journal Volume: 6; Journal Issue: 3; Journal ID: ISSN 2053-1583

- Publisher:
- IOP Publishing

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

### Citation Formats

```
Hsu, Chuang-Han, Zhou, Xiaoting, Ma, Qiong, Gedik, Nuh, Bansil, Arun, Pereira, Vitor M., Lin, Hsin, Fu, Liang, Xu, Su -Yang, and Chang, Tay -Rong. Purely rotational symmetry-protected topological crystalline insulator $\alpha$ -Bi4 Br4. United States: N. p., 2019.
Web. doi:10.1088/2053-1583/ab1607.
```

```
Hsu, Chuang-Han, Zhou, Xiaoting, Ma, Qiong, Gedik, Nuh, Bansil, Arun, Pereira, Vitor M., Lin, Hsin, Fu, Liang, Xu, Su -Yang, & Chang, Tay -Rong. Purely rotational symmetry-protected topological crystalline insulator $\alpha$ -Bi4 Br4. United States. doi:10.1088/2053-1583/ab1607.
```

```
Hsu, Chuang-Han, Zhou, Xiaoting, Ma, Qiong, Gedik, Nuh, Bansil, Arun, Pereira, Vitor M., Lin, Hsin, Fu, Liang, Xu, Su -Yang, and Chang, Tay -Rong. Wed .
"Purely rotational symmetry-protected topological crystalline insulator $\alpha$ -Bi4 Br4". United States. doi:10.1088/2053-1583/ab1607.
```

```
@article{osti_1515095,
```

title = {Purely rotational symmetry-protected topological crystalline insulator $\alpha$ -Bi4 Br4},

author = {Hsu, Chuang-Han and Zhou, Xiaoting and Ma, Qiong and Gedik, Nuh and Bansil, Arun and Pereira, Vitor M. and Lin, Hsin and Fu, Liang and Xu, Su -Yang and Chang, Tay -Rong},

abstractNote = {Rotational-symmetry-protected topological crystalline insulators (TCIs) are expected to host unique boundary modes, in that the surface normal to the rotational axis can feature surface states with ‘unpinned’ Dirac points, which are not constrained to lie on high symmetry points or lines, but can lie at any general k point in the Brillouin zone. Also, as a higher order bulk boundary correspondence is involved here, a three-dimensional (3D) TCI can support one-dimensional (1D) helical edge states. Using first-principles band structure calculations, we identify the van der Waals material $\alpha$ -Bi4 Br4 as a purely rotation symmetry protected TCI. We show that the (0 1 0) surface of Bi4 Br4 exhibits a pair of unpinned topological Dirac fermions which are related to the presence of a two-fold rotation axis. These unpinned Dirac fermions possess an exotic spin texture which will be highly favorable for spin transport, and a band structure that consists of van Hove singularities due to a Lifshitz transition. We also identify 1D topological hinge states along the edges of an $\alpha$ -Bi4 Br4 rod. We comment on how the predicted topological features in $\alpha$ -Bi4 Br4 could be accessed experimentally.},

doi = {10.1088/2053-1583/ab1607},

journal = {2D Materials},

number = 3,

volume = 6,

place = {United States},

year = {2019},

month = {5}

}

DOI: 10.1088/2053-1583/ab1607