Type-II Dirac semimetals are quantum materials with unique energy level structures, such as a bulk Dirac point (BDP). However, these semimetals are unsuitable for real-life applications because their BDP is far off the “Fermi energy level.” Nickel ditelluride (NiTe₂), a newly reported type-II Dirac semimetal has a BDP slightly above the Fermi level. Now, researchers have substituted tellurium with selenium in NiTe₂ to alter the strength of its spin-orbit coupling and further tune its BDP.   

Quantum materials, such as topological semimetals, are materials whose electronic properties are linked to exotic quantum effects. While their interior is a semimetal (a metal with properties between a conductor and semiconductor), the surface behaves like a conductor. This unique electronic behavior arises due to the topology (special geometric properties) of the energy levels occupied by the electrons on the surface of these materials. Specifically, the energy levels closest to the Fermi level (E_F)—the highest energy level that an electron can occupy at 0K—form up and down Dirac cones whose tips touch at Dirac points.

Materials with tilted Dirac cones are known as type-II Dirac topological semimetals and have potential applications in topological quantum computing. However, there is a bottleneck. Quantum computers need type-II Dirac semimetals whose bulk Dirac point (BDP) is close to the E_F—a rarity. Scientists recently reported that nickel ditelluride (NiTe₂) has a BDP slightly above the E_F, making it an ideal candidate for quantum computing.

Recently, a research team, led by Associate Professor Jaekwang Lee of Pusan National University, developed a novel technique for further tuning the BDP around the E_F in NiTe₂. Their work was made available online on 15 July 2022 and published in Volume 16, Issue 7 of the ACS Nano journal on 26 July 2022.

Using density functional theory calculations, the researchers show that substituting tellurium (Te) with selenium (Se) reduces the strength of the spin-orbit coupling (SOC)—the interaction between the electron’s spin and its orbital motion around the atomic nucleus—in NiTe₂. This shifts the BDP while preserving the type-II Dirac band.

“DFT calculations shows that the SOC strength and the BDP are almost linearly tunable. Scanning tunneling microscopy and angle-resolved photoemission spectroscopy confirm that the BDP in the NiTe_2−xSe_x alloy moves from +0.1 eV (NiTe₂) to −0.3 eV (NiTeSe) about the E_F. Further, the BDP is at the exact E_F for NiTe_1.4Se_0.6,” highlights Prof. Lee.

Hence, NiTe_2−xSe_x alloys offer a versatile platform for facilitating numerous technologies based on topological effects, including next-generation electronics, spintronics devices, efficient electrocatalysis, topological superconductivity, and quantum computers.

In conclusion, Prof. Lee discusses the longer-term implications of this work. “It provides insights into SOC control to tailor type-II Dirac bands, and will open up new avenues for exploring and developing materials with unconventional electronic properties.”

Here’s hoping for topological quantum computers!

***

Authors: Nguyen Huu Lam¹, Phuong Lien Nguyen², Byoung Ki Choi^3,4, Trinh Thi Ly¹, Ganbat Duvjir¹, Tae Gyu Rhee⁵, Yong Jin Jo¹, Tae Heon Kim¹, Chris Jozwiak³, Aaron Bostwick³, Eli Rotenberg³, Younghun Hwang⁶, Young Jun Chang⁵, Jaekwang Lee^2,*, and Jungdae Kim¹

Title of original paper: Controlling Spin−Orbit Coupling to Tailor type-II Dirac Bands

Journal: ACS Nano

¹Department of Physics, University of Ulsan, Republic of Korea

²Department of Physics, Pusan National University, Republic of Korea

³Advanced Light Source (ALS), E. O. Lawrence Berkeley National Laboratory, United States

⁴Department of Physics, University of Seoul, Republic of Korea

⁵Department of Physics and Department of Smart Cities, University of Seoul, Republic of Korea

⁶Electricity and Electronics and Semiconductor Applications, Ulsan College, Republic of Korea

Lab website address: http://cpmd.pusan.ac.kr

ORCID id: 0000-0002-7854-4329

*Corresponding author’s email: jaekwangl@pusan.ac.kr

Jaekwang Lee

Email id: jaekwangl@pusan.ac.kr

Pusan National University, located in Busan, South Korea, was founded in 1946, and is now the no. 1 national university of South Korea in research and educational competency. The multi-campus university also has other smaller campuses in Yangsan, Miryang, and Ami. The university prides itself on the principles of truth, freedom, and service, and has approximately 30,000 students, 1200 professors, and 750 faculty members. The university is composed of 14 colleges (schools) and one independent division, with 103 departments in all.

Website: https://www.pusan.ac.kr/eng/Main.do

Prof. Jaekwang Lee is an associate professor at the Department of Physics at Pusan National University (PNU), Korea. He received Ph.D. in Physics from the University of Texas in Austin, USA in 2010. He was awarded the Young Scientist Award by Physics and Chemistry of Surface and Interfaces (PCSI) in 2011. Prof. Lee worked as a postdoctoral researcher at the Oak Ridge National Laboratory, USA from 2010 to 2014. In 2015, he joined the Department of Physics at PNU. His research interests include first-principle study of interfaces, spectroscopy simulations, and materials design for energy applications.