The canonical topological insulator Bi₂Se₃ has played a central role in the development of ‘topological spintronics,’ a framework that seeks to exploit the inherent spin-momentum correlation in helical Dirac surface states for spin transport devices that could be used for non-volatile magnetic random access memory. Prior experiments have focused on studying efficient interconversion between spin current and charge current in devices that interface a topological insulator with a ferromagnet. This project studied the thermal generation of spin current in the topological insulator Bi₂Se₃, thereby completing measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents. This was accomplished by comparing the spin Nernst magneto-thermopower to the spin Hall magnetoresistance for bilayers of Bi₂Se₃/CoFeB. We find that Bi₂Se₃ generates substantial thermally driven spin currents efficiently, with a metric (the spin Nernst ratio) that is the largest among all materials measured to date, two to three times larger compared to previous measurements for the heavy metals Pt and W. This work also provided new fundamental insights into thermally generated spin currents in Bi₂Se₃ via Mott relations, revealing the role played by overall large spin Hall conductivity and its dependence on electron energy. 

2DCC Role: The 2DCC facility was used for epitaxial growth of high-quality Bi₂Se₃ and their characterization using in vacuo angle resolved photoemission spectroscopy (ARPES). 

What Has Been Achieved: This work demonstrated efficient generation of spin current in a topological insulator using a thermal gradient. Importance of the Achievement: Understanding thermally generated spin currents in topological insulators is important for characterizing the effect of Joule heating on measurements of current-induced spin-orbit torques. If the thermal spin currents are sufficiently strong, they could in principle be applied for generating useful torques. The study also showed how the Mott relation, which connects the spin Nernst effect to the spin Hall effect, provides insight into the origin of the large effect we measure and also suggests how even stronger thermal-gradient to spin-current conversion might be achieved by optimizing topological insulators. 

Unique Feature(s) of the MIP that Enabled this Achievement: Synthesis of high-quality Bi₂Se₃ thin films by MBE and their in vacuo characterization by angle resolved photoemission spectroscopy. 

Publication: Rakshit Jain, Max Stanley, Arnab Bose, Anthony R. Richardella, Xiyue Zhang, Tim Pillsbury, David Muller, Nitin Samarth, Daniel C. Ralph, Science Advances (2023), 9, adj4540. 

Acknowledgments: The research at Cornell performed by R.J. and A.B. was supported by the U.S. Department of Energy (DE-SC0017671) and made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC program (DMR-1719875), and the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (grant NNCI-2025233). The research at Penn State was supported by the National Science Foundation through the Penn State 2D Crystal Consortium–Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreement DMR-1539916 and DMR-2039351. D.A.M. and X.S.Z. acknowledge NSF through the Cornell Center for Materials Research (DMR-1719875).