In-situ Spectroscopic Ellipsometry as a Tool to Monitor Growth and to Optimize Structures

Thursday February 23, 2023 from 12-1pm ET

Speaker: Dr. Frank Peiris, Kenyon College 

Abstract: Using immediate feedback, the ability to control and optimize growth-parameters during the deposition of a structure is crucial for producing high-quality samples. In this regard, in-situ spectroscopic ellipsometry (IS-SE) not only complements conventional techniques such as RHEED and laser-interferometry, but it also provides vital information that is difficult to access via other in-situ methods. In this talk, focusing on the growth of thin films and heterostructures of topological insulators, the advantages of using IS-SE will be highlighted. Initially, the growth of Bi2Se3 thin films will be discussed, elucidating how thickness and temperature parameters are manifested in the spectra obtained via IS-SE. Finally, the growth of multilayer structures will be discussed, focusing on how the IS-SE spectra obtained after the deposition of each layer produce important growth details of the heterostructure.


Layered magnetic topological materials (MnBi2Te4)(Bi2Te3)m

Thursday January 26, 2023 from 12-1pm ET 

Speaker: Dr. Zhiqiang Mao, Penn State University

Abstract: The combination of magnetism and non-trivial band topology can generate potentially useful exotic quantum states of technological relevance, e.g., the quantum anomalous Hall insulator (QAHI). In this talk, I will present our recent studies on layered magnetic topological insulators (MnBi2Te4)(Bi2Te3)m (m=0, 1 & 2). This material system has recently attracted a great deal of interest, since it is predicted to provide access to a rich verity of novel topological quantum states, such as QAHI, axion insulators, high-order topological insulators and ideal Weyl semimetals [1-4]. Experimentally, there have been significant advancements in the studies of these materials.  Intrinsic antiferromagnetic (AFM) topological insulators have been demonstrated in bulk single crystals of MnBi2Te4 (m=0) [3], MnBi4Te7 (m=1) [5] and MnBi6Te10 [6].  Furthermore, both QAHI [7] and axion insulators have been observed in 2D thin layers of MnBi2Te4 [8]. Our work [9-11] in this area has focused on the study of competing magnetic interactions of (MnBi2Te4)(Bi2Te3)m and the field-driven topological phase transition from the topological insulator to the ideal Weyl semimetal state in Mn(Bi1-xSbx)2Te4. While theory predicts the AFM phases are more stable than the FM phases in (MnBi2Te4)(Bi2Te3)m (m=0, 1 & 2), we have succeeded in synthesizing both AFM and FM phases for these compositions through finely tuning growth conditions and tuned their chemical potential to be close to their charge neutral points via Sb substitution for Bi. These materials provide rich opportunities for observing novel magnetic topological phases. From magnetotransport measurements on Mn(Bi1-xSbx)2Te4, we have observed clear evidence of an ideal type-II FM Weyl state in the polarized FM phase. We find the field-driven AFM-to-FM transition induces an electronic structure reconstruction in lightly hole-doped samples, which results in a large intrinsic anomalous Hall effect and negative c-axis longitudinal magnetoresistance attributable to the chiral anomaly [11]. These results establish a promising platform for exploring the underlying physics of the long-sought, ideal time-reversal-symmetry breaking type II Weyl semimetal.


[1] D. Zhang et al., Phys. Rev. Lett. 122, 206401 (2019).
[2] J. Li et al., Science Advances 5, eaaw5685 (2019).

[3] M. M. Otrokov et al., Nature 576, 416 (2019).
[4] R.X. Zhang, F.C.Wu & S.D. Sarma, Phys. Rev. Lett. 124, 136407 (2020)
[5] C. Hu, et al., Nat. Commun. 11, 1 (2020).
[6] S. Tian et al., Phys. Rev. B 102, 035144 (2020
[7] Y. Deng et al., Science 367, 895 (2020)
[8] C. Liu et al., Nat. Mater. 19, 5 (2020
[9] Lee et al., Phys. Rev. Research 1, 012011(R) (2019).
[10] Y.Y. Chen et al., Phys. Rev. Materials 4, 064411(2020).
[11] Lee et al., Phys. Rev. X 11, 031032 (2021).