Theory of finite-temperature two-dimensional structural transformations in group-IV monochalcogenide monolayers

Journal Article · Tue May 05 00:00:00 EDT 2020 · Physical Review B

DOI:https://doi.org/10.1103/PhysRevB.101.184101· OSTI ID:1617375

Villanova, John W. ^[1]; Kumar, Pradeep ^[1]; Barraza-Lopez, Salvador  ^[1]

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1. Univ. of Arkansas, Fayetteville, AR (United States)

An account of two-dimensional (2D) structural transformations in 2D ferroelectrics predicts an evolution from a structure with ${\mathrm{Pnm}2}_1$ symmetry into a structure with square P4/nmm symmetry and is consistent with experimental evidence, while another argues for a transformation into a structure with rectangular Pnmm symmetry. An analysis of the assumptions made in these models is provided here, and six fundamental results concerning these transformations are contributed as follows: (i) Softened phonon modes produce rotational modes in these materials. (ii) The transformation to a structure with P4/nmm symmetry occurs at the lowest critical temperature $T_c$. (iii) The hypothesis that one unidirectional optical vibrational mode underpins the 2D transformation is unwarranted. (iv) Being successively more constrained, a succession of critical temperatures $\left(T_c<T_c^{\text{'}}<T_c^{"}\right)$ occurs in going from molecular dynamics calculations with the NPT and NVT ensembles onto models with unidirectional oscillations. (v) The choice of exchange-correlation functional impacts the estimate of the critical temperature. (vi) Importantly, the correct physical picture of these transformations is one in which rotational modes confer a topological character to the 2D transformation via the proliferation of vortices.

• Research Organization: Univ. of Arkansas, Fayetteville, AR (United States); Univ. of California, Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division
• Grant/Contract Number: SC0016139; AC02-05CH11231
• OSTI ID: 1617375
• Journal Information: Physical Review B, Vol. 101, Issue 18; ISSN 2469-9950
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

77 NANOSCIENCE AND NANOTECHNOLOGY
2d ferroelectrics
phase transitions
2d materials