LaN structural and topological transitions driven by temperature and pressure
Graphical abstract
Introduction
Semimetallic lanthanum monopnictides such as LaAs, LaSb, and LaBi have attracted much attention because of their unusual extreme magnetoresistance due to electron–hole compensation [1], [2], [3], [4]. These materials typically assume stable rock-salt structures, and they are promising for magnetic sensor and spintronics applications. Different from the above semimetals, LaP is found theoretically to be semiconducting with a narrow band gap ( eV), and it displays high thermoelectric performance tunable by strain and pressure [5], [6]. In addition to the intriguing magnetic and electronic behaviors of lanthanum monopnictides, researchers have recently paid attention to their non-trivial topological properties.
In the single-particle picture, topological bulk materials can be roughly categorized into topological semimetals (TSMs) and topological insulators (TIs). TSMs are characterized by topologically protected band crossings [7]; they include nodal semimetals such as Weyl and Dirac semimetals [8], [9], as well as more exotic varieties like nodal-line semimetals [10], type-II Weyl semimetals [11], and multifold fermions [12], [13], [14]. For insulators with the time-reversal symmetry (TRS), topology arises from band inversion at TR-invariant momenta caused by strong spin–orbit coupling and can be classified by the indices [15], [16]. The topologically non-trivial phases exhibit robust helical surface states with spin-momentum locking, which forbids back-scattering and thus renders new potential applications in low-dissipation devices.
LaX (X = N, P, As, Sb, and Bi) lacks conducting f-electrons and preserves TRS, and their band structures also show an energy gap between valence and conduction bands at each k-point in the Brillouin zone (BZ). Hence, LaX still permits the classifications, even though some lanthanum monopnictides are semimetallic. Experimentally, the signature of topologically nontrivial (bulk) insulating phases can be determined by the conducting surface states near high symmetry points in the surface BZ, and they can be observed directly by angle-resolved photoemission spectroscopy (ARPES). In ambient conditions, LaBi is confirmed to be topologically nontrivial by several experimental groups [2], [17], [18], [19], [20], [21]. In contrast, LaAs and LaSb are found to be topologically trivial in ARPES experiments [2], [21], [22]. Recently, topological phase transitions (TPTs) are especially intriguing, because of the possibility to control different topological phases [23], [24], [25], [26], [27], [28], [29]. In LaAs and LaSb, pressure-induced TPTs have been proposed [30], [31]. Since LaN and LaP are expected to show similar crystal and electronic structures as other lanthanum monopnictides, TPTs also may be achieved by uniaxial strain or hydrostatic pressure.
In this paper, we use first-principles calculations to study the structural, electronic, and topological phase transitions of LaN. By re-examining the dynamic stabilities of all rock-salt LaX, we find that only LaN is dynamically unstable with imaginary phonon modes at zero temperature. Using evolutionary crystal structure prediction techniques, we find a new, dynamically stable structure of LaN with symmetry. In our calculations, the -LaN also exhibits spontaneous electronic polarization, and it can undergo concomitant structural and ferroelectric transitions driven by temperature or pressure. The rest of the paper is organized as follows. In Sec. II, we discuss the computational methods based on density functional theory, evolutionary crystal structure prediction, and ab initio molecular dynamics. In Sec. III, we present the theoretical results of structural, electronic, and topological properties of LaN under different temperature and pressure. Sec. IV concludes the paper by summarizing the conditions for stabilizing different phases under study. The Appendix discusses additionally topological phase transitions in rock-salt LaP, LaAs, and LaSb under hydrostatic pressure.
Section snippets
Methods
Our density functional theory (DFT) [32], [33] calculations are conducted using the Vienna Ab initio Simulation Package (VASP) [34], [35], [36], [37]. In particular, the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) [38] functional and the projector augmented wave (PAW) method [39], [40] are adopted. The wavefunctions are expanded in plane-wave basis sets with a kinetic energy cutoff of 500 eV, and a -centered 11 × 11 × 11 Monkhorst–Pack k mesh [41] is used for the
Results and discussion
Several rare-earth monopnictides are known to be stabilized in the cubic rock-salt structure (space group No. 225, symmetry), as shown in Fig. 1(a). Indeed, a previous X-ray diffraction (XRD) experiment has reported the observation of rock-salt LaN [51]. However, a recent experiment using magnetron sputtering found only wurtzite and zinc blende structures for LaN [52]. As a result, we begin by using DFT to examine the structural stability. We first fully relax the rock-salt structures for
Conclusion
Using density functional theory and evolutionary crystal searches, we have discovered a new, low-temperature structure of LaN belonging to space group No. 1 with the symmetry. This lower-symmetry phase, which can be regarded as a distorted rock-salt structure, displays ferroelectricity with a spontaneous polarization of 36 C/cm2. Using ab initio molecular dynamics, we found that when the temperature is increased, the -LaN will undergo simultaneous ferroelectric and structural transitions
Data availability statement
The data that support the findings of this study are available on request from the corresponding author.
CRediT authorship contribution statement
Wei-Chih Chen: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft. Chia-Min Lin: Investigation, Validation. Joseph Maciejko: Supervision, Writing - review & editing. Cheng-Chien Chen: Supervision, Project administration, Resources, Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The calculations were performed on the Frontera computing system at the Texas Advanced Computing Center. Frontera is made possible by NSF award OAC-1818253. J.M. was supported by NSERC Discovery Grant Nos. RGPIN-2020-06999 and RGPAS-2020-00064; the CRC Program; CIFAR; a Government of Alberta MIF Grant; a Tri-Agency NFRF Grant (Exploration Stream); and the PIMS CRG program.
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