Session D53: Novel Phases and Phase Transitions in 2D Materials

Imaging spinons in a 2D gapless quantum spin liquid

Two-dimensional triangular-lattice antiferromagnets are predicted under some conditions to exhibit a quantum spin liquid ground state whose low-energy behavior is described by a spinon Fermi surface. This “ghost” Fermi surface (in an otherwise insulating material) is a key concept for understanding spin liquids and their relationship to other quantum phases. Directly imaging the spinon Fermi surface, however, is difficult due to the fractional and chargeless nature of spinons. I will discuss how we have used scanning tunneling microscopy (STM) to image density fluctuations arising from a spin liquid Fermi surface in single-layer 1T-TaSe2, a two-dimensional Mott insulator. Quantum spin liquid behavior was observed in isolated single layers of 1T-TaSe2 through long-wavelength modulations of the local density of states at Hubbard band energies. These modulations reflect a spinon Fermi surface instability in single-layer 1T-TaSe2 and allow direct experimental measurement of the spinon Fermi wavevector, in good agreement with theoretical predictions for a 2D quantum spin liquid. Our results establish single-layer 1T-TaSe2 as an ideal platform for studying novel two-dimensional quantum spin liquid phenomena.   

Intrinsic magnetic topological insulators: MnBi2Te4 and beyond

Intrinsic magnetic topological insulators are novel states of quantum matter possessing both inherent magnetic order and topological electronic states, which offer a fertile playground to explore emergent quantum physics. The antiferromagnetic topological insulator MnBi₂Te₄ [1-4] is a rapidly rising star in the research field. The material is theoretically predicted to host rich topological quantum states (e.g., topological axion states, magnetic Weyl semimetal, and quantum anomalous Hall (QAH) effect). In addition to theoretical proposals, I will also introduce recent experimental findings, including the discoveries of antiferromagnetic topological insulator states [1,4], QAH effect [5], robust axion insulator and Chern insulator phases [6], high-Chern-number and high-temperature QAH effect [7], helical Chern insulator phase [8], etc. An outlook for future work will be given. Importantly, the working temperature of MnBi₂Te₄ is limited by its rather weak ferromagnetic exchange, making superior material candidates desirable. In this perspective, I will briefly report an unexpected theoretical finding of room-temperature ferromagnetism and large-gap QAH insulators in lithium-decorated iron-based superconductor materials LiFeX (X=S, Se, Te) [9], which is awaiting for experimental proof.
Reference:
[1] Y. Gong, et al. Chin. Phys. Lett. 36, 076801 (2019).
[2] J. Li, et al. Sci. Adv. 5, eaaw5685 (2019).
[3] D. Zhang, et al. Phys. Rev. Lett. 122, 206401 (2019).
[4] M. M. Otrokov, et al. Nature 576, 416 (2019).
[5] Y. Deng, et al. Science, eaax8156 (2020).
[6] C. Liu, et al. Nature Mater. (2020).
[7] J. Ge, et al. arXiv:1907.09947.
[8] C. Liu, et al. arXiv:2001.08401.
[9] Y. Li, et al. arXiv:1912.07461.   

Optically probing tunable topology in group V monolayers

Experiments on Bi monolayers on a SiC substrate reveal an interplay between a huge topologically nontrivial gap ~0.8 eV and strong spin-orbit coupling (SOC), leading to striking transport properties such as a robust quantum spin Hall effect (QSHE) [1]. With a suitable choice of substrates it is also possible to remove valley degeneracy and realize multiple Hall effects in a single materials system [2]. In contrast to transport properties, much less is known about how an optical response could yield topological signatures in these group V monolayers. By combining first-principles calculations for different substrates and a careful inclusion of strong SOC in effective models as well as the Coulomb interaction in these monolayers, we show that the changes in optical response reveal topological properties inherent to these systems. We explain how these findings offer new opportunities for proximitized materials [3].
[1] F. Reis, et al., Science 357, 287-290 (2017); F. Dominguez, et al., Phys. Rev. B 98, 161407(R) (2018).
[2] T. Zhou, et al., npj Quant. Mater. 3, 39 (2018).
[3] I. Zutic, et al., Mater. Today 22, 85 (2019).   

Topological aspects of the band structure of monolayer \beta-Sb in flat and buckled form : Nodal line to unpinned Dirac cones.

Monolayer antimonene in the beta structure has a buckled honeycomb
structure with a semiconducting band structure. DFT calculations show that
a flat honeycomb structure is a metastable state which has been stabilized
epitaxially on Ag. In the flat form of Sb, the Fermi level occurs near
linear dispersion band crossings, formed by the intersection of two warped
Dirac cones, one derived from p_z orbitals and the other from p_x, p_y orbitals.
This conical intersection corresponds to a nodal line
with a Lissajous pattern in k-space leading to an interesting Fermi
surface with electron and hole pockets in different in-plane
directions, in other words an in-plane angle dependent carrier
polarity (goniopolarity). Tight-binding as well as DFT
calculations show that the nodal line breaks
up into six individual symmetry protected Dirac points
in the slightly buckled form. These Dirac cones are unpinned and can
be moved with increasing buckling until Dirac points of
opposite winding number annihilate beyond a critical buckling angle.
Band structure calculations in the quasiparticle self-consistent GW approximation including
spin-orbit coupling show that a gap opens at the various Dirac crossings
but lead to an indirect zero gap situation.   

Ab initio study of pressure-driven phase transition in FePS3 and FePSe3

In spite of recent findings about the pressure-driven insulator-to-metal phase transition, and emerging superconductivity of FePS₃ and FePSe₃, the knowledge about the atomic structures of them is still vague. Here, we investigate the pressure-driven structural phase transitions of FePS₃ and FePSe₃ from 0 to 35 GPa by using ab initio calculations. We find that FePS₃ B-I structure transforms to FePS₃ B-II phase at about 5 GPa. Then above 17 GPa, FePS₃ B-III phase becomes energetically favored. For FePSe₃, with increasing pressure, FePSe₃ transforms to B-II phase at around 6 GPa and further to B-III phase at about 15 GPa. Our calculation results are consistent with experimentally observed high-pressure induced cell volume collapse, spin-crossovers and insulator-metal transition in FePS₃ and FePSe₃, which shed new light on understanding the high-pressure physics and phase transitions of FePS₃ and FePSe₃.   

Van-der-Waals layered ferroelectric CuInP2S6 I: a quadruple-well potential

CuInP₂S₆ (CIPS) is a van der Waals solid that is ferrielectric below room temperature and has polarization in the stacking direction. Using density-functional-theory calculations, we discovered that, instead of the usual double-well potential, CIPS features a unique quadruple-well potential, with two low-polarization and two high-polarization states, with the latter corresponding to large Cu displacements to the layer surfaces, where they bond to the adjacent layers.[1] The quadruple well is tunable by strain, which can eliminate one or the other of the polar states. Quantum molecular dynamics shed light on the nature of polarization switching in the CIPS environment. The predicted features have been verified by experiments utilizing scanning probe microscopy (next abstract, Maksymovych et al.). The new results and the propensity of CIPS for ionic substitution opens new opportunities to control and generate ferroelectric properties in layered materials.
[1] J. A. Brehm et al. “Tunable quadruple-well ferroelectric van-der-Waals crystals”, Nature Mater., in press.   

Van-der-Waals layered ferroelectric CuInP2S6 II: negative electrostriction and pressure-induced switching

Layered ferroelectric CuInP₂S₆ (CIPS) is highly unusual in light of its negative longitudinal piezoelectric coefficient the recently predicted quadruple-well potential (previous abstract, Brehm et al.). Here we show an even larger spectrum of peculiar behaviors, obtained via systematic analysis of nanoscale electromechanical properties with scanning probe microscopy [1]: giant negative electrostriction, coexistence of high and low polarization phases with four distinct polarization orientations, and intrinsic pressure-induced polarization switching. These properties derive from the quadruple potential well for polar displacements, verifying its existence. The combination of unique properties enable deeper insight from microscopic measurements, such as imaging of inhomogeneous strain distribution. More broadly, the peculiarities of CIPS are in large part engendered by its van der Waals structure, thus opening new prospects at the intersection of ferroelectric and 2D materials. [1] J. A. Brehm et al. “Tunable quadruple-well ferroelectric van-der-Waals crystals” Nature Mater., in press.   

Reversible electrical control of stacking order phase transition in few-layer graphene

The layer stacking order has profound effects on physical properties of two-dimensional (2D) van der Waals heterostructures. For example, graphene multilayers can have distinct electronic band structures and behaviors depending on their stacking orders. Fascinating physical phenomena -- such as correlated insulators, superconductors, and ferromagnetism -- can also emerge with a periodic variation of the layer stacking order, known as the moiré superlattice in van der Waals materials. In this work, we demonstrate that a reversible phase transition between different layer stacking orders can be induced globally in few-layer graphene by electrostatic gating. We directly image the gate-induced stacking orders phase transition with infrared near-field optical microscopy. We reveal that both the carrier doping and the vertical electrical field can drive the stacking order phase transition, but with different mechanisms, through a systematic study of dual-gated few-layer graphene. Our findings provide a reversible and non-invasive method to globally control the stacking orders of few-layer graphene, and they have important implications for the understanding of gate-dependent quantum phenomena in graphene moiré superlattices.   

Electronic Properties and Polarization Profiles of Janus Transition Metal Dichalcogenides

In this talk, we will present our recent studies on the structural and electronic properties of Janus transition metal dichalcogenides by density functional theory calculations. For the structural properties, we identify a novel reaction path in the materials conversion from transition metal dichalcogenides monolayers to its Janus form. For the electronic properties, we calculate the electronic band structures and polarization profile of the Janus monolayer, and discuss how these electronic properties can be modified through interlayer interactions. We further connect our theoretical works to experimental measurements.   

Crystallography and Properties of Atomic Chirality in Two-dimensional Helical Crystal Tellurium

Chirality is a fundamental property of nature which is extensively studied in various of fields such as chemistry, material science and biology. In condensed matter physics, chiral crystals bring exotic optical, electrical and magneto properties due to the lack of mirror and inversion symmetry. Tellurium (Te) as one of the chiral materials is formed by van der Waals interaction between each one-dimensional helical chiral atom chains which determine the chirality of the whole crystal, and Te with opposite chiralities are described by different space groups: P3₁21 (right-handed) and P3₂21 (left-handed). Here we report that two dimensional tellurium twin flakes were identified with opposite chirality by electron crystallographic techniques (high-resolution transmission electron microscope image along different primary axes) and sulfuric acid etching method. These Te twin flakes offers an ideal platform to study the chirality-dependent material properties which can potentially be distinguished in electronic structure and transport properties. Our results pave a way for the research in fundamental properties of chiral materials in two-dimensional limit.