2D Quantum materials: Magnetism and superconductivity

One of the most exciting recent developments in quantum materials is the discovery and characterization of magnetism and superconductivity in exfoliated single-layer materials. These developments pave the way for an enormous influx of new quantum materials designed layer by layer through mechanical stacking. Today, given the dramatic advances in nano-characterization tools driven primarily by the study of graphene, the science of 2D quantum materials is advancing very quickly.

Although the study of quasi-2D magnetism is well-established in bulk materials, the study of literal 2D magnets—consisting of a single layer of magnetic ions—is quite limited. In the 1970s and 1980s, there were studies of adsorbed monolayers of magnetic atoms as well as investigations of Langmuir–Blodgett films of manganese stearate. This early work is nicely reviewed by Pomerantz.¹ This paper also contains a good discussion of why the Mermin–Wagner theorem is unlikely to apply to very many real materials.

The materials we consider are layered, cleavable transition metal chalcogenides and halides. Typically, these materials will have a layer of metal ions sandwiched between layers of chalcogens or halides. The weak bonding between the layers allows mechanical cleavage of single crystals down to few- and even single-layer materials. These van der Waals (vdW) materials can be viewed as the bridge between the bulk and the nano-world. The electronic properties can change strikingly with the layer number. The easy cleavage and lack of dangling bonds make it possible to create nearly perfect surfaces. These surfaces are ideal for surface-sensitive spectroscopies such as quasiparticle interference and angle resolved photoemission. The crystalline perfection of the materials also makes it possible to stack single layers into heterostructures, the possibilities of which are just beginning to be imagined.² 

The magnetism in these systems is very rich, including ferromagnetic insulators such as CrSiTe₃,³ itinerant ferromagnets such as Fe₃GeTe₂,⁴ and insulating antiferromagnets such as MPX₃ materials (M = transition metal, X = S or Se).⁵ There are also materials with strong Kitaev interactions such as α-RuCl₃.⁶ The spin Hamiltonian and magneto-crystalline anisotropy varies widely from material to material and can be tuned with chemical doping, strain, or proximity effects.

As it is much easier to create 2D magnets in silico than in the laboratory, theoretical predictions have abounded and most have proven quite reliable. For example, the experimental work on MBE-grown 1T-MnSe₂⁷ was motivated by DFT calculations.⁸ One of the first theory papers was by Sachs et al. on the layered perovskite K₂CoF₄.⁹ This paper was pathbreaking in several ways, not least because it calculated the cleavage energy and Young's modulus of the monolayers. These quantities can be compared to graphite to provide an estimate of the cleavability of a given material. In some cases, theoretical predictions are made on doped materials. A good example is GaSe, which is semiconducting but was predicted in Ref. 10 to become ferromagnetic if hole-doped to 3 × 10¹³holes/cm². In some cases, DFT predicts stable materials that (so far, anyway) have not been synthesizable in the lab. A good example is CrSnTe3,¹¹ which is stable according to DFT but seems to lack a thermodynamic pathway for its synthesis. There have been several high-throughput studies of 2D materials, and some have identified potential 2D magnets. In Ref. 12, for example, the 2D magnets CrSiTe₃ and VSe₂ were identified.

The proximity effect refers to the transformation of a given material due to its close proximity to a neighboring material. Proximity effects in superconductors have been known for nearly 90 years,¹³ but until recently proximity effects in magnets have been harder to study due to the much smaller range of the proximity effect in magnets compared to superconductors. Although historically the proximity effect referred to the transfer of an ordered state to another material, we now understand that spin–orbit coupling and topological properties can also be transferred by the proximity effect.^14,15 The proximity effect offers an exciting new way to tune 2D materials as the short range of the interaction is not a problem. Some recent examples involve placing graphene on ferromagnetic insulators such as EuS and YIG.^16,17 In these experiments, the magnetic exchange interaction was transferred to graphene via the proximity effect.

The wide variety of papers appearing in the “2D Quantum Materials: Magnetism and Superconductivity” Special Topic in Journal of Applied Physics is emblematic of the breadth of the field of 2D quantum materials. There are six theory papers with differing approaches.^18–23 The paper by Ko and Son¹⁸ is focused on understanding the magnetism in bulk and single-layer CrGeTe₃, an important model system, from first principles. The magnetic properties of two known by little investigated materials, MnBr₂ and MnI₂, are studied in Ref. 19 by Luo et al. The paper by Meng et al.²⁰ uses an evolutionary algorithm to search for new 2D materials among gallium and indium oxides. The paper by Aperis and Varelogiannis²¹ develops a generalized mean-field theory of charge density waves under Zeeman fields and argue that 2D materials are good platforms to search for the predicted novel physics. The paper by Chaves et al.²² models Andreev scattering in superconductor-graphene devices. Finally, the paper by Möckli et al.²³ develops the theory of magnetic impurities in 2D Ising superconductors.

Four of the contributed papers report experimental work on nanostructures: three of these involve superconductivity and one is on magnetism. The paper by Kunakova et al.²⁴ reports on the properties of Josephson junctions formed between Bi₂Se₃ nanoribbons and Al electrodes. The paper by Kononov et al.²⁵ reports the emergence of superconductivity at the interface between flakes of WTe₂ and Pd. The contribution by Romanin et al.²⁶ determines the scattering lifetime in gated MoS2 nanolayers as a function of electron doping and temperature. Finally, the paper by Kim et al.²⁷ reports on the observation of unexpected plateaus magnetoresistance of a twisted junction of Fe₃GeTe₂/Fe₃GeTe₂.

Two of the contributed articles focus on the properties of bulk 2D materials (i.e., the crystals from which atomically thin flakes can be exfoliated). The paper by Wildes et al.²⁸ involves inelastic neutron scattering studies on FePS₃ to better understand the magnetic exchange in this material. The article by Xing et al.²⁹ reports magnetotransport measurements on bulk TbTe₃ as well as thin flakes. This material undergoes a charge-density wave transition at 334 K and an antiferromagnetic transition at 6 K.

Three of the contributions are review articles. The paper by Haruyama³⁰ discusses how 2D topological insulating states and quantum spin Hall phases can be created and used in spintronic devices. The paper by McGuire³¹ discusses the crystal-chemistry of a wide variety of cleavable magnetic materials. Finally, the paper by Brahlek et al.³² discusses the role of molecular beam epitaxy in the creation and control of superconducting and magnetic topological phases.

In summary, the “2D Quantum Materials: Magnetism and Superconductivity Special Topic” in Journal of Applied Physics highlights several exploration avenues that advance theoretical and experimental research into novel 2D quantum materials with engineered unique magnetic and superconducting properties.

The authors thank the editors and staff of Journal of Applied Physics for support in organizing the “2D Quantum Materials: Magnetism and Superconductivity” Special Topic.