[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Reactive oxygen species (ROS) play an essential role in regulating various physiological functions of living organisms. The intrinsic biochemical properties of ROS, which underlie the mechanisms ne...

Reactive Oxygen Species (ROS)-Based Nanomedicine.

The family of two-dimensional (2D) materials grows day by day, hugely expanding the scope of possible phenomena to be explored in two dimensions, as well as the possible van der Waals (vdW) heterostructures that one can create. Such 2D materials currently cover a vast range of properties. Until recently, this family has been missing one crucial member: 2D magnets. The situation has changed over the past 2 years with the introduction of a variety of atomically thin magnetic crystals. Here we will discuss the difference between magnetic states in 2D materials and in bulk crystals and present an overview of the 2D magnets that have been explored recently. We will focus on the case of the two most studied systems-semiconducting CrI3 and metallic Fe3GeTe2-and illustrate the physical phenomena that have been observed. Special attention will be given to the range of new van der Waals heterostructures that became possible with the appearance of 2D magnets, offering new perspectives in this rapidly expanding field.

Magnetic 2D materials and heterostructures.

The family of 2D materials grows day by day, drastically expanding the scope of possible phenomena to be explored in two dimensions, as well as the possible van der Waals heterostructures that one can create. Such 2D materials currently cover a vast range of properties. Until recently, this family has been missing one crucial member - 2D magnets. The situation has changed over the last two years with the introduction of a variety of atomically-thin magnetic crystals. Here we will discuss the difference between magnetic states in 2D materials and in bulk crystals and present an overview of the 2D magnets that have been explored recently. We will focus, in particular, on the case of the two most studied systems - semiconducting CrI$_3$ and metallic Fe$_3$GeTe$_2$ - and illustrate the physical phenomena that have been observed. Special attention will be given to the range of novel van der Waals heterostructures that became possible with the appearance of 2D magnets, offering new perspectives in this rapidly expanding field.

Magnetic 2D materials and heterostructures

Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examined and their properties are discussed, with particular attention to their charge density wave, superconductive and topological phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties. Two-dimensional transition metal dichalcogenides (TMDCs) exhibit attractive electronic and mechanical properties. In this Review, the charge density wave, superconductive and topological phases of TMCDs are discussed, along with their synthesis and applications in devices with enhanced mobility and with the use of strain engineering to improve their properties.

2D transition metal dichalcogenides

Atomically thin rhenium disulphide (ReS2) is a member of the transition metal dichalcogenide family of materials. This two-dimensional semiconductor is characterized by weak interlayer coupling and a distorted 1T structure, which leads to anisotropy in electrical and optical properties. Here we report on the electrical transport study of mono- and multilayer ReS2 with polymer electrolyte gating. We find that the conductivity of monolayer ReS2 is completely suppressed at high carrier densities, an unusual feature unique to monolayers, making ReS2 the first example of such a material. Using dual-gated devices, we can distinguish the gate-induced doping from the electrostatic disorder induced by the polymer electrolyte itself. Theoretical calculations and a transport model indicate that the observed conductivity suppression can be explained by a combination of a narrow conduction band and Anderson localization due to electrolyte-induced disorder.

/pdf/disorder-engineering-and-conductivity-dome-in-res2-with-583gbn0ytv.pdf

Disorder engineering and conductivity dome in ReS2 with electrolyte gating.

The metastable 1$T$ polymorph of NbSe${}_{2}$ was recently successfully synthesized in monolayer form. The observation of a superlattice and insulating behavior suggests an analogy with 1$T$-TaS${}_{2}$, an extensively investigated system known for its complex phase diagram that has recently been suggested to realize a possible quantum spin-liquid ground state. Here, a series of $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ techniques, from density functional theory (DFT) to dynamical mean-field theory (DMFT), finds that the star-of-David phase in 1$T$-NbSe${}_{2}$ is the most stable commensurate charge density wave phase and that its electronic character is a Mott insulator. DFT calculations also predict weak nearest-neighbor ferromagnetic coupling, which would disfavor a quantum spin liquid phase but suggest a flat-band ferromagnetism scenario.

Charge density wave phase, Mottness, and ferromagnetism in monolayer 1 T − NbSe 2

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exist in two polymorphs, referred to as 1T and 1H, depending on the coordination sphere of the transition metal atom. The broken octahedral and trigonal prismatic symmetries lead to different crystal and ligand field splittings of the d electron states, resulting in distinct electronic properties. In this work, we quantify the crystal and ligand field parameters of 2D TMDs using a Wannier-function approach. We adopt the methodology proposed by Scaramucci et al (2015 J. Phys.: Condens. Matter 27 175503) that allows to separate various contributions to the ligand field by choosing different manifolds in the construction of the Wannier functions. We discuss the relevance of the crystal and ligand fields in determining the relative stability of the two polymorphs as a function of the filling of the d-shell. Based on the calculated parameters, we conclude that the ligand field, while leading to a small stabilizing factor for the 1H polymorph in the d 1 and d 2 TMDs, plays mostly an indirect role and that hybridization between different d orbitals is the dominant feature. We investigate trends across the periodic table and interpret the variations of the calculated crystal and ligand fields in terms of the change of charge-transfer energy, which allows developing simple chemical intuition.

https://iopscience.iop.org/article/10.1088/2053-1583/ab0188/pdf

Crystal field, ligand field, and interorbital effects in two-dimensional transition metal dichalcogenides across the periodic table

Transition metal dichalcogenides (TMDs), whether in bulk or in monolayer form, exhibit a rich variety of charge-density-wave (CDW) phases and stronger periodic lattice distortions. While the actual role of nesting has been under debate, it is well understood that the microscopic interaction responsible for the CDWs is the electron-phonon coupling. The case of $1T\ensuremath{-}{\mathrm{TiSe}}_{2}$ is, however, unique in this family in that the normal state above the critical temperature ${T}_{\mathrm{CDW}}$ is characterized by a small quasiparticle band gap as measured by angle-resolved photoemission spectroscopy, so that no nesting-derived enhancement of the susceptibility is present. It has therefore been argued that the mechanism responsible for this CDW should be different and that this material realizes the excitonic insulator phase proposed by W. Kohn. On the other hand, it has also been suggested that the whole phase diagram can be explained by a sufficiently strong electron-phonon coupling. In this paper, in order to estimate how close this material is to the pure excitonic insulator instability, we quantify the strength of electron-hole interactions by computing the exciton band structure at the level of hybrid density functional theory, focusing on the monolayer. We find that in a certain range of parameters the indirect gap at ${q}_{\mathrm{CDW}}$ is significantly reduced by excitonic effects. We also stress the important role of the spin-orbit coupling in significantly reducing the band gap. We discuss the consequences of those results regarding the debate on the physical mechanism responsible for this CDW. Based on the dependence of the calculated exciton binding energies as a function of the mixing parameter of hybrid density functional theory, we conjecture that a necessary condition for a pure excitonic insulator is that its noninteracting electronic structure is metallic.

Diego Pasquier

Papers

2D transition metal dichalcogenides

Disorder engineering and conductivity dome in ReS2 with electrolyte gating.

Charge density wave phase, Mottness, and ferromagnetism in monolayer 1 T − NbSe 2

Crystal field, ligand field, and interorbital effects in two-dimensional transition metal dichalcogenides across the periodic table

Excitonic effects in two-dimensional TiSe 2 from hybrid density functional theory