constructed/established and are combined with atomic scale characterization facilities in- and ex-situ, where atomistic details of the interactions of electrons would be disclosed. We believe that the combination of such in-/ex-situ fabrication and measurements can provide breakthroughs in both the old problems and new challenges in low dimensional electronic systems. Furthermore, we also purse the bulk materials synthesis of both new organic and inorganic systems which host intriguing phases and properties by themselves and can be utilized as starting materials for fabricating new low dimensional heterostructured materials such as 1D or 2D epixatically-grown heterostructures and stacked heterostructures of exfoliated monolayers. At present, we are focusing on the former systems of the heteroepitaxial growth.
We develop the state-of-the-art instruments such as the lowest temperature and high magnetic field scanning tunneling microscopy and spectroscopy (STM/STS) and the highest-efficiency spin- and angle-resolved photoelectron spectroscopy system (ARPES), which will be described further in the next section. In addition, we construct the combination of STM and ARPES with molecular beam epitaxy (MBE) systems for atomically controlled 1D/2D materials. For the chemical-vapor-deposition (CVD) grown monolayers, we also establish the STM measurement schemes using a specially designed STM system and clean transfer techniques, based on a glove box and/or a vacuum suitcase. The STM instruments for MBE- or CVD-grown materials with the special schemes for alignment and contact allow us to perform high-quality STM/STS measurements on microcrystalline patches and intrinsic domains of low dimensional materials. For the ARPES band mapping for those microcrystals, a nanoscale focus of excitation sources is very important. We are strategically collaborating with Advanced Light Source on the construction of the nano ARPES beamline, which is almost ready for active use. At the same time, a few 100 nm beam size will be achieved in our spin ARPES beamline in the next stage.
We access directly edge states of various new 1D/2D topological insulators. Edge states in 1D topological insulators (TI’s) are soliton excitations with spin-charge separation and possible charge fractionalization. This particularresearch field is largely initiated by our center. In 2D TI’s, the edge channels are quantum spin Hall channels, which have rarely been accessed in atomic resolution. These new materials and direct atomic scale access would make it possible to have the atomic scale control over their topologically protected electron channels. Indeed, the atomic scale control and manipulation over quantum spin Hall channels has not been exploited yet, while there exist various exciting theoretical proposals. Furthermore, the exploitation of individual soliton excitations would open a totally new research field. At the same time, we also fabricate new types of low dimensional topological materials such as 2D Weyl semimetals among layered materials with strong interactions. This part of research is in its infant stage with large uncertain impacts and prospects. Not only restricted to topological materials, we investigate new 1D/2D systems, where their spin, orbital, lattice degrees of freedom together at low dimension lead to new emerging phenomena. In particular, we are interested in 2D materials with strong interplay of different-interactions expected such as that of spin-orbit and electron-phonon interactions, electron-phonon and electron-electron interactions, and electron-phonon and spin interactions. These interplays have been investigated mostly in bulk systems and we believe the lower dimensional atomic scale systems with such complex interactions would bring a new chapter into the current 2D physics research.
We fabricate various types of heterointerfaces of different materials with distinct symmetry, topology, and interactions. These interfaces can be fabricated through the atomic-layer heteroepitaxial growth. Since most of the current low dimensional systems of interest are van der Waals materials, the possibility of heteroepitaxial growth is unlimited. In addition, naturally inhomogeneous 2D materials or partially-grown heteroepitaxial monolayers also offer various types of lateral heterointerfaces as interesting platforms for scanning probe studies. We propose that the nanoscale inhomogeneity, by themselves or as manipulated by scanning probe techniques, provides further variety in the heterointerface formation. We are also working on the synthesis of heterostructured bulk crystals.
Atomic-scale characterization of heterointerfaces and, in particular measuring their physical properties poses a challenging technical issue. We develop novel scanning probe tools to reveal not only atomic and electronic structures but also thermal, magnetic, and optical properties of lateral heterointerfaces of atomic layer 1D/2D materials. Spin-polarized STM, thermovolatic STM, and photon STM (using tip-enhanced Raman and tunneling-induced photoemission) are important tools under development and construction (see further below in the instrument section). The combination of ultralow-temperature-high magnetic field STM with the aforementioned physical property measurements would make the center’s research unique and competitive in wide range of materials.
We create new 1D/2D electronic systems through interfacing 1D/2D materials of distinct symmetry, topology, and interactions. These systems straightforwardly include topological edge electronic states in 2D/3D topological insulators and also various topological semimetals. The research opportunity in this field is growing as physics scope and variety of topological materials is rapidly widening. Particularly for those new types of topological systems, physics and technical implication of edge channels are largely unexplored. We further propose that various interesting and unprecedented low dimensional electronic systems can be created by interfacing topological systems themselves or interfacing them with other systems of largely different interactions. At these proximity heterointerfaces, the proximity fields naturally bring different interactions and symmetry/topology to interplay for emerging physics. One well-known example is the superconductor/TI interfaces for Majorana fermion physics, but the 2D TI edge channel in the proximity of superconductivity is not clearly established yet. A similar concept can be extended to the proximity interface of a strongly correlated electronic system and TI, where the interplay of strong electron correlation with the strong spin-orbit interaction has been known to bring up new Mott physics and novel topological systems. Our original approach is to generate such interplay by the proximity field of atomic scale heterointerfaces. We test various possibilities of novel phenomena and unprecedented properties of low dimensional electronic systems at diverse heterointerfaces with different types of interactions.
Searching for new physics often requests developing new instruments and improving their performances. In turn, new and better instruments can substantially widen the capabilities of physics research and provide unforeseen insight. During the development of the research on atomic scale low dimensional electronic systems, STM and ARPES have played, unarguably the most important roles. In order to become a world-leading group in this field, we develop new STM and ARPES instruments with possibly the highest performance in the world. We develop the lowest temperature STM with high magnetic field, which can reach sub 10 mK temperature. At the same time, various types of STM-based probes are constructed for atomic scale measurements of not only electronic but also optical, spin, and thermal properties of heterointerfaces. Eventually, these new STM-based measurement techniques developed will be combined with the state-of-the-art low temperature STM to really open up unprecedented possibilities. As a flagship case, our new sub 10 mK STM in its final form would be the first such system with the access of laser-in and-out. This machine would make it possible to probe excitations of the smallest energy scale in atomic scale spatial resolution both by direct tunnelling and inelastic, photon-involved, processes. We also construct a high-efficiency spin-polarized ARPES system, which is an essential tool for the investigation of new topological materials. We integrate new types of high-efficiency spin detector with high flux undulator synchrotron radiation and newly developed electron energy-momentum analyzer. This system aims the highest throughput machine among spin-resolved ARPES systems in the world. The above systems will be fully open for domestic and internationally users and collaborations to position our center as an important international research hub. In particular, we expect to attract a huge number of international users for the spin ARPES system.
Diverse reaction chemistry between transition-metal ions and chalcogen ions leads to a variety of transition-metal chalcogenides (TMCs) with vdW layered crystal structures. We exploit this versatility toward designs of new 2D heterostructures by atomic vdW heteroepitaxy in vapor phases, where each vdW crystal can be selectively a band insulator, a semiconductor or an exotic metal with superconductivity, charge density wave or Mott transition. In other words, we aim to achieve synthetic integration of such atomically thin materials toward new 2D electronic/photonic systems. For the initial 5-years, we have mainly focused on synthetic creation of semiconductor heterostructures, composed of dissimilar TMC monolayers (MLs), with distinct 2D crystallographic relations. For example, we demonstrated vertical heteroepitaxy of two hexagonal TMC (MoS2 and WS2) MLs without interlayer rotation misfits, i.e., the coherent hexagon-on-hexagon unit cell stack, from which we reported a new low-energy interlayer excitation between each ML valley. Another class of a new 2D superstructure was demonstrated by lateral heteroepitaxy of distinct TMC polymorphs (metallic 1T’-MoTe2 and semiconducting 2H-MoTe2), namely “polymorphic epitaxy”, where the monoclinic 1T’ unit cell is seamlessly stitched to the 2H one with deterministic crystallographic variants. These coplanar metal-semiconductor contacts are atomically coherent, showing the lowest contact barrier height ever-reported, contributing to the substantial device outperformance, when incorporated into atomically thin field-effect transistors. In parallel, we have also made substantial efforts on robust growth of such heterostructures on 4 in. wafers by metalorganic chemical vapor deposition to scale up to potential technology platforms. In the later stage of the program, we plan to expand our 2D heteroepitaxy schemes to diverse heterojunctions, composed of correlated 2D materials, such as topological insulators, superconductors and ferromagnets.
In establishing new 2D electronic systems, it is essential to properly adjoin two 2D crystals in crystallographic and momentum spaces. We have achieved such 2D heterostructures by atomic epitaxy, and unveiled new functions by investigating variation in properties at those deterministic heterointerfaces. Particularly, our Group employs local “light” probes to investigate such interfacial phenomena. In this regard, we have established laser spectroscopy systems combined with electrical measurement tools, by incorporating various lasers (including supercontinuum lasers and femtosecond lasers) into confocal microscopy systems, enabling to provide spatially, spectrally, time and polarization resolved information upon local photo-excitations on our synthetic 2D heterostructures devices. We have exploited these tools for explorations into interactions among light, heat and electricity at our vdW heterointerfaces. As for TMC semiconductor heterostructures, we have reported tunable light absorption/emission in MoS2/WS2 ML stacks with interlayer rotational attributes and anisotropic excitonic transitions in ReS2 MLs. We also investigated photoinduced thermoelectric conversions at topological edge-states in atomically thin Bi2 (Sb2)Te3. Later, we will expand this research thrust to single-photon emission processes at diverse atomic point defects that we can synthetically imbed in wide band-gap semiconductor MLs by a deterministic manner, for a new type of 2D quantum photonics platforms.
It is focused on spin and orbital dynamics in low-dimensional electronic systems, based on the recognition that a full description of a low-dimensional electron system requires not only accurate measurements of charge distributions and their spatiotemporal evolutions but also the momentum- and energy-resolved information on the spin and orbital dynamics, and how they couple to charge and lattice degrees of freedom. With such a comprehensiveset of information, we will be able to acquire a microscopic understanding of the physics of low dimensional electronic systems, in which electron interactions are enhanced and thus the tendency toward developing a collective symmetry-breaking order, such as unconventional superconductivity, metal-insulator transition, and spin/charge density waves are increased. Such quantum orders often lead to colossal responses to external stimuli, providing a basis for future quantum electronic devices. One of our major goals is to investigate how effective electron interactions are modified by low-energy dynamical fluctuations of spin and orbital degrees of freedom, how these processes lead to novel quantum phases of matter, and what the physical properties of such new phases are.
A particular focus will be given on low-dimensional systems based on heavy transition-metal compounds, in which spin-orbit coupling enhances strong correlation effects and thus drives otherwise weakly-correlated, Fermi liquid metals into correlated magnetic insulators. These “spin-orbit Mott insulators” serve as parent compounds from which myriad electronic phases emerge upon application of external perturbations such as doping, high pressure, electric/magnetic field, etc., whose unconventional nature is indicated by nonlinear optical responses, anomalous transport properties, and momentum- and temperature-dependent gap opening in spectral functions. To gain comprehensive information encompassing charge, spin, and orbital degrees of freedom, we will use resonant inelastic x-ray scattering, inelastic neutron scattering, and Raman spectroscopy to probe spin and orbital dynamics; complementary information on charge dynamics will be provided by ARPES and scanning tunneling spectroscopy already established in the director’s group.
These spectroscopic tools, in parallel with discovery and synthesis of new materials, will be our main arsenal in performing the mission of CALDES, to create new low-dimensional systems with atomic-level precision, to understand fundamental physics behind their electronic properties, and ultimately to control them for applications to electronic devices. It should also be noted that the x-ray and soft x-ray free electron laser is available from last year in Pohang Accelerator Laboratory. We will gradually start our activity in both x-ray and soft x-ray free electron laser beamlines firstly at the feasibility test level and to develop ideas for another major research program. With the combination of powerful STMs, spin-resolved ARPES, RIXS, and free electron laser, our center in Pohang will definitely become a unique international research hub.