Synthesis and application of nanocrystals

**Discovery of the new 1D topological insulator and its chiral soliton edge states for topological informatics (Director and PI S. H. Lee, Science 2015, Nature Physics 2017, Inchon Award 2016, Kyung-Am Award 2017)**

The existence of robustly protected chiral edge modes, such as the unidirectional edge charge (spin) current in quantum Hall (spin Hall) systems is the essence of the physics and applications of topological materials. While such a concept has been successfully introduced and confirmed at 2D or 3D systems, there is no direct analogue in 1D systems. Peierls-distorted molecular chains such as polyacetylene are prototypical 1D topological insulators and their topological edge states, known as Jackiw-Rebbi solitons, exhibit novel features as a topologically protected zero- energy mode such as charge-spin separation and charge fractionalization. However, they do not possess chirality, and none of these solitons is observed directly.

In this work, we discover that the 1D charge density waves (CDW) of indium atomic wires self-assembled on silicon surfaces have chiral solitons and show that they are inherited from the chiral edge states of a new class of Z4 topological symmetry. The indium atomic wire composed of two Peierls- distorted atomic chains has significant interchain coupling, which induces dynamical breaking of the sublattice symmetry and results in the Z4 topological class with four degenerate ground states. In this distinct and unprecedented topological system, three types of topological edge states emerge: right-chiral, left-chiral, and nonchiral solitons. The chiral solitons make the charge pumping possible, providing an important material platform for transferring information robustly. In fact, the use of chiral topological excitations to carry information robustly has been demonstrated in a few different systems.

For instance, reading, writing, and transfer of binary information were demonstrated with chiral topological excitations in magnetic systems such as skyrmion. However, the next step, the logic or algebraic operation of such topological bits has not been realized yet. Based on the chiral solitons with Z4 topological symmetry, we discover the switching between chiral topological excitations of different chirality in a 1D electronic system. We found that a fast-moving achiral soliton merges with chiral solitons to switch their handedness. This is shown to be the realization of the algebraic operation of Z4 topological charges. Chiral solitons could thus be a platform for storage and operation of robust topological multi-digit (4 digits) information. This work opens a new avenue to topological information processing in electronic systems.

**Discovery of a new 2D Wyle semimetal and a new 2D topological insulator (PI K. S. Kim and Director, Science 2015, Phys. Rev. Lett. 2017, Sci. Rep. 2017):** Based on an atomic layer 2D semiconductor material, black phosphorous, we succeeded in artificial control of the band gap, which leaded to a discovery of, possibly, the first 2D Wyle semimetal. Black phosphorus has a similar structural motif with graphene, a hexagonal lattice, but buckled along one of the crystallographic orientations. This induces an intrinsic band gap of 0.3 eV, which might be modulated by external electric or strain fields and thus offers various possibilities for tunable electrical mobility and optoelectronic properties. In this study, we introduce a large electric field on the surface of black phosphorus using a potassium adsorbate layer and modulate the bandgap continuously in a wide range from zero to 0.6 eV. Particularly, in the zero-gap state, we observed unconventional low energy quasiparticles having a linear energy dispersion in one direction and a quadratic one in the other direction. Such a hybrid nature of energy dispersions induces a quasiparticle being massless and massive with a strong moment-dependence. This is highly distinct from the low energy state of graphene or the surface state of a topological insulator, which may offer a new platform to study unconventional quantum Hall effect and many-body interactions in a 2D system.

Our recent study with an even stronger electric field at a higher K dose and high-resolution ARPES directly reveals the pair creation of Dirac points and their movement along the axis of the glide-mirror symmetry. Unlike graphene, the Dirac points of black phosphorus are stable, as protected by space-time inversion symmetry, even in the presence of spin-orbit coupling. Our results establish black phosphorus in the inverted regime as a simple model system of 2D symmetry-protected (topological) Dirac semimetals, offering an unprecedented opportunity for the discovery of a 2D Weyl semimetal.

In addition to a topological system based on black phosphorus, we also investigate possible 2D topological insulator phases in expitaxially-grown and atomically-thin Bi and Sb. While the 2D TI phase was established in HgTe/CdTe and InAs/GaSb quantum well structures through transport measurements, its realization in atomic layer materials is rare, making the direct access to its edge channel difficult. We report the epitaxial growth of an atomic layer 2D topological insulator, Bi monolayer film and identified directly their edge channels with scanning tunneling microscopy/spectroscopy (Phys. Rev. B 2014). Moreover, we succeeded in the realization of the 2D topological insulator phase in another atomic layer materials Sb, which was theoretically proposed to have a series of topological quantum phase transitions in ultrathin films, from trivial to the quantum spin Hall phase between 3 and 4~BL and to 3D topological semimetal in 22 BL thickness. We successfully grew ultrathin Sb films on Bi2Te2Se and identified clearly their edge states by scanning tunneling spectroscopy and ab initio calculations (Sci. Rep. 2017). Further atomic scale exploitation of quantum spin Hall edge states of Bi and Sb ultrathin films is expected.

**Violation of Ohm’s law in a Weyl semimetal (PI J. H. Kim, Nature Materials 2017):** The role of a topological structure on anomalous transport phenomena is currently one of the challenging questions. Particularly in topological semimetals possessing various types of point or line nodes singularities in the momentum space, the resulting Berry curvatures produce a fictitious magnetic field and significantly affect electron’s motion. While the chiral anomaly was observed as a negative longitudinal magnetoresistivity, its consequence in electronic transport has been unclear. So far, all existing discussions have been limited to the linear response regime, governed by Ohm’s law. We found, however, that in a Weyl metal, Ohm’s law breaks down due to a topological structure of the chiral anomaly with the charge pumping effect. We take Bi0.96Sb0.04 single crystal as a model 3D Dirac semimetal, which becomes a Weyl semimetal under magnetic field. The nonlinear I–V characteristics in the diffusive limit occurs only for a magnetic-
field aligned to an electric field, in which dissipation-less conduction channel connecting two paired Weyl points
contributes to the electrical transport. The Boltzmann transport theory with the charge pumping effect reveals the topological nonlinear conductivity, and it leads to a universal scaling function of the longitudinal magnetoconductivity, which completely describes our experimental observations. As a hallmark of Weyl metals, the nonlinear conductivity provides a venue for nonlinear electronics, optical applications, and the development of a topological Fermi-liquid theory beyond the Landau Fermi-liquid theory. This work thus puts an important stepstone in understanding the physical consequence of topology in a new class of topological materials.

**Discovering a new type of topological semimetal in a ferromagnetic van der Waals material (Transport group leader J. S. Kim and Director, Nature Materials, under revision):** Topological semimetals possess several band contact points or lines, called nodes, and the interplay of symmetry and topology determine the node degeneracy and its dimension. While various types of topological semimetals have been theoretically proposed, most of the previous studies, especially those with nodal lines, are limited to non-magnetic materials with time-reversal symmetry. In the course of exploring new topological states, we focused on magnetic materials, which can also be endowed with a topological band structure with broken time-reversal symmetry. Here, the interplay of magnetism and band topology can generate novel correlated topological phenomena which are potentially important for spin-related electronic
applications. Despite interesting theoretical proposals, the discovery of ferromagnetic topological semimetals, especially those with line nodes near the Fermi level, has remained elusive because of the lack of suitable materials.
We propose a ferromagnetic van der Waals metal Fe3GeTe2 as a candidate for a magnetic topological semimetal. In this system, the spin degree of freedom is fully quenched by the large ferromagnetic polarization, while the line
degeneracy is protected by crystalline symmetries connecting two different orbitals in neighboring layers. This orbital-
driven nodal line is tunable by changing the spin orientation due to spin-orbit coupling and can produce large Berry curvature, leading to a large anomalous Hall current. This turns out to make this system have the largest anomalous Hall angle and anomalous Hall factor among metallic ferromagnets, demonstrating that ferromagnetic topological semimetal possesses a great potential for various spin and orbital-dependent electronic functionalities.

**Creating an atomically sharp topological p-n heterostructure (Director and Group leader J. S. Kim, ACS Nano 2017):** In order to put a TI into any practical use, it has to be fabricated into devices whose basic units are often p-n junctions. Moreover, unique electronic properties of such a ’topological’ p-n junction were proposed theoretically such as the junction electronic state and the spin rectification. However, the fabrication of a lateral topological p-n junction has been challenging because of materials, process, and fundamental reasons. We demonstrate an innovative approach to realize a p-n junction of topological surface states of a 3D topological insulator with an atomically abrupt interface. We used a heterointerface of Sb and a 3D topological insulator Bi2Se3 When an ultrathin Sb film is grown on Bi2Se3 with a typical n-type topological surface state, the surface develops a strongly p-type topological surface state through the substantial hybridization between the 2D Sb film and the Bi2Se3 surface. Thus, the Bi2Se3 surface covered partially with Sb films bifurcates into areas of n- and p-type topological surface states as separated by atomic step edges with a lateral electronic junction of as short as 2 nm. This approach opens a new avenue toward various electronic and spintronic devices based on well defined topological p-n junctions with the scalability down to atomic dimension.

**Synthetic creation of coplanar circuitry based on vdW heterostructures (Associate Director M. H. Jo, Director, Group leader J. S. Kim, and PI J. W. Park, Nature Electronics under review 2018, Nature Nanotechnology 2017, Nano Letters 2015, Adv. Mater. 2015):** Crystal polymorphism selectively stabilizes the electronic phase of atomically thin transition-metal dichalcogenides (TMDCs) as metallic or semiconducting, suggesting the potential to integrate these polymorphs as circuit components in 2D circuitry. Developing a selective and sequential growth strategy for such 2D polymorphs in the vapor phase is a critical step in this endeavor. In this work, we reported on the polymorphic integration of distinct metallic (1T′) and semiconducting (2H) MoTe2 crystals within the same atomic planes by heteroepitaxy. The realized polymorphic coplanar contact is atomically coherent, and its barrier potential is spatially tight-confined over a length of only a few nanometers, with a lowest contact barrier height of ~ 25 meV. We also demonstrated the generality of our synthetic integration approach for other TMDC polymorph films with large areas.

**Investigation of interlayer optical excitations and photoinduced thermoelectric conversion in synthetic 2D heterointerfaces (Associate Director M. H. Jo, Nature Comm. 2018, Nano Letters 2018, Nature Comm. (2) 2016, Nature Comm. 2015, Nano Letters 2014):** We demonstrated vertical heteroepitaxy of MoS2 and WS2 MLs without interlayer rotation misfits, i.e., the coherent hexagon-on-hexagon unit cell stacking, by manipulation of 2D nucleation kinetics during the sequential vapor phase growth. Therein, we reported a new low-energy interlayer
excitation between each ML valley, directed by the rotational misfit-free stacking. Specifically, we reported that light absorption/emission in such coherent ML stacks can be tunable from indirect-to direct-gap transitions in both spectral and dynamic characteristics. This study suggests that the interlayer rotational attributes can determine tunable interlayer excitation as a new set of the basis for investigating optical phenomena in a 2D ML system.

Bi2Te3 and Sb2Te3, displaying the highest thermoelectric power at room temperature, are also known as topological insulators (TIs) whose electronic structures are modified by electronic confinements and strong spin−orbit interaction near the ML thickness regime. We explored novel thermoelectric conversion in the atomic ML steps of Bi2Te3 (n-type) and Sb2Te3 (p-type). Specifically, by scanning photoinduced thermoelectric current imaging at the ML steps, we showed that efficient thermoelectric conversion is accomplished by optothermal motion of hot electrons (Bi2Te3) and holes (Sb2Te3) through 2D subbands and topologically protected surface states. Our discovery suggests that the thermoelectric conversion can be interiorly achieved at the atomic steps of a homogeneous medium by direct exploiting of quantum nature of TIs, thus providing a new design rule for the compact thermoelectric circuitry at the ultimate size limit. We have also found a strategy to maximize the thermoelectric figure of merit in 2D SnS2. Specifically, we observed that as the thickness of SnS2 decreased, electrical conductivity increased whereas thermal conductivity decreased. This approach leads to a thermoelectric figure of merit increase to 0.13 at 300 K, a factor ~ 1,000 times greater than previously reported bulk single-crystal SnS2. The Seebeck coefficient obtained for our 2D SnS2 was 34.7 mV K 1 for 16-nm-thick crystals at 300 K.

**Creating Mott insulator heterostructure in 2D transition metal dichalcogenide (Director, Nature Commun. 2016 & 2017):** The control and manipulation over correlated electronic states of solids have been challenging but provide exciting opportunity to develop novel electronic devices. For example, the metal-insulator transition of a Mott insulating state can lead to a new type of non-volatile memory devices. As one of the most exciting candidates in this direction, the Mott insulating state of a layered transition metal dichalcogenide of 1T-TaS2 was shown to be switchable in an ultrafast time-scale, paving an avenue to ultrafast non-volatile memory devices based on correlated electrons. However, the electronic switching for realistic devices has been challenging and the nature of the metallic state stemming from Mott insulator was elusive. In this work, we provide two important breakthroughs in this field. We demonstrate that the Mott phase of this material can be manipulated electronically in nanoscale, switched into nanoscale metallic patches reversibly. We believe this is the first clear demonstration of the reversible nanoscale manipulation of the Mott transition, providing an essential stepstone to nanoscale ultrafast memory devices based on correlated electrons.

Moreover, using scanning tunneling spectroscopy with unprecedented spatial resolution, we found that the emergent metallic state is the incommensurate charge density wave state with domain wall network, another novel correlated electronic state near the Mott criticality, as controlled by the degree of the charge order coherence. While the domain walls has been believed to play crucial roles in this quantum phase transition and also the emerging superconductivity, there has been no clear microscopic understanding of domain walls themselves and their roles. In this work, we for the first time resolve out the atomic structures and electronic states localized on domain walls in a Mott-CDW insulator 1T-TaS2 using scanning tunneling microscopy and spectroscopy. We establish that the domain wall state decomposes into two well localized but nonconducting states at the center of domain walls and edges of domains. Theoretical calculations reveal their atomistic origin as the local reconstruction of domain walls under the strong influence of electron correlation. Our result introduces a new concept of the domain wall’s own internal degrees of freedom and their interplay, which is potentially related to the controllability of electronic properties in low dimensional correlated electronic systems.

**Discovery of new types of quantum matter originating from magnetic proximity coupling in a superconductor/Mott-insulator heterostructure (Group leader J. S. Kim, Nat. Commun. 2017):** One of the central interests of our center is the interplay of distinct symmetries, topologies, and interaction as realized in the proximity field of artificial heterointerfaces. In the spin systems such as iron-based superconductors (FeSCs) with itinerant electrons forming magnetic moments, various types of Fermi surface instabilities are often frustrated, leading to highly degenerate magnetic ground states, which stabilizes complex magnetic phases with broken time-reversal or lattice symmetries or even hidden phases without breaking any of them.

Particularly, when the proximity-coupled layer is strongly correlated and magnetically active, the additional interfacial spin interaction may induce distinct ground states in FeSCs, which however has not been explored so far. In order to create new heterostructures including both correlated itinerant electrons and localized spins, we take a heterostructured FeSC, Sr2VO3FeAs as a model system, consisting of a high-Tc superconducting iron pnictide layer and a Mott-insulating vanadium oxide layer. Each constituent layer usually hosts distinct magnetic orders, the stripe antiferromagnetic or nematic order in iron pnictide layer and the Neel antiferromagnetic order in vanadium oxide layer. We find however that an unusual charge/orbital order in the iron pnictide layers, without either static magnetism or broken C4-symmetry while suppressing the Neel antiferromagnetism in the vanadium oxide layers. Such an unprecedented C4-symmetric hidden order is due to the frustration of the otherwise dominant iron stripe and vanadium Neel fluctuations via interfacial magnetic proximity coupling, which also induces various unusual properties such as band-selective pseudogap or magnetically switchable superconductivities. Our findings, therefore, manifest that the physics of itinerant correlated magnetic system can become even richer in the proximity of other correlated systems and also offer a new avenue for exploring unusual ground state in the correlated heterostructures.

Construction of high-performance spin-resolved ARPES system: The fourth quantum number, spin, is important to understand the nature quantum materials, and spin-dependent band structure becomes crucial in the cases of topological materials. The spin measurement, thus, in ARPES becomes more demanded than ever as the most powerful and the unique tool to determine spin-dependent band structures. However, spin-resolved ARPES has been a difficult
and low-yield experiment due mainly to the low detection efficiency of conventional spin detectors using Mott scattering based on spin-orbit interaction. High brightness incident photon beams have compensated this low efficiency in advanced undulator synchrotron radiation beamlines, but only very few such systems have been available around the world. On the other hand, a new type of higher efficiency (about 100 times higher) spin detectors (so-called VLEED detector)
was developed by T. Okuda in Hiroshima University, Japan, which is based on exchange scattering mechanism using magnetic targets. We adopted this detector in collaboration with T. Okuda to combine it with a high brightness undulator in Pohang Light Source (PLS). This machine can be the highest efficiency spin-resolved ARPES system in the world due to the first combination of the high-efficiency spin detector with a highly bright undulator light source. Recently, we realized that a parallel project is also underway in Italian synchrotron radiation source, VESPA beamline in Elletra [1]. While we are behind on the construction schedule, far better performance is expected because our photon source is several times brighter with a much smaller beam size of 30μm × 30μm and covers a wider energy range (4A2: 10 eV ~ 1000 eV, VESPA: 8 eV ~ 120 eV). A smaller beam size below 1 μm will also be achieved in the next stage.

The construction of spin-resolved ARPES is composed of four different parts, the replacement of the photon source of the beamline with a brighter elliptically polarized undulator (EPU), the modification of beamline optics to admit higher power from the EPU, the construction of ARPES experimental system, and the development of highly efficient spin detectors.

The first two were done in collaboration with PLS. The design of new EPU and X-ray optics was done in 2013, and all the installation were completed in 2016. Figure 15 shows two ARPES spectra taken with right and left circularly polarized lights and their dichroism, which show well the helicity of the electron band. While the undulator upgrade and beamline modification were going on, the assembly of ARPES endstation was done in 2016, and the complete system was moved to the beamline at the end of 2016 (Fig. 16). It was commissioned with synchrotron radiation and opened to the user experiments in the middle of 2017. It is equipped with Scienta DA30-L analyzer, which allows for the simultaneous photoelectron acquisition within the solid angle of 30-degree cone from the sample, without moving the sample. This is indispensable for investigate the orbital symmetry of a specific band with variously polarized photons and the secure high momentum-resolution in SARPES. The system is equipped with a six-axis manipulator and the lowest temperature of 10 K has been achieved.

As the last part of this project, the development of highly efficient spin detectors is underway. For 3D vectorial spin analysis, we adapted double VLEED detectors in orthogonal configuration as sketched in Fig. E18. As of January 2018, all the components of the detectors were assembled, and the commissioning with synchrotron radiation will be finished before June 2018. The whole SARPES system can be available for external users from 2019.

[1] The number of magnet periods of the VESPA undulator is 17. That of 4A2 is 30. The brilliance of beamline is proportional to the square of the number of periods. Sqr (30/17)~3 and the beam currents of storage rings are 300 mA (Elettra) and 400 mA (PLS).

**Construction of ultra-low temperature high magnetic field scanning the tunneling microscope (ULT HMF STM):** Since its invention over 30 years ago, STM has become one of the most powerful scientific tools in condensed matter physics. It provides not only structural information of sample surfaces but also various physical properties, such as quasiparticle, optical, vibrational and thermoelectric properties, at the atomic scale. Its role is even more important for atomic scale low dimensional materials, which are the targets of the Center. The energy resolution of STM-based spectroscopy is limited by the thermal broadening of the Fermi edge given by ΔE ~ 3.5 kBT, where T is the electronic temperature of tunneling electrons. This limits access to the exotic electronic phases expected at lower temperatures as well as the lowest lying energy scales such as the hyperfine splitting.

We aim to build the STM operating at the lowest temperature below 10 mK while only few STM systems in the worlds can reach the operating temperature below 100 mK. Also, we set more challenging requirement on our STM: 1) It must keep both the sample and the tip below 10 mK in ultra-high vacuum (UHV) condition. 2) It must be capable of applying a three-axis high magnetic field to the sample. 3) It should have an easy tip and sample transferring mechanism and be able to do in situ deposition. 4) It must have ultimate-condition vibration isolation facility. We have been constructing an ultra-low temperature high magnetic field (ULTHMF) STM with an innovative STM scanner design to meet those criteria. The dilution refrigeration cryostat for the ULTHMF STM was constructed in late 2016 by the manufacturer and installed in Pohang in May, 2017 (Figure 18, left panel). We have confirmed intensively the overall cooling performance, which gives us a base temperature less than 6 mK. The cryostat has the highest cooling power ever for an STM and provides a full UHV environment with a 9(z)-2(x)-2(y) T vector magnetic field. The new STM head is designed to allow in situ deposition and easy transfer through the manipulator. It can be integrated to a rigid structure of vibration isolation facility (Figure 18, right panel). Currently, we are working on machining and assembling our STM head (Figure 19) and expect the whole system will be under test in this summer.