Kondo interaction in FeTe and its potential role in the magnetic order.

Finding d-electron heavy fermion states has been an important topic as the diversity in d-electron materials can lead to many exotic Kondo effect-related phenomena or new states of matter such as correlation-driven topological Kondo insulator. Yet, obtaining direct spectroscopic evidence for a d-electron heavy fermion system has been elusive to date. Here, we report the observation of Kondo lattice behavior in an antiferromagnetic metal, FeTe, via angle-resolved photoemission spectroscopy, scanning tunneling spectroscopy and transport property measurements. The Kondo lattice behavior is represented by the emergence of a sharp quasiparticle and Fano-type tunneling spectra at low temperatures. The transport property measurements confirm the low-temperature Fermi liquid behavior and reveal successive coherent-incoherent crossover upon increasing temperature. We interpret the Kondo lattice behavior as a result of hybridization between localized Fe 3dxy and itinerant Te 5pz orbitals. Our observations strongly suggest unusual cooperation between Kondo lattice behavior and long-range magnetic order.

Quantum electron liquid and its possible phase transition.

Purely quantum electron systems exhibit intriguing correlated electronic phases by virtue of quantum fluctuations in addition to electron-electron interactions. To realize such quantum electron systems, a key ingredient is dense electrons decoupled from other degrees of freedom. Here, we report the discovery of a pure quantum electron liquid that spreads up to ~3 Å in a vacuum on the surface of an electride crystal. Its extremely high electron density and weak hybridization with buried atomic orbitals show the quantum and pure nature of the electrons, which exhibit a polarized liquid phase, as demonstrated by our spin-dependent measurement. Furthermore, upon enhancing the electron correlation strength, the dynamics of the quantum electrons change to that of a non-Fermi liquid along with an anomalous band deformation, suggestive of a transition to a hexatic liquid crystal phase. Our findings develop the frontier of quantum electron systems and serve as a platform for exploring correlated electronic phases in a pure fashion.

Impact of the gate geometry on adiabatic charge pumping in InAs double quantum dots.

We compare the adiabatic quantized charge pumping performed in two types of InAs nanowire double quantum dots (DQDs), either with tunnel barriers defined by closely spaced narrow bottom gates, or by well-separated side gates. In the device with an array of bottom gates of 100 nm pitch and 10 µm lengths, the pump current is quantized only up to frequencies of a few MHz due to the strong capacitive coupling between the bottom gates. In contrast, in devices with well-separated side gates with reduced mutual gate capacitances, we find well-defined pump currents up to 30 MHz. Our experiments demonstrate that high frequency quantized charge pumping requires careful optimization of the device geometry, including the typically neglected gate feed lines.

Breakdown of chiral recognition of amino acids in reduced dimensions.

The homochirality of amino acids in living organisms is one of the great mysteries in the phenomena of life. To understand the chiral recognition of amino acids, we have used scanning tunnelling microscopy to investigate the self-assembly of molecules of the amino acid tryptophan (Trp) on Au(111). Earlier experiments showed only homochiral configurations in the self-assembly of amino acids, despite using a mixture of the two opposite enantiomers. In our study, we demonstrate that heterochiral configurations can be favored energetically when L- and D-Trp molecules are mixed to form self-assembly on the Au surface. Using density functional theory calculations, we show that the indole side chain strongly interacts with the Au surface, which reduces the system effectively to two-dimension, with chiral recognition disabled. Our study provides important insight into the recognition of the chirality of amino acid molecules in life.

Phase Controlled Growth of Cd3As2 Nanowires and Their Negative Photoconductivity.

The bottom-up synthesis process often allows the growth of metastable phase nanowires instead of the thermodynamically stable phase. Herein, we synthesized Cd3As2 nanowires with a controlled three-dimensional Dirac semimetal phase using a chemical vapor transport method. Three different phases such as the body centered tetragonal (bct), and two metastable primitive tetragonal (P42/nbc and P42/nmc) phases were identified. The conversion between three phases (bct → P42/nbc → P42/nmc) was achieved by increasing the growth temperature. The growth direction is [110] for bct and P42/nbc and [100] for P42/nmc, corresponding to the same crystallographic axis. Field effect transistors and photodetector devices showed the nearly same electrical and photoelectrical properties for three phases. Differential conductance measurement confirms excellent electron mobility (2 × 104 cm2/(V s) at 10 K). Negative photoconductance was first observed, and the photoresponsivity reached 3 × 104 A/W, which is ascribed to the surface defects acting as trap sites for the photogenerated electrons.

Visualization of Local Magnetic Moments Emerging from Impurities in Hund's Metal States of FeSe.

Understanding the origin of the magnetism of high temperature superconductors is crucial for establishing their unconventional pairing mechanism. Recently, theory predicts that FeSe is close to a magnetic quantum critical point, and thus weak perturbations such as impurities could induce local magnetic moments. To elucidate such quantum instability, we have employed scanning tunneling microscopy and spectroscopy. In particular, we have grown FeSe film on superconducting Pb(111) using molecular beam epitaxy and investigated magnetic excitation caused by impurities in the proximity-induced superconducting gap of FeSe. Our study provides deep insight into the origin of the magnetic ordering of FeSe by showing the way local magnetic moments develop in response to impurities near the magnetic quantum critical point.

Controllable p-n junctions in three-dimensional Dirac semimetal Cd3As2 nanowires.

We demonstrate a controllable p-n junction in a three-dimensional Dirac semimetal (DSM) Cd3As2 nanowire with two recessed bottom gates. The device exhibits four different conductance regimes with gate voltages, the unipolar (n-n and p-p) and bipolar (n-p and n-p) regimes, where p-n junctions are formed. The conductance in the p-n junction regimes decreases drastically when a magnetic field is applied perpendicular to the nanowire. In these regimes, the device shows quantum dot behavior, whereas the device exhibits conductance plateaus in the n-n regime at high magnetic fields. Our experiment shows that the ambipolar tunability of DSM nanowires can enable the realization of quantum devices based on quantum dots and electron optics.

Quantum Dots Formed in Three-dimensional Dirac Semimetal Cd3As2 Nanowires.

We demonstrate quantum dot (QD) formation in three-dimensional Dirac semimetal Cd3As2 nanowires using two electrostatically tuned p-n junctions with a gate and magnetic fields. The linear conductance measured as a function of gate voltage under high magnetic fields is strongly suppressed at the Dirac point close to zero conductance, showing strong conductance oscillations. Remarkably, in this regime, the Cd3As2 nanowire device exhibits Coulomb diamond features, indicating that a clean single QD forms in the Dirac semimetal nanowire. Our results show that a p-type QD can be formed between two n-type leads underneath metal contacts in the nanowire by applying gate voltages under strong magnetic fields. Analysis of the quantum confinement in the gapless band structure confirms that p-n junctions formed between the p-type QD and two neighboring n-type leads under high magnetic fields behave as resistive tunnel barriers due to cyclotron motion, resulting in the suppression of Klein tunneling. The p-type QD with magnetic field-induced confinement shows a single hole filling. Our results will open up a route to quantum devices such as QDs or quantum point contacts based on Dirac and Weyl semimetals.

Observation of enhanced superconductivity in the vicinity of Ar-induced nano-cavities in Pb(111).

Local variations of superconductivity have been studied using scanning tunneling microscopy around nano-cavities formed by Ar ions embedded in Pb(111). Various factors including the density of states at Fermi energy, electron-phonon couplings, and quantum well states, which are known to affect superconductivity, have been examined. We show that the superconductivity is enhanced near the nano-cavities and propose that quantum effects such as quantum confinement, proximity effect and multi-gap effect are possibly involved in determining the superconducting gap of this system. These results have important implications for the characterization and understanding of superconductivity at a nanometer scale.

Thickness-dependent Dirac dispersions of few-layer topological insulators supported by metal substrate.

The surface states protected by time-reversal symmetry in 3-dimensional topological insulators have recently been confirmed by angle-resolved photoemission spectroscopy, scanning tunneling microscopy, quantum transport and so on. However, the electronic properties of ultra-thin topological insulator films have not been extensively studied, especially when the films are grown on metal substrates. In this paper, we have elucidated the local behaviors of the electronic states of ultra-thin topological insulator Bi2Se3 grown with molecular beam epitaxy on Au(111) using scanning tunneling microscopy/spectroscopy. We have observed linear dispersion of electron interference patterns at higher energies than the Fermi energy that were not accessible by conventional angle-resolved photoemission spectroscopy. Moreover, the dispersion of the interference patterns varies with the film thickness, which is explained by band bending near the interface between the topological insulator and the metal substrate. Our experiments demonstrate that interfacial effects in thin topological insulator films on metal substrate can be sensed using scanning tunneling spectroscopy.

Local modification of intermolecular interactions at a sub-molecule level.

The local modification of intermolecular interactions in nickel-phthalocyanine molecules (NiPCs) is investigated on an Au(111) substrate using scanning tunneling microscopy. When the molecules are physisorbed on the substrate, they repel each other due to induced charge dipole moments. However, when the NiPC is chemisorbed on the substrate through the dehydrogenation of one of its ligands by a bias pulse, we find that a nearby physisorbed NiPC is attracted to the dehydrogenated ligand and trapped. Using our experimental results in combination with density functional theory calculations, we show that the observed attraction can be ascribed to the local charge redistribution around the dehydrogenated ligand of the chemisorbed NiPC. Furthermore, we demonstrate that desorption of the attracted NiPC from the trapped site can be readily controlled by changing the density of NiPCs around the dehydrogenated ligand.

Topological matter. Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor.

Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

Pedunculated serrated polyp with histologic features of sessile serrated adenoma: a clinicopathologic and molecular study.

In this study, we describe a previously undescribed pedunculated serrated polyp of the colon showing typical features of sessile serrated adenoma/polyp (SSA/P). All polyps were pedunculated, located in the proximal colon, small in size, and occurred in elderly patients. Histologically, the polyps showed typical features of SSA/P in the basal crypts with irregular, asymmetric expression of Ki-67. All polyps showed the BRAF-V600E mutation. The cells in the polyps did not show obvious cytologic dysplasia, prominent serration, or diffuse cytoplasmic eosinophilia with any occurrence of the so-called "ectopic crypt formation." We consider pedunculated serrated polyp showing features of SSA/P as a previously undescribed form of serrated adenoma/polyp in the spectrum of serrated neoplasia, which might represent a pedunculated variant of SSA/P or a precursor lesion of proximally located traditional serrated adenomas in the earliest stage.

Transmission of topological surface states through surface barriers.

Topological surface states are a class of novel electronic states that are of potential interest in quantum computing or spintronic applications. Unlike conventional two-dimensional electron states, these surface states are expected to be immune to localization and to overcome barriers caused by material imperfection. Previous experiments have demonstrated that topological surface states do not backscatter between equal and opposite momentum states, owing to their chiral spin texture. However, so far there is no evidence that these states in fact transmit through naturally occurring surface defects. Here we use a scanning tunnelling microscope to measure the transmission and reflection probabilities of topological surface states of antimony through naturally occurring crystalline steps separating atomic terraces. In contrast to non-topological surface states of common metals (copper, silver and gold), which are either reflected or absorbed by atomic steps, we show that topological surface states of antimony penetrate such barriers with high probability. This demonstration of the extended nature of antimony's topological surface states suggests that such states may be useful for high current transmission even in the presence of atomic-scale irregularities-an electronic feature sought to efficiently interconnect nanoscale devices.

Topological surface states protected from backscattering by chiral spin texture.

Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated because of the strong spin-orbit coupling inherent to these systems. These materials are distinguished from ordinary insulators by the presence of gapless metallic surface states, resembling chiral edge modes in quantum Hall systems, but with unconventional spin textures. A key predicted feature of such spin-textured boundary states is their insensitivity to spin-independent scattering, which is thought to protect them from backscattering and localization. Recently, experimental and theoretical efforts have provided strong evidence for the existence of both two- and three-dimensional classes of such topological insulator materials in semiconductor quantum well structures and several bismuth-based compounds, but so far experiments have not probed the sensitivity of these chiral states to scattering. Here we use scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy to visualize the gapless surface states in the three-dimensional topological insulator Bi(1-x)Sb(x), and examine in detail the influence of scattering from disorder caused by random alloying in this compound. We show that, despite strong atomic scale disorder, backscattering between states of opposite momentum and opposite spin is absent. Our observations demonstrate that the chiral nature of these states protects the spin of the carriers. These chiral states are therefore potentially useful for spin-based electronics, in which long spin coherence is critical, and also for quantum computing applications, where topological protection can enable fault-tolerant information processing.