Spinful hinge states in the higher-order topological insulators WTe2.

Higher-order topological insulators are recently discovered quantum materials exhibiting distinct topological phases with the generalized bulk-boundary correspondence. Td-WTe2 is a promising candidate to reveal topological hinge excitation in an atomically thin regime. However, with initial theories and experiments focusing on localized one-dimensional conductance only, no experimental reports exist on how the spin orientations are distributed over the helical hinges-this is critical, yet one missing puzzle. Here, we employ the magneto-optic Kerr effect to visualize the spinful characteristics of the hinge states in a few-layer Td-WTe2. By examining the spin polarization of electrons injected from WTe2 to graphene under external electric and magnetic fields, we conclude that WTe2 hosts a spinful and helical topological hinge state protected by the time-reversal symmetry. Our experiment provides a fertile diagnosis to investigate the topologically protected gapless hinge states, and may call for new theoretical studies to extend the previous spinless model.

Heteroepitaxial van der Waals semiconductor superlattices.

A broad range of transition metal dichalcogenide (TMDC) semiconductors are available as monolayer (ML) crystals, so the precise integration of each kind into van der Waals (vdW) superlattices (SLs) could enable the realization of novel structures with previously unexplored functionalities. Here we report the atomic layer-by-layer epitaxial growth of vdW SLs with programmable stacking periodicities, composed of more than two kinds of dissimilar TMDC MLs, such as MoS2, WS2 and WSe2. Using kinetics-controlled vdW epitaxy in the near-equilibrium limit by metal-organic chemical vapour depositions, we achieved precise ML-by-ML stacking, free of interlayer atomic mixing, which resulted in tunable two-dimensional vdW electronic systems. As an example, by exploiting the series of type II band alignments at coherent two-dimensional vdW heterointerfaces, we demonstrated valley-polarized carrier excitations-one of the most distinctive electronic features in vdW ML semiconductors-which scale with the stack numbers n in our (MoS2/WS2)n SLs on optical excitations.

Ultrafast non-excitonic valley Hall effect in MoS2/WTe2 heterobilayers.

The valley Hall effect (VHE) in two-dimensional (2D) van der Waals (vdW) crystals is a promising approach to study the valley pseudospin. Most experiments so far have used bound electron-hole pairs (excitons) through local photoexcitation. However, the valley depolarization of such excitons is fast, so that several challenges remain to be resolved. We address this issue by exploiting a unipolar VHE using a heterobilayer made of monolayer MoS2/WTe2 to exhibit a long valley-polarized lifetime due to the absence of electron-hole exchange interaction. The unipolar VHE is manifested by reduced photoluminescence at the MoS2 A exciton energy. Furthermore, we provide quantitative information on the time-dependent valley Hall dynamics by performing the spatially-resolved ultrafast Kerr-rotation microscopy; we find that the valley-polarized electrons persist for more than 4 nanoseconds and the valley Hall mobility exceeds 4.49 × 103 cm2/Vs, which is orders of magnitude larger than previous reports.

Near-field sub-diffraction photolithography with an elastomeric photomask.

Photolithography is the prevalent microfabrication technology. It needs to meet resolution and yield demands at a cost that makes it economically viable. However, conventional far-field photolithography has reached the diffraction limit, which imposes complex optics and short-wavelength beam source to achieve high resolution at the expense of cost efficiency. Here, we present a cost-effective near-field optical printing approach that uses metal patterns embedded in a flexible elastomer photomask with mechanical robustness. This technique generates sub-diffraction patterns that are smaller than 1/10th of the wavelength of the incoming light. It can be integrated into existing hardware and standard mercury lamp, and used for a variety of surfaces, such as curved, rough and defect surfaces. This method offers a higher resolution than common light-based printing systems, while enabling parallel-writing. We anticipate that it will be widely used in academic and industrial productions.

Light Polarization-Controlled Conversion of Ultrafast Coherent-Incoherent Exciton Dynamics in Few-Layer ReS2.

Coherent light-matter interaction can transiently modulate the quantum states of matter under nonresonant laser excitation. This phenomenon, called the optical Stark effect, is one of the promising candidates for realizing ultrafast optical switches. However, the ultrafast modulations induced by the coherent light-matter interactions usually involve unwanted incoherent responses, significantly reducing the overall operation speed. Here, by using ultrafast pump-probe spectroscopy, we suppress the incoherent response and modulate the coherent-to-incoherent ratio in the two-dimensional semiconductor ReS2. We selectively convert the coherent and incoherent responses of an anisotropic exciton state by solely using photon polarizations, improving the control ratio by 3 orders of magnitude. The efficient modulation was enabled by transient superpositions of differential spectra from two nondegenerate exciton states due to the light polarization dependencies. This work provides a valuable contribution toward realizing ideal ultrafast optical switches.

Generation, transport and detection of valley-locked spin photocurrent in WSe2-graphene-Bi2Se3 heterostructures.

Quantum optoelectronic devices capable of isolating a target degree of freedom (DoF) from other DoFs have allowed for new applications in modern information technology. Many works on solid-state spintronics have focused on methods to disentangle the spin DoF from the charge DoF1, yet many related issues remain unresolved. Although the recent advent of atomically thin transition metal dichalcogenides (TMDs) has enabled the use of valley pseudospin as an alternative DoF2,3, it is nontrivial to separate the spin DoF from the valley DoF since the time-reversal valley DoF is intrinsically locked with the spin DoF4. Here, we demonstrate lateral TMD-graphene-topological insulator hetero-devices with the possibility of such a DoF-selective measurement. We generate the valley-locked spin DoF via a circular photogalvanic effect in an electric-double-layer WSe2 transistor. The valley-locked spin photocarriers then diffuse in a submicrometre-long graphene layer, and the spin DoF is measured separately in the topological insulator via non-local electrical detection using the characteristic spin-momentum locking. Operating at room temperature, our integrated devices exhibit a non-local spin polarization degree of higher than 0.5, providing the potential for coupled opto-spin-valleytronic applications that independently exploit the valley and spin DoFs.

Pseudoxanthomonas putridarboris sp. nov. isolated from rotten tree.

A Gram-staining-negative, rod-shaped, motile bacterium, designated WD12T, was isolated from a rotten tree at Chungbuk National University, South Korea. WD12T grew optimally at 30-37 °C and pH 7.0-7.5 and could assimilate arbutin and potassium-5-ketogluconate. The major cellular fatty acid were iso-C16 : 0, C16 : 0, cyclo C17 : 0, iso-C15 : 0, summed features 3 (comprising C16 : 1ω7c/iso-C15 : 0 2-OH) and anteiso-C15 : 0. The major polar lipids consisted of phosphatidylethanolamine, diphosphatidylglycerol and phosphatidylglycerol. The major respiratory quinone was ubiquinone-8 (Q-8). The G+C content of the genomic DNA was 69.1 %. The results of phylogenetic and comparative analysis based on the 16S rRNA gene sequence indicated that WD12T formed a tight phylogenetic lineage with Pseudoxanthomonas mexicana AMX 26BT and Pseudoxanthomonas japonensis 12-3T of the the genus Pseudoxanthomonas in the family Xanthomonadaceae. Sequence similarity to other members of the genus Pseudoxanthomonasranged from 98.6 % (P. mexicana AMX 26BT) to 95.1 % (Pseudoxanthomonas taiwanensis CB-226T). DNA-DNA relatedness between WD12T and eight type strains of species of the genus Pseudoxanthomonasshowing more than 97 % 16S rRNA sequence similarity were 6±0-26±1 %. On the basis of the evidence from this polyphasic study, WD12T represents a novel species of the genus Pseudoxanthomonas, for which the name Pseudoxanthomonas putridarboris sp. nov. is proposed. The type strain is WD12T (=KACC 15045T=LMG 25968T).

Reichenbachiella faecimaris sp. nov., isolated from a tidal flat, and emended descriptions of the genus Reichenbachiella and Reichenbachiella agariperforans.

A taxonomic study was carried out on two bacterial strains, PCP11(T) and PCP104, isolated from a tidal flat of the Yellow Sea, Korea. Comparative 16S rRNA gene sequence studies showed that these strains belonged to the family Cytophagaceae, phylum Bacteroidetes. Strains PCP11(T) and PCP104 shared 99.4 % sequence similarity and were related most closely to Reichenbachiella agariperforans KMM 3525(T) (95.8 and 96.0 % sequence similarity, respectively). Members of the genera Fulvivirga, Roseivirga, Fabibacter and Marinoscillum were the next closest relatives of the new isolates, with sequence similarities ≤ 91 %. The two isolates were Gram-staining-negative, strictly aerobic, gliding bacteria. They grew in the presence of 1-5 % NaCl, at pH 5.5-8.5 and at 4-35 °C. Strains PCP11(T) and PCP104 shared a number of physiological and biochemical properties with Reichenbachiella agariperforans KMM 3525(T), but they differed from this strain in the hydrolysis of biopolymers and in the production of carotenoid and flexirubin-type pigments. Both strains possessed iso-C(15 : 0), summed feature 4 (C(16 : 1)ω7c and/or iso-C(15 : 0) 2-OH) and C(15 : 0) as major cellular fatty acids. The major respiratory quinone was menaquinone 7 (MK-7). The G+C contents of the genomic DNA of strains PCP11(T) and PCP104 were 39.6 and 41.9 mol%, respectively. On the basis of phenotypic data and phylogenetic inference, it is proposed that the two isolates represent a novel species, Reichenbachiella faecimaris sp. nov., with strain PCP11(T) ( = KACC 14523(T)  = JCM 16588(T)) as the type strain. Emended descriptions of the genus Reichenbachiella and Reichenbachiella agariperforans are also proposed.

Pseudoalteromonas donghaensis sp. nov., isolated from seawater.

A Gram-negative, rod-shaped, motile and aerobic bacterium, designated strain HJ51(T), was isolated from a seawater sample from the East Sea, near South Korea. The isolate grew slowly at 4 °C, was able to grow at 40 °C, required NaCl and grew optimally at pH 6.5-7.0. The G+C content of the genomic DNA was 41.8 mol%. The major fatty acids were summed feature 4 (C(16 : 1)ω7c and/or iso-C(15 : 0) 2-OH), C(16 : 0) and summed feature 7 (C(18 : 1)ω7c, C(18 : 1)ω9t and/or C(18 : 1)ω12t). Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain HJ51(T) belonged to the genus Pseudoalteromonas and had 91.7-98.9 % 16S rRNA gene sequence similarity with type strains of species of the genus Pseudoalteromonas. Strain HJ51(T) had 7.2 % DNA-DNA relatedness with Pseudoalteromonas mariniglutinosa DSM 15203(T) and 12.9 % with Pseudoalteromonas prydzensis DSM 14232(T). On the basis of the phenotypic, phylogenetic and genomic data, strain HJ51(T) represents a novel species of the genus Pseudoalteromonas, for which the name Pseudoalteromonas donghaensis sp. nov. is proposed. The type strain is HJ51(T) (=KCTC 22219(T)=LMG 24469(T)).