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We demonstrate van der Waals double quantum well (vDQW) devices based on few-layer WSe

Few-layer transition metal dichalcogenides (TMDs) exhibit out-of-plane wave function confinement with subband quantization. This phenomenon is totally absent in monolayer crystals and is regarded a...

Organic–inorganic hybrid materials (OIHMs) represented by methylammonium lead triiodide (MAPbI3) and formamidinium lead triiodide (FAPbI3), which are the absorption layer of perovskite solar cells,...

Organic-inorganic hybrid materials (OIHMs), such as methylammonium lead triiodide (MAPbI3), have a wide composition space because of the various potential combinations of organic molecules and inorganic cages. However, for unknown OHIMs, it is difficult to predict what kind of crystal structure will be stable without any experimental data. In this work, we report an efficient scheme for predicting crystal structures and phase diagrams of MA-Pb-I systems from first-principles calculations and genetic algorithms. In our scheme, OIHMs are divided into organic molecules and inorganic clusters. A pseudobinary phase diagram of MAI-PbI2 was obtained by predicting structures at each composition. These results indicated that only MAPbI3 and MA2PbI4 are stable phases, consistent with the experiments. In addition, the electronic and optical properties of the predicted structures were calculated and the solar cell performance was evaluated. Thus, our method allowed us to search for unknown OIHMs without any experimental data.

The van der Waals (vdW) materials with low dimensions have been extensively studied as a platform to generate exotic quantum properties. Advancing this view, a great deal of attention is currently paid to topological quantum materials with vdW structures. Here, we provide a new concept of designing topological materials by the vdW stacking of quantum spin Hall insulators (QSHIs). Most interestingly, a slight shift of inversion center in the unit cell caused by a modification of stacking is found to induce the topological variation from a trivial insulator to a higher-order topological insulator (HOTI). Based on that, we present the first experimental realization of a HOTI by investigating a bismuth bromide Bi4Br4 with angle-resolved photoemission spectroscopy (ARPES). The unique feature in bismuth halides capable of selecting various topology only by differently stacking chains, combined with the great advantage of the vdW structure, offers a fascinating playground for engineering topologically non-trivial edge-states toward future spintronics applications.

In underdoped cuprates, the interplay of the pseudogap, superconductivity, and charge and spin ordering can give rise to exotic quantum states, including the pair density wave (PDW), in which the superconducting (SC) order parameter is oscillatory in space. However, the evidence for a PDW state remains inconclusive and its broader relevance to cuprate physics is an open question. To test the interlayer frustration, the crucial component of the PDW picture, we perform transport measurements on charge- and spin-stripe-ordered La1.7Eu0.2Sr0.1CuO4 and La1.48Nd0.4Sr0.12CuO4 in perpendicular magnetic fields (H⊥), and also with an additional field applied parallel to CuO2 layers (H∥). We detect several phenomena predicted to arise from the existence of a PDW, including an enhancement of interlayer SC phase coherence with increasing H∥. These data also provide much-needed transport signatures of the PDW in the regime where superconductivity is destroyed by quantum phase fluctuations., Among the exotic phases in underdoped cuprates, the evidence of a pair density wave (PDW) remains inconclusive. Here, Shi et al. report transport signatures consistent with the presence of PDW pairing correlations that compete with uniform superconductivity in two underdoped cuprate superconductors.

Deep-learning algorithms enable precise image recognition based on high-dimensional hierarchical image features. Here, we report the development and implementation of a deep-learning-based image segmentation algorithm in an autonomous robotic system to search for two-dimensional (2D) materials. We trained the neural network based on Mask-RCNN on annotated optical microscope images of 2D materials (graphene, hBN, MoS2, and WTe2). The inference algorithm is run on a 1024 × 1024 px2 optical microscope images for 200 ms, enabling the real-time detection of 2D materials. The detection process is robust against changes in the microscopy conditions, such as illumination and color balance, which obviates the parameter-tuning process required for conventional rule-based detection algorithms. Integrating the algorithm with a motorized optical microscope enables the automated searching and cataloging of 2D materials. This development will allow researchers to utilize a large number of 2D materials simply by exfoliating and running the automated searching process. To facilitate research, we make the training codes, dataset, and model weights publicly available.

A van der Waals coupled Weyl semimetal NbIrTe4 is investigated by combining scanning tunneling microscopy/spectroscopy and first-principles calculations. We observe a sharp peak in the tunneling conductance near the Fermi energy (E-F). Comparison with calculations indicates that the peak originates from a van Hove singularity (vHs) associated with a Lifshitz transition of the surface Fermi-arc state. Interestingly, our tunneling spectroscopy also shows signatures of strong electron-boson coupling. This is potentially due to an anomalously enhanced charge susceptibility coming from the near-E-F vHs formation.

Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological viewpoints. Here we report possible signatures of the pressure-induced topological phase transition from a semiconductor to a Weyl semimetal in elemental tellurium probed by transport measurements. Pressure variation of the periods of Shubnikov–de Haas oscillations, as well as oscillation phases, shows an anomaly around the pressure theoretically predicted for topological phase transition. This behavior is consistent with the pressure-induced band deformation and resultant band-crossing effect. Moreover, effective cyclotron mass is reduced toward the critical pressure, potentially reflecting the emergence of massless linear dispersion. The present result paves the way for studying the electronic band topology in well-known compounds and topological phase transition by the external field.

Nonreciprocal or even-order nonlinear responses in symmetry-broken systems are powerful probes of emergent properties in quantum materials, including superconductors, magnets, and topological materials. Recently, vortex matter has been recognized as a key ingredient of giant nonlinear responses in superconductors with broken inversion symmetry. However, nonlinear effects have been probed as excess voltage only under broken time-reversal symmetry. In this study, we report second harmonic transport under time-reversal symmetry in the noncentrosymmetric trigonal superconductor PbTaSe2. The magnitude of anomalous nonlinear transport is two orders of magnitude larger than those in the normal state, and the directional dependence of nonlinear signals are fully consistent with crystal symmetry. The enhanced nonlinearity is semiquantitatively explained by the asymmetric Hall effect of vortex-antivortex string pairs in noncentrosymmetric systems. This study enriches the literature on nonlinear phenomena by elucidating quantum transport in noncentrosymmetric superconductors.

The Weyl semimetal $\mathrm{Mo}{\mathrm{Te}}_{2}$ offers a rare opportunity to study the interplay between Weyl physics and superconductivity. Recent studies have found that Se substitution can boost the superconductivity up to 1.5 K, but suppresses the ${T}_{d}$ structure phase that is essential for the emergence of the Weyl state. A microscopic understanding of the possible coexistence of enhanced superconductivity and the ${T}_{d}$ phase has not been established so far. Here, we use scanning tunneling microscopy to study an optimally doped superconductor $\mathrm{Mo}{\mathrm{Te}}_{1.85}{\mathrm{Se}}_{0.15}$ with bulk ${T}_{c}\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. By means of quasiparticle interference imaging, we identify the existence of a low-temperature ${T}_{d}$ phase with broken inversion symmetry where superconductivity globally coexists. Furthermore, we find that the superconducting coherence length, extracted from both the upper critical field and the decay of density of states near a vortex, is much larger than the characteristic length scale of the existing chemical disorder. Our findings of robust superconductivity arising from a Weyl semimetal normal phase in $\mathrm{Mo}{\mathrm{Te}}_{1.85}{\mathrm{Se}}_{0.15}$ make it a promising candidate for realizing topological superconductivity.

We present x-ray spectral evidence for the evolution of valence- and spin-specific tetragonal distortions in single-layer cobaltates. Measurements of Co ${L}_{3}$-edge resonant inelastic x-ray scattering reveal the ${t}_{2g}$ electronic structure of Co in hole-doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{4}$ ($x=0.5$, 0.7, and 0.8). As the Sr-doping $x$ increases, the tetragonal splitting of the ${t}_{2g}$ states of high-spin ${\mathrm{Co}}^{2+}$ decreases, whereas that of low-spin ${\mathrm{Co}}^{3+}$ increases and the population fraction of high-spin ${\mathrm{Co}}^{3+}$ increases. The results enable us to clarify the origin of the change of magnetic anisotropy and in-plane resistivity in the mixed-valence cobaltates caused by the interplay of spin-orbit coupling and tetragonal distortion.

The major breakthroughs in understanding of topological materials over the past decade were all triggered by the discovery of the Z2-type topological insulator-a type of material that is insulating in its interior but allows electron flow on its surface. In three dimensions, a topological insulator is classified as either 'strong' or 'weak'1,2, and experimental confirmations of the strong topological insulator rapidly followed theoretical predictions3-5. By contrast, the weak topological insulator (WTI) has so far eluded experimental verification, because the topological surface states emerge only on particular side surfaces, which are typically undetectable in real three-dimensional crystals6-10. Here we provide experimental evidence for the WTI state in a bismuth iodide, β-Bi4I4. Notably, the crystal has naturally cleavable top and side planes-stacked via van der Waals forces-which have long been desirable for the experimental realization of the WTI state11,12. As a definitive signature of this state, we find a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. We also find that a crystal transition from the β-phase to the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator at roughly room temperature. The weak topological phase-viewed as quantum spin Hall insulators stacked three-dimensionally13,14-will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering.

Topological insulators with broken time-reversal symmetry and the Fermi level within the magnetic gap at the Dirac cone provides exotic topological magneto-electronic phenomena. Here, we introduce an improved magnetically doped topological insulator, Fe-doped BiSbTe2Se (Fe-BSTS) bulk single crystal, with an ideal Fermi level. Scanning tunneling microscopy and spectroscopy (STM/STS) measurements revealed that the surface state possesses a Dirac cone with the Dirac point just below the Fermi level by 12 meV. The normalized dI/dV spectra suggest a gap opening with Δmag ∼55 meV, resulting in the Fermi level within the opened gap. Ionic-liquid gated-transport measurements also support the Dirac point just below the Fermi level and the presence of the magnetic gap. The chemical potential of the surface state can be fully tuned by ionic-liquid gating, and thus the Fe-doped BSTS provides an ideal platform to investigate exotic quantum topological phenomena.

Few-layer transition metal dichalcogenides (TMDs) exhibit out-of-plane wave function confinement with subband quantization. This phenomenon is totally absent in monolayer crystals and is regarded as resulting from a naturally existing van der Waals quantum-well state. Because the energy separation between the subbands corresponds to the infrared wavelength range, few-layer TMDs are attractive for their potential to facilitate the application of TMD semiconductors as infrared photodetectors and emitters. Here, we report a few-layer WSe

Topological insulators with broken time-reversal symmetry and the Fermi level within the magnetic gap at the Dirac cone provides exotic topological magneto-electronic phenomena. Here, we introduce an improved magnetically doped topological insulator, Fe-doped BiSbTe

Organic-inorganic hybrid materials (OIHMs), such as methylammonium lead triiodide (MAPbI

This paper investigates the bulk rectification effect under out-of-plane magnetic field in a noncentrosymmetric three-dimensioanl superconductor PbTaSe${}_{2}$. The authors show a giant enhancement of the nonreciprocal magnetotransport during the superconducting transition and a sign reversal.

We report the electronic properties of single crystals of candidate nodal-line semimetal CaAgP. The transport properties of CaAgP are understood within the framework of a hole-doped nodal-line semimetal. In contrast, Pd-doped CaAgP shows a drastic increase of magnetoresistance at low magnetic fields and a strong decrease of electrical resistivity at low temperatures probably due to weak antilocalization. Hall conductivity data indicated that the Pd-doped CaAgP has not only hole carriers induced by the Pd doping, but also high-mobility electron carriers in proximity of the Dirac point. Electrical resistivity of Pd-doped CaAgP also showed a superconducting transition with onset temperature of 1.7-1.8 K., 6 pages, 3 figures, accepted for publication in Phys. Rev. B

Surface arcs (SAs) or Fermi arcs connecting pairs of bulk Weyl points with opposite chiralities are the signatures of Weyl semimetals in angle-resolved photoemission spectroscopy (ARPES) studies. The nontrivial topology of the bulk band structure guarantees the existence of these exotic Fermi arcs with connectivity that is strongly dependent on the surface. It has been theoretically proposed and experimentally confirmed that Fermi arcs at opposite surfaces can complete an unusual closed cyclotron orbit called a Weyl orbit, which leads to various intriguing transport properties. In this paper, a systematic ARPES study on opposite terminations (001) of type-II Weyl semimetal ${\mathrm{NbIrTe}}_{4}$ reveals different Fermi arc connections which result in a unique closed intersurface Fermi arc loop configurations (combining both projections of SAs) containing two pairs of Weyl points. In particular, the top surface ARPES data and corresponding ab initio calculation suggests that a topological Lifshitz transition occurs by tuning the chemical potential. SA rewiring on the top surface opens the intersurface arc loop at the Weyl node energy level into an open line, challenging the close-orbit description and leading to an unexplored scenario. Our results demonstrate the intrinsic alteration of Fermi arc connections and propose ${\mathrm{NbIrTe}}_{4}$ as a potential platform to examine Fermi-arc related phenomenon.

Exotic surface states of topological insulators have long attracted the attention of researchers. Recently, surface-dominant electrical transport in topological insulators has been observed; however, surface conduction in topological insulators is still not fully understood. To address this knowledge gap, we measured the transport properties of a thin flake of a highly bulk-resistive topological insulator, Sn

The dynamics of the charge-order domains has been investigated in La1.48Nd0.4Sr0.12CuO4, a prototypical stripe-ordered cuprate, using pulsed current injection. We first identify the regime in which nonthermal effects dominate over simple Joule heating and then demonstrate that, for small enough perturbation, pulsed current injection allows access to nonthermally induced resistive metastable states. The results are consistent with the pinning of the fluctuating charge order, with fluctuations being most pronounced at the charge-order onset temperature. The nonequilibrium effects are revealed only when the transition is approached from the charge-ordered phase. Our experiment establishes pulsed current injection as a viable and effective method for probing the charge-order dynamics in various other materials.

A bulk superconductor possessing a topological surface state at the Fermi level is a promising system to realise long-sought topological superconductivity. Although several candidate materials have been proposed, experimental demonstrations concurrently exploring spin textures and superconductivity at the surface have remained elusive. Here we perform spectroscopic-imaging scanning tunnelling microscopy on the centrosymmetric superconductor β-PdBi2 that hosts a topological surface state. By combining first-principles electronic-structure calculations and quasiparticle interference experiments, we determine the spin textures at the surface, and show not only the topological surface state but also all other surface bands exhibit spin polarisations parallel to the surface. We find that the superconducting gap fully opens in all the spin-polarised surface states. This behaviour is consistent with a possible spin-triplet order parameter expected for such in-plane spin textures, but the observed superconducting gap amplitude is comparable to that of the bulk, suggesting that the spin-singlet component is predominant in β-PdBi2., Although several materials have been proposed as topological superconductors, spin textures and superconductivity at the surface remain elusive. Here, Iwaya et al. determine the spin textures at the surface of a superconductor β-PdBi2 and find the superconducting gap opening in all spin-polarised surface states.

The chiral crystal is characterized by a lack of mirror symmetry and an inversion center, resulting in the inequivalent right- and left-handed structures. In the noncentrosymmetric crystal structure, the spin and momentum of electrons are locked in the reciprocal space with the help of the spin-orbit interaction. To reveal the spin textures of chiral crystals, here we investigate the spin and electronic structure in p-type semiconductor elemental tellurium with a chiral crystal structure by using spin- and angle-resolved photoemission spectroscopy. Our data demonstrate that the highest valence band crossing the Fermi level has a spin component parallel to the electron momentum around the BZ corners. Significantly, we have also confirmed that the spin polarization is reversed in the crystal with the opposite chirality. The results indicate that the spin textures of the right- and left-handed chiral crystals are hedgehog-like, leading to unconventional magnetoelectric effects and nonreciprocal phenomena., Comment: 15 pages, 4 figures

An electron and a hole created in cuprate Mott insulators attract each other via spin-spin interactions similar to a Cooper pair., Coupling of charge and spin degrees of freedom is a critical feature of correlated electron oxides, as represented by the spin-related mechanism of a Cooper pair under high-Tc superconductivity. A doublon-holon pair generated on an antiferromagnetic spin background is also predicted to attract each other via the spin-spin interaction J, similar to a Cooper pair, while its evidence is difficult to obtain experimentally. Here, we investigate such an excitonic effect by electroreflectance spectroscopy using terahertz electric field pulses in undoped cuprates: Nd2CuO4, Sr2CuO2Cl2, and La2CuO4. Analyses of the spectral changes of reflectivity under electric fields reveal that the splitting of odd-parity and even-parity excitons, a measure of doublon-holon binding energy, increases with J. This trend is reproduced by t-J–type model calculations, providing strong evidence of the spin-related doublon-holon pairing. Agreement with the calculations supports the s-wave symmetry of the doublon-holon pair in contrast to the d-wave Cooper pair in doped cuprates.

We studied magnetized topological insulator/superconductor junctions with the expectation of unconventional superconductive states holding Majorana fermions induced by superconductive proximity effects on the surface states of magnetized topological insulators (TIs), attached by conventional superconductors. We introduced Fe-doped BiSbTe2Se as an ideal magnetic TI and used the developed junction fabrication process to access the proximity-induced surface superconducting states. The bulk single crystals of the Fe-doped TI showed excellent bulk-insulating properties and ferromagnetism simultaneously at a low temperature. Meanwhile, the fabricated junctions also showed an insulating behavior above 100 K, as well as metallic conduction at a low temperature, which reflects bulk carrier freezing. In addition, we observed a proximity-induced gap structure in the conductance spectra. These results indicate that the junctions using the established materials and process are preferable to observe unconventional superconducting states which are induced via the surface channels of the magnetized TI. We believe that the developed process can be applied for the fabrication of complicated junctions and suites for braiding operations.

The origin of the weak insulating behavior of the resistivity, i.e. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\rho }_{xx}\propto {\mathrm{ln}}\,(1/T)$$\end{document}ρxx∝ln(1/T), revealed when magnetic fields (H) suppress superconductivity in underdoped cuprates has been a longtime mystery. Surprisingly, the high-field behavior of the resistivity observed recently in charge- and spin-stripe-ordered La-214 cuprates suggests a metallic, as opposed to insulating, high-field normal state. Here we report the vanishing of the Hall coefficient in this field-revealed normal state for all \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T\, The Hall effect has been used as a probe of the normal state of cuprates, when superconductivity is supressed by a magnetic field. Here, the authors report the vanishing of the Hall coefficient at high magnetic field in cuprates with stripe order and interpret it as a signature of the stripe-ordered phase.

Dirac fermions display a singular response against magnetic and electric fields. A distinct manifestation is large diamagnetism originating in the interband effect of Bloch bands, as observed in bismuth alloys. Through $^{209}$Bi NMR spectroscopy, we extract diamagnetic orbital susceptibility inherent to Dirac fermions in the semiconducting bismuth alloys Bi$_{1-x}$Sb$_x$ ($x = 0.08 - 0.16$). The $^{209}$Bi hyperfine coupling constant provides an estimate of the effective orbital radius. In addition to the interband diamagnetism, Knight shift includes an anomalous temperature-independent term originating in the enhanced intraband diamagnetism under strong spin-orbit coupling. The nuclear spin-lattice relaxation rate $1/T_1$ is dominated by orbital excitation and follows cubic temperature dependence in the extensive temperature range. The result demonstrates the robust diamagnetism and low-lying orbital excitation against the small gap opening, whereas $x$-dependent spin excitation appears at low temperatures., Comment: 10 pages, 4 figures

The control of the optoelectronic properties of 2D organic-inorganic hybrid perovskite (2D-OIHP) lead halides is an increasingly prevalent topic. Herein, the observation of the circular photogalvanic effect (CPGE) in new enantiomorphic 2D-OIHP lead iodides is reported, which are synthesized as a first OIHP-related system belonging to a chiral space group by incorporating organic chiral cations into the inorganic layers of lead iodides. The CPGE is an optoelectronic phenomenon associated with the spin-orbit coupling of heavy atoms in noncentrosymmetric systems. Owing to the CPGE, light-helicity-dependent steady photocurrents are generated without an external bias voltage under the irradiation of circularly polarized light. Furthermore, the sign reversal of the CPGE photocurrent depending on the chirality of the designed 2D-OIHP lead iodides is observed. This result indicates formation of the theoretically predicted radial spin-polarized texture in k-space of chiral systems owing to spin-momentum locking. Hence, chiral 2D-OIHP lead halides can be a promising platform for engineering opto-spintronic functionalities.

To realize the single-surface conduction in a topological insulator, a microscale Corbino device was fabricated from a single crystal of a highly bulk-resistive topological insulator Sn0.02Bi1.08Sb0.9Te2S (Sn-BSTS). The four-terminal resistance was measured in 2 K–300 K, and it was found that the temperature dependence of the resistance of the Corbino device was clearly distinct from that of Sn-BSTS bulk samples. We showed that the unconventional behavior of the Corbino device was quantitatively understood by considering the top surface, bottom surface, and bulk as independent conduction paths in parallel. Furthermore, the current flow distribution was studied by introducing the resistance network model, which revealed that the top-surface dominant conduction was realized in a low temperature. This study demonstrates the usefulness of the Corbino geometry for transport measurement in topological insulators.

ACrX2 (A = Cu, Ag; X = S, Se) compounds have a layered triangular structure similar to that of delafossites. In addition to a large variety of physical and chemical properties found in centrosymmetric delafossites, the broken inversion symmetry along the c-axis in this system leads to the emergence of additional functionalities such as piezoelectric, pyroelectric, and/or nonlinear optical properties. We found that the chemical vapor transport (CVT) technique using CrCl3 was the best growth method for ACrX2. In the case of A = Cu, however, a standard CVT procedure always produced a certain amount of a secondary phase, identified as the CuCr2X4 spinel, which significantly affected magnetic and transport properties. By a modified CVT technique with appropriate heat treatments, pure single crystals of CuCrX2 were successfully grown. The resistivity of ACrX2 was systematically changed by the combinations of A and X atoms. In contrast to metallic selenides, sulfides were confirmed to be insulators with giant an...

Magnetoresistance (MR) was measured in a La1.48Nd0.4Sr0.12CuO4 single crystal in perpendicular magnetic fields H up to 9 T in the region of the structural transition from the low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase. The hysteretic MR exhibits discrete jumps or avalanches only when the transition is approached from the LTT phase, and only during the first field sweep. The properties of the hysteresis are found to be independent of the field driving rate. The results are consistent with the presence of magnetostructural domains coupled with the charge degrees of freedom.