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For the inverted organic solar cells (OSCs), the interface contacts between the ZnO electron transporting layer and the organic active layer play an important role in the device performance and stability. Since the solution-processed ZnO surface always contains some base or zinc salt contaminants, we explored how the surface pH conditions influence the performance and stability of the nonfullerene acceptor (NFA) cells. A tight relationship between the surface pH condition and the device performance and stability was established. Specifically, device performance and stability were improved by treating the ZnO films with acid solutions but worsened after base treatment. The large number of hydroxyl groups on the surface of the solution-processed ZnO films was proved to be the main reason for the surface pH condition-related performance, which caused oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindered the photogenerated charge transfer and collection, and consequently resulted in low short-circuit current density and power conversion efficiency (PCE). The surface -OH groups also boosted the photocatalytic activity and led to fast degradation of the nonfullerene acceptor. Removal of the surface -OH groups can alleviate such problems. Different acid solutions, ZrAcac, 2-phenylethylmercaptan (PET), and glutamic acid (GC), were used to treat the ZnO films, and PET treatment was the most effective treatment for performance improvement. An efficiency of 16.46% was achieved for the PM6:Y6 cells and the long-term stability under continuous illumination conditions was significantly improved with a T80 lifetime of over 4000 h (4410 h), showing the excellent long-term stability of this heterojunction solar cell. Our understanding of the surface pH condition-related device performance and stability would guide the development of a feasible method for solving the interface problems in OSCs. We also provide a practical strategy to modify ZnO with acid solutions for high-performance and stable NFA OSCs.

Although perovskite solar cells have achieved great development in the device performance, the device stability is still a crucial issue hinder the further development. The ion migration is a main reason for the poor stability of the perovskite solar cells. Here, we use a cross-linking material, polydimethylsiloxane (PDMS) to dope the perovskite layer to improve the operational stability. We systematically studied the effect of curing agent content in PDMS on the device performance and stability. The characterization of perovskite film indicates that PDMS primarily doped in the perovskite layer and as well as enriched at the surface of perovskite film. Doping of PDMS promoted the crystallization of perovskite phase, leading to increase of short circuit density (JSC) and fill factor (FF) of the solar cells. In addition, PDMS doping improved the operational stability of device due to the suppressing the migration of iodide ions during aging. Moreover, the higher content of curing agents used in PDMS, the better stability of the devices. This work indicates PDMS doping the perovskite layer with a suitable content of curing agent can produce a better operational stability of the perovskite solar cells.

n–i–p Type perovskite solar cells generally require air oxidation of the Spiro-OMeTAD layer to achieve high power conversion efficiency (PCE). However, the detailed oxidation mechanism is still not fully understood. In this paper, oxidation of Spiro-OMeTAD was demonstrated via a non-contact electrochemical route using UV-Vis absorption, laser beam induced current (LBIC) imaging and secondary ion mass spectrometry (SIMS) profiling of the Spiro-OMeTAD films. At the cathode, oxygen is reduced to form OH− with the help of H2O, while the anodic reaction is the oxidation of Spiro-OMeTAD to form Spiro-OMeTAD+. Diffusion of Li+ towards the surface of the Ag electrode completes the electrochemical cycle and increases the conductivity of the hole-transporting layer. SIMS analyses of the completed devices demonstrate that the oxidation of Spiro-OMeTAD also leads to migration of Li+ through the perovskite layer into SnO2, which supposedly leads to an increase of the built-in voltage. We verify these results by incorporation of the experimentally measured Li+ concentration into a numerical drift-diffusion simulation, to replicate solar cell J–V-curves. This work provides a new insight into the oxidation of Spiro-OMeTAD in perovskite solar cells, and demonstrates that Li+ migration is involved in the oxidation of Spiro-OMeTAD.

Microscopic imaging of molecules is important for determination of molecular structures by providing real space snapshots. The scanning tunneling microscope (STM) is able to offer local perturbatio...

The growth of graphene on Ni magnetic films is of great significance for graphene spintronics, whereas the existence of grain boundaries and twin crystal structures in Ni films is an obstacle for obtaining the large-scale and uniform graphene. In this paper, an epitaxial wafer-scale single-crystalline Ni(111) film with the flat and clean surface was successfully prepared on the commercial α-Al2O3(0001) substrate by a two-step method, which was demonstrated with several characterization methods. According to the abnormal grain growth mechanism, the clean and uniform sapphire surface plays a key role for the single crystallization of Ni films as it induces a weak interface energy difference between two atomic stacking structures (ABC and ACB), thus stimulating the evolution of Ni films from (111) out-of-plane textures to single crystals. Furthermore, an ultra-flat and wrinkle-free graphene monolayer was synthesized on the prepared Ni film, which further verified its high quality and effectiveness.

Ni-rich layered oxides (LiNixMnyCozO2, x ≥ 0.6, x + y + z = 1) are promising positive electrode materials for high energy density lithium-ion batteries thanks to their high specific capacity. Howev...

Interfacial electron-phonon coupling in ultrathin films has attracted much interest recently. Here, by combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we report quantized electronic states and strong interfacial electron-phonon coupling in ultrathin Yb films on graphite. We observed clear kinks in the energy-momentum dispersion of quantum well states, and the kink positions agree well with the energies of optical phonons of graphite. The extracted coupling strength $\ensuremath{\lambda}$ is largest for the thinnest film with a preferred (``magic'') thickness of four monolayers and exhibits a strong band dependence, which can be qualitatively accounted for by a simple model. The interfacial electron-phonon coupling also gives rise to characteristic steplike structures in the $dI/dV$ spectra, implying dominant coupling with the phonons with zero in-plane momentum. A Lifshitz transition occurs at higher coverage, where quantum well states derived mainly from $5d$ electrons dominate near the Fermi level and possess large effective mass (up to $\ensuremath{\sim}19\phantom{\rule{0.28em}{0ex}}{m}_{e}$). Our results highlight the potentially important role of interfacial electron-phonon interaction for ultrathin films and provide spectroscopic insight to understand this cross-interface fermion-boson interaction.

The scattering process induced by impurities in graphene plays a key role in transport properties. Especially, the disorder impurities can drive the ordered state with a hexagonal superlattice on graphene by electron-mediated interaction at a transition temperature. Using angle-resolved photoemission spectroscopy (ARPES), we reveal that the epitaxial monolayer and bilayer graphene with various impurities display global elastic intervalley scattering and quantum interference below the critical temperature (34 K), which leads to a set of new folded Dirac cones at the Brillouin-zone center by mixing two inequivalent Dirac cones. The Dirac electrons generated from intervalley scattering without chirality can be due to the breaking of the sublattice symmetry. In addition, the temperature-dependent ARPES measurements indicate the thermal damping of quantum interference patterns from Dirac electron scattering on impurities. Our results demonstrate that the electron scattering and interference induced by impurities can completely modulate the Dirac bands of graphene.

As a special order of electronic correlation induced by spatial modulation, the charge density wave (CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning-tunneling microscopy in various temperatures, we observe a new (2×1) CDW phase besides the (√7×√3) CDW phase in epitaxial monolayer 1T-VSe2 film. Combining the variable-temperature angle-resolved photoemission spectroscopic (ARPES) measurements, we discover an anisotropic CDW gap and a two-step transition associated with the different CDW phases, which were observed below 135 K for the (√7×√3) CDW phase and between 135 K to 330 K for the (2×1) CDW phase respectively. The (√7×√3) CDW phase results a full gap, while the (2×1) CDW phase shows highly momentum dependence and results a partial gap structure at the Fermi surface. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure. Our findings provide significant information and deep understanding on the CDW phases in monolayer 1T-VSe2 film as a 2D material.

Selective C(sp3) −H activation is of fundamental importance in processing alkane feedstocks to produce high-value-added chemical products. By virtue of an on-surface synthesis strategy, we report selective cascade dehydrogenation of n-alkane molecules under surface constraints, which yields monodispersed all-trans conjugated polyenes with unprecedented length controllability. We are also able to demonstrate the generality of this concept for alkyl-substituted molecules with programmable lengths and diverse functionalities, and more importantly its promising potential in molecular wiring., Direct transformation of n-alkanes and alkyl-substituted derivatives has been achieved under surface constraints to yield conjugated polyenes with programmed length and functionalities.

The phenomenon of oxygen incorporation induced superconductivity in iron telluride (Fe1+yTe, with antiferromagnetic (AFM) orders) is intriguing and quite different from the case of FeSe. Until now, the microscopic origin of the induced superconductivity and the role of oxygen are far from clear. Here, by combining in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and x-ray photoemission spectroscopy (XPS) on oxygenated FeTe, we found physically adsorbed O2 molecules crystallized into c(2/3x2) structure as an oxygen overlayer at low temperature, which was vital for superconductivity. The O2 overlayer were not epitaxial on the FeTe lattice, which implied weak O2-FeTe interaction but strong molecular interactions. Energy shift observed in the STS and XPS measurements indicated hole doping effect from the O2 overlayer to the FeTe layer, leading to a superconducting gap of 4.5 meV opened across the Fermi level. Our direct microscopic probe clarified the role of oxygen on FeTe and emphasized the importance of charge transfer effect to induce superconductivity in iron-chalcogenide thin films., Comment: 15 pages,4 figures

Two-dimensional honeycomb crystals have inspired intense research interest for their novel properties and great potential in electronics and optoelectronics. Here, through molecular beam epitaxy on SrTiO3(001), we report successful epitaxial growth of metal-rich chalcogenide Fe2Te, a honeycomb-structured film that has no direct bulk analogue, under Te-limited growth conditions. The structural morphology and electronic properties of Fe2Te are explored with scanning tunneling microscopy and angle resolved photoemission spectroscopy, which reveal electronic bands cross the Fermi level and nearly flat bands. Moreover, we find a weak interfacial interaction between Fe2Te and the underlying substrates, paving a newly developed alternative avenue for honeycomb-based electronic devices., Comment: 12 pages,5 figures

The nature of the parent compound is intimately connected to the doping-induced emergence of superconductivity and the pairing mechanism in high-temperature superconductors. In high-temperature cuprate superconductors, the parent phase is believed to be a Mott insulator and superconductivity is realized by doping the Mott insulator. In iron-based superconductors, on the other hand, the parent compounds discovered so far are all bad metals which are either antiferromagnetic or nematic. Here we report direct spectroscopic evidence that the parent phase of the single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_{3}$ (FeSe/STO) films is insulating. It represents a case in the iron-based superconductors in which the parent compound is an insulator. We also find that the single-layer FeSe/STO films exhibit a distinct doping evolution to superconductivity that is similar to doping a Mott insulator in cuprate superconductors. These observations provide key insights in understanding the record high- ${T}_{C}$ superconductivity in single-layer FeSe/STO films.

Recently, the observations of spontaneous out-of-plane electric polarization and ferroelectric switching in WTe2 related devices with semimetal property a have generated considerable attentions. The polar structure of WTe2 was proposed to be vital, however there is still no consensus about the physical origin, which deserves detailed surface analysis. Here, by using angle-dependent X-ray photoelectron spectroscopy (ADXPS), we observed an obvious downward band bending in WTe2 after slightly surface oxidation, which can't be observed on the freshly cleaved WTe2. And it will disappear after fully oxidation, demonstrating strong correlation between band bending and surface oxidation. Our study suggests surface oxidation and resulted band bending in WTe2 should be taken into account in the WTe2-related devices, such as ferroelectric.

The stability of the self-assembled RS–Au–SR (R = CH2CH3)/Au(111) interface at room temperature has been investigated using scanning tunneling microscopy (STM) in conjunction with density functiona...

The accuracy and error propagation for determining the low specific contact resistance of Ohmic contacts on III-V wide band-gap semiconductors based on the circular transmission line model have been analyzed and the validity of this method is discussed in detail. The accuracy is more susceptible to the factors including data fitting method, electrical measurement technique and contact area correction. By using the equations of the original circular transmission line model to extract the fitting parameters, the calculation accuracy is much improved and the inapplicability of the linear least-square fitting is prevented. To further improve the accuracy, a four-probe current-voltage measurement technique was adopted to reduce the parasitic series resistances and the uncertainty bound, especially for the Ohmic contact with low sheet resistance of the semiconductor. Moreover, we have studied the size effect of contact pads of patterns and demonstrated that contact area correction is necessary for the semiconductor with high sheet resistance. A comprehensive error analysis is also performed to fully understand all the impact factors on this advanced method of specific contact resistance measurement, which is benefit for device performance evaluation and failure analysis.

Surface band bending and composition of Ga-polar n-GaN with different surface treatments were characterized by using angular dependent X-ray photoelectron spectroscopy. Upward surface band bending of varying degree was observed distinctly upon to the treatment methods. Besides the nitrogen vacancies, we found that surface states of oxygen-containing absorbates (O-H component) also contribute to the surface band bending, which lead the Fermi level pined at a level further closer to the conduction band edge on n-GaN surface. The n-GaN surface with lower surface band bending exhibits better linear electrical properties for Ti/GaN Ohmic contacts. Moreover, the density of positively charged surface states could be derived from the values of surface band bending.

We investigate electrical manipulation of magnetism in Ta/Pd/Co/HfO2 films probed by anomalous Hall Effect (AHE). The coercivity of the one monolayer cobalt and the transition temperature for the disappearance of usual AHE curves could be modified by a low voltage below ±2 V via the usage of ionic liquid. The unexpected emergence of inverted Hall loops above the transition temperature but far below the Curie temperature performed on the magnetometer could be explained by the competition between skew scattering and intrinsic/side-jump contributions of AHE. Our finding provides a different avenue for exploring the origin of AHE by electrical manipulation.

We present a systematic study of surface band bending in Ga-polar n-GaN with different Si doping concentrations by angular dependent X-ray photoelectron spectroscopy (ADXPS). The binding energies of Ga 3d and N 1 s core levels in n-GaN films increase with increasing the emission angle, i. e., the probing depth, suggesting an upward surface band bending. By fitting the Ga 3d core level spectra at different emission angles and considering the integrated effect of electrostatic potential, the core level energy at the topmost surface layer is well corrected, therefore, the surface band bending is precisely evaluated. For moderately doped GaN, the electrostatic potential can be reflected by the simply linear potential approximation. However, for highly doped GaN samples, in which the photoelectron depth is comparable to the width of the space charge region, quadratic depletion approximation was used for the electrostatic potential to better understand the surface band bending effect. Our work improves the knowledge of surface band bending determination by ADXPS and also paves the way for studying the band bending effect in the interface of GaN based heterostructures.

Efficient electrical switching of antiferromagnets (AFMs) is key to their use in high-density, ultrafast, nonvolatile spintronic memory. Mn${}_{2}$Au, an AFM with opposite spin sublattices, is a unique metallic material, in that fieldlike spin torque can switch its AFM moments. However, switching induced by antidamping torque remains to be verified in metallic AFMs. Here the authors demonstrate current-induced switching of AFM moment in both a (103)-oriented Mn${}_{2}$Au single layer and a Mn${}_{2}$Au/Pt heterojunction by fieldlike torque and antidamping torque respectively. The simultaneous realization of both torque types in metallic Mn2Au makes it a promising candidate for AFM spintronics.

Current-driven domain-wall motion has attracted intense attention, due to its rich physics and promising application in ``racetrack'' nonvolatile memory. However, the interplay between antiferromagnetic/ferromagnetic (AFM/FM) phase boundaries and current has remained elusive. To investigate, the authors use pulses of current to induce the formation of anisotropic FM domains in an AFM matrix. The phase boundaries can be reversibly oriented by the applied (writing) current, giving rise to the orthogonal alignment of phase boundaries and current, which could pave the way for multistage digital memory based on AFM/FM boundaries.

The WSe2 monolayer in 1T' phase is reported to be a large-gap quantum spin Hall insulator, but is thermodynamically metastable and so far the fabricated samples have always been in the mixed phase of 1T' and 2H, which has become a bottleneck for further exploration and potential applications of the nontrivial topological properties. Based on first-principle calculations in this work, it is found that the 1T' phase could be more stable than 2H phase with enhanced interface interactions. Inspired by this discovery, SrTiO3 (100) is chosen as substrate and WSe2 monolayer is successfully grown in a 100% single 1T' phase using the molecular beam epitaxial method. Combining in situ scanning tunneling microscopy and angle-resolved photoemission spectroscopy measurements, it is found that the in-plane compressive strain in the interface drives the 1T'-WSe2 into a semimetallic phase. Besides providing a new material platform for topological states, the results show that the interface interaction is a new approach to control both the structure phase stability and the topological band structures of transition metal dichalcogenides.

Superconducting and topological states are two most intriguing quantum phenomena in solid materials. The entanglement of these two states, the topological superconducting state, will give rise to even more exotic quantum phenomena. While many materials are found to be either a superconductor or a topological insulator, it is very rare that both states exist in one material. Here, we demonstrate by first-principles theory as well as scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy experiments that the recently discovered 'two-dimensional (2D) superconductor' of single-layer FeSe also exhibits 1D topological edge states within an energy gap of ∼40 meV at the M point below the Fermi level. It is the first 2D material that supports both superconducting and topological states, offering an exciting opportunity to study 2D topological superconductors through the proximity effect.

A monolayer 1T′-WTe2 film is grown on SrTiO3(001) with in-plane tensile strain. A height of ∼0.7 nm, obvious charge transfer, and incommensurate charge fluctuations in 1T′-WTe2 suggest strong coupling to the STO substrate. Scanning tunneling spectroscopy on the surface reveals that a large energy gap opens at the Fermi level with nearly zero conductance. The opened energy gap decreases with the increase in the WTe2 island size. The lack of the metallic edge state on monolayer 1T′-WTe2/SrTiO3(001) indicates the absence of the quantum spin Hall (QSH) state. Our study here demonstrates that the energy gap of monolayer 1T′-WTe2 can be tuned by lattice strain and illustrates the importance of interface coupling to realize the metallic edge state and QSH in monolayer 1T′-WTe2.

Silver nanowires (Ag NWs) network is an excellent candidate as flexible transparent electrode applying in flexible perovskite solar cells, owing to their excellent electrical and optical properties. However, several problems including large surface roughness, chemical reaction between Ag and perovskite precursor, and migration of Ag limited the application in high performance perovskite solar cells. Aiming to solve these problems, the composite electrode combining the spray coated Ag nanowires and the low-temperature sol-gel zinc oxide was developed in this work. The optimized concentration and annealing temperature of sol-gel ZnO were 0.45 M and 150 °C. The introduction of zinc oxide with suitable concentration caused slight impact on the transmittance and sheet resistant of transparent composite electrode, and promoted mechanical and chemical stability in air relative to the pristine Ag NWs electrode. The use of the composite flexible electrode could decrease the surface roughness of the Ag NWs electrode, passivate the reaction of perovskite and silver electrode, and prevent the migration of Ag. As a consequence, the performance of the flexible device significantly improved from 9.51% to 13.12%. Under the AM 1.5G constant illumination, the optimization device has a remarkably improved stability than pristine device. This study demonstrates that spray Ag NWs as bottom electrode is suitable for flexible perovskite solar cells. Meanwhile, it's an effective method using sol-gel ZnO to construct composite electrode to promote the device performance and stability.

Contact property is now becoming to be a key factor for achieving high performance and high reliability in GaN-based III-V semiconductor devices. Energetic ion sputter, as an effective interface probe, is widely used to profile the metal/GaN contacts for interfacial analysis and process optimization. However, the details of ion-induced interfacial reaction, as well as the formation of sputter by-products at the interfaces are still unclear. Here by combining state-of-the-art Ar+ ion sputter with in-situ X-ray photoelectron spectroscopy (XPS) and ex-situ high resolution transmission electron microscopy (HRTEM), we have observed clearly not only the ion-induced chemical state changes at interface, but also the by-products at the prototypical Ti/GaN system. For the first time, we identified the formation of a metallic Ga layer at the GaOx/GaN interface. At the Ti/GaOx interface, TiCx components were also detected due to the reaction between metal Ti and surface-adsorbed C species. Our study reveals that the corresponding core level binding energy and peak intensity obtained from ion sputter depth profile should be treated with much caution, since they will be changed due to ion-induced interface reactions and formation of by-products during ion bombardment.

With zero net magnetic moment for ultrafast switching and high resistance to interference, antiferromagnets are of keen interest for next-generation data storage. This study demonstrates and analyzes current-induced switching of antiferromagnetic moment in Mn${}_{2}$Au films at room temperature, with different film orientations exhibiting various switching characteristics. This strong orientation dependence of switching adds another dimension to our thinking about spin-orbit torque, and makes this alloy a versatile basis for antiferromagnetic spintronics.

We investigate an unconventional resistive switching (RS) behavior in La0.67Sr0.33MnO3/BaTiO3/metal (LSMO/BTO) ferroelectric tunnel junctions (FTJs), which is dominated by the variation of the barrier potential profile modulated by the migration of oxygen vacancies in the p-LSMO/n-BTO junction. The LSMO/BTO/Co junction exhibits a remarkable self-rectifying effect ascribed to the high-density interface state at the BTO/Co interface, in contrast to the symmetric conductivity when the top metal electrode is inert Pt. The effects of ferroelectric polarization on the RS behavior are also emphasized. Our work builds a bridge between FTJs and resistive random access memory devices.

In high temperature cuprate superconductors, it is now generally agreed that the parent compound is a Mott insulator and superconductivity is realized by doping the antiferromagnetic Mott insulator. In the iron-based superconductors, however, the parent compound is mostly antiferromagnetic metal, raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture. It has been proposed theoretically that the parent compound of the iron-based superconductors may be on the verge of a Mott insulator, but so far no clear experimental evidence of doping-induced Mott transition has been available. Here we report an electronic evidence of an insulator-superconductor transition observed in the single-layer FeSe films grown on the SrTiO3 substrate. By taking angle-resolved photoemission measurements on the electronic structure and energy gap, we have identified a clear evolution of an insulator to a superconductor with the increasing doping. This observation represents the first example of an insulator-superconductor transition via doping observed in the iron-based superconductors. It indicates that the parent compound of the iron-based superconductors is in proximity of a Mott insulator and strong electron correlation should be considered in describing the iron-based superconductors., 15 pages, 4 figures

Spin current generated by spin Hall effect in the heavy metal would diffuse up and down to adjacent ferromagnetic layers and exert torque on their magnetization, called spin-orbit torque. Antiferromagnetically coupled trilayers, namely the so-called synthetic antiferromagnets (SAF), are usually employed to serve as the pinned layer of spintronic devices based on spin valves and magnetic tunnel junctions to reduce the stray field and/or increase the pinning field. Here we investigate the spin-orbit torque in MgO/CoFeB/Ta/CoFeB/MgO perpendicularly magnetized multilayer with interlayer antiferromagnetic coupling. It is found that the magnetization of two CoFeB layers can be switched between two antiparallel states simultaneously. This observation is replicated by the theoretical calculations by solving Stoner-Wohlfarth model and Landau-Lifshitz-Gilbert equation. Our findings combine spin-orbit torque and interlayer coupling, which might advance the magnetic memories with low stray field and low power consumption., Comment: 21 pages, 4 figures, Accepted by Phys. Rev. B

Determination of the pairing symmetry in monolayer FeSe films on SrTiO3 is a requisite for understanding the high superconducting transition temperature in this system, which has attracted intense theoretical and experimental studies but remains controversial. Here, by introducing several types of point defects in FeSe monolayer films, we conduct a systematic investigation on the impurity-induced electronic states by spatially resolved scanning tunneling spectroscopy. Ranging from surface adsorption, chemical substitution to intrinsic structural modification, these defects generate a variety of scattering strength, which renders new insights on the pairing symmetry., Comment: to be published in Phys. Rev. B Regular Article

The frustration of magnetization and conductivity properties of ultrathin manganite is detrimental to their device performance, preventing their scaling down process. Here we demonstrate that the magnetism of ultrathin $\mathrm{LaMn}{\mathrm{O}}_{3}$ films can be restored by a $\mathrm{SrTi}{\mathrm{O}}_{3}$ capping layer, which engineers the surface from a symmetry breaking induced out-of-plane orbital occupancy to the recovered in-plane orbital occupancy. The stabilized in-plane orbital occupancy would strengthen the intralayer double exchange and thus recovers the robust magnetism. This method is proved to be effective for films as thin as 2 unit cells, greatly shrinking the critical thickness of 6 unit cells for ferromagnetic $\mathrm{LaMn}{\mathrm{O}}_{3}$ as demonstrated previously [Wang et al., Science 349, 716 (2015)]. The achievement made in this work opens up new perspectives to an active control of surface states and thereby tailors the surface functional properties of transition metal oxides.

The surface band bending in Ga-polar n-type GaN surfaces, as well as the effect of Si doping levels and in situ Ar+ ion processing on band bending, was systematically investigated. To precisely determine the valence band maximum (VBM) of GaN beyond instrumental and material surface environments by XPS, a valence band feature fitting procedure based on photoemission spectra and theoretical densities of states has been developed. Poisson calculation with quadratic depletion approximation on surface potential has been used to model the band bending and further correct the VBM energy. Then, the actual surface band bending was correctly evaluated. Upward band bending of 1.55 ± 0.03 eV with highly Si doped n-GaN, which is about 0.88 eV higher than that of the moderately doped sample, was found. After in situ Ar+ plasma treatment, the varying degree of band bending was observed distinctly depending on the Si doping density. The surface components associated with the Ga/N ratio and Ga–O bonding concentration on the n-GaN surface have been used to evaluate the contribution to surface band bending.The surface band bending in Ga-polar n-type GaN surfaces, as well as the effect of Si doping levels and in situ Ar+ ion processing on band bending, was systematically investigated. To precisely determine the valence band maximum (VBM) of GaN beyond instrumental and material surface environments by XPS, a valence band feature fitting procedure based on photoemission spectra and theoretical densities of states has been developed. Poisson calculation with quadratic depletion approximation on surface potential has been used to model the band bending and further correct the VBM energy. Then, the actual surface band bending was correctly evaluated. Upward band bending of 1.55 ± 0.03 eV with highly Si doped n-GaN, which is about 0.88 eV higher than that of the moderately doped sample, was found. After in situ Ar+ plasma treatment, the varying degree of band bending was observed distinctly depending on the Si doping density. The surface components associated with the Ga/N ratio and Ga–O bonding concentration on t...

An abnormal aging phenomenon is reported for GaN-based near-ultraviolet laser diodes (LDs). Under an electrical stress for several minutes, the threshold current of the LDs decreased, while the light output power and the operation voltage increased. The amplitude of the abnormal aging phenomena was found to be mainly related to the excess Mg concentration in the p-AlGaN electron blocking layer (EBL). It almost disappeared when the Mg concentration in the AlGaN EBL was reduced to 2??×??1019 cm?3. We propose that this phenomenon was related to the Mg?VN?H complex defects formed in highly doped AlGaN EBL ([Mg]??>??3??×??1019 cm?3).

As a member of ZrHM (H = Si/Ge/Sn; M = O/S/Se/Te) family materials, which were predicted to be the candidates of topological Dirac nodal-line semimetals, ZrGeSe exhibited particular properties, such as magnetic breakdown effect in the transport measurement, different from its other isostructural compounds, informing an unique topology of the electronic band structure. However, the related experimental research is insufficient until now. Here, we present a systematic study of the band structure and Fermi surfaces (FS) of ZrGeSe by angle-resolved photoemission spectroscopy (ARPES). Our Brillouin zone (BZ) mapping shows multiple Fermi pockets such as the diamond-shaped FS around the zone center Γ point, small electron pocket encircling the X point of the BZ, and lenses-shaped FS in the Γ-M direction. The obtained Fermi velocities and effective masses were up to 9.2 eV·A and 0.42 me, and revealing an anisotropic electronic property along different high-symmetry k-space directions. Moreover, a kink appears near the Fermi level in the linear Dirac bands along the M-X direction, probably originated from the band hybridization and has not been reported in other ZrHM-type materials. Our findings support that the ZrHM-type material family can be a new platform on which to explore exotic states of quantum matter.

The spin valve effect driven by the magnetic field in sandwich structures stands out as a seminal phenomenon in the emerging field of spintronics, but it has not been realized in low-power-dissipation devices operated via electrical means. Here, we investigate spin-orbit torque switching in a perpendicularly magnetized CoFeB/Mo/CoFeB sandwich, where the spin current generated in the Mo layer flows upward and downward to the ferromagnetically coupled CoFeB layers. When scanning the current, two CoFeB layers can be switched from one antiparallel state to a parallel state and then to the other antiparallel state, producing an unprecedentedly current-dependent spin valve effect. Such a spin valve effect is sensitive to the additional magnetic field direction due to the small spin torque efficiency and efficiency difference of the two CoFeB layers. The experimental observations are supported by the dynamics simulation based on the Landau-Lifshitz-Gilbert equation. Besides the fundamental interest, our finding would add a different dimension to energy efficient memory devices and sensors.

We investigate the film thickness dependent metal-to-insulator transition temperature $({T}_{\mathrm{MIT}})$ of $\mathrm{NdNi}{\mathrm{O}}_{3}$ films under tensile and compressive strain states. For the films exceeding the critical thickness for strain relaxation, ${T}_{\mathrm{MIT}}$ varies gradually with the film thickness caused by strain relaxation. The variation tendency differs dramatically for the films below the critical thickness: an increase (decrease) of ${T}_{\mathrm{MIT}}$ with increasing the film thickness for the case of tensile (compressive) strain, which is attributed to the decaying of orbital polarization. As the overlap of $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2{p}_{x,y}$ orbits with $\mathrm{Ni}\phantom{\rule{0.16em}{0ex}}3d{x}^{2}\ensuremath{-}{y}^{2}$ orbits determines ${T}_{\mathrm{MIT}}$, a decrease of ${x}^{2}\ensuremath{-}{y}^{2}$ orbital occupation with increasing film thickness would reduce the orbital overlap and resultant enhanced ${T}_{\mathrm{MIT}}$ for tensile strained films, while their compressive counterparts do the opposite. Our findings identify the importance of orbital polarization in regulating the metal-to-insulator transitions, opening up a new perspective for orbital physics in transition metal oxides.

The significant role of interfacial coupling on the superconductivity enhancement in FeSe films on SrTiO3 has been widely recognized. But the explicit origination of this coupling is yet to be identified. Here by surface phonon measurements using high resolution electron energy loss spectroscopy, we found electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3 can penetrate into FeSe films and strongly interact with electrons therein. The mode-specific electron-phonon coupling (EPC) constant for the ~92 meV F-K phonon is ~0.25 in the single-layer FeSe on SrTiO3. With increasing FeSe thickness, the penetrating field intensity decays exponentially, which matches well the observed exponential decay of the superconducting gap. It is unambiguously shown that the SrTiO3 F-K phonon penetrating into FeSe is essential in the interfacial superconductivity enhancement., Main text: 6 pages, 3 figures; Supplementary Material: 8 pages, 6 figures. An error in the EPC constant calculation was corrected in this version and several new references are added. The authors would like to thank Dr. Yan Wang (University of Tennessee) for pointing out the error

Alkali-metal (potassium) adsorption on FeSe thin films with thickness from 2 unit cells (UC) to 4 UC on $\mathrm{SrTi}{\mathrm{O}}_{3}$ grown by molecular beam epitaxy is investigated with a low-temperature scanning tunneling microscope. At appropriate potassium coverage (0.20--0.25 monolayer), the tunneling spectra of the films all exhibit a superconductinglike gap which is overall larger than 11 meV (five times the gap value of bulk FeSe) and decreases with increasing thickness, and two distinct features of characteristic phonon modes at \ensuremath{\sim}11 and \ensuremath{\sim}21 meV. The results reveal the critical role of the interface-enhanced electron-phonon coupling for possible high-temperature superconductivity in ultrathin FeSe films on $\mathrm{SrTi}{\mathrm{O}}_{3}$ and is consistent with recent theories. Our study provides compelling evidence for the conventional pairing mechanism for this type of heterostructure superconducting system.

We report the formation of an ordered two-dimensional gold–silicon (Au/Si) alloy by deposition of a sub-monolayer of Si onto Au(1 1 1). Scanning tunneling microscope (STM) images show that at room temperature the Au/Si alloy consists of parallel rows with row-spacing 0.47 nm. These rows appear to float on Au(1 1 1) in bundles. Each bundle of rows can bend quite freely forming arches of arbitrary curvature. By annealing to 300 °C in ultra-high vacuum, the alloy changes its structure by forming straight rows with row spacing 0.35 nm.

Thermal stability of metal/n-GaN contact is critical for its applications in microelectronic and optoelectronic devices. Metal Ti is generally used to make Ohmic contact on n-GaN after high temperature annealing, and the key factor is to form TiN at the interface. To reduce the processing temperature and improve the reliability, metallic titanium nitride (TiN) thin film has been proposed to substitute traditional metals (such as Ti) in the contact structures, due to its low work function and high blocking effect. For this novel approach, the first step is to fabricate high quality TiN films. Here, the authors adopted remote plasma-enhanced atomic layer deposition method to deposit TiN films under well-controlled conditions. Stoichiometric TiN films (Ti:N ∼ 1:1) with low oxygen contamination (

A single unit cell FeTe${}_{1-x}$Se${}_{x}$ film consisting of a flat square Fe layer sandwiched between two Se/Te monolayers shows a fully gapped tunneling spectrum with superconducting gaps up to ~16.5 meV, nearly ten times the gap value of an optimally doped bulk single crystal. Compared with the ${T}_{c}$~14.5 K of bulk FeTe${}_{0.6}$Se${}_{0.4}$, a gap of ~16.5 meV could correspond to a transition temperature higher than the boiling temperature of liquid-nitrogen (77 K). Initial $e\phantom{\rule{0}{0ex}}x$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ transport measurements reveal an onset superconducting transition temperature above 40 K, nearly two times higher than that of the optimally doped bulk single crystal FeTe${}_{0.6}$Se${}_{0.4}$.