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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.

Self-assembly represents a typical example of bottom-up nanotechnology where the formation of macroscopic structures is driven by forces operating at the molecular level. Under situations where several forces of comparable strength are competing with one another, the stable structure achieved depends on the fine balance of all the forces. A good example is the formation of self-assembled monolayers (SAMs) of non-branched alkanethiol molecules, CH3–(CH2)n–SH, on the surface of gold, where the interaction between the S headgroup and the gold substrate competes with the force acting between the alkane tails. Here we demonstrate that, if the tail is short, n ≤ 2, the S–Au interaction dictates the structure of the SAM. As the tail becomes longer than n = 2, the enhanced tail–tail interaction is able to displace the S headgroup. Our finding highlights the importance of the “weak” van der Waals interaction for molecular assembly even in the presence of chemical bonding.

The cis-trans isomerization of CH

Antiferromagnetic (AFM) materials, with robust rigidity to magnetic field perturbations and ultrafast spin dynamics, show great advantages in information storage and have developed into a fast-emerging field of AFM spintronics. However, a direct characterization of spin alignments in AFM films has been challenging, and their manipulation by lattice distortion and magnetic proximity is inevitably accompanied by ``ferromagnetic'' features within the AFM matrix. Here we resolve the $G$-type AFM structure of $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ and find that the interfacial AFM structure could be modulated intrinsically from in plane to out of plane with a canted angle of ${60}^{\ensuremath{\circ}}$ by the charge transfer and orbital reconstruction in $\mathrm{SrCo}{\mathrm{O}}_{2.5}/\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ heterostructures both experimentally and theoretically. Such an interfacial AFM reconfiguration caused by electronic reconstruction does not cause the ferromagnetic feature and changes the magnetization switching process of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ from in plane to perpendicular to the plane, in turn. Our study not only reveals the coupling between charge, orbital, and AFM structure, but also provides a unique approach to manipulating AFM structure.

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.

The cis–trans isomerization of CH3S-Au-SCH3 driven by the tip of the scanning tunneling microscope is investigated at 77 K. CH3S-Au-SCH3 anchored on the Au(111) surface with the S–Au–S axis parallel to the substrate functions as a molecular switch due to the flipping of the CH3 groups. The bonding between CH3S-Au-SCH3 and Au(111) leads to asymmetric isomerization where one of the two methyl groups flips much more effectively than the other, despite the symmetry of CH3S-Au-SCH3. Our findings suggest the possibility of constructing similar molecular switches that can be operated at room temperature and a potential route for fine-tuning of molecular switches in future nanoscale electro-mechanical devices.

Dissociation of ethanethiol and the formation of Au-adatom-diethylthiolate rows on the Au(111) surface were investigated using scanning tunneling microscopy (STM) at low temperature. Ethanethiol molecules physisorb on Au(111) at 120 K by sequentially occupation of the elbow site, the fcc domain before covering the whole surface with a semiliquid layer without long-range order. Scanning the physisorbed layer with a sample bias higher than +1.2 V leads to dissociation via cleaving the H-S bond. One of the dissociation products, ethylthiolate, forms a double-row structure with the rows aligned in one of the [112(-)] directions. These double rows arise from the Au-adatom-dithiolate species: CH(3)CH(2)S-Au-SCH(2)CH(3).

Surface structures of self-assembled methylthiolate and ethylthiolate monolayers on Au(111) have been imaged with STM. For saturation coverage of 0.33 ML at room temperature, the well-known (√3 × √3)R30° phase routinely observed for longer chain alkanethiolates does not appear under any conditions for adsorbed methylthiolate and ethylthiolate. Instead, both thiolate species organize themselves into a well-ordered 3 × 4 structure. We thus conclude that the stable structure for saturation coverage of methylthiolate/ethylthiolate on Au(111) at RT is 3 × 4, not (√3 × √3)R30° as generally believed. For coverage less than 0.33 ML, a striped-phase with short-range order is observed for methylthiolate. Fourier transform of the STM image from the striped-phase produces a clear (√3 × √3)R30° “diffraction” pattern. This strongly indicates that the (√3 × √3)R30° diffraction pattern for methylthiolate monolayers reported in literature is likely from the striped-phase, rather than from a true (√3 × √3)R30° lattice in real space. Consequently, theoretical modeling that reproduces the (√3 × √3)R30° structure for methylthiolate monolayers should be re-examined.

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.

Physical vapor deposition of gold onto a self-assembled monolayer (SAM) of octanethiol on Au(111) has been studied at the molecular level in ultra-high vacuum (UHV) using atomic-resolution scanning tunneling microscopy (STM). A specially prepared SAM with not only the usual etch pits but also co-existing phases and domain boundaries is used for the purpose of studying details of the nucleation process. Etch pits are found to be filled by deposited Au atoms. At the same time, preferential nucleation and growth of gold islands at intersections of different domains, as well as inside the domains of the less dense striped phase, is observed. We find no gold islands within the densely-packed (√3 × √3)R30° phase. High-resolution STM imaging shows that the SAM over the newly formed gold islands adopts the same structure as that in the immediate surroundings.

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 (

Self-assembled monolayers (SAMs) of Au-octanethiolate on Au(111) have been studied using scanning tunneling microscopy (STM). Thermal annealing of the dense (square root(3) x square root(3))R30 degrees layer at 353 K for 1 h leads to the formation of a (5 square root(3) x square root(3))R30 degrees striped phase coexisting with the (square root(3) x square root(3))R30 degrees phase. High-resolution STM imaging shows that the unit cell of the (5 square root(3) x square root(3))R30 degrees phase consists of four adsorbed Au-thiolate species giving rise to an adsorbate coverage of 0.27 ML. The four Au-thiolate species take the standing-up orientation and occupy inequivalent adsorption sites: one on a bridge site and three on the hollow sites. By drawing connections between the (5 square root(3) x square root(3))R30 degrees and the (square root(3) x square root(3))R30 degrees phases, it is found that the adsorption site for Au-thiolate inside the (square root(3)3 x square root(3))R30 degrees phase must be either the fcc hollow or the hcp hollow site.

The release of gold atoms from an octanethiol monolayer on Au(1 1 1) and the subsequent formation of single-layer-high gold islands have been investigated using a scanning tunnelling microscope (STM) in air. When the bias voltage between the STM tip and the sample is above the threshold for water electrolysis, reactive desorption of the thiol molecules takes place leading to the release of gold adatoms. The number of released atoms has been evaluated as a function of exposure to the tip current under both positive and negative bias voltages. Tip-induced ripening of the gold islands, and more interestingly, tip-induced disintegration of small islands are observed.

Confined C60 islands have been grown within octanethiol self-assembled monolayers (SAMs) on the Au(111) surface. The structure of the C60 islands is investigated by use of scanning tunneling microscopy (STM), and a single ordered phase of the close-packed C60 islands with a (2√3 × 2√3)R30° structure is found. In the vicinity of the close-packed C60 island there are loosely packed C60 molecules which are mixed with the thiolate species. Floating C60 on top of the SAM, in the form of either compact islands or individual molecules, is not found at room temperature and above. The diffusion of C60 molecules is strongly controlled by the structure of the SAM, and the (√3 × √3)R30° phase of the SAM is not permissible to diffusing C60 molecules.

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${}_{0.4}$.

With the recent realization of hybrid improper ferroelectricity and room-temperature multiferroic by tilt engineering, “functional” octahedral tilting has become a novel concept in multifunctional perovskite oxides, showing great potential for property manipulation and device design. However, the control of magnetism by octahedral tilting has remained a challenging issue. Here a qualitative and quantitative tilt engineering of exchange coupling, one of the magnetic properties, is demonstrated at compensated G-type antiferromagnetic/ferromagnetic (SrMnO3/La2/3Sr1/3MnO3) interfaces. According to interfacial Hamiltonian, exchange bias (EB) in this system originates from an in-plane antiphase rotation (a−) in G-type antiferromagnetic layer. Based on first-principles calculation, tilt patterns in SrMnO3 are artificially designed in experiment with different epitaxial strain and a much stronger EB is attained in the tensile heterostructure than the compressive counterpart. By controlling the magnitude of octahedral tilting, the manipulation of exchange coupling is even performed in a quantitative manner, as expected in the theoretical estimation. This work realized the combination of tilt engineering and exchange coupling, which might be significant for the development of multifunctional materials and antiferromagnetic spintronics.

Interface-enhanced high-temperature superconductivity in one unit-cell (UC) FeSe films on SrTiO3(001) (STO) substrate has recently attracted much attention in condensed matter physics and material science. By combined in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and ex-situ scanning transmission electron microscopy (STEM) studies, we report on atomically resolved structure including both lattice constants and actual atomic positions of the FeSe/STO interface under both non-superconducting and superconducting states. We observed TiO2 double layers (DLs) and significant atomic displacements in the top two layers of STO, lattice compression of the Se-Fe-Se triple layer, and relative shift between bottom Se and topmost Ti atoms. By imaging the interface structures under various superconducting states, we unveil a close correlation between interface structure and superconductivity. Our atomic-scale identification of FeSe/STO interface structure provides useful information on investigating the pairing mechanism of this interface-enhanced high-temperature superconducting system.

The recent observation of high-temperature superconductivity in one unit cell thick FeSe on SrTiO3 (1UC FeSe/STO)1 initiated a new pathway to realizing even higher transition temperature (Tc) superconductors and established a promising different paradigm to understand the mechanism of high temperature superconductivity2-10. However, independent confirmation on the high Tc value, the role of capping layer in reducing Tc, and the underlying mechanism of the novel superconductivity remain open questions11. Here we report a time-resolved study of the excited state ultrafast dynamics of 1UC FeSe/STO protected by 2UC FeTe capping layer, and identify the superconducting Tc to be 68 (-5/+2) K. We found a coherent acoustic phonon mode in the capping layer, which provides an additional decay channel to the gluing bosons and explains the reduced Tc value. Our investigation supports the phonon pairing scenario.

Layered manganate Ba 6 Mn 5 O 16 was prepared by a traditional solid-state reaction method and its microstructure at atomic level was investigated in detail by means of high-resolution transmission electron microscopy (HRTEM). Although the sample shows, from the XRD data, a nearly single-phase n =5 layered Ba 6 Mn 5 O 16 phase of the hexagonal Ba n +1 Mn n O 3 n +1 homologous series, the presence of numerous structural defects, especially intergrowth faults of the hexagonal Ba n +1 Mn n O 3 n +1 homologous series with different n in it, was revealed by HRTEM. Furthermore, a minor 2H BaMnO 3 phase was also found to coexist with the layered Ba 6 Mn 5 O 16 phase. These defects could have a correlation with the magnetic properties of the sample, i.e. the T N being very broad and the appearance of the Curie tail in the susceptibility.

The structural and electrical properties of infinite-layer CaCuO2 (IL CaCuO2) under high pressure at room temperature were studied using a diamond anvil cell by in situ high pressure energy-dispersive X-ray diffraction with synchrotron radiation and by simultaneous resistance and electrical capacitance measurements. The results indicate that the primary crystal structure of IL CaCuO2 is stable under pressure up to 30 GPa with an anisotropic compressibility. The equation of state of IL CaCuO2 was obtained from the V/V0−P relationship based on the Birch–Murnaghan equation, which gives rise to a bulk modulus B0=96 GPa in the low pressure range below 6 GPa, and B0=186 GPa at pressures from 6 to 30 GPa for IL CaCuO2. The resistance and capacitance measurements of IL CaCuO2 up to 20 GPa revealed several unusual changes. There is an abrupt resistance drop in the pressure range of 3–6 GPa followed by an abnormal hump occurring around 12 GPa with increasing pressure. Corresponding changes were also observed in the dependence of capacitance on pressure. The former drop is attributed to an isostructural phase transition as observed in the synchrotron radiation experiments. The latter is considered to be related to an electronic structure transition resulting from the anisotropic compression of the IL CaCuO2 unit cell under high pressure.

High-quality interface between high-mobility III–V compound semiconductor and high-k dielectrics is critical for achieving high electrical performance of the devices. Elemental diffusion through the high-k dielectrics from III–V semiconductors (e.g., InP, InAs, and InGaAs) upon the atomic layer deposition process and during the postdeposition annealing (PDA) at low temperature rises a concern with the reliability of III–V compound semiconductor devices. In this work, the thermal stability of GaP/high-k dielectrics interface has been studied in terms of elemental diffusion upon PDA, using angle-resolved X-ray photoelectron spectroscopy. The Ga and P oxides are below the detection limit of X-ray photoelectron spectroscopy for the GaP/HfO2 stack after PDA at 400 °C, and for the GaP/Al2O3 stack after PDA at 500 °C. Outdiffusion of Ga and P atoms through HfO2 film is detected by time-of-flight secondary ions mass spectrometry after PDA at 500 and 600 °C. Highly stable GaP/high-k dielectrics interfaces suggest that a thin barrier layer of wide bandgap GaP between the III–V semiconductor channel materials and high-k dielectrics would benefit the reliability of the devices.

Metal-insulator transition features as a transformation, from a highly charge conductive state to another state where charge conductivity is greatly suppressed when decreasing the temperature. Here we demonstrate two consecutive transitions in NdNiO3 films with CoFe2O4 capping, in which the metal-insulator transition occurs at 85 K, followed by an unprecedented insulator-metal transition below 40 K. The emerging insulator-metal transition associated with a weak antiferromagnetic behavior is observed in 20 unit cell-thick NdNiO3 with more than 5 unit cell CoFe2O4 capping. Differently, the NdNiO3 films with thinner CoFe2O4 capping only exhibit metal-insulator transition at 85 K, accompanied by a strong antiferromagnetic state below 40 K. Charge transfer from Co to Ni, instead of from Fe to Ni, formulates the ferromagnetic interaction between Ni-Ni and Ni-Co atoms, thus suppressing the antiferromagnetic feature and producing metallic conductive behavior. Furthermore, a phase diagram for the metal-insulator-metal transition in this system is drawn., Comment: 14 pages, 5 figures, to appear in Appl. Phys. Lett

We investigate the spin Hall magnetoresistance (SMR) in $\mathrm{SrMn}{\mathrm{O}}_{3}\phantom{\rule{0.16em}{0ex}}(\mathrm{SMO})/\mathrm{Pt}$ hybrids, where SMO is an antiferromagnetic (AFM) insulator. The AFM moments partially rotate with out-of-plane magnetic fields, producing room-temperature SMR. By manipulating the electron spins in Pt, we observe Larmor precession-induced oscillating SMR, reaffirming the spin current transport determined by the relative arrangement between the Pt electron spins and AFM moments. The use of the AFM with no net moments annihilates the magnetic proximity effect and thus confirms the SMR origination from AFM-controlled spin current transport, with significant spin mixing conductance of $\ensuremath{\sim}{10}^{17}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$. Our findings provide an interesting perspective to detecting AFM moments and represent a significant step towards AFM spintroincs.

We verify that the exchange bias effect unexpectedly emerges in a single ${\mathrm{LaMnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ film, one of the most studied correlated oxides. We combine x-ray absorption spectroscopy results, which serves as a fingerprint of the electronic structure, with microstructure characterizations and magnetization data to explore the origin of the exchange bias behavior. Taken together, these measurements provide compelling evidence that the formation of a Mn${}^{2+}$ component associated with the double exchange between Mn${}^{2+}$-O-Mn${}^{3+}$ produces robust ferromagnetism in the upper part of the film, which is exchanged coupled with the antiferromagnetic bottom part that is dominated by Mn${}^{3+}$. Thus, the Mn ions are found to be distributed unevenly in the depth profile, with vertical electronic phase separation, in contrast to the often accepted view of the lateral phase separation in manganites. The observation of exchange bias in a chemically single film paves the way for interface engineering induced by vertical electronic phase separation towards technological applications.

We prepare single-unit-cell FeSe films on insulating SrTiO3 substrates by molecular beam epitaxy and investigate the evolution of their superconducting properties with annealing by in situ scanning tunneling microscopy and scanning tunneling spectroscopy and ex situ transport measurements. We find that through an annealing process, the superconductivity of 1-uc FeSe films on SrTiO3 substrates develops with the formation of stoichiometric FeSe films and is further enhanced by charge transfer from SrTiO3 substrates to FeSe films. Moreover, the superconductivity is independent of the bulk property of the SrTiO3 substrate, regardless of whether it is insulating or conductive. Our results reveal that the high-temperature superconductivity of 1-uc FeSe films on SrTiO3 substrates indeed occurs at the FeSe/SrTiO3 interface, where the electron doping at FeSe films plays an important role in this interfacial superconductivity.

The latest discovery of possible high temperature superconductivity in the single-layer FeSe film grown on a SrTiO3 substrate, together with the observation of its unique electronic structure and nodeless superconducting gap, has generated much attention. Initial work also found that, while the single-layer FeSe/SrTiO3 film exhibits a clear signature of superconductivity, the double-layer FeSe/SrTiO3 film shows an insulating behavior. Such a dramatic difference between the single-layer and double-layer FeSe/SrTiO3 films is surprising and the underlying origin remains unclear. Here we report our comparative study between the single-layer and double-layer FeSe/SrTiO3 films by performing a systematic angle-resolved photoemission study on the samples annealed in vacuum. We find that, like the single-layer FeSe/SrTiO3 film, the as-prepared double-layer FeSe/SrTiO3 film is insulating and possibly magnetic, thus establishing a universal existence of the magnetic phase in the FeSe/SrTiO3 films. In particular, the double-layer FeSe/SrTiO3 film shows a quite different doping behavior from the single-layer film in that it is hard to get doped and remains in the insulating state under an extensive annealing condition. The difference originates from the much reduced doping efficiency in the bottom FeSe layer of the double-layer FeSe/SrTiO3 film from the FeSe-SrTiO3 interface. These observations provide key insights in understanding the origin of superconductivity and the doping mechanism in the FeSe/SrTiO3 films. The property disparity between the single-layer and double-layer FeSe/SrTiO3 films may facilitate to fabricate electronic devices by making superconducting and insulating components on the same substrate under the same condition., 19 pages, 4 figures

The interplay between orbital, charge, spin, and lattice degrees of freedom is at the core of correlated oxides. This is extensively studied at the interface of heterostructures constituted of two-layer or multilayer oxide films. Here, we demonstrate the interactions between orbital reconstruction and charge transfer in the surface regime of ultrathin (La,Sr)MnO3, which is a model system of correlated oxides. The interactions are manipulated in a quantitative manner by surface symmetry-breaking and epitaxial strain, both tensile and compressive. The established charge transfer, accompanied by the formation of oxygen vacancies, provides a conceptually novel vision for the long-term problem of manganites--the severe surface/interface magnetization and conductivity deterioration. The oxygen vacancies are then purposefully tuned by cooling oxygen pressure, markedly improving the performances of differently strained films. Our findings offer a broad opportunity to tailor and benefit from the entanglements between orbit, charge, spin, and lattice at the surface of oxide films.