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[Display omitted] Magnetic impurities in superconductors are of increasing interest due to emergent Yu-Shiba-Rusinov (YSR) states and Majorana zero modes for fault-tolerant quantum computation. However, a direct relationship between the YSR multiple states and magnetic anisotropy splitting of quantum impurity spins remains poorly characterized. By using scanning tunneling microscopy, we systematically resolve individual transition-metal (Fe, Cr, and Ni) impurities induced YSR multiplets as well as their Zeeman effects in the K 3 C 60 superconductor. The YSR multiplets show identical d orbital-like wave functions that are symmetry-mismatched to the threefold K 3 C 60 (1 1 1) host surface, breaking point-group symmetries of the spatial distribution of YSR bound states in real space. Remarkably, we identify an unprecedented fermion-parity-preserving quantum phase transition between ground states with opposite signs of the uniaxial magnetic anisotropy that can be manipulated by an external magnetic field. These findings can be readily understood in terms of anisotropy splitting of quantum impurity spins, and thus elucidate the intricate interplay between the magnetic anisotropy and YSR multiplets. [ABSTRACT FROM AUTHOR]

We demonstrate fluorine intercalation into Bi2Sr2CaCu2O8+x (BSCCO) flakes, as thick as 100 nm, by back-gating a single crystalline LaF3 substrate. Element-sensitive analysis not only confirms the presence of fluorine in BSCCO after gating but also reveals lateral diffusion of fluorine in BSCCO. Transport measurements further demonstrate the hole-doping effect of fluoride ions in BSCCO through effective modulation of the superconducting transitions. Our work introduces a distinct type of ions (F−) that can be intercalated into materials via solid state back-gating. It broadens the toolbox of ion back-gating and is useful for addressing exotic phenomena in the heavily hole doped regime. [ABSTRACT FROM AUTHOR]

Superconductivity transition temperature (T c) marks the inception of a macroscopic quantum phase-coherent paired state in fermionic systems. For 2D superconductivity, the paired electrons condense into a coherent superfluid state at T c, which is usually lower than the pairing temperature, between which intrinsic physics including Berezinskii–Kosterlitz–Thouless transition and pseudogap state are hotly debated. In the case of monolayer FeSe superconducting films on SrTiO3(001), although the pairing temperature (T p) is revealed to be 65–83 K by using spectroscopy characterization, the measured zero-resistance temperature (⁠|${{T}}_{{\rm c}}^0$|⁠) is limited to 20 K. Here, we report significantly enhanced superconductivity in monolayer FeSe films by δ-doping of Eu or Al on SrTiO3(001) surface, in which |${{T}}_{{\rm c}}^0$| is enhanced by 12 K with a narrowed transition width Δ T c ∼ 8 K, compared with non-doped samples. Using scanning tunneling microscopy/spectroscopy measurements, we demonstrate lowered work function of the δ-doped SrTiO3(001) surface and enlarged superconducting gaps in the monolayer FeSe with improved morphology/electronic homogeneity. Our work provides a practical route to enhance 2D superconductivity by using interface engineering. [ABSTRACT FROM AUTHOR]

The intrinsic magnetic topological insulator MnBi2Te4 provides a feasible pathway to the high-temperature quantum anomalous Hall (QAH) effect as well as various novel topological quantum phases. Although quantized transport properties have been observed in exfoliated MnBi2Te4 thin flakes, it remains a big challenge to achieve molecular beam epitaxy (MBE)-grown MnBi2Te4 thin films even close to the quantized regime. In this work, we report the realization of quantized anomalous Hall resistivity in MBE-grown MnBi2Te4 thin films with the chemical potential tuned by both controlled in situ oxygen exposure and top gating. We find that elongated post-annealing obviously elevates the temperature to achieve quantization of the Hall resistivity, but also increases the residual longitudinal resistivity, indicating a picture of high-quality QAH puddles weakly coupled by tunnel barriers. These results help to clarify the puzzles in previous experimental studies on MnBi2Te4 and to find a way out of the big difficulty in obtaining MnBi2Te4 samples showing quantized transport properties. [ABSTRACT FROM AUTHOR]

The article focuses on a research conducted by scientist Chris Jozwiak and colleagues related to spin-polarized photoelectrons. According to the research, topological insulators can manipulate the spin orientation of spin-polarized photoelectrons which have been ejected from protected surface electrons through light polarization. It also mentions that scientists used spin and angle resolved photoemission spectroscopy (spin-ARPES) to measure the electron energy and momentum during the process.

The microscopic understanding of high-temperature superconductivity in cuprates has been hindered by the apparent complexity of crystal structures in these materials. We used scanning tunneling microscopy and spectroscopy to study the electron-doped copper oxide compound Sr1−x Nd x CuO2, which has only bare cations separating the CuO2 planes and thus the simplest infinite-layer structure of all cuprate superconductors. Tunneling conductance spectra of the major CuO2 planes in the superconducting state revealed direct evidence for a nodeless pairing gap, regardless of variation of its magnitude with the local doping of trivalent neodymium. Furthermore, three distinct bosonic modes are observed as multiple peak-dip-hump features outside the superconducting gaps and their respective energies depend little on the spatially varying gaps. As well as the bosonic modes, with energies identical to those of the external, bending and stretching phonons of copper oxides, our findings reveal the origin of the bosonic modes in lattice vibrations rather than spin excitations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It is widely believed that the undoped Mott insulator is crucial to understand the origin of high-TC superconductivity and the complex phase diagram in cuprates. Tetragonal CuO, the simplest structure among cuprates, is regarded as an archetypal model to explore the ground state physics of Mott insulator. Here, we report on the epitaxial growth and surface characterization of tetragonal CuO thin film by combining ozone-assisted molecular beam epitaxy and multi-analytical techniques. In situ scanning tunneling microscopy measurement reveals two sets of Cu2+ square lattices with a reversed contrast of their apparent heights between neighboring domains. A spectroscopy study demonstrates an upper bound of the charge transfer gap as 3.68 eV, and quantitatively agrees with the previous photoemission and x-ray photoelectron results. Our work will deepen the understanding of high-energy electronic structures in parent cuprates. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, it is demonstrated the defects play a very important role in determining the quality of artificial nanoclusters grown. The surface strain (stress) distribution around defects in Al artificial nanocluster is analyzed by Moiré fringes pattern. The moiré fringes generated by scanning lines in monitor and nanocluster array can be used as a “magnifier” to study surface imperfections with scanning tunneling microscopy (STM). As moiré fringes’ exist in many ordered nanostructures and adsorbate systems, the method is expected to have wide applications. [Copyright &y& Elsevier]

Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic s-wave superconductivity in epitaxial Rb3C60 films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Yu-Shiba-Rusinov (YSR) states, induced by deliberately incurred Fe adatoms that act as magnetic scatterers, have also been observed. The YSR bound states show abrupt spatial decay and vary in energy with the Fe adatom registry. These results and a doping-dependent study of superconductivity point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes. Doped-fullerenes are a class of organic superconductors where disorder can be used to tune the superconducting temperature as well as the presence of subgap excitations such as Yu-Shiba-Rusinov states. Here, the authors investigate how structural disorder and non-magnetic impurities affect the superconductivity of Rb-doped fullerenes and what information this can provide about the underlying mechanisms. [ABSTRACT FROM AUTHOR]

We report transport measurements on Josephson junctions consisting of Bi2Te3 topological insulator (TI) thin films contacted by superconducting Nb electrodes. For a device with junction length L = 134 nm, the critical supercurrent Ic can be modulated by an electrical gate which tunes the carrier type and density of the TI film. Ic can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state. In the p-type regime the Josephson current can be well described by a short ballistic junction model. In the n-type regime the junction is ballistic at 0.7 K < T < 3.8 K while for T < 0.7 K the diffusive bulk modes emerge and contribute a larger Ic than the ballistic model. We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p–n junctions. Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices. [ABSTRACT FROM AUTHOR]

Interface superconductivity, realized in multiple artificial crystalline heterostructures, is one of the most exciting directions to search for high-temperature superconductivity. In this work, we prepare bulk (FeSe)η(SrTiO3)1−η multi-grain composites by a simple facile liquid-phase compaction method using a spark-plasma-sintering technique. Combining transmission electron microscopy/scanning electron microscopy and x-ray diffraction investigations, we demonstrate that the composites consist of micron-scale SrTiO3 grains surrounded by [001]-compressed β-FeSe grains. Transport measurements for the composites with FeSe mole fraction η > 0.06 reveal that two superconducting channels, one Tc ∼ 13 K phase from FeSe/SrTiO3 interfaces and another Tc ∼ 7 K phase from FeSe grains, cooperatively induce macroscopic superconducting behavior with isotropic upper critical fields above 40 T. This work points out a straightforward method to enhance Tc in the multi-grain (FeSe)η(SrTiO3)1−η composites by reducing the crystalline grains to nanoscale and finely tuning the stoichiometries of FeSe and SrTiO3. [ABSTRACT FROM AUTHOR]

Low-dimensional materials exhibit exotic properties and have attracted widespread attention. However, many low-dimensional materials are highly sensitive to air, making it challenging to investigate their intrinsic properties with ex situ measurements. To overcome such challenges, here, we developed a system combined with sample growth, electrode deposition, and in situ electrical transport measurement under ultra-high vacuum condition. The in situ deposition of electrodes enables desired ohmic electrical contacts between the probes and samples, which allows continuous temperature dependent resistance (R–T) measurements. Combined with a scanning tunneling microscope, surface morphology, electronic structure, and electrical transport properties of the same sample can be systematically investigated. We demonstrate the performance of this in situ electrical transport measurement system with three-unit-cell thick FeSe films grown on Nb-doped SrTiO3(001) substrates, where a low-noise R–T curve with a zero-resistance superconducting transition temperature of ∼30 K is observed. [ABSTRACT FROM AUTHOR]

Molecular beam epitaxy (MBE) is a powerful technique to grow high quality thin films with atomic precision, and multiprobe scanning tunneling potentiometry (STP) is an ideal tool to probe electronic transportation in nanometer scale. We combine the two advanced techniques together and successfully construct a unique system of MBE, STP, and four-probe in situ transport measurement. Excellent functions of this system have been demonstrated by experiments on several materials under ultrahigh vacuum conditions. The system provides an ideal platform for in situ study of electronic transport properties of various thin films, such as two-dimensional superconductors and topological insulators. [ABSTRACT FROM AUTHOR]

We investigate the atomic structure and electronic properties of monolayers of copper phthalocyanines (CuPc) deposited on epitaxial graphene substrate. We focus in particular on hexadecafluorophthalocyanine (F16CuPc), using both theoretical and experimental (scanning tunneling microscopy - STM) studies. For the individual CuPc and F16CuPc molecules, we calculated the electronic and optical properties using density functional theory (DFT) and time-dependent DFT and found a red-shift in the absorption peaks of F16CuPc relative to those of CuPc. In F16CuPc, the electronic wavefunctions are more polarized toward the electronegative fluorine atoms and away from the Cu atom at the center of the molecule. When adsorbed on graphene, the molecules lie flat and form closely packed patterns: F16CuPc forms a hexagonal pattern with two well-ordered alternating α and β stripes while CuPc arranges into a square lattice. The competition between molecule-substrate and intermolecular van der Waals interactions plays a crucial role in establishing the molecular patterns leading to tunable electron transfer from graphene to the molecules. This transfer is controlled by the layer thickness of, or the applied voltage on, epitaxial graphene resulting in selective F16CuPc adsorption, as observed in STM experiments. In addition, phthalocyanine adsorption modifies the electronic structure of the underlying graphene substrate introducing intensity smoothing in the range of 2-3 eV below the Dirac point (ED) and a small peak in the density of states at ∼0.4 eV above ED. [ABSTRACT FROM AUTHOR]

The self-assembled structure of submonolayer manganese phthalocyanine (MnPc) on Pb(111) surface is investigated by using low-temperature scanning tunneling microscopy (STM). A 'holelike' superlattice, which is superimposed on the self-assembled nearly quadratic network, is observed. High resolution STM images reveal that there are two distinct azimuthal orientations of MnPc molecules. It is found that by taking the two different orientations the self-assembly can further be optimized energetically by maximizing intermolecular orbital overlapping. It is this intralayer energy minimization process that leads to the characteristic holelike superlattice. [ABSTRACT FROM AUTHOR]

The transport property of a superconducting Pb nanobridge, which is carved by focus ion beam technique from an atomically flat single-crystal Pb thin film grown on Si(111) substrate, is investigated. Below the superconducting transition temperature TC, the nanobridge exhibits a series of sharp voltage steps as a function of current. The multiple voltage steps are interpreted as a consequence of spatially localized phase slip centers or hot-spot formation in the bridge. Just below the critical current, the voltages versus current curve shows a power-law behavior in the low temperature region, but Ohmic near the TC. The thermally activated phase slip, quantum phase slip, and imhomogeneity in a one-dimensional superconducting system may contribute to the observed results. [ABSTRACT FROM AUTHOR]

Highlights from the article: Superconductivity above 28 K in single unit cell FeSe films interfaced with GaO2- layer on NdGaO3(1 1 0) The discovery of high temperature superconductivity in single unit cell (UC) FeSe on TiO SB 2- sb SB sb terminated perovskite SrTiO SB 3 sb (0 0 1) substrates [1] has attracted intensive attention on searching for new superconducting systems with engineered interfaces as well as understanding the mechanism of interface high temperature superconductivity. In stark contrast to bulk FeSe - a superconductor with transition temperature T SB c sb ~ 8 K at ambient pressure [2], the single UC FeSe on SrTiO SB 3 sb (0 0 1) becomes superconducting at T SB c sb ~ 65 K [3], [4], [5] or higher [6], [7], holding the record T SB c sb among all known Fe-based superconductors. (a) and (b) The temperature dependent resistance under various perpendicular magnetic fields for FeTe/FeSe/NdGaO3(1 1 0) hetero-structures that consist of 1 UC and 4 UC FeSe films, respectively.

It has been 30 years since the Nobel Prize was awarded for scanning tunneling microscope (STM) in the year 1986, and there have been many instrumental developments and experimental achievements based on STM. In consideration of the strong capability and the extreme versatility in imaging, manipulating, and spectroscopy at the atomic level, STM has witnessed remarkable breakthroughs in many disciplines of condensed matter physics. In this paper, we will focus on recent STM studies on the interface superconductivity, which demonstrate a novel platform for exploring two dimensional superconductors and even high temperature superconductors by means of interface engineering. [ABSTRACT FROM AUTHOR]

The inherent susceptibility of low-dimensional materials to thermal fluctuations has long been expected to pose a major challenge to achieve intrinsic long-range ferromagnetic order in two-dimensional materials. The recent explosion of interest in atomically thin materials and their assembly into van der Waals heterostructures has renewed interest in two-dimensional ferromagnetism, which is interesting from a fundamental scientific point of view and also offers a missing ingredient necessary for the realization of spintronic functionality in van der Waals heterostructures. Recently, several atomically thin materials have been shown to be robust ferromagnets. Such ferromagnetism is thought to be enabled by magnetocrystalline anisotropy which suppresses thermal fluctuations. In this article, we review recent progress in two-dimensional ferromagnetism in detail and predict new possible two-dimensional ferromagnetic materials. We also discuss the prospects for applications of atomically thin ferromagnets in novel dissipationless electronics, spintronics, and other conventional magnetic technologies. Particularly, atomically thin ferromagnets are promising to realize time reversal symmetry breaking in two-dimensional topological systems, providing a platform for electronic devices based on the quantum anomalous Hall effect showing dissipationless transport. Our proposed directions will assist the scientific community to explore novel two-dimensional ferromagnetic families which can spawn new technologies and further improve the fundamental understanding of this fascinating area. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Recent experimental and theoretical studies of single-layer FeSe film grown on SrTiO 3 have revealed interface enhanced superconductivity, which opens up a pathway to promote the superconducting transition temperature. Here, to investigate the role of SrTiO 3 substrate in epitaxial superconducting film, we grew a conventional superconductor β-Sn (bulk T c ∼ 3.72 K) onto SrTiO 3 substrate by molecular beam epitaxy. By employing scanning tunneling microscope and spectroscopic measurements, an enhanced T c of 8.2 K is found for epitaxial β-Sn islands, deduced by fitting the temperature dependence of the gap values using the BCS formula. The observed interfacial charge injection and enhanced electron–phonon coupling are responsible for this T c enhancement. Moreover, the critical field of 8.3 T exhibits a tremendous increase due to the suppression of the vortex formation. Therefore, the coexistence of enhanced superconductivity and high critical field of Sn islands demonstrates a feasible and effective route to improve the superconductivity by growing the islands of conventional superconductors on perovskite-type titanium oxide substrates. [ABSTRACT FROM AUTHOR]

Abstract: The exploration of intriguing topological quantum physics in stanene has attracted enormous interest but is challenged by lacking desirable material samples. The successful fabrication of monolayer stanene on PbTe(111) films with low‐temperature molecular beam epitaxy and thorough characterizations of its atomic and electronic structures are reported here. In situ angle‐resolved photoemission spectroscopy together with first‐principles calculations identify two hole bands of p xy orbital with a spin‐orbit coupling induced band splitting and meanwhile reveal an automatic passivation of p z orbital of stanene. Importantly, material properties are tuned by substrate engineering, realizing a decorated stanene sample with truly insulating bulk on Sr‐doped PbTe. This finding paves a road for studies of stanene‐based topological quantum effects and electronics. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Interface enhanced superconductivity over 50 K has been discovered in monolayer FeSe films grown on several TiO 2 -terminated oxide substrates. Whether such phenomenon exists in other oxide substrates remains an extremely interesting topic. Here we report enhanced superconductivity with an onset transition temperature of 18 K in monolayer FeSe on MgO(001) substrate by transport measurement. Scanning transmission electron microscopy investigation on the interface structure indicates that FeSe films grow epitaxially on MgO(001) and that overlayer Fe atoms diffuse into the top two layers of MgO and substitute Mg atoms. Our density functional theory calculations reveal that this substitution promotes the charge transfer from the MgO substrate to the FeSe films, an essential process that also occurs in monolayer FeSe on TiO 2 -terminated oxides and contributes to the enhanced superconductivity therein. Our finding suggests that superconductivity enhancement in monolayer FeSe films on oxides substrates is rather general as long as charge transfer is allowed at the interface, thus pointing out an explicit direction for searching for new high temperature superconductivity by interface engineering. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract The search for Majorana fermions in topological superconductors is one of paramount research targets in physics today. Using a cryogenic scanning tunneling microscopy, we here report the signature of topologically nontrivial superconductivity on a single material of β-Bi 2 Pd films grown by molecular beam epitaxy. The superconducting gap associated with spinless odd-parity pairing opens on the surface and appears much larger than the bulk one due to the Dirac-fermion enhanced parity mixing of surface pair potential. Zero bias conductance peaks, probably from Majorana zero modes supported by such superconducting states, are identified at magnetic vortices. The superconductivity exhibits resistance to nonmagnetic defects, characteristic of time-reversal-invariant topological superconductors. Our study reveals β-Bi 2 Pd as a prime platform to generate, manipulate and braid Majorana zero modes for quantum computation. [ABSTRACT FROM AUTHOR]

The pairing mechanism of high-temperature superconductivity in cuprates remains the biggest unresolved mystery in condensed matter physics. To solve the problem, one of the most effective approaches is to investigate directly the superconducting CuO layers. Here, by growing CuO monolayer films on BiSrCaCuO substrates, we identify two distinct and spatially separated energy gaps centered at the Fermi energy, a smaller U-like gap and a larger V-like gap on the films, and study their interactions with alien atoms by low-temperature scanning tunneling microscopy. The newly discovered U-like gap exhibits strong phase coherence and is immune to scattering by K, Cs and Ag atoms, suggesting its nature as a nodeless superconducting gap in the CuO layers, whereas the V-like gap agrees with the well-known pseudogap state in the underdoped regime. Our results support an s-wave superconductivity in BiSrCaCuO, which, we propose, originates from the modulation-doping resultant two-dimensional hole liquid confined in the CuO layers. [ABSTRACT FROM AUTHOR]

Recent experiments on FeSe films grown on SrTiO3 (STO) suggest that interface effects can be used as a means to reach superconducting critical temperatures (Tc) of up to 80 K (ref. ). This is nearly ten times the Tc of bulk FeSe and higher than the record value of 56 K for known bulk Fe-based superconductors. Together with recent studies of superconductivity at oxide heterostructure interfaces, these results rekindle the long-standing idea that electron pairing at interfaces between two different materials can be tailored to achieve high-temperature superconductivity. Subsequent angle-resolved photoemission spectroscopy measurements of the FeSe/STO system revealed an electronic structure distinct from bulk FeSe (refs , ), with an energy gap vanishing at around 65 K. However, ex situ electrical transport measurements have so far detected zero resistance-the key experimental signature of superconductivity-only below 30 K. Here, we report the observation of superconductivity with Tc above 100 K in the FeSe/STO system by means of in situ four-point probe electrical transport measurements. This finding confirms FeSe/STO as an ideal material for studying high-Tc superconductivity. [ABSTRACT FROM AUTHOR]

Itis crucial for the studies of the transport properties and quantumeffects related to Dirac surface states of three-dimensional topologicalinsulators (3D TIs) to be able to simultaneously tune the chemicalpotentials of both top and bottom surfaces of a 3D TI thin film. Wehave realized this in molecular beam epitaxy-grown thin films of 3DTIs, as well as magnetic 3D TIs, by fabricating dual-gate structureson them. The films could be tuned between n-type and p-type by eachgate alone. Combined application of two gates can reduce the carrierdensity of a TI film to a much lower level than with only one of themand enhance the film resistance by 10 000%, implying that Fermilevel is tuned very close to the Dirac points of both top and bottomsurface states without crossing any bulk band. The result promisesapplications of 3D TIs in field effect devices. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) Dirac cones are predicted to reside in semimetals A 3 Bi (A = Na, K). By using molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM), we have successfully established the growth conditions for Na 3 Bi thin films on Si(1 1 1)-7 × 7, and determined that the lattice of Na 3 Bi is rotated by 30 degree with respect to that of Si(1 1 1)-7 × 7. The Na 3 Bi/Si(1 1 1)-7 × 7 thin film was further used as the substrate for the growth of K 3 Bi. The 3D Dirac-cone-like electronic band structures of Na 3 Bi and K 3 Bi have been clearly revealed by angle resolved photoelectron spectroscopy (ARPES). [ABSTRACT FROM AUTHOR]

Abstract: We present the growth of atomically flat topological crystalline insulator (TCI) SnTe films on Si(111) substrate by molecular beam epitaxy (MBE). The growth condition for achieving high quality SnTe film was established by a combination of reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) studies. In-situ angle resolved photoemission spectroscopy (ARPES) measurements elucidate the topological nature of the SnTe films. The electronic structure of SnTe films can be tuned by film thickness and Pb doping. The success in growing high quality SnTe thin films with tunable electronic structure is crucial for potential device applications. [Copyright &y& Elsevier]

Ultrathin Pb films with a thickness of three monolayers (ML) were prepared on α-[formula]Pb/Si(111) (Pb-SIC) substrate by molecular beam epitaxy. Despite significant defect scattering, low temperature scanning tunneling microscopy reveals a high superconducting transition temperature Tc of 6.9 K, compared with the bulk Tc (7.2 K). By applying external magnetic field, magnetic vortices were directly imaged, which demonstrates the robustness of superconductivity. By comparing to nearly free-standing Pb films on graphitized SiC (0001) substrate, we suggest that the higher Tc of 3 ML Pb films on Pb-SIC originates from the combined effects of quantum confinement and substrate-enhanced electron-phonon coupling. [ABSTRACT FROM AUTHOR]

Abstract: The electronic structure of reconstructed surface is measured by angle-resolved photoemission spectroscopy. By the first time, band splitting at and points in the surface Brillouin zone (SBZ) of this system is observed, which could be simply interpreted in the framework of Rashba splitting. Furthermore, first principle calculation is carried out and compared with our experimental results. [Copyright &y& Elsevier]

Abstract: Using angle-resolved photoemission spectroscopy (ARPES), we investigate the electronic structure of Cu-doped topological insulator Bi2Te3 on Si(111) substrate prepared by molecular beam epitaxy (MBE) to clarify the doping nature of Cu atoms in the films. By systematic studying the structural and electronic properties of the Cu-doped Bi2Te3 films by different doping methods, we find that Cu acts as electron donors when deposited onto the surface of Bi2Te3 films while behave as holes when doped in the process of Bi2Te3 thin film growth. The model of the formation of Cu+, five/seven layer lamella structures and Cu2−x Te is proposed to explain the different doping mechanisms. The robustness of topological surface states and insensitivity to non-magnetic impurities is indicated. [Copyright &y& Elsevier]

Topological insulators are a new class of material, that exhibit robust gapless surface states protected by time-reversal symmetry. The interplay of such symmetry-protected topological surface states and symmetry-broken states (for example, superconductivity) provides a platform for exploring new quantum phenomena and functionalities, such as one-dimensional chiral or helical gapless Majorana fermions, and Majorana zero modes that may find application in fault-tolerant quantum computation. Inducing superconductivity on the topological surface states is a prerequisite for their experimental realization. Here, by growing high-quality topological insulator Bi2Se3 films on a d-wave superconductor Bi2Sr2CaCu2O8+δ using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi2Se3 films with a large pairing gap up to 15 meV. Interestingly, distinct from the d-wave pairing of Bi2Sr2CaCu2O8+δ, the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant s-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes. [ABSTRACT FROM AUTHOR]

Abstract: We report results from scanning tunneling microscopy and transport measurements on a series of crystalline lead films containing an integer number of atomic layers, and find that the observed features in sufficiently thin films are consistent with Berezinskii–Kosterlitz–Thouless (BKT) physics. Specifically, Cooper pairing and superconductivity disappear at two distinct temperatures; the current–voltage characteristics in the intermediate phase are non-Ohmic; and the temperature and current dependences of resistance agree with the expectation from the BKT theory. [Copyright &y& Elsevier]

Quantum Hall effect (QHE), as a class of quantum phenomena that occur in macroscopic scale, is one of the most important topics in condensed matter physics. It has long been expected that QHE may occur without Landau levels so that neither external magnetic field nor high sample mobility is required for its study and application. Such a QHE free of Landau levels, can appear in topological insulators (TIs) with ferromagnetism as the quantized version of the anomalous Hall effect, i.e., quantum anomalous Hall (QAH) effect. Here we review our recent work on experimental realization of the QAH effect in magnetically doped TIs. With molecular beam epitaxy, we prepare thin films of Cr-doped (Bi,Sb)2Te3 TIs with well-controlled chemical potential and long-range ferromagnetic order that can survive the insulating phase. In such thin films, we eventually observed the quantization of the Hall resistance at h/e² at zero field, accompanied by a considerable drop in the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes, whereas the Hall resistance remains at the quantized value. The realization of the QAH effect provides a foundation for many other novel quantum phenomena predicted in TIs, and opens a route to practical applications of quantum Hall physics in low-power-consumption electronics. [ABSTRACT FROM AUTHOR]

We report on molecular beam epitaxy growth of bilayer Bi(111) films on topological insulator Bi2Te3. In situ scanning tunneling microscopy/spectroscopy shows that Bi growth mode changes from quasi bilayer-by-bilayer to step-flow with increasing substrate temperature. Bilayer Bi(111) exhibits an electron donor behavior, causing an 80 meV downshift of the Dirac point of Bi2Te3. Local work function difference between the bilayer films and Bi2Te3 films is measured to be 390 meV. Based on the observations, we propose a schematic energy-band diagram which reveals band bending effect at the Bi/Bi2Te3 interface. Our work paves a way to explore the exotic topological properties of bilayer islands and thin films of Bi. [ABSTRACT FROM AUTHOR]

We report high transition temperature superconductivity in one unit-cell (UC) thick FeSe films grown on a Seetched SrTiO3(001) substrate by molecular beam epitaxy (MBE). A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy (STM) suggest that the superconductivity of the 1 UC FeSe films could occur around 77 K. The control transport measurement shows that the onset superconductivity temperature is well above 50 K. Our work not only demonstrates a powerful way for finding new superconductors and for raising TC, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates. [ABSTRACT FROM AUTHOR]

The newly discovered alkali-doped iron selenide superconductors not only reach a superconducting transition temperature as high as 32?K, but also exhibit unique characteristics that are absent from other iron-based superconductors, such as antiferromagnetically ordered insulating phases, extremely high Néel transition temperatures and the presence of Fe vacancies and ordering. These features have generated considerable excitement as well as confusion, regarding the delicate interplay between Fe vacancies, magnetism and superconductivity. Here we report on molecular beam epitaxy growth of high-quality KxFe2?ySe2 thin films and in situ low-temperature scanning tunnelling microscope measurement of their atomic and electronic structures. We demonstrate that a KxFe2?ySe2 sample contains two distinct phases: an insulating phase with well-defined order of Fe vacancies, and a superconducting KFe2Se2 phase containing no Fe vacancies. An individual Fe vacancy can locally destroy superconductivity in a similar way to a magnetic impurity in conventional superconductors. Measurement of the magnetic-field dependence of the Fe-vacancy-induced bound states reveals a magnetically related bipartite order in the tetragonal iron lattice. These findings elucidate the existing controversies on this new superconductor and provide atomistic information on the interplay between magnetism and superconductivity in iron-based superconductors. [ABSTRACT FROM AUTHOR]

Abstract: Surface superstructures are studied with scanning tunneling microscopy during the growth of In on In/Si(111). On the inhomogeneous substrate of Si(111) 4×1/-In coexisting surface, the deposition of 1.5 monolayer (ML) In at about 0°C leads to reconstruction on surface and the formation of one-dimensional nanowires on 4×1 surface (1.0ML=7.8×1014 atoms/cm2). Subsequent deposition of In at −100°C gives rise to the appearance of a hexagonal superstructure with periodicity on reconstructed surface, while self-alignment of In dots along one-dimensional nanowires is observed on the initial 4×1 surface. On Si(111) -In surface, the growth of 2.5ML In at about −100°C yields 6×6 superstructure. The strain in the epitaxial In thin films provides a driving force for the formation of self-organized surface structures. [Copyright &y& Elsevier]

The photoabsorption edge shifting from UV to visible region accompanied with increased intensity and Raman features change was observed for N+ implanted TiO2. Raman spectra prove the formation of TiNx and oxygen vacancies in the implanted TiO2, which is considered as the origin of absorption edge shifting. The work may give a hint to increase the photocatalytic activity of TiO2 in the visible region at the molecule level. [ABSTRACT FROM AUTHOR]

Ultra-thin Pb films with magic thicknesses of 2 monolayer (ML), 4 ML and 6 ML were prepared of atomically flat on the substrate of Si(111)-α-√3 × √3 (or SIC phase) at 145 K. Their surface morphologies and stability were studied by low temperature scanning tunneling microscopy and temperature-dependent angle resolved photoemission spectroscopy. We found that the well ordered SIC interface can lower the diffusion barrier and enhance the interface charge transfer, leading to different critical thickness compared to Pb/Si(111)-7 × 7 grown under same conditions. Enhanced thermal expansion coefficients were also observed in ultra-thin Pb films at low temperature. [ABSTRACT FROM AUTHOR]

We have studied the superconductivity of Pb ultra-thin films with thickness from 1 monolayer (ML) to 7 ML grown on Si(111) by molecular beam epitaxy. In situ low temperature scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) measurements were performed on the films. It is suggested that the interface effect plays a critical role in enhancing the electron-phonon coupling, which consequently increases the superconducting transition temperature when a film reaches to the two-dimensional limit. [ABSTRACT FROM AUTHOR]

Abstract: An angle-resolved photoemission study of epitaxied Bi(111) film on Si(111)-(7×7) surface has been carried out at a temperature from 10K to 210K. The results show that there exists strong temperature dependence in the valence band of Bi(111) film. The variations in the spectral intensity and shape indicate the importance of phonon-assisted indirect transition in the photoemission process. The thermal sensitivity coefficients and effective Debye temperatures for main spectral peaks are determined from the experimental data. [Copyright &y& Elsevier]

Abstract: The surface dynamics of Fe deposited onto the template surface of -Al adatom-layer on Si(111) has been studied using scanning tunneling microscopy (STM). By imaging at both positive and negative sample bias, we identify the locations of Fe, Al and Si atoms by the susceptibility differences of these sites in receiving electrons and holes from the STM tip. When 0.1ML of Fe atoms are deposited at room temperature, some Fe atoms incorporate themselves into the adlayer as dispersed atoms or small Fe clusters with 2–10 atoms. A smaller amount of Fe atoms choose to sit on the adlayer, once again as dispersed atoms or small Fe clusters. As the Fe coverage increases beyond 0.1ML, both Fe clusters in and on the adlayer grow into 3-dimensional islands. Raising the growth temperature to 100°C is enough to enhance the diffusivity of Fe atoms and initiate a small degree of cluster coalescence. However, these clusters still do not show any crystallinity by STM observation until they are annealed. Deposition of Fe atoms at 400°C or post-deposition annealing above this temperature leads to the formation of iron silicide islands with a 2×2 surface lattice and a CsCl-type iron silicide structure, which are surrounded by -Al/Si(111) adlayer domains with a depletion of Si atoms near the silicide islands. Most of these islands have a triangular shape, and grow preferentially along the step edges of the Si(111) surface. The -Al adlayer appears to passivate the Si surface and enhances the diffusivity of Fe atoms but the passivation is not thermally stable. [Copyright &y& Elsevier]

Abstract: We theoretically analyzed the complex probability amplitudes of three states in a V-type system with two orthogonal energy sub-states. With rotating-wave approximation, the non-damped analytic solutions of the complex probability amplitudes equations with resonant excitation were deduced. The effective transition dipole moments and effective input pulse area were introduced and their expressions were given. It''s revealed that both amplitudes and frequencies of the population oscillations on the two orthogonal sub-states could be manipulated by the polarization angle of the excitation and the initial distribution of populations. [Copyright &y& Elsevier]

Abstract: Using the annealed vicinal Si(001) surface with 4° miscut toward the [110] direction as a substrate, single-domain monatomic In chain arrays have been fabricated. High-resolution STM images reveal that deposited In atoms preferentially form In dimers between two neighboring Si dimer rows along the step edges on the lower terrace. Formation of In dimers removes the surface dangling bonds and saturates the In valency. With increasing coverage, the In dimers develop into straight monatomic In chains along the step running direction. It is found that the ordered narrow terrace and rebonded double-layer (DB) step edge are the keys for the formation of monatomic In chains. [Copyright &y& Elsevier]

Domain boundaries of superlattices formed on the highly oriented pyrolytic graphite surface are studied by scanning tunneling microscopy. Twist and glide boundaries are identified with help of theoretical simulations. The apparent vertical corrugation difference of the superlattices with the same lattice periodicity is explained in terms of overlayer attenuation. Our study reveals that the intra-layer strain has important effect on the electronic structure of the graphite surface. [Copyright &y& Elsevier]

We report the use of an interrupted growth method in metalorganic chemical vapor deposition (MOCVD) to control the growth of InGaN layers and to grow nanoscale InGaN self-assembled quantum dots (QDs). With a 12-s growth interrupt, we successfully formed InGaN QDs with a typical lateral size of 25 nm and an average height of 4.1 nm. The QDs density is about 2×1010 cm−2. Strong photoluminescence (PL) emission of InGaN nanostructure was observed at a room temperature with a full-width-half-maximum (FWHM) of about 92 meV. These results suggest that such QDs are potentially useful in nitride-based optoelectronic devices. [Copyright &y& Elsevier]