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Bismuth-based binary alloy β-Bi2Pd has recently been suggested as a connate topological superconductor with immense promise for Majorana zero modes. However, the epitaxial thin films down to the ultimate level of single unit-cell have been challenging. Here, we employed the state-of-the-art molecular beam epitaxy to prepare single unit-cell superconducting β-Bi2Pd films with lateral dimensions of hundreds of nanometers. Utilizing in situ scanning tunneling microscopy/spectroscopy, we uncover evidence of anisotropic superconductivity whose spectra are well fit by anisotropic s-wave gap functions and demonstrate that the epitaxial growth of β-Bi2Pd films relies substantially on the substrate temperature, flux ratio, growth rate, and coverage. The high-quality epitaxial films provide a promising platform to investigate the topological superconductivity at the two-dimensional limit. [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]

We report measurements of low-temperature magnetoresistance in Sr1âˆ' x La x CuO2 (x = 0.1) epitaxial thin film grown on (110) DyScO3 single crystal. A positive magnetoresistance which is anisotropic and hysteretic with respect to the in-plane direction of magnetic field appears in Sr1âˆ' x La x CuO2 below T = 5 K, coinciding with antiferromagnetic ordering and strong magnetic anisotropy in DyScO3. The interplay of magnetotransport in epitaxial Sr1âˆ' x La x CuO2 with magnetism in the substrate is discussed based on magnetostriction and magnetic relaxation in DyScO3. [ABSTRACT FROM AUTHOR]

Superconductivity is significantly enhanced in monolayer FeSe grown on SrTiO3, but not for multilayer films, in which large strength of nematicity develops. However, the link between the high-transition temperature superconductivity in monolayer and the correlation related nematicity in multilayer FeSe films is not well understood. Here, we use low-temperature scanning tunneling microscopy to study few-layer FeSe thin films grown by molecular beam epitaxy. We observe an incommensurate long-range smectic phase, which solely appears in bilayer FeSe films. The smectic order still locally exists and gradually fades away with increasing film thickness, while it suddenly vanishes in monolayer FeSe, indicative of an abrupt smectic phase transition. Surface alkali-metal doping can suppress the smectic phase and induce high-Tc superconductivity in bilayer FeSe. Our observations provide evidence that the monolayer FeSe is in close proximity to the smectic phase, and its superconductivity is likely enhanced by this electronic instability as well. The relation between enhanced superconductivity in monolayer FeSe grown on SrTiO3 and the large nematicity in multilayer FeSe on SrTiO3 remains not well understood. Here, the authors observe a long-range smectic phase in bilayer FeSe films but vanishes in monolayer FeSe, providing a new instability to help enhance the superconductivity. [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]

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]

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]

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]

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]

Bi2Se3 is a typical three-dimensional topological insulator but always exhibits metallic behavior due to heavy n-type doping. With in situ angle-resolved photoemission spectroscopy, we have systematically studied the doping effects of Sb and Pb on the electronic structure of Bi2Se3 films prepared by molecular beam epitaxy. The surface chemical potential of Bi2Se3 can be tuned by 110 and 145 meV by doping Sb and Pb atoms, respectively. By codoping Pb and Sb, the Fermi level can be shifted from above to below the Dirac point. The underlying mechanism in different doping effects of Sb and Pb is discussed. [ABSTRACT FROM AUTHOR]

We have calculated the energy barriers for an adatom climbing up onto a Pb mesa top either over a facet-facet edge or through a facet-step joint, using a modified embedded atom method. We found that the second process is not only thermodynamically more favorable than the first one but also much faster with a diminishing barrier. Our results provide a plausible explanation for the experimentally observed intriguing growth behavior of a Pb mesa. The underlying mechanisms can be generally applicable to other systems. [ABSTRACT FROM AUTHOR]

Varied temperature photoemission study is performed to investigate the quantum size effects on the thermal property of atomically flat Pb films grown on Si(111). The binding energies of the quantum well states for the films with thicknesses from 10 to 24 ML exhibit a linear increase with increasing temperature from 75 to 270 K. Under free electron approximation, thermal expansion coefficients of the thin films are determined, which manifest a large enhancement and oscillation behavior. The large enhancement is interpreted by a model based on the quantum confinement along the film normal direction. The oscillation is shown to be closely related to the structural instability and is a result of the formation of the quantized electronic states in thin films. [ABSTRACT FROM AUTHOR]

The thickness dependence of the local work function (LWF) and its relationship with the quantum well states (QWSs) are studied. The measured LWF shows an oscillatory behavior between adjacent layers with a period of 2 ML and, in addition, an envelope beating pattern with a period of 9 ML. Scanning tunneling spectroscopy investigations reveal that the oscillatory LWF correlates perfectly with the formation of the QWSs: the higher the occupied QWS is, the smaller the LWF is. Through the role of the LWF, this study establishes the importance of quantum size effects in thin films for surface reactions and catalysis. [ABSTRACT FROM AUTHOR]

The cuprate superconductors exhibit ubiquitous instabilities toward charge-ordered states. These unusual electronic states break the spatial symmetries of the host crystal, and have been widely appreciated as essential ingredients for constructing a theory for high-temperature superconductivity in cuprates. Here, we report real-space imaging of the doping-dependent charge orders in the epitaxial thin films of a canonical cuprate compound La2–xSrxCuO4 using scanning tunneling microscopy. As the films are moderately doped, we observe a crossover from incommensurate to commensurate (4a0, where a0 is the Cu–O–Cu distance) stripes. Furthermore, at lower and higher doping levels, the charge orders occur in the form of distorted Wigner crystal and grid phase of crossed vertical and horizontal stripes. We discuss how the charge orders are stabilized, and their interplay with superconductivity. [ABSTRACT FROM AUTHOR]

Atomically flat thin films of topological semimetal Na3 Bi are grown on double-layer graphene formed on 6H–SiC(0001) substrates by molecular beam epitaxy. By combined techniques of molecular beam epitaxy, scanning tunneling microscopy and angle resolved photoelectron spectroscopy, the growth conditions for Na3 Bi thin films on double-layer graphene are successfully established. The band structure of Na3 Bi grown on graphene is mapped along Γ̄–M̄ and Γ̄–K̄ directions. Furthermore, the energy band of Na3 Bi at higher energy is uncovered by doping Cs atoms on the surface. [ABSTRACT FROM AUTHOR]

We report comprehensive x-ray diffraction studies of the crystal structure and epitaxy of thin films of the topological insulator Bi2Te3 grown on Si (1 1 1). The films are single crystals of high crystalline quality, which strongly depend on that of their substrates, with in-plane epitaxial relationships of Bi2Te3 [2 1 -3 0] | Si [1 -1 0] and Bi2Te3 [0 1 -1 0] | Si [1 1 -2] along which the lattices of 1 × 3 Bi2Te3 and 2 × 2 Si supercells are well matched. As the samples age, we observe loss of crystalline Bi2Te3 film thickness accompanied with roughening of the crystalline interfaces, formation of new crystalline phases as well as compositional and structural modification of the Si substrate, consistent with the diffusion of Te into the Si substrate. [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]

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]

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]