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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 Rb$_3$C$_{60}$ films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Also observed have been Yu-Shiba-Rusinov (YSR) states induced by deliberately incurred Fe adatoms that act as magnetic scatters. The bound states display abrupt spatial decay and vary in energy with the Fe adatom registry. Our results and the universal optimal superconductivity at half-filling point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes., Comment: 25 pages, 5 figures, 5 supplemental figures

Low-dimensional superconductors have been at the forefront of physics research due to their rich physical properties such as high-temperature (Tc) superconductivity. In this article, we review the field of emergent high-Tc superconductivity at interfaces of heterostructures, focusing on the experimental advances and its physical mechanism. Charge transfer between constituent materials leads to two-dimensional carrier confinement that facilitates occurrence of superconductivity at the interface. We discuss the similarities between bulk high-Tc superconductors and interface systems, as well as implications from a survey of interface superconductors. We expect that the hybrid heterostructures and the ability to manipulate them on an atomic scale could be an enormously fertile ground to explore superconductivity with higher critical temperature Tc.

A charge density wave (CDW) is a collective quantum phenomenon in metals and features a wavelike modulation of the conduction electron density. A microscopic understanding and experimental control of this many-body electronic state in atomically thin materials remain hot topics in materials physics. By means of material engineering, we realized a dimensionality and Zr intercalation induced semiconductor-metal phase transition in 1

Interconversion of charge and spin currents via spin Hall effect is one of the key physical phenomena in spintronics. However, the spin polarizations are orthogonal to both the charge and spin flows due to the restricted symmetry conditions. Here, we use noncollinear antiferromagnet ${\mathrm{Mn}}_{3}\mathrm{Pt}$, which has different magnetic mirror planes in different orientations, to investigate magnetic symmetry dependent atypical spin-orbit torques. We observed weak generation of out of plane spin polarization $({\ensuremath{\sigma}}_{z})$ in ${\mathrm{Mn}}_{3}\mathrm{Pt}$/permalloy bilayers when current was applied perpendicular to the magnetic mirror plane of ${\mathrm{Mn}}_{3}\mathrm{Pt}$, and strong generation of ${\ensuremath{\sigma}}_{z}$ when current was applied parallel to the magnetic mirror plane. All three oriented ${\mathrm{Mn}}_{3}\mathrm{Pt}$ films show the same pattern, indicating the generation of unconventional ${\ensuremath{\sigma}}_{z}$ has strong dependence on magnetic symmetry. Controlling spin-orbit torques by changing current directions in differently oriented ${\mathrm{Mn}}_{3}\mathrm{Pt}$ films provides a strategy for optimizing antiferromagnetic spintronics.

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various localized phonon modes, of which the extended and interfacial modes act as bridges to connect the bulk AlN modes and bulk Si modes, and are expected to boost the inelastic phonon transport thus substantially contribute to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

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 Sr

Reactive molecular beam epitaxy has been employed to prepare ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ cuprate films via a Co-deposition method. Precise control of flux ratios among the solid sources leads us to establish a very narrow growth window for the cation stoichiometry. The hole doping by divalent ${\mathrm{Sr}}^{2+}$ cation is found to occur in a substitution-limited manner, accompanied by partial oxidation of Sr flux into peroxide $\mathrm{Sr}{\mathrm{O}}_{2}$ on the top surface. Through interface engineering, we explore the impact of substrate strain on the crystal parameters and superconductivity of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ thin films. The present study provides a time-saving, effective approach to preparing high-quality cuprate films, and further understanding the effect of surface/interface microstructures on cuprate superconductivity.

Charge density wave (CDW) is a collective quantum phenomenon in metals and features a wave-like modulation of the conduction electron density. A microscopic understanding and experimental control of this many-body electronic state in atomically thin materials remain hot topics in materials physics. By means of material engineering, we realized a dimensionality and Zr intercalation induced semiconductor-metal phase transition in 1T-ZrX$_2$ (X = Se, Te) ultra-thin films, accompanied by a commensurate 2 $\times$ 2 CDW order. Furthermore, we observed a CDW energy gap up to 22 meV around the Fermi level. Fourier-transformed scanning tunneling microscopy and angle-resolved photoemission spectroscopy reveal that 1T-ZrX$_2$ films exhibit the simplest Fermi surface among the known CDW materials in TMDCs, consisting only of Zr 4d-derived elliptical electron conduction band at the corners of the Brillouin zone., 29 pages, 4 figures, 4 Supplementary notes

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

Materials with reduced dimensionality often exhibit exceptional properties that are different from their bulk counterparts. Here we report the emergence of a commensurate 2 $\times$ 2 charge density wave (CDW) in monolayer and bilayer SnSe$_2$ films by scanning tunneling microscope. The visualized spatial modulation of CDW phase becomes prominent near the Fermi level, which is pinned inside the semiconductor band gap of SnSe$_2$. We show that both CDW and Fermi level pinning are intimately correlated with band bending and virtual induced gap states at the semiconductor heterointerface. Through interface engineering, the electron-density-dependent phase diagram is established in SnSe$_2$. Fermi surface nesting between symmetry inequivalent electron pockets is revealed to drive the CDW formation and to provide an alternative CDW mechanism that might work in other compounds., 5 pages, 5 figures

Scanning tunneling microscopy and spectroscopy are utilized to study the atomic-scale structure and electronic properties of infinite-layer ${\mathrm{Sr}}_{0.94}{\mathrm{La}}_{0.06}\mathrm{Cu}{\mathrm{O}}_{2+y}$ films prepared on $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}$-buffered $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$(001) substrate by ozone-assisted molecular beam epitaxy. Incommensurate structural supermodulation with a period of 24.5 $\text{\AA{}}$ is identified on the $\mathrm{Cu}{\mathrm{O}}_{2}$-terminated surface, leading to characteristic stripes running along the ${45}^{\text{o}}$ direction with respect to the Cu-O-Cu bonds. Spatially resolved tunneling spectra reveal substantial inhomogeneity on a nanometer-length scale and emergence of in-gap states at sufficient doping. Despite the Fermi level shifting up to 0.7 eV, the charge-transfer energy gap of the $\mathrm{Cu}{\mathrm{O}}_{2}$ planes remains fundamentally unchanged at different doping levels. The occurrence of the $\mathrm{Cu}{\mathrm{O}}_{2}$ superstructure is constrained in the surface region and its formation is found to link with oxygen intake that serves as the doping agent of holes in the epitaxial films.

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

Identifying the essence of doped Mott insulators is one of the major outstanding problems in condensed matter physics and the key to understanding the high-temperature superconductivity in cuprates. We report real space visualization of Mott transition in Sr1-xLaxCuO2+y cuprate films that cover the entire electron- and hole-doped regimes. Tunneling conductance measurements directly on the cooper-oxide (CuO2) planes reveal a systematic shift in the Fermi level, while the fundamental Mott-Hubbard band structure remains unchanged. This is further demonstrated by exploring atomic-scale electronic response of CuO2 to substitutional dopants and intrinsic defects in a sister compound Sr0.92Nd0.08CuO2. The results could be better explained in the framework of self-modulation doping, similar to that in semiconductor heterostructures, and form a basis for developing any microscopic theories for cuprate superconductivity., Comment: 5 pages, 4 figures, See Supplemental Material at https://journals.aps.org/prl/supplemental/10.1103/PhysRevLett.125.077002

Cryogenic scanning tunneling microscopy is employed to investigate the stoichiometry and defects of epitaxial FeSe thin films on SrTiO3(001) substrates under various post-growth annealing conditions. Low-temperature annealing with an excess supply of Se leads to formation of Fe vacancies and superstructures, accompanied by a superconductivity (metal)-to-insulator transition in FeSe films. By contrast, high-temperature annealing could eliminate the Fe vacancies and superstructures, and thus recover the high-temperature superconducting phase of monolayer FeSe films. We also observe multilayer FeSe during low-temperature annealing, which is revealed to link with Fe vacancy formation and adatom migration. Our results document very special roles of film stoichiometry and help unravel several controversies in the properties of monolayer FeSe films., Comment: 5 pages, 4 figures

We report on the epitaxial growth and surface structure of infinite-layer cuprate ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Nd}}_{x}{\mathrm{CuO}}_{2}$ films on ${\mathrm{SrTiO}}_{3}$(001) substrates by combining ozone-assisted molecular beam epitaxy and in situ scanning tunneling microscopy. Careful substrate temperature and flux control has been used to achieve single-phase, stoichiometric, and $c$-axis oriented films. The surface of the films is usually characterized by a mixed ${\mathrm{CuO}}_{2}$ surface and gridlike superstructure. The superstructure exhibits a periodicity of 3.47 nm that corresponds to a coincidence lattice between the overlayer peroxide ${\mathrm{SrO}}_{2}$ and underlying ${\mathrm{CuO}}_{2}$ plane, and gives rise to a conductance spectrum that is distinct from the Mott-Hubbard band structure of ${\mathrm{CuO}}_{2}$. At a higher Nd composition $xg0.1$, a $(2\ifmmode\times\else\texttimes\fi{}2)$ surface characteristic of the hole-doped ${\mathrm{CuO}}_{2}$ emerges, which we ascribe to the intake of apical oxygens in the intervening Sr planes.

Alkali-doped fullerides exhibit a wealth of unusual phases that remain controversial by nature. Here we report a cryogenic scanning tunneling microscopy study of the submolecular structural and electronic properties of expanded fullerene ${\mathrm{C}}_{60}^{n\ensuremath{-}}$ films with various cesium dopings. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of ${\mathrm{C}}_{60}^{n\ensuremath{-}}$ anions, yet the tunneling conductance spectra are all robustly characteristic of energy gaps, hallmarks of Jahn-Teller instability and electronic correlations. The Fermi level lies halfway within the insulating gap for stoichiometric Cs doping level of $n\phantom{\rule{4pt}{0ex}}=1$, 2, 3, and 4, apart from which it moves toward band edges with concomitant electronic states within the energy gap. Our findings establish the universality of Jahn-Teller instability and clarify the relationship between the doping and structural and electronic properties in ${\mathrm{Cs}}_{n}{\mathrm{C}}_{60}$ fullerides.

Using in situ angle-resolved photoemission spectroscopy, we systematically studied molecular-beam-epitaxy-grown topological crystalline insulator SnTe(111) thin films with varied thicknesses and substrate conditions. An oscillation in the band gap size with an increase in thickness was observed to depend on the electric field perpendicular to the films. The observations are consistent with the theoretically predicted thickness- and electric-field-dependent topological phase transitions between the normal insulator and the quantum spin Hall insulator phases in SnTe(111) films and demonstrate them to be an excellent and electrically tunable quantum spin Hall system.

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.

Using a cryogenic scanning tunneling microscopy, we report the observation of topologically nontrivial superconductivity on a single material of \beta-Bi2Pd 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. Majorana zero modes (MZMs), supported by such superconducting states, are identified at magnetic vortices. The superconductivity and MZMs exhibit resistance to nonmagnetic defects, characteristic of time-reversal-invariant topological superconductors. Our results demonstrate a simple platform to generate, manipulate and braid MZMs for quantum computation.

In this paper, we review our recent experimental developments on antiferromagnet (AFM) spintronics mainly comprising Mn-based noncollinear AFM metals. IrMn-based tunnel junctions and Hall devices have been investigated to explore the manipulation of AFM moments by magnetic fields, ferromagnetic materials and electric fields. Room-temperature tunneling anisotropic magnetoresistance based on IrMn as well as FeMn has been successfully achieved, and electrical control of the AFM exchange spring is realized by adopting ionic liquid. In addition, promising spin-orbit effects in AFM as well as spin transfer via AFM spin waves reported by different groups have also been reviewed, indicating that the AFM can serve as an efficient spin current source. To explore the crucial role of AFM acting as efficient generators, transmitters, and detectors of spin currents is an emerging topic in the field of magnetism today. AFM metals are now ready to join the rapidly developing fields of basic and applied spintronics, enriching this area of solid-state physics and microelectronics.

The requirement for low-power consumption advances the development of antiferromagnetic (AFM) spintronics manipulated by electric fields. Here we report an electrical manipulation of metallic AFM moments within IrMn/[Co/Pt] by interface engineering, where ultrathin non-magnetic metals are highlighted between IrMn and ferroelectric substrates. Ultrathin Pt seed layers are proved to be vital in elevating the blocking temperature and enhancing the perpendicular exchange coupling through modulating the domain structures of as-prepared IrMn AFM. Further electrical manipulations of perpendicular magnetic anisotropy crucially verify the indispensable role of pre-deposited ultrathin Pt layers in modulating IrMn antiferromagnetic moments, which is confirmed by the intimate correlation between the electrically manipulating AFM and improving its blocking temperature. Instead of immediate contact between IrMn AFM and ferroelectric substrates in a conventional way, interface engineering by adopting ultrathin seed layers here adds a new twist to the electrical modulation of AFM metals. This would provide scientific basis on how to manipulate AFM moments and optimize the design of practical AFM spintronics.

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

Quantum spin Hall (QSH) effect is an intriguing phenomenon arising from the helical edge states in two-dimensional topological insulators. We use molecular beam epitaxy (MBE) to prepare FeSe films with atomically sharp nematic domain boundaries, where tensile strains, nematicity suppression, and topological band inversion are simultaneously achieved. Using scanning tunneling microscopy (STM), we observe edge states at the Fermi level that spatially distribute as two distinct strips in the vicinity of the domain boundaries. At the end point of the boundaries, a bound state at the Fermi level is further observed. The topological origin of the edge states is supported by density functional theory calculations. Our findings not only demonstrate a candidate for QSH states but also provide a new pathway to realize topological superconductivity in a single-component film.

High-temperature ( T c) superconductors have been at the forefront of physics research for more than 30 years due to their rich physical properties and potential applications. Exploring and discovering new high- T c superconductors, as well as hereby clarifying the microscopic mechanism of unconventional superconductivity are among the long-desired goals of the world’s physicists. In this article, starting from the history and current challenges of high- T c superconductors, we review the atomic-scale understanding of conventional high- T c superconductivity based on heteroepitaxial films, especially focusing on the recent experimental advances on molecular beam epitaxy growth and quantum manipulation of high- T c superconducting thin films. We emphasize the atomic-scale exploration of epitaxial superconducting films by means of scanning tunneling microscopy, while a strategy of designing interfacial superconductors between two dissimilar materials is also introduced. We mainly devote to the experimental wisdom, methods, the microscopic understanding of high- T c superconductivity, and hope to inspire the related scientific researchers.

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.

The properties of iron-based superconductors (Fe-SCs) can be varied dramatically with the introduction of dopants and atomic defects. As a pressing example, FeSe, parent phase of the highest-$T_c$ Fe-SC, exhibits prevalent defects with atomic-scale "dumbbell" signatures as imaged by scanning tunneling microscopy (STM). These defects spoil superconductivity when their concentration exceeds 2.5%. Resolving their chemical identity is prerequisite to applications such as nanoscale patterning of superconducting/nonsuperconducting regions in FeSe, as well as fundamental questions such as the mechanism of superconductivity and the path by which the defects destroy it. We use STM and density functional theory to characterize and identify the dumbbell defects. In contrast to previous speculations about Se adsorbates or substitutions, we find that an Fe-site vacancy is the most energetically favorable defect in Se-rich conditions, and reproduces our observed STM signature. Our calculations shed light more generally on the nature of Se capping, the removal of Fe vacancies via annealing, and their ordering into a $\sqrt{5}$$\times$$\sqrt{5}$ superstructure in FeSe and related alkali-doped compounds., 7 pages, 5 figures

To overcome the practical issues existed in RRAMs application, complementary resistive switching (CRS) and self-compliance effect are investigated to alleviate sneak current issue and avoid external compliance current ( I comp ), respectively. It is remarkable that the two resistive switching (RS) behaviors are attempted to achieve in the same system of Pt/HfO 2 (7.5− x )/Ti(2 x )/HfO 2 (7.5− x )/Pt ( x = 0, 2.5, 3.5 nm) device fabricated by magnetron sputtering, i.e., multi-mode resistive switching behaviors are modulated by varying Ti interlayer thickness and operation scheme, such as initial I comp and applied voltage during reset switching. As indicated by the results, typical bipolar resistive switching (BRS) and CRS behaviors exhibit in Pt/HfO 2 (∼4 nm)/Ti(∼7 nm)/HfO 2 (∼4 nm)/Pt devices, and self-compliance effect appears in Pt/HfO 2 (∼5 nm)/Ti(∼5 nm)/HfO 2 (∼5 nm)/Pt devices. After investigated the conductive mechanisms, it is proposed adopting conceivable switching mechanism associated with the formation of oxygen vacancy (V O ) and metallic conduction filament (CF) to understand CRS and self-compliance effect. The exploration of multi-mode RS behaviors paves the way to improve the flexibility and convenience of RRAMs in actual application.

We investigate the influences of film composition, tuned by argon growth pressure and palladium doping, on antiferromagnetic to ferromagnetic transition temperatures of FeRh films. Employing complementary characterizations we show that the CsCl-type FeRh grows on MgO (100) substrate epitaxially with a controllable transition temperature. Lower argon pressure, a suitable palladium doping are found to effectively decrease the transition temperature. In addition, the exploration about the influence of post-annealing time on un-doped FeRh films indicates that annealing procedure is helpful to improve the growth quality. The optimized growth parameter provides an opportunity to deposit ultrathin FeRh films (5 nm) with a clear antiferromagnetic to ferromagnetic transition. The manipulation of the transition temperature of FeRh would advance its use in antiferromagnetic spintronics.

We report high-resolution scanning tunneling microscopy (STM) study of nano-sized Pb islands grown on SrTiO3, where three distinct types of gaps with different energy scales are revealed. At low temperature, we find that the superconducting gap (Δ s) in nano-sized Pb islands is significantly enhanced from the one in bulk Pb, while there is no essential change in superconducting transition temperature T c, giving rise to a larger BCS ratio 2Δ s/k B T c ∼ 8.31 and implying stronger electron-phonon coupling. The stronger coupling can originate from the interface electron-phonon interactions between Pb islands and SrTiO3. As the superconducting gap is totally suppressed under applied magnetic field, the Coulomb gap with apparent V-shape emerges. Moreover, the size of Coulomb gap (Δ C) depends on the lateral size of Pb islands (R) with Δ C ∼ 1/R 0.35, indicating that quantum size effect can significantly influence electronic correlations. Our experimental results shall shed important light on the interplay among superconductivity, quantum size effect and correlations in nano-sized strong-coupling superconductors.

We report on a bottom-up approach via molecular-beam epitaxy to grow high-quality crystalline two-dimensional films of ${\mathrm{PdTe}}_{2}$ on ${\mathrm{SrTiO}}_{3}$(001), and investigation on the electronic and superconducting properties therein by using scanning tunneling microscopy and transport experiments in combination with first-principles calculations. We observed a transition from the narrow-gap semiconducting phase in the monolayer to the metallic phase in the multilayer films. Importantly, all the multilayer films exhibited robust superconductivity, whose transition temperatures in the ultrathin limit were obviously higher than expected from the empirical inverse-of-thickness dependence and could be further enhanced through magnesium intercalation. Our first-principles studies revealed thickness-tuned band topology caused by the significant orbital-dependent interlayer coupling, which explained the experimentally observed thickness-dependent behaviors of atomic and electronic properties. Combined with the prediction of topologically nontrivial states introduced by honeycomb lattice of $p$ orbitals, ${\mathrm{PdTe}}_{2}$ thin films are promising material candidates for exploring the interplay between superconductivity and topology in the two-dimensional limit.

The structural or electronic symmetry breaking of the host lattice is a recurrent phenomenon in many quantum materials, including superconductors. Yet, how these broken symmetry states affect the electronic pair wave function of superconductivity have been rarely elucidated. Here, using low-temperature scanning tunneling microscopy and first-principles calculations, we identify the broken rotational symmetry via stripe ordering on the (001) surface of perovskite BaBi3 films grown by molecular beam epitaxy, and show that it consequently leads to anisotropic superconductivity with twofold symmetry. In contrast, the structural disorder smears out the anisotropy of electron pairing and fills superconducting subgap density of states as the film thickness is reduced. A quasi-long range model of superconducting fluctuations is revealed to describe the tunneling conductance spectra of thin BaBi3 films well, and to exemplify how disorders contribute to the low-energy quasiparticle excitations in superconductors. Our findings help understand the effects of symmetry breaking states and disorders on superconductivity, particularly the existing tunneling conductance spectra there., 38 pages, 15 figures

We report on atomic-scale visualization of the structure of infinite-layer cuprate SrCuO2 thin films grown on Nb-doped SrTiO3 substrates by molecular beam epitaxy. In-situ scanning tunneling microscopy study reveals stoichiometric copper oxide (CuO2) plane with a 2 x 2 surface reconstruction, prompted by preferential clustering of four adjacent CuO2 plaquettes. By imaging the subsurface Sr atoms, intra-unit-cell rotational symmetry breaking is observed, which, together with the adjacent CuO2 clustering, can be well accounted for by a periodic up-down buckling of oxygen ions on the CuO2 plane. Further post-annealing leads to an incommensurate stripe structure of the surface layer. Our findings provide important structural information for deeply understanding the electronic structure of superconducting CuO2 plane as well as high temperature superconductivity in cuprates., Comment: 15 pages, 4 figures

Antiferromagnets with zero net magnetic moment, strong anti-interference and ultrafast switching speed have potential competitiveness in high-density information storage [1], [2]. Electrical switching of antiferromagnets is at the heart of device application [3]. We will present our recent progress in the current-driven magnetization switching through spin-orbit torque (SOT) in three antiferromagnetic systems, including Mn 2 Au, and [Co/Pd]/Ru/[Co/Pd] synthetic antiferromagnets (SAF). Body centered tetragonal antiferromagnet Mn 2 Au with opposite spin sub-lattices is a unique metallic material for Neelorder spin-orbit torque (SOT) switching. The SOT switching in quasi-epitaxial (103), (101) and (204) Mn 2 Au films prepared by a simple magnetron sputtering method will be discussed. We demonstrate current induced antiferromagnetic moment switching in all the prepared Mn 2 Au films by a short current pulse at room temperature, whereas different orientated films exhibit distinguished switching characters. A direction-independent reversible switching is attained in Mn 2 Au (103) films due to negligible magnetocrystalline anisotropy energy, while for Mn 2 Au (101) and (204) films, the switching is invertible with the current applied along the in-plane easy axis and its vertical axis, but becomes attenuated seriously during initially switching circles when the current is applied along hard axis, because of the existence of magnetocrystalline anisotropy energy [4]. SAF were proposed to replace ferromagnets in magnetic memory devices to reduce the stray field, increase the storage density and improve the thermal stability. We will discuss the SOT in a perpendicularly magnetized Pt/[Co/Pd]/Ru/[Co/ Pd] SAF structure, which exhibits completely compensated magnetization and a high exchange coupling field of 2200 Oe. The magnetizations of two Co/Pd layers can be switched by spin-orbit torque between two antiparallel states simultaneously. The magnetization switching can be read out due to much stronger spin-orbit coupling at bottom Pt/[Co/Pd] interface compared to its upper counterpart without Pt. Both experimental and theoretical analyses unravel that the torque efficiency of antiferromagnetic coupled stacks is significantly higher than the ferromagnetic counterpart, which conquers the exchange coupling field, leading to the critical switching current of SAF comparable to the ferromagnetic coupled one [5], [6]. Besides the fundamental significance, the efficient switching of antiferromagnets by current would advance magnetic memory devices with high density, high speed and low power consumption.

We report on the direct observation of interface superconductivity in single-unit-cell SnSe2 films grown on graphitized SiC(0001) substrate by means of van der Waals epitaxy. Tunneling spectrum in the superconducting state reveals rather conventional character with a fully gapped order parameter. The occurrence of superconductivity is further confirmed by the presence of vortices under external magnetic field. Through interface engineering, we unravel the mechanism of superconductivity that originates from a two-dimensional electron gas formed at the interface of SnSe2 and graphene. Our finding opens up novel strategies to hunt for and understand interface superconductivity based on van der Waals heterostructures., 7 pages, 7 figures

We report scanning tunneling microscopy investigation on epitaxial ultrathin films of pyrite-type copper disulfide. Layer by layer growth of CuS2 films with a preferential orientation of (111) on SrTiO3(001) and Bi2Sr2CaCu2O8+{\delta} substrates is achieved by molecular beam epitaxy growth. For ultrathin films on both kinds of substrates, we observed symmetric tunneling gap around Fermi level that persists up to ~ 15 K. The tunneling gap degrades with either increasing temperature or increasing thickness, suggesting new matter states at the extreme two dimensional limit., Comment: 4 Figures

We report low temperature scanning tunneling microscopy and spectroscopy studies of Ni-Bi films grown by molecular beam epitaxy. Highly anisotropic and twofold symmetric superconducting gaps are revealed in two distinct composites, Bi-rich NiBi3 and near-equimolar NixBi, both sharing quasi-one-dimensional crystal structure. We further reveal axially elongated vortices in both phases, but Caroli-de Gennes-Matricon states solely within the vortex cores of NiBi3. Intriguingly, although the localized bound state splits energetically off at a finite distance ~10 nm away from a vortex center along the minor axis of elliptic vortex, no splitting is found along the major axis. We attribute the elongated vortices and unusual vortex behaviors to the combined effects of twofold superconducting gap and Fermi velocity. The findings provide a comprehensive understanding of the electron pairing and vortex matter in quasi-one-dimensional superconductors, Comment: 7 pages, 7 figures

Quantum anomalous Hall (QAH) effect is a quantum Hall effect that occurs without the need of external magnetic field. A system composed of multiple parallel QAH layers is an effective high Chern number QAH insulator and the key to the applications of the dissipationless chiral edge channels in low energy consumption electronics. Such a QAH multilayer can also be engineered into other exotic topological phases such as a magnetic Weyl semimetal with only one pair of Weyl points. This work reports the first experimental realization of QAH multilayers in the superlattices composed of magnetically doped (Bi,Sb)$_2$Te$_3$ topological insulator and CdSe normal insulator layers grown by molecular beam epitaxy. The obtained multilayer samples show quantized Hall resistance $h/Ne$$^2$, where $h$ is the Planck's constant, $e$ is the elementary charge and $N$ is the number of the magnetic topological insulator layers, resembling a high Chern number QAH insulator., Comment: 9 pages, 3 figures

Electron energy-loss magnetic chiral dichroism (EMCD) is a novel technique that allows magnetic information determination down to the nanoscale. However, constrained by the predefined diffraction g ...

The newly discovered iron-based superconductors in 2008 have become the second class of high temperature superconductor besides the cuprates. Among all these iron-based compounds, β-FeSe has the simplest chemical structure and can be an archetype system for unraveling the mechanism of superconductivity. By using state-of-the-art molecular beam epitaxy (MBE) technique, high-quality superconducting FeSe single crystalline films with finely controllable topography and composition have been successfully prepared, with their superconducting properties extensively investigated. The low-temperature scanning tunneling spectra reveal that the superconducting gap in the quasiparticle density of states is visible down to two unit cells of FeSe films, and evidence for a gap function with nodal lines. Electron pairing with two-fold symmetry has also been demonstrated by direct imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms and Se vacancies. The two-fold pairing symmetry is supported by our observation of striped electronic nanostructures in the slightly Se-doped samples, primarily due to the orbital ordering. Twin boundaries run at approximately 45° to the Fe-Fe bond directions, and noticeably suppress the superconducting gap and pin magnetic vortices. This is likely caused by the increased Se height in the vicinity of twin boundaries, providing the first local evidence for the importance of this height to the mechanism of superconductivity. Furthermore, we reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films, whose energy reduces with decreasing gap magnitude Ω . Recently, the growth recipe has been further extended to grow FeSe film on SrTiO3(001) substrate, leading to the high- T c superconductivity in the very FeSe/SrTiO3 interface. A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy suggest that the superconductivity of single-unit-cell FeSe films could occur around 77 K. Our subsequent transport and measurements of the one-unit-cell thick FeSe films on insulating SrTiO3 substrates with non-superconducting FeTe protection layers provide definitive evidence for high temperature superconductivity with an onset T c above 40 K which are much higher than T c~9.4 K for bulk FeSe, respectively. The finding provides a completely new avenue and method to reveal the pairing mechanism of high- T c superconductors, and to further enhance T c, stirring up worldwide increasing attention in the field of high- T c superconductors and material sciences.

Z. Wang, J. Wang, Y. Zang, Dr. T. Jiang, Y. Gong, Prof. C.-L. Song, Prof. S.-H. Ji, Prof. L.-L. Wang, Prof. K. He, Prof. W. Duan, Prof. X. Ma, Prof. X. Chen, Prof. Q.-K. Xue State Key Laboratory of Low-Dimensional Quantum Physics Department of Physics Tsinghua University Beijing 100084 , China E-mail: kehe@tsinghua.edu.cn; qkxue@tsinghua.edu.cn Q. Zhang, J.-A. Shi, Prof. L. Gu Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 , China Dr. T. Jiang College of Opto-Electronic Science and Engineering National University of Defense Technology Changsha , Hunan 410073 , China Prof. S.-H. Ji, Prof. L.-L. Wang, Prof. L. Gu, Prof. K. He, Prof. W. Duan, Prof. X. Ma, Prof. X. Chen, Prof. Q.-K. Xue Collaborative Innovation Center of Quantum Matter Beijing 100084 , P. R. China

We investigate the spin Hall magnetoresistance (SMR) in niobium (Nb) attached to Y3Fe5O12 near the superconducting critical temperature (Tc) of Nb. The SMR vanishes after cooling the sample below Tc, and recovers if the temperature is raised. When a magnetic field larger than the critical field of Nb is applied, the SMR re-emerges with an enhanced magnitude even if the temperature is below Tc. The experimental results demonstrate that the SMR could be completely suppressed by the coupling between superconducting condensation and spin–orbit interaction in superconductors. In addition to the fundamental physics on the charge–spin interactions in superconductors, our work adds a different dimension to superconducting spintronics. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)