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The Mott state in 1T-TaS2 is predicted to host quantum spin liquids (QSLs). However, its insulating mechanism is controversial due to complications from interlayer coupling. Here, we study the charge transfer state in monolayer 1T-NbSe2, an electronic analogue to TaS2 exempt from interlayer coupling, using spectroscopic imaging scanning tunneling microscopy and first-principles calculations. Monolayer NbSe2 surprisingly displays two types of star of David (SD) motifs with different charge transfer gap sizes, which are interconvertible via temperature variation. In addition, bilayer 1T-NbSe2 shows a Mott collapse by interlayer coupling. Our calculation unveils that the two types of SDs possess distinct structural distortions, altering the effective Coulomb energies of the central Nb orbital. Our calculation suggests that the charge transfer gap, the same parameter for determining the QSL regime, is tunable with strain. This finding offers a general strategy for manipulating the charge transfer state in related systems, which may be tuned into the potential QSL regime.

As a single-elemental system, tellurium can exist stably in the form of layers with an intriguing multivalence character, which constructs a new member of the 2D family. However, the growth and electronic structure of tellurium films are still far from known at present. Here, combined with molecular beam epitaxy, scanning tunneling microscopy/spectroscopy measurements and density functional theory calculations, we report the geometric and electronic structures of tellurium grown on NbSe2 from sub-monolayer to few-layer films. At the sub-monolayer coverage, we obtain two types of adatom-induced ordered superstructures that are strongly coupled with NbSe2. With the increase in coverage, the few-layer tellurium films adopt the α-phase form, showing internal strain-induced ripple patterns in the few-layers and bulk-like in thick layers with distinct edge geometries. The band gap of α-tellurium films decreases with the increase in thickness, which is associated with notable in-gap states. These observations, corroborated with DFT calculations, emphasize the important role of the NbSe2 substrate in modulating the structural and electronic properties of tellurium films. Moreover, the interaction between tellurium adatoms and tellurium films leads to √2 × √2 surface reconstruction prior to a new monolayer, conforming to our theoretical calculations. Our work clarifies the kinetic growth of tellurium films on NbSe2 and reveals the tunability of electronic properties via substrate modulation or surface decoration.

Yu-Shiba-Rusinov (YSR) states arise when magnetic impurities interact with superconductivity. The intricacy of coupling and the nature of the superconductivity determine the behavior of the YSR state, whose detailed correlations are not yet fully understood. Here, we study the YSR state of a single Fe adatom on the surface of 2H-NbSe2 with combined low temperature scanning tunneling microscopy/spectroscopy, density functional theory calculations and tight-binding modeling. It is found that the Fe adatom occupies the hollow site of the Se surface layer. A prominent YSR state close to the Fermi level is observed. The YSR state exhibits a threefold symmetry along the diagonal direction of the Se lattice. The spatial decay of the YSR state follows a behavior in three-dimensional superconductivity. This behavior contrasts with a previous study of imbedded Fe impurities, whose YSR state shows a six-fold symmetry and a two-dimensional long-range decay. According to our theoretical modeling, the coupling configurations affect the adatom-substrate hopping and the interlayer coupling of the substrate. Both factors are crucial for the consequent behavior of the YSR state.

2D ferroelectrics have received wide interest due to the remarkable quantum states of emerging physics at reduced dimensionality, associated with their exotic properties in high-performance and nonvolatile functional devices. Here, by combing molecular beam epitaxy synthesis and scanning tunneling microscopy characterization, two metastable phases of layered In

The Mott state in 1T-TaS

Three-dimensional (3D) topological Dirac semimetal, when thinned down to 2D few layers, is expected to possess gapped Dirac nodes via quantum confinement effect and concomitantly display the intriguing quantum spin Hall (QSH) insulator phase. However, the 3D-to-2D crossover and the associated topological phase transition, which is valuable for understanding the topological quantum phases, remain unexplored. Here, we synthesize high-quality Na3Bi thin films with R3*R3 reconstruction on graphene, and systematically characterize their thickness-dependent electronic and topological properties by scanning tunneling microscopy/spectroscopy in combination with first-principles calculations. We demonstrate that Dirac gaps emerge in Na3Bi films, providing spectroscopic evidences of dimensional crossover from a 3D semimetal to a 2D topological insulator. Importantly, the Dirac gaps are revealed to be of sizable magnitudes on 3 and 4 monolayers (72 and 65 meV, respectively) with topologically nontrivial edge states. Moreover, the Fermi energy of a Na3Bi film can be tuned via certain growth process, thus offering a viable way for achieving charge neutrality in transport. The feasibility of controlling Dirac gap opening and charge neutrality enables realizing intrinsic high-temperature QSH effect in Na3Bi films and achieving potential applications in topological devices., 28 pages, 4 figures

Exploring new two-dimensional (2D) van der Waals (vdW) systems is at the forefront of materials physics. Here, through molecular beam epitaxy on graphene-covered SiC(0001), we report successful growth of AlSb in the double-layer honeycomb (DLHC) structure, a 2D vdW material which has no direct analogue to its 3D bulk and is predicted kinetically stable when freestanding. The structural morphology and electronic structure of the experimental 2D AlSb are characterized with spectroscopic imaging scanning tunneling microscopy and cross-sectional imaging scanning transmission electron microscopy, which compare well to the proposed DLHC structure. The 2D AlSb exhibits a bandgap of 0.93 eV versus the predicted 1.06 eV, which is substantially smaller than the 1.6 eV of bulk. We also attempt the less-stable InSb DLHC structure; however, it grows into bulk islands instead. The successful growth of a DLHC material here opens the door for the realization of a large family of novel 2D DLHC traditional semiconductors with unique excitonic, topological, and electronic properties., 21 pages, 6 figures

Correlated states emerge in low-dimensional systems owing to enhanced Coulomb interactions. Elucidating these states requires atomic scale characterization and delicate control capabilities. In this study, spectroscopic imaging-scanning tunneling microscopy was employed to investigate the correlated states residing in the one-dimensional electrons of the monolayer and bilayer MoSe2 mirror twin boundary (MTB). The Coulomb energies, determined by the wire length, drive the MTB into two types of ground states with distinct respective out-of-phase and in-phase charge orders. The two ground states can be reversibly converted through a metastable zero-energy state with in situ voltage pulses, which tunes the electron filling of the MTB via a polaronic process, as substantiated by first-principles calculations. Our modified Hubbard model reveals the ground states as correlated insulators from an on-site U-originated Coulomb interaction, dubbed Hubbard-type Coulomb blockade effect. Our work sets a foundation for understanding correlated physics in complex systems and for tailoring quantum states for nano-electronics applications., Comment: 4 figures, 19 pages

We present an investigation on the controlled growth of epitaxial iron telluride films of multiple phases by molecular beam epitaxy. By optimizing the substrate temperature, we fabricate different phases of \ensuremath{\alpha}-FeTe, \ensuremath{\beta}-FeTe, and $\mathrm{Fe}{\mathrm{Te}}_{2}$, respectively, whose crystalline morphologies are determined by means of in situ scanning tunneling microscopy/spectroscopy and ex situ scanning transmission electron microscopy. While both \ensuremath{\alpha}- and \ensuremath{\beta}-FeTe films are metallic, we uncover a \ensuremath{\sim}185- and 65-mV semiconducting band gap for the (100) and (011) facets of $\mathrm{Fe}{\mathrm{Te}}_{2}$ film, respectively, with the former one being compatible with the first principles calculations. Moreover, for $\mathrm{Fe}{\mathrm{Te}}_{2}$, we observe reduced gaps with enhanced conductance in the vicinity of edge boundaries, which are dependent on the geometries of step orientation and possibly in correlation to the magnetic anisotropy with structural distortion. Our study provides insight into the controllable synthesis of Fe-Te compounds with variation of stoichiometries and surface terminations, which may generalize to other epitaxial Fe chalcogenide films.

Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals enriches our understanding of two-dimensional (2D) magnetic orders and presents several advantages over ferromagnetism in spintronic applications. However, studies of 2D intrinsic antiferromagnetism are sparse, owing to the lack of net magnetisation. Here, by combining spin-polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe2, which has been successfully grown through molecular-beam epitaxy. We observe a stable antiferromagnetic (AFM) order at the atomic scale in the ML crystal, whose bulk is ferromagnetic, and correlate its imaged zigzag spin texture with the atomic lattice structure. The AFM order exhibits an intriguing noncollinear spin reorientation under magnetic fields, consistent with its calculated moderate magnetic anisotropy. The findings of this study demonstrate the intricacy of 2D vdW magnetic materials and pave the way for their in-depth analysis., In two dimensions magnetic order without magnetic anisotropy is forbidden, making 2D magnetic systems a rich playground for interesting physics. Here, Xian et al. fabricate monolayers of CrTe2, and demonstrate antiferromagnetic ordering, with spin reorientation at finite magnetic fields.

In one-dimensional (1D) metallic systems, the diverging electron susceptibility and electron-phonon coupling collaboratively drive the electrons into a charge density wave (CDW) state. However, a strictly 1D system is unstable against perturbations, whose effect on CDW order requires clarification ideally with altered coupling to surroundings. Here, we fabricate such a system with nanowires of Mo6Se6 bundles, which are either attached to edges of monolayer MoSe2 or isolated freely, by postannealing the preformed MoSe2. Using scanning tunneling microscopy, we visualize charge modulations and CDW gaps with prominent coherent peaks in the edge-attached nanowires. Astonishingly, the CDW order becomes suppressed in the isolated nanowires, showing CDW correlation gaps without coherent peaks. The contrasting behavior, as revealed with theoretical modeling, is interpreted as the effect of phason polarons on the 1D CDW state. Our work elucidates a possibly unprecedented many-body effect that may be generic to strictly 1D system but undermined in a quasi-1D system.

The authors uncover a valley dependent superconducting proximity effect in a heterostructure, realized by growth of multi-domain Bi(111) films on NbSe${}_{2}$ substrate, with twisted overlapping.

As a single-elemental system, tellurium can exist stably in the form of layers with an intriguing multivalence character, which constructs a new member of the 2D family. However, the growth and electronic structure of tellurium films are still far from known at present. Here, combined with molecular beam epitaxy, scanning tunneling microscopy/spectroscopy measurements and density functional theory calculations, we report the geometric and electronic structures of tellurium grown on NbSe2 from sub-monolayer to few-layer films. At the sub-monolayer coverage, we obtain two types of adatom-induced ordered superstructures that are strongly coupled with NbSe2. With the increase in coverage, the few-layer tellurium films adopt the α-phase form, showing internal strain-induced ripple patterns in the few-layers and bulk-like in thick layers with distinct edge geometries. The band gap of α-tellurium films decreases with the increase in thickness, which is associated with notable in-gap states. These observations, corroborated with DFT calculations, emphasize the important role of the NbSe2 substrate in modulating the structural and electronic properties of tellurium films. Moreover, the interaction between tellurium adatoms and tellurium films leads to √2 × √2 surface reconstruction prior to a new monolayer, conforming to our theoretical calculations. Our work clarifies the kinetic growth of tellurium films on NbSe2 and reveals the tunability of electronic properties via substrate modulation or surface decoration.

2D ferroelectrics have received wide interest due to the remarkable quantum states of emerging physics at reduced dimensionality, associated with their exotic properties in high-performance and nonvolatile functional devices. Here, by combing molecular beam epitaxy synthesis and scanning tunneling microscopy characterization, two metastable phases of layered In2 Se3 films: β'- and β*-In2 Se3 are reported, which develop different types of in-plane spontaneous polarizations, thus resulting in different striped morphologies. The anti-ferroelectric order in β'-In2 Se3 and ferroelectric order of β*-In2 Se3 are identified, respectively, down to the 2D limit by comprehensive investigations of structural and spectroscopic signatures, including the lattice distortion, the spatial profile of images, the formation of domain structure, and the electronic band-bending by polarization charges at edges. The ferroelectric switching between those two phases are further controlled via applying an electric field generated from the scanning tunneling microscopy tip in a reversible manner. The intriguing tunability between the (anti-)ferroelectric orders in the 2D limit provides a promising platform for studying the interplay between electronic structure and ferroelectricity in van der Waals materials, and promotes potential development of miniaturized transistors and memory devices based on electric polarizations.

The synthesis and integration of core-shell materials have been extensively explored in three-dimensional nanostructures, while they are hardly ever extended into the emerging two-dimensional (2D) research field. Herein, demonstrated by graphene (G) and hexagonal boron nitride (h-BN) and via a sequential chemical vapor deposition method, we succeed for the first time in synthesizing 2D h-BN-G core-shell arrays (CSA), which possess extremely high uniformity in shapes, sizes and distributions. Each of the core-shell units is composed of G ring-shaped shell internally filled with h-BN circular core. In addition, we perform simulations to further explain the self-symmetrical etching growth mechanism of the h-BN-G CSA, demonstrating its potential to be used as an efficient synthetic method suitable for other 2D CSA systems.

Topological states emerge at the boundary of solids as a consequence of the nontrivial topology of the bulk. Recently, theory predicts a topological edge state on single layer transition metal dichalcogenides with 1T' structure. However, its existence still lacks experimental proof. Here, we report the direct observations of the topological states at the step edge of WTe2 by spectroscopic-imaging scanning tunneling microscopy. A one-dimensional electronic state residing at the step edge of WTe2 is observed, which has a spatial extension of about 2.5 nm. First principles calculations rigorously verify the edge state has a topological origin, and its topological nature is unaffected by the presence of the substrate. Our study supports the existence of topological edge states in 1T'-WTe2, which may envision in-depth study of its topological physics and device applications., Comment: 17pages, 4 figures in main text, supplementary included

Three-dimensional (3D) topological Dirac semimetal, when thinned down to 2D few layers, is expected to possess gapped Dirac nodes

Heterostructures of two-dimensional layered materials can be functionalized with exotic phenomena that are unpresented with each constituting component. The interface effect plays a key role in determining the electronic properties of the heterostructure, whose characterization requires a correlation with the morphology with atomic-scale precision. Here, we report an investigation on the electronic properties of few-layer Bi(110) films mediated by a NbSe2 substrate. By utilizing scanning tunneling microscopy and spectroscopy, we show a significant variation of the density of states at different Bi film thicknesses, resulting in an unusual superconducting proximity effect that deviates from the conventional monotonous decay behavior. Moreover, the electronic states of the Bi films are also prominently modulated by the Moire pattern spatially. With first-principles calculations, we illuminate these findings as the results of covalent-like quasi-bonds formed at the Bi/NbSe2 interface, which profoundly alter ...

Heterostructures of two-dimensional layered materials can be functionalized with exotic phenomena that are unpresented with each constituting component. The interface effect plays a key role in determining the electronic properties of the heterostructure, whose characterization requires a correlation with the morphology with atomic-scale precision. Here, we report an investigation on the electronic properties of few-layer Bi(110) films mediated by a NbSe

Two-dimensional materials offer opportunities for unravelling unprecedented ordered states at single layer limit. Among such ordered states, Mott phase is rarely explored. Here, we report the Mott phase in van der Waals chromium (II) iodide (CrI2) films. High quality CrI2 films with atomically flat surface and macro size are grown on graphitized 6H-SiC(0001) substrate by molecular beam epitaxy. By in situ low temperature scanning tunneling microscopy and spectroscopy (STM/STS), we reveal that the film has a band gap as large as ~3.2 eV, which is nearly thickness independent. Density functional plus dynamic mean field theory calculations suggest that CrI2 films may be a strong Mott insulator with a ferromagnetically ordered ground state. The Mott phase is corroborated by the spectral band splitting, that is consistent with the extended Hubbard model, and gap reduction at charge dopants. Our study provides a platform for studying correlated electron states at single layer limit., Comment: 18 pages, 4 figures

Freestanding single-bilayer Bi(111) is a two-dimensional topological insulator with edge states propagating along its perimeter. Given the interlayer coupling experimentally, the topological nature of Bi(111) thin films and the impact of the supporting substrate on the topmost Bi bilayer are still under debate. Here, combined with scanning tunneling microscopy and first-principles calculations, we systematically study the electronic properties of Bi(111) thin films grown on a NbSe2 substrate. Two types of non-magnetic edge structures, i.e., a conventional zigzag edge and a 2x1 reconstructed edge, coexist alternately at the boundaries of single bilayer islands, the topological edge states of which exhibit remarkably different energy and spatial distributions. Prominent edge states are persistently visualized at the edges of both single and double bilayer Bi islands, regardless of the underlying thickness of Bi(111) thin films. We provide an explanation for the topological origin of the observed edge states that is verified with first-principles calculations. Our paper clarifies the long-standing controversy regarding the topology of Bi(111) thin films and reveals the tunability of topological edge states via edge modifications., 36 pages, 10 figures

Weyl semimetals possess linear dispersions through pairs of Weyl nodes in three-dimensional momentum spaces, whose hallmark arclike surface states are connected to Weyl nodes with different chirality. $\mathrm{WT}{\mathrm{e}}_{2}$ was recently predicted to be a new type of Weyl semimetal. Here, we study the quasiparticle interference (QPI) of its Fermi arc surface states by combined spectroscopic-imaging scanning tunneling spectroscopy and density functional theory calculations. We observed the electron scattering on two types of $\mathrm{WT}{\mathrm{e}}_{2}$ surfaces unambiguously. Its scattering signal can be ascribed mainly to trivial surface states. We also address the QPI feature of nontrivial surface states from theoretical calculations. The experimental QPI patterns show some features that are likely related to the nontrivial Fermi arc states, whose existence is, however, not conclusive. Our study provides an indispensable clue for studying the Weyl semimetal phase in $\mathrm{WT}{\mathrm{e}}_{2}$.

Exploring the exciton-phonon interaction of cesium lead bromide (CsPbBr3) perovskite nanowires (NWs) is not only important to the fundamental understanding of phonon-assisted carrier dynamics but also critical for the development of high performance miniaturized light sources. Herein, we systematically investigated the temperature (T) dependent spontaneous emission (SE) and lasing spectra of chemical vapor deposited CsPbBr3 NWs. We demonstrated that both the SE and the lasing energy were mainly determined by two facts: the electron-phonon interaction (EP) and the lattice thermal expansion (TE). The lattice TE effect plays a key role at T ∼ 78–170 K, therefore both the lasing and SE peaks exhibit a nearly linear blueshift with increasing T. As T > 195 K, the EP interaction becomes dominant, which reduces the blueshift rate of the SE peak and causes a redshift of lasing peaks. These findings will further advance the understanding of the exciton-phonon interaction and the T-dependence of emission properties of lasers and related optoelectronic devices.Exploring the exciton-phonon interaction of cesium lead bromide (CsPbBr3) perovskite nanowires (NWs) is not only important to the fundamental understanding of phonon-assisted carrier dynamics but also critical for the development of high performance miniaturized light sources. Herein, we systematically investigated the temperature (T) dependent spontaneous emission (SE) and lasing spectra of chemical vapor deposited CsPbBr3 NWs. We demonstrated that both the SE and the lasing energy were mainly determined by two facts: the electron-phonon interaction (EP) and the lattice thermal expansion (TE). The lattice TE effect plays a key role at T ∼ 78–170 K, therefore both the lasing and SE peaks exhibit a nearly linear blueshift with increasing T. As T > 195 K, the EP interaction becomes dominant, which reduces the blueshift rate of the SE peak and causes a redshift of lasing peaks. These findings will further advance the understanding of the exciton-phonon interaction and the T-dependence of emission properties ...

By using a one-step approach, In-filled MgO nanotubes with diameters of 200-300 nm and lengths of several micrometers are synthesized. A Gatan heating experiment in a transmission electron microscope (TEM) shows that In-filling begins to melt far below the melting point 156.6 degrees C of In and abnormally shrinks upon increasing temperature above 270 degrees C. Massive In-filled MgO nanotubes were pressed into a disk and its resistance starts dramatically decrease upon increasing temperature above -150 degrees C, suggesting a temperature switch with a protective point -150 degrees C. The expansion and I-V characteristics were discussed.

The helical Dirac fermions on the surface of topological insulators host novel relativistic quantum phenomena in solids. Manipulating spins of topological surface state (TSS) represents an essential step towards exploring the theoretically predicted exotic states related to time reversal symmetry (TRS) breaking via magnetism or magnetic field. Understanding Zeeman effect of TSS and determining its g-factor are pivotal for such manipulations in the latter form of TRS breaking. Here, we report those direct experimental observations in Bi2Se3 and Sb2Te2Se by spectroscopic imaging scanning tunneling microscopy. The Zeeman shifting of zero mode Landau level is identified unambiguously by judiciously excluding the extrinsic influences associated with the non-linearity in the TSS band dispersion and the spatially varying potential. The g-factors of TSS in Bi2Se3 and Sb2Te2Se are determined to be 18 and -6, respectively. This remarkable material dependence opens a new route to control the spins in the TSS., main text: 17 pages, 4 figures; supplementary: 15 pages, 7 figures

The magnetic state and the Kondo effect in Mn phthalocyanine (MnPc) molecules on Pb(111) were investigated using low-temperature scanning tunneling microscopy (STM), density-functional theory, and numerical renormalization group calculations. We found that a unique spin state induced by the strong $\ensuremath{\pi}\ensuremath{-}d$ interaction between the Mn $d$-orbitals and ligand \ensuremath{\pi}-orbitals causes an underscreened Kondo effect observed both at Mn and the ligand. STM observations show that a Kondo resonance extends over the pyrrole ring in the Pc ligand rather than remaining localized at Mn. In gas-phase MnPc, we reveal that the $\ensuremath{\pi}\ensuremath{-}d$ interaction result in an $S=3/2$ collective spin state extended over the Pc ligand with a complex spin polarization inside the molecule. On Pb(111), charge transfer from the substrate reduces the total spin of MnPc from 3/2 to 1 while the intramolecular $\ensuremath{\pi}\ensuremath{-}d$ interaction is maintained. Partial screening of this collective $S=1$ spin state by the conduction electron in Pb(111) makes the Kondo resonance also observed at the ligand part, which is not directly hybridized with the substrate.

Massless Dirac electrons in condensed matter have attracted considerable attention. Unlike conventional electrons, Dirac electrons are described in the form of two-component wave functions. In the surface state of topological insulators, these two components are associated with the spin degrees of freedom, hence governing the magnetic properties. Therefore, the observation of the two-component wave function provides a useful clue for exploring the novel spin phenomena. Here we show that the two-component nature is manifested in the Landau levels (LLs) whose degeneracy is lifted by a Coulomb potential. Using spectroscopic-imaging scanning tunneling microscopy, we visualize energy and spatial structures of LLs in a topological insulator Bi2Se3. The observed potential-induced LL splitting and internal structures of Landau orbits are distinct from those in a conventional electron system and are well reproduced by a two-component model Dirac Hamiltonian. Our model further predicts non-trivial energy-dependent spin-magnetization textures in a potential variation. This provides a way to manipulate spins in the topological surface state., Comment: Revised version to appear in Nature Physics. Supplementary information is available at http://www.riken.jp/epmrt/Hanaguri/SI/Bi2Se3_2comp/SI.html

We demonstrate the controllable local manipulation of the Dirac surface state in a topological insulator, Bi2Te2Se, which has suppressed bulk carrier density. Using scanning tunneling microscopy/spectroscopy under magnetic fields, we observe Landau levels of the Dirac surface state in the conductance spectra. The Landau levels start to shift in their energy once the bias voltage between the tip and the sample exceeds a threshold value. The amount of shift depends on the history of bias ramping. As a result, conductance spectra show noticeable hysteresis, giving rise to a memory effect. The conductance images exhibit spatially inhomogeneous patterns which can also be controlled by the bias voltage in a reproducible way. On the basis of these observations, we argue that the memory effect is associated with the tip-induced local charging effect which is pinned by the defect-generated random potential. Our study opens up a new avenue to controlling the topological surface state.

A key challenge in the field of molecular spintronics, and for the design of single-molecule magnet-based devices in particular, is the understanding and control of the molecular coupling at the electrode interfaces. It was demonstrated for the field of molecular electronics that the characterization of the molecule-metal-interface requires the precise knowledge of the atomic environment as well as the molecular orbitals being involved in electron transport. To extend the field of molecular electronics towards molecular spintronics, it is of utmost importance to resolve the spin character of molecular orbitals interacting with ferromagnetic leads. Here we present first direct real-space images of spin-split molecular orbitals of a single-molecule magnet adsorbed on a ferromagnetic nanostructure. Moreover, we are able to determine quantitatively the magnitude of the spin-splitting as well as the charge state of the adsorbed molecule.

Superconductor films on semiconductor substrates have drawn much attention recently since the derived superconductor-based electronics have been shown to be promising for future data processing and storage technologies. By growing atomically uniform single-crystal epitaxial Pb films of several nanometers thick on Si wafers to form a sharp superconductor-semiconductor heterojunction, we have obtained an unusual magnetoresistance effect when the Pb film is superconducting. In addition to the large fundamental interest in this effect, the simple structure, and compatibility and scalability with current Si-based semiconductor technology offer a great opportunity for integrating superconducting circuits and detectors in a single chip.

We have performed spin-resolved measurements on a Kondo impurity in the presence of RKKY-type exchange coupling. By placing manganese phthalocyanine (MnPc) molecules on Fe-supported Pb islands, a Kondo system is devised which is exchange coupled to a magnetic substrate via conduction electrons in Pb, inducing spin splitting of the Kondo resonance. The spin-polarized nature of the split Kondo resonance and a spin filter effect induced by spin-flip inelastic electron tunneling are revealed by spin-polarized scanning tunneling microscopy and spectroscopy.