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Spatially controlling the Fermi level of topological insulators and keeping their electronic states stable are indispensable processes to put this material into practical use for semiconductor spintronics devices. So far, however, such a method has not been established yet. Here we show a novel method for doping a hole into n-type topological insulators Bi2X3 (X= Se, Te) that overcomes the shortcomings of the previous reported methods. The key of this doping is to adsorb H2O on Bi2X3 decorated with a small amount of carbon, and its trigger is the irradiation of a photon with sufficient energy to excite the core electrons of the outermost layer atoms. This method allows controlling the doping amount by the irradiation time and acts as photolithography. Such a tunable doping makes it possible to design the electronic states at the nanometer scale and, thus, paves a promising avenue toward the realization of novel spintronics devices based on topological insulators.

We report the growth process, structure, and electronic states of 1 unit layer (u.l.) of a Ce-Pt intermetallic compound on Pt(111) using scanning tunneling microscopy/spectroscopy (STM/STS) and low-energy electron diffraction. An ordered $(2\ifmmode\times\else\texttimes\fi{}2)$ structure was observed in the form of films or nanoislands depending on Ce dose by annealing at around 700 K. A structural model constructed from atomically resolved STM images and quasiparticle interference (QPI) patterns indicates the formation of a new surface compound 1 u.l. ${\mathrm{CePt}}_{6}$ on Pt(111) terminated by a Pt layer. A lateral lattice constant of the 1 u.l. ${\mathrm{CePt}}_{6}$ on Pt(111) is expanded from the value of a bulk ${\mathrm{CePt}}_{5}$ crystal to match the Pt(111) substrate. By measuring $dI/dV$ spectra and QPI, we observed an onset energy of the surface state found on Pt(111) above Fermi energy (${E}_{F}$) shifts below ${E}_{F}$ on the Pt layer of the 1 u.l. ${\mathrm{CePt}}_{6}$ due to charge transfer from the underneath ${\mathrm{CePt}}_{2}$ layer. We discuss a possible two-dimensional coherent Kondo effect with the observed spectra on the 1 u.l. ${\mathrm{CePt}}_{6}$.

Interesting electronic properties of a-few-degree-twisted bilayer graphene are caused by the formation of a flat band due to the interlayer interaction and electronic correlation. The authors quantitatively investigated the band structure of wide 3${}^{\mathrm{deg}}$-twisted bilayer graphene with clean interface by angle-resolved photoelectron spectroscopy, and compare the results with those of a band calculation using a recently-developed band unfolding method. The observed band structure indicates strong interlayer coupling that renormalizes the band structure including partial flat band features and gap formation. The observed band structure is in good agreement with the calculated results, indicating the importance of the interlayer coupling for the band renormalization.

Mapping of the local lattice distortion is required to understand the details of the chemical and physical properties of thin films. However, the resolution by the direct microscopic methods was insufficient to detect the local distortion. Here, we have demonstrated that the local distortion of a monatomic film on a substrate is estimated with high resolution using the moire pattern of the topographic scanning tunneling microscopy image. The analysis focused on the apparently low centers of the moire pattern of the hexagonal iron nitride monolayer on the Cu(111) substrate. The local distortion was visualized by estimating the displacement of the experimentally detected center position from the ideal position that is extracted from the adjacent center positions. The map of the local distortion revealed that the subsurface impurities are preferentially located on the largely distorted areas. This approach is widely applicable to other thin films on the substrates.

The electronic structure and the domain wall width of industrial (Nd,Dy)$_{2}$Fe$_{14}$B hard magnets were investigated using low-temperature, spin-polarized scanning tunneling microscopy (STM) in ultra-high vacuum. In a first step, atomically clean and flat surfaces were prepared. The flat terraces were separated by monatomic steps. Surface termination was identified as the Fe c layer from atomically resolved STM imaging. The electronic density of states and its spin polarization agree well with ab initio predictions of the Fe c layer. High-resolution spin-polarized STM images allowed to finally resolve the domain wall width w of only 3.2 ± 0.4 nm.

The robust bonding between Fe and N atoms has the potential to fabricate a ferromagnetic Fe

Twisted bilayer graphene (TBG), in which two monolayer graphene are stacked with an in-plane rotation angle, has recently become a hot topic due to unique electronic structures. TBG is normally produced in air by the tear-and-stack method of mechanical exfoliation and transferring graphene flakes, by which a sizable, millimeter-order area, and importantly clean interface between layers are hard to obtain. In this study, we resolved these problems by directly transferring the easy-to-exfoliate CVD-grown graphene on SiC substrate to graphene in a high vacuum without using any transfer assisting medium and observed electronic band modulations due to the strong interlayer coupling.

Detection of lattice strain is crucial for various studies in a nanometer scale because it largely modifies the local electronic states and thus various physical and chemical properties. Here, we demonstrate that the Kondo effect in a single magnetic atom on a metal surface can be a quantum sensor for the local lattice strain. Using low-temperature scanning tunneling spectroscopy, we measured the Kondo resonance in a Co adatom on partially N-adsorbed Cu(001) surfaces, which consist of nanoislands of the Cu 2N monolayer and the clean Cu(001) surface compressed by the surrounding Cu 2N nanoislands. The observed Kondo temperature at the compressed clean surface depends on the area size of the surface, i . e ., the strength of the local lattice strain. This behavior is attributed to the change in the distance between the Co adatom and Cu surface due to the surface lattice strain, which is supported by our density functional calculations. These results provide a way to detect the local strain on the sub-angstrom scale by using the sensitivity of quantum many-body effects.

Rectangular iron nitride monatomic layers are fabricated on the threefold symmetric Cu(111) substrate by taking advantage of the stability of the square nitride film. Two different ordered structures are observed on the Cu(111) substrate by scanning tunneling microscopy after annealing at 510 K and 580 K. Their chemical composition and lattice symmetry are investigated by x-ray photoelectron spectroscopy and low energy electron diffraction. The monolayer film prepared at 580 K is a distorted ${\mathrm{Fe}}_{2}\mathrm{N}$ monolayer analogous to a ferromagnetic square ${\mathrm{Fe}}_{2}\mathrm{N}$ monolayer with a clock reconstruction on the Cu(001) substrate. The lattice deformation of the square ${\mathrm{Fe}}_{2}\mathrm{N}$ monolayer is induced by using Cu(111) with threefold symmetry.

Spins in magnetic nanostructures can be excited by an electric current. In the inelastic scattering events, the spin of a tunneling electron provided by a scanning tunneling microscope may be flipped and angular momentum is transferred to the magnetic system. We used this technique to determine magnetic anisotropies and exchange couplings in 3d transition and 4f rare earth metal atoms and clusters on surfaces. Further, we discuss that the magnetic anisotropy, i.e. the zero field splitting, in rare earth atoms requires a fully relativistic description, in which not only the spin of the atoms but also their orbital angular momentum is taken into account. When describing the magnetic anisotropy with the Hamiltonian including all Stevens operators and combining it with spin flip scattering by conduction and tunneling electrons, we find that long spin lifetimes are theoretically expected for specific total angular momenta and crystal field symmetries and show experimental evidence that such a situation is realized in Ho on Pt(111).

We experimentally prepared a ferromagnet with an L10-FeCo ordered structure by inserting a periodic buffer layer to suppress the B2 structural transition and to maintain the L10 structure. The sample was fabricated by utilizing a technique involving the deposition of alternating monoatomic layers using pulsed laser deposition (PLD). This technique was used to deposit a thin film of (7 ML-FeCo/3 ML-buffer)3, in which either Cu or Ni was utilized as buffer layer. We characterized the surface roughness, surface morphology, lattice structure, and magnetic properties of the specimens by RHEED, AFM, SR-XRD, and SQUID, respectively. As a result, we successfully confirmed the construction of the L10-FeCo superstructure with a periodic Ni buffer layer for the first time. Both the magnetic moment and magnetic anisotropy were also increased by replacing Cu with Ni.

We demonstrated the fabrication of a rare-earth-free ferromagnetic L10-type Fe–Ni alloy (L10-FeNi) by pulsed laser deposition (PLD). We deposited Fe and Ni on Cu(001) by alternating monoatomic deposition via automatically stabilized laser ablation. We examined the structural properties, magnetic properties, and surface morphology of the alloy specimens as the growth temperature (Ts) was varied. We adequately confirmed the construction of the most prominent L10-FeNi phase at 300 °C, which is significantly higher than previously reported growth temperatures, indicating that PLD followed by thermal treatment promoted two-dimensional growth of the adsorbent. The formation process of L10-FeNi was investigated from the standpoint of surface thermodynamics, and the results suggest that the surface free energy of PLD and its highly instantaneous deposition process by PLD played key roles. Our findings are expected to lead to advanced methods for the fabrication of L10-FeNi.

Single magnetic atoms on non-magnetic surfaces have magnetic moments that are usually destabilized within a microsecond, too speedily to be useful, but here the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. The magnetic moments of individual magnetic atoms are attractive components for both memory and quantum computing applications. But interactions between such atoms and the substrates on which they are mounted tend to destabilize the magnetic moments, giving them lifetimes of typically less than a few milliseconds. Toshio Miyamachi and colleagues have now identified a system consisting of single atoms of the lanthanide series rare earth element holmium on a highly conductive surface, in which intrinsic symmetries related to the properties of both the atom and the substrate combine to minimize these destabilizing interactions. As a result, the magnetic moments of the atoms can achieve lifetimes of several minutes. Single magnetic atoms, and assemblies of such atoms, on non-magnetic surfaces have recently attracted attention owing to their potential use in high-density magnetic data storage and as a platform for quantum computing1,2,3,4,5,6,7,8. A fundamental problem resulting from their quantum mechanical nature is that the localized magnetic moments of these atoms are easily destabilized by interactions with electrons, nuclear spins and lattice vibrations of the substrate3,4,5. Even when large magnetic fields are applied to stabilize the magnetic moment, the observed lifetimes remain rather short5,6 (less than a microsecond). Several routes for stabilizing the magnetic moment against fluctuations have been suggested, such as using thin insulating layers between the magnetic atom and the substrate to suppress the interactions with the substrate’s conduction electrons2,3,5, or coupling several magnetic moments together to reduce their quantum mechanical fluctuations7,8. Here we show that the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. The necessary decoupling from the thermal bath of electrons, nuclear spins and lattice vibrations is achieved by a remarkable combination of several symmetries intrinsic to the system: time reversal symmetry, the internal symmetries of the total angular momentum and the point symmetry of the local environment of the magnetic atom.

Structural and electronic properties of the SmB6(001) single-crystal surface prepared by Ar+ ion sputtering and controlled annealing are investigated by scanning tunneling microscopy. In contrast to the cases of cleaved surfaces, we observe a single phase surface with a non-reconstructed p(1 × 1) lattice on the entire surface at an optimized annealing temperature. The surface is identified as Sm-terminated on the basis of spectroscopic measurements. On a structurally uniform surface, the emergence of the in-gap state, a robust surface state against structural variation, is further confirmed inside a Kondo hybridization gap at 4.4 K by temperature and atomically-resolved spatial dependences of the differential conductance spectrum near the Fermi energy.

The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of PbSi chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.

Magnetic thin film heterostructures have been widely studied for fundamental interests in the emergence of novel phenomena accompanied by the heterointerface formation as well as their promising practical potential. Combining x-ray magnetic circular dichroism with scanning tunneling microscopy, we show for Mn/Fe thin film heterostructures that the interfacial factors dominating electronic and magnetic properties of the entire system dynamically change with the amount of the Mn overlayer. Element specific magnetization curves of the Fe layer exhibit a two-step spin reorientation transition from out-of-plane to in-plane direction with increasing the Mn coverage. Atomic-scale characterizations of structural and electronic properties in combination with the first-principles calculations successfully unravel the roles of the entangled interfacial factors, and clarify the driving forces of the transition. The first step of the transition at a low Mn coverage is dominantly induced by the formation of FeMn disordered alloy at the heterointerface, and the electronic hybridization with interfacial FeMn ordered alloy is dominant as the origin of the second step of the transition at a high Mn coverage.

We show a new way to stabilize epitaxial structures against transforming bulk stable phases for Fe thin films on a vicinal Cu(001) surface. Atomically-resolved observations by scanning tunneling microscopy reveal that high-density Cu steps serve as strain relievers for keeping epitaxially-stabilized Fe fcc(001) lattice even at a transient thickness towards the bulk stable bcc(110) lattice. Spectroscopic measurements further clarify the intrinsic electronic properties of the fcc Fe thin film in real space, implying electronic differences between 6 and 7 monolayer thick films induced by the modification of the lattice constant in the topmost layers., Comment: 5 pages, 5 figures

In scanning tunneling microscopy, orbital selectivity of the tunneling process can make the topographic image dependent on a tip-surface distance. We have found reproducible dependence of the images on the distance for a monatomic layer of iron nitride formed on a Cu(001) surface. Observed atomic images systematically change between a regular dot array and a dimerized structure depending on the tip-surface distance, which turns out to be the only relevant parameter in the image variation. An accompanied change in the weight of Fe-3d local density of states to a tunneling background was detected in dI/dV spectra. These have been attributed to a shift in surface orbitals detected by the tip from the d states to the s/p states with increasing the tip-surface distance, consistent with an orbital assignment from first-principles calculations.

Sn atomic layers attract considerable interest owing to their spin-related physical properties caused by their strong spin–orbit interactions. We performed Sn intercalation into the graphene/SiC(0001) interface and found a new type of Sn atomic layer. Sn atoms occupy on-top sites of Si-terminated SiC(0001) with in-plane Sn–Sn bondings, resulting in a triangular lattice. Angle-resolved photoemission spectroscopy revealed characteristic dispersions at and points, which agreed well with density functional theory calculations. The Sn triangular lattice atomic layer at the interface showed no oxidation upon exposure to air, which is useful for characterization and device fabrication ex situ.

Metal nitride has robust bonding between metal and nitrogen atoms. We have synthesized a monoatomic layer of a hexagonal-type iron nitride with strong Fe-N bond on the Cu(001) substrate with a fourfold lattice symmetry. From atomic-scale observation using scanning tunneling microscopy, we have found that the Fe-N film exhibits a strip structure consisting of regular and deformed hexagonal lattices. The lattices are deformed along a hypothetical Moir\'e pattern made by a perfect hexagonal lattice on the square Cu(001) lattice. With increasing the lattice deformation, the strain energy increases, whereas the periodic deformation of the Fe-N lattice can minimize the total lattice energy including the strain energy of the Fe-N in-plane bonding and interfacial bonding energy between the Fe-N layer and the Cu substrate. In $\mathrm{d}I/\mathrm{d}V$ spectra, periodic spatial change of the unoccupied density of states due to the lattice strain was observed. This is attributed to an energy shift of the antibonding states due to compressive strain.

Local electron-phonon coupling of a one-dimensionally nanorippled graphene is studied on a SiC(0001) vicinal substrate. We have characterized local atomic and electronic structures of a periodically nanorippled graphene (3.4 nm period) prepared on a macrofacet of the 6H-SiC crystal using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES). The rippled graphene on the macrofacets distributes homogeneously over the 6H-SiC substrate in a millimeter scale, and thus replica bands are detected by the macroscopic ARPES. The STM/STS results indicate the strength of electron-phonon coupling to the out-of-plane phonon at the K̅ points of graphene is periodically modified in accordance with the ripple structure. We propose an interface carbon nanostructure with graphene nanoribbons between the surface rippled graphene and the substrate SiC that periodically modifies the electron-phonon coupling in the surface graphene.

In this review, we give an overview on the spin crossover of Fe(phen)(2)(NCS) (2) complexes adsorbed on Cu(100), Cu2N/Cu(100), Cu(111), Co/Cu(111), Co(100), Au(100), and Au(111) surfaces. Depending on the strength of the interaction of the molecules with the substrates, the spin crossover behavior can be drastically changed. Molecules in direct contact with non-magnetic metallic surfaces coexist in both the high-and low-spin states but cannot be switched between the two. Our analysis shows that this is due to a strong interaction with the substrate in the form of a chemisorption that dictates the spin state of the molecules through its adsorption geometry. Upon reducing the interaction to the surface either by adding a second molecular layer or inserting an insulating thin film of Cu2N, the spin crossover behavior is restored and molecules can be switched between the two states with the help of scanning tunneling microscopy. Especially on Cu2N, the two states of single molecules are stable at low temperature and thus allow the realization of a molecular memory. Similarly, the molecules decoupled from metallic substrates in the second or higher layers display thermally driven spin crossover as has been revealed by X-ray absorption spectroscopy. Finally, we discuss the situation when the complex is brought into contact with a ferromagnetic substrate. This leads to a strong exchange coupling between the Fe spin in the high-spin state and the magnetization of the substrate as deduced from spin-polarized scanning tunneling spectroscopy and ab initio calculation. Published

We studied magnetism and morphology of metastable bcc Co nanostructures on Au(001) by x-ray magnetic circular dichroism in combination with scanning tunneling microscopy and photoelectron emission microscopy. While room-temperature deposition of Co onto Au(001) leads to the formation of bcc Co thin films with pure in-plane magnetization, postannealing of these thin films at 500 K drastically changes the morphology to bcc Co islands embedded in Au. In accordance with this morphological change, we find that an out-of-plane magnetization emerges additionally and the coexistence of in-plane and out-of-plane remanent magnetizations is observed for the islands. The nanostructure-size dependence of magnetic moments suggests that the magnetization easy axis of the island changes from the in-plane to the out-of-plane direction with decreasing nanostructure size. Such a spin reorientation transition is likely due to the increased fractional population of rim atoms generating the out-of-plane magnetization in the smaller islands. The observed out-of-plane remanent magnetization of the smaller islands indicates their large magnetic anisotropy.

The growth of spin-crossover Fe(1,10-phenanthroline)${}_{2}$(NCS)${}_{2}$ molecules on Cu(100) surfaces in the coverage range from 0.1 to 1.8 molecular layers was studied using a scanning tunneling microscope (STM) operated in ultrahigh vacuum at low temperature ($\ensuremath{\approx}$4 K). STM imaging allowed us to extract the molecular adsorption geometry. While the first-layer molecules point their NCS groups toward the surface and their phenanthroline groups upwards, the adsorption geometry is reversed for the molecules in the second layer. For submonolayer coverages, a coexistence of molecules in the high- and low-spin states was found that is not correlated with the coverage. This coexistence is reduced for second-layer molecules, leading to a dominant spin state at low temperatures. Differential conductance spectra acquired at negative bias voltage on first- and second-layer molecules suggest that second-layer molecules are in the high-spin state and are partially electronically decoupled from the substrate. Furthermore, increasing the tip-to-sample voltage reduces the distance between the two lobes of the molecule. The current dependence of this effect suggests that a smooth spin crossover from a high- to a low-spin state occurs with increasing sample voltage. This analog spin-state switching is well described within a simple transition-state model involving modifications to the energy barriers between low- and high-spin states due to a tip-induced electric field through the Stark effect.

We report a combined experimental and theoretical study of the Kondo effect in a series of binuclear metal-organic complexes of the form [(Me(hfacac)_2)_2(bpym)]^0, with Me = Nickel (II), Manganese(II), Zinc (II); hfacac = hexafluoroacetylacetonate, and bpym = bipyrimidine, adsorbed on Cu(100) surface. While Kondo-features did not appear in the scanning tunneling spectroscopy spectra of non-magnetic Zn_2, a zero bias resonance was resolved in magnetic Mn_2 and Ni_2 complexes. The case of Ni_2 is particularly interesting as the experiments indicate two adsorption geometries with very different properties. For Ni_2-complexes we have employed density functional theory to further elucidate the situation. Our simulations show that one geometry with relatively large Kondo temperatures T_K ~ 10K can be attributed to distorted Ni_2 complexes, which are chemically bound to the surface via the bipyrimidine unit. The second geometry, we assign to molecular fragmentation: we suggest that the original binuclear molecule decomposes into two pieces, including Ni(hexafluoroacetylacetonate)_2, when brought into contact with the Cu-substrate. For both geometries our calculations support a picture of the (S=1)-type Kondo effect emerging due to open 3d shells of the individual Ni^{2+} ions., Comment: 11 pages, 10 figures, Supplementary Information is attached as a separate PDF file, submitted to Phys. Rev. B

The dynamics of the soft x-ray induced excited spin state trapping (SOXIESST) effect of Fe(phen)(2)(NCS)(2) (Fe-phen) powder have been investigated by x-ray absorption spectroscopy (XAS) using the total electron yield method, in a wide temperature range. The low-spin (LS) state is excited into the metastable high-spin (HS) state at a rate that depends on the intensity of the x-ray illumination it receives, and both the temperature and the intensity of the x-ray illumination will affect the maximum HS proportion that is reached. We find that the SOXIESST HS spin state transforms back to the LS state at a rate that is similar to that found for the light induced excited spin state trapping (LIESST) effect. We show that it is possible to use the SOXIESST effect in combination with the LIESST effect to investigate the influence of cooperative behavior on the dynamics of both effects. To investigate the impact of molecular cooperativity, we compare our results on Fe-phen with those obtained for Fe[Me(2)Pyrz](3)BH(2) (Fe-pyrz) powder, which exhibits a similar thermal transition temperature but with a hysteresis. We find that, while the time constant of the dynamic is identical for both molecules, the SOXIESST effect is less efficient at exciting the HS state in Fe-pyrz than in Fe-phen. (C) 2013 AIP Publishing LLC.

We present a view on inelastic scanning tunneling spectroscopy of magnetic impurities relying on states of the total angular momentum J=L+S in the presence of a crystal field. We show that the selection rules for spin-flip scattering within the J-multiplet agree with the simple selection rules for the effective spin model, but also show the deviations from the latter for the transition probabilities. A reinterpretation of some recent experimental findings in a description based on the total angular momentum J is done. © 2011 American Physical Society.

An epitaxial silicon-oxide monolayer of chemical composition of Si2O3 (the Si2O3 layer) formed on hexagonal SiC(0001¯) surfaces has been observed by scanning tunneling microscopy (STM). Filled- and empty-state STM images with atomic resolution support the previously reported model. Typical structural defects in the Si2O3 layer are found to be missing SiOn (n = 1, 2, 3) molecules. The band gap of the Si2O3 layer obtained by point tunneling spectroscopy is 5.5±0.5 eV, exhibiting considerable narrowing from that of bulk SiO2, 8.9 eV. It is proposed that the Si2O3 layer is suitable as a relevant interface material for formation of SiC-based metal-oxide-semiconductor devices.