Scanning Tunneling Spectra Research Articles

Spin‐State Switching of Spin‐Crossover Complexes on Cu(111) Evidenced by Spin‐Flip Spectroscopy

Spin‐crossover compounds can be switched between two stable states with different magnetic moments, conformations, electronic, and optical properties, which opens appealing perspectives for technological applications including miniaturization down to the scale of single molecules. Although control of the spin states is crucial their direct identification is challenging in single‐molecule experiments. Here we investigate the spin‐crossover complex [Fe(HB(1,2,4‐triazol‐1‐yl)3)2] on a Cu(111) surface with scanning tunneling microscopy and density functional theory calculations. Spin crossover of single molecules in dense islands is achieved via electron injection. Spin‐flip excitations are resolved in scanning tunneling spectra in a magnetic field enabling the direct identification of the molecular spin state, and revealing the existence of magnetic anisotropy in the HS molecules.

Spin-State Switching of Spin-Crossover Complexes on Cu(111) Evidenced by Spin-Flip Spectroscopy.

Spin-crossover compounds can be switched between two stable states with different magnetic moments, conformations, electronic, and optical properties, which opens appealing perspectives for technological applications including miniaturization down to the scale of single molecules. Although control of the spin states is crucial their direct identification is challenging in single-molecule experiments. Here we investigate the spin-crossover complex [Fe(HB(1,2,4-triazol-1-yl)3)2] on a Cu(111) surface with scanning tunneling microscopy and density functional theory calculations. Spin crossover of single molecules in dense islands is achieved via electron injection. Spin-flip excitations are resolved in scanning tunneling spectra in a magnetic field enabling the direct identification of the molecular spin state, and revealing the existence of magnetic anisotropy in the HS molecules.

Bosonic excitation spectra of superconducting Bi2Sr2CaCu2O8+δ and YBa2Cu3O6+x extracted from scanning tunneling spectra

A detailed interpretation of scanning tunneling spectra obtained on unconventional superconductors enables one to gain information on the pairing boson. Decisive for this approach are inelastic tunneling events. Due to the lack of momentum conservation in tunneling from or to the sharp tip, those are enhanced in the geometry of a scanning tunneling microscope compared to planar tunnel junctions. This work extends the method of obtaining the bosonic excitation spectrum by deconvolution from tunneling spectra to nodal d-wave superconductors. In particular, scanning tunneling spectra of slightly underdoped Bi2Sr2CaCu2O8+δ with a Tc of 82 K and optimally doped YBa2Cu3O6+x with a Tc of 92 K reveal a resonance mode in their bosonic excitation spectrum at Ωres≈63 meV and Ωres≈61 meV respectively. In both cases, the overall shape of the bosonic excitation spectrum is indicative of predominant spin scattering with a resonant mode at Ωres<2Δ and overdamped spin fluctuations for energies larger than 2Δ. To perform the deconvolution of the experimental data, we implemented an efficient iterative algorithm that significantly enhances the reliability of our analysis.

Two-dimensional heavy fermions in the van der Waals metal CeSiI.

Heavy-fermion metals are prototype systems for observing emergent quantum phases driven by electronic interactions1-6. A long-standing aspiration is the dimensional reduction of these materials to exert control over their quantum phases7-11, which remains a significant challenge because traditional intermetallic heavy-fermion compounds have three-dimensional atomic and electronic structures. Here we report comprehensive thermodynamic and spectroscopic evidence of an antiferromagnetically ordered heavy-fermion ground state in CeSiI, an intermetallic comprising two-dimensional (2D) metallic sheets held together by weak interlayer van der Waals (vdW) interactions. Owing to its vdW nature, CeSiI has a quasi-2D electronic structure, and we can control its physical dimension through exfoliation. The emergence of coherenthybridization of f and conduction electrons at low temperature is supported by the temperature evolution of angle-resolved photoemission and scanning tunnelling spectra near the Fermi level andby heat capacity measurements. Electrical transport measurements on few-layer flakes reveal heavy-fermion behaviour and magnetic order down to the ultra-thin regime. Our work establishes CeSiI and related materials as a unique platform for studying dimensionally confined heavy fermions in bulk crystals and employing 2D device fabrication techniques and vdW heterostructures12 to manipulate the interplay between Kondo screening, magnetic order and proximity effects.

On-Surface Synthesis and Characterization of a High-Spin Aza-[5]-Triangulene.

Triangulenes are open-shell triangular graphene flakes with total spin increasing with their size. In the last years, onsurface- synthesis strategies have permitted fabricating and engineering triangulenes of various sizes and structures with atomic precision. However, direct proof of the increasing total spin with their size remains elusive. In this work, we report the combined in-solution and on-surface synthesis of a large nitrogen-doped triangulene (aza-[5]-triangulene) and the detection of its high spin ground state on a Au(111) surface. Bond-resolved scanning tunneling microscopy images uncovered radical states distributed along the zigzag edges, which were detected as weak zero-bias resonances in scanning tunneling spectra. These spectral features reveal the partial Kondo screening of a high spin state. Through a combination of several simulation tools, we find that the observed distribution of radical states is explained by a quintet ground state (S = 2), instead of the expected quartet state (S = 3/2), confirming the positively charged state of the molecule on the surface.We further provide a qualitative description of the change of (anti)aromaticity introduced by N-substitution, and its role in the charge stabilization on a surface, resulting in a S = 2 aza-[5]-triangulene on Au(111).

Thickness-dependent energy gap of van der Waals epitaxial Cr1+δTe2 films: An insulator-metal transition

Recent research on ferromagnetic 2D materials renewed the interest in Cr-Te-system films [Cr1+δTe2 (0 ≤ δ ≤ 1)], a kind of pseudo-layer compounds which were found to be an alternative to pure van der Waals materials. Here, the phase structure and an insulator–metal transition (IMT) induced by interface effect of Cr-Te-system films grown epitaxially on mica substrates were investigated. In situ reflection high energy electron diffraction, scanning tunneling microscopy, x-ray diffraction, scanning transmission electron microscopy, and scanning tunneling microscope measurements showed the phase structure of the high-quality as-grown films was Cr3Te4 (δ = 0.5). By combining scanning tunneling spectrum and ultraviolet–visible absorption spectroscopy, we found that the energy gap of the film is remarkably thickness-dependent and an IMT phenomenon was observed at a critical film thickness of about 4 nm (the bandgap decreased from 2.5 eV to 0 eV as the thickness increased from 2 nm to 24 nm). These findings will promote the development of practical applications of Cr1+δTe2 in photoelectronics and electronics devices.

Critical behavior in the epitaxial growth of two-dimensional tellurium films on SrTiO3 (001) substrates

Materials’ properties may differ in the thin-film form, especially for epitaxial ultra-thin films, where the substrates play an important role in their deviation from the bulk quality. Here by molecular beam epitaxy (MBE) and scanning tunneling microscopy/spectroscopy, we investigate the growth kinetics of ultra-thin tellurium (Te) films on SrTiO3 (STO) (001). The MBE growth of Te films usually exhibits Volmer–Weber (VW) island growth mode and no a-few-monolayer film with full coverage has been reported. The absence of wetting-layer formation in the VW growth mode of Te on STO (001) is resulted from its low diffusion barriers as well as its relatively higher surface energy compared with those of the substrate and the interface. Here we circumvent these limiting factors and achieve the growth of ultra-thin β-Te films with near-complete coverages by driving the growth kinetics to the extreme condition. There is a critical thickness (3 monolayer) above which the two-dimensional Te films can form on the STO (001) substrate. In addition, the scanning tunneling spectra on the ultra-thin Te film grown on STO exhibits an enormously large forbidden gap compared with that grown on the graphene substrate. Our work establishes the necessary conditions for the growth of ultra-thin materials with similar kinetics and thermodynamics.

Critical behavior in the epitaxial growth of two-dimensional tellurium films on SrTiO3 (001) substrates

Materials’ properties may differ in the thin-film form, especially for epitaxial ultra-thin films, where the substrates play an important role in their deviation from the bulk quality. Here by molecular beam epitaxy (MBE) and scanning tunneling microscopy/spectroscopy, we investigate the growth kinetics of ultra-thin tellurium (Te) films on SrTiO3 (STO) (001). The MBE growth of Te films usually exhibits Volmer–Weber (VW) island growth mode and no a-few-monolayer film with full coverage has been reported. The absence of wetting-layer formation in the VW growth mode of Te on STO (001) is resulted from its low diffusion barriers as well as its relatively higher surface energy compared with those of the substrate and the interface. Here we circumvent these limiting factors and achieve the growth of ultra-thin β-Te films with near-complete coverages by driving the growth kinetics to the extreme condition. There is a critical thickness (3 monolayer) above which the two-dimensional Te films can form on the STO (001) substrate. In addition, the scanning tunneling spectra on the ultra-thin Te film grown on STO exhibits an enormously large forbidden gap compared with that grown on the graphene substrate. Our work establishes the necessary conditions for the growth of ultra-thin materials with similar kinetics and thermodynamics.

Real-space BCS-BEC crossover in FeSe monolayers

The quantum many-body states in the Bardeen-Cooper-Schrieffer--Bose-Einstein condensation (BCS-BEC) crossover regime are of long-lasting interest. Here we report direct spectroscopic evidence of BCS-BEC crossover in real space in a FeSe monolayer thin film by using spatially resolved scanning tunneling spectra. The crossover is driven by the shift of band structure relative to the Fermi level. The theoretical calculation based on a two-band model qualitatively reproduces the measured spectra in the whole crossover range. In addition, the Zeeman splitting of the quasiparticle states is found to be consistent with the characteristics of a condensate. Our work paves the way to study the exotic states of BCS-BEC crossover in a two-dimensional crystalline material at the atomic scale.

Topological quantum phase transition of nickelocene on Cu(100)

Local quantum phase transitions driven by Kondo correlations have been theoretically proposed in several magnetic nanosystems; however, clear experimental signatures are scant. Modeling a nickelocene molecule on a Cu(100) substrate as a two-orbital Anderson impurity with single-ion anisotropy coupled to two conduction bands, we find that recent scanning tunneling spectra reveal the existence of a topological quantum phase transition from the usual local Fermi liquid with high zero-bias conductance to a non-Landau Fermi liquid, characterized by a non-trivial quantized Luttinger integral, with a small conductance. The effects of intermediate valence, finite temperature, and structural relaxation of the molecule position allow us to explain the different observed behaviors.

Resolving Site-Specific Energy Levels of Small-Molecule Donor-Acceptor Heterostructures Close to Metal Contacts.

The active material of optoelectronic devices must accommodate for contacts which serve to collect or inject the charge carriers. It is the purpose of this work to find out to which extent properties of organic optoelectronic layers change close to metal contacts compared to known properties of bulk materials. Bottom-up fabrication capabilities of model interfaces under ultrahigh vacuum and single-atom low temperature (LT)-STM spectroscopy with density functional theory (DFT) calculations are used to detect the spatial modifications of electronic states such as frontier-orbitals at interfaces. The system under consideration is made of a silver substrate covered with a blend of C60 and ZnPc molecules of a few monolayers. When C60 and ZnPc are separately adsorbed on Ag(111), they show distinct spectroscopic features in STM. However, when C60 is added to the ZnPc monolayer, it shows scanning tunneling spectra similar to ZnPc, revealing a strong interaction of C60 with the ZnPc induced by the substrate. DFT calculations on a model complex confirm the strong hybridization of C60 with ZnPc layer upon adsorption on Ag(111), thus highlighting the role of boundary layers where the donor-acceptor character is strongly perturbed. The calculation also reveals a significant charge transfer from the Ag to the complex that is likely responsible for a downward shift of the molecular LUMO in agreement with the experiment.

Lattice-Matched Metal-Semiconductor Heterointerface in Monolayer Cu2Te.

The interface between metals and semiconductors plays an essential role in two-dimensional electronic heterostructures, which has provided an alternative opportunity to realize next-generation electronic devices. Lattice-matched two-dimensional heterointerfaces have been achieved in polymorphic 2D transition-metal dichalcogenides MX2 with M = (W, Mo) and X = (Te, Se, S) through phase engineering; yet other transition-metal chalcogenides have been rarely reported. Here we show that a single layer of hexagonal Cu2Te crystal could be synthesized by one-step liquid-solid interface growth and exfoliation. Characterizations of atomically resolved scanning tunneling microscope reveal that the Cu2Te monolayer consists of two lattice-matched distinct phases, similar to the 1T and 1T' phases of MX2. The scanning tunneling spectra identify the coexistence of the metallic 1T and semiconducting 1T' phases within the chemically homogeneous Cu2Te crystals, as confirmed by density functional theory calculations. Moreover, the two phases could form nanoscale lattice-matched metal-semiconductor junctions with atomically sharp interfaces. These results suggest a promising potential for exploiting atomic-scale electronic devices in 2D materials.

Enhancement of maximum superconducting temperature by applying pressure and reducing the charge transfer gap

Recent Scanning Tunneling Spectra(STS) measurements on underdoped cuprates have found that the maximum superconducting transition temperature Tc increases when the size of charge transfer gap (CTG) is reduced. Applying pressure is another well-known method to increase the maximum Tc. However, these pressure experiments also found that Tc is enhanced in underdoped and optimally-doped samples but suppressed in overdoped ones. Here we present a possible mechanism based on the charge fluctuation to explain both these observed effects. Starting from the 3-band Hubbard model, we retrieve the charge fluctuation (CF) between the oxygen 2p6 band and copper 3d10 band, which is ignored in the t−J model. This model is investigated using the variational Monte Carlo method(VMC).

Theoretical explanation of scanning tunneling spectrum of cleaved (110) surface of InGaAs

The cross-sectional (110) surface of In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As/InP hetero-structure grown by molecular beam epitaxy on an InP (001) substrate is characterized by the cross-sectional scanning tunneling microscopy (XSTM). The cleaved (110) surface across the interface between the In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As layer and InP layer is atomically flat but displays slight different image contrast between the two neighbor regions. The scanning tunneling spectroscopy (STS) is used to measure the current/voltage (&lt;i&gt;I-V&lt;/i&gt;) spectra. The &lt;i&gt;I-V&lt;/i&gt; data of the InGaAs surface and InP (110) surface show the different characteristics. The voltage range of zero-current plateau (apparent band gap) in the &lt;i&gt;I-V&lt;/i&gt; spectrum of InP displays the values close to its energy band gaps whereas the plateau ranges in the spectra of In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As are by contrast generally 50% larger than the energy band gap of In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As. The above phenomenon implies the different physical pictures on the tunneling of two surfaces. In the case of InP, the flat band model is feasible since the band edge states existing in the InP (110) surface can prevent the surface from being affected by the tip –induced band bending (TIBB) effect. In contrast, the TIBB effect must be taken into account to explain the &lt;i&gt;I-V&lt;/i&gt; spectra of the In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As (110) surface. A statistical analysis of the &lt;i&gt;I-V&lt;/i&gt; data of In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As reveals that the width of current plateau in the &lt;i&gt;I-V&lt;/i&gt; spectrum is generally between 1.05 eV and 1.20 eV and the current onset points (turn-points) with the plateau for the different spectra are slightly different from each other. We are able to explain quantitatively the above features based on the three-dimensional TIBB model given by Feenstra (&lt;ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1116/1.1606466"&gt;2003 &lt;i&gt;J.&lt;/i&gt; &lt;i&gt;Vac. Sci. Technol. B&lt;/i&gt; &lt;b&gt;21&lt;/b&gt; 2080&lt;/ext-link&gt;). Our calculation reveals that the parameter of density of surface states (DOSS) is a sensitive parameter responsible for the &lt;i&gt;I-V&lt;/i&gt; features mentioned above. According to an appropriate assignment of the value of DOSS, which is generally taken in the scope of (0.8–3.0) × 10&lt;sup&gt;12&lt;/sup&gt; (cm&lt;sup&gt;2&lt;/sup&gt;·eV)&lt;sup&gt;–1&lt;/sup&gt;, we well predict both the width and the onset points of the current-plateau. Moreover, the model also reproduces the line-shapes of the &lt;i&gt;I-V&lt;/i&gt; spectra measured on In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As.

Germanium/MoS2 : Competition between the growth of germanene and intercalation

We have scrutinized the growth of germanium (Ge) on molybdenum disulfide (${\mathrm{MoS}}_{2}$) using scanning tunneling microscopy and density functional theory calculations in order to resolve the still outstanding question whether Ge atoms prefer to intercalate between the ${\mathrm{MoS}}_{2}$ layers or rather form germanene islands on top of the ${\mathrm{MoS}}_{2}$ substrate. We found that, at a fixed growth temperature, germanene islands are formed on top of the ${\mathrm{MoS}}_{2}$ substrate at high deposition rates, whereas at low deposition rates the Ge intercalates between the ${\mathrm{MoS}}_{2}$ layers. Scanning tunneling spectra recorded on the germanene islands reveal a $V$-shaped density of states, which is one of the hallmarks of a two-dimensional Dirac material. The intercalated Ge clusters have a band gap of 0.5--0.6 eV. Density functional theory calculations have been conducted in order to study the stability and electronic band structure of several intercalated Ge cluster configurations. Based on these calculations we are able to identify two promising stable configurations that have a band gap that compares favorably well with the experimental observations. Scanning tunneling spectroscopy measurement recorded on the intercalated Ge clusters reveals signatures of Coulomb blockade.

Spin-Dependent Tunneling Barriers in CoPc/VSe2 from Many-Body Interactions.

Mixed-dimensional magnetic heterostructures are intriguing, newly available platforms to explore quantum physics and its applications. Using state-of-the-art many-body perturbation theory, we predict the energy level alignment for a self-assembled monolayer of cobalt phthalocyanine (CoPc) molecules on magnetic VSe2 monolayers. The predicted projected density of states on CoPc agrees with experimental scanning tunneling spectra. Consistent with experiment, we predict a shoulder in the unoccupied region of the spectra that is absent from mean-field calculations. Unlike the nearly spin-degenerate gas-phase frontier molecular orbitals, the tunneling barriers at the interface are spin-dependent, a finding of interest for quantum information and spintronics applications. Both the experimentally observed shoulder and the predicted spin-dependent tunneling barriers originate from many-body interactions in the interface-hybridized states. Our results showcase the intricate many-body physics that governs the properties of these mixed-dimensional magnetic heterostructures and suggests the possibility of manipulating the spin-dependent tunneling barriers through modifications of interface coupling.

Direct molecular quantification of electronic disorder in N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine on Au(111)

Organic light-emitting diodes are important in display applications, but thin films used in these devices often exhibit complex and highly disordered structures. We have studied the adsorption of a typical hole transport material used in such devices, N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD), on the Au(111) surface. Scanning tunneling microscopy images reveal the appearance of different conformations in the first monolayer with submolecular resolution. Scanning tunneling spectra identify the highest occupied molecular orbital on several different adsorption structures. We directly compare the statistical distribution of this orbital energy between an ordered monolayer structure and a disordered bilayer structure of α-NPD on Au(111). The disordered structure exhibits a very broad distribution that is consistent with inferences from prior organic device studies and that we propose arises from minor conformational variations.

Room-temperature quantum spin Hall phase in laser-patterned few-layer 1T\u2032- MoS2

The quantum-spin-Hall (QSH) phase of 2D topological insulators has attracted increased attention since the onset of 2D materials research. While large bulk gaps with vanishing edge gaps in atomically thin layers have been reported, verifications of the QSH phase by resistance measurements are comparatively few. This is partly due to the poor uniformity of the bulk gap induced by the substrate over a large sample area and/or defects induced by oxidation. Here, we report the observation of the QSH phase at room-temperature in the 1T′-phase of few-layer MoS2 patterned onto the 2H semiconducting phase using low-power and short-time laser beam irradiation. Two different resistance measurements reveal hallmark transport conductance values, ~e2/2 h and e2/4 h, as predicted by the theory. Magnetic-field dependence, scanning tunneling spectra, and calculations support the emergence of the room-temperature QSH phase. Although further experimental verification is still desirable, our results provide feasible application to room-temperature topological devices.

On the mystery of the absence of a spin-orbit gap in scanning tunneling microscopy spectra of germanene

Germanene, the germanium analogue of graphene, shares many properties with its carbon counterpart. Both materials are two-dimensional materials that host Dirac fermions. There are, however, also a few important differences between these two materials: (1) graphene has a planar honeycomb lattice, whereas germanene’s honeycomb lattice is buckled and (2) the spin-orbit gap in germanene is predicted to be about three orders of magnitude larger than the spin-orbit gap in graphene (24 meV for germanene versus 20 μeV for graphene). Surprisingly, scanning tunneling spectra recorded on germanene layers synthesized on different substrates do not show any sign of the presence of a spin-orbit gap. To date the exact origin of the absence of this spin-orbit gap in the scanning tunneling spectra of germanene has remained a mystery. In this work we show that the absence of the spin-orbit gap can be explained by germanene’s exceptionally low work function of only 3.8 eV. The difference in work function between germanene and the scanning tunneling microscopy tip (the work functions of most commonly used STM tips are in the range of 4.5 to 5.5 eV) gives rise to an electric field in the tunnel junction. This electric field results in a strong suppression of the size of the spin-orbit gap.

Spectroscopic signature of two superconducting gaps and their unusual field dependence in RuB2

Recently RuB2 was shown to be a possible two-gap, type-I superconductor. Temperature dependent heat capacity measurements revealed a two-gap superconducting ground state, while magnetic field dependent magnetization measurements indicated surprizing type-I superconductivity with a very low experimental critical field (Hc) ∼120 Oe. In this paper, we report direct spectroscopic evidence of two superconducting energy gaps in RuB2. We have measured scanning tunnelling spectra exhibiting signature of two gaps on different grains of polycrystalline RuB2, possibly originating from multiple bands. Analysis of the temperature dependent tunnelling spectra revealed that the gaps from different bands evolve differently with temperature before disappearing simultaneously at a single Tc. Interestingly, our experiments also reveal that the gaps in quasiparticle density of states survive up to magnetic fields much higher than the bulk Hc and they evolve smoothly with field, unlike what is expected for a type-I superconductor, indicating the existence of a ‘mixed state’.