Differential Conductance Spectra Research Articles

Dominant s -wave superconducting gap in PdTe2 observed by tunneling spectroscopy on side junctions

We have fabricated superconductor-normal metal side-junctions with different barrier transparencies out of PdTe$_2$ crystalline flakes and measured the differential conductance spectra. Modeling our measurements using a modified Blonder Tinkham Klapwijk (BTK) formalism confirms that the superconductivity is mostly comprised of the conventional $s$-wave symmetry. We have found that for junctions with very low barrier transparencies, the junctions can enter a thermal regime, where the critical current becomes important. Adding this to the BTK-model allows us to accurately fit the experimental data, from which we conclude that the superconductivity in the $a$-$b$ plane of PdTe$_2$ is dominated by conventional $s$-wave pairing.

Unraveling the Structural and Electronic Properties of Graphene/Ge(110)

The direct growth of graphene on a semiconducting substrate opens a new avenue for future graphene-based applications. Understanding the structural and electronic properties of the graphene on a semiconducting surface is key for realizing such structures; however, these properties are poorly understood thus far. Here, we provide insight into the structural and electronic properties of graphene grown directly on a Ge(110) substrate. Our scanning tunneling microscopy (STM) study reveals that overlaying graphene on Ge(110) promotes the formation of a new Ge surface reconstruction, i.e., a (6 × 2) superstructure, which has been never observed for a bare Ge(110) surface. The electronic properties of the system exhibit the characteristics of both graphene and Ge. The differential conductance (d I/d V) spectrum from a scanning tunneling spectroscopy (STS) study bears a parabolic structure, corresponding to a reduction in the graphene Fermi velocity, exhibiting additional peaks stemming from the p-orbitals of Ge. The density functional theory (DFT) calculations confirm the existence of surface states due to the p-orbitals of Ge.

The Kondo resonance line shape in scanning tunnelling spectroscopy: instrumental aspects

In the scanning tunnelling microscope, the many-body Kondo effect leads to a zero-bias feature of the differential conductance spectra of magnetic adsorbates on surfaces. The intrinsic line shape of this Kondo resonance and its temperature dependence in principle contain valuable information. We use measurements on a molecular Kondo system, all- trans retinoic acid on Au(1 1 1), and model calculations to discuss the role of instrumental broadening. The modulation voltage used for the lock-in detection, noise on the sample voltage, and the temperature of the microscope tip are considered. These sources of broadening affect the apparent line shapes and render difficult a determination of the intrinsic line width, in particular when variable temperatures are involved.

Optical Stark effect of a local defect on the TiO2(110) surface

Probing optical Stark effect at the single-molecule or atomic scale is crucial for understanding many photo-induced chemical and physical processes on surfaces. Here we report a study about optical Stark effect of single atomic defects on TiO2(110) surface with photo-assisted scanning tunneling spectroscopy. When a laser is coupled into the tunneling junction, the mid-gap state of OH-O2 defects changes remarkably in the differential conductance spectra. As laser power gradually increases, the energy of the mid-gap state shifts away from the Fermi level with increase in intensity and broadening of peak width. The observation can be explained as optical Stark effect with the Autler-Townes formula. This large optical Stark effect is due to the tip-enhancement and the strong dipole moment in the transient charged state during electron tunneling. Our study provides new aspects in exploring electron-photon interactions at the microscopic scale.

Linear scanning tunneling spectroscopy over a large energy range in black phosphorus

We reveal the unique electronic characteristics of the conduction band (CB) of black phosphorus (BP) by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), density functional theory calculations, analytic fitting, and model simulations. We discover that the differential conductance spectrum, which represents the local density of states (LDOS) of BP, exhibits a linear character over a large energy range in the unoccupied electronic state region. Combining theoretical calculations, we demonstrate that the linear character right above the conduction band minimum originates from a specific combination of the anisotropic band dispersions of BP's CB. In particular, the wave function of BP's CB possesses a pronounced density between BP layers and extends into the vacuum significantly, which is in sharp contrast to those of adjacent bands. This makes the CB dominate STS signals even when the energy is sufficiently high to involve other bands, and maintains the linearity of the STS spectrum over a wide energy range. The fact that the CB provides linear DOS and possesses pronounced wave function density in BP interlayers provides new insights for engineering the electronic structures and properties of BP and BP based materials.

Exchange-correlation functionals of i-DFT for asymmetrically coupled leads

A recently proposed density functional approach for steady-state transport through nanoscale systems (called i-DFT) is used to investigate junctions which are asymmetrically coupled to the leads and biased with asymmetric voltage drops. In the latter case, the system can simply be transformed to a physically equivalent one with symmetric voltage drop by a total energy shift of the entire system. For the former case, known exchange correlation gate and bias functionals have to be generalized to take into account the asymmetric coupling to the leads. We show how differential conductance spectra of the constant interaction model evolve with increasing asymmetry of both voltage drops and coupling to the leads.

Spin-selective coupling to Majorana zero modes in mixed singlet and triplet superconducting nanowires

We theoretically investigate the transport properties of a quasi one dimensional ferromagnet-superconductor junction where the superconductor consists of mixed singlet and triplet pairings. We show that the relative orientation of the stoner field ($\bf{\tilde{h}}$) in the ferromagnetic lead and the $\bf{d}$ vector of the superconductor acts like a on-off switch for the zero bias conductance of the device. In the regime, where triplet pairing amplitude dominates over the singlet counterpart (topological phase), a pair of Majorana zero modes appear at each end of the superconducting part of the nanowire. When $\bf{\tilde{h}}$ is parallel or anti-parallel to the $\bf{d}$ vector, transport gets completely blocked due to blockage in pairing while, when $\bf{\tilde{h}}$ and $\bf{d}$ are perpendicular to each other, the zero energy two terminal differential conductance spectra exhibits sharp transition from $4e^2/h$ to $2e^2/h$ as the magnetization strength in the lead becomes larger than the chemical potential indicating the spin selective coupling of pair of Majorana zero modes to the lead.

Nanoscale High-Tc YBCO/GaN Super-Schottky Diode

We demonstrate a high-temperature nanoscale super-Schottky diode based on a superconducting tunnel junction of pulsed-laser-deposited YBCO on GaN thin films. A buffer-free direct growth of nanoscale YBCO thin films on heavily doped GaN was performed to realize a direct high-Tc superconductor-semiconductor junction. The junction shows strongly non-linear I-V characteristics, which have practical applications as a low-voltage super-Schottky diode for microwave mixing and detection. The V-shaped differential conductance spectra observed across the junction are characteristic of the c-axis tunneling into a cuprate superconductor with a certain disorder level. This implementation of the super-Schottky diode, supported by the buffer-free direct growth of nanoscale high-Tc thin films on semiconductors, paves the way for practical large-scale fabrication and integration of high-Tc-superconductor devices in future technologies.

Electronic confining effects in Sierpiński triangle fractals

Electron confinement in fractal Sierpi\ifmmode \acute{n}\else \'{n}\fi{}ski triangles (STs) on Ag(111) is investigated using scanning tunneling spectroscopy and theoretically simulated by employing an improved two-dimensional (2D) multiple scattering theory in which the energy-dependent phase shifts are explicitly calculated from the electrostatic potentials of the molecular building block of STs. Well-defined bound surface states are observed in three kinds of triangular cavities with their sides changing at a scale factor of 2. The decrease in length of the cavities results in an upshift of the resonances that deviates from an expected inverse quadratic dependence on the cavity length due to the less efficient confinement of smaller triangular cavities. Differential conductance maps at some specific biases present a series of alternative bright and dark rounded triangles preserving the symmetry of the boundary. Our improved 2D multiple scattering model reproduces the characteristics of the standing wave patterns and all features in the differential conductance spectra measured in experiments, illustrating that the elastic loss boundary scattering dominates the resonance broadening in these ST quantum corrals. Moreover, the self-similar structure of STs, that a larger central cavity is surrounded by three smaller ones with a half side length, gives rise to interactions of surface states confined in neighboring cavities, which are helpful for the suppression of the linewidth in differential conductance spectra.

Band‐Engineered PbS Nanoparticles in CH3NH3PbI3 Solar Cells to Extend Activity Toward Near‐Infrared Region

Here, a route is reported to extend the activity of perovskite solar cells toward the near‐infrared (NIR) region through incorporation of band‐engineered PbS nanoparticles in the form of a bilayer structure with the perovskite layer. The nanoparticles were doped with a heterovalent element (bismuth) to shift their Fermi energy towards the conduction band‐edge. With a controlled level of doping and thereby an appropriate shift of Fermi energy, a type‐II band‐alignment could be achieved at the perovskite/doped‐PbS interface, so that the charge‐separation becomes possible upon photogeneration in both the materials. In this work, conduction and valence band‐edges of the nanoparticles upon different extent of bismuth‐doping were monitored through differential tunnel conductance spectra, which have a correspondence to density of states of the semiconductors. The perovskite solar cells with the band‐engineered bismuth‐doped PbS‐layer could be seen to offer NIR activities due to presence of the nanoparticle layer. The NIR activities were in addition to normal responses of perovskite solar cells in the UV‐visible region. The overall power conversion efficiency hence augmented from 6.5 to 10.1% in the inverted planar structure.

Moiré structure of MoS2 on Au(111): Local structural and electronic properties

Monolayer islands of molybdenum disulfide (MoS2) on Au(111) form a characteristic moiré structure, leading to locally different stacking sequences at the S-Mo-S-Au interface. Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we find that the moiré islands exhibit a unique orientation with respect to the Au crystal structure. This indicates a clear preference of MoS2 growth in a regular stacking fashion. We further probe the influence of the local atomic structure on the electronic properties. Differential conductance spectra show pronounced features of the valence band and conduction band, some of which undergo significant shifts depending on the local atomic structure. We also determine the tunneling decay constant as a function of the bias voltage by a height-modulated spectroscopy method. This allows for an increased sensitivity of states with non-negligible parallel momentum k∥ and the identification of the origin of the states from different areas in the Brillouin zone.

Investigation of Transport Parameters of Graphene-Based Nanostructures

The paper presents results of computer simulation of the main transport parameters of nanostructures obtained through the row-by-row removal of carbon atoms from graphene ribbon. Research into the electrical parameters is carried out within the density functional theory using the non-equilibrium Green functions in the local-density approximation. Virtual NanoLab based on Atomistix ToolKit is used to construct structures and analyze simulation results. Current-voltage characteristics, differential conductivity and transmittance spectra of nanostructures are calculated at different values of bias voltage. It is found that there is a large region of negative differential resistance in current-voltage characteristics of nanostructures caused by resonant tunneling of quasi-particles. Differential (dI/dV) characteristic also has similar changes. The obtained results can be useful for building novel electronic devices in the field of nanoelectronics.

Renormalization of single-ion magnetic anisotropy in the absence of the Kondo effect

Inelastic spin flip excitations associated with single-ion magnetic anisotropy of quantum spins, can be strongly renormalized by Kondo exchange coupling to the conduction electrons in the substrate, as shown recently for the case of Co adatoms on CuN$_2$ islands. In this case differential conductance spectra show zero-bias anomalies due to a Kondo effect of the doubly degenerate ground state, and finite-bias step features due to spin flip excitations. Here I consider spin-1 quantum magnets with positive uniaxial anisotropy, where the ground state is non-degenerate and hence the Kondo effect does not take place. Nevertheless the renormalization of inelastic spin excitations due to exchange coupling by hybridization of the quantum spin with the conduction electrons still takes place despite the complete absence of the Kondo effect in the ground state. Additionally, I show that away from particle-hole symmetry, charge fluctuations have a similar effect to Kondo exchange coupling, leading to the renormalization of spin flip excitations. However, in contrast to the renormalization by Kondo exchange, charge fluctuations lead to asymmetric spectra, which for strong charge fluctuations can mimic Fano behavior.

Evidence for in-gap surface states on the single phase SmB6(001) surface

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.

Low-temperature hydrogen absorption in metallic nanocontacts studied by point-contact spectroscopy measurements

We report on hydrogen (H) and deuterium (D) atoms absorption below T = 20 K in metallic palladium (Pd) via quantum tunnelling (QT). When a small bias voltage is applied between Pd nanocontacts that are immersed in liquid H2 (D2), the differential conductance spectra measured by point-contact spectroscopy change enormously. The results indicate H (D) absorption in Pd nanocontacts at the temperature where H (D) absorption due to thermal hopping process is not expected, and can be explained by QT. The QT occurs when the energy level of the potential well trapping the H (D) atom coincides with those not trapping the H (D) atom, and is assisted by phonons induced by ballistic electrons.

Transport characteristics of a silicene nanoribbon on Ag(110)

We present the transport characteristics of individual silicene nanoribbons (SiNRs) grown on Ag(110). By lifting up a single SiNR with a low-temperature scanning tunneling microscope tip, a nanojunction consisting of tip, SiNR and Ag is fabricated. In the differential conductance spectra of the nanojunctions fabricated by this methodology, a peak appears at the Fermi level which is not observed in the spectra measured either for the SiNRs before being lifted up or the clean Ag substrate. We discuss the origin of the peak as it relates to the SiNR.

Origin of bias-independent conductance plateaus and zero-bias conductance peaks in Bi2Se3/NbSe2 hybrid structures

Superconducting proximity effect (SPE) in topological insulator (TI) and superconductor (SC) hybrid structure has attracted intense attention in recent years in an effort to search for mysterious Majorana fermions (MFs) in condensed matter systems. Here we report on the SPE in a Bi2Se3/NbSe2 junction fabricated with an all-dry transfer method. Resulting from the highly transparent interface, two sharp resistance drops are observed at 7 K and 2 K, respectively, corresponding to the superconducting transition of NbSe2 flake and the SPE induced superconductivity in Bi2Se3 flake. Experimentally measured differential conductance spectra exhibit a bias-independent conductance plateau (BICP) in the vicinity of zero bias below 7 K. As temperatures further decrease a zero bias conductance peak (ZBCP) emerges from the plateau and becomes more enhanced and sharpened at lower temperatures. Our numerically simulated differential conductance spectra reproduce the observed BICP and ZBCP and show that the SPE in topological surface states (TSS) is much stronger than that in the bulk states of Bi2Se3. The SPE induced superconducting gap for the TSS of Bi2Se3 is comparable to that of NbSe2 and gives rise to the observed BICP below 7 K. In contrast, the SPE induced superconducting gap for the bulk states of Bi2Se3 is an order of magnitude smaller than that of NbSe2 and superconducting TSS. These weakly paired bulk states in Bi2Se3 give rise to the ZBCP below 2 K. Our study has clearly unveiled the different roles of TSS and bulk stats in SPE, clarified the physical origin of the SPE induced features, and shined light on further investigation of SPE and MF in TI/SC hybrid structures.

Coexistence of Weak and Strong Coupling Mechanism, in an Iron-Based Superconductor FeSe 0.5 Te 0.5: Possible Signature of BCS-BEC Crossover

In this work, we present the scanning tunneling microscopy/spectroscopy (STM/STS) studies of an iron-based superconductor, FeSe0.5Te0.5. We obtained differential conductance (dI/dV) spectra and extracted the value of the superconducting order parameter Δ using extended Bardeen Cooper Schieffer (BCS) phenomenology for anisotropic s-wave pairing. We observed widespread values of Δ from ∼0.6 to ∼4.5 meV, with distinct two peaks in the histogram around 1 and 3 meV. The corresponding values of 2Δ/kBTc for the peaks are ∼1.5 and ∼5.0, respectively, indicating coexistence of weak and strong coupling mechanism. We also found two different coupling mechanisms through Δ to Fermi energy ratio (Δ/Ef) of the material; which infers to the possible composite superconductivity in the realm of BCS-Bose Einstein Condensate (BEC) crossover.

Differential conductance (dI/dV) imaging of a heterojunction-nanorod

Through scanning tunneling spectroscopy, we envisage imaging a heterostructure, namely a junction formed in a single nanorod. While the differential conductance spectrum provides location of conduction and valence band edges, dI/dV images record energy levels of materials. Such dI/dV images at different voltages allowed us to view p- and n-sections of heterojunction nanorods and more importantly the depletion region in such a junction that has a type-II band alignment. Viewing of selective sections in a heterojunction occurred due to band-bending in the junction and is correlated to the density of states spectrum of the individual semiconductors. The dI/dV images recorded at different voltages could be used to generate a band diagram of a pn junction.

Doping effects on the hybridization gap and antiferromagnetic order in the Kondo semiconductor CeOs2Al10 studied by break-junction experiments

The Kondo semiconductors $\mathrm{Ce}{T}_{2}\mathrm{A}{\mathrm{l}}_{10}$ ($T=\mathrm{Ru}$ and Os) exhibit antiferromagnetic (AFM) orders at unexpectedly high temperatures ${T}_{\mathrm{N}}=27.0$ and 28.5 K, respectively, whose mechanism remains in debate. We report the break-junction experiments on $4f/5d$-hole and $5d$-electron doped $\mathrm{CeO}{\mathrm{s}}_{2}\mathrm{A}{\mathrm{l}}_{10}$ as well as nondoped $\mathrm{CeR}{\mathrm{u}}_{2}\mathrm{A}{\mathrm{l}}_{10}$. The differential conductance spectra $dI/dV$ for $T=\mathrm{Os}$ and Ru show three gap structures; two hybridization gaps ${V}_{1}, {V}_{2}$ and an AFM gap ${V}_{\mathrm{AF}}$, whose magnitudes for $T=\mathrm{Os}$ are $15\ensuremath{-}50%$ larger than for $T=\mathrm{Ru}$. Doping of $4f/5d$ holes and $5d$ electrons in $\mathrm{CeO}{\mathrm{s}}_{2}\mathrm{A}{\mathrm{l}}_{10}$ changes the $dI/dV$ spectrum in very different ways. Nevertheless, in all cases, the suppression of ${V}_{1}$ is well correlated with those of ${V}_{\mathrm{AF}}$ and ${T}_{\mathrm{N}}$. Furthermore, the zero-bias conductance decreases on cooling below ${T}^{*}$ ($g{T}_{\mathrm{N}}$) only in the doping region where ${V}_{1}$ and ${V}_{\mathrm{AF}}$ coexist. This fact indicates that the unusual AFM order is preceded by the decrease in the density of states in the presence of ${V}_{1}$.