Zero-bias Feature Research Articles

Large Orbital Moment and Dynamical Jahn-Teller Effect of AlCl-Phthalocyanine on Cu(100).

Submonolayer amounts of chloroaluminum-phthalocyanine on Cu(100) were studied with scanning tunneling spectroscopy. The molecule can be prepared in a fourfold symmetric state whose conductance spectrum exhibits a zero-bias feature similar to a Kondo resonance. In magnetic fields, however, this resonance splits far more than expected from the spin of a single electron. Density functional theory calculations reveal a charge transfer of 1.3 electrons to the degenerate lowest unoccupied molecular orbitals. These orbitals are mixed by the orbital momentum operator L[over ^]_{z} with a large matrix element corresponding to m_{L}≈2.7. Dehydrogenation of a ligand lifts the degerenracy of the lowest unoccupied molecular orbital, reduces the splitting in magnetic fields, and induces a polarity dependence of the spectra. Using model calculations of the spin, orbital, and vibrational degrees of freedom we show that a dynamical Jahn-Teller effect reproduces the main experimental observations.

Predicting Finite-Bias Tunneling Current Properties from Zero-Bias Features: The Frontier Orbital Bias Dependence at an Exemplar Case of DNA Nucleotides in a Nanogap.

The electrical current properties of single-molecule sensing devices based on electronic (tunneling) transport strongly depend on molecule frontier orbital energy, spatial distribution, and position with respect to the electrodes. Here, we present an analysis of the bias dependence of molecule frontier orbital properties at an exemplar case of DNA nucleotides in the gap between H-terminated (3, 3) carbon nanotube (CNT) electrodes and its relation to transversal current rectification. The electronic transport properties of this simple single-molecule device, whose characteristic is the absence of covalent bonding between electrodes and a molecule between them, were obtained using density functional theory and non-equilibrium Green’s functions. As in our previous studies, we could observe two distinct bias dependences of frontier orbital energies: the so-called strong and the weak pinning regimes. We established a procedure, from zero-bias and empty-gap characteristics, to estimate finite-bias electronic tunneling transport properties, i.e., whether the molecular junction would operate in the weak or strong pinning regime. We also discuss the use of the zero-bias approximation to calculate electric current properties at finite bias. The results from this work could have an impact on the design of new single-molecule applications that use tunneling current or rectification applicable in high-sensitivity sensors, protein, or DNA sequencing.

Robust, carbon related, superconducting nanostructure at the apex of a tungsten STM tip

By pulsing the tunneling voltage between the Tungsten (W) tip of a Scanning Tunneling Microscope (STM) and a graphene-covered metal surface, a superconducting (SC) nanostructure is formed at the apex of the STM tip. We have characterized the SC properties of the resulting nanotip as a function of temperature and magnetic field, obtaining a transition temperature of 3.3 K and a critical field well above 3 T. The SC nanotip is robust and stable and achieves atomic resolution. A non-SC tip can be easily recovered by controlled voltage pulsing on a clean metal surface. The present result should be taken into account when studying zero-bias features like Kondo resonances, zero-bias-conductance peaks, or superconductivity on graphene-based systems by means of STM using tungsten tips.

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.

Interconnected Cobaltocene Complexes on Metal Surfaces

Interconnected molecular magnetic centers on metallic surfaces are of interest for molecular spintronics. Complexes composed of two or three cobaltocene units linked by naphthalene or benzene groups are successfully deposited on Au(111) and Cu(111) by sublimation and electrospray deposition. Low-temperature scanning tunneling microscopy and spectroscopy are employed to investigate the deposited compounds and their spin state. Although all molecules are composed of the same magnetic cobaltocene unit, only one compound shows a zero-bias feature compatible with a Kondo resonance, whose amplitude varies from molecule to molecule. The amplitude variation and its absence for the other investigated complexes are attributed to different molecule–substrate coupling, which is strongly influenced by the linker. Parameters influencing the molecule–substrate coupling and molecular properties are extracted from the experimental data. These key parameters should be considered for future strategies of interconnected magn...

Quasiclassical Eilenberger theory of the topological proximity effect in a superconducting nanowire

We use the quasiclassical Eilenberger theory to study the topological superconducting proximity effects between a segment of a nanowire with a p-wave order parameter and a metallic segment. This model faithfully represents key qualitative features of an experimental setup, where only a part of a nanowire is in immediate contact with a bulk superconductor, inducing topological superconductivity. It is shown that the Eilenberger equations represent a viable alternative to the Bogoliubov-de Gennes theory of the topological superconducting heterostructures and provide a much simpler quantitative description of some observables. For our setup, we obtain exact analytical solutions for the quasiclassical Green's functions and the density of states as a function of position and energy. The correlations induced by the boundary involve terms associated with both p-wave and odd-frequency pairing, which are intertwined and contribute to observables on an equal footing. We recover the signatures of the standard Majorana mode near the end of the superconducting segment, but find no such localized mode induced in the metallic segment. Instead, the zero-bias feature is spread out across the entire metallic part in accordance with the previous works. In shorter wires, the Majorana mode and delocalized peak split up away from zero energy. For long metallic segments, non-topological Andreev bound states appear and eventually merge together, giving rise to a gapless superconductor.

Switching and Sensing Spin States of Co–Porphyrin in Bimolecular Reactions on Au(111) Using Scanning Tunneling Microscopy

Controlling and sensing spin states of magnetic molecules at the single-molecule level is essential for spintronic molecular device applications. Here, we demonstrate that spin states of Co-porphyrin on Au(111) can be reversibly switched over by binding and unbinding of the NO molecule and can be sensed using scanning tunneling microscopy and spectroscopy (STM and STS). Before NO exposure, Co-porphryin showed a clear zero-bias peak, a signature of Kondo effect in STS, whereas after NO exposures, it formed a molecular complex, NO-Co-porphyrin, that did not show any zero-bias feature, implying that the Kondo effect was switched off by binding of NO. The Kondo effect could be switched back on by unbinding of NO through single-molecule manipulation or thermal desorption. Our density functional theory calculation results explain the observations with pairing of unpaired spins in dz(2) and ppπ* orbitals of Co-porphyrin and NO, respectively. Our study opens up ways to control molecular spin state and Kondo effect by means of enormous variety of bimolecular binding and unbinding reactions on metallic surfaces.

Superconductor-nanowire devices from tunneling to the multichannel regime: Zero-bias oscillations and magnetoconductance crossover

We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above $\ensuremath{\sim}$0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including the Kondo effect and analogs of 0.7 structure in a disordered nanowire.

Zero bias features in self-assembling tunnel junctions

A marked suppression of the conductance near zero bias voltage is a feature common to many electron tunneling systems and has been attributed to several different mechanisms including Coulomb blockade and non-equilibrium tunneling. We observe zero bias features (ZBFs) in self-assembling tunnel junctions in which the oxide layer of a traditional metal–insulator–metal junction is replaced by an atomically-thin gas film. We also find small conductance steps superimposed on the otherwise smoothly varying ZBF. Examination of the derivative of the junction conductance in this region reveals peaks, similar to those observed in point contact spectroscopy, that appear to be correlated with the phonon density of states in the metal electrodes.

Observations of Kondo scattering without magnetic impurities: A point contact study of two-level tunneling systems in metals.

We study singularities in the differential resistance of metal point contacts containing structural disorder. We find zero-bias features which are well described by theories in which two-level tunneling systems couple strongly to the conduction electrons to produce a Kondo resonance whose origin is not magnetic impurities. Away from zero bias, we observe abrupt resistance steps which indicate transitions in the multichannel Kondo condensate. We suggest that two-level systems are the explanation for many of the zero-bias anomalies seen in tunnel junctions and point contacts.