STM Experiments Research Articles

Nodal-antinodal dichotomy from pairing disorder ind-wave superconductors

We study the basic features of the local density of states (LDOS) observed in STM experiments on high-T$_c$ d-wave superconductors in the context of a minimal model of a d-wave superconductor which has {\it weakly} modulated off-diagonal disorder. We show that the low and high energy features of the LDOS are consistent with the observed experimental patterns and in particular, the anisotropic local domain features at high energies. At low energies, we obtain not only the scattering peaks predicted by the octet model, but also weak features that should be experimentally accessible. Finally, we show that the emerging features of the LDOS lose their correspondence with the features of the imposed disorder, as its complexity increases spatially.

Performing spin-polarized STM experiments theoretically

By introducing paths on the hypersurface of the band-energy contribution free energy to the magnetic anisotropy energy and of corresponding differences in the zz-like elements of the electric conductivity tensor, viewed as an implicit function of the free energy, experimental spin-polarized scanning tunnel microscope STM data, namely, the recording of a differential current as a function of an externally applied field, can be analyzed and facilitate a direct comparison between theory and experiment. It is shown that along different paths rather different flipping times of the direction of the magnetization can occur. In particular, discussed are in terms of “theoretical experiments” horizontal and vertical movements of the tip, the influence of the magnetic properties of the tip as well as of typical samples. Furthermore, it is claimed that because of different time scales in a presently still fictional time-resolved mode for spin-polarized STM experiments even particular paths on these hypersurfaces could be mapped out.

Recursive Method for the Density of States in One Dimension

We derive a powerful yet simple method for analyzing the local density of states (DOS) in gapless one-dimensional fermionic systems, including extensions such as momentum dependent interaction parameters and hard-wall boundaries. We study the crossover of the local DOS from individual density waves to the well-known asymptotic power laws and identify characteristic signs of spin charge separation in possible STM experiments. For semi-infinite systems a closed analytic expression is found in terms of hypergeometric functions.

The role of van der Waals interactions in surface-supported supramolecular networks

The development of a detailed theoretical understanding of surface-supported supramolecular networks is important for designing novel organic nanodevices. By comparing with STM experiments, we show that van der Waals corrections to density functional theory (DFT) in the generalized gradient approximation (GGA) are mandatory to correctly reproduce the electronic and geometric structure of a prototypical system of this kind, the self-assembled hydrogen bonded network formed by 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and 4,4''-diamino-p-terphenyl (DATP) deposited on Au(111). Our results reproduce both the network structure and its higher stability with respect to homomolecular networks. By successful comparison with the experiments, we demonstrate that dispersive interactions must be taken into account when rationally designing organic semiconductor nanostructures on a metallic substrate. DFT-GGA alone would fail in predicting geometric and electronic properties for weakly bounded large organic adsorbates on coinage metal surfaces.

Influence of Structural Defects on the Electronic Properties of Carbon Nanotubes Examined by Scanning Tunnelling Microscopy

AbstractThe electronic properties of carbon nanotubes are quite often drastically affected by the presence of defects that can develop during nanotube growth, processing or characterization too. Some of these defects such as pentagon-heptagon rings, substitutional impurities, vacancies and dislocations are of topological nature, and can sometimes create on-tube intramolecular junctions, as found by previous scanning tunnelling microscopy studies.Our recent STM experiments reveal for the first time a much more complicated junction structure, a hybrid single-walled carbon nanotube consisting of a distinct coiled structure located between two straight segments, each of different helicity. We characterise the hybrid junction at the atomic level and describe its electronic behaviour that has important implications in the practical design of functional components for nanoelectronic applications.

Classical spins in topological insulators

Following the recent theoretical proposal and experiment on quantum spin Hall effect in HgTe/CdTe quantum wells, we consider a single magnetic impurity localized in the bulk of the system, which we treat as a classical spin. It is shown that there are always localized excited states in the bulk energy gap for arbitrarily strong impurity strength in inverted region, while the localized excited states vanish for very strong impurity strength in normal region. Similar conclusion also applies to three-dimensional topological insulators. This distinct difference serves as another novel criterion for the conventional and topological insulating phases when the time-reversal symmetry is broken, and can be easily experimentally observed through the STM and/or ARPES experiments.

Theoretical prediction of the antiferromagnetic ground state of a C defect on Si(001)

We have performed first-principles density-functional calculations for the C defect on Si(001), which has been experimentally identified in terms of the dissociative adsorption of a single water molecule on two adjacent dimers. Unlike the hitherto accepted nonmagnetic structure that involves the buckling of two Si dangling bonds (DBs) within the C defect, the presently proposed structure shows that the two DBs are antiferromagnetically coupled with each other, thereby giving rise to a suppression of their buckling. This antiferromagnetic ground state of C defect reproduces the key features observed in the STM and STS experiments.

Fully relativisticab initiodescription of spin-polarized STM experiments in terms of magnetic-field-dependent tunneling currents

An approach is introduced to interpret spin-polarized scanning tunnel microscope experiments in terms of differences in the tunneling conductivity with respect to different magnetic configurations and the corresponding band energy contribution to the magnetic anisotropy energy, the former being considered to be proportional to the applied external magnetic field. This approach is illustrated for Cu 111 coated with two monolayers of Co considering two types of Cr/W tips. It is found that the reorientation transition of the magnetization in the Co layers can unambiguously be identified even in cases when the tip contributes considerably to the tunneling current difference.

Molecular Staples on Si(001)-2 × 1: Dual-Head Primary Amines

The present study aims at determining the bonding configurations of bifunctional primary amines, ethylenediamine (H2N-[CH2]2-NH2) and 1,4-diaminobutane (H2N-[CH2]4-NH2), on Si(001)-2 × 1. We carry out Δ Kohn−Sham ionization potential calculations of various plausible geometries and compare the calculation outputs to synchrotron radiation core-level photoemission (XPS) data. Besides the primary motivation of chemical identification, the DFT calculations point to interesting and unexpected issues, related to the propagation of energy shifts throughout the molecular chain, or to the influence of intramolecular hydrogen bonds on ionization energies. Thanks to the theoretical/experimental combined approach, we can determine that a majority of ethylenediamine molecules adopts a dual-head dissociated geometry at room temperature and high coverage. In the very low coverage limit, complementary STM experiments indicate that ethylenediamine bridges two Si dimers over the trench possibly in a dual dative bond configuration. Such dative bonds are only detected by XPS after molecular adsorption at low temperature. Despite an aliphatic spacer length longer than that of ethylenediamine, 1,4-diaminobutane also adopts a dual-head dissociative geometry at room temperature.

First Preparations and Characterization of Conductive Polymer Crystalline Nanoneedles

AbstractSingle crystalline nanoneedles of three families of the most studied conductive organic polymers ‐ polythiophene, polyaniline and polypyrrole ‐ were synthesized for the first time using an interfacial polymerization process that takes place with simultaneous crystallization. As the crystal growth is concurrent with polymerization, more ordered crystal packing can be expected. Most of the bulk conducting‐polymer systems studied contains regions that are inhomogeneous. Single nanocrystals of conducting polymers have not been reported, although needle‐shaped bulk crystals of the quarterphenyl cation radical salt have previously been studied. The investigation of processes in a nanodomain of a single crystal is critical in ascertaining the inherent electronic properties of polymer nanoelements. The organic conductive nanoneedles were characterized using TEM, HRTEM, electron diffraction, EDS, and EPR to establish their crystal structure and composition. Scanning tunneling microscopy/spectroscopy (STM/STS) investigation were conducted to examine their electronic behaviors, leading to the discovery of a field‐induced conductance switching with response times on the millisecond level. The switch voltages are in the range of 3 to 4 volts in STM experiments, consistent with the trend of the band gap of the three polymers. The organic conductive nanoneedles with nano‐tip having high density of mobile electron may serve as interesting elements for nanoscale electronics.

Magnetic Impurities on the Surface of a Topological Insulator

The surface states of a topological insulator are described by an emergent relativistic massless Dirac equation in 2 + 1 dimensions. In contrast with graphene, there is an odd number of Dirac points, and the electron spin is directly coupled to the momentum. We show that a magnetic impurity opens up a local gap and suppresses the local density of states. Furthermore, the Dirac electronic states mediate an RKKY interaction among the magnetic impurities which is always ferromagnetic, whenever the chemical potential lies near the Dirac point. Through this exchange mechanism, magnetic atoms uniformly deposited on the surface of a topological insulator could naturally form a ferromagnetically ordered film. These effects can be directly measured in STM experiments. We also study the case of quenched disorder through a renormalization group analysis.

Weak ferromagnetism in Mn nanochains on the CuN surface

We investigate the electronic and magnetic structures of Mn chains supported on a CuN surface, using first-principles LSDA and $\text{LDA}+U$ methods. The isotropic exchange integrals and anisotropic Dzyaloshinskii-Moriya interactions between Mn atoms are calculated using the Green's function technique and total energies difference method. It is shown that lattice relaxation and on-site Coulomb interaction are important for an accurate description of the magnetic properties of the investigated nanosystems. Based on a classical spin Hamiltonian we conclude that the Mn antiferromagnetic nanochains on the CuN surface demonstrate weak ferromagnetism. The net magnetic moment and direction of the spin canting are estimated in the framework of a classical spin Hamiltonian. We show that some aspects of the experimental spectrum can be explained using a quantum spin Hamiltonian, with parameters defined from first-principles calculations and extracted from recent STM experiments.

A Multivalent Hexapod: Conformational Dynamics of Six-Legged Molecules in Self-Assembled Monolayers at a Solid−Liquid Interface

A molecular hexapod having a benzene core and six oligo(p-phenylene vinylene) (OPV) legs is an ideal system to probe various types of (intramolecular) dynamics of individual molecules in physisorbed self-assembled monolayers at a solid-liquid interface. Scanning tunneling microscopy reveals that molecules adsorb in 2D crystalline as well as disordered domains. Strikingly, not all molecules have the six OPV units in contact with the graphite substrate: 4% of the molecules in the 2D crystalline domains and up to 80% of the molecules in the disordered domains have one or two OPV units desorbed. In addition, the presence of such a defect promotes the coexistence of another defect adjacent to it. Time-dependent STM experiments and molecular dynamics simulations reveal in detail the different dynamics involved and the multivalent nature of the interactions between hexapod and surface.

Effect of external magnetic field on superconducting and spin density wave gaps of high- Tc superconductors

A theoretical model is addressed here to study the interplay of the superconductivity (SC) and the spin density wave (SDW) long range orders in underdoped region in the vicinity of on-set of superconductivity in presence of an external magnetic field. The order parameters are calculated by using Zubarev’s technique of Green’s functions and determined numerically self-consistently. The gap parameters are found to be strongly coupled to each other through their coupling constants. The interplay displays BCS type two gaps in the quasi-particle density of states (DOS) which resemble the tunneling conductance of STM experiments. The gap edges in the DOS appear at ± ( z + z 1 ) and ± ( z - z 1 ) . The applied magnetic field further induces Zeeman splitting which is explained on the basis of spin-filter effect of tunneling experiment.

Field angle dependence of the zero-energy density of states in unconventional superconductors: analysis of the borocarbide superconductor YNi2B2C

We investigate the field-angle-dependent zero-energy density of states for YNi2B2C with using realistic Fermi surfaces obtained by band calculations. Both the 17th and 18th bands are taken into account. For calculating the oscillating density of states, we adopt the Kramer-Pesch approximation, which is found to improve accuracy in the oscillation amplitude. We show that superconducting gap structure determined by analyzing STM experiments is consistent with thermal transport and heat capacity measurements.

Solvent Effects on the Self-Assembly of 1-Bromoeicosane on Graphite. Part II. Theory

The phase ordering of 1-bromoeicosane (C20H41Br) at the liquid-graphite and vacuum-graphite interfaces is examined through a joint experimental (part I) and theoretical effort (part II). The stable morphologies under solvent and ultrahigh vacuum conditions are revealed by STM experiments to be the head-to-head structures with 90° and 60° lamella−backbone angles, respectively. At 90° and 60° close packing is attained, independent of the corrugation of the graphite lattice. The potential energy of the minimized 60° structure is slightly lower than that of the 90° structure under vacuum conditions. In addition, the basin of the potential energy surface about the 90° form is very narrow. All-atom molecular dynamics simulations depict a 90°-to-60° phase transition in vacuum. Both morphologies are stable when an explicit solvent model is included. We speculate that the choice of the 90° form under solvent is driven by symmetry considerations and the self-assembly pathway. For example, the 90° structure may serv...

Edge states for the n = 0 Laudau level in graphene

In the anomalous quantum Hall effect (QHE), a hallmark of graphene, nature of the edge states in magnetic fields poses an important question, since the edge and bulk should be intimately related in QHE. Here we have theoretically studied the edge states, focusing on the E = 0 edge mode, which is unusual in that the mode is embedded right within the n = 0 bulk Landau level, while usual QHE edge modes reside across adjacent Landau levels. Here we show that the n = 0 Landau level, including the edge mode, has a wave function amplitude accumulated along zigzag edges whose width scales with the magnetic length, lB. This contrasts with the usual QHE where the charge is depleted from the edge. The implications are: (i) The E = 0 edge states in strong magnetic fields have a topological origin in the honeycomb lattice, so that they are outside the continuum ("massless Dirac") model. (ii) The edge-mode contribution decays only algebraically into the bulk, but this is "topologically" compensated by the bulk contribution, resulting in the accumulation over lB. (iii) The real space behavior obtained here should be observable in STM experiments.

Acetone on silicon (001): ambiphilic molecule meets ambiphilic surface

Using density functional theory, we report detailed reaction path calculations for the reaction of acetone with the silicon (001) surface. We identify the key reaction intermediates of dissociative adsorption and the transition states between them. This resolves the identity of the one-dimer intermediate observed in STM experiments and its role in the formation of several two-dimer-wide end products of dissociation. Key to the understanding of the dissociation mechanism is the ambiphilic character of the two reactants, that is the simultaneous expression of electrophilic and nucleophilic reactivities in both the surface and the acetone molecule.

Charge-memory polaron effect in molecular junctions

The charge-memory effect, bistability and switching between charged and neutral states of a molecular junction, as observed in recent STM experiments, is considered within a minimal polaron model. We show that in the case of strong electron-vibron interaction the rate of spontaneous quantum switching between charged and neutral states is exponentially suppressed at zero bias voltage but can be tuned through a wide range of finite switching timescales upon changing the bias. We further find that, while junctions with symmetric voltage drop give rise to random switching at finite bias, asymmetric junctions exhibit hysteretic behavior enabling controlled switching. Lifetimes and charge-voltage curves are calculated by the master equation method for weak coupling to the leads and at stronger coupling by the equation-of-motion method for nonequilibrium Green functions.

Oxygen-induced restructuring of Cu(19 19 1) studied by scanning tunneling microscopy

Oxidation and the formation of a Cu-CuO stripe phase on the Cu(19 19 1) surface was investigated by means of STM. Since this surface is vicinal to the (110) orientation with step edges oriented perpendicular to the CuO stripes, it is especially well suited to study the changes in surface morphology induced by small amounts of oxygen. It is shown, that the tendency of the CuO stripes to grow lengthwise is strong enough to extend or displace certain terraces. Since this effect involves the transport of large amounts of Cu atoms, it strongly depends on surface mobility as shown by STM experiments carried out at different temperatures. On the clean Cu(19 19 1) surface a transition from multiple to single steps was observed after annealing at temperatures close to the roughening transition of Cu(110).