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Ferdinand Evers

Publications and source records attributed to Ferdinand Evers.

48 records · Page 3Linked to original sources
arXiv2015

Impact of electrode density of states on transport through pyridine-linked single molecule junctions

We study the impact of electrode band structure on transport through single-molecule junctions by measuring the conductance of pyridine-based molecules using Ag and Au electrodes. Our experiments are carried out using the scanning tunneling microscope based break-junction technique and are supported by density functional theory based calculations. We find from both experiments and calculations that the coupling of the dominant transport orbital to the metal is stronger for Au-based junctions when compared with Ag-based junctions. We attribute this difference to relativistic effects, which results in an enhanced density of d-states at the Fermi energy for Au compared with Ag. We further show that the alignment of the conducting orbital relative to the Fermi level does not follow the work function difference between two metals and is different for conjugated and saturated systems. We thus demonstrate that the details of the molecular level alignment and electronic coupling in metal-organic interfaces do not follow simple rules, but are rather the consequence of subtle local interactions.

cond-mat.mes-hall↗
arXiv2014

Ab initio quantum transport through armchair graphene nanoribbons: Streamlines in the current density

We calculate the local current density in pristine armchair graphene nanoribbons (AGNRs) with varying width, $N_\mathrm{C}$, employing a density-functional-theory-based ab initio transport formalism. We observe very pronounced current patterns (streamlines) with threefold periodicity in $N_\mathrm{C}$. They arise as a consequence of quantum confinement in the transverse flow direction. Neighboring streamlines are separated by stripes of almost vanishing flow. As a consequence, the response of the current to functionalizing adsorbates is very sensitive to their placement: adsorbates located within the current filaments lead to strong backscattering, while adsorbates placed in other regions have almost no impact at all.

cond-mat.mes-hall↗
arXiv2013

Statistics of Conductances and Subleading Corrections to Scaling near the Integer Quantum Hall Plateau Transition

We study the critical behavior near the integer quantum Hall plateau transition by focusing on the multifractal (MF) exponents $X_q$ describing the scaling of the disorder-average moments of the point contact conductance $T$ between two points of the sample, within the Chalker-Coddington network model. Past analytical work has related the exponents $X_q$ to the MF exponents $Δ_q$ of the local density of states (LDOS). To verify this relation, we numerically determine the exponents $X_q$ with high accuracy. We thereby provide, at the same time, independent numerical results for the MF exponents $Δ_q$ for the LDOS. The presence of subleading corrections to scaling makes such determination directly from scaling of the moments of $T$ virtually impossible. We overcome this difficulty by using two recent advances. First, we construct pure scaling operators for the moments of $T$ which have precisely the same leading scaling behavior, but no subleading contributions. Secondly, we take into account corrections to scaling from irrelevant (in the renormalization group sense) scaling fields by employing a numerical technique ("stability map") recently developed by us. We thereby numerically confirm the relation between the two sets of exponents, $X_q$ (point contact conductances) and $Δ_q$ (LDOS), and also determine the leading irrelevant (corrections to scaling) exponent $y$ as well as other subleading exponents. Our results suggest a way to access multifractality in an experimental setting.

cond-mat.mes-hall↗
arXiv2013

Invariants of the single impurity Anderson model and implications for conductance functionals

An exact relation between the conductance maximum $G_0$ at zero temperature and a ratio of lead densities is derived within the framework of the single impurity Anderson model: $G_0={\mathfrak R}[n] \frac{2e^2}{h}$, where ${\mathfrak R}[n]=4ΔN_{{\cal L},x} ΔN_{{\cal R},x}/(ΔN_{{\cal L},x}+ΔN_{{\cal R},x})^2$ and $ΔN_{{\cal L},x}$, $ΔN_{{\cal R},x}$ denote the excess density in the left/right lead at distance $x$ due to the presence of the impurity at the origin, $x=0$. The relation constitutes a parameter-free expression of the conductance of the model in terms of the ground state density that generalizes an earlier result to the generic case of asymmetric lead couplings. It turns out that the specific density ratio, ${\mathfrak R}[n]$, is independent of the distance to the impurity $x$, the (magnetic) band-structure and filling fraction of the contacting wires, the strength of the onsite interaction, the gate voltage and the temperature. Disorder induced backscattering in the contacting wires has an impact on ${\mathfrak R}$ that we discuss. Our result suggests that it should be possible, in principle, to determine experimentally the peak conductance of the Anderson impurity by performing a combination of measurements of ground-state densities.

cond-mat.str-el↗
arXiv2013

Spin locking at the apex of nano-scale platinum tips

Nanostructures based on platinum, such as small clusters or STM-tips, often exhibit an atomistic structure that relies upon one or very few strongly under-coordinated platinum atoms. Here, we analyze a paradigmatic example, an apex atom on a pyramidal platinum cluster employing the density functional theory. We show that such a pristine platinum tip exhibits a spin polarization of the apex atom with a remarkable robustness. Due to a depletion of the projected density of states at the apex position, the apex-magnetization is efficiently locked to about 0.6$μ_\text{B}$.

cond-mat.mes-hall↗
arXiv2013

C58 on Au(111): a scanning tunneling microscopy study

C58 fullerenes were adsorbed onto room temperature Au(111) surface by low-energy (~6 eV) cluster ion beam deposition under ultrahigh vacuum conditions. The topographic and electronic properties of the deposits were monitored by means of scanning tunnelling microscopy (STM at 4.2 K). Topographic images reveal that at low coverages fullerene cages are pinned by point dislocation defects on the herringbone reconstructed gold terraces (as well as by step edges). At intermediate coverages, pinned monomers, act as nucleation centres for the formation of oligomeric C58 chains and 2D islands. At the largest coverages studied, the surface becomes covered by 3D interlinked C58 cages. STM topographic images of pinned single adsorbates are essentially featureless. The corresponding local densities of states are consistent with strong cage-substrate interactions. Topographic images of [C58]n oligomers show a stripe-like intensity pattern oriented perpendicular to the axis connecting the cage centers. This striped pattern becomes even more pronounced in maps of the local density of states. As supported by density functional theory, DFT calculations, and also by analogous STM images previously obtained for C60 polymers (M. Nakaya et al., J. Nanosci. Nanotechnol. 11, 2829 (2011)), we conclude that these striped orbital patterns are a fingerprint of covalent intercage bonds. For thick C58 films we have derived a band gap of 1.2 eV from scanning tunnelling spectroscopy data, STS, confirming that the outermost C58 layer behaves as a wide band semiconductor.

cond-mat.mes-hall↗
arXiv2012

Finite Size Effects and Irrelevant Corrections to Scaling near the Integer Quantum Hall Transition

We present a numerical finite size scaling study of the localization length in long cylinders near the integer quantum Hall transition (IQHT) employing the Chalker-Coddington network model. Corrections to scaling that decay slowly with increasing system size make this analysis a very challenging numerical problem. In this work we develop a novel method of stability analysis that allows for a better estimate of error bars. Applying the new method we find consistent results when keeping second (or higher) order terms of the leading irrelevant scaling field. The knowledge of the associated (negative) irrelevant exponent $y$ is crucial for a precise determination of other critical exponents, including multifractal spectra of wave functions. We estimate $|y| > 0.4$, which is considerably larger than most recently reported values. Within this approach we obtain the localization length exponent $2.62 \pm 0.06$ confirming recent results. Our stability analysis has broad applicability to other observables at IQHT, as well as other critical points where corrections to scaling are present.

cond-mat.dis-nn↗
arXiv2011

Magnetoresistance through a single molecule

The use of single molecules to design electronic devices is an extremely challenging and fundamentally different approach to further downsizing electronic circuits. Two-terminal molecular devices such as diodes were first predicted [1] and, more recently, measured experimentally [2]. The addition of a gate then enabled the study of molecular transistors [3-5]. In general terms, in order to increase data processing capabilities, one may not only consider the electron's charge but also its spin [6,7]. This concept has been pioneered in giant magnetoresistance (GMR) junctions that consist of thin metallic films [8,9]. Spin transport across molecules, i.e. Molecular Spintronics remains, however, a challenging endeavor. As an important first step in this field, we have performed an experimental and theoretical study on spin transport across a molecular GMR junction consisting of two ferromagnetic electrodes bridged by a single hydrogen phthalocyanine (H2Pc) molecule. We observe that even though H2Pc in itself is nonmagnetic, incorporating it into a molecular junction can enhance the magnetoresistance by one order of magnitude to 52%.

cond-mat.mes-hall↗
arXiv2009

Molecular switch controlled by pulsed bias voltages

It was observed in recent experiments that the current-voltage characteristics (IV) of BPDN-DT (bipyridyl- dinitro- oligophenylene- ethynylene- dithiol) can be switched in a very controlled manner between "on" and "off" traces by applying a pulse in a bias voltage, V_bias. We have calculated the polaron formation energies to check a frequently held belief, namely, that the polaron formation can explain the observed bistability. Our results are not consistent with such a mechanism. Instead, we propose a conformational reorientation. The molecule carries an intrinsic dipole moment which couples to V_bias. Ramping V_bias exerts a force on the dipole that can reorient ("rotate") the molecule from the ground state ("off") into a metastable configuration ("on") and back. By elaborated electronic structure calculations, we identify a specific path for this rotation through the molecule's conformational phase space. We show that this path has sufficiently high barriers to inhibit thermal instability but still the molecule can be switched in the voltage range of the junction stability. The theoretical IVs reproduce qualitatively the key experimental observations. We propose, how the alternative mechanism of conductance switching can be experimentally verified.

cond-mat.mes-hall↗
arXiv2008

Charge Transport in Single Au|Alkanedithiol|Au Junctions: Coordination Geometries and Conformational Degrees of Freedom

Recent STM molecular break-junction experiments have revealed multiple series of peaks in the conductance histograms of alkanedithiols. To resolve a current controversy, we present here an in-depth study of charge transport properties of Au|alkanedithiol|Au junctions. Conductance histograms extracted from our STM measurements unambiguously confirm features showing more than one set of junction configurations. Based on quantum chemistry calculations, we propose that certain combinations of different sulfur-gold couplings and trans/gauche conformations act as the driving agents. The present study may have implications for experimental methodology: whenever conductances of different junction conformations are not statistically independent, the conductance histogram technique can exhibit a single series only, even though a much larger abundance of microscopic realizations exists.

cond-mat.mtrl-sci↗
arXiv2007

Exact ground state density functional theory for impurity models coupled to external reservoirs and transport calculations

A method is presented, which employs the density matrix renormalization group technique in order to construct exact ground state exchange correlation functionals for models of correlated electron systems coupled to external reservoirs. The technique is applied to the M-site resonant level model. We calculate its exact Kubo-conductance, which is available within DMRG, and compare to the single-particle conductance obtained from Kohn-Sham energies and orbitals of the exact ground state density functional theory (DFT). It is found that the position of transport resonances is reproduced essentially exactly, while deviations in the level broadening can be less than 1% and do not exceed 10%. Our findings lend strong support to a recently held point of view, namely that approximations in the ground state functionals used in DFT based transport calculations can lead to drastic errors in transport calculations while exchange correlation contributions to the induced effective potential tend to be less significant.

cond-mat.mes-hall↗