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Ioan Baldea

Publications and source records attributed to Ioan Baldea.

36 records · Page 2Linked to original sources
arXiv2015

A quantum chemical study from a molecular perspective: ionization and electron attachment energies for species often used to fabricate single-molecule junctions

The accurate determination of the lowest electron attachment ($EA$) and ionization ($IP$) energies for molecules embedded in molecular junctions is important for correctly estimating, \emph{e.g.}, the magnitude of the currents ($I$) or the biases ($V$) where an $I-V$-curve exhibits a significant non-Ohmic behavior. Benchmark calculations for the lowest electron attachment and ionization energies of several typical molecules utilized to fabricate single-molecule junctions characterized by n-type conduction (4,4'-bipyridine, 1,4-dicyanobenzene, and 4,4'-dicyano-1,1'-biphenyl) and p-type conduction (benzenedithiol, biphenyldithiol, hexanemonothiol, and hexanedithiol] based on the EOM-CCSD (equation-of-motion coupled-cluster singles and doubles) state-of-the-art method of quantum chemistry are presented. They indicate significant differences from the results obtained within current approaches to molecular transport. The present study emphasizes that, in addition to a reliable quantum chemical method, basis sets much better than the ubiquitous double-zeta set employed for transport calculations are needed. The latter is a particularly critical issue for correctly determining $EA$'s, which is impossible without including sufficient diffuse basis functions. The spatial distribution of the dominant molecular orbitals (MO's) is another important issue, on which the present study draws attention, because it sensitively affects the MO-energy shifts $Φ$ due to image charges formed in electrodes. The present results cannot substantiate the common assumption of a point-like MO midway between electrodes, which substantially affects the actual $Φ$-values.

physics.chem-ph↗
arXiv2015

Concurrent conductance and transition voltage spectroscopy study of scanning tunneling microscopy vacuum junctions. Does it unravel new physics?

We show that the conductance ($G$) and transition voltage ($V_t$) spectroscopy data for standard vacuum nanogaps reported in the preceding paper by Sotthewes \emph{et al} cannot be understood within the framework of the existing theories/models whatever realistic effects are incorporates. However, if we include an additional ("ghost") current, the trends of the dependencies of $G$ and $V_t$ on the nanogap size $d$ can be explained. The ghost current is very small. Therefore, effects related to the ghost current can only be revealed at larger $d$'s, where the ghost current becomes comparable to or overcomes the tunneling current. Although we are not able to unravel the origin of this ghost contribution, we can and do refer to experimental $G$- and $V_t$-data reported for molecular junctions, which exhibit similarities to the presently considered vacuum nanojunctions. Analyzing notable difficulties related to the regime assigned as transport via hopping, we speculate that a ghost contribution may also exist in molecular junctions.

cond-mat.mes-hall↗
arXiv2015

An important impact of the molecule-electrode couplings asymmetry on the efficiency of bias-driven redox processes in molecular junctions

Two recent experimental (Li, J.~\emphj{et al}, \emph{Proc.\ Natl.\ Acad.\ Sci.\ U.~S.~A.} {\bf 2014}, 111, 1282-1287) and theoretical studies (Bâldea, I, \emph{Phys.\ Chem.\ Chem.\ Phys.}\ {\bf 2014}, 16, 25942-25949) have addressed the problem of tuning molecular charge and vibrational properties of single molecules embedded in nanojunctions. These are molecular characteristics escaping so far to an efficient experimental control in broad ranges. Here, we present a general argument demonstrating why, out of various experimental platforms possible, those wherein active molecules are asymmetrically coupled to electrodes are to be preferred to those symmetrically coupled for achieving a(n almost) complete redox process, and why electrochemical environment has advantages over "dry" setups. This study aims at helping to nanofabricate molecular junctions using the most appropriate platforms enabling the broadest possible bias-driven control of the redox state and vibrational modes of single molecules linked to electrodes.

physics.chem-ph↗
arXiv2015

Important issues facing model-based approaches to tunneling transport in molecular junctions

Extensive studies on thin films indicated a generic cubic current-voltage $I-V$ dependence as a salient feature of charge transport by tunneling. A quick glance at $I-V$ data for molecular junctions suggests a qualitatively similar behavior. This would render model-based studies almost irrelevant, since, whatever the model, its parameters can always be adjusted to fit symmetric (asymmetric) $I-V$ curves characterized by two (three) expansion coefficients. Here, we systematically examine popular models based on tunneling barrier or tight-binding pictures and demonstrate that, for a quantitative description at biases of interest ($V$ slightly higher than the transition voltage $V_t$), cubic expansions do not suffice. A detailed collection of analytical formulae as well as their conditions of applicability are presented to facilitate experimentalists colleagues to process and interpret their experimental data by obtained by measuring currents in molecular junctions. We discuss in detail the limits of applicability of the various models and emphasize that uncritically adjusting model parameters to experiment may be unjustified because the values deduced in this way may fall in ranges rendering a specific model invalid or incompatible to ab initio estimates. We exemplify with the benchmark case of oligophenylene-based junctions, for which results of ab initio quantum chemical calculations are also reported. As a specific issue, we address the impact of the spatial potential profile and show that it is not notable up to biases V somewhat larger than V_t, unlike at higher biases, where it may be responsible for negative differential resistance effects.

physics.chem-ph↗
arXiv2012

Extending the Newns-Anderson model to allow nanotransport studies through molecules with floppy degrees of freedom

The Newns-Anderson model is ubiquitous in studies of the molecular transport in the presence of solvent (outer) reorganization. The present work demonstrates that intramolecular reorganization can also be significant for the transport through molecules with floppy degrees of freedom, for which the Newns-Anderson model can be extended. The expressions of the model parameters deduced from electronic structure calculations for (4, 4')-bipyridine (44BPY) quantitatively differ from those characteristic for outer reorganization due to strong intramolecular anharmonicities. These expressions can be utilized as input in transport calculations for 44BPY-based molecular junctions of experimental interest [Science 301 (2003) 203, J. Am. Chem. Soc. 130 (2008) 16045, Nano Lett. 12 (2012) 354].

physics.chem-ph↗
arXiv2012

Ambipolar transition voltage spectroscopy: analytical results and experimental agreement

This work emphasizes that the transition voltages $V_{t \pm}$ for both bias polarities ($V${\raisebox{1.2ex}{$_{\stackrel{>}{<}}$}0}) should be used to properly determine the energy offset $\varepsilon_{0}$ of the molecular orbital closest to electrodes' Fermi level and the bias asymmetry $γ$ in molecular junctions. Accurate analytical formulas are deduced to estimate $\varepsilon_{0}$ and $γ$ solely in terms of $V_{t \pm}$. These estimates are validated against experiments, by showing that full experimental $I$-$V$-curves measured by Beebe et al [\prl 97, 026801 (2006)] and Tan et al [Appl. Phsy. Lett. 96, 013110 (2010)] for both bias polarities can be excellently reproduced.

cond-mat.mes-hall↗
arXiv2011

Reply to Comment on "Critical analysis of a variational method used to describe molecular electron transport"

We show that the failure of the Delaney-Greer (DG) variational ansatz for transport demonstrated by us in Phys.\ Rev.\ B {\bf 80}, 165301 (2009) (I) is not related to an unsuitable constraint that prevents a broken time-reversal symmetry or to real orbitals, as DG incorrectly claim. The complex orbitals suggested by them as a way-out solution merely represent a particular case of the general case considered by us in I, which do not in the least affect our conclusion.

cond-mat.mes-hall↗
arXiv2011

Critical analysis of a variational method used to describe the molecular electron transport

In a recent paper [I.\ Bâldea and H.\ Köppel, \prb {\bf 78}, 115315 (2008)], we showed that a variational approach [P.\ Delaney and J.\ C.\ Greer, \prl {\bf 93}, 036805 (2004)] proposed to compute the electron transport through molecules, which is based on boundary constraints of the Wigner function, is unable to correctly describe the zero-bias conductance of the simplest uncorrelated and correlated systems. In the present paper, we extend our previous analysis of the linear response limit of that approach, by considering, instead of the Wigner function, general constraints. We demonstrate that, if, as usual in transport theories, the quasi-particle distributions in electrodes are constrained, this method yields the completely unphysical result that the zero-bias conductance vanishes. Therefore, we conclude that the variational approach itself is defective.

cond-mat.mes-hall↗
arXiv2011

Transition voltage spectroscopy: a challenge for vacuum tunneling models at nanoscale

Several studies on the transition voltage ($V_t$) are based on calculations of the tunneling current within the Simmons model. In this paper devoted to vacuum nano-junctions, we compare the Simmons results for $V_t$ with those obtained from the exact Schrodinger equation by exactly including the classical image effects. The comparison reveals that the Simmons estimates for $V_t$ are completely unacceptable for nanogap sizes ($d$) at which image effects are important. The Simmons treatment drastically overestimates these effects, because it misses the famous 1/2 factor related to the fact that the image interaction energy is a self energy. The maximum of the Simmons curve $V_t$ vs. $d$ turns out to be merely an artefact of an inappropriate approximation. Unlike the Simmons approach, the "exact" WKB method yields results, which qualitatively agree with the exact ones; quantitative differences are important, demonstrating that the transmission prefactor has a significant impact on $V_t$. Further, we show that a difference between the work functions of the left and right electrodes, which gives rise to a Volta field, may be important for the ubiquitous asymmetry of the measured $I$-$V$-characteristics in general, and for the different $V_t$-values at positive and negative biases reported in vacuum nanojunctions in particular. The weak dependence $V_t = V_t(d)$ found experimentally in vacuum nanojunctions contrasts to the pronounced dependence obtained by including the exact electrostatic image contribution into the Schrodinger equation. This demonstrates that not only the molecular transport, but also the transport through a vacuum nanogap represents a nontrivial problem, which requires further refinements, e.g., a realistic description of contacts' geometry, retardation effects due to the finite tunneling time, local phonons, surface plasmons or electron image states.

cond-mat.mes-hall↗
arXiv2011

A physical limitation of the Wigner "distribution" function in transport

We present an example revealing that the sign of the "momentum" $P$ of the Wigner "distribution" function $f(q, P)$ is not necessarily associated with the direction of motion in the real world. This aspect, which is not related to the well known limitation of the Wigner function that traces back to the Heisenberg's uncertainty principle, is particularly relevant in transport studies, wherein it is helpful to distinguish between electrons flowing from electrodes into devices and vice versa.

cond-mat.mes-hall↗
arXiv2010

Revealing molecular orbital gating by transition-voltage spectroscopy

Recently, Song et al [Nature 462, 1039 (2009)] employed transition-voltage spectroscopy to demonstrate that the energy $\varepsilon_H$ of the highest occupied molecular orbital (HOMO) of single-molecule transistors can be controlled by a gate potential $V_G$. To demonstrate the linear dependence $\varepsilon_H-V_G$, the experimental data have been interpreted by modeling the molecule as an energy barrier spanning the spatial source-drain region of molecular junctions. Since, as shown in this work, that crude model cannot quantitatively describe the measured $I$-$V$-characteristics, it is important to get further support for the linear dependence of $\varepsilon_H$ on $V_G$. The results presented here, which have been obtained within a model of a point-like molecule, confirm this linear dependence. Because the two models rely upon complementary descriptions, the present results indicate that the interpretation of the experimental results as evidence for a gate controlled HOMO is sufficiently general.

cond-mat.mes-hall↗
arXiv2010

Reply to Comment on "Electron transport through correlated molecules computed using the time-independent Wigner function: Two critical tests''

In their Comment, Greer et al (i) put us in charge of a pretended wrong claim, which we never made in Phys.~Rev.~B {\bf 78}, 115315 (2008), where we criticized a method (DG)proposed by two of them, (ii) incorrectly claim that the DG method can reproduce the conductance quantum $g_0$, but (iii) to deduce $g_0$ for a toy model, they carry out calculations within the standard Landauer method, which has nothing to do with the DG's. We present results for their model obtained within the DG method, which demonstrate that the DG method fails as lamentably as in the examples we presented in our earlier work. We also analyze the physical reasons why the DG method fails.

cond-mat.mes-hall↗
arXiv2010

Applying the extended molecule approach to correlated electron transport: important insight from model calculations

Theoretical approaches of electronic transport in correlated molecules usually consider an extended molecule, which includes, in addition to the molecule itself, parts of electrodes. In the case where electron correlations remain confined within the molecule, and the extended molecule is sufficiently large, the current can be expressed by means of Laudauer-type formulae. Electron correlations are embodied into the retarded Green function of a sufficiently large but isolated extended molecule, which represents the key quantity that can be accurately determined by means of ab initio quantum chemical calculations. To exemplify these ideas, we present and analyze numerical results obtained within full CI calculations for an extended molecule described by the interacting resonant level model. Based on them, we argue that for organic electrodes the transport properties can be reliably computed, because the extended molecule can be chosen sufficiently small to be tackled within accurate ab initio methods. For metallic electrodes, larger extended molecules have to be considered in general, but a (semi-)quantitative description of the transport should still be possible particularly in the typical cases where electron transport proceeds by off-resonant tunneling. Our numerical results also demonstrate that, contrary to the usual claim, the ratio between the characteristic Coulomb strength and the level width due to molecule-electrode coupling is not the only quantity needed to assess whether electron correlation effects are strong or weak.

cond-mat.mes-hall↗
arXiv2010

Sources of negative differential resistance in electric nanotransport

A negative differential resistance (NDR) in the nanotransport is often ascribed to electron correlations. We present a simple example which demonstrates that finite electrode bandwidths and energy dependent electrode density of states are responsible for significant NDR effects, which occur even in uncorrelated nanotransport. Therefore, special care is needed in assessing the contribution of electron correlations to the NDR.

cond-mat.mes-hall↗
arXiv2009

Intriguing electron correlation effects in the photoionization of metallic quantum--dot nanorings

We report detailed results on ionization in metallic quantum--dot (QD) nanorings described by the extended Hubbard model at half filling obtained by exact numerical diagonalization. In spite of very strong electron correlations, the ionization spectra are astonishingly scarce. We attribute this scarcity to a hidden quasi--symmetry, generalizing thereby similar results on optical absorption recently reported [I. Baldea and L. S. Cederbaum, \prb {\bf 75}, 125323 (2007); {\bf 77}, 165339 (2008)]. Numerical results indicate that this hidden quasi--symmetry of the extended Hubbard model does not evolve into a true (hidden) symmetry but remains a quasi--symmetry in the case of the restricted Hubbard model as well. Based on the observation on the number of significant ionization signals per each spatial symmetry, we claim the existence of a one--to--one map between the relevant ionization signals of the correlated half-filled nanorings and the one-hole and two-hole--one-particle processes possible in the noninteracting case. Similar to the case of optical absorption, numerous avoided crossings (anticrossings) are present in the ionization spectra, which often involve more than two states. The present results demonstrate that ionization could be a useful tool to study electron correlations in metallic QD--nanoarrays, providing information that is complementary to optical absorption.

cond-mat.str-el↗
arXiv2009

Studying the single--electron transistor by photoionization

We present theoretical results demonstrating that photoionization can be a useful tool for investigating single--electron transistors. Suggestions are given on how to conduct experiments using photoionization alone or in combination with transport measurements.

cond-mat.other↗
arXiv2008

On a method to calculate conductance by means of the Wigner function: two critical tests

We have implemented the linear response approximation of a method proposed to compute the electron transport through correlated molecules based on the time-independent Wigner function [P. Delaney and J. C. Greer, \prl {\bf 93}, 36805 (2004)]. The results thus obtained for the zero-bias conductance through quantum dot both without and with correlations demonstrate that this method is either quantitatively nor qualitatively able to provide a correct physical escription of the electric transport through nanosystems. We present an analysis indicating that the failure is due to the manner of imposing the boundary conditions, and that it cannot be simply remedied.

cond-mat.other↗