## Molecular Electronics

June 1 - August 31, 2012

Organizer:  Amnon Aharony (Ben-Gurion University)

Molecular electronics, one of the major fields in nanoscience, studies electronic devices based on single molecules, and on molecular networks connected to other electronic components. Its potential applications include sensors, displays, smart materials, molecular motors, logic and memory devices, molecular scale transistors and energy transduction devices. Besides being the next step in device miniaturization, molecules are able to bind to one another, recognize each other, assemble into larger structures, and exhibit dynamical stereochemistry. In addition to its technological potential, molecular electronics has raised many new fundamental questions, e.g. concerning the interactions of molecular systems with their environment and their functioning far from equilibrium.  Also, fluctuations and noise constitute an important part of the physics of such microscopic systems. At the moment there already exist several ingenious experimental realizations of transport through molecular bridges. There also exist a variety of different theoretical tools (both in chemistry and in physics) to attack the above important issues. This interdisciplinary workshop will bring together physicists and chemists, experimentalists and theoreticians, senior and young scientists, aiming to understand existing experiments, to propose new experiments (possibly combining various experimental tools) and new technological devices, using combinations of  various theoretical and experimental methods.

Research Group Conference

July 16-20, 2012

Molecular Electronics in Jerusalem, International Meeting (Click for details)

Group Seminars:

Seminars are held at the IAS, located at the Edmond J. Safra , Givat Ram campus of The Hebrew University. All researchers in related fields, including students, are invited.

June 20, 2pm:

Fast DNA sequencing: a physicist’s perspective

Prof. Massimiliano Di Ventra, Department of Physics University of California San Diego La Jolla, CA, USA

Fast and low-cost DNA sequencing methods would revolutionize medicine: a person could have his/her full genome sequenced so that drugs could be tailored to his/her specific illnesses; doctors could know in advance patients’ likelihood to develop a given ailment; cures to major diseases could be found faster [1]. However, this goal of “personalized medicine” is hampered today by the high cost and slow speed of DNA sequencing methods. In this talk, I will discuss the sequencing protocol we suggest which requires the measurement of the distributions of transverse currents during the translocation of single-stranded DNA into nanopores [2-5]. I will support our conclusions with a combination of molecular dynamics simulations coupled to quantum mechanical calculations of electrical current in experimentally realizable systems [2-5]. I will also discuss recent experiments that support these theoretical predictions. In addition, I will show how this relatively unexplored area of research at the interface between solids, liquids, and biomolecules at the nanometer length scale is a fertile ground to study quantum phenomena that have a classical counterpart, such as ionic quasi-particles, “quantized” conductance [6,7] and Coulomb blockade [8].

References 1. M. Zwolak, M. Di Ventra, Rev. Mod. Phys. 2008, 80, 141. 2. M. Zwolak and M. Di Ventra, Nano Lett. 5, 421 (2005). 3. J. Lagerqvist, M. Zwolak, and M. Di Ventra, Nano Lett., 2006 6, 779. 4. J. Lagerqvist, M. Zwolak, and M. Di Ventra, Biophys. J. 2007, 93, 2384. 5. M. Krems, M. Zwolak, Y.V. Pershin, and M. Di Ventra, Biophys. J. 2009, 97, 1990. 6. M. Zwolak, J. Lagerqvist, and M. Di Ventra, Phys. Rev.Lett. 2009, 103, 128102. 7. M. Zwolak, J. Wilson, and M. Di Ventra, J. Phys. Cond. Matt. 2011, 22, 454126. 8. M. Krems and M. Di Ventra, Arxiv preprint arXiv:1103.2749 (2011).

June 25, 3pm:

Fundamental aspects of transport in nanoscale systems and ultra-cold atoms

Massimiliano Di Ventra, UC San Diego

I will discuss some of the most basic questions in fermionic and bosonic transport, such as the conditions for the existence of a steady-state current, its uniqueness, the role of interactions and spin statistics, its entanglement entropy, etc. This will lead me to introduce an alternative viewpoint to conduction - the micro-canonical formalism of transport - which is ideal to study the above issues [1].

I will point out the similarities and differences with the widely used Landauer formalism, and advance a series of predictions that can be verified by loading ultra-cold atoms into artificial optical lattices.

[1] M. Di Ventra, Electrical Transport in Nanoscale Systems (Cambridge University Press, 2008).

July 2, 3pm:

Optical Phonon Lasing in Semiconductor Double Quantum Dot

Mikio Eto, Keio University, Japan

We theoretically study the nonequilibrium transport through a double quantum dot (DQD) fabricated on semiconductors coupled to LO phonons, and DQD in a carbon nanotube (CNT) coupled to a vibron, longitudinal stretching mode of phonon [1]. In a DQD on semiconductors, the phonon-assisted tunneling was observed when the level spacing $\Delta$ between the quantum dots is tuned to the energy of LO phonon $\omega_0$ in a recent experiment [2]. We show that (i) the DQD effectively couples to only two modes of LO phonons which work as a natural cavity. (ii) The pumping to the upper level is realized by an electric current under a finite bias. The phonon lasing takes place at the resonance of $\Delta=n \omega_0$ (n: positive integer) when the tunnel rate between leads and DQD is much larger than the phonon decay rate. (iii) The phonon anti-bunching is observed in an opposite case of small tunnel rate. In a DQD fabricated on a suspended CNT [3], the electron-phonon coupling is so strong that neither phonon lasing nor anti-bunching take place. In this case, the Franck-Condon effect induces an effective thermalization of phonons and destroys the coherent phenomena.

[1] R. Okuyama, M. Eto, and T. Brandes, arXiv:1205.6955v1.
[2] S. Amaha and K. Ono (Riken, Japan), private communications.
[3] R. Leturcq et al., Nat. Phys. 5, 327 (2009).

July 9, 3pm:

Fundamental aspects of density functional transport theories

Ferdinand Evers, Physics Dept., University of Karlsruhe

The most wide spread technology in order to calculate, conductances of single molecules relies on the Landauer-formalism applied to Kohn-Sham scattering states (non-equilibrium Green's function formalism).

Originally, the approach was formulated as an ad hoc procedure motivated by the fact that relatively large systems could be treated.

During the last years significant progress has been made in understanding under what conditions the method can give quantitative results, qualitative results and where is it is bound to fail. The talk offers a review about the current situation.

An emphasis will be on insights obtained from DFT calculations with exact XC-potentials  reconstructed from DMRG-simulations.

This latter part of the talk relies on published  and unpublished work obtained in intensive collaborations with Peter Schmitteckert.

July 12, 10am to 5:30 pm: A Tutorial day

10-12:  J. C. Cuevas, U. Madrid: Basics of Quantum Transport in Single Electron Junctions:  Coherent Transport, Coulomb Blockade

1-2: O. Tal , Weizmann Inst.: Electron-Vibration Interactions in Single-molecular Junctions

2-3: A. Nitzan, Tel Aviv U: Theory of Molecular Electron Transfer

3:30-4:30: D. Neuhauser, UCLA: Nanopolaritonics Transport

4:30-5:30: J. C. Cuevas, U. Madrid: Basics of Quantum Transport in Single Electron Junctions:   Role of Vibrational Modes

July 23, 11 am (note unusual hour): Experimental Seminar:

Electron transport in atomic and single-molecule junctions beyond linear conductance

András Halbritter,  Solid State Physics Laboratory, Budapest University of Technology and Economics

Break junction techniques are among the most widely used methods to create atomic-scale and single molecule structures along the controlled mechanical elongation of a metallic wire in molecular environment. The atomic resolution imaging of such structures is highly demanding, thus the microscopic behavior is usually traced back from indirect conductance measurements and their comparison with ab initio simulations. In the most common experiment the evolution of the conductance is monitored during the repeated opening and closing of the nanojunction, and the captured conductance vs. electrode-separation traces are analyzed by conductance histograms. Peaks in the histogram reflect the conductance of stable, frequently occurring junction configurations. It is, however, obvious that conductance histograms supply a very limited information about the system under study. To gain more information one can either perform an advanced statistical analysis of the conductance traces, or apply special measurement techniques going beyond the linear conductance. The talk will address the latter experimental techniques with a special focus on conductance fluctuation measurements, inelastic spectroscopy measurements and the spectroscopy based on superconducting Andreev reflections.

July 23, 1:30 pm, Seminar:

Thermoelectric transport in three-terminal junctions

O. Entin-Wohlman, Department of Physics, Ben Gurion University, Beer Sheva 84105, Israel

Heat and Charge currents through a junction bridging two electronics baths (of possibly different temperatures and different chemical potentials), and coupled to a third thermal terminal are considered. The role of inelastic processes between the charge carriers and the thermal terminal are considered.

The main idea is to try  and forcr electrons transported through the junction (e.g., a molecular bridge) to take relatively large energy from the thermal bath and deliver it to another bath or to an electronic reservoir, as a heat or a charge current, attempting to achieve a significant figure of merit for the process.

July 23, 2:15 pm, Seminar:

From molecular thermoelectrics till carbon-based nanophononics

Gianaurelio Cuniberti, Technical University, Dresden

The problem of electrical transport at the molecular scale in strongly interconnected with heat transfer and thermal management. In this lecture, we will introduce the basic equations governing charge and phonon transport through molecular junctions and generalize these results to

the case of larger pure carbon nanostructures. Main observables in these systems are allowing us to infer on their thermoelectric figure of merit, ZT, and its dependence of structural or electronic disorder and atomistic dynamics.

July 25, 11 am, Seminar:

IAS Room 128

The role of molecular polarizability in tuning semiconductor interfacial electronic properties

Roie Yerushalmi, Institute of Chemistry and the Center for Nanoscience and Nanotechnology,
The Hebrew University of Jerusalem

Abstract
Self assembled monolayers of polar molecules are commonly used to modify the electronic properties of semiconductor interfaces and surfaces. Such layers are used to tune semiconductors and metals work function, for surface passivation, and for a vast set of applications. The adsorption of polar molecules on a semiconductor surface results in several changes. The molecule intrinsic dipole and the interface bond dipole cause a change in the substrate work function. The relationship between the change in work function and the gas phase dipole of the adsorbed molecules was the subject of intense research. It was demonstrated that by experiment and theory that the work function change is related to the molecule gas phase dipole through the Helmholtz relation. The present study addresses molecular layer formation at interfaces involving non-covalent surface interactions H-bond formation, relying on polar interactions at the interface with polarizable molecular head groups. In order to study the contribution of polarizability, various molecules having X=O (X= P, As or C) head groups and bulky phenyl ring as tail group were selected with diverse dipole and polarizability values while maintaining similar type of surface interactions and similar molecular footprint. The formation of the molecular layer was investigated by various surface analytical techniques such as ellipsometry, XPS and contact angle measurements. The electronic structures at the Si/SiO2/molecule interfaces were studied by Kelvin probe techniques and a theoretical model for generalized Helmholtz equation is presented.
The results provide a general and broad approach for the formation of molecular layers by neutral molecules bearing a polar and polarizable head groups and semiconductor surface and quantitative description of the electronic effects.

July 26, 2 pm, Seminar:

Avraham Schiller.  Racah Institute of Physics, the Hebrew University

The promise of molecular electronics has generated intense interest in the interplay of molecular vibrations and resonant electronic transport. Two customary limits of phonon-assisted tunneling are the anti-adiabatic and adiabatic regimes, where the phonon frequency is either sufficiently large or sufficiently small as compared to the bare electronic hopping rate. In the former case the phonon can efficiently respond to hopping events by forming a polaron, thus suppressing the bare tunneling rate. In the latter case the phonon is too slow to track the electronic motion, having little effect on its rate. While each of these extreme limits is rather well controlled theoretically, the crossover between them is far less understood as it lacks a small parameter.

Focusing on strong electron-phonon coupling, we conducted a systematic study of the crossover between the two regimes in the framework of the resonant-level model in order to (1) reveal the physical parameters governing the transition, (2) expose its manifestations in linear transport, and (3) track the evolution of the low-energy scale corresponding to the renormalized resonance width. In particular, we find an extended regime where the bare electronic tunneling rate is large as compared to the phonon frequency and yet the physics is essentially that of the anti-adiabatic limit. The effective low-energy Hamiltonian in this regime is found to be the interacting resonant-level model, whose parameters we extract numerically. In this talk we shall expand on details omitted in the conference presentation, including a discussion of charging properties and the phonon distribution function. A brief discussion of Wislon's numerical renormalization group will also be included.

July 30, 1:30 pm, Seminar:

Adsorbates in graphene: Impurity states in quantizing magnetic field

P.G. Silvestrov, FU Berlin

Covalent bonding of impurity atoms to graphene, in many cases leads to creation of unusual (singular) zero energy localized electron states. Existence of such states would lead to interesting phenomena, actively discussed recently.  In this talk I consider the behavior of localized impurity levels in graphene in quantizing magnetic field. In the magnetic field the impurity level effectively hybridizes with one of the n=0 Landau level states and splits into two close opposite-energy states. In turn, mixing of localized and Landau levels changes a spin content of a quantum Hall ferromagnet and modifies, via the exchange interaction, the spectrum of electrons surrounding the impurity.   Existing theories investigate graphene uniformly covered by adatoms, though some experiments indicate the tendency towards their clusterization. To address this "unpleasant" possibility, I consider the case of a dense bunch of the impurity atoms, and show how such bunch changes dynamics and spin polarization of a large dense electron droplet surrounding it.