arXiv subjects

Yonatan Dubi

Publications and source records attributed to Yonatan Dubi.

At least 55 records · Page 3Linked to original sources

arXiv2015

Negative differential conductance in molecular junctions: an overview of experiment and theory

One of the ultimate goals of molecular electronics is to create technologies that will complement - and eventually supersede - Si-based microelectronics technologies. To reach this goal, electronic properties that mimic at least some of the electrical behaviors of today's semiconductor components must be recognized and characterized. AN outstanding example for one such behavior is negative differential conductance (NDC), in which an increase in voltage across the device terminals results in a decrease in the electrical current passing through the device. This overview focuses on the NDC phenomena observed in metal-single molecule-metal junctions, and is roughly divided into two parts. In the first part, the central experiments which demonstrate NDC in single-molecule junctions are critically overviewed, with emphasis on the main observations and their possible physical origins. The second part is devoted to the theory of NDC in single-molecule junctions, where simple models are employed to shed light on possible mechanisms leading to NDC.

cond-mat.mes-hall↗

arXiv2015

Mechanical Tuning of Conductance and Thermopower in Helicene Molecular Junctions

Helicenes are inherently chiral polyaromatic molecules composed of all-ortho fused benzene rings possessing a spring-like structure. Here, using a combination of density functional theory and tight-binding calculations, it is demonstrated that controlling the length of the helicene molecule by mechanically stretching or compressing the molecular junction can dramatically change the electronic properties of the helicene, leading to a tunable switching behavior of the conductance and thermopower of the junction with on/off ratios of several orders of magnitude. Furthermore, control over the helicene length and number of rings is shown to lead to more than an order of magnitude increase in the thermopower and thermoelectric figure-of-merit over typical molecular junctions, presenting new possibilities of making efficient thermoelectric molecular devices. The physical origin of the strong dependence of the transport properties of the junction is investigated, and found to be related to a shift in the position of the molecular orbitals.

cond-mat.mes-hall↗

arXiv2015

The Molecular Photo-Cell: Quantum Transport and Energy Conversion at Strong Non-Equilibrium

Non-equilibrium transport properties and energy conversion performance of a molecular photo-voltaic cell are analyzed using the Lindblad master equation within the open quantum systems approach. The method allows us to calculate the dynamics of a system driven by several non-equilibrium sources (a situation we call "strong non-equilibrium"), which is the natural operating condition of photovoltaic cells. We include both coherent and incoherent processes and treat electrons, photon, and phonons on an equal footing. We find that decoherence plays a crucial role in determining both the overall efficiency of the photovoltaic conversion and the optimal energy configuration of the system. Specifically, decoherence leads to better performance, due to a faster relaxation of the excited electrons to the electrodes. We also examine the effect of coherent interference on the efficiency. The approach we propose in this letter is suitable for studying transport and energy conversion in other nanoscale systems at non-equilibrium, where both coherent and incoherent processes take place.

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arXiv2014

Transport Through Self-Assembled Monolayer Molecular Junctions: Role of In-Plane Dephasing

Self-assembled-monolayer (SAM) molecular junctions (MJs) constitute a promising building block candidate for future molecular electronic devices. Transport properties of SAM-MJs are usually calculate using either the phenomenological Simmons model, or a fully-coherent transport theory, employing the SAMs periodicity. We suggest that dephasing plays an important role in determining the transport properties of SAM-MJs. We present an approach for calculating the transport properties of SAM-MJs that inherently takes into account in-plane dephasing in the electron motion as it traverses the SAM plane. The calculation is based on the non-equilibrium Green's function formalism, with a local dynamics approximation that describes incoherent motion along the SAM plane. Our approach describes well the two hallmarks of transport through SAM-MJs, namely the exponential decay of current with molecular chain length and the reduction of the current per molecule as compared to single-molecule junctions. Specifically, we show that dephasing leads to an exponential decay of the current as a function of molecular length, even for resonant tunneling, where the fully coherent calculation shows little or no length-dependence of the current. The dephasing is also shown to lead to a substantial reduction of the current in a SAM-MJ as compared to the single molecule junction, in a realistic parameter regime, where the coherent calculation shows only a very small reduction of the current. Finally, we discuss the effect of dephasing on more subtle transport phenomena such as the conductance even-odd effect and negative differential resistance.

cond-mat.mes-hall↗

arXiv2013

Dynamical coupling and negative differential resistance from interactions across the molecule-electrode interface in molecular junctions

Negative differential resistance - a decrease in current with increasing bias voltage - is a counter-intuitive effect that is observed in various molecular junctions. Here, we present a novel mechanism that may be responsible for such an effect, based on strong Coulomb interaction between electrons in the molecule and electrons on the atoms closest to the molecule. The Coulomb interaction induces electron-hole binding across the molecule-electrode interface, resulting in a renormalized and enhanced molecule-electrode coupling. Using a self-consistent non-equilibrium Green's function approach, we show that the effective coupling is non-monotonic in bias voltage, leading to negative differential resistance. The model is in accord with recent experimental observations that showed a correlation between the negative differential resistance and the coupling strength. We provide detailed suggestions for experimental tests which may help to shed light on the origin of the negative differential resistance. Finally, we demonstrate that the interface Coulomb interaction affects not only the I-V curves but also the thermoelectric properties of molecular junctions.

cond-mat.mes-hall↗

arXiv2013

The effect of fluctuations - thermal and otherwise - on the temperature dependence of thermopower in aromatic chain single-molecule junctions

We report a theoretical study of the thermopower of single-molecule junctions, with focus on phenyl-based molecular junctions. In contrast to prior studies, thermal fluctuations of the torsional angle between the phenyl rings and variations in the position of the molecular level alignment with respect to the electrode Fermi energy were both taken into account. Full thermopower histograms were obtained, and their dependence on the magnitude of the fluctuations was studied. We found that at large molecular orbital variations the thermopower becomes strongly dependent on the torsion angle and can even change sign. This results in a marked effect of fluctuations on the thermopower distribution, yielding an average thermopower at high temperatures that differs (smaller or larger) from the fluctuation-free value, depending on the strength of fluctuations. We therefore conclude that fluctuations should be taken into account both when extracting single-molecule parameters, such as the molecular level-Fermi level offset, and in predictions of the thermopower of molecular junctions.

cond-mat.mes-hall↗

arXiv2012

Tunable Thermal Switching via DNA-Based Nano Devices

DNA has a well-defined structural transition -- the denaturation of its double-stranded form into two single strands -- that strongly affects its thermal transport properties. We show that, according to a widely implemented model for DNA denaturation, one can engineer DNA "heattronic" devices that have a rapidly increasing thermal conductance over a narrow temperature range across the denaturation transition (~350 K). The origin of this rapid increase of conductance, or "switching", is the softening of the lattice and suppression of nonlinear effects as the temperature crosses the transition temperature and DNA denatures. Most importantly, we demonstrate that DNA nanojunctions have a broad range of thermal tunability due to varying the sequence and length, and exploiting the underlying nonlinear behavior. We discuss the role of disorder in the base sequence, as well as the relation to genomic DNA. These results set the basis for developing thermal devices out of materials with nonlinear structural dynamics, as well as understanding the underlying mechanisms of DNA denaturation.

cond-mat.soft↗

arXiv2012

Origin of thermoelectric response fluctuations in single-molecule junctions

The thermoelectric response of molecular junctions exhibits large fluctuations, as observed in recent experiments [e.g. Malen J. A. {\sl et al.}, Nano Lett. {\bf 10}, 3406 (2009)]. These were attributed to fluctuations in the energy alignment between the highest occupied molecular orbital (HOMO) and the Fermi level at the electrodes. By analyzing these fluctuations assuming resonant transport through the HOMO level, we demonstrate that fluctuations in the HOMO level alone cannot account for the observed fluctuations in the thermopower, and that the thermo-voltage distributions obtained using the most common method, the Non-equilibrium Green's function method, are qualitatively different than those observed experimentally. We argue that this inconsistency between the theory and experiment is due to the level broadening, which is inherently built-in to the method, and smears out any variations of the transmission on energy scales smaller than the level broadening. We show that although this smearing only weakly affects the transmission, it has a large effect on the calculated thermopower. Using the theory of open quantum systems we account for both the magnitude of the variations and the qualitative form of the distributions, and show that they arise not only from variations in the HOMO-Fermi level offset, but also from variations of the local density of states at the contact point between the molecule and the electrode.

cond-mat.mes-hall↗

arXiv2012

Microwave-mediated heat transport through a quantum dot

The thermoelectric effect in a quantum dot (QD) attached to two leads in the presence of microwave fields is studied by using the Keldysh nonequilibrium Green function technique. When the microwave is applied only on the QD and in the linear-response regime, the main peaks in the thermoelectric figure of merit and the thermopower are found to decrease, with the emergence of a set of photon-induced peaks. Under this condition the microwave field can not generate heat current or electrical bias voltage. Surprisingly, when the microwave field is applied only to one (bright) lead and not to the other (dark) lead or the QD, heat flows mostly from the dark to the bright lead, almost irrespectively to the direction of the thermal gradient. We attribute this effect to microwave-induced opening of additional transport channels below the Fermi energy. The microwave field can change both the magnitude and the sign of the electrical bias voltage induced by the temperature gradient.

cond-mat.mes-hall↗

arXiv2011

How Kondo Holes Create Intense Nanoscale Heavy-Fermion Hybridization Disorder

Replacing a magnetic atom by a spinless atom in a heavy fermion compound generates a quantum state often referred to as a 'Kondo-hole'. No experimental imaging has been achieved of the atomic-scale electronic structure of a Kondo-hole, or of their destructive impact (Lawrence JM, et al. (1996) Kondo hole behavior in Ce0. 97La0. 03Pd3. Phys Rev B 53:12559-12562; Bauer ED, et al. (2011) Electronic inhomogeneity in a Kondo lattice. Proc Natl Acad Sci. 108:6857-6861) on the hybridization process between conduction and localized electrons which generates the heavy fermion state. Here we report visualization of the electronic structure at Kondo-holes created by substituting spinless Thorium atoms for magnetic Uranium atoms in the heavy-fermion system URu2Si2. At each Thorium atom, an electronic bound state is observed. Moreover, surrounding each Thorium atom we find the unusual modulations of hybridization strength recently predicted to occur at Kondo-holes (Figgins J, Morr DK (2011) Defects in heavy-fermion materials: unveiling strong correlations in real space. Phys Rev Lett 107:066401). Then, by introducing the 'hybridization gapmap' technique to heavy fermion studies, we discover intense nanoscale heterogeneity of hybridization due to a combination of the randomness of Kondo-hole sites and the long-range nature of the hybridization oscillations. These observations provide direct insight into both the microscopic processes of heavy-fermion forming hybridization and the macroscopic effects of Kondo-hole doping.

cond-mat.str-el↗

arXiv2011

Driving denaturation: Nanoscale thermal transport as a probe of DNA melting

DNA denaturation has long been a subject of intense study due to its relationship to DNA transcription and its fundamental importance as a nonlinear, structural transition. Many aspects of this phenomenon, however, remain poorly understood. Existing models fit quite well with experimental results on the fraction of unbound base pairs versus temperature. Yet, these same models give incorrect results for other essential quantities. For example, the predicted base pair fluctuation timescales - relevant to transcription - are orders of magnitude different from those observed experimentally. Here, we demonstrate that nanoscale thermal transport can serve as a sensitive probe of the underlying microscopic mechanisms responsible for the dynamics of DNA denaturation. Specifically, we show that the heat transport properties of DNA are altered significantly and abruptly as it denaturates, and this alteration encodes detailed information on the dynamics of thermal fluctuations and their interaction along the chain. This finding allows for the unambiguous discrimination between models of DNA denaturation. Measuring the thermal conductance will thus shed new light on the nature of this important molecule.

cond-mat.mes-hall↗

arXiv2010

Energy flow and thermo-electricity in atomic and molecular junctions

Advances in the fabrication and characterization of nanoscale systems now allow for a deeper understanding of one of the most basic issues in science and technology: the flow of energy at the microscopic level. In this Colloquium we survey recent advances and present understanding of physical mechanisms of energy transport in nanostructures, focusing mainly on molecular junctions and atomic wires. We examine basic issues such as thermal conductivity, thermoelectricity, local temperature and heating, and the relation between energy current density and temperature gradient - known as Fourier's law. We critically report on both theoretical and experimental progress in each of these issues, and discuss future research opportunities in the field.

cond-mat.mes-hall↗

arXiv2010

Charge density wave in hidden order state of URu$_2$Si$_2$

We argue that the hidden order state in URu$_2$Si$_2$ will induce a charge density wave. The modulation vector of the charge density wave will be twice that of the hidden order state, $Q_{CDW} = 2Q_{HO}$. To illustrate how the charge density wave arises we use a Ginzburg-Landau theory that contains a coupling of the charge density wave amplitude to the square of the HO order parameter $Δ_{HO}$. This simple analysis allows us to predict the intensity and temperature dependence of the charge density wave order parameter in terms of the susceptibilities and coupling constants used in the Ginzburg-Landau analysis.

cond-mat.str-el↗

arXiv2009

Dimer mean-field model for the Ising spin glass

A dimer mean-field model for the Ising spin-glass is presented. Despite its simplicity it captures some of the essential features of the spin-glass physics. The distribution of the single-spin magnetization is determined from a self-consistent integral equation. By solving the self-consistency condition numerically, we find that there are two temperature scales characterizing the glass transition. At the first, higher temperature, the glass order parameter becomes non-vanishing, and at the second, freezing temperature, it saturates to its maximal value. The effect of magnetic field and the existence of the Almeida-Thouless line are discussed. Finally, it is shown that the information compressibility, defined as the derivative of entropy with respect to energy, diverges at the freezing temperature. This indicates a zero internal temperature and true glassy dynamics with diverging relaxation times.

cond-mat.dis-nn↗

arXiv2009

Maintaining the local temperature below the critical value in thermally out of equilibrium superconducting wires

A generalized theory of open quantum systems combined with mean-field theory is used to study a superconducting wire in contact with thermal baths at different temperatures. It is shown that, depending on the temperature of the colder bath, the temperature of the hotter bath can greatly exceed the equilibrium critical temperature, and still the local temperature in the wire is maintained below the critical temperature and hence the wire remains in the superconducting state. The effects of contact areas and disorder are studied. Finally, an experimental setup is suggested to test our predictions.

cond-mat.mes-hall↗

arXiv2007

Nature of the superconductor-insulator transition in disordered superconductors

As a superconducting thin film becomes disordered and subject to an increasing magnetic field, a point is reached when it undergoes a transition from a superconducting to an insulating state. We use the Bogoliubov-De-Gennes equations and a novel Monte-Carlo approach to study this transition numerically, starting from a microscopic hamiltonian. The key effect of disorder is to create 'islands' of strong superconductivity, coupled by regions that are only weakly superconducting. In the case of weak disorder, an increasing magnetic field eventually destroys the superconducting state throughout the material, leading to an insulator. On the other hand, when disorder is strong, superconductivity persists in the islands, and the effect of a magnetic field is to suppress the coupling between them, resulting in strong superconducting phase fluctuations, again leading to an insulating state. These findings may be relevant to the high-temperature superconductors, where intrinsic disorder may play a role.

cond-mat.supr-con↗

arXiv2007

Pair correlations and the survival of superconductivity in and around a super-conducting impurity

The problem of the survival of superconductivity in a small super-conducting grain placed in a metal substrate is addressed. For this aim the pair correlations and super-conducting gap around and inside a negative-U impurity in one and two dimensions is calculated, in a discrete tight-binding model and a continuous model. Using the Hartree-Fock-Gorkov mean-field decomposition, it is found that pairing inside the gap occurs when the system has a degeneracy between successive number of electron pairs, and is highly sensitive to the chemical potential. For finite pairing in the island, pair correlations in the normal part may either decay exponentially or be long-ranged, depending on the strength of interaction. In addition, it is shown analytically that there is a minimal island size under-which pairing vanishes, and that it scales as a power-law, rather then exponentially as in isolated grains. These results are interpreted in terms of screening of the negative-U impurity by the electron gas.

cond-mat.mes-hall↗

arXiv2005

Theory of Magneto-resistance of Disordered Superconducting Films

Recent experimental studies of magneto-resistance in disordered superconducting thin films reveal a huge peak (about 5 orders of magnitude). While it may be expected that magnetic field destroys superconductivity, leading to an enhanced resistance, attenuation of the resistance at higher magnetic fields is surprising. We propose a model which accounts for the experimental results in the entire range of magnetic fields, based on the formation of superconducting islands due to fluctuations in the superconducting order parameter amplitude. At strong magnetic fields Coulomb blockade in these islands gives rise to negative magneto-resistance. As the magnetic field is reduced the effect of Coulomb blockade diminishes and eventually the magneto-resistance changes sign. Numerical calculations show good qualitative agreement with experimental data.

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