Session W20: Focus Session: Electron, Ion, Exciton Transport in Nanostructures: Charge Transport in Functional Nanostructures

Two important physical models for resistance switching phenomena

Resistance switching (RS) phenomena refer to reversible resistance changes between two metastable resistance states driven by an external voltage. Recently, there has been a flurry of investigations into RS due to their inherent scientific interest and potentials for memory applications. In spite of extensive efforts, the basic mechanisms of RS still remain to be elucidated. One of the reasons is that RS usually occurs in very dirty materials, where defects should play important roles. In this talk, I will present two models for RS phenomena, which are material independent and can be used to make quantitative predictions. The first model is for unipolar RS, where the corresponding current-voltage ($I-V)$ curves are quite symmetric. We introduced a new kind of percolation model, called the random circuit breaker (RCB) network model, which allows reversible changes between two resistance states. This model can describe the formation of conducting channels due to dielectric breakdown and make quantitative predictions especially for scaling behaviors. We will show that collective behavior of conducting channels plays an important role in most aspects of unipolar RS, including the wide distribution of set and reset voltages, scaling behaviors, and large 1/$f$ noise. The second model is for bipolar RS, where the corresponding $I-V$ curves are quite asymmetric. We introduced a quantitative model which can describe motion of mobile defects under electric field. We will show that oxygen vacancy migration near the interface region could determine important features of bipolar RS, including two switching directions of $I-V$ curves. We also showed that important aspects of these two models can be combined successfully in a unified scheme by putting interface effects into the RCB network model.   

Fractal dynamics in chaotic quantum transport

Despite several experiments on chaotic quantum transport, corresponding ab initio quantum simulations have been out of reach so far. Here we carry out quantum transport calculations in real space and real time for a two-dimensional stadium cavity that shows chaotic dynamics. Applying a large set of magnetic fields yields a complete picture of the magnetoconductance that indicates fractal scaling on intermediate time scales. Two methods that originate from different fields of physics are used to analyze the scaling exponent and the fractal dimension. They lead to consistent results that, in turn, qualitatively agree with the previous experimental data.   

Schottky Barrier Transport for Multiphase Gallium Nitride Nanowire

Our group has shown that gallium nitride nanowires grown by catalyst-free vapor deposition at 850$^{\mathrm{o}}$C have multiple internal crystalline regions that may be zinc blende or wurtzite phase. Stability is enabled by one or more totally coherent (0001)/(111) internal interfaces. Cross-section HRTEM has further demonstrated that, while the transverse nanowire profile appears triangular, it is actually made up of two or more surface orientations corresponding to the multi-phase internal regions. We present results of a transport investigation of these multiphase nanowires within a nanoFET circuit architecture, focusing on injection from the contacts into the nanowires. Experimental results demonstrated that a variety of surface state derived Schottky barriers could be present at the contact-nanowire interfaces. Transport across the Schottky barriers was modeled using a combined thermionic emission-tunnelling approach, leading to information about barrier height, carrier concentrations, and expected temperature behavior. The experimental and theoretical results indicate that with optimal design taking surface and internal structures into account, high current densities can be supported.   

Non-adiabatic excitation and detection of coherent oscillations of single electrons

Surface acoustic waves (SAWs) are used to drive single-electron quantum dots along a complex depleted channel defined by various split gates. As the electron moves through this potential landscape at the SAW velocity (2800m/s), the evolution of the electron's wavefunction may be probed by detecting oscillations in the probability of tunnelling through a narrow barrier on one side of the channel. Coherent oscillations of the wavefunction are generated by non-adiabatic potential changes on a time-scale of tens of ps. We present here results of work in which this phenomenon is observed in two separate tunnelling regions, indicating a charge coherence time $> 500$ picoseconds. Additionally, we show that the initial state of the oscillations may be determined a significant distance from the tunnelling region through the use of suitably tuned gate voltages.   

Non-Radiative Energy Transfer Into Nanometer-Scale Thin Semiconducting Films

\noindent Non-radiative energy transfer (NRET) has gained a lot of attention recently due to its possible utility in new generations of light-emitting and photovoltaic devices. In this process, a ``donor'' species in an excited state transfers its excitation energy resonantly to an ``acceptor'' species. A classical realization of NRET is F\"{o}rster ET between two point-like species. Our interest is in ET between a small donor and an ultrathin acceptor layer. The layers can be realized as planar ensembles of molecules or QDs or as a thin crystalline semiconductor slab. We use two complementary approaches to study the effects of dielectric polarization in thin layers on ET rates: (1) The classical macroscopic electrodynamics treating the acceptor layer as a continuum of certain dielectric permittivity; (2) A direct modeling utilizing planar acceptor lattices, each of the acceptors treated as a polarizable point dipole. Comparison of the results allows us to establish salient qualitative features as well as to clarify the role of local-field factors. Of particular interest is our finding a broad region of the dielectric responses where ET into thinner films \textit{counter-intuitively} turns out to be more efficient than ET into thicker films.   

Quantum decrease of capacitance in a nanometer-sized tunnel junction

We have studied the capacitance of the tunnel junction defined by the tip and sample of a Scanning Tunnelling Microscope through the measurement of the electrostatic forces and impedance of the junction. A decrease of the capacitance when a tunnel current is present has shown to be a more general phenomenon as previously reported in other systems [1]. On another hand, an unexpected reduction of the capacitance is also observed when increasing the applied voltage above the work function energy of the electrodes to the Field Emission (FE) regime, and the decrease of capacitance due to a single FE-Resonance has been characterized. All these effects should be considered when doing measurements of the electronic characteristics of nanometer-sized electronic devices and have been neglected up to date.\\[4pt] [1] J.G. Hou, B. Wang, J. Yang, X.R. Wang, H.Q. Wang, Q. Zhu, and X. Xiao. Phys. Rev. Lett. 86, 5321 (2001)   

Detecting stray microwaves and nonequilibrium quasiparticles in thin films by single-electron tunneling

Superconducting thin films and tunnel junctions are the building blocks of many state-of-the-art technologies related to quantum information processing, microwave detection, and electronic amplification. These devices operate at millikelvin temperatures, and -- in a naive picture -- their fidelity metrics are expected to improve as the temperature is lowered. However, very often one finds in the experiment that the device performance levels off around 100--150 mK. In my presentation, I will address three common physical mechanisms that can cause such saturation: stray microwaves, nonequilibrium quasiparticles, and sub-gap quasiparticle states. The new experimental data I will present is based on a series of studies on quasiparticle transport in Coulomb-blockaded normal-insulator-superconductor tunnel junction devices. We have used a capacitively coupled SET electrometer to detect individual quasiparticle tunneling events in real time. We demonstrate the following record-low values for thin film aluminum: quasiparticle density $n_{\mathrm{qp}} < 0.033 / \mu\mathrm{m}^3$, normalized density of sub-gap quasiparticle states (Dynes parameter) $\gamma < 1.6 \times 10^{-7}$. I will also discuss some sample stage and chip designs that improve microwave shielding.   

Frequency Regimes of Kondo Dynamics in a Single-Electron Transistor

It has been theoretically predicted that the Kondo temperature, T$_K$, serves as the intrinsic timescale for the formation of Kondo correlations between conduction electrons and local spin moments. To probe this timescale, we have measured the time averaged differential conductance, $\langle$G$\rangle$=d$\langle$I$\rangle$/dV$_{ds}$, of a single electron transistor in the spin 1/2 Kondo regime in presence of an oscillating bias voltage, V(t)=V$_{ds}$+V$_{AC}$ sin(2$\pi$ft). We present the amplitude dependent conductance over select frequencies spanning several orders of magnitude below T$_K$ to twice T$_K$ (T$_K \sim$ 16GHz). At frequencies above T$_K$, we find good agreement with theory [Kaminski, et al. Phys. Rev. B 62, 8154 (2000)] in both the low (V$_{AC} \sim$ T$_K$/10) and high (V$_{AC} \sim$ 10T$_K$) amplitude regimes. The onset of non-adiabatic conductance behavior occurs well below prediction, f $\sim$ T$_K$, and becomes more apparent as the frequency nears T$_K$.   

Treatment of High Conductance Kondo Transport in Single Molecule Devices

A single molecule break junction device serves as a tunable model system for probing the many body Kondo state. There are predictions of universality across many realizations of the Kondo model in which the response of the system to different perturbations is characterized by a single emergent energy scale, $k_BT_K$. Comparisons between different experimental systems have shown issues with numerical consistency. With a new constrained analysis examining the response of conductance to temperature, bias, and magnetic field perturbations simultaneously, we show that these deviations from universality can be resolved by properly accounting for background, non-Kondo contributions to the conductance that are often neglected. We clearly demonstrate the importance of these non-Kondo conduction channels by examining transport in devices with total conductances exceeding the theoretical maximum due to Kondo-assisted tunneling alone.   

Constrained-DFT method for energy level alignment of metal-molecule interfaces

The electron transport properties of molecular junctions depend strongly on the alignment of the molecule's ionization potential (IP) and electron affinity (EA) with respect to the metal Fermi energy. It has been demonstrated experimentally\footnote{M. T. Greiner et al., Nature Mater. \textbf{11}, 76 (2011)} and theoretically \textbf{J. Neaton et al., Phys. Rev. Lett. \textbf{97}, 216405 (2006).} that the IP and the EA of molecules change when they are absorbed on a polarizable substrate, due to the formation of an image charge in the surface when an electron is either removed or added to the molecule. While within the GW approximation such a renormalization can be described, the energy levels of standard density functional theory (DFT) fail to capture it. However, DFT total energy differences between charged and neutral systems can usually describe IP and EA of molecules rather well. Here we therefore apply constrained DFT (CDFT) to calculate charge transfer energies between molecules and a metallic substrate in the weak coupling limit. We present CDFT results for the IP and EA of a benzene molecule as function of molecule-surface separation, and find good agreement with GW calculations. Within the CDFT approach we also evaluate the image plane height as function of separation.   

Vibrationally Induced Decoherence in Single-Molecule Junctions: The Role of Electron-Hole Pair Creation Processes

We investigate quantum interference effects and vibrationally induced decoherence in single-molecule junctions, employing nonequilibrium Green's function theory [1]. Molecular junctions often exhibit quasidegenerate electronic states that allow an electron to tunnel through the junction in different ways [2,3]. The respective outgoing wavefunctions interfere constructively or destructively, leading to an increase or decrease of the tunnel current, respectively. Interaction of the tunneling electrons with the vibrational degrees of freedom of the junction, however, gives 'which-path' information about the corresponding tunneling pathways because of the state-specific nature of electronic-vibrational coupling [2,3,4]. We demonstrate how this interplay between interference and vibrationally induced decoherence results in a strong temperature dependence of the current and highlight the role of electron-hole pair creation processes in this context [3,4]. To this end, we employ both generic models of single-molecule junctions as well as realistic models that are based on first-principles electronic structure calculations. [1] Phys. Rev. Lett. 102, 146801 (2009), [2] Phys. Rev. Lett. 107, 046802 (2011), [3] Phys. Rev. Lett. 109, 056801 (2012), [4] arXiv:1209.5619 (2012).   

Charge Transport in Azobenzene-Based Single-Molecule Junctions

The azobenzene class of molecules has become an archetype of molecular photoswitch research, due to their simple structure and the significant difference of the electronic system between their \textit{cis} and \textit{trans} isomers. However, a detailed understanding of the charge transport for the two isomers, when embedded in a junction with electrodes is still lacking. In order to clarify this issue, we investigate charge transport properties through single Azobenzene-ThioMethyl (AzoTM) molecules in a mechanically controlled break junction (MCBJ) system at 4.2 K. Single-molecule conductance, \textit{I}-\textit{V} characteristics, and IETS spectra of molecular junctions are measured and compared with first-principles transport calculations. Our studies elucidate the origin of a slightly higher conductance of junctions with \textit{cis} isomer and demonstrate that IETS spectra of \textit{cis} and \textit{trans} forms show distinct vibrational fingerprints that can be used for identifying the isomer.[1] \\ 1. Y. Kim, A. Garcia-Lekue, D. Sysoiev, T. Frederiksen, U. Groth, E. Scheer, Phys. Rev. Lett. (accepted).   

High bias shot noises measurement and electronic heating in STM style gold junctions at room temperature

Shot noise is a powerful tool in transport measurements, which encodes individual transmission channel's behavior; thus shot noise provides more information than solely conductance measurements. Using a STM-style gold break junction method, we can measure shot noise and conductance simultaneously at room temperature to study its bias-dependence and the distribution of noise and so on. Quantum suppression of shot noise remains very robust even at room temperature. The standard Landauer-Buttiker treatment of shot noise in nanoscale junctions at finite temperature assumes that the electronic temperature in the source and drain electrodes is unaffected by the applied bias. That is, the applied bias is assumed to shift the relative chemical potentials of the electrodes without broadening the electronic distributions. We perform noise measurements at biases as high as 0.5 V (an energy scale much larger than room temperature) and analyze the noise to determine if its bias dependence shows evidence of electronic heating. We will discuss the evolution of shot noise with bias voltage in detail and the role of electronic heating in this experiment.