Session A16: Transport in Nanostructures -- Thin Films, Heterostructures and Nanodevices

Pascal-liquid phases in ballistic one-dimensional LaAlO3/SrTiO3 channels

The challenge of understanding strongly interacting composite fermionic phases of matter spans many fields in physics, ranging from neutron stars to solid-state materials to quark-gluon plasmas. We report experimental evidence of a new family of degenerate quantum liquids formed from bound states of n = 2, 3, 4, … electrons, which are stabilized within quasi-one-dimensional electron waveguides formed at the LaAlO₃/SrTiO₃ interface. The key signature of this phase is the existence of quantized conduction that follows a characteristic sequence within Pascal’s triangle: (1, 3, 6, 10, 15,…), where is the electron charge and is the Planck constant. The ability to create and investigate composite fermionic phases opens new avenues for the investigation of strongly correlated quantum matter.
  

Development of single-electron sources in LaAlO3/SrTiO3 nanostructures

The 2D electron gas at the LaAlO₃/SrTiO₃ interface can be patterned using conductive atomic force microscope (c-AFM) lithography [1], which has been used to create quantum dots and single electron transistors [2]. We aim to use this technique to create an on-demand single-electron source by sketching a linear array of quantum dots and applying out-of-phase excitation across the dot array. We will discuss both double-dot and triple-dot devices and their associated electron tunneling phenomenon and issues related to the the individual tunabilities of the dots.
[1] Cen et al, Nature Mater. 7, 298 (2008).
[2] Cheng et al, Nature Nanotechnology 6, 343 (2011).   

Quantum Transport of Excitons in Nano-Devices

An indirect exciton is a bound pair of an electron and a hole in spatially separated layers. Such bosonic particles can be created optically in semiconductor double quantum wells or layered two-dimensional materials. Indirect excitons couple to external electric field via their permanent dipole moment. This allows one to engineer arbitrary potential landscapes for the excitons. We study theoretically the problem of quantum transport of indirect excitons through constrictions, including quantum-point contacts. We show that this problem provides a novel counterpart to previous investigations of electron transport in nano-devices. We calculate analytically and numerically the conductance and the real-space current density of excitons propagating through single, double, and multiple slits. Our calculations illustrate that hallmark ballistic quantum transport phenomena, such as diffraction, interference, and the Talbot effect, are experimentally observable with excitons by spatially resolved photoluminescence imaging.
  

Probing energy transport in atomic and nanoscale junctions

Understanding energy transport at the atomic and nanoscale junctions is of great interest to reveal novel transport phenomena at the fundamental limit, and is expected to be critical to developing a variety of technologies ranging from nanoelectronics, photonics, to thermoelectrics and photovoltaics. In contrast to the extensive studies of charge and electronic properties at the nanoscale, the energy transport properties of materials and devices ranging in size from the atomic and molecular scale to the realm of a few nanometers, have remained largely unexplored due to experimental challenges. In my presentation, I will describe the experimental progress we made on the development of the experimental technique for reliably accessing energy transport properties in nanojunctions down to the atomic and single molecule scale.   

Stark Control of Electrons along Nanojunctions

Ultrafast control of currents on the nanoscale is essential for future innovations in nanoelectronics. Recently it was experimentally demonstrated that strong non-resonant few-cycle 4 fs laser pulses can be used to induce phase-controllable currents along gold–silica–gold nanojunctions in the absence of a bias voltage. However, since the effect depends on a highly non-equilibrium state of matter, its microscopic origin is unclear and the subject of recent controversy. Here we present atomistically detailed (time-dependent non-equilibrium Green’s function) electronic transport simulations that recover the main experimental observations and offer a simple intuitive picture of the effect. The photoinduced currents are seen to arise due to a difference in effective silica-metal coupling for negative and positive field amplitudes induced by lasers with low temporal symmetry. These insights can be employed to interpret related experiments, and advance our ability to control electrons in matter using lasers.
L. Chen, Y. Zhang, G. Chen and I. Franco, Nature Communications 9, 2070 (2018)   

Stark Control of Electrons Across Interfaces

We introduce a laser control scenario to transiently transform an insulating heterojunction into a conducting one on a femtosecond timescale. The scenario is based on opening Landau-Zener quantum tunneling channels for electron transfer between two adjacent semiconductors via Stark shifts induced by non-resonant lasers of intermediate intensity (non-perturbative but non-ionizing). Through quantum dynamics simulations we demonstrate the robustness of the approach and its utility for controlling electron dynamics at interfaces.

A. J. Garzon-Ramirez and I. Franco, Phys. Rev. B, 98, 121305(R) (2018)   

Photothermoelectric effects at and near individual grain boundaries in gold

Thermoelectricity is best known for thermocouples, where a voltage is generated when heating the interface of two materials with different Seebeck coefficients. In metals, the electronic Seebeck coefficient depends on the electrical conductivity which can be manipulated at the nanoscale to create single metal thermocouples. We will present scanning photothermoelectric measurements of the simplest single metal thermocouple: a single grain boundary between two single-crystal gold nanowires. Unlike a traditional thermocouple, where heating the grain boundary results in the largest voltage, the photovoltage as a function of laser position changes polarity at the grain boundary, varying on length scales much larger than the laser size. Modeling suggests that these results are consistent with long-scale Seebeck coefficient gradients within the crystals. Electron back-scatter diffraction relates the voltages to the relative crystallographic orientation across the boundary and x-ray probes provide insight of strain within the device. We propose how thermovoltages can probe areas of impurities, strain, and other intrinsic irregularities that may not otherwise be detected using traditional electronic transport measurements.   

Hierarchical Kinetics in 1/f Noise in Amorphous and Nanocrystalline Semiconductor Thin Films

We report studies of conductance fluctuations in hydrogenated amorphous germanium (a-Ge:H) that have an inverse frequency (1/f) spectral density with non-Gaussian statistics, as reflected in (1) histograms of the noise power per octave that are not described by Gaussian distributions, (2) strong correlations of the noise power in frequency-space and (3) power-law second spectra. In particular, histograms of the 1/f noise power per octave for a-Ge:H are well described by a log-normal distribution. The correlation coefficients across frequencies are non-zero and larger than expected for independently modulated fluctuators, and grow with averaging time with a logarithmic time-dependence. In contrast, the 1/f noise for polycrystalline Ge, and free-standing nanocrystalline thin films display Gaussian statistics. These results are discussed in terms of a model of filamentary conduction, where the conductance is modulated by hydrogen motion governed by hierarchical kinetics.   

Quantum dynamics of single-photon detection using functionalized quantum transport electronic channels

Single photon detectors have historically consisted of macroscopic-sized materials, but recent experimental and theoretical progress opens new approaches based on nanoscale and molecular electronics. Here we present a theoretical study of photodetection in a system composed of a quantum electronic transport channel functionalized by a photon absorber. Notably, the photon field, absorption process, transduction mechanism, and measurement process are all treated as part of one fully-coupled quantum system, with explicit interactions. Using non-equilibrium, time-dependent quantum transport simulations, we reveal the unique temporal signatures of the single photon detection process, and show that the system can be described using optical Bloch equations, with a new non-linearity as a consequence of time-dependent detuning caused by the back-action from the transport channel. We compute the photodetector signal-to-noise ratio and demonstrate that single photon detection is possible for realistic parameters.   

Cross-plane thermal conductance of Au/graphene/Au heterojunction

Thermal management has become a critical issue for microelectronics, as the characteristic sizes of these devices shrink into the nanometer region. Interesting phenomena were observed for heat conduction through ultrathin interfacial layers, such as thickness dependent thermal boundary conductivity. Here, we use the Frequency Domain Thermoreflectance technique to investigate the heat conduction across the heterojunction composed of few-layer graphene as the interfacial layer between gold. By varying the graphene layer number to control the thickness of the interfacial layer, we measure the thickness dependent thermal conductance across this sandwiched structure. We found that electron transmission dominates the thermal conductance for monolayer graphene, and a two-orders of magnitude decrease in thermal conductance through bilayer graphene. For more layers graphene, phonon thermal conductivity is suggested to be the major contributor.
  

Quantum oscillations under photoexcitation involve direct and indirect excitons and quantum photocapacitance in GaAs/AlAs/InAs p-i-n diode

We explore the quantum dynamics at the interface of a single barrier GaAs/AlAs p-i-n device having InAs quantum dots embedded inside AlAs layer. We observe systematic quantum oscillations in photocapacitance measurements when the sample is optically excited at 10 K. Two sharp peaks in the photocapacitance spectra imply the presence of direct and indirect excitons formed inside the InAs quantum dot and at the interface of quantum dot and the triangular GaAs quantum well, respectively. Spectral peak shifts with increasing applied bias complies with the change of energy levels of triangular quantum well due to effective electric field. Presence of two-dimensional electron and hole gases near the GaAs/AlAs interface certainly point towards the involvement of quantum capacitance. In addition, we also discuss how the formation of excitonic dipoles affects such quantum capacitance. Moreover, periodic variation of negative differential resistance in photocurrent oscillations also correlate with photoconductance oscillations. Understanding the quantum dynamics of carriers involving quantum capacitance and negative differential resistance will help us to explain the many-body physics of these interfacial excitons and its applications.   

non-classical noise and light emission of an ac-driven tunnel junction

The non-symmetrized noise is crucial for the analysis of light emission in nanojunctions. The latter represent non-classical photon emitters whose description requires a full quantum approach [1]. It was found experimentally that light emission can occur with a photon energy exceeding the applied dc voltage [2], which intuitively should be forbidden due to Pauli principle. This overbias light emission cannot be described by the single-electron physics, but can be explained by 2-electron or even 3-electron process, correlated by a local antenna mode in analogy to the well-known dynamical Coulomb blockade (DCB) [3]. Here, we obtain the non-symmetrized noise for junctions driven by an arbitrarily shaped periodic voltage. We find that when the junction is driven, overbias light emission occurs and exhibits intriguing different features compared to the dc case. For example, multiple peaks appear at integers of the ac driving frequency. Our work generalizes the DCB theory to light emission in driven tunnel junctions and opens the avenue to engineered quantum light sources, which can be tuned purely by applied voltages.

References:
1. M. H. Devoret et al. PRL 1990
2. G. Schull et al. PRL 2009
3. F. Xu et al. PRL 2014, PRB 2016; P.-J. Peters et al. PRL 2018   

Long-Range Frictional Drag in Coupled LaAlO3/SrTiO3 Nanowires

Frictional drag, where current in one nanowire induces a voltage across a nearby nanowire, is a powerful tool to study electron interactions. Here we investigate long-range electron interactions in coupled nanowires at the LaAlO₃/SrTiO₃ interface via frictional drag. In the normal state (B > 0.3T) regime, the antisymmetric drag resistance of double-wire devices is independent of their separation, ruling out the Coulomb interaction as the dominant coupling mechanism. In triple-wire devices this separation independence is corroborated. In the superconducting (B < 0.3T, T < 300mK) regime, a symmetric component is identified in the drag resistance and its separation independence also shows the coupling is predominantly non-Coulombic. These results provide strong evidence for a new long-range non-Coulombic electron interaction that must be accounted for in description of electron transport at oxide interfaces.