Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B55: Electron Transport in Nanoscale Devices and MaterialsFocus Live

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Sponsoring Units: DMP Chair: TzuMing Lu, Sandia National Laboratories 
Monday, March 15, 2021 11:30AM  12:06PM Live 
B55.00001: Twistronic structures and moire superlattices in homo & heterobilayers of transition metal dichalcogenides (TMDs) Invited Speaker: Vladimir Falko We apply a multiscale modelling approach to study moiré superlattice in twisted homo and heterobilayers of transition metal dichalcogenides (TMD), taking into account the interlayer hybridisation of the electronic orbital and lattice reconstruction due to stackingdependent adhesion. First of all, we develop DFTparametrized interpolation formulae for interlayer adhesion energies of TMDs with both parallel and antiparallel orientation of their unit cells and arbitrary offset of the honeycomb lattices in the adjacent layers. Then, we combine those interpolation formulae with elasticity theory and analyze the bilayer lattice relaxation into mesoscale domain structures. We find that 3R and 2H stacking domains develop for, respectively, bilayers with parallel (P) and antiparallel (AP) orientation of the monolayer unit cells, separated by a network of dislocations, for twist angles θ <θ_{P}∼2.5 and θ <θ_{AP}∼1. Such lattice reconstruction has been verified by STEM imaging. We also show that the triangular domain structures of Poriented homobilayers would manifest itself in local tunnelling characteristics of marginally twisted bilayers: these domains feature the layer asymmetry of bandedge wave functions and also the ferroeletric interlayer polarisation. For AP bilayer, we show that the deformation of the lattices around domain walls (which resemble twist dislocations oriented along the planes of in bulk 2H crystals) generate piezoelectric charges at the junctions of the honeycomb domain wall network. Finally, we establish the electronic structure of the bilayer, taking into account the ferroelectric and piezolectric charge transfers. 
Monday, March 15, 2021 12:06PM  12:18PM Live 
B55.00002: Enhanced thermoelectricity and steadystate thermodynamics of a quantum dot embedded in networked reservoirs Nobuhiko Taniguchi Thermoelectricity is substantially affected by quantum coherence. In nanostructures, one can control it, say, by changing how a dot is coupled with external leads or embedded into networked reservoirs. We theoretically demonstrate how we can improve linear and nonlinear thermoelectric performance (the efficiency and the power) by embedding a dot into a ring geometry. We find, even for the temperature much smaller than the resonant width where one cannot usually anticipate good thermoelectricity, one can achieve reasonably good thermoelectric performance by adjusting parameters. We argue how we can treat the effect of a generic network of reservoirs in the steadystate thermodynamic description for nonlinear thermoelectric transport. We also develop a scattering description, possibly with some strong correlation on the dot. 
Monday, March 15, 2021 12:18PM  12:30PM Live 
B55.00003: Taylor series of Landauer conductance: Towards closedform solutions Carlos Ramírez, Mauricio J. Rodríguez, Bryan D. Gomez We present a novel method to calculate the exact Taylor series of the scattering matrix in general multiterminal tightbinding systems to arbitrary order N, which allows us to find the Taylor expansion of Landauer conductance in mesoscopic systems [1]. The method is based on the recursive scattering matrix method, that permits us to find the scattering matrix of a system from the scattering matrices of its subsystems [2]. By extending this study to the complex domain, we find the presence of complexconjugated pairs of simple poles that are responsible for transmission peaks in the realdomain evaluations. This leads us to formulate a method to find the closedform expression for the transmission function in general systems [3]. 
Monday, March 15, 2021 12:30PM  12:42PM Live 
B55.00004: Room Temperature Operation of DonorBased Atomically Precise Devices Jeffrey Ivie, Lisa A Tracy, Juan Mendez, Suzey Gao, Evan Anderson, Scott W Schmucker, DeAnna Campbell, David Scrymgeour, Aaron Katzenmeyer, Daniel R Ward, TzuMing Lu, Shashank Misra Atomic precision (AP) electrical devices, fabricated using hydrogen depassivation lithography in a scanning tunneling microscope, offer a way to explore device physics with atomic control. However, these devices are isolated through freezing out leakage pathways at cryogenic temperatures and cannot function at room temperature (RT), making them incompatible with metaloxide semiconductor (MOS) technologies. To address this, we have developed a MOS compatible counterdoping scheme, providing significant leakage current isolation. RT electrical measurements on AP devices demonstrate electrical properties on par with devices measured at low temperatures, aligning with electrical properties extracted from RT spectroscopic ellipsometry. This demonstration of a MOS doping scheme enabling RT operation enables the integration of AP devices with MOS technology. 
Monday, March 15, 2021 12:42PM  12:54PM Live 
B55.00005: Transport in Electrostatically Confined Bilayer Graphene Nanostructures Angelika Knothe, Vladimir Falko Quantum nanostructures, e.g., quantum wires and quantum dots, are needed for applications in quantum information processing devices, such as transistors or qubits. In gapped bilayer graphene (BLG), one can confine charge carriers purely electrostatically, inducing smooth confinement potentials and thereby limiting edgeinduced perturbances, while allowing gatedefined control of the confined structure. I will report on a series of works on electrostatically confined nanostructure in gapped BLG. We demonstrated, e.g., how some of BLG's unusual properties, i.e., its states' Berry curvatureinduced orbital magnetic moment and the mini valleys and band inversions of its nonparabolic lowenergy dispersion, translate into a BLG quantum wire's transport properties and a quantum dot's single and twoelectron states. We investigated both theoretically, and in collaboration with experiments, how to tune these features of BLG nanostructures externally to make them useful in future quantum technology applications. 
Monday, March 15, 2021 12:54PM  1:06PM Live 
B55.00006: AC photoconductivity of typeII InAs/AlAs_{1x}Sb_{x} multiquantum well structure Herath Piyathilaka, Rishmali Sooriyagoda, Hamidreza Esmaielpour, Vincent R. Whiteside, Tetsuya D Mishima, Michael B Santos, Ian R Sellers, Alan D Bristow InAs/AlAs_{1x}Sb_{x} multiquantum wells (MQW) are a promising way to understand solar cells design through engineering of long excitedstate lifetime and inhibition of phonon interactions. A typeII MQW structure is investigated using timeresolved terahertz spectroscopy as a function of lattice temperature to determine the dynamics of hot carriers. For hotcarrier densities above the Mott density and at lowtointermediate temperatures, metastability is observed during early times of the transient absorption signal, which exhibits a plateau in the excitationphotonenergy dependence. Meanwhile, AC photoconductivity spectra, analyzed using the DrudeSmith model, shows that the mobility of hotcarriers remains constant above the Mott density and increases with increased carrier recombination. Under the latter excitation conditions, the mobility is consistent with higher scattering rates, attributed to a contribution of Lvalley scattering. Finally, the carrier mobility reduces with increasing lattice temperature, inhibited by a phonon bottleneck that keeping both hot carrier and phonon densities high. Therefore, understanding AC photoconductivity of typeII InAs/AlAsSb MQW structures will reveal potential pathways for the development of efficient hotcarrierbased solar cells. 
Monday, March 15, 2021 1:06PM  1:18PM Live 
B55.00007: Effect of ElectronPhonon and ElectronIonized Impurity Interactions on Electronic Transport in Si/Ge Superlattices Sanghamitra Neogi, Manoj Settipalli, Vitaly Proshchenko Firstprinciples predictions of electronic transport properties of ndoped Si/Ge superlattices (SLs) revealed significant modulations with varying strain, period, and composition, indicating improvements in their thermoelectric performance. Such modulations are predicted employing the Boltzmann transport equation (BTE) framework with constant relaxation approximation (CRTA). However, recent modeling studies for bulk Si demonstrated the importance of including energy dependent relaxation times (EDRT) to improve the prediction, compared to experimental data. We compute electronic transport coefficients of ndoped Si/Ge SLs with EDRT considering both electronphonon and electronionized impurity scattering mechanisms. We find that rigorous inclusion of impurity scattering mechanisms, considering the density functional theory obtained wavefunctions and anisotropic potentials improves predictions, especially at higher carrier concentrations. We observe that the strainrelated modulations in transport properties are exhibited by CRTABTE calculations as well, including the scattering mechanisms. Our study highlights the role of different scattering mechanisms that dictate the relaxation times of strained Si/Ge SLs. 
Monday, March 15, 2021 1:18PM  1:30PM Live 
B55.00008: Electron hydrodynamics  microscopic origins and effects of nanoscale geometries Georgios Varnavides, Adam Jermyn, Yaxian Wang, Uri Vool, Assaf Hamo, Amir Yacoby, Polina Anikeeva, Prineha Narang Electrons in condensed matter can flow collectively when their momentum is conserved during microscopic scattering processes. Recently, this hydrodynamic regime has been observed in a handful of materials classes at moderately low temperatures and small lengthscales.^{12} The reason for this narrow temperature/lengthscale regime is the competition between the ballistic (low temperatures/small lengthscales) and diffusive (high temperatures/large lengthscales) regimes. 
Monday, March 15, 2021 1:30PM  1:42PM Live 
B55.00009: " Energy dissipation in electric field driven graphene" Ishiaka Mansaray, Jiajun Li, Jong E Han One of the promises of graphene as a replacement in nanoelectronics relies on the unusually high drift velocities that can be achieved in graphenebased devices. The drift velocity can however be degraded through energy losses with the substrate the graphene device is coupled with, and this becomes even more pronounced in high electric fields. This makes it difficult to determine the intrinsic saturation velocity of graphene which will help to better understand its full potential for use in nanoelectronics. This work gives a full quantum treatment of the energy dissipation mechanism in graphene and sheds light on the nature of velocity saturation in graphene under high dc fields. We study the energy dissipation of a dc field driven graphene system coupled to fermionic and phononic Ohmic baths, including electronphonon coupling. We treat this problem within the nonequilibrium DMFT framework with the Keldysh formalism. The electron and phonon transport properties are investigated and we demonstrate that in the case of large energy dissipation to the surrounding phonon baths, the drift velocity saturates to a much lower value, whereas with minimal energy losses, the drift velocity saturates to a substantially higher value. 
Monday, March 15, 2021 1:42PM  2:18PM Live 
B55.00010: From Quantum Dot Heat Engines to HotCarrier Photovoltaics Invited Speaker: Heiner Linke It has been known for some time that a perfect (deltafunction) energy filter allows, in principle, thermaltoelectric energy conversion near ideal (Carnot) efficiency. [1,2] I will introduce this concept and report on a recent experiment where we realized a nearideal quantumdot heat engine in devices based on single nanowires, realizing power production at maximum power with CurzonAhlborn efficiency, and reaching more than 70% of Carnot efficiency at maximum efficiency settings [3]. 
Monday, March 15, 2021 2:18PM  2:30PM Live 
B55.00011: Dependence of nonequilibrium electron energy dynamics on electronelectron and electronphonon interaction strengths Richard Wilson, Xinping Shi, Sinisa Coh Photoexcitation of a metal results in hot electrons that cool on femto to picosecond timescales because of electronelectron (ee) and electronphonon (ep) interactions. In this talk, we report on the parametric dependence of the energy relaxation time on the ratio of ee to ep interactino strengths. We performed timedomain thermoreflectance, picosecond acoustic, and timeresolved magneto optical Kerr effect experiments to measure the hot electron energy relaxation time in various metals. We study Au (weak ee interactions, weak ep interactions), Al (weak ee interactions, strong ep interactions), Pd (strong ee interactions, medium ep interactions)and their alloys. We find that, for metals with strong ee interactions like Pd, the two temperature model provides an accurate description of the dynamics. For metals with weak ee interactions, the twotemperature model fails. 
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