Session B55: Electron Transport in Nanoscale Devices and Materials

Live

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 stacking-dependent adhesion. First of all, we develop DFT-parametrized 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 P-oriented homobilayers would manifest itself in local tunnelling characteristics of marginally twisted bilayers: these domains feature the layer asymmetry of band-edge 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 piezo-electric 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.   

Live

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 steady-state thermodynamic description for nonlinear thermoelectric transport. We also develop a scattering description, possibly with some strong correlation on the dot.

[1] NT: Quantum control of nonlinear thermoelectricity at the nanoscale, arXiv:1912.11562 [PRB 101, 115404 (2020)]
[2] NT: Quantum thermodynamics of nanoscale steady states far from equilibrium, arXiv:1710.07385 [PRB 97, 115404 (2018)].   

Live

We present a novel method to calculate the exact Taylor series of the scattering matrix in general multiterminal tight-binding 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 complex-conjugated pairs of simple poles that are responsible for transmission peaks in the real-domain evaluations. This leads us to formulate a method to find the closed-form expression for the transmission function in general systems [3].

[1] C. Ramírez, M.J. Rodríguez and B.D. Gomez, Physica E 124, 114213 (2020).
[2] C. Ramírez, Ann. Phys. (Berlin) 530, 1700170 (2018).
[3] M.J. Rodríguez and B.D. Gomez, C. Ramírez, arXiv:2008.01923.   

Live

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 metal-oxide semiconductor (MOS) technologies. To address this, we have developed a MOS compatible counter-doping 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.   

Live

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 edge-induced perturbances, while allowing gate-defined 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 curvature-induced orbital magnetic moment and the mini valleys and band inversions of its non-parabolic low-energy dispersion, translate into a BLG quantum wire's transport properties and a quantum dot's single- and two-electron 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.   

Live

InAs/AlAs_1-xSb_x multi-quantum wells (MQW) are a promising way to understand solar cells design through engineering of long excited-state lifetime and inhibition of phonon interactions. A type-II MQW structure is investigated using time-resolved terahertz spectroscopy as a function of lattice temperature to determine the dynamics of hot carriers. For hot-carrier densities above the Mott density and at low-to-intermediate temperatures, metastability is observed during early times of the transient absorption signal, which exhibits a plateau in the excitation-photon-energy dependence. Meanwhile, AC photoconductivity spectra, analyzed using the Drude-Smith model, shows that the mobility of hot-carriers 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 L-valley 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 type-II InAs/AlAsSb MQW structures will reveal potential pathways for the development of efficient hot-carrier-based solar cells.   

Live

First-principles predictions of electronic transport properties of n-doped 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 n-doped Si/Ge SLs with EDRT considering both electron-phonon and electron-ionized 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 strain-related modulations in transport properties are exhibited by CRTA-BTE 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.   

Live

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.^1-2 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.

In this work, we compute temperature-resolved viscosity tensors for a broad range of materials from first principles and identify diagnostic tools for hydrodynamic behavior from microscopic scattering lifetimes. We then investigate the effect of nanoscale geometries on these viscous electron fluids, by comparing the results of the electronic Navier stokes equation with ab-initio viscosity tensors,³ to an independent calculation of the spatially-resolved Boltzmann transport equation using the same ab-initio scattering matrix.<sup>4

1</sup> Sulpizio, J. A. et al. Nature 576, 75–79 (2019).
² Vool, U., Hamo, A., Varnavides, G., Yaxian, W. et al arXiv:2009.04477 (2020)
³ Varnavides, G., Jermyn, A.S., Anikeeva, P. et al. Nat Commun 11, 4710 (2020)
⁴ Varnavides G., Jermyn A.S., Anikeeva P. et al. Phys. Rev. B 100 115402 (2019)   

Live

One of the promises of graphene as a replacement in nanoelectronics relies on the unusually high drift velocities that can be achieved in graphene-based 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 electron-phonon 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.   

Live

It has been known for some time that a perfect (delta-function) energy filter allows, in principle, thermal-to-electric energy conversion near ideal (Carnot) efficiency. [1,2] I will introduce this concept and report on a recent experiment where we realized a near-ideal quantum-dot heat engine in devices based on single nanowires, realizing power production at maximum power with Curzon-Ahlborn efficiency, and reaching more than 70% of Carnot efficiency at maximum efficiency settings [3].
This proof-of-performance of efficient energy harvesting from electrons is directly relevant to the concept of hot-carrier solar cells or thermophotovoltaics, where the aim is to boost energy conversion efficiency by harvesting heat from non-equilibrium electrons [4]. I will present our progress towards implementing this principle in heterostructure nanowires. [4]

References
[1] Mahan, G. D., & Sofo, J. O. (1996). The best thermoelectric. Proceedings of the National Academy of Sciences of the United States of America, 93(15), 7436–7439.
[2] Humphrey, T. E., Newbury, R., Taylor, R. P., & Linke, H. (2002). Reversible Quantum Brownian Heat Engines for Electrons. Physical Review Letters, 89(11), 116801.
[3] Martin Josefsson, Artis Svilans, Adam M. Burke, Eric A. Hoffmann, Sofia Fahlvik, Claes Thelander, Martin Leijnse, Heiner Linke: A quantum-dot heat engine operated close to thermodynamic efficiency limits. Nature Nanotechnology (2018)
[4] S. Limpert, A. Burke, I-Ju Chen, N. Anttu, S. Lehmann, S. Fahlvik S. Bremner, G. Conibeer, C. Thelander, M.E, Pistol and H. Linke: Single-nanowire, low-bandgap hot carrier solar cells with tunable open-circuit voltage Nanotechnology 28, 43 (2017)
[5] I-Ju Chen, S. Limpert, W. Metaferia, C. Thelander, L. Samuelson, F. Capasso, A.M. Burke, and H. Linke: Hot-Carrier Extraction in Nanowire-Nanoantenna Photovoltaic Devices. Nano Lett. 20, 4064 (2020)   

Live

Photoexcitation of a metal results in hot electrons that cool on femto to picosecond time-scales because of electron-electron (e-e) and electron-phonon (e-p) interactions. In this talk, we report on the parametric dependence of the energy relaxation time on the ratio of e-e to e-p interactino strengths. We performed time-domain thermoreflectance, picosecond acoustic, and time-resolved magneto optical Kerr effect experiments to measure the hot electron energy relaxation time in various metals. We study Au (weak e-e interactions, weak e-p interactions), Al (weak e-e interactions, strong e-p interactions), Pd (strong e-e interactions, medium e-p interactions)and their alloys. We find that, for metals with strong e-e interactions like Pd, the two temperature model provides an accurate description of the dynamics. For metals with weak e-e interactions, the two-temperature model fails.