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Bilayer graphene (BLG) has multiple internal degrees of freedom and a constant density of states down to the charge neutrality point when trigonal warping is ignored. Consequently, it is susceptible to various competing ground states. However, a coherent experimental determination of the ground state has been challenging due to the interaction-disorder interplay. Here we present an extensive transport study in a series of dually gated freestanding BLG devices and identify the layer-antiferromagnet as the ground state with a continuous strength across all devices. This strength correlates with the width of the state in the electric field. We systematically identify electric-field disorder─spatial variations in the interlayer potential difference─as the main source responsible for the observations. Our results pinpoint for the first time the importance of electric-field disorder on spontaneous symmetry breaking in BLG and solve a long-standing debate on its ground state. The electric-field disorder should be universal to all 2D materials.

Electrolyte-gated organic transistors (EGOTs) are promising candidates as a new class of neuromorphic devices in hardware-based artificial neural networks that can outperform their complementary metal oxide semiconductor (CMOS) counterparts regarding processing speed and energy consumption. Several ways in which to implement such networks exist, two prominent methods of which can be implemented by nanoscopic vertical EGOTs, as we show here. First, nanoscopic vertical electrolyte-gated transistors with a donor-acceptor diketopyrrolopyrrole-terthiophene polymer as an active material can be used to reversibly switch the channel conductivity over five orders of magnitude (3.8 nS to 392 μS) and perform switching at low operation voltages down to -1 mV. Second, nanoscopic EGOTs can also mimic fundamental synaptic functions, and we show an interconnection of up to three transistors, highlighting the possibility to emulate biological nerve cells.

Even though charge transport in semiconducting polymers is of relevance for a number of potential applications in (opto-)electronic devices, the fundamental mechanism of how charges are transported through organic polymers that are typically characterized by a complex nanostructure is still open. One of the challenges which we address here, is how to gain controllable experimental access to charge transport at the sub-100 nm lengthscale. To this end charge transport in single poly(diketopyrrolopyrrole-terthiophene) fiber transistors, employing two different solid gate dielectrics, a hybrid Al2O3/self-assembled monolayer and hexagonal boron nitride, is investigated in the sub-50 nm regime using electron-beam contact patterning. The electrical characteristics exhibit near ideal behavior at room temperature which demonstrates the general feasibility of the nanoscale contacting approach, even though the channels are only a few nanometers in width. At low temperatures, we observe nonlinear behavior in the current–voltage characteristics in the form of Coulomb diamonds which can be explained by the formation of an array of multiple quantum dots at cryogenic temperatures.

An established way of realising topologically protected states in a two-dimensional electron gas is by applying a perpendicular magnetic field thus creating quantum Hall edge channels. In electrostatically gapped bilayer graphene intriguingly, even in the absence of a magnetic field, topologically protected electronic states can emerge at naturally occurring stacking domain walls. While individually both types of topologically protected states have been investigated, their intriguing interplay remains poorly understood. Here, we focus on the interplay between topological domain wall states and quantum Hall edge transport within the eight-fold degenerate zeroth Landau level of high-quality suspended bilayer graphene. We find that the two-terminal conductance remains approximately constant for low magnetic fields throughout the distinct quantum Hall states since the conduction channels are traded between domain wall and device edges. For high magnetic fields, however, we observe evidence of transport suppression at the domain wall, which can be attributed to the emergence of spectral minigaps. This indicates that stacking domain walls potentially do not correspond to a topological domain wall in the order parameter.

Until now, organic semiconductors have failed to achieve high performance in highly integrated, sub-100 nm transistors. Consequently, single-crystalline materials such as single-walled carbon nanotubes, MoS2 or inorganic semiconductors are the materials of choice at the nanoscale. Here we show, using a vertical field-effect transistor design with a channel length of only 40 nm and a footprint of 2 × 80 × 80 nm2, that high electrical performance with organic polymers can be realized when using electrolyte gating. Our organic transistors combine high on-state current densities of above 3 MA cm−2, on/off current modulation ratios of up to 108 and large transconductances of up to 5,000 S m−1. Given the high on-state currents at such large on/off ratios, our novel structures also show promise for use in artificial neural networks, where they could operate as memristive devices with sub-100 fJ energy usage. A vertical, electrolyte-gated organic transistor shows high on-state current densities, large on/off ratio and the potential for use in artificial neural networks.

Miniaturization of electronic circuits increases their overall performance. So far, electronics based on organic semiconductors has not played an important role in the miniaturization race. Here, we show the fabrication of liquid electrolyte gated vertical organic field effect transistors with channel lengths down to 2.4 nm. These ultrashort channel lengths are enabled by using insulating hexagonal boron nitride with atomically precise thickness and flatness as a spacer separating the vertically aligned source and drain electrodes. The transistors reveal promising electrical characteristics with output current densities of up to 2.95 MA cm-2 at -0.4 V bias, on-off ratios of up to 106, a steep subthreshold swing of down to 65 mV dec-1 and a transconductance of up to 714 S m-1. Realizing channel lengths in the sub-5 nm regime and operation voltages down to 100 μV proves the potential of organic semiconductors for future highly integrated or low power electronics.

The quantum anomalous Hall (QAH) effect-a macroscopic manifestation of chiral band topology at zero magnetic field-has been experimentally realized only by the magnetic doping of topological insulators

In organic electronics, local crystalline order is of critical importance for the charge transport. Grain boundaries between molecularly ordered domains are generally known to hamper or completely suppress charge transfer and detailed knowledge of the local electronic nature is critical for future minimization of such malicious defects. However, grain boundaries are typically hidden within the bulk film and consequently escape observation or investigation. Here, a minimal model system in form of monolayer-thin films with sub-nm roughness of a prototypical n-type organic semiconductor is presented. Since these films consist of large crystalline areas, the detailed energy landscape at single grain boundaries can be studied using Kelvin probe force microscopy. By controlling the charge-carrier density in the films electrostatically, the impact of the grain boundaries on charge transport in organic devices is modeled. First, two distinct types of grain boundaries are identified, namely energetic barriers and valleys, which can coexist within the same thin film. Their absolute height is found to be especially pronounced at charge-carrier densities below 10

The quantum anomalous Hall (QAH) effect—a macroscopic manifestation of chiral band topology at zero magnetic field—has been experimentally realized only by the magnetic doping of topological insulators1–3 and the delicate design of moire heterostructures4–8. However, the seemingly simple bilayer graphene without magnetic doping or moire engineering has long been predicted to host competing ordered states with QAH effects9–11. Here we explore states in bilayer graphene with a conductance of 2 e2 h−1 (where e is the electronic charge and h is Planck’s constant) that not only survive down to anomalously small magnetic fields and up to temperatures of five kelvin but also exhibit magnetic hysteresis. Together, the experimental signatures provide compelling evidence for orbital-magnetism-driven QAH behaviour that is tunable via electric and magnetic fields as well as carrier sign. The observed octet of QAH phases is distinct from previous observations owing to its peculiar ferrimagnetic and ferrielectric order that is characterized by quantized anomalous charge, spin, valley and spin–valley Hall behaviour9. Bilayer graphene states are observed at anomalously small magnetic fields and show magnetic hysteresis, providing evidence for a quantum anomalous Hall effect driven by orbital magnetism.

In crystalline small molecule organic semiconductors, the interplay between the charge transport mechanism and the crystal and molecular structure is nowadays comparably well understood due to the clearly defined morphology. Charge transport in polymeric semiconductors on the other hand is rather complex, for example, due to the substantial amount of conformational freedom of the polymer chains. In macroscopic devices, charge transport is characterized by alternating ordered and disordered phases with varying interconnections and structural defects, which implies that the influence of molecular weight and side-chains, polymer fiber alignment, and backbone rigidity has to be considered, since different transport mechanisms at various length scales from single chains to the macroscale can overlap. To fully understand transport in these systems, ideally, each length scale would be addressed individually before different processes can be joined in a macroscopic picture. In this Perspective, we focus on charge transport properties of polymeric semiconductors at the shortest possible length scales and discuss approaches that aim to make the short length scales still accessible for charge transport experiments.

All-inorganic halide perovskite materials have recently emerged as outstanding materials for optoelectronic applications. However, although critical for developing novel technologies, the influence of charge traps on charge transport in all-inorganic systems still remains elusive. Here, the charge transport properties in cesium lead bromide, nanowire films are probed using a field-effect transistor geometry. Field-effect mobilities of μFET = 4 × 10−3 cm−2 V−1 s−1 and photoresponsivities in the range of R = 25 A W−1 are demonstrated. Furthermore, charge transport both with and without illumination is investigated down to cryogenic temperatures. Without illumination, deep traps dominate transport and the mobility freezes out at low temperatures. Despite the presence of deep traps, when illuminating the sample, the field-effect mobility increases by several orders of magnitude and even phonon-limited transport characteristics are visible. This can be seen as an extension to the notion of “defect tolerance” of perovskite materials that has solely been associated with shallow traps. These findings provide further insight in understanding charge transport in perovskite materials and underlines that managing deep traps can open up a route to optimizing optoelectronic devices such as solar cells or phototransistors operable also at low light intensities Deutsche Forschungsgemeinschaft | Ref. EXC‐2111‐390814868 Deutsche Forschungsgemeinschaft | Ref. EXC 2089/1‐390776260 Bayerisches Staatsministerium für Bildung und Kultus, Wissenschaft und Kunst | Ref. Solar Technologies go Hybrid

Ionic liquids enable efficient gating of materials with nanoscale morphology due to the formation of a nanoscale double layer that can also follow strongly vaulted surfaces. On carbon nanotubes, this can lead to the formation of a cylindrical gate layer, allowing an ideal control of the drain current even at small gate voltages. In this work, we apply ionic liquid gating to chirality-sorted (9, 8) carbon nanotubes bridging metallic electrodes with gap sizes of 20 nm and 10 nm. The single-tube devices exhibit diameter-normalized current densities of up to 2.57 mA/μm, on-off ratios up to 104, and a subthreshold swing down to 100 mV/dec. Measurements after long vacuum storage indicate that the hysteresis of ionic liquid gated devices depends not only on the gate voltage sweep rate and the polarization dynamics but also on charge traps in the vicinity of the carbon nanotube, which, in turn, might act as trap states for the ionic liquid ions. The ambipolar transfer characteristics are compared with calculations based on the Landauer–Buttiker formalism. Qualitative agreement is demonstrated, and the possible reasons for quantitative deviations and possible improvements to the model are discussed. Besides being of fundamental interest, the results have potential relevance for biosensing applications employing high-density device arrays.

The crystal structure of solid-state matter greatly affects its electronic properties. For example, in multilayer graphene, precise knowledge of the lateral layer arrangement is crucial, since the most stable configurations, Bernal and rhombohedral stacking, exhibit very different electronic properties. Nevertheless, both stacking orders can coexist within one flake, separated by a strain soliton that can host topologically protected states. Clearly, accessing the transport properties of the two stackings and the soliton is of high interest. However, the stacking orders can transform into one another, and therefore, the seemingly trivial question of how reliable electrical contact can be made to either stacking order can a priori not be answered easily. Here, we show that manufacturing metal contacts to multilayer graphene can move solitons by several μm, unidirectionally enlarging Bernal domains due to arising mechanical strain. Furthermore, we also find that during dry transfer of multilayer graphene onto hexagonal boron nitride, such a transformation can happen. Using density functional theory modeling, we corroborate that anisotropic deformations of the multilayer graphene lattice decrease the rhombohedral stacking stability. Finally, we have devised systematics to avoid soliton movement, and how to reliably realize contacts to both stacking configurations, which will aid to reliably access charge transport in both stacking configurations., F.R.G., F.W., D.P., J.L., and R.T.W. acknowledge funding from the excellence initiative Nanosystems Initiative Munich (NIM), the Center for Nanoscience (CeNS) and the Solar Technologies go Hybrid (SolTech) initiative. We additionally acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy − EXC-2111−390814868 (MCQST) and EXC 2089/1−390776260.“ (e-conversion). We also thank Leonid S. Levitov, Nicola Mazzar,i and Nicolas Mounet for discussions and Jochen Feldmann for using his scanning Raman setup. R.G.-A., M.P., and A.A. thank the Project FIS2016-76617-P of the Spanish Ministry of Economy and Competitiveness MINECO, the Basque Government under the ELKARTEK project (SUPER), and the University of the Basque Country (Grant No. IT-756-13) for partial funding of this work. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, A3 Foresight by JSPS and the CREST (JPMJCR15F3), JST.

Until now, organic semiconductors have failed to achieve high performance in highly integrated, sub-100 nm transistors. Consequently, single-crystalline materials such as single-walled carbon nanotubes, MoS

Determining the electronic properties of nanoscopic, low-dimensional materials free of external influences is key to the discovery and understanding of new physical phenomena. An example is the suspension of graphene, which has allowed access to their intrinsic charge transport properties. Furthermore, suspending thin films enables their application as membranes, sensors, or resonators, as has been explored extensively. While the suspension of covalently bound, electronically active thin films is well established, semiconducting thin films composed of functional molecules only held together by van der Waals interactions could only be studied supported by a substrate. In the present work, it is shown that by utilizing a surface-crystallization method, electron conductive films with thicknesses of down to 6 nm and planar chiral optical activity can be freely suspended across several hundreds of nanometers. The suspended membranes exhibit a Young's modulus of 2-13 GPa and are electronically decoupled from the environment, as established by temperature-dependent field-effect transistor measurements.

Bounding flight is a flight mode consisting of repeated cycles with alternating phases involving flapping and non-flapping. The bounding flight mode is treated as a periodic optimal control problem using an efficient optimization procedure to construct solutions. It is shown that bounding flight yields performance enhancements in the energy expenditure required for flying. In zero wind, bounding flight is superior to steady-state cruise wind at higher speeds. When flying against a headwind, the superiority of bounding flight is enlarged. It is shown that bounding flight requires less energy per range for a greater speed range. Particularly, the minimum energy per range of bounding flight is smaller than that possible with continuous flapping flight.

A simulation model for aerobatic aircraft is derived that accounts for a proper inclusion of static, convective and time-dependent wind fields. Two different depths of modeling are regarded, namely a point-mass simulation model and a full, nonlinear 6-DoF simulation model. Both simulation models make use of the aerodynamic flight path angles as translational states to describe the respective aircraft trajectories since the aerodynamic quantities physically determine the motion of the aircraft. The kinematic quantities are then a function of the aerodynamic quantities and the wind influence. The 6-DoF simulation model is based on a sequential structure, where the aircraft’s attitude and rotational dynamics are given with respect to its aerodynamic trajectory. Thus, the aerodynamic angle of attack and the aerodynamic sideslip angle are used to describe the attitude of the aircraft with respect to the Aerodynamic Frame instead of Euler Angles that would describe the aircraft’s attitude with respect to the North-East-Down Frame. In order to avoid the singularity that occurs for aerodynamic flight-path inclination angles of , quaternions are utilized to replace the aerodynamic flight-path course angle, the aerodynamic flight-path inclination angle and the aerodynamic bank angle instead of replacing the Euler Angles or the kinematic flight-path angles as it is commonly the case.

The maximum-range flight of a motor glider equipped with a retractable jet engine is treated as a periodic optimal control problem. The maximum-range flight is of periodic nature because it consists of cycles where a climbing phase with the engine extended is followed by a gliding phase with the engine retracted. This kind of flight is termed saw-tooth mode. It is shown that the maximum range of optimal saw-tooth flight is larger than the greatest range achievable with the best steady-state cruise. An efficient optimization method is applied for solving the periodic optimal control problem, using a realistic mathematical modeling.

The maximum-range flight of a powered sailplane with a retractable electric motor is treated as a periodic optimal control problem. The periodic nature of the maximum-range flight, known as saw-tooth mode, is due to the high drag when the motor is extended and the low drag when the motor is retracted. An optimization treatment based on energy considerations is performed to develop analytical solutions and to deepen the insight into the physical mechanism underlying the superiority of the saw-tooth mode. This is complemented by a treatment using a modeling based on point mass dynamics and an efficient optimization method to construct solutions for maximum-range saw-tooth flight. As a main result concerning the range performance, it is shown that the maximum range achievable with optimal saw-tooth flight is considerably larger than the greatest range possible with the best steady-state cruise.

Flap-bounding flight which is a flight mode consisting of continually repeated cycles with flapping and non-flapping phases is treated as a periodic optimal control problem. Two modelings of flap-bounding flight are developed. One modeling is based on energy considerations. The goal is to derive analytical solutions using an appropriate non-dimensionalization. The other model accounts for the unsteady behavior of the state and control variables. Furthermore, an efficient optimization procedure is used to construct solutions. It is shown where flap-bounding flight is superior in the required energy per range when compared to the best continuous flapping flight.

Periodic optimal flight is a means contributing to achieve an unlimited endurance performance of solar-powered UAVs. The optimization goal of the treatment presented in this paper is to attain the capability to stay aloft with a minimum or even no solar energy to be stored in batteries for the flight during the night. It is shown that this is possible with an appropriate trajectory control. The periodicity of the optimized flight implies that the trajectory can be continued after a complete day-night cycle in the same manner as before. Realistic mathematical models of the UAV dynamics and the energy management system for an optimal solar power use are applied. Employing an efficient optimization method, results on periodic optimal UAV flight yielding the performance goal aimed at are presented. With a minimum or zero electric energy to be stored in batteries, the related weight penalty can be reduced or even avoided.

The range of a powered sailplane employing a retractable propeller-engine combination is maximized. The maximum range trajectory is of periodic nature because there are alternating flight segments in terms of the best operating condition of the powered phase as opposed to that with zero thrust where the propeller-engine combination is retracted. It is shown that a significant enlargement of the maximum range can be achieved when compared with the best steady-state cruise. Solutions of the periodic optimal flight problem are constructed, using an efficient optimization method and a realistic mathematical model for describing the motion of the vehicle as well as the procedure for extending and retracting the propeller-engine combination.

The paper describes the development and implementation of a MATLAB based object oriented computational tool that allows numerical t rimming of complex high fidelity simulation models implemented in SIMULINK. Equilibrium points for the simulation model are determined numerically by a nonlinear equation solver. The nonlinear trim equations to be solved are defined by so called trim templates, which are built around the simulation model, enforcing the desired flight condition by su perimposing numerical constraints to the model. Trim templates are implemented as objects on various levels that can be accessed by several methods. To trim complex simulation models with rigid body dynamics and a variable number of different subsystems, child trim template objects are specified that inherit from the parent trim template class. They a ddress rigid body dynamics and different subsystems and are combined in the model trim template assembling the trim condition for the complete simulation model. Objects of the class model trim template handle the correlation between the different subsystems and co ntrol possible trim procedures like integrated or nested trim. The object oriented impl ementation and the trim algorithm are applied to a complex simulation model featuring a high system order as many subsystem states (e.g. those of the propulsion system, actuat ors or sensors) augment the original rigid body states. The aircraft rigid body trim template allows to specify different flight maneuvers, e.g. steady-state straight horizontal fl ight, climbing and turning flight with the implementation of several constraints or more complex cases like steady heading sideslips as well as failure cases or quasi-steady maneuvers, e. g. the determination of the maximum achievable steady-state roll rate or acceleration.

The paper describes a novel approach for the optimization of air race trajectories taking into account the highly non-linear nature of the dynamics of the participating aircraft. Therefore, no point-mass model is utilized for the optimization task but an enhanced, scalable multi-fidelity simulation model that is a sequential model extending the translation dynamics by different representations for the attitude and the rotational dynamics of the flight system. The inner loop can either contain linear transfer functions for the load factors and the roll rate, linear state-space models for the longitudinal and the lateral motion of the aircraft or fully non-linear rotational and attitude dynamics. Thus, the full dynamic order of the flight system considered is taken into account such that the resulting optimal race trajectory is actually achievable. Inversion controllers for the different loops are incorporated into the simulation model. With this sequential structure of the model, the complexity level of the rotational dynamics and thus the optimization time and quality can easily be switched to the required level. Furthermore, a procedure for generating robust and suitable initial guesses for the optimization with full, non-linear 6-degree of freedom simulation models is established. This novel approach allows for the solution of highly complex trajectory optimization problems where classical methods failed due to stiffness problems.

The problem of computing optimal approach or landing trajectories for civil passenger aircraft taking into account aircraft configuration changes is considered. For the solution of the approach trajectory optimization problems, the different configuration settings are regarded as controls that can only take discrete values (e.g. flaps or gear extended or retracted) so that mixed-integer trajectory optimization problems result. At this, the necessity to introduce auxiliary phases due to the configuration changes is avoided. Moreover, one does not a priori have to guess the structure of the optimal solution (this means the optimal sequence of the configuration changes) so that the true optimal sequence of configuration changes is to be found by the optimization algorithm without being limited by a pre-defined sequence of phases. Two transformation methods are presented that transform the resulting mixed-integer trajectory optimization problems into standard trajectory optimization problems featuring solely continuous controls, namely the variable time transformation method and a transformation method utilizing the tangens hyperbolicus function. Both transformation methods are applied to a benchmark approach trajectory optimization problem. Furthermore, the variable time transformation method is enhanced in a novel way that allows for taking into account constraints on the sequence in which the discrete controls are to be operated.

The paper describes the development and implementation of a MATLAB tool for executing performance calculations on the basis of complex high-fidelity simulation models featuring a high level of automation and a high computation speed. The tool is utilized for a large range of applications, including performance chart generation, automated checking of certification cri teria, parameter sensitivity studies or flight perf ormance optimizations. The basic concept is that the desire d system condition is expressed by a set of nonline ar constraints imposed on the nonlinear SIMULINK simulation model. The approach also accounts for differential algebraic systems, i.e. the case when the nonlinear simulation model is only available as implicit model and not explicitly solved for the state derivatives. The se t of nonlinear constraints is programmed in a templ ate with an embedded call to the simulation model. As the size of the nonlinear equation system could significantl y increase for complex simulation models when compared to a system only consisting of the rigid-body aircraft, it can be chosen between an integral, a nested or a staged ap proach to optimize the computational performance of the system based on the actual application case. The system also features a high numeric efficiency due to a large number of algorithmic optimizations, like a custom-tailored nonlinear equation solver or a high-level trim strategy for multi-point and grid trimming.

The possibility of an unlimited flight endurance of solar aircraft is treated with the objective to maximize their performance. This is a periodic optimal control problem because the trajectory is continued after a one-day flight in the same manner as before. Focus of the performance goal is the minimization of the required battery capacity so that the related weight penalty can be kept as small as possible. Realistic mathematical models of the dynamics of the aircraft and the energy management system for an optimal utilization of the solar energy are applied. With the use of an efficient optimization method, results on periodic optimal solar aircraft trajectories yielding the performance enhancement aimed at are presented.

Results from in-flight measurements of dynamic soaring of albatrosses over land are presented. Using a new computational method based on L1 phase measurement as well as miniaturized and low cost GPS logging devices featuring a sufficiently high data sampling rate, it was possible to determine precisely the flight path and the motion quantities of the birds. The recorded data of the flight over land show the characteristic pattern of dynamic soaring that consists of continually repeated cycles comprising a windward climb, an upper curve, a leeward descent and a lower curve. A further point is the mechanism of the energy transfer from the wind to the bird. It is shown that the upper curve of a dynamic soaring cycle is the decisive flight phase of the energy gain.