Electrostatic Disorder Research Articles

Automated tuning of inter-dot tunnel coupling in double quantum dots

Semiconductor quantum dot arrays defined electrostatically in a 2D electron gas provide a scalable platform for quantum information processing and quantum simulations. For the operation of quantum dot arrays, appropriate voltages need to be applied to the gate electrodes that define the quantum dot potential landscape. Tuning the gate voltages has proven to be a time-consuming task, because of initial electrostatic disorder and capacitive cross-talk effects. Here, we report on the automated tuning of the inter-dot tunnel coupling in gate-defined semiconductor double quantum dots. The automation of the tuning of the inter-dot tunnel coupling is the next step forward in scalable and efficient control of larger quantum dot arrays. This work greatly reduces the effort of tuning semiconductor quantum dots for quantum information processing and quantum simulation.

Self-Assembled Photochromic Molecular Dipoles for High-Performance Polymer Thin-Film Transistors.

The development of high-performance multifunctional polymer-based electronic circuits is a major step toward future flexible electronics. Here, we demonstrate a tunable approach to fabricate such devices based on rationally designed dielectric super-lattice structures with photochromic azobenzene molecules. These nanodielectrics possessing ionic, molecular, and atomic polarization are utilized in polymer thin-film transistors (TFTs) to realize high-performance electronics with a p-type field-effect mobility (μFET) exceeding 2 cm2 V-1 s-1. A crossover in the transport mechanism from electrostatic dipolar disorder to ionic-induced disorder is observed in the transistor characteristics over a range of temperatures. The facile supramolecular design allows the possibility to optically control the extent of molecular and ionic polarization in the ultrathin nanodielectric. Thus, we demonstrate a 3-fold increase in the capacitance from 0.1 to 0.34 μF/cm2, which results in a 200% increase in TFT channel current.

Quantum simulation of a Fermi-Hubbard model using a semiconductor quantum dot array.

Interacting fermions on a lattice can develop strong quantum correlations, which are the cause of the classical intractability of many exotic phases of matter. Current efforts are directed towards the control of artificial quantum systems that can be made to emulate the underlying Fermi-Hubbard models. Electrostatically confined conduction-band electrons define interacting quantum coherent spin and charge degrees of freedom that allow all-electrical initialization of low-entropy states and readily adhere to the Fermi-Hubbard Hamiltonian. Until now, however, the substantial electrostatic disorder of the solid state has meant that only a few attempts at emulating Fermi-Hubbard physics on solid-state platforms have been made. Here we show that for gate-defined quantum dots this disorder can be suppressed in a controlled manner. Using a semi-automated and scalable set of experimental tools, we homogeneously and independently set up the electron filling and nearest-neighbour tunnel coupling in a semiconductor quantum dot array so as to simulate a Fermi-Hubbard system. With this set-up, we realize a detailed characterization of the collective Coulomb blockade transition, which is the finite-size analogue of the interaction-driven Mott metal-to-insulator transition. As automation and device fabrication of semiconductor quantum dots continue to improve, the ideas presented here will enable the investigation of the physics of ever more complex many-body states using quantum dots.

Polymer-dielectric molecular interactions in defect-free poly(3-hexylthiophene): dependence and consequences of regioregularity on transistor charge transport properties

We investigate the molecular interaction of poly(3-hexylthiophene-2,5-diyl) (P3HT) molecules with polar functional groups of the dielectric surface, and its dependence on the regioregularity of P3HT. With this aim, we consider thickness-dependent molecular order of 100% regioregular defect-free P3HT (DF-P3HT) and 93% regioregular P3HT (LT-P3HT), deposited on top of cross-linked poly(vinyl alcohol) (cr-PVA) substrates. Intimate contact of P3HT molecules and cr-PVA surface defects affects the molecular order of P3HT differently, depending on the regioregularity. Consequently, these molecular order changes on the charge transport properties of organic field-effect transistors (OFETs) are investigated using four thicknesses (20, 40, 80 and 120 nm) of P3HT. As compared to other thicknesses, μsat for 20 nm DF-P3HT OFETs shows further improvement, while the opposite occurs for 20 nm LT-P3HT OFETs. Depending on the regioregularity (and thus the chain orientation), P3HT molecules exhibit a difference in dipole moments. Consequently, the interaction of edge-on or face-on P3HT molecules with cr-PVA surface dipoles has different contributions towards the electrostatic energetic disorder at cr-PVA/P3HT interface. This subtle difference of behavior helps one to understand the huge spread of characteristics of P3HT based transistors found in literature.

Disorder engineering and conductivity dome in ReS2 with electrolyte gating.

Atomically thin rhenium disulphide (ReS2) is a member of the transition metal dichalcogenide family of materials. This two-dimensional semiconductor is characterized by weak interlayer coupling and a distorted 1T structure, which leads to anisotropy in electrical and optical properties. Here we report on the electrical transport study of mono- and multilayer ReS2 with polymer electrolyte gating. We find that the conductivity of monolayer ReS2 is completely suppressed at high carrier densities, an unusual feature unique to monolayers, making ReS2 the first example of such a material. Using dual-gated devices, we can distinguish the gate-induced doping from the electrostatic disorder induced by the polymer electrolyte itself. Theoretical calculations and a transport model indicate that the observed conductivity suppression can be explained by a combination of a narrow conduction band and Anderson localization due to electrolyte-induced disorder.

Impact of a Low Concentration of Dopants on the Distribution of Gap States in a Molecular Semiconductor

We investigate the distribution of valence and tail states in copper phthalocyanine (CuPc) upon the introduction of minute amounts of the p-dopant molybdenum tris[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), using a combination of electron spectroscopy and carrier transport measurements. Density of gap states, conductivity, and hole-hopping activation energy are measured. We observe the progressive filling (and deactivation) of the deepest tail states by charges introduced by the dopants, as well as significant broadening of the CuPc density of states. Simulations relate this broadening to the electrostatic and structural disorder induced by the dopant in the CuPc matrix.

Signatures of Rashba spin-orbit interaction in charge and spin properties of quantum Hall systems

We study the local equilibrium properties of two-dimensional electron gases at high magnetic fields in the presence of random smooth electrostatic disorder, Rashba spin-orbit coupling, and the Zeeman interaction. Using a systematic magnetic length (${l}_{B}$) expansion within a Green's function framework we derive quantum functionals for the local spin-resolved particle and current densities which can be useful for future studies combining disorder and mean-field electron-electron interaction in the quantum Hall regime. We point out that the spin polarization presents a peculiar spatial dependence which can be used to determine the strength of the Rashba coupling by local probes. The spatial structure of the current density, consisting of both compressible and incompressible contributions, also essentially reflects the effects of Rashba spin-orbit interaction on the energy spectrum. We show that in the semiclassical limit ${l}_{B}\ensuremath{\rightarrow}0$ the local Hall conductivity remains, however, still quantized in units of ${e}^{2}/h$ for any finite strength of the spin-orbit interaction. In contrast, it becomes half-integer quantized when the latter is infinite, a situation which corresponds to a disordered topological insulator surface consisting of a single Dirac cone. Finally, we argue how to define at high magnetic fields a spin Hall conductivity related to a dissipationless angular momentum flow, which is characterized by a sequence of plateaus as a function of the inverse magnetic field (thus free of resonances).

Disorder and magnetic-field-induced breakdown of helical edge conduction in an inverted electron-hole bilayer

We calculate the conductance of a two-dimensional bilayer with inverted electron-hole bands, to study the sensitivity of the quantum spin Hall insulator (with helical edge conduction) to the combination of electrostatic disorder and a perpendicular magnetic field. The characteristic breakdown field for helical edge conduction splits into two fields with increasing disorder, a field $B_{c}$ for the transition into a quantum Hall insulator (supporting chiral edge conduction) and a smaller field $B'_{c}$ for the transition to bulk conduction in a quasi-metallic regime. The spatial separation of the inverted bands, typical for broken-gap InAs/GaSb quantum wells, is essential for the magnetic-field induced bulk conduction --- there is no such regime in HgTe quantum wells.

Quantum phase transitions of a disordered antiferromagnetic topological insulator

We study the effect of electrostatic disorder on the conductivity of a three-dimensional antiferromagnetic insulator (a stack of quantum anomalous Hall layers with staggered magnetization). The phase diagram contains regions where the increase of disorder first causes the appearance of surface conduction (via a topological phase transition), followed by the appearance of bulk conduction (via a metal-insulator transition). The conducting surface states are stabilized by an effective time-reversal symmetry that is broken locally by the disorder but restored on long length scales. A simple self-consistent Born approximation reliably locates the boundaries of this socalled "statistical" topological phase.

Charge separation energetics at organic heterojunctions: on the role of structural and electrostatic disorder

Improving the performance of organic photovoltaic cells requires the individuation of the specific factors limiting their efficiency, by rationalizing the relationship between the chemical nature of the materials, their morphology, and the electronic processes taking place at their interface. In this contribution, we present recent theoretical advances regarding the determination of the energetics and dynamics of charge carriers at organic-organic interfaces, highlighting the role of structural and electrostatic disorder in the separation of electron-hole pairs. The influence of interfacial electrostatic interactions on charge carrier energetics is first illustrated in model aggregates. Then, we review some of our recent theoretical studies in which we combined molecular dynamics, quantum-chemical and classical micro-electrostatic methods to evaluate the energy landscape explored by the mobile charges in the vicinity of donor-acceptor interfaces with realistic morphologies. Finally, we describe the theoretical challenges that still need to be overcome in order to gain a complete overview of the charge separation processes at the molecular level.

Spectral properties and local density of states of disordered quantum Hall systems with Rashba spin-orbit coupling

We theoretically investigate the spectral properties and the spatial dependence of the local density of states (LDoS) in disordered two-dimensional electron gases (2DEG) in the quantum Hall regime, taking into account the combined presence of electrostatic disorder, random Rashba spin-orbit in- teraction, and finite Zeeman coupling. To this purpose, we extend a coherent-state Green's function formalism previously proposed for spinless 2DEG in the presence of smooth arbitrary disorder, that here incorporates the nontrivial coupling between the orbital and spin degrees of freedom into the electronic drift states. The formalism allows us to obtain analytical and controlled nonperturbative expressions of the energy spectrum in arbitrary locally flat disorder potentials with both random electric fields and Rashba coupling. As an illustration of this theory, we derive analytical microscopic expressions for the LDoS in different temperature regimes which can be used as a starting point to interpret scanning tunneling spectroscopy data at high magnetic fields. In this context, we study the spatial dependence and linewidth of the LDoS peaks and explain an experimentally-noticed correlation between the spatial dispersion of the spin-orbit splitting and the local extrema of the potential landscape.

Robustness of topological order in semiconductor–superconductor nanowires in the Coulomb blockade regime

Semiconductor–superconductor hybrid systems are promising candidates for the realization of Majorana fermions and topological order, i.e. topologically protected degeneracies, in solid state devices. We show that the topological order is mirrored in the excitation spectra and can be observed in nonlinear Coulomb blockade transport through a ring-shaped nanowire. Especially, the excitation spectrum is almost independent of magnetic flux in the topologically trivial phase but acquires a characteristic h/e magnetic flux periodicity in the non-trivial phase. The transition between the trivial and non-trivial phase is reflected in the closing and reopening of an excitation gap. We show that the signatures of topological order are robust against details of the geometry, electrostatic disorder and the existence of additional subbands and only rely on the topology of the nanowire and the existence of a superconducting gap. Finally, we show that the coherence length in the non-trivial phase is much longer than in the trivial phase. This opens the possibility to coat the nanowire with superconducting nanograins and thereby significantly reduce the current due to cotunnelling of Cooper pairs and to enhance the Coulomb charging energy without destroying the superconducting gap.

Chemical potential of inhomogeneous single-layer graphene

In this paper, we present measurements of the chemical potential of single-layer graphene as a function of carrier density and temperature, including near the Dirac point. Far from the charge neutrality point, the graphene is homogenous with a single carrier type. However, as the Dirac point is approached, puddles form, and electrons and holes coexist. Hall effect analyses based on two charge carriers are not adequate in this regime. Hence, a new methodology is introduced, and by using the chemical potential and the transport data self-consistently, we were able to extract the density of each carrier. We obtained very good agreement with a recent theory that assumes a Gaussian distribution of the electrostatic disorder potential. Surprisingly, the temperature dependence of the minimum conductivity of graphene was primarily attributed to the temperature dependence of the disorder potential itself through the carrier charge densities, and not to the temperature dependence of the carrier's mobility. .

Relation between Structure and Electronic Properties of Amorphous MEH-PPV Polymers

Classical molecular dynamics simulations were used to build several large models of the amorphous polymeric semiconductor MEH-PPV. A balanced set of approximations was determined to evaluate the electronic structure of these large systems, providing quantitative information for the understanding of the charge transport properties. We have verified that the electronic structure is largely determined by the conformational disorder of the individual chains, with little effect of electrostatic disorder and interchain coupling. The disorder is essentially static, although thermal motions cause an evolution of the single chain orbital energies. The localization length of the orbitals relevant for transport is energy-dependent, unlike what is normally assumed in variable range hopping methods. Although we have found evidence of correlation between planarity of the polymer chain and localization of the highest valence band orbitals, the correlation is moderate and exceptions are frequent. All observations are discussed in terms of the desirable characteristics that should be included in a model of transport for amorphous polymeric semiconductors.

Hopping charge transport in organic materials

General properties of the transport of charge carriers (electrons and holes) in disordered organic materials are discussed. It was demonstrated that the dominant part of the total energetic disorder in organic material is usually provided by the electrostatic disorder, generated by randomly located and oriented dipoles and quadrupoles. For this reason this disorder is strongly spatially correlated. Spatial correlation directly governs the field dependence of the carrier drift mobility. Shape of the current transients, which is of primary importance for a correct determination of the carrier mobility, is considered. A notable feature of the electro-static disorder is its modification in the vicinity of the electrode, and this modification takes place without modification of the structure of the material. It is shown how this phenomenon affects characteristics of the charge injection. We consider also effect of inter-charge interaction on charge transport.

Organic glasses: cluster structure of the random energy landscape and its effect on charge transport and injection

AbstractAn appropriate model for the random energy landscape in organic glasses is a spatially correlated Gaussian field, generated by randomly located and oriented dipoles and quadrupoles. Correlation properties of energetic disorder directly dictates the mobility dependence on the applied electric field. Electrostatic disorder is significantly modified in the vicinity of the electrode that affects injection properties. Correlated Gaussian field forms clusters. We suggest a simple method to estimate an asymptotics of the cluster distribution on size for deep clusters where a value of the field on each site is much greater than the rms disorder. Hopping transport in organic glasses in the case of high carrier density could be described in terms of the effective density‐dependent temperature. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Variable-range-hopping conductivity in polygrain samples of icosahedral Al–Re–Pd

We further develop a model, based on an analogy with granular electronic conductors, which explains the nature of the variable-range-hopping conductivity in polygrain samples of icosahedral Al–Re–Pd with resistivity ratios of in an excess of ∼13. The electrical conductivity of the model is determined by intergranular tunnelling and electrostatic energy. In polygrain icosahedral Al–Re–Pd, pores and precipitates of secondary phases create the barriers. Depending on the barrier height and electrostatic disorder, either an insulating variable-range-hopping regime of an Efros–Shklovskii or Mott type with elastic co-tunnelling or a metallic regime typical of homogeneously disordered conductors is possible.

Ab initiostudy of electron transport in dry poly(G)-poly(C)A-DNA strands

The bias-dependent transport properties of short poly(G)-poly(C) A-DNA strands attached to Au electrodes are investigated with first principles electronic transport methods. By using the non- equilibrium Green's function approach combined with self-interaction corrected density functional theory, we calculate the fully self-consistent coherent I-V curve of various double-strand polymeric DNA fragments. We show that electronic wave-function localization, induced either by the native electrical dipole and/or by the electrostatic disorder originating from the first few water solvation layers, drastically suppresses the magnitude of the elastic conductance of A-DNA oligonucleotides. We then argue that electron transport through DNA is the result of sequence-specific short-range tunneling across a few bases combined with general diffusive/inelastic processes.

Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Explicit Defects and Electrostatics

The effects of defects and electrostatics on charge transport in realistic organic field effect transistors were studied using a combination of first principles quantum chemistry calculations and Monte Carlo simulations with explicit introduction of defect sites. The results show that electrostatic interactions dramatically affect the field and carrier concentration dependence of charge transport in devices that include a significant number of traps, as well as its “switch-on” characteristics. Our results also show that charge transport decreases linearly as a function of neutral defect concentration as conduction pathways are turned off. For charged defects, mobility of imperfect devices is lower relative to defect-free devices but is surprisingly unaffected by the concentration of charged defects. The exact statistics of electrostatic disorder introduced by charged defects is found to obey a Poisson distribution rather than Gaussian or exponential as previously assumed. We also demonstrate that without ...

Effect of Electrostatic Interactions and Dynamic Disorder on the Distance Dependence of Charge Transfer in Donor−Bridge−Acceptor Systems

Using a tight-binding model of charge transport in systems with static and dynamic disorder, we present a theoretical study of the positive charge transfer in molecular assemblies that involve a hole donor and an acceptor connected by fluorene and phenyl bridges. Two parameters that determine the rate of charge transfer within the proposed model are the charge transfer integral between neighboring units and the site energies. Fluctuations in the values of the charge transfer integral and the energy landscape for hole transport were calculated by taking into account variations of the dihedral angle between neighboring units and electrostatic interaction of positive charge moving along the bridge and the negative charge that remains on the hole donor. Analysis of the dynamics of hole transfer and the distribution of the positive charge during this process allows the conclusion that the rapid fall of the hole transfer rate coefficient observed in experiments with short bridges (three and four structural units for systems with fluorene and phenyl bridges, respectively) can be attributed to the electrostatic interaction. This interaction is responsible for the formation of the effective barrier between donor and acceptor with the height that increases as the number of structural bridge units remains less than 3 (fluorene bridge) or 4 (phenyl bridge). For longer bridges, however, the effective barrier changes only weakly and now the charge transport is mostly dominated by the fluctuation-assisted incoherent hole migration along the bridge. The latter mechanism exhibits much weaker dependence of the rate coefficient on the bridge length in agreement with the available experimental results.