Coulomb Island Research Articles

Waiting time distribution and current correlations via a Majorana single-charge transistor

We investigate the electron transport and the waiting time distribution (WTD) in a topological superconducting Coulomb island system weakly coupled to two metallic leads. A pair of Majorana bound states are hosted in the island with a finite charging energy. By employing the Markovian master equation, we study the effect of interplay between Majorana energy splitting and the charging energy on the current correlations and the relevant electron WTDs. It is found that the super-Poissonian shot noise could be induced when the Majorana energy splitting is larger than the charging energy. The reason is that the degeneracy of the ground states in the island could be lifted by a finite Majorana energy splitting, which produces the asymmetry between tunneling channels and leads to the dynamical channel blockade effect. We show that the WTD for electron tunneling through two Majorana bound states is equivalent to the WTD in a single-resonant-level device. For comparison, we also discuss the WTD for electron tunneling through a non-Majorana device. It is found that the WTD sensitively depends on the length of the island and indicates oscillation behaviors in the absence of Majorana bound states. The particular behaviors of the WTDs in a Majorana island device can be useful in identifying the existence of Majorana bound states. Furthermore, we study the randomness parameter of the waiting time to describe the fluctuations of waiting times. It is shown that the randomness parameter indicates behavior similar to that indicated by the noise Fano factor, and both of them reflect the information of tunneling dynamics.

Towards single electron transistor-based photon detection with microplasma-enabled graphene quantum dots

Single-electron transistors (SETs) represent a new generation of electronic devices with high charge sensitivity, high switching speed, and low power consumption. Here a simple and controlled fabrication of graphene quantum dot (GQD)-based SETs for photon detectors has been demonstrated. The plasma-synthesized GQDs exhibit stable photoluminescence and are successfully used as the Coulomb islands between heteroepitaxial spherical-gold/platinum (HS-Au/Pt) nanogap electrodes. The as-fabricated GQD-SETs enable photon detection with 410 nm excitation owing to the ability of GQDs to generate photoluminescence emission.

Charge control of blockade of Cooper pair tunneling in highly disordered TiN nanowires in an inductive environment

A central problem in understanding the superconductor-insulator transition in disordered superconductors is that the properties of grains and intergrain medium cannot be independently studied. Here we demonstrate an approach to the study of strongly disordered superconducting films by relying on the stochastic nature of the disorder probed by electrostatic gating in a restricted geometry. Charge tuning and magnetotransport measurements in quasihomogeneous TiN nanowires embedded in a superinductor environment allow us to classify different devices and distinguish between spontaneously formed Coulomb islands (with typical blockade voltage in the mV range) and homogeneous wires showing behavior indicative of coherent quantum phase slips (with significantly smaller blockade voltage).

Hybridization of Subgap States in One-Dimensional Superconductor-Semiconductor Coulomb Islands.

We present measurements of one-dimensional superconductor-semiconductor Coulomb islands, fabricated by gate confinement of a two-dimensional InAs heterostructure with an epitaxial Al layer. When tuned via electrostatic side gates to regimes without subgap states, Coulomb blockade reveals Cooper-pair mediated transport. When subgap states are present, Coulomb peak positions and heights oscillate in a correlated way with magnetic field and gate voltage, as predicted theoretically, with (anti)crossings in (parallel) transverse magnetic field indicating Rashba-type spin-orbit coupling. Overall results are consistent with a picture of overlapping Majorana zero modes in finite wires.

Parity transitions in the superconducting ground state of hybrid InSb\u2013Al Coulomb islands

The number of electrons in small metallic or semiconducting islands is quantised. When tunnelling is enabled via opaque barriers this number can change by an integer. In superconductors the addition is in units of two electron charges (2e), reflecting that the Cooper pair condensate must have an even parity. This ground state (GS) is foundational for all superconducting qubit devices. Here, we study a hybrid superconducting–semiconducting island and find three typical GS evolutions in a parallel magnetic field: a robust 2e-periodic even-parity GS, a transition to a 2e-periodic odd-parity GS, and a transition from a 2e- to a 1e-periodic GS. The 2e-periodic odd-parity GS persistent in gate-voltage occurs when a spin-resolved subgap state crosses zero energy. For our 1e-periodic GSs we explicitly show the origin being a single zero-energy state gapped from the continuum, i.e., compatible with an Andreev bound states stabilized at zero energy or the presence of Majorana zero modes.

Coherent Resonant Electron Tunneling at 9 and 300 K through a 4.5 nm Long, Rigid, Planar Organic Molecular Wire.

Organic molecular wires that operate stably at ambient temperatures are a necessary first step toward practical and useful molecular-scale electronic devices, which have thus far been hampered by many factors, including the structural and electron configurational instability of organic molecules. We report here that a single disulfanyl carbon-bridged oligo(phenylenevinylene) (COPV6) molecule embedded between thermally stable electroless Au-plated electrodes of a 4 nm nanogap undergoes coherent resonant tunneling at both 9 and 300 K and functions even after storage in air at room temperature. Such enormous stability is ascribed to the unique structural characteristics of COPV6, that is, rigidity, planarity, thermal stability, resistivity against oxidation and reduction, and an organic insulating sheath that protects the π-system. When sandwiched between the gaps without pinning, this molecule behaves as a Coulomb island with sequential single-electron tunneling at 9 K that disappears at 300 K while maintaining a stable electron flow.

Gold Nanoparticles on Functionalized Silicon Substrate under Coulomb Blockade Regime: An Experimental and Theoretical Investigation.

Single charge electronics offer a way for disruptive technology in nanoelectronics. Coulomb blockade is a realistic way for controlling the electric current through a device with the accuracy of one electron. In such devices the current exhibits a step-like increase upon bias which reflects the discrete nature of the fundamental charge. We have assembled a double tunnel junction on an oxide-free silicon substrate that exhibits Coulomb staircase characteristics using gold nanoparticles (AuNPs) as Coulomb islands. The first tunnel junction is an insulating layer made of a grafted organic monolayer (GOM) developed for this purpose. The GOM also serves for attaching AuNPs covalently. The second tunnel junction is made by the tip of an STM. We show that this device exhibits reproducible Coulomb blockade I-V curves at 40 K in vacuum. We also show that depending on the doping of the silicon substrate, the whole Coulomb staircase can be adjusted. We have developed a simulation approach based on the orthodox theory that was completed by calculating the bias dependent tunnel barriers and by including an accurate calculation of the band bending. This model accounts for the experimental data and the doping dependence of Coulomb oscillations. This study opens new perspectives toward designing new kind of single electron transistors (SET) based on this dependence of the Coulomb staircase with the charge carrier concentration.

Three-input gate logic circuits on chemically assembled single-electron transistors with organic and inorganic hybrid passivation layers

Single-electron transistors (SETs) are sub-10-nm scale electronic devices based on conductive Coulomb islands sandwiched between double-barrier tunneling barriers. Chemically assembled SETs with alkanethiol-protected Au nanoparticles show highly stable Coulomb diamonds and two-input logic operations. The combination of bottom-up and top-down processes used to form the passivation layer is vital for realizing multi-gate chemically assembled SET circuits, as this combination enables us to connect conventional complementary metal oxide semiconductor (CMOS) technologies via planar processes. Here, three-input gate exclusive-OR (XOR) logic operations are demonstrated in passivated chemically assembled SETs. The passivation layer is a hybrid bilayer of self-assembled monolayers (SAMs) and pulsed laser deposited (PLD) aluminum oxide (AlO), and top-gate electrodes were prepared on the hybrid passivation layers. Top and two-side-gated SETs showed clear Coulomb oscillation and diamonds for each of the three available gates, and three-input gate XOR logic operation was clearly demonstrated. These results show the potential of chemically assembled SETs to work as logic devices with multi-gate inputs using organic and inorganic hybrid passivation layers.

Molecular floating-gate single-electron transistor

We investigated reversible switching behaviors of a molecular floating-gate single-electron transistor (MFG-SET). The device consists of a gold nanoparticle-based SET and a few tetra-tert-butyl copper phthalocyanine (ttbCuPc) molecules; each nanoparticle (NP) functions as a Coulomb island. The ttbCuPc molecules function as photoreactive floating gates, which reversibly change the potential of the Coulomb island depending on the charge states induced in the ttbCuPc molecules by light irradiation or by externally applied voltages. We found that single-electron charging of ttbCuPc leads to a potential shift in the Coulomb island by more than half of its charging energy. The first induced device state was sufficiently stable; the retention time was more than a few hours without application of an external voltage. Moreover, the device exhibited an additional state when irradiated with 700 nm light, corresponding to doubly charged ttbCuPc. The life time of this additional state was several seconds, which is much shorter than that of the first induced state. These results clearly demonstrate an alternative method utilizing the unique functionality of the single molecule in nanoelectronics devices, and the potential application of MFG-SETs for investigating molecular charging phenomena.

Coulomb blockade and Coulomb staircase behavior observed at room temperature

A single-electron transistor (SET) consists of source, drain, Coulomb island, and gate to modulate the number of electrons and control the current. For practical applications, it is important to operate a SET at room temperature. One proposal towards the ability to operate at room temperature is to decrease Coulomb island size down to a few nanometres. We investigate a SET using Sn-porphyrin (Sn-por) protected gold nanoparticles (AuNPs) with 1.4 nm in core diameter as a Coulomb island. The fabrication method of nanogap electrodes uses the combination of a top-down technique by electron beam lithography (EBL) and a bottom-up process through electroless gold plating (ELGP) as our group have described before. The electrical measurement was conducted at room temperature (300 K). From current–voltage (Id–Vd) characteristics, we obtained clear Coulomb blockade phenomena together with a Coulomb staircase due to a Sn-por protected gold NP as a Coulomb island. Experimental results of Id–Vd characteristics agree with a theoretical curve based on using the orthodox model. Clear dId/dVd peaks are observed in the Coulomb staircase at 9 K which suggest the electron transports through excited energy levels of Au NPs. These results are a big step for obtaining SETs that can operate at room temperature.

Room temperature Coulomb blockade mediated field emission via self-assembled gold nanoparticles

Coulomb blockade mediated field-emission current was observed in single-electron tunneling devices based on self-assembled gold nanoparticles at 300 K. According to Raichev's theoretical model, by fixing a proper geometric distribution of source, island and drain, the transfer characteristics can be well explained through a combination of Coulomb blockade and field emission. Coulomb blockade and field emission alternately happen in our self-assembled devices. The Coulomb island size derived from the experimental data is in good agreement with the average size of the gold nanoparticles used in the device. The integrated tunneling can be adjusted via a gate electrode.

Observation of charge transport through CdSe/ZnS quantum dots in a single-electron transistor structure

We fabricated single-electron transistors (SETs) having CdSe/ZnS core-shell-type quantum dots (CdSe/ZnS-QDs) as a Coulomb island using a wet chemistry technique. The CdSe/ZnS-QDs were deposited onto Au electrodes with or without the assistance of a self-assembled monolayer of octane(di)thiols. The CdSe/ZnS-QDs were adsorbed onto the Au electrodes even without the interlayer of thiol molecules depending on the concentration of the CdSe/ZnS-QD solution. The electron-transport characteristics through the CdSe/ZnS-QDs were examined in an SET structure at 13 K. Coulomb blockade behavior with typical gate voltage dependence was clearly observed. The estimated charge addition energies of a CdSe/ZnS-QD ranged from 70 to 280 meV. Moreover, additional structures, including negative differential conductance, appeared in the stability diagram in the source-drain bias region beyond 100 mV; these structures are specific to single-charge transport through the discrete energy levels in the Coulomb island.

Exponential protection of zero modes in Majorana islands.

Majorana zero modes are quasiparticle excitations in condensed matter systems that have been proposed as building blocks of fault-tolerant quantum computers. They are expected to exhibit non-Abelian particle statistics, in contrast to the usual statistics of fermions and bosons, enabling quantum operations to be performed by braiding isolated modes around one another. Quantum braiding operations are topologically protected insofar as these modes are pinned near zero energy, with the departure from zero expected to be exponentially small as the modes become spatially separated. Following theoretical proposals, several experiments have identified signatures of Majorana modes in nanowires with proximity-induced superconductivity and atomic chains, with small amounts of mode splitting potentially explained by hybridization of Majorana modes. Here, we use Coulomb-blockade spectroscopy in an InAs nanowire segment with epitaxial aluminium, which forms a proximity-induced superconducting Coulomb island (a 'Majorana island') that is isolated from normal-metal leads by tunnel barriers, to measure the splitting of near-zero-energy Majorana modes. We observe exponential suppression of energy splitting with increasing wire length. For short devices of a few hundred nanometres, sub-gap state energies oscillate as the magnetic field is varied, as is expected for hybridized Majorana modes. Splitting decreases by a factor of about ten for each half a micrometre of increased wire length. For devices longer than about one micrometre, transport in strong magnetic fields occurs through a zero-energy state that is energetically isolated from a continuum, yielding uniformly spaced Coulomb-blockade conductance peaks, consistent with teleportation via Majorana modes. Our results help to explain the trivial-to-topological transition in finite systems and to quantify the scaling of topological protection with end-mode separation.

Fabrication and Characterization of a Single Electron Transistor Based on a Silicon-on-Insulator**Supported by the National Natural Science Foundation of China under Grant Nos 11074280 and 11403084, the Instrument Developing Project of Chinese Academy of Sciences under Grant No YZ201152, the Fundamental Research Funds for Central Universities under Grant Nos JUSRP51323B and JUDCF12032, the Joint Innovation Project of Jiangsu Province under

A single electron transistor based on a silicon-on-insulator is successfully fabricated with electron-beam nanolithography, inductively coupled plasma etching, thermal oxidation and other techniques. The unique design of the pattern inversion is used, and the pattern is transferred to be negative in the electron-beam lithography step. The oxidation process is used to form the silicon oxide tunneling barriers, and to further reduce the effective size of the quantum dot. Combinations of these methods offer advantages of good size controllability and accuracy, high reproducibility, low cost, large-area contacts, allowing batch fabrication of single electron transistors and good integration with a radio-frequency tank circuit. The fabricated single electron transistor with a quantum dot about 50nm in diameter is demonstrated to operate at temperatures up to 70 K. The charging energy of the Coulomb island is about 12.5 meV.

Control of charging energy in chemically assembled nanoparticle single-electron transistors

We show the control of a charging energy in chemically assembled nanoparticle single-electron transistors (SETs) by altering the core diameter of Au nanoparticles. The charging energy is a fundamental parameter that decides the operating temperature of SETs. Practical application of SETs requires us to regulate the value of the charging energy by tuning the diameter of quantum dots. In this study, we used 3.0, 5.0 and 6.2 nm diameter synthesized Au nanoparticles as a quantum dot in the SETs. The total capacitances and charging energy of the SETs were evaluated from the rhombic Coulomb diamonds attributed to a single Coulomb island. The capacitance and charging energy matched with a concentric sphere model much better than with a simple sphere model. The operating temperatures of the SETs suggested that a charging energy 2.2 times greater than the thermal energy was required for stable operation, in theory. These results will help us to select an appropriate core diameter for the Au nanoparticles in practical SETs.

Spin regulation in composite spin-filter barrier devices

Magnetic insulators are known to provide large effective Zeeman fields that are confined at an interface, making them especially powerful in modifying adjacent one- or two-dimensional electronic structures. Utilizing this phenomenon and the other important property of magnetic insulators--spin filtering--here we report the generation and subsequent detection of a large interface field, as large as tens of tesla in EuS/Al/EuS heterostructures with metallic coulomb islands confined within a magnetic insulator barrier. The unique energy profile across this sandwich structure produces spin-assisted charge transfer across the device, generating a spontaneous spin current and voltage. These unique properties can be practical for controlling spin flows in electronic devices and for energy harvesting.

DNA Origami: DNA Origami Directed Large‐Scale Fabrication of Nanostructures Resembling Room Temperature Single‐Electron Transistors (Small 21/2013)

Large-scale fabrication of room-temperature single-electron transistors core nanostructures in an unprecedented yield is achieved by using DNA self-assembly strategy, in which three gold nanorods (13 nm × 38 nm) are precisely positioned on the DNA origami surface as source, drain, and gate electrodes, with the distance between the source and drain electrodes smaller than 10 nm. As reported on page 3567 by Q. Wang and co-workers, a gold nanoparticle (4.5 nm) can be located in the center of the source and drain electrodes as Coulomb island. This delicate nanostructure cannot be achieved using state-of-art nanofabrication techniques.

DNA Origami Directed Large‐Scale Fabrication of Nanostructures Resembling Room Temperature Single‐Electron Transistors

Room temperature single-electron transistor core nanostructures are fabricated on a large scale with DNA origami as a template with an unprecedented yield. The accuracy of DNA origami enables precise positioning of a Coulomb island in the center of a 10 nm gap between the source and the drain electrodes, which can not be realized by using state-of-the-art nanofabrication techniques.

Single electron transistors with ultra-thin Au nanowires as a single Coulomb island

Single electron transistors exhibiting transport properties based on a single Coulomb island have been fabricated using ultra-thin gold nanowires (AuNWs), which are synthesized via a chemical reduction process. The AuNWs are bottom-contacted with source and drain electrodes to avoid damaging the AuNWs under fabrication processes. We investigate the transport properties in the fabricated devices as a function of the bias and gate voltages at room and low temperatures. At 0.23 K, the periodical Coulomb oscillations and diamonds are clearly observed indicating that an individual AuNW acts as a single Coulomb island. These transport properties can be explained by the orthodox Coulomb blockade theory.

Single-hole tunneling through a two-dimensional hole gas in intrinsic silicon

In this letter we report single-hole tunneling through a quantum dot in a two-dimensional hole gas, situated in a narrow-channel field-effect transistor in intrinsic silicon. Two layers of aluminum gate electrodes are defined on Si/SiO2 using electron-beam lithography. Fabrication and subsequent electrical characterization of different devices yield reproducible results, such as typical MOSFET turn-on and pinch-off characteristics. Additionally, linear transport measurements at 4 K result in regularly spaced Coulomb oscillations, corresponding to single-hole tunneling through individual Coulomb islands. These Coulomb peaks are visible over a broad range in gate voltage, indicating very stable device operation. Energy spectroscopy measurements show closed Coulomb diamonds with single-hole charging energies of 5–10 meV and lines of increased conductance as a result of resonant tunneling through additional available hole states.