Ballistic Channel Research Articles

Decoherence in electron transport: back-scattering, effect on interference and rectification

Decoherence is an undesirable, but ubiquitous phenomenon in quantum systems. Here, we study the effect of partial decoherence, induced via a Büttiker probe, on two-terminal electronic transport across one-dimensional quantum wires and rings, in both the linear and non-linear regimes. We find that dephasing causes backscattering when introduced locally in a ballistic channel. Further, we find that decoherence results in rectification when inversion is broken in the two-terminal transport set-up by a combination of a local dephasing centre and a static impurity. Interestingly, the rectification strength and even its direction varies strongly with the relative distance between the probe and the scatterer. We further analyze how decoherence affects characteristic quantum effects in electronic transport, such as, Fabry-Pérot oscillations in double-barrier setups, and Aharonov–Bohm interference in one-dimensional rings, and find that the amplitude of oscillations in conductance is reduced by decoherence.

Electronic structure of ultrathin single-walled platinum nanotubes

Using Density Functional Theory based on the plane wave method and linearized augmented cylindrical wave method, and taking into account the spin–orbit interaction, the structural, electronic and spin properties of ultrathin single-walled nanotubes consisting of six platinum rows are obtained. The nanotubes are found to possess metallic electronic mobility, including the nonchiral Pt(6,0) with the largest number (15G0) of ballistic channels. Analysis of the density of electronic states at Fermi level revealed that the chiral Pt(5,1) has a difference in concentration of mobile electrons with opposite spins of 1.42 times, and up to 3–4 times under a weak torsional strain.

Universality and the thermoelectric transport properties of a double quantum dot system: Seeking for conditions that improve the thermoelectric efficiency

Employing universal relations for the Onsager coefficients in the linear regime at the symmetric point of the single impurity Anderson model, we calculate the conditions under which the quantum scattering phase shift should satisfy to produce the asymptotic Carnot’s limit for the thermoelectric efficiency. We show that a single quantum dot connected by metallic leads at the Kondo regime cannot achieve the best thermoelectric efficiency. We study a serial double quantum dot system, with the quantum dots immersed in ballistic conduction channels. Each QD exhibits a strong but finite local electronic correlation UU. We show that maintaining one dot in the electron-hole symmetric point and allowing charge fluctuations in the other QD makes it possible to drive the system to the maximum thermoelectric efficiency. We also show that the bound states in the continuum (BICs) and quasi-BICs associated with the quantum scattering interference process drive the DQD to the maximum thermoelectric efficiency. We identify two types of quasi-BICs that occur at low and high temperatures: The first is associated with single Fano resonances, and the last is with several Fano processes. We also discussed possible temperature values and conditions that could be linked with the experimental realization of the results.

Irreversible entropy transport enhanced by fermionic superfluidity

The nature of particle and entropy flow between two superfluids is often understood in terms of reversible flow carried by an entropy-free, macroscopic wavefunction. While this wavefunction is responsible for many intriguing properties of superfluids and superconductors, its interplay with excitations in non-equilibrium situations is less understood. Here we observe large concurrent flows of both particles and entropy through a ballistic channel connecting two strongly interacting fermionic superfluids. Both currents respond nonlinearly to chemical potential and temperature biases. We find that the entropy transported per particle is much larger than the prediction of superfluid hydrodynamics in the linear regime and largely independent of changes in the channel’s geometry. By contrast, the timescales of advective and diffusive entropy transport vary significantly with the channel geometry. In our setting, superfluidity counterintuitively increases the speed of entropy transport. Moreover, we develop a phenomenological model describing the nonlinear dynamics within the framework of generalized gradient dynamics. Our approach for measuring entropy currents may help elucidate mechanisms of heat transfer in superfluids and superconducting devices.

Lateral junctions of transition metal dichalcogenides as ballistic channels for straintronic applications

In the context of advanced nanoelectronics, two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are gaining considerable interest due to their ultimate thinness, clean surface and high carrier mobility. The engineering prospects offered by those materials are further enlarged by the recent realization of atomically sharp TMD-based lateral junctions, whose electronic properties are governed by strain effects arising from the constituents lattice mismatch. Although most theoretical studies considered only misfit strain, first-principles simulations are employed here to investigate the transport properties under external deformation of a three-terminal device constructed from a MoS2/WSe2/MoS2 junction. Large modulation of the current is reported owing to the change in band offset, illustrating the importance of strain on the p–n junction characteristics. The device operation is demonstrated for both local and global deformations, even for ultra-short channels, suggesting potential applications for ultra-thin body straintronics.

Robust Majorana bound states in magnetic topological insulator nanoribbons with fragile chiral edge channels

Magnetic topological insulators in the quantum anomalous Hall regime host ballistic chiral edge channels. When proximitized by an s-wave superconductor, these edge states offer the potential for realizing topological superconductivity and Majorana bound states without the detrimental effect of large externally applied magnetic fields on superconductivity. Realizing well-separated unpaired Majorana bound states requires magnetic topological insulator ribbons with a width of the order of the transverse extent of the edge state, however, which is expected to bring the required ribbon width down to around 100nm. In this regime, it is known to be extremely difficult to retain the ballistic nature of chiral edge channels and realize a quantized Hall conductance. In this paper, we study the impact of disorder in such magnetic topological insulator nanoribbons and compare the fragility of ballistic chiral edge channels with the stability of Majorana bound states when the ribbon is covered by a superconducting film. We find that the Majorana bound states exhibit greater robustness against disorder than the underlying chiral edge channels. Published by the American Physical Society 2024

STUDY OF MAGNETIC FIELD GENERATION IN CHIRAL COPPER NANOTUBESS

The magnetic fields generated by chiral copper nanotubes are calculated. The number of ballistic transport channels, low-temperature electron currents, and magnetic fields in nanosolenoids based on copper nanotubes of various structures are determined. The results indicate that chiral nanotubes can be used to create nanosolenoids with desired characteristics.

Isolated ballistic non-abelian interface channel.

The topological order of a quantum Hall state is mirrored by the gapless edge modes owing to bulk-edge correspondence. The state at the filling of ν = 5/2, predicted to host non-abelian anyons, supports a variety of edge modes (integer, fractional, neutral). To ensure thermal equilibration between the edge modes and thus accurately determine the state's nature, it is advantageous to isolate the fractional channel (1/2 and neutral modes). In this study, we gapped out the integer modes by interfacing the ν = 5/2 state with integer states ν = 2 and ν = 3 and measured the thermal conductance of the isolated-interface channel. Our measured half-quantized thermal conductance confirms the non-abelian nature of the ν = 5/2 state and its particle-hole Pfaffian topological order. Such an isolated channel may be more amenable to braiding experiments.

Probing Edge State Conductance in Ultra‐Thin Topological Insulator Films

AbstractQuantum spin Hall (QSH) insulators have unique electronic properties, comprising a band gap in their 2D interior and 1D spin‐polarized edge states in which current flows ballistically. In scanning tunneling microscopy (STM), the edge states manifest themselves as an enhanced local density of states (LDOS). However, there is a significant research gap between the observation of edge states in nanoscale spectroscopy, and the detection of ballistic transport in edge channels which typically relies on transport experiments with microscale lithographic contacts. Here, few‐layer films of the 3D topological insulator (BixSb)2Te3 are studied, for which a topological transition to a 2D topological QSH insulator phase has been proposed. Indeed, an edge state in the LDOS is observed within the band gap. Yet, in nanoscale transport experiments with a four‐tip STM, two‐quintuple‐layer films do not exhibit a ballistic conductance in the edge channels and thus no QSH edge states. This demonstrates that the detection of edge states in spectroscopy can be misleading with regard to the identification of a QSH phase. In contrast, nanoscale multi‐tip transport experiments are a robust method for effectively pinpointing ballistic edge channels, as opposed to trivial edge states, in quantum materials.

Effect of a Carbon Nanotube on Ballistic Conduction through Single-Quantum-Dot

We will study the competitive effect between the transport of a quantum dot adsorbed to a ballistic channel and laterally coupled to a single-walled carbon nanotube (SWNT). We will use the “tight-binding" approach to analytically write the SWNT Green's function and the quantum dot will be solved by the atomic method for. We will present curves of the electronic density of states for some different sizes of nanotubes. The results for the conductance curves will be presented as a function of and for different values of n and hopping between the nanotube and the quantum dot.

Fractionalization and anyonic statistics in the integer quantum Hall collider

One remarkable feature of strongly correlated systems is the phenomenon of fractionalization where quasiparticles carry only a fraction of the charge or spin of the elementary constituents. Such quasiparticles often present anyonic statistics in two dimensions and lie at the heart of the fractional quantum Hall effect. We discuss the observation of fractionalization and anyonic statistics already in the integer quantum Hall effect coupled to a metallic island. A continuous fractional emitter is proposed, which sends dilute beams of noninteger charges, and its full counting statistics is obtained. The fractional charge is governed solely by the number of ballistic channels covered by the island and it is one half of the electron charge for a single ballistic channel. We further characterize the mixing of two such fractional beams through a quantum point contact beam splitter. We predict negative cross-correlations, in strong contrast with free electrons, that depend on the double exchange phase between electrons and the fractional charges emulating anyons. The result is similar to a genuine fractional edge state as recently measured at filling $\ensuremath{\nu}=1/3$. We revisit the physical interpretation of this experiment and point towards a direct braiding measurement rather than a deviation from fermionic antibunching.

Valley and spin accumulation in ballistic and hydrodynamic channels

A theory of the valley and spin Hall effects and resulting accumulation of the valley and spin polarization is developed for ultraclean channels made of two-dimensional semiconductors where the electron mean free path due to the residual disorder or phonons exceeds the channel width. Both ballistic and hydrodynamic regimes of the electron transport are studied. The polarization accumulation is determined by interplay of the anomalous velocity, side-jump and skew scattering effects. In the hydrodynamic regime, where the electron–electron scattering is dominant, the valley and spin current generation and dissipation by the electron–electron collisions are taken into account. The accumulated polarization magnitude and its spatial distribution depend strongly on the transport regime. The polarization is much larger in the hydrodynamic regime as compared to the ballistic one. Significant valley and spin polarization arises in the immediate vicinity of the channel edges due to the side-jump and skew scattering mechanisms.

Modeling of electrotransport properties of Li-intercalated graphene film

In this work, within the framework of density functional theory combined with the method of nonequilibrium Green’s functions the density of states, transmission spectrum, current-voltage characteristics, and differential conductivity of Li-intercalated graphene (LiC6) have been determined. It is shown that in the energy range of -1.3÷-1.05 eV the quasiparticle transport through the nanostructure is disable. The features of IV- and dI/dV-characteristics of LiC6 in the form of decreasing of resistance in the range of -0.4÷0.4 V were revealed, and in the interval of 0.4÷1.4 V formation of negative differential resistance area, related to scattering of quasiparticles. It is established, that LiC6 nanodevice has 12÷13 ballistic channels and has the maximum amount of conductance 12÷13G0 , where Go is the conductance quantum.

(Invited) Experimental Quantum Transport in Strained Graphene and SWCNTs - Towards Noems

We measure ballistic charge conductivity in strained suspended graphene and observe the theoretically predicted [1] strain-induced scalar and vector potentials. To do so, we built an experimental platform for in-situ quantum transport strain engineering in 2D materials. This instrumentation also permits low temperature (0.3 K) transport in 0 to 9 Tesla magnetic fields. The tunable uniaxial strain (up to 3%) is completely decoupled from the gate-tunable charge density, permitting quantitative understanding of strain effects. We show slippage-free mechanical clamping of high aspect-ratio graphene crystals, where atomically ordered edges are unnecessary for quantitative straintronics. We study in detail transport in a ballistic graphene channel whose length is 90 nm and width is 600 nm. By applying strain up to 0.6 % in this device, we observe that the strain-induced scalar potential shifts its low energy band structure downward by up to 30 meV. We show precise control of the gauge vector potential which reversibly suppresses the conductance. We discuss our progress towards strain engineering in SWCNTs, and calculations showing its potential in valley filtering and tuning electron-hole asymmetric quantum transistors.We conclude with an overview of another project to integrate suspended bilayer graphene devices in planar optical cavities towards achieving fully tunable nano-opto-electro-mechanical (NOEMS) carbon systems. To do so, we discuss a nitrocellulose-based dry-stamping method to manipulate and suspend individual 2D materials [2]. Using this method we assembled optical cavities able to a widely tune Raman scattering and absorption in graphene. [1] A. C. McRae, G. Wei, and A. R. Champagne, Phys. Rev. Applied 11, 054019 (2019). [2] I. G. Rebollo, F. C. Rodrigues-Machado, W. Wright, G. J. Melin, A. R. Champagne, ArXiv:2011.14166 Figure 1

Simulation of low-noise amplifier with quantized ballistic nanowire channel

The concept of a cryogenic low-noise amplifier (LNA) using quantized conduction in a 1D channel is proposed and explored through simulations. A MOSFET with a 1D channel (nanowire) will exhibit the formation of plateaus in the current due to sub-band quantization at temperatures of below about 100 K and low drive bias. Using a ballistic 1D top-of-the-barrier model, we show that the current plateaus significantly enhance the transconductance, enabling low noise and high gain at low DC power dissipation—key characteristics for LNA applications such as quantum computing. The results indicate that cryogenic 1D ballistic MOSFETs may offer a significant improvement in noise performance and power dissipation (40x reduction) over traditional devices for cryogenic applications with low input power, such as quantum computing.

(Invited) Experimental Quantum Transport in Strain Engineered Graphene - Towards NOEMS

We measure ballistic charge conductivity in strained suspended graphene and observe the theoretically predicted [1] strain-induced scalar and vector potentials. To do so, we built an experimental platform for quantum transport strain engineering in 2D materials. This instrumentation permits low temperature (0.3 K- 70 K) transport in 0 to 9 Tesla magnetic fields. The tunable uniaxial strain (up to 3%) is completely decoupled from the gate-tunable charge density, permitting quantitative understanding of strain effects. We show slippage-free mechanical clamping of high aspect-ratio graphene crystals, where atomically ordered edges are unnecessary for quantitative straintronics. We study in detail transport in a ballistic graphene channel whose length is 90 nm and width is 600nm. By applying strain up to 0.6 % in this device, we observe that the strain-induced scalar potential shifts its low energy band structure downward by up to 30 meV. We show precise control of the gauge vector potential which reversibly suppresses the conductance by up to 14 %.We conclude with a brief overview of other projects aimed at integrating both RF dynamic strain control and optically resonant cavities in suspended bilayer graphene devices to achieve fully tunable nano-opto-electro-mechanical (NOEMS) carbon systems. We report for instance a nitrocellulose-based dry-stamping method to manipulate and suspend individual 2D materials. Using this method we assembled optical cavities able to a widely tune light absorption in graphene. [1] A. C. McRae, G. Wei, and A. R. Champagne, Phys. Rev. Applied 11, 054019 (2019) Figure 1

Nonequilibrium physics in integrable systems and spin-flip non-invariant conserved quantities

Recently found spin-flip non-invariant (SFNI) conserved quantities play important roles in discussing nonequilibrium physics of the XXZ model. The representative examples are the generalized Gibbs ensemble (GGE) and the ballistic transport of the spin current. In spite of big progress in understanding nonequilibrium physics of integrable systems, the general framework to determine a minimal complete set of conserved quantities which describes the long-time steady state has not yet been found. This paper shows that the GGE of the gapless XXZ model consists of functionally independent conserved quantities rather than linearly independent. At the same time, the physical meaning of SFNI conserved quantities is provided. We also discuss that there exist ballistic channels of the spin current supported by non-quasilocal conserved quantities. The saturation of the lower bound for the Drude weight by quasilocal conserved quantities reads the linear dependence of non-quasilocal conserved quantities on quasilocal ones. We show that their (generalized) linearly dependence relation is consistent with the statement that the GGE consists of functionally independent conserved quantities without containing all linearly independent conserved quantities.

One-step fabrication of 1D p-NiO nanowire/n-Si heterojunction: Development of self-powered ultraviolet photodetector

Even though there are reports on NiO-p/Si-n based photodetector, work along 1D NiO/Si self-powered photodetector remains unexplored. 1D materials offer better electron mobility and can provide ballistic transport channel, which helps in achieving high responsivity. In this work, direct single-step deposition of p-type NiO nanowires on n-type Si as a self-powered photodetector was successfully fabricated by engaging simple electrospinning technique. The structural properties as observed from XRD indicate highly crystalline NiO nanowires and FESEM images revealed 60-75 nm diameter with uniform distribution. Growth of nanowires by using stimuli polymer and its completely removal at high temperature was confirmed using TGA and XPS analysis. The responsivity of the fabricated photodetector was calculated to be 9.1 mA W−1 at zero bias, which can be attributed to the p-type NiO interface with the n-type Silicon, which creates an internal electrical field thereby assisting in the effective separation of the photogenerated carriers. Further, under illumination, at zero bias, the photogenerated current still exists suggesting a generation of internal voltage, which makes the fabricated device as self-powered. This fabrication method will enhance the photodetection properties, and it can be implemented in the fields of optoelectronic devices, sensors, and flexible electronics.

Observation of room temperature electronic localization through a single graphene layer on sapphire

A He+ ion beam is used to induce electronic localization, on a single graphene layer exfoliated on an ultra-flat sapphire substrate, based on crystalline defects and with a lateral irradiation precision of 1 nm. Inducing a 1% carbon defect density by step by step irradiation of a 100-nm-wide band on the supported graphene increases its electrical resistance from 0.90 to 133 kΩ. The resistance build-up was monitored in situ and in real time by measuring the I(t) current intensity through the graphene monolayer flake during its irradiation. The whole process takes place on an ultra-polished sapphire surface used to retain the planarity of the graphene. We propose that local heating of the graphene by irradiation promotes the migration of the created atomic carbon defects to the edge of the flake. This inherently moderates the increase of resistance with time during He+ irradiation due to the gradual re-opening of low-voltage graphene ballistic channels.

Magnetic-field-induced transport asymmetry in two-terminal circular ballistic bends

A recent experiment [Hortelano et al., Semicond. Sci. Technol. 32, 125005 (2017)] reported a rectification effect that appeared in curved narrow ballistic channels of a two-dimensional electron gas when the strength of a magnetic field applied to the channels was tuned. The phenomenon was reproduced by classical billiard simulations as resulting from a transmission asymmetry caused by diffuse boundary scattering. However, this manifests breakdown of a commonly used simple model for diffuse boundary since the magnetic-field dependence of the transmission in two-terminal geometries has to be symmetric in equilibrium. We demonstrate here that this tendency of the system predicted by the billiard simulations is a real transmission asymmetry effect that emerges in the nonequilibrium transport. We perform nonequilibrium quantum-mechanical simulations with taking into account Coulomb repulsion. Experimental observations are presented to demonstrate consistencies with the numerical results.