Ballistic Transport Research Articles

Hall-Bulk photovoltaic effect in BiFeO3/SrTiO3 at low temperatures

The bulk photovoltaic effect is considered promising for the next generation of solar cells as it could exceed the Shockley-Queisser limit. Studies have shown many materials have the bulk photovoltaic effect, however, the inner properties such as carrier mobility, relaxation time, etc., have been barely reported, largely hindering further understanding and utilization of the bulk photovoltaic effect. In this paper, we employ a Hall effect to study the transport properties of the bulk photovoltaic effect in a BiFeO3/SrTiO3 heterostructure. We interpret the magnetic field-dependent bulk photovoltaic currents using the fundamental Lorentz force law acting on photocarriers. A carrier transport with ~3 ps relaxation time, a mobility of 3.3times {10}^{3},{{{{{{rm{cm}}}}}}}^{2}{{{{{{rm{V}}}}}}}^{-1}{{{{{{rm{s}}}}}}}^{-1} for the non-thermalised carrier and a mean free path larger than 50 nm are revealed. This is in good agreement with the ballistic transport mechanism. This work provides a generic approach to acquiring the basic characteristics of the bulk photovoltaic effect.

Bouncing Spallation Bombs During the 2021 La Palma Eruption, Canary Islands, Spain

Incandescent pyroclasts of more than 64 mm in diameter erupted from active volcanoes are known as bombs and pose a significant hazard to life and infrastructure. Volcanic ballistic projectile hazard assessment normally considers fall as the main transport process, estimating its intensity from bomb location and impact cratering. We describe ballistically ejected bombs observed during the late October 2021 episode of eruption at La Palma (Canary Islands) that additionally travelled downhill by rolling and bouncing on the steep tephra-dominated cone. These bouncing bombs travelled for distances >1 km beyond their initial impact sites, increasing total travel distance by as much as 100%. They left multiple impact craters on their travel path and frequently spalled incandescent fragments on impact with substrate, leading to significant fire hazard for partially buried trees and structures far beyond the range of ballistic transport. We term these phenomena as bouncing spallation bombs. The official exclusion zone encompassed this hazard at La Palma, but elsewhere bouncing spallation bombs ought to be accounted for in risk assessment, necessitating awareness of an increased hazard footprint on steep-sided volcanoes with ballistic activity.

Ballistic transport of interacting Bose particles in a tight-binding chain.

It is known that the quantum transport of noninteracting Bose particles across a tight-binding chain is ballistic in the sense that the current does not depend on the chain length. We address the question whether the transport of strongly interacting bosons can be ballistic as well. We find such a regime and show that, classically, it corresponds to the synchronized motion of local nonlinear oscillators. It is also argued that, unlike the case of noninteracting bosons, the transporting state responsible for the ballistic transport of interacting bosons is metastable, i.e., the current decays in the course of time. An estimate for the decay time is obtained.

From dissipationless to normal diffusion in the easy-axis Heisenberg spin chain

The anomalous spin diffusion of the integrable easy-axis Heisenberg chain originates in the ballistic transport of symmetry sectors with nonzero magnetization. Ballistic transport is replaced by normal dissipative transport in all magnetization sectors upon introducing the integrability-breaking perturbations, including external driving. Such behavior implies that the diffusion constant obtained for the integrable model is relevant for the spread of spin excitations but not for the spin conductivity. We present numerical results for closed systems and driven open systems, indicating that the diffusion constant shows a discontinuous variation as the function of perturbation strength.

Spin Hall conductivity of interacting two-dimensional electron systems

We consider a two-dimensional electron system subjected to a short-ranged nonmagnetic disorder potential, Coulomb interactions, and Rashba spin-orbit coupling. The path-integral approach incorporated within the Keldysh formalism is used to derive the kinetic equation for the semiclassical Green's function and applied to compute the spin current within the linear response theory. We discuss the frequency dependence of the spin Hall conductivity and further elucidate the role of electron interactions at finite temperatures for both the ballistic and diffusive regimes of transport. We argue that interaction corrections to the spin Hall effect stem from the quantum interference processes whose magnitude is estimated in terms of parameters of the considered model.

Half-Metallic Heusler Alloy/MoS2 Based Magnetic Tunnel Junction.

Integration of half-metallic materials and 2D spacers into vertical magnetoresistive spin valves may pave the way for effective low-power consumption storage and memory technologies. Driven by the recent successful growth of graphene/half-metallic Co2Fe(Ge1/2Ga1/2) (CFGG) heterostructure, here we report a theoretical investigation of magnetic tunnel junction (MTJ) based on the ferromagnetic CFGG Heusler alloy and the MoS2 spacer of different thicknesses. Using ab initio approach, we demonstrate that the inherent ferromagnetism of CFGG is preserved at the interface, while its half-metallicity is recovering within few atomic layers. Ballistic transport in CFGG/MoS2/CFGG MTJ is studied within the nonequilibrium Green's function formalism, and a large magnetoresistance value up to ∼105% is observed. These findings support the idea of effective spintronics devices based on half-metallic Heusler alloys and highly diversified transition metal dichalcogenide family.

High-performance self-driven near-infrared photodetector based on single-walled carbon nanotube/graphene/Al2O3/InP

Carbon nanotube (CNT) is one of the typical materials for near-infrared photodetectors. Combining its ultra-high mobility, excellent infrared absorption, and suitable energy band structure with graphene and semiconductors, it greatly improves the photodetector performance. This paper reports a self-driven near-infrared photodetector of single-walled carbon nanotubes (SWCNTs)/single-layer graphene/Al 2 O 3 /P-type InP structure, the photocurrent of the device is greatly improved with the help of SWCNTs, and the Al 2 O 3 layer improves the Schottky barrier between InP and graphene, which can suppress the carrier recombination on the semiconductor surface. This device structure exhibits a highly sensitive photoresponse at 808 nm, and at zero under the bias condition, the responsivity reaches 154.3 mA·W -1 , and the detection rate reaches 1.27 × 10 12 Jones. It has a fast response time of microsecond level, and can meet the detection requirements in the mid-frequency range. This result is expected to provide ideas for further optimization on single-walled carbon nanotubes in near-infrared photodetectors. • A monolayer graphene/InP Schottky junction near-infrared photodetector with SWCNTs is fabricated. • The combination of SWCNTs and graphene greatly improves the responsivity, detectivity, and quantum efficiency of the device. • The interface passivation technology of Al 2 O 3 effectively reduces dark current. • The ballistic transport mechanism of SWCNTs enables the device with microsecond response time.

Study on interfacial thermal resistance between single wall carbon nanotubes and nickel by molecular dynamics

The interfacial thermal resistance of the nanocontact system of carbon nanotubes and nickel crystals was investigated using molecular dynamics. It was found that with the increase in temperature, the interface thermal resistance gradually increased. In addition, the interfacial thermal resistance also increases gradually with the increase of the contact distance. The ballistic transport of phonons is proposed to be the main reason for the interfacial thermal resistance in this case.

Band Structure and Quantum Conductance of Surface-unsaturated and Hydrogenated Sb and Bi Monolayer Nanoribbons

Electronic structure and quantum conductance of surface-unsaturated and hydrogenated Sb and Bi monolayer nanoribbons are theoretically investigated by first-principles calculations combined with non-equilibrium Green’s function method. Band structures, electronic transmission spectra and current-voltage curves of these Sb and Bi monolayer derived nanoribbons along zigzag crystallographic orientations are calculated to explore their potential applications in topological nanoelectronics. It is verified that extremely high conductivity under low bias voltage is acquired from the scattering-forbidden topological edge-states of these nanoribbons, as indicated by Dirac-point-like energy dispersion of band-edges near Fermi level, which also provides an evident negative differential conductance under 0.2 ∼ 0.3 V voltage bias when the ballistic conductance peak at Fermi level shifting out of bias window. The present study suggests Sb and Bi monolayers after acquiring chemical stability by hydrogenation are prospective candidates to be applied for ultrahigh power and zero-loss nanotransistors.

Quantum transport of strongly interacting fermions in one dimension far out of equilibrium

In the study of quantum transport, much has been known about dynamics near thermal equilibrium. However, quantum transport far away from equilibrium is much less well understood---the linear response approximation does not hold for physics far out of equilibrium in general. In this work, motivated by recent cold atom experiments on probing quantum many-body dynamics of a one-dimensional XXZ spin chain, where a transition from ballistic to diffusive dynamics has been established by increasing the interaction strengths, we study the strong interaction limit of the one-dimensional spinless fermion model, which is dual to the XXZ spin chain. We develop a highly efficient computation algorithm for simulating the nonequilibrium dynamics of this system exactly, and examine the nonequilibrium dynamics starting from a density modulation quantum state. We find ballistic transport in this strongly correlated setting and show that a plane-wave description emerges at long-time evolution. We also observe a sharp distinction between transport velocities in short and long times as induced by interaction effects and provide a quantitative interpretation for the long-time transport velocity. We expect our results to shed light on the understanding of the dynamics of the XXZ spin chain in the strong interaction regime.

DESIGN OF LOW POWER HIGH SPEED CNTFET ADDER SUBTRACTOR

Multi-valued logic design itself is not sufficient to achieve the performance improvement in digital system design in nanotechnology. The need to replace silicon devices with some more efficient new devices are required so as to meet the power improvements in the nanoscale region. Among various devices explored, CNTFET is the most promising alternative to replace conventional MOS transistors. These nano devices have the benefits of low power consumption due to reduced leakage component, ballistic transport operation even at low supply voltages makes them suitable for high frequency and low voltage applications. MVL circuitry has soared in popularity because the threshold voltage of the CNFET device is somewhat adjustable by the diameter of carbon nanotubes. CNFET is used for high performance, high stability and low-power circuit designs as an alternative material to silicon in recent years. Carbon Nanotube Filed-Effect Transistor (CNFET) is one of the promising alternatives to the MOS transistors. The geometry- dependent threshold voltage is one of the CNFET characteristics, which is used in the proposed design. In this paper, we present a novel high speed Adder-subtractor cell using CNFETs based on XOR gates and multiplexer. Proposed design uses fourteen transistors, ten for full adder and four to modify the cell for subtraction. Simulation results show significant improvement in terms of delay and area saving with respectively compared to the latest design. Simulations were carried out using HSPICE based on CNFET model with optimized design parameters. All designs are simulated at 32nm CNFET with HSPICE. The simulation results show that on average, speed enhancement and area saving of 48% and 11% can be achieved with optimized parameters design over default values of these parameters. The cumulative benefits of the novel adder-subtractor design based CNFET result in an PDP reduction by a factor of 41%.

High Responsivity Vacuum Nano-Photodiode Using Single-Crystal CsPbBr3 Micro-Sheet.

Field electron emission vacuum photodiode is promising for converting free-space electromagnetic radiation into electronic signal within an ultrafast timescale due to the ballistic electron transport in its vacuum channel. However, the low photoelectric conversion efficiency still hinders the popularity of vacuum photodiode. Here, we report an on-chip integrated vacuum nano-photodiode constructed from a Si-tip anode and a single-crystal CsPbBr3 cathode with a nano-separation of ~30 nm. Benefiting from the nanoscale vacuum channel and the high surface work function of the CsPbBr3 (4.55 eV), the vacuum nano-photodiode exhibits a low driving voltage of 15 V with an ultra-low dark current (50 pA). The vacuum nano-photodiode demonstrates a high photo responsivity (1.75 AW-1@15 V) under the illumination of a 532-nm laser light. The estimated external quantum efficiency is up to 400%. The electrostatic field simulation indicates that the CsPbBr3 cathode can be totally depleted at an optimal thickness. The large built-in electric field in the depletion region facilitates the dissociation of photoexcited electron-hole pairs, leading to an enhanced photoelectric conversion efficiency. Moreover, the voltage drop in the vacuum channel increases due to the photoconductive effect, which is beneficial to the narrowing of the vacuum barrier for more efficient electron tunneling. This device shows great promise for the development of highly sensitive perovskite-based vacuum opto-electronics.

Bi2O2Se nanowires presenting high mobility and strong spin–orbit coupling

Systematic electrical transport characterizations were performed on high-quality Bi2O2Se nanowires to illustrate its great transport properties and further application potentials in spintronics. Bi2O2Se nanowires synthesized by chemical vapor deposition method presented a high field-effect mobility up to ∼1.34 × 104 cm2 V−1 s−1 and exhibited ballistic transport in the low back-gate voltage (Vg) regime where conductance plateaus were observed. When further increasing the electron density by increasing Vg, we entered the phase coherent regime and weak antilocalization (WAL) was observed. The spin relaxation length extracted from the WAL was found to be gate tunable, ranging from ∼100 nm to ∼250 nm and reaching a stronger spin–obit coupling (SOC) than the two-dimensional counterpart (flakes). We attribute the strong SOC and the gate tunability to the presence of a surface accumulation layer, which induces a strong inversion asymmetry on the surface. Such scenario was supported by the observation of two Shubnikov–de Haas oscillation frequencies that correspond to two types of carriers, one on the surface and the other in the bulk. The high-quality Bi2O2Se nanowires with a high mobility and a strong SOC can act as a very prospective material in future spintronics.

Ballistic Thermal Transport at Sub-10nm Laser-Induced Hot Spots in GaN Crystal.

Ballistic thermal transport at nanoscale hotspots will greatly reduce the performance of a Gallium nitride (GaN) device when its characteristic length reaches the nanometer scale. In this work, the authors develop a tip-enhanced Raman thermometry approach to study ballistic thermal transport within the range of 10nm in GaN, simultaneously achieving laser heating and measuring the local temperature. The Raman results show that the temperature increase from an Au-coated tip-focused hotspot up to two times higher (40 K) than that in a bare tip-focused region (20 K). To further investigate the possible mechanisms behind this temperature difference, the authors perform electromagnetic simulations to generate a highly focused heating field, and observe a highly localized optical penetration, within a range of 10nm. The phonon mean free path (MFP) of the GaN substrate can thus be determined by comparing the numerical simulation results with the experimentally measured temperature increase which is in good agreement with the average MFP weighted by the mode-specific thermal conductivity, as calculated from first-principles simulations. The results demonstrate that the phonon MFP of a material can be rapidly predicted through a combination of experiments and simulations, which can find wide application in the thermal management of GaN-based electronics.

Nanoscale Mapping of Carrier Mobilities in the Ballistic Transports of Carbon Nanotube Networks.

Much progress has been made in the nanoscale analysis of nanostructures, while the mapping of key charge transport properties such as a carrier mobility remains a challenge, especially for one-dimensional systems. Here, we report the nanoscale mapping of carrier mobilities in carbon nanotube (CNT) networks and show that charge transport behaviors varied depending on network structures. In this work, the spatial distribution of localized charge transport properties such as mobilities and charge trap densities in CNT networks were mapped via a scanning noise microscopy. The mobility map was obtained from the conductivity maps measured at different back-gate biases, showing up to two orders of mobility variations depending on localized network structures. Furthermore, from the maps, correlations between mobility/conductivity and charge trap density were analyzed to determine charge transport mechanisms. In metallic CNT networks, the regions with rather high (low) or low (high) charge trap densities (mobilities) exhibited a diffusive or ballistic transport behavior, respectively. Interestingly, semiconducting CNT networks also exhibited a gradual transition from a diffusive to a ballistic transport behavior as the CNT mobility was increased by reaching the on-state with negative gate biases. The mapping of the cross-patterned CNT network showed that metallic CNT electrodes could achieve a good electrical contact with semiconducting CNTs without high contact resistance regions. Since this method allowed one to map versatile charge transport properties such as mobility, conductivity, and charge trap density, it can be a powerful tool for basic research about charge transport phenomena and practical device applications.

Spin-valley polarized transport in a field-controllable bilayer silicene superlattice

We have theoretically investigated the spin-valley asymmetric transport of massive Dirac fermions in the field-controllable bilayer silicene superlattices. The spin-valley dependent ballistic transmission, conductance, and polarization have been systematically calculated by formulating the scattering matrix method for the completed four-band low-energy effective Hamiltonian. Our results uncover that for a single valley transport, near-perfect spin polarization and its perfect switching could be efficiently modulated by the gate field engineering. Under the one-dimensional periodic field modulation, two types of flat bands with less dispersion and, importantly, the perfect contrast in the spin-dependent subbands are observed for the bilayer silicene superlattice. Together with its larger spin-orbit coupling and better stability, these spin-valley asymmetric characteristics engineered by the gate field indicate that the field-controllable bilayer silicene could be a potential component candidate to achieve a fully spin-valley polarized beam for quantum logic applications.

Determination of tunnelling magneto resistance of magnetic tunnel junction designed using Co2TiAl Heusler alloy with MgO spacer layer

A magnetic tunnel junction (MTJ) is an integral component of spintronic devices like complex magnetic sensor and magnetic random access memory (MRAM) etc. This significance motivated to design an MTJ using Heusler alloy Co2TiAl and MgO as barrier layer. In order to check the suitability of Heusler alloy its structural, electronic and magnetic characteristics are investigated. Band structures and density of states are determined by GGA and GGA + mBJ approaches. Curie temperature is assessed using mean field approximation (MFA). After confirming the suitability, three different MTJs are designed and calculations are carried out using Quantum Espresso (QE) code. The tunnelling magneto-resistance (TMR) is determined through ballistic conductance for both spins (up and down) in parallel and antiparallel configurations. The calculated values of TMR are reasonably high and comparable with literature. It is envisaged that the findings of this work will be helpful in the fabrication of MTJ for spintronic devices.

Charge filling induced quasi-two dimensional charge/spin density wave channel in a ballistic transport device

Charge filling controlled mean field metal–insulator phase transition is examined in the context of two dimensional Fermi surface nesting and van Hove singularity induced charge density wave (CDW), spin density wave (SDW) condensates. In the framework of a coherent ballistic transport model utilizing the Non-Equilibrium Green Function approach (NEGF), a three terminal device with metallic gate, source, drain and CDW/SDW channel is simulated and studied. Within the validity of mean field approximation, we exposit the commensurability and boundary effects. The efficacy of the Hubbard model for (quasi) two dimensional Charge and Spin Density Wave materials is discussed. A two orbital generalization of the effective Hamiltonian is proposed for transport calculations in rare earth Tellurides RTe 3.

Ballistic guided electrons against disorder in graphene nanoribbons

Graphene nanoribbons (GNRs) are natural waveguides for electrons in graphene. Nevertheless, unlike micrometer-sized samples, conductance is nearly suppressed in these narrow graphene stripes, mainly due to scattering with edge disorder generated during synthesis or cut. A possible way to circumvent this effect is to define an internal waveguide that isolates specific modes from the edge disorder and allows ballistic conductance. There are several proposals for defining waveguides in graphene; in this manuscript, we consider strain folds and scalar potentials and numerically evaluate these proposals’ performance against edge and bulk disorder. Using the Green’s function approach, we calculate conductance and the local density of states of zigzag GNRs and characterize the performance of these different physical waveguiding effects in both types of disorders. We found a general improvement in the electronic conductance of GNR due to the presence of the internal waveguiding, with the emergence of plateaus with quasi-ballistic properties and robustness against edge disorder. These findings are ready to be applied in modern nanotechnology and are being experimentally tested.

Multiterminal epitaxial tungsten nanostructures on MgO/GaAs(001) substrates: Temperature effects in ballistic electron transport

High-quality single-crystalline multiterminal tungsten nanostructures were fabricated on MgO/GaAs (001) substrates using subtractive lithography. Single-crystalline tungsten films with a thickness of d = 80 nm and low roughness were grown using sequential epitaxy of MgO (001) and W (001) layers on GaAs (001) via pulsed laser deposition. The temperature dependence of bridge-type nanostructure electron conductivity indicates that they are high-quality metal conductors. The electron mean free path reached 760 nm at low temperatures and was approximately an order of magnitude greater than the tungsten film thickness. Strong non-local effects resulting from ballistic electron transport were observed in the multiterminal cross-type W (001) nanostructures with an arm width Wc = 400 nm below T = 80 K. Such effects can be explained by the exponential damping of ballistic properties of nanostructures as a function of the electron mean free path in the wide temperature range 4.2–100 K. Simulations predict that the ballistic effects in such nanostructures can be significant even at room temperature with an arm width approaching 10 nm and a size ratio of Wc/d ∼ 1.