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We report dynamic magnetotransport measurements in a ballistic quantum point contact (QPC) at gigahertz frequencies. We observed a gate-controlled crossover of the magnetoadmittance from the partially diffusive boundary scattering effect to weak localization. Moreover, the charge relaxation time of this mesoscopic circuit shows similar behaviors, indicating that the density of states of QPC channels is also sensitively adjusted by the boundary scattering in addition to their transmission. Our experiment demonstrates that boundary scattering plays a dominant role in the dynamic transport in a ballistic QPC and could influence the coherent dynamics in other QPC-based phase-coherent systems such as the quantum dots and qubits. [ABSTRACT FROM AUTHOR]

The interactions of vortex domain walls with corners in planar magnetic nanowires are probed using magnetic soft X-ray transmission microscopy. We show that when the domain walls are propagated into sharp corners using applied magnetic fields above a critical value, their chiralities are rectified to either clockwise or anticlockwise circulation depending on whether the corners turn left or right. Single-shot focused magneto-optic Kerr effect measurements are then used to demonstrate how, when combined with modes of domain propagation that conserve vortex chirality, this allows us to dramatically reduce the stochasticity of domain pinning at artificial defect sites. Our results provide a tool for controlling domain wall chirality and pinning behavior both in further experimental studies and in future domain wall-based memory, logic and sensor technologies. [ABSTRACT FROM AUTHOR]

We consider a ballistic detector formed in an interferometer manner which operational principle relies on Josephson vortex scattering at a measurement potential. We propose an approach to symmetrize the detector scheme and explore arising advantages in the signal-to-noise ratio and in the back-action on a measured object by means of recently presented numerical and analytical methods for modeling of a soliton scattering dynamics in the presence of thermal fluctuations. The obtained characteristics for experimentally relevant parameters reveal practical applicability of the considered schemes including possibility of coupling with standard digital rapid single flux quantum circuits. [ABSTRACT FROM AUTHOR]

Room temperature negative differential resistance is demonstrated in a unipolar GaN-based tunneling hot electron transistor. Such a device employs tunnel-injected electrons to vary the electron energy and change the fraction of reflected electrons, and shows repeatable negative differential resistance with a peak to valley current ratio of 7.2. The device was stable when biased in the negative resistance regime and tunable by changing collector bias. Good repeatability and double-sweep characteristics at room temperature show the potential of such device for high frequency oscillators based on quasi-ballistic transport. [ABSTRACT FROM AUTHOR]

It is demonstrated how the correction factor α used in Hall magnetometry of localized magnetic field profiles depends on the sample geometry and the electron mean free path, in the quasiballistic and ballistic regimes. The frequently used approximation of a constant correction factor close to 1 is generally not justified, and may lead to large errors in the determination of the magnitude of the magnetic fields. α depends in a nontrivial way on the magnetic structure and the Hall cross and may vary considerably. The sensitivity peaks when the characteristic cyclotron diameter equals the size of the Hall cross. [ABSTRACT FROM AUTHOR]

We consider a magnetic moment with an easy axis anisotropy energy, switched by an external field applied along this axis. Additional small, time-independent bias field is applied perpendicular to the axis. It is found that the magnet's switching time is a non-monotonic function of the rate at which the field is swept from 'up' to 'down.' Switching time exhibits a minimum at a particular optimal sweep time. This unusual behavior is explained by the admixture of a ballistic (precessional) rotation of the moment caused by the perpendicular bias field in the presence of a variable switching field. We derive analytic expressions for the optimal switching time, and for the entire dependence of the switching time on the field sweep time. The existence of the optimal field sweep time has important implications for the optimization of magnetic memory devices. [ABSTRACT FROM AUTHOR]

Ballistic electron transport at nanoscale dimensions is investigated and exploited in a nanoscale three-terminal, all-electrical spintronic semiconductor device. Charge current cancellation under appropriate device biasing yields a large, spin-dependent current signal even with modest spin injection efficiency into the semiconductor, while reliance on ballistic, rather than diffusive, carrier transport is expected to enable robust scalability to smaller dimensions. Magnetocurrent in excess of 200% is measured with spin injection efficiency of 5%, and a spin-dependent ballistic carrier transport model is shown to yield accurate, quantitative predictions of current-voltage behavior. [ABSTRACT FROM AUTHOR]

In this work we show that junctionless nanowire transistor (JNT) exhibits lower degree of ballisticity in subthreshold and higher ballisticity above threshold compare to conventional inversion-mode transistors, according to quantum mechanical simulations. The lower degradation of the ballisticity above threshold region gives the JNT near-ballistic transport performance and hence a high current drive. On the other hand, lower ballisticity in subthreshold region helps reducing the off-current and improves the subthreshold slope. A three-dimensional quantum mechanical device simulator based on the nonequilibrium Green's function formalism in the uncoupled mode-space approach has been developed to extract the physical parameters of the devices. [ABSTRACT FROM AUTHOR]

We show that the tilt angle of nanostructures obtained by glancing angle sputtering is finely tuned by selecting the adequate argon pressure. At low pressures, a ballistic deposition regime dominates, yielding high directional atoms that form tilted nanocolumns. High pressures lead to a diffusive regime which gives rise to vertical columnar growth. Monte Carlo simulations reproduce the experimental results indicating that the loss of directionality of the sputtered particles in the gas phase, together with the self-shadowing mechanism at the surface, are the main processes responsible for the development of the columns. [ABSTRACT FROM AUTHOR]

A band-to-band tunneling field-effect transistor (FET) can achieve a subthreshold slope steeper than 60 mV/dec at room temperature, but the on-current is low due to existence of the tunneling barrier. Graphene has a monolayer-thin body which is amenable to strain. By using self-consistent quantum transport simulations, we show that with local strain applied at the tunneling junction between the source and the channel in a graphene nanoribbon tunneling FET, the on-current can be significantly improved by over a factor of 10 with the same off-current, no matter at the ballistic limit or in the presence of inelastic phonon scattering. [ABSTRACT FROM AUTHOR]

Local phase transition from ferromagnetic to paramagnetic state in the region of the ballistic Ni nanocontacts (NCs) has been experimentally observed. We found that contact size reduction leads to an increase in the bias voltage at which the local phase transition occurs. Presented theoretical interpretation of this phenomenon takes into account the specificity of the local heating of the ballistic NC and describes the electron’s energy relaxation dependences on the applied voltage. The experimental data are in good qualitative and quantitative agreement with the theory proposed. [ABSTRACT FROM AUTHOR]

Ballistic electron emission microscopy is utilized to investigate the hot-electron scattering properties of Cu through Cu/Si(001) Schottky diodes. A Schottky barrier height of 0.64±0.02 eV and a hot-electron attenuation length of 33.4±2.9 nm are measured at a tip bias of 1.0 eV and a temperature of 80 K. The dependence of the attenuation length with tip bias is fit to a Fermi liquid model that allows extraction of the inelastic and elastic scattering components. This modeling indicates that elastic scattering due to defects, grain boundaries, and interfaces is the dominant scattering mechanism in this energy range. [ABSTRACT FROM AUTHOR]

We report on the observation of inertial-ballistic transport in nanoscale cross junctions fabricated from epitaxial graphene grown on SiC(0001). Ballistic transport is indicated by a negative bend resistance of R12,43≈-170 Ω, which is measured in a nonlocal, four-terminal configuration at 4.2 Κ and which vanishes as the temperature is increased above 80 K. [ABSTRACT FROM AUTHOR]

We demonstrate that in graphitic nanocarbon materials, combination of ballistic conductivity and strong electron photon coupling opens a unique opportunity to observe transfer of momentum of the electromagnetic radiation to free carriers. The resulting drag of quasiballistically propagating electrons can be employed, in particular, to visualize the temporal profile, polarization, and propagation direction of the laser pulse. In this letter, we report the giant photon drag effect in yarns made of multiwall carbon nanotubes. [ABSTRACT FROM AUTHOR]

We investigate ballistic transport and quantum interference in a nanoscale quantum wire loop fabricated as a GaAs/AlGaAs field-effect heterostructure. Four-terminal measurements of current and voltage characteristics as a function of top gate voltages show negative bend resistance as a clear signature of ballistic transport. In perpendicular magnetic fields, phase-coherent transport leads to Aharonov–Bohm conductance oscillations, which show equal amplitudes in the local and the nonlocal measurement at a temperature of 1.5 K and above. We attribute this observation to the symmetry of the orthogonal cross junctions connecting the four quantum wire leads with the asymmetric quantum wire ring. [ABSTRACT FROM AUTHOR]

We studied ballistic transport across a quantum point contact (QPC) defined in a high-quality GaAs (311)A two-dimensional hole system using shallow etching and top gating. The QPC conductance exhibits up to 11 quantized plateaus. The ballistic one-dimensional subbands are tuned by changing the lateral confinement and the Fermi energy of the holes in the QPC. We demonstrate that the positions of the plateaus (in gate voltage), the source-drain data, and the negative magnetoresistance data can be understood in a simple model that takes into account the variation, with gate bias, of the hole density and the width of the QPC conducting channel. [ABSTRACT FROM AUTHOR]

In periodic graphene structures formed by mechanical undulation or by patterning an array of metallic electrodes on graphene, Bloch oscillations up to tens of terahertz can be generated due to ballistic transport and the low scattering rate. [ABSTRACT FROM AUTHOR]

In this letter, the channel backscattering characteristics and ballistic efficiency of gate-all-around silicon nanowire transistors (SNWTs) are experimentally investigated. A modified experimental extraction methodology for SNWTs is proposed, which takes into account the impact of contact resistance. It was found that without this correction, the characterized results will severely overestimate the channel backscattering in SNWTs due to the temperature sensitivity of the source contact resistance. The highest ballistic efficiency is observed in sub-40-nm SNWTs due to their quasi-one-dimensional properties, demonstrating their potential as an alternative device structure for near-ballistic transport from top-down approach. [ABSTRACT FROM AUTHOR]

In this paper, we focus on a graphene heterojunction device: a Z-shaped graphene nanoribbon, which consists of two armchair leads and a zigzag junction. Based on the Landauer–Büttiker formula and the tight binding model, we found that the rectifying behavior can be achieved by applying an external gate voltage in the heterjunction region. We also found that the rectification effect is independent of junction width and length, it is an intrinsic property of the Z-junction graphene nanoribbon. This platform can be used to design and study functional graphene nanoscale devices. [ABSTRACT FROM AUTHOR]

The authors report on photomixing terahertz sources that overcome the transit time versus RC-time trade-off and allow for independent optimization of both of them, using a n-i-p-n-i-p superlattice. Furthermore, they take advantage of ballistic transport for reduced transit times. Apart from more favorable material parameters, In(Al)GaAs photomixers benefit from the advanced telecommunication laser technology around 1.55 μm as compared to GaAs. In such devices, a terahertz-power output of 1 μW has been achieved at 0.4 THz at a photocurrent of 3.8 mA. A comparison between corresponding GaAs- and InGaAs-based n-i-p-n-i-p photomixers reveals an improvement of performance by at least an order of magnitude for the latter one. [ABSTRACT FROM AUTHOR]

Ballistic thermal conductance in a three-dimensional quantum wire with a stub structure is presented under both stress-free and hard wall boundary conditions at low temperatures. A comparative analysis for two-dimensional and three-dimensional models is made. The results show that when stress-free boundary conditions are applied, the universal quantum thermal conductance can be observed regardless of the geometry details in the limit T→0, and the behavior of the thermal conductance is qualitatively similar to that calculated by two-dimensional model. However, when hard wall boundary conditions are applied, the thermal conductance displays different behaviors in both two-dimensional and three-dimensional models. [ABSTRACT FROM AUTHOR]

Implementation of the universal Toffoli gate in a ballistic nanowire based structure has been discussed in this letter. The gate operation is dependent on spin manipulation in nanowires. Spin injection into the Toffoli gate can be done using a similar intertwined ballistic nanowire based setup. The device inputs are digital voltages and the output is obtained by spin readout. Realization of NAND gate, fan out, and half-adder based on the Toffoli gate has also been presented. [ABSTRACT FROM AUTHOR]

The authors investigated the current-voltage characteristics of a 100 nm AlGaAs/InGaAs pseudomorphic high electron mobility transistor as a function of temperature. The drain current in the linear region showed a dramatic increase when the temperature was lowered below a critical value (TB) and the drain voltage is increased. A quantitative analysis based on self-consistent Schrödinger-Poisson and simple electrostatic band potential profile calculations was performed to model the transmission coefficient. The modeled results are consistent with the measured data, confirming that the main transport mechanism switches from a classical drift-diffusion transport into a quasiballistic transport when decreasing temperature below TB and increasing drain voltage. [ABSTRACT FROM AUTHOR]

The authors report an atomistic study of the ballistic current through silicon nanowire metal-oxide-semiconductor transistors. A self-consistent quantum ballistic transport model is used to calculate the current in gate-all-around nanowire transistors, taking into account the full-band structure of the quantum wire with a sp3 tight-binding approach. The authors demonstrate the occurrence of an optimal wire cross section for which the on-state/off-state current ratio is maximum, a result which cannot be obtained in a standard bulk effective mass description. [ABSTRACT FROM AUTHOR]

The authors discuss a generalized transport model including memory and nonlocal effects, which aims to describe the transition of heat transport from the diffusive regime to the ballistic regime. By using an effective thermal conductivity depending on the Knudsen number, they describe in a single equation the behavior of conductivity in terms of the system size and a reduction in the limit flux through nanoscale devices. [ABSTRACT FROM AUTHOR]

The authors show that both real and imaginary parts of the admittance of short (ballistic or quasiballistic) semiconductor structures are oscillatory functions of the frequency with the period determined by the inverse electron transit time with the Fermi velocity. This oscillatory dependence is caused by the phase difference between the electrons injected from the opposite contacts. [ABSTRACT FROM AUTHOR]

The authors have fabricated and studied a ballistic one-dimensional p-type quantum wire using an undoped AlGaAs/GaAs heterostructure. The absence of modulation doping eliminates remote ionized impurity scattering and allows high mobilities to be achieved over a wide range of hole densities and, in particular, at very low densities where carrier-carrier interactions are strongest. The device exhibits clear quantized conductance plateaus with highly stable gate characteristics. These devices provide opportunities for studying spin-orbit coupling and interaction effects in mesoscopic hole systems in the strong interaction regime where rs>10. [ABSTRACT FROM AUTHOR]

We have studied ballistic transport in a one-dimensional (1D) channel formed using surface gate techniques on a back-gated, high-mobility, bilayer two-dimensional hole system. At millikelvin temperatures, robust conductance quantization is observed in the quantum wire formed in the top layer of the bilayer system, without the gate instabilities that have hampered previous studies of 1D hole systems. Using source drain bias spectroscopy, we have measured the 1D subband spacings, which are 5–10 times smaller than in comparable GaAs electron systems, but 2–3 times larger than in previous studies of 1D holes. We also report the first observation of the anomalous conductance plateau at G=0.7×2e2/h in a 1D hole system. [ABSTRACT FROM AUTHOR]

In this letter, we explore the band structure effects on the performance of ballistic silicon nanowire transistors (SNWTs). The energy dispersion relations for silicon nanowires are evaluated with an sp3d5s* tight binding model. Based on the calculated dispersion relations, the ballistic currents for both n-type and p-type SNWTs are evaluated by using a seminumerical ballistic model. For large diameter nanowires, we find that the ballistic p-SNWT delivers half the ON-current of a ballistic n-SNWT. For small diameters, however, the ON-current of the p-type SNWT approaches that of its n-type counterpart. Finally, the carrier injection velocity for SNWTs is compared with those for planar metal-oxide-semiconductor field-effect transistors, clearly demonstrating the impact of quantum confinement on the performance limits of SNWTs. [ABSTRACT FROM AUTHOR]

We study ballistic electron transport through a finite chain of quantum circular rings in the presence of spin–orbit interaction of strength α. For a single ring, the transmission and reflection coefficients are obtained analytically and from them the conductance for a chain of rings as a function of α and of the wave vector k of the incident electron. We show that due to destructive spin interferences, the chain can be totally opaque for certain ranges of k, the width of which depends on the value of α. A periodic modulation of the strength α or of the ring radius widens the gaps considerably and produces a nearly binary conductance output. [ABSTRACT FROM AUTHOR]

We discuss an implementation of ballistic electron emission microscopy (BEEM), in which the metallic or metal–insulator “stack” of interest is formed directly over an avalanche p–n diode. This allows nanometer-resolution studies of hot-electron transport through technologically important device stacks with up to single electron sensitivity and >10 kHz measurement bandwidth when the avalanche diode is cooled to <200 K. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the switching properties of a T-shaped quantum waveguide. Transient analysis of ballistic transport; Spectral contributions in the terahertz range; Accounts on the pronounced mixing effects; Sharing of electron wave function between the bound and metastables states of the initial and final eigenspectra.

Indicates the role of high voltage electroforming in the one-dimensional conductance quantization (1DCQ). Formation of quantum point contacts during devices deposition or electron injection; Details on the result in relation to ballistic point contact; Observation on a 1DCQ room temperature; Manifestation on the onset of conductance quantization.

This letter revises the interpretation of recent experiments showing quantized conductance on GaAs/GaAlAs constrictions or point contacts in terms of coherent ballistic and sequential transport. Our calculations show by using a first iterative self-consistent procedure that the experimental data can also be reconciled with sequential transport. The mean free path concept in the constriction is discussed in comparison with that of the reservoirs. We conclude that, for the higher modes, the former is much smaller than the latter, implying that in the constriction the transport may be sequential. [ABSTRACT FROM AUTHOR]

We introduce three basic components for the manipulation of quasi-classical ballistic electrons in very high mobility two-dimensional electron systems and evaluate their effectiveness using the transverse electron focusing technique. We present an electron absorber or electron randomizer, demonstrate electron focusing in an area of tunable electron density, and present a simplified version of an injector/collector for ballistic carriers in two dimensions. [ABSTRACT FROM AUTHOR]

Recent experimental studies of ballistic electron transport have pushed the transit time well into the subpicosecond dynamical regime. By applying the quantum dynamical theory to study ultrafast transient transport, we report results that are dramatically different from those predicted by the classical Boltzmann theory. [ABSTRACT FROM AUTHOR]