Ballistic Transport Research Articles

Unsteady ballistic heat transport in a 1D harmonic crystal due to a source on an isotopic defect

In the paper we apply asymptotic technique based on the method of stationary phase and obtain the approximate analytical description of thermal motions caused by a source on an isotopic defect of an arbitrary mass in a 1D harmonic crystal. It is well known that localized oscillation is possible in this system in the case of a light defect. We consider the unsteady heat propagation and obtain formulae, which provide continualization (everywhere excepting a neighbourhood of a defect) and asymptotic uncoupling of the thermal motion into the sum of the slow and fast components. The slow motion is related to ballistic heat transport, whereas the fast motion is energy oscillation related to transformation of the kinetic energy into the potential one and in the opposite direction. To obtain the propagating component of the fast and slow motions we estimate the exact solution in the integral form at a moving point of observation. We demonstrate that the propagating parts of the slow and the fast motions are “anti-localized” near the defect. The physical meaning of the anti-localization is a tendency for the unsteady propagating wave-field to avoid a neighbourhood of a defect. The effect of anti-localization increases with the absolute value of the difference between the alternated mass and the mass of a regular particle, and, therefore, more energy concentrates just behind the leading wave-front of the propagating component. The obtained solution is valid in a wide range of a spatial co-ordinate (i.e. a particle number), everywhere excepting a neighbourhood of the leading wave-front.

Development of a cryogenic passive-scattering-type near-field optical microscopy system.

Passive scattering-type, scanning near-field optical microscopy (s-SNOM) has been employed to study localized, long-wavelength infrared (LWIR) surface waves without external illumination. Here, we develop a cryogenic passive s-SNOM instrument in a vacuum chamber with 4K liquid-helium cooling. Notably, the extremely low-temperature environment inside the chamber enables the realization of passive near-field detection with low background thermal noise. The technique mainly utilizes a highly sensitive LWIR confocal optical system and a tuning fork-based atomic force microscope, and the near-field detection was performed at a wavelength of 10.2 ± 0.9 µm. In this paper, we discuss the cryogenic s-SNOM implementation in detail and report the investigation of thermally excited surface electromagnetic fields on a self-heated NiCr wire deposited on SiO2 at a temperature of 5K. The origin of the surface electromagnetic fields was established to be the thermally excited fluctuating charges of the conduction electrons. The cryogenic s-SNOM method presented herein shows significant promise for application in a variety of spheres, including hot-carrier dissipation in ballistic conductors.

Effect of Structural Defects and Adsorbates on the Ballistic Conductivity of Carbon Nanotubes

Effect of Structural Defects and Adsorbates on the Ballistic Conductivity of Carbon Nanotubes

Unsteady thermal transport in an instantly heated semi-infinite free end Hooke chain

We consider unsteady ballistic heat transport in a semi-infinite Hooke chain with free end and arbitrary initial temperature profile. An analytical description of the evolution of the kinetic temperature is proposed in both discrete (exact) and continuum (approximate) formulations. By comparison of the discrete and continuum descriptions of kinetic temperature field, we reveal some restrictions to the latter. Specifically, the far-field kinetic temperature is well described by the continuum solution, which, however, deviates near and at the free end (boundary). We show analytically that, after thermal wave reflects from the boundary, the discrete solution for the kinetic temperature undergoes a jump near the free end. A comparison of the descriptions of heat propagation in the semi-infinite and infinite Hooke chains is presented. Results of the current paper are expected to provide insight into non-stationary heat transport in the semi-infinite lattices.

Quasiballistic transport in the long-range anisotropic Heisenberg model

Purely ballistic transport is a rare feature even for integrable models. By numerically studying the Heisenberg chain with the power-law exchange, $J\ensuremath{\propto}1/{r}^{\ensuremath{\alpha}}$, where $r$ is a distance, we show that for spin anisotropy $\mathrm{\ensuremath{\Delta}}\ensuremath{\simeq}exp(\ensuremath{-}\ensuremath{\alpha}+2)$ the system exhibits a quasiballistic spin transport and the presence of fermionic excitation, which do not decay up to extremely long times $\ensuremath{\sim}{10}^{3}/J$. This conclusion is reached on the basis of the dynamics of spin domains, the dynamical spin conductivity, inspection of the matrix elements of the spin-current operator, and analysis of most conserved operators. Our results smoothly connect two models in which fully ballistic transport is present: free particles with nearest-neighbor hopping and the isotropic Haldane-Shastry model.

Resistivity minima in disordered Co2FeAl0.5Si0.5 Heusler alloy thin films

An exhaustive study of the effect of anti-site disorder on the ‘zero-field’, ρ(T, H = 0), and ‘in-field’, ρ(T, H = 80 kOe), electrical resistivity in 50 nm thick Co2FeAl0.5Si0.5 (CFAS) Heusler alloy thin films (deposited on the Si(100) or SiO2/Si(100) substrates at fixed substrate temperatures in the range 27°C≤TS≤550°C), has been carried out. Irrespective of the strength of disorder, resistivity goes through a minimum as a function of temperature at T = Tmin. With increasing substrate temperature, the crystalline order of the CFAS thin films improves so much so that the films deposited at 500 °C have the lowest anti-site disorder within the B2 structure and the least residual resistivity. A quantitative comparison of our results with the predictions of the existing theoretical models permits us to unambiguously identify the diffusive and ballistic transport mechanisms, responsible for ρ(T, H = 0) and ρ(T, H = 80 kOe) in different temperature ranges and accurately determine their relative magnitudes. The electron-diffuson (e – d) scattering and weak localization (WL) mechanisms, responsible for negative temperature coefficient of resistivity (TCR) for T < Tmin, compete with the positive TCR mechanisms, electron-magnon (e – m) and electron–phonon (e – p) scattering, to produce the resistivity minimum at Tmin. The e – d and WL contributions to ρ, ρe-d and ρwl, dominate over the e – m and e – p contributions, ρe-m and ρe-p, for T<Tmin whereas the reverse is true for T>Tmin. At any given temperature, ρe-d, ρwl and ρe-m decrease while ρe-p increases as the atomic order improves with increasing substrate temperature, TS. ρe-p and ρe-m originate from the phonon-induced non-spin-flip two-band (s↑↓- d↑↓) scattering and magnon - induced spin-flip s - d interband (s↑↓- d↓↑) transitions, respectively. In all the CFAS films, except for the one deposited at TS = 27°C, the thermal renormalization of the spin-wave stiffness, due to the electron - magnon interaction, contributes significantly to ρe-m(T). Furthermore, we demonstrate that the negative magnetoresistance (MR) is a consequence of a progressive suppression of the WL effect and e – m scattering by external magnetic field. However, the WL contribution to MR turns out to be negligibly small as the e – m contribution almost entirely accounts for MR over the temperature range 5K≤T≤300K.

Competition Pathways of Energy Relaxation of Hot Electrons through Coupling with Optical, Surface, and Acoustic Phonons

The hot electrons in carbon-based materials exhibit interesting ballistic transport behaviors for designing high-performance single-electron transistors. However, the cooling of such hot electrons back to the equilibrium state may be slowed down by the excessively populated optical phonons, thus limiting their ballistic transport. Therefore, a thorough understanding of the coupling between hot electrons and optical phonons is of critical importance. Here, by varying the thickness of a multilayer graphene film on a supporting substrate, we investigated the competition pathways of the energy relaxation of the photoinduced hot electrons through coupling with the optical, surface, and acoustic phonons. The difference in the τ2 values indicates that thickness plays an important role in the optical phonon population. For the multilayer graphene film thickness less than 3 nm, the super-collision model describes the hot electron cooling dynamics that is strongly affected by the surface phonons from the supporting substrate. As the multilayer graphene film thickness increases from 3 to 20 nm, the accumulation of the optical phonons induces a hot optical phonon effect, resulting in a bottleneck of cooling of the hot electrons. As the multilayer graphene film thickness further increases from 20 to 40 nm, a direct coupling between the hot electrons and acoustic phonons starts to dominate. The surface states of the interfaces at the inner layers of the multilayer graphene film contribute to this direct coupling as an additional cooling channel. The quantitative understanding of the energy relaxation pathways of the hot electrons offers insights into designing high-performance single-electron transistors.

Recent progress in the theory of bulk photovoltaic effect

The bulk photovoltaic effect (BPVE) occurs in solids with broken inversion symmetry and refers to DC generation due to uniform illumination, without the need of heterostructures or interfaces, a feature that is distinct from the traditional photovoltaic effect. Its existence has been demonstrated almost 50 years ago, but predictive theories only appeared in the last ten years, allowing for the identification of different mechanisms and the determination of their relative importance in real materials. It is now generally accepted that there is an intrinsic mechanism that is insensitive to scattering, called shift current, where first-principles calculations can now give highly accurate predictions. Another important but more extrinsic mechanism, called ballistic current, is also attracting a great deal of attention, but due to the complicated scattering processes, its numerical calculation for real materials is only made possible quite recently. In addition, an intrinsic ballistic current, usually referred to as injection current, will appear under circularly polarized light and has wide application in experiments. In this review, experiments that are pertinent to the theory development are reviewed, and a significant portion is devoted to discussing the recent progress in the theories of BPVE and their numerical implementations. As a demonstration of the capability of the newly developed theories, a brief review of the materials' design strategies enabled by the theory development is given. Finally, remaining questions in the BPVE field and possible future directions are discussed to inspire further investigations.

From enhanced diffusion to ultrafast ballistic motion of hybrid light–matter excitations

Transport of excitons and charge carriers in molecular systems can be enhanced by coherent coupling to photons, giving rise to the formation of hybrid excitations known as polaritons. Such enhancement has far-reaching technological implications; however, the enhancement mechanism and the transport nature of these hybrid excitations remain elusive. Here we map the ultrafast spatiotemporal dynamics of polaritons formed by mixing surface-bound optical waves with Frenkel excitons in a self-assembled molecular layer, resolving polariton dynamics in energy/momentum space. We find that the interplay between the molecular disorder and long-range correlations induced by coherent mixing with light leads to a mobility transition between diffusive and ballistic transport, which can be controlled by varying the light-matter composition of the polaritons. Furthermore, we show that coupling to light enhances the diffusion coefficient of molecular excitons by six orders of magnitude and even leads to ballistic flow at two-thirds the speed of light.

Disorder effects on the ballistic transport of gated phosphorene superlattices

In this paper, we carry out a theoretical study of the electronic ballistic transport in gated phosphorene superlattices. Our work is focused on the effects by the random introduction of structural disorder in the electrostatic potential barriers. Specifically, we consider various degrees of disorder in the width and height of the barriers. To explain the transport of the charge carriers (electrons and holes) in the armchair direction of phosphorene, we employ a low-energy two-band Hamiltonian derived from the tight-binding model. The transmission and conductance are calculated by the transfer matrix technique and the Landauer-B\"uttiker formalism, respectively. We have found that both types of disorder affect the transmission and conductance in different ways, and we also found distinct effects on the charge carriers. In general, the disorder significantly reduces the transmission as well as the peak zones associated with the tunneling effect. Particularly, for disorder in the width, the transmission miniband structure is greatly degraded, while for disorder in the height, the miniband structure is preserved. The variation in the width of the barriers interferes directly with the Fabry-P\'erot resonances of the propagating waves, while the variation in the height mainly reduces the magnitude of the transmission due to the dominance of evanescent waves. Consequently, the conductance loses its oscillatory characteristic and experiences flattening under the influence of disorder of the width, in contrast with the disorder of the height in which the oscillation prevails. Finally, we found that both types of disorder induce a redistribution of electron states. Specifically, the allowed energy bands are increased, and the occupation number as well as the accumulation of states are greatly reduced, which is in accordance with the effects of disorder observed in the transport properties.

Quantitative Analysis of Nonequilibrium Phonon Transport Near a Nanoscale Hotspot

Understanding thermal transport by phonons near nanoscale hotspots is critical for many engineering applications, especially for nanoscale thermal measurements and nanoelectronics. Previous studies usually adopted the gray phonon Boltzmann transport equation to consider ballistic phonon transport or the multitemperature model to consider selective phonon excitation. However, nonequilibrium phonon transport near a nanoscale hotspot cannot be fully captured by either method. In this work, we employ the more rigorous nongray phonon Boltzmann transport equation to investigate phonon transport near nanoscale hotspots. We first consider hotspots in a one-dimensional system with two phonon modes to extract the underlying physics. Thermal transport is found to be less efficient near these hotspots, leading to significantly smaller effective thermal conductivity. The mechanism behind this is that different phonon modes have different temperatures and are therefore not in equilibrium. Nonequilibrium is caused by both the selective electron-phonon interaction and ballistic phonon transport. For relatively large hotspots, nonequilibrium is primarily contributed to by the selective excitation effect. When the hotspot size further reduces, the contribution from ballistic phonon transport becomes increasingly more important. Furthermore, we quantitatively study the thermal transport of laser-heated hotspots in Raman experiments and Joule heating in fin field-effect transistors (FETs) using the nongray phonon Boltzmann transport equation. We find that the measured thermal conductivity of single-layer graphene can be significantly underestimated if the nonequilibrium effect is ignored. Additionally, the peak temperature rise in a silicon fin FET is much larger than that calculated by the heat-diffusion equation.

Direct Measurement of Ballistic and Diffusive Electron Transport in Gold

We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from pump-probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm, which is higher than the average value of 30 nm predicted from our parameter-free density functional perturbation theory. Our first-principles calculations of electron-phonon coupling reveal that the increase in the mean free path due to interband excitation is a consequence of drastically reduced electron-phonon coupling from lattice stiffening, thus providing the microscopic understanding of our experimental findings.

Approaching PetaVolts per Meter Plasmonics Using Structured Semiconductors

A newly uncovered class of plasmons in the strongly excited limit opens access to unprecedented Petavolts per meter electromagnetic fields with wide-ranging, transformative impact. Unlike conventional plasmons, such plasmons are constituted by non-perturbative, large-amplitude oscillations of the ultradense, delocalized free electron Fermi gas inherent in conductive media. Here structured semiconductors doped to have an appropriate conduction electron density are introduced to tune the properties of the Fermi gas for matched excitation of large-amplitude plasmons using readily available electron beams which enables immediate experimental validation. Specifically, an electrostatic, surface “crunch-in” plasmon is collisionlessly excited by the beam launched inside a tube. Strong excitation due to matching results in relativistic oscillations of the electron gas and unravels unique phenomena. Relativistically induced ballistic electron transport comes about due to relativistic multifold increase in the mean free path and also leads to unconventional heat deposition beyond Ohm’s law. This explains the absence of observed damage or solid-plasma formation in past experiments on conductive samples interacting with electron bunches shorter than 10−13 seconds. Furthermore, relativistic momentum leads to copious tunneling of electron gas across the surface, which then crunches inside the tube. Relativistic effects along with large, localized electron density variations underlying these modes necessitate kinetic approach to theoretical and computational modeling. Kinetic model presented here demonstrates experimental viability of observing tens of gigavolts per meter plasmonic fields excited by matching readily available electron beams to plasmons in semiconductors with 1018cm−3 free electron density, and paves the way for Petavolts per meter plasmonics.

Ballistic Thermal Transport at Sub‐10 nm Laser‐Induced Hot Spots in GaN Crystal (Adv. Sci. 2/2023)

Ballistic Thermal Transport In this article number 2204777, Yanan Yue and co-workers develop a tip-enhanced Raman thermometry to study ballistic thermal transport within 10 nm in GaN, simultaneously achieving local laser heating and temperature probing. The phonon mean free path is predicted which can find wide application in the thermal management of GaN-based electronics.

A Fully Coupled Scheme for a Self-Consistent Poisson-NEGF Solver

This work presents a fully coupled scheme for a self-consistent Poisson-nonequilibrium Green function (NEGF) solver. In this solver, responses of the Green functions to the potential perturbation are rigorously calculated. As a result, the detailed dependence of the electron density on the electrostatic potential can be accurately considered. When applied to the steady-state (DC) simulation, the convergence behavior is much improved compared with the conventional Gummel-type approach. As a numerical example, real-space NEGF results are shown for a highly scaled nanosheet MOSFET under the ballistic transport approximation.

Josephson effect in topological semimetal-superconductor heterojunctions

Topological semimetals are exotic phases of quantum matter with gapless electronic excitation protected by symmetry. Benefitting from its unique relativistic band dispersion, topological semimetals host abundant quantum states and quantum effects, such as Fermi-arc surface states and chiral anomaly. In recent years, due to the potential application in topological quantum computing, the hybrid system of topology and superconductivity has aroused wide interest in the community. Recent experimental progress of topological semimetal-superconductor heterojunctions is reviewed in two aspects: 1) Josephson current as a mode filter of different topological quantum states; 2) detection and manipulation of topological superconductivity and Majorana zero modes. For the former, utilizing Josephson interference, ballistic transport of Fermi-arc surface states is revealed, higher-order topological phases are discovered, and finite-momentum Cooper pairing and superconducting diode effect are realized. For the latter, by detecting a.c. Josephson effect in Dirac semimetals, the 4π-periodic supercurrent is discovered. By all-electric gate control, the topological transition of superconductivity is obtained. Outlooks of future research on topological semimetal-superconductor heterojunctions and their application in Majorana braiding and topological quantum computing are discussed.

Isolated conductance channels inside the bandgap of GaAs nanowires of zincblende and wurtzite heterostructures

Electronic properties of polytypes (i.e., the co-existence of zincblende and wurtzite structures in III–V compound semiconductor nanowires) are still highly controversial. In an effort to explore fundamental insights of this topic, we investigate the electronic conductance for GaAs nanowires having arbitrary sequence of zincblende and wurtzite heterostructures by a five-nearest-neighbor tight-binding model. The transmission function χ(E) of the zincblende–wurtzite heterostructure is calculated in the mixture from the complete zincblende to the complete wurtzite configuration by non-equilibrium Green’s function formulae. The properties of the ballistic conductance of the system are analyzed by convolving χ(E) with the thermal broadening function. Numerical results show that more inclusion of the wurtzite-type segments into the GaAs nanowires can establish isolated conductance channels in the neighborhood of the bandgap edge, providing potential optoelectronic applications for mid-infrared optics.

Mobility and quasi-ballistic charge carrier transport in graphene field-effect transistors

The optimization of graphene field-effect transistors (GFETs) for high-frequency applications requires further understanding of the physical mechanisms concerning charge carrier transport at short channel lengths. Here, we study the charge carrier transport in GFETs with gate lengths ranging from 2 μm down to 0.2 μm by applying a quasi-ballistic transport model. It is found that the carrier mobility, evaluated via the drain–source resistance model, including the geometrical magnetoresistance effect, is more than halved with decreasing the gate length in the studied range. This decrease in mobility is explained by the impact of ballistic charge carrier transport. The analysis allows for evaluation of the characteristic length, a parameter of the order of the mean-free path, which is found to be in the range of 359–374 nm. The mobility term associated with scattering mechanisms is found to be up to 4456 cm2/Vs. Transmission formalism treating the electrons as purely classical particles allows for the estimation of the probability of charge carrier transport without scattering events. It is shown that at the gate length of 2 μm, approximately 20% of the charge carriers are moving without scattering, while at the gate length of 0.2 μm, this number increases to above 60%.

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.

Ballistic transport of a polariton ring condensate with spin precession

It is now routine to make Bose-Einstein condensates of polaritons with long enough lifetime and low enough disorder to travel ballistically for hundreds of microns in quasi-one-dimensional (1D) wires. We present observations of a nonequilibrium polariton condensate injected at one point in a quasi-1D ring, with a well-defined initial velocity and direction. A clear precession of the circular polarization is seen, which arises from an effective spin-orbit-coupling term in the Hamiltonian. Our theoretical model accurately predicts the experimentally observed behavior and shows that ``zitterbewegung'' behavior plays a role in the motion of the polaritons.