Low-frequency Transport Research Articles

Determining the Electron Scattering from Interfacial Coulomb Scatterers in Two-Dimensional Transistors.

Two-dimensional (2D) transistors are promising for potential applications in next-generation semiconductor chips. Owing to the atomically thin thickness of 2D materials, the carrier scattering from interfacial Coulomb scatterers greatly suppresses the carrier mobility and hampers transistor performance. However, a feasible method to quantitatively determine relevant Coulomb scattering parameters from interfacial long-range scatterers is largely lacking. Here, we demonstrate a method to determine the Coulomb scattering strength and the density of Coulomb scattering centers in InSe transistors by comprehensively analyzing the low-frequency noise and transport characteristics. Moreover, the relative contributions from long-range and short-range scattering in the InSe transistors can be distinguished. This method is employed to make InSe transistors consisting of various interfaces a model system, revealing the profound effects of different scattering sources on transport characteristics and low-frequency noise. Quantitatively accessing the scattering parameters of 2D transistors provides valuable insight into engineering the interfaces of a wide spectrum of ultrathin-body transistors for high-performance electronics.

On Molecular Dynamics and Charge Transport in a Flexible Epoxy Resin Network.

An epoxy based on diglycidyl ether of bisphenol A was reacted with a long-chain poly(oxypropylene diamine) hardener in the presence of an accelerator, resulting in a flexible epoxy network. Tensile properties were tested as a function of accelerator concentration. All systems exhibited high levels of extensibility, with strain at failure values in excess of 65%. Molecular dynamics in a formulation containing 10 phr of accelerator were then examined using dielectric spectroscopy over the temperature range of 103–433 K. At low temperatures, a molecular relaxation process (γ relaxation) was observed and shown to conform well to both the Arrhenius equation and activated tunnelling. A stronger relaxation appeared (203–303 K) just before the onset of charge transport, which dominated the behaviour at higher temperatures. The former takes an unusual bimodal form, which we consider a result of overlapping β and α relaxations, consequently termed αβ mode. Analysis of this mechanism revealed a Vogel–Fulcher–Tammann (VFT) behaviour. The temperature-dependent DC conductivity, σDC (deduced from the low-frequency charge transport contribution to εr″), also revealed VFT behaviour with an onset statistically equivalent to that of the αβ mode, therefore suggesting that charge transport, at this temperature regime, is strongly affiliated with cooperative molecular motion.

Critical roles of H2O and O2 in H2O2 photoproduction over biomass derived metal-free catalyst

In this paper, in-situ transient photoinduced-voltage combined with the continuous wavelet transform was used for the first time to study the critical roles of H2O and O2 in the H2O2 photoproduction. A metal-free photocatalyst containing carbon dots was synthesized by a one-step hydrothermal method using hesperidin as raw material. The photocatalytic H2O2 yield of the catalyst can reach 4480 μmol·L−1 under visible light without sacrificial agent. In addition to being raw materials, we have proved that H2O and O2 can regulate the photogenerated electron transport kinetics, namely, O2 can decelerate low-frequency electron transport, while H2O can accelerate the low-frequency electron transport. This work provides an in-depth understanding on photocatalytic production of H2O2 over composite metal-free photocatalysts and new insight for their design.

Phonon resonant effect in silicon membranes with different crystallographic orientations

Engineering low-frequency phonon transport in nanostructures with the phonon resonant mechanism has become an important research direction. On the basis of non-equilibrium molecular dynamics simulations, the thermal transport in pristine and resonant Si-membranes bounded with {100}, {110} and {111} facets is investigated. It is found that the creation of surfaces can introduce anisotropic thermal transport due to the lattice symmetry breaking. Besides, ballistic phonon transport is found in pristine membranes with lengths up to 500 nm at low-frequencies with a critical frequency mainly dependent on the crystallographic orientation. Moreover, although surface resonances can dramatically reduce the thermal conductivity of all membranes, the resonant effect strongly relies on membrane orientation. Among the three studied membrane orientations, the resonant effect is maximized in the {111}-membrane, where the thermal conductivity is tuned from the largest one to the smallest one among the three membrane types by resonant pillars. The large thermal conductivity reduction in the {111}-membranes by resonances originated from the reduced spectral heat flux between 3 and 12 THz. Furthermore, the resonant coupling strength can be tuned by the interface vacancy between resonant pillars and the base material, which can enhance phonon transport at an intermediate frequency range. Our work provides further insights on thermal transport engineering by phonon resonances and could be useful for thermal conductivity engineering with surface orientations and resonances.

Spin Pumping of an Easy-Plane Antiferromagnet Enhanced by Dzyaloshinskii-Moriya Interaction.

Recently, antiferromagnets have received revived interest due to their significant potential for developing next-generation ultrafast magnetic storage. Here, we report dc spin pumping by the acoustic resonant mode in a canted easy-plane antiferromagnet α-Fe_{2}O_{3} enabled by the Dzyaloshinskii-Moriya interaction. Systematic angle and frequency-dependent measurements demonstrate that the observed spin-pumping signals arise from resonance-induced spin injection and inverse spin Hall effect in α-Fe_{2}O_{3}-metal heterostructures, mimicking the behavior of spin pumping in conventional ferromagnet-nonmagnet systems. The pure spin current nature is further corroborated by reversal of the polarity of spin-pumping signals when the spin detector is switched from platinum to tungsten which has an opposite sign of the spin Hall angle. Our results reveal the intriguing physics underlying the low-frequency spin dynamics and transport in canted easy-plane antiferromagnet-based heterostructures.

THz electrodynamics of mixed-valent YbAl$$_3$$ and LuAl$$_3$$ thin films

We present THz measurements of thin films of mixed-valent YbAl$_3$ and its structural analogue LuAl$_3$. Combined with traditional Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the low-frequency transport of these materials. We find that LuAl$_3$ demonstrates conventional Drude transport whereas at low temperatures YbAl$_3$ demonstrates a sharply renormalized Drude peak and a mid-infrared (MIR) peak in the conductivity, indicative of the formation of a heavy Fermi liquid. In YbAl$_3$ the extended Drude framework shows a consistency of the scattering rate with Fermi-liquid behavior below $T < 40$ K and a moderate mass enhancement. While a $\omega^2$ Fermi liquid-like frequency dependence is not clearly exhibited, the temperature dependence of the Drude scattering rate and effective mass is consistent with the formation of a low-temperature moderately heavy Fermi liquid, albeit one with a smaller mass than observed in single crystals. The extended Drude analysis also supports a slow crossover between the Fermi liquid state and the normal state in YbAl$_3$.

Structure and dynamics of aromatic and alkyl substituted Imidazolium-based ionic liquids

The structure and dynamics of two ionic liquids based on imidazolium cations with the common anion dicyanamide, [N(CN)2]-, were studied and compared: one with an aromatic side chain, 1-benzyl-3-methylimidazolium dicyanamide [BzC1Im][N(CN)2], and another with a normal alkyl chain, 1-n-heptyl-3-methylimidazolium dicyanamide [C7C1Im][N(CN)2]. We use classical molecular dynamics (MD) simulations in order to unveil subtle structural differences between these two systems. The collective dynamics were analyzed by considering the spectra of the mass current correlation functions, which were analyzed within the framework of the Zwanzig-Mori formalism. This allowed to compare the effect of aromatic or non-aromatic substituents on the high-frequency collective dynamics and its relationship to a low-frequency transport property such as viscosity. It is found that even though the aromatic-substituted system is a denser and more viscous liquid, the high-frequency dynamics at finite momentum transfers and frequencies, i.e. within the length and timescales of the intermediate-range order (IRO), is only slightly altered. In the low-wavevector limit, however, there are significant differences in the vibrational density of states and viscosity. The scenario put forward by the MD simulations is that even though the role of ionic packing dominates over specific interactions, the different structural motifs within the IRO range imply signatures on the macroscopic dynamics of aromatic and aliphatic substituted imidazolium ionic liquids.

Improved accuracy in high-frequency AC transport measurements in pulsed high magnetic fields.

We show theoretically and experimentally that accurate transport measurements are possible even within the short time provided by pulsed magnetic fields. For this purpose, a new method has been devised, which removes the noise component of a specific frequency from the signal by taking a linear combination of the results of numerical phase detection using multiple integer periods. We also established a method to unambiguously determine the phase rotation angle in AC transport measurements using a frequency range of tens of kilohertz. We revealed that the dominant noise in low-frequency transport measurements in pulsed magnetic fields is the electromagnetic induction caused by mechanical vibrations of wire loops in inhomogeneous magnetic fields. These results strongly suggest that accurate transport measurements in short-pulsed magnets are possible when mechanical vibrations are well suppressed.

INFORMATIVE SIGNS IN TASKS OF RECOGNITION OF TRANSPORT OBJECTS BY NOISE

The study relates to the problem of assessing and preventing geoecological risks arising from exposure to traffic noise. This paper explores the noise of large vehicles. Spectral, spatio-temporal characteristics of noise of tracked and wheeled heavy equipment, railway transport were obtained. The patterns of attenuation of the low-frequency transport noise level by distance are determined, the main modes of heavy equipment noise are highlighted.

Zero-field edge plasmons in a magnetic topological insulator

Incorporating ferromagnetic dopants into three-dimensional topological insulator thin films has recently led to the realisation of the quantum anomalous Hall effect. These materials are of great interest since they may support electrical currents that flow without resistance, even at zero magnetic field. To date, the quantum anomalous Hall effect has been investigated using low-frequency transport measurements. However, transport results can be difficult to interpret due to the presence of parallel conductive paths, or because additional non-chiral edge channels may exist. Here we move beyond transport measurements by probing the microwave response of a magnetised disk of Cr-(Bi,Sb)2Te3. We identify features associated with chiral edge plasmons, a signature that robust edge channels are intrinsic to this material system. Our results provide a measure of the velocity of edge excitations without contacting the sample, and pave the way for an on-chip circuit element of practical importance: the zero-field microwave circulator.

Low-frequency ac transport in silicene nanosystem with normal-metal electrodes

Low-frequency ac transport properties are investigated theoretically in the zigzag silicene nanosystem with normal-metal electrode/silicene nanoribbon/normal-metal electrode (NSN) structure. Based on the tight-binding approach and ac transport theory, we numerically compute the dc conductance and ac emittance in the nanosystem, by considering the nearest-neighbor hopping, second-nearest-neighbor spin–orbit interaction (SOI) and external electric field. The results show that the anti-resonance effect, caused by the interface scattering between the silicene ribbon and the normal-metal electrodes, is offset by the relatively strong SOI. The SOI induces a quantum phase transition and establishes a topological edge channel in the real space revealed by the local density of states (LDOS), resulting in a constant dc conductance and a vanishing ac emittance around the Dirac point. Further study suggests that the transport property is topologically protected by the SOI from the geometrical size change of the nanosystem.

Testing the high turbulence level breakdown of low-frequency gyrokinetics against high-frequency cyclokinetic simulationsa)

This paper presents numerical simulations of the nonlinear cyclokinetic equations in the cyclotron harmonic representation [R. E. Waltz and Zhao Deng, Phys. Plasmas 20, 012507 (2013)]. Simulations are done with a local flux-tube geometry and with the parallel motion and variation suppressed using a newly developed rCYCLO code. Cyclokinetic simulations dynamically follow the high-frequency ion gyro-phase motion which is nonlinearly coupled into the low-frequency drift-waves possibly interrupting and suppressing gyro-averaging and increasing the transport over gyrokinetic levels. By comparing the more fundamental cyclokinetic simulations with the corresponding gyrokinetic simulations, the breakdown of gyrokinetics at high turbulence levels is quantitatively tested over a range of relative ion cyclotron frequency 10 < Ω* < 100 where Ω* = 1/ρ*, and ρ* is the relative ion gyroradius. The gyrokinetic linear mode rates closely match the cyclokinetic low-frequency rates for Ω* > 5. Gyrokinetic transport recovers cyclokinetic transport at high relative ion cyclotron frequency (Ω* ≥ 50) and low turbulence level as required. Cyclokinetic transport is found to be lower than gyrokinetic transport at high turbulence levels and low-Ω* values with stable ion cyclotron (IC) modes. The gyrokinetic approximation is found to break down when the density perturbations exceed 20%. For cyclokinetic simulations with sufficiently unstable IC modes and sufficiently low Ω* ∼ 10, the high-frequency component of cyclokinetic transport level can exceed the gyrokinetic transport level. However, the low-frequency component of the cyclokinetic transport and turbulence level does not exceed that of gyrokinetics. At higher and more physically relevant Ω* ≥ 50 values and physically realistic IC driving rates, the low-frequency component of the cyclokinetic transport and turbulence level is still smaller than that of gyrokinetics. Thus, the cyclokinetic simulations do not account for the so-called “L-mode near edge short fall” seen in some low-frequency gyrokinetic transport and turbulence simulations.

Evaluation of the arbitrary Lagrangian–Eulerian vertical coordinate method in the MPAS-Ocean model

The vertical coordinate of the Model for Prediction Across Scales-Ocean (MPAS-Ocean) uses the Arbitrary Lagrangian–Eulerian (ALE) method, which offers a variety of configurations. When fully Eulerian, the vertical coordinate is fixed like a z-level ocean model; when fully Lagrangian there is no vertical transport through the interfaces so that the mesh moves with the fluid; additional options for vertical coordinates exist between these two extremes, including z-star, z-tilde, sigma, and isopycnal coordinates. Here we evaluate spurious diapycnal mixing in MPAS-Ocean in several idealized test cases as well as real-world domains with full bathymetry. Mixing data is compared to several other ocean models, including the Parallel Ocean Program (POP) z-level and z-star formulations. In three-dimensional domains, MPAS-Ocean has lower spurious mixing that other ocean models. A series of simulations show that this is likely due to MPAS-Ocean’s hexagon-type horizontal grid cells combined with a flux-corrected transport tracer advection scheme designed for these unstructured meshes.The frequency-filtered vertical coordinate of Leclair and Madec (2011) (also called z-tilde) has been implemented and analyzed in MPAS-Ocean. This addition allows low-frequency vertical transport to pass through the vertical interface in an Eulerian manner, while high-frequency vertical oscillations, such as internal gravity waves, are treated in a Lagrangian manner. Z-tilde leads to a substantial reduction in vertical transport across layer interfaces, and a reduction in spurious diapycnal mixing.

Optically induced oxygen desorption from graphene measured using femtosecond two-pulse correlation

Recently, there has been a great deal of interest in the effect of atmospheric gases on the properties of graphene. We investigate the electrical and optical response of graphene-based field effect transistors that have been exposed to high purity oxygen gas using a combination of ultrafast two-pulse correlation (to give high temporal resolution) and low-frequency transport measurements (to monitor the photoinduced changes in the Fermi level). By measuring the Fermi level shifts, we are only sensitive to the oxygen atoms that interact directly with the surface. We compare our results to predictions of the empirical friction model for molecular desorption. We show the time scale of the relaxation associated with oxygen desorption to be $\ensuremath{\sim}100$ fs, suggesting the desorption proceeds through hot electron generation in the graphene rather than heating of the lattice through hot phonon generation.

Understanding Thickness-Dependent Charge Transport in Pentacene Transistors by Low-Frequency Noise

Close correlation between low-frequency noise and charge transport in pentacene transistors is observed. The trap density evolving with pentacene deposition reveals transformation of growth phase and different transport mechanisms. It explains the greatly altered mobility and contact resistance in which the upper surface's trapping plays an important role. Inspecting contact noise shows high density of traps at contacts that result possibly from pyramid like growth of pentacene or contact damage or both.

Strongly correlated dynamics in multichannel quantum RC circuits

We examine dissipation effects in a multichannel quantum RC circuit, comprising a cavity or single-electron box capacitively coupled to a gate and connected to a reservoir lead via several conducting channels. Depending on the engineering details of the quantum RC circuit, the number of channels contributing to transport vary, as do the form of the interchannel couplings. For low-frequency AC transport, the charge-relaxation resistance ($R_{q}$) is a nontrivial function of the parameters of the system. However, in the vicinity of the charge degeneracy points and for weak tunneling, we find as a result of cross-mode mixing or channel asymmetry that $R_q$ becomes universal for a metallic cavity at low temperatures, and equals the unit of quantum resistance. To prove this universality we map the system to an effective one-channel Kondo model, and construct an analogy with the Coulomb gas. Next, we probe the opposite regime of near-perfect transmission using a bosonization approach. Focussing on the two-channel case, we study the effect of backscattering at the lead-dot interface, more specifically, the role of an asymmetry in the backscattering amplitudes, and make a connection with the weak tunneling regime near the charge degeneracy points.

Relative Contributions of Synoptic and Low-Frequency Eddies to Time-Mean Atmospheric Moisture Transport, Including the Role of Atmospheric Rivers

The relative contributions to mean global atmospheric moisture transport by both the time-mean circulation and by synoptic and low-frequency (periods greater than 10 days) anomalies are evaluated from the vertically integrated atmospheric moisture budget based on 40 yr of “chi corrected” NCEP–NCAR reanalysis data. In the extratropics, while the time-mean circulation primarily moves moisture zonally within ocean basins, low-frequency and synoptic anomalies drive much of the mean moisture transport both from ocean to land and toward the poles. In particular, during the cool-season low-frequency variability is the largest contributor to mean moisture transport into southwestern North America, Europe, and Australia. While some low-frequency transport originates in low latitudes, much is of extratropical origin due to large-scale atmospheric anomalies that extract moisture from the northeast Pacific and Atlantic Oceans. Low-frequency variability is also integral to the Arctic (latitudes &gt; 70°N) mean moisture budget, especially during summer, when it drives mean poleward transport from relatively wet high-latitude continental regions. Synoptic variability drives about half of the mean poleward moisture transport in the midlatitudes of both hemispheres, consistent with simple “lateral mixing” arguments. Extratropical atmospheric transport is also particularly focused within “atmospheric rivers” (ARs), relatively narrow poleward-moving moisture plumes associated with frontal dynamics. AR moisture transport, defined by compositing fluxes over those locations and times where column-integrated water vapor and poleward low-level wind anomalies are both positive, represents most of the total extratropical meridional moisture transport. These results suggest that understanding potential anthropogenic changes in the earth ’s hydrological cycle may require understanding corresponding changes in atmospheric variability, especially on low-frequency time scales.

Low-Frequency Ultrasound-Induced Transport across Non-Raft-Forming Ternary Lipid Bilayers

We examined the effect of bilayer composition on membrane sensitivity to low-frequency ultrasound (LFUS) in bilayers composed of ternary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), dipalmitoyl-phosphocholine (DPPC), and cholesterol. The phase diagram of this system does not display macroscopic phase coexistence between liquid phases (although there are suggestions that there is coexistence between a liquid and a solid phase). Samples from across the composition space were exposed to 20 kHz, continuous wave ultrasound, and the response of the bilayer was quantified using steady-state fluorescence spectroscopy to measure the release of a self-quenching dye, calcein, from large unilamellar vesicles. Dynamic light scattering measurements indicate that, in this system, release proceeds primarily by transport through the vesicle bilayer. While vesicle destruction might account, at least in part, for the light scattering trends observed, evidence of destruction was not as obvious as in other lipid systems. Values for bilayer permeability are obtained by fitting release kinetics to a two-film theory mathematical model. The permeability due to LFUS is found to increase with increasing DPPC content, as the bilayer tends toward the solid-ordered phase. Permeability, and thus sensitivity to LFUS, decreases with either POPC or cholesterol mole fractions. In the liquid regime of this system, there is no recorded phase transition; thus cholesterol is the determining factor in release rates. However, the presence of domain boundaries between distinctly differing phases of liquid and solid is found to cause release rates to more than double. The correlation of permeability with phase behavior might prove useful in designing and developing therapies based on ultrasound and membrane interactions.

Electron Waiting Times in Mesoscopic Conductors

Electron transport in mesoscopic conductors has traditionally involved investigations of the mean current and the fluctuations of the current. A complementary view on charge transport is provided by the distribution of waiting times between charge carriers, but a proper theoretical framework for coherent electronic systems has so far been lacking. Here we develop a quantum theory of electron waiting times in mesoscopic conductors expressed by a compact determinant formula. We illustrate our methodology by calculating the waiting time distribution for a quantum point contact and find a crossover from Wigner-Dyson statistics at full transmission to Poisson statistics close to pinch-off. Even when the low-frequency transport is noiseless, the electrons are not equally spaced in time due to their inherent wave nature. We discuss the implications for renewal theory in mesoscopic systems and point out several analogies with level spacing statistics and random matrix theory.

Dressed spectral densities for heavy quark diffusion in holographic plasmas

We analyze the large frequency behavior of the spectral densities that govern the generalized Langevin diffusion process for a heavy quark in the context of the gauge/gravity duality. The bare Langevin correlators obtained from the trailing string solution have a singular short-distance behavior. We argue that the proper dressed spectral functions are obtained by subtracting the zero-temperature correlators. The dressed spectral functions have a sufficiently fast fall-off at large frequency so that the Langevin process is well defined and the dispersion relations are satisfied. We identify the cases in which the subtraction does not modify the associated low-frequency transport coefficients. These include conformal theories and the non-conformal, non-confining models. We provide several analytic and numerical examples in conformal and non-conformal holographic backgrounds.