Nonlinear Transport Properties Research Articles

An improved continuum model for hypersonic thermal nonequilibrium flow in the near-continuum regime

In this work, the rarefied Couette flow of diatomic gases with thermal nonequilibrium effects is investigated by the direct simulation Monte Carlo (DSMC) method, and a macroscopic computational model is developed to consider the local rarefaction effects for diatomic gases in the near-continuum regime. The nonlinear transport properties of the diatomic gases are studied, indicating that effective viscosity and effective translational thermal conductivity in the shear nonequilibrium state are affected by translational nonequilibrium effects, which obey the same laws for both monatomic and diatomic gases. The transport coefficients of internal energy modes are affected by both translational nonequilibrium and internal energy relaxation, therefore, the effective rotational and vibrational thermal conductivities are related to the effective viscosity through a modified Eucken relation that accounts for internal energy relaxation. Conclusively, effective constitutive relations are newly established as a function of the shear nonequilibrium parameter and the modified Eucken factors for thermal nonequilibrium flows, and these are integrated into the macroscopic two-temperature model. Subsequently, it is assessed in the simulation of hypersonic flows over flat plates and cylinders at various Knudsen numbers. The results show that the surface shear stress and heat flux obtained by the proposed model agree well with the DSMC results, indicating significantly improved performance compared to the conventional Navier–Stokes two-temperature model for hypersonic flows in the near-continuum regime.

Enzymatic cleaving of entangled DNA rings drives scale-dependent rheological trajectories.

DNA, which naturally occurs in linear, ring, and supercoiled topologies, frequently undergoes enzyme-driven topological conversion and fragmentation in vivo, enabling it to perform a variety of functions within the cell. In vitro, highly concentrated DNA polymers form entanglements that yield viscoelastic properties dependent on the topologies and lengths of the DNA. Enzyme-driven alterations of DNA size and shape therefore offer a means of designing active materials with programmable viscoelastic properties. Here, we incorporate multi-site restriction endonucleases into dense DNA solutions to linearize and fragment circular DNA molecules. We pair optical tweezers microrheology with differential dynamic microscopy and single-molecule tracking to measure the linear and nonlinear viscoelastic response and transport properties of entangled DNA solutions over a wide range of spatiotemporal scales throughout the course of enzymatic digestion. We show that, at short timescales, relative to the relaxation timescales of the polymers, digestion of these 'topologically-active' fluids initially causes an increase in elasticity and relaxation times followed by a gradual decrease. Conversely, for long timescales, linear viscoelastic moduli exhibit signatures of increasing elasticity. DNA diffusion, likewise, becomes increasingly slowed, in direct opposition to the short-time behavior. We hypothesize that this scale-dependent rheology arises from the population of small DNA fragments, which increases as digestion proceeds, driving self-association of larger fragments via depletion interactions, giving rise to slow relaxation modes of clusters of entangled chains, interspersed among shorter unentangled fragments. While these slow modes likely dominate at long times, they are presumably frozen out in the short-time limit, which instead probes the faster relaxation modes of the unentangled population.

Non-Linear Optical and Charge Transport Properties of Oxa[n]circulenes: A Theoretical Insight

Non-Linear Optical and Charge Transport Properties of Oxa[n]circulenes: A Theoretical Insight

Significant effects of epitaxial strain on the nonlinear transport properties in Ca2RuO4 thin films with the current-driven transition

Control of the nonlinear transport properties in strongly correlated materials with metal–insulator transitions has become an important task in the development of next-generation computing elements. Here, we demonstrate that the nonlinear transport properties in the Mott insulator Ca2RuO4, which are caused by the current-driven transition with a nonthermal-type mechanism, can be significantly affected by epitaxial strain in thin films. Ca2RuO4 epitaxial thin films were grown on various single-crystal substrates by the solid-phase epitaxy method under nonvacuum conditions, and the effects of epitaxial strain on the transport properties were investigated. While nonlinear transport properties with negative differential resistance behavior were observed in [001]-oriented Ca2RuO4/LaAlO3 (001) thin films, the current transport nonlinearity was critically diminished in the current–voltage characteristics of Ca2RuO4/NdCaAlO4 (100) thin films with the [110] orientation. Through structural characterization of the thin films, a possible correlation between the strain states and nonlinear transport properties was discussed.

Current induced second-order nonlinear Hall effect in bulk WTe2

The observation of second-order nonlinear Hall effect generally requires broken inversion symmetry and other spatial symmetries (such as out-of-plane C2 symmetry) rather than time-reversal symmetry for the conventional Hall effect, which brings fascinating opportunities for the fundamental investigation and innovative concepts for device applications. However, the imposed symmetry constraints severely limit the accessibility of the second-order nonlinear Hall effect in many materials. Here, we report the observation of the second-order nonlinear Hall effect in bulk Td-WTe2 with symmetry constraints under the application of direct current (DC). The observed second-order nonlinear Hall effect is strongly dependent on the external DC with its magnitude being vanished at zero driving DC and linearly increased with increasing driving DC. Furthermore, the driving DC induced second-order nonlinear Hall effect in bulk Td-WTe2 exhibits the same twofold rotational symmetry as that of the intrinsic third-order nonlinear Hall effect, which is distinct from the second-order nonlinear Hall effect in few-layer Td-WTe2. In addition, similar driving DC induced second-order nonlinear features are also observed in the longitudinal direction. These imply that the driving DC induced second-order nonlinear transport properties may share the same physical origins as that of the intrinsic third-order nonlinear transport. Scaling relationship analyses further demonstrate that extrinsic scattering-related mechanism plays a dominant role in the observed high-order nonlinear transport properties. Our work outlines a potential roadmap to control the second-order nonlinear Hall effect in materials where crystal symmetry forbids this effect with both fundamental research interest and technological device applications.

Nonlinear transport properties of atomic copper point contacts

We report studies on the nonlinear electronic transport properties of copper point contacts. Utilizing the mechanically controllable break junction technique, various contact sizes can be realized to study ensemble-averaged differential conductance spectra at low temperatures. We investigate signatures of phonon excitations for contact sizes down to the atomic scale, where conductance fluctuations arise superimposing the phonon signatures. Applying high bias voltages to atomic-size copper contacts reveal additional features caused by atomic rearrangements.

Rashba spin–orbit coupling induced rectified currents in monolayer graphene with exchange field and sublattice potential

We investigate the effect of Rashba spin–orbit coupling (RSOC) on photoconductivities of rectified currents in monolayer graphene with exchange field and sublattice potential. The system shows that the photoconductivities of resonant shift and injection current contributions are nonzero, while the photoconductivities of non-resonant shift current contribution are zero. We find that the RSOC induces a warping term, which leads to the nonzero rectified currents. Moreover, the photoconductivities of resonant injection (shift) current contribution are (not) related to the relaxation rate. The similar behavior can be found in other Dirac materials, and our findings provide a way to tune the nonlinear transport properties of Dirac materials.

Theoretical Investigation of the Nonlinear Optical and Charge Transport Properties of N-(4-Methoxybenzylidene) Isonicotinohydrazone and Some of Its Derivatives: A DFT and TD-DFT Study

Here, we report the findings from a study on the charge transport and nonlinear optical (NLO) properties of N-(4-methoxybenzylidene) isonicotinohydrazone (INH) and some of its derivatives named INH1-INH15. The density functional theory (DFT) approach was used for ground state computations at the B3LYP-D/6-311G (d,p) level of theory, while the time-dependent density functional theory (TD-DFT) was carried out at the CAM-B3LYP/6-311G (d,p) level. The results show that the energy gaps of all the studied compounds range from 3.933 to 4.645 eV. INH3 and INH4 have the lowest electron and hole reorganization energies (i.e., 0.409 and 0.634 eV, respectively) and can thus be classified as moderate electron and hole-carrying materials for organic light-emitting diode (OLED) applications. TD-DFT computations demonstrate that an extension of the conjugation (in INH2 and INH3) increases the oscillator strength, improving the NLO response. According to the NLO data, INH2 and INH3 have higher static isotropic polarizabilities (38.509 and 37.986 × 10−24 esu, respectively) and second hyperpolarizabilities (54.440 and 57.598 × 10−36 esu, respectively), while INH4 and INH13 have higher first hyperpolarizability values (11.944 and 10.939 × 10−30 esu, respectively). The results reveal that INH derivatives with different groups are viable alternatives for OLED and NLO applications.

The effect of covalent binding of phthalocyanine macrocycles with a cyclotriphosphazene spacer on the aggregation, NLO, and CT properties

This work is devoted to studying the aggregation effect on the nonlinear optical (NLO) and charge transport (CT) properties of low-symmetry phthalocyanines bearing a cyclotriphosphazene moiety, a monomer, and clamshell-type bis-phthalocyanine. FE-SEM and AFM studies revealed ordered nanoaggregation in these compounds, represented as repeating objects of regular configuration, globules (340–480 nm), which contain small aggregates inside their cavity, similar to smoothed polygons (ca. 40 nm). Such specificity is not characteristic of other functionalized phthalocyanines, which we dealt with earlier. A new spectral method for estimating the aggregation threshold based on the change in extinction with increasing concentration of solutions was also implemented, and the effect of aggregation on optical limiting was demonstrated. The behavior of isomers and rotamers of bis-phthalocyanine in the presence of electric fields of various strengths was investigated with the DFT calculations, and that partially corresponds to the impact of laser radiation on molecules. It was shown that the unusual behavior of the trans-isomer in FF-DFT calculations is the reason for the worsening of the NLO and CT characteristics of bis-phthalocyanine, in which the macrocycles are strapped through a phosphazene spacer. An advanced method of multiparametric analysis has been implemented to evaluate the NLO and CT properties of the dyes in general.

Nondivergent chiral charge pumping in Weyl semimetals

Recent studies suggest that the nonlinear transport properties in Weyl semimetal may be a measurable consequence of its chiral anomaly. Nonlinear responses in transport are estimated to be substantial, because in real materials such as TaAs or ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$, the Fermi level resides near the Weyl nodes where the chiral charge pumping is said to diverge. However, this work presents semiclassical Boltzmann analysis that indicates that the chiral charge pumping is nondivergent even at the zero-temperature limit. We demonstrate that the divergence in a common semiclassical calculation scheme is not a problem of the scheme itself, but occurs because a commonly used approximation of the change in particle number breaks down near the Weyl nodes. Our result suggests the possibility that the nonlinear properties in Weyl semimetals can be overestimated, and it provides the validity condition for the conventional approximation. We also show the distinct Fermi level dependencies of the chiral magnetic effect and the negative longitudinal magnetoresistance, as a consequence of nondiverging chiral charge pumping.

Quantum third-order nonlinear Hall effect of a four-terminal device with time-reversal symmetry

The third-order nonlinear Hall effect induced by Berry-connection polarizability tensor has been observed in Weyl semimetals T$_d$-MoTe$_2$ as well as T$_d$-TaIrTe$_4$. The experiments were performed on bulk samples, and the results were interpreted with the semiclassical Boltzmann approach. Beyond the bulk limit, we develop a quantum nonlinear transport theory to investigate the third-order Hall response of a four-terminal setup with time-reversal symmetry in quantum regime. The quantum nonlinear theory is verified on a model system of monolayer MoTe$_2$, and numerical results on the angle-resolved Hall currents are qualitatively consistent with the experiment. More importantly, quantum signatures of the third-order Hall effect are revealed, which are independent of the system symmetry. The first quantum signature is quantum enhancement of the third-order Hall current, which is characterized by sharp current peaks whose magnitudes are three orders larger than the first-order Hall current. Such quantum enhancement originates from quantum interference in coherent transport, and it can be easily destroyed by dephasing effect. The second quantum signature is disorder-induced enhancement of the third-order Hall current for weak disorders. Our findings reveal quantum characteristics of the third-order Hall effect, and we propose feasible ways to enhance it in nanoscale systems. The quantum third-order theory developed in this work provides a general formalism for describing nonlinear coherent transport properties in multi-terminal devices, regardless of the system symmetry.

Linear and nonlinear thermoelectric transport in a quantum spin Hall insulators coupled with a nanomagnet

Thermoelectric effects in quantum systems have been focused in recent years. Thermoelectric energy conversion study of systems with edge states, such as quantum Hall insulators and quantum spin Hall insulators, is one of the most important frontier topics in material science and condensed-matter physics. Based on the previous paper (Gresta in Phys Rev Lett 123:186801, 2019), we further investigated the linear and nonlinear thermoelectric transport properties of helical edge states of the quantum spin Hall insulators coupled with double nanomagnet, calculated the Seebeck coefficients S_c and the thermoelectrical figure of merit ZT, discussed the influence of the length of the nanomagnet and the relative tilt angle of component of the magnetization perpendicular on the thermoelectric coefficients (S_c and ZT), and summarized some meaningful conclusions in the linear response regime. In the nonlinear regime, we calculated the equivalent figure of merit ZT_M and the power-generation efficiency eta in different length of the nanomagnet, obtain the temperature difference of achieving optimal thermoelectricity. The results of this paper further confirm that the setup can indeed be used as a device for achieving high performance thermoelectric.

Enhanced Berry Curvature Dipole and Persistent Spin Texture in the Bi(110) Monolayer.

Nonvanishing Berry curvature dipole (BCD) and persistent spin texture (PST) are intriguing physical manifestations of electronic states in noncentrosymmetric 2D materials. The former induces a nonlinear Hall conductivity while the latter offers a coherent spin current. Based on density-functional-theory (DFT) calculations, we demonstrate the coexistence of both phenomena in a Bi(110) monolayer with a distorted phosphorene structure. Both effects are concurrently enhanced due to the strong spin-orbit coupling of Bi while the structural distortion creates internal in-plane ferroelectricity with inversion asymmetry. We further succeed in fabricating a Bi(110) monolayer in the desired phosphorene structure on the NbSe2 substrate. Detailed atomic and electronic structures of the Bi(110)/NbSe2 heterostructure are characterized by scanning tunneling microscopy/spectroscopy and angle-resolved-photoemission spectroscopy. These results are consistent with DFT calculations which indicate the large BCD and PST are retained. Our results suggest the Bi(110)/NbSe2 heterostructure as a promising platform to exploit nonlinear Hall and coherent spin transport properties together.

Diffusion and thermalization in a boundary-driven dephasing model

We study a model of noninteracting spinless fermions coupled to local dephasing and boundary drive and described within a Lindblad master equation. The model features an interplay between infinite temperature thermalization due to bulk dephasing and a nonequilibrium stationary state due to the boundary drive and dissipation. We revisit the linear and nonlinear transport properties of the model, featuring a crossover from ballistic to diffusive scaling, and compute the spectral and occupation properties encoded in the single-particle Green's functions, that we compute exactly using the Lindblad equations of motion in spite of the interacting nature of the dephasing term. We show that the distribution function in the bulk of the system becomes frequency independent and flat, consistent with infinite temperature thermalization, while near the boundaries it retains strong nonequilibrium features that reflect the continuous injection and depletion of particles due to driving and dissipation.

Ballistic transport through quantum point contacts of multiorbital oxides

Linear and nonlinear transport properties through a quantum point contact based on oxides two-dimensional electron gas is examined using the tight-binding method and the $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ approach. The ballistic transport is analyzed in contacts realized at the (001) interface between band insulators ${\text{LaAlO}}_{3}$ and ${\text{SrTiO}}_{3}$ by using the Landauer-B\"uttiker method for many sub-bands derived from three Ti $3d$ orbitals (${d}_{yz}$, ${d}_{zx}$, and ${d}_{xy}$) in the presence of an out-of-plane magnetic field. We focus especially on the role played by the atomic spin-orbit coupling and the inversion-symmetry-breaking term. Three different transport regimes stem out: The first, at low energies, involving the first ${d}_{xy}$-like sub-bands, where the conductance quantization is clearly observed; a second one, at intermediate energies, entailing further ${d}_{xy}$-like sub-bands, where the sub-band splitting induced by the magnetic field is quenched; the third one, where the mixing between light ${d}_{xy}$-like, heavy ${d}_{yz}$-like, and ${d}_{zx}$-like sub-bands is so strong that the conductance plateaus turn out to be very narrow. Very good agreement is found with recent experiments exploring the transport properties at low energies.

Linear and nonlinear thermal spin transport properties of zigzag α-graphyne nanoribbons with sp2–sp3 edges

In this paper, we investigated the electronic structures and thermal spin transport properties of zigzag α-graphyne nanoribbons (N-ZαGYNRs) with sp2–sp3 edges. As N increased, the results showed that ZαGYNRs undergo a transformation from indirect-band-gap bipolar magnetic semiconductors into half-metallic materials. We then designed two spin caloritronics devices based on two representative ZαGYNRs with N = 4 and N = 12. Both linear and nonlinear thermal spin transport properties were found in the 4-ZαGYNRs device, but only linear thermal spin transport properties could be found in the 12-ZαGYNRs device. These discoveries indicate that ZαGYNRs are potential candidate materials for spin caloritronics device applications.

Metal modulated effects on the optoelectronic and charge transport properties of some graphene nanoribbons

In this study, we have investigated the effects of adding a metal on the nonlinear, mechanical and charge transport properties of some small organic graphene nanoribons (GNRs) molecule. A theoretical investigation is done using the ab initio (rhf) and DFT (B3LYP and BPBE) methods with the 6-31+g(d,p) basis set integrated in Gaussian 09. Results reveals that the metalated GNRs have higher nonlinear optical activity than the virgin counterpart. Interestingly these GNRs metal derivatives also have remarkable nonlinear optical effect in comparison to the classical push–pull organic crystal; para-nitroaniline and urea. Other remarkable results show that as compared to the starting GNRs the metalated counterparts are excellent ambipolar charge transport carriers better than the prototypic electron transport Alq3 (tris(8-hydroxyquinolinato)aluminum(III), and hole tranport TPD (N,N0-diphenyl-N,N0-bis(3-methlphenyl)-(1,10-biphenyl)-4,40-diamine. However, the metal derivative have diminished mechanical stability than the starting molecule. Metalated GNRs could therefore serve as potential candidates for the fabrication of optoelectronic and photonic devices for second and third-order NLO application.

Enhancement of YIG|Pt spin conductance by local Joule annealing

We report that Joule heating can be used to enhance the interfacial spin conductivity between a metal and an oxide. We observe that local annealing of the interface at about 550 K, when injecting large current densities (>1012A/m2) into a pristine 7 nm thick Pt nanostrip evaporated on top of yttrium iron garnet (YIG), can improve the effective spin transmission up to a factor of 3. This result is of particular interest when interfacing ultrathin garnet films to avoid strong chemical etching of the surface. The effect is confirmed by the following methods: spin Hall magnetoresistance, spin pumping, and non-local spin transport. We use it to study the influence of the YIG|Pt coupling on the non-linear spin transport properties. We find that the crossover current from a linear to a non-linear spin transport regime is independent of this coupling.

Nonuniform Carrier Heating Induced Nonlinear Electron Transport Properties in Asymmetrically Necked InAs Mesa Structures

IntroductionTerahertz (THz) wave has been very attractive light for scientific and practical applications such as light source for characterizing fundamental physics on ultra-fast carrier dynamics in semiconductors and imaging applications in the submillimeter wavelength regime. Therefore, there have been studied on various kind of emitting and detecting methods on THz waves in this quarter century. So far, we also have proposed simple and low-cost THz wave source with InAs thin films grown on GaAs substrates combining with a time-domain spectroscopy system [1, 2]. In case of THz detectors, there are not so many techniques which enable to detect THz wave at room temperature with high responsivity without biasing. For example, square-law field rectifier detectors, such as Schottky diodes and InGaAs-based bow-tie diodes, respond to THz electric field at room temperature with no external power supply [3]. Nonlinear current-voltage characteristics of the latter technique is based on unique operation principle which is generated by nonuniform carrier heating in asymmetric bow-tie shaped planar structures. Since InAs possesses one of the highest electron mobility at room temperature among semiconductors, this material based thin films will be hopeful candidate for detecting THz waves by fabricating this unique nonlinear device structures on them. In this paper, we devise utilizing InAs THz wave emitter thin films to base material for asymmetrically necked bow-tie mesa structures for the THz detector. As a first step of this idea, nonlinear current-voltage characteristics of these devices are presented. We especially discuss on the curvature of current-voltage characteristic at zero-bias point which is closely related to the sensitivity on square-law detection for THz wave.ExperimentalNominally undoped InAs thin films were epitaxially grown on semi-insulating GaAs (100) substrate by molecular beam epitaxy. For improving the crystal quality of InAs channel layer, 150 nm thick InAs buffer layer was directly grown with gradually changing growth temperature from 300 to 500°C on GaAs substrate. An 1 µm think InAs channel layer was grown on this buffer layer at 450°C. This growth condition is same as that of THz emitter InAs films we have reported [2]. The patterning of asymmetric bow-tie mesa structure was carried out by photo lithography and phosphoric acid based wet chemical etching. After defining mesa structures, in order to investigate the film thickness dependence on nonlinear current properties, some samples were performed recess etching for InAs channel, which were made each thickness to be 265 nm, 435 nm, 625 nm and 835 nm. Finally, non-alloyed ohmic contact metals of Ti/Pt/Au (50/30/170 nm) were deposited by electron-beam evaporation technique and defined by lift-off. The structure of asymmetric bow-tie diode and the measurement configuration are shown in Fig. 1. Current-voltage characteristics were measured by using Keithley 4200A-SCS system at room temperature.Results and Conclusion Typical current-voltage characteristic of the investigated InAs asymmetric bow-tie diode is represented in Fig. 2, which was the result measured with the sample of 265 nm thick InAs channel. It showed clear asymmetric current-voltage characteristic depended on the polarity of applied voltage, which was considered originated from nonlinear carrier transport related to non-uniform carrier heating in asymmetric mesa structure. In order to evaluate the nonlinearity of current-voltage curve at zero-biased point, the zero-biased curvature coefficient γ was calculated, which is given by the formula written in Fig. 3. A fifth order polynomial fit was utilized to current-voltage curves in determining the curvature. As shown in Fig. 3, the zero-bias curvature clearly depended on thickness of InAs thin film. The highest zero-bias curvature of 0.12 V-1 was obtained in the thinnest sample. In addition, this value was about 2.2 times larger than that of the GaAs/AlGaAs heterostructure-based asymmetric bow-tie diode which had the same shape and size (not shown here). These results indicate that the InAs thin film for THz radiation has possibility to be applied to square-law detection device for THz wave. The details of nonlinearity which depends on mesa shape and size and comparison between InAs based structures and that of GaAs/AlGaAs will be discussed at the conference.

Nonlinear transport of rarefied Couette flows from low speed to high speed

The nonlinear transport properties and macroscopic flow features of rarefied plane Couette flows from low speed to high speed for a monatomic gas are investigated in detail using the direct simulation Monte Carlo (DSMC) method. The effective viscosity and thermal conductivity are directly computed from the DSMC results according to the linear constitutive relations. The detailed structure of the Knudsen layer (KL) and the functional dependence of the effective transport coefficients on local Knudsen numbers in the whole system are presented and compared with existing theoretical models. The results show that the effective viscosity and thermal conductivity distributions in the KL for different Mach number flows can be recast into the same profile (i.e., isothermal scaling function) in terms of a scaled wall distance η=∫0y1/λ(y)dy, though the local flow is nonisothermal. For all cases, the shear-stress Knudsen number distributions across the channel show a well opposite trend to the effective transport coefficient profiles. The functional dependence between them in the bulk region always coincides with the normal solution that is derived from the Boltzmann model equations for unbounded shear flows, while that in the KL for low-speed cases shows a large difference with the normal solution. As the Mach number increases, the DSMC data in the KL can also agree approximately with the normal solution at a large shear-stress Knudsen number. These results can be very useful for developing phenomenological models to describe a wall-bounded rarefied shear flow, showing a good prospect in both microflow and high-altitude applications.