Anomalous Transport Phenomena Research Articles

Non-trivial evolution of the Dirac cone in chromium doped Dirac semimetal Cd3As2

The magnetic doping of Dirac semimetals breaks their time-reversal symmetry thus giving rise to anomalous transport phenomena promising for applications. While the Dirac cone (DC) manifestations were observed in the electron transport of (Cd1−xCrx)3As2 alloys up to x = 0.06, their mechanism remains challenging. To address this challenge, we performed DFT calculations of the (Cd0.96Cr0.04)3As2 alloy with the non-magnetic (NM), ferromagnetic (FM), and antiferromagnetic (AFM) spin orders. It was found that the NM state is unstable, while the FM state has the lowest energy. The band structure analysis revealed that Cr 3d states are distributed in energy not uniformly, but with Cr-free windows, in which the Dirac spectrum can survive. The only exception is the Dirac point vicinity, where a narrow gap is formed. Outside the window, the DC band strongly hybridizes with the Cr 3d states and disappears. Near EF, such a Cr-free window with the DC band exists only in the FM↓ state and is absent in the NM, FM↑, and AFM states. The Fermi surface of FM (Cd1−xCrx)3As2 has two parts: the DC↓ sheet with the velocity vF ≈ 1∙106 m/s and several sheets with low vF, originating from Cr 3d↑ states. As an example of transport properties, the dc conductivity of FM (Cd1−xCrx)3As2 was estimated. We found that at T→0 K the DC↓ electrons have a very large transport lifetime and therefore dominate in the conductivity. In this dominance, the role of the Cr-free window is double: it ensures the DC↓ surviving and greatly reduces an admixture of Cr 3d↓ orbitals to DC↓ states, so suppressing the scattering of DC↓ electrons by doped Cr atoms. This mechanism looks rather general and may be applied to the design of magnetic topological alloys.

Intermittent subdiffusion of short nuclear actin rods due to interactions with chromatin.

The interior of cellular nuclei, the nucleoplasm, is a crowded fluid that is pervaded by protein-decorated DNA polymers, the chromatin. Due to the complex architecture of chromatin and a multitude of associated nonequilibrium processes, e.g., DNA repair, the nucleoplasm can be expected to feature nontrivial material properties and hence anomalous transport phenomena. Here, we have used single-particle tracking on nuclear actin rods to probe such transport phenomena. Our analysis reveals that short actin rods in the nucleus show an intermittent, antipersistent subdiffusion with clear signatures of fractional Brownian motion. Moreover, the diffusive motion is heterogeneous with clear signatures of an intermittent switching of trajectories between at least two different mobilities, most likely due to transient associations with chromatin. In line with this interpretation, hyperosmotic stress is seen to stall the motion of nuclear actin rods, whereas hypo-osmotic conditions yield a reptationlike motion. Our data highlights the heterogeneity of transport in the nucleoplasm that needs to be taken into account for an understanding of nucleoplasmic organization and the mechanobiology of nuclei.

A robust numerical scheme for solving Riesz-tempered fractional reaction–diffusion equations

The Fractional Diffusion Equation (FDE) is a mathematical model that describes anomalous transport phenomena characterized by non-local and long-range dependencies which deviate from the traditional behavior of diffusion. Solving this equation numerically is challenging due to the need to discretize complicated integral operators which increase the computational costs. These complexities are exacerbated by nonlinear source terms, nonsmooth data and irregular domains. In this study, we propose a second order Exponential Time Differencing Finite Element Method (ETD-RDP-FEM) to efficiently solve nonlinear FDE, posed in irregular domains. This approach discretizes matrix exponentials using a rational function with real and distinct poles, resulting in an L-stable scheme that damps spurious oscillations caused by non-smooth initial data. The method is shown to outperform existing second-order methods for FDEs with a higher accuracy and faster computational time.

Lattice Boltzmann model for incompressible flows through porous media with time-fractional effects

Anomalous transport has been commonly observed in the fluid flow through a complex porous medium, where the evolution exhibits a complex memory-like behavior. Classical integer-order models fail to depict anomalous transport phenomena, while fractional calculus has been proved effective in describing such behavior due to its ability to characterize long memory processes. In this paper, the time-fractional generalized Navier-Stokes (N-S) equations are formulated to model anomalous transport in porous media at the representative elementary volume (REV) scale by incorporating the Caputo fractional derivative of time into the widely employed generalized N-S equations. An innovative lattice Boltzmann (LB) model is presented for solving the time-fractional generalized N-S equations. We validate the proposed LB model by conducting a numerical example with analytical solutions, and observe a good agreement between the LB results and the analytical solutions. The proposed LB model is utilized for simulating Poiseuille flow, Couette flow, and cavity flow in porous media. Unlike the previous studies, which focus on the steady state of these flows, the present work focuses on the unsteady process from the initial state to the final steady state. It is found that time-fractional effects play a significant role in the unsteady processes of these flows. As the fractional order decreases, the flows undergo a slower evolution process and the impact of Darcy number on the unsteady process becomes increasingly noticeable. The Reynolds number has a significant impact on the velocity profile in the steady state, but it has relatively little effect on the duration of the unsteady process.

High-throughput calculations of antiferromagnets hosting anomalous transport phenomena

High-throughput calculations of antiferromagnets hosting anomalous transport phenomena

Modeling anomalous transport in fractal porous media: A study of fractional diffusion PDEs using numerical method

Abstract Fractional diffusion partial differential equation (PDE) models are used to describe anomalous transport phenomena in fractal porous media, where traditional diffusion models may not be applicable due to the presence of long-range dependencies and non-local behaviors. This study presents an efficient hybrid meshless method to the compute numerical solution of a two-dimensional multiterm time-fractional convection-diffusion equation. The proposed meshless method employs multiquadric-cubic radial basis functions for the spatial derivatives, and the Liouville-Caputo derivative technique is used for the time derivative portion of the model equation. The accuracy of the method is evaluated using error norms, and a comparison is made with the exact solution. The numerical results demonstrate that the suggested approach achieves better accuracy and computationally efficient performance.

The Wiedemann-Franz law in doped Mott insulators without quasiparticles.

Many metallic quantum materials display anomalous transport phenomena that defy a Fermi liquid description. Here, we use numerical methods to calculate thermal and charge transport in the doped Hubbard model and observe a crossover separating high- and low-temperature behaviors. Distinct from the behavior at high temperatures, the Lorenz number [Formula: see text] becomes weakly doping dependent and less sensitive to parameters at low temperatures. At the lowest numerically accessible temperatures, [Formula: see text] roughly approaches the Wiedemann-Franz constant [Formula: see text], even in a doped Mott insulator that lacks well-defined quasiparticles. Decomposing the energy current operator indicates a compensation between kinetic and potential contributions, which may help to clarify the interpretation of transport experiments beyond Boltzmann theory in strongly correlated metals.

Pseudogap behavior in charge density wave kagome material ScV6Sn6 revealed by magnetotransport measurements

Over the last few years, significant attention has been devoted to studying the kagome materials A V3Sb5 (A = K, Rb, Cs) due to their unconventional superconductivity and charge density wave (CDW) ordering. Recently {{{{rm{ScV}}}}}_{6}{{{{rm{Sn}}}}}_{6} was found to host a CDW below ≈ 90 K, and, like A V3Sb5, it contains a kagome lattice comprised only of V ions. Here we present a comprehensive magnetotransport study on {{{{rm{ScV}}}}}_{6}{{{{rm{Sn}}}}}_{6}. We discovered several anomalous transport phenomena above the CDW ordering temperature, including insulating behavior in interlayer resistivity, a strongly temperature-dependent Hall coefficient, and a violation of Kohler’s rule. All these anomalies can be consistently explained by a progressive decrease in carrier densities with decreasing temperature, suggesting the formation of a pseudogap. Our findings suggest that high-temperature CDW fluctuations play a significant role in determining the normal state electronic properties of {{{{rm{ScV}}}}}_{6}{{{{rm{Sn}}}}}_{6}.

Thermosensitive Polyurethane-Based Hydrogels as Potential Vehicles for Meloxicam Delivery.

Meloxicam (MX) is a nonsteroidal anti-inflammatory drug (NSAID) used mainly to reduce pain, inflammation, and fever. In the present study, thermosensitive polyurethane (PU)-based hydrogels with various excipients (PEG, PVP, HPC, and essential oil) were prepared and loaded with MX. Rheological investigations were carried out on the PU-based formulations in various shear regimes, and their viscoelastic characteristics were determined. The average size of the PU micelles was 35.8 nm at 37 °C and slightly increased at 37 nm in the presence of MX. The zeta potential values of the hydrogels were between -10 mV and -11.5 mV. At pH = 6 and temperature of 37 °C, the formulated PU-based hydrogels loaded with MX could deliver significant amounts of the active substance, between 60% and 80% over 24-48 h and more than 90% within 2 weeks. It was found that anomalous transport phenomena dominated MX's release mechanism from the PU-based networks. The results are encouraging for further studies aiming to design alternative carriers to commercial dosage forms of nonsteroidal anti-inflammatory drugs.

Chiral Instabilities and the Fate of Chirality Imbalance in Non-Abelian Plasmas.

We present a microscopic study of chiral plasma instabilities and axial charge transfer in non-Abelian plasmas with a strong gauge-matter coupling g^{2}N_{f}=64, by performing 3+1D real-time classical-statistical lattice simulation with dynamical fermions. We explicitly demonstrate for the first time that-unlike in an Abelian plasma-the transfer of chirality from the matter sector to the gauge fields occurs predominantly due to topological sphaleron transitions. We elaborate on the similiarities and differences of the axial charge dynamics in cold Abelian U(1) and non-Abelian SU(2) plasmas, and comment on the implications of our findings for the study of anomalous transport phenomena, such as the chiral magnetic effect in QCD matter.

Carrier‐Modulated Anomalous Electron Transport in Electrolyte‐Gated SrTiO3

The anomalous transport phenomena and underlying mechanism of SrTiO3 have been long term concerned, including the Kondo physics and anomalous Hall effects to be discussed herein. Nevertheless, a clarification of the physical origins for these phenomena and their intertransitions upon varying carrier density remain to be an issue, due to the limited modulation of carrier density. Herein, the intriguing transport behaviors of electrolyte‐gated SrTiO3 in the field‐effect transistor geometry are investigated, which allows a relatively wide carrier density window. It is revealed that the remarkable Kondo‐like resistance upturn, identified at relatively low carrier density, becomes gradually disappeared and replaced by the emerging nonlinear Hall resistance with increasing gating voltage, indicating the interesting gating‐controlled transport behaviors. The detailed discussions on the underlying physics suggest that the gating‐controlled transport can be well described by the multiband transport model taking into account magnetic scattering in the channel layer, a comprehensive scenario for the emergent transport effects in carrier‐doped SrTiO3. The present work provides a promising platform for exploring novel quantum materials phenomena in SrTiO3 and analogous systems.

Solvent-Assisted Filling of Liquid Metal and Its Selective Dewetting for the Multilayered 3D Interconnect in Stretchable Electronics.

As stretchable electronics are rapidly developing and becoming complex, the requirement for stretchable, multilayered, and large-area printed circuit boards (PCBs) is emerging. This demands a stretchable electrode and its vertical interconnect access (via) for 3-dimensional (3D) connectivity between layers. Here, we demonstrate solvent-assisted liquid metal (LM) filling into the submicrometer channel (∼400 nm), including via-hole filling and selective dewetting of LM. We provide the theoretical background of solvent-assisted LM filling and selective dewetting and reveal the osmotic pressure arising from anomalous mass transport phenomena, case II diffusion, which drives negative pressure, the spontaneous pulling of LM into the open channel. Also, we suggest design criteria for the geometry and dimension of LM interconnects to obtain structural stability without dewetting, based on the theoretical and computational background. We demonstrate a simple stretchable near-field communication (NFC) device including transferred micrometer-size light-emitting diodes (LEDs) with only 230 μm to the stretchable liquid metal PCB, without any soldering process. The device operates stably under repetitive stretching and releasing (∼50% uniaxial strain) due to the stable connection through the LM via between the upper and lower layers. Finally, we propose a concept for modular-type stretchable electronics, based on the cohesive liquid nature of LM. As a building block, the functional module can be easily removed from a mainframe, and replaced by another functional module, to suit user demand.

An investigation of space distributed-order models for simulating anomalous transport in a binary medium

Recent studies highlight that diffusion processes in highly heterogeneous, fractal-like media can exhibit anomalous transport phenomena, which motivates us to consider the use of generalised transport models based on fractional operators. In this work, we harness the properties of the distributed-order space fractional potential to provide a new perspective on dealing with boundary conditions for nonlocal operators on finite domains. Firstly, we consider a homogeneous space distributed-order model with Beta distribution weight. An a priori estimate based on the L2 norm is presented. Secondly, utilising finite Fourier and Laplace transform techniques, the analytical solution to the model is derived in terms of Kummer’s confluent hypergeometric function. Moreover, the finite volume method combined with Jacobi-Gauss quadrature is applied to derive the numerical solution, which demonstrates high accuracy even when the weight function shows near singularity. Finally, a one-dimensional two-layered problem involving the use of the fractional Laplacian operator and space distributed-order operator is developed and the correct form of boundary conditions to impose is analysed. Utilising the idea of ‘geometric reconstruction’, we introduce a ‘transition layer’ whereby the fractional operator index varies from fractional order to integer order across a fine layer at the boundary of the domain when transitioning from the complex internal structure to the external conditions exposed to the medium. An important observation is that for a fractional dominated case, the diffusion behaviour in the main layer is similar to fractional diffusion, while near the boundary the behaviour transitions to the case of classical diffusion.

Anomalous Thermoelectric Transport Phenomena from First‐Principles Computations of Interband Electron–Phonon Scattering

AbstractThe Seebeck coefficient and electrical conductivity are two central quantities to be optimized simultaneously in designing thermoelectric materials, and they are determined by the dynamics of carrier scattering. Here a new regime is uncovered where the presence of multiple electron bands with different effective masses, crossing near the Fermi level, leads to strong energy‐dependent carrier lifetimes due to intrinsic electron–phonon scattering. In this anomalous regime, electrical conductivity decreases with carrier concentration, Seebeck coefficient reverses sign even at high doping, and power factor exhibits an unusual second peak. The origin and magnitude of this effect is explained using a general simplified model as well as first‐principles Boltzmann transport calculations in recently discovered half‐Heusler alloys. General design rules for using this paradigm to engineer enhanced performance in thermoelectric materials are identified.

Anomalous transport in low-dimensional systems: A pedagogical overview

This manuscript contains the notes of the lectures given in July 2021 at the Summer School “Fundamental Problems in Statistical Physics XV”, Bruneck, Italy. A pedagogical approach has been adopted by first providing the students a summary of the basic concepts of non-equilibrium statistical mechanics associated with transport processes. A detailed account about anomalous heat transport in anharmonic lattices is the first research topic described in these lecture notes. The interest of these results for material science is then highlighted, in order to point out the applicative potentials of anomalous transport in nanostructures, polymers etc. Further details about anomalous transport phenomena are reported in the presence of a magnetic field or of long-range interactions. A large section of these lecture notes is devoted to the theoretical approaches adopted for obtaining a consistent description of anomalous transport phenomena, including recent achievements that point out the crucial importance of finite-size effects on theoretical predictions and, accordingly, on experimental verifications. The final section is devoted to a summary about coupled transport in low-dimensional systems and the consequences of the presence of anomalous diffusion processes in this complex scenario. The appendices provide additional information about transport in basic 1d models (harmonic chains and rotor model), in 2d lattices and about integrable nonlinear models.

Anomalous Thermoelectric Performance in Asymmetric Dirac Semimetal BaAgBi.

Multiple-band degeneracy has been widely recognized to be beneficial for high thermoelectric performance. Here, we discover that the p-type Dirac bands with lower degeneracy synergistically produce a higher Seebeck coefficient and electrical conductivity in topological semimetal BaAgBi. The anomalous transport phenomenon intrinsically originated from the asymmetric electronic structures: (i) complete p-type Dirac bands near the Fermi level facilitate high and strong energy-dependent hole relaxation time; (ii) the presence of additional parabolic conduction valleys allows for a large density of states to accept scattered electrons, leading to an enlarged hole-electron relaxation time ratio and, thus, weakened bipolar effect. In combination with the strong lattice anharmonicity, an exceptional p-type average ZT of 0.42 is achieved from 300 to 600 K, which can be dramatically enhanced to 1.38 via breaking the C3v symmetry. This work uncovers the underlying mechanisms governing the abnormal transport behavior in Dirac semimetal BaAgBi and highlights the asymmetric electronic structures as target features to discover/design high-performance thermoelectric materials.

Interplay of orbital magnetic moment and chiral magnetic effect in Shubnikov–de Haas quantum oscillations

Emergent Lorentz symmetry and chiral anomaly are well known for playing an essential role in anomalous transport phenomena of Weyl metals. In particular, the former causes a Berry-curvature-induced orbital magnetic moment to modify the group velocity of Weyl electrons, and the latter results in the chiral magnetic effect to be responsible for a ``dissipationless'' longitudinal current channel of the bulk. In this study, we verify that these two effects can be measured in Shubnikov--de Haas (SdH) quantum oscillations, where a double-peak structure of the SdH oscillation appears to cause a kink in the Landau fan diagram. We examine three different cases which cover all possible experimental situations under external electric/magnetic fields and identify the experimental condition for the existence of the double-peak structure. We claim that interplay of the orbital magnetic moment and the chiral magnetic effect in SdH quantum oscillations is an interesting feature of the Weyl metal state.

Anomalous transport of a classical wave-particle entity in a tilted potential.

A classical wave-particle entity in the form of a millimetric walking droplet can emerge on the free surface of a vertically vibrating liquid bath. Such wave-particle entities have been shown to exhibit hydrodynamic analogs of quantum systems. Using an idealized theoretical model of this wave-particle entity in a tilted potential, we explore its transport behavior. The integro-differential equation of motion governing the dynamics of the wave-particle entity transforms to a Lorenz-like system of ordinary differential equations that drives the particle's velocity. Several anomalous transport regimes such as absolute negative mobility, differential negative mobility, and lock-in regions corresponding to force-independent mobility are observed. These observations motivate experiments in the hydrodynamic walking-droplet system for the experimental realizations of anomalous transport phenomena.

Dynamics of Langmuir Wave Spectra in Randomly Inhomogeneous Solar Wind Plasmas

Abstract Solar coronal and wind plasmas often contain density fluctuations of various scales and amplitudes. The scattering of Langmuir wave turbulence on these inhomogeneities modifies the properties of the radiated electromagnetic emissions traveling from the Sun to the Earth. This paper shows the similarities between the physical results obtained by (i) a model based on the Zakharov equations, describing the self-consistent dynamics of Langmuir wave turbulence spectra in a plasma with external density fluctuations, and (ii) a modeling, within the framework of geometric optics approximation, of quasi-particles (representing plasmon quanta) moving in a fluctuating potential. It is shown that the dynamics of the Langmuir spectra is governed by anomalous diffusion processes, as a result of multiple scattering of waves on the density fluctuations; the same dynamics are observed in the momenta distributions of quasi-particles moving in potential structures with random inhomogeneities. These spectra and distributions are both characterized by a fast broadening during which energy is transported to larger wavevectors and momenta, exhibiting nonlinear time dependence of the average squares of wavevectors and quasi-particle momenta as well as non-Gaussian tails in the asymptotic stage. The corresponding diffusion coefficients depend on the time and are proportional to the square of the average level of density (or potential) fluctuations. It appears that anomalous transport and superdiffusion phenomena are responsible for the spectral broadening.

Anomalous transport phenomenon of a charged Brownian particle under a thermal gradient and a magnetic field.

There is a growing interest in the stochastic processes of nonequilibrium systems subject to nonconserved forces, such as the magnetic forces acting on charged particles and the chiral self-propelled force acting on active particles. In this paper, we consider the stationary transport of noninteracting Brownian particles under a constant magnetic field in a position-dependent temperature background. We demonstrate the existence of the Nernst-like stationary density current perpendicular to both the temperature gradient and magnetic field, induced by the intricate coupling between the nonconserved force and the multiplicative noises due to the position-dependent temperature.