Incoherent Transport Research Articles

Heat conduction of multilayer nanostructures with consideration of coherent and incoherent phonon transport

Heat conduction of multilayer nanostructures with consideration of coherent and incoherent phonon transport

Controlling Exciton Propagation in Organic Crystals through Strong Coupling to Plasmonic Nanoparticle Arrays.

Exciton transport in most organic materials is based on an incoherent hopping process between neighboring molecules. This process is very slow, setting a limit to the performance of organic optoelectronic devices. In this Article, we overcome the incoherent exciton transport by strongly coupling localized singlet excitations in a tetracene crystal to confined light modes in an array of plasmonic nanoparticles. We image the transport of the resulting exciton–polaritons in Fourier space at various distances from the excitation to directly probe their propagation length as a function of the exciton to photon fraction. Exciton–polaritons with an exciton fraction of 50% show a propagation length of 4.4 μm, which is an increase by 2 orders of magnitude compared to the singlet exciton diffusion length. This remarkable increase has been qualitatively confirmed with both finite-difference time-domain simulations and atomistic multiscale molecular dynamics simulations. Furthermore, we observe that the propagation length is modified when the dipole moment of the exciton transition is either parallel or perpendicular to the cavity field, which opens a new avenue for controlling the anisotropy of the exciton flow in organic crystals. The enhanced exciton–polariton transport reported here may contribute to the development of organic devices with lower recombination losses and improved performance.

Coherent and incoherent phonon thermal transport in group-III nitride monolayer superlattices with Tersoff type interatomic potential

The existence of a crossover from coherent to incoherent phonon transport for different monolayer superlattices such as h-BN/h-AlN, h-BN/h-GaN, h-AlN/h-GaN, and h-BN/h-AlN/h-GaN using large-scale non-equilibrium molecular dynamics simulations is unveiled. In this context, the lattice thermal conductivities of considered superlattice structures are obtained for varying period lengths, sample lengths, and temperatures. It is predicted that the thermal conductivities of superlattice structures can be lowered even up to 99% than that of their pristine counterparts. Also, it is revealed that a minimum thermal conductivity resulting from the competition between coherent, wave-like, and incoherent, particle-like, phonon transport can be achieved by altering the period length for all the superlattice structures. Moreover, it is observed that the thermal conductivity minimum at higher temperatures compared to low temperatures occurs at shorter period lengths for binary superlattices, and the minimum depths decrease with increasing temperature. Results point out that group-III nitride superlattices can be considered as an alternative option in controlling the heat flow for thermal management and thermoelectric device applications.

Global Oceanic Mass Transport by Coherent Eddies

Abstract Mesoscale eddies are one of the most prominent processes in the world’s ocean. The eddy-induced transport of water mass, heat, and energy has a great impact on the ocean and atmosphere. The study of global mass transport by mesoscale eddies is important. However, most existing studies have used Eulerian eddy detection methods. Compared with Lagrangian methods, Eulerian methods fail to distinguish the coherent transport from the incoherent transport induced by eddies. Using a Lagrangian-averaged vorticity deviation (LAVD)-based coherent eddy detection method, this study identifies global coherent mesoscale eddies in the upper 1000 m of the ocean. Based on the eddy dataset, the eddy-induced coherent mass transport is calculated. Compared with Eulerian estimates, the Lagrangian results shown in this study are one order of magnitude smaller. This means that roughly only about 10% of eddy-induced global water mass transport is coherent. The cumulative eddy-induced coherent transport across each latitude or longitude is only around 1 Sv (1 Sv ≡ 106 m3 s−1), which is much less than the transport induced by wind-driven gyres and thermohaline circulation.

Coherent and incoherent contributions to molecular electron transport.

We numerically isolate the limits of validity of the Landauer approximation to describe charge transport along molecular junctions in condensed phase environments. To do so, we contrast Landauer with exact time-dependent non-equilibrium Green's function quantum transport computations in a two-site molecular junction subject to exponentially correlated noise. Under resonant transport conditions, we find Landauer accuracy to critically depend on intramolecular interactions. By contrast, under nonresonant conditions, the emergence of incoherent transport routes that go beyond Landauer depends on charging and discharging processes at the electrode-molecule interface. In both cases, decreasing the rate of charge exchange between the electrodes and molecule and increasing the interaction strength with the thermal environment cause Landauer to become less accurate. The results are interpreted from a time-dependent perspective where the noise prevents the junction from achieving steady-state and from a fully quantum perspective where the environment introduces dephasing in the dynamics. Using these results, we analyze why the Landauer approach is so useful to understand experiments, isolate regimes where it fails, and propose schemes to chemically manipulate the degree of transport coherence.

Thermal transport in periodic and quasiperiodic graphene-hBN superlattice ribbons

Nanostructured superlattices are expected to play a significant role in the next generation of technological devices, specially due to their adjustable physical properties. In terms of heat transport, materials with low thermal conductivities can be useful in thermoelectric devices or heat shields, while materials with high thermal conductivities are fundamental for heat dissipation in miniaturized electronic devices. In general, transport properties are dominated by translational symmetry and the presence of unconventional symmetries might lead to unusual transport characteristics. In this work, we report our results from nonequilibrium molecular dynamics simulations to investigate phonon heat transport in periodic and quasiperiodic graphene-hBN superlattices. The periodic superlattices are built with alternating equal-sized domains of graphene and hBN, while the quasiperiodic case follows the Fibonacci sequence, which lies between periodic and disordered structures. Periodic superlattices can facilitate coherent phonon transport due to constructive interference at the boundaries between the materials. Nonetheless, it is possible to induce a crossover from a coherent to an incoherent transport regime by increasing the length of individual domains, thus adjusting the superlattice period. We also show that the quasiperiodicity can suppress coherent phonon transport in these superlattices. We attribute this behavior to the increased inhomogeneity in the distribution of interfaces, which increases for each Fibonacci generation, hindering coherent phonon transport in the superlattices. The suppression of coherent thermal transport enables a higher degree of control on heat conduction at the nanoscale, and shows potential for application in thermoelectric devices and heat management.

Relaminarization of spanwise-rotating viscoelastic plane Couette flow via a transition sequence from a drag-reduced inertial to a drag-enhanced elasto-inertial turbulent flow

Direct numerical simulation of spanwise-rotation-driven flow transitions in viscoelastic plane Couette flow from a drag-reduced inertial to a drag-enhanced elasto-inertial turbulent flow state followed by full relaminarization is reported for the first time. Specifically, this novel flow transition begins with a drag-reduced inertial turbulent flow state at a low rotation number $0\leqslant Ro \leqslant 0.1$ , and then transitions to a rotation/polymer-additive-driven drag-enhanced inertial turbulent regime, $0.1\leqslant Ro \leqslant 0.3$ . In turn, the flow transitions to a drag-enhanced elasto-inertial turbulent state, $0.3\leqslant Ro \leqslant 0.9$ , and eventually relaminarizes at $Ro=1$ . In addition, two novel rotation-dependent drag enhancement mechanisms are proposed and substantiated. (1) The formation of large-scale roll cells results in enhanced convective momentum transport along with significant polymer elongation and stress generated in the extensionally dominated flow between adjacent roll cells at $Ro\leqslant 0.2$ . (2) Coriolis-force-generated turbulent vortices cause strong incoherent transport and homogenization of significant polymer stress in the bulk via their vortical circulations at $Ro=0.5 - 0.9$ .

Superconducting order parameters in overdoped BaFe1.86Ni0.14As2 revealed by multiple Andreev reflection spectroscopy of planar break junctions

Using multiple Andreev reflection (MAR) spectroscopy, we studied superconductor - normal metal - superconductor (SnS) semiballistic contacts with incoherent transport in overdoped pnictide superconductors BaFe$_{1.86}$Ni$_{0.14}$As$_2$ with critical temperature $T_c \approx 12$ K. We directly determined an anisotropic large gap with the characteristic ratios $2\Delta_L^{\rm out}(0)/k_BT_c \approx 6$ and $2\Delta_L^{\rm in}(0)/k_BT_c \approx 4$, almost isotropic small gap with $2\Delta_S(0)/k_BT_c \approx 2$, and the temperature dependence of the resolved superconducting order parameters. Above the large gap position $2\Delta_L$, a fine dI(V)/dV structure fully vanishing at $T_c$ and possibly caused by a resonant coupling with a bosonic mode was observed.

Contacts, equilibration, and interactions in fractional quantum Hall edge transport

We study electron transport through a multichannel fractional quantum Hall edge in the presence of both interchannel interaction and random tunneling between channels, with emphasis on the role of contacts. The prime example in our discussion is the edge at filling factor 2/3 with two counterpropagating channels. Having established a general framework to describe contacts to a multichannel edge as thermal reservoirs, we particularly focus on the line-junction model for the contacts and investigate incoherent charge transport for an arbitrary strength of interchannel interaction beneath the contacts and, possibly different, outside them. We show that the conductance does not explicitly depend on the interaction strength either in or outside the contact regions (implicitly, it only depends through renormalization of the tunneling rates). Rather, a long line-junction contact is characterized by a single parameter which defines the modes that are at thermal equilibrium with the contact and is determined by the interplay of various types of scattering beneath the contact. This parameter -- playing the role of an effective interaction strength within an idealized model of thermal reservoirs -- is generically nonzero and affects the conductance. We formulate a framework of fractionalization-renormalized tunneling to describe the effect of disorder on transport in the presence of interchannel interaction. Within this framework, we give a detailed discussion of charge equilibration for arbitrarily strong interaction in the bulk of the edge and arbitrary effective interaction characterizing the line-junction contacts.

Phonon heat transport in two-dimensional phagraphene-graphene superlattice

In this study, we perform non-equilibrium molecular dynamics simulations to investigate phonon heat transport in a two-dimensional superlattice with equal-sized domains of graphene and phagraphene. Effects on conductivity are examined in relation to modifications of domain sizes, the length of employed nanoribbons and temperature differences between the thermal baths used with the superlattices. We have determined that effective thermal conductivity reaches a minimum value of 155W/mK for ribbons with a superlattice period of 12.85nm. This minimum thermal conductivity of graphene-phagraphene superlattices at infinite length is approximately 5%, of pure graphene thermal conductivity, and ≈50% of phagraphene thermal conductivity. Minimum thermal conductivity occurs at the transition from coherent to incoherent phonon transport, where the superlattice period is comparable to the phonon coherence length.

Incoherent transport across the strange-metal regime of overdoped cuprates.

Strange metals possess highly unconventional electrical properties, such as a linear-in-temperature resistivity1-6, an inverse Hall angle that varies as temperature squared7-9 and a linear-in-field magnetoresistance10-13. Identifying the origin of these collective anomalies has proved fundamentally challenging, even in materials such as the hole-doped cuprates that possess a simple bandstructure. The prevailing consensus is that strange metallicity in the cuprates is tied to a quantum critical point at a doping p* inside the superconducting dome14,15. Here we study the high-field in-plane magnetoresistance of two superconducting cuprate families at doping levels beyond p*. At all dopings, the magnetoresistance exhibits quadrature scaling and becomes linear at high values of the ratio of the field and the temperature, indicating that the strange-metal regime extends well beyond p*. Moreover, the magnitude of the magnetoresistance is found to be much larger than predicted by conventional theory and is insensitive to both impurity scattering and magnetic field orientation. These observations, coupled with analysis of the zero-field and Hall resistivities, suggest that despite having a single band, the cuprate strange-metal region hosts two charge sectors, one containing coherent quasiparticles, the other scale-invariant 'Planckian' dissipators.

Possible superconductivity from incoherent carriers in overdoped cuprates

There is now compelling evidence that the normal state of superconducting overdoped cuprates is a strange metal comprising two distinct charge sectors, one governed by coherent quasiparticle excitations, the other seemingly incoherent and characterized by non-quasiparticle (Planckian) dissipation. The zero-temperature superfluid density n_s(0)ns(0) of overdoped cuprates exhibits an anomalous depletion with increased hole doping pp, falling to zero at the edge of the superconducting dome. Over the same doping range, the effective zero-temperature Hall number n_{\rm H}(0) transitions from pp to 1 + pp. By taking into account the presence of these two charge sectors, we demonstrate that in the overdoped cuprates Tl_22Ba_22CuO_{6+\delta}6+δ and La_{2-x}2−xSr_xxCuO_44, the growth in n_s(0)ns(0) as pp is decreased from the overdoped side may be compensated by the loss of carriers in the coherent sector. Such a correspondence is contrary to expectations from conventional BCS theory and implies that superconductivity in overdoped cuprates emerges uniquely from the sector that exhibits incoherent transport in the normal state.

Excitation Energy Transfer Efficiency in Fluctuating Environments: Role of Quantum Coherence in the Presence of Memory Effects.

Several recent studies have interrogated the role of quantum coherence in affecting the transfer efficiency of an optical excitation to the designated "trap" state where the energy can be used for subsequent reactions, as in photosynthesis. However, these studies invoke a Markovian approximation for the time correlation function describing the environment-induced stochastic fluctuations. Here, we employ Kubo's quantum stochastic Liouville equation (QSLE) to include memory effects. We extend the existing QSLE scheme to introduce decay of a newly created excitation due to radiative and nonradiative channels and also by desired trapping toward the targeted chromophore. We show that the theoretical formalism based on the QSLE correctly reproduces the rate equation description in the Markovian limit, with the rate constants determined by an appropriate quantum limiting procedure. We find that under certain conditions, the efficiency of excitation transfer to the trap gains from the combined presence of quantum coherence and temporally correlated stochastic fluctuations. We work out different limiting situations in order to discover and quantify the optimum conditions for the energy transfer to the trapped state. We find that maximum energy transfer efficiency is achieved in the intermediate limit between coherent and incoherent transport.

Coherent and incoherent effects of nanopores on thermal conductance in silicene

A fundamentally new approach of manipulating thermal properties is utilizing the wave nature of phonons, which can be realized by introducing secondary artificial periodicity in nanostructures. In this paper, we have studied the heat transport in silicene nanomesh (SNM, a silicene sheet with periodically arranged nanopores), using first-principles calculations and molecular dynamics simulations. The results show the thermal conductivity of SNM is obviously lower than that of silicene. Combining with the analysis of wave and particle characteristics of phonons, we reveal the main reasons responsible for the reduction in thermal conductivity of SNM. The coherent Bragg scattering from the secondary artificial periodicity leads to the reduction in phonon group velocities of LA and optical phonons. Different from the graphene nanomesh reported previously, multiple phonon bandgaps in the optical phonon spectrum of SNM are found for the first time. Moreover, it is noticed that, phonons are scattered not only at the hole boundaries, but also at the interface between the surface and interfacial regions in SNMs. In particular, the out-of-plane vibrations are easier to be scattered at the interface than the in-plane vibrations. This study uncovers the coherent and incoherent phonon transport in SNMs, which would deepen our understanding on heat conduction and shed light on the development of Si-based nanoelectronic devices.

Electron-phonon hydrodynamics

We develop the theory of hydrodynamics of an isotropic Fermi liquid of electrons coupled to isotropic acoustic phonons, assuming that umklapp processes may be neglected. At low temperatures, the fluid is approximately Galilean invariant; at high temperatures, the fluid is nearly relativistic; at intermediate temperatures, there are seven additional temperature regimes with unconventional thermodynamic properties and hydrodynamic transport coefficients in a three-dimensional system. We predict qualitative signatures of electron-phonon fluids in incoherent transport coefficients, shear and Hall viscosity, and plasmon dispersion relations. Our theory may be relevant for numerous quantum materials where strong electron-phonon scattering has been proposed to underlie a hydrodynamic regime, including $\mathrm{WTe}_2$, $\mathrm{WP}_2$, and $\mathrm{PtSn}_4$.

Incoherent transport in a classical spin liquid

We study the energy and spin transport of the classical spin liquid hosted by the pyrochlore Heisenberg antiferromagnet in the large $S$ limit. Molecular dynamics calculation suggests that both the energy and spin diffusion constants approach finite limits as the temperature tends to zero. We explain our results in terms of an effective disorder model, where the energy/spin-carrying normal modes propagate in a quasi-static disordered spin background. The finite zero temperature limits of the diffusion constants are then naturally understood as a result of the finite mean free path of the normal modes due to the effective disorder.

Prediction of Bi2Te3-Sb2Te3 Interfacial Conductance and Superlattice Thermal Conductivity Using Molecular Dynamics Simulations.

Bismuth telluride (Bi2Te3) and its alloys with antimony telluride (Sb2Te3) have long been considered to be the best room-temperature bulk thermoelectric (TE) materials. In recent decades, proof-of-concept demonstrations on Bi2Te3-Sb2Te3 nanostructures have shown high TE performance due to reduction in lattice thermal conductivities. Particularly, ultra-low thermal conductivities have been observed in Bi2Te3-Sb2Te3 1D superlattices, leading to thermoelectric figures of merit (ZT) as high as 2.4. In contrast, very few computational studies have been performed to provide insight into the phonon transport across these nanostructures. In this work, we use non-equilibrium molecular dynamics simulations with previously developed force fields to simulate thermal transport across Bi2Te3-Sb2Te3 interfaces and superlattices. We first calculate the thermal conductance associated with a Bi2Te3-Sb2Te3 interface across a temperature range of 200-400 K. The values are also compared with thermal conductances calculated by a modified Landauer transport formalism using phonon transmission coefficients obtained from the diffuse mismatch model. Our results show that inelastic scattering processes contribute to an increase in interfacial thermal conductance at higher temperatures. Finally, we calculate the thermal conductivities of Bi2Te3-Sb2Te3 superlattices with varying period lengths from 2 to 18 nm. A minimum thermal conductivity of 0.27 W/mK is observed at a period length of 4 nm, which is attributed to the competition between incoherent and coherent phonon transport regimes. In comparison with previous experimental measurements in the literature, our results show good agreement with respect to the range of thermal conductivity values and the period length corresponding to the minimum superlattice thermal conductivity.

Evolution of crystal structure of dual layered molecular conductor (ET)4ZnBr4(C6H4Cl2) with temperature

Disordering of ET in dual-layered organic metal with incoherent interlayer electron transport (ET)4ZnBr4(C6H4Cl2) begins already in the LT-phase when approaching Tp.

Complex temperature dependence of coherent and incoherent lattice thermal transport in superlattices

Currently, it is still unclear how and to what extent a change in temperature impacts the relative contributions of coherent and incoherent phonons to thermal transport in superlattices. Some seemingly conflicting computational and experimental observations of the temperature dependence of lattice thermal conductivity make the coherent–incoherent thermal transport behaviors in superlattices even more elusive. In this work, we demonstrate that incoherent phonon contribution to thermal transport in superlattices increases as the temperature increases due to elevated inelastic interfacial transmission. On the other hand, the coherent phonon contribution decreases at higher temperatures due to elevated anharmonic scattering. The competition between these two conflicting mechanisms can lead to different trends of lattice thermal conductivity as temperature increases, i.e. increasing, decreasing, or non-monotonic. Finally, we demonstrate that the neural network-based machine learning model can well capture the coherent–incoherent transition of lattice thermal transport in the superlattice, which can greatly aid the understanding and optimization of thermal transport properties of superlattices.

Quantum mechanical modeling of anharmonic phonon-phonon scattering in nanostructures

The coherent quantum effect becomes increasingly important in the heat dissipation bottleneck of semiconductor nanoelectronics with the characteristic size shrinking down to few nano-meters scale nowadays. However, the quantum mechanical model remains elusive for anharmonic phonon-phonon scattering in extremely small nanostructures with broken translational symmetry. It is a long-term challenging task to correctly simulate quantum heat transport including anharmonic scattering at a scale relevant to practical applications. In this article, we present a clarified theoretical formulation of anharmonic phonon non-equilibrium Green function (NEGF) formalism for both 1D and 3D nanostructures, through a diagrammatic perturbation expansion and an introduction of Fourier representation to both harmonic and anharmonic terms. A parallelized computational framework with first-principle force constants input is developed for large-scale quantum heat transport simulation. Some crucial approximations in numerical implementation are investigated to ensure the balance between numerical accuracy and efficiency. A quantitative validation is demonstrated for the anharmonic phonon NEGF formalism and computational framework by modeling cross-plane heat transport through silicon thin film. The phonon-phonon scattering is shown to be appreciable and to introduce about 20% reduction of thermal conductivity at room temperature even for a film thickness around 10 nm. The present methodology provides a robust platform for the device quantum thermal modeling, as well as the study on the transition from coherent to incoherent heat transport in nano-phononic crystals. This work thus paves the way to understand and to manipulate heat conduction via the wave nature of phonons.