Wiedemann-Franz Law Research Articles

Coupling Electronic and Phonon Thermal Transport in Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) Nanofibers.

The thermal transport of Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) nanofiber is contributed by the electronic component of thermal conduction and the phonon component of thermal conduction. The relationship between the electrical conductivity and thermal conductivity of these conducting polymers is of great interest in thermoelectric energy conversation. In this work, we characterized the axial electrical conductivities and thermal conductivities of the single PEDOT:PSS nanofibers and found that the Lorenz number L is larger than Sommerfeld value L0 at 300 K. In addition, we found that the L increased significantly in the low-temperature region. We consider that this trend is due to the bipolar contribution of conducting polymers with low-level electrical conductivity and the increasing trend of the electronic contribution to thermal conductivity in low-temperature regions.

Quantum states from mixtures of equilibrium distributions

We construct and explore a family of states for quantum systems in contact with two or more heath reservoirs. The reservoirs are described by equilibrium distributions. The interaction of each reservoir with the bulk of the system is encoded in a probability, which characterises the particle exchange among them and depends in general on the particle momentum. The convex combination of the reservoir distributions, weighted with the aforementioned probabilities, defines a new distribution. We establish the existence of an emission–absorption regime in which the new distribution generates a non-equilibrium quantum state. We develop a systematic field theory framework for constructing this state and illustrate its physical properties on a simple model. In this context we derive the particle current full counting statistics, the heat current and the Lorenz number. The entropy production and the relative quantum fluctuations are also determined.

Hydrodynamic thermoelectric transport in Corbino geometry

We study hydrodynamic electron transport in Corbino graphene devices. Due to the irrotational character of the flow, the forces exerted on the electron liquid are expelled from the bulk. We show that in the absence of Galilean invariance, force expulsion produces qualitatively new features in thermoelectric transport: (i) it results in drops of both voltage and temperature at the system boundaries and (ii) in conductance measurements in pristine systems, the electric field is not expelled from the bulk. We obtain thermoelectric coefficients of the system in the entire crossover region between charge neutrality and high electron density regime. The thermal conductance exhibits a sensitive Lorentzian dependence on the electron density. The width of the Lorentzian is determined by the fluid viscosity. This enables determination of the viscosity of electron liquid near charge neutrality from purely thermal transport measurements. In general, the thermoelectric response is anomalous: it violates the Matthiessen's rule, the Wiedemann-Franz law, and the Mott relation.

Breakdown of the Wiedemann-Franz law at the Lifshitz point of strained Sr2RuO4

Strain tuning Sr$_{2}$RuO$_{4}$ through the Lifshitz point, where the Van Hove singularity of the electronic spectrum crosses the Fermi energy, is expected to cause a change in the temperature dependence of the electrical resistivity from its Fermi liquid behavior $\rho\sim T^{2}$ to $\rho\sim T^{2}{\rm log}\left(1/T\right)$, a behavior consistent with experiments by Barber et al. [Phys. Rev. Lett. 120, 076601 (2018)]. This expectation originates from the same multi-band scattering processes with large momentum transfer that were recently shown to account for the linear in $T$ resistivity of the strange metal Sr$_{3}$Ru$_{2}$O$_{7}$. In contrast, the thermal resistivity $\rho_{Q}\equiv T/\kappa$, where $\kappa$ is the thermal conductivity, is governed by qualitatively distinct processes that involve a broad continuum of compressive modes, i.e. long wavelength density excitations in Van Hove systems. While these compressive modes do not affect the charge current, they couple to thermal transport and yield $\rho_{Q}\propto T^{3/2}$. As a result, we predict that the Wiedemann-Franz law in strained Sr$_{2}$RuO$_{4}$ should be violated with a Lorenz ratio $L\propto T^{1/2}{\rm log}\left(1/T\right)$. We expect this effect to be observable in the temperature and strain regime where the anomalous charge transport was established.

Topological charge-entropy scaling in kagome Chern magnet TbMn6Sn6

In ordinary materials, electrons conduct both electricity and heat, where their charge-entropy relations observe the Mott formula and the Wiedemann-Franz law. In topological quantum materials, the transverse motion of relativistic electrons can be strongly affected by the quantum field arising around the topological fermions, where a simple model description of their charge-entropy relations remains elusive. Here we report the topological charge-entropy scaling in the kagome Chern magnet TbMn6Sn6, featuring pristine Mn kagome lattices with strong out-of-plane magnetization. Through both electric and thermoelectric transports, we observe quantum oscillations with a nontrivial Berry phase, a large Fermi velocity and two-dimensionality, supporting the existence of Dirac fermions in the magnetic kagome lattice. This quantum magnet further exhibits large anomalous Hall, anomalous Nernst, and anomalous thermal Hall effects, all of which persist to above room temperature. Remarkably, we show that the charge-entropy scaling relations of these anomalous transverse transports can be ubiquitously described by the Berry curvature field effects in a Chern-gapped Dirac model. Our work points to a model kagome Chern magnet for the proof-of-principle elaboration of the topological charge-entropy scaling.

Comparison of the electronic transport of ThN against ThC

We compared the electronic heat capacity coefficient, thermal conductivity and resistivity of ThN and ThC. We found that the electronic heat capacity coefficient of both compounds was very low as evaluated using Quantum Espresso and GGA of the Perdew, Burke, and Ernzerhof (PBE) functional. The calculated electronic heat coefficient was further reduced (∼three times for ThN and four times for ThC) when the non-local hybrid functional (B3LYP) was used. QE combined with EPW code were used to calculate integrated electron-phonon coupling strength, the Eliashberg transport coupling function and electron-phonon scattering rates. The electronic resistivity was calculated using Ziman's formula for metals as implemented in EPW code and compared to the values derived from experimental data correlations. Furthermore, the effective number (neff) of electronic carriers was calculated. The electronic thermal conductivity was evaluated from the calculated electronic resistivity via Wiedemann-Franz law using the derived number of mobility electrons: nav (averaged over the temperature range 300 K–1000 K). We found that the only way to reproduce exactly the experimental electrical resistivity and electronic thermal conductivity in both ThN and ThC was to incorporate the number of electrons contributing to thermal mobility (neff), which increase with temperature. We estimated that in ThN the number of electronic carriers is two times larger and electron-phonon coupling strength lower by a factor ∼ two, which led to about five times higher electronic thermal conductivity than calculated for ThC.

Superconducting aluminum heat switch with 3 nΩ equivalent resistance

Superconducting heat switches with extremely low normal state resistances are needed for constructing continuous nuclear demagnetization refrigerators with high cooling power. Aluminum is a suitable superconductor for the heat switch because of its high Debye temperature and its commercial availability in high purity. We have constructed a high quality Al heat switch whose design is significantly different than that of previous heat switches. In order to join the Al to Cu with low contact resistance, we plasma etched the Al to remove its oxide layer and then immediately deposited Au without breaking the vacuum of the e-beam evaporator. In the normal state of the heat switch, we measured a thermal conductance of 8T W/K2, which is equivalent to an electrical resistance of 3 nΩ according to the Wiedemann–Franz law. In the superconducting state, we measured a thermal conductance that is 2 × 106 times lower than that of the normal state at 50 mK.

Spin-resolved thermal signatures of Majorana-Kondo interplay in double quantum dots

We investigate theoretically the thermoelectric transport properties of a T-shaped double quantum dot side-coupled to a topological superconducting nanowire hosting Majorana zero-energy modes. The calculations are performed using the numerical renormalization group method focusing on the transport regime, where the system exhibits the two-stage Kondo effect. It is shown that the leakage of Majorana quasiparticles into the double dot system results in a half-suppression of the second stage of the Kondo effect, which is revealed through fractional values of the charge and heat conductances and gives rise to new resonances in the Seebeck coefficient. The heat conductance is found to satisfy a modified Wiedemann-Franz law. Finally, the interplay of Majorana-induced interference with strong electron correlations is discussed in the behavior of the spin Seebeck effect, a unique feature of the considered setup.

Microscale thermal characteristics of cold-rolled aluminum alloy using a thermoreflectance method

Abstract In this study, the microscale thermal propagation behavior of cold-rolled aluminum alloy sheets was characterized using a thermoreflectance (TR) method from the micro and macro perspectives, and the relationship between the crystallite sizes of cold-rolled aluminum sheets with different rolling reduction rates and their thermal propagation characteristics was described. The crystallite sizes were analyzed by X-ray diffraction using Scherrer’s equation. The microscale thermal propagation characteristics of these specimens were measured using a TR method with a high spatial resolution of several micrometers through a focused laser beam and by controlling the thermal diffusion length. The macroscale thermal propagation characteristics of these specimens were then measured using two methods: the spot periodic heating method and the electrical resistance measurement method with the Wiedemann–Franz law. Experimental results showed that the microscale thermal propagation correlated with a change in the crystallite size. However, the macroscale thermal conductivity decreased with an increase in the rolling reduction rate regardless of the crystallite size. It is expected that the thermal propagation characteristics at the microscale can be controlled by a change in the crystallite size.

Violation of the Wiedemann-Franz law in the topological Kondo model

We study the thermal transport through a Majorana island connected to multiple external quantum wires. In the presence of a large charging energy, we find that the Wiedemann-Franz law is nontrivially violated at low temperature, contrarily to what happens for the overscreened Kondo effect and for nontopological junctions. For three wires, we find that the Lorenz ratio is rescaled by a universal factor 2/3 and we show that this behavior is due to the presence of localized Majorana modes on the island.

Charge-neutral fermions and magnetic field-driven instability in insulating YbIr3Si7

Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr3Si7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr3Si7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.

Multiparticle scattering and breakdown of the Wiedemann-Franz law at a junction of N interacting quantum wires

We analyze the charge and thermal transport at a junction of interacting quantum wires close to equilibrium. Within the framework of Tomonaga-Luttinger liquids, we compute the thermal conductance for a wide class of boundary conditions and detail the physical processes leading to the breakdown of the Wiedemann-Franz law at the junction. We show how connecting external reservoirs to the quantum wires affects the conductance tensors close to the various fixed points of the phase diagram of the junction. We therefore distinguish two types of violation of the Wiedemann-Franz law: a ``trivial'' one, independent of the junction dynamics and arising from the breakdown of the Fermi-liquid picture in the wire, and a junction-related counterpart, arising from multiparticle scattering processes at the junction.

Experimental estimation of the Lorenz number and scattering parameter for p-type bismuth antimony telluride via multiple doping under constant temperature conditions

The effects of the addition of lead as one of the multiple dopants in p-type Bi0.3−xSb1.7Te3.0+0.01Pbx (x = 0, 0.001, 0.002, 0.003) fabricated by mechanical alloying followed by hot pressing were investigated. Measurements by X-ray diffraction (XRD), differential thermal analysis (DTA), and scanning electron microscopy (SEM) showed the same matrix morphology. The second phase by doped elements was not confirmed by transmission electron microscopy (TEM). By using the lead addition results and previously studied tellurium doping effects, the Lorenz number L was evaluated to be 0.73–1.18 × 10−8 W S−1 K−2. The scattering parameter γ and reduced Fermi energy η were estimated by using expressions on the basis of a one-electron approximation, measured Seebeck coefficients, and the estimated L at room temperature. The γ ranged approximately from −1.06 to −0.60 and showed a mutual effect of acoustic and optical phonon scattering. The relationship between a dimensionless figure of merit ZT and η was clarified. The optimum η was determined as −1.25 at ZT = 1.26. From these results, multi-doped Bi0.3Sb1.7Te3.0 could be applied to evaluate L, γ, and η at a constant temperature.

Quantum Confinement Suppressing Electronic Heat Flow below the Wiedemann-Franz Law.

The Wiedemann–Franz law states that the charge conductance and the electronic contribution to the heat conductance are proportional. This sets stringent constraints on efficiency bounds for thermoelectric applications, which seek a large charge conduction in response to a small heat flow. We present experiments based on a quantum dot formed inside a semiconducting InAs nanowire transistor, in which the heat conduction can be tuned significantly below the Wiedemann–Franz prediction. Comparison with scattering theory shows that this is caused by quantum confinement and the resulting energy-selective transport properties of the quantum dot. Our results open up perspectives for tailoring independently the heat and electrical conduction properties in semiconductor nanostructures.

Electronic structure and engineered thermoelectric properties of SnSe

Thermoelectric properties of SnSe like the maximum thermoelectric performance and the Lorenz number are obtained from density functional theory and Boltzmann transport theory. The transport properties have been engineered by assuming a rigid band shift for modeling the n- and p-type SnSe. In consistent with the experimental data close to the Fermi-level a two-valley valence-band structure along the Γ-Z direction has been observed. Furthermore, our calculated lattice thermal conductivity along the three axes shows the same trend as the experimental data (i.e. increasing from the x to the y to the z-axis). The engineered power factor enables us to obtain the maximum figure of merit of 2.89 at 850 K in the n-type SnSe which is higher than experimentally achieved value of 2.6. Finally, our calculated Lorenz number L for the p-doped SnSe at 300 K is close to the value of 2.44 × 10−8 WσK−2 from fundamental theory.

Electronic transport and Wiedemann-Franz law in 2D phosphorene

We have investigated electrical conductivity, electronic thermal conductivity and Wiedemann-Franz law in 2D phosphorene considering electrons to be scattered by single in-plane and two out-of-plane flexural phonon processes. The variation of Lorenz number in 2D phosphorene is investigated in the temperature range, 5 K to 300 K and its deviation from actual Lorenz number is noticed with increase in temperature. The behavior of electrical conductivity and electronic thermal conductivity of armchair and zigzag 2D phosphorene are investigated and found to be anisotropic in nature. The effective Lorenz number for 2D phosphorene is observed to vary by 32% from its actual value at room temperature T = 300 K.

Four-phonon and electron-phonon scattering effects on thermal properties in two-dimensional 2H-TaS2.

Thermal transport characteristics of monolayer trigonal prismatic tantalum disulfide (2H-TaS2) are investigated using first-principles calculations combined with the Boltzmann transport equation. Due to a large acoustic-optical phonon gap of 1.85 THz, the four-phonon (4ph) scattering significantly reduces the room-temperature phononic thermal conductivity (κph). With the further inclusion of phonon-electron scattering, κph reduces to 1.78 W mK-1. Nevertheless, the total thermal conductivity (κtotal) of 7.82 W mK-1 is dominated by the electronic thermal conductivity (κe) of 6.04 W mK-1. Due to the electron-phonon coupling, κe differs from the typical estimation based on the Wiedemann-Franz law with a constant Sommerfeld value. This work provides new insights into the physical mechanisms for thermal transport in metallic 2D systems with strong anharmonic and electron-phonon coupling effects. The phonon scattering beyond three-phonon (3ph) scattering and even κe are typically overlooked in computations, and the constant Sommerfeld value is widely used for separating κe and κph from the experimental thermal conductivity. These conclusions have implications for both the computational and experimental measurements of the thermal properties of transition metal dichalcogenides.

Strong Correlation between Electrical and Thermal Transport Properties in Graphene/Graphite Films Beyond the Wiedemann-Franz Law

Strong Correlation between Electrical and Thermal Transport Properties in Graphene/Graphite Films Beyond the Wiedemann-Franz Law

Investigation of the Lorenz number and the carrier concentration of the GaAs semiconductor depending on temperature

As is known, semiconductors are insulators under normal conditions but can become conductive with external excitation. Considering the effects of acting on these materials, the number of free electrons and the electrical conductivity will increase with increasing temperature. The increase in the concentration of free electrons in the semiconductor can be shown as the increase in electrical conductivity. If a semiconductor is exposed to an electric field with increasing concentration, we can have an idea about how the number of free electrons or the speed of free electrons will be affected. It is well known that it is necessary to calculate two-parameter Fermi functions to solve the properties of kinetic effects and electron transport phenomena in semiconductors. Effective methods have been developed for the calculation of two-parameter Fermi functions in literature. In this study, analytical calculations for the Lorenz number and the carrier concentration of the GaAs semiconductor were made using the two-parameter Fermi function.

Inhomogeneous Conformal Field Theory Out of Equilibrium

We study the non-equilibrium dynamics of conformal field theory (CFT) in 1+1 dimensions with a smooth position-dependent velocity v(x) explicitly breaking translation invariance. Such inhomogeneous CFT is argued to effectively describe 1+1-dimensional quantum many-body systems with certain inhomogeneities varying on mesoscopic scales. Both heat and charge transport are studied, where, for concreteness, we suppose that our CFT has a conserved U(1) current. Based on projective unitary representations of diffeomorphisms and smooth maps in Minkowskian CFT, we obtain a recipe for computing the exact non-equilibrium dynamics in inhomogeneous CFT when evolving from initial states defined by smooth inverse-temperature and chemical-potential profiles β(x)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta (x)$$\\end{document} and μ(x)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu (x)$$\\end{document}. Using this recipe, the following exact analytical results are obtained: (i) the full time evolution of densities and currents for heat and charge transport, (ii) correlation functions for components of the energy–momentum tensor and the U(1) current as well as for any primary field, and (iii) the thermal and electrical conductivities. The latter are computed by direct dynamical considerations and alternatively using a Green–Kubo formula. Both give the same explicit expressions for the conductivities, which reveal how inhomogeneous dynamics opens up the possibility for diffusion as well as implies a generalization of the Wiedemann–Franz law to finite times within CFT.