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We reviewed some recent ideas to improve the efficiency and power output of thermoelectric nano-devices. We focused on two essentially independent aspects: (i) increasing the charge current by taking advantage of an interplay between the material and the thermodynamic parameters, which is only available in the non-linear regime; and (ii) decreasing the heat current by using nanowires with surface disorder, which helps excite localized phonons at random positions that can strongly scatter the propagating phonons carrying the thermal current.

The nature of the pseudogap phase of cuprates remains a major puzzle1,2. One of its signatures is a large negative thermal Hall conductivity3, whose origin is as yet unknown. This is observed even in the undoped Mott insulator La2CuO4, in which the charge carriers are localized and therefore cannot be responsible. Here, we show that the thermal Hall conductivity of La2CuO4 is roughly isotropic; that is, for heat transport parallel and normal to the CuO2 planes, it is nearly the same. This shows that the Hall response must come from phonons, as they are the only heat carriers that are able to move with the same ease both normal and parallel to the planes4. For doping levels higher than the critical doping level at which the pseudogap phase ends, both La1.6−xNd0.4SrxCuO4 and La1.8−xEu0.2SrxCuO4 show no thermal Hall signal for a heat current normal to the planes, which establishes that phonons have zero Hall response outside the pseudogap phase. Inside the pseudogap phase, the phonons must become chiral to generate the Hall response, but the mechanism by which this happens remains to be identified. It must be intrinsic (from a coupling of phonons to their electronic environment) rather than extrinsic (from structural defects or impurities), as these are the same on both sides of critical doping. Thermal transport measurements show that there is a thermal Hall effect in the out-of-plane direction in two cuprates in the pseudogap regime. This indicates that phonons are carrying the heat and that they have a handedness of unknown origin.

We study hydrodynamic phonon heat transport in two-dimensional (2D) materials. Starting from the Peierls-Boltzmann equation with the Callaway model approximation, we derive a 2D Guyer-Krumhansl-like equation describing hydrodynamic phonon transport, taking into account the quadratic dispersion of flexural phonons. In addition to Poiseuille flow, second sound propagation, the equation predicts heat current vortices and negative non-local thermal conductance in 2D materials, which are common in classical fluids but have not yet been considered in phonon transport. Our results also illustrate the universal transport behaviors of hydrodynamics, independent of the type of quasi-particles and their microscopic interactions.

Interfaces often govern the thermal performance of nanoscale devices and nanostructured materials. As a result, accurate knowledge of thermal interface conductance is necessary to model the temperature response of nanoscale devices or nanostructured materials to heating. Here, we report the thermal boundary conductance between metals and insulators that are commonly used in spin-caloritronic experiments. We use time-domain thermoreflectance to measure the interface conductance between metals such as Au, Pt, Ta, Cu, and Al with garnet and oxide substrates, e.g., NiO, yttrium iron garnet (YIG), thulium iron garnet (TmIG), ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$, and sapphire. We find that, at room temperature, the interface conductance in these types of material systems range from 50 to $300\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. We also measure the interface conductance between Pt and YIG at temperatures between 80 and 350 K. At room temperature, the interface conductance of Pt/YIG is $170\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ and the Kapitza length is \ensuremath{\sim}40 nm. A Kapitza length of 40 nm means that, in the presence of a steady-state heat current, the temperature drop at the Pt/YIG interface is equal to the temperature drop across a 40-nm-thick layer of YIG. At 80 K, the interface conductance of Pt/YIG is $60\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, corresponding to a Kapitza length of \ensuremath{\sim}300 nm.

We study dissipation in holographic superfluids at finite temperature and zero chemical potential. The zero overlap with the heat current allows us to isolate the physics of the conserved current corresponding to the broken global $U(1)$. By using analytic techniques we write constitutive relations including the first non-trivial dissipative terms. The corresponding transport coefficients are determined in terms of thermodynamic quantities and the black hole horizon data. By analysing their behaviour close to the phase transition we show explicitly the breakdown of the hydrodynamic expansion. Finally, we study the pseudo-Goldstone mode that emerges upon introducing a perturbative symmetry breaking source and we determine its resonant frequency and decay rate., Comment: 31 pages, 5 figures, version accepted at JHEP

Three different experiments were performed in order to determine the mechanism of pillars formation using four-beam interference lithography. The experimental results demonstrate that pillars, fabricated in argon gas, were wider and higher compared with the pillars fabricated in nitrogen gas, low vacuum or air. It clearly indicates that the pillar bottom widening effect is not affected by the depletion of atmospheric oxygen as in all environments the fabricated pillars have a wider bottom part. Moreover, the shape of the fabricated pillars is not affecting by the back reflection from the positioning stage and by the light irradiation conditions. These results clearly indicate that the photopolymerization process is enhanced by the heat current and it determines the pillar bottom widening effect.

We study the steady-state entanglement and heat current of two coupled qubits, in which one qubit is connected with either two baths with the different temperature or a bath. We construct the master equation in the eigenstate representation of two coupled qubits to describe the dynamics and derive the solutions in the steady-state with strong coupling regime between them. We have demonstrated the variations of the steady-state entanglement with respect to various parameters of the system in both equilibrium and nonequilibrium cases. We find that the coupling strengths and the energy detuning as well as the temperature gradient are beneficial to the enhancement of the entanglement in equilibrium case. In the nonequilibrium case, the temperature gradient has different effects on entanglement due to the temperature of the bath(s) which the qubits touch. We also study the heat current and their variations with the energy detuning, coupling strengths and diverse low temperature. The energy detuning has a positive (negative) effect on the heat current in the low (high) temperature; heat current also have different trends with coupling strengths increases for a given temperature. The temperature of cool bath is lower, the heat current is larger.

In this paper, we theoretically investigate the Fano resonance and thermoelectric transport in a nanoscale Aharonov-Bohm interferometer consisting of an InAs quantum dot coupled to two electrodes under THz irradiation. In the non-linear regime, the charge current shows a symmetric line shape at asymmetric THz frequency even in the presence of the Fano effect. Under asymmetric THz irradiation, the heat current presents the cooling effect. Interestingly, the heat current will change its sign and the cooling effect appears in different V G region if we switch the THz frequency asymmetric ratio of the two electrodes. This characteristic provides a new way of manipulating thermoelectric devices. In addition, with increasing strength or frequency of the THz field, a restricted zone of the output power would appear around the region V G = 0.0 . These results have potential for the application of thermoelectric devices under THz irradiation.

The development of microwave photon detectors is paving the way for a wide range of quantum technologies and fundamental discoveries involving single photons. Here, we investigate the photon emission from a microwave cavity and find that distribution of photon waiting times contains information about few-photon processes, which cannot easily be extracted from standard correlation measurements. The factorial cumulants of the photon counting statistics are positive at all times, which may be intimately linked with the bosonic quantum nature of the photons. We obtain a simple expression for the rare fluctuations of the photon current, which is helpful in understanding earlier results on heat transport statistics and measurements of work distributions. Under non-equilibrium conditions, where a small temperature gradient drives a heat current through the cavity, we formulate a fluctuation-dissipation relation for the heat noise spectra. Our work suggests a number of experiments for the near future, and it offers theoretical questions for further investigation., 16 pages, 3 figures, final version as published in Phys. Rev. B

International audience; We consider heat transport across a harmonic chain of charged particles, with transverse degrees of freedom, in the presence of a uniform magnetic field. For an open chain connected to heat baths at the two ends we obtain the nonequilibrium Green's function expression for the heat current. This expression involves two different Green's functions which can be identified as corresponding respectively to scattering processes within or between the two transverse waves. The presence of the magnetic field leads to two phonon bands of the isolated system and we show that the net transmission can be written as a sum of two distinct terms attributable to the two bands. Exact expressions are obtained for the current in the thermodynamic limit, for the the cases of free and fixed boundary conditions. In this limit, we find that at small frequency $\omega$, the effective transmission has the frequency-dependence $\omega^{3/2}$ and $\omega^{1/2}$ for fixed and free boundary conditions respectively. This is in contrast to the zero magnetic field case where the transmission has the dependence $\omega^2$ and $\omega^0$ for the two boundary conditions respectively, and can be understood as arising from the quadratic low frequency phonon dispersion.

This study investigated the thermal transport behaviors of branched carbon nanotubes (CNTs) with cross and T-junctions through non-equilibrium molecular dynamics (NEMD) simulations. A hot region was created at the end of one branch, whereas cold regions were created at the ends of all other branches. The effects on thermal flow due to branch length, topological defects at junctions, and temperature were studied. The NEMD simulations at room temperature indicated that heat transfer tended to move sideways rather than straight in branched CNTs with cross-junctions, despite all branches being identical in chirality and length. However, straight heat transfer was preferred in branched CNTs with T-junctions, irrespective of the atomic configuration of the junction. As branches became longer, the heat current inside approached the values obtained through conventional prediction based on diffusive thermal transport. Moreover, directional thermal transport behaviors became prominent at a low temperature (50 K), which implied that ballistic phonon transport contributed greatly to directional thermal transport. Finally, the collective atomic velocity cross-correlation spectra between branches were used to analyze phonon transport mechanisms for different junctions. Our findings deeply elucidate the thermal transport mechanisms of branched CNTs, which aid in thermal management applications.

Heat current generated by electronic transport through a quantum dot (QD) coupled to both a phonon bath and a Majorana nanowire hosting Majorana bound states (MBSs) is theoretically studied in the framework of non-equilibrium Green’s function technique. The calculated numerical results show that electrical current can be either enhanced or suppressed by the combined influences of the phonon bath and the MBSs at certain bias voltage regimes. The enhancement and suppression of the current’s magnitude for a fixed bias voltage will be reversed due to the direct hybridization between the MBSs. The simultaneous coupling between both MBSs will amplify the function of the MBSs on the current, with the same unchanged and essential qualitative impacts. Heat generation by the electrical current can be fully adjusted by the dot–MBS coupling, direct hybridization between the MBSs, and positions of the dot level. By properly choosing the above parameters, heat generation can be suppressed even for increased electrical current, which is favorable in removing waste heat generated by electrical current flowing through low-dimensional circuits.

Heat current J that flows through a few typical two-dimensional nonlinear lattices is systematically studied. Each lattice consists of two identical segments that are coupled by an interface with strength k_{int}. It is found that the two-universality-class scenario that is revealed in one-dimensional systems is still valid in the two-dimensional systems. Namely, J may follow k_{int} in two entirely different ways, depending on whether or not the interface potential energy decays to zero. We also study the dependence of J on lattice width N_{Y} and transverse interaction strength k_{Y}. Universal power-law decay or divergence is observed. Finally, we check the equipartition theorem in the systems since it is the basis of all our theoretical analyses. Surprisingly, it holds perfectly even at the interface where there is a finite temperature jump, which makes the system far from equilibrium. However, the equipartition of potential energy, which is observed in one-dimensional systems, is no longer satisfied due to the interaction between different dimensions.

In this paper we consider a bipartite system composed of two subsystems each coupled to its own thermal environment. Based on a collision model, we mainly study whether the approximation (i.e., the inter-system interaction is ignored when modeling the system-environment coupling) is valid or not. We also address the problem of heat transport unitedly for both conventional energy-preserving system-environment interactions and non-energy preserving system-environment interactions. For the former interaction, as the inter-system interaction strength increases, at first this approximation gets worse as expected, but then counterintuitively gets better even for a stronger inter-system coupling. For the latter interaction with asymmetry, this approximation gets progressively worse. In this case we realize a perfect thermal rectification, and we can not find apparent rectification effect for the former interaction. Finally and more importantly, our results show that whether this approximation is valid or not is closely related to the quantum correlations between the subsystems, i.e., the weaker the quantum correlations, the more justified the approximation and vice versa., 9 pages, 10 figures

The thermodynamic uncertainty relation, originally derived for classical Markov-jump processes, provides a trade-off relation between precision and dissipation, deepening our understanding of the performance of quantum thermal machines. Here, we examine the interplay of quantum system coherences and heat current fluctuations on the validity of the thermodynamics uncertainty relation in the quantum regime. To achieve the current statistics, we perform a full counting statistics simulation of the Redfield quantum master equation. We focus on steady-state quantum absorption refrigerators where nonzero coherence between eigenstates can either suppress or enhance the cooling power, compared with the incoherent limit. In either scenario, we find enhanced relative noise of the cooling power (standard deviation of the power over the mean) in the presence of system coherence, thereby corroborating the thermodynamic uncertainty relation. Our results indicate that fluctuations necessitate consideration when assessing the performance of quantum coherent thermal machines., 14 pages, 11 figures, comments and suggestions are welcome

Heat conduction through a disordered Fermi-Pasta-Ulam-$\ensuremath{\beta}$ (DFPU-$\ensuremath{\beta}$) chain is studied. The presence of disorder makes the heat current behave significantly different from that of the ordered Fermi-Pasta-Ulam-$\ensuremath{\beta}$ (FPU-$\ensuremath{\beta}$) chain. Thanks to the interplay between disorder and anharmonicity, a nonmonotonic-monotonic transition occurs when the disorder strength increases. That is, a peak for the heat current emerges for weak disorder; however, monotonic increasing of the heat current shows up for strong disorder. This can be understood based on the competition between two effects of anharmonicity on phonons, namely, delocalization and phonon-phonon scattering, which is shown by the spectral decomposition of heat current.

We present a semi-analytical approach for studying quantum thermal energy transport beyond the weak system-bath coupling regime. Our treatment, which results in a renormalized, effective Hamiltonian model is based on the reaction coordinate method. In our technique, applied to the nonequilibrium spin-boson model, a collective coordinate is extracted from each environment and added into the system to construct an enlarged system. After performing additional Hamiltonian's truncation and transformation, we attain an effective two-level system with renormalized parameters, which is weakly coupled to its environments, thus can be simulated using a perturbative Markovian quantum master equation approach. We compare the heat current characteristics in our method to other techniques, and demonstrate that we properly capture strong system-bath signatures such as the turnover behavior of the heat current as a function of system-bath coupling strength. We further investigate the thermal diode effect and demonstrate that strong couplings moderately improve the rectification ratio relative to the weak coupling limit. The effective Hamiltonian method that we developed here offers fundamental insight into the strong coupling behavior, and is computationally economic. Applications of the method towards studying quantum thermal machines are anticipated., 19 pages, 10 figures

We develop a non-Markovian approach to study quantum dissipation and thermal transport in the nonequilibrium two-level system based on a canonical transformation and full-counting statistics. Specifically, we analytically study dissipative dynamics of population difference, quantum coherence, and heat current in terms of the canonical transformed Redfield approach. It demonstrates that the dynamics of both the population difference and the energy flow show monotonic decaying behaviors with the identical decay rate, whereas the quantum coherence is dominated by the oscillation showing the non-Markovian effect. Moreover, through tuning the temperature bias of thermal baths, negative differential thermal conductance is clearly exhibited beyond the weak system-bath coupling limit and Markovian approximation. The results may provide guidance for the efficient control of energy and the information transport in nanodevices.

Using the standard fluctuational electrodynamics framework, we analytically calculate the radiative heat current between two thin metallic layers, separated by a vacuum gap. We analyze different contributions to the heat current (traveling or evanescent waves, transverse electric or magnetic polarization) and reveal the crucial qualitative role played by the dc conductivity of the metals as compared to the speed of light. For poorly conducting metals, the heat current may be dominated by evanescent waves even when the separation between the layers greatly exceeds the thermal photon wavelength, and the coupling is of an electrostatic nature. For well-conducting metals, the evanescent contribution dominates at separations smaller than the thermal wavelength and is mainly due to magnetostatic coupling, in agreement with earlier works on bulk metals.

openaire: EC/H2020/742559/EU//SQH | openaire: EC/H2020/766025/EU//QuESTech We discuss the nonzero frequency noise of heat current with the explicit example of energy carried by thermal photons in a circuit. Instead of the standard circuit modeling that gives a convenient way of predicting timeaveraged heat current, we describe a setup composed of two resistors forming the heat baths by collections of bosonic oscillators. In terms of average heat transport this model leads to identical results with the conventional one, but besides this, it yields a convenient way of dealing with noise as well. The nonzero-frequency heat current noise does not vanish in equilibrium even at zero temperature, a result that is known for, e.g., electron tunneling. We present a modulation method that can convert the difficult-to-measure high-frequency quantum noise down to zero frequency.

The fluctuations of the heat current in a quantum dot coupled to electron reservoirs are calculated at finite frequency, voltage and temperature using the nonequilibrium Green function technique. The non-symmetrized heat noise is expressed as an integral on energy containing four contributions, each of which includes transmission amplitudes, electron-hole pair distribution functions and energy difference factors. The effect of the asymmetry of the couplings between the quantum dot and the reservoirs is studied. Features of the heat noise are highlighted and discussed for an equilibrium and an out-of-equilibrium quantum dot. In the latter case and within the high transmission limit, the heat noise is closely related to the radiative power spectrum, leading to an out-of-equilibrium Planck's law. Proposals for the measurement of the heat noise are discussed., Comment: 7 pages, 4 figures

We consider a harmonic chain of N oscillators in the presence of a disordered magnetic field. The ends of the chain are connected to heat baths and we study the effects of the magnetic field randomness on heat transport. The disorder, in general, causes localization of the normal modes, due to which a system becomes insulating. However, for this system, the localization length diverges as the normal mode frequency approaches zero. Therefore, the low frequency modes contribute to the transmission, T N ( ω ) , and the heat current goes down as a power law with the system size, N. This power law is determined by the small frequency behaviour of some Lyapunov exponents, λ(ω), and the transmission in the thermodynamic limit, T ∞ ( ω ) . While it is known that in the presence of a constant magnetic field T ∞ ( ω ) ∼ ω 3 / 2 , ω 1 / 2 depending on the boundary conditions, we find that the Lyapunov exponent for the system behaves as λ(ω) ∼ ω for B ≠ 0 and λ(ω) ∼ ω 2/3 for B = 0 . Therefore, we obtain different power laws for current vs N depending on B and the boundary conditions.

When a quantum system is subject to a thermal gradient it may sustain a steady non-equilibrium heat current, by entering into a so-called non equilibrium steady state (NESS). Here we show that NESS constitute a thermodynamic resource that can be exploited to fuel a quantum heat engine. This adds to the list of recently reported sources available at the nano-scale, such as coherence, entanglement and quantum measurements. We elucidate this concept by showing analytic and numerical studies of a two-qubits quantum battery that is alternatively charged by a thermal gradient and discharged by application of a properly chosen unitary gate. The presence of a NESS for the charging step guarantees steady operation with positive power output. Decreasing the duration of the charging step results in a time periodic steady state accompanied by increased efficiency and output power. The device is amenable to implementation with different nanotechnology platforms., 6 pages, 4 figures

Photoisomeric molecules rearrange their structure when exposed to light, which alters their chemical, electronic, mechanical, as well as vibrational properties. The present study explores the possibilities to tune the thermal transport across molecular junctions by using photoisomeric molecules. The effect of isomeric switching on phonon transport through single-molecule junctions linking two macroscopic reservoirs is investigated using density-functional-theory-based tight-binding calculations and Green-function formalism. The junctions are built using azobenzene and its derivatives (azobiphenyl and azotriphenyl) that display photoisomeric behavior. Effects of system setup on the heat current and the switching coefficient are studied systematically. Dependence on the molecular species, the choice of reservoir, as well as the type of linkers that bind the molecules to the reservoir are investigated with calculating the phonon-transmission spectra and temperature-dependent thermal conductance values. The results show that thermal conductance can be altered significantly by switching the molecule from trans- to cis-configuration since all molecules yield higher conductances in trans-configurations than their cis-configurations at temperatures higher than 50 K. In the low-temperature range, results reveal considerable switching coefficients exceeding $50\mathrm{%}$. At room temperature, the switching coefficient can be as high as $20\mathrm{%}$. It is shown that the effect is robust under the variation of both the molecular species and the linkers.

The spin Nernst effect (SNE) and anomalous Nernst effect (ANE) convert a heat current generated by a temperature gradient to a spin current in nonmagnetic metals and ferromagnets, respectively. We present a diffusive spin transport theory of the combination of these thermoelectric effects in ferromagnet (F)/nonmagnet (N) heterostructures and their respective output signals. In a F/N bilayer, in the presence of an in-plane temperature gradient, electrical voltages are induced by the SNE in N and the ANE in the F layers, via the inverse spin Hall effect in the N. The analytical expression of the thermally driven spin Hall magnetoresistance (TSMR) output is derived, which captures both the local SNE contribution in the N and the nonlocal mixing contribution due to the ANE in the adjacent F. Interestingly, the SNE and ANE give additive contributions to the transverse TSMR voltages but subtract from each other for the longitudinal component. This anisotropic response suggests a possible means to extract the individual SNE and ANE contributions, as well as spin-dependent polarization parameters. In a F1/N/F2 trilayer, both SNE and ANE contribute to spin torques on the free F2 layer. We analyze the ferromagnetic resonance (FMR) of the free layer and show that both thermoelectric effects contribute additively to the FMR magnitude and linewidth for the considered magnetization configuration. Finally, we show that TSMR can provide a more sensitive experimental detection of SNE and ANE down to a threshold thermal gradient of the order of ${10}^{\ensuremath{-}6}$ K/nm.

We discuss the nonradiative heat transfer in a nonequilibrium double-layer graphene system. We show that at the neutrality point the heat exchange is dominated by the interlayer plasmon modes and derive analytic expressions for the heat current as a function of temperature and the interlayer separation. These results show that for a range of low temperatures the two graphene layers are a much more efficient heat exchanger than conventional metals. The physical reason behind this phenomenon is the presence of interband excitations with a large energy, and a small momentum in the graphene spectrum. The plasmonic mechanism of the heat transfer is sharply suppressed by electrostatic doping. This allows for tuning of the heat exchange by a small applied voltage between the two layers.

We propose a way to properly interpret the apparent thermal conductivity obtained for finite systems using equilibrium molecular dynamics simulations (EMD) with fixed or open boundary conditions in the transport direction. In such systems the heat current autocorrelation function develops negative values after a correlation time which is proportional to the length of the simulation cell in the transport direction. Accordingly, the running thermal conductivity develops a maximum value at the same correlation time and eventually decays to zero. By comparing EMD with nonequilibrium molecular dynamics (NEMD) simulations, we conclude that the maximum thermal conductivity from EMD in a system with domain length 2L is equal to the thermal conductivity from NEMD in a system with domain length L. This facilitates the use of nonperiodic-boundary EMD for thermal transport in finite samples in close correspondence to NEMD., 7pages, 8 figures

Prevailing nanostructuring strategies focus on increasing phonon scattering and reducing the mean-free-path of phonons across the spectrum. In nanoporous Si materials, for example, boundary scattering reduces thermal conductivity drastically. In this work, we identify an unusual anticorrelated specular phonon scattering effect which can result in additional reductions in thermal conductivity of up to ~ 80% for specific nanoporous geometries. We further find evidence that this effect has its origin in heat trapping between large pores with narrow necks. As the heat becomes trapped between the pores, phonons undergo multiple specular reflections such that their contribution to the thermal conductivity is partly undone. We find this effect to be wave-vector dependent at low temperatures. We use large-scale molecular dynamics simulations, wave packet analysis, as well as an analytical model to illustrate the anticorrelation effect, evaluate its impact on thermal conductivity, and detail how it can be controlled to manipulate phonon transport in nanoporous materials., 24 pages, 5 figures

We study heat rectification in a minimalistic model composed of two unequal atoms subjected to linear forces and in contact with effective Langevin baths induced by Doppler lasers. Analytic expressions of the heat currents in the steady state are spelled out. Asymmetric heat transport is found in this linear system if both the bath temperatures and the temperature-dependent bath-system couplings are exchanged. The model can be realized with two ions in either common or individual traps. This physical setting allows for a natural temperature dependence of the coupling to the baths. We also explore the parameter space of the model to optimize asymmetric heat current and find conditions for maximal rectification. High rectification corresponds to a good match of the power spectra of the ions for forward temperature bias and mismatch for reverse bias, which may be understood by the behavior of dissipative normal modes.

The appearance of half-quantized thermal Hall conductivity in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ in the presence of in-plane magnetic fields has been taken as a strong evidence for the Kitaev spin liquid. Apart from the quantization, the observed sign structure of the thermal Hall conductivity is also consistent with predictions from the exact solution of the Kitaev honeycomb model. Namely, the thermal Hall conductivity changes sign when the field direction is reversed with respect to the heat current, which is perpendicular to one of the three nearest neighbor bonds on the honeycomb lattice. On the other hand, the thermal Hall conductivity is almost zero when the field is applied along the bond direction. Here, we theoretically demonstrate that such a peculiar sign structure of the thermal Hall conductivity is a generic property of the polarized state in the presence of in-plane magnetic fields. In this case, the thermal Hall effect arises from topological magnons with finite Chern numbers, and the sign structure follows from the symmetries of the momentum space Berry curvature. Using a realistic spin model with bond-dependent interactions, we show that the thermal Hall conductivity can have a magnitude comparable to that observed in the experiments. Hence, the sign structure alone cannot make a strong case for the Kitaev spin liquid. The quantization at very low temperatures, however, will be a decisive test as the magnon contribution vanishes in the zero temperature limit.

We explore transport properties in a disordered nonlinear chain of classical harmonic oscillators and thereby identify a regime exhibiting behavior analogous to that seen in quantum many-body-localized systems. Through extensive numerical simulations of this system connected at its ends to heat baths at different temperatures, we computed the heat current and the temperature profile in the nonequilibrium steady state as a function of system size $N$, disorder strength $\Delta$, and temperature $T$. The conductivity $\kappa_N$, obtained for finite length ($N$) systems, saturates to a value $\kappa_\infty >0$ in the large $N$ limit, for all values of disorder strength $\Delta$ and temperature $T>0$. We show evidence that for any $\Delta>0$ the conductivity goes to zero faster than any power of $T$ in the $(T/\Delta) \to 0$ limit, and find that the form $\kappa_\infty \sim e^{-B |\ln(C \Delta/T)|^3}$ fits our data. This form has earlier been suggested by a theory based on the dynamics of multi-oscillator chaotic islands. The finite-size effect can be $\kappa_N < \kappa_{\infty}$ due to boundary resistance when the bulk conductivity is high (the weak disorder case), or $\kappa_N > \kappa_{\infty}$ due to direct bath-to-bath coupling through bulk localized modes when the bulk is weakly conducting (the strong disorder case). We also present results on equilibrium dynamical correlation functions and on the role of chaos on transport properties. Finally, we explore the differences in the growth and propagation of chaos in the weak and strong chaos regimes by studying the classical version of the Out-of-Time-Ordered-Commutator., Comment: To appear in Phys. Rev. E

The emerging of spin caloritronics leads to a series of new spin-thermal related effects, such as spin Seebeck effect (SSE), spin Nernst effect (SNE) and their corresponding inverse effects. Anomalous Righi–Leduc effect (ARLE) describes that a transverse temperature gradient can be induced by a longitudinal heat flow in ferromagnets. The driving force and the response of the ARLE are all involved with heat. It is curious if spin effects mediate the heat transport and provide extra influence. In this work, we investigate the ARLE and the interplay between the heat current, charge current, and spin current via linear response theory. We identified that spin effects do have clear roles in heat transport, which can be confirmed by phase shifts of voltage output varying with the direction of magnetization. Our formulas fit the experimental data very well. Moreover, we discuss more configuration of magnetization which is expected to be tested in the future. It should be emphasized that the present formalism including spin effects is out of the theory based on magnon transport, which may be conspicuous in the devices within the spin diffusion length.

International audience; We study the heat current flowing between two baths consisting of harmonic oscillators interacting with a qubit through a spin-boson coupling. An explicit expression for the generating function of the total heat flowing between the right and left baths is derived by evaluating the corresponding Feynman-Vernon path integral by performing the non-interacting blip approximation (NIBA). We recover the known expression, obtained by using the polaron transform. This generating function satisfies the Gallavotti-Cohen fluctuation theorem, both before and after performing the NIBA. We also verify that the heat conductance is proportional to the variance of the heat current, retrieving the well known fluctuation dissipation relation. Finally, we present numerical results for the heat current.

We study the heat current through two capacitively coupled quantum dots coupled in series with two conducting leads at different temperatures T L and T R in the spinless case (valid for a high applied magnetic field). Our results are also valid for the heat current through a single quantum dot with strongly ferromagnetic leads pointing in opposite directions (so that the electrons with given spin at the dot can jump only to one lead) or through a quantum dot with two degenerate levels with destructive quantum interference and high magnetic field. Although the charge current is always zero, the heat current is finite when the interdot Coulomb repulsion U is taken into account due to many-body effects. We study the thermal conductance as a function of temperature and the dependence of the thermal current with the couplings to the leads, T L − T R , energy levels of the dots and U , including conditions for which an orbital Kondo regime takes place. When the energy levels of the dots are different, the device has rectifying properties for the thermal current. We find that the ratio between the thermal current resulting from a thermal bias T L > T R and the one from T L < T R is maximized for particular values of the energy levels, one above and the other below the Fermi level. Fil: Aligia, Armando Angel. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina Fil: Perez Daroca, Diego Raul. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Arrachea, Liliana del Carmen. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología. Centro Internacional de Estudios Avanzados; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Roura Bas, Pablo Gines. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina

We investigate the microscopic mechanisms of ultralow lattice thermal conductivity (${\ensuremath{\kappa}}_{l}$) in ${\mathrm{Tl}}_{3}{\mathrm{VSe}}_{4}$ by combining a first principles density functional theory based framework of anharmonic lattice dynamics with the Peierls-Boltzmann transport equation for phonons. We include contributions of the three- and four-phonon scattering processes to the phonon lifetimes as well as the temperature dependent anharmonic renormalization of phonon energies arising from an unusually strong quartic anharmonicity in ${\mathrm{Tl}}_{3}{\mathrm{VSe}}_{4}$. In contrast to a recent report by Mukhopadhyay et al. [Science 360, 1455 (2018)] which suggested that a significant contribution to ${\ensuremath{\kappa}}_{l}$ arises from random walks among uncorrelated oscillators, we show that particlelike propagation of phonon excitations can successfully explain the experimentally observed ultralow ${\ensuremath{\kappa}}_{l}$. Our findings are further supported by explicit calculations of the off-diagonal terms of the heat current operator, which are found to be small and indicate that wavelike tunneling of heat carrying vibrations is of minor importance. Our results (i) resolve the discrepancy between the theoretical and experimental ${\ensuremath{\kappa}}_{l}$, (ii) offer new insights into the minimum ${\ensuremath{\kappa}}_{l}$ achievable in ${\mathrm{Tl}}_{3}{\mathrm{VSe}}_{4}$, and (iii) highlight the importance of high order anharmonicity in low-${\ensuremath{\kappa}}_{l}$ systems. The methodology demonstrated here may be used to resolve the discrepancies between the experimentally measured and the theoretically calculated ${\ensuremath{\kappa}}_{l}$ in skutterides and perovskites, as well as to understand the glasslike ${\ensuremath{\kappa}}_{l}$ in complex crystals with strong anharmonicity, leading towards the goal of rational design of new materials.

We perform a theoretical study of the near-field heat transfer between two atomically thin metallic layers, isolated galvanically but coupled by the Coulomb interaction, within the framework of fluctuational electrodynamics in the Coulomb limit. We clarify the role of disorder and spatial dispersion, and identify several distinct regimes of the heat transfer. We find that the plasmon contribution to the heat current is suppressed both in the clean and diffusive limits, but dominates in a parametrically wide crossover region at sufficiently high temperatures., Addition of Sec. IIC. discussing the generality of the results - in particular clarifying the validity conditions for the near-field limit. Addition of Sec. VI. comparing the theoretical results to experiments. Addition of an illustrative panel in Fig. 5(b)

When assembling individual quantum components into a mesoscopic circuit, the interplay between Coulomb interaction and charge granularity breaks down the classical laws of electrical impedance composition. Here we explore experimentally the thermal consequences, and observe an additional quantum mechanism of electronic heat transport. The investigated, broadly tunable test-bed circuit is composed of a micron-scale metallic node connected to one electronic channel and a resistance. Heating up the node with Joule dissipation, we separately determine, from complementary noise measurements, both its temperature and the thermal shot noise induced by the temperature difference across the channel (`delta-$T$ noise'). The thermal shot noise predictions are thereby directly validated, and the electronic heat flow is revealed. The latter exhibits a contribution from the channel involving the electrons' partitioning together with the Coulomb interaction. Expanding heat current predictions to include the thermal shot noise, we find a quantitative agreement with experiments., Minor differences with published article: additional reference [29], more explicit identification 'thermal shot noise' - 'delta-T noise', includes the supplementary figures

Developing quantum machines which can outperform their classical counterparts, thereby achieving quantum supremacy or quantum advantage, is a major aim of the current research on quantum thermodynamics and quantum technologies. Here we show that a fast-modulated cyclic quantum heat machine operating in the non-Markovian regime can lead to significant heat-current and power boosts induced by the anti-Zeno effect. Such boosts signify a quantum advantage over almost all heat-machines proposed thus far that operate in the conventional Markovian regime, where the quantumness of the system-bath interaction plays no role. The present effect owes its origin to the time-energy uncertainty relation in quantum mechanics, which may result in enhanced system-bath energy exchange for modulation periods shorter than the bath correlation-time., Comment: 16 pages, 10 figures

We propose a three-qubit setup for the implementation of a variety of quantum thermal machines where all heat fluxes and work production can be controlled. An important configuration that can be designed is that of an absorption refrigerator, extracting heat from the coldest reservoir without the need of external work supply. Remarkably, we achieve this regime by using only two-body interactions instead of the widely employed three-body interactions. This configuration could be more easily realised in current experimental setups. We model the open-system dynamics with both a global and a local master equation thermodynamic-consistent approach. Finally, we show how this model can be employed as a heat valve, in which by varying the local field of one of the two qubits allows one to control and amplify the heat current between the other qubits., Close to published version

The half-Heusler rare-earth intermetallic GdPtBi has recently gained attention due to peculiar magnetotransport phenomena that have been associated with the possible existence of Weyl fermions, thought to arise from the crossings of spin-split conduction and valence bands. On the other hand, similar magnetotransport phenomena observed in other rare-earth intermetallics have often been attributed to the interaction of itinerant carriers with localized magnetic moments stemming from the $4f$-shell of the rare-earth element. In order to address the origin of the magnetotransport phenomena in GdPtBi, we performed a comprehensive study of the magnetization, electrical and thermal magnetoresistivity on two single-crystalline GdPtBi samples. In addition, we performed an analysis of the Fermi surface via Shubnikov-de Haas oscillations in one of the samples and compared the results to \emph{ab initio} band structure calculations. Our findings indicate that the electrical and thermal magnetotransport in GdPtBi cannot be solely explained by Weyl physics and is strongly influenced by the interaction of both itinerant charge carriers and phonons with localized magnetic Gd-ions and possibly also paramagnetic impurities., Comment: 11 figures

Counter-rotating-wave terms (CRWTs) are traditionally viewed to be crucial in open small quantum systems with strong system-bath dissipation. Here by exemplifying in a nonequilibrium qubit-phonon hybrid model, we show that CRWTs can play the significant role in quantum heat transfer even with weak system-bath dissipation. By using coherent phonon states, we obtain the quantum master equation with heat exchange rates contributed by rotating-wave-terms (RWTs) and CRWTs, respectively. We find that including only RWTs, steady state heat current and current fluctuations will be significantly suppressed at large temperature bias, whereas they are strongly enhanced by considering CRWTs in addition. Furthermore, for the phonon statistics, the average phonon number and two-phonon correlation are nearly insensitive to strong qubit-phonon hybridization with only RWTs, whereas they will be dramatically cooled down via the cooperative transitions based on CRWTs in addition. Therefore, CRWTs in quantum heat transfer system should be treated carefully., Comment: 8 pages, 4 figures

Thermal transport through nanosystems is central to numerous processes in chemistry, material sciences, and electrical and mechanical engineering, with classical molecular dynamics as the key simulation tool. Here, we focus on thermal junctions with a molecule bridging two solids that are maintained at different temperatures. The classical steady state heat current in this system can be simulated in different ways, either at the interfaces with the solids, which are represented by thermostats, or between atoms within the conducting molecule. We show that while the latter, intramolecular definition feasibly converges to the correct limit, the molecule–thermostat interface definition is more challenging to converge to the correct result. The problem with the interface definition is demonstrated by simulating heat transport in harmonic and anharmonic one-dimensional chains illustrating unphysical effects such as thermal rectification in harmonic junctions.

We study the constraints on reciprocal and non-reciprocal many-body radiative heat transfer imposed by symmetry and the second law of thermodynamics. We show that the symmetry of such a many-body system in general forms a magnetic group, and the constraints of the magnetic group on the heat transfer can be derived using a generalized reciprocity theorem. We also show that the second law of thermodynamics provides additional constraints in the form of a nodal conservation law of heat flow at equilibrium. As an application, we provide a systematic approach to determine the existence of persistent heat current in arbitrary many-body systems., Comment: 8 pages, 3 figures

We show that in mesoscopic four-terminal thermoelectric devices with two electrodes (the source and the drain) and two heat baths, inelastic scattering processes can lead to unconventional thermoelectric transport. The source (or the drain) can be cooled by passing a thermal current between the two heat baths, with no net heat exchange between the heat baths and the electrodes. This effect, termed as cooling by heat current, is a mesoscopic heat drag effect. In addition, there is a transverse thermoelectric effect where electrical current and power can be generated by a transverse temperature bias (i.e., the temperature bias between the two heat baths). This transverse thermoelectric effect, originates from inelastic scattering processes, may have advantages for improved figures of merit and power factor due to spatial separation of charge and heat transport. We study the Onsager current-affinity relations, the linear-response transport properties, and the transverse thermoelectric figure of merit of the four-terminal thermoelectric devices for various system parameters. In addition, we investigate the efficiency and power of the cooling by transverse current effect in both linear and nonlinear transport regimes. We also demonstrate that by exploiting the inelastic transport in the quantum-dot four-terminal systems, a type of Maxwell's demon can be realized using nonequilibrium heat baths., Comment: 11 pages, 8 figures

Using the non-equilibrium Green's function method, we theoretically investigate the thermoelectric transport properties in an InAs quantum dot subjected to THz irradiation and an external magnetic field. Very interestingly, we get the cooling effect under the asymmetric THz irradiation, i.e., the heat current shows a negative value through particular photon-assisted tunneling channels, and we can switch the direction of heat current effectively just by changing both the THz frequency and the gate voltage. In addition, when a THz field is applied to two leads symmetrically, we reveal the dependence of the charge and spin figure of merit on the temperature in various magnetic fields. It is found that the spin figure of merit is sensitive to the magnetic field. These results shed some light on potential applications of the cooling effect in controlling the heat current from THz irradiation.

We investigate Brownian heat current induced by noise and driving in the heat conduction of Frenkel–Kontorova (FK) lattices, whose two ends contact respectively with two baths with temporally. Our numerical results indicate that thermal current of the system versus environmental reference temperature exhibits two peaks in symmetric cases, i.e., thermal SR. In asymmetric cases, the phenomenon also takes place, depending on on-site potentials of the atoms exerted on by the periodic signal, but disappears for harmonic lattices. Moreover, its intrinsic physical mechanism is analyzed in detail. The above results play important roles in controlling thermal current of nonlinear lattices.

Calcite based materials are being used in a range of new energy technologies for power generation and energy storage; in addition to their traditional use in manufacturing, agri-food industry and medicine. The optimum design of these new energy technology platforms requires an in-depth knowledge of the thermo-physical properties of calcite. In this study, the lattice thermal conductivity of calcite (CaCO 3 ) is investigated in detail over a wide temperature range. Calculations are performed within the framework of equilibrium molecular dynamics simulations in conjunction with the Green-Kubo formalism. To describe the interatomic interactions, the potential parameters of the shell model potentials for calcite developed by Pavese et al. are used. Within the framework of this model we calculate the heat current autocorrelation function over the temperature range of 200–800 K for averages over the NPT , NVT and NVE ensembles in the a- and c- directions. It is revealed that the lattice thermal conductivity can be decomposed into three contributions due to the optical, acoustic short- and long-range phonon modes. Finally, results from this study can be compared with previous related dielectric materials and experimental studies, with good agreement.

Size effects are discussed for several non-linear heat conduction models with flux-limited behaviors, including the phonon hydrodynamic, Lagrange multiplier, hierarchy moment, nonlinear phonon hydrodynamic, tempered diffusion, thermon gas and generalized nonlinear models. For the phonon hydrodynamic, Lagrange multiplier and tempered diffusion models, heat flux will not exist in problems with sufficiently small scale. The existence of heat flux needs the sizes of heat conduction larger than their corresponding critical sizes, which are determined by the physical properties and boundary temperatures. The critical sizes can be regarded as the theoretical limits of the applicable ranges for these non-linear heat conduction models with flux-limited behaviors. For sufficiently small scale heat conduction, the phonon hydrodynamic and Lagrange multiplier models can also predict the theoretical possibility of violating the second law and multiplicity. Comparisons are also made between these non-Fourier models and non-linear Fourier heat conduction in the type of fast diffusion, which can also predict flux-limited behaviors.

Liquid layering is considered to be one of the factors contributing to the often anomalous enhancement in thermal conductivity of nanoparticle suspensions. The extent of this layering was found to be significant at lower particle sizes, as reported in an earlier work by the authors. In continuation to that work, an investigation was conducted to better understand the fundamental parameters impacting the reported anomalous enhancement in thermal conductivity of nanoparticle suspensions (nanofluids), utilizing equilibrium molecular dynamics simulations in a copper-argon system. Nanofluids containing nanoparticles of size less than 6 nm were investigated and studied analytically. The heat current auto-correlation function in the Green-Kubo formulation for thermal conductivity was decomposed into self-correlations and cross-correlations of different species and the kinetic, potential, collision and enthalpy terms of the dominant portion of the heat current vector. The presence of liquid layering around the nanoparticle was firmly established through simulations that show the dominant contribution of Ar-Ar self-correlation and the trend displayed by the kinetic-potential cross-correlation within the argon species.

Photon heat transport through a mesoscopic Josephson junction (MJJ) device under the perturbation of dc and ac voltages has been investigated, where the MJJ device is coupled to photon reservoirs, and the Caldeira–Leggett circuit theory has been used. The photon heat current and differential heat conductance have been evaluated to show the dynamic behaviors governed by the applied fields. The dc voltage V induces time t oscillating supercurrent with frequency ω = 2 e V ∕ ħ . The ac voltage V cos ( Ω 0 t ) generates a series of supercurrent branches relating to the ac voltage frequency Ω 0 and its magnitude V . The photon heat current is determined by the superposition of different heat current branches induced by the dc and ac fields. The frequencies ω and Ω 0 relating to dc and ac fields play important role in controlling the photon heat current and conductance. The detailed magnitude and oscillation structure are strongly dependent on the frequency ratio ω ∕ Ω 0 , and the scaled magnitude of ac field Λ = 2 e V ħ Ω 0 . Resonant heat current appears when ω and Ω 0 possess commensurate relations, where the superposition of heat current branches displays coherent interference. As ω and Ω 0 possess incommensurate relations, heat current is much smaller, and it fluctuates fiercely compared with the commensurate cases. Beat-like resonance emerges by tuning the frequencies ω and Ω 0 at some definite values of Coulomb energy.