Showing papers on "Heat current published in 2022"

Quantum mechanical modeling of magnon-phonon scattering heat transport across three-dimensional ferromagnetic/nonmagnetic interfaces

Abstract: Magnon-phonon scattering (MPS) has attracted widespread attention in quantum heat/spin transport across the ferromagnetic/nonmagnetic (F/N) interfaces, with the rapid progress of experiments on spin caloritronics in recent years. However, the lack of theoretical methods, accounting for the MPS rigorously, has seriously hindered investigations on the quantum heat transport in magnetic nanostructures with broken translational symmetry, such as F/N interfaces. In this paper, we propose a theoretical formalism of the nonequilibrium Green function to incorporate the MPS into the quantum heat transport for three-dimensional ferromagnetic nanostructures, rigorously, through a diagrammatic perturbation analysis. A computational scheme is developed for the first-principles simulation of quantum heat transport in practical magnetic nanostructures, and a generalized formalism of heat flow is presented for the analysis of the elastic and inelastic process of heat transport. A thermal rectification driven by MPS is observed in the numerical simulation of heat transport across the F/N interface based on the ${\text{CrI}}_{3}$ monolayer, which is consistent with recent studies. In this paper, we open the gate to first-principles investigations of quantum heat transport in magnetic nanostructures and pave the way for the theoretical design of magnetic thermal nanodevices.

Many-body Green's function approach to lattice thermal transport

Abstract: Recent progress in understanding thermal transport in complex crystals has highlighted the prominent role of heat conduction mediated by interband tunneling processes, which emerge between overlapping phonon bands (i.e., with energy differences smaller than their broadenings). These processes have recently been described in different ways, relying on the Wigner or Green-Kubo formalism, leading to apparently different results, which question the definition of the heat-current operator. Here, we implement a full quantum approach based on the Kubo formula, elucidating analogies and differences with the recently introduced Wigner or Green-Kubo formulations, and extending the description of thermal transport to the overdamped regime of atomic vibrations, where the phonon quasiparticle picture breaks down. We rely on first-principles calculations on complex crystals with ultralow conductivity to compare numerically the thermal conductivity obtained within the aforementioned approaches, showing that at least in the quasiparticle regime the differences are negligible for practical applications.

Phonon thermal Hall effect in a metallic spin ice

Abstract: Abstract It has become common knowledge that phonons can generate thermal Hall effect in a wide variety of materials, although the underlying mechanism is still controversial. We study longitudinal κ x x and transverse κ x y thermal conductivity in Pr 2 Ir 2 O 7 , which is a metallic analog of spin ice. Despite the presence of mobile charge carriers, we find that both κ x x and κ x y are dominated by phonons. A T / H scaling of κ x x unambiguously reveals that longitudinal heat current is substantially impeded by resonant scattering of phonons on paramagnetic spins. Upon cooling, the resonant scattering is strongly affected by a development of spin ice correlation and κ x x deviates from the scaling in an anisotropic way with respect to field directions. Strikingly, a set of the κ x x and κ x y data clearly shows that κ x y correlates with κ x x in its response to magnetic field including a success of the T / H scaling and its failure at low temperature. This remarkable correlation provides solid evidence that an indispensable role is played by spin-phonon scattering not only for hindering the longitudinal heat conduction, but also for generating the transverse response.

Tuning nonequilibrium heat current and two-photon statistics via composite qubit-resonator interaction

Abstract: Quantum thermal transport and two-photon statistics serve as two representative nonequilibrium features in circuit quantum electrodynamics (cQED) systems. Here we investigate quantum heat flow and two-photon correlation function at steady state in a composite qubit-resonator model, where one qubit shows both transverse and longitudinal couplings to a single-mode optical resonator. With weak qubit-resonator interaction, we unravel two microscopic transport pictures, i.e., cotunneling and cyclic heat exchange processes corresponding to transverse and longitudinal couplings, respectively. The nonmonotonic behavior of the heat current is exhibited by tuning the temperature bias with the weak longitudinal coupling. At strong qubit-resonator coupling, the heat current also exhibits a nonmonotonic feature by increasing qubit-resonator coupling strength, which tightly relies on the scattering processes between the qubit and the corresponding thermal bath. Furthermore, the longitudinal coupling is preferred to enhance heat current in the strong qubit-resonator coupling regime. For two-photon correlation function, it exhibits an antibunching-to-bunching transition by tuning the composite angle, which is mainly dominated by the modulation of the energy gap between the first and the second excited eigenstates. Our results are expected to deepen the understanding of nonequilibrium thermal transport and nonclassical photon radiation based on the cQED platform.

Computational Study on the Thermal Conductivity of a Protein.

Abstract: Protein molecules are thermally fluctuating and tightly packed amino acid residues strongly interact with each other. Such interactions are characterized in terms of heat current at the atomic level. We calculated the thermal conductivity of a small globular protein, villin headpiece subdomain, based on the linear response theory using equilibrium molecular dynamics simulation. The value of its thermal conductivity was 0.3 ± 0.01 [W m-1 K-1], which is in good agreement with experimental and computational studies on the other proteins in the literature. Heat current along the main chain was dominated by local vibrations in the polypeptide bonds, with amide I, II, III, and A bands on the Fourier transform of the heat current autocorrelation function.

Net heat current at zero mean temperature gradient

Abstract: The existence of a net heat current of conductive thermal waves is demonstrated even in the absence of a mean temperature gradient. This effect, which we called heat shuttling, is generated by the temperature-dependent thermal conductivity of materials excited with a thermal excitation periodically modulated in time. We show that this modulation gives rise to a heat current superimposed on the one generated by the mean temperature gradient, which enhances the heat transport when the thermal conductivity increases with temperature. By contrast, if the thermal conductivity decreases as temperature increases, the thermal-wave heat current inverts its direction and reduces the total heat flux. The reported shutting effect is sensitive to the amplitude of the periodic thermal excitation, which can facilitate its observation and application to harvest energy from the temperature variations of the environment.

Heat transfer in transversely coupled qubits: optically controlled thermal modulator with common reservoirs

Abstract: This paper systematically studied heat transfer through two transversely coupled qubits in contact with two types of heat reservoirs. One is the independent heat reservoir which essentially interacts with only a single qubit, the other is the common heat reservoir which is allowed to simultaneously interact with two qubits. Compared to independent heat reservoirs, common reservoirs always suppress heat current in most cases. However, the common environment could enhance heat current, if the dissipation rate corresponding to the higher eigenfrequency is significantly higher than that corresponding to the lower eigenfrequency. In particular, in the case of resonant coupling of two qubits and the proper dissipations, the steady state can be decomposed into a stationary dark state which does not evolve and contributes zero heat current, and a residual steady state which corresponds to the maximal heat current. This dark state enables us to control steady-state heat current with an external control field and design a thermal modulator. In addition, we find that inverse heat currents could be present in the dissipative subchannels between the system and reservoirs, which interprets the suppression roles of common heat reservoirs. We also calculate the concurrence of assistance (COA) of the system and find that heat current and COA have the same trend with temperature, which further indicates that entanglement can be regarded as a resource to regulate heat transport.

Nonequilibrium quantum thermodynamics in non-Markovian adiabatic speedup

Abstract: Understanding heat transfer between a quantum system and its environment is of undisputed importance if reliable quantum devices are to be constructed. Here, we investigate the heat transfer between system and bath in non-Markovian open systems in the process of adiabatic speedup. Using the quantum state diffusion equation method, the heat current, energy current, and power are calculated during free evolution and under external control of the system. While the heat current increases with increasing system–bath coupling strength and bath temperature, it can be restricted by the non-Markovian nature of the bath. Without pulse control, the heat current is nearly equal to the energy current. On the other hand, with pulse control, the energy current turns out to be nearly equal to the power. In this scenario, we show that non-Markovianity is a useful tool to drive the system through an approximate adiabatic dynamics, with pulse control acting in the conversion between heat current and power throughout the evolution.

Ballistic heat conduction characteristics of graphene nanoribbons

Abstract: One of the fundamental physical properties of graphene nanoribbons is its ability to conduct heat. Unfortunately, the precise microscopic mechanisms of heat conduction have also not been fully elucidated at room temperature when heat transport occurs in the two-dimensional material ballistically. The transport mechanism of ballistic heat conduction in graphene nanoribbons was investigated experimentally and theoretically. The dimensions of the two-dimensional material are substantially equal to or shorter than the average length that the phonons can travel freely. The thermal conductivity was determined experimentally and predicted theoretically under different dimension conditions, and the ability of the graphene nanoribbons to conduct heat ballistically was characterized. The boundary scattering effect arising from the crystallographic edge structure was evaluated, and the ballistic and diffusive transport characteristics were investigated. The results indicated that the heat conduction can be ballistic or diffusive, depending on the dimensions. With the decrease of the dimensions, heat conduction may begin to transition from diffusive to ballistic. This will inevitably lead to a considerable decrease in thermal conductivity. Transport can be ballistic even at room temperature, and ballistic effects may be noticeable. Graphene nanoribbons with larger dimensions may disadvantageously be used for applications in thermal management.

Characterizing the performance of heat rectifiers

Abstract: A physical system connected to two thermal reservoirs at different temperatures is said to act as a heat rectifier when it is able to bias the heat current in a given direction, similarly to an electronic diode. We propose to quantify the performance of a heat rectifier by mapping out the trade-off between heat currents and rectification. By optimizing over the system's parameters, we obtain Pareto fronts, which can be efficiently computed using general coefficients of performance. This approach naturally highlights the fundamental trade-off between heat rectification and conduction, and allows for a meaningful comparison between different devices for heat rectification. We illustrate the practical relevance of these ideas on three minimal models for spin-boson nanoscale rectifiers, i.e., systems consisting of one or two interacting qubits coupled to bosonic reservoirs biased in temperature. Our results demonstrate the superiority of two strongly-interacting qubits for heat rectification.

Fluctuations in Heat Current and Scaling Dimension

Abstract: In this work, we theoretically study the heat flow between two $1+1\mathrm{D}$ chiral gapless systems connected by a point contact. With a small temperature gradient between the two, we find that the ratio between fluctuations of the heat current and the heat current itself is proportional to the scaling dimension---a universal number that characterizes the distribution of the particles tunneling through the point contact. We adopt two different approaches, scattering theory and conformal field theory, to calculate this ratio and see that their results agree. Our findings are useful for probing not only fractional charge excitations in fractional quantum Hall states but also neutral ones.

Heat current in non-Markovian open systems

Abstract: We generalize time-evolving matrix product operators method to nonequilibrium quantum transport problems. The nonequilibrium current is obtained via numerical differentiation of the generating functional which is represented as a tensor network. The approach is numerically exact and the non-Markovian effects are fully taken into account. In the transport process, a part of the heat that flows out from a bath flows into the system and other baths, and the rest is stored in the system-bath coupling part. We take the spin-boson model as a demonstration to show the details of this heat flowing and the establishment of a steady current between two baths.

Universal Scaling Bounds on a Quantum Heat Current

Abstract: We derive new bounds on a heat current flowing into a quantum $L$-particle system coupled with a Markovian environment. By assuming that a system Hamiltonian and a system-environment interaction Hamiltonian are extensive in $L$, we show that the absolute value of the heat current scales at most as $\Theta (L^3)$ in a limit of large $L$. Also, we present an example that saturates this bound in terms of scaling: non-interacting particles globally coupled with a thermal bath. However, the construction of such system requires many-body interactions induced by the environment, which may be difficult to realize with the current technology. To consider more feasible cases, we focus on a class of system where any non-diagonal elements of the noise operator (derived from the system-environment interaction Hamiltonian) become zero in the system energy basis, if the energy difference is beyond a certain value $\Delta E$. Then, for $\Delta E = \Theta (L^0)$, we derive another scaling bound $\Theta (L^2)$ on the absolute value of the heat current, and the so-called superradiance belongs to a class to saturate this bound. Our results are useful to evaluate the best achievable performance of quantum-enhanced thermodynamic devices, which contain far-reaching applications for such as quantum heat engines, quantum refrigerators and quantum batteries.

Theory of shift heat current and its application to electron-phonon coupled systems

Abstract: We propose a heat current analog of the shift current, ``shift heat current.'' We study nonlinear heat current responses to an applied ac electric field by a diagrammatic method and derive a microscopic expression for the second-order dc heat current response. As a result, we find that the shift heat current is related to the shift vector, a geometric quantity that also appears in the expression for the shift current. The shift heat current directly depends on and can be controlled through the chemical potential. In addition, we apply the diagrammatic method to electron-phonon coupled systems, and we find that even if only the phonons are excited by an external field, the amplitude of the shift heat current is determined by the energy scale of electrons, not of phonons.

On the definition of heat current for periodic systems and its implications for simulations of thermal conductivity in solids

Abstract: We re-derive the expression for the heat current for a classical system subject to periodic boundary conditions and show that it can be written as a sum of two terms. The first term is a time derivative of the first moment of the system energy density while the second term is expressed through the energy transfer rate through the periodic boundary. We show that in solids the second term alone leads to the same thermal conductivity as the full expression for the heat current when used in the Green-Kubo approach. More generally, energy passing though any surface formed by translation of the original periodic boundary can be used to calculate thermal conductivity. These statements are verified for two systems: crystalline argon and crystals of argon and krypton forming an interface.

Characterizing the performance of heat rectifiers

Abstract: A physical system connected to two thermal reservoirs at different temperatures is said to act as a heat rectifier when it is able to bias the heat current in a given direction, similarly to an electronic diode. We propose to quantify the performance of a heat rectifier by mapping out the trade-off between heat currents and rectification. By optimizing over the system's parameters, we obtain Pareto fronts, which can be efficiently computed using general coefficients of performance. This approach naturally highlights the fundamental trade-off between heat rectification and conduction, and allows for a meaningful comparison between different devices for heat rectification. We illustrate the practical relevance of these ideas on three minimal models for spin-boson nanoscale rectifiers, i.e., systems consisting of one or two interacting qubits coupled to bosonic reservoirs biased in temperature. Our results demonstrate the superiority of two strongly-interacting qubits for heat rectification.

Effect of nano-scale grain dimensions on the ballistic and diffusive phonon transport properties of graphene nanoribbons

Abstract: Abstract A phonon is a unit of vibrational energy that arises from oscillating atoms within a crystal. The study of phonons is an important part of condensed matter physics. The present study relates to the ballistic and diffusive phonon transport properties of graphene nanoribbons deposited on a silicon carbide substrate. The two-dimensional crystal has dimensions comparable to or smaller than the mean free path of phonons. The effect of nano-scale grain dimensions on the phonon transport properties of the two-dimensional crystal was investigated theoretically and experimentally. The characteristics of ballistic and diffusive heat conduction in the two-dimensional crystal were investigated. The edge-scattering effects arising from the crystal edges were determined in order to increase phonon transport and reduce phonon scattering in the two-dimensional crystal. The results indicated that phonons play a major role in the ballistic and diffusive phonon transport properties of graphene nanoribbons. Heat conduction is mediated by the combination of propagation and collisions of phonons. The heat flux is carried almost entirely by phonon vibrations. The transport mode of phonons depends upon the length scales of the two-dimensional crystal. As the dimensions of the two-dimensional crystal decrease, there is a transition from the diffusive transport mode to the ballistic transport mode. The method of dimensional and structural design is effective to reduce phonon scattering and increase phonon transport in the two-dimensional crystal, thereby allowing the thermal conductivity approaching as closely as possible the theoretical limit.

Heat Transport in an Ordered Harmonic Chain in Presence of a Uniform Magnetic Field

Abstract: 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.

Multiscale heat transport with inertia and thermal vortices

Abstract: In this paper, we present a Hamiltonian and thermodynamic theory of heat transport on various levels of description. Transport of heat is formulated within kinetic theory of polarized phonons, kinetic theory of unpolarized phonons, hydrodynamics of polarized phonons, and hydrodynamics of unpolarized phonons. These various levels of description are linked by Poisson reductions, where no linearizations are made. Consequently, we obtain a new phonon hydrodynamics that contains convective terms dependent on vorticity of the heat flux, which are missing in the standard theories of phonon hydrodynamics. Moreover, the equations are hyperbolic and Galilean invariant, unlike current theories for beyond-Fourier heat transport. The vorticity-dependent terms violate the alignment of the heat flux with the temperature gradient even in the stationary state, which is expressed by a Fourier-Crocco equation. The new terms also cause that temperature plays in heat transport a similar role as pressure in aerodynamics.

Theory of shift heat current and its application to electron-phonon coupled systems

Abstract: We propose a heat current analog of the shift current, "shift heat current". We study nonlinear heat current responses to an applied ac electric field by a diagrammatic method and derive a microscopic expression for the second order dc heat current response. As a result, we find that the shift heat current is related to the shift vector, a geometric quantity that also appears in the expression for the shift current. The shift heat current directly depends on and can be controlled through the chemical potential. In addition, we apply the diagrammatic method to electron-phonon coupled systems, and we find that even if only the phonons are excited by an external field, the amplitude of the shift heat current is determined by the energy scale of electrons, not of phonons.

Photonic heat transport from weak to strong coupling

Abstract: Superconducting circuits provide a favorable platform for quantum thermodynamic experiments. An important component for such experiments is a heat valve, i.e. a device which allows one to control the heat power flowing through the system. Here we theoretically study the heat valve based on a superconducting quantum interference device (SQUID) coupled to two heat baths via two resonators. The heat current in such system can be tuned by magnetic flux. We investigate how does the heat current modulation depend on the coupling strength g between the SQUID and the resonators. In the weak coupling regime the heat current modulation grows as g2, but, surprisingly, at the intermediate coupling it can be strongly suppressed. This effect is linked to the resonant nature of the heat transport at weak coupling, where the heat current dependence on the magnetic flux is a periodic set of narrow peaks. At the intermediate coupling, the peaks become broader and overlap, thus reducing the heat modulation. At very strong coupling the heat modulation grows again and finally saturates at a constant value.

Heat Current in Non-Markovian Open Systems

Abstract: We generalize time-evolving matrix product operators method to nonequilibrium quantum transport problems. The nonequilibrium current is obtained via numerical differentiation of the generating functional which is represented as a tensor network. The approach is numerically exact and the non-Markovian effects are fully taken into account. In the transport process, a part of the heat that flows out from a bath flows into the system and other baths, and the rest is stored in the system-bath coupling part. We take the spin-boson model as a demonstration to show the details of this heat flowing and the establishment of a steady current between two baths.