In a multi-terminal device the (electronic) heat and charge currents can follow different paths. In this paper we introduce and analyse a class of multi-terminal devices where this property is pushed to its extreme limits, with charge $and$ heat currents flowing in different reservoirs. After introducing the main characteristics of such $heat-charge$ $current$ $separation$ regime we show how to realise it in a multi-terminal device with normal and superconducting leads. We demonstrate that this regime allows to control independently heat and charge flows and to greatly enhance thermoelectric performances at low temperatures. We analyse in details a three-terminal setup involving a superconducting lead, a normal lead and a voltage probe. For a generic scattering region we show that in the regime of heat-charge current separation both the power factor and the figure of merit $ZT$ are highly increased with respect to a standard two-terminal system. These results are confirmed for the specific case of a system consisting of three coupled quantum dots.
Lack of materials which are thermoelectrically efficient and economically attractive is a challenge in thermoelectricity. Silicon could be a good thermoelectric material offering CMOS compatibility, harmlessness and cost reduction but it features a too high thermal conductivity. High harvested power density of 7W/cm2 at deltaT=30K is modeled based on a thin-film lateral architecture of thermo-converter that takes advantage of confinement effects to reduce the thermal conductivity. The simulation leads to the conclusion that 10nm thick Silicon has 10 times higher efficiency than bulk.
Half-Heusler compounds (space group Fm3m) has garnered increasing attention in recent years in the thermoelectric community. Three decades ago, refractory RNiSn half-Heusler compounds (R represents refractory metals such as Hf, Zr, Ti) were found to be narrow-gap semiconductors with large Seebeck coefficients in 100s of micro-volt per Kelvin. Today, half-Heusler (HH) compounds have emerged as promising thermoelectric materials in the intermediate temperature range (400-800oC). HH materials are endowed with good thermal stability and scalability. Thermoelectric n-p modules based on HH materials…
We demonstrate high-temperature thermoelectric conversion in InAs/InP nanowire quantum dots by taking advantage of their strong electronic confinement. The electrical conductance G and the thermopower S are obtained from charge transport measurements and accurately reproduced with a theoretical model accounting for the multi-level structure of the quantum dot. Notably, our analysis does not rely on the estimate of co-tunnelling contributions since electronic thermal transport is dominated by multi-level heat transport. By taking into account two spin-degenerate energy levels we are able to eva…
Quantum thermocouples: nonlocal conversion and control of heat in nanostructures
Universidad Autónoma de Madrid（马德里自治大学）；Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid（马德里自治大学凝聚态物理中心 (IFIMAC)）；Instituto Nicolás Cabrera (INC), Universidad Autónoma de Madrid（马德里自治大学尼古拉斯·卡布雷拉研究所 (INC)）
Electronic and Thermoelectric Properties of Molecular Junctions Incorporating Organometallic Complexes: Implications for Thermoelectric Energy Conversion
Molecular junctions comprise small molecules on the order of one or a few nanometers in length, chemically attached to two electrodes of metal or semiconductors. They have recently been identified as promising candidates for materials/devices for thermoelectric energy conversion. Molecules have the potential to act as sharp energy filters for electrical currents and could outperform other materials considered for thermoelectric energy conversion so far, thereby providing a wide-spread technological relevance thermoelectrics could not achieve to date. However, there is a clear gap between predi…
Solid-state thermoelectric devices are currently used in applications ranging from thermocouple sensors to power generators in satellites, to portable air-conditioners and refrigerators. With the ever-rising demand throughout the world for energy consumption and CO2 reduction, thermoelectric energy conversion has been receiving intensified attention as a potential candidate for waste-heat harvesting as well as for power generation from renewable sources. Efficient thermoelectric energy conversion critically depends on the performance of thermoelectric materials and devices. In this review, we …
Decades of research on thermoelectrics stimulated by the fact that nano- and meso-scale thermoelectric transport could yield higher energy conversion efficiency and output power has recently uncovered a new direction on inelastic thermoelectric effects. We introduce the history, motivation, and perspectives on mesoscopic inelastic thermoelectric effects.
This review focuses on the investigation and enhancement of the thermoelectric properties of semiconducting nanowires (NWs). NWs are nanostructures with typical diameters between few to hundreds of nm and length of few to several microns, exhibiting a high surface-to-volume ratio. Nowadays an extraordinary control over their growth has been achieved, enabling also the integration of different types of heterostructures, which can lead to the engineering of the functional properties of the NWs. In this review, we discuss all concepts which have been presented and achieved so far for the improvem…
It was recently suggested that molecular junctions would be excellent elements for efficient and high-power thermoelectric energy conversion devices. However, experimental measurements of thermoelectric conversion in molecular junctions have indicated rather poor efficiency, raising the question of whether it is indeed possible to design a setup for molecular junctions that will exhibit enhanced thermoelectric performance. Here we suggest that hybrid single-molecule nanoparticle junctions can serve as efficient thermoelectric converters. The introduction of a semiconducting nanoparticle introd…
Thermoelectric effects, such as the generation of a particle current by a temperature gradient, have their origin in a reversible coupling between heat and particle flows. These effects are fundamental probes for materials and have applications to cooling and power generation. Here we demonstrate thermoelectricity in a fermionic cold atoms channel, ballistic or diffusive, connected to two reservoirs. We show that the magnitude of the effect and the efficiency of energy conversion can be optimized by controlling the geometry or disorder strength. Our observations are in quantitative agreement w…
The usually negligibly small thermoelectric effects in superconducting heterostructures can be boosted dramatically due to the simultaneous effect of spin splitting and spin filtering. Building on an idea of our earlier work [Phys. Rev. Lett. $\textbf{110}$, 047002 (2013)], we propose realistic mesoscopic setups to observe thermoelectric effects in superconductor heterostructures with ferromagnetic interfaces or terminals. We focus on the Seebeck effect being a direct measure of the local thermoelectric response and find that a thermopower of the order of $\sim200$ $μV/K$ can be achieved in a …
Thermoelectric energy conversion is perhaps the most promising of the potential applications of molecular electronics. Ultimately, it is desirable for this technology to operate at around room temperature, and it is therefore important to consider the role of dissipative effects in these conditions. Here, we develop a theory of thermoelectricity which accounts for the vibrational coupling within the framework of Marcus theory. We demonstrate that the inclusion of lifetime broadening is necessary in the theoretical description of this phenomenon. We further show that the Seebeck coefficient and…
Thermocurrent flowing through a single-molecule device contains valuable information about the quantum properties of the molecular structure and, in particular, on its electronic and phononic excitation spectra, and entropy. Furthermore, accessing the thermoelectric heat-to-charge conversion efficiency experimentally can help to select suitable molecules for future energy conversion devices, which - predicted by theoretical studies - could reach unprecedented efficiencies. However, one of the major challenges in quantifying thermocurrents in nanoscale devices is to determine the exact temperat…
The possibility of using quantum Hall systems for thermoelectric energy conversion is investigated. It is shown that the massive degeneracy and the metallicity of a partially-filled Landau level enable thermoelectric cooling and power generation with unprecedented efficiency at low temperature. The figure of merit is explicitly derived for a transverse thermoelectric device using the $ν=0$ quantum Hall state of Dirac materials at charge neutrality, where due to electron-hole symmetry electrical Hall effect vanishes but thermoelectric Hall effect peaks.
Thermoelectric three-terminal hopping transport through one-dimensional nanosystems
一个双位纳米结构（例如，一个“分子”）桥接两个导电引线和连接到声子浴被认为是。最接近费米能的两个相关能级各自与其引线相连。引线具有略微不同的温度和化学势，并且纳米结构还耦合到热（第三）声子浴。评估了3 X 3线性传输（“Onsager”）矩阵，以及随之而来的新的品质因数，并且发现其非常有利于热电能量转换。
Spin caloritronics is the science and technology to control spin, charge, and heat currents in magnetic nanostructures. The spin degree of freedom provides new strategies for thermolelectric power generation that have not yet been fully explored. After an elementary introduction into conventional thermoelectrics and spintronics, we give a brief review of the physics of spin caloritronics. We discuss spin-dependent thermoelectrics based on the the two-current model in metallic magnets as well as the spin Seebeck and Peltier effects that are based on spin wave excitations in ferromagnets. We der…
In recent years, the study of heat to work conversion has been re-invigorated by nanotechnology. Steady-state devices do this conversion without any macroscopic moving parts, through steady-state flows of microscopic particles such as electrons, photons, phonons, etc. This review aims to introduce some of the theories used to describe these steady-state flows in a variety of mesoscopic or nanoscale systems. These theories are introduced in the context of idealized machines which convert heat into electrical power (heat-engines) or convert electrical power into a heat flow (refrigerators). In t…
The power factor of a thermoelectric device is a measure of the heat-to-energy conversion efficiency in nanoscopic devices. Yet, even as interest in low-dimensional thermoelectric materials has increased, experimental research on what influences the power factor in these systems is scarce. Here, we present a detailed thermoelectric study of graphene quantum dot devices. We show that spin-degeneracy of the quantum dot states has a significant impact on the zero-bias conductance of the device and leads to an increase of the power factor. Conversely, we demonstrate that non-ideal heat exchange wi…
The low thermal conductivity of silicon nanostructures, with respect to bulk silicon, opens excellent possibilities for thermoelectric applications because it will enable the use of silicon for the high efficient direct conversion of wasted heat into electrical power. This paves the way for the application of silicon devices for energy scavenging and green energy harvesting. We present a device with a large number of nanostructures suspended on a silicon substrate. Top-down techniques for device fabrication based on advanced lithography and anisotropic etching will be discussed. FEM simulation…
Thermoelectric devices are heat engines, which operate as generators or refrigerators using the conduction electrons as a working fluid. The thermoelectric heat-to-work conversion efficiency has always been typically quite low, but much effort continues to be devoted to the design of new materials boasting improved transport properties that would make them of the electron crystal-phonon glass type of systems. On the other hand, there are comparatively few studies where a proper thermodynamic treatment of the electronic working fluid is proposed. The present article aims to contribute to bridge…
We study thermoelectric effects in Coulomb-coupled quantum-dot (CCQD) systems beyond lowest-order tunneling processes and the often applied wide-band approximation. To this end, we present a master-equation (ME) approach based on a perturbative $T$-matrix calculation of the charge and heat tunneling rates and transport currents. Applying the method to transport through a non-interacting single-level QD, we demonstrate excellent agreement with the Landauer-B{ü}ttiker theory when higher-order (cotunneling) processes are included in the ME. Next, we study the effect of cotunneling and energy-depe…
We discuss thermal rectification and thermoelectric energy conversion from the perspective of nonequilibrium statistical mechanics and dynamical systems theory. After preliminary considerations on the dynamical foundations of the phenomenological Fourier law in classical and quantum mechanics, we illustrate ways to control the phononic heat flow and design thermal diodes. Finally, we consider the coupled transport of heat and charge and discuss several general mechanisms for optimizing the figure of merit of thermoelectric efficiency.
We present a comprehensive analysis of a dual, micro-gap thermionic-thermoelectric hybrid energy converter by developing a detailed theoretical model of the system. The Space-charge and near-field effects in thermionic conversion and the temperature-dependent effects in thermoelectric conversion are considered while studying the energy flow through the cascade system. The temperatures and energy exchange channels in the different parts of the system are quantified with a self-consistent iterative algorithm considering the energy balance condition. The model is deployed to simulate the hybrid s…
The energy efficiency and power of a quantum thermoelectric system with multiple electric currents and only one heat currents are studied. The system is connected to the hot heat bath with one terminal but the cold bath with multiple terminals or vice versal. We find that the cooperative effects can be a potentially useful tool in improving the energy efficiency and output power in multi-terminal mesoscopic thermoelectric systems. As an example, we show that the cooperation between the two thermoelectric effects in three-terminal thermoelectric systems leads to markedly improved performance of…
We study the thermoelectric properties and heat-to-work conversion performance of an interacting, multi-level quantum dot (QD) weakly coupled to electronic reservoirs. We focus on the sequential tunneling regime. The dynamics of the charge in the QD is studied by means of master equations for the probabilities of occupation. From here we compute the charge and heat currents in the linear response regime. Assuming a generic multi-terminal setup, and for low temperatures (quantum limit), we obtain analytical expressions for the transport coefficients which account for the interplay between inter…
We study thermoelectric transport in ultrathin topological insulators under the application of circularly polarized off-resonant light of frequency Ω and amplitude A. We derive analytical expressions for the band structure, orbital magnetization Morb, and the thermal (\k{appa}xy) and Nernst (αxy) conductivities. Reversing the light polarization from right to left leads to an exchange of the conduction and valence bands of the symmetric and antisymmetric surface states and to a sign change in Morb,αxy, and \k{appa}xy. Varying the sample thickness or A/Ω leads to a strong enhancement of Morb and…
We review theoretical approaches to analyzing efficiency of steady state heat to work conversion which is crucial in the timely problem of optimizing efficiency of small-scale heat engines and refrigerators. A rather abstract perspective of non-equilibrium statistical mechanics and dynamical system's theory is taken to view at this very practical problem. Several recently discovered general mechanisms of optimizing the figure of merit of thermoelectric efficiency are discussed, also in connection to breaking time-reversal symmetry of the microscopic equations of motion. Applications of these t…
The close relationship between the noise and the thermoelectric conversion is studied in a quantum dot using a quantum approach based on the non-equilibrium Green function technique. We show that both the figure of merit and the efficiency can be written in term of noise and we highlight the central role played by the correlator between the charge current and the heat current that we call the mixed noise. After giving the expression of this quantity as an integral over energy, we calculate it, first in the linear response regime, next in the limit of small transmission through the barriers (Sc…
A theoretical study of the thermoelectric current and energy harvesting in an interacting double quantum dot system, connected to reservoirs held at different chemical potentials and temperatures is presented. Using a rate-equation approach, the current is evaluated for different energetic configurations of the double quantum dot. We discuss in detail the current-temperature gradient relations (the thermoelectric analog to current-voltage relations), and demonstrate that, due to interactions, the current is non-monotonically dependent on thermal bias. This interaction-induced non-monotonicity …
The spin-Seebeck effect mediated thermoelectric energy conversion can provide efficient alternative to traditional thermoelectrics for waste heat recovery. To achieve this goal, efficient spin to charge conversion using earth-abundant materials is essential. Proximity induced Rashba effect arises from the charge potential mediated by structural inversion asymmetry, which has been reported in Si thin films and can be manipulated by controlling the thickness of Rashba layer. We demonstrate a giant Rashba spin-Seebeck effect in NiFe/p-Si (polycrystalline) bilayer thin films. The bilayer thin film…
We investigate the thermoelectric properties of a quantum dot coupled to ferromagnetic and superconducting electrodes. The combination of spin polarized tunneling at the ferromagnetic-quantum dot interface and the application of an external magnetic field that Zeeman splits the dot energy level leads to large values of the thermopower (Seebeck coefficient). Importantly, the thermopower can be tuned with an external gate voltage connected to the dot. We compute the figure of merit that measures the efficiency of thermoelectric conversion and find that it attains high values. We discuss the diff…
Miniaturization of electronic devices aims at manufacturing ever smaller products, from mesoscopic to nanoscopic sizes. This trend is challenging because the increased levels of dissipated power demands a better understanding of heat transport in small volumes. A significant amount of the consumed energy is transformed into heat and dissipated to the environment. Thermoelectric materials offer the possibility to harness dissipated energy and make devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high elect…
The spin Seebeck effect (SSE) refers to the generation of a spin current as a result of a temperature gradient in magnetic materials including insulators. The SSE is applicable to thermoelectric generation because the thermally generated spin current can be converted into a charge current via spin-orbit interaction in conductive materials adjacent to the magnets. The insulator-based SSE device exhibits unconventional characteristics potentially useful for thermoelectric applications, such as simple structure, device-design flexibility, and convenient scaling capability. In this article, we rev…
Thermoelectric response of a ferroelectric insulator
National Institute for Materials Science（物质材料研究机构）；Okayama University（冈山大学）；Institute of Science Tokyo（东京科学大学）；Murata Manufacturing Co., Ltd.（村田制作所）；Tohoku University（东北大学）；The University of Tokyo（东京大学）
Focus of the chapter is on the theoretical approaches aimed to analyze thermoelectric properties at the nanoscale. We discuss several relevant theoretical approaches for different set-ups of nano-devices providing estimations of the thermoelectric parameters in the linear and non-linear regime, in particular the thermoelectric figure of merit and the power-efficiency trade-off. Moreover, we analyze the role of not only electronic, but also of vibrational degrees of freedom. First, nanoscale thermoelectric phenomena are considered in the quantum coherent regime using the Landauer-Büttiker metho…
We analyze the noise properties of both electric charge and heat currents as well as their correlations in a quantum-dot based thermoelectric engine. The engine is a three-terminal conductor with crossed heat and charge flows where heat fluctuations can be monitored by a charge detector. We investigate the mutual influence of charge and heat dynamics and how it is manifested in the current and noise properties. In the presence of energy-dependent tunneling, operating conditions are discussed where a charge current can be generated by heat conversion. In addition, heat can be pumped into the ho…
In this work, we aim to design a heterostructure based nanoscale thermoelectric generator that can maximize the waste-heat conversion efficiency at a given output power. The primary objective to be achieved for this is to realize a boxcar-shaped (bandpass) electronic transmission function (R. S. Whitney, Phys. Rev. Lett. 112, 130601 (2014)). In order to achieve that, we propose the use of an electronic analog of optical Fabry-Pérot cavity over a central resonant tunneling structure. We further explore the optimum design possibilities by varying the geometry of the cavity wall to ensure a nearl…
Two-dimensional thermoelectric materials with a figure of merit $ZT$, which is greater than 2.0 at room temperature, would be highly desirable in energy conversion since the efficiency is competitive to conventional energy conversion techniques. Here, we propose that the indium triphosphide (InP$_3$) monolayer offers an extraordinary $ZT$ of 2.2 at 300 K by using quantum calculations within the ballistic thermal transport region. A remarkably low and isotropic phononic thermal conductivity is founded, which is due to flat lattice vibration modes. This low thermal conductivity takes a major res…
Thermoelectric devices that utilize the Seebeck effect convert heat flow into electrical energy and are highly desirable for the development of portable, solid state, passively-powered electronic systems. The conversion efficiencies of such devices are quantified by the dimensionless thermoelectric figure of merit (ZT), which is proportional to the ratio of a device's electrical conductance to its thermal conductance. High ZT (>2) has been achieved in materials via all-scale hierarchical architecturing. This efficiency holds at high temperatures (700K~900K) but quickly diminishes at lower temp…
