Although there are enormous numbers of high-temperature thermoelectric materials present, designing a near-room-temperature especially n-type thermoelectric material with high zT is extremely challenging. Generally, pristine Ag2Se exhibits unusual low thermal conductivity along with high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudoternary phase, Ag2Se0.5Te0.25S0.25, which shows improved thermoelectric performance (zT ~ 2.1 at 400 K). Density functional theory reveals that the Rashba type of spin-dependent band spitting originated because of Te-doping, enhancing carrier mobility. Using density functional perturbation theory, we hereby realize that the intrinsic carrier mobility is not only controlled by carrier effective mass, neither deformation potential theory, instead it is substantially limited by longitudinal optical phonon scattering. In fact, locally off-centered S atoms and rising configurational entropy via substitution of Te and S atoms in Ag2Se significantly reduce the lattice thermal conductivity (klat ~ 0.34 at 400 K). In order to accurately obtain electrical as well as thermal transport coefficient, we adopt deformation potential theory based on Boltzmann transport formalism. The combined consequence of the Rashba effect coupled with configurational entropy synergistically results in such high thermoelectric performance with the development of new n-type thermoelectric material working at the near-room-temperature regime.
Establishing strong electron-correlations not only shed lights on overcoming the trade-off limitations for optimizing thermoelectric materials, but can also introduce new functionalities that extend the vision of conventional thermoelectric applications. Here, we demonstrate that the high thermoelectric and thermistor functionalities coexist in lattice distorted SrNbxTi1-xO3 films with electron correlations between carriers and ordering aligned lattice dipoles. As-grown SrNbxTi1-xO3/SrTiO3 with effectively preserved interfacial strains exhibits cross-plane charge ordering and orbital anisotrop…
Realizing record-high transverse thermoelectric figure of merit at room temperature in artificially tilted multilayers based on high power factor NiFe alloy
National Institute for Materials Science（物质材料研究机构）；University of Tsukuba（筑波大学）；The University of Tokyo（东京大学）；Resonac Corporation（瑞萨科株式会社）
The valence band of a variety of few-layer, two-dimensional materials consists of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a `Mexican hat' or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III-VI …
We investigate the possibility of realizing quantum anomalous Hall effect in graphene. We show that a bulk energy gap can be opened in the presence of both Rashba spin-orbit coupling and an exchange field. We calculate the Berry curvature distribution and find a non-zero Chern number for the valence bands and demonstrate the existence of gapless edge states. Inspired by this finding, we also study, by first principles method, a concrete example of graphene with Fe atoms adsorbed on top, obtaining the same result.
The momentum-dependent Rashba and Dresselhaus spin-splitting has gained much attention for its highly promising applications in spintronics. In the present work, ab initio density functional theory calculations are performed to study the spin-splitting effect in ferroelectric oxide perovskite KIO3. Our calculations are additionally supported by symmetry adapted two-band k.p Hamiltonian. Non-negligible spin-splitting effect is observed at conduction band minimum (CBm) and valence band maximum (VBM) for rhombohedral R3m and R3c phases. Linear Rashba terms successfully explain the splitting at VB…
A major impediment towards realizing technologies based on the emerging principles of antiferromagnetic spintronics is the shortage of suitable materials. In this paper, we propose a design of polar|nonpolar heterostructures of perovskite oxides, where a single unit cell of SrIrO3 is sandwiched between a thin film of LaAlO3 and a substrate of SrTiO3. Our calculations within the framework of density-functional theory + Hubbard U + spin-orbit coupling reveal a two-dimensional conducting layer with electron and hole pockets at the interface, exhibiting a strong anisotropic Rashba-Dresselhaus effe…
A caloritronic device setup is proposed that harnesses the intrinsic spin-valley locking of two-dimensional honeycomb lattices with graphene-like valleys, for instance, silicene and stanene. Combining first-principles and analytic calculations, we quantitatively show that when sheets of such materials are placed on a ferromagnetic substrate and held between two contacts at different temperatures, an interplay between the electron degrees-of-freedom of charge, spin, and valley arises. A manifestation of this interplay are finite charge, spin, and valley currents. Uniaxial strain that adjusts th…
Thermoelectric and Magnetic Properties in Doped Fe$_2$VAl within a Bipolar Random Anderson Model
Department of Applied Physics, Tokyo University of Science（东京理科大学应用物理系）；Research Institute for Science and Technology, Tokyo University of Science（东京理科大学科学技术研究院）
We report the discovery of a very large thermoelectric power over -400 microV K-1 in the whisker crystals of a one-dimensional telluride Ta4SiTe4, while maintaining a low electrical resistivity of rho = 2 mohm cm, yielding a very large power factor of P = 80 microW cm-1 K-2 at an optimum temperature of 130 K. This temperature is widely controlled from the cryogenic temperature of 50 K to room temperature by chemical doping, resulting in the largest P of 170 microW cm-1 K-2 at 220-280 K. These P values far exceed those of the Bi2Te3-Sb2Te3 alloys at around room temperature, offering an avenue f…
This work describes through semiclassical Boltzmann transport theory and simulation a novel nanostructured material design that can lead to unprecedentedly high thermoelectric power factors, with improvements of more than an order of magnitude compared to optimal bulk material power factors. The design is based on a specific grain/grain-boundary (potential well/barrier) engineering such that: i) carrier energy filtering is achieved using potential barriers, combined with ii) higher than usual doping operating conditions such that high carrier velocities and mean-free-paths are utilized, iii) m…
Realizing topological magnetism and its electric control are intensive topics in the spintronics due to their promising applications in information storage and logic technologies. Here we unveil that both can be achieved in the two-dimensional (2D) magnetoelectric multiferroics. Using first-principles calculations, we show that strong Dzyaloshinskii-Moriya interaction (DMI), which is the key ingredient for the formation of exotic chiral magnetism, could be induced in 2D multiferroics with vertical electric polarization via Rashba effect. We verify that such significant DMI can promote sub-10 n…
Nanostructured composites and nanowire arrays of traditional thermoelectrics like Bi, Bi(1-x)Sb(x) and Bi(2)Te(3) have metallic Rashba surface spin-orbit bands featuring high mobilities rivaling that of the bulk for which topological insulator behavior has been proposed. Nearly pure surface electronic transport has been observed at low temperatures in Bi nanowires with diameter around the critical diameter, 50 nm, for the semimetal-to semiconductor transition. The surface contributes strongly to the thermopower, actually dominating for temperatures T < 100 K in these nanowires. The surface the…
Stannous selenide is a layered semiconductor that is a polar analogue of black phosphorus, and of great interest as a thermoelectric material. Unusually, hole doped SnSe supports a large Seebeck coefficient at high conductivity, which has not been explained to date. Angle resolved photo-emission spectroscopy, optical reflection spectroscopy and magnetotransport measurements reveal a multiple-valley valence band structure and a quasi two-dimensional dispersion, realizing a Hicks-Dresselhaus thermoelectric contributing to the high Seebeck coefficient at high carrier density. We further demonstra…
We present a systematic study on the influence of Rashba spin-orbit coupling, interlayer potential difference and exchange field on the topological properties of bilayer graphene. In the presence of only Rashba spin-orbit coupling and interlayer potential difference, the band gap opening due to broken out-of-plane inversion symmetry offers new possibilities of realizing tunable topological phase transitions by varying an external gate voltage. We find a two-dimensional $Z_2$ topological insulator phase and a quantum valley Hall phase in $AB$-stacked bilayer graphene and obtain their effective …
V-telluride superlattice thin films have shown promising performance for on-chip cooling devices. Recent experimental studies have indicated that device performance is limited by the metal/semiconductor electrical contacts. One challenge in realizing a low resistivity contacts is the absence of fundamental knowledge of the physical and chemical properties of interfaces between metal and V-telluride materials. Here we present a combination of experimental and theoretical efforts to understand, design and harness low resistivity contacts to V-tellurides. Ab initio calculations are used to explor…
Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing nontrivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, a…
It was recently shown that a spatially modulated Rashba spin-orbit coupling in a quantum wire drives a transition from a metallic to an insulating state when the wave number of the modulation becomes commensurate with the Fermi wave length of the electrons in the wire. It was suggested that the effect may be put to practical use in a future spin transistor design. In the present article we revisit the problem and present a detailed analysis of the underlying physics. First, we explore how the build-up of charge density wave correlations in the quantum wire due to the periodic gate configuratio…
Crystals exhibiting glass-like and low lattice thermal conductivity ($κ_{\rm L}$) are not only scientifically intriguing but also practically valuable in various applications, including thermal barrier coatings, thermoelectric energy conversion, and thermal management. However, such unusual $κ_{\rm L}$ are typically observed only in compounds containing heavy elements, with large unit cells, or at high temperatures, primarily due to significant anharmonicity. In this study, we utilize chemical bonding principles to weaken the Ag-Ag bonds within the Ag$_6$ octahedron by introducing a ligand in …
Reducing thermal conductivity ($κ$) is an efficient way to boost the thermoelectric performance to achieve direct solid-state conversion to electrical power from thermal energy, which has lots of valuable applications in reusing waste resources. In this study, we propose an effective approach for realizing low $κ$ by introducing lone-pair electrons or making the lone-pair electrons stereochemically active through bond nanodesigning. As a case study, by cutting at the (111) cross section of the three-dimensional cubic boron arsenide (c-BAs), the $κ$ is lowered by more than one order of magnitud…
Topological Lifshitz transition-induced bipolarity of anomalous Nernst effect in kagome magnet YCo3
Zhejiang University of Technology（浙江工业大学）；Hefei National Laboratory（合肥国家实验室）；Collaborative Innovation Center of Advanced Microstructures, Nanjing University（南京大学先进微结构协同创新中心）
We report the calculation results of specific heat ($c_v$), and thermal conductivity ($κ$) by using Einstein, and Debye models about rock salt (NaCl), oxides: Na$_x$CoO$_2$, SrTiO$_3$, and LiNbO$_3$. In calculation, the longitudinal (L), and transverse (T) sound velocities ($v_T$, $v_L$) were estimated from acoustic phonons' dispersion's (~$ω$/$K$) in above materials, and the average sound velocities ($v_a$) were input into Debye model for $κ$ equation, and results were compared with that of Einstein model. In some oxides, $v_a$ is relatively reduced at slight high $v_T$ ($v_T$/$v_L$=0.3-0.5).…
Symmetry-Protected Phonon Topology and Low Lattice Thermal Conductivity in Square-Octagonal Chalcogenides
Khalifa University（哈利法大学）；Seoul National University（首尔国立大学）；Indian Institute of Technology, Bombay（印度理工学院孟买分校）；Research and Innovation Center for Graphene and 2D materials (RIC2D)（石墨烯与二维材料研究与创新中心）
