Despite the usefulness of the anomalous Nernst conductivity ($α^{A}_{xy}$) for studying electronic band structures and exploring magnetic materials with large transverse thermopower, there has not been a straightforward way to obtain $α^{A}_{xy}$ in the experiment. Here, we propose a simple and versatile method enabling direct electrical probing of $α^{A}_{xy}$, which is realized by creating a closed circuit consisting of a target magnetic material and a non-magnetic conductor. This method was experimentally demonstrated on a thin film of magnetic Weyl semimetal Co$_{2}$MnGa, where the closed circuit was formed simply by connecting both ends of the Co$_{2}$MnGa film with a Au wire. A good approximation of $α^{A}_{xy}$ was obtained in a wide temperature range, validating the proposed method and exhibiting its potential for aiding the further development of topological materials science and transverse thermoelectrics.
We report the planar and anomalous Nernst effect in epitaxial thin films of spin polarized La2/3Sr1/3MnO3. The thermal counterpart of the anomalous Hall effect in this material (i.e. the anomalous Nernst effect) shows a extreme sensitivity to any parasitic thermal gradient, resulting in large asymmetric voltages under small temperature differences. This should be considered when interpreting the magnitude of the electrical response in nanostructures and devices that operate under high current densities. Finally, none of the observed magneto-thermoelectric signals is compatible with the observa…
The anomalous Nernst effect (ANE) is a thermomagnetic phenomenon with potential applications in thermal energy harvesting. While many recent works studied the approaches to increase the ANE coefficient of materials, relatively little effort was devoted to increasing the power supplied by the effect. Here we demonstrate a nanofabricated device with record power density generated by the ANE. To accomplish this, we fabricate micrometer-sized devices in which the thermal gradient is three orders of magnitude higher than conventional macroscopic devices. In addition, we use Co/Pt multilayers, a sys…
The quantum Hall effect in a 2D electron system expresses a topological invariant, leading to a quantized conductivity. The thermal Hall and thermoelectric Nernst conductances in two dimensions are also reported to be quantized in specific systems. However, a comprehensive study of these quantities within one formalism for quantum Hall systems is so far elusive. In this work, we investigate the Hall, thermal Hall, and Nernst effects analytically and numerically in 2D and 3D. In addition to the quantized values for the Hall and thermal Hall conductances in two dimensions, we also find quasi-qua…
Thermal transport is less appreciated in probing quantum materials in comparison to electrical transport. This article aims to show the pivotal role that thermal transport may play in understanding quantum materials: the longitudinal thermal transport reflects the itinerant quasiparticles even in an electrical insulating phase, while the transverse thermal transport such as thermal Hall and Nernst effect are tightly linked to nontrivial topology. We discuss three types of examples: quantum spin liquids where thermal transport identifies its existence, superconductors where thermal transport re…
The Mott relation between the electrical and thermoelectric transport coefficients normally holds for phenomena involving scattering. However, the anomalous Hall effect (AHE) in ferromagnets may arise from intrinsic spin-orbit interaction. In this work, we have simultaneously measured AHE and the anomalous Nernst effect (ANE) in Ga1-xMnxAs ferromagnetic semiconductor films, and observed an exceptionally large ANE at zero magnetic field. We further show that AHE and ANE share a common origin and demonstrate the validity of the Mott relation for the anomalous transport phenomena.
We report the anomalous Nernst effect (ANE), i.e. creation of electrical voltage transverse to the direction of applied temperature gradient and magnetic field in bulk samples of La1-xNaxMnO3 (x = 0.02, 0.05, 0.1 and 0.3) whose ferromagnetic Cure temperature(TC) varies from 251 K for x = 0.02 to 310 K for x = 0.3. We measured both the magnetic field and temperature dependences of ANE. While ANE is negligible much above TC, it increases rapidly at TC and exhibits a peak just below it. The field dependence of ANE closely follows magnetization. The maximum value of ANE at TC increases from x = 0.…
Applying a temperature gradient in a magnetic material generates a voltage that is perpendicular to both the heat flow and the magnetization. This is the anomalous Nernst effect (ANE) which was thought to be proportional to the value of the magnetization for a long time. However, more generally, the ANE has been predicted to originate from a net Berry curvature of all bands near the Fermi level. Subsequently, a large anomalous Nernst thermopower has recently been observed in topological materials with no net magnetization but large net Berry curvature around E$_F$. These experiments clearly fa…
Thermoelectric modules are a promising approach to energy harvesting and efficient cooling. In addition to the longitudinal Seebeck effect, recently transverse devices utilizing the anomalous Nernst effect (ANE) have attracted interest. For high conversion efficiency, it is required that the material should have a large ANE thermoelectric power and low electrical resistance, the product of which is the ANE conductivity. ANE is usually explained in terms of intrinsic contributions from Berry curvature. Our observations suggest that extrinsic contributions also matter. Studying single-crystal Mn…
Ferromagnetic metallic oxides have potential applications in spincaloric devices which utilize the spin property of charge carriers for interconversion of heat and electricity through the spin Seebeck or the anomalous Nernst effect or both. In this work, we synthesized polycrystalline La0.5S0.5CoO3 by microwave irradiation method and studied its transverse thermoelectric voltage (Nernst thermopower) and change in the linear dimension of the sample (Joule magnetostriction) in response to external magnetic fields. In addition, magnetization, temperature dependences of electrical resistivity, and…
Transverse thermoelectric devices produce electric fields perpendicular to an incident heat flux. Classically, this process is driven by the Nernst effect in bulk solids, wherein a magnetic field generates a Lorentz force on thermally excited electrons. The spin Seebeck effect (SSE) also produces magnetization-dependent transverse electric fields. SSE is traditionally observed in thin metallic films deposited on electrically insulating ferromagnets, but the films' high resistance limits thermoelectric conversion efficiency. Combining Nernst and SSE in bulk materials would enable devices with s…
Resolving the structure of energy bands in transport experiments is a major challenge in condensed matter physics and material science. Sometimes, however, traditional electrical conductance or resistance measurements only provide very small signals, and thus limit the ability to obtain direct band structure information. In this case, it has been proven beneficial to employ thermoelectric measurements which are sensitive to the first derivative of the density of states with respect to energy, rather than to its value itself. Due to the large interest in topological effects these days, it is im…
Exchange splitting as a descriptor for giant anomalous Hall and Nernst effects in ferromagnets
National Institute for Materials Science (NIMS)（物质材料研究机构）；Zhejiang Key Laboratory of Magnetic Materials and Applications, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences（中国科学院宁波材料技术与工程研究所磁材料与应用浙江省重点实验室）；Kyoto Institute of Technology（京都工艺纤维大学）
We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe$_2$O$_{\textrm{4-x}}$ ($4\geq x \geq 0$) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect contributions and even disentangle the intrinsic anomalous Nernst effect (ANE) in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to its proximity to the FM. Further, we probe the dependence of these effects on the electrical conductivity and the band gap energy of the FM film varying from nearly insulating NiFe$_2$O$_4$ to metalli…
The electrical and thermo-electrical transport effects of the TaAs$_2$ semimetal were measured in a magnetic field applied along [-2 0 1] direction. The resulting field dependences of the resistivity as well as the Hall, Seebeck and Nernst coefficient below T ~ 100 K can be satisfactory described within the two-band model consisting of the electron and hole pockets. At low temperature all the measured effects exhibit significant contribution from quantum oscillations. The fast Fourier transform (FFT) of the oscillatory Nernst signal shows two fundamental frequencies, Fa = 105 T and Fb = 221 T,…
The electrical impedance response of Gelatin based solid polymer electrolyte to gamma irradiation is investigated by impedance spectroscopy. An analysis based on Poisson-Nernst-Plank model, incorporating fractional time derivatives is carried out. A detailed derivation for anomalous impedance function is given.The model involves boundary conditions with convolution of the fractional time derivative of ion density and adsorption desorption relaxation kinetics. A fractional diffusion-drift equation is used to solve the bulk behavior of the mobile charges in the electrolyte. The complex adsorptio…
The effect of grain boundaries and wrinkles on the electrical properties of polycrystalline graphene is pronounced. Here we investigate the stitching between grains of polycrystalline graphene, specifically, overlapping of layers at the boundaries, grown by chemical vapor deposition (CVD) and subsequently doped by the oxidized Cu substrate. We analyze overlapped regions between 60 and 220 nm wide via Raman spectroscopy, and find that some of these overlapped boundaries contain AB stacked bilayers. The Raman spectra from the overlapped grain boundaries are distinctly different from bilayer grap…
Bismuth is an archetypal semimetal with gigantic spin-orbit coupling and it has been a major source material for the discovery of seminal phenomena in solid state physics for more than a century. In recent years, spin current transports in bismuth have also attracted considerable attention. In this paper, we theoretically study both spin Hall effect (SHE) and spin Nernst effect (SNE) in bismuth, based on relativistic band structure calculations. First, we find that there are three independent tensor elements of spin Hall conductivity (SHC) and spin Nernst conductivity (SNC), namely, $Z_{yx}^z$…
An accurate estimate of the ferroelectric polarization in ferroelectric-dielectric stacks is important from a materials science perspective, and it is also crucial for the development of ferroelectric based electron devices. This paper revisits the theory and application of the PUND technique in Metal-Ferroelectric-Dielectric-Metal (MFDM) structures by using analytical derivations and numerical simulations. In an MFDM structure the results of the PUND technique may largely differ from the polarization actually switched in the stack, which in turn is different from the remnant polarization of t…
Motivated by the experimental measurement of electrical and hall conductivity, thermopower and Nernst effect, we calculate the longitudinal and transverse electrical and heat transport in graphene in the presence of unitary scatterers as well as charged impurities. The temperature and carrier density dependence in this system display a number of anomalous features that arise due to the relativistic nature of the low energy fermionic degrees of freedom. We derive the properties in detail including the effect of unitary and charged impurities self-consistently, and present tables giving the anal…
The scaling of active photonic devices to deep-submicron length-scales has been hampered by the fundamental diffraction limit and the absence of materials with sufficiently strong electro-optic effects. Here, we demonstrate a solid state electro-optical switching mechanism that can operate in the visible spectral range with an unparalleled active volume of less than 5 nm cube, comparable to the size of the smallest electronic components. The switching mechanism relies on electrochemically displacing metal atoms inside the nanometer-scale gap to electrically connect two crossed metallic wires f…
In calculations of transport quantities, such as the electrical conductivity, thermal conductivity, Seebeck, Peltier, Nernst, Ettingshausen, Righi-Leduc, or Hall coefficients, sums over the Brillouin zone of wave-vector derivatives of the dispersion relation commonly appear. When the self-energy depends only on frequency, as in single-site dynamical mean-field theory, it is advantageous to perform these sums once and for all. We show here that in the case of a hypercubic lattice in d dimensions, the sums needed for any of the transport coefficients can be expressed as integrals over powers of …
Electrical double layers play a key role in a variety of electrochemical systems. The mean free path of secondary electrons in aqueous solutions is on the order of a nanometer, making them suitable for probing of ultrathin electrical double layers at solid-liquid electrolyte interfaces. Employing graphene as an electron-transparent electrode in a two-electrode electrochemical system, we show that the secondary electron yield of the graphene-liquid interface depends on the ionic strength and concentration of electrolyte and applied bias at the remote counter electrode. These observations have b…
Time-resolved electrical spin transport is used to generate and probe spin currents in GaAs electrically. We use high bandwidth current pulses to inject phase-coherent spin packets from Fe into n-GaAs. By means of time-resolved Faraday rotation we demonstrate that spins are injected with a clearly defined phase by the observation of multiple Larmor precession cycles. We furthermore show that spin precession of optically created spin packets in n-GaAs can be probed electrically by spin-polarized photo-current pulses. The injection and detection experiments are not direct reciprocals of each oth…
Thermoelectric (TE) conversion in conducting materials is of eminent importance for providing renewable energy and solid-state cooling. Although traditionally, the Seebeck effect plays a key role for the TE figure of merit zST, it encounters fundamental constraints hindering its conversion efficiency. Most notably, there are the charge compensation of electrons and holes that diminishes this effect, and the intertwinement of the corresponding electrical and thermal conductivities through the Wiedemann-Franz (WF) law which makes their independent optimization in zST impossible. Here, we demonst…
Electrical Probing of Dark Excitons through Microwave Permittivity
University of Texas at Austin（德克萨斯大学奥斯汀分校）；Washington University in St Louis（华盛顿大学圣路易斯分校）；University of Wisconsin-Madison（威斯康星大学麦迪逊分校）；Texas Materials Institute, The University of Texas at Austin（德克萨斯大学奥斯汀分校德克萨斯材料研究所）；Walker Department of Mechanical Engineering, Materials Science and Engineering Program, University of Texas at Austin（德克萨斯大学奥斯汀分校沃克机械工程系材料科学与工程项目）；National Institute for Materials Science（日本国立物质材料研究所）
Probing the Effects of Heat Treatment Atmosphere on the Structural and Electrical Properties of NBT via Eu Photoluminescence
State Key Laboratory of Integrated Optoelectronics, Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, School of Physics, Northeast Normal University（东北师范大学物理学院集成光电子学国家重点实验室教育部紫外发光材料与技术重点实验室）
The question whether ferroelectricity (FE) may coexist with a metallic or highly conducting state, or rather it must be suppressed by the screening from the free charges, is the focus of a rapidly increasing number of theoretical studies and is finally receiving positive experimental responses. The issue is closely related to the thermoelectric and multiferroic (also magnetic) applications of FE materials, where the electrical conductivity is required or spurious. In these circumstances, the traditional methods for probing ferroelectricity are hampered or made totally ineffective by the free c…
Here, in an effort towards facile and fast screening/diagnosis of novel coronavirus disease 2019 (COVID-19), we combined the unprecedently sensitive graphene field-effect transistor (Gr-FET) with highly selective antibody-antigen interaction to develop a coronavirus immunosensor. The Gr-FET immunosensors can rapidly identify (about 2 mins) and accurately capture the COVID-19 spike protein S1 (which contains a receptor binding domain, RBD) at a limit of detection down to 0.2 pM, in a real-time and label-free manner. Further results ensure that the Gr-FET immunosensors can be promisingly applied…
Electrical Probing of Sub-Néel Spin Dynamics in Two-Dimensional Antiferromagnets Using Graphene Heterostructures
Universidad Autónoma de Madrid（马德里自治大学）；Universidad de Sevilla（塞维利亚大学）；Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC—Universidad de Zaragoza（阿拉贡纳米科学与材料研究所，西班牙国家研究委员会—萨拉戈萨大学）；Condensed Matter Physics Center (IFIMAC) and Instituto Universitario de Ciencia de Materiales “Nicolás Cabrera” (INC), Universidad Autónoma de Madrid（凝聚态物理中心及“尼古拉斯·卡布雷拉”材料科学大学研究所，马德里自治大学）
The Seebeck-driven transverse thermoelectric generation in magnetic/thermoelectric hybrid materials (STTG) has been investigated in all-bulk hybrid materials. The transverse thermopower in a ferromagnetic Co$_2$MnGa/thermoelectric $n$-type Si hybrid bulk material with the adjusted dimensions reaches 16.0 $μ$V/K at room temperature with the aid of the STTG contribution, which is much larger than the anomalous Nernst coefficient of the Co$_2$MnGa slab (6.8 $μ$V/K). Although this transverse thermopower is smaller than the value for previously reported thin-film-based hybrid materials, the hybrid …
Using the linear response formalism for the spin-orbit torque (SOT) we compute from first principles the SOT in a system of two layers of L1$_0$-FePt(001) deposited on an fcc Pt(001) substrate of varying thickness. We find that at room temperature the values of the SOTs that are even and odd with respect to magnetization generally lie in the range of values measured and computed for Co/Pt bilayers. We also observe that the even SOT is much more robust with respect to changing the number of layers in the substrate, and as a function of energy it follows the general trend of the even SOT exerted…
Low-frequency noise with the spectral density S(f)~1/f^g (f is the frequency and g~1) is a ubiquitous phenomenon, which hampers operation of many devices and circuits. A long-standing question of particular importance for electronics is whether 1/f noise is generated on the surface of electrical conductors or inside their volumes. Using high-quality graphene multilayers we were able to directly address this fundamental problem of the noise origin. Unlike the thickness of metal or semiconductor films, the thickness of graphene multilayers can be continuously and uniformly varied all the way dow…
Magnetic insulators are a key resource for next-generation spintronic and topological devices. The family of layered metal halides promises ultrathin insulating multiferroics, spin liquids, and ferromagnets, but new characterization methods are required to unlock their potential. Here, we report tunneling through the layered magnetic insulator CrI3 as a function of temperature and applied magnetic field. We electrically detect the magnetic ground state and inter-layer coupling and observe a field-induced metamagnetic transition. The metamagnetic transition results in magnetoresistances of 95%,…
