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Register a new userObservation of thermoelectric currents in high-field superconductor-ferromagnet tunnel junctions
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We report on the experimental observation of thermoelectric currents in superconductor-ferromagnet tunnel junctions in high magnetic fields. The thermoelectric signals are due to a spin-dependent lifting of particle-hole symmetry, and are found to be in excellent agreement with recent theoretical predictions. The maximum Seebeck coefficient inferred from the data is about $-100~mathrm{mu V/K}$, much larger than commonly found in metallic structures. Our results directly prove the coupling of spin and heat transport in high-field superconductors.
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Thermoelectric effects result from the coupling of charge and heat transport, and can be used for thermometry, cooling and harvesting of thermal energy. The microscopic origin of thermoelectric effects is a broken electron-hole symmetry, which is usually quite small in metal structures, and vanishes at low temperatures. We report on a combined experimental and theoretical investigation of thermoelectric effects in superconductor/ferromagnet hybrid structures. We investigate the depencence of thermoelectric currents on the thermal excitation, as well as on the presence of a dc bias voltage across the junction. Large thermoelectric effects are observed in superconductor/ferromagnet and superconductor/normal-metal hybrid structures. The spin-independent signals observed under finite voltage bias are shown to be reciprocal to the physics of superconductor/normal-metal microrefrigerators. The spin-dependent thermoelectric signals in the linear regime are due to the coupling of spin and heat transport, and can be used to design more efficient refrigerators
We show that a huge thermoelectric effect can be observed by contacting a superconductor whose density of states is spin-split by a Zeeman field with a ferromagnet with a non-zero polarization. The resulting thermopower exceeds $k_B/e$ by a large factor, and the thermoelectric figure of merit $ZT$ can far exceed unity, leading to heat engine efficiencies close to the Carnot limit. We also show that spin-polarized currents can be generated in the superconductor by applying a temperature bias.
Superconductivity and magnetism are generally incompatible because of the opposing requirement on electron spin alignment. When combined, they produce a multitude of fascinating phenomena, including unconventional superconductivity and topological superconductivity. The emergence of two-dimensional (2D)layered superconducting and magnetic materials that can form nanoscale junctions with atomically sharp interfaces presents an ideal laboratory to explore new phenomena from coexisting superconductivity and magnetic ordering. Here we report tunneling spectroscopy under an in-plane magnetic field of superconductor-ferromagnet-superconductor (S/F/S) tunnel junctions that are made of 2D Ising superconductor NbSe2 and ferromagnetic insulator CrBr3. We observe nearly 100% tunneling anisotropic magnetoresistance (AMR), that is, difference in tunnel resistance upon changing magnetization direction from out-of-plane to inplane. The giant tunneling AMR is induced by superconductivity, particularly, a result of interfacial magnetic exchange coupling and spin-dependent quasiparticle scattering. We also observe an intriguing magnetic hysteresis effect in superconducting gap energy and quasiparticle scattering rate with a critical temperature that is 2 K below the superconducting transition temperature. Our study paves the path for exploring superconducting spintronic and unconventional superconductivity in van der Waals heterostructures.
We demonstrate both theoretically and experimentally two limiting factors in cooling electrons using biased tunnel junctions to extract heat from a normal metal into a superconductor. Firstly, when the injection rate of electrons exceeds the internal relaxation rate in the metal to be cooled, the electrons do no more obey the Fermi-Dirac distribution, and the concept of temperature cannot be applied as such. Secondly, at low bath temperatures, states within the gap induce anomalous heating and yield a theoretical limit of the achievable minimum temperature.
Topological spin configurations in proximity to a superconductor have recently attracted great interest due to the potential application of the former in spintronics and also as another platform for realizing non-trivial topological superconductors. Their application in these areas requires precise knowledge of the existing exchange fields and/or the stray-fields which are therefore essential for the study of these systems. Here, we determine the effective stray-field and the Meissner currents in a Superconductor/Ferromagnet/Superconductor (S/F/S) junction produced by various nonhomogenous magnetic textures in the F. The inhomogeneity arises either due to a periodic structure with flat domain walls (DW) or is caused by an isolated chiral magnetic skyrmion (Sk). We consider both Bloch- and N{e}el-type Sk and also analyze in detail the periodic structures of different types of DWs-- that is Bloch-type DW (BDW) and N{e}el-type DW (NDW) of finite width with in- and out-of-plane magnetization vector. The spatial dependence of the fields and Meissner currents are shown to be qualitatively different for the case of Bloch- and N{e}el-type magnetic textures. While the spatial distributions in the upper and lower S are identical for Bloch-type Sk and DWs they are asymmetric for the case of N{e}el-type magnetic textures. The depairing factor, which determines the critical temperature and which is related to vector potential of the stray-field, can have its maximum at the center of a magnetic domain but also, as we show, above the DW. For Sks the maximum is located at a finite distance within the Sk radius. Based on this, we study the nucleation of superconductivity in the presence of DWs. Because of the asymmetry for N{e}el-type structures, the critical temperature in the upper and lower S is expected to be different. The obtained results can also be applied to S/F bilayers.
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