A Nickel Dissolution Process for Multilayer Electroforming to Achieve Ultrahigh Adhesion Strength.

Multilayer electroforming has a high potential to produce Ni/Ni layer structured metal walls with excellent material properties and a high thickness uniformity. However, Ni is easily oxidized in air, which fundamentally leads to a low adhesion strength between the Ni layers. Here, a novel in situ treatment is proposed for improving the adhesion performance between Ni layers. This treatment integrated the steps of electrochemical dissolution, surface protection, and electroforming. A study of the polarization behavior implied the electroformed Ni layer was dissolved efficiently in the NH2SO3H solution, beginning at a dissolution current density of 5 A·cm-2, which could remove the oxide film. A smooth substrate surface with a good surface hydrophilicity was obtained starting at 8 A·cm-2, helping to protect the activated substrate from being contaminated and oxidized. The experimental results showed that ultrahigh normal and shear adhesion strengths over 400 MPa between the Ni layers were achieved.

Tuning the anomalous Nernst and Hall effects with shifting the chemical potential in Fe-doped and Ni-doped Co3Sn2S2.

Co3Sn2S2is believed to be a magnetic Weyl semimetal. It displays large anomalous Hall, Nernst and thermal Hall effects with a remarkably large anomalous Hall angle. Here, we present a comprehensive study of how substituting Co by Fe or Ni affects the electrical and thermoelectric transport. We find that doping alters the amplitude of the anomalous transverse coefficients. The maximum decrease in the amplitude of the low-temperature anomalous Hall conductivityσijAis twofold. Comparing our results with theoretical calculations of the Berry spectrum assuming a rigid shift of the Fermi level, we find that given the modest shift in the position of the chemical potential induced by doping, the experimentally observed variation occurs five times faster than expected. Doping affects the amplitude and the sign of the anomalous Nernst coefficient. Despite these drastic changes, the amplitude of theαijA/σijAratio at the Curie temperature remains close to≈0.5kB/e, in agreement with the scaling relationship observed across many topological magnets.

Evidence for Anisotropic Superconductivity Beyond Pauli Limit in Infinite-Layer Lanthanum Nickelates.

After being expected to be a promising analog to cuprates for decades, superconductivity has recently been discovered in infinite-layer nickelates, providing new opportunities to explore mechanisms of high-temperature superconductivity. However, in sharp contrast to the single-band and anisotropic superconductivity in cuprates, nickelates exhibit a multi-band electronic structure and an unexpected isotropic superconductivity as reported recently, which challenges the cuprate-like picture in nickelates. Here, it is shown that strong anisotropic magnetotransport behaviors exist in La-based nickelate films with enhanced crystallinity and superconductivity ( T c onset $T_{\rm{c}}^{{\rm{onset}}}$ = 18.8 K, T c zero $T_{\rm{c}}^{{\rm{zero}}}$ = 16.5 K). The upper critical fields are anisotropic and violate the estimated Bardeen-Cooper-Schrieffer (BCS) Pauli limit ( H Pauli , µ = 1 µ B = 1.86 × T c , H = 0 ${H}_{\mathrm{Pauli},\mu =1{\mu}_{B}}=1.86\ensuremath{\times{}}{T}_{\mathrm{c},H=0}$ ) for in-plane magnetic fields. Moreover, the anisotropic superconductivity is further manifested by the cusp-like peak of the angle-dependent Tc and the vortex motion anisotropy under external magnetic fields.

Field-linear anomalous Hall effect and Berry curvature induced by spin chirality in the kagome antiferromagnet Mn3Sn.

During the past two decades, it has been established that a non-trivial electron wave-function topology generates an anomalous Hall effect (AHE), which shows itself as a Hall conductivity non-linear in magnetic field. Here, we report on an unprecedented case of field-linear AHE. In Mn3Sn, a kagome magnet, the out-of-plane Hall response, which shows an abrupt jump, was discovered to be a case of AHE. We find now that the in-plane Hall response, which is perfectly linear in magnetic field, is set by the Berry curvature of the wavefunction. The amplitude of the Hall response and its concomitant Nernst signal exceed by far what is expected in the semiclassical picture. We argue that magnetic field induces out-of-plane spin canting and thereafter gives rise to nontrivial spin chirality on the kagome lattice. In band structure, we find that the spin chirality modifies the topology by gapping out Weyl nodal lines unknown before, accounting for the AHE observed. Our work reveals intriguing unification of real-space Berry phase from spin chirality and momentum-space Berry curvature in a kagome material.

The phonon thermal Hall angle in black phosphorus.

The origin of phonon thermal Hall Effect (THE) observed in a variety of insulators is yet to be identified. Here, we report on the observation of a thermal Hall conductivity in a non-magnetic elemental insulator, with an amplitude exceeding what has been previously observed. In black phosphorus (BP), the longitudinal (κii), and the transverse, κij, thermal conductivities peak at the same temperature and at this peak temperature, the κij/κjj/B is ≈ 10-4-10-3 T-1. Both these features are shared by other insulators displaying THE, despite an absolute amplitude spreading over three orders of magnitude. The absence of correlation between the thermal Hall angle and the phonon mean-free-path imposes a severe constraint for theoretical scenarios of THE. We show that in BP a longitudinal and a transverse acoustic phonon mode anti-cross, facilitating wave-like transport across modes. The anisotropic charge distribution surrounding atomic bonds can pave the way for coupling between phonons and the magnetic field.

Onsager Reciprocal Relation between Anomalous Transverse Coefficients of an Anisotropic Antiferromagnet.

Whenever two irreversible processes occur simultaneously, time-reversal symmetry of microscopic dynamics gives rise, on a macroscopic level, to Onsager's reciprocal relations, which impose constraints on the number of independent components of any transport coefficient tensor. Here, we show that in the antiferromagnetic YbMnBi_{2}, which displays a strong temperature-dependent anisotropy, Onsager's reciprocal relations are strictly satisfied for anomalous electric (σ_{ij}^{A}) and anomalous thermoelectric (α_{ij}^{A}) conductivity tensors. In contradiction with what was recently reported by Pan et al. [Nat. Mater. 21, 203 (2022)NMAACR1476-112210.1038/s41563-021-01149-2], we find that σ_{ij}^{A}(H)=σ_{ji}^{A}(-H) and α_{ij}^{A}(H)=α_{ji}^{A}(-H). This equality holds in the whole temperature window irrespective of the relative weights of the intrinsic or extrinsic mechanisms. The α_{ij}^{A}/σ_{ij}^{A} ratio is close to k_{B}/e at room temperature but peaks to an unprecedented magnitude of 2.9k_{B}/e at ∼150 K, which may involve nondegenerate carriers of small Fermi surface pockets.

Electrochemical 3D printing of Ni-Mn and Ni-Co alloy with FluidFM.

Additive manufacturing can realize almost any designed geometry, enabling the fabrication of innovative products for advanced applications. Local electrochemical plating is a powerful approach for additive manufacturing of metal microstructures; however, previously reported data have been mostly obtained with copper, and only a few cases have been reported with other elements. In this study, we assessed the ability of fluidic force microscopy to produce Ni-Mn and Ni-Co alloy structures. Once the optimal deposition potential window was determined, pillars with relatively smooth surfaces were obtained. The printing process was characterized by printing rates in the range of 50-60 nm s-1. Cross-sections exposed by focused ion beam showed highly dense microstructures, while the corresponding face scan with energy-dispersive x-ray spectroscopy spectra revealed a uniform distribution of alloy components.

Influence of ultrasonic in low thermal expansion Fe-Ni electrodeposition process for micro-electroforming.

The electrochemical mechanism of Fe-Ni electrodeposition under ultrasonic was investigated by electrochemistry methods. Linear scanning voltammetry and cyclic voltammetry were used to show that the deposition process changed from the diffusion control under static conditions to an electrochemical control under ultrasonic conditions. Chronoamperometry curves showed that the Fe-Ni deposit occurred by a mechanism that instantaneous nucleation is followed by three-dimensional growth under charge transfer control. Chronopotentiogram indicated that because of the intensity of the ultrasound stripping effect, high ultrasonic power is unsuitable for electroforming Fe-Ni alloy, and a high current density is also not appropriate. Thus, the optimum parameters for Fe-Ni electrodeposition under ultrasonic conditions are ultrasonic power between 80 and 100 W (power density 0.28-0.35 W/cm2), and a current density lower than 10 mA/cm2 with temperature 323 K and pH 3. Experiments were performed to verify that the Fe-Ni masks prepared by ultrasonic-assisted electroforming had a good surface quality. The increase in ultrasonic power can obtain a larger grain size, thus got a low thermal expansion coefficient and a high hardness. Therefore, ultrasonic-assisted electrodeposition technology provides an effective and practically feasible manufacturing method for OLED Fe-Ni mask preparation.

A Monomaterial Nernst Thermopile with Hermaphroditic Legs.

A large transverse thermoelectric response, known as the anomalous Nernst effect (ANE) has been recently observed in several topological magnets. Building a thermopile employing this effect has been the subject of several recent propositions. Here, a thermopile is designed and built with an array of tilted adjacent crystals of Mn3 Sn. The design employs a single material and replaces pairs of P and N thermocouples of the traditional design with hermaphroditic legs. The design exploits the large lag angle between the applied field and the magnetization, which is attributed to the interruption of magnetic octupoles at the edge of the xy-plane. Eliminating extrinsic contact between the legs will boost the efficiency, simplify the process, and pave the way for a new generation of thermopiles.

Unconventional quantum vortex matter state hosts quantum oscillations in the underdoped high-temperature cuprate superconductors.

A central question in the underdoped cuprates pertains to the nature of the pseudogap ground state. A conventional metallic ground state of the pseudogap region has been argued to host quantum oscillations upon destruction of the superconducting order parameter by modest magnetic fields. Here, we use low applied measurement currents and millikelvin temperatures on ultrapure single crystals of underdoped [Formula: see text] to unearth an unconventional quantum vortex matter ground state characterized by vanishing electrical resistivity, magnetic hysteresis, and nonohmic electrical transport characteristics beyond the highest laboratory-accessible static fields. A model of the pseudogap ground state is now required to explain quantum oscillations that are hosted by the bulk quantum vortex matter state without experiencing sizable additional damping in the presence of a large maximum superconducting gap; possibilities include a pair density wave.

Observation of the antiferromagnetic spin Hall effect.

The discovery of the spin Hall effect1 enabled the efficient generation and manipulation of the spin current. More recently, the magnetic spin Hall effect2,3 was observed in non-collinear antiferromagnets, where the spin conservation is broken due to the non-collinear spin configuration. This provides a unique opportunity to control the spin current and relevant device performance with controllable magnetization. Here, we report a magnetic spin Hall effect in a collinear antiferromagnet, Mn2Au. The spin currents are generated at two spin sublattices with broken spatial symmetry, and the antiparallel antiferromagnetic moments play an important role. Therefore, we term this effect the 'antiferromagnetic spin Hall effect'. The out-of-plane spins from the antiferromagnetic spin Hall effect are favourable for the efficient switching of perpendicular magnetized devices, which is required for high-density applications. The antiferromagnetic spin Hall effect adds another twist to the atomic-level control of spin currents via the antiferromagnetic spin structure.

Critical point for Bose-Einstein condensation of excitons in graphite.

An exciton is an electron-hole pair bound by attractive Coulomb interaction. Short-lived excitons have been detected by a variety of experimental probes in numerous contexts. An excitonic insulator, a collective state of such excitons, has been more elusive. Here, thanks to Nernst measurements in pulsed magnetic fields, we show that in graphite there is a critical temperature (T = 9.2 K) and a critical magnetic field (B = 47 T) for Bose-Einstein condensation of excitons. At this critical field, hole and electron Landau subbands simultaneously cross the Fermi level and allow exciton formation. By quantifying the effective mass and the spatial separation of the excitons in the basal plane, we show that the degeneracy temperature of the excitonic fluid corresponds to this critical temperature. This identification would explain why the field-induced transition observed in graphite is not a universal feature of three-dimensional electron systems pushed beyond the quantum limit.

Anisotropic Transport and Quantum Oscillations in the Quasi-One-Dimensional TaNiTe5: Evidence for the Nontrivial Band Topology.

The past decade has witnessed the burgeoning discovery of a variety of topological states of matter with distinct nontrivial band topologies. Thus far, most materials that have been studied possess two-dimensional or three-dimensional electronic structures, with only a few exceptions that host quasi-one-dimensional (quasi-1D) topological electronic properties. Here we present clear-cut evidence for Dirac Fermions in the quasi-1D telluride TaNiTe5. We show that its transport behaviors are highly anisotropic, and we observe nontrivial Berry phases via quantum oscillation measurements. The nontrivial band topology is further corroborated by first-principles calculations. Our results may help to guide the future quest for topological states in this new family of quasi-1D ternary chalcogenides.

Finite-temperature violation of the anomalous transverse Wiedemann-Franz law.

The Wiedemann-Franz (WF) law has been tested in numerous solids, but the extent of its relevance to the anomalous transverse transport and the topological nature of the wave function, remains an open question. Here, we present a study of anomalous transverse response in the noncollinear antiferromagnet Mn3Ge extended from room temperature down to sub-kelvin temperature and find that the anomalous Lorenz ratio remains close to the Sommerfeld value up to 100 K but not above. The finite-temperature violation of the WF correlation is caused by a mismatch between the thermal and electrical summations of the Berry curvature and not by inelastic scattering. This interpretation is backed by our theoretical calculations, which reveals a competition between the temperature and the Berry curvature distribution. The data accuracy is supported by verifying the anomalous Bridgman relation. The anomalous Lorenz ratio is thus an extremely sensitive probe of the Berry spectrum of a solid.

Anomalous Hall Effect, Robust Negative Magnetoresistance, and Memory Devices Based on a Noncollinear Antiferromagnetic Metal.

We report the successful fabrication of noncollinear antiferromagnetic D019 Mn3Ge thin films on insulating oxide substrates. The anomalous Hall effect and the large parallel negative magnetoresistance that is robust up to 53 T are observed in the thin films, which may provide evidence for the recent theoretical prediction of the existence of Weyl fermions in antiferromagnetic Mn3Ge. More importantly, we integrate the Mn3Ge thin films onto ferroelectric PMN-PT substrates and manipulate the longitudinal resistance reversibly by electric fields at room temperature, demonstrating the anisotropic magnetoresistance effect in noncollinear antiferromagnets, which thus illustrates the potential of antiferromagnetic Mn3Ge for information storage applications.

Phonon Thermal Hall Effect in Strontium Titanate.

It has been known for more than a decade that phonons can produce an off-diagonal thermal conductivity in the presence of a magnetic field. Recent studies of thermal Hall conductivity, κ_{xy}, in a variety of contexts, however, have assumed a negligibly small phonon contribution. We present a study of κ_{xy} in quantum paraelectric SrTiO_{3}, which is a nonmagnetic insulator and find that its peak value exceeds what has been reported in any other insulator, including those in which the signal has been qualified as "giant." Remarkably, κ_{xy}(T) and κ(T) peak at the same temperature and the former decreases faster than the latter at both sides of the peak. Interestingly, in the case of La_{2}CuO_{4} and α-RuCl_{3}, κ_{xy}(T) and κ(T) peak also at the same temperature. We also studied KTaO_{3} and found a small signal, indicating that a sizable κ_{xy}(T) is not a generic feature of quantum paraelectrics. Combined to other observations, this points to a crucial role played by antiferrodistortive domains in generating κ_{xy} of this solid.

Surface superconductivity in the type II Weyl semimetal TaIrTe4.

The search for unconventional superconductivity in Weyl semimetal materials is currently an exciting pursuit, since such superconducting phases could potentially be topologically non-trivial and host exotic Majorana modes. The layered material TaIrTe4 is a newly predicted time-reversal invariant type II Weyl semimetal with the minimum number of Weyl points. Here, we report the discovery of surface superconductivity in Weyl semimetal TaIrTe4. Our scanning tunneling microscopy/spectroscopy (STM/STS) visualizes Fermi arc surface states of TaIrTe4 that are consistent with the previous angle-resolved photoemission spectroscopy results. By a systematic study based on STS at ultralow temperature, we observe uniform superconducting gaps on the sample surface. The superconductivity is further confirmed by electrical transport measurements at ultralow temperature, with an onset transition temperature (T c) up to 1.54 K being observed. The normalized upper critical field h*(T/T c) behavior and the stability of the superconductivity against the ferromagnet indicate that the discovered superconductivity is unconventional with the p-wave pairing. The systematic STS, and thickness- and angular-dependent transport measurements reveal that the detected superconductivity is quasi-1D and occurs in the surface states. The discovery of the surface superconductivity in TaIrTe4 provides a new novel platform to explore topological superconductivity and Majorana modes.

Chiral domain walls of Mn3Sn and their memory.

Magnetic domain walls are topological solitons whose internal structure is set by competing energies which sculpt them. In common ferromagnets, domain walls are known to be of either Bloch or Néel types. Little is established in the case of Mn3Sn, a triangular antiferromagnet with a large room-temperature anomalous Hall effect, where domain nucleation is triggered by a well-defined threshold magnetic field. Here, we show that the domain walls of this system generate an additional contribution to the Hall conductivity tensor and a transverse magnetization. The former is an electric field lying in the same plane with the magnetic field and electric current and therefore a planar Hall effect. We demonstrate that in-plane rotation of spins inside the domain wall would explain both observations and the clockwise or anticlockwise chirality of the walls depends on the history of the field orientation and can be controlled.

Unconventional Antiferromagnetic Quantum Critical Point in Ba(Fe_{0.97}Cr_{0.03})_{2}(As_{1-x}P_{x})_{2}.

We have systematically studied physical properties of Ba(Fe_{0.97}Cr_{0.03})_{2}(As_{1-x}P_{x})_{2}, where superconductivity in BaFe_{2}(As_{1-x}P_{x})_{2} is fully suppressed by just 3% of Cr substitution of Fe. A quantum critical point is revealed at x∼0.42, where non-Fermi-liquid behaviors similar to those in BaFe_{2}(As_{1-x}P_{x})_{2} are observed. Neutron diffraction and inelastic neutron scattering measurements suggest that the quantum critical point is associated with the antiferromagnetic order, which is not of conventional spin-density-wave type as evidenced by the ω/T scaling of spin excitations. On the other hand, no divergence of low-temperature nematic susceptibility is observed when x is decreased to 0.42 from higher doping level, demonstrating that there are no nematic quantum critical fluctuations. Our results suggest that non-Fermi-liquid behaviors in iron-based superconductors can be solely resulted from the antiferromagnetic quantum critical fluctuations, which cast doubts on the role of nematic fluctuations played in the normal-state properties in iron-based superconductors.

A piezoelectric, strain-controlled antiferromagnetic memory insensitive to magnetic fields.

Spintronic devices based on antiferromagnetic (AFM) materials hold the promise of fast switching speeds and robustness against magnetic fields1-3. Different device concepts have been predicted4,5 and experimentally demonstrated, such as low-temperature AFM tunnel junctions that operate as spin-valves6, or room-temperature AFM memory, for which either thermal heating in combination with magnetic fields7 or Néel spin-orbit torque8 is used for the information writing process. On the other hand, piezoelectric materials were employed to control magnetism by electric fields in multiferroic heterostructures9-12, which suppresses Joule heating caused by switching currents and may enable low-energy-consuming electronic devices. Here, we combine the two material classes to explore changes in the resistance of the high-Néel-temperature antiferromagnet MnPt induced by piezoelectric strain. We find two non-volatile resistance states at room temperature and zero electric field that are stable in magnetic fields up to 60 T. Furthermore, the strain-induced resistance switching process is insensitive to magnetic fields. Integration in a tunnel junction can further amplify the electroresistance. The tunnelling anisotropic magnetoresistance reaches ~11.2% at room temperature. Overall, we demonstrate a piezoelectric, strain-controlled AFM memory that is fully operational in strong magnetic fields and has the potential for low-energy and high-density memory applications.