The quantum Hall effect allows the international standard for resistance to be defined in terms of the electron charge and Planck's constant alone. The effect comprises the quantization of the Hall resistance in two-dimensional electron systems in rational fractions of R(K) = h/e(2) = 25,812.807557(18) Omega, the resistance quantum. Despite 30 years of research into the quantum Hall effect, the level of precision necessary for metrology--a few parts per billion--has been achieved only in silicon and iii-v heterostructure devices. Graphene should, in principle, be an ideal material for a quantum resistance standard, because it is inherently two-dimensional and its discrete electron energy levels in a magnetic field (the Landau levels) are widely spaced. However, the precisions demonstrated so far have been lower than one part per million. Here, we report a quantum Hall resistance quantization accuracy of three parts per billion in monolayer epitaxial graphene at 300 mK, four orders of magnitude better than previously reported. Moreover, by demonstrating the structural integrity and uniformity of graphene over hundreds of micrometres, as well as reproducible mobility and carrier concentrations across a half-centimetre wafer, these results boost the prospects of using epitaxial graphene in applications beyond quantum metrology.

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Topological superconductivity is central to a variety of novel phenomena involving the interplay between topologically ordered phases and broken-symmetry states. The key ingredient is an unconventional order parameter, with an orbital component containing a chiral p x + ip y wave term. Here we present phase-sensitive measurements, based on the quantum interference in nanoscale Josephson junctions, realized by using Bi2Te3 topological insulator. We demonstrate that the induced superconductivity is unconventional and consistent with a sign-changing order parameter, such as a chiral p x + ip y component. The magnetic field pattern of the junctions shows a dip at zero externally applied magnetic field, which is an incontrovertible signature of the simultaneous existence of 0 and π coupling within the junction, inherent to a non trivial order parameter phase. The nano-textured morphology of the Bi2Te3 flakes, and the dramatic role played by thermal strain are the surprising key factors for the display of an unconventional induced order parameter.

Heterostructures composed of ferromagnetic La 0.7 Sr 0.3 MnO 3 , ferromagnetic SrRuO 3 , and superconducting YBa 2 Cu 3 O 6+x were studied experimentally. Structures of composition Au/La 0.7 Sr 0.3 MnO 3 /SrRuO 3 / YBa 2 Cu 3 O 6+x were prepared by pulsed laser deposition, and their high quality was confirmed by x-ray diffraction and reflectometry. A noncollinear magnetic state of the heterostructures was revealed by means of superconducting quantum interference device magnetometry and polarized neutron reflectometry. We have further observed superconducting currents in mesa structures fabricated by deposition of a second superconducting Nb layer on top of the heterostructure, followed by patterning with photolithography and ion-beam etching. Josephson effects observed in these mesa structures can be explained by the penetration of a triplet component of the superconducting order parameter into the magnetic layers.

can allow the discovery of basic new physical phenomena and the development of new device concepts. [1] The discovery of new vdW quantum materials and their heterostructures starting from graphene, insulators, semiconductors, superconductors, and topological materials has revolutionized both fundamental and applied research. [2,3] The most recent addition to this vdW family is magnets, which have offered various advantages over conventional magnets and opened new perspectives in vdW heterostructure designs. [4][5][6] In addition to the atomically thin and flat nature of vdW magnets, flexibility, gate tunability, strong proximity interactions, and twist angle between the layers can offer a unique degree of freedom and an innovative platform for device functionalities. [4,5] Recently, several vdW magnets have emerged with the discovery of insulating Cr 2 Ge 2 Te 6 , [7] semiconducting (CrI 3 , [8] CrBr 3 [9] ), and metallic Fe x GeTe 2 . [10,11] The insulating vdW magnets are useful for spin-filter tunneling [9,12] and proximityinduced magnetism, [13][14][15] whereas the metallic magnets can be used as electrodes in magnetic tunnel junctions, [16] observationThe discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe 5 GeTe 2 in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe 5 GeTe 2 are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe 5 GeTe 2 /graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe 5 GeTe 2 along with the presence of negative spin polarization at the Fe 5 GeTe 2 / graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

In the quest for developing novel and efficient batteries, a great interest has been raised for sustainable K-based honeycomb layer oxide materials, both for their application in energy devices as well as for their fundamental material properties. A key issue in the realization of efficient batteries based on such compounds, is to understand the K-ion diffusion mechanism. However, investigation of potassium-ion (K$$^+$$ + ) dynamics in materials using e.g. NMR and related techniques has so far been very challenging, due to its inherently weak nuclear magnetic moment, in contrast to other alkali ions such as lithium and sodium. Spin-polarised muons, having a high gyromagnetic ratio, make the muon spin rotation and relaxation ($$\mu ^+$$ μ + SR) technique ideal for probing ions dynamics in these types of energy materials. Here we present a study of the low-temperature magnetic properties as well as K$$^+$$ + dynamics in honeycomb layered oxide material $${\hbox {K}_2\hbox {Ni}_2\hbox {TeO}_6}$$ K 2 Ni 2 TeO 6  using mainly the $$\mu ^+$$ μ + SR technique. Our low-temperature $$\mu ^+$$ μ + SR results together with complementary magnetic susceptibility measurements find an antiferromagnetic transition at $$T_{\mathrm{N}}\approx 27$$ T N ≈ 27  K. Further $${\mu}^{+}$$ μ + SR studies performed at higher temperatures reveal that potassium ions (K$$^+$$ + ) become mobile above 200 K and the activation energy for the diffusion process is obtained as $$E_{\mathrm{a}}=121 (13)$$ E a = 121 ( 13 )  meV. This is the first time that K$$^+$$ + dynamics in potassium-based battery materials has been measured using $$\mu ^+$$ μ + SR. Assisted by high-resolution neutron diffraction, the temperature dependence of the K-ion self diffusion constant is also extracted. Finally our results also reveal that K-ion diffusion occurs predominantly at the surface of the powder particles. This opens future possibilities for potentially improving ion diffusion as well as K-ion battery device performance using nano-structuring and surface coatings of the particles.

Complex oxides show extreme sensitivity to structural distortions and defects, and the intricate balance of competing interactions which emerge at atomically defined interfaces may give rise to unexpected physics. In the interfaces of non-magnetic complex oxides, one of the most intriguing properties is the emergence of magnetism which is sensitive to chemical defects. Particularly, it is unclear which defects are responsible for the emergent magnetic interfaces. Here, we show direct and clear experimental evidence, supported by theoretical explanation, that the B-site cation stoichiometry is crucial for the creation and control of magnetism at the interface between non-magnetic ABO 3-perovskite oxides, LaAlO 3 and SrTiO 3. We find that consecutive defect formation, driven by atomic charge compensation, establishes the formation of robust perpendicular magnetic moments at the interface. Our observations propose a route to tune these emerging magnetoelectric structures, which are strongly coupled at the polar-nonpolar complex oxide interfaces.

We present noise measurements performed on a YBa 2 Cu 3 O 7−δ nanoscale weak-linkbased magnetometer consisting of a Superconducting QUantum Interference Device (SQUID) galvanically coupled to a 3.5 × 3.5 mm 2 pick-up loop, reaching white flux noise levels and magnetic noise levels as low as 6 µΦ 0 / √ Hz and 100 fT/ √ Hz at T = 77 K, respectively. The low noise is achieved by introducing Grooved Dayem Bridges (GDBs), a new concept of weak-link. A fabrication technique has been developed for the realization of nanoscale grooved bridges, which substitutes standard Dayem bridge weak links. The introduction of these novel key blocks reduces the parasitic inductance of the weak links and increases the differential resistance of the SQUIDs. This greatly improves the device performance, thus resulting in a reduction of the white noise.