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The existence of a quantum spin liquid (QSL) in which quantum fluctuations of spins are sufficiently strong to preclude spin ordering down to zero temperature was originally proposed theoretically more than 40 years ago, but its experimental realization turned out to be very elusive. Here we report on an almost ideal spin liquid state that appears to be realized by atomic-cluster spins on the triangular lattice of a charge-density wave state of 1T-TaS 2 . In this system, the charge excitations have a well-defined gap of ∼0.3 eV, while nuclear quadrupole resonance and muon-spin-relaxation experiments reveal that the spins show gapless QSL dynamics and no long-range magnetic order at least down to 70 mK. Canonical T 2 power-law temperature dependence of the spin relaxation dynamics characteristic of a QSL is observed from 200 K to T f = 55 K. Below this temperature, we observe a new gapless state with reduced density of spin excitations and high degree of local disorder signifying new quantum spin order emerging from the QSL. In the charge-density wave state of tantalum sulfide, tantalum atoms group into a Star-of-David arrangement. Experiments show that the polaron spins associated with these atomic clusters can form a quantum spin liquid.

Directed motion of liquid droplets is of considerable importance in various water and thermal management technologies. Although various methods to generate such motion have been developed at low temperature, they become rather ineffective at high temperature, where the droplet transits to a Leidenfrost state. In this state, it becomes challenging to control and direct the motion of the highly mobile droplets towards specific locations on the surface without compromising the effective heat transfer. Here we report that the wetting symmetry of a droplet can be broken at high temperature by creating two concurrent thermal states (Leidenfrost and contact-boiling) on a topographically patterned surface, thus engendering a preferential motion of a droplet towards the region with a higher heat transfer coefficient. The fundamental understanding and the ability to control the droplet dynamics at high temperature have promising applications in various systems requiring high thermal efficiency, operational security and fidelity. Controlled motion of a droplet on a hot surface is hampered by the formation of an evaporation layer below the droplet (Leidenfrost effect). But a cleverly patterned surface induces a Leidenfrost–contact-boiling state, directing the droplet’s motion.

High-temperature superconductivity in the iron-based materials emerges from, or sometimes coexists with, their metallic or insulating parent compound states. This is surprising, as these undoped states exhibit dramatically different antiferromagnetic spin arrangements and Néel temperatures. Although there is a general consensus that magnetic interactions are important for superconductivity, much remains unknown concerning the microscopic origin of the magnetic states. In this review, we summarize the progress in this area, focusing on recent experimental and theoretical results, and their microscopic implications. We conclude that the parent compounds are in a state that is more complex than that implied by a simple Fermi surface nesting scenario, and a dual description including both itinerant and localized degrees of freedom is needed to properly describe these fascinating materials. The magnetic states found in iron-based superconductors are more complex than originally thought. This Review argues that the magnetism arises from both itinerant and localized electrons.

The unconventional superconductivity associated with iron pnictide materials has been the subject of intense interest. Using an annealing procedure to control the charge-carrier concentration, the behaviour of an FeSe monolayer deposited on SrTiO 3 is now investigated, and indications of superconductivity at temperatures up to 65 K observed. The recent discovery of possible high-temperature superconductivity in single-layer FeSe films 1 , 2 has generated significant experimental and theoretical interest 3 , 4 . In both the cuprate 5 , 6 and the iron-based 7 , 8 , 9 , 10 , 11 high-temperature superconductors, superconductivity is induced by doping charge carriers into the parent compound to suppress the antiferromagnetic state. It is therefore important to establish whether the superconductivity observed in the single-layer sheets of FeSe—the essential building blocks of the Fe-based superconductors—is realized by undergoing a similar transition. Here we report the phase diagram for an FeSe monolayer grown on a SrTiO 3 substrate, by tuning the charge carrier concentration over a wide range through an extensive annealing procedure. We identify two distinct phases that compete during the annealing process: the electronic structure of the phase at low doping (N phase) bears a clear resemblance to the antiferromagnetic parent compound of the Fe-based superconductors, whereas the superconducting phase (S phase) emerges with the increase in doping and the suppression of the N phase. By optimizing the carrier concentration, we observe strong indications of superconductivity with a transition temperature of 65±5 K. The wide tunability of the system across different phases makes the FeSe monolayer ideal for investigating not only the physics of superconductivity, but also for studying novel quantum phenomena more generally.

In the search for the mechanism of high-temperature superconductivity, intense research has been focused on the evolution of the spin excitation spectrum on doping from the antiferromagnetic insulating to the superconducting state of the cuprates. Because of technical limitations, the experimental investigation of doped cuprates has been largely focused on low-energy excitations in a small range of momentum space. Here we use resonant inelastic X-ray scattering to show that a large family of superconductors, encompassing underdoped YBa 2 Cu 4 O 8 and overdoped YBa 2 Cu 3 O 7 , exhibits damped spin excitations (paramagnons) with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. The comprehensive experimental description of this surprisingly simple spectrum enables quantitative tests of magnetic Cooper pairing models. A numerical solution of the Eliashberg equations for the magnetic spectrum of YBa 2 Cu 3 O 7 reproduces its superconducting transition temperature within a factor of two, a level of agreement comparable to that of Eliashberg theories of conventional superconductors. In the copper oxide superconductors, spin fluctuations might be involved in the electronic pairing mechanism. The case for such magnetically mediated superconductivity is now strengthened by the discovery of high-energy magnetic excitations that are not affected by chemical doping levels within several cuprates.

A thermodynamic probe of the recently discovered charge-density-wave order in YBa 2 Cu 3 O y reveals a biaxial modulation in magnetic fields up to 40 T. The interplay between superconductivity and any other competing order is an essential part of the long-standing debate on the origin of high-temperature superconductivity in cuprate materials 1 , 2 . Akin to the situation in the heavy fermions, organic superconductors and pnictides, it has been proposed that the pairing mechanism in the cuprates comes from fluctuations of a nearby quantum phase transition 3 . Recent evidence for charge modulation 4 and its associated fluctuations 5 , 6 , 7 in the pseudogap phase of YBa 2 Cu 3 O y makes charge order a likely candidate for a competing order. However, a thermodynamic signature of the charge-ordering phase transition is still lacking. Moreover, whether the charge modulation is uniaxial or biaxial remains controversial. Here we address both issues by measuring sound velocities in YBa 2 Cu 3 O 6.55 in high magnetic fields. We provide the first thermodynamic signature of the competing charge-order phase transition in YBa 2 Cu 3 O y and construct a field–temperature phase diagram. The comparison of different acoustic modes indicates that the charge modulation is biaxial, which differs from a uniaxial stripe charge order.

Boosting large-scale superconductor applications require nanostructured conductors with artificial pinning centres immobilizing quantized vortices at high temperature and magnetic fields. Here we demonstrate a highly effective mechanism of artificial pinning centres in solution-derived high-temperature superconductor nanocomposites through generation of nanostrained regions where Cooper pair formation is suppressed. The nanostrained regions identified from transmission electron microscopy devise a very high concentration of partial dislocations associated with intergrowths generated between the randomly oriented nanodots and the epitaxial YBa 2 Cu 3 O 7 matrix. Consequently, an outstanding vortex-pinning enhancement correlated to the nanostrain is demonstrated for four types of randomly oriented nanodot, and a unique evolution towards an isotropic vortex-pinning behaviour, even in the effective anisotropy, is achieved as the nanostrain turns isotropic. We suggest a new vortex-pinning mechanism based on the bond-contraction pairing model, where pair formation is quenched under tensile strain, forming new and effective core-pinning regions. It is well known that to reduce dissipation in a superconductor it is necessary to introduce artificial pinning centres, that is, small regions in which superconductivity is suppressed. This is usually achieved by introducing small regions of non-superconducting phases. A new concept of pinning centres, the local suppression of superconductivity induced by strain, is now demonstrated.

The discovery of charge-density-wave order in the high-temperature superconductor YBa2Cu3O6+y places charge order centre stage with superconductivity, suggesting that they are intertwined rather than competing.

A new metallurgical strategy, high-entropy alloying (HEA), was used to explore new composition and phase spaces in the development of new refractory alloys with reduced densities and improved properties. Combining Mo, Ta, and Hf with “low-density” refractory elements (Nb, V, and Zr) and with Ti and Al produced six new refractory HEAs with densities ranging from 6.9 g/cm 3 to 9.1 g/cm 3 . Three alloys have single-phase disordered body-centered cubic (bcc) crystal structures and three other alloys contain two bcc nanophases with very close lattice parameters. The alloys have high hardness, in the range from H v  = 4.0 GPa to 5.8 GPa, and compression yield strength, σ 0.2  = 1280 MPa to 2035 MPa, depending on the composition. Some of these refractory HEAs show considerably improved high temperature strengths relative to advanced Ni-based superalloys. Compressive ductility of all the alloys is limited at room temperature, but it improves significantly at 800°C and 1000°C.

Coupling a ferromagnetic insulator to a topological insulator induces a robust magnetic state at the interface, resulting from the large spin-orbit interaction and the spin-momentum locking property of Dirac fermions, and leads to an extraordinary enhancement of the magnetic ordering (Curie) temperature. Electric field control for spintronics Topological insulators are materials with conducting surface states with unusual properties as a result of topological protection. They are of particular interest for spintronic applications because the surface state electron spins are locked to their momentum, an effect that could be exploited by interfacing with other materials. This study reports that coupling a ferromagnetic insulator (europium sulfide, EuS) to a topological insulator (bismuth selenide, Bi 2 Se 3 ) induces ferromagnetic order at the interface at up to room temperature, even though EuS orders ferromagnetically only below 17 kelvin. The observed topological magneto-electric response in this hybrid structure opens up a possible new method of controlling magnetization dynamics using an electric field, a long-standing goal in spintronics. Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry 1 , 2 , wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices 3 , 4 , 5 . Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena 6 , 7 . In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi 2 Se 3 ) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin–orbit interaction and the spin–momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi 2 Se 3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator 2 , 8 could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.