Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100-nm room-temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively. We thus establish a platform for investigating functional sub-50-nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.

Spin-orbit coupling (SOC) describes the relativistic interaction between the spin and momentum degrees of freedom of electrons, and is central to the rich phenomena observed in condensed matter systems. In recent years, new phases of matter have emerged from the interplay between SOC and low dimensionality, such as chiral spin textures and spin-polarized surface and interface states. These low-dimensional SOC-based realizations are typically robust and can be exploited at room temperature. Here we discuss SOC as a means of producing such fundamentally new physical phenomena in thin films and heterostructures. We put into context the technological promise of these material classes for developing spin-based device applications at room temperature.

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We study the low-energy spin fluctuations and superfluid density of a series of pure and Zn-substituted high-T c superconductors ͑HTS͒ using the muon spin relaxation and ac-susceptibility techniques. At a critical doping state, p c , we find ͑i͒ simultaneous abrupt changes in the magnetic spectrum and in the superconducting ground state and ͑ii͒ that the slowing down of spin fluctuations becomes singular at Tϭ0. These results provide experimental evidence for a quantum transition that separates the superconducting phase diagram of HTS into two distinct ground states. DOI: 10.1103/PhysRevB.66.064501 PACS number͑s͒: 74.72.Ϫh, 74.25.Ha, 75.40.Ϫs, 76.75.ϩi Quantum phase transitions occur at zero temperature at a critical electron density separating distinct ground states. Near a quantum critical point, electrons in metals are highly correlated and the diverging fluctuations may induce unconventional superconductivity. [1][2][3][4][5][6][7][8] For example, in certain heavy fermion compounds a ''bubble'' of superconductivity occurs around the quantum critical point at which itinerant antiferromagnetism is suppressed by applied pressure. 9 The search for an underlying quantum phase transition in high-T c superconductors ͑HTS͒ is motivated by the potential for quantum fluctuations to bind electronic carriers into superconducting Cooper pairs and also to cause the celebrated linear temperature dependence of their electrical resistivity. [1][2][3][4][5][6][7][8]10 HTS exhibit a common generic phase diagram in which the superconducting transition temperature, T c , rises to a maximum at an optimal doping of approximately 0.16 holes per planar copper atom and then falls to zero on the overdoped side. In addition the underdoped normal state exhibits correlations, which introduce a gap in the density of states that strongly affects all physical properties. There is no phase transition associated with the opening of this gap and so it is called a pseudogap. Analysis of specific heat data, for example, suggests that the pseudogap energy decreases with doping and falls to zero at a critical doping of p c Ӎ0.19, just beyond optimal doping, 10,11 a behavior rather analogous to the quantum-critical heavy-fermion materials. 9 Many fundamental physical quantities such as the superconducting condensation energy, 10,11 the superfluid density, 12,13 and the quasiparticle weight, 10,14 show abrupt changes as p→p c . While compelling in their totality, 10,11 none of the results can be considered as evidence of a quantum transition. In particular there is no evidence for an associated order parameter and slowing down of the relevant fluctuations. With this in mind we examined the evolution with doping of the low-energy spin fluctuation spectrum using muon spin relaxation ( SR) combined with low-field ac-susceptibility measurements of the superfluid density.The samples studied were: ͑i͒ La 2Ϫx Sr x Cu 1Ϫy Zn y O 4 ͑LSCO͒ (xϭ0.03-0.24 and yϭ0, 0.01, and 0.02͒. Samples were synthesized using solid-state reaction and where necessary follow...

The doping dependence of the superfluid density, ρ s ≡ λ ab -2 ∝ n s /m*, of high-T c superconductors is usually considered in the context of the Uemura relation, namely T c proportional to ρ s , which is generally assumed to apply in the underdoped regime. We show that a modified plot of T c /∆ 0 versus ρ s , where ∆ 0 is the maximum d-wave gap at T=0, exhibits universal features that point to an alternative interpretation of the underlying physics. In the underdoped region this plot exhibits the canonical negative curvature expected when a ground-state correlation competes with superconductivity (SC) by opening up a gap in the normal-state DOS. In particular ρ s is suppressed much faster than T c /∆ 0 or indeed T c . The pseudogap is found to strongly modify the SC ground state. PACS numbers 74.25.Ha, 74.25.Bt, 76.75.+i

The absolute values and temperature, T, dependence of the in-plane magnetic penetration depth, λ ab , of La2−xSrxCuO4 and HgBa2CuO 4+δ have been measured as a function of carrier concentration. We find that the superfluid density, ρs, changes substantially and systematically with doping. The values of ρs(0) are closely linked to the available low energy spectral weight as determined by the electronic entropy just above Tc and the initial slope of ρs(T )/ρs(0) increases rapidly with carrier concentration. The results are discussed in the context of a possible relationship between ρs and the normal-state (or pseudo) energy gap.

Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO 3 has been considered to remain cubic down to low temperature. Here we present high-resolution synchrotron x-ray powder-diffraction data showing the existence of a structural phase transition, from cubic P m-3m to tetragonal I 4/mcm, involving TiO 6 octahedra tilting, in analogy to the case of SrTiO 3 . The temperature evolution of the tilting angle and of the full width at half maximum of the (200) cubic reflection family indicate a critical temperature T c = 235 K. This critical temperature is well below the recent anomaly reported by specific-heat measurement at T A ∼ 282 K. By performing atomic pair distribution function analysis on diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated T c , the average model symmetry is fully compatible with the local environment distortion, but the former is characterized by a reduced value of the tilting angle compared to the latter. At T = 240 K, data show the presence of local octahedra tilting identical to the low-temperature one, while the average crystallographic structure remains cubic. On this basis, we propose that intrinsic lattice disorder is of fundamental importance in the understanding of EuTiO 3 properties.

The study of interacting spin systems is of fundamental importance for modern condensed matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states 1 . The frustrated 2D ShastrySutherland lattice 2 realized by SrCu 2 (BO 3 ) 2 3 is an important test to our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated 2-12 . We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu 2 (BO 3 ) 2 . It is observed by inelastic neutron scattering after pressure tuning at 21.5 kbar. This gapped plaquette singlet state with strong 4-spin correlations leads to a transition to an ordered Néel state above 40 kbar, which can realize a deconfined quantum critical point.In the field of quantum magnetism, geometrically frustrated lattices generally imply major difficulties in analytical and numerical studies. For very few particular topologies however, it has been shown that the ground state, at least, can be calculated exactly as for the Majumdar-Gosh model 13 that solves the J 1 -J 2 zigzag chain when J 1 = 2J 2 . In 2D, the Shastry-Sutherland model 2 consisting of an orthogonal dimer network of spin S=1/2 was developed in order to be exactly solvable. For an inter-dimer J to intra-dimer J exchange ratio α ≡ J /J ≤ 0.5 the ground state is a product of singlets on the strong bond J. Numerical calculations have further shown that this remains valid up to α ≤∼ 0.7 and for small values of 3D couplings J between dimer layers. At the other end, for ∼ 0.9 ≤ α ≤ ∞ the system approaches the well known 2D square lattice, which is antiferromagnetically (AFM) ordered, albeit with significant quantum fluctuations that are believed to include resonating singlet correlations resulting in fractional excitations 14 . The phase diagram of the Shastry-Sutherland model, both with and without applied magnetic field, has been intensively studied by numerous theoretical and numerical approaches 3 . In the presence of magnetic field, magnetization plateaus at fractional values of the saturation magnetization corresponding to Mott insulator phases of dimer states, as well as possible superfluid and supersolid phases have been extensively studied 6,15,16 . At zero field, the main unsolved issue is the existence and nature arXiv:1603.02039v1 [cond-mat.str-el]