Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems 1,2 , and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr 2 RuO 4 for example 3-5 , leading to a pronounced peak in T c vs. strain whose origin is still under active debate 4,6,7 . Here, we develop a simple and compact method to apply large uniaxial pressures passively in restricted sample environments, and utilize this to study the evolution of the electronic structure of Sr 2 RuO 4 using angle-resolved photoemission. We directly visualize how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in T c 7 . Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr 2 RuO 4 . More generally, our novel experimental approach opens the door to future studies of straintuned phase transitions not only using photoemission, but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.The layered perovskite Sr 2 RuO 4 has been extensively studied both because of its celebrated unconventional superconductivity 5,8-11 and the accuracy with which its normal state properties can be measured 12-15 and analysed [16][17][18][19] . In spite of a quarter of a century of work, there is still no consensus on the symmetry of its superconducting order parameter, or the mechanism by which the superconductivity condenses 5 . This is a major unsolved problem because its electronic structure, which is relatively simple compared to that of many other unconventional superconductors, is now known in considerable detail and its metallic state is firmly established to be a Fermi liquid below approximately 30 K 13 . A full understanding of the Sr 2 RuO 4 problem is therefore a benchmark for the progress of the fields of strongly interacting systems and unconventional superconductivity.

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In a recent work devoted to the magnetism of Li 2 CuO 2 , Shu et al (2017 New J. Phys. 19, 023026) have proposed a 'simplified' unfrustrated microscopic model that differs considerably from the models refined through decades of prior work. We show that the proposed model is at odds with known experimental data, including the reported magnetic susceptibility χ(T) data up to 550K. Using an 8th order high-temperature expansion for χ(T), we show that the experimental data for Li 2 CuO 2 are consistent with the prior model derived from inelastic neutron scattering studies. We also establish the T-range of validity for a Curie-Weiss law for the real frustrated magnetic system. We argue that the knowledge of the long-range ordered magnetic structure for T<T N and of χ(T) in a restricted Trange provides insufficient information to extract all of the relevant couplings in frustrated magnets; the saturation field and INS data must also be used to determine several exchange couplings, including the weak but decisive frustrating antiferromagnetic interchain couplings.Li 2 CuO 2 takes a special place among the still increasing family of frustrated chain compounds with edge-sharing CuO 4 plaquettes and a ferromagnetic (FM) nearest neighbor (NN) inchain coupling J 1 [1]. This unique position is due to its ideal planar CuO 2 chain structure and its well-defined ordering characterized by a 3D Neél-type arrangement of adjacent chains whose magnetic moments are aligned ferromagnetically along the chains (b-axis). Li 2 CuO 2 is well studied in both experiment and theory (see e.g. [2][3][4][5][6][7][8][9][10][11]) and serves nowadays as a reference system for more complex and structurally less ideal systems. In particular, it is accepted in the quantum magnetism community that the leading FM coupling is the NN inchain coupling J 1 . (J 1 is also dominant but antiferromagnetic (AFM) in the special spin-Peierls case of CuGeO 3 [12].) There is always also a finite frustrating AFM next-nearest neighbor (NNN) coupling J 2 >0, see figure 1, left. This inchain frustration is quantified by J J 2 1 a = | |. In the present case, and in that of the related Ca 2 Y 2 Cu 5 O 10 , there are only frustrating OPEN ACCESS RECEIVED

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We report a combined experimental and theoretical study of the surface and bulk electronic structure of aluminium diboride, a nonsuperconducting sister compound of the superconductor MgB 2. We perform angleresolved photoemission measurements with variable photon energy, and compare them to density functional theory calculations to disentangle the surface and bulk contributions to the measured spectra. Aluminium diboride is known to be aluminium deficient, Al 1−δ B 2 , which would be expected to lead to a hole doping as compared to the nominally stoichimoetric compound. Nonetheless, we find that the bulk σ states, which mediate superconductivity in MgB 2 , remain more than 600 meV below the Fermi level. However, we also observe σ states originating from the boron terminated surface, with an order of magnitude smaller binding energy of 70 meV, and demonstrate how surface hole-doping can bring these across the Fermi level.

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