The mysterious pseudogap phase of cuprate superconductors ends at a critical hole doping level p* but the nature of the ground state below p* is still debated. Here, we show that the genuine nature of the magnetic ground state in La 2x Sr x CuO 4 is hidden by competing effects from superconductivity: applying intense magnetic fields to quench superconductivity, we uncover the presence of glassy antiferromagnetic order up to the pseudogap boundary p* ≈ 0.19, and not above. There is thus a quantum phase transition at p*, which is likely to underlie highfield observations of a fundamental change in electronic properties across p*. Furthermore, the continuous presence of quasi-static moments from the insulator up to p* suggests that the physics of the doped Mott insulator is relevant through the entire pseudogap regime and might be more fundamentally driving the transition at p* than just spin or charge ordering.

The low energy dynamical properties of the multiferroic hexagonal perovskite ErMnO3 have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon only excitable by the electric field of an electromagnetic wave. Using comparison with the isostructural YMnO3 compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electro-active crystal field transition. The term magnetoelectric (ME) makes reference to a variety of phenomena in which electric dipoles and magnetic moments are mutually linked [1]. ME processes attract a considerable research interest, largely driven by their potential use in future information technologies [1, 2] and, more recently, by their interpretation in terms of exotic magnetic and ME monopoles [3]. A ME process that is particularly striking is the electric-charge dressing of spin-waves that gives rise to electrically active magnons (or electromagnons) [4].This dynamical ME effect has been most clearly demonstrated in multiferroic materials such as Several microscopic mechanisms have been identified behind the ME character of these hybrid excitations. In essence, they all trace back to the specific couplings between spins and the deformable lattice that can produce ferroelectricity. In orthorhombic RMnO 3 , for example, both the so-called inverse Dzyaloshinksii-Moriya and Heisenberg exchange mechanisms contribute to the static polarization [12] and also generate different electromagnons [5]. In hexagonal YMnO 3 , a phonon-magnon anticrossing has been observed revealing that lattice and spins are dynamically coupled in this system too [13,14]. However, the ME nature of this feature was not been proven and, to the best of our knowledge, no electromagnon has been reported so far in the other members of this family of prominent multiferroics [15][16][17]. In thisFIG. 1: Schematic illustration of ErMnO3 crystallographic and magnetic structure in the temperature range T N < T < TN where Mn magnetic moments are ordered at 120 • (a) and Er moments on the 4b site only are polarized in an antiferromagnetic arrangement (b). This multiferroic phase presents an electric polarisation P along the c-axis. The exchange interactions involved in the Mn magnetic order are also shown.Letter, we report on the presence of a new type of ME excitation in the hexagonal ErMnO 3 multiferroic compound. By means of complementary spectroscopic tools and the comparison with hexagonal YMnO 3 , we show evidence of the transmutation of a regular magnon to an electro-active excitation in ErMnO 3 . We ascribe this effect to a distinct hybridization mechanism between Mn 3+ spin-waves and crystal field (CF) excitations of the Er 3+ rare earth.In ErMnO 3 , the ferroelectric state occurs below T c = 833 K with a spontaneous polarization along the c-axis (pola...