We report a sequence of continuous phase transformations in iron telluride, Fe1+yTe (y~0.1), which is observed by combining neutron diffraction, magnetic susceptibility, and specific heat measurements on single crystal samples. While a gradual increase of magnetic scattering near the wave vector (0.5, 0, 0.5) is seen below T = 70 K, a temperature where the discontinuous first order magneto-structural phase transition is found in systems with small y (< 0.06), the reduction of the lattice symmetry in Fe1.1Te only occurs at Ts = 63 K. Below TN = 57.5 K the long-range magnetic order develops, whose incommensurate wave vector Qm varies with temperature. Finally, at Tm ~ 45 K the system enters the low-T phase, where Qm is locked at (0.48, 0, 0.5). We conclude that these instabilities are weak compared to the strength of the underlying interactions, and we suggest that the impact of the Fe interstitials on the transitions can be treated with random-field models.Comment: revised resubmission, 8 pages, 5 figure

A key question in condensed-matter physics is to understand how high-temperature superconductivity emerges on adding mobile charged carriers to an antiferromagnetic Mott insulator. We address this question using angle-resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the Mott insulator and the d-wave superconductor in Bi 2 Sr 2 CaCu 2 O 8+δ . Despite a temperature-dependent resistivity characteristic of an insulator, the excitations in this intermediate state have a highly anisotropic energy gap that vanishes at four points in momentum space. This nodal-liquid state has the same gap structure as that of the d-wave superconductor but no sharp quasiparticle peaks. We observe a smooth evolution of the excitation spectrum, along with the appearance of coherent quasiparticles, as one goes through the insulator-tosuperconductor transition as a function of doping. Our results suggest that high-temperature superconductivity emerges when quantum phase coherence is established in a nonsuperconducting nodal liquid.High-temperature superconductivity in the cuprates occurs by doping a Mott insulator for which the antiferromagnetic ground state and low-energy excitations are well understood 1 . By adding carriers, the parent insulator turns into a superconductor for dopings that exceed 0.05 holes per CuO 2 plane. The d-wave nature of the superconducting ground state 2 and its low-lying excitations are also well understood. Between these phases lies an electronic ground state that is poorly understood. As the temperature is raised, this intermediate 'pseudogap' state occupies a larger and larger region of the phase diagram (Fig. 1a). It is from this phase that superconductivity emerges for all but the most highly doped samples. Consequently, the nature of this phase holds the key to the origin of high-temperature superconductivity.Whereas the electronic excitations in the high-temperature pseudogap region have been studied extensively, there is little spectroscopic data at low temperatures, as there is only a very narrow window of dopings where neither superconducting nor antiferromagnetic order occurs. Here, we present angleresolved photoemission spectroscopy (ARPES) data on single crystals and thin films 3 with doping levels that range all the way from the insulator to the over-doped superconductor. We focus in particular on non-superconducting thin films, just to the left of the superconducting transition temperature T c dome (see Fig. 1a), with an estimated hole doping ∼0. 04 (ref. 3). It is normally quite difficult to span the insulator-superconductor transition in Bi 2 Sr 2 CaCu 2 O 8+δ single crystals. However, it is possible to obtain very underdoped thin films by removing oxygen without film decomposition, as their large surface/volume ratio allows much lower annealing temperatures than those required for crystals. The integrity of the insulating films was confirmed by re-oxygenating them and checking their resistivity R(T ) and X-ray diffracti...

We present Raman experiments on underdoped and overdoped Bi2Sr2CaCu2O 8+δ (Bi-2212) single crystals. We reveal the pseudogap in the electronic Raman spectra in the B1g and B2g geometries. In these geometries we probe respectively, the antinodal (AN) and nodal (N) regions corresponding to the principal axes and the diagonal of the Brillouin zone. The pseudogap appears in underdoped regime and manifests itself in the B1g spectra by a strong depletion of the low energy electronic continuum as the temperature decreases. We define a temperature T * below which the depletion appears and the pseudogap energy, ωP G the energy at which the depeletion closes.The pseudogap is also present in the B2g spectra but the depletion opens at higher energy than in the B1g spectra. We observe the creation of new electronic states inside the depletion as we enter the superconducting phase. This leads us to conclude (as proposed by S. Sakai et al. [1]) that the pseudogap has a different structure than the superconducting gap and competes with it. We show that the nodal quasiparticle dynamic is very robust and almost insensitive to the pseudogap phase contrary to the antinodal quasiparticle dynamic. We finally reveal, in contrast to what it is usually admitted,an increase of the nodal quasiparticle spectral weight with underdoping. We interpret this result as the consequence of a possible Fermi surface disturbances in the doping range p = 0.1 − 0.2.

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