The nature of the pseudogap phase of cuprate high-temperature superconductors is one of the most important unsolved problems in condensed matter physics. We studied the commencement of the pseudogap state at temperature T * using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally-doped Bi2201 crystals. We observe the coincident onset at T * of a particle-hole asymmetric antinodal gap, a non-zero Kerr rotation, and a change in the relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T c ), entangled in an energy-momentum dependent fashion with the pre-existing pseudogap features.As complex oxides, cuprate superconductors belong to a class of materials which exhibit many broken-symmetry states; unravelling the relationship between superconductivity in the cuprates and other possible broken-symmetry states has been a major challenge of condensed matter physics. A possibly related issue concerns the nature of the pseudogap in the cuprates and its relationship with superconductivity. Angle-resolved photoemission spectroscopy (ARPES) studies have shown that the pseudogap develops below a temperature T * near the Brillouin zone boundary while preserving a gapless Fermi arc near the zone diagonal (1). A key issue is the extent to which the pseudogap is a consequence of superconducting fluctuations (2-5), which should exhibit a rough particle-hole symmetry, or another form of (incipient) order (6-12), which typically should induce particle-hole asymmetric spectral changes. Candidate orders include various forms of density wave, nematic or unconventional magnetic orders that break different combinations of lattice translational (6-8, 13-19), rotational (6, 9, 15, 17, 20-22), and time-reversal (7, 9, 23-26) symmetries.We have focused on crystals of nearly optimally-doped (OP) Pb 0.55 Bi 1.5 Sr 1.6 La 0.4 CuO 6+δ (PbBi2201, T c = 38 K, T * = 132 ± 8 K) (27), and combined the ARPES measurements of the evolution of the band structure over a wide range of temperature, momentum and energy, with high-precision measurements of the polar Kerr effect (PKE) and time-resolved reflectivity (TRR).Bi2201 was chosen to avoid the complications resulting from bilayer splitting and strong antinodal bosonic mode coupling inherent to Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) (1). Whereas ARPES is a surface probe, PKE enables us to monitor a bulk, thermodynamic (via the fluctuation-dissipation theorem) 2 property which has proven (28) to be a sensitive probe of the onset of a broken-symmetry state, and TRR gives complementary information on the bulk, near-equilibrium dynamics of the system. We will first analyze our ARPES data collected in different temperature regions. Above T * , PbBi2201 has a simple one-band band structure (right side of Fig. 1). For each cut in momentum space Fig. 1), the only distinct feature in the ...

A detailed phenomenology of low energy excitations is a crucial starting point for microscopic understanding of complex materials, such as the cuprate high-temperature superconductors. Because of its unique momentum-space discrimination, angle-resolved photoemission spectroscopy (ARPES) is ideally suited for this task in the cuprates, where emergent phases, particularly superconductivity and the pseudogap, have anisotropic gap structure in momentum space. We present a comprehensive doping-and temperaturedependence ARPES study of spectral gaps in Bi 2 Sr 2 CaCu 2 O 8+δ , covering much of the superconducting portion of the phase diagram. In the ground state, abrupt changes in near-nodal gap phenomenology give spectroscopic evidence for two potential quantum critical points, p = 0.19 for the pseudogap phase and p = 0.076 for another competing phase. Temperature dependence reveals that the pseudogap is not static below T c and exists p > 0.19 at higher temperatures. Our data imply a revised phase diagram that reconciles conflicting reports about the endpoint of the pseudogap in the literature, incorporates phase competition between the superconducting gap and pseudogap, and highlights distinct physics at the edge of the superconducting dome.quantum materials | correlated electrons | laser ARPES T he momentum-resolved nature of angle-resolved photoemission spectroscopy (ARPES) makes it a key probe of the cuprates, the interesting phases of which have anisotropic momentumspace structure (1-4): both the d-wave superconducting gap and the pseudogap above T c have a maximum at the antinode [AN, near (π, 0)] and are ungapped at the node, although the latter phase also exhibits an extended ungapped arc (5-8). Ordering phenomena often result in gapping of the quasiparticle spectrum, and distinct quantum states produce spectral gaps with characteristic temperature, doping, and momentum dependence. These phenomena were demonstrated by recent ARPES experiments that argued that the pseudogap is a distinct phase from superconductivity based on their unique phenomenology (8-15): the pseudogap dominates near the AN (8, 11), and its magnitude increases with underdoping (11, 12), whereas near-nodal (NN) gaps have a different doping dependence and can be attributed to superconductivity because they close at T c (8, 12). Previous measurements focused on AN or intermediate (IM) momenta, but laser-ARPES, with its superior resolution and enhanced statistics, allows for precise gap measurements near the node where the gap is smallest. Our work is unique in its attention to NN momenta using laser-ARPES, and we demonstrate, via a single technique, that three distinct quantum phases manifest in different NN phenomenology as a function of doping. ResultsGaps at parallel cuts were determined by fitting symmetrized energy distribution curves (EDCs) at k F to a minimal model (16).The Fermi wavevector, k F , is defined by the minimum gap locus. Example spectra, raw and symmetrized EDCs at k F , and fits are shown for UD92 (underdoped, T c = 92) ...

Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current d2I/dV2 corresponds to each phonon mode. For high-transition-temperature (high-T(c)) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron-boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in d2I/dV2 spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale. Here we report d2I/dV2 imaging studies of the high-T(c) superconductor Bi2Sr2CaCu2O8+delta. We find intense disorder of electron-boson interaction energies at the nanometre scale, along with the expected modulations in d2I/dV2 (refs 9, 10). Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of 18O for 16O throughout the material reduces the average mode energy by approximately 6 per cent--the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity.

In-plane microwave penetration depth λ ab and quaiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor PrFeAsO1−y (Tc ≈ 35 K) by using a sensitive superconducting cavity resonator. λ ab (T ) shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap ∆(k). The temperature dependence of the superfluid density demonstrates that the gap is non-zero (∆/kBTc 1.6) all over the Fermi surface. The microwave conductivity below Tc exhibits an enhancement larger than the coherence peak, reminiscent of high-Tc cuprate superconductors.PACS numbers: 74.25. Nf, 74.20.Rp, 74.25.Fy Since the discovery of superconductivity in, high transition temperatures (T c ) up to 56 K have been reported in the doped Fe-based oxypnictides [2,3,4,5,6,7,8,9]. The nature of superconductivity and the pairing mechanism in this system are fundamental physical problem of crucial importance. The first experimental task to this problem is to elucidate the superconducting pairing symmetry, which is intimately related to the pairing interaction.The NMR Knight-shift measurements appear to indicate the spin-singlet pairing [10,11]. However, the superconducting gap structure, particulary the presence or absence of nodes in the gap, is highly controversial. The specific heat shows a nonlinear magnetic field dependence [12]. The NMR relaxation rate shows the absence of the coherence peak and the T 3 -dependence below T c [10,11,13,14]. The lower critical field exhibits a T -linear dependence at low temperatures [15]. The µSR experiments report an unusual field-dependence of the penetration depth [16]. In the point-contact spectroscopy, a zero-bias conductance peak is reported [17,18]. These results have been interpretated as an indication of unconvensional superconductivity with line nodes. On the other hand, the Andreev reflection data are found to be consistent with an isotropic gap [19]. All of these experiments have been performed by using polycrystalline samples. Definitely, measurements using single crystals are highly desired to obtain unambiguous conclusions on the superconducting gap structure.In this paper, we report on the measurements of the complex surface impedance in underdoped single crystals of the oxypnictide superconductor PrFeAsO 1−y (T c ≈ 35 K), from which properties of thermally excited quasiparticles can be directly deduced. Since the recent NMR experiments of the Pr-based iron oxypnictide suggest the non-magnetic state in the superconducting samples [11], PrFeAsO 1−y seems suitable for the penetration depth study [20,21]. Moreover, PrFeAsO 1−y has a higher T c than that of La-compounds, which enables the measurements in a wider temperature range. We observe flat temperature dependence of the in-plane penetration depth λ ab (T ) at low temperatures, indicating exponentially small quasiparticle excitations, which clearly contradicts the presence of nodes in the gap. The quasiparticle conductivity is enhanced compared with ...

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.