We present an experimental review of the nature of the pseudogap in the cuprate superconductors. Evidence from various experimental techniques points to a common phenomenology. The pseudogap is seen in all high temperature superconductors and there is general agreement on the temperature and doping range where it exists. It is also becoming clear that the superconducting gap emerges from the normal state pseudogap. The d-wave nature of the order parameter holds for both the superconducting gap and the pseudogap. Although an extensive body of evidence is reviewed, a consensus on the origin of the pseudogap is as lacking as it is for the mechanism underlying high temperature superconductivity.

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The c-axis polarized phonon spectra of single crystals of YBa2Cu3O6+x, were measured for the doping range x = 0.5 → 0.95, between 10 K and 300 K. The low background electronic conductivity, determined by Kramers-Kronig analysis of the reflectance, leads to a rich phonon structure. With decreased doping the five normally-active B1u modes broaden and the high-frequency apical oxygen mode splits into two components. We associate the higher of these with the two-fold coordinated copper "sticks". The 155 cm −1 low-frequency mode, which involves the apical and the chainoxygens, splits into at least three components with decreasing doping. Some phonon anomalies which occur near Tc in the highly-doped material occur well above Tc in the oxygen-reduced systems. An unusual broad phonon band develops in the normal state at ≈ 400 cm −1 , which becomes more intense at low doping and low temperatures, borrowing oscillator strength from apical and plane oxygen modes resulting in a major transformation of the phonon spectrum below ≈ 150 K.Résumé : Les spectres de phonons polarisés suivant l'axe c, dans des monocristaux de YBa2Cu3O6+x ontété measurés, pour l'intervalle de dopage x = 0, 5à 0, 95, entre 10 et 300 K. La faible condutivitéélectronique de fond, déterminée par analyse Kramers-Kronig de la réflectance, donne lieuà une riche structure de phonons. Lorsqu'on diminue le dopage, les cinque modes B1u normalement actifs s'élargissent, et le mode apicalà haute fréquence de l'oxygène se divise en deux composantes. Nous associons la plus haute de ces composantes aux « bâtons » de cuivreà coordinence deux. Le mode de basse fréquence,à 155 cm −1 , qui implique les oxygènes apical et en chaîne, se sépare en au moins trois composantes lorsque le dopage diminue. Certaines anomalies de phonons, présentes au voisinage de Tc dans les cristaux faiblement dopés, apparaissentà des températures bien plus grandes que Tc dans les systèmes avec moins de l'óxygène. A environ 400 cm −1 , il se développe dans l'état normal une bande large insolite qui devient plus intenseà faible dopage et a basse température, avec une force d'ocsillateur empruntée aux modes apical et plan de l'oxygène, ce quie entraîne une transformation majeure des spectres de phonons au-dessous d'environ 150 K.[Traduit par la rédaction]

We have studied spin excitation spectra in the Shastry-Sutherland model compound SrCu2(BO3)2 in magnetic fields using far-infrared Fourier spectroscopy. The transitions from the ground singlet state to the triplet state at 24 cm −1 and to several bound triplet states are induced by the electric field component of the far-infrared light. To explain the light absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM) mechanism where light couples to a phonon mode, allowing the DM interaction. Two optical phonons couple light to the singlet to triplet transition in SrCu2(BO3)2. One is a-polarized and creates an intra-dimer dynamic DM along the c axis. The other is c-polarized and creates an intra-dimer dynamic DM interaction, it is in the (ab) plane and perpendicular to the dimer axis. Singlet levels at 21.5 and 28.6 cm −1 anti-cross with the first triplet as is seen in far-infrared spectra. We used a cluster of two dimers with a periodic boundary condition to perform a model calculation with scaled intra-and inter-dimer exchange interactions. Two static DM interactions are sufficient to describe the observed triplet state spectra. The static inter-dimer DM in the c-direction d1 = 0.7 cm −1 splits the triplet state sub-levels in zero field [Cépas et al., Phys. Rev. Lett. 87, 167205 (2001)]. The static intra-dimer DM in the (ab) plane (perpendicular to the dimer axis) d2 = 1.8 cm −1 , allowed by the buckling of CuBO3 planes, couples the triplet state to the 28.6 cm −1 singlet as is seen from the avoided crossing.