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Register a new userThe pseudogap in high-temperature superconductors: an experimental survey
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T. Timusk
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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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By re-examining recently-published data from angle-resolved photoemission spectroscopy we demonstrate that, in the superconducting region of the phase diagram, the pseudogap ground state is an arc metal. This scenario is consistent with results from Raman spectroscopy, specific heat and NMR. In addition, we propose an explanation for the Fermi pockets inferred from quantum oscillations in terms of a pseudogapped bilayer Fermi surface.
We investigate infrared manifestations of the pseudogap in the prototypical cuprate and pnictide superconductors: YBa2Cu3Oy and BaFe2As2 (Ba122) systems. We find remarkable similarities between the spectroscopic features attributable to the pseudogap in these two classes of superconductors. The hallmarks of the pseudogap state in both systems include a weak absorption feature at about 500 cm-1 followed by a featureless continuum between 500 and 1500 cm-1 in the conductivity data and a significant suppression in the scattering rate below 700 - 900 cm-1. The latter result allows us to identify the energy scale associated with the pseudogap $Delta_{PG}$. We find that in the Ba122-based materials the superconductivity-induced changes of the infrared spectra occur in the frequency region below 100 - 200 cm-1, which is much lower than the energy scale of the pseudogap. We performed theoretical analysis of the scattering rate data of the two compounds using the same model which accounts for the effects of the pseudogap and electron-boson coupling. We find that the scattering rate suppression in Ba122-based compounds below $Delta_{PG}$ is solely due to the pseudogap formation whereas the impact of the electron-boson coupling effects is limited to lower frequencies. The magnetic resonance modes used as inputs in our modeling are found to evolve with the development of the pseudogap, suggesting an intimate correlation between the pseudogap and magnetism.
We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y) and Pr(1.85)Ce(0.15)CuO(4-y) as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature Tc. In contrast, below Tc for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to the superconducting energy gap below Tc. Possible interpretations of this observation are the existence of a normal state pseudogap in the electron doped HTS or the existence of a spatially non-uniform superconducting state.
We report on infrared studies of charge dynamics in a prototypical pnictide system: the BaFe2As2 family. Our experiments have identified hallmarks of the pseudogap state in the BaFe2As2 system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.
The pseudogap (PG) regime of the underdoped cuprates arguably remains one of the most enigmatic phenomena of correlated quantum matter. Recent theoretical ideas suggest that fractionalized bosonic fields can lead to the PG phase, by opening a gap in the anti-nodal (AN) region of the Brillouin zone. Such fractionalized boson can originate from modulated particle-particle pairs or pair density wave (PDW), a magnetic stripe, or a modulated spin one particle-hole pair like a spin density wave (SDW) boson, among others. The main picture goes as follows. Electrons under strong coupling tend to form different types of unstable bosons at high temperatures. As the temperature goes down, the compact object gets extremely unstable, and to minimize the entropy, it finally fractionalizes into elementary components, linked by a constraint. The process of fractionalization involves, in this way, an emergent gauge field directly linked to the constraint. This, in turn, couples to the Fermi surface of electronic carriers and opens a gap in the AN region, which is partly responsible for the PG phase. Alternative theoretical approaches invoke a simple coexistence between the multiple quasi-degenerate orders like charge density wave (CDW), superconductivity (SC), and magnetic orders at low temperatures. This scenario attributes the PG formation as a vestigial order showing up at $T^{*}$, which acts as a precursor to the zero temperature orders. This intricate situation calls for a key experimental test, enabling us to discriminate between the various theoretical scenarios. In this paper, we focus on the case where the PDW boson has fractionalized into a CDW and SC order below $T^{*}$, and we compare this to the situation where the two orders simply coexist.
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