Pseudogap from ARPES experiment: Three gaps in cuprates and topological superconductivity (Review Article)

Abstract

A term first coined by Mott back in 1968 a "pseudogap" is the depletion of the electronic density of states at the Fermi level, and pseudogaps have been observed in many systems. However, since the discovery of the high-temperature superconductors (HTSC) in 1986, the central role attributed to the pseudogap in these systems has meant that by many researchers now associate the term pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a lot of new attention with the rediscovery of two distinct energy scales ("two-gap scenario") and charge density waves patterns in the cuprates. Despite many excellent reviews on the pseudogap phenomenon in HTSC, published from its very discovery up to now, the mechanism of the pseudogap and its relation to superconductivity are still open questions. The present review represents a contribution dealing with the pseudogap, focusing on results from angle resolved photoemission spectroscopy (ARPES) and ends up with the conclusion that the pseudogap in cuprates is a complex phenomenon which includes at least three different "intertwined" orders: spin and charge density waves and preformed pairs, which appears in different parts of the phase diagram. The density waves in cuprates are competing to superconductivity for the electronic states but, on the other hand, should drive the electronic structure to vicinity of Lifshitz transition, that could be a key similarity between the superconducting cuprates and iron-based superconductors. One may also note that since the pseudogap in cuprates has multiple origins there is no need to recoin the term suggested by Mott. V C 2015 AIP Publishing LLC. [http://dx.

Specific temperature dependence of pseudogap in YBa2Cu3O7−δ nanolayers

“…At T pair << T * SBBs transform into fluctuating Cooper pairs (FCPs) [22,23,32,58], thus demonstrating the BEC-BCS transition [9,54] predicted by the theory [50][51][52][53]. This is a specific property of the HTSCs, which is the consequence of the extremely short coherence length in cuprates [2,3,9,10,50]. σ ′ (T ), found with a help of Eq.…”

“…However, the corresponding theories [14,15] as well as neutron measurements [16,17] show that, after the long-range AF order breaks down, the short-range AF correlations persist up to the rather high doping level. In FePns the AF order of spin-density-wavetype (SDW-type) is found to coexist with superconductivity in a wide range of doping [9][10][11][12]. This leaves little doubt that pairing and scattering in these materials are both affected by the low-energy AF fluctuations [8,[18][19][20].…”

“…We believe the PG to be due to the formation of preformed pairs [local pairs (LPs)] [2,3,22,23,32]. Nevertheless, the pairing mechanism being responsible for the electron coupling at very high temperatures very likely can be of a magnetic type [1,8,10,30,33]. But the issue as for the PG physics still remains controversial, suggesting the study of the interplay between superconductivity and magnetism to be one of the challenging problems of high-temperature superconductivity [34].…”

“…Various models have been put forward to explain both the pairing mechanism and PG phenomenon in HTSC's [2,3,[8][9][10][21][22][23], including various forms of electron pairing [3,[22][23][24][25], spin-fluctuations [26][27][28], interplay with charge fluctuations [4,29] and even spin-charge separation scenarios [30,31]. We believe the PG to be due to the formation of preformed pairs [local pairs (LPs)] [2,3,22,23,32].…”

“…Gradually, it has become clear that usual electron-phonon interaction, proposed by the BCS theory [5], is hardly possible to account for formation of the superconducting (SC) Cooper pairs at such high temperatures [6] and any additional interaction mechanism is to be taken into consideration [4,[7][8][9][10].…”