Coexistence of Fermi arcs and Fermi pockets in a high Tc copper oxide superconductor

Abstract: In the pseudogap state of the high-Tc copper-oxide (cuprate) superconductors, angle-resolved photoemission (ARPES) measurements have seen an Fermi arc, i.e., an open-ended gapless section in the large Fermi surface, rather than a closed loop expected of an ordinary metal. This is all the more puzzling because Fermi pockets (small closed Fermi surface features) have been suggested from recent quantum oscillation measurements. The Fermi arcs have worried the high-Tc community for many years because they cannot be understood in terms of existing theories. Theorists came up with a way out in the form of conventional Fermi surface pockets associated with competing order, with a back side that is for detailed reasons invisible by photoemission. Here we report ARPES measurements of La-Bi2201 that give direct evidence of the Fermi pocket. The charge carriers in the pocket are holes and the pockets show an unusual dependence upon doping, namely, they exist in underdoped but not overdoped samples. A big surprise is that these Fermi pockets appear to coexist with the Fermi arcs. This coexistence has not been expected theoretically and the understanding of the mysterious pseudogap state in the high-Tc cuprate superconductors will rely critically on understanding such a new finding.

Coexistence of Fermi Arcs and Fermi Pockets in High-Temperature Cuprate Superconductors

The study "Coexistence of Fermi Arcs and Fermi Pockets in High Temperature Cuprate Superconductors" explores a significant peculiarity in the electronic structure of high-$T_c$ superconductors, particularly focusing on the La-Bi2201 compound. High-temperature superconductors, such as the copper-oxide cuprates, have long been the subject of intense research efforts due to their complex and not well-understood properties under high-$T_c$ conditions. One of the enigmatic phenomena in these materials is the pseudogap state, within which conventional theoretical models have failed to adequately describe the observed electronic behavior, such as the emergence of Fermi arcs instead of closed Fermi surfaces.

In this investigation, the researchers employed angle-resolved photoemission spectroscopy (ARPES) to elucidate the nature of the Fermi surface in underdoped La-Bi2201 samples. The presence of Fermi arcs, characterized by gapless segments on the expected large Fermi surface, has historically been contrasted with the quantum oscillation evidence of Fermi pockets. Here, the authors provide direct ARPES evidence of the coexistence of small closed Fermi pockets alongside these Fermi arcs, a finding that adds complexity to the understanding of the pseudogap state in high-$T_c$ cuprate superconductors.

The study identifies distinct Fermi surface features labeled as LP, LS, and LPS, in addition to the main Fermi surface denoted as LM, within the momentum space of the La-Bi2201 UD18K sample. Particularly noteworthy is the Fermi pocket (LP), a closed loop near the nodal region, which does not conform to known band structures such as umklapp or shadow bands. The consistency of these pockets observed via both VUV laser and Helium discharge lamp measurements reinforces their intrinsic nature.

A significant aspect of the findings is the doping dependence of these Fermi pockets. They are present in certain underdoped samples (UD18K and UD26K) but absent in the optimally doped or extremely underdoped samples. This narrow range for Fermi pocket observation suggests intrinsic properties tied to specific doping levels. The holes within the Fermi pockets contrast with prior suggestions of electron-like pockets, faced with separate challenges in theoretical accommodation.

The coexistence of Fermi arcs and pockets poses theoretical challenges and prompts speculation regarding the reconstruction of the Fermi surface due to possible interactions such as charge or spin density waves. The lack of evidence for sample inhomogeneity further emphasizes that these are not mere experimental artifacts but genuine coexistence within the material system.

The implications of these findings are profound, requiring reevaluation of pseudogap models and consideration of new variables in the high-$T_c$ cuprate superconductors. The presence of Fermi pockets alongside arcs implies additional factors at play, potentially involving resonant valence bond states or density wave orders.

Future research should explore the mechanisms allowing such coexistence and align theoretical models with empirical observations. These insights could pave the way towards more comprehensive models of superconductivity and electronic behavior in complex oxides, potentially influencing the development of new materials and technologies. The study raises pertinent questions about the interplay of electronic phases in high-$T_c$ regimes and calls upon further experimental and theoretical scrutiny to reconcile these intriguing observations.