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

Abstract

1In the pseudogap state of the high−T c copper-oxide (cuprate) superconductors [1], angle-resolved photoemission (ARPES) measurements have seen an Fermi arc, i.e., an open-ended gapless section in the large Fermi surface [2,3,4,5,6,7,8], 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 [9,10,11,12,13,14]. The Fermi arcs have worried the high−T c 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 [15]. 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-T c cuprate superconductors will rely critically on understanding such a new finding.The high resolution Fermi surface mapping (Fig. 1a) on the underdoped La-Bi2201 UD18K sample using VUV laser reveals three Fermi surface sheets with low spectral weight (labeled as LP, LS and LPS in Fig. 1a) in the covered momentum space, in addition to the prominent main Fermi surface (LM). One particular Fermi surface sheet LP crosses the main band LM, forming an enclosed loop, an Fermi pocket, near the nodal region. Quantitative Fermi surface data measured from both VUV laser (Fig. 2a) and Helium discharge lamp ( (Fig. 2a) and HP band observed in Helium lamp measurement (Fig. 2b) are intrinsic; they can not be attributed to any of the umklapp bands or shadow bands (Fig. 2c). The location of the three bands can be well connected by the same superlattice vector, indicating that the HP and LPS bands correspond to the first order umklapp bands of the main Fermi pocket LP. The shape 2 and area of the Fermi pockets are also consistent in these two independent measurements, making a convincing case on the presence of the Fermi pocket. We note that in both the laser (Fig. 2a) and Helium lamp (Fig. 2b and SFig. 2 in the Supplementary) measurements, all the observed bands except for the "Fermi pocket bands" can be assigned by only one regular superstructure wavevector (0.24,0.24). The presence of additional superstructure, which would give rise to new bands, appears to be unlikely because there is no indication of such additional bands observed in our measurements.The Fermi pocket is observed both in the normal state and superconducting state, as shown in Fig. 3 for the La-Bi2201 UD18K sample. Moreover, its location, shape and area show little change with temperature (Figs. 3a and 3f). Below T c , the openi...

Fermi surface reconstruction of superoxygenated La2CuO4superconductors with ordered oxygen interstitials

“…42,45,46 It is also realized that portions "e" and parts of "c" will be gone at high doping. This is because the original M-centered FS circle will increase its radius for higher doping; the FS crossing that occurs on the M-X line for low doping will reach the X point at optimal doping, and ultimately cross the -X line when the doping is increased further.…”

Quantum oscillations in the high- T c cuprates

“…We focus on an aspect of complementary experiments that has been thought to be particularly challenging to reconcile with results of quantum oscillation experiments, and examine to what extent these can be explained within a common framework. An antinodal energy gap (known as a pseudo-gap) is indicated directly by ARPES experiments that do not observe coherent antinodal quasi-particle states at the Fermi energy [6,[113][114][115], and less directly by optical conductivity experiments [11,116]. It has been suggested from the observation of quantum oscillations in a state characterized by a negative Hall coefficient that the associated Fermi surface pocket is an electron pocket, located at the antinode for symmetry reasons [27].…”

Compositional evolution of the anti-phase stripe superstructure in Bi2Sr2−xLaxCuO6(0 <x≤ 1.1) revealed by transmission electron microscopy

“…It is highly unlikely that the Fermi pocket observed in ARPES [1] can be explained by the new superstructure modulation q 2 observed in our TEM. In the ARPES measurements, the Fermi pocket was observed only in the underdoped region, but not in the optimally-doped and overdoped samples [1].…”

“…The origin of this supermodulation is mainly attributed to the distortions of the BiO bilayers [12,16,17]. Substitutions in Bi or Sr sites usually modify the distortions of the BiO bilayers, and may lead to a change of the modulation vector q 1 . There have been several TEM studies on La-Bi2201 with different compositions.…”

“…Since LEED is a surface sensitive technique, this LEED q 2 should be related to a modulation on the surface of La-Bi2201. This LEED q 2 was proposed to be structural [20] and was attributed to be the origin of the Fermi pocket observed in ARPES [1]. A further study combining surface-sensitive probes like ARPES and LEED, and bulk-sensitive probes like resonant (REXS) and non-resonant (XRD) x-ray diffraction, suggested that the new incommensurate superstructure q 2 behaves differently on the surface and in the bulk [21].…”