Completed-Band Dispersion Relation in Sodium Cobaltates

Abstract

We utilize fine-tuned polarization selection coupled with excitation-energy variation of photoelectron signal to image the complete d-band dispersion relation in sodium cobaltates. A hybridization gap anticrossing is observed along the Brillouin zone corner and the full quasiparticle band is found to emerge as a many-body entity lacking a pure orbital polarization. At low dopings, the quasiparticle bandwidth (Fermion scale, many-body E F 0:25 eV) is found to be smaller than most known oxide metals. The low-lying density of states is found to be in agreement with bulk-sensitive thermodynamic measurements for nonmagnetic dopings where the 2D Luttinger theorem is also observed to be satisfied. DOI: 10.1103/PhysRevLett.97.186405 PACS numbers: 71.20.Be, 71.30.+h, 73.20.At, 74.90.+n Sodium cobaltate (Na 1=3 CoO 2 yH 2 O) is the only known example of an oxide superconductor, besides high T c cuprates, which is realized in the vicinity of a spin-1=2 Mott state. However, unlike the single band cuprates, the layered cobaltate class Na x CoO 2 is a prime example of a multiorbital interacting electron system. Observations of superconductivity, magnetic order, spin-thermopower, charge-order, non-Fermi-liquid transport, and many other unusual properties [1][2][3] have led to an extensive body of theories to model the electron behavior [4 -18]. The symmetries proposed for the superconducting order parameter [4 -7] critically depend on the orbital character of the bands and the Fermi surface topology. Moreover, a measure of the quasiparticle bandwidth is crucial for the theories of enhanced thermopower and anomalous Hall effect [6,8].Previous angle-resolved photoemission spectroscopy (ARPES) measurements [9][10][11] have focused on the Fermi surface topology, Luttinger theorem, and lowenergy physics. However, many important issues such as the orbital polarization of the quasiparticles, the reason for the sinking of the e g 0 bands, and the effects of hydration have not yet been explored. The experimental map of the Fermi surface (FS) [9-11] of these compounds revealed only one FS. The band associated with the corner pockets predicted in local-density approximation (LDA) [12] was observed to have sunk [10,11] below the Fermi level. Three different intrinsic many-body explanations have been advanced to account for this unusual phenomenon. One scenario argues that the corner pockets sink due to a delicate balance of interorbital Coulomb correlations [16,17]. A second scenario [18] suggests that the effect of the random potential of the sodium layer leads to a weak localization of the pocket states. Finally, it has been suggested that the polarization employed in all previous ARPES studies precludes the detection of the pockets [15]. In this Letter, we report high-resolution measurements of the complete d-band multiplet, which enables a comparison of the data with theoretical models in the literature.We have extended the electronic structure determination problem in three regards: First, in order to accurately image the band d...

Evolution of electron Fermi surface with doping in cobaltates

“…The preceding ARPES experimental results confirmed that the top postion of the e g band is doping independent, sinking below the Fermi energy, suggesting that it plays no role in the lowenergy phenomena [13][14][15][16][17][18][19][20] . The combined these ARPES experimental results [13][14][15][16][17][18][19][20] and the transport experimental data 6-10 therefore show that the essential low-energy physics of the cobaltates is dominated by the strong electron correlation. The strong-coupling Hubbard model and its equivalent, the t-J model, are prototypes to study the strong correlation effects in solids, especially in connection with the unconventional superconductivity 21,22 .…”

“…The first principle band calculations predict that the cobaltates have a large EFS centered around the Γ point of the Brillouin zone (BZ) and six small pockets near the K points 11 . However, the angleresolved photoemission spectroscopy (ARPES) measurements on the cobaltates reveal only the large EFS [12][13][14][15][16][17][18][19][20] , while six small EFS pockets near the K points are absent. In particular, these ARPES experimental data also indicate that the area of EFS contains 1 + δ electrons, and therefore fulfills Luttinger's theorem 15,18,20 , where δ is the electron doping concentration.…”

“…However, the angleresolved photoemission spectroscopy (ARPES) measurements on the cobaltates reveal only the large EFS [12][13][14][15][16][17][18][19][20] , while six small EFS pockets near the K points are absent. In particular, these ARPES experimental data also indicate that the area of EFS contains 1 + δ electrons, and therefore fulfills Luttinger's theorem 15,18,20 , where δ is the electron doping concentration. In particular, there is a remarkable resemblance in the normal-and SC-state properties between the cobaltates and cuprates [1][2][3][4][5][6][7][8][9][10] , reflecting a fact that the strong electron correlation is common for both these materials 6,21, 22 .…”

“…Although the evolution of EFS with doping in the cobaltates has been well established by now [12][13][14][15][16][17][18][19][20] , its full understanding is still a challenging issue. Within the framework of the local-spin-density approximation, the EFS topology in the cobaltates has been studied numerically by taking into account the onsite Coulomb interaction 23 , however, the obtained EFS area is twice as large, and therefore is inconsistent with the ARPES observations.…”

Spin fluctuations, magnetic long-range order, and Fermi surface gapping inNaxCoO2

“…One study sees a significant z-dispersion resulting in additional FS segments 15 while in most cases the system seems to remain quasi-2D 14,30 . If additional Fermi surface caused by threedimensionality was present, a higher DOS is consequently to be expected.…”