Laser-excited ultrahigh-resolution photoemission spectroscopy ofNaxCoO2·yH2O

Abstract

We have studied the temperature-dependent electronic structure near the Fermi level (E F ) of the layered cobaltate superconductor, Na 0.35 CoO 2 ·1.3H 2 O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap with an energy scale of ~ 20 meV in Na 0.35 CoO 2 ·1.3H 2 O and Na 0.35 CoO 2 ·0.7H 2 O, which is clearly absent in Na 0.7 CoO 2 . The energy scale of the pseudogap is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low temperatures. We discuss implications of the pseudogap in relation to available transport and magnetic susceptibility results. 2 Theoretically, recent studies on the Co oxide superconductors have proposed novel superconductivity with various types of order parameters depending on the model used.3 And, several experimental studies have been performed to clarify the symmetry of the superconducting gap. 4 However, agreement on the order parameter has not been reached so far.In the cuprate high temperature superconductors (high-T c 's), whose electronic structure is often described in terms of a quasi-2 dimensional correlated system, understanding the unusual normal state properties has lead to deeper understanding of the superconductivity. Even for the layered Co oxides, recent studies have indicated a rich phase diagram in the non-hydrated Na x CoO 2 system. 5 For superconducting samples, as has been also suggested from theoretical studies, [6][7][8] importance of well-separated Co 3+ and Co 4+ in the superconducting Co oxides has been reported experimentally very recently. 9Coexistence of superconductivity and ordered states have been discussed in cuprate high-T c superconductors.10 In this regard, Na 0.35 CoO 2 ·1.3H 2 O provides another opportunity to investigate charge and/or spin order competing or cooperating with superconductivity in superconducting oxides.Thus, it is essential to study the normal-state electronic structure of the superconducting Co oxides using photoemission spectroscopy (PES), which provides direct information on the electronic structure. Very recent PES studies 9 using hard x-rays (escape depth of ~ 50 Å) have provided bulk fundamental physical parameters that describe the new superconductor in terms of, on-site coulomb energy U dd , charge transfer energy ∆, and hybridization strength V.More importantly, the double-peak structure in the Co 2p core-level spectra revealed charge disproportionation in Na 0.7 CoO 2 and Na 0.35 CoO 2 ·1.3H 2 O samples, which suggest existence of remnant charge order in the superconducting samples. For the states near the Fermi level (E F ), while angle-resolved photoemission spectroscopy (ARPES) studies 11-13 on non hydrated samples showed a large hole pocket around the Γ point in the Brillouin zone, and renormalization on the energy scales of J of ~ 10 meV, 12 no PES study reporting the electronic structure near E F of superconducting Na 0.35 CoO 2 ·1.3H 2 O is reported. Therefore, it is extr...

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.…”

“…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.…”

s-wave superconductivity due to Suhl-Kondo mechanism inNaxCoO2⋅yH2

“…For example, the uniform magnetic susceptibility decreases with decreasing temperature for x ≤ 0.6, while it shows a Curie-Weiss-like behavior for x ≥ 0.6 [3,4]. The quasiparticle spectra in photoemission spectroscopy [5] and optical conductivity in infrared spectroscopy [6] also decrease with decreasing temperature for x ≤ 0.6. It is noteworthy that these "weak pseudogap" behavior is also observed in the normal state of superconducting Na x CoO 2 · yH 2 O.…”

Spin Fluctuations and Weak Pseudogap Behaviors in Na0.35CoO2: Renormalization of Band Structure

“…In this case, the degree of reduction of DOS is greater when the top of the e ′ g band is just below the Fermi level. In experimental measurements, both uniform spin susceptibility [6] and DOS in the photoemission spectroscpy [7] decrease at lower temperatures. These pseudogap behaviors are consistent with the latter result.…”