Superconductivity of cobalt oxide hydrate, Nax(H3O)zCoO2 · yH2O

doi:10.1016/j.physc.2015.02.010

Physica C: Superconductivity and its Applications

Volume 514, 15 July 2015, Pages 378-387

https://doi.org/10.1016/j.physc.2015.02.010 Get rights and content

Highlights

•
The experimental results of the Na_x(H₃O)_z ${CoO}_{2} \cdot y$ H₂O superconductivity was reviewed.
•
The superconductivity is unconventional.
•
Contradictory interpretations and results of physical properties of the substance are discussed in details.

Abstract

Na_x(H₃O)_z ${CoO}_{2} \cdot y$ H₂O has a highly two-dimensional triangular lattice of Co ions and exhibits the superconducting transition at $T_{C} = 4.7$ K. It is classified as an extreme type-II unconventional superconductor and the superconducting gap function has a line-nodal structure. The superconducting phase diagram shows that the superconductivity appears on the verge of a magnetic phase. In addition to the phase diagram, microscopic experiments on the normal state and the magnetic phase suggest that the superconductivity is induced by magnetic fluctuations. However, despite intensive study on Na_x(H₃O)_z ${CoO}_{2} \cdot y$ H₂O and related compounds, there remain some contradictory interpretations and experimental results regarding the Fermi surface topology and superconducting pairing symmetry. They are summarized and discussed in details.

Introduction

The discovery of the superconductivity of the Co oxide hydrate was made in 2003, following a fortuitous encounter by two researchers in different research fields [1]. The compound was discovered during an attempt to synthesize CoO₂ nanosheets, which involved the reduction of the bonding energy between the CoO₂ layers in Na_0.7CoO₂ by extracting Na ions between the layers. The second researcher then noticed that the compound could exhibit superconductivity and interesting electronic properties.

The superconductivity of the Co oxide hydrate immediately attracted considerable attention from condensed matter physicists and chemists, because of the following features: (1) It is the first and still the only known Co oxide superconductor. Since the discovery of high- $T_{C}$ cuprates, the superconductivity of new transition metal oxides, especially $3 d$ substances, has been expected to lead to new insights in physics. In addition, the further discovery of similar materials has become a major aim in chemistry. (2) CoO₂ exhibits the first and only superconductivity found on a regular triangular lattice of 3d transition metal ions. It is well known that the resonating-valence-bond (RVB) state can be strongly related to the high- $T_{C}$ superconductivity mechanism [2], and since the RVB state was originally proposed for a triangular lattice [3], the superconductivity on such a lattice is expected to be exotic. (3) Finally, synthesis was not the prevailing approach among researchers searching for new superconductors as a means of controlling carrier contents and electronic dimensionality. The control of these characteristics is recognized as being crucial to superconductivity.

There is still some controversy regarding understanding of the Co oxide hydrate superconductivity. This is mainly because the compound is not very stable and, therefore, it is sometimes difficult to prevent the water molecules in the compound from evaporating under certain experimental conditions. Moreover, samples properties show a large aging effect following synthesis. In addition, the compound was found to contain H₃O ions, which were not identified at first even though the electronic properties of the substance are very sensitive to the H₃O ion number. Hence, this causes a large property sample dependence. Nevertheless, careful and intensive study of this compound and related substances has revealed a great deal of unusual properties, some of which are most likely related to the occurrence of superconductivity. The purpose of this review is to give a perspective on the current understanding of Co oxide hydrate superconductivity by providing information on well-established properties and interpretations. Furthermore, contradictory experimental results and interpretations are shown in detail, because they must be resolved through future study. However, a detailed introduction of theoretical work on superconductivity is beyond the scope of this article, and those who are interested in this topic may refer to Refs. [4], [5], [6], [7], [8], [9] and references therein. Finally, in this article, $T, H, M, χ$ , and $C_{P}$ represent temperature, magnetic field, magnetization, magnetic susceptibility, and specific heat, respectively.

Section snippets

Chemical compositions and crystal structures

Sodium cobalt oxide hydrate is usually synthesized from sodium cobalt oxide, γ-Na_0.7CoO₂, by soft-chemical methods. The γ-Na_0.7CoO₂ is immersed in a bromine acetonitrile solution so that some of Na ions are extracted through oxidation caused by the bromine. After the immersion, the sodium content is approximately 0.4. Then, the product is immersed in water to intercalate the water molecules. Therefore, the chemical composition was at first thought to be Na_x ${CoO}_{2} \cdot y$ H₂O ( $x \sim 0.35$ and $y \sim 1.3$ ). However,

Electronic band structure

The Fermi surface of anhydrous Na_0.5CoO₂ (as shown in Fig. 2 [34]) had been calculated when the superconductivity of BLH was discovered. Reflecting the two-dimensional crystal structure, the dispersion along the $k_{z}$ direction is small. There are two kinds of Fermi surface; one is a large cylindrical surface around the Γ–A line and the other is a small oval surface near the K–H line. Both are of hole type. The trigonal distortion of the CoO₆ octahedron partially lifts the $t_{2 g}$ orbitals into the $a_{1 g}$

Superconducting phase diagram

Before the presence and importance of the H₃O⁺ ions were recognized to be important, three contradictory compositional phase diagrams were proposed [61], [62], [63]. In two of them [61], [62], the superconducting transition temperature, $T_{C}$ , exhibited a dome shape as a function of the Na content, x, or the Co valence, s. On the other hand, in the other phase diagram [63], the superconducting region had a trapezoid shape. This inconsistency is obviously due to ignorance concerning the presence of

Specific heat

Unfortunately, most of the previous research on the macroscopic properties of BLH was performed without awareness of the existence of the H₃O ions, or by neglecting them. Also, some properties display large sample dependence, and it is therefore difficult to correlate one property with another.

The specific heat of the BLH has been analyzed by a number of groups [54], [85], [86], [87], [88], [89], [90], [91] and clear peaks have been observed at $T_{C}$ , indicating the bulk superconductivity of BLH.

Proximity to magnetic phase

For a situation in which the Knight shift measurement cannot be applied to determine the pairing symmetry, other information must be used to elucidate the superconducting mechanism. As the magnetic ordering has been observed by various experiments, magnetic properties observed near the superconducting phases can be used to identify the component required for superconductivity to be induced.

Acknowledgements

We would like to thank Prof. H.-D. Yang (National Sun Yat-sen University, Taiwan) and Prof. J.-Y. Lin (National Chiao Tung University, Taiwan) for kindly permitting us to show the specific heat data in Fig. 6. We would also like to thank all our collaborators, colleagues, and competitors for their discussion and criticism of experimental results, which have made a significant contribution towards the current understanding of superconductivity. Among them, Dr. E. Takayama-Muromachi, Dr. T.

References (125)

P.W. Anderson
Mater. Res. Bull.
(1973)
K. Takada et al.
J. Solid State Chem.
(2004)
R.J. Balsys et al.
Solid State Ionics
(1997)
H. Nakatsugawa et al.
J. Solid State Chem.
(2004)
M.L. Foo et al.
Solid State Commun.
(2005)
S. Park et al.
Solid State Commun.
(2005)
S. Park et al.
Solid State Commun.
(2005)
S. Park et al.
Solid State Commun.
(2007)
J.H. Roudebush et al.
J. Solid State Chem.
(2013)
A. Lerf et al.
Mat. Res. Bull.
(1979)

K. Kuroki et al.

Phys. Rev. Lett.

(2007)

K. Yada et al.

Phys. Rev. B

(2008)

H. Sakurai et al.

Chemical and macroscopic physical properties of the sodium cobalt oxyhydrate superconductor

Y. Ihara et al.

(2005)

Y.G. Shi et al.

Phys. Rev. B

M.Z. Hasan et al.

Phys. Rev. Lett.

(2004)

Cited by (21)

From charge- and spin-ordering to superconductivity in the organic charge-transfer solids
2019, Physics Reports
We review recent progress in understanding the different spatial broken symmetries that occur in the normal states of the family of charge-transfer solids (CTS) that exhibit superconductivity (SC), and discuss how this knowledge gives insight to the mechanism of the unconventional SC in these systems. A great variety of spatial broken symmetries occur in the semiconducting states proximate to SC in the CTS, including charge ordering, antiferromagnetism and spin-density wave, spin-Peierls state and the quantum spin liquid. We show that a unified theory of the diverse broken symmetry states necessarily requires explicit incorporation of strong electron–electron interactions and lattice discreteness, and most importantly, the correct bandfilling of one-quarter, as opposed to the effective half-filled band picture that is often employed. Uniquely in the quarter-filled band, there is a very strong tendency to form nearest neighbor spin–singlets, in both one- and two-dimension. The spin–singlet in the quarter-filled band is necessarily charge-disproportionated, with charge-rich pairs of nearest neighbor sites separated by charge-poor pairs of sites in the insulating state. Thus the tendency to spin–singlets, a quantum effect, drives a commensurate charge-order in the correlated quarter-filled band. This charge-ordered spin–singlet, which we label as a paired-electron crystal (PEC), is different from and competes with both the antiferromagnetic (AFM) state and the Wigner crystal (WC) of single electrons. Further, unlike these classical broken symmetries, the PEC is characterized by a spin gap. The tendency to the PEC in two dimension is enhanced by lattice frustration. The concept of the PEC mirrors parallel development of the idea of a density wave of Cooper pairs in the superconducting high T $_{c}$ cuprates, where also the existence of a charge-ordered state in between the antiferromagnetic and the superconducting phase has now been confirmed. Following this characterization of the spatial broken symmetries, we critically reexamine spin-fluctuation and resonating valence bond theories of frustration-driven SC within half-filled band Hubbard and Hubbard–Heisenberg Hamiltonians for the superconducting CTS. We present numerical evidence for the absence of SC within the half-filled band correlated-electron Hamiltonians for any degree of frustration. We then develop a valence-bond theory of SC within which the superconducting state is reached by the destabilization of the PEC by additional pressure-induced lattice frustration that makes the spin–singlets mobile. We present limited but accurate numerical evidence for the existence of such a charge order–SC duality. Our proposed mechanism for SC is the same for CTS in which the proximate semiconducting state is antiferromagnetic instead of charge-ordered, with the only difference that SC in the former is generated via a fluctuating spin–singlet state as opposed to static PEC. In Appendix B we point out that several classes of unconventional superconductors share the same band-filling of one-quarter with the superconducting CTS. In many of these materials there are also indications of similar intertwined charge order and SC. We discuss the transferability of our valence-bond theory of SC to these systems.
Superconducting materials classes: Introduction and overview
2015, Physica C: Superconductivity and its Applications
No superconductivity in Pb<inf>9</inf>Cu<inf>1</inf>(PO<inf>4</inf>)<inf>6</inf>O found in orbital and spin fluctuation exchange calculations
2023, arXiv
Coulomb Correlations and Electronic Structure of CuCo<inf>2</inf>S<inf>4</inf>: a DFT + DMFT Study
2023, JETP Letters
No superconductivity in Pb<inf>9</inf>Cu<inf>1</inf>(PO<inf>4</inf>)<inf>6</inf>O found in orbital and spin fluctuation exchange calculations
2023, SciPost Physics
Electronic properties of single-layer CoO<inf>2</inf>/Au(111)
2021, 2D Materials

View all citing articles on Scopus

View full text

Superconductivity of cobalt oxide hydrate, Nax(H3O)zCoO2 · yH2O

Highlights

Abstract

Introduction

Section snippets

Chemical compositions and crystal structures

Electronic band structure

Superconducting phase diagram

Specific heat

Proximity to magnetic phase

Acknowledgements

Mater. Res. Bull.

J. Solid State Chem.

Solid State Ionics

J. Solid State Chem.

Solid State Commun.

Solid State Commun.

Solid State Commun.

Solid State Commun.

J. Solid State Chem.

Mat. Res. Bull.

J. Less-Common Met.

Physica B

Physica B

Physical C

Physica C

J. Phys. Chem. Solids

Nature

Science

J. Phys. Soc. Jpn.

J. Phys. Soc. Jpn.

J. Phys. Soc. Jpn.

J. Phys. Soc. Jpn.

Phys. Rev. Lett.

Phys. Rev. B

Chemical and macroscopic physical properties of the sodium cobalt oxyhydrate superconductor

Phase diagram for Nax(H3O)zCoO2·yH2O

J. Mater. Chem.

J. Jpn. Soc. Powder Powder Metall.

Chem. Mater.

Adv. Mater.

Chem. Commun.

Chem. Mater.

Phys. Rev. B

Chem. Mater.

Inorg. Chem.

Phys. Rev. B

Phys. Rev. B

Phys. Rev. B

Phys. Rev. B

Phys. Rev. B

Phys. Rev. B

Phys. Rev. B

Phys. Rev. Lett.

Phys. Rev. B

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Superconductivity of cobalt oxide hydrate, Na_x(H₃O)_zCoO₂ · yH₂O

Phase diagram for Na_x(H₃O)_z ${CoO}_{2} \cdot y$ H₂O