More or different?
Exploring analogs of the cuprates in other oxides
1. Pseudogap of metallic layered nickelate R2−x Srx NiO4 (R=Nd, Eu) crystals
measured using angle-resolved photoemission spectroscopyAuthors: M. Uchida, K. Ishizaka, P. Hansmann, Y. Kaneko, Y. Ishida, X.
Yang, R. Kumai, A. Toschi, Y. Onose, R. Arita, K. Hel, O. K. Andersen, S.
Shin, Y. Tokura
arXiv:1012.5516
2. Phase-Sensitive Observation of a Spin-Orbital Mott State in Sr2 IrO4Authors: B. J Kim, H. Ohsumi, T. Komesu, S. Sakai, T. Morita, H. Takagi,T. Arima
ArXiv:0907.0956
Science, 323 1329, 2009.
3. Twisted Hubbard Model for Sr2 IrO4 : Magnetism and Possible High Temperature Superconductivity
Authors: Fa Wang and T. Senthil
ArXiv:1011.3500
Recommendation and Commentary by Leon Balents, KITP, UCSB
The cuprate family of high temperature superconductors have fascinatedcondensed matter physicists for about 25 years. A consensus explanation
for the many intriguing phenomena (not just superconductivity!) in thesematerials continues to elude us. An important question is whether there
are other materials with properties similar to the cuprates which might shedlight upon on the essential microscopic physics. Two recent investigations
exploring this question are discussed here.
Paper 1 above describes very recent angle-resolved photoemission studies
which reveal strikingly “cuprate-like” behavior in nickelates, R2−x Srx NiO4 .
The idea that nickelates might behave similarly to cuprates is not new. Nickelates form the same perovskite-type layered structures, with an underlying
square lattice, as do the cuprates. Formally, the Ni3+ ionization state is analogous to that of the Cu2+ ion in the cuprates, with one unpaired electron in
the so-called eg orbital manifold. However, there are important differences(for example the absence of any binary compounds with Ni3+ ), so that theidea died out for some time.[1] Recently, it has revived in the context of artificial oxide heterostructures grown as thin films,[2] but this proposal too has
so far not been successful.
1The material studied in Paper 1 is not a heterostructure but a bulk single
crystal of the “214” material, which is the single-layer perovskite structure,
where the most two-dimensional behavior would be expected. The paper
focuses on the sample with x = 1.1, which is analogous to an underdoped
cuprate. Several features observed are strikingly similar to the cuprates. Theenergy integrated spectrum displays a large, hole-like Fermi surface, which
seems to fade on approaching the “anti-nodal” points, very reminiscent of
the Fermi arcs seen in the cuprates. This picture is reinforced by the energy
distribution curves in the anti-nodal region, which shows a broad pseudogap
feature over about 0.1eV. Some evidence for kinks in the hole dispersion isalso reported, another prominent cuprate feature.
These observations raise many questions. LDA predicts an additional
small electron Fermi surface in this material, associated with the two-folddegeneracy of the eg orbitals. Its absence in the experiment may indicate an
unexpectedly large eg splitting, or simply that the electron pocket is “hiding”
at a different kz not accessible at this photon energy. The mechanism forthe pseudogap and arcs is another key issue. Given the predominance ofstripe physics in the nickelates, charge ordering or fluctuations may play animportant role. Note that superconductivity has not been observed here or
anywhere else in this material. Why?
Papers 2 and 3 address a much more unlikely candidate for an ersatz
cuprate. While Sr2 IrO4 also has the single-layer cuprate structure, one usually expects very different physics from 5d transition metals like iridium. Asa rule, the size of the d orbitals increases on progressing downward in thetransition metals, consequently increasing the bandwidth and decreasing theon-site Coulomb repulsion U . Thus Mott insulators and strong correlations
are primarily the domain of the 3d’s. However, because of its large atomic
number (Z = 77), iridium also has very strong spin-orbit coupling, of theorder of half an electron volt. The spin-orbit coupling in this case splits theorbital degeneracy present in its absence, creating a j = 1/2 ionic groundstate Kramer’s doublet. In the band picture, the usual spaghetti of statesnear the Fermi level is largely split. The spin-orbit split bands are muchnarrower than the “bare” 5d bands, and indeed narrow enough in Sr2 IrO4
that the weak U is sufficient to stabilize a Mott insulator.
Paper 2 directly verifies this picture of a Sr2 IrO4 as a spin-orbit assisted
Mott insulator using resonant x-ray scattering. Based on the selection rules
associated with the 2p to 5d transition being probed, B. J. Kim et al weredirectly able to verify the j = 1/2 nature of the Ir4+ state. Moreover, because2the wavelength of light at this energy in iridium is of order 1Å, spatial information can be obtained. In this way, Sr2 IrO4 was found to be essentiallya Néel ordered two-sublattice antiferromagnet as in the undoped cuprates
such as La2 CuO4 , but with a large canting angle of about 10◦ . This work has
shown that iridium oxides are nearly ideal systems for study with resonant
x-ray scattering, and further elastic and inelastic studies of these materials
are proceeding rapidly in several labs worldwide.
From the above discussion, it is apparent that Sr2 IrO4 has a single j =1/2 band near the Fermi energy, and has predominantly antiferromagnetic
interactions like the cuprates. The authors of Paper 3 push this argument
further, and construct a one-band Hubbard model for the j = 1/2 states.
The most interesting question is the remaining role of the large spin orbit
interaction. Wang and Senthil argue that the spin orbit effects appear in
the Hubbard model in a form that is “pure gauge”, so that the model canbe transformed back into the “usual” Hubbard model without spin-orbit
coupling. In this way they conclude that electron doped Sr2 IrO4 should
should display high-Tc superconductivity.
Predicting superconductivity is a bold and often frustrating game. Veryrecent experiments reported in Ref.[3] do not find superconductivity in holedoped Sr2 IrO4−δ , but this could perhaps be blamed on oxygen deficiency in
the IrO2 planes. Alternatively, perhaps the “pure gauge” terms in the WangSenthil Hubbard model do not capture all the spin-orbit effects, leading toessential differences between the physics of the cuprates and this iridate.
The experiments and theory described here are just a small part of growing efforts to explore novel correlated oxides, especially off the beaten track.
Comparing and contrasting the behaviors of more materials that are like thecuprates but different will surely lead to a better understanding of the physicsof strong correlations and superconductivity.

References[1] V. I. Anisimov, D. Bukhvalov, and T. M. Rice. Electronic structure ofpossible nickelate analogs to the cuprates. Phys. Rev. B, 59(12):7901–7906, Mar 1999.
[2] Jiri Chaloupka and Giniyat Khaliullin. Orbital Order and Possible Superconductivity in LaN iO3 /LaM O3 Superlattices. Phys. Rev. Lett.,3

100(1):016404, Jan 2008.
[3] O. B. Korneta, Tongfei Qi, S. Chikara, S. Parkin, L. E. De Long,
P. Schlottmann, and G. Cao. Electron-doped Sr2 IrO4−δ (0 ≤ δ ≤ 0.04) :Evolution of a disordered Jef f = 12 Mott insulator into an exotic metallicstate. Phys. Rev. B, 82(11):115117, Sep 2010.

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