|Pseudogaps, Polarons, and the Mystery of High-Tc Superconductivity|
Working at the ALS, a multi-institutional collaboration led by researchers at ALS and Stanford University has identified a pseudogap phase with a nodal-antinodal dichotomy in ferromagnetic manganese oxide materials (manganites). Even though ferromagnetism and superconductivity do not exist together, the pseudogap state found in these manganites is remarkably similar to that found in high-temperature superconducting copper oxide materials (cuprates). This discovery casts new doubts on any direct link between the pseudogap phase and high-temperature superconductivity and adds fire to the debate over one of the great scientific mysteries of our time: What causes high-temperature superconductivity?
Beginning in 1986, the cuprates were discovered to have much higher superconducting transition temperatures (Tc) than known metallic superconductors. Among the possible explanations for high-Tc superconductivity, the pseudogap state has played a prominent role. In metals, electrons occupy a continuous spectrum of quantum states up to a maximum energy known as the Fermi level. When the metal is superchilled to become a superconductor, a forbidden energy gap opens up at the Fermi energy.
Experiments in recent years have shown that the spectrum in the superconducting state of high-Tc cuprates is characterized by a “nodal-antinodal dichotomy": when plotted in reciprocal (momentum) space, the spectrum is shaped like a cloverleaf, exhibiting a gap in the so-called antinodal direction (parallel to the Cu-O bonds), which vanishes along the nodal direction (from the origin running diagonal to the Cu-O chemical bonds). This dichotomous character of the excitation spectrum is generally assumed to be linked to the ”d-wave” symmetry of the superconducting state.
In the high-Tc cuprates, this dichotomous gap structure persists even above Tc and gives rise to the so-called “pseudogap state” with finite “Fermi arcs” replacing the original nodal points of the superconducting state. More intriguingly, the pseudogap appears to have two energy scales, with a low-energy pseudogap comparable to that of the superconducting gap and a much larger high-energy pseudogap. Naturally, the high-energy pseudogap is taken to represent the dominating physics.
The investigators used angle-resolved photoemission (ARPES) at ALS Beamline 10.0.1 to study a two-layer manganite compound consisting of a mixture of lanthanum, strontium, manganese, and oxygen (LSMO). Such manganese oxide materials become ferromagnetic below a certain critical temperature and display colossal magnetoresistance (CMR), meaning their electrical resistance can change by orders of magnitude in the presence of a magnetic field.
The investigators were quite surprised when the LSMO manganites displayed the characteristic signatures of the pseudogap state as found in the cuprates, including the nodal and antinodal dichotomy. These findings therefore cast doubt on the assumption that the pseudogap state and the nodal-antinodal dichotomy are hallmarks of the superconductivity state and suggests they are a more general phenomenon characteristic of transition-metal oxides.
Moreover, these ARPES experiments suggest the occurrence of a polaronic metal with anisotropic band structure. A polaron is a "quasiparticle" formed when an electron causes a large distortion around it in the atomic lattice of a crystal. Since the surrounding atoms are much more massive than the bare electron, the polaron behaves as a negatively charged particle with a larger mass and lower mobility than an isolated electron. Earlier research at the ALS had linked the formation of polarons in LMSO to the CMR effect ("Polaron Behavior in CMR Manganites").
This latest research goes farther and indicates that polaron formation is crucial to CMR. What might be even more interesting is the now emerging picture that polarons are the basic constituents of a poorly understood mysterious metallic phase that is present in both CMR materials and so-called underdoped high-Tc cuprates and likely ubiquitous to all transition-metal oxides. A proper understanding of this new phase will go a long way in capturing the essence of important phenomena such as colossal magnetoresistance and high-temperature superconductivity.
Research conducted by N. Mannella, W. Yang, and X.J. Zhou (Stanford University, Stanford Synchrotron Radiation Laboratory, and ALS); H. Zheng and J.F. Mitchell (Argonne National Laboratory); J. Zaanen (Stanford University, Stanford Synchrotron Radiation Laboratory, and Leiden University, The Netherlands); T.P. Devereaux (University of Waterloo, Canada); N. Nagasosa (University of Tokyo and Correlated Electron Research Center, Tsukuba, Japan); Z. Hussain (ALS); and Z.-X. Shen (Stanford University and Stanford Synchrotron Radiation Laboratory).
Research Funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); National Science Foundation; and Office of Naval Research. Operation of the ALS is supported by BES.
Publication about this research: N. Mannella, W. Yang, X.J. Zhou, H. Zheng, J.F. Mitchell, J. Zaanen, T.P. Devereaux, N. Nagasosa, Z. Hussain, and Z.-X. Shen, “Nodal quasiparticles in pseudo-gapped colossal magnetoresistive manganites,” Nature 438, 474 (2005).