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Polaron Coherence Condensation in Layered Colossal Resistive Manganites Print
Wednesday, 30 July 2008 00:00

Novel quantum phenomena, such as high-temperature superconductivity (HTSC) and colossal magnetoresistance (CMR), arise in certain materials where the interactions between electrons are very strong, but the mechanism driving their appearance remains a major puzzle. Now, angle-resolved photoemission findings from an international team led by researchers from Stanford University and the ALS provide the first direct spectroscopic evidence that the transition from insulator to metal in CMR manganese oxides (manganites) results from coherent "polaron condensation." The new findings also suggest that coherence-driven transitions are a generic controlling factor for novel quantum phenomena in doped transition-metal oxides.

Coherence Rules the Day

When physicists speak of coherence, they are referring to a property of waves. The coherence between multiple light waves in a laser is the property that gives the light its extreme brightness. In the case of the laser, the waves all have the same frequency and phase. Similarly, the quantum mechanical wave functions that describe particles like electrons in solids can also have a coherence that manifests in various ways, including superconductivity, where a metal cooled to a very low temperature loses all its resistance to the flow of electricity.

Quantum-mechanical coherence now seems to be an essential ingredient in at least two examples of "novel quantum pheonomena" in materials called transition-metal oxides: colossal magnetoresistance (CMR), in which an insulator becomes a metal when cooled to a lower temperature or placed in a magnetic field, and high-temperature superconductivity (HTSC), in which the loss of resistance to current flow appears at much higher temperatures than in normal metal superconductors. It is well known that HTSC is driven by the onset of phase coherence involving pairs of electrons, although how this occurs is still a mystery. Now, Mannella et al. have uncovered a parallel behavior in which CMR is driven by coherence involving composite "quasiparticles" known as polarons. The parallels between these two disparate phenomena suggest a common underlying coherence-based mechanism.

Temperature-dependence of the quasiparticle peak in LSMO and BiSCCO2212. The quasiparticle peak at near the Fermi energy (EF) collapses when the temperature is close to the Curie point, TC = 120 K, for LSMO and the critical point, Tc = 83 K, for BiSCCO2212. [BiSCCO2212 data from D.L. Feng et al., Science 289, 277 (2000).]

Describing a condensed system composed of 1023 electrons is a formidable task. The standard (mean-field Fermi-liquid) theory comes to the rescue with the concept of quasiparticle. A system composed of many strongly interacting particles can sometimes be described as if it were composed of a few weakly interacting "quasi-electrons," which one can roughly imagine as the electrons surrounded by a cloud of "screening" electrons. Angle-resolved photoelectron spectroscopy (ARPES) allows measuring quasiparticles directly, providing effective "single-particle" spectra from which one can construct a multidimensional map of the particle energy–momentum relationship (dispersion).

In strongly interacting materials like the CMR manganites, the quasiparticle concept can be more problematic. For example, earlier in ARPES experiments at Beamline 10.0.1 on the CMR manganite LSMO, a lanthanum-strontium-manganese oxide (La1.2Sr1.8Mn2O7), the Stanford–ALS team observed polarons (see Pseudogaps, Polarons, and the Mystery of High-Tc Superconductivity). A polaron forms when an electron is strongly coupled to the atoms in a crystal. The polaron thus has a larger effective mass as it moves and a lower mobility than an isolated electron. However, the spectra exhibited only a small "polaronic quasiparticle" peak below the ferromagnetic transition temperature (Curie point) of about 120 K, accounting for only 10% of the total spectral weight (intensity), indicating that the interactions are so strong that quasiparticles barely survive in manganites.

The temperature dependence of the integrated intensity of the quasiparticle peak (QP) for LMSO tracks extremely well the DC conductivity curve σDC as measured on samples grown in the same laboratory (Q.A. Li, K.E. Gray, and J.F. Mitchell, Phys. Rev. B 63, 024417 [2000]). Right: In analogy, with BiSCCO2212, the temperature dependence of the quasiparticle peak resembles that of the superfluid density (latter not shown in figure).

Nonetheless, in their current detailed measurements of the temperature dependence of the ARPES spectra, the team was extremely surprised to find that the temperature dependence of the small quasiparticle peak in the ARPES spectra tracked extremely well the DC electrical conductivity. Remarkably, despite being such a minority, the polaronic quasiparticle was still able to track the evolution of the whole system, a behavior completely unexpected from conventional Fermi-liquid theory.

The temperature-dependence of the quasiparticle intensity closely resembles the state of affairs in the "underdoped" (i.e., d is smaller than needed for maximum superconductivity) HTSC Bi2Sr2CaCu2O8+d (BiSCCO2212). Here, the superconducting transition is controlled by the condensation of phase-coherent paired electrons (Cooper pairs); however, the temperature dependence of the quasiparticle peak intensity resembles that of various collective properties related to the superfluid density, again an unexpected behavior, since quasiparticles should provide information about the strength of the pairing, but not reflect the coherence of cooper pairs.

The physical picture for the quasiparticle formation emerging from the new work is that the coherent polaronic metallic ground state appears in the ARPES spectra very rapidly as the wavefunctions of the individual "small" polarons that exist at higher temperature overlap and the polarons condense into a phase-coherent "many-body" quantum state. The proposed driving force for this transition is the sudden reduction of kinetic energy induced by the synergy of polaron condensation and the magnetic double-exchange interaction (see Polaron Behavior in CMR Manganites).

In sum, the commonality between the quasiparticle behavior in LSMO and underdoped BiSCCO2212 has to be identified with the coherence-driven nature of the transitions and with the polarons that are the basic constituents of the poorly understood metallic phase present in both CMR manganites and HTSC cuprates. Coherence-driven transitions may thus provide a rationale for understanding high-temperature quantum phenomena.

Research Conducted by N. Mannella, W.L. Yang, K. Tanaka, and X.J. Zhou (Stanford University and ALS); H. Zheng and J.F. Mitchell (Argonne National Laboratory); J. Zaanen (Leiden University, The Netherlands); T.P. Devereaux (University of Waterloo, Canada); N. Nagaosa (University of Tokyo); Z. Hussain (ALS); and Z.-X. Shen (Stanford University).

Research Funding: U.S. National Science Foundation, Office of Naval Research, and U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: N. Mannella, W.L. Yang, K. Tanaka, X.J. Zhou, H. Zheng, J.F. Mitchell, J. Zaanen, T.P. Devereaux, N. Nagaosa, Z. Hussain, and Z.-X. Shen, "Nodal quasiparticle and polaron coherence condensation in layered colossal resistive manganites," Phys. Rev. B 76, 233102 (2007).