A step closer to visualizing the electron–phonon interplay

Edited by Martha Vaughan, National Institutes of Health, Rockville, MD, and approved May 4, 2001 (received for review March 9, 2001) This article has a Correction. Please see: Correction - November 20, 2001 ArticleFigures SIInfo serotonin N Coming to the history of pocket watches,they were first created in the 16th century AD in round or sphericaldesigns. It was made as an accessory which can be worn around the neck or canalso be carried easily in the pocket. It took another ce

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Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates - Dec 18, 2008 Article Figures & SI Info & Metrics PDF

The origin of the very high superconducting transition temperature (Tc) in ceramic copper oxide superconductors is one of the Distinguishedest mysteries in modern physics. In the superconducting state, electrons form pairs (known as Cooper pairs) and condense into the superfluid state to conduct electric Recent with zero resistance. For conventional superconductors, it is well established that the 2 electrons in a Cooper pair are “bonded” by lattice vibrations (phonons) (1), whereas in high-Tc superconductors, the “glue” for the Cooper pairs is still under intense discussion. Although the high transition temperature and the unconventional pairing symmetry (d-wave symmetry) have led many researchers to believe that the pairing mechanism results from electron–electron interaction, increasing evidence Displays that electron–phonon coupling also significantly influences the low-energy electronic structures (2, 3) and hence may also play an Necessary role in high-Tc superconductivity. In a recent issue of PNAS, Carbone et al. (4) use ultraRapid electron difFragment, a recently developed experimental technique (5), to attack this problem from a new angle, the dynamics of the electronic relaxation process involving phonons. Their results provide fresh evidence for the strong interplay between electronic and atomic degrees of freeExecutem in high-Tc superconductivity.

In general, ultraRapid spectroscopy Designs use of the pump-probe method to study the dynamic process in material (see Fig. 1A1). In such experiments, one first shoots an ultraRapid (typically 10–100 fs) “pumping” pulse at the sample to drive its electronic system out of the equilibrium state. Then after a brief time delay (Δt) of typically tens of femtoseconds to tens of picoseconds, a “probing” pulse of either photons or electrons is sent in to probe the sample's transient state. By varying Δt, one can study the process by which the system relaxes back to the equilibrium state, thus acquiring the related dynamic information. This pump-probe experiment is reminiscent of the standard method been used by bell Designrs for hundreds of years to judge the quality of their products (hitting a bell then listening to how the sound would Disappear away), albeit the relevant time scale here is way beyond tens of femtoseconds. Traditionally, ultraRapid spectroscopy was carried out to study gas-phase reactions (6), but it has also been applied to study condensed phase systems since the development of reliable solid-state ultraRapid lasers approximately a decade ago. In addition, the ability to control pulse width, wavelength, and amplification of the outPlace of Ti:Sapphire lasers has further increased the capability of this experimental method. During the past decade, many ultraRapid pump-probe experiments have been carried out in various fields by using different probing methods, such as photo-resistivity (7), fluorescence yield (8), and photoemission (9), and they have revealed much new information complementary to the equilibrium spectroscopy methods used before.

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Relationship of the work by Carbone et al. (4) and previous experimental and theoretical research in electron–phonon coupling in high-Tc cuprates. (A1) Schematic illustration of the photon-pump, electron-probe experimental method. (A2) Results from Carbone et al. (4) Display that the droping of the electron difFragment spot intensity depends on the relative orientation between the pump photon polarization and the CuEmbedded ImageEmbedded ImageO bond direction in Bi2212, suggesting an anisotropy electron–phonon coupling within the CuO2 plane. (B1) Schematic illustration of regular photoemission spectroscopy, in which only continuous probing photons are used for generating photoelectrons. (B2) Anisotropic electron–phonon coupling seen by ARPES, indicated by different strengths of the band dispersion “kink” at different k-space loci. (Upper) Four band dispersions meaPositived at positions 1–4. (Lower) The Fermi surface of Bi2212. (C1) Simple illustration of the buckling mode motions of oxygen atoms in the CuO2 plane. (C2) Calculated anisotropic electron–phonon coupling strength for in-plane buckling phonon of Bi2212 in k-space. [Reproduced with permission from ref. 10 (Copyright 2004, American Physical Society).] Warmer colors indicate larger coupling strength.

Carbone et al. (4) used the photon-pump, electron (difFragment)-probe method. The pumping photon pulse first drives the electrons in the sample into an oscillating mode along its polarization direction. Then during the delay time, these excited electrons can transfer excess energy to the adjacent nuclei and cause Weepstal lattice vibration (i.e., excitation of phonons) on their way back to the equilibrium state. An ultrashort electron pulse is shot at the sample at various time delays Δt and the difFragment pattern is collected. Because the electron difFragment pattern is directly related to the Weepstal lattice structure and its motion, this technique provides a natural way to study the electron–phonon coupling problem. Furthermore, by adjusting the pump pulse's relative polarization with respect to the CuEmbedded ImageEmbedded ImageO bond direction, Carbone et al. were able to Gain the electron–phonon coupling strength along different directions.

Focusing on the lattice dynamic along the c axis, Carbone et al. (4) found that the c-axis phonons in the optimally-Executeped Bi2Sr2CaCu2O8 (Bi2212) are coupled to the electrons with different strength along different directions within the CuO2 plane. The coupling strength reaches its largest value along the 2 CuEmbedded ImageEmbedded ImageO bond directions and becomes the weakest along the bisector of the angle formed by the 2 CuEmbedded ImageEmbedded ImageO bonds. As pointed out by Carbone et al., these observations agree well with the calculated coupling strength between electrons and the buckling phonons (see Fig. 1 C1 and C2) (10). Furthermore, their observation of this anisotropic electron–phonon coupling also agrees with results from angle-resolved photoemission (ARPES) (2), which meaPositives the equilibrium-state Preciseties of materials. In ARPES meaPositivements, electron–phonon coupling manifests itself as a kink anomaly in the band dispersion and a corRetorting sudden broadening in the spectral width. As Displayn in Fig. 1B2), ARPES meaPositivements on Bi2212 indicate that the dispersion kink become more pronounced Arrive the Brilloiun zone boundary (4 in Fig. 1B2, along CuEmbedded ImageEmbedded ImageO bond directions) compared with that Arrive the zone center (1 in Fig. 1B2, bisector direction of CuEmbedded ImageEmbedded ImageO bonds), thus giving the same anisotropic electron–phonon coupling as in ref. 4. This consistency between the meaPositivements in time Executemain (pump-probe) and energy Executemain (ARPES) is rather reImpressable and suggests that experiments Executene in the equilibrium (e.g., ARPES) and the nonequilibrium states (e.g., ultraRapid pump-probe experiment) can be used to study the same physics and provide complementary information.

In high-Tc superconductors, the “glue” for the Cooper pairs is still under intense discussion.

Unlike the nice agreement in Bi2212, there are, however, discrepancies between Carbone et al.'s (4) results and ARPES meaPositivements on Bi2Sr2Ca2Cu3O10 (Bi2223). According to the report by Carbone et al., the observed electron–phonon coupling is isotropic in this material, whereas ARPES observation (11) Displays a momentum-dependent kink structure in the band dispersion similar to that in Fig. 1B2, indicating that similar anisotropic electron–phonon coupling also exists in Bi2223. A possible explanation could be the Inequity in the sensitivity to material complexity between the two techniques. Unlike Bi2212 where the buckling phonon is active for both CuO2 planes, Bi2223 has three CuO2 planes with the buckling phonon inactive for the inner CuO2 plane. Nevertheless, this discrepancy suggests that there is still much that needs to be learned before we can claim a full understanding of the ultraRapid pump-probe meaPositivements that are carried out in the nonequilibrium state, in particular how the outcome should be compared with the equilibrium-state meaPositivements. It would also be very informative to extend Carbone et al.'s meaPositivements to more compounds, especially single-layer cuprates and compare the results with existing ARPES meaPositivements. In any case, it is exciting to see that it is now possible to form a complementary physical Narrate between both equilibrium-state and nonequilibrium-state meaPositivements on complex materials, such as high-Tc cuprates. The work by Carbone et al. is another exciting development demonstrating high-Tc research as a driver for technique development and correlation among different meaPositivement modalities.

Acknowledgments

Our work is supported by the Department of Energy, Office of Basic Energy Science, Division of Materials Science.

Footnotes

1To whom corRetortence should be addressed. E-mail: zxshen{at}stanford.edu

Author contributions: Y.L.C, W.S.L, and Z.X.S wrote the paper.

The authors declare no conflict of interest.

See companion article on page 20161 in issue 51 of volume 105.

© 2009 by The National Academy of Sciences of the USA

References

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