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Topic 19: Correlation between Superconductivity and Lattice Vibrations / Electron Excitations

Dreaming of Room Temperature Superconductors

Superconductivity is a phenomenon where the electrical resistance of a conductor vanishes, and is typically observed at extremely low temperatures. Successfully developing room temperature superconductors, a researcher's dream, would begin a new era in the energy revolution. For example, superconductive electric power transmission systems would enable electric power networks to be established with negligible loss in electric transmission, and they could store electric current itself in superconductive rings without external energy. Additionally, if superconductive circuit elements can be used in supercomputers, power consumption and heat generation would be drastically reduced, allowing further downsizing. Moreover, the construction of super fast, super energy-efficient magnetically-levitated trains would be possible. Although the mechanisms for superconductivity are not well understood, SPring-8 is playing a crucial role in solving these mysteries through the various types of superconductivity research conducted using synchrotron radiation.

Striving to Solve the Mysteries of Superconductivity using Inelastic X-Ray Scattering

 

Although electrons normally repel one another, superconductivity is a singular phenomenon where the electrical resistance vanishes due to electron pairing through an attractive force. Although electron pairing in high-temperature copper-oxide superconductor was discovered in 1986, its mechanisms were not elucidated. However, several hypotheses have been proposed. One is the BCS theory which postulates that lattice vibrations (vibrations of atoms situated at the vertices of a lattice) between atoms comprising a superconductor are the major cause of superconductivity. Another theory postulates that superconductivity is due to ordered spin or excitation motion (fluctuation) of electric charge. Yet others are a mixture of these two theories. To date, the highest critical temperature (Tc) below which superconductivity is present has been reported for a copper oxide (Tc of ~150 K (~ -123 °C)), but this value is still quite low. Therefore, determining the mechanisms of superconductivity and synthesizing materials with a high Tc based on these mechanisms are immediate measures to develop room temperature superconductors.

SPring-8's High-Resolution Inelastic Scattering Beamline (BL35XU), which boasts the world-best energy resolution at a few electron volts, has significantly contributed to unraveling the mechanisms of lattice vibrations and charge excitations of superconductors through inelastic X-ray scattering experiments. Inelastic X-ray scattering experiments measure tiny differences between the energies of the incident X-rays onto a material and the scattered X-rays. Thus, they require highly brilliant X-rays to examine the excitation states of atoms and electrons in materials.

Demonstrating the Correlation between Lattice Vibrations and Superconductivity

Dr. Alfred Q. R. Baron (Associate Chief Scientist, RIKEN, Japan), Dr. Hiroshi Uchiyama (Research Scientist, ditto), and colleagues examined the dispersion relations (relations among vibration direction, momentum, and energy) of the lattice vibrations in MgB2 (a new high-temperature superconducting material discovered by Japanese researchers in 2001). They employed an inelastic X-ray scattering technique that they helped develop. Their research revealed that MgB2 is an existing type of superconducting materials and its lattice vibrations are closely related to the onset of superconductivity. Additionally, they observed the softening (decrease) in the lattice vibrational energy of a copper-oxide high-temperature superconducting material, HgBa2CuO4+δ. These achievements were published in Physical Review Letters (2004).

Moreover, Dr. Jun-ichiro Mizuki (Deputy Director, the Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA)), Drs. Moritz Hoesch, Tatsuo Fukuda, Kenji Ishii, Shuichi Wakimoto (collaborative researchers with Dr. Mizuki at JAEA), and colleagues at JAEA also studied superconductivity mechanisms at BL35XU. Through experiments on newly discovered iron-arsenic compound high-temperature superconducting materials (LaFeAsO1-xFx and PrFeAsO1-y), Dr. Fukuda and colleagues revealed that lattice vibrational energy induced by the motions of iron and arsenic atoms is lower than the theoretical prediction, questioning the validity of the common understanding. This finding was published in the Journal of the Physical Society of Japan (2008).

Dr. Mizuki and colleagues found that diamond, which is the same material used in jewelry, plays an important role in their research. Surprisingly, when a high concentration of boron is doped (injected) into diamond, which is a good insulator, it becomes superconductive. Since its discovery in 2004 by Russian researchers, boron-doped diamond has received much attention as a potential material to decipher the superconductivity mechanisms and to increase Tc. However, single crystal samples must be prepared to examine the lattice vibrations of boron-doped superconducting diamond using inelastic X-Ray scattering. Dr. Hiroshi Kawarada (Professor, Waseda University, Japan) and colleagues developed ideal single crystal samples (100-μm thick) of such diamond using a vapor phase method (a method to synthesize diamonds from hydrogen and methane), and Dr. Yoshihiko Takano (the National Institute for Materials Science, Japan) demonstrated that Tc of this diamond is 4.2 K.

The longitudinal optical lattice vibrational mode (LO　mode) is a type of lattice vibration in which atoms repeatedly move closer and then farther from each other (Fig. 1). Dr. Mizuki and colleagues examined the LO modes of superconducting and non-superconducting diamonds. They found that the energy of the superconducting LO modes is significantly softened. Hence, this lattice vibration plays an important role in electron pairs responsible for superconductivity. These research achievements, which were realized through a series of experiments, were published in Physical Review B (2007).

Additionally, research on the lattice vibrations of other high-temperature superconductors has also progressed. Figure 2 shows the relationship between Tc and the concentration (x) of strontium (Sr) in a superconducting lanthanum strontium copper oxide (La2-xSrxCuO4). The distribution reaches a maximum near x = 0.15 (B). La2-xSrxCuO4 is nonconductive when x < 0.04, but becomes superconductive when 0.04 fig x fig 0.3 (Tc gradually increases as x increases passing the peak, and then gradually decreases), and lose superconductivity when x > 0.3. To promote further studies on La2-xSrxCuO4, Dr. Kazuyoshi Yamada (Professor, the Institute for Materials Research, Tohoku University, Japan) has fabricated a special rod-like single crystal of La2-xSrxCuO4 in which the strontium concentration gradually increases from one end at a constant rate.

As Dr. Mizuki has acknowledged, “This special crystal has enabled systematic measurements of the dependence of lattice vibrations on the strontium concentration while simultaneously reducing experimental errors.” Research in this field has gained momentum since then. Experiments using this crystal have revealed that the softening of lattice vibrations do not match the expectation that the softening of lattice vibrations is proportional to the strontium concentration (C in Fig. 2). Additionally, Fig. 2 shows that the increase/decrease patterns in the relationships between strontium concentration and lattice vibration softening (A) and strontium concentration and Tc (B) are clearly correlated. This finding indicates that the longitudinal wave expansion and contraction of the lattice vibrations between copper and oxygen, which are the constituent elements of La2-xSrxCuO4, are strongly correlated with the mechanisms of superconductivity. Their research results were published in Physical Review B (2005).

 

Fig. 1. Schematic diagrams of the crystal structures and longitudinal optical lattice vibration modes (LO modes) of diamonds. (a) Crystal structure of a diamond. (b) 3D view of a LO mode.
Fig. 1. Schematic diagrams of the crystal structures and longitudinal optical lattice vibration modes (LO modes) of diamonds. (a) Crystal structure of a diamond. (b) 3D view of a LO mode.

(c) Top view of a LO mode. Eight atoms at the vertices of a cube and six atoms at the center of each side of a cube vibrate in the opposite directions from other atoms. Softening of lattice vibrations is a phenomenon where this vibration energy is unusually weak.

 

 

Fig. 2. Relationships among strontium (Sr) concentration, softening of lattice vibrations, and superconductivity critical temperature (Tc).
Fig. 2. Relationships among strontium (Sr) concentration, softening of lattice vibrations, and superconductivity critical temperature (Tc).

Relationships between the magnitude of the softening of lattice vibrations (decrease in vibration energy) and strontium concentration (A: blue), and between Tc and strontium concentration (B: purple). When plot A increases (decreases), plot B increases (decreases). Plot A deviates from the expected linear dependence on x when there is no enhancement (proportional plot C: green).

 

Elucidating the Superconductivity Mechanisms through Studies on Electron Excitations

Dr. Ishii and colleagues have also conducted resonant inelastic X-ray scattering experiments at the JAEA Quantum Dynamics Beamline (BL11XU), and directly observed the electron excitation states of copper-oxide superconducting materials, YBa2Cu3O7-δ and Nd2-xCexCuO4. They revealed that the strong coupling of electrons, which carry charge and induce unusual metallic states, are partly responsible for superconductivity. Additionally, they demonstrated that theoretical calculations obtained from the electron correlation theory proposed by the Tohoku University group reproduce the experimental data. Moreover, Dr. Wakimoto and colleagues are the first to observe the collective excitation of a group of electrons with strong couplings in copper-oxide high-temperature superconducting materials and related nickel oxides using resonant inelastic X-ray scattering (Fig. 3). These research achievements were published three times in Physical Review Letters (2005 and 2009). “Although a complete understanding of these unusual electron behaviors and paring mechanisms of electrons has yet to be achieved, I am hopeful that our research will pave the way toward the development of room temperature superconductors,” expresses Dr. Mizuki with anticipation.

 

Fig. 3. Resonant inelastic X-ray scattering spectra.
Fig. 3. Resonant inelastic X-ray scattering spectra.

Comparison of the resonant inelastic X-ray scattering spectrum measured by the momentum variations corresponding to the periodic structure of the aligned electrons in a striped pattern (blue) and that measured by the momentum variations independent of the periodic structure (red). Signals induced by the collective excitations of electrons are observed as an enhancement in the former data (yellow circle).

 

References
1. A. Q. R. Baron, H. Uchiyama, Y. Tanaka, S. Tsutsui, D. Ishikawa, S. Lee, R. Heid, K.-P. Bohnen, S. Tajima and T. Ishikawa; Phys. Rev. Lett., 92, 197004 (2004)
2. H. Uchiyama, A. Q. R. Baron, S. Tsutsui, Y. Tanaka, W.-Z. Hu, A. Yamamoto, S. Tajima and Y. Endoh; Phys. Rev. Lett., 92, 197005 (2004)
3. T. Fukuda, A. Q. R. Baron, S. Shamoto, M. Ishikado, H. Nakamura, M. Machida, H. Uchiyama, S. Tsutsui, A. Iyo, H. Kito, J. Mizuki, M. Arai, H. Eisaki and H. Hosono; J. Phys. Soc. Jpn., 77, 103715 (2008)
4. M. Hoesch, T. Fukuda, J. Mizuki, T. Takenouchi, H. Kawarada, J. P. Sutter, S. Tsutsui, A. Q. R. Baron, M. Nagao, and Y. Takano; Phys. Rev. B, 75, 140508(R) (2007)
5. T. Fukuda1, J. Mizuki, K. Ikeuchi, K. Yamada, A. Q. R. Baron and S. Tsutsui; Phys. Rev. B, 71, 060501(R) (2005)
6. K. Ishii1, K. Tsutsui, Y. Endoh, T. Tohyama, K. Kuzushita, T. Inami, K. Ohwada, S. Maekawa, T. Masui, S. Tajima, Y. Murakami and J. Mizuki; Phys. Rev. Lett., 94, 187002 (2005)
7. K. Ishii1, K. Tsutsui, Y. Endoh, T. Tohyama, S. Maekawa, M. Hoesch, K. Kuzushita, M. Tsubota, T. Inami, J. Mizuki, Y. Murakami and K. Yamada; Phys. Rev. Lett., 94, 207003 (2005)
8. S. Wakimoto, H. Kimura, K. Ishii, K. Ikeuchi, T. Adachi, M. Fujita, K. Kakurai, Y. Koike, J. Mizuki, Y. Noda, K. Yamada, A. H. Said and Yu. Shvyd'ko; Phys. Rev. Lett., 102, 157001 (2009)