Principal Research Results

Addressing the Mechanism of High-Temperature Superconductivity

BackgroundIf a “room-temperature superconductor” (a material which conducts electrical current without any loss at room temperature)
is discovered, it will revolutionize a wide range of industries, including the electric power industry. Hence, CRIEPI has been actively
advancing the basic research of high-temperature superconductivity (HTS) to elucidate its mechanism, with the ultimate goal of finding a recipe for synthesizing a room-temperature superconductor.

Objectives
Currently, the group at CRIEPI is leading the HTS research community in terms of producing high-quality single crystals ofhigh-temperature superconductors and making best measurements of their electrical transport properties; at the same time, a worldwide network of collaborations has been established, placing CRIEPI at the “hub” of the network, to employ various experimental
tools to study the HTS from all possible aspects. Here, as examples of such collaborations, we present (i) ultra-low-temperature measurements of the Hall coefficient under very high magnetic fields, done in collaboration with the National High Magnetic Field
Laboratory of the United States, and (ii) scanning-tunneling-microscopy (STM) experiments to directly observe the electronic states,
done in collaboration with Prof. Yazdani’s group at University of Illinois at Urbana-Champaign.

Principal Results(1) The Hall coefficient, RH, reflects the collective state of electrons in materials. We measured the RH in a series of Bi-based hightemperature superconductors by applying very high magnetic field of 550,000 Gauss (about a million times larger than the geomagnetic field) to suppress superconductivity down to ultra-low temperatures below 1 K. We discovered that at low temperatures
the doping dependence of RH presents a sharp cusp at the optimum doping level (15%), where the superconducting transition temperature is maximal (Fig. 1). This result indicates that the electronic states to host the superconductivity are of different natureacross the optimum doping. Since collective electron systems are known to become unstable near a “quantum critical point” (QCP)
where two different ground states meet, the present result suggests that the quantum fluctuations associated with a QCP are the driving force of the high-temperature superconductivity. This result was published in Nature1).
(2) By directly observing the spatial arrangement of the electronic density at the surface of a Bi-based high-temperature superconductor above the superconducting transition temperature, we discovered that the electrons self-organize into a peculiar ordered state(Fig. 2) before the superconductivity sets in. Normally, the flow of the electrical current is hindered when the electrons are ordered,
but the present result suggests that in high-temperature superconductors an ordered state of electrons is a precursor to superconductivity and leads to the state where the current flow becomes the easiest. This result demonstrates an unexpected intimacy between the electron self-organization and the superconductivity, and was published in Science2).

Future Developments
Now that such peculiar phenomena as the quantum fluctuations and the electron self-organization are found to be fundamentally related to the occurrence of the superconductivity, we will sort out the exact role of these phenomena and elucidate the mechanism of the HTS.
Main Researcher: Yoichi Ando, Ph.D.,
Sector Leader, Materials Physics and Synthesis, Materials Science Research Laboratory.
References
1) F. F. Balakirev, J. B. Betts, A. Migliori, S. Ono, Y. Ando, G. S. Boebinger, 2003, “Signature of optimal doping in Hall-effect measurements on a high-temperature superconductor”, Nature 424, 912.
2) M. Vershinin, S. Misra, S. Ono, Y. Abe, Y. Ando, and A. Yazdani, 2004, “Local Ordering in the Pseudogap State of the High-Tc
Superconductor Bi2Sr2CaCu2O8+δ”, Science 303, 1995.

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10. Advanced Basic Technologies - High temperature super conductivity

Fig.1 3D plot of the temperatureand carrier-doping dependencesof the Hall density, which isessentially an inverse of the RH
and reflects the collective state ofelectrons, measured under very
high magnetic field of 550,000
Gauss that is strong enough todestroy the superconducting
state. The carrier doping of 0.15
(i.e., 15% per Cu) corresponds tothe optimum doping level where
the superconductivity becomes
the strongest. As a result of acompetition
betweentwo
different collective states ofelectrons that alternate acrossthe optimum doping, the Halldensity shows a sharp cusp (redarrow)nearabsolute-zerotemperature.

Fig.2 Spatial distribution of the electronic density of a Bi-based high-temperature superconductor
measured at -173℃, which is above the superconducting transition temperature, using a scanningtunneling microscope (STM). In this measurement, the change of the tunneling current isregistered while keeping the distance between the STM tip and the specimen, so that the regionswith higher electronic density show up as bright spots. As is enclosed by yellow dashed circles in
the figure, the electrons in this material appear to form
up into a two-dimensional ordered state.

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blobs

of about 1 nm in diameter and line