Towards higher-Tc superconductors

Abstract

New superconductors discovered in the Akimitsu laboratory are reviewed here. These materials can be categorized into two groups:1) Cu-oxide superconductors.1-1 Cu-oxide system having CuO₂ planes.1-2 Ladder lattice superconductor.2) Exploration of new metal-based superconductors.2-1 MgB₂ and its application.2-2 Y₂C₃.2-3 Carrier-doped wide-gap semiconductors.2-4 New superconductor with a cage-type structure: R₅T₆Sn₁₈ (R = Sc, Y, Lu; T = Rh, Ir).Finally, all of the new superconductors discovered in our laboratory are summarized. The outlook for the high-T_c superconductors and our present work are also described.

Keywords: Cu-oxide system; Ir oxide; MgB2; doped semiconductors; high-Tc superconductors; ladder superconductor.

Figure 1.

Phase diagram of the electron-phonon coupling constant V vs. superconducting critical temperature T_c. (N. Tsuda, K. Nasu, J. Fujimori and K. Siratori: Electronic Conduction in Oxide (in Japanese) p. 120.).

Figure 2.

(a) Normalized resistance as a function of the temperature for Nb_1−xTa_xSe₃ with several Ta concentrations. (b) Ta concentration dependence of T_c. Substituting Ta to CDW material NbSe₃, forming Nb_1−xTa_xSe₃ to be superconductive.¹⁾ ○ K. Kawabata,²⁾ ● Our data.¹⁾

Figure 3.

Temperature dependence of resistance in Bi₂Sr₂CuO₆. The inset shows the crystal structure.⁵⁾

Figure 4.

Layered structure of (a) Bi₂Sr₂CuO₆ and (b) Bi₂Sr₂CaCu₂O₈.

Figure 5.

Superconductivity in (Nd, Sr, Ce)₂CuO₄: T* structure. (a) Crystal structure of T* structure. (b) Temperature dependence of resistivity in T* structure.^8,9)

Figure 6.

Three types of “214” structure: (a) (La_1−xSr_x)₂CuO₄: T structure, (b) (Nd_1−xCe_x)₂CuO₄: T′ structure, (c) (Nd, Sr, Ce)₂CuO₄: T* structure.

Figure 7.

Concept of the block layer. CuO₂ layers are inserted between block layers.¹¹⁾

Figure 8.

Phase diagram of Sr₂CuO₄(CO₃)_1−x(BO₃)_x. Carriers can be controlled by (BO₃) content.¹³⁾

Figure 9.

Three types of superconductor composed of different block layers including the carbonate layers. (a) Sr₂CuO₂(CO₃)_1−x(BO₃)_x (T_c = 55 K) (b) Sr₂CaCu₂O₄(CO₃)_1−x(BO₃)_x (T_c = 105 K) (c) Sr₂Ca₂CuO₄(CO₃)_1−x (BO₃)_x (T_c = 115 K).¹⁵⁾

Figure 10.

Two types of Cu-O planes. (a) Two-dimensional CuO₂ planes, (b) Two-leg ladder Cu₂O₃ planes.

Figure 11.

Crystal structure of telephone number compound Sr₁₄Cu₂₄O₄₁, being composed of 1D-chain layer and 2-leg ladder layer.

Figure 12.

(a) Temperature dependence of electrical resistivity in Sr_14−xCa_xCu₂₄O₄₁. With increasing x (Ca content), resistivity is decreased. (b) Pressure dependence of Sr_0.4Ca_13.6Cu₂₄O_41.84. Superconductivity appears at 3 GPa.¹⁹⁾

Figure 13.

Electrical resistivity and magnetic susceptibility of MgB₂ as a function of the temperature.²¹⁾

Figure 14.

Crystal structure of MgB₂.²¹⁾

Figure 15.

The band structure and the Fermi surface of MgB₂.²²⁾

Figure 16.

Two fabrication methods to make a MgB₂ wire. 1) In situ powder-in-tube (PIT) method. 2) Internal Mg diffusion (IMD) method.²³⁾

Figure 17.

Temperature dependence of B_c2 in MgB₂ wires.²³⁾

Figure 18.

Present status of critical current density J_c (A/cm²) vs. magnetic field B (T).²³⁾

Figure 19.

Temperature dependence of resistivity and susceptibilities in Y₂C₃.^25,27)

Figure 20.

Crystal structure of Y₂C₃ to be the Pu₂C₃-type with (b.c.c.) structure.

Figure 21.

Temperature dependence of the muon spin relaxation rates of La₂C₃ and Y₂C₃. Solid and dashed lines are fitted with the s-wave’s phenomenologial double-gap model.³⁰⁾

Figure 22.

Crystal structure of the 3C-SiC and 6H-SiC, respectively.

Figure 23.

Temperature dependences of the electrical resistivities and magnetic susceptibilities.³⁶⁾

Figure 24.

H-T phase diagram for (a) B-doped 3C-SiC(3C-SiC:B) and (b) B-doped 6H-SiC(6H-SiC:B), determined from the onset of superconductivity during a T-scan and H-scan of resistivity.

Figure 25.

Temperature dependences of the electrical resistivity and magnetic susceptibility in SiC(3C-SiC:Al), where 3C-SiC was doped with Al.³⁶⁾

Figure 26.

Temperature dependence of the upper critical field H_c2 and the irreversibility field H_irr in 3C-SiC:Al.³⁶⁾

Figure 27.

Cage-type crystal structure of R₅T₆Sn₁₈ (R = Sc, Y, Lu; T = Rh, Ir).³⁸⁾

Figure 28.

Temperature dependence of the magnetic susceptibility in R₅Rh₆Sn₁₈ (R: Sc, Y and Lu).³⁸⁾

Figure 29.

Quasiparticle state density (γ) vs. magnetic field.³⁸⁾

Figure 30.

Typical examples of superconductivities and crystal structures chosen from Table 5. (a) Cr₂Re₃B,⁶¹⁾ (b) NaAlSi⁶²⁾ and (c) Li_xIrSi₂.

Figure 31.

(a) Crystal structure of La₂CuO₄ and Sr₂IrO₄. (b) Schematic diagram of the 5d energy levels split by the crystal field (Δ), spin–orbit coupling (SOC), and on-site Coulomb repulsion (U) leading to the formation of unoccupied (upper) and occupied (lower) Hubbard bands at around the Fermi level from the J_eff = 1/2 band and the fully occupied J_eff = 3/2 band.

Figure 32.

(a)–(c) Second derivative of ARPES intensity plots as a function of the binding energy and wave vector on Sr_2−xLa_xIrO₄ (x = 0, 0.04 and 0.08) measured along the k direction. (d) Schematic graph of the doping-induced change in the electronic structure of Sr_2−xLa_xIrO₄ derived from our ARPES study.⁶⁹⁾

Figure 33.

Magnetic phase diagram of Sr_2−xLa_xIrO₄ determined from magnetic susceptibility and µSR measurements.⁷⁰⁾