K. Takita

Origin and control of high-temperature ferromagnetism in semiconductors

The extensive experimental and computational search for multifunctional materials has resulted in the development of semiconductor and oxide systems, such as (Ga,Mn)N, (Zn,Cr)Te and HfO(2), which exhibit surprisingly stable ferromagnetic signatures despite having a small or nominally zero concentration of magnetic elements. Here, we show that the ferromagnetism of (Zn,Cr)Te, and the associated magnetooptical and magnetotransport functionalities, are dominated by the formation of Cr-rich (Zn,Cr)Te metallic nanocrystals embedded in the Cr-poor (Zn,Cr)Te matrix. Importantly, the formation of these nanocrystals can be controlled by manipulating the charge state of the Cr ions during the epitaxy. The findings provide insight into the origin of ferromagnetism in a broad range of semiconductors and oxides, and indicate possible functionalities of these composite systems. Furthermore, they demonstrate a bottom-up method for self-organized nanostructure fabrication that is applicable to any system in which the charge state of a constituent depends on the Fermi-level position in the host semiconductor.

Crystal chemistry and physical properties of La2−xSrxNiO4 (0 ≤ x ≤ 1.6)

Abstract Structural, transport, and magnetic data for the system La 2−x Sr x NiO 4 , 0 ≤ x ≤ 1.6, are reported. The oxygen content of the system was controlled by annealing under various oxygen partial pressures below 150 atm and was determined by iodometric titration. A Rietveld analysis of the room-temperature powder X-ray diffraction data showed that the tetragonal distortion of the NiO 6 octahedra decreases monotonically with increasing x for 0 ≤ x ≤ 1.4, but the La(Sr)O(2) bond length increases anomalously in the interval 0 ≤ x ≤ 0.6. A semiconductor-metal transition decreases monotonically with increasing x from about 675 K(x=0) to 20 K(x=1.2). Samples with x > 1.0 exhibited a weak ferromagnetic moment at 5 K and the onset of a positive component in the Hall effect. Comparison of the La 2−x Sr x CuO 4 and La 2−x Sr x NiO 4 systems shows that the superconductive copper oxides have the equilibrium Cu 3+ + O 2− = Cu 2+ + O − shifted to the right and, for x = 0, antiferromagnetic correlation in a half-filled σ x 2 − y 2 band whereas the weakly ferromagnetic nickel oxides have the equilibrium Ni 4+ + O 2− = Ni 3+ + O − shifted to the right, but ferromagnetic correlations in a quarter-filled σ∗ band.

X-Ray Diffraction Study on the Crystal Structure of Nd1+xBa2-xCu3O7-δ

Crystal structures in the Nd1+xBa2-xCu3O7-δ system have been investigated by X-ray powder diffraction combined with Rietveld analysis. The compound NdBa2Cu3O7-δ is isomorphic with orthorhombic YBa2Cu3O7-δ. With the increase of x the orthorhombic distortion decreases, and the compound becomes tetragonal at x=0.2. The tetragonal structure is isomorphic with the tetragonal form of YBa2Cu3O7-δ. Tc of the compounds with x=0 and 0.05 is above 90 K, which decreases monotonically with x and the compound becomes semiconducting at x=0.4.

Hole Concentration Compensation Effect and Superconducting Properties of Nd1+xBa2-xCu3O7-δ

The superconducting transition temperature Tc and normal-state resistivity have been investigated as a function of x for the Nd1+xBa2-xCu3O7-? system, whose crystal structures have been determined at room temperature by X-ray powder diffraction combined with the Rietveld method. Tc of the samples with x=0 and 0.05 is above 90 K, which decreases monotonically with x except for around x=0.25?0.30 (Tc?35 K) and the sample becomes semiconducting at x=0.40. The orthorhombic distortion of the compounds decreases with x and the compound becomes tetragonal at x=0.20. The variation of Tc with x is insensitive to the orthorhombic-tetragonal phase transition. A correlation of Tc with hole concentration in copper-oxide-type superconductors is discussed based on a possible hole concentration compensation effect of excess Nd.

Structure and properties of YBaSrCu3Oy (y=6.2−7.3)

Abstract Sintered YBaSrCu 3 O y was treated at 300–950°C under various oxygen pressures of P O 2 =0.2−5×10 4 atm. The structure as observed by powder X-ray diffraction (XRD) changed with increasing oxygen content from tetragonal ( y ⩽6.6) to orthorhombic ( y ⩽7.0) to tetragonal ( y ⩾7.0). The maximum orthorhombic distortion and the highest superconducting transition temperature where reached at y =6.90−6.95. The orthorhombic structure for y =6.95 refined by a Reitveld analysis of the XRD data was essentially the same as that of YBa 2 Cu 3 O 6.9 . On the other hand, transmission electron microscopic studies of the latter tetragonal structure strongly suggested ordering of Cu(1)-O chains in microdomains with sizes of 50×50 A 2 , the XRD “tetragonal” symmetry being only an averaged one. The transition temperature, T c , was 82 K at y =6.90, while 80 K at y =7.20. The concentration of carrier holes as estimated from the Hall effect was 4.9×10 21 cm -3 at y =6.90 and 3.9×10 21 cm -3 at y =7.20 at 100 K. Excess oxygen has been supposed to be concentrated in microdomain boundaries where Cu ions get octahedrally coordinated, leading to the reduction of the carrier concentration.

Charge transfer and superconductivity in Nd(Ba1−xNdx)2Cu3O7+x prepared in high pressure oxygen

Abstract By annealing in oxygen at high pressure around 600 ≈ 1000 atm, we have been able to make a series of the single-phase solid solution system Nd(Ba 1− x Nd x ) 2 Cu 3 O 7+ x i.e. σ = x in Nd(Ba 1− x Nd x ) 2 Cu 3 O 7+ x . x -dependences of both hole concentration and superconductivity have been investigated in this system. In this series of samples, the hole concentration deduced from the Hall effect measurement changes monotonically with x while the average charge of a [CuO] unit, i.e. the total hole concentration, deduced from a chemical analysis is not changed for all samples. This clearly demonstrates that the mobile hole concentration in this system is determined by a charge transfer between CuO 2 sheet-layer and CuO 1+ x chain-layer. This charge transfer is understood by consideration of the electrostatic energy based on a simple model. x -dependence of the superconductivity transition temperature T c is interpreted as the dependence of T c on the mobile hole concentration of the CuO 2 plane, which is expressed by an almost linear relation up to the highest T c of 95 K.

Relation between Superconducting Tc and Hole Concentration: Hall Effect Measurement and Chemical Analysis in Charge-Doped and Oxygen-Doped System Nd1+xBa2-xCu3O7-δ

Using single-phase samples of Nd1+xBa2-xCu3O7-δ sintered under identical conditions, both the Hall effect measurement and the chemical analysis of the average charge of a [Cu-O] unit have been performed. The plot of the superconducting Tc against effective hole concentration pH (=1/RHe) deduced from the Hall coefficient just above Tc, RH, shows a simple relation between them. A similar plot against hole concentration estimated from the average charge shows a different relation. An estimation of hole concentration in two-dimensional CuO2 sheets, however, based on an assumption proposed by Tokura et al., gives almost the same value to pH, at least below 80 K. This strongly suggests that the superconductivity of YBa2Cu3O7-δ-like superconductors is determined uniquely by the hole concentration in CuO2 sheets which can be deduced from the Hall coefficient just above Tc.

Comment on the de Haas-van Alphen Effect in the Superconducting Mixed State of 2H–NbSe 2

We present the de Haas-van Alphen (dHvA) effect in the superconducting mixed state of 2H–NbSe 2 . Our data are consistent with the dHvA results obtained by Graebner and Robbins, which have been the subject of controversy from the viewpoint of experiment and theory. We discuss the reason why the theoretically expected cylindrical Fermi surfaces are not detected in the dHvA experiment and also the mechanism of the dHvA effect in the mixed state.

Suppression of ferromagnetism due to hole doping in Zn1−xCrxTe grown by molecular-beam epitaxy

Electric and magnetic properties were investigated on p-type Zn1−xCrxTe doped with nitrogen (N) as an acceptor. Thin films of p-Zn1−xCrxTe(x≦0.09) were grown by molecular-beam epitaxy with the supply of N2 gas excited by rf plasma. With the increase of Cr composition x at an almost fixed N concentration of the order of 1020cm−3, the temperature dependence of resistivity changed from metallic behavior to an insulating one, accompanied with a significant decrease of the hole concentration. The magnetization measurements revealed that ferromagnetic behaviors observed in undoped Zn1−xCrxTe were suppressed due to the nitrogen doping; with N concentrations of the order of 1020cm−3, hysteresis loops in the magnetization curve disappeared, the magnitude of magnetization decreased, and the ferromagnetic transition were not observed down to 2 K according to the Arrott plot analysis. These experimental findings are discussed on the basis of the ferromagnetic double exchange interaction which is considered to work on...

Peak effect and dynamic melting of vortex matter in NbSe2 crystals

We present a mode locking (ML) phenomenon of vortex matter observed around the peak effect regime of 2H-NbSe2 pure single crystals. The ML features allow us not only to trace how the shear rigidity of driven vortices persists on approaching the second critical field, but also to demonstrate a dynamic melting transition of driven vortices at a given velocity. We observe the velocity dependent melting signatures in the peak effect regime, which reveal a crossover between the disorder-induced transition at small velocity and the thermally induced transition at large velocity. This uncovers the relationship between the peak effect and the thermal melting.