Metallic Side Of Metal-insulator Transition Research Articles

Observation of disorder induced weak localization in Gd:ZnO thin films

We have observed weak localization induced by disorder in thin films of Gd doped ZnO (Gd:ZnO) deposited by pulsed laser deposition (PLD). Disorder parameter (Kfl) of all the Gd:ZnO thin films was found to be greater than 1, which indicates that thin films of Gd:ZnO in this study are in metallic side of metal-insulator transition. However, temperature dependent resistivity studies of all the Gd:ZnO thin films exhibits negative temperature coefficient of resistance (TCR) showing semiconductor like nature at low temperatures. This anomaly in temperature dependent resistivity behaviour has been attributed to the dominant presence of weak localization. Magnetoresistance measurements confirms the dominant influence of weak localization in the conducting mechanism of Gd:ZnO films. Phase coherent length was found to be varied with the concentration of Gd and reports the highest value at ~360 nm for Gd:ZnO thin film with 1 at.% of Gd concentration. .

Critical fields and features of electromagnetic transport of Bi2Se3 whiskers at low temperatures

The temperature dependences of resistance of n-type conductivity Bi2Se3 whiskers with doping concentration (1–2)⋅1019 cm−3 were studied in the temperature range 1.6–300 K. The sharp drop of the resistance was detected that is a result of a partial transition to the superconductive state at the critical temperature Tc = 5.3 K and probably due to the inclusion of β-PdBi2 phase in the studied samples. The transverse magnetoresistance of Bi2Se3 whiskers with various concentration of Pd doping impurity corresponding to the metal side of metal-insulator transition was studied in a magnetic field of 0–10 T. Superconductivity suppression effect by the magnetic field was found that permits to create the basic parameters such as the upper critical field Bc2 = 1.5 T, the coherence length of the superconductor ξ (0) = 15 nm and the superconducting gap Δ ≈ 0.8 meV. Studies of the n-type conductivity Bi2Se3 whiskers allow to create on their basis magnetic field sensors with a sensitivity of about 3.5% / T, capable in the temperature range of 4.2–77 K. The investigated crystals can also be used in the magnetic switch-control sensors magnetic switches due to the transition to the superconducting state at the critical temperature Tc, depending on a magnetic field induction.

Tunneling anomalous Hall effect in nanogranular CoFe-B-Al-O films near the metal-insulator transition

We present results of experimental studies of structural, magneto-transport and magnetic properties of CoFe-B-Al-O films deposited onto a glass ceramic substrate by the ion-beam sputtering of the target composed of Co40Fe40B20 and Al2O3 plates. The system consists on the strained crystalline CoFe metallic nanogranules with the size 2-5 nm which are embedded into the B-Al-O oxide insulating matrix. Our investigations are focused on the anomalous Hall effect (AHE) resistivity Rh and longitudinal resistivity R at T=5-200 K on the metallic side of metal-insulator transition in samples with the metal content x=49-56 at.%, that nominally corresponds to (Co40Fe40B20)x(Al2O3)100-x in the formula approximation. The conductivity at T > 15 K follows the lnT behavior that matches a strong tunnel coupling between nanogranules. It is shown that the scaling power-laws between AHE resistivity and longitudinal resistivity strongly differ, if temperature T or metal content x are variable parameters: Rh(T)~R(T)^0.4-0.5 obtained from the temperature variation of R and Rh at fixed x, while Rh(x)/x~R(x)^0.24, obtained from measurements at the fixed low temperature region (10-40 K) for samples with different x. We qualitatively describe our experimental data in the frame of phenomenological model of two sources of AHE e.m.f. arising from metallic nanogranules and insulating tunneling regions, respectively, at that the tunneling AHE (TAHE) source is strongly shunted due to generation of local circular Hall currents. We consider our experimental results as the first experimental proof of the TAHE manifestation.

Magnetic Properties of Doped Si<B,Ni> Whiskers for Spintronics

Study the magnetic properties of Si<B,Ni> whiskers, the concentration of which corresponds to a dielectric and metal side of metal-insulator transition, is performed. Percolation laws of the magnetic clusters formation, that is important for development of spintronic devices, are considered.

PIEZORESITIVE CHARACTERISTICS OF SILICON MICROCRYSTALS AT CRYOGENIC TEMPERATURES

To create strain sensors operating at cryogenic temperatures piezoresistive characteristics of boron doped p-type Si whiskers, mounted on spring elements fabricated of steel, were studied at fixed temperatures 4.2, 77 and 300 K. It was shown that the effect of thermal strain that appears due to the difference between thermal expansion coefficients of silicon and material of the spring element (steel), when the crystals are mounted on the beam, on the piezoresistive characteristics of silicon crystals is very strong, especially at cryogenic temperatures. This fact should be accounted to develop sensors for strain measurements of constructions, fabricated from different materials. P-type Si whiskers with ρ300К =0.010 Ohm×cm with boron concentration near metalinsulator transition (MIT) from the metallic side of MIT with gauge factor GF4.2K =–1020 at the compressive strain are most suitable to create strain sensors for cryogenic temperatures, particularly, for 4.2 K. To create strain sensors for the temperature of liquid nitrogen the most suitable are p-Si whiskers with ρ300К =0.025 Ohm×cm with gauge factor GF77K =263.

Insulator-like electrical transport in Al65−x Si x Cu20Ru15 (x=0.0, 0.5, 1.0, 1.5, 2.0) icosahedral quasicrystals

Low-temperature resistivity and magneto-resistance have been measured from 2.8 to 300 K for stable icosahedral quasicrystals Al65–x Si x Cu20Ru15 (x = 0.0, 0.5, 1.0, 1.5, 2.0). The low-temperature σ–T behavior, in the temperature range 2.8–15 K, for the Si-substituted samples are drastically different from undoped samples. The in-field σ–T for the Si-undoped sample has been found to follow ln σ vs. T −1/4 below 5 K, indicating that this sample lies in the insulating side of the metal–insulator (MI) transition. The substitution of silicon at the aluminum site drives the Al–Cu–Ru quasicrystals to the metallic side of MI transition.