Low Temperature Conductivity in n

We investigate the transport properties of n-type noncompensated silicon below the insulator-metal transition by measuring the electrical and magnetoresistances as a function of temperature T for the interval 2–300 K. Experimental data are analyzed taking into account possible simple activation and hopping mechanisms of the conductivity in the presence of two impurity bands, the upper and lower Hubbard bands (UHB and LHB, resp.). We demonstrate that the charge transport develops with decreasing temperature from the band edge activation (110–300 K) to the simple activation with much less energy associated with the activation motion in the UHB (28–90 K). Then, the Mott-type variable range hopping (VRH) with spin dependent hops occurs (5–20 K). Finally, the VRH in the presence of the hard gap (HG) between LHB and UHB (2–4 K) takes place. We propose the empiric expression for the low T density of states which involves both the UHB and LHB and takes into account the crossover from the HG regime to the Mott-type VRH with increasing temperature. This allows us to fit the low T experimental data with high accuracy

Low temperature conductivity of silicon doped with antimony

A detailed analysis of the experimental temperature dependences of the resistivity of silicon doped with arsenic with a concentration of 10*18 cm–3 is performed for the region 1.8 K < T < 25 K. It is shown that, as a result of cooling to a temperature lower than 4.5 K, a transition from the Mott mode with variable hopping length to the mode of hopping conduction via nearest neighbors is observed, while, at T < 2.5 K, a transition to the Shklovskii–Efros mechanism is possible. A model for such a temperature crossover is suggested; the model is based on simplified solution of the percolation problem with the use of an interpolation expression for the density of states. Performed estimates show that the model is in satisfactory agreement with experimental data when the minimum number of adjustable parameters are used

Low temperature conductivity in n-type noncompensated silicon below insulator-metal transition

We investigate the transport properties of -type noncompensated silicon below the insulator-metal transition by measuring the electrical and magnetoresistances as a function of temperature for the interval 2–300K. Experimental data are analyzed taking into account possible simple activation and hopping mechanisms of the conductivity in the presence of two impurity bands, the upper and lower Hubbard bands (UHB and LHB, resp.). We demonstrate that the charge transport develops with decreasing temperature from the band edge activation (110–300K) to the simple activation with much less energy associated with the activation motion in the UHB (28–90 K). Then, the Mott-type variable range hopping (VRH) with spin dependent hops occurs (5–20 K). Finally, the VRH in the presence of the hard gap (HG) between LHB and UHB (2–4 K) takes place. We propose the empiric expression for the low density of states which involves both the UHB and LHB and takes into account the crossover from the HG regime to the Mott-type VRH with increasing temperature. This allows us to fit the low experimental data with high accuracy

Magnetic relaxation experiments in CNT-based magnetic nanocomposite

In this work, we discuss the relaxation of the magnetic moments in a novel carbon nanotube (CNT)-based nanocomposite synthesized by using chemical vapor deposition process. The material consists of a matrix of CNT filled by Fe-based nanoparticles. This structure is seen clearly by scanning and transmission. X-ray diffraction and Raman spectroscopy are used to detect the predominant Fe3C phase and the CNT presence in the sample, respectively. The results obtained from both hysteresis cycles, M(H), and zero field cooled-field cooled (ZFC-FC) measurements confirm that the material is characterized by both a strong ferromagnetic exchange and random magnetic anisotropy. For the first time, we have been able to fit the magnetic relaxation data, M(t), by using both the two distributions of nanoparticles data deduced from the ZFC-FC data and the temperature dependence of the magnetic anisotropy obtained from the law of approach to saturation in random magnets

Manifestation of coherent magnetic anisotropy in a carbon nanotube matrix with low ferromagnetic nanoparticle content

The influence of the magnetic medium can lead to peculiar interaction between ferromagnetic nanoparticles (NPs). Most research in this area involves analysis of the interplay between magnetic anisotropy and exchange coupling. Increasing the average interparticle distance leads to the dominant role of the random magnetic anisotropy. Here we study the interparticle interaction in a carbon nanotube (CNT) matrix with low ferromag netic NP content. Samples were synthesized by fl oating catalyst chemical vapor deposition. We fo und that below some critical NP concentration, when NPs are intercalated only inside CNTs, and at low temperatures, th eextendedmagnetic order, of up to 150 nm, presents in our samples. It is shown by analyzing the correlation functions of the magnetic anisotropy axes that the extended order is not simply due to random anisotropy but is associated with the coherent magnetic anisotropy, which is strengthened by the CNT alignment. With increasing temperature the extended magnetic order is lost. Above the critical NP concentration, when NPs start to be intercalated not only into inner CNT channels, but also outside CNTs, the coherent anisotropy weakens and the exchange coupling dominates in the whole temperature range. We can make a connection with the various correlation functions using the generalized expression for the law of the approach to saturation and show that these different correlation functions re fl ect the peculiarities in the interparticle interaction inside CNTs. Moreover, we can extract such important micromagnetic parameters like the exchange field, local fields of random and coherent anisotropies, as well as their temperature and NP concentration dependencies

Manifestation of coherent magnetic anisotropy in a carbon nanotube matrix with low ferromagnetic nanoparticle content

The influence of the magnetic medium can lead to peculiar interaction between ferromagnetic nanoparticles (NPs). Most research in this area involves analysis of the interplay between magnetic anisotropy and exchange coupling. Increasing the average interparticle distance leads to the dominant role of the random magnetic anisotropy. Here we study the interparticle interaction in a carbon nanotube (CNT) matrix with low ferromagnetic NP content. Samples were synthesized by floating catalyst chemical vapor deposition. We found that below some critical NP concentration, when NPs are intercalated only inside CNTs, and at low temperatures, the extended magnetic order, of up to 150 nm, presents in our samples. It is shown by analyzing the correlation functions of the magnetic anisotropy axes that the extended order is not simply due to random anisotropy but is associated with the coherent magnetic anisotropy, which is strengthened by the CNT alignment. With increasing temperature the extended magnetic order is lost. Above the critical NP concentration, when NPs start to be intercalated not only into inner CNT channels, but also outside CNTs, the coherent anisotropy weakens and the exchange coupling dominates in the whole temperature range. We can make a connection with the various correlation functions using the generalized expression for the law of the approach to saturation and show that these different correlation functions reflect the peculiarities in the interparticle interaction inside CNTs. Moreover, we can extract such important micromagnetic parameters like the exchange field, local fields of random and coherent anisotropies, as well as their temperature and NP concentration dependencies

Charge properties of a MOS transistor structure with a channel made of a two-dimensional crystal

For further improvement of efficiency and speed of field transistors the application of semiconductor two-dimensional crystals is possible. Such transistors are devoid of some negative effects, appearing in traditional MOS transistors while decreasing their dimensions. In this paper the model has been proposed and the charge properties of the transistor MOS structure with the channel made of two-dimensional crystal have been investigated. The numerical modeling of such characteristics has been performed in the range of variation of electro-physical properties of 2D-crystals, typical for MoSe, WS, WSe, ZrSe, HfSe, PtTe. A self-consistent relationship through the chemical potential between the electrophysical parameters of the structure has been established, as well as the influence of potential of field electrode and the gate dielectric potential on them. The performed calculations of the steepness of the transfer characteristic and the amplification coefficient of such transistor structure have shown that for the channel made of the transition metals dichalcogenides with the bandgap in the range of 0.25-2.1 eV the values of given parameters can reach 0.1 mA/V and 1000, respectively

Development of aerogel Cherenkov counters at Novosibirsk

Abstract The work on aerogel Cherenkov counters was started in Novosibirsk in 1986. Production of aerogels with refractive indices of 1.006–1.13 and thicknesses of blocks up to 50 mm was developed. The light absorption length at 400 nm is 5–7 m, the scattering length is 4–5 cm. By these parameters, the Novosibirsk aerogel is one of the best in the world. The ASHIPH Cherenkov counters with light collection on wavelength shifters have been developed. The ASHIPH system of the KEDR detector contains 1000 l of aerogel. The π / K separation is 4.5 σ . A project of ASHIPH counters for the SND detector has been developed. Aerogel RICH for LHCb gives a possibility to identify hadrons in the momentum range of 2–10 GeV/c. The Novosibirsk group is developing an aerogel RICH for the endcap for the SuperBaBar project. Calculations performed by a group of physicists from Novosibirsk and DESY-Zeuthen have shown that aerogel radiators enable to achieve time resolution up to 20 fs

Development of aerogel Cherenkov detectors at Novosibirsk

Abstract The development of aerogel Cherenkov counters with the light collection using a wavelength shifter is described. 80 counters of this type are working in the KEDR detector. A project of similar counters for the SND detector based on "heavy" aerogel with n = 1.13 has been developed. Aerogel with a refractive index of 1.006–1.13 and dimensions of blocks up to 200 × 200 × 50 mm 3 is produced by the Novosibirsk group for use in Cherenkov counters of different types. The Novosibirsk group is participating in the development of LHCb RICH as well as a beam diagnostics for a photo-injector test facility at DESY–Zeuthen. Recently we started development of RICH based on focusing aerogel (FARICH) for the endcap of the SuperBaBar. For the first time in the world the focusing aerogel with layers of different refractive indices has been produced