B. K. Cho

Superconducting state magnetization and critical fields of single- crystal YNi2B2C

Abstract Single crystals of YNi 2 B 2 C have been grown via high-temperature flux growth, using Ni 2 B as a solvent. Magnetization measurements have been carried out over a large portion of the magnetic field-temperature plane on a single crystal of YNi 2 B 2 C for field applied parallel to the c -axis in order to determine the range of thermodynamic reversibility and the critical magnetic fields. The magnetization is thermodynamically reversible within 2% of the magnetization for fields above 0.5 T and temperatures above 3 K. Near T c , the upper critical field has a slope of -0.32 T/K. The zero-temperature Ginzburg-Landau parameter κ is 13≈15, the penetration depth λ ab is about 150 nm, and the coherence distance λ ab is about 10 nm. The irreversibility line H irr ( T ) obeys H irr ( T ) = H irr (0) (1- T / T c ) 1.16 . The magnetization data below T c closely follow a logarithmic dependence on applied field, in the intermediate region, as predicted by the London model. The magnetization versus temperature curves shift uniformly to lower temperature as the field increases. Based on these measurements, YNi 2 B 2 C seems more similar to conventional low- T c superconductors than to high- T c superconductors.

Specific heat and anisotropic superconducting and normal-state magnetization of HoNi2B2C

Abstract The magnetization of single-crystal HoNi 2 B 2 C has been measured as a function of applied field ( H ) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi 2 B 2 C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c -axis and with a much weaker temperature dependence for H applied parallel to the c -axis, indicating that the Ho +3 magnetic moments lie predominately in the tetragonal a − b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T N =5.0 K. Remarkably, in 20 Oe applied field both superconductivity ( T c =8.0 K) and antiferromagnetism ( T N =5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H − T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H c2 with a deep minimum at T N =5 K. The high-field magnetization data for H applied perpendicular to the c -axis also reveal a cascade of three phase transitions for T H H versus T phase diagram for HoNi 2 B 2 C at low temperatures.

Silicon Porosification: State of the Art

This review is devoted to the analysis of the problems related to fabrication of the Si porous layers. The review was motivated by a great interest to Si-based porous materials from nano- to macro-scale for various applications in electronics, optoelectronics, photonics, chemical sensors, biosensors, etc. The peculiarities of the silicon porosification and the principles of preparing porous layers are considered in the present article. Various methods used for Si porosification such as chemical stain etching, chemical vapor etching, laser-induced etching, metal-assisted etching, spark processing and reactive ion (plasma) etching were analyzed. However, the main attention was focused on electrochemical porosification of Si. The review discusses in detail the influence of parameters such as electrolyte composition and pH, current density, etching time, temperature, wafer doping and orientation, lighting, magnetic field, and ultrasonic agitation on the process of Si porosification. It was shown that the structure of porous silicon strongly depends on both technological parameters of electrochemical etching and the parameters of the semiconductor subject to treatment. This review also addresses the main properties of porous silicon, porous multilayer and 3D structure formation, oxidation of porous Si, release of the porous layer, drying, storage, etching, filling and surface functionalizing of porous Si. Features of III-V compound porosification are also briefly analyzed.

Grain Size Effects in Sensor Response of Nanostructured SnO2- and In2O3-Based Conductometric Thin Film Gas Sensor

Based on the experimental results, obtained by studying both structural and gas-sensing properties of the SnO2 and In2O3 films deposited by the spray pyrolysis method, we analyzed the influence of crystallite size on the parameters of the SnO2- and In2O3-based thin film solid-state gas sensors. For comparison, the behavior of ceramic-type gas sensors was considered as well. In particular, we examined the correlation between the grain size and parameters of conductometric-type gas sensors such as the magnitude of sensor signal, the rate of sensor response, thermal stability, and the sensitivity of sensor signal to air humidity. Findings confirmed that that grain size is one of the most important parameters of metal oxides, controlling almost all operating characteristics of the solid state gas sensors fabricated using both the ceramic and thin film technologies. However, it was shown that there is no single universal requirement for the grain size, because changes in grain size could either improve, or worsen of operating characteristics of gas sensors. Therefore, the choice of optimal grain size should be based on the detailed consideration of all possible consequences of their influence on the parameters of sensors designed.

Possible co-existence of superconductivity and weak ferromagnetism in ErNi2B2C

Abstract Detailed measurements of the field H and temperature T dependent, anisotropic magnetization M ( T , H ) on single crystal ErNi 2 B 2 C indicate that there is a phase transition to a magnetically ordered ground state that has a weak ferromagnetic component of 0.33 μ B /Er for T 2 B 2 C, but, unlike TbNi 2 B 2 C, the ferromagnetic component exists in ErNi 2 B 2 C for temperatures and fields that are well below the T c and H c2 . Therefore, in ErNi 2 B 2 C there appears to be the coexistence of superconductivity and weak ferromagnetism for T H

Porous Semiconductors: Advanced Material for Gas Sensor Applications

The present review article is devoted to the analysis of the problems related to the design of gas sensors based on porous semiconductors (PS). The peculiarities of the semiconductor porosification by anodic etching and the principles of gas sensor design based on porous semiconductors, including gas sensor construction and main operating characteristics, are considered in the article. It is shown that the influence of the surrounding atmosphere on such parameters of porous semiconductors as refractive index, the intensity of photoluminescence, electroconductivity, dielectric constant, and surface potential might be used for gas sensor design. Based on the conducted analysis it is concluded that porous semiconductors have a great potential for the above-mentioned applications. However, the realization of those opportunities is restrained by such factors as bad reproducibility, increased temporal drift of characteristics, low selectivity, and an unsatisfactory level of understanding of the operating mechanism of sensors fabricated on the basis of porous semiconductors.

Specific heat of single-crystal YNi2B2C and TmNi2B2C superconductors

Abstract We have measured the specific heat of single crystals of superconducting YNi 2 B 2 C and TmNi 2 B 2 C. Analysis of YNi 2 B 2 C data in zero and 5 T magnetic fields indicates modestly strong electron-phonon coupling and a superconducting energy gap Δ ( O ) = 29 K . Superconductivity coexists with antiferromagnetic order below 1.5 K in TmNi 2 B 2 C. An interpretation of very low temperature specific heat indicates substantial magnetic anisotropy, with ferromagnetic planes weakly coupled antiferromagnetically to each other. Two-dimensional ferromagnetic spin waves produce a large linear-in-temperature contribution to the specific heat. We find no compelling evidence for an unconventional pairing mechanism in these systems.

Fabrication of high-density arrays of individually isolated nanocapacitors using anodic aluminum oxide templates and carbon nanotubes

We have fabricated high-density arrays of individually isolated semispherical nanocapacitors consisting of porous anodic aluminum oxide (AAO) layers as dielectric materials and carbon nanotubes (CNTs) as electrodes. It is shown that the nanocapacitors made with the CNT electrodes exhibit much better C-V behaviors than those without the CNT electrodes. The improved electrical behavior is explained in terms of the use of the CNT electrodes deposited within the porous AAO layers. The capacitance calculated using semispherical capacitor formula is in agreement with the experimental value.

Phenomenological Theory of Superconductivity and Magnetism in Ho 1 − x Dy x Ni 2 B 2 C

The coexistence of the superconductivity and magnetism in the

Magnetic field sensing scheme using CoFeB∕MgO∕CoFeB tunneling junction with superparamagnetic CoFeB layer

{\mathrm{Ho}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}