马克·加格布斯:美式风情

在这个大选之年,关于美国是什么,不是什么的话题已经很多.对于作家、音乐家、艺术家、时装设计师,尤其是那些喜欢从各种复杂、矛盾的角度来思考美国生活和历史的人来说,这个大选就是一处可以催生无数话题的肥沃土壤.     

Features of the conduction mechanisms of the n-HfNiSn semiconductor heavily doped with the Co acceptor impurity

The crystalline structure, electron density distribution, energy and kinetic parameters of a HfNi_{1 ? x} Co _x Sn semiconductor heavily doped with a Co acceptor impurity are studied in the ranges T = 80?C1620 K and N _A ^Co from 9.5 × 10²⁰ cm^?3 (at x = 0.05) to 7.6 × 10²¹ cm^?3 (at x = 0.40). It is shown that variations in the activation energy of hopping conduction ? ₃ ^?? (x) and the modulation amplitudes of continuous energy bands ? ₃ ^?? (x) are caused by the appearance of a donor source in the n-type HfNi_{1 ? x} Co _x Sn semiconductor. It is shown that the doping of n-HfNiSn with a Co acceptor impurity is accompanied by a change in the degree of compensation of the semiconductor due to the simultaneous generation of both structural acceptor-type defects during Ni atom substitution with Co atoms and structural donor-type defects during the partial occupation of Ni sites by Sn atoms. The results are discussed within the Shklovskii-Efros model for a heavily doped and compensated semiconductor.     

Features of conductivity mechanisms in heavily doped compensated V<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Ti<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>FeSb Semiconductor

The crystal and electronic structure and also the energy and kinetic properties of n-VFeSb semiconductor heavily doped with the Ti acceptor impurity are investigated in the temperature and Ti concentration ranges of T = 4.2–400 K and N_A^Ti ≈ 9.5 × 10¹⁹–3.6 × 10²¹ cm^–3 (x = 0.005–0.20), respectively. The complex mechanism of the generation of acceptor and donor structural defects is established. It is demonstrated that the presence of vacancies at Sb atomic sites in n-VFeSb gives rise to donor structural defects (“a priori doping”). Substitution of the Ti dopant for V in VFeSb leads simultaneously to the generation of acceptortype structural defects, a decrease in the number of donor defects, and their removal in the concentration range of 0 ≤ x ≤ 0.03 via the occupation of vacancies by Sb atoms, and the generation of donor defects due to the occurrence of vacancies and an increase in their number. The result obtained underlies the technique for fabricating new n-VFeSb-based thermoelectric materials. The results are discussed in the context of the Shklovsky–Efros model for a heavily doped compensated semiconductor.     

Magnetic field-induced superconductivity in the ferromagnetic state of HoMo6S8

The Chevrel-phase HoMo₆S₈ is one of the few known ferromagnetic reentrant superconductors, having an upper superconductive critical temperatureT _c1 =1.78 K, a Curie temperatureT _m =0.75 K, and reentering into a normal ferromagnetic state belowT _c2 =0.64 K. After a brief survey of the characteristic properties of HoMo₆S₈, we present new results obtained by simultaneous resistance and magnetization measurements on a single crystal of HoMo₆S₈, performed in the whole temperature range 35mKT4.2 K and for several orientations of the easy magnetization axis with respect to the applied magnetic field. Magnetic field-induced superconductive transitions are observed well belowT _c2 in the ferromagnetic state. Those transitions can be understood as the first experimental observation of a purely electromagnetic effect earlier suggested by Ginzburg: if the internal field is antiparallel to the spontaneous magnetization, the magnetic induction may be reduced below its critical value and a superconductive transition may take place. It is possible if exchange interactions are negligible; our results concerning the anisotropy of the internal upper critical fieldH _c2 are consistent with that requirement. We also discuss the possible origins for the low-resistance state in zero field: it might be due to domain wall superconductivity.     

Features of the band structure and conduction mechanisms of <Emphasis Type="Italic">n</Emphasis>-HfNiSn heavily doped with Y

The crystalline and electronic structures, energy, kinetic, and magnetic characteristics of n-HfNiSn semiconductor heavily doped with Y acceptor impurity are studied in the ranges: T = 80–400 K, N _A ^Y ≈ 1.9 × 10²⁰–5.7 × 10²¹ cm^–3 (x = 0.01–0.30), and H ≤ 10 kG. The nature of the mechanism of structural defect generation is determined, which leads to a change in the band gap and the degree of semiconductor compensation, the essence of which is the simultaneous reduction and elimination of structural donor-type defects as a result of the displacement of ~1% of Ni atoms from the Hf (4a) site, and the generation of structural acceptor-type defects by substituting Hf atoms with Y atoms at the 4a site. The results of calculations of the electronic structure of Hf_1–x Y _x NiSn are in agreement with the experimental data. The discussion is performed within the Shklovskii–Efros model of a heavily doped and compensated semiconductor.     

Features of conductivity of the intermetallic semiconductor n-ZrNiSn heavily doped with a Bi donor impurity

The crystal structure, distribution of the electron density of states, and the energy, kinetic, and magnetic properties of the intermetallic semiconductor n-ZrNiSn heavily doped with a Bi donor impurity have been investigated in the ranges T = 80?400 K, N _D ^Bi ?? 9.5 × 10¹⁹cm^?3 (x = 0.005)?1.9 × 10²¹ cm^?3 (x = 0.10), and H ?? 0.5 T. It has been established that such doping generates two types of donor-like structural defects in the crystal, which manifest themselves in both the dependence of the variation in the unit cell parameter a(x) and temperature dependence of resistivity ln??(1/T) of ZrNiSn_{1 ? x} Bi _x (x = 0.005). It is shown that ZrNiSn_{1 ? x} Bi _x is a new promising thermoelectric material, which converts thermal energy to electric energy much more effectively as compared to n-ZrNiSn. The results obtained are discussed within the Shklovskii-Efros model of a heavily doped and strongly compensated semiconductor.     

Effect of the accumulation of excess Ni atoms in the crystal structure of the intermetallic semiconductor n-ZrNiSn

The crystal structure, electron density distribution, and energy, kinetic, and magnetic properties of the n-ZrNiSn intermetallic semiconductor heavily doped with a Ni impurity are investigated. The effect of the accumulation of an excess number of Ni_{1 + x} atoms in tetrahedral interstices of the crystal structure of the semiconductor is found and the donor nature of such structural defects that change the properties of the semiconductor is established. The results obtained are discussed within the Shklovskii-Efros model of a heavily doped and strongly compensated semiconductor.     

Features of conduction mechanisms in n-HfNiSn semiconductor heavily doped with a Rh acceptor impurity

The crystal structure and electron-density distribution, as well as the energy, kinetic, and magnetic characteristics of n-HfNiSn intermetallic semiconductor heavily doped with a Rh acceptor impurity in the temperature range T = 80–400 K, in the acceptor-concentration range N _A ^Rh ≈ 9.5 × 10¹⁹?1.9 × 10²¹ cm^?3 (x = 0.005–0.10), and in magnetic fields H ≤ 10 kG are investigated. It is established that doping is accompanied by a simultaneous decrease in concentration, the elimination of donor-type structural defects (to x ≈ 0.02), and an increase in the concentration of acceptor-type structural defects (0 < x ≤ 0.10). The dependence of the degree of semiconductor compensation on temperature is revealed. A model of the spatial arrangement of atoms in HfNi_{1 ? x} Rh _x Sn is proposed, and the results of calculating the electron structure based on this model agree with the results of investigations of the kinetic and magnetic characteristics of the semiconductor. The results are discussed within the context of the Shklovskii-Efros model for a heavily doped and compensated semiconductor.