Aluminum Single Electron Transistors with Islands Isolated from a Substrate

The low-frequency noise figures of single-electron transistors (electrometers) of traditional planar and new stacked geometry were compared. We observed a correlation between the charge noise and the contact area of the transistor island with a dielectric substrate in the set of Al transistors located on the same chip and having almost similar electric parameters. We have found that the smaller the contact area the lower the noise level of the transistor. The lowest noise value 8*10E-6 e/sqrt(Hz) at f = 10 Hz. has been measured in a stacked transistor with an island which was completely isolated from a substrate. Our measurements have unambiguously indicated that the dominant source of the background charge fluctuations is associated with a dielectric substrateComment: Review paper, latex, 10 pages, 7 figures, to be publ. in JLTP, 2000; Proceeding of "Electron Transport in Mesoscopic Systems", August 12-15, 1999 Geteborg, Sweden, http://fy.chalmers.se/meso_satellite/index.html See also LT22 manuscript: http://lt22.hut.fi/cgi/view?id=S1113

Noise in Al single electron transistors of stacked design

We have fabricated and examined several Al single electron transistors whose small islands were positioned on top of a counter electrode and hence did not come into contact with a dielectric substrate. The equivalent charge noise figure of all transistors turned out to be surprisingly low, (2.5 - 7)*10E-5 e/sqrt(Hz) at f = 10 Hz. Although the lowest detected noise originates mostly from fluctuations of background charge, the noise contribution of the tunnel junction conductances was, on occasion, found to be dominant.Comment: 4 pages of text with 1 table and 5 figure

Metallic single-electron transistor without traditional tunnel barriers

We report on a new type of single-electron transistor (SET) comprising two highly resistive Cr thin-film strips (~ 1um long) connecting a 1 um-long Al island to two Al outer electrodes. These resistors replace small-area oxide tunnel junctions of traditional SETs. Our transistor with a total asymptotic resistance of 110 kOhm showed a very sharp Coulomb blockade and reproducible, deep and strictly e-periodic gate modulation in wide ranges of bias currents I and gate voltages V_g. In the Coulomb blockade region (|V| < 0.5 mV), we observed a strong suppression of the cotunneling current allowing appreciable modulation curves V-V_g to be measured at currents I as low as 100 fA. The noise figure of our SET was found to be similar to that of typical Al/AlOx/Al single-electron transistors.Comment: 5 pages incl. 4 fig

Highly mobile carriers in orthorhombic phases of iron-based superconductors FeSe1−x{}_{1-x}Sx{}_{x}

The field and temperature dependencies of the longitudinal and Hall resistivity have been measured for FeSe${}_{1-x}$S${}_{x}$ (x=0.04, 0.09 and 0.19) single crystals. The sample FeSe${}_{0.81}$S${}_{0.19}$ does not show a transition to an orthorhombic phase and exhibits at low temperatures the transport properties quite different from those of orthorhombic samples. The behavior of FeSe${}_{0.81}$S${}_{0.19}$ is well described by the simple two band model with comparable values of hole and electron mobility. In particular, at low temperatures the transverse resistance shows a linear field dependence, the magnetoresistance follow a quadratic field dependence and obeys to Kohler's rule. In contrast, Kohler's rule is strongly violated for samples having an orthorhombic low temperature structure. However, the transport properties of the orthorhombic samples can be satisfactory described by the three band model with the pair of almost equivalent to the tetragonal sample hole and electron bands, supplemented with the highly mobile electron band which has two order smaller carrier number. Therefore, the peculiarity of the low temperature transport properties of the orthorhombic Fe(SeS) samples, as probably of many other orthorhombic iron superconductors, is due to the presence of a small number of highly mobile carriers which originate from the local regions of the Fermi surface, presumably, nearby the Van Hove singularity points

Magnetotransport properties of FeSe in fields up to 50T

Magnetotransport properties of the high-quality FeSe crystal, measured in a wide temperature range and in magnetic fields up to 50 T, show the symmetry of the main holelike and electronlike bands in this compound. In addition to the main two bands, there is also a tiny, highly mobile, electronlike band which is responsible for the non-linear behavior of $\rho_{xy}$(B) at low temperatures and some other peculiarities of FeSe. We observe the inversion of the $\rho_{xx}$ temperature coeficient at a magnetic field higher than about 20 T which is an implicit conformation of the electron-hole symmetry in the main bands.Comment: MISM 201

Majority carrier type inversion in FeSe family and "doped semimetal" scheme in iron-based superconductors

The field and temperature dependencies of the longitudinal and Hall resistivity have been studied for high-quality FeSe${}_{1-x}$S${}_{x}$ (x up to 0.14) single crystals. Quasiclassical analysis of the obtained data indicates a strong variation of the electron and hole concentrations under the studied isovalent substitution and proximity of FeSe to the point of the majority carrier-type inversion. On this basis, we propose a `doped semimetal' scheme for the superconducting phase diagram of the FeSe family, which can be applied to other iron-based superconductors. In this scheme, the two local maxima of the superconducting temperature can be associated with the Van Hove singularities of a simplified semi-metallic electronic structure. The multicarrier analysis of the experimental data also reveals the presence of a tiny and highly mobile electron band for all the samples studied. Sulfur substitution in the studied range leads to a decrease in the number of mobile electrons by more than ten times, from about 3\% to about 0.2\%. This behavior may indicate a successive change of the Fermi level position relative to singular points of the electronic structure which is consistent with the `doped semimetal' scheme. The scattering time for mobile carriers does not depend on impurities, which allows us to consider this group as a possible source of unusual acoustic properties of FeSe

Multiband effect in elastoresistance of Fe(Se,Te)

We have investigated the elastoresistance of two FeSe${}_{1-x}$Te${}_{x}$ (x about 0.4 - 0.5) compounds that have a close chemical composition but differ significantly in electronic properties. The first compound has a negative temperature coefficient of resistance and does not show any phase transitions other than superconducting. The elastoresistance of this compound approximately follows $1/T$ low as it usually occurs in Fe(Se,S) with metallic conductivity. The second compound has a metallic type of conductivity and in addition to the superconducting transition, there is also a phase transition at a temperature of about 30 K. The elastoresistance of the second compound is sign-reversing and can be approximated with the sum of two Curie-Weiss type terms with opposite signs and different critical temperatures which suggest a competition of contributions to the elastoresistance from different band valleys

Multiband Effect in Elastoresistance of Fe(Se,Te)

We have investigated the elastoresistance of two compounds that have a close chemical composition but differ significantly in electronic properties. The first compound has a negative temperature coefficient of resistance and does not show any phase transitions other than a superconducting one. The elastoresistance of this compound approximately follows the law which is a special case of the Curie-Weiss law, which is usually observed for Fe(Se,S) with metallic conductivity. The second compound has a metallic type of conductivity and, in addition to the superconducting transition, there is also a phase transition at a temperature of about 30 K. The elastoresistance of the second compound is sign-reversing and can be approximated with the sum of two Curie-Weiss-type terms with opposite signs and different critical temperatures. We attribute this behavior to the competition of contributions to the elastoresistance from different band valleys. These competing contributions may appear since the composition of our compound is close to the critical point at which the low-temperature ground state in the 11 series of iron-based superconductors changes from electronic nematic order to magnetic order. © Copyright2020 EPLA.This work was supported in part from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (K2-2020-008) and by Act 211 of the Government of Russian Federation, agreements 02.A03.21.0004 and 02.A03.21.0006 and by the Program of Competitive Growth of Kazan Federal University. We acknowledge support from Russian Foundation for Basic Research (Grants 20-02-00561 and ofi-m 17-29-10007) and Russian Science Foundation (Grant 19-42-02010)