A. Nikolaeva

Confinement effects and surface-induced charge carriers in Bi quantum wires

We present measurements of Shubnikov–de Haas oscillations in arrays of bismuth nanowires. For 80 nm wires, the hole concentration is less than 30% that of bulk Bi, a finding that is consistent with current models of quantum confinement effects. However, 30-nm-diam nanowires which are predicted to be semiconductors show a nearly isotropic short period of 0.025 T−1, consistent with a heavy carrier concentration five times that of bulk Bi. These results are discussed in terms of surface-induced charge carriers in a spherical Fermi surface pocket that are uniformly distributed in the 30 nm nanowire volume and that inhibit the semimetal-to-semiconductor transition.

Role of boundary roughness in the electronic transport of Bi nanowires

We present a study of electronic transport in 200 nm diameter bismuth nanowire arrays embedded in an alumina matrix where the nanowires are oriented preferentially with the trigonal crystalline axis parallel to the wire length. The study is based on measurements of the resistance and thermopower over a wide range of temperatures (4–300 K) as well as of magnetoresistance for fields of up to 9 T. The Fermi energies are obtained from the Landau level spectrum; results show that the wires have the intrinsic electron and hole concentrations. At high temperatures, the mobilities are temperature dependent and the electron mobility is several orders of magnitude larger than that of holes. This nanowire mobility behavior, which is also observed in the bulk, is attributed to carrier-phonon scattering. At low temperatures, the mobilities are temperature independent and roughly the same for electrons and holes. An interpretation in terms of boundary roughness scattering is proposed.

Thermoelectric Properties of Films and Monocrystalline Whiskers of Tellurium

Tellurium is an interesting semiconductor for thermoelectric applications because its thermopower is high (500 muV/K). Its figure of merit is low in the bulk because it has high thermal conductivity. However, in confined geometries, the phonon conductivity decreases due to phonon scattering. Therefore we have investigated the thermoelectric properties of Te films and whiskers. We report a study of the thermoelectric properties of Te films obtained by vacuum-condensation method on glass, polyamide, mica at various temperatures of condensation. The crystallographic structure of the films is characterized by the distribution function of the grain major axis. It is shown that the films condensed at 433 K possess the highest mobility. Te whiskers were grown from the paragas phase and they were single crystals in the form of hexahedral cylinder having cavities inside. When deposited on cold substrate, the cylinders had no cavities. The direction of the whisker crystal growth was always [0001] with respect to the whiskers length. It is shown that in films with the thickness of ~1 mum, the room temperature thermopower S is 330 muV/K and increases with temperature increasing (S = 450 muV/K at 450 K). The resistivity decreases with the temperature growth, as a result the power factor alpha2sigma increases by almost a factor of two. The 77 K electric conductivity of Te whiskers is 2-to-3 order-of-magnitude higher than in bulk crystals, presumably due to high structural perfection of whisker crystals. The thermopower in the range of 300 K is S = 400-750 muV/K

Nonequilibrium galvanomagnetic properties of quasi-one-dimensional superconductors

Abstract The transport properties of thin single-crystalline tin and indium filaments in glass cover were studied. The length of the samples was much greater than the quasiparticle relaxation length. The observed step-like current-voltage characteristics could not be described by the model of non-interacting phase-slip centers.

Glass-encapsulated single-crystal nanowires and filiform nanostructures fabrication

This paper reports on the production technology, features and practical application of capabilities of glass-coated micro- and nanowires fabricated by casting from the liquid phase. Micro- and nanowires with diameters from 80 nanometers to 1 micron have been received using improved technology of microwire casting by Ulitovsky method. The authors offer a technology of thinning such wires, as well as new manufacturing techniques of the filiform composite nanostructures on their basis. Some physical properties of the received micro- and nanowires have been investigated.

GALVANOMAGNETIC PROPERTIES OF QUASI-ONE-DIMENSIONAL SUPERCONDUCTORS

The transport properties of thin single‐crystalline tin and indium filaments in glass cover were studied. The length of the samples was much greater than the quasiparticle relaxation length. The width of the resistive transition and the electron free path of the samples studied were comparable with the corresponding values of the perfect whiskers. The observed step‐like current‐voltage characteristic could be described by the model of noninteracting phase‐slip centers.

Temperature dependencies of the electric field effect resistivity and thermoelectric properties of thin Bi-alloy wires

The temperature dependencies (4.2 - 300 K) of electrical field effect (EFE) in bismuth monocrystal wires were investigated. From our experiments on EFE the temperature ranges where /spl mu//sub n/ > /spl mu//sub p/ and /spl mu//sub n/ < /spl mu//sub p/ were defined. The influence of EFE on the thermoelectric power was studied. On the thin Bi wires have been detected essential altering of the thermoelectric power.

Galvano-thermomagnetic properties of Bi/sub 1-x/Sb/sub x/ nanowires at semiconductor-semimetal transition induced by elastic elongation

In the work thin single crystal (0.3 < d < 5 /spl mu/m) wires Bi/sub 1-x/Sb/sub x/ obtained by the liquid phase casting in a glass coating were investigated under elastic deformations up to 2-3% relative elongation in the temperature range 4.2-300 K. In the nondeformed state the wires are semiconductor with minimal indirect gap and have a characteristic semiconductor dependence on T. Under elastic stretch, the semiconductor-semimetal transition was realized, accompanied by the sample metallization and appearance of the Shubnikov-de Haas oscillations from L charge carriers. The decrease of the resistance here 40-50% is observed, and of the thermopower absolute value near 15-20% decreases. Both values depend on wire diameter d. The longitudinal magneto-Seebeck coefficient in the magnetic field up to 0.3 T was measured. It is shown that elastic stretch and longitudinal magnetic field lead to 30-40% increase of the power factor /spl alpha//sup 2//spl sigma/ in the temperature range 150-200 K and at 77 K in the longitudinal magnetic field up to 0.07 T. The temperature region wherein the power factor increase takes place, depends significantly on wire diameter d.

Anisotropic Thermoelectric Generator Made from Single Crystal Bi Microwire

Currently, for thermoelectric conversion of heat most widely used is the approach based on the Seebeck and Peltier effects created at the interface of two materials with different values S of the Seebeck coefficient. Another type of thermoelectric converter is the anisotropic thermoelement (AT) using anisotropy of thermoelectric power. AT has some advantages: - the transverse thermopower, unlike a conventional thermocouple, is proportional to the temperature gradient (T1- T2)/h instead of the temperature difference T1-T2; - Voltage V is proportional to the length l of AT. To increase the AT output voltage we need either increase the length l of AT or decrease its thickness h. According to our experimental data, to obtain a thermoelectric voltage of 1 V at a transverse temperature gradient of 5 K, the microwire with a diameter of 2 μm and a length of 8 m must be used. In our experimental sample the long wire in glass coating was wound into a flat spiral.

Berry's phase manifestation in Aharonov-Bohm oscillations in single Bi nanowires

Here we report on Aharonov-Bohm oscillations of magnetoresistance (MR) of the single Bi nanowires with diameter d<80 nm. The samples were prepared by Ulitovsky technique and represented cylindrical single crystals with the 1011 orientation along the wire axis. Due to semimetal-to-semiconductor transformation and big density of surface states with strong spin-orbit interactions Bi nanowire should effectively become a conducting tube. The equidistant oscillations of the MR have been observed in a wide range of magnetic fields up to 14 T at various temperatures (1.5 K< T< 4.2 K) and angles θ (0< θ < 90°) of the sample orientation relative to the magnetic field. We have obtained longitudinal MR oscillations with periods ΔB1=Φ0/S and ΔB2=Φ0/2S, where Φ0=h/e is the flux quantum and S is the wire cross section. From B ≈ 8 T down to B=0 the extremums of Φ0/2S oscillations are shifted up to 3π at B=0 which is the manifestation of Berry phase shift due to carriers moving in inhomogeneous magnetic field. An interpretation of the MR oscillations in terms of a subband structure in the surface state band caused by quantum interference is presented.