T. E. Huber

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 0.025u2002T−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.

Longitudinal magnetoresistance of -diameter Bismuth nanowires

Abstract We study the longitudinal magnetoresistance of 270-nm diameter bismuth nanowire arrays embedded in an alumina matrix which are capped with layers of pure Bi that have low contact resistance. At intermediate fields, the LMR presents a broad maximum that is discussed in terms of the interplay between the carriers cyclotron radius and scattering at the wire walls and the onset of Shubnikov–de Haas oscillations.

Negative magnetoresistance in transverse and longitudinal magnetic fields in Bi nanowires

In this work the measurements of longitudinal (LMR) and transverse magnetoresistance (TMR) in Bi wires with 50 R0 at T<5 K. This result was unexpected, because it is known that in bulk Bi samples the giant increase of magnetoresistance in the case H⊥I is observed. We calculate the conductivity of nanowires in longitudinal and transverse magnetic fields on the basis of the Cubo formula for the conductivity. We use the model of quantum wires with a parabolic potential in the plane orthogonal to the axis of the size-quantized system. According to the model of such TMR and LMR were experimentally observed in the Bi quantum wires (QW) with d<80nm.

Magnetic Anisotropy and de Haas–van Alphen Oscillations in a Bi Microwire Array Studied via Cantilever Magnetometry at Low Temperatures

We report measurements of the low temperature (T=0.5 K) oscillatory magnetization in a high-density array of 50μm diameter wires of polycrystalline Bi utilizing a high sensitivity silicon cantilever magnetometer. We find that the magnetic response is strongly anisotropic, being much larger for magnetic field perpendicular than for fields parallel to the wire-axis. We argue that this is a geometric effect caused by the large aspect ratio of the individual microwires in the array. The magnetic response of the microwires is dominated by the light electrons due to the larger cyclotron orbits in comparison with the heavier holes. We find that de Haas–van Alphen oscillations are easily resolved, and discuss the application of this technique to the study of Bi nanowire arrays.

Anisotropic Thermoelectric Devices Made from Single-Crystal Semimetal Microwires in Glass Coating

Thermoelectric heat conversion based on the Seebeck and Peltier effects generated at the junction between two materials of type-n and type-p is well known. Here, we present a demonstration of an unconventional thermoelectric energy conversion that is based on a single element made of an anisotropic material. In such materials, a heat flow generates a transverse thermoelectric electric field lying across the heat flow. Potentially, in applications involving miniature devices, the anisotropic thermoelectric (AT) effect has the advantage over traditional thermoelectrics that it simplifies the thermoelectric generator architecture. This is because the generator can be made of a single thermoelectric material without the complexity of a series of contacts forming a pile. A feature of anisotropic thermoelectrics is that the thermoelectric voltage is proportional to the element length and inversely proportional to the effective thickness. The AT effect has been demonstrated with artificial anisotropic thin film consisting of layers of alternating thermoelectric type, but there has been no demonstration of this effect in a long single-crystal. Electronic transport measurements have shown that the semimetal bismuth is highly anisotropic. We have prepared an experimental sample consisting of a 10-m-long glass-insulated single-crystal tin-doped bismuth microwire (du2009=u20094xa0μm). Crucial for this experiment is the ability to grow the microwire as a single-crystal using a technique of recrystallization with laser heating and under a strong electric field. The sample was wound as a spiral, bonded to a copper disk, and used in various experiments. The sensitivity of the sample to heat flow is as high as 10−2xa0V/W with a time constant τ of about 0.5xa0s.

Lifshitz topological transitions, induced by doping and deformation in single-crystal bismuth wires

The features associated with the manifestation of Lifshitz electron topological transitions (ETT) in glass-insulated bismuth wires upon qualitative changes to the topology of the Fermi surface are investigated. The variation of the energy spectrum parameters was implemented by doping Bi with an acceptor impurity Sn and using elastic strain of up to 2%, relative to the elongation in the weakly-doped p-type Bi wires. Pure and doped glass-insulated single-crystal bismuth with different diameters and (1011) orientations along the axis were prepared by the Ulitovsky liquid phase casting method. For the first time, ETT-induced anomalies are observed along the temperature dependences of the thermoemf α(T) as triple-changes of the α sign (given heavy doping of Bi wires with an acceptor impurity Sn). The concentration and energy position of the Σ-band given a high degree of bismuth doping with Sn was assessed using the Shubnikov-de Haas effect oscillations, which were detected both from L-electrons and from T-hole...

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.

Aharonov-Bohm Oscillations in Single Bi Nanowires

We report on the study of the magnetoresistance (MR) of 55 nm diameter single bismuth nanowire at a temperature of 1.5 K. For a wide range of magnetic fields and different sample orientations the MR exhibit field-periodic modulations. We have observed 3 periods of MR oscillations; one of them is consistent with theoretical predictions for the Aharonov-Bohm oscillations in disordered cylinders. Two others may be connected with Dingle’s predictions for oscillations resulting from quantization of the electron energy spectrum.

Thermoelectric Power of Bi Microwires at Low Temperatures

The temperature dependence (4.2–300 K) of the thermopower of single‐crystal Bi and Bi doped with Te (0.0005 at.% – 0.02 at. %) microwires was investigated. For temperatures below ∼12 K the thermopower is dominated by phonon drag and a maximum is observed at around 6 K. The position of this maximum and its value depends on the wire’s diameter and the length of the samples. In the temperature range 8 – 11 K the phonon drag contribution follows an exponential temperature dependence which is characteristic of a two‐stage mechanism. The positive thermopower maxima of the order of 10 μVK−1 are explained in terms of the phonon drag of carriers. The phonon drag contribution for the doped samples is strongly suppressed.