Haas Magnetoresistance Oscillations Research Articles

Surface superconductivity on Weyl semimetal induced by nonmagnetic and ferromagnetic tips

Topological superconductors showing Majorana fermions or bound states at the edge or surface are promising components for potential quantum computation and thus attract much attention in physical sciences. Recently, topological Weyl fermion states have been discovered in nonsuperconducting TaP crystals, which may offer a great platform to investigate topological nontrivial superconductivity if superconductivity can be induced wherein. Here we report the discovery of nonmagnetic and ferromagnetic tips induced unconventional interface superconductivity around the nanoscale point contact region on TaP crystals. The tip-induced superconductivity (TISC) at the contact is enhanced with the ferromagnetic Ni tip, comparing with nonmagnetic Au and PtIr tips. The point contact spectroscopy shows evidences of anisotropic superconductivity with the mixed spin-triplet pairing, indicating unconventional superconducting nature. Moreover, the Shubnikov--de Haas oscillations in magnetoresistance are detected from the tip-point-contact measurement, which demonstrate that the nonzero Berry phase or topological nontrivial property of TaP maintains under the tip point contact. The theoretical analysis shows that surface superconductivity plays a major role in this TISC and there exists a large spin-triplet mixing in the gap function. Therefore, our work suggests an intriguing Weyl surface superconducting state on TaP.

Spin-orbit coupling in strained Ge whiskers

The field dependences of the magnetoresistance for germanium whiskers with gallium doping concentration of 2 ⋅ 1017 cm-3 were studied in the magnetic field range 0–14 T at temperature 4.2 K under compressive strain up to 0.2%. The strain influence on a spin-orbit splitting on the valence band spectrum was studied. As a result, the effective mass and the energies of spin-orbit splitting for light and heavy holes were found under the compressive strain according to kp method. From an analysis of the Shubnikov—de Haas magnetoresistance oscillations the effective mass of heavy holes mc = 0.25 m0 was calculated. The spin-orbit splitting energy of heavy holes ΔHH = 15 meV, the Lande factor in direction [111] g* = 4.8 as well as the Rashba cubic parameter of spin-orbit interaction βSO=1⋅10−28eV⋅m3 were found due to the studying of longitudinal magnetoconductance in strained Ge whiskers at low temperatures. The appearance of negative magnetoresistance in the magnetic field range up to 7 T likely results from the effect of charge carrier interaction.

Theoretical Investigation of the Shubnikov-de Haas Magnetoresistance Oscillations in a Quantum well under the Influence of Confined Acoustic Phonons

The Shubnikov – de Haas magnetoresistance oscillations in the Quantum well (QW) under the influence of confined acoustic phonons, The theoretical results show that the conductivity tensor, the complex magnetic impedance of the magnetic field, the frequency, the amplitude of the laser radiation, the QW width, the temperature of the system and especially the quantum index m characterizes the confinement of the phonon. The amplitude of the oscillations of the Shubnikov-de Haas impedance decreases with the increase of the influence of the confined acoustic phonons. The results for bulk phonons in a QW could be achieved, when m goes to zero. We has been compared with other studies when perform the numerical calculations are also achieved for the GaAs/AlGaAs in the QW. Results show that The Shubnikov-de Haas magnetoresistance oscillations amplitude decrease when phonon confinement effect increasing and when width L of the QW increases to a certain value, The Shubnikov – de Haas magnetoresistance oscillations amplitude completely disappears can not be observed.

Origin and Magnitude of ‘Designer’ Spin-Orbit Interaction in Graphene on Semiconducting Transition Metal Dichalcogenides

We use a combination of experimental techniques to demonstrate a general occurrence of spin-orbit interaction (SOI) in graphene on transition metal dichalcogenide (TMD) substrates. Our measurements indicate that SOI is ultra-strong and extremely robust, despite it being merely interfacially-induced, with neither graphene nor the TMD substrates changing their structure. This is found to be the case irrespective of the TMD material used, of the transport regime, of the carrier type in the graphene band, and of the thickness of the graphene multilayer. Specifically, we perform weak antilocalization measurements as the simplest and most general diagnostic of SOI, and show that the spin relaxation time is very short in all cases regardless of the elastic scattering time. Such a short spin-relaxation time strongly suggests that the SOI originates from a modification of graphene band structure. We confirmed this expectation by measuring a gate-dependent beating, and a corresponding frequency splitting, in the low-field Shubnikov-de Haas magneto-resistance oscillations in high quality bilayer graphene on WSe$_2$. These measurements provide an unambiguous diagnostic of a SOI-induced splitting in the electronic band structure, and their analysis allows us to determine the SOI coupling constants for the Rashba term and the so-called spin-valley coupling term, i.e., the terms that were recently predicted theoretically for interface-induced SOI in graphene. The magnitude of the SOI splitting is found to be on the order of 10 meV, more than 100 times greater than the SOI intrinsic to graphene. Both the band character of the interfacially induced SOI, as well as its robustness and large magnitude make graphene-on-TMD a promising system to realize and explore a variety of spin-dependent transport phenomena, such as, in particular, spin-Hall and valley-Hall topological insulating states.

Low temperature magnetoresistance of InSb whiskers

Transverse and longitudinal magnetoresistance in InSb whiskers with different impurity concentration (4.4×1016–7.16×1017)сm−3 were studied at temperature range (4.2–77)K and in magnetic fields (0–14)T. Shubnikov-de Haas oscillations in magnetoresistance for n-type InSb whiskers with impurity concentration in the vicinity to the metal–insulator transition (MIT) at low temperature were revealed. Every peak of longitudinal and transverse magnetoresistance (MR) split as the doublet for concentration 3.26×1017сm−3. The giant g-factor was defined, that rises from 46 to 60 with increasing of the magnetic field up to 14T. The oscillation period differs in the range (0.095–0.11)T−1 at various doping concentration in InSb whiskers. The effective mass of electrons mс=0.03m0 and Dingle temperature with range (3–12)K were found in InSb whiskers with impurity concentration in the vicinity to MIT. The observed giant magnetoresistance (increasing in order of magnitude in magnetic field induction 14T) was used to design the sensitive sensor of magnetic field induction operated at the range of cryogenic temperatures.

Surface state band mobility and thermopower in semiconducting bismuth nanowires

Many thermoelectrics such as Bi exhibit Rashba spin-orbit surface bands for which topological insulator behavior consisting of ultrahigh mobilities and enhanced thermopower has been predicted. Bi nanowires realize surface-only electronic transport since they become bulk insulators when they undergo the bulk semimetal-semiconductor transition as a result of quantum confinement for diameters close to 50 nm. We studied 20, 30, 50, and 200 nm trigonal Bi wires. Shubnikov--de Haas magnetoresistance oscillations caused by surface electrons and bulklike holes enable the determination of their densities and mobilities. Surface electrons have high mobilities exceeding 2 ${\mathrm{m}}^{2}$ sec${}^{\ensuremath{-}1}$ ${\mathrm{V}}^{\ensuremath{-}1}$ and contribute strongly to the thermopower, dominating for temperatures $T$ 100 K. The surface thermopower is \ensuremath{-}1.2 $T$ \ensuremath{\mu}V/K${}^{2}$, a value that is consistent with theory, raising the prospect of developing nanoscale thermoelectrics based on surface bands.

The Landé‐ g factor in heterostructures

We have studied the behavior of the Shubnikov-de Haas oscillations in magnetoresistance under applied magnetic fields with an arbitrary orientation. Different procedures to measure the Landé-g factor have been used in the literature. We propose four unambiguous methods and formulas for the evaluation of this quantity. The Landé-g factor obtained using these formulas compares within the 5% with the reported results of Brosig et al. [1] in InAs – AlSb quantum wells, Bompadre et al. [2] for Bismuth crystals, Kobayashi et al. [3] measuring magnetoresistance as a function of the gate Voltage and F. Fang et al. [4] for InAs – GaSb heterostructure. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The high-order cyclotron modes in Fermi liquid of Q2D layered conductors

Experimental studies of galvanomagnetic phenomena in a large family of tetrathiafulvalene based ion-radical salts of the form (BEDT TTF)2X (X stands for a set of various anions) indicate that these layered compounds possess the metal type electrical conductivity. This permits to describe electron processes in such conductors based on the well-developed concept of quasiparticles, analogous to conduction electrons in metals. Observation of Shubnikov-de Haas magnetoresistance oscillations �,p �

Characterization of an AlGaN/GaN two-dimensional electron gas structure

An AlxGa1−xN/GaN two-dimensional electron gas structure with x=0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. X-ray diffraction maps of asymmetric reciprocal lattice points confirmed that the thin AlGaN layer was coherently strained to the thick GaN layer. Methods for computing the aluminum mole fraction in the AlGaN layer by x-ray diffraction are discussed. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm−2 and a mobility of 1.9×104 cm2/V s at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov–de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov–de Haas oscillations. The quantum lifetime was about one-fifth of the transport lifetime of 2.3×10−12 s.

Electrical transport of an AlGaN/GaN two-dimensional electron gas

An AlxGa1−xN/GaN two-dimensional electron gas structure with x = 0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm−2 and a mobility of 1.9 × 104 cm2/Vs at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov-de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov-de Haas oscillations. The quantum lifetime was about one-fifth of the transport lifetime of 2.3 × 10−12 s.

Electrical Transport of an AlGaN/GaN Two-Dimensional Electron Gas

AbstractAn AlxGa1-xN/GaN two-dimensional electron gas structure with x = 0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm-2 and a mobility of 1.9 × 104 cm2/Vs at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov-de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov-de Haas oscillations. The quantum lifetime was about one-fifth of the transport lifetime of 2.3 × 10-12 s.

Shubnikov‐de Haas investigations on n‐type Bi2(SexTe1−x)3

AbstractShubnikov‐de Haas magnetoresistance oscillations are observed in the mixed crystal system Bi2(SexTe1−x)3 for 0 < x ≦ 0.1 and 0.8 ≦ x < 1 at temperatures between 1.7 and 4 2 K in magnetic fields up to 10.5 T flux density. The electron concentrations ranged from 5 × 1017 to 6.5 × 1019 cm−3. In both the separate ranges (Te‐rich and Se‐rich composition) the electronic mass parameters are essentially unchanged compared to pure Bi2Te3 and Bi2Se3, respectively. For the Te‐rich side the energy gap between the lowest conduction band and the postulated second type of conduction band valleys decreases with increasing selenium content. The evaluated g‐tensor ellipsoids correspond to the results for n‐Bi2Te3 in the region 0 < x ≦ 0.1.