Magnetoresistance in dilute p‐Si/SiGe in parallel and tilted magnetic fields

We report the results of an experimental study of the magnetoresistance ρ_xx and ρ_xy in two samples of p‐Si/SiGe with low carrier concentrations p = 8.2 × 10¹⁰ cm^‐2 and p = 2 × 10¹¹ cm^‐2. The research was performed in the temperature range of 0.3–2 K and in the magnetic fields of up to 18 T, parallel or tilted with respect to the two‐dimensional (2D) channel plane. The large in‐plane magnetoresistance can be explained by the influence of the in‐plane magnetic field on the orbital motion of the charge carriers in the quasi‐2D system. The measurements of ρ_xx and ρ_xy in the tilted magnetic field showed that the anomaly in ρ_xx, observed at filling factor ν = 3/2 is practically nonexistent in the conductivity σ_xx. The anomaly in σ_xx at ν = 2 might be explained by overlapping of the levels with different spins 0 ↑ and 1 ↓ when the tilt angle of the applied magnetic field is changed. The dependence of g‐factor g^*(Θ)/g^*(0⁰) on the tilt angle Θ was determined.     

Hall effect plateaus and giant Shubnikov-de Haas oscillations in (BiSb)Telayered single crystals near the quantum limit

On bulk layered single crystals (Bi_0.25Sb_0.75)₂Te₃ with a hole concentration cm^-3 and a mobility cm²/Vs magnetoresistance and Hall effect investigations were performed in the temperature range T = 1.4 K ... 20 K in magnetic fields up to 18 T. For the magnetic field perpendicular to the layered structure giant Shubnikov-de Haas oscillations are measured; the positions of the maxima are triplets in the reciprocally scaled magnetic field. From the damping of the amplitudes with increasing temperature the cyclotron mass m _c = 0.12m ₀ is evaluated. Correlated with the SdH oscillations doublets of Hall effect plateaus (or kinks in low fields) are found. The weak well known Shubnikov-de Haas oscillations from the generally accepted multivallied highest valence band can be detected as a modulation on the giant oscillation. The high anisotropy of the SdH oscillations and their triplet structure in connection with the layered crystal structure lead us to suggest that the effects are caused by hole carrier pairing (mediated by the bipolaron mechanism) in quasi 2D sheets parallel to the crystal layer stacks. The measured Hall plateau resistances coincide with the quantum Hall effect values considering the number of layer stacks and the valley degeneracy of the 3D hole carrier reservoir. The ratio of spin splitting to Landau (cyclotron) splitting is observed to be . Received: 12 September 1997 / Revised: 8 January 1998 / Accepted: 22 January 1998     

Beating patterns in the oscillatory magnetoresistance of an AlGaN/GaN heterostructure

Magneto-transport measurements have been carried out on a modulation-doped Al_0.22Ga_0.78N/GaN heterostructure in a temperature range between 1.5 and 25 K with a rather high carrier density, 1.1×10¹³ cm⁻². Striking beating patterns in magnetoresistance vs magnetic field are observed in the vicinity of a special temperature. Theoretical simulation is performed and the comparison between numerical simulations and the experimental data reveals that the beating patterns are due to the interference of the magneto-intersubband scattering and the SdH oscillator of first subband.     

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the AlGaN/AlN/GaN in an SiC heterostructure, which demonstrates the existence of the high-quality two-dimensional electron gas (2DGE) at the AlN/GaN interface. While the carrier concentration reaches 1.32 × 10¹³ cm ^{- 2} and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21 × 10⁴ cm²/(V·s) at 2 K. The Shubnikov—de Haas (SdH) oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEG is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.     

Large unsaturated positive and negative magnetoresistance in Weyl semimetal TaP

After successfully growing single-crystal Ta P, we measured its longitudinal resistivity(ρxx) and Hall resistivity(ρyx) at magnetic fields up to 9 T in the temperature range of 2-300 K. At 8 T, the magnetoresistance(MR) reached 3.28 × 105% at 2 K, 176%at 300 K. Neither value appeared saturated. We confirmed that Ta P is a hole-electron compensated semimetal with a low carrier concentration and high hole mobility of μh=3.71 × 10~5cm~2/V s, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, because a magnetic field(H) was applied in parallel to the electric field(E), a negative MR due to a chiral anomaly was observed and reached-3000% at 9 T without any sign of saturation, either, which is in contrast to other Weyl semimetals(WSMs). The analysis of the Shubnikov-de Haas(Sd H) oscillations superimposed on the MR revealed that a nontrivial Berry's phase with a strong offset of 0.3958, which is the characteristic feature of charge carriers enclosing a Weyl node. These results indicate that Ta P is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications.     

Magnetotransport in modulation-doped In0.53Ga0.47As/In0.52Al0.48As heterojunctions: in-plane effective mass,quantum and transport mobilities of 2D electrons

Shubnikov–de Haas (SdH) and Hall effect measurements, performed in the temperature range between 3.3 and 20 K and at magnetic fields up to 2.3 T, have been used to investigate the electronic transport properties of lattice-matched In_0.53Ga_0.47As/In_0.52Al_0.48As heterojunctions. The spacer layer thickness (t_S) in modulation-doped samples was in the range between 0 and 400 Å. SdH oscillations indicate that two subbands are already occupied for all samples except for that witht_S =  400 Å. The carrier density in each subband, Fermi energy and subband separation have been determined from the periods of the SdH oscillations. The in-plane effective mass (m ^* ) and the quantum lifetime (τ_q) of 2D electrons in each subband have been obtained from the temperature and magnetic field dependences of the amplitude of SdH oscillations, respectively. The 2D carrier density (N₁) in the first subband decreases rapidly with increasing spacer thickness, while that (N₂) in the second subband, which is much smaller thanN₁ , decreases slightly with increasing spacer thickness from 0 to 200 Å. The in-plane effective mass of 2D electrons is similar to that of electrons in bulk In_0.53Ga_0.47As and show no dependence on spacer thickness. The quantum mobility of 2D electrons is essentially independent of the thickness of the spacer layer in the range between 0 and 200 Å. It is, however, markedly higher for the samples with a 400 Å thick spacer layer. The quantum mobility of 2D electrons is substantially smaller than the transport mobility which is obtained from the Hall effect measurements at low magnetic fields. The transport mobility of 2D electrons in the first subband is substantially higher than that of electrons in the second subband for all samples with double subband occupancy. The results obtained for transport-to-quantum lifetime ratios suggest that the scattering of electrons in the first subband is, on average, forward displaced in momentum space, while the electrons in the second subband undergo mainly large-angle scattering.     

Observation of Microwave-Induced Magnetoresistance Oscillations in a ZnO/Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>O Heterojunction

Microwave-induced magnetoresistance and magnetoconductance oscillations in the two-dimensional electron system have been detected in a ZnO/Mg _x Zn_1–x O heterojunction. The structure has a high electron density of n = 6.5 × 10¹¹ cm^–2 and a very low mobility of 10⁵ cm² V^–1 s^–1. This clearly indicates that the mobility of two-dimensional electrons is not a critical parameter for the observation of microwave-induced oscillations. The samples were both Hall bars and Corbino disks. At least four oscillations have been resolved. Their amplitude increases with the magnetic field and all oscillations with numbers >1 have the same phase of 0.25. It has been shown that the microwave-induced photoelectromotive force is also a periodically oscillating function of the inverse magnetic field.     

Low temperature subband 2D electron mobilities in heavy delta- and modulation doped GaAs/GaAlAs heterostructures

We synthesised high-2D electron-density GaGs/GaAlAs heterostructures with different distance L_σ of Si delta-layer in GaAs from the heterojunction and uniform doped GaAlAs. The quantum Hall effect and Shubnikov-de Haas effect were measured for temperatures down to 0.4 K in magnetic fields up to 40 T. The enhanced 2D electron concentration achieved was 1.1^*10¹³ cm^?2 in six filled subbands. The Hall mobility depends on L_σ and has maximum for L_σ=600÷750Å. From the amplitudes of the SdH oscillations and Fourier transforms the subband mobilities and electron concentration in each subband have been extracted. According to calculations intersubband electron scattering appears to be important and reduces mobilities in subbands.     

Magnetothermal factor of intersubband relaxation of 2D electrons from a highly doped heterotransition in AlGaAs (Si)/GaAs

Characteristics of the Shubnikov-de Haas transverse magnetoresistance oscillations of 2D electrons in highly dopedAlGaAs(Si)/GaAs heterostructures are investigated in the present paper. Anomalies caused by the occupation of two quantization subbands are revealed for samples with 2D-electron density n_s>7·10¹¹ cm⁻² at T=1.7–16 K and magnetic field induction B up to 7.4 T. The dependences of the normalized oscillation amplitude on the magnetic field show bends that typically displace toward weaker magnetic fields with decreasing temperature and electron density n_s. A nonmonotonic (oscillating) dependence of the Dingle temperature on the experimental temperature is found. These anomalies are interpreted for a model of the occupation of two quantization subbands with electrons. They are caused by the competitive character, of intersubband 2D-electron scattering. Small-angle relaxation times are estimated for 2D electrons of the zero and first quantization subbands. S. A. Esenin Ryazan' State Pedagogical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 52–57, January, 2000.     

Temperature relaxation and energy loss of hot carriers in graphene

The temperature relaxation and energy loss of hot Dirac fermions are investigated theoretically in graphene with carrier–optical phonon scattering. The time evolutions of temperature and energy loss for hot Dirac fermions in graphene are calculated self-consistently. It shows that the carrier–optical phonon coupling results in the energy relaxation of hot carriers excited by an electric field, and the relaxation time for temperature is about 0.5–1 ps and the corresponding energy loss is about 10–25 nW per carrier for typically doped graphene samples with a carrier density range of 1–5×10¹² cm^?2. Moreover, we analyze the dependence of temperature and energy relaxation on initial hot carrier temperature, lattice temperature and carrier density in detail.     

Observation of π Berry phase in quantum oscillations of three‐dimensional Fermi surface in topological insulator Bi2Se3

We report the quantum transport studies on Bi₂Se₃ single crystal with bulk carrier concentration of ～10¹⁹ cm^–3. The Bi₂Se₃ crystal exhibits metallic character, and at low temperatures, the field dependence of resistivity shows clear Shubnikov–de Haas (SdH) oscillations above 6 T. The analysis of these oscillations through Lifshitz–Kosevich theory reveals a non‐trivial π Berry phase coming from three‐dimensional (3D) Fermi surface, which is a strong signature of Dirac fermions with three‐dimensional dispersion. The large Dingle temperature and non zero slope of Williamson–Hall plot suggest the presence of enhanced local strain field in our system which possibly transforms the regions of topological insulator to 3D Dirac fermion metal state. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Magneto-acoustic attenuation in AuSb2

Quantum oscillations in the ultrasonic attenuation in AuSb₂ were studied as a function of temperature, magnetic field and crystal orientation. The effective masses of the carriers associated with the F₅ and F₆ oscillations were measured in a (110) plane. For the F₅ oscillations, the Dingle temperature and apparent magnetic breakdown field appear to depend strongly upon orientation. For the F₆ oscillations, however, there were no signs of magnetic breakdown up to the highest magnetic fields available (70 kOe) and the Dingle temperature was roughly independent of orientation. From the acoustic velocities, the elastic constants were determined at 77 K: C₁₁ = (14·7 ± 0·9) × 10¹¹ dyne/cm², C₁₂ = (6·0 ± 0·9) × 10¹¹ dyne/cm², and C₄₄ = (2·59 ± 0·06) × 10¹¹ dyne/cm². These elastic constants give an adiabatic compressibility K_s = (1·13 ± 0·12) × 10^?12 cm²/dyne and a Debye temperature ?_D = (203 ± 15) K.     

Enhancement of the colossal magnetoresistance in Eu1 − x Ca x B6

The transport and magnetic properties of single crystal samples of substitutional solid solutions Eu_{1 ? x} Ca _x B₆ (0 ≤ x ≤ 0.26) have been studied at temperatures 1.8–300 K in magnetic fields up to 80 kOe. It has been shown that an increase in the calcium concentration results in the suppression of the charge transport accompanied by an increase in the amplitude of the colossal magnetoresistance (CMR) up to the value (ρ(0) ? ρ(H))/ρ(H) ≈ 7 × 10⁵ detected for x = 0.26 at liquid-helium temperature in a field of 80 kOe. The transition from the hole-like conductivity to the electron-like conductivity has been observed in the Eu_0.74Ca_0.26B₆ solid solution in the CMR regime at T < 40 K. The Hall mobility values μH = 200?350 cm²/(V s) estimated for charge carriers in the strongly disordered matrix of the Eu_0.74Ca_0.26B₆ solid solution are comparable with the charge carrier mobility μH = 400?600 cm²/(V s) for the undoped EuB₆ compound. The anomalous behavior of the transport and magnetic parameters of the Eu_{1 ? x} Ca _x B₆ solid solutions is discussed in terms of a metal-insulator transition predicted within the double exchange model for this system with low carrier density.     

Magnetotransport coefficients of Sm0.55Sr0.45MnO3

Measurements of resistivity, magnetization, Seebeck coefficient in 0 and 1.5 T and Nernst coeffecient in 0.3, 0.9 and 1.8 T as a function of temperature on a polycrystalline sample Sm_0.55Sr_0.45MnO₃ are presented. The data demonstrate that an increase in both the carrier density and the mobility of the charge carriers is responsible for the observed colossal magnetoresistance in this compound.     

Transport evidence of 3D topological nodal-line semimetal phase in ZrSiS

Topological nodal-line semimetal is a new emerging material, which is viewed as a three-dimensional (3D) analog of graphene with the conduction and valence bands crossing at Dirac nodes, resulting in a range of exotic transport properties. Herein, we report on the direct quantum transport evidence of the 3D topological nodal-line semimetal phase of ZrSiS with angular-dependent magnetoresistance (MR) and the combined de Hass-van Alphen (dHvA) and Shubnikov-de Hass (SdH) oscillations. Through fitting by a two-band model, the MR results demonstrate high topological nodal-line fermion densities of approximately 6×10²¹ cm⁻³ and a perfect electron/hole compensation ratio of 0.94, which is consistent with the semi-classical expression fitting of Hall conductance Gxy and the theoretical calculation. Both the SdH and dHvA oscillations provide clear evidence of 3D topological nodal-line semimetal characteristic.     

Hopping conductivity of carbynes modified under high pressures and temperatures: Galvanomagnetic and thermoelectric properties

The conductivity, thermopower, and magnetoresistance of carbynes structurally modified by heating under a high pressure are investigated in the temperature range 1.8–300 K in a magnetic field up to 70 kOe. It is shown that an increase in the synthesis temperature under pressure leads to a transition from 1D hopping conductivity to 2D and then to 3D hopping conductivity. An analysis of transport data at T ≤ 40 K makes it possible to determine the localization radius a ～ (56?140) Å of the wave function and to estimate the density of localized states _g(E _F) for various dimensions d of space: _g(E _F) ≈ 5.8 × 10⁷ eV^?1 cm^?1 (d=1), _g(E _F) ≈5×10¹⁴ eV^?1 cm ^?2 (d=2), and _g(E _F)≈1.1×10²¹ eV^?1 cm_?3 (d=3). A model for hopping conductivity and structure of carbynes is proposed on the basis of clusterization of sp ² bonds in the carbyne matrix on the nanometer scale.     

Magnetoresistivity in a tilted magnetic field in p-Si/SiGe/Si heterostructures with an anisotropic g-factor. Part II

The magnetoresistance components ??xx and ??xy are measured in two p-Si/SiGe/Si quantum wells that have an anisotropic g-factor in a tilted magnetic field as a function of the temperature, field, and tilt angle. Activation energy measurements demonstrate the existence of a ferromagnetic-paramagnetic (F-P) transition for the sample with the hole density p = 2 × 10¹¹ cm^?2. This transition is due to the crossing of the 0?? and 1?? Landau levels. However, in another sample with p = 7.2 × 10¹⁰ cm^?2, the 0?? and 1?? Landau levels coincide for angles ?? = 0?C70°. Only for ?? > 70° do the levels start to diverge which, in turn, results in the energy gap opening.     

X-ray-line plasma satellites of ions in solid target plasmas produced by picosecond laser pulse

The X-ray lines of ions in a solid target interacting with picosecond laser pulses of moderate intensity (2×10¹⁷ W/cm²) were measured on the “Neodim” laser facility. X-ray Ly _α emission spectra of hydrogen-like fluorine ions were observed. Satellite lines were also observed, evidencing the presence of intense plasma oscillations. The positions and separation between the satellites allow their assignment to the intense electrostatic oscillations with an amplitude larger than 10⁸ V/cm and a frequency of about 7× 10¹⁴ s^?1 that is noticeably lower than the laser frequency ω_las～1.8×10¹⁵ s^?1. It is suggested that these oscillations may be due to strong plasma turbulence caused by the development of plasma oscillations of the Bernstein-mode type under the action of a strong magnetic field generated in plasma. The experimental results are compared with the calculated spectra of multicharged ions.     

Raman scattering and electrical studies of the phase stability in the Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te

Raman scattering is performed to access phase stability in the boron-implanted Hg_0.7Cd_0.3Te with fluences ranging from 1 × 10¹² to 1 × 10¹⁵ cm^?2. Threshold fluence for the formation of an amorphous phase is invoked here using Thomas–Fermi statistical model. Asymmetric broadening and red shift of the Raman active HgTe-like LO phonon mode are observed with varying fluencies. Electrical properties such as sheet carrier concentration and mobility are also changed with the fluence and reach their saturated values beyond threshold fluence of 5 × 10¹³ cm^?2. Threshold fluence for the formation of amorphous phase is also validated by the Raman measurements and electrical transport properties in the implanted layers. The excess free energy of 6.8 kJ/mole is found corresponding to the threshold fluence for phase transition.     

Oxygen plasma and high pressure H<Subscript>2</Subscript>O vapor heat treatments used to fabricate polycrystalline silicon thin film transistors

Oxygen plasma and high pressure H₂O vapor heat treatment were applied to fabrication of n-channel polycrystalline silicon thin film transistors (poly-Si TFTs). 13.56 MHz-oxygen-plasma treatment at 250 °C, 100 W for 5 min effectively reduced defect states of 25-nm-thick silicon films crystallized by 30 ns-pulsed XeCl excimer laser irradiation. 1.3×10⁶-Pa-H₂O vapor heat treatment at 260 °C for 3 h was carried out in order to improve electrical properties of SiO_x gate insulators and SiO_x/Si interfaces. A carrier mobility of 470 cm²/V s and a low threshold voltage of 1.8 V were achieved for TFTs fabricated with crystallization at 285 mJ/cm². Received: 18 November 2002 / Accepted: 25 November 2002 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +81-42/388-7109, E-mail: tsamesim@cc.tuat.ac.jp