Strong Shubnikov-de Haas Oscillations Research Articles

Quantum interference effects in a 3D topological insulator with high-temperature bulk-insulating behavior

The Bi2Se3-family of 3D topological insulators (3DTI) exhibit insulating bulk states and surface states presenting a Dirac cone. At low temperatures, the conduction channels through the bulk of the material are fully gapped, making 3DTIs perfect systems to study the 2D transport behavior of Dirac fermions. Here, we report a 3DTI Bi1.1Sb0.9STe2 with a reduced level of defects, and thus, high-temperature insulating behavior in its bulk states. The insulator-to-metal transition occurs at ∼250 K, below which the bulk contributions are negligible. Even at room temperature, the conductivity contribution from the bulk channel is less than 20%. Quantum transport properties of topological surface states are observed in the Bi1.1Sb0.9STe2 nanoflake devices, e.g., high Hall mobility (∼1150 cm2/V s at 3 K), strong Shubnikov–de Haas oscillations with π Berry phase, weak antilocalization, and electron–electron interaction. Notably, additional oscillation patterns with quasi-periodicity-in-B and field-independent amplitude features are observed. The surface dominant transport behavior up to room temperature suggests that Bi1.1Sb0.9STe2 is a room temperature topological insulator for electronic/spintronic applications.

Anomalous Hall effect in Nd-doped Bi1.1Sb0.9STe2 topological insulator single crystals.

Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantum anomalous Hall effect, which has been observed in magnetic-topological-insulator-based devices. In this work, we report successful doping of rare-earth element Nd into Bi1.1Sb0.9STe2 bulk-insulating topological insulator single crystals, in which the Nd moments are ferromagnetically ordered at ∼100 K. Benefiting from the in-bulk-gap Fermi level, electronic transport behaviors dominated by the topological surface states are observed in the ferromagnetic region. At low temperatures, strong Shubnikov-de Haas oscillations with a nontrivial Berry phase are observed. The topological insulator with long range magnetic ordering in Nd-doped Bi1.1Sb0.9STe2 single crystals provides a good platform for quantum transport studies and spintronic applications.

Scattering-induced positive unsaturated linear magnetoresistance in LaAlO3/SrTiO3 two-dimensional electron gas system

In this paper, positive and unsaturated linear magnetoresistance (LMR) in an LaAlO3/SrTiO3 two-dimensional electron gas system is reported. LMR appears in this system over a wide range of temperatures and magnetic fields and does not tend to saturate at magnetic field up to 14 T. The spatial fluctuation in mobility is the most likely origin of positive unsaturated LMR because the existence of strong Shubnikov-de Haas oscillations rules out the extreme quantum origin. Moreover, LMR ∝ μ and HC ∝ 1/μ are confirmed and meet the requirements of Δμ/μ < 1 in the classical model proposed by Parish and Littlewood. This suggests that the observed LMR is the classical LMR effect induced by a narrow mobility distribution. These findings are of great interest for the practical application of magnetoresistive devices such as magnetic sensors, magnetometers, and magnetic storage devices.

Transversal magnetoresistance and Shubnikov–de Haas oscillations in Weyl semimetals

We explore theoretically the magnetoresistance of Weyl semimetals in transversal magnetic fields away from charge neutrality. The analysis within the self-consistent Born approximation is done for the two different models of disorder: (i) short-range impurties and (ii) charged (Coulomb) impurities. For these models of disorder, we calculate the conductivity away from charge neutrality point as well as the Hall conductivity, and analyze the transversal magnetoresistance (TMR) and Shubnikov-de Haas oscillations for both types of disorder. We further consider a model with Weyl nodes shifted in energy with respect to each other (as found in various materials) with the chemical potential corresponding to the total charge neutrality. In the experimentally most relevant case of Coulomb impurities, we find in this model a large TMR in a broad range of quantizing magnetic fields. More specifically, in the ultra-quantum limit, where only the zeroth Landau level is effective, the TMR is linear in magnetic field. In the regime of moderate (but still quantizing) magnetic fields, where the higher Landau levels are relevant, the rapidly growing TMR is supplemented by strong Shubnikov-de Haas oscillations, consistent with experimental observations.

Microwave radiation absorption and Shubnikov-de Haas oscillations in semimetal InAs/GaSb/AlSb composite quantum wells

Strong Shubnikov-de Haas (SdH) oscillations were observed in the derivative of microwave absorption (f=10 GHz) in InAs/GaSb/AlSb composite quantum wells (CQWs) using electron-paramagnetic resonance spectroscopy at low temperatures (2.7 to 20 K) and in the magnetic field up to 14 kOe. CQWs were grown on n-GaSb:Te(100) and n-InAs:Mn(100) substrates with various widths of QWs by MOVPE. A predominant contribution to the bulk n-GaSb substrate in SdH oscillations was manifested. Two frequencies of the SdH oscillations connected with warping of the Fermi surface of GaSb were found from Fourier analysis. An unusual angular indicatrix was observed in dependence on the orientation of the samples grown on n-GaSb in the magnetic field. The obtained results can be explained by bulk inversion asymmetry, which is a feature of substances with a lack of inversion centers. For CQWs grown on n-InAs:Mn substrate, only several SdH oscillations with higher period were observed. We succeeded in extracting a contribution of the two-dimensional carriers of InAs QW∼H⊥, where H⊥=H·cos Θ, from bulk substrate oscillations using a special spline interpolation from the angular dependence of SdH oscillatory amplitudes in the angle range of 0 to 90 deg.

Microwave radiation absorption by 2D-electrons in the type II composite InAs/GaSb/AlSb quantum wells in a magnetic field

Strong Shubnikov - de Haas (SdH) oscillations were observed in the derivative of microwave absorption (f = 10 GHz) in the InAs/GaSb/AlSb composite quantum wells (CQWs) using electron-paramagnetic-resonance spectroscopy at low temperatures (2.7-20 K) and in the magnetic field up to 14 kOe. CQWs were grown on the n-GaSb:Te(100) and n-InAs:Mn(100) substrates with various width of QWs by MOVPE. Predominance contribution of the bulk n- GaSb substrate in SdH oscillations was manifested. Two frequencies of the SdH oscillations were found from Fourier analysis, which is connected to warping of the Fermi surface of GaSb. Unusual angular indicatrix was observed in dependence on orientation of the samples in the magnetic field. Obtained results can be explained by inversion asymmetry, which is a feature of the substances with lack of inversion centres. For CQWs grown on n-InAs:Mn (ns = 1.1 × 1017 cm-3) substrate, only several SdH oscillations with higher period were observed. Taking into account isotropic Fermi surface of bulk InAs, we succeeded to extract a contribution of the 2D carriers of InAs QW ∼ H⊥, where H⊥= HconstcosΘ, from bulk substrate oscillations using special spline interpolation from angular dependence of SdH oscillatory amplitudes in the angle range 0-90°. 2D electron concentration in the InAs QW ns ≈ 1 - 3 × 1011 cm-2 was evaluated from oscillatory period.

Nearly massless Dirac fermions hosted by Sb square net in BaMnSb2.

Layered compounds AMnBi2 (A = Ca, Sr, Ba, or rare earth element) have been established as Dirac materials. Dirac electrons generated by the two-dimensional (2D) Bi square net in these materials are normally massive due to the presence of a spin-orbital coupling (SOC) induced gap at Dirac nodes. Here we report that the Sb square net in an isostructural compound BaMnSb2 can host nearly massless Dirac fermions. We observed strong Shubnikov-de Haas (SdH) oscillations in this material. From the analyses of the SdH oscillations, we find key signatures of Dirac fermions, including light effective mass (~0.052m0; m0, mass of free electron), high quantum mobility (1280 cm2V−1S−1) and a π Berry phase accumulated along cyclotron orbit. Compared with AMnBi2, BaMnSb2 also exhibits much more significant quasi two-dimensional (2D) electronic structure, with the out-of-plane transport showing nonmetallic conduction below 120 K and the ratio of the out-of-plane and in-plane resistivity reaching ~670. Additionally, BaMnSb2 also exhibits a G-type antiferromagnetic order below 283 K. The combination of nearly massless Dirac fermions on quasi-2D planes with a magnetic order makes BaMnSb2 an intriguing platform for seeking novel exotic phenomena of massless Dirac electrons.

Size-Quantization Semimetal–Semiconductor Transition in Bi0.98Sb0.02 Nanowires: Thermoelectric Properties

We fabricated Bi0.98Sb0.02 wires with diameters in the range between 200 nm and 2 μm and studied their electronic transport and thermoelectric properties. Rotational diagrams and strong Shubnikov–de Haas oscillations show that the wires have a high degree of crystallinity. The temperature dependence of the resistance evidences a semimetal–semiconductor transition at a critical diameter of roughly 300 nm. Our BiSb nanowire samples feature both electrons and holes that contribute to negative and positive thermopower, respectively; we find evidence of the interplay between the two types of carriers according to wire diameter and temperature. An evaluation of the power factor, and of its dependence on wire diameter, temperature, and the magnitude and direction of the magnetic field, is presented.

Transport/Magnetotransport of High-Performance Graphene Transistors on Organic Molecule-Functionalized Substrates

In this article, we present the transport and magnetotransport of high-quality graphene transistors on conventional SiO(2)/Si substrates by modification with organic molecule octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs). Graphene devices on OTS SAM-functionalized substrates with high carrier mobility, low intrinsic doping, suppressed carrier scattering, and reduced thermal activation of resistivity at room temperature were observed. Most interestingly, the remarkable magnetotransport of graphene devices with pronounced quantum Hall effect, strong Shubnikov-de Haas oscillations, a nonzero Berry's phase, and a short carrier scattering time also confirms the high quality of graphene on this ultrasmooth organic SAM-modified platform. The high-performance graphene transistors on the solution-processable OTS SAM-functionalized SiO(2)/Si substrates are promising for the future development of large-area and low-cost fabrications of graphene-based nanoelectronics.

Observation of oscillatory magnetoresistance periodic in1∕BandBinCa3Ru2O7

We report magnetoresistance oscillations in high magnetic fields, B, up to 45 T and over a wide range of temperature in the Mott-like system Ca3Ru2O7. For B rotating within the ac-plane, slow and strong Shubnikov-de Haas (SdH) oscillations periodic in 1/B are observed for T≤1.5 K in the presence of metamagnetism. These oscillations are highly angular dependent and intimately correlated with the spin-polarization of the ferromagnetic state. For B||[110], oscillations are also observed but periodic in B (rather than 1/B) which persist up to 15 K. While the SdH oscillations are a manifestation of the presence of small Fermi surface (FS) pockets in the Mott-like system, the B-periodic oscillations, an exotic quantum phenomenon, may be a result of anomalous coupling of the magnetic field to the t2g-orbitals that makes the extremal cross-section of the FS field-dependent.

Transport properties of two-dimensional electron gas in different subbands in triangular quantum wells at AlxGa1−xN/GaN heterointerfaces

Magnetotransport properties of modulation-doped Al0.22Ga0.78N/GaN heterostructures were investigated by means of magnetoresistance measurements at low temperatures and high magnetic fields. Strong Shubnikov–de Haas oscillations with the double periodicity are observed. The mobility spectrum is obtained, which demonstrates that the mobilities of the two-dimensional electron gas (2DEG) in the two subbands in the triangular quantum well at heterointerface. It is found that the mobility of the 2DEG in the second subband is much higher than that in the first one. This is explained that interface roughness scattering and alloy disorder scattering have much stronger influence on transport properties of the 2DEG in the first subband than that in the second subband in AlxGa1−xN/GaN heterostructures.

Minigaps in strained silicon quantum wells on tilted substrates

The two-dimensional electron gas formed at the inverted surface of a tilted silicon substrate shows unusual magnetotransport properties due to the presence of a minigap in the density of states. For metal–oxide–semiconductor inversion layers the strong scattering at the interface limits the mobility to values μ<10–20 000 cm2/V s. To achieve mobilities approaching 105 cm2/V s we have used strained Si:SiGe quantum wells grown on substrates tilted away from the (001) normal by 0°, 2°, 4°, 6°, and 10°. Their transport properties have been measured in the temperature range of 20–500 mK. All the samples show strong Shubnikov–de Haas oscillations. For the 2° and 4° samples the envelope of the fast oscillations is modulated by a longer period oscillation at low magnetic fields. We attribute the slow oscillation in the 2° and 4° samples to the presence of a minigap. For the 6° and 10° samples the minigap is higher than the Fermi energy and is not expected to influence the transport properties.

Interface roughness scattering in thin quantum wells

Electronic transport coefficients of very thin undoped GaInP/GaAs quantum wells have been measured at low temperatures by the low field Hall effect and by the Shubnikov-de Haas effect. Unlike the case in undoped AlGaAs/GaAs, strong Shubnikov-de Haas oscillations were observed in this structure after increasing the carrier concentration via the persistent photoconductivity effect. Low temperature mobilities close to 70 000 cm2 V-1 s-1 at a carrier concentration of 6.5 × 1011 cm-2 were observed in a 25 Å quantum well. The results are compared with the theory of interace roughness scattering and we find that the mobility of our samples goes as L1.3 where L is the well width. We conclude that the barrier height of the confining potential plays a very important role in determining the effect of interface roughness scattering on the mobility of very thin quantum wells.

Interface roughness scattering in thin, undoped GaInP/GaAs quantum wells

Electronic transport properties of very thin undoped GaInP/GaAs quantum wells have been measured by temperature dependent low field Hall effect and by Shubnikov–de Haas effect. Strong Shubnikov–de Haas oscillations were observed after increasing the electron concentration via the persistent photocurrent effect. Low temperature mobilities of up to 70 000 cm2/V s at carrier concentrations of 6.5×1011 cm−2 were observed in a 20 Å quantum well. The results are compared with the theory of interface roughness scattering which indicates extremely smooth interfaces; however, discrepancies between experiment and theory are observed.

Quantum and semi-classical magnetoresistance oscillations in a new organic metal (BEDT-TTF) 2T1Hg(SeCN) 4

Magnetoresistance studies of a new quasi-two- dimensional organic metal, (BEDT-TTF) 2T1Hg(SeCN) 4, are presented. Angular oscillations of the semi-classical magnetoresistance and very strong Shubnikov-de Haas (SdH) oscillations have been found both associated with an extremely weakly warped Fermi surface cylinder with the cross-sectional area amounting to 16% of the first Brillouin zone. The shape of the Fermi surface cross-section has been reconstructed using the measured periods of the angular magnetoresistance oscillations. The spin-splitting effect manifested in a non-monotonous angular dependence of the SdH oscillations points to the importance of many-body interactions in the system.

An InAlAs/InAs MODFET

An InAs modulation-doped field-effect transistor (MODFET) using an epitaxial heterostructure based entirely on arsenides is reported. The heterostructure was grown by MBE on InP and contains a 30-AA InAs channel. A 2- mu m-gate-length device displays well-behaved characteristics, showing sharp pinch-off (V/sub th/=0.8 V) and small output conductance (5 mS/mm) at 300 K. The maximum transconductance is 170 mS mm with a maximum drain current of 312 mA/mm. Strong channel quantization results in a breakdown voltage of -9.6 V, a severalfold improvement over previous InAs MODFETs based on antimonides. Low-temperature magnetic field measurements show strong Shubnikov-de Haas oscillations which, over a certain range of gate voltage, strongly indicate that the electron channel resides in the InAs layer.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High mobility AlInAs/InP high electron mobility transistor structures grown by organometallic vapor phase epitaxy

We have grown single and double-channel AlInAs/InP modulation doped heterostructures with electron mobilities as high as 5000 and 27 000 cm2/V s at 300 and 77 K, respectively. The sheet electron concentrations for these structures range from 1.5×1012 to 5×1012 cm−2. The layers exhibit strong Shubnikov de Haas oscillations, from which we determined two-dimensional electron gas mobilities at 1.8 K of 40 000 cm2/V s. The electrical properties of the AlInAs/InP heterostructures are the best reported for any device structures with InP as the active layer material.