Shubnikov-de Haas Research Articles

Charged Impurity Scattering and Electron-Electron Interactions in Large-Area Hydrogen Intercalated Bilayer Graphene.

Intercalation is a promising technique to modify the structural and electronic properties of 2D materials on the wafer scale for future electronic device applications. Yet, few reports to date demonstrate 2D intercalation as a viable technique on this scale. Spurred by recent demonstrations of mm-scale sensors, we use hydrogen intercalated quasi-freestanding bilayer graphene (hQBG) grown on 6H-SiC(0001), to understand the electronic properties of a large-area (16 mm2) device. To do this, we first analyze Shubnikov-de Haas (SdH) oscillations and weak localization, permitting determination of the Fermi level, cyclotron effective mass, and quantum scattering time. Our transport results indicate that at low temperature, scattering in hQBG is dominated by charged impurities and electron-electron interactions. Using low- temperature scanning tunneling microscopy and spectroscopy (STS), we investigate the source of the charged impurities on the nm-scale via observation of Friedel oscillations. Comparison to theory suggests that the Friedel oscillations we observe are caused by hydrogen vacancies underneath the hQBG. Furthermore, STS measurements demonstrate that hydrogen vacancies in the hQBG have an extremely localized effect on the local density of states, such that the Fermi level of the hQBG is only affected directly above the location of the defect. Hence, we find that the calculated Fermi level from SdH oscillations on the millimeter scale agrees with the value measured locally on the nanometer scale with STS measurements.

Obtained Berry phase and cyclotron mass of Bi2Se3 topological insulator thin film through weak anti-localization and Shubnikov-de haas oscillation studies

Bi-based binary alloys have drawn enormous attention in modern condensed matter research for their novel topological property. Here, we have explored the quantum-transport properties of a 100 nm Bi2Se3 topological insulator thin film grown by an indigenously developed electron-beam-evaporator through co-deposition technique. A detailed study about the structural, elemental, and morphological analysis has been presented through the GI-XRD, Raman spectroscopy, XPS, EDX, SEM, and AFM characterization. Finally, we have investigated the angle and temperature-dependent magneto-conductance properties of our deposited films, which indicate the surface-electron dominated quantum-transport has occurred. Interestingly, our Bi2Se3 film exhibits 2D weak anti-localization and Subnikov-de Hass oscillation features. From which some novel topological parameters are explored, such as, Berry phase (β), phase-coherence-length (l ϕ ), Fermi velocity (vF), wave vector (kF), Dingle temperature (TD), quantum mobility (μ q), and cyclotron mass (mc). The estimated β = 0.7π and mc = 0.17me, indicate that the topologically protected massless Dirac particles can be achieved in this kind of system.

Characterization of induced quasi-two-dimensional transport in n-type InxGa1−xAs1 − yBiy bulk layer

The temperature-dependent transport properties of n-type InGaAsBi epitaxial alloys with various doping densities are investigated by conducting magnetoresistance (MR) and Hall Effect (HE) measurements. The electronic band structure of the alloys and free electron distribution were calculated using Finite Element Method (FEM). Analysis of the oscillations in the transverse (Hall) resistivity shows that quasi-two-dimensional electron gas (Q-2D) in the bulk InGaAsBi epitaxial layer (three-dimensional, 3D) forms at the sample surface under magnetic field even though there is no formation of the spacial two-dimensional electron gas (2DEG) at the interface between InGaAs and InP:Fe interlayer. The formation of Q-2D in the 3D epitaxial layer was verified by temperature and magnetic field dependence of the resistivity and carrier concentration. Analysis of Shubnikov-de Haas (SdH) oscillations in longitudinal (sample) resistivity reveals that the electron effective mass in InGaAsBi alloys are not affect by Bi incorporation into host InGaAs alloys, which verifies the validity of the Valence Band Anti-Crossing (VBAC) model. The Hall mobility of the nondegenerate samples shows the conventional 3D characteristics while that of the samples is independence of temperature for degenerated samples. The scattering mechanism of the electrons at low temperature is in long-range interaction regime. In addition, the effects of electron density on the transport parameters such as the effective mass, and Fermi level are elucidated considering bandgap nonparabolicity and VBAC interaction in InGaAsBi alloys.

Optical Shubnikov–de Haas oscillations in two-dimensional electron systems

We report on dynamic Shubnikov–de Haas (SdH) oscillations that are measured in the optical response, subterahertz transmittance of two-dimensional systems, and reveal two distinct types of oscillation nodes: “universal” nodes at integer ratios of radiation and cyclotron frequencies and “tunable” nodes at positions sensitive to all parameters of the structure. The nodes in both real and imaginary parts of the measured complex transmittance are analyzed using a dynamic version of the static Lifshitz-Kosevich formula. These results demonstrate that the node structure of the dynamic SdH oscillations provides an all-optical access to quantization- and interaction-induced renormalization effects, in addition to parameters one can obtain from the static SdH oscillations. Published by the American Physical Society 2024

High-mobility spin-polarized quasi-two-dimensional electron gas and large low-field magnetoresistance at the interface of EuTiO3/SrTiO3 (110) heterostructures

High-mobility spin-polarized two-dimensional electron gas (2DEG) at the interfaces of complex oxide heterostructures provide great potential for spintronic device applications. Unfortunately, the interfacial ferromagnetism and its associated spin polarization of mobile electrons and negative magnetoresistance (MR) are too weak. As of now, obtaining enhanced interfacial ferromagnetism and MR and strong spin-polarized 2DEG is still a great challenge. In this paper, we report on the realization of strong spin-polarized 2DEG at the interface of EuTiO3/SrTiO3 (110) heterostructures, which were prepared by directly depositing 39-nm EuTiO3 films onto as-received SrTiO3 (110) substrates. Hall and Kondo effects, low-field MR, Shubnikov–de Haas (SdH) oscillation, and magnetic hysteresis loop measurements demonstrate that high mobility electrons (1.4 × 104 cm2 V−1 s−1) accumulate at the interface of the heterostructures, which are not only highly conducting and show SdH oscillations with a non-zero Berry phase but also show a large out-of-plane and in-plane butterfly-like negative low-field MR whose magnitude is unprecedentedly large (46%–59% at 500 Oe and 1.8 K), approximately one to two orders higher than those of previously reported spin-polarized 2DEG systems. The strong spin polarization of the interfacial 2DEG is attributed to the presence of interfacial Eu2+ 4f (3.6–4 μB/f.u.) and Ti3+ 3d moments. Our results may provide guidance for exploring strong spin-polarized 2DEG at the interface of rare-earth titanate–strontium titanate heterostructures.

Quantum interference in superposed lattices

Charge transport in solids at low temperature reveals a material's mesoscopic properties and structure. Under a magnetic field, Shubnikov-de Haas (SdH) oscillations inform complex quantum transport phenomena that are not limited by the ground state characteristics and have facilitated extensive explorations of quantum and topological interest in two- and three-dimensional materials. Here, in elemental metal Cr with two incommensurately superposed lattices of ions and a spin-density-wave ground state, we reveal that the phases of several low-frequency SdH oscillations in [Formula: see text] and [Formula: see text] are no longer identical but opposite. These relationships contrast with the SdH oscillations from normal cyclotron orbits that maintain identical phases between [Formula: see text] and [Formula: see text] . We trace the origin of the low-frequency SdH oscillations to quantum interference effects arising from the incommensurate orbits of Cr's superposed reciprocal lattices and explain the observed [Formula: see text]-phase shift by the reconnection of anisotropic joint open and closed orbits.

Shubnikov–de Haas oscillations of biaxial-strain-tuned superconductors in pulsed magnetic field up to 60 T

Two-dimensional (2D) materials have gained increasing prominence not only in fundamental research but also in daily applications. However, to fully harness their potential, it is crucial to optimize their properties with an external parameter and track the electronic structure simultaneously. Magnetotransport over a wide magnetic field range is a powerful method to probe the electronic structure and, for metallic 2D materials, quantum oscillations superimposed on the transport signals encode Fermi surface parameters. In this manuscript, we utilize biaxial strain as an external tuning parameter and investigate the effects of strain on the electronic properties of two quasi-2D superconductors, MoTe2 and RbV3Sb5, by measuring their magnetoresistance in pulsed magnetic fields up to 60 T. With a careful selection of insulating substrates, we demonstrate the possibility of both the compressive and tensile biaxial strains imposed on MoTe2 and RbV3Sb5, respectively. For both systems, the applied strain has led to superconducting critical temperature enhancement compared to their free-standing counterparts, proving the effectiveness of this biaxial strain method at cryogenic temperatures. Clear quantum oscillations in the magnetoresistance—the Shubnikov–de Haas (SdH) effect—are obtained in both samples. In strained MoTe2, the magnetoresistance exhibits a nearly quadratic dependence on the magnetic field and remains non-saturating even at the highest field, whereas in strained RbV3Sb5, two SdH frequencies showed a substantial enhancement in effective mass values, hinting at a possible enhancement of charge fluctuations. Our results demonstrate that combining biaxial strain and pulsed magnetic field paves the way for studying 2D materials under unprecedented conditions.

The comprehensive investigation of barrier layers on power loss mechanisms in AlGaN/GaN HEMT structures

The electron relaxation mechanism of the electrons in the pseudo triangle quantum well located in the Al0.3Ga0.7N/GaN heterostructure interface grown by the Molecular Beam Epitaxy (MBE) is studied by Shubnikov de-Haas (SdH) oscillations. Four different sample types were used in the experiments; the carrier density in the quantum well is the same for all samples, whereas the top layer structures are different. The effects of a spacer layer placed between the barrier layer and the quantum well layer, a doped barrier layer placed behind the undoped barrier layer, and a passivation layer placed as a top layer on the energy relaxation processes of the carriers in the quantum well were investigated. The SdH oscillation measurements were carried out under a magnetic field of up to 11 T at a temperature range of 1.8–20 K. The electron temperatures and energy relaxation mechanisms are obtained by comparing the change in the relative amplitude of SdH oscillations in different applied electric fields and lattice temperatures. It is deduced that the samples with a passivation layer and doped barrier layer have higher electron temperatures at lower applied electric fields. The hot electrons relax with acoustic phonon scattering for these samples, including deformation and piezoelectric interactions in the low-temperature region determined theoretically. However, the hot electrons relax with acoustic phonon scattering for samples with spacer layer and spacer/passivation layers in the high-temperature region determined theoretically. Furthermore, it was observed that the spacer and passivation layers lead to faster relaxation of hot electrons.

Tunable non-Lifshitz–Kosevich temperature dependence of Shubnikov–de Haas oscillation amplitudes in SmSb

The Lifshitz–Kosevich (LK) theory is the pillar of magnetic quantum oscillations, which have been extensively applied to characterise a wide range of metallic states. In this study, we focus on the Shubnikov–de Haas (SdH) effect observed in SmSb, a rare-earth monopnictide. We observed a significant departure from the expected LK theory near TN = 2.4 K: both a peak-like anomaly and an enhancement in the temperature dependence of quantum oscillation amplitude are seen in SmSb. Moreover, we discovered a remarkable sensitivity of the SdH amplitudes to sample purity. By adjusting the sample purity, we were able to tune the temperature dependence of the α band’s SdH amplitudes from a peak-like anomalous behaviour to an enhancement. Therefore, SdH oscillations from the α band connect the two well-known non-LK behaviours, controllable through varying the sample purity, paving the way for developing further understanding of the mechanism leading to the anomalous quantum oscillations.

Magneto-electronic coupling to the anomalous lattice expansion in Bi1.8Sb0.2Te[formula omitted] crystal

Our low-temperature synchrotron diffraction studies reveal an anomalous thermal expansion below ∼ 25 K (Tm). Below Tm, the Shubnikov-de Haas (SdH) oscillations are evident and anisotropic with respect to magnetic field, suggesting an anisotropic Fermi surface. Thermal variation of magnetization result shows a minimum around Tm, pointing to a significant magnetoelastic coupling. Paramagnetic singularities in the magnetization curve indicate the robust topologically-protected surface state. Careful comparison of the resistivity and magnetization results link the elastic coupling to the magnetic and electronic structure in Bi1.8Sb0.2Te3, which has not been explored in the well-studied pristine compound Bi2Te3.

Transport properties of a 1000 nm HgTe film: the interplay of surface and bulk carriers

We report on systematic study of transport properties of a 1000 nm HgTe film. Unlike thinner and strained HgTe films, which are known as high-quality three-dimensional topological insulators, the film under study is much thicker than the limit of pseudomorphic growth of HgTe on a CdTe substrate. Therefore, the 1000 nm HgTe film is expected to be fully relaxed and has the band structure of bulk HgTe, i.e. a zero gap semiconductor. Additionally, the system is characterized by the bands inversion, so that the two-dimensional topological surface states (TSSs) are expected to exist. To check this claim we studied classical and quantum transport response of the system. We demonstrate that by tuning the top-gate voltage one can change the electron-dominating transport to the hole one. The highest electron mobility is found to be more than cm2 Vs−1. The system exhibits Shubnikov-de Haas (SdH) oscillations with a complicated pattern and shows up to five independent frequencies in corresponding Fourier spectra. These Fourier peaks are attributed to the TSSs, Volkov-Pankratov states and spin-degenerate bulk states in the accumulation layer near the gate. The observed peculiarities of the quantum transport are the strong SdH oscillations of the Hall resistance, and the suppressed oscillatory response of the TSSs.

Nonlinear transport phenomena and current-induced hydrodynamics in ultrahigh mobility two-dimensional electron gas

We report on nonlinear transport phenomena at high filling factor and DC current-induced electronic hydrodynamics in an ultrahigh mobility $\text{(}\ensuremath{\mu}=20\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}\text{)}$ two-dimensional electron gas in a narrow ($15\ensuremath{-}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wide) GaAs/AlGaAs Hall bar for DC current densities reaching 0.67 A/m. The various phenomena and the boundaries between the phenomena are captured together in a two-dimensional differential resistivity map as a function of magnetic field (up to 250 mT) and DC current. This map, which resembles a phase diagram, demarcate distinct regions dominated by Shubnikov-de Haas (SdH) oscillations (and phase inversion of these oscillations) around zero DC current; negative magnetoresistance and a double-peak feature (both ballistic in origin) around zero field; and Hall field-induced resistance oscillations (HIROs) radiating out from the origin. From a detailed analysis of the data near zero field, we show that increasing the DC current suppresses the electron-electron scattering length that drives a growing hydrodynamic contribution to both the differential longitudinal and transverse (Hall) resistivities. Our approach to induce hydrodynamics with DC current differs from the more usual approach of changing the temperature. We also find a significant (factor of two to four) difference between the quantum lifetime extracted from SdH oscillations, and the quantum lifetime extracted from HIROs. In addition to observing HIRO peaks up to the seventh order, we observe an unexpected HIRO-like feature close to midway between the first-order and the second-order HIRO maxima at high DC current.

Weak antilocalization and topological edge states in PdSn4

Here, we report a successful synthesis of single crystals of a topological semimetal (TSM) candidate, PdSn4, using a self-flux route. The synthesized crystal is well characterized through x-ray diffraction (XRD), field emission scanning electron microscopy, and x-ray photoelectron spectroscopy. A detailed Rietveld analysis of the powder XRD pattern of PdSn4 confirmed the same to crystallize in an Aea2 space group instead of reported Ccce. A large magnetoresistance (MR) along with Shubnikov–de Haas (SdH) oscillations have been observed in magneto-transport measurements at 2 K. The presence of weak antilocalization (WAL) effect in the synthesized PdSn4 crystal is confirmed and analyzed using Hikami Larkin Nagaoka formalism, being applied on magnetoconductivity of the same at a low magnetic field. An extended Kohler's rule is implemented on MR data to determine the role of scattering processes and temperature-dependent carrier density on the transport phenomenon in PdSn4. Furthermore, the non-trivial band topology and the presence of edge states are shown through density functional theory-based theoretical calculations. All calculations are performed considering the Aea2 space group symmetry. The calculated Z2 invariants (0; 0 1 0) suggest the presence of weak topological insulating properties in PdSn4. A clear evidence of topological edge states at Γ point is visible in calculated edge states spectra. This is the first report on PdSn4, showing the presence of SdH oscillation in magneto-transport measurements. Also, the presence of a weak insulating phase in PdSn4 is described for the first time on the basis of calculated Z2 invariants.

Electrical conductivity of strongly magnetized dense quark matter - possibility of quantum Hall effect

We have pointed out the possibilities of quantum Hall effect (QHE) and Shubnikov-de Haas (SdH) oscillation in the transportation of strongly magnetized degenerate quark matter- expected inside the highly dense compact star, such as the neutron star. The anisotropic pattern of electrical conductivity and resistivity tensor is explored in classical to quantum cases by considering the Landau quantization of quarks in the presence of a finite magnetic field. With increasing magnetic field, classical to quantum transitions are realized through enhanced (reduced) resistivity (conductivity) with QHE/SdH type oscillatory patterns. Estimating QCD relaxation time for degenerate quark matter from Ref. [57], eB≈(1−4)mπ2 can be considered a strong magnetic field for massless and degenerate quark matter with quark chemical potential μ≈0.2−0.4 GeV. We discovered exotic quantum magnetic oscillating patterns associated with the QHE and SdH phenomena within the mentioned regimes.

Anisotropic magnetoresistance and quantum oscillation in Sb2Te3 single crystal synthesized by a Te-flux method

Topological insulators (TIs) have attracted the widespread attentions due to their unique electronic structures and novel physical properties in past decades. Here, we investigate the magnetoresistance (MR) and Shubnikov-de Haas (SdH) oscillation in topological Sb2Te3. It is found that the angle-dependent MR shows the obvious anisotropy, and the maximal MR reaches ∼220% with the magnetic field rotated in ab plane at 2.5 K and 9 T. The Landau-level fan diagram indicates that Sb2Te3 possesses a nontrivial Berry phase. Comparing the SdH oscillations measured by the magnetic fields rotated in bc and ab planes, we find that the oscillation frequencies are similar, and the Berry phase shift is very slight. The first-principles calculations demonstrate that Sb vacancy can introduce an additional band across the Fermi level. The anisotropy of the Fermi surfaces reveals the anisotropic MR.

Shubnikov-de Haas (SdH) Oscillation in Self-Flux Grown Rhombohedral Single-Crystalline Bismuth

The historic de Haas-van Alphen effect observed in the late 1950s in CSIR-NPL by J.S. Dhillon and D. Shoenberg in pure bismuth and zinc metal is revisited in this article through a single-crystalline phase of bismuth crystal, which is observed in terms of resistivity as predicted by Shubnikov-de Haas (SdH) oscillations. The single crystal of bismuth is grown through solid-state reaction under an optimized heat treatment whose purity and structural phase are confirmed through XRD, SEM, and EDAX. The transport properties of single crystal show the presence of SdH oscillations from a temperature range of 2 to 10 K. The occurrence of oscillations in the transverse magnetic field confirms the presence of the Fermi surface. Landau level (LL) fan diagram reveals the presence of topological surface states and the Berry’s phase confirmation from temperature-dependent SdH oscillations, concluding the presence of a noble topological phase of bismuth exhibiting SdH oscillations.

Microstructuring YbRh2Si2 for resistance and noise measurements down to ultra-low temperatures

The discovery of superconductivity in the quantum critical Kondo-lattice system YbRh2Si2 at an extremely low temperature of 2 mK has inspired efforts to perform high-resolution electrical resistivity measurements down to this temperature range in highly conductive materials. Here we show that control over the sample geometry by microstructuring using focused-ion-beam techniques allows to reach ultra-low temperatures and increase signal-to-noise ratios (SNRs) tenfold, without adverse effects to sample quality. In five experiments we show four-terminal sensing resistance and magnetoresistance measurements which exhibit sharp phase transitions at the Néel temperature, and Shubnikov–de-Haas (SdH) oscillations between 13 T and 18 T where we identified a new SdH frequency of 0.39 kT. The increased SNR allowed resistance fluctuation (noise) spectroscopy that would not be possible for bulk crystals, and confirmed intrinsic -type fluctuations. Under controlled strain, two thin microstructured samples exhibited a large increase of from 67 mK up to 188 mK while still showing clear signatures of the phase transition and SdH oscillations. Superconducting quantum interference device-based thermal noise spectroscopy measurements in a nuclear demagnetization refrigerator down to 0.95 mK, show a sharp superconducting transition at mK. These experiments demonstrate microstructuring as a powerful tool to investigate the resistance and the noise spectrum of highly conductive correlated metals over wide temperature ranges.

Electron transport, transverse and longitudinal magnetoresistance, and Shubnikov–de Haas oscillations in Bi0.83Sb0.17 topological insulator semiconductor wires

The paper describes results of experimental studies of the dependence of the resistance, transverse and longitudinal magnetoresistance, as well as the Shubnikov–de Haas (SdH) oscillations of Bi0.83Sb0.17 semiconductor single-crystal wires with diameters of (75–1100) nm, as a function of the thickness, in a temperature range of (3.1–300) K at magnetic fields of up to 14 T. The wires were prepared by liquid-phase casting. It has been found that the temperature dependences of the resistance of wires with d < 0.5 m have two regions exhibiting a semiconductor and a metallic behavior of the resistance, the two regions being separated by a maximum, which is shifted to the high-temperature region with a decrease in the wire diameter d. It has been revealed that the energy gap E increases by a factor of 2 with a decrease in wire diameter d, due to the occurrence of the quantum size effect. The “metallic” behavior of conductivity is attributed to surface states characteristic of topological insulators, which is most clearly evident in thin wires at temperatures of T < 50 K. It has been shown that, in the presence of a uniform magnetic field H, the field dependences of the longitudinal and transverse magnetoresistance in quasi-one-dimensional systems can undergo a significant change depending on the ratio of quantum wire radius to the magnetic length R = (ch/eH)1/2, as well as on the decrease in the mean free path of carriers due to scattering on the wire surface. The SdH oscillation periods exhibit anomalies that are typical neither to bulk Bi1xSbx samples nor to semimetallic wires based on Bi1xSbx alloys. This fact points to the essential role of surface states of topological insulators in 1D-systems, which lead to the occurrence of new effects that are not characteristic of other systems.

Nontrivial Berry's phase and exotic quantum phenomena in the vicinity of the quantum limit in the three-dimensional Dirac system Bi0.97Sb0.03

Quantum oscillations are known as the manifestation of the Fermi surface of metals in the presence of a strong magnetic field and of associated geometrical phases such as Berry's phase, playing a critical role in topological quantum materials. This study reports quantum oscillations on the three-dimensional (3D) Dirac material Bi0.97Sb0.03 single crystals in pulsed magnetic fields up to 50 T with three possible orientations. Under a magnetic field, we observe very pronounced Shubnikov-de Haas (SdH) oscillations originating from the Landau quantization of T-point holes. The SdH oscillations were well indexed with a large Landé g factor. The anisotropic behaviors of the Landé g factor were also deduced based on a detailed analysis of SdH oscillations. The Landau fan diagram revealed a nontrivial Berry's phase. Beyond the quantum limit (QL) under an extremely strong magnetic field, unusual quantum oscillations (UQO) were found. The possible origin of this phenomenon may be electronic instability caused by interactions. These results offer insights into the unusual electronic properties under the extremely strong fields caused by the unique band topology in this case.

Shubnikov–de Haas oscillations and nontrivial topological states in Weyl semimetal candidate SmAlSi

The RAlX (R = Light rare earth; X = Ge, Si) compounds, as a family of magnetic Weyl semimetal, have recently attracted growing attention due to the tunability of Weyl nodes and its interactions with diverse magnetism by rare-earth atoms. Here, we report the magnetotransport evidence and electronic structure calculations on nontrivial band topology of SmAlSi, a new member of this family. At low temperatures, SmAlSi exhibits large non-saturated magnetoresistance (MR) (as large as ∼5500% at 2 K and 48 T) and distinct Shubnikov–de Haas (SdH) oscillations. The field dependent MRs at 2 K deviate from the semiclassical (μ 0 H)2 variation but follow the power-law relation MR (μ 0 H) m with a crossover from m ∼ 1.52 at low fields (μ 0 H< 15 T) to m ∼ 1 under high fields (μ 0 H> 18 T), which is attributed to the existence of Weyl points and electron–hole compensated characteristics with high mobility. From the analysis of SdH oscillations, two fundamental frequencies originating from the Fermi surface pockets with non-trivial π Berry phases and small cyclotron mass can be identified, this feature is supported by the calculated electronic band structures with two Weyl pockets near the Fermi level. Our study establishes SmAlSi as a paradigm for researching the novel topological states of RAlX family.