Shubnikov-de Haas Measurements Research Articles

Magnetotransport and high-resolution angle-resolved photoelectron spectroscopy studies of palladium-doped Bi2Te3

We have performed magnetotransport and high-resolution angle-resolved photoelectron spectroscopy (ARPES) measurements on topological insulator ${\mathrm{Pd}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}(0\ensuremath{\le}x\ensuremath{\le}0.20)$ single crystals. We have observed unusually high values of magnetoresistance ($\ensuremath{\sim}1500%$) and mobility $(\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{--1}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{--1})$ at low temperatures for pristine ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ that decrease upon Pd doping. Shubnikov--de Haas (SdH) oscillations were detected for $x=0.05$, 0.10, confirming the presence of two-dimensional topological surface states (TSSs) for these samples. Hall measurement shows the crossover from $n$-type charge carriers in pristine ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ to $p$-type charge carriers upon Pd doping. The ARPES measurements show that the conduction band crosses the Fermi level for pristine ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, and the Dirac point of the TSSs and bulk-derived valence bands indicated a shift to lower binding energy upon Pd doping in a rigid-band-like way up to $x\ensuremath{\sim}0.10$. Based on the comparison of the parameters obtained from the SdH and ARPES measurements, the reduction in the ${k}_{\mathrm{F}}$ value in the magnetotransport measurements is likely due to the band bending induced by the Schottky barrier.

Quantum magneto-transport properties of GaAs/AlxGa1-xAs Weakly coupled quantum wells for near wavelength infrared detection

We investigated here the GaAs(d1 = 184 Å)/Al0.18Ga0.82As (d2 = 42 Å) multiple quantum wells based on our enhanced numerical calculations in the envelope function formalism. We calculated the band structures and the effective mass mi* of charge carriers. When the well thickness d1 increases, the bandgap Eg decreases like m*. The calculated Fermi level energy and the density of states show n-type conductivity and a quasi-two-dimensional transport character. When the barrier thickness d2 increases the bandwidth decreases as a result of the weakening of interlayer tunnelling along the growth direction in the first Brillouin zone. We also interpreted theoretically the Shubnikov-de Haas and quantum Hall effects measurements reported by B. G. Tavares et al. These results are necessary for the design and engineering of multiple quantum wells infrared detectors.

Puzzle of non-surface related 2D electron gas in n-InN epitaxial samples

We have studied multicarrier contributions to the conductivity in low concentration (with Hall concentration below 1×1018 cm−3) nondoped bulk n-InN samples using Shubnikov-de Haas (SdH) in the tilted magnetic field as well as variable field resistivity tensor measurements. In some samples, the Shubnikov-de Haas effect also revealed, besides 3D electron gas, high mobility 2D electron contributions (with transport and quantum mobilities reaching the values of 5060 cm2/V s and 1800 cm2/V s, respectively, for one of our samples), which could hardly be assigned to the surface electrons as the latter are commonly believed to have mobility too low to be detected at magnetic fields not exceeding 12 T in our SdH measurements. The values of the effective masses derived from the temperature dependences of the SdH oscillation amplitudes scale with the concentration of 2D channels and are typical for low concentration InN, thus confirming that these contributions are presumably located on the InN side of some interfaces. This is one of the first experimental evaluations of the effective mass of nonsurface-related 2D electron gas in InN grown on GaN.

Anomalous quantum oscillations and evidence for a non-trivial Berry phase in SmSb

Topologically non-trivial electronic structures can give rise to a range of unusual physical phenomena, and the interplay of band topology with other effects such as electronic correlations and magnetism requires further exploration. The rare earth monopnictides X(Sb,Bi) (X = lanthanide) are a large family of semimetals where these different effects may be tuned by the substitution of rare-earth elements. Here we observe anomalous behavior in the quantum oscillations of one member of this family, antiferromagnetic SmSb. The analysis of Shubnikov-de Haas (SdH) oscillations provides evidence for a non-zero Berry phase, indicating a non-trivial topology of the α-band. Furthermore, striking differences are found between the temperature dependence of the amplitudes of de Haas-van Alphen effect oscillations, which are well fitted by the Lifshitz-Kosevich (LK) formula across the measured temperature range, and those from SdH measurements which show a significant disagreement with LK behavior at low temperatures. Our findings of unusual quantum oscillations in an antiferromagnetic, mixed valence semimetal with a possible non-trivial band topology can provide an opportunity for studying the interplay between topology, electronic correlations and magnetism.

Electron transport in N-polar GaN-based heterostructures

Electron transport in N-polar GaN-based high-electron-mobility transistor (HEMT) structures with a combination of In0.18Al0.82N-AlN as the barrier was studied via temperature-dependent van der Pauw Hall and Shubnikov de Haas measurements. In contrast to Ga-polar HEMT structures, no persistent photoconductivity could be detected. In a sample with 10 nm thick InAlN, only one oscillation frequency was observed, demonstrating that a single sublevel is present. From the oscillations, a two-dimensional electron gas carrier density of 8.54 × 1012 cm−2 and a mobility of 4970 cm2/V s were extracted at 1.7 K. This sample was further investigated using ionic liquid gating. The charge density was varied from 7.5 × 1012 cm−2 to 9.6 × 1012 cm−2. The electron mobility significantly declined with decreasing charge density. This is in contrast to Ga-polar HEMT structures, where the electron mobility typically increases slightly as the charge density decreases.

Possible pressure-induced topological quantum phase transition in the nodal line semimetal ZrSiS

ZrSiS has recently gained attention due to its unusual electronic properties: nearly perfect electron-hole compensation, large, anisotropic magneto-resistance, multiple Dirac nodes near the Fermi level, and an extremely large range of linear dispersion of up to 2 eV. We have carried out a series of high pressure electrical resistivity measurements on single crystals of ZrSiS. Shubnikov-de Haas measurements show two distinct oscillation frequencies. For the smaller orbit, we observe a change in the phase of 0.5, which occurs between 0.16 - 0.5 GPa. This change in phase is accompanied by an abrupt decrease of the cross-sectional area of this Fermi surface. We attribute this change in phase to a possible topological quantum phase transition. The phase of the larger orbit exhibits a Berry phase of pi and remains roughly constant up to 2.3 GPa. Resistivity measurements to higher pressures show no evidence for pressure-induced superconductivity to at least 20 GPa.

Experimental determination of Rashba and Dresselhaus parameters and g*-factor anisotropy via Shubnikov-de Haas oscillations

The spin splitting of conduction band electrons in inversion-asymmetric InGaAs/InP quantum wells (QWs) is studied by Shubnikov-de Haas measurements combining the analysis of beating patterns and coincidence measurements in doubly tilted magnetic fields. The method allows us to determine the absolute values of the Rashba and linear Dresselhaus spin–orbit interaction (SOI) coefficients, their relative sign and the full Landé g-tensor. This is achieved by analyzing the anisotropy of the beat node positions with respect to both polar and azimuthal angles between the magnetic field direction and the QW normal. We show that the SOI is dominated by a large Rashba coefficient together with a linear Dresselhaus coefficient that is 10% of the Rashba coefficient. Their relative sign is found to be positive. The g-tensor is found to have a marked out-of-plane anisotropy and a smaller but distinct in-plane anisotropy due to SOI.

Shubnikov–de Haas measurements on a high mobility monolayer graphene flake sandwiched between boron nitride sheets

A flake of monolayer graphene was sandwiched between boron nitride sheets. Temperature dependent Shubnikov–de Haas measurements were performed to access how this technique influences the electronic properties of the graphene sample. The maximum mobility found in this configuration was approximately 105 cm2 Vs −1. From the phase of the oscillations a Berry phase β of 1/2 was obtained indicating the presence of Dirac fermions. We obtained Fermi velocities around m s−1 which imply hopping energies close to 2.5 eV. Furthermore, the carrier lifetime is typically higher than that found in graphene supported by SiO2, reaching values higher than 700 fs.

Temperature dependence of electron density and electron–electron interactions in monolayer epitaxial graphene grown on SiC

We report carrier density measurements and electron–electron (e–e) interactions in monolayer epitaxial graphene grown on SiC. The temperature (T)-independent carrier density determined from the Shubnikov–de Haas (SdH) oscillations clearly demonstrates that the observed logarithmic temperature dependence of the Hall slope in our system must be due to e–e interactions. Since the electron density determined from conventional SdH measurements does not depend on e–e interactions based on Kohn’s theorem, SdH experiments appear to be more reliable compared with the classical Hall effect when one studies the T dependence of the carrier density in the low T regime. On the other hand, the logarithmic T dependence of the Hall slope δRxy/δB can be used to probe e–e interactions even when the conventional conductivity method is not applicable due to strong electron–phonon scattering.

Fermi surface evolution of Na-doped PbTe studied through density functional theory calculations and Shubnikov–de Haas measurements

We present a combined experimental and theoretical study of the evolution of the low-temperature Fermi surface of lead telluride, PbTe, when holes are introduced through sodium substitution on the lead site. Our Shubnikov-de-Haas measurements for samples with carrier concentrations up to $9.4\times10^{19}$cm$^{-3}$ (0.62 Na atomic $\%$) show the qualitative features of the Fermi surface evolution (topology and effective mass) predicted by our density functional (DFT) calculations within the generalized gradient approximation (GGA): we obtain perfect ellipsoidal L-pockets at low and intermediate carrier concentrations, evolution away from ideal ellipsoidicity for the highest doping studied, and cyclotron effective masses increasing monotonically with doping level, implying deviations from perfect parabolicity throughout the whole band. Our measurements show, however, that standard DFT calculations underestimate the energy difference between the L-point and $\Sigma$-line valence band maxima, since our data are consistent with occupation of a single Fermi surface pocket over the entire doping range studied, whereas the calculations predict an occupation of the $\Sigma$-pockets at higher doping. Our results for low and intermediate compositions are consistent with a non-parabolic Kane-model dispersion, in which the L-pockets are ellipsoids of fixed anisotropy throughout the band, but the effective masses depend strongly on Fermi energy.

Chiral Weyl Pockets and Fermi Surface Topology of the Weyl Semimetal TaAs.

Tantalum arsenide is a member of the noncentrosymmetric monopnictides, which are putative Weyl semimetals. In these materials, three-dimensional chiral massless quasiparticles, the so-called Weyl fermions, are predicted to induce novel quantum mechanical phenomena, such as the chiral anomaly and topological surface states. However, their chirality is only well defined if the Fermi level is close enough to the Weyl points that separate Fermi surface pockets of opposite chirality exist. In this Letter, we present the bulk Fermi surface topology of high quality single crystals of TaAs, as determined by angle-dependent Shubnikov-de Haas and de Haas-van Alphen measurements combined with abinitio band-structure calculations. Quantum oscillations originating from three different types of Fermi surface pockets were found in magnetization, magnetic torque, and magnetoresistance measurements performed in magnetic fields up to 14T and temperatures down to 1.8K. Of these Fermi pockets, two are pairs of topologically nontrivial electron pockets around the Weyl points and one is a trivial hole pocket. Unlike the other members of the noncentrosymmetric monopnictides, TaAs is the first Weyl semimetal candidate with the Fermi energy sufficiently close to both types of Weyl points to generate chiral quasiparticles at the Fermi surface.

Nonmonotonic pressure evolution of the upper critical field in superconducting FeSe

The pressure dependence of the upper critical field, $H_\textrm{c2,c}$, of single crystalline FeSe was studied using measurements of the inter-plane resistivity, $\rho_{\textrm{c}}$ in magnetic fields parallel to tetragonal $c$-axis. $H_\textrm{c2,c}(T)$ curves obtained under hydrostatic pressures up to $1.56$ GPa, the range over which the superconducting transition temperature, $T_\textrm{c}$, of FeSe exhibits a non-monotonic dependence with local maximum at $p_1\approx$ 0.8 GPa and local minimum at $p_2\approx$ 1.2 GPa. The slope of the upper critical field at $T_\textrm{c}$, $\left(\textrm{d}H_\text{c2,c}/\textrm{d}T\right)_{T_\textrm{c}}$, also exhibits a non-monotonic pressure dependence with distinct changes at $p_1$ and $p_2$. For $p<p_1$ the slope can be described within multi-band orbital model. For both $p_1<p <p_2$ and $p>p_2$ the slope is in good quantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov-de Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of $T_\textrm{c}(p)$ at $p_2\approx$ 1.2 GPa.

Splitting of the Fermi Contour of Quasi-2D Electrons in Parallel Magnetic Fields.

In a quasi-two-dimensional electron system with nonzero layer thickness, a parallel magnetic field can couple to the out-of-plane electron motion and lead to a severe distortion and eventual splitting of the Fermi contour. Here we directly and quantitatively probe this evolution through commensurability and Shubnikov-de Haas measurements on electrons confined to a 40-nm-wide GaAs (001) quantum well. We are able to observe the Fermi contour splitting phenomenon, in good agreement with the results of semiclassical calculations. Experimentally, we also observe intriguing features, suggesting magnetic-breakdown-type behavior when the Fermi contour splits.

Influence of the Al concentration on the electronic properties of coupled and uncoupled AlxGa1−xAs/AlAs/AlyGa1−yAs double quantum wells

Shubnikov-de Haas (SdH) and Hall measurements have been used to investigate the electron densities and mobilities in a pair of adjacent two-dimensional electron gases (2DEGs), which were confined in coupled and uncoupled AlxGa1−xAs/AlAs/AlyGa1−yAs double quantum wells (DQWs), as a function of front-gate voltage (Vg). The electron densities and mobilities of the first and second sub-bands were determined using the fast Fourier transform (FFT) in both coupled and uncoupled samples, named S1, S2 and S3, which have different aluminum (Al) content in each well. Sample S1 was illuminated in order to reach the coupled behavior. The electronic properties of the samples have been compared, showing that the electron density and mobility decrease with increasing Al content within the quantum wells (QWs). The last fact was attributed to the alloy scattering in the AlxGa1−xAs quantum well.

Bulk Fermi surface and electronic properties of Cu0.07Bi2Se3

The electronic properties of Cu$_{0.07}$Bi$_{2}$Se$_{3}$ have been investigated using Shubnikov-de Haas and optical reflectance measurements. Quantum oscillations reveal a bulk, three-dimensional Fermi surface with anisotropy $k^{c}_{F}/k^{ab}_{F}\approx$ 2 and a modest increase in free-carrier concentration and in scattering rate with respect to the undoped Bi$_{2}$Se$_{3}$, also confirmed by reflectivity data. The effective mass is almost identical to that of Bi$_{2}$Se$_{3}$. Optical conductivity reveals a strong enhancement of the bound impurity bands with Cu addition, suggesting that a significant number of Cu atoms enter the interstitial sites between Bi and Se layers or may even substitute for Bi. This conclusion is also supported by X-ray diffraction measurements, where a significant increase of microstrain was found in Cu$_{0.07}$Bi$_{2}$Se$_{3}$, compared to Bi$_{2}$Se$_{3}$.

Gate voltage control of the Rashba effect in a p-type GaSb quantum well and application in a complementary device

The gate voltage dependence of the Rashba effect in a p-type quantum well was investigated by using Shubnikov–de Haas measurements. The GaSb-based p-type quantum well has a large Rashba spin–orbit interaction parameter of 1.71×10−11eVm for a zero gate voltage and exhibits gate controllability. We also propose a complementary logic device using n- and p-type spin transistors that simultaneously utilize charge and spin currents to improve the signal margin.

Terahertz imaging using strained-Si MODFETs as sensors

We report on non-resonant (broadband) and resonant detection of terahertz radiation using strained-Si modulation doped field effect transistors. The devices were excited at room temperature by two types of terahertz sources (an electronic source based on frequency multipliers at 0.292THz and a pulsed parametric laser at 1.5THz). In both cases, a non-resonant response with maxima around the threshold voltage was observed. Shubnikov-de Haas and photoresponse measurements were performed simultaneously and showed a phase-shift of π/2 in good agreement with the theory, which demonstrates that the observed response is related to the plasma waves oscillation in the channel. The non-resonant features were used to demonstrate the capabilities of such devices in terahertz imaging. We also cooled our device down to 4.2K to increase the quality factor and resonant detection was observed by using a tunable source of terahertz radiation.

Effective mass of electron in monolayer graphene: Electron-phonon interaction

Shubnikov-de Haas (SdH) and Hall effect measurements performed in a temperature range between 1.8 and 275 K, at an electric field up to 35 kV m−1 and magnetic fields up to 11 T, have been used to investigate the electronic transport properties of monolayer graphene on SiC substrate. The number of layers was determined by the use of the Raman spectroscopy. The carrier density and in-plane effective mass of electrons have been obtained from the periods and temperature dependencies of the amplitude of the SdH oscillations, respectively. The effective mass is in good agreement with the current results in the literature. The two-dimensional (2D) electron energy relaxations in monolayer graphene were also investigated experimentally. The electron temperature (Te) of hot electrons was obtained from the lattice temperature (TL) and the applied electric field dependencies of the amplitude of SdH oscillations. The experimental results for the electron temperature dependence of power loss indicate that the energy relaxation of electrons is due to acoustic phonon emission via mixed unscreened piezoelectric interaction and deformation-potential scattering.

Shubnikov-de Haas measurements on LuRh2Si2

We present Shubnikov-de Haas measurements on LuRh2Si2, the non-magnetic reference compound to the prototypical heavy-fermion system YbRh2Si2. We find an extensive set of orbits with clear angular dependences. Surprisingly, the agreement with non-correlated band structure calculations is limited. This may be related to an uncertainty in the calculations arising from a lack of knowledge about the exact Si atom position in the unit cell. The data on LuRh2Si2 provide an extensive basis for the interpretation of measurements on YbRh2Si2 indicative of discrepancies between the high-field Fermi surface of YbRh2Si2 and the "small" Fermi surface configuration.

Temperature dependent energy relaxation time in AlGaN/AlN/GaN heterostructures

The two-dimensional (2D) electron energy relaxation in Al0.25Ga0.75N/AlN/GaN heterostructures was investigated experimentally by using two experimental techniques; Shubnikov-de Haas (SdH) effect and classical Hall Effect. The electron temperature (Te) of hot electrons was obtained from the lattice temperature (TL) and the applied electric field dependencies of the amplitude of SdH oscillations and Hall mobility. The experimental results for the electron temperature dependence of power loss are also compared with the current theoretical models for power loss in 2D semiconductors. The power loss that was determined from the SdH measurements indicates that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. In addition, the power loss from the electrons obtained from Hall mobility for electron temperatures in the range Te>100K is associated with optical phonon emission. The temperature dependent energy relaxation time in Al0.25Ga0.75N/AlN/GaN heterostructures that was determined from the power loss data indicates that hot electrons relax spontaneously with MHz to THz emission with increasing temperatures.