Shubnikov-de Haas Research Articles

Interference evidence for Rashba-type spin splitting on a semimetallicWTe2surface

Semimetallic tungsten ditelluride displays an extremely large nonsaturating magnetoresistance, which is thought to arise from the perfect $n\ensuremath{-}p$ charge compensation with low carrier densities in $\mathrm{WT}{\mathrm{e}}_{2}$. We find a strong Rashba spin-orbit effect in density functional calculations due to the noncentrosymmetric structure. This lifts twofold spin degeneracy of the bands. A prominent umklapp interference pattern is observed by our scanning tunneling microscopic measurements at 4.2 K, which differs distinctly from the surface atomic structure demonstrated at 77 K. The energy dependence of umklapp interference shows a strong correspondence with densities of states integrated from ARPES measurement, manifesting a fact that the bands are spin-split on the opposite sides of \ensuremath{\Gamma}. Spectroscopic survey reveals the electron/hole asymmetry changes alternately with lateral locations along the $b$ axis, providing a microscopic picture for double-carrier transport of semimetallic $\mathrm{WT}{\mathrm{e}}_{2}$. The conclusion is further supported by our ARPES results and Shubnikov--de Haas (SdH) oscillations measurements.

Complex quantum transport in a modulation doped strained Ge quantum well heterostructure with a high mobility 2D hole gas

The complex quantum transport of a strained Ge quantum well (QW) modulation doped heterostructure with two types of mobile carriers has been observed. The two dimensional hole gas (2DHG) in the Ge QW exhibits an exceptionally high mobility of 780 000 cm2/Vs at temperatures below 10 K. Through analysis of Shubnikov de-Haas oscillations in the magnetoresistance of this 2DHG below 2 K, the hole effective mass is found to be 0.065 m0. Anomalous conductance peaks are observed at higher fields which deviate from standard Shubnikov de-Haas and quantum Hall effect behaviour due to conduction via multiple carrier types. Despite this complex behaviour, analysis using a transport model with two conductive channels explains this behaviour and allows key physical parameters such as the carrier effective mass, transport, and quantum lifetimes and conductivity of the electrically active layers to be extracted. This finding is important for electronic device applications, since inclusion of highly doped interlayers which are electrically active, for enhancement of, for example, room temperature carrier mobility, does not prevent analysis of quantum transport in a QW.

Effective mass of two-dimensional electrons in InGaAsN/GaAsSb type II quantum well by Shubnikov-de Haas oscillations

In order to develop optical devices for 2–3 μm wavelength regions, the InP-based InGaAs/GaAsSb type II multiple quantum well system has been investigated. By doping nitrogen into InGaAs layers, the system becomes effective in creating the optical devices with a longer wavelength. In this report, electrical transport properties are reported on the InGaAsN/GaAsSb type II system. The epitaxial layers with the single hetero or multiple quantum well structure on InP substrates are grown by the molecular beam epitaxy. The electrical resistance of samples with different nitrogen concentrations has been measured as a function of the magnetic field up to 9 Tesla at several temperatures between 2 and 6 K. The oscillation of the resistance due to the Shubnikov-de Haas (SdH) effect has been observed at each temperature. The effective mass is obtained from the temperature dependence of the amplitude of the SdH oscillations. The value of the effective mass increases from 0.048 for N = 0.0% to 0.062 for N = 1.2 and 1.5% as the nitrogen concentration increases. The mass enhancement occurs with corresponding to the reduction of the bandgap energy. These results are consistent with the band anticrossing model.

(Invited) Physical Properties of Novel 2D Semimetal WTe2: From Microscopic Study to Devices

An increasing number of alternative two-dimensional (2D) materials are being explored in the “post-graphene age”, such as transition metal dichalcogenides (TMDs), silicene, and germanene. In this study, multiple measurements, including scanning tunneling microscopy (STM), electrical transport and Raman spectroscopy are performed on freshly cleaved surfaces of semimetallic tungsten ditelluride (WTe2) crystals. High sample quality with very low density of defects is confirmed by STM observation and four Fermi pockets can be revealed by Shubnikov–de Haas (SdH) oscillations based on transport measurement. Furthermore, we investigate Raman spectroscopy of thin-layer WTe2. Our results reveal that few-layer WTe2 could be an ideal candidate for future 2D electronic, optoelectronic devices.

Shubnikov–de Haas Effect and Angular-Dependent Magnetoresistance in Layered Organic Conductor β′′-(ET)(TCNQ)

This paper reports the experimental results of the Shubnikov–de Haas (SdH) effect and angular-dependent magnetoresistance oscillation (AMRO) for the organic conductor β′′-(ET)(TCNQ). We observed several two dimensional (2D) SdH frequencies, whose cross-sectional areas of the Fermi surfaces (FSs) correspond to only a few percent of the first Brillouin zone. Such small 2D FSs are not predicted by band-structure calculations, suggesting that these FS pockets are created by an imperfect nesting of FSs at low temperatures. It is found that the AMRO consists of a long-period oscillation and a short-period one. The long-period oscillation is associated with the Yamaji oscillation corresponding to the α orbit, whose shape and area are consistent with previous magneto-optical measurement. The short-period oscillation is not caused by peaks instead but dips. The dip structure is discussed in terms of the AMRO of a quasi-2D electron system with a periodic potential caused by the possible density-wave related to the ...

Magneto-transport properties of As-implanted highly oriented pyrolytic graphite

We report on magneto-transport experiments in a high-quality sample of highly-oriented pyrolytic graphite (HOPG). Magneto-resistance and Hall resistivity measurements were carried out in magnetic inductions up to B=9T applied parallel to the c-axis at fixed temperatures between T=2K and T=12K. The sample was submitted to three subsequent irradiations with As ions. The implanted As contents were 2.5, 5 and 10at% at the maximum of the distribution profile. Experiments were performed after each implantation stage. Shubnikov-de Haas (SdH) oscillations were observed in both the magneto-resistance and Hall-effect measurements. Analyses of these results with fast Fourier transform (FFT) lead to fundamental frequencies and effective masses for electrons and holes that are independent of the implantation fluences. The Hall resistivity at low temperatures shows a sign reversal as a function of the field in all implanted states. We interpret the obtained results with basis on a qualitative model that supposes the existence of an extrinsic hole density associated to the defect structure of our sample. We conclude that the As implantation does not produce a semiconductor-type doping in our HOPG sample. Instead, an increase in the extrinsic hole density is likely to occur as a consequence of disorder induced by implantation.

Spatial mobility fluctuation induced giant linear magnetoresistance in multilayered graphene foam

Giant, positive, and near-temperature-independent linear magnetoresistance (LMR), as large as 340%, was observed in graphene foam with a three-dimensional flexible network. Careful analysis of the magnetoresistance revealed that Shubnikov--de Haas (SdH) oscillations occurred at low temperatures and decayed with increasing temperature. The average classical mobility ranged from 300 (2 K) to 150 (300 K) $\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, which is much smaller than that required by the observed SdH oscillations. To understand the mechanism behind the observation, we performed the same measurements on the microsized graphene sheets that constitute the graphene foam. Much more pronounced SdH oscillations superimposed on the LMR background were observed in these microscaled samples, which correspond to a quantum mobility as high as $26,500\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Moreover, the spatial mobility fluctuated significantly from $64,200\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ to $1370\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, accompanied by a variation of magnetoresistance from near 20,000% to less than 20%. The presence of SdH oscillations actually excludes the possibility that the observed LMR originated from the extreme quantum limit, because this would demand all electrons to be in the first Landau level. Instead, we ascribe the large LMR to the second case of the classical Parish and Littlewood model, in which spatial mobility fluctuation dominates electrical transport. This is an experimental confirmation of the Parish and Littlewood model by measuring the local mobility randomly (by measuring the microsized graphene sheets) and finding the spatial mobility fluctuation.

Large transverse Hall-like signal in topological Dirac semimetal Cd3As2.

Cadmium arsenide (Cd3As2) is known for its inverted band structure and ultra-high electron mobility. It has been theoretically predicted and also confirmed by ARPES experiments to exhibit a 3D Dirac semimetal phase containing degenerate Weyl nodes. From magneto-transport measurements in high quality single crystals of Cd3As2, a small effective mass m* ≈ 0.05 me is determined from the Shubnikov-de Haas (SdH) oscillations. In certain field orientations, we find a splitting of the SdH oscillation frequency in the FFT spectrum suggesting a possible lifting of the double degeneracy in accord with the helical spin texture at outer and inner Fermi surfaces with opposite chirality predicted by our ab initio calculations. Strikingly, a large antisymmetric magnetoresistance with respect to the applied magnetic fields is uncovered over a wide temperature range in needle crystal of Cd3As2 with its long axis along [112] crystal direction. It reveals a possible contribution of intrinsic anomalous velocity term in the transport equation resulting from a unique 3D Rashba-like spin splitted bands that can be obtained from band calculations with the inclusion of Cd antisite defects.

Studies on proximity effect in Mo/Bi1.95Sb0.05Se3 hybrid structure

Proximity effect in a mechanically exfoliated Bi1.95Sb0.05Se3 topological insulator (TI) single crystal partially covered with disordered superconducting (SC) Mo thin film is reported. Magneto-transport measurement was performed simultaneously across three different regions of the sample viz. SC, TI and SC / TI junction. Resistance measured across SC shows a TC at 4.3 K concomitantly the resistance measurement on TI showed a metallic trend with a steep upturn at TC. Magneto-resistance (MR) measurement on TI exhibit a positive MR with Shubnikov-de Haas (SdH) oscillations, whereas on SC a positive MR superimposed with steep cusp close to TC is observed. Across SC/TI junction both SdH oscillation and the cusp were observed. The frequency of SdH oscillation on SC / TI junction is found to be lesser (~125 T) as compared to a reference Bi1.95Sb0.05Se3 sample (~174 T). Upper critical field HC2 deduced from WHH fit was found to be 17.14 T for a reference Mo film whereas Mo film deposited on TI showed a decreased HC2 of 4.05 T. The coherence length for the former was found to be 4.38 nm and for the latter 9.01 nm. The interaction between the spin-less Cooper pairs in SC with the spin-momentum locked carriers on the surface of TI is believed to cause such changes in transport properties.

Fermi surface topology and negative longitudinal magnetoresistance observed in the semimetalNbAs2

We report transverse and longitudinal magneto-transport properties of NbAs2 single crystals. Attributing to the electron-hole compensation, non-saturating large transverse magnetoresistance reaches up to 8000 at 9 T at 1.8 K with mobility around 1 to 2 m^2V^-1S^-1. We present a thorough study of angular-dependent Shubnikov-de Haas (SdH) quantum oscillations of NbAs2. Three distinct oscillation frequencies are identified. First-principles calculations reveal four types of Fermi pockets: electron alpha pocket, hole beta pocket, hole gamma pocket and small electron delta pocket. Although the angular dependence of alpha, beta and delta agree well with the SdH data, it is unclear why the gamma pocket is missing in SdH. Negative longitudinal magnetoresistance is observed which may be linked to novel topological states in this material, although systematic study is necessary to ascertain its origin.

Beating patterns in the Shubnikov-de Haas oscillations originated from spin splitting in In0.52Al0.48As/In0.65Ga0.35As heterostructures: Experiment and calculation

Shubnikov-de Haas (SdH) oscillation measurements at 1.5K were carried out for In0.52Al0.48As/In0.65Ga0.35As heterostructures with different Si delta-doping concentration and spacer thickness. The dominant zero-magnetic field spin splitting mechanism is attributed to the contribution by the Rashba term due to the structure inversion asymmetry (SIA) in the In0.65Ga0.35As quantum well. The origin of the beating pattern in the SdH oscillations is investigated through the calculation of the transverse magnetoresistance versus magnetic field B by considering the Zeeman splitting and zero-magnetic field spin splitting in Si delta-doped In0.65Ga0.35As quantum wells. The good agreement between the theoretical and experimental curves suggest that the origin of beating patterns of the SdH oscillations is due to interplay of Rashba spin splitting, Zeeman splitting and Landau splitting.

(Invited) 2D Transport and Velocity Modulation in Black Phosphorus Field Effect Transistors

Black phosphorus (bP) is an elemental allotrope with a layered crystal structure of puckered honeycombs that can be mechanically exfoliated down to atomic layer thickness. First synthesized over a century ago, bP has been previously studied as a bulk semiconductor [1,2] and is subject to a resurgence of interest as a 2D semiconductor with properties complementary to the 2D semi-metal graphene [3]. bP is the most thermodynamically stable allotrope of phosphorus but is nonetheless subject to photo-oxidation in the presence of water, oxygen and visible light with a reaction rate that increases as bP layer thickness decreases [4]. We have fabricated bP quantum wells in both back-gated and dual-gated field effect transistor (FET) geometries with bP thicknesses ranging from 6±1 nm to 47±1 nm. Stability under ambient conditions was achieved in back-gated FETs by encapsulation with a layer of poly methyl methacrylate (PMMA), while dual-gated FETs were additionaly protected by a combination of Al2O3 deposited by atomic layer deposition and optically opaque gold metallization. Charge transport measurements reveal ambipolar conduction, field effect mobilities of up to 900 cm2/Vs for holes at low temperatures ( T< 100 K ), current saturation as expected of a bona fide semiconductor, and on/off current ratios exceeding 105 at room temperature [5]. Shubnikov-de Haas (SdH) oscillations were observed in bP FETs at 300 mK to 30 K and magnetic fields of up to 35 T. Lifshitz-Kosevich analysis of the SdH oscillations indicates the presence of a 2-D hole gas with Schrödinger fermion character in an accumulation layer at the bP surface. A simple Schrödinger-Poisson analysis predicts accumulation of holes in a single 2-D sub-band at 300 mK over the range of applied electric fields used in our experiments. Analysis of the thermal activation of SdH oscillations yields a hole effective mass of m*=0.36±0.03m0. Improvements to bP device quality by the independent Y. Zhang group has led to the observation of the quantum Hall effect [6].The top gate of dual-gate bP FETs was found to be effective at modulating both carrier mobility and contact resistance [7]. The mobility modulation effect enables operation of the dual-gate bP FET as a velocity modulated transistor (VMT), first proposed by Sakaki [8] to overcome the limitation on transistor switching frequency imposed by the channel transit time of charge carriers. The mobility of charge carriers within an assymetric bP FET structure is dependent upon the charge carrier density distribution, and is thereby dependent upon the externally applied electric field of the top gate. A two-fold mobility modulation and a two-fold contact resistance modulation leads to a four-fold modulation of transconductance at a fixed hole density. The observation of mobility modulation effects in dual-gate bP FETs demonstrates the capacity for bP to function as a room temperature VMT. Charge density distribution plays an important role in the charge transport properties of 2D atomic crystals, where the exposed surfaces of naked quantum well structures can lead to a strong spatial dependence of charge carrier scattering rates. The recent observation in angle resolved photo-emission spectroscopy of bandgap tuning and a semiconductor to semi-metal transition in K-doped bP via a giant Stark effect [9] suggests the possibility of electric field tuning of bP bandgap in bP FETs. Further work is required to improve the quality of bP/dielectric interfaces and to increase the applied electric field strength to access electric field tunable bandgap devices with bP.

Fermi surface reconstruction in FeSe under high pressure

We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to $P$ = 16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at $P$ $\sim$ 8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative d$T_c$/d$P$ observed in a range between $P$ $\sim$ 8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: $k_BT_c/E_F$ $\sim$ 0.1 even at $P$ = 16.1 kbar.

Large unsaturated positive and negative magnetoresistance in Weyl semimetal TaP

After growing successfully TaP single crystal, we measured its longitudinal resistivity (rhoxx) and Hall resistivity (rhoyx) at magnetic fields up to 9T in the temperature range of 2-300K. It was found that at 2K its magnetoresistivity (MR) reaches to 328000 percent, at 300K to 176 percent at 8T, and both do not appear saturation. We confirmed that TaP is indeed a low carrier concentration, hole-electron compensated semimetal, with a high mobility of hole muh=371000 cm2V-1s-1, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, as a magnetic field (H) is applied in parallel to the electric field (E), the negative MR due to chiral anomaly is observed, and reaches to -3000 percent at 9T without any signature of saturation, too, which distinguishes with other Weyl semimetals (WSMs). The analysis on the Shubnikov-de Haas (SdH) oscillations superimposing on the MR reveals that a nontrivial Berry phase with strong offset of 0.3958 realizes in TaP, which is the characteristic feature of the charge carriers enclosing a Weyl nodes. These results indicate that TaP is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications.

Shubnikov-de Haas Oscillations of High-Mobility Holes in Monolayer and Bilayer WSe_{2}: Landau Level Degeneracy, Effective Mass, and Negative Compressibility.

We study the magnetotransport properties of high-mobility holes in monolayer and bilayer WSe_{2}, which display well defined Shubnikov-de Haas (SdH) oscillations, and quantum Hall states in high magnetic fields. In both mono- and bilayer WSe_{2}, the SdH oscillations and the quantum Hall states occur predominantly at even filling factors, evincing a twofold Landau level degeneracy. The Fourier transform analysis of the SdH oscillations in bilayer WSe_{2} reveals the presence of two subbands localized in the top or the bottom layer, as well as negative compressibility. From the temperature dependence of the SdH oscillations we determine a hole effective mass of 0.45m_{0} for both mono- and bilayer WSe_{2}.

Investigation of the magnetoresistivity in compositional superlattices under the influence of an intense electromagnetic wave

The magnetoresistivity (MR) is theoretically calculated in a compositional semiconductor superlattice (CSSL), subjected to a crossed DC electric field and magnetic field, in the presence of an intense electromagnetic wave (EMW). The magnetic field is oriented along the growth direction of the CSSL and the electron–acoustic phonon interaction is taken into account at low temperature. Numerical results for the GaN/AlGaN CSSL show the Shubnikov–de Haas (SdH) oscillations in the MR whose period does not depend on the temperature and amplitude decreases with increasing temperature. The temperature dependence of the relative amplitude of these oscillations is in good agreement with other theories and experiments in some two-dimensional (2D) electron systems. The influence of the EMW as well as superlattice structure on the MR is discussed and compared with available theoretical and experimental results.

Magnetoresistance (MR) of twisted bilayer graphene on electron transparent substrate

We studied the magnetoresistance (MR) of twisted bilayer graphene (tBLG) on electron transparent substrate. Samples of tBLG were assembled on free-standing silicon nitride (SiNx) membranes (<100nm thick) by transferring chemical vapor deposition (CVD)-grown single layer graphene (SLG) twice; this allowed the measurement of the angle of rotation between the two layers, the twist angle, by electron diffraction using a transmission electron microscope (TEM). To compare with the previous reports on tBLG, we performed Raman spectroscopy on our samples. We measured the MR of tBLG for two different twist angles: 2° (small) and 18° (large). The MR showed superposition of two Shubnikov de Haas (SdH) oscillations for both angles. An analysis of the oscillation peaks by Landau fan diagrams showed difference as twist angle. While the large twist angle (18°) sample had two anomalous π Berry’s phases, the small twist angle (2°) sample had conventional 2π and anomalous π Berry’s phase depending on carrier density.

Quantum magnetotransport properties of aMoS2monolayer

We study transport properties of a ${\text{MoS}}_{2}$ monolayer in the presence of a perpendicular magnetic field $B$. We derive and discuss its band structure and take into account spin and valley Zeeman effects. Compared to a conventional two-dimensional electron gas, these effects lead to new quantum Hall plateaus and new peaks in the longitudinal resistivity as functions of the magnetic field. The field $B$ leads to a significant enhancement of the spin splitting in the conduction band, to a beating of the Shubnikov--de Haas (SdH) oscillations in the low-field regime, and to their splitting in the high-field regime. The Zeeman fields suppress significantly the beating of the SdH oscillations in the low-field regime and strongly enhance their splitting at high fields. The spin and valley polarizations show a similar beating pattern at low fields and are clearly separated at high fields in which they attain a value higher than $90%$.

π Berry phase and Zeeman splitting of Weyl semimetal TaP.

The recent breakthrough in the discovery of Weyl fermions in monopnictide semimetals provides opportunities to explore the exotic properties of relativistic fermions in condensed matter. The chiral anomaly-induced negative magnetoresistance and π Berry phase are two fundamental transport properties associated with the topological characteristics of Weyl semimetals. Since monopnictide semimetals are multiple-band systems, resolving clear Berry phase for each Fermi pocket remains a challenge. Here we report the determination of Berry phases of multiple Fermi pockets of Weyl semimetal TaP through high field quantum transport measurements. We show our TaP single crystal has the signatures of a Weyl state, including light effective quasiparticle masses, ultrahigh carrier mobility, as well as negative longitudinal magnetoresistance. Furthermore, we have generalized the Lifshitz-Kosevich formula for multiple-band Shubnikov-de Haas (SdH) oscillations and extracted the Berry phases of π for multiple Fermi pockets in TaP through the direct fits of the modified LK formula to the SdH oscillations. In high fields, we also probed signatures of Zeeman splitting, from which the Landé g-factor is extracted.

Lifshitz topological impurity transitions in bismuth wires doped with acceptor and donor impurities

This paper reports the results of an experimental study of electronic topological transitions in bismuth glass-covered wires doped with acceptor (Sn) and donor (Te) impurities. The temperature dependences of the thermoelectric power and resistance are measured within the temperature range from 1.5 to 300 K and magnetic fields up to 14T. The position of the Fermi level eF and the concentration of charge carriers at doping are estimated from the Shubnikov de Haas (SdH) oscillations which are clearly visible from both L electrons and L and T holes in all crystallographic directions. We demonstrate anomalies in the temperature dependences of the thermopower in Bi wires doped with acceptor (Sn) and donor (Te) impurities in the form of a triple (doping by Sn) and double (doping by Te) change in the sign of the thermopower. The effect is interpreted with relation to the manifestation of impurity Lifshitz topological transitions. The SdH oscillation method was used to determine the energy position of the Σ band by doping Bi wires with the acceptor impurity Sn and the T band conduction by doping with Te. It is shown that the appearance of the Σ and T bands in Bi wires doped with the acceptor and donor impurities is responsible for the anomalies in the diffusive thermoelectric power, which gives a good conform with to the theoretical models and predictions.