Shubnikov-de Haas Research Articles

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.

Angular-Dependent Phase Factor of Shubnikov-de Haas Oscillations in the Dirac Semimetal Cd_{3}As_{2}.

We measure the magnetotransport properties of the three-dimensional Dirac semimetal Cd_{3}As_{2} single crystal under magnetic fields up to 36T. Shubnikov-de Haas (SdH) oscillations are clearly resolved and the n=1 Landau level is reached. A detailed analysis on the intercept of the Landau index plot reveals a significant dependence of the SdH phase factor on the orientation of the applied magnetic field. When the magnetic field is applied in the [001] direction, i.e., along the fourfold screw axis of the tetragonal crystal structure, a nontrivial π Berry phase, as predicted for the Dirac fermions, is observed. However, in a magnetic field tilted away from the [001] direction, the π Berry phase is evidently reduced, and a considerable enhancement of the effective mass is also revealed. Our observations demonstrate that the Dirac dispersion in Cd_{3}As_{2} is effectively modified in a tilted magnetic field, whereas the preserved π Berry phase in a magnetic field along the [001] direction can be related to the realization of the Weyl fermions. The sudden change of the SdH phase also indicates a possible topological phase transition induced by the symmetry-breaking effect.

Quantum oscillations of the topological surface states in low carrier concentration crystals of Bi2−xSbxTe3−ySey

We report a high-field magnetotransport study on selected low-carrier crystals of the topological insulator Bi$_{2-x}$Sb${_x}$Te$_{3-y}$Se$_{y}$. Monochromatic Shubnikov - de Haas (SdH) oscillations are observed at 4.2~K and their two-dimensional nature is confirmed by tilting the magnetic field with respect to the sample surface. With help of Lifshitz-Kosevich theory, important transport parameters of the surface states are obtained, including the carrier density, cyclotron mass and mobility. For $(x,y)=(0.50,1.3)$ the Landau level plot is analyzed in terms of a model based on a topological surface state in the presence of a non-ideal linear dispersion relation and a Zeeman term with $g_s = 70$ or $-54$. Input parameters were taken from the electronic dispersion relation measured directly by angle resolved photoemission spectroscopy on crystals from the same batch. The Hall resistivity of the same crystal (thickness of 40~$\mu$m) is analyzed in a two-band model, from which we conclude that the ratio of the surface conductance to the total conductance amounts to 32~\%.

Effect of Halopyridine Guest Molecules on the Structure and Superconducting Prop­erties of β″‐[Bis(ethylenedithio)tetra­thiafulvalene]4(H3O)[Fe(C2O4)3]·Guest Crystals

AbstractFour new superconducting and metallic salts of the family of organic molecular conductors (BEDT‐TTF)4AI[FeIII(C2O4)3]G [A = H3O+; G = 2‐chloropyridine (1), 2‐bromopyridine (2), 3‐chloropyridine (3), and 3‐bromopyridine (4); BEDT‐TTF = bis(ethylenedithio)tetrathiafulvalene] have been prepared, and their crystal structures, electronic structures, transport properties, and magnetotransport properties have been studied. All of the crystals obtained belong to the monoclinic group of the family with β″‐type packing of the conducting BEDT‐TTF layers. Crystals 1 and 2 are superconductors with Tc = 2.4–4.0 and 4.3 K, respectively, and possess structural phase transitions from monoclinic to triclinic symmetry at temperatures of 190–215 K, whereas 3 and 4 retain monoclinic symmetry at 90–300 K and do not show a superconducting transition above 0.5 K. Shubnikov–de Haas (SdH) oscillations were found in crystals of 1, 2, and 4. The structures and conducting properties of 1–4 are compared with those of the known monoclinic phases of the family containing FeIII ions and different monohalobenzenes and pyridine as “guest” solvent molecules (G).

Multiple Fermi pockets revealed by Shubnikov-de Haas oscillations in WTe2

The recently discovered non-saturating and parabolic magnetoresistance and the pressure-induced superconductivity at low temperature in WTe2 imply its rich electronic structure and possible practical applications. Here we use magnetotransport measurements to investigate the electronic structure of WTe2 single crystals. A non-saturating and parabolic magnetoresistance is observed from low temperature to high temperature up to 200 K with magnetic fields up to 8 T. Shubnikov-de Haas (SdH) oscillations with beating patterns are observed, the fast Fourier transform of which reveals three oscillation frequencies, corresponding to three pairs of Fermi pockets with comparable effective masses, . By fitting the Hall resistivity, we infer that they can be attributed to one pair of electron pockets and two pairs of hole pockets, together with nearly perfect compensation of the electron-hole carrier concentration. These magnetotransport measurements reveal the complex electronic structure in WTe2, explaining the non-saturating magnetoresistance.

Shubnikov–de Haas oscillations in n and p type Bi2Se3 flakes

Shubnikov–de Haas (SdH) oscillation, describing the magnetic quantum oscillation in material resistivity at low temperatures, is recently observed in topological insulators and plays an important role in clarifying the existence of topological surface states. Here, we report the SdH oscillations observed in both n and p type Bi2Se3 flake samples. The angle dependent magneto-resistance results reveal the 2D nature of SdH oscillations and the Landau level fan diagrams exhibit π Berry phase. These indicate that the observed SdH oscillations originate from the topological surface states. Moreover, the cyclotron mass, Dingle temperature, Fermi velocity, and other parameters of the systems are deduced from the analysis of temperature dependent SdH oscillations. The obtained Fermi velocities of surface electrons and holes in Bi2Se3 show a good agreement with those found in the typical angle-resolved photoemission spectroscopy experiments. This further confirms that the topological surface electrons/holes are responsible for observed SdH oscillations.

Radiation-assisted magnetotransport in two-dimensional electron gas systems: appearance of zero resistance states

Zero-resistance states (ZRS) are normally associated with superconducting and quantum Hall phases. Experimental detection of ZRS in two-dimensional electron gas (2DEG) systems irridiated by microwave(MW) radiation in a magnetic field has been quite a surprise. We develop a semiclassical transport formalism to explain the phenomena. We find a sequence of Zero-Resistance States (ZRS) inherited from the suppression of Shubnikov–de Haas (SdH) oscillations under the influence of high-frequency and large amplitude microwave radiation. Furthermore, the ZRS are well pronounced and persist up to broad intervals of magnetic field as observed in experiments on microwave illuminated 2DEG systems.

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

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

Evidence of strong spin–orbit interaction in strained epitaxial germanium

Spin–orbit interaction effects are of great interest, primarily for the ability to modulate spin transport in a semiconductor channel for device applications. In particular, the Rashba spin–orbit (S–O) interaction allows for a tuneable spin modulation response dependent on the applied electric field, which can be achieved using standard semiconductor gate technology. We present evidence of the Rashba S–O interaction in two modulation doped germanium (Ge) quantum well (QW) heterostructures with different layer structures, and consequently different band structures and internal electric fields across the QW region. We show that two complementary low temperature magnetotransport analyses can be used to identify and quantify the Rashba S–O parameter in these materials, although quantification is limited in one case due to significant parallel conduction. We highlight that both techniques (Weak Antilocalisation and Shubnikov de-Haas oscillations) should be used when analysing a new material system as other conduction and magnetoresistive effects can obscure evidence of the Rashba S–O interaction in one or both of these regimes.

Role of spin-orbit coupling and evolution of the electronic structure ofWTe2under an external magnetic field

Here, we present a detailed study on the temperature and angular dependence of the Shubnikov-de-Haas (SdH) effect in the semi-metal WTe$_2$. This compound was recently shown to display a very large non-saturating magnetoresistance which was attributed to nearly perfectly compensated densities of electrons and holes. We observe four fundamental SdH frequencies and attribute them to spin-orbit split, electron- and hole-like, Fermi surface (FS) cross-sectional areas. Their angular dependence seems consistent with ellipsoidal FSs with volumes suggesting a modest excess in the density of electrons with respect to that of the holes. We show that density functional theory (DFT) calculations fail to correctly describe the FSs of WTe$_2$. When their cross-sectional areas are adjusted to reflect the experimental data, the resulting volumes of the electron/hole FSs obtained from the DFT calculations would imply a pronounced imbalance between the densities of electrons and holes. We find evidence for field-dependent Fermi surface cross-sectional areas by fitting the oscillatory component superimposed onto the magnetoresistivity signal to several Lifshitz-Kosevich components. We also observe a pronounced field-induced renormalization of the effective masses. Taken together, our observations suggest that the electronic structure of WTe$_2$ evolves with the magnetic field. This evolution might be a factor contributing to its pronounced magnetoresistivity.

Measurement of topological Berry phase in highly disordered graphene

We have observed the quantum Hall effect (QHE) and Shubnikov-de Haas (SdH) oscillations in highly disordered graphene at magnetic fields up to 65 T. Disorder was introduced by hydrogenation of graphene up to a ratio H/C $\approx 0.1\%$. The analysis of SdH oscillations and QHE indicates that the topological part of the Berry phase, proportional to the pseudo-spin winding number, is robust against introduction of disorder by hydrogenation in large scale graphene.

Influence of the Spin–Orbit Interaction on the Magnetotransport Properties of a Two-Dimensional Electron System

In this paper, we review the contribution of the Rashba spin–orbit coupling to the magnetoconduction of a two-dimensional electron system (2DES) confined in an inversion layer under quantum Hall regime (low temperature and low defects and impurities). The study is based on a semi-classical model for the magnetoconductivities of the 2DES. This model reproduces the measurements of the Shubnikov-de Haas (SdH) oscillations obtained in systems confined in III–V heterostructures, and also the quantum Hall magnetoconductivity (magnetoresistivity). We also discuss the Rashba and Zeeman competition and its effect on the magnetoconductivity.

Magnetotransport Properties of Cd3As2 Nanostructures.

Three-dimensional (3D) topological Dirac semimetal has a linear energy dispersion in 3D momentum space, and it can be viewed as an analogue of graphene. Extensive efforts have been devoted to the understanding of bulk materials, but yet it remains a challenge to explore the intriguing physics in low-dimensional Dirac semimetals. Here, we report on the synthesis of Cd3As2 nanowires and nanobelts and a systematic investigation of their magnetotransport properties. Temperature-dependent ambipolar behavior is evidently demonstrated, suggesting the presence of finite-size of bandgap in nanowires. Cd3As2 nanobelts, however, exhibit metallic characteristics with a high carrier mobility exceeding 32,000 cm(2) V(-1) s(-1) and pronounced anomalous double-period Shubnikov-de Haas (SdH) oscillations. Unlike the bulk counterpart, the Cd3As2 nanobelts reveal the possibility of unusual change of the Fermi sphere owing to the suppression of the dimensionality. More importantly, their SdH oscillations can be effectively tuned by the gate voltage. The successful synthesis of Cd3As2 nanostructures and their rich physics open up exciting nanoelectronic applications of 3D Dirac semimetals.

Magnetoresistance quantum oscillations in a magnetic two-dimensional electron gas

Magneto-transport measurements of Shubnikov-de Haas (SdH) oscillations have been performed on two-dimensional electron gases (2DEGs) confined in CdTe and CdMnTe quantum wells. The quantum oscillations in CdMnTe, where the 2DEG interacts with magnetic Mn ions, can be described by incorporating the electron-Mn exchange interaction into the traditional Lifshitz-Kosevich formalism. The modified spin splitting leads to characteristic beating pattern in the SdH oscillations, the study of which indicates the formation of Mn clusters resulting in direct anti-ferromagnetic Mn-Mn interaction. The Landau level broadening in this system shows a peculiar decrease with increasing temperature, which could be related to statistical fluctuations of the Mn concentration.

Surface origin of quasi-2D Shubnikov–de Haas oscillations in Bi2Te2Se

Transport measurements at liquid helium temperatures were done on a number of Bi2Te2Se samples with thicknesses ranging from 30 to 200 μm in order to detect surface states. In each sample we observed Shubnikov–de Haas (SdH) oscillations and sublinear dependence of off-diagonal component of magnetoresistance tensor on magnetic field. The periods of SdH oscillations in inverse magnetic field were found to be the same within 15%. The positions of SdH oscillations are determined by the normal to surface component of magnetic field. We found that the measured conductivity can be well described by a model with two groups of electrons, 2D and 3D. The conductivity of 2D electrons was found to be relatively weakly varying from sample to sample and not depending on thickness in a systematic manner. This behavior can be explained only by their localization on the surface. Comparison of the results of magnetotransport measurements with our scanning tunneling spectroscopy results on atomically smooth Bi2Te2Se surface in ultrahigh vacuum led us to conclude that the surface electrons are separated from the bulk electrons by a depletion layer approximately 100 nm thick. This effect could provide the dominant contribution of surface electrons to conductivity in samples with thicknesses less than 200 nm.

Quantum oscillations and interference effects in strained n- and p-type modulation doped GaInNAs/GaAs quantum wells

We have performed magnetoresistance measurements on n- and p-type modulation doped GaInNAs/GaAs quantum well (QW) structures in both the weak (B < 0.08 T) and the high magnetic field (up to 18 T) at 75 mK and 6 K. We observe that the quantum oscillations in and quantum Hall effect (QHE) plateaus in are affected from the presence of the nitrogen in the III–V lattice. The enhancement of N-related scatterings and electron effective mass with increasing nitrogen causes lower electron mobility and higher two-dimensional (2D) electron density, leading to suppressed QHE plateaus in up to 7 T at 6 K. The Shubnikov de Haas (SdH) oscillations develop at lower magnetic fields for higher mobility samples at 6 K and the amplitude of SdH oscillations decreases with increasing nitrogen composition. The well-pronounced QHE plateaus are observed at 75 mK and at higher magnetic fields up to 18 T, for the p-type sample. For n-type samples, the observed anomalies in the characteristic of QHE is attributed the nitrogen-related disorders and overlapping of fluctuating Landau levels. The low magnetic field measurements at 75 mK reveal that the n-type samples exhibit weak antilocalization, whereas weak localization is observed for the p-type sample. The observation of weak antilocalization is an indication of strong electron spin–orbit interactions. The low field magnetoresistance traces are used to extract the spin coherence, phase coherence and elastic scattering times as well Rashba parameters and spin-splitting energy. The calculated Rashba parameters for nitrogen containing samples reveal that the nitrogen composition is a significant parameter to determine the degree of the spin–orbit interactions. Consequently, GaInNAs-based QW structures with various nitrogen compositions can be beneficial to adjust the spin–orbit coupling strength and may be used as a candidate for spintronics applications.

Growth and characterization of (110) InAs quantum well metamorphic heterostructures

An understanding of the growth of (110) quantum wells (QWs) is of great importance to spin systems due to the observed long spin relaxation times. In this article, we report on the metamorphic growth and characterization of high mobility undoped InAs (110) QWs on GaAs (110) substrates. A low-temperature nucleation layer reduces dislocation density, results in tilting of the subsequent buffer layer and increases the electron mobility of the QW structure. The mobility varies widely and systematically (4000–16 000 cm2/Vs at room temperature) with deposition temperature and layer thicknesses. Low-temperature transport measurements exhibit Shubnikov de-Haas oscillations and quantized plateaus in the quantum Hall regime.

Enhanced Shubnikov-De Haas Oscillation in Nitrogen-Doped Graphene.

N-doped graphene displays many interesting properties compared with pristine graphene, which makes it a potential candidate in many applications. Here, we report that the Shubnikov-de Haas (SdH) oscillation effect in graphene can be enhanced by N-doping. We show that the amplitude of the SdH oscillation increases with N-doping and reaches around 5k Ω under a field of 14 T at 10 K for highly N-doped graphene, which is over 1 order of magnitude larger than the value found for pristine graphene devices with the same geometry. Moreover, in contrast to the well-established standard Lifshitz-Kosevich theory, the amplitude of the SdH oscillation decreases linearly with increasing temperature and persists up to a temperature of 150 K. Our results also show that the magnetoresistance (MR) in N-doped graphene increases with increasing temperature. Our results may be useful for the application of N-doped graphene in magnetic devices.

Shubnikov-de Haas Oscillation in the cubic Γ3-based heavy fermion superconductor PrV2Al20

PrV2Al20 is the cubic Γ3 doublet system that exhibits strong hybridization effects and heavy fermion superconductivity. We measured the magnetoresistance using a high quality single crystal of PrV2Al20 under the DC high magnetic field up to 35 T below ~ 0.4 K. We have succeeded in detecting the Shubnikov-de Haas (SdH) oscillation with a frequency of 250 T. Our analysis of this SdH oscillation yields the cyclotron effective mass (m*c/m0 ~ 5) and the mean-free path (l = 170 Å).

Quantum oscillations in the Luttinger model with quadratic band touching: Applications to pyrochlore iridates

Motivated by recent experiments on ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, we provide a theory of quantum oscillations in the Luttinger model with quadratic band touching, modeled for the spin-orbit-coupled conduction electrons in pyrochlore iridates. The magneto- and Hall resistivities are computed for electron- and hole-doped systems, and the corresponding Shubnikov--de Haas (SdH) signals are investigated. The SdH signals are characterized by aperiodic behaviors that originate from the unconventional Landau level structures of the Luttinger model near the neutrality point, such as inter-Landau-level crossing, nonuniform Landau level spacings, and nonparabolic dispersions along the applied magnetic-field direction. The aperiodic SdH signals observed in the paramagnetic state of ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ are shown to be consistent with such behaviors, justifying the use of the Luttinger model and the quadratic band touching spectrum as excellent starting points for physics of pyrochlore iridates. The implications of these results are discussed in light of recent theoretical and experimental developments in these systems.