Maximum Entropy Mobility Spectrum Research Articles

Doping-enhanced robustness of anomaly-related magnetoresistance in WTe2±α flakes

We study systematically the negative magnetoresistance (MR) effect in WTe2±α flakes with different thicknesses and doping concentrations. The negative MR is sensitive to the relative orientation between electrical-/magnetic-field and crystallographic orientation of WTe2±α . The analysis proves that the negative MR originates from chiral anomaly and is anisotropic. Maximum entropy mobility spectrum is used to analyze the electron and hole concentrations in the flake samples. It is found that the negative MR observed in WTe2±α flakes with low doping concentration is small, and the high doping concentration is large. The doping-induced disorder obviously inhibits the positive MR, so the negative MR can be more easily observed. In a word, we introduce disorder to suppress positive MR by doping, and successfully obtain the negative MR in WTe2±α flakes with different thicknesses and doping concentrations, which indicates that the chiral anomaly effect in WTe2 is robust.

Transport properties and phase diagrams of FeSe1−S (0 ≤ x ≤ 1) single crystals

Based on the high quality FeSe1−xSx (0 ≤ x ≤ 1) single crystals synthesized via a hydrothermal method, we carried out systematic measurement of the normal state magnetoresistance and Hall effect. Through the maximum entropy mobility spectrum analysis (MEMSA), we investigate the evolution of mobility spectrum, which reflects the multi-band structures for different doping content x and temperature T. The relationships between Tc and carrier properties calculated by MEMSA, as well as bond angles and anion height calculated by first-principle method are investigated. Nonlinear to linear transition of Hall resistivity(ρxy(B)) is mainly attributed to the decreasing mobilities and merging of mobility peaks with the reductive x or increasing T. The relationship between Tc and carrier density n show weak interaction for the orthogonal phases when x ≤ 0.2, but nearly positive correlation for the tetragonal phases when x > 0.2. Bond angle as well as the Se/S anion height change almost linearly with x within the tetragonal phases, and the V-shaped relationships with Tc indicate that structural factors play a key role in the regulation of superconductivity. Our researches are helpful for understanding the multi-band and complex phase diagram of iron-based superconductors.

Maximum entropy mobility spectrum analysis for the type-I Weyl semimetal TaAs

Due to non-saturating magnetoresistance (MR) and the special compensation mechanism, the Weyl semimetal TaAs single crystal has attracted considerable attention in condensed matter physics. Herein, we use maximum entropy mobility spectrum analysis (MEMSA) to extract charge carrier information by fitting the experimentally measured longitudinal and transverse electric transport curves of TaAs. The carrier types and the number of bands are obtained without any hypothesis. Study of the temperature dependence shows details of carrier property evolution. Our quantitative results explain the non-saturated magnetoresistance and Hall sign change phenomena of TaAs.

Transport property of multi-band topological material PtBi_2 studied by maximum entropy mobility spectrum analysis (MEMSA)

Electrical transport of both longitudinal and transverse directions carries rich information. Mobility spectrum analysis (MSA) is capable of extracting charge information from conductivity tensor, including charge types, concentration and mobilities. Using a numerical method based on maximum entropy principle, i.e., maximum entropy mobility spectrum analysis (MEMSA), mobility spectrum for beta -type PtBi_2 is studied. Three hole-pockets and two electron-pockets were found, including a small hole pocket with very high mobility, which is very likely corresponding to Dirac Fermions. Benefiting from our high resolution result, we studied temperature dependence of carrier properties and explained the sign change phenomenon of Hall conductivity. We further compared the results with band structure obtained by our first principle calculation. The present results prove MEMSA is a useful tool of extracting carries’ information in recently discovered Iron-based superconductors, and topological materials.

Large linear magnetoresistance caused by disorder in WTe2−δ thin film

Weyl semimetal WTe2 has attracted considerable attention owing to its extremely large, unsaturated and quadratic magnetoresistance. Here, we study the magnetotransport properties of WTe2−δ thin film, which shows an unsaturated and linear magnetoresistance of up to ∼1650%. A more complex and accurate method, known as the maximum entropy mobility spectrum, is used to analyze the mobility and density of carriers. The results show that linear magnetoresistance can be explained by the classical disorder model because the slope of linear magnetoresistance and the crossover field are proportional to the mobility and inverse mobility, respectively. Furthermore, the validity of the maximum entropy mobility spectrum is validated by the Shubnikov–de Haas oscillations. Moreover, at low temperature, we determined that the unsaturated and near-quadratic magnetoresistance in the WTe1.93 thin film can be explained by charge compensation. Note that the electron–hole compensation is broken in the WTe1.42 thin film, which indicates that the carrier scattering induced by the disorder may suppress the charge compensation in the WTe2 sample with defects/dopants. To summarize, the discovery of disorder-induced linear magnetoresistance allows us to explain different magnetoresistance behaviors of WTe2.

Maximum entropy mobility spectrum analysis of LPE-grown and anodic oxidated Hg1-xCdxTe(x=0.237)

In this paper, magneto-transport properties of the LPE-grown and anodic oxidated p-type Hg1-xCdxTe(x=0.237) films have been studied by using maximum entropy mobility spectrum analysis (ME-MSA) technique. It can be found that the high-mobility electron (μe∼2 × 104cm2/Vs) has considerable contributions to the conduction of anodic oxidated Hg1-xCdxTe(x=0.237) film, but not in LPE-grown Hg1-xCdxTe(x=0.237) film. The high-mobility electron maintains dominant contributions from 11k to 150k, which can be attributed to two-dimensional electron gas in the inversion layer of anodic oxidated p-type Hg1-xCdxTe(x=0.237) film. In addition, we also observe the nonphysical contributions of low mobility electrons (μe∼0.08 × 104cm2/Vs) in mobility spectrum of both LPE-grown and anodic oxidated p-type HgCdTe films. The low-mobility electrons, so-called mirror peaks, can be interpreted as a consequence of magnetic freeze-out of holes in vacancy-doped HgCdTe, which disappeared at T=150k.

Coexistence of free holes and electrons in InN:Mg with In- and N-growth polarities

The coexistence of two types of carriers (free electrons and free holes) in InN:Mg and their competition is demonstrated by the temperature and magnetic-field-induced change of the sign of thermopower (α) as well as the maximum entropy mobility spectrum analysis. The results confirm the existence of alternative carrier channels in addition to the n-type surface inversion layer and p-type bulk. They also show that In-polarity can be propitious for occurrence of p-type conductivity.

Interpretation of transport measurements in ZnO-thin films

In order to interpret results of temperature dependent Hall measurements in heteroepitaxial ZnO-thin films, we adopted a multilayer conductivity model considering carrier-transport through the interfacial layer with degenerate electron gas as well as the upper part of ZnO layers with lower conductivity. This model was applied to the temperature dependence of the carrier concentration and mobility measured by Hall effect in a ZnO-layer grown on c-sapphire with conventional high-temperature MgO and low-temperature ZnO buffer. We also compared our results with the results of maximum entropy mobility-spectrum analysis (MEMSA). The formation of the highly conductive interfacial layer was explained by analysis of transmission electron microscopy (TEM) images taken from similar layers.

Multicarrier analysis of magnetotransport data at low and high electric fields

AbstractWe present some results on multicarrier analysis of magnetotransport data. Both synthetic as well as data from narrow gap Hg0.8Cd0.2Te samples are used to demonstrate applicability of various algorithms vs. nonlinear least square fitting, Quantitative Mobility Spectrum Analysis (QMSA) and Maximum Entropy Mobility Spectrum Analysis (MEMSA). Comments are made from our experience on these algorithms, and, on the inversion procedure from experimental R/σ‐B to S‐μ specifically with least square fitting as an example. Amongst the conclusions drawn are: (i) Experimentally measured resistivity (Rxx, Rxy) should also be used instead of just the inverted conductivity (σxx, σxy) to fit data to semiclassical expressions for better fits especially at higher B. (ii) High magnetic field is necessary to extract low mobility carrier parameters. (iii) Provided the error in data is not large, better estimates to carrier parameters of remaining carrier species can be obtained at any stage by subtracting highest mobility carrier contribution to σ from the experimental data and fitting with the remaining carriers. (iv)Even in presence of high electric field, an approximate multicarrier expression can be used to guess the carrier mobilities and their variations before solving the full Boltzmann equation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evidence of different conduction channels in bulk ZnO using f-MEMSA Analysis of transport properties

AbstractThe characterization of transport properties in Zn0 is known to be challenging, particularly due to surface (in the case of bulk) or interface (in the case of heteroepitaxial layers) conduction channels, which puts severe limitations on the interpretation of Hall Effect measurements. In this communication, we report on the study of transport properties of n-type ZnO bulk material using Hall mobility spectrum analysis estimated through the algorithm known as full Maximum Entropy Mobility Spectrum Analysis, f-MEMSA. The electrical properties of bulk Zn0 are measured using a Hall setup for applied magnetic fields µ0H in the range 0T-9T and for temperatures between 50K and 400K. The f-MEMSA analysis highlights the existence of two types of conduction channels in the considered ZnO substrate. We also show that surface conductive channel can be suppressed using appropriate annealing conditions.

Mobility spectrum computational analysis using a maximum entropy approach.

A method to calculate a smooth electrical conductivity versus mobility plot ("mobility spectrum") from the classical magnetoconductivity tensor in heterogeneous structures with the help of a "maximum entropy principle" has been developed. In this approach the closeness of the fit and the entropy of the mobility spectrum are optimized. The spectrum is then the most probable one with the least influence of the personal bias of the investigator for any given set of experimental data and is maximally noncommittal with regard to the unmeasured data. The advantages of the maximum entropy mobility spectrum analysis as compared to the conventional mobility spectrum analysis are demonstrated using a synthetic dataset.

Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures

To extract the room-temperature drift mobility and sheet carrier density of two-dimensional hole gas (2DHG) that form in Ge strained channels of various thicknesses in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures, the magnetic field dependences of the magnetoresistance and Hall resistance at temperature of 295 K were measured and the technique of maximum entropy mobility spectrum analysis was applied. This technique allows a unique determination of mobility and sheet carrier density of each group of carriers present in parallel conducting multilayers semiconductor heterostructures. Extremely high room-temperature drift mobility (at sheet carrier density) of 2DHG 2940 cm2 V−1 s−1 (5.11×1011 cm−2) was obtained in a sample with a 20 nm thick Ge strained channel.