Coexistence Of Electrons Research Articles

Interface Dependent Coexistence of Two‐Dimensional Electron and Hole Gases in Mn‐doped InAs/GaSb

AbstractThe interface of common III‐V semiconductors InAs and GaSb can be utilized to realize a two‐dimensional (2D) topological insulator state. The 2D electronic gas at this interface can yield Hall quantization from coexisting electrons and holes. This anomaly is a determining factor in the fundamental origin of the topological state in InAs/GaSb. Here, the coexistence of electrons and holes in InAs/GaSb is tied to the chemical sharpness of the interface. Magnetotransport, in samples of Mn‐doped InAs/GaSb cleaved from wafers grown at a spatially inhomogeneous substrate temperature, is studied. It is reported that the observation of quantum oscillations and a quantized Hall effect whose behavior, exhibiting coexisting electrons and holes, is tuned by this spatial nonuniformity. Through transmission electron microscopy measurements, it is additionally found that samples that host this co‐existence exhibit a chemical intermixing between group III and group V atoms that extends over a larger thickness about the interface. The issue of intermixing at the interface is systematically overlooked in electronic transport studies of topological InAs/GaSb. These findings address this gap in knowledge and shed important light on the origin of the anomalous behavior of quantum oscillations seen in this 2D topological insulator.

A First‐Principles Investigation of Monolayer MoS2 Doped with Mn and V as an Excellent NO2 Sensor with High Selectivity

This article conducts a comprehensive investigation into the adsorption properties of both pristine MoS2 monolayers and those doped with Mn and V, wherein these dopants replace Mo atoms. By the first‐principles calculations, the research reveals a noteworthy reduction in the adsorption energy of NO2 on Mn‐doped and V‐doped MoS2 monolayers compared to pure MoS2 monolayers. Specifically, at the most favorable adsorption site, the adsorption energy decreases from −5.83 to 5.908 eV for Mn‐doped MoS2 and from −5.58 to −6.17 eV for V‐doped MoS2. Furthermore, computations of the band structure, work function, and charge density difference highlight a significant degree of charge transfer and coexistence of electrons within these systems. Additionally, post‐adsorption of NO2 molecules, a distinct shift in the density of states (DOS) toward lower energy states is observed at the adsorption sites of both MoS2/Mn and MoS2/V monolayers, underscoring their exceptional sensitivity in detecting NO2. In summary, these results demonstrate the substantial potential of Mn‐doped and V‐doped MoS2 monolayers as highly sensitive materials for NO2 detection. Their lower adsorption energies and noteworthy shifts in DOS following NO2 adsorption enhance their capabilities as NO2 sensors.

Coexistence of electron and phonon topology in conjunction with quantum transport device modeling

The escalating research in the field of topology necessitates an understanding of the underlying rich physics behind the materials possessing unique features of non-trivial topology in both electronic and phononic states. Due to the interaction between electronic quasiparticles and spin degrees of freedom, the realization of magnetic topological materials has opened up a new frontier with unusual topological phases, however, these are rarely reported alongside phononic quasiparticle excitations. In this work, by first-principles calculations and symmetry analysis, the intermetallic ferromagnetic compounds MnGaGe and MnZnSb with the coexistence of exceptional topological features in the electronic and phononic states are proposed. These compounds host nodal surface on ky=π plane in bulk Brillouin zone in the electronic and phononic spectra protected by the combination of time-reversal symmetry and nonsymmorphic two-fold screw-rotation symmetry. In the former case, a spin-polarized nodal surface is present in the majority and minority spin channels and found to be robust to ground-state magnetic polarization. The presence of nodal line features is analyzed in both the quasiparticle spectra, whose non-trivial nature is confirmed by the Berry phase calculation. The incorporation of spin–orbit coupling in the electron spectra introduces distinctive characteristics in the transport properties, facilitating the emergence of anomalous Hall conductivity through Berry curvature in both bulk and monolayer. Furthermore, the monolayer has been proposed as a two-terminal device model to investigate the quantum transport properties using the non-equilibrium Green’s function approach. This superlative combination of observations and modeling sets the path for a greater level of insight into the behavior and aspects of topological materials at the atomic scale.

Odnovremennoe nablyudenie tsiklotronnogo rezonansa dyrok i elektronov v dvoynoy kvantovoy yame HgTe/CdHgTe v usloviyakh effekta “opticheskogo zatvora”

Spectral studies of the photoconductivity in the temperature range of T = 5–70 K, as well as studies of the magneto-absorption and magnetotransport at T = 4.2 K, have been performed in a HgTe/CdHgTe heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap (~80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.

Simultaneous Observation of the Cyclotron Resonances of Electrons and Holes in a HgTe/CdHgTe Double Quantum Well under “Optical Gate” Effect

Spectral studies of the photoconductivity in the temperature range of T = 5–70 K, as well as studies of the magneto-absorption and magnetotransport at T = 4.2 K, have been performed in a HgTe/CdHgTe heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap (~80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.

Possible evidence of Weyl fermion enhanced thermal conductivity under magnetic fields in the antiferromagnetic topological insulator Mn(Bi1−xSbx)2Te4

We report thermal conductivity and Seebeck effect measurements on $\text{Mn}{({\text{Bi}}_{1\ensuremath{-}x}{\text{Sb}}_{x})}_{2}{\text{Te}}_{4}$ (MBST) with $x=0.26$ under applied magnetic fields below $50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Our data shows clear indications of the electronic structure transition induced by the antiferromagnetic (AFM) to ferromagnetic (FM) transition driven by applied magnetic field as well as significant positive magnetothermal conductivity in the Weyl semimetal state of MBST. Further, by examining the dependence of magnetothermal conductivity on field orientation for MBST and comparison with the magnetothermal conductivity of ${\text{MnBi}}_{2}{\text{Te}}_{4}$, we see possible evidence of a contribution to thermal conductivity due to Weyl fermions in the FM phase of MBST. From the temperature dependence of the Seebeck coefficient under magnetic fields for MBST, we also observed features consistent with the Fermi surface evolution from a hole pocket in the paramagnetic state to a Fermi surface with coexistence of electron and hole pockets in the FM state. These findings provide further evidence for the field-driven topological phase transition from an AFM topological insulator to a FM Weyl semimetal.

Fractional-unit-cell-doped spinel/perovskite oxide interfaces with switchable carrier conduction

The two-dimensional hole gas (2DHG) at the polar LaAlO3/SrTiO3 interface remains elusive. Different from isostructural perovskite-type interfaces, the spinel/perovskite heterointerface of γ-Al2O3/SrTiO3 (GAO/STO) enables us to control interfacial states with sub-unit-cell precision. Herein, we present the epitaxial growth of fractionally doped GAO/STO heterointerfaces, where GAO is precisely doped on the scale of 1/4-unit-cell (0.2 nm) by ferromagnetic Fe3O4 and nonmagnetic ZnO atomic layers. Notably, the conduction of the engineered interfaces depends critically on the position of the dopant, where a coexistence of electron and hole conduction is measured at even sublayer-doped GAO/STO interfaces. First-principles density functional theory calculations indicate that electron conductivity is from the interfacial TiO2 layers of the STO substrate, while the hole conductivity is from the Zn-doped GAO film. The presence of hole conduction can be explained from the alternating structural feature of a doped layer without oxygen vacancies. This work sheds additional insight on the emergence of 2DHG at oxide interfaces and provides opportunities for atomically engineered oxide interfaces with non-isostructural layers.

Weyl Semimetal States Generated Extraordinary Quasi‐Linear Magnetoresistance and Nernst Thermoelectric Power Factor in Polycrystalline NbP

AbstractUnsaturated and extremely large magnetoresistance (MR), as well as the giant Nernst effect, are intriguing transport phenomena in Weyl semimetals, which are technically appealing for potential applications in magneto‐electric sensors and transverse thermoelectric conversion. The prominent properties are originated from Weyl semimetal states, i.e., the coexistence of electron and hole pockets combined with linear band dispersion. However, previous studies have been focused on small‐sized single crystals, rendering the practical applications of Weyl semimetals. Here, it is reported an unsaturated, quasi‐linear MR as well as a very large Nernst power factor PFxy in the prepared centimeter‐sized and polycrystalline Weyl semimetal NbP. An extraordinary MR of ≈2 × 104% is observed below 60 K with a magnetic field up to 55 T and persists to elevated temperatures. The unusual quasi‐linear MR behavior is explained by the theory of classical linear MR arising from structural disorder. The polycrystalline NbP exhibits state‐of‐the‐art PFxy that reaches a maximum value of 74.81 μW cm–1 K–2 at 9 T and 220 K, which is 1.5 times larger than its longitudinal thermoelectric power factor PFxx. Given that polycrystalline Weyl semimetal, NbP is suitable for large‐scale production, the results pave the way for its practical applications in magneto‐electric sensors and transverse thermoelectric conversion.

Anisotropic giant magnetoresistanceand de Hass–van Alphen oscillations in layered topological semimetal crystals

Here, we report an anisotropic giant magnetoresistance (GMR) effect and de Hass–van Alphen (dHvA) oscillation phenomena in nominal TaNiTe5 single crystals. TaNiTe5 exhibits the GMR effect with the maximum value of ∼3 × 103% at T = 1.7 K and B = 31 T, with no sign of saturation. The two-band model fitting of Hall resistivity indicates that the anomalous GMR effect was derived from the coexistence of electron and hole carriers. When the external magnetic field is applied to the electron–hole resonance, the GMR effect is enhanced. The dHvA oscillation data at multiple frequencies reveal the topological characteristics of high carrier mobility, low carrier effective mass, and a small Fermi surface pocket with a nontrivial Berry phase. Our work provides a new platform for the study of topological semimetals with significant anisotropic GMR effect.

Large anomalous Hall effect in the kagome ferromagnet LiMn6Sn6

Kagome magnets are believed to have numerous exotic physical properties due to the possible interplay between lattice geometry, electron correlation and band topology. Here, we report the large anomalous Hall effect in the kagome ferromagnet LiMn$_6$Sn$_6$, which has a Curie temperature of 382 K and easy plane along with the kagome lattice. At low temperatures, unsaturated positive magnetoresistance and opposite signs of ordinary Hall coefficient for $\rho_{xz}$ and $\rho_{yx}$ indicate the coexistence of electrons and holes in the system. A large intrinsic anomalous Hall conductivity of 380 $\Omega^{-1}$ cm$^{-1}$, or 0.44 $e^2/h$ per Mn layer, is observed in $\sigma_{xy}^A$. This value is significantly larger than those in other $R$Mn$_6$Sn$_6$ ($R$ = rare earth elements) kagome compounds. Band structure calculations show several band crossings, including a spin-polarized Dirac point at the K point, close to the Fermi energy. The calculated intrinsic Hall conductivity agrees well with the experimental value, and shows a maximum peak near the Fermi energy. We attribute the large anomalous Hall effect in LiMn$_6$Sn$_6$ to the band crossings closely located near the Fermi energy.

Transport signatures of temperature-induced chemical potential shift and Lifshitz transition in layered type-II Weyl semimetal TaIrTe4

Temperature-induced Lifshitz transitions have been identified in several materials. Their chemical potential shows a substantial shift with changing temperature. The common feature of these materials is the coexistence of electron and hole pockets in the vicinity of the chemical potential. Here, we report the observation of temperature-induced chemical potential shift and Lifshitz transition in a layered type-II Weyl semimetal, TaIrTe4. The reversal of the polarity of the Hall resistivity and thermoelectric power (TEP) as the temperature increases clearly signal an appreciable shift of the chemical potential and change of the Fermi surface. It is corroborated by the improving agreement between the experimental TEP and the one calculated with temperature-dependent chemical potential. The complete disappearance of an electron pocket, consistent with the change of the Fermi surface when the chemical potential moves downwards, provides an evident signature of a temperature-induced Lifshitz transition in TaIrTe4.

Controlling magnetic order, magnetic anisotropy, and band topology in the semimetals Sr(Mn0.9Cu0.1)Sb2 and Sr(Mn0.9Zn0.1)Sb2

Neutron diffraction and magnetic susceptibility studies show that orthorhombic single-crystals of topological semimetals ${\rm Sr(Mn_{0.9}Cu_{0.1})Sb_2}$ and ${\rm Sr(Mn_{0.9}Zn_{0.1})Sb_2}$ undergo three dimensional C-type antiferromagnetic (AFM) ordering of the Mn$^{2+}$ moments at $T_N = 200\pm10$ and $210\pm12$ K, respectively, significantly lower than that of the parent SrMnSb$_2$ with $T_N=297 \pm 3$ K. Magnetization versus applied magnetic field (perpendicular to MnSb planes) below $T_N$ exhibits slightly modified de Haas van Alphen oscillations for the Zn-doped crystal as compared to that of the parent compound. By contrast, the Cu-doped system does not show de Haas van Alphen magnetic oscillations, suggesting that either Cu substitution for Mn changes the electronic structure of the parent compound substantially, or that the Cu sites are strong scatterers of carriers that significantly shorten their mean free path thus diminishing the oscillations. Density functional theory (DFT) calculations including spin-orbit coupling predict the C-type AFM state for the parent, Cu-, and Zn-doped systems and identify the $a$-axis (i.e., perpendicular to the Mn layer) as the easy magnetization direction in the parent and 12.5% of Cu or Zn substitutions. In contrast, 25% of Cu content changes the easy magnetization to the $b$-axis (i.e., within the Mn layer). We find that the incorporation of Cu and Zn in SrMnSb$_2$ tunes electronic bands near the Fermi level resulting in different band topology and semi-metallicity. The parent and Zn-doped systems have coexistence of electron and hole pockets with opened Dirac cone around the Y-point whereas the Cu-doped system has dominant hole pockets around the Fermi level with a distorted Dirac cone. The tunable electronic structure may point out possibilities of rationalizing the experimentally observed de Haas van Alphen magnetic oscillations.

Synthetic Semimetals with van der Waals Interfaces.

The assembly of suitably designed van der Waals (vdW) heterostructures represents a new approach to produce artificial systems with engineered electronic properties. Here, we apply this strategy to realize synthetic semimetals based on vdW interfaces formed by two different semiconductors. Guided by existing ab initio calculations, we select WSe2 and SnSe2 mono- and multilayers to assemble vdW interfaces and demonstrate the occurrence of semimetallicity by means of different transport experiments. Semimetallicity manifests itself in a finite minimum conductance upon sweeping the gate over a large range in ionic liquid gated devices, which also offer spectroscopic capabilities enabling the quantitative determination of the band overlap. The semimetallic state is additionally revealed in Hall effect measurements by the coexistence of electrons and holes, observed by either looking at the evolution of the Hall slope with sweeping the gate voltage or with lowering temperature. Finally, semimetallicity results in the low-temperature metallic conductivity of interfaces of two materials that are themselves insulating. These results demonstrate the possibility to implement a state of matter that had not yet been realized in vdW interfaces and represent a first step toward using these interfaces to engineer topological or excitonic insulating states.

(Invited) Characteristics of Si Single-Electron Transistor under Illumination

Photo-excited carriers in Si quantum islands and their surrounded area exhibit interesting features due to electron-hole generation. We show Si single- hole transistor (SHT) characteristics in a Si single-electron transistor (SET) comprising n-type source and drain electrodes under illumination. Holes excited in the Si are expected to flow through the SET island by applying negative gate voltages. Interesting phenomenon is the co-existence of electrons and holes around the island. In a SET, when holes generated by photo-excitation are trapped in the one side (either top or bottom side) of the SET island under the vertical electric field, the SET characteristics are changed depending on the number of trapped holes because effective total number of electrons can be negative. Here, we show the characteristics of photo-excited SETs without strong vertical electric fields.

First−principles study: Structural, mechanical, electronic and thermodynamic properties of simple−cubic−perovskite (Ba0.62K0.38)(Bi0.92Mg0.08)O3

A new simple cubic perovskite superconductor (Ba0.62K0.38)(Bi0.92Mg0.08)O3 with a Tcmag ∼30 K has been recently synthesized by utilizing facile hydrothermal conditions at 220 °C. Here, we have employed the pseudo-potential plane-wave (PP–PW) approach based on the density functional (DFT) theory, within the generalized gradient approximation (GGA) to calculate and investigate the physical and electronic properties of this new superconductor. The calculated elastic constants satisfy the Born mechanical stability criterion. Based on the calculated data of Cauchy pressure, Pugh ratio and Poisson's ratio, the compound under investigation is found to be brittle nature. The calculated Peierls stress indicates easier movement of dislocations between glide planes. The energy bands crossing the Fermi level (EF) in electronic band structure reveal metallic behavior of the newly synthesized compound. The contribution of O-2p orbital electrons at the EF is higher than those of other orbital electrons as seen from partial density of states. The formation of saddle point owing to the flatness of the energy band may enhance the electron pairing which subsequently leads to high-TC superconductivity. The valence electron charge density map shows the isotropic nature of charge distribution. Coexistence of electron and hole like pockets form the multi sheets Fermi surfaces and exhibit the multi-band nature of the compound. Importantly, Fermi surfaces with flat faces are considered as favorable condition for Fermi surface nesting. The calculated value of electron-phonon coupling constant (λ = 1.61) implies that the studied compound can be treated as a strongly coupled superconductor. Further, the thermodynamic properties at elevated temperature and pressure are calculated and analyzed for the first time by using quasi-harmonic model.

Charge puddles in a completely compensated topological insulator

Compensation of intrinsic charges is widely used to reduce the bulk conductivity of 3D topological insulators (TIs). Here we use low temperature electron irradiation-induced defects paired with in situ electrical transport measurements to fine-tune the degree of compensation in Bi2Te3. The coexistence of electrons and holes at the point of optimal compensation can only be explained by bulk carriers forming charge puddles. These need to be considered to understand the electric transport in compensated TI samples, irrespective of the method of compensation.

Semimetallic transport properties of epitaxially stabilized perovskite CaIrO3 films

We report on the synthesis and transport properties of perovskite (Pv) CaIrO3 thin films. The Pv phase of CaIrO3 was stabilized by epitaxial growth on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and LaAlO3 substrates with strong tensile, weak tensile, and compressive strains, respectively. The resistivity of these films showed a poorly metallic behavior. The Hall resistivity exhibited a sign change as a function of temperature and a nonlinear magnetic-field dependence, which clearly indicated the coexistence of electrons and holes and hence supported that Pv CaIrO3 films are semimetallic. The observed robustness of the semimetallic ground state against tensile and compressive strains is consistent with the presence of symmetry-protected Dirac points (nodes) around the Fermi level that prohibits the system from becoming a band insulator.

Experimental developments of A2RAM memory cells on SOI and bulk substrates

A2RAM prototype devices have been demonstrated in both SOI and bulk technologies. The fabrication process has successfully achieved the characteristic retrograde doping profile of the channel which allows the coexistence of electrons and holes in the same body while maintaining low-voltage single-gate operation. The different prototypes have been electrically characterized, all of them exhibiting memory effect. The SOI samples present the best performance, showing very attractive current margin between states, competitive retention time, reasonable variability, immunity to disturbance events and no endurance issues even in the short-channel devices fabricated in the most advanced 22nm process.

Ambipolar Charge Photogeneration and Transfer at GaAs/P3HT Heterointerfaces.

Recent work on hybrid photovoltaic systems based on conjugated polymers and III-V compound semiconductors with relatively high power conversion efficiency revived fundamental questions regarding the nature of charge separation and transfer at the interface between organic and inorganic semiconductors with different degrees of delocalization. In this work, we studied photoinduced charge generation and interfacial transfer dynamics in a prototypical photovoltaic n-type GaAs (111)B and poly(3-hexyl-thiophene) (P3HT) bilayer system. Ultrafast spectroscopy and density functional theory calculations indicate the coexistence of electron and hole transfer at the GaAs/P3HT interface, leading to the generation of long-lived species and photoinduced absorption upon creation of hybrid interfacial states. This opens up new avenues for the use of low-dimensional III-V compounds (e.g., nanowires or quantum dots) in hybrid organic/inorganic photovoltaics, where advanced bandgap and density of states engineering may also be exploited as design parameters.

Giant Magnetodrag in Graphene at Charge Neutrality

We report experimental data and theoretical analysis of Coulomb drag between two closely positioned graphene monolayers in a weak magnetic field. Close enough to the neutrality point, the coexistence of electrons and holes in each layer leads to a dramatic increase of the drag resistivity. Away from charge neutrality, we observe nonzero Hall drag. The observed phenomena are explained by decoupling of electric and quasiparticle currents which are orthogonal at charge neutrality. The sign of magnetodrag depends on the energy relaxation rate and geometry of the sample.