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We investigate the use of Raman spectroscopy to measure carrier concentrations in n -type GaSb epilayers to aid in the development of this technique for the nondestructive characterization of transport properties in doped semiconductors. The carrier concentration is quantified by modeling the measured coupled optical phonon-free carrier plasmon mode spectra. We employ the Lindhard–Mermin optical susceptibility model with contributions from carriers in the two lowest GaSb conduction-band minima, the Γ and L minima. Furthermore, we evaluate three conduction-band models: (1) both minima parabolic and isotropic, (2) the Γ minimum non-parabolic and isotropic and the L minima parabolic and isotropic, and (3) the Γ minimum non-parabolic and isotropic and the L minima parabolic and ellipsoidal. For a given epilayer, the carrier concentration determined from the spectral simulations was consistently higher for the ellipsoidal L minima model than the other two models. To evaluate the conduction-band models, we calculated the L to Γ electron mobility ratio necessary for the electron concentrations from the Raman spectral measurements to reproduce those from the Hall effect measurements. We found that the model with the ellipsoidal L minima agreed best with reported carrier-dependent mobility-ratio values. Hence, employing isotropic L minima in GaSb conduction-band models, a common assumption when describing the GaSb conduction band, likely results in an underestimation of carrier concentration at room temperature and higher doping levels. This observation could have implications for Raman spectral modeling and any investigation involving the GaSb conduction band, e.g., modeling electrical measurements or calculating electron mobility. [ABSTRACT FROM AUTHOR]

Partially magnetized low-temperature plasmas (LTP) in an E × B configuration, where the applied magnetic field is perpendicular to the self-consistent electric field, have become increasingly relevant in industrial applications. Hall thrusters, a type of electrostatic plasma propulsion, are one of the main LTP technologies whose advancement is hindered by the not-fully-understood underlying physics of operation, particularly, with respect to the plasma instabilities and the associated electron cross field transport. The development of Hall thrusters with unconventional magnetic field topologies has imposed further questions regarding the instabilities' characteristics and the electrons' dynamics in these modern cross field configurations. Accordingly, we present in this effort a detailed parametric study of the influence of three factors on the plasma processes in the radial-azimuthal coordinates of a Hall thruster, namely, the magnetic field gradient, secondary electron emission, and plasma number density. The studies are carried out using the reduced-order particle-in-cell code developed by the authors. The setup of the radial-azimuthal simulations largely follows a well-defined benchmark case from the literature in which the magnetic field is oriented along the radius, and a constant axial electric field is applied perpendicular to the simulation plane. The salient finding from our investigations is that, in the studied cases corresponding to elevated plasma densities, a long-wavelength azimuthal mode with the frequency of about 1 MHz is developed. Moreover, in the presence of strong magnetic field gradients, this mode results from an inverse energy cascade and induces a significant electron cross field transport as well as a notable heating of the ions. [ABSTRACT FROM AUTHOR]

Recently, the study of quantum materials through thermal characterization methods has attracted much attention. These methods, although not as widely used as electrical methods, can reveal intriguing physical properties in materials that are not detectable by electrical methods, particularly in electrical insulators. A fundamental understanding of these physical properties is critical for the development of novel applications for energy conversion and storage, quantum sensing and quantum information processing. In this review, we introduce several commonly used thermal characterization methods for quantum materials, including specific heat, thermal conductivity, thermal Hall effect, and Nernst effect measurements. Important theories for the thermal properties of quantum materials are discussed. Moreover, we introduce recent research progress on thermal measurements of quantum materials. We highlight experimental studies on probing the existence of quantum spin liquids, Berry curvature, chiral anomaly, and coupling between heat carriers. We also discuss the work on investigating the quantum phase transitions and quasi-particle hydrodynamics using thermal characterization methods. These findings have significantly advanced knowledge regarding novel physical properties in quantum materials. In addition, we provide some perspectives on further investigation of novel thermal properties in quantum materials. [ABSTRACT FROM AUTHOR]

Antiperovskite compounds with abundant magnetic phase transitions provide an ideal platform for exploring nontrivial magnetotransport responses. In this study, the anomalous Hall effect (AHE) in an antiperovskite (110)-oriented Mn3.19Ga0.81N1−δ film up to room temperature was observed, and an unusual sign reversal was detected in the Hall measurements. The AHE reversal suggests that the carrier reversal corresponds to a magnetic transition from a ferrimagnetic order to noncollinear antiferromagnetic order at about 240 K. Analysis of the scaling relation of AHE indicates that the sign reversal originates from the transition from the skew scattering dominated AHE to the intrinsic mechanism dominated AHE. These findings will inspire effective control of the magnetic configuration and innovative applications for manipulating carrier transport in spintronic memory devices based on antiperovskite films. [ABSTRACT FROM AUTHOR]

Nodal link semimetals as a type of topological semimetals are characterized by the nodal link rings in the Brillouin zone. The magneto-optical conductivity of nodal link semimetals is investigated numerically based on a system with a magnetic field. We find that in the system, the Landau levels (LLs) and magnetic field show a relationship of three halves power, which can lead to an intricate curved LLs. Due to the rich structure of LLs, we divided the LLs into four different regions to calculate the magneto-optical conductivity. We calculated the absorption peaks of longitudinal and Hall conductance of a neutral system and with different chemical potential. We find that the absorption peaks of longitudinal conductance are in a curved background in the whole range of magnetic field. When chemical potential is tuned to pass through the LLs, there are not only the intraband transition but also weak redistributed peaks in the low frequency. From the transition peaks of Hall conductance, we can find that there are several negative peaks because of the striking LLs spectrum structure. This will provide a feasible way to distinguish nodal link semimetals from other materials. [ABSTRACT FROM AUTHOR]

Anomalous Nernst effect of epitaxial Fe4N films on MgO(001), MgAl2O4(MAO)(001), and SrTiO3(STO)(001) substrates grown by molecular beam epitaxy was investigated. Moderately large anomalous Nernst coefficients (SANE) of 1.4 and 1.7 μV/K were obtained in the Fe4N films on the MgO(001) and MAO(001) substrates, respectively, and large anomalous Hall angles (∼0.06) and transverse thermoelectric conductivities [∼1.3 A/(m K)] were derived from the experimental results. On the other hand, a large effective SANE of 2.8 μV/K was obtained in the Fe4N film on the STO(001) substrate. The origin of the enhanced effective SANE is the negatively large Seebeck coefficient (SSE) in an oxygen deficient STO layer near the surface of the STO substrate. This indicates that it is possible to enhance the effective SANE of ferromagnetic materials by utilizing adjacent materials with large |SSE| such as the oxygen deficient STO layer. [ABSTRACT FROM AUTHOR]

During the standard ac lock-in measurement of the resistance of a two-dimensional electron gas (2DEG) applying an ac current I = 2 I 0 sin (ω t) , the electron temperature T e oscillates with the angular frequency 2 ω due to the Joule heating ∝ I 2. We have shown that the highest (T H ) and the lowest (T L ) temperatures during a cycle of the oscillations can be deduced, at cryogenic temperatures, exploiting the third-harmonic (3 ω) component of the voltage drop generated by the ac current I and employing the amplitude of the Shubnikov–de Haas oscillations as the measure of T e. The temperatures T H and T L thus obtained allow us to roughly evaluate the thermal conductivity κ x x of the 2DEG via the modified 3 ω method, in which the method originally devised for bulk materials is modified to be applicable to a 2DEG embedded in a semiconductor wafer. κ x x thus deduced is found to be consistent with the Wiedemann–Franz law. The method provides a convenient way to access κ x x using only a standard Hall-bar device and the simple experimental setup for the resistance measurement. [ABSTRACT FROM AUTHOR]

Switching of perpendicular magnetization via spin–orbit torque (SOT) is of particular interest in the development of non-volatile magnetic random access memory (MRAM) devices. We studied current-induced magnetization switching of Ir/GdFeCo/Cu/Pt heterostructures in a Hall cross geometry as a function of the in-plane applied magnetic field. Remarkably, magnetization switching is observed at zero applied field. This is shown to result from the competition between SOT, the Oersted field generated by the charge current, and the material's coercivity. Our results show a means of achieving zero-field switching that can impact the design of future spintronics devices, such as SOT-MRAM. [ABSTRACT FROM AUTHOR]

We examine the thermal origin of the detected "saw-tooth" shaped Hall resistance (R xy ) signals in the spin–orbit torque switching experiment for antiferromagnetic MnN. Compared with the results of the heavy metal/antiferromagnet bilayers (MnN/Ta), the qualitatively same "saw-tooth" shaped signals also appear in the samples with the heavy metal layer alone (either Ta or Pt) without the MnN layer. In addition, the R xy signal changes oppositely in the devices with Ta and Pt, due to the opposite temperature coefficient of resistivity of the two materials. All those results are consistent with the "localized Joule heating" mechanism in devices with Hall crosses geometry. Moreover, by utilizing a structure with separated writing current paths and Hall cross area, the quadratic relationship between Δ R xy and the writing current's amplitude is observed, which provides quantitative evidence of the thermal contribution. These results reveal the dominant thermal artifact in the widely used Hall crosses geometry for Néel vector probing and also provide a strategy to semi-quantitatively evaluate the thermal effect, which can shed light on a more conclusive experiment design. [ABSTRACT FROM AUTHOR]

Low-frequency axial oscillations in the range of 5–50 kHz stand out as a pervasive feature observed in many types of Hall thrusters. While it is widely recognized that the ionization effects play the central role in this mode, as manifested via the large-scale oscillations of neutral and plasma density, the exact mechanism(s) of the instabilities remain unclear. To gain further insight into the physics of the breathing mode and evaluate the role of kinetic effects, a one-dimensional time-dependent full nonlinear low-frequency model describing neutral atoms, ions, and electrons is developed in full fluid formulation and compared to the hybrid model in which the ions and neutrals are kinetic. Both models are quasi-neutral and share the same electron fluid equations that include the electron diffusion, mobility across the magnetic field, and the electron energy evolution. The ionization models are also similar in both approaches. The predictions of fluid and hybrid simulations are compared for different test cases. Two main regimes are identified in both models: one with pure low-frequency behavior and the other one, where the low-frequency oscillations coexist with high-frequency oscillations in the range of 100–200 kHz, with the characteristic time scale of the ion channel fly-by time, 100–200 kHz. The other test case demonstrates the effect of a finite temperature of injected neutral atoms, which has a substantial suppression effect on the oscillation amplitude. [ABSTRACT FROM AUTHOR]