Hall Resistance Research Articles

Realization of Graphene-based Quantum Hall Resistance Standard

Quantum Hall Resistance (QHR) standard replaced the physical resistance standard as a primary standard. Traditional QHR standard needs liquid helium to operate, which limits the application of the QHR standard. A graphene-based QHR standard with G-M cryocooler is realized, which does not need liquid helium and has been in operation continuously with high performance. It can meet extensive needs of accurate measurement of precision resistors and helps to promote the development of traceability flattening.

Exchange-Biased Quantum Anomalous Hall Effect.

The quantum anomalous Hall (QAH) effect is characterized by a dissipationless chiral edge state with a quantized Hall resistance at zero magnetic field. Manipulating the QAH state is of great importance in both the understanding of topological quantum physics and the implementation of dissipationless electronics. Here, the QAH effect is realized in the magnetic topological insulator Cr-doped (Bi,Sb)2 Te3 (CBST) grown on an uncompensated antiferromagnetic insulator Al-doped Cr2 O3 . Through polarized neutron reflectometry (PNR), a strong exchange coupling is found between CBST and Al-Cr2 O3 surface spins fixing interfacial magnetic moments perpendicular to the film plane. The interfacial coupling results in an exchange-biased QAH effect. This study further demonstrates that the magnitude and sign of the exchange bias can be effectively controlled using a field training process to set the magnetization of the Al-Cr2 O3 layer. It demonstrates the use of the exchange bias effect to effectively manipulate the QAH state, opening new possibilities in QAH-based spintronics.

Enhanced magnetic anisotropy induced by praseodymium ferromagnetic order and anomalous transport properties in PrMn2Ge2 single crystals

Anisotropic magnetization and anomalous electrical transport properties of flux-grown $\mathrm{Pr}{\mathrm{Mn}}_{2}{\mathrm{Ge}}_{2}$ single crystals were investigated. The grown crystals exhibit strong magnetic anisotropy with easy magnetization along the $c$ axis. Linear magnetoresistance (MR) and a conventional anomalous Hall effect (AHE) are observed for $H\text{//}c$ and $I\text{//}b$ in contrast to a nonlinear positive MR and a large planar topological Hall effect (PTHE) observed for $H\text{//}a$ and $I\text{//}b$. The anisotropy of the magnetic and electrical transport properties is increased as temperature decreases because magnetic ordering of Pr atoms brings about an increase of the saturation magnetic field of nonlinear MR and the critical field for the maximum values of PTHE. A scaling analysis using the Tian-Ye-Jin model suggests that the AHE in this compound has a larger intrinsic contribution compared to the extrinsic contribution. The nonlinear positive MRs are attributed to the field-induced nontrivial spin textures. The PTHE is related to the real-space skyrmionic bubbles and the noncoplanar spin texture with nonzero spin chirality. These results demonstrate a magnetic field induced anisotropic magnetoresistance and Hall resistivity and its tunability by the ferromagnetic ordering of rare earth atoms in conical-ordered magnets.

Fermi-level dependence of magnetism and magnetotransport in the magnetic topological insulators Bi2Te3 and BiSbTe3 containing self-organized MnBi2Te4 septuple layers

The magnetic coupling mechanisms underlying ferromagnetism and magnetotransport in magnetically doped topological insulators (TIs) have been a central issue to gain controlled access to the magnetotopological phenomena such as the quantum anomalous Hall effect (AHE) and the topological axion insulating state. Here, we focus on the role of bulk carriers in magnetism of the family of magnetic TIs, in which the host material is either ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ or $\mathrm{BiSb}{\mathrm{Te}}_{3}$, containing Mn self-organized in $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ septuple layers. We tune the Fermi level using the electron irradiation technique and study how magnetic properties vary through the change in carrier density; the role of the irradiation defects is also discussed. Ferromagnetic resonance spectroscopy and magnetotransport measurements show no effect of the Fermi-level position on the magnetic anisotropy field and the Curie temperature, respectively, excluding bulk magnetism based on a carrier-mediated process. Furthermore, the magnetotransport measurements show that the AHE is dominated by the intrinsic and dissipationless Berry-phase-driven mechanism, with the Hall resistivity enhanced near the bottom/top of the conduction/valence band, due to the Berry curvature which is concentrated near the avoided band crossings. These results demonstrate that the AHE can be effectively managed, maximized, or turned off by adjusting the Fermi level.

Phonon-mediated superconductivity in the Sb square-net compound LaCuSb2

We investigated the electronic structure and superconducting properties of single-crystalline LaCuSb$_2$. The resistivity, magnetization, and specific heat measurements showed that LaCuSb$_2$ is a bulk superconductor. The observed Shubnikov-de Haas oscillation and magnetic field dependence of the Hall resistivity can be reasonably understood assuming a slightly hole-doped Fermi surface. Electron-phonon coupling calculation clarified the difference from the isostructural compound LaAgSb$_2$ indicating that (i) low-frequency vibration modes related to the interstitial layer sandwiched between the Sb-square nets significantly contribute to the superconductivity and (ii) carriers with sizable electron--phonon coupling distribute isotropically on the Fermi surface. These are assumed to be the origin of higher superconducting transition temperature compared with LaAgSb$_2$. We conclude that the superconducting properties of LaCuSb$_2$ can be understood within the framework of conventional phonon-mediated mechanism.

Carrier‐Modulated Anomalous Electron Transport in Electrolyte‐Gated SrTiO3

The anomalous transport phenomena and underlying mechanism of SrTiO3 have been long term concerned, including the Kondo physics and anomalous Hall effects to be discussed herein. Nevertheless, a clarification of the physical origins for these phenomena and their intertransitions upon varying carrier density remain to be an issue, due to the limited modulation of carrier density. Herein, the intriguing transport behaviors of electrolyte‐gated SrTiO3 in the field‐effect transistor geometry are investigated, which allows a relatively wide carrier density window. It is revealed that the remarkable Kondo‐like resistance upturn, identified at relatively low carrier density, becomes gradually disappeared and replaced by the emerging nonlinear Hall resistance with increasing gating voltage, indicating the interesting gating‐controlled transport behaviors. The detailed discussions on the underlying physics suggest that the gating‐controlled transport can be well described by the multiband transport model taking into account magnetic scattering in the channel layer, a comprehensive scenario for the emergent transport effects in carrier‐doped SrTiO3. The present work provides a promising platform for exploring novel quantum materials phenomena in SrTiO3 and analogous systems.

Interplay between Magnetism and Topology: Large Topological Hall Effect in an Antiferromagnetic Topological Insulator, EuCuAs.

Magnetic interactions in combination with nontrivial band structures can give rise to several exotic physical properties such as a large anomalous Hall effect, the anomalous Nernst effect, and the topological Hall effect (THE). Antiferromagnetic (AFM) materials exhibit the THE due to the presence of nontrivial spin structures. EuCuAs crystallizes in a hexagonal structure with an AFM ground state (Néel temperature ∼ 16 K). In this work, we observe a large topological Hall resistivity of ∼7.4 μΩ-cm at 13 K which is significantly higher than the giant topological Hall effect of Gd2PdSi3 (∼3 μΩ-cm). Neutron diffraction experiments reveal that the spins form a transverse conical structure during the metamagnetic transition, resulting in the large THE. In addition, by controlling the magnetic ordering structure of EuCuAs with an external magnetic field, several fascinating topological states such as Dirac and Weyl semimetals have been revealed. These results suggest the possibility of spintronic devices based on antiferromagnets with tailored noncoplanar spin configurations.

A model heterostructure with engineered Berry curvature

Molecular-beam epitaxy enables ultrathin functional materials to be combined in heterostructures to create emergent phenomena at the interface. Magnetic skyrmions are an example of an exciting phase found in such heterostructures. SrRuO3 and SrRuO3-based heterostructures have been at the center of the debate on whether a hump-like feature appearing in Hall resistivities is sufficient evidence to prove the presence of skyrmions in a material. To address the ambiguity, we synthesize a model heterostructure with engineered Berry curvature that combines, in parallel, a positive anomalous Hall effect (AHE) channel (a Sr0.6Ca0.4RuO3 layer) with a negative AHE channel (a SrRuO3 layer). We demonstrate that the two opposite AHE channels can be combined to artificially reproduce a “hump-like” feature, which closely resembles the hump-like feature typically attributed to the topological Hall effect and the presence of chiral spin textures, such as skyrmions. We compare our heterostructure with a parallel resistor model, where the inputs are the AHE data from individual Sr0.6Ca0.4RuO3 and SrRuO3 films. To check for the presence of skyrmions, we measure the current dependence, angle dependence, and minor loop dependence of Rhump in the heterostructure. Despite the clear hump, no evidence of skyrmions is found.

An evolution of hyperbolic magneto-hydrodynamic (HMHD) wave propagating through radiative quantum plasma including cosmic ray pressure

The gravitational (Jeans) instability of radiative quantum plasma including cosmic ray (CR) pressure and diffusion is theoretically investigated using a generalized hyperbolic magneto-hydrodynamic model. It concurrently includes the impacts of the finite electrical resistivity, the Hall parameter, and the Coriolis force. The application of normal mode technique yields a unique form of a generalized dispersion relation. This dispersion relation is further discussed in the different modes of propagation with the different axis of rotation along the direction of the magnetic field. It has been noted that all the considered parameters affect the system's growth rate in both directions, but the Hall parameter does not affect it in the transverse direction. We also explored that together with the CRs, the Hall parameter, resistivity, rotation, and quantum parameter suppressed the Jeans instability's growth rate. Thus, these parameters act as stabilizing agents to the instability. The study identified radiative instability and analyzed the impact of an arbitrary heat-loss function on the system. The current findings provide new theoretical support to the existing various astronomic observations on the cosmic plasma and in the development of unique galactic formations of distinct scale lengths.

Scattering-dependent transport of SrRuO3 films: From Weyl fermion transport to hump-like Hall effect anomaly

Recent observation of quantum transport phenomena of Weyl fermions has brought much attention to $4d$ ferromagnetic perovskite ${\mathrm{SrRuO}}_{3}$ as a magnetic Weyl semimetal. Additionally, the hump-like Hall effect anomaly, which might have a topological origin, has been reported for this material. Here, we show that the emergence of such phenomena is governed by the degree of scattering determined by the defect density (Ru-deficiency- and/or interface-driven-defect scattering) and measurement temperature (phonon scattering), where the former is controlled by varying the growth conditions of the ${\mathrm{SrRuO}}_{3}$ films in molecular beam epitaxy as well as the film thickness. The resulting electronic transport properties can be classified into three categories: clean, intermediate, and dirty regimes. The transport of Weyl fermions emerges in the clean regime, whereas that of topologically trivial conduction electrons in the ferromagnetic metal state prevails in the intermediate and dirty regimes. In the clean and intermediate regimes, anomalous Hall resistivity obeys a scaling law incorporating the intrinsic Karplus-Luttinger and extrinsic side-jump mechanisms. The hump-like Hall effect anomaly is observed only in the dirty regime, which is contrary to the scaling law between anomalous Hall resistivity and longitudinal resistivity. Hence, we conclude that this anomaly is not inherent to the material and does not have a topological origin. We also provide defect- and temperature-dependent transport phase diagrams of stoichiometric ${\mathrm{SrRuO}}_{3}$ and Ru-deficient ${\mathrm{SrRu}}_{0.7}{\mathrm{O}}_{3}$ where the appearance of Weyl fermions and hump-like Hall effect anomaly is mapped. These diagrams may serve as a guideline for designing $\mathrm{Sr}\phantom{\rule{0.16em}{0ex}}{\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{O}}_{3}$-based spintronic and topological electronic devices.

Anomalous Hall effect in ferromagnetic LaCo2As2 and ferrimagnetic NdCo2As2

We conducted a comparative study of the magnetic and transport properties of single-crystalline LaCo2As2 and NdCo2As2. LaCo2As2 is a soft metallic ferromagnet which exhibits purely intrinsic anomalous Hall effect (AHE) due to Co-3d electrons. With Nd-4f electronic magnetism, ferrimagnetic NdCo2As2 manifests pronounced sign reversal and multiple hysteresis loops in temperature- and field-dependent magnetization, Hall resistivity, and magnetoresistance, due to complicated magnetic structural changes. We reveal that the AHE for NdCo2As2 is stemming from the Co sub-lattice and deduce its phase diagram which includes magnetic compensation and two meta-magnetic phase transitions. The sensitivity of the Hall effect on the details of the magnetic structures in ferrimagnetic NdCo2As2 provides a unique opportunity to explore the magnetic interaction between 4f and 3d electrons and its impact on the electronic structure.

Hall effect of RCo2 (R= Er and Lu) compounds

The Hall effect of RCo2 with R = Er and Lu has been studied. LuCo2 is an exchange-enhanced Pauli paramagnet. ErCo2 is ferrimagnetic below TC = 32 K and undergoes a first-order magnetic transition at TC. The Hall resistivity ρxy was measured as a function of the magnetic field B at various temperatures. We estimated the normal (ordinary) Hall coefficient RN and the anomalous Hall coefficient RA from the analyses of the ρxy – B curves and the magnetization curves. For LuCo2, ρxy increases linearly with increasing field and its slope decreases rapidly with increasing temperature below 70 K. This is attributable to strong temperature dependence of RN. The ρxy – B curve of ErCo2 depends on a magnetic state. In the ferrimagnetic state, both RN and RA have positive values, while RN is positive and RA is negative in the paramagnetic state. We calculated RN's of nonmagnetic and ferromagnetic LuCo2 from the energy band structures and compared them with the experimental values. The origin of a sign change in RA of ErCo2 is briefly discussed.

Growth and Variable Temperature Hall Effect Study of SrPb3Br8 Crystal with Low Phonon Energy

AbstractTransparent and homogeneous SrPb3Br8 single crystals with the size of Ø25 mm × 50 mm are grown by the modified vertical Bridgman method. X‐ray diffraction results reveal that the middle part of the crystal is pure in chemical compositions, while the tip part contains low content of Sr(PbBr3)2(H2O)5. X‐ray photoelectron spectroscopy analysis points out the absorption to C, O‐containing substances of the pristine surfaces of SrPb3Br8 wafers. The Raman results prove the low phonon energy of the SrPb3Br8 (116 cm−1). Transmission spectra demonstrate its wide transparent range (0.37–25 µm) and high transmittance in the mid‐infrared range except for several absorption lines. Moreover, Hall measurements show that the SrPb3Br8 sample is n‐type conductive at 110–300 K, and the Hall coefficient RH, resistivity ρ and mobility µH decrease as temperature increases while the carrier concentration n has an opposite trend. The conduction mechanism and scattering mechanism of SrPb3Br8 crystal at different temperatures has been studied by analyzing ln(|RH|)−T−1, n−T and µH−T curves, and the dominant donor defect ionization energy ED is calculated to be 0.077 eV.

Transport properties of a 1000 nm HgTe film: the interplay of surface and bulk carriers

We report on systematic study of transport properties of a 1000 nm HgTe film. Unlike thinner and strained HgTe films, which are known as high-quality three-dimensional topological insulators, the film under study is much thicker than the limit of pseudomorphic growth of HgTe on a CdTe substrate. Therefore, the 1000 nm HgTe film is expected to be fully relaxed and has the band structure of bulk HgTe, i.e. a zero gap semiconductor. Additionally, the system is characterized by the bands inversion, so that the two-dimensional topological surface states (TSSs) are expected to exist. To check this claim we studied classical and quantum transport response of the system. We demonstrate that by tuning the top-gate voltage one can change the electron-dominating transport to the hole one. The highest electron mobility is found to be more than cm2 Vs−1. The system exhibits Shubnikov-de Haas (SdH) oscillations with a complicated pattern and shows up to five independent frequencies in corresponding Fourier spectra. These Fourier peaks are attributed to the TSSs, Volkov-Pankratov states and spin-degenerate bulk states in the accumulation layer near the gate. The observed peculiarities of the quantum transport are the strong SdH oscillations of the Hall resistance, and the suppressed oscillatory response of the TSSs.

Nonlinear transport phenomena and current-induced hydrodynamics in ultrahigh mobility two-dimensional electron gas

We report on nonlinear transport phenomena at high filling factor and DC current-induced electronic hydrodynamics in an ultrahigh mobility $\text{(}\ensuremath{\mu}=20\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}\text{)}$ two-dimensional electron gas in a narrow ($15\ensuremath{-}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wide) GaAs/AlGaAs Hall bar for DC current densities reaching 0.67 A/m. The various phenomena and the boundaries between the phenomena are captured together in a two-dimensional differential resistivity map as a function of magnetic field (up to 250 mT) and DC current. This map, which resembles a phase diagram, demarcate distinct regions dominated by Shubnikov-de Haas (SdH) oscillations (and phase inversion of these oscillations) around zero DC current; negative magnetoresistance and a double-peak feature (both ballistic in origin) around zero field; and Hall field-induced resistance oscillations (HIROs) radiating out from the origin. From a detailed analysis of the data near zero field, we show that increasing the DC current suppresses the electron-electron scattering length that drives a growing hydrodynamic contribution to both the differential longitudinal and transverse (Hall) resistivities. Our approach to induce hydrodynamics with DC current differs from the more usual approach of changing the temperature. We also find a significant (factor of two to four) difference between the quantum lifetime extracted from SdH oscillations, and the quantum lifetime extracted from HIROs. In addition to observing HIRO peaks up to the seventh order, we observe an unexpected HIRO-like feature close to midway between the first-order and the second-order HIRO maxima at high DC current.

Cryogenic sensor enabling broad-band and traceable power measurements

Recently, great progress has been made in the field of ultrasensitive microwave detectors, reaching even the threshold for utilization in circuit quantum electrodynamics. However, cryogenic sensors lack the compatibility with broad-band metrologically traceable power absorption measurements at ultralow powers, which restricts their range of applications. Here, we demonstrate such measurements using an ultralow-noise nanobolometer, which we extend by an additional direct-current (dc) heater input. The tracing of the absorbed power relies on comparing the response of the bolometer between radio frequency and dc-heating powers traced to the Josephson voltage and quantum Hall resistance. To illustrate this technique, we demonstrate two different methods of dc-substitution to calibrate the power that is delivered to the base temperature stage of a dilution refrigerator using our in situ power sensor. As an example, we demonstrate the ability to accurately measure the attenuation of a coaxial input line between the frequencies of 50MHz and 7GHz with an uncertainty down to 0.1 dB at a typical input power of -114 dBm.

Anomalous Hall effect and topological Hall effect in the noncollinear antiferromagnetV0.3NbS2

The recent discoveries of large anomalous Hall effect (AHE) in antiferromagnets (AFMs) have brought intense study in theories and experiments. Special attention has also been given to the topic of topological Hall effect (THE). Both of them provide a good platform for understanding the interplay between magnetism and transport characteristics. Here, we report the magnetic and electrical transport properties of a layered AFM ${\mathrm{V}}_{0.3}{\mathrm{NbS}}_{2}$ single crystal. It orders antiferromagnetically below ${T}_{N1}\ensuremath{\sim}53\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and exhibits weak magnetic hysteresis loop with a tiny net magnetization ($\ensuremath{\approx}16\phantom{\rule{0.16em}{0ex}}m{\ensuremath{\mu}}_{B}/\mathrm{V}$) along the $c$ axis as a result of canted antiferromagnetic configuration. Of particular interest is the realization of AHE in such AFMs with a near-perfect cancellation of moment, which probably originates from the contribution of nonzero Berry curvature. THE is detected at low temperatures, which acts as the probe of spin reorientation in the magnetization process induced by field. The magnetoresistance shows a weak irreversible behavior near zero field, consistent with the hysteresis loop in magnetization and Hall resistivity. It presents a negative value below 7 T, which is ascribed to the spin-dependent scattering. The observation of anomalous transport in ${\mathrm{V}}_{0.3}{\mathrm{NbS}}_{2}$ serves as a platform for studying AHE within AFMs.

Kondo scattering in underdoped Nd1-xSrxNiO2 infinite-layer superconducting thin films.

The recent discovery of superconductivity in infinite-layer nickelates generates tremendous research endeavors, but the ground state of their parent compounds is still under debate. Here, we report experimental evidence for the dominant role of Kondo scattering in the underdoped Nd1-xSrxNiO2 thin films. A resistivity minimum associated with logarithmic temperature dependence in both longitudinal and Hall resistivities are observed in the underdoped Nd1-xSrxNiO2 samples before the superconducting transition. At lower temperatures down to 0.04K, the resistivities become saturated, following the prediction of the Kondo model. A linear scaling behavior [Formula: see text] between anomalous Hall conductivity [Formula: see text] and conductivity [Formula: see text]is revealed, verifying the dominant Kondo scattering at low temperature. The effect of weak (anti-)localization is found to be secondary. Our experiments can help in clarifying the basic physics in the underdoped Nd1-xSrxNiO2 infinite-layer thin films.

Coherent backscattering in the topological Hall effect

The mutual interplay between electron transport and magnetism has attracted considerable attention in recent years, primarily motivated by strategies to manipulate magnetic degrees of freedom electrically, such as spin–orbit torques and domain wall motion. Within this field the topological Hall effect, which originates from scalar spin chirality, is an example of inter-band quantum coherence induced by real-space inhomogeneous magnetic textures, and its magnitude depends on the winding number and chiral spin features that establish the total topological charge of the system. Remarkably, in the two decades since its discovery, there has been no research on the quantum correction to the topological Hall effect. Here we will show that, unlike the ordinary Hall effect, the inhomogeneous magnetization arising from the spin texture will give additional scattering terms in the kinetic equation, which result in a quantum correction to the topological Hall resistivity. We focus on two-dimensional systems, where weak localization is strongest, and determine the complicated gradient corrections to the Cooperon and kinetic equation. Whereas the weak localization correction to the topological Hall effect is not large in currently known materials, we show that it is experimentally observable in dilute magnetic semiconductors. Our theoretical results will stimulate experiments on the topological Hall effect and fill the theoretical knowledge gap on weak localization corrections to transverse transport.

Planar Hall effect in the quasi-two-dimensional topological semimetal candidate In0.93TaSe2

Here, we report the systematic study on the planar transport properties of the quasi-two-dimensional (quasi-2D) topological nodal-line semimetal candidate In0.93TaSe2. When rotating magnetic field in the plane, the anisotropic longitudinal resistance and planar Hall resistance are clearly observed and can be well described by the theoretical formulation of the planar Hall effect (PHE). Further analysis demonstrates that the anisotropic orbital resistance rather than the topological-nontrivial chiral anomaly plays a dominant role on the PHE in In0.93TaSe2. Our study provides another platform for understanding the mechanism of PHE, which may also be valuable for future planar Hall sensors based on quasi-2D materials.