Hall Bar Research Articles

Levitons in correlated nano-scale systems.

In this short review (written to celebrate David Campbell's 80th birthday), we provide a theoretical description of quantum transport in nanoscale systems in the presence of single-electron excitations generated by Lorentzian voltage drives, termed Levitons. These excitations allow us to realize the analog of quantum optics experiments using electrons instead of photons. Importantly, electrons in condensed matter systems are strongly affected by the presence of different types of non-trivial correlations, with no counterpart in the domain of photonic quantum optics. After providing a short introduction about Levitons in non-interacting systems, we focus on how they operate in the presence of two types of strong electronic correlations in nanoscale systems, such as those arising in the fractional quantum Hall effect or in superconducting systems. Specifically, we consider Levitons in a quantum Hall bar of the fractional quantum Hall effect, pinched by a quantum point contact, where anyons with fractional charge and statistics tunnel between opposite edges. In this case, a Leviton-Leviton interaction can be induced by the strongly correlated background. Concerning the effect of superconducting correlations on Levitons, we show that, in a normal metal system coupled to BCS superconductors, half-integer Levitons minimize the excess noise in the Andreev regime. Interestingly, energy-entangled electron states can be realized on-demand in this type of hybrid setup by exploiting crossed Andreev reflection. The results exposed in this review have potential applications in the context of quantum information and computation with single-electron flying qubits.

Anomalous hall and skyrmion topological hall resistivity in magnetic heterostructures for the neuromorphic computing applications

Topologically protected spin textures, such as magnetic skyrmions, have shown the potential for high-density data storage and energy-efficient computing applications owing to their particle-like behavior, small size, and low driving current requirements. Evaluating the writing and reading of the skyrmion’s magnetic and electrical characteristics is crucial to implementing these devices. In this paper, we present the magnetic heterostructure Hall bar device and study the anomalous Hall and topological Hall signals in these devices. Using different measurement techniques, we investigate the magnetic and electrical characteristics of the magnetic structure. We measure the skyrmion topological resistivity and the magnetic field at different temperatures. MFM imaging and micromagnetic simulations further explain the anomalous Hall and topological Hall resistivity characteristics at various magnetic fields and temperatures. The study is extended to propose a skyrmion-based synaptic device showing spin-orbit torque-controlled plasticity. The resistance states are read using the anomalous Hall measurement technique. The device integration in a neuromorphic circuit is simulated in a 3-layer feedforward artificial neural network ANN. Based on the proposed synapses, the neural network is trained and tested on the MNIST data set, where a recognition accuracy performance of about 90% is achieved. Considering the nanosecond reading/writing time scale and a good system level performance, these devices exhibit a substantial prospect for energy-efficient neuromorphic computing.

Origin of hole mobility anisotropy in 4H-SiC

Hole mobility anisotropy in 4H-SiC was investigated based on both experimental and theoretical approaches. First, the authors established a complete database of the anisotropic hole mobility along both directions parallel and perpendicular to the c-axis in 4H-SiC over the wide acceptor density and temperature ranges by preparing Hall bar structures on p-type SiC(112¯0) epitaxial layers. Empirical equations for the mobility along each direction vs the acceptor density and temperature were determined, which should be useful for the simulation and designing of any SiC devices. In addition to that, the anisotropy in the hole mobility was extracted from the experimental results, and its origin was discussed focusing on that in the effective mass (m∗) of holes. The obtained mobility ratio was far from the m∗ ratio at the valence band maximum, and an averaged m∗ along each direction was determined by theoretical calculation taking into account the energy distribution of holes. Consequently, the authors revealed that the anisotropic hole mobility is explained quantitatively by the anisotropic m∗ considering the E–k dispersion over the entire first Brillouin zone.

Enhancing damping-like efficiency by low-energy mixed ions bombardment

Pursuing enhanced spin–orbit torque (SOT) has become a significant focus for achieving energy-efficient spintronics devices. Researchers have explored different materials, layer engineering, and various post-deposition modification methods to realize higher SOT. Here, we have utilized the bombardment of mixed ions (Ar+ and He+ in various ratios) with 0.6 kV bias voltage to enhance the SOT efficiency in a Pt/Co/W stack. The bombardment modifies the physical properties (magnetic, electric, and structural) due to cascade collision, knocking off atoms (from the surface), and ion implantation. The increased percentage of He+ suppresses the drastic behavior of cascade collisions, and hence, the properties are tunable by changing the gas ratio. We exploited this behavior to design a material stack with enhanced SOT efficiency. We fabricated Hall bars to quantify SOT and observed a 4.5× increase in the effective damping-like efficiency. The efficiency and switching current density in the stack could be tuned by the variation of Ar:He ion ratio and the exposure duration. This study offers an easy route to tune the energy efficiency of spin devices, contributing to the advancement of next-generation spintronics.

Emulation of neuron and synaptic functions in spin-orbit torque domain wall devices.

Neuromorphic computing (NC) architecture has shown its suitability for energy-efficient computation. Amongst several systems, spin-orbit torque (SOT) based domain wall (DW) devices are one of the most energy-efficient contenders for NC. To realize spin-based NC architecture, the computing elements such as synthetic neurons and synapses need to be developed. However, there are very few experimental investigations on DW neurons and synapses. The present study demonstrates the energy-efficient operations of neurons and synapses by using novel reading and writing strategies. We have used a W/CoFeB-based energy-efficient SOT mechanism to drive the DWs at low current densities. We have used the concept of meander devices for achieving synaptic functions. By doing this, we have achieved 9 different resistive states in experiments. We have experimentally demonstrated the functional spike and step neurons. Additionally, we have engineered the anomalous Hall bars by incorporating several pairs, in comparison to conventional Hall crosses, to increase the sensitivity as well as signal-to-noise ratio (SNR). We have performed micromagnetic simulations and transport measurements to demonstrate the above-mentioned functionalities.

Skyrmion Based Synaptic Device Having Perpendicular Magnetic-Tunnel-Junction Spin-Valve Sensor

Perpendicular spin-transfer-torque magnetic random access memory (p-STT MRAM) has been reported as an application of binary neural networks (BNN) [1]. In addition, magnetic skyrmions have been reported as information carriers for neuromorphic devices due to their small nanoscale diameter, topological stability, and complete linearity [2-3]. A previous study has demonstrated the synaptic characteristics of skyrmion based synapses using a Hall bar measurement [4]. However, skyrmion based synapses using the Hall bar measurement were not able to detect electrically a synaptic behavior. Thus, for the first time, the skyrmion based synaptic device detected resistance change via sensing the perpendicular magnetic-tunnel-junction (p-MTJ) spin-valve, as shown in Fig.1(a). The detailed synaptic behavior and mechanism will be presented.In this study, we introduced the skyrmion based synaptic device having p-MTJ spin-valve sensor designed by CoFeB free layer, MgO tunneling barrier, CoFeB pinned layer, SyAF layer, and interface reducing W layer between the CoFeB free layer and MgO tunneling barrier, as shown in Fig.1(a). The spin direction of the CoFeB free layer was changed by the generated skyrmions, and the resistance of the synaptic device changed depending on the parallel or anti-parallel state between CoFeB free and pinned layer. In particular, the skyrmion generation was determined critically by the combination of DMI and PMA (i.e., saturation magnetization (Ms), squareness), and the interface reducing W layer was designed to reduce PMA. The M-H loop of the synaptic device with the interface reducing W layer thickness of 0.110 nm showed reduced saturation magnetization (Ms ~ 60 μemu) and squareness (Hc ~ 8 Oe) via a vibrating-sample magnetometer (VSM), as shown in [Figs.1(b), 1(c), and 1(d)]. In addition, uniform state, generation of skyrmions, and stripe domains in the synaptic device were achieved at the external magnetic field of 0, 3.5, and 7 Oe, respectively, as shown in [Figs.1(e), 1(f), and 1(g)]. Moreover, we observed that the number of skyrmions in the synaptic device with a cell diameter of 5μm changed depending on the external magnetic field via the magneto-optical Kerr effect (MOKE) image, as shown in [Figs.1(h), 1(i), and 1(j)]. Finally, we demonstrated a multi-level operation in which the electrical resistance of the synaptic device changed as the number of skyrmions. In our presentation, we will review in detail the synaptic behavior and mechanism for the synaptic device. Acknowledgements This research was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2021M3F3A2A01037733) References [1] Thi-Nhan Pham et al., IEEE JOURNAL ON EMERGING AND SELECTED TOPICS IN CIRCUITS AND SYSTEMS, 12, 569-579 (2022)[2] Nagaosa, N. & Tokura, Y. Nat. Nanotechnol. 8, 899–911 (2013).[3] C Back, V Cros et al., J. phys. D. Appl. phys. 53, 363001 (2020).[4] Kyung Mee Song et al., Nature Electronics. 3, 148–155 (2020) Figure 1

Above-Room-Temperature Ferromagnetism in Copper-Doped Two-Dimensional Chromium-Based Nanosheets.

Two-dimensional ferromagnetic materials (2D-FMs) are expected to become ideal candidates for low-power, high-density information storage in next-generation spintronics devices due to their atomically ultrathin and intriguing magnetic properties. However, 2D-FMs with room-temperature Curie temperatures (Tc) are still rarely reported, which greatly hinders their research progress and practical applications. Herein, ultrathin Cu-doped Cr7Te8 FMs were successfully prepared and can achieve above-room-temperature ferromagnetism with perpendicular magnetic anisotropy via a facile chemical vapor deposition (CVD) method, which can be controlled down to an atomic thin layer of ∼3.4 nm. STEM-EDX quantitative analysis shows that the proportion of Cu to metal atoms is ∼5%. Moreover, based on the anomalous Hall effect (AHE) measurements in a six-terminal Hall bar device without any encapsulation as well as an out-of-plane magnetic field, the maximum Tc achieved ∼315 K when the thickness of the sample is ∼28.8 nm; even the ultrathin 7.6 nm sample possessed a near-room-temperature Tc of ∼275 K. Meanwhile, theoretical calculations elucidated the mechanism of the ferromagnetic enhancement of Cu-doped Cr7Te8 nanosheets. More importantly, the ferromagnetism of CVD-synthesized Cu-doped CrSe nanosheets can also be maintained above room temperature. Our work broadens the scope on room-temperature ferromagnets and their heterojunctions, promoting fundamental research and practical applications in next-generation spintronics.

Highly Sensitive Hall Sensors Based on Chemical Vapor Deposition Graphene.

In this work, we demonstrate highly sensitive and scalable Hall sensors fabricated by adopting arrays of monolayer single-crystal chemical vapor deposition (CVD) graphene. The devices are based on graphene Hall bars with a carrier mobility of >12000 cm2 V-1 s-1 and a low residual carrier density of ∼1 × 1011 cm-2, showing Hall sensitivity higher than 5000 V A-1 T-1, which is a value previously only achieved when using exfoliated graphene encapsulated with flakes of hexagonal boron nitride. We also implement a facile and scalable polymeric encapsulation, allowing the performance of graphene Hall bars to be stabilized when measured in an ambient environment. We demonstrate that this capping method can reduce the degradation of electrical transport properties when the graphene devices are kept in air over 10 weeks. State-of-the-art performance of the realized devices, based on scalable synthesis and encapsulation, contributes to the proliferation of graphene-based Hall sensors.

Electrical switching of the edge current chirality in quantum anomalous Hall insulators.

A quantum anomalous Hall (QAH) insulator is a topological phase in which the interior is insulating but electrical current flows along the edges of the sample in either a clockwise or counterclockwise direction, as dictated by the spontaneous magnetization orientation. Such a chiral edge current eliminates any backscattering, giving rise to quantized Hall resistance and zero longitudinal resistance. Here we fabricate mesoscopic QAH sandwich Hall bar devices and succeed in switching the edge current chirality through thermally assisted spin-orbit torque (SOT). The well-quantized QAH states before and after SOT switching with opposite edge current chiralities are demonstrated through four- and three-terminal measurements. We show that the SOT responsible for magnetization switching can be generated by both surface and bulk carriers. Our results further our understanding of the interplay between magnetism and topological states and usher in an easy and instantaneous method to manipulate the QAH state.

A way of determination of axion mass with quantum Hall effect

Axion dark matter is converted to electromagnetic radiations in the presence of strong magnetic field. The radiations possibly give rise to non trivial phenomena in condensed matter physics. Especially, we discuss that saturation of plateau-plateau transition width observed at low temperature in integer quantum Hall effect is caused by the axion. The radiations from axions are inevitably present in the experiment. Although the radiations generated by axion are extremely weak, Hall conductivity jumps up to next plateau even if only a single electron occupies an extended state; a localized electron is transited to the extended state by absorbing the radiation. According to our analysis, previous experiment [33] of the saturation in detail suggests that the axion mass is in the range 10−5eV∼10−6 eV. We propose a way of the determination of the axion mass by imposing microwaves on Hall bar and also a way of the confirmation that the axion really causes the saturation of the width.

Side charge propagation in simultaneous KPFM and transport measurement of humidity exposed graphene FET sensor

The surface diffusion (dissipation) of charge carriers enhanced by water molecules in solution-based biosensors and ambient operating gas sensors strongly influence their resistance response, sensitivity, and stability in time. Therefore, the information on the charge distribution at interfaces of conductive and insulating parts is essential for the operating sensors. This work presents the simultaneous measurement of the longitudinal macroscopic resistance response and local surface potential (SP) mapping by Kelvin probe force microscopy (KPFM) on a graphene Hall bar sensor. The results show the propagation of an electric charge from the main graphene channel onto the neighboring SiO2 surface. The charge propagation strongly increases with the relative humidity and can be controlled by a bottom-gate voltage used in most sensors based on a field effect transistor (FET) architecture. As proved by the longitudinal resistance measurements, the resulting side charge accumulation has a very small impact on the 2D resistivity of the graphene channel. It has been explained by an application of the Thomas-Fermi theory, proving an efficient screening of side accumulated charge potential caused by a redistribution of the charge inside the wide graphene channel. The combination of a transport resistance response and KPFM provides a deeper understanding of sensors/biosensors functionality and their design features than a simple resistance response usually observed.

The topological in-equivalence of Hall bar and Corbino geometries in coordinate space: Screening theory and direct transport experiments

The topological in-equivalence of Hall bar and Corbino geometries in coordinate space: Screening theory and direct transport experiments

(Invited) Hybrid Nanodevices for Technology and Fun

Two-dimensional materials like graphene or transition metal dichalcogenides have remarkable properties, making them very attractive from a technological point of view and incredibly fun to work with. The intrinsic properties of 2D materials change drastically when going from the bulk into the true 2D limit, where they exhibit exotic quantum properties. While many groups have focused on studying and exploiting these materials, very little work has been devoted to engineering their intrinsic properties. The “all-surface” nature makes device fabrication complex, and so far, only the most conventional nanofabrication or tailoring routes have been explored. In this talk, I will present novel routes currently under exploration in our group to tailor and exploit the properties of TMDS and MOF devices based on mechanical deformation or physical nanopatterning.Also, the integration of 2D materials into superconducting circuits can lead to novel technological applications of interest for photon and radiation detectors or quantum computing.In this talk, I will present some results concerning the development of retroreflectors based on MoS2 oscillating membranes and how they can be turned into electroluminescent diodes. I will also present the tailoring of conductive channels on MoS2 FET devices. Preliminary results on lateral PN homo-junctions, memristive field-effect transistors and photonic crystal cavities will also be presented. I will present preliminary results on the integration of MOF into Hall bar structures for metrology applications. To finish, I will also present results on the integration of Graphene and MoS2 onto Superconducting nanowire single photon detectors. Figure 1

Sub-micrometer four-point probe transport measurements on graphene

Usually, electronic transport measurements on two-dimensional materials, such as graphene and transition metal dichalcogenides, require deposition of electrodes on top of the material, in, for instance, the form of a Hall bar device. In this work, we show that by making use of a collinear micro-four-point probe, electrical transport measurements on small flakes of graphene can be performed without having to fabricate electrodes on top of the flakes. Using probes with probe pitches down to sub-micrometer scale, we show back-gate tuned transport measurements in graphene on silicon oxide and on hexagonal boron nitride. The charge carrier mobilities and the minimum conductivity of graphene are in good agreement with conventional transport measurements. In addition, we assess the possible damage caused by landing these probes on graphene.

Non-volatile reconfigurable spin logic functions in a two-channel Hall bar by spin–orbit torque-based magnetic domains and directional read current

A long-standing goal of CMOS-based logic devices is to meet the needs of key markets, including ultralow-power operation and high operation speed, along with the continuing miniaturization of the architecture. However, despite significant progress in their development, conventional CMOS-based devices still suffer from drawbacks such as introducing large unintended leakage currents and volatile behavior. Thus, reconfigurable logic gates based on magnetic domain (MD) have emerged as a highly promising option because they offer fast operation speeds, nonvolatility, and diverse logic functions in a single-device configuration. Here, we address multiple reconfigurable MD logic gates in a single two-channel Hall bar device by varying the voltage-driven read-current directions and selecting a non-inverting or inverting comparator in W/CoFeB/MgO/Ta stacks. The non-volatile MD switching behavior induced by spin–orbit torque significantly affects our logic gate functions, which are not necessarily synchronized to a single clock. By adapting MD switching by spin-orbit torque and anomalous Hall effect voltage outputs, we identified eight reconfigurable logic gates, including AND, NAND, NOR, OR, INH, Converse INH, Converse IMP, and IMP, in a single device. These experimental findings represent a significant step forward in a wide range of MD-based logic applications in the near future.

Angular dependence of antisymmetric magnetoresistance in Co-Tb thin films with artificial domain wall inclination

We report the observation of angular-dependent antisymmetric magnetoresistance (MR) in artificially deformed Co-Tb Hall bar structures with perpendicular magnetization. Simultaneous transport measurements and domain imaging show that the antisymmetric MR results from the generation of a single domain wall (DW) inclination due to the restricted geometry and is further proportional to the inclination-associated geometry factor. The results are well described by a theoretical model that is supported by analytic and numerical calculations of the nonequilibrium current and Hall voltage distribution in the vicinity of the inclined DW. This finding provides a straightforward and effective approach to control DW geometries, leading to various DW-based spintronic device applications.

Enhanced Hall mobility in graphene-on-electronic-grade diamond

The outstanding electronic properties of graphene make this material a candidate for many applications, for instance, ultra-fast transistors. However, self-heating and especially the detrimental influence of available supporting substrates have impeded progress in this field. In this study, we fabricate graphene-diamond heterostructures by transferring graphene to an ultra-pure single-crystalline diamond substrate. Hall-effect measurements were conducted at 80 to 300 K on graphene Hall bars to investigate the charge transport properties in these devices. Enhanced hole mobility of 2750 cm2 V−1 s−1 could be observed at room-temperature when using diamond with reduced nitrogen (Ns0) impurity concentration. In addition, by electrostatically varying the carrier concentration, an upper limit for mobility is determined in the devices. The results are promising for enabling carbon–carbon (C-C) devices for room-temperature applications.

An in-situ thermoelectric measurement apparatus inside a thermal-evaporator

At the ultra-thin limit below 20 nm, a film’s electrical conductivity, thermal conductivity, or thermoelectricity depends heavily on its thickness. In most studies, each sample is fabricated one at a time, potentially leading to considerable uncertainty in later characterizations. We design and build an in-situ apparatus to measure thermoelectricity during their deposition inside a thermal evaporator. A temperature difference of up to 2 K is generated by a current passing through an on-chip resistor patterned using photolithography. The Seebeck voltage is measured on a Hall bar structure of a film deposited through a shadow mask. The measurement system is calibrated carefully before loading into the thermal evaporator. This in-situ thermoelectricity measurement system has been thoroughly tested on various materials, including Bi, Te, and Bi2Te3, at high temperatures up to 500 K. Working reliably and precisely, the in-situ measurement system would help to study physics during film growth or speedup our search for better thermoelectric materials.

Patterning of Superconducting Two‐Dimensional Electron Gases based on AlOx/KTaO3 (111) Interfaces

AbstractThe versatility of properties displayed by two‐dimensional electron gases (2DEGs) at oxide interfaces has fostered intense research in hope of achieving exotic electromagnetic effects in confined systems. Of particular interest is the recently discovered superconducting state appearing in (111)‐oriented KTaO3 interfaces, with a critical temperature Tc≈2 K, almost 10 times higher than that of SrTiO3‐based 2DEGs. Just as in SrTiO3‐based 2DEGs, fabricating devices in this new system is a technical challenge due to the fragility of the 2DEG and the propensity of bulk KTaO3 to become conducting outside the devices upon adventitious oxygen vacancy doping. Here, three different techniques are presented for patterning Hall bars in AlOx/KTaO3 (111) heterostructures. The devices show superconducting transitions ranging from 1.3 to 1.78 K, with limited degradation from the unpatterned thin film, and enable an efficient tuning of the carrier density by electric field effect. The array of techniques allows for the definition of channels with a large range of dimensions for the design of various kinds of devices to explore the properties of this system down to the nanoscale.

Spin Hall rectification effect in heavy metal/ferromagnetic metal bilayers

The rectification effect is a fundamental physical property of heterojunctions, which has wide applications in microelectronic and spintronic devices. Here we report a spin Hall rectification effect (SHRE) in heavy-metal/ferromagnetic metal (FM) bilayers under current-in-plane geometry, in which a pure sinusoidal alternating current along the current channel of Hall bar can generate a longitudinal rectification direct-current voltage. The rectification mechanism stems from spin Hall effect of heavy metals as well as spin-dependent asymmetric scattering associated with current polarity. Experimental results show that SHRE couples with magnetic information, and thus is non-volatile and controllable depending on the magnetization direction of FM layer, which opens up a new avenue for spintronics diode engineering based on SHRE.