Micrometer-Scale Ballistic Transport in Encapsulated Graphene at Room Temperature

How Magical Is Magic-Angle Graphene?

Topologically nontrivial electronic states can lead to novel anomalous Hall effects, with room temperature manifestations promising for applications in magnetic sensing, spintronics, and energy harvesting. The anomalous in-plane Hall effect is expected in topological magnetic materials under an in-plane magnetic field, but its detection has been challenging because of strict symmetry requirements. Here, we combine molecular beam epitaxy of the kagome metal Fe3Sn, electric Hall effect measurements, and theoretical calculations to propose and demonstrate that the kagome lattice motif combined with spin-orbit coupling and canted ferromagnetism induces the anomalous in-plane Hall effect at room temperature via topological Weyl points. Additionally, we synthesize a topological thin-film heterostructure with Fe3Sn and ferromagnetic CoFeB, showing enhanced anomalous in-plane Hall effect amplitude due to CoFeB's magnetic stray field. This work establishes a design framework for topological magnets and heterostructures aimed at discovering and controlling anomalous Hall effects for technological applications.

Transistor Z. C. Yang, D. N. Nath, and S. Rajan Department of Electrical and Computer Engineering, The Ohio State University Columbus, OH, 43210 Phone: (614)-717-8245 | E-mail: yangzh@ece.osu.edu In this work, we use ballistic quantum transport in a III-nitride to realize room temperature negative differential resistance (NDR) in a GaN-based Tunneling Hot Electron Transistor. The results showed reproducible double-sweep characteristics, with peak-to-valley ratio (PVCR) of 7.2 and peak current density (PCD) about 143 A/cm. This is the first report of repeatable room temperature negative differential resistance in a III-nitride device. GaN-based materials have attracted considerable interests because of several advantages such as larger band gap, larger conduction band offset, better thermal stability, and higher breakdown field. In the reported GaN-based RTDs, reproducibility has been poor and the reverse-sweep NDR is usually not observed. The results are highly influenced by dislocation density, interface roughness and particularly polarization effect. In this work, we show that repeatable NDR can be measured at room temperature in a GaN-based tunneling hot electron transistor[1]. In a tunneling hot electron transistor, electrons are injected from the emitter across the base. The collector barrier forms a high pass filter for the injected hot electrons. When the injected electrons have lower energy than the collector barrier, the flow into the base. When the injected energy exceeds the collector barrier, they flow ballistically into the collector, inducing NDR in the base current. We have done detailed Monte Carlo simulations of transport in this device, and find that ballistic transport across thin base layers (< 7 nm) can enable coherent quantum phenomena to be observed. The investigated epitaxial structure consists of a 3.8 nm Al0.6Ga0.4N emitter tunneling barrier, a 22 nm n+ GaN base transit layer (Si doping on the order of 10 cm), and a collector barrier of 5.3/7.9/9 nm AlxGa1-xN (graded from 30% to 15%)/Al0.3Ga0.7N/AlxGa1-xN (graded from 50% to 32%), as confirmed by XRD ω-2θ scan. The structure was grown in Ga-rich conditions by plasma-assisted MBE on free-standing GaN substrates (St. Gobain, n-type doped, TDD ~5x10 cm). Devices have an emitter contact of Ti/Au/Ni (20/50/30 nm), and two base contacts of Al/Ni/Au/Ni (20/20/50/30 nm). The active emitter area is about 12 μm, and mesa area around 100 μm. The investigated structure shows low base-collector leakage, moderate emitter-base injection current, ohmic base-base characteristic. High composition tunneling barrier is applied to narrow the electron energy distribution and reduce percolative transport in low composition barrier, which is important for the observation of NDR. The devices were measured by sweeping emitter bias with both base and collector grounded. At baseemitter bias from 1.5 to 2.8 V, the I-V characteristics shows PVCR of 7.2, and PCD of 143 A/cm at 1.5 V. The repeatability is demonstrated by multiple forward and reverse sweeps, with different voltage steps. The voltage at which NDR emerges is consistent among different devices, and is close to the collector barrier height, which is also expected from the energy distribution. The good repeatability and double-sweep characteristics indicate the potential of such device in high frequency applications We acknowledge funding from ONR N00014-11-1-0721 DATE MURI (Program manager: Dr. Paul Maki).

The anomalous Hall effect (AHE) has emerged as a powerful probe of the interplay between magnetism, electronic band topology, and spin-dependent quantum transport in two-dimensional (2D) magnetic materials. While extensive efforts have been devoted to the AHE in 2D magnets, most studies have been confined to ultra-low temperatures or regimes far below room temperature, limiting both fundamental understanding and practical applicability. The recent discovery of several high-temperature van der Waals (vdW) ferromagnets has opened up a promising platform for investigating the underlying mechanisms and device opportunities of the AHE at and above room temperature. In this review, we provide a comprehensive overview of recent advancements of the AHE in room-temperature 2D vdW ferromagnets. We first summarize the macroscopic manifestations of the AHE in representative vdW ferromagnets operating at room temperature. Then, we provide an overview of typical vdW ferromagnetic systems exhibiting room-temperature AHE, with a particular focus on thickness dependence, Curie temperature, magnetic anisotropy, and materials engineering strategies such as chemical doping, strain engineering, and interface design, together with state-of-the-art synthesis techniques. Subsequently, we discuss and compare the key microscopic mechanisms underlying the AHE in these materials, including intrinsic Berry-curvature-driven contributions and extrinsic contributions. Based on this understanding, recent progress in electrically tuning the AHE via external electric fields and applied currents is systematically reviewed, highlighting their impacts on anomalous Hall resistance, coercivity, and critical temperature. We further summarize advances in different types of electrically controlled vdW magnetic prototype devices, including current-tunable AHE devices and magnetic tunnel junctions. Finally, we outline the significant progress achieved so far, identify the key remaining challenges, and propose promising directions for future research toward robust and functional room-temperature AHE.

Mn3Sn is an AHE antiferromagnet exhibiting a hysteretic anomalous Hall effect (AHE) originating from the cluster octupole moments of inverse triangular antiferromagnetic spins at room temperature, whereas the AHE disappears at low temperatures. In this study, we evaluated the systematic doping effect on the AHE of Mn3Sn to stabilize the AHE at low temperatures. We grew single crystals of M0.1Mn2.9Sn and Mn3Sn0.9X0.1 (M = Cr and Fe; X = Si and Ge) and evaluated their transport and magnetic properties. Compared with doping at the Sn site, doping at the Mn site drastically changes the AHE at low temperatures. Reproducible magnetic structures were not observed at low temperatures owing to low Fe content fluctuation near x ∼ 0.1 in FexMn3–xSn. Some crystals showed a large hysteretic-type AHE, unlike the AHE at 300 K, while others did not show a finite AHE below 50 K. Conversely, Cr doping stabilizes the AHE with additional magnetoresistance that hinders observing the AHE. Considering these features, we prepared Cr and Fe codoped Mn3Sn crystals. The Fe0.05/Cr0.1-doped Mn3Sn crystals showed the AHE from room temperature to 2 K without magnetic transitions. This finding provides a new route to access octupole-driven phenomena of Mn3Sn-based devices at low temperatures.

Topological semimetals have been a focal point of research in recent years. One of the salient characters of topological semimetals is the anomalous Hall effect arising from the nontrivial momentum-space Berry phase. In contrast, the anomalous transverse thermoelectric coefficient and thermal Hall effect have been much less explored to date. In this paper, we report thermoelectric and thermal transport properties of ${\mathrm{Fe}}_{3}{\mathrm{Sn}}_{2}$, a prototypical Dirac semimetal. We find that ${\mathrm{Fe}}_{3}{\mathrm{Sn}}_{2}$ exhibits a large topological Nernst effect which is associated with the nonzero spin chirality of the skyrmion bubble phase. Furthermore, ${\mathrm{Fe}}_{3}{\mathrm{Sn}}_{2}$ shows a prominent anomalous thermal Hall effect and the anomalous Nernst effect with features similar to the anomalous Hall effect. The anomalous Nernst signal is about $2.1\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{V}/\mathrm{K}$ at room temperature, which is comparable to the largest value reported thus far. These results highlight the synergic effects of the Berry phase in both real space and momentum space of ${\mathrm{Fe}}_{3}{\mathrm{Sn}}_{2}$, and this study demonstrates an effective strategy of investigating topological materials by measuring their transverse thermal and thermoelectric responses.

Recently, Heusler ferromagnets have been found to exhibit unconventional anomalous electric, thermal, and thermoelectric transport properties. In this study, we employed first-principles density functional theory calculations to systematically investigate both intrinsic and extrinsic contributions to the anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and anomalous thermal Hall effect (ATHE) in two Heusler ferromagnets: ${\mathrm{Fe}}_{2}\mathrm{CoAl}$ and ${\mathrm{Fe}}_{2}\mathrm{NiAl}$. Our analysis reveals that the extrinsic mechanism originating from disorder dominates the AHE and ATHE in ${\mathrm{Fe}}_{2}\mathrm{CoAl}$, primarily due to the steep band dispersions across the Fermi energy and corresponding high longitudinal electronic conductivity. Conversely, the intrinsic Berry phase mechanism, physically linked to nearly flat bands around the Fermi energy and gapped by spin-orbit interaction band crossings, governs the AHE and ATHE in ${\mathrm{Fe}}_{2}\mathrm{NiAl}$. With respect to ANE, both intrinsic and extrinsic mechanisms are competing in ${\mathrm{Fe}}_{2}\mathrm{CoAl}$ as well as in ${\mathrm{Fe}}_{2}\mathrm{NiAl}$. Furthermore, ${\mathrm{Fe}}_{2}\mathrm{CoAl}$ and ${\mathrm{Fe}}_{2}\mathrm{NiAl}$ exhibit tunable and remarkably pronounced anomalous transport properties. For instance, the anomalous Nernst and anomalous thermal Hall conductivities in ${\mathrm{Fe}}_{2}\mathrm{NiAl}$ attain giant values of 8.29 A/Km and 1.19 W/Km, respectively, at room temperature. To provide a useful comparison, we also thoroughly investigated the anomalous transport properties of ${\mathrm{Co}}_{2}\mathrm{MnGa}$. Our findings suggest that Heusler ferromagnets ${\mathrm{Fe}}_{2}\mathrm{CoAl}$ and ${\mathrm{Fe}}_{2}\mathrm{NiAl}$ are promising candidates for spintronics and spin caloritronics applications.

A recent experiment reported a large anomalous Hall effect in Magic Angle Twisted Bilayer Graphene (TBG) aligned with a hexagonal boron nitride(h-BN) substrate at $\frac{3}{4}$ filling of the conduction band. In this paper we study this system theoretically, and propose explanations of this observation. We emphasize that the physics for this new system is qualitatively different from the pure TBG system. The aligned h-BN breaks in-plane two-fold rotation symmetry and gaps out the Dirac crossings of ordinary TBG. The resulting valence and conduction bands of each valley carry equal and opposite Chern numbers $C=\pm 1$. A useful framework is provided by a lattice extended Hubbard model for this system which we derive. An obvious possible explanation of the anomalous Hall effect is that at $3/4$-filling the system is a spin-valley polarized ferromagnetic insulator where the electrons completely fill a Chern band. We also examine an alternate more radical proposal of a compressible valley polarized but spin unpolarized composite ferm liquid metallic state. We argue that either state is compatible with current experiments, and propose ways to distinguish between them in the future. We also briefly discuss the physics at $1/2$ filling.

Hexagonal boron nitride (BN), one of the very few layered insulators, plays a crucial role in 2D materials research. In particular, BN grown with a high pressure technique has proven to be an excellent substrate material for graphene and related 2D materials, but at the same time very hard to replace. Here we report on a method of growth at atmospheric pressure as a true alternative for producing BN for high quality graphene/BN heterostructures. The process is not only more scalable, but also allows to grow isotopically purified BN crystals. We employ Raman spectroscopy, cathodoluminescence, and electronic transport measurements to show the high-quality of such monoisotopic BN and its potential for graphene-based heterostructures. The excellent electronic performance of our heterostructures is demonstrated by well developed fractional quantum Hall states, ballistic transport over distances around 10 µm at low temperatures and electron-phonon scattering limited transport at room temperature.

We report on the observation of inertial-ballistic and mode-controlled rectification in mesoscopic GaAs/AlGaAs cross junctions. The cross junctions are composed of two current injecting branches which oppositely merge under an injection angle φ (90° ≥ φ ≥ 30°) into a straight central voltage stem. Ballistic electron transport is indicated by negative bend resistance which develops for φ = 90° in bend resistance configuration. Rectification is observed at both ends of the voltage stem. The mode-controlled signal at the upper end of the stem is found to be nearly independent from φ. The inertial-ballistic signal obtained from the potential difference between both ends of the stem vanishes for φ = 90° and increases with decreasing φ. Rectification is studied for different top-gate voltages and temperatures.

In this letter, we study the origin of the anomalous Hall effect (AHE) in ferrimagnetic insulator ${\mathrm{Tm}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (TmIG)/Pt heterostructures. A monotonic decrease of the anomalous Hall resistivity ($\mathrm{\ensuremath{\Delta}}{\ensuremath{\rho}}_{\mathrm{AHE}}$) with decreasing temperature is observed for TmIG/Pt, and a sign reversal of $\mathrm{\ensuremath{\Delta}}{\ensuremath{\rho}}_{\mathrm{AHE}}$ occurs at around 80 K. With the addition of a Cu interlayer, the $\mathrm{\ensuremath{\Delta}}{\ensuremath{\rho}}_{\mathrm{AHE}}$ similarly decreases as a function of temperature, but maintains the same sign across the full temperature range. This indicates that both the magnetic-proximity effect and spin Hall effect in the TmIG/Pt bilayer contribute to the AHE signal with opposing signs. The spin-Hall contribution to the AHE is dominant at room temperature but decreases with decreasing temperature. Meanwhile, the magnetic-proximity contribution to the AHE becomes dominant with decreasing temperatures, leading to a change of sign for $\mathrm{\ensuremath{\Delta}}{\ensuremath{\rho}}_{\mathrm{AHE}}$. We exclude a dominant influence of a ferrimagnetic compensation point in the temperature region by complementary magnetic hysteresis and neutron diffraction measurements. Our work, based on a simple method, sheds light on the origin of the AHE in magnetic insulator heterostructures, where the competition between the magnetic-proximity effect and spin Hall effect governs the sign and amplitude of the AHE.

The past decade has seen a new paradigm in solid state physics, where a new class of layered crystals can be thinned down to a monolayer and exhibit drastic changes in their electronic and optical properties in comparison to their bulk counterpart. Graphene was the first, and certainly most outstanding, of this set of so called two-dimensional (2D) materials. Aside from its obvious appeal which earnt its discovery the 2010 Nobel Prize, the electronic properties of graphene are truly unique. Perhaps the most familiar is its linear electron dispersion which hosts quasi-particles that obey the Dirac equation. This has enabled the study of a plethora of transport phenomena, as well as the realisation of novel device architectures that will be used in the next generation electronics. In general, experimental signatures of electron transport are most prominent at liquid helium temperatures when lattice vibrations are weak, for example in quantum hall physics. In this Thesis, we explore the regime of intermediate temperatures where the physics of interest is strongest between 100 and 300 K. Equipped with the state of the art high quality graphene samples, we demonstrate novel electron transport unique to graphene. The experimental work consists of two themes. In the first work, we study hydrodynamic electron flow in graphene encapsulated with hexagonal boron nitride devices. At elevated temperatures, electron-electron collisions become significant, and the electron viscosity starts to influence the steady state current distribution in a variety of surprising ways. In the first work, we perform transport experiments on standard graphene hall bars in a unique measurement geometry which allows the detection of negative non-local voltages intrinsic to viscous flow. In another experiment, we study viscous electron flow through graphene nano-constrictions/classical point contacts. Here, we observed anomalous temperature dependence in the conductance measured across the constriction. Specifically, the conductance increases with increasing temperature and even exceeded the semi-classical limit which is expected for single-particle ballistic transport. The underlying mechanism originates from electron-electron collisions, which, counter-intuitively, act to enhance current flow. In the second work, we slightly change our experimental system by studying magneto transport in a graphene/hexagonal boron nitride superlattice. Owed to the large periodicity of the superlattice unit cell, these devices have allowed experimental observation of the long sought Hofstadter butterfly, which addresses the electronic dispersion of electrons in a periodic potential and magnetic field. Here, we again go to elevated temperatures, where all the spectral gaps related to Hofstadter butterflies are completely smeared, and instead find a new type of quantum oscillation. These new oscillations are periodic in 1/B with a frequency corresponding to one flux quantum piercing the superlattice unit cell. Whilst these oscillations are related to Hofstadter physics, they are in fact more primal in origin. The most fascinating feature is their robustness with respect to increasing temperature. The oscillations are easily observable at room temperature in fields as low as 3 T and still remained prominent at 373 K, the boiling point of water

Stabilizing Solid Electrolyte-Anode Interface in Li-Metal Batteries by Boron Nitride-Based Nanocomposite Coating

Piezomagnetism couples strain linearly to magnetic order, implying that it can produce and control magnetization. However, unlike magnetostriction, which couples magnetization quadratically to strain, it enables bidirectional control of a net magnetic moment. If this effect becomes large at room temperature, it may be technologically relevant, similar to its electric analogue, piezoelectricity. However, current studies of the piezomagnetic effect have been primarily restricted to antiferromagnetic insulators at cryogenic temperatures. Here we report the observation of large piezomagnetism in the antiferromagnetic Weyl semimetal Mn3Sn at room temperature. This material is known for its nearly magnetization-free anomalous Hall effect. We find that a small uniaxial strain on the order of 0.1% can control both the sign and size of the anomalous Hall effect. Our experiment and theory show that the piezomagnetism can control the anomalous Hall effect, which will be useful for spintronics applications. Control of magnetization is important for applications in spintronics. Now, the piezomagnetic effect allows strain to control the anomalous Hall effect in a metal at room temperature by rotating its antiferromagnetic order.

Evidence of ballistic thermal transport in lithium niobate at room temperature