Electronic Transport Measurements Research Articles

KMg4Bi3: A Narrow Band Gap Semiconductor with a Channel Structure.

The creation of new families of intermetallic or Zintl-phase compounds with high-spin orbit elements has attracted a considerable amount of interest due to the presence of unique electronic, magnetic, and topological phenomena in these materials. Here, we establish the synthesis and structural and electronic characterization of KMg4Bi3 single crystals having a new structure type. KMg4Bi3 crystallizes in space group Cmcm having unit cell parameters a = 4.7654(11) Å, b = 15.694(4) Å, and c = 13.4200(30) Å and features an edge-sharing MgBi4 tetrahedral framework that forms cage-like one-dimensional channels around K+ ions. Diffuse reflectance absorption measurements indicate that this material has a narrow band gap of 0.27 eV, which is in close agreement with the electronic structure calculations that predict it to be a trivial insulator. Electronic transport measurements from 80 to 380 K indicate this material behaves like a narrow band gap semiconductor doped to ∼1018 holes/cm-3, with thermopowers of ∼100 μV/K and appreciable magnetoresistance. Electronic structure calculations indicate this material is close to a topological phase transition and becomes a topological insulator when the lattice is uniformly expanded by 3.5%. Overall, this unique structure type expands the landscape of potential quantum materials.

Graphene conductance modulation through controlled molecular release in a hybrid coordination polymer/graphene field-effect transistor

Coordination polymers, including metal-organic frameworks (MOFs), present a wide range of functionalities ranging from sensing to energy storage. Despite this, many coordination polymers are typically insulating which prevents their implementation into nanoscale electronic devices. Besides, the hybridization of MOFs with other conducting materials, like graphene, is rather unexplored. Here we introduce the grafting of a soft non-porous coordination polymer, the 1·2CH3CN, on a single-layer graphene to form a hybrid 1·2CH3CN/graphene field effect transistor. Electron transport measurements show that the sharp structural transformations in 1·2CH3CN, triggered at two specific temperatures, are detected in graphene as two sharp increments in the electrical current. Gate spectroscopy measurements show that the origin of the electrical modulation is an abrupt p-doping due to the sudden release of acetonitrile right at the structural transition temperatures. Reciprocally, the sensitivity of graphene to external perturbations allows to detect additional transformations in the coordination polymer not observed in direct transport measurements across crystals. Finally, the results have been reproduced in in-situ grown 1·2CH3CN nanoscale films on CVDG FETs. This simple method optimizes the contact area between graphene and 1·2CH3CN and facilitates the integration of coordination polymers with van der Waals materials and electronic devices. Further, a selective doping at specific temperatures could be obtained by modifying the nature of the interstitial molecule in 1.

Higher order gaps in the renormalized band structure of doubly aligned hBN/bilayer graphene moiré superlattice

This paper presents our findings on the recursive band gap engineering of chiral fermions in bilayer graphene doubly aligned with hBN. Using two interfering moiré potentials, we generate a supermoiré pattern that renormalizes the electronic bands of the pristine bilayer graphene, resulting in higher order fractal gaps even at very low energies. These Bragg gaps can be mapped using a unique linear combination of periodic areas within the system. To validate our findings, we use electronic transport measurements to identify the position of these gaps as a function of the carrier density. We establish their agreement with the predicted carrier densities and corresponding quantum numbers obtained using the continuum model. Our study provides strong evidence of the quantization of the momentum-space area of quasi-Brillouin zones in a minimally incommensurate lattice. It fills important gaps in the understanding of band structure engineering of Dirac fermions with a doubly periodic superlattice spinor potential.

Diffusive and ballistic transport in thin InSb nanowire devices using a few-layer-graphene-AlO x gate

Quantum devices based on InSb nanowires (NWs) are a prime candidate system for realizing and exploring topologically-protected quantum states and for electrically-controlled spin-based qubits. The influence of disorder on achieving reliable quantum transport regimes has been studied theoretically, highlighting the importance of optimizing both growth and nanofabrication. In this work, we consider both aspects. We developed InSb NW with thin diameters, as well as a novel gating approach, involving few-layer graphene and atomic layer deposition-grown AlO x . Low-temperature electronic transport measurements of these devices reveal conductance plateaus and Fabry–Pérot interference, evidencing phase-coherent transport in the regime of few quantum modes. The approaches developed in this work could help mitigate the role of material and fabrication-induced disorder in semiconductor-based quantum devices.

Interfacial modulation of magnetic relaxation and electrical characteristic in RuO2/CrO2 antiferromagnet-half metal bilayer

Half-metallic chromium dioxide (CrO2) is a promising candidate in magnetic random-access memory (MRAM) devices due to its nearly full spin polarization and ultralow Gilbert damping at room temperature. In this study, we succeeded in fabricating (110) and (100)-oriented RuO2/CrO2 bilayers with different thicknesses of CrO2 film using two-step method. The CrO2 film epitaxially grown on a RuO2 layer exhibits a perfect in-plane magnetic uniaxial anisotropy. The magnetic relaxation of CrO2 films can be strongly modulated via the proximity to RuO2 layer, which may relate to the enhancement of the two-magnon scattering due to effects of antiferromagnetic order in RuO2 film. In addition, electronic transport measurements indicate that the electronic characteristic is strongly related to the film growth orientation, and a linear magnetoresistance is observed stemming from intergrain tunneling magnetoresistance and double-exchange mechanism. Angle-dependent ferromagnetic resonance measurements indicates that the uniaxial anisotropy in CrO2 film is strongly related to the strain effect, which is consistent with reciprocal space mapping results. This work lays a foundation for current-drive MRAM based on RuO2/CrO2 bilayer and provides a feasible damping modulation method for CrO2 film.

Experimental observation of two-dimensional phase in compressed FeF2

Iron difluoride (FeF2) has attracted considerable attention for its physical characteristics and practical applications, and its compression behaviors usually play a key role in the in-depth understanding of this compound. Since its high-pressure crystal structure evolution determining a more profound comprehension remains disputable, we carried out extensive experiments to focus on the pressure-induced structural phase transitions of FeF2. Through in situ high-pressure synchrotron x-ray diffraction measurements, we not only confirmed a reported high-pressure orthorhombic Pbca phase at 11 GPa but also identified an interesting two-dimensional structure with hexagonal close packed symmetry (P-3m1) that appears above 25 GPa at room temperature. Furthermore, the spontaneous strain fitting and electronic transport measurements suggest that its ambient rutile-type structure (P42/mnm) evolves into an orthorhombic structure (Pnnm) through a second-order phase transition at 5 GPa. These experimental results elaborate on the pressure-induced phase transitions of FeF2 on the order of P42/mnm → Pnnm → Pbca → P-3m1, shedding light on a rare three-dimensional to two-dimensional configuration transition in difluorides.

Tip Growth of Quasi-Metallic Bilayer Graphene Nanoribbons with Armchair Chirality.

Graphene nanoribbons (GNRs), quasi one-dimensional (1D) narrow strips of graphene, have shown promise for high-performance nanoelectronics due to their exceptionally high carrier mobility and structurally tunable bandgaps. However, producing chirality-uniform GNRs on insulating substrates remains a big challenge. Here, we report the successful growth of bilayer GNRs with predominantly armchair chirality and ultranarrow widths (<5 nm) on insulating hexagonal boron nitride (h-BN) substrates using chemical vapor deposition (CVD). The growth of GNRs is catalyzed by transition metal nanoparticles, including Fe, Co, and Ni, through a unique tip-growth mechanism. Notably, GNRs catalyzed by Ni exhibit a high purity (97.3%) of armchair chirality. Electron transport measurements indicate that the ultrathin bilayer armchair GNRs exhibit quasi-metallic behavior. This quasi-metallicity is further supported by density functional theory (DFT) calculations, which reveal a significantly reduced bandgap in bilayer armchair GNRs. The chirality-specific GNRs reported here offer promising advancements for the application of graphene in nanoelectronics.

The NAD + Metabolic Dependency Restriction Overcomes Drug-Resistance Phenotype of Multiple Myeloma CELLS

Introduction: We previously reported that Nicotinamide Adenine Dinucleotide (NAD +), a master regulator of catabolic metabolism and several enzymatic activities, plays a crucial role for Multiple Myeloma (MM) cells growth; indeed, small molecules targeting the rate-limiting NAD + producer enzyme NAMPT, have potent anti-MM activity. However, targeting dysregulated metabolic landscape of tumors has proven to be a major challenge as suggested by existence of alternative NAD + production routes which confer resistance to NAMPT inhibitors. As result here we focused on whole NAD + metabolic dependency of MM cells by investigating relevance of a comprehensive NAD + metabolic restriction as strategy to enhance activity of currently used anti-MM strategies. Methods: whole transcriptomic analysis of the NADome signature was performed on MM patients by probing MMRF CoMMpass study; its correlation with clinical data (OS and PFS) was also investigated. Next, GSEA algorithm on biological modules was run on MM patients carrying low- vs high-NADome signature. Chemical and genome editing-approaches using CRISPR-Cas9-based strategy were both used to investigate relevance of our findings. Then, an anti-MM drugs screen was used to challenge fully-NAD depleted MM cells using different viability tools. Mechanistic studies were also performed using qPCR, western blot, immunofluorescence analyses, targeted enzymatic assays and in-vivo xenograft MM models. Finally, clinical benefits of our findings were accomplished by challenging publicly available dataset on ASCT-treated MM patients. Results: RNAseq analysis on CoMMPass dataset sharply divided MM patients according to their comprehensive NADome signaling landscape: high-NAD producers showed poorer outcome than those expressing low mRNA levels, both in terms of PFS and OS as well. In line with these data, a panel of MM cell lines showed that pervasive NAPRT levels reduced NAMPT inhibitors activity thus supporting a complete NAD +-depletion for more effective strategies also in in-vivo xenograft models. PROGENy analysis revealed an increased NF-KB activity in NAPRT-depleted cells which in turn resulted in higher oxygen reactive species generation from these cells. Indeed, NAPRT KO cells showed oxidative damage accumulation, with increased malondialdehyde (MDA) activity and lower endogenous antioxidant defenses as suggested by glutathione reductase, glutathione peroxidase and ratio of reduced GSH to oxidized glutathione (GSSG) ratio measurements. Next, mitochondrial-energetic status was also tested by measuring electron transport chain (ETC) Complex I and Complex II activities: a shift towards oxidative metabolism was observed. Overall these data phenocopy the energetic status observed in drug-resistant cells; as result NAPRT-KO makes MM cells resistant to Melphalan. In such a context, the use of NAMPT-inhibitors was able to overcome drug-resistance by re-sensitizing cells to anti-MM activity of DNA damaging agents. Finally, a focused analysis on ASCT-treated MM patients, revealed poor outcome for “high-NAD + producers” thus supporting clinical relevance for NAD + metabolic restriction in these patients. Conclusions. Our data provide evidence that NAD + biosynthesis shapes metabolic phenotypes of MM cells, with its impairment resulting in metabolic reprogramming and cells viability injury. As result, a comprehensive NAD + depletion with dual NAPRT and NAMPT inhibition primes MM cells to alkylating agents which still represent the backbone strategy for ASCT-eligible patients.

Modulating the Electrochemical Intercalation of Graphene Interfaces with α-RuCl3 as a Solid-State Electron Acceptor.

Intercalation reactions modify the charge density in van der Waals (vdW) materials through coupled electronic-ionic charge accumulation and are susceptible to modulation by interlayer hybridization in vdW heterostructures. Here, we demonstrate that charge transfer between graphene and α-RuCl3, which hole-dopes the graphene, greatly favors the intercalation of lithium ions into graphene-based vdW heterostructures. We systematically tune this effect on Li+ ion intercalation, modulating the intercalation potential, by using varying thicknesses of hexagonal boron nitride (hBN) as spacer layers between graphene and α-RuCl3. Confocal Raman spectroscopy and electronic transport measurements are used to monitor electrochemical intercalation, and density functional theory computations help quantify charge transfer to both α-RuCl3 and graphene upon Li intercalation. This work demonstrates a versatile approach for systematically modulating the electrochemical intercalation behavior of two-dimensional layers akin to electron donating/withdrawing substituent effects used to tune molecular redox potentials.

Correlated insulator and Chern insulators in pentalayer rhombohedral-stacked graphene.

Rhombohedral-stacked multilayer graphene hosts a pair of flat bands touching at zero energy, which should give rise to correlated electron phenomena that can be tuned further by an electric field. Moreover, when electron correlation breaks the isospin symmetry, the valley-dependent Berry phase at zero energy may give rise to topologically non-trivial states. Here we measure electron transport through hexagonal boron nitride-encapsulated pentalayer graphene down to 100 mK. We observed a correlated insulating state with resistance at the megaohm level or greater at charge density n = 0 and displacement field D = 0. Tight-binding calculations predict a metallic ground state under these conditions. By increasing D, we observed a Chern insulator state with C = -5 and two other states with C = -3 at a magnetic field of around 1 T. At high D and n, we observed isospin-polarized quarter- and half-metals. Hence, rhombohedral pentalayer graphene exhibits two different types of Fermi-surface instability, one driven by a pair of flat bands touching at zero energy, and one induced by the Stoner mechanism in a single flat band. Our results establish rhombohedral multilayer graphene as a suitable system for exploring intertwined electron correlation and topology phenomena in natural graphitic materials without the need for moiré superlattice engineering.

Detection of Nontrivial Topology Driven by Charge Density Wave in a Semi‐Dirac Metal

AbstractThe presence of electron correlations in a system with topological order can lead to exotic ground states. Considering single crystals of LaAgSb2 which has a square net crystal structure, one finds multiple charge density wave transitions (CDW) as the temperature is lowered. Large planar Hall (PHE) signals are found in the CDW phase, which are still finite in the high‐temperature phase though they change sign. Optimizing the structure within first‐principles calculations, one finds an unusual chiral metallic phase. This is because as the temperature is lowered, the separation between the Ag/Sb atoms on different layers decreases, leading to stronger repulsions between electrons associated with atoms on different layers. This leads to successive layers sliding with respect to each other, thereby stabilizing a chiral structure in which inversion symmetry is also broken. The large Berry curvature associated with the low‐temperature structure explains the low‐temperature PHE. At high temperature, the PHE arises from the changes induced in the anisotropic Dirac cone in presence of a magnetic field. This work represents a route toward detecting and understanding the mechanism in a correlation‐driven topological transition through electron transport measurements, complemented by ab‐initio electronic structure calculations.

Annealing effects on the magnetic and magnetotransport properties of iron oxide nanoparticles self-assemblies

In magnetic tunnel junctions based on iron oxide nanoparticles the disorder and the oxidation state of the surface spin as well as the nanoparticles functionalization play a crucial role in the magnetotransport properties. In this work, we report a systematic study of the effects of vacuum annealing on the structural, magnetic and transport properties of self-assembled ∼10 nm Fe3O4 nanoparticles. The high temperature treatment (from 573 to 873 K) decomposes the organic coating into amorphous carbon, reducing the electrical resistivity of the assemblies by 4 orders of magnitude. At the same time, the 3.Fe2+/(Fe3++Fe2+) ratio is reduced from 1.11 to 0.13 when the annealing temperature of the sample increases from 573 to 873 K, indicating an important surface oxidation. Although the 2 nm physical gap remains unchanged with the thermal treatment, a monotonous decrease of tunnel barrier width was obtained from the electron transport measurements when the annealing temperature increases, indicating an increment in the number of defects and hot-spots in the gap between the nanoparticles. This is reflected in the reduction of the spin dependent tunneling, which reduces the interparticle magnetoresistance. This work shows new insights about influence of the nanoparticle interfacial composition, as well their the spatial arrangement, on the tunnel transport of self-assemblies, and evidence the importance of optimizing the nanostructure fabrication for increasing the tunneling current without degrading the spin polarized current.

Electron transport measurements in liquid xenon with Xenoscope, a large-scale DARWIN demonstrator

The DARWIN observatory is a proposed next-generation experiment with 40 tonnes of liquid xenon as an active target in a time projection chamber. To study challenges related to the construction and operation of a multi-tonne scale detector, we have designed and constructed a vertical, full-scale demonstrator for the DARWIN experiment at the University of Zurich. Here, we present the first results from a several-months run with 343,hbox {kg} of xenon and electron drift lifetime and transport measurements with a 53,hbox {cm} tall purity monitor immersed in the cryogenic liquid. After 88,hbox {days} of continuous purification, the electron lifetime reached a value of (664,pm ,23),upmu hbox {s}. We measured the drift velocity of electrons for electric fields in the range (25–75) V/cm, and found values consistent with previous measurements. We also calculated the longitudinal diffusion constant of the electron cloud in the same field range, and compared with previous data, as well as with predictions from an empirical model.

Sn/InAs Josephson Junctions on Selective Area Grown Nanowires with in Situ Shadowed Superconductor Evaporation.

Superconductor-semiconductor nanowire hybrid structures are useful in fabricating devices for quantum information processing. While selective area growth (SAG) offers the flexibility to grow semiconductor nanowires in arbitrary geometries, in situ evaporation of superconductors ensures pristine superconductor-semiconductor interfaces, resulting in strong induced superconductivity in the semiconducting nanowire. In this work, we used high-aspect-ratio SiOx dielectric walls to in situ evaporate islands of superconductor tin on in-plane InAs SAG nanowires. Our technique enables customization in the designs of such hybrid nanostructures, while simultaneously performing the nanowire and superconductor growth without breaking vacuum. Using this technique, we grew super(S)-normal(N)-super(S), NS, and SNSNS junctions. We performed cryogenic electron transport measurements revealing the presence of gate and field tunable supercurrents. We further measured the superconducting gap and critical fields in the hybrid nanostructures and the crossover from 2e to 1e periodicity in the SNSNS junctions as a proof of the usability of these hybrid nanostructures.

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.

Tuning and interpretation of electronic transport properties with

Electronic transport coefficients such as the electrical conductivity, the termo-power, and the charge carrier concentration are routinely measured in a variety of application areas such as electronics or thermoelectric power-generation and cooling. Using fitting procedures, those measurements are used to infer microscopic features of the samples. The code ▪ facilitates the estimation of electronic structure parameters such as the effective masses and band energies improving the current approach by increasing the complexity of the band structure representation. The software is designed with a server-client architecture to enhance performance and scalability. The server is implemented in the Julia programming language. We illustrate the design and the efficiency of the code with selected applications, which are relevant to the optimization of thermoelectric materials. Program summaryProgram Title:▪CPC Library link to program files:https://doi.org/10.17632/494nj6ftbv.1Developer's repository link:https://github.com/marcofornari/etransport.gitLicensing provisions: GPLv3Programming language: Julia, Python.Nature of problem: Electronic transport measurements are routinely used to link the functional properties exploited in electronics and energy sciences applications with the underlying electronic structure of the material under investigation. Experimentalists collect data of derived and integrated physical quantities, from which they estimate properties and parameters, e.g., nature of the conduction mechanisms and effective mass. The interpretation is based on simplistic models that rarely represent the complexity of the band structure. The code ▪ improves the current state of the art and facilitates the estimation of effective masses and energies from experimental measurements of electrical conductivity, Seebeck coefficients, and carrier concentration using a multi-valley anisotropic parabolic band structure as a model for the unknown real band structure. Experimental data can be imported and visualized to facilitate the comparison with theoretical analysis.Solution method: The Boltzmann equation within the relaxation-time approximation is used to compute the electronic transport tensors from the electronic density of states characterized in terms of a set of effective masses tensors mi⁎ and critical energies ϵi. Numerical and analytical techniques are adopted to integrate the Boltzmann equation efficiently. By tuning the features of the density of states, temperature, Fermi level, and the functional expression for the relaxation time, the software streamlines the reconstruction of the underlying electronic properties from experimental data.Additional comments including restrictions and unusual features: The software is designed with a server-client architecture to enhance performance and scalability. The server is implemented in the Julia programming language as a stand-alone library. It performs all the numerical operations to compute the transport tensors from a set of multi-valley parabolic band structures. The client is implemented either as a command-line user interface (CLI) or a graphical user interface (GUI) written in the Python programming language. The communications between the server and clients are handled by a REST API, where the data are exchanged using JSON payloads.

Anomalous electronic transport in high-mobility Corbino rings

We report low-temperature electronic transport measurements performed in two multi-terminal Corbino samples formed in GaAs/Al-GaAs two-dimensional electron gases (2DEG) with both ultra-high electron mobility ( ≳ 20 × 106 cm2/ Vs) and with distinct electron density of 1.7 and 3.6 × 1011 cm−2. In both Corbino samples, a non-monotonic behavior is observed in the temperature dependence of the resistance below 1 K. Surprisingly, a sharp decrease in resistance is observed with increasing temperature in the sample with lower electron density, whereas an opposite behavior is observed in the sample with higher density. To investigate further, transport measurements were performed in large van der Pauw samples having identical heterostructures, and as expected they exhibit resistivity that is monotonic with temperature. Finally, we discuss the results in terms of various lengthscales leading to ballistic and hydrodynamic electronic transport, as well as a possible Gurzhi effect.

Schottky barrier diode consisting of van der Waals heterojunction of MoS2 film and PtCoO2 contact

We investigated the synthesis of delafossite PtCoO2 crystals, performed electronic transport measurements, and researched the formation of heterojunction with layered films. The crystal was deposited onto a c-plane sapphire substrate using the vapor growth method. The in-plane resistivity (460–475 µΩcm) is comparable to that of layered materials, such as few-layer graphite, applied to electrodes. Schottky contacts were obtained between PtCoO2 and n-type MoS2; therefore, a Schottky barrier diode with a low reverse saturation current was fabricated. The Schottky barrier height (ΦB) was estimated to be 0.896 eV. The atomic scale electric dipole at the interface may contributed to achieving a large ΦB. In addition, Fermi level pinning was suppressed owing to the formation of an interface by the dry transfer method.

Effect of Manganese Alloying on Infrared Detectors Made of Pb1-xMnxTe/CdTe Multilayer Composite.

The properties of Pb1-xMnxTe/CdTe multilayer composite grown by molecular beam epitaxy on a GaAs substrate were studied. The study included morphological characterization by X-ray diffraction, scanning electron microscopy, secondary ion mass spectroscopy, as well as electron transport and optical spectroscopy measurements. The main focus of the study was on the sensing properties of photoresistors made of Pb1-xMnxTe/CdTe in the infrared spectral region. It was shown that the presence of Mn in the Pb1-xMnxTe conductive layers shifted the cut-off wavelength toward blue and weakened the spectral sensitivity of the photoresistors. The first effect was due to an increase in the energy gap of Pb1-xMnxTe with an increase in Mn concentration, and the second was due to a pronounced deterioration in the crystal quality of the multilayers owing to the presence of Mn atoms, as shown by the morphological analysis.

Transport measurement of fractional charges in topological models

The static topological fractional charge (TFC) in condensed matter systems is related to the band topology and thus has potential applications in topological quantum computation. However, the experimental measurement of these TFCs in electronic systems is quite challenging. We propose an electronic transport measurement scheme in which both the charge amount and the spatial distribution of the TFC can be extracted from the differential conductance through a quantum dot coupled to the topological system being measured. For one-dimensional Su–Schrieffer–Heeger (SSH) model, both the e/2 charge of the TFC and its distribution can be verified. As for the disorder effect, it is shown that the Anderson disorder, which breaks certain symmetry related to the TFC, is significant in higher-dimensional systems while having little effect on the one-dimensional SSH chain. Nonetheless, our measurement scheme can still work well for specific higher-order topological insulator materials, for instance, the 2e/3 TFC in the breathing kagome model could be confirmed even in the presence of disorder effect. These conclusions about spatial dimension and disorder effect are quite universal, which also applies to other topological systems such as topological classic wave system.