Interplay of bulk and interface effects in the electric-field driven transition in magnetite

Contact effects in devices incorporating strongly-correlated electronic materials are comparatively unexplored. We have investigated the electrically-driven phase transition in magnetite (100) thin films by four-terminal methods. In the lateral configuration, the channel length is less than 2 $\mu$m, and voltage-probe wires $\sim$100 nm in width are directly patterned within the channel. Multilead measurements quantitatively separate the contributions of each electrode interface and the magnetite channel. We demonstrate that on the onset of the transition contact resistances at both source and drain electrodes and the resistance of magnetite channel decrease abruptly. Temperature dependent electrical measurements below the Verwey temperature indicate thermally activated transport over the charge gap. The behavior of the magnetite system at a transition point is consistent with a theoretically predicted transition mechanism of charge gap closure by electric field.Comment: 6 pages, 5 figures, to appear in PR

Nanogaps with very large aspect ratios for electrical measurements

For nanoscale electrical characterization and device fabrication it is often desirable to fabricate planar metal electrodes separated by large aspect ratio gaps with interelectrode distances well below 100 nm. We demonstrate a self-aligned process to accomplish this goal using a thin Cr film as a sacrificial etch layer. The resulting gaps can be as small as 10 nm and have aspect ratios exceeding 1000, with excellent interelectrode isolation. Such Ti/Au electrodes are demonstrated on Si substrates and are used to examine a voltage-driven transition in magnetite nanostructures. This shows the utility of this fabrication approach even with relatively reactive substrates.Comment: 4 pages, 4 figure

The origin of hysteresis in resistive switching in magnetite is Joule heating

In many transition metal oxides the electrical resistance is observed to undergo dramatic changes induced by large biases. In magnetite, Fe$_3$O$_4$, below the Verwey temperature, an electric field driven transition to a state of lower resistance was recently found, with hysteretic current-voltage response. We report the results of pulsed electrical conduction measurements in epitaxial magnetite thin films. We show that while the high- to low-resistance transition is driven by electric field, the hysteresis observed in $I-V$ curves results from Joule heating in the low resistance state. The shape of the hysteresis loop depends on pulse parameters, and reduces to a hysteresis-free "jump" of the current provided thermal relaxation is rapid compared to the time between voltage pulses. A simple relaxation time thermal model is proposed that captures the essentials of the hysteresis mechanism.Comment: 7 pages, 6 figure

Statistical distribution of the electric field-driven switching of the Verwey state in Fe3O4

The insulating state of magnetite (Fe3O4) can be disrupted by a sufficiently large dc electric field. Pulsed measurements are used to examine the kinetics of this transition. Histograms of the switching voltage show a transition width that broadens as the temperature is decreased, consistent with trends seen in other systems involving “unpinning” in the presence of disorder. The switching distributions are also modified by an external magnetic field on a scale comparable to that required to reorient the magnetization

Field effect enhancement in buffered quantum nanowire networks

III-V semiconductor nanowires have shown great potential in various quantum transport experiments. However, realizing a scalable high-quality nanowire-based platform that could lead to quantum information applications has been challenging. Here, we study the potential of selective area growth by molecular beam epitaxy of InAs nanowire networks grown on GaAs-based buffer layers. The buffered geometry allows for substantial elastic strain relaxation and a strong enhancement of field effect mobility. We show that the networks possess strong spin-orbit interaction and long phase coherence lengths with a temperature dependence indicating ballistic transport. With these findings, and the compatibility of the growth method with hybrid epitaxy, we conclude that the material platform fulfills the requirements for a wide range of quantum experiments and applications

Interfacial charge transfer in nanoscale polymer transistors

Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred in a number of photoemission experiments. We present electronic transport measurements in very short channel ($L < 100$ nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact resistance and the zero-gate-voltage conductance are consistent with such charge transfer. Short channel conduction in devices with Pt contacts is greatly enhanced compared to analogous devices with Au contacts, consistent with charge transfer expectations. Alternating current scanning tunneling microscopy (ACSTM) provides further evidence that holes are transferred from Pt into P3HT, while much less charge transfer takes place at the Au/P3HT interface.Comment: 19 preprint pages, 6 figure

Electrically-driven phase transition in magnetite nanostructures

Magnetite (Fe$_{3}$O$_{4}$), an archetypal transition metal oxide, has been used for thousands of years, from lodestones in primitive compasses[1] to a candidate material for magnetoelectronic devices.[2] In 1939 Verwey[3] found that bulk magnetite undergoes a transition at T$_{V}$ $\approx$ 120 K from a high temperature "bad metal" conducting phase to a low-temperature insulating phase. He suggested[4] that high temperature conduction is via the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering upon cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial.[5-11] Here we show that magnetite nanocrystals and single-crystal thin films exhibit an electrically driven phase transition below the Verwey temperature. The signature of this transition is the onset of sharp conductance switching in high electric fields, hysteretic in voltage. We demonstrate that this transition is not due to local heating, but instead is due to the breakdown of the correlated insulating state when driven out of equilibrium by electrical bias. We anticipate that further studies of this newly observed transition and its low-temperature conducting phase will shed light on how charge ordering and vibrational degrees of freedom determine the ground state of this important compound.Comment: 17 pages, 4 figure

Sequential Electron Transport and Vibrational Excitations in an Organic Molecule Coupled to Few-Layer Graphene Electrodes

Graphene electrodes are promising candidates to improvereproducibility and stability in molecular electronics through new electrode−molecule anchoring strategies. Here we report sequentialelectron transport in few-layer graphene transistors containing individualcurcuminoid-based molecules anchored to the electrodes via π −π orbital bonding. We show the coexistence of inelastic co-tunneling excitations with single-electron transport physics due to an intermediate molecule−electrode coupling; we argue that an intermediate electron−phononcoupling is the origin of these vibrational-assisted excitations. Theseexperimental observations are complemented with density functionaltheory calculations to model electron transport and the interaction between electrons and vibrational modes of thecurcuminoid molecule. We find that the calculated vibrational modes of the molecule are in agreement with theexperimentally observed excitation

Determinants of Comhetitive Development of National Defence Industrial Complexes

One of the main priorities of the socio-economic policy of the state in the conditions of globalization and instability of the competitive environment is the formation of a competitive economy. The competitiveness of economies is looked at from the point of view for the achievement of positive results as well as the development of society. The task of theoretical generalization of the experience and leaders of the present day, as well as the development of effective development strategies, all contribute to the growth of economies. At the present stage of development, one of the most urgent issues is the fulfillment of this task. It consists of ensuring the economic, political and military security of the states as the basis for the competitive development of the economies of the countries. Attempts to change geopolitics, are the forms and means of resolving international issues and conflicts, contradictions between states. As well as the contour of regions with the help of new technologies of influence to necessitate a revision of the traditional means of security of the states. The armaments market is an important segment of the world market. Successful operations in the market of armaments can only be those states that implement high-tech and advanced production in the militarily-industrial sector or have a political influence in a separate region