Structure of Tetranuclear Iron(III) d- and dl-Tartrates in Aqueous Solutions

The geometric parameters of dl- and d-tetranuclear iron(III) tartrates [Fe₄(d-L)₂(l-L)₂(H₂O)₈]^4–and [Fe₄(d-L)₄(H₂O)₈]^4–(H₄L is tartaric acid) were optimized using the molecular mechanics method (MIND program, Dashevskii–Plyamovatyi model).     

Influence of high magnetic fields on the superconducting transition of one-dimensional Nb and MoGe nanowires

The effects of a strong magnetic field on superconducting Nb and MoGe nanowires with diameter approximately 10 nm have been studied. We have found that the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase slips is applicable in a wide range of magnetic fields and describes well the temperature dependence of the wire resistance, over 11 orders of magnitude. The field dependence of the critical temperature, T(c), extracted from the LAMH fits is in good quantitative agreement with the theory of pair-breaking perturbations that takes into account both spin and orbital contributions. The extracted spin-orbit scattering time agrees with an estimate tau(s.o.) approximately tau(variant Planck's over 2pic/Ze(2))(4), where tau is the elastic scattering time and Z is the atomic number.     

Nuclear energy conversion with stacks of graphene nanocapacitors

The efficiency of conventional techniques used to harvest energy in nuclear reactors lies around 35%. This limit exists, because the nuclear energy is converted to electrical energy via heat engines. We study an alternative approach where the kinetic energy of nuclear reaction products is directly converted into electric energy in a stack of charged capacitors with a gap size of 500 nm and graphene electrodes. Graphene is expected to be chemically and mechanically stable in high‐radiation environments, because its tensile strength of 130 GPa is very large, about 100 times larger than most metals. The dielectric strength of such nanocapacitors exceeds 1 GV/m, because avalanching is suppressed at small gap sizes. In a 1 GV/m electric field charged nuclear reaction products, such as 5.6 MeV alpha particles, come to rest in of a stack with 5000 nanocapacitors. We show that during the deceleration process more than 90% of kinetic energy of charged nuclear reaction products is converted to electric energy and stored as electric energy in the stack. Each stack is 2.5‐mm thick and produces a high‐voltage DC current. A device with a 1‐Ci²⁴¹Am source is expected to generate 22 mW of electric power.     

Distribution of supercurrent switching in graphene under the proximity effect

We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor junctions. We find that the dispersion of the switching current distribution scales with temperature as σ(I) proportional to T(α(G)) with α(G) as low as 1/3. This observation is in sharp contrast to the known Josephson junction behavior where σ(I) proportional to T(α(J)) with α(J)=2/3. We propose an explanation using a generalized version of Kurkij?rvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.     

Negative and positive magnetoresistance in bilayer graphene: Effects of weak localization and charge inhomogeneity

We report measurements of magnetoresistance in bilayer graphene as a function of gate voltage (carrier density) and temperature. We examine multiple contributions to the magnetoresistance, including those of weak localization (WL), universal conductance fluctuations (UCF), and inhomogeneous charge transport. A clear WL signal is evident at all measured gate voltages (in the hole doped regime) and temperature ranges (from 0.25 to 4.3 K), and the phase coherence length extracted from the WL data does not saturate at low temperatures. The WL data is fit to demonstrate that the electron-electron Nyquist scattering is the major source of phase decoherence. A decrease in UCF amplitude with increase in gate voltage and temperature is shown to be consistent with a corresponding decrease in the phase coherence length. In addition, a weak positive magnetoresistance at higher magnetic fields is observed, and attributed to inhomogeneous charge transport.     

Magnetic-field enhancement of superconductivity in ultranarrow wires

We study the effect of an applied magnetic field on sub-10-nm wide MoGe and Nb superconducting wires. We find that magnetic fields can enhance the critical supercurrent at low temperatures, and do so more strongly for narrower wires. We conjecture that magnetic moments are present, but their pair-breaking effect, active at lower magnetic fields, is suppressed by higher fields. The corresponding microscopic theory, which we have developed, quantitatively explains all experimental observations, and suggests that magnetic moments have formed on the wire surfaces.     

Evolution of avalanche conducting states in electrorheological liquids

Charge transport in electrorheological fluids is studied experimentally under strongly nonequilibrium conditions. By injecting an electrical current into a suspension of conducting nanoparticles we are able to initiate a process of self-organization which leads, in certain cases, to formation of a stable pattern which consists of continuous conducting chains of particles. The evolution of the dissipative state in such a system is a complex process. It starts as an avalanche process characterized by nucleation, growth, and thermal destruction of such dissipative elements as continuous conducting chains of particles as well as electroconvective vortices. A power-law distribution of avalanche sizes and durations, observed at this stage of the evolution, indicates that the system is in a self-organized critical state. A sharp transition into an avalanche-free state with a stable pattern of conducting chains is observed when the power dissipated in the fluid reaches its maximum. We propose a simple evolution model which obeys the maximum power condition and also shows a power-law distribution of the avalanche sizes.     

Electron microscopy investigation of GaAs(100)-(Ga2Se3)-GaAs nanostructures

The GaAs(100)-Ga₂Se₃ heterostructures have been investigated by transmission and scanning electron microscopy. The sequence of structural transformations at the GaAs(100) surface during treatment in selenium vapor has been determined. It has been found that the preliminary treatment of the GaAs(100) surface in selenium vapor increases the orienting effect of the GaAs(100) substrate on the GaAs film growth.