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Formation of confined crystalline tetra-silicene (t-silicene) from crystalline hexa-silicene (h-silicene) via compression, and the reverse transition from the obtained t-silicene to h-silicene via ...

The geometric, electronic and magnetic properties of strained graphene nanoribbons were investigated using spin polarized calculations within the framework of density functional theory . Cases of compressive stress along the longer axis of a nanoribbon and tensile stress at the midpoint and perpendicular to the plane of the nanoribbon were considered. Significant structural changes were observed including the formation of nanoripples. The calculated electronic and magnetic properties strongly depend on the size and shape of nanoribbons. The tunable magnetic properties of strained nanoribbons can be employed for designing magnetic nano-switches.

We consider the transport properties of a single-electron transistor consisting of a diffusive normal-metal island and superconducting electrodes. We show that the interplay of a dissipative impedance of the gate electrode, a weak magnetic field, and a charging energy of the island causes a suppression of the critical supercurrent of the transistor. When the device is voltage biased, pumping of single electrons can be achieved by a periodic modulation of the gate voltage. We derive the analytic expressions for the error rates and show that they are suppressed by the applied magnetic field.

We present schemes for quantum teleportation and entanglement swapping of electronic spin states in hybrid superconductor–normal-metal systems. The proposed schemes employ subgap transport whereby the lowest order processes involve Cooper pair-electron and double Cooper-pair cotunneling in quantum teleportation and entanglement swapping protocols, respectively. The competition between elastic cotunneling and Cooper-pair splitting results in the success probability of 25% in both cases. Described implementations of these protocols are within reach of present-day experimental techniques.

We consider the influence of the electromagnetic fluctuations on the transport properties of a hybrid single electron transistor, consisting of superconducting electrodes and a normal-metal island, when operated as a turnstile. We derive the analytic expressions for the rates near the thresholds of single electron tunneling, Andreev reflection, and Cooper-pair–electron cotunneling processes. These results show that the dissipative on-chip impedance suppresses the rates of the undesirable higher-order tunneling processes much stronger than the single electron tunneling which can therefore be utilized to increase the accuracy of such a device in quantum metrological applications.

We consider a three-terminal Cooper-pair splitting device with a superconducting electrode tunnel coupled to two normal metal electrodes. We employ the Nambu-Gor'kov and Schwinger-Keldysh formalisms to describe the nonequilibrium transport properties of the device for arbitrary transmissions of the barriers and for a general electromagnetic environment. We derive the analytic expressions for the current and the nonlocal differential conductance, and analyze the limits of clean and dirty superconductivity.

The problem considered in this paper arises in the design of a high-temperature superconducting cryogenic current comparator (CCC). The CCC consists of two currents flowing in opposite directions inside a toroidal superconducting shield. The shield has a radial cut, necessary for the measurement of the current ratio, but causing an error in the obtained ratio. The problem of interest is the dependence of the error on the geometric parameters of the device: the major and minor radii of the shield, the cut width, the material thickness, and the location of the currents. In the first part of the paper, a toroidal shield with an infinitesimal cut is considered and analytic expressions are derived for the magnetic field and the surface-current distribution. In the second part, a cut of finite width is introduced. Since all the perturbing currents are present in the narrow region around the cut, a shield of cylindrical shape is assumed. Expressions are derived for the flux through the cut and the magnetic field around the cut. Analytical results are in good agreement with the numerical results obtained by a finite-element method. In the final part, the expression for the ratio error is derived, which shows that in order to minimize the error, currents should be concentrated around the shield axis, the major radius of the shield should be maximized and the bore radius minimized. The error depends logarithmically on the cut width.

We consider the 3-junction single electron pump with on-chip resistors (R-pump) driven by harmonic gates. We calculate the cotunneling error of the device as a function of the external resistance. It is shown that for suitable values of the parameters metrological accuracy can be achieved.

We study the effect of the finite RC line on the electron cotunneling through two serially connected small normal-metal tunnel junctions. We obtain the analytic expressions for the cotunneling current in the cases of small and large line resistance compared to the quantum resistance.

We study the effects of the circuit impedance on tunneling of Cooper pairs from a superconductor into two normal-metal leads. We derive the expressions for the crossed and direct Andreev current and consider the limits of ballistic and diffusive electron motion in the electrodes. The derived analytic expressions show that voltage cross correlations, induced by the capacitive coupling of normal-metal leads, narrow the region where the crossed Andreev current dominates over the direct Andreev current, thereby reducing the efficiency of the entangler.

Using non-equilibrium Green's function technique with auxiliary-particle operators, we study co-tunneling effects on transport through double quantum dots in the spin blockade regime. We construct the Luttinger-Ward functional by summing infinite number of Feynman diagrams under the non-crossing approximation and, thereby, tunneling currents are calculated in conservative ways for a finite Coulomb potential U. We find that co-tunneling results in Kondo resonance peaks along the chemical potential even in high bias voltages and leads to relatively large occupation probabilities of states which would be prohibited in sequential tunneling cases. As a result, conductance peaks are broadened and large leakage currents are found in the spin blockade regime, which may lead to errors in the spin read-out.

The large-scale behavior of surface-interacting self-avoiding polymer chains placed on finitely ramified fractal lattices is studied using exact recursion relations. It is shown how to obtain surface susceptibility critical indices and how to modify a scaling relation for these indices in the case of fractal lattices. We present the exact results for critical exponents at the point of adsorption transition for polymer chains situated on a class of Sierpinski gasket-type fractals. We provide numerical evidence for a critical behavior of the type found recently in the case of bulk self-avoiding random walks at the fractal to Euclidean crossover.

We formulate the theory of electron transport through coupled-quantum dots by extending the auxiliary operator representation. By using the generating functional technique, we derive the exact expressions for currents, dot-occupation numbers and spin correlations, and examine them based on the non-equilibrium Green's function method under the non-crossing approximation (NCA). Our formulation generalizes the previous NCA approaches by allowing full occupation numbers with a finite Coulomb repulsion., 16pages, 5 figures

The time-of-swing experiment for measuring the gravitational constant is considered. The expression for the gravitational attraction of the cylinder is derived. The cases of no damping, viscous damping and a general damping are studied. Modeling the environment as a bath of non-interacting modes the equation of motion of the pendulum is derived in terms of the experimentally obtainable spectral function.

We consider the electron transport in the capacitively coupled single-electron transistor with an ultrasmall Tomonaga-Luttinger liquid island. The charging effects, as well as the Tomonaga-Luttinger liquid nature, are treated by a self-consistent theory of the Coulomb blockade using the open boundary bosonization technique. Analytical expressions for conductance are derived in the limits of low and high voltages and temperatures, for bulk and edge island contact geometries, and for arbitrary environmental impedance. For an infinite system, we obtain the power law of the conductance with the exponent changed from the usual Tomonaga-Luttinger exponent due to the effects of the electromagnetic environment. For a finite system, we obtain expressions for the conductance as a function of voltage near the Coulomb blockade boundary and as a function of temperature for low temperatures; these expressions differ from the usual power-law behavior. The results show the potential for improving the accuracy of single-electron devices such as those used in electrical metrology.

We study the dissipation in the time-of-swing method of measuring the gravitational constant. Modelling the environment as a bath of non-interacting modes we derive the equation of motion of the pendulum in terms of the experimentally obtainable spectral function.

We consider a double junction system in the Coulomb blockade regime with an island with discrete energy levels and analyze the effect of electromagnetic fluctuations on elastic cotunneling. We obtain the analytic expression for the elastic cotunneling current at zero temperature, which shows the power-law suppression of cotunneling at low voltages due to circuit impedance $I\ensuremath{\sim}{V}^{1+2\text{ }\text{Re}\text{ }Z(0)/{R}_{K}}$. The results will be useful for calculating the accuracy of single-electron devices used in metrology.

We have investigated the influence of the dissipative circuit impedance on the cotunneling process in a double junction system and on the accuracy of the 3-junction R-pump.

We consider two serially connected tunnel junctions embedded in a dissipative electromagnetic environment. Within the Coulomb blockade region where the sequential single electron tunneling is suppressed, electron transport through the whole system is achieved via higher-order tunneling processes. The standard approach using perturbation theory to fourth order in the tunnel Hamiltonian gives divergent rates of these processes at the Coulomb blockade boundary. By using a diagrammatic technique, we perform an analytic summation over a perturbative expansion of infinite order. In this way, we remove the divergence and derive an expression for the cotunneling current applicable over the whole Coulomb blockade region.

We study a single-electron room temperature memory device. The device was fabricated by a pulse mode atomic force microscopy nano-oxidation process on an atomically flat α-Al2O3 substrate. We model the operation using a Monte Carlo simulation. There is a general agreement between theory and experiment. The observed differences stem from noise and the uncertainty in determining the geometry of the fabricated circuit.

A planar-type single electron transistor (SET) was fabricated by the atomic force microscopy (AFM) nano-oxidation process. The fabricated SET showed the Coulomb oscillation characteristic with the period of about 2 V at room temperature. From the three-dimensional simulation, it is found out that the smaller the SET island size, the smaller the tunnel junction capacitance, and the tunnel junction capacitance shows a weak dependence on the tunnel junction width. Using the analytical model, the reason for this weak dependence was clarified.