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We study current-voltage (V-I) characteristics of short superconducting nanowires of length &3x223C; 100 nm exposed to microwave (MW) radiation of frequencies between 2 and 15 GHz. The radiation causes a decrease of the average switching current of the wire. This suppression of the switching current is modeled assuming that there is one-to-one correspondence between Little's phase slips, which are microscopic stochastic events induced by thermal and quantum fluctuations, and the experimentally observed switching events.We also find that at some critical power P_ of the radiation a dissipative dynamic superconducting state occurs as an extra step on the V-I curve. It is identified as a phase slip center (PSC), which is essentially a deterministic and periodic in-time phase rotation. With the dependence of the switching currents and the standard deviations observed at the transitions: (i) from the supercurrent state to the normal state and (ii) from the supercurrent state to the PSC regime, we conclude that both of the two types of switching events are triggered by the same microscopic event, namely a single-phase slip. We show that the Skocpol- Beasley-Tinkham model is not applicable to ourMW-driven PSCs, probably due to the tendency of the PSC to synchronize with the MW. Through the analysis of the switching current distributions at a sufficiently low temperature, we also present evidence that quantum phase slips play a role in switching events even under MWs. [ABSTRACT FROM AUTHOR]

The performance and scaling of graphene-based electronics is limited by the quality of contacts between the graphene and metal electrodes. However, the nature of graphene-metal contacts remains incompletely understood. Here, we use atomic force microscopy to measure the temperature distributions at the contacts of working graphene transistors with a spatial resolution of ∼10 nm (refs 5, , , 8), allowing us to identify the presence of Joule heating, current crowding and thermoelectric heating and cooling. Comparison with simulation enables extraction of the contact resistivity (150-200 Ω µm2) and transfer length (0.2-0.5 µm) in our devices; these generally limit performance and must be minimized. Our data indicate that thermoelectric effects account for up to one-third of the contact temperature changes, and that current crowding accounts for most of the remainder. Modelling predicts that the role of current crowding will diminish and the role of thermoelectric effects will increase as contacts improve. [ABSTRACT FROM AUTHOR]

We directly image hot spot formation in functioning mono- and bilayer graphene field effect transistors (GFETs) using infrared thermal microscopy. Correlating with an electrical−thermal transport model provides insight into carrier distributions, fields, and GFET power dissipation. The hot spot corresponds to the location of minimum charge density along the GFET; by changing the applied bias, this can be shifted between electrodes or held in the middle of the channel in ambipolar transport. Interestingly, the hot spot shape bears the imprint of the density of states in mono- vs bilayer graphene. More broadly, we find that thermal imaging combined with self-consistent simulation provide a noninvasive approach for more deeply examining transport and energy dissipation in nanoscale devices. [ABSTRACT FROM AUTHOR]

We report the thermal conductance Gof Au/Ti/graphene/SiO2interfaces (graphene layers 1 ≤ n≤ 10) typical of graphene transistor contacts. We find G≈ 25 MW m−2K−1at room temperature, four times smaller than the thermal conductance of a Au/Ti/SiO2interface, even when n= 1. We attribute this reduction to the thermal resistance of Au/Ti/graphene and graphene/SiO2interfaces acting in series. The temperature dependence of Gfrom 50 ≤ T≤ 500 K also indicates that heat is predominantly carried by phonons through these interfaces. Our findings suggest that metal contacts can limit not only electrical transport but also thermal dissipation from submicrometer graphene devices. [ABSTRACT FROM AUTHOR]

Phase slips are topological fluctuations that carry the superconducting order-parameter field between distinct current-carrying states. Owing to these phase slips, superconducting nanowires acquire electrical resistance. In such wires, it is well known that at higher temperatures phase slips occur through the process of thermal barrier-crossing by the order-parameter field. At low temperatures, the general expectation is that phase slips should proceed through quantum tunnelling events, which are known as quantum phase slips. However, resistive measurements have produced evidence both for and against the occurrence of quantum phase slips. Here, we report evidence for the observation of individual quantum phase-slip events in homogeneous ultranarrow wires at high bias currents. We accomplish this through measurements of the distribution of switching currents for which the width exhibits a rather counter-intuitive, monotonic increase with decreasing temperature. Importantly, measurements show that in nanowires with larger critical currents, quantum fluctuations dominate thermal fluctuations up to higher temperatures. [ABSTRACT FROM AUTHOR]

We observed subbranch splitting in the low-bias vortex-flow branch of a dense-Josephson-vortex state in current-voltage characteristics of Bi2Sr2CaCu2O8+x intrinsic Josephson junctions in the long-junction limit. We suggest that each subbranch corresponds to a plasma mode in serially coupled Josephson junctions, manifested in the Josephson vortex-flow current-voltage characteristics resonating with the plasma modes. [ABSTRACT FROM AUTHOR]

Detailed tunneling characteristics of a stack of intrinsic Josephson junctions in a Bi2Sr2CaCu2O8+x single crystal were obtained both in spin-degenerate and spin-polarized bias configurations. Injection of the spin-polarized quasiparticles effectively weakened the in-plane superconducting strength and the interlayer Jospehson coupling. The superconducting gap and the Josephson critical current of the stack were, in general, further suppressed by an external field applied in parallel with the c-axis direction. Study of the spin-dependent tunneling characteristics may provide valuable information on clarifying the mechanism of high-Tc superconductivity as well as the interlayer phase fluctuation induced by the "pancake vortices" on CuO2 layers. [ABSTRACT FROM AUTHOR]

We examine mobility and saturation velocity in graphene on SiO2 above room temperature (300–500 K) and at high fields (∼1 V/μm). Data are analyzed with practical models including gated carriers, thermal generation, “puddle” charge, and Joule heating. Both mobility and saturation velocity decrease with rising temperature above 300 K, and with rising carrier density above 2×1012 cm-2. Saturation velocity is >3×107 cm/s at low carrier density, and remains greater than in Si up to 1.2×1013 cm-2. Transport appears primarily limited by the SiO2 substrate but results suggest intrinsic graphene saturation velocity could be more than twice that observed here. [ABSTRACT FROM AUTHOR]

We examine mobility and saturation velocity in graphene on SiO2 above room temperature (300–500 K) and at high fields (∼1 V/μm). Data are analyzed with practical models including gated carriers, thermal generation, “puddle” charge, and Joule heating. Both mobility and saturation velocity decrease with rising temperature above 300 K, and with rising carrier density above 2×1012 cm-2. Saturation velocity is >3×107 cm/s at low carrier density, and remains greater than in Si up to 1.2×1013 cm-2. Transport appears primarily limited by the SiO2 substrate but results suggest intrinsic graphene saturation velocity could be more than twice that observed here. [ABSTRACT FROM AUTHOR]

We expose superconducting nanowires to microwave radiation in order to study phase lock-in effects in quasi-one-dimensional superconductors. For sufficiently high microwave powers a resistive branch with Shapiro steps appears in the voltage-current characteristics. At frequencies in the range of 0.9–4 GHz these steps are of integer order only. At higher frequencies steps of 1/2, 1/3, 1/4, and even 1/6 order appear. We numerically model this behavior using a multivalued current-phase relationship for nanowires. [ABSTRACT FROM AUTHOR]

We confirmed the existence of the collective transverse plasma modes excited by the motion of the Josephson vortex lattice in stacked intrinsic Josephson junctions of Bi2Sr2CaCu2O8+x by observing the multiple subbranches in the Josephson-vortex-flow current-voltage characteristics. We also observed the symptom of the microwave emission from the resonance between the Josephson vortex lattice and the collective transverse plasma modes, which provides the possibility of developing Josephson-vortex-flow electromagnetic oscillators. [ABSTRACT FROM AUTHOR]

In highly anisotropic layered cuprates such as Bi2Sr2CaCu2O8+x tunneling measurements on a stack of intrinsic junctions (IJs) in a high-bias range are often susceptible to self-heating. In this study we monitored the temperature variation of a stack (“sample stack”) of IJs by measuring the resistance change of a nearby stack of IJs, which was strongly thermal coupled to the sample stack. We then adopted a proportional-integral-derivative scheme incorporated with a substrate-holder heater to compensate the temperature variation. This in situ temperature monitoring and controlling technique allows one to get rid of spurious tunneling effects arising from the self-heating. [ABSTRACT FROM AUTHOR]

The microwave distribution inside a rectangular stack (15 μm×0.72 μm×60 nm) of Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8+x] intrinsic Josephson junctions (IJJs) was studied. The stack was microfabricated into a transmission-line geometry, with a-few-hundred-nm-thick Au layers deposited on the top and bottom of the stack. The microwave distribution was monitored by measuring the anomalous suppression of the tunneling critical current of the IJJs with varied microwave power at frequencies in the W band. This technique can provide valuable information on the microwave transmission modes inside the sandwiched stack of IJJs, which is utterly important for the high-frequency device applications using IJJs, such as fluxon-flow THz oscillators. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated the accuracy dependence of a single-electron pump on the confinement-potential shape of a quantum dot (QD) pump, A uniquely designed QD, which employs multiple gates to control the shape of the QD potential well, is utilized for electron pumping. It has been observed that the accuracy of the pump can be dramatically enhanced by achieving smaller QD size and greater decoupling from the electrodes, which is supported by the so-called decay-cascade model. The accuracy of the pump current is estimated, based on the decay-cascade model, to be close to 0.1 ppm for 80-pA pump current when the confinement-potential shape is optimally tuned. This is the highest reported accuracy, although it is a theoretically estimated value, among QD-based pumps measured at 4.2 K in the absence of an external magnetic field. Our numerical calculations show that the enhancement of the estimated accuracy is mainly due to the increase of the electron addition energy in the QD as well as the increase of the height and thickness of the QD potential barrier. [ABSTRACT FROM AUTHOR]

We report nonequilibrium transport measurements of gate-tunable Andreev bound states in a carbon nanotube quantum dot coupled to two superconducting leads. In particular, we observe clear features of two types of Kondo ridges, which can be understood in terms of the interplay between the Kondo effect and superconductivity. In the first type (type I), the coupling is strong and the Kondo effect is dominant. Levels of the Andreev bound states display anticrossing in the middle of the ridge. On the other hand, crossing of the two Andreev bound states is shown in the second type (type II) together with the 0-π transition of the Josephson junction. Our scenario is well understood in terms of only a single dimensionless parameter, kBTKmin/Δ, where TKmin/Δ and Δ are the minimum Kondo temperature of a ridge and the superconducting order parameter, respectively. Our observation is consistent with measurements of the critical current, and is supported by numerical renormalization group calculations. [ABSTRACT FROM AUTHOR]