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We show experimentally that, in the same Si devices, we can demonstrate multiple two-gate pumping modes but not single-gate mode. We contrast this with GaAs devices, which do show single-gate pumping at a high yield. We propose four mechanisms to explain the lack of plateaus in the Si devices in single-gate ratchet mode: operating the dot with a large number of electrons, a large ratio between the change in electrochemical potential energy and the change in the energy of the barrier (plunger-to-barrier ratio, Δptb) compared to the charging energy (Δptb/EC), nonlinear tunnel barriers, and phase offset leading to nonequilibrium heating. Our analysis shows that each of these could contribute to the lack of plateaus in single-gate ratchet pumping on Si devices but allow two-gate pumping methods to work with robust plateaus. It is easier for GaAs pumps to avoid these failure mechanisms due to their different architectures and cleaner gate turnoff curves. We propose several methods to reduce these sources of error, including reducing cross capacitances between gates. These recommendations may prove useful to other researchers in producing more robust, higher yield single-gate ratchet pumps. [ABSTRACT FROM AUTHOR]

Pauli-spin blockade (PSB) is a transport phenomenon in double quantum dots that allows for a type of spin to charge conversion often used to probe fundamental physics such as spin relaxation and singlet-triplet coupling. In this paper, we theoretically explore Pauli-spin blockade as a function of magnetic field B applied parallel to the substrate. In the well-studied low magnetic field regime, where PSB occurs in the forward (1, 1) → (0, 2) tunneling direction, we highlight some aspects of PSB that are not discussed in detail in existing literature, including the change in size of both bias triangles measured in the forward and reverse biasing directions as a function of B. At higher fields, we predict a crossover to "reverse PSB" in which current is blockaded in the reverse direction due to the occupation of a spin singlet as opposed to the traditional triplet blockade that occurs at low fields. The onset of reverse PSB coincides with the development of a tail like feature in the measured bias triangles and occurs when the Zeeman energy of the polarized triplet equals the exchange energy in the (0, 2) charge configuration. In Si quantum dots, these fields are experimentally accessible; thus, this work suggests a way to observe a crossover in magnetic field to qualitatively different behavior. [ABSTRACT FROM AUTHOR]

Proximity and coupling effects between contiguous regions of 4He are expected to occur over a distance on the order of the temperature-dependent correlation length ξ(t). We present recent measurements of 4He confined in arrays of (2 μm)³ boxes linked through a 4He film. The specific heat and superfluid fraction of the 4He in these geometries reveal effects which occur at distances much larger than expected. These effects include enhancements in specific heat and superfluid fraction as well as shifts in the temperatures of the superfluid onset and specific-heat maximum. Our analysis gives evidence for the relevance of the finite-size correlation length ξ(t,L) in explaining these effects even though the effects occur at distances over an order of magnitude larger than ξ(t,L). These results can be used to deduce the finite-size scaling locus for ξ(t,L). The spatial distance over which coupling and proximity effects are observed raises questions regarding the physical mechanism for these effects and the interpretation of ξ(t). Lastly, these results are discussed in connection to other measurements of 4He in confined geometries, and the relevance to other critical systems. [ABSTRACT FROM AUTHOR]

We report measurements of the superfluid fraction ρs/ρ and specific heat cp near the superfluid transition of 4He when confined in an array of (2 μm)3 boxes at a separation of S = 2 μm and coupled through a 32.5 nm film. We find that cp is strongly enhanced when compared with data where coupling is not present. An analysis of this excess signal shows that it is proportional to the finite-size correlation length in the boxes &xgr;(t, L), and it is measurable as far as S/&xgr; ∼ 30 -- 50. We obtain &xgr;(0, L) and the scaling function (within a constant) for &xgr;(t, L) in an L3 box geometry. Furthermore, we find that ρs/ρ of the film persists a full decade closer to the bulk transition temperature Tλ than a film uninfluenced by proximity effects. This excess in ρs/ρ is measurable even when S/&xgr; > 100, which cannot be understood on the basis of mean field theory. [ABSTRACT FROM AUTHOR]

The superfluid transition in 4He is associated with the onset of an order parameter that is a macroscopic wavefunction. This is similar to the onset of superconductivity in metals and alloys. Yet, some of the hallmark phenomena that are associated with wavefunctions, and are evident in superconductors, have not been identified in 4He. Here we report proximity effects on the specific heat and superfluid density of a thin 4He film in equilibrium with an array of larger boxes of 4He. Comparison with previous data also enables us, by scaling, to deduce an excess specific heat associated with a collective behaviour of weakly coupled boxes. These effects should be relevant to the understanding of experiments where helium has been studied when confined in packed powders, porous materials or other inhomogeneous confinements, where proximity and collective effects must be present but not readily quantifiable. Our work also forms a point of comparison and contrast with the case of superconductors, and is relevant to studies of Josephson effects in superfluids, coupling effects in arrays of Bose-condensed atoms, and magnetic systems where, because of chemical composition, two separate but interacting ordering transitions take place. [ABSTRACT FROM AUTHOR]

In this work, we present a master equation approach to simulating DC transport through single electron transistors and quantum dots suitable for an upper-division undergraduate computational physics project. After introducing the basic theory describing transport through quantum dots, we present a simulation of the simple case of a metallic dot including effects due to the finite temperature of the leads. Building on this example, we simulate published data with orbital and spin states. We envision students building on these simulations to replicate other data in the published literature. Projects of this type would be suitable for an undergraduate independent study or computational project and will give students a strong introduction to the topic of transport through quantum dots without the need for expensive cryogenic and electrical measurement systems or device fabrication necessary for experimental work. [ABSTRACT FROM AUTHOR]

Electrical transport in double quantum dots (DQDs) illuminates many interesting features of the dots' carrier states. Recent advances in silicon quantum information technologies have renewed interest in the valley states of electrons confined in silicon. Here we show measurements of dc transport through a mesa-etched silicon double quantum dot. Comparing bias triangles (i.e., regions of allowed current in DQDs) at positive and negative bias voltages we find a systematic asymmetry in the size of the bias triangles at the two bias polarities. Asymmetries of this nature are associated with blocked tunneling events due to the occupation of a metastable state. Several features of our data lead us to conclude that the states involved are not simple spin states. Rather, we develop a model based on selective filling of valley states in the DQD that is consistent with all of the qualitative features of our data. [ABSTRACT FROM AUTHOR]

Previous measurements of the superfluid density ρs and specific heat for 4He have identified effects that are manifest at distances much larger than the correlation length ξ3D [Perron et al., Nat. Phys. 6, 499 (2010); Perron and Gasparini, Phys. Rev. Lett. 109, 035302 (2012); Perron et al., Phys. Rev. B 87, 094507 (2013)]. We report here measurements of the superfluid density which are designed to explore this phenomenon further. We determine the superfluid fraction ρs/ρ from the resonance of 34-nm films of varying widths 4=W=100μm. The films are formed across a Corbino ring separating two chambers where a thicker 268-nm film is formed. This arrangement is realized using lithography and direct Si-wafer bonding. We identify two effects in the behavior of ρs/ρ: one is hydrodynamic, for which we present an analysis, and the other is a correlation-length effect which manifests as a shift in the transition temperature Tc relative to that of a uniform 34-nm film uninfluenced by proximity effects. We find that one can collapse both ρs/ρ and the quality factor of the resonance onto universal curves by shifting Tc as ΔTc ~ W-ν. This scaling is a surprising result on two counts: it involves a very large length scale W relative to the magnitude of ξ3D and the dependence on W is not what is expected from correlation-length finite-size scaling which would predict ΔTc ~ W-1/ν. [ABSTRACT FROM AUTHOR]

We have measured the low-frequency time instability known as charge offset drift of Si/SiO 2 single electron devices (SEDs) with and without an overall poly-Si top gate. We find that SEDs with a poly-Si top gate have significantly less charge offset drift, exhibiting fewer isolated jumps and a factor of two reduction in fluctuations about a stable mean value. The observed reduction can be accounted for by the electrostatic reduction in the mutual capacitance C m between defects and the quantum dot and increase in the total defect capacitance C d due to the top gate. These results depart from the prominent interpretation that the level of charge offset drift in SEDs is determined by the intrinsic material properties, forcing consideration of the device design as well. We expect these results to be of importance in developing SEDs for applications from quantum information to metrology or wherever charge noise or integrability of devices is a challenge. [ABSTRACT FROM AUTHOR]