When two superconductors are connected by a weak link, a supercurrent flows,
the magnitude of which is determined by the difference in the macroscopic
quantum phases of the superconductors. This ...phenomenon was discovered by Josephson for the case of a weak link formed by a thin tunnel barrier: the
supercurrent, I, is related to the phase difference, , through
the Josephson current-phase relation, I =
Icsin , with Ic being the critical
current which depends on the properties of the weak link. A similar relation
holds for weak links consisting of a normal metal, a semiconductor or a constriction. In all cases, the phase difference is zero when no supercurrent
flows through the junction, and increases monotonically with increasing supercurrent
until the critical current is reached. Here we use nanolithography techniques
to fabricate a Josephson junction with a normal-metal weak link in which we
have direct access to the microscopic current-carrying electronic states inside
the link. We find that the fundamental Josephson relation can be changed from
I = Icsin to I
= Icsin( + π)-that is, a π-junction-by
controlling the energy distribution of the current-carrying states in the
normal metal. This fundamental change in the way these Josephson junctions
behave has potential implications for their use in superconducting electronics
as well as in (quantum) logic circuits based on superconductors.
Shot noise in ballistic graphene Danneau, R; Wu, F; Craciun, M F ...
Physical review letters,
05/2008, Letnik:
100, Številka:
19
Journal Article
Recenzirano
Odprti dostop
We have investigated shot noise in graphene field effect devices in the temperature range of 4.2-30 K at low frequency (f=600-850 MHz). We find that for our graphene samples with a large width over ...length ratio W/L, the Fano factor F reaches a maximum F ~ 1/3 at the Dirac point and that it decreases strongly with increasing charge density. For smaller W/L, the Fano factor at Dirac point is significantly lower. Our results are in good agreement with the theory describing that transport at the Dirac point in clean graphene arises from evanescent electronic states.
We report on a study of magnetotransport in LaAlO3 /SrTiO3 interfaces characterized by mobilities of the order of several thousands cm2/V s. We observe Shubnikov-de Haas oscillations whose period ...depends only on the perpendicular component of the magnetic field. This observation directly indicates the formation of a two-dimensional electron gas originating from quantum confinement at the interface. From the temperature dependence of the oscillation amplitude we extract an effective carrier mass m* ≃ 1.45 m(e). An electric field applied in the back-gate geometry increases the mobility, the carrier density, and the oscillation frequency.
The superconducting proximity effect in single-walled carbon nanotubes connected to niobium electrodes was controlled with the use of nearby gates that tune the niobium-nanotube transparency. At 4.2 ...kelvin, when the transparency was tuned to be high, a dip in the low-bias differential resistance was observed, indicating a proximity effect mediated by Andreev reflection. When the transparency was tuned to be low, signatures of Andreev reflection disappeared and only tunneling conduction was observed. Below ∼4 kelvin, a narrow peak in differential resistance around zero bias appeared superimposed on the Andreev dip, probably as a result of electron-electron interaction competing with the proximity effect.
Transport measurements for different electron‐doped metal phthalocyanine materials demonstrate the possibility to control their electronic properties through alkali intercalation. In these materials, ...increasing the potassium concentration results in the formation of a doping‐induced metallic state (see Figure and Inside Cover). Further doping brings the materials back into the insulating state.
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