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Ultimately short ballistic vertical graphene Josephson junctions.

Lee GH, Kim S, Jhi SH, Lee HJ - Nat Commun (2015)

Bottom Line: To overcome the obstacles, we vertically sandwiched a cleaved graphene monoatomic layer as the normal-conducting spacer between superconducting electrodes.The atomically thin single-crystalline graphene layer serves as an ultimately short conducting channel, with highly transparent interfaces with superconductors.This vertical stacking scheme for extremely thin transparent spacers would open a new pathway for exploring the exotic coherence phenomena occurring on an atomic scale.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea [2].

ABSTRACT
Much efforts have been made for the realization of hybrid Josephson junctions incorporating various materials for the fundamental studies of exotic physical phenomena as well as the applications to superconducting quantum devices. Nonetheless, the efforts have been hindered by the diffusive nature of the conducting channels and interfaces. To overcome the obstacles, we vertically sandwiched a cleaved graphene monoatomic layer as the normal-conducting spacer between superconducting electrodes. The atomically thin single-crystalline graphene layer serves as an ultimately short conducting channel, with highly transparent interfaces with superconductors. In particular, we show the strong Josephson coupling reaching the theoretical limit, the convex-shaped temperature dependence of the Josephson critical current and the exceptionally skewed phase dependence of the Josephson current; all demonstrate the bona fide short and ballistic Josephson nature. This vertical stacking scheme for extremely thin transparent spacers would open a new pathway for exploring the exotic coherence phenomena occurring on an atomic scale.

No MeSH data available.


Related in: MedlinePlus

Temperature dependence of the junction critical current.(a) Colour-coded plot of dV/dI, measured with a current sweep from negative to positive as a function of temperature T. Tc,b(t) was determined as the temperature where the critical current Ic,b(t) vanished at the interface between the bottom (top) electrode and graphene. Above the critical temperatures of Tc,b and Tc,t at the bottom and top electrodes, respectively, dV/dI became equal to the normal-state resistance, RN. (b) Experimentally measured Ic (blue symbols) of monolayer graphene vGJJ (JJ2), along with the best-fit curve to the short ballistic junction characters (blue line). Temperature dependences of Ic for vGJJs made of five-layer graphene (red symbols) and 43-nm-thick graphite (green symbols). Lines are provided as guides.
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f3: Temperature dependence of the junction critical current.(a) Colour-coded plot of dV/dI, measured with a current sweep from negative to positive as a function of temperature T. Tc,b(t) was determined as the temperature where the critical current Ic,b(t) vanished at the interface between the bottom (top) electrode and graphene. Above the critical temperatures of Tc,b and Tc,t at the bottom and top electrodes, respectively, dV/dI became equal to the normal-state resistance, RN. (b) Experimentally measured Ic (blue symbols) of monolayer graphene vGJJ (JJ2), along with the best-fit curve to the short ballistic junction characters (blue line). Temperature dependences of Ic for vGJJs made of five-layer graphene (red symbols) and 43-nm-thick graphite (green symbols). Lines are provided as guides.

Mentions: Having established strong Josephson coupling in the vGJJ, we now discuss its temperature (T) dependence in detail. The T dependence of Ic, identified as the bright local maximum curves of dV/dI in Fig. 3a, showed two uncommon features. One is that Ic appeared immediately below the electrode critical temperatures (in this case, Tc,b), that is, the junction critical temperature (Tc,JJ) is identical to Tc,b, indicating the establishment of strong Josephson coupling in the vGJJ. In ordinary proximity JJs, Tc,JJ is noticeably below the critical temperature of the electrodes (ΔT~0.3 K, ~0.1 K and ~1.6 K for Nb-2DEG-Nb (ref. 19), Al-nanowire-Al20 and Pb-graphene-Pb JJs23, respectively) because sufficiently strong superconductivity of the electrodes is required for discernible Josephson coupling to be established. Another uncommon feature is that Ic decreases with a convex-shaped T dependence, that is, d2Ic(T)/dT2<0, up to Tc,JJ. Whereas, diverse proximity JJs studied to date have shown a concave-shaped or exponentially decaying Ic(T) for T close to Tc,JJ, which is a typical long-junction behaviour. As L is increased with thicker graphite flakes, the convex-shaped Ic(T) gradually changed to a concave-shaped or exponential decay (Fig. 3b and Supplementary Fig. 5). The convex-shaped Ic(T), especially close to Tc,JJ, was predicted uniquely for JJs in the short-junction limit22 in the 1970s; however, its experimental observation has seldom been reported except for a few recent reports in high-Tc edge JJs, although values of the IcRN product of the junctions fall far below the gap value.


Ultimately short ballistic vertical graphene Josephson junctions.

Lee GH, Kim S, Jhi SH, Lee HJ - Nat Commun (2015)

Temperature dependence of the junction critical current.(a) Colour-coded plot of dV/dI, measured with a current sweep from negative to positive as a function of temperature T. Tc,b(t) was determined as the temperature where the critical current Ic,b(t) vanished at the interface between the bottom (top) electrode and graphene. Above the critical temperatures of Tc,b and Tc,t at the bottom and top electrodes, respectively, dV/dI became equal to the normal-state resistance, RN. (b) Experimentally measured Ic (blue symbols) of monolayer graphene vGJJ (JJ2), along with the best-fit curve to the short ballistic junction characters (blue line). Temperature dependences of Ic for vGJJs made of five-layer graphene (red symbols) and 43-nm-thick graphite (green symbols). Lines are provided as guides.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4317505&req=5

f3: Temperature dependence of the junction critical current.(a) Colour-coded plot of dV/dI, measured with a current sweep from negative to positive as a function of temperature T. Tc,b(t) was determined as the temperature where the critical current Ic,b(t) vanished at the interface between the bottom (top) electrode and graphene. Above the critical temperatures of Tc,b and Tc,t at the bottom and top electrodes, respectively, dV/dI became equal to the normal-state resistance, RN. (b) Experimentally measured Ic (blue symbols) of monolayer graphene vGJJ (JJ2), along with the best-fit curve to the short ballistic junction characters (blue line). Temperature dependences of Ic for vGJJs made of five-layer graphene (red symbols) and 43-nm-thick graphite (green symbols). Lines are provided as guides.
Mentions: Having established strong Josephson coupling in the vGJJ, we now discuss its temperature (T) dependence in detail. The T dependence of Ic, identified as the bright local maximum curves of dV/dI in Fig. 3a, showed two uncommon features. One is that Ic appeared immediately below the electrode critical temperatures (in this case, Tc,b), that is, the junction critical temperature (Tc,JJ) is identical to Tc,b, indicating the establishment of strong Josephson coupling in the vGJJ. In ordinary proximity JJs, Tc,JJ is noticeably below the critical temperature of the electrodes (ΔT~0.3 K, ~0.1 K and ~1.6 K for Nb-2DEG-Nb (ref. 19), Al-nanowire-Al20 and Pb-graphene-Pb JJs23, respectively) because sufficiently strong superconductivity of the electrodes is required for discernible Josephson coupling to be established. Another uncommon feature is that Ic decreases with a convex-shaped T dependence, that is, d2Ic(T)/dT2<0, up to Tc,JJ. Whereas, diverse proximity JJs studied to date have shown a concave-shaped or exponentially decaying Ic(T) for T close to Tc,JJ, which is a typical long-junction behaviour. As L is increased with thicker graphite flakes, the convex-shaped Ic(T) gradually changed to a concave-shaped or exponential decay (Fig. 3b and Supplementary Fig. 5). The convex-shaped Ic(T), especially close to Tc,JJ, was predicted uniquely for JJs in the short-junction limit22 in the 1970s; however, its experimental observation has seldom been reported except for a few recent reports in high-Tc edge JJs, although values of the IcRN product of the junctions fall far below the gap value.

Bottom Line: To overcome the obstacles, we vertically sandwiched a cleaved graphene monoatomic layer as the normal-conducting spacer between superconducting electrodes.The atomically thin single-crystalline graphene layer serves as an ultimately short conducting channel, with highly transparent interfaces with superconductors.This vertical stacking scheme for extremely thin transparent spacers would open a new pathway for exploring the exotic coherence phenomena occurring on an atomic scale.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea [2].

ABSTRACT
Much efforts have been made for the realization of hybrid Josephson junctions incorporating various materials for the fundamental studies of exotic physical phenomena as well as the applications to superconducting quantum devices. Nonetheless, the efforts have been hindered by the diffusive nature of the conducting channels and interfaces. To overcome the obstacles, we vertically sandwiched a cleaved graphene monoatomic layer as the normal-conducting spacer between superconducting electrodes. The atomically thin single-crystalline graphene layer serves as an ultimately short conducting channel, with highly transparent interfaces with superconductors. In particular, we show the strong Josephson coupling reaching the theoretical limit, the convex-shaped temperature dependence of the Josephson critical current and the exceptionally skewed phase dependence of the Josephson current; all demonstrate the bona fide short and ballistic Josephson nature. This vertical stacking scheme for extremely thin transparent spacers would open a new pathway for exploring the exotic coherence phenomena occurring on an atomic scale.

No MeSH data available.


Related in: MedlinePlus