Proximity- Induced Superconductivity in graphene - imaginenano 2013

Proximity- Induced Superconductivity ingraphene:SSC. Li, K. Komatsu, S. Guéron, H. BouchiatGroupe de Physique MésoscopiqueLaboratoire de Physique des Solides, Orsay

Outline• Superconducting proximity effect in graphene at low field(long junction limit): evidence of specificities of graphene.• Sensitivity to doping with Porphyrin molecules• Revealing gate dependent magnetismSNeh• Superconducting proximity effect in the Quantum Hall regime:Injecting Cooper pairs through edge states?eBzEdge stateEdge statesh

Current injection through a SNS junctionSingle electron or holetransport possible only for eV > 2∆-e+e2eS,∆NS current at V=0:one electron reflected into a hole(Andreev reflection)two electrons passing from N to S

Coherent propagation of pairs through a nonsuperconducting materialAttractive interactionNothing to « cement » pairsAttractive interactionS,∆e iϕ 1 S,∆e iϕ 2Propagation of Andreev pairs occurs only if nothing breaks the pairduring traversal. L < L T , L ϕTypically, 1 micron, T

Specificity of Andreev reflection in graphenevalence bandElectron and hole haveopposite velocities along y

Specificity of Andreev reflection in grapheneE F < ∆valence bandPossible to constructe/h pairs belongingto different bands…Velocity along yis conserved!AccumulatedPhase differencebetween e-htrajectoriesBeenakker 06

Superconducting proximity effect in longgraphene junctions at low fieldPrevious work:Ojeda-Aristizabal (2010): Pt/TaMorpurgo(2007), Andrei (2008): Ti/AlJoeng (2011): PbIn, Borzenets (2011): Pd/PbL = 200 to 500 nmThis work: Long junction, L=1200 nm >> ξ S ~ 100nm,Contacts Pd/Nb (Tc ≈ 8K)More than 3 times longer thanprevious worksLong junction may be a way to observe specular reflection...

Proximity effect at zero field (Nb, L=1200 nm)V (V)I (µA)Zero resistance state at high doping.Supercurrent (~200nA) at 200 mK, tunable by back gateCritical current suppressed at low doping.

Finding: I c suppressed around Dirac PointTheory for long junction:I c =10 E Th /R N with E Th =ħD/L² Thouless energy• I c decreases much fasterthan E Th /R N near the Diracpoint.Komatsu et al. 2012Question: is this becauseof the specular Andreevreflexion specific tographene?

Graphene near the Dirac point (zero doping)At half filling: No carriers? No charge?n c =0 on average. Locally…

Graphene near the Dirac point (zero doping)A. Yacoby et al, 2008Puddles, size ~ 100 nmDistribution of doping, E F ~10-100KDistribution of differently doped pockets

Influence of charge inhomogeneity on critical current1)Doped graphene: homogeneous dopingSuperconductorL < L TW > LTSSUsual Andreev retro-reflection: Coherent conterpropagation ofAndreev pairs; constructive interference (zero phaseaccumulation): Usual critical current expected

Influence of charge inhomogeneity on critical current2) Graphene near Dirac PointZero charge regionSSRandom dephasing upon specular reflection and diffusion.Destructive interference: Suppressed critical current near DiracPoint.

Variations on the Proximity effect (recent)Pt• Add metal-Porphyrine Molecules(coll. S. Campidelli, A. Filoramo CEA Saclay)• At room temperature after deposition:Dirac point shifts:• Pophyrines be have as donors• Hysteretic behaviour with gate voltage• Disapear at low temperatureii) Add moleculesi) No molecules

The Proximity effect with ionized porphyrinson grapheneporphyrinesPtLi et al. 2013• No supercurrent atpositive gate voltage!

Hysteresis in magnetoresistanceat V g >0 but not V g

Why gate dependent magnetism?BS+ + + + +- - - - - - - - - - - - - - -SiO2+++++++++++++++++++SiThe number of ionized porphyrins does notDepend on the gate voltageHysteresis in Magnetic field:magnetic correlations??

Localised Magnetic impurities on GrapheneExchange coupling depends on the polarity of carriers

Exchange coupling in graphene can be controlledby gatingUchoa et al. 2008Kondo effect and RKKY interactions are gate voltage dependent

Unipolar supercurrent trough graphene grafted with porphyrinsNormal stateSuperconducting stateGate dependent magnetism revealed by proximity effect

The Superconducting proximity effect in theQuantum Hall regime:Can Cooper pairs travel via edge states?BzEdge stateehEdge states

What about with superconducting contacts?In magnetic field : Partners in S pair are no longer time-reversed!Random dephasing during propagation.Supercurrent disapears…S,∆e iϕ 1S,∆e iϕ 2But in Quantum Hall regime only a few number of channels->Interference should not be washed out BzSupercurrent recovered !Theoretically possible for edge states to carry a supercurrent

Theoretical predictions for supercurrent• Ma, Zyuzin (1994):small contact < ξ S =(hD/∆) 1/2eBh• supercurrent α 1/perimeter• Should be independent of S contact position

Superconducting electrodes with high Hc2Electrodes: ReW (Hc > 7 T, Tc ~ 5 K), (H. Raffy)Contact distance ~ 0.7µm(Also tried NbN and NbTi)ReWgrapheneReW wire resistance2 wire measurement

Proximity effect with High Hc2 electrodes(Pd/ReW, L=0.7µm) in zero fieldH=0TIc tuned by back gate, similar to sample with Pd/Nbelectrodes

Quantum Hall regime (but with only two contacts!)SS• No obvious sign ofsupercurrent in QHEregime.• ρ xy is mixed with ρ xx(2wire measurement)Rsquare∝ρ + ρ2xx2xyVisible interference effects

Supercurrent in QH regime??2 wire measurement (can’t see zero resistance state)7.5 T0 T• Resistance oscillation related todip in dV/dI: Interference ofAndreev pairs.

Conclusion• Supercurrent through graphene at low field, with both Pd/Nb and Pd/ReWelectrodes in long SGS junctions (~1µm).• Suppression of critical current near zero doping a sign of specular AndreevreflectionSensitivity to doping with porphyrins: signature of gate dependent magnetism• At high field, Andreev interference in QH regime.But need clearer signature of supercurrent than is achieved in two wireresistance measurement.SQUID geometryVia magnetisation measurement?

Why is the proximity effect (in a long junction) interesting?l eS,∆e iϕ N 1 S,∆e iϕ 2= ξ SPair correlations diffuseinto the N metaldos dos dos∆« N » acquires S propertiesBut is neither a normal metal nor a superconductor