Carbon nanotube superconducting quantum interference device

Transcription

1 nture nnotehnology VOL 1 OCTOBER 26 p.53 / Crbon nnotube superonduting quntum interferene devie J.-P. CLEUZIOU, W. WERNSDORFER, V. BOUCHIAT, T. ONDARÇUHU AND M. MONTHIOUX 1 Centre d Elbortion des Mtériux et d Etudes Struturles, CEMES-CNRS, 29 rue Jenne Mrvig, 3155 Toulouse Cedex 4, Frne 2 Lbortoire L. Néel, LLN-CNRS, ssoie ` l UJF, BP 166, 3842 Grenoble Cedex 9, Frne 3 Centre de Reherhes sur les Tre ` s Bsses Tempe rtures, CRTBT-CNRS, ssoie l UJF, BP 166, 3842 Grenoble Cedex 9, Frne

2 Crbon nnotube superonduting quntum interferene devie J.-P. CLEUZIOU 1, W. WERNSDORFER 2 *,V.BOUCHIAT 3,T.ONDARÇUHU 1 AND M. MONTHIOUX 1 1 Centre d Elbortion des Mtériux et d Etudes Struturles, CEMES-CNRS, 29 rue Jenne Mrvig, 3155 Toulouse Cedex 4, Frne 2 Lbortoire L. Néel, LLN-CNRS, ssoié à l UJF, BP 166, 3842 Grenoble Cedex 9, Frne 3 Centre de Reherhes sur les Très Bsses Tempértures, CRTBT-CNRS, ssoié à l UJF, BP 166, 3842 Grenoble Cedex 9, Frne *e-mil: wolfgng.wernsdorfer@grenoble.nrs.fr ARTICLES Published online: 4 Otober 26; doi:1.138/nnno A superonduting quntum interferene devie (SQUID) with single-wlled rbon nnotube (CNT) Josephson juntions is presented. Quntum onfinement in eh juntion indues disrete quntum dot (QD) energy level struture, whih n be ontrolled with two lterl eletrostti gtes. In ddition, bkgte eletrode n vry the trnspreny of the QD brriers, thus permitting hnge in the hybridiztion of the QD sttes with the superonduting ontts. The gtes re lso used to diretly tune the quntum phse interferene of the Cooper pirs irulting in the SQUID ring. Optiml modultion of the swithing urrent with mgneti flux is hieved when both QD juntions re in the on or off stte. In prtiulr, the SQUID design estblishes tht these CNT Josephson juntions n be used s gte-ontrolled p-juntions; tht is, the sign of the urrent phse reltion ross the CNT juntions n be tuned with gte voltge. The CNT-SQUIDs re sensitive lol mgnetometers, whih re very promising for the study of mgnetiztion reversl of n individul mgneti prtile or moleule pled on one of the two CNT Josephson juntions. The superonduting quntum interferene devie (SQUID) hs been used very suessfully for mgnetometry nd voltge or urrent mesurements in the fields of mediine, metrology nd other fields of reserh 1,2. It ombines two quntum properties of superondutivity: the tunnelling of Cooper pirs through nonsuperonduting medium (the Josephson effet 3 ) nd flux quntiztion in superonduting loop. The direturrent version of this devie (DC-SQUID) is omposed of superonduting loop hving two Josephson juntions. Its most striking property is tht the mximum superonduting urrent flowing through the devie n be periodilly modulted by the mgneti flux entering the loop 4, with period equl to the flux quntum. Reently, miniturized versions of these devies hve been used to implement phse qubits 5 or to mesure quntum mgnetiztion reversl of nnoprtiles 6 nd singlemoleule mgnets 7. There hs lso been enormous progress in the understnding of the eletroni nd mgneti properties of isolted moleulr systems, reveling intriguing new physis. In the erly experiments, the mesuring devies were omposed of two nnoeletrodes nd bridging moleule, llowing the mesurement of the eletroni trnsport through single moleules 8. In suh devies, mesurements re performed t the moleulr level, the observbles re referred to moleulr orbitls, nd not to Bloh wves, s in mssive mterils. New rules were then found for these systems nd it beme possible to diretly probe the quntum properties of the moleule. For exmple, the tunnelling of eletrons through moleulr juntions n show the presene of Coulomb-blokde 8, Kondo oupling 9,1 nd shell filling 11, depending on the intensity of oupling between the moleule nd the eletrodes. Reent progress in improving the ontt of CNTs to superonduting nnoeletrodes hs shown tht suh moleules n ommodte superonduting urrent 12 nd behve s gte-ontrolled Josephson juntions 13,14. These CNT juntions re in the quntum-onfined regime nd n thus t s gte-ontrolled QDs, tht is, devies reminisent of the well-studied superonduting single-eletron trnsistors 15, but with strong quntum onfinement. It ws lso predited tht reverse superonduting urrent (Josephson urrent) would tke ple in juntion involving tunnelling through QD populted with n odd number of eletrons Suh juntion is lso lled p-juntion beuse the spin ordering of the Cooper pir tunnelling through the juntion is reversed; tht is, the Cooper pir wve funtion is multiplied by phse ftor e ip. We ombine here the reserh re of SQUIDs with tht of eletroni trnsport through moleules in order to design new detetor: SQUID with moleulr Josephson juntions mde out of CNTs. This system llows us to ddress the problem of resonnt tunnelling through QD hving disrete energy levels nd oupled to superonduting eletrodes. The SQUID hs the dvntge of linking the phse ross both juntions with the mgneti flux entering the loop. It then yields insight into the urrent phse reltion ross QD oupled to superondutors. In prtiulr, gte-ontrolled trnsition from the norml to the p-juntion n be observed. Owing to the geometril spets of CNTs, suh SQUIDs re lso very promising in the study of spin sttes of n individul mgneti moleule pled on one of the two CNT Josephson juntions. RESULTS AND DISCUSSION SQUID DESIGN AND CNT JUNCTION We designed nd built the first CNT-SQUID s presented in Fig. 1 nd desribed in the Method setion. The superonduting SQUID nture nnotehnology VOL 1 OCTOBER Nture Publishing Group

3 G1 CNT b Off On ΔE 2Δ g G2 5 nm V G V G1 V G1 V G1 I II III On On Off On Off Off V G2 V G2 V G2 Figure 1 Devie nd opertion sheme., Typil devie geometry of the CNT-SQUID with two lterl gtes G1 nd G2 (oloured in gold). The tomi fore mirosope (AFM) imge shows the SQUID loop (grey), whih is interrupted by the two CNT Josephson juntions with length of bout 2 nm. The single-wlled CNT with dimeter of bout 1 nm ws loted using n AFM, nd Pd/Al (3/5 nm) ligned eletrodes were deposited over the tube using eletron-bem lithogrphy. b, Energy level shemtis of QD between two superonduting leds. The energy level sping DE nd the superonduting gp 2D g re indited. The position of the quntum levels n be tuned with gte voltge V G. Only when quntum level is djusted to the Fermi energies of the superonduting leds (green urve) n strong superurrent flow between the leds., Shemtis of the CNT-SQUID with two nnotube juntions, whih n be tuned with the gte voltges V G1 nd V G2. In se I, both juntions hve quntum level djusted to the Fermi energy of the leds (on-resonne) nd mximl superurrent n flow through the devie. In ses II nd III, one nd two juntions re tuned off-resonne, respetively. loop is mde from Al, nd hs ritil temperture T 1.2 K. If not stted differently, ll mesurements were performed t ryostt temperture of bout 35 mk. The CNT-SQUID ontins two CNT-bsed superonduting trnsistors, whih hve been desribed previously 13,14 nd n be modelled by QD between two superonduting leds (Fig. 1b). The position of the quntum levels n be tuned with gte voltges V G1 nd V G2. When quntum level is ligned with respet to the Fermi energy of the superonduting leds ( on stte in Fig. 1b), superurrent of few nnomperes n flow by resonnt tunnelling through the CNT. When the quntum levels re fr from the Fermi energy ( off stte in Fig. 1b), the superurrent is strongly redued (typilly by ftor of 1 to 1 3 ). However, we show in the following tht the superurrent is not ompletely suppressed. This effet, not seen before 13, might be due to Josephson effet vi non-zero eletron density between the levels of the CNT nd higher-order tunnel proesses. Further studies re needed to understnd this effet. ELECTRONIC TRANSPORT PROPERTIES AND KONDO EFFECT Before turning our ttention to the CNT-SQUIDs, we hve to hrterize the eletroni trnsport properties of our CNT juntions. Prtiulrly importnt is the interply between Kondo orreltions nd superondutivity, whih hs reently motivted theoretil nd experimentl studies The Kondo orreltions re due to mgneti exhnge intertion between lolized mgneti moment nd free ondution eletrons. In order to minimize the exhnge energy, the ondution eletrons 54 tend to sreen the mgneti moment nd the ensemble forms spin singlet. It hs been predited tht Kondo resonne in spin 1/2 QD with n odd eletron number n oexist with superonduting stte. Indeed, for strongly hybridized QD sttes, it hs been theoretilly predited 16 tht the Josephson oupling ould be enhned by the Kondo resonne. This effet prtilly offsets the redution in Josephson oupling due to the Coulomb repulsion energy U. The Kondo resonnes of eh QD n be studied independently by mesuring the differentil ondutne di/dv s funtion of soure drin voltge V sd nd lterl gte voltge V G while keeping the hemil potentil of the other QD t onstnt vlue. Wheres V sd shifts the Fermi energy of the left led with respet to the right one, the lterl gte voltge V G tunes the position of the quntum levels with respet to both Fermi energies of the superonduting leds. The mesured ondutne mp of di/dv versus V sd nd V G exhibits the typil fetures of Coulomb dimonds (Fig. 2b), whih re onneted by Kondo ridges 23 of enhned ondutne. The enhned ondutne t zero bis results from the Kondo resonne when there re n odd number of eletrons on the QD. Note tht the Kondo ridges pper t the sme gte voltges s the ourrene of superonduting stte (Fig. 2). Moreover the mximum superurrent oinides with the most prominent Kondo ridge (lbelled d in Fig. 2b), ft tht supports the enhnement of superondutivity by the Kondo resonne 16. The tunnel rte G 1 mev/h (where h is Plnk s onstnt), the Coulomb energy U 6 mev, nd the energy-level sping DE 9 mev re nture nnotehnology VOL 1 OCTOBER Nture Publishing Group

4 b 6 4 V sd (mv) I (na) 2 α β γ δ e 2 /h kω Figure 2 Correltion between Kondo effet nd superondutivity., Colour-sle representtion of differentil ondutivity di/dv mp mesured t zero bis t 34 mk s funtion of the lterl gte voltges V G1 nd V G2 nd t bkgte voltge V BG ¼ V. A mgneti field of H z ¼ 5 mt ws pplied perpendiulr to the SQUID plne in order to suppress the superondutivity of the leds. The effet of ross-pitne ws subtrted in situ. The dotted line indites the gte voltge rnge of V G1 tht is further studied in b nd, wheres the dotted squre enloses the region tht is further studied in Fig. 5b nd. b, di/dv mp s funtion of V G1 nd soure drin voltge V sd. V G2 ¼ 6 V nd H z ¼ 5 mt. Note tht nd b hve the sme olour ode for di/dv. The dotted lines in b indite the Coulomb dimonds, whih re onneted by Kondo ridges lbelled, b, g nd d. The ondutnes in the middle of the Kondo ridges re 1.2,.72,.46 nd 1.45 e 2 /h for, b, g nd d, respetively. (See Supplementry Informtion for disussion of the determintion of the Kondo temperture.), Differentil resistivity dv/di mp of the sme V G1 region nd for V G2 ¼ 26 V, but for H z ¼. A superurrent is observed in the blk regions, whih orrespond to the Kondo ridges in b. 3 b I II III e 2 /h na Figure 3 Correltion between norml-stte ondutne nd superonduting swithing urrent., Colour-sle representtion of typil differentil ondutivity di/dv mp t 34 mk nd bkgte voltge V BG ¼ 26 V s funtion of the lterl gte voltges V G1 nd V G2.AmgnetifieldofH z ¼ 5 mt ws pplied perpendiulr to the SQUID plne in order to suppress the superondutivity of the leds. The effet of ross-pitne ws subtrted in situ. b,, Colour-sle representtions of the swithing urrent I sw t 3 mk s funtion of V G1 nd V G2 t the sme bkgte voltge. Mgneti fields H z ¼ nd1.3mt were pplied in b nd, respetively. The ltter orresponds to hlf flux quntum (h/4e ). I sw is mximl or miniml when both juntions re tuned on- or off-resonne, respetively (the three ses I, II nd III of Fig. 1 re indited). Eh pixel orresponds to single mesurement of I sw. obtined from the size nd shpe of the Coulomb dimonds. Using DE ¼ hv F /2kL, where v F ¼ ms 21 is the Fermi veloity in the CNT, k ¼ 1 for orbitl nd spin degeneries, k ¼ 2 for only spin degenery, nd L is the length of the CNT. We obtined L ¼ 186 nm for k ¼ 1, whih is in greement with the length of 2 nm estimted using tomi fore mirosopy nture nnotehnology VOL 1 OCTOBER Nture Publishing Group

5 (AFM). This suggests tht our CNTs hve orbitl nd spin degeneries. However, the gte dependene does not exhibit ler fourfold symmetry s expeted for suh degenerte QDs 11 but rther n even odd behviour 26, whih my hve two origins. First, hnnel mixing might our t the trnsprent ontts. More probble, however, is the influene of defets indued either by the intertion of the CNT with the substrte, or by struturl imperfetions inside the nnotube. These defets lower the symmetry of the system nd thereby lift the orbitl degenery 27. CNT-SQUID CHARACTERISTICS The opertion of the CNT-SQUID is bsed on the quntum phse interferene of the superurrent flowing through two CNT-bsed superonduting trnsistors 13 in superonduting ring (Fig. 1). The position of the quntum levels in eh juntion n be individully tuned using the two lterl gte voltges V G1 nd V G2, nd the trnspreny of the CNT ontt brriers n be globlly djusted using the bkgte voltge V BG. In order to find the gte voltges required to djust the levels of eh CNT juntion with respet to the Fermi energy of their ontts, we mesured the differentil ondutne di/dv t zero bis s funtion of V G1, V G2 nd V BG in field of H z ¼ 5 mt to drive the superonduting leds to the norml stte (Figs. 2 nd 3). The mps follow the stbility digrm of two QDs in prllel, tht is, hequerbord pttern with high-ondutne sttes hving vlues of the order 4e 2 /h (where e 2 /h is the ondutne quntum) entred on points where both CNTs re on-resonne, nd low-ondutne sttes (typilly between.1 nd.5 e 2 /h), where both CNTs re off-resonne. At smll pplied fields, the CNT juntions trnsmit superurrent. We define the swithing urrent I sw s the mximum dissiptionless urrent tht the Josephson juntions n pss through the devie. I sw is mesured by rmping the urrent t onstnt sweep rte from zero to I sw, t whih point voltge drop is mesured ross the juntion. Fig. 3b, presents I sw s funtion of V G1 nd V G2 in zero field nd in perpendiulr mgneti field H z ¼ 1.3 mt orresponding to mgneti flux, penetrting the SQUID loop, of F e ¼ F /2 ¼ h/4e where F is the mgneti flux quntum. Compring the di/dv mp (Fig. 3) with the I sw mp (Fig. 3b,) shows tht I sw is mximl (bout 6 na) t mximl ondutne in the norml stte, tht is, when both CNTs re on-resonne. I sw is bout two times smller when only one CNT is on-resonne. Typil voltge versus urrent hrteristis nd field modultions of I sw re presented in Fig. 4. The overll flux modultion of the CNT-SQUID n be understood by using stndrd SQUID model 2, whih predits the field dependene of the mximl superurrent, lled the ritil urrent I : I ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ði 1 I 2 Þ 2 þ 4I 1 I 2 os 2 ðp F e =F Þ Here, I i re the ritil urrents of eh Josephson juntion (i ¼ 1 nd 2), whih n be tuned independently with the gte voltges V G1 nd V G2. I is n upper bound of the experimentlly observed swithing urrent I sw, beuse the former neglets environmentl effets like temperture, eletroni noise nd quntum effets. The field modultion depth is mximl when I 1 nd I 2 re omprble (Figs. 3 nd 4), but it is miniml when the differene between both urrent mgnitudes is mximl. For short juntions, the mximl superurrent t zero temperture for single-hnnel juntion is given by I i ¼ ed g /h, where 2D g is the superonduting gp nd h is the redued Plnk onstnt. 56 ð1þ V (μv) V (μv) I (na) I (pa) Beuse of proximity effets between the Al nd the Pd ontt lyers, the effetive gp of the ontting bilyer is redued to 2D g.12 mev (dt extrted from the temperture dependene of di/dv(t); see Supplementry Informtion). We yield I i ¼ 15 na, whih is further redued when the symmetry of the brrier is tken into ount 28. Experimentlly, we do not mesure I i, but mximl swithing urrent I sw 3 na per juntion for the devie presented here. The highest vlue hieved so fr with our single-wlled CNT juntion is I sw 5 na. The disrepny between I i nd I sw is due to the ft tht the ltter is stohsti vrible influened by temperture, eletroni noise nd quntum effets. The effet of the eletromgneti environment n drstilly redue I sw, espeilly for smll, underdmped, urrent-bised Josephson juntions 29. Another mnifesttion of the eletromgneti environment is tht the superonduting brnh exhibits smll bending towrds non-zero voltge (Fig. 4,) due to retrpping effets 3. CNT p-junction A loser investigtion of the field modultion shows strong devitions from eqution (1). In prtiulr, when the ontt brriers re inresed with the bkgte voltge, redued Kondo effet is found (see Supplementry Informtion) nd, under ertin gte voltges, the ontrst of the interferene fringes drops to zero nd finlly exhibits phse shift, leding to minimum swithing urrent t zero field (Fig. 5). Suh n effet, in whih the minimum energy stte of one of the Josephson juntions (the so-lled p-juntion) is obtined for phse differene of p insted of zero, hs been the objet of intense studies in the pst dede. It hs been reported experimentlly s onsequene of d-wve superondutivity 31, b I SW (pa) I SW (na) d μ H z (mt) µ H z (mt) Figure 4 CNT-SQUID hrteristis.,, Voltge versus urrent urves t three pplied fields orresponding to zero (red), qurter (green) nd hlf (blue) flux quntum for the situtions when both juntions re tuned onresonne () or off-resonne () nd orrespond to mximl nd miniml swithing urrents in Figs. 3b nd 2, respetively. The urrent ws swept t sweep rtes of.5 na s 1 nd 5 pa s 1 for nd, respetively. Note tht eh urve is single sweep (no dt verging ws performed). b,d, Field modultion of the swithing urrent I sw for the situtions in nd, respetively. Eh point in b nd d orresponds to single mesurement tken t frequenies of 1 nd.1 Hz, respetively. The field periodiity times the re of the SQUID loop is in good greement with flux quntum (h/2e ). nture nnotehnology VOL 1 OCTOBER Nture Publishing Group

6 1. b I (na) na.4.2. V 9.7 V 1. V 1.9 V μ H z (mt) π % 8% d e π π π.2.4 na π 5% 75% Figure 5 Gte-ontrolled p-juntion CNT-SQUID hrteristis., Field modultion of the swithing urrent I sw for grounded bkgte (V BG ¼ V),where wek Kondo oupling ws observed. Using the lterl gte voltge V G1 while keeping the gte voltge V G2 t 12.5 V, the flux modultion n be driven from n even (-juntion SQUID, blk) to n odd (p-juntion SQUID, blue) urve. In the viinity of the phse reversl, the field modultion exhibits ler distortion from sine dependene, suggesting peulir urrent phse reltion in tht se. Eh point orresponds to single I sw mesurement (no verging). b e, Colour-sle representtions of the mximum swithing urrent I sw (b nd d) nd normlized field modultion mps ( nd e) s funtion of the lterl gte voltges V G1 nd V G2 for two different bkgte voltges V BG ¼ ( nd b) nd1v(d nd e). Eh pixel orresponds to single mesurement of I sw. The V G1 V G2 zone in b nd is indited by the dotted squre in Fig. 2. Red nd blue regions in nd e orrespond to even () nd odd (p) flux modultion, respetively. b, nd d,e show the situtions when only one or two juntions re tuned vi p-trnsition, respetively. Note tht in the entre of e, both juntions hve p-shift, leding to n even flux modultion. The sizes nd positions of the orresponding Kondo ridges in the norml stte (see Supplementry Informtion) re shemtilly indited by dotted lines in nd e. when tunnelling ours through ferromgneti lyer 32,33. Lol ontrol of the sign of Josephson urrent ws lso proved possible 34 by tuning the lol density of stte of n SNS juntion 35. Reminisent of ferromgneti impurity in Josephson juntion, it ws predited tht reverse Josephson urrent would tke ple in juntion involving tunnelling through QD populted with n odd number of eletrons For strong Kondo effet (Fig. 3), the Josephson oupling is expeted to be positive (-juntion) beuse the lolized spin is sreened due to the Kondo effet. On the other hnd, for wek Kondo effet (Fig. 5), the lrge on-site intertion only llows the eletrons in Cooper pir to tunnel one by one vi virtul proesses in whih the spin ordering of the Cooper pir is reversed, leding to negtive Josephson oupling (p-juntion). This predition n be demonstrted with our CNT-SQUID beuse it links the phse ross both juntions to the mgneti flux entering the loop. When one of the two juntions hs negtive Josephson oupling, the flux modultion is odd, tht is, I sw hs minimum t zero field. However, when both juntions hve positive or negtive Josephson oupling, the flux modultion is even, tht is, I sw hs mximum t zero field. Beuse the number of eletrons in the QD is odd in the middle of Kondo ridge nd even t the outside, gte voltge llows us to vry the QD juntion from positive to negtive Josephson oupling. Fig. 5 detils the situtions when only one (Fig. 5b,) or two (Fig. 5d,e) juntions re tuned vi p-trnsition. Suh CNT devies re therefore very promising in irumstnes where gte-ontrolled phse-reltions re needed. For exmple, theoretil works hve shown tht p-josephson juntion hs nture nnotehnology VOL 1 OCTOBER Nture Publishing Group

7 protetion ginst deoherene 36. Consequently, p-juntion SQUID n be well suited to implementing qubit with long oherene time 37. PROSPECTS FOR DETECTING SINGLE MAGNETIC MOMENTS In designing CNT-SQUID, our motivtion is to use it s detetor for mgnetiztion swithing of the mgneti moment of single moleule. The spet rtio of CNTs mkes them idel for oupling to single nnometre-sized objets. For exmple, it hs been shown tht semionduting nnotube properly funtionlized nd operted t the ondution threshold hs the bility to sense the binding of single moleule by eletrostti oupling 38. In this pper, we present devie tht we wnt to use to detet the swithing of few mgneti moments. Indeed, SQUIDs re the most sensitive mgneti flux detetors 2,39. The mgneti flux vrition DF is relted to the mgnetiztion hnge DM ssoited with the reversl of mgneti moments: DF ¼ DM, where is the flux oupling ftor determined by the geometry of the devie nd the smple. The flux sensitivity of SQUID is limited by the quntum limit, whih hs been hieved experimentlly by severl groups 2,4. However, the high sensitivity of suh SQUIDs nnot be used to detet the mgnetiztion reversl of single nnoprtiles or moleules beuse of the very smll flux oupling ftor. An improvement ws hieved with plnr mirobridge DC-SQUIDs (Fig. 6). A 3-nm prtile ws pled on top of mirobridge with rosssetion of 5 2 nm 2 (Fig. 6b). The oupling ws strong enough to study 1 3 mgneti moments 41,42. However, the flux oupling ftor ws rther poor (Fig. 6b). In the se of CNT juntion (Fig. 6), nnometre-sized moleule ould be pled diretly on the CNT, whih hs ross-setion of bout 1 nm 2. We therefore expet nerly optimized oupling ftor, beuse the moleule size nd the juntion ross-setion re omprble (Fig. 6d). The preise lultion of is diffiult in suh nerfield sitution for whih the dipolr pproximtion is not vlid. However, rough estimte of the mgneti signl of Mn 12 moleule, sitting on the CNT, yields flux vrition of 1 24 flux quntum, whih should be within the sensitivity of our mesurements (see Supporting Informtion for the first estimtion of the flux sensitivity of CNT-SQUIDs). In order to detet rpidly the swithing of mgnetiztion, we often use the SQUID s threshold detetor 41,42. This method, lled the old mode, onsists in bising the SQUID lose to the swithing urrent. The mgnetiztion reversl triggers trnsition of the SQUID from the superonduting to the norml stte beuse of the fst mgnetiztion hnge nd/or the ssoited dissiption. A dv/dt pulse n then be esily deteted on the urrent led bising the SQUID beuse of the hystereti behviour of the SQUID. The sensitivity of the old mode sles roughly with the inverse of the swithing urrent; we therefore expet strong improvement with the CNT-SQUIDs, whih hve tunble swithing urrent in the nno nd piompere regions insted of the mirompere region of previous SQUIDs 41,42. Another importnt feture of the CNT-SQUIDs onerns the bility to tune the oupling between the detetor nd smple. Indeed, the superurrent through the juntion n be swithed on nd off using the lterl gte. In the off stte, the mgneti moleule is deoupled from the mesuring devie nd it n evolve without deoherene oming from the devie. In order to mesure the mgnetiztion stte of the moleule, the SQUID is then swithed on. This should hve importnt onsequenes s it llows us to limit the bk-tion of the CNT-SQUID on the quntum stte of single moleule mgnet. 58 Prtile Further improvement of the flux sensitivity ould be hieved with new redout sheme, whih probes the SQUID s nonliner indutne rther thn its resistne 43. The SQUID is driven with mirowve frequeny.. urrent ner bifurtion point where two osilltion sttes exist 44. These superonduting dynmil sttes differ in mplitude nd phse, nd re ssoited with zero d.. voltge. They n be distinguished by mesuring hnge in mplitude or phse of the.. voltge ross the SQUID 43.Inthis non-dissiptive, dispersive SQUID mgnetometer, the swithing from one dynmil stte to the other would signl hnge in spin stte of the moleule. As the working temperture of our devies ws limited to few hundred millikelvin, further improvements ould be hieved by using other superonduting mterils with higher T, suh s Nb, Sn nd NbTi. In onlusion, the CNT-SQUIDs provide new genertion of ultrsensitive mgnetometers of nnometre-sized smples. Suh devies lso offer the opportunity to test interesting physil phenomen rnging from Kondo physis to p-juntions, nd pve the wy for non-lolity experiments by generting pirs of entngled eletrons in nnotube 457. METHODS SC CNT juntion Mirobridge Josephson juntions Moleule SC 1, nm In order to build CNT-SQUID s presented in Fig. 1, we strted from degenertely n-doped silion substrte with 35-nm-thik thermlly grown SiO 2 lyer on top, whih ws used s bkgte. Single-wlled CNTs were prepred by the lser vporiztion method 48 t Rie University. They were dispersed in wter by sonition using sodium dodeyl sulphte s surftnt. The CNTs were deposited using ombing tehnique, whih llows good ontrol of the CNT density nd orienttion on the substrte 49. The sili surfe ws first funtionlized using stndrd silniztion tehnique, leding to self-ssembled monolyer of minopropyltriethoxysilne (Aldrih). The substrte ws then dipped for 5 min in the dispersion of CNTs nd withdrwn t onstnt veloity of 2 mm s 21. The smple ws thoroughly wshed in distilled wter in order to remove the surftnt from the nnotubes. The nnotube lotion ws imged by AFM, nd ligned e-bem lithogrphy ws rried out to pttern the SQUID loops nd the ontts. The fork geometry for the loop llowed us to fbrite both juntions from the sme nnotube. Metl eletrodes b d 2 nm.6 nm 1 nm 5 nm Substrte Substrte Nnoprtile Juntion Moleule Nnotube Figure 6 Shemtis of single-moleule studies using CNT-SQUIDs., Shemti of plnr mirobridge DC-SQUID on whih ferromgneti prtile is pled. The SQUID detets the flux through its loop, produed by the smple mgnetiztion. b, Cross-setion (5 2 nm 2 ) of mirobridge juntion on whih 3-nm prtile is pled. The mgneti field lines re shown in green. The flux oupling is poor beuse of the lrge mismth between the prtile size nd the juntion ross-setion., Shemti of one of the two juntions of CNT-SQUID seprting the superonduting (SC) leds. A nnometre-sized moleule sits on top of the CNT. d, Cross-setion (1 nm 2 ) of CNT juntion on whih.6-nm moleule is pled. The flux oupling is optimized beuse the moleule size nd the juntion ross-setion re omprble. nture nnotehnology VOL 1 OCTOBER Nture Publishing Group

8 were deposited using eletron-gun evportion nd thikness of 3 nm Pd followed by 5 nm Al ws used. Pd provides high-trnspreny ontts to the CNTs 5. Al is superondutor widely used in nnosle devies, hving ritil temperture of bout 1.2 K. Only devies with resistne below 3 kv nd no signifint gte effet t room temperture were used for our studies. In ddition to the bkgte, two lterl gtes G1 nd G2 were ligned to eh devie, llowing us to tune independently the eletroni properties of eh CNT juntion (Fig. 1). We fbrited bout 1 CNT-SQUIDs nd 3 CNT superonduting trnsistors using CNTs, ropes of CNTs nd multiwlled CNTs. Only devies hving individul CNTs re presented here. About 3% of ll devies worked similrly to the presented one. Fbrition filure ws used minly beuse of mislignment of the ontt pds nd lterl gtes, lowondutne nnotube metl ontts, nd semionduting CNTs. The properties of the superonduting ontts, nd our filtering system re disussed in the Supplementry Informtion. Reeived 22 My 26; epted 3 August 26; published 4 Otober 26. Referenes 1. Clrke, J., Clelnd, A. N., Devoret, M. H., Esteve, D. & Mrtinis, J. M. Quntum mehnis of mrosopi vrible: the phse differene of Josephson juntion. Siene 239, (1988). 2. Clrke, J. & Brginski, A. I. (eds) The SQUID Hndbook (Wiley-VCH, Weinheim, 24). 3. Josephson, B. D. Possible new effets in superondutive tunnelling. Phys. Lett. 1, (1962). 4. Jklevi, R. C., Lmbe, J., Silver, A. H. & Merereu, J. E. Quntum interferene effets in Josephson tunneling. Phys. Rev. Lett. 12, (1964). 5. Chioresu, I., Nkmur, Y., Hrmns, C. J. P. M. & Mooij, J. E. Coherent quntum dynmis of superonduting flux qubit. Siene 299, (23). 6. Wernsdorfer, W. et l. Mrosopi quntum tunneling of mgnetiztion of single ferrimgneti nnoprtiles of brium ferrite. Phys. Rev. Lett. 79, (1997). 7. Wernsdorfer, W. & Sessoli, R. Quntum phse interferene nd prity effets in mgneti moleulr lusters. Siene 284, (1999). 8. Tns, S. J. et l. 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A. & Dekker, C. Enzyme-oted rbon nnotubes s single-moleule biosensors. Nno Lett. 3, (23). 39. Kethen, M. B. & Kirtley, J. R. Design nd performne spets of pikup loop strutures for miniture SQUID mgnetometry. IEEE Appl. Superond. 5, (1995). 4. Mük, M., Welzel, C. & Clrke, J. Superonduting quntum interferene devie mplifiers t gighertz frequenies. Appl. Phys. Lett. 82, (23). 41. Wernsdorfer, W. Clssil nd quntum mgnetiztion reversl studies in nnometer-sized prtiles nd lusters. Adv. Chem. Phys. 188, (21). 42. Jmet, M. et l. Mgneti nisotropy of single oblt nnoluster. Phys. Rev. Lett. 86, (21). 43. Siddiqi, I. et l. RF-driven Josephson bifurtion mplifier for quntum mesurement. Phys. Rev. Lett. 93, 272 (24). 44. Siddiqi, I. et l. Diret observtion of dynmil bifurtion between two driven osilltion sttes of Josephson juntion. Phys. Rev. Lett. 94, 275 (25). 45. Reher, P. & Loss, D. Superondutor oupled to two Luttinger liquids s n entngler for eletron spins. Phys. Rev. B 65, (22). 46. Ben, C., Vishveshwr, S., Blents, L. & Fisher, M. P. A. Quntum entnglement in rbon nnotubes. Phys. Rev. Lett. 89, 3791 (22). 47. Bouhit, V. et l. Single-wlled rbon nnotube superondutor entngler: noise orreltions nd Einstein Podolsky Rosen sttes. Nnotehnology 14, 775 (23). 48. Thess, A. et l. Crystlline ropes of metlli rbon nnotubes. Siene 273, (1996). 49. Gerdes, S., Ondruhu, T., Cholet, S. & Johim, C. Combing rbon nnotube on flt metl insultor metl nnojuntion. Europhys. Lett. 48, (1999). 5. Jvey, A., Guo, J.,Wng, Q., Lundstrom, M. & Di, H. Bllisti rbon nnotube field-effet trnsistors. Nture 424, (23). Aknowledgements We thnk the TEAM group of LAAS (Toulouse) for their help in len room proesses, nd knowledge our prtiiption in the Interntionl GDR #2756 CNRS Siene nd Applitions of Nnotubes. We thnk F. Blestro, B. Brbr, H. Bouhit, E. Eyrud, I. Siddiqi nd C. Thirion for importnt ontributions nd disussions. This work ws supported by the EC-TMR Network QuEMolN (MRTN-CT ), the NoE Network MAGMANet, CNRS, nd Rhône-Alpes funding. Correspondene nd requests for mterils should be ddressed to W.W. Supplementry Informtion ompnies this pper on Author ontributions J.-P.C. fbrited the devies, nd W.W. oneived nd performed the experiments with help from J.-P.C. nd V.B. All uthors disussed the results nd ommented on the mnusript. Competing finnil interests The uthors delre tht they hve no ompeting finnil interests. Reprints nd permission informtion is vilble online t nture nnotehnology VOL 1 OCTOBER Nture Publishing Group