CNR IMM-Bologna Università dell!Aquila How to make graphene superconducting Gianni Profeta Physics Department, University of L`Aquila, Italy & Max-Planck Institute of Microstructure Physics, Halle, Germany Matteo Calandra and Francesco Mauri IMPMC, University of Paris 6, CNRS, Jussieu , Paris France Graphene: physical realization of fundamental phenomena in solid-state-physics Graphene: physical realization of fundamental phenomena in solid-state-physics • Ultra-relativistic particles near EF E = ±vF p, vF = c/300 A. H. Castro Neto et al. RMP (2009) Novoselov et al. Nature (2005) Lee et al., Science (2008) Novoselov et al, Science (2007) Graphene: physical realization of fundamental phenomena in solid-state-physics • Ultra-relativistic particles near EF E = ±vF p, vF = c/300 • Effective theory around EF −ivF σ · ∇ψ(r) = Eψ(r) A. H. Castro Neto et al. RMP (2009) Novoselov et al. Nature (2005) Lee et al., Science (2008) Novoselov et al, Science (2007) plateaux in r xy and therefore occur at half-integer n; see Figs 2 and 4), in agreement with theory14–17. Note, however, that from another perspective the phase shift can be viewed as the direct manifestation of Berry’s phase acquired by Dirac fermions moving in magnetic field20,21. Finally, we return to zero-field behaviour and discuss another feature related to graphene’s relativistic-like spectrum. The spectrum E = vanishing ±vF p,concentrations vF = c/300 implies of both carriers near the Dirac point E ¼ 0 (Fig. 3e), which suggests that low-T resistivity of the emerge only at expon absence of localization, explained qualitatively free path l of charge carr wavelength lF. Then, j cannot be smaller than is known to have failed which disorder leads t behaviour22,23. For 2D D and, accordingly, Mott’ broad theoretical conse should exhibit a minim tion was not expected value of ,e 2/ph, in dis Thus, graphene exhi for a 2D gas of particles the Schro¨dinger equatio Graphene: physical realization of fundamental phenomena in solid-state-physics • Ultra-relativistic particles near EF • Effective theory around EF −ivF σ · ∇ψ(r) = Eψ(r) • Cyclotron mass with carriers √ mc ∝ n Figure 3 | Dirac fermions of graphene. a, Dependence of B F on carrier concentration n (positive n corresponds to electrons; negative to holes). N isH. theCastro number b, Examples of fan diagrams used in our analysis7 to find B F.A. Neto et al. RMP (2009) associated with different minima of oscillations. The lower and upper curves are for graphene (sample of Fig. 2a) and a 5-nm-thick film ofNovoselov graphite with et al.a Nature (2005) similar value of B F, respectively. Note that the curves extrapolate to different origins, namely to N ¼ 1/2 and N ¼ 0. In graphene, curvesLee foretallal., n Science (2008) extrapolate to N ¼ 1/2 (compare ref. 7). This indicates a phase shift of p with al, Science (2007) respect to the conventional Landau quantization in metals.Novoselov The shift isetdue 14,20 to Berry’s phase . c, Examples of the behaviour of SdHO amplitude Dj plateaux in r xy and therefore occur at half-integer n; see Figs 2 and 4), in agreement with theory14–17. Note, however, that from another perspective the phase shift can be viewed as the direct manifestation of Berry’s phase acquired by Dirac fermions moving in magnetic field20,21. Finally, we return to zero-field behaviour and discuss another feature related to graphene’s relativistic-like spectrum. The spectrum E = vanishing ±vF p,concentrations vF = c/300 implies of both carriers near the Dirac point E ¼ 0 (Fig. 3e), which suggests that low-T resistivity of the emerge only at expon absence of localization, explained qualitatively free path l of charge carr wavelength lF. Then, j cannot be smaller than is known to have failed which disorder leads t behaviour22,23. For 2D D and, accordingly, Mott’ broad theoretical conse should exhibit a minim tion was not expected value of ,e 2/ph, in dis Thus, graphene exhi for a 2D gas of particles the Schro¨dinger equatio Graphene: physical realization of fundamental phenomena in solid-state-physics • Ultra-relativistic particles near EF • Effective theory around EF −ivF σ · ∇ψ(r) = Eψ(r) • Cyclotron mass with carriers √ Castro Neto mc ∝ net al.: The electronic properties of graphene 116 Energy !I!r" = D • Klein paradox E x D I φ III II θ 1 2 se i# ei!kxx+kyy" + # $ r 2 se 1 i!$−#" % ei!−kxx+kyy" , !25" V0 y # $ % 1 with # = arctan!ky / kx", kx = kF cos #, ky = kF sin #, and kF the Fermi momentum. In region II, we have !II!r" = φ % # $ a 1 2 s !e i" ei!qxx+kyy" + # $ b 2 s !e 1 i!$−"" % ei!−qxx+kyy" , !26" x with " = arctan!ky / qx" and FIG. 6. !Color online" Klein tunneling in graphene. Top: sche!27" qx = #!V0 − E"2/!v2F" − k2y , matic of the scattering of Dirac electrons by a square potential. Bottom: definition of the angles # and " used in the scattering and finally in region III we have a transmitted wave only, formalism in regions I, II, andFigure III. 3 | Dirac fermions of graphene. a, Dependence of B F on carrier $ % concentration n (positive n corresponds to electrons; to holes). 1 t negative i!kxx+kyy" 7 ! !r" = e , !28" III to N isH. theCastro number b, Examples of fan diagrams used in our analysis Neto et al. RMP (2009) #2find 1 1 seiB# F.A. with different minima !K!k" = !24" of oscillations. The lower and upper curves #2 ±ei"k . associated are for graphene (sample of Fig. 2a) and filmsof with a Nature =graphite sgn!E − V The coefficients witha s5-nm-thick = sgn!E" and !Novoselov et0".al. (2005)r, a, respectively. curves extrapolate to different similar value of B F, not andthe t are determined from the continuity of the wave We further assume that the scattering does mix the Noteb,that namely to 6, N we ¼ 1/2 and N ¼function, 0. In graphene, forthat nthe Lee etallal., Science (2008) has to which curves implies wave function In Fig. depict momenta around K and Korigins, ! points. p with to Nbarrier ¼ 1/2 (compare 7). This indicates a phase!Ishift !x = 0of, y" = !II!x = 0 , y" and !II!x the conditions the scattering process due extrapolate to the square of width ref.obey Novoselov et al, Science (2007) respect to the conventional Landau quantization in metals. The shift is Schrödinger equation, = D , y" = !III!x = D , y". Unlike the due D. 14,20 . c, Examples the behaviour amplitude Djfunction but not its Berry’s phase only needoftoSdHO match the wave The wave function in thetodifferent regions can be writ- of we $ % plateaux in r xy and therefore occur at half-integer n; see Figs 2 and 4), in agreement with theory14–17. Note, however, that from another perspective the phase shift can be viewed as the direct manifestation of Berry’s phase acquired by Dirac fermions moving in magnetic field20,21. Finally, we return to zero-field behaviour and discuss another feature related to graphene’s relativistic-like spectrum. The spectrum E = vanishing ±vF p,concentrations vF = c/300 implies of both carriers near the Dirac point E ¼ 0 (Fig. 3e), which suggests that low-T resistivity of the emerge only at expon absence of localization, explained qualitatively free path l of charge carr wavelength lF. Then, j cannot be smaller than is known to have failed which disorder leads t behaviour22,23. For 2D D and, accordingly, Mott’ broad theoretical conse should exhibit a minim tion was not expected value of ,e 2/ph, in dis Thus, graphene exhi for a 2D gas of particles the Schro¨dinger equatio Graphene: physical realization of fundamental phenomena in solid-state-physics • Ultra-relativistic particles near EF • Effective theory around EF −ivF σ · ∇ψ(r) = Eψ(r) • Cyclotron mass with carriers √ Castro Neto mc ∝ net al.: The electronic properties of graphene 116 Energy !I!r" = D • Klein paradox E x D I • High stiffness and high mobilityφ III II θ 1 2 se i# ei!kxx+kyy" + # $ r 2 se 1 i!$−#" % ei!−kxx+kyy" , !25" V0 y # $ % 1 with # = arctan!ky / kx", kx = kF cos #, ky = kF sin #, and kF the Fermi momentum. In region II, we have !II!r" = φ % # $ a 1 2 s !e i" ei!qxx+kyy" + # $ b 2 s !e 1 i!$−"" % ei!−qxx+kyy" , !26" x with " = arctan!ky / qx" and FIG. 6. !Color online" Klein tunneling in graphene. Top: sche!27" qx = #!V0 − E"2/!v2F" − k2y , matic of the scattering of Dirac electrons by a square potential. Bottom: definition of the angles # and " used in the scattering and finally in region III we have a transmitted wave only, formalism in regions I, II, andFigure III. 3 | Dirac fermions of graphene. a, Dependence of B F on carrier $ % concentration n (positive n corresponds to electrons; to holes). 1 t negative i!kxx+kyy" 7 ! !r" = e , !28" III to N isH. theCastro number b, Examples of fan diagrams used in our analysis Neto et al. RMP (2009) #2find 1 1 seiB# F.A. with different minima !K!k" = !24" of oscillations. The lower and upper curves #2 ±ei"k . associated are for graphene (sample of Fig. 2a) and filmsof with a Nature =graphite sgn!E − V The coefficients witha s5-nm-thick = sgn!E" and !Novoselov et0".al. (2005)r, a, respectively. curves extrapolate to different similar value of B F, not andthe t are determined from the continuity of the wave We further assume that the scattering does mix the Noteb,that namely to 6, N we ¼ 1/2 and N ¼function, 0. In graphene, forthat nthe Lee etallal., Science (2008) has to which curves implies wave function In Fig. depict momenta around K and Korigins, ! points. p with to Nbarrier ¼ 1/2 (compare 7). This indicates a phase!Ishift !x = 0of, y" = !II!x = 0 , y" and !II!x the conditions the scattering process due extrapolate to the square of width ref.obey Novoselov et al, Science (2007) respect to the conventional Landau quantization in metals. The shift is Schrödinger equation, = D , y" = !III!x = D , y". Unlike the due D. 14,20 . c, Examples the behaviour amplitude Djfunction but not its Berry’s phase only needoftoSdHO match the wave The wave function in thetodifferent regions can be writ- of we $ % plateaux in r xy and therefore occur at half-integer n; see Figs 2 and 4), in agreement with theory14–17. Note, however, that from another perspective the phase shift can be viewed as the direct manifestation of Berry’s phase acquired by Dirac fermions moving in magnetic field20,21. Finally, we return to zero-field behaviour and discuss another feature related to graphene’s relativistic-like spectrum. The spectrum E = vanishing ±vF p,concentrations vF = c/300 implies of both carriers near the Dirac point E ¼ 0 (Fig. 3e), which suggests that low-T resistivity of the emerge only at expon absence of localization, explained qualitatively free path l of charge carr wavelength lF. Then, j cannot be smaller than is known to have failed which disorder leads t behaviour22,23. For 2D D and, accordingly, Mott’ broad theoretical conse should exhibit a minim tion was not expected value of ,e 2/ph, in dis Thus, graphene exhi for a 2D gas of particles the Schro¨dinger equatio Graphene: physical realization of fundamental phenomena in solid-state-physics ! • Ultra-relativistic particles near EF • Effective theory around EF −ivF σ · ∇ψ(r) = Eψ(r) • Cyclotron mass with carriers √ Castro Neto mc ∝ net al.: The electronic properties of graphene LETTERS 116 Energy NATURE|Vol 438|10 November 2005 # $ % 1 • Klein paradox # $ 1 1 % r i!kxx+kyy" i!−kxx+kyy" The unusual response of massless fermions to a magnetic field is! !r" = e + , i# i!$−#" e D highlighted further by their behaviour in the high-field limit, at I se se 2 2 which SdHOs evolve into the quantum Hall effect (QHE). Figure 4 shows the Hall conductivity j xy of graphene plotted as a function of !25" electron and V hole 0 concentrations in a constant B. Pronounced QHE plateaux are visible, but E they do not occur in the expected sequence with # = arctan!ky / kx", kx = kF cos #, ky = kF sin #, and kF On the contrary, the plateaux j xy ¼ (4e 2/h)N, where N is integer. x correspond to half-integer n so that the first plateau occurs at 2e 2/hFermi momentum. In region II, we have the 2 and the D sequence is (4e /h)(N þ 1/2). The transition from the lowest hole (n ¼ 21/2) to the lowest electron (n ¼ þ1/2) Landau level (LL) 1 1 a b in graphene requires III 22 the same number of carriers (Dn ¼ 4B/ II i!qxx+kyy" i!−qxx+kyy" 12 ! !r" = e + , f 0 < 1.2 £ 10 cm ) as the transition between other nearest levels II i" i!$−"" e s !e s !e 2 2 (compare the distances between minima in r xx). This results in a φ ladderθof equidistant steps in j xy that are not interrupted when !26" passing through zero. To emphasize this highly unusual behaviour, Fig. 4 also shows j xy for a graphite film consisting of only two x graphene layers, in which the sequence of plateaux returns to with normal " = arctan!k / q " and y x and the first plateau is at 4e 2/h, as in the conventional QHE. We FIG. 6. !Color online" Klein tunneling in graphene. Top:graphene sche- and its twoattribute this qualitative transition between 2 2 layer counterpart to the fact that fermions in the latter exhibit a finiteqx = !V0 − E"2/!v " − k , !27" matic of the scattering of Dirac electrons by a square potential. F y mass near n < 0 and can no longer be described as massless Dirac Bottom: definition of theparticles. angles # and " used in the scattering and finally in region III we have a transmitted wave only, half-integer QHE in graphene has recently been suggested bya, Dependence of B on carrier formalism in regions I, II, The andFigure III. 315,16 | Dirac fermions of graphene. F two theory groups , stimulated by our work on thin graphite films7 concentration n (positive n corresponds to holes). 1 but unaware of the present experiment. The effect is single-particleto electrons; t negative i!kxx+kyy" 7 ! !r" = e , !28" and is b, intimately related to subtle properties of massless Dirac III to find of fan diagrams used in our analysis 1 fermions, Examples 1 seiB# F. N is the number in particular to the existence of both electron-like and 2 14–17 different minima of oscillations. The lower and upper curves !K!k" = . associated !24" . The latter can be Landau stateswith at exactly zero energy i"k hole-like ±e 2 viewedare as a direct consequence of the Atiyah–Singer index theorem for graphene (sample of Fig. 2a) and filmsof with a sgn!E − V coefficients r, a, witha s5-nm-thick = sgn!E" and ! =graphite 0". The that is important in quantum field theory and the theory of super18,19 respectively. curves extrapolate to different similar B F,fermions, .scattering For 2Dvalue masslessof Dirac the theorem guarantees strings b,that andthe t are determined from the continuity of the wave We further assume that the does not mix the Note the existence of Landau states at E ¼ 0 by relating the difference in namely to 6, N we ¼ 1/2 and N ¼function, 0. In graphene, all nthe wave function has to which curves impliesforthat In with Fig. depict momenta around K and Korigins, ! points. the number of such states opposite chiralities to the total flux p with to Nbarrier ¼ 1/2 (compare 7). This indicates a phase!Ishift system (magnetic field can be !x = 0of, y" = !II!x = 0 , y" and !II!x the conditions the scattering process through due extrapolate tothethe square ofinhomogeneous). width ref.obey where S(E) ¼ pk 2 is the area in k-space of the orbits at the Fermi energy E(k) (ref. 13). If these expressions are combined with the experimentally found dependences m c / n 1/2 and B F ¼ (h/4e)n it is straightforward to show that S must be proportional to E 2, which yields E / k. The data in Fig. 3 therefore unambiguously prove the linear dispersion E ¼ h! kc * for both electrons and holes with a common origin at E ¼ 0 (refs 11, 12). Furthermore, the above equations also imply m c ¼ E/c 2* ¼ (h 2n/4pc 2* )1/2 and the best fit to our data yields c * < 106 m s21, in agreement with band structure y calculations11,12. The semiclassical model employed is fully justified by a recent theory for graphene14, which shows that SdHO’s I amplitude can indeed be described by the above expression T/sinh(2p2k BTm c/!heB) with m c ¼ E/c 2* . Therefore, even though the linear spectrum of fermions in graphene (Fig. 3e) implies zero φ rest mass, their cyclotron mass is not zero. • High stiffness and high mobility # $ % # $ %A. H. Castro Neto et al. RMP (2009) # $ % # • Ambipolar # $ % Novoselov et al. Nature (2005) Lee et al., Science (2008) al, Science (2007) respect to the conventional Landau quantization in=metals. The shiftthe isetdue D , y". Novoselov Unlike Schrödinger equation, = D , y" = !III!x D. 14,20 . c, Examples the behaviour amplitude Djfunction but not its Berry’s phase only needoftoSdHO match the wave The wave function in thetodifferent regions can be writ- of we plateaux in r xy and therefore occur at half-integer n; see Figs 2 and 4), in agreement with theory14–17. Note, however, that from another perspective the phase shift can be viewed as the direct manifestation of Berry’s phase acquired by Dirac fermions moving in magnetic field20,21. Finally, we return to zero-field behaviour and discuss another feature related to graphene’s relativistic-like spectrum. The spectrum E = vanishing ±vF p,concentrations vF = c/300 implies of both carriers near the Dirac point E ¼ 0 (Fig. 3e), which suggests that low-T resistivity of the emerge only at expon absence of localization, explained qualitatively free path l of charge carr wavelength lF. Then, j cannot be smaller than is known to have failed which disorder leads t behaviour22,23. For 2D D and, accordingly, Mott’ broad theoretical conse should exhibit a minim tion was not expected value of ,e 2/ph, in dis Thus, graphene exhi for a 2D gas of particles the Schro¨dinger equatio Graphene: physical realization of fundamental phenomena in solid-state-physics ! • Ultra-relativistic particles near EF Room-Temperature Quantum Hall Effect in Graphene −iv σ , ·G.S.∇ψ(r) =& Eψ(r) theory around EF, H.L. Stormer , U. Zeitler •K.S.Effective Novoselov , Z. Jiang , Y. Zhang , S.V. Morozov , J.C.F Maan Boebinger , P. Kim A.K. Geim 1 2, 3 2 1 2 4 4 3 2* 1* 1Department of Physics, University of Manchester, M13 9PL, Manchester, UK of Physics, Columbia University, New York, New York 10027, USA 3National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA 4High Filed Magnet Laboratory, Radboud University Nijmegen, 6525 ED Nijmegen, Netherlands 116 Castro Neto et al.: The electronic properties of graphene 2Department √ monc a ∝ n scale, has been attracting intense Cyclotron mass with The • quantum Hall effect (QHE), one example of a carriers quantum phenomenon that occurs truly macroscopic LETTERS NATURE|Vol 438|10 November 2005 $ % $ % interest since its discovery in 1980 (1). The QHE is exclusive to two-dimensional (2D) metals and has elucidated many important aspects of quantum physics and deepened our understanding of interactingEnergy systems. It has also led to the establishment of a new metrological standard,1 1 1 r i!kxx+kyy" where S(E) ¼ pk 2 is2 the area in k-space of the orbits at the Fermi The unusual response of massless fermions to a magnetic field is! !r" = e + ei!−kxx+kyy" , that contains only fundamental constants of the electron charge e and the Planck constant h (2). As many other the resistance quantum h/e i # i! $ − # " D energy E(k) (ref. 13). If these expressions are combined with the highlighted further by their behaviour in the high-field limit, at I # #2 se 1/2 usually requires low temperatures T, typically below the boiling point of liquid helium (1).2 se quantum phenomena, observation ofmthe QHE and B F ¼ (h/4e)n it is experimentallythe found dependences which SdHOs evolve into the quantum Hall effect (QHE). Figure 4 c / n , whichsemiconductors as a function straightforward show that S must be by, proportional to E 2using shows the Hallwith conductivity j xy of graphene Efforts to extend the QHEto temperature range for example, small effective massesplotted of charge carriersofhave so far !25" yields E / k. The data in Fig. 3 therefore unambiguously prove the electron and hole concentrations in a constant B. Pronounced QHE V failed to reach T above 30K (3,4). These efforts are driven by both innate desire to0 observe apparently fragile quantum phenomena under linear dispersion E ¼ h! kc * for both electrons and holes with a plateaux are visible, but not occur in the expected sequence E they dotemperatures. ambient conditions to perform metrology at roomjor,¼at(4e least, liquid-nitrogen More robust quantum 2 # =states, arctan!ky / kx", kx = kF cos #, ky = kF sin #, and kF with /h)N, where N is integer. On the contrary, the plateaux common and originthe at pragmatic E ¼ 0 (refsneed 11, 12). Furthermore, the above x xy 2 2 2 1/2 2 implied by equations their persistence higher would added to investigate finer features and, possibly, allow higher (h n/4also pc * ) provide and the best freedom fit /h correspond to half-integer n so that of thethe firstQHE plateau occurs at 2e also implyto m c ¼ E/cT, * ¼ the Fermi momentum. In region II, we have 6 21 m sshow , in that agreement with band structure /h)(N þ 1/2). The transition from the lowestlattice – the to our data yields cHere, andof the sequence is (4e 2tightly quantization accuracy (2). in graphene – a single layer carbon atoms packed in a honeycomb crystal * < 10we y D 11,12 The at semiclassical model employed fully tojusticalculations hole (n ¼ 21/2)nature to the lowest electron (n ¼ þ1/2) Landau level (LL) behave as QHE can be observed .even room temperature. This isis due the highly unusual of charge carriers in graphene, which 1 1 a b fied by a recent theory for graphene14, which shows that SdHO’s in graphene requires the same number of carriers (Dn ¼ 4B/ III I II i!qxx+kyy" 12 22 massless relativistic particles (Dirac fermions) and move with little scattering under ambient conditions (5). ! !r" = e + ei!−qxx+kyy" , amplitude can indeed be described by the above expression f 0 < 1.2 £ 10 cm ) as the transition between other nearest levels II i " i! $ − " " 2 2 #2 s ! e #2 s ! e p kroom-T heB) withinmgraphene. though V(compare theclear distances between minima in r xx). This results in a T/sinh(2the Figure 1 shows QHE The Hall even conductivity xy reveals BTm c/! c ¼ E/c * . Therefore, φ θ that are not interrupted when the linear spectrum of fermions in graphene (Fig. 3e) implies zero ladder of equidistant steps in j xy conductivity Uxx plateaux at 2e2/h for both electrons and holes, while the longitudinal φ passing !26" through zero. To emphasize this highly unusual behaviour, rest mass, their cyclotron mass is not zero. approaches zero (<10:) exhibiting an activation energy 'E |600K Fig. (Fig.4 also 1B).shows The j xy for a graphite film consisting of only two x graphene layers, in which the sequence of plateaux returns to with normal " = arctan!k / q " and Fig. 1C). The quantization in Vxy is exact within an experimental accuracy of |0.2% (see 2 y x /h, as in the conventional QHE. We and the first plateau is at 4e survival of the QHE to such high temperatures can be attributed to the large cyclotron FIG. 6. !Color online" Klein tunneling in graphene. Top:graphene sche- and its twoattribute this qualitative transition between quantization into the fact that fermions in the latter exhibit a finiteq = !V − E"2/!v2 " − k2 , gaps !Z characteristic to Dirac fermions in graphene. Their energy layer counterpart !27" x 0 matic of the scattering of Dirac electrons by a square potential. F y mass near n < 0 and can no longer be described as massless Dirac 6 magnetic field B is described by E N v FBottom: is the # Fermi | 2e!BNdefinition | where vFof|10 them/s angles and " used in the scattering • Klein paradox $ % • High stiffness and high mobility $ % # and finally in region III we have a transmitted wave only, velocity and N an integer Landau level (LL)formalism number (5). in Theregions expression yields an energy3QHE The half-integer in graphene has recently been suggested bya, Dependence of B on carrier I, II, and III. Figure | Dirac fermions of graphene. F 15,16 , stimulated by our work on thin graphite films7 twoLandau theory groups gap 'E |2800K at B =45T if the Fermi energy EF lies between the lowest level concentration n (positive n corresponds to holes). 1 but unaware of the present experiment. The effect is single-particleto electrons; t negative i!kxx+kyy" N=0 and the first excited one N=±1 (inset, Fig. 1B). This implies that, in our 7 ! !r" = e , !28" and experiments is b, intimately related to subtle properties of massless Dirac III to N isH. theCastro number Examples of fan diagrams used in our analysis Neto et al. RMP (2009) #2find 1 offermions, 1 a factor seiB# F.A. 10. Importantly, at room temperature, !Z exceeded the thermal energy kBT by in particular to the existence of both electron-like and 14–17 different minima of oscillations. The lower and upper curves !k" = of other factors . thatassociated !24" . The latter can be hole-like Landau at exactly zero energy in addition to the large cyclotron gap, there are!aKnumber help thestateswith #2 ±ei"k viewed as a direct consequence of the Atiyah–Singer index theorem are forforgraphene (sample of Fig. 2a) and filmsof with a Nature =graphite sgn!E − V The coefficients witha s5-nm-thick = sgn!E" and QHE in graphene to survive to so high temperatures. First, graphene devices allow !Novoselov et0".al. (2005)r, a, that is important in quantum field theory and the theory of super13 -2 18,19 respectively. curves extrapolate to different similar B F,fermions, cm )further with only aassume single 2D that subband occupied, very high carrier concentrations (up to 10 We .scattering For 2Dvalue masslessof Dirac the theorem guarantees strings b,that andthe t are determined from the continuity of the wave the does not mix the Note existence of Landau states at E ¼ 0 by relating the difference in which is essential to fully populate the lowest LL even in ultra-high B. Thisthe is in contrast to namely to 6, N we ¼ 1/2 and N ¼function, 0. In graphene, forthat nthe Lee etallal., Science (2008) has to which curves implies wave function In with Fig. depict momenta around K and Korigins, ! points. the number of such states opposite chiralities to the total flux traditional 2D systems (for example, GaAs heterostructures) which are either depopulated p with to N field ¼ 1/2 (compare ref.obey 7). This indicates a phase!Ishift throughextrapolate system (magnetic can be inhomogeneous). !x = 0of, y" = !II!x = 0 , y" and !II!x the conditions the scattering tothethe already in moderate B or exhibit multiple subband occupation process leading to due the reduction ofsquare barrier of width Novoselov et al, Science (2007) respect to the conventional Landau quantization in metals. The shift is Schrödinger equation, = D , y" = !III!x = D , y". Unlike the due the effective energy gap to values well D. below !Z . Second, the mobility P of Dirac 14,20 . c, Examples the behaviour amplitude Djfunction but not its Berry’s phase only needoftoSdHO match the wave wave function in thetodifferent regions can be writ- of we fermions in our samples does not changeThe appreciably from liquid-helium to room particles. • Ambipolar • Room Temperature QHE $ % $ % ...but in the long list of graphene remarkable properties, a fundamental block is missing: SUPERCONDUCTIVITY Vol 446 | 1 March 2007 | doi:10.1038/nature05555 ...but in the long list of graphene remarkable properties, a fundamental block is missing: LETTERS NATURE | Vol 446 | 1 March 2007 mesoscopic samples, the QHE can be used to identify single layer devices. 75 30 Cooling down the devices below the critical temperature of the electrodes (Tc < 1.3 K) leads to proximity-induced superconductivity in the graphene layer. A direct proof of induced superconductivity is 20 0 I the observation of a Josephson supercurrent20. Figure 2a shows the current–voltage (I–V) characteristics of a single layer device. The curVG = –50 V 10 rent flows without resistance (no voltage drop at finite current) below VG = –7.5 V –7 5 the critical current, Ic (what we actually measure is the switching VG = +1 V current; the intrinsic Ic may be higher20,21). In our devices, Ic ranged 0 – 250 0 250 –10 0 10 from ,10 nA to more than 800 nA (at high VG). Remarkably, we have B (mT) I (n A ) measured proximity-induced supercurrents in all the devices that we c 1.2 d 60 100 tested (17 flakes in total, with several devices on some flakes), including four flakes that were unambiguously identified as single layer 2 ∆/3 30 graphene via QHE (the rest being probably two to four layers thick). 1 1 0 1.0 This clear observation demonstrates the robustness of the Josephson 0 effect in graphene junctions. (The data shown are representative of the –100 general behaviour observed; the measurements shown in Figs 1, 2a, c, = 4 . 5 G H z f 0.8 – 30 P1/2 (a.u.) d and 3 have been taken on the same device, whereas those in Figs 2b and 4 correspond to a different single layer device, shown in Fig. 1a.) ∆ 2∆ – 60 To investigate further the superconducting properties of our – 150 – 75 0 75 – 700 – 350 0 350 700 V (µV) I (n A) devices, we measured the dependence of Ic on magnetic field (Fig. 2b). The critical current exhibits an oscillatory Fraunhofer-like density only in the bottom one or two layers. For single Figure layers, the 2 | Josephson effect in graphene. a, Voltage, V, versus current bias, pattern, with at least six visible side lobes, which is indicative of a I, characteristics at various VG, showing a modulation of the critical current. position of the resistance maximum corresponds to the gate voltage Vol 446 | 1 March 2007 | doi:10.1038/nature05555 uniform supercurrent density distribution22. The periodicity of the Inset, current bias sweeps from negative to positive (red) and vice versa wethat the asymmetry in the main panel is due to hystereticPUBLISHED at which the Fermi energy is located at the Dirac point, V(blue), oscillations is in the ideal case expected to be equal to a flux quantum showing D, and ONLINE: 6 FEBRUARY 2011 | DOI: 10.1038/NPHYS1911 W divided by the junction area. The area that corresponds to the behaviour (the retrapping current is smaller than the switching current, as o the typically find that jVDj , 20 V. We unambiguously determine is typical for an underdamped junction). b, Colour-scale representation of 2.5 6 0.5 mT period is 0.8 6 0.2 mm2, which is in good agreement dV/dI(I,B) at T 5 30 mK (yellow-orange is zero, that is, the supercurrent single layer character of a device by quantum Hall effect (QHE) with the measured device area (0.7 6 0.2 mm2) determined from region, and red corresponds to finite dV/dI). The critical current exhibits a the atomic force microscope image. Applying a radio-frequency field measurements. Because the superconducting proximity series ofeffect oscillations described by a Fraunhofer-like pattern. c, Differential to the sample results in the observation of quantized voltage steps, resistance, dV/dI, versus V, showing multiple Andreev reflection dips below requires two closely spaced electrodes, we can only perform magneknown as Shapiro steps, in the I–V characteristics20. The voltage the superconducting energy gap. The dips in dV/dI occur at values of steps, of amplitude "v/2e (v is the microwave frequency), are a toconductance measurements in a two terminal configuration. In n is an integer number. d, a.c. Josephson effect. The V 5 2D/en, where manifestation of the a.c. Josephson effect, and are evident in Shapiro steps in the I–V characteristics appear when the sample is irradiated general, the conductance, G, measured in this way is a mixture of In the example, we applied 4.5 GHz microwaves, resulting Fig. 2d.Taylor The induced superconductivity itselfUchoa, also at finite with microwaves. Travis Dirks, L. Hughes, Siddharthamanifests Lal † , Bruno Yung-Fu Chen, Cesar Chialvo, 1 1 1 1 Hubert B. Heersche *, Pablo Jarillo-Herrero *, Jeroen B. Oostinga , Lieven M. K. Vandersypen in 9.3 mV voltage steps. Inset, colour-scale plot showing the characteristic bias in the form of subgap structure in the differential resistance due ongitudinal and Hall 1 signals, but at high fields G < jGHallj (this † Paul M.toGoldbart and Nadya Mason 23 * microwave amplitude (P1/2; a.u., arbitrary units) dependence of the a.c. & Alberto F. Morpurgo multiple Andreev reflections , as shown in Fig. 2c. This subgap 19 measureapproximation is exact at the Hall plateaus ). Indeed, theJosephson effect (orange and red correspond respectively to zero and finite structure consists of a series of minima at source–drain voltages When a low-energy is incident on anenables interfaceus between Fig. 2a the (discussed dV/dI). V 5 2D/enelectron (n 5 1,2…), which to determine super-below). The devices consist of normal-metal end ment of G versus VG at B 5 10 T shows clearly identifiable Hall plaa metal and superconductor, it causes the injection of a contacts (2 nm/50 nm Cr/Au bilayer) and SC tunnel probes18,19 conducting D. We findand D 5the 125generation meV, as expected ournm Ti/Al in the 2007 bottom one or two layers. For single layers, the recently discovered form of graphite only one Cooper pair into 18the gap, superconductor of (200for nm/30 Pb/In bilayer) (see Methods for details). The | Voldensity 446 | 1only March LETTERS (Fig. 1d), characteristic of the teaus atGraphene—a half-integer multiples of 4e2/hNATURE bilayers . For . 2D/e, the differential resistancetunnelling returns resistances to the through the SC probes were typically 10–100 position of the resistance maximum corresponds to the gate voltage atomic layer thick1—constitutes a new model system in condensed a hole that reflects backVinto the metal—a process known a b V (µV) I (nA) V SUPERCONDUCTIVITY Some experimental discoveries V (µV) I (nA) ostinga , Lieven M. K. Vandersypen dV/dI (kΩ) phene LETTERS LETTERS Transport through Andreev bound states in a graphene quantum dot Bipolar supercurrent in graphene LETTERS 2,3 at which thethat, Fermi energy matter physics, layer because itgraphene is the first material in which charge . This demonstrates evenis located in at the Dirac point, V QHE in single dI/dV (µS) IcRn (µV) I (nA) I (nA) µV) V (µV) (kΩ) T = 30 mK G (mS) 2.0 as Andreev reflection. In confined geometries, this process times larger than the end-to-end resistances. In all samples, the charge neutral point, as seen in the end-to-end can give rise to discrete Andreev bound states (ABS), which can b enable transport of supercurrents through non- conductance versus back-gate measurements, shows a large offset 2 mesoscopic the QHE canbeen beproposed useda to toidentify single the positive-gate sidelayer (see Fig. 1c,d for samples A and B, respecsuperconducting samples, materials and have recently 120realizing solid-state qubits1–3 . Here, we report tively). For example, sample A shows an asymmetric cone around as a2means of devices. 4 transport measurements of sharp, gate-tunable ABS formed in the Dirac point at Vg ∼ +17.5 V. The asymmetry and direction of are consistent with work-function mismatch (�W ) at the a superconductor–quantum (QD)–normal system Cooling down thedotdevices below therealized criticaloffset temperature of athe on an exfoliated graphene sheet. The QD is formed in graphene Cr/Au–graphene interface, which results in a transfer of charge that electrodes (Tc < 1.3 K)contact leadsastoaproximity-induced 3 superconductivity the surface potentials4 ; similar charge transfer effects have beneath a superconducting result of a work- equalizes 80 4,5 . Individual ABS form when the discrete previously been predicted4 and function mismatch in the graphene layer. A direct proof of induced superconductivity isobserved5 . Because of its low density 1 QD levels are proximity-coupled to the superconducting20 of states, graphene is efficiently doped by this charge transfer. The the observation of density a Josephson Figure 2a shows 2 interfacethe of the end contacts is dominated by the contact. Owing to the low of states ofsupercurrent graphene and the . Cr/Au–graphene work function of the Au. cur(This is expected because the Cr layer is so sensitivity of the QD levels to an applied gate voltage, the ABS layer current–voltage (I–V) characteristics of a single device. The VG = –50 V spectra are narrow be continuously tuned down to zero thin as to be discontinuous. It is also similar to the Ti/Au–graphene 10 0 40 and can a b rent flows resistance (no voltage drop atcontacts finite current) VG = –7.5 1 µmV measured in below ref. 5.) Therefore, �W ranges from 0.14 to energy by thewithout gate voltage. 1 0critical 1.04 eV (see Methods for calculation). The positive �W indicates Although signatures of Andreev reflection and bound states in the current, I (what we actually measure is the switching c VG = +1 V conductance have been widely reported6 , it has been difficult to hole (that is, p-type) doping of the graphene under the end leads. VD 20,21 current; the intrinsic be higher ). In our0The devices, Ic ranged c may 0 superconducting tunnel probe also leads to a local doping of directly probe individual ABS. IIn superconductor (SC)–graphene 0 250 –10 0 10 – 1 00 graphene. However, for the Pb–graphene interface, �W structures, most previous work has focused on the nature of 0 from ,10 nA to more than 800 nA (at high V ). Remarkably, we have 0 2 4 is −1.15 6 to ¬6 ¬4 ¬2 G 20the supercurrent in well-coupled – 40 – 2Josephson 0 0junctions7–10 20 . SC–QD – 4 0 B (mT 0 – 2) 0 I (n A ) −0.25 eV and is negative, implying local electron (n-type) doping. V (mV) V ( V ) V G (V ) measured proximity-induced supercurrents in all the devices g As illustrated in Figs that 1b andwe 2a,tunnel the �W doping generates a potential hybrids in graphene have not been studied, although recent work d 60 11–14 15 well underneath the SC tunnel probe, which acts as a confining has predicted and demonstrated that(V ABS) is can bedevices isolated by 100 tested (17 flakes in total, several on some flakes), includb Figure 3 | Bipolar supercurrent transistor behaviour and finite finite. Note that the critical current is supercurrent at the Diracwith point D 0.8 for a QD—that is, a pn junction. Quantum dots formed by coupling them to discrete QD energy levels. However, the ABS peaks potential asymmetry isas notsingle layer notflakes symmetric withwere respect unambiguously to VD. The origin of this supercurrent at the Dirac point. a, Colour-scale plot of dV/dI(VG,I). Yellow ing four that have been observed in carbon nanotubes20 , by means in previous SC–QD experiments were strongly broadened, eitheridentified by pn junctions B=1 0 T means zero, that is, the supercurrent region, and finite dV/dI increases via known, but a similar asymmetry is seen in the normal state conductance 15 16 2 ∆/3 3 0 the large lead density of states of work-function doping, and in graphene21 , where Klein tunnelling or by the lack of a tunnel barrier . In d graphene via QHE (the rest being probably two to four layers thick). c b, Letter, Product of the critical current times the normal state orange swept and (blue curve). 0 negative to positive values, = 10 Tfrom B =to 35dark mT red. The current is B 2.0 the work described in this sharp subgap conductance peaks through smooth barriers can lead to the confinement of carriers22,23 . 0.6 .5 the hysteresis associated with an underdamped normal state resistance is measured at resistance versus into VG. The is asymmetric owing2to This clear observation the robustness ofzero are obtained by tunnelling ademonstrates proximity-coupled QD formed The QD isthe alsoJosephson proximity coupled to the superconducting lead. T = 30 mK D, and we typically find that jVDj , 20 V. We unambiguously determine the 12 –2 a Hall effect (QHE)n (10 cm ) single layer character of a device by quantum –2 0 measurements. Because the superconducting proximity effect 200 b we can only requires two closely spaced electrodes, perform magnetoconductance measurements in a two terminal configuration. In 30this way is a mixture of general, the conductance, G, measured in longitudinal and Hall signals, but at high fields G < jGHallj (this approximation is exact at the Hall plateaus19). Indeed, 100 the measurement of G versus VG at B 5 10 T shows clearly 20 identifiable Hall pla2 teaus I at half-integer multiples of 4e /h (Fig. 1d), characteristic of the QHE in single layer graphene2,3. This demonstrates that, even in I (nA) c V a carriers behave as massless chiral relativistic particles. The anomalous quantization of the Hall conductance2,3, which is now understood theoretically4,5, is one of the experimental signatures of the peculiar transport properties of relativistic electrons b ain graphene. Other unusual phenomena, like 1 theµm finite conductivity 7 of5order 4e2/h (where e is the electron charge and h is Planck’s constant) at the charge neutrality (or Dirac) point2, have come as a surprise and remain to be explained5–13. Here we experimentally study the Josephson effect14 in mesoscopic junctions consisting of a gra0 phene layer contacted by two closely spaced superconducting electrodes15. The charge density in the graphene layer can be controlled by means of a gate electrode. We observe a supercurrent that, depending on the gate voltage, is carried by either electrons in the conduction band or by holes in the valence band. More impor–7 5 of tantly, we find that not only the normal state conductance graphene is finite, but also a finite supercurrent can flow at zero charge density. Our observations shed light on the special– role 2 5 0of time reversal symmetry in graphene, and demonstrate phase coherent electronic transport at the Dirac point. Owing to the Josephson effect14,16, a supercurrent can flow through c 1.2 a normal conductor placed between two closely spaced superconducting electrodes. For this to happen, transport indthe normal conductor must be phase coherent and time reversal symmetry (TRS) B = 35 mT must be present. In graphene, the Josephson effect can be investigated 2 .5 in the ‘relativistic’ regime15, where the supercurrent is carried 1.0 by Superconductivity: Is it expected? Superconductivity: Is it expected? The total electron-phonon coupling is : λ= N (0)D 2 M ω2 Superconductivity: Is it expected? The total electron-phonon coupling is : N (0) is vanishingly small, however it can be varied by rigid doping but it increases very slowly λ= N (0)D 2 M ω2 Superconductivity: Is it expected? The total electron-phonon coupling is : λ= N (0) is vanishingly small, however it can be varied by rigid doping but it increases very slowly The deformation potential, D, for in-plane modes is high, but 1/ω2 is low N (0)D 2 M ω2 Superconductivity: Is it expected? The total electron-phonon coupling is : λ= N (0) is vanishingly small, however it can be varied by rigid doping but it increases very slowly The deformation potential, D, for in-plane modes is high, but 1/ω2 is low By symmetry out-of-plane modes do not couple with πelectrons N (0)D 2 M ω2 Superconductivity in Graphite Superconductivity in Graphite Superconductivity in Graphite The interlayer state Superconductivity in Graphite The interlayer state The electron-phonon coupling: Cz coupling M. Calandra and F. Mauri, Phys. Rev. Lett (2005) Superconductivity in Graphite The interlayer state The Cambridge criterion M. S. Dresselhaus and G. Dresselhaus, Adv. in Phys. (2002) G. Csanyi et al, Nat. Phys. (2005) I. MazinPhilosophical and A. V. Balatsky Mag. Lett. (2010) MagazinePhil. Letters 735 The electron-phonon coupling: Cz coupling M. Calandra and F. Mauri, Phys. Rev. Lett (2005) 4. Correlation between the position of nearly free band, brought down to chemical The interlayer state It is a strongly 3D dispersive state living between carbon layers* * M. Posternak et al. Phys. Rev. Lett. (1983) L. Boeri et al. Phys. Rev. B (2007) 10 5 Energy (eV) 0 -5 -10 -15 -20 K M K K The interlayer state It is a strongly 3D dispersive state living between carbon layers* * M. Posternak et al. Phys. Rev. Lett. (1983) L. Boeri et al. Phys. Rev. B (2007) But it exists even in multiwall nanotubes, fullerenes, BN, MgB2 and......... 10 10 5 5 0 0 Energy (eV) Energy (eV) .......graphene -5 -5 -10 -10 -15 -15 -20 Γ≡Γ’ Γ≡K’ -20 K M K K K M K K Use the interlayer state: put it at EF Interlayer state is a truly 2D free-electron state in xy direction, ψ(x, y) = ei(kx x+ky y) However, in the z direction is not exactly “free”. a Kronig-Penney model of bulk GIC a En = 2 �2 π 2 n 2m a2 The interlayer state energy decreases removing the quantum confinement along z a→∞ that is Metal adatoms on Graphene % dpotential !k +(r, q,k" lascreened namely, termined intothe so-calle 0) in the golden rule, ij!k,k + q"dV jiSCF tions is identical the numbe G. Phonon frequencies interpolation over 'q(Fermi full width half own equation for the phonon linewidth Specifically, if the band-structure is !T " N this work we choose to with well controlled error. k ph group of bands, then the mat phonon momentum: Practical imple inN2kEq. 29, we obtain Allen ng ωqνa!FWHM", ximum that is, f !T " − f !T " posite group of bands and the number (T ) ki ph k+qj phdoped Graphene Metal ! Wannier functions (ML conditions illustrated in Refs. 29,30, the − certain' 4πωqν trix. On the contrary if the d y,= Under ν 2 The practical implementation of the abo tions is identical to the number of band Nk!Tph" |gkijnm (k, k#ki+−q)| #k+qjδ(εkn )δ(εk+qm ) nierization schemes are phonon linewidth can be reduced to: Nk!T" in a larger manifold, a prelim N (T ) mulation for the phonon frequencies proce k group of bands, then the matrix U (k We performed first-principles Density Functional Theory calculation of structural, electronic, mn k,n,m s 4# r Nk (T ) Fermi ( matrices Ulinear-response (k) are m ob 2 nm !k + q,k" !23" cedure from the manifold ! 'q(% d=ij!k,k + q"d ji !1" Perform a standard (g !k,k + q"( % nmNk (T ) trix. On the contrary if the desired bag (30) ν 2 ! N !T" of Refs. 14,28, which dynamical andk properties of)metal adatoms on Graphene |gnm (k, + k,m,n q)| 4πωδ(ε k superconducting the square matrix Umna(k) is p given phonon momentum q using Nk!T k+qm qν kn )δ(ε ν 2 2,29 heAllen-formula following γ definition of the deformation potential in a larger manifold, a preliminary dise ˜qν = is widely used, |gnmand (k, krepresents + q)| ×and a smearing calization of the final W for the electronic inte T k,n,m The deformation potential ph N (T ) k element: of"the !f knphonon-linewidth − f k+qm"k,m,n %!&k+qm − &due !q(electron", cedure !28" from the manifold will must be pe estimation kn − to quirement be essen !q , 0 , T ". C The electron-phonon matrix element (30) sr ph q, k) (seeU Eq.(k)24) are calcu2 the square matrix is obtained n effects, the absence of anharmonic effects and mn 2,29 inδ(% !2" In order to perform the secon of the electron-phonon )δ(% − % − ω ) (29) & v kn k+qm kn qν SCF widely used, a mula sand represents ( ds is s scheme. In this calculation q,k" qm& , kprocesses. + q" =! %=s(k eq(+In ·d !k + q&kn) , k"the / -2M ere gnm!k The potential matrix-e q Specifically, mn!k scattering addition, Allen-formula s!q!24" (. , ! , T0" #Eq. !23"$, calculateif deformat "sr!qdeformation mn the ba & u of the phonon-linewidth due to electronfor the deformation potenti qs absence of the electron-electron aobtain Eq. interaction 29, by weinelastic Allen neglecting ωqν asin measured element of24) Eq.are !24" on of thebands electron q,similar k)trix(see Eq. calculated usin snInFinally, the phonon-linewidth, posite group a s the absence of anharmonic effects and k+q d (k, k), as explained in se 29 here is the periodic the Bloch wave func-external pression involving only part the of derivative of the bare k points using a the smen composed ofthis Ntions formula namely, scheme. Inmn particula k!Tcalculation ph" is on or&kn) X-ray measurements, to the electron-phonon identical to ik·r processes. In&kn) addition, the Allen-formula *Nk andusing It is wave crucial to note that t Phonon linewidth ' ) = e / u is the Fourier trans.e., & !3" Generate functions &kn) and en ential is easily obtained Fermi golden rule. In the kn qs k for the deformation potential at zon ng as: group of bands, then th s N (T ) on-linewidth, as measured by inelastic |k + qm# entering in d (k + k . The integration in Eq. of the phonon displacement u !T " electron-momentum k-points N s denser Ls sence of electron-electron interaction our formalism jusmn k 0 ! dmn (k, k), as explained in sec. III B. if 4πωqν trix. On the contrary ν 2 γ is understood to be on the unit cell. The quantity vbare y the measurements, to the the|g electron-phonon !4")Perform a second nonself-consistent same wavefunctions used for qν (k, k + = q)| δ(ε )δ(ε γqν SCF replacement of derivative of the external kn k+qm nm Itdeformation-potential is crucial to thatmanifold, the period λthe , (31) in note a larger ap qν Nofk (T ) = eriodic part static self-consistent potential, 2 matrix element on dure. If this is not the case, s 2πω N k,n,m ential by the derivative of the static screened potential s s qν entering in d (k + q, k) hav cedure from the manifo mn y, VSCF!r" = vSCF!r"eiq·r. This static self-consistent po-|k +Nqm# !T " k-points grid using smearing T k 0(30) 0 and appear in the |kn#’s (i.e. a !r , 0" in the Fermi golden rule with a well-controlled CF γqν standard same wavefunctions used for the Wan the square matrix U ( is calculated using linear response at tem!T " grid to obtain the first the N mn 2,29 k ph N is the electronic density of states per spin at λ , (31) = routine or other c is widely used, and representsnalization a sThe Allen-formula or. 2 . re Tphqν deformation dure. the case, spuriouspot (u !qthis , ! , Tis "srIf 2πω N 0". notThe s qν rmi energy. good certain estimation of the illustrated phonon-linewidth to electronUnder conditions in Refs. due 31 and 32, the localization properties o q, k) (see 24) are appear in the |kn#’s (i.e. Eq. a phase add hphonon this definition, the isotropic Eliashberg-function phononlinewidth effects, in absence of anharmonic effects and real space are completely lo canthe reduced electronic density ofbestates pertospin at 165111-5 scheme. In computati this calcu nalization routine or other other scattering processes. In addition, the Allen-formula condition is to use a uniform y. The Eliashberg functionN !T" for the deformation po the localization properties of the Wan ! ! relates the phonon-linewidth, as measured by inelastic k k + q = ! s k + G with k sti 4 #! 1 d (k, k), as explained nition, q( Eliashberg-function real space are completely lost. A wa 2 the isotropic ( ω 2 1.04(1+λ) mn log Neutron X-ray measurements, to the electron-phonon ˜'or α F (ω) = λ ω δ(ω − ω ) (32) = (g !k,k + q"( grid. In this way |k+qn# can qνnmqν qν % T = exp{− } q( ∗ c It is crucial to note 1.20 λ−µ (1+0.62λ) 2N condition is to use a uniform grid cent q qν k!T" k,m,n coupling as: N ply by a phase sd ! multiplying !entering |k + qm# in d k + q = k + G with k still belongim 1 ! translation and no additional Metal doped Graphene • Ca is the first choice (CaC6 has Tc=11.5 K) • Ca adsorbs in the hollow site at 2.24 Å • IL at EF ? Metal doped Graphene • Ca is the first choice (CaC6 has Tc=11.5 K) • Ca adsorbs in the hollow site at 2.24 Å • IL at EF ? Monolayer 2 2 1 1 Energy (eV) Energy (eV) Bulk 0 -1 -2!K 0 -1 M K !K -2!K M K !K Ca on graphene: The interlayer state CaC6 mono CaC6 bulk |sK!IL| 2 0.6 0.4 0.2 00 1 2 3 4 5 6 7 8 z distance (Å) |sK!IL| 2 0.6 0.4 LiC6 mono LiC6 bulk 0.2 The IL moves away from graphene, reducing the electron-phonon coupling 00 7 1 2 3 4 8 5 6 with Cz modes z distance (Å) Ca on graphene: electron-phonon coupling 1600 1400 _ F(t"#$h%t" 1000 800 ! -1 Frequency (cm ) 1200 600 1.5 1 0.5 0 400 CaC6 0 1000 500 -1 Frequency (cm ) 0!K M K !K _ F(t"#$h%t" 200 1.5 1 LiC6 1500 ! _ F Bulk ! _ F Monolayer h%t" Bulk h%t" Monolayer ! The coupling with Cz modes, present in the bulk compound, is small in the monolayer case 0.5 The total electron-phonon coupling is λ= 0.40 with an extimated Tc= 1.4 K ( λ= 0.70 and Tc= 11.5 K) 0 0 1000 500 1500 -1 Frequency (cm ) Removal of quantum confinement reduces λ: Bulk better than Film Lithium doped Graphene • There is at least one case among GICs that can be further explored, stage-1 Lithium GIC, LiC6. • IL state is completely empty: the strong quantum confinement prevents its occupation • It is not superconducting Removal of confinement along c direction should bring the IL at the Fermi level Energy (eV) 2 1 0 -1 -2!K M K !K Lithium doped Graphene • There is at least one case among GICs that can be further explored, stage-1 Lithium GIC, LiC6. • IL state is completely empty: the strong quantum confinement prevents its occupation • It is not superconducting 2 2 1 1 Energy (eV) Energy (eV) Removal of confinement along c direction should bring the IL at the Fermi level 0 0 -1 -1 -2!K -2!K M K !K M K !K Lithium doped Graphene 400 200 0 !K M K !K Phonon DOS 2 Energy (eV) -1 Frequency (cm ) 600 1 • IL state at the Fermi level 0.8 0.6 0 M K • Softening of Li and Cz modes 0.4 0.2 0 -1 • Increase of DOS at EF -0.2 -0.4 -2!K -0.6 -0.8 M K !K ! _ F(t"#$h%t" 0 Lithium doped Graphene 1000 500 Electron-phonon coupling -1 Frequency (cm ) 0 1.5 LiC6 1500 ! _ F Bulk ! _ F Monolayer h%t" Bulk h%t" Monolayer 1 0.5 0 0 1000 500 -1 Frequency (cm ) 1500 • Bulk LiC6 is not superconducting (no interlayer state) •The interlayer state allows the coupling with “dormant” Cz modes • Increases the DOS at the Fermi level • Allows an additional intra-band and 2 inter-band scattering channels • Increases the total electron-phonon coupling to λ= 0.61 and Tc= 8.1 K Lithium on both sides of Graphene 2 1600 Energy (eV) 1 -1 Frequency (cm ) 1200 800 0 -1 -2 -3 -4 !K M K DOS (states/eV) 400 λ= 1.0 and Tc= 18 K 0 !K M K !K 0.5 1 1.5 " !_ F(t#$!h%t# !K 2 4 Conclusions • Rigid doping can not induce electron-phonon driven superconductivity • Metal adatoms on graphene promotes the interlayer state at the Fermi level • The coupling depends on the particular adatom, very different from the bulk counterpart • Lithium on graphene has a sizable electron-phonon coupling • and can induce superconductivity. “How to make graphene superconducting” G. Profeta, M. Calandra and F. Mauri cond-mat arXiv: xxx.xxxx Thank you for your attention
© Copyright 2024