Publications (77)452.85 Total impact
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ABSTRACT: We report a sudden reversal in the pressure dependence of Tc in the ironbased superconductor RbFe2As2, at a critical pressure Pc = 11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal Vshaped phase diagram for Tc vs P in these fully holedoped 122 materials, when measured relative to the critical point (Pc, Tc). From measurements of the upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe the same twofold jump in (1/Tc)(dHc2/dT) across Pc, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of Pc. We discuss a possible link between scattering and pairing, and a scenario where a dwave state favored by highQ scattering at low pressure changes to a state with s+ symmetry favored by lowQ scattering at high pressure.12/2014;  [Show abstract] [Hide abstract]
ABSTRACT: The Fermi surface of a metal is the fundamental basis from which its properties can be understood. In underdoped cuprate superconductors, the Fermi surface undergoes a reconstruction that produces a small electron pocket, but whether there is another, as yet undetected portion to the Fermi surface is unknown. Establishing the complete topology of the Fermi surface is key to identifying the mechanism responsible for its reconstruction. Here we report the discovery of a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a small quantum oscillation frequency in the thermoelectric response and in the caxis resistance. The fieldangle dependence of the frequency demonstrates that it is a distinct Fermi surface and the normalstate thermopower requires it to be a hole pocket. A Fermi surface consisting of one electron pocket and two hole pockets with the measured areas and masses is consistent with a Fermisurface reconstruction caused by the chargedensitywave order observed in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are removed by a separate mechanism, possibly the pseudogap.09/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Geometrical Berry phase is recognized as having profound implications for the properties of electronic systems. Over the last decade, Berry phase has been essential to our understanding of new materials, including graphene and topological insulators. The Berry phase can be accessed via its contribution to the phase mismatch in quantum oscillation experiments, where electrons accumulate a phase as they traverse closed cyclotron orbits in momentum space. The hightemperature cuprate superconductors are a class of materials where the Berry phase is thus far unknown despite the large body of existing quantum oscillations data. In this report we present a systematic Berry phase analysis of Shubnikov  de Haas measurements on the holedoped cuprates YBa$_2$Cu$_3$O$_{y}$, YBa$_2$Cu$_4$O$_8$, HgBa$_2$CuO$_{4 + \delta}$, and the electrondoped cuprate Nd$_{2x}$Ce$_x$CuO$_4$. For the holedoped materials, a trivial Berry phase of 0 mod $2\pi$ is systematically observed whereas the electrondoped Nd$_{2x}$Ce$_x$CuO$_4$ exhibits a significant nonzero Berry phase. These observations set constraints on the nature of the highfield normal state of the cuprates and points towards contrasting behaviour between holedoped and electrondoped materials. We discuss this difference in light of recent developments related to charge densitywave and broken timereversal symmetry states.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We report a sudden reversal in the pressure dependence of Tc in the ironbased superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at the zero temperature limit, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and investigate what parameters control this transition. Comparing samples of different residual resistivity, we find that a 6fold increase in impurity scattering does not shift Pc. From a study of Xray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and AsFeAs bond angle. The pressure dependence of these lattice parameters suggests that Pc should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2. Resistivity measurements under pressure reveal a change of regime across Pc, suggesting a possible link between inelastic scattering and pairing symmetry.Physical Review B 03/2014; 89(13). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In the quest to increase the critical temperature Tc of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field Hc2 is a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect Hc2 in the cuprates YBa2Cu3Oy, YBa2Cu4O8 and Tl2Ba2CuO6+δ, allowing us to map out Hc2 across the doping phase diagram. It exhibits two peaks, each located at a critical point where the Fermi surface of YBa2Cu3Oy is known to undergo a transformation. Below the higher critical point, the condensation energy, obtained directly from Hc2, suffers a sudden 20fold collapse. This reveals that phase competitionassociated with Fermisurface reconstruction and chargedensitywave orderis a key limiting factor in the superconductivity of cuprates.Nature Communications 02/2014; 5:3280. · 10.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Proximity to an antiferromagnetic phase suggests that pairing in ironbased superconductors is mediated by spin fluctuations, but orbital fluctuations have also been invoked. The former typically favour a pairing state of extended swave symmetry with a gap that changes sign between electron and hole Fermi surfaces (s+/), whereas the latter yield a standard swave state without sign change (s++). Here we show that applying pressure to KFe2As2 induces a sudden change in the critical temperature Tc, from an initial decrease with pressure to an increase above a critical pressure Pc. The smooth evolution of the resistivity and Hall coefficient through Pc rules out a change in the Fermi surface. We infer that there must be a change of pairing symmetry at Pc. Below Pc, there is compelling evidence for a dwave state. Above Pc, the high sensitivity to disorder rules out an s++state. Given the near degeneracy of dwave and s+/ states found theoretically, we propose an s+/ state above Pc. A change from dwave to swave would probably proceed through an intermediate s+id state that breaks timereversal symmetry.Nature Physics 06/2013; 9(6):349352. · 20.60 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report superconductivity in the ternary halfHeusler compound LuPtBi, with Tc=1.0 K and Hc2=1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spinorbit interaction. The combination of strong spinorbit coupling and noncentrosymmetric crystal structure make LuPtBi a strong candidate for 3D topological superconductivity.Physical review. B, Condensed matter 05/2013; 87(18). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The thermal conductivity of the cuprate superconductor YBa2Cu3Oy was measured at temperatures down to T ˜50 mK on highquality single crystals with a hole doping ranging from p = 0.06 to p = 0.18. The residual linear term at T >0 is a direct measure of the ratio of nodal quasiparticle velocities. When combined with published data on Tl2Ba2CuO6+δ [1], our data spans the full superconducting phase. The ratio of quasiparticle velocities agrees well with recent, highresolution ARPES measurements of the Fermi velocity and gap velocity at the nodes as a function of doping, in the related cuprate superconductor Bi2Sr2CaCu2O8+δ [2,3]. [4pt] [1] D.G. Hawthorn et al., Phys. Rev. B 75, 104518 (2007). [0pt] [2] I.M. Vishik et al., Phys. Rev. Lett. 104, 207002 (2010). [0pt] [3] I.M. Vishik et al., ArXiv, 1209.6514 (2012).03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: It is generally thought that the upper critical field Hc2 of underdoped cuprate superconductors is far greater than the vortexsolid melting field Hvs at which the state of zero resistance ends, even at T = 0 [1]. Here we compare electrical measurements of Hvs and thermal measurements of Hc2 [2] in the cuprate YBCO and show that Hc2= Hvs at T >0, strong evidence that there is no vortex liquid phase at T = 0. We then present extensive measurements of the electrical resistivity in high magnetic fields over a wide doping range, from which we obtain Hc2 as a function of doping in YBCO. We find that Hc2 collapses to remarkably low values in the underdoped regime, which we attribute to the competing effect of a phase with chargedensitywave order [3, 4], also responsible for a reconstruction of the Fermi surface [5, 6].[4pt] [1] T. Senthil and P.A. Lee, Phys. Rev. B 79, 245116 (2009).[0pt] [2] See APS talk by G. Grissonnanche.[0pt] [3] T. Wu et al., Nature 477, 191 (2011).[0pt] [4] G. Ghiringhelli et al., Science 337, 821 (2012).[0pt] [5] D. LeBoeuf et al., Phys. Rev. B 83, 054506 (2011).[0pt] [6] N. DoironLeyraud and L. Taillefer, Physica C 481, 161 (2012).03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We present highpressure magnetotransport data on single crystals of LuPtBi, a member of the ternary halfHeusler family. Recent band structure calculations show that LuPtBi is a topological semimetal at ambient pressure due to strong spinorbit coupling [1]. By decreasing the lattice parameter, equivalent to increasing pressure, the system should become a trivial insulator We have grown single crystals of LuPtBi and studied both the field dependence and the pressure dependence of their resistivity. The field dependence shows typical semimetal behaviour, namely a weak temperature dependence and a large magnetoresistance. The pressure dependence shows a significant increase of resistivity and a decrease of magnetoresistance with increasing pressure. We compare our experimental results to the available theoretical work on the transport properties of topological semimetals [2]. [4pt] [1] Stanislav Chadov, et al. Nature, 9, 541 (2010)[0pt] [2] W. AlSawai, et al. PRB, 82, 125208 (2010)03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: The behavior of the thermal conductivity in the ironarsenide KFe2As2 at low temperature provides compelling evidence of dwave superconductivity [1]. Here we report a detailed study of the thermal conductivity in KFe2As2 as a function of magnetic field, for two field orientations: perpendicular and parallel to the FeAs planes. The data are in excellent quantitative agreement with theoretical calculations for a dwave superconductor [2]. Our study also highlights the power of thermal conductivity as a technique to directly measure the upper critical field Hc2 in a clean typeII superconductor. [1] J.Ph. Reid et al., Phys. Rev. Lett. 109, 087001 (2012). [2] A. B. Vorontsov and I. Vekhter, Phys. Rev. B 75, 224502 (2007).03/2013; 
Article: Thermal conductivity as a direct probe of the upper critical field Hc2 in cuprate superconductors
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ABSTRACT: The value of the upper critical field Hc2 in cuprate superconductors is an open question, subject to much debate [1]. Owing to its sensitivity to vortex scattering, the thermal conductivity is a powerful technique to directly measure the upper critical field Hc2 in a clean typeII superconductor [2]. Here we report measurements of the thermal conductivity in the underdoped cuprate superconductor YBCO in magnetic fields up to 45 T, from which we can directly extract Hc2. We find that Hc2 is remarkably low at a doping p = 0.11, showing that quantum oscillations [3, 4] are observed above Hc2, in a normal state without vortices.[4pt] [1] J. Chang et al., Nat. Phys. 8, 751 (2012).[0pt] [2] A. B. Vorontsov and I. Vekhter, Phys. Rev. B 75, 224502 (2007).[0pt] [3] N. DoironLeyraud et al., Nature 447, 565 (2007).[0pt] [4] S. C. Riggs et al., Nat. Phys. 7, 332 (2011).03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Charge densitywave order has been observed in cuprate superconductors whose crystal structure breaks the square symmetry of the CuO2 planes, such as orthorhombic YBa2Cu3Oy (YBCO), but not so far in cuprates that preserve that symmetry, such as tetragonal HgBa2CuO4+d (Hg1201). We have measured the Hall (R_H), Seebeck (S), and Nernst coefficients of underdoped Hg1201 in magnetic fields large enough to suppress superconductivity. The highfield R_H(T) and S(T) are found to drop with decreasing temperature and become negative, as also observed in YBCO at comparable doping. In YBCO, the negative R_H and S are signatures of a small electron pocket caused by Fermisurface reconstruction, attributed to charge densitywave modulations observed in the same range of doping and temperature. We deduce that a similar Fermisurface reconstruction takes place in Hg1201, evidence that densitywave order exists in this material. A striking similarity is also found in the normalstate Nernst coefficient, further supporting this interpretation. Given the model nature of Hg1201, Fermisurface reconstruction appears to be common to all holedoped cuprates, suggesting that densitywave order is a fundamental property of these materials.Physical Review X 10/2012; 3(2). · 8.39 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The electrical resistivity rho of the ironarsenide superconductor Ba1xKxFe2As2 was measured in applied pressures up to 2.6 GPa for four underdoped samples, with x = 0.16, 0.18, 0.19 and 0.21. The antiferromagnetic ordering temperature T_N, detected as a sharp anomaly in rho(T), decreases linearly with pressure. At pressures above around 1.0 GPa, a second sharp anomaly is detected at a lower temperature T_0, which rises with pressure. We attribute this second anomaly to the onset of a phase that causes a reconstruction of the Fermi surface. This new phase expands with increasing x and it competes with superconductivity. We discuss the possibility that a second spindensity wave orders at T_0, with a Q vector distinct from that of the spindensity wave that sets in at T_N.Physical review. B, Condensed matter 10/2012; 86(14):140502. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The transition temperature Tc of cuprate superconductors falls when the doping p is reduced below a certain optimal value. It is unclear whether this fall is due to strong phase fluctuations or to a decrease in the pairing gap. Different interpretations of photoemission data disagree on the evolution of the pairing gap and different estimates of the upper critical field Hc2 are in sharp contradiction. Here we resolve this contradiction by showing that superconducting fluctuations in the underdoped cuprate EuLSCO, measured via the Nernst effect, have a characteristic field scale that falls with underdoping. The critical field Hc2 dips at p = 0.11, showing that superconductivity is weak where stripe order is strong. In the archetypal cuprate superconductor YBCO, Hc2 extracted from other measurements has the same doping dependence, also with a minimum at p = 0.11, again where stripe order is present. We conclude that competing states such as stripe order weaken superconductivity and this, rather than phase fluctuations, causes Tc to fall as cuprates become underdoped.Nature Physics 08/2012; 8(10). · 20.60 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The nature of the pairing state in ironbased superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a dwave pairing state, characterized by symmetryimposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is swave and the Tc is ten times higher. The transition from dwave to swave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal ironbased superconductor LaFePO, for which the pairing symmetry is probably not dwave, but more likely swave with accidental line nodes.Superconductor Science and Technology 07/2012; 25(2012). · 2.80 Impact Factor 
Article: Quantum critical point for stripe order: An organizing principle of cuprate superconductivity
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ABSTRACT: A spin densitywave quantum critical point (QCP) is the central organizing principle of organic, ironpnictide, heavyfermion and electrondoped cuprate superconductors. It accounts for the superconducting Tc dome, the nonFermiliquid resistivity, and the Fermisurface reconstruction. Outside the magnetically ordered phase above the QCP, scattering and pairing decrease in parallel as the system moves away from the QCP. Here we argue that a similar scenario, based on a stripeorder QCP, is a central organizing principle of holedoped cuprate superconductors. Key properties of EuLSCO, NdLSCO and YBCO are naturally unified, including stripe order itself, its QCP, Fermisurface reconstruction, the linearT resistivity, and the nematic character of the pseudogap phase.Physica C Superconductivity 04/2012; 481. · 1.11 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The thermal conductivity of the ironbased superconductor KFe2As2 was measured at temperatures down to 50 mK in magnetic fields up to 15 T, as a way to probe the superconducting gap structure. A large residual linear term in the T=0 limit is observed in zero field, showing that the gap structure contains nodes, consistent with a previous report [J.K. Dong et al., PRL 104, 087005 (2010)]. We discuss the possible interpretations for the nature of these nodes, in light of three different theoretical proposals: accidental line nodes in an extended swave state which are either horizontal [K. Suzuki et al., arXiv:1108.0657] or vertical [S. Maiti et al., arXiv:1111.0306], or symmetryimposed vertical line nodes in a dwave state [R. Thomale et al., PRL 107, 117001 (2011)].02/2012;  [Show abstract] [Hide abstract]
ABSTRACT: The thermal conductivity κ of Ba1xKxFe2As2 was measured down to 50 mK in magnetic fields up to 15 T, for heat current both parallel and perpendicular to the tetragonal c axis, across a range of K concentrations from optimal doping (Tc=38 K) down to Tc=7 K, deep into the region of coexistence with antiferromagnetic order. From optimal doping down to Tc˜15 K, well into the coexistence region, there is no residual linear term in κ(T) as T>0, showing that there are no nodes in the superconducting gap anywhere on the Fermi surface. For concentrations in a narrow range such that 9 K<Tc<13 K, a large residual linear term appears, signaling the onset of nodes in the superconducting gap, most likely vertical line nodes running along the c axis. For Tc<9 K, the gap is again nodeless. We propose that these changes in the superconducting gap structure are triggered by changes in the Fermi surface as it is reconstructed by the growing antiferromagnetic order.02/2012; 
Article: Thermal Hall effect in YBCO: Probing Fermisurface reconstruction inside the superconducting state
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ABSTRACT: The thermal Hall (RighiLeduc) effect was measured in the cuprate superconductor YBCO at a doping p = 0.11, as a function of magnetic field H up to 29 T. At temperatures well below the zerofield superconducting Tc, the thermal Hall conductivity κxy is positive at low field and then turns over to become negative at fields above 15 T. The negative κxy is consistent with the negative Hall and Seebeck coefficients observed in the normal state above 25 T [1,2]. This further supports our interpretation: the Fermi surface of YBCO contains a small electronlike pocket [3] in that region of the phase diagram, the result of a Fermisurface reconstruction attributed to stripe order [4]. In the T = 0 limit at H = 29 T, we find reasonable agreement with the WiedemannFranz law, κxy/T=L0σxy. The fact that κxy changes sign at H 15 T is consistent with a scenario of phase competition whereby stripe order emerges only at finite field, in agreement with recent NMR studies that detect the onset of chargestripe order above 15 T [5]. [4pt] [1] LeBoeuf et al., PRB 83, 054056 (2011); [2] Lalibert'e et al., Nat. Commun. 2, 432 (2011); [3] LeBoeuf et al., Nature 450, 533 (2007); [4] Chang et al., PRL 104, 057005 (2010); [5] Wu et al., Nature 477, 191 (2011).02/2012;
Publication Stats
2k  Citations  
452.85  Total Impact Points  
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Institutions

2014

McGill University
Montréal, Quebec, Canada


2012–2014

Canadian Institute For Advanced Research
Toronto, Ontario, Canada


2004–2014

Université de Sherbrooke
 Department of Physics
Sherbrooke, Quebec, Canada


2003

University of Cambridge
 Department of Physics: Cavendish Laboratory
Cambridge, ENG, United Kingdom
