N. Doiron-Leyraud

Université de Sherbrooke, Шербрук, Quebec, Canada

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Publications (92)468.95 Total impact

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    ABSTRACT: In the temperature-concentration phase diagram of most iron-based superconductors, antiferromagnetic order is gradually suppressed to zero at a critical point, and a dome of superconductivity forms around that point. The nature of the magnetic phase and its fluctuations is of fundamental importance for elucidating the pairing mechanism. In Ba{1-x}KxFe2As2 and Ba{1-x}NaxFe2As2, it has recently become clear that the usual stripe-like magnetic phase, of orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal symmetry, near the critical point, between x = 0.24 and x = 0.28. Here we report measurements of the electrical resistivity of Ba{1-x}KxFe2As2 under applied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24 and 0.28. We track the onset of the tetragonal magnetic phase using the sharp anomaly it produces in the resistivity. In the temperature-concentration phase diagram of Ba{1-x}KxFe2As2, we find that pressure greatly expands the tetragonal magnetic phase, while the stripe-like phase shrinks. This raises the interesting possibility that the fluctuations of the former phase might be involved in the pairing mechanism responsible for the superconductivity.
    Full-text · Article · Dec 2015
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    ABSTRACT: Nematicity has emerged as a key feature of cuprate superconductors, but its link to other fundamental properties such as superconductivity, charge order and the pseudogap remains unclear. Here we use measurements of transport anisotropy in YBa$_2$Cu$_3$O$_y$ to distinguish two types of nematicity. The first is associated with short-range charge-density-wave modulations in a doping region near $p = 0.12$. It is detected in the Nernst coefficient, but not in the resistivity. The second type prevails at lower doping, where there are spin modulations but no charge modulations. In this case, the onset of in-plane anisotropy - detected in both the Nernst coefficient and the resistivity - follows a line in the temperature-doping phase diagram that tracks the pseudogap energy. We discuss two possible scenarios for the latter nematicity.
    No preview · Article · Dec 2015 · Physical Review B
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    ABSTRACT: The Seebeck coefficient $S$ of the cuprate superconductor La$ _{2-x} $Sr$_{x} $CuO$ _{4} $ (LSCO) was measured in magnetic fields large enough to suppress superconductivity, for a range of Sr concentrations from $x = 0.07$ to $x = 0.15$. For $x = 0.11, 0.12, 0.125$ and $0.13$, $S/T$ decreases upon cooling to become negative at low temperature. The same behavior is observed in the Hall coefficient $R_{\rm H}$. In analogy with other hole-doped cuprates at similar hole concentrations $p$, the negative $S$ and $R_{\rm H}$ show that the Fermi surface of LSCO undergoes a reconstruction caused by the onset of charge-density-wave modulations. Such modulations have indeed been detected in LSCO by X-ray diffraction in precisely the same doping range. Our data show that in LSCO this Fermi-surface reconstruction is confined to $0.085 < p < 0.15$. We argue that in the field-induced normal state of LSCO, spin-density-wave order extends beyond $p = 0.15$ and it may thus be associated with the quantum critical behavior observed in the resistivity of LSCO around $p \simeq 0.2$.
    Full-text · Article · Dec 2015
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    ABSTRACT: The pseudogap is a central puzzle of cuprate superconductors. Its connection to the Mott insulator at low doping $p$ remains ambiguous and its relation to the charge order that reconstructs the Fermi surface at intermediate $p$ is still unclear. Here we use measurements of the Hall coefficient in magnetic fields up to 88 T to show that Fermi-surface reconstruction by charge order in YBa$_2$Cu$_3$O$_y$ ends sharply at a critical doping $p = 0.16$, distinctly lower than the pseudogap critical point at $p^* = 0.19$. This shows that pseudogap and charge order are separate phenomena. We then find that the change of carrier density from $n = 1 + p$ in the conventional metal at high p to $n = p$ at low $p$ - a signature of the lightly doped cuprates - starts at $p^*$. This shows that pseudogap and antiferromagnetic Mott insulator are linked.
    Full-text · Article · Nov 2015
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    ABSTRACT: Quantum oscillations and negative Hall and Seebeck coefficients at low temperature and high magnetic field have shown the Fermi surface of underdoped cuprates to contain a small closed electron pocket. It is thought to result from a reconstruction by charge order, but whether it is the order seen by NMR and ultrasound above a threshold field or the short-range modulations seen by X-ray diffraction in zero field is unclear. Here we use measurements of the thermal Hall conductivity in YBCO to show that Fermi-surface reconstruction occurs only above a sharply defined onset field, equal to the transition field seen in ultrasound. This reveals that electrons do not experience long-range broken translational symmetry in the zero-field ground state, and hence in zero field there is no quantum critical point for the onset of charge order as a function of doping.
    Full-text · Article · Aug 2015
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    ABSTRACT: Nematicity has emerged as a key feature of cuprate superconductors, but its link to other fundamental properties such as superconductivity, charge order and the pseudogap remains unclear. Here we use measurements of transport anisotropy in YBa$_2$Cu$_3$O$_y$ to distinguish two sources of anisotropy : one associated with charge modulations, known to be anisotropic, and another associated with the pseudogap. In the second case, the onset of in-plane anisotropy follows a line in the temperature-doping phase diagram that tracks the pseudogap energy. This nematic line decreases with doping until it hits the dome of charge order at its summit. It also hits the dome of superconductivity at its summit, once the competing effect of charge order has been suppressed by pressure. This is compelling evidence for a pseudogap phase with nematic character that shapes the domes of both superconductivity and charge order.
    Full-text · Article · Apr 2015
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    ABSTRACT: The recent detection of charge-density modulations in YBa2Cu3Oy and other cuprate superconductors raises new questions about the normal state of underdoped cuprates. In one class of theories, the modulations are intertwined with pairing in a dual state, expected to persist up to high magnetic fields as a vortex liquid. In support of such a state, specific heat and magnetisation data on YBa2Cu3Oy have been interpreted in terms of a vortex liquid persisting above the vortex-melting field Hvs at T = 0. Here we report high-field measurements of the electrical and thermal Hall conductivities in YBa2Cu3O6.54 that allow us to probe the Wiedemann-Franz law, a sensitive test of the presence of superconductivity in a metal. In the T = 0 limit, we find that the law is satisfied for fields immediately above Hvs. This rules out the existence of a vortex liquid and it places strict constraints on the nature of the normal state in underdoped cuprates.
    Full-text · Article · Mar 2015
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    ABSTRACT: It has recently become clear that cuprate superconductors have a universal tendency to form charge-density-wave order. A fundamental question is the relation between this charge order and the pseudogap phase. A key feature is that this tendency is strongest at a doping $p \simeq 0.12$, irrespective of the modulation period. Here we show that pressure suppresses charge order in YBa$_{2}$Cu$_{3}$O$_{y}$, but does not affect the pseudogap phase. The latter is therefore not simply a precursor of the former. Looking at high-pressure data, we find that when charge order is suppressed, the superconducting dome in the phase diagram of YBa$_{2}$Cu$_{3}$O$_{y}$ is transformed so that it no longer dips but instead now peaks at $p \simeq 0.12$. The fact that in the absence of mutual competition the domes of superconductivity and of charge order both peak at the same doping is strong evidence for the existence of a third phase that competes with both orders at low doping, thereby shaping the phase diagram of cuprates.
    Full-text · Article · Mar 2015
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    ABSTRACT: We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor RbFe2As2, at a critical pressure Pc = 11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal V-shaped phase diagram for Tc vs P in these fully hole-doped 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 two-fold 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 d-wave state favored by high-Q scattering at low pressure changes to a state with s+- symmetry favored by low-Q scattering at high pressure.
    Full-text · Article · Dec 2014 · Physical Review B
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    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 c-axis resistance. The field-angle dependence of the frequency demonstrates that it is a distinct Fermi surface and the normal-state 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 Fermi-surface reconstruction caused by the charge-density-wave order observed in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are removed by a separate mechanism, possibly the pseudogap.
    Full-text · Article · Sep 2014 · Nature Communications
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    N. Doiron-Leyraud · T. Szkopek · T. Pereg-Barnea · C. Proust · G. Gervais
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    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 high-temperature 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 hole-doped cuprates YBa$_2$Cu$_3$O$_{y}$, YBa$_2$Cu$_4$O$_8$, HgBa$_2$CuO$_{4 + \delta}$, and the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$. For the hole-doped materials, a trivial Berry phase of 0 mod $2\pi$ is systematically observed whereas the electron-doped Nd$_{2-x}$Ce$_x$CuO$_4$ exhibits a significant non-zero Berry phase. These observations set constraints on the nature of the high-field normal state of the cuprates and points towards contrasting behaviour between hole-doped and electron-doped materials. We discuss this difference in light of recent developments related to charge density-wave and broken time-reversal symmetry states.
    Preview · Article · Jul 2014 · Physical Review B
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    ABSTRACT: We report a sudden reversal in the pressure dependence of Tc in the iron-based 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 6-fold increase in impurity scattering does not shift Pc. From a study of X-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As 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.
    Full-text · Article · Mar 2014 · Physical Review B
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    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 20-fold collapse. This reveals that phase competition-associated with Fermi-surface reconstruction and charge-density-wave order-is a key limiting factor in the superconductivity of cuprates.
    Full-text · Article · Feb 2014 · Nature Communications
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    ABSTRACT: Proximity to an antiferromagnetic phase suggests that pairing in iron-based superconductors is mediated by spin fluctuations, but orbital fluctuations have also been invoked. The former typically favour a pairing state of extended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces (s+/-), whereas the latter yield a standard s-wave 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 d-wave state. Above Pc, the high sensitivity to disorder rules out an s++state. Given the near degeneracy of d-wave and s+/- states found theoretically, we propose an s+/- state above Pc. A change from d-wave to s-wave would probably proceed through an intermediate s+id state that breaks time-reversal symmetry.
    Full-text · Article · Jun 2013 · Nature Physics
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    ABSTRACT: We report superconductivity in the ternary half-Heusler 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 spin-orbit interaction. The combination of strong spin-orbit coupling and noncentrosymmetric crystal structure make LuPtBi a strong candidate for 3D topological superconductivity.
    Full-text · Article · May 2013 · Physical review. B, Condensed matter
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    ABSTRACT: It is generally thought that the upper critical field Hc2 of underdoped cuprate superconductors is far greater than the vortex-solid 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 charge-density-wave 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. Doiron-Leyraud and L. Taillefer, Physica C 481, 161 (2012).
    No preview · Article · Mar 2013
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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 type-II 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. Doiron-Leyraud et al., Nature 447, 565 (2007).[0pt] [4] S. C. Riggs et al., Nat. Phys. 7, 332 (2011).
    No preview · Article · Mar 2013
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    ABSTRACT: The thermal conductivity of the cuprate superconductor YBa2Cu3Oy was measured at temperatures down to T ˜50 mK on high-quality 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, high-resolution 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).
    No preview · Article · Mar 2013
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    ABSTRACT: We present high-pressure magneto-transport data on single crystals of LuPtBi, a member of the ternary half-Heusler family. Recent band structure calculations show that LuPtBi is a topological semi-metal at ambient pressure due to strong spin-orbit 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 semi-metal behaviour, namely a weak temperature dependence and a large magneto-resistance. The pressure dependence shows a significant increase of resistivity and a decrease of magneto-resistance with increasing pressure. We compare our experimental results to the available theoretical work on the transport properties of topological semi-metals [2]. [4pt] [1] Stanislav Chadov, et al. Nature, 9, 541 (2010)[0pt] [2] W. Al-Sawai, et al. PRB, 82, 125208 (2010)
    No preview · Article · Mar 2013
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    ABSTRACT: The behavior of the thermal conductivity in the iron-arsenide KFe2As2 at low temperature provides compelling evidence of d-wave 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 d-wave 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 type-II 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).
    No preview · Article · Mar 2013