arXiv subjects

Cyril Proust

Publications and source records attributed to Cyril Proust.

At least 37 records · Page 2Linked to original sources

arXiv2012

Vortex Lattice Melting and Hc2 in underdoped YBa2Cu3Oy

Vortices in a type-II superconductor form a lattice structure that melts when the thermal displacement of the vortices is an appreciable fraction of the distance between vortices. In an anisotropic high-Tc superconductor, such as YBa2Cu3Oy, the magnetic field value where this melting occurs can be much lower than the mean-field critical field Hc2. We examine this melting transition in YBa2Cu3Oy with oxygen content y from 6.45 to 6.92, and fit the data to a theory of vortex-lattice melting. The quality of the fits indicates that the transition to a resistive state is indeed the vortex lattice melting transition, with the shape of the melting curves being consistent with the known change in penetration depth anisotropy from underdoped to optimally doped YBa2Cu3Oy. From the fits we extract Hc2(T = 0) as a function of hole doping. The unusual doping dependence of Hc2(T =0) points to some form of electronic order competing with superconductivity around 0.12 hole doping.

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arXiv2012

Thermodynamic phase diagram of static charge order in underdoped YBCO

The interplay between superconductivity and any other competing order is an essential part of the long-standing debate on the origin of high temperature superconductivity in cuprates. Akin to the situation of heavy fermions, organic superconductors and pnictides, it has been proposed that the pairing mechanism in cuprates comes from fluctuations of a nearby quantum phase transition. Recent evidence of charge modulation and the associated fluctuations in the pseudogap phase of YBa_2Cu_3O_y make charge order a likely candidate for a competing order. However, a thermodynamic signature of the charge ordering phase transition is still lacking. Moreover, whether such charge order is one- or two-dimensional is still controversial but pivotal for the understanding the topology of the reconstructed Fermi surface. Here we address both issues by measuring sound velocities in YBCO_6.55 in high magnetic fields, a powerful thermodynamic probe to detect phase transitions. We provide the first thermodynamic signature of the field-induced charge ordering phase transition in YBCO allowing construction of a field-temperature phase diagram, which reveals the competing nature of this charge order. The comparison of different acoustic modes indicates that the charge modulation has a two-dimensional character, thus imposing strong constraints on Fermi surface reconstruction scenarios.

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arXiv2012

Coherent c-axis transport in the underdoped cuprate superconductor YBCO

The electrical resistivity rho_c of the underdoped cuprate superconductor YBCO was measured perpendicular to the CuO_2 planes on ultra-high quality single crystals in magnetic fields large enough to suppress superconductivity. The incoherent insulating-like behavior of rho_c at high temperature, characteristic of all underdoped cuprates, is found to cross over to a coherent regime of metallic behavior at low temperature. This crossover coincides with the emergence of the small electron pocket detected in the Fermi surface of YBCO via quantum oscillations, the Hall and Seebeck coefficients and with the detection of a unidirectional modulation of the charge density as seen by high-field NMR measurements. The low coherence temperature is quantitatively consistent with the small hopping integral t_perp inferred from the splitting of the quantum oscillation frequencies. We conclude that the Fermi-surface reconstruction in YBCO at dopings from p = 0.08 to at least p = 0.15, attributed to stripe order, produces a metallic state with 3D coherence deep in the underdoped regime.

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arXiv2011

Nernst effect in the cuprate superconductor YBCO: Broken rotational and translational symmetries

The Nernst coefficient of the cuprate superconductor YBa2Cu3Oy was recently shown to become strongly anisotropic within the basal plane when cooled below the pseudogap temperature T*, revealing that the pseudogap phase breaks the four-fold rotational symmetry of the CuO2 planes. Here we report on the evolution of this Nernst anisotropy at low temperature, once superconductivity is suppressed by a magnetic field. We find that the anisotropy drops rapidly below 80 K, to vanish in the T=0 limit. We show that this loss of anisotropy is due to the emergence of a small high-mobility electron-like pocket in the Fermi surface at low temperature, a reconstruction attributed to a low-temperature state that breaks the translational symmetry of the CuO2 planes. We discuss the sequence of broken symmetries - first rotational, then translational - in terms of an electronic nematic-to-smectic transition such as could arise when unidirectional spin or charge modulations order. We compare YBa2Cu3Oy with iron-pnictide superconductors where the process of (unidirectional) antiferromagnetic ordering gives rises to the same sequence of broken symmetries.

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arXiv2011

Lifshitz critical point in the cuprate superconductor YBa2Cu3Oy from high-field Hall effect measurements

The Hall coefficient R_H of the cuprate superconductor YBa2Cu3Oy was measured in magnetic fields up to 60 T for a hole concentration p from 0.078 to 0.152, in the underdoped regime. In fields large enough to suppress superconductivity, R_H(T) is seen to go from positive at high temperature to negative at low temperature, for p > 0.08. This change of sign is attributed to the emergence of an electron pocket in the Fermi surface at low temperature. At p < 0.08, the normal-state R_H(T) remains positive at all temperatures, increasing monotonically as T \to 0. We attribute the change of behaviour across p = 0.08 to a Lifshitz transition, namely a change in Fermi-surface topology occurring at a critical concentration p_L = 0.08, where the electron pocket vanishes. The loss of the high-mobility electron pocket across p_L coincides with a ten-fold drop in the conductivity at low temperature, revealed in measurements of the electrical resistivity $ρ$ at high fields, showing that the so-called metal-insulator crossover of cuprates is in fact driven by a Lifshitz transition. It also coincides with a jump in the in-plane anisotropy of $ρ$, showing that without its electron pocket the Fermi surface must have strong two-fold in-plane anisotropy. These findings are consistent with a Fermi-surface reconstruction caused by a unidirectional spin-density wave or stripe order.

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arXiv2010

Angle-dependence of quantum oscillations in YBa2Cu3O6.59 shows free spin behaviour of quasiparticles

Measurements of quantum oscillations in the cuprate superconductors afford a new opportunity to assess the extent to which the electronic properties of these materials yield to a description rooted in Fermi liquid theory. However, such an analysis is hampered by the small number of oscillatory periods observed. Here we employ a genetic algorithm to globally model the field, angular, and temperature dependence of the quantum oscillations observed in the resistivity of YBa2Cu3O6.59. This approach successfully fits an entire data set to a Fermi surface comprised of two small, quasi-2-dimensional cylinders. A key feature of the data is the first identification of the effect of Zeeman splitting, which separates spin-up and spin-down contributions, indicating that the quasiparticles in the cuprates behave as nearly free spins, constraining the source of the Fermi surface reconstruction to something other than a conventional spin density wave with moments parallel to the CuO2 planes.

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arXiv2010

Nernst and Seebeck Coefficients of the Cuprate SuperconductorYBa$_2$Cu$_3$O$_{6.67}$: A Study of Fermi Surface Reconstruction

The Seebeck and Nernst coefficients $S$ and $ν$ of the cuprate superconductor YBa$_2$Cu$_3$O$_y$ (YBCO) were measured in a single crystal with doping $p = 0.12$ in magnetic fields up to H = 28 T. Down to T=9 K, $ν$ becomes independent of field by $H \simeq 30$ T, showing that superconducting fluctuations have become negligible. In this field-induced normal state, $S/T$ and $ν/T$ are both large and negative in the $T \to 0$ limit, with the magnitude and sign of $S/T$ consistent with the small electron-like Fermi surface pocket detected previously by quantum oscillations and the Hall effect. The change of sign in $S(T)$ at $T \simeq 50$ K is remarkably similar to that observed in La$_{2-x}$Ba$_x$CuO$_4$, La$_{2-x-y}$Nd$_y$Sr$_x$CuO$_4$ and La$_{2-x-y}$Eu$_y$Sr$_x$CuO$_4$, where it is clearly associated with the onset of stripe order. We propose that a similar density-wave mechanism causes the Fermi surface reconstruction in YBCO.

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arXiv2010

Quantum oscillations in underdoped YBa_2Cu_3O_6.5

Shubnikov-de Haas and de Haas-van Alphen effects have been measured in the underdoped high temperature superconductor YBa$_2$Cu$_3$O$_{6.51}$. Data are in agreement with the standard Lifshitz-Kosevitch theory, which confirms the presence of a coherent Fermi surface in the ground state of underdoped cuprates. A low frequency $F = 530 \pm 10$ T is reported in both measurements, pointing to small Fermi pocket, which corresponds to 2% of the first Brillouin zone area only. This low value is in sharp contrast with that of overdoped Tl$_2$Ba$_2$CuO$_{6+δ}$, where a high frequency $F = 18$ kT has been recently reported and corresponds to a large hole cylinder in agreement with band structure calculations. These results point to a radical change in the topology of the Fermi surface on opposing sides of the cuprate phase diagram.

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arXiv2010

Multiple Quantum Oscillations in the de Haas van Alphen Spectra of the Underdoped High Temperature Superconductor YBa_2Cu_3O_6.5

By improving the experimental conditions and extensive data accumulation, we have achieved very high-precision in the measurements of the de Haas-van Alphen effect in the underdoped high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{6.5}$. We find that the main oscillation, so far believed to be single-frequency, is composed of three closely spaced frequencies. We attribute this to bilayer splitting and warping of a single quasi-2D Fermi surface, indicating that \emph{c}-axis coherence is restored at low temperature in underdoped cuprates. Our results do not support the existence of a larger frequency of the order of 1650 T reported recently in the same compound [S.E. Sebastian {\it et al}., Nature {\bf 454}, 200 (2008)].

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arXiv2009

An electronic instability in bismuth far beyond the quantum limit

We present a transport study of semi-metallic bismuth in presence of a magnetic field applied along the trigonal axis extended to 55 T for electric conductivity and to 45 T for thermoelectric response. The results uncover a new field scale at about 40 T in addition to the previously detected ones. Large anomalies in all transport properties point to an intriguing electronic instability deep in the ultraquantum regime. Unexpectedly, both the sheer magnitude of conductivity and its metallic temperature dependence are enhanced by this instability.

cond-mat.str-el↗

arXiv2008

Electron pockets in the Fermi surface of hole-doped high-Tc superconductors

High-temperature superconductivity occurs as copper oxides are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of underdoped YBa2Cu3Oy and YBa2Cu4O8 was recently shown to include small pockets in contrast with the large cylinder characteristic of the overdoped regime1, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic field-induced normal state of YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like. We propose that electron pockets arise most likely from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped materials near the onset of antiferromagnetic order Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-Tc superconductors.

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arXiv2008

Quantum oscillations and the Fermi surface in an underdoped high-Tc superconductor

Despite twenty years of research, the phase diagram of high transition- temperature superconductors remains enigmatic. A central issue is the origin of the differences in the physical properties of these copper oxides doped to opposite sides of the superconducting region. In the overdoped regime, the material behaves as a reasonably conventional metal, with a large Fermi surface. The underdoped regime, however, is highly anomalous and appears to have no coherent Fermi surface, but only disconnected "Fermi arcs". The fundamental question, then, is whether underdoped copper oxides have a Fermi surface, and if so, whether it is topologically different from that seen in the overdoped regime. Here we report the observation of quantum oscillations in the electrical resistance of the oxygen-ordered copper oxide YBa2Cu3O6.5, establishing the existence of a well-defined Fermi surface in the ground state of underdoped copper oxides, once superconductivity is suppressed by a magnetic field. The low oscillation frequency reveals a Fermi surface made of small pockets, in contrast to the large cylinder characteristic of the overdoped regime. Two possible interpretations are discussed: either a small pocket is part of the band structure specific to YBa2Cu3O6.5 or small pockets arise from a topological change at a critical point in the phase diagram. Our understanding of high-transition temperature (high-Tc) superconductors will depend critically on which of these two interpretations proves to be correct.

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arXiv2006

Total suppression of superconductivity by high magnetic fields in YBa2 Cu3O6.6

We have studied in fields up to 60T the variation of the transverse magnetoresistance (MR) of underdoped YBCO6.6 crystals either pure or with Tc reduced down to 3.5K by electron irradiation. We evidence that the normal state MR is restored above a threshold field H'c(T), which is found to vanish at T'c>>Tc. In the pure YBCO6.6 sample a 50 Tesla field is already required to completely suppress the superconducting fluctuations at Tc. While disorder does not depress the pseudogap temperature, it reduces drastically the phase coherence established at Tc and weakly H'c(0), T'c and the onset Tnu of the Nernst signal which are more characteristic of the 2D local pairing.

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arXiv2005

Heat transport in Bi_{2+x}Sr_{2-x}CuO_{6+δ}: departure from the Wiedemann-Franz law in the vicinity of the metal-insulator transition

We present a study of heat transport in the cuprate superconductor Bi_{2+x}Sr_{2-x}CuO_{6+δ} at subkelvin temperatures and in magnetic fields as high as 25T. In several samples with different doping levels close to optimal, the linear-temperature term of thermal conductivity was measured both at zero-field and in presence of a magnetic field strong enough to quench superconductivity. The zero-field data yields a superconducting gap of reasonable magnitude displaying a doping dependence similar to the one reported in other families of cuprate. The normal-state data together with the results of the resistivity measurements allows us to test the Wiedemann-Franz(WF) law, the validity of which was confirmed in an overdoped sample in agreement with previous studies. In contrast, a systematic deviation from the WF law was resolved for samples displaying either a lower doping content or a higher disorder. Thus, in the vicinity of the metal-insulator cross-over, heat conduction in the zero-temperature limit appears to become significantly larger than predicted by the WF law. Possible origins of this observation are discussed.

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arXiv2004

Test of the Wiedemann-Franz law in an optimally-doped cuprate

We present a study of heat and charge transport in Bi$_{2+x}$Sr$_{2-x}$CuO$_{6+δ}$ focused on the size of the low-temperature linear term of the thermal conductivity at optimal doping level. In the superconducting state, the magnitude of this term implies a d-wave gap with an amplitude close to what has been reported. In the normal state, recovered by the application of a magnetic field, measurement of this term and residual resistivity yields a Lorenz number $L = κ_{N}ρ_{0}/T = 1.3 \pm 0.2 L_{0}$. The departure from the value expected by the Wiedemann-Franz law is thus slightly larger than our estimated experimental resolution.

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arXiv2003

Transport in Ultraclean YBa$_2$Cu$_3$O$_7$: neither Unitary nor Born Impurity Scattering

The thermal conductivity of ultraclean YBa$_2$Cu$_3$O$_7$ was measured at very low temperature in magnetic fields up to 13 T. The temperature and field dependence of the electronic heat conductivity show that two widespread assumptions of transport theory applied to unconventional superconductors fail for clean cuprates: impurity scattering cannot be treated in the usual unitary limit (nor indeed in the Born limit), and scattering of quasiparticles off vortices cannot be neglected. Our study also sheds light on the long-standing puzzle of a sudden onset of a "plateau" in the thermal conductivity of Bi-2212 versus field.

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arXiv2002

Heat Transport in a Strongly Overdoped Cuprate: Fermi Liquid and Pure d-wave BCS Superconductor

The transport of heat and charge in the overdoped cuprate superconductor Tl_2Ba_2CuO_(6+delta) was measured down to low temperature. In the normal state, obtained by applying a magnetic field greater than the upper critical field, the Wiedemann-Franz law is verified to hold perfectly. In the superconducting state, a large residual linear term is observed in the thermal conductivity, in quantitative agreement with BCS theory for a d-wave superconductor. This is compelling evidence that the electrons in overdoped cuprates form a Fermi liquid, with no indication of spin-charge separation.

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arXiv2002

Highly Anisotropic Gap Function in Borocarbide Superconductor LuNi_2B_2C

The thermal conductivity of borocarbide superconductor LuNi_2B_2C was measured down to 70 mK (T_c/200) in a magnetic field perpendicular to the heat current from H = 0 to above H_c2 = 7 T. As soon as vortices enter the sample, the conduction at T -> 0 grows rapidly, showing unambiguously that delocalized quasiparticles are present at the lowest energies. The field dependence is very similar to that of UPt_3, a heavy-fermion superconductor with a line of nodes in the gap, and very different from the exponential dependence characteristic of s-wave superconductors. This is strong evidence for a highly anisotropic gap function in LuNi_2B_2C, possibly with nodes.

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