Une fable de phases en interaction dans les cuprates supraconducteurs contée par le transport thermique

Cette thèse traite l'interaction d'ordres en compétition dans les cuprates supraconducteurs dopés en trous; il sera question de supraconductivité et d'ordre de charge. Dans une première étude, la conductivité thermique $\kappa_{\rm xx}$ sous forts champs magnétiques du cuprate YBCO est utilisée pour mesurer le champ critique $H_{\rm c2}$. Cette expérience révèle la forte compétition entre la supraconductivité et l'ordre de charge dans ces matériaux dopés en trous. Ce résultat représente la première mesure directe de champ critique $H_{\rm c2}$ dans cette famille de supraconducteurs et démontre l'absence de liquide de vortex à température nulle. Dans une deuxième étude, la combinaison de l'effet Hall thermique $\kappa_{\rm xy}$ et de mesures électriques sous forts champs magnétiques permet l'exploration de la loi de Wiedemann-Franz dans le cuprate YBCO. En démontrant que cette loi est satisfaite au-dessus du champ magnétique critique $H_{\rm c2}$ déterminé lors du premier projet, cette expérience montre qu'il ne reste pas de supraconductivité au-dessus du champ magnétique critique et que l'état normal des cuprates sous-dopés est métallique. Dans une troisième étude, l'effet Hall thermique $\kappa_{\rm xy}$ est utilisé pour sonder la surface de Fermi d'un matériau dans sa phase supraconductrice. Pour les cuprates sous-dopés en trous, ce projet révèle qu'il n'y a pas de reconstruction de la surface de Fermi en champ nul par l'ordre de charge à courte portée. Cette expérience pionnière représente ainsi le trait d'union manquant entre de nombreux résultats qui pourtant parurent contradictoires au premier abord

Role of magnetic ions in the thermal Hall effect of the paramagnetic insulator TmVO4_{4}

In a growing number of materials, phonons have been found to generate a thermal Hall effect, but the underlying mechanism remains unclear. Inspired by previous studies that revealed the importance of Tb$^{3+}$ ions in generating the thermal Hall effect of Tb$_{2}$Ti$_{2}$O$_{7}$, we investigated the role of Tm$^{3+}$ ions in TmVO$_{4}$, a paramagnetic insulator with a different crystal structure. We observe a negative thermal Hall conductivity in TmVO$_{4}$ with a magnitude such that the Hall angle, $|\kappa_{xy}$/$\kappa_{xx}|$, is approximately 1 x 10$^{-3}$ at $H$ = 15 T and $T$ = 20 K, typical for a phonon-generated thermal Hall effect. In contrast to the negligible $\kappa_{xy}$ found in Y$_{2}$Ti$_{2}$O$_{7}$, we observe a negative $\kappa_{xy}$ in YVO$_{4}$ with a Hall angle of magnitude comparable to that of TmVO$_{4}$. This shows that the Tm$^{3+}$ ions are not essential for the thermal Hall effect in this family of materials. Interestingly, at an intermediate Y concentration of 30 % in Tm$_{1-x}$Y$_{x}$VO$_{4}$, $\kappa_{xy}$ was found to have a positive sign, pointing to the possible importance of impurities in the thermal Hall effect of phonons

Anisotropic Seebeck coefficient of Sr2RuO4\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4 in the incoherent regime

Intuitive entropic interpretations of the thermoelectric effect in metals predict an isotropic Seebeck coefficient at high temperatures in the incoherent regime even in anisotropic metals since entropy is not directional. $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ is an enigmatic material known for a well characterised anisotropic normal state and unconventional superconductivity. Recent ab-initio transport calculations of $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ that include the effect of strong electronic correlations predicted an enhanced high-temperature anisotropy of the Seebeck coefficient at temperatures above 300 K, but experimental evidence is missing. From measurements on clean $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ single crystals along both crystallographic directions, we find that the Seebeck coefficient becomes increasingly isotropic upon heating towards room temperature as generally expected. Above 300 K, however, $S$ acquires a new anisotropy which rises up to the highest temperatures measured (750 K), in qualitative agreement with calculations. This is a challenge to entropic interpretations and highlights the lack of an intuitive framework to understand the anisotropy of thermopower at high temperatures.Comment: 5 pages, 2 figure

Thermal Hall conductivity of electron-doped cuprates: Electrons and phonons

It has recently become clear that phonons generate a sizable thermal Hall effect in cuprates, whether they are undoped, electron-doped or hole-doped (inside the pseudogap phase). At higher doping, where cuprates are reasonably good metals, mobile electrons also generate a thermal Hall effect, the thermal equivalent of the standard electrical Hall effect. Here we show that in the cleanest crystals of the electron-doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4}$, at high doping, the phonon and electron contributions to the thermal Hall conductivity $\kappa_{\rm {xy}}$ are of comparable magnitude, but of opposite sign. In samples of lower quality, phonons dominate $\kappa_{\rm {xy}}$, resulting in a negative $\kappa_{\rm {xy}}$ at all temperatures. The fact that the negative phononic $\kappa_{\rm {xy}}$ in the metallic state is similar in magnitude and temperature dependence to that found in the insulating state at lower doping rules out any mechanism based on skew scattering of phonons off charged impurities, since a local charge should be screened in the metallic regime. The phononic $\kappa_{\rm {xy}}$ is found to persist over the entire doping range where antiferromagnetic correlations are known to be significant, suggesting that such correlations may play a role in generating the phonon thermal Hall effect in electron-doped cuprates. If the same mechanism is also at play in hole-doped cuprates, the presence of a phononic $\kappa_{\rm {xy}}$ below (and only below) the critical doping $p^{\star}$ would be evidence that spin correlations are a property of the pseudogap phase

Une fable de phases en interaction dans les cuprates supraconducteurs contée par le transport thermique

Cette thèse traite l'interaction d'ordres en compétition dans les cuprates supraconducteurs dopés en trous; il sera question de supraconductivité et d'ordre de charge. Dans une première étude, la conductivité thermique $\kappa_{\rm xx}$ sous forts champs magnétiques du cuprate YBCO est utilisée pour mesurer le champ critique $H_{\rm c2}$. Cette expérience révèle la forte compétition entre la supraconductivité et l'ordre de charge dans ces matériaux dopés en trous. Ce résultat représente la première mesure directe de champ critique $H_{\rm c2}$ dans cette famille de supraconducteurs et démontre l'absence de liquide de vortex à température nulle. Dans une deuxième étude, la combinaison de l'effet Hall thermique $\kappa_{\rm xy}$ et de mesures électriques sous forts champs magnétiques permet l'exploration de la loi de Wiedemann-Franz dans le cuprate YBCO. En démontrant que cette loi est satisfaite au-dessus du champ magnétique critique $H_{\rm c2}$ déterminé lors du premier projet, cette expérience montre qu'il ne reste pas de supraconductivité au-dessus du champ magnétique critique et que l'état normal des cuprates sous-dopés est métallique. Dans une troisième étude, l'effet Hall thermique $\kappa_{\rm xy}$ est utilisé pour sonder la surface de Fermi d'un matériau dans sa phase supraconductrice. Pour les cuprates sous-dopés en trous, ce projet révèle qu'il n'y a pas de reconstruction de la surface de Fermi en champ nul par l'ordre de charge à courte portée. Cette expérience pionnière représente ainsi le trait d'union manquant entre de nombreux résultats qui pourtant parurent contradictoires au premier abord

Anisotropic Seebeck coefficient of Sr2_2RuO4_4 in the incoherent regime

5 pages, 2 figures, accepted paperInternational audienceIntuitive entropic interpretations of the thermoelectric effect in metals predict an isotropic Seebeck coefficient at high temperatures in the incoherent regime even in anisotropic metals since entropy is not directional. Sr$_2$RuO$_4$ is an enigmatic material known for a well characterised anisotropic normal state and unconventional superconductivity. Recent ab-initio transport calculations of Sr$_2$RuO$_4$ that include the effect of strong electronic correlations predicted an enhanced high-temperature anisotropy of the Seebeck coefficient at temperatures above 300 K, but experimental evidence is missing. From measurements on clean Sr$_2$RuO$_4$ single crystals along both crystallographic directions, we find that the Seebeck coefficient becomes increasingly isotropic upon heating towards room temperature as generally expected. Above 300 K, however, S acquires a new anisotropy which rises up to the highest temperatures measured (750 K), in qualitative agreement with calculations. This is a challenge to entropic interpretations and highlights the lack of an intuitive framework to understand the anisotropy of thermopower at high temperatures

Anisotropic Seebeck coefficient of Sr2_2RuO4_4 in the incoherent regime

5 pages, 2 figures, accepted paperInternational audienceIntuitive entropic interpretations of the thermoelectric effect in metals predict an isotropic Seebeck coefficient at high temperatures in the incoherent regime even in anisotropic metals since entropy is not directional. Sr$_2$RuO$_4$ is an enigmatic material known for a well characterised anisotropic normal state and unconventional superconductivity. Recent ab-initio transport calculations of Sr$_2$RuO$_4$ that include the effect of strong electronic correlations predicted an enhanced high-temperature anisotropy of the Seebeck coefficient at temperatures above 300 K, but experimental evidence is missing. From measurements on clean Sr$_2$RuO$_4$ single crystals along both crystallographic directions, we find that the Seebeck coefficient becomes increasingly isotropic upon heating towards room temperature as generally expected. Above 300 K, however, S acquires a new anisotropy which rises up to the highest temperatures measured (750 K), in qualitative agreement with calculations. This is a challenge to entropic interpretations and highlights the lack of an intuitive framework to understand the anisotropy of thermopower at high temperatures

Anisotropic Seebeck coefficient of Sr2_2RuO4_4 in the incoherent regime

5 pages, 2 figures, accepted paperInternational audienceIntuitive entropic interpretations of the thermoelectric effect in metals predict an isotropic Seebeck coefficient at high temperatures in the incoherent regime even in anisotropic metals since entropy is not directional. Sr$_2$RuO$_4$ is an enigmatic material known for a well characterised anisotropic normal state and unconventional superconductivity. Recent ab-initio transport calculations of Sr$_2$RuO$_4$ that include the effect of strong electronic correlations predicted an enhanced high-temperature anisotropy of the Seebeck coefficient at temperatures above 300 K, but experimental evidence is missing. From measurements on clean Sr$_2$RuO$_4$ single crystals along both crystallographic directions, we find that the Seebeck coefficient becomes increasingly isotropic upon heating towards room temperature as generally expected. Above 300 K, however, S acquires a new anisotropy which rises up to the highest temperatures measured (750 K), in qualitative agreement with calculations. This is a challenge to entropic interpretations and highlights the lack of an intuitive framework to understand the anisotropy of thermopower at high temperatures

Data for Linear-in temperature resistivity from an isotropic Planckian scattering rate

A variety of ‘strange metals’ exhibit resistivity that decreases linearly with temperature as the temperature decreases to zero1,2,3, in contrast to conventional metals where resistivity decreases quadratically with temperature. This linear-in-temperature resistivity has been attributed to charge carriers scattering at a rate given by ħ/τ = αkBT, where α is a constant of order unity, ħ is the Planck constant and kB is the Boltzmann constant. This simple relationship between the scattering rate and temperature is observed across a wide variety of materials, suggesting a fundamental upper limit on scattering—the ‘Planckian limit’4,5—but little is known about the underlying origins of this limit. Here we report a measurement of the angle-dependent magnetoresistance of La1.6−xNd0.4SrxCuO4—a hole-doped cuprate that shows linear-in-temperature resistivity down to the lowest measured temperatures6. The angle-dependent magnetoresistance shows a well defined Fermi surface that agrees quantitatively with angle-resolved photoemission spectroscopy measurements7 and reveals a linear-in-temperature scattering rate that saturates at the Planckian limit, namely α = 1.2 ± 0.4. Remarkably, we find that this Planckian scattering rate is isotropic, that is, it is independent of direction, in contrast to expectations from ‘hotspot’ models8,9. Our findings suggest that linear-in-temperature resistivity in strange metals emerges from a momentum-independent inelastic scattering rate that reaches the Planckian limit