Collapse of ferromagnetism and Fermi surface instability near reentrant superconductivity of URhGe

We present thermoelectric power and resistivity measurements in the ferromagnetic superconductor URhGe for magnetic field applied along the hard magnetization b axis of the orthorhombic crystal. Reentrant superconductivity is observed near the the spin reorientation transition at $H_{R}$=12.75 T, where a first order transition from the ferromagnetic to the polarized paramagnetic state occurs. Special focus is given to the longitudinal configuration, where both electric and heat current are parallel to the applied field. The validity of the Fermi-liquid $T^2$ dependence of the resistivity through $H_R$ demonstrates clearly that no quantum critical point occurs at $H_R$. Thus the ferromagnetic transition line at $H_R$ becomes first order implying the existence of a tricritical point at finite temperature. The enhancement of magnetic fluctuations in the vicinity of the tricritical point stimulates the reentrance of superconductivity. The abrupt sign change observed in the thermoelectric power with the thermal gradient applied along the b axis together with the strong anomalies in the other directions is a definitive macroscopic evidence that in addition a significant change of the Fermi surface appears through $H_R$.Comment: 6 pages, 5 figure

Multiple nodeless superconducting gaps in optimally-doped SrTi1−x_{1-x}Nbx_{x}O3_{3}

We present the first study of thermal conductivity in superconducting SrTi$_{1-x}$Nb$_{x}$O$_{3}$, sufficiently doped to be near its maximum critical temperature. The bulk critical temperature, determined by the jump in specific heat, occurs at a significantly lower temperature than the resistive T$_{c}$. Thermal conductivity, dominated by the electron contribution, deviates from its normal-state magnitude at bulk T$_{c}$, following a Bardeen-Rickayzen-Tewordt (BRT) behavior, expected for thermal transport by Bogoliubov excitations. Absence of a T-linear term at very low temperatures rules out the presence of nodal quasi-particles. On the other hand, the field dependence of thermal conductivity points to the existence of at least two distinct superconducting gaps. We conclude that optimally-doped strontium titanate is a multigap nodeless superconductor.Comment: 6 pages including a supplemen

Critical doping for the onset of a two-band superconducting ground state in SrTiO3−δ_{3-\delta}

In doped SrTiO$_{3}$ superconductivity persists down to an exceptionally low concentration of mobile electrons. This restricts the relevant energy window and possible pairing scenarios. We present a study of quantum oscillations and superconducting transition temperature, $T_{c}$ as the carrier density is tuned from $10^{17}$ to $10^{20}$ $cm^{-3}$ and identify two critical doping levels corresponding to the filling thresholds of the upper bands. At the first critical doping, which separates the single-band and the two-band superconducting regimes in oxygen-deficient samples, the steady increase of T$_{c}$ with carrier concentration suddenly stops. Near this doping level, the energy dispersion in the lowest band displays a downward deviation from parabolic behavior. The results impose new constraints for microscopic pairing scenarios.Comment: 5 pages of main article and 4 pages of supplemen

Seebeck Coefficient in a Cuprate Superconductor: Particle-Hole Asymmetry in the Strange Metal Phase and Fermi Surface Transformation in the Pseudogap Phase

We report measurements of the Seebeck effect in both the ab plane (Sa) and along the c axis (Sc) of the cuprate superconductor La1.6−xNd0.4SrxCuO4 (Nd-LSCO), performed in magnetic fields large enough to suppress superconductivity down to low temperature. We use the Seebeck coefficient as a probe of the particle-hole asymmetry of the electronic structure across the pseudogap critical doping p ⋆ ¼ 0.23. Outside the pseudogap phase, at p ¼ 0.24 > p ⋆, we observe a positive and essentially isotropic Seebeck coefficient as T →0. That S > 0 at p ¼ 0.24 is at odds with expectations given the electronic band structure of Nd-LSCO above p ⋆ and its known electronlike Fermi surface. We can reconcile this observation by invoking an energy-dependent scattering rate with a particle-hole asymmetry, possibly rooted in the non-Fermi-liquid nature of cuprates just above p ⋆. Inside the pseudogap phase, for p < p⋆, S a is seen to rise at low temperature as previously reported, consistent with the drop in carrier density n from n ≃1þ p to n ≃p across p ⋆ as inferred from other transport properties. In stark contrast, S c at low temperature becomes negative below p ⋆, a novel signature of the pseudogap phase. The sudden drop in S c reveals a change in the electronic structure of Nd-LSCO upon crossing p ⋆. We can exclude a profound change of the scattering across p ⋆ and conclude that the change in the out-of-plane Seebeck coefficient originates from a transformation of the Fermi surface.L. T. acknowledges support from the Canadian Institute for Advanced Research (CIFAR) as a CIFAR Fellow and funding from the Institut Quantique, the Natural Sciences and Engineering Research Council of Canada (PIN:123817), the Fonds de Recherche du Quebec—Nature et Technologies (FRQNT), the Canada Foundation for Innovation (CFI), and a Canada Research Chair. J. M. is supported by the Slovenian Research Agency (ARRS) under Program No. P1-0044 and Projects No. J1-2458, No. N1-0088, and No. J1-2455-1. J.-S. Z. was supported by NSF MRSEC under Cooperative Agreement No. DMR-1720595. This research was under- taken thanks in part to funding from the Canada First Research Excellence Fund and the Gordon and Betty Moore Foundation’s EPiQS Initiative (Grant No. GBMF5306 to L. T.). The Flatiron Institute is a division of the Simons Foundation.Center for Dynamics and Control of Material

Effect of pressure on the pseudogap and charge density wave phases of the cuprate Nd-LSCO probed by thermopower measurements

We report thermopower measurements under hydrostatic pressure on the cuprate superconductor La1.6−xNd0.4SrxCuO4 (Nd-LSCO), at low temperature in the normal state accessed by suppressing superconduc- tivity with a magnetic field up to H = 31 T. Using an ac thermopower measurement technique suitable for high pressure and high field, we track the pressure evolution of the Seebeck coefficient S. At ambient pressure and low temperature, S/T in Nd-LSCO was recently found to suddenly increase below the pseudogap critical doping p ⋆ = 0.23, consistent with a drop in carrier density n from n = 1 + p above p⋆ to n = p below. Under a pressure of 2.0 GPa, we observe that the large S/T value just below p ⋆ is suppressed. This confirms a previous pressure study based on electrical resistivity and Hall effect, which found that pressure lowers p ⋆, thereby reinforcing the interpretation that this effect is driven by the pressure-induced shift of the van Hove point. It implies that the pseudogap only exists when the Fermi surface is hole-like, which puts strong constraints on theories of the pseudogap phase. We also report thermopower measurements on Nd-LSCO and La1.8−xEu0.2SrxCuO4 in the charge density wave phase near p ∼ 1/8, which reveals a weakening of this phase under pressure.L.T. acknowledges support from the Canadian Institute for Ad- vanced Research (CIFAR) as a CIFAR Fellow and funding from the Institut Quantique, the Natural Sciences and Engi- neering Research Council of Canada (PIN:123817), the Fonds de Recherche du Québec–Nature et Technologies (FRQNT), the Canada Foundation for Innovation (CFI), and a Canada Research Chair. This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund and the Gordon and Betty Moore Foundation’s EPiQS Ini- tiative (Grant No. GBMF5306 to L.T.). The National High Magnetic Field Laboratory is supported by the National Sci- ence Foundation through Grant No. NSF/DMR-1644779 and the State of Florida. J.-S.Z. was supported by NSF MRSEC under Cooperative Agreement No. DMR-1720595.Center for Dynamics and Control of Material

Thermopower across the phase diagram of the cuprate La1.6−xNd0.4SrxCuO4 : signatures of the pseudogap and charge-density-wave phases

The Seebeck coefficient (thermopower) S of the cuprate superconductor La1.6−xNd0.4SrxCuO4 was measured across its doping phase diagram (from p = 0.12 to p = 0.25), at various temperatures down to T ≃ 2 K, in the normal state accessed by suppressing superconductivity with a magnetic field up to H = 37.5 T. The magnitude of S/T in the T = 0 limit is found to suddenly increase, by a factor ≃ 5, when the doping is reduced below p ⋆ = 0.23, the critical doping for the onset of the pseudogap phase. This confirms that the pseudogap phase causes a large reduction of the carrier density n, consistent with a drop from n = 1 + p above p ⋆ to n = p below p⋆, as previously inferred from measurements of the Hall coefficient, resistivity and thermal conductivity. When the doping is reduced below p = 0.19, a qualitative change is observed whereby S/T decreases as T → 0, eventually to reach negative values at T = 0. In prior work on other cuprates, negative values of S/T at T → 0 were shown to result from a reconstruction of the Fermi surface caused by charge- density-wave (CDW) order. We therefore identify pCDW ≃ 0.19 as the critical doping beyond which there is no CDW-induced Fermi surface reconstruction. The fact that pCDW is well separated from p ⋆ reveals that there is a doping range below p⋆ where the transport signatures of the pseudogap phase are unaffected by CDW correlations, as previously found in YBa2Cu3Oy and La2−xSrxCuO4.Center for Dynamics and Control of Material

Phonons become chiral in the pseudogap phase of cuprates

The nature of the pseudogap phase of cuprates remains a major puzzle. One of its new signatures is a large negative thermal Hall conductivity κxy, which appears for dopings p below the pseudogap critical doping p∗, but whose origin is as yet unknown. Because this large κxy is observed even in the undoped Mott insulator La2CuO4, it cannot come from charge carriers, these being localized at p=0. Here we show that the thermal Hall conductivity of La2CuO4 is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO2 planes, i.e. κzy(T)≈κxy(T). This shows that the Hall response must come from phonons, these being the only heat carriers able to move as easily normal and parallel to the planes . At p>p∗, in both La1.6−xNd0.4SrxCuO4 and La1.8−xEu0.2SrxCuO4 with p=0.24, we observe no c-axis Hall signal, i.e. κzy(T)=0, showing that phonons have zero Hall response outside the pseudogap phase. The phonon Hall response appears immediately below p∗=0.23, as confirmed by the large κzy(T) signal we find in La1.6−xNd0.4SrxCuO4 with p=0.21. The microscopic mechanism by which phonons become chiral in cuprates remains to be identified. This mechanism must be intrinsic - from a coupling of phonons to their electronic environment - rather than extrinsic, from structural defects or impurities, as these are the same on both sides of p∗. This intrinsic phonon Hall effect provides a new window on quantum materials and it may explain the thermal Hall signal observed in other topologically nontrivial insulators.Center for Dynamics and Control of Material

Instabilités de surface de Fermi avec et sans transitions magnétiques : étude de URhGe, UPd2AI3, UCoGe et CeIrIn5

In this thesis, we have studied the evolution of the Fermi surface under the influence of a magnetic field in bulk materials that can be easily polarized at low temperature. The first part was devoted to the cases of the ferromagnetic superconductor UCoGe with a magnetic field applied along the easy magnetization c-axis and the paramagnetic superconductor CeIrIn5 with the field along the c-axis. In UCoGe, several successive anomalies were detected in resistivity, Hall effect and thermoelectric power, without any thermodynamic transition being detected in magnetization. The direct observation of quantum oscillations showed that these anomalies are related to topological changes of the Fermi surface, also known as Lifshitz transitions. In CeIrIn5, the thermoelectric power detected an anomaly at HM = 28 T and the quantum oscillations observed in torque magnetometry showed that a Lifshitz transition occurs at this field.In the second part of this thesis, we studied the evolution of the Fermi surface through first order magnetic transitions induced by magnetic field. In the ferromagnetic superconductor URhGe with the field applied along the hard magnetization b-axis and the antiferromagnetic superconductor UPd2Al3 with the field in the basal plane. In URhGe, the thermoelectric power allowed to observe a change in the Fermi surface at the spin reorientation transition at HR = 11.75 T defining the ferromagnetic state and along with resistivity confirmed the first order character of the transition as well as give a location of the tricritical point. In UPd2Al3, a new branch was observed in de Haas-van Alphen experiment in the antiferromagnetic phase and the thermoelectric power showed that the Fermi surface is reconstructed at the metamagnetic transition at HM = 18 T where the antiferromagnetic state is suppressed and could suggest that the Fermi surface changes before this transition. Additionally, four new branches were observed in the polarized paramagnetic phase, above HM, that cannot be associated with calculated branches in the paramagnetic of antiferromagnetic states.Dans cette thèse, j'ai étudié l'évolution de la surface de Fermi sous l'influence d'un champ magnétique dans des systèmes massifs facilement polarisables à basse température. La première partie est dévouée aux cas du supraconducteur ferromagnétique UCoGe et du supraconducteur paramagnétique CeIrIn5, où la surface de Fermi peut être modifiée sans transition magnétique. Dans UCoGe, plusieurs anomalies successives ont été détectées dans l'effet Seebeck, la résistivité et l'effet Hall, sans transition nette dans l'aimantation. L'observation d'oscillations quantiques montre que ces anomalies sont reliées à des changements de topologie de la surface de Fermi, aussi appelés transitions de Lifshitz. Dans CeIrIn5, une anomalie est détectée dans l'effet Seebeck à HM = 28 T et les oscillations quantiques observées en magnétométrie torque montrent qu'une transition de Lifshitz à lieu à ce champ.Dans la deuxième partie, j'ai étudié comment varie la surface de Fermi à travers une transition magnétique du premier ordre induite par le champ magnétique dans le supraconducteur ferromagnétique URhGe avec le champ selon l'axe de difficile aimantation b et le supraconducteur antiferromagnétique UPd2Al3 avec le champ dans le plan basal. Dans URhGe, l'effet Seebeck permet d'observer un changement de la surface de Fermi à la transition de réorientation des spins à HR = 11.75 T et avec la résistivité confirme le caractère premier ordre de la transition en plus de fournir la localisation dans le diagramme de phase du point tricritique. Dans UPd2Al3, une nouvelle branche de la surface de Fermi est observée dans les oscillations quantiques de de Haas-van Alphen dans l'état antiferromagnétique et l'effet Seebeck montre que la surface de Fermi change à la transition métamagnétique à HM = 18 T. En outre, quatre nouvelles branches sont observées dans la phase polarisée au delà de HM et qui ne peuvent être associées à celles calculées dans les états paramagnétique et antiferromagnétique