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

Degradation of Phenol, Benzene and Cyanides in an Innovative Semi- industrial Ozonation Pilot Plant: The Effects of Operating Parameters

International audienc

Transport Spectroscopy of the Field Induced Cascade of Lifshitz Transitions in YbRh2Si2

A series of strong anomalies in the thermoelectric power is observed in the heavy fermion compound YbRh$_2$Si$_2$ under the effect of magnetic field varying in the range from 9.5~T to 13~T. We identify these features with a sequence of topological transformations of the sophisticated Fermi surface of this compound, namely a cascade of Lifshitz topological transitions. In order to undoubtedly attribute these anomalies to the specific topological changes of the Fermi surface, we employ the renormalized band method. Basing on its results we suggest a simplified model consisting of the large peripheral Fermi surface sheet and the number of continuously appearing (disappearing) small "voids" or "necks". We account for the multiple electron scattering processes between various components of the Fermi surface, calculate the corresponding scattering times, and, finally, find the magnetic field dependence of the Seebeck coefficient. The obtained analytical expression reproduces reasonably the observed positions of the maxima and minima as well as the overall line shapes and allows us to identify the character of corresponding topological transformations.Comment: 7 pages, 6 figure, to appear in J. Phys. Soc. Jp

Unveiling the double-peak structure of quantum oscillations in the specific heat

Quantum oscillation phenomenon is an essential tool to understand the electronic structure of quantum matter. Here we report the first systematic study of quantum oscillations in the electronic specific heat $C_{el}$ in natural graphite. We show that the crossing of a single spin Landau level and the Fermi energy give rise to a double-peak structure, in striking contrast to the single peak expected from Lifshitz-Kosevich theory. Intriguingly, the double-peak structure is predicted by the kernel term for $C_{el}/T$ in the free electron theory. The $C_{el}/T$ represents a spectroscopic tuning fork of width 4.8 $k_B T$ which can be tuned at will to resonance. Using a coincidence method, the double-peak structure can be used to accurately determine the Lande $g$-factor of quantum materials. More generally, the tuning fork can be used to reveal any peak in fermionic density of states tuned by magnetic field, such as Lifshitz transition in heavy-fermion compounds.Comment: 22 pages, 5 figure

Field-induced compensation of magnetic exchange as the origin of superconductivity above \texorpdfstring{40\,T}{40~T} in \texorpdfstring{\UTe}{UTe2}

The potential spin-triplet heavy-fermion superconductor \UTe exhibits signatures of multiple distinct superconducting phases. For field aligned along the $b$ axis, a metamagnetic transition occurs at $\mu_0$\Hm$\approx35\,$T. It is associated with magnetic fluctuations that may be beneficial for the field-enhanced superconductivity surviving up to \Hm. Once the field is tilted away from the $b$ towards the $c$ axis, a reentrant superconducting phase emerges just above \Hm. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of $\mu_0$\Hc$\approx 73\,$T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above \Hm in the angular range around $30^\circ$ caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a possible mechanism for the reentrant superconductivity above \Hm.Comment: Main text: 27 pages, 4 figure, supplement: 10 pages, 5 figure

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

We report the magnetoresistance in the novel spin-triplet superconductor UTe2 under pressure close to the critical pressure Pc, where the superconducting phase terminates, for field along the three a, b and c-axes in the orthorhombic structure. The superconducting phase for H // a-axis just below Pc shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields. The upper critical field Hc2 for H // c-axis shows a quasi-vertical increase in the H-T phase diagram just below Pc, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20T. Increasing pressure leads to the disappearance of superconductivity at zero field with the emergence of magnetic order. Surprisingly, field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable. The huge Hc2 in the spin-polarized state seems to imply a spin-triplet state. Contrary to the a- and c-axes, no field-reinforcement of superconductivity occurs for magnetic field along the b-axis. We compare the results with the field-reentrant superconductivity above the metamagnetic field, Hm for the field direction tilted by about 30 deg. from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe.Comment: 7 pages, 5 figures, submitted to J. Phys. Soc. Jp

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

L'effet Nernst dans les systèmes corrélés : étude des fluctuations supraconductrices dans NbxSi1-x et des ordres électroniques dans PrFe4P12

Although the Nernst effect was poorly used since its discovery in 1886, it acquired recently an important place in the field of strongly correlated systems.In this PhD. work, we use the Nernst effect to study two examples of correlated systems, the superconductor NbxSi1−x and the heavy fermion PrFe4P12.A study of superconducting amorphous thin films of Nb0.15Si0.85 shows that, above the superconducting transition, the Nernst signal observed is in perfect agreement with the theoretical prediction of Ussiskhin, Sondhi and Huse (USH) within the limit of weak magnetic field and close to Tc. The USH theory, based on the existence of short lived Cooper pairs above Tc, directly connects the Nernst coefficient to the zero field correlation length. This correlation length sets the size of the fluctuating Cooper pairs. More generally, the Nernst signal is shown to be determined by a unique length at arbitrary temperature and magnetic field. Consequently, the USH theory appears to be the low field limit of amore general theory which would connect the Nernst coefficient to the correlation length. These results show that the Nernst signal observed above Tc until very high temperature (30 × Tc) and very high magnetic field (3 × Bc2) in amorphous films of NbxSi1−x is generated by fluctuating Cooper pairs. The second study, carried out on the heavy fermion compound PrFe4P12, enabled us to characterize the phases appearing in this material at low temperature. The exceptional amplitude of the Nernst effect observed in the ordered phase at low magnetic field of PrFe4P12 is the consequence of three independent factors : a low carriers density, an increase in the effective mass and a large mean free path. This behavior is characteristic of a semi-metallic heavy fermion. The important increase in the thermoelectric power in the ordered phase is revealing of an important destruction of the Fermi surface. The phase which appears in high field for the direction [111] seems also related to a reorganization of the Fermi surface, in a less important way though, with a non Fermi liquid behavior. The sign change of the Nernst coeficient with the onset of the ordering at high magnetic field appears to be the consequence of a metamagnetic transition.L'effet Nernst, bien que peu exploité depuis sa découverte en 1886, a acquis récemment une place importante dans le domaine des électrons corrélés. Au cours de cette thèse, nous avons utilisé l'effet Nernst afin d'étudier deux exemples de systèmes corrélés, un supraconducteur NbxSi1-x et un fermion lourd PrFe4P12. Dans l'étude des films amorphes supraconducteurs de Nb0.15Si0.85, le signal Nernst observé est en parfait accord avec la prédiction théorique de Ussiskhin, Sondhi et Huse (USH) dans la limite de faible champ magnétique et près de Tc. La théorie USH qui se fonde sur l'existence de paires de Cooper au temps de vie fini au-dessus de Tc, relie directement le coefficient Nernst à la longueur de corrélation à champ nul, c-à-d la taille des paires de Cooper fluctuantes. L'étude approfondie des données a montré que, de façon plus générale, le signal Nernst est déterminé par une seule longueur, la longueur de corrélation à toute température et tout champ magnétique. La théorie USH n'est que la limite bas champ d'une théorie plus générale qui relierait le coefficient Nernst à la longueur de corrélation. Ces résultats démontrent que le signal Nernst observé au-dessus de Tc jusqu'à très haute température (30 xTc) et jusqu'à très haut champ magnétique (3 X Bc2) dans les films amorphes de Nb0.15Si0.85 est généré par les fluctuations supraconductrices de type paires de Cooper fluctuantes. La seconde étude que nous avons effectuée dans le composé fermion lourd PrFe4P12 nous a permis de caractériser les phases qui apparaissent dans ce matériau à basse température. L'amplitude exceptionnelle de l'effet Nernst observée dans la phase ordonnée à bas champ magnétique est la conséquence de trois facteurs indépendants : une faible densité de porteurs, une augmentation de la masse effective et un grand libre parcours moyen. Ce comportement est caractéristique d'un fermion lourd semi-métallique. L'augmentation importante du pouvoir thermoélectrique dans la phase ordonnée est révélatrice d'une destruction importante de la surface de Fermi. La phase qui apparaît à haut champ magnétique pour la direction [111] semble également liée à une restructuration de la surface de Fermi, bien que moins importante, associée à un comportement non liquide de Fermi. Le changement de signe de l'effet Nernst lors de l'apparition de la phase à haut champ magnétique pourrait s'interpréter comme le signe d'une transition métamagnétique