33 research outputs found
Higgs-mode radiance and charge-density-wave order in 2H-NbSe
Despite being usually considered two competing phenomena, charge-density-wave
and superconductivity coexist in few systems, the most emblematic one being the
transition metal dichalcogenide 2H-NbSe. This unusual condition is
responsible for specific Raman signatures across the two phase transitions in
this compound. While the appearance of a soft phonon mode is a well-established
fingerprint of the charge-density-wave order, the nature of the sharp sub-gap
mode emerging below the superconducting temperature is still under debate. In
this work we use the external pressure as a knob to unveil the delicate
interplay between the two orders, and consequently the nature of the
superconducting mode. Thanks to an advanced extreme-conditions Raman technique
we are able to follow the pressure evolution and the simultaneous collapse of
the two intertwined charge density wave and superconducting modes. The
comparison with microscopic calculations in a model system supports the
Higgs-type nature of the superconducting mode and suggests that
charge-density-wave and superconductivity in 2H-NbSe involve mutual
electronic degrees of freedom. These findings fill knowledge gap on the
electronic mechanisms at play in transition metal dichalcogenides, a crucial
step to fully exploit their properties in few-layers systems optimized for
devices applications
Collapse of critical nematic fluctuations in FeSe under pressure
We report the evolution of the electronic nematic susceptibility in FeSe via
Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the
superconducting transition temperature reaches its maximum. The
critical nematic fluctuations observed at low pressure vanish above 1.6 GPa,
indicating they play a marginal role in the four-fold enhancement of at
higher pressures. The collapse of nematic fluctuations appears to be linked to
a suppression of low energy electronic excitations which manifests itself by
optical phonon anomalies at around 2 GPa, in agreement with lattice dynamical
and electronic structure calculations using local density approximation
combined with dynamical mean field theory. Our results reveal two different
regimes of nematicity in the phase diagram of FeSe under pressure: a d-wave
Pomeranchuk instability of the Fermi surface at low pressure and a magnetic
driven orthorhombic distortion at higher pressure.Comment: 7 pages, 4 figures. Supplementary Material available upon reques
Pressure-Induced Collapse of the Charge Density Wave and Higgs Mode Visibility in 2HâTaS2
The pressure evolution of the Raman active electronic excitations of the transition metal dichalcogenides 2H-TaS2 is followed through the pressure phase diagram embedding incommensurate charge-density-wave and superconducting states. At high pressure, the charge-density wave is found to collapse at 8.5 GPa. In the coexisting charge-density-wave and superconducting orders, we unravel a strong in-gap superconducting mode, attributed to a Higgs mode, coexisting with the expected incoherent Cooper-pair breaking signature. The latter remains in the pure superconducting state reached above 8.5 GPa. Our report constitutes a new observation of such Raman active Higgs mode since the long-standing unique case 2H-NbSe2
Electronic dispersion, correlations and stacking in the photoexcited state of 1T-TaS
Here we perform angle and time-resolved photoelectron spectroscopy on the
commensurate Charge Density Wave (CDW) phase of 1T-TaS. Data with different
probe pulse polarization are employed to map the dispersion of electronic
states below and above the chemical potential. Upon photoexcitation, the
fluctuations of CDW order erase the band dispersion near to the chemical
potential and halve the charge gap size. This transient phase sets within half
a period of the coherent lattice motion and is favored by strong electronic
correlations. The experimental results are compared to Density-Functional
Theory (DFT) calculations with a self-consistent evaluation of the Coulomb
repulsion. Our simulations indicate that the screening of Coulomb repulsion
depends on the stacking order of the TaS layers. The entanglement of such
degrees of freedom suggest that both the structural order and electronic
repulsion are locally modified by the photoinduced CDW fluctuations
Dynamics of electronic states in the Intermediate phase of 1T-TaS
This article reports a comparative study of bulk and surface properties in
the transition metal dichalcogenide 1T-TaS. When heating the sample, the
surface displays an intermediate insulating phase that persists for K
on top of a metallic bulk. The weaker screening of Coulomb repulsion and
stiffer Charge Density Wave (CDW) explain such resilience of a correlated
insulator in the topmost layers. Both time resolved ARPES and transient
reflectivity are employed to investigate the dynamics of electrons and CDW
collective motion. It follows that the amplitude mode is always stiffer at the
surface and displays variable coupling to the Mott-Peierls band, stronger in
the low temperature phase and weaker in the intermediate one
Investigation of Planckian behavior in a high-conductivity oxide : PdCrO2
Funding: JFMV and DC are supported by faculty startup grants at Cornell University. ET and EB were supported by the European Research Council (ERC) under grant HQMAT (Grant Agreement No. 817799), the Israel-US Binational Science Foundation (BSF), and the Minerva Foundation.The layered delafossite metal PdCrO2 is a natural heterostructure ofhighly conductive Pd layers Kondo coupled to localized spins in the adjacentMott insulating CrO2 layers. At high temperatures T it has a T-linearresistivity which is not seen in the isostructural but non-magnetic PdCoO2.The strength of the Kondo coupling is known, as-grown crystals are extremelyhigh purity and the Fermi surface is both very simple and experimentally known.It is therefore an ideal material platform in which to investigate 'Planckianmetal' physics. We do this by means of controlled introduction of pointdisorder, measurement of the thermal conductivity and Lorenz ratio and studyingthe sources of its high temperature entropy. The T-linear resistivity is seento be due mainly to elastic scattering and to arise from a sum of severalscattering mechanisms. Remarkably, this sum leads to a scattering rate within10% of the Planckian value of kBT/â.Publisher PDFPeer reviewe
Controllable suppression of the unconventional superconductivity in bulk and thin-film SrRuO via high-energy electron irradiation
In bulk Sr RuO, the strong sensitivity of the superconducting transition temperature c to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film Sr RuO, the relationship between c and the residual resistivity 0, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film Sr RuO, we show that c is suppressed in both systems at nearly identical rates. This suggests that part of 0 in films comes from defects that do not contribute to superconducting pairbreaking and establishes a quantitative link between the superconductivity of bulk and thin-film samples
Superconductivity and exotic orders : the quest for the Higgs boson
En 1980, une excitation Ă©lectronique inĂ©dite des Ă©lectrons supraconducteurs a Ă©tĂ© mise en Ă©vidence par spectroscopie Raman dans le composĂ© 2H-NbSe2. Cette excitation semble pouvoir ĂȘtre associĂ©e Ă un mode collectif dâamplitude du paramĂštre dâordre supraconducteur.Ce mode, analogue au boson de Higgs dans le modĂšle standard, est normalement non couplĂ© Ă la lumiĂšre mais serait observable dans le cas de 2H-NbSe2 via un couplage de la supraconductivitĂ© avec une autre phase exotique appelĂ©e onde de densitĂ© de charge.Cette thĂšse a consistĂ© Ă utiliser la spectroscopie Raman sous haute pression pour mettre cette thĂ©orie Ă lâĂ©preuve de deux façons. Dâune part, en dĂ©truisant lâonde de densitĂ© de charge dans 2H-NbSe2 sous haute pression et voir si celle-ci est rĂ©ellement nĂ©cessaire Ă lâobservation du mode Higgs. Dâautre part, en cherchant de nouveaux exemples dâobservation du mode de Higgs dans dâautres composĂ©s (2H-TaS2, 2H-TaSe2,...) oĂč la supraconductivitĂ© coexiste avec une onde de densitĂ© de chargeIn 1980, a unique electronic excitation of the superconducting electrons was observed in 2H-NbSe2 by Raman spectroscopy. This excitation can be associated to a collective mode of the amplitude of the superconducting order parameter. This mode, analogous to the Higgs boson in the standard model, is not coupled to light but would be visible in 2H-NbSe2through a coupling to another exotic phase called charge density wave.This thesis consisted in using Raman spectroscopy under high pressures to tackle thistheory in two different ways. Firstly, by inducing a collapse of the charge density wave athigh pressure and see if the coupling to this phase really is a requisite to the visibility of the Higgs mode. Secondly, by searching for new exemples of Higgs modes in other compounds (2H-TaS2, 2H-TaSe2,...) where superconductivity coexists with a charge density wav
Supraconductivité et ordres exotiques : à la recherche du Boson de Higgs
In 1980, a unique electronic excitation of the superconducting electrons was observed in 2H-NbSe2 by Raman spectroscopy. This excitation can be associated to a collective mode of the amplitude of the superconducting order parameter. This mode, analogous to the Higgs boson in the standard model, is not coupled to light but would be visible in 2H-NbSe2through a coupling to another exotic phase called charge density wave.This thesis consisted in using Raman spectroscopy under high pressures to tackle thistheory in two different ways. Firstly, by inducing a collapse of the charge density wave athigh pressure and see if the coupling to this phase really is a requisite to the visibility of the Higgs mode. Secondly, by searching for new exemples of Higgs modes in other compounds (2H-TaS2, 2H-TaSe2,...) where superconductivity coexists with a charge density waveEn 1980, une excitation Ă©lectronique inĂ©dite des Ă©lectrons supraconducteurs a Ă©tĂ© mise en Ă©vidence par spectroscopie Raman dans le composĂ© 2H-NbSe2. Cette excitation semble pouvoir ĂȘtre associĂ©e Ă un mode collectif dâamplitude du paramĂštre dâordre supraconducteur.Ce mode, analogue au boson de Higgs dans le modĂšle standard, est normalement non couplĂ© Ă la lumiĂšre mais serait observable dans le cas de 2H-NbSe2 via un couplage de la supraconductivitĂ© avec une autre phase exotique appelĂ©e onde de densitĂ© de charge.Cette thĂšse a consistĂ© Ă utiliser la spectroscopie Raman sous haute pression pour mettre cette thĂ©orie Ă lâĂ©preuve de deux façons. Dâune part, en dĂ©truisant lâonde de densitĂ© de charge dans 2H-NbSe2 sous haute pression et voir si celle-ci est rĂ©ellement nĂ©cessaire Ă lâobservation du mode Higgs. Dâautre part, en cherchant de nouveaux exemples dâobservation du mode de Higgs dans dâautres composĂ©s (2H-TaS2, 2H-TaSe2,...) oĂč la supraconductivitĂ© coexiste avec une onde de densitĂ© de charg
Planification et replanification pour une constellation de satellites agiles d'observation de la Terre
Le contexte dans lequel s'inscrivent ces travaux est le projet europĂ©en MUSIS, et plus prĂ©cisĂ©ment la gestion de satellites Ă©quipĂ©s d'instruments d'observation optique Ă haute rĂ©solution. Un premier objectif Ă©tait de percevoir un algorithme capable de construire, en une seule passe et en un temps limitĂ©, un plan qui couvre toutes les activitĂ©s de satellites agiles d'observation de la Terre (observation de zones au sol, manĆuvres orbitales, pointage hĂ©liocentriques et gĂ©ocentriques, vidage de donnĂ©es en parallĂšle, activations des instruments), qui respecte toutes les contraintes physiques (y compris celles liĂ©es Ă la trajectoire en attitude), et qui satisfasse autant que possible les requĂȘtes d'utilisateurs. Le second objectif Ă©tait de traiter la version dynamique du problĂšme qui se pose lorsque des requĂȘtes urgentes d'observation arrivent en cours d'exĂ©cution du plan journalier, en tenant compte des exigences de qualitĂ© et de stabilitĂ© des plans et de temps de calcul cette fois trĂšs limitĂ©. L'algorithme de planification Ă©laborĂ© est une succession de recherches chronologiques en avant, avec des rĂšgles de dĂ©cision dĂ©diĂ©es et des mĂ©canismes de backtrack en cas de violation de contraintes. Une approche pragmatique a Ă©galement Ă©tĂ© proposĂ©e pour rĂ©soudre le problĂšme de replanification. Elle consiste Ă utiliser l'algorithme de planification journaliĂšre en jouant sur les prioritĂ©s, les poids et les observations candidates. Un environnement expĂ©rimental intĂ©grant les mĂ©canismes de dĂ©cision retenus a Ă©tĂ© dĂ©veloppĂ©. Les expĂ©rimentations effectuĂ©es sur des instances rĂ©alistes attestent des bonnes performances de l'algorithme en terme de calcul et de qualitĂ© des rĂ©sultats, en modes planification et replanification.TOULOUSE-ISAE (315552318) / SudocSudocFranceF