19 research outputs found
Conductivité thermique dans les cuprates dopés en électrons
Dans cette thĂšse, la mesure de lâeffet Hall thermique a Ă©tĂ© utilisĂ©e pour Ă©tudier diffĂ©rents matĂ©riaux quantiques, plus particuliĂšrement les cuprates. Cette nouvelle sonde prometteuse est sensible aux excitations Ă©lectroniques, magnĂ©tiques et possiblement topologiques de la matiĂšre. Ătonnamment, il sâest avĂ©rĂ© que les phonons peuvent gĂ©nĂ©rer un effet Hall thermique non nul dans la phase pseudogap des cuprates dopĂ©s en trous, et ce jusquâĂ la phase isolante. Les phonons, particules sans charge, se sont avĂ©rĂ©s ĂȘtre les porteurs de chaleur responsables de ce signal transverse. La question premiĂšre Ă laquelle cette thĂšse veut rĂ©pondre est : quel est le mĂ©canisme responsable de lâeffet Hall thermique des phonons dans les cuprates, ou les isolants en gĂ©nĂ©ral.
Cette thĂšse par articles prĂ©sente les rĂ©sultats de trois projets distincts. Le premier est lâeffet Hall thermique dans les cuprates isolants de Mott. Cette Ă©tude a confirmĂ© lâorigine phononique de lâeffet Hall thermique dans les cuprates, tout en Ă©liminant divers mĂ©canismes pouvant gĂ©nĂ©rer un effet Hall thermique dans les isolants, comme les domaines structuraux et magnĂ©tiques ou la diffusion sur des impuretĂ©s de terres rares. La deuxiĂšme Ă©tude porte sur lâeffet Hall thermique dans les cuprates dopĂ©s en Ă©lectrons, de la phase isolante Ă la phase mĂ©tallique. Un signal dâeffet Hall thermique attribuĂ© aux phonons est toujours prĂ©sent, et ce Ă tous les dopages. Les fluctuations antiferromagnĂ©tiques bidimensionnelles Ă©tant prĂ©sentes Ă tous les dopages mesurĂ©s, cela semble ĂȘtre un Ă©lĂ©ment Ă examiner dans la recherche du mĂ©canisme responsable de lâeffet Hall thermique des phonons. Finalement, le dernier projet prĂ©sentĂ© dans cette thĂšse porte sur lâĂ©tude dâun isolant antiferromagnĂ©tique cubique, Cu3TeO6. Lâeffet Hall thermique de cet isolant est dâamplitude relative similaire aux cuprates dopĂ©s en trous, et ne montre aucune anomalie lors de la transition magnĂ©tique Ă TN. Ainsi, lâordre magnĂ©tique Ă longue portĂ©e nâest pas nĂ©cessaire Ă la conductivitĂ© thermique de Hall des phonons et la physique particuliĂšre des cuprates non plus
Ătude de la conductivitĂ© thermique dans SmB6, un isolant de Kondo topologique
Lâhexaborure de samarium, SmB6, est un isolant de Kondo topologique dĂ©couvert il y a plus de 50 ans. Tout rĂ©cemment, lâobservation dâoscillations quantiques dans SmB6 suggĂšre quâil pourrait y avoir des excitations fermioniques sans charge Ă basse tempĂ©rature dans le volume isolant de ce matĂ©riau. Il sâagit dâune proposition rĂ©volutionnaire ! Une façon de dĂ©tecter de telles excitations est via leur capacitĂ© Ă transporter lâentropie, ce quâune mesure de conductivitĂ© thermique devrait dĂ©tecter comme un terme linĂ©aire rĂ©siduel dans la limite oĂč la tempĂ©rature tend vers zĂ©ro.
Ce mĂ©moire porte sur des mesures de conductivitĂ© thermique dans SmB6 jusquâĂ 70 mK sur divers monocristaux et dans des champs magnĂ©tiques allant jusquâĂ 15 T. Pour tous les Ă©chantillons, en extrapolant les donnĂ©es Ă basse tempĂ©rature, nous observons que le terme rĂ©siduel est nul. Nous ne trouvons donc aucune preuve directe de la prĂ©sence de fermions. Cependant, nous observons une augmentation importante de la conductivitĂ© thermique lors de lâapplication dâun champ magnĂ©tique. Nous proposons un scĂ©nario oĂč la chaleur est portĂ©e principalement par des phonons qui sont diffusĂ©s par des impuretĂ©s magnĂ©tiques, un mĂ©canisme qui serait attĂ©nuĂ© par lâapplication dâun champ. Les donnĂ©es prĂ©sentĂ©es ici seront comparĂ©es Ă deux Ă©tudes semblables publiĂ©es rĂ©cemment. Les rĂ©sultats de ce mĂ©moire ont Ă©tĂ© publiĂ©s en juin 2018 dans la revue Physical Review B
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 NdCeCuO, at
high doping, the phonon and electron contributions to the thermal Hall
conductivity are of comparable magnitude, but of opposite
sign. In samples of lower quality, phonons dominate ,
resulting in a negative at all temperatures. The fact that
the negative phononic 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 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
below (and only below) the critical doping would be evidence that
spin correlations are a property of the pseudogap phase
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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
La1.6âxNd0.4SrxCuO4 (Nd-LSCO), at low temperature in the normal state accessed by suppressing superconduc-
tivity with a magnetic ïŹeld up to H = 31 T. Using an ac thermopower measurement technique suitable for high
pressure and high ïŹeld, we track the pressure evolution of the Seebeck coefïŹcient S. At ambient pressure and
low temperature, S/T in Nd-LSCO was recently found to suddenly increase below the pseudogap critical doping
p
â = 0.23, consistent with a drop in carrier density n from n = 1 + p above pâ to n = p below. Under a pressure
of 2.0 GPa, we observe that the large S/T value just below p
â is suppressed. This conïŹrms a previous pressure
study based on electrical resistivity and Hall effect, which found that pressure lowers p
â, 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 La1.8âxEu0.2SrxCuO4 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 QuĂ©becâNature 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 Foundationâs 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
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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 Îșxy, which appears for dopings p below the pseudogap critical doping pâ, but whose origin is as yet unknown. Because this large Îșxy 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. Îșzy(T)âÎșxy(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>pâ, in both La1.6âxNd0.4SrxCuO4 and La1.8âxEu0.2SrxCuO4 with p=0.24, we observe no c-axis Hall signal, i.e. Îșzy(T)=0, showing that phonons have zero Hall response outside the pseudogap phase. The phonon Hall response appears immediately below pâ=0.23, as confirmed by the large Îșzy(T) signal we find in La1.6âxNd0.4SrxCuO4 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 pâ. 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
Ătude de la conductivitĂ© thermique dans SmB6, un isolant de Kondo topologique
Lâhexaborure de samarium, SmB6, est un isolant de Kondo topologique dĂ©couvert il y a plus de 50 ans. Tout rĂ©cemment, lâobservation dâoscillations quantiques dans SmB6 suggĂšre quâil pourrait y avoir des excitations fermioniques sans charge Ă basse tempĂ©rature dans le volume isolant de ce matĂ©riau. Il sâagit dâune proposition rĂ©volutionnaire ! Une façon de dĂ©tecter de telles excitations est via leur capacitĂ© Ă transporter lâentropie, ce quâune mesure de conductivitĂ© thermique devrait dĂ©tecter comme un terme linĂ©aire rĂ©siduel dans la limite oĂč la tempĂ©rature tend vers zĂ©ro.
Ce mĂ©moire porte sur des mesures de conductivitĂ© thermique dans SmB6 jusquâĂ 70 mK sur divers monocristaux et dans des champs magnĂ©tiques allant jusquâĂ 15 T. Pour tous les Ă©chantillons, en extrapolant les donnĂ©es Ă basse tempĂ©rature, nous observons que le terme rĂ©siduel est nul. Nous ne trouvons donc aucune preuve directe de la prĂ©sence de fermions. Cependant, nous observons une augmentation importante de la conductivitĂ© thermique lors de lâapplication dâun champ magnĂ©tique. Nous proposons un scĂ©nario oĂč la chaleur est portĂ©e principalement par des phonons qui sont diffusĂ©s par des impuretĂ©s magnĂ©tiques, un mĂ©canisme qui serait attĂ©nuĂ© par lâapplication dâun champ. Les donnĂ©es prĂ©sentĂ©es ici seront comparĂ©es Ă deux Ă©tudes semblables publiĂ©es rĂ©cemment. Les rĂ©sultats de ce mĂ©moire ont Ă©tĂ© publiĂ©s en juin 2018 dans la revue Physical Review B
Transport signatures of surface states in a Weyl semimetal: evidence of field driven Fermi arc interferometry
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