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Louis Taillefer

Publications and source records attributed to Louis Taillefer.

At least 19 recordsLinked to original sources
arXiv2026

Anomalously high quasiparticle thermal conductivity in the underdoped cuprate superconductor HgBa$_{2}$CuO$_{4+\delta}$

The single-layer cuprate superconductor HgBa$_{2}$CuO$_{4+\delta}$ (Hg1201) is an ideal candidate for investigating many properties of cuprates with minimal disorder and without the complication of multiple CuO$_2$ layers. Here we measure the in-plane longitudinal thermal conductivity $\kappa$ of underdoped Hg1201 ($T_c$ = 76 K, $p$ = 0.11) at dilution refrigerator temperatures to extract the nodal quasiparticle velocity ratio $v_F/v_\Delta$. Assuming contributions from only a single line node per quadrant on the Fermi surface leads to a value of $v_F/v_\Delta$ = $23 \pm 3$, anomalously large compared to other cuprates at similar dopings. In conjunction with the anomalously high quasiparticle specific heat of Hg1201 in the normal state reported previously at a similar doping, this points to more than one Fermi surface sheet crossing the nodal line, suggesting the presence of more than the single small electron pocket detected by quantum oscillations.

cond-mat.supr-con↗
arXiv2025

Electronic mean free path of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ from thermal Hall conductivity

We use thermal transport to access the electronic mean free path of $d$-wave quasiparticles in one of the most widely studied cuprate superconductors, Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212). We have measured the thermal conductivity $\kappa_{\rm xx}$ and the thermal Hall conductivity $\kappa_{\rm xy}$ of three single crystals across a range of dopings. In the overdoped and optimally-doped samples, a clear enhancement is observed in both $\kappa_{\rm xx}$ and $\kappa_{\rm xy}$ upon cooling below the critical temperature $T_{\rm c}$, due to a suppression of the inelastic electron-electron scattering as electrons condense into pairs. The underdoped sample shows no enhancement in either, pointing to a high degree of disorder in that sample. For the two highest dopings, the magnitude of the enhancement in $\kappa_{\rm xy}$ is controlled by the strength of the elastic impurity scattering. Using a prior model to estimate the mean free path from $\kappa_{\rm xy}$ data, we find that the mean free path in Bi2212 is approximately 7 times shorter than in YBa$_2$Cu$_3$O$_7$, considered to be one of the least disordered cuprates. We conclude that the thermal Hall technique is a good way to compare the mean free path of $d$-wave quasiparticles in various cuprate materials.

cond-mat.supr-con↗
arXiv2025

Thermal Hall conductivity in the strongest cuprate superconductor: Estimate of the mean free path in the trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8 + \delta}$

The thermal Hall conductivity of the trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg1223) - the superconductor with the highest critical temperature $T_c$ at ambient pressure - was measured at temperatures down to 2 K for three dopings in the underdoped regime ($p$ = 0.09, 0.10, 0.11). By combining a previously introduced simple model and prior theoretical results, we derive a formula for the inverse mean free path, $1 / \ell$, which allows us to estimate the mean free path of $d$-wave quasiparticles in Hg1223 below $T_c$. We find that $1 / \ell$ grows as $T^3$, in agreement with the theoretical expectation for a clean $d$-wave superconductor. Measurements were also conducted on the single layer mercury-based cuprate HgBa$_2$CuO$_{6+\delta}$ (Hg1201), revealing that the mean free path in this compound is roughly half that of its three-layered counterpart at the same doping ($p$ = 0.10). This observation is be attributed to the protective role of the outer planes in Hg1223, which results in a more pristine inner plane. We also report data in an ultraclean crystal of YBa$_2$Cu$_3$O$_y$ (YBCO) with full oxygen content $p$ = 0.18, believed to be the cleanest of any cuprate, and find that $\ell$ is not longer than in Hg1223.

cond-mat.supr-con↗
arXiv2025

Comment on Unusual violation of the Wiedemann-Franz law at ultralow temperatures in topological compensated semimetals

Recently, Wang et al. [1] reported on an unusual violation of Wiedemann-Franz law in three semimetals. We compare their observations to our observations in a variety of systems, where the apparent WF law violations in the same temperature range arise as a consequence of electron-phonon decoupling. Given the empirical similarity of their data with these cases, the most plausible explanation for the reported violation is an experimental artefact.

cond-mat.str-el↗
arXiv2023

Planar parallel phonon Hall effect and local symmetry breaking

Y-kapellasite [Y3Cu9(OH)19Cl8] is a frustrated antiferromagnetic insulator which remains paramagnetic down to a remarkably low N\'eel temperature of about 2 K. Having studied this material in the paramagnetic regime, in which phonons are the only possible heat carriers, we report the observation of a planar parallel thermal Hall effect coming unambiguously from phonons. This is an advantage over the Kitaev quantum spin liquid candidates {\alpha}-RuCl3 and Na2Co2TeO6 where in principle other heat carriers can be involved [1-4]. As it happens, Y-kapellasite undergoes a structural transition attributed to the positional freezing of a hydrogen atom below about 33 K. Above this transition, the global crystal symmetry forbids the existence of a planar parallel signal - the same situation as in Na2Co2TeO6 and cuprates [3-5]. This points to the notion of a local symmetry breaking at the root of the phonon Hall effect. In this context, the advantage of Y-kapellasite over Na2Co2TeO6 (with high levels of Na disorder and stacking faults) and cuprates (with high levels of disorder coming from dopants and oxygen vacancies) is its clean structure, where the only degree of freedom available for local symmetry breaking is this hydrogen atom randomly distributed over six equivalent positions above 33 K. This provides a specific and concrete case for the general idea of local symmetry breaking leading to the phonon Hall effect in a wide range of insulators.

cond-mat.str-el↗
arXiv2023

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.

cond-mat.str-el↗
arXiv2023

Planar thermal Hall effect from phonons in cuprates

A surprising "planar" thermal Hall effect, whereby the field is parallel to the current, has recently been observed in a few magnetic insulators, and this has been attributed to exotic excitations such as Majorana fermions or chiral magnons. Here we investigate the possibility of a planar thermal Hall effect in three different cuprate materials, in which the conventional thermal Hall conductivity $\kappa_{\rm {xy}}$ (with an out-of-plane field perpendicular to the current) is dominated by either electrons or phonons. Our measurements show that the planar $\kappa_{\rm {xy}}$ from electrons in cuprates is zero, as expected from the absence of a Lorentz force in the planar configuration. By contrast, we observe a sizable planar $\kappa_{\rm {xy}}$ in those samples where the thermal Hall response is due to phonons, even though it should in principle be forbidden by the high crystal symmetry. Our findings call for a careful re-examination of the mechanisms responsible for the phonon thermal Hall effect in insulators.

cond-mat.str-el↗
arXiv2023

Planar thermal Hall effect from phonons in a Kitaev candidate material

Kitaev materials are a promising platform for the realization of quantum spin liquid states. The thermal Hall effect has emerged as a potential probe of exotic excitations within such states. In the Kitaev candidate material $\alpha$-RuCl$_{3}$, however, the thermal Hall conductivity $\kappa_{xy}$ has been attributed not only to exotic Majorana fermions or chiral magnons, but also to phonons. It has been shown theoretically that the former two types of heat carriers can generate a "planar" thermal Hall effect, whereby the magnetic field is parallel to the heat current, as observed experimentally, but it is unknown whether phonons also could. Here we show that a planar thermal Hall effect is present in another Kitaev candidate material, Na$_{2}$Co$_{2}$TeO$_{6}$. On the basis of a striking similarity between the temperature and field dependence of $\kappa_{xy}$ and that of the phonon-dominated thermal conductivity $\kappa_{xx}$, we argue that the planar thermal Hall effect in Na$_{2}$Co$_{2}$TeO$_{6}$ is generated by phonons. The phonon contributed planar $\kappa_{xy}$ also shows a strong sample dependence, which indicates an extrinsic origin of the mechanism. By conducting a complete study with different in-plane configurations of heat current $J$ and magnetic field $H$, $i.e.$ $H$ $\|$ $J$ and $H$ $\bot$ $J$, we observe a large difference in $\kappa_{xy}$ between these two configurations, which reveals that the direction of the heat current $J$ may play an important role in determining the planar thermal Hall effect. Our observation calls for a re-evaluation of the planar thermal Hall effect observed in $\alpha$-RuCl$_{3}$.

cond-mat.str-el↗
arXiv2023

Role of magnetic ions in the thermal Hall effect of the paramagnetic insulator TmVO$_{4}$

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 in a family of pyrochlores, 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 the nonmagnetic pyrochlore analog (where the Tb$^{3+}$ ions are replaced with Y$^{3+}$), 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 $x$ = 0.3 in Tm$_{1-x}$Y$_{x}$VO$_{4}$, $\kappa_{xy}$ was found to have a positive sign, pointing to the importance of impurities in the thermal Hall effect of phonons.

cond-mat.mtrl-sci↗
arXiv2023

Anisotropic Seebeck coefficient of $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ 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.

cond-mat.str-el↗
arXiv2023

Impurity-induced phonon thermal Hall effect in the antiferromagnetic phase of Sr2IrO4

A thermal Hall effect is observed in an increasing number of insulators often attributed to phonons, but the underlying mechanism is in most cases unknown. A coupling of phonons to spins has been invoked and scattering of phonons by impurities or defects has been proposed, but there is no systematic evidence to support either scenario. Here we present a study on the effect of adding Rh impurities to the antiferromagnetic insulator Sr2IrO4, substituting for the spin-carrying Ir atoms. We find that adding small concentrations of Rh impurities increases the thermal Hall conductivity dramatically, but adding enough Rh to suppress the magnetic order eventually decreases the thermal Hall conductivity until it nearly vanishes. We conclude that the thermal Hall effect in this material is caused by the scattering of phonons by impurities embedded within a magnetic environment.

cond-mat.str-el↗
arXiv2023

Oscillations in the magnetothermal conductivity of $\boldsymbolα$-RuCl$_3$: Evidence of transition anomalies

The 2D layered insulator $α$-RuCl$_3$ is a candidate material for a quantum spin-liquid state, which may be realized when a magnetic field suppresses the antiferromagnetic order present at low temperature. Oscillations in the field dependence of the thermal conductivity, observed for an in-plane magnetic field $B$ up to a critical field $B^{\star}$, have been attributed to exotic charge-neutral fermions, viewed as evidence of a quantum spin-liquid state between the critical field $B_{c}$ $\simeq 7$ T at which the antiferromagnetic phase ends and $B^{\star}$. Here we report measurements of the thermal conductivity of $α$-RuCl$_3$ as a function of magnetic field up to 15 T applied in two distinct in-plane directions: parallel and perpendicular to the Ru-Ru bond. We find that the number of oscillations between $B_{c}$ and $B^{\star}$ is the same for the two field directions even though the field interval between $B_{c}$ and $B^{\star}$ is different. In other words, the period of the oscillations is controlled by the transition fields $B_{c}$ and $B^{\star}$. We conclude that these are not true oscillations -- coming from putative fermions in a spin-liquid state -- but anomalies associated with a sequence of magnetic transitions.

cond-mat.str-el↗
arXiv2022

Fermi surface transformation at the pseudogap critical point of a cuprate superconductor

The nature of the pseudogap phase remains a major barrier to our understanding of cuprate high-temperature superconductivity. Whether or not this metallic phase is defined by any of the reported broken symmetries, the topology of its Fermi surface remains a fundamental open question. Here we use angle-dependent magnetoresistance (ADMR) to measure the Fermi surface of the cuprate Nd-LSCO. Above the critical doping $p^*$ -- outside of the pseudogap phase -- we fit the ADMR data and extract a Fermi surface geometry that is in quantitative agreement with angle-resolved photoemission. Below $p^*$ -- within the pseudogap phase -- the ADMR is qualitatively different, revealing a clear transformation of the Fermi surface. Changes in the quasiparticle lifetime across $p^*$ are ruled out as the cause of this transformation. Instead we find that our data are most consistent with a reconstruction of the Fermi surface by a $Q=(π, π)$ wavevector.

cond-mat.str-el↗
arXiv2022

Sub-Phases in the Superconducting State of CeIrIn$_5$ Revealed by Low Temperature $c$-axis Heat Transport

Low-temperature (down to $\sim$ 50 mK) thermal conductivity measurements with the heat flow direction along the inter-plane tetragonal $c$-axis, $κ_c$, were used to study the superconducting state of heavy fermion CeIrIn$_5$. Measurements were performed in the magnetic fields both parallel to the heat flow direction, $H\parallel c$, and transverse to it, $H \parallel a$. Inter-plane heat conductivity in $H \parallel c$ configuration shows negligible initial increase with magnetic field and a rapid rise on approaching $H_{c2}$ from below, similar to the expectations for the superconducting gap without line nodes. This observation is in stark contrast to monotonic increase found in the previous in-plane heat transport measurements. In the configuration with the magnetic field breaking the tetragonal symmetry of the lattice, $H \parallel a$, $κ_c$ reveals non-monotonic evolution with temperature and magnetic field suggesting sub-phase boundary in the superconducting state. The characteristic temperature $T_{kink} \sim$ 0.07~K of the sub-boundary is well within the domain of bulk superconductivity $T_c \sim$ 0.4~K and $H_{c2}\sim$ 1.0~T. These results are consistent with a superconducting gap with an equatorial line node and polar point nodes, a gap symmetry of the D$_{4h}$ point group, for which magnetic field along the tetragonal plane breaks the degeneracy of the multi-component order parameter and induces a phase transition with nodal topology change.

cond-mat.supr-con↗
arXiv2022

Thermal Hall conductivity of electron-doped cuprates

Measurements of the thermal Hall conductivity in hole-doped cuprates have shown that phonons acquire chirality in a magnetic field, both in the pseudogap phase and in the Mott insulator state. The microscopic mechanism at play is still unclear. A number of theoretical proposals are being considered, including skew scattering of phonons by various defects, the coupling of phonons to spins, and a state of loop-current order with the appropriate symmetries, but more experimental information is required to constrain theoretical scenarios. Here we present our study of the thermal Hall conductivity $κ_{\rm {xy}}$ in the electron-doped cuprates Nd$_{2-x}$Ce$_x$CuO$_4$ and Pr$_{2-x}$Ce$_x$CuO$_4$, for dopings across the phase diagram, from $x$ = 0, in the insulating antiferromagnetic phase, up to $x$ = 0.17, in the metallic phase above optimal doping. We observe a large negative thermal Hall conductivity at all dopings, in both materials. Since heat conduction perpendicular to the CuO$_2$ planes is dominated by phonons, the large thermal Hall conductivity we observe in electron-doped cuprates for a heat current in that direction must also be due to phonons, as in hole-doped cuprates. Measurements with a heat current perpendicular to the CuO$_2$ planes confirm that phonons are responsible for this thermal Hall signal, as in hole-doped cuprates. However, the degree of chirality, measured as the ratio $|κ_{\rm {xy}}$ / $κ_{\rm {xx}} |$, where $κ_{\rm {xx}}$ is the longitudinal thermal conductivity, is much larger in the electron-doped cuprates. We discuss various factors that may be involved in the mechanism that confers chirality to phonons in cuprates, including short-range spin correlations.

cond-mat.supr-con↗
arXiv2021

Anomalous thermoelectric effects and quantum oscillations in the kagome metal CsV$_3$Sb$_5$

The kagome metal compounds $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) feature a wealth of phenomena including nontrivial band topology, charge density wave (CDW), and superconductivity. One intriguing property is the time-reversal symmetry breaking in the CDW state without local moments, which leads to anomalous transport responses. Here, we report the investigation of magneto-thermoelectric effects on high-quality CsV$_3$Sb$_5$ single crystals. A large anomalous Nernst effect is observed at temperatures below 30 K. Multiple Fermi surfaces with small effective masses are revealed by quantum oscillations in Nernst and Seebeck signals under high magnetic field. Furthermore, we find an unknown frequency, and attribute it to the magnetic breakdown across two smaller Fermi surfaces. A gap around 20 meV can be resolved from the breakdown threshold field, which we propose to be introduced by the CDW. These results shed new light on the CDW-related phenomena, particularly in $A$V$_3$Sb$_5$ compounds.

cond-mat.mtrl-sci↗
arXiv2021

Large Phonon Thermal Hall Conductivity in a Simple Antiferromagnetic Insulator

Phonons are known to generate a thermal Hall effect in certain insulators, including oxides with rare-earth impurities, quantum paraelectrics, multiferroic materials and cuprate Mott insulators. In each case, a special feature of the material is presumed relevant for the underlying mechanism that confers chirality to phonons in a magnetic field. The question is whether a phonon Hall effect is an unusual occurrence - linked to special characteristics such as skew scattering off rare-earth impurities, structural domains, ferroelectricity, ferromagnetism - or a much more common property of insulators than hitherto believed. To help answer this question, we have turned to a simple insulator, with none of the previously encountered special features: the cubic antiferromagnet Cu$_3$TeO$_6$. We find that it has the largest thermal Hall conductivity $κ_{\rm{xy}}$ of any insulator so far. We show that this record-high $κ_{\rm{xy}}$ signal is due to phonons and it does not require the presence of magnetic order, as it persists above the ordering temperature. We conclude that the phonon Hall effect is likely to be a fairly common property of solids.

cond-mat.str-el↗
arXiv2021

The 2021 Quantum Materials Roadmap

In recent years, the notion of Quantum Materials has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and cold atom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topological quantum matter, two-dimensional materials and their van der Waals heterostructures, Moire materials, Floquet time crystals, as well as materials and devices for quantum computation with Majorana fermions. In this Roadmap collection we aim to capture a snapshot of the most recent developments in the field, and to identify outstanding challenges and emerging opportunities. The format of the Roadmap, whereby experts in each discipline share their viewpoint and articulate their vision for quantum materials, reflects the dynamic and multifaceted nature of this research area, and is meant to encourage exchanges and discussions across traditional disciplinary boundaries. It is our hope that this collective vision will contribute to sparking new fascinating questions and activities at the intersection of materials science, condensed matter physics, device engineering, and quantum information, and to shaping a clearer landscape of quantum materials science as a new frontier of interdisciplinary scientific inquiry.

cond-mat.mes-hall↗