David Hawthorn

• PhD
• Professor (Associate) at University of Waterloo

In unconventional superconductors, coupled charge and lattice degrees of freedom can manifest in ordered phases of matter that are intertwined. In the cuprate family, fluctuating short-range charge correlations can coalesce into a longer-range charge density wave (CDW) order which is thought to intertwine with superconductivity, yet the nature of t...

We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO$_2$. A combination of oxide molecular-beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconductin...

The discovery of superconductivity in La$_3$Ni$_2$O$_7$ under pressure has motivated the investigation of a parent spin density wave (SDW) state which could provide the underlying pairing interaction. Here, we employ resonant soft x-ray scattering and polarimetry on thin films of bilayer La$_3$Ni$_2$O$_7$ to determine that the magnetic structure of...

Understanding the interplay between charge, nematic, and structural ordering tendencies in cuprate superconductors is critical to unraveling their complex phase diagram. Using pump–probe time-resolved resonant X-ray scattering on the (0 0 1) Bragg peak at the Cu L 3 and O K resonances, we investigate nonequilibrium dynamics of Q a = Q b = 0 nematic...

A hallmark of many unconventional superconductors is the presence of many-body interactions that give rise to broken-symmetry states intertwined with superconductivity. Recent resonant soft X-ray scattering experiments report commensurate 3a0 charge density wave order in infinite-layer nickelates, which has important implications regarding the univ...

Spin and charge density wave order in the cuprates are known to compete with superconductivity. In the stripe order (La,M)2CuO4 family of cuprates, spin and charge order occur as unidirectional order that can be stabilized by symmetry-breaking structural distortions, such as the low-temperature tetragonal (LTT) phase. Here we examine the interplay...

A hallmark of many unconventional superconductors is the presence of many-body interactions which give rise to broken symmetry states intertwined with superconductivity. Recent resonant soft x-ray scattering experiments report commensurate $3a_0$ charge density wave order in the infinite layer nickelates, which has important implications regarding...

Unidirectional spin and charge density wave order in the cuprates is known to compete with superconductivity. In the stripe order (La,M)$_2$CuO$_4$ family of cuprates, spin and charge order occur as unidirectional order that can be stabilized by symmetry breaking structural distortions, such as the low temperature tetragonal (LTT) phase. Here we ex...

Understanding the interplay between charge, nematic, and structural ordering tendencies in cuprate superconductors is critical to unraveling their complex phase diagram. Using pump-probe time-resolved resonant x-ray scattering on the (0 0 1) Bragg peak at the Cu L3 and oxygen K resonances, we investigate non-equilibrium dynamics of Qa = Qb = 0 nema...

Superconductivity and charge density waves (CDWs) are competitive, yet coexisting, orders in cuprate superconductors. To understand their microscopic interdependence, a probe capable of discerning their interaction on its natural length and time scale is necessary. We use ultrafast resonant soft x-ray scattering to track the transient evolution of...

Significance Understanding the character of the enigmatic pseudogap phase is a fundamental problem in the physics of the cuprate superconductors. Key to this problem is identifying what types of order, if any, are associated with pseudogap. Here, we establish using resonant X-ray scattering that electronic nematicity—a rotational symmetry breaking...

In the last decade, rotational and translational symmetry-breaking phases -- electronic nematicity and density wave order -- have been established as generic and distinct features in the cuprate phase diagram. However, their relationship to the pseudogap phase has remained unclear. In this study, we employ resonant x-ray scattering in the cuprate h...

The electronic structure of LiNiO$_2$, a promising Li-ion battery cathode material, has remained a challenge to understand due to its highly covalent yet correlated nature. Here we elucidate the electronic structure in LiNiO$_2$ and the related compound NaNiO$_2$ using x-ray absorption spectra (XAS) and quantum many-body calculations. Notably, we u...

Charge density wave (CDW) order has been shown to compete and coexist with superconductivity in underdoped cuprates. Theoretical proposals for the CDW order include an unconventional d -symmetry form factor CDW, evidence for which has emerged from measurements, including resonant soft x-ray scattering (RSXS) in YBa 2 Cu 3 O 6+ x (YBCO). Here, we re...

Quantum materials are often characterized by intertwined and competitive orders. The competition between superconductivity and charge density waves (CDW) in high-T$_{C}$ cuprates is a paradigmatic example. External tuning knobs, such as high-magnetic-fields and uniaxial strain, have been successfully employed to investigate their interaction. Howev...

Charge density wave (CDW) order has been shown to compete and co-exist with superconductivity in underdoped cuprate superconductors. Theoretical proposals for the CDW order include an unconventional d-symmetry form factor CDW -- a quadrupolar modulation of the charge on oxygen sites surrounding each Cu site -- evidence for which has emerged from ST...

It is known that solid-state reaction in high-pressure oxygen can stabilize high-oxidation phases of Y-Ba-Cu-O superconductors in powder form. We extend this superoxygenation concept of synthesis to thin films which, due to their large surface-to-volume ratio, are more reactive thermodynamically. Epitaxial thin films of $\rm{YBa_2Cu_3O_{7-\delta}}$...

We report optical pump–soft X-ray scattering probe studies of YBCO single crystals. The experiments reveal a picosecond relaxation dynamics of charge density waves, which is in turn strongly modified by the onset of superconductivity.

It is known that solid-state reaction in high-pressure oxygen can stabilize high-oxidation phases of Y-Ba-Cu-O superconductors in powder form. We extend this superoxygenation concept of synthesis to thin films which, due to their large surface-to-volume ratio, are more reactive thermodynamically. Epitaxial thin films of $\rm{YBa_2Cu_3O_{7-\delta}}$...

Sr 14‐x Ca x Cu 24 O 41 is a fascinating member in the family of cuprates, not only because of its peculiar crystal structure where two distinct units, corner‐shared CuO 2 chains and edge‐shared Cu 2 O 3 ladders, coexist within the unit cell but also because it is the only known superconductor with a non two‐dimensional CuO 2 plane structure. Indee...

Understanding the physical properties of the chain-ladder Sr3Ca11Cu24O41 hole-doped superconductor has been precluded by the unknown hole distribution among chains and ladders. We use electron energy-loss spectrometry (EELS) in a scanning transmission electron microscope (STEM) at atomic resolution to directly separate the contributions of chains a...

Disentangling intertwined orders In copper oxide superconductors, several types of order compete for supremacy. In addition to superconductivity, researchers have found periodic patterns in charge density (CDW order), as well as an asymmetry in the electronic density within the unit cell of some cuprates (nematicity). CDW order has been detected in...

In underdoped cuprate superconductors, a rich competition occurs between superconductivity and charge density wave (CDW) order. Whether rotational symmetry breaking (nematicity) occurs intrinsically and generically or as a consequence of other orders is under debate. Here we employ resonant x-ray scattering in stripe-ordered (La,M)_2CuO_4 to probe...

The superconducting critical temperature (T c ) of hole-doped cuprates tends to increase with their lattice complexity, which is generally correlated with higher states of oxidation. For YBa2Cu3O7−δ (YBCO-123), it is known that solid-state reaction in ultrahigh-pressure oxygen can induce the formation of more complex and oxidized phases such as Y2B...

Aberration corrected STEM-EELS study of the hole distribution in cuprate superconductors - Volume 21 Issue S3 - M. Bugnet, G. Radtke, S. Loffler, P. Schattschneider, D. Hawthorn, G. A. Sawatzky, G. A. Botton

We utilize intense, single-cycle terahertz pulses to induce collective excitations in the charge-density-wave-ordered underdoped cuprate YBa2Cu3O6+x. These excitations manifest themselves as pronounced coherent oscillations of the optical reflectivity in the transient state, accompanied by minimal incoherent quasiparticle relaxation dynamics. The o...

The structure, transport, thermodynamic properties, x-ray absorption spectra (XAS), and electronic structure of a new barium titanate suboxide, Ba1+δTi13−δO12 (δ = 0.11), are reported. It is a paramagnetic poor metal with hole carriers dominating the transport. Fermi liquid behavior appears at low temperature. The oxidization state of Ti obtained b...

Recent theories of charge density wave (CDW) order in high temperature superconductors have predicted a primarily d CDW orbital symmetry. Here, we report on the orbital symmetry of CDW order in the canonical cuprate superconductors La1.875Ba0.125CuO4 (LBCO) and YBa2Cu3O6.67 (YBCO), using resonant soft x-ray scattering and a model mapped to the CDW...

The competition between superconductivity and charge density wave (CDW) order in underdoped cuprates has now been widely reported, but the role of disorder in this competition has yet to be fully resolved. A central question is whether disorder sets the length scale of the CDW order, for instance by pinning charge density fluctuations or disrupting...

The high-temperature superconductor YBa2Cu3O6+δ consists of two main structural units-a bilayer of CuO2 planes that are central to superconductivity and a CuO2+δ chain layer. Although the functional role of the planes and chains has long been established, most probes integrate over both, which makes it difficult to distinguish the contribution of e...

Clear experimental evidence of charge density wave correlations competing with superconducting order in YBCO have thrust their relationship with the pseudogap regime into the spotlight. To aid in characterizing the pseudogap regime, we propose a dimensionless ratio of the diamagnetic susceptibility to the correlation length of the charge density wa...

A Comment on the Letter by S. I. Bokarev et al., Phys. Rev. Lett. 111, 083002 (2013). The authors of the Letter offer a Reply.

The Cuprate Pseudogap The properties of copper-oxide superconductors are changed by chemical doping, but, if doping is suboptimal, the transition temperature T c drops. Conversely, the so-called pseudogap, a depression in the density of states around the Fermi level that may or may not be related to superconductivity, gains strength. The cuprate YB...

We report the study of magnetic and orbital order in epitaxial thin films grown on –. Resonant soft x-ray scattering revealed significant modifications of the magnetic order in the film as compared to the bulk. Namely (i) a different magnetic ordering wave vector, (ii) different spin directions and (iii) an additional magnetic reordering transition...

Charge density wave (CDW) order in underdoped YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) has been reported to coexist and compete with superconductivity. Here we investigate the sensitivity of the CDW order to quenched oxygen disorder in the chain layer of YBCO using resonant soft x-ray scattering. We find that disordering the chains in o-V and o-VIII ordered Y...

The fundamental electronic structure of the widely used battery material Li_{x}CoO_{2} still remains a mystery. Soft x-ray absorption spectroscopy of Li_{x}CoO_{2} reveals that holes with strong O 2p character play an essential role in the electronic conductivity of the Co^{3+}/Co^{4+} mixed valence CoO_{2} layer. The oxygen holes are bound to the...

Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive tool for the study of subtle charge, orbital, and spin superlattice orders driven by the valence electrons, which therefore escape detection in conventional x-ray diffraction (XRD). Although the power of REXS has been demonstrated by numerous studies of complex oxides perfor...

We investigated the electronic structure of layered Mn oxide Bi3Mn4O12(NO3) with a Mn honeycomb lattice by x-ray absorption spectroscopy and model calculations. The valence of Mn was determined to be 4+4+ with a small charge-transfer energy of ∼∼ 1 eV. The values of (J1, J2, J3, Jc, Jc1, and Jc2) obtained by unrestricted Hartree–Fock calculations o...

Scientific Reports is an online, open access journal from the publishers of Nature. The 2014 Impact Factor for Scientific Reports is 5.578.

Recently, charge density wave order with Q = [0.3 0 L] and [0 0.3 L] was detected for the first time in underdoped YBCO using resonant soft x-ray scattering at the Cu L3 absorption edge. Here, we explore the energy and polarization dependence of the resonant scattering intensity in detwinned YBa2Cu3O6.75 with ortho-III oxygen ordering in the chain...

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate on a microscopic scale into hole-rich and hole-poor regions. We report resonant elastic x-ray scattering measurements of stripe-ordered La_{1.475}Nd_{0.4}Sr_{0.125}CuO_{4} at the Cu L and O K absorption edges that i...

Recently, charge density wave (CDW) order in the CuO_{2} planes of underdoped YBa_{2}Cu_{3}O_{6+δ} was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinned samp...

10.1038/nchem.1430

Colossal magnetoresistance, whereby the application of a magnetic field reduces the resistivity of a manganite by orders of magnitude, is generally believed to occur because of coexisting phases. Development of a complete theory to explain the phenomenon requires that the exact nature of these phases be known. We used resonant elastic soft x-ray sc...

Recently, charge density wave (CDW) order in the CuO$_2$ planes of underdoped YBa$_2$Cu$_3$O$_{6+\delta}$ was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinn...

The concept that superconductivity competes with other orders in cuprate superconductors has become increasingly apparent, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of ~3.2 lattice units in th...

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate on a microscopic scale into hole rich and hole poor regions. We report resonant elastic x-ray scattering measurements of stripe-ordered La$_{1.475}$Nd$_{0.4}$Sr$_{0.125}$CuO$_4$ at the Cu $L$ and O $K$ absorption ed...

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate into hole rich regions that form anti-phase boundaries between regions of anti-ferromagnetic order. I will present resonant elastic x-ray scattering measurements of stripe-ordered La1.475Nd0.4Sr0.125CuO4 at the Cu L...

We report on the ab-plane optical properties of the magnetic field induced normal state of La1.94Sr0.06CuO4 (Tc=5.5 K), the first such study. We apply strong magnetic fields (4 T and 16 T) along the c-axis . We find that at 4 T fields, which are strong enough to destroy superconductivity, the normal state at 1.4 K is very similar to the normal stat...

X-ray absorption spectroscopy (XAS) is one of the most widely used experimental techniques to study the electronic and spatial structure of materials. Fluorescence yield mode is bulk-sensitive, but has several serious problems coming from saturation effects. In this study, we show the usefulness of partial fluorescence yields in addressing these pr...

The concept that superconductivity competes with other orders in cuprate superconductors has become increasingly apparent, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of ∼3.2 lattice units in th...

We report the study of magnetic and orbital order in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ epitaxial thin films grown on (LaAlO$_3$)$_{0.3}$-(SrAl$_{0.5}$Ta$_{0.5}$O$_3$)$_{0.7}$ (LSAT) (011)$_c$. In a new experimental approach, the polarization and energy dependence of resonant soft x-ray scattering are used to reveal significant modifications of the magnet...

Interfaces in heterostructures composed of two different transition-metal oxides are now a strongly growing research topic in condensed-matter physics due to the anomalous electronic properties very different from those of the component materials. Here we present resonant soft x-ray scattering studies of SrTiO3/LaAlO3 superlattices. From the existe...

Static charge-density-wave (CDW) and spin-density-wave (SDW) order has been convincingly observed in La-based cuprates for some time. However, more recently it has been suggested by quantum oscillation, transport, and thermodynamic measurements that density-wave order is generic to underdoped cuprates and plays a significant role in YBa2Cu3O6+δ (YB...

We describe the design, construction, and performance of a 4-circle in-vacuum diffractometer for resonant elastic soft x-ray scattering. The diffractometer, installed on the resonant elastic and inelastic x-ray scattering beamline at the Canadian Light Source, includes 9 in-vacuum motions driven by in-vacuum stepper motors and operates in ultra-hig...

We have studied the electronic structure of multiferroic BiCoO3 using x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and subsequent model calculations. The XAS results show that the Co3+ ion takes the high-spin d6 configuration which usually prefers G-type antiferromagnetic state. The XPS results and model Hartree-Fock...

An alternative measure of x-ray absorption spectroscopy (XAS) called inverse partial fluorescence yield (IPFY) has recently been developed that is both bulk sensitive and free of saturation effects. Here we show that the angle dependence of IPFY can provide a measure directly proportional to the total x-ray absorption coefficient, $\mu(E)$. In cont...

DOI:https://doi.org/10.1103/PhysRevB.83.129901

Eu doped rare-earth orthovanadates are known to be good red phosphor materials. In particular, LaVO4:Eu is a promising candidate due to the low Eu-site point symmetry, and thus high dipole transition probability within Judd–Ofelt theory. However, the low solubility limit (<3 mol %) of Eu in LaVO4 prevents its efficient use as a phosphor. We present...

We investigated the electronic structure of layered Mn oxide Bi3Mn4O12(NO3) with a Mn honeycomb lattice by x-ray absorption spectroscopy. The valence of Mn was determined to be 4+ with a small charge-transfer energy. We estimated the values of superexchange interactions up to the fourth nearest neighbors (J1, J2, J3, and J4) by unrestricted Hartree...

X-ray absorption spectroscopy on oxygen-annealed, self-flux-grown single crystals of Tl2Ba2CuO6-delta suggests a microscopic doping mechanism whereby interstitial oxygens are attracted to copper substituted on the thallium site, contributing holes to both the planes and to these coppers, and typically promoting only one hole to the plane rather tha...

A Reply to the Comment by Philip Phillips and Mark Jarrell.

We derive diffusion equations, which describe spin-charge coupled transport on the helical metal surface of a three-dimensional topological insulator. The main feature of these equations is a large magnitude of the spin-charge coupling, which leads to interesting and observable effects. In particular, we predict a new magnetoresistance effect, whic...

A recent article suggested that the saturation of low energy spectral weight observed by X-ray absorption spectroscopy in the cuprates at high hole doping could be explained within the single-band Hubbard model. We show that this result is an artifact of inappropriate integration limits.

We present an x-ray absorption study of the oxidation states of transition-metal-ions of LiMnO2 and its related materials, widely used as cathodes in Li-ion batteries. The comparison between the obtained spectrum and the configuration-interaction cluster-model calculations showed that the Mn3+ in LiMnO2 is a mixture of the high-spin and low-spin st...

We demonstrate a new method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy sensitive photon detector to...

We present resonant soft x-ray scattering studies of Ti 3d and O 2p states at the interfaces of SrTiO ₃/ LaAlO ₃ superlattices. From reflectivity analyses, focusing on the (003) peak which is forbidden for our “ideal” superlattice structure, we concluded that the LaO | TiO ₂/ SrO and the SrO | AlO ₂/ LaO...

We present an x-ray absorption study of the dependence of the V oxidation state on the thickness of La V O ₃ (LVO) and capping La Al O ₃ (LAO) layers in the multilayer structure of LVO sandwiched between LAO. We found that the change of the valence of V as a function of LAO layer thickness can be qualitatively explained by a t...

X-ray absorption spectra on the overdoped high-temperature superconductors Tl2Ba2CuO(6+delta) and La(2-x)SrxCuO(4+/-delta) reveal a striking departure in the electronic structure from that of the underdoped regime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygen K edge, while the lowest-energy prepe...

We present an x-ray absorption study of the dependence of the V oxidation state on the thickness of LaVO$_3$ (LVO) and capping LaAlO$_3$ (LAO) layers in the multilayer structure of LVO sandwiched between LAO. We found that the change of the valence of V as a function of LAO layer thickness can be qualitatively explained by a transition between elec...

We present a temperature-dependent resonant elastic soft x-ray scattering (REXS) study of the metal-insulator transition in Sr3(Ru1-xMnx)2O7, performed at both Ru and Mn L-edges. Resonant magnetic superstructure reflections, which indicate an incipient instability of the parent compound, are detected below the transition. Based on modelling of the...

X-ray absorption spectra on the overdoped high-temperature superconductors Tl_2Ba_2CuO_{6+delta} (Tl-2201) and La_{2-x}Sr_xCuO_{4+delta} (LSCO) reveal a striking departure in the electronic structure from that of the underdoped regime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygen K edge, while th...

Recently resonant elastic soft x-ray scattering (RSXS) has emerged as a powerful new tool to study electronic ordering in materials like cuprates and manganites. The power of this technique is to combine x-ray scattering, which is sensitive to spatial order, with x-ray spectroscopy, which is sensitive to the valence, spin and orbital symmetry of sp...

Resonant soft x-ray scattering with photon energies near the O K and the Cu L-3 edges was used to study the charge ordering in the system La1.8-xEu0.2SrxCuO4 as a function of temperature for x=0.125 and 0.15. From the superstructure diffraction intensities, a charge ordering with a doping-dependent wave vector is derived which is in this system wel...

We investigated the electronic structure of the SrTiO$_3$/LaAlO$_3$ superlattice (SL) by resonant soft x-ray scattering. The (003) peak, which is forbidden for our "ideal" SL structure, was observed at all photon energies, indicating reconstruction at the interface. From the peak position analyses taking into account the effects of refraction, we o...

We present low-temperature thermal conductivity measurements on the cuprate Tl2Ba2CuO6+delta throughout the overdoped regime. In the T-->0 limit, the thermal conductivity due to d -wave nodal quasiparticles provides a bulk measurement of the superconducting gap Delta . We find Delta to decrease with increasing doping, with a magnitude consistent wi...

We present calculations for resonant inelastic x-ray scattering (RIXS) in edge-shared copper oxide systems, such as CuGeO$_{3}$ and Li$_{2}$CuO$_{2}$, appropriate for hard x-ray scattering where the photoexcited electron lies above oxygen 2p and copper 3d orbital energies. We perform exact diagonalizations of the multi-band Hubbard and determine th...

We present low-temperature thermal conductivity measurements on the cuprate Tl2Ba2CuO6+δ throughout the overdoped regime. In the T→0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap Δ. We find Δ to decrease with increasing doping, with a magnitude consistent with spectroscopic...

Heat and charge transport were used to probe the magnetic field-tuned quantum critical point in the heavy-fermion metal CeCoIn5. A comparison of electrical and thermal resistivities reveals three characteristic energy scales. A Fermi-liquid regime is observed below T(FL), with both transport coefficients diverging in parallel and T(FL) -->0 as H --...

Thermal conductivity of Sr 3 Ru 2 O 7 was measured down to 40 mK and at magnetic fields through the quantum critical end point at H c = 7.85 T . A peak in the electrical resistivity as a function of the field was mimicked by the thermal resistivity. In the limit as T → 0 K , we find that the Wiedemann-Franz law is satisfied to within 5% at all fiel...

Thermal conductivity of Sr3Ru2O7 was measured down to 40 mK and at magnetic fields through the quantum critical end point at Hc=7.85 T. A peak in the electrical resistivity as a function of the field was mimicked by the thermal resistivity. In the limit as T-->0 K, we find that the Wiedemann-Franz law is satisfied to within 5% at all fields, implyi...

Heat and charge transport were used to probe the magnetic field-tuned quantum critical point in the heavy-fermion metal CeCoIn$_5$. A comparison of electrical and thermal resistivities reveals three characteristic energy scales. A Fermi-liquid regime is observed below $T_{FL}$, with both transport coefficients diverging in parallel and $T_{FL}\to 0...

The family of RBaCo2O5+delta cobaltates is known to exhibit a rich variety of magnetic behavior as a function of oxygen content and temperature. We present x-ray absorption measurements on detwinned single crystals of GdBaCo2O5.5, where the structure is comprised of alternating rows of CoO6 octahedra and CoO5 pyramids. GdBaCo2O5.5 exhibits successi...

We present detailed polarization dependent x-ray absorption spectroscopy (XAS) measurements of the cuprate YBa2Cu3O6+delta as a function of oxygen concentration for (0

We elucidate the nature of the thermal metal-to-insulator transition in La2-xSrxCuO4 (LSCO) [1] through measurements of the thermal conductivity kappa performed very close to the transition, down to temperatures as low as 50 mK and in magnetic fields H up to 17 T. For a single crystal with x = 0.15, a monotonic increase in the residual linear term...