Abstract The signature feature of the ‘strange metal’ state of high-T c cuprates—its linear-in-temperature resistivity—has a coefficient α 1 that correlates with T c , as expected were α 1 derived from scattering off the same bosonic fluctuations that mediate pairing. Recently, an anomalous linear-in-field magnetoresistance (=γ 1 H) has also been observed, but only over a narrow doping range, leaving its relation to the strange metal state and to the superconductivity unclear. Here, we report in-plane magnetoresistance measurements on three hole-doped cuprate families spanning a wide range of temperatures, magnetic field strengths and doping. In contrast to expectations from Boltzmann transport theory, γ 1 is found to correlate universally with α 1. A phenomenological model incorporating real-space inhomogeneity is proposed to explain this correlation. Within this picture, superconductivity in hole-doped cuprates is governed not by the strength of quasiparticle interactions with a bosonic bath, but by the concentration of strange metallic carriers.

Despite obvious similarities in their electronic and crystallographic structures, it remains unclear whether the interactions that shape the normal and superconducting (SC) state properties of high-Tc cuprates and infinite-layer nickelates (ILNs) have the same origin. This question has been brought into sharper focus with recent studies on ILNs of improved crystallinity that reveal a SC dome of comparable extent and similar transport properties above Tc as the hole-doped cuprates. The evolution of these properties in the magnetic-field-induced normal state, however, has yet to be determined. Here, we examine the magnetotransport properties of new-generation Nd1-xSrxNiO2 films in the T -> 0 limit across the phase diagram in fields up to 54 T. This extensive study reveals that the limiting low-T form of the normal-state resistivity in ILNs exhibits non-Fermi-liquid behaviour over an extended doping range inside the SC dome, rather than at a singular quantum critical point. While there are clear differences in the charge dynamics of ILNs and cuprates, most notably in the magnetoresistance, our findings reveal that both systems exhibit anomalous metallicity characteristic of a quantum critical phase. Recent advancements in the sample quality of infinite-layer nickelates have made systematic comparisons with cuprates increasingly relevant. Hsu et al. report anomalous electronic transport in high-crystallinity (Nd,Sr)NiO2 at large magnetic fields, map out the transport phase diagram, and discuss similarities and differences with cuprates.

High-temperature superconductivity in cuprates emerges upon doping the parent Mott insulator. Robust signatures of the low-doped electronic state include a Hall carrier density that initially tracks the number of doped holes and the emergence of an anisotropic pseudogap; the latter characterised by disconnected Fermi arcs, closure at a critical doping level $p^* \approx 0.19$, and, in some cases, a strongly enhanced carrier effective mass. In Sr$_2$IrO$_4$, a spin-orbit-coupled Mott insulator often regarded as a 5$d$ analogue of the cuprates, surface probes have revealed the emergence of an anisotropic pseudogap and Fermi arcs under electron doping, though neither the corresponding $p^*$ nor bulk signatures of pseudogap closing have as yet been observed. Here, we report electrical transport and specific heat measurements on Sr$_{2-x}$La$_x$IrO$_4$ over an extended doping range 0 $\leq x \leq$ 0.20. The effective carrier density $n_{\rm H}$ at low temperatures exhibits a crossover from $n_{\rm H} \approx x$ to $n_{\rm H} \approx 1+x$ near $x$ = 0.16, accompanied by \textcolor{blue}{a five-orders-of-magnitude increase in conductivity} and a six-fold enhancement in the electronic specific heat. These striking parallels in the bulk pseudogap phenomenology, coupled with the absence of superconductivity in electron-doped Sr$_2$IrO$_4$, disfavour the pseudogap as a state of precursor pairing and thereby narrow the search for the key ingredient underpinning the formation of the superconducting condensate in doped Mott insulators.

Following allegations of scientific misconduct in high-temperature superconductivity, Caitlin Duffy says that if trust in science is to be maintained, researchers must be able to report their concerns.

The search for superconductivity in infinite-layer nickelates was motivated by analogy to the cuprates, and this perspective has framed much of the initial consideration of this material. However, a growing number of studies have highlighted the involvement of rare-earth orbitals; in that context, the consequences of varying the rare-earth element in the superconducting nickelates have been much debated. Here, we show notable differences in the magnitude and anisotropy of the superconducting upper critical field across the La-, Pr-, and Nd-nickelates. These distinctions originate from the 4f electron characteristics of the rare-earth ions in the lattice: They are absent for La3+, nonmagnetic for the Pr3+ singlet ground state, and magnetic for the Nd3+ Kramer's doublet. The unique polar and azimuthal angle-dependent magnetoresistance found in the Nd-nickelates can be understood to arise from the magnetic contribution of the Nd3+ 4f moments. Such robust and tunable superconductivity suggests potential in future high-field applications.

Upon cooling, condensed-matter systems typically transition into states of lower symmetry. The converse-i.e., the emergence of higher symmetry at lower temperatures-is extremely rare. In this work, we show how an unusually isotropic magnetoresistance in the highly anisotropic, one-dimensional conductor Li0.9Mo6O17 and its temperature dependence can be interpreted as a renormalization group (RG) flow toward a so-called separatrix. This approach is equivalent to an emergent symmetry in the system. The existence of two distinct ground states, Mott insulator and superconductor, can then be traced back to two opposing RG trajectories. By establishing a direct link between quantum field theory and an experimentally measurable quantity, we uncover a path through which emergent symmetry might be identified in other candidate materials.

High Field Magnet Laboratory (HFML-EMFL) and Institute for Molecules and Materials, Radboud University, Toernooiveld 7, 6525 ED Nijmegen, Netherlands H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom LNCMI-EMFL, CNRS UPR3228, Univ. Grenoble Alpes, Univ. Toulouse, INSA-T, Grenoble and Toulouse, France Van der Waals-Zeeman Institute, University of Amsterdam, Postbus 94485, 1090 GL Amsterdam, Netherlands Institute for Solid State Physics, University of Tokyo, Kashiwa-no-ha, Kashiwa, Japan Toyota Technological Institute, Nagoya 468-8511, Japan Department of Physics, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, United Kingdom

Unlike their cuprate counterparts, the undoped nickelates are weak insulators without long-range antiferromagnetic order. Identifying the origin of this insulating behavior, found on both sides of the superconducting dome, is potentially a crucial step in the development of a coherent understanding of nickelate superconductivity. In this work, we study the normal-state resistivity of infinite-layer nickelates using high magnetic fields to suppress the superconductivity and examine the impact of disorder and doping on its overall temperature ( T ) dependence. In superconducting samples, the resistivity of Nd- and La-based nickelates continues to exhibit weakly insulating behavior with a magnitude and functional form similar to that found in underdoped electron-doped cuprates. We find a systematic evolution of the insulating behavior as a function of nominal hole doping across different rare-earth families, suggesting a pivotal role for strong electron interactions, and uncover a correlation between the suppression of the resistivity upturn and the robustness of the superconductivity. By contrast, we find very little correlation between the level of disorder and the magnitude and onset temperature of the resistivity upturn. Combining these experimental observations with previous Hall effect measurements on these two nickelate families, we consider various possible origins for this correlated insulator behavior and its evolution across their respective phase diagrams.

A key goal of superconductivity research is to identify the mechanism by which electrons pair.Clues to the pairing mecha-nism lie in the resistive behavior(that is itself governed by elec-trons scattering off impurities and dynamical fluctuations within the material)above the superconducting transition temperature Tc.In conventional,i.e.,BCS,superconductors where pairing is mediated by a retarded electron-phonon interaction,one of the key parameters affecting Tc is the electron-phonon coupling strength λ.This in turn is closely related to λtr,the coupling param-eter that defines the transport scattering rate hr=2πλtrkBT asso-ciated with the T-linear resistivity found in all conventional metals and superconductors at intermediate temperatures(typi-cally in the range 50-500 K).Here,h is Planck's constant and kB the Boltzmann constant.This link between the coefficient of the T-linear resistivity and Tc in conventional superconductors is thus well established,a principle enshrined in the old adage that"good metals make bad superconductors".

Caitlin Duffy argues why it is important for PhD students to develop interests outside the lab and not just solely focus on their research.

It has long been recognized that the key to unlocking the mystery of cuprate high-Tc superconductivity lies in understanding the anomalous normal state from which pairs form and condense. While many of its defining properties have been identified, they are often considered either at a singular doping level or as an isolated phenomenon as a function of doping. As a result, their relation to each other and to the pseudogap (PG), strange metal (SM) and non-superconducting (non-SC) regimes that define the cuprate phase diagram has yet to be elucidated. Here, we report a high-field in-plane MR study on several cuprate families spanning all 3 regimes that reveal a complex yet nonetheless systematic evolution of the form of the MR, with each regime possessing its own distinct scaling behavior. In the PG regime, the MR exhibits pure H/T^2 scaling at low fields and H-linearity at the highest field strengths. While the H-linearity persists inside the SM regime, the scaling changes abruptly to H/T. The size of the H-linear slope, meanwhile, is found to be correlated with both the T-linear resistivity coefficient and Tc, strengthening the characterization of the SM regime as a quantum critical phase. We interpret the omnipresence of H-linear MR across both regimes as a signature of highly anisotropic, possibly discontinuous features on the Fermi surface. Finally, within the non-SC, Fermi-liquid regime, we observe a recovery of conventional Kohler scaling. This comprehensive study establishes the distinct nature of the magnetotransport within each regime and identifies power-law scaling of the normal state MR as a defining feature of SC hole-doped cuprates. The incompatibility of such power-law scaling with any known variant of Boltzmann transport theory motivates the quest for an altogether new theoretical framework, one in which the MR is entirely decoupled from elastic impurity scattering.

Once doped away from their parent Mott insulating state, the hole-doped cuprates enter into many varied and exotic phases. The onset temperature of each phase is then plotted versus p-the number of doped holes per copper atom-to form a representative phase diagram. Apart from differences in the absolute temperature scales among the various families, the resultant phase diagrams are strikingly similar. In particular, the p values corresponding to optimal doping (p(opt) similar to 0.16) and to the end of the pseudogap phase (p* similar to 0.19-0.20) are essentially the same for all cuprate families bar one: the single-layer Bi-based cuprate Bi2+z-yPbySr2-x-zLaxCuO6+delta (Bi2201). This anomaly arises partly due to the complex stoichiometry of this material and also to the different p values inferred from disparate (e.g., bulk or surface) measurements performed on samples with comparable superconducting transition temperatures T-c. Here, by combining measurements of the in-plane resistivity in zero and high magnetic fields with angle-resolved photoemission spectroscopy studies in the superconducting and normal state, we argue that the phase diagram of Bi2201 may in fact be similar to that realized in other families. This study therefore brings Bi2201 into the fold and supports the notion of universality of p(opt) and p* in all hole-doped cuprates.

We report a systematic magnetotransport study of superconducting infinite-layer nickelate thin films Nd_{1−x}Sr_{x}NiO_{2} with 0.15≤x≤0.225. By suppressing superconductivity with out-of-plane magnetic fields up to 37.5 T, we find that the normal state resistivity of Nd_{1−x}Sr_{x}NiO_{2} is characterized by a crossover from a metallic T^{2} behavior to an insulating log(1/T) behavior for all x except x=0.225, at which the resistivity is predominantly metallic. The log(1/T) behavior is found to be robust against magnetic fields, inconsistent with scenarios involving localization or Kondo scattering, and points to an anomalous insulating state possibly driven by strong correlations. In the metallic state, we find no evidence for non-Fermi-liquid behavior arising from proximity to a putative quantum critical point located inside the superconducting dome.

Caitlin Duffy says it is more important than ever to consider your next career move despite the challenges posed by the COVID-19 pandemic

Caitlin Duffy says that physicists must recognize that failure is part and parcel of science.

There is now compelling evidence that the normal state of superconducting overdoped cuprates is a strange metal comprising two distinct charge sectors, one governed by coherent quasiparticle excitations, the other seemingly incoherent and characterized by non-quasiparticle (Planckian) dissipation. The zero-temperature superfluid density n_s(0)ns(0) of overdoped cuprates exhibits an anomalous depletion with increased hole doping pp, falling to zero at the edge of the superconducting dome. Over the same doping range, the effective zero-temperature Hall number n_{\rm H}(0) transitions from pp to 1 + pp. By taking into account the presence of these two charge sectors, we demonstrate that in the overdoped cuprates Tl_22Ba_22CuO_{6+\delta}6+δ and La_{2-x}2−xSr_xxCuO_44, the growth in n_s(0)ns(0) as pp is decreased from the overdoped side may be compensated by the loss of carriers in the coherent sector. Such a correspondence is contrary to expectations from conventional BCS theory and implies that superconductivity in overdoped cuprates emerges uniquely from the sector that exhibits incoherent transport in the normal state.

We report a systematic magnetotransport study of superconducting infinite-layer nickelate thin films Nd_1-xSr_xNiO_2 with 0.15 ≤ x ≤ 0.225. By suppressing superconductivity with out-of-plane magnetic fields up to 37.5 T, we find that the normal state resistivity of Nd_1-xSr_xNiO_2 is characterized by a crossover from a metallic T^2-behavior to an insulating log(1/T)-behavior for all x except x = 0.225, at which the resistivity is predominantly metallic. The log(1/T)-behavior is found to be robust against magnetic fields, inconsistent with scenarios involving localization or Kondo scattering, and points to an anomalous insulating state possibly driven by strong correlations. In the metallic state, we find no evidence for non-Fermi-liquid behavior arising from proximity to a putative quantum critical point located inside the superconducting dome.

With COVID-19 fostering anti-science conspiracies, Caitlin Duffy says that scientists have a duty to speak up and challenge misinformation

Yu-Te Hsu,1, ∗ Bai Yang Wang,2, 3 Maarten Berben,1 Danfeng Li,3, 4 Kyuho Lee,2, 3 Caitlin Duffy,1 Thom Ottenbros,1 Woo Jin Kim,3, 4 Motoki Osada,3, 5 Steffen Wiedmann,1 Harold Y. Hwang,3, 4, † and Nigel E. Hussey1, 6, ‡ High Field Magnet Laboratory (HFML-EMFL) and Institute for Molecules and Materials, Radboud University, Toernooiveld 7, 6525 ED Nijmegen, Netherlands Department of Physics, Stanford University, Stanford, CA 94305, United States Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, Stanford, CA 94025, United States Department of Applied Physics, Stanford University, Stanford, CA 94305, United States Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, United States H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom (Dated: May 18, 2021)

As an undergraduate student at the University of St Andrews from 2015 to 2019, my thirst for knowledge, and fine beverages, was quenched by two part-time jobs. The first was as a tour guide at Kingsbarns whisky distillery, and the second was as a retail assistant in a local drinks shop.