arXiv subjects

Nigel E. Hussey

Publications and source records attributed to Nigel E. Hussey.

14 recordsLinked to original sources

arXiv2026

Extended strange metal regime in a pure nematic quantum critical superconductor

High-temperature superconductivity often emerges from a strange metallic state, where the electrical resistivity exhibits a linear-in-temperature dependence over an anomalously extended temperature range. The prevailing belief is that magnetic critical fluctuations gives rise to strange metallicity, enhancing the superconducting transition temperature. Here, using high pulsed magnetic fields, we have uncovered the strange metallic ground state hidden below the superconducting dome of nonmagnetic FeSe$_{1-x}$Te$_{x}$, which harbors a quantum critical point (QCP) of pure electronic nematicity, characterized by spontaneous rotational symmetry breaking. Unlike the conventional quantum criticality, this strange metallic state does not appear in a fan-shaped region above the singular QCP but spans a wide compositional range, where pairing interactions are strengthened by nonmagnetic nematic critical fluctuations. This stands in sharp contrast to the much cleaner system FeSe$_{1-x}$S$_{x}$, which displays a quantum critical fan above nematic QCP, indicating that disorder-induced spatial randomness of the nematic fluctuations likely enlarges the QCP of FeSe$_{1-x}$Te$_{x}$ into an extended region of criticality, as suggested by relevant hybrid Quantum Monte Carlo simulations. These observations highlight superconductivity promoted by a unique interplay between pure nematic critical fluctuations, strange metallicity and disorder, providing new insight into the emergence of non-Fermi-liquid transport in various correlated materials.

cond-mat.supr-con↗

arXiv2026

Inferring resource selection and utilization distributions from irregular and error-prone animal tracking data

Habitat selection and space use are fundamental to understanding animal distribution. Traditional methods for quantifying habitat preferences from telemetry data assume regular sampling and negligible measurement error. However, these assumptions are routinely violated in marine systems. Practitioners typically regularize and filter the data before fitting models, but these two-step procedures do not propagate uncertainty from the filtering stage and can yield biased estimates. Habitat-driven Langevin diffusion models offer an elegant alternative, naturally accommodating irregular sampling. However, incorporating measurement error via a state-space formulation is challenging because habitat covariates depend on the latent true locations. We address this using the Laplace approximation to simultaneously integrate over true locations and account for habitat covariates along latent paths, yielding a single-stage framework efficiently implemented in Template Model Builder (TMB). By doing so, we provide the first TMB implementation capable of handling covariates that depend on latent variables, allowing inference via fast and efficient maximum likelihood estimation. Simulations show that our approach outperforms the two-step method, recovering habitat-selection parameters even under substantial measurement error and missing data, with more accurate utilization distributions and trajectory reconstructions. Applied to narwhal (Monodon monoceros) telemetry data, the two-step method substantially shrinks the habitat selection coefficient towards zero, while our unified approach recovers a much stronger signal. Our framework offers a computationally efficient solution to long-standing challenges of measurement error and temporal irregularity in habitat selection inference, applicable across a wide range of taxa and environments.

stat.ME↗

arXiv2026

Magnetothermopower and particle-hole symmetry in a cuprate strange metal

Here, we report magnetothermopower measurements on overdoped (Bi,Pb)2(Sr,La)2CuO6+delta (Bi2201) single crystals in magnetic fields up to 35 T. Whereas the temperature dependence of the zero-field Seebeck coefficient S(T) can be captured using Boltzmann transport theory, the field-dependent response S(H) cannot. Instead, the magnetothermopower contains a large additional contribution whose field and temperature dependence is consistent with the presence of short-range superconducting order well above Tc. Combined with earlier Hall and magnetoresistance results, these data imply that the overdoped cuprate strange metal contains two transport sectors with distinct particle-hole symmetry: a conventional particle-hole-asymmetric Fermi-liquid (FL) contribution governing the Hall effect and the zero-field thermopower, and a nearly particle-hole-symmetric sector dominating the anomalous longitudinal magnetotransport. We formulate a phenomenological real-space model in which disconnected FL islands are embedded in a compensated Dirac liquid of phase-incoherent d-wave Bogoliubov quasiparticles. This picture reconciles conventional zero-field transport with anomalous magnetothermopower and magnetoresistance and offers a concrete framework for thinking about strange metallicity in overdoped cuprates.

cond-mat.supr-con↗

arXiv2026

Estimating the distance at which narwhal respond to disturbance: a penalized threshold hidden Markov model

Understanding behavioural responses to disturbances is vital for wildlife conservation. For example, in the Arctic, the decrease in sea ice has opened new shipping routes, increasing the need for impact assessments that quantify the distance at which marine mammals react to vessel presence. This information can then guide targeted mitigation policies, such as vessel slow-down regulations and delineation of avoidance areas. Using telemetry data to determine distances linked to deviations from normal behaviour requires advanced statistical models, such as threshold hidden Markov models (THMMs). While these are powerful tools, they do not assess whether the estimated threshold reflects a meaningful behavioural shift. We introduce a lasso-penalized THMM that builds on computationally efficient methods to impose penalties on HMMs and present a new, efficient penalized quasi-restricted maximum-likelihood estimator. Our framework is capable of estimating thresholds and assessing whether the disturbance effects are distinguishable from baseline behaviour. With simulations, we demonstrate that our lasso method effectively shrinks spurious threshold effects towards zero. When applied to narwhal movement data, our analysis suggests that narwhal react to vessels up to 3.4 kilometres away by decreasing movement persistence and spending more time in deeper waters (average maximum depth of 356m). Overall, we provide a broadly applicable framework for quantifying behavioural responses to stimuli, with applications ranging from determining reaction thresholds to disturbance to estimating the distances at which terrestrial species, such as elephants, detect water. We also provide a tutorial and an open-source implementation to facilitate the application of our framework.

stat.AP↗

arXiv2023

Iron-based superconductors: teenage, complex, challenging

The advent of iron-based superconductors in 2008 came as a complete surprise to the condensed matter community. Now 15 years later, they are beginning to impart some of their new-found wisdom on a slew of emerging superconductors that boast similar traits.

cond-mat.supr-con↗

arXiv2022

Superfluid density and two-component conductivity in hole-doped cuprates

While the pseudogap dominates the phase diagram of hole-doped cuprates, connecting the antiferromagnetic parent insulator at low doping to the strange metal at higher doping, its origin and relation to superconductivity remains unknown. In order to proceed, a complete understanding of how the single hole - initially localized in the Mott state - becomes mobile and ultimately evolves into a coherent quasiparticle at the end of the superconducting dome is required. In order to affect this development, we examine recent transport and spectroscopic studies of hole-doped cuprates across their phase diagram. In the process, we highlight a set of empirical correlations between the superfluid density and certain normal state properties of hole-doped cuprates that offer fresh insights into the emergence of metallicity within the CuO_2 plane and its influence on the robustness of the superconducting state. We conclude by arguing that the overall behavior is best understood in terms of two distinct current-carrying fluids, only one of which dominates the superconducting condensate and is gapped out below the pseudogap endpoint at a critical hole concentration p^\ast.

cond-mat.supr-con↗

arXiv2022

Rare-Earth Control of the Superconducting Upper Critical Field in Infinite-Layer Nickelates

The consequences of varying the rare-earth element in the superconducting infinite-layer nickelates have been much debated. Here we show striking differences in the magnitude and anisotropy of the superconducting upper critical field across the La-, Pr-, and Nd-nickelates. These 5 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. In the absence of rare-earth effects, we find that the nickelates broadly violate the Pauli limit. Such robust and tunable superconductivity suggests potential in future high-field applications.

cond-mat.supr-con↗

arXiv2022

Stranger than Metals

Although the resistivity in traditional metals increases with temperature, its $T$ dependence vanishes at low or high temperature, albeit for different reasons. Here, we review a class of materials, known as \lq strange' metals, that can violate both principles. In materials exhibiting such behavior, the change in slope of the resistivity as the mean free path drops below the lattice constant, or as $T \rightarrow 0$, can be imperceptible, suggesting complete continuity between the charge carriers at low and high $T$. Since particles cannot scatter at length scales shorter than the interatomic spacing, strange metallicity calls into question the relevance of locality and a particle picture of the underlying current. This review focuses on transport and spectroscopic data on candidate strange metals with an eye to isolate and identify a unifying physical principle. Special attention is paid to quantum criticality, Planckian dissipation, Mottness, and whether a new gauge principle, which has a clear experimental signature, is needed to account for the non-local transport seen in strange metals. For the cuprates, strange metallicity is shown to track the superfluid density, thereby making a theory of this state the primary hurdle in solving the riddle of high-temperature superconductivity.

cond-mat.str-el↗

arXiv2021

Evidence for strong electron correlations in a non-symmorphic Dirac semimetal

Metallic iridium oxides (iridates) provide a fertile playground to explore new phenomena resulting from the interplay between topological protection, spin-orbit and electron-electron interactions. To date, however, few studies of the low energy electronic excitations exist due to the difficulty in synthesising crystals with sufficiently large carrier mean-free-paths. Here, we report the observation of Shubnikov-de Haas quantum oscillations in high-quality single crystals of monoclinic SrIrO$_3$ in magnetic fields up to 35~T. Analysis of the oscillations reveals a Fermi surface comprising multiple small pockets with effective masses up to 4.5 times larger than the calculated band mass. \textit{Ab-initio} calculations reveal robust linear band-crossings at the Brillouin zone boundary, due to its non-symmorphic symmetry, and overall we find good agreement between the angular dependence of the oscillations and the theoretical expectations. Further evidence of strong electron correlations is realized through the observation of signatures of non-Fermi liquid transport as well as a large Kadowaki-Woods ratio. These collective findings, coupled with knowledge of the evolution of the electronic state across the Ruddlesden-Popper iridate series, establishes monoclinic SrIrO$_3$ as a topological semimetal on the boundary of the Mott metal-insulator transition.

cond-mat.str-el↗

arXiv2021

Insulator-to-Metal Crossover near the Edge of the Superconducting Dome in Nd$_{1-x}$Sr$_x$NiO$_2$

We report a systematic magnetotransport study of superconducting infinite-layer nickelate thin films Nd$_{1-x}$Sr$_x$NiO$_2$ with $0.15 \leq x \leq 0.225$. 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.

cond-mat.supr-con↗

arXiv2021

Decoupled nematic and magnetic criticality in FeSe$_{1-x}$S$_{x}$

Electronic nematicity in correlated metals often occurs alongside another instability such as magnetism. As a result, the question remains whether nematicity alone can drive unconventional superconductivity or anomalous (quantum critical) transport in such systems. In FeSe, nematicity emerges in isolation, providing a unique opportunity to address this question. Studies to date, however, have proved inconclusive; while signatures of nematic criticality are observed upon sulfur substitution, they appear to be quenched under the application of pressure due to the emergent magnetism. Here, we study the temperature and pressure dependence of the low-temperature resistivity of FeSe$_{1-x}$S$_{x}$ crystals at $x$ values just beyond the nematic quantum critical point. Two distinct components to the resistivity are revealed; one whose magnitude falls with increasing pressure and one which grows upon approaching the magnetic state at higher pressures. These findings indicate that nematic and magnetic critical fluctuations in FeSe$_{1-x}$S$_{x}$ are completely decoupled, in marked contrast to other Fe-based superconductors, and that nematic fluctuations alone may be responsible for the transport signatures of quantum criticality found in FeSe$_{1-x}$S$_{x}$ at ambient pressure.

cond-mat.str-el↗

arXiv2020

Reduced Hall carrier density in the overdoped strange metal regime of cuprate superconductors

Efforts to understand the microscopic origin of superconductivity in the cuprates are dependent on knowledge of the normal state. The Hall number in the low temperature, high field limit $n_{\rm H}(0)$ has a particular significance because within conventional transport theory it is simply related to the number of charge carriers, and so its evolution with doping gives crucial information about the nature of the charge transport. Here we report a study of the high field Hall coefficient of the single-layer cuprates Tl$_2$Ba$_2$CuO$_{6+δ}$ (Tl2201) and (Pb/La) doped Bi$_2$Sr$_2$CuO$_{6+δ}$ (Bi2201) which shows how $n_{\rm H}(0)$ evolves in the overdoped, so-called strange metal, regime of cuprates. We find that $n_{\rm H}(0)$ increases smoothly from $p$ to $1+p$, where $p$ is the number of holes doped into the parent insulating state, over a wide range of doping. The evolution of $n_{\rm H}$ correlates with the emergence of the anomalous linear-in-$T$ term in the low-$T$ in-plane resistivity. The results could suggest that quasiparticle decoherence extends to dopings well beyond the pseudogap regime.

cond-mat.supr-con↗

arXiv2016

Anomalous Magnetothermopower in a Metallic Frustrated Antiferromagnet

We report the temperature $T$ and magnetic field $H$ dependence of the thermopower $S$ of an itinerant triangular antiferromagnet PdCrO$_2$ in high magnetic fields up to 32 T. In the paramagnetic phase, the zero-field thermopower is positive with a value typical of good metals with a high carrier density. In marked contrast to typical metals, however, $S$ decreases rapidly with increasing magnetic field, approaching zero at the maximum field scale for $T >$ 70 K. We argue here that this profound change in the thermoelectric response derives from the strong interaction of the 4$d$ correlated electrons of the Pd ions with the short-range spin correlations of the Cr$^{3+}$ spins that persist beyond the Néel ordering temperature due to the combined effects of geometrical frustration and low dimensionality.

cond-mat.str-el↗

arXiv2012

Transport properties of the metallic state of overdoped cuprate superconductors from an anisotropic marginal Fermi liquid model

We consider the implications of a phenomenological model self-energy for the charge transport properties of the metallic phase of the overdoped cuprate superconductors. The self-energy is the sum of two terms with characteristic dependencies on temperature, frequency, location on the Fermi surface, and doping. The first term is isotropic over the Fermi surface, independent of doping, and has the frequency and temperature dependence characteristic of a Fermi liquid. The second term is anisotropic over the Fermi surface (vanishing at the same points as the superconducting energy gap), strongly varies with doping (scaling roughly with $T_c$, the superconducting transition temperature), and has the frequency and temperature dependence characteristic of a marginal Fermi liquid. Previously it has been shown this self-energy can describe a range of experimental data including angle-dependent magnetoresistance (ADMR) and quasi-particle renormalisations determined from specific heat, quantum oscillations, and angle-resolved photo-emission spectroscopy (ARPES). Without introducing new parameters and neglecting vertex corrections we show that this model self-energy can give a quantitative description of the temperature and doping dependence of a range of reported transport properties of Tl2201 samples. These include the intra-layer resistivity, the frequency dependent optical conductivity, the intra-layer magnetoresistance, and the Hall coefficient. The temperature dependence of the latter two are particularly sensitive to the anisotropy of the scattering rate and to the shape of the Fermi surface. In contrast, the temperature dependence of the Hall angle is dominated by the Fermi liquid contribution to the self-energy that determines the scattering rate in the nodal regions of the Fermi surface.

cond-mat.str-el↗