arXiv subjects

J. L. Tallon

Publications and source records attributed to J. L. Tallon.

At least 19 recordsLinked to original sources

arXiv2025

Questioning the cuprate paradigm -- absence of superfluid density loss in several overdoped cuprates I

It is long established that overdoped cuprate superconductors experience a loss of superfluid density (SFD) with increasing doping, p, along with the decline in T_c. Such behavior is unconventional and suggests a depletion of the condensate by increasing pairbreaking or the growth of a second non-pairing channel. This led to a recent suggestion that the condensate arises from an incoherent charge channel which progressively gives way with overdoping to a second, coherent non-pairing channel. Contra these ideas, we report analysis of the field-dependent electronic specific heat of several cuprates from which we find no apparent loss of SFD with overdoping. The SFD per CuO_2 plaquette is found to rise progressively with overdoping from p towards (1+p), undiminished and much the same as the Hall number, thus implying that all available carriers contribute to the condensate. We suggest this could be the underlying intrinsic behavior for all cuprates. Our samples include (Y,Ca)Ba_2Cu_3O_{7-δ}, Bi_2Sr_2CaCu_2O_{8+δ}, La_{2-x}Sr_xCuO_4 and Tl_2Ba_2CuO_6, with the latter being the only exception. Our results signal a possible return to a more conventional picture.

cond-mat.supr-con↗

arXiv2022

Peak in the critical current density in (Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ tuned towards the structural quantum critical point

(Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ is a rare system that has been shown to display an interesting interplay between structural quantum criticality and superconductivity. A putative structural quantum critical point, which is hidden beneath a broad superconducting dome, is believed to give rise to optimized superconducting properties in (Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$. However, the presence of the superconducting dome itself hinders the examination of the quantum critical point through electrical transport, as the transport coefficients vanish in the superconducting state. Here, we use critical current density to explore within the superconducting dome. Our measurements reveal a large enhancement of the critical current density at the zero-temperature limit when the system is tuned towards the structural quantum critical point.

cond-mat.supr-con↗

arXiv2016

Possible very high transition temperatures in the infinite-layer ACuO$ _{2} $ cuprate superconductor for A={Mg, Ca, Sr, Ba}: A DFT study

We show from a bond valence sum correlation that very high superconducting $ T_{c} $ values should be found in optimally hole-doped infinite-layer ACuO$ _{2} $ cuprates - up to 160~K for A = Ba. The projected increase in $ T_{c} $ across the series arises from "internal pressure" effects as A runs from Mg to Ba. We then use density functional theory to investigate these pressure effects on the band structure in an attempt to understand this progressive increase in $ T_{c} $. Where these materials have been synthesised we find good agreement between our calculated structural parameters and the experimental ones. We find that internal pressure associated with increasing ion size does indeed enhance the superconducting energy gap, as observed, via modifications to the electronic dispersion. Furthermore, in our calculations, pressure alters the dispersion independently of how it is applied (internal or external) so that the superconducting energy gap correlates with the unit-cell volume and a Fermi-surface shape-parameter describing ratio of next-nearest-neighbor to nearest-neighbor hopping integrals. We infer an energy scale for the pairing interaction of the order of 1~eV, well above the magnetic energy scale.

cond-mat.supr-con↗

arXiv2016

Muon spin rotation and infrared spectroscopy study of magnetism and superconductivity in Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $

Using muon spin rotation and infrared spectroscopy we study the relation between magnetism and superconductivity in Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $ single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the Néel-temperature, $ T^{\mathrm{N}} $. This applies for both the total Fe moment obtained from muon spin rotation and for the itinerant component that is deduced from the spectral weight of the spin-density-wave pair breaking peak in the infrared response. In the moderately underdoped region, superconducting and static magnetic orders co-exist on the nano-scale and compete for the same electronic states. The static magnetic moment disappears rather sharply near optimal doping, however, in the slightly overdoped region there is still an enhancement or slowing down of spin fluctuations in the superconducting state. Similar to the gap magnitude reported from specific heat measurements, the superconducting condensate density is nearly constant in the optimally- and slightly overdoped region, but exhibits a rather pronounced decrease on the underdoped side. Several of these observations are similar to the phenomenology in the electron doped counterpart Ba(Fe$ _{1-y} $Co$ _{y} $)$ _{2} $As$ _{2} $.

cond-mat.supr-con↗

arXiv2014

The pseudogap in YBa$_2$Cu$_3$O$_{6+δ}$ is not bounded by a line of phase transitions - thermodynamic evidence

We discuss a recent resonant ultrasound spectroscopy (RUS) study of YBa$_2$Cu$_3$O$_{6+δ}$, which infers a line of phase transitions bounding the pseudogap phase and argue that this scenario is not supported by thermodynamic evidence. We show that the anomalies in RUS, heat capacity and thermal expansion at the superconducting transition temperatures agree well. But there are large discrepancies between RUS and thermodynamic measurements at $T^*$ where the pseudogap phase transitions are purported to occur. Moreover, the frequency and temperature dependence of the RUS data for the crystal with $δ= 0.98$, interpreted in terms of critical slowing down near an electronic phase transition, is five orders of magnitude smaller than what is expected. For this crystal the RUS data near $T^*$ are more consistent with non-equilibrium effects such as oxygen relaxation.

cond-mat.supr-con↗

arXiv2013

Dielectric versus magnetic pairing mechanisms in high-temperature cuprate superconductors investigated using Raman scattering

We suggest, and demonstrate, a systematic approach to the study of cuprate superconductors, namely, progressive change of ion size in order to systematically alter the interaction strength and other key parameters. R(Ba,Sr)$_2$Cu$_3$O$_y$ (R={La, ... Lu,Y}) is such a system where potentially obscuring structural changes are minimal. We thereby systematically alter both dielectric and magnetic properties. Dielectric fluctuation is characterized by ionic polarizability while magnetic fluctuation is characterized by exchange interactions measurable by Raman scattering. The range of transition temperatures is 70 to 107 K and we find that these correlate only with the dielectric properties, a behavior which persists with external pressure. The ultimate significance may remain to be proven but it highlights the role of dielectric screening in the cuprates and adds support to a previously proposed novel pairing mechanism involving exchange of quantized waves of electronic polarization.

cond-mat.supr-con↗

arXiv2013

Large low-temperature magnetoresistance in SrFe2As2 single crystals

We present the first report on a large low-temperature magnetoresistance (MR) of more than 1600% in a SrFe2As2 single crystal and 1300% in a low-energy Ca ion-implanted SrFe2As2 single crystal that occurs before the emergence of crystallographic strain-induced bulk superconductivity arising from a sample aging effect. In accordance to band structure calculations from literature, which consitently show more than 2 bands are involved in the transport, we have modeled this large MR at high fields using a 3-carrier scenario rather than solely on quantum linear MR model generally used to explain the MR in iron-pnictides. At and below 20 K the large MR may be due to 3-carrier transport in an inhomogeneous state where there are superconducting and metallic regions.

cond-mat.supr-con↗

arXiv2013

Electron pockets and pseudogap asymmetry observed in the thermopower of underdoped cuprates

We calculate the diffusion thermoelectric power of high-Tc cuprates using the resonating-valence-bond spin-liquid model developed by Yang, Rice and Zhang (YRZ). In this model, reconstruction of the energy-momentum dispersion results in a pseudogap in the density of states that is heavily asymmetric about the Fermi level. The subsequent asymmetry in the spectral conductivity is found to account for the large magnitude and temperature dependence of the thermopower observed in underdoped cuprates. In addition we find evidence in experimental data for electron pockets in the Fermi surface, arising from a YRZ-like reconstruction, near the onset of the pseudogap in the slightly overdoped regime.

cond-mat.supr-con↗

arXiv2012

Two-component electron fluid in underdoped high-$T_c$ cuprate superconductors

Evidence from NMR of a two-component spin system in cuprate high-$T_c$ superconductors is shown to be paralleled by similar evidence from the electronic entropy so that a two-component quasiparticle fluid is implicated. We propose that this two-component scenario is restricted to the optimal and underdoped regimes and arises from the upper and lower branches of the reconstructed energy-momentum dispersion proposed by Yang, Rice and Zhang (YRZ) to describe the pseudogap. We calculate the spin susceptibility within the YRZ formalism and show that the doping and temperature dependence reproduces the experimental data for the cuprates.

cond-mat.supr-con↗

arXiv2012

Tc is insensitive to magnetic interactions in high-Tc superconductors

A quarter of a century after their discovery the mechanism that pairs carriers in the cuprate high-Tc superconductors (HTS) still remains uncertain. Despite this the general consensus is that it is probably magnetic in origin [1] so that the energy scale for the pairing boson is governed by J, the antiferromagnetic exchange interaction. Recent studies using resonant inelastic X-ray scattering strongly support these ideas [2]. Here as a further test we vary J (as measured by two-magnon Raman scattering) by more than 60% by changing ion sizes in the model HTS system LnA2Cu3O7-δ where A=(Ba,Sr) and Ln=(La, Nd, Sm, Eu, Gd, Dy, Yb, Lu). Such changes are often referred to as "internal" pressure. Surprisingly, we find Tcmax anticorrelates with J where internal pressure is the implicit variable. This is the opposite to the effect of external pressure and suggests that J is not the dominant energy scale governing Tcmax.

cond-mat.supr-con↗

arXiv2010

Evidence of precursor superconductivity as high as 180 K from infrared spectroscopy

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O7-d (R=Y, Gd, Eu) provides important new information about the anomalous normal state properties of underdoped cuprate high temperature superconductors. Besides competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of ~180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions.

cond-mat.supr-con↗

arXiv2009

Fluctuations and $T_c$ reduction in cuprate superconductors

We analyse fluctuations about $T_c$ in the specific heat of (Y,Ca)Ba$_2$Cu$_3$O$_{7-δ}$, YBa$_2$Cu$_4$O$_8$ and Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. The mean-field transition temperature, $T_c^{mf}$, in the absence of fluctuations lies well above $T_c$ especially at low doping where it reaches as high as 150K. We show that phase and amplitude fluctuations set in simultaneously and $T_c^{mf}$ scales with the gap, $Δ_0$, such that $2Δ_0/k_BT_c^{mf}$ is comparable to the BCS weak-coupling value, 4.3, for d-wave superconductivity. We also show that $T_c^{mf}$ is unrelated to the pseudogap temperature, $T^*$.

cond-mat.supr-con↗

arXiv2009

Energy gaps in high-$T_c$ superconductors: BCS after all?

A major impediment to solving the problem of high-$T_c$ superconductivity is the ongoing confusion about the magnitude, structure and doping dependence of the superconducting gap, $Δ_0$, and of the mysterious pseudogap found in underdoped samples\cite{TallonLoram}. The pseudogap opens around the ($π$,0) antinodes below a temperature $T^*$ leaving Fermi arcs across the remnant Fermi surface\cite{Kanigel} on which the superconducting gap forms at $T_c$. One thing that seems agreed is that the ratio $2Δ_0/k_BT_c$ well exceeds the BCS value and grows with underdoping\cite{Miyakawa1,Miyakawa2}, suggesting unconventional, non-BCS superconductivity. Here we re-examine data from many spectroscopies, especially Raman $B_{1g}$ and $B_{2g}$ scattering\cite{Sacuto,Guyard}, and reconcile them all within a two-gap scenario showing that the points of disagreement are an artefact of spectral-weight loss arising from the pseudogap. Crucially, we find that $Δ_0(p)$, or more generally the order parameter, now scales with the mean-field $T_c$ value, adopting the weak-coupling BCS ratio across the entire phase diagram.

cond-mat.supr-con↗

arXiv2009

Thermoelectric power of high-Tc superconductors calculated from the electronic structure

We have calculated the thermopower of the Bi2Sr2CuO6 and Bi2Sr2CaCu2O8 superconductors using an ARPES-derived dispersion, with a model pseudogap, and a marginal-Fermi liquid scattering rate that has a minimum with respect to energy at the van Hove singularity (vHs). Good fits with data are achieved across the entire phase diagram, thus confirming the dispersions, the locations of the vHs and the dominance of the diffusion thermopower over the phonon drag contribution.

cond-mat.supr-con↗

arXiv2008

The pseudogap ground state in high temperature superconductors

By re-examining recently-published data from angle-resolved photoemission spectroscopy we demonstrate that, in the superconducting region of the phase diagram, the pseudogap ground state is an arc metal. This scenario is consistent with results from Raman spectroscopy, specific heat and NMR. In addition, we propose an explanation for the "Fermi pockets" inferred from quantum oscillations in terms of a pseudogapped bilayer Fermi surface.

cond-mat.supr-con↗

arXiv2007

Pressure dependence of the oxygen isotope effect in YBa$_2$Cu$_4$O$_8$

We have carried out measurements of the pressure dependence to 1.2 GPa of the oxygen isotope effect on $T_c$ in the high-$T_c$ superconductor YBa$_2$Cu$_4$O$_8$ using a clamp cell in a SQUID magnetometer. This compound lies close to, but just above, the 1/8$^{th}$ doping point where in La$_{2-x}$Sr$_x$CuO$_4$ marked anomalies in isotope effects occur. Both isotopes show the same very large pressure dependence of $T_c$ with the result that the isotope exponent remains low ($\sim$0.08) but increases slightly with increasing pressure. This is discussed in terms of stripe suppression, a competing pseudogap and the effect of superconducting fluctuations.

cond-mat.supr-con↗

arXiv2007

Measurement of Newtonian fluid slip using a torsional ultrasonic oscillator

The composite torsional ultrasonic oscillator, a versatile experimental system, can be used to investigate slip of Newtonian fluid at a smooth surface. A rigorous analysis of slip-dependent damping for the oscillator is presented. Initially, the phenomenon of finite surface slip and the slip length are considered for a half-space of Newtonian fluid in contact with a smooth, oscillating solid surface. Definitions are revisited and clarified in light of inconsistencies in the literature. We point out that, in general oscillating flows, Navier's slip length b is a complex number. An intuitive velocity discontinuity parameter of unrestricted phase is used to describe the effect of slip on measurement of viscous shear damping. The analysis is applied to the composite oscillator and preliminary experimental work for a 40 kHz oscillator is presented. The Non-Slip Boundary Condition (NSBC) has been verified for a hydrophobic surface in water to within ~60 nm of |b|=0 nm. Experiments were carried out at shear rate amplitudes between 230 and 6800 /s, corresponding to linear displacement amplitudes between 3.2 and 96 nm.

physics.flu-dyn↗

arXiv2007

Saddle-point van Hove singularity and the phase diagram of high-Tc cuprates

We examine the generic phase behavior of high-Tc cuprate superconductors in terms a universal van Hove singularity in the strongly overdoped region. Using a rigid ARPES-derived dispersion we solve the BCS gap equation and show that the pairing interaction or pairing energy cutoff must be a rapidly declining function of doping. This result is prejudicial to a phonon-based pairing interaction and more consistent with a magnetic or magnetically enhanced interaction.

cond-mat.supr-con↗