arXiv subjects

Jeffery L. Tallon

Publications and source records attributed to Jeffery L. Tallon.

18 recordsLinked to original sources

arXiv2026

Comment on "Distinct Behaviors of Inner and Outer CuO$_2$ Planes in Quadruple-Layer Cuprate (Cu,C)Ba$_2$Ca$_3$Cu$_4$O$_{11 + δ}$"

In a recent Letter, Sun et al. report photoemission spectroscopy measurements on the four-layer cuprate (Cu,C)Ba2Ca3Cu4O11 (CuC-1234) in which they resolve two superconducting gaps associated with the inner (IP) and outer (OP) CuO2 planes. CuC-1234 has a Tc as 117 K in a sample previously structurally characterized using neutron powder diffraction. From the Luttinger sum, the beta-band of the IP was found to be heavily underdoped (p~0.07) while the OP alpha2 bonding band was strongly overdoped (p~0.25). Each revealed a gap with very different momentum and temperature dependences. A large gap on the beta band was found to close at the bulk Tc, while a smaller gap on the alpha2 band closed at 70 K. Such two-gap behavior would indicate weak-coupling between the IP and OP, resulting in a second gap-opening temperature, Tc2, and implying a two-step development of the superfluid density on cooling, first on the IPs then on the OPs. Here, we point out that the observed values of Tc and Tc2 are fully consistent with a long-standing correlation of Tc with a bond-valence sum parameter, V+, calculated from the crystallographic bond lengths. This suggests that Tc2 in the OP is entirely consistent with the local structure of the OP where the value of V+(OP) is reduced by the presence of the apical oxygens and Tc2 reduced accordingly. It is as though two largely decoupled superconductors are present in the single 1234 compound and their individual pairing temperatures are determined by their local nearest-neighbour structure, reflecting the very short-range physics.

cond-mat.supr-con↗

arXiv2026

Best practices for a proper evaluation and conversion of physical property equations in superconductors: the examples of WHH formulation, Bean model and other cases of interest

In recent years there have been growing concerns about the proper evaluation of physical properties of superconductors, in particular for quantities extracted from magnetic characterizations. Errors can and often do occur due to the following issues: i) several measurement instruments still use Gaussian & cgs-emu units instead of the preferable International System (SI) units; ii) there are decades of valuable publications where, however, equations were expressed in Gaussian & cgs-emu or other unit systems or where constants were normalized to unity, which requires proper understanding and unit conversion in order to correctly evaluate the measured physical quantities; iii) the conversion between unit systems sometimes appears challenging and may not be properly performed. In this paper we will describe how to properly convert physical quantities relevant for the evaluation of magnetic and other properties focusing on the still most used unit systems, SI and Gaussian & cgs-emu. We will provide examples of how to properly verify and understand the physical formulae. We will include examples for the correct method to determine the critical current density Jc of a superconductor from the measurement of its magnetic hysteresis loop through the Bean model, and the correct conversion to SI of the equations for Hc2(T) according to the Werthamer-Helfand-Hohenberg (WHH) formulation. The goals of this paper are to make the readers aware of the unit conversion issue, to provide useful hands-on tools for proper conversion and to strongly encourage future exclusive use of the SI units and formulae.

cond-mat.supr-con↗

arXiv2025

Giant critical current peak induced by pressure in kagome superconductor RbV$_{3}$Sb$_{5}$

Superconductivity can coexist or compete with other orders such as magnetism or density waves. Optimizing superconductivity requires identifying competing orders that may disrupt Cooper pair coherence. Here, we use the self-field critical current ($I_{\rm c,sf}$) to probe pressure-tuned superconductivity in the kagome superconductor RbV$_3$Sb$_5$. As pressure destabilizes the charge-density wave (CDW) state, $I_{\rm c,sf}$ drastically enhances, peaking near the critical pressure where the CDW state is completely suppressed at zero temperature. Surprisingly, a weaker $I_{\rm c,sf}$ peak emerges within the CDW phase. Near the pressure of the weaker peak, the superconducting phase transition temperature shifts from an increasing trend with pressure to a near plateau. Our analysis suggests the possibility of a sudden change in the CDW pattern or a Lifshitz transition, highlighting the need for microscopic examinations of the CDW state for understanding the pressure evolution of superconductivity in RbV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2024

Quantum Phase Transition as a Promising Route to Enhance the Critical Current in Kagome Superconductor CsV$_{3}$Sb$_{5}$

Developing strategies to systematically increase the critical current, the threshold current below which the superconductivity exists, is an important goal of materials science. Here, the concept of quantum phase transition is employed to enhance the critical current of a kagome superconductor CsV$_3$Sb$_5$, which exhibits a charge density wave (CDW) and superconductivity that are both affected by hydrostatic pressure. As the CDW phase is rapidly suppressed under pressure, a large enhancement in the self-field critical current ($I_{\rm c,sf}$) is recorded. The observation of a peak-like enhancement of $I_{\rm c,sf}$ at the zero-temperature limit ($I_{\rm c,sf}(0)$) centred at $p^*\approx 20$~kbar, the same pressure where the CDW phase transition vanishes, further provides strong evidence of a zero-temperature quantum anomaly in this class of pressure-tuned superconductor. Such a peak in $I_{\rm c,sf}(0)$ resembles the findings in other well-established quantum-critical superconductors, hinting at the presence of enhanced quantum fluctuations associated with the CDW phase in CsV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2024

Fundamental nature of the self-field critical current in superconductors

The ability to conduct electric current without dissipating energy is a property of superconductors that is used in magnetic systems utilized in healthcare, natural sciences, and ongoing global projects in nuclear fusion and aviation. The highest dissipation-less current is named the critical current, and this is one of the prime practical properties of superconductors (together with the critical current density, Jc). Recently, Goyal et al reported a record high Jc~190 MA/cm2 at 4.2 K in (RE)BCO films, exceeding the highest Jc in the best commercial (RE)BCO wires by a factor of five. Based on the huge potential practical impact of this high Jc, we examined the raw experimental data and found that this high value originates from an error in the conversion of units. The real Jc is 10 times smaller than the reported Jc, consistent with values currently achieved by many manufacturers.

cond-mat.supr-con↗

arXiv2024

Flux-trapping experiments on ultra-high pressure hydrides as evidence of superconductivity

Flux-trapping magnetization studies on H3S at ultra-high pressure have been reported by Minkov et al. as definitive evidence of superconductivity in this hydride system. This is very helpful in a field that has become somewhat controversial. However, this conclusion has been questioned based on an apparent zero-field cooled (ZFC) linear magnetization at low field. The standard Bean model would require an approximately quadratic dependence. In support, we note that the reported ZFC magnetization is indeed super-linear and consistent both with model calculations for thin discs and with the ZFC magnetization reported for YBa2Cu3Oy films. We conclude that the reported high-pressure magnetization data is fully consistent with superconductivity and that there is no reason, in this particular data set, to reject the original inference of hydride superconductivity.

cond-mat.supr-con↗

arXiv2020

Field-dependent specific heat of the canonical underdoped cuprate superconductor YBa$_2$Cu$_4$O$_8$

The cuprate superconductor YBa$_2$Cu$_4$O$_8$, in comparison with most other cuprates, has a stable stoichiometry, is largely free of defects and may be regarded as the canonical underdoped cuprate, displaying marked pseudogap behaviour and an associated distinct weakening of superconducting properties. This cuprate `pseudogap' manifests as a partial gap in the electronic density of states at the Fermi level and is observed in most spectroscopic properties. After several decades of intensive study it is widely believed that the pseudogap closes, mean-field like, near a characteristic temperature, $T^*$, which rises with decreasing hole concentration, $p$. Here, we report extensive field-dependent electronic specific heat studies on YBa$_2$Cu$_4$O$_8$ up to an unprecedented 400 K and show unequivocally that the pseudogap never closes, remaining open to at least 400 K where $T^*$ is typically presumed to be about 150 K. We show from the NMR Knight shift and the electronic entropy that the Wilson ratio is numerically consistent with a weakly-interacting Fermion system for the near-nodal states. And, from the field-dependent specific heat, we characterise the impact of fluctuations and impurity scattering on the thermodynamic properties.

cond-mat.supr-con↗

arXiv2020

Locating the pseudogap closing point in cuprate superconductors: absence of entrant or reentrant behavior

Current descriptions of the pseudogap in underdoped cuprates envision a doping-dependent transition line $T^*(p)$ which descends monotonically towards zero just beyond optimal doping. There is much debate as to the location of the terminal point $p^*$ where $T^*(p)$ vanishes, whether or not there is a phase transition at $T^*$ and exactly how $T^*(p)$ behaves below $T_c$ within the superconducting dome. One perspective sees $T^*(p)$ cutting the dome and continuing to descend monotonically to zero at $p_{crit} \approx 0.19$ holes/Cu $-$ referred to here as `entrant behavior'. Another perspective derived from photoemission studies is that $T^*(p)$ intersects the dome near $p_{crit} \approx 0.23$ holes/Cu then turns back below $T_c$, falling to zero again around $p_{crit} \approx 0.19$ $-$ referred to here as `reentrant behavior'. By examining thermodynamic data for Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ we show that neither entrant nor reentrant behavior is experimentally supported. Rather, $p_{crit} \approx 0.19$ sharply delimits the pseudogap regime and for $p < 0.19$ the pseudogap is always present, independent of temperature. Similar results are found for Y$_{0.8}$Ca$_{0.2}$Ba$_2$Cu$_3$O$_{7-δ}$. For both materials $T^*(p)$ is not a temperature but a crossover scale, $\approx E^*(p)/2k_B$, reflecting instead the underlying pseudogap energy $E^*(p)$ which vanishes as $p \rightarrow 0.19$.

cond-mat.supr-con↗

arXiv2019

Relevance of magnetism to cuprate superconductivity: Lanthanides versus charge-compensated cuprates

We address what seemed to be a contradiction between the lanthanide series REBa$_2$Cu$_3$O$_y$ (RE123) and the charge-compensated series (Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x} $)Cu$_{3}$O$_{y}$ (CLBLCO) regarding the superexchange ($J$) dependence of the maximum superconductivity (SC) critical temperature $T_c^{max}(J)$; RE and $x$ are implicit variables. This is done by measuring the Néel temperature and the temperature dependence of the magnetic order parameter for RE=Nd, Sm, Eu, Gd, Dy, Yb, Y, and for Y(BaSr)Cu$_3$O$_y$, at various very light dopings. The doping is determined by thermopower, and the magnetic properties by muon spin rotation. We find that the normalized-temperature dependence of the order parameter is identical for all RE123 in the undoped limit (with the exception of Gd123) implying identical out-of-plane magnetic coupling. The extrapolation of $T_N$ to zero doping suggests that, despite the variations in ionic radii, $J$ varies too weakly in this system to test the relation between SC and magnetism. This stands in contrast to CLBLCO where both $T_c^{max}$ and $T_N^{max}$ vary considerably in the undoped limit, and a positive correlation between the two quantities was observed.

cond-mat.supr-con↗

arXiv2019

Systematic Raman study of optical phonons in $R$Ba$_2$Cu$_3$O$_{6+δ}$ ($R$ = Y, Dy, Gd, Sm, Nd): Antiferromagnetic coupling strength versus lattice parameters

We present a systematic study of the interplay between lattice parameters and the energy of the optical phonons as well as the antiferromagnetic coupling strength, $J$, in the high-$T_{\text{c}}$ superconducting cuprate $R$Ba$_2$Cu$_3$O$_{6+δ}$ (\mbox{R-123}, $R =$ Y, Dy, Gd, Sm, Nd) with hole-doping $p$ ($0.00<p \lesssim 0.04$). The energy of the $B_{1g}$-mode at $ν_{B1g}\approx~335$ cm$^{-1}$ has been found to relate systematically to the inverse of the lattice parameter $a$. Our results confirm the temperature dependent phonon splitting for Nd-123 at low doping, which has been reported for optimally-doped Nd-123. Surprisingly, $J$ is independent of $a$ for the first four $R$ families, and a general consistency between $T_{\text{c}}^{\text{max}}$ and $J$, as suggested in a previous investigation, could not be confirmed.

cond-mat.str-el↗

arXiv2018

Universal scaling of the self-field critical current in superconductors: from sub-nanometre to millimetre size

Universal scaling behaviour in superconductors has significantly elucidated fluctuation and phase transition phenomena in these materials. However, universal behaviour for the most practical property, the critical current, was not contemplated because prevailing models invoke nucleation and migration of flux vortices. Such migration depends critically on pinning, and the detailed microstructure naturally differs from one material to another, even within a single material. Through microstructural engineering there have been ongoing improvements in the field-dependent critical current, thus illustrating its nonuniversal behaviour. But here we demonstrate the universal size scaling of the self-field critical current for any superconductor, of any symmetry, geometry or band multiplicity. Key to our analysis is the huge range of sample dimensions, from single-atomic-layer to mm-scale. These have widely variable microstructure with transition temperatures ranging from 1.2 K to the current record, 203 K. In all cases the critical current is governed by a fundamental surface current density limit given by the relevant critical field divided by the penetration depth.

cond-mat.supr-con↗

arXiv2018

Current distribution across type II superconducting films: a new vortex-free critical state

The current distribution across the thickness of a current-carrying rectangular film in the Meissner state was established long ago by the London brothers. The distribution across the width is more complicated but was later shown to be highly non-uniform, diverging at the edges. Accordingly, the standard view for type II superconductors is that vortices enter at the edges and, with increasing current, are driven inwards until they self-annihilate at the centre, causing dissipation. This condition is presumed to define the critical current. However we have shown that, under self-field (no external field), the transport critical current is a London surface current where the surface current density equals the critical field divided by λ, across the entire width. The critical current distribution must therefore be uniform. Here we report studies of the current and field distribution across commercial YBa2Cu3O7 conductors and confirm the accepted non-uniform distribution at low current but demonstrate a radical crossover to a uniform distribution at critical current. This crossover ends discontinuously at a singularity and calculations quantitatively confirm these results in detail. The onset of self-field dissipation is, unexpectedly, thermodynamic in character and the implied vortex-free critical state seems to require new physics.

cond-mat.supr-con↗

arXiv2018

The onset of dissipation in high-temperature superconductors: magnetic hysteresis and field dependence

Recently, we showed that the self-field transport critical current, Ic(sf), of a superconducting wire can be defined in a more fundamental way than the conventional (and arbitrary) electric field criterion, Ec = 1 microV/cm. We defined Ic(sf) as the threshold current, Ic,B, at which the perpendicular component of the local magnetic flux density, measured at any point on the surface of a high-temperature superconducting tape, abruptly crosses over from a non-linear to a linear dependence with increasing transport current. This effect results from the current distribution across the tape width progressively transitioning from non-uniform to uniform. The completion of this progressive transition was found to be singular. It coincides with the first discernible onset of dissipation and immediately precedes the formation of a measureable electric field. Here, we show that the same Ic,B definition of critical currents applies in the presence of an external applied magnetic field. In all experimental data presented here Ic,B is found to be significantly (10-30%) lower than Ic,E determined by the common electric field criterion of Ec = 1 microV/cm, and Ec to be up to 50 times lower at Ic,B than at Ic,E.

cond-mat.supr-con↗

arXiv2017

Thermodynamic parameters of single- or multi-band superconductors derived from self-field critical currents

Key questions for any superconductor include: what is its maximum dissipation-free electrical current (its `critical current') and can this be used to extract fundamental thermodynamic parameters? Present models focus on depinning of magnetic vortices and implicate materials engineering to maximise pinning performance. But recently we showed that the self-field critical current for thin films is a universal property, independent of microstructure, controlled only by the penetration depth. Here we generalise this observation to include thin films, wires or nanowires of single- or multi-band s-wave and d-wave superconductors. Using extended BCS theory we consider dissipation-free self-field transport currents as Meissner-London currents, avoiding the concept of pinning altogether. We find quite generally, for type I or type II superconductors, the current is limited by the relevant critical field divided by the penetration depth. Our fits to 62 available data sets, from zinc nanowires to compressed sulphur hydride with critical temperatures of 0.65 to 203 K, respectively, are excellent. Extracted London penetration depths, superconducting energy gaps and specific heat jumps agree well with reported bulk values. For multiband or multiphase samples we accurately recover individual band contributions and phase fractions.

cond-mat.supr-con↗

arXiv2017

London penetration depth and thermal fluctuations in the sulphur hydride 203 K superconductor

Recently, compressed H$_2$S has been shown to become superconducting at 203 K under a pressure of 155 GPa. One might expect fluctuations to dominate at such temperatures. Using the magnetisation critical current, we determine the ground-state London penetration depth, $λ_0$=189 nm, and the superconducting energy gap, $Δ_0$=27.8 meV, and find these parameters are similar to those of cuprate superconductors. We also determine the fluctuation temperature scale, $T_{\textrm{fluc}}=1470$ K, which shows that, unlike the cuprates, $T_c$ of the hydride is not limited by fluctuations. This is due to its three dimensionality and suggests the search for better superconductors should refocus on three-dimensional systems where the inevitable thermal fluctuations are less likely to reduce the observed $T_c$.

cond-mat.supr-con↗

arXiv2016

Universal self-field critical current for thin-film superconductors

For any practical superconductor the magnitude of the critical current density, $J_\textrm{c}$, is crucially important. It sets the upper limit for current in the conductor. Usually $J_\textrm{c}$ falls rapidly with increasing external magnetic field but even in zero external field the current flowing in the conductor generates a self-field which limits $J_\textrm{c}$. Here we show for thin films of thickness less than the London penetration depth, $λ$, this limiting $J_\textrm{c}$ adopts a universal value for all superconductors - metals, oxides, cuprates, pnictides, borocarbides and heavy Fermions. For type I superconductors, it is $H_{\textrm{c}}/λ$ where $H_\textrm{c}$ is the thermodynamic critical field. But surprisingly for type II superconductors we find the self-field $J_\textrm{c}$ is $H_{\textrm{c}1}/λ$ where $H_{\textrm{c}1}$ is the lower critical field. $J_\textrm{c}$ is thus fundamentally determined and this provides a simple means to extract absolute values of $λ(T)$ and, from its temperature dependence, the symmetry and magnitude of the superconducting gap.

cond-mat.supr-con↗

arXiv2013

Coexistence of the superconducting energy gap and pseudogap above and below the transition temperature of superconducting cuprates

We express the superconducting gap, $Δ(T)$, in terms of thermodynamic functions in both $s$- and d-wave symmetries. Applying to Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ and Y$_{0.8}$Ca$_{0.2}$Ba$_2$Cu$_3$O$_{7-δ}$ we find that for all dopings $Δ(T)$ persists, as a partial gap, high above $T_c$ due to strong superconducting fluctuations. Therefore in general two gaps are present above $T_c$, the superconducting gap and the pseudogap, effectively reconciling two highly polarized views concerning pseudogap physics.

cond-mat.supr-con↗

arXiv2009

Synthesis and structure of Na+ intercalated WO3(4,4'-bipyridyl)0.5

We have prepared single crystals of WO3(4,4'-bipyridyl)0.5 and doped these by Na-ion implantation. The structure of the resultant NaxWO3(4,4'-bipyridyl)0.5 was determined by single-crystal x-ray diffraction to comprise atomic layers of corner-shared WO5N octahedra linked by the 4,4'-bypyridine via the apical nitrogen. In the observed space group of Pbca, the fully ordered bipyridyl molecules define cage-shaped structures, not the channels erroneously reported previously for the Cmca polymorph. The Na ions are disordered bimodally about the cage centre, displaced in the c-direction so as to lie closer to the apical oxygens.

cond-mat.mtrl-sci↗