arXiv subjects

William Sacks

Publications and source records attributed to William Sacks.

12 recordsLinked to original sources

arXiv2026

Condensation mechanism of high-$T_c$ cuprates: the key role of pairon excitations

In this article we show that the condensation mechanism in cuprates involves the strong coupling of the condensate to pairon excited states. We present an accessible formalism that significantly extends our previous work, providing a theoretical basis for the energy-dependent gap function $\Delta(E)$. The latter is proportional to the effective spin exchange energy, $J_{eff}$, with no retardation effects, such as the case of spin-fluctuation or phonon mediated couplings. The fundamental parameters of the superconducting (SC) state are the condensation energy per pair, $\beta_c$, and the antinodal energy gap, $\Delta_p$, which are quantitatively extracted by fitting the cuprate quasiparticle spectrum from tunneling experiments. An explicit formula for the critical temperature is also derived in the model. Valid for any doping, we find $T_c$ to be proportional to $\beta_c$, and not the gap $\Delta_p$, in sharp contrast to conventional SC. The numerical factor $\beta_c/k_BT_c\simeq 2.24$ originates from pair excitations of the condensate, following Bose statistics, with a mini-gap $\delta_M \simeq 1\,$meV in the excitation spectrum. These results strongly suggest that the same `all-electron' mechanism is at work all along the $T_c$-dome.

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arXiv2025

Magnetic phase diagram of cuprates and universal scaling laws

In this article we consider the magnetic field phase diagram of hole-doped high-$T_c$ cuprates, which has been given much less attention than the temperature diagram. In the framework of the {\it pairon model}, we show that the two characteristic energies, the pair binding energy (the gap $\Delta_p$) and the condensation energy ($\beta_c$) resulting from pair correlations, give rise to two major magnetic fields, the upper critical field $B_{c2}$ and a second field, $B_{pg}$, associated with the pseudogap (PG). The latter implies a second length scale in addition to the coherence length, characteristic of incoherent pairs. Universal scaling laws for both $B_{c2}$ and $B_{pg}$ are derived: $B_{c2}$ scales with the critical temperature, $B_{c2}/T_c\simeq 1.65$ T/K, in agreement with many experiments, and $B_{pg}$ has a similar scaling with respect to $T^*$. Finally, Fermi arcs centered on the nodal directions are predicted to appear as a function of magnetic field, an effect testable experimentally.

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arXiv2024

Unraveling pairon excitations and the antiferromagnetic contributions in the cuprate specific heat

Thermal measurements, such as the entropy and the specific heat, reveal key elementary excitations for understanding the cuprates. In this paper, we study the specific heat measurements on three different compounds La$_{2-x}$Sr$_x$CuO$_4$, Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and YBa$_2$Cu$_3$O$_{7-\delta}$ and show that the data are compatible with `pairons' and their excitations. However, the precise fits require the contribution of the antiferromagnetic entropy deduced from the magnetic susceptibility $\chi(T)$. Two temperature scales are involved in the excitations above the critical temperature $T_c$: the pseudogap $T^*$, related to pairon excitations, and the magnetic correlation temperature, $T_{max}$, having very different dependencies on the carrier density ($p$). In agreement with our previous analysis of $\chi(T)$, the $T_{max}(p)$ line is not the signature of a gap in the electronic density of states, but is rather the temperature scale of strong local antiferromagnetic correlations which dominate for low carrier concentration. These progressively evolve into paramagnetic fluctuations in the overdoped limit. Our results are in striking contradiction with the model of J. L. Tallon and J. G. Storey [Phys. Rev. B {\bf 107}, 054507 (2023)], who reaffirm the idea of a $T$-independent gap $E_g$, whose temperature scale $T_g=E_g/k_B$ decreases linearly with $p$ and vanishes at a critical value $p_c \sim 0.19$. Finally, we discuss the unconventional fluctuation regime above $T_c$, which is associated with a mini-gap $\delta\sim$ 2\,meV in the pairon excitation spectrum. This energy scale is fundamental to the condensation mechanism.

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arXiv2023

Statistics of the cuprate pairon states on a square lattice

In this paper the fundamental parameters of high-$T_c$ superconductivity are shown to be connected to the statistics of pairons (hole pairs in their antiferromagnetic environment) on a square lattice. In particular, we study the density fluctuations and the distribution of the area surrounding each pairon on the scale of the antiferromagnetic correlation length $\xi_{AF}$, for the complete range of hole concentration. We show that the key parameters of the phase diagram, the $T_c$ dome, and the pseudogap temperature $T^*$, emerge from the statistical properties of the pairon disordered state. In this approach, the superconducting and the pseudogap states appear as inseparable phenomena. The condensation energy, which fixes the critical temperature, is directly proportional to the {\it correlation energy} between pairons and {\it not} to the energy gap, contrary to conventional superconductors. When the correlation energy between pairons is suppressed by fluctuations, either thermally, by disorder, or in the vortex core, the pseudogap state of disordered pairons is obtained. We attribute the unique features of cuprate superconductivity to this order-disorder transition in real space, which clearly differs from the BCS mechanism. Our predictions are in quantitative agreement with low-temperature tunneling and photoemission spectroscopy experiments.

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arXiv2022

Superconductivity in cuprates governed by topological constraints

The remarkable universality of the cuprate $T_c$ dome suggests a very fundamental unifying principle. Moreover, the superconducting gap is known to persist above $T_c$ in the pseudogap phase of all cuprates. So, contrary to BCS, the gap cannot be the order parameter of the transition. In this work, we show that both the $T_c$-dome and the pseudogap line $T^*(p)$ arise from a unique and identifiable principle: the interaction of localized `pairons' on an antiferromagnetic square lattice. The topological constraints on such preformed pairons give rise to both the $T_c$ dome and the pairing energy {\it simultaneously}. It also provides a natural explanation for the critical doping points of the phase diagram. The model matches perfectly both the $T^*$ and $T_c$ experimental lines, with only one adjustable parameter.

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arXiv2022

Cuprates phase diagram deduced from magnetic susceptibility: what is the `true' pseudogap line?

Two contradictory phase diagrams have dominated the literature of high-$T_c$ cuprate superconductors. Does the pseudogap line cross the superconducting $T_c$-dome or not? To answer, we have revisited the experimental magnetic susceptibility and knight shift of four different compounds, La$_{1-x}$Sr$_x$CuO$_4$, Bi$_2$Sr$_2$Ca$_{1-x}$Y$_x$Cu$_2$O$_8$, Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$, and YBa$_2$Cu$_3$O$_{6+y}$, as a function of temperature and doping. The susceptibility can be described by the same function for all materials, having a magnetic and an electronic contributions. The former is the 2D antiferromagnetic (AF) square lattice response, with a characteristic temperature of magnetic correlations $T_{max}$. The latter is the `Pauli' term, revealing the gap opening in the electronic density of states at the pseudogap temperature $T^*$. From precise fits of the data, we find that $T_{max}(p)$ decreases linearly as a function of doping ($p$) over a wide range, but saturates abruptly in the overdoped regime. Concomitantly, $T^*(p)$ is {\it linear and tangent} to the dome, either crossing or approaching $T_{max}(p)$ at the top of the dome, indicating a qualitative change of behavior from underdoped to overdoped regimes. Contrary to the idea that the pseudogap terminates just above optimal doping, our analysis suggests that the gap exists throughout the phase diagram. It is consistent with a pseudogap due to hole pairs, or `pairons', above $T_c$. We conclude that $T_{max}$, reflecting the AF magnetic correlations, has often been misinterpreted as the pseudogap temperature $T^*$.

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arXiv2020

How 'pairons' are revealed in the electronic specific heat of cuprates

Understanding the thermodynamic properties of high-$T_c$ cuprate superconductors is a key step to establish a satisfactory theory of these materials. The electronic specific heat is highly unconventional, distinctly non-BCS, with remarkable doping-dependent features extending well beyond $T_c$. The pairon concept, bound holes in their local antiferromagnetic environment, has successfully described the tunneling and photoemission spectra. In this article, we show that the model explains the distinctive features of the entropy and specific heat throughout the temperature-doping phase diagram. Their interpretation connects unambiguously the pseudogap, existing up to $T^*$, to the superconducting state below $T_c$. In the underdoped case, the specific heat is dominated by pairon excitations, following Bose statistics, while with increasing doping, both bosonic excitations and fermionic quasiparticles coexist.

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arXiv2018

Origin of the Fermi arcs in cuprates: a dual role of quasiparticle and pair excitations

ARPES mesurements in cuprates have given key information on the temperature and angle dependence of the gap ($d$-wave order parameter, Fermi arcs and pseudogap). We show that these features can be understood in terms of a Bose condensation of interacting {\it pairons} (preformed hole pairs which form in their local antiferromagnetic environment). Starting from the basic properties of the pairon wavefunction, we derive the corresponding k-space spectral function. The latter explains the variation of the ARPES spectra as a function of temperature and angle up to T*, the onset temperature of pairon formation. While Bose excitations dominate at the antinode, the fermion excitations dominate around the nodal direction, giving rise to the Fermi arcs at finite temperature. This dual role is the key feature distinguishing cuprate from conventional superconductivity.

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arXiv2017

Cooper pairs without 'glue' in high-$T_c$ superconductors

We address the origin of the Cooper pairs in high-$T_c$ cuprates and the unique nature of the superconducting (SC) condensate. Itinerant holes in an antiferromagnetic background form pairs spontaneously, without any `glue', defining a new quantum object the `pairon'. In the incoherent pseudogap phase, above $T_c$ or within the vortex core, the pairon binding energies are distributed statistically, forming a `Cooper-pair glass'. Contrary to conventional SC, it is the mutual pair-pair interaction that is responsable for the condensation. We give a natural explanation for the {\it ergodic rigidity} of the excitation gap, being uniquely determined by the carrier concentration $p$ and $J$. The phase diagram can be understood, without spin fluctuations, in terms of a single energy scale $\sim J$, the exchange energy at the metal-insulator transition.

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arXiv2016

From Cooper-pair glass to unconventional superconductivity: a unified approach to cuprates and pnictides

We report a microscopic model wherein the unconventional superconductivity emerges from an incoherent `Cooper-pair glass' state. Driven by the pair-pair interaction, a new type of quasi-Bose phase transition is at work. The interaction leads to the unconventional coupling of the quasiparticles to excited pair states, or `super-quasiparticles', with a non-retarded energy-dependent gap. The model describes quantitatively the quasiparticle excitation spectra of both cuprates and pnictides, including the universal `peak-dip-hump' signatures, and for the pseudogap phase above $T_c$. The results show that instantaneous pair-pair interactions account for the SC condensation without a collective mode.

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arXiv2016

Unconventional temperature dependence of the cuprate excitation spectrum

Key properties of the cuprates, such as the pseudogap observed above the critical temperature $T_c$, remain highly debated. Given their importance, we recently proposed a novel mechanism based on the Bose-like condensation of mutually interacting Cooper pairs [W. Sacks, A. Mauger, Y. Noat, Superconduct. Sci. Technol. 28 105014, (2015)]. In this work, we calculate the temperature dependent DOS using this model for different doping levels from underdoped to overdoped. In all situations, due to the presence of excited pairs, a pseudogap is found above $T_c$ while the normal DOS is recovered at $T^*$, the pair formation temperature. A similar behavior is found as a function of magnetic field, crossing a vortex, where a pseudogap exists in the vortex core. We show that the precise DOS shape depends on combined pair (boson) and quasiparticle (fermion) excitations, allowing for a deeper understanding of the SC to the PG transition.

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arXiv2015

Pair-pair interactions as a mechanism for high-T$_c$ superconductivity

The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above $T_c$, pre-existing but fluctuating Cooper pairs give rise to the unconventional {\it pseudogap} (PG) state, well-characterized by experiment. At the critical temperature, the pair-pair interaction induces a Bose-like condensation of these preformed pairs leading to the superconducting (SC) state. Below $T_c$, both the condensation energy and the pair-pair interaction $β$ are proportional to the condensate density $N_{oc}(T)$, whereas the usual Fermi-level spectral gap $Δ_p$ is independent of temperature. The new order parameter $β(T)$, can be followed as a function of temperature, carrier concentration and disorder - i.e. the phase diagrams. The complexity of the cuprates, revealed by the large number of parameters, is a consequence of the {\it coupling of quasiparticles to Cooper-pair excitations}. The latter interpretation is strongly supported by the observed quasiparticle spectral function.

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