Research

8 pages, 373 KiB

Open AccessArticle

Possible Benefits from Phonon/Spin-Wave Induced Gaps below or above E_F for Superconductivity in High-T_C Cuprates

by Thomas Jarlborg

Condens. Matter 2022, 7(2), 41; https://doi.org/10.3390/condmat7020041 - 11 Jun 2022

Viewed by 1404

A phonon of appropriate momentum

k_{F}

will open a band gap at the Fermi energy

E_{F}

. The gap within the electronic density-of-states (DOS),

N (E_{F})

, leads to a gain in electronic energy and a loss of [...] Read more.

A phonon of appropriate momentum

k_{F}

will open a band gap at the Fermi energy

E_{F}

. The gap within the electronic density-of-states (DOS),

N (E_{F})

, leads to a gain in electronic energy and a loss of elastic energy because of the gap-generating phonon. A BCS-like simulation shows that the energy gain is larger than the loss for temperatures below a certain transition temperature,

T_{C}

. Here, it is shown that the energy count can be almost as favorable for gaps a little below or above

E_{F}

. Such gaps can be generated by auxiliary phonons (or even spin- and charge-density waves) with k-vectors slightly different from

k_{F}

. Gaps not too far from

E_{F}

will add to the energy gain at the superconducting transition. In addition, a DOS-peak can appear at

E_{F}

and thereby increase

N (E_{F})

and

T_{C}

. A dip in the DOS below

E_{F}

will result for temperatures below

T_{C}

, which is similar to what often is observed in cuprate superconductors. The roles of spin waves and thermal disorders are discussed. Full article

(This article belongs to the Special Issue Quantum Complex Matter from Charge Density Waves to Superstripes)

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14 pages, 843 KiB

Open AccessArticle

Euclidean Q-Balls of Fluctuating SDW/CDW in the ‘Nested’ Hubbard Model of High-T_c Superconductors as the Origin of Pseudogap and Superconducting Behaviors

by Sergei Mukhin

Condens. Matter 2022, 7(2), 31; https://doi.org/10.3390/condmat7020031 - 31 Mar 2022

Cited by 7 | Viewed by 2015

Abstract

The origin of the pseudogap and superconducting behaviors in high-

T_{c}

superconductors is proposed, based on the picture of Euclidean Q-balls formation that carry Cooper/local-pair condensates inside their volumes. Euclidean Q-balls that describe bubbles of collective spin-/charge density fluctuations (SDW/CDW) [...] Read more.

The origin of the pseudogap and superconducting behaviors in high-

T_{c}

superconductors is proposed, based on the picture of Euclidean Q-balls formation that carry Cooper/local-pair condensates inside their volumes. Euclidean Q-balls that describe bubbles of collective spin-/charge density fluctuations (SDW/CDW) oscillating in Matsubara time are found as a new self-consistent solution of the Eliashberg equations in the ‘nested’ repulsive Hubbard model of high-

T_{c}

superconductors. The Q-balls arise due to global invariance of the effective theory under the phase rotation of the Fourier amplitudes of SDW/CDW fluctuations, leading to conservation of the ‘Noether charge’ Q in Matsubara time. Due to self-consistently arising local minimum of their potential energy at finite amplitude of the density fluctuations, the Q-balls provide greater binding energy of fermions into local/Cooper pairs relative to the usual Frohlich mechanism of exchange with infinitesimal lattice/charge/spin quasiparticles. We show that around some temperature

T^{*}

the Q-balls arise with a finite density of superconducting condensate inside them. The Q-balls expand their sizes to infinity at superconducting transition temperature

T_{c}

. The fermionic spectral gap inside the Q-balls arises in the vicinity of the ‘nested’ regions of the bare Fermi surface. Solutions are found analytically from the Eliashberg equations with the ‘nesting’ wave vectors connecting ‘hot spots’ in the Brillouin zone. The experimental ‘Uemura plot’ of the linear dependence of

T_{c}

on superconducting density

n_{s}

in high-

T_{c}

superconducting compounds follows naturally from the proposed theory. Full article

(This article belongs to the Special Issue Quantum Complex Matter from Charge Density Waves to Superstripes)

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17 pages, 1177 KiB

Open AccessArticle

The Strange-Metal Behavior of Cuprates

by Giovanni Mirarchi, Götz Seibold, Carlo Di Castro, Marco Grilli and Sergio Caprara

Condens. Matter 2022, 7(1), 29; https://doi.org/10.3390/condmat7010029 - 14 Mar 2022

Cited by 4 | Viewed by 3048

Abstract

Recent resonant X-ray scattering experiments on cuprates allowed to identify a new kind of collective excitations, known as charge density fluctuations, which have finite characteristic wave vector, short correlation length and small characteristic energy. It was then shown that these fluctuations provide a [...] Read more.

Recent resonant X-ray scattering experiments on cuprates allowed to identify a new kind of collective excitations, known as charge density fluctuations, which have finite characteristic wave vector, short correlation length and small characteristic energy. It was then shown that these fluctuations provide a microscopic scattering mechanism that accounts for the anomalous transport properties of cuprates in the so-called strange-metal phase and are a source of anomalies in the specific heat. In this work, we retrace the main steps that led us to attributing a central role to charge density fluctuations in the strange-metal phase of cuprates, discuss the state of the art on the issue and provide an in-depth analysis of the contribution of charge density fluctuations to the specific heat. Full article

(This article belongs to the Special Issue Quantum Complex Matter from Charge Density Waves to Superstripes)

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7 pages, 1927 KiB

Open AccessArticle

Out-of-Plane Sulfur Distortions in the Bi₄O₄S₃ Superconductor

by Sharon S. Philip, Anushika Athauda, Yosuke Goto, Yoshikazu Mizuguchi and Despina Louca

Condens. Matter 2021, 6(4), 48; https://doi.org/10.3390/condmat6040048 - 26 Nov 2021

Viewed by 2161

Abstract

The local atomic structure of the non-magnetic layered superconductor Bi

_{4}

O

_{4}

S

_{3}

was investigated using neutron diffraction and pair density function (PDF) analysis. Although on average, the crystal structure is well ordered, evidence for local, out–of–plane sulfur distortions is provided, [...] Read more.

The local atomic structure of the non-magnetic layered superconductor Bi

_{4}

O

_{4}

S

_{3}

was investigated using neutron diffraction and pair density function (PDF) analysis. Although on average, the crystal structure is well ordered, evidence for local, out–of–plane sulfur distortions is provided, which may act as a conduit for charge transfer from the SO

_{4}

blocks into the superconducting BiS

_{2}

planes. In contrast with LaO

_{1 - x}

F

_{x}

BiS

_{2}

, no sulfur distortions were detected in the planes, which indicates that charge density wave fluctuations are not supported in Bi

_{4}

O

_{4}

S

_{3}

. Full article

(This article belongs to the Special Issue Quantum Complex Matter from Charge Density Waves to Superstripes)

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11 pages, 1700 KiB

Open AccessArticle

Nanoscale Phase Separation of Incommensurate and Quasi-Commensurate Spin Stripes in Low Temperature Spin Glass of La_2−xSr_xNiO₄

by Gaetano Campi, Antonio Bianconi and Alessandro Ricci

Condens. Matter 2021, 6(4), 45; https://doi.org/10.3390/condmat6040045 - 23 Nov 2021

Cited by 3 | Viewed by 2397

Abstract

While spin striped phases in La_2−xSr_xNiO_4+y for 0.25 < x < 0.33 are the archetypal case of a 1D spin density wave (SDW) phase in doped antiferromagnetic strongly correlated perovskites, few information is available on the SDW spatial [...] Read more.

While spin striped phases in La_2−xSr_xNiO_4+y for 0.25 < x < 0.33 are the archetypal case of a 1D spin density wave (SDW) phase in doped antiferromagnetic strongly correlated perovskites, few information is available on the SDW spatial organization. In this context, we have measured the spatial variation of the wave vector of the SDW reflection profile by scanning micro X-ray diffractions with a coherent beam. We obtained evidence of a SDW order–disorder transition by lowering a high temperature phase (T > 50 K) to a low temperature phase (T < 50 K). We have identified quasi-commensurate spin stripe puddles in the ordered phase at 50 < T < 70 K, while the low temperature spin glassy phase presents a nanoscale phase separation of T = 30 K, with the coexistence of quasi-commensurate and incommensurate spin stripe puddles assigned to the interplay of quantum frustration and strong electronic correlations. Full article

(This article belongs to the Special Issue Quantum Complex Matter from Charge Density Waves to Superstripes)

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22 pages, 1320 KiB

Open AccessArticle

Connecting Complex Electronic Pattern Formation to Critical Exponents

by Shuo Liu, Erica W. Carlson and Karin A. Dahmen

Condens. Matter 2021, 6(4), 39; https://doi.org/10.3390/condmat6040039 - 04 Nov 2021

Cited by 4 | Viewed by 2516

Abstract

Scanning probes reveal complex, inhomogeneous patterns on the surface of many condensed matter systems. In some cases, the patterns form self-similar, fractal geometric clusters. In this paper, we advance the theory of criticality as it pertains to those geometric clusters (defined as connected [...] Read more.

Scanning probes reveal complex, inhomogeneous patterns on the surface of many condensed matter systems. In some cases, the patterns form self-similar, fractal geometric clusters. In this paper, we advance the theory of criticality as it pertains to those geometric clusters (defined as connected sets of nearest-neighbor aligned spins) in the context of Ising models. We show how data from surface probes can be used to distinguish whether electronic patterns observed at the surface of a material are confined to the surface, or whether the patterns originate in the bulk. Whereas thermodynamic critical exponents are derived from the behavior of Fortuin–Kasteleyn (FK) clusters, critical exponents can be similarly defined for geometric clusters. We find that these geometric critical exponents are not only distinct numerically from the thermodynamic and uncorrelated percolation exponents, but that they separately satisfy scaling relations at the critical fixed points discussed in the text. We furthermore find that the two-dimensional (2D) cross-sections of geometric clusters in the three-dimensional (3D) Ising model display critical scaling behavior at the bulk phase transition temperature. In particular, we show that when considered on a 2D slice of a 3D system, the pair connectivity function familiar from percolation theory displays more robust critical behavior than the spin-spin correlation function, and we calculate the corresponding critical exponent. We discuss the implications of these two distinct length scales in Ising models. We also calculate the pair connectivity exponent in the clean 2D case. These results extend the theory of geometric criticality in the clean Ising universality classes, and facilitate the broad application of geometric cluster analysis techniques to maximize the information that can be extracted from scanning image probe data in condensed matter systems. Full article

(This article belongs to the Special Issue Quantum Complex Matter from Charge Density Waves to Superstripes)

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11 pages, 564 KiB

Open AccessArticle

Character of Doped Holes in Nd_1−xSr_xNiO₂

by Tharathep Plienbumrung, Michael Thobias Schmid, Maria Daghofer and Andrzej M. Oleś

Condens. Matter 2021, 6(3), 33; https://doi.org/10.3390/condmat6030033 - 19 Aug 2021

Cited by 6 | Viewed by 2922

Abstract

We investigate charge distribution in the recently discovered high-

T_{c}

superconductors, layered nickelates. With increasing value of charge-transfer energy, we observe the expected crossover from the cuprate to the local triplet regime upon hole doping. We find that the

d - p

[...] Read more.

We investigate charge distribution in the recently discovered high-

T_{c}

superconductors, layered nickelates. With increasing value of charge-transfer energy, we observe the expected crossover from the cuprate to the local triplet regime upon hole doping. We find that the

d - p

Coulomb interaction

U_{d p}

makes Zhang-Rice singlets less favorable, while the amplitude of local triplets at Ni ions is enhanced. By investigating the effective two-band model with orbitals of

x^{2} - y^{2}

and s symmetries we show that antiferromagnetic interactions dominate for electron doping. The screened interactions for the s band suggest the importance of rare-earth atoms in superconducting nickelates. Full article

(This article belongs to the Special Issue Quantum Complex Matter from Charge Density Waves to Superstripes)

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7 pages, 1422 KiB

Open AccessCommunication

Charge Order and Suppression of Superconductivity in HgBa₂CuO_4+d at High Pressures

by Manuel Izquierdo, Daniele C. Freitas, Dorothée Colson, Gastón Garbarino, Anne Forget, Helène Raffy, Jean-Paul Itié, Sylvain Ravy, Pierre Fertey and Manuel Núñez-Regueiro

Condens. Matter 2021, 6(3), 25; https://doi.org/10.3390/condmat6030025 - 23 Jul 2021

Cited by 4 | Viewed by 2144

Abstract

New insight into the superconducting properties of HgBa₂CuO₄ (Hg-1201) cuprates is provided by combined measurements of electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20 GPa in optimally doped [...] Read more.

New insight into the superconducting properties of HgBa₂CuO₄ (Hg-1201) cuprates is provided by combined measurements of electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20 GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to ~10 GPa that gradually passes into an insulating state, typical of charge ordering, which totally suppresses superconductivity above 13 GPa. The changes in resistivity are accompanied by the apparition of sharp Bragg peaks in the X-ray diffraction patterns, indicating that the charge ordering is accompanied by a 3D oxygen ordering. Considering that pressure induces a charge transfer of about 0.02 at 10 GPa, our results are the first observation of charge order competing with superconductivity developed in the overdoped region of the phase diagram of a Hg-based cuprate. Full article

(This article belongs to the Special Issue Quantum Complex Matter from Charge Density Waves to Superstripes)

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