high-Tc Cuprate Superconductors Research Articles

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

A phonon of appropriate momentum kF will open a band gap at the Fermi energy EF. The gap within the electronic density-of-states (DOS), N(EF), 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, TC. Here, it is shown that the energy count can be almost as favorable for gaps a little below or above EF. Such gaps can be generated by auxiliary phonons (or even spin- and charge-density waves) with k-vectors slightly different from kF. Gaps not too far from EF will add to the energy gain at the superconducting transition. In addition, a DOS-peak can appear at EF and thereby increase N(EF) and TC. A dip in the DOS below EF will result for temperatures below TC, which is similar to what often is observed in cuprate superconductors. The roles of spin waves and thermal disorders are discussed.

High critical current density and high-tolerance superconductivity in high-entropy alloy thin films

High-entropy alloy (HEA) superconductors—a new class of functional materials—can be utilized stably under extreme conditions, such as in space environments, owing to their high mechanical hardness and excellent irradiation tolerance. However, the feasibility of practical applications of HEA superconductors has not yet been demonstrated because the critical current density (Jc) for HEA superconductors has not yet been adequately characterized. Here, we report the fabrication of high-quality superconducting (SC) thin films of Ta–Nb–Hf–Zr–Ti HEAs via a pulsed laser deposition. The thin films exhibit a large Jc of >1 MA cm−2 at 4.2 K and are therefore favorable for SC devices as well as large-scale applications. In addition, they show extremely robust superconductivity to irradiation-induced disorder controlled by the dose of Kr-ion irradiation. The superconductivity of the HEA films is more than 1000 times more resistant to displacement damage than that of other promising superconductors with technological applications, such as MgB2, Nb3Sn, Fe-based superconductors, and high-Tc cuprate superconductors. These results demonstrate that HEA superconductors have considerable potential for use under extreme conditions, such as in aerospace applications, nuclear fusion reactors, and high-field SC magnets.

Antinodal kink in the band dispersion of electron-doped cuprate La2\u2212xCexCuO4\xb1\u03b4

Angle-resolved photoemission spectroscopy (ARPES) measurements have established the phenomenon of kink in band dispersion of high-Tc cuprate superconductors. However, systematic studies of the kink in electron-doped cuprates are still lacking experimentally. We performed in situ ARPES measurements on La2−xCexCuO4±δ (LCCO) thin films over a wide electron doping (n) range from 0.05 to 0.23. While the nodal kink is nearly invisible, an antinodal kink around 45 meV, surviving above 200 K, is observed for n ~ 0.05–0.19, whose position is roughly independent of doping. The fact that the antinodal kink observed at high temperatures and in the highly overdoped region favors the phonon mechanism with contributions from the Cu–O bond-stretching mode and the out-of-plane oxygen buckling mode.

Anomalous phase separation in a correlated electron system: Machine-learning–enabled large-scale kinetic Monte Carlo simulations

SignificancePhase separation is crucial to the functionalities of many correlated electron materials with notable examples including colossal magnetoresistance in manganites and high-Tc superconductivity in cuprates. However, the nonequilibrium phase-separation dynamics in such systems are poorly understood theoretically, partly because the required multiscale modeling is computationally very demanding. With the aid of machine-learning methods, we have achieved large-scale dynamical simulations in a representative correlated electron system. We observe an unusual relaxation process that is beyond the framework of classical phase-ordering theories. We also uncover a correlation-induced freezing behavior, which could be a generic feature of phase separation in correlated electron systems.

Cuprates phase diagram deduced from magnetic susceptibility: What is the ‘true’ pseudogap line?

Two contradictory phase diagrams have dominated the literature of high-Tc cuprate superconductors. Does the pseudogap line cross the superconducting Tc-dome or not? To answer, we have revisited the experimental magnetic susceptibility and knight shift of four different compounds, La1−xSrxCuO4, Bi2Sr2Ca1−xYxCu2O8, Bi2Sr2CaCu2O8+y, YBa2Cu3O6+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 Tmax. 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 Tmax(p) decreases linearly as a function of doping (p) over a wide range, but saturates abruptly in the overdoped regime. Concomitantly, T∗(p) is linear and tangent to the dome, either crossing or approaching Tmax(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 Tc. We conclude that Tmax, reflecting the AF magnetic correlations, has often been misinterpreted as the pseudogap temperature T∗.

63,65Cu, 139La-NMR study of Electron-doped High Temperature Cuprate Superconductor T’-La1.8Eu0.2CuO4–y F y

T’-La1.8Eu0.2CuO4–y F y (0 ≤ y ≥ 0.125) was measured by 63,6BCu and 139La NMR. This material is an electron-doped high-Tc cuprate superconductor which has Nd2CuO4-type structure (so-called T’-structure). The nuclear spin-lattice relaxation rate 1/T 1 revealed that pseudogap behavior exists in the lightly electron-doped region of y ≤ 0.075 and electron doping suppresses the antiferromagnetic spin flutuations. These behaviors in electron-doped high-T c cuprate superconductor are analogous with the optimum- and over-doped region in hole-doped type. 139 La NMR spectra shows almost no antiferromagnetic region in the samples of T’-La1.8Eu0.2CuO4–y F y .

The Superconductivity in Fe-Based Family of Superconductors and Its Electronic Structure Analysis in Presence of Dopants Rh and Pd

The discovered in 2008 Fe-based superconductors (SC) are a paramagnetic semimetal at ambient temperature and in some cases they become superconductor upon doping. In spite of so many years since its discovery it is still not known the mechanism that leads to superconductivity. The electronic structure study is used for determining key features of the SC mechanism in these materials. The calculations were performed using the modern suite of programs MOLPRO 2021. We performed quantum calculations of a cluster embedded in a background charge distribution that represents the infinite crystal. The Natural Population Analysis was used for determining the charge and spin distribution in the studied materials. As follows from our results, the possible mechanism for superconductivity corresponds to the RVB theory proposed by Anderson for high Tc superconductivity in cuprates.

Observation of Cu Spin Fluctuations in High-Tc Cuprate Superconductor Nanoparticles Investigated by Muon Spin Relaxation.

The nano-size effects of high-Tc cuprate superconductor La2−xSrxCuO4 with x = 0.20 are investigated using X-ray diffractometry, Transmission electron microscopy, and muon-spin relaxation (μSR). It is investigated whether an increase in the bond distance of Cu and O atoms in the conducting layer compared to those of the bulk state might affect its physical and magnetic properties. The μSR measurements revealed the slowing down of Cu spin fluctuations in La2−xSrxCuO4 nanoparticles, indicating the development of a magnetic correlation at low temperatures. The magnetic correlation strengthens as the particle size reduces. This significantly differs from those observed in the bulk form, which show a superconducting state below Tc. It is indicated that reducing the particle size of La2−xSrxCuO4 down to nanometer size causes the appearance of magnetism. The magnetism enhances with decreasing particle size.

Spectral weight of hole-doped cuprates across the pseudogap critical point

One of the most widely discussed features of the cuprate high Tc superconductors is the presence of a pseudogap in the normal state. Recent transport and specific heat measurements have revealed an abrupt transition at the pseudogap critical point, denoted p*, characterized by a drop in carrier density and a strong mass enhancement. In order to give more details about this transition at p*, we performed low-temperature infrared spectroscopy in the normal state of cuprate superconductors La2-xSrxCuO4 (LSCO) and La1.8-xEu0.2SrxCuO4 (Eu-LSCO) for doping contents across the pseudogap critical point p* (from p = 0.12 to 0.24). Through the complex optical conductivity we can extract the spectral weight, K*, of the narrow Drude peak due the coherent motion of the quasi-particles, and the spectral weight enclosed inside the mid-infrared (MIR) band, KMIR, caused by coupling of the quasi-particles to collective excitations of the many-body system. K* is smaller than a third of the value predicted by band calculations, and KMIR forms a dome as a function of doping. We observe a smooth doping dependence of K* through p*, and demonstrate that this is consistent with the observed doping dependence of the carrier density and the mass enhancement. We argue that the superconducting dome is the result of the confluence of two opposite trends, namely the increase of the density of the quasi-particles and the decrease of their coupling to the collective excitations as a function of doping.

Roton pair density wave in a strong-coupling kagome superconductor.

The transition metal kagome lattice materials host frustrated, correlated and topological quantum states of matter1-9. Recently, a new family of vanadium-based kagome metals, AV3Sb5 (A = K, Rb or Cs), with topological band structures has been discovered10,11. These layered compounds are nonmagnetic and undergo charge density wave transitions before developing superconductivity at low temperatures11-19. Here we report the observation of unconventional superconductivity and a pair density wave (PDW) in CsV3Sb5 using scanning tunnelling microscope/spectroscopy and Josephson scanning tunnelling spectroscopy. We find that CsV3Sb5 exhibits a V-shaped pairing gap Δ ~ 0.5 meV and is a strong-coupling superconductor (2Δ/kBTc ~ 5) that coexists with 4a0 unidirectional and 2a0 × 2a0 charge order. Remarkably, we discover a 3Q PDW accompanied by bidirectional 4a0/3 spatial modulations of the superconducting gap, coherence peak and gap depth in the tunnelling conductance. We term this novel quantum state a roton PDW associated with an underlying vortex-antivortex lattice that can account for the observed conductance modulations. Probing the electronic states in the vortex halo in an applied magnetic field, in strong field that suppresses superconductivity and in zero field above Tc,reveals that the PDW is a primary state responsible for an emergent pseudogap and intertwined electronic order. Our findings show striking analogies and distinctions to the phenomenology of high-Tc cuprate superconductors, and provide groundwork for understanding the microscopic origin of correlated electronic states and superconductivity in vanadium-based kagome metals.

Synthesis of murunskite single crystals: A bridge between cuprates and pnictides

Numerous contemporary investigations in condensed matter physics are devoted to high temperature (high-Tc) cuprate superconductors. Despite its unique effulgence among research subjects, the enigma of the high-Tc mechanism still persists. One way to advance its understanding is to discover and study new analogous systems. Here we begin a novel exploration of the natural mineral murunskite, K2FeCu3S4, as an interpolation compound between cuprates and ferropnictides, the only known high-Tc superconductors at ambient pressure. Because in-depth studies can be carried out only on single crystals, we have mastered the synthesis and growth of high quality specimens. Similar to the cuprate parent compounds, these show semiconducting behavior in resistivity and optical transmittance, and an antiferromagnetic ordering at 100 K. Spectroscopy (XPS) and calculations (DFT) concur that the sulfur 3p orbitals are partially open, making them accessible for charge manipulation, which is a prerequisite for superconductivity in analogous layered structures. DFT indicates that the valence band is more cuprate-like, while the conduction band is more pnictide-like. With appropriate doping strategies, this parent compound promises exciting future developments.

Magnetic field effects on the transport properties of high-Tc cuprates

Starting from a recently proposed comprehensive theory for the high-Tc superconductivity in cuprates, we derive a general analytic expression for the planar resistivity, in the presence of an applied external magnetic field and explore its consequences in the different phases of these materials. As an initial probe of our result, we show it compares very well with experimental data for the resistivity of La2-xSrx CuO4 (LSCO) at different values of the applied field. We also apply our result to Bi2201 and show that the magnetoresistivity (MR) in the strange metal (SM) phase of this material, exhibits the H 2 to H crossover, as we move from the weak to the strong field regime. Yet, despite of that, the MR does not present a quadrature scaling. Remarkably, the resistivity H-field derivative does scale as a function of , in complete agreement with recent magneto-transport measurements made in the SM phase of cuprates (Ayres et al 2020 arXiv:2012.01208). We, finally, address the issue of the T-power-law dependence of the resistivity of overdoped cuprates and compare our results with experimental data for Tl2201. We show that this provides a simple method to determine whether the quantum critical point associated to the pseudogap temperature T *(x) belongs to the superconducting dome or not.

Suppression of Pseudogap Behavior and Antiferromagnetic Fluctuations by Electron Doping in T′-La1.8Eu0.2CuO4−yFy Studied by 63,65Cu NMR

We performed 63,65Cu NMR measurements of an electron-doped high-Tc cuprate superconductor T′-La1.8Eu0.2CuO4−yFy (0 ≤ y ≤ 0.125) having Nd2CuO4-type structure (so-called T′-structure). Pseudogap beh...

Superfluid weight and Berezinskii-Kosterlitz-Thouless transition temperature of strained graphene

We obtain the superfluid weight and Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for highly unconventional superconducting states with the coexistence of chiral d-wave superconductivity, charge density waves and pair density waves in the strained graphene. Our results show that the strain-induced flat bands can promote the superconducting transition temperature approximately 50% compared to that of the original doped graphene, which suggests that the flat-band superconductivity is a potential route to get superconductivity with higher critical temperatures. In particular, we obtain the superfluid weight for the pure superconducting pair-density-wave states from which the deduced superconducting transition temperature is shown to be much lower than the gap-opening temperature of the pair density wave, which is helpful to understand the phenomenon of the pseudogap state in high-Tc cuprate superconductors. Finally, we show that the BKT transition temperature versus doping for strained graphene exhibits a dome-like shape and it depends linearly on the spin-spin interaction strength.

Microscopic evidence for the intra-unit-cell electronic nematicity inside the pseudogap phase in YBa2Cu4O8

Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-Tc superconductors. Here, we report 17O nuclear magnetic resonance (NMR) studies on two planar oxygen sites in stoichiometric cuprate YBa2Cu4O8 to investigate the symmetry breaking inside the pseudogap phase. We observe that the Knight shifts of the two oxygen sites are identical at high temperatures but different below Tnem ∼ 185 K, which is close to the pseudogap temperature T*. Our result provides a microscopic evidence for intra-unit-cell electronic nematicity. The difference in quadrupole resonance frequency between the two oxygen sites is unchanged below Tnem, which suggests that the observed nematicity does not directly stem from the local charge density modulation. Furthermore, a short-range charge density wave (CDW) order is observed below T ≃ 150 K. The additional broadening in the 17O-NMR spectra because of this CDW order is determined to be inequivalent for the two oxygen sites, which is similar to that observed in case of nematicity. These results suggest a possible connection between nematicity, CDW order, and pseudogap.

Interplay between nematicity and Bardasis-Schrieffer modes in the short-time dynamics of unconventional superconductors

Motivated by the recent experiments suggesting the importance of nematicity in the phase diagrams of ironbased and cuprate high-Tc superconductors, we study the influence of nematicity on the collective modes inside the superconducting state in a non-equilibrium. In particular, we consider the signatures of collective modes in short-time dynamics of a system with competing nematic and s- and d-wave superconducting orders. In the rotationally symmetric state, we show that the Bardasis-Schrieffer mode, corresponding to the subdominant pairing, hybridizes with the nematic collective mode and merges into a single in-gap mode, with the mixing vanishing only close to the phase boundaries. For the d-wave ground state, we find that nematic interaction suppresses the damping of the collective oscillations in the short-time dynamics. Additionally, we find that even inside the nematic s+d-wave superconducting state, a Bardasis-Schrieffer-like mode leads to order parameter oscillations that strongly depend on the competition between the two pairing symmetries. We discuss the connection of our results to the recent pump-probe experiments on high-Tc superconductors.

Quantum Mechanical Calculations of High-T<sub>c</sub> Fe-Superconductors

In introduction we presented a short historical survey of the discovery of superconductivity (SC) up to the Fe-based materials that are not superconducting in a pure state. For this type of material, the transition to SC state occurs in presence of different dopants. Recently in the Fe-based materials at high pressures, the SC was obtained at room critical temperature. In this paper, we present the results of calculations of the isolated cluster representing infinitum crystal with Rh and Pd as dopants. All calculations are performed with the suite of programs Gaussian 16. The obtained results are compared with our previous results obtained for embedded cluster using Gaussian 09. In the case of embedded cluster our methodology of the Embedded Cluster Method at the MP2 electron correlation level was applied. In the NBO population analysis two main features are revealed: the independence of charge density transfer from the spin density transfer and, the presence of orbitals with electron density but without spin density. This is similar to the Anderson’s spinless holon and confirms our conclusions in previous publications that the possible mechanism for superconductivity can be the RVB mechanism proposed by Anderson for high Tc superconductivity in cuprates.

Spin-fluctuation-driven B1g and B2g Bond Order and Induced In-plane Anisotropy in Magnetic Susceptibility in Cuprate Superconductors: Impact of Hot-/Cold-spot Structure on Bond-order Symmetry

Electronic nematic states in cuprate high-Tc superconductors are not only ubiquitous but also very diverse. Recently, a B1g-symmetry nematic transition around the pseudogap temperature T* has been ...

Intermolecular Resonance Correlates Electron Pairs Down a Supermolecular Chain: Antiferromagnetism in K-Doped p-Terphenyl.

Recent interest in potassium-doped p-terphenyl has been fueled by reports of superconductivity at Tc values surprisingly high for organic compounds. Despite these interesting properties, studies of the structure-function relationships within these materials have been scarce. Here, we isolate a phase-pure crystal of potassium-doped p-terphenyl: [K(222)]2[p-terphenyl3]. Emerging antiferromagnetism in the anisotropic structure is studied in depth by magnetometry and electron spin resonance. Combining these experimental results with density functional theory calculations, we describe the antiferromagnetic coupling in this system that occurs in all 3 crystallographic directions. The strongest coupling was found along the ends of the terphenyls, where the additional electron on neighboring p-terphenyls antiferromagnetically couple. This delocalized bonding interaction is reminiscent of the doubly degenerate resonance structure depiction of polyacetylene. These findings hint toward magnetic fluctuation-induced superconductivity in potassium-doped p-terphenyl, which has a close analogy with high Tc cuprate superconductors. The new approach described here is very versatile as shown by the preparation of two additional salts through systematic changing of the building blocks.

How ‘pairons’ are revealed in the electronic specific heat of cuprates

Understanding the thermodynamic properties of high-Tc 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 Tc. 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 Tc. 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.