high-Tc Cuprates Research Articles

Two distinctive temperature dependences of the London penetration depth in high-Tc cuprate superconductors as support for the theory of Bose-liquid superconductivity

In the framework of the theory of Bose-liquid superconductivity, we show that in high-Tc cuprate superconductors the magnetic field penetration depth (known as the London penetration depth λL(T)) decreases first exponentially below the superconducting transition temperature Tc with decreasing the temperature T and then λL(T) decreases like a certain power of T at lower temperatures due to the vanishing of the energy gap Δg(T) in the excitation spectrum of a superfluid Bose-liquid at a characteristic temperature Tc⁎ lower than Tc. Below Tc, the London penetration depth λL(T) has an exponential temperature dependence down to Tc⁎, which is different from a BCS- like exponential temperature dependence, and then λL(T) will follow a power law temperature dependence down to T=0, which is also different from power-law temperature dependences predicted by some BCS-like (d-wave) pairing theories. In unconventional (bosonic) cuprate superconductors characterized by the strong interboson coupling constant (γB≳1), λL(T) exhibits the distinctly different temperature dependences in different temperature intervals 0≤T≤Tc⁎<<Tc and Tc⁎<T<Tc when the energy gap Δg(T) vanishes at a temperature Tc⁎ well below Tc. In the weak interboson coupling (γB<<1) the energy gap Δg(T) vanishes at a temperature Tc⁎ close to Tc and the reduced London penetration depth λL(T)/λL(0) has an unusual Tn power-law temperature dependence in a wide temperature range 0<T<Tc⁎ below Tc, i.e. the expression λL(T)/λL(0)≃[1−aL(T)(T/Tc)n]−1/2 (where n≳4 and aL(T)≲1) is valid only below Tc⁎ and is similar to the well-known empirical Gorter-Casimir law. Our theoretical predictions of the distinctive power-law and exponential temperature dependences of λL(T)/λL(0) are in good agreement with the experimental results reported for GdBa2Cu3O7−δ ceramics and YBa2Cu3O7−δ films. The peculiar temperature dependences of λL(T)/λL(0) observed in these high-Tc materials at low and high temperatures do not follow the predictions of the s- and d-wave BCS-like theories of Fermi-liquid superconductivity and give evidence for the predictions of the theory of Bose-liquid superconductivity.

Hot spots along the Fermi contour of high-Tc cuprates analyzed by s-d exchange interaction

We perform a thorough theoretical study of the electron properties of a generic CuO_2 plane in the framework of Shubin–Kondo–Zener s-d exchange interaction that simultaneously describes the correlation between Tc and the Cu4s energy. To this end, we apply the Pokrovsky theory (J Exp Theor Phys 13:447–450, 1961) for anisotropic gap BCS superconductors. It takes into account the thermodynamic fluctuations of the electric field in the dielectric direction perpendicular to the conducting layers. We microscopically derive a multiplicatively separable kernel able to describe the scattering rate in the momentum space, as well as the superconducting gap anisotropy within the BCS theory. These findings may be traced back to the fact that both the Fermi liquid and the BCS reductions lead to one and the same reduced Hamiltonian involving a separable interaction, such that a strong electron scattering corresponds to a strong superconducting gap and vice versa. Moreover, the superconducting gap and the scattering rate vanish simultaneously along the diagonals of the Brillouin zone. We would like to stress that our theoretical study reproduces the phenomenological analysis of other authors aiming at describing Angle Resolved Photoemission Spectroscopy measurements. Within standard approximations one and the same s-d exchange Hamiltonian describes gap anisotropy of the superconducting phase and the anisotropy of scattering rate of charge carriers in the normal phase.

Pressure-induced monotonic enhancement of Tc to over 30 K in superconducting Pr0.82Sr0.18NiO2 thin films

The successful synthesis of superconducting infinite-layer nickelate thin films with the highest Tc ≈ 15 K has ignited great enthusiasm for this material class as potential analogs of the high-Tc cuprates. Pursuing a higher Tc is always an imperative task in studying a new superconducting material system. Here we report high-quality Pr0.82Sr0.18NiO2 thin films with Tconset ≈ 17 K synthesized by carefully tuning the amount of CaH2 in the topotactic chemical reduction and the effect of pressure on its superconducting properties by measuring electrical resistivity under various pressures in a cubic anvil cell apparatus. We find that the onset temperature of the superconductivity, Tconset, can be enhanced monotonically from ~17 K at ambient pressure to ~31 K at 12.1 GPa without showing signatures of saturation upon increasing pressure. This encouraging result indicates that the Tc of infinite-layer nickelates superconductors still has room to go higher and it can be further boosted by applying higher pressures or strain engineering in the heterostructure films.

Resonant Inelastic X-Ray Scattering Study of Electron-Exciton Coupling in High- Tc Cuprates

Explaining the mechanism of superconductivity in the high-Tc cuprates requires an understanding of what causes electrons to form Cooper pairs. Pairing can be mediated by phonons, the screened Coulomb force, spin or charge fluctuations, excitons, or by a combination of these. An excitonic pairing mechanism has been postulated, but experimental evidence for coupling between conduction electrons and excitons in the cuprates is sporadic. Here we use resonant inelastic x-ray scattering to monitor the temperature dependence of the d_d exciton spectrum of Bi2Sr2CaCu2O8−x crystals with different charge carrier concentrations. We observe a significant change of the d_d exciton spectra when the materials pass from the normal state into the superconductor state. Our observations show that the d_d excitons start to shift up (down) in the overdoped (underdoped) sample when the material enters the superconducting phase. We attribute the superconductivity-induced effect and its sign reversal from underdoped to overdoped to the exchange coupling of the site of the d_d exciton to the surrounding copper spins.3 MoreReceived 7 January 2022Revised 24 May 2022Accepted 7 June 2022Corrected 30 December 2022DOI:https://doi.org/10.1103/PhysRevX.12.021068Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasExcitonsPairing mechanismsSuperconductivityTechniquesResonant inelastic x-ray scatteringCondensed Matter, Materials & Applied Physics

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.

Three studies in high-Tc cuprates

We investigate three fundamental issues in the physics of high-Tc cuprates, from the perspective of a recently proposed comprehensive theory for these materials. (a) Orbital ordering × superconductivity. The first issue is the detailed microscopic mechanism that produces an attractive interaction between holes in high-Tc cuprates. (b) Dispersion relation × pseudogap order parameter. The second issue refers to the existence of a pseudogap order parameter, which would be different from zero all over the pseudogap phase and would vanish elsewhere. (c) Chemical potential × quantum critical point location. The third issue concerns the debate as to whether the quantum critical point, located where the pseudogap transition line T*(x) meets the T = 0 axis is inside the superconducting dome or at its boundary. We obtain clearcut solutions for the three issues.

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.

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∗.

Infinite-Layer Nickelate Superconductors: A Current Experimental Perspective of the Crystal and Electronic Structures

After the reward of more than 2 decades of pursuit on the high-Tc cuprate analog with the hope to obtain a better understanding of the mechanism of high-Tc superconductivity, the discovery of superconductivity in the infinite-layer nickelate brings more mystery to the picture than expected. Tops in the list of questions are perhaps 1) absence of superconductivity in the bulk nickelate and limited thickness of the infinite-layer phase in thin film, 2) absence of superconductivity in the La-nickelate despite it being the earliest studied rare-earth nickelate, and the role of 4 f orbital in the recipe of superconductivity, 3) absence of Meissner effect and suspect of the origin of superconductivity from the interface, 4) whether nickelate hosts similar pairing symmetry to the single-band high-Tc cuprates or multiband iron-based superconductor. In this perspective article, we will discuss the following aspects: 1) stabilization of the infinite-layer phase on the SrTiO3(001) substrate and the thickness dependency of observables; 2) rare-earth dependence of the superconducting dome and phase diagram of the (La/Pr/Nd)- infinite-layer nickelate thin film; 3) experimental aspects of the measurement of Meissner effect; 4) theoretical framework and experimental study of the pairing symmetry of infinite-layer nickelate superconductor.

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 .

Characterization of photoinduced normal state through charge density wave in superconducting YBa 2 Cu 3 O 6.67

The normal state of high-Tc cuprates has been considered one of the essential topics in high-temperature superconductivity research. However, compared to the high magnetic field study of it, understanding a photoinduced normal state remains elusive. Here, we explore a photoinduced normal state of YBa2Cu3O6.67 through a charge density wave (CDW) with time-resolved resonant soft x-ray scattering, as well as a high magnetic field x-ray scattering. In the nonequilibrium state where people predict a quenched superconducting state based on the previous optical spectroscopies, we experimentally observed a similar analogy to the competition between superconductivity and CDW shown in the equilibrium state. We further observe that the broken pairing states in the superconducting CuO2 plane via the optical pump lead to nucleation of three-dimensional CDW precursor correlation. Ultimately, these findings provide a critical clue that the characteristics of the photoinduced normal state show a solid resemblance to those under magnetic fields in equilibrium conditions.

Thin-Film Aspects of Superconducting Nickelates

The discovery of superconductivity in infinite-layer nickelates has attracted much attention due to their association to the high-Tc cuprates. Cuprate superconductivity was first demonstrated in bulk samples and subsequently in thin films. In the nickelates, however, the situation has been reversed: although surging as a bulk phenomenon, nickelate superconductivity has only been reported in thin films so far. At the same time, the specifics of infinite-layer nickelates yield distinct interface and surface effects that determine their bulk vs thin-film behavior. In this paper, we provide an overview on these important aspects.

Unconventional spectral signature of Tc in a pure d-wave superconductor.

In conventional superconductors, the phase transition into a zero-resistance and perfectly diamagnetic state is accompanied by a jump in the specific heat and the opening of a spectral gap1. In the high-transition-temperature (high-Tc) cuprates, although the transport, magnetic and thermodynamic signatures of Tc have been known since the 1980s2, the spectroscopic singularity associated with the transition remains unknown. Here we resolve this long-standing puzzle with a high-precision angle-resolved photoemission spectroscopy (ARPES) study on overdoped (Bi,Pb)2Sr2CaCu2O8+δ (Bi2212). We first probe the momentum-resolved electronic specific heat via spectroscopy and reproduce the specific heat peak at Tc, completing the missing link for a holistic description of superconductivity. Then, by studying the full momentum, energy and temperature evolution of the spectra, we reveal that this thermodynamic anomaly arises from the singular growth of in-gap spectral intensity across Tc. Furthermore, we observe that the temperature evolution of in-gap intensity is highly anisotropic in the momentum space, and the gap itself obeys both the d-wave functional form and particle-hole symmetry. These findings support the scenario that the superconducting transition is driven by phase fluctuations. They also serve as an anchor point for understanding the Fermi arc and pseudogap phenomena in underdoped cuprates.

Visualizing the unusual spectral weight transfer in DyBa2Cu3O7–δ thin film

We report a Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) study of a DyBa2Cu3O7-δ (DBCO) thin film (Tc ~ 79 K) synthesized by the molecular beam epitaxy (MBE). We observed an unusual transfer of spectral weight in the local density of states (LDOS) spectra occurring only within the superconducting gap. By a systematic control of the tip-sample distance and the junction resistance, we demonstrate that the spectral weight transfer can be switched at a nano-meter length scale. These results suggest that an interaction between the STM tip and the sample alters the electronic configurations in the film. This probably originates from a combination of an intrinsic band bending at the interface between the surface and the bulk, and a tip-induced band bending. These results may open a new avenue for band engineering and applications of thin films of high-Tc cuprates.

Coexistence of antiferromagnetism, ferromagnetism and superconductivity in Iron based superconductors

Although the coexistence of Superconductivity (SC) and magnetism in one compound is rare as both phenomena are antagonistic to each other but many examples of such materials like high Tc cuprates, heavy fermion materials and Uranium based compounds are also known to exist. The study of this interplay of SC and magnetism might be very important as high Tc cuprates are antiferromagnetic insulators in its pristine form and upon doping, theirs transition temperature (Tc) could enhance up to 183 K in Hg based cuprates at high pressure. Iron based Superconductors (IBSCs) are also showing magnetic ground state and doping with rare earth ions change its state and it starts superconducting. In this paper, I have theoretically studied the Hamiltonian of Iron based superconductors especially Eu1-xLaxFeAs2 having superconducting, antiferromagnetic and ferromagnetic interactions. I have also tried to find the exact mechanism responsible for Cooper pairing in these compounds by finding superconducting order parameter (Δ), ferromagnetic (∅↑↑) and antiferromagnetic (∅↑↓) order parameters. In the pretext of this finding, we would able to find many Fe based materials which could super conduct even at room temperature.

Pseudogap state and unusual metallic conductivity in high-Tc cuprate superconductors

The BCS-like pairing of polaronic carriers in underdoped and optimally doped high-Tc cuprates above the superconducting transition temperature Tc is considered. Such a BCS-like pairing correlation leads to the formation of bosonic Cooper pairs and the appearance of a pseudogap on the Fermi surface at a characteristic temperature T* &amp;gt; Tc. It is shown that the calculated doping dependence of the BCS-like pseudogap agrees quite well with the experimentally observed doping dependence of the pseudogap in La2–xSrxCuO4 (where x is the doping level). The mechanisms of the unusual metallic transports of different types of charge carriers above and below T* along the CuO2 layers (i.e., ab planes) in high-Tc cuprates are identified. The conductivity of the three types of charge carriers (large polarons, excited Fermi components of bosonic Cooper pairs, and bosonic Cooper pairs themselves) at their scattering by lattice vibrations is considered. It is established that the linear temperature dependence of the in-plane resistivity ρab (T) observed, as a rule, in underdoped and optimally doped cuprates above T* is associated with the scattering of polaronic carriers by acoustic and optical phonons. Theoretical results compared with the experimental data allow for confirming that the different (upward and downward) deviations from the linearity in ρab (T) below T* and the most interesting resistive transitions [i.e., a clear jump and a sharp drop in ρab (T)] at T = T* are caused by the pseudogap effect on the conductivity of the excited Fermi components of bosonic Cooper pairs and by the excessive conductivity of bosonic Cooper pairs in high-Tc cuprates above Tc.

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.

Band-selective gap opening by a C4-symmetric order in a proximity-coupled heterostructure Sr2VO3FeAs

Complex electronic phases in strongly correlated electron systems are manifested by broken symmetries in the low-energy electronic states. Some mysterious phases, however, exhibit intriguing energy gap opening without an apparent signature of symmetry breaking (e.g., high-TC cuprates and heavy fermion superconductors). Here, we report an unconventional gap opening in a heterostructured, iron-based superconductor Sr2VO3FeAs across a phase transition at T 0 ∼150 K. Using angle-resolved photoemission spectroscopy, we identify that a fully isotropic gap opens selectively on one of the Fermi surfaces with finite warping along the interlayer direction. This band selectivity is incompatible with conventional gap opening mechanisms associated with symmetry breaking. These findings, together with the unusual field-dependent magnetoresistance, suggest that the Kondo-type proximity coupling of itinerant Fe electrons to localized V spin plays a role in stabilizing the exotic phase, which may serve as a distinct precursor state for unconventional superconductivity.

Ubiquitous Charge Order Correlations in High-Temperature Superconducting Cuprates

The presence of charge order in high-transition-temperature copper oxides (high-Tc cuprates) was identified a decade ago. Now it is a universally observed order like the antiferromagnetic and the superconducting orders of the cuprates. The charge order shows up in various forms depending on materials, and it overlaps other orders in the phase diagram. Because of this diversity and complexity it has been far from clear whether or not the charge order has a direct relevance to superconductivity and to the mysterious pseudogap phenomenon. However, the research development in the last few years has been successful in revealing universal aspects of the charge order and its fluctuations. It turns out that the charge order is compatible with superconductivity, and that its fluctuations have high onset temperature and high energy scale comparable to those of the pseudogap. The charge order correlations might be indispensable for creating the pseudogap and possibly generating high-Tc superconductivity by collaborating with spin and pairing correlations.