Searching for superconductivity in high entropy oxide Ruddlesden?Popper cuprate films
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Mazza, Alessandro R.↗
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Progress and prospects for cuprate high temperature superconductors under pressure
Since the discovery of high temperature superconductivity in La 5–x Ba x Cu 5 O 5(3–y) at an unprecedented T c of 35K over 35 years ago, high pressure experiments have played a critical role in understanding cuprate superconductivity. Soon after its discovery, compression experiments on La 5–x Ba x Cu 5 O 5(3–y) revealed a large dT c /dP, motivating the study of ‘chemical pre-compression’ in the material that led to the 90K Superconductor YBa 2 Cu 3 O 7–y . Cuprate superconductors discovered subsequently exhibited a range of Tc from 30K-140K, including the commonly studied Bi, Tl, and Hg based families. Here, pressure has large effects on superconductivity in these materials, including raising T c in the Hg based cuprates from 140K to 164K at 30 GPa. Reviewing past experiments indicate that pressure dopes holes into the CuO 2 planes common to all cuprates. Further detailed high-pressure studies of these materials should deepen our understanding of cuprate superconductivity and the possibility of reaching still higher T c in cuprates.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Thermal Hall conductivity of electron-doped cuprates
Measurements of the thermal Hall conductivity in hole-doped cuprates have shown that phonons acquire chirality in a magnetic field both in the pseudogap phase and in the Mott insulator state. The microscopic mechanism at play is still unclear. A number of theoretical proposals are being considered including skew scattering of phonons by various defects, the coupling of phonons to spins, and a state of loop-current order with the appropriate symmetries, but more experimental information is required to constrain theoretical scenarios. Here we present our study of the thermal Hall conductivity κ xy in the electron-doped cuprates Nd 2–x Ce x CuO 4 and Pr 2–x Ce x CuO 4 for dopings across the phase diagram, from x = 0 in the insulating antiferromagnetic phase up to x = 0.17 in the metallic phase above optimal doping. We observe a large negative thermal Hall conductivity at all dopings in both materials. Since heat conduction perpendicular to the CuO 2 planes is dominated by phonons, the large thermal Hall conductivity we observe in electron-doped cuprates for a heat current in that direction must also be due to phonons, as in hole-doped cuprates. However, the degree of chirality, measured as the ratio |κ xy /κ xx | where κ xx is the longitudinal thermal conductivity, is much larger in the electron-doped cuprates. Here, we discuss various factors that may be involved in the mechanism that confers chirality to phonons in cuprates, including short-range spin correlations.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Universal Stripe Symmetry of Short-Range Charge Density Waves in Cuprate Superconductors
The omnipresence of charge density waves (CDWs) across almost all cuprate families underpins a common organizing principle. However, a longstanding debate of whether its spatial symmetry is stripe or checkerboard remains unresolved. While CDWs in lanthanum- and yttrium-based cuprates possess a stripe symmetry, distinguishing these two scenarios is challenging for the short-range CDW in bismuth-based cuprates. Here, high-resolution resonant inelastic x-ray scattering is employed to uncover the spatial symmetry of the CDW in Bi 2 Sr 2-x La x CuO 6 + δ . Across a wide range of doping and temperature, anisotropic CDW peaks with elliptical shapes are found in reciprocal space. Based on Fourier transform analysis of real-space models, the results are interpreted as evidence of unidirectional charge stripes, hosted by mutually 90°-rotated anisotropic domains. This work paves the way for a unified symmetry and microscopic description of CDW order in cuprates.
36 MATERIALS SCIENCE↗
Doping dependence of the electron-phonon coupling in two families of bilayer superconducting cuprates
While electron-phonon coupling (EPC) is crucial for Cooper pairing in conventional superconductors, its role in high-Tc superconducting cuprates is debated. Here, using resonant inelastic x-ray scattering at the oxygen K edge, we study the EPC in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) and Nd 1+x Ba 2-x Cu 3 O 7-δ (NBCO) at different doping levels ranging from heavily underdoped (p=0.07) to overdoped (p=0.21). We analyze the data with a localized Lang-Firsov model that allows for the coherent excitations of two phonon modes. While electronic band dispersion effects are non-negligible, we are able to perform a study of the relative values of EPC matrix elements in these cuprate families. In the case of NBCO, the choice of the excitation energy allows us to disentangle modes related to the CuO chains and the CuO 2 planes. Combining the results from the two families, we find the EPC strength decreases with doping at q∥=(-0.25, 0) r.l.u., but has a nonmonotonic trend as a function of doping at smaller momenta. This behavior is attributed to the screening effect of charge carriers. We also find that the phonon intensity is enhanced in the vicinity of the charge-density-wave excitations while the extracted EPC strength appears to be less sensitive to their proximity. By performing a comparative study of two cuprate families, we are able to identify general trends in the EPC for the cuprates and provide experimental input to theories invoking a synergistic role for this interaction in d -wave pairing.
36 MATERIALS SCIENCE↗
Superfluid density in overdoped cuprates: Thin films versus bulk samples
Recent study of overdoped La 2–x Sr x CuO 4 cuprate superconductor thin films by Božovic et al. has revealed several unexpected findings, most notably the violation of the BCS description which was believed to adequately describe overdoped cuprates. In particular, it was found that the superfluid density in La 2–x Sr x CuO 4 films decreases on the overdoped side as a linear function of critical temperature T c , which was taken as evidence for the violation of Homes' law. Here we show explicitly that the law is indeed violated, and as the main reason for violation we find that the superfluid density in Božovic's films is suppressed more strongly than in bulk samples. Based on the existing literature data, we show that the superfluid density in bulk cuprate samples does not decrease with doping, but instead tends to saturate on the overdoped side. The result is also supported by our recent measurement of a heavily overdoped bulk La 2–x Sr x CuO 4 sample. Moreover, this saturation of superfluid density might not be limited to cuprates, as we find evidence for similar behavior in two pnictide superconductor families. We argue that quantum phase fluctuations play an important role in suppressing the superfluid density in thin films.
2-dimensional systems↗
Acoustic plasmons and conducting carriers in hole-doped cuprate superconductors
The layered crystal structures of cuprates enable collective charge excitations fundamentally different from those of three-dimensional metals. Acoustic plasmons have been observed in electron-doped cuprates by resonant inelastic X-ray scattering (RIXS); in contrast, whether acoustic plasmons exist in hole-doped cuprates is under debate, despite extensive measurements. This contrast led us to investigate the charge excitations of hole-doped cuprate La 2-x Sr x CuO 4 . Here we present incidentenergy-dependent RIXS measurements and calculations of collective charge response via the loss function to reconcile the aforementioned issues. Our results provide evidence for the acoustic plasmons of Zhang-Rice singlet (ZRS), which has a character of the Cu 3d x 2 -y 2 strongly hybridised with the O 2p orbitals; the metallic behaviour is implied to result from the movement of ZRS rather than the simple hopping of O 2p holes.
36 MATERIALS SCIENCE↗
Phonon-Mediated Long-Range Attractive Interaction in One-Dimensional Cuprates
Establishing a minimal microscopic model for cuprates is a key step towards the elucidation of a high-T c mechanism. By a quantitative comparison with a recent in situ angle-resolved photoemission spectroscopy measurement in doped 1D cuprate chains, our simulation identifies a crucial contribution from long-range electron-phonon coupling beyond standard Hubbard models. Using reasonable ranges of coupling strengths and phonon energies, we obtain a strong attractive interaction between neighboring electrons, whose strength is comparable to experimental observations. Additionally, nonlocal couplings play a significant role in the mediation of neighboring interactions. Considering the structural and chemical similarity between 1D and 2D cuprate materials, this minimal model with long-range electron-phonon coupling will provide important new insights on cuprate high-T c superconductivity and related quantum phases.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Beyond-Hubbard Pairing in a Cuprate Ladder
The Hubbard model is believed to capture the essential physics of cuprate superconductors. However, recent theoretical studies suggest that it fails to reproduce a robust and homogeneous superconducting ground state. Here, using resonant inelastic x-ray scattering and density matrix renormalization group calculations, we show that magnetic excitations in the prototypical cuprate ladder Sr 14 Cu 24 O 41 are inconsistent with those of a simple Hubbard model. The magnetic response of hole carriers, contributing to an emergent branch of spin-flip excitations, is strongly suppressed. This effect is the consequence of strong d -wavelike pairing, enhanced by nearly an order of magnitude through a large nearest-neighbor attractive interaction and persisting up to at least 260 K. The close connection between the physics of cuprate ladders and that of the two-dimensional compounds suggests that such an enhanced hole pairing may be a universal feature of superconducting cuprates. Published by the American Physical Society 2025
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Anomalously strong near-neighbor attraction in doped 1D cuprate chains
In the cuprates, one-dimensional (1D) chain compounds provide a distinctive opportunity to understand the microscopic physics, owing to the availability of reliable theories. However, progress has been limited by the challenge of controllably doping these materials. We report the synthesis and spectroscopic analysis of the 1D cuprate Ba 2-x Sr x CuO 3+δ over a wide range of hole doping. Our angle-resolved photoemission experiments reveal the doping evolution of the holon and spinon branches. We identify a prominent folding branch whose intensity fails to match predictions of the simple Hubbard model. An additional strong near-neighbor attraction, which may arise from coupling to phonons, quantitatively explains experiments for all accessible doping levels. Furthermore, considering structural and quantum chemistry similarities among cuprates, this attraction may play a similarly important role in high-temperature cuprate superconductors.
1D Chain↗
Gapped commensurate antiferromagnetic response in a strongly underdoped model cuprate superconductor
It is a distinct possibility that spin fluctuations are the pairing interactions in numerous unconventional superconductors. In the high-transition-temperature (high-T c ) cuprates, superconductivity emerges upon doping antiferromagnetic Mott insulators, and spin fluctuations might furthermore drive unusual pseudogap phenomena. Here we use magnetic neutron scattering to study the highly underdoped cuprate HgBa 2 CuO 4+δ (hole concentration p ≈ 0.064). In contrast to prior results for other underdoped cuprates, we find no evidence of incommensurate magnetic order associated with spin-density-wave or stripe correlations. Instead, the antiferromagnetic response in both the superconducting and pseudogap states is gapped below Δ AF ≈ 6 meV, commensurate over a wide energy range, and disperses above about 55 meV. Given the pristine nature of HgBa 2 CuO 4+δ , which exhibits high structural symmetry and minimal point disorder effects, this behavior likely signifies the unmasked response of the underlying CuO 2 planes near the Mott-insulating state. These results serve as a benchmark for a refined theoretical understanding of the cuprates.
Materials science↗
Particle-hole asymmetry in the dynamical spin and charge responses of corner-shared 1D cuprates
Although many experiments imply that oxygen orbitals play an essential role in the high-temperature superconducting cuprates, their precise role in collective spin and charge excitations and superconductivity is not yet fully understood. Here, we study the doping-dependent dynamical spin and charge structure factors of single and multi-orbital (pd) models for doped one-dimensional corner-shared spin-chain cuprates using several numerically exact methods. In doing so, we determine the orbital composition of the collective spin and charge excitations of cuprates, with important implications for our understanding of these materials. For example, we observe a particle-hole asymmetry in the orbital-resolved charge excitations, which is directly relevant to resonant inelastic x-ray scattering experiments and not captured by the single-band Hubbard model. Our results imply that one must explicitly include the oxygen degrees of freedom in order to fully understand some experimental observations on cuprate materials.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Superconductivity Reinforces Charge-Density-Wave Phase Coherence across Cuprates
For decades, superconductivity in high-𝑇 c cuprates has been viewed as a competitor that suppresses charge-density-wave (CDW) order by reducing its amplitude and spatial extent. Here, in this study, we show that this picture is incomplete, as superconductivity is accompanied by a systematic enhancement of CDW phase coherence across multiple cuprate families. Using resonant soft x-ray scattering combined with a coherence-sensitive momentum-profile analysis, we uncover a BCS-like growth of phase coherence below 𝑇 c , which phenomenologically manifests as the absence of CDW peak broadening and near-perfect wave-vector locking. This enhancement remains visible even in a disorder-dominated regime created by long-term crystal aging and follows a common trend when compared with published data on Bi-, Hg-, Y-, and Nd-based cuprates. These results indicate that superconductivity reshapes CDW order in two distinct ways, suppressing its amplitude while strengthening its phase coherence, and reveal an additional phase-level interplay with lattice coupling in high-𝑇 c cuprates.
Lee, H. [SLAC National Accelerator Laboratory (SLA↗
Nanoscale structural correlations in a model cuprate superconductor
Understanding the extent and role of inhomogeneity is a pivotal challenge in the physics of cuprate superconductors. While it is known that structural and electronic inhomogeneity is prevalent in the cuprates, it has proven difficult to disentangle compound-specific features from universally relevant effects. Here, in this study, we combine advanced neutron and x-ray diffuse scattering with numerical modeling to obtain insight into bulk structural correlations in HgBa 2 CuO 4+δ . This cuprate exhibits a high optimal transition temperature of nearly 100 K, pristine charge-transport behavior, and a simple average crystal structure without long-range structural instabilities, and is therefore uniquely suited for investigations of intrinsic inhomogeneity. We uncover diffuse reciprocal-space patterns that correspond to prominent nanoscale correlations of atomic displacements perpendicular to the CuO 2 planes. The real-space nature of the correlations is revealed through three-dimensional pair distribution function analysis and complementary numerical refinement. We find that relative displacements of ionic and CuO 2 layers play a crucial role, and that the structural inhomogeneity is not directly caused by the presence of conventional point defects. The observed correlations are therefore intrinsic to HgBa 2 CuO 4+δ , and thus likely important for the physics of cuprates more broadly. It is possible that the structural correlations are closely related to the unusual superconducting fluctuations and Mott-localization in these complex oxides. As advances in scattering techniques yield increasingly comprehensive data, the experimental and analysis tools developed here for large volumes of diffuse scattering data can be expected to aid future investigations of a wide range of materials.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Anomalous normal-state gap in an electron-doped cuprate
In the underdoped n-type cuprate Nd 2-x Ce x CuO 4 , long-range antiferromagnetic order reconstructs the Fermi surface, resulting in a putative antiferromagnetic metal with small Fermi pockets. Using angle-resolved photoemission spectroscopy, here we observe an anomalous energy gap, an order of magnitude smaller than the antiferromagnetic gap, in a wide portion of the underdoped regime and smoothly connecting to the superconducting gap at optimal doping. After considering all the known ordering tendencies in tandem with the phase diagram, we hypothesize that the normal-state gap in the underdoped n-type cuprates originates from Cooper pairing. In conclusion, the high temperature scale of the normal-state gap raises the prospect of engineering higher transition temperatures in the n-type cuprates comparable to those of the p-type cuprates.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Generic character of charge and spin density waves in superconducting cuprates
Charge density waves (CDWs) have been observed in nearly all families of copper-oxide superconductors. But the behavior of these phases across different families has been perplexing. In La-based cuprates, the CDW wavevector is an increasing function of doping, exhibiting the so-called Yamada behavior, while in Y- and Bi-based materials the behavior is the opposite. Here, we report a combined resonant soft X-ray scattering (RSXS) and neutron scattering study of charge and spin density waves in isotopically enriched La 1.8–x Eu 0.2 Sr x CuO 4 over a range of doping 0.07 ≤ x ≤ 0.20. In this work, we find that the CDW amplitude is temperature independent and develops well above experimentally accessible temperatures. Further, the CDW wavevector shows a nonmonotonic temperature dependence, exhibiting Yamada behavior at low temperature with a sudden change occurring near the spin ordering temperature. We describe these observations using a Landau–Ginzburg theory for an incommensurate CDW in a metallic system with a finite charge compressibility and spin-CDW coupling. Extrapolating to high temperature, where the CDW amplitude is small and spin order is absent, our analysis predicts a decreasing wavevector with doping, similar to Y and Bi cuprates. Our study suggests that CDW order in all families of cuprates forms by a common mechanism.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Pseudogap in electron-doped cuprates: Strong correlation leading to band splitting
The pseudogap phenomena have been a long-standing mystery of the cuprate high-temperature superconductors. The pseudogap in the electron-doped cuprates has been attributed to band folding due to antiferromagnetic (AFM) long-range order or short-range correlation. We performed an angle-resolved photoemission spectroscopy study of the electron-doped cuprates Pr 1.3−x La 0.7 Ce x CuO 4 showing spin-glass, disordered AFM behaviors, and superconductivity at low temperatures and, by measurements with fine momentum cuts, found that the gap opens on the unfolded Fermi surface rather than the AFM Brillouin zone boundary. The gap did not show a node, following the full symmetry of the Brillouin zone, and its magnitude decreased from the zone-diagonal to (π,0) directions, opposite to the hole-doped case. These observations were reproduced by cluster dynamical-mean-field-theory calculation, which took into account electron correlation precisely within a (CuO 2 ) 4 cluster. In conclusion, the present experimental and theoretical results are consistent with the mechanism that electron or hole doping into a Mott insulator creates an in-gap band that is separated from the upper or lower Hubbard band by the pseudogap.
74 ATOMIC AND MOLECULAR PHYSICS↗
Suppressed superexchange interactions in the cuprates by bond-stretching oxygen phonons
We study a multiorbital Hubbard–Su-Schrieffer-Heeger model for the one-dimensional (1D) corner-shared cuprates in the adiabatic and nonadiabatic limits using exact diagonalization and determinant quantum Monte Carlo. At half filling and in the adiabatic limit, lattice dimerization can be achieved only over a narrow range of couplings slightly below a critical coupling g c . Beyond this value, the sign of the effective hopping changes, and the lattice becomes unstable. Strong lattice fluctuations replace the dimerization state in the nonadiabatic case. We also examine the model's temperature-dependent uniform and dynamical magnetic susceptibilities and compare them to the results of an effective spin-1/2 Heisenberg model. In doing so, we demonstrate that lattice fluctuations induced by the e-ph interaction suppress the effective superexchange interaction when g < g c . Furthermore, our results elucidate the effect of bond-stretching phonons in the parent cuprate compounds in general and are particularly relevant to 1D cuprates, where strong e-ph interactions have recently been inferred.
36 MATERIALS SCIENCE↗