Band-structure trend in hole-doped cuprates and correlation with T(c max)

Variational Monte Carlo study of stripes as a function of doping in thet-t'Hubbard model

We perform variational Monte Carlo simulations of the single-band Hubbard model on the square lattice with both nearest (t) and next-nearest (t') neighbor hoppings. Our work investigates the consequences of increasing hole doping on the instauration of stripes and the behavior of the superconducting order parameter, with a discussion on how the two phenomena affect each other. We consider two different values of the next-nearest neighbor hopping parameter, that are appropriate for describing cuprate superconductors. We observe that stripes are the optimal state in a wide doping range; the stripe wavelength reduces at increasing doping, until stripes melt into a uniform state for large values of doping. Superconducting pair-pair correlations, indicating the presence of superconductivity, are always suppressed in the presence of stripes. Our results suggest that the phase diagram for the single-band Hubbard model is dominated by stripes, with superconductivity being possible only in a narrow doping range between striped states and a nonsuperconducting metal.

Superconductivity due to fluctuating loop currents

Orbital magnetism and the loop currents (LCs) that accompany it have been proposed to emerge in many systems, including cuprates, iridates, and kagome superconductors. In the case of cuprates, LCs have been put forward as the driving force behind the pseudogap, strange-metal behavior, and dx2-y2-wave superconductivity. Here, we investigate whether fluctuating intra-unit-cell LCs can cause unconventional superconductivity. For odd-parity LCs, we find that they are repulsive in all pairing channels near the underlying quantum-critical point (QCP). For even-parity LCs, their fluctuations give rise to unconventional pairing, which is not amplified in the vicinity of the QCP, in sharp contrast to pairing mediated by spin-magnetic, nematic, or ferroelectric fluctuations. Applying our formalism to the cuprates, we conclude that fluctuating intra-unit-cell LCs are unlikely to yield dx2-y2-wave superconductivity. If LCs are to be relevant for the cuprates, they must break translation symmetry.

Collective Nature of Orbital Excitations in Layered Cuprates in the Absence of Apical Oxygens

We have investigated the 3d orbital excitations in CaCuO_{2} (CCO), Nd_{2}CuO_{4} (NCO), and La_{2}CuO_{4} (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the d_{xy} orbital clearly disperses, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of the orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. Finally, we show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy.

Growth and Electronic Structure of Copper Oxide Monolayer Epitaxial Films

Copper-based high-temperature superconductors share a common feature in their crystal structure, which is the presence of a CuO2 plane, where superconductivity takes place. Therefore, important questions arise as to whether superconductivity can exist in a single layer of the CuO2 plane and, if so, how such superconductivity in a single CuO2 plane differs from that in a bulk cuprate system. To answer these questions, studies of the superconductivity in cuprate monolayers are necessary. In this study, we constructed a heterostructure system with a La2-xSrxCuO4 (LSCO) monolayer containing a single CuO2 plane and measured the resulting electronic structures. Monolayer LSCO has metallic and bulk-like electronic structures. The hole doping ratio of the monolayer LSCO is found to depend on the underlying buffer layer due to the interface effect. Our work will provide a platform for research into ideal two-dimensional cuprate systems.

Transport properties and doping evolution of the Fermi surface in cuprates

Measured transport properties of three representative cuprates are reproduced within the paradigm of two electron subsystems, itinerant and localized. The localized subsystem evolves continuously from the Cu 3d[Formula: see text] hole at half-filling and corresponds to the (pseudo)gapped parts of the Fermi surface. The itinerant subsystem is observed as a pure Fermi liquid (FL) with material-independent universal mobility across the doping/temperature phase diagram. The localized subsystem affects the itinerant one in our transport calculations solely by truncating the textbook FL integrals to the observed (doping- and temperature-dependent) Fermi arcs. With this extremely simple picture, we obtain the measured evolution of the resistivity and Hall coefficients in all three cases considered, including LSCO which undergoes a Lifshitz transition in the relevant doping range, a complication which turns out to be superficial. Our results imply that prior to evoking polaronic, quantum critical point, quantum dissipation, or even more exotic scenarios for the evolution of transport properties in cuprates, Fermi-surface properties must be addressed in realistic detail.

Frustration- and doping-induced magnetism in a Fermi-Hubbard simulator

Geometrical frustration in strongly correlated systems can give rise to a plethora of novel ordered states and intriguing magnetic phases, such as quantum spin liquids1-3. Promising candidate materials for such phases4-6 can be described by the Hubbard model on an anisotropic triangular lattice, a paradigmatic model capturing the interplay between strong correlations and magnetic frustration7-11. However, the fate of frustrated magnetism in the presence of itinerant dopants remains unclear, as well as its connection to the doped phases of the square Hubbard model12. Here we investigate the local spin order of a Hubbard model with controllable frustration and doping, using ultracold fermions in anisotropic optical lattices continuously tunable from a square to a triangular geometry. At half-filling and strong interactions U/t ≈ 9, we observe at the single-site level how frustration reduces the range of magnetic correlations and drives a transition from a collinear Néel antiferromagnet to a short-range correlated 120° spiral phase. Away from half-filling, the triangular limit shows enhanced antiferromagnetic correlations on the hole-doped side and a reversal to ferromagnetic correlations at particle dopings above 20%, hinting at the role of kinetic magnetism in frustrated systems. This work paves the way towards exploring possible chiral ordered or superconducting phases in triangular lattices8,13 and realizing t-t' square lattice Hubbard models that may be essential to describe superconductivity in cuprate materials14.

Correlating the charge-transfer gap to the maximum transition temperature in Bi2Sr2Can-1CunO2n+4+δ

As the number of CuO₂ layers, n, in each unit cell of a cuprate family increases, the maximum transition temperature (T_c,max) exhibits a universal bell-shaped curve with a peak at n = 3. The microscopic mechanism of this trend remains elusive. In this study, we used advanced electron microscopy to image the atomic structure of cuprates in the Bi₂Sr₂Ca_n-1Cu_nO_2n+4+δ family with 1 ≤ n ≤ 9; the evolution of the charge-transfer gap size (Δ) with n can be measured simultaneously. We determined that the n dependence of Δ follows an inverted bell-shaped curve with the minimum Δ value at n = 3. The correlation between Δ, n, and T_c,max may clarify the origin of superconductivity in cuprates.

Reconciling scaling of the optical conductivity of cuprate superconductors with Planckian resistivity and specific heat

Materials tuned to a quantum critical point display universal scaling properties as a function of temperature T and frequency ω. A long-standing puzzle regarding cuprate superconductors has been the observed power-law dependence of optical conductivity with an exponent smaller than one, in contrast to T-linear dependence of the resistivity and ω-linear dependence of the optical scattering rate. Here, we present and analyze resistivity and optical conductivity of La_2-xSr_xCuO₄ with x = 0.24. We demonstrate ℏω/k_BT scaling of the optical data over a wide range of frequency and temperature, T-linear resistivity, and optical effective mass proportional to [Formula: see text] corroborating previous specific heat experiments. We show that a T, ω-linear scaling Ansatz for the inelastic scattering rate leads to a unified theoretical description of the experimental data, including the power-law of the optical conductivity. This theoretical framework provides new opportunities for describing the unique properties of quantum critical matter.

Optimizing Superconductivity: From Cuprates via Nickelates to Palladates

Motivated by cuprate and nickelate superconductors, we perform a comprehensive study of the superconducting instability in the single-band Hubbard model. We calculate the spectrum and superconducting transition temperature T_{c} as a function of filling and Coulomb interaction for a range of hopping parameters, using the dynamical vertex approximation. We find the sweet spot for high T_{c} to be at intermediate coupling, moderate Fermi surface warping, and low hole doping. Combining these results with first principles calculations, neither nickelates nor cuprates are close to this optimum within the single-band description. Instead, we identify some palladates, notably RbSr_{2}PdO_{3} and A_{2}^{'}PdO_{2}Cl_{2} (A^{'}=Ba_{0.5}La_{0.5}), to be virtually optimal, while others, such as NdPdO_{2}, are too weakly correlated.

Exploring the 4d1analogue of cuprates: theoretical studies on bulk NbF4and NbF4monolayer stabilized on MgO (001) plane

The recent research in infinite-layer nickelates has inspired new effort in finding the cuprate analogs. Here we propose that NbF₄, which contains niobium-centered fluorine octahedra, is a promising 4d¹analogue of cuprates. Using the density functional theory, we first show that bulk NbF₄is in close proximity tod¹configuration, with Nb4dxyorbital nearly half-filled. A single band with dominating4dxycharacter crosses the Fermi level, forming a square-like Fermi surface. The intralayer G-type antiferromagnetic (AFM) order is energetically favored and the Coulomb interaction drives the system into an AFM insulator. Next we demonstrate that the NbF₄layer can be stabilized on MgO substrate with main electronic and magnetic features retained, offering an alternative route to realize the NbF₄-related high-T_csuperconductors. Furthermore, we derive effective single orbital models for both systems and investigate the electron correlation effects via functional renormalization group. We find that the G-type AFM dominates near half-filling butdx2-y2-wave superconductivity (SC) prevails upon suitable hole/electron doping. Based on the striking similarities between NbF₄and cuprates, we suggest that NbF₄-related compounds may be exotic candidates for searching new high-T_csuperconductors.

A Review on Strain Study of Cuprate Superconductors

Cuprate superconductors have attracted extensive attention due to their broad promising application prospects. Among the factors affecting superconductivity, the effect of strain cannot be ignored, which can significantly enhance or degrade superconductivity. In this review, we discuss and summarize the methods of applying strain to cuprate superconductors, strain measurement techniques, and the influence of strain on superconductivity. Among them, we pay special attention to the study of strain in high-temperature superconducting (HTS) films and coating. We expect this review can guide further research in the field of cuprate superconductors.

On the electron pairing mechanism of copper-oxide high temperature superconductivity

The elementary CuO₂ plane sustaining cuprate high-temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO₅ pyramids. Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate t/ℏ and across the charge-transfer energy gap [Formula: see text], generate "superexchange" spin-spin interactions of energy [Formula: see text] in an antiferromagnetic correlated-insulator state. However, hole doping this CuO₂ plane converts this into a very-high-temperature superconducting state whose electron pairing is exceptional. A leading proposal for the mechanism of this intense electron pairing is that, while hole doping destroys magnetic order, it preserves pair-forming superexchange interactions governed by the charge-transfer energy scale [Formula: see text]. To explore this hypothesis directly at atomic scale, we combine single-electron and electron-pair (Josephson) scanning tunneling microscopy to visualize the interplay of [Formula: see text] and the electron-pair density n_P in Bi₂Sr₂CaCu₂O_8+x. The responses of both [Formula: see text] and n_P to alterations in the distance δ between planar Cu and apical O atoms are then determined. These data reveal the empirical crux of strongly correlated superconductivity in CuO₂, the response of the electron-pair condensate to varying the charge-transfer energy. Concurrence of predictions from strong-correlation theory for hole-doped charge-transfer insulators with these observations indicates that charge-transfer superexchange is the electron-pairing mechanism of superconductive Bi₂Sr₂CaCu₂O_8+x.

Systematic electronic structure in the cuprate parent state from quantum many-body simulations

The quantitative description of correlated electron materials remains a modern computational challenge. We demonstrate a numerical strategy to simulate correlated materials at the fully ab initio level beyond the solution of effective low-energy models and apply it to gain a detailed microscopic understanding across a family of cuprate superconducting materials in their parent undoped states. We uncover microscopic trends in the electron correlations and reveal the link between the material composition and magnetic energy scales through a many-body picture of excitation processes involving the buffer layers. Our work illustrates a path toward a quantitative and reliable understanding of more complex states of correlated materials at the ab initio many-body level.

Differentiated roles of Lifshitz transition on thermodynamics and superconductivity in La2-xSrxCuO4

The effect of Lifshitz transition on thermodynamics and superconductivity in hole-doped cuprates has been heavily debated but remains an open question. In particular, an observed peak of electronic specific heat is proposed to originate from fluctuations of a putative quantum critical point p* (e.g., the termination of pseudogap at zero temperature), which is close to but distinguishable from the Lifshitz transition in overdoped La-based cuprates where the Fermi surface transforms from hole-like to electron-like. Here we report an in situ angle-resolved photoemission spectroscopy study of three-dimensional Fermi surfaces in La_2-xSr_xCuO₄ thin films (x = 0.06 to 0.35). With accurate k_z dispersion quantification, the said Lifshitz transition is determined to happen within a finite range around x = 0.21. Normal state electronic specific heat, calculated from spectroscopy-derived band parameters, reveals a doping-dependent profile with a maximum at x = 0.21 that agrees with previous thermodynamic microcalorimetry measurements. The account of the specific heat maximum by underlying band structures excludes the need for additionally dominant contribution from the quantum fluctuations at p*. A d-wave superconducting gap smoothly across the Lifshitz transition demonstrates the insensitivity of superconductivity to the dramatic density of states enhancement.

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$$_2$$ 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.

Gapped Collective Charge Excitations and Interlayer Hopping in Cuprate Superconductors

We use resonant inelastic x-ray scattering to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr_{0.9}La_{0.1}CuO_{2}. We detect a plasmon gap of ∼120  meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in Sr_{0.9}La_{0.1}CuO_{2} are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered t-J-V model calculations. A similar analysis performed on recent resonant inelastic x-ray scattering data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping t_{z}. Our work signifies the three dimensionality of the charge dynamics in layered cuprates and provides a new method to determine t_{z}.

Paramagnons and high-temperature superconductivity in a model family of cuprates

Cuprate superconductors have the highest critical temperatures (T_c) at ambient pressure, yet a consensus on the superconducting mechanism remains to be established. Finding an empirical parameter that limits the highest reachable T_c can provide crucial insight into this outstanding problem. Here, in the first two Ruddlesden-Popper members of the model Hg-family of cuprates, which are chemically nearly identical and have the highest T_c among all cuprate families, we use inelastic photon scattering to reveal that the energy of magnetic fluctuations may play such a role. In particular, we observe the single-paramagnon spectra to be nearly identical between the two compounds, apart from an energy scale difference of ~30% which matches their difference in T_c. The empirical correlation between paramagnon energy and maximal T_c is further found to extend to other cuprate families with relatively high T_c’s, hinting at a fundamental connection between them.

Finding a parameter that limits the critical temperature of cuprate superconductors can provide crucial insight on the superconducting mechanism. Here, the authors use inelastic photon scattering on two Ruddlesden-Popper members of the model Hg-family of cuprates to reveal that the energy of magnetic fluctuations may play such a role, and suggest that the Cooper pairing is mediated by paramagnons.

Non-Fermi liquid phase and linear-in-temperature scattering rate in overdoped two-dimensional Hubbard model

SignificanceMost metals display an electron-scattering rate [Formula: see text] that follows [Formula: see text] at low temperatures, as prescribed by Fermi liquid theory. But there are important exceptions. One of the most prominent examples is the "strange" metal regime in overdoped cuprate supercondcutors, which exhibits a linear T dependence of the scattering rate [Formula: see text] that reaches a putative Planckian limit. Here, using cutting-edge computational approaches, we show that T-linear scattering rate can emerge from the overdoped Hubbard model at low temperatures. Our results agree with cuprate experiments in various aspects but challenge the Planckian limit. Finally, by identifying antiferromagnetic fluctuations as the physical origin of the T-linear scattering rate, we discover the microscopic mechanism of strange metallicity in cuprates.

Magnetotransport signatures of antiferromagnetism coexisting with charge order in the trilayer cuprate HgBa2Ca2Cu3O8+δ

Multilayered cuprates possess not only the highest superconducting temperature transition but also offer a unique platform to study disorder-free CuO2 planes and the interplay between competing orders with superconductivity. Here, we study the underdoped trilayer cuprate HgBa2Ca2Cu3O8+δ and we report quantum oscillation and Hall effect measurements in magnetic field up to 88 T. A careful analysis of the complex spectra of quantum oscillations strongly supports the coexistence of an antiferromagnetic order in the inner plane and a charge order in the outer planes. The presence of an ordered antiferromagnetic metallic state that extends deep in the superconducting phase is a key ingredient that supports magnetically mediated pairing interaction in cuprates.

High-T c Cuprates: a Story of Two Electronic Subsystems

A review of the phenomenology and microscopy of cuprate superconductors is presented, with particular attention to universal conductance features, which reveal the existence of two electronic subsystems. The overall electronic system consists of 1 + p charges, where p is the doping. At low dopings, exactly one hole is localized per planar copper-oxygen unit, while upon increasing doping and temperature, the hole is gradually delocalized and becomes itinerant. Remarkably, the itinerant holes exhibit identical Fermi liquid character across the cuprate phase diagram. This universality enables a simple count of carrier density and yields comprehensive understanding of the key features in the normal and superconducting state. A possible superconducting mechanism is presented, compatible with the key experimental facts. The base of this mechanism is the interaction of fast Fermi liquid carriers with localized holes. A change in the microscopic nature of chemical bonding in the copper oxide planes, from ionic to covalent, is invoked to explain the phase diagram of these fascinating compounds.

Superconductivity and the Jahn–Teller Polaron

In this article, we review the essential properties of high-temperature superconducting cuprates, which are unconventional isotope effects, heterogeneity, and lattice responses. Since their discovery was based on ideas stemming from Jahn–Teller polarons, their special role, together with the Jahn–Teller effect itself, is discussed in greater detail. We conclude that the underlying physics of cuprates cannot stem from purely electronic mechanisms, but that the intricate interaction between lattice and charge is at its origin.

Robust d-Wave Superconductivity in the Square-Lattice t-J Model

Unravelling competing orders emergent in doped Mott insulators and their interplay with unconventional superconductivity is one of the major challenges in condensed matter physics. To explore the possible superconducting state in a doped Mott insulator, we study the square-lattice t-J model with both the nearest-neighbor and next-nearest-neighbor electron hoppings and spin interactions. By using the state-of-the-art density matrix renormalization group calculation with imposing charge U(1) and spin SU(2) symmetries on the six-leg cylinders, we establish a quantum phase diagram including three phases: a stripe charge density wave phase, a superconducting phase without static charge order, and a superconducting phase coexistent with a weak charge stripe order. Crucially, we demonstrate that the superconducting phase has a power-law pairing correlation that decays much slower than the charge density and spin correlations, which is a quasi-1D descendant of the uniform d-wave superconductor in two dimensions. These findings reveal that enhanced charge and spin fluctuations with optimal doping is able to produce robust d-wave superconductivity in doped Mott insulators, providing a foundation for connecting theories of superconductivity to models of strongly correlated systems.

Superconductivity in the three-band model of cuprates: nodal direction characteristics and influence of intersite interactions

The three-band Emery model is applied to study the selected principal features of thed-wavesuperconducting phase in the copper-based compounds. The electron-electron correlations are taken into account by the use of the diagrammatic expansion of the Guztwiller wave function (DE-GWF method). The nodal Fermi velocity, Fermi momentum, and effective mass are all determined in the paired state and show relatively good agreement with the available experimental data, as well as with the corresponding single-band calculations. Additionally, the influence of the next-nearest neighbor oxygen-oxygen hopping and intersite Coulomb repulsion terms on the superconducting phase is analyzed.

Machine-Learning-Guided Prediction Models of Critical Temperature of Cuprates

Cuprates have been at the center of long debate regarding their superconducting mechanism; therefore, predicting the critical temperatures of cuprates remains elusive. Herein, using machine learning and first-principles calculations, we predict the maximum superconducting transition temperature (T_c,max) of hole-doped cuprates and suggest the functional form for T_c,max with the root-mean-square-error of 3.705 K and R² of 0.969. We have found that the Bader charge of apical oxygen, the bond strength between apical atoms, and the number of superconducting layers are essential to estimate T_c,max. Furthermore, we predict the T_c,max of hypothetical cuprates generated by replacing apical cations with other elements. Among the hypothetical structures, the cuprates with Ga show the highest predicted T_c,max values, which are 71, 117, and 131 K for one, two, and three CuO₂ layers, respectively. These findings suggest that machine learning could guide the design of new high-T_c superconductors in the future.

First-principles calculations of effects of pressure on paramagnetic, ferromagnetic, and antiferromagnetic spin-web Cu3TeO6

The structure, electronic, and magnetic properties have been investigated by the first-principles calculations for paramagnetic, ferromagnetic, and antiferromagnetic Cu₃TeO₆ under pressure from 0 to 100 GPa. The calculated lattice parameters at 0 GPa are in excellent agreement with the available calculated and experimental values. With increasing pressure, the lattice parameters and volume decrease, but Cu₃TeO₆ keeps a stable cubic structure. The electronic calculations show that paramagnetic and ferromagnetic Cu₃TeO₆ are metallic, and antiferromagnetic Cu₃TeO₆ is non-metallic with a direct band gap which decreases with the increasing pressure. Under the pressure, their non-locality of density of states enhances and the electrons become more active. Moreover, for antiferromagnetic Cu₃TeO₆, the spin moments of Cu atoms are affected obviously by pressures, and Te atoms show nonmagnetic performance. The total magnetic moment, which is mainly contributed by Cu, reaches the maximum at 20 GPa, and decreases with the increasing pressure. The knowledge of these properties will provide reference and guidance for the subsequent study of Cu₃TeO₆.

Standard Behaviour of Bi2Sr2CaCu2O8+δ Overdoped

I calculated the critical temperature and superconducting gap in the framework of one band d wave Eliashberg theory with only one free parameter in order to reproduce the experimental data relative to Bi2Sr2CaCu2O8+δ(BSCCO) in the overdoped regime. The theoretical calculations are in excellent agreement with the experimental data and indicate that cuprates in the overdoped regime are well described by standard d-wave Eliashberg theory with coupling provided by antiferromagnetic spin fluctuations.

Detection of Acoustic Plasmons in Hole-Doped Lanthanum and Bismuth Cuprate Superconductors Using Resonant Inelastic X-Ray Scattering

High T_{c} superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic x-ray scattering to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors (La_{1.84}Sr_{0.16}CuO_{4} and Bi_{2}Sr_{1.6}La_{0.4}CuO_{6+δ}), crucially completing the picture. Interestingly, in contrast to the quasistatic charge ordering which manifests at both Cu and O sites, the observed acoustic plasmons are predominantly associated with the O sites, revealing a unique dichotomy in the behavior of valence charges in hole-doped cuprates.

The local structure of self-doped BiS2-based layered systems as a function of temperature

We have studied the local structure of layered Eu(La,Ce)FBiS2 compounds by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements as a function of temperature. We find that the BiS2 sub-lattice is largely distorted in EuFBiS2, characterized by two different in-plane Bi-S1 distances. The distortion is marginally affected by partial substitutions of Ce (Eu0.5Ce0.5FBiS2) and La (Eu0.5La0.5FBiS2). The temperature dependence of the local structure distortion reveals an indication of possible charge density wave like instability in the pristine self-doped EuFBiS2 and Ce substituted Eu0.5Ce0.5FBiS2 while it is suppressed in La substituted Eu0.5La0.5FBiS2. In compounds with higher superconducting transition temperature, the axial Bi-S2 bond distance is elongated and the related bond stiffness decreased, suggesting some important role of this in the charge transfer mechanism for self-doping in the active BiS2-layer. In-plane Bi-S1 distances are generally softer than the axial Bi-S2 distance and they suffer further softening by the substitutions. The results are discussed in relation to an important role of the Bi defect chemistry driven asymmetric local environment in the physical properties of these materials.

Local strain-driven migration of oxygen vacancies to apical sites in YBa2Cu3O7-x

It is well known that in the high-temperature superconductor YBa2Cu3O7-x (YBCO), oxygen vacancies (VO) control the carrier concentration, its critical current density and transition temperature. In this work, it is revealed that VO also allows the accommodation of local strain fields caused by large-scale defects within the crystal. We show that the nanoscale strain associated with Y2Ba4Cu8O16 (Y124) intergrowths-that are common defects in YBCO-strongly affect the venue and concentration of VO. Local probe measurements in conjunction with density-functional-theory calculations indicate a strain-driven reordering of VO from the commonly observed CuO chains towards the bridging apical sites located in the BaO plane and bind directly to the superconducting CuO2 planes. Our findings have strong implications on the physical properties of the YBCO, as the presence of apical VO alters the transfer of carriers to the CuO2 planes, confirmed by changes in the Cu and O core-loss edge probed using electron energy loss spectroscopy, and creates structural changes that affect the Cu-O bonds in the superconducting planes. In addition, the revelation of apical VO also has implications on modulating critical current densities and enhancing vortex pinning.

Magnetic signatures of 120 K superconductivity at interfaces in La2CuO4+δ

In self-assembled vertically aligned nanocomposite (VAN) thin films of La2CuO4+δ + LaCuO3, we find from DC magnetic susceptibility measurements, weak signatures of superconductivity at ∼120 K. This compares to a maximum TC of 40 K in bulk La2CuO4+δ. The 120 K signature occurs only when both c-axis and a-axis oriented La2CuO4+δ grains are present in the films. The superconductivity was lost after 3 months of storage but was recovered by annealing in oxygen. From lattice parameter analyses undertaken close to the c/a grain boundaries, it was determined that expansion of the La perovskite block in c-La2CuO4+δ enables the differently oriented grains to join at the interface. This expansion is consistent with the higher TC interfacial region. The work shows a new direction for increasing TC in cuprates - namely careful strain engineering of the crystal structure independently in-plane and out-of-plane.

Effect of Structural Supermodulation on Superconductivity in Trilayer Cuprate Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ}

We investigate the spatial and doping evolutions of the superconducting properties of trilayer cuprate Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ} by using scanning tunneling microscopy and spectroscopy. Both the superconducting coherence peak and gap size exhibit periodic variations with structural supermodulation, but the effect is much more pronounced in the underdoped regime than at optimal doping. Moreover, a new type of tunneling spectrum characterized by two superconducting gaps emerges with increasing doping, and the two-gap features also correlate with the supermodulation. We propose that the interaction between the inequivalent outer and inner CuO_{2} planes is responsible for these novel features that are unique to trilayer cuprates.

Magnetic force theory combined with quasi-particle self-consistent GW method

We report a successful combination of magnetic force linear response theory with quasiparticle self-consistent GW method. The self-consistently determined wavefunctions and eigenvalues can just be used for the conventional magnetic force calculations. While its formulation is straightforward, this combination provides a way to investigate the effect of GW self-energy on the magnetic interactions which can hardly be quantified due to the limitation of current GW methodology in calculating the total energy difference in between different magnetic phases. In ferromagnetic 3d elements, GW self-energy slightly reduces the d bandwidth and enhances the interactions while the same long-range feature is maintained. In antiferromagnetic transition-metal monoxides, QSGW significantly reduces the interaction strengths by enlarging the gap. Orbital-dependent magnetic force calculations show that the coupling between e _g and the nominally-empty 4s orbital is noticeably large in MnO which is reminiscent of the discussion for cuprates regarding the role of Cu-4s state. This combination of magnetic force theory with quasiparticle self-consistent GW can be a useful tool to study various magnetic materials.

Theory of angle-dependent marginal Fermi liquid self-energy and its existence at all dopings in cuprates

Various angle-dependent measurements in hole-doped cuprates suggested that non-Fermi liquid (NFL) and Fermi-liquid (FL) self-energies coexist in the Brillouin zone. Moreover, it is also found that NFL self-energies survive up to the overdoped region where the resistivity features a global FL-behavior. To address this problem, we compute the momentum dependent self-energy from a single band Hubbard model. The self-energy is calculated self-consistently by using a momentum-dependent density-fluctuation (MRDF) method. One of our main results is that the computed self-energy exhibits a marginal-FL (MFL)-like frequency dependence only in the antinodal region, and FL-like behavior elsewhere at all dopings. The MFL self-energy stems from the fluctuations between the itinerant and localized densities-a result that appears when self-energy is calculated self-consistently and features an intermediate coupling behavior of cuprates. We also calculate the DC conductivity by including the full momentum dependent self-energy. We find that the resistivity-temperature exponent n becomes 1 near the optimal doping, while the MFL self-energy occupies largest momentum-space volume. Surprisingly, even in the NFL state near the optimal doping, the nodal region contains FL-like self-energies; while in the under- and over-dopings ([Formula: see text]), the antinodal region remains NFL-like. These results highlight the non-local correlation physics in cuprates and in other similar intermediately correlated materials, where a direct link between the microscopic single-particle spectral properties and the macroscopic transport behavior can not be well established.

Tc and Other Cuprate Properties in Relation to Planar Charges as Measured by NMR

Nuclear magnetic resonance (NMR) in cuprate research is a prominent bulk local probe of magnetic properties. NMR also, as was shown over the last years, actually provides a quantitative measure of local charges in the CuO 2 plane. This has led to fundamental insights, e.g., that the maximum T c is determined by the sharing of the parent planar hole between Cu and O. Using bonding orbital hole contents on planar Cu and O measured by NMR, instead of the total doping x, the thus defined two-dimensional cuprate phase diagram reveals significant differences between the various cuprate materials. Even more importantly, the reflected differences in material chemistry appear to set a number of electronic properties as we discuss here, for undoped, underdoped and optimally doped cuprates. These relations should advise attempts at a theoretical understanding of cuprate physics as well as inspire material chemists towards new high- T c materials. Probing planar charges, NMR is also sensitive to charge variations or ordering phenomena in the CuO 2 plane. Thereby, local charge order on planar O in optimally doped YBCO could recently be proven. Charge density variations seen by NMR in both planar bonding orbitals with amplitudes between 1% to 5% appear to be omnipresent in the doped CuO 2 plane, i.e., not limited to underdoped cuprates and low temperatures.

Superconductivity in a unique type of copper oxide

The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics. High-T c cuprates crystallize into a layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high-T c cuprates are elongated along the c axis, leading to a 3dx 2-y 2 orbital at the top of the band structure wherein the doped holes reside. This scenario gives rise to 2D characteristics in high-T c cuprates that favor d-wave pairing symmetry. Here, we report superconductivity in a cuprate Ba2CuO4-y , wherein the local octahedron is in a very exceptional compressed version. The Ba2CuO4-y compound was synthesized at high pressure at high temperatures and shows bulk superconductivity with critical temperature (T c ) above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the T c for the isostructural counterparts based on classical La2CuO4 X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron, the 3d3z 2-r 2 orbital will be lifted above the 3dx 2-y 2 orbital, leading to significant 3D nature in addition to the conventional 3dx 2-y 2 orbital. This work sheds important light on advancing our comprehensive understanding of the superconducting mechanism of high T c in cuprate materials.

3D strain-induced superconductivity in La2CuO4+δ using a simple vertically aligned nanocomposite approach

A long-term goal for superconductors is to increase the superconducting transition temperature, T _C. In cuprates, T _C depends strongly on the out-of-plane Cu-apical oxygen distance and the in-plane Cu-O distance, but there has been little attention paid to tuning them independently. Here, in simply grown, self-assembled, vertically aligned nanocomposite thin films of La₂CuO_4+δ + LaCuO₃, by strongly increasing out-of-plane distances without reducing in-plane distances (three-dimensional strain engineering), we achieve superconductivity up to 50 K in the vertical interface regions, spaced ~50 nm apart. No additional process to supply excess oxygen, e.g., by ozone or high-pressure oxygen annealing, was required, as is normally the case for plain La₂CuO_4+δ films. Our proof-of-concept work represents an entirely new approach to increasing T _C in cuprates or other superconductors.

Correlated materials design: prospects and challenges

The design of correlated materials challenges researchers to combine the maturing, high throughput framework of DFT-based materials design with the rapidly-developing first-principles theory for correlated electron systems. We review the field of correlated materials, distinguishing two broad classes of correlation effects, static and dynamics, and describe methodologies to take them into account. We introduce a material design workflow, and illustrate it via examples in several materials classes, including superconductors, charge ordering materials and systems near an electronically driven metal to insulator transition, highlighting the interplay between theory and experiment with a view towards finding new materials. We review the statistical formulation of the errors of currently available methods to estimate formation energies. We formulate an approach for estimating a lower-bound for the probability of a new compound to form. Correlation effects have to be considered in all the material design steps. These include bridging between structure and property, obtaining the correct structure and predicting material stability. We introduce a post-processing strategy to take them into account.

Apical Charge Flux-Modulated In-Plane Transport Properties of Cuprate Superconductors

For copper-based superconductors, the maximum superconducting transition temperature T_{c,max} of different families measured from experiment can vary from 38 K in La_{2}CuO_{4} to 135 K in HgBa_{2}Ca_{2}Cu_{3}O_{8} at the optimal hole doping concentration. We demonstrate herein, using ab initio computations, a new trend suggesting that the cuprates with stronger out-of-CuO_{2}-plane chemical bonding between the apical anion (O, Cl) and apical cation (e.g., La, Hg, Bi, Tl) are generally correlated with higher T_{c,max} in experiments. We then show the underlying fundamental phenomena of coupled apical charge flux and lattice dynamics when the apical oxygen oscillates vertically. This triggers the charge flux among the apical cation, apical anion, and the in-plane CuO_{4} unit. The effect not only dynamically modulates the site energy of the hole at a given Cu site to control the in-plane charge transfer energy, but also can modulate the in-plane hole hopping integral simultaneously in a dynamic way by the cooperative apical charge fluxes.

Simple analytical model of the effect of high pressure on the critical temperature and other thermodynamic properties of superconductors

Within the general conformal transformation method a simplified analytical model is proposed to study the effect of external hydrostatic pressure on low- and high-temperature superconducting systems. A single fluctuation in the density of states, placed away from the Fermi level, as well as external pressure are included in the model to derive equations for the superconducting gap, free energy difference, and specific heat difference. The zero- and sub-critical temperature limits are discussed by the method of successive approximations. The critical temperature is found as a function of high external pressure. It is shown that there are four universal types of the response of the system, in terms of dependence of the critical temperature on increasing external pressure. Some effects, which should be possible to be observed experimentally in s-wave superconductors, the cuprates (i.e. high-T_c superconductors) and other superconducting materials of the new generation such as two-gap superconductors, are revealed and discussed. An equation for the ratio \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{ {\mathcal R} }}}_{{\bf{1}}}$$\end{document}ℛ1 ≡ 2Δ(0)/T_c, as a function of the introduced parameters, is derived and solved numerically. Analysis of other thermodynamic quantities and the characteristic ratio \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{ {\mathcal R} }}}_{{\bf{2}}}$$\end{document}ℛ2 ≡ ΔC(T_c)/C_N(T_c) is performed numerically, and mutual relations between the discussed quantities are investigated. The simple analytical model presented in the paper may turn out to be helpful in searching for novel superconducting components with higher critical temperatures induced by pressure effects.

Direct observation of orbital hybridisation in a cuprate superconductor

The minimal ingredients to explain the essential physics of layered copper-oxide (cuprates) materials remains heavily debated. Effective low-energy single-band models of the copper–oxygen orbitals are widely used because there exists no strong experimental evidence supporting multi-band structures. Here, we report angle-resolved photoelectron spectroscopy experiments on La-based cuprates that provide direct observation of a two-band structure. This electronic structure, qualitatively consistent with density functional theory, is parametrised by a two-orbital (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d_{x^2 - y^2}$$\end{document}dx2-y2 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d_{z^2}$$\end{document}dz2) tight-binding model. We quantify the orbital hybridisation which provides an explanation for the Fermi surface topology and the proximity of the van-Hove singularity to the Fermi level. Our analysis leads to a unification of electronic hopping parameters for single-layer cuprates and we conclude that hybridisation, restraining d-wave pairing, is an important optimisation element for superconductivity.

The essential physics of cuprate superconductors is often described by single-band models. Here, Matt et al. report direct observation of a two-band electronic structure in La-based cuprates.

Strain control of oxygen kinetics in the Ruddlesden-Popper oxide La1.85Sr0.15CuO4

Oxygen defect control has long been considered an important route to functionalizing complex oxide films. However, the nature of oxygen defects in thin films is often not investigated beyond basic redox chemistry. One of the model examples for oxygen-defect studies is the layered Ruddlesden-Popper phase La2-xSr x CuO4-δ (LSCO), in which the superconducting transition temperature is highly sensitive to epitaxial strain. However, previous observations of strain-superconductivity coupling in LSCO thin films were mainly understood in terms of elastic contributions to mechanical buckling, with minimal consideration of kinetic or thermodynamic factors. Here, we report that the oxygen nonstoichiometry commonly reported for strained cuprates is mediated by the strain-modified surface exchange kinetics, rather than reduced thermodynamic oxygen formation energies. Remarkably, tensile-strained LSCO shows nearly an order of magnitude faster oxygen exchange rate than a compressively strained film, providing a strategy for developing high-performance energy materials.

Quasiparticle Interference on Cubic Perovskite Oxide Surfaces

We report the observation of coherent surface states on cubic perovskite oxide SrVO_{3}(001) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a d_{xy}-derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO_{2} plane play a critical role in controlling the coherent surface state via modulating orbital state.

Entropic Origin of Pseudogap Physics and a Mott-Slater Transition in Cuprates

We propose a new approach to understand the origin of the pseudogap in the cuprates, in terms of bosonic entropy. The near-simultaneous softening of a large number of different q-bosons yields an extended range of short-range order, wherein the growth of magnetic correlations with decreasing temperature T is anomalously slow. These entropic effects cause the spectral weight associated with the Van Hove singularity (VHS) to shift rapidly and nearly linearly toward half filling at higher T, consistent with a picture of the VHS driving the pseudogap transition at a temperature ~T*. As a byproduct, we develop an order-parameter classification scheme that predicts supertransitions between families of order parameters. As one example, we find that by tuning the hopping parameters, it is possible to drive the cuprates across a transition between Mott and Slater physics, where a spin-frustrated state emerges at the crossover.