Temperature Cuprate Superconductors Research Articles

Quantum interference in a high-transition-temperature superconductor based on nanoslits on SrTiO3 substrate

The Josephson junction, a key component of superconducting quantum circuits, is much less mature, especially for the high-transition (high-Tc) temperature cuprate superconductors. Herein, we report on the realization of high-Tc Josephson junction based on nanoslits on a SrTiO3 substrate. We demonstrate the ability to manipulate the current–voltage characteristics of YBa2Cu3O7−x bridges continuously from superconducting current-dominated to Josephson junction behavior by changing the irradiation parameters of the focused Ga+ ion beam on a single-crystal SrTiO3 substrate. The periodic critical current that depends on the magnetic flux coupled into the superconducting quantum interference devices was observed, which exactly reflects the effects of quantum tunneling and flux quantization. To some extent, this weak link within the framework of the semiconductor manufacturing process shows the potential to provide a cost-effective, highly efficient, and reliable pathway for scaling up quantum mechanical superconducting circuits, which is promising for the fabrication process.

Non-Fermi liquid behavior in a simple model of Fermi arcs and pseudogap

In this paper, we consider a perturbed version of a very simple and exactly solvable model that supports Fermi arcs and pseudogap in its ground state and excitation spectrum, which includes Hubbard-like interactions in both momentum and real spaces. We find that the combined effects give rise to non-Fermi liquid behavior in the electron self-energy. This points to a novel mechanism that leads to non-Fermi liquid behavior, which is of strong current interest in the context of strongly correlated metals, that often become superconductors. Comparison will be made with phenomenology of high- temperature cuprate superconductors.

Momentum-resolved visualization of electronic evolution in doping a Mott insulator

High temperature superconductivity in cuprates arises from doping a parent Mott insulator by electrons or holes. A central issue is how the Mott gap evolves and the low-energy states emerge with doping. Here we report angle-resolved photoemission spectroscopy measurements on a cuprate parent compound by sequential in situ electron doping. The chemical potential jumps to the bottom of the upper Hubbard band upon a slight electron doping, making it possible to directly visualize the charge transfer band and the full Mott gap region. With increasing doping, the Mott gap rapidly collapses due to the spectral weight transfer from the charge transfer band to the gapped region and the induced low-energy states emerge in a wide energy range inside the Mott gap. These results provide key information on the electronic evolution in doping a Mott insulator and establish a basis for developing microscopic theories for cuprate superconductivity.

Evolution of Charge and Pair Density Modulations in Overdoped Bi2Sr2CuO6+δ

One of the central issues concerning the mechanism of high temperature superconductivity in cuprates is the nature of the ubiquitous charge order and its implications to superconductivity. Here we use scanning tunneling microscopy to investigate the evolution of charge order from the optimally doped to strongly overdoped Bi2Sr2CuO6+{\delta} cuprates. We find that with increasing hole concentration, the long-range checkerboard order gradually evolves into short-range glassy patterns consisting of diluted charge puddles. Each charge puddle has a unidirectional nematic internal structure, and exhibits clear pair density modulations as revealed by the spatial variations of superconducting coherence peak and gap depth. Both the charge puddles and the nematicity vanish completely in the strongly overdoped non-superconducting regime, when another type of short-range order with root2 * root2 periodicity emerges. These results shed important new lights on the intricate interplay between the intertwined orders and the superconducting phase of cuprates.

Electronic Evolution from the Parent Mott Insulator to a Superconductor in Lightly Hole-Doped Bi2Sr2CaCu2O8 + δ**Supported by the National Natural Science Foundation of China (Grant Nos. 11888101, 11922414, and 11534007), the National Key Research and Development Program of China (Grant Nos. 2016YFA0300300 and 2017YFA0302900), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB25000000), the

High temperature superconductivity in cuprates is realized by doping the Mott insulator with charge carriers. A central issue is how such an insulating state can evolve into a conducting or superconducting state when charge carriers are introduced. Here, by in situ vacuum annealing and Rb deposition on the Bi2Sr2Ca0.6Dy0.4Cu2O8 + δ (Bi2212) sample surface to push its doping level continuously from deeply underdoped (Tc = 25 K, doping level p ∼ 0.066) to the near-zero doping parent Mott insulator, angle-resolved photoemission spectroscopy measurements are carried out to observe the detailed electronic structure evolution in the lightly hole-doped region for the first time. Our results indicate that the chemical potential lies at about l eV above the charge transfer band for the parent state at zero doping, which is quite close to the upper Hubbard band. With increasing hole doping, the chemical potential moves continuously towards the charge transfer band and the band structure evolution exhibits a rigid band shift-like behavior. When the chemical potential approaches the charge transfer band at a doping level of ∼0.05, the nodal spectral weight near the Fermi level increases, followed by the emergence of the coherent quasiparticle peak and the insulator–superconductor transition. Our observations provide key insights in understanding the insulator–superconductor transition in doping the parent cuprate compound and for establishing related theories.

Structural, Electronic and Magnetic Properties of a Few Nanometer-Thick Superconducting NdBa2Cu3O7 Films.

Epitaxial films of high critical temperature () cuprate superconductors preserve their transport properties even when their thickness is reduced to a few nanometers. However, when approaching the single crystalline unit cell (u.c.) of thickness, decreases and eventually, superconductivity is lost. Strain originating from the mismatch with the substrate, electronic reconstruction at the interface and alteration of the chemical composition and of doping can be the cause of such changes. Here, we use resonant inelastic x-ray scattering at the Cu edge to study the crystal field and spin excitations of NdBaCuO ultrathin films grown on SrTiO, comparing 1, 2 and 80 u.c.-thick samples. We find that even at extremely low thicknesses, the strength of the in-plane superexchange interaction is mostly preserved, with just a slight decrease in the 1 u.c. with respect to the 80 u.c.-thick sample. We also observe spectroscopic signatures for a decrease of the hole-doping at low thickness, consistent with the expansion of the c-axis lattice parameter and oxygen deficiency in the chains of the first unit cell, determined by high-resolution transmission microscopy and x-ray diffraction.

Orbital Approach to High Temperature Superconductivity

High temperature superconductivity in cuprates is explained in terms of 3d-orbital capture in copper. In elemental Cu 3d-orbital capture abstracts an electron from the 4 s2 valence orbital, and leaves it as 4 s1. This is known since Cu occurs in Group IB of the Periodic Table. This forms an electron vacancy, or hole, in the valence shell. Therefore, the energy of 3d-orbital capture is stronger than the energy of unpairing of a paired-spin 4 s2 orbital. In cuprates 3d-orbital capture abstracts an electron from a Cu-O covalent bond, and leaves a hole in the excited state orbital. By electron-hole migration the excited state orbital leads to a coordinate covalent bond. This leads to superconductivity. The 3d-orbital process accounts for superconductivity and insulator behavior in cuprates. These results lend credence to the statement that 3d-orbital capture in copper is the cause of high temperature superconductivity.

Experimental observation of copper oxide layers in cuprates by scanning tunneling microscopy

The nature of high temperature superconductivity in cuprates has currently been one of major scientific problems in condensed matter physics. In cuprate superconductors, the d -wave pairing symmetry with gap nodes on the Fermi surface and the pseudogap phenomenology have been well documented and regarded as the most important findings in the 30-year study. Due to the very complicated layered structure of cuprates, which consist of alternative superconducting (copper oxide) and non-superconducting (charge reservoir) layers, almost all macroscopic experiments previously performed probe the combined properties of both superconducting layers and non-superconducting layers. Meanwhile, surface-sensitive techniques, including scanning tunneling microscopy and spectroscopy, have mostly conducted on the cleaved reservoir layers, while the copper oxide layer is usually sandwiched between the reservoir layers. Because copper oxide layers are the main building blocks of cuprates, a direct probe of the copper oxide layer should be most desirable, although it is highly challenging experimentally. However, the significant progress has been made in the recent years. In this article we review the recent efforts on the preparation and direct observation of copper oxide layers in cuprates, and shed insight on the relationship between the pseudogap and superconductivity. These results might open new avenue for building up new high temperature superconductors and understanding the pairing mechanism of high temperature superconductivity in cuprates.

X-Rays Writing/Reading of Charge Density Waves in the CuO2 Plane of a Simple Cuprate Superconductor

It is now well established that superconductivity in cuprates competes with charge modulations giving an electronic phase separation at nanoscale. More specifically, superconducting electronic current takes root in the available free space left by electronic charge ordered domains, called charge density wave (CDW) puddles. This means that CDW domain arrangement plays a fundamental role in the mechanism of high temperature superconductivity in cuprates. Here we report about the possibility of controlling the population and spatial organization of the charge density wave puddles in a single crystal La2CuO4+y through X-ray illumination and thermal treatments. We apply a pump-probe method—based on X-ray illumination as a pump and X-ray diffraction as a probe—setting a writing/reading procedure of CDW puddles. Our findings are expected to allow new routes for advanced design and manipulation of superconducting pathways in new electronics.

High temperature superconductivity in cuprates and in organics: Where do we stand?

Refinement of materials and of experimental tools over the last two decades has led to broad agreement on many aspects of the phase diagram of cuprate high-temperature superconductors. Similarly, theoretical tools have been improving steadily. In this series of four talks, I give an overview of experimental results and then focus on two classes of theoretical methods and what they teach us on the physics of high-temperature superconductors and of related strongly correlated materials, such as...

The high temperature superconductivity in cuprates: physics of the pseudogap region

We discuss the physics of the high temperature superconductivity in hole doped copper oxide ceramics in the pseudogap region. Starting from an effective reduced Hamiltonian relevant to the dynamics of holes injected into the copper oxide layers proposed in a previous paper, we determine the superconductive condensate wavefunction. We show that the low-lying elementary condensate excitations are analogous to the rotons in superfluid $^4He$. We argue that the rotons-like excitations account for the specific heat anomaly at the critical temperature. We discuss and compare with experimental observations the London penetration length, the Abrikosov vortices, the upper and lower critical magnetic fields, and the critical current density. We give arguments to explain the origin of the Fermi arcs and Fermi pockets. We investigate the nodal gap in the cuprate superconductors and discuss both the doping and temperature dependence of the nodal gap. We suggest that the nodal gap is responsible for the doping dependence of the so-called nodal Fermi velocity detected in angle resolved photoemission spectroscopy studies. We discuss the thermodynamics of the nodal quasielectron liquid and their role in the low temperature specific heat. We propose that the ubiquitous presence of charge density wave in hole doped cuprate superconductors in the pseudogap region originates from instabilities of the nodal quasielectrons driven by the interaction with the planar $CuO_2$ lattice. We investigate the doping dependence of the charge density wave gap and the competition between charge order and superconductivity. We discuss the effects of external magnetic fields on the charge density wave gap and elucidate the interplay between charge density wave and Abrikosov vortices. Finally, we examine the physics underlying quantum oscillations in the pseudogap region.

Unconventional superconductivity in low density electron systems and conventional superconductivity in hydrogen metallic alloys

In a short review-article we first discuss the results, which are mainly devoted to the generalizations of the famous Kohn-Luttinger mechanism of superconductivity in purely repulsive fermion systems at low electron densities. In the context of repulsive-U Hubbard model and Shubin-Vonsovsky model we consider briefly the superconducting phase diagrams and the symmetries of the order parameter in novel strongly correlated electron systems including idealized monolayer and bilayer graphene. We stress that purely repulsive fermion systems are mainly the subject of unconventional low-temperature superconductivity. To get the high temperature superconductivity in cuprates (with T_C of the order of 100 K) we should proceed to the t-J model with the Van der Waals interaction potential and the competition between short-range repulsion and long-range attraction. Finally we stress that to describe superconductivity in metallic hydrogen alloys under pressure (with T_C of the order of 200 K) it is reasonable to reexamine more conventional mechanisms connected with electron-phonon interaction. These mechanisms arise in the attractive-U Hubbard model with static onsite or intersite attractive potential or in more realistic theories (which include retardation effects) such as Migdal-Eliashberg strong coupling theory or even Fermi-Bose mixture theory of Ranninger et al., and its generalizations.

Observation of a d-wave gap in electron-doped Sr2IrO4

High temperature superconductivity in cuprates emerges out of a highly enigmatic `pseudogap' metal phase. The mechanism of high temperature superconductivity is likely encrypted in the elusive relationship between the two phases, which spectroscopically is manifested as Fermi arcs---disconnected segments of zero-energy states---collapsing into $d$-wave point nodes upon entering the superconducting phase. Here, we reproduce this distinct cuprate phenomenology in the 5$d$ transition-metal oxide Sr$_2$IrO$_4$. Using angle-resolved photoemission, we show that clean, low-temperature phase of 6-8$\%$ electron-doped Sr$_2$IrO$_4$ has gapless excitations only at four isolated points in the Brillouin zone with a predominant $d$-wave symmetry of the gap. Our work thus establishes a connection between the low-temperature $d$-wave instability and the previously reported high-temperature Fermi arcs in electron-doped Sr$_2$IrO$_4$. Although the physical origin of the $d$-wave gap remains to be understood, Sr$_2$IrO$_4$ is a first non-cuprate material to spectroscopically reproduce the complete phenomenology of the cuprates, thus offering a new material platform to investigate the relationship between the pseudogap and the $d$-wave gap.

Sr2IrO4: Gateway to cuprate superconductivity?

High temperature superconductivity in cuprates remains a defining challenge in condensed matter physics. Recently, a new set of related compounds based on Ir rather than Cu has been discovered that may be on the verge of superconductivity themselves or be able to shed new light on the underlying interactions responsible for superconductivity in the cuprates.

ARPES experiment in fermiology of quasi-2D metals (Review Article)

Angle-resolved photoemission spectroscopy (ARPES) enables direct observation of the Fermi surface and underlying electronic structure of crystals, which are the basic concepts necessary to describe all the electronic properties of solids and to reveal the nature of key electronic interactions involved. ARPES proved to be the most efficient for studies of quasi-2D metals, to which the most challenging and hence exciting compounds belong. This stimulated tremendously the development of ARPES in the recent years. The aim of this paper is to introduce the reader to the state-of-the-art ARPES experiment and to review the results of its application to such highly topical problems in solid state physics as high temperature superconductivity in cuprates and iron-based superconductors and electronic ordering in the transition metal dichalcogenides and manganites.

Thermodynamic phase diagram of static charge order in underdoped YBa2Cu3Oy

The interplay between superconductivity and any other competing order is an essential part of the long-standing debate on the origin of high temperature superconductivity in cuprates. Akin to the situation of heavy fermions, organic superconductors and pnictides, it has been proposed that the pairing mechanism in cuprates comes from fluctuations of a nearby quantum phase transition. Recent evidence of charge modulation and the associated fluctuations in the pseudogap phase of YBa_2Cu_3O_y make charge order a likely candidate for a competing order. However, a thermodynamic signature of the charge ordering phase transition is still lacking. Moreover, whether such charge order is one- or two-dimensional is still controversial but pivotal for the understanding the topology of the reconstructed Fermi surface. Here we address both issues by measuring sound velocities in YBCO_6.55 in high magnetic fields, a powerful thermodynamic probe to detect phase transitions. We provide the first thermodynamic signature of the field-induced charge ordering phase transition in YBCO allowing construction of a field-temperature phase diagram, which reveals the competing nature of this charge order. The comparison of different acoustic modes indicates that the charge modulation has a two-dimensional character, thus imposing strong constraints on Fermi surface reconstruction scenarios.

Unconventional magnetic phase diagram of cuprate superconductor La2−xSrxCuO4 at quantum critical point x=1/9

We propose a new magnetic phase diagram of La2−xSrxCuO4 around a quantum critical point x=1/9 based on field-cooled magnetization measurements and critical fittings. A new phase boundary Tm2(H) is discovered which buries deeply below the first order vortex melting line in the vortex solid phase. The coupling between superconductivity and antiferromagnetism is found to be attractive below Tm2(H) while repulsive above. The attractive coupling between superconducting order and static antiferromagnetic order provides compelling experimental evidence that the antiferromagnetism microscopically coexists and collaborates with the high temperature superconductivity in cuprates.

Theory of High Temperature Superconductivity in Cuprates and Other Doped Polar Insulators

In the last two decades, there have been tremendous attempts to build an adequate theory of high-temperature superconductivity. Most studies used some model Hamiltonians with input parameters not directly related to the material. The dielectric response function of electrons in strongly correlated high-temperature superconductors is a priori unknown. Hence, one has to start with the generic Hamiltonian including unscreened Coulomb and Frohlich electron–phonon interactions operating on the same scale since any ad-hoc assumption on their range and relative magnitude might fail. Using such a generic Hamiltonian, the analytical theory of high-temperature superconductivity in doped polar insulators predicting the critical temperature in excess of a hundred Kelvin without any adjustable parameters has been built. The many-particle electron system is described by an analytically solvable polaronic t–Jp Hamiltonian with reduced hopping integral, t, allowed double on-site occupancy, large phonon-induced antiferromagnetic exchange, Jp>t, and a high-temperature superconducting state of small superlight bipolarons protected from clustering. Here, major steps of the theory are outlined suggesting that the true origin of high-temperature superconductivity is found in a proper combination of strong electron–electron correlations with a significant finite-range (Frohlich) EPI, and that the theory is fully compatible with the key experiments.

Quantum oscillations and the Fermi surface of high-temperature cuprate superconductors

Abstract Over 20 years since the discovery of high temperature superconductivity in cuprates (Bednorz and Muller, 1986 [1] ), the first convincing observation of quantum oscillations in underdoped YBa2Cu3O6.5 (Doiron-Leyraud et al., 2007 [2] ) has deeply changed the theoretical landscape relevant to these materials. The Fermi surface is a basic concept of solid state physics, which underpins most physical properties (electrical, thermal, optical, etc.) of a metal. Even in the presence of interactions, this fundamental concept remains robust. While there was little doubt about the existence of a Fermi surface on the overdoped side of the phase diagram of the cuprates, the discovery of quantum oscillations in the underdoped regime was a surprise. The small pockets inferred from the measurements in underdoped YBa2Cu3Oy contrast with the large orbit found in overdoped Tl2Ba2CuO6 + δ. A central issue in understanding the phase diagram of high temperature superconductors is the origin of this difference at opposite sides of the superconducting dome. This review aims to shed light on this issue by bringing together recent results of quantum oscillation and transport measurements under high magnetic fields in hole-doped cuprates.

Effect of Short Antiferromagnetic Correlations on the Normal and Superconducting Properties in Copper Oxides

The abnormal “normal” state and origin of high temperature superconductivity in cuprates are still not clear [1, 2]. The strong electron correlations (SEC) are known to be one of the main difficulties for the theory of high- cuprates. The conventional LDA (local density approximation) approach to the band structure fails in the regime of SEC. Various realistic multiband models of a layer at low energy result in the effective Hubbard or models [3 7]. The hybrid LDA+GTB scheme [8] generates the low-energy effective model with all parameters calculated ab initio.