Showing papers on "High-temperature superconductivity published in 2015"
TL;DR: In this article, the interplay between different order parameters in high temperature superconductors is discussed, and the intertwining of these orders leads to new experimentally observable consequences, shedding new light into the physics of these fascinating materials.
Abstract: Understanding high temperature superconductors is a central problem in condensed matter physics. Many experiments have uncovered ordering tendencies which are responsible for the complex phase diagram of high temperature superconductors. This Colloquium discusses the interplay between different order parameters in these materials. Considering the intertwining of these orders leads to new experimentally observable consequences, shedding new light into the physics of these fascinating materials.
837 citations
TL;DR: In this paper, an overview of the neutron scattering results on iron-based superconductors is presented, focusing on the evolution of spin excitation spectra as a function of electron/hole-doping and isoelectronic substitution.
Abstract: High-transition temperature (high-$T_c$) superconductivity in the iron pnictides/chalcogenides emerges from the suppression of the static antiferromagnetic order in their parent compounds, similar to copper oxides superconductors. This raises a fundamental question concerning the role of magnetism in the superconductivity of these materials. Neutron scattering, a powerful probe to study the magnetic order and spin dynamics, plays an essential role in determining the relationship between magnetism and superconductivity in high-$T_c$ superconductors. The rapid development of modern neutron time-of-flight spectrometers allows a direct determination of the spin dynamical properties of iron-based superconductors throughout the entire Brillouin zone. In this review, we present an overview of the neutron scattering results on iron-based superconductors, focusing on the evolution of spin excitation spectra as a function of electron/hole-doping and isoelectronic substitution. We compare spin dynamical properties of iron-based superconductors with those of copper oxide and heavy fermion superconductors, and discuss the common features of spin excitations in these three families of unconventional superconductors and their relationship with superconductivity.
573 citations
TL;DR: It is demonstrated that deposition of potassium onto FeSe films markedly expands the accessible doping range towards the heavily electron-doped region and provides a new strategy to enhance and optimize T(c) in ultrathin films of iron-based superconductors.
Abstract: The evolution of the superconductivity as a function of film thickness and doping is systematically studied in FeSe films. A high-temperature superconducting phase is found to arise in multilayer films.
254 citations
TL;DR: In this article, the authors present a brief review of the investigation of the electronic structure and superconductivity of the FeSe superconductor and related systems, with a particular focus on FeSe films.
Abstract: FeSe superconductors and their related systems have attracted much attention in the study of iron-based superconductors owing to their simple crystal structure and peculiar electronic and physical properties. The bulk FeSe superconductor has a superconducting transition temperature (Tc) of ~8 K and it can be dramatically enhanced to 37 K at high pressure. On the other hand, its cousin system, FeTe, possesses a unique antiferromagnetic ground state but is non-superconducting. Substitution of Se with Te in the FeSe superconductor results in an enhancement of Tc up to 14.5 K and superconductivity can persist over a large composition range in the Fe(Se,Te) system. Intercalation of the FeSe superconductor leads to the discovery of the AxFe2-ySe2 (A = K, Cs and Tl) system that exhibits a Tc higher than 30 K and a unique electronic structure of the superconducting phase. A recent report of possible high temperature superconductivity in single-layer FeSe/SrTiO3 films with a Tc above 65 K has generated much excitement in the community. This pioneering work opens a door for interface superconductivity to explore for high Tc superconductors. The distinct electronic structure and superconducting gap, layer-dependent behavior and insulator-superconductor transition of the FeSe/SrTiO3 films provide critical information in understanding the superconductivity mechanism of iron-based superconductors. In this paper, we present a brief review of the investigation of the electronic structure and superconductivity of the FeSe superconductor and related systems, with a particular focus on the FeSe films.
149 citations
TL;DR: In this paper, a sodalite-like structure of MgH6 was predicted with first-principles electronic structure calculations and the calculated formation enthalpy suggests that it is thermodynamically stable above 263 GPa.
Abstract: Recently, an experimental work reported a very high Tc of ∼190 K in hydrogen sulphide (H2S) at 200 GPa. The search for new superconductors with high superconducting critical temperatures in hydrogen-dominated materials has attracted significant attention. Here we predict a candidate phase of MgH6 with a sodalite-like framework in conjunction with first-principles electronic structure calculations. The calculated formation enthalpy suggests that it is thermodynamically stable above 263 GPa relative to MgH2 and solid hydrogen (H2). Moreover, the absence of imaginary frequency in phonon calculations implies that this MgH6 structure is dynamically stable. Furthermore, our electron–phonon coupling calculation based on BCS theory indicates that this MgH6 phase is a conventional superconductor with a high superconducting critical temperature of ∼260 K under high pressure, which is even higher than that of the recently reported compressed H2S. The present results offer insight in understanding and designing new high-temperature superconductors.
141 citations
TL;DR: In this paper, the effects of hydrogen zero-point motion and the multiband electronic structure relevant for multigap superconductivity near Lifshitz transitions are investigated. But the authors focus on the effect of zero point motion on the topology of the Fermi surfaces.
Abstract: While 203 K high temperature superconductivity in H3S has been interpreted by BCS theory in the dirty limit here we focus on the effects of hydrogen zero-point-motion and the multiband electronic structure relevant for multigap superconductivity near Lifshitz transitions. We describe how the topology of the Fermi surfaces evolves with pressure giving different Lifshitz-transitions. A neck-disrupting Lifshitz-transition (type 2) occurs where the van Hove singularity, vHs, crosses the chemical potential at 210 GPa and new small 2D Fermi surface portions appear with slow Fermi velocity where the Migdal-approximation becomes questionable. We show that the neglected hydrogen zero-point motion ZPM, plays a key role at Lifshitz transitions. It induces an energy shift of about 600 meV of the vHs. The other Lifshitz-transition (of type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new Fermi surfaces appear at the Gamma point of the Brillouin zone here the Migdal-approximation breaks down and the zero-point-motion induces large fluctuations. The maximum Tc=203K occurs at 160 GPa where Ef/w0=1 in the small Fermi surface pocket at Gamma point. A Feshbach-like resonance between a possible BEC-BCS condensate at Gamma and the BCS condensate in different k-space spots is proposed.
93 citations
TL;DR: In this paper, the diversity of the crystal structure and the physical properties of the BiS2-based superconductors are reviewed and notable characteristics of superconductivity in the Bi-S2 family are introduced.
93 citations
TL;DR: In this article, a scanning tunneling microscopy/spectroscopy (STM/STS) study of Sr2IrO4 with surface electron doping by depositing potassium (K) atoms was conducted.
Abstract: Sr2IrO4 was predicted to be a high temperature superconductor upon electron doping since it highly resembles the cuprates in crystal structure, electronic structure and magnetic coupling constants. Here we report a scanning tunneling microscopy/spectroscopy (STM/STS) study of Sr2IrO4 with surface electron doping by depositing potassium (K) atoms. At the 0.5-0.7 monolayer (ML) K coverage, we observed a sharp, V-shaped gap with about 95% loss of density of state (DOS) at EFand visible coherence peaks. The gap magnitude is 25-30 meV for 0.5-0.6 ML K coverage and it closes around 50 K. These behaviors exhibit clear signature of superconductivity. Furthermore, we found that with increased electron doping, the system gradually evolves from an insulating state to a normal metallic state, via a pseudogap-like state and possible superconducting state. Our data suggest possible high temperature superconductivity in electron doped Sr2IrO4, and its remarkable analogy to the cuprates.
77 citations
TL;DR: In this article, a quasi-one-dimensional superconductor with frustrated magnetism is shown to possess strong frustrated magnetic fluctuations and is very sensitive to c-axis lattice constant and can be suppressed by increasing pressure.
Abstract: We predict that the recently discovered quasi-one dimensional superconductors, A$_2$Cr$_3$As$_3$(A=K,Rb), possess strong frustrated magnetic fluctuations and are nearby a novel in-out co-planar magnetic ground state. The frustrated magnetism is very sensitive to c-axis lattice constant and can thus be suppressed by increasing pressure. Our results qualitatively explain strong non-Fermi liquid behaviors observed in the normal state of the superconductors as the intertwining between the magnetism and superconductivity can create a large quantum critical region in quasi-one dimensional systems and also suggest that the materials share similar phase diagrams and superconducting mechanism with other unconventional superconductors, such as cuprates and iron-based superconductors.
62 citations
TL;DR: In this article, a newly discovered iron-based high-temperature superconductor, CaFeAs2, is a staggered intercalation compound that integrates topological quantum spin Hall (QSH) and superconductivity (SC).
Abstract: We predict that CaFeAs2, a newly discovered iron-based high-temperature (T-c) superconductor, is a
staggered intercalation compound that integrates topological quantum spin Hall (QSH) and
superconductivity (SC). CaFeAs2 has a structure with staggered CaAs and FeAs layers. While the
FeAs layers are known to be responsible for high T-c superconductivity, we show that with spin orbital
coupling each CaAs layer is a Z(2) topologically nontrivial two-dimensional QSH insulator and the bulk
is a three-dimensional weak topological insulator. In the superconducting state, the edge states in the
CaAs layer are natural one-dimensional topological superconductors. The staggered intercalation of
QSH and SC provides us a unique opportunity to realize and explore physics, such as Majorana modes
and Majorana fermion chains
55 citations
TL;DR: This study replaces single LaO planes with SrO dopant planes using atomic-layer-by-layer molecular beam epitaxy for two-dimensional doping of superconducting oxide systems and demonstrates its power in this field.
Abstract: The exploitation of interface effects turned out to be a powerful tool for generating exciting material properties. Such properties include magnetism, electronic and ionic transport and even superconductivity. Here, instead of using conventional homogeneous doping to enhance the hole concentration in lanthanum cuprate and achieve superconductivity, we replace single LaO planes with SrO dopant planes using atomic-layer-by-layer molecular beam epitaxy (two-dimensional doping). Electron spectroscopy and microscopy, conductivity measurements and zinc tomography reveal such negatively charged interfaces to induce layer-dependent superconductivity (Tc up to 35 K) in the space-charge zone at the side of the planes facing the substrate, where the strontium (Sr) profile is abrupt. Owing to the growth conditions, the other side exhibits instead a Sr redistribution resulting in superconductivity due to conventional doping. The present study represents a successful example of two-dimensional doping of superconducting oxide systems and demonstrates its power in this field.
TL;DR: In this article, high temperature superconductivity in one unitcell (1-UC) FeSe films grown on STO(110) substrate by molecular beam epitaxy was reported. And the authors pointed out the important roles of interface related charge transfer and electron-phonon coupling in the high-temperature supercondivity of FeSe/STO.
Abstract: We report high temperature superconductivity in one unit-cell (1-UC) FeSe films grown on STO(110) substrate by molecular beam epitaxy. By in-situ scanning tunneling spectroscopy measurement, we observed a superconducting gap as large as 17 meV. Transport measurements on 1-UC FeSe/STO(110) capped with FeTe layers reveal superconductivity with an onset TC of 31.6 K and an upper critical magnetic field of 30.2 T. We also find that the TC can be further increased by an external electric field, but the effect is smaller than that on STO(001) substrate. The study points out the important roles of interface related charge transfer and electron-phonon coupling in the high temperature superconductivity of FeSe/STO.
TL;DR: In this article, the authors discuss the topological and non-topological flat bands discussed in different systems, and show that graphite is a good candidate for showing high-temperature flat-band interface superconductivity.
Abstract: Superconductivity is traditionally viewed as a low-temperature phenomenon. Within the BCS theory this is understood to result from the fact that the pairing of electrons takes place only close to the usually two-dimensional Fermi surface residing at a finite chemical potential. Because of this, the critical temperature is exponentially suppressed compared to the microscopic energy scales. On the other hand, pairing electrons around a dispersionless (flat) energy band leads to very strong superconductivity, with a mean-field critical temperature linearly proportional to the microscopic coupling constant. The prize to be paid is that flat bands can generally be generated only on surfaces and interfaces, where high-temperature superconductivity would show up. The flat-band character and the low dimensionality also mean that despite the high critical temperature such a superconducting state would be subject to strong fluctuations. Here we discuss the topological and non-topological flat bands discussed in different systems, and show that graphite is a good candidate for showing high-temperature flat-band interface superconductivity.
TL;DR: In this paper, it was shown that hole superconductivity is driven by pairing of heavily dressed hole carriers to lower their kinetic energy, which can be interpreted as a BCS-electron-phonon-driven mechanism.
Abstract: Superconductivity at temperatures up to 190 K at high pressures has recently been observed in H 2 S and interpreted as conventional BCS-electron–phonon-driven superconductivity (Drozdov et al., 2014). Instead we propose that it is another example of the mechanism of hole superconductivity at work. Within this mechanism high temperature superconductivity arises when holes conduct through negatively charged anions in close proximity. We propose that electron transfer from H to S leads to conduction by holes in a nearly full band arising from direct overlap of S = p orbitals in a planar structure. The superconductivity is non-phononic and is driven by pairing of heavily dressed hole carriers to lower their kinetic energy. Possible explanations for the observed lower critical temperature of D 2 S are discussed. We predict that high temperature superconductivity will also be found in other sulfides under high pressure such as Li 2 S, Na 2 S and K 2 S .
Book•
07 Jul 2015
TL;DR: In this article, a much-needed update on complex high-temperature superconductors focusing on materials aspects is presented, which coincides with a recent major break-through of the discovery of iron-based supercondors.
Abstract: A much-needed update on complex high-temperature superconductors, focusing on materials aspects; this timely book coincides with a recent major break-through of the discovery of iron-based superconductors. It provides an overview of materials aspects of high-temperature superconductors, combining introductory aspects, description of new physics, material aspects, and a description of the material properties This title is suitable for researchers in materials science, physics and engineering. Also for technicians interested in the applications of superconductors, e.g. as biomagnets
TL;DR: In this paper, a comparative study of pinning properties in iron-chalcogenides is presented, investigating the flux pinning mechanisms in optimized Fe(SeTe x ) and FeSe samples by currentvoltage characterization, magneto-resistance and magnetization measurements.
Abstract: Among the families of iron-based superconductors, the 11-family is one of the most attractive for high field applications at low temperatures. Optimization of the fabrication processes for bulk, crystalline and/or thin film samples is the first step in producing wires and/or tapes for practical high power conductors. Here we present the results of a comparative study of pinning properties in iron-chalcogenides, investigating the flux pinning mechanisms in optimized Fe(SeTe x ) and FeSe samples by current–voltage characterization, magneto-resistance and magnetization measurements. In particular, from Arrhenius plots in magnetic fields up to 9 T, the activation energy is derived as a function of the magnetic field, whereas the activation energy as a function of temperature, is derived from relaxation magnetization curves. The high pinning energies, high upper critical field versus temperature slopes near critical temperatures, and highly isotropic pinning properties make iron-chalcogenide superconductors a technological material which could be a real competitor to cuprate high temperature superconductors for high field applications.
TL;DR: In this paper, the authors show a route to a low-temperature superconducting state with broken time-reversal symmetry that may accommodate currently conflicting experiments, characterized by an unusual vortex pattern in the form of a necklace of fractional vortices around the perimeter of the material.
Abstract: Conventional superconductors are strong diamagnets that, through the Meissner effect, expel magnetic fields. It would therefore be surprising if a superconducting ground state would support spontaneous magnetics fields. Such time-reversal symmetry-broken states have been proposed for the high-temperature superconductors, but their identification remains experimentally controversial. Here we show a route to a low-temperature superconducting state with broken time-reversal symmetry that may accommodate currently conflicting experiments. This state is characterized by an unusual vortex pattern in the form of a necklace of fractional vortices around the perimeter of the material, where neighbouring vortices have opposite current circulation. This vortex pattern is a result of a spectral rearrangement of current-carrying states near the edges.
TL;DR: In this article, the authors discuss several classes of conventional magnetic superconductors including the ternary rhodium borides and molybdenum chalcogenides, and the quaternary nickel-borocarbides.
Abstract: We discuss several classes of conventional magnetic superconductors including the ternary rhodium borides and molybdenum chalcogenides (or Chevrel phases), and the quaternary nickel-borocarbides. These materials exhibit some exotic phenomena related to the interplay between superconductivity and long-range magnetic order including: the coexistence of superconductivity and antiferromagnetic order; reentrant and double reentrant superconductivity, magnetic field induced superconductivity, and the formation of a sinusoidally-modulated magnetic state that coexists with superconductivity. We introduce the article with a discussion of the binary and pseudobinary superconducting materials containing magnetic impurities which at best exhibit short-range “glassy” magnetic order. Early experiments on these materials led to the idea of a magnetic exchange interaction between the localized spins of magnetic impurity ions and the spins of the conduction electrons which plays an important role in understanding conventional magnetic superconductors. These advances provide a natural foundation for investigating unconventional superconductivity in heavy-fermion compounds, cuprates, and other classes of materials in which superconductivity coexists with, or is in proximity to, a magnetically-ordered phase.
TL;DR: In this paper, the physical properties of the superconducting and normal states of the first family of organic superconductors, the quasi-one dimensional Bechgaard salts (TMTSF)2X, as well as the quasi two dimensional compounds κ -(BEDT-TTF) 2X, were analyzed.
Abstract: Organic conductors were originally considered a route to achieving high temperature superconductivity. While that goal could not be met, what came to be was a class of materials in which the interplay between correlations and dimensionality, and sometimes geometric frustration, lead to a spectacular diversity of phases and phenomena that are tuned by magnetic field, pressure, and temperature. Highlighted here are the physical properties of the superconducting and normal states of the first family of organic superconductors, the quasi-one dimensional Bechgaard salts (TMTSF)2X, as well as the quasi-two dimensional compounds κ -(BEDT-TTF)2X. In both cases, the preponderance of experiments indicate that the superconductivity is nodal. As well, the importance of correlations is evident in the temperature/pressure phase diagrams, and the influence of low-energy magnetic fluctuations over the normal state properties above the superconducting transition temperature is substantial.
TL;DR: In this article, the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO1−xFx with 0.25 ≤ x ≤ 0.75 was reported, where the maximal critical temperature Tc at xopt = 0.5−0.55 is even higher than that at x ≥ 0.2.
Abstract: High-temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO1−xFx (x ≤ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO1−xFx with 0.25 ≤ x ≤ 0.75, where the maximal critical temperature Tc at xopt = 0.5−0.55 is even higher than that at x ≤ 0.2. By nuclear magnetic resonance and transmission electron microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for x > 0.5 above Tc. Our results point to a new paradigm of high temperature superconductivity.
TL;DR: It is shown that the specific heat saturates in high magnetic fields, consistent with a normal state without any significant superconducting contribution and a total Sommerfeld coefficient γN∼6.5±1.5 mJ mol−1 K−2 putting strong constraints on the theoretical models for the Fermi surface reconstruction.
Abstract: The recent discovery of a charge order in underdoped YBa2Cu3Oy raised the question of the interplay between superconductivity and this competing phase. Understanding the normal state of high-temperature superconductors is now an essential step towards the description of the pairing mechanism in those materials and determining the upper critical field is therefore of fundamental importance. We present here a calorimetric determination of the field-temperature phase diagram in underdoped YBa2Cu3Oy single crystals. We show that the specific heat saturates in high magnetic fields. This saturation is consistent with a normal state without any significant superconducting contribution and a total Sommerfeld coefficient γN∼6.5±1.5 mJ mol(-1) K(-2) putting strong constraints on the theoretical models for the Fermi surface reconstruction.
TL;DR: In this paper, the sign change mechanism of the superconducting order parameter (OP) is discussed in the minimal two-orbital model of iron-based superconductors, where the interband Cooper pairs hopping interaction can lead to a change of the sign of the OP regardless of its symmetry.
Abstract: Iron based superconductors are characterized by the gap symmetry, where the gap changes its sign between pockets of the Fermi surface. We discuss another sign change mechanism of the superconducting order parameter (OP)?the interband Cooper pairs hopping interaction. In the minimal two-orbital model of iron based superconductors we show that this interaction can lead to a change of the sign of the intraband superconducting OP regardless of its symmetry.
TL;DR: In this paper, the quasi-adiabatic normal zone propagation velocity and quench energies of a SuperPower SCS4050 copper stabilised ReBCO superconducting tape are presented over a temperature range of 23 − 47 K; in parallel applied magnetic fields of 6, 10 and 14 T; and over a current range from 50% to 100% of Ic.
TL;DR: In this article, angle-resolved photoemission spectroscopy (ARPES) was used to investigate the inherent electronic structure of the NdFeAsO0.6F0.4 compound.
Abstract: In the family of the iron-based superconductors, the REFeAsO-type compounds (with RE being a rare-earth metal) exhibit the highest bulk superconducting transition temperatures (Tc) up to 55 K and thus hold the key to the elusive pairing mechanism. Recently, it has been demonstrated that the intrinsic electronic structure of SmFe0.92Co0.08AsO (Tc = 18 K) is highly nontrivial and consists of multiple band-edge singularities in close proximity to the Fermi level. However, it remains unclear whether these singularities are generic to the REFeAsO-type materials and if so, whether their exact topology is responsible for the aforementioned record Tc. In this work, we use angle-resolved photoemission spectroscopy (ARPES) to investigate the inherent electronic structure of the NdFeAsO0.6F0.4 compound with a twice higher Tc = 38 K. We find a similarly singular Fermi surface and further demonstrate that the dramatic enhancement of superconductivity in this compound correlates closely with the fine-tuning of one of the band-edge singularities to within a fraction of the superconducting energy gap Δ below the Fermi level. Our results provide compelling evidence that the band-structure singularities near the Fermi level in the iron-based superconductors must be explicitly accounted for in any attempt to understand the mechanism of superconducting pairing in these materials.
TL;DR: Perturbative linear-response calculations for C2/m silane at 610 GPa reveal a high superconducting critical temperature that beyond the order of 102 K.
Abstract: Crystal structures of silane have been extensively investigated using ab initio evolutionary simulation methods at high pressures. Two metallic structures with P21/c and C2/m symmetries are found stable above 383 GPa. The superconductivities of metallic phases are fully explored under BCS theory, including the reported C2/c one. Perturbative linear-response calculations for C2/m silane at 610 GPa reveal a high superconducting critical temperature that beyond the order of 102 K.
TL;DR: In this article, an electrochemical co-intercalation of Na and propylene carbonate (PC) into FeSe was performed, and successfully synthesized a new superconductor, Na x(PC) y Fe 2 Se 2, with T c = 43 k.
Abstract: Iron-chalcogenide-based superconductors have attracted much attention due to their relatively high superconducting transition temperatures ( T c ) and their simple layered crystal structures. We have performed electrochemical co-intercalation of Na and propylene carbonate (PC) into FeSe, and successfully synthesized a new superconductor, Na x (PC) y Fe 2 Se 2 , with T c = 43 K. The type and amount of intercalated metal, and the electrolyte used in the intercalation affected the superconductivity. Our electrochemical intercalation method should be a useful tool for discovering new superconductors by controlling the intercalation conditions.
TL;DR: In this article, the authors proposed to apply an additional screen made of a coated superconducting tape in order to increase the magnetic field interval of full shielding region, which was created in Matlab and validated experimentally.
Abstract: Recent advances in the measurements with sensitive magnetic field sensors made the issue of magnetic shielding important. The application of high temperature superconductors allows to obtain full shielding in zero field cooling conditions for DC and low frequency magnetic fields by the means of the Meissner effect. However, currently used conventional bulk magnetic shields maintain full shielding only in low magnetic fields—up to approximately 10 mT. In this paper, it is proposed to apply an additional screen made of a coated superconducting tape in order to increase the magnetic field interval of full shielding region. Computer model of such set of screens was created in Matlab and validated experimentally. Improvement of shielding quality was observed experimentally and calculated with the model.
TL;DR: In this paper, the thickness-dependent strain-relaxation behavior and the associated impacts upon the superconductivity in epitaxial La1.85Sr0.15CuO4 films were investigated.
Abstract: We report the thickness-dependent strain-relaxation behavior and the associated impacts upon the superconductivity in epitaxial La1.85Sr0.15CuO4 films grown on different substrates, which provide a range of strain. We have found that the critical thickness for the onset of superconductivity in La1.85Sr0.15CuO4 films is associated with the finite thickness effect and epitaxial strain. In particular, thin films with tensile strain greater than ∼0.25% revealed no superconductivity. We attribute this phenomenon to the inherent formation of oxygen vacancies that can be minimized via strain relaxation.
TL;DR: In this paper, the authors reported the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO${1-x}$F$_{x} with 0.75, where the maximal critical temperature $T_c$ at $x{opt}$ = 0.5.
Abstract: High temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO$_{1-x}$F$_{x}$ ($x \leq$ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO$_{1-x}$F$_{x}$ with 0.25$\leq x \leq$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ = 0.5$\sim$0.55 is even higher than that at $x \leq$ 0.2. By nuclear magnetic resonance and Transmission Electron Microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for $x>$ 0.5 above $T_c$. Our results point to a new paradigm of high temperature superconductivity.
TL;DR: The theory of emergent gauge fields in insulators was studied in this paper, where the existence of long-lived electron-like quasiparticles around a Fermi surface enclosing a volume determined by the total density of electrons was investigated.
Abstract: The quantum entanglement of many states of matter can be represented by electric and magnetic fields, much like those found in Maxwell's theory. These fields "emerge" from the quantum structure of the many-electron state, rather than being fundamental degrees of freedom of the vacuum. I review basic aspects of the theory of emergent gauge fields in insulators in an intuitive manner. In metals, Fermi liquid theory relies on adiabatic continuity from the free electron state, and its central consequence is the existence of long-lived electron-like quasiparticles around a Fermi surface enclosing a volume determined by the total density of electrons, via the Luttinger theorem. However long-range entanglement and emergent gauge fields can also be present in metals. I focus on the "fractionalized Fermi liquid" (FL*) state, which also has long-lived electron-like quasiparticles around a Fermi surface; however the Luttinger theorem on the Fermi volume is violated, and this requires the presence of emergent gauge fields, and the associated loss of adiabatic continuity to the free electron state. Finally, I present a brief survey of some recent experiments in the hole-doped cuprate superconductors, and interpret the properties of the pseudogap regime in the framework of the FL* theory.