Signatures of Electronic Correlations in Optical Properties of LaFeAsO$_{1-x}$F$_x$

Structural and electronic properties of LaFeAsO$_{0.85}$F$_{0.15}$ superconductor modified under neutron irradiation

The effect of atomic disorder induced by neutron irradiation on the crystal structure and electronic states near $E_F$ of the lightly overdoped LaFeAsO$_{0.85}$F$_{0.15}$ ($T_c=21K$) was studied by X-ray diffraction and $^{75}$As NMR. The irradiation of the polycrystalline sample by "moderate" neutron fluence of $Φ=(0;0.5;1.6)\cdot10^{19}cm^{-2}$ at $T=50^{\circ}C$ leads to the suppression of superconductivity. It is shown that neutron irradiation produces an anisotropic expansion of the tetragonal lattice almost due to an increase of the Fe-As distance. A partial loss of the 2D character of t…

Electron-phonon interaction in Fe-based superconductors: Coupling of magnetic moments with phonons in LaFeAsO$_{1-x}$F$_{x}$

The coupling of Fe magnetic moments in LaFeAsO$_{1-x}$F$_{x}$ with the As $A_{1g}$ phonon is calculated. We present first principles calculations of the atomic and electronic structure of LaFeAsO as a function of electron doping. We perform calculations using the virtual crystal approximation as well as supercell calculations with F substitutional impurity atoms. The results validate the virtual crystal approximation for the electronic structure near the Fermi level. Its is found that the electronic density of states at the Fermi level is maximum for x=0.125, enhancing the electron-phonon inte…

Optical spectroscopy study on single crystalline LaFeAsO

Millimeter-sized single crystals of LaFeAsO were grown from NaAs flux and the in-plane optical properties were studied over a wide frequency range. A sizable electronic correlation effect was indicated from the analysis of the free-carrier spectral weight. With decreasing temperature from 300 K, we observed a continuous suppression of the spectral weight near 0.6 eV. But a spin-density-wave gap formation at lower energy scale was seen only in the broken-symmetry state. We elaborate that both the itinerancy and local spin interactions of Fe\emph{3d} electrons are present for the FeAs-based syst…

Anisotropic electrical resistivity of LaFeAsO: evidence for electronic nematicity

Single crystals of LaFeAsO were successfully grown out of KI flux. Temperature dependent electrical resistivity was measured with current flow along the basal plane, ρ_perpend(T), as well as with current flow along the crystallographic c-axis, ρ_parallel(T), the latter one utilizing electron beam lithography and argon ion beam milling. The anisotropy ratio was found to lie between ρ_parallel/ρ_perpend = 20 - 200. The measurement of ρ_perpend(T) was performed with current flow along the tetragonal [1 0 0] direction and along the [1 1 0] direction and revealed a clear in-plane anisotropy already…

Dynamical Mean-Field Theory within an Augmented Plane-Wave Framework: Assessing Electronic Correlations in the Iron Pnictide LaFeAsO

We present an approach that combines the local density approximation (LDA) and the dynamical mean-field theory (DMFT) in the framework of the full-potential linear augmented plane waves (FLAPW) method. Wannier-like functions for the correlated shell are constructed by projecting local orbitals onto a set of Bloch eigenstates located within a certain energy window. The screened Coulomb interaction and Hund's coupling are calculated from a first-principle constrained RPA scheme. We apply this LDA+DMFT implementation, in conjunction with continuous-time quantum Monte-Carlo, to study the electroni…

Coulomb Parameter U and Correlation Strength in LaFeAsO

First principles constrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund's exchange J parameters for iron 3d electrons in LaFeAsO. Results strongly depend on the basis set used in calculations: when O-2p, As-4p, and Fe-3d orbitals and corresponding bands are included, computation results in U=3-4 eV, however, with the basis set restricted to Fe-3d orbitals and bands only, computation gives parameters corresponding to F^0=0.8 eV, J=0.5 eV. LDA+DMFT (the Local Density Approximation combined with the Dynamical Mean-Field …

Crystalline anisotropy induces a second antiferromagnetic phase in the absence of SDW in the heavily hydrogen-doped LaFeAsO$_{1-x}$H$_x$ $(x\sim0.5 )$

Electronic and magnetic properties of the heavily H-doped LaFeAsO$_{1-x}$H$_x$ $(x\sim0.5 )$ were studied in the framework of the density functional theory combined with the dynamical mean field theory (DFT+DMFT). We found a stripe-like-ordered structure of hydrogen and oxygen atoms, as a ground state, with the same configuration as the antiferromagnetic (AF) order. The new configuration could explain the existing experimental results related to the heavily H-doped LaFeAsO$_{1-x}$H$_x$, such as an in-plane electronic anisotropy and a non-uniform magnetic behavior. A significant anisotropy was …

Electron-conduction mechanism and specific heat above transition temperature in LaFeAsO and BaFe(2)As(2) superconductors

The ionization energy theory is used to calculate the evolution of the resistivity and specific heat curves with respect to different doping elements in the recently discovered superconducting Pnictide materials. Subsequently, the electron-conduction mechanism in the Pnictides above the structural transition temperature is explained unambiguously, which is also consistent with other strongly correlated matter, such as cuprates, manganites, titanates and magnetic semiconductors. Therefore, the superconductivity is not uniquely corresponds to the electronic properties above the structural transi…

Electronic Structure of Novel Superconductor Ca4Al2O6Fe2As2

We have performed the first-principles electronic structure calculation for the novel superconductor Ca4Al2O6Fe2As2 which has the smallest a lattice parameter and the largest As height from the Fe plane among the Fe-As superconductors. We find that one of the hole-like Fermi surfaces is missing around the Gamma point compared to the case of LaFeAsO. Analysis using the maximally-localized-Wannier-function technique indicates that the xy orbital becomes more localized as the As-Fe-As angle decreases. This induces rearrangement of bands, which results in the change of the Fermi-surface topology o…

LDA + Negative U Solves a Puzzle of too Large Calculated Magnetic Moment in Iron-based Superconductor LaFeAsO_{1-x}F_x

A puzzle in the iron-based superconductor LaFeAsO_{1-x}F_x is that the magnetic moment obtained by first-principle electronic structure calculations is unexpectedly much larger than the experimentally observed one. For example, the calculated value is ~ 2.0 μ_B in the mother compound, while it is ~ 0.3 μ_B in experiments. We find that the puzzle is solved within the framework LDA + U by expanding the U value into a slightly negative range. We show U dependence of the obtained magnetic moment in both the undoped x=0.0 and doped x = 0.125. These results reveal that the magnetic moment is drastic…

Two superconducting gaps in LaFeAsO0.92F0.08 revealed by 75As nuclear quadrupole resonance

We report $^{75}$As nuclear quadrupole resonance (NQR) studies on superconducting oxypnictide LaFeAsO$_{0.92}$F$_{0.08}$ ($T_{\rm c}$ = 23 K). The temperature dependence of the spin lattice relaxation rate (1/$T_1$) decreases below $T_{\rm c}$ without a coherence (Hebel-Slichter) peak and shows a temperature dependence that is not simple power-law nor exponential. We show that the result can be understood in terms of two superconducting gaps of either $d$- or ${\pm}s$-wave symmetry, with the larger gap $Δ_1\sim 4 k_{\rm B}T_{\rm c}$ and the smaller one $Δ_2 \sim 1.5 k_{\rm B}T_{\rm c}$. Our re…

Coulomb repulsion and correlation strength in LaFeAsO from Density Functional and Dynamical Mean-Field Theories

LDA+DMFT (Local Density Approximation combined with Dynamical Mean-Field Theory) computation scheme has been used to calculate spectral properties of LaFeAsO -- the parent compound for new high-T$_c$ iron oxypnictides. Coulomb repulsion $U$ and Hund's exchange $J$ parameters for iron 3d electrons were calculated using \textit {first principles} constrained density functional theory scheme in Wannier functions formalism. Resulting values strongly depend on the number of states taken into account in calculations: when full set of O-$2p$, As-$4p$, and Fe-3d orbitals with corresponding bands are i…

Fermi-Suface Evolution by Transition-metal Substitution in the Iron-based Superconductor LaFeAsO

We study how Co- and Ni-substitution affect the electronic structure of the iron-based superconductor, LaFeAsO. We perform {\it ab initio} supercell calculations and unfold the first Brillouin zone (BZ) to calculate the spectral function in the BZ for the normal cell. The charge density distribution in real space shows that doped extra electrons are trapped around Co (Ni) atom. This seems to mean that Co(Ni)-substitution does not work as carrier doping. However, the present momentum-space analysis indicates that the Fermi-surface volume indeed expands by substitutions, which can be well descri…

The intrinsic electronic phase diagram of iron-oxypnictide superconductors

We present the first comprehensive derivation of the intrinsic electronic phase diagram of the iron-oxypnictide superconductors in the normal state based on the analysis of the electrical resistivity $ρ$ of both LaFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$ for a wide range of doping. Our data give clear-cut evidence for unusual normal state properties in these new materials. In particular, the emergence of superconductivity at low doping levels is accompanied by distinct anomalous transport behavior in $ρ$ of the normal state which is reminiscent of the spin density wave (SDW) signature in th…

Large decrease in the critical temperature of superconducting LaFeAsO0.85 compounds doped with 3% atomic weight of nonmagnetic Zn impurities

We observed a large decrease of Tc by no more than 3 at.% of Zn doped to the optimized superconductor LaFeAsO0.85 (Tc = 26 K), confirmed by measurements of electrical resistivity, magnetic susceptibility, specific heat, Mössbauer spectroscopy, Hall coefficient, and an electron probe micro-analysis. The rate ~9 K/% is remarkably higher than observations regarding nonmagnetic impurities. The Tc suppression is likely due to pair-breaking caused by scatterings associated with highly localized electronic state of Zn doped into the Fe2As2 layer. If this is true, the Zn result well accords with the t…

Momentum dependent $d_{xz/yz}$ band splitting in LaFeAsO

We performed angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of the nematic phase in LaFeAsO. Degeneracy breaking between the dxz and dyz hole bands near the Γ and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the Γ and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the Γ point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo devi…

Crystallographic Phase Transition and High-Tc Superconductivity in LaFeAsO:F

Undoped LaFeAsO, parent compound of the newly found high-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160 K. The anomaly can be suppressed by F doping and the superconductivity appears correspondingly, suggesting a close associate of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and superconductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO (Tc = 20 K) by synchrotron X-ray diffraction, DC magnetic measurements, and ab initio calculations to demonstrate that the anomaly is associated …

Multiple-pseudogap phases in hydrogen-doped LaFeAsO system

The low energy electronic structure of LaFeAsO1-xHx (0.0 < x < 0.60), the system which exhibits two superconducting domes in its phase diagram, is investigated by utilizing the laser photoemission spectroscopy. From the precise temperature-dependent measurement of the spectra near the Fermi level, we find the suppression of the density of states with cooling, namely the pseudogap formation, for all doping range. The pseudogap in the low x range (i.e. the first superconducting dome regime) gets suppressed with increasing x, more or less similarly to the previous results in F-doped LaFeAsO syste…

Unusual Pseudogap-like Features Observed in Iron Oxypnictide Superconductors

We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical transition temperatures (Tc's) of 26 K and 5 K, respectively. We find that high-Tc LaFeAsO:F exhibits a temperature-dependent pseudogap-like feature extending over ~0.1 eV about the Fermi level at 250 K, whereas such a feature is absent in low-Tc LaFePO:F. We also find ~20-meV pseudogap-like features and signatures of superconducting gaps both in LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogap-like f…

The challenge of unravelling magnetic properties in LaFeAsO

First principles calculations of magnetic and, to a lesser extent, electronic properties of the novel LaFeAsO-based superconductors show substantial apparent controversy, as opposed to most weakly or strongly correlated materials. Not only do different reports disagree about quantitative values, there is also a schism in terms of interpreting the basic physics of the magnetic interactions in this system. In this paper, we present a systematic analysis using four different first principles methods and show that while there is an unusual sensitivity to computational details, well-converged full-…

Influence of electronic correlations on orbital polarizations in the parent and doped iron pnictides

Orbital polarization and electronic correlation are two essential aspects in understanding the normal state and superconducting properties of multiorbital FeAs-based superconductors. In this Letter, we present a systematical study on the orbital polarization of iron pnictides from weak to strong Coulomb correlations within the KRSB approach. The magnetic phase diagram of the two-orbital model for LaFeAsO clearly shows that the striped antiferromagnetic metallic phase with considerable orbital polarization exists over a wide doping range. The reversal of the orbital polarization occurs in the i…

单晶LaFeAsO在同时Sm和P替代下的超导性涌现

Emergence of superconductivity in single-crystalline LaFeAsO under simultaneous Sm and P substitution

本文通过高压生长Sm和P共替代的单晶LaFeAsO，发现较小尺寸等价替代产生的内部化学压力诱导了超导性，并详细表征了其超导态参数，为按需设计超导体提供了新途径。

LaFeAsO$_{1-x}M_x$（$M={\rm F,H}$）第二超导穹顶中混合态线宽的异常场演化

Anomalous field evolution of the mixed-state linewidth in the second superconducting dome of LaFeAsO$_{1-x}M_x$ ($M={\rm F,H}$)

通过横向场μ子自旋旋转/弛豫研究LaFeAsO$_{1-x}M_x$混合态内场分布，发现低于$T_{\rm c}$时线宽增加，高于$T_{\rm c}$时与场有关。减去正常态贡献后，SC1和SC2样品超导线宽场依赖性不同，SC2有异常场演化，揭示了两超导穹顶不同电子特性。

Heat capacity measurements on FeAs-based compounds: a thermodynamic probe of electronic and magnetic states

We report heat capacity measurements of the pnictide materials SmFeAsO$_{1-x}$F$_x$, NdFeAsO, LaFeAsO$_{1-x}$F$_x$ and LiFeAs. For SmFeAsO$_{1-x}$F$_x$, with x close to 0.1, we use 3 He measurements to demonstrate a transfer of entropy from the peak at TN to a previously unidentified ~2 K feature which grows with increasing doping. Our results on the Sm samples are compared with a similarly doped La sample to elucidate the crystal field levels of the Sm3+ ion at 0, 23, and 56 meV which lead to a Schottky-like anomaly, and also show that there is a significant increase in the Sommerfeld coeffic…

Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor

In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc=55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-base…

Anisotropic spin and charge excitations in superconductors: signature of electronic nematic order

We study spin and charge susceptibilities in the d-wave superconducting state whose underlying electronic dispersion is anisotropic due to the formation of the electronic nematic order. We show that the amplitude of the incommensurate peaks in the spin susceptibility near (π,π) reveals a pronounced anisotropy in the momentum space. The relevance of our findings to the magnetic scattering pattern observed in a recent neutron scattering measurement on untwinned YBa_2Cu_3O_{6+x} is discussed. In the charge channel, we identify a well-defined collective mode at small momentum transfer with strong …

Multiband one-dimensional electronic structure and spectroscopic signature of Tomonoga-Luttinger liquid behavior in K$_2$Cr$_3$As$_3$

We present Angle-Resolved Photoemission Spectroscopy measurements of the quasi-one dimensional superconductor K$_2$Cr$_3$As$_3$. We find that the Fermi surface contains two Fermi surface sheets, with linearly dispersing bands not displaying any significant band renormalizations. The one-dimensional band dispersions display a suppression of spectral intensity approaching the Fermi level according to a linear power law, over an energy range of ~200 meV. This is interpreted as a signature of Tomonoga-Luttinger liquid physics, which provides a new perspective on the possibly unconventional superco…

Signatures of unconventional pairing in near-vortex electronic structure of LiFeAs

A major question in Fe-based superconductors remains the structure of the pairing, in particular whether it is of unconventional nature. The electronic structure near vortices can serve as a platform for phase-sensitive measurements to answer this question. By solving Bogoliubov-de Gennes equations for LiFeAs, we calculate the energy-dependent local electronic structure near a vortex for different nodeless gap-structure possibilities. At low energies, the local density of states (LDOS) around a vortex is determined by the normal-state electronic structure. However, at energies closer to the ga…

A Novel Dielectric Anomaly in Cuprates and Nickelates: Signature of an Electronic Glassy State

The low-frequency dielectric response of hole-doped insulators La_{2}Cu_{1-x}Li_{x}O_{4} and La_{2-x}Sr_{x}NiO_{4} shows a large dielectric constant ε^{'} at high temperature and a step-like drop by a factor of 100 at a material-dependent low temperature T_{f}. T_{f} increases with frequency and the dielectric response shows universal scaling in a Cole-Cole plot, suggesting that a charge glass state is realized both in the cuprates and in the nickelates.

Effect of pressure cycling on Iron: Signatures of an electronic instability and unconventional superconductivity

High pressure electrical resistivity and x-ray diffraction experiments have been performed on Fe single crystals. The crystallographic investigation provides direct evidence that in the martensitic $bcc \rightarrow hcp$ transition at 14 GPa the $\lbrace 110\rbrace_{bcc}$ become the $\lbrace 002\rbrace_{hcp}$ directions. During a pressure cycle, resistivity shows a broad hysteresis of 6.5 GPa, whereas superconductivity, observed between 13 and 31 GPa, remains unaffected. Upon increasing pressure an electronic instability, probably a quantum critical point, is observed at around 19 GPa and, clos…

Anisotropic signatures of the electronic correlations in the electrical resistivity of UTe$_2$

Multiple unconventional superconducting phases are suspected to be driven by magnetic fluctuations in the heavy-fermion paramagnet UTe$_2$, and a challenge is to identify the signatures of the electronic correlations, including the magnetic fluctuations, in the bulk physical quantities. Here, we investigate thoroughly the anisotropy of the electrical resistivity of UTe$_2$ under intense magnetic fields up to 70~T, for different electrical-current and magnetic-field configurations. Two characteristic temperatures and an anisotropic low-temperature Fermi-liquid-like coefficient $A$, controlled b…

Electronic Structure of Electron-doped Sm1.86Ce0.14CuO4: Strong `Pseudo-Gap' Effects, Nodeless Gap and Signatures of Short Range Order

Angle resolved photoemission (ARPES) data from the electron doped cuprate superconductor Sm$_{1.86}$Ce$_{0.14}$CuO$_4$ shows a much stronger pseudo-gap or "hot-spot" effect than that observed in other optimally doped $n$-type cuprates. Importantly, these effects are strong enough to drive the zone-diagonal states below the chemical potential, implying that d-wave superconductivity in this compound would be of a novel "nodeless" gap variety. The gross features of the Fermi surface topology and low energy electronic structure are found to be well described by reconstruction of bands by a $\sqrt{…

The Puzzle of High Temperature Superconductivity in Layered Iron Pnictides and Chalcogenides

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at Tc = 26 K in the Fe-based compound LaFeAsO_{1-x}F_x has been very enthusiastic. In short order, other Fe-based superconductors with the same or related crystal structures were discovered with Tc up to 56 K. Many experiments were carried out and theories formulated to try to understand the basic properties of these new materials and the mechanism for Tc. In this selective critical review of the experimental literature, we distill some of this extensive body of work, and discuss relationships betw…

Logarithmic Upturn in Low-Temperature Electronic Transport as a Signature of d-Wave Order in Cuprate Superconductors

In cuprate superconductors, high magnetic fields have been used extensively to suppress superconductivity and expose the underlying normal state. Early measurements revealed insulating-like behavior in underdoped material versus temperature $T$, in which resistivity increases on cooling with a puzzling $\log(1/T)$ form. We instead use microwave measurements of flux-flow resistivity in YBa$_2$Cu$_3$O$_{6+y}$ and Tl$_2$Ba$_2$CuO$_{6+δ}$ to study charge transport deep inside the superconducting phase, in the low temperature and low field regime. Here, the transition from metallic low-temperature …

Systematic Study on Fluorine-doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx

We have investigated the fluorine-doping dependence of lattice constants, transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly and monotonically decreases the in-plane lattice parameter. In the normal state, electrical resistivity at low temperature is proportional to the square of temperature and the electronic specific heat coefficient has large value, indicating the existence of moderate electron-electron correlation in this system. Hall coefficient has large magnitude, and shows large temperature dependence, indicating the low carrier density and multiple carrier…

Roton Induced Modulations in Underdoped Cuprates as a Signature of Incipient Electronic Order

In the last years there have been measurements of energy independent modulations in underdoped cuprates via scanning tunelling microscopy. These modulations of around 4 lattice constants occur near vortex cores in samples with and without superconductivity, and in the pseudogap regime of some superconducting samples. Recently, it was proposed that the origin of the modulations is electronic charge ordered phases intrinsic to doped Mott insulators. We show that even if the system has not charge ordered, proximity to such an order will induce modulations at the same ordering wavevector. Such mod…

Strong Electronic Interaction and Signatures of Nodal Superconductivity in Zr$_5$Pt$_3$C$_x$

The physical properties of the Zr$_5$Pt$_3$ compound with interstitial carbon in hexagonal D8$_8$-structure was investigated. A set of macroscopic measurements reveal a bulk superconducting at approximately 7 K for Zr$_5$Pt$_3$C$_{0.3}$ close to Zr$_5$Pt$_3$, also with a correlate anomalous resistivity behavior. However, both the signatures of strong electron-electron interaction, and the electronic contribution to specific heat, increase dramatically with the C doping. For the first time the x-ray photoelectron spectra compared with DFT/PWLO calculations of electronic structure show a complex…

通过共振非弹性X射线散射探测电荷响应函数：YBa$_2$Cu$_3$O$_{7-δ}$电子能隙的特征

Charge response function probed by resonant inelastic x-ray scattering: the signature of electronic gaps of YBa$_2$Cu$_3$O$_{7-δ}$

本文提出从欠掺杂YBa$_2$Cu$_3$O$_{7-δ}$的Cu L$_3$边RIXS谱温度依赖性中提取电荷动力学磁化率信息，通过低能玻色激发连续谱偏离统计温度依赖性的行为识别超导能隙和赝能隙，为系统研究铜氧化物赝能隙本质提供了不受表面敏感限制的新方法。

Signatures of a momentum independent pseudogap in the electronic density of states and Raman spectroscopy of the underdoped cuprates

We propose a hybridization phenomenology to describe the pseudogap state of the underdoped cuprates. We show how a momentum independent pseudogap opens asymmetrically from the Fermi-surface but symmetric to the zeroes of the hybridized bonding dispersion, which results in false d-wave characteristics of the pseudogap at the Fermi level. By comparing against a d-wave form factor we illustrate the difficulty in identifying a momentum independent order in momentum averaged quantities such as the electronic Raman response. We identify a suppression in the single-particle density of states which pr…

Electronic and vibrational signatures of ruthenium vacancies in Sr$_2$RuO$_4$ thin films

The synthesis of stoichiometric Sr$_2$RuO$_4$ thin films has been a challenge because of the high volatility of ruthenium oxide precursors, which gives rise to ruthenium vacancies in the films. Ru vacancies greatly affect the transport properties and electronic phase behavior of Sr$_2$RuO$_4$, but their direct detection is difficult due to their atomic dimensions and low concentration. We applied polarized X-ray absorption spectroscopy at the oxygen K-edge and confocal Raman spectroscopy to Sr$_2$RuO$_4$ thin films synthesized under different conditions. The results show that these methods can…

Signatures of a Quantum Griffiths Phase close to an Electronic Nematic Quantum Phase Transition

In the vicinity of a quantum critical point, quenched disorder can lead to a quantum Griffiths phase, accompanied by an exotic power-law scaling with a continuously varying dynamical exponent that diverges in the zero-temperature limit. Here, we investigate a nematic quantum critical point in the iron-based superconductor FeSe$_{0.89}$S$_{0.11}$ using applied hydrostatic pressure. We report an unusual crossing of the magnetoresistivity isotherms in the non-superconducting normal state which features a continuously varying dynamical exponent over a large temperature range. We interpret our resu…

Neutron-irradiation effects in LaO0.9F0.1FeAs superconductor

The effect of atomic disorder induced by neutrons irradiation on superconducting and normal state properties of polycrystalline LaFeAsO_0.9F_0.1 was investigated. The irradiation of the sample by a moderate neutron fluence F = 1.6*1019 cm^-2 at Tirr = 50 +- 10 C leads to the suppression of superconductivity which recovers almost completely after annealing at temperatures Tann < 750 C. It is shown that the reduction of superconducting transition temperature Tc under atomic disordering is not determined solely by the value of Hall concentration nH, i.e. doping level, but is governed by the reduc…

剧烈的磁场诱导电阻上升作为铁基超导体中非常规磁性特征的标志

Drastic field-induced resistivity upturns as signatures of unconventional magnetism in superconducting iron chalcogenides

本研究通过高压和磁场调控揭示了铁基超导体中非常规磁性与超导性的复杂相互作用。

Indications for Topological Physics in High-$T_c$ Materials

Research on high-$T_c$ superconductors has generally not focused on analysis of the topological structure of electronic bands in these materials. In this article we collate and discuss several well-known experimental observables that signal that certain signatures of topology may be present. The topological signatures suggested by experiment include dimensional reduction, electronic transport determined by loop statistics, a winding number, Weyl fermions, and others. We suggest that topology, in a wider sense than band topology, may be key to the mechanism of high-$T_c$ superconductivity.

Thermoelectric signature of the nematic phase in iron-based superconductor

Studies of the copper-based superconductors demonstrate how their phase diagram becomes more complex as experimental probes improve, able to distinguish among subtly different electronic phases. One of those phases, nematicity, has become the matter of great interest also in the iron-based superconductors, where it is detected deep in the tetragonal state. Here we present the evolution of the in-plane Nernst effect anisotropy in the strain detwinned Ca(Fe1-xCox)2As2 single crystals, whose behaviour can be explained within the approach developed to describe the nematic order parameter in liquid…

Electronic Raman Scattering and Phonon Self-Energy Effects in the R-123 System: Signatures of Gap and Pseudogap

Using Raman spectroscopy and transport measurements we investigate thin epitaxial films of Y{1-x}(Pr,Ca)xBa2Cu3O{6+y}. We explore the electronic Raman responses obtained after subtraction of phononic excitations, and the 2Delta peaks that form out of the electronic background below Tc. We find that the energy of the B1g 2Delta peak increases monotonically with decreasing doping until the peaks become unresolvable. In contrast, the peaks in A1g symmetry follow Tc being resolvable in the Pr-doped films. The B2g responses are weak and a 2Delta peak is only detected at the highest doping level. As…

Band inversion and topology of the bulk electronic structure in FeSe${}_{0.45}$Te${}_{0.55}$

FeSe${}_{0.45}$Te${}_{0.55}$ (FeSeTe) has recently emerged as a promising candidate to host topological superconductivity, with a Dirac surface state and signatures of Majorana bound states in vortex cores. However, correlations strongly renormalize the bands compared to electronic structure calculations, and there is no evidence for the expected bulk band inversion. We present here a comprehensive angle resolved photoemission (ARPES) study of FeSeTe as function of photon energies ranging from 15 - 100 eV. We find that although the top of bulk valence band shows essentially no $k_z$ dispersion…

扭曲三层石墨烯中热力学关联特征与超导性之间的联系

Link between thermodynamic correlation signatures and superconductivity in twisted trilayer graphene

研究扭曲三层石墨烯中热力学关联特征与超导性的联系，结合局部探针与输运测量，发现超导性与关联绝缘体无直接关联，其临界温度与压缩性锯齿强度密切相关，揭示两者联系及对层间扭曲角的依赖。

在硼掺杂超薄碳纳米管网络中高温度超导性的巨压效应特征

Signatures of a high-temperature collective electronic phase with superconductivity-like characteristics and a giant pressure effect in networks of boron-doped ultrathin carbon nanotubes

研究通过硼掺杂超薄碳纳米管网络在常压下发现高温度超导性，压力效应显著提升临界温度并改变电阻特性。

无限层镍酸盐中强关联和多带超导性的光学电导特征

Optical conductivity signatures of strong correlations and multiband superconductivity in infinite-layer nickelates

研究通过光学电导揭示无限层镍酸盐中多带超导性和强关联电子结构的特征。