The interplay between different ordered phases, such as superconducting, charge or spin ordered phases, is of central interest in condensed matter physics. The very recent discovery of superconductivity with a remarkable T$_c$= 26 K in Fe-based oxypnictide La(O$_{1-x}$F$_x$)FeAs is a surprise to the scientific community\cite{Kamihara08}. The pure LaOFeAs itself is not superconducting but shows an anomaly near 150 K in both resistivity and dc magnetic susceptibility. Here we provide combined experimental and theoretical evidences showing that the anomaly is caused by the spin-density-wave (SDW) instability, and electron-doping by F suppresses the SDW instability and recovers the superconductivity. Therefore, the La(O$_{1-x}$F$_x$)FeAs offers an exciting new system showing competing orders in layered compounds.
Using the unrestricted Hartree-Fock approximation and Landau theory we identify possible phases competing with superconductivity in FeAs layers. We find that close to half-filling the transition from the paramagnet to the magnetically ordered phase is first-order making anharmonicities relevant and leading to a rich phase diagram. Between the already known one dimensionally modulated magnetic stripe phase and the paramagnet we find a new phase which has the same structure factor as the former but in which magnetic moments at nearest-neighbor sites are at right angles making electrons to acquir…
We use inelastic neutron scattering to show that long-range spin waves arising from the static bicollinear antiferromagnetic (AF) order in FeTe, which have twofold rotational symmetry in a fully detwinned crystal, rapidly dissolve above $E\approx 26$ meV into ridges of scattering with fourfold rotational symmetry and a nearly isotropic magnetic fluctuation spectrum. With increasing temperature above $T_N\approx 68$ K, the twofold spin waves change into broad regions of scattering with fourfold symmetry. Since the scattering patterns from plaquette magnetic order generated within a bilinear biq…
A series of layered CeO$_{1-x}$F$_x$FeAs compounds with x=0 to 0.20 are synthesized by solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F-doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. The very high superconducting transition temperature strongly challenges the classic BCS theory based on the electron-phonon interaction. The very closeness of the …
The Kondo lattice model describes a quantum phase transition between the antiferromagnetic state and heavy-fermion states. Applying the dual-fermion approach, we explore possible superconductivities emerging due to the critical antiferromagnetic fluctuations. The d-wave pairing is found to be the leading instability only in the weak-coupling regime. As the coupling is increased, we observe a change of the pairing symmetry into a p-wave spin-singlet pairing. The competing superconductivities are ascribed to crossover between small and large Fermi surfaces, which occurs with formation of heavy q…
Copper oxides become superconductors rapidly upon doping with electron holes, suggesting a fundamental pairing instability. The Cooper mechanism explains normal superconductivity as an instability of a fermi-liquid state, but high-temperature superconductors derive from a Mott-insulator normal state, not a fermi liquid. We show that precocity to pair condensation with doping is a natural property of competing antiferromagnetism and d-wave superconductivity on a singly-occupied lattice, thus generalizing the Cooper instability to doped Mott insulators, with significant implications for the high…
To understand the origin of unconventional charge-density-wave (CDW) states in cuprate superconductors, we establish the self-consistent CDW equation, and analyze the CDW instabilities based on the realistic Hubbard model, without assuming any $q$-dependence and the form factor. Many higher-order many-body processes, which are called the vertex corrections, are systematically generated by solving the CDW equation. When the spin fluctuations are strong, the uniform $q=0$ nematic CDW with $d$-form factor shows the leading instability. The axial nematic CDW instability at $q = Q_a = (δ,0)$ ($δ\ap…
It has long been known that two-dimensional metals with antiferromagnetic exchange interactions have a weak-coupling instability to the superconductivity of spin-singlet, d-wave electron pairs. We examine additional possible instabilities in the spin-singlet particle-hole channel, and study their interplay with superconductivity. We perform an unrestricted Hartree-Fock-BCS analysis of bond order parameters in a single band model on the square lattice with nearest-neighbor exchange and repulsion, while neglecting on-site interactions. The dominant particle-hole instability is found to be an inc…
The representative cuprate, La$_{2-x}$M$_x$CuO$_4$, with M = Sr and $x = 1/8$ is studied via first-principles calculations in the high-temperature tetragonal (HTT), low-temperature orthorhombic (LTO), and low-temperature less-orthorhombic (LTLO) structures. By suppressing the magnetism and superconductivity, the LTLO phase, which has rarely been observed in La$_{2-x}$Sr$_x$CuO$_4$, is found to be the ground state, where the structural phase transitions, HTT$\rightarrow$LTO$\rightarrow$LTLO, can be understood via phonon instability. While the La-O composition is identified to be responsible for…
We develop a theory for light-induced superconductivity in underdoped cuprates in which the competing bond-density wave order is suppressed by driving phonons with light. Close to a bond-density wave instability in a system with a small Fermi surface, such as a fractionalized Fermi liquid, we show that the coupling of electrons to phonons is strongly enhanced at the bond-density wave ordering wavevectors, leading to a strong softening of phonons at these wavevectors. For a model of classical phonons with anharmonic couplings, we show that the combination of strong softening and driving can lea…
By first-principles calculations, we present a doping-dependent phase diagram of LaO{\it M}As ({\it M}=V--Cu) family. It is characterized as antiferromagnetic semiconductor around LaOMnAs side and ferromagnetic metal around LaOCoAs. Both LaOFeAs and LaONiAs, where superconductivity were discovered, are located at the borderline of magnetic phases. Extensive Fermi surface analysis suggests that the observed superconductivity is of electron-type in its origin. We discuss possible pairing mechanisms in the context of competing ferromagnetic phases found in this work and the ferromagnetic spin flu…
At a temperature of roughly 1\,K, \ce{Sr2RuO4} undergoes a transition from a normal Fermi liquid to a superconducting phase. Even while the former is relatively simple and well understood, the superconducting state is not even after 25 years of study. More recently it has been found that critical temperatures can be enhanced by application of uniaxial strain, up to a critical strain, after which it falls off. In this work, we take an `instability' approach and seek for divergences in susceptibilities. This provides an unbiased way to distinguish tendencies to competing ground states. We show t…
We use the weak coupling renormalization group method to examine the interplay between charge-density-wave and s-wave superconducting orders in a quasi-one-dimensional model of electrons interacting with acoustic phonons. The relative stability of both types of order is mapped out at arbitrary nesting deviations and Debye phonon frequency $ω_D$. We singled out a power law increase of the superconducting $T_c\sim ω_D^{0.7}$ from a quantum critical point of charge-density-wave order triggered by nesting alterations. The results capture the key features shown by the proximity between the two type…
We show that the so-called kinks and waterfalls observed in angle-resolved photoemission spectra of La2-xSrxCuO4, a prototypical high-Tc superconducting cuprate, result from the coupling of quasiparticles with two distinct nearly critical collective modes with finite characteristic wave vectors, typical of charge and spin fluctuations near a stripe instability. Both phonon-like charge and spin collective modes are needed to account for the kinked quasiparticle dispersions. This clarifies the long-standing question whether kinks are due to phonons or spin waves and the nature of the bosonic med…
We study the quantum many-body instabilities of interacting electrons with SU(2)$\times$SU(2) symmetry in spin and orbital degrees of freedom on the triangular lattice near van-Hove filling. Our work is motivated by effective models for the flat bands in hexagonal moiré heterostructures like twisted bilayer boron nitride and trilayer graphene-boron nitride systems. We consider an extended Hubbard model including onsite Hubbard and Hund's couplings, as well as nearest-neighbor exchange interactions and analyze the different ordering tendencies with the help of an unbiased functional renormaliza…
We propose a simple effective model to describe FeAs superconductors. This model is based on the assumption of a local spin-density-wave (SDW) order, with its magnetization direction allowed to fluctuate. It is shown that the long-range order with momentum Q=(π,π) is generally unstable in competing with the kinetic energy of the charge carriers. A true weak SDW order is formed in the undoped case with an additional momentum shift Q_s=(π, 0) due to the peculiar Fermi surface nesting. In the doped case, the unstable long-range order driven by kinetic energy can naturally result in a d-wave super…
The phase diagram of the two-dimensional extended one-band U-V-J Hubbard model is considered within a mean-field approximation and two- and many-patch renormalization group (RG) approaches near the van Hove band fillings. At small t' and J>0 mean-field and many-patch RG approaches give similar results for the leading spin-density-wave (SDW) instability, while the two-patch RG approach, which predicts a wide region of charge-flux (CF) phase becomes unreliable due to nesting effect. At the same time, there is a complex competition between SDW, CF phases, and d-wave superconductivity in two- and …
Using \emph{ab initio} band structure methods and DFT+dynamical mean-field theory approach we explore the possible formation of spin and charge stripes in the Ni-O plane of hole-doped infinite-layer nickelates, $R$NiO$_2$. Our results reveal a remarkable instability of the $C$-type $(110)$ spin state with undistorted lattice towards the formation of the spin-density, charge- and bond-disproportionation stripe phases accompanied by in-plane``breathing''-like distortions of the crystal structure. Our work gives a comprehensive picture of competing charge and spin stripe states, with possible fru…
We show that the competition between magnetism and superconductivity can be used to determine the pairing state in the iron arsenides. To this end we demonstrate that the itinerant antiferromagnetic phase (AFM) and the unconventional $s^{+-}$ sign-changing superconducting state (SC) are near the borderline of microscopic coexistence and macroscopic phase separation, explaining the experimentally observed competition of both ordered states. In contrast, conventional $s^{++}$ pairing is not able to coexist with magnetism. Expanding the microscopic free energy of the system with competing orders …
On Degeneracies of Density, Magnetic, and Pairing Responses: How Competing Orders Echo Underlying Symmetries in the Hubbard Model
Max-Planck-Institut für Festkörperforschung（马克斯·普朗克固体研究所）；Institute of Solid State Physics, TU Wien（维也纳工业大学固体物理研究所）；Dipartimento di Fisica, Università di Trieste（的里雅斯特大学物理系）
We report a $^{23}$Na and $^{75}$As nuclear magnetic resonance (NMR) investigation of Na$_{x}$FeAs series ($x=1$, 0.9, 0.8) exhibiting a spin-density wave (SDW) order below $T_{\rm SDW}=45$, 50 and 43 K for $x=1$, 0.9, 0.8, respectively, and a bulk superconductivity below $T_c\approx 12$ K for x=0.9. Below $T_{\rm SDW}$, a spin-lattice relaxation reveals the presence of gapless particle-hole excitations in the whole $x$ range, meaning that a portion of the Fermi surface remains gapless. The superconducting fraction as deduced from the bulk susceptibility scales with this portion, while the SDW…
Strong-coupling approach to temperature dependence of competing orders of superconductivity: Possible time-reversal symmetry breaking and nontrivial topology
The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied magnetic field of 9 T, consistent with the proposal that SDW and superconductivity are competing in the laminar materials. While our inelastic measurements offer no constraints on the spin dynamic response from d-wave pairing, an upper limit for the magnetic resonance peak predicted from an extended s-wave pairing mechanism is provided. Our measure…
Magnetism in the orthorhombic metal CaFe$_4$As$_3$ was examined through neutron diffraction for powder and single crystalline samples. Incommensurate (${\bm q}_{\rm m}\approx (0.37-0.39)\times{\bm b}^{\ast}$) and predominantly longitudinally ($\parallel b$) modulated order develops through a 2nd order phase transition at $T_{\rm N}=89.63(6)$ K with a 3D Heisenberg-like critical exponent $β=0.365(6)$. A 1st order transition at $T_2=25.6(9)$ K is associated with the development of a transverse component, locking ${\bm q}_{\rm m}$ to $0.375(2){\bm b}^{\ast}$, and increasing the moments from 2.1(1…
Very recently a new family of layered materials, containing BiS2 planes was discovered to be superconducting at temperatures up to Tc=10 K, raising questions about the mechanism of superconductivity in these systems. Here, we present state-of-the-art first principles calculations that directly address this question and reveal several surprising findings. The parent compound LaOBiS2 possesses anharmonic ferroelectric soft phonons at the zone center with a rather large polarization of $\approx 10 μC/cm^2$, which is comparable to the well-known ferroelectric BiFeO3. Upon electron doping, new unst…
The recently discovered high-temperature superconductivity in doped quaternary iron oxypnictides correlates experimentally with a magnetic instability. We have used first-principles calculations to determine a magnetic phase diagram of ReO$_{1-δ}$FeAs (Re=La--Dy) as a function of the doping $δ$, of the FeAs in-plane lattice constant $a$, and of the distance between the Fe and As planes, that is qualitatively consistent with recent experimental findings on the doping, internal (chemical) and external pressure dependence. The existence of a tricritical point (TCP) in the phase diagram suggests n…
Specific heat, resistivity, susceptibility and Hall coefficient measurements were performed on high-quality single crystalline Na$_{1-δ}$FeAs. This compound is found to undergo three successive phase transitions at around 52, 41, and 23 K, which correspond to structural, magnetic and superconducting transitions, respectively. The Hall effect result indicates the development of energy gap at low temperature due to the occurrence of spin-density-wave instability. Our results provide direct experimental evidence of the magnetic ordering in the nearly stoichiometric NaFeAs.
We performed optical spectroscopy measurement on single crystals of BaFe$_2$As$_2$ and SrFe$_2$As$_2$, the parent compounds of FeAs based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the spin-density-wave (SDW) state, formation of partial energy gaps was clearly observed with the presence of surprisingly two different energy scales. A large part of the Drude component was removed by the gapping of Fermi surfaces (FS). Meanwhile, the carrier scattering rate was even more dramatically reduced. We elaborate that the S…
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping from a pseudogap state in the underdoped cuprates to a superconducting state at optimal and overdoping reflects an underlying SO(6) instability structure of the (pi,0) saddle points. The pseudogap is probably not associated with superconductivity, but is related to competing nesting instabilities, which are responsible for the stripe phases. We earlier introduced a simple Ansatz of this competition in terms of a pinned Balseiro-Falicov (pBF) model of competing charge density wave and (s-wave) superconductivity. This mod…
We discuss the electronic structure, lattice dynamics and electron-phonon interaction of newly discovered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ using density functional based calculations. A strong Fermi surface nesting at $\mathbf{k}$=($π$,$π$,0) suggests a proximity to charge density wave instability and leads to imaginary harmonic phonons at this $\mathbf{k}$ point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions t…
