When either electron or hole doped at concentrations $x\sim 0.1$, the LaOFeAs family displays remarkably high temperature superconductivity with T$_c$ up to 55 K. In the most energetically stable $\vec Q_M = (π,π)$ antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy, and very strong magnetophonon coupling is uncovered. Doping (of either sign) beyond $x \sim 0.1$ results in Fe 3d heavy mass carriers ($m^*\sim 4-8$) with a large Fermi surface. Calculated Fe-Fe transverse exchange couplings $J_{ij}(R)$ reveal that exchange coupling is strongly dependent on the AFM symmetry and Fe-As distance.
From first-principles calculations, we have studied the electronic and magnetic structures of the ground state of LaOFeAs. The Fe spins are found to be collinear antiferromagnetic ordered, resulting from the interplay between the strong nearest and next-nearest neighbor superexchange antiferromagnetic interactions. The structure transition observed by neutron scattering is shown to be magnetically driven. Our study suggests that the antiferromagnetic fluctuation plays an important role in the Fe-based superconductors. This sheds light on the understanding of the pairing mechanism in these mate…
From first principles calculations we find that the nearest and next nearest neighbor superexchange interactions between Fe ions in LaOFeAs are large, antiferromagnetic (AF), and give rise to a frustrated magnetic ground state which consists of two interpenerating AF square sublattices with M(Fe)=0.48$μ_B$. The system lowers its energy further by removing the frustration via a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaOFeAs recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange inte…
We have studied the newly found superconductor compound LaO$_{1-x}$F$_x$FeAs through the first-principles density functional theory calculations. We find that the parent compound LaOFeAs is a quasi-2-dimensional antiferromgnetic semimetal with most carriers being electrons and with a magnetic moment of $2.3μ_B$ located around each Fe atom on the Fe-Fe square lattice. Furthermore this is a commensurate antiferromagnetic spin density wave due to the Fermi surface nesting, which is robust against the F-doping. The observed superconduction happens on the Fe-Fe antiferromagnetic layer, suggesting a…
We establish the general form of effective interacting Hamiltonian for LaOFeAs system based on the symmetry consideration. The peculiar symmetry property of the electron states yields unusual form of electron-electron interaction. Based on the general effective Hamiltonian, we determine all the ten possible pairing states. More physical considerations would further reduce the list of the candidates for the pairing state.
The electronic and magnetic properties of the mother material LaOFeAs of new superconductors have been carefully studied using first-principles electronic structure calculations based on the generalized gradient approximation in the density functional theory. The present calculation predicts that the ground state of LaOFeAs is antiferromagnetic with a stripe type magnetic moment alignment leading to orthorhombic symmetry of the crystal. In this particular magnetic state, the density of states at the Fermi level is very small. On the other hand, LaOFeP has turned out to be paramagnetic and a go…
All the possible super-conducting order parameters for the LaOFeAs system are classified by their transformation under the complete crystal symmetry. The general forms of the super-conducting gap functions for each class are discussed. We find that the gap functions in such a multi-band system belong to three types, full gap, nodal type and finite {}``Fermi arc'' type. Possible physical consequences caused by different types of gap functions are also discussed.
We present first principles calculations of the atomic and electronic structure of electron-doped LaOFeAs. We find that whereas the undoped compound has an antiferromagnetic arrangement of magnetic moments at the Fe atoms, the doped system becomes non magnetic at a critical electron concentration. We have studied the electron-phonon interaction in the doped paramagnetic phase. For the $A_{1g}$ phonon, the separation between the As and Fe planes induces a non-collinear arrangement of the Fe magnetic moments. This arrangement is anti parallel for interactions mediated by As, and perpendicular fo…
In high-transition temperature (high-Tc) copper oxides, it is generally believed that antiferromagnetism plays a fundamental role in the superconducting mechanism because superconductivity occurs when mobile electrons or holes are doped into the antiferromagnetic parent compounds. The recent discovery of superconductivity in the rare-earth (R) iron-based oxide systems [RO1-xFxFeAs] has generated enormous interest because these materials are the first noncopper oxide superconductors with Tc exceeding 50 K. The parent (nonsuperconducting) LaOFeAs material is metallic but shows anomalies near 150…
We show that the combined effects of spin-orbit, monoclinic distortion, and p-d hybridization in tetrahedrally coordinated Fe in LaOFeAs invalidates the naive Hund's rule filling of the Fe d-levels. The two highest occupied levels have one electron each but as a result of the p-d hybridization have very different on-site repulsions. As a result, electrons in the upper level are more itinerant while those in the lower level are more localized. It is the xy-projection of the spin in the lower level that orders antiferromagnetically as the z-components of the spins in the two levels is shown to b…
Zero-field (ZF) muon spin relaxation ($μ$SR) measurements have revealed static commensurate magnetic order of Fe moments in NdOFeAs below $T_{N} \sim 135$ K, with the ordered moment size nearly equal to that in LaOFeAs, and confirmed similar behavior in BaFe$_{2}$As$_{2}$. In single crystals of superconducting (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$, $μ$SR spectra indicate static magnetism with incommensurate or short-ranged spin structure in $\sim$ 70 % of volume below $T_{N} \sim$ 80 K, coexisting with remaining volume which exhibits superfluid-response consistent with nodeless gap below $T_…
The structural, electronic, magnetic, and vibrational properties of LaFeSiH$_x$ for $x$ between 0 and 1 are investigated using density functional calculations. We find that the electronic and magnetic properties are strongly controlled by the hydrogen concentration $x$ in LaFeSiH$_x$. While fully hydrogenated LaFeSiH has a striped antiferromagnetic ground state, the underdoped LaFeSiH$_x$ for $x\leq0.75$ is not magnetic within the virtual crystal approximation or with explicit doping of supercells. The antiferromagnetic configuration breaks the symmetry of Fe $d$ orbitals and increases electro…
We calculate the expected finite frequency neutron scattering intensity based on the two-sublattice collinear antiferromagnet found by recent neutron scattering experiments as well as by theoretical analysis on the iron oxypnictide LaOFeAs. We consider two types of superexchange couplings between Fe atoms: nearest-neighbor coupling J1 and next-nearest-neighbor coupling J2. We show how to distinguish experimentally between ferromagnetic and antiferromagnetic J1. Whereas magnetic excitations in the cuprates display a so-called resonance peak at (pi,pi) (corresponding to a saddlepoint in the magn…
Temperature vs. chemical-potential phase diagrams of an SO(5) model for high-(T_c) cuprates are calculated by Monte Carlo simulation. There is a bicritical point where the second-order antiferromagnetism (AF) and superconductivity transition lines merge tangentially into a first-order line, and the SO(5) symmetry is achieved. In an external magnetic field, the AF ordering is first order in the region where the first-order melting line of flux lattice joins in. There is a tricritical point on the AF transition line from which the AF ordering becomes second order.
As reported by Q. Huang et al. [Phys. Rev. Lett. 101, 257003 (2008)], neutron diffraction studies show an onset of antiferromagnetic order in BaFe2As2 associated with a tetragonal-to-orthorhombic distortion. We determine the group Cmca as the space group of antiferromagnetic BaFe2As2 and identify a roughly half-filled energy band of BaFe2As2 with Bloch functions of special symmetry as magnetic band. As explained by the group-theoretical nonadiabatic Heisenberg model, the electrons in this narrow band may lower their Coulomb correlation energy by producing just the experimentally observed antif…
Motivated by recent experimental studies on superconductivity found in nickelate-based materials, we study the temperature dependence of the spin correlation and the superconducting pairing interaction within an effective two-band Hubbard model by the quantum Monte Carlo method. Based on parameters extracted from first-principles calculations, our intensive numerical results reveal that the pairing with a $d_{xy}$-wave symmetry firmly dominates over other pairings at low temperature, which is mainly determined by the Ni 3$d$ orbital. It is also found that the effective pairing interaction is e…
Pairing symmetry in the superconducting state coexisting with antiferromagnetic order is studied based on a microscopic theory. We calculate the linearized Eliashberg's equation within the random phase approximation in the Hubbard model with a staggered field. We find that odd-frequency spin-triplet (equal-spin) s-wave pairing state can be realized. This result contradicts a naive expectation that antiferromagnetic order induces antiferromagnetic spin fluctuation and favors spin-singlet d-wave pairing as in the standard strongly correlated systems.
We present the investigation results of the in-plane \{rho}(T) resistivity tensor at the temperature range 0.4-40 K in magnetic fields up to 90kOe (H||c, J||ab) for electron-doped Nd{2-x}Ce{x}CuO{4+δ} with different degree of disorder near antiferromagnetic - superconducting phase boundary. We have experimentally found that for optimally doped compound both the upper critical field slope and the critical temperature decrease with increasing of the disorder parameter (d-wave pairing) while in the case of the underdoped system the critical temperature remains constant and (dHc2/dT)|Tc increases …
The problem of the symmetry of the superconducting pairing and the form of the gap function in the electron-doped superconductors is reconsidered within the temperature-cutoff functional renormalization group approach combined with the Bethe-Salpeter equations. The momentum dependence of the order parameter for antiferromagnetic and superconducting instabilities in these compounds is analyzed. The gap function in the antiferromagnetic (particle-hole) channel has its maxima at the hot-spots, or at the diagonal of the Brilloin zone in their absence. The wavefunction in the singlet superconductin…
A two-dimensional time-reversal symmetric topological superconductor is a fully gapped system possessing a helical Majorana mode on the edges. This helical Majorana edge mode (HMEM), which is a Kramer's pair of two chiral Majorana edge modes in the opposite propagating directions, is robust under time-reversal symmetry protection. We propose a feasible setup and accessible measurement to provide the preliminary step of the HMEM realization by studying superconducting antiferromagnetic quantum spin Hall insulators. Since this antiferromagnetic topological insulator hosts a helical electron edge…
We calculate the superconducting phase diagram as a function of temperature and z-axis anisotropy in a model for tetragonal and cubic systems having strong antiferromagnetic fluctuations. The formal basis for our calculations is the fluctuation exchange approximation (FLEX) applied to the single-band Hubbard model near half-filling. For nearly cubic lattices, two superconducting phase transitions are observed as a function of temperature with the low-temperature state having the time-reversal symmetry-breaking form, $d_{x^2 - y^2} \pm id_{3z^2 -r^2}$. With increasing tetragonal distortion the …
In the layered cuprate perovskites, the occurence of high-temperature superconductivity seems deeply related to the unusual nature of the hole excitations. The limiting case of a very small number of holes diffusing in the antiferromagnetic (AF) background may provide important insights into this problem. We have investigated the transport properties in a series of crystals of $\rm YBa_2Cu_3O_y$, and found that the temperature dependences of the Hall coefficient $R_H$ and thermopower $S$ change abruptly as soon as the AF phase boundary is crossed. In the AF state at low temperatures $T$, both …
We propose a route to achieve odd-parity spin-triplet superconductivity in metallic collinear antiferromagnets with inversion symmetry. Owing to the existence of hidden antiunitary symmetry, which we call the effective time-reversal symmetry (eTRS), the Fermi surfaces of ordinary antiferromagnetic metals are generally spin-degenerate, and spin-singlet pairing is favored. However, by introducing a local inversion symmetry breaking perturbation that also breaks the eTRS, we can lift the degeneracy to obtain spin-polarized Fermi surfaces. In the weak-coupling limit, the spin-polarized Fermi surfa…
We investigate the interplay between charge order and superconductivity near an antiferromagnetic quantum critical point using sign-problem-free Quantum Monte Carlo simulations. We establish that, when the electronic dispersion is particle-hole symmetric, the system has an emergent SU(2) symmetry that implies a degeneracy between $d$-wave superconductivity and charge order with $d$-wave form factor. Deviations from particle-hole symmetry, however, rapidly lift this degeneracy, despite the fact that the SU(2) symmetry is preserved at low energies. As a result, we find a strong suppression of ch…
We present the group-thoretical classification of gap functions in superconductors coexisting with some magnetic order in non-symmorphic magnetic space groups. Based on the weak-coupling BCS theory, we show that UCoGe-type ferromagnetic superconductors must have horizontal line nodes on either $k_z=0$ or $\pmπ/c$ plane. Moreover, it is likely that additional Weyl point nodes exist at the axial point. On the other hand, in UPd$_2$Al$_3$-type antiferromagnetic superconductors, gap functions with $A_g$ symmetry possess horizontal line nodes in antiferromagnetic Brillouin zone boundary perpendicul…
We present an unrestricted Hartree-Fock computation of charge-ordering instabilities of two-dimensional metals with antiferromagnetic exchange interactions, allowing for arbitrary ordering wavevectors and internal wavefunctions of the particle-hole pair condensate. We find that the ordering has a dominant d symmetry of rotations about lattice points for a range of ordering wavevectors, including those observed in recent experiments at low temperatures on YBCO. This d symmetry implies the charge ordering is primarily on the bonds of the Cu lattice, and we propose incommensurate bond order param…
We report on measurements of the temperature dependence of the magnetic penetration depth of a very high quality single crystal of nonmagnetic superconductor LaPt3Si without inversion symmetry. The results are compared with those previously reported for the isostructural antiferromagnetic superconductor CePt3Si. At low temperatures, the penetration depth follows a BCS exponential behavior that implies an isotropic energy gap in LaPt3Si, in contrast to a linear response that indicates line nodes in CePt3Si. These line nodes have been argued to be protected by symmetry or accidentally generated …
Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh$_{2}$As$_{2}$ (SC transition temperature $T_{\rm SC} \sim 0.37$~K), wherein the Ce site breaks the local inversion symmetry. The evidence is based…
We argue the emergence of nodal excitations due to the coupling with static antiferromagnetic order in fully-gapped superconducting states in both cases with and without inversion symmetry. This line node structure is not accompanied with the sign change of the superconducting gap, in contrast to usual unconventional Cooper pairs with higher angular momenta. In the case without inversion symmetry, the stability of the nodal excitations crucially depends on the direction of the antiferromagnetic staggered magnetic moment. A possible realization of this phenomenon in CePt$_3$Si is discussed.
The Hubbard model is reformulated in terms of different ``colored'' fermion species for the electrons or holes at different lattice sites. Antiferromagnetic ordering or d-wave superconductivity can then be described in terms of translationally invariant expectation values for colored composite scalar fields. A suitable mean field approximation for the two dimensional colored Hubbard model shows indeed phases with antiferromagnetic ordering or d-wave superconductivity at low temperature. At low enough temperature the transition to the antiferromagnetic phase is of first order. The present formu…
We study excitations of the predicted antiferromagnetically ordered vortex cores in the superconducting phase of the newly proposed SO(5) model of strongly correlated electrons. Using experimental data from the literature we show that the susceptibilities in the spin sector and the charge sector are nearly equal, and likewise for the stiffnesses. In the case of strict equality SO(5) symmetry is possible, and we find that if present the vortices give rise to an enhanced neutron scattering cross section near the so called pi resonance at 41 meV. In the case of broken SO(5) symmetry two effects a…
We develop an approach to describe antiferromagnetic magnons on a bipartite lattice supporting the Néel state using fractionalized degrees of freedom typically inherent to quantum spin liquids. In particular we consider a long-range magnetically ordered state of interacting two-dimensional quantum spin$-1/2$ models using the Chern-Simons (CS) fermion representation of interacting spins. The interaction leads to Cooper instability and pairing of CS fermions, and to CS superconductivity which spontaneously violates the continuous $\mathrm{U}(1)$ symmetry generating a linearly-dispersing gapless …
We investigate the competition between antiferromagnetism and triplet superconductivity in quasi one-dimensional electron systems. We show that the two order parameters can be unified using a SO(4) symmetry and demonstrate the existence of such symmetry in one dimensional Luttinger liquids of interacting electrons. We argue that approximate SO(4) symmetry remains valid even when interchain hopping is strong enough to turn the system into a strongly anisotropic Fermi liquid. For unitary triplet superconductors SO(4) symmetry requires a first order transition between antiferromagnetic and superc…
Quasi-two dimensional CeIrIn5 contains two distinct domes with different heavy fermion superconducting states in its phase diagram. Here we pinned down the superconducting gap structure of CeIrIn5 in the second dome, located away from the antiferromagnetic quantum critical point, by the thermal transport measurements in magnetic fields rotated relative to the crystal axes. Clear fourfold oscillation was observed when the field is rotated within the ab-plane, while no oscillation was observed within the bc-plane. In sharp contrast to previous reports, our results are most consistent with d_{x^2…
The out-of-plane resistivity ($ρ_c$) and magnetoresistivity (MR) are studied in antiferromangetic (AF) $Nd_{2-x}Ce_xCuO_{4}$ single crystals, which have three types of noncollinear antiferromangetic spin structures. The apparent signatures are observed in $ρ_c(T)$ measured at the zero-field and 14 T at the spin structure transitions, giving a definite evidence for the itinerant electrons directly coupled to the localized spins. One of striking feature is an anisotropy of the MR with a fourfold symmetry upon rotating the external field (B) within ab plane in the different phases, while twofold …
The mechanism that leads to high-temperature superconductivity in cuprates remains an open question despite intense study for nearly two decades. Here, we introduce an SU(4) model for cuprate systems having many similarities to dynamical symmetries known to play an important role in nuclear structure physics and in elementary particle physics. Analytical solutions in three dynamical symmetry limits of this model are found: an SO(4) limit associated with antiferromagnetic order; an SU(2) limit that may be interpreted as a d-wave pairing condensate; and an SO(5) limit that may be interpreted as …
