Iron-based Superconductor LaFeAsO Research Articles

A bulk form Cu-based ferromagnetic semiconductor (La,Ba)(Cu,Mn)SO with the Curie temperature up to 170 K

We report the ferromagnetism in a new bulk form Cu-based magnetic semiconductor (La,Ba)(Cu,Mn)SO, which is iso-structural to the prototypical iron-based 1111-type superconductor LaFeAsO. Starting from the parent compound LaCuSO, carriers are introduced via the substitutions of La for Ba while spins are introduced via the substitutions of Cu for Mn. Spins are mediated by carriers, which develops into the long range ferromagnetic ordering. The maximum Curie temperature T_{C} reaches up to sim 170 K with the doping levels of 10% Ba and 5% Mn. By comparing to the (La,Sr)(Cu,Mn)SO where Sr and Mn are co-doped into LaCuSO, we demonstrate that negative chemical pressure would suppress the ferromagnetic ordering.

Thermodynamic electric quadrupole moments of nematic phases from first-principles calculations

The electronic nematic phase emerging with spontaneous rotation symmetry breaking is a central issue of modern condensed matter physics. In particular, various nematic phases in iron-based superconductors and high-$T_{\rm c}$ cuprate superconductors are extensively studied recently. Electric quadrupole moments (EQMs) are one of the order parameters characterizing these nematic phases in a unified way, and elucidating EQMs is a key to understanding these nematic phases. However, the quantum-mechanical formulation of the EQMs in crystals is a nontrivial issue because the position operators are non-periodic and unbound. Recently, the EQMs have been formulated by local thermodynamics, and such {\it thermodynamic EQMs} may be used to characterize the fourfold rotation symmetry breaking in materials. In this paper, we calculate the thermodynamic EQMs in iron-based superconductors LaFeAsO and FeSe as well as a cuprate superconductor La$_2$CuO$_4$ by a first-principles calculation. We show that owing to the orbital degeneracy the EQMs in iron-based superconductors are mainly determined by the geometric properties of wave functions. This result is in sharp contrast to the cuprate superconductor, in which the EQMs are dominated by distortion of the Fermi surface.

(La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>)(Zn<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>)SbO: A novel 1111-type diluted magnetic semiconductor

Diluted magnetic semiconductor (DMS) that combines the properties of spin and charge degrees of freedom, which has potential applications in the field of spintronic devices. In the 1990s, due to the breakthrough of low-temperature molecular beam epitaxy technology, scientists successfully synthesized III-V DMS (Ga, Mn)As, and developed some spintronics devices accordingly. However, the maximum Curie temperature of (Ga, Mn)As is only 200 K, which is still below room temperature that is required for practical applications. Searching for diluted magnetic semiconductors with higher Curie temperature and the exploring of their magnetism is still one of the focuses at present. In recent years, developed from iron-based superconductors, a series of novel magnetic semiconductors have been reported. These new DMSs have the advantages of decoupled charge and spin doping, and each concentration can be precisely controlled. In this paper, novel bulk diluted magnetic semiconductors (La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>)(Zn<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>)SbO (<i>x</i> = 0.025, 0.050,0.075, 0.10) are successfully synthesized, with the highest <i>T</i><sub>c</sub> ~ 27.1 K for the doping level of <i>x</i> = 0.10. We dope Sr<sup>2+</sup> and Mn<sup>2+</sup> into the parent semiconductor material LaZnSbO to introduce holes and moments, respectively. The ferromagnetic ordered phase transition can be observed in the samples with various doping concentrations. A relatively large coercive field is observed to be ~ 5000 Oe from the iso-thermal magnetization measurement at 2 K. The (La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>)(Zn<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>)SbO has the same crystal structure as the “1111-type” iron-based superconductor LaFeAsO, and the lattice parameter difference is very small. It provides a possible material choice for preparing the multifunctional heterojunction devices.

Pressure-induced quantum critical point in the heavily hydrogen-doped iron-based superconductor LaFeAsO

An iron-based superconductor ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{H}}_{x}\phantom{\rule{0.16em}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}0.6)$ undergoes two antiferromagnetic (AF) phases upon H doping. We investigated the second AF phase ($x=0.6$) using NMR techniques under pressure. At pressures up to 2 GPa, the ground state is a spin-density-wave state with a large gap; however, the gap closes at 4.0 GPa, suggesting a pressure-induced quantum critical point. Interestingly, the gapped excitation coexists with gapless magnetic fluctuations at pressures between 2 and 4 GPa. This coexistence is attributable to the lift up of the ${d}_{xy}$ orbital to the Fermi level, a Lifshitz transition under pressure.

Quantum phase transition under pressure in the heavily hydrogen-doped iron-based superconductor LaFeAsO

Hydrogen (H)-doped LaFeAsO is a prototypical iron-based superconductor. However, its phase diagram extends beyond the standard framework, where a superconducting (SC) phase follows an antiferromagnetic (AF) phase upon carrier doping; instead, the SC phase is sandwiched between two AF phases appearing in lightly and heavily H-doped regimes. We performed nuclear magnetic resonance (NMR) measurements under pressure, focusing on the second AF phase in the heavily H-doped regime. The second AF phase is strongly suppressed when a pressure of 3.0 GPa is applied, and apparently shifts to a highly H-doped regime, thereby a "bare" quantum critical point (QCP) emerges. A quantum critical regime emerges in a paramagnetic state near the QCP, however, the influence of the AF critical fluctuations to the SC phase is limited in the narrow doping regime near the QCP. The optimal SC condition ($T_c \sim$ 48 K) is unaffected by AF fluctuations.

Origin of doping-induced suppression and reemergence of magnetism in LaFeAsO1−xHx

We investigate the evolution of magnetic properties as a function of hydrogen doping in iron based superconductor LaFeAsO$_{1-x}$H$_x$ using the dynamical mean-field theory combined with the density-functional theory. We find that two independent consequences of the doping, the increase of the electron occupation and the structural modification, have the opposite effects on the strength of electron correlation and magnetism, resulting in the minimum of the calculated magnetic moment around the intermediate doping level as a function of $x$. Our result provides a natural explanation for the puzzling recent experimental discovery of the two separated antiferromagnetic phases at low and high doping limits. Furthermore, the increase of orbital occupation and correlation strength with the doping results in reduced orbital polarization of $d_{xz/yz}$ orbitals and the enhanced role of $d_{xy}$ orbital in the magnetism at high doping levels, and their possible implications to the superconductivity are discussed in line with the essential role of the magnetism.

Experimental evidence ofTcenhancement without the influence of spin fluctuations: NMR study onLaFeAsO1−xHxunder a pressure of 3.0 GPa

The electron-doped high-transition-temperature (T_c) iron-based pnictide superconductor LaFeAsO_{1-x}H_x has a unique phase diagram: superconducting (SC) double domes are sandwiched by antiferromagnetic phases at ambient pressure and they turn to a single dome with a maximum T_c that exceeds 45K at a pressure of 3.0 GPa. We studied whether spin fluctuations are involved in increasing T_c under a pressure of 3.0 GPa by using ^{75}As nuclear magnetic resonance (NMR) technique. The ^{75}As-NMR results for the powder samples show that T_c increases up to 48 K without the influence of spin fluctuations. The fact indicates that spin fluctuations are not involved in raising T_c, which implies that other factors, such as orbital degrees of freedom, may be important for achieving a high T_c of almost 50 K.

Superconductivity and its mechanism in an ab initio model for electron-doped LaFeAsO.

Two families of high-temperature superconductors whose critical temperatures are higher than 50 K are known. One are the copper oxides and the other are the iron-based superconductors. Comparisons of mechanisms between these two in terms of common ground as well as distinctions will greatly help in searching for higher T(c) superconductors. However, studies on mechanisms for the iron family based on first principles calculations are few. Here we first show that superconductivity emerges in the state-of-the-art numerical calculations for an ab initio multi-orbital model of an electron-doped iron-based superconductor LaFeAsO, in accordance with experimental observations. Then the mechanism of the superconductivity is identified as enhanced uniform density fluctuations by one-to-one correspondence with the instability towards inhomogeneity driven by first-order antiferromagnetic and nematic transitions. Despite many differences, certain common features with the copper oxides are also discovered in terms of the underlying orbital-selective Mottness found in the iron family.

Progressive slowing down of spin fluctuations in underdoped LaFeAsO1−xFx

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO$_{1-x}$F$_x$ was studied over a broad doping, temperature, and magnetic field range (x = 0 - 0.15, T up to 480K, H up to 30T) by means of As nuclear magnetic resonance (NMR). An enhanced spin-lattice relaxation rate divided by temperature, 1/T1T, in underdoped superconducting samples (x = 0.045, 0.05 and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally-doped (x = 0.10) and completely absent in overdoped (x = 0.15) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a BPP-type model is used to describe the occurrence of a peak in 1/T1T clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.

Orbital ordering and charge profile deformation in iron-based superconductors

We investigate electronic structures of a typical iron-based superconductor LaFeAsO by first-principles calculations in order to clarify the ground-state of the mother compounds. With use of around mean field (AMF) for LDA+U formalism, we obtain an orbital ordered state showing relatively low magnetic moment widely for intermediate U values. In the low moment state, we find that electric quadrupole moment develops around iron atom, i.e., the charge profile deforms from a spherical to a collapsed one in contrast to the high moment state with almost zero quadrupole moment. Moreover, we examine As height dependence of the low moment state and find that the low moment state is also widely stable for a variation of As height compared to the high moment one.

Effects of transition-metal substitution in the iron-based superconductor LaFeAsO: Momentum- and real-space analysis from first principles

We study how transition-metal substitution changes the electronic structure of the iron-based superconductor LaFeAsO in real and momentum space. We first perform ab initio density functional calculation for various sizes of supercells with one transition-metal impurity. For various substitutes (Mn, Co, Ni, Zn, and Ru), we derive effective tight-binding models by constructing the maximally localized Wannier functions from the d bands around the Fermi level. The local electronic structure around the impurity site is quantitatively characterized by their onsite potential and transfer hoppings to neighboring sites. We find that the impurities are classified into three groups according to the derived parameters. For Mn, Co, and Ni, their impurity 3d levels measured from the Fe 3d level are ∼0.3, −0.3, and −0.8 eV, respectively, while, for the Zn case, its d level is extremely deep as ∼8 eV. For the Ru case, although the onsite-level difference is much smaller (∼O(0.1) eV), the transfer integrals around the impurity ion are larger than those of the pure system by 20% ∼ 30%, due to the large spatial spread of the Ru 4d orbitals. We also show that the charge distribution of the extra d electrons is confined around the impurity ion. We then unfold the first Brillouin zone (BZ) for the supercell to calculate the spectral function in the BZ for the normal cell for the case of Co and Ni doping. While the charge distribution seems to suggest that Co and Ni impurities do not change the amount of mobile carriers in the system, the momentum-space analysis clearly shows that the Fermi-surface volume indeed expands by Co and Ni substitutions, which can be well described by the rigid-band shift approximation.

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 described by the rigid-band shift. By taking into account this effective doping, we discuss whether the sign-reversing s-wave ($s_{\pm}$-wave) scenario is compatible with experiments.

First-Principles Studies for the Hydrogen Doping Effects on Iron-Based Superconductors

We study hydrogen doping effects in an iron-based superconductor LaFeAsO_(1-y) by using the first-principles calculation and explore the reason why the superconducting transition temperature is remarkably enhanced by the hydrogen doping. The present calculations reveal that a hydrogen cation stably locating close to an iron atom attracts a negatively-charged FeAs layer and results in structural distortion favorable for further high temperature transition. In fact, the lattice constant a averaged over the employed supercell shrinks and then the averaged As-Fe-As angle approaches 109.74 degrees with increasing the hydrogen doping amount. Moreover, the calculations clarify electron doping effects of the solute hydrogen and resultant Fermi-level shift. These insights are useful for design of high transition-temperature iron-based superconductors.

Fast Spin Fluctuation Viewed by Muon Spin Relaxation in Optimally Doped and Overdoped Iron-Based Oxypnictide Superconductors LaFeAsO1-xFx

Zero field and longitudinal field muon spin relaxation measurements have been performed in optimally doped and overdoped superconductors LaFeAsO$_{1-x}$F$_x$ in order to investigate the magnetic fluctuation over a wide range of temperature and longitudinal field. We have observed no sign of magnetic fluctuation against temperature in the muons' time window (10$^{-5}\sim$ 10$^{-9}$s). Considering the current results and other results, i.e., spin fluctuation observed by neutron scattering, pseudogap-like behaviors by NMR and photoemission spectroscopy, it is suggested that not only the spin fluctuation but also the multiband character with several different orbital contributions at the Fermi surface may play an important role in the superconducting pairing mechanism of LaFeAsO$_{1-x}$F$_x$.

Pressure effects on iron-based superconductors: A first-principles study

We investigate pressure effects on an iron-based superconductor LaFeAsO using first-principles calculations. As the pressure increases, we find a structural transition from the orthorhombic to the tetragonal crystal with disappearance of the antiferromagnetically ordered magnetic moment at 24GPa in agreement with an experimental observation. Moreover, the calculations reveal that the structural transition is almost of the second order.

First-principles calculations of the effect of pressure on the iron-based superconductor LaFeAsO

We perform first-principles calculations for an iron-based superconductor LaFeAsO in order to study its pressure effects. As the pressure increases, we find a change from monotonic to nonmonotonic shrinking manner in lattice constants and related crystalline parameters at 20 GPa and a structural transition from the orthorhombic to the tetragonal crystal with a disappearance of the antiferromagnetically ordered moment at 24 GPa in an excellent agreement with an experimental observation. Moreover, the calculations reveal that the structural transition is almost the phase transition of the second order. We further explore a very high-pressure range and find that the contraction of the distance between As and Fe plane stops above 100 GPa although the $c$-axis lattice constant continues to shrink.

First-principle electronic structure calculations for magnetic moment in iron-based superconductors: An LSDA + negative U study

In order to resolve a discrepancy of the magnetic moment on Fe between the experimental and calculation results, we perform first-principle electronic structure calculations for iron-based superconductors LaFeAsO 1 - x F x in which x = 0.0 and x = 0.125 by using the LSDA + U framework. Consequently, we confirm in both the mother and doped compounds that negative U correction is crucial in matching the calculated magnetic moment with the observed one. A reason of the negative correction is that the Coulomb interaction on Fe orbitals is unexpectedly screened than LSDA’s expectation. We discuss which type of situation emerges when the negative U is a good correction in these compounds.

Pressure studies of (La, Sm)FeAsO 1−xF x and LaFePO

Pressure dependence on superconducting transition temperature ( T c) was examined for iron-based superconductor LaFeAsO 1− x F x , SmFeAsO 1− x F x and LaFePO. The T c’s increase largely for LaFePO and LaFeAsO 1− x F x with a small increase of pressure. The electrical resistivity measurements reveal the pressure-induced superconductivity in undoped LaFeAsO and SmFeAsO. X-ray diffraction measurements were also performed under high pressure up to 10 GPa for LaFePO and LaFeAsO 1− x F x system, where the anisotropic decrease of the lattice constants was observed with applying pressure.

Spin Ordering in LaFeAsO and Its Suppression in Superconductor LaFeAsO0.89F0.11 Probed by Mössbauer Spectroscopy

57 Fe Mössbauer spectroscopy was applied to an iron-based layered superconductor LaFeAsO 0.89 F 0.11 with a transition temperature of 26 K and to its parent material LaFeAsO. Throughout the temperature range from 4.2 to 298 K, a singlet pattern with no magnetic splitting was observed in the Mössbauer spectrum of the F-doped superconductor. Furthermore, no additional internal magnetic field was observed for the spectrum measured at 4.2 K under a magnetic field of 7 T. On the other hand, magnetically split spectra were observed in the parent LaFeAsO below 140 K, and this temperature is slightly lower than that of a structural phase transition from tetragonal to orthorhombic phase, which accompanies the electrical resistivity anomaly at around 150 K. The magnetic moment is estimated to be ∼0.35 µ B /Fe from the internal magnetic field of 5.3 T at 4.2 K in the orthorhombic phase, and the spin disorder appears to remain in the magnetically ordered state even at 4.2 K. The lack of a magnetic transition in LaFeAsO 0.89 F 0.11 down to 4.2 K suggests that this system exhibits a paramagnetic state or that the magnetic moment is small. The present results show that F doping effectively suppresses the magnetic and structural transitions in the parent material, leading to the emergence of superconductivity in the F-doped system.

Electronic Structure and Electron Correlation in LaFeAsO1-xFx and LaFePO1-xFx

Photoemission spectroscopy is used to investigate the electronic structure of the newly discovered iron-based superconductors LaFeAsO 1- x F x and LaFePO 1- x F x . Line shapes of the Fe 2 p core-level spectra suggest an itinerant character of Fe 3 d electrons. The valence-band spectra are generally consistent with band-structure calculations except for the shifts of Fe 3 d -derived peaks toward the Fermi level. From the spectra taken in the Fe 3 p → 3 d core-absorption region, we have obtained the experimental Fe 3 d partial density of states, and explained it in terms of a band-structure calculation with a phenomenological self-energy correction, yielding a mass renormalization factor of \({\lesssim}2\).