Coexistence of magnetism and superconductivity in the iron-based compound Cs0.8(FeSe0.98)2

2D gapless topological superfluids generated by pairing phases

We systematically investigate the ground state phase diagram and the finite temperature phase transitions for a Rydberg-dressed Fermi gas loaded in a bilayer optical lattice. When an effective finite-ranged attraction is induced, our self-consistent mean-field calculation shows that the gapped topological (p-wave) superfluids in each layer are coupled together by thes-wave pairing in an intermediate inter-layer distance with a spontaneously modulated phases between these two order parameters. The obtained ground state is a gapless topological superfluid with quantized topological charges characterizing the gapless points, leading to a zero energy flat band at the edges. Finally, we calculate the finite temperature phase diagrams of this two-dimensional gapless superfluid and observe two distinct critical temperatures, demonstrating the fruitful many-body effects on a paired topological superfluids.

A Retrospective of Materials Synthesis at the Paul Scherrer Institut (PSI)

The availability of high-quality and well characterized materials is a key factor for condensed-matter research [...]

Finite temperature effects in quantum systems with competing scalar orders

The study of the competition or coexistence of different ground states in many-body systems is an exciting and actual topic of research, both experimentally and theoretically. Quantum fluctuations of a given phase can suppress or enhance another phase depending on the nature of the coupling between the order parameters, their dynamics and the dimensionality of the system. The zero temperature phase diagrams of systems with competing scalar order parameters with quartic and bilinear coupling terms have been previously studied for the cases of a zero temperature bicritical point and of coexisting orders. In this work, we apply theMatsubara summationtechnique from finite temperature quantum field theory to introduce the effects of thermal fluctuations on the effective potential of these systems. This is essential to make contact with experiments. We consider two and three-dimensional materials characterized by a Lorentz invariant quantum critical theory, i.e., with dynamic critical exponentz= 1, such that time and space scale in the same way. We obtain that in both cases, thermal fluctuations lead to weak first-order temperature phase transitions, at which coexisting phases arising from quantum corrections become unstable. We show that above this critical temperature (Tc), the system presents scaling behavior consistent with that approaching a quantum critical point. Below the transition the specific heat has a thermally activated contribution with a gap related to the size of the domains of the ordered phases. We obtain thatTcdecreases as a function of the distance to the zero temperature classical bicritical point (ZTCBP) in the coexistence region, implying that in our approach, the system attains the highestTcabove the fine tuned value of this ZTCBP.

Highly-Tunable Crystal Structure and Physical Properties in FeSe-Based Superconductors

Here, crystal structure, electronic structure, chemical substitution, pressure-dependent superconductivity, and thickness-dependent properties in FeSe-based superconductors are systemically reviewed. First, the superconductivity versus chemical substitution is reviewed, where the doping at Fe or Se sites induces different effects on the superconducting critical temperature (Tc). Meanwhile, the application of high pressure is extremely effective in enhancing Tc and simultaneously induces magnetism. Second, the intercalated-FeSe superconductors exhibit higher Tc from 30 to 46 K. Such an enhancement is mainly caused by the charge transfer from the intercalated organic and inorganic layer. Finally, the highest Tc emerging in single-unit-cell FeSe on the SrTiO3 substrate is discussed, where electron-phonon coupling between FeSe and the substrate could enhance Tc to as high as 65 K or 100 K. The step-wise increment of Tc indicates that the synergic effect of carrier doping and electron-phonon coupling plays a critical role in tuning the electronic structure and superconductivity in FeSe-based superconductors.

Enhancement of superconducting properties and flux pinning mechanism on Cr0.0005NbSe2 single crystal under Hydrostatic pressure

Superconducting properties of Cr_0.0005NbSe₂ (T_c~6.64 K) single crystals have been investigated through the temperature dependent resistivity (~8 GPa) and DC magnetization (~1 GPa) measurements. Further, the critical current density (J_c) as a function of applied magnetic field has been studied from magnetic isotherms. The vortex pinning mechanisms have also been systematically analyzed using weak collective pinning theory as a function of pressure. The J_c corresponds to the flux flow enhanced by the application of pressure due to increase of T_c and vortex changes. We found that the pressure is responsible for the spatial variations in the charge carrier mean free path (δl pinning). We find that core point pinning is more dominant than surface pinning which is caused by the application of pressure. In addition, J_c(H = 0) increases from 3.9 × 10⁵ (0 GPa) to 1.3 × 10⁶ (1.02 GPa) A/cm² at 2 K as the pressure is increased from normal pressure to 1.02 GPa. The pressure dependence of T_c (dT_c/dP) becomes 0.91 K/GPa and 0.75 K/GPa from magnetization and resistivity measurements respectively. We found that the pressure promotes the anisotropy nature, and decrease of coherence length and resulting in pathetic interface of the vortex core with pinning centers.

Spin dynamics of the block orbital-selective Mott phase

Iron-based superconductors display a variety of magnetic phases originating in the competition between electronic, orbital, and spin degrees of freedom. Previous theoretical investigations of the multi-orbital Hubbard model in one-dimension revealed the existence of an orbital-selective Mott phase (OSMP) with block spin order. Recent inelastic neutron scattering (INS) experiments on the BaFe₂Se₃ ladder compound confirmed the relevance of the block-OSMP. Moreover, the powder INS spectrum revealed an unexpected structure, containing both low-energy acoustic and high-energy optical modes. Here we present the theoretical prediction for the dynamical spin structure factor within a block-OSMP regime using the density-matrix renormalization-group method. In agreement with experiments, we find two dominant features: low-energy dispersive and high-energy dispersionless modes. We argue that the former represents the spin-wave-like dynamics of the block ferromagnetic islands, while the latter is attributed to a novel type of local on-site spin excitations controlled by the Hund coupling.

Exploring the orbital-selective Mott phase (OSMP) addresses the central issue of electron correlations in the iron-based superconductors. Here the authors theoretically study the dynamical spin structure factor in the block-OSMP regime and unveil momentum dependent characteristics for different spin excitation modes.

Quantum corrections for the phase diagram of systems with competing order

We use the effective potential method of quantum field theory to obtain the quantum corrections to the zero temperature phase diagram of systems with competing order parameters. We are particularly interested in two different scenarios: regions of the phase diagram where there is a bicritical point, at which both phases vanish continuously, and the case where both phases coexist homogeneously. We consider different types of couplings between the order parameters, including a bilinear one. This kind of coupling breaks time-reversal symmetry and it is only allowed if both order parameters transform according to the same irreducible representation. This occurs in many physical systems of actual interest like competing spin density waves, different types of orbital antiferromagnetism, elastic instabilities of crystal lattices, vortices in a multigap SC and also applies to describe the unusual magnetism of the heavy fermion compound URu₂Si₂. Our results show that quantum corrections have an important effect on the phase diagram of systems with competing orders.

Superconductivity in alkali metal intercalated iron selenides

Alkali metal intercalated iron selenide superconductors A x Fe2-y Se2 (where A  =  K, Rb, Cs, Tl/K, and Tl/Rb) are characterized by several unique properties, which were not revealed in other superconducting materials. The compounds crystallize in overall simple layered structure with FeSe layers intercalated with alkali metal. The structure turned out to be pretty complex as the existing Fe-vacancies order below ~550 K, which further leads to an antiferromagnetic ordering with Néel temperature fairly above room temperature. At even lower temperatures a phase separation is observed. While one of these phases stays magnetic down to the lowest temperatures the second is becoming superconducting below ~30 K. All these effects give rise to complex relationships between the structure, magnetism and superconductivity. In particular the iron vacancy ordering, linked with a long-range magnetic order and a mesoscopic phase separation, is assumed to be an intrinsic property of the system. Since the discovery of superconductivity in those compounds in 2010 they were investigated very extensively. Results of the studies conducted using a variety of experimental techniques and performed during the last five years were published in hundreds of reports. The present paper reviews scientific work concerning methods of synthesis and crystal growth, structural and superconducting properties as well as pressure investigations.

Signature of coexistence of superconductivity and ferromagnetism in two-dimensional NbSe2 triggered by surface molecular adsorption

Ferromagnetism is usually deemed incompatible with superconductivity. Consequently, the coexistence of superconductivity and ferromagnetism is usually observed only in elegantly designed multi-ingredient structures in which the two competing electronic states originate from separate structural components. Here we report the use of surface molecular adsorption to induce ferromagnetism in two-dimensional superconducting NbSe₂, representing the freestanding case of the coexistence of superconductivity and ferromagnetism in one two-dimensional nanomaterial. Surface-structural modulation of the ultrathin superconducting NbSe₂ by polar reductive hydrazine molecules triggers a slight elongation of the covalent Nb–Se bond, which weakens the covalent interaction and enhances the ionicity of the tetravalent Nb with unpaired electrons, yielding ferromagnetic ordering. The induced ferromagnetic momentum couples with conduction electrons generating unique correlated effects of intrinsic negative magnetoresistance and the Kondo effect. We anticipate that the surface molecular adsorption will be a powerful tool to regulate spin ordering in the two-dimensional paradigm.

Ferromagnetism and superconductivity possess inherently incompatible electronic spin ordering, and their coexistence requires elaborate engineering of material components. Here, the authors induce ferromagnetism in a two-dimensional superconducting crystal by the adsorption of hydrazine molecules.

Magnetic (CoFe2O4)0.1(CeO2)0.9 nanocomposite as effective pinning centers in FeSe0.1Te0.9 thin films

Magnetic epitaxial (CoFe2O4)0.1(CeO2)0.9 nanocomposite layers were incorporated into superconducting FeSe0.1Te0.9 thin films as either a cap layer or a buffer layer. Both capped and buffered samples show an enhancement of the superconducting property compared to the reference sample without the incorporated layer, while the capped one shows the best pinning properties of all the samples. Specifically for the capped sample, the critical temperature Tc is ~12.5 K, while the self-field critical current density J(c)(sf )increases to as high as 1.20 MA cm(-2) at 4 K. Its J(c)(in-field) value shows a slower decrease with increasing applied magnetic field, with the lowest power-law exponent α values (derived following Jc[formula: see text](μ0H)(-α) by the log(Jc) − log(μ0H) plot) of 0.20, 0.23 and 0.33 at 2 K, 4 K and 8 K, respectively. This nanocomposite capped sample also exhibits a high upper critical field Hc2(0) of 166 T, which indicates its potential in high field applications. This pinning method provides an effective way of enhancing the superconducting property of iron chalcogenide thin film.

Structure and superconductivity of (Li1-x Fe x )OHFeSe single crystals grown using A x Fe2-y Se2 (A = K, Rb, and Cs) as precursors

We present results on the hydrothermal growth of ([Formula: see text])OHFeSe single crystals using floating-zone-grown [Formula: see text] (A  =  K, Rb, and Cs) as precursors. The growth proceeds by the hydrothermal ion exchange of Li/Fe-O-H for K, Rb, and Cs, resulting in a stacking layer of ([Formula: see text])OH sandwiched between the FeSe layers. Optimal growth parameters are achieved using high quality A 0.80Fe1.81Se2 single crystals added to the mixtures of LiOH, H2O, Fe and C(NH2)2Se in an autoclave and subsequently heated to 120 °C for 2 d, to obtain highest quality single crystals. The obtained crystals have lateral dimensions up to centimeters, with the final size related to that of the precursor crystal used. All ([Formula: see text])OHFeSe single crystals show a superconducting transition temperature T c  >  42 K, regardless of the phase of the precursor such as superconducting K0.80Fe1.81Se2 (T c  =  29.31 K) or non-superconducting Rb0.80Fe1.81Se2 or poor-superconducting Cs0.80Fe1.81Se2 (T c  =  28.67 K). The T c and transition width ΔT vary in the obtained single crystals, due to the inhomogeneity of the ionic exchange. X-ray diffraction analysis demonstrates that the 245 insulating phase is absent in the ion-exchanged single crystals, while it is observed in the [Formula: see text] precursors. Comparative studies of the structure, magnetization, and superconductivity on the parent A 0.80Fe1.81Se2 and the ion-exchanged ([Formula: see text])OHFeSe crystals are discussed. A phase diagram including antiferromagnetic spin density wave and superconducting phases is also proposed.

Effect of ferromagnetic ordering on phonons in KCo2Se2

Results of the density functional theory studies of the phonon dynamics in the ternary layered cobalt diselenide are reported. The partial phonon densities of states due to vibrations of K, Co, and Se atoms are analysed in detail. They indicate that phonons associated with the dynamics of Co and Se ions within the [Co2Se2] structural blocks span the entire spectral range extending to 260 cm(-1), whereas phonons from the K-sublattice remain limited to the frequency range of 80-150 cm(-1). The phonons conform with structural features of the quasi-2D layered structure of KCo2Se2. Ferromagnetic order in the Co-sublattice is shown to determine to a great extent the phonon densities of states, the Raman and infrared spectra of KCo2Se2. The in-planar magnetic interactions are responsible for pronounced softening of the high-frequency phonon modes and lead to disappearance of the low-frequency Raman-active mode of the E g symmetry. The observed behavior of the Raman-active and infrared-active modes suggests rather strong spin-phonon coupling in KCo2Se2. Results of the present investigations allow to clarify the origin of substantial differences between dynamical properties of the ferromagnetic Co-based and the paramagnetic Ni-based ternary layered dichalcogenides, both adopting the ThCr2Si2-type structure.

Pressure-induced electronic phase separation of magnetism and superconductivity in CrAs

The recent discovery of pressure (p) induced superconductivity in the binary helimagnet CrAs has raised questions on how superconductivity emerges from the magnetic state and on the mechanism of the superconducting pairing. In the present work the suppression of magnetism and the occurrence of superconductivity in CrAs were studied by means of muon spin rotation. The magnetism remains bulk up to p  3.5 kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at p  7 kbar. At 3.5 kbar superconductivity abruptly appears with its maximum T_c  1.2 K which decreases upon increasing the pressure. In the intermediate pressure region (3.5  p  7 kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature (T_c) and of the superfluid density (ρ_s). A scaling of ρ_s with as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs.

Interplay of disorder and antiferromagnetism in TlFe(1.6+δ)(Se(1-x)S(x))2 probed by neutron scattering

The effect of selenium substitution by sulphur on the structural and physical properties of antiferromagnetic TlFe1.6+δSe2 has been investigated via neutron, x-ray and electron diffraction, and transport measurements. The √5a×√5a×c super-cell related to the iron vacancy ordering found in the pure TlFe1.6Se2 selenide is also present in the S-doped TlFe1.6+δ(Se1-xSx)2 compounds. Neutron scattering experiments show the occurrence of the same long range magnetic ordering in the whole series i.e. the 'block checkerboard' antiferromagnetic structure. In particular, this is the first detailed study where the crystal structure and the √5a×√5a antiferromagnetic structure is characterized by neutron powder diffraction for the pure TlFe1.6+δS2 sulphide over a large temperature range. We demonstrate the strong correlation between occupancies of the crystallographic iron sites, the level of iron vacancy ordering and the occurrence of block antiferromagnetism in the sulphur series. Introducing S into the Se sites also increases the Fe content in TlFe1.6+δ(Se1-xSx)2 which in turn leads to the disappearance of the Fe vacancy ordering at x = 0.5 ± 0.15. However, by reducing the nominal Fe content, the same √5a×√5a×c vacancy ordering and antiferromagnetic order can be recovered also in the pure TlFe1.6+δS2 sulphide with a simultaneous reduction in the Néel temperature from 435 K in the selenide TlFe1.75Se2 to 330 K in the sulphide TlFe1.5S2. The magnetic moment remains high at low temperature throughout the full substitution range, which contributes to the absence of superconductivity in these compounds.

Effects of Co and Mn doping in K0.8Fe2-ySe2 revisited

Accumulated evidence indicates that phase separation occurs in potassium intercalated iron selenides, a superconducting phase coexisting with the antiferromagnetic phase K2Fe4Se5, the so-called '245 phase'. Here, we report a comparative study of substitution effects by Co and Mn for Fe sites in K0.8Fe2-ySe2 within the phase separation scenario. Our results demonstrate that Co and Mn dopants have distinct differences in occupancy and hence in the suppression mechanism of superconductivity upon doping of Fe sites. In K0.8Fe2-xCoxSe2, Co prefers to occupy the lattice of the superconducting phase and suppresses superconductivity very quickly, obeying the magnetic pair-breaking mechanism or the collapse of the Fermi surface nesting mechanism. In contrast, in K0.8Fe1.7-xMnxSe2, Mn shows no preferential occupancy in the superconducting phase or the 245 phase. The suppression of superconductivity can be attributed to restraining of the superconducting phase and meanwhile inducing another non-superconducting phase by Mn doping.

Two-dimensional superfluid density in an alkali metal-organic solvent intercalated iron selenide superconductor Li(C5H5N)0.2Fe2Se2

We report the low-temperature electronic and magnetic properties of the alkali metal-organic solvent intercalated iron selenide superconductor Li(C5H5N)0.2Fe2Se2 using muon-spin-spectroscopy measurements. The zero-field muon spin relaxation (μSR) results indicate that nearly half of the sample is magnetically ordered and spatially phase separated from the superconducting region. The transverse-field μSR results reveal that the superfluid density of Li(C5H5N)0.2Fe2Se2 is two dimensional in nature. The temperature dependence of the penetration depth λ(T) can be explained using a two-gap s-wave model. This implies that, despite the 2D nature of the superfluid density, the symmetry of the superconducting gap remains unaltered to the parent compound FeSe.

Spin wave excitations in AFe1.5Se2 (A = K, Tl): analytical study

By generalizing the equation of motion method, we can analytically solve the spin wave excitations for the intercalated ternary iron-selenide AFe(1.5)Se(2) (A = K, Tl) in a complex 4 × 2 collinear antiferromagnetic order. It is found that there are one acoustic branch (gapless Goldstone mode) and two gapful optical branches of spin wave excitations with each in double degeneracy. By examining the non-imaginary excitation frequency condition, we can determine the corresponding phase boundary. The exchange couplings between Fe moments in AFe(1.5)Se(2) are derived based on the first-principles total energy calculations. The Fe spin is found to be S = 3/2 through computing the antiferromagnetic quantum fluctuation. It is also found that a very small spin-orientation anisotropy can remarkably suppress the antiferromagnetic quantum fluctuation. The spin dynamical structure factors are calculated and discussed in association with neutron inelastic scattering experiment.

Magnetic excitations in iron chalcogenide superconductors

Nuclear magnetic resonance and neutron scattering experiments in iron chalcogenide superconductors are reviewed to make a survey of the magnetic excitations in FeSe, FeSe1-x Te x and alkali-metal-doped Ax Fe2-y Se2 (A = K, Rb, Cs, etc). In FeSe, the intimate relationship between the spin fluctuations and superconductivity can be seen universally for the variations in the off-stoichiometry, the Co-substitution and applied pressure. The isovalent compound FeTe has a magnetic ordering with different wave vector from that of other Fe-based magnetic materials. The transition temperature Tc of FeSe increases with Te substitution in FeSe1-x Te x with small x, and decreases in the vicinity of the end member FeTe. The spin fluctuations are drastically modified by the Te substitution. In the vicinity of the end member FeTe, the low-energy part of the spin fluctuation is dominated by the wave vector of the ordered phase of FeTe; however, the reduction of Tc shows that it does not support superconductivity. The presence of same wave vector as that of other Fe-based superconductors in FeSe1-x Te x and the observation of the resonance mode demonstrate that FeSe1-x Te x belongs to the same group as most of other Fe-based superconductors in the entire range of x, where superconductivity is mediated by the spin fluctuations whose wave vector is the same as the nesting vector between the hole pockets and the electron pockets. On the other hand, the spin fluctuations differ for alkali-metal-doped Ax Fe2-y Se2 and FeSe or other Fe-based superconductors in their wave vector and strength in the low-energy part, most likely because of the different Fermi surfaces. The resonance mode with different wave vector suggests that Ax Fe2-y Se2 has an exceptional superconducting symmetry among Fe-based superconductors.

Spin reorientation in TlFe1.6Se2 with complete vacancy ordering

The relationship between vacancy ordering and magnetism in TlFe(1.6)Se(2) has been investigated via single crystal neutron diffraction, nuclear forward scattering, and transmission electron microscopy. The examination of chemically and structurally homogeneous crystals allows the true ground state to be revealed, which is characterized by Fe moments lying in the ab plane below 100 K. This is in sharp contrast to crystals containing regions of order and disorder, where a competition between c axis and ab plane orientations of the moments is observed. The properties of partially disordered TlFe(1.6)Se(2) are, therefore, not associated with solely the ordered or disordered regions. This contrasts the viewpoint that phase separation results in independent physical properties in intercalated iron selenides, suggesting a coupling between ordered and disordered regions may play an important role in the superconducting analogues.

KFe2Se2 is the parent compound of K-doped iron selenide superconductors

We elucidate the existing controversies in the newly discovered K-doped iron selenide (K(x)Fe(2-y)Se(2-z)) superconductors. The stoichiometric KFe(2)Se(2) with √2 × √2 charge ordering was identified as the parent compound of K(x)Fe(2-y)Se(2-z) superconductor using scanning tunneling microscopy and spectroscopy. The superconductivity is induced in KFe(2)Se(2) by either Se vacancies or interacting with the antiferromagnetic K(2)Fe(4)Se(5) compound. In total, four phases were found to exist in K(x)Fe(2-y)Se(2-z): parent compound KFe(2)Se(2), superconducting KFe(2)Se(2) with √2 × √5 charge ordering, superconducting KFe(2)Se(2-z) with Se vacancies, and insulating K(2)Fe(4)Se(5) with √5 × √5 Fe vacancy order. The phase separation takes place at the mesoscopic scale under standard molecular beam epitaxy conditions.

Theoretical investigation on the structural and thermodynamic properties of FeSe at high pressure and high temperature

A theoretical investigation on structural and thermodynamic properties of 11-type iron-based superconductor FeSe at high pressure and high temperature was performed by employing the first-principles method based on the density functional theory. Some structural parameters of FeSe in both tetragonal and hexagonal phases are reported. According to the fourth-order Birch-Murnaghan equation of states, the transition pressure P(t) of FeSe from the PbO-type phase to the NiAs-type phase was determined. The calculated results are found to be in good agreement with the available experimental data. Based on the quasi-harmonic Debye model, the pressure and temperature dependence of the thermodynamic properties for hexagonal phase FeSe were investigated. Our theoretical calculations suggest that the pressure and temperature have significant effects on the heat capacity, vibrational internal energy, vibrational entropy, vibrational Helmholtz free energy, thermal expansion coefficient and Debye temperature. Even though few theoretical reports on the structural properties of FeSe are found in the current literature, to our knowledge, this is a novel theoretical investigation on the structural and thermodynamic properties of FeSe at high temperature. We hope that the theoretical results reported here can give more insight into the structural and thermodynamic properties of other iron-based superconductors at high temperature.

Pressure-driven quantum criticality in iron-selenide superconductors

We report a finding of a pressure-induced quantum critical transition in K0.8Fe(x)Se2 (x = 1.7 and 1.78) superconductors through in situ high-pressure electrical transport and x-ray diffraction measurements in diamond anvil cells. Transitions from metallic Fermi liquid behavior to non-Fermi liquid behavior and from antiferromagnetism to paramagnetism are found in the pressure range of 9.2-10.3 GPa, in which superconductivity tends to disappear. The change around the quantum critical point from the coexisting antiferromagnetism state and the Fermi liquid behavior to the paramagnetism state and the non-Fermi liquid behavior in the iron-selenide superconductors demonstrates a unique mechanism for their quantum critical transition.

The superconducting transition temperatures of Fe(1+x)Se(1-y), Fe(1+x)Se(1-y)Te(y) and (K/Rb/Cs)(z)Fe(2-x)Se2

In a recent contribution to this journal, it was shown that the transition temperatures of optimal high-T(C) compounds obey the algebraic relation T(C0) = k(-1)(B)/ℓζ, where ℓ is related to the mean spacing between interacting charges in the layers, ζ is the distance between interacting electronic layers, β is a universal constant and k(B) is Boltzmann's constant. The equation was derived assuming pairing based on interlayer Coulomb interactions between physically separated charges. This theory was initially validated for 31 compounds from five different high-T(C) families (within an accuracy of ±1.37 K). Herein we report the addition of Fe(1+x)Se(1-y) and Fe(1+x)Se(1-y)Te(y) (both optimized under pressure) and A(z)Fe(2-x)Se(2) (for A = K, Rb or Cs) to the growing list of Coulomb-mediated superconducting compounds in which T(C0) is determined by the above equation. Doping in these materials is accomplished through the introduction of excess Fe and/or Se deficiency, or a combination of alkali metal and Fe vacancies. Consequently, a very small number of vacancies or interstitials can induce a superconducting state with a substantial transition temperature. The confirmation of the above equation for these Se-based Fe chalcogenides increases to six the number of superconducting families for which the transition temperature can be accurately predicted.

Nanoscale phase separation of antiferromagnetic order and superconductivity in K(0.75)Fe(1.75)Se(2)

We report an in-plane optical spectroscopy study on the iron-selenide superconductor K_0.75Fe_1.75Se₂. The measurement revealed the development of a sharp reflectance edge below T_c at frequency much smaller than the superconducting energy gap on a relatively incoherent electronic background, a phenomenon which was not seen in any other Fe-based superconductors so far investigated. Furthermore, the feature could be noticeably suppressed and shifted to lower frequency by a moderate magnetic field. Our analysis indicates that this edge structure arises from the development of a Josephson-coupling plasmon in the superconducting condensate. Together with the transmission electron microscopy analysis, our study yields compelling evidence for the presence of nanoscale phase separation between superconductivity and magnetism. The results also enable us to understand various seemingly controversial experimental data probed from different techniques.

Electronic and magnetic phase diagram in K(x)Fe(2-y)Se(2) superconductors

The correlation and competition between antiferromagnetism and superconductivity are one of the most fundamental issues in high temperature superconductors. Superconductivity in high temperature cuprate superconductors arises from suppressing an antiferromagnetic (AFM) Mott insulator1 while in iron-pnictide superconductors arises from AFM semimetals and can coexist with AFM orders23456789. This difference raises many intriguing debates on the relation between the two classes of high temperature superconductors. Recently, superconductivity at 32 K has been reported in iron-chalcogenide superconductors A_xFe_2−ySe₂ (A = K, Rb, and Cs)101112. They have the same structure as that of iron-pnictide 122-system131415. Here, we report electronic and magnetic phase diagram of K_xFe_2−ySe₂ system as a function of Fe valence. We find a superconducting phase sandwiched between two AFM insulating phases. The two insulating phases are characterized by two distinct superstructures caused by Fe vacancy orders with modulation wave vectors of q₁ = (1/5, 3/5, 0) and q₂ = (1/4, 3/4, 0), respectively.

Magnetic resonant mode in the low-energy spin-excitation spectrum of superconducting Rb2Fe4Se5 single crystals

We have studied the low-energy spin-excitation spectrum of the single-crystalline Rb(2)Fe(4)Se(5) superconductor (T(c)=32 K) by means of inelastic neutron scattering. In the superconducting state, we observe a magnetic resonant mode centered at an energy of ℏω(res)=14 meV and at the (0.5 0.25 0.5) wave vector (unfolded Fe-sublattice notation), which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors. Our finding suggests that the 245-iron selenides are unconventional superconductors with a sign-changing order parameter, in which bulk superconductivity coexists with the √5×√5 magnetic superstructure. The estimated ratios of ℏω(res)/k(B)T(c)≈5.1±0.4 and ℏω(res)/2Δ≈0.7±0.1, where Δ is the superconducting gap, indicate moderate pairing strength in this compound, similar to that in optimally doped 1111 and 122 pnictides.

Phase diagram and a possible unified description of intercalated iron selenide superconductors

We propose a theoretical description of the phase diagram and physical properties in A(2)Fe(4)Se(5)-type (A=K, Tl) compounds based on a coexistent local moment and itinerant electron picture. Using neutron scattering and angle-resolved photoemission spectroscopy measurements to fix the general structure of the local moment and itinerant Fermi pockets, we find a superconducting phase with s-wave pairing at the M pockets and an incipient sign-change s wave near the Γ point, which is adjacent to the insulating phases. The uniform susceptibility and resistivity are found to be consistent with the experiment. The main distinction with iron pnictide superconductors is also discussed.

The synthesis, and crystal and magnetic structure of the iron selenide BaFe2Se3 with possible superconductivity at Tc = 11 K

We report on the synthesis of single crystals of BaFe(2)Se(3) and study their crystal and magnetic structures by means of synchrotron single-crystal x-ray and neutron powder diffraction. The crystal structure has orthorhombic symmetry and consists of double chains of FeSe(4) edge connected tetrahedra intercalated with barium. Below 240 K, long range spin-block checkerboard antiferromagnetic order is developed. The magnetic structure is similar to one observed in A(0.8)Fe(1.6)Se(2) (A = K, Rb or Cs) superconductors. The crystals exhibit a transition to the diamagnetic state with an onset transition temperature of T(c) ∼ 11 K. Though we observe FeSe as an impurity phase (<0.8% mass fraction) it is not likely that the diamagnetism is attributable to the FeSe superconductor, which has T(c) ≈ 8.5 K.

Temperature and pressure evolution of the crystal structure of A(x)(Fe(1-y)Se)2 (A = Cs, Rb, K) studied by synchrotron powder diffraction

Temperature-dependent synchrotron powder diffraction on Cs(0.83)(Fe(0.86)Se)(2) revealed first-order I4/m to I4/mmm structural transformation around 216 °C associated with a disorder of the Fe vacancies. Irreversibility observed during the transition is likely associated with a mobility of the intercalated alkali atoms. Pressure-dependent synchrotron powder diffraction on Cs(0.83)(Fe(1-y)Se)(2), Rb(0.85)(Fe(1-y)Se)(2), and K(0.8)(Fe(1-y)Se)(2) (y ~ 0.14) indicated that the I4/m superstructure reflections are present up to pressures of 120 kbar. This may indicate that the ordering of the Fe vacancies is present in both superconducting and nonsuperconductive states.

Common crystalline and magnetic structure of superconducting A2Fe4Se5 (A=K,Rb,Cs,Tl) single crystals measured using neutron diffraction

Single-crystal neutron diffraction studies on superconductors A(2)Fe(4)Se(5), where A=Rb, Cs, (Tl, Rb), and (Tl, K) (T(c) ∼ 30 K), uncover the same Fe vacancy ordered crystal structure and the same block antiferromagnetic order as in K(2)Fe(4)Se(5). The Fe order-disorder transition occurs at T(S)=500-578 K, and the antiferromagnetic transition at T(N) = 471-559 K with an ordered magnetic moment ∼3.3μ(B)/Fe at 10 K. Thus, all recently discovered A intercalated iron selenide superconductors share the common crystalline and magnetic structure, which are very different from previous families of Fe-based superconductors, and constitute a distinct new 245 family.

Room temperature antiferromagnetic order in superconducting X(y)Fe(2-x)Se₂ (X = Rb, K): a neutron powder diffraction study

Magnetic and crystal structures of superconducting X(y)Fe(2-x)Se₂ (X = Rb and K with T(c) = 31.5 and 29.5 K) have been studied by neutron powder diffraction at room temperature. Both crystals show an ordered iron vacancy pattern and the crystal structure is well described by the I4/m space group with the lattice constants a = 8.799, c = 14.576 and a = 8.730, c = 14.115 Å and the refined stoichiometry x = 0.30(1), y = 0.83(2) and x = 0.34(1), y = 0.83(1) for Rb and K crystals, respectively. The structure contains one fully occupied iron position and one almost empty vacancy position. Assuming that the iron moment is ordered only on the fully occupied site we have sorted out all eight irreducible representations (irreps) for the propagation vector k = 0 and have found that irreps τ₂ and τ₇ fit the experimental data well with the moments along the c axis. The moment amplitudes amounted to 2.15(3) µ(B), 2.55(3) μ(B) for τ₂ and 2.08(6) μ(B), 2.57(3) μ(B) for τ₇ for Rb and K crystals, respectively. Irrep τ₂ corresponds to the Shubnikov group I4/m' and gives a constant moment antiferromagnetic configuration, whereas τ₇ does not have a Shubnikov counterpart and allows two different magnetic moments in the structure.