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The past decade has witnessed the burgeoning discovery of a variety of topological states of matter with distinct nontrivial band topologies. Thus far, most materials that have been studied possess two-dimensional or three-dimensional electronic structures, with only a few exceptions that host quasi-one-dimensional (quasi-1D) topological electronic properties. Here we present clear-cut evidence for Dirac Fermions in the quasi-1D telluride TaNiTe5. We show that its transport behaviors are highly anisotropic, and we observe nontrivial Berry phases via quantum oscillation measurements. The nontrivial band topology is further corroborated by first-principles calculations. Our results may help to guide the future quest for topological states in this new family of quasi-1D ternary chalcogenides.

A spontaneous vortex structure in magnetic domains (vortex domains) has been visualized by the decoration technique near the ferromagnetic transition temperature (TC = 18 K) in a ferromagnetic superconductor EuFe2(As0.79P0.21)2 single crystal with the superconducting transition temperature (TSC = 22 K). Meissner domains previously observed by magnetic force microscopy have also been observed in a narrow temperature range (ΔT ∼ 1 K) near TC. A domain structure with the alternating direction of the magnetic flux has been observed upon cooling below TC. Experimental results are in reasonable agreement with a theoretical estimate of the periods of domain structures and the temperature interval below TC in which they are observed.

Understanding the interplay among different magnetic exchange interactions and its physical consequences, especially in the presence of itinerant electrons and disorder, remains one of the central themes in condensed matter physics. In this vein, the coupling between antiferromagnetic and spin-glass orders may lead to large exchange bias, a property with potential broad technological applications. In this paper, we report the coexistence of antiferromagnetic order and spin-glass behaviors in the quasi-one-dimensional iron telluride ${\mathrm{TaFe}}_{1+x}{\mathrm{Te}}_{3}$ ($x=0.25$). Its antiferromagnetism is believed to arise from the antiferromagnetic interchain coupling between the ferromagnetically aligned FeTe chains along the $b$ axis, while the spin-glassy state stems from the disordered Fe interstitials. This dichotomic role of chain and interstitial sublattices is responsible for the large exchange bias observed at low temperatures, with the interstitial Fe acting as the uncompensated moment and its neighboring Fe chain providing the source for its pinning. This iron-based telluride may thereby represent a paradigm to study the large family of transition metal chalcogenides whose magnetic order or even dimensionality can be tuned to a large extent, forming a fertile playground to manipulate or switch the spin degrees of freedom thereof.

We report on the solid-state synthesis and the strongly anisotropic transport properties of the ternary telluride TaNi2Te3, whose three orthogonal resistivity coefficients exhibit a large ratio of 1.4:1:2294 (14:1:2303) at 300 K (2 K), thereby demonstrating its quasi-one-dimensional (q1D) electronic structure. The Kohler’s rule in different current/field configurations shows a moderate violation. Its one dimensionality manifests itself in the needle-like shape of crystal, the large anisotropic resistivity and the flat Fermi surface normal to the chain direction. Moreover, the first-principles calculations also provide evidence for the existence of the nontrivial topological carriers in this q1D system. Our calculation demonstrates that TaNi2Te3 is a strong topological nontrivial material with topological indices (1; 1 0 1) and its nontrivial topology is also evidenced from its bulk-surface correspondence. Our study may therefore offer a new platform for engineering the topologically nontrivial phase in low-dimensional materials, in analogy to the recently discovered q1D topological TaNiTe5.

We report the synthesis and systematic studies of a new layered ternary telluride ${\mathrm{TaPdTe}}_{5}$ with quasi-one-dimensional ${\mathrm{PdTe}}_{2}$ chains. This compound crystalizes in a layered orthorhombic structure with space group $Cmcm$. Analysis of its curved field-dependent Hall resistivity, using the two-band model, indicates the hole-dominated transport with a high mobility ${\ensuremath{\mu}}_{h}=2.38\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at low temperatures. The in-plane magnetoresistance (MR) displays significant anisotropy with field applied along the crystallographic $b$ axis. The MR with the current applied along the $c$ axis is also measured in high magnetic fields up to 51.7 T. Remarkably, it follows a power-law dependence and reaches $(9.5\ifmmode\times\else\texttimes\fi{}{10}^{3})%$ at 2.1 K without any signature of saturation. The de Haas--van Alphen oscillations show a small Fermi surface pocket with a nontrivial Berry phase. The Shubnikov--de Haas (SdH) oscillations are detected at low temperatures and under magnetic fields above 28.5 T. Two effective masses ${m}^{*}$ ($0.26{m}_{e}$ and $0.41{m}_{e}$) are extracted from the oscillatory SdH data. Our first-principles calculations unveil a topological Dirac cone in its surface states, and, in particular, the topological index indicates that ${\mathrm{TaPdTe}}_{5}$ is a topologically nontrivial material.

Superconductivity in topological materials, either at ambient or extreme conditions, has continued to intrigue scientists as a promising candidate for realizing topological superconductivity, a platform to host the long-sought Majorana fermions in condensed matter. The recent discovery of extremely large magnetoresistance (XMR) in the rare-earth monopnictides opens a new avenue to search for topologically nontrivial states therein, although contrasting opinions argue that it is the carrier compensation effect that is responsible for the observed large, nonsaturating magnetoresistance. Here we study the quantum oscillations and pressure-induced superconductivity in the topologically nontrivial candidate YBi. While the magnetotransport and quantum oscillations do reveal nearly compensated charge carriers, first-principles calculations clearly show that the electronic surface states manifest topologically nontrivial features. Upon applying external hydrostatic pressures, the magnetoresistance is found to decrease and at $P\ensuremath{\sim}2.5$ GPa, superconductivity emerges. There exists, however, a regime where XMR and superconductivity coexist in the phase diagram. YBi may therefore represent a rare system for studying the interplay between XMR, topological states, and superconductivity.

Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe2-class of transition metal dichalcogenides. However, the Dirac nodes observed in PtSe2, PdTe2, and PtTe2 candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here, we report the synthesis of a new type-II Dirac semimetal NiTe2 in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry’s phase associated with these Dirac fermions. Our first-principles calculations f...

In this study, we measure the in-plane transport properties of high-quality Ba(Fe0.914Co0.086)2As2 single crystals. Signatures of vortex unbinding Berezinskii–Kosterlitz–Thouless (BKT) transition are shown from both the conventional approach and the Fisher–Fisher–Huse dynamic scaling analysis, in which a characteristic Nelson–Kosterlitz jump is demonstrated. We also observe a non-Hall transverse signal exactly at the superconducting transition, which is explained in terms of guided motion of unbound vortices.

This paper presents a microwave analysis of the interplay between magnetism and superconductivity in an iron-based ferromagnetic superconductor. By comparing the complex rf susceptibility with magnetic force images, the authors discuss the nature of the observed phase transitions and the possible presence of a quantum critical point.

The study of universal critical behavior is a crucial issue in a continuous phase transition, which groups various critical phenomena into universality classes for revealing microscopic electronic behaviors. The understanding of the nature of magnetism in Eu-based ferromagnetic superconductors is largely impeded by the infeasibility of performing inelastic neutron scattering measurements to deduce the microscopic magnetic behaviors and the effects on the superconductivity, due to the significant neutron absorption effect of natural $^{152}\mathrm{Eu}$ and unavailability of large single crystals. However, by systematically combining the neutron diffraction experiment, the first-principles calculations, and the quantum Monte Carlo simulations, we have obtained a perfectly consistent universal critical exponent value of $\ensuremath{\beta}=0.385(13)$ experimentally and theoretically for $\mathrm{Eu}{({\mathrm{Fe}}_{0.75}{\mathrm{Ru}}_{0.25})}_{2}{\mathrm{As}}_{2}$, from which the magnetism in the Eu-based ferromagnetic superconductors is identified as the universal class of a three-dimensional anisotropic quantum Heisenberg model with long-range magnetic exchange coupling. This systematic study points out a suitable microscopic theoretical model for describing the nature of magnetism in the intriguing Eu-based ferromagnetic superconductors.

The iron-based superconductors represent a promising platform for high-temperature superconductivity, but the interactions underpinning their pairing present a puzzle. The EuFe$_2$As$_2$ family is unique among these materials for having magnetic order which onsets within the superconducting state, just below the superconducting transition. Superconductivity and magnetic order are normally antagonistic and often vie for the same unpaired electrons, but in this family the magnetism arises from largely localized Eu moments and they coexist, with the competition between these evenly-matched opponents leading to reentrant superconducting behavior. To help elucidate the physics in this family and the interactions between the magnetic order and superconductivity, we investigate the $H$--$T$ phase diagram near optimal Rh doping through specific heat, resistivity, and magnetization measurements, and study the electronic structure by angular-resolved photoemission spectroscopy. The competition between the Eu and FeAs layers may offer a route to directly accessing the electronic structure under effective magnetic fields via ARPES, which is ordinarily a strictly zero-field technique.

We examined the physical properties of the quasi-one-dimensional superconductor Ta

The interplay between superconductivity and magnetism is one of the oldest enigmas in physics. Usually, the strong exchange field of ferromagnet suppresses singlet superconductivity via the paramagnetic effect. In EuFe2(As0.79P0.21)2, a material that becomes not only superconducting at 24.2 K but also ferromagnetic below 19 K, the coexistence of the two antagonistic phenomena becomes possible because of the unusually weak exchange field produced by the Eu subsystem. We demonstrate experimentally and theoretically that when the ferromagnetism adds to superconductivity, the Meissner state becomes spontaneously inhomogeneous, characterized by a nanometer-scale striped domain structure. At yet lower temperature and without any externally applied magnetic field, the system locally generates quantum vortex-antivortex pairs and undergoes a phase transition into a domain vortex-antivortex state characterized by much larger domains and peculiar Turing-like patterns. We develop a quantitative theory of this phenomenon and put forth a new way to realize superconducting superlattices and control the vortex motion in ferromagnetic superconductors by tuning magnetic domains—unprecedented opportunity to consider for advanced superconducting hybrids.

Superconductivity in noncentrosymmetric compounds has attracted sustained interest in the last decades. Here we present a detailed study on the transport, thermodynamic properties and the band structure of the noncentrosymmetric superconductor La 7 Ir 3 (T c ~ 2.3 K) that was recently proposed to break the time-reversal symmetry. It is found that La7Ir3 displays a moderately large electronic heat capacity (Sommerfeld coefficient γ n ~ 53.1 mJ/mol K2) and a significantly enhanced Kadowaki-Woods ratio (KWR ~32 μΩ cm mol2 K2 J−2) that is greater than the typical value (~10 μΩ cm mol2 K2 J−2) for strongly correlated electron systems. The upper critical field Hc2 was seen to be nicely described by the single-band Werthamer-Helfand-Hohenberg model down to very low temperatures. The hydrostatic pressure effects on the superconductivity were also investigated. The heat capacity below T c reveals a dominant s-wave gap with the magnitude close to the BCS value. The first-principles calculations yield the electron-phonon coupling constant λ = 0.81 and the logarithmically averaged frequency ω ln = 78.5 K, resulting in a theoretical T c = 2.5 K, close to the experimental value. Our calculations suggest that the enhanced electronic heat capacity is more likely due to electron-phonon coupling, rather than the electron-electron correlation effects. Collectively, these results place severe constraints on any theory of exotic superconductivity in this system.

We report evidence for a reentrant spin glass phase in electron-doped $\text{EuFe}_2\text{As}_2$ single crystals and first traces of the superconductivity re-entrance in optics. In the close-to-optimal doped $\text{Eu}(\text{Fe}_{0.91}\text{Ir}_{0.09})_2\text{As}_2$ and $\text{Eu}(\text{Fe}_{0.93}\text{Rh}_{0.07})_2\text{As}_2$ samples two magnetic transitions are observed below the superconducting critical temperature $T_c \approx 21$~K: the canted $A$-type antiferromagnetic order of the $\text{Eu}^{2+}$ ions sets in around $17\,\text{K}$; the spin glass behavior occurs another $2\,\text{K}$ lower in temperature. In addition, strong evidence for an additional transition is found far below the spin glass temperature. Our extensive optical and magnetic investigations provide new insight into the interplay of local magnetism and superconductivity in these systems and elucidate the effect of the spin-glass phase on the reentrant superconducting state.

Electron-doped $\mathrm{Eu}{({\mathrm{Fe}}_{0.93}{\mathrm{Rh}}_{0.07})}_{2}{\mathrm{As}}_{2}$ has been systematically studied by high-pressure investigations of the magnetic and electrical transport properties in order to unravel the complex interplay of superconductivity and magnetism. The compound reveals an exceedingly broad reentrant transition to the superconducting state between ${T}_{\mathrm{c},\mathrm{on}}=19.8$ K and ${T}_{\mathrm{c},0}=5.2$ K due to a canted A-type antiferromagnetic ordering of the ${\mathrm{Eu}}^{2+}$ moments at ${T}_{\mathrm{N}}=16.6$ K and a reentrant spin glass transition at ${T}_{\mathrm{SG}}=14.1$ K. At ambient pressure evidences for the coexistence of superconductivity and ferromagnetism could be observed, as well as a magnetic-field-induced enhancement of the zero-resistance temperature ${T}_{\mathrm{c},0}$ up to 7.2 K with small magnetic fields applied parallel to the $ab$ plane of the crystal. We attribute the field-induced-enhancement of superconductivity to the suppression of the ferromagnetic component of the ${\mathrm{Eu}}^{2+}$ moments along the $c$ axis, which leads to a reduction of the orbital-pair-breaking effect. Application of hydrostatic pressure suppresses the superconducting state around 14 kbar along with a linear temperature dependence of the resistivity, implying that a non-Fermi liquid region is located at the boundary of the superconducting phase. At intermediate pressure, an additional feature in the resistivity curves is identified, which can be suppressed by external magnetic fields and competes with the superconducting phase. We suggest that the effect of negative pressure by the chemical Rh substitution in $\mathrm{Eu}{({\mathrm{Fe}}_{0.93}{\mathrm{Rh}}_{0.07})}_{2}{\mathrm{As}}_{2}$ is partially reversed, leading to a reactivation of the spin density wave.

We examined the physical properties of the quasi-one-dimensional superconductor Ta4Pd3Te16 in the normal state by detailed measurements of susceptibility, in-plane anisotropic resistivity, magnetoresistance, Hall resistivity, and Seebeck coefficient. The large Wilson ratio, as inferred from normal-state susceptibility, indicates strong electron-electron interaction. The Hall and Seebeck coefficients show not only significant temperature-dependent behavior, indicating the multiband effect, but also an obvious anomaly around T 1 = 40 K. Analyses of both the Hall resistivity and thermopower using a two-band model indicate that the electrons dominate the electrical transport at low temperatures. Our results imply that it is the quantum fluctuations of the charge order taking place in the temperature range 30-50 K that may result in the abnormal normal-state properties of Ta4Pd3Te16.

We report $^{125}\mathrm{Te}$ nuclear magnetic resonance and $^{181}\mathrm{Ta}$ nuclear quadrupole resonance studies on single-crystal ${\mathrm{Ta}}_{4}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{16}$, which has a quasi-one-dimensional structure and superconducts below ${T}_{c}=4.3$ K. $^{181}\mathrm{Ta}$ with spin $I=7/2$ is sensitive to quadrupole interactions, while $^{125}\mathrm{Te}$ with spin $I=1/2$ can only relax by magnetic interactions. By comparing the spin-lattice relaxation rate ($1/{T}_{1}$) of $^{181}\mathrm{Ta}$ and $^{125}\mathrm{Te}$, we found that electric-field-gradient (EFG) fluctuations develop below 80 K. The EFG fluctuations are enhanced with decreasing temperature due to the fluctuations of a charge density wave that sets in at ${T}_{\mathrm{CDW}}=20$ K, below which the spectra are broadened and $1/{T}_{1}T$ drops sharply. In the superconducting state, $1/{T}_{1}$ shows a Hebel-Slichter coherence peak just below ${T}_{c}$ for $^{125}\mathrm{Te}$, indicating that ${\mathrm{Ta}}_{4}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{16}$ is a full-gap superconductor without nodes in the gap function. The coherence peak is absent in the $1/{T}_{1}$ of $^{181}\mathrm{Ta}$ due to the strong EFG fluctuations.

We have synthesized a new oxypnictide, Ba2Ti2Fe2As4O, via a solid-state reaction under a vacuum. The compound crystallizes in a body-centered tetragonal lattice, which can be viewed as an intergrowth of BaFe2As2 and BaTi2As2O, thus containing Fe2As2 layers and Ti2O sheets. Bulk superconductivity at 21 K is observed after annealing the as-prepared sample at 773 K for 40 h. In addition, an anomaly in resistivity and magnetic susceptibility around 125 K is revealed, suggesting a charge- or spin-density wave transition in the Ti sublattice.

EuRu2As2 was characterized by structural, resistivity, magnetization and Mossbauer measurements. It crystallizes in collapsed ThCr2Si2-type structure with c/a=2.58 and space group I4/mmm. The temperature and field dependences of magnetization indicate that the material is ferromagnetically ordered at T C=17.3±0.5 K, where the electrical resistivity shows a kink. Mossbauer spectroscopy studies determine that the Eu2+ moments align basically along the crystallographic c-axis, which is in contrast to the case in EuFe2As2 in which the Eu2+ spins lie in the basal planes.

Superconductivity (SC) and ferromagnetism (FM) are in general antagonistic, which makes their coexistence very rare. Following our recent discovery of robust coexistence of SC and FM in RbEuFe$_4$As$_4$ [Y. Liu et al., arXiv: 1605.04396 (2016)], here we report another example of such a coexistence in its sister compound CsEuFe$_4$As$_4$, synthesized for the first time. The new material exhibits bulk SC at 35.2 K and Eu$^{2+}$-spin ferromagnetic ordering at 15.5 K, demonstrating that it is a new robust ferromagnetic superconductor., Comment: 7 pages, 6 figures

We measured the low-temperature thermal conductivity of a layered superconductor with quasi-one-dimensional characteristics, the ternary telluride ${\mathrm{Ta}}_{4}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{16}$ with a transition temperature ${T}_{c}\ensuremath{\approx}4.3$ K. The significant residual linear term of thermal conductivity in zero magnetic field and its rapid field dependence provide evidence for nodes in the superconducting gap. By measuring resistivity under pressure, we reveal a superconducting dome in the temperature-pressure phase diagram. The existence of gap nodes and a superconducting dome suggest unconventional superconductivity in ${\mathrm{Ta}}_{4}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{16}$, which may relate to a charge-density-wave instability in this low-dimensional compound.

The magnetic order of the localized Eu$^{2+}$ spins in optimally-doped Eu(Fe$_{1-x}$Ir$_{x}$)$_{2}$As$_{2}$ ($\mathit{x}$ = 0.12) with superconducting transition temperature $\mathit{T_{SC}}$ = 22 K was investigated by single-crystal neutron diffraction. The Eu$^{2+}$ moments were found to be ferromagnetically aligned along the $\mathit{c}$-direction with an ordered moment of 7.0(1) $\mu_{B}$ well below the magnetic phase transition temperature $\mathit{T_{C}}$ = 17 K. No evidence of the tetragonal-to-orthorhombic structural phase transition was found in this compound within the experimental uncertainty, in which the spin-density-wave (SDW) order of the Fe sublattice is supposed to be completely suppressed and the superconductivity gets fully developed. The ferromagnetic groud state of the Eu$^{2+}$ spins in Eu(Fe$_{0.88}$Ir$_{0.12}$)$_{2}$As$_{2}$ was supported by the first-principles density functional calculation. In addition, comparison of the electronic structure calculations between Eu(Fe$_{0.875}$Ir$_{0.125}$)$_{2}$As$_{2}$ and the parent compound EuFe$_{2}$As$_{2}$ indicates stronger hybridization and more expanded bandwith due to the Ir substitution, which together with the introduction of electrons might work against the Fe-SDW in favor of the superconductivity.

We have performed polarized Raman scattering measurements on the newly discovered superconductor Ta$_{4}$Pd$_{3}$Te$_{16}$ ($T_c = 4.6$ K). We observe twenty-eight out of thirty-three Raman active modes, with frequencies in good accordance with first-principles calculations. Although most of the phonons observed vary only slightly with temperature and do not exhibit any asymmetric profile that would suggest strong electron-phonon coupling, the linewidth of the A$_{g}$ phonon mode at 89.9 cm$^{-1}$ shows an unconventional increase with temperature decreasing, which is possibly due to a charge-density-wave transition or the emergence of charge-density-wave fluctuations below a temperature estimated to fall in the 140-200 K range., Comment: 6 pages, 4 figures, 1 table

We investigate the anisotropic magnetic transports in topological semimetal TaSb2. The compound shows the large magnetoresistance (MR) without saturation and the metal-insulator-like transition no matter whether the magnetic field is parallel to c-axis or a-axis, except that the MR for B||c is almost twice as large as that of B||a at low temperatures. The adopted Kohler's rule can be obeyed by the MR at distinct temperatures for B||c, but it is slightly violated as B||a. The angle-dependent MR measurements exhibit the two-fold rotational symmetry below 70 K, consistent with the monoclinic crystal structure of TaSb2. The dumbbell-like picture of angle-dependent MR in TaSb2 suggests a strongly anisotropic Fermi surface at low temperatures. However, it finally loses the two-fold symmetry over 70 K, implying a possible topological phase transition at around the temperature where Tm is related to a metal-insulator-like transition under magnetic fields.

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states, which have received sustained interest for decades. Here, we report that, in a layered europium bismuth sulfofluoride, ${\mathrm{EuBiS}}_{2}\mathrm{F}$, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of a possible dynamic CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane $\mathrm{Bi}6p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by $\ensuremath{\sim}50$ times, owing to the significant hybridizations between $\mathrm{Eu}4f$ and $\mathrm{Bi}6p$ electrons, as verified by band-structure calculations. Thus ${\mathrm{EuBiS}}_{2}\mathrm{F}$ manifests itself as an unprecedented material that simultaneously accommodates SC, CDW, and $f$-electron valence instability.

Ta4Pd3Te16 crystals are prepared by a self-flux method from mixtures of the elements in an atomic ratio of Ta:Pd:Te = 2:3:15 (quartz ampule, 1223 K, 24 h).

Electronic nematics, an electron orientational order which breaks the underlying rotational symmetry, have been observed in iron pnictide superconductors several years after their discovery. However, the universality of the doping dependence of this phase and its relation to other symmetry-breaking orders (such as superconductivity) in distinct families of iron pnictides, remain outstanding questions. Here we use torque magnetometry as a probe to study the rotational symmetry breaking in EuFe$_2$(As$_{1-x}$P$_x$)$_2$ without introducing external pressure. The nematic phase is found to proliferate well above the structural transition and to persist into the superconducting regime at optimal doping, after which it becomes absent or very weak, in sharp contrast to the behaviour observed in BaFe$_2$(As$_{1-x}$P$_x$)$_2$. These measurements suggest a putative quantum nematic transition near optimal doping under the superconducting dome.

The magnetic structure of superconducting Eu(Fe${}_{0.82}$Co${}_{0.18}$)${}_{2}$As${}_{2}$ is unambiguously determined by single-crystal neutron diffraction. A long-range ferromagnetic order of the Eu${}^{2+}$ moments along the $c$ direction is revealed below the magnetic phase transition temperature ${T}_{C}=17$ K. In addition, the antiferromagnetism of the Fe${}^{2+}$ moments still survives and the tetragonal-to-orthorhombic structural phase transition is also observed, although the transition temperatures of the Fe spin-density-wave (SDW) order and the structural phase transition are significantly suppressed to ${T}_{N}=70$ K and ${T}_{S}=90$ K, respectively, compared to the parent compound EuFe${}_{2}$As${}_{2}$. We present microscopic evidence for the coexistence of the Eu ferromagnetism and the Fe SDW in the superconducting crystal. The superconductivity competes with the Fe SDW in Eu(Fe${}_{0.82}$Co${}_{0.18}$)${}_{2}$As${}_{2}$. Moreover, the comparison between Eu(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ and Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ indicates a considerable influence of the rare-earth element Eu on the magnetism of the Fe sublattice.

We report a detailed calorimetric study on single crystals of the 5$d$-transition metal pnictide SrPt2As2 with a superconducting critical temperature $T_c$ $\sim$5K. The peculiar field dependence of the electronic specific heat coefficient $\gamma$ can be decomposed into two linear components. Moreover, the temperature evolution of the electronic specific heat below $T_c$ is best described by a two-gap model. These findings suggest that two energy gaps are associated with the superconductivity. In parallel, we show that the spin-lattice relaxation time $T_1$, through nuclear magnetic resonance measurement, obeys the so-called Korringa relation well. This, along with the $T^2$ dependence of resistivity at low temperatures, points to a Fermi liquid ground state in this material.

We have performed an isovalent substitution study in a layered titanium oxypnictide system BaTi${}_{2}$(Sb${}_{1\ensuremath{-}x}$Bi${}_{x}$)${}_{2}$O ($0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.40$) by measurements of x-ray diffraction, electrical resistivity, and magnetic susceptibility. The parent compound BaTi${}_{2}$Sb${}_{2}$O is confirmed to exhibit superconductivity at 1.5 K as well as charge- or spin-density wave (CDW/SDW) ordering below 55 K. With the partial substitution of Sb by Bi, the lattice parameters $a$, $c$, and $c/a$ all increase monotonically, indicating a negative chemical pressure and lattice distortion for the (super)conducting Ti${}_{2}$Sb${}_{2}$O layers. Bi doping elevates the superconducting transition temperature to its maximum ${T}_{c}=3.7$ K at $x=0.17$, and then ${T}_{c}$ decreases gradually with further Bi doping. A metal-to-nonmetal transition takes place around $x=0.3$, and superconductivity at $\ensuremath{\sim}$1 K survives on the nonmetal side. The CDW/SDW anomaly, in comparison, is rapidly suppressed by Bi doping, and vanishes for $x\ensuremath{\geqslant}0.17$. The results are discussed in terms of negative chemical pressure and the disorder effect.

Large single crystals of optimally doped Eu(Fe0.91Rh0.09)2As2 were grown by the self-flux method. Resistivity and magnetization measurements show that the compound undergoes a superconducting transition at 19.6 K, followed by a ferromagnetic ordering of the Eu2+ moments at 16.8 K. The upper critical fields at zero temperature are estimated to be 12.5 T and 15.1 T for fields along the ab plane and the c-axis, respectively. Moreover, anisotropy reversal of the upper critical field is observed near the magnetic ordering temperature, which may be related to the anisotropic magnetization of Eu ions and the demagnetization effect. Both ferromagnetic and superconducting features are displayed by the magnetic susceptibility and peculiar isothermal magnetization loop. Our combined results unambiguously demonstrate the coexistence of ferromagnetism and superconductivity in the title compound.

authoren We present a broadband optical-conductivity study of superconducting single-crystalline EuFe(AsP) with three different substitutional levels. We analyze the normal-state electrodynamics by decomposing the conductivity spectra using a Drude–Lorentz model with two Drude terms representing two groups of carriers with different scattering rates. The analysis reveals that the scattering rate of at least one of the Drude components develops linearly with temperature for each doping level. This points toward strong electron–electron correlations and a non-Fermi-liquid behavior in the P-substituted superconducting Eu-122 pnictides. We also detect a transfer of the spectral weight from mid-infrared to higher frequencies and assign it to the Hund's-rule coupling between itinerant and localized carriers. The conductivity spectra below the superconducting transition show no sharp features to be associated with the dirty-limit superconducting s-wave gaps. We interpret these results in terms of clean-limit superconductivity in EuFe(AsP). The best parametrization fit can be achieved using a two-gap model. We find that the larger gap at the hole pockets of the Fermi surface is likely to be isotropic, while the smaller gap at the electron pockets is anisotropic or even nodal. Schematic phase diagram of EuFe(AsP). The indicated phases are: spin density wave (SDW), superconductivity (SC), magnetic ordering of Eu spins (, brown), and a spin-glass state of the Eu subsystem (, green). Above the SC dome, a non-Fermi-liquid behavior (NFL) points toward a possible quantum critical point (QCP). The black arrows indicate the approximate positions of the compositions investigated in this study.

Millimeter sized ThCr$_2$Si$_2$-type CeNi$_2$As$_2$ single crystal was synthesized by NaAs flux method and its physical properties were investigated by magnetization, transport and specific heat measurements. In contrast to the previously reported CaBe$_2$Ge$_2$-type CeNi$_2$As$_2$, the ThCr$_2$Si$_2$-type CeNi$_2$As$_2$ is a highly anisotropic uniaxial antiferromagnet with the transition temperature $T_N$=4.8 K. A field induced spin flop transition was seen below $T_N$ when the applied $\textbf{B}$ is parallel to the $\textbf{c}$-axis, the magnetic easy axis, together with a huge frustration parameter $f=\theta_W/T_N$. A pronounced Schottky-like anomaly in specific heat was also found around 160 K, which could be attributed to the crystalline electric field effect with the excitation energies being fitted to $\Delta_1=$325 K and $\Delta_2=$520 K, respectively. Moreover, the in-plane resistivity anisotropy and low temperature X-ray diffractions suggest that this compound is a rare example exhibiting a possible structure distortion induced by the $4f$-electron magnetic frustration.

We report a systematic angle-resolved photoemission spectroscopy study on Ba(Fe1-xRux)(2)As-2 for a wide range of Ru concentrations (0.15

Heavily doped Ba${}_{1\ensuremath{-}x}$K${}_{x}$Mn${}_{2}$As${}_{2}$ ($x=0.19$ and 0.26) single crystals were successfully grown and investigated by the measurements of resistivity and anisotropic magnetization. In contrast to the antiferromagnetic insulating ground state of the undoped BaMn${}_{2}$As${}_{2}$, the K-doped crystals show metallic conduction with weak ferromagnetism below $\ensuremath{\sim}$50 K and Curie--Weiss-like in-plane magnetic susceptibility above $\ensuremath{\sim}$50 K. Under high pressures up to 6 G Pa, the low-temperature metallicity changes into a state characterized by a Kondo-like resistivity minimum. Electronic structure calculations for $x=0.25$ using a $2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}1$ supercell reproduce the hole-doped metallic state. The density of states at Fermi energy has significant As-4$p$ components, indicating that the 4$p$ holes are mainly responsible for the metallic conduction. Our results suggest that the interplay between the itinerant 4$p$ holes and the local 3$d$ moments is mostly responsible for the novel metallic state.

Eu(Fe(0.79)Ru(0.21))2As2 is suggested to be a nodeless superconductor based on the empirical correlation between pnictogen height (hPn) and superconducting gap behavior, in contrast to BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2. We studied the low-lying electronic structure of Eu(Fe(0.79)Ru(0.21))2As2 with angle-resolved photoemission spectroscopy (ARPES). By photon energy dependence and polarization dependence measurements, we resolved the band structure in the three-dimensional momentum space and determined the orbital character of each band. In particular, we found that the dz2 -originated ζ band does not contribute spectral weight to the Fermi surface around Z, unlike BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2. Since BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2 are nodal superconductors and their hPn's are less than 1.33 Å, while the hPn of Eu(Fe(0.79)Ru(0.21))2As2 is larger than 1.33 Å, our results provide more evidence for a direct relationship between nodes, dz2 orbital character and hPn. Our results help to provide an understanding of the nodal superconductivity in iron-based superconductors.

Ba0.6K0.4Fe2Se3 (BKFS) single crystals were investigated by means of measurements of powder x-ray diffraction, temperature-dependent resistivity, anisotropic dc magnetization, ac magnetic susceptibility and specific heat. The powder x-ray diffraction indicates staggered iron displacements along the ladders with short and long Fe-Fe bond lengths (2.64(2) and 2.91(2) Å) variation. The resistivity of BKFS exhibits variable range hopping behavior with ln(ρ) ~ T(-1/2) at low temperature. The magnetic susceptibility χ(T) exhibits a sharp cusp at around 20 K in a zero-field-cooled process. The frequency-dependent ac magnetic susceptibility reveals that the cusp feature is attributable to spin glass behavior. The anisotropic ac magnetic susceptibility indicates that BKFS is probably an anisotropic Heisenberg-like spin glass with its easy magnetization plane perpendicular to the chain direction. The specific heat also supports an insulating and spin glass ground state. Extended Curie-Weiss behavior above 40 K was observed with a reduced effective moment (μ(eff) = 1.66 μ(B)/Fe for H is perpendicular to b and μ(eff) = 1.82 μB/Fe for H is parallel to b) in BKFS, which is close to the spin-only magnetism with S=1/2.

We report electrical and magnetic measurements of an optimally doped Eu(Fe0.88Ir0.12)2As2 crystal which shows superconductivity at Tsc?=?22?K and magnetic ordering of the Eu2+ spins below 20?K. The results suggest that the Eu2+ spins lie flat ferromagnetically in the ab plane at Tabm?=?20?K, and then tilt toward the c-axis at Ttiltm?=?17.4?K. The isothermal magnetization loop at low temperatures shows both ferromagnetic behavior and superconducting characteristics, unambiguously demonstrating the coexistence of ferromagnetism and superconductivity. The upper critical field measured is remarkably reduced, as compared to other Fe-based superconductors with the same Tsc, and it exhibits abnormal temperature dependence featured by the existence of an inflection point around Tabm, where the anisotropy ratio ? (??H?abc2/H?cc2) shows a minimum value smaller than 1.0. These observations can be explained by a ferromagnetic exchange field of ?30?T which tilts its direction toward the c-axis below Ttiltm. The strong internal field, much higher than the intrinsic lower critical field expected, leads to the absence of Meissner state, which is confirmed by the magnetic measurements under ultra-low fields.

Eu(Fe1−xRux)2As2 crystals were studied by resistivity and magnetization measurements. It is found that the SDW transition is gradually suppressed with the Ru doping and superconductivity with Tsc ~22 K emerges when x ≥0.2. The magnetic order of the Eu sublattice changes from A-type antiferromagnetic to ferromagnetic within x ≤0.2, making superconductivity and ferromagnetism coexist in a broad regime of 0.2 < x < 0.55.

Superconductivity (SC) and long-range ferromagnetism (FM) are mutually antagonistic, thus in general SC does not coexist with FM. In this mini-review, however, we will show that such a coexistence can inhabit several iron pnictide systems, including EuFe2(As1−xPx)2, Eu(Fe1−xCox)2As2, Eu(Fe1−xRux)2As2, CeFe(As1−xPx)O0.95F0.05 and Sr2VFeAsO3. We will briefly discuss the possible reasons and consequences of the coexistence of 3d-electron SC and 4f-electron FM.

We report an unprecedented anisotropic superconductivity in Eu(Fe0.75Ru0.25)2As2 single crystals. The in-plane resistivity (ρab) and vertical magnetization (Mc) show a superconducting transition at TSC=23 K with zero resistance and magnetic shielding, respectively. In comparison, the out-of-plane resistivity (ρc) does not go to zero, neither does the horizontal magnetization (Mab) show the shielding against external fields. Magnetization and Mossbauer data indicate that the Eu2+ spins order ferromagnetically below 19.5 K with the moments tilted 20° from the c-axis. Our result suggests spontaneous-vortex phase and/or Fulde-Ferrell-Larkin-Ovchinnikov state in the ferromagnetic superconductor.