Observation of Zero-Energy Modes with Possible Time-Reversal Symmetry Breaking on Step Edge of CaKFe_(4)As_(4)

Topologically nontrivial Fe-based superconductors attract extensive attentions due to their ability of hosting Majorana zero modes(MZMs)which could be used for topological quantum computation.Topological defects such as vortex lines are required to generate MZMs.Here,we observe the robust edge states along the surface steps of CaKFe_(4)As_(4).Remarkably,the tunneling spectra show a sharp zero-bias peak(ZBP)with multiple integerquantized states at the step edge under zero magnetic field.We propose that the increasing hole doping around step edges may drive the local superconductivity into a state with possible spontaneous time-reversal symmetry breaking.Consequently,the ZBP can be interpreted as an MZM in an effective vortex in the superconducting topological surface state by proximity to the center of a tri-junction with different superconducting order parameters.Our results provide new insights into the interplay between topology and unconventional superconductivity,and pave a new path to generate MZMs without magnetic field.

Superconductivity with peculiar upper critical fields in quasi-one-dimensional Cr-based pnictides

The discovery of superconductivity in quasi-one-dimensional Cr-based pnictides A2Cr3As3（A = alkali metals） has generated considerable research interest, primarily owing to their reduced dimensionality, significant electron correlations,and possible unconventional superconductivity. The upper critical field（Hc2） provides important information on the superconducting pairing. In this paper, we first briefly overview the latest research progress on the Cr-based superconductors.Then, we introduce typical Hc2（T） behaviors of type-II superconductors in relation with the pair-breaking mechanisms.After a description of the measurement method for Hc2, we focus on the analysis of Hc2 data, especially for the temperature and angle dependence, in K2Cr3As3 crystals. The result indicates（i） an absence of Pauli-paramagnetic pair breaking for field perpendicular to the Cr3As3 chains, and（ii） a unique threefold modulation for the in-plane Hc2Φ profile. Finally we conclude with remarks on the possible unconventional superconducting pairing symmetry.

Coexistence of Polaronic States and Superconductivity in Iron-Pnictide Compound Ba_2Ti_2Fe_2As_4O

The electronic structure of iron-pnictide compound superconductor Ba_2Ti_2Fe_2As_4O, which has metallic intermediate Ti_2O layers, is studied using angle-resolved photoemission spectroscopy. The Ti-related bands show a‘peak-dip-hump＇ line shape with two branches of dispersion associated with the polaronic states at temperatures below around 120 K. This change in the spectra occurs along with the resistivity anomaly that was not clearly understood in a previous study. Moreover, an energy gap induced by the superconducting proximity effect opens in the polaronic bands at temperatures below T_c（~21 K）. Our study provides the spectroscopic evidence that superconductivity coexists with polarons in the same bands near the Fermi level, which provides a suitable platform to study interactions between charge, lattice and spin freedoms in a correlated system.

Neutron Powder Diffraction Study on the Non-Superconducting Phases of ThFeAsN1-xOx（x=0.15,0.6）Iron Pnictide

We use neutron powder diffraction to study the non-superconducting phases of ThFeAsN1-xOx with x = 0.15,0.6.In our previous results of the superconducting phase ThFeAsN with Tc = 30 K,no magnetic transition is observed by cooling down to 6 K,and possible oxygen occupancy at the nitrogen site is shown in the refinement[Europhys.Lett.117(2017)57005].Here in the oxygen doped system ThFeAsN1-xOx,two superconducting regions(0≤x≤0.1 and 0.25≤x≤0.55)are identified by transport experiments[J.Phys.:Condens.Matter30(2018)255602].However,within the resolution of our neutron powder diffraction experiment,neither the intermediate doping x= 0.15 nor the heavily overdoped compound x = 0.6 shows any magnetic order from 300 K to 4 K.Therefore,while it shares the common phenomenon of two superconducting domes as most 1111-type iron-based superconductors,the magneticall.y ordered parent compound may not exist in this nitride family.

Combined Study of Structural, Magnetic and Transport Properties of Eu_(0.5)Ln_(0.5)BiS_(2)F Superconductor

Superconductivity below 0.3 K and a charge-density-wave-like(CDW-like) anomaly at 280 K were observed in EuBiS_(2)F recently.Here we report a systematic study of structural and transport properties in Eu_(0.5)Ln_(0.5)BiS_(2)F(Ln=La,Ce,Pr,Nd,Sm) by electrical resistivity,magnetization,and specific heat measurements.The lattice constants have a significant change upon rare earth substitution for Eu,suggesting an effective doping.As Ln is changed from Sm to La,the superconducting transition temperature T_(c) increases from 1.55 K to 2.8 K.In contrast to the metallic parent compound,the temperature dependence of electrical resistivity displays semiconductinglike behavior for all the Eu_(0.5)Ln_(0.5)BiS_(2)F samples.Meanwhile,the CDW-like anomaly observed in EuBiS2F is completely suppressed.Unlike the mixed valence state in the undoped compound,Eu ions in these rareearth-doped samples are mainly divalent.A specific anomaly at 1.3 K resembling that in EuBiS_(2)F suggests the coexistence of superconductivity and spin glass state for Eu_(0.5)Ln_(0.5)BiS_(2)F.Coexistence of ferromagnetic order and superconductivity is found below 2.2 K in Eu_(0.5)Ln_(0.5)BiS_(2)F samples.Our results supplies a rich diagram showing that many interesting properties can be induced in BiS_(2)-based compounds.

Annealing-induced long-range charge density wave order in magnetic kagome FeGe:Fluctuations and disordered structure

Charge density wave(CDW) in kagome materials with the geometric frustration is able to carry unconventional characteristics.Recently, a CDW has been observed below the antiferromagnetic order in kagome FeGe, in which magnetism and CDW are intertwined to form an emergent quantum ground state. However, the CDW is only short-ranged and the structural modulation originating from it has yet to be determined experimentally. Here we realize a long-range CDW order by post-annealing process,and resolve the structure model through single crystal X-ray diffraction. Occupational disorder of Ge resulting from short-range CDW correlations above T_(CDW) is identified from structure refinements. The partial dimerization of Ge along the c axis is unveiled to be the dominant distortion for the CDW. Occupational disorder of Ge is also proved to exist in the CDW phase due to the random selection of partially dimerized Ge sites. Our work provides useful insights for understanding the unconventional nature of the CDW in FeGe.

碳添加引起一种超导高熵合金的中心对称到非中心对称结构转变

四方σ相是过渡金属合金中最常见的物相之一.然而,σ型高熵合金中尚未有关于碳添加效应的报道.在本文中,我们发现在超导的Ta_(10)Mo_(5)W_(30)Re_(35)Ru_(20)高熵合金(Tc=4.87K)中添加碳会引起结构转变.结果表明在0≤x≤2时Ta_(10)Mo_(5)W_(30)Re_(35)Ru_(20)Cx高熵合金具有中心对称的σ型结构,但在16≤x≤20时转变为非中心对称的β-Mn型结构.转变后的β-Mn型高熵合金仍表现出体超导电性且具有更高的Tc(5.34 K)和接近泡利顺磁极限的上临界场.此外,该结构转变归因于碳添加引起价电子浓度的降低以及晶格畸变和混合熵的增加.我们的研究结果不仅提供了高熵合金中中心对称到非中心对称结构转变的罕见例子,而且夯实了非金属碳是调节高熵合金结构和物理性质的有效添加剂.

Antiferromagnetic insulating state in quasi-one-dimensional K_(2)Cr_(3)As_(3)H

Quasi-one-dimensional(Q1D) Cr-based pnictide K_(2)Cr_(3)As_(3)exhibits superconductivity probably with spin-triplet pairing. It is of fundamental importance to explore the parent compound from which superconductivity emerges. Here we report the synthesis,crystal structure, physical properties, and density functional theory(DFT) calculations of(nearly) fully hydrogenized K_(2)Cr_(3)As_(3)H.It is found that the intercalation of hydrogen in K_(2)Cr_(3)As_(3)leads to absence of metallicity as well as superconductivity. An antiferromagnetic transition nearby room temperature is evidenced from the measurements of magnetic susceptibility and heat capacity.The antiferromagnetic insulating state can be reproduced by the DFT calculations, which show a novel non-collinear co-planar magnetic order. Our result sheds light on the mechanism of unconventional superconductivity in Q1D Cr-based superconductors.

Anisotropic transport in a possible quasi-one-dimensional topological candidate:TaNi_(2)Te_(3)

We report on the solid-state synthesis and the strongly anisotropic transport properties of the ternary telluride TaNi_(2)Te_(3),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 TaNi_(2)Te_(3) 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 TaNiTe_(5).

Structural evolution and superconductivity tuned by valence electron concentration in the Nb-Mo-Re-Ru-Rh high-entropy alloys

The crystal structure and physical properties of Nb_(25)Mo_(5+x)Re_(35)Ru_(25-x)Rh_(10)(0≤x≤10)and Nb_(5)Mo_(35-y)Re_(15+y)Ru_(35)Rh_(10)(0≤y≤15)high-entropy alloys(HEAs)have been studied by X-ray diffraction,electrical resistivity,magnetic susceptibility,and specific heat measurements.The results show that the former HEAs with valence electron concentration(VEC)values of 6.7-6.9 crystallize in a noncentrosymmetric cubicα-Mn structure,while the latter ones with VEC values of 7.1-7.25 adopt a centrosymmetric hexagonal close-packed(hcp)structure.Despite different structures,both series of HEAs are found to be bulk superconductors with a full energy gap,and the superconducting transition temperature Tc tends to decrease with the increase of VEC.Nevertheless,the Tc values of the hcp-type HEAs are higher than those of theα-Mn-type ones,likely due to a stronger electron phonon coupling.Furthermore,we show that VEC and electronegativity difference are two key parameters to control the stability ofα-Mn and hcp-type HEAs.These results not only are helpful for the design of such HEAs,but also represent the first realization of structurally different HEA superconductors without changing the constituent elements.

Interstitially carbon-alloyed refractory high-entropy alloys with a body-centered cubic structure

The introduction of carbon interstitials into high-entropy alloys(HEAs)provides an effective way to improve their properties.However,all such HEA systems explored so far are limited to those with the face-centered-cubic(fcc)structure.Here we report the structural,mechanical and physical properties of the refractory(Nb_(0.375)Ta_(0.25)Mo_(0.125)W_(0.125)Re_(0.125))_(100−x)C_(x) HEAs over a wide x range of 0≤x≤20.It is found that,whereas the starting HEA(x=0)is composed of a major body-centered-cubic(bcc)phase with significant impurities,the bcc phase fraction increases with the C concentration and achieves almost 100%at x=20.Moreover,the increase of C content x results in an expansion of the bcc lattice,an enhancement of the microhardness,an increase in residual resistivity and a small variation of density of states at the Fermi level.All these features are consistent with the expectation that carbon atoms occupy the interstitial site.For x≥11.1,the X-ray photoelectron spectroscopy indicates the bond formation between the carbon and metal atoms,suggesting that some carbon atoms may also reside in the lattice site.In addition,a semiquantitative analysis shows that the enhanced mixing entropy caused by carbon addition plays a key role in stabilizing the(nearly)single solid-solution phase.Our study not only provides the first series of carbon interstitial HEAs with a bcc structure,but also helps to better understand the alloying behavior of carbon in refractory HEAs.

BaTh2Fe4As4(N0.7O0.3)2: An iron-based superconductor stabilized by inter-block-layer charge transfer

The discovery of superconductivity at Tc = 26 K in fluorine-doped LaFeAsO [1] has led to the subsequent findings of dozens of new superconducting materials in the class of Fe-based superconductors (FeSCs)[2,3]. All those FeSCs were found to possess antifluorite-like Fe2X2 (X = As, Se) layers which were believed to be the crucial structural motif for the emergence of high-temperature superconductivity. This common feature makes it possible to design a new FeSC with the crystal chemistry [3-5]. For example, with the consideration of the intergrowth of BaFe2As2 and BaTi2As2O, the new FeSC Ba2Fe2Ti2As4O (Tc=21 K) was discovered [6]. The block-layer-design approach was also demonstrated to be effective for other class of layered materials, such as Eu3Bi2S4F4 [7], LaPb- BiS3O [8], La5Cu4As4O4C12 [9], Pb5BiFe3O10Cl2 [10], and Bi4Q4CU1.7Se2.7CI03 [11].

A possible family of Ni-based high temperature superconductors

We suggest that a family of Ni-based compounds, which contain [Ni_2M_2O]~2à(M = chalcogen) layers with an antiperovskite structure constructed by mixed-anion Ni complexes, Ni M_4O_2, can be potential high temperature superconductors(high-Tc) upon doping or applying pressure. The layer structures have been formed in many other transitional metal compounds such as La_2B_2Se_2O_3(B = Mn, Fe, Co). For the Ni-based compounds, we predict that the parental compounds host collinear antiferromagnetic states similar to those in iron-based high temperature superconductors. The electronic physics near Fermi energy is controlled by two egd-orbitals with completely independent in-plane kinematics. We predict that the superconductivity in this family is characterized by strong competition between extended s-wave and d-wave pairing symmetries.

Effect of impurity scattering on superconductivity in K2Cr3As3

Impurity scattering in a superconductor may serve as an important probe for the nature of superconducting pairing state. Here we re- port the impurity effect on superconducting transition temperature Te in the newly discovered Cr-based superconductor K2Cr3As3. The resistivity measurements show that the crystals prepared using high-purity Cr metal （≥99.99%） have an electron mean free path much larger than the superconducting coherence length. For the crystals prepared using impure Cr that contains various non- magnetic impurities, however, the Tc decreases significantly, in accordance with the generalized Abrikosov-Gor＇kov pair-breaking theory. This finding supports a non-s-wave superconductivity in K2Cr3As3.

The As-surface of an iron-based superconductor CaKFe_(4)As_(4)

As a new type of iron-based superconductor, CaKFe_(4)As_(4) has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe_(4)As_(4) plays an important role in realizing the MZM since it hosts superconducting topological surface states. However, due to the complicated crystal structure, the terminal surface of CaKFe_(4)As_(4) has not been determined yet. Here, by using scanning tunneling microscopy/spectroscopy (STM/S), we find that there are two types of surface structure in CaKFe_(4)As_(4). Bias-dependent atomic resolution images show an evolvement from 2–√×2–√ superstructure with respect to the As lattice into 1 × 1 when the tip is brought close to the surface, revealing the sublattice of missing As atoms. Together with the first-principles calculations, we show that the surface As layer has a buckled structure. Our findings provide insight to future surface study of CaKFe_(4)As_(4) as well as other iron-pnictide superconductors.

Superconductivity and strong spin-orbit coupling in a new noncentrosymmetric compound ThIrP

A new noncentrosymmetric phosphide ThIrP has been synthesized and characterized. X-ray difraction analysis shows that this compound crystallizes in a LaPtSi-type tetragonal lattice(space group I41md, Z = 4), whose lattice parameters are a = b =4.0676(1) ?, c = 14.3354(2) ?, and V = 237.191(8) ?3. Moreover, ThIrP is discovered to be an intermediately coupled,type-Ⅱ superconductor with possibly multiple gaps below Tc= 5.07 K. The upper critical magnetic field, Sommerfield coefficient, and Ginzburg-Laudau parameter are determined based on physical property measurements, which are Bc2= 0.83 T, γ =7.5 mJ mol-1 K-2, and κGL= 7.5, respectively. The electronic band structure calculations point out nearly equal contributions of Ir and Th atoms on the density of states around the Fermi surface. In addition, the spin-orbit coupling induced band splitting reaches as large as 270 meV along the Γ-Z line. Our results suggest that ThIrP provides a platform to study the interplay between inversion-symmetry breaking, strong spin-orbit coupling, and superconductivity.

Unveiling pairing mechanism in quasi-one-dimensional Cr-based superconductors

Superconductivity is a macroscopic quantum phenomenon in which electron pairing(or Cooper pairing)plays a crucial role for its emergence.The Cooper pairing is mediated by electron-phonon interactions in most of simple metals and alloys which are widely called conventional superconductors.There are a few classes of superconductors,however,in which the electron pairing

Synthesis,Structure and Properties of Layered Phosphide Nitrides AkTh_(2)Mn_(4)P_(4)N_(2)(Ak=Rb,Cs)

Main observation and conclusion We report the design,synthesis,structure,and properties of two complex layered phosphide nitrides,AkTh_(2)Mn_(4)P_(4)N_(2)(Ak=Rb,Cs),which contain anti-fluorite-type[Mn_(2)P_(2)]bilayers separated by fluorite-type[Th_(2)N_(2)]layers as a result of the intergrowth between AkMn2P2 and ThMnPN.The new compounds are featured with an intrinsic hole doping associated with the interlayer charge transfer and a built-in chemical pressure from the[Th_(2)N_(2)]layers,both of which are reflected by the changes in the lattice and the atomic position of phosphorus.The measurements of magnetic susceptibility,electrical resistivity,and specific heat indicate existence of local moments as well as itinerant electrons in relation with d-p hybridizations.The expected dominant antiferromagnetic interactions with enhanced d-p hybridizations were demonstrated by the first-principles calculations only when additional Coulomb repulsions are included.The density of states at the Fermi level derived from the specific-heat analysis are 3.5 and 7.5 times of the calculated ones for Ak=Rb and Cs,respectively,suggesting strong electron correlations in the title compounds.

Coexistence of superconductivity and antiferromagentic order in Er_(2)O_(2)Bi with anti-ThCr_(2)Si_(2) structure

We investigated the coexistence of superconductivity and antiferromagnetic order in the compound Er_(2)O_(2)Bi with anti-ThCr_(2)Si_(2)-type structure through resistivity,magnetization,specific heat measurements and first-principle calculations.The superconducting transition temperature Tc of 1.23 K and antiferromagnetic transition temperature TN of 3 K are observed in the sample with the best nominal composition.The superconducting upper critical field H_(c2)(0)and electron-phonon coupling constantλe−ph in Er_(2)O_(2)Bi are similar to those in the previously reported non-magnetic superconductor Y_(2)O_(2)Bi with the same structure,indicating that the superconductivity in Er_(2)O_(2)Bi may have the same origin as in Y_(2)O_(2)Bi.The first-principle calculations of Er_(2)O_(2)Bi show that the Fermi surface is mainly composed of the Bi 6p orbitals both in the paramagnetic and antiferromagnetic state,implying minor effect of the 4f electrons on the Fermi surface.Besides,upon increasing the oxygen incorporation in Er_(2)O_(x)Bi,Tc increases from 1 to 1.23 K and TN decreases slightly from 3 K to 2.96 K,revealing that superconductivity and antiferromagnetic order may compete with each other.The Hall effect measurements indicate that hole-type carrier density indeed increases with increasing oxygen content,which may account for the variations of Tc and TN with different oxygen content.

New high-T_c iron-selenide superconductor with hydroxide spacer layers

Iron-based superconductors include pnictides and chalcogenides.So far,the chalcogenides are much fewer in number[1],thus it is particularly valuable to find new superconductors in iron chalcogenides,for the'ultimate'understanding of iron-based superconductivity.The tetragonal binary iron selenide(β-Fe1+δSe)is the structurally simplest iron-based superconductor with a superconducting critical temperature(Tc)of 8.5 K at ambient pressure[2].By intercalation of alkali metal ions into