Multiband nodeless superconductivity near the charge-density-wave quantum critical point in ZrTe_(3-x)Se_x

It was found that selenium doping can suppress the charge-density-wave(CDW) order and induce bulk superconductivity in ZrTe_3. The observed superconducting dome suggests the existence of a CDW quantum critical point(QCP) in ZrTe_3-xSex near x ≈ 0.04. To elucidate the superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe_(3-x)Se_x single crystals(x = 0.044 and 0.051) down to 80 m K. For both samples, the residual linear term κ_0/T at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of κ_0/T manifests a multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe_(3-x)Se_x,which indicates conventional superconductivity despite of the existence of a CDW QCP.     

Observation of a Ubiquitous(π,π)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe_(2-x)Ni_xAs_2 Single Crystals

In iron-based superconductors,the(0,π) or(π,0) nematicity,which describes an electronic anisotropy with a fourfold symmetry breaking,is well established and believed to be important for understanding the superconducting mechanism.However,how exactly such a nematic order observed in the normal state can be related to the superconducting pairing is still elusive.Here,by performing angular-dependent in-plane magnetoresistivity using ultra-thin flakes in the steep superconducting transition region,we unveil a nematic superconducting order along the(π,π) direction in electron-doped BaFe_(2-x)Ni_xAs_2 from under-doped to heavily overdoped regimes with x=0.065-0.18.It shows superconducting gap maxima along the(π,π) direction rotated by 45° from the nematicity along(0,π) or(π,0) direction observed in the normal state.A similar(π,π)-type nematicity is also observed in the under-doped and optimally doped hole-type Ba_(1-y)K_yFe_2 As_2,with y=0.2-0.5.These results suggest that the(π,π) nematic superconducting order is a universal feature that needs to be taken into account in the superconducting pairing mechanism in iron-based superconductors.     

Low temperature specific heat of 12442-type KCa_2Fe_4As_4F_2 single crystals

Low-temperature specific heat(SH) is measured for the 12442-type KCa_2Fe_4As_4F_2 single crystal under different magnetic fields.A clear SH jump with the height of ?C/T|_(T_c)= 130 mJ mol~(-1) K~(-2) is observed at the superconducting transition temperature T_c.It is found that the electronic SH coefficient ?γ(H) quickly increases when the field is in the low-field region below 3T and then considerably slows down the increase with a further increase in the field, which indicates a rather strong anisotropy or multi-gap feature with a small minimum in the superconducting gap(s). The temperature-dependent SH data indicate the presence of the T~2 term, which supplies further information and supports the picture with a line-nodal gap structure. Moreover, the onset point of the SH transition remains almost unchanged under the field as high as 9 T, which is similar to that observed in cuprates, and places this system in the middle between the BCS limit and the Bose-Einstein condensation.     

First-principles Study on the Magnetic,Half-metal and Thermoelectric Transport Properties of Inorganic-Organic Hybrid Compounds [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3]

The electronic structure,magnetic and half-metal properties of inorganic-organic hybrid compound [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3] are investigated by using the full-potential linearized augmented plane wave (FPLAPW) method within density-functional theory (DFT) calculations.The density of states (DOS),the total energy of the cell and the spontaneous magnetic moment of [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3] are calculated.The calculation results reveal that the low-temperature phase of [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3] exhibits a stable ferromagnetic (FM) ground state,and we find that this organic compound is a half-metal in FM state.In addition,we have calculated antiferromagnetically coupled interactions,revealing the existence of antiferromagnetic (AFM),which is in agreement with the experiment.We have also found that [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3] is a semiconductor in the AFM state with a band gap of about 0.40 eV.Subsequently,the transport properties for potential thermoelectric applications have been studied in detail based on the Boltzmann transport theory.     

Realization of low-energy type-Ⅱ Dirac fermions in(Ir_(1-x)Pt_x)Te_2 superconductors

Topological Dirac semimetals(DSMs) present a kind of topologically nontrivial quantum state of matter, which has massless Dirac fermions in the bulk and topologically protected states on certain surfaces. In superconducting DSMs, the effects of their nontrivial topology on superconducting pairing could realize topological superconductivity in the bulk or on the surface. As superconducting pairing takes place at the Fermi level E_F, to make the effects possible, the Dirac points should lie in the vicinity of E_F so that the topological electronic states can participate in the superconducting paring. Here,we show using angle-resolved photoelectron spectroscopy that in a series of(Ir_(1-x)Pt_x)Te_2 compounds, the type-Ⅱ Dirac points reside around E_F in the superconducting region, in which the bulk superconductivity has a maximum T_c of ～ 3 K.The realization of the coexistence of bulk superconductivity and low-energy Dirac fermions in(Ir_(1-x)Pt_x)Te_2 paves the way for studying the effects of the nontrivial topology in DSMs on the superconducting state.     

Multiple Nodeless Superconducting Gaps in (Ba0.6K0.4)Fe2As2 Superconductor from Angle-Resolved Photoemission Spectroscopy

High resolution angle-resolved photoemission measurements have been carried out to study the superconducting gap in the (Ba_0.6K_0.4)Fe₂As₂ superconductor with T_c=35 K. Two hole-like Fermi surface sheets around the Γ point exhibit different superconducting gaps. The inner Fermi surface sheet shows larger (10~12 meV) and slightly momentum-dependent gap while the outer one has smaller (7~8meV) and nearly isotropic gap. The lack of gap node in both Fermi surface sheets favours s-wave superconducting gap symmetry. Superconducting gap opening is also observed at the M(π,π) point. The two Fermi surface spots near the M point are gapped below T_c but the gap persists above T_c. The rich and detailed superconducting gap information will provide key insights and constraints in understanding pairing mechanism in the iron-based superconductors.     

Structural modulation and physical properties of cobalt-doped layered La_2M_5As_3O_2(M=Cu,Ni) compounds

We investigate the structural variation and physical properties of layered La_2M_5As_3O_2(M=Cu,Ni) compound upon Co doping. It is found that the substitution of Co ion just induces the monotonous change of lattice constants without observing the anomalous kink in superconducting La_2(Cu~(1-x)Nix)_5 As_3O_2 solid-solutions. Meanwhile, this doping barely changes As–As bond length in [M_5As_3]~(2-) subunit(±2%), being significantly smaller than 7% shrinkage of that in La_2(Cu~(1-x)Nix)_5 As_3 O_2. Therefore, the doping dependence of crystal structure exhibits similar trend with Ba~(1-x)K_xFe_2 As_2 without the interference of As1–As 2 bonding, implying that the Co substitution for Cu/Ni is hole-doped. In terms of physical property, La2(Cu1-xCox)~5As_3O_2 turns into itinerant ferromagnetic metal, while La2(Ni1-x Cox)5 As3 O2 shows paramagnetism and suppressed structural phase transition upon Co-doping. The distinct structural variation and absence of superconductivity provide important clues to understand the effect of As–As bond in [M_5 As_3]~(2-) subunit.     

NMR and NQR studies on transition-metal arsenide superconductors LaRu_2As_2,KCa_2Fe_4As_4F_2,and A_2Cr_3As_3

We report ~(75)As-nuclear magnetic resonance(NMR) and nuclear quadrupole resonance(NQR) measurements on transition-metal arsenides LaRu_2As_2, KCa_2Fe_4As_4F_2, and A_2Cr_3As_3. In the superconducting state of La Ru_2As_2, a Hebel–Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T1 just below Tc, which indicates that LaRu_2As_2 is a full-gap superperconducor. For KCa_2Fe_4As_4F_2, antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate RAF= Tc1/Tab1 is small and temperature independent,implying that the low energy spin fluctuations are isotropic in spin space. Our results indicate that KCa_2Fe_4As_4F_2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition. For A_2Cr_3As_3(A = Na, K, Rb, Cs), we calculate the electric field gradient by first-principle method and assign the ~(75)As-NQR peaks to two crystallographically different As sites, paving the way for further NMR investigation.     

Possible Nodeless Superconducting Gaps in Bi_2Sr_2CaCu_2O_(8+δ) and YBa_2Cu_3O_(7-x) Revealed by Cross-Sectional Scanning Tunneling Spectroscopy

Pairing in the cuprate high-temperature superconductors and its origin remain among the most enduring mysteries in condensed matter physics.With cross-sectional scanning tunneling microscopy/spectroscopy we clearly reveal the spatial-dependence or inhomogeneity of the superconducting gap structure of Bi_2Sr_2CaCu_2O_(8+δ)(Bi2212) and YBa_2Cu_3O_(7-x)(YBCO) along their c-axes on a scale shorter than the interlayer spacing.By tunneling into the(100) plane of a Bi2212 single crystal and a YBCO film,we observe both U-shaped tunneling spectra with extended flat zero-conductance bottoms,and V-shaped gap structures,in different regions of each sample.On the YBCO film,tunneling into a(110) surface only reveals a U-shaped gap without any zero-bias peak.Our analysis suggests that the U-shaped gap is likely a nodeless superconducting gap.The V-shaped gap has a very small amplitude,and is likely proximity-induced by regions having the larger U-shaped gap.     

Revealing the A_(1g)-type strain effect on superconductivity and nematicity in FeSe thin flake

The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate.Recently,a dominant B_(1 g)-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe_(1-x)Co_x)_2 As_2,suggesting a strong interplay between nematicity and superconductivity.Since the long-range spin order is absent in FeSe superconductor,whether a similar strain effect could be also observed or not is an interesting question.Here,by utilizing a flexible film as substrate,we successfully achieve a wide-range-strain tuning of FeSe thin flake,in which both the tensile and compressive strain could reach up to ～0.7%,and systematically study the strain effect on both superconducting and nematic transition(T_c and T_s) in the FeSe thin flake.Our results reveal a predominant A_(1 g)-type strain effect on T_c.Meanwhile,T_s exhibits a monotonic anti-correlation with T_c and the maximum T_c reaches to 12 K when T_s is strongly suppressed under the maximum compressive strain.Finally,in comparison with the results in the underdoped Ba(Fe_(1-x)Co_x)_2 As_2,the absence of B_(1 g)-type strain effect in FeSe further supports the role of stripe-type spin fluctuations on superconductivity.In addition,our work also supports that the orbital degree of freedom plays a key role to drive the nematic transition in FeSe.     

Distinct Superconducting Gap on Two Bilayer-Split Fermi Surface Sheets in Bi_2Sr_2CaCu_2O_(8+δ) Superconductor

High resolution laser-based angle-resolved photoemission measurements are carried out on an overdoped superconductor Bi_2Sr_2CaCu_2O_(8+)with a_(c )of 75 K.Two Fermi surface sheets caused by bilayer splitting are clearly identified with rather different doping levels:the bonding sheet corresponds to a doping level of 0.14,which is slightly underdoped while the antibonding sheet has a doping of 0.27 that is heavily overdoped,giving an overall doping level of 0.20 for the sample.Different superconducting gap sizes on the two Fermi surface sheets are revealed.The superconducting gap on the antibonding Fermi surface sheet follows a standard d-wave form while it deviates from the standard d-wave form for the bonding Fermi surface sheet.The maximum gap difference between the two Fermi surface sheets near the antinodal region is～2 meV.These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity,and the layer-dependent superconductivity in high temperature cuprate superconductors.     

Superconductivity in an intermetallic oxide Hf_3Pt_4Ge_2O

Discovery of a new superconductor with distinct crystal structure and chemistry often provides great opportunity for further expanding superconductor material base, and also leads to better understanding of superconductivity mechanisms. Here, we report the discovery of superconductivity in a new intermetallic oxide Hf_3Pt_ Ge_2O synthesized through a solid-state reaction. The Hf_3Pt_ Ge_2O crystallizes in a cubic structure(space group Fm-3 m) with a lattice constant of a = 1.241 nm, whose stoichiometry and atomic structure are determined by electron microscopy and x-ray diffraction techniques. The superconductivity at 4.1 K and type-II superconducting nature are evidenced by the electrical resistivity, magnetic susceptibility, and specific heat measurements. The intermetallic oxide Hf_3Pt_ Ge_2O system demonstrates an intriguing structural feature that foreign oxygen atoms can be accommodated in the interstitial sites of the ternary intermetallic framework. We also successfully synthesized a series of Hf_3Pt_ Ge_2O1+δ(-0.25 ≤δ≤ 0.5), and found theδ-dependent superconducting transition temperature Tc. The atomic structure and the electronic structure are also substantiated by first-principles calculations. Our results present an entirely new family of superconductors with distinct structural and chemical characteristics, and could attract research interest in further finding new superconductors and exploring novel physics pertaining to the 5 d-electron in these intermetallic compound systems.     

Nodal superconducting gap in LiFeP revealed by NMR:Contrast with LiFeAs

Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides.Here,we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms.The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth λ_L.We find that λ_L saturates below T～0.2 T_C in LiFeAs,where T_C is the superconducting transition temperature,indicating nodeless superconducting gaps.Furthermore,by using a two-gaps model,we simulate the temperature dependence of λ_L and obtain the superconducting gaps of LiFeAs,as Δ_1=1.2 k_B T_C and Δ_2=2.8 k_BT_C,in agreement with previous result from spin-lattice relaxation.For LiFeP,in contrast,λ_L does not show any saturation down to T～0.03 T_C,indicating nodes in the superconducting gap function.Finally,we demonstrate that strong spin fluctuations with diffusive characteristics exist in LiFeP,as in some cuprate high temperature 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 I4_1md, 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 T_c= 5.07 K. The upper critical magnetic field, Sommerfield coefficient, and Ginzburg-Laudau parameter are determined based on physical property measurements, which are B_(c2)= 0.83 T, γ =7.5 mJ mol~(-1) K~(-2), and κ_(GL)= 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.     

Tunneling spectra of underdoped Bi_2Sr_2Ca_(1-x)Y_xCu_2O_(8+δ) intrinsic Josephson junctions

<正>Mesa-structured submicron intrinsic Josephson junctions are successfully fabricated and well characterized on underdoped Bi_2Sr_2Ca_(1-x)Y_xCu-2O_(8+δ) single crystals with a T_c of 80 K.Tunneling spectra at the temperatures ranging from 4.2 K to 295 K are measured.A pulse technique is used to reduce sample heating for the measurement near pseudogap opening temperature T~*~280 K.Our experimental results show that the superconducting gap,the peak-dip separation,and the pseudogap opening temperature are all increased as compared with those from near optimally doped samples,which requires further theoretical analysis in the future.     

Grüneisen ratio quest for self-duality of quantum criticality in a spin-1/2 XY chain with Dzyaloshinskii–Moriya interaction

The quantum phase transition(QPT) and quantum criticality of an anisotropic spin-1/2 XY chain under the interplay of magnetic field and Dzyaloshinskii–Moriya(DM) interaction, which is interpreted as an electric field, are investigated, wherein the anisotropic parameter plays a similar role as the superconducting pairing gap in the interacting Kitaev topological superconductor model that protects the topological order. It is shown that the thermal Drude weight is a good quantity to characterize the gapped(D_(th) = 0) and gapless(D_(th) 0) ground states. The continuous QPT is marked by a quantum critical point(QCP) associated with entropy accumulation, which is manifested by a characteristic Güneisen ratio(GR) with or without selfduality symmetry. It is shown that at a self-dual QCP, the GR keeps a finite value as T→0,while at a general QCP without self-duality symmetry, it displays a power-law temperature dependent divergence: Γ(T,r_c)～±T~(-1),which provides a novel thermodynamic means for probing QPT.     

Temperature Evolution of Energy Gap and Band Structure in the Superconducting and Pseudogap States of Bi_2Sr_2CaCu_2O_(8+δ) Superconductor Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

We carry out detailed momentum-dependent and temperature-dependent measurements on Bi_2Sr_2CaCu_2O_(8+δ)(Bi2212) superconductor in the superconducting and pseudogap states by super-high resolution laser-based angleresolved photoemission spectroscopy. The precise determination of the superconducting gap for the nearly optimally doped Bi2212(T_c= 91 K) at low temperature indicates that the momentum-dependence of the superconducting gap deviates from the standard d-wave form(cos(2Φ)). It can be alternatively fitted by including a high-order term(cos(6Φ)) in which the next nearest-neighbor interaction is considered. We find that the band structure near the antinodal region smoothly evolves across the pseudogap temperature without a signature of band reorganization which is distinct from that found in Bi_2Sr_2CuO_(6+δ) superconductors. This indicates that the band reorganization across the pseudogap temperature is not a universal behavior in cuprate superconductors.These results provide new insights in understanding the nature of the superconducting gap and pseudogap in high-temperature cuprate superconductors.     

快淬(Er_(l-x)Sm_x)_2Fe_(17)C_y化合物的结构与磁性

用快速急冷方法制备了(Er_(1-x)Sm_x)_2Fe_(17)C_y(x≤0.8,1.5≤y≤2.5)化合物,它们具有菱方Th_2Zn_(17)型结构,并且在高温是稳定的。与相应的不含C的(Er_(1-x)Sm_x)_2Fe_(17)化合物比较,当C含量y=2.5时,引起单胞体积增加～6.3％,居里温度增加约330—360K,并使室温饱和磁化强度显著提高。当y≥1.5时,含Sm样品在室温时均显示出轴各向异性,(Er_(0.2)Sm_(0.8)_2Fe_(17)C_(1.5)化合物的室温各向异性场H_α=8T。     

Magnetostriction and spin reorientation in ferromagnetic Laves phase Pr(Ga_xFe_(1-x))_(1.9) compounds

The magnetostriction, magnetization, and spin reorientation properties in Pr(Ga_xFe_(1-x))_(1.9) alloys have been investigated by high-precision x-ray diffraction(XRD) step scanning, magnetization, and Mo¨ssbauer spectra measurements. Ga substitution reduces the magnetostriction(λ_(||)) with magnetic field H ≥ 8 kOe(1 Oe = 1.33322×10~2 Pa), but it also increases the λ|| value when H ≤ 8 kOe at 5 K. Spin-reorientations(SR) are observed in all the alloys investigated, as determined by the step scanned XRD, Mo¨ssbauer spectra, and the abnormal temperature dependence of magnetization. An increase of the spin reorientation temperature(T_(SR)) due to Ga substitution is found in the phase diagram, which is different from the decrease one in many R(T_x Fe_(1-x))_(1.9)(T = Co, Al, Mn) alloys. The present work provides a method to control the easy magnetization direction(EMD) or T_(SR) for developing an anisotropic compensation system.     

Fe_2Ga_2S_5 as a 2D Antiferromagnetic Semiconductor

We theoretically investigate physical properties of two-dimensional(2D) Fe_2Ga_2S_5 by employing first-principles calculations.It is found that it is an antiferromagnet with zigzag magnetic configuration orienting in the inplane direction,with Neel temperatures around 160 K.The band structure of the ground state shows that it is a semiconductor with the indirect band gap of about 0.9 eV,which could be effectively tuned by the lattice strain.We predict that the carrier transport is highly anisotropic,with the electron mobility up to the order of～10~3 cm~2/(V·s) much higher than the hole.These fantastic electronic properties make 2D Fe_2Ga_2S_5 a promising candidate for the future spintronics.