Interplay of superconductivity and d-f correlation in CeFeAslPOl_yFy

The recent discovery of high-temperature superconductivity in iron-based pnictides （chalcogenides） not only trig- gers tremendous enthusiasm in searching for new superconducting materials, but also opens a new avenue to the study of the Kondo physics. CeFeAsO is a parent compound of the 1111-type iron-based superconductors. It shows 3d- antiferromagnetic （AFM） ordering below 139 K and 4f-AFM ordering below 4 K. On the other hand, the phosphide CeFePO is a ferromagnetically corelated heavy-fermion （HF） metal with Kondo scale TK 10 K. These properties set up a new platform for research of the interplay among magnetism, Kondo effect, and superconductivity （SC）. In this review, we present the recent progress in the study of chemical pressure effect in CeFeAsOl_yFy （y = 0 and 0.05）. This P/As-doping in CeFeAsO serves as an effective controlling parameter which leads to two magnetic critical points, Xcl -- 0.4 and Xc2 - 0.92, associated with suppression of 3d and 4f magnetism, respectively. We also observe a turning point of AFM-FM ordering of Ce3＋ moment at Xc3 - 0.37. The SC is absent in the phase diagram, which is attributed to the destruction to Cooper pair by Ce-FM fluctuations in the vicinity of Xcl. We continue to investigate CeFeAsl-xPxO0.95Fo.os. With the separation of xcl and xc3, this chemical pressure results in a broad SC region 0〈 x 〈 0.53, while the original HF behavior is driven away by 5% F- doping. Different roles of P and F dopings are addressed, and the interplay between SC and Ce-4f magnetism is also discussed.     

Raman scattering studies on the collapsed phase of CaCo_2As_2

In this work,Raman scattering measurements have been performed on the collapsed phase CaCo_2As_2 crystals.At least 8 Raman modes were observed at room temperature though CaCo_2As_2 is structurally similar to other 122 compounds like BaFe_2As_2.Two Raman modes are assigned to the intrinsic A_(1g)and B_(1g)of this material system respectively.The other ones are considered to originate from the local vibrations relevant to cobalt vacancies.Careful polarized measurements allow us to clearly resolve the four-fold symmetry of the B_(1g)mode,which put strong constraints on possible point group symmetries of the system with Co vacancies.The temperature-dependent measurements demonstrate that the anomalies in both frequency and width of the B_(1g)mode occur around Neel temperature T_N.The anomalies are considered to be related to the gap opening near the magnetic transition.The study may shed light on the structural and magnetic changes and their correlations with superconductivity in 122 systems.     

Interplay of superconductivity and d-f correlation in CeFeAs_(1-x)P_xO_(1-y)F_y

The recent discovery of high-temperature superconductivity in iron-based pnictides (chalcogenides) not only triggers tremendous enthusiasm in searching for new superconducting materials, but also opens a new avenue to the study of the Kondo physics. CeFeAsO is a parent compound of the 1111-type iron-based superconductors. It shows 3d-antiferromagnetic (AFM) ordering below ～ 139 K and 4f-AFM ordering below ～ 4 K. On the other hand, the phosphide CeFePO is a ferromagnetically correlated heavy-fermion (HF) metal with Kondo scale T K ～ 10 K. These properties set up a new platform for research of the interplay among magnetism, Kondo effect, and superconductivity (SC). In this review, we present the recent progress in the study of chemical pressure effect in CeFeAsO 1-y F y (y = 0 and 0.05). This P/As-doping in CeFeAsO serves as an effective controlling parameter which leads to two magnetic critical points, x c1 0.4 and x c2 0.92, associated with suppression of 3d and 4f magnetism, respectively. We also observe a turning point of AFM-FM ordering of Ce 3+ moment at x c3 0.37. The SC is absent in the phase diagram, which is attributed to the destruction to Cooper pair by Ce-FM fluctuations in the vicinity of x c1 . We continue to investigate CeFeAs 1-x P x O 0.95 F 0.05 . With the separation of x c1 and x c3 , this chemical pressure results in a broad SC region 0≤ x ≤ 0.53, while the original HF behavior is driven away by 5% F doping. Different roles of P and F dopings are addressed, and the interplay between SC and Ce-4f magnetism is also discussed.     

Optical conductivity as a probe of a hidden Fermi-liquid behavior in BaFe_(1.904) Ni_(0.096) As_2

We measured the infrared reflectivity of BaFe1.904Ni0.096As2 single crystal from room temperature down to 20 K.Two Drude terms and a Lorentz term well describe the real part of the optical conductivity σ1(ω). We fit the reciprocal of static optical conductivity 1/σ1(0) by the power law ρ(T)=ρ0+ATnwith n=1.6. The broad Drude component exhibits an incoherent background with a T-independent scattering rate 1/τb, while the other narrow one reveals a T-quadratic scattering rate 1/τn, indicating a hidden Fermi-liquid behavior in BaFe1.904Ni0.096As2 compound.     

铁基材料 KxFe2-ySe2中一种可能的超导基态相K2Fe7Se8的发现*

铁基超导体的发现开启了探索超导材料的新一轮热潮。最新发现的铁基超导体 KxFe2-ySe2有着许多不同寻常的性质。此材料中明显存在相分离,材料中主要的部分是具有反铁磁性质的绝缘相 K2Fe4Se5,超导相只占少量体积。但是超导相在物理上很重要,因为根据理论计算,该材料中不存在空穴型费米面,这样该领域中被广泛接受的S ±配对图像似乎受到挑战。相分离的存在致使此材料中相关研究的难度非常大。该文介绍了此材料中超导相的探索和研究,确认了此材料的超导相是以三维网络状的细丝形态存在,并提出超导的母体相可能是每8个 Fe 位置有一个 Fe 空位形成的8×10这种有序平行四边形结构,称为K2Fe7Se8相。     

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.     

Superconductivity in Sm-doped CaFe_2As_2 single crystals

In this article,the Sm-doping single crystals Ca_(1-x)Sm_xFe_2As_2(x = 0 ~0.2) were prepared by the Ca As flux method,and followed by a rapid quenching treatment after the high temperature growth.The samples were characterized by structural,resistive,and magnetic measurements.The successful Sm-substitution was revealed by the reduction of the lattice parameter c,due to the smaller ionic radius of Sm~(3+)than Ca~(2+).Superconductivity was observed in all samples with onset T_c varying from 27 K to 44 K upon Sm-doping.The coexistence of a collapsed phase transition and the superconducting transition was found for the lower Sm-doping samples.Zero resistivity and substantial superconducting volume fraction only happen in higher Sm-doping crystals with the nominal x 0.10.The doping dependences of the c-axis length and onset T_c were summarized.The high-T_c observed in these quenched crystals may be attributed to simultaneous tuning of electron carriers doping and strain effect caused by lattice reduction of Sm-substitution.     

Two-coupled structural modulations in charge-density-wave state of SrPt_2As_2 superconductor

Transmission electron microscopy(TEM) study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave(CDW) state below TCDW≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1= 0.62a*on the a*–b*plane and q2=0.23a*on the a*–c*plane. The atomic displacements occur along the b-axis direction for q1and along the c-axis direction for q2, respectively. Moreover, the correlation between q1and q2can be generally written as q1=(q2+ a*)/2 in the CDW state, suggesting the presence of essential coupling between q1and q2. A small fraction of Ir doping on the Pt site in Sr(Pt1-xIrx)2As2(x ≤ 0.06) could moderately change these CDW modulations and also affect their superconductivities.     

The c-axis complex permittivity and electrical impedance in BaFe_2As_2: Experimental examination on transformation validity

Complex permittivity and electrical impedance have been measured along the c-axis in single crystals BaFe_2 As_2,which are the conductors known as the parent compound of 122-type iron superconductor. The resultant relative errors defined in the study indicate the existence of the transformation between complex permittivity and electrical impedance in the conductors, and these two physics quantities possibly reveal different aspects of the consistent superconductivityrelevant physics picture.     

Fermi liquid breakdown and evidence for superconductivity in YFe2Ge2

In the d‐electron system YFe₂Ge₂, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ～ 100 mJ/(mol K²) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close‐by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe₂Ge₂ superconduct below 1.8 K. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of residual stress on nematic domains in BaFe_(2-x)Ni_xAs_2 studied by angular magnetoresistance

We have studied the angular magnetoresistance of iron pnictides BaFe_(2-x)Ni_xAs_2, which shows clear 180 degree periodicity as fitted by a cosine function. In the x = 0.065 sample, the phase of the two-fold symmetry changes 90 degrees above the tetragonal-to-orthorhombic structural transition temperature Ts. Since the phase at low temperature is associated with the rotation of orthorhombic domains by magnetic field, we show that even vacuum grease can push the presence of orthorhombic domains at temperatures much higher than Ts. Our results suggest that residual stress may have significant effects in studying the nematic orders and its fluctuations in iron pnictides.     

Coexistence of filamentary and homogeneous resistive switching in graded WOx thin films

Reversible clockwise and counter‐clockwise resistive switching in a Pt/graded WO_x /W stack is reported. Two stable switching modes with opposite switching polarity and different electrical characteristics are found to coexist in the same memory cell. Clockwise switching shows filamentary characteristics that lead to relatively faster switching with excellent retention at high temperature. Counter‐clockwise switching exhibits homogeneous conduction with slower switching and moderate retention. The field‐induced switching reversal might be due to inhomogeneous expansion of W during thermal oxidation. Our results provide a clue to modulate the switching type in Pt/WO_x /W memory cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simulations of filamentary structure in an edge-localized mode burst on EAST using BOUT++ code

In this paper, simulation on the filamentary structure for the low-hybrid wave heating H-mode on experimental advanced superconducting tokamak (EAST) is carried out for the first time using BOUT++, and the speed and width of edge localized mode (ELM) filaments have been evaluated during the simulation. The evolutions of the ELM filaments are illustrated temporally and spatially. Then the results are compared with the experimental observations. It is found that at the peak gradient region in the outer midplane, the radial speed of the filaments is changed frequently and varied from 0.016 to 0.38 km/s. For the contrast, the poloidal speed oscillates in a narrower range of 0.68–0.88 km/s. The calculation results show the width of ELM filaments oscillates during the simulation, the minimum width is around 8.3 mm, and the maximum value is 41.24 mm. The distributions of the ELM filament widths with the radial position indicate the width is decreased with the radial position. Furthermore, by tracing one single filament, the radial width is decreased gradually when the filament is moving outward, which is also consistent with experiment conclusion. The consistencies also indicate the availability and practicability of the six-field two-fluid model of BOUT++ on the study of ELMs.     

In-plane optical spectral weight redistribution in the optimally doped Ba_(0.6)K_(0.4)Fe_2As_2 superconductor

We performed detailed temperature-dependent optical measurements on optimally doped Ba0.6K0.4Fe2As2 single crystal, We examine the changes of the in-plane optical conductivity spectral weight in the normal state and the evolution of the superconducting condensate in the superconducting state. In the normal state, the low-frequency spectral weight shows a metallic response with an arctan （T） dependence, indicating a T-linear scattering rate behavior for the carriers. A high energy spectral weight transfer associated with the Hund＇s coupling occurs from the low frequencies below 4000 cm^-1 5000 cm^-1 to higher frequencies up to at least 104 cm^-1. Its temperature dependence analysis suggests that the Hund＇s coupling strength is continuously enhanced as the temperature is reduced. In the superconducting state, the FGT sum rule is conserved according to the spectral weight estimation within the conduction bands, only about 40% of the conduction bands participates in the superconducting condensate indicating that Ba0.6K0.4Fe2As2 is in dirty limit.     

Structure and superconductivity in the ternary silicide CaAlSi

Using the linear response-linearized Muffin-tin orbital (LR-LMTO) method, we study the electronic band structure, phonon spectra, electron-phonon coupling and superconductivity for c-axis ferromagnetic-like (F-like) and antiferromagnetic-like (AF-like) structures in ternary silicide CaAlSi. The following conclusions are drawn from our calculations. If Al and Si atoms are assumed to arrange along the c axis in an F-like long-range ordering (-Al-Al-Al-and-Si-Si-Si-), one could obtain the ultrasoft B_1g phonon mode and thus very strong electron-phonon coupling in CaAlSi. However, the appearance of imaginary frequency phonon modes indicates the instability of such a structure. For Al and Si atoms arranging along the c axis in an AF-like long-range ordering (-Al-Si-Al-), the calculated electron-phonon coupling constant is equal to 0.8 and the logarithmically averaged frequency is 146.8 K. This calculated result can correctly yield the superconducting transition temperature of CaAlSi by the standard BCS theory in the moderate electron-phonon coupling strength. We propose that an AF-like superlattice model for Al (or Si) atoms along the c direction may mediate the inconsistency estimated from theory and experiment, and explain the anomalous superconductivity in CaAlSi.      

Low temperature specific heat in BaFe_(1.9)Ni_(0.1)As_2 single crystals

Low-temperature specific heat was measured on the BaFe1.9Ni0.1As2 single crystals with critical transition temperature Tc = 20.1 K.A clear specific heat jump with the value ΔC/T|Tc ≈ 23 mJ/mol K2 was observed.In addition,a roughly linear magnetic field dependence of the electronic specific heat coefficient Δγ(H) was found in the zero-temperature limit,suggesting that at least one Fermi pocket,probably the hole derivative one,was fully gapped with a small anisotropy in the present sample.A slight curvature of the curve Δγ(H) may suggest a complex gap structure(anisotropic gap or nodes) at other Fermi surfaces.     

Unconventional superconductivity and magnetism in Sr2RuO4 and related materials

We review the normal and superconducting state properties of the unconventional triplet superconductor Sr₂RuO₄ with an emphasis on the analysis of the magnetic susceptibility and the role played by strong electronic correlations. In particular, we show that the magnetic activity arises from the itinerant electrons in the Ru d‐orbitals and a strong magnetic anisotropy occurs (χ^+‐ < χ^zz) due to spin‐orbit coupling. The latter results mainly from different values of the g‐factor for the transverse and longitudinal components of the spin susceptibility (i.e. the matrix elements differ). Most importantly, this anisotropy and the presence of incommensurate antiferromagnetic and ferromagnetic fluctuations have strong consequences for the symmetry of the superconducting order parameter. In particular, reviewing spin fluctuation‐induced Cooper‐pairing scenario in application to Sr₂RuO₄ we show how p‐wave Cooper‐pairing with line nodes between neighboring RuO₂‐planes may occur. We also discuss the open issues in Sr₂RuO₄ like the influence of magnetic and non‐magnetic impurities on the superconducting and normal state of Sr₂RuO₄. It is clear that the physics of triplet superconductivity in Sr₂RuO₄ is still far from being understood completely and remains to be analyzed more in more detail. It is of interest to apply the theory also to superconductivity in heavy‐fermion systems exhibiting spin fluctuations.     

Nonperturbative QCD vacuum and colour superconductivity

We discuss the possibility of existence of colour superconducting state in real QCD vacuum with nonzero _sGG. We argue, that nonperturbative gluonic fields might play a crucial role in colour superconductivity scenario.     

Fluorescent filamentary complexes between semiconductor nanoparticles and polyallylamine

We obtained complexes between core/shell type CdSe/ZnS semiconductor nanoparticles (NP) and the synthetic polyelectrolyte polyallylamine (PAA) as a result of ionic interactions in aqueous solutions and subsequent stretching on flat substrates using the "molecular combing" technology. By varying the size of the nanoparticles and the stoichiometric ratios of the concentrations of nanoparticles and PAA molecules in solution, we determined the factors ensuring formation of filamentary fluorescent nanoparticle–PAA complexes. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 428–433, May–June, 2009.