Josephson and quasiparticle currents in tunneling junctions between partially dielectrized (partially gapped) superconductors with spin density waves

The current-voltage characteristics (CVC) are calculated for the Josephson, interference, and quasiparticle components of the current through a tunneling junction formed by two superconductors with spin density waves (SDW). The treatment is based on the model of partial dielectrization (gapping) of the Fermi surface and the assumption of pinning of the spin density waves. The following particular cases are studied in detail: asymmetric SDW superconductor-ordinary superconductor junctions and symmetric junctions between two identical SDW superconductors. The positions and nature of the singularities in the CVC are determined. For a symmetric contact the possibility of the existence of asymmetric CVC’s is predicted. The calculations are in qualitative agreement with the experimentally observed behavior of the CVC’s of tunneling junctions and microcontacts containing the SDW superconductor with heavy fermions URu₂Si₂. Fiz. Tverd. Tela (St. Petersburg) 41, 1743–1749 (October 1999)     

Photoexcited Eu2+ spin dynamics in EuFe2As2

Employing temperature dependent time-resolved optical femtosecond spectroscopy, we investigated the quasiparticle and Eu²⁺ spin relaxation dynamics in EuFe₂As₂ (EFA). As previously reported in other undoped iron-based pnictides, we observe the quasiparticle relaxation bottleneck due to the charge gap opening in the spin density wave (SDW) state below T _SDW = 189 K. Below the Eu²⁺ antiferromagnetic (AFM) spin ordering temperature, T _AFM = 19 K, we observe another slower relaxation component, which we attribute to the Eu²⁺ AFM order dynamics. The slow dynamics of this component suggests a weak coupling between the Eu²⁺ spins and the carriers in the Fe-d derived bands.     

Competing energy scales in high‐temperature superconductors: Ultrafast pump–probe studies

We present a review of photoexcited quasiparticle dynamics of cuprate and pnictide high‐temperature superconductors in regimes (temperature, doping) where different phases such as superconductivity, spin‐density‐wave (SDW) and pseudogap phases coexist or compete with one another. We start with the overdoped cuprate superconductor Y_1–xCa_x Ba₂Cu₃O_7–δ, where the superconducting gap and pseudogap coexist in the superconducting state. In another cuprate Tl₂Ba₂Ca₂Cu₃O_y, we ob‐ serve a competition between SDW and superconducting orders deep in the superconducting state. Finally, in the underdoped iron pnictide superconductor (Ba,K)Fe₂As₂, SDW order forms at 85 K, followed by superconductivity at 28 K. We also find the emergence of a normal‐state order that suppresses SDW at a temperature T * ～ 60 K and argue that this normal‐state order is a precursor to superconductivity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dipole-dipole spin anisotropy energy for spin density waves in highly anisotropic crystals: (TMTSF) 2X

The dipole-dipole spin anisotropy energy for a spin density wave (SDW) is compatible with experimental easy, intermediate, and hard spin polarization axes in the (TMTSF) ₂X class of organic conductors only when electron-hole correlations of the SDW are atomic in size. Magnitudes of the spin anisotropy energy and the spin-flip magnetic field in this case agree with experiment for a SDW amplitude comparable to 10^-2 Bohr magnetons/TMTSF molecule.     

Mössbauer measurements in CuFeTe2

The Mössbauer measurements at variable temperature (from liquid helium to room temperature) on the layered compound CuFeTe₂ have been able to determine the existence of a magnetically ordered state below T _N= 254 ± 15 K. The general shape of the spectra and the magnitudes of the hyperfine fields let us to propose a Spin Density Wave (SDW) behavior for this compound.     

AC-DC coupling experiments in the spin density wave state of tetramethyltetraselenofulvalene-hexafluorophosphate [(TMTSF)2PF6]

Experiments performed on (TMTSF)₂PF₆ in the spin density wave (SDW) state in the presence of joint ac and dc excitations do not show evidence for coupling between dc and ac response. Irradiation by microwaves does not lead to strong dc nonlinearity, and attempts to resurrect the ESR signal by applied dc currents were also unsuccessful. While the strongly frequency dependent response is indicative of collective effects, the response of the SDW ground state is fundamentally different from that of a charge density wave condensate.     

Evidence of partial gap opening and Ce-site dilution effects in a heavy fermion compound CeNiGe2

We report the results of magnetisation, non-linear dc susceptibility, electrical transport, and heat capacity measurements on Y-substituted heavy fermion CeNiGe₂. Investigations are carried out on the compounds CeNiGe₂, Ce_0.9Y_0.1NiGe₂, Ce_0.8Y_0.2NiGe₂ and Ce_0.6Y_0.4NiGe₂. It is observed that with the increase in Y-concentration, the magnetic ordering temperature decreases. For CeNiGe₂, below ordering temperature Arrott plots suggest the presence of spin density wave (SDW). Third- and fifth-order dc susceptibility indicates magnetic instability which possibly leads to partial gap opening resulting in the observation of SDW. These observations are further investigated through resistivity and heat capacity measurements which also point toward partial gap opening in CeNiGe₂. Interestingly, with the increase in Y-substitution, it is noted that the gap opening is suppressed and also shifted towards lower temperature. Moreover, our investigations reveal absence of non-Fermi liquid behaviour or zero field quantum critical point even after 40% dilution of Ce-site.     

Low-frequency dielectric response in the quasi-one-dimensional organic conductor (TMTSF)2PF6

A unique and rare opportunity of revealing the nature of the spin-density-wave (SDW) fluctuation in the quasi-one-dimensional organic conductor (TMTSF)₂PF₆ has been provided by our AC impedance measurements above the SDW transition temperature. A close resemblance of the low-frequency dielectric response in the range between 20 K and the temperature of maximum resistivity T_max ∼ 80 K to that in the SDW ground state was revealed, with quite compatible exponents for their power law dependencies. The SDW fluctuation has thus been identified in the 2D Fermi liquid regime, which can be regarded as a precursor phase of the SDW ground state.     

SDW in the 2D Compound CuFeTe2

In a previous work (ICAME'97) we presented the Mössbauer results for a non-stoichiometric sample of the quasi-two-dimensional (2D) dichalcogenide CuFeTe₂, where a Spin Density Wave (SDW) ground state with T _SDW=256±15 K was proposed. Here we report the study of the magnetic and electric properties determined by magnetic susceptibility, Mössbauer spectroscopy and resistance measurements, of an almost stoichiometric sample prepared by the vertical Bridgman growth technique. The SDW behavior is supported by the results obtained by the following different techniques: Magnetic susceptibility: A magnetic transition is observed at T _SDW=308 K with a Pauli paramagnetic behavior above this temperature. Mössbauer effect: The shape of the spectra and the thermal evolution of the hyperfine field are characteristic of the SDW's in quasi-2D systems. Electrical resistance: There is a metal–semiconductor transition along the layers as the temperature decreases indicating the opening of a gap at the Fermi level.     

Spin-density-wave and superconducting states in thermally quenched (TMTSF)2ClO4

Successive transitions from metal to the spin-density-wave (SDW) state and the three-dimensional superconducting (3D-SC) state were found in thermally quenched bis-tetramethyltetraselenafulvalenium perchlorate, (TMTSF)₂ClO₄. The SDW state was suppressed after the sample was warmed above ～25 K and cooled slowly. The appearance of the SDW state in addition to the 3D-SC state in the quenched sample is interpreted by the decrease in interchain coupling which could result from the thermal quenching of a high temperature configuration with disordered (ClO₄)-anions. The effect of thermal quenching on quantum oscillations in transverse magnetoresistance was also studied.     

Enhancement of band magnetism and features of the magnetically ordered state in the CeB<Subscript>6</Subscript> compound with strong electron correlations

Precision measurements of transport and magnetic parameters of high-quality CeB₆ single crystals are performed in the temperature range 1.8—300 K. It is shown that their resistivity in the temperature interval 5 K < T < T* ≈ 80 K obeys not a logarithmic law, which is typical of the Kondo mechanism of charge carrier scattering, but the law ρ ∝ T ^?1/η corresponding to the weak localization regime with a critical index 1/η = 0.39 ± 0.02. Instead of the Curie-Weiss dependences, the asymptotic form χ(T) ∝ T ^?0.8 is obtained for magnetic susceptibility of CeB₆ in a temperature range of 15–300 K. Analysis of the field dependences of magnetization, magnetoresistance, and the Hall coefficient in the paramagnetic and magnetically ordered phases of CeB₆ and comparison with the results of measurements of Seebeck coefficient, the inelastic neutron scattering coefficient, and EPR spectroscopy lead to the conclusion that the Kondo lattice model and skew scattering model cannot be used for describing the transport and thermodynamic parameters of this compound with strong electron correlations. On the basis of detailed analysis of experimental data, an alternative approach to interpreting the properties of CeB₆ is proposed using (1) the assumption concerning itinerant paramagnetism and substantial renormalization of the density of electron states upon cooling in the vicinity of the Fermi energy, which is associated with the formation of heavy fermions (spin-polaron states) in the metallic CeB₆ matrix in the vicinity of Ce sites; (2) the formation of ferromagnetic nanosize regions from spin polarons at 3.3 K < T < 7 K and a transition to a state with a spin density wave (SDW) at T _Q ≈ 3.3 K; and (3) realization of a complex magnetic phase H-T diagram of CeB₆, which is associated with an increase in the SDW amplitude and competition between the SDW and antiferromagnetism of localized magnetic moments of cerium ions.     

Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF) 2 PF 6

The phase diagram of the organic superconductor (TMTSF)₂PF₆has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T _SDW and T _SC and the superconducting critical current. The onset temperature T _SC is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below T _SC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T _SC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings. Received 3 September 2001 and Received in final form 9 November 2001     

Structural and physical properties of undoped and nickel-doped BaFe2-xNixAs2（x = 0,0.04） single crystals

<正>Single crystals of undoped and nickel-doped BaFe_(2-x)Ni_xAs_2(x=0,0.04) have been grown by FeAs self-flux method.The maximum dimension of the crystal is as large as ~ 1 cm along the ab plane.The crystalline topography of a cleaved crystal surface is examined by scanning electron microscope(SEM).By x-ray powder diffraction(XRD) experiments using pure silicon as an internal standard,precise unit cell parameters(tetragonal at room temperature) are determined:a = 3.9606(4) A(1 A=0.1 nm),c = 13.015(2) A for BaFe_(1.96)Ni_(0.04)As_2 and a = 3.9590(5) A,c = 13.024(1) A for BaFe_2As_2.DC magnetization and transport measurements are performed to check superconducting transition(T_c=15 K for x=0.04) and other subtle anomalies.For BaFe_(0.96)Ni_(0.04)As_2 crystal,the resistance curve at normal state shows two distinct anomalies associated with spin and structure transitions,and its magnetization data above ~ 91 K exhibit a linear temperature dependence due to spin density wave(SDW) instability.     

Dissimilarity between metallic-like transport in the dielectric spin density wave and field-induced spin density wave states in (TMTSF)<Subscript>2</Subscript>PF<Subscript>6</Subscript>

We report similarities and differences of the transport features in the spin density wave (SDW) and in the field-induced SDW (FISDW) phases of the quasi-one-dimensional compound (TMTSF)₂PF₆. As temperature decreases below ≈2 K, the resistance in both phases exhibits a maximum and a subsequent strong drop. However, the characteristic temperature of the R(T) maximum and its scaling behavior in different magnetic fields B are evidence that the nonmonotonic R(T) dependences have different origin in SDW and FISDW regions of the phase diagram. We also found that the borderline T₀(B, P) which divides the FISDW region of the P-B-T phase diagram into the hysteresis and nonhysteresis domains terminates in the N=1 subphase; the borderline thus has no extension to the SDW N=0 phase.     

Charge dynamics of the spin-density-wave state in BaFe<Subscript>2</Subscript>As<Subscript>2</Subscript>

We report on a thorough optical investigation of BaFe₂As₂ over a broad spectral range and as a function of temperature, focusing our attention on its spin-density-wave (SDW) phase transition at T_SDW = 135 K. While BaFe₂As₂ remains metallic at all temperatures, we observe a depletion in the far infrared energy interval of the optical conductivity below T_SDW, ascribed to the formation of a pseudogap-like feature in the excitation spectrum. This is accompanied by the narrowing of the Drude term consistent with the dc transport results and suggestive of suppression of scattering channels in the SDW state. About 20% of the spectral weight in the far infrared energy interval is affected by the SDW phase transition.     

Differential thermal analysis of the field induced phase transitions of (TMTSF)2ClO4 above 1.2 K

We present some results of a differential thermal analysis of the magnetic field induced phase transitions in the organic conductor (TMTSF)₂ClO₄ above 1.2 K. This study shows that transitions between different spin density wave states are first order and that the total entropy change involved in the two detected transitions (in the temperature range 1.2–2 K) is close to that of the quasi-one-dimensional electron gas. Above 2 and 4.2 K, only a single transition has been detected in our measurements. The entropy of that transition decreases and extrapolates to zero near 5 K. We present some arguments suggesting that if longitudinal nesting (2k_F, 0, 0) is to take place in the semi-metallic SDW phase at high fields it exists only above 2 K or so.     

Effects of spin structures on Fermi surface topologies in BaFe2As2

The effects of spin structures on the Fermi surface topologies of BaFe₂As₂ were calculated using the first-principles approach. Here, we considered the nonmagnetic, Checkerboard, Stripe, and SDW (spin-density-wave) structures as well as a tetragonal structure labeled as STR17. By comparing the calculated results with the published angle-resolved photoemission spectroscopy from the literature, we propose that most of the experimentally observed Fermi surfaces of BaFe₂As₂ are the thermal mixture of those of the SDW, STR17, and Stripe structures.     

Gd3+ electron spin resonance spectroscopy on LaO1 ? x F x FeAs superconductors

We report an electron spin resonance (ESR) spectroscopy study on polycrystalline samples of the LaO_{1 ? x} F _x FeAs (x = 0 and 0.1) compound with small levels of Gd doping (2% and 5%). The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent LaO_{1 ? x} F _x FeAs compounds at T _SDW ?? 130 K. Interestingly, the analysis of the low-temperature ESR spectra of the c-axis oriented Gd_{1 ? y} La _y OFeAs samples gives evidence for the magnetically nonequivalent Gd sites and also for sites having a different local charge environment. The analysis of the temperature dependence of the ESR linewidths gives evidence for a coupling of the localized 4f electrons of Gd to the conduction electrons in the FeAs layers. The ESR data reveal that the fluorine substitution, which provides electron doping, suppresses the SDW order and enhances the density of states in the electronic bands stemming from the xz and yz orbital states of Fe to which the 4f electrons are most strongly coupled.     

Divergency of SDW and structure transition in Fe1−xNixSe0.82 superconductors

The effect of the Ni-substitution on the superconductivity of α-FeSe_0.82 system has been investigated. It is found that the superconductivity of α-FeSe_0.82 can be quickly suppressed when a small quantity of Fe is substituted by Ni element. The resistivity and moment of the sample at normal state increase with increasing Ni-substitution content. Moreover, it is found that the Ni-substitution does not change the structure transition temperature, but increases the spin density wave (SDW) order temperature. It is suggested that the quick suppression of superconductivity of α-FeSe_0.82 caused by the Ni-substitution is related to the increase of SDW order temperature. Therefore, Fe atom plays a key role for the appearance of the superconductivity in FeSe system.     

Magnetization and electron spin resonance of Fe impurities in (TMTSF)2AsF6: magnetic-field-dependent interaction between impurity spins and spin-density waves

A new spin-density-wave (SDW) system with magnetic impurities (TMTSF)₂(AsF₆)_1−x(FeCl₄)_x was prepared and its magnetic properties were studied by means of magnetization and electron-spin-resonance measurements. The anisotropic g-value and comparison of the Fe concentration with the Curie constant indicate that the Fe³⁺ ions are in a low-spin state. We also found that the magnetization curve of the impurity spins in this compound shows an anomalous behavior. This behavior can be explained if one assumes a field-dependent magnetic interaction between the Fe³⁺ spins and the SDW moment. We suppose that the field dependence of the SDW pinning potential is responsible for this phenomenon.