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.     

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.     

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.     

Frequency- and bias-dependent ac conductivity of charge-density waves in TaS3

The charge-density wave contribution to the dc and the small-signal ac conductivity of orthorhombic TaS₃ has been completely characterized for a single crystal. The ac conductivity is found to be independent of dc bias below threshold. The tunneling theory of charge-density wave depinning successfully predicts the real and imaginary parts of the ac conductivity as functions of both frequency and applied dc bias, using only a fit to the dc I–V data and one adjustable parameter.     

Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr0.25Ti0.75]O3 ceramic

In this work, we report the Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr_0.25Ti_0.75]O₃ ceramic synthesized by solid state reaction. This ceramic was characterized by X-ray diffraction, Rietveld refinement, scanning electron microscopy and energy dispersive X-ray spectrometry. Impedance spectroscopy analyses reveals a non-Debye relaxation phenomenon being its relaxation frequency moving toward to positive side with increase of temperature. A significant shift in impedance loss peaks toward higher frequency side indicates conduction in material and favoring the long range motion of mobile charge carriers. The frequency dependent ac conductivity at different temperatures indicates that the conduction process is thermally activated. The variation of dc conductivity exhibited a negative temperature coefficient of resistance behavior. The ac conductivity data are used to evaluate the density of states at Fermi level and activation energy of this ceramic. The dc electrical and thermal conductivities of grain and grain boundary have been discussed.     

Magnetic and transport properties of Pr2Pt3Si5

We have investigated the magnetic and transport properties of a polycrystalline Pr₂Pt₃Si₅ sample through the dc and ac magnetic susceptibilities, electrical resistivity, and specific heat measurements. The Rietveld refinement of the powder X-ray diffraction data reveals that Pr₂Pt₃Si₅ crystallizes in the U₂Co₃Si₅-type orthorhombic structure (space group Ibam). Both the dc and ac magnetic susceptibility data measured at low fields exhibit sharp anomaly near 15 K. In contrast, the specific heat data exhibit only a broad anomaly implying no long range magnetic order down to 2 K. The broad Schottky-type anomaly in low temperature specific heat data is interpreted in terms of crystal electric field (CEF) effect, and a CEF-split singlet ground state is inferred. The absence of the long range order is attributed to the presence of nonmagnetic singlet ground state of the Pr³⁺ ion. The electrical resistivity data exhibit metallic behavior and are well described by the Bloch–Grüniesen–Mott relation.     

Non-equilibrium transport in NbSe3: Effects of a temperature gradient

We have measured the dc response characteristics of NbSe₃ in the charge density wave (CDW) state, in the presence of an applied temperature gradient. The threshold field E_T for the onset of nonlinear conduction remains sharp, and is determined by the average temperature of the specimen. No change is observed in the amplitude or quality factor of the coherent current oscillations in the nonlinear conductivity region, other than that expected from a change in average temperature. We interpret our results as evidence for macroscopic dynamical coherence throughout the specimen.     

The magnetism for NN AFM ground state in Fe-based superconductor: Sr1−xKxFe2As2

The crystal structure, magnetism properties, and density of states for FeAs layered compound SrFe₂As₂ have been investigated by using the density functional theory (DFT) method. The magnetism under a checkerboard nearest neighbor anti-ferromagnetic (NN AFM) and ferromagnetic (FM) order ground-state have been analyzed with substitution for Sr with K ion in Sr_1−xK_xFe₂As₂. The results indicate that the distortion of FeAs tetrahedrons is sensitive to the electron doping concentration. The system magnetism was suppressed by K doping in NN-AFM ground state instead of FM. The density of states at Fermi level N(E_F) under NN AFM ground state would be regarded as a driving force for the increased T_c of Sr_1−xK_xFe₂As₂ system as observed experimentally. Our calculation reflects that NN AFM type spin fluctuation may still exist in the Sr_1−xK_xFe₂As₂ system and it may be an origin of strong spin fluctuation in this system besides the spin density wave (SDW) states.     

Magnetic study of CaMn0.96Mo0.04O3, canting vs. phase separation

CaMn_0.96Mo_0.04O₃ is an example of Mn⁴⁺ rich perovskite manganites, which exhibits a net ferromagnetic component at low temperature, observed by dc magnetization and ac susceptibility. To characterize the magnetic state of this compound, neutron powder diffraction was carried out in the 2-400 K temperature range, showing that it is necessary to use three components (ferromagnetic and G- and A-type antiferromagnetic) to describe it. This particular state is in agreement with the unusual magnetic behaviour observed by macroscopic measurements and is compared to the one observed for manganites with similar Mn valence but obtained by A-site substitution.     

Spin density wave of itinerant heavy electrons in Ce(Ru0.85Rh0.15)2Si2

The magnetic properties of Ce(Ru_0.85Rh_0.15)₂Si₂ were studied by neutron scattering and measurements of magnetization, susceptibility, specific heat and thermal expansion as a function of temperature. We observe a crossover from a high temperature localized spin to a low temperature heavy electron state. Spin density wave (SDW) behavior appears in the heavy electron state below T_N = 5.5 K and the volume change due to spin quantum fluctuations associated with the SDW and the Kondo screening is reminiscent of moment-volume instabilities of the INVAR and anti-INVAR behavior of 3 d transition metal alloys.     

Room temperature ferromagnetism and magnetotransport properties of the layered manganite system La1.2Ba1.8Mn2−xRuxO7 (0≤x≤1.0)

The dc magnetization and ac susceptibility measurements on two dimensional layered manganite La_1.2Ba_1.8Mn₂O₇ samples reveal the occurrence of ferromagnetism above room temperature with ferromagnetic (FM) to paramagnetic (PM) transitions at 338 K. The bifurcation temperatures shown by the zero-field cooled (ZFC) and field cooled (FC) dc magnetization curves at high temperatures shift towards lower temperatures as the applied field is increased from 100 to 2500 Oe. The data are suggestive of a large magnetic anisotropy due to the strong competing ferromagnetic and antiferromagnetic interactions resulting in a spin-glass-like state. Ru doping is found to enhance the ferromagnetism and metallicity of the system in a remarkable way. The magnetoresistance (MR) values obtained are very high and about 40% even at 260 K for the undoped sample.     

Electronic structure reconstruction of CaFe2As2 in the spin density wave state

The electronic structure of CaFe₂As₂, a parent compound of iron-based superconductors, is studied with high-resolution angle-resolved photoemission spectroscopy. The electronic structure of CaFe₂As₂ in the paramagnetic state is consistent with that of density-functional theory calculations. We show that the electronic structure of this compound is significantly reconstructed when entering the spin density wave state. We could resolve two hole-like pockets and four electron-like pockets around the (0, 0) point, and one electron-like pocket surrounded with a pair of electron- and hole-like pockets around the (π, π) point in the spin density wave state. Therefore, the complicated Fermi surface topology and electronic structure near Fermi surface of CaFe₂As₂ illustrate that there exists unconventional electronic reconstruction in the spin density wave state, which cannot be explained by the band folding and Fermi surface nesting pictures.     

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.     

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)     

Spin- and charge-density-wave orders in La1.87Sr0.13Cu0.99Fe0.01O4

We have performed elastic neutron-scattering measurements on La_1.87Sr_0.13Cu_0.99Fe_0.01O₄ to study the magnetic impurity effects on the stripe correlations in high-T_c cuprates. Both charge-density-wave (CDW) and spin-density-wave (SDW) orders are observed in the low-temperature-orthorhombic (LTO) phase for the first time. With decreasing the temperature, CDW order first appears at followed by the SDW order at lower temperature of . The incommensurability for the CDW order (ε) was found to be 0.224±0.002 r.l.u., which is approximately twice of that for SDW order (δ) of 0.115±0.003 r.l.u. Both ordering sequence and relation of ε≈2δ are the same for SDW and CDW orders observed in the low-temperature-tetragonal (LTT) phase of La_1.60-xNd_0.4Sr_xCuO₄ (LNSCO), suggesting that the similar stripe correlations exist in the cuprate oxides, irrespective of crystal structure.     

Dephasing of Andreev pairs entering a charge density wave

A Cooper pair from a s-wave superconductor (S) entering a conventional charge density wave (CDW) below the Peierls gap dephases on the Fermi wavelength while one particle states are localized on the CDW coherence length ξ_CDW. It is thus practically impossible to observe a Josephson current through a CDW. The paths following different sequences of impurities interfere destructively, due to the different electron and hole densities in the CDW. The same conclusion holds for averaging over the conduction channels in the ballistic system. We apply two microscopic approaches to this phenomenon: (i) a Blonder, Tinkham, Klapwijk (BTK) approach for a single highly transparent S-CDW interface; and (ii) the Hamiltonian approach for the Josephson effect in a clean CDW and a CDW with non magnetic disorder. The Josephson effect through a spin density wave (SDW) is limited by the coherence length ξ_SDW, not by the Fermi wave-length. A Josephson current through a SDW might be observed in a structure with contacts on a SDW separated by a distance ξ_SDW.     

The influence of impurities on the quasiparticle tunneling current between weakly coupled quasi-one-dimensional charge density waves

The dc and ac conductivity of a tunneling junction between two impure quasi-one-dimensional charge density wave (CDW) systems is calculated. The non-magnetic impurities are considered in the self-consistent Born approximation (SCBA). Impurities modify the density of states (DOS) of the pure CDW system for quasiparticles inside the energy region of the gap 2(T). As in the pure case, the theory predicts in addition to a tunneling current which is proportional to the product of the DOS a term proportional to the cosine of the order parameter phase difference. In the case of a normal state/CDW junction, analytical expressions are obtained forT=0 showing deviations from the pure case. The linear ac conductivity is obtained by the scaling relation between the dc and the ac response.     

A neutron diffraction investigation of the sinusoidal atomic displacement wave in pure chromium

A neutron diffraction investigation on a single crystal of chromium, which is approximately single-Q, confirms the existence of a sinusoidal atomic displacement wave in the antiferromagnetic state. The wave is observed to be polarized parallel to the spin density wave (SDW) wavevector, Q, and to have a wavevector 2Q in both the transversely and longitudinally polarized SDW phases. The amplitude of the displacement wave is found to be proportional to the square of the amplitude of the SDW. We obtain for the displacement wave amplitude (0.0017 ± 0.0002)a at 130 K, where a is the lattice parameter.     

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.