Coexistence and coupling of superconductivity and magnetism in thin film structures

Superconducting and magnetic order are usually mutually exclusive, and are found to coexist in relatively few materials. We have obtained direct evidence for a spin-density wave (SDW) coexisting with bulk superconductivity in a ferromagnetic-superconducting trilayer. In the superconducting state the amplitude of the SDW is enhanced and modeling the data also suggests a pi/2 phase shift of one component of the SDW, implying a profound coupling of these two forms of order.     

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.     

Interface-induced states with an incommensurate spin-density wave in Fe/Cr-type multilayers

A model is proposed for magnetic ordering in Fe/Cr-type multilayers substantially above the Néel temperature of bulk chromium. Redistribution of the charge (and, hence, spin) density near the Fe/Cr interfaces gives rise to the formation of an essentially inhomogeneous spin-density-wave (SDW) state in the chromium spacer. The spatial structure of the antiferromagnetic order parameter in thick spacers is described. The SDW contribution to the effective exchange coupling between the moments in adjacent iron layers is calculated. The data obtained are used in the interpretation of experimental data on the tunneling spectroscopy of trilayers and neutron diffraction from Fe/Cr superlattices.     

Contact-induced magnetism in nanostructures based on chromium with nonmagnetic metal monolayers

Magnetic ordering is studied in Cr/X (1 ML) layer structures with monatomic layers (ML) of nonmagnetic metals (X = Sn, V, Ag, Au) incorporated into a chromium matrix. The mechanism associated with a redistribution of the charge and spin densities near the Cr/X interface and through which a spin-density wave (SDW) is induced and modified in these systems is analyzed. A semiphenomenological model is considered in detail in terms of which SDW structure near a single planar nonmagnetic defect can be described qualitatively using the Ginzburg-Landau functional. The spatial SDW configuration in a [Cr(t/X(1 ML)] superlattice is calculated, and the dependences of the SDW parameters on the temperature T, type of metal X, and superlattice period t are established. Based on the results of the study, experimental Mössbauer spectroscopy data are interpreted.     

Form-factor measurements on chromium with high-energy synchrotron radiation

Results of high-energy magnetic X-ray diffraction on pure antiferromagnetic chromium are presented. The temperature dependence of the propagation vector of the spin-density wave (SDW) and the strain-wave (SW) could be reproduced. The temperature dependence of the magnetic integrated intensity could be measured in the transversally as well as in the longitudinally polarised SDW phase. The magnetic form-factor has been determined in the transversally polarised SDW phase with five magnetic satellites. For the first time a spin-orbit separation has been performed by comparing X-ray to neutron data. The small orbital contribution to the magnetisation density turns out to be negligible, in agreement to our relativistic band-structure calculations. In addition, measurements of strain-wave reflections have been undertaken, and the results complement previous studies. Received 17 August 1998 and Received in final form 10 August 1999     

Hysteretic spin-density-wave ordering in confined geometries

We have measured the antiferromagnetic spin-density-wave (SDW) order in Cr/Cr(97.5)Mn2.5(001) superlattices. The Mn doping creates a high Néel temperature layer that confines the incommensurate SDW order within the Cr layers. With temperature cycling we observe a transition from commensurate to incommensurate SDW order and discrete changes in the SDW period. We find that these transitions show significant hysteresis (up to 75 K) when the number of SDW nodes within the Cr layer changes by an odd number, while there is no hysteresis for changes of an even number of nodes. This results from the competition between maintaining the spin structure at the interfaces and introducing a spin slip at the nodes of the Cr SDW.     

Coexistence of superconductivity and antiferromagnetism probed by simultaneous nuclear magnetic resonance and electrical transport in (TMTSF)2PF6 system

We report simultaneous NMR and electrical transport experiments in the pressure range near the boundary of the antiferromagnetic spin density wave (SDW) insulator and the metallic/superconducting (SC) phase in (TMTSF)2PF6. Measurements indicate a tricritical point separating a line of second-order SDW/metal transitions from a line of first-order SDW/metal(SC) transitions with coexistence of macroscopic regions of SDW and metal(SC) order, with little mutual interaction but strong hysteretic effects. NMR results quantify the fraction of each phase.     

Growth and transport properties of CaFeAsF_(1-x) single crystals

Using self-flux method,we have successfully grown the parent phase of the single crystals of CaFeAsF1-x.The X-ray di?raction indicates good crystallinity.In-plane resistivity shows a bad metallic behavior with a sharp drop of resistivity at about T SDW=119K.This anomaly is associated with the possible spin density wave(SDW)order.Interestingly near T SDW,the resistivity exhibits a cusp-like feature,which may be understood as the strong coupling effect between the electrons and the antiferromagnetic(AF)spin fluctuations.A reduction of fluorine or application of a high pressure will suppress the SDW feature and induce superconductivity.Hall effect measurements reveal a positive Hall coefficient below T SDW indicating a dominant role of the hole-like charge carriers in the parent phase.Strong magnetoresistance has been observed below T SDW suggesting multiple conduction channels of the charge carriers.     

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.     

Theory for Difference Between Photoinduced Phase and Thermally Excited Phase

A possible difference between the photoinduced phase and the thermally excited one is studied by using a two-dimensional extended Peierls-Hubbard model, which includes a strong electron-phonon coupling and a on-site interelectron repulsion, as well as an anharmonic lattice potential. Because of this anharmonicity, the system undergoes a first order phase transition from an insulating CDW state to a metallic one at a high temperature. Although some sign of an SDW order is expected to appear due to this repulsion, it is always hidden in any equilibrium phase of the present system. In fact, it is hidden, not only in the CDW ground state, but also in this metallic one, since the high temperature itself destroys the SDW order, far before the CDW-metal transition occurs, while a photo-excitation at low enough temperature is shown to generate a local metastable SDW domain. Therefore, to observe the presence of such Coulomb interaction and the resultant broken symmetry, a nonequilibrium photoinduced phase is shown to be most straightforward. Thus, the photoinduce phase transition can make an interaction appear as a broken symmetry only in this phase, even though this interaction is almost completely hidden in all the equilibrium phases from low temperature to high ones.     

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.     

An adaptive squared-distance-weighted interpolation for volume reconstruction in 3D freehand ultrasound

Volume reconstruction is a key procedure in 3D ultrasound imaging. An algorithm named as squared-distance-weighted (SDW) interpolation has been earlier proposed to reduce the blurring effect in the 3D ultrasonic images caused by the conventional distance weighted (DW) interpolation. However, the SDW parameter alpha, which controls the weight distribution, is a constant assigned by operators so that the interpolation effect is invariant for both sharp edges and speckle noises. In this paper, we introduced a new adaptive algorithm based on SDW interpolation for volume reconstruction of 3D freehand ultrasound. In the algorithm, the local statistics of pixels surrounding each voxel grid were used to adaptively adjust the parameter alpha in SDW. The voxel grids with a higher ratio of local variance and mean in their neighbourhoods would have a smaller alpha to make the image details sharper, while the voxel grids locating in regions with a lower ratio of local variance and mean would have a larger alpha to smooth image content in homogeneous regions, where speckle noise is usually observed and damages the image quality. By comparing the simulation results using the SDW and new adaptive algorithm, it was demonstrated that this new algorithm worked well in both edge preservation and speckle reduction.     

Triangular lattices of charge-and spin-density waves in chemisorbed metal monolayers on the surface of diamond structure semiconductors

The electronic spectrum of a planar triangular lattice is analyzed, and the possible occurrence of charge (CDWs) and spin density waves (SDWs) in the lattice is discussed. Commensurate CDW and SDW structures of two types are considered in the weak and strong electron-electron interaction approximations. The CDW and SDW models are applied to the specific case of chemisorbed metal monolayers on the (111) surface of diamond structure semiconductors with a coverage close to 1/3.     

Model of vortex states in hole-doped iron-pnictide superconductors

Based on a phenomenological model with competing spin-density-wave (SDW) and extended s-wave superconductivity, the vortex states in Ba(1-x)K(x)Fe2As2 are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap.     

EFFECTS OF ELECTRON-ELECTRON INTERACTION AND ELECTRON-PHONON COUPLING ON THE SPIN-DENSITY-WAVE OF QUASI-ONE-DIMENSIONAL ORGANIC POLYMER FERROMAGNETS

Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferro-magnet poly-BIPO, the effects of electron-electron interaction and electron-phonon coupling on the spin-density-wave (SDW) state of poly-BIPO system are studied. It is found that the SDW state is the stable ground state of the system, in which the system shows ferromagnetic order. And the larger electron-electron interaction and the smaller electron-phonon coupling will make the SDW state more stable than the nonmagnetic state.     

Competing spin waves and superconducting fluctuations in strongly correlated electron systems

A special diagram technique recently proposed for strongly correlated electron systems is used to study the peculiarities of a spin-density wave (SDW) in competition with superconductivity. This method allows formulation of the Dyson equations for the renormalized electron propagators of the co-existing phases of SDW antiferromagnetism and superconductivity. We find the surprising result that triplet superconductivity appears provided that we have the co-existence of singlet superconductivity and SDW antiferromagnetism. A special ansatz, which takes into account the full Green's functions as well as the dynamical structure of the correlations, is used to establish the equations determining the gap functions and order parameters.     

Possible evidence for an incommensurate spin density wave in the stanford electron tunneling studies into LaSrCuO

The Stanford studies include a remarkable dI/dV versus voltage curve with not just one BCS-like peak on each of the positive and negative voltage curves, but with three peaks on the positive-V side, and almost the same structure on the negative-V side. This structure may be caused by an incommensurate spin density wave (SDW) with T_CSCW comparable to 125 K. Incommensurability of the SDW would replicate the usual structure at E_F with two peaks at the edges of a 2Δ gap centred not at E_F, but at displaced positions ± 2v_FδQ away from E_F, where δQ is the incommensurability of the SDW wave vector.     

The competition between CDW and SDW in quasi-one-dimensional organic magnetic polymer with interchain coupling interaction

Based on a theoretical model proposed for interchain-coupled quasi-one-dimensional organic magnetic polymer, the effects of the interchain couplings and electron–electron interactions on the charge density wave (CDW) and spin density wave (SDW) that exist in the system are studied. It is found that the amplitude of the SDW along the main chain will decrease with increasing of the oscillatory term of the interchain couplings in the system, which is unfavorable to the ferromagnetic ground state of the system. Moreover, with different interchain couplings, there will all exist a critical value of the inter-site electron–electron Coulomb repulsion, and at this value, the system will experience a transformation from strong SDW state to strong CDW one, which will weaken the mediating function of the antiferromagnetic SDW along the main chain. As a result, the ferromagnetic correlation intensity between the spins of the side radicals will be affected and consequently the stability of the ferromagnetic state in the system will be weakened.     

STUDIES ON THE NEXT-NEAREST NEIGHBOR HOPPING INTERACTIONS OF π-ELECTRONS IN QUASI-ONE-DIMENSIONAL ORGANIC FERROMAGNETIC MODEL

Based on a theoretical model proposed for quasi-one-dimensional organic polymer fer-romagnets, the next-nearest neighbor hopping interactions of sr-electrons are considered. Allowing for full lattice relaxation, a set of self-consistent equations is established to study the system. The spin-density-wave (SDW) and the possible ferromagnetic ground state of the system are investigated in detail. It is found that the next-nearest neighbor hopping in-teractions will make the SDW stronger and consequently make the ferromagnetic state more stable as compared with the nonmagnetic reference state.     

Divergence of the grüneisen ratio at quantum critical points in heavy fermion metals

We present low-temperature volume thermal expansion, beta, and specific heat, C, measurements on high-quality single crystals of CeNi2Ge2 and YbRh2(Si0.95Ge0.05)(2) which are located very near to quantum critical points. For both systems, beta shows a more singular temperature dependence than C, and thus the Grüneisen ratio Gamma proportional to beta/C diverges as T-->0. For CeNi2Ge2, our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spin fluctuations. By contrast, the observed singularity in YbRh2(Si0.95Ge0.05)(2) cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.