Field and temperature dependence of the superfluid density in LaFeAsO1-xFx superconductors: a muon spin relaxation study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda ab(0)=254(2) nm for x=0.1 and lambda ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.     

Two-magnon Raman scattering in a spin density wave antiferromagnet

We present the results for a model calculation of resonant two-magnon Raman scattering in a spin density wave (SDW) antiferromagnet. The resonant enhancement of the two-magnon intensity is obtained from a microscopic analysis of the photon-magnon coupling vertex. By combining magnon-magnon interactions with ‘triple resonance’ phenomena in the vertex function the resulting intensity line shape is found to closely resemble the measured two-magnon Raman signal in antiferromagnetic cuprates. Both, resonant and non-resonant Raman scattering are discussed for the SDW antiferromagnet and a comparison is made to the conventional Loudon-Fleury theory of two-magnon light scattering.     

Short-range order in spin density wave antiferromagnets

The model of the short-range order state for itinerant antiferromagnets with SDW is developed. The thermodynamic transverse spin density fluctuation are shown to influence essentially on the formation of the long-range magnetic order. In the wide temperature range these occurs the shortprange, magnetic order regim. The properties of the low-frequency transverse excitations of the spin density are analized. The existence of a fully diffusive mode at small wave vectors is predicted.     

Dynamical group model of a spin density wave system

We obtain the dynamical group for a mean field quasi-one-dimensional model of an interacting many electron system exhibiting spin density waves. The corresponding Lie algebra is a sub-algebra of su(8); we use it to obtain the spectrum and define the corresponding order parameters. The algebraic treatment is extended to cover the presence of an external magnetic field.     

Resonant Raman scattering from a spin density wave insulator

We present a calculation of the electronic Raman cross section for the scattering of light across the energy gap of an antiferromagnetic insulator. The antiferromagnet is described in terms of a spin density wave state for the Hubbard model at half filling. We consider the coupling of the light to the current density and the inverse mass tensor on equal footing. A comparison of the cross section for different scattering geometries is given.     

Short-range magnetic order in a spin density wave in Cr-based multilayers

A mechanism of the formation of the short antiferromagnetic order with a spin density wave (SDW) in the vicinity of the interfaces in the Fe/Cr type multilayers is proposed. The main reason behind the emergence of magnetic ordering with SDWs is the redistribution of charge (and, hence, spin) density in the vicinity of Fe/Cr interfaces, which leads to the paramagnetic phase instability at a temperature considerably higher than the Néel temperature in chromium. The Ginzburg-Landau expansion for the free energy of the system is used for determining the inhomogeneous collinear structures of CDWs and for constructing the phase diagram (the dependence of the transition temperature on the thickness of the antiferromagnetic interlayer). The obtained results are used for discussing the experimental data on neutron scattering and tunnel microscopy.     

Commensurate and incommensurate states of a spin density wave in a quasi-two-dimensional system with an anisotropic energy spectrum in an external magnetic field of arbitrary direction relative to magnetization

A theory of thermodynamic properties of a spin density wave (SDW) in a quasi-two-dimensional system (with a preset impurity concentration x) is constructed. We choose an anisotropic dispersion relation for the electron energy and assume that external magnetic field H has an arbitrary direction relative to magnetic moment M _Q . The system of equations defining order parameters M _Q ^z , M _Q ^σ , M _z , and M ^σ is constructed and transformed with allowance for the Umklapp processes. Special cases when H ‖ M _Q and H ⊥ M _Q (H _Z H ^σ = 0) are considered in detail as well as cases of weak fields H of arbitrary direction. The condition for the transition of the system to the commensurate and incommensurate states of the SDW is analyzed. The concentration dependence of magnetic transition temperature T _M is calculated, and the components of the order parameter for the incommensurate phase are determined. The phase diagram (T,~x) is constructed. The effect of the magnetic field on magnetic transition temperature T _M is analyzed for H _Z H ^σ = 0, and longitudinal magnetic susceptibility χ‖ is calculated; this quantity demonstrates the temperature dependence corresponding to a system with a gap for x < x _c and to a gapless state for x > x _c . In the immediate vicinity of the critical impurity concentration (x ～ x _c ), the temperature dependence of the magnetic susceptibility acquires a local maximum. The effect of anisotropy of the electron energy spectrum on the investigated physical quantities is also analyzed.     

Quasiparticle states at a d-wave vortex core in high- T(c) superconductors: induction of local spin density wave order

The local density of states (LDOS) at the vortex lattice cores in a high- T(c) superconductor is studied by using a self-consistent mean-field theory including interactions for both antiferromagnetism (AF) and d-wave superconductivity (DSC). In a zero-field optimally doped sample the AF order is completely suppressed while DSC prevails. In the mixed state, we show that the local AF-like spin density wave order appears near the vortex core and acts as an effective local magnetic field on electrons via Zeeman coupling. As a result, the LDOS at the core exhibits a double-peak structure near the Fermi level that is in good agreement with recent scanning tunneling microscopy observations.     

Short-range order in spin density wave antiferromagnets with chemical dimerization

The spin density wave model in a quasi-one-dimensional itinerant antiferromagnet with staggered potential at finite temperature is studied. Only short-range ordering exists in this system above the Néel temperature. The local-band theory of spin fluctuations is developed to calculate the spin density wave amplitude and the effective exchange integral. The one-electron spectrum and magnon spectrum are obtained in the short-range ordering regime. Zh. éksp. Teor. Fiz. 114, 1346–1364 (October 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Ultrasonic attenuation in the transverse spin density wave phase of chromium

A strong hysteretic attenuation peak is seen when a polarizing field itH_p is applied perpendicular to both the longitudinal ultrasonic wavevector itq and the wavevector itQ of the transverse spin density wave (TSDW) in chromium, i.e., itH_p⊥itq⊥ itQ, but not when (itH_p∥itq)⊥itQ. This effect is discussed in relation to the various models for the configuration of the polarization that have been proposed to explain the anomalous attenuation seen in the TSDW phase for itq⊥itQ.     

Changes of the Q-vector of the spin density wave in chromium

Changes of the Q-vector with temperature and pressure occur in the electron-hole $\text{k}$-state pairing model even when no changes occur in the Fermi surface nesting vectors. These changes agree with experimental results which have previously been regarded as contradictory to the electron-hole pairing model.     

Magnitude of spin and charge density wave amplitudes in underdoped cuprates

Self-consistent calculation of spin (charge) density wave (S(C)DW) order parameters have been performed for bilayered cuprates on the basis of a singlet-correlated band model. Evolution of the Fermi surface in the strongly underdoped regime is described by a two-band approach. The smooth development of the pseudogap formation temperature from underdoped to overdoped states is explained and the Fourier amplitudes 〈s_q〉 (spin) and 〈e_q〉 (charge) modulations are calculated. We have found a maximum of the incommensurability for doping 0.09 ÷ 0.11 holes per copper site.     

Hall mobility in quasi one-dimensional conductors with charge density or spin density wave ground state

High mobilities found for free carriers below the phase transition in quasi one-dimensional crystals such as TTF-TCNQ and (TMTSF)₂PF₆ indicate that defect scattering is unimportant. We calculate the Hall mobility due to phonon scattering and find good agreement with the measured value of 10⁴cm²/Vsec for (TMTSF)₂PF₆ at 4K.     

The chromium spin density wave: magnetic X-ray scattering studies with polarisation analysis

We report on X-ray magnetic diffraction studies of the spin density wave antiferromagnetism formed in the conduction electron band of chromium. Non-resonant X-ray magnetic scattering was used to directly determine that chromium has zero orbital magnetisation. Furthermore, the azimuthal dependence of this scattering provides unique evidence that chromium forms a linearly polarised wave. In the vicinity of the K absorption edge, resonant X-ray magnetic scattering was observed. A consistent model of the magnetic scattering has been derived from the resonant and non-resonant magnetic amplitudes. The enhancement of the magnetic intensity arises primarily from dipole transitions from the core 1s level to 4p states. Quadrupole transitions to the magnetic 3d states are essentially non-existent due to their sensitivity to (and the absence of) orbital moment. This effect is predicted from atomic considerations of the 3d⁵ ( = 0) transition metal ions. Received 22 September 2000     

Lattice distortion and multiple spin density wave state in CsCl type AuMn

The relation between the tetragonal lattice distortion and the multiple-Q antiferromagnetic ordering and possible phase diagram in the temperature vs. composition plane of CsCl type ordered alloys Au_xMn_1−x (x≈0.5) are discussed on the basis of Landau's phenomenological theory.     

Commensurate spin dynamics in the superconducting state of an electron-doped cuprate superconductor

We report neutron scattering studies on two single crystal samples of the electron-doped (n-type) superconducting (SC) cuprate Nd2-xCexCuO4 (x=0.15) with T(c)=18 and 25 K. Unlike the hole-doped (p-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the n-type cuprate shows commensurate magnetic fluctuations at the tetragonal (1/2 1/2 0) reciprocal points both in the SC and in the normal state. A spin gap opens up when the n-type cuprate becomes SC, as in the optimally doped p-type La2-xSrxCuO4. The gap energy, however, increases gradually up to about 4 meV as T decreases from T(c) to 2 K, which contrasts with the spin pseudogap behavior with a T-independent gap energy in the SC state of p-type cuprates.     

Phase and amplitude collective modes of an incommensurate spin density wave

We examine the collective modes of an incommensurate quasi-one-dimensional spin density wave associated with oscillations in the phase and amplitude of its complex order parameter. Using a linear response formalism that ensures gauge- and translational-invariance the effects of the Coulomb repulsion in the particle-particle channel are shown to simply renormalise the velocity of the massless phase mode to a value higher than the Fermi velocity. Analytic results for the frequency and damping of the massive amplitude mode are presented. These two longitudinal collective modes remain decoupled for arbitrary wavevector q.     

Muon depolarization by the spin density wave hyperfine fields of lead

Muon depolarization arising from hyperfine fields created by the twelve SDWs in Pb is studied. Since the SDW wave vectors, ={210}, are commensurate with the lattice, there are no hyperfine fields at the octahedral interstitial sites (where muons ordinarily reside). Instead, the SDWs have antinodes at the tetrahedral interstitial sites and generate ⁺ hyperfine fields estimated to be 3000 G. Recent data show that the depolarization rate in Pb falls sharply towards zero at 35K (on account of ⁺ diffusion), but then slowly recovers to half its low temperature value near 200 K. Such recovery of the depolarization rate can be attributed to growth of the thermally activated occupation probability for muons at tetrahedral sites, where their spins undergo (motionally narrowed) transient precession caused by the static 3000 G fields. SDW depolarization can be distinguished from recovery caused by deep traps. A 10 G longitudinal field will quench the latter but not the former. Such an experiment could provide a crucial confirmation of the exotic magnetism of Pb.