Antiferromagnetic Stripe Research Articles

Stripes disorder and correlation lengths in doped antiferromagnets

For stripes in doped antiferromagnets, we find that the ratio of spin and charge correlation lengths ${\ensuremath{\xi}}_{s}/{\ensuremath{\xi}}_{c}$ provides a sharp criterion for determining the dominant form of disorder in the system. If stripes are disordered predominantly by topological defects then ${\ensuremath{\xi}}_{s}/{\ensuremath{\xi}}_{c}\ensuremath{\lesssim}1.$ In contast, if stripes correlations are disordered primarily by nontopological elastic deformations (i.e., a Bragg-glass type of disorder) then $1<{\ensuremath{\xi}}_{s}/{\ensuremath{\xi}}_{c}\ensuremath{\lesssim}4$ is expected. Therefore, the observation of ${\ensuremath{\xi}}_{s}/{\ensuremath{\xi}}_{c}\ensuremath{\approx}4$ in $(\mathrm{LaNd}{)}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ and ${\ensuremath{\xi}}_{s}/{\ensuremath{\xi}}_{c}\ensuremath{\approx}3$ in ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{NiO}}_{4}$ invariably implies that the stripes are in a Bragg-glass-type state, and topological defects are much less relevant than commonly assumed. Expected spectral properties are discussed. Thus, we establish the basis for any theoretical analysis of the experimentally observed glassy state in these materials.

Magnetic domains and stripes in a spin-fermion model for cuprates

Monte Carlo simulations applied to a model of interacting fermions and classical spins show the existence of antiferromagnetic spin domains and charge stripes upon hole doping. The stripes have a filling of approximately 1/2 hole per site, and they separate spin domains with a pi phase shift among them. The observed stripes run either along the x or y axes. No particular boundary conditions or external fields are needed to stabilize these structures. When magnetic incommensurate peaks are observed at momentum pi(1,1-delta), charge incommensurate peaks appear at (0,2delta). The charge fluctuations responsible for the stripe formation also induce a pseudogap in the density of states.

Charge fluctuations in YBa2Cu3O7-x high-temperature superconductors

Understanding the behaviour of the electrons in the high-temperature copper oxide superconductors remains a challenging problem. An important class of models1,2,3,4,5 argues that the distribution of electronic charge and spin is not homogeneous: rather, spin and charge adopt a dynamic arrangement in which the spins on the copper form antiferromagnetic stripes, separated by domain walls containing the charge1,2,3,4,5. The dynamic behaviour of the spins has been extensively studied by neutron scattering, and recent results6 have shown that the low-frequency fluctuations for different classes of materials display a universal spatial behaviour that is consistent with the stripe picture. But arguments for the existence of the stripe phases are difficult to sustain without a demonstration that charge is distributed in the domain walls. Here we report phonon measurements for the YBa2Cu3O7-x high-temperature superconductors, which reveal the presence of charge fluctuations. The inferred periodicity is that expected if the charge is located in the domain walls separating the spin stripes. Our results therefore provide strong support for the existence of a dynamic stripe phase in the high-temperature superconductors.

Dynamics of stripes in doped antiferromagnets

We study the dynamics of the striped phase, which has previously been suggested to be the ground state of a doped antiferromagnet. Starting from the $t\ensuremath{-}J$ model, we derive the classical equation governing the motion of the charged wall by using a fictitious spin model as an intermediate step. A wavelike equation of motion is obtained and the wall elasticity and mass density constants are derived in terms of the $t$ and $J$ parameters. The wall is then regarded as an elastic string that will be trapped by the pinning potential produced by randomly distributed impurities. We evaluate the pinning potential and estimate the threshold electric field that has to be applied to the system in order to release the walls. Besides, the dynamics of the stripe in the presence of a bias field below the threshold is considered and the high- and low-temperature relaxation rates are derived.

Luttinger stripes in antiferromagnets

We propose a model for the physics of stripes in antiferromagnets in which the stripes are described by Luttinger liquids hybridized with antiferromagnetic domains. Using bosonization techniques we study the model in the limit where the magnetic correlation length is larger than the inter-stripe distance and propose an explanation for the commensurate-incommensurate phase transition seen in neutron scattering in the underdoped regime of La_{2-x} Sr_x Cu O_4. The explanation is based on a phase to anti-phase domain transition in the spin configuration which is associated with the transverse motion of the stripes. Using a non-linear sigma model to describe the antiferromagnetic regions we conjecture the crystalization of the stripes in the magnetically ordered phase.

Stripe correlations of spins and holes in cuprates and nickelates

Recent neutron diffraction studies of coupled spatial modulations of the spin and charge densities within the CuO2 planes of layered cuprates (and NiO2 planes of nickelate analogs) are reviewed. Analysis of superlattice peaks that appear at low temperatures (≤ 100 K) in hole-doped La2NiO4 indicates that the holes tend to localize in periodically-ordered antiphase domain walls separating antiferromagnetic stripes of Ni spins. Evidence for a similar ordering has now been found in a copper-oxide compound. La1.6-xNd0.4SrxCuO4 with ≈ 1/8, in which the superconductivity is anomalously suppressed. It is proposed that dynamical correlations of the same nature occur in superconducting cuprates. The relevance to the low-energy electrodynamics is discussed.

Ordering of holes and spins in La 2NiO 4.125 and La 1.8Sr 0.2NiO 4

Single-crystal neutron diffraction studies of La 2NiO 4.125 and La 1.8Sr 0.2NiO 4 reveal evidence for a long-period-commensurate coupled ordering of spins and charges below approximately 110 K. Analysis of the superlattice peaks indicates that the dopant-induced holes order in domain walls that form antiphase boundaries between antiferromagnetic stripes.

Magnetic-domain structure of Ni1-xMnx films inferred from resistance fluctuations.

In films of partially ordered ${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$, with x\ensuremath{\approxeq}0.28, large electrical-resistance noise from magnetic effects was found. Individual fluctuating domains with large volumes but low magnetic moments were observed. These domains are apparently clusters of constituent ferromagnetic domains which have a net systematic antiferromagnetic alignment. The clusters are larger in the ``reentrant spin-glass'' regime than in the ferromagnetic regime. Ferromagnetic domains arranged in antiferromagnetic stripes were also imaged by scanning electron microscopy.