[en] A spin-density wave (SDW) is shown to mediate a strongly temperature-dependent magnetic coupling between magnetic proximity layers. For parallel or antiparallel moments in the proximity layers, the order parameters of the SDW oscillate as a function of the spacer thickness with a two monolayer period. The SDW phase transition between incommensurate and commensurate phases can be controlled by flipping the magnetization of one of the proximity layers. copyright 1997 The American Physical Society

[en] The dispersion of a single hole in the t-J model obtained by the exact result of 32 sites and the results obtained by self-consistent Born approximation and the Green function Monte Carlo method can be simply derived by a mean-field theory with d-wave resonating-valence-bond (d-RVB) and antiferromagnetic order parameters. In addition, it offers a simple explanation for the difference observed between those results. The presence of the extended van Hove region at (π,0) is a consequence of the d-RVB pairing instead of the antiferromagnetic order. Results including t^' and t^double-prime are also presented and explained consistently in a similar way. copyright 1997 The American Physical Society

[en] A high-frequency pulsed electron paramagnetic resonance electron-nuclear double-resonance (ENDOR) study on a ¹³C-enriched 6H-SiC single crystal is reported. This type of spectroscopy, owing to its superior spectral resolution, allows us to obtain detailed information about the electronic structure of the shallow boron acceptor centers in 6H-SiC. It is concluded that around 40% of the spin density is localized in the p_z orbital of a carbon which is nearest to boron. The p_z orbital is directed along the C-B connection line and is parallel to the c axis for the hexagonal site and 70 degree away from the c axis for the two quasicubic sites. We conclude that the C-B bond is a dangling bond, that boron is neutral, and that there is no direct spin density on boron. There is a relaxation of the carbon atom, carrying the spin density, and the boron atom away from each other. From a ²⁹Si and ¹³C ENDOR study it is further concluded that around 60% of the spin density is distributed in the crystal with a Bohr radius of 2.2 Angstrom. copyright 1997 The American Physical Society

[en] A local, second-order (truncated) approximation is applied to the Hubbard model in three dimensions. Lowering the temperature, at half-filling, the paramagnetic ground state becomes unstable towards the formation of a commensurate spin-density-wave (SDW) state (antiferromagnetism) and sufficiently far away from half-filling towards the formation of incommensurate SDW states. The incommensurate-ordering wave vector does not deviate much from the commensurate one, which is in accordance with the experimental data for the SDW in chromium alloys. copyright 1997 The American Physical Society

[en] The competition between long-range and short-range interactions among holes moving in an antiferromagnet (AF) is studied within a model derived from the spin-density-wave picture of layered transition metal oxides.A novel numerical approach is developed that allows one to solve the problem at finite hole densities in very large systems (of the order of hundreds of lattice spacings), albeit in a quasiclassical limit, and to correctly incorporate the long-range part of the Coulomb interaction. The focus is on the problem of charge ordering and the charge phase diagram: at low temperatures four different phases are found, depending on the strength of the magnetic (dipolar) interaction generated by the spin-wave exchange and the density of holes. The four phases are the Wigner crystal, diagonal stripes, a grid phase (horizontal-vertical stripe loops), and a glassy-clumped phase. In the presence of both in-plane and out-of-plane charged impurities the stripe ordering is suppressed, although finite stripe segments persist. At finite temperatures multiscale (intermittency) dynamics is found, reminiscent of that in glasses. The dynamics of stripe melting and its implications for experiments is discussed. (c) 2000 The American Physical Society

[en] The coupling of spin waves with charge- and spin-density waves is shown to be induced by a spin-dependent interaction in a quantum well, which is spin polarized by a dc magnetic field at an angle θ to the symmetry axis. The mixing of the plasmonic and magnonic modes, which occurs for both intra- and intersubband transitions, depends on the coupling constant of the spin-spin interaction, the tilt angle θ, and the initial spin polarization ζ. (c) 1999 The American Physical Society

[en] The subband levels of quantum wells grown in a periodic array form minibands whose bandwidth Δ depends on the probability of interlayer tunneling. In the presence of a strong magnetic field, this system of minibands can exhibit various Coulomb-interaction-driven spin polarization instabilities at an integral value of the filling factor ν. We investigate in particular the Hartree-Fock phase diagram in the case in which the n=0 spin-up and n=1 spin-down Landau levels are separated by an energy smaller than Δ. A spin-density-wave ground state is shown to occur at filling factor ν=2. (c) 2000 The American Physical Society

[en] Neutron-diffraction measurements have been carried out to investigate the crystal structure, magnetic structures, and magnetic phase transitions in RNi₂B₂C(R= Y, Ce, Pr, Nd, Tb, Dy, Ho, Er, Tm, and Yb). The materials that order magnetically exhibit a wide variety of both commensurate and incommensurate magnetic structures, which argues strongly that the dominant exchange interactions are of the indirect Ruderman-Kittel-Kasuya-Yosida type. The Nd system exhibits a commensurate antiferromagnetic ordering at 4.8 K, with wave vector δ=(1/2,0,1/2) and moment direction along a (or equivalently with δ=(0,1/2,1/2) and moment direction along b in this tetragonal system). For Dy (T_N=10.6K), Pr (T_N=4.0K), and the low-temperature phase of Ho, the magnetic structure is also a commensurate antiferromagnet that consists of ferromagnetic sheets of rare-earth moments in the a-b plane, with the sheets coupled antiferromagnetically along the c axis [δ=(0,0,1)]. Pr is not superconducting, while for Dy (T_c=6K) and Ho (T_c=8K) this magnetic order coexists with superconductivity. For Ho, though, the magnetic state that initially forms at T_N∼8.5K is an incommensurate spiral antiferromagnetic state along the c axis in which the direction of these ferromagnetic sheets are rotated in the a-b plane by ∼17 degree from their low-temperature antiparallel configuration [δ=(0,0,0.91)]. The intensity for this spiral state reaches a maximum near the reentrant superconducting transition (∼5K); the spiral state then collapses at lower temperature in favor of the commensurate antiferromagnetic state. An incommensurate a-axis modulation, with δ=(0.55,0,0), is also observed above the spiral-antiferromagnetic transition, but it exists over a narrower temperature range than the spiral state, and also collapses near the reentrant superconducting transition. (Abstract Truncated)

[en] The nesting of the Fermi surfaces of an electron pocket and a hole pocket separated by a wave vector Q interacting via a repulsive potential gives rise to itinerant antiferromagnetism. The order can gradually be suppressed by mismatching the nesting and a quantum critical point is obtained as T_N→0. The renormalization group flow leads to a strong coupling fixed point. We study the linear response to spin- and charge-density waves, to singlet and triplet superconducting fluctuations, the low-T specific heat, and the magnetic susceptibility. All quantities increase on a logarithmic scale when the temperature is lowered. The results are discussed in the context of non-Fermi-liquid behavior observed in some heavy fermion compounds. (c) 2000 American Institute of Physics

[en] When the Ni concentration exceeds about 18%, Mn-Ni alloys were expected to support two different noncollinear spin-density wave (SDW) phases. A triple-Q SDW with moments along the crystal diagonals was believed to appear in the fcc phase between T_N and T_t. Below T_t, the fct phase with c>a was believed to contain a double-Q SDW with moments in the ab plane and at 45 degree sign angles from the crystal axes. Based on resistivity, neutron-scattering, and susceptibility measurements, we show that the structural and magnetic phase transitions in a Mn_1-xNi_x alloy with x≅0.20 are actually distinct, with the structural phase transition at T_t≅250 K lying far above the magnetic transition at T_m≅125 K. A Hamiltonian which includes elastic, magnetoelastic, and noncollinearity energies is used to describe these two transitions. In the tetragonal phase between T_t and T_m, our model predicts a new SDW phase with moments tilted away from the crystal diagonals toward the ab plane. The energy gap in the spin-wave spectrum is predicted to change discontinuously at T_m. (c) 2000 The American Physical Society