Temperature renormalization of the magnetic excitations in S = 1/2 KCuCl3

A complete temperature characterization of the spin dynamics in the unconventional S =1/2 antiferromagnet KCuCl₃ is presented from single crystal inelastic neutron scattering studies. KCuCl₃ features a quantum disordered singlet ground state with a finite spin gap to triplet excitations of dimer origin. Three dimensional magnetic correlations support the dispersive propagation of the excitations in the whole reciprocal space. Upon increasing the temperature, a renormalization in the energy, in the intensity and in the damping rate of the triplet modes is reported. The experimental observations can be described within the framework of a selfconsistent dimer RPA theory, with no free parameters. The driving mechanism behind the model is the thermally activated decrease of the occupation difference n ₀ - n ₁ between singlet and triplet dimer states. This is the expression of kinematic constraints which are of minor importance for classical magnons in Néel ordered antiferromagnets. Implications for the temperature dependence of macroscopic quantities are discussed.     

Screened self-energy correction to the 2<Emphasis Type="Italic">p</Emphasis><Subscript>3/2</Subscript>-2<Emphasis Type="Italic">s</Emphasis> transition energy in Li-like ions

We present an ab initio calculation of the screened self-energy correction for 1s² 2p_3/2 and 1s² 2s states of Li-like ions with nuclear charge numbers in the range Z = 12?100. The evaluation is carried out to all orders in the nuclear strength parameter Zα. This investigation concludes our calculations of all two-electron QED corrections for the 2p_3/2-2s transition energy in Li-like ions and thus considerably improves theoretical predictions for this transition for high-Z ions.     

Two-electron self-energy contribution to the ground-state energy of helium-like ions

The two-electron self-energy contribution to the ground-state energy of helium-like ions is calculated both for a point nucleus and an extended nucleus in a wide interval of Z. All the two-electron contributions are compiled to obtain most accurate values for the two-electron part of the ground-state energy of helium-like ions in the range Z = 20–100. The theoretical value of the ground-state energy of ²³⁸U⁹⁰⁺, based on currently available theory, is evaluated to be −261382.9(8) eV, without higher-order one-electron QED corrections.     

Polarization 1/<Emphasis Type="Italic">f</Emphasis> noise in ferroelectric PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> with a diffused phase transition

The dispersion of complex permittivity in ferroelectric PbFe_1/2Nb_1/2O₃ with diffused phase transition at frequencies of 20 Hz-10 kHz was studied. Dielectric measurements revealed that polarization 1/f noise occurs in the region of coexistence of polar and nonpolar phases. It was found that the temperature dependence of parameter α exhibits a singularity when the temperature of the local order parameter rises. Our analysis of the experimental data enables us to say that the source of the noise is the fluctuations of spontaneous polarization.     

Application of the dynamic renormalization group to non-equilibrium instabilities: Thek=0 hard mode transition

The behaviour of correlation functions of a complex fieldW close to thek=0 hard mode transition of a non-equilibrium system are investigated using the dynamic renormalization group. The Langevin equation forW is closely related to then=2 time-dependent Ginzburg-Landau equation, but in this case one finds two relevant scaling fields. This leads to a behavior which is more closely related to that expected at tricritical points.     

Intensity of cross-peaks in hyscore spectra of S = 1/2, I = 1/2 spin systems

The cross-peak intensity for a S = 1/2, I = 1/2 spin system in two-dimensional HYSCORE spectra of single-crystals and powders is analyzed. There is a fundamental difference between these two cases. For single crystals, the cross-peak intensity is distributed between the two (+, +) and (+, -) quadrants of the hyperfine sublevel correlation (HYSCORE) spectrum by the ratio c(2):s(2) (C. Gemperle, G. Aebli, A. Schweiger, and R. R. Ernst, J. Magn. Reson. 88, 241 (1990)). However, for powder spectra another factor becomes dominant and governs cross-peak intensities in the two quadrants. This factor is the phase interference between modulation from different orientations of the paramagnetic species. This can lead to essentially complete disappearance of the cross-peak in one of the two (+, +) or (+, -) quadrants. In the (+, +) quadrant, cross-peaks oriented parallel to the main (positive) diagonal of the HYSCORE spectrum are suppressed, while the opposite is true in the (+, -) quadrant where cross-peaks nearly perpendicular to the main (negative) diagonal of HYSCORE spectra are suppressed. Analytical expressions are derived for the cross-peak intensity profiles in powder HYSCORE spectra for both axial and nonaxial hyperfine interactions (HFI). The intensity is a product of two terms, one depending only on experimental parameter (tau) and the other only on the spin Hamiltonian. This separation provides a rapid way to choose tau for maximum cross-peak intensity in a region of interest in the spectrum. For axial HFI, the Hamiltonian-dependent term has only one maximum and decreases to zero at the canonical orientations. For nonaxial HFI, this term produces three separate ridges which outline the whole powder lineshape. These three ridges have the majority of the intensity in the HYSCORE spectrum. The intensity profile of each ridge resembles that observed for axial HFI. Each ridge defines two principal values of the HFI similar to the ridges from an axial HFI.     

Evidence of Landau levels and interactions in low-lying excitations of composite fermions at 1/3<or= nu <or=2/5

Excitation modes in the range 2/5>or=nu>or=1/3 of the fractional quantum Hall regime are observed by resonant inelastic light scattering. Spectra of spin-reversed excitations suggest a structure of lowest spin-split Landau levels of composite fermions that is similar to that of electrons. Spin-flip energies determined from spectra reveal significant composite fermion interactions. The filling factor dependence of mode energies displays an abrupt change in the middle of the range when there is partial population of a composite fermion level.     

Migdal-Kadanoff renormalization group approach to the spin-1/2 anisotropic Heisenberg model

The spin-1/2 anisotropic Heisenberg model is studied by generalizing the Migdal-Kadanoff renormalization transformations to quantum spin systems. An approximate one-dimensional decimation is employed besides the potential-moving approximation in this generalization. It is shown that these approximations are valid at high temperatures. The results obtained from these approximations suggest that the two-dimensional spin-1/2X-Y model shows the critical behavior similar to that expected for the classicalX-Y and planar models.