Electronic structure and Jahn-Teller effect in GaN:Mn and ZnS:Cr

We present an ab-initio and analytical study of the Jahn-Teller effect in two diluted magnetic semiconductors (DMS) with d4 impurities, namely Mn-doped GaN and Cr-doped ZnS. We show that only the combined treatment of Jahn-Teller distortion and strong electron correlation in the 3d shell may lead to the correct insulating electronic structure. Using the LSDA+U approach we obtain the Jahn-Teller energy gain in reasonable agreement with the available experimental data. The ab-initio results are completed by a more phenomenological ligand field theory.Comment: 15 pages, 5 figure

Electronic Structure of Copper Impurities in ZnO

We have measured the near infrared absorption, Zeeman effect, and electron spin resonance of Cu2+ ions introduced as a substitutional impurity into single-crystal ZnO. From the g values of the lowest Γ6 component of the T2 state (the ground state), gII=0.74 and g⊥=1.531, and from the g values of the Γ4Γ5 component of the E state, gII=1.63 and g⊥=0, we have determined the wave functions of Cu2+ in terms of an LCAO MO model in which overlap only with the first nearest neighbor oxygen ions is considered. These wave functions indicate that the copper 3d (t2) hole spends about 40% of its time in the oxygen orbitals, and that the copper t2 orbitals are expanded radially with respect to the e orbitals. Corroboration for the radial expansion of the t2 orbitals is obtained from an analysis of the hyperfine splitting. It is concluded from our model that the large values of the hyperfine constants, |A|=195×10^-4 cm^-1 and |B|=231×10^-4 cm^-1, are due to the contribution from the orbital motion of the t2 hole

The null energy condition and instability

We extend previous work showing that violation of the null energy condition implies instability in a broad class of models, including gauge theories with scalar and fermionic matter as well as any perfect fluid. Simple examples are given to illustrate these results. The role of causality in our results is discussed. Finally, we extend the fluid results to more general systems in thermal equilibrium. When applied to the dark energy, our results imply that w is unlikely to be less than -1.Comment: 11 pages, 5 figures, Revte

Hidden symmetry and magnetospectroscopy of quantum wells near filling factor v = 2

he magnetoluminescence spectra of symmetric quantum wells containing an electron gas show an abrupt changeover from Landau-level behavior (i.e., linear shift of energy with magnetic field) to quadratic (excitonlike) behavior as the field is increased. This so-called "Mott transition" occurs when the electron filling factor v(e) is 2, i.e., when the lowest Landau level is just filled. We show that the changeover is a natural consequence of the hidden symmetry that has been shown to hold in two-dimensional systems at high fields. This symmetry is broken when the integer parts of v(e)/2 and V-h//2 differ (v(h), the hole filling factor, is very small in these experiments), This symmetry breaking can also account for the new emission bands that are observed in such spectra when v(e) becomes larger than 2, and these bands can be described, at least qualitatively, in the framework of the magnetoexciton theory. These bands include the red-shifted magnetoplasmon and shake-up satellites and the blue-shifted cyclotron satellites. We also discuss the possible existence of a new type of trion (chargcd exciton), associated with the electron n(e) = 1 Landau level, and conclude that these trions should be observable.Physic

Crystal Distortion and the Two-Channel Kondo Effect

We study a simple model of the two-channel Kondo effect in a distorted crystal. This model is then used to investigate the interplay of the Kondo and Jahn-Teller effects, and also the Kondo effect in an impure crystal. We find that the Jahn-Teller interaction modifies the characteristic energy scale of the system below which non-Fermi-liquid properties of the model become apparent. The modified energy scale tends to zero as the limit of a purely static Jahn-Teller effect is approached. We find also that the non-Fermi-liquid properties of the quadrupolar Kondo effect are not stable against crystal distortion caused by impurities.Comment: 11 page

Cooperative Jahn-Teller Effect and Electron-Phonon Coupling in La1−xAxMnO3La_{1-x}A_xMnO_3

A classical model for the lattice distortions of \lax is derived and, in a mean field approximation, solved. The model is based on previous work by Kanamori and involves localized Mn d-electrons (which induce tetragonal distortions of the oxygen octahedra surrounding the Mn) and localized holes (which induce breathing distortions). Parameters are determined by fitting to the room temperature structure of $LaMnO_3$. The energy gained by formation of a local lattice distortion is found to be large, most likely $\approx 0.6$ eV per site, implying a strong electorn-phonon coupling and supporting polaronic models of transport in the doped materials. The structural transition is shown to be of the order-disorder type; the rapid x-dependence of the transition temperature is argued to occur because added holes produce a "random" field which misaligns the nearby sites.Comment: 24 pages. No figures. One Table. Late

Ultrahigh Bandwidth Spin Noise Spectroscopy: Detection of Large g-Factor Fluctuations in Highly n-Doped GaAs

We advance all optical spin noise spectroscopy (SNS) in semiconductors to detection bandwidths of several hundred gigahertz by employing an ingenious scheme of pulse trains from ultrafast laser oscillators as an optical probe. The ultrafast SNS technique avoids the need for optical pumping and enables nearly perturbation free measurements of extremely short spin dephasing times. We employ the technique to highly n-doped bulk GaAs where magnetic field dependent measurements show unexpected large g-factor fluctuations. Calculations suggest that such large g-factor fluctuations do not necessarily result from extrinsic sample variations but are intrinsically present in every doped semiconductor due to the stochastic nature of the dopant distribution.Comment: 5 pages, 3 figure

Zero Temperature Insulator-Metal Transition in Doped Manganites

We study the transition at T=0 from a ferromagnetic insulating to a ferromagnetic metallic phase in manganites as a function of hole doping using an effective low-energy model Hamiltonian proposed by us recently. The model incorporates the quantum nature of the dynamic Jahn-Teller(JT) phonons strongly coupled to orbitally degenerate electrons as well as strong Coulomb correlation effects and leads naturally to the coexistence of localized (JT polaronic) and band-like electronic states. We study the insulator-metal transition as a function of doping as well as of the correlation strength U and JT gain in energy E_{JT}, and find, for realistic values of parameters, a ground state phase diagram in agreement with experiments. We also discuss how several other features of manganites as well as differences in behaviour among manganites can be understood in terms of our model.Comment: To be published in Europhysics Letter

Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction

High real-space-resolution atomic pair distribution functions of La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using high-energy x-ray powder diffraction to study the size and shape of the MnO_6 octahedron as a function of temperature and doping. In the paramagnetic insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95 and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and T=20K, we find a single Mn-O bond-length; however, as the metal-insulator transition is approached either by increasing T or decreasing x, intensity progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong evidence that charge localized and delocalized phases coexist close to the metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.