Elementary spin excitations in ultrathin itinerant magnets

Elementary spin excitations (magnons) play a fundamental role in condensed matter physics, since many phenomena e.g. magnetic ordering, electrical (as well as heat) transport properties, ultrafast magnetization processes, and most importantly electron/spin dynamics can only be understood when these quasi-particles are taken into consideration. In addition to their fundamental importance, magnons may also be used for information processing in modern spintronics.     

Bose-Einstein condensation of dilute magnons in TlCuCl3

The recent observation [A. Oosawa et al., J. Phys. Condens. Matter 11, 265 (1999)] of the field-induced Neel ordering in the spin-gap magnetic compound TlCuCl3 is interpreted as a Bose-Einstein condensation of magnons. A Hartree-Fock-type calculation based on this picture is shown to describe the temperature dependence of the magnetization well.     

Symmetry of elementary excitations in incommensurate crystals

A generalized notion of symmetry in quantum mechanics is used for the classification of elementary excitations in incommensurate crystal phases.     

Magnetoelastic coupling in the spin-dimer system TlCuCl3

Based on high-resolution measurements of the thermal expansion and the magnetostriction we analyze the magnetoelastic coupling of TlCuCl3. We find that the phase boundary between the low-field spin liquid phase and the magnetic-field-induced Néel phase is extremely sensitive to pressure and would change by about +/-185%/GPa depending on the direction of uniaxial pressure. This drastic effect can unambiguously be traced back to the pressure dependencies of the intradimer coupling, whereas changes of the interdimer couplings under pressure play a minor role. The observed strong decrease of the interdimer coupling, when Tl is substituted by K, is therefore not a consequence of chemical pressure.     

Localized spin excitations in magnets with triangular spin ordering

Some types of localized spin excitations in hexagonal antiferromagnets with triangular spin ordering are investigated. The character of relaxation of these spin excitations is studied.     

Entanglement and elementary excitations in quantum spin chain

The entanglement induced by elementary excitations in the XX spin chain is investigated by Bethe ansatz method. The reduced density matrix and correlation function between any pair of spins can be obtained for ground and all excited states with N qubits. Rely on them we show the explicit and general relations between entanglement and elementary excitations in XX spin chain. We further show our method can be extend to other integrable models.     

Effective interactions and elementary excitations in nuclear matter

We present a polarization potential theory for nuclear matter. Starting from a realistic two-nucleon interaction, position space Pseudopotentials which describe effective nucleon-nucleon interactions are constructed in a manner similar to that used by Aldrich and Pines in the helium liquids. Important modifications which result from tensor forces and three-body interactions are incorporated; exchange and screening effects are included. The resulting momentum-dependent quasiparticle interactions are used to calculate higher-order Fermi liquid parameters, dynamic and static structure functions, and static polarizabilities. Comparisons are made with recent microscopic calculations.     

Photoinduced magnetism caused by charge-transfer excitations in tetracyanoethylene-based organic magnets

The photoinduced magnetism in Mn-tetracyanoethylene (TCNE) molecule-based magnets is ascribed to charge-transfer excitations from manganese to TCNE. Charge-transfer energies are calculated using density functional theory; photoinduced magnetization is described using a model Hamiltonian based on a double-exchange mechanism. Photoexciting electrons from the manganese core spins into the lowest unoccupied orbital of TCNE with photon energies around 3 eV increase the magnetization through a reduction of the canting angle of the manganese core spins for an average electron density on TCNE less than one. When photoexciting with a smaller energy, divalent TCNE molecules are formed. The delocalization of the excited electron causes a local spin flip of a manganese core spin.     

Pressure-induced quantum phase transition in the spin-liquid TlCuCl3

The condensation of magnetic quasiparticles into the nonmagnetic ground state has been used to explain novel magnetic ordering phenomena observed in quantum spin systems. We present neutron scattering results across the pressure-induced quantum phase transition and for the novel ordered phase of the magnetic insulator TlCuCl3, which are consistent with the theoretically predicted two degenerate gapless Goldstone modes, similar to the low-energy spin excitations in the field-induced case. These novel experimental findings complete the field-induced Bose-Einstein condensate picture and support the recently proposed field-pressure phase diagram common for quantum spin systems with an energy gap of singlet-triplet nature.     

Energy excitations in three-axial magnets with single-ion and exchange anisotropies

The energy spectra of three-axial ferromagnets like U₃P₄ and ferrimagnets like U₃Sb₄ are presented and their properties are discussed. In case of U₃Sb₄ we find an interesting example of a magnetic structure which, due to the single-ion anisotropy, is unstable in the spin wave approximation. The k² dependence of magnon energies requires re-examination of the temperature dependence of the magnetisation and of the specific heat.     

Renormalized dispersion of elementary excitations in spinor polariton condensates

We analyse the polarization of spinor polariton condensates and corresponding dispersions of elementary excitations. We have considered the effects of magnetic field induced splitting in circular polarizations and residual splitting in linear polarizations in the ground state provided by the cavity asymmetry. We show that anisotropic polariton–polariton interactions fully compensate the Zeeman splitting in circular polarizations below the critical magnetic field, thus leading to the spin-Meissner effect for the polariton condensates. We also analyzed the effect of polariton–polariton interactions on the stability of the gap in linear polarizations characteristic for anisotropic microcavities. It was shown that in realistic systems this gap increases with concentration of the particles, thus contributing to the stability of the pinning of linear polarization of photoemission in semiconductor microcavities for pump intensities above the stimulation threshold.     

Coherence of elementary excitations in a disordered electron system

The localization properties of the single-particle and collective electron excitations were investigated in the intentionally disordered GaAs/AlGaAs superlattices by weak-field magnetoresistance and Raman scattering. The localization length of the individual electron was found to be considerably larger than that of the collective excitations. This suggests that the disorder has a weaker effect on the electrons than on their collective motion and that the interaction which gives rise to the collective effects increases localization.