Branches Of Excitations Research Articles

Optical conductivity and the sum rule in the DDW state

The density wave with d-wave order parameter (DDW) is possibly realized in the underdoped regime of high- T c cuprates. The DDW state is characterized by two branches of low-lying electronic excitations, and the quantum mechanical current has in particular an inter-branch contribution. The latter component causes a finite-frequency response in the optical conductivity and a reduction of the Drude contribution. We show that this redistribution of the spectral weight leaves the optical sum mostly intact, so that the restricted optical sum rule is only weakly violated.

Photoisomerization Mechanism of 11-cis-Locked Artificial Retinal Chromophores: Acceleration and Primary Photoproduct Assignment

CASPT2//CASSCF/6-31G photochemical reaction path computations for two 4-cis-nona-2,4,6,8-tetraeniminium cation derivatives, with the 4-cis double bond embedded in a seven- and eight-member ring, are carried out to model the reactivity of the corresponding ring-locked retinal chromophores. The comparison of the excited state branches of the two reaction paths with that of the native chromophore, is used to unveil the factors responsible for the remarkably short (60 fs) excited state (S(1)) lifetime observed when an artificial rhodopsin containing an eight member ring-locked retinal is photoexcited. Indeed, it is shown that the strain imposed by the eight-member ring on the chromophore backbone leads to a dramatic change in the shape of the S(1) energy surface. Our models are also used to investigate the nature of the primary photoproducts observed in different artificial rhodopsins. It is seen that only the eight member ring-locked retinal model can access a shallow energy minimum on the ground state. This result implies that the primary, photorhodopsin-like, transient observed in artificial rhodopsins could correspond to a shallow excited state minimum. Similarly, the second, bathorhodopsin-like, transient species could be assigned to a ground state structure displaying a nearly all-trans conformation.

Magnetic Excitation in an Anisotropic Bond-AlternatingS= 1 One Dimensional Antiferromagnet Ni(C9D24N4)(NO2)ClO4

We report the results of inelastic neutron scattering (INS) experiments on the S = 1 bond-alternating quasi-one-dimensional (1D) antiferromagnet Ni(C 9 D 24 N 4 )(NO 2 )ClO 4 under magnetic' fields below and above the critical field H c at which the energy gap closes. Normal field dependence of Zeeman split of the excited triplet state below H c has been observed, but the highest mode is unusually small and smears out with increasing field. This is probably explained by the highest branch buried in a magnon continuum, which was never observed in the S = 1 antiferromagnetic uniform chain. Above H c , we observed only one excitation branch, which is completely different from the Haldane material NDMAP [A. Zheludev et al., Phys. Rev. B 68 (2003), 134438] where three gapped branches were clearly observed.

High Field Properties of Geometrically Frustrated Magnets

Above the saturation field, geometrically frustrated quantum antiferromagnets have dispersionless low-energy branches of excitations corresponding to localized spin-flip modes. Transition into a partially magnetized state occurs via condensation of an infinite number of degrees of freedom. The ground state below the phase transition is a magnon crystal, which breaks only translational symmetry and preserves spin-rotations about the field direction. We give a detailed review of recent works on physics of such phase transitions and present further theoretical developments. Specifically, the low-energy degrees of freedom of a spin-1/2 kagom\'e antiferromagnet are mapped to a hard hexagon gas on a triangular lattice. Such a mapping allows to obtain a quantitative description of the magnetothermodynamics of a quantum kagom\'e antiferromagnet from the exact solution for a hard hexagon gas. In particular, we find the exact critical behavior at the transition into a magnon crystal state, the universal value of the entropy at the saturation field, and the position of peaks in temperature- and field-dependence of the specific heat. Analogous mapping is presented for the sawtooth chain, which is mapped onto a model of classical hard dimers on a chain. The finite macroscopic entropies of geometrically frustrated magnets at the saturation field lead to a large magnetocaloric effect.

Hall conductivity in unconventional charge-density-wave systems

Charge density waves with unconventional order parameters, for instance, with $d$-wave symmetry (DDW), may be relevant in the underdoped regime of high-${T}_{c}$ cuprates or other quasione- or two-dimensional metals. A DDW state is characterized by two branches of low-lying electronic excitations. The resulting quantum mechanical current has an interbranch component which leads to an additional mass term in the expression for the Hall conductivity. This extra mass term is parametrically enhanced near the ``hot spots'' of fermionic dispersion and is non-neglegible as is shown by numerical calculations of the Hall number in the DDW state.

Role of correlation and exchange for quasiparticle spectra of magnetic and diluted magnetic semiconductors

Theoretical foundation and applications of the generalized spin-fermion (sp–d) exchange lattice model to various magnetic systems, e.g., rare-earth metals and compounds, and magnetic semiconductors are discussed. The capabilities of the model to describe spin quasiparticle spectra are investigated. The main emphasis is put on the dynamic behavior of two interacting subsystems, the localized spins and spin density of itinerant carriers. A nonperturbative many-body approach is used to describe the quasiparticle dynamics. Scattering states are investigated and three branches of magnetic excitations are calculated in the regime characteristic of a magnetic semiconductor. For a simplified version of the model (Kondo lattice model) we study the spectra of quasiparticle excitations with special attention given to diluted magnetic semiconductors in a simple approximation to demonstrate the role of disorder effects. For this, to include the effects of disorder, a modified mean field is determined self-consistently. The approach permits to investigate and clarify the role of various interactions and disorder effects in unified and coherent fashion.

Rotational oscillations of a molecular chain with quadrupolar interaction

Molecular chains with rotational degree of freedom in the plane of an adsorbing surface and quadrupolar interaction between linear molecules are investigated theoretically. It is found that alternation ordering of the molecules is preferable. Equations of rotational movement are derived and solved for linear oscillations. It is shown that dispersion relation has two optic branches of rotational excitations. The normal coordinates are found to be strongly dependent on dispersion. In the long wave limit normal coordinates are symmetric and antisymmetric. The heat capacity of the molecular chain is found at low temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An investigation of the influence of the pairing correlations on the properties of the isobar analog resonances inA = 208 nuclei

Within the quasi-particle random phase approximation (QRPA), the method of the self-consistent determination of the isovector effective interaction which restores a broken isotopic symmetry for the nuclear part of the Hamiltonian is given. The effect of the pairing correlations between nucleons on the following quantities were investigated for theA = 208 nuclei: energies of the isobar analog 0+ states, the isospin admixtures in the ground state of the even-even nuclei, and the differential cross-section for the208Pb(3He,t)208Bi reaction atE(3He)=450 MeV. Both couplings of the excitation branches withT z = T0 ± 1, and the analog state with isovector monopole resonance (IVMR) in the quasi-particle representation were taken into account in our calculations. As a result of these calculations, it was seen that the pairing correlations between nucleons have no considerable effect on theT = 23 isospin admixture in the ground state of the208Pb nucleus, and they cause partially an increase in the isospin impurity of the isobar analog resonance (IAR). It was also established that these correlations have changed the isospin structure of the IAR states, and shifted the energies of the IVMR states to the higher values.

New level schemes with high-spin states ofTc105,107,109

New level schemes of odd-$Z$ $^{105,107,109}\mathrm{Tc}$ are proposed based on the $^{252}\mathrm{Cf}$ spontaneous-fission-gamma data taken with Gammasphere in 2000. Bands of levels are considerably extended and expanded to show rich spectroscopic information. Spin/parity and configuration assignments are made based on determinations of multipolarities of low-lying transitions and the level analogies to the previously reported levels, and to those of the neighboring $\mathrm{Rh}$ isotopes. A non-yrast negative-parity band built on the $3∕{2}^{\ensuremath{-}}[301]$ orbital is observed for the first time in $^{105}\mathrm{Tc}$. A positive-parity band built on the $1∕{2}^{+}[431]$ intruder orbital originating from the $\ensuremath{\pi}({g}_{7∕2}∕{d}_{5∕2})$ subshells and having a strong deformation-driving effect is observed for the first time in $^{105}\mathrm{Tc}$, and assigned in $^{107}\mathrm{Tc}$. A positive-parity band built on the excited $11∕{2}^{+}$ level, which has rather low excitation energy and predominantly decays into the $9∕{2}^{+}$ level of the ground state band, provides evidence of triaxiality in $^{107,109}\mathrm{Tc}$, and probably also in $^{105}\mathrm{Tc}$. Rotational constants are calculated and discussed for the $K=1∕2$ intruder bands using the Bohr-Mottelson formula. Level systematics are discussed in terms of the locations of proton Fermi levels and deformations. The band crossings of yrast positive-parity bands are observed, most likely related to ${h}_{11∕2}$ neutron alignment. Triaxial-rotor-plus-particle model calculations performed with $\ensuremath{\epsilon}=0.32$ and $\ensuremath{\gamma}=\ensuremath{-}22.5\ifmmode^\circ\else\textdegree\fi{}$ on the prolate side of maximum triaxiality yielded the best reproduction of the excitation energies, signature splittings, and branching ratios of the positive-parity bands (except for the intruder bands) of these $\mathrm{Tc}$ isotopes. The significant discrepancies between the triaxial-rotor-plus-particle model calculations and experiment for the $K=1∕2$ intruder bands in $^{105,107}\mathrm{Tc}$ need further theoretical studies.

SELECTION OF THE GROUND STATE FOR NONLINEAR SCHRÖDINGER EQUATIONS

We prove for a class of nonlinear Schrödinger systems (NLS) having two nonlinear bound states that the (generic) large time behavior is characterized by decay of the excited state, asymptotic approach to the nonlinear ground state and dispersive radiation. Our analysis elucidates the mechanism through which initial conditions which are very near the excited state branch evolve into a (nonlinear) ground state, a phenomenon known as ground state selection. Key steps in the analysis are the introduction of a particular linearization and the derivation of a normal form which reflects the dynamics on all time scales and yields, in particular, nonlinear master equations. Then, a novel multiple time scale dynamic stability theory is developed. Consequently, we give a detailed description of the asymptotic behavior of the two bound state NLS for all small initial data. The methods are general and can be extended to treat NLS with more than two bound states and more general nonlinearities including those of Hartree–Fock type.

Many-body dispersions in interacting ballistic quantum wires

We have measured the collective excitation spectrum of interacting electrons in one-dimension. The experiment consists of controlling the energy and momentum of electrons tunneling between two clean and closely situated, parallel quantum wires in a GaAs/AlGaAs heterostructure while measuring the resulting conductance. We measure excitation spectra that clearly deviate from the non-interacting spectrum, attesting to the importance of Coulomb interactions. Notable is an observed 30% enhancement of the velocity of the main excitation branch relative to non-interacting electrons with the same density. In short wires, finite size effects resulting from broken translational invariance are observed. Spin–charge separation is manifested through moiré patterns reflecting different spin and charge excitation velocities.

Quantum Numbers for Excitations of Bose–Einstein Condensates in 1D Optical Lattices

The excitation spectrum and the band structure of a Bose-Einstein condensate in a periodic potential are investigated. Analyses within full 3D systems, finite 1D systems, and ideal periodic 1D systems are compared. We find two branches of excitations in the spectra of the finite 1D model. The band structures for the first and (part of) the second band are compared between a finite 1D and the fully periodic 1D systems, utilizing a new definition of a effective wavenumber and a phase-slip number. The upper and lower edges of the first gap coincide well between the two cases. The remaining difference is explained by the existence of the two branches due to the finite-size effect.

Chasing ghosts in reciprocal space—a novel inelastic neutron multiple scattering process

We have discovered that a recently reported weak excitation branch in the spin-Peierls material CuGeO 3 is in fact a ghost image of the primary magnetic excitation shifted in reciprocal space by a novel multiple scattering process. A model is developed that predicts the occurrence of such multiple scattering and accounts for the observations in CuGeO 3. New ‘ghostons’ can occur when the magnetic unit cell is smaller than the structural, while mixing of intensities from different reciprocal space zones jeopardize accurate polarisation analysis and the study of weak modes in general.

Self-consistent mean-field theory for spin 1 and spin [formula omitted] ferrimagnetic chain

We use the self-consistent mean-field theory method to study the ground states of the quantum ferrimagnetic chain. This one-dimensional chain can be described by the Hamiltonian: H=∑ i [(S i·s j) λ+(s i·S j) λ] , where ( S i · s j ) λ = λ( S i x s j x + S i y s j y )+ S i z s j z . At the Heisenberg point ( λ=1), we observe two branches of the low lying excitation. We calculate the gap between the two excitation branches, the spin reduction and the spin fluctuation at T=0 K . We also give the correlation length at T=0 K . These results agree with the established numerical results quite well. We also calculate the ground-state energy and the excitation gap varying with the Ising anisotropy λ. It agrees quite well with quantum Monte Carlo approaches and fourth-order perturbation approaches.

Ordering and excitations in the field-induced magnetic phase of Cs3Cr2Br9.

Field-induced magnetic order has been investigated in detail in the interacting spin 3/2 dimer system Cs3Cr2Br9. Elastic and inelastic neutron scattering measurements were performed up to H=6 T, well above the critical field H(c1) approximately 1.5 T. The ordering displays incommensurabilities and a large hysteresis before a commensurate structure is reached. This structure is fully determined. Surprisingly, the lowest excitation branch never closes. Above H(c1), the gap increases slowly with the field. An analysis in terms of projected pseudospins is given.

Spin order and orbital disorder in LaTiO 3

A mechanism for the removal of orbital degeneracy in LaTiO 3 is proposed. Assuming an antiferromagnetic spin ordering (presumably induced by some deviations from the fully symmetric Kugel–Khomskii (KK) model Hamiltonian), we show that the two-magnon excitations play a crucial role in the formation of the orbital liquid. The strongly damped orbital excitations (orbitons) merge into a common continuum with the spin excitations. As a result, the orbital dynamics do modify the stiffness of the spin waves but do not participate as a separate branch of excitations in the low-temperature thermodynamics. Therefore, the direct experimental observation of the orbital dynamics at low temperatures is hardly possible.

Spontaneous rotational symmetry breaking and roton like excitations in gauged σ-model at finite density

The linear σ-model with a chemical potential for hypercharge is a toy model for the description of the dynamics of the kaon condensate in high density QCD. We analyze the dynamics of the gauged version of this model. It is shown that spontaneous breakdown of SU(2)×U(1)Y symmetry, caused by the chemical potential, is always accompanied by spontaneous breakdown of both rotational symmetry and electromagnetic U(1)em. The spectrum of excitations in this model is rich and, because of rotational symmetry breakdown, anisotropic. It is shown that there exist excitation branches that behave as phonon like quasiparticles for small momenta and as roton like ones for large momenta. This suggests that this model can be relevant for anisotropic superfluid systems.

Imaging the pair-correlated excitation function: The F+CH4-->HF(v')+CH3(nu=0) reaction.

The velocity map ion imaging technique was applied to measure the reaction excitation function for the first time. It was found that the "raw" excitation function was significantly distorted by the density-to-flux transformation of the title reaction. Through a systematic investigation, possible reasons for such a dramatic effect are outlined. In addition, the state-resolved, pair-correlated excitation functions and branching ratios are presented. Effects of imperfect time slicing in the time-sliced velocity imaging technique in general are also discussed.

Kelvin modes of a fast rotating Bose-Einstein condensate

Using the concept of diffused vorticity and the formalism of rotational hydrodynamics we calculate the eigenmodes of a harmonically trapped Bose-Einstein condensate containing an array of quantized vortices. We predict the occurrence of a new branch of anomalous excitations, analogous to the Kelvin modes of the single vortex dynamics. Special attention is devoted to the excitation of the anomalous scissors mode.

Massive triplet excitations in a magnetized anisotropic Haldane spin chain

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet $\mathrm{Ni}({\mathrm{C}}_{5}{\mathrm{D}}_{14}{\mathrm{N}}_{2}{)}_{2}{\mathrm{N}}_{3}({\mathrm{PF}}_{6})$ are performed at mK temperatures in magnetic fields of almost twice the critical field ${H}_{c}$ applied perpendicular to the spin chains. Above ${H}_{c}$ a reopening of the spin gap is clearly observed. In the high-field N\'eel-ordered state the spectrum is dominated by three distinct excitation branches. A theoretical model consistently describing the experimental data is proposed.