Surface excitations in magnetic systems with a singlet ground state

We have investigated surface excitations in a system wherein the ionic ground state is a magnetic singlet. The pseudospin formalism is employed to account for the crystal-field and exchange interactions between the ions in a Heisenberg ferromagnet with the singlet-triplet crystal-field-only level scheme. The Hamiltonian was studied in the molecular field approximation to find the possible ground states. Surface excitations for the simple cubic structure were investigated for the (001) surface in the random phase approximation. Analytic expressions have been obtained for the thermodynamic Green functions. For a fully magnetized molecular field ground state, there are in general two bulk bands, the spin-wave and the excitonic. Surface modes were found to exist below the spin-wave band, above the excitonic band and between the bands. The dispersion curves can exist only over one or two limited regions of the two-dimensional wave-vector parallel to the crystal surface.     

Crystal field effects in the resistivity of the singlet ground state PrIn3 compound

Results of the experimentally found temperature dependence of the magnetic resistivity for PrIn₃ are compared with theoretical data for two possible scattering mechanisms. A good quantitative agreement between theory and experiment is observed at low temperature region in the case of the pure crystal field effected exchange scattering.     

Zeeman effect on magnetic susceptibility in singlet ground state PrNi5

The Zeeman effect on the crystal electric field (CEF) magnetic susceptibility in 4f-electron singlet ground state (SGS) systems is studied analytically in the framework of the molecular-field model and experimentally by measuring the magnetic susceptibility of SGS compound PrNi₅. The most striking result of the analysis is the shift of the zero-field magnetic susceptibility maximum to lower temperatures in magnetic field. This thoroughly CEF effect is a direct consequence of the convergence of the SGS and the low lying Zeeman branch of the first excited state in an applied magnetic field. The observed shift of the susceptibility maximum from 14 K to 9 K in polycrystalline PrNi₅ by applying a magnetic field of 9T confirms the foregone conclusion.     

Magnetism in singlet ground states

The spin‐gap phenomenon is a key characteristic of high T_c superconducting cuprates, whose experimental results are summarized and discussed in the context of the slave‐boson mean field theory of the extended t–J model. It is seen that there exists satisfactory agreement between theory and experiments in bilayer cuprates. As regards the apparent absence of the spin‐gap in single layer La_2-xSr_xCuO₄ (LSCO), the possible roles played by disorder have been pointed out in view of the recent neutron scattering experiment on other spin singlet systems, the spin‐Peierls CuGeO₃ and two‐leg ladders. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the theory of superconductors with magnetic impurities in a singlet ground state

We attempt a theoretical axplanation of recent measurements of Bucher et al. on superconducting systems containing rare earth impurities in a single ground state and propose an additional experiment to demonstrate the importance of crystalline fields.     

From ferromagnetic to non-magnetic singlet ground state in CePd1-x Ni x

Magnetic, resistivity and specific heat measurements were performed on the CePd_1-x Ni _x system, which ranges between a ferromagnetic (CePd) and non-magnetic (CeNi) singlet ground state. The Curie temperature (T _c ) grows with Ni concentration, reaching a maximum value ofT _c =(10.5±0.1) K for 0.5x0.8. Both, the concentration dependence ofT _c and the Curie-Weiss temperature ( _P ) follow the predicted competition between the magnetic exchange interactions and the Kondo effect. In the crossover region (x0.9) the entropy gain shows that only the doublet crystal field ground state is involved in the singlet ground state formation.Partially supported by the Direction de la Coopération Scientifique et Technique du Ministère des Relations Extérieures of France     

Temperature and magnetic field dependent correlations in the singlet ground state system CsFeBr3

The magnetic excitations in CsFeBr₃ have been measured with inelastic scattering of cold neutrons to high precision at 80 mK. The fact that the lowest frequency mode softens with decreasing temperature but stabilizes at 0.11 THz below 2.5 K is the indication that CsFeBr₃ remains a SGS system forT0.From dispersion curves measured earlier in a magnetic field along the chain axis experimental intensities were derived and in turn correlation lengths. Correlation lengths were also calculated using the new parameters for exchange and anisotropy. The experimental results and the calculations both show that the correlation lengths increase for increasing magnetic field, flatten off around the phase transformation at 4.1 Tesla with a maximum of about 35 Å and then decrease again for higher fields. No critical phenomena connected with this phase transformation could be detected.The Mirror Mode published earlier was found to be of spurious origin due to double scattering. It does not exist.     

Unexpected magnetic behaviour in a singlet crystal field ground state induced by Tb 4f-instability

The magnetic susceptibility () of Th_1-x Tb _x alloys (with 5<x<55a/o Tb) in the range of 1 K<T<350K and the specific heat (forx=1.5a/o Tb) for 0.5<T<10 K were measured. A maximum in (T) was observed at a temperature which depends on concentration. The crystal field level scheme that fits the specific heat measurements shows two non-magnetic singlets as lowest levels. Such ambiguity is discussed within the framework of the 4f-Tb instability. The results are in agreement with the matrix superconductiveT _c decrease. The contribution of the interconfigurational mixing on and the specific heat is also discussed.This work was partially supported by the KfA-CNEA cooperation program and by the Deutsche Farschungsgemeinschaft through SFB125     

Collective excitations in the 1-D-ferromagnet CsFeCl3 with singlet ground state

By means of inalastic neutron scattering we have determined the dispersion relation of the magnetic excitations in CsFeCl₃ at different temperatures.The dispersion in c-direction, along the Fe-chains is typically ferromagnetic and in the hexagonal plane antiferromagnetic. Due to the lack of an applicable theory the data were parametrized by the simple heuristic formula:?ω = [2J[1 - cos πq_c] [A + 2J(1 - cos π)q_c)] + [C + J' (1.5 + γ(q))]²]^{$\text{1}\text{2}$}The gap was found to be C = 0.148 THz, the easy plane anisotropy A = 0.308 THz, the ferromagnetic interaction J = 0.148 THz and the antiferromagnetic interaction J' = -0.04 THz. At 1.25 K all excitations had a width smaller than the instrumental resolution ΔE = 0.025 THz. These results can be interpreted as follows: CsFeCl₃ is a singlet ground state system with strong ferromagnetic interaction J along the crystallographic c-axis and weak antiferromagnetic interaction J' in the plane perpendicular to c.In addition we have measured the influence of a magnetic field along the hexagonal c-axis. The splitting found agrees with the assumed level scheme yielding g = 2.5 for the first excited level.     

Theory of ground state factorization in quantum cooperative systems

We introduce a general analytic approach to the study of factorization points and factorized ground states in quantum cooperative systems. The method allows us to determine rigorously the existence, location, and exact form of separable ground states in a large variety of, generally nonexactly solvable, spin models belonging to different universality classes. The theory applies to translationally invariant systems, irrespective of spatial dimensionality, and for spin-spin interactions of arbitrary range.     

Accurate ground state wavefunctions for several three-body systems

The angularly correlated basis functions proposed by Gasaneo and Ancarani (Phys Rev, A 77:012705, 2008) are used to construct approximated wavefunctions, satisfying all two-body cusp conditions, for several three-body systems. The focus here is on the study of the following negatively charged hydrogen-like ions: ^?∞?H^???, ¹H^???, D^???, T^??? and Mu^???, the negative positronium ion Ps^???, and the exotic systems $e^{-}e^{-}(nm_{e})^{+}$ in which one of the particles is heavier than the other two. Accurate results for the mean of the ground state energy and of other radial quantities are compared with those given in the literature, when available.     

Spin-incoherent behavior in the ground state of strongly correlated systems

It is commonly believed that strongly interacting one-dimensional Fermi systems with gapless excitations are effectively described by Luttinger liquid theory. However, when the temperature of the system is high compared to the spin energy, but small compared to the charge energy, the system becomes "spin incoherent." We present numerical evidence showing that the one-dimensional "t-J-Kondo" lattice, consisting of a t-J chain interacting with localized spins, displays all the characteristic signatures of spin-incoherent physics, but in the ground state. We argue that similar physics may be present in a wide range of strongly interacting systems.     

The dynamical effects in the ground state of nuclei

Zeitschrift für Physik A Hadrons and nuclei - A method of including the dynamical effects of the multipole vibrations is proposed. The influence of the collective ground state wave function of...     

Branching ratio of para- and ortho- helium in the electron impact excitation from the ground singlet state

The ratios of differential and total cross sections for electron-impact excitation of singlet- and triplet- states helium atoms from its ground state are studied both in the threshold and in the high energy regions.     

Uniqueness of the translationally invariant ground state in quantum spin systems

We introduce a class of quantum spin systems on ^d. We show that the translationally invariant ground state is unique for this system if it is in a strong external field.Supported by the Science and Engineering Research Council     

Nonadiabatic effects in the ground state of the hydrogen molecule

A sum-over-states procedure is used in the ab initio evaluation of the matrix elements needed for a nonadiabatic treatment of the ground electronic state of H₂. Bunker's (J. Mol. Spectrosc.42, 478 (1972)) semiempirical value for the nonadiabatic parameter l₁ = 0.130 ± 0.006 is in agreement with the present calculation, which therefore supports the interpretation of the difference between theoretical adiabatic and experimental vibrational energies as being due to nonadiabatic effects. The same techniques are used to evaluate the expectation value of $\text{?}{}^2\text{?R}{}^2$ in the H₂ ground electronic state. Finally, it is pointed out that an average energy denominator used in several recent treatments of nonadiabatic effects in H₂ is in error by roughly a factor of two. The correct average energy denominator reflects that the sums have large contributions from states in the electronic continuum.