Kondo tunneling through real and artificial molecules

When a cerocene molecule is chemisorbed on metallic substrate, or when an asymmetric double dot is hybridized with itinerant electrons, its singlet ground state crosses its lowly excited triplet state, leading to a competition between the Zhang-Rice mechanism of singlet-triplet splitting in a confined cluster and the Kondo effect (which accompanies the tunneling through quantum dot under a Coulomb blockade restriction). The rich physics of an underscreened S=1 Kondo impurity in the presence of low-lying triplet/singlet excitations is exposed. Estimates of the magnetic susceptibility and the electric conductance are presented.Comment: 4 two-column revtex pages including 1 eps figur

Solution of the two impurity, two channel Kondo Model

We solve the two-impurity two-channel Kondo model using a combination of conformal invariance and bosonisation techniques. The odd-even symmetric case is analysed in detail. The RKKY interaction turns out to be exactly marginal, resulting in a line of non-Fermi liquid fixed points. Explicit formulae are given for the critical exponents and for the finite-size spectrum, which depend continuously on a single parameter. The marginal line spans a range of values of the RKKY coupling $I$ which goes from the infinitely strong ferromagnetic point $I=-\infty$ (associated with a 4-channel spin-1 Kondo model) to a finite antiferromagnetic critical value $I_c>0$ beyond which a Fermi liquid is recovered. We also find that, when the odd-even symmetry is broken, the marginal line is unstable for ferromagnetic $I$, while for antiferromagnetic $I$ it extends into a manifold of fixed points.Comment: 9 pages, preprint LPTENS 94/1

The Excitation Spectrum of a Bose-Einstein Condensate

We report the first measurement of the excitation spectrum and the static structure factor of a Bose-Einstein condensate. The excitation spectrum displays a linear phonon regime, as well as a parabolic single-particle regime. The linear regime provides an upper limit for the superfluid critical velocity, by the Landau criterion. The excitation spectrum agrees well with the Bogoliubov spectrum, in the local density approximation. This agreement continues even for excitations close to the long-wavelength limit of the region of applicability of the approximation. Feynman's relation between the excitation spectrum and the static structure factor is verified, within an overall constant

\eta-superconductivity in the Hubbard chain with pair hopping

The ground state phase diagram of the 1D Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies. At half-filling the phase diagram is shown to consist of two superconducting states with Cooper pair center-of-mass momentum Q=0 (BCS-\eta_0 phase) and Q=\pi (\eta_\pi-phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator as well as an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting BCS-\eta_0 and \eta_\pi phases and the metallic Luttinger-liquid phase. The BCS-\eta_0 phase exhibits smooth crossover from a weak-coupling BCS type to a strong coupling local-pair regime. The \eta_\pi phase shows properties of the doublon (zero size Cooper pair) superconductor with Cooper pair center-of-mass momentum Q=\pi. The transition into the \eta_\pi- paired state corresponds to an abrupt change in the groundstate structure. After the transition the conduction band is completely destroyed and a new \eta_\pi-pair band corresponding to the strongly correlated doublon motion is created.Comment: 15 pages Revtex, 15 embedded eps figure

Quantum liquids resulting from quark systems with four-quark interaction

Quark ensembles influenced by strong stochastic vacuum gluon fields are investigated within the four-fermion interaction approximation. The comparative analysis of several quantum liquid models is performed and this analysis leads to the conclusion that the presence of a gas–liquid phase transition is their characteristic feature. The problem of the instability of small quark number droplets is discussed and it is argued that it is rooted in the chiral soliton formation. The existence of a mixed phase of the vacuum and baryon matter is proposed as a possible explanation of the latter stability

On the Emery-Kivelson Solution of the two channel Kondo problem

We consider the two channel Kondo model in the Emery-Kivelson approach, and calculate the total susceptibility enhancement due to the impurity $\chi_{imp}=\chi-\chi_{bulk}$. We find that $\chi_{imp}$ exactly vanishes at the solvable point, in a completely analogous way to the singular part of the specific heat $C_{imp}$. A perturbative calculation around the solvable point yields the generic behaviour $\chi_{imp} \sim \log {1 \over T}$, $C_{imp} \sim T\log T$ and the known universal value of the Wilson ratio $R_W={8 \over 3}$. From this calculation, the Kondo temperature can be identified and is found to behave as the inverse-square of the perturbation parameter. The small field, zero-temperature behaviour $\chi_{imp}\sim log {1 \over h}$ is also recovered.Comment: 7 pages, REVTEX, 1 figure available on request, LPTENS preprint 93/4