The Dicke model phase transition in the quantum motion of a Bose-Einstein condensate in an optical cavity

We show that the motion of a laser-driven Bose-Einstein condensate in a high-finesse optical cavity realizes the spin-boson Dicke-model. The quantum phase transition of the Dicke-model from the normal to the superradiant phase corresponds to the self-organization of atoms from the homogeneous into a periodically patterned distribution above a critical driving strength. The fragility of the ground state due to photon measurement induced back action is calculated.Comment: 5 pages, 2 figure

Photonic tuning of quasi-particle decay in a superfluid

We show that the damping rate of elementary excitations of hybrid systems close to a phase transition can undergo a remarkable resonance like enhancement before mode softening takes place. In particular, we consider the friction of a collective density wave in a homogeneous superfluid of weakly interacting bosonic atoms coupled to the electromagnetic field of a single mode optical resonator. Here the Beliaev damping can thus be controlled by an external laser drive and be enhanced by several orders of magnitude

Graphical Frobenius representations of non-abelian groups

A group G has a Frobenius graphical representation (GFR) if there is a simple graph Γ whose full automorphism group is isomorphic to G acting on the vertices as a Frobenius group. In particular, any group G with a GFR is a Frobenius group and Γ is a Cayley graph. By very recent results of Spiga, there exists a function f such that if G is a finite Frobenius group with complement H and |G| > f(|H|) then G admits a GFR. This paper provides an infinite family of graphs that admit GFRs despite not meeting Spiga's bound. In our construction, the group G is the Higman group A(f, q0) for an infinite sequence of f and q0, having a nonabelian kernel and a complement of odd order

A Clockwise P-T Path from the Variscan Basement of the Tisza Unit, Pannonian Basin, Hungary

The polymetamorphic basement of the Tisza Unit forms a detached fragment of the Variscan European foreland of the Neotethyan realm. A clockwise evolution path of a gneiss-amphibolite complex of the Tisza Unit was reconstructed, investigating the polymetamorphic rocks of the borehole Baksa-2, SE Transdanubia, Hungary. The results obtained by microstructural and mineral paragenetic observations, mineral chemical analyses, and thermobarometric calculations define a P-T loop which suggests a complex Variscan polyphase model rather than a pre-Variscan - Variscan polycyclic one. The early part of the prograde path with kyanite is characterized by T-P conditions of 480±50°C and 470±70 MPa, respectively. The metamorphism reached its peak at 660±25°C and 750±50 MPa, when both kyanite and staurolite were stable. This metamorphic climax was followed by a nearly isothermal decompression to 440±20 MPa at 650±40°C. This event is marked by the presence of sillimanite and a second generation of garnet, and is closely related to the collisional Variscan granitoid magmatism observed in considerable parts of the Tisza Unit. In amphibolites intercalated with gneisses, only this last event was preserved, providing T-P estimates of ca. 650-690°C/400-500 MPa. The present paper provides the first demonstration of a continuous, clockwise P-T path from the metamorphic basement of the Hungarian part of the Tisza Uni

Parametrization of semi-dynamical quantum reflection algebra

We construct sets of structure matrices for the semi-dynamical reflection algebra, solving the Yang-Baxter type consistency equations extended by the action of an automorphism of the auxiliary space. These solutions are parametrized by dynamical conjugation matrices, Drinfel'd twist representations and quantum non-dynamical $R$-matrices. They yield factorized forms for the monodromy matrices.Comment: LaTeX, 24 pages. Misprints corrected, comments added in Conclusion on construction of Hamiltonian

QCD corrections to e+ e- --> 4 jets

We report on the next-to-leading order QCD calculation for e+ e- --> 4 jets. We explain some modern techniques which have been used to calculate the one-loop amplitudes efficiently. We further report on the general purpose numerical program ``Mercutio'', which can be used to calculate any infrared safe four-jet quantity in electron-positron annihilation at next-to-leading order.Comment: 4 pages, talk given at the UK Phenomenology Workshop on Collider Physics, Durham, 19-24 September 199