Many-body ionization in a frozen Rydberg gas

In a dense gas of 300 microK 85Rb atoms of n approximately 50 ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 K blackbody radiation. Rapid ionization is accompanied by spectral broadening, with the spectrum becoming continuous at n=88 at a density of 5x10(10)cm(-3). The atomic transitions broaden both smoothly and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms.     

Ground state of a spin system with two- and four-spin exchange interactions on the triangular lattice

We study a spin system with both two- and four-spin exchange interactions on the triangular lattice as a possible model for the nuclear magnetism of solid ³He layers adsorbed on grafoil. The ground state is analyzed by the use of the mean-field approximation. It is shown that the four-sublattice state is favored by introduction of the fourspin exchange interaction. A possible phase transition at a finite temperature into a phase with the scalar chirality is predicted. Application of a magnetic field is shown to cause a variety of phase transitions.     

中性碳原子基态关联效应的多体理论计算

应用原子多体理论对开壳层原子中电子关联效应的有效哈密顿量及波算符的Goldston图进行了分析,在此基础上得到了计算有效哈密顿量的简单方法;以HFS模型为零级近似对中性碳原子基态的关联效应进行了计算并通过与实验结果及其他文献基于HF模型进行的计算相比较,得到了令人满意的结论。     

Classical lattice gas model with a unique nondegenerate but unstable periodic ground state configuration

We construct a classical lattice gas model with a unique periodic ground state configuration such that the Peierls' condition is not satisfied. The ground state configuration is nondegenerate, which means that for any fixed energyE and any integern, the diameter of the support of alln-connected local excitations, with energy less thanE, is bounded. Nevertheless the configuration is not stable: it does not give rise to a low temperature phase. Any translation invariant Gibbs state of our model corresponds to quasiperiodic ground state configurations. This requires the modification of a recent hypothesis of Dobrushin and Shlosman.     

Structure of the ground state of a nonsuperfluid dense quark-gluon plasma

The single-particle spectrum and momentum distribution of quasiparticles in a cold dense quark-gluon plasma are calculated within the Fermi liquid approach. It is shown that this system does not behave as a standard Fermi liquid: at zero temperature, the single-particle spectrum has a plateau at the Fermi surface, while the Fermi surface itself has a nonzero volume in momentum space.     

Antiblockade in Rydberg excitation of an ultracold lattice gas

It is shown that the two-step excitation scheme typically used to create an ultracold Rydberg gas can be described with an effective two-level rate equation, greatly reducing the complexity of the optical Bloch equations. This allows us to efficiently solve the many-body problem of interacting cold atoms with a Monte Carlo technique. Our results reproduce the observed excitation blockade effect. However, we demonstrate that an Autler-Townes double peak structure in the two-step excitation scheme, which occurs for moderate pulse lengths as used in the experiment, can give rise to an antiblockade effect. It is most pronounced for atoms arranged on a lattice. Since the effect is robust against a large number of lattice defects it should be experimentally realizable with an optical lattice created by CO2 lasers.     

Many-body spin Berry phases emerging from the pi-flux state: competition between antiferromagnetism and the valence-bond-solid state

We uncover new topology-related features of the pi-flux saddle-point solution of the D=2+1 Heisenberg antiferromagnet. We note that symmetries of the spinons sustain a built-in competition between antiferromagnetic (AFM) and valence-bond-solid (VBS) orders, the two tendencies central to recent developments on quantum criticality. An effective theory containing an analogue of the Wess-Zumino-Witten term is derived, which generates quantum phases related to AFM monopoles with VBS cores, and reproduces Haldane's hedgehog Berry phases. The theory readily generalizes to pi-flux states for all D.     

Ferromagnetic ground state in the Kondo lattice model

We present a series of rigorous examples of the Kondo lattice model that exhibit full ferromagnetism in the ground state. The models are defined in one-, two- and three-dimensional lattices, and are characterized by a range of hopping terms, specific electron filling, and large ferromagnetic coupling. Our examples show that a sufficient strong but finite exchange coupling between conduction electrons and localized spins could overcome the competition from mobility of a finite density of electrons and drive the system from a paramagnetic phase to a ferromagnetic phase. We also establish a relation of ferromagnetism between the Hubbard model and Kondo lattice model. Meanwhile some rigorous results on ferromagnetism in the corresponding Hubbard model are presented. Received: 10 September 1997 / Revised: 15 October 1997 / Accepted: 17 October 1997     

Rabi oscillations between ground and Rydberg states with dipole-dipole atomic interactions

We demonstrate Rabi oscillations of small numbers of 87Rb atoms between ground and Rydberg states with n< or =43. Coherent population oscillations are observed for single atoms, while the presence of two or more atoms decoheres the oscillations. We show that these observations are consistent with van der Waals interactions of Rydberg atoms.     

Entanglement of two ground state neutral atoms using Rydberg blockade

We report on our recent progress in trapping and manipulation of internal states of single neutral rubidium atoms in optical tweezers. We demonstrate the creation of an entangled state between two ground state atoms trapped in separate tweezers using the effect of Rydberg blockade. The quality of the entanglement is measured using global rotations of the internal states of both atoms.     

Exact ground state and finite-size scaling in a supersymmetric lattice model

We study a model of strongly correlated fermions in one dimension with extended N = 2 supersymmetry. The model is related to the spin S = 1/2 XXZ Heisenberg chain at anisotropy Delta = -1/2 with a real magnetic field on the boundary. We exploit the combinatorial properties of the ground state to determine its exact wave function on finite lattices with up to 30 sites. We compute several correlation functions of the fermionic and spin fields. We discuss the continuum limit by constructing lattice observables with well defined finite-size scaling behavior. For the fermionic model with periodic boundary conditions we give the emptiness formation probability in closed form.     

Meson-meson interactions and the equation of state for dense nucleons

The effect of the interaction among mesons upon the equation of state for a high-density system of nucleons is studied, and it is found that such interactions do not significantly alter the asymptotic behaviour of the equation of state.     

The ground state energy of a classical gas

In this paper we study the ground state energy of a classical gas. Our interest centers mainly on Coulomb systems. We obtain some new lower bounds for the energy of a Coulomb gas. As a corollary of our results we can show that a fermionic system with relativistic kinetic energy and Coulomb interaction is stable. More precisely, letH _N (α) be theN particle Hamiltonian $$H_N (\alpha ) = \alpha \sum\limits_{i = 1}^N {( - \Delta _i )^{1/2} + } \sum\limits_{i< j} {\left| {x_i - x_j } \right|^{ - 1} } - \sum\limits_{i,j} {\left| {x_i - R_j } \right|^{ - 1} } + \sum\limits_{i< j} {\left| {R_i - R_j } \right|^{ - 1} } $$ where Δ _i is the Laplacian in the variablex _i ∈?³ andR ₁, ...,R _N are fixed points in ?³. We show that for sufficiently large α, independent ofN, the HamiltonianH _N (α) is nonnegative on the space of square integrable functions ψ(x ₁, ...,x _N ), antisymmetric in the variablesx _i , 1≦i≦N.     

Critical phenomena for a triangular spin lattice I: Triplet interactions

Critical phenomena in an Ising-like spin system on a triangular lattice are studied by means of the renormalization theory. The critical indices of the critical point due to triplet interactions are determined approximately, in fair agreement with the exponents, known from the exact solution due to Baxter and Wu. The cross-over from triplet critical behavior to ferromagnetic (nearest neighbor) critical behavior is calculated.     

The ground state of the Bose gas

The mathematical formalism describing the Bose gas at zero temperature is analysed with the aid of methods that have recently been successful in relativistic quantum field theory. First the spectrum conditions for an infinitely extended system are given and the algebra of observables and the algebra of field operators are defined. General properties of states over these algebras are discussed and theorems are given which connect the linked cluster property, translation invariance and the purity of the states. It is proved that pure states over the algebra of observables have the property of factorisable off-diagonal long range order. The class of quasi free states is defined and of these states those which are translation invariant and possess the linked cluster property are analysed. It is shown that this class of states contains a subclass of pure states of the Bogoliubov type and a subclass of states which are mixtures of non-translationally invariant pure states. The applications of these quasi free states to the interacting Bose gas are summarized.     

Degenerate ferrimagnetic ground state of frustrated kagome lattice

The frustrated Ising model on kagome lattice with nearest-neighboring antiferromagnetic interaction is investigated by using Monte Carlo simulation of the Wang-Landau algorithm and Glauber dynamics. The geometrical frustration leads to a particularly high degeneracy of ground states in this system. A small magnetic field applied can lift the degeneracy partially, and produce the magnetization plateau of 1/3 saturate value (Ms), which is analogous to the magnetic behavior in triangular antiferromagnetic system. However, different from the long-range ferrimagnetic state responsible for 1/3 Ms plateau in triangular lattice, the ferrimagnetic ground state corresponding to 1/3 Ms plateau in kagome lattice is short-ranged and still highly degenerate. Furthermore, the spin configuration of these degenerate ferrimagnetic ground states show an inherent characteristic that the spins along the magnetic field must be aligned on the closed loops, which can be well understood in terms of geometrical frustration.     

The ground state spin ordering and lowest energy excitations of a model organic ferrimagnet-polyallyl spin chain

We studied zero-temperature magnetic ordering in a model organic ferrimagnet- polyallyl spin-1/2 chain as a function of the exchange integral λ describing the interaction between spins of radical centers and the main chain. Exact diagonalization and density matrix renormalization group (DMRG) methods demonstrated that for small applied fields, the difference in zero-temperature magnetizations of the sublattices reaches a maximum value at λ=0.70. It was shown that there is a critical value of magnetic field, above which this ferrimagnetic ordering is destroyed. We also present the results of DMRG calculations that show the gap character of excitations without decrease of the total spin for λ⩾0.4.