We present calculations of the ground state and excitations of an anisotropic dipolar Bose gas in two dimensions, realized by a non-perpendicular polarization with respect to the system plane. For sufficiently high density an increase of the polarization angle leads to a density instability of the gas phase in the direction where the anisotropic interaction is strongest. Using a dynamic many-body theory, we calculate the dynamic structure function in the gas phase which shows the anisotropic dispersion of the excitations. We find that the energy of roton excitations in the strongly interacting direction decreases with increasing polarization angle and almost vanishes close to the instability. Exact path integral ground state Monte Carlo simulations show that this instability is indeed a quantum phase transition to a stripe phase, characterized by long-range order in the strongly interacting direction.Strongly correlated dipolar Bose gases in two dimensions (2D) polarized along the direction normal to the system plane have been extensively investigated in recent years [1][2][3][4]. The ratio between the dipolar length r 0 = mC dd /(4πh 2 ) and the average interparticle distance provides a measure of the strength of the interaction. C dd is the coupling constant proportional to the square of the (magnetic µ or electric d) dipole moment, resulting in a dipolar length that can range from a fewÅ for magnetic dipolar systems like 52 Cr (µ = 6µ B , with µ B the Bohr magneton), to thousands ofÅ for heteronuclear polar molecules like KRb, LiCs [5], or RbCs [6]. However, chemical reactions and three-body losses impose limitations on what can be measured in experiments with polar molecules. Therefore, recent efforts focus also on exotic lanthanide magnetic systems like 164 Dy or 168 Er, [7] where the combined effect of a large magnetic moment (µ = 10µ B for 164 Dy and µ = 7µ B for 168 Er) and a large mass, lead to dipolar length scales that, although still significantly lower than the corresponding value for polar molecules, is several times larger than that of 52 Cr. Er 2 with µ = 14µ B and twice the mass of Er would reach even higher values of r 0 [8].A 2D dipolar Bose gas polarized along the normal direction to the confining plane develops a roton excitation at high density due to the strong repulsion between dipoles at short distances [3]. Other works have revealed competing effects in a quasi-2D geometry due to the head-to-tail attraction of the dipole-dipole interaction when the third spatial dimension is added, to the point that the system becomes unstable against density fluctuation below a critical trapping frequency in that direction [9][10][11]. This leads to the question of whether a similar situation can hold in a purely 2D geometry when a head-to-tail component to the dipole-dipole interaction is added by tilting the polarization with respect to the direction normal to the system plane. The interaction becomes anisotropic, V (r) = V (x, y) = C dd 4πr 3 1 − 3 x 2 r 2 sin 2 α , with particles moving in the x, y...

We have recorded the R(0)nu(CO) = 1 <- 0 IR spectrum of CO and its isotopomers in superfluid helium nanodroplets. For droplets with average size N greater than or similar to 2000 helium atoms, the transition exhibits a Lorentzian shaped linewidth of 0.034 cm(-1), indicating a homogeneous broadening mechanism. The rotational constants could be deduced and were found to be reduced to about 60% of the corresponding gas-phase values (63% for the reference C-12 O-16 species). Accompanying calculations of the pure rotational spectra were carried out using the method of correlated basis functions in combination with diffusion Monte Carlo (CBF/DMC). These calculations show that both the reduction of the rotational B constant and the line broadening can be attributed to phonon-rotation coupling. The reduction in B is confirmed by path integral correlation function calculations for a cluster of 64 He-4 atoms, which also reveal a non-negligible effect of finite size on the collective modes. The phonon-rotation coupling strength is seen to depend strongly on the strength and anisotropy of the molecule-helium interaction potential. Comparison with other light rotors shows that this coupling is particularly high for CO. The CBF/DMC analysis shows that the J = 1 rotational state couples effectively to phonon states, which are only present in large helium droplets or bulk. In particular, they are not present in small clusters with n <= 20, thereby accounting for the much narrower linewidths and larger B constant measured for these sizes

We present a path integral Monte Carlo (PIMC) methodology for quantum simulation of molecular rotations in superfluid environments such as helium and para-hydrogen that combines the sampling of rotational degrees of freedom for a molecular impurity with multilevel Metropolis sampling of Bose permutation exchanges for the solvating species. We show how the present methodology can be applied to the evaluation of imaginary time rotational correlation functions of the molecular impurity, from which the effective rotational constants can be extracted. The combined rotation/permutation sampling approach allows for the first time explicit assessment of the effect of Bose permutations on molecular rotation dynamics, and the converse, i.e., the effect of molecular rotations on permutation exchanges and local superfluidity. We present detailed studies showing that the effect of Bose permutations in the solvating environment is more significant for the dynamics of heavy than light molecules in helium, and that Bose permutation exchanges are slightly enhanced locally by molecular rotation. Finally, the examples studied here reveal a size dependence of rotational excitations for molecules possessing a strongly anisotropic interaction with helium in 4HeN clusters between N approximately 20 and N approximately 10(3).

Rotation of molecules embedded in He nanodroplets is explored by a combination of fs laserinduced alignment experiments and angulon quasiparticle theory. We demonstrate that at low fluence of the fs alignment pulse, the molecule and its solvation shell can be set into coherent collective rotation lasting long enough to form revivals. With increasing fluence, however, the revivals disappear -instead, rotational dynamics as rapid as for an isolated molecule is observed during the first few picoseconds. Classical calculations trace this phenomenon to transient decoupling of the molecule from its He shell. Our results open novel opportunities for studying non-equilibrium solute-solvent dynamics and quantum thermalization.

We present calculations of rotational absorption spectra of the molecules HCN and DCN in superfluid helium-4, using a combination of the Diffusion Monte Carlo method for ground state properties and an analytic many-body method (Correlated Basis Function theory) for the excited states. Our results agree with the experimentally determined effective moment of inertia which has been obtained from the J = 0 → 1 spectral transition. The correlated basis function analysis shows that, unlike heavy rotors such as OCS, the J = 2 and higher rotational excitations of HCN and DCN have high enough energy to strongly couple to rotons, leading to large shifts of the lines and accordingly to anomalous large spectroscopic distortion constants, to the emergence of roton-maxon bands, and to secondary peaks in the absorption spectra for J = 2 and J = 3. In microwave helium nanodroplet isolation spectroscopy experiments, Conjusteau et. al. [1] have measured the rotational excitation energy J = 0 → 1 of HCN and DCN embedded in 4 He clusters. Their results show a reduction of this excitation energy by factors of 0.815 and 0.827 with respect to gas phase HCN and DCN, respectively. Infrared spectroscopy experiments of HCN by Nauta et. al. [2] yield similar results from analysis of the ro-vibrational excitation of the C-H stretching mode, namely a reduction of 0.795 in the J = 0 → 1 energy. These fractional reductions are considerably smaller than those observed for heavier molecules such as SF 6 and OCS, where reductions by factors of ∼ 0.3 are seen [3]. The gas phase rotational constants, B = 1.478222 cm −1 for HCN and B = 1.207780 cm −1 for DCN, are also much larger than the corresponding values for the heavier molecules (e.g. B = 0.2029 cm −1 and 0.0911 cm −1 for OCS and SF 6 respectively). The widely observed reduction in B is understood to be due to the interaction of the molecule with the surrounding 4 He atoms [4]. For the heavier molecules it has been found that calculations based on the microscopic 2-fluid theory [5] can reproduce the effective rotational constant B eff [4, 6]. For some heavy linear rotors, a semiclassical hydrodynamical analysis that combines a classical treatment of the molecular rotation with a quantum calculation of helium solvation density approximately reproduced the moment of inertia increase measured in experiments (see table I in Ref. 7), although no agreement is found for the octahedral SF 6 molecule [4,8,9]. The hydrodynamic contribution to the effective moment of inertia is found to be considerably decreased when the molecular rotation is treated quantum mechanically [10].These models for heavier molecules are based on analysis of partial or complete adiabatic following of the molecular rotational motion by helium and cannot describe the dynamics of light rotors like HCN and DCN in helium for which adiabatic following does not hold [11]. Furthermore, infrared spectra of HCN [2] and acetylene, C 2 H 2 [12], and other light molecules show a small splitting of the ro-vibrational R(0)-line which cannot be...

An analytical potential energy surface for a rigid Rb₂ in the ³Σ(u)⁺ state interacting with one helium atom based on accurate ab initio computations is proposed. This 2-dimensional potential is used, together with the pair approximation approach, to investigate Rb₂ attached to small helium clusters He(N) with N = 1-6, 12, and 20 by means of quantum Monte Carlo studies. The limit of large clusters is approximated by a flat helium surface. The relative orientation of the dialkali axis and the helium surface is found to be parallel. Dynamical investigations of the pendular and of the in-plane rotation of the rigid Rb₂ molecule on the surface are presented.

Optimized variational calculations have been carried out for clusters of 4 He between Nϭ20 and Nϭ1000 atoms. For small cluster sizes with less or equal to 112 particles, where comparisons with existing diffusion Monte Carlo results are possible, we find good agreement for the ground state energies and densities. Using a somewhat simpler approximation, we also calculate the bound state energies of 3 He atoms attached to these clusters. We then calculate excitations and the dynamic structure function. The complex and nonlocal self-energy introduced for that purpose gives access to the calculation of both elastic and inelastic scattering processes for 4 He and 3 He atoms impinging on the clusters.

Rotational absorption spectra of acetylene in superfluid 4He calculated using a path-integral correlation function approach are seen to result in an anomalously large distortion constant in addition to a reduced rotational constant, with values in excellent agreement with recent experiments. Semianalytic treatment of the dynamics with a combined correlated basis function-diffusion Monte Carlo method reveals that this anomalous behavior is due to strong coupling of the higher rotational states of the molecule with the roton and maxon excitations of 4He, and the associated divergence of the 4He density of states in this region.