Identifying topological edge states in 2D optical lattices using light scattering

We recently proposed in a Letter [Physical Review Letters 108 255303] a novel scheme to detect topological edge states in an optical lattice, based on a generalization of Bragg spectroscopy. The scope of the present article is to provide a more detailed and pedagogical description of the system - the Hofstadter optical lattice - and probing method. We first show the existence of topological edge states, in an ultra-cold gas trapped in a 2D optical lattice and subjected to a synthetic magnetic field. The remarkable robustness of the edge states is verified for a variety of external confining potentials. Then, we describe a specific laser probe, made from two lasers in Laguerre-Gaussian modes, which captures unambiguous signatures of these edge states. In particular, the resulting Bragg spectra provide the dispersion relation of the edge states, establishing their chiral nature. In order to make the Bragg signal experimentally detectable, we introduce a "shelving method", which simultaneously transfers angular momentum and changes the internal atomic state. This scheme allows to directly visualize the selected edge states on a dark background, offering an instructive view on topological insulating phases, not accessible in solid-state experiments.Comment: 17 pages, 10 figures. Revised and extended version, to appear in EJP Special Topic for the special issue on "Novel Quantum Phases and Mesoscopic Physics in Quantum Gases". Extended version of arXiv:1203.124

Short-Term Enrichment Makes Male Rats More Attractive, More Defensive and Alters Hypothalamic Neurons

Innate behaviors are shaped by contingencies built during evolutionary history. On the other hand, environmental stimuli play a significant role in shaping behavior. In particular, a short period of environmental enrichment can enhance cognitive behavior, modify effects of stress on learned behaviors and induce brain plasticity. It is unclear if modulation by environment can extend to innate behaviors which are preserved by intense selection pressure. In the present report we investigate this issue by studying effects of relatively short (14-days) environmental enrichment on two prominent innate behaviors in rats, avoidance of predator odors and ability of males to attract mates. We show that enrichment has strong effects on both the innate behaviors: a) enriched males were more avoidant of a predator odor than non-enriched controls, and had a greater rise in corticosterone levels in response to the odor; and b) had higher testosterone levels and were more attractive to females. Additionally, we demonstrate decrease in dendritic length of neurons of ventrolateral nucleus of hypothalamus, important for reproductive mate-choice and increase in the same in dorsomedial nucleus, important for defensive behavior. Thus, behavioral and hormonal observations provide evidence that a short period of environmental manipulation can alter innate behaviors, providing a good example of gene-environment interaction

Monitored open fermion dynamics: Exploring the interplay of measurement, decoherence, and free Hamiltonian evolution

The interplay of unitary evolution and local measurements in many-body systems gives rise to a stochastic state evolution and to measurement-induced phase transitions in the pure state entanglement. In realistic settings, however, this dynamics may be spoiled by decoherence, e.g., dephasing, due to coupling to an environment or measurement imperfections. We investigate the impact of dephasing and the inevitable evolution into a non-Gaussian, mixed state, on the dynamics of monitored fermions. We approach it from three complementary perspectives: (i) the exact solution of the conditional master equation for small systems, (ii) quantum trajectory simulations of Gaussian states for large systems, and (iii) a renormalization group analysis of a bosonic replica field theory. For weak dephasing, constant monitoring preserves a weakly mixed state, which displays a robust measurement-induced phase transition between a critical and a pinned phase, as in the decoherence-free case. At strong dephasing, we observe the emergence of a new scale describing an effective temperature, which is accompanied with an increased mixedness of the fermion density matrix. Remarkably, observables such as density-density correlation functions or the subsystem parity still display scale invariant behavior even in this strongly mixed phase. We interpret this as a signature of gapless, classical diffusion, which is stabilized by the balanced interplay of Hamiltonian dynamics, measurements, and decoherence

Ab initio studies of pi-water tetramer complexes: Evolution of optimal structures, binding energies, and vibrational spectra of pi-(H2O)(n) (n=1-4) complexes

The optimal structures, binding energies, and harmonic vibrational frequencies of clusters containing a substituted benzene molecule microsolvated by four water molecules, termed as pi-(water tetramer) clusters (pi: p-difluorobenzene, fluorobenzene, benzene, toluene) have been evaluated at the second order perturbation level of theory (MP2) using both the 6-31+G* and aug-cc-pVDZ basis sets. In sharp contrast to the complexes of smaller water clusters with these pi systems, wherein the water subcluster is most strongly bound to toluene, the water tetramer is most strongly bound to fluorobenzene. This exceptionally high binding energy results from both a pi . . . OH H-bond and a competing sigma F . . . OH bond between the water tetramer moiety and the aromatic molecule. The magnitudes of the many-body energy terms and their contribution to the binding energies of these pi-(water tetramer) systems indicates that the contributions of three- and higher-order terms are much smaller when compared to the neutral water clusters. The two-body terms associated with the pi- and sigma -type of interaction indicates that in both the fluorobenzene and p-difluorobenzene complexes, the increase in the size of the water cluster enhances the pi -H-bonding interaction and weakens the sigma F . . .H interaction. This observation is in consonance with the calculated and experimentally observed redshifts of the OH vibrational frequencies. Thus, with an increase in the size of a water cluster bound to the fluorinated pi system, there is a lowering of the redshift induced by the sigma F . . .H interaction and an increase in the redshift due to the pi -H interaction. The calculated redshift of the pi H-bonded OH mode is very much dependent on the basis set, with larger basis sets yielding shifts which are in better agreement with the experimentally determined shifts. (C) 2001 American Institute of Physics.open1136sciescopu

Anisole-(H2O)(n) (n=1-3) complexes: An experimental and theoretical investigation of the modulation of optimal structures, binding energies, and vibrational spectra in both the ground and first excited states

We present the results obtained from spectroscopic investigations and quantum chemical calculations of the interaction of anisole (methoxybenzene) with small water clusters. The experiments have been carried out using resonant two-photon ionization (R2PI) and IR-UV double-resonance vibrational spectroscopy (IR/R2PI) in the region of the OH stretches. Apart from the vibrational spectra of the water moiety in the clusters, their intermolecular vibrations in the electronically excited S-1 state are identified by IR/R2PI hole burning spectroscopy and assigned according to the vibrations calculated for the S-1 state and compared with the vibrations calculated for the S-0 state. The calculations for the S-0 state were carried out at the second order Moller-Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets and for the S-1 state at the configuration interaction singles (CIS) level with the 6-31+G(*) basis set. In the electronic ground state (S-0), the interaction of a water monomer to anisole is mediated through its oxygen atom, and that of a water dimer both through the oxygen atom (sigma type of interaction) and the arene ring (pi type of interaction). Thus in contrast to the interaction of fluorinated benzenes with water clusters, wherein a conformational transition from an in-plane sigma to a on-top pi bonding emerges starting with a water trimer, this conformational transition appears in case of anisole already with a water dimer. In the excited state (S-1) of the investigated systems, there is a pronounced weakening of the interaction of the water cluster with the aromatic chromophore, which is also responsible for the blue shift of the electronic transitions. Consequently, the structures of the complexes of anisole with a water monomer or dimer are very different in both states. The weakening results from a diminished electron density of the oxygen atom and of the pi system of anisole in the excited state. The calculated binding energies of the ground-state conformers indicate that these small water clusters are bound more strongly to anisole than to other pi systems like benzene, toluene, fluorobenzene, and p-difluorobenzene. The many-body decomposition of the binding energy (S-0) reveals that a progressive increase in the size of the water clusters results in a weakening of the sigma O...H interaction and a concurrent strengthening of the pi-H interaction. The complex containing a cyclic water trimer exhibits in the excited state also a pi-type H-bonding interaction, but its stability emerges from a delocalization of the electron density from the water trimer to the anisole oxygen. Excepting the water dimer complexes, there is a good agreement between the calculated OH vibrational frequencies and the experimental IR spectra recorded for the ground state. The surprisingly good agreement of the calculated and the experimentally observed intermolecular modes in the excited state of these complexes provides convincing evidence that the experimental spectra emerge from structures similar to those predicted at the CIS/6-31+G* level. (C) 2002 American Institute of Physics.open116565sciescopu

On the microscopic interaction of para-chlorofluorobenzene with water

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