The search for the neutron electric dipole moment at PSI

The existence of a nonzero permanent electric dipole moment (EDM) of the neutron would reveal a new source of CP violation and shed light on the origin of the matter–antimatter asymmetry of the Universe. The sensitivity of current experiments using stored ultracold neutrons (UCN) probe new physics beyond the TeV scale. Using the UCN source at the Paul Scherrer Institut, the nEDM collaboration has performed the most sensitive measurement of the neutron EDM to date, still compatible with zero (|d_n|<1.8\times 10^{-26} \, e {cm}|dn|<1.8×10−26ecm, C.L.,90%). A new experiment designed to improve the sensitivity by an order of magnitude, n2EDM, is currently under construction.

Evidence of ideal excitonic insulator in bulk MoS2 under pressure

Spontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons—electron (e) and hole (h) pairs interacting by Coulomb force—overcomes the band gap, giving rise to a new phase: the “excitonic insulator” (EI). Transition metal dichalcogenides are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon. Here we focus on bulk MoS2 and demonstrate theoretically that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. Starting from first-principles many-body perturbation theory, we also predict that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an antiferroelectric texture in the layer plane: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a unique Raman fingerprint for the EI formation. Finally, we identify such fingerprint in a Raman feature that was previously observed experimentally, thus providing direct spectroscopic confirmation of an ideal excitonic insulator phase in bulk MoS2 above 30 GPa.

Propagation of optically tunable coherent radiation in a gas of polar molecules

Coherent, optically dressed media composed of two-level molecular systems without inversion symmetry are considered as all-optically tunable sources of coherent radiation in the microwave domain. A theoretical model and a numerical toolbox are developed to confirm the main finding: the generation of low-frequency radiation, and the buildup and propagation dynamics of such low-frequency signals in a medium of polar molecules in a gas phase. The physical mechanism of the signal generation relies on the permanent dipole moment characterizing systems without inversion symmetry. The molecules are polarized with a DC electric field yielding a permanent electric dipole moment in the laboratory frame; the direction and magnitude of the moment depend on the molecular state. As the system is resonantly driven, the dipole moment oscillates at the Rabi frequency and, hence, generates microwave radiation. We demonstrate the tuning capability of the output signal frequency with the drive amplitude and detuning. We find that even though decoherence mechanisms such as spontaneous emission may damp the output field, a scenario based on pulsed illumination yields a coherent, pulsed output of tunable temporal width. Finally, we discuss experimental scenarios exploiting rotational levels of gaseous ensembles of heteronuclear diatomic molecules.

Effect of Chemical Order in the Structural Stability and Physicochemical Properties of B12N12 Fullerenes

The effect of chemical order in the structural and physicochemical properties of B12N12 [4,6]-fullerene (BNF) isomers was evaluated using density functional theory and molecular dynamic calculations. The feasibility to find stable BNF isomers with atomic arrangement other than the well-known octahedral Th-symmetry was explored. In this study, the number of homonuclear bonds in the modeled nanostructures was used as categorical parameter to describe and quantify the degree of structural order. The BNF without homonuclear bonds was identified as the most energetically favorable isomer. However, a variety of BNF arrays departing from Th-symmetry was determined as stable structures also. The calculated vibrational spectra suggest that isomers with chemical disorder can be identified by infrared spectroscopy. In general, formation of homonuclear bonds is possible meanwhile the entropy of the system increases, but at expense of cohesive energy. It is proposed that formation of phase-segregated regions stablishes an apparent limit to the number of homonuclear bonds in stable B12N12 fullerenes. It was found that formation of homonuclear bonds decreases substantially the chemical hardness of BNF isomers and generates zones with large charge density, which might act as reactive sites. Moreover, chemical disorder endows BNF isomers with a permanent electric dipole moment as large as 3.28 D. The obtained results suggest that by manipulating their chemical order, the interaction of BNF’s with other molecular entities can be controlled, making them potential candidates for drug delivery, catalysis and sensing.

Electro-optic sensor for static fields

A sensor has been developed for low frequency and DC electric fields E. The device is capable of measuring fields with $$\varDelta \mathrm{E}= 4$$ΔE=4 (1) V/cm resolution. It is based on a Y-cut Z-propagation lithium niobate electro-optic crystal. For a particular commercially available bare crystal, we achieved an in air time constant $$\tau _\mathrm{c}(\mathrm air)= 6.4(1.8)$$τc(air)=6.4(1.8) h for the decay of the electro-optic signal. This enables field monitoring for several hours. As an application, we demonstrated that a constant electric field $$E^{\mathrm{ext}} = 640$$Eext=640 V/cm applied via external electrodes to a particular spherical glass container holding an Xe/He gas mixture decays inside this cell with a time constant $$\tau _{E}^{\mathrm{glass}} = 2.5(5)$$τEglass=2.5(5) h. This is sufficient for the needs of experiments searching for a permanent electric dipole moment in $$^{129}$$129Xe. An integrated electric field sensor has been constructed which is coupled to a light source and light detectors via optical fibers. The sensor head does not contain any electrically conducting material.

Solute-solvent electronic interaction is responsible for initial charge separation in ruthenium complexes [Ru(bpy)3]2+ and [Ru(phen)3]2+

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D3 symmetry has remained a disputed issue for decades. Here we measure the femtosecond two-photon absorption (2PA) cross section spectra of [Ru(2,2′-bipyridine)3]2 and [Ru(1,10-phenanthroline)3]2 in a series of solvents with varying polarity and show that for vertical transitions to the lower-energy 1MLCT excited state, the permanent electric dipole moment change is nearly solvent-independent, Δμ = 5.1–6.3 D and 5.3–5.9 D, respectively. Comparison of experimental results with quantum-chemical calculations of complexes in the gas phase, in a polarizable dielectric continuum and in solute-solvent clusters containing up to 18 explicit solvent molecules indicate that the non-vanishing permanent dipole moment change in the nominally double-degenerate E-symmetry state is caused by the solute-solvent interaction twisting the two constituent dipoles out of their original opposite orientation, with average angles matching the experimental two-photon polarization ratio.