The Sauter-Schwinger effect predicts the creation of electron-positron pairs out of the quantum vacuum by a strong and slowly varying electric field. This effect can be dynamically assisted by an additional weaker time-dependent field, which may drastically enhance the pair-creation probability. In previous studies, it has been found that the enhancement may crucially depend on the temporal shape of this weaker pulse, e.g., a Gaussian profile exp{−(ωt) 2 } or a Sauter pulse 1/ cosh 2 (ωt) behave quite differently. In order to understand this difference, we make a perturbative expansion in terms of the weaker field while treating the strong electric field non-perturbatively. For a large class of profiles including the Sauter pulse, already the sum of the zeroth-order and the first-order amplitudes of this perturbative expansion yields good agreement. For other cases, such as a Gaussian or sinusoidal profile, this is not true in general and higher orders can yield the dominant contribution -where the dominant order depends on the chosen parameters. Our findings are confirmed by numerical simulations and help us to sort previous results into a bigger picture.

The host–guest system TCNQ@Cu3BTC2 is a striking example of how semiconductivity can be introduced by guest incorporation in an otherwise insulating parent material.

While the Sauter-Schwinger effect describes nonperturbative electron-positron pair creation from vacuum by a strong and slowly varying electric field Estrong via tunneling, the dynamically assisted Sauter-Schwinger effect corresponds to a strong (exponential) enhancement of the pair-creation probability by an additional weak and fast electric or electromagnetic pulse E weak . Using the WKB and worldline instanton method, we find that this enhancement mechanism strongly depends on the shape of the fast pulse. For the Sauter profile 1/ cosh 2 (ωt) considered previously, the threshold frequency ωcrit (where the enhancement mechanism sets in) is basically independent of the magnitude E weak of the weak pulse-whereas for a Gaussian pulse exp(−ω 2 t 2 ), an oscillating profile cos(ωt) or a standing wave cos(ωt) cos(kx), the value of ωcrit does depend (logarithmically) on E weak /Estrong.

We report the results of a direct search for the 229 Th (I p = 3/2 + ← 5/2 + ) nuclear isomeric transition, performed by exposing 229 Th-doped LiSrAlF6 crystals to tunable vacuum-ultraviolet synchrotron radiation and observing any resulting fluorescence. We also use existing nuclear physics data to establish a range of possible transition strengths for the isomeric transition. We find no evidence for the thorium nuclear transition between 7.3 eV and 8.8 eV with transition lifetime (1-2) s τ (2000-5600) s. This measurement excludes roughly half of the favored transition search area and can be used to direct future searches.

Via the world-line instanton method, we study electron-positron pair creation by a strong (but sub-critical) electric field of the profile E/ cosh 2 (kx) superimposed by a weaker pulse E / cosh 2 (ωt). If the temporal Keldysh parameter γ ω = mω/(qE) exceeds a threshold value γ crit ω which depends on the spatial Keldysh parameter γ k = mk/(qE), we find a drastic enhancement of the pair creation probability -reporting on what we believe to be the first analytic non-perturbative result for the interplay between temporal and spatial field dependences E(t, x) in the Sauter-Schwinger effect. Finally, we speculate whether an analogous effect (drastic enhancement of tunneling probability) could occur in other scenarios such as stimulated nuclear decay, for example.

We study electron-positron pair creation by a strong and constant electric field superimposed with a weaker transversal plane wave which is incident perpendicularly (or under some angle). Comparing the fully nonperturbative approach based on the world-line instanton method with a perturbative expansion into powers of the strength of the weaker plane wave, we find good agreement-provided that the latter is carried out to sufficiently high orders. As usual for the dynamically assisted Sauter-Schwinger effect, the additional plane wave induces an exponential enhancement of the pair-creation probability if the combined Keldysh parameter exceeds a certain threshold.

Erratum: Radiative lifetime and energy of the low-energy isomeric level in 229 Th[Phys. Rev. C 92, 054324 (2015)]

The modular building principle of metal− organic frameworks (MOFs) presents an excellent platform to explore and establish structure−property relations that tie microscopic to macroscopic properties. Negative thermal expansion (NTE) is a common phenomenon in MOFs and is often ascribed to collective motions that can move through the structure at sufficiently low energies. Here, we show that the introduction of additional linkages in a parent framework, retrofitting, is an effective approach to access lattice dynamics experimentally, in turn providing researchers with a tool to alter the NTE behavior in MOFs. By introducing TCNQ (7,7,8,8-tetracyanoquinodimethane) into the prototypical MOF Cu 3 BTC 2 (BTC = 1,3,5-benzenetricarboxylate; HKUST-1), NTE can be tuned between α V = −15.3 × 10 −6 K −1 (Cu 3 BTC 2 ) and α V = −8.4 × 10 −6 K −1 (1.0TCNQ@ Cu 3 BTC 2 ). We ascribe this phenomenon to a general stiffening of the framework as a function of TCNQ loading due to additional network connectivity, which is confirmed by computational modeling and far-infrared spectroscopy. Our findings imply that retrofitting is generally applicable to MOFs with open metal sites, opening yet another way to fine-tune properties in this versatile class of materials.