Dominik Vogel

Experimental reconstruction of the Berry curvature in a Floquet Bloch band

Cold atoms do geometry Electrons in solids populate energy bands, which can be simulated in cold atom systems using optical lattices. The geometry of the corresponding wave functions determines the topological properties of the system, but getting a direct look is tricky. Fläschner et al. and Li et al. measured the detailed structure of the band wave functions in hexagonal optical lattices, one resembling a boron-nitride and the other a graphene lattice. These techniques will make it possible to explore more complex situations that include the effects of interactions. Science, this issue pp. 1091 and 1094 Berry curvature is engineered and measured in a simulated boron-nitride optical lattice filled with fermionic K atoms. Topological properties lie at the heart of many fascinating phenomena in solid-state systems such as quantum Hall systems or Chern insulators. The topology of the bands can be captured by the distribution of Berry curvature, which describes the geometry of the eigenstates across the Brillouin zone. Using fermionic ultracold atoms in a hexagonal optical lattice, we engineered the Berry curvature of the Bloch bands using resonant driving and show a full momentum-resolved measurement of the ensuing Berry curvature. Our results pave the way to explore intriguing phases of matter with interactions in topological band structures.

Observation of dynamical vortices after quenches in a system with topology

Topological phases constitute an exotic form of matter characterized by non-local properties rather than local order parameters1. The paradigmatic Haldane model on a hexagonal lattice features such topological phases distinguished by an integer topological invariant known as the first Chern number2. Recently, the identification of non-equilibrium signatures of topology in the dynamics of such systems has attracted particular attention3–6. Here, we experimentally study the dynamical evolution of the wavefunction using time- and momentum-resolved full state tomography for spin-polarized fermionic atoms in driven optical lattices7. We observe the appearance, movement and annihilation of dynamical vortices in momentum space after sudden quenches close to the topological phase transition. These dynamical vortices can be interpreted as dynamical Fisher zeros of the Loschmidt amplitude8, which signal a so-called dynamical phase transition9,10. Our results pave the way to a deeper understanding of the connection between topological phases and non-equilibrium dynamics.Non-equilibrium signatures of topology—the appearance, movement and annihilation of vortices in a cold-atom system—are identified, showing that topological phase can emerge dynamically from a non-topological state.Phase transitions are a fundamental concept in science describing diverse phenomena ranging from, e.g., the freezing of water to Bose-Einstein condensation. While the concept is well-established in equilibrium, similarly fundamental concepts for systems far from equilibrium are just being explored, such as the recently introduced dynamical phase transition (DPT). Here we report on the first observation of a DPT in the dynamics of a fermionic many-body state after a quench between two lattice Hamiltonians. With time-resolved state tomography in a system of ultracold atoms in optical lattices, we obtain full access to the evolution of the wave function. We observe the appearance, movement, and annihilation of vortices in reciprocal space. We identify their number as a dynamical topological order parameter, which suddenly changes its value at the critical times of the DPT. Our observation of a DPT is an important step towards a more comprehensive understanding of non-equilibrium dynamics in general.

Observation of Topological Bloch-State Defects and Their Merging Transition

Topological defects in Bloch bands, such as Dirac points in graphene, and their resulting Berry phases play an important role for the electronic dynamics in solid state crystals. Such defects can arise in systems with a two-atomic basis due to the momentum-dependent coupling of the two sublattice states, which gives rise to a pseudospin texture. The topological defects appear as vortices in the azimuthal phase of this pseudospin texture. Here, we demonstrate a complete measurement of the azimuthal phase in a hexagonal optical lattice employing a versatile method based on time-of-flight imaging after off-resonant lattice modulation. Furthermore, we map out the merging transition of the two Dirac points induced by beam imbalance. Our work paves the way to accessing geometric properties in optical lattices also with spin-orbit coupling and interactions.

Prosocial Attitudes in the Public and Private Sector: Exploring Behavioral Effects and Variation across Time

This paper will contribute to a growing body of research on the concept of public service motivation and its effects. It addresses two important though still largely unexplored questions: How stable or dynamic are prosocial attitudes, and do differences among employees or the individual changes in their attitudes over time matter, in order to explain the performance of prosocial behavior? To learn more about public employees in this respect, the paper will compare them with their counterparts in the private sector and explore multiple waves of data from the German Socio-Economic Panel Study that covers a time period of sixteen years. We find that prosocial attitudes are mostly stable, there are no socialization effects in either sector, and differences among employees matter a great deal. We also detect longitudinal effects, suggesting that increases in prosocial motivation trigger related behaviors, however, at the risk of neglecting others.

Changes in Prosocial Motivation over Time: A Cross-Sector Analysis of Effects on Volunteering and Work Behavior

ABSTRACT A gap in research on prosocial motivation is that very little is known about its change across time, let alone, how such changes affect employee behavior. Using multiple waves of panel data, covering a period of sixteen years, this article finds that prosocial motivation is mostly stable, and there are no broader socialization effects in the private and public sector. However, when prosocial motivation increases, it leads to increases in either work or volunteering behavior, suggesting that public employees may use alternative outlets to realize their motivation if such motivational capital cannot be linked to the mission of their organizations.

Prosocial Attitudes in the Public and Private Sector

This paper will contribute to a growing body of research on the concept of public service motivation and its effects. It addresses two important though still largely unexplored questions: How stable or dynamic are prosocial attitudes, and do differences among employees or the individual changes in their attitudes over time matter, in order to explain the performance of prosocial behavior? To learn more about public employees in this respect, the paper will compare them with their counterparts in the private sector and explore multiple waves of data from the German Socio-Economic Panel Study that covers a time period of sixteen years. We find that prosocial attitudes are mostly stable, there are no socialization effects in either sector, and differences among employees matter a great deal. We also detect longitudinal effects, suggesting that increases in prosocial motivation trigger related behaviors, however, at the risk of neglecting others.