RESUMEN
Crystal lattice fluctuations, which are known to influence phase transitions of quantum materials in equilibrium, are also expected to determine the dynamics of light-induced phase changes. However, they have only rarely been explored in these dynamical settings. Here we study the time evolution of lattice fluctuations in the quantum paraelectric SrTiO3, in which mid-infrared drives have been shown to induce a metastable ferroelectric state. Crucial in these physics is the competition between polar instabilities and antiferrodistortive rotations, which in equilibrium frustrate the formation of long-range ferroelectricity. We make use of high-intensity mid-infrared optical pulses to resonantly drive the Ti-O-stretching mode at 17 THz, and we measure the resulting change in lattice fluctuations using time-resolved X-ray diffuse scattering at a free-electron laser. After a prompt increase, we observe a long-lived quench in R-point antiferrodistortive lattice fluctuations. Their enhancement and reduction are theoretically explained by considering the fourth-order nonlinear phononic interactions to the driven optical phonon and third-order coupling to lattice strain, respectively. These observations provide a number of testable hypotheses for the physics of light-induced ferroelectricity.
RESUMEN
Ultrafast strong-field physics provides insight into quantum phenomena that evolve on an attosecond time scale, the most fundamental of which is quantum tunneling. The tunneling process initiates a range of strong field phenomena such as high harmonic generation (HHG), laser-induced electron diffraction, double ionization and photoelectron holography-all evolving during a fraction of the optical cycle. Here we apply attosecond photoelectron holography as a method to resolve the temporal properties of the tunneling process. Adding a weak second harmonic (SH) field to a strong fundamental laser field enables us to reconstruct the ionization times of photoelectrons that play a role in the formation of a photoelectron hologram with attosecond precision. We decouple the contributions of the two arms of the hologram and resolve the subtle differences in their ionization times, separated by only a few tens of attoseconds.
RESUMEN
CONTEXT: A working model of caregiving has been defined as an internalized schema of caregiving that guides a mother's behavior with her infant. This schema is strongly influenced by the mother's first attachment relationship with her own mother, which frequently operates outside her conscious awareness. OBJECTIVE: To examine doula support at birth as an intervention that might correct distortions of a mother's unconscious schema of caregiving, promoting more secure caregiving following childbirth. DESIGN: The effects of doula support versus Lamaze birth preparation on the working models of caregiving of 35 first-time pregnant mothers were compared. INTERVENTIONS: An interview was used to assess the mothers' working models in the third trimester of pregnancy and again 4 months after the mothers gave birth. Mood state and self-esteem were measured at these times. A post hoc analysis was performed on the mothers' birth experiences, the quality of their postpartum support, and their evaluations of their infants' temperaments. RESULTS: The prebirth to postbirth rating analysis of the interviews revealed that the mothers in the doula group were significantly less rejecting and helpless in their working models of caregiving than were the mothers in the Lamaze group. The mothers in the doula group also showed greater security than did mothers in the Lamaze group, a difference that approached but did not reach significance. In addition, the women in the doula group were less emotionally distressed and had higher self-esteem than did the women in the Lamaze group, and the women in the doula group rated their infants as significantly less fussy than the mothers in the Lamaze group rated their infants. CONCLUSIONS: When viewed in the context of working models of caregiving, mothers who used a doula were significantly less rejecting and helpless, and approached significantly higher scores on the secure scale, than did mothers who used Lamaze birth preparation. Mothers rated as secure showed significantly less mood disturbance and higher prebirth and postbirth self-esteem than did mothers rated as insecure, providing construct validity for the caregiving interviews.