The coupling of ordered electronic phases with lattice, spin, and orbital degrees of freedom are of central interest in strongly correlated systems. Their interplay has been intensively studied from ...femtosecond to picosecond time scales, while their dynamics beyond nanoseconds are usually assumed to follow lattice cooling. Here, we report an unusual slowing down of the recovery of an electronic phase across a first-order phase transition. Following optical excitation, the recovery time of both transient optical reflectivity and X-ray diffraction intensity from the charge-ordered superstructure in a La
Sr
FeO
thin film increases by orders of magnitude as the sample temperature approaches the phase transition temperature. In this regime, the recovery time becomes much longer than the lattice cooling time. The combined experimental and theoretical investigation shows that the slowing down of electronic recovery corresponds to the pseudo-critical dynamics that originates from magnetic interactions close to a weakly first-order phase transition.
Semiconductor nanocrystals (NCs), largely important for their photophysical properties, are known to exhibit photoluminescence (PL) quenching upon subjection to elevated temperatures, where many of ...their proposed applications are most relevant. Here, the mechanisms that result in the PL quenching of CdSe, InP, and Si NCs are investigated using both time-integrated and time-resolved emission spectroscopies as well as transient absorption spectroscopy. In combination, these techniques permit the determination of whether changes in electron, hole, or exciton dynamics lie at the heart of changes in radiative recombination. In this manner, we ascertain that although the changes in thermally-induced PL quenching are comparable in CdSe and InP, the former quenches as a result of hole trapping, likely at surface sites where ligand degradation has occurred, whereas we attribute quenching in the latter composition to electron trapping. By changing the composition and binding strength of the surface ligand and by examining the effects of wide bandgap shells (ZnS) on semiconductor cores, the thermally-induced origin of both electron and hole traps in semiconductor NCs at elevated temperature is investigated. The addition of the ZnS shell to both CdSe and InP NCs markedly improves thermal robustness of PL as well as recovery of PL upon return to ambient temperatures. Additionally, substitution of an organic capping ligand on a core-only NC for an inorganic ligand such as (Sn2S6)4- onto InP provides comparable levels of thermal robustness as the ZnS shell, as does the use of covalently bound ligands on Si NCs. These experiments all point to the thermal robustness of the surface as key in retention of PL at elevated temperatures and suggest that it is thermal degradation of surface ligands or, more likely, the breaking of ligand-NC ionic bonds, that leads to the availability of the trap states attributed to thermally-driven PL quenching.
Nature Communications, 9, 1799 (2018) Ordered electronic phases are intimately related to emerging phenomena such
as high Tc superconductivity and colossal magnetoresistance. The coupling of
...electronic charge with other degrees of freedom such as lattice and spin are of
central interest in correlated systems. Their correlations have been
intensively studied from femtosecond to picosecond time scales, while the
dynamics of ordered electronic phases beyond nanoseconds are usually assumed to
follow a trivia thermally driven recovery. Here, we report an unusual slowing
down of the recovery of an electronic phase across a first-order phase
transition, far beyond thermal relaxation time. Following optical excitation,
the recovery time of both transient optical reflectivity and x-ray diffraction
intensity from a charge-ordered superstructure in a La$_{1/3}$Sr$_{2/3}$FeO$_3$
thin film increases by orders of magnitude longer than the independently
measured lattice cooling time when the sample temperature approaches the phase
transition temperature. The combined experimental and theoretical
investigations show that the slowing down of electronic recovery corresponds to
the pseudo-critical dynamics that originates from magnetic interactions close
to a weakly first-order phase transition. This extraordinary long electronic
recovery time exemplifies an interplay of ordered electronic phases with
magnetism beyond thermal processes in correlated systems.
Fetal structural anomalies, which are detected by ultrasonography, have a range of genetic causes, including chromosomal aneuploidy, copy number variations (CNVs; which are detectable by chromosomal ...microarrays), and pathogenic sequence variants in developmental genes. Testing for aneuploidy and CNVs is routine during the investigation of fetal structural anomalies, but there is little information on the clinical usefulness of genome-wide next-generation sequencing in the prenatal setting. We therefore aimed to evaluate the proportion of fetuses with structural abnormalities that had identifiable variants in genes associated with developmental disorders when assessed with whole-exome sequencing (WES).
In this prospective cohort study, two groups in Birmingham and London recruited patients from 34 fetal medicine units in England and Scotland. We used whole-exome sequencing (WES) to evaluate the presence of genetic variants in developmental disorder genes (diagnostic genetic variants) in a cohort of fetuses with structural anomalies and samples from their parents, after exclusion of aneuploidy and large CNVs. Women were eligible for inclusion if they were undergoing invasive testing for identified nuchal translucency or structural anomalies in their fetus, as detected by ultrasound after 11 weeks of gestation. The partners of these women also had to consent to participate. Sequencing results were interpreted with a targeted virtual gene panel for developmental disorders that comprised 1628 genes. Genetic results related to fetal structural anomaly phenotypes were then validated and reported postnatally. The primary endpoint, which was assessed in all fetuses, was the detection of diagnostic genetic variants considered to have caused the fetal developmental anomaly.
The cohort was recruited between Oct 22, 2014, and June 29, 2017, and clinical data were collected until March 31, 2018. After exclusion of fetuses with aneuploidy and CNVs, 610 fetuses with structural anomalies and 1202 matched parental samples (analysed as 596 fetus-parental trios, including two sets of twins, and 14 fetus-parent dyads) were analysed by WES. After bioinformatic filtering and prioritisation according to allele frequency and effect on protein and inheritance pattern, 321 genetic variants (representing 255 potential diagnoses) were selected as potentially pathogenic genetic variants (diagnostic genetic variants), and these variants were reviewed by a multidisciplinary clinical review panel. A diagnostic genetic variant was identified in 52 (8·5%; 95% CI 6·4–11·0) of 610 fetuses assessed and an additional 24 (3·9%) fetuses had a variant of uncertain significance that had potential clinical usefulness. Detection of diagnostic genetic variants enabled us to distinguish between syndromic and non-syndromic fetal anomalies (eg, congenital heart disease only vs a syndrome with congenital heart disease and learning disability). Diagnostic genetic variants were present in 22 (15·4%) of 143 fetuses with multisystem anomalies (ie, more than one fetal structural anomaly), nine (11·1%) of 81 fetuses with cardiac anomalies, and ten (15·4%) of 65 fetuses with skeletal anomalies; these phenotypes were most commonly associated with diagnostic variants. However, diagnostic genetic variants were least common in fetuses with isolated increased nuchal translucency (≥4·0 mm) in the first trimester (in three 3·2% of 93 fetuses).
WES facilitates genetic diagnosis of fetal structural anomalies, which enables more accurate predictions of fetal prognosis and risk of recurrence in future pregnancies. However, the overall detection of diagnostic genetic variants in a prospectively ascertained cohort with a broad range of fetal structural anomalies is lower than that suggested by previous smaller-scale studies of fewer phenotypes. WES improved the identification of genetic disorders in fetuses with structural abnormalities; however, before clinical implementation, careful consideration should be given to case selection to maximise clinical usefulness.
UK Department of Health and Social Care and The Wellcome Trust.
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