arXiv subjects

William Schneider

Publications and source records attributed to William Schneider.

6 recordsLinked to original sources

arXiv2025

Cash Transfers in the Perinatal Period and Child Welfare System Involvement Among Infants: Evidence from the Rx Kids Program in Flint, Michigan

Infants are most vulnerable to child maltreatment, which may be due in part to economic instability during the perinatal period. In 2024, Rx Kids was launched in Flint, Michigan, achieving near 100% aggregate take up and providing every expectant mother with unconditional cash transfers during pregnancy and infancy. Synthetic difference-in-differences was used to compare changes in allegations of maltreatment within the first six months of life in Flint before and after implementation of Rx Kids relative to the corresponding change in control cities without the program. In the three years prior to the implementation of Rx Kids, the proportion of infants with a maltreatment allegation within the first six months of life was 21.7% in Flint and 19.5% among control cities. After implementation of Rx Kids in 2024, the maltreatment allegation rate dropped to 15.5% in Flint, falling below the maltreatment allegation rate of 20.6% among the control cities. Rx Kids was associated with a statistically significant 7.0 percentage-point decrease in the maltreatment allegation rate (p = 0.021), corresponding to a 32% decrease relative to the pre-intervention period. There was a decrease in the rate of neglect-related, non-neglect-related, and substantiated allegations; these were directionally consistent with the primary outcome but not statistically significant. Results were robust to alternative model specifications. The Rx Kids prenatal and infant cash prescription program led to a significant reduction in allegations of maltreatment among infants. These findings provide important evidence about the role of economic stability in preventing child welfare system involvement.

econ.GN↗

arXiv2015

Band Anticrossing in Dilute Germanium Carbides Using Hybrid Functionals

Dilute germanium carbides (Ge1-xCx) offer a direct bandgap for compact silicon photonics, but widely varying results have been reported. This work uses ab initio simulations with HSE06 hybrid functionals and spin-orbit coupling to study the Ge1-xCx band structure behavior in the absence of defects. Contrary to Vegard's law, the conduction band minimum at k=0 is consistently found to decrease with increasing C content, while L and X valleys change much more slowly. A vanishing bandgap was observed for all alloys with x>0.017. Conduction bands deviate from a constant-potential band anticrossing model except near the center of the Brillouin zone.

physics.comp-ph↗

arXiv2012

Colliding clouds of strongly interacting spin-polarized fermions

Motivated by a recent experiment at MIT, we consider the collision of two clouds of spin-polarized atomic Fermi gases close to a Feshbach resonance. We explain why two dilute gas clouds, with underlying attractive interactions between their constituents, bounce off each other in the strongly interacting regime. Our hydrodynamic analysis, in excellent agreement with experiment, gives strong evidence for a metastable many-body state with effective repulsive interactions.

cond-mat.quant-gas↗

arXiv2010

Universal Short-Distance Structure of the Single-Particle Spectral Function of Dilute Fermi Gases

We show that the universal $1/k^4$ tail in the momentum distribution of dilute Fermi gases implies that the spectral function $A(\kk,ω)$ must have weight below the chemical potential for large momentum $k \gg k_F$, with observable consequences in RF spectroscopy experiments. We find that this incoherent spectral weight is centered about $ω\simeq - ε(\kk)$ in a range of energies of order $v_F k$. This "bending back" in the dispersion, while natural for superfluids, is quite surprising for normal gases. This universal structure is present in the hard-sphere gas as well as the Fermi liquid ground state of the highly imbalanced, attractive gas near unitarity. We argue that, even in the BCS superfluid, this bending back at large $k$ is dominated by interaction effects which do not reflect the pairing gap.

cond-mat.quant-gas↗

arXiv2009

Theory of Radio Frequency Spectroscopy of Polarized Fermi Gases

We present two exact results for singular features in the radio frequency intensity $I(ω)$ for ultracold Fermi gases. First, in the absence of final state interactions, $I(ω)$ has a universal high frequency tail $ Cω^{-3/2}$ for \emph{all} many-body states, where $C$ is Tan's contact. Second, in a \emph{normal} Fermi liquid at T=0, $I(ω)$ has a jump discontinuity of $Z/(1 - m/m^{*})$, where $Z$ is the quasiparticle weight and $m^*/m$ the mass renormalization. We then describe various approximations for $I(ω)$ in polarized normal gases. We show why an approximation that is exact in the $n_\dn=0$ limit, fails qualitatively for $n_{\dn} > 0$: there is no universal tail and sum rules are violated. The simple ladder approximation is qualitatively correct for very small $n_{\dn}$, but not quantitatively.

cond-mat.other↗

arXiv2007

Breakdown of the Thomas Fermi approximation for polarized Fermi gases

We use Bogoliubov de-Gennes theory to show that the commonly used Thomas-Fermi approximation (TFA) can fail in describing polarized unitary gases in anisotropic harmonic traps. We find a magnetized superfluid region inside the trap, with order parameter oscillations, even though there is no such stable bulk phase. This leads to magnetization profiles that deviate from contours of constant potential energy. We determine how this violation scales with trap anisotropy and number of particles, and show that we are able to account for important differences between the MIT and Rice experiments.

cond-mat.supr-con↗