Wigner function for noninteracting fermions in hard wall potentials – Archive ouverte HAL

Benjamin de Bruyne ¹ David S. Dean ² Pierre Le Doussal ³ Satya N. Majumdar ¹ Gregory Schehr ⁴

Benjamin de Bruyne, David S. Dean, Pierre Le Doussal, Satya N. Majumdar, Gregory Schehr. Wigner function for noninteracting fermions in hard wall potentials. Physical Review A, American Physical Society 2021. ⟨hal-03301503⟩

The Wigner function $W_N({\bf x}, {\bf p})$ is a useful quantity to characterize the quantum fluctuations of an $N$-body system in its phase space. Here we study $W_N({\bf x}, {\bf p})$ for $N$ noninteracting spinless fermions in a $d$-dimensional spherical hard box of radius $R$ at temperature $T=0$. In the large $N$ limit, the local density approximation (LDA) predicts that $W_N({\bf x}, {\bf p}) \approx 1/(2 \pi \hbar)^d$ inside a finite region of the $({\bf x}, {\bf p})$ plane, namely for $|{\bf x}| < R$ and $|{\bf p}| < k_F$ where $k_F$ is the Fermi momentum, while $W_N({\bf x}, {\bf p})$ vanishes outside this region, or "droplet", on a scale determined by quantum fluctuations. In this paper we investigate systematically, in this quantum region, the structure of the Wigner function along the edge of this droplet, called the Fermi surf. In one dimension, we find that there are three distinct edge regions along the Fermi surf and we compute exactly the associated nontrivial scaling functions in each regime. We also study the momentum distribution $\hat \rho_N(p)$ and find a striking algebraic tail for very large momenta $\hat \rho_N(p) \propto 1/p^4$, well beyond $k_F$, reminiscent of a similar tail found in interacting quantum systems (discussed in the context of Tan's relation). We then generalize these results to higher $d$ and find, remarkably, that the scaling function close to the edge of the box is universal, i.e., independent of the dimension~$d$.

• 1. LPTMS – Laboratoire de Physique Théorique et Modèles Statistiques
• 2. LOMA – Laboratoire Ondes et Matière d’Aquitaine
• 3. LPENS (UMR_8023) – Laboratoire de physique de l’ENS – ENS Paris
• 4. LPTHE – Laboratoire de Physique Théorique et Hautes Energies