arXiv subjects

X. L. Wu

Publications and source records attributed to X. L. Wu.

6 recordsLinked to original sources

arXiv2022

Spin-phonon coupling driven Charge density wave in a Kagome Magnet

The intertwining between spin, charge, and lattice degrees of freedom can give rise to unusual macroscopic quantum states, including high-temperature superconductivity and quantum anomalous Hall effects. Recently, a charge density wave (CDW) is observed in the kagome antiferromagnet FeGe, indicative of possible intertwining physics. An outstanding question is that whether magnetic correlation is fundamental for the spontaneous spatial symmetry breaking orders. Here, utilizing elastic and high-resolution inelastic x-ray scattering, we discover a charge dimerization superlattice that coexists with the 2$\times$2$\times$1 CDW in the kagome sublattice. Most interestingly, between the magnetic and CDW transition temperature, the phonon dynamical structure factor shows a giant phonon-energy hardening and a substantial phonon linewidth broadening near the charge-dimerization wavevectors, both signaling the spin-phonon coupling. By first principles calculations, we show that both the static and dynamic spin excitations intertwine with the phonon to drive the spatial symmetry breaking.

cond-mat.str-el↗

arXiv2018

Tunneling of Micro-sized Droplets Through a Flowing Soap Film

When a micron-sized water droplet impacts on a freely suspended soap film with speed $v_{i}$, there exists a critical impact velocity of penetration $v_{C}$. For the droplet with $v_{i} v_{C}$, it tunnels through. In all cases, the film remains intact despite the fact that the droplet radius ($R_{0}=26\,μm$) is much greater than the film thickness ($0<h\lesssim10\,μm$). The critical velocity $v_{C}$ was measured as a function of $h$, and interestingly $v_{C}$ approaches an asymptotic value $v_{C0}\simeq520\,$ cm/s in the limit $h\rightarrow0$. This indicates that in addition to an inertial effect, a deformation or stretching energy of the film is required for penetration. Quantitatively, we found that this deformation energy corresponds to the creation of $\sim14$ times of the cross-sectional area of the droplet ($14πR_{0}^{2}$) or a critical Weber number ${\rm We_{C} } (\equiv2R_{0}ρ_{w}v_{C0}^{2}/σ)\simeq44$, where $ρ_{w}$ and $σ$ are respectively the density and the surface tension of water. Key results: The interaction between liquid droplet and soap films is studied. When the impact velocity is higher than a critical velocity, the droplet penetrates the soap film without breaking it. The experimental results are rationalized using the mechanical collision model with the film stretching.

physics.flu-dyn↗

arXiv2017

A Unified Strouhal-Reynolds Number Relationship for Laminar Vortex Streets Generated by Different Shaped Obstacles

A new Strouhal-Reynolds number relationship, $St=1/(A+B/Re)$, has been recently proposed based on observations of laminar vortex shedding from circular cylinders in a flowing soap film. Since the new $St$-$Re$ relation was derived from a general physical consideration, it raises the possibility that it may be applicable to vortex shedding from bodies other than circular ones. The work presented herein provides experimental evidence that this is the case. Our measurements also show that in the asymptotic limit ($Re\rightarrow\infty$), $St_{\infty}=1/A\simeq0.21$ is constant independent of rod shapes, leaving $B$ the only parameter that is shape dependent.

physics.flu-dyn↗

arXiv2008

Power-law scaling in protein synthesis of a stochastic regulon

We investigate the protein expression pattern of the lamB gene in Escherichia coli LE392. The gene product LamB is an important membrane protein for maltose transport into cells but it is also exploited by bacteriophage lambda for infection. Although our bacterial population is clonal, stochastic gene expression leads to a majority population with a large receptor number and a minority population with a small receptor number. We find that the LamB receptor distribution p(n) of the small-n population is scale invariant with the exponent depending on growth conditions. A heuristic model is proposed that relates the observed exponent to the protein production rate.

q-bio.CB↗

arXiv2005

Swimming Efficiency of Bacterium Escherichia Coli

We use in vivo measurements of swimming bacteria in an optical trap to determine fundamental properties of bacterial propulsion. In particular, we determine the propulsion matrix, which relates the angular velocity of the flagellum to the torques and forces propelling the bacterium. From the propulsion matrix dynamical properties such as forces, torques, swimming speed and power can be obtained from measurements of the angular velocity of the motor. We find significant heterogeneities among different individuals even though all bacteria started from a single colony. The propulsive efficiency, defined as the ratio of the propulsive power output to the rotary power input provided by the motors, is found to be 0.2%.

physics.bio-ph↗

arXiv2000

Hysteresis at low Reynolds number: the onset of 2D vortex shedding

Hysteresis has been observed in a study of the transition between laminar flow and vortex shedding in a quasi-two dimensional system. The system is a vertical, rapidly flowing soap film which is penetrated by a rod oriented perpendicular to the film plane. Our experiments show that the transition from laminar flow to a periodic Kármán vortex street can be hysteretic, i.e. vortices can survive at velocities lower than the velocity needed to generate them.

cond-mat.soft↗