Yusheng Li — Page 2

arXiv2015

Anomalous valley polarization in monolayer MoSe2

Modern electronic devices heavily rely on the accurate control of charge and spin of electrons. The emergence of controllable valley degree of freedom brings new possibilities and presents a promising prospect towards valleytronics. Recently, valley excitation selected by chiral optical pumping has been observed in monolayer MoS2. In this work, we report polarized photoluminescence (PL) measurements for monolayer MoSe2, another member of the family of transition-metal-dichalcogenides (MX2), and observe drastic difference from the outcomes of MoS2. In particular, we identify a valley polarization (VP) up to 70% for B exciton, while that for A exciton is less than 3%. Besides, we also find a small but finite negative VP for A- trion. These results reveal several new intra- and inter-valley scattering processes which significantly affect valley polarization, hence provide new insights into exciton physics in monolayer MX2 and possible valleytronic applications.

cond-mat.mtrl-sci↗

arXiv2014

A Folkman Linear Family

For graphs $F$ and $G$, let $F\to (G,G)$ signify that any red/blue edge coloring of $F$ contains a monochromatic $G$. Define Folkman number $f(G;p)$ to be the smallest order of a graph $F$ such that $F\to (G,G)$ and $ω(F) \le p$. It is shown that $f(G;p)\le cn$ for graphs $G$ of order $n$ with $Δ(G)\le Δ$, where $Δ\ge 3$, $c=c(Δ)$ and $p=p(Δ)$ are positive constants.

math.CO↗

arXiv2014

Ramsey numbers of paths and graphs of the same order

For graphs $F_n$ and $G_n$ of order $n$, if $R(F_n, G_n)=(χ(G_n)-1)(n-1)+σ(G_n)$, then $F_n$ is said to be $G_n$-good, where $σ(G_n)$ is the minimum size of a color class among all proper vertex-colorings of $G_n$ with $χ(G_n)$ colors. Given $Δ(G_n)\le Δ$, it is shown that $P_n$ is asymptotically $G_n$-good if $α(G_n)\le\frac{n}{4}$.

math.CO↗