arXiv subjects

Samuel Y. Mensah

Publications and source records attributed to Samuel Y. Mensah.

3 recordsLinked to original sources

arXiv2015

Anomalous viscosity of vortex hall states in graphene

We study temperature effect on anomalous viscosity of Graphene Hall fluid within quantum many-vortex hydrodynamics. The commonly observed filling fractions, $ν$ in the range $0 < ν< 2$ is considered. An expression for anomalous viscosity dependent on a geometric parameter-Hall expansion coefficient, is obtained at finite temperatures. It arises from strained induced pseudo-magnetic field in addition to an anomalous term in vortex velocity, which is responsible for re-normalization of vortex-vortex interactions. We observed that both terms greatly modify the anomalous viscosity as well as an enhancement of weakly observed v fractions. Finite values of the expansion coefficient produce a constant and an infinite viscosity at varying temperatures. The infinities are identified as energy gaps and suggest temperatures at which new stable quantum hall filling fractions could be seen. This phenomenon is used to estimate energy gaps of already measured fractional quantum Hall states in Graphene.

cond-mat.mes-hall↗

arXiv2015

Amplification of Hypersound in Graphene with degenerate energy dispersion

Hypersound amplification/absorption of acoustic phonons in Graphene with degenerate energy dispersion $\varepsilon(p)$ near the Fermi level was theoretically studied. For $k_βT << 1$ and $ql >> 1$, the dependence of the absorption coefficient $Γ/Γ_0$ on ${V_D\over V_s}$ was studied where the results satisfied the Cerenkov effect. That is when ${V_D\over V_s} > 1$, an amplification was obtained but for ${V_D\over V_s} < 1$, an absorption was obtained which could lead to Acoustoelectric Effect (AE) in Graphene. A linear dependence of the $Γ/Γ_0$ on $ω_q$ was observed where the result obtianed qualitatively agreed with an experimentally observed acoustoelectric current in Graphene via the Weinrich relation. It is interesting to note from this study that, frequencies above $10THz$ can be attained for $V_D = 1.1ms^{-1}$. This study permit the use of Graphene as hypersound phonon laser (SASER).

cond-mat.mes-hall↗

arXiv2011

Amplification of acoustic phonons in superlattice

The amplification of acoustic phonons in a superlattice in the presence of an electric field $E =E_0 + E_1 cos(ωt)$ has been investigated theoretically and numerically by computational methods. The calculation is done in the hypersound regime $(ql\gg 1)$ where the attenuation coefficient depends on the phonon wave vector $ q =[-\fracπ{q} \quad \frac{2pi}{q}]$ and the frequency $ω_q = 10^{12} s^{-1}$ . An inversion is attained where amplification far exceeds absorption and the ratio $ frac\mid\frac{Γ/Γ_0\mid_{min}}{\midΓ/Γ_0\mid_{max}}\approx 3$ . A high frequency build up of acoustic energy from noise (phonon spectrum) is obtained by using specialized spectral techniques. This indicates an amplification of the phonons generated in the terahertz range leading to the possibility of obtaining a hypersound MASER.

cond-mat.mes-hall↗