arXiv subjects

A. Almoalem

Publications and source records attributed to A. Almoalem.

5 recordsLinked to original sources

arXiv2020

Link between superconductivity and a Lifshitz transition in intercalated Bi$_2$Se$_3$

Topological superconductivity is an exotic phase of matter in which the fully gapped superconducting bulk hosts gapless Majorana surface states protected by topology. Intercalation of copper, strontium or niobium between the quintuple layers of the topological insulator Bi$_2$Se$_3$ increases the carrier density and leads to superconductivity that is suggested to be topological. Here we study the electronic structure of strontium-intercalated Bi$_2$Se$_3$ using angle resolved photoemission spectroscopy (ARPES) and Shubnikov-de Haas (SdH) oscillations. Despite the apparent low Hall number of $\sim2 \times 10 ^{19}$cm$^{-3}$, we show that the Fermi surface is shaped as an open cylinder with a larger carrier density of $\sim 10 ^{20}$cm$^{-3}$. We suggest that superconductivity in intercalated Bi$_2$Se$_3$ emerges with the appearance of a quasi-2D open Fermi surface.

cond-mat.supr-con↗

arXiv2019

Superconductivity and phonon self-energy effects in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$

We study Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ multi-band superconductor with $T_c=14$K by polarization-resolved Raman spectroscopy. Deep in the superconducting state, we detect pair-breaking excitation at 45cm$^{-1}$ ($2Δ=5.6$meV) in the $XY$($B_{2g}$) scattering geometry, consistent with twice of the superconducting gap energy (3 meV) revealed by ARPES on the hole-like Fermi pocket with $d_{xz}/d_{yz}$ character. We analyze the superconductivity induced phonon self-energy effects for the $B_{1g}$(Fe) phonon and estimate the electron-phonon coupling constant $λ^Γ\approx 0.026$, which is insufficient to explain superconductivity with $T_c=14$K.

cond-mat.supr-con↗

arXiv2018

Chiral magnetic textures in Ir/Fe/Co/Pt multilayers: Evolution and topological Hall signature

Skyrmions are topologically protected, two-dimensional, localized hedgehogs and whorls of spin. Originally invented as a concept in field theory for nuclear interactions, skyrmions are central to a wide range of phenomena in condensed matter. Their realization at room temperature (RT) in magnetic multilayers has generated considerable interest, fueled by technological prospects and the access granted to fundamental questions. The interaction of skyrmions with charge carriers gives rise to exotic electrodynamics, such as the topological Hall effect (THE), the Hall response to an emergent magnetic field, a manifestation of the skyrmion Berry-phase. The proposal that THE can be used to detect skyrmions needs to be tested quantitatively. For that it is imperative to develop comprehensive understanding of skyrmions and other chiral textures, and their electrical fingerprint. Here, using Hall transport and magnetic imaging, we track the evolution of magnetic textures and their THE signature in a technologically viable multilayer film as a function of temperature ($T$) and out-of-plane applied magnetic field ($H$). We show that topological Hall resistivity ($ρ_\mathrm{TH}$) scales with the density of isolated skyrmions ($n_\mathrm{sk}$) over a wide range of $T$, confirming the impact of the skyrmion Berry-phase on electronic transport. We find that at higher $n_\mathrm{sk}$ skyrmions cluster into worms which carry considerable topological charge, unlike topologically-trivial spin spirals. While we establish a qualitative agreement between $ρ_\mathrm{TH}(H,T)$ and areal density of topological charge $n_\mathrm{T}(H,T)$, our detailed quantitative analysis shows a much larger $ρ_\mathrm{TH}$ than the prevailing theory predicts for observed $n_\mathrm{T}$.

cond-mat.mes-hall↗

arXiv2018

Stray field signatures of Néel textured skyrmions in Ir/Fe/Co/Pt multilayer films

Skyrmions are nanoscale spin configurations with topological properties that hold great promise for spintronic devices. Here, we establish their Néel texture, helicity, and size in Ir/Fe/Co/Pt multilayer films by constructing a multipole expansion to model their stray field signatures and applying it to magnetic force microscopy (MFM) images. Furthermore, the demonstrated sensitivity to inhomogeneity in skyrmion properties, coupled with a unique capability to estimate the pinning force governing dynamics, portends broad applicability in the burgeoning field of topological spin textures.

physics.app-ph↗

arXiv2016

Diamagnetic Vortex Barrier Stripes in Underdoped $BaFe_2(As_{1-x}P_x)_2$

We report magnetic force microscopy (MFM) measurements on underdoped $BaFe_2(As_{1-x}P_x)_2$ ($x=0.26$) that show enhanced superconductivity along stripes parallel to twin boundaries. These stripes of enhanced diamagnetic response repel vortices when we cool in a finite magnetic field and act as barriers when we drag vortices with the magnetic MFM tip. The stripes disappear when we warm the sample towards the superconducting transition temperature. We show that stripes can move when we warm the sample and comment on the relationship between the stripes and similar stripes observed previously in $Ba(Fe_{1-x}Co_x)_2As_2$.

cond-mat.supr-con↗