arXiv subjects

Harsojo

Publications and source records attributed to Harsojo.

7 recordsLinked to original sources

arXiv2024

Anisotropic spin-split states with canted persistent spin textures in two-dimensional Janus $1T^{'}$ $MXX'$ ($M$ = Mo, W; $X\neq X'$= S, Se, Te) controlled by surface alloying

Two-dimensional tungsten-based transition metal dichalcogenides (TMDCs), $MX_{2}$ ($M$: W, Mo; $X$: S, Se, Te) monolayers (MLs) with a $1T'$ structure, serve as significant-gap quantum spin Hall insulators. However, due to the centrosymmetric nature of these crystals, spin degeneracy persists throughout their electronic band structures, limiting their potential for spintronic applications. By modifying the chalcogen ($X$) atoms in the TMDCs ML surface to create a higly stable Janus $MXX'$ MLs structure, we demonstrate through density-functional theory calculations that substantial spin splitting of the electronic states can be achieved. Taking the Janus $1T'$ WSTe ML as a representative case, we identify pronounced anisotropic spin-splitting bands, with maximum spin splittings of 0.14 eV and 0.10 eV occurring at the highest occupied states and lowest unoccupied states, respectively. These significant band splittings give rise to canted persistent spin textures (PST) in the spin polarization, which differ significantly from those in commonly studied PST materials. We demonstrate that this intricate spin splitting and unique spin textures stem from strong in-plane $p-d$ orbital interactions between tungsten (W) and the chalcogen atoms (Te and Se), driven by the reduced symmetry of the crystal's point group. Further analysis using a $\vec{k}\cdot\vec{p}$ model derived from symmetry considerations corroborates the origins of the observed anisotropic spin splitting and canted PST. the spin-split states are highly sensitive to surface imperfections caused by surface alloying effects, such as variations in the chalcogen composition on the monolayer surface. These findings underscore the potential of Janus $1T'$ $MXX'$ MLs as promising candidates for next-generation spintronic devices.

cond-mat.mes-hall↗

arXiv2023

Strong unidirectional Rashba state induced by extended vacancy line defects in a $1T'$-WTe$_{2}$ monolayer

The correlation between spin-orbit coupling and low crystal symmetry in the $1T'$ phase of the tungsten ditellurides (WTe$_{2}$) monolayer (ML) plays a significant role in its electronic and topological properties. However, the centrosymmetric nature of the crystal maintains Kramer's spin degeneracy in its electronic states, which limits its functionality in spintronics. In this paper, through a systematic study using first-principles calculations, we show that significant spin splitting can be induced in the $1T'$-WTe$_{2}$ ML by introducing one dimensional (1D) vacancy line defect (VLD). We examine six configurations of the 1D VLD, which consist of three VLDs extended in the armchair direction including a Te$_{1}$ armchair-VLD ($ACV_{\texttt{Te}_{1}}$), Te$_{2}$ armchair-VLD ($ACV_{\texttt{Te}_{2}}$), and W armchair-VLD ($ACV_{\texttt{W}}$); and three VLDs elongated along the zigzag direction comprising a Te$_{1}$ zigzag-VLD ($ZZV_{\texttt{Te}_{1}}$), Te$_{2}$ zigzag-VLD ($ZZV_{\texttt{Te}_{2}}$), and W zigzag-VLD ($ZZV_{\texttt{W}}$), where Te$_{1}$ and Te$_{2}$ are two nonequivalent Te atoms located at the lower and higher sites in the top layer, respectively. We find that both the $ACV_{\texttt{Te}_{1}}$ and $ACV_{\texttt{W}}$ systems have the lowest formation energy. Concerning these two most stable VLD systems, we identify large spin splitting in the defect states near the Fermi level driven by a strong coupling of the in-plane $p-d$ orbitals, displaying highly unidirectional Rashba states with perfectly collinear spin configurations in the momentum space. This unique spin configuration gives rise to a specific spin mode that protects the spin from decoherence and leads to an exceptionally long spin lifetime...........

cond-mat.mes-hall↗

arXiv2023

Emergence of Rashba spin valley state in two-dimensional strained bismuth oxychalcogenides Bi$_{2}$O$_{2}$Se

The experimental evidence of the ultra-high electron mobility and strong spin-orbit coupling in the two-dimensional (2D) layered bismuth-based oxyselenide, Bi$_{2}$O$_{2}$Se, makes it a potential material for spintronic devices. However, its spin-related properties have not been extensively studied due to the centrosymmetric nature of its crystal structure. By using first-principles density-functional theory calculation, this study reports the emergence of Rashba-spin-valley states in Bi$_{2}$O$_{2}$Se monolayer (ML). Breaking the crystal inversion symmetry of Bi$_{2}$O$_{2}$Se ML using an external electric field enables the Rashba-spin-valley formation, causing the appearance of the Rashba-type splitting around the $\Gamma$ valley and spin-valley coupling at the $D$ valleys located near the middle of $\Gamma-M$ line. In addition to the typical Rashba-type spin textures around the $\Gamma$ valley, the study also observed in-plane unidirectional spin textures around the $D$ valleys, which is a rare phenomenon in 2D materials. The observed Rashba-spin-valley states are driven by the lowering point group symmetry of the crystal from $D_{4h}$ to $C_{4v}$ enforced by the electric field, as clarified through $\vec{k}\cdot\vec{p}$ model derived from symmetry analysis. More importantly, tuning the Rashba and spin-valley states by using biaxial strain offers a promising route to regulate the spin textures and spin splitting preventing the electron from back-scattering in spin transport. Finally, we proposed a more realistic system, namely, Bi$_{2}$O$_{2}$Se ML/SrTiO$_{3}$ (001) heterointerface that supports the strong Rashba-spin-valley states and highlighting the potential of the Bi$_{2}$O$_{2}$Se ML for future spintronics and valleytronics-based devices.

cond-mat.str-el↗

arXiv2021

Highly persistent spin textures with giant tunable spin splitting in the two-dimensional germanium monochalcogenides

The ability to control the spin textures in semiconductors is a fundamental step toward novel spintronic devices, while seeking desirable materials exhibiting persistent spin texture (PST) remains a key challenge. The PST is the property of materials preserving a unidirectional spin orientation in the momentum space, which has been predicted to support an extraordinarily long spin lifetime of carriers. Herein, by using first-principles density functional theory calculations, we report the emergence of the PST in the two-dimensional (2D) germanium monochalcogenides (GeMC). By considering two stable formations of the 2D GeMC, namely the pure GeX and Janus Ge2XY monolayers (X, Y = S, Se, and Te), we observed the PST around the valence band maximum where the spin orientation is enforced by the lower point group symmetry of the crystal. In the case of the pure GeX monolayers, we found that the PST is characterized by fully out-of-plane spin orientation protected by C2v point group, while the canted PST in the y-z plane is observed in the case of the Janus Ge2XY monolayers due to the lowering symmetry into Cs point group. More importantly, we find large spin-orbit coupling (SOC) parameter in which the PST sustains, which could be effectively tuned by in-plane strain. The large SOC parameter observed in the present systems leads to the small wavelength of the spatially periodic mode of the spin polarization, which is promising for short spin channel in the spin Hall transistor devices.

cond-mat.str-el↗

arXiv2018

Strong Rashba effect in the localized impurity states of halogen-doped monolayer PtSe2

The recent epitaxial growth of 1T-phase of PtSe2 monolayer (ML) has opened a possibility for its novel applications, in particular for spintronics device. However, in contrast to 2H-phase of transition-metal dichalcogenides (TMDs), the absence of spin splitting in the PtSe2 ML may limit the functionality for spintronics application. Through fully-relativistic density-functional theory calculations, we show that large spin splitting can be induced in the PtSe2 ML by introducing a substitutional halogen impurity. Depending on the atomic number (Z) of the halogen dopants, we observe an enhancement of the spin splitting in the localized impurity states (LIS), which is due to the increased contribution of the p-d orbitals coupling. More importantly, we identify very large Rashba splitting in the LIS near Fermi level around the Gamma point characterized by hexagonal warping of the Fermi surface. We show that the Rashba splitting can be controlled by adjusting the doping concentration. Therefore, this work paves a possible way to induce the significant Rashba splitting in the two-dimensional TMDs, which is useful for spintronic devices operating at room temperature.

cond-mat.str-el↗

arXiv2017

Defect-induced large spin-orbit splitting in the monolayer of PtSe$_2$

The effect of spin-orbit coupling (SOC) on the electronic properties of monolayer (ML) PtSe$_2$ is dictated by the presence of the crystal inversion symmetry to exhibit spin polarized band without characteristic of spin splitting. Through fully-relativistic density-functional theory calculations, we show that large spin-orbit splitting can be induced by introducing point defects. We calculate stability of native point defects such as a Se vacancy (V$_{\texttt{Se}}$), a Se interstitial (Se$_{i}$), a Pt vacancy (V$_{\texttt{Pt}}$), and a Pt interstitial (Pt$_{i}$), and find that both the V$_{\texttt{Se}}$ and Se$_{i}$ have the lowest formation energy. We also find that in contrast to the Se$_{i}$ case exhibiting spin degeneracy in the defect states, the large spin-orbit splitting up to 152 meV is observed in the defect states of the V$_{\texttt{Se}}$. Our analyses of orbital contributions to the defect states show that the large spin splitting is originated from the strong hybridization between Pt-$d_{x{^2}+y{^2}}+d_{xy}$ and Se-$p_{x}+p_{y}$ orbitals. Our study clarifies that the defects play an important role in the spin splitting properties of the PtSe$_2$ ML, which is important for designing future spintronic devices.

cond-mat.str-el↗

arXiv2017

Polarity tuning of spin-orbit-induced spin splitting in two-dimensional transition metal dichalcogenides semiconductors

The established spin splitting in monolayer (ML) of transition metal dichalcogenides (TMDs) that is caused by inversion symmetry breaking is dictated by mirror symmetry operations to exhibit fully out-of-plane direction of spin polarization. Through first-principles density functional theory calculations, we show that polarity-induced mirror symmetry breaking leads to new sizable spin splitting having in-plane spin polarization. These splittings are effectively controlled by tuning the polarity using biaxial strain. Furthermore, the admixtures of the out-of-plane and in-plane spin-polarized states in the strained polar systems are identified, which is expected to influence the spin relaxation through the Dyakonov-Perel mechanism. Our study clarified that the polarity-induced mirror symmetry breaking plays an important role in controlling the spin splitting and spin relaxation in the TMDs ML, which is useful for designing future spintronic devices.

cond-mat.str-el↗