arXiv subjects

Moh. Adhib Ulil Absor

Publications and source records attributed to Moh. Adhib Ulil Absor.

17 recordsLinked to original sources

arXiv2025

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↗

arXiv2025

One-dimensional confined Rashba states in a two-dimensional Si$_{2}$Bi$_{2}$ induced by vacancy line defects

Advanced defect engineering techniques have enabled the creation of unique quantum phases from pristine materials. One-dimensional (1D) atomic defects in low-dimensional systems are particularly intriguing due to their distinct quantum properties, such as 1D Rashba states that allow for the generation of nondissipative spin currents, making them ideal for spintronic devices. Using density-functional calculations and model-based symmetry analysis, we report the emergence of 1D Rashba states in a two-dimensional Si$_{2}$Bi$_{2}$ monolayer (ML) with vacancy line defects (VLDs). We show that introducing VLDs in the Si$_{2}$Bi$_{2}$ ML induces 1D confined defect states near the Fermi level, which are strongly localized along the extended defect line. Notably, we observed 1D Rashba spin-split bands in these defect states with significant spin splitting originating mainly from the strong $p-p$ coupling orbitals between Si and Bi atoms near the defect sites. These spin-split defect states exhibit perfectly collinear spin polarization in momentum $\vec{k}$-space, which is oriented perpendicularly to the VLD orientation. Moreover, using $\vec{k}\cdot\vec{p}$ perturbation theory supplemented with symmetry analysis, we show that the 1D Rashba states with collinear spin polarization are enforced by the lowering of symmetry of the VLDs into the $C_{s}$ point group, which retains the $M_{xz}$ mirror symmetry along with the 1D nature of the VLDs. The observed 1D Rashba states in this system protect carriers against spin decoherence and support an exceptionally long spin lifetime, which could be promising for developing highly efficient spintronic devices.

cond-mat.str-el↗

arXiv2024

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

Anisotropic Rashba splitting dominated by out-of-plane spin polarization in two-dimensional Janus $XA_{2}Y$ ($A$= Si, Sn, Ge; $X,Y$= Sb, Bi) with surface imperfection

The anisotropic Rashba effect allows for the manipulation of electron spins in a more precise and tunable manner since the magnitude of the Rashba splitting and orientation of the spin textures can be simply controlled by tuning the direction of the externally applied electric field. Herein, we predict the emergence of the anisotropic Rashba effect in the two-dimensional (2D) Janus $XA_{2}Y$ constructed from the group IV ($A$= Si, Sn, Ge) and group V ($X,Y$ = Sb, Bi) elements having trigonal prismatic structures but lacking in-plane mirror symmetry. Due to the lowering point group symmetry of the crystal, the Rashba spin splitting is enforced to becomes anisotropic around certain high symmetry points in the Brillouin zone and preserves the out-of-plane spin textures. We illustrate this behavior using density functional theory calculations supplemented with $\vec{k}\cdot\vec{p}$ analysis on the Janus SbSi$_{2}$Bi monolayer as a representative example. Specifically, we observed large and anisotropic Rashba splitting with prominence contribution of the out-of-plane spin textures in the conduction band minimum around the $M$ point and valence band maximum around the $Γ$ point. More importantly, the anisotropic spin splitting and out-of-plane spin polarization are sensitively affected by surface imperfections, depending on the concentration and configuration of the $X$ and $Y$ elements in the 2D Janus $XA_{2}Y$ surface. Our study offer the possibility to realize the present systems for spintronics applications.

cond-mat.mtrl-sci↗

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 $Γ$ valley and spin-valley coupling at the $D$ valleys located near the middle of $Γ-M$ line. In addition to the typical Rashba-type spin textures around the $Γ$ 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↗

arXiv2023

Reversible canted persistent spin textures with large spin splitting in two-dimensional ferroelectric bilayer WTe$_{2}$

The recent discovery of materials hosting persistent spin texture (PST) opens an avenue for the realization of energy-saving spintronics since they support an extraordinarily long spin lifetime. However, the stability of the PST is sensitively affected by symmetry breaking of the crystal induced by external perturbation such as the electric field. In this paper, through first-principles calculations supplemented by symmetry analysis, we report the emergence of the robust and stable PST with large spin splitting in the two-dimensional ferroelectric bilayer WTe$_{2}$. Due to the low symmetry of the crystal ($C_{s}$ point group), we observe a canted PST in the spin-split bands around the Fermi level displaying a unidirectional spin configuration tilted along the $yz$ plane in the first Brillouin zone. Such a typical PST can be effectively reversed by out-of-plane ferroelectric switching induced by interlayer sliding along the in-plane direction. We further demonstrated that the reversible PST is realized by the application of an out-of-plane external electric field. Thus, our findings uncover the possibility of an electrically tunable PST in the 2D materials, offering a promising platform for highly efficient spintronics devices.

cond-mat.str-el↗

arXiv2022

Full-zone Persistent Spin Textures with Giant Spin Splitting in Two-dimensional Group IV-V Compounds

Persistent spin texture (PST), a property of solid-state materials maintaining unidirectional spin polarization in the momentum $k$-space, offers a route to deliver the necessary long carrier spin lifetimes through the persistent spin helix (PSH) mechanism. However, most of the discovered PST locally occurred in the small part around certain high symmetry $k$-points or lines in the first Brillouin zone (FBZ), thus limiting the stability of the PSH state. Herein, by symmetry analysis and first-principles calculations, we report the emergence of full-zone PST (FZPST), a phenomenon displaying the PST in the whole FBZ, in the two-dimensional group IV-V $A_{2}B_{2}$ ($A$ = Si, Sn, Ge; $B$ = Bi, Sb) compounds. Due to the existence of the in-plane mirror symmetry operation in the wave vector point group symmetry for the arbitrary $\vec{k}$ in the whole FBZ, fully out-of-plane spin polarization is observed in the $k$-space, thus maintaining the FZPST. Importantly, we observed giant spin splitting in which the PST sustains, supporting large SOC parameters and small wavelengths of the PSH states. Our $\vec{k}\cdot\vec{p}$ analysis demonstrated that the FZPST is robust for the non-degenerate bands, which can be effectively controlled by the application of an external electric field, thus offering a promising platform for future spintronic applications.

cond-mat.str-el↗

arXiv2021

Reversible giant out-of-plane Rashba effect in two-dimensional Ga$XY$ ($X$= Se, Te; $Y$= Cl, Br, I) compounds for persistent spin helix

The coexistence of ferroelectricity and spin-orbit coupling (SOC) in noncentrosymmetric systems may allow for a nonvolatile control of spin degrees of freedom by switching the ferroelectric polarization through the well-known ferroelectric Rashba effect (FRE). Although the FER has been widely observed for bulk ferroelectric systems, its existence in two-dimensional (2D) ferroelectric systems is still very rarely discovered. Based on first-principles calculations, supplemented with $\vec{k}\cdot\vec{p}$ analysis, we report the emergence of the FRE in the Ga$XY$ ($X$= Se, Te; $Y$= Cl, Br, I) monolayer compounds, a new class of 2D materials having in-plane ferroelectricity. Due to the large in-plane ferroelectric polarization, a giant out-of-plane Rashba effect is observed in the topmost valence band, producing unidirectional out-of-plane spin textures in the momentum space. Importantly, such out-of-plane spin textures, which can host a long-lived helical spin mode known as a persistent spin helix, can be fully reversed by switching the direction of the in-plane ferroelectric polarization. Thus, our findings can open avenues for interplay between the unidirectional out-of-plane Rashba effect and the in-plane ferroelectricity in 2D materials, which is useful for efficient and non-volatile spintronic 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↗

arXiv2021

Large band splitting with tunable spin polarization in two-dimensional ferroelectric GaXY (X= Se, Te; Y= Cl, Br, I) family

It has been generally accepted that the spin-orbit coupling effect in noncentrosymmetric materials leads to the band splitting and non-trivial spin polarization in the momentum space. However, in some cases, zero net spin polarization in the split bands may occurs, dubbed as the band splitting with vanishing spin polarization (BSVSP) effect, protected by non-pseudo-polar point group symmetry of the wave vector in the first Brillouin zone [Liu et. al., Nat. Commun. \textbf{10}, 5144 (2019)]. In this paper, by using first-principles calculations, we show that the BSVSP effect emerges in two-dimensional (2D) nonsymmorphic Ga$XY$ ($X$= Se, Te; $Y$= Cl, Br, I) family, a new class of 2D materials having in-plane ferroelectricity. Taking the GaTeCl monolayer as a representative example, we observe the BSVSP effect in the split bands along the $X-M$ line located in the proximity of the conduction band minimum. By using $\vec{k}\cdot\vec{p}$ Hamiltonian derived based on the symmetry analysis, we clarify that such effect is originated from the cancellation of the local spin polarization, enforced by non-pseudo-polar $C_{2v}$ point group symmetry of the wave vector along the $X-M$ line. Importantly, we find that the spin polarization can be effectively induced by applying an external out-of-plane electric field, indicating that an electrically tunable spin polarization for spintronic applications is plausible.

cond-mat.mtrl-sci↗

arXiv2020

Spin splitting with persistent spin textures induced by the line defect in 1T-phase of monolayer transition metal dichalcogenides

The spin splitting driven by spin-orbit coupling in monolayer (ML) transition metal dichalcogenides (TMDCs) family has been widely studied only for the 1H-phase structure, while it is not profound for the 1T-phase structure due to the centrosymmetric of the crystal. Based on first-principles calculations, we show that significant spin splitting can be induced in the ML 1T-TMDCs by introducing the line defect. Taking the ML PtSe2 as a representative example, we considered the most stable form of the line defects, namely Se-vacancy line defect (Se-VLD). We find that large spin splitting is observed in the defect states of the Se-VLD, exhibiting a highly unidirectional spin configuration in the momentum space. This peculiar spin configuration may yield the so-called persistent spin textures (PST), a specific spin structure resulting in protection against spin-decoherence and supporting an extraordinarily long spin lifetime. Moreover, by using k.p perturbation theory supplemented with symmetry analysis, we clarified that the emerging of the spin splitting maintaining the PST in the defect states is originated from the inversion symmetry breaking together with one-dimensional nature of the Se-VLD engineered ML PtSe2. Our findings pave a possible way to induce the significant spin splitting in the ML 1T-TMDCs, which could be highly important for designing spintronic devices.

cond-mat.str-el↗

arXiv2019

Intrinsic persistent spin helix in two-dimensional group-IV monochalcogenide MX (M : Sn, Ge; X: S, Se, Te) monolayer

Energy-saving spintronics are believed to be implementable on the systems hosting persistent spin helix (PSH) since they support an extraordinarily long spin lifetime of carriers. However, achieving the PSH requires a unidirectional spin configuration in the momentum space, which is practically non-trivial due to the stringent conditions for fine-tuning the Rashba and Dresselhaus spin-orbit couplings. Here, we predict that the PSH can be intrinsically achieved on a two-dimensional (2D) group-IV monochalcogenide M X monolayer, a new class of the noncentrosymmetric 2D materials having in-plane ferroelctricity. Due to the C2v point group symmetry in the MX monolayer, a unidirectional spin configuration is preserved in the out-of-plane direction and thus maintains the PSH that is similar to the [110] Dresselhaus model in the [110]-oriented quantum well. Our first-principle calculations on various MX (M : Sn, Ge; X: S, Se, Te) monolayers confirmed that such typical spin configuration is observed, in particular, at near the valence band maximum where a sizable spin splitting and a substantially small wavelength of the spin polarization are achieved. Importantly, we observe reversible out-of-plane spin orientation under opposite in-plane ferroelectric polarization, indicating that an electrically controllable PSH for spintronic applications is plausible.

cond-mat.str-el↗

arXiv2018

Doping-induced persistent spin helix with large spin splitting in monolayer SnSe

Finding a new class of materials supporting a long spin lifetime is essential in development of energy-saving spintronics, which is achievable by using a persistent spin helix (PSH) materials. However, for spintronic devices, the PSH states with large spin splitting is required for operation at room temperature. By employing first-principles calculations, we show that the PSH states with large spin splitting are achieved in the SnSe monolayer (ML) functionalized by a substitutional halogen impurity. We find the PSH states in the Fermi level where k-space Fermi surface is characterized by the shifted two loops, dominated by out-of-plane spin orientations. We clarify the PSH states in term of an effective k.p Hamiltonian obtained from symmetry consideration. Finally, large spin-orbit strength in the PSH states with a substantially small wavelength are found, rendering that this system is promising for the development of an efficient and high-density scalable spintronic devices operating at room temperatures.

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↗

arXiv2016

Strain-controlled spin splitting in the conduction band of monolayer WS2

Spin splitting bands that arises in conduction band minimum (CBM) of WS2 monolayer (ML) play an important role in the new spin-orbit phenomena such as spin-valley coupled electronics. However, application of strain strongly modifies electronic properties of the WS2 ML, which is expected to significantly affect to the properties of the spin splitting bands. Here, by using fully-relativistic first-principles calculations based on density-functional theory, we show that a substantial spin spliting bands observed in the CBM is effectively controlled and tuned by applying the biaxial strain. We also find that these spin splitting bands induce spin textures exhibiting fully out-of-plane spin polarization in the opposite direction between the K and Q points and their time reversals in the first Brillouin zone. Our study clarify that the strain plays an significant role in the spin-orbit coupling of the WS2 ML, which has very important implications in designing future spintronics devices.

cond-mat.str-el↗