Query: Valley polarization in magnetically doped single-layer transition-metal dichalcogenides

Candidate papers:

abstract: The fields of electronics and spintronics require an active control and manipulation of the charge and spin degrees of freedom [1] Valleytronics, on the other hand, is very new field that relies on the property that the conduction/valence bands have two or more minima/maxima at equal energy but different momenta. For valleytronics devices it is necessary to induce valley polarization, i.e. control the number of electrons in each valley, typically by strain [2] or a magnetic field [3, 4] In general, twodimensional materials have raised a lot of interest both for fundamental and applied reasons. Examples are semiconductor quantum wells [5] noble metal surfaces [6] graphene [7] and topological insulators [8] Semiconducting single layer transition metal dichalcogenides MX2 with M Mo, W and X S, Se, Te and D3h point group have caught attention, because they display distinctively different physical properties as compared to their bulk compounds with D6h point group. There exists a crossover from an indirect band gap in multilayers to a direct band gap in the single layer limit [9 12] see Fig. 1(a) In the latter the conduction and valence band edges are located at the K points of the two dimensional hexagonal Brillouin zone, see Fig. 1(c) These two inequivalent valleys constitute a binary index for low energy carriers, which gives rise to a valley Hall effect and valley dependent optical selection rules for interband transitions at the K points [13 15] It has been demonstrated that optical pumping with circularly polarized light can achieve a dynamic valley polarization in single layer MoS2 [16 18] From application point of view, the equilibrium valley polarization in single layer MX2 is more important, which has not been investigated till now experimentally or theoretically. The interplay between spin orbit coupling and ferromagnetism in two dimensional materials gives rise to a variety of unconventional phenomena, such as the quantum anomalous Hall effect [19 22] In addition, it recently has been put forward that dichalcogenides doped with magnetic transition metal atoms form a promising platform for two dimensional dilute magnetic semiconductors [23 25] However, these studies did not take into account the spin orbit coupling, which interconnects the spin and valley physics, making it desirable to investigate the effects of the exchange field induced by magnetic doping. We propose in this work a method to control the valley polarization by magnetic doping, see Fig. 1(b,d, and e) Various possible Mn doping sites in single layer MoS2 are studied to investigate the influence of the exchange field on the electronic structure by first principles calculations. We will argue that the strength of the spin orbit coupling together with the exchange energy determine the valley polarization, which can be inverted by modifying the spin polarization.
author_names: Yingchun Cheng

Qingyun Zhang

Udo Schwingenschlogl
title: Valley polarization in magnetically doped single layer transition metal dichalcogenides
year: 2014

abstract: We use a variational technique to study the many body phase diagram of electrons and holes in $n$ doped and $p$ doped monolayer transition metal dichalcogenides (TMDs) We find a total of four different phases. $i$ A fully spin polarized and valley polarized ferromagnetic state. $ii$ A state with no global spin polarization but with spin polarization in each valley separately, i.e. spin valley locking. $iii$ A state with spin polarization in one of the valleys and little to no spin polarization in the other valley. $iv$ A paramagnetic state with no valley polarization. These phases are separated by first order phase transitions and are determined by the particle density and the dielectric constant of the substrate. We find that in the presence of a perpendicular magnetic field the four different phases persist. In the case of $n$ doped MoS$_2$ a fifth phase, which is completely valley polarized but not spin polarized, appears for magnetic fields larger than 7 T and for magnetic fields larger than 23 T completely replaces the second phase.
author_names: Matthias Van der Donck

Francois M Peeters
title: Rich many body phase diagram of electrons and holes in doped monolayer transition metal dichalcogenides
year: 2018

abstract: Engineering the substrate of 2D transition metal dichalcogenides can couple the quasiparticle interaction between the 2D material and substrate, providing an additional route to realize conceptual quantum phenomena and novel device functionalities, such as realization of a 12 time increased valley spitting in single layer WSe2 through the interfacial magnetic exchange field from a ferromagnetic EuS substrate, and band to band tunnel field effect transistors with a subthreshold swing below 60 mV dec 1 at room temperature based on bilayer n MoS2 and heavily doped p germanium, etc. Here, it is demonstrated that epitaxially grown single layer MoS2 on a lattice matched GaN substrate, possessing a type I band alignment, exhibits strong substrate induced interactions. The phonons in GaN quickly dissipate the energy of photogenerated carriers through electron phonon interaction, resulting in a short exciton lifetime in the MoS2 /GaN heterostructure. This interaction enables an enhanced valley helicity at room temperature (0.33 0.05) observed in both steady state and time resolved circularly polarized photoluminescence measurements. The findings highlight the importance of substrate engineering for modulating the intrinsic valley carriers in ultrathin 2D materials and potentially open new paths for valleytronics and valley optoelectronic device applications.
author_names: Yi Wan

Jun Xiao

Jingzhen Li

X Z Fang

Kun Zhang

Lei Fu

Pan Li

Zhigang Song

Hui Zhang

Yilun Wang

Mervin Zhao

Jing Lu

Ning Tang

Guangzhao Ran

Xiang Zhang

Yu Ye

Lun Dai
title: Epitaxial Single Layer MoS2 on GaN with Enhanced Valley Helicity.
venue: Advanced materials
year: 2018

abstract: Giant spin Hall effect (GSHE) has been observed in heavy metal materials such as Ta, Pt, and W, where spins are polarized in the surface plane and perpendicular to the charge current direction. Spins generated in these materials have successfully switched magnets with in plane magnetic anisotropy (IMA) and perpendicular magnetic anisotropy (PMA) through spin orbit torque (SOT) mechanism. It is generally accepted that PMA magnets are preferred over IMA magnets in data storage applications owing to their large thermal stability even at ultra scaled dimensions. However, SOT switching of PMA magnets by conventional GSHE materials requires either a small external magnetic field, a local dipolar field, or introducing tilted anisotropy to break the symmetry with respect to the magnetization. To deterministically switch a PMA without any additional assistance, nonconventional GSHE materials that can generate spins with polarization perpendicular to the surfaces are needed. Several monolayer transition metal dichalcogenides (TMDs) have been predicted to generate such out of plane spins due to their 2D nature and unique band structures. Interestingly, opposite spins are locked to their respective sub band in each K valley of the TMD valence band with substantially large energy splitting, which enables polarized spins to be accessible through electrical gating and spatially separated by electric field through the valley Hall effect (VHE) Therefore, spatial separation and accumulation of spins in these 2D TMDs are uniquely referred to as coupled valley and spin Hall effect. Here, we report an experiment of electrical generation of spin current with out of plane polarization in monolayer WSe2 and detection of spin signals through a nonlocal spin valve structure built on a lateral graphene spin diffusion channel that partially overlaps with WSe2.
author_names: Terry Y T Hung

Avinash Rustagi

Shengjiao Zhang

Pramey Upadhyaya

Zhihong Chen
title: Experimental observation of coupled valley and spin Hall effect in p doped WSe 2 devices
year: 2019

abstract: We study the valley dependent magnetic and transport properties of massive Dirac fermions in multivalley systems such as the transition metal dichalcogenides. The asymmetry of the zeroth Landau level between valleys and the enhanced magnetic susceptibility can be attributed to the different orbital magnetic moment tied with each valley. This allows the valley polarization to be controlled by tuning the external magnetic field and the doping level. As a result of this magnetic field induced valley polarization, there exists an extra contribution to the ordinary Hall effect. All these effects can be captured by a low energy effective theory with a valley orbit coupling term.
author_names: Tianyi Cai

Shengyuan A Yang

Xiao Li

Fan Zhang

Junren Shi

Wang Yao

Qian Niu
title: Magnetic control of the valley degree of freedom of massive Dirac fermions with application to transition metal dichalcogenides
year: 2013

abstract: Very recently, the centimeter scale MoSi2N4 monolayer was synthesized experimentally and exhibited a semiconducting nature with high mobility (Hong et al. Science, 2020, 369, 670 674) Here, we show that MoSi2N4 and its analogues, MoSi2P4 and MoSi2As4, are potential two dimensional (2D) materials for valleytronics based on first principles calculations. We demonstrate that the intrinsic inversion symmetry breaking and strong spin orbital coupling lead to the remarkable spin valley coupling in the inequivalent valleys at K and K' points, which result in not only the valley contrasting transport properties, but also the spin and valley coupled optical selection rules. Moreover, the in plane strain can tune the bandgaps and spin splitting or even induce an indirect to direct bandgap transition for promising application in the strain tunable valleytronics. We find that the valley polarization can be generated by doping magnetic element. Our findings offer theoretical insight into the exotic physical properties of novel MoSi2N4 family materials beyond transition metal dichalcogenides.
author_names: Haoqiang Ai

Di Liu

Jiazhong Geng

Shuangpeng Wang

Kin Ho Lo

Hui Pan
title: Theoretical evidence of the spin valley coupling and valley polarization in two dimensional MoSi2X4 (X N, P, and As)
venue: Physical chemistry chemical physics PCCP
year: 2021

abstract: The emerging field of valleytronics has boosted intensive interests in investigating and controlling valley polarized light emission of monolayer transition metal dichalcogenides (1L TMDs) However, so far, the effective control of valley polarization degree in monolayer TMDs semiconductors is mostly achieved at liquid helium cryogenic temperature (4.2 K) with the requirements of high magnetic field and on resonance laser, which are of high cost and unwelcome for applications. To overcome this obstacle, it is depicted that by electrostatic and optical doping, even at temperatures far above liquid helium cryogenic temperature (80 K) and under off resonance laser excitation, a competitive valley polarization degree of monolayer WS2 can be achieved (more than threefold enhancement) The enhanced polarization is understood by a general doping dependent valley relaxation mechanism, which agrees well with the unified theory of carrier screening effects on intervalley scattering process. These results demonstrate that the tunability corresponds to an effective magnet field of 10 T at 4.2 K. This work not only serves as a reference to future valleytronic studies based on monolayer TMDs with various external or native carrier densities, but also provides an alternative approach toward enhanced polarization degree, which denotes an essential step toward practical valleytronic applications.
author_names: Shun Feng

Chunxiao Cong

Satoru Konabe

Jing Zhang

Jingzhi Shang

Yu Chen

Chenji Zou

Bingchen Cao

Lishu Wu

Namphung Peimyoo

Baile Zhang

Ting Yu
title: Engineering Valley Polarization of Monolayer WS2 A Physical Doping Approach.
venue: Small
year: 2019

abstract: Manipulation of spin degree of freedom (DOF) of electrons is the fundamental aspect of spintronic and valleytronic devices. Two dimensional transition metal dichalcogenides (2D TMDCs) exhibit an emerging valley pseudospin, in which spin up down) electrons are distributed in a +K K) valley. This valley polarization gives a DOF for spintronic and valleytronic devices. Recently, magnetic exchange interactions between graphene and magnetic insulator yttrium iron garnet (YIG) have been exploited. However, the physics of 2D TMDCs with YIG have not been shown before. Here we demonstrate strong many body effects in a heterostructure geometry comprising a MoS2 monolayer and YIG. High order trions are directly identified by mapping absorption and photoluminescence at 12 K. The electron doping density is up to ~1013 cm 2, resulting in a large splitting of ~40 meV between trions and excitons. The trions exhibit a high circular polarization of ~80% under optical pumping by circularly polarized light at ~1.96 eV; it is confirmed experimentally that both phonon scattering and electron hole exchange interaction contribute to the valley depolarization with temperature; importantly, a magnetoresistance (MR) behavior in the MoS2 monolayer was observed, and a giant MR ratio of ~30% is achieved, which is 1 order of magnitude larger than the reported ratio in MoS2/CoFe2O4 heterostructures. Our experimental results confirm that the giant MR behaviors are attributed to the interfacial spin accumulation due to YIG substrates. Our work provides an insight into spin manipulation in a heterostructure of monolayer materials and magnetic substrates.
author_names: Bo Peng

Qi Li

Xiao Liang

Peng Song

Jian Li

Keliang He

Deyi Fu

Yue Li

Chao Shen

Hailong Wang

Chuangtang Wang

Tao Liu

Lei Zhang

Haipeng Lu

Xuefeng Wang

Jianhua Zhao

Jianliang Xie

Mingzhong Wu

Lei Bi

Longjiang Deng

Kian Ping Loh
title: Valley Polarization of Trions and Magnetoresistance in Heterostructures of MoS2 and Yttrium Iron Garnet.
venue: ACS nano
year: 2017

abstract: Atomically thin transition metal dichalcogenides (TMDCs) with the chemical stoichiometry MX2, where M is group VI transition metal atoms including Mo, W; and X is chalcogen atoms of S, Se and Te, have been emerging as a fast growing research field. Similar to graphene, a layered structure with weak van der Waals coupling between neighboring layers allows for them to be easily separated into single to few layer thick samples. Different from graphene of zero band gap, they exhibit a band gap with magnitude between 1.5 and 2.0 eV and show current on off ratios exceeding 108 in field effect transistors. It has been discovered that an indirect to direct band gap transition occurs in the monolayer limit at the K point of the Brillouin zone, which significantly enhances the photoluminescence quantum yield. Moreover, monolayer TMDCs are non centrosymmetric arising from A and B sublattices being occupied by M and X atoms, respectively, which makes them an ideal laboratory to study valleytronics. The electronic structure of atomically thin TMDCs, mainly focusing on MoS2 and MoSe2, is investigated with optical absorption, photoluminescence and ultrafast pump probe spectroscopic techniques in this thesis. First, we demonstrate continuous tuning of the electronic structure of atomically thin MoS2 on flexible substrates by applying a uniaxial tensile strain. A redshift at a rate of ~70 meV per percent applied strain for both mono and bilayer MoS2 direct gap transitions, and at a rate 1.6 times larger for bilayer MoS2 indirect gap transitions, have been determined.Second, we investigate the optical response of monolayer MoS2 as a function of carrier density. Tightly bound negative trions, a quasi particle composed of two electrons and a hole, are identified in doped monolayer MoS2. They possess a large binding energy 20 meV) rendering trion effects significant even at room temperature. Third, we demonstrate that optical pumping with circularly polarized light can achieve complete dynamic valley polarization in monolayer MoS2. Moreover, we found that this polarization is retained for longer than 1 ns. Finally, preliminary results on the ultrafast dynamics in mono and bilayer MoSe2 are presented. The photo excited carrier dynamics is investigated via optical pump probe technique.
author_names: Keliang He
title: Optical Spectroscopy of Two Dimensional Transition Metal Dichalcogenides (TMDCs)
year: 2014

abstract: Manipulation of spin degree of freedom (DOF) of electrons is the fundamental aspect of spintronic and valleytronic devices. Two dimensional transition metal dichalcogenides (2D TMDCs) exhibit an emerging valley pseudospin, in which spin up down) electrons are distributed in a +K K) valley. This valley polarization gives a DOF for spintronic and valleytronic devices. Recently, magnetic exchange interactions between graphene and magnetic insulator yttrium iron garnet (YIG) have been exploited. However, the physics of 2D TMDCs with YIG have not been shown before. Here we demonstrate strong many body effects in a heterostructure geometry comprising a MoS2 monolayer and YIG. High order trions are directly identified by mapping absorption and photoluminescence at 12 K. The electron doping density is up to ~10 cm 2, resulting in a large splitting of ~40 meV between trions and excitons. The trions exhibit a high circular polarization of ~80% under optical pumping by circularly polarized light at ~1.96 eV; it is confirmed experimentally that both phonon scattering and electron hole exchange interaction contribute to the valley depolarization with temperature; importantly, a magnetoresistance (MR) behavior in the MoS2 monolayer was observed, and a giant MR ratio of ~30% is achieved, which is 1 order of magnitude larger than the reported ratio in MoS2/CoFe2O4 heterostructures. Our experimental results confirm that the giant MR behaviors are attributed to the interfacial spin accumulation due to YIG substrates. Our work provides an insight into spin manipulation in a heterostructure of monolayer materials and magnetic substrates.
author_names: Bo Peng

Qi Li

Xiao Liang

Peng Song

Jian Li

Keliang He

Deyi Fu

Yue Li

Chao Shen

Hailong Wang

Chuangtang Wang

Tao Liu

Li Zhang

Haipeng Lu

Xin Wang

Jianhua Zhao

Jianliang Xie

Mingzhong Wu

Lei Bi

Longjiang Deng

Kian Ping Loh
title: Valley Polarization of Trions and Magnetoresistance in Heterostructures of MoS 2 and Yttrium Iron
year: 2017