Query: Probing and controlling magnetic states in 2D layered magnetic materials

Candidate papers:

abstract: The discovery of atomic monolayer magnetic materials has triggered significant interest in the magnetism/spintronics and 2D van der Waals materials communities. Here we review recent progress in this rapidly growing field. We survey the physical properties of the large class of layered magnetic materials, and discuss recent advances in the study of these materials in the 2D limit. We then overview the optical and electrical techniques used for probing 2D magnetic materials (for reading their magnetic states) and the mechanisms for reorienting and/or switching 2D magnets by electric fields (for writing) Emerging device concepts based on magnetic van der Waals heterostructures are also discussed. We conclude with the future challenges and opportunities in this area of research.The emergence of 2D magnetic materials presents a unique opportunity to study magnetism and spintronics devices in new regimes. This Review surveys the basic properties of these materials, methods to read and write their magnetic states, and emerging device concepts.Key pointsThere is a large class of layered magnetic materials with unique magnetic properties, which provides an ideal platform to study magnetism and spintronics device concepts in the 2D limit.Magneto optical and electrical probes are powerful techniques for probing or reading the magnetic states of these materials.Because these materials are atomically thin, their magnetic states can be effectively controlled or switched by external perturbations other than magnetic fields, such as electric fields, free carrier doping and strain.New materials concepts, such as magnetizing 2D semiconductors by magnetic proximity coupling, and new devices, such as spin tunnel field effect transistors, are rapidly emerging.Although rapid progress has already been made, there are many opportunities and challenges remaining in this young field.
author_names: Kin Fai Mak

Jie Shan

Daniel C Ralph
title: Probing and controlling magnetic states in 2D layered magnetic materials
venue: Nature Reviews Physics
year: 2019

abstract: Abstract Two dimensional (2D) materials are intrinsically heterogeneous. Both localized defects, such as vacancies and dopants, and mesoscopic boundaries, such as surfaces and interfaces, give rise to compositional or structural heterogeneities. The presence of defects and boundaries can break lattice symmetry, modify the energy landscape, and create quantum confinement, leading to fascinating electronic properties different from the "ideal" 2D sheets. This review summarizes recent progress in understanding the roles of defects and boundaries in electronic, magnetic, thermoelectric, and transport properties of 2D layered materials. The focus is on the understanding of correlation of atomic scale structural information with electronic functions by interrogating heterogeneities individually. The materials concerned are graphene, transition metal dichalcogenides (TMDs) hexagonal boron nitride (hBN) and topological insulators (TIs) The experimental investigations benefit from new methodologies and techniques in scanning tunneling microscopy (STM) including spin polarized STM, scanning tunneling potentiometry (STP) scanning tunneling thermopower microscopy, and multi probe STM. The experimental effort is complemented by the computational and theoretical approaches, capable of discriminating between closely competing states and achieving the length scales necessary to bridge across features such as local defects and complex heterostructures. The goal is to provide a general view of current understanding and challenges in studying the heterogeneities in 2D materials and to evaluate the potential of controlling and exploiting these heterogeneities for novel functionalities and electron devices.
author_names: Saban M Hus

An-Ping Li
title: Spatially resolved studies on the role of defects and boundaries in electronic behavior of 2D materials
year: 2017

abstract: Author(s) Li, Zhenglu Advisor(s) Louie, Steven G. Abstract: Condensed matter physics is a very broad and fast developing field, which studies emerging phenomena, interactions, phases, and symmetries in materials, such as solids. Predictive first principles, or ab initio, methodologies play a significant role in understanding various phenomena and new physics. This dissertation is aimed at developing new ab initio methodologies for the investigation of important novel phenomena and applying various ab initio methods combined with analytical approaches to a broad range of condensed matter systems, including the high transition temperature superconductor Ba1 xKxBiO3, the two dimensional (2D) ferromagnet Cr2Ge2Te6, Dirac fermions generated in few layer black phosphorus, defects in hexagonal boron nitride, and non trivial topological surface states of antimony.This dissertation is divided into two parts. Part I is focused on methods development, and Part II is a collection of theoretical and computational studies of novel materials. The dissertation is organized as follows:Part I: Electronic structure methodologies for condensed matterIn Chapter 1, we review some important ab initio methods to lay the foundation for the development of a new ab initio method named GW perturbation theory (GWPT) in Chapter 2, and for various applications to the materials studied in Part II. In Chapter 1, we review the basics of density functional theory (DFT) the GW method, the general phonon formalism and electron phonon (e ph) coupling formalism, density functional perturbation theory (DFPT) and the Wannier representation of e ph coupling.In Chapter 2, we present a new ab initio method, which we named the GW perturbation theory (GWPT) This method is a linear response theory of the GW method, and it gives efficient and accurate access to all e ph matrix elements at the many electron level in the full Brillouin zone and between any pairs of electronic states. We discuss its general formalism, implementation and verification in this Chapter.In Chapter 3, we develop a general renormalized spin wave theory (RSWT) by including full sublattice dependence. This RSWT method includes magnon magnon interactions, and therefore can give quantitative predictions of magnetic transition temperatures, especially in 2D. This method is solved numerically and self consistently. We discuss its formalism, implementation, and behavior in this Chapter.Part II: Studies of superconductivity, and electronic and magnetic interactions in novel materialsIn Chapter 4, we apply our newly developed GWPT method to study superconductivity in Ba1 xKxBiO3, which shows an experimental superconducting transition temperature (Tc) of 30 32 K at optimal doping. Our GWPT calculations show that many electron correlations significantly enhance the e ph interactions compared to DFPT values for states near the Fermi surface and renormalize the e ph coupling constant lambda by a factor of 2.4, nicely explaining the high Tc as well as the doping dependence observed in this family of material.In Chapter 5, we present a collaborative work with experimental groups on the discovery of the 2D van der Waals ferromagnet Cr2Ge2Te6, probed using the scanning magneto optic Kerr effect (MOKE) technique. We apply our RSWT method to this system, and our calculation nicely reproduces and explains the experimentally observed strong dimensionality effect in this 2D ferromagnet. Furthermore, our theory reveals an intriguing interplay between anisotropy and dimensionality, which leads to an unprecedented magnetic field control of ferromagnetism in this system.In Chapter 6, we propose a strategy for the generation of novel anisotropic Dirac fermions in few layer black phosphorus by applying inversely designed superlattice potentials. We show that these novel quasiparticles exhibit asymmetric Klein tunneling, in which the perfect transmission direction significantly differs from the normal incidence direction. These unusual states are highly tunable and accessible with experimentally achievable conditions. The findings revealed in this Chapter provide new platforms for device design.In Chapter 7, we present a collaborative work with an experimental group to study the electron irradiation induced triangular and hexagonal defects in hexagonal boron nitride, observed in transmission electron microscopy (TEM) measurements. We use DFT to calculate the formation enthalpy of different structures (as well as the edges and corners) to provide an overall diagram of preferred structures under different conditions at equilibrium. Our theory provides important insights into the formation of these defects.In Chapter 8, we present a collaborative work with experimental groups to study the unusual behavior of photoelectrons from the topological surface states of Sb(111) measured with spin and angle resolved photoemission spectroscopy (spin ARPES) Our theory, using the ab initio tight binding method, reproduces well the observed spin textures. Our theoretical analysis shows that the unexpected spin polarization behavior comes from the interplay between strong spin orbit coupling (SOC) and the symmetry requirement of the electron wavefunction in high symmetry regions of the Brillouin zone.
author_names: Zhenglu Li
title: Electron Phonon Coupling from GW Perturbation Theory and Electronic and Magnetic Properties of Novel Two Dimensional Materials
year: 2019

abstract: Despite recent advances in layered ferromagnets, ferromagnetic interactions in these materials are rather weak. Here, we report pressure enhanced ferromagnetism in layered CrSiTe3 flakes revealed by high pressure magnetic circular dichroism measurements. Below ~3 GPa, CrSiTe3 undergoes a paramagnetic to ferromagnetic phase transition at ~32 K, and the field induced spin flip in the ferromagnetic phase produces nearly zero hysteresis loops, demonstrating soft ferromagnetism. Above ~4 GPa, a soft to hard ferromagnetic transition occurs, signaled by rectangular shaped hysteresis loops with finite coercivity and remanent magnetization. Interestingly, as pressure increases, the Curie temperature and coercivity dramatically increase up to ~138 K and 0.17 T at 7.8 GPa, respectively, in contrast to ~36 K and 0.02 T at 4.6 GPa. It indicates a remarkable influence of pressure on exchange interactions, which is consistent with DFT calculations. The effective interaction between magnetic couplings and external pressure offers new opportunities in pursuit of high temperature layered ferromagnets.
author_names: Yue Gu

Le Wang

Lianglong Huang

Ying Fu

Cai Liu

Shanmin Wang

Jia Wei Mei

Xiaolong Zou

Junfeng Dai
title: Pressure Enhanced Ferromagnetism in Layered CrSiTe3 Flakes.
venue: Nano letters
year: 2021

abstract: Remarkable properties of two dimensional (2D) layer magnetic materials, which include spin filtering in magnetic tunnel junctions and the gate control of magnetic states, were demonstrated recently 1 12 Whereas these studies focused on static properties, dynamic magnetic properties, such as excitation and control of spin waves, remain elusive. Here we investigate spin wave dynamics in antiferromagnetic CrI 3 bilayers using an ultrafast optical pump/magneto optical Kerr probe technique. Monolayer WSe 2 with a strong excitonic resonance was introduced on CrI 3 to enhance the optical excitation of spin waves. We identified subterahertz magnetic resonances under an in plane magnetic field, from which the anisotropy and interlayer exchange fields were determined. We further showed tuning of the antiferromagnetic resonances by tens of gigahertz through electrostatic gating. Our results shed light on magnetic excitations and spin dynamics in 2D magnetic materials, and demonstrate their potential for applications in ultrafast data storage and processing. Gating dependent laser induced spin dynamics in an antiferromagnetic bilayer are observed and explained, with implications for future spintronic applications.
author_names: Xiao-Xiao Zhang

Lizhong Li

Daniel Weber

Joshua E Goldberger

Kin Fai Mak

Jie Shan
title: Gate tunable spin waves in antiferromagnetic atomic bilayers
venue: Nature Materials
year: 2020

abstract: Optically addressable spins in wide bandgap semiconductors have become one of the most prominent platforms for exploring fundamental quantum phenomena. While several candidates in 3D crystals including diamond and silicon carbide have been extensively studied, the identification of spindependent processes in atomically thin 2D materials has remained elusive. Although optically accessible spin states in hBN are theoretically predicted, they have not yet been observed experimentally. Here, employing rigorous electron paramagnetic resonance techniques and photoluminescence spectroscopy, we identify fluorescence lines in hexagonal boron nitride associated with a particular defect the negatively charged boron vacancy and determine the parameters of its spin Hamiltonian. We show that the defect has a triplet (S 1) ground state with a zero field splitting of 3.5 GHz and establish that the centre exhibits optically detected magnetic resonance (ODMR) at room temperature. We also demonstrate the spin polarization of this centre under optical pumping, which leads to optically induced population inversion of the spin ground state a prerequisite for coherent spin manipulation schemes. Our results constitute a leap forward in establishing twodimensional hBN as a prime platform for scalable quantum technologies, with extended potential for spin based quantum information and sensing applications, as our ODMR studies on hBN NV diamonds hybrid structures show. The emergence of 2D materials and van der Waals (vdW) crystals has enabled the observation and realisation of unique optoelectronic and nanophotonic effects such as unconventional superconductivity, Moire excitons and quantum spin Hall effects at elevated temperatures, to name a few.1 3 Amidst the large variety of studied vdW crystals, hexagonal Boron Nitride (hBN) offers a combination of unique physical, chemical and optical properties4. Most relevant to this work, is the ability of hBN to host atomic impurities (or point defects) that give rise to quantized optical transitions, well below its bandgap.5,6 hBN colour centres are ultrabright with narrow and tuneable linewidth,7 9 and photostability up to 800 K.10 Whilst the nature of many of the defects is still uncertain,11 15 they are being extensively studied as promising candidates for quantum photonic applications requiring on demand, ultrabright single photon emission. A step forward, which will significantly extend the functionality of hBN emitters for quantum applications, is to interface their optical properties with spin transitions, and realise spin polarization and optical spinreadout schemes.16, 17 The concept of spin photon interface has been extensively studied in quantum dots18 and the nitrogen vacancy (NV) centre in diamond.19, 20 The latter has been harnessed to realise basic two node quantum networks19 and a plethora of advanced quantum sensing schemes.21 23 The basic principle is that the triplet spin ground state of the defect can be polarised, manipulated and read out optically owing to the spin dependent excitation, decay and intersystem crossing pathways available to the system during the optical excitation recombination cycle.24 Yet, extending the optical control of single spin states beyond defects in 3D crystals, to 2D systems, has remained elusive. If achieved, it will open up a stretch of novel possibilities both fundamental and technological. The two dimensional nature of these materials inherently allows for seamless integration with heterogeneous, opto electronic devices where the hosted solid state qubits can be readily interfaced with cavities, resonators and nanophotonic components from foreign materials. Further, it naturally grants nanoscale proximity of the spin probe to target samples for high resolution quantum sensing realizations. Reliable and deterministic transfer of hBN layers on stacks of other 2D materials is well established and is part of one of the currently most relevant endeavours of condensed matter physics engineering heterostructures made by purposefully chosen sequences of atomically thin 2Dmaterials.25 Here we report on the optical initialisation and readout of an ensemble of spins in hBN. We perform rigorous electron paramagnetic resonance (EPR) spectroscopy and optically detected magnetic resonance (ODMR) measurements to establish that the defect has a triplet ground state with zero field splitting (ZFS) of D/h 3.5 GHz and almost isotropic Lande factor g 2.000. From the analysis of the angular dependence and nitrogen hyperfine structure, we confirm the intrinsic nature of the defect and assign it to the negatively charged boron vacancy The alternative nitrogen vacancy structure was also considered, but discarded upon analysis of the experimental data (see discussion below) Figure 1a is a schematic illustration of the proposed defect. The defect is a negatively charged boron vacancy centre consisting of a missing boron atom surrounded by three equivalent nitrogen atoms in the hBN lattice. The defect has D3h point group symmetry, typical for a substitutional defect in hBN, and exhibits a strong room temperature photoluminescence (PL) emission at lmax 850 nm, under lexc 532 nm laser excitation (Figure 1b) Figure 1. ODMR of an hBN single crystal at room temperature, T 300 K. a) Schematic of an hBN monolayer and its crystalline hexagonal structure with alternating boron (red) and nitrogen (blue) atoms. The green arrows indicate the spins of the negatively charged boron vacancy defects, b) Photoluminescence spectrum of the sample at room temperature displaying a pronounced emission at 850 nm. c) ODMR spectra measured with zero magnetic field (bottom) and with magnetic field B 10 mT (top) d) Dependence of ODMR frequencies and on the magnetic field (BIIc) Experimental (red) and fit (blue line) obtained using Equation 2 with parameters D/h 3.48 GHz, E/h 50 MHz and g=2.000. Most interestingly, we find that the PL from this hBN colour centre is spin dependent. Figure 1c shows the ODMR spectrum recorded for an hBN single crystal at T 300 K. In ODMR experiments, microwaveinduced magnetic dipole transitions between spin sublevels manifest as changes in PL intensity (DPL) The prerequisite for optical (PL) detection of EPR is thus the existence of a dependence between the optical excitation recombination cycle and the defect's spin orientation. Figure 1c shows the spectrum of the investigated sample as normalized change of PL intensity (DPL/PL) i.e. ODMR contrast as a function of the applied microwave frequency for two static magnetic fields 0 and 10 Even without external magnetic field, the ODMR spectrum shows two distinct resonances and located symmetrically around the frequency We tentatively assign them to the DmS 1 spin transitions between triplet energy sublevels with completely lifted three fold degeneracy, due to a splitting induced by dipolar interaction between the unpaired electron spins, forming the triplet. This so called zero field splitting is described by parameters and which can be derived from the spectrum as /h and /h. In order to verify this assignment, we have studied the dependence of the and resonant microwave frequencies on the magnitude of the external static magnetic field. The evolution of the ODMR spectrum with the field applied parallel to the hexagonal c axis (B II c) of hBN is presented in Figure 1d. To explain the observed transitions and their variation with magnetic field, we use the standard spin Hamiltonian given by Equation 1 with z as the principle symmetry axis oriented perpendicular to the plane (collinear with the c axis of the hBN crystal) 1)/3) " (1) where D and E are the ZFS parameters, S 1 is the total spin, g is the Lande factor, is the Bohr magneton, is the static magnetic field and are the spin 1 operators. According to Equation 1 and for applied parallel to the c axis the resonant microwave frequencies at which the transitions occur
author_names: Andreas Gottscholl

Mehran Kianinia

Victor A Soltamov

Carlo Bradac

Christian Kasper

Klaus Krambrock

Andreas Sperlich

Milos Toth

Igor Aharonovich

Vladimir Dyakonov
title: Room Temperature Initialisation and Readout of Intrinsic Spin Defects in a Van der Waals Crystal
year: 2019

abstract: Delving deep into electronic properties The spectral function of a material, which reflects the distribution of its electronic states as a function of momentum and energy, carries a wealth of information on its properties. However, measuring the spectral function directly is tricky, particularly in systems inaccessible to surface probes or in insulators. Jang et al. introduce a method dubbed momentum and energy resolved tunneling spectroscopy, in which electrons tunnel from a probe layer to unoccupied states in a target layer deep in a heterostructure. Because the momentum and energy of the electrons are tightly controlled, the measured tunneling probability is proportional to the spectral function of the target system. Science, this issue p. 901 A method for measuring the spectral function of a material is demonstrated using gallium arsenide quantum wells. The single particle spectral function measures the density of electronic states in a material as a function of both momentum and energy, providing central insights into strongly correlated electron phenomena. Here we demonstrate a high resolution method for measuring the full momentum and energy resolved electronic spectral function of a two dimensional (2D) electronic system embedded in a semiconductor. The technique remains operational in the presence of large externally applied magnetic fields and functions even for electronic systems with zero electrical conductivity or with zero electron density. Using the technique on a prototypical 2D system, a GaAs quantum well, we uncover signatures of many body effects involving electron phonon interactions, plasmons, polarons, and a phonon analog of the vacuum Rabi splitting in atomic systems.
author_names: Joonho Jang

Heun Mo Yoo

Loren N Pfeiffer

Ken W West

Kirk W Baldwin

Raymond C Ashoori
title: Full momentum and energy resolved spectral function of a 2D electronic system
venue: Science
year: 2017

abstract: The electric field induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor [1 4] In this scheme an electric field can switch 'on' the ballistic flow of charge and spin along dissipationless edges of the two dimensional (2D) quantum spin Hall insulator [5 9] and when 'off' is a conventional insulator with no conductive channels. Such a topological transistor is promising for low energy logic circuits [4] which would necessitate electric field switched materials with conventional and topological bandgaps much greater than room temperature, significantly greater than proposed to date [6 8] Topological Dirac semimetals (TDS) are promising systems in which to look for topological field effect switching, as they lie at the boundary between conventional and topological phases [3,10 16] Here we use scanning probe microscopy/spectroscopy (STM/STS) and angle resolved photoelectron spectroscopy (ARPES) to show that monoand bilayer films of TDS Na3Bi [3,17] are 2D topological insulators with bulk bandgaps >400 meV in the absence of electric field. Upon application of electric field by doping with potassium or by close approach of the STM tip, the bandgap can be completely closed then re opened with conventional gap greater than 100 meV. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy kT 25 meV at room temperature, suggest that ultrathin Na3Bi is suitable for room temperature topological transistor operation. Two dimensional Quantum Spin Hall (QSH) insulators are characterized by an insulating interior with bulk bandgap Eg, and topologically protected conducting edge channels that are robust to backscattering by non magnetic disorder. The QSH effect was first realized in HgTe quantum wells [5] where the small Eg prevents device applications above cryogenic temperatures. This has led to efforts to find new materials with Eg 25 meV (thermal energy at room temperature) for room temperature topological electronic devices. Recent reports of QSH insulators bismuthene on SiC (Eg 0.8 eV) [6] and monolayer 1T' WTe2 (Eg 50 meV) [7] are promising, with the quantum spin Hall effect measured in monolayer WTe2 up to 100K [8,9] However, a predicted electric field effect in WTe2 has not yet been reported, and a substantial field effect in atomically two dimensional bismuthene is unlikely due to the completely in plane structure that lacks inversion symmetry meaning any Stark effect would most likely be small. Ultrathin films of topological Dirac semimetals (TDS) are a promising material class to realise the electric field tuned topological phase transition, with such a transition predicted in few layer films of TDS Na3Bi and Cd3As2 [3] Bulk TDSs are zero bandgap semimetals with a linear band dispersion in all three dimensions around pairs of Dirac points [10 13] whilst few layer TDSs are predicted to be non trivial insulators with bulk bandgaps up to ~300 meV for monolayer Na3Bi [17] However, experiments on few layer TDSs are presently lacking [14 16] To unambiguously demonstrate electric field control over the magnitude, electric field dependence, and topological nature of the bandgap in ultrathin Na3Bi, we employ two independent experimental techniques. First, we utilize ARPES to directly measure the electronic bandstructure and its modification as a result of doping the surface with potassium to generate an electric field. Second, we use scanning tunnelling spectroscopy (STS) which measures the local density of states (DOS) as a function of energy, to directly probe the energy gap while varying the tip sample separation and consequently the induced electric field caused by the potential difference between tip and sample. STS also resolves the topological edge state in Na3Bi at low electric field, demonstrating the topological nature of this phase. The unit cell of Na3Bi contains two stacked triple layers in the z direction, comprising Na and Bi atoms that form a honeycomb lattice, with interleaved Na atoms as shown in the crystal structures of Fig. 1(a)(b) One triple layer and two stacked triple layers correspond to monolayer (ML) and bilayer (BL) Na3Bi respectively as illustrated in Fig. 1(b) Figure 1(c) shows the Brillouin zone of 2D Na3Bi. In Fig. 1(d) STM on few layer Na3Bi(001) epitaxial films grown via molecular beam epitaxy (MBE) on Si(111) (see methods and SM1 for details) reveals coexisting regions of ML and BL Na3Bi islands that are atomically flat and up to 40 nm in size, along with small areas of bare substrate. Monolayer regions are identified by an additional 0.22 nm distance to the underlying substrate, due to interfacial spacing or structural relaxation which has been observed previously in other atomically thin materials [18] Figure 1(e) shows the overall band structure of few layer Na3Bi films along the M K surface directions measured with ARPES at hv 48 eV. Figure 1(f) shows the second derivative of the spectra in order to enhance low intensity features. This has been overlaid with density functional theory (DFT) calculations for ML (blue) and BL (green) Na3Bi showing good agreement, consistent with the STM topography which shows coexisting ML and BL regions. Photon energy dependent ARPES (see SM2) demonstrates that the film is electronically 2D, with no dispersion in kz unlike its bulk or 15 nm thin film counterparts [10,15] Depth dependent X ray photoelectron spectroscopy (XPS) (See SM1) revealed no additional components observed in either the Si2p core level corresponding to the Si(111) substrate or the Na and Bi core levels of Na3Bi, verifying Na3Bi is free standing on Si(111) We first measure the size of the bulk bandgap for ML and BL regions of Na3Bi by probing the electronic structure with STS, which measures the differential conductance dI/dV as a function of sample bias V, and is proportional to the local density of states. Figure 2(a) shows typical dI/dV spectra for ML (red) and BL (black) with bandgaps corresponding to 0.44 0.01 eV and 0.41 0.01 eV respectively (see SM3 for details on extracting bandgap values) All STS curves in Fig. 2(a) were taken more than 5 nm inwards from step edges. Figure 2(b) plots the experimental bandgap (blue squares) in comparison to DFT calculated values for pristine Na3Bi (black circles) and Na3Bi layers that contain an Na(2) surface vacancy (red circles) (see Methods for details, with associated bandstructures found in SM4) The ~0.15 eV discrepancy between experiment and DFT for ML is likely a result of the underestimation of electronic bandgap well known to occur in DFT calculations [19] The minimal change in bandgap from ML to BL observed experimentally is well explained by the DFT calculations that include Na(2) surface vacancies; this vacancy gives rise to a delocalized resonance feature and enhancement of the electronic bandgap [20] resulting in only a small layer dependent evolution in bandgap. To verify the prediction that ML and BL Na3Bi are large bandgap QSH insulators (see Fig. S4) we probe the step edge of these islands to the underlying Si(111) substrate to look for the conductive edge state signature of a QSH insulator (STM topography shown in Fig. 2(c) Figure 2(d) shows local dI/dV spectra taken on BL Na3Bi far away from the edge (black curve) and at the edge (blue curve) In contrast to the gap in the bulk, the dI/dV at the edge is quite different, with states filling the bulk gap along with a characteristic dip at 0 mV bias. Similar features observed in other QSH insulators 1T' WTe2 [7] and bismuthene [6] have been attributed to one dimensional (1D) non trivial edge states and the emergence of a Luttinger liquid [21] Figure 2(e) probes dI/dV as a function of distance away from the edge, demonstrating the extended nature of the edge state feature, with Fig. 2(f) showing that the dI/dV signal within the bulk bandgap moving away from the edge follows the expected exponential decay for a 1D topological non trivial state [6] With ML and BL Na3Bi verified as large bandgap QSH insulators, we now examine the role of electric field on modifying the size and nature of the bandgap. First, we utilize ARPES to measure the bandstructure after doping the surface with K to generate an electric field. Details on calculating the displacement field are in SM5. Figure 3(a) (d) shows the bandstructure along K for values of the electric field of 0.0, 0.72, 1.44 and 2.18 Vnm respectively, with the green and blue dots reflecting the extracted maxima from energy distribution curve (EDC) and momentum distribution curve (MDC) analysis (see SM5 for details) The right panel in (a) (d) represents a model of a 2D gapped Dirac system (see SM5) In Fig. 3(a) only the hole band is observable, which away from displays a linear band dispersion (hole Fermi velocity of vF 3x10 ms) The band dispersion near displays the clear cusp of a band edge indicating a gapped system, with 140 meV separation between the valence band edge and the Fermi energy EF. The effect of K dosing in Figs. 3(b) (d) is to n type dope the sample and consequently increase the displacement field. At a displacement field of 0.7 Vnm the separation from the valence band edge to EF has increased to ~257 meV. The bandgap must be at least this amount, consistent with STS, though we cannot determine its exact magnitude since the conduction band lies above EF (although it can be estimated see Fig. S7(a) Upon increasing the displacement field, a Diraclike electron band emerges (electron Fermi velocity of vF 10 ms) At 1.4 Vnm the extracted gap between the two band edges is ~100 meV, and reduces to ~90 meV at 2.2 Vnm (see Supplementary Materials S5 for calculation and comparison with STS values) Whilst a significant reduction in bandgap with displacement field occurs, due to the finite energy width of the bands ~100 meV) we c
author_names: James L Collins

A Tadich

Weikang Wu

Lidia C Gomes

Joao N B Rodrigues

Chang Liu

Jack Hellerstedt

Hyejin Ryu

Shujie Tang

S K Mo

Shaffique Adam

Shengyuan A Yang

Michael S Fuhrer

Mark T Edmonds
title: Electric Field Tuned Topological Phase Transition in Ultra Thin Na3Bi Towards a Topological Transistor
year: 2018

abstract: All electrical (magnetic material free) spin injection is one of the outstanding goals in spintronics. Topological insulators (TIs) have been recognized as a promising electrically controlled spin source thanks to the strong spin orbit coupling and in particular, the spin momentum locked topological surface states (TSS) supporting helically spin polarized currents. Many TI materials such as Bi based chalcogenides are also layered 2D materials and can be incorporated into van der Waals (vdW) coupled heterostructures, opening the possibility of the utilization of TIs for electrical spin injection into other 2D materials. Here, we demonstrate electrical injection of helically spin polarized current into graphene through a 3D TI in a mechanically stacked heterostructure between Bi2Te2Se (a TI) and chemical vapor deposition (CVD) grown graphene, using the spin potentiometric measurement. When a dc current is flowing from the TI to graphene, we detect a striking step like voltage change (spin signal) in both the TI and graphene using ferromagnetic (FM) probes. The sign of the spin signal can be reversed by reversing the direction of the dc bias current, and the corresponding amplitude of the spin signal increases linearly with the bias current, indicative of a current induced helical spin polarization in both TI and graphene. In contrast, the graphene itself exhibits usual nonlocal spin valve signal when the spins are injected using an FM electrode. We discuss possible origins of the helical spin polarization injected into the graphene in our TI/graphene heterostructure that may include TSS as well as the spin orbit coupled Rashba states. Our findings show electrical injection of a spin helical current into graphene through a TI and demonstrate TIs as potential spin sources for future spintronic devices wherein spin manipulation is achieved electrically.
author_names: Jifa Tian

Ting-Fung Chung

Ireneusz Miotkowski

Yong P Chen
title: Electrical spin injection into graphene from a topological insulator in a van der Waals heterostructure
year: 2016

abstract: Optical properties of semiconductors are dominated by excitons, which are electron hole pairs bound by Coulomb interactions. Excitons can be bright or dark depending on the spin orientation of the individual carriers. In a bright exciton the electron and the hole have antiparallel spins and can recombine easily through the emission of a photon. In a dark exciton the spins are parallel and the recombination cannot occur via direct emission of a photon as this would not allow for spin momentum conservation. Thus, dark excitons have much longer radiative lifetimes than bright excitons. This large difference in recombination time can potentially be harnessed to create 'fast' or 'slow' light. Besides, the highly stable, non radiative nature of dark excitons makes them attractive for optically controlled information processing. To take advantage of the features of dark excitons, it is essential to create conditions in which these excitonic species can exist. It turns out that the formation of dark excitons occurs in WS2 or WSe2, where rotational and inversion symmetrybreaking together with strong spin orbit coupling leads to spin split energy states. In these materials dark excitons are lower in energy than the bright excitons in the conduction band1. However, in order to access the stored spin information, the dark exciton needs to have the ability to respond to light. Making use of the optical selection rules in 2D transition metal dichalcogenides (TMDs)2, it is possible to selectively control spin and valley excitation to initialize this state (Fig. 1) This has piqued the interest of researchers and efforts are ongoing to find ways to optically detect and manipulate dark excitons. Writing in Nature Nanotechnology, two teams, Xiao Xiao Zhang and coworkers3, and You Zhou and co workers4, independently describe two approaches to induce light emission from dark excitons in monolayer WSe2, thus opening new possibilities to store and manipulate valley and spin information. Applying a magnetic field to a semiconductor can mix electronic wave functions and shift the spectral weight between bright and dark excitons depending on the magnetic field direction5. An in plane magnetic field does not couple to the in plane motion of charge carriers in 2D materials and hence, does not perturb the electronic structure. Instead, it alters the spin alignment of carriers, which relaxes the spin selection rule and makes dark excitons optically detectable by photoluminescence (PL) spectroscopy. Zhang et al.3 demonstrated that by applying an in plane magnetic field greater than 10 T, a magnetic brightening of the dark exciton can be achieved in WSe2. Using circularly polarized light to excite the dark state, the researchers show that the dark exciton is defined by its spin state as well as the chiral information characteristic of the valley it resides in. By measuring the polarization of the light emitted from the dark exciton, the team discovered that the dark exciton, in addition to having an opposite chirality to that of the bright exciton, also has a much longer valley recombination lifetime. Although the exact mechanism is not clear, Zhang et al. suggest that it could be due to the transfer of electrons from one photo excited valley to another. This result in particular could have implications for valley selective spin injection. Besides, because dark excitons in WSe2 emit light of certain chirality, this process is also attractive for developing photon emitters for chiral optics6. It is worth investigating if, instead of applying a high magnetic field, a similar effect can be achieved by coupling the 2D TMD to a ferromagnetic layer. Although magnetic brightening is fundamentally important for the understanding of, for example, the structure of the spin split conduction bands in 2D TMDs, it is not easy to make use of this effect in conventional device fabrication schemes. Luckily, an alternative way of probing the dark exciton is available, which simply involves placing WSe2 monolayers on patterned silver surfaces the approach pioneered by Zhou and colleagues4. Silver surfaces have a high density of free electrons, which can be collectively excited to form plasmons. When light interacts with these species, a combined electromagnetic radiation called the surface plasmon polariton (SPP) is generated. Coupling with SPPs can be used to selectively enhance optical transitions with dipole moments normal to the 2D plane. Dark excitons in WSe2 have a nonzero dipole moment along the z direction, allowing them to couple to the SPP modes, which are also z polarized. This 'switches on' the PL response from the dark excitons, yielding a PL intensity equivalent to that achieved by a 30 T inplane magnetic field. The advantage of the SPP approach is its ease of integration with atomically thin 2D materials. Silver mirrors can be patterned in the form of subwavelength structures called metasurfaces to create optical interference effects, which enables spatial variation of the optical response 2D MATERIALS
author_names: Kian Ping Loh
title: Brightening the dark excitons Two independent methods using near field coupling to surface plasmon polaritons and magnetic brightening allow the observation of dark excitons in WSe
year: 2017