First-principles study of adsorption-induced magnetic properties of InSe monolayers

Accepted Manuscript Title: First-principles study of adsorption-induced magnetic properties of InSe monolayers Authors: Zhaoming Fu, Bowen Yang, Na Zhang, Dongwei Ma, Zongxian Yang PII: DOI: Reference:

S0169-4332(17)33579-1 https://doi.org/10.1016/j.apsusc.2017.11.286 APSUSC 37868

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APSUSC

Received date: Revised date: Accepted date:

21-7-2017 22-10-2017 30-11-2017

Please cite this article as: Fu Z, Yang B, Zhang N, Ma D, Yang Z, First-principles study of adsorption-induced magnetic properties of InSe monolayers, Applied Surface Science (2010), https://doi.org/10.1016/j.apsusc.2017.11.286 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

First-principles study of adsorption-induced magnetic properties of InSe monolayers

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Zhaoming Fu1, Bowen Yang1, Na Zhang 1, Dongwei Ma*2, and Zongxian Yang*1

College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan, 453007,

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China.

School of Physics, Anyang Normal University, Anyang, Henan 455000, China.

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GRAPHICAL ABSTRACT

Highlights

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Corresponding author. E-mail: [email protected] (Dongwei Ma), [email protected] (Z. Yang).

1. The nonmagnetic InSe monolayer can obtain the local magnetic moments and long-range ordering by chemical modification. 2. In chemically modified InSe, the strength of magnetic exchange interaction can be modulated by changing the coverage of adsorbates. 3. The orbital hybridization between the magnetic adatoms and nonmagnetic InSe

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monolayer could play a vital role for this kind of induced magnetism.

ABSTRACT: In this work we studied the adsorption-induced magnetic behaviors on the two-dimensional InSe monolayer. Six kinds of adatoms (H, B, C, N, O and F) are

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taken into account. It is found that the InSe with adsorbing C and F have nonzero

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magnetic moments and good stability. Importantly, the magnetism of C and F

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modified InSe monolayers completely comes from p electrons of adatoms and

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substrates. The strength of magnetic exchange interaction can be controlled by

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changing the coverage of adsorbates. This p-electron magnetic material is thought to have obvious advantages compared to conventional d- or f-electron magnets. Our

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research is meaningful for practical applications in spintronic electronics and two

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dimensional magnetic semiconductors.

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Keywords: 2D monolayers; Adsorption; Magnetism; Electronic structure.

1. Introduction Two-dimensional (2D) materials such as graphene, silylene, and boron nitride monolayer are always a hot topic for their exotic electronic properties.[1, 2] The doping and adsorbing are two important means used to improve the inherent attributes

of 2D materials for potential applications in electronics.[3-6] It has been found that the transport property, magnetism, catalytic activity and etc. can be tuned delicately by introducing dopants or adsorbing adsorbates.[7, 8] Taking the magnetic property as an example, magnetism can be introduced into nonmagnetic 2D materials via chemical modification of single atom or cluster.[5, 7, 9] The theoretical simulations

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based on DFT calculations have suggested that the absorption can induce local moments and even possible long-range magnetic order into graphene and MoS2

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monolayer, two representative two-dimensional crystals.[5, 7, 10, 11] These magnetic nanostructures are not only scientifically interesting but also technologically

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important, with possible applications in magnetic recording and spintronics.

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In this work, we focus on the InSe monolayer, which is a new 2D material and has

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a higher environmental stability than the few-layer black phosphorous.12 Importantly,

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it exhibits very useful physical properties such as high electron mobility, quantum Hall effect and anomalous optical response.[12] Three-dimensional InSe bulk belongs

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to the layered metal chalcogenide semiconductors. Each layer has a honeycomb lattice, which just corresponds to the structure of 2D InSe monolayer. The pristine

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InSe monolayer is nonmagnetic. However, the magnetism is observed in the

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chemically modified InSe in our simulations. The mechanism for inducing magnetism is studied detailed based on first-principles calculations. Six kinds of nonmetal atoms (H, B, C, N, O and F) as adsorbates are taken into account on the InSe monolayer. As

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known, the magnetism of materials usually originates from the unpaired d or f electrons. However, in InSe systems the adsorption induced magnetism comes from the p electrons of the nonmetal elements, which is very important for both basic physics and practical applications compared to d-electron magnets. Some authors have claimed that s- or p-electron magnetic materials have obvious advantages, such

as stronger long-range exchange coupling interactions and no clustering of magnetic ions.[13] As for the induced magnetism, the InSe monolayer has significant differences from the graphene and the MoS2 monolayer. Firstly, the induced magnetism on InSe monolayers only comes from p electrons, different from that on MoS2. For MoS2 monolayer, the 4d electrons of Mo also make a contribution to the

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induced magnetic moments by adsorption, apart from the s or p electrons.[11] Secondly, for graphene, though the s or p-electron magnetism can be induced by

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adsorbing Nitrogen atom and etc,[10] both the adsorption stability and magnetic

moments need to be enhanced. Take N-adsorbed graphene as an example, only 0.84

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μB magnetic moment is obtained per N atom with a small absorption energy (0.88 eV). For C/InSe (hereinafter nonmetal atom adsorbed InSe is marked by Atom/InSe),

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our calculations show that the induced magnetic moment is 2μB per C atom with an absorption energy of 2.64 eV, significantly larger than those of N/graphene. The

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stable adsorption and large magnetic moment are very important for practical

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applications at room temperature and the formation of long-range magnetic order.

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2. Model and methods

All calculations are performed using the VASP package.[14] The ionic cores are

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represented by the projector augmented wave (PAW) potentials.[15] The Kohn-Sham orbitals are expanded using plane waves with the well converged cutoff energy of 500 eV. The exchange-correlation functional are employed to describe the interaction

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among electrons at the level of the generalized gradient approximation (GGA), using the PerdeweBurkeeErnzerhof (PBE) formula.[16] The sampling of the Brillouin zone is done using a 4×4×1 Monkhorst–Pack grid.[17] The studied model consists of a 4×4 InSe supercell including 32 In and 32 Se atoms, and an nonmetal atom. In order test the effects of supercell sizes and shapes on induced magnetism, 3×3, 4×8, 8×8

and 4 × 𝟐√𝟑 supercells are also calculated (see Supplementary material). All adsorption are relaxed until the Hellmann–Feynman force less than 0.01eV/Å. The Gaussian smearing method is used to determine electron occupancies with a smearing parameter of 0.05 eV. To testify whether the spin-orbital coupling (SOC) has a vital effect, DFT+SOC methods are also used for all adsorption systems. It is found that the

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correction of adsorption energies is less than 0.001 eV for all adsorbing systems, and

the correction of magnetic moment is less than 0.03 μB for all magnetic systems. The

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detailed comparison is given in Supplementary material. 3. Results and discussion

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3.1 Adsorption energies and structures

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In order to quantify the binding strength of a single adatom on the InSe

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monolayer, the adsorption energies are calculated, which is defined as

(1),

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Eads = Etot - EInSe - Eatom

where Etot and EInSe are the total energies of the InSe monolayer with and without

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adatoms, respectively, and Eatom stands for the energy of a single adatom. To search for the most stable configuration, four different initial positions labeled as top (at the

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top of Se atom), bridge (above the middle of In and Se atoms), hcp (above the center of the In-Se hexagonal ring) and fcc (at the top of In atom) are considered, as shown

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in Fig. 1(a). The calculated adsorption energies for the obtained stable configurations are listed in Table I.

Table 1. The absorbed energy (Eads, in eV) at each adsorption site, and the total magnetic moment of the supercell (in μB) listed in bracket. H

B

C

N

O

F

1.03 _ _ _

_ 2.78 2.43 _

1.65 (1.9) 2.47 (0.0) _ 2.64 (2.0)

0.35 0.06 (3.0) 0.06 (3.0) _

3.57 _ _ 3.57

1.97 (0.78) 2.57 _ _

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top hcp fcc bridge

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Fig. 1 (Color online) (a) The 4×4 supercell of InSe monolayer with four adsorption sites marked.

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The spin density distributions (isosurface value=0.001 Å-3) of C/InSe for the top and bridge sites

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are shown in (b) and (c), respectively, and that of F/InSe for the top site shown in (d). The blue

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and red colors represent the net spin-up and spin-down electron density, respectively.

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It is found that, for various adatoms, the most stable adsorption sites are completely different. H/InSe system has only one stable adsorption structure with an Eads of 1.03

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eV, in which H atom is adsorbed on the top site of Se atom, similar to the case of

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H/MoS2 and H/Graphene systems.[11, 18] B preferentially adsorbs on two sorts of three-fold hollow sites (hcp and fcc) surrounded by three Se atoms, forming three equivalent B-Se bonds. C atoms at the fcc sites are not stable, and spontaneously

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move to the In-Se bridge sites bonded with Se and In atoms. Additionally, the C can also be adsorbed on top site vertically (see Fig. 1(b)), and on hcp site with C leaning to two Se atoms. Comparatively, the In-Se bridge site has a best stability with an Eads of 2.64 eV, shown in Fig. 1(c). Both N and O atoms tend to bond with Se on the top with an Eads of 0.35 eV and 3.57 eV, respectively. The N/InSe belongs to the physical

adsorption with very small adsorption energies, so we will leave out the discussion on the N adsorption. The O atoms at hcp and fcc sites are not stable and move to In-Se bridge-like sites. F atoms can only localize at two sites (top and hcp) steadily. 3.2 Adsorption induced magnetism

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For H, B and O/InSe monolayers, all 4×4 supercells are nonmagnetic. In contrast, for C and F absorbed InSe monolayers there exists nonzero magnetic moments, and

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the magnetism depends on the adsorption structures, as listed in brackets of Table I. These results are significantly different with the results on the MoS2 monolayer and graphene.[11, 18] For MoS2 all adatoms (H, B, C, N, O and F) can induce

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magnetization due to the contributions of Mo_4d electrons. However, for InSe

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monolayers the adsorption induced magnetism is selective for adatoms. Herein three

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magnetic adsorption systems are discussed, i.e. C/InSe with C on the top of Se,

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C/InSe with C on the In-Se bridge, and F/InSe with F on the top of Se atom, as shown in Fig. 1(b), (c), and (d). For F/InSe systems only the adsorption on top sites has

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nonzero magnetic moments (0.78 μB). For C/InSe systems the adsorption on both top

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and bridge sites give magnetic states. Interestingly, the absorptions of C has an integer number (about 2.0 μB per C) of unpaired spin in the supercell, which is

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similar to the H-absorbed graphene (1.0 μB per H);[5] but the absorptions of F have fractional magnetic moments, similar to the cases of N and P absorbed graphene.[10, 19] Besides the Kekule supercells shown in Fig. 1, we also use the orthorhombic

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supercells to calculate the induced magnetic moments. The results are very similar (see Supplementary material). To investigate the contributions of the adatoms and InSe monolayer on the magnetism respectively, we use Bader’s analysis to calculate the magnetic moment localized on the individual atoms, and the distributions of spin densities. It is found

that the contribution of F atom to the total magnetic moments is 23.8%, and other moments come from the neighboring Se of upper surface and In of lower surface with the spin density distributing in two sublayers of InSe, shown in Fig. 1(d). The contributions of C atom to magnetism are 72.8% and 77.5% on In-Se bridge and top sites respectively, and other moments come from the neighboring Se of the substrate,

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shown in Fig. 1(b) and (c). These results are also different from the cases of C

absorbed MoS2 monolayers, where the magnetic moments mainly come from the

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substrates, and the contributions of adatoms is very small (only 1.7% for adsorbed C).[11] The differences indicate that the formation mechanism of the magnetic

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moment could be different. In MoS2 the Mo_4d electrons of the substrate make the

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significant contributions to the total magnetic moment. In contrast, in the modified

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InSe systems all magnetism comes from the p electrons. In addition, relatively large spatial extensions of the spin density are observed in the adsorption systems with C on

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In-Se bridge site and with F on Se top site, as shown in Fig. 1(b) and (d), which is

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important to achieve long-range magnetic coupling interaction at low adsorbate concentrations. While the adsorption with C on Se top site has relatively small spatial

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extensions of the spin density. For this case, the magnetic coupling can be enhanced

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by increasing the coverage of adatoms, as discussed below. 3.3 Electronic structures To understand the origin of magnetism induced by the adsorption, the total and

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partial density of states (PDOS) is calculated for the adsorption configurations with maximum stability, as shown in Fig. 2.

Density of states (1/eV)

1

H

0

100

Se_p

-3

0

C

C_p

-3

Se_p

1 0

O

-1

O_p

3

Se_p

0 -3 2

0

0

0

100

100

100

0

0

0

0

-100

-100

-100

-100

0

0

-100 3 -3

0

Energy (eV)

-100 -3 3

0

Energy (eV)

0 -100 -3 3

F

F_p Se_p

0

0

Energy (eV)

3 -100-3

0

Energy (eV)

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Energy (eV)

3 0

In_p

Se_s 100 pure InSe

0

B_p

Se_p

0

0 -100modified InSe

-100 -3

B

0

-1 -8 -7 0

3

H_s

Fig. 2 (Color online) The total DOS and PDOS. (a) the H, B, C, O and F absorbed InSe

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monolayers with the best stability; Positive and negative values correspond to spin-up and spindown states. The first row is the PDOS of adatoms; the second row is the PDOS of the nearest Se

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(black) and In (red) atoms; the third row is the total DOS of pure InSe monolayer; and the last row

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is the total DOS of adatom/InSe, respectively. The insert picture displays the deep energy levels of

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H/InSe.

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In B, C, and O/InSe systems, the PDOS plots suggest that there exist the significant orbital hybridization of the adatom and neighboring Se (or In) atoms in the

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substrate, indicating the strong interaction between them. For B/InSe system the 2p states of B hybridize with both Se_4p and In_5p states; for C/InSe and O/InSe the 2p

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electrons of adatoms (C and O) only hybridize with Se_4p electrons. For the H/InSe

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(top) and F/InSe (hcp) systems, the small bond lengths of 1.488 Å for H-In bond and 2.03 Å for three F-In bonds as well as the large bonding energies (see Table 1) also indicate forming chemical adsorption, although the hybridization between the states

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of adatoms and the nearest Se and In is not obvious near the Fermi energy. In the O/InSe system no new gap states are observed with little change of the band gap, despite the very large adsorption energy. The absorption of B and C atoms can induce deep-level defect states in the band gap, which causes an evident decrease of the energy gaps of the InSe monolayer. Therefore, the optical and transport properties

may be significantly affected. It’s worth noting that, in the C/InSe adsorption system, the spin-splitting of the energy level of gap states is observed between the occupied spin-up states and unoccupied spin-down states, leading to the nonzero magnetic moments. The Bader charge analysis suggest that all adatoms obtain electrons from the InSe substrates, which is similar to the 2D antimonene adsorbing H, B and C,[6]

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but is in contrast with the H, B and C modified MoS2. To understand this result, we compare the charges of In and Se of the InSe monolayer before and after the atom

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adsorption, and find the excess charges on the adatoms mainly come from the In ions

instead of Se. As known, In atom has three valence electrons in the 5s and 5p orbitals, but the Bader calculations show that the In ion has a charge state of “+2” in

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the pristine InSe monolayer. In2+ ion still has reducibility and can lose one electron.

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Additionally, In atom has a lower electronegativity than all the adatoms. The B, C and

which contribute to the gap states.

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F absorbers obtain about 1, 2 and 1 electron from the InSe substrate, respectively,

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Besides the most stable systems, the F and C adsorption on the Se top sites is also discussed due to their nonzero magnetic moment. The total and partial DOS have

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been shown in Fig. 3(a) and (b). Here the gap states of the C/InSe system mainly

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come from the adatom (C atom), and the contribution of the InSe substrate is very small. Therefore, the magnetic moments largely localize on the C atom with a ratio of 77.5%, similar to the case of the most stable C/InSe system with the C atom on the In-

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Se bridge site. As for F/InSe, there exist significant differences of electronic structures between the metastable configuration and the most stable one. The former has the gap states in the PDOS of F with a strong hybridization between adatom and substrate, and the latter does not.

As known, the single H, B, C, O, and F atoms have nonzero magnetic moments due to the unpaired electrons. The orbital hybridization between the magnetic adatoms and nonmagnetic InSe monolayer will play an interesting dual role for the magnetism of A/InSe. For the adsorption of H, B, and O, the hybridization (and charge transfer) can lead to the magnetic disappearance. But for the adsorption of C and F, hybridizing

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with the states of adatoms can induce the magnetism of the InSe substrate. In a word, orbital hybridization between adatoms and substrate is very important for inducing

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magnetism.

In addition, previous theoretical studies [20, 21] have pointed that, for the band

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gap of monolayer InSe, GW approximation and the Bethe-Salpeter equation for

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excitonic corrections would give a better description. However, it requires a

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computationally much more expensive calculation for our adsorption models

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including 65 atoms, and then is not adopted in the present work, similar to previous

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works.

3.4 Possible long-range ordering

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At last, we suppose the long-range magnetic ordering could be formed in these adsorbates modified InSe monolayer. Two sorts of high coverages (25% and 100%)

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for C and F atoms adsorption on the InSe monolayers are considered. F/InSe systems can retain the local magnetic moments with the coverage of 25% for the

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ferromagnetic and antiferromagnetic arrangements, but the total energies of both magnetic configurations are nearly degenerate, indicating that the exchange interaction is too weak to get the long-range order. For the case of 100% coverage of F, the local magnetic moments vanish, and the F/InSe becomes a paramagnetic system. For C/InSe systems, different adsorption patterns are considered firstly (see Supplementary material). It is found that the adatoms prefer uniform distribution

instead of clusterisation. For the C adsorption with a 25% coverage, the antiferromagnetic state has a better stability than that of ferromagnetic state with an energy difference of 14 meV (EAFM − EFM= −14 meV) for a 4×4 supercell including 4 C atoms, shown in Fig. 3(c) amd (d). As known, the two-dimensional magnetic lattice models exhibit the extremely abundant physics, such as frustration phenomenon, spin

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liquid and spin ice.[22-24] However it is difficult to design an actual 2D magnetic material to match these theoretical lattice models. Our research could provide an idea

DOS (States/eV)

2

-1

Se_p

0

0

0

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0

Energy (eV)

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(a)

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Se_p

-100

0 -3

F_p

A

100

0 100

-100

F

0

-100

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1

C_p

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0

C

N

2

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to prepare 2D magnetic systems in experiments based on the theoretical models.

(c)

0

3 -100-3

0

Energy (eV)

3

(b)

(d)

Fig. 3 (Color online) The total DOS and PDOS of C/InSe (a) and F/InSe (b) with C and F

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adsorbed on the Se top sites. (c) and (d) are ferromagnetic and antiferromagnetic order of C/InSe system with a 25% coverage, respectively.

4. Conclusions

In conclusion, our studies suggest that the adsorption of C and F atoms on the InSe monolayer can induce magnetic moments of p electrons and modify the electronic band structures, which means that the adsorption can be served as an efficient method to tune the physical properties of InSe. In addition, the calculated results show that the coverage of adatoms also plays a vital role to form both the local

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moments and long-range order. The strength of magnetic exchange interaction can be

enhanced by increasing the coverage of adsorbates. The current work on InSe and

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previous studies on MoS2 and graphene indicate that it could be a universal behavior for two-dimensional semiconductor materials that the magnetism can be induced by

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the adsorption of nonmetal atoms.

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Acknowledgements

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This work was supported by National Natural Science Foundation of China

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(Grant Nos. 11247012, U1504108, 11474086 and 11504093). The computational

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resources were provided by the Supercomputing Center of Henan Normal University. References

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