Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P31: Magnetism in 2D Materials IIFocus
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Sponsoring Units: DMP Chair: Efren Navarro-Moratalla, Massachusetts Institute of Technology Room: 294 |
Wednesday, March 15, 2017 2:30PM - 3:06PM |
P31.00001: 2D Magnets Invited Speaker: Dave Mandrus |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P31.00002: Magnetic, transport, and electronic properties of V-intercalated 2H-NbS2 Deepak Sapkota, Nick Sirica, Rupam Mukherjee, Michael Koehler, Ganesh Pokharel, Paolo Vilmercati, Norman Mannella, David Mandrus Intercalation of transition metal dichalcogenides (TMDs) by 3d or 4d transition metal elements is of considerable interest because the foreign atoms play an important role in the physical properties of host TMDs. The intercalation can cause profound change in magnetic, transport, thermal, and electronic properties of the host TMDs. One of the well-studied intercalated systems is Cr$_{\mathrm{1/3}}$NbS$_{\mathrm{2}}$, which belongs to a non-centrosymmetric hexagonal space group $P$6$_{\mathrm{3}}$22, and it exhibits helical spin order below transition temperature 127 K. To further understand the effect of intercalated species, we synthesized and investigated magnetic, transport, thermal, and electronic properties of V-intercalated NbS$_{\mathrm{2}}$ (V$_{\mathrm{0.3}}$NbS$_{\mathrm{2}})$. V$_{\mathrm{0.3}}$NbS$_{\mathrm{2}}$ belongs to the $P$-31m ({\#}162) space group and shows ferromagnetic ordering. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P31.00003: Defect induced magnetism in transition metal dichalcogenides. Mahtab Khan Single layer (SL) transition metal dichalcogenides (TMDCs) (MX$_{\mathrm{2}}$; M$=$ transition metal such as Mo, W and X $=$ S, Se, Te) have attracted a lot of attention due to their intriguing electronic and optical properties. SL TMDCs are direct band gap semiconductors, which can be used to produce smaller and more energy efficient devices such as transistors and integrated circuits. Moreover, the band gap lies in the visible region, which makes them highly responsive when exposed to visible light, a property with potential applications in optical detection. Despite their success as a fascinating SL semiconductor, magnetism in TMDCs has remained almost unexplored. Due to their technological importance wafer scale production of TMDCs is required. The types of defect observed in TMDCs depends on the fabrication process. The most common experimental techniques used to produce large chunks of SL MoS2 are i) mechanical exfoliation, ii) chemical vapor deposition, and iii) physical vapor deposition. Defects usually play an important role in tailoring electronic, optical and magnetic properties. We performed standard first principle calculations to show that certain defects induce magnetism in TMDCs. In particular we study two types of defects: i) the M vacancy defect and ii) the antisite defect M$_{\mathrm{X}}$. We find that certain TMDCs exhibit an exceptionally large magnetic moment in the presence of these defects. In addition, we show that the value of their magnetic moment can be tuned by changing the defect density. Our findings considerably improve the understanding of defect induced magnetism in SL TMDCs and should benefit their potential applications in spintronic devices. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P31.00004: RKKY interaction in graphene with Rashba spin-orbit coupling Diego Mastrogiuseppe, Sergio Ulloa We present a study of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities embedded in graphene in the presence of Rashba spin-orbit coupling (SOC). It is well known that a combination of SOC and broken inversion symmetry results in an anisotropic exchange interaction between magnetic species, including a twisted Dzyaloshinskii-Moriya (DM) component [1]. In graphene, a large Rashba SOC can be induced with Au intercalation in Ni or BN substrates [2]. Using the Matsubara Green’s function formalism, we calculate the RKKY interaction at zero and finite temperature. We present results for varying SOC strength and graphene Fermi energy, paying special attention to the case where the Fermi level lies close to the secondary band edge. We provide angular dependent factors that modulate the interaction for different relative orientation of the impurities. We also compare our results to those of bilayer graphene without SOC [3], which features similar band structure as our system, with the interlayer hopping parameter playing the role of the Rashba parameter. Our results can be tested by spin polarized STM experiments. [1] PRB 69, 121303 (2004) [2] Nat. Commun. 3, 1232 (2012); PRL 117, 076603 (2016) [3] PRB 92, 205414 (2015) [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P31.00005: Exploring Proximity-Induced Ferromagnetism in Graphene/Cr$_{2}$Ge$_{2}$Te$_{6}$ Heterostructures Aaron Sharpe, Wenmin Yang, Menyoung Lee, Kenji Watanabe, Takashi Taniguchi, David Goldhaber-Gordon Due to its tunability and high electron mobility, graphene is a promising platform for spintronics. While isolated graphene is non-magnetic, ferromagnetism can be induced by controlling its local environment. Through proximity effects, 2D materials are known to inherit order parameters from a substrate when the two are placed in intimate contact. Proximity-induced ferromagnetism has been seen in graphene/yttrium iron garnet (YIG) heterostructures. Several other ferromagnetic insulators could be used instead. Unlike YIG, Cr$_{2}$Ge$_{2}$Te$_{6}$ (CGT) is a layered ferromagnetic insulator which makes it an ideal candidate substrate to produce ferromagnetism in graphene while maintaining its high electron mobility. Here we study proximity-induced ferromagnetism in graphene placed on exfoliated flakes of CGT. CGT is unfortunately sensitive to air, a major challenge for making a clean interface. Therefore, we measured graphene/CGT heterostructures fabricated in a controlled environment. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P31.00006: Towards valley transistor in MoS2/EuS through interfacial magnetic exchange field Peng Wei, Ahmad Zubair, Tomas Palacios, Jagadeesh Moodera Intense Zeeman field (ZF) in transitional metal dichalcogenides (TMD) can lift valley degeneracy and give rise to novel transistor devices carrying pure valley polarization. Such ZF has been shown to exist at the interface between 2D materials, for example graphene, and ferromagnetic insulator (FI) EuS due to magnetic proximity effect. Taking this approach, our research work focuses on investigating new valley transistor phenomena in TMD coupled to FI. We have successfully built high quality MoS2/EuS heterostructures and fabricated mesoscopic MoS2/EuS transistors. We found appreciable charge transferring effect that is taking place between EuS and MoS2, which significantly modifies the transistor behavior compared to pristine device. Furthermore, we have achieved ambipolar gating in such devices, and point out to the successful tuning into the hole doped regime. Such conductivity gating demonstrated fine features inside MoS2 band gap that might be associated to the neighboring EuS layer. Our results thus serve to achieve the yet to be reached full-valley-operating nano devices, and will lead to the discovery of unexplored physics phenomena on valley controlled charge and spin transport. Work supported by NSF and ONR grants. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P31.00007: Resonant to bound state transitions in superconducting graphene: roles of local spin-orbit coupling and magnetic moments. Denis Kochan, Lucia Komendova, Jaroslav Fabian The unique electronic band structure of graphene allows formation of resonant states near its charge neutrality point. Resonant impurities that locally enhance spin dynamics, via adatom induced local spin-orbit coupling (SOC), or local magnetic moments, turn those resonances into spin hot spots, that strongly affect spin-relaxation. It is of great practical and theoretical importance to disentangle effects originating from the local SOC and the local magnetic moments. \newline Superconducting graphene offers such a possibility. The proximity induced gap can significantly affect the formation of resonances. Moreover, the interplay between s-wave pairing and local spin dynamics creating the spin hot spots causes formation of bound states (Shiba states). Those can be detected and analyzed by STM experiments. We discuss the formation of such bound states focusing on LDOS and spatial variation of the superconducting pairing, exploring potential imprints distinguishing local SOC features from local magnetic moments. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P31.00008: Resonant scattering in graphene in a strong magnetic field Jeongsu Lee, Denis Kochan, Jaroslav Fabian Spin relaxation in graphene is strongly affected by the resonant scattering mechanism due to magnetic impurities forming spin hot spots even with little contribution to the momentum scattering process. Recent experiments demonstrate that these magnetic moments can be induced either by vacancies or adsorbents such as hydrogen atoms. Meanwhile, unlike in the non-relativistic two dimensional electron gas, a strong magnetic field in graphene induces a Landau level overlapping with the resonance peak in the density of states near the Dirac point. We study the interplay between resonance scattering and strong magnetic fields in the presence of magnetic moments and local spin-orbit coupling to elucidate the spin dependent transport characteristics. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P31.00009: Valley selective excitations and the Kondo effect in two dimensional transition metal dichalcogenides Vivek Aji, Michael Phillips Lack of inversion, strong spin orbit coupling, and nontrivial Berry curvature lead to interesting correlated phenomena in the two dimensional transition metal dichalcogenides (TMDCs). In this talk we report on Kondo effect in hole doped systems, focussing on the nature of the resonance for unequal chemical potential in the two valleys. The imbalance is generated by circularly polarized light that couples only to one of the valleys which is a property endowed by the Berry curvature of the bands. Using variational wave-function and numerical renormalization group approaches we establish the properties of the Kondo resonance namely the impurity spectral function, magnetization, entropy and susceptibility. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P31.00010: Understanding the magnetic and electronic properties of transition-metal chalcogenides T. LaMartina, A.V. Balatsky, J.T. Haraldsen This study computationally examines the magnetic and electronic properties for various transition-metal chalcogenides (M$_2$X$_2$ with M = Cr, Mn, Fe, and Co and X = S, Se) using density functional theory. The M$_2$X$_2$ structure is a quasi two-dimensional honeycomb lattice. Through spin-polarized general gradient approximation with onsite potential (SGGA + U), we determine and compare the net electronic structure and density of states for the various compounds. Examination of the net magnetic moment and structure suggest a distorted tetrahedral crystal-field symmetry, and an analysis of the electronic structure shows the presence of nodal points that resemble Dirac nodes. Overall, it is predicted that all materials demonstrate a ferromagnetic metallic state, while some structures (mainly Cr-based) have a half-metallic state. This leads to the possibility of Dirac half-metal states. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P31.00011: The Magnetism in 3d and 4f Intercalated Graphene-Surface Systems Nicolae Atodiresei, Vasile Caciuc, Stefan Bl\"ugel Graphene adsorbed on Ir(111) [1] is a widely used two-dimensional template which can be functionalized via metal intercalation [2, 3] or molecular adsorption [4]. Our first principles density functional theory (DFT) calculations employing a non-local correlation vdW-DF functional unveiled that the bonding mechanism of graphene on Ir(111) is physisorption with a local chemical modulation. Furthermore, our theoretical investigations of a 3d Co and Fe monolayer intercalated between graphene and Ir(111) demonstrated that the strong hybridization at the interface drastically modifies the magnetic properties of both graphene and substrate. With the prospect of using graphene in spintronics, we will show how the spin-splitting of the graphene $\pi $ system can be tailored through a fine interplay between the weak hybridization, electrostatic and vdW interactions by intercalating 4f Eu between graphene and Ni(111). \textit{E-mail address: n.atodiresei@fz-juelich.de ; } [1] C. Busse et al. Phys. Rev. Lett. \textbf{107 }036101 (2011); [2] R. Decker et al. Phys. Rev. B \textbf{87}, 041403(R) (2013); [3] J. Brede et al. Nature Nanotech. \textbf{9}, 1018 (2014); [4] F. Huttmann et al. Phys. Rev. Lett. \textbf{115 }236101 (2015) [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P31.00012: Quantum oscillations of the magnetic moment of graphene and graphite. Ivana Petkovic, Anthony Lollo, Ke Wang, Philip Kim, Jack Harris Quantum oscillations of the magnetic moment, so called de Haas - van Alphen (dHvA) oscillations, are a powerful tool for the investigation of the Fermi surface. In graphene with a fixed carrier density, the magnetic moment is predicted to oscillate as function of increasing perpendicular field B every time the uppermost Landau level empties out, yielding the characteristic 1/B dependence. To date, it has been challenging to measure the equilibrium magnetic moment of isolated samples of graphene. In graphite a more complex oscillation sequence is observed, due to its complicated Fermi surface with both electron and hole carriers. Historically graphite was one of the first materials in which dHvA oscillations were studied, but recently interest was revived due to an observation of carriers with relativistic dynamics. We have used cantilever torque magnetometry to study diamagnetism and dHvA oscillations of isolated samples of graphene and graphite between 400 mK and 20 K. For graphite, we observe dHvA oscillations which are used to study the composition and nature of carriers. For graphene, we discuss the results in relation to relativistic dispersion and disorder. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P31.00013: Magnetic gap opening in rhombohedral stacked multilayer graphene from first principles Matteo Calandra, Betul Pamuk, Jacopo Baima, Francesco Mauri We investigate the occurrence of magnetic and charge density wave instabilities in rhombohedral stacked multilayer graphene by using hybrid functionals. Neglecting spin-polarization, an extremely flat surface band centered at the special point ${\bf K}$ of the Brillouin zone occurs at the Fermi level. Spin polarization opens a gap in the surface state by stabilizing an antiferromagnetic state. The top and the bottom surface layers are weakly ferrimagnetic in-plane and are antiferromagnetic coupled to each other. This coupling is propagated by the out-of-plane antiferromagnetic coupling between the nearest neighbors. The gap is very small in spin polarized generalized gradient approximation, it increases with larger amount of exact exchange. For trilayer rhombohedral graphene it is $38.6$ meV in PBE0, in agreement with the $42$ meV gap found in experiments. We study the temperature and doping dependence of the magnetic gap. Charge density wave instabilities with $\sqrt{3}\times\sqrt{3}$ periodicity do not occur.\newline\\ References: Betul Pamuk, Jacopo Baima, Francesco Mauri, Matteo Calandra arXiv:1610.03445 [Preview Abstract] |
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