Metal-insulator transition and phase separation in doped AA-stacked graphene bilayer (original) (raw)
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Magnetic Field-Controlled Electrical Conductivity in AA Bilayer Graphene
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We consider the effect of the external magnetic field on the in-plane conductivity in the AA-stacked bilayer graphene system in the strong excitonic condensate regime. We include the effects of the applied inter-layer electric field and the Coulomb interactions. The on-site and inter-layer Coulomb interactions were treated via the bilayer Hubbard model. Using the solutions for the physical parameters in the system, we calculate the in-plane conductivity of the bilayer graphene. By employing the Green-Kubo formalism for the polarization function in the system, we show that the conductivity in the AA bilayer system is fully controlled by the applied magnetic field. For the partial filling in the layers, the electrical conductivity is different for different spin orientations, and, at the high values of the magnetic field, only one component remains with the given spin orientation. Meanwhile, for the half-filling limit, there is no spin-splitting observed in the conductivity function. ...
Materials Today: Proceedings, 2019
We communicate a microscopic theoretical model study of electronic and thermal properties of AA-stacked bi-layer graphene in a transverse electric field. In order to describe the system, we have considered a Hamiltonian consisting of nearest-neighbor electron hopping in both the layers in presence of a transverse electric field between the two layers created by a gate electrode. The inter-layer electron hopping from atom of the first layer to atom of the second layer is considered with a hopping energy , = | ()|. Due to strong on-site Coulomb correlation between the electrons of sub-lattices of both the layers, antiferromagnetic order is developed in the system. The electron Green's functions of both sub-lattices in the two layers are solved by Zubarev's Green's function technique. The anti-ferromagnetic (AFM) gap equation is found from the electron correlations of the corresponding Green's functions. It is assumed that the A-site magnetization dominates over the B-site magnetization giving rise to the anti-ferromagnetic order. The temperature dependent AFM gap exhibits high Neel's temperature where electronic specific heat shows a sharp jump. Besides this, the specific heat exhibits another flat peak arising due to the onset of AFM order in A and B sub-lattices of the layer. The evolution of the gap equation and specific heat are reported by varying the model parameters like Coulomb energy, electric field, transverse electron hopping integral and chemical potential.
Semiconducting behavior of substitutionally doped bilayer graphene
Physica B-condensed Matter, 2018
In the framework of the Green's functions approach, random tight-binding model and using the coherent potential approximation, electronic characteristics of the bilayer graphene are investigated by exploring various forms of substitutional doping of a single or both layers of the system by either boron and (or) nitrogen atoms. The results for displacement of the Fermi level resemble the behavior of acceptor or donor doping in a conventional semiconductor, dependent on the impurity type and concentration. The particular pattern of doping of just one layer with one impurity type is most efficient for opening a gap within the energy bands which could be tuned directly by impurity concentration. Doping both layers at the same time, each with one impurity type, leads to an anomaly whereby the gap decreases with increasing impurity concentration.
Low density ferromagnetism in a biased bilayer graphene
Physical Review Letters, 2008
We compute the phase diagram of a biased graphene bilayer. The existence of a ferromagnetic phase is discussed with respect both to carrier density and temperature. We find that the ferromagnetic transition is first order, lowering the value of UUU relatively to the usual Stoner criterion. We show that in the ferromagnetic phase the two planes have unequal magnetization and that the electronic density is hole like in one plane and electron like in the other.
Metal-insulator transitions in graphene
The experimental observation of a non standard sequence of integer quantum Hall plateaus in graphene has renewed the interest for the study of the quantum phase transitions (QPTs). We have measured the plateau-insulator (PI) QPT in monolayer graphene, observing a ν = 0 plateau and obtaining the value of the critical exponent k = 0.58 ± 0.03 . More recently we extended our study to a wide temperature range and different gate voltage (VG), and the results question the universality of the critical exponents in graphene. Actually, this study can help to clarify the controversy about the nature of ν = 0 state. Indeed, previous experiments have shown that at ν ∼ 0 the longitudinal resistivity may either decrease or increase [5] with decreasing temperature, fueling the debate concerning the existence and the origin of an insulator phase at the charge neutrality point (CNP) at high magnetic fields.
Low-Density Ferromagnetism in Biased Bilayer Graphene
Physical Review Letters, 2008
We compute the phase diagram of a biased graphene bilayer. The existence of a ferromagnetic phase is discussed with respect both to carrier density and temperature. We find that the ferromagnetic transition is first order, lowering the value of U relatively to the usual Stoner criterion. We show that in the ferromagnetic phase the two planes have unequal magnetization and that the electronic density is hole like in one plane and electron like in the other.
Functionalized graphene as a model system for the two-dimensional metal-insulator transition OPEN
Reports of metallic behavior in two-dimensional (2D) systems such as high mobility metal-oxide field effect transistors, insulating oxide interfaces, graphene, and MoS 2 have challenged the well-known prediction of Abrahams, et al. that all 2D systems must be insulating. The existence of a metallic state for such a wide range of 2D systems thus reveals a wide gap in our understanding of 2D transport that has become more important as research in 2D systems expands. A key to understanding the 2D metallic state is the metal-insulator transition (MIT). In this report, we explore the nature of a disorder induced MIT in functionalized graphene, a model 2D system. Magneto-transport measurements show that weak-localization overwhelmingly drives the transition, in contradiction to theoretical assumptions that enhanced electron-electron interactions dominate. These results provide the first detailed picture of the nature of the transition from the metallic to insulating states of a 2D system. The excitement generated by the achievement of metallic single layer graphene has obscured the fact that seminal theoretical work predicted that purely two-dimensional (2D) systems should not be metallic 1. A possible explanation for the metallic behavior in graphene is that massless Dirac electrons exhibit Klein tunneling and are thus, immune to the effects of disorder 2,3. This argument is contradicted by reports that the carriers often have mass 4–6 , possibly due to disorder and/or the underlying substrate breaking lattice symmetry or the fact that the Fermi energy is far from the Dirac point 2. Thus, graphene should be described by the theory presented in reference 1 if there is disorder in the potential binding the electrons. The situation is confounded further by later theoretical work showing that Dirac Fermionic systems with no spin-orbit interactions and Gaussian correlated disorder exhibit scaling behavior but should always be metallic 3. The observed metallic behavior is an unquestionable addition to a series of systems such as high mobility metal-oxide field effect transistors (HMFET) 7 and interface oxides 8 that have demonstrated a 2D metallic state (although the nature of that state for the HMFET's is not well understood). These systems are presumed to be 2D due to their geometry but might have some three-dimensional character since the charge regions extend over finite distances 9,10 that could explain their metallic transport properties. The experimental conditions are also confounded by the fact that the thickness and shape of the charge layer varies with the application of a gate voltage. In contrast, graphene is a model system for studying the 2D metal-insulator transition (MIT) as it is a pure 2D system (with a constant thickness of 0.335 nm) like MoS 2 (which has recently been shown to also have an MIT 11–14). In this work, we increase the resistivity of epitaxial graphene through surface functionalization by exposure to low energy plasmas. These results reveal the existence of a 2D MIT in epitaxial graphene where the pre-functionalization values of carrier concentrations and mobili-ties are ~10 12 –10 13 cm −2 and ~700–900 cm 2 V −1 s −1 , values that are out of the range of applicability for the models developed to describe the previous results on the HMFETs 15,16 where the disorder is thought to be screened by high mobility electrons. Recent theoretical work treated the transition density for the apparent MIT observed in the HMFETs 17. Since these models treat a transition that occurs at finite temperature, rather than the true MIT quantum phase transition that occurs at T = 0, they are not applicable to this work. It is possible that a more recent general scaling model that was developed for the high mobility case, and allows for the existence of a 2D MIT 18 , can be used to model the graphene system as well. The results presented here demonstrate that the strongly localized state is separated from the metallic state by a weakly localized phase with conductivity, σ , ~log(T) similar to results recently reported for thin films of RuO 2 19 .
Emergence of magnetic behavior in AB-stacked bilayer graphene via Fe-doping
Vacuum, 2020
Bilayer graphene (BLG) as an electronic device has distinctive electronic properties in contrast to the monolayer graphene (MLG). The electronic and magnetic properties of BLG can be tailored effortlessly by authorizing an electric field, tacking dopant, and adatom. We have studied these properties of AB-stacked BLG on Fe-doping using a first-principles study with projector augmented wave (PAW) method under generalized gradient approximation (GGA). This doping is canvassed in three different configurations: Top, Bridge, and Hollow type. Our results manifest that the doping generates spin polarization and induces substantial magnetic moments in all three configurations. In the midst of all, the top configuration turns up energetically favorable. The genesis of magnetism begins with the interplay of Fe-3d states interacting with C-2p states in the environs of the doping site, which leads to band shifting at Fermi level (E F) and thus inducing the magnetic response in bilayer graphene.
Tight-Binding Model Study of Anti-ferromagnetic Order in AA-Stacked Bi-layer Graphene
Journal of Superconductivity and Novel Magnetism, 2017
We address here the anti-ferromagnetic order present in AA-stacked bi-layer graphene in a transversely applied electric field. The system is described by kinetic energy with nearest-neighbor electron hopping with same hopping integral t 1 for both the layers. Besides this, Coulomb interaction exists at A and B sub-lattices with same Coulomb correlation energy. The electron Green's functions are calculated by Zubarev's Green's technique. The temperature-dependent anti-ferromagnetic magnetization is calculated from the Green's function and is computed numerically and self-consistently. The strong on-site Coulomb interaction stabilizes the anti-ferromagnetic order in graphene. We assume that the electron spin at A site in the first layer is directed in the opposite direction to that of A site electron in the second layer. Similar spin order is observed for electrons in B site atom in reversed order. It is observed that anti-ferromagnetic (AFM) magnetization in the first layer nearly remains constant up to certain temperature and then increases with temperature, while the AFM magnetization in the second layer remains nearly constant and then rapidly decreases with temperature. The net AFM magnetization in bi-layer graphene remains constant and then rapidly increases with temperature. The evolution G. C.
Electronic transport and Klein tunneling in gapped AA-stacked bilayer graphene
Journal of Applied Physics
We demonstrate that AA-stacked bilayer graphene (AA-BLG) encapsulated by dielectric materials can possess an energy gap due to the induced mass term. Using the four-band continuum model, we evaluate transmission and reflection probabilities along with the respective conductances. Considering interlayer mass-term difference opens a gap in the energy spectrum and also couples the two Dirac cones. This cone coupling induces an inter-cone transport that is asymmetric with respect to the normal incidence in the presence of asymmetric mass-term. The energy spectrum of the gapped AA-BLG exhibits electron-hole asymmetry that is reflected in the associated intraand inter-cone channels. We also find that even though Klein tunneling exists in gated and biased AA-BLG, it is precluded by the interlayer mass-term difference and instead Febry-Pérot resonances appear.