| 1. |
- Pournaghavi, Nezhat, et al.
(författare)
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Realization of the Chern-insulator and axion-insulator phases in antiferromagnetic MnTe/Bi2(Se, Te)3/MnTe heterostructures
- 2021
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Ingår i: Physical review B (PRB). - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:19
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Tidskriftsartikel (refereegranskat)abstract
- Breaking time-reversal symmetry in three-dimensional topological insulator thin films can lead to different topological quantum phases, such as the Chern insulator (CI) phase and the axion insulator (AI) phase. Using first-principles density functional theory methods, we investigate the onset of these two topological phases in a trilayer heterostructure consisting of a Bi2Se3 (Bi2Te3) TI thin film sandwiched between two antiferromagneticMnTe layers. We find that an orthogonal exchange field from the MnTe layers, stabilized by a small anisotropy barrier, opens an energy gap of the order of 10 meV at the Dirac point of the TI film. A topological analysis demonstrates that, depending on the relative orientation of the exchange field at the two interfaces, the total Chern number of the system is either C = 1 or C = 0, characteristic of the CI and AI phases, respectively. Nontopological surface states inside the energy-gap region, caused by the interface potential, complicate this identification. Remarkably though, the calculation of the anomalous Hall conductivity shows that such nontopological surface states do not affect the topology-induced transport properties. Given the size of the exchange gap, we estimate that gapless chiral edge states, leading to the quantum anomalous Hall effect, should emerge on the sidewalls of these heterostructures in the CI phase for widths > 200 nm. We also discuss the possibility of inducing transitions between the CI and the AI phases by means of the spin-orbit torque caused by the spin Hall effect in an adjacent conducting layer.
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| 2. |
- Islam, Fhokrul, et al.
(författare)
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Ab initio calculations of the magnetic properties of Mn impurities on GaAs (110) surfaces
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; B 85, s. Article ID: 155306-
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Tidskriftsartikel (refereegranskat)abstract
- We present a computational study of individual and pairs of substitutional Mn impurities on the (110) surface of GaAs samples based on density functional theory. We focus on the anisotropy properties of these magnetic centers and their dependence on on-site correlations, spin-orbit interaction, and surface-induced symmetry-breaking effects. For a Mn impurity on the surface, the associated acceptor-hole wave function tends to be more localized around the Mn than for an impurity in bulk GaAs. The magnetic anisotropy energy for isolated Mn impurities is of the order of 1 meV, and can be related to the anisotropy of the orbital magnetic moment of the Mn acceptor hole. Typically Mn pairs have their spin magnetic moments parallel aligned, with an exchange energy that strongly depends on the pair orientation on the surface. The spin magnetic moment and exchange energies for these magnetic entities are not significantly modified by the spin-orbit interaction, but are more sensitive to on-site correlations. Correlations in general reduce the magnetic anisotropy for most of the ferromagnetic Mn pairs.
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| 3. |
- Islam, Fhokrul, et al.
(författare)
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First-principles study of spin-electric coupling in a {Cu3} single molecular magnet
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:15
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Tidskriftsartikel (refereegranskat)abstract
- We report on a study of the electronic and magnetic properties of the triangular antiferromagnetic {Cu3} single-molecule magnet, based on spin-density-functional theory. Our calculations show that the low-energy magnetic properties are correctly described by an effective three-site spin s = 1/2 Heisenberg model, with an antiferromagnetic exchange coupling J approximate to 5 meV. The ground-state manifold of the model is composed of two degenerate spin S = 1/2 doublets of opposite chirality. Due to lack of inversion symmetry in the molecule these two states are coupled by an external electric field, even when spin-orbit interaction is absent. The spin-electric coupling can be viewed as originating from a modified exchange constant delta J induced by the electric field. We find that the calculated transition rate between the chiral states yields an effective electric dipole moment d = 3.38 x 10(-33) C m approximate to e10(-4)a, where a is the Cu separation. For external electric fields epsilon approximate to 10(8) V/m this value corresponds to a Rabi time tau approximate to 1 ns and to a delta J on the order of a few mu eV.
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| 4. |
- Islam, Fhokrul, et al.
(författare)
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Impurity potential induced gap at the Dirac point of topological insulators with in-plane magnetization
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 99:15, s. 1-6
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Tidskriftsartikel (refereegranskat)abstract
- The quantum anomalous Hall effect (QAHE), characterized by dissipationless quantized edge transport, relies crucially on a nontrivial topology of the electronic bulk band structure and a robust ferromagnetic order that breaks time-reversal symmetry. Magnetically doped topological insulators (TIs) satisfy both these criteria, and are the most promising quantum materials for realizing the QAHE. Because the spin of the surface electrons aligns along the direction of the magnetic-impurity exchange field, only magnetic TIs with an out-of-plane magnetization are thought to open a gap at the Dirac point (DP) of the surface states, resulting in the QAHE. Using a continuum model supported by atomistic tight-binding and first-principles calculations of transition-metal doped Bi2Se3, we show that a surface-impurity potential generates an additional effective magnetic field which spin polarizes the surface electrons along the direction perpendicular to the surface. The predicted gap-opening mechanism results from the interplay of this additional field and the in-plane magnetization that shifts the position of the DP away from the Γ point. This effect is similar to the one originating from the hexagonal warping correction of the band structure but is one order of magnitude stronger. Our calculations show that in a doped TI with in-plane magnetization the impurity-potential-induced gap at the DP is comparable to the one opened by an out-of-plane magnetization.
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| 5. |
- Islam, Fhokrul, et al.
(författare)
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On the enhancement of magnetic anisotropy in cobalt clusters via non-magnetic doping
- 2014
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 26:125303
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Tidskriftsartikel (refereegranskat)abstract
- We show that the magnetic anisotropy energy (MAE) in cobalt clusters can be significantlyenhanced by doping them with group IV elements. Our firstprincipleselectronic structurecalculations show that Co4C2 and Co12C4 clusters have MAEs of 25 K and 61 K, respectively. The large MAE is due to controlled mixing between Co dandC pstatesand can be furthertuned by replacing C by Si. Larger assemblies of such primitive units are shown to be stablewith MAEs exceeding 100 K in units as small as 1.2 nm, in agreement with the recentobservation of large coercivity. These results may pave the way for the use of nanoclustersinhigh density magnetic memory devices for spintronics applications
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| 6. |
- Islam, Fhokrul
(författare)
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Stable magnetic order and charge induced rotation of magnetizationin nano-clusters
- 2014
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 105:152409
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Tidskriftsartikel (refereegranskat)abstract
- Efficient control of magnetic anisotropy and the orientation of magnetization are of centralimportance for the application of nanoparticles in spintronics. Conventionally, magnetization iscontrolled directly by an external magnetic field or by an electric field via spin-orbit coupling.Here, we demonstrate a different approach to control magnetization in small clusters. We firstshow that the low magnetic anisotropy of a Co5 cluster can be substantially enhanced by attachingbenzene molecules due to the mixing between p states of C and the d states of Co sites. We thenshow that the direction of magnetization vector of Co5 sandwiched between two benzene moleculesrotates by 90 when an electron is added or removed from the system. An experimental set up torealize such effect is also suggested.
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| 7. |
- Islam, Fhokrul, et al.
(författare)
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Systematics of electronic and magnetic properties in the transition metal doped Sb2Te3 quantum anomalous Hall platform
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:15
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Tidskriftsartikel (refereegranskat)abstract
- The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here, we report on a detailed and systematic investigation of transition metal (TM) doped Sb2Te3. By combining density functional theory calculations with complementary experimental techniques, i.e., scanning tunneling microscopy, resonant photoemission, and x-raymagnetic circular dichroism, we provide a complete spectroscopic characterization of both electronic and magnetic properties. Our results reveal that the TM dopants not only affect the magnetic state of the host material, but also significantly alter the electronic structure by generating impurity-derived energy bands. Our findings demonstrate the existence of a delicate interplay between electronic and magnetic properties in TM doped topological insulators. In particular, we find that the fate of the topological surface states critically depends on the specific character of the TM impurity: while V-and Fe-doped Sb2Te3 display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected. The single-ion magnetic anisotropy energy and easy axis, which control the magnetic gap opening and its stability, are also found to be strongly TM impurity dependent and can vary from in plane to out of plane depending on the impurity and its distance from the surface. Overall, our results provide general guidelines for the realization of a robust QAHE in TM doped Sb2Te3 in the ferromagnetic state.
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| 8. |
- Johnson, Alexander I., et al.
(författare)
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A multiferroic molecular magnetic qubit
- 2019
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 151:17, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- The chiral Fe3O(NC5H5)3(O2CC6H5)6 molecular cation, with C-3 symmetry, is composed of three six-fold coordinated spin-carrying Fe3+ cations that form a perfect equilateral triangle. Experimental reports demonstrating the spin-electric effect in this system also identify the presence of a magnetic uniaxis and suggest that this molecule may be a good candidate for an externally controllable molecular qubit. Here, we demonstrate, using standard density-functional methods, that the spin-electric behavior of this molecule could be even more interesting as there are energetically competitive reference states associated with both high and low local spins (S = 5/2 vs S = 1/2) on the Fe3+ ions. Each of these structures allow for spin-electric ground states. We find that qualitative differences in the broadening of the Fe(2s) and O(1s) core levels, shifts in the core-level energies, and the magnetic signatures of the single-spin anisotropy Hamiltonian may be used to confirm whether a transition between a high-spin manifold and a low spin manifold occurs.
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| 9. |
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| 10. |
- Mahani, Mohammad Reza, et al.
(författare)
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Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:16, s. Article ID: 165408-
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Tidskriftsartikel (refereegranskat)abstract
- Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically theelectronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the [Mn2+](0) plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity d electrons are in good agreement with approaches that treat the spin ofthe impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic [Fe3+](0) and the ionized [Fe2+](-)states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of d orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state [Fe2+, h](0), the magnetic-anisotropy dependence on the impurity sublayer resembles the case of [Mn2+, h](0). In contrast, for [Fe3+](0) electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e. g., by an external magnetic field, with detectable consequences in the local density of states probed by STM.
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