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Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Tue, 18 Apr 2023

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1.Anomalous impact of thermal fluctuations on spintransfer torque induced ferrimagnetic switching

Authors:Zhengping Yuan, Jingwei Long, Zhengde Xu, Yue Xin, Lihua An, Jie Ren, Xue Zhang, Yumeng Yang, Zhifeng Zhu

Abstract: The dynamics of a spin torque driven ferrimagnetic (FiM) system is investigated using the two-sublattice macrospin model. We demonstrate an ultrafast switching in the picosecond range. However, we find that the excessive current leads to the magnetic oscillation. Therefore, faster switching cannot be achieved by unlimitedly increasing the current. By systematically studying the impact of thermal fluctuations, we find the dynamics of FiMs can also be distinguished into the precessional region, the thermally activated region, and the cross-over region. However, in the precessional region, there is a significant deviation between FiM and ferromagnet (FM), i.e., the FM is insensitive to thermal fluctuations since its switching is only determined by the amount of net charge. In contrast, we find that the thermal effect is pronounced even a very short current pulse is applied to the FiM. We attribute this anomalous effect to the complex relation between the anisotropy and overdrive current. By controlling the magnetic anisotropy, we demonstrate that the FiM can also be configured to be insensitive to thermal fluctuations. This controllable thermal property makes the FiM promising in many emerging applications such as the implementation of tunable activation functions in the neuromorphic computing.

2.Role of magnetic field on the electronic properties of an $α$-$T_3$ ring

Authors:Mijanur Islam, Tutul Biswas, Saurabh Basu

Abstract: We consider a quantum ring of a certain radius R built from a sheet of the $\alpha$-$T_3$ lattice and solve for its spectral properties in presence of an external magnetic field. The energy spectrum consists of a conduction band, a valence band and a zero energy flat band, all having a number of discrete levels therein which can be characterized by the angular momentum quantum number, m. The energy levels in the flat band are infinitely degenerate irrespective of the value of $\alpha$. We reveal a two-fold degeneracy of the levels in the conduction band as well as in the valence band for $\alpha$ = 0 and $\alpha$ = 1. However, the m = 0 level for $\alpha$ = 1 is an exception. Corresponding to an intermediate value of $\alpha$, namely, 0 <$\alpha$< 1, the energy levels become nondegenerate. The scenario remains unaltered when the ring is threaded by a magnetic flux which is an integer multiple of the flux quantum. We also calculate the persistent current which exhibits quantum oscillations as a function of the magnetic field with a period of one flux quantum at a particular Dirac point, which is often referred to as a valley. The total current oscillates with a periodicity of one flux quantum for any intermediate value of $\alpha$. We have also explored the effect of a mass term (that breaks the sublattice symmetry) in the Hamiltonian. In the absence of a magnetic field, the energy levels in the flat band become dispersive, except for the m = 0 level in the case of $\alpha$ = 1. In presence of the field, each of the flat band levels becomes dispersive for any $\alpha \neq$ 0. Finally, we also see the effect of the mass term on the behaviour of the persistent current, which shows periodicity of one flux quantum, but the total current remains finite for all values of $\alpha$.

3.Efficient characteristics of exchange coupling and spin-flop transition in Py/Gd bilayer using anisotropic magnetoresistance

Authors:Kaiyuan Zhou, Xiang Zhan, Zishuang Li, Haotian Li, Chunjie Yan, Lina Chen, Ronghua Liu

Abstract: The interlayer antiferromagnetic coupling rare-earth/transition-metal bilayer ferrimagnet systems have attracted much attention because they present variously unusual temperature-and field-dependent nontrivial magnetic states and dynamics. These properties and the implementation of their applications in spintronics highly depend on the significant temperature dependence of the magnetic exchange stiffness constant A. Here, we quantitatively determine the temperature dependence of magnetic exchange stiffness A_{Py-Gd} and A_{Gd} in the artificially layered ferrimagnet consisting of a Py/Gd bilayer, using a measurement of anisotropic magnetoresistance (AMR) of the bilayer thin film at different temperatures and magnetic fields. The obtained temperature dependence of A_{Py-Gd} and A_{Gd} exhibit a scaling power law with the magnetization of Gd. The critical field of spin-flop transition and its temperature dependence can also be directly obtained by this method. Additionally, the experimental results are well reproduced by micromagnetic simulations with the obtained parameters A_{Py-Gd} and A_{Gd}, which further confirms the reliability of this easily accessible technique.

4.Feedback enhanced Dyakonov-Shur instability in Graphene-FET

Authors:Pedro Cosme, Diogo Simões

Abstract: Graphene devices are known to have the potential to operate THz signals. In particular, graphene field-effect transistors have been proposed as devices to host plasmonic instabilities in the THz realm; for instance, Dyakonov-Shur instability which relies upon dc excitation. In this work, starting from a hydrodynamical description of the charge carriers, we extend the transmission line description of graphene field-effect transistors to a scheme with a positive feedback loop, also considering the effects of delay, which leads to the transcendental transfer function with terms of the form $e^{as}{\rm sech}^k(s)/s$. Applying the conditions for the excitation of Dyakonov-Shur instability, we report an enhanced voltage gain in the linear regime that is corroborated by our simulations of the nonlinear hydrodynamic model for the charge carriers. This translates to both greater saturation amplitude -- often up to 50% increase -- and fastest growth rate of the self-oscillations. Thus, we bring forth a prospective concept for the realization of a THz oscillator suitable for future plasmonic circuitry.

5.Defects in Graphene : A Topological Description

Authors:Amit Goft, Yuval Abulafia, Nadav Orion, Claude L. Schochet, Eric Akkermans

Abstract: Specific types of spatial defects or potentials can turn monolayer graphene into a topological material. These topological defects are classified by a spatial dimension $D$ and they are systematically obtained from the Hamiltonian by means of its symbol $\mathcal{H} (\boldsymbol{k}, \boldsymbol{r}) $, an operator which generalises the Bloch Hamiltonian and contains all topological information. This approach, when applied to Dirac operators, allows to recover the tenfold classification of insulators and superconductors. The existence of a stable $\mathbb{Z}$-topology is predicted as a condition on the dimension $D$, similar to the classification of defects in thermodynamic phase transitions. Kekule distortions, vacancies and adatoms in graphene are proposed as examples of such defects and their topological equivalence is discussed.

6.Anisotropic linear and non-linear excitonic optical properties of buckled monolayer semiconductors

Authors:M. F. C. Martins Quintela, T. Garm Pedersen

Abstract: The optical properties of two-dimensional materials are exceptional in several respects. They are highly anisotropic and frequently dominated by excitonic effects. Dipole-allowed second order non-linear optical properties require broken inversion symmetry. Hence, several two-dimensional materials show strong in-plane (IP) non-linearity but negligible out-of-plane (OOP) response due to vertical symmetry. By considering buckled hexagonal monolayers, we analyze the critical role of broken vertical symmetry on their excitonic optical response. Both linear as well as second order shift current and second harmonic response are studied. We demonstrate that substantial OOP non-linear response can be obtained, in particular, through off-diagonal tensor elements coupling IP excitation to OOP response. Our findings are explained by excitonic selection rules for OOP response and the impact of dielectric screening on excitons is elucidated.

7.Edge-selective extremal damping from topological heritage of dissipative Chern insulators

Authors:Suraj S. Hegde, Toni Ehmcke, Tobias Meng

Abstract: One of the most important practical hallmarks of topological matter is the presence of topologically protected, exponentially localised edge states at interfaces of regions characterised by unequal topological invariants. Here, we show that even when driven far from their equilibrium ground state, Chern insulators can inherit topological edge features from their parent Hamiltonian. In particular, we show that the asymptotic long-time approach of the non-equilibrium steady state, governed by a Lindblad Master equation, can exhibit edge-selective extremal damping. This phenomenon derives from edge states of non-Hermitian extensions of the parent Chern insulator Hamiltonian. The combination of (non-Hermitian) topology and dissipation hence allows to design topologically robust, spatially localised damping patterns.

8.The Casimir effect for stack of graphenes

Authors:Natalia Emelianova, Rashid Kashapov, Nail Khusnutdinov

Abstract: We consider a stack of parallel sheets composed of conducting planes with tensorial conductivities. Using the scattering matrix approach, we derive explicit formulas for the Casimir energy of two, three, and four planes, as well as a recurrence relation for arbitrary planes. Specifically, for a stack of graphene, we solve the recurrence relations and obtain formulas for the Casimir energy and force acting on the planes within the stack. Moreover, we calculate the binding energy in the graphene stack with graphite interplane separation, which amounts to $E_{ib} = 9.9$ meV/atom. Notably, the Casimir force on graphene sheets decreases rapidly for planes beyond the first one. In particular, for the second graphene layer in the stack, the force is $35$ times smaller than that experienced by the first layer.

9.Magneto-optical induced supermode switching in quantum fluids of light

Authors:Magdalena Furman, Rafał Mirek, Mateusz Król, Wojciech Pacuski, Helgi Sigurðsson, Jacek Szczytko, Barbara Piętka

Abstract: The insensitivity of photons towards external magnetic fields forms one of the hardest barriers against efficient magneto-optical control, aiming at modulating the polarization state of light. However, there is even scarcer evidence of magneto-optical effects that can spatially modulate light. Here, we demonstrate the latter by exploiting strongly coupled states of semimagnetic matter and light in planar semiconductor microcavities. We nonresonantly excite two spatially adjacent exciton-polariton condensates which, through inherent ballistic near field coupling mechanism, spontaneously synchronise into a dissipative quantum fluidic supermode of definite parity. Applying a magnetic field along the optical axis, we continuously adjust the light-matter composition of the condensate exciton-polaritons, inducing a supermode switch into a higher order mode of opposite parity. Our findings set the ground towards magnetic spatial modulation of nonlinear light.