Two-dimensional Magnetic Lattice Research Articles

Atypical breathing driven two-dimensional valley multiferroicity.

Valley multiferroicity, coupled with ferro-valleytricity and primary ferroicities in a single phase, is of fundamental significance in condensed-matter physics and materials science, as it provides a convenient route to reverse the anomalous valley Hall (AVH) effect. Current research in this field focuses mainly on ferromagnetic ferro-valleytricity, whereas ferroelectric ferro-valleytricity is seldom explored. Here, using symmetry arguments and tight-binding model analysis, we report a novel mechanism of coupling ferro-valleytricity with ferroelectricity, i.e., single-phase valley multiferroicity, in a two-dimensional magnetic lattice. This mechanism correlates to the atypical breathing nature of the magnetic lattice. Importantly, the valley physics, associated with Berry curvature, can be reversed under a ferroelectric transition, thereby guaranteeing the ferroelectrically reversible AVH effect. The underlying physics are discussed in detail. Based on first-principles calculations, we further confirm valley multiferroicity in a real 2D magnetic material of single-layer Gd2CO2. The explored phenomena and mechanism are not only useful for fundamental research in valley multiferroics but also enable a wide range of applications in nanodevices.

Classification of emerging patterns in self-assembled two-dimensional magnetic lattices.

Self-assembled granular materials can be utilized in many applications such as shock absorption and energy harvesting. Such materials are inherently discrete with an easy path to tunability through external applied forces such as stress or by adding more elements to the system. However, the self-assembly process is statistical in nature with no guarantee for repeatability, stability, or order of emergent final assemblies. Here we study both numerically and experimentally the two-dimensional self-assembly of free-floating disks with repulsive magnetic potentials confined to a boundary with embedded permanent magnets. Six different types of disks and seven boundary shapes are considered. An agent-based model is developed to predict the self-assembled patterns for any given disk type, boundary, and number of disks. The validity of the model is experimentally verified. While the model converges to a physical solution, these solutions are not always unique and depend on the initial position of the disks. The emerging patterns are classified into monostable patterns (i.e., stable patterns that emerge regardless of the initial conditions) and multistable patterns. We also characterize the emergent order and crystallinity of the emerging patterns. The developed model along with the self-assembly nature of the system can be key in creating re-programmable materials with exceptional nonlinear properties.

Direct Observation of Magnon-Phonon Strong Coupling in Two-Dimensional Antiferromagnet at High Magnetic Fields.

We report the direct observation of strong coupling between magnons and phonons in a two-dimensional antiferromagnetic semiconductor FePS_{3}, via magneto-Raman spectroscopy at magnetic fields up to 30Tesla. A Raman-active magnon at 121 cm^{-1} is identified through Zeeman splitting in an applied magnetic field. At a field-driven resonance with a nearby phonon mode, a hybridized magnon-phonon quasiparticle is formed due to strong coupling between the two modes. We develop a microscopic model of the strong coupling in the two-dimensional magnetic lattice, which enables us to elucidate the nature of the emergent quasiparticle. Our polarized Raman results directly show that the magnons transfer their spin angular momentum to the phonons and generate phonon spin through the strong coupling.

Nonlinear localized modes in two-dimensional hexagonally-packed magnetic lattices

We conduct an extensive study of nonlinear localized modes (NLMs), which are temporally periodic and spatially localized structures, in a two-dimensional array of repelling magnets. In our experiments, we arrange a lattice in a hexagonal configuration with a light-mass defect, and we harmonically drive the center of the chain with a tunable excitation frequency, amplitude, and angle. We use a damped, driven variant of a vector Fermi–Pasta–Ulam–Tsingou lattice to model our experimental setup. Despite the idealized nature of this model, we obtain good qualitative agreement between theory and experiments for a variety of dynamical behaviors. We find that the spatial decay is direction-dependent and that drive amplitudes along fundamental displacement axes lead to nonlinear resonant peaks in frequency continuations that are similar to those that occur in one-dimensional damped, driven lattices. However, we observe numerically that driving along other directions results in asymmetric NLMs that bifurcate from the main solution branch, which consists of symmetric NLMs. We also demonstrate both experimentally and numerically that solutions that appear to be time-quasiperiodic bifurcate from the branch of symmetric time-periodic NLMs.

Magnetic charge's relaxation propelled electricity in two-dimensional magnetic honeycomb lattice

SummaryEmerging new concepts, such as magnetic charge dynamics in two-dimensional magnetic material, can provide novel mechanism for spin-based electrical transport at macroscopic length. In artificial spin ice of single domain elements, magnetic charge's relaxation can create an efficient electrical pathway for conduction by generating fluctuations in local magnetic field that couple with conduction electron spins. In a first demonstration, we show that the electrical conductivity is propelled by more than an order of magnitude at room temperature due to magnetic charge defects sub-picosecond relaxation in artificial magnetic honeycomb lattice. The direct evidence to the proposed electrical conduction mechanism in two-dimensional frustrated magnet points to the untapped potential for spintronic applications in this system.

Low Dimensional Magnetic Lattice and Room Temperature Magneto(di)electric Effect in Polyanion Ruddlesden-Popper Iron Oxides.

We have shown a new design strategy which exploits different oxyanions in a Ruddlesden-Popper (RP)-type phase to modulate the local crystal structure and magnetic lattice. Material (Sr4Fe2(SO4)0.5O6.5) with the larger voluminous oxyanion (SO4, S-O distance = 1.49 Å) as separating blocks between magnetic FeO layers shows a two-dimensional magnetic lattice. A three-dimensional magnetic lattice and spin reorientation transition is observed for the Sr4Fe2(CO3)O6, having CO3 (C-O distance = 1.25 Å), a smaller oxyanion, as a separating layer. Using mixed oxyanions (SO4 and CO3) in the central perovskite block of the RP3 phase, we have demonstrated a facile strategy to modulate the local crystal structure. The modulated displacement of the magnetic cations, which can break the local centrosymmetry, is suggested to originate the magnetodielectric effect near the magnetic ordering temperatures (higher than room temperature). Further, all CO3 containing samples show magnetodielectric coupling below room temperature due to the spin reorientation transition. The room temperature magnetodielectric effect coupled to the targeted local modulation of the crystal structure by oxyanions (in the absence of second-order Jahn-Teller active "distortion centers") opens a new door to the design of new multifunctional materials with the possibility for the room temperature application.

Control of ultracold atoms with a chiral ferromagnetic film

We show that the magnetic field produced by a chiral ferromagnetic film can be applied to control ultracold atoms. The film will act as a magnetic mirror or a reflection grating for ultracold atoms when it is in the helical phase or the skyrmion crystal phase respectively. By applying a bias magnetic field and a time-dependent magnetic field, one-dimensional or two-dimensional magnetic lattices including honeycomb, Kagome, triangular types can be created to trap the ultracold atoms. We have also discussed the trapping height, potential barrier, trapping frequency, and Majorana loss rate for each lattice. Our results suggest that the chiral ferromagnetic film can be a platform to develop artificial quantum systems with ultracold atoms based on modern spintronics technologies.

Observation of topological phenomena in a programmable lattice of 1,800 qubits.

The work of Berezinskii, Kosterlitz and Thouless in the 1970s1,2 revealed exotic phases of matter governed by thetopological properties of low-dimensional materials such as thin films of superfluids and superconductors. A hallmark of this phenomenon is the appearance and interaction of vortices and antivortices in an angular degree of freedom-typified by the classical XY model-owing to thermal fluctuations. In the two-dimensional Ising model this angular degree of freedom is absent in the classical case, but with the addition of a transverse field it can emerge from the interplay between frustration and quantum fluctuations. Consequently, a Kosterlitz-Thouless phase transition has been predicted in the quantum system-the two-dimensional transverse-field Ising model-by theory and simulation3-5. Here we demonstrate a large-scale quantum simulation of this phenomenon in a network of 1,800 in situ programmable superconducting niobium flux qubits whose pairwise couplings are arranged in a fully frustrated square-octagonal lattice. Essential to the critical behaviour, we observe the emergence of a complex order parameter with continuous rotational symmetry, and the onset of quasi-long-range order as the system approaches a critical temperature. We describe and use a simple approach to statistical estimation with an annealing-based quantum processor thatperforms Monte Carlo sampling in a chain of reverse quantum annealing protocols. Observations are consistent with classical simulations across a range of Hamiltonian parameters. We anticipate that our approach of using a quantum processor as a programmable magnetic lattice will find widespread use in the simulation and development of exotic materials.

Cooper pair induced frustration and nematicity of two-dimensional magnetic adatom lattices

We propose utilizing the Cooper pair to induce magnetic frustration in systems of two-dimensional (2D) magnetic adatom lattices on s-wave superconducting surfaces. The competition between singlet electron correlations and the RKKY coupling is shown to lead to a variety of hidden order states that break the point-group symmetry of the 2D adatom lattice at finite temperature. The phase diagram is constructed using a newly developed effective bond theory [M. Schecter et al., Phys. Rev. Lett. 119, 157202 (2017)], and exhibits broad regions of long-range vestigial nematic order.

Generating an effective magnetic lattice for ultracold atoms

We present a general scheme for synthesizing a spatially periodic magnetic field, or a magnetic lattice (ML), for ultracold atoms using pulsed gradient magnetic field (GMF). Our scheme is immune to atomic spontaneous emission often encountered in optical lattices, and has the additional benefits of easy tunability for both the lattice period and depth. Technical requirements for the experimental protocol implementing our scheme is estimated and shown to be readily available in today’s cold atom laboratories. The effective Hamiltonian for atoms interacting with the synthesized two-dimensional ML has not been studied in quantum condensed matter physics previously. Its band structure shows interesting features reminiscent of lattice models in p-orbit physics. Realization of our proposal will significantly expand the repertoire for quantum simulation with ultracold atoms.

A two-dimensional permanent magnetic lattice for ultracold atoms

We propose a permanent magnetic lattice for creating a two-dimensional array of Ioffe–Pritchard permanent magnetic microtraps for holding and controlling ultracold atoms and Bose–Einstein condensates. This atom chip may be fabricated using laser carving on two separate magnetic films such as Tb6Gd10Fe80Co4 with thicknesses of 500 and 50 nm, respectively, and a periodicity of 1 μm. The trap depth and frequencies are controlled via an external bias field to handle tunneling rates between lattice sites. We present analytical expressions and compare them with numerical calculations.

Dynamic control of magnetically trapped indirect excitons using external magnetic bias: Transition from two- to one-dimensional lattice

We demonstrate an on-demand spatial control of excitonic magnetic lattices for the potential applications of excitonic-based quantum optical devices. A two-dimensional magnetic lattice of indirect excitons can form a transition to a one-dimensional lattice configuration under the influence of external magnetic bias fields. The transition is identified by measuring the spatial distribution of two-dimensional photoluminescence for several values of the external magnetic bias fields. The number of trapped excitons is found to increase between sites along a perpendicular direction exhibiting a two- to one-dimensional lattice transition. This work may apply for various controllable quantum simulations, such as superfluid-Mott-insulators, in quantum optical devices.

Observations of indirect exciton trapping in one- and two-dimensional magnetic lattices

A simple method to create and control magnetic potentials onto coupled quantum wells is demonstrated for indirect-exciton magnetic confinement. Localized inhomogeneous magnetic potentials with periodically distributed local minima and maxima, known as magnetic lattices, are projected into the plane of the coupled quantum wells and used for the spatially distributed two-dimensional indirect-exciton trapping, in which case localized indirect-excitons are observed. The indirect-exciton trapping mechanism is examined by controlling external magnetic field bias resulting in a shift of the localized excitonic lattice.

Magnetic properties of two dimensional silicon carbide triangular nanoflakes-based kagome lattices

Two-dimensional (2D) magnetic kagome lattices are constructed using silicon carbide triangular nanoflakes (SiC-TNFs). Two types of structures with alternating Si and C atoms are studied: the first one is constructed using the C-edged SiC-TNFs as the building blocks and C atoms as the linkers of kagome sites (TNFN–C–TNFN) while the second one is composed of the Si-edged SiC-TNFs with Si atoms as linkers (TNFN–Si–TNFN). Using density functional theory-based calculations, we show that the fully relaxed TNFN–C–TNFN retains the morphology of regular kagome lattice and is ferromagnetism. On the other hand, the TNFN–Si–TNFN structure is deformed and antiferromagnetic. However, the ground state of TNFN–Si–TNFN structure can be transformed from the antiferromagnetic to ferromagnetic state by applying tensile strain. Monte Carlo simulations indicate that the SiC-TNFs-based kagome lattices can be ferromagnetic at room temperature.

Dispersion relations and low relaxation of spin waves in thin magnetic films

We study spin excitations in thin magnetic films in the Heisenberg model with magnetic dipole and exchange interactions by the spin operator diagram technique and make comparison of their parameters with characteristics of spin waves in thick films. Dispersion relations of spin waves in thin magnetic films (in two-dimensional magnetic monolayer and bilayer lattices) and the spin-wave resonance spectrum in $N$-layer structures are found. For thick magnetic films, spin excitations are determined by simultaneous solution of the generalized Landau-Lifshitz equations and the equation for the magnetostatic potential. Generalized Landau-Lifshitz equations are derived from first principles and have the integral (pseudodifferential) form. It is found that dispersion relations of spin waves in monolayers and in bilayers differ from dispersion relations of spin waves in continuous thick magnetic films. For normal magnetized ferromagnetic films, the spin-wave damping is calculated in the one-loop approximation for a diagram expansion of the Green functions at low temperature. In thick magnetic films, the magnetic dipole interaction makes a major contribution to the relaxation of long-wavelength spin waves. Thin films have a region of the low relaxation of long-wavelength spin waves. In thin magnetic films, four-spin-wave processes take place and the exchange interaction makes a major contribution to the damping. It is found that the damping of spin waves propagating in a magnetic monolayer is proportional to the quadratic dependence on the temperature and is very low for spin waves with small wave vectors.

The macroscopic quantum phase of ultracold degenerate gases in the asymmetrical two-dimensional magnetic lattice

We investigate the existence of the macroscopic quantum phase in trapped ultracold quantum degenerate gases in an asymmetrical two-dimensional magnetic lattice. We show the key to adiabatically control the tunneling in the new two-dimensional magnetic lattice by means of external magnetic bias fields. In solving the system of coupled time-dependent differential equations, described here by the Boson Josephson Junctions (BJJs), we used an order parameter that includes both time-dependent variational parameters to describe the fractional population at each lattice site and the phase difference to quantify the macroscopic quantum phase signature. A dynamical oscillation of the fractional population and the phase difference at each individual lattice site is observed when solving the BJJs system.

Critical behavior of two-dimensional magnetic lattice gas model

Using Monte Carlo simulations and finite-size scaling, we investigate the critical behavior of two-dimensional magnetic lattice gas at densities rho = 0.90, 0.95, 1.0. There is a ferromagnetic phase transition at each density. As expected, the critical temperature T-c depends on system density rho. Unexpectedly, there is a density dependence of the critical exponent of correlation length v. For densities rho = 0.90, 0.95, 1.0, we obtain the inverse of critical exponent 1/v = 0.835(5), 0.905(5), 1.00(1) respectively. It is found that the ratios of critical exponent beta/v and gamma/v of magnetization and susceptibility are independent of density.

Asymmetrical two-dimensional magnetic lattices for ultracold atoms

A simple method for implementing an asymmetrical two-dimensional magnetic lattice is proposed. The asymmetrical two-dimensional magnetic lattice is created by periodically distributing nonzero magnetic minima across the surface of a magnetic thin film, where the magnetic patterns are formed by milling $n\ifmmode\times\else\texttimes\fi{}n$ square holes on the surface of the film. The method is proposed for trapping and confining quantum degenerate gases, such as Bose-Einstein condensates and ultracold Fermi gases, prepared in low-magnetic-field-seeking states. Analytical expressions and numerical simulation results of the magnetic local minima are shown where we analyze the effect of changing the magnetic lattice parameters, such as the separation of the holes, the hole size, and external bias magnetic fields, to maintain and locate the nonzero local minima at a suitable distance above the film surface to avoid the effect of Majorana spin flips and the Casimir-Polder potential.

Adiabatically induced coherent Josephson oscillations of ultracold atoms in an asymmetric two-dimensional magnetic lattice

We propose a new method to create an asymmetric two-dimensional magnetic lattice which exhibits magnetic band gap structure similar to semiconductor devices. The quantum device is assumed to host bound states of collective excitations formed in a magnetically trapped quantum degenerate gas of ultracold atoms such as a Bose-Einstein condensate (BEC) or a degenerate Fermi gas. A theoretical framework is established to describe possible realization of the exciton-Mott to discharging Josephson states oscillations in which the adiabatically controlled oscillations induce ac and dc Josephson atomic currents where this effect can be used to transfer n Josephson qubits across the asymmetric two-dimensional magnetic lattice. We consider second-quantized Hamiltonians to describe the Mott insulator state and the coherence of multiple tunneling between adjacent magnetic lattice sites where we derive the self consistent non-linear Schrödinger equation with a proper field operator to describe the exciton Mott quantum phase transition via the induced Josephson atomic current across the n magnetic bands.

Have we been here before? Inorganic precursors for collective electronic behaviour of molecular crystals

The community focused on collective electronic properties of organic and metal-organic molecular crystals sometimes assumes that these are uniquely a consequence of the fact that the lattice is composed of molecular building blocks. However, quantum mechanics has a wider horizon and precursors for some of the phenomena currently occupying our field were observed and investigated a while ago in various inorganic lattices. We recall some of the latter, which serve to highlight what really is unique to the molecular solid state. We also recall a simplified classification scheme to correlate crystal structures and physical properties. Examples from magnetism include one- and two-dimensional ferromagnetism and complex magnetic lattice topologies; from electron transport we mention low-dimensional superconductors incorporating localized magnetic moments