Anisotropic Magnetic Response Research Articles

Large-scale simulations of vortex Majorana zero modes in topological crystalline insulators

Topological crystalline insulators are known to support multiple Majorana zero modes (MZMs) at a single vortex, their hybridization is forbidden by a magnetic mirror symmetry MT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$M_{T}$\\end{document}. Due to the limited energy resolution of scanning tunneling microscopes and the very small energy spacing of trivial bound states, it remains challenging to directly probe and demonstrate the existence of multiple MZMs. In this work, we propose to demonstrate the existence of MZMs by studying the hybridization of multiple MZMs in a symmetry breaking field. The different responses of trivial bound states and MZMs can be inferred from their spatial distribution in a vortex. However, the theoretical simulations are very demanding since it requires an extremely large system in real space. By utilizing the kernel polynomial method, we can efficiently simulate large lattices with over 108 orbitals to compute the local density of states which bridges the gap between theoretical studies based on minimal models and experimental measurements. We show that the spatial distribution of MZMs and trivial vortex bound states differs drastically in tilted magnetic fields. The zero-bias peak elongates when the magnetic field preserves MT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$M_{T}$\\end{document}, while it splits when MT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$M_{T}$\\end{document} is broken, giving rise to an anisotropic magnetic response. Since the bulk of SnTe are metallic, we also study the robustness of MZMs against the bulk states, and clarify when can the MZMs produce a pronounced anisotropic magnetic response.

Extremely large magnetoresistance and anisotropic transport in the multipolar Kondo system PrTi2Al20

Multipolar Kondo systems offer unprecedented opportunities to design astonishing quantum phases and functionalities beyond spin-only descriptions. A model material platform of this kind is the cubic heavy fermion system PrT2Al20 (T=Ti, V), which hosts a nonmagnetic crystal-electric-field ground state and substantial Kondo entanglement of the local quadrupolar and octopolar moments with the conduction electron sea. Here, we explore the magnetoresistance (MR) and Hall effect of PrTi2Al20, which develops ferroquadrupolar (FQ) order below TQ∼2 K, and compare their behavior with that of the MR and Hall effect of the non-4f analog, LaTi2Al20. In the FQ ordered phase, PrTi2Al20 displays extremely large magnetoresistance (XMR) of ∼103%. The unsaturated, quasilinear field (B) dependence of the XMR violates Kohler's scaling and defies description based on carrier compensation alone. By comparing the MR and the Hall effect observed in PrTi2Al20 and LaTi2Al20, we conclude that the open-orbit topology on the electron-type Fermi surface sheet is key for the observed XMR. The low-temperature MR and the Hall resistivity in PrTi2Al20 display pronounced anisotropy in the [111] and [001] magnetic fields, which is absent in LaTi2Al20, suggesting that the transport anisotropy ties in with the anisotropic magnetic field response of the quadrupolar order parameter. Published by the American Physical Society 2024

Anisotropic Superconducting Nb2 CTx MXene Processed by Atomic Exchange at the Wafer Scale.

Superconductivty has recently been induced in MXenes through surface modification. However, the previous reports have mostly been based on powders or cold-pressed pellets, with no known reports on the intrinsic superconsucting properties of MXenes at the nanoale. Here, it is developed a high-temperature atomic exchange process in NH3 atmosphere which induces superconductivity in either singleflakes or thin filmsof Nb2 CTx MXene. The exchange process between nitrogen atoms and fluorine, carbon, and oxygen atoms in the MXene lattice and related structural adjustments are studied using both experiments and density functional theory. Using either single-flake or thin-film devices, an anisotropic magnetic response of the 2D superconducting transformation has beensuccessfully revealed. The anisotropic superconductivity is further demonstrated using superconducting thin films uniformly deposited over a 4 in. wafers, which opens up the possibility of scalable MXene-based superconducting devices.

Magnetic anisotropy and magnetic phase diagram of a kagome antiferromagnet Fe1-xCoxSn

We investigate the anisotropic magnetic response in a kagome antiferromagnetic compound Fe1-xCoxSn (x = 0, 0.04, 0.06, 0.14). The magnetic behaviors of three distinct spin orderings, namely the planar, tilted, and axial spin states, are examined under various conditions, including doping rate, temperature, and magnetic fields. We confirm that the magnetic susceptibility of all three spin states has significant anisotropy, which depends on both the magnitude and direction of the applied magnetic field. The planar and axial states show scaling behavior in magnetic susceptibility, while the tilted state does not. Furthermore, employing the differential magnetic susceptibility, we observe that the axial spins display discontinuous spin-flop transitions, while the planar and tilted spins exhibit a continuous spin reorientation. The magnetic phase diagrams, summarizing our observations, highlight the anisotropic nature of magnetic behaviors in the Fe1-xCoxSn system. These diagrams clearly show that the magnetic responses are not overlapping and follow distinct mechanisms in their interaction with magnetic fields. We suggest that the axial state may arise due to the dopants as an obstacle within the kagome lattice, as evidenced by similar magnetic behaviors in Fe0.96Ni0.06Sn.

Anisotropic optical and magnetic response in self-assembled TiN–CoFe2 nanocomposites

Transition metal nitrides (e.g., TiN) have shown tremendous promise in optical metamaterials for nanophotonic devices due to their plasmonic properties comparable to noble metals and superior high temperature stability. Vertically aligned nanocomposites (VANs) offer a great platform for combining two dissimilar functional materials with a one-step deposition technique toward multifunctionality integration and strong structural/property anisotropy. Here, we report a two-phase nanocomposite design combining ferromagnetic CoFe2 nanosheets in the plasmonic TiN matrix as a new hybrid plasmonic metamaterial. The hybrid metamaterials exhibit anisotropic optical and magnetic responses, as well as a pronounced magneto-optical coupling response evidenced by Magneto-optic Kerr Effect measurement, owing to the novel vertically aligned structure. This work demonstrates a new TiN-based metamaterial with anisotropic properties and multifunctionality toward light polarization modulation, optical switching, and integrated optics.

Two-dimensional superconducting nature of Bi2Sr2CaCu2O8+δ thin films revealed by BKT transition.

High-quality Bi2Sr2CaCu2O8+δ superconducting thin films are successfully grown on a SrTiO3 substrate by the Pulsed Laser Deposition technique. Superconducting critical transition temperatures Tc,zero have reached up to 85 K by using optimized growth parameters. In addition, we demonstrated the two-dimensional nature of the superconductivity of thin films by virtue of exhibiting Berezinskii-Kosterlitz-Thouless (BKT) physics and anisotropic magnetic response. Furthermore, three distinct regimes are identified based on the analysis of direct current resistance. The non-Fermi liquid phase and BKT phase fluctuation zone almost perfectly merge together, which implies that the system undergoes a unique topological state that is determined by the BKT phase fluctuation preceding the onset of the superconducting state. The emergence of such a topological state radically differentiates from the three-dimensional superconducting transition, which spontaneously breaks the gauge symmetry. The current studies on the Bi2Sr2CaCu2O8+δ superconducting thin films provide some new insights for understanding the rich quantum states of matter that emerge in the vicinity of the superconducting phase transition and highlight the significant role of BKT fluctuation on two-dimensional superconducting transition.

Unveiling the Complex Magnetization Reversal Process in 3D Nickel Nanowire Networks

AbstractUnderstanding the interactions among magnetic nanostructures is one of the key factors to predict and control the advanced functionalities of 3D integrated magnetic nanostructures. In this work, the focus is on different interconnected Ni nanowires forming an intricate, but controlled, and ordered magnetic system: Ni 3D Nanowire Networks (3DNNs). These self‐ordered systems present striking anisotropic magnetic responses, depending on the interconnections’ position between nanowires. To understand their collective magnetic behavior, the magnetization reversal processes are studied within different Ni 3D Nanowire Networks compared to the 1D nanowire 1DNW array counterparts. The systems are characterized at different angles using first magnetization curves, hysteresis loops, and First Order Reversal Curves techniques, which provided information about the key features that enable macroscopic tuning of the magnetic properties of the 3D nanostructures. In addition, micromagnetic simulations endorse the experiments, providing accurate modeling of their magnetic behavior. The results reveal a plethora of magnetic interactions, neither evident nor intuitive, which are the main role players controlling the collective response of the system. The results pave the way for the design and realization of 3D novel metamaterials and devices based on the nucleation and propagation of ferromagnetic domain walls both in 3D self‐ordered systems and future nano‐lithographed devices.

Magnetic response of Majorana Kramers pairs with an order-two symmetry

We study an intrinsic relation between the topology of bulk electronic states and magnetic responses of Majorana Kramers pairs, Kramers pairs of Majorana fermions, on a surface of time-reversal-invariant topological superconductors. Majorana Kramers pairs respond to an applied magnetic field anisotropically due to the interplay between time-reversal and crystal symmetries. In this paper, we propose a systematic procedure to determine such surface magnetic responses in systems with an order-two symmetry. From the analysis of topological invariants associated with an order-two symmetry, it is found that magnetic responses are classified into four types, which are attributed to different topological invariants and exhibit distinguishable, characteristic magnetic responses. For a Kramers pair of Majorana fermions protected by $\mathbb{Z}_2$ topological invariants, we clarify that types of magnetic responses are determined only from Fermi--surface topology and symmetry of Cooper pairs. Finally, we apply our theory to the topological nonsymmorphic crystalline superconducting state in UCoGe, which exhibits a biaxially anisotropic magnetic response.

Controllable electric field tuning of anisotropic magnetic response of Ni/PMN-PT heterostructures

Epitaxial Ni films with various thicknesses were grown on (001)-oriented 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) substrates. Electric field tuning (E-tuning) of ferromagnetic resonance field (Hr) was utilized to investigate the magnetic response of Ni films along different film directions. Butterfly-like Hr vs E curves are shown when magnetic field is applied along film edge. While the combination of both loop-like and butterfly-like Hr vs E curves are observed for Ni film with thicknesses of 180 nm and 240 nm, and merely loop-like curves can be seen for film thicknesses of 330 nm and 510 nm, as magnetic field is applied along film diagonal. These interesting phenomena are assumed to be related to ferroelectric and ferroelastic domain switching in PMN-PT, film strain status, as well as magnetic anisotropy in Ni film. This work demonstrates the feasibility of control of anisotropic magnetic response via electric field in multiferroic heterostructure.

Magnetic filaments for anisotropic composite polymers

The use of contactless magnetic forces meets numerous needs in microelectromechanical systems (MEMS) or microfluidic devices. In this view, heterogeneous materials integrating magnetic nanostructures within a non-magnetic matrix such as polymer can produce local variations of magnetic field, at the sub-micrometer scale. Here we report on the synthesis of magnetic composites using electrospun nanofilaments and a polydimethylsiloxane (PDMS) matrix. Varying the precursor nature and heat treatment conditions, we obtained single phase filaments of Fe, FeNi, and MFe2O4 (M = Co, Fe, Ni). Thanks to a fine investigation of their structure and morphology, it was possible to measure from magnetically-soft (µ0HC ⩽ 5 mT) to relatively hard (µ0HC up to 93 mT, MR/MS up to 0.5) behaviors. The common one-dimensional shape of these filaments leads to an anisotropic magnetic response. This can be exploited to achieve self-organization of the filaments in arrays within the non-magnetic matrix. We show the first step towards the development of magnetically anisotropic membranes of PDMS with 0.23 wt% Fe filaments. These composite materials are promising for implementing magnetic functions in microsystems while circumventing complex micro-fabrication steps.

Anisotropic magnetic response of Weyl semimetals in a topological insulator multilayer

Magnetic susceptibility of Weyl semimetals, placed in a tilted magnetic field, is studied. Weyl semimetals have three contributions: spin, orbital, and spin–orbital; all three contributions are investigated for a tilted magnetic field. We find that spin susceptibility at a low temperature varies linearly with the Fermi energy and is oscillatory function of tilt angle; the orbital part shows diamagnetic divergence at the zero Fermi energy like graphene, and spin–orbital contribution is an even function of the Fermi energy and the tilt angle. Due to the anisotropic nature of the magnetic response, we propose that spin, orbital, and spin–orbital magnetic response can be distinguished by changing the orientation of the applied magnetic field.

Magnetic Response of Majorana Kramers Pairs Protected by \(\mathbb{Z}_{2}\) Invariants

On the surface of time-reversal-invariant topological superconductors, Kramers pairs of Majorana fermions with chiral and crystalline symmetries exhibit completely uniaxial or octupole anisotropic magnetic response. This paper reports possible types of magnetic responses of Majorana Kramers pairs with one-dimensional Z2 invariants defined by crystalline symmetry. In particular, the general theory predicts a new type of magnetic response where two Majorana Kramers pairs associated with the Z2 invariant show biaxially (quadrupolar) anisotropic magnetic response, which is a novel type of response that is rarely observed in conventional and Majorana fermions.

Room-temperature anisotropic magnetoresistance of magnetic carbon films with vertically-grown graphene nanocrystallites

In this study, graphene nanocrystallited carbon films were deposited with electron irradiation induced deposition in electron cyclotron resonance plasma. The orientation of nanocrstallites was revealed with transmission electron microscope (TEM) images. The TEM results showed that the graphene nanocrystallites aligned perpendicular to substrate surface. The anisotropic-magneto-resistive performances of the films were investigated under diowed that from 400 to 2 K, the film resistance varied periodically as the film rotated along its surface axial, exhibiting a sinusoidal feature. In room temperature, the average angular resolution of the film was better than 0.5%/degree, and the largest relative changing rate of the film MR ratio reached over 400% as magnetic field increased from 3 to 9 T. The mechanism of this performance was investigated through electrical transport measurement and magnetic hysteresis loops, which revealed the spin-enhanced magnetism of the nanocrystallites were the main origin of this anisotropic magnetic response behaviors.

Electric and magnetic transitions with 90° turning of polarizations in a layered perovskite of [NH4Cl]2[Ni(HCOO)2(NH3)2

A hybrid layered perovskite compound of [NH4Cl]2[Ni(HCOO)2(NH3)2] consists of alternatively stacked perovskite layers of [Ni(HCOO)2(NH3)2] and CsCl-type slabs of [(NH3)2(NH4Cl)2]. The material undergoes a ferro- to ferro-electric transition around 110 K, from a high temperature, high polarized phase to a low temperature, low polarized one via an intermediate incommensurate phase and then a para- to antiferro-magnetic transition at 25 K. It displays significant anisotropic dielectric and magnetic responses, and both electric and magnetic polarizations turn from the direction perpendicular to the layers to the direction parallel to the layers during the transitions.

Tunable magnetic anisotropy of self-assembled Fe nanostructures within a La0.5Sr0.5FeO3 matrix

Metallic nanostructures within ceramic matrices provide a unique platform for integrating magnetic, optical, and electrical properties for device applications. Currently, hurdles still exist for the integration of metallic nanostructures within conventional devices, including the incompatible growth conditions between metals and ceramics and control of the overall physical properties. In this study, we demonstrate the tunability of a one-step growth method to fabricate magnetic and metallic nanostructures embedded within an oxide matrix, La0.5Sr0.5FeO3:Fe, from a composite target using pulsed laser deposition. The metal-ceramic nanocomposite films demonstrate tunable nanostructures and anisotropic magnetic response by varying deposition energy, presenting a mechanism for tuning the physical properties of vertically aligned ferromagnetic metallic nanopillars in an oxide matrix. This study also opens avenues towards the integration of nanoscale, vertical, metallic ferromagnetic contacts for anisotropic magnetic tunneling junctions which may not be easily realized by single-phase thin films.

Direct observation of anisotropic magnetic field response of the spin helix in FeGe thin films

Spiral spin orders often produce various fascinating phenomena via interaction with electronic structures. Examples include, but are not limited to, multiferroicity and skyrmion formation. Among them, a deformed spiral spin structure, which can be viewed as a periodic sequence of 2$\ensuremath{\pi}$ spin-rotation kinks, can exhibit the functionality of information transmission owing to its phase coherence. This is the chiral magnetic soliton lattice (CSL). Here, the authors have succeeded in building a spin spiral in a thin film of the chiral magnet FeGe, and revealed its characteristic magnetic responses. Magnetic anisotropy and boundary conditions inherent to the epitaxial thin film secure the alignment of the spiral winding direction along the film normal, even under external magnetic fields. This constraint leads to the anisotropic deformation of the spin spiral with respect to the magnetic field direction. Especially, the CSL, which is realized under an in-plane magnetic field, transforms with field between states with different numbers of kinks. The authors clearly demonstrate the deformation by direct observation of the spiral propagation vector by use of the small-angle neutron scattering technique.

Anisotropic magnetic responses of a 2D-superconducting Bi2Te3/FeTe heterostructure

We have investigated the anisotropic magnetic responses of a 2D-superconducting Bi2Te3/FeTe heterostructure. Cross-sectional STEM imaging revealed that the excess Fe atoms in the FeTe layer occupy specific interstitial sites. They were found to show strong anisotropic magnetic responses under a magnetic field either perpendicular or parallel to the sample surface. Under perpendicular magnetic fields within 1000 Oe, conventional paramagnetic Meissner effect, superconducting diamagnetism, and anomalous enhancement of magnetization successively occur as the magnetic field increases. In contrast, under parallel magnetic fields, superconducting diamagnetism was not observed explicitly in the magnetization measurements and the anomalous enhancement of magnetization appears only for fields higher than 1000 Oe. The observed strong magnetic anisotropy provides further evidence that the induced superconductivity at the interface of the Bi2Te3/FeTe heterostucture has a 2D nature.

Heat capacity measurements on UBe13 in rotated magnetic fields: Anisotropic response in the normal state and absence of nodal quasiparticles

In order to gain insight into the superconducting (SC) gap of UBe13, we studied its quasiparticle excitations by means of heat-capacity measurements. Quite unexpectedly, we found the isotropic C(H)∝H behavior in low fields at low temperatures, implying the absence of nodal quasiparticle excitations. This result indicates that the SC gap in UBe13 is fully open over the Fermi surfaces. Furthermore, we observed a characteristic oscillation of heat capacity both in the SC and non-Fermi-liquid normal states above ∼2T, and the angular variation of heat capacity possibly originates from anisotropic magnetic response of the heavy-electron state. Our result regarding the low-energy quasiparticle excitations in the SC and normal states will be a clue to understand the unusual nature of UBe13.

Optically active quantum dots in monolayer WSe2.

Semiconductor quantum dots have emerged as promising candidates for the implementation of quantum information processing, because they allow for a quantum interface between stationary spin qubits and propagating single photons. In the meantime, transition-metal dichalcogenide monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large bandgap with degenerate valleys and non-zero Berry curvature. Here, we report the observation of zero-dimensional anharmonic quantum emitters, which we refer to as quantum dots, in monolayer tungsten diselenide, with an energy that is 20-100 meV lower than that of two-dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host transition-metal dichalcogenide. The large ∼1 meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement that is most probably induced by impurities or defects. The possibility of achieving electrical control in van der Waals heterostructures and to exploit the spin-valley degree of freedom renders transition-metal-dichalcogenide quantum dots interesting for quantum information processing.

Anisotropic Magnetic Response in Kondo Lattice with Antiferromagnetic Order

Magnetic properties are investigated for the Kondo lattice by using the continuous time quantum Monte Colro (CT-QMC) and the dynamical mean field theory (DMFT). The DMFT+CT-QMC approach is extended so as to derive the anisotropic magnetic response in the antiferromagnetic phase. The longitudinal and transverse magnetic susceptibilities are numerically derived in the antiferromagnetic phase. For the RKKY regime with a small Kondo coupling, the transverse susceptibility does not decrease below the transition temperature while the longitudinal susceptibility decreases as expected from the mean field picture. In the competing region between the RKKY interaction and the Kondo effect, however, both longitudinal and transverse susceptibilities decrease below the transition temperature. The results obtained naturally explain the temperature dependence of the magnetic susceptibility observed in CeT$_2$Al$_{10}$ ($T$=Ru,Os,Fe) family.