Exotic Properties Research Articles

Systematic researches on the ground state properties of the medium-mass neutron-rich nuclei

Abstract The recently developed relativistic-mean-field in complex momentum representation with the functional NL3* was used to explore the exotic properties of neutron-rich Pd, Cd, Te, and Xe isotopes. The results were compared with those obtained using the relativistic Hartree-Bogoliubov (RHB) calculations, and available experimental data. The single-particle levels were obtained for the bound and resonant states. The two neutron separation energies S2n and root mean square (rms) radii, agree with the experimental data. It is shown that a halo structure in the extremely neutron-rich 164-180Te and 164-182Xe, as well as a thick neutron skin in the extremely neutron-rich Pd and Cd isotopes. From the numbers of neutrons (Nλ) and (N0 ) occupying the levels above the Fermi surface and the zero-potential energy level, it was found that pairing correlations play an important role in the formation of halo phenomena. These findings are further supported by investigating S2n, rms radii, occupation probabilities, contributions of single-particle levels to the neutron rms radii, and density distributions. The neutron rms radii increased sharply, obviously deviating from the traditional rule r ∝A1/3, and the density distributions were very diffuse. Finally, the contributions of different single-particle levels to the total neutron density and wavefunction were discussed. It was found that the sudden increase in the neutron rms radii and diffuse density distributions mainly come from the resonant levels with a lower orbital angular momentum near the continuum threshold.

Direct Three-Dimensional Observation of the Plasmonic Near-Fields of a Nanoparticle with Circular Dichroism.

Characterizing the spatial distribution of the electromagnetic fields of a plasmonic nanoparticle is crucial for exploiting its strong light-matter interaction for optoelectronic and catalytic applications. However, observing the near-fields in three dimensions with a high spatial resolution is still challenging. To realize efficient three-dimensional (3D) nanoscale mapping of the plasmonic fields of nanoparticles with complex shapes, this work established autoencoder-embedded electron energy loss spectroscopy (EELS) tomography. A 432-symmetric chiral gold nanoparticle, a nanoparticle with a high optical dissymmetry factor, was analyzed to relate its geometrical features to its exotic optical properties. Our deep-learning-based feature extraction method discriminated plasmons with different energies in the EEL spectra of the nanoparticle in which signals from multiple plasmons were intermixed; this component was key for acceptable 3D visualization of each plasmonic field separately using EELS tomography. With this methodology, the electric field of the plasmon that induces far-field circular dichroism was observed in 3D. The field linked to this chiroptical property was strong along the swirling edges of the particle, as predicted by a numerical calculation. This study provides insight into the correlation between structural and optical chiralities through direct 3D observation of the plasmonic fields. Furthermore, the strategy of implementing an autoencoder for EELS tomography can be generalized to achieve competent 3D analysis of other features, including the optical properties of the dielectrics and chemical states.

Programmable viscosity metamaterials: Designing fluid properties using temporal superposition of shear and acoustics

Metamaterials are composite structures whose extraordinary properties arise from a mesoscale organization of their constituents. Here, we introduce a different material class—viscosity metafluids. Specifically, we demonstrate that we can rapidly drive large viscosity oscillations in shear-thickened fluids using acoustic perturbations with kHz to MHz frequencies. Because the timescale for these oscillations can be orders of magnitude smaller than the timescales associated with the global material flow, we can construct metafluids whose resulting time-averaged viscosity is a composite of the thickened, high-viscosity and dethickened, low-viscosity states. We show that viscosity metafluids can be used to engineer a variety of unique properties including zero, infinite, and negative viscosities. The high degree of control over the resulting viscosity, the ease with which they can be accessed, and the variety of exotic properties achievable make viscosity metafluids attractive for uses in technologies ranging from coatings to cloaking to 3D printing. Published by the American Physical Society 2024

Chirality- and Rashba- related effects in the spin texture of a two-dimensional centrosymmetric ferromagnet: The case of the CrI3 bilayer

The newly discovered two-dimensional (2D) magnetic semiconductors such as CrI3 have triggered a surge of interest stemming from their exotic spin-dependent properties and potential applications in spintronics and magneto-optoelectronics. Using first-principle density-functional-theory calculations, we investigate the properties of the spin-polarization texture in momentum space in the prototype 2D centrosymmetric ferromagnetic (FM) bilayer of CrI3 with perpendicular magnetization, with a goal of identifying general features due to interlayer interaction and their microscopic origins in 2D centrosymmetric FM materials. The FM CrI3 bilayer displays a rich in-plane spin texture in its highest valence bands. We show the existence of two distinct spin canting effects induced by the coupling of the two FM layers in establishing the in-plane spin texture. The first effect is generated by the mirror-related chirality of the layer stacking and the spin–orbit-polarized nature of the valence states, and yields the same canting on both layers. The second effect is a Rashba-related effect, which in a centrosymmetric ferromagnet induces in a single electronic state two opposite spin-canted components on the two layers, resulting in a notable frustration effect on the energy of the bonding states. Finally, we show that the above effects can be effectively used to manipulate the spin texture via compressive vertical strain, which induces in the FM CrI3 bilayer valence-band-edge states with canted spins.

Two-Dimensional Topological Ferroelectric Metal with Giant Shift Current.

The pursuit for "ferroelectric metal," which combines seemingly incompatible spontaneous electric polarization and metallicity, has been assiduously ongoing but remains elusive. Unlike traditional ferroelectrics with a wide band gap, ferroelectric (FE) metals can naturally incorporate nontrivial band topology near the Fermi level, endowing them with additional exotic properties. Here, we show first-principles evidence that the metallic PtBi_{2} monolayer is an intrinsic two-dimensional (2D) topological FE metal, characterized by out-of-plane polarization and a moderate switching barrier. Moreover, it exhibits a topologically nontrivial electronic structure with Z_{2} invariant equal to 1, leading to a significant FE bulk photovoltaic effect. A slight strain can further enhance this effect to a remarkable level, which far surpasses that of previously reported 2D and 3D FE materials. Our Letter provides an important step toward realizing intrinsic monolayer topological FE metals and paves a promising way for future nonlinear optical devices.

Incoherence-to-coherence crossover observed in charge-density-wave material 1T-TiSe2

Analogous to the condensation of Cooper pairs in superconductors, the Bose–Einstein condensation (BEC) of electron–hole pairs in semiconductors and semimetals leads to an emergence of an exotic ground state — the excitonic insulator state. In this paper, we study the electronic structure of 1T-TiSe2 utilizing angle-resolved photoemission spectroscopy and alkali-metal deposition. Alkali-metal adatoms are deposited in-situ on the sample surface, doping the system with electrons. The conduction bands of 1T-TiSe2 are thereby pushed down below the Fermi energy, which enables us to characterize its temperature dependence with precision. We found that the formation of the charge density wave (CDW) in 1T-TiSe2 at ~ 205 K is accompanied by a significant increase of the band gap, supporting the existence of excitonic pairing in the CDW state of 1T-TiSe2. More importantly, by analyzing the linewidth of the single-particle excitation spectrum, we unveiled an incoherence-to-coherence crossover at 165 K, which could be attributed to a possible exciton condensation that occurs beneath the CDW transition in 1T-TiSe2. Our results not only explain the exotic transport properties of 1T-TiSe2, but also highlight the possible existence of an excitonic condensate in this semiconducting material.

Linearly programmable two-dimensional halide perovskite memristor arrays for neuromorphic computing.

The exotic properties of three-dimensional halide perovskites, such as mixed ionic-electronic conductivity and feasible ion migration, have enabled them to challenge traditional memristive materials. However, the poor moisture stability and difficulty in controlling ion transport due to their polycrystalline nature have hindered their use as a neuromorphic hardware. Recently, two-dimensional (2D) halide perovskites have emerged as promising artificial synapses owing to their phase versatility, microstructural anisotropy in electrical and optoelectronic properties, and excellent moisture resistance. However, their asymmetrical and nonlinear conductance changes still limit the efficiency of training and accuracy of inference. Here we achieve highly linear and symmetrical conductance changes in Dion-Jacobson 2D perovskites. We further build a 7 × 7 crossbar array based on analogue perovskite synapses, achieving a high device yield, low variation with synaptic weight storing capability, multi-level analogue states with long retention, and moisture stability over 7 months. We explore the potential of such devices in large-scale image inference via simulations and show an accuracy within 0.08% of the theoretical limit. The excellent device performance is attributed to the elimination of gaps between inorganic layers, allowing the halide vacancies to migrate homogeneously regardless of grain boundaries. This was confirmed by first-principles calculations and experimental analysis.

Emerging two-dimensional ferromagnetic semiconductors.

Two-dimensional (2D) semiconductors have attracted considerable attention for their potential in extending Moore's law and advancing next-generation electronic devices. Notably, the discovery and development of 2D ferromagnetic semiconductors (FMSs) open exciting opportunities in manipulating both charge and spin, enabling the exploration of exotic properties and the design of innovative spintronic devices. In this review, we aim to offer a comprehensive summary of emerging 2D FMSs, covering their atomic structures, physical properties, preparation methods, growth mechanisms, magnetism modulation techniques, and potential applications. We begin with a brief introduction of the atomic structures and magnetic properties of novel 2D FMSs. Next, we delve into the latest advancements in the exotic physical properties of 2D FMSs. Following that, we summarize the growth methods, associated growth mechanisms, magnetism modulation techniques and spintronic applications of 2D FMSs. Finally, we offer insights into the challenges and potential applications of 2D FMSs, which may inspire further research in developing high-density, non-volatile storage devices based on 2D FMSs.

Quasi-one-dimensional bismuth chains with topological properties

One-dimensional (1D) systems have received a lot of attention due to their exotic physical phenomena such as quantization of conductance, Peierls instability, and Luttinger liquid behavior. Here, we present the synthesis of self-assembled quasi-1D bismuth (Bi) chains on an Au(111) substrate, highlighting their topological properties. Scanning tunneling microscopy and spectroscopy measurements reveal that the high-quality large area array is composed of Bi atomic chains with a bandgap of 0.5 eV. Density functional theory (DFT) calculations confirm the experimental observation and reveal the topological properties of the system, providing evidence of the spin texture of the Bi atomic chains. Our DFT results predict that a single Bi chain hosts exotic topological properties. The atomically precise synthesis of Bi atomic chains overcomes the limitation of the top-down approach toward integration of devices. The work sheds light on the realization of 1D system, providing a platform for further investigation into the intriguing physics in 1D systems.

Spin dynamics and Phonons of the Chromites CoCr2O4 and MnCr2O4

Abstract Due to the geometric magnetic frustration inherent to their lattice and the resulting complex magnetic states, the spinel compounds are of great interest in both fundamental and application-oriented perspectives. Here, we applied x-ray diffraction, magnetization, heat capacity and powder inelastic neutron scattering measurements, along with theoretical calculations, to study the exotic properties of chromite-spinel oxides, CoCr2O4 and MnCr2O4. The temperature dependence of the phonon spectra provides an insight into the correlation between oxygen motion and the magnetic order, as well as the magnetoelectric effect in the ground state of MnCr2O4. Moreover, spinwave excitations in CoCr2O4 and MnCr2O4 are compared with Heisenberg model calculations. This approach enables the precise determination of exchange energies and offers a comprehensive understanding of the spin dynamics and relevant exchange interactions in complicated spiral spin ordering.

Realization of a 2D Lieb Lattice in a Metal-Inorganic Framework with Partial Flat Bands and Topological Edge States.

Flat bands and Dirac cones in materials are the source of the exotic electronic and topological properties. The Lieb lattice is expected to host these electronic structures, arising from quantum destructive interference. Nevertheless, the experimental realization of a 2D Lieb lattice remained challenging to date due to its intrinsic structural instability. After computationally designing a Platinum-Phosphorus (Pt-P) Lieb lattice, it has successfully overcome its structural instability and synthesized on a gold substrate via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy verify the Lieb lattice's morphology and electronic flat bands. Furthermore, topological Dirac edge states stemming from pronounced spin-orbit coupling induced by heavy Pt atoms are predicted. These findings convincingly open perspectives for creating metal-inorganic framework-based atomic lattices, offering prospects for strongly correlated phases interplayed with topology.

Growth and electronic structure of the nodal line semimetal in monolayer Cu2Si on Cu(111)

Cu2Si, a single-layer two-dimensional material with a honeycomb structure, has been proposed to have Dirac nodal line fermions. In this study, the synchrotron radiation X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (SR-XPS, SR-UPS, and SR-ARPES) techniques were used to investigate the dynamic process of in situ deposition of single-layer Cu2Si on a Cu(111) crystal surface via molecular beam epitaxy (MBE). Cu2Si existed as a monolayer (ML) alloy, and there were competing mechanisms of distinct chemical states of silicon in different growth periods, according to a detailed examination of the experimental SR-XPS and SR-UPS spectra. Additionally, a weak interaction between the Cu2Si ML and Cu(111) was demonstrated via SR-ARPES and first-principles computations. The unique electronic structure of the Cu2Si ML was not destroyed by either this weak interaction or the disordered silicon produced on the surface during the growth process. The study of the Cu2Si growth kinetics provides a guarantee and a basis for the future exploration of the exotic properties of Cu2Si.

A coherent microscopic picture for the exotic structure of 11Be

The neutron-rich nucleus 11Be is one of the most enigmatic exotic nuclei in the periodic table. It exhibits a multitude of unique and intriguing phenomena within a single nucleus, including the halo phenomenon, clustering structure, and parity inversion. A unified description of these exotic properties represents a significant challenge for the nuclear physics field. By employing the axially deformed relativistic Hartree-Fock-Bogoliubov (D-RHFB) model, we have successfully reproduced the parity inversion and the one-neutron halo in 11Be, and revealed the underlying cluster structure. It is found that the intrusion of the sd-shell, which results in the parity inversion, is coherently connected with the halo and cluster structures. In particular, the s-wave is responsible for the halo formation, and the mixing of 2s1/2- and 1d5/2-waves enhances the cluster structure. This work elucidates the coherence between the parity inversion and the halo/cluster structures of 11Be, and addresses the long-standing challenge of providing a unified explanation for the multiple exotic phenomena observed in 11Be.

Accessing bands with extended quantum metric in kagome Cs 2 Ni 3 S 4 through soft chemical processing

Flat bands that do not merely arise from weak interactions can produce exotic physical properties, such as superconductivity or correlated many-body effects. The quantum metric can differentiate whether flat bands will result in correlated physics or are merely dangling bonds. A potential avenue for achieving correlated flat bands involves leveraging geometrical constraints within specific lattice structures, such as the kagome lattice; however, materials are often more complex. In these cases, quantum geometry becomes a powerful indicator of the nature of bands with small dispersions. We present a simple, soft-chemical processing route to access a flat band with an extended quantum metric below the Fermi level. By oxidizing Ni-kagome material Cs 2 Ni 3 S 4 to CsNi 3 S 4 , we see a two orders of magnitude drop in the room temperature resistance. However, CsNi 3 S 4 is still insulating, with no evidence of a phase transition. Using experimental data, density functional theory calculations, and symmetry analysis, our results suggest the emergence of a correlated insulating state of unknown origin.

Ultrasensitive integrated circuit sensors based on high-order non-Hermitian topological physics.

High-precision sensors are of fundamental importance in modern society and technology. Although numerous sensors have been developed, obtaining sensors with higher levels of sensitivity and stronger robustness has always been expected. Here, we propose theoretically and demonstrate experimentally an alternative class of sensors with superior performances based on exotic properties of high-order non-Hermitian topological physics. The frequency shift induced by perturbations for these sensors can show an exponential growth with respect to the size of the device, which can grow well beyond the limitations of conventional sensors. The fully integrated circuit chips have been designed and fabricated in a standard 65-nanometer complementary metal-oxide semiconductor process technology. Not only has the sensitivity of systems less than 10-3 femtofarad been experimentally verified, but these systems are also robust against disorders. Our proposed ultrasensitive integrated circuit sensors can have a wide range of applications in various fields and show an exciting prospect for next-generation sensing technologies.

Combined 3d-4f magnetic interactions to manipulate long-range ferromagnetism in monoclinic double perovskites Gd-substituted [formula omitted][formula omitted]

Oxide double perovskites (DPs) with half-filled 4f Rare-earth (RE) elements along with partially-filled 3d transition-metals (TM) are rich source of exotic spin-orbital properties. Herein, we have investigated explicitly, the structural properties, spin-orbital ground states of the bulk phases and electronic structure of the RE-double perovskites Sm2−xGdxCoMnO6x=0.0,0.5,1.0,1.5 by synthesizing high quality single-phase samples and undertaking high precision experimental measurements of the transport and magnetic properties down to 8 K. In fact, we are able to show that with Gd-substitution, it lowers the value of the magnetoresistance in Sm2CoMnO6 (SCMO) and it is consistent with the observed Curie-Weiss temperature (TCW) which is lower than the observed ordering temperature (TC) upon Gd-substitution of Sm3+. Native presence of magnetic impurities (Co3+ and Mn3+) with low-spin S = 0 configurations in the SCMO vs. the nominal higher spin-state (Co2+ and Mn4+) with S = 3/2 is predicted. It essentially tunes super-exchange mechanism and relative ground states as compared to Gd2CoMnO6 (GCMO). The synergy of crystal structure and electronic structure to the magnetic properties are thus, established with first-principles calculations data, employing self-consistent Hubbard U (Uself) determination for both 3d- and 4f-site’s electronic correlation. The computed quenched spin-moments (∼20–40 %) in the experimental observation at the Sm3+(4f5) and Gd3+(4f7) sites from the ideal value 10.0 μB and 14.0 μB per f.u. is established, respectively. It could be due to magnetic impurities in their ferri-magnetic ground state ordering (FiM3), as also confirmed from present first-principles calculations in the pristine limit. In SCMO x=0.0, the valence band is dominated with the RE(4 f)-orbitals along with the TM(3d)- and O(2p)-orbitals, resulting it as a charge-transfer type semiconductor, while in GCMO x=1.5, Gd(4 f)-orbitals are localized well below 6.5 eV from Fermi level, EF = 0 eV, resulting it as a Mott-insulator-like, mediated via 3d-3d transitions. The long-range ferro-magnetic spin-ordering ground states in SCMO is established through non-collinear spin-orbital ground states determinations. Indeed, the long-range insulating FM ordering in SCMO from combined 3d-4f-sub-lattice’s magnetic interactions, but found to be broken with Gd-substitution (x = 0.5–1.5) aligning all 3d-4f site’s spins into the in-plane. Observed coercive field (HC) and remanence asymmetry (Mr), the nature of exchange-bias transformed from inverse to conventional type, as soon as it contains a small fraction of the Gd-substitution (x = 0.5).

Anomalous Raman Response in 2D Magnetic FeTe Under Uniaxial Strain: Tetragonal and Hexagonal Polymorphs.

2D Fe-chalcogenides emerge with rich structures, magnetisms, and superconductivities, which spark the growing research interests in the torturous transition mechanism and tunable properties for their potential applications in nanoelectronics. Uniaxial strain can produce a lattice distortion to study symmetry breaking induced exotic properties in 2D magnets. Herein, the anomalous Raman spectrum of 2D tetragonal (t-) and hexagonal (h-) FeTe is systematically investigated via uniaxial strain engineering strategy. It is found that both t- and h-FeTe keep the structural stability under different uniaxial tensile or compressive strain up to ± 0.4%. Intriguingly, the lattice vibrations along both in-plane and out-of-plane directions exceptionally harden (softened) under tensile (compressive) strain, distinguished from the behaviors of many conventional 2D systems. Further, the difference in thickness-dependent strain effect can be well explained by their structural discrepancy between two polymorphs of FeTe. These results can supply a unique platform to explore the vibrational properties of many novel 2D materials.

Understanding disorder in monolayer graphene devices with gate-defined superlattices

Engineering superlattices (SLs)-which are spatially periodic potential landscapes for electrons-is an emerging approach for the realization of exotic properties, including superconductivity and correlated insulators, in two-dimensional materials. While moiré SL engineering has been a popular approach, nanopatterning is an attractive alternative offering control over the pattern and wavelength of the SL. However, the disorder arising in the system due to imperfect nanopatterning is seldom studied. Here, by creating a square lattice of nanoholes in the SiO2dielectric layer using nanolithography, we study the SL potential and the disorder formed in hBN-graphene-hBN heterostructures. Specifically, we observe that while electrical transport shows distinct SL satellite peaks, the disorder of the device is significantly higher than graphene devices without any SL. We use finite-element simulations combined with a resistor network model to calculate the effects of this disorder on the transport properties of graphene. We consider three types of disorder: nanohole size variations, adjacent nanohole mergers, and nanohole vacancies. Comparing our experimental results with the model, we find that the disorder primarily originates from nanohole size variations rather than nanohole mergers in square SLs. We further confirm the validity of our model by comparing the results with quantum transport simulations. Our findings highlight the applicability of our simple framework to predict and engineer disorder in patterned SLs, specifically correlating variations in the resultant SL patterns to the observed disorder. Our combined experimental and theoretical results could serve as a valuable guide for optimizing nanofabrication processes to engineer disorder in nanopatterned SLs.

Magnetotransport and angle-resolved photoemission spectroscopy of MnSb12Te19: a new member of MnSb2nTe3n+1 family

The quest for intrinsically ferromagnetic topological materials is a focal point in the study of topological phases of matter, as intrinsic ferromagnetism plays a vital role in realizing exotic properties such as the anomalous Hall effect (AHE) in quasi-two-dimensional materials, and this stands out as one of the most pressing concerns within the field. Here, we investigate a novel higher order member of the MnSb2nTe3n+1family, MnSb12Te19, for the first time combining magnetotransport and angle-resolved photoemission spectroscopy (ARPES) measurements. Our magnetic susceptibility experiments identify ferromagnetic transitions at temperatureTc= 18.7 K, consistent with our heat capacity measurements (T= 18.8 K). The AHE is observed for the field along thec-axis belowTc. Our study of Shubinikov-de-Haas oscillations provides evidence for Dirac fermions withπBerry phase. Our comprehensive investigation reveals that MnSb12Te19exhibits a FM ground state along with AHE, and hole-dominated transport properties consistent with ARPES measurements.

Current developments in elastic and acoustic metamaterials science.

The concept of metamaterial recently emerged as a new frontier of scientific research, encompassing physics, materials science and engineering. In a broad sense, a metamaterial indicates an engineered material with exotic properties not found in nature, obtained by appropriate architecture either at macro-scale or at micro-/nano-scales. The architecture of metamaterials can be tailored to open unforeseen opportunities for mechanical and acoustic applications, as demonstrated by an impressive and increasing number of studies. Building on this knowledge, this theme issue aims to gather cutting-edge theoretical, computational and experimental studies on elastic and acoustic metamaterials, with the purpose of offering a wide perspective on recent achievements and future challenges. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 1)'.