Search

We report first principles numerical discovery of hydrostatic pressure driven tetragonal to collapsed tetragonal transition in 1111-type material ThMnAsN accompanied by simultaneous magneto-structural, insulator to metal transition together with complete collapse of Mn moment. We present detailed evolution of various structural parameters, magnetism and electronic structures of ThMnAsN with increasing hydrostatic pressure. All the structural parameters show anomalies at a critical pressure P$_c \sim$ 9 GPa; c-lattice parameter, out of plane As-As bond length, anion height (h$_{As}$) undergo drastic modification compared to the in-plane parameters which is manifested in an iso-structural phase transition from tetragonal to a collapsed tetragonal (cT) phase. These modifications in "local structural correlations" due to pressure destroys usually localized nature of Mn moments and gets completely quenched. Apart from that the elastic constant, the electronic structures also bear the finger prints of insulator-metal and magneto-structural transition at higher pressures accompanying a total collapse of magnetic moment at the vicinity of 9 GPa. The critical value of the pressure P$_c$ at which tetragonal to collapse tetragonal phase transition occurs, remains robust with respect to the on-site Hubbard correlation (U). The dynamical stability of the compound at higher pressures are affirmed through detailed computations of phonon dispersion curves endowed with positive phonon frequency through out the Brillouin zone. The effect of magnetic spin structure on the electronic band structures are obtained through band unfolding. The electronic structure of ThMnAsN at higher pressures "orbital selectively" influences bands, band gap and closely resembles with the electronic structure of Fe-based superconductors with the occurrences of orbital selective Lifshitz transition., Comment: 13 pages, 21 figures

We report the magnitude of the induced magnetic moment in CVD-grown epitaxial and rotated-domain graphene in proximity with a ferromagnetic Ni film, using polarized neutron reflectivity (PNR) and X-ray magnetic circular dichroism (XMCD). The XMCD spectra at the C K-edge confirms the presence of a magnetic signal in the graphene layer and the sum rules give a magnetic moment of up to $\sim\,0.47\,\mu$_B/C atom induced in the graphene layer. For a more precise estimation, we conducted PNR measurements. The PNR results indicate an induced magnetic moment of $\sim$ 0.53 $\mu$_B/C atom at 10 K for rotated graphene and $\sim$ 0.38 $\mu$_B/C atom at 10 K for epitaxial graphene. Additional PNR measurements on graphene grown on a non-magnetic Ni_9Mo_1 substrate, where no magnetic moment in graphene is measured, suggest that the origin of the induced magnetic moment is due to the opening of the graphene's Dirac cone as a result of the strong C pz-3d hybridization., Comment: 21 pages, 7 figures

The local deformation behavior and dynamic recrystallization of a shock compressed Mg-1 Zn alloy was investigated through EBSD and TEM. Since dislocation slipping and twinning were locally suppressed during high strain-rate deformation, a more flexible kinking deformation was activated to adjusted local orientation and facilitate slipping and twinning within the kinks. Meanwhile, due to the slow heat dissipation that resulted in a local temperature elevating, the kink bands were evolved into deformation bands with recrystallized nano-grains. Such a finding provides a new perspective for kinking-facilitated nanocrystallization in Mg alloys and other anisotropic metallic materials.

Two essential ingredients for the quantum anomalous Hall (QAH) effect, i.e. topological and magnetic orders, can be combined by doping magnetic ions into a topological insulator (TI) film. Through this approach, the QAH effect has been realized in chromium (Cr)- and/or vanadium (V)-doped TI (Bi,Sb)2Te3 thin films. In this work, we synthesize both V- and Cr-doped Bi2Te3 thin films with controlled dopant concentration using molecular beam epitaxy (MBE). By performing magneto-transport measurements, we find that both systems show an unusual but yet similar ferromagnetic response with respect to magnetic dopant concentration, specifically the Curie temperature does not increase monotonically but shows a local maximum at a critical dopant concentration. Our angle-resolved photoemission spectroscopy (ARPES) measurements show that the Cr/V doping introduces hole carriers into Bi2Te3, which consequently move the chemical potential toward the charge neutral point. In addition, the Cr/V doping also reduces the spin-orbit coupling of Bi2Te3 which drives it from a nontrivial TI to a trivial semiconductor. The unusual ferromagnetic response observed in Cr/V-doped Bi2Te3 thin films is attributed to the dopant-concentration-induced magnetic exchange interaction, which displays the evolution from the van Vleck-type ferromagnetism in a nontrivial magnetic TI to the Ruderman-Kittel-Kasuya-Yosida (RKKY)-type ferromagnetism in a trivial diluted magnetic semiconductor. Our work provides insights into the ferromagnetic properties of magnetically doped TI thin films and facilitates the pursuit of high-temperature QAH effect., 17 pages, 4 figures. Comments are welcome

In density functional theory (DFT) based total energy studies, the van der Waals (vdW) and zero-point vibrational energy (ZPVE) correction terms are included to obtain energy differences between polymorphs. We propose and compute a new correction term to the total energy, due to electron-phonon interactions (EPI). We rely on Allen's general formalism, which goes beyond the Quasi-Harmonic Approximation (QHA), to include the free energy contributions due to quasiparticle interactions. We show that, for semiconductors and insulators, the EPI contributions to the free energies of electrons and phonons are the corresponding zero-point energy contributions. Using an approximate version of Allen's formalism in combination with the Allen-Heine theory for EPI corrections, we calculate the zero-point EPI corrections to the total energy for cubic and hexagonal polytypes of Carbon, Silicon and Silicon Carbide. The EPI corrections alter the energy differences between polytypes. In SiC polytypes, the EPI correction term is more sensitive to crystal structure than the vdW and ZPVE terms and is thus essential in determining their energy differences. It clearly establishes that the cubic SiC-3C is metastable and hexagonal SiC-4H is the stable polytype. Our results are consistent with the experimental results of Kleykamp. Our study enables the inclusion of EPI corrections as a separate term in the free energy expression. This opens the way to go beyond the QHA by including the contribution of EPI on all thermodynamic properties., Comment: 30 pages and 2 figures

Excitons, Coulomb-bound electron-hole pairs, play a fundamental role in both optical excitation and correlated phenomena in solids. When an exciton interacts with other quasi-particles, few- and many-body excited states, such as trions, exciton Fermi-polarons, Mahan excitons can appear. Here, we report a new interaction between exciton and charges enabled by unusual quantum confinement in 2D moir\'e superlattices, which results in novel exciton many-body ground states composed of moir\'e excitons and correlated electron lattices. Unique to H-stacked (or 60o-twisted) WS2/WSe2 heterobilayer, we found that the interlayer atomic registry and moir\'e structural reconstruction leads to an interlayer moir\'e exciton (IME) whose hole in one layer is surrounded by its partner electron's wavefunction spread among three adjacent moir\'e traps in the other layer. This 3D excitonic structure can enable large in-plane electrical quadrupole moments in addition to the vertical dipole. Upon doping, the electric quadrupole facilitates the binding of IME to the charges in neighboring moir\'e cells, forming an intercell charged exciton complex. The exciton complex is unveiled by the IME photoluminescence energy jumps when the electron lattices form at both fractional and integer-filled moir\'e minibands, with replica-like spectral features between successive integer moir\'e fillings. Our work provides the framework in understanding and engineering emergent exciton many-body states in correlated moir\'e charge orders.

The different magnetic behaviors of LaCoO$_3$ films grown on LaAlO$_3$ and SrTiO$_3$ are related to the Co-O-Co bond angles and the constraints imposed on the Co-O bond lengths by the substrate geometries. Long-range magnetic order occurs below T ~ 90 K when the Co-O-Co bond angle is greater than 163 degrees, consistent with the behavior of bulk and nanoparticles forms of LaCoO$_3$. A LaAlO$_3$ substrate prevents magnetic long-range order at low temperatures near the film-substrate interface and collinear antiferromagnetic sublattices away from the interface. At low temperatures, the antiferromagnetically ordered sublattices are non-collinear in films grown on SrTiO$_3$ substrates, leading to a significant net moment., Comment: 28 pages, 8 figures, 6 supplementary figures

To overcome the limitation in formability at room temperature, manufacturers have developed magnesium alloys with remarkable properties by adding rare-earth elements. The rare-earth magnesium alloys behave differently from the conventional alloys, especially with respect to their coupled anisotropic and strain rate sensitive behavior. In the current work, such behavior of the rare-earth Mg alloy ZEK100 sheet at room temperature is investigated with the aid of the elastic viscoplastic self-consistent polycrystal plasticity model. Different strain rate sensitivities (SRSs) for various deformation modes are employed by the model to simulate the strain rate sensitive behaviors under different loading directions and loading rates. Good agreement between the experiments and simulations reveals the importance and necessity of using different SRSs for each deformation mode in hexagonal close-packed metals. Furthermore, the relative activities of each deformation mode and the texture evolution during different loadings are discussed. The anisotropic and strain rate sensitive behavior is ascribed to the various operating deformation modes with different SRSs during loading along different directions.

The low dimensional inorganic–organic (IO) hybrid semiconductors become a fascinating subject of interest due to their unique crystal structural packing and tunable optical exciton emission features. These naturally self-assembled 2D IO hybrid semiconductors form multiple quantum wells (MQWs) like structure due to the quantum widths of both organic moiety and inorganic network , enabling them to show stable room temperature Mott-type excitons with large exciton binding energy. A systematic co-existence of two kinds of excitons observable from one-(ħω ≥ Eg) and two-photon (2ħω ≥ Eg) excitations is studied to probe into deeper details of sensitive structural changes in the IO-hybrids thin film and single crystals.

We revisit some recents results inspired by the Peierls-Nabarro model on edge dislocations for crystals which rely on the fractional Laplace representation of the corresponding equation. In particular, we discuss results related to heteroclinic, homoclinic and multibump patterns for the atom dislocation function, the large space and time scale of the solutions of the parabolic problem, the dynamics of the dislocation points and the large time asymptotics after possible dislocation collisions.

Semiconductor Quantum Dots have unique properties, such as the quantum size effect that provide the breakthrough in optoelectronic devices. The small effective mass of electron and hole open-up an avenue of wide bandgap tunability in InAs Quantum Dots (QDs), making them an attractive material for solar cell application. Therein, 4x4 Luttinger Kohn Hamiltonian method is used to calculate the absorption co-efficient of epitaxially grown InAs QDs. Further, we studied the size-dependent absorption co-efficient and the barrier material model of InAs QDs capping with InP and GaAs. Our theoretical simulation predicts the efficiency of InAs/GaAs and InAs/InP system in the intrinsic layer of p-i-n structure solar cells.

For single-walled carbon nanotubes (SWCNTs) with a length of 1-50 nm, the surface plasmon-polariton (SPP) resonance is within the terahertz frequency range; therefore, SWCNT lattices can be used to design frequency-selective surface (FSS). The numerical model of electromagnetic wave diffraction on a two-dimensional periodic SWCNT lattice can be described by an integro-differential equation of the second-order with respect to the surface current along SWCNT. The equation can be solved by the Bubnov-Galerkin method. Frequency dependence of reflecting and transmitting electromagnetic waves for FSSs near the SPP resonance are studied numerically. It is shown that the resonances are within the lower-frequency part of the terahertz range. Also, we estimate the relaxation frequency of an individual SWCNT and demonstrate the applicability of the Kubo formula for graphene conductivity to array of strips similar in size with SWCNTs under consideration., Comment: 8 pages, 10 figures

Aluminum alloys are commonly used in the aerospace industry because of their good specific strength and corrosion resistance. However, reducing residual stresses and machining deformation of aluminum alloy components are the focus of researches. In this paper, a material model of 2A14 aluminum alloy during solution treatment and aging process is established with thermal expansion, high temperature tensile and stress relaxation experiments. The stress relaxation during aging is described with the Zener-Wert-Avrami model based on the experimental data. Quenching and aging experiments of 2A14 aluminum alloy C-ring samples were carried out. Temperature changes of C-rings were measured during quenching, residual stresses in the samples after solution, and aging are measured by XRD, respectively. Based on measured temperature variations, surface heat transfer coefficients of a C-ring during quenching were calculated by inverse heat transfer calculation and a novel optimization method. The changes of temperature, stresses, and notch opening distance of a C-ring during solution and aging process were calculated, and calculated results were in good agreement with the measured results.

A large number of experimental observations show that under two-dimensional (2-D) rotational magnetization, there is a complicated relationship between the magnetostrictive strain and magnetic flux density. In this paper, the magnetostrictive characteristics in a non-oriented electrical steel sheet under 2-D magnetization are measured and modelled based on the experimental system developed by the laboratory. The in-plane magnetostrictive strain tensor under the 2-D magnetization is expressed as nonlinear superposition of dynamic magnetostrictive strain in the rolling direction (RD) and transverse direction (TD), respectively. The magnetostriction’s first derivative with respect to magnetization depicts the pinning effect and the interaction among magnetic domains which result in the hysteresis behaviour. And based on the particle swarm optimization (PSO), the model parameters are identified. The validity of the proposed model is verified by comparing the calculation results with the experimental ones. Studies have shown that the magnetostrictive properties in an electrical steel sheet have strong hysteresis behaviour under the 2-D magnetization, and the model proposed here can well characterize these characteristics.

Understanding and controlling the interaction between the excitonic states of a quantum emitter and the plasmonic modes of a nanocavity is one of the most relevant current scientific challenges, key for the development of many applications, from quantum information processing devices to polaritonic catalysts. In this paper we demonstrate that the tunnel electroluminescence of C60 nanocrystals enclosed in the plasmonic nanocavity between a metallic surface and the tip of a Scanning Tunnelling Microscope, and isolated from the metal surface by a thin NaCl film, can be switched from a broad emission spectrum, revealing the plasmonic modes of the cavity, to a narrow band emission, displaying only the excitonic states of the C60 molecules by changing the bias voltage applied to the junction. Plasmonic emission is found in the same voltage region in which the rate of inelastic tunnel transitions is large and, thus, vanishes for large voltages. Excitonic emission, on the other hand, dominates the spectra in the high-voltage region in which the inelastic rate is low, demonstrating that the excitons cannot be created by an inelastic tunnel process. These results point towards new possible mechanisms to explain the tunnel electroluminescence of quantum emitters and offer new avenues to develop electrically tuneable nanoscale light sources.

We consider a nanoelectromechanical system composed of a carbon nanotube suspended between two normal leads and coupled to a superconducting scanning tunneling microscope (STM) tip via vacuum tunnel barrier. Treating the nanotube as a single-level quantum dot, it is shown that an applied voltage between the superconducting STM tip and normal leads gives rise to a pumping or a cooling of the mechanical subsystem depending on the direction of the electronic flow. It is also demonstrated that the transition between these two regimes is controlled by the strength of the tunnel coupling between the nanotube and superconducting STM tip and the relative position of the electronic level. Such phenomena are realized due to a specific electromechanical coupling that is fully governed by the quantum dynamics of the Cooper pairs. The amplitude of the self-sustained oscillations in the pumping regime is analyzed numerically, and the effective temperature of the mechanical subsystem in the cooling regime is obtained.

A topological insulator (TI) interfaced with an s-wave superconductor has been predicted to host an unusual form of superconductivity known as topological superconductivity (TSC). Molecular beam epitaxy (MBE) has been the primary approach in the scalable synthesis of the TI/superconductor heterostructures. Although the growth of epitaxial TI films on s-wave superconductors has been achieved, it remains an outstanding challenge for synthesizing atomically thin TI/superconductor heterostructures, which are critical for engineering the TSC phase. Here, we used MBE to grow Bi2Se3 films with the controlled thickness on monolayer NbSe2 and performed in-situ angle-resolved photoemission spectroscopy and ex-situ magneto-transport measurements on these Bi2Se3/monolayer NbSe2 heterostructures. We found that the emergence of Rashba-type bulk quantum well bands and spin-nondegenerate surface states coincides with a marked suppression of the in-plane upper critical magnetic field of the superconductivity in Bi2Se3/monolayer NbSe2 heterostructures. This is the signature of a crossover from Ising- to Rashba-type superconducting pairings, induced by altering Bi2Se3 film thickness. Our work opens a new route for exploring a robust TSC phase in TI/Ising superconductor heterostructures., 27 pages, 4 pages, comments are welcome

International audience; We propose accurate computable error bounds for quantities of interest in plane-wave electronic structure calculations, in particular ground-state density matrices and energies, and interatomic forces. These bounds are based on an estimation of the error in terms of the residual of the solved equations, which is then efficiently approximated with computable terms. After providing coarse bounds based on an analysis of the inverse Jacobian, we improve on these bounds by solving a linear problem in a small dimension that involves a Schur complement. We numerically show how accurate these bounds are on a few representative materials, namely silicon, gallium arsenide and titanium dioxide.

2D materials have intriguing quantum phenomena that are distinctively different from their bulk counterparts. Recently, epitaxially synthesized wafer-scale 2D metals, composed of elemental atoms, are attracting attention not only for their potential applications but also for exotic quantum effects such as superconductivity. By mapping momentum-resolved electronic states using time-resolved and angle-resolved photoemission spectroscopy (ARPES), we reveal that monolayer Ag confined between bilayer graphene and SiC is a large gap (> 1 eV) 2D semiconductor, consistent with GW-corrected density functional theory. The measured valence band dispersion matches the DFT-GW quasiparticle band. However, the conduction band dispersion shows an anomalously large effective mass of 2.4 m0. Possible mechanisms for this large enhancement in the apparent mass are discussed.

Cavity magnonics deals with the interaction of magnons - elementary excitations in magnetic materials - and confined electromagnetic fields. We introduce the basic physics and review the experimental and theoretical progress of this young field that is gearing up for integration in future quantum technologies. Much of its appeal is derived from the strong magnon-photon coupling and the easily-reached nonlinear regime in microwave cavities. The interaction of magnons with light as detected by Brillouin light scattering is enhanced in magnetic optical resonators, which can be employed to manipulate magnon distributions. The cavity photon-mediated coupling of a magnon mode to a superconducting qubit enables measurements in the single magnon limit., Comment: review article, 54 pages

Topological antiferromagnets have a significant role to play in the modern day electronics and spintronics due to the presence of peculiar symmetries and magnetically driven large anomalous transport behaviour. We report the observation of anomalous Hall effect in Mn3Sn polycrystalline thin films deposited on Al2O3 substrate with a large anomalous Hall conductivity of 65 (Ωcm)−1 at 3 K. The Hall and magnetic measurements show a very small hysteresis owing to a weak ferromagnetic moment present in this material. The longitudinal resistivity decreases sufficiently for the thin films as compared to the polycrystalline bulk sample used as the target for the film deposition. The anomalous Hall resistivity and conductivity decreases almost linearly with the increase in the temperature. A negative magnetoresistance is observed for all the measured temperatures with the negative decrease in the magnitude with the increase in temperature.

In this paper, recent results regarding rare earth element doped with chalcogenide glasses for optical gain media such as amplifiers and lasers are reviewed. Rare earth Chalcogenide glasses are formed when chalcogen elements (S, Se, Te etc.) are mixed with new additive elements (Ge, Ga, Sb, As etc.) and also with rare earth elements (Er, Dy, Pd, Pr etc.) from periodic table and prepared by melt quench technique (MQT). Rare earth element doped with chalcogenide glasses has been emerged as a suitable candidate for applications in the numeral passive and active. Among various materials, chalcogenide glasses show significant improvement in their emission properties in the optical communication range and spectroscopic properties. Finally, rare earth doped chalcogenide and optical fibers show potential for high power mid-infrared gain medium and their spectroscopic properties as well.

Thin films of Topological insulators (TIs) coupled with ferromagnets (FMs) are excellent candidates for energy-efficient spintronics devices. Here, the effect of crystalline structural disorder of TI on interfacial and magnetic properties of sputter-deposited TI/FM, Bi2Te3/Ni80Fe20, heterostructures is reported. Ni and a smaller amount of Fe from Py was found to diffuse across the interface and react with Bi2Te3. For highly crystalline c-axis oriented Bi2Te3 films, a giant enhancement in Gilbert damping is observed, accompanied by an effective out-of-plane magnetic anisotropy and enhanced damping-like spin-orbit torque (DL-SOT), possibly due to the topological surface states (TSS) of Bi2Te3. Furthermore, a spontaneous exchange bias is observed in hysteresis loop measurements at low temperatures. This is because of an antiferromagnetic topological interfacial layer formed by reaction of the diffused Ni with Bi2Te3 which couples with the FM, Ni80Fe20. For increasing disorder of Bi2Te3, a significant weakening of exchange interaction in the AFM interfacial layer is found. These experimental results Abstract length is one paragraph.

The relative energetic stability of the structural phases of common antiferromagnetic transition-metal oxides (MnO, FeO, CoO, and NiO) within the semilocal and hybrid density functionals are fraught with difficulties. In particular, MnO is known to be the most difficult case for almost all common semilocal and hybrid density approximations. Here, we show that the meta-generalized gradient approximation (meta-GGA) constructed from the cuspless hydrogen model and Pauli kinetic energy density (MGGAC) can lead to the correct ground state of MnO. The relative energy differences of zinc-blende (zb) and rock-salt (rs) structures as computed using MGGAC are found to be in nice agreement with those obtained from high-level correlation methods like the random phase approximation or quantum Monte Carlo techniques. Besides, we have also applied the onsite hybrid functionals (closely related to DFT+U ) based on GGA and meta-GGA functionals, and it is shown that a relatively high amount of Hartree-Fock exchange is necessary to obtain the correct ground-state structure. Our present investigation suggests that the semilocal MGGAC and onsite hybrids, both being computationally cheap, as methods of choice for the calculation of the relative stability of antiferromagnetic transition-metal oxides having potential applications in solid-state physics and structural chemistry.

Piezomagnetism couples strain linearly to magnetic order producing magnetization. Thus, unlike magnetostriction, it enables bidirectional control of a net magnetic moment. If this effect becomes large at room temperature, it may be technologically relevant, similar to its electric analogue, piezoelectricity. To date, however, the studies of the piezomagnetic effect have been primarily restricted to antiferromagnetic (AF) insulators at cryogenic temperatures. Here we report the discovery of a large piezomagnetism in a metal at room temperature. Strikingly, by using the AF Weyl semimetal Mn$_3$Sn, known for its nearly magnetization-free anomalous Hall effect (AHE), we find that an application of small uniaxial strain of the order of 0.1 % can control both the sign and size of the AHE. Our experiment and theory show that the piezomagnetism can control the AHE distinctly from the magnetization, which will be useful for spintronics applications., Comment: 25 pages, 4 figures, To appear in Nature Physics

Moir\'e superlattices in atomically thin van-der-Waals heterostructures hold great promise for an extended control of electronic and valleytronic lifetimes, the confinement of excitons in artificial moir\'e lattices, and the formation of novel exotic quantum phases. Such moir\'e-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure. In order to exploit the full potential of correlated moir\'e and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement in the moir\'e potential is indispensable. However, direct experimental access to these parameters is limited since most excitonic quasiparticles are optically dark. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moir\'e interlayer excitons. We find that interlayer excitons are dominantly formed on the sub-50~fs timescale via interlayer tunneling at the K valleys of the Brillouin zones. In addition, we directly measure energy-momentum fingerprints of the moir\'e interlayer excitons by mapping their spectral signatures within the mini Brillouin zone that is built up by the twisted heterostructure. From these momentum-fingerprints, we gain quantitative access to the modulation of the exciton wavefunction within the moir\'e potential in real-space. Our work provides the first direct access to the interlayer moir\'e exciton formation dynamics in space and time and reveals new opportunities to study correlated moir\'e and exciton physics for the future realization of exotic quantum phases of matter., Comment: 7 pages main text, 11 pages SI

We have investigated the coupling of lattice with spin via strain interactions in the CoFe2O4/PMN-PT composite system. X-ray diffraction and Raman spectroscopic studies illustrate a remarkable modification in CoFe2O4lattice across Curie temperature (450 K) of PMN-PT. Subsequently, CoFe2O4/PMN-PT composite reveals a sudden drop in magnetic moment across Tc of PMN-PT (450 K). However,theindependent CoFe2O4phasedisplaystypical ferromagnetic behaviour across this temperature. These findings establish spin-lattice coupling owing to th interfacial strain transfer between CoFe2O4 and PMN-PT in composite. The strain intractions leads to magneto-electric coupling, evidenced by measuring magentization and magneto-electric coefficient for the electric field poled and unploed CoFe2O4/PMN-PT composite samples. X-ray magnetic circular dichroism (XMCD) analysis establishes that the cation (Fe3+/Co2+) redistribution occurs on tetrahedral and octahedral site in the electrically poled CoFe2O4/PMN-PT composite, confirming the coupling between magnetic and electric ordering in the composite. The magneto-electric coupling coefficient alpha vs dc magnetic field curves revealed hysteretic behavior and enhanced {\alpha} values after electric poling, which originates from the strain induced modifications in the magnetic domains configuration of composite in the poled samples. These findings suggest that the existence of spin lattice coupling may leads to the mechanism of strong magneto-electric effects via strain interactions in CoFe2O4/PMN-PT composite., Comment: 26 pages, 9 figures

Recently, fabricating type-II vertical van der Waals (vdWs) heterostructure is a promising material for hydrogen production. The absorption capability, charge density distributions, band alignments and electronic properties of the monolayers and heterostructures are systematically investigated using computational studies. Using ab initio molecular dynamics, binding energy and phonon calculations, the stability of the heterostructures are verified. Both heterostructures are type-II materials, which can increase the separation of charge carriers. Moreover, the charge density difference and the potential drop across the interface of MSe2/BSe creates a high built-in electric field that can prevent the recombination of charge carriers. We found that the visible-light optical properties of both heterostructures are much enhanced with suitable bandgap energy for water splitting. The band alignment suggests that the heterostructures straddle water redox potentials in acid solutions (0

Energy carrier evolution is crucial for material performance. Ultrafast microscopy has been widely applied to visualize the spatiotemporal evolution of energy carriers. However, direct imaging of small amounts of energy carriers on nanoscale remains difficult due to extremely weak transient signals. Here we present a method for ultrasensitive and high-throughput imaging of energy carrier evolution in space and time. This method combines femtosecond pump-probe techniques with interferometric scattering microscopy (iSCAT), named Femto-iSCAT. The interferometric principle and unique spatially-modulated contrast enhancement increase the transient image contrast by >2 orders of magnitude and enable the exploration of new science. We address three important and challenging problems: transport of different energy carriers at various interfaces, heterogeneous hot electron distribution and relaxation in single plasmonic resonators, and distinct structure-dependent edge state dynamics of carriers and excitons in optoelectronic semiconductors. Femto-iSCAT holds great potential as a universal tool for ultrasensitive imaging of energy carrier evolution in space and time.

We consider a mathematical model for the gas flow through a one-way valve and focus on two issues. First, we propose a way to eliminate the chattering (the fast switch on and off of the valve) by slightly modifying the design of the valve. This mathematically amounts to the construction of a coupling Riemann solver with a suitable stability property, namely, coherence. We provide a numerical comparison of the behavior of the two valves. Second, we analyze, both analytically and numerically, for several significative situations, the maximization of the flow through the modified valve according to a control parameter of the valve and time.

We study crystalline surface evolution in extreme environments, where high-energy atoms impinge on a crystalline surface to cause sputtering, growth and surface roughening. For our model system, we study the evolution of the Ni(111) surface under Ni atom bombardment, using Molecular Dynamics (MD) simulation and a Sutton-Chen force field, where the uppermost surface layers are free to move, supported by thermostatically controlled layers above a rigid template. The MD statistics of sputtering and sticking are used to aid the development of a computationally efficient kinetic Monte Carlo (KMC) code. Comparisons between the simulation surface morphologies are used to tune the KMC growth rules so that the key statistical features of the MD structures are captured by the KMC model. This model is then employed to explore the predicted behaviour over length and times scales much larger than those accessible to MD. While the MD-KMC approach is well known, this application to the complex surface growth encountered in energetic atomic bombardment illuminates the complexities of relating atomistic events to morphological evolution. The work shows how simulation methodology can be extended to provide predictive capabilities, paving the way for design tools for engineering processes such as plasma deposition film growth.

Using localized surface plasmon resonance (LSPR) as an optical probe we demonstrate the presence of free carriers in phosphorus doped silicon nanocrystals (SiNCs) embedded in a silica matrix. In small SiNCs, with radius ranging from 2.6 to 5.5 nm, the infrared spectroscopy study coupled to numerical simulations allows us to determine the number of electrically active phosphorus atoms with a precision of a few atoms. We demonstrate that LSP resonances can be supported with only about 10 free electrons per nanocrystal, confirming theoretical predictions and probing the limit of the collective nature of plasmons. We reveal the appearance of an avoided crossing behavior linked to the hybridization between the localized surface plasmon in the doped nanocrystals and the silica matrix phonon modes. Finally, a careful analysis of the scattering time dependence versus carrier density in the small size regime allows us to detect the appearance of a new scattering process at high dopant concentration, which can be explained by P clustering inside the SiNCs.

While it is widely used to represent hysteresis phenomena with unidirectional-oriented loops, we study in this article the use of Preisach operator for describing hysteresis behavior with multidirectional-oriented loops. This complex hysteresis behavior is commonly found in advanced materials, such as, shape-memory alloys or piezoelectric materials, that are used for high-precision sensor and actuator systems. We provide characterization of the Preisach operators exhibiting such input-output behaviors and we show the richness of the operators that are capable of producing intricate loops.

We prove an existence and uniqueness result for solutions $\varphi$ to a weighted pendulum equation in $\mathbb{R}$ where the weight is non-smooth and coercive. We also establish (in)stability results for $\varphi$ according to the monotonicity of the weight. These results are applied in a reduced model for thin ferromagnetic nanowires with notches to obtain existence, uniqueness and stability of domain walls connecting two opposite directions of the magnetization.

Ta–Si–C–N single-layer films have been synthesized by direct current magnetron sputtering on fused quartz substrates. The structural perfection of the films has been studied using X-ray diffraction, scanning electron microscopy and glow discharge optical emission spectroscopy. The optical parameters of the films have been studied using multi-angle spectrophotometry. The spectral dependences of the transmission coefficients of substrates and structures were measured for normal incidence of light in the wavelength range 200–2500 nm. We show that the transmittance spectrum of the specimen has an oscillating pattern due to interference phenomena that are typical for layered structures. The reflectance of the films and the substrates has been measured in the 200–2500 nm region for small incidence angles. The difference between the reflection coefficient at the maximum of the interference of the film and the corresponding reflection coefficient of the substrate at the same wavelength shows that the absorption in the film is low. A formula for determination of the film absorption index based on the measured parameters has been derived. The experimental and calculated data have been used for plotting the absorption spectra of the substrate, structure and film. Discrete refractive indices in the 400–1200 nm region have been calculated for reflections at two incidence angles by determining the positions of interference maxima in the reflectivity spectral responses. The results have been approximated using the Cauchy equation. The film thickness has been estimated to be df = 1046 nm ± 13%. The spectra of film extinction coefficients have been plotted with and without allowance for reflection. Obtained values of the refractive and absorption indices with and without reflection have been summarized in a table.

Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photoluminescence, optical reflection measurements and ab initio theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP2), where the bound electron–hole pair is composed of electrons confined within one-dimensional phosphorus–phosphorus chains and holes extended in two-dimensional SiP2 layers. The excitonic state and emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Our ab initio many-body calculations confirm that the observed phonon sideband results from the correlated interaction between excitons and optical phonons. With these results, we propose layered SiP2 as a platform for the study of excitonic physics and many-particle effects.

Transition metal dichalcogenide monolayers and heterostructures are highly tunable material systems that provide excellent models for physical phenomena at the two-dimensional (2D) limit. While most studies to date have focused on electrons and electron-hole pairs, phonons also play essential roles. Here, we apply ultrafast electron diffraction and diffuse scattering to directly quantify, with time and momentum resolution, electron-phonon coupling (EPC) in monolayer molybdenum disulfide (MoS$_{2}$) and phonon transport from the monolayer to a silicon nitride (Si$_3$N$_4$) substrate. Optically generated hot carriers result in a profoundly anisotropic distribution of phonons in the monolayer on the $\sim$ 5 ps time scale. A quantitative comparison with ab-initio ultrafast dynamics simulations reveals the essential role of dielectric screening in weakening EPC. Thermal transport from the monolayer to the substrate occurs with the phonon system far from equilibrium. While screening in 2D is known to strongly affect equilibrium properties, our findings extend this understanding to the dynamic regime.

Single crystalline NbMnP was grown by the high-temperature solution growth technique and characterized by room temperature X-ray diffraction, temperature- and field-dependent magnetization, temperature-dependent resistivity, and heat capacity measurements. NbMnP is isostructural to TiNiSi with the space group of Pnma. Physical characterizations suggest that NbMnP is metallic and goes through an anti-ferromagnet transition at around 230 K with a weak magnetic anisotropy. A small ferromagnetic component is found to be perpendicular to [010].

The discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni$^{1+}$ cations possess nominally the same 3$d^9$ configuration as Cu$^{2+}$ in cuprates, the electronic structure variances remain elusive. Here, we present a soft x-ray photoemission spectroscopy study on parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison to density functional theory + U (on-site Coulomb repulsion energy) calculations, we estimate U ~5 eV, smaller than the charge transfer energy $\Delta$ ~8 eV, confirming the Mott-Hubbard electronic structure in contrast to charge-transfer cuprates. Near the Fermi level ($E_F$), we observe a signature of occupied rare-earth states in the parent compound, which is consistent with a self-doping picture. Our results demonstrate a correlation between the superconducting transition temperature and the oxygen 2$p$ hybridization near $E_F$ when comparing hole-doped nickelates and cuprates., Comment: 28 pages, 10 figures

In recent years, it has been found that the flexibility of structure and diversity of the components endow quite an amount of the organic-inorganic hybrid perovskites with novel properties, i.e., structural phase transitions. Considering the natural advantage of the perovskite-type structure in generation of stimuli-responsive or smart materials, we synthesized an organic-inorganic hybrid rare-earth double perovskite-type compound, (DMP)2LaRb(NO3)6 (DMP = N,N-dimethylpyrrolidinium cation, 1). It shows reversible phase transition at 219/209 K (heating/cooling). Variable-temperature single-crystal structure analysis and dielectric constant measurements reveal that the thermal vibrations of the polar cation guests and the distortion of the anionic cage-like framework are the origin of the phase transition. Meanwhile, the movement of polar cation in crystal lattices arouses dielectric transition between the low- and high-dielectric states, resulting in a switchable property of dielectric constant. The results reveal that the rare-earth double perovskite provides a promising platform for achieving switchable physical/chemical properties.

We study the effects of electrostatic gating on the lateral distribution of charge carriers in two dimensional devices, in a non-linear dielectric environment. We compute the charge distribution using the Thomas-Fermi approximation to model the electrostatics of the system. The electric field lines generated by the gate are focused at the edges of the device, causing an increased depletion near the edges, compared to the center of the device. This effect strongly depends on the dimensions of the device, and the non-linear dielectric constant of the substrate. We experimentally demonstrate this effect using scanning superconducting interference device (SQUID) microscopy images of current distributions in gated LaAlO$_3$/SrTiO$_3$ heterostructures., Comment: 6 pages, 4 figures

The application of gate voltages in heavy metal/ferromagnet/Oxide multilayer stacks has been identified as one possible candidate to manipulate their anisotropy at will. However, this method has proven to show a wide variety of behaviours in terms of reversibility, depending on the nature of the metal/oxide interface and its degree of oxidation. In order to shed light on the microscopic mechanism governing the complex magneto-ionic behaviour in $\text{Ta/CoFeB/}\text{HfO}_2$, we perform ab-initio simulations on various setups comprising $\text{Fe/O, Fe/HfO}_2$ interfaces with different oxygen atom interfacial geometries. After the determination of the more stable interfacial configurations, we calculate the magnetic anisotropy energy on the different unit cell configurations and formulate a possible mechanism that well describes the recent experimental observations in $\text{Ta/CoFeB/}\text{HfO}_2$., Comment: 8 pages, 5 figures

In this letter, an asymmetric spin wave scattering behaviors caused by vortex chirality are investigated in cross-shaped ferromagnetic system by using the micromagnetic simulations. In the system, four scattering behaviors are found, 1) asymmetric skew scattering, depending on the polarity of vortex core, 2) back scattering (reflection), depending on the vortex core stiffness, 3) side deflection scattering, depending on structural symmetry of the vortex circulation, and 4) geometrical scattering, depending on waveguide structure. The first and second scattering behaviors are attributed to nonlinear topological magnon spin Hall effect related to magnon spin-transfer torque effect, which has value for magnonic exploration and application.

Spin-orbit torques (SOTs) have opened a novel way to manipulate the magnetization using in-plane current, with a great potential for the development of fast and low power information technologies. It has been recently shown that two-dimensional electron gases (2DEGs) appearing at oxide interfaces provide a highly efficient spin-to-charge current interconversion. The ability to manipulate 2DEGs using gate voltages could offer a degree of freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for spin-orbitronics, in which the sign and amplitude of SOTs at a given current are fixed by the stack structure. Here, we report the non-volatile electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We demonstrate that the 2DEG is controlled using a back-gate electric-field, providing two remanent and switchable states, with a large resistance contrast of 1064%. The SOTs can then be controlled electrically in a non-volatile way, both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO heterostructures with large perpendicular magnetization further validates the compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT oscillators, skyrmion and domain-wall-based devices, and magnonic circuits., Nature Communications, 14 (1), ISSN:2041-1723

Dynamic manipulation of magnetism in topological materials is demonstrated here via a Floquet engineering approach using circularly polarized light. Increasing the strength of the laser field, besides the expected topological phase transition, the magnetically doped topological insulator thin film also undergoes a magnetic phase transition from ferromagnetism to paramagnetism, whose critical behavior strongly depends on the quantum quenching. In sharp contrast to the equilibrium case, the non-equilibrium Curie temperatures vary for different time scale and experimental setup, not all relying on change of topology. Our discoveries deepen the understanding of the relationship between topology and magnetism in the non-equilibrium regime and extend optoelectronic device applications to topological materials., Comment: 4 figures

Singlet fission may boost photovoltaic efficiency [by transforming a singlet exciton into two triplet excitons and thereby doubling the number of excited charge carriers. The primary step of singlet fission is the ultrafast creation of the correlated triplet pair. While several mechanisms have been proposed to explain this step, none has emerged as a consensus. The challenge lies in tracking the transient excitonic states. Here we use time- and angle-resolved photoemission spectroscopy to observe the primary step of singlet fission in crystalline pentacene. Our results suggest a charge-transfer mediated mechanism with a hybridization of Frenkel and charge-transfer states in the lowest bright singlet exciton. We gained intimate knowledge about the localization and the orbital character of the exciton wave functions recorded in momentum maps. This allowed us to directly compare the localization of singlet and bitriplet excitons and decompose energetically overlapping states based on their orbital character. Orbital- and localization- resolved many-body dynamics promise deep insights into the mechanics governing molecular systems and topological materials., 24 pages, 4 main figures, 9 supplementary figures

Bose-Einstein condensation of electron-hole pairs, exciton condensation, has been effortfully investigated since predicted 60 years ago. Irrefutable evidence has still been lacking due to experimental difficulties in verifying the condensation of the charge neutral and non-magnetic spin-singlet excitons. Whilst, condensation of spin-triplet excitons is a promising frontier because spin supercurrent and spin-Seebeck effects will be observable. A canonical cobaltite LaCoO$_{3}$ under very high magnetic fields is a propitious candidate, yet to be verified. Here, we unveil the exotic phase diagram of LaCoO$_{3}$ up to 600 T generated using the electromagnetic flux compression method and the state-of-the-art magnetostriction gauge. We found the continuous magnetostriction curves and a bending structure, which suggest the emergence of two distinct spin-triplet exciton condensates. By constructing a phenomenological model, we showed that quantum fluctuations of excitons are crucial for the field-induced successive transitions. The spin-triplet exciton condensation in a cobaltite, which is three-dimensional and thermally equilibrated, opens up a novel venue for spintronics technologies with spin-supercurrent such as a spin Josephson junction., 6 pages, 4 figures plus supplementary material

As a fundamental physical quantity of thermal phonons, temporal coherence participates in a broad range of thermal and phononic processes, while a clear methodology for the measurement of phonon coherence is still lacking. In this Lettter, we derive a theoretical model for the experimental exploration of phonon coherence based on spectroscopy, which is then validated by comparison with Brillouin light scattering data and direct molecular dynamic simulations of confined modes in nanostructures. The proposed model highlights that confined modes exhibit a pronounced wavelike behavior characterized by a higher ratio of coherence time to lifetime. The dependence of phonon coherence on system size is also demonstrated from spectroscopy data. The proposed theory allows for reassessing data of conventional spectroscopy to yield coherence times, which are essential for the understanding and the estimation of phonon characteristics and heat transport in solids in general., Comment: 4 pages, 3 figures

The past decade's discovery of topological excitations in nanoscale ferroelectrics has turned the prevailing view that the polar ground state in these materials is uniform. However, the systematic understanding of the topological polar structures in ferroelectrics is still on track. Here we study stable vortex-like textures of polarization in the nanocylinders of ferroelectric PbTiO$_3$, arising due to the competition of the elastic and electrostatic interactions. Using the phase-field numerical modeling and analytical calculations, we show that the orientation of the vortex core with respect to the cylinder axis is tuned by the geometrical parameters and temperature of the system., Comment: 21 pages, 9 figures

The effect of spin fluctuations on the $\alpha$ (bcc) - $\gamma$ (fcc) - $\delta$ (bcc) structural phase transitions in iron is investigated with a tight-binding (TB) model. The orthogonal $d$-valent TB model is combined with thermodynamic integration, spin-space averaging and Hamiltonian Monte Carlo to compute the temperature-dependent free-energy difference between bcc and fcc iron. We demonstrate that the TB model captures experimentally observed phonon spectra of bcc iron at elevated temperatures. Our calculations show that spin fluctuations are crucial for both, the $\alpha$ - $\gamma$ and the $\gamma$ - $\delta$ phase transitions but they enter through different mechanisms. Spin fluctuations impact the $\alpha$ - $\gamma$ phase transition mainly via the magnetic/electronic free-energy difference between bcc and fcc iron. The $\gamma$ - $\delta$ phase transition, in contrast, is influenced by spin fluctuations only indirectly via the spin-lattice coupling. Combining the two mechanisms, we obtain both, the $\alpha$ - $\gamma$ and the $\gamma$ - $\delta$ phase transitions with our TB model. The calculated transition temperatures are in very good agreement with experimental values.