Critical External Field Research Articles

On the magnetization of an antiferromagnetic film with uniaxial magnetocrystalline anisotropy

In this work finite element micromagnetic simulations were performed in order to study the magnetic behavior of an antiferromagnetic film with uniaxial magnetocrystalline anisotropy under the influence of external magnetic fields. The stability of the antiferromagnetic phase and metamagnetic phases arising from it, as well as the features and the evolution of the magnetic microstructure are studied in three different combinations of the easy axis and applied field directions. The investigation was carried out in a wide range of geometrical and intrinsic film’s parameters in order to derive analytical relations regarding the critical external fields.

Magnetization Process in Bilayer Honeycomb Spin Lattice

The magnetization process of the AA-stacked bilayer honeycomb spin lattice in the out-of-plane applied field is examined in the framework of the Ising model and mean field approximation. Competition between different kinds (antiferromagnetic: AF, or ferromagnetic: FM) of intra-layer and inter-layer spin exchange couplings could lead to the first order magnetization process, characterized by sharp jumps in magnetization curves occurring in critical external fields (spin-flop and spin- flip fields). There is only the spin-flip field at very low temperature and its magnitude depends not only on the intra-layer frustration level but also exchange coupling between spin layers. During the magnetization process, the initial AF bilayer honeycomb spin lattice may undergo ferrimagnetic or week-ferromagnetic states before reaching full ferromagnetic saturation state by the spin-flip field. The results of this work can be applied for an explanation of the spin-flip phenomenon registered in the AF bilayer CrI3.

The frequency of current fluctuations in two-valley semiconductors in an external electric and strong magnetic (

Boltzmann's kinetic equations have not been used to date to study nonequilibrium phenomena in semiconductors, and therefore, to obtain analytical expressions for the oscillation frequency inside the semiconductor and the critical external electric field, it is of theoretical interest. In this theoretical work, the frequency of oscillations occurring inside a two-valley semiconductor of the GaAs type in an external constant electric field and in an external strong magnetic field (

Ferroelectric phase-transition frustration near a tricritical composition point

Phase transition describes a mutational behavior of matter states at a critical transition temperature or external field. Despite the phase-transition orders are well sorted by classic thermodynamic theory, ambiguous situations interposed between the first- and second-order transitions were exposed one after another. Here, we report discovery of phase-transition frustration near a tricritical composition point in ferroelectric Pb(Zr1-xTix)O3. Our multi-scale transmission electron microscopy characterization reveals a number of geometrically frustrated microstructure features such as self-assembled hierarchical domain structure, degeneracy of mesoscale domain tetragonality and decoupled polarization-strain relationship. Associated with deviation from the classic mean-field theory, dielectric critical exponent anomalies and temperature dependent birefringence data unveil that the frustrated transition order stems from intricate competition of short-range polar orders and their decoupling to long-range lattice deformation. With supports from effective Hamiltonian Monte Carlo simulations, our findings point out a potentially universal mechanism to comprehend the abnormal critical phenomena occurring in phase-transition materials.

Glauber dynamics for Ising models on random regular graphs: cut-off and metastability

Consider random $d$-regular graphs, i.e., random graphs such that there are exactly $d$ edges from each vertex for some $d\ge 3$. We study both the configuration model version of this graph, which has occasional multi-edges and self-loops, as well as the simple version of it, which is a $d$-regular graph chosen uniformly at random from the collection of all $d$-regular graphs. In this paper, we discuss mixing times of Glauber dynamics for the Ising model with an external magnetic field on a random $d$-regular graph, both in the quenched as well as the annealed settings. Let $\beta$ be the inverse temperature, $\beta_c$ be the critical temperature and $B$ be the external magnetic field. Concerning the annealed measure, we show that for $\beta > \beta_c$ there exists $\hat{B}_c(\beta)\in (0,\infty)$ such that the model is metastable (i.e., the mixing time is exponential in the graph size $n$) when $\beta> \beta_c$ and $0 \leq B \beta_c$ and $B>\hat{B}_c(\beta)$. Interestingly, $\hat{B}_c(\beta)$ coincides with the critical external field of the Ising model on the $d$-ary tree (namely, above which the model has a unique Gibbs measure). Concerning the quenched measure, we show that there exists $B_c(\beta)$ with $B_c(\beta) \leq \hat{B}_c(\beta)$ such that for $\beta> \beta_c$, the mixing time is at least exponential along some subsequence $(n_k)_{k\geq 1}$ when $0 \leq B \hat{B}_c(\beta)$. The quenched results also hold for the model conditioned on simplicity, for the annealed results this is unclear.

Heteroborospherene nanoclusters as high-performance nonlinear optical materials

Today, the design of new compounds with giant nonlinear optical responses is attracted to many researchers. Inspired by an interesting finding of a new class of heteroborospherenes which were formed by doping four carbon atoms in the B364- nanocluster (C4B32), we suggest the alkali metal-doped C4B32 (M@C4B32, M=Li, Na, and K) nanoclusters as high-performance nonlinear optical materials. Our results show that the alkali metal atoms have a considerable effect on the structural and electronic properties of the C4B32 nanocluster. We found that the doping alkali metal can remarkably decrease the HOMO-LUMO gap and significantly increases the first hyperpolarizability of the C4B32 nanocluster. Also, our results reveal that the first hyperpolarizability of the M@C4B32 nanoclusters can be progressively enhanced by increasing the atomic number of alkali metals. The effect of external electric fields on the nonlinear optical responses of the M@C4B32 has been systematically explored. We found that the first hyperpolarizability of the M@C4B32 compounds can be gradually increased by increasing the imposed external electric field from zero to the critical external electric field along the charge transfer direction (M→C4B32). Accordingly, this work presents an efficient strategy to improve the nonlinear optical responses of the heteroborospherenes.

Anisotropic avalanches and critical depinning of three-dimensional magnetic domain walls.

Simulations with more than 10^{12} spins are used to study the motion of a domain wall driven through a three-dimensional random-field Ising magnet (RFIM) by an external field H. The interface advances in a series of avalanches whose size diverges at a critical external field H_{c}. Finite-size scaling is applied to determine critical exponents and test scaling relations. Growth is intrinsically anisotropic with the height of an avalanche normal to the interface ℓ_{⊥} scaling as the width along the interface ℓ_{∥} to a power χ=0.85±0.01. The total interface roughness is consistent with self-affine scaling with a roughness exponent ζ≈χ that is much larger than values found previously for the RFIM and related models that explicitly break orientational symmetry by requiring the interface to be single-valued. Because the RFIM maintains orientational symmetry, the interface develops overhangs that may surround unfavorable regions to create uninvaded bubbles. Overhangs complicate measures of the roughness exponent but decrease in importance with increasing system size.

Field-tuned spin excitation spectrum of kπ skyrmion

We study spin wave excitation modes of kπ skyrmion (k = 1, 2, 3) in a magnetic nanodot under an external magnetic field along the z direction using micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation. We find that a transition of kπ skyrmion to other skyrmion-like structures appears under some critical external fields, the corresponding spin wave excitations are simulated for each state under magnetic field. For skyrmion, the frequencies of excitation modes increases and then decreases with the low frequency mode splitting at a critical magnetic field. In addition to the well-known two in-plane rotation modes and an out-of-plane breathing mode of skyrmion, more excitation modes are found with a higher k (k = 2, 3). The excitation modes vary as a function of magnetic field, and the excitation frequencies for different modes exhibit a rapid or slight change depending on the field-induced change of magnetization profile. Our study indicates the rich spin wave excitations for kπ skyrmion and opens up the possibility for theoretical or experimental investigation of magnonics application.

Generalized spin-orbit torques in two-dimensional ferromagnets with spin-orbit coupling

Based on the spinor Boltzmann equation, we obtain the continuity equations of charge and spin accumulation in two-dimensional ferromagnets with Rashba or Dresselhaus spin-orbit coupling, in which the spin diffusion equation (the continuity equation of spin accumulation) gives the spin-orbit torques that have an extra term of charge density gradient that is paid little attention in previous studies. Numerical results indicate this term plays an important role in inhomogeneous systems. In order to study the magnetization dynamics of ferromagnets, we solve the Landau-Lifshitz-Gilbert equation together with the spin diffusion equation. Our results indicate that the magnetization in systems with Rashba SOI is easier to be switched by an external electric field than the system with Dresselhaus SOI, and for anisotropic ferromagnets, we study the critical external electric field that manipulate the reversal of magnetization.

Electric field-induced structural transition of domain walls in nanoconfined nematic liquid crystal systems

ABSTRACTThe electric field-induced structural transition of domain walls in nanoconfined nematic liquid crystal systems was investigated on the basis of Landau–de Gennes theory. Two models of nanoconfined domain wall systems were established as splay–bend and twist wall systems under Fréedericksz transitions with two different rotation directions under the effect of electric field E/E0. Results showed that two structural transition processes occur in both models. Pincement transition occurs under a critical external field Ec1/E0. In pincement transition, walls change into two squeezed symmetric surface defects with opposite charges. Surface defects spread along the direction of the substrates and attain surface order reconstruction states as E/E0 is enhanced to Ec2/E0. The increment in cell gap d from the nanoscale to the microscale will not affect Ec2/E0 in both models.

Design of a Novel Series of Donor-Acceptor Frameworks via Superalkali-Superhalogen Assemblage to Improve the Nonlinear Optical Responses.

Presently, many researches are directed toward the design of novel superatoms with high nonlinear optical responses. Inspired by a fascinating finding of superatoms which were designed by bonding superhalogen (Al13 nanocluster) with superalkalis (M3O, M = Na and K), we suggest an effective strategy to form a series of typical donor-acceptor frameworks with high nonlinear optical responses via bonding the superalkalis M3O (Li3O, Na3O, K3O, Li2NaO, Li2KO, Na2LiO, Na2KO, K2LiO, K2NaO, and LiNaKO) with low ionization potential to the superhalogen Al13 with large electron affinity. The ionization potential, electronic spatial extent, electric field gradient tensors of 17O nuclei, and natural bond orbital charge values of the superalkalis M3O were also calculated. We found that the M ligands have the remarkable effect on the ionization potential as well as 17O nuclear quadrupole resonance parameters of the superalkalis M3O. Our results also represented that the bonding superalkalis can efficiently narrow wide HOMO-LUMO gap and considerably enhance first hyperpolarizability of the pristine Al13, due to electron transfer in this type of superatom. Also, the effect of oriented external electric fields on the nonlinear optical responses of the superatoms M3O-Al13 has been systematically explored. We found that the first hyperpolarizability of the superatom compounds can be gradually increased by increasing the imposed oriented external electric field from zero to the critical external electric field along the charge transfer direction (M3O → Al13). In this respect, this work reveals an effective approach to gradually enhance the nonlinear optical responses of the superatoms through applying oriented external electric fields.

Superatom Compounds under Oriented External Electric Fields: Simultaneously Enhanced Bond Energies and Nonlinear Optical Responses

It has recently been demonstrated that oriented external electric fields (OEEFs) can serve as future smart reagents and effectors in chemistry. Consequently, the effect of OEEFs on the geometric structures, electronic properties, bonding properties, and NLO responses of three typical superatom compounds, i.e., (NLi4)(BF4) and (BLi6)X (X = BeF3 and BF4) has been systemically investigated by ab initio computations in this work. The computational results reveal that the (NLi4)(BF4) possesses almost the same performance as the traditional alkali metal halide under the same OEEFs. It is found that the vertical ionization potential, bond energy, and first hyperpolarizability of (NLi4)(BF4) can be gradually enhanced by increasing the imposed OEEF from zero to the critical external electric field (Fc) along the charge transfer direction (NLi4 → BF4). In particular, the first hyperpolarizability of (NLi4)(BF4) is greatly enlarged from 2.84 × 103 to 1.36 × 107 au (4790-fold) by increasing OEEF from 0 to 121 × 10–4 ...

Role of surface-bound hole states in electric-field-driven superconductivity at the (110)-surface of diamond

Hole states in electric-field-driven superconductivity at the (110)-surface of diamond are examined by means of first-principles calculations and one-dimensional tight-binding model calculations. It is found that surface-bound hole states confined near the surface by application of an electric field $E$ play a key role in superconductivity. Indeed, there is a critical external electric field $|{E}_{\mathrm{c}}|$ ($\ensuremath{\simeq}0.4$ V/\AA{}) for observing the superconductivity, which can be attributed to the second surface-bound hole state. With McMillan's formula and calculated phonon-electron coupling constants, we demonstrate that, in electric fields $l|{E}_{\mathrm{c}}|$ which correspond to a surface carrier density of $\ensuremath{\sim}2.3\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, superconductivity may not be practically observed while the superconductivity transition temperature suddenly increases at $|{E}_{\mathrm{c}}|$.

Stability of the phase transition of critical-field Ising model on Cayley trees under inhomogeneous external fields

We consider the ferromagnetic Ising model with spatially dependent external fields on a Cayley tree, and we investigate the conditions for the existence of the phase transition for a class of external fields, asymptotically approaching a homogeneous critical external field. Our results extend earlier results by Rozikov and Ganikhodjaev.

Electric field-driven acid-base transformation: proton transfer from acid(HBr/HF) to base(NH3/H2O)

The proton-transfer between ammonia/water and HF/HBr without and with the stimulus of external electric fields(Eext) was investigated with the ab initio calculations. When external electric field is applied, the proton transfer occurs, resulting in ion-paired H4N+X- and H3O+X-(X=Br and F) from hydrogen-bonded complexes in view of the great changes of geometrical structures, dipole moments, frontier molecular orbitals and potential energy surfaces in the critical external electric fields(Ec) of 1.131×107 V/cm for H3N-HBr, 1.378×108 V/cm for H3N-HF, 9.358×107 V/cm for H2O-HBr and 2.304×108 V/cm for H2O-HF, respectively. Furthermore, one or three excess electrons can trigger the proton transfer from H3N-HBr and H3N-HF to H4N+Br- and H4N+F-, while two and four excess electrons can induce the proton transfer from H2O-HBr and H2O-HF to H3O+Br- and H3O+F-, respectively. Compared with that of the analogous NH3/H2O-HCl systems, the strength of Ec of proton transfer increases from HBr to HCl and HF for either H3N-HX or H2O-HX series, which is understandable by the fact that the acidity sequence is HBr>HCl>HF. And the larger of acidity of conjugated acid, the smaller of needed Ec. On the other hand, the Ec for the systems of NH3 with a stronger basicity is generally smaller than that of H2O systems for the same conjugated acid.

Scaling Relations for Two-Dimensional Ising Percolation

We consider the percolation problem in the high-temperature Ising model on the two-dimensional square lattice at or near critical external fields. We show that all scaling relations, except for a single hyperscaling relation, hold under the power law assumptions for the one-arm path and the four-arm paths.

Amplification of hippersound in graphene under external direct current electric field

We present a theoretical investigation on the hippersound generation and amplification in n-type graphene in the presence of a dc driving electric field. By investigating the consequences of electron-acoustic phonon interaction in a gated graphene, we have found the conditions for phonon amplification to be obtained, namely vD>ṽs,vD being the drift velocity of electrons in the dc field and and ṽs being the effective sound velocity in the graphene sheet and Rq→>Γq→, where Rq→ is the phonon generation rate and Γq→ is the phonon linear losses. By considering two extreme values of the electron mobility in graphene, namely μ = 20 000 cm2/Vs and 1.2 V/cm for μ = 190 000 cm2/Vs, the critical external electric fields for hippersound amplification were obtained as much as 10 V/cm 1.2 V/cm, respectively. These threshold dc fields are smaller when compared with the corresponding applied fields for phonon amplification in semiconducting bulk and nanostructures. The study is relevant to the application of graphene as acoustoelectric devices as well as in the construction of a high-frequency phonon spectrometer.

Edge saturation effects on the magnetism and band gaps in multilayer graphene ribbons and flakes

Using a density functional theory based electronic structure method and semi-local density approximation, we study the interplay of geometric confinement, magnetism and external electric fields on the electronic structure and the resulting band gaps of multilayer graphene ribbons whose edges are saturated with molecular hydrogen (H$_2$) or hydroxyl (OH) groups. We discuss the similarities and differences of computed features in comparison with the atomic hydrogen (or H-) saturated ribbons and flakes. For H$_2$ edge-saturation, we find \emph{shifted} labeling of three armchair ribbon classes and magnetic to non-magnetic transition in narrow zigzag ribbons whose critical width changes with the number of layers. Other computed characteristics, such as the existence of a critical gap and external electric field behavior, layer dependent electronic structure, stacking-dependent band gap induction and the length confinement effects remain qualitatively same with those of H-saturated ribbons.

Streamer‐ and leader‐like processes in the upper atmosphere: Models of red sprites and blue jets

Models of red sprites and blue jets based on the concept of the streamer‐leader mechanism of the air breakdown are presented. Red sprites are interpreted as the downward streamers of varying cross sections launched from plasma patches in the lower ionosphere, which are caused by the electric field generated by the lightning discharge. Results of this streamer model are in good agreement with the observations of sprites. Blue jets are interpreted as produced by a bileader, whose top part is seen on photos of blue jets as a “trunk of a tree.” The upward leader is capped by its streamer zone seen as the tall and narrow “branches” of “the tree.” It is shown that the long upward streamers of blue jets can be sustained by the moderate cloud charge due to transfer of the high potential of the thundercloud edge by the upward leader tip. It is revealed that the critical external field ES required for an unlimited streamer growth satisfies the similarity law ES/N ∼ constant for a wide range of the molecular number density N. In addition, the streamer length is estimated along with the height from which the streamers can reach the ionosphere. The main unresolved problems in the field are discussed.

New examples of ternary rare-earth metal boride carbides containing finite boron–carbon chains: The crystal and electronic structure of RE15B 6C 20 ( RE=Pr, Nd)

The ternary rare-earth metal boride carbides RE 15B 6C 20 ( RE=Pr, Nd) were synthesized by co-melting the elements. They exist above 1270 K. Their crystal structures were determined from single-crystal X-ray diffraction data. Both crystallize in the space group P1¯, Z=1, a=8.3431(8) Å, b=9.2492(9) Å, c=8.3581(8) Å, α=84.72(1)°, β=89.68(1)°, γ =84.23(1)° ( R1=0.041 (w R2=0.10) for 3291 reflections with I o>2 σ( I o)) for Pr 15B 6C 20, and a=8.284(1) Å, b=9.228(1) Å, c=8.309(1) Å, α=84.74(1)°, β=89.68(1)°, γ=84.17(2)° ( R1=0.033 (w R2=0.049) for 2970 reflections with I o>2 σ( I o)) for Nd 15B 6C 20. Their structure consists of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with unprecedented B 2C 4 finite chains, disordered C 3 entities and isolated carbon atoms, respectively. Structural and theoretical analyses suggest the ionic formulation ( RE 3+) 15([B 2C 4] 6−) 3([C 3] 4−) 2(C 4−) 2·11ē. Accordingly, density functional theory calculations indicate that the compounds are metallic. Both structural arguments as well as energy calculations on different boron vs. carbon distributions in the B 2C 4 chains support the presence of a CBCCBC unit. Pr 15B 6C 18 exhibits antiferromagnetic order at T N=7.9 K, followed by a meta-magnetic transition above a critical external field B>0.03 T. On the other hand, Nd 15B 6C 18 is a ferromagnet below T C≈40 K.