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The operation of fundamental forces in quantum field theory is explicated here as the exchange of particles, consistently with the standard methodology of particle physics. The particles involved are seen to bear little relation to any classical particle but, rather, comprise unified collections of compresent, conserved quantities indicated by propagators. The exchange particles, which supervene upon quantum fields, are neither more fundamental than fields nor replace them as has often previously been assumed in models of exchange forces. It is argued that this explication of quantum field-theoretic exchange forces definitively improves upon its predecessors.

At present there is a renewed interest in theories of ”modified” gravity. Here, under a more drastic modification enforced by Galilei group, we obtain a completely new gravitational structure, and which exists in addition to the already available general relativity of today. Correlated with this, we show that in addition, there is a new ”modified” quantum mechanics, in as much as it exists as an independent and new ”pure” non-relativistic quantum me- chanics, and which has no relativistic counterpart. This is in addition to the present quantum mechanics, where the relativistic and non-relativistic structures are counterparts of each other. The above holds, firstly due to the correlation between Galilei group and quantum mechanics. These math- ematical conclusions are consolidated by the fact that there exists a physical Majorana interaction between each neutron- proton pairs in nuclei. Galilei invariance of Majorana exchange in Majorana interaction, shows that the mass here is of pure gravitational nature, and which is immune to the other three forces. This makes an amazing connection between the gravitational force and the quantum mechanics. This pure gravitational mass would man- ifest itself as dark matter of the universe. It is our new modified gravity that generates the dark matter.

Cubic double perovskites that host heavy ions with total angular momentum $J$ = 2 can exhibit a singular magnetic state epitomized by a lone octupole and bulk ferro-type magnetism. It exists in the Chen-Balents Hamiltonian with a quadrupole interaction and competing exchange forces between the ions. Our symmetry inspired analysis mirrors the Dzyaloshinskii-Man'ko theory of latent antiferromagnetic ordering, and a 3k collinear structure. Experimental tests for the singular state include neutron or resonant x-ray Bragg diffraction.

This work predicts that individual Bloch points can be created and stabilized by magnetostatic and chiral interactions in nanocuboids, confined in between two chiral bobbers of opposing polarity. The Bloch point can be moved by an external magnetic field of moderate strength but only if the field strength is enough to overcome a pinning potential that results from intrinsic exchange forces and extrinsic surface effects. The Bloch point can be driven by the external field reversibly, in a direction opposing the field, and it remains stable up to moderate field strengths. At a critical field strength the Bloch point escapes through one of the surfaces, leaving behind a collinear magnetization configuration, and upon removing the field a new Bloch point is formed. These findings highlight the topological diversity in nanostructures and show that a Bloch point, despite its zero-dimensionality, couples to external fields via a substantial magnetic volume around it. The control of topological point defects has technological implications with regards to reversibly movable nanomagnetic textures and their associated emergent electrodynamics., Comment: 7 pages, 6 figures

The exchange of two neutrinos at one loop leads to a long-range parity-violating force between fermions. We explore the two-neutrino force in the backdrop of atomic physics. We point out that this is the largest parity-violating long-range force in the Standard Model and calculate the effect of this force in experiments that probe atomic parity violation by measuring optical rotation of light as it passes through a sample of vaporized atoms. We perform explicit calculations for the hydrogen atom to demonstrate this effect. Although we find that the effect is too small to be observed in hydrogen in the foreseeable future, our approach may be applied to other setups where long-range parity violation is large enough to be probed experimentally., 34 pages, 9 figures. Content in Section IIID updated

Generation of bispecific antibodies (bsAbs) requires a combination of compatible binders in formats that support desired functionalities. Here, we report that bsAb-matrices can be generated by Format Chain Exchange (FORCE), enabling screening of combinatorial binder/format spaces. Input molecules for generation of bi/multi-valent bsAbs are monospecific entities similar to knob-into-hole half-antibodies, yet with complementary CH3-interface-modulated and affinity-tagged dummy-chains. These contain mutations that lead to limited interface repulsions without compromising expression or biophysical properties of educts. Mild reduction of combinations of educts triggers spontaneous chain-exchange reactions driven by partially flawed CH3-educt interfaces resolving to perfect complementarity. This generates large bsAb matrices harboring different binders in multiple formats. Benign biophysical properties and good expression yields of educts, combined with simplicity of purification enables process automation. Examples that demonstrate the relevance of screening binder/format combinations are provided as a matrix of bsAbs that simultaneously bind Her1/Her2 and DR5 without encountering binder or format-inflicted interferences., Bispecific antibodies have been generated in many different formats and it is becoming clear that rational design alone cannot create optimal functionalities. Here the authors introduce the high throughput methodology, Format Chain Exchange (FORCE), to enable combinatorial generation of bispecific antibodies.

In this work, we have synthesized nickel oxide (NiO) nanoparticles using a simple wet chemical method. The synthesized nanoparticles were annealed at different temperatures ranging from 573 to 973 K. The structural, magnetic, and optical properties were characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM), Superconducting Quantum Interference Device (SQUID), UV–VIS and Photoluminescence (PL) spectrophotometer. The optical properties depict that as NiO particles grow bigger, the intensity of the UV emission increases. The broad emission band can be attributed to the direct recombination of photogenerated charge carriers from the conduction band with the holes in the valence band. This kind of emission due to the transition of 3d8 electrons of the Ni2+ ions was also observed in the PL spectra. NiO nanoparticles exhibit antiferromagnetic behaviour with varied magnetic moment values of 3.08–5.72 μB. The ZFC curves of all samples show cusp and sharp decrease in magnetization at temperatures in the region of 10–21 K. This is due to the surface cluster spins whose thermal fluctuations freeze in a cluster-glass-like state. The bifurcation temperature between ZFC-FC curves increases with respect to the particle size due to the exchange field which is related to the overlap of electron wave functions of two magnetic ions (Ni2+). This field acts indirectly through the neighbouring anions O2−, as Ni2+ ions are too far apart for direct exchange forces mechanism called super exchange and originates from Pauli exclusion principle and electrostatic force.

Propylene oxide is one of the simplest organic chiral molecules and has attracted considerable interest from the scientific community a few years ago, when it was discovered in the interstellar medium. Here, we report a preliminary study on the interaction between propylene oxide and rare-gas atoms, specifically He, Ne, and Ar. The interaction potentials as a function of the distance between the center-of-mass of propylene oxide and the rare-gas-atom are calculated for fourteen leading configurations at CCSD(T)/aug-cc-pVDZ level of theory. Symmetry Adapted Perturbation Theory has been employed for the analysis of the intermolecular potential, revealing that most of the contribution is given by dispersion and exchange forces.

The exchange interaction is investigated theoretically for electrons confined to a 2-D sample placed in a linearly varying magnetic field perpendicular to the plane. Unusual and interesting behavior is predicted: starting from zero, as one adds electrons to the system, its size will increase continuously but will then collapse. However this collapse will be reversed as the number crosses another critical value, which we estimate here. For electron parameters typical for 2DEG's, the instability could be observable at sufficiently low electron densities. A Hartree Fock equation is derived and investigated. We also show that in an appropriate asymptotic limit an approximate local potential can be derived. One key lesson is that the exchange interaction is large and cannot be reasonably excluded from any valid theoretical investigation., Comment: 9 pages, 3 figures. Revised for greater clarity

The large interest in chiral magnetic structures for realization of nanoscale magnetic storage or logic devices has necessitated methods which can quantify magnetic interactions at the atomic scale. To overcome the limitations of the typically used current-based sensing of atomic-scale exchange interactions, a force-based detection scheme is highly advantageous. Here, we quantify the atomic-scale exchange force field between a ferromagnetic tip and a cycloidal spin spiral using our developed combination of current and exchange force detection. Compared to the surprisingly weak spin polarization, the exchange force field is more sensitive to atomic-scale variations in the magnetization. First-principles calculations reveal that the measured atomic-scale variations in the exchange force originate from different contributions of direct and indirect (Zener type) exchange mechanisms, depending on the chemical tip termination. Our work opens the perspective of quantifying different exchange mechanisms of chiral magnetic structures with atomic-scale precision using 3D magnetic exchange force field measurements., Atomic-scale spin order is commonly studied using spin polarized scanning tunneling microscopy. Here, the authors report on a combination of current and exchange force detection together with first-principles calculations demonstrating increased resolution and sensing different exchange mechanisms.

Exchange forces on the movable dot ("shuttle") in a magnetic shuttle device depend on the parity of the number of shuttling electrons. The performance of such a device can therefore be tuned by changing the strength $U$ of Coulomb correlations to block or unblock parity fluctuations. We show that by increasing $U$ the spintro-mechanics of the device crosses over, at $U=U_c(T)$, from a mechanically stable regime to a regime of spin-induced shuttle instabilities. This is due to enhanced spin-dependent mechanical forces as parity fluctuations are reduced by a Coulomb blockade of tunneling and demonstrates that single-electron manipulation of single-spin controlled nano-mechanics is possible., 5 pages, 2 figures and a supplementary information file

On the basis of the shell model with an extended basis, the structure of 9Li-9Be to 11Li-11Be nuclei is examined with allowance for the competition of jj coupling and Majorana exchange forces via considering the sequential addition of neutrons, and the respective wave functions are determined. A formalism for calculating the spectroscopic factor for a dineutron and for individual neutrons in nuclei whose wave functions incorporate the mixing of shell configurations is developed. The reactions 9Li(t, p)11Li and 9Be(t, p)11Be treated with allowance for the mechanisms of dineutron stripping and a sequential transfer of two neutrons are considered as an indicator of the proposed structure of lithium and berylliumisotopes. The parameters of the optical potentials, the wave functions for the bound states of transferred particles, and the interaction potentials corresponding to them are determined from a comparison of the theoretical angular distribution of protons from the reaction 9Be(t, p)11Be with its experimental counterpart. It is shown that a dineutron periphery of size about 6.4 fm is present in the 11Li nucleus and that a single-neutron periphery of size about 8 fm is present in the 11Be nucleus.

For translationally invariant one-band lattice models, we exploit the ab initio knowledge of the natural orbitals to simplify reduced density matrix functional theory (RDMFT). Striking underlying features are discovered: First, within each symmetry sector, the interaction functional $\mathcal{F}$ depends only on the natural occupation numbers $\bf{n}$. The respective sets $\mathcal{P}^1_N$ and $\mathcal{E}^1_N$ of pure and ensemble $N$-representable one-matrices coincide. Second, and most importantly, the exact functional is strongly shaped by the geometry of the polytope $\mathcal{E}^1_N \equiv \mathcal{P}^1_N $, described by linear constraints $D^{(j)}(\bf{n})\geq 0$. For smaller systems, it follows as $\mathcal{F}[\bf{n}]=\sum_{i,i'} \overline{V}_{i,i'} \sqrt{D^{(i)}(\bf{n})D^{(i')}(\bf{n})}$. This generalizes to systems of arbitrary size by replacing each $D^{(i)}$ by a linear combination of $\{D^{(j)}(\bf{n})\}$ and adding a non-analytical term involving the interaction $\hat{V}$. Third, the gradient $\mathrm{d}\mathcal{F}/\mathrm{d}\bf{n}$ is shown to diverge on the boundary $\partial\mathcal{E}^1_N$, suggesting that the fermionic exchange symmetry manifests itself within RDMFT in the form of an "exchange force". All findings hold for systems with non-fixed particle number as well and $\hat{V}$ can be any $p$-particle interaction. As an illustration, we derive the exact functional for the Hubbard square., Comment: published version

Storing and accessing information in atomic-scale magnets requires magnetic imaging techniques with single-atom resolution. Here, we show simultaneous detection of the spin-polarization and exchange force, with or without the flow of current, with a new method, which combines scanning tunneling microscopy and non-contact atomic force microscopy. To demonstrate the application of this new method, we characterize the prototypical nano-skyrmion lattice formed on a monolayer of Fe/Ir(111). We resolve the square magnetic lattice by employing magnetic exchange force microscopy, demonstrating its applicability to non-collinear magnetic structures, for the first time. Utilizing distance-dependent force and current spectroscopy, we quantify the exchange forces in comparison to the spin-polarization. For strongly spin-polarized tips, we distinguish different signs of the exchange force which we suggest arises from a change in exchange mechanisms between the probe and a skyrmion. This new approach may enable both non-perturbative readout combined with writing by current-driven reversal of atomic-scale magnets.

The composition dependence of gravitational constant $G$ is measured at the millimeter scale to test the weak equivalence principle, which may be violated at short range through new Yukawa interactions such as the dilaton exchange force. A torsion balance on a turning table with two identical tungsten targets surrounded by two different attractor materials (copper and aluminum) is used to measure gravitational torque by means of digital measurements of a position sensor. Values of the ratios $\tilde{G}_{Al-W}/\tilde{G}_{Cu-W} -1$ and $\tilde{G}_{Cu-W}/G_{N} -1$ were $(0.9 \pm 1.1_{\mathrm{sta}} \pm 4.8_{\mathrm{sys}}) \times 10^{-2}$ and $ (0.2 \pm 0.9_{\mathrm{sta}} \pm 2.1_{\mathrm{sys}}) \times 10^{-2}$ , respectively; these were obtained at a center to center separation of 1.7 cm and surface to surface separation of 4.5 mm between target and attractor, which is consistent with the universality of $G$. A weak equivalence principle (WEP) violation parameter of $\eta_{Al-Cu}(r\sim 1\: \mathrm{cm})=(0.9 \pm 1.1_{\mathrm{sta}} \pm 4.9_{\mathrm{sys}}) \times 10^{-2} $ at the shortest range of around 1 cm was also obtained., Comment: submitted to Class. Quantum Grav

In scattering theory the effect associated with the downward crossing of the phase-shift δ l ( k ) ( l being the orbital angular momentum and k the momentum) through δ l = π / 2 (mod π ) is called echo . In the standard nuclear theory (Breit–Wigner theory) the echo is described and evaluated in terms of scattering by an impenetrable sphere. However, this model holds only at sufficiently high energy, while it is inadequate at low energy. In this paper we show that the echo effect can be associated with two different regimes acting at low and high energy, respectively. At high energy the hard-sphere scattering model seems to describe appropriately the phenomenon. At low energy we propose a mechanism due to the exchange forces induced by the Pauli exclusion principle in the fermionic interaction, which leads to nonlocal potentials. These potentials admit for the scattering amplitude pole singularities in the fourth quadrant of the complex angular momentum plane. This paper analyzes the role played by this class of poles in the description of the low energy regime of echoes. A specific phenomenological analysis is performed, taking as typical example the α – α elastic scattering.

The flexible-boundary (FB) quantum mechanical/molecular mechanical (QM/MM) scheme accounts for partial charge transfer between the QM and MM subsystems. Previous calculations have demonstrated excellent performance of FB-QM/MM in geometry optimizations. This article reports an implementation to extend FB-QM/MM to molecular dynamics simulations. To prevent atoms from getting unreasonably close, which can lead to polarization catastrophe, empirical correcting functions are introduced to provide additive penalty energies for the involved atom pairs and to improve the descriptions of the repulsive exchange forces in FB-QM/MM calculations. Test calculations are carried out for chloride, lithium, sodium, and ammonium ions solvated in water. Comparisons with conventional QM/MM calculations suggest that the FB treatment provides reasonably good results for the charge distributions of the atoms in the QM subsystems and for the solvation shell structural properties, albeit smaller QM subsystems have been used in the FB-QM/MM dynamics simulations.

Density functional theory-based molecular dynamics calculations of condensed phase systems often benefit from the use of hybrid functionals. However, their use is computationally very demanding and severely limits the system size and time scale that can be simulated. Several methods have been introduced to accelerate hybrid functional molecular dynamics including Schwarz screening and the auxiliary density matrix method (ADMM). Here we present a simple screening scheme that can be applied in addition to these methods. It works by examining Hartree-Fock exchange (HFX) integrals and subsequently excluding those that contribute very little to any nuclear force component. The resultant force error is corrected by a history-dependent extrapolation scheme. We find that for systems where the calculation of HFX forces is a major bottleneck, a large fraction of the integrals can be neglected without introducing significant errors in the nuclear forces. For instance, for a 2 × 2 × 2 unit cell of CoO, 92% of the HFX integrals that have passed Schwarz screening within the ADMM approach can be neglected leading to a performance gain of a factor of 3 at a negligible error in nuclear forces (≤5 × 10

We study nano-sized spherically symmetric plasma structures which are radial nonlinear oscillations of electrons in plasma. The effective interaction of these plasmoids via quantum exchange forces between ions is described. We calculate the energy of this interaction for the case of a dense plasma. The conditions when the exchange interaction is attractive are examined and it is shown that separate plasmoids can form a single object. The application of our results to the theoretical description of stable atmospheric plasma structures is considered., Comment: 11 pages in LaTeX2e, two columns, 2 eps figures; paper was significantly revised, more realistic values for plasma parameters are used, several references were added; matches version to be published in J. Atm. Solar-Terr. Phys

An alternative exchange energy density (eed) in DFT is first proposed, depending on the optimized exchange force (OEF). It is evaluated using a numerical form of OEF for the non-relativistic Ne-like atomic ion series for atomic number Z = 92. Comparison is made with the Dirac form of eed available analytically. Related results are also presented for Ar-like atomic ions.

Quantum dynamical calculations are presented for the scattering of electron beams from gaseous carbon monosulfide in its ground electronic state (1Σ), and considered as occupying its ground rotovibrational level. The work focuses on the various symmetry components which contribute to the total integral, elastic cross sections and shows how different treatments of the short-range exchange effects are essentially producing the same partial cross section values and shapes over a very broad range of collision energies (from 1.0 to 100.0 eV) which are well above the very-low energy region. The further analysis of the very-low energy (∼0.7 eV) shape resonance of 2Π symmetry firstly confirms the quality of the exchange forces employed in the present work and additionally indicates the essential role of short-range correlation effects in locating that resonance.

Dependence of the exchange bias and coercivity as surface magnetic anisotropy and ferromagnet materials for and /FeMn multilayers with perpendicular magnetic anisotropy were investigated. The coercivity was proportionally increased to 670 Oe by increasing stack number N in Ta(2.1 nm)/[Pd(3.1/N)//Ta(2.1) multilayers with perpendicular magnetic anisotropy. Also, the coercivity in exchange biased multilayer was tend to increased by increasing stack number N. But coercivity of each materials have been in order of Co (600 Oe), (520 Oe) and (320 Oe) as function of the ferromagnet materials. The other side, exchange force of each materials is 300 Oe when the reiteration layer number N is 3. In over number of reiteration layer 3, they maintained coercivity between 200 Oe and 300 Oe.

Many studies in psychology have documented how the behaviour of verbally communicating pairs is affected by social factors such as the partner’s gaze. However, few studies have examined whether physically interacting pairs are influenced by social factors. Here, we asked two partners to exchange forces with one another, where the goal was to accurately replicate the force back onto the other. We first measured an individual’s accuracy in reproducing a force from a robot. We then tested pairs who knowingly exchanged forces whilst separated by a curtain. These separated pairs exchanged forces as two independent individuals would, hence the force reproduction accuracy of partners is not affected by knowingly reproducing a force onto a nonvisible partner. On the other hand, pairs who exchanged forces whilst facing one another consistently under-reproduced the partner’s force in comparison to separated partners. Thus, the force reproduction accuracy of subjects is strongly biased by facing a partner.

The premartensitic phase transformation in Ni 49.0 Mn 23.5 Ga 27.5 (wt pet) has been established by means of neutron powder diffraction and measurements of elastic constants of C 44 and C'. The commensurate, modulated premartensitic phase has been verified by stiffening of C 44 prior to the martensitic phase transformation. The slope change of C' at the Curie temperature positively confirms that the precursor phenomena in NiMnGa are enhanced by the magnetoelastic coupling. Magnetically, Ni 49.0 Mn 23.5 Ga 27.5 premartensite is characterized by the coexistence of a finite dc magnetic susceptibility and a vanishing magnetocrystalline anisotropy, which distinguishes bcc NiMnGa from the typical magnetic soft materials. This property arises from the competition between the exchange forces of the host lattice and the strong local crystal fields stemming from the tweed.

Realization of pulsed-current selectable domain configurations of small permalloy islands is reported. In the islands with 65–100 nm thickness and micron-sized lateral dimensions, the magnetic domain pattern can be uniquely set into either a four or seven closure domain configuration by applying either a positive or negative 10 ns current pulse at the density of 10 7 A/cm 2 . The transition is completely reversible. This phenomenon appears to belong to a family of current-induced magnetization reordering, whose main mechanism appears to be domain wall motion due to s - d exchange force directed along the electron flow, and partially influenced by the Amperian field produced at the contact regions. This paper presents two cases, namely, bi-stable and tri-stable configurations, and discusses the mechanism as well as power requirements in using this approach for memory device applications.

The nature and strength of the cation-π interactions betweenand toluene, p-cresol, or Me-indole were studied in terms of the topological properties of molecular charge density and binding energy decomposition. The results display that the diversity in the distribution pattern of bond and cage critical points reflects the profound influence of the number and nature of substituent on the electron density of the aromatic rings. On the other hand, the energy decomposition shows that dispersion and repulsive exchange forces play an important role in the organic cation ()-π interaction, although the electrostatic and induction forces dominate the interaction. In addition, it is intriguing that there is an excellent correlation between the electrostatic energy and ellipticity at the bond critical point of the aromatic π systems, which would be helpful to further understand the electrostatic interaction in the cation-π complexes.

The characteristic features of ortho- and para-helium are investigated within the framework of Relativistic Schrodinger Theory (RST). The emphasis lies on the conceptual level, where the geometric and physical properties of both RST field configurations are inspected in detail. From the geometric point of view, the striking feature consists in the splitting of the $$\mathfrak{u}(2)$$ -valued bundle connection $$\mathcal{A}_{\mu}$$ into an abelian electromagnetic part (organizing the electromagnetic interactions between the two electrons) and an exchange part, which is responsible for their exchange interactions. The electromagnetic interactions are mediated by the usual four-potentials A μ and thus are essentially the same for both types of field configurations, where naturally the electrostatic forces (described by the time component A 0 of A μ) dominate their magnetostatic counterparts (described by the space part A of A μ). Quite analogously to this, the exchange forces are as well described in terms of a certain vector potential (B μ), again along the gauge principles of minimal coupling, so that also the exchange forces split up into an “electric” type ( $$\rightsquigarrow B_{0}$$ ) and a “magnetic” type ( $$\rightsquigarrow {\bf B}$$ ). The physical difference of ortho- and para-helium is now that the first (ortho-) type is governed mainly by the “electric” kind of exchange forces and therefore is subject to a stronger influence of the exchange phenomenon; whereas the second (para-) type has vanishing “electric” exchange potential (B 0 ≡ 0) and therefore realizes exclusively the “magnetic” kind of interactions ( $$\rightsquigarrow {\bf B}$$ ), which, however, in general are smaller than their “electric” counterparts. The corresponding ortho/para splitting of the helium energy levels is inspected merely in the lowest order of approximation, where it coincides with the Hartree–Fock (HF) approximation. Thus RST may be conceived as a relativistic generalization of the HF approach where the fluid-dynamic character of RST implies many similarities with the density functional theory.

Single-molecule transistors provide a unique experimental tool to investigate the coupling between charge transport and the molecular degrees of freedom in individual molecules. One interesting class of molecules for such experiments are the single-molecule magnets, since the intramolecular exchange forces present in these molecules should couple strongly to the spin of transport electrons, thereby providing both new mechanisms for modulating electron flow and also new means for probing nanoscale magnetic excitations. Here we report single-molecule transistor measurements on devices incorporating Mn12 molecules. By studying the electron-tunneling spectrum as a function of magnetic field, we are able to identify clear signatures of magnetic states and their associated magnetic anisotropy. A comparison of the data to simulations also suggests that electron flow can strongly enhance magnetic relaxation of the magnetic molecule.

Exchange force of a ferromagnetic Fe probe on antiferromagnetic NiO(0 0 1) surface has been investigated by means of a first-principles calculation. Calculated exchange force images show a clear spin image when the probe is located within 1 A above the contact point. We can see antiferromagnetic pattern of the surface Ni atoms along the [1 1 0] direction, and asymmetric feature around surface O sites. The main contrast of Ni comes from the direct exchange interaction between the Fe probe and the surface Ni atom, while the asymmetric image possibly comes from the super exchange interaction between the Fe probe and the second layer Ni atom via the surface O. Such asymmetric feature is a key proof of the exchange force microscope image on observation.

Recent first-principles studies on antiferromagnetic oxide surfaces of MnO(001) and NiO(001) are reviewed. By structural optimization for the surface relaxation, small surface contraction and rampling relaxation where the surface transition-metal ions are displaced outwards are obtained. Surface electronic structures of the both oxides are still insulating and the spin magnetic moment at the transition-metal site is almost unchanged from its bulk value. The most interesting feature is the existence of finite spin magnetic moment at the surface oxygen site despite of no moment at oxygen in bulk by symmetry. The surface spin density distribution may be possibly observed by using exchange force microscopy technique recently proposed.

Physicists often claim that there is an effective repulsion between fermions, implied by the Pauli principle, and a corresponding effective attraction between bosons. We examine the origins of such exchange force ideas, the validity for them, and the areas where they are highly misleading. We propose that future explanations of quantum statistics should avoid the idea of a effective force completely and replace it with more appropriate physical insights, some of which are suggested here., Comment: 26 pages, 3 figures. Submitted to American Journal of Physics

We study propagation of electromagnetic waves in an antiferromagnetic medium by solving Maxwell equations coupled with a nonlinear spin equation for magnetization of the medium. In the case of isotropic medium, the propagation is governed by perturbed electromagnetic soliton of the perturbed modified Korteweg–de Vries equation. When anisotropy is added as a small correction to the exchange force, we do not observe any qualitative change in the propagation of EMW. However, when anisotropy is treated as a dominant force the propagation is governed by electromagnetic soliton of the derivative nonlinear Schrodinger equation. In both isotropic and anisotropic cases the magnetization of the antiferromagnetic medium is also found to exhibit solitonic behaviour (electromagnetic spin soliton) similar to the propagating electromagnetic soliton.