Weak Spin-exchange Interaction Research Articles

Enhancing Magnetic Ordering in Two-Dimensional Metal-Organic Frameworks via Frontier Molecular Orbital Engineering.

Two-dimensional (2D) metal-organic frameworks (MOFs) have promise for use in lightweight permanent magnets in contrast to inorganic solid- or molecule-based magnets, but the realization of 2D MOF magnets with a high ordering temperature is limited by the typically weak spin exchange interactions. Here, we have proposed a frontier molecular orbital engineering strategy for modulating magnetism in 2D MOFs. It shows that the magnetic ground state can be mediated by two intra-atomic spin exchange pathways in organic ligands, akin to the Bloch and Heisenberg models, depending on the shape of the frontier orbitals of the organic ligands. By engineering the shape of the lowest unoccupied molecular orbital (LUMO) via chemical hydrogenation, we achieved a nearly 11-fold increase in the ordering temperature. In particular, a quantitative analysis shows that the ordering temperature increases linearly with the orbital delocalization index of the ligands' LUMO. This work suggests a general frontier orbital engineering approach for modulating the spin exchange interaction in 2D MOF magnets.

Surface-strain-dependent room-temperature ferromagnetism in hexagonal MIn2S4 monolayers

The electronic occupation at 3d transition metal and crystal symmetry play a critical role in realizing spin polarization, but the weak spin exchange interaction makes it difficult to obtain room-temperature ferromagnetism, especially in two-dimensional (2D) materials. Different from traditional bulk phase with semiconducting feature, the free-standing hexagonal MIn2S4 (M = Fe, Co, Ni, Mn) monolayers are proposed to discuss the strain-dependent magnetic behavior, in which the 3d-orbital hybridization and charge transfer can be regulated by structural deformation to enforce their spin exchange interaction, finally leading to a room-temperature 2D magnetic characteristic. The calculations disclose that the structural deformation not only regulates the spin polarization feature but also leads to an electronic reconfiguration. Our findings provide a new insight into designing 2D spintronic devices by strain engineering in hexagonal MIn2S4 monolayers.

Vortex-bright soliton complexes in [formula omitted] rotating Bose–Einstein condensates

We investigate a novel class of states associated with rotating bosonic F=2 Bose–Einstein condensates (BECs). In the cyclic phase with weak spin-exchange interaction, no vortex core is observed when rotation is considered. However, strong spin-exchange interaction can induce vortex-bright soliton complex, inside which the massive solitons locate within the cores of vortices. Anticlockwise current of the aforementioned excitation is obtained through analysing the particle current of every component. Furthermore, various axisymmetric vortex-bright soliton textures are excited via stronger density-density interaction. Remarkably, all of the vortex-bright soliton complexes rotate following the rotation rate, as in scalar condensates. Subsequently, upon sweeping density-density interaction and rotation frequency in numerical calculation, first-order phase transitions are generated when angular momentum changes discontinuously. With the aid of these findings, nontrivial topological excitations are investigated and a new gate is opened for manipulating topological phase transition in high-spin systems.

Room-Temperature Magnetic Field Effect on Excitonic Photoluminescence in Perovskite Nanocrystals.

Magnetic-field-enhanced spin-polarized electronic/optical properties in semiconductors are crucial for fabricating various spintronic devices. However, this spin polarization is governed by weak spin exchange interactions and easily randomized by thermal fluctuations; therefore, it is only produced at cryogenic temperatures, which severely limits the applications. Herein, a room-temperature intrinsic magnetic field effect (MFE) on excitonic photoluminescence is achieved in CsPbX3 :Mn (X = Cl, Br) perovskite nanocrystals. Through moderate Mn doping, the MFE is enhanced by exciton-Mn interactions, and through partial Br substitution, the MFE is stabilized at room temperature by exciton orbital ordering. The orbital ordering significantly enhances the g-factor difference between electrons and holes, which is evidenced by a parallel orbit-orbit interaction among excitons generated by circular polarized laser excitation. This study provides a clear avenue for engineering spintronic materials based on orbital interactions in perovskites.

The weakening of fermionization of one dimensional spinor Bose gases induced by spin-exchange interaction

We investigate the ground state density distributions of anti-ferromagnetic spin-1 Bose gases in one dimensional harmonic potential in the full interacting regimes. The ground state is obtained by diagonalizing the Hamiltonian in the Hilbert space composed of the lowest eigenstates of noninteracting Bose gas and spin components. The study reveals that in the situation of weak spin-dependent interaction the total density profiles evolve from Gaussian-like distribution to a Fermi-like shell structure of $N$ peaks with the increasing of spin-independent interaction. While the increasing spin-exchange interaction always weaken the fermionization of density distribution such that the total density profiles show shell structure of less peaks and even show single peak structure in the limit of strong spin-exchange interaction. The weakening of fermionization results from the formation of composite atoms induced by spin-exchange interaction. It is also shown that phase separation occurs for the spinor Bose gas with weak spin-exchange interaction, meanwhile strong spin-independent interaction.

One-Dimensional Electron Liquid in an Antiferromagnetic Environment: Spin Gap from Magnetic Correlations

We study a one-dimensional electron liquid coupled by a weak spin-exchange interaction to an antiferromagnetic spin-S ladder with n legs. A perturbative renormalization group analysis in the semiclassical limit reveals the opening of a spin gap, driven by the local magnetic correlations on the ladder. The effect, which we argue is present for any gapful ladder or gapless ladder with $nS\gg 1$, is enhanced by the repulsive interaction among the conduction electrons but is insensitive to the sign of the spin exchange interaction with the ladder. Possible implications for the striped phases of the cuprates are discussed.

Very weak electron–electron exchange interactions in paramagnetic dinuclear tris(pyrazolyl)boratomolybdenum centres with extended bridging ligands: estimation of the exchange coupling constant J by simulation of second-order EPR spectra †

Two series of dinuclear complexes have been prepared in which paramagnetic nitrosylmolybdenum(I) or oxomolybdenum(V) units have been attached to either end of very long bis-pyridyl or bis-phenolate bridging ligands respectively. The first series of complexes is [{MoI(TpMe,Me)(NO)Cl}2{µ-L}] (L = 4,4′-bis[2-(4-pyridyl)ethen-1-yl]biphenyl; 4,4″-bis[2-(4-pyridyl)ethen-1-yl]terphenyl 2; 4,4′-bis[2-(4-pyridyl)ethen-1-yl]benzophenone 3; 4,4′-bis[2-(4-pyridyl)ethen-1-yl]benzil 4; or 6,6′-bis[2-(4-pyridyl)ethen-1-yl]-2,2′-bipyridine 5). The second series of complexes is [{MoV(TpMe,Me)(O)Cl}2(µ-L)] (H2L = HOC6H4OC(S)OC6H4OH 6; HOC6H4OS(O)OC6H4OH 7; or HOC6H4OC(O)C6H4C(O)OC6H4OH 8, with all-para substitution for the C6H4 units in each case). The very weak spin exchange interactions between the remote paramagnetic centres result in many cases in second-order EPR spectra, because |J| ≈ A (where J is the exchange coupling constant, and A the electron–nucleus hyperfine coupling). In these cases the appearance of the EPR spectra is complicated and sensitive to small changes in the magnitude of J, which could be exploited to estimate values for |J| by comparing the measured spectra with computer simulations calculated using a range of values of |J|. For the first series of complexes the spin exchange interactions decrease in the order 1 (|J| ≥ 4000), 2 (1000), 3 (150), 4 (43), 5 (|J| ≤ 10 MHz) which is readily explicable in terms of the lengths, conformations and substitution patterns of the bridging ligands. For the second series of complexes, 6 and 7 both gave second-order spectra with |J| = 2000 MHz, whereas 8, with a much longer bridging ligand, has |J| ≤ 10 MHz. Crucially, these spin-exchange interactions are much too weak to be determined by conventional magnetic susceptibility measurements (|J| 1 cm–1), and therefore simulation of second-order EPR spectra provides a simple route to providing useful information about the relative magnitudes of very weak spin exchange interactions which is not available by any other route.

Synthesis and properties of polynuclear complexes containing {Mo(NO)[HB(dmpz)3]Cl} metal centres axially bound to a Ru(tpp) core (dmpz = 3,5-dimethylpyrazol-1-yl; tpp =meso-5,10,15,20-tetraphenylporphyrinate)

A series of mononuclear complexes [Mo(NO){HB(dmpz)3}Cl(L)][dmpz = 3,5-dimethylpyrazol-1-yl; L = a potentially bridging ligand of which one terminus (pyridyl or phenolate) is attached to the Mo and the second terminus (pyridyl) is pendant] were attached to the axial positions of a Ru(tpp) core (tpp =meso-5,10,15,20-tetraphenylporphyrinate)via the pendant pyridyl groups. By using [Ru(tpp)(CO)(EtOH)], of which only the axial EtOH ligand is substitution labile, binuclear complexes [{(OC)(tpp)Ru}(µ-L){Mo(NO)[HB(dmpz)3]Cl}] were prepared. With [Ru(tpp)(thf)2](thf = tetrahydrofuran), in which both thf ligands are labile, the trinuclear complexes [{Cl[HB(dmpz)3](ON)Mo}(µ-L){Ru(tpp)}(µ-L){Mo(NO)[HB(dmpz)3]Cl}] were prepared. The new complexes have been thoroughly characterised by 1H NMR, fast atom bombardment mass spectrometry, IR, UV/VIS and EPR spectroscopy and electrochemistry as appropriate. Weak electrochemical interactions between the molybdenum groups and the Ru(tpp) core are apparent. In the trinuclear complexes with two axial paramagnetic molybdenum groups (17 valence-electron configuration) a weak spin-exchange interaction between the two remote molybdenum centres can be detected when the intermediate bridging ligands L are 4,4′-bipyridine but not when L is 3,3′-dimethyl-4,4′-bipyridine, possibly due to an increased dihedral twist of the two bridging ligands in the latter case.