Superexchange-mediated magnetization dynamics with ultracold alkaline-earth atoms in an optical lattice

Superexchange and inter-orbital spin-exchange interactions are key ingredients for understanding（orbital） quantum magnetism in strongly correlated systems and have been realized in ultracold atomic gases.Here we study the spin dynamics of ultracold alkaline-earth atoms in an optical lattice when the two exchange interactions coexist.In the superexchange interaction dominating regime,we find that the time-resolved spin imbalance shows a remarkable modulated oscillation,which can be attributed to the interplay between local and nonlocal quantum mechanical exchange mechanisms.Moreover,the filling of the long-lived excited atoms affects the collapse and revival of the magnetization dynamics.These observations can be realized in state-dependent optical lattices combined with the state-of-the-art advances in optical lattice clock spectroscopy.

Double spin-glass-like behavior and antiferromagnetic superexchange interaction between Fe^(3+) ions in α-Ga_(2-x)Fe_xO_3(0≤x≤0.4)

Single phase of Fe^3＋-doped α-Ga2-xFexO3（α-GF x O, x = 0.1, 0.2, 0.3, 0.4） is synthesized by treating the β-Ga2-x Fe x O3（β-GF x O） precursors at high temperatures and high pressures. Rietveld refinements of the X-ray diffraction data show that the lattice constants increase monotonically with the increase of Fe^3＋content. Calorimetric measurements show that the temperature of the phase transition from α-GF x O to β-GF x O increases, while the associated enthalpy change decreases upon increasing Fe^3＋content. The optical energy gap deduced from the reflectance measurement is found to decrease monotonically with the increase in Fe3＋content. From the measurements of magnetic field-dependent magnetization and temperature-dependent inverse molar susceptibility, we find that the superexchange interaction between Fe^3＋ions is antiferromagnetic. Remnant magnetization is observed in the Fe^3＋-doped α-GF x O and is attributed to the spin glass in the magnetic sublattice. At high Fe^3＋doping level（x = 0.4）, two evident peaks are observed in the image part of the AC susceptibility χ ac. The frequency dependence in intensity of these two peaks as well as two spin freezing temperatures observed in the DC magnetization measurements of α-GF0.4O is suggested to be the behavior of two spin glasses.

Superexchange interaction enhancement of the quantum transport in a DNA-type molecule

We use the transfer matrix method and the Green function technique to theoretically study the quantum tunnelling through a DNA-type molecule. Ferromagnetic electrodes are used to produce the spin-polarized transmission probability and therefore the spin current. The distance-dependent crossover comes from the topological variation from the one- dimensional to the two-dimensional model transform as we switch on the interstrand coupling; a new base pair will present N - 1 extrachannels for the charge and spin as N being the total base pairs. This will restrain the decay of the transmission and improve the stability of the quantum transport. The spin and charge transfer through the DNA-type molecule is consistent with the quantum tunneling barrier.

Superexchange and charge transfer in the nickelate superconductor La_(3)Ni_(2)O_(7) under pressure

Recently, a bulk nickelate superconductor La_(3)Ni_(2)O_(7) is discovered at pressures with a remarkable high transition temperature T_(c)~ 80 K. Here, we study a Hubbard model with tight-binding parameters derived from ab initio calculations of La_(3)Ni_(2)O_(7),by employing large scale determinant quantum Monte Carlo and cellular dynamical mean-field theory. Our result suggests that the superexchange couplings in this system are comparable to that of cuprates. The system is a charge transfer insulator as the hole concentration becomes four per site at large Hubbard U. Upon hole doping, two low-energy spin-singlet bands emerge in the system exhibiting distinct correlation properties: while the one composed of the out-of-plane Ni-d_(3z^(2)-r^(2)) and O-p_(z) orbitals demonstrates strong antiferromagnetic correlations and narrow effective bandwidth, the in-plane singlet band consisting of the Ni-d_(x^(2)-y^(2)) and O-p_(x)/p_(y) orbitals is in general more itinerant. Over a broad range of hole doping, the doped holes occupy primarily the d_(x^(2)-y^(2)) and p_(x)/p_(y) orbitals, whereas the d_(3z^(2)-r^(2)) and p_(z) orbitals retain underdoped. We propose an effective t-J model to capture the relevant physics and discuss the implications of our result for comprehending the La_(3)Ni_(2)O_(7) superconductivity.

Li_(2)NiSe_(2):A new-type intrinsic two-dimensional ferromagnetic semiconductor above 200 K

By using first-principles electronic structure calculations,we propose a two-dimensional ferromagnetic semiconductor Li_(2)NiSe_(2)with a Curie temperature above 200 K.The structure of monolayer Li_(2)NiSe_(2)is dynamically stable,which is derived from the synthesized prototype compound Li_(2)Ni O_(2)and can be denoted as Li-decorated 1T-type NiSe_(2).The Ni–Se–Ni ferromagnetic superexchange dominates the magnetic couplings between the Ni atoms,which can be understood in the frame of the Goodenough–Kanamori–Anderson(GKA)rules.Our systematic study of monolayer Li_(2)NiSe_(2)enables its promising applications in spintronics and suggests a new choice to design two-dimensional ferromagnetic semiconductors.

Room-Temperature Magnetism of Ceria Nanocubes by Inductively Transferring Electrons to Ce Atoms from Nearby Oxygen Vacancy

Ceria(CeO_2) nanocubes were synthesized by a hydrothermal method and weak ferromagnetism was observed in room temperature. After ultraviolet irradiation, the saturation magnetization was significantly enhanced from*3.18×10^(-3) to *1.89×10^(-2) emug^(-1). This is due to the increase of oxygen vacancies in CeO_2 structure which was confirmed by X-ray photoelectron spectra. The first-principle calculation with Vienna ab-initio simulation package was used to illustrate the enhanced ferromagnetism mechanism after calculating the density of states(DOSs) and partial density of states(PDOSs) of CeO_2 without and with different oxygen vacancies. It was found that the increase of oxygen vacancies will enlarge the PDOSs of Ce 4f orbital and DOSs. Two electrons in one oxygen vacancy are respectively excited to 4f orbital of two Ce atoms neighboring the vacancy, making these electron spin directions on 4f orbitals of these two Ce atoms parallel. This superexchange interaction leads to the formation of ferromagnetism in CeO_2 at room temperature. Our work indicates that ultraviolet irradiation is an effective method to enhance the magnetism of CeO_2 nanocube, and the firstprinciple calculation can understand well the enhanced magnetism.

Theoretical analysis of the anisotropy of the magnetization of the Ho^(3+) ion in holmium iron garnet single crystals

The spontaneous magnetization of the Ho^3＋ ion in holmium iron garnet （HoIG） single crystals in the temperature range of 4.2-294K along the directions [111], [110], and [100] are calculated, taking into account the effects of six magnetically inequivalent sites occupied by the Ho^3＋ ions based on the quantum theory. The calculated results show that the magnetization of the Ho^3＋ ion in HoIG is obviously anisotropic. The theoretical results ave in agreement with those of experiments. A primary interpretation of the anisotropy of magnetization of the Ho^3＋ ion in HoIG is put forward.

Photoinduced Intramolecular Charge Transfer in Donor-acceptor Dyad and Donor-bridge-acceptor Triad

The ground and excited state properties of the [60]fullerene, diphenylbenzothiadiazole-triphenylamine （PBTDP-TPA） dyad and fullerene-diphenylbenzothiadiazole-triphenylamine （fullerene-PBTDP-TPA） triad were investigated theoretically using density functional theory with B3LYP functional and 3-21G basis set and time-dependent density functional theory with B3LYP functional and STO-3G basis set as well as 2D and 3D real space analysis methods. The 2D site representation reveals the electron-hole coherence on excitation. The 3D transition density shows the orientation and strength of the transition dipole moment, and the 3D charge difference density gives the orientation and result of the intramolecular charge transfer. Also photoinduced intermolecular charge transfer （ICT） in PBTDP-TPA-fullerene triad are identified with 2D and 3D representations, which reveals the mechanisms of ICT in donor-bridge-acceptor triad on excitation. Besides that we also found that the direct superexchange ICT from donor to acceptor （tunneling through the bridge） strongly promotes the ICT in the donor-bridge-acceptor triad.

An investigation on the terbium gallium garnet magneto-optic properties of under high magnetic field

The superexchange interaction on a magnetic ion may be represented by an effective field Hm =λM in some paramagnetic materials, here λis the coefficient of effective field and M = XHe with X being the magnetic susceptibility and He being the applied field. The variation of the equivalent λX with the dynamic applied field is given and the crystal field-splitting levels of the excited configuration 4f75d1 of the Tb3＋ ion are calculated in the Tb3Ga5O12. By means of the effective field Hm and the applied field He, the Faraday rotation of Tb3CasO12 at 6 K and 41 K, under the high magnetic field and at 0.63μm wavelength, are presented. Our calculated results are in agreement with the experimental data.

Renormalization of spin polarised itinerant electron bands in the normal state of a model ferromagnetic superconductor

This paper studies the normal state properties of itinerant electrons in a toy model, which is constructed according to the model for coexisting ferromagnetism and superconductivity proposed by Suhl [Suhl H 2001 Phys. Rev. Lett. 87 167007]. In this theory with ferromagnetic ordering based on localized spins, the exchange interaction J between conduction electrons and localized spin is taken as the pairing glue for s-wave superconductivity. It shows that this J term will first renormalize the normal state single conduction electron structures substantially. It finds dramatically enhanced or suppressed magnetization of itinerant electrons for positive or negative J. Singlet Cooper pairing can be ruled out due to strong spin polarisation in the J 〉 0 case while a narrow window for s-wave superconductivity is opened around some ferromagnetic J.

Strong Ferromagnetic Coupling of {[Cu（Hpht）（N3）]·H2O}n with Simultaneous End-on Azido and Carboxylato Bridges： a First Principle Study

The compound {[Cu(Hpht)(N_3)]·H_2O}_n (Hpht = hydrogen phthalate) is formedby chains of copper atoms bridged simultaneously by syn-syn carboxylato and end-on azido bridges.Taking into account the large Cu-O(1)-C(7) bond angle of the single carboxylato bridge (131°), orthe large Cu-N(11)-Cu bond angle of the azido bridge (111.9°), a moderately intrachainantiferromagnetic behavior should be expected for the compound. This paper is devoted to examiningthe apparently anomalous intrachain ferromagnetic behavior of {[Cu(Hpht)(N_3)]·H_2O}_n, using firstprinciples within the full potential linearized augmented plane wave (FP-LAPW) method. The totalenergy, the density of states (DOS), and the spin distributions are obtained. The atomic spindistribution has been analyzed as resulting from the interplay of electron delocalization and spinpolarization. The DOS reveals a surprisingly strong exchange interaction between the d type orbitalsof the copper and the π molecular orbitals of the two ligands.

Crystal growth and magnetic properties of quantum spin liquid candidate KErTe_(2)

Recently rare-earth chalcogenides have been revealed as a family of quantum spin liquid(QSL)candidates hosting a large number of members.In this paper we report the crystal growth and magnetic measurements of KErTe_(2),which is the first member of telluride in the family.Compared to its cousins of oxides,sulfides and selenides,KErTe_(2) retains the high symmetry of R3m and Er3+ions still sit on a perfect triangular lattice.The separation between adjacent magnetic layers is expectedly increased,which further enhances the two dimensionality of the spin system.Specific heat and magnetic susceptibility measurements on KErTe_(2) single crystals reveal no structural and magnetic transition down to 1.8 K.Most interestingly,the absorption spectrum shows that the charge gap of KErTe_(2) is roughly 0.93±0.35 eV,which is the smallest among all the reported members in the family.This immediately invokes the interest towards metallization even superconductivity using the compound.

Thickness-dependent magnetic order and phase transition in V5S8

V5S8 is an ideal candidate to explore the magnetism at the two-dimensional(2D)limit.A recent experiment has shown that the V5S8 thin films exhibit an antiferromagnetic(AFM)to ferromagnetic(FM)phase transition with reducing thickness.Here,for the first time,using density functional theory calculations,we report the antiferromagnetic order of bulk V5S8,which is consistent with the previous experiments.The specific antiferromagnetic order is reproduced when Ueff=2 eV is applied on the intercalated vanadium atoms within LDA.We find that the origin of the magnetic ordering is from superexchange interaction.We also investigate the thickness-dependent magnetic order in V5S8 thin films.It is found that there is an antiferromagnetic to ferromagnetic phase transition when V5S8 is thinned down to 2.2 nm.The main magnetic moments of the antiferromagnetic and ferromagnetic states of the thin films are located on the interlayered vanadium atoms,which is the same as that in the bulk.Meanwhile,the strain in the thin films also influences the AFM-FM phase transition.Our results not only reveal the magnetic order and origin in bulk V5S8 and thin films,but also provide a set of parameters which can be used in future calculations.

Identifying the genes of unconventional high temperature superconductors

We elucidate a recently emergent framework in unifying the two families of high temperature （high To） superconductors, cuprates and iron-based superconductors. The unification suggests that the latter is simply the counterpart of the former to realize robust extended s-wave pairing symmetries in a square lattice. The unification identifies that the key ingredients （gene） of high Tc superconductors is a quasi two dimensional electronic environment in which the d-orbitals of cations that partic- ipate in strong in-plane couplings to the p-orbitals of anions are isolated near Fermi energy. With this gene, the superexchange magnetic interactions mediated by anions could maximize their contributions to superconductivity. Creating the gene requires special arrangements between local electronic structures and crystal lattice structures. The speciality explains why high Tc superconductors are so rare. An explicit prediction is made to realize high Tc superconductivity in Co/Ni-based materials with a quasi two dimensional hexagonal lattice structure formed by trigonal bipyramidal complexes.

Magnetic properties of M-type strontium ferrites with different heat treatment conditions

The effects of heat treatment conditions on the magnetic properties and microstructure of M-type strontium ferrite according to calcination temperature were analyzed.Strontium ferrite Sr0.06Ca0.52La0.52Fe11.68Co0.22O19magnetic powder was prepared by a standard ceramic process.During experiments,the calcination temperature was varied from 1180 to 1260℃,and sintering temperature was fixed.While the M-phase(SrFe12O19)existed with hematite(Fe2 O3)in the powder calcined at below 1220℃,the pure M-phase was observed in the powder calcined at over1240℃.With an increase in the calcination temperature,the magnetization of the calcined powder increases,meanwhile,the coercivity decreases.The magnetization is improved by decreasing the lattice constant c and activating the Fe3+-OFe3+superexchange interaction,and the coercivity decreases by the large particle sizes due to the grain growth.

Stability of the Fe12O12 cluster

Superexchange effects play an important role in the determination of crystal structures; however, there has been much less reported on how they determine the stability of dusters. Using evolutionary search strategies and DFT＋U （density functional theory with the Hubbard U correction） calculations, we investigate the global minimum-energy structures of Fe12O12 clusters. Among predicted Fe12O12 dusters, a cage-shaped Fe12O12 cluster with unexpected stability was observed. In addition, the bare Fe12O12 cluster is shown to possess an extremely large energy gap （2.00 eV）, which is greater than that of C60, Au20 and Al13- clusters. Using a Heisenberg model, we traced the origin of the unexpected stability of the bare Fe12O12 cluster to magnetic competition between the nearestneighbor exchange constant h and the next-nearest neighbor exchange constant J2 that was induced by the superexchange interactions. The bare Fe12O12 cluster is thus a unique molecule that is stable and chemically inert.

Mossbauer and magnetization studies of magnetic phase transitions in 0.5BiFeO_(3)-0.5NaNbO_(3)and 0.5LaFeO_(3)-0.5NaNbO_(3)solid solutions

0.5AFeO_(3)-0.5NaNbO_(3)(A=Bi,La)solid solution compositions were prepared using a solid phase reaction route from high-purity starting oxides.Mossbauer studies have shown that while for 0.5BiFeO_(3)-0.5NaNbO_(3)the magnetic phase transition temperature T_(M) value is about 150 K,for 0.5LaFeO_(3)-0.5NaNbO_(3),it is only≈25 K.This dramatic difference in T_(M) values seems to be due to additional contribution of the magnetic superexchange between Fe^(3+)ions via the empty 6p-states of Bi^(3+)ions to the overall superexchange.

Investigation of Possible Half-Metallic Antiferromagnets on Double Perovskites A_(2)BB′O_(6)(A=Ca,Sr Ba;B,B′=Transition Metal Elements)

A search was made for possible half-metallic(HM)antiferromagnet(AFM)in all the(C_(2)^(92)=406)double perovskites structures of Sr2BB′O6 where BB′pairs are any combination of 3d,4d or 5d transition elements with the exception of La.Sr can also be replaced by Ca or Ba whenever HM-AFM was found and similar calculations were then performed in order to probe further possibilities.It was found that A_(2)MoOsO_(6),A_(2)TcReO_(6),A_(2)CrRuO_(6),where A=Ca,Sr,Ba,are all potential candidates for HM-AFM.The AFM of A2BB′O6 comes from both the superexchange mechanism and the generalized double exchange mechanism via the B(t2g)-O2pp-B′(t2g)coupling,With the latter also being the origin of their HM.Also considered were the effects of spin-orbit coupling(SOC)and correlation(+U)by introducing+SOC and+U corrections.It is found that the SOC effect has much less influence than the correlation effect on the HMproperty of the compounds.For A_(2)TcReO_(6)and A_(2)CrRuO_(6),after+U,they become nearlyMott-Insulators.In the future,it is hoped that therewill be further experimental confirmation for these possible HM-AFMcandidates.