Density Functional Theory Calculations of Structural,Electronic, and Magnetic Properties of the 3d Metal Trifluorides MF3 (M = Ti-Ni) in the Solid State

We employ density functional theory with Hubbard U correction or hybrid functionals to study the series of magnetic 3d metal trifluorides MF₃ (M = Ti-Ni). Experimental lattice parameters are reproduced with an error margin of 0.5%–4.3%. Cooperative Jahn–Teller distortions are reproduced for MnF₃, but also found in TiF₃ and CoF₃ at smaller levels compared to MnF₃. Trends in electronic structure with respect to positions of the d bands are linked to the magnetic properties where M = Ti-Cr are weak magnetic Mott–Hubbard insulators, M = Fe-Ni are strong magnetic charge-transfer insulators and MnF₃ falls in between. Our work contributes to the characterization of the relatively unknown NiF₃, since FeF₃ and CoF₃ have similar electronic and magnetic properties. However, NiF₃ does not show a Jahn–Teller distortion as present in CoF₃. © 2019 Wiley Periodicals, Inc.     

Structural and Magnetic Properties of FexTiSe2 Intercalation Compounds

Abstract

Magnetic and neutron studies are done about iron-intercalated titanium diselenide, Fe^xTiSe² with 0<x≤0.5. A neutron diffraction measurement shows that Fe-atoms are located between neighboring selenium layers. Magnetic measurements show that FexTiSe² are spin glasses for 0. 15≤^x≤0.22 and antiferromagnets for 0.25≤^x≤0.5. T-x magnetic phase diagram is determined. This diagram is slightly different from that determined by Huntley et al. The difference may be caused by a formation of Fe-clusters in their samples. The x-dependence of the parameters of paramagnetic susceptibilities is determined.     

Effects of the Next Nearest Neighbor Hopping on Superconductivity and Antiferromagnetism of Gossamer Superconductivity

The two dimensions hole-doped t-t '-J-U model was studied based on the Gutzwiller approach and the renormalized mean-field theory.The phase diagrams of gossamer superconductors and the effects of the next-nearestneighbor hopping(t ') on superconductivity and antiferromagnetism based on the t-t '-J-U model were investigated.The results show that the qualitative feature of the phase diagrams in the t-t '-J-U model is the same as in the case of the t-J-U model.The antiferromagnetic order coexists with the d-wave superconductivity(dSC) in the underdoped region below the doping δ≈ 0.1 and is enhanced by the t '.The dSC order is slightly suppressed by t ' in the underdoped region and greatly enhanced in the overdoped region.The dSC order is pushed to a larger doping region and the coexistence region of the AF and dSC extends to higher doping.     

A Copper–Formate Framework Showing a Simple to Helical Antiferroelectric Transition with Prominent Dielectric Anomalies and Anisotropic Thermal Expansion,and Antiferromagnetism

We present here the compound [NH₄][Cu(HCOO)₃], a new member of the [NH₄][M(HCOO)₃] family. The Jahn–Teller Cu²⁺ ion leads to a distorted 4⁹?6⁶ chiral Cu–formate framework. In the low‐temperature (LT) orthorhombic phase, the Cu²⁺ is in an elongated octahedron, and the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions in the framework channel are off the channel axis. From 94 to 350 K the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ion gradually approaches the channel axis and the related modulation of the framework and the hydrogen‐bond system occurs. The LT phase is simple antiferroelectric (AFE). The material becomes hexagonal above 355 K. In the high‐temperature (HT) phase, the Cu²⁺ octahedron is compressed, and the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions are arranged helically along the channel axis. Therefore, the phase transition is one from LT simple AFE to HT helical AFE. The temperature‐dependent structure evolution is accompanied by significant thermal and dielectric anomalies and anisotropic thermal expansion, due to the different status of the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions and the framework modulations, and the structure–property relationship was established based on the extensive variable‐temperature single‐crystal structures. The material showed long range ordering of antiferromagnetism (AFM), with low dimensional character and a Néel temperature of 2.9 K. Therefore, within the material AFE and AFM orderings coexist in the low‐temperature region.     

Superconductivity and spin density waves

In the electron-electron interaction the r-space structure caused by magnetic fluctuations at the phase transition from a nonmagnetic metal to an antiferromagnetic metal gives rise to a d-wave attractive interaction for Cooper pairing. This is a contribution to some total electron-electron interaction which in total may or may not give rise to Cooper pairing and superconductivity.     

Synthesis and characterization of carboxylate-bridged copper(II) dinuclear complex

The dinuclear copper(II) complex {[Cu(pmea)Cl][Cu(H₂O)₃Cl]}(Cl)·H₂O (1) (pmea =bis(2-pyridylmethyl)amino-2-ethanoic acid) has been synthesized and characterized. It crystallizes in the monoclinic system P2₁/c, a = 9.0057(11), b = 28.031(3), c = 8.5669(10) Å, β = 104.267(2)°, V = 2095.9(4) ų, Z = 4. Crystal structure of 1 reveals a syn-anti carboxylate-bridged dinuclear complex, in which the coordination environment around each copper atom exhibits a distorted square-pyramid. Magnetic susceptibility measurement for 1 exhibits a weak intermolecular antiferromagnetic exchange interaction with 2J = −1.17(9) cm⁻¹. The cyclic voltammogram of 1 gives one oxidation and two reduction processes.     

基于End-On叠氮桥联的混合价Co(Ⅱ/Ⅲ)和一维Cu(II)链-席夫碱化合物的合成、结构及磁学性质

合成了2个化合物[Co^ⅡCo₄^Ⅲ（salhn）₄（N₃）₆（CH₃OH）₂（H₂O）₂]·4CH₃OH·2H₂O（1）和[Cu₂（salhn）（N₃）₂]_n（2）（H₂salhn=N,N''-bis（salicylidene） hydrazine）,并用X射线单晶衍射进行结构表征。化合物1是一个五核的[Co^ⅡCo₄^Ⅲ]钴簇,而化合物2是一个具有结构单元为[Cu₂（salhn）（N₃）₂]的一维链结构。2个化合物中叠氮均具有end-on（EO,μ-1,1）的配位模式。化合物1和2的磁学性质测试表明它们都具有反铁磁行为。     

Superconductivity in self-flux-synthesized single crystalline R_2Pt_3Ge_5(R=La,Ce,Pr)

In order to study the basic superconductivity properties of R_2Pt_3Ge_5, we synthesized the single crystalline samples by the Pt–Ge self-flux method. R_2Pt_3Ge_5(R = La, Ce) were also grown for a systematic study. Zero-resistivity was observed in both the La-and Pr-based samples below the reported superconducting transition temperatures. However, magnetic susceptibility measurements showed low superconductivity volume fractions in both La_2Pt_3Ge_5 and R_2Pt_3Ge_5(less than2%). Ce_2Pt_3Ge_5 did not show any signature of superconductivity. From the specific heat measurements, we did not observe a superconducting transition peak in R_2Pt_3Ge_5, suggesting that it is not a bulk superconductor. The magnetic susceptibility and heat capacity measurements revealed two antiferromagnetic(AFM) orders in R_2Pt_3Ge_5 at T_(N1)= 4.2 K and T_(N2)= 3.5K, as well as a single AFM transition at TN= 3.8 K in Ce_2Pt_3Ge_5.     

Ultra‐Fast Control of Magnetic Relaxation in a Periodically Driven Hubbard Model

Motivated by cold atom and ultra‐fast pump‐probe experiments we study the melting of long‐range antiferromagnetic order of a perfect Néel state in a periodically driven repulsive Hubbard model. The dynamics is calculated for a Bethe lattice in infinite dimensions with non‐equilibrium dynamical mean‐field theory. In the absence of driving melting proceeds differently depending on the quench of the interactions to hopping ratio from the atomic limit. For decay occurs due to mobile charge‐excitations transferring energy to the spin sector, while for it is governed by the dynamics of residual quasi‐particles. Here we explore the rich effects that strong periodic driving has on this relaxation process spanning three frequency ω regimes: (i) high‐frequency , (ii) resonant with integer l, and (iii) in‐gap away from resonance. In case (i) we can quickly switch the decay from quasi‐particle to charge‐excitation mechanism through the suppression of ν₀. For (ii) the interaction can be engineered, even allowing an effective regime to be reached, giving the reverse switch from a charge‐excitation to quasi‐particle decay mechanism. For (iii) the exchange interaction can be controlled with little effect on the decay. By combining these regimes we show how periodic driving could be a potential pathway for controlling magnetism in antiferromagnetic materials. Finally, our numerical results demonstrate the accuracy and applicability of matrix product state techniques to the Hamiltonian DMFT impurity problem subjected to strong periodic driving.