Negative Magnetoresistance in Antiferromagnetic Topological Insulator EuSn_2As_2

The measurements of magnetization,longitudinal and Hall resistivities are carried out on the intrinsic antiferromagnetic(AFM) topological insulator ElSn_2 As_2.It is confirmed that our ElSn_2 As_2 crystal is a heavily hole doping A-type AFM metal with the Neel temperature T_N=24 K,with a metamagnetic transition from an AFM to a ferromagnetic(FIM) phase occurring at a certain critical magnetic Held for the different Held orientations.Meanwhile,we also find that the carrier concentration does not change with the evolution of magnetic order,indicating that the weak interaction between the localized magnetic moments from Eu~(2+)4f~7 orbits and the electronic states near the Fermi level.Although the quantum anomalous Hall effect(AHE) is not observed in our crystals,it is found that a relatively large negative magnetoresistance (-13%) emerges in the AFM phase,and exhibits an exponential dependence upon magnetic Held,whose microscopic origin is waiting to be clarified in future research.     

Magnetic Proximity Effect in an Antiferromagnetic Insulator/Topological Insulator Heterostructure with Sharp Interface

We report an experimental study of electron transport properties of MnSe/(Bi,Sb)_2Te_3 heterostructures,in which MnSe is an antiferromagnetic insulator,and(Bi,Sb)_2Te_3 is a three-dimensional topological insulator(TI).Strong magnetic proximity effect is manifested in the measurements of the Hall effect and longitudinal resistances.Our analysis shows that the gate voltage can substantially modify the anomalous Hall conductance,which exceeds 0.1 e~2/h at temperature T=1.6 K and magnetic field μ_0H=5 T,even though only the top TI surface is in proximity to MnSe.This work suggests that heterostructures based on antiferromagnetic insulators provide a promising platform for investigating a wide range of topological spintronic phenomena.     

Pressure-Induced Topological and Structural Phase Transitions in an Antiferromagnetic Topological Insulator

Recently,natural van der Waals heterostructures of(MnBi₂ Te₄)_m(Bi₂ Te₃)_n have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states.We systematically investigate both the structural and electronic responses of MnBi₂ Te₄ and MnBi₄ Te₇ to external pressure.In addition to the suppression of antiferromagnetic ord...     

C-Type Antiferromagnetic Structure of Topological Semimetal CaMnSb2

<正>Determination of the magnetic structure and confirmation of the presence or absence of inversion (P) and time reversal (Τ) symmetry is imperative for correctly understanding the topological magnetic materials. Here highquality single crystals of the layered manganese pnictide CaMnSb₂ are synthesized using the self-flux method.     

Large Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn_2Bi_2Te_5

The dynamical axion field is a new state of quantum matter where the magnetoelectric response couples strongly to its low-energy magnetic fluctuations.It is fundamentally different from an axion insulator with a static quantized magnetoelectric response.The dynamical axion field exhibits many exotic phenomena such as axionic polariton and axion instability.However,these effects have not been experimentally confirmed due to the lack of proper topological magnetic materials.Combining analytic mode...     

Quantum Description of the Magnetic Polaron in Antiferromagnetic Semiconductors

On the basis of exchange interaction of impurity electrons with localized moments, the magneticpolaron Hamiltonian for antiferromagneticsemiconductorsis derived. By themethod of canonical transformation, the properties of magnetic polarons for the intermediate-coupling case are discussed. Especially in the strong-coupling case, those properties are considered by using the generalized Landau-Pekar method, and the analytical results such as self-energy, effective mass and the space location are given in the long wave-length approximation.     

Topological Phase in Non-centrosymmetric Material NaSnBi

We predict that a non-centrosymmetric material NaSnBi locates in a three-dimensional non-trivial topological phase under ambient pressure based on first-principle calculations.By deriving the effective model around the Γpoint,we find that the topological phase transition is driven by a Rashba spin-orbital coupling through an odd number of pairs of band touch due to a small anisotropic gap caused by quintic dispersion terms.In contrast to conventional topological insulators,the spin texture of the surface Dirac cone is right-handed and the surface states are strikingly different for different surface terminations.     

Superconducting anisotropy and vortex pinning in CaKFe_4As_4 and KCa_2Fe_4As_4F_2

The vortex pinning determining the current carrying capacity of a superconductor is an important property to the applications of superconducting materials.For layered superconductors,the vortex pinning can be enhanced by a strong interlayer interaction in accompany with a suppression of superconducting anisotropy,which remains to be investigated in iron based superconductors(FeSCs)with the layered structure.Here,based on the transport and magnetic torque measurements,we experimentally investigate the vortex pinning in two bilayer FeSCs,CaKFe₄As₄(Fe1144)and KCa₂Fe₄As4F₂(Fe12442),and compare their superconducting anisotropyγ.While the anisotropyγ≈3 for Fe1144 is much smaller thanγ≈15 in Fe12442 around Tc,a higher flux pinning energy as evidenced by a higher critical current density is found in Fe1144,as compared with the case of Fe12442.In combination with the literature data of Ba_0.72K_0.28Fe2As₂ and Nd Fe As_O0.82F_0.18,we reveal an anti-correlation between the pinning energy and the superconducting anisotropy in these Fe SCs.Our results thus suggest that the interlayer interaction can not be neglected when considering the vortex pinning in Fe SCs.     

Antiferromagnetic Ordering in TmCu2

Intermetallic compound TmCu₂ crystalizes in the orthorhombic structure, orders antiferromagnetically (AF) and exhibits four different AF phases below T _N = 6.5 K. In all these phases the Tm magnetic moment is parallel to the b-axis. The ground-state AF1 (T < 3.4 K) corresponds to the squared-up AF structure with propagation vector ₀₁ = (5/8, 0, 0) and magnetic unit cell 8a × b × c. Narrow phases AF2 and AF3 (3.3 K < T < 3.7 K and 3.6 K < T < 4.6 K) are 'spin-slip'-derivatives from the AF1 structure, yielding propagation vectors ₀₂ = (0.633, 0, 0.02) and ₀₃ = (0.618, 0, 0), respectively. The phase AF4 (4.4 K < T < T _N) is then incommensurate transversal spin-wave with the propagation ₀₄ = (0.64, 0, 0). The co-existence of AF phases indicates the first-order transitions between them.     

Magnetic Relaxation Coupling in Heterogeneous Systems

The effects of magnetization transfer hold considerable potential for providing new information in a magnetic resonance image. Recent applications and discussions, however, have neglected to utilize the full potential of the experiment by ignoring its sensitivity to tissue water content and to the nature of the macromolecular components. The magnetization-transfer experiment where one spin population is irradiated with a preparation pulse and its effects are detected in the solvent resonance is examined in the context of the simplified equations of Grad and Bryant. Specific effects on the magnetic relaxation properties of the solids and on the water content are examined. Recent developments in modeling such systems are discussed, and it is demonstrated that reports of magnetization-transfer rates may be underestimated in some cases. Complications of mobility within the solid macromolecular phase of the system are also addressed in the context of the currently employed model that assumes heterogeneous systems may be partitioned into two or more motionally distinct proton populations.     

一维钻石链反铁磁Ising模型磁化的模拟

采用Monte Carlo模拟方法对自旋为1/2的一维钻石链反铁磁Ising系统的磁化行为进行研究.在这个系统中,反铁磁的交换作用和三角结构导致存在自旋阻挫.重点研究不同的反铁磁自旋交换作用对系统磁行为和自旋构型的影响.模拟磁化曲线中M=M_S/3磁化平台,得到平台宽度随不同的自旋交换相互作用强弱的变化关系,以及出现磁化平台时格点的自旋构型;给出亚稳态存在的条件及亚稳态时的微观构型.研究磁滞回线随温度的变化关系.结果表明,随着温度的升高,磁滞回线逐渐减小,最终消失.     

NMR and NQR studies on transition-metal arsenide superconductors LaRu_2As_2,KCa_2Fe_4As_4F_2,and A_2Cr_3As_3

We report ~(75)As-nuclear magnetic resonance(NMR) and nuclear quadrupole resonance(NQR) measurements on transition-metal arsenides LaRu_2As_2, KCa_2Fe_4As_4F_2, and A_2Cr_3As_3. In the superconducting state of La Ru_2As_2, a Hebel–Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T1 just below Tc, which indicates that LaRu_2As_2 is a full-gap superperconducor. For KCa_2Fe_4As_4F_2, antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate RAF= Tc1/Tab1 is small and temperature independent,implying that the low energy spin fluctuations are isotropic in spin space. Our results indicate that KCa_2Fe_4As_4F_2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition. For A_2Cr_3As_3(A = Na, K, Rb, Cs), we calculate the electric field gradient by first-principle method and assign the ~(75)As-NQR peaks to two crystallographically different As sites, paving the way for further NMR investigation.     

Tip-Pressure-Induced Incoherent Energy Gap in CaFe_2As_2

In CaFe_2As_2,superconductivity can be achieved by applying a modest c-axis pressure of several kbar.Here we use scanning tunneling microscopy/spectroscopy(STM/S) to explore the STM tip pressure effect on single crystals of CaFe_2As_2.When performing STM/S measurements,the tip-sample interaction can be controlled to act repulsive with reduction of the junction resistance,thus to apply a tip pressure on the sample.We End that an incoherent energy gap emerges at the Fermi level in the differential conductance spectrum when the tip pressure is increased.This energy gap is of the similar order of magnitude as the superconducting gap in the chemical doped compound Ca_(0.4)Na_(0.6)Fe_2As_2 and disappears at the temperature well below that of the bulk magnetic ordering.Moreover,we also observe the rhombic distortion of the As lattice,which agrees with the orthorhombic distortion of the underlying Fe lattice.These Endings suggest that the STM tip pressure can induce the local Cooper pairing in the orthorhombic phase of CaFe_2As_2.     

Soliton Lattice State of Spin—1／2 Antiferromagnetic Chain in an External Magnetic Field

One-dimensional spin-1-2 anisotropic Heisenberg antiferromagnet in a longitudinal external magnetic field is studied using bosonization method and Gaussian wave functional techniques which take account of the spatial structure.The magnetization and the energy of the ground state which depend on the external magnetic field are calculated.For the case of anisotropic parameter △＞△0,increasing of the external magnetic field above the threshold value leads to the appearance of the soliton lattice state in the ground state,until to an another critical field where the ground state changes to the canted state phase.Therefore,with increasing external magnetic field,the ground state experiences four different phases successively,namely,antiferromagnetic Ising,soliton lattice state canted state,and magnetization saturated phases.When △＜△0,the soliton lattice state phase does not appear,with increasing external field,the paramagnetic phase smoothly evolves into the canted state phase,finally reaches magnetization saturated phase.     

Superconductivity in Undoped CaFe_2As_2 Single Crystals

Single crystals of undoped CaFe_2As_2 are grown by an FeAs self-flux method,and the crystals are quenched in ice-water rapidly after high-temperature growth.The quenched crystal undergoes a collapsed tetragonal structural phase transition around 80 K revealed by the temperature-dependent x-ray diffraction measurements.Superconductivity below 25 K is observed in the collapsed phase by resistivity and magnetization measurements.The isothermal magnetization curve measured at 2K indicates that this is a typical type-Ⅱ superconductor.For comparison,we systematically characterize the properties of the furnace-cooled,quenched,and post-annealed single crystals,and find strong internal crystallographic strain existing in the quenched samples,which is the key for the occurrence of superconductivity in the undoped CaFe_2As_2 single crystals.     

Magnetic Skyrmions and Phase Transitions in Antiferromagnetic/Ferroelectric Bilayers

JETP Letters - We study in this paper the ground state and the properties of a skyrmions in magnetoelectric films, namely antiferromagnetic/ferroelectric superlattices in a support by steepest...     

Magnetic Properties of XXZ Heisenberg Antiferromagnetic and Ferrimagnetic Nanotubes

The spin-1/2 antiferromagnetic and spin-(1/2, 1) ferrimagnetic single-walled nanotubes are described by XXZ Heisenberg model. The sublattice magnetization and the critical temperature of the system are calculated by using the double-time spin Green's function method. At zero temperature, with the increase of the exchange interaction in the circumferential direction, a maximum value appears in the sublattice magnetization curves of antiferromagnetic and ferrimagnetic systems. As the diameter of the tube increases, the spin quantum fluctuations and thermal fluctuations are suppressed. In addition, the spin quantum fluctuation of the spin-1/2 antiferromagnetic system is greater than that of the spin-(1/2, 1) ferrimagnetic system. The critical temperature of the system increases firstly and then tends to a constant with the increase of the diameter of tube, and it decreases to zero as the exchange anisotropy of the system disappears.     

Universal Long-Time Relaxation on Lattices of Classical Spins: Markovian Behavior on Non-Markovian Timescales

The long-time behavior of certain fast-decaying infinite temperature correlation functions on one-, two-, and three-dimensional lattices of classical spins with various kinds of nearest-neighbor interactions is studied numerically, and evidence is presented that the functional form of this behavior is either simple exponential or exponential multiplied by cosine. Due to the fast characteristic timescale of the long-time decay, such a universality cannot be explained on the basis of conventional Markovian assumptions. It is suggested that this behavior is related to the chaotic properties of the spin dynamics.     

Superconductivity in Sm-doped CaFe_2As_2 single crystals

In this article,the Sm-doping single crystals Ca_(1-x)Sm_xFe_2As_2(x = 0 ~0.2) were prepared by the Ca As flux method,and followed by a rapid quenching treatment after the high temperature growth.The samples were characterized by structural,resistive,and magnetic measurements.The successful Sm-substitution was revealed by the reduction of the lattice parameter c,due to the smaller ionic radius of Sm~(3+)than Ca~(2+).Superconductivity was observed in all samples with onset T_c varying from 27 K to 44 K upon Sm-doping.The coexistence of a collapsed phase transition and the superconducting transition was found for the lower Sm-doping samples.Zero resistivity and substantial superconducting volume fraction only happen in higher Sm-doping crystals with the nominal x 0.10.The doping dependences of the c-axis length and onset T_c were summarized.The high-T_c observed in these quenched crystals may be attributed to simultaneous tuning of electron carriers doping and strain effect caused by lattice reduction of Sm-substitution.