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 R■m and Er~(3+) 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 e V, which is the smallest among all the reported members in the family. This immediately invokes the interest towards metallization even superconductivity using the compound.     

Frustration effect and possibility of spin quantum liquid state in a tetrahedral spin ½ molecule

In this work, we studied the magnetic properties of a regularly tetrahedral molecule made of four spin-½, interconnected by exchange. For this purpose, we used the Heisenberg model and performed an exact resolution. In the case of a ferromagnetic coupling among the spins, the system orders itself under a magnetic field without displaying any net spontaneous magnetization. It behaves as a high spin-2 unique magnet.Under antiferromagnetic exchange, some exotic behaviour has been revealed due to the frustration governing the spin edifice. Thus, the magnetization is quantized into plateaus, which are separated by quantum phase transitions. In particular, we have argued the possibility of a quantum spin liquid (QSL) state that can occur at low temperature.This contribution intends mainly to bring single-molecular magnets to the front line of innovative nanoscale applications.     

Mott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe_2

The Mott transition is one of the fundamental issues in condensed matter physics,especially in the system with antiferromagnetic long-range order.However,such a transition is rare in quantum spin liquid(QSL) systems without long-range order.Here we report the experimental pressure-induced insulator to metal transition followed by the emergence of superconductivity in the QSL candidate NaYbSe_2 with a triangular lattice of 4 f Yb~(3+) ions.Detail analysis of transport properties in metallic state shows an evolution from non-Fermi liquid to Fermi liquid behavior when approaching the vicinity of superconductivity.An irreversible structure phase transition occurs around 11 GPa,which is revealed by the x-ray diffraction.These results shed light on the Mott transition in the QSL systems.     

Understanding spin configuration in the geometrically frustrated magnet TbB4: A resonant soft X-ray scattering study

The frustrated magnet has been regarded as a system that could be a promising host material for the quantum spin liquid (QSL). However, it is difficult to determine the spin configuration and the corresponding mechanism in this system, because of its geometrical frustration (i.e., crystal structure and symmetry). Herein, we systematically investigate one of the geometrically frustrated magnets, the TbB₄ compound. Using resonant soft x-ray scattering (RSXS), we explored its spin configuration, as well as Tb's quadrupole. Comprehensive evaluations of the temperature and photon energy/polarization dependences of the RSXS signals reveal the mechanism of spin reorientation upon cooling down, which is the sophisticated interplay between the Tb spin and the crystal symmetry rather than its orbit (quadrupole). Our results and their implications would further shed a light on the search for possible realization of QSL.     

Spin liquid state in an organic Mott insulator with a triangular lattice

1H NMR and static susceptibility measurements have been performed in an organic Mott insulator with a nearly isotropic triangular lattice, kappa-(BEDT-TTF)2Cu2(CN)(3), which is a model system of frustrated quantum spins. The static susceptibility is described by the spin S=1/2 antiferromagnetic triangular-lattice Heisenberg model with the exchange constant J approximately 250 K. Regardless of the large magnetic interactions, the 1H NMR spectra show no indication of long-range magnetic ordering down to 32 mK, which is 4 orders of magnitude smaller than J. These results suggest that a quantum spin liquid state is realized in the close proximity of the superconducting state appearing under pressure.     

From Claringbullite to a New Spin Liquid Candidate Cu_3Zn(OH)_6FCl

The search for quantum spin liquid(QSL) materials has attracted significant attention in the field of condensed matter physics in recent years, however so far only a handful of them are considered as candidates hosting QSL ground state. Owning to their geometrically frustrated structures, Kagome materials are ideal systems to realize QSL. We synthesize the kagome structured material claringbullite(Cu_4(OH)_6FCl) and then replace inter-layer Cu with Zn to form Cu_3Zn(OH)_6FCl. Comprehensive measurements reveal that doping Zn~(2+) ions transforms magnetically ordered Cu_4(OH)_6FCl into a non-magnetic QSL candidate Cu_3Zn(OH)_6FCl. Therefore,the successful syntheses of Cu_4(OH)_6FCl and Cu_3Zn(OH)_6FCl provide not only a new platform for the study of QSL but also a novel pathway of investigating the transition between QSL and magnetically ordered systems.     

量子顺电EuTiO_3材料基态磁性的第一性原理研究

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。     

Magnetic anisotropy,exchange coupling and Dzyaloshinskii–Moriya interaction of two-dimensional magnets

The two-dimensional (2D) magnets provide novel opportunities for understanding magnetism and investigating spin related phenomena in several atomic thickness. Multiple features of 2D magnets, such as critical temperatures, magnetoelectric/magneto-optic responses, and spin configurations, depend on the basic magnetic terms that describe various spins interactions and cooperatively determine the spin Hamiltonian of studied systems. In this review, we present a comprehensive survey of three types of basic terms, including magnetic anisotropy that is intimately related with long-range magnetic order, exchange coupling that normally dominates the spin interactions, and Dzyaloshinskii−Moriya interaction (DMI) that favors the noncollinear spin configurations, from the theoretical aspect. We introduce not only the physical features and origin of these crucial terms in 2D magnets but also many correlated phenomena, which may lead to the advance of 2D spintronics.     

Anisotropic nanocomposite soft/hard multilayer magnets

Experimental and theoretical researches on nanostructured exchange coupled magnets have been carried out since about 1988. Here, we review the structure and magnetic properties of the anisotropic nanocomposite soft/hard multilayer magnets including some new results and phenomena from an experimental point of view. According to the different component of the oriented hard phase in the nanocomposite soft/hard multilayer magnets, three types of magnets will be discussed:1) anisotropic Nd_2Fe_(14)B based nanocomposite multilayer magnets, 2) anisotropic SmCo_5 based nanocomposite multilayer magnets, and 3) anisotropic rare-earth free based nanocomposite multilayer magnets. For each of them, the formation of the oriented hard phase, exchange coupling, coercivity mechanism, and magnetic properties of the corresponding anisotropic nanocomposite multilayer magnets are briefly reviewed, and then the prospect of realization of bulk magnets on new results of anisotropic nanocomposite multilayer magnets will be carried out.     

量子自旋液体候选材料的缪子自旋弛豫研究

量子自旋液体是一种新奇的磁性物态。由于极强的量子涨落，直至零温都不会出现长程序。量子自旋液体的基态不能用序参量描述，并且缺少对称性破缺，因此该物态的实现打破朗道理论的范式。对于量子自旋液体的研究有助于理解高温超导的机理，并且可以被应用在量子计算和量子信息中。目前，尽管理论上有了长足的发展，但仍旧没有任何一个材料被证实为量子自旋液体。因此，探测和确认一个真正的量子自旋液体材料是当前的研究重点。缪子自旋弛豫是一个对磁场极为敏感的实验技术，被广泛应用于量子自旋液体候选材料的研究中。该技术可以观测基态中是否存在磁有序，测量系统中的涨落频率，这两点都是表征量子自旋液体的重要性质。本文简要介绍了量子自旋液体态和缪子自旋弛豫技术，回顾了近期在不同体系的量子自旋液体候选材料中的实验结果，特别是缪子自旋弛豫的成果。这些体系包括一维反铁磁海森堡链（苯甲酸铜），三角格子（YbMgGaO4，NaYbO2 和TbInO3），笼目格[ZnCu3(OH)6Cl2 和 m3Sb3Zn2O14]，蜂窝状格子（Na2IrO3 和 α-RuCl3），以及烧绿石结构（Tb2Ti2O7，Pr2Ir2O7 和Ce2Zr2O7）。     

Quantum Speed Limit for Dynamics of Central Spin Model with Initial System-Bath Correlations

We investigate the quantum speed limit (QSL) time of an electronic spin coupled to a bath of nuclear spins. We consider three types of initial states with different correlations between the system and bath, i.e., quantum correlation, classical correlation, and no any correlation. Interestingly, we show that the QSL times of the central spin for these three types of initial correlations are identical when the couplings are homogeneous. However, it is remarkable different for inhomogenous couplings. The QSL time of the central spin is sensitive to the initial states, the average coupling strength, the distribution of the couplings between the system and bath and the number of the nuclear spins in the bath. Furthermore, we find that the coherence in the initial state has significant influences on the QSL time of the system, and can lead to the increase of QSL time for homogeneous couplings.     

Quantum ice: a quantum Monte Carlo study

Ice states, in which frustrated interactions lead to a macroscopic ground-state degeneracy, occur in water ice, in problems of frustrated charge order on the pyrochlore lattice, and in the family of rare-earth magnets collectively known as spin ice. Of particular interest at the moment are "quantum spin-ice" materials, where large quantum fluctuations may permit tunnelling between a macroscopic number of different classical ground states. Here we use zero-temperature quantum Monte Carlo simulations to show how such tunnelling can lift the degeneracy of a spin or charge ice, stabilizing a unique "quantum-ice" ground state-a quantum liquid with excitations described by the Maxwell action of (3+1)-dimensional quantum electrodynamics. We further identify a competing ordered squiggle state, and show how both squiggle and quantum-ice states might be distinguished in neutron scattering experiments on a spin-ice material.     

Magnetic structure of geometrically frustrated compound Co2Cl(OH)3 determined by proton NMR

The magnetic structure of a geometrically frustrated system Co₂Cl(OH)₃ is determined by comparing the observed proton NMR spectrum with numerical calculations based on various magnetic models. The best fit is obtained with a model that the magnetic moments of Co²⁺ ions in the triangular plane are parallel to the principal axis of local crystal field and those of Co²⁺ ions in the kagome lattice plane are randomly disordered in the a-b plane, which nearly bisects the angle between the principal axis of the local field and a line pointing towards the body center of the tetrahedron. The coexistence of the ferromagnetic order in the triangular plane and the random disorder in the kagome plane is consistent with the results of measurements by Zheng et al. However, the magnetic moments of Co²⁺ ions are not directed towards the body center of the tetrahedron as characteristic in the “spin ice” magnetic structure. Furthermore, the Co²⁺ ions in the triangular plane have a smaller magnitude of magnetic moment than those in the kagome plane. Thus, our result suggests that the transition metal compound Co₂Cl(OH)₃ is different from the “spin ice” in magnetic structure, although it is similar to rare-earth pyrochlores in crystal structure.     

Thermal control magnetic switching dominated by spin reorientation transition in Mn-doped PrFeO3 single crystals

Spin reorientation transition (SRT) has attracted substantial attention due to its important role in the ultrafast control of spins. However, the transition temperature is usually too low for its practical applications. Here, we demonstrate the ability to modulate the SRT temperature in PrFe_1−xMnxO₃ single crystals from 196 K to 317 K across the room temperature by varying the Mn concentration. Interestingly, the Γ₄ to Γ₁ spin reorientation of the Mn-doped PrFeO₃ is distinct from the Γ₄ to Γ₂ spin reorientation transition as in the parent material. Because of the coupling between rare-earth ions and transition-metal ions in determining the SRT temperature, the demonstrated control scheme of spin reorientation transition temperature by Mn-doping is expected to be used in temperature control magnetic switching devices and applicable to many other rare-earth orthoferrites.     

Simulation of magnetic properties of the two-dimensional magnetic with anisotropic antiferromagnetic interactions and cluster ordering by quantum Monte Carlo

Magnetic with anisotropic antiferromagnetic exchange interactions and special topology of coupling in the square lattice with spins pairs ordering is studied by quantum Monte Carlo method. The antiferromagnetic order is found to be more stable as compared to spin liquid state. Exchange interactions and wave vector of structure modulation for Cu₃B₂O₆ is estimated. Neel temperature versus strength of exchange in spin pair is calculated. Plateau and modulation of magnetic structure in field magnetization dependence is revealed.     

Origin of Anisotropy in Gadolinium Crystal Using a New Spin Hamiltonian

Single crystal rare-earth magnets,such as hexagonal-close-packed gadolinium,usually have a large second order anisotropy K_2 and a negative first order anisotropy K_1 at low temperatures,which are difficult to explain using microscopic theories.An atomic scale effective spin Hamiltonian F|{S_i}] is proposed,which,apart from the usual isotropic nearest neighbor coupling J,consists of two new terms that are different for in-plane and out-of-plane neighbors and which are characterized by two new couplings C_1 and C_2,respectively.The hybrid Monte-Carlo method is utilized to sample this system to the desired Boltzmann-like distribution exp(-F/k_BT).It is found that K_2 and K_1 are compatible with the experimental values and arise naturally from the exchange anisotropy C_1 and C_2,which are less than 0.01% in magnitude of the isotropic exchange energy J.This new model spin Hamiltonian can also be applied to study other magnetic properties.     

Spin relaxation of Mn ions in (CdMn)Te/(CdMg)Te quantum wells under picosecond optical pumping

Spin relaxation of Mn ions in a Cd_0.97Mn_0.03Te/Cd_0.75Mg_0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (_B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively. Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997)     

Structure and frustrated magnetism of the two-dimensional triangular lattice antiferromagnet Na2BaNi(PO4)2

A new frustrated triangular lattice antiferromagnet Na₂BaNi(PO₄)₂ was synthesized by high temperature flux method. The two-dimensional triangular lattice is formed by the Ni²⁺ ions with S =1. Its magnetism is highly anisotropic with the Weiss constants θ_CW =-6.615 K (H⊥c) and -43.979 K (H||c). However, no magnetic ordering is present down to 0.3 K, reflecting strong geometric spin frustration. Our heat capacity measurements show substantial residual magnetic entropy existing below 0.3 K at zero field, implying the presence of low energy spin excitations. These results indicate that Na₂BaNi(PO₄)₂ is a potential spin liquid candidate with spin-1.     

Algebraic vortex liquid in spin-1/2 triangular antiferromagnets: scenario for Cs2CuCl4

Motivated by inelastic neutron scattering data on Cs2CuCl4, we explore spin-1/2 triangular lattice antiferromagnets with both spatial and easy-plane exchange anisotropies, the latter due to an observed Dzyaloshinskii-Moriya interaction. Exploiting a duality mapping followed by a fermionization of the dual vortex degrees of freedom, we find a novel critical spin-liquid phase described in terms of Dirac fermions with an emergent global SU(4) symmetry minimally coupled to a noncompact U(1) gauge field. This "algebraic vortex liquid" supports gapless spin excitations and universal power-law correlations in the dynamical spin structure factor which are consistent with those observed in Cs2CuCl4. We suggest future neutron scattering experiments that should help distinguish between the algebraic vortex liquid and other spin liquids and quantum critical points previously proposed in the context of Cs2CuCl4.