We present a study of the magnetic susceptibility $χ_{mol}$ under variable hydrostatic pressure on single crystals of Cs$_2$CuCl$_{4-x}$Br$_x$. This includes the border compounds \textit{x} = 0 and 4, known as good realizations of the distorted triangular-lattice spin-1/2 Heisenberg antiferromagnet, as well as the isostructural stoichiometric systems Cs$_2$CuCl$_{3}$Br$_1$ and Cs$_2$CuCl$_{2}$Br$_2$. For the determination of the exchange coupling constants $J$ and $J^{\prime}$, $χ_{mol}$ data were fitted by a $J-J^{\prime}$ model \cite{Schmidt2015}. Its application, validated for the border compounds, yields a degree of frustration $J^{\prime}$/$J$ = 0.47 for Cs$_2$CuCl$_3$Br$_1$ and $J^{\prime}$/$J$ $\simeq$ 0.63 - 0.78 for Cs$_2$CuCl$_2$Br$_2$, making these systems particular interesting representatives of this family. From the evolution of the magnetic susceptibility under pressure up to about 0.4\,GPa, the maximum pressure applied, two observations were made for all the compounds investigated here. First, we find that the overall energy scale, given by $J_c = (J^2$ + $J^{\prime 2}$)$^{1/2}$, increases under pressure, whereas the ratio $J^{\prime}$/$J$ remains unchanged in this pressure range. These experimental observations are in accordance with the results of DFT calculations performed for these materials. Secondly, for the magnetoelastic coupling constants, extraordinarily small values are obtained. We assign these observations to a structural peculiarity of this class of materials.
We report on low-temperature heat-transport properties of the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuCl$_4$. Broad maxima in the thermal conductivity along the three principal axes, observed at about 5 K, are interpreted in terms of the Debye model, including the phonon Umklapp scattering. For thermal transport along the $b$ axis, we observed a pronounced field-dependent anomaly, close to the transition into the three-dimensional long-range-ordered state. No such anomalies were found for the transport along the $a$ and $c$ directions. We argue that this anisotropic behavior is rel…
We report an experimental and theoretical study of the low-temperature specific heat $C$ and magnetic susceptibility $χ$ of the layered anisotropic triangular-lattice spin-1/2 Heisenberg antiferromagnets Cs$_2$CuCl$_{4-x}$Br$_x$ with $x$ = 0, 1, 2, and 4. We find that the ratio $J'/J$ of the exchange couplings ranges from 0.32 to $\approx 0.78$, implying a change (crossover or quantum phase transition) in the materials' magnetic properties from one-dimensional (1D) behavior for $J'/J < 0.6$ to two-dimensional (2D) behavior for $J'/J \approx 0.78$ behavior. For $J'/J < 0.6$, realized for $x$ = …
Hard-core boson approach to the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuCl$_4$ at finite temperatures in magnetic fields higher than the saturation field
We investigate the electronic structure of Cs$_2$CuCl$_4$, a material discussed in the framework of a frustrated quantum antiferromagnet, by means of resonant inelastic x-ray scattering (RIXS) and Density Functional Theory (DFT). From the non-dispersive highly localized dd excitations, we resolve the crystal field splitting of the Cu$^{2+}$ ions in a strongly distorted tetrahedral coordination. This allows us to model the RIXS spectrum within the Crystal Field Theory (CFT), assign the dd orbital excitations and retrieve experimentally the values of the crystal field splitting parameters D$_q$,…
Quantum triangular-lattice antiferromagnets are important prototype systems to investigate phenomena of the geometrical frustration in condensed matter. Apart from highly unusual magnetic properties, they possess a rich phase diagram (ranging from an unfrustrated square lattice to a quantum spin liquid), yet to be confirmed experimentally. One major obstacle in this area of research is the lack of materials with appropriate (ideally tuned) magnetic parameters. Using Cs$_2$CuCl$_4$ as a model system, we demonstrate an alternative approach, where, instead of the chemical composition, the spin Ha…
The quasi-two-dimensional Heisenberg spin $S=1/2$ dimer system bis(2-amino-5-fluoro-pyridinium) tetrachlorocuprate(II) is studied by means of inelastic neutron scattering, calorimetry and nuclear magnetic resonance (NMR) experiments. In the absence of an applied magnetic field we find dispersive triplet excitations with a spin gap of $Δ=1.112(15)$ meV and a bandwidth of $0.715(15)$ meV within the layers and $0.116(15)$ meV between the layers, respectively. In an applied magnetic field of $μ_0H_c\approx 8.5$ T the spin gap is closed and we find a field induced antiferromagnetically ordered phas…
The objective of the present work was to analyze the possibility of realization of quantum spin liquid in three volcanic minerals - averievite (Cu5O2(VO4)2(CuCl)), ilinskite (NaCu5O2(SeO3)2Cl3), and avdononite (K2Cu5Cl8(OH)4 2H2O - from the crystal chemistry point of view. Based on the structural data, the sign and strength of magnetic interactions have been calculated and the geometric frustrations serving as the main reason of the existence of spin liquids have been investigated. According to our calculations, the magnetic structures of averievite and ilinskite are composed of antiferromagne…
Large-N solutions of the Heisenberg and Hubbard-Heisenberg models on the anisotropic triangular lattice: application to Cs$_2$CuCl$_4$ and to the layered organic superconductors $κ$-(BEDT-TTF)$_2$X
Atomic and magnetic structures of (CuCl)LaNb$_2$O$_7$ and (CuBr)LaNb$_2$O$_7$: Density functional calculations
用密度泛函方法研究了（Cu $X$）LaNb $_2$ O $_7$（$X$ =Cl和Br）的原子结构和磁性.在几十个被检查的结构中，正交畸变的$2\times 2$结构，其中$X$卤素的位移模式类似于先前基于经验信息所描述的模型，被确定为最稳定的一个。$X$卤素的置换与Cu离子的置换一起导致在两种材料中形成$X$ -Cu-$X$-Cu-$X$之字形链。Z字形链中的最近邻相互作用对（CuCl）LaNb $_2$ O $_7$是反铁磁的（AFM），而对（CuBr）LaNb $_2$ O $_7$是铁磁的（FM）.另一方面，前两个相邻的铜阳离子之间的相互作用从相邻的链被发现是AFM和FM分别为这两个化合物。在（CuBr）LaNb $_2$ O $_7$中所有这些面内交换耦合的大小被估计为在（CuCl）LaNb $_2$ O $_7$中的那些的大约三倍。此外，一个相当大的AFM面间相互作用被发现由两个NbO $_6$八面体分离的Cu离子之间。本研究强烈地表明，为了正确地解释（Cu $X)$ LaNb $_2$ O $_7$）的磁性，有必要超越正方形$J_1-J_2$模型。
$μ$SR studies of the frustrated quasi-2d square-lattice spin system Cu(Cl,Br)La(Nb,Ta)$_{2}$O$_{7}$: evolution from spin-gap to antiferromagnetic state
我们报道了μ子自旋弛豫（$μ$ SR）和对Cu（Cl，Br）La（Nb，Ta）$_{2}$ O $_{7}$的磁化率测量，其证明：（a）在低至15 mK的（CuCl）LaNb $_{2}$ O $_{7}$中不存在静态磁性，证实了自旋能隙基态;（b）在（CuCl）La（Nb，Ta）$_{2}$ O $_{7}$中具有自旋能隙和静态磁性的部分体积之间的相分离;（c）（Nb，Ta）和（Cl，Br）替代系统中的历史相关磁化;（d）（CuBr）LaNb $_{2}$ O $_{7}$中的均匀长程共线反铁磁态;以及（e）Cu（Cl $_{1-x}$ Br $_{x}$）LaNb $_{2}$ O $_{7}$中的Néel温度随Br浓度的降低而降低，而$0.33 \leq x \leq 1$的有序Cu矩大小没有变化。与其他几个$μ$ SR研究的量子相变的几何挫折自旋系统，目前的结果表明，从一个自旋能隙到磁有序状态的演变往往与相分离和/或一级相变。
Magnetoresistivity in the Antiferromagnetic Hubbard Model
Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales（布宜诺斯艾利斯大学，理学院）；Université Paris-Saclay（巴黎萨克雷大学）；CNRS Laboratoire de Physique des Solides（法国国家科学研究中心，固体物理实验室）；Université Paris-Cité（巴黎西岱大学）；CNRS Integrative Neuroscience and Cognition Center（法国国家科学研究中心，整合神经科学与认知中心）；CONICET-Universidad de Buenos Aires, Instituto de Física de Buenos Aires (IFIBA)（阿根廷国家科学研究委员会-布宜诺斯艾利斯大学，布宜诺斯艾利斯物理研究所）
Gigantic magnetoelectric effect caused by magnetic-field-induced canted antiferromagnetic-paramagnetic transition in quasi-two-dimensional Ca$_2$CoSi$_2$O$_7$ crystal
研究了Ca $_2$ CoSi $_2$ O $_7$晶体的磁性和介电性能。Ca $_2$ CoSi $_2$ O $_7$的介电性和磁性在倾斜反铁磁转变温度（$T_{\rm N}$）以下强烈耦合：磁场在低于$T_{\rm N}$的温度下诱导电极化的强度。有趣的是，即使没有极化电场，也能检测到磁场诱导的电极化的强度。在$T_{\rm N}$以下，由磁场诱导倾斜的反铁磁-顺磁转变。在$T_{\rm N}$周围观察到大的磁电容。大的磁电容是由磁场引起的反铁磁-顺磁转变引起的。
The helimagnetic order describes a non-collinear spin texture of antiferromagnets, arising from competing exchange interactions. Although collinear antiferromagnets are elemental building blocks of antiferromagnetic (AFM) spintronics, the potential of implementing spintronic functionality in non-collinear antiferromagnets has not been clarified thus far. Here, we propose an AFM helimagnet of EuCo2As2 as a novel single-phase spintronic material that exhibits a remarkable sign reversal of anisotropic magnetoresistance (AMR). The contrast in the AMR arises from two electrically distinctive magnet…
The magnetic $H$-$T$ phase diagram and magnetocaloric effect in the recently discovered high-temperature heavy-fermion compound YbCoC$_2$ have been studied. With the increase in the external magnetic field YbCoC$_2$ experiences the metamagnetic transition and then transition to the ferromagnetic state. The dependencies of magnetic entropy change -$ΔS_m (T)$ have segments with positive and negative magnetocaloric effects for $ΔH \leq 6$~T. For $ΔH = 9$~T magnetocaloric effect becomes positive with a maximum value of -$ΔS_m (T)$ is 4.1 J / kg K and a refrigerant capacity is 56.6 J / kg.
We study the anisotropic quantum Heisenberg antiferromagnet for spin-1/2 that interpolates smoothly between the one-dimensional (1D) and the two-dimensional (2D) limits. Using the spin Hartree-Fock approach we construct a quantitative theory of heat capacity in the quasi-1D regime with a finite coupling between spin chains. This theory reproduces closely the exact result of Bethe Ansatz in the 1D limit and does not produces any spurious phase transitions for any anisotropy in the quasi-1D regime at finite temperatures in agreement with the Mermin-Wagner theorem. We study the static spin-spin c…
The electrical resistance, Hall resistance and thermoelectric power of the Ising-like antiferromagnet UIrSi3 were measured as functions of temperature and magnetic field. We have observed that the unequivocally different characters of first-order and second-order magnetic phase transitions lead to distinctly different magnetotransport properties in the neighborhood of corresponding critical temperatures and magnetic fields, respectively. The magnetic contributions to the electrical and Hall resistivity in the antiferromagnetic state, and the polarized and normal regimes of paramagnetic state a…
We apply torque equilibrium spin wave theory (TESWT) to investigate an anisotropic XXZ antiferromagnetic model with Dzyaloshinskii-Moriya (DM) interaction in a triangular lattice. Considering the quasiparticle vacuum as our reference, we provide an accurate analysis of the non-collinear ground state of a frustrated triangular lattice magnet using the TESWT formalism. We elucidate the effects of quantum fluctuations on the ordering wave vector based on model system parameters. We study the single magnon dispersion, the two-magnon continuum using the spectral function, and the Raman spectrum of …
Short-range order of Br and three-dimensional magnetism in (CuBr)LaNb2O7
我们提出了一个全面的研究（CuBr）LaNb 2 O 7的晶体结构，磁结构和微观磁模型，Br模拟的自旋能隙量子磁体（CuCl）LaNb 2 O 7。尽管晶体结构和自旋晶格相似，但Cl和Br化合物的磁性行为甚至原子排列的特殊性都非常不同。高分辨率的X射线和中子数据揭示了Br原子在（CuBr）LaNb 2 O 7中的分裂位置。这种分裂起源于两种可能的配置开发的[CuBr]锯齿带。虽然Br原子在'ab'平面中局部有序，但它们沿'c'方向的排列仍然部分无序。占主导地位的和积极的更有利的配置功能的“c”晶格参数，没有观察到（CuCl）LaNb 2 O 7的额外的两倍。（CuBr）LaNb 2 O 7在TN=32 K时发生了长程反铁磁有序化，约为J 4 ~48 K时的70%。Br化合物并没有表现出任何低维磁性的实验特征，因为底层的自旋晶格是三维的。沿'c'方向的耦合与'ab'平面中的耦合相当，尽管沿'c'方向的最短Cu-Cu距离（11.69 A）比'ab'平面中的最近邻距离（3.55 A）大三倍。实验证实了条纹反铁磁长程序的特点列的平行自旋在'ab'平面和反平行自旋沿'c'的实验和微观分析证实。
The antiferromagnet Mn$_5$Si$_3$ has recently attracted attention because a noncollinear spin arrangement has been shown to produce a topological anomalous Hall effect and an inverse magnetocaloric effect. Here we synthesize single crystals of Mn$_5$Si$_3$ using flux growth. We determine the phase diagram through magnetization and measure the magnetoresistance and the Hall effect. We find the collinear and noncollinear antiferromagnetic phases at low temperatures and, in addition, a third magnetic phase, in between the two antiferromagnetic phases which has ferromagnetic character. The latter …
Antiferromagnetic spintronics actively introduces new principles of magnetic memory, in which the most fundamental spin-dependent phenomena, i.e. anisotropic magnetoresistance effects, are governed by an antiferromagnet instead of a ferromagnet. A general scenario of the antiferromagnetic anisotropic magnetoresistance effects mainly stems from the magnetocrystalline anisotropy related to spin-orbit coupling. Here we demonstrate magnetic field driven contour rotation of the fourfold anisotropic magnetoresistance in bare antiferromagnetic Sr2IrO4/SrTiO3 (001) thin films hosting a strong spin-orb…
We consider magnetostriction in magnetic materials. Starting from a microscopic lattice model of the magneto-elastic coupling, we derive the continuum magnetostriction Hamiltonian in cubic ferromagnets and antiferromagnets. In ferromagnets, our results agree with established results. In antiferromagnets, we find a new central contribution which reflects the spin ordering. The additional term is essential in certain antiferromagnets where magnetic moments order in antiparallel ferromagnetic planes characterized by a normal axis. In such antiferromagnets, we predict that the magnetostrictive str…
