Pulsed-Field Ultrasonic Experiments in the Quasi-2D Antiferromagnet Cs2CuBr4

The magnetic insulators Cs₂CuCl₄ and Cs₂CuBr₄ are model systems for a frustrated quasi-two-dimensional triangular-lattice spin-1/2 Heisenberg antiferromagnet with a weak interlayer coupling. The quasi-two-dimensional character manifests itself in a broad maximum at T _max in the magnetic susceptibility, which for Cs₂CuCl₄ has been found to mark the upper bound for a spin-liquid regime (R. Coldea et al. in Phys. Rev. B 68:134424, 2003). Motivated by the observation of characteristic B ² dependencies in the elastic constants and ultrasound attenuation in the spin-liquid regime for this material, we look for corresponding signatures in the related Cs₂CuBr₄ compound at low temperatures T≤4.2 K and fields B≤50 T. We observe a softening of the elastic constant up to B _s ～32 T (B∥a) and a maximum in the sound attenuation at 0.86 B _s . Both quantities exhibit the same characteristic B ² dependence as found for Cs₂CuCl₄, suggesting the existence of a spin-liquid phase in Cs₂CuBr₄ as well.     

Spin Properties of Electrons in Low-Dimensional SiGe Structures

Using ESR, we investigate g-factor and spin coherence time of electrons confined in 2D Si_1–xGe_x {channels} (x < 0.1) by barriers with x > 0.2 and in SiGe quantum dots grown on prepatterned Si substrates. The quantum wells exhibit 2D-anisotropy of both g and which can be explained in terms of the Bychkov–Rashba field. The latter increases with increasing Ge content in the well indicating that the increasing spin-orbit coupling is more important than interface properties. The narrow ESR permits selective spin manipulation already for x > 0.02. Large, regular arrays of Ge quantum dots (about 10⁹) were grown on prepatterned substrates. Strain in the Si capping layer lowers the conduction band relative to that of Ge causing confinement. The g-shift observed implies the possibility of g-tuning by confinement. The line width shows substantial inhomogeneous broadening whereas the longitudinal spin lifetime is hardly changed with respect to 2D structures.     

High-Field Magnetization of the Distorted Spin Frustration System Ni5(TeO3)4Br2

We performed high-field magnetization measurements, M(H), on the Ni₅(TeO₃)₄Br₂ by using a pulse magnet. The Ni²⁺ ions (S = 1) of this compound form a two dimensional distorted spin frustrated system (distorted kagome lattice), and undergo a Neel transition at T _N ～29 K, due to the anisotropy. Below T _N , a complex spin arrangement which contains ten spin sub-lattices was suggested by neutron scattering and electron spin resonance studies. We observed a step-like transition at H _c ～11 T when T<T _N . By contrast, at T>T _N , the field-dependent magnetization curves behaved like a straight line. The H _c is close to those obtained by previous spin resonance studies of Ni₅(TeO₃)₄Br₂ and Ni₅(TeO₃)₄Cl₂ in which a spin-flop-like transition was proposed to explain the field-dependent resonance spectra. Interestingly, the observed change in magnetization (ΔM) at H _c corresponds to nearly one-half of the magnetic moment of Ni²⁺ ion, suggesting that the observed step-like behavior possibly corresponds to a spin-flip-like transition (i.e. from antiferromagnetic state to ferrimagnetic state). In addition, in the high-field region (H>H _c ), the M(H) curves did not show plateau behavior, but exhibited the straight lines having finite slopes without any sign of saturation up to 55 T.     

Spin Properties of Electrons in Low-Dimensional SiGe Structures

Using ESR, we investigate g-factor and spin coherence time of electrons confined in 2D Si_1–xGe_x {channels} (x < 0.1) by barriers with x > 0.2 and in SiGe quantum dots grown on prepatterned Si substrates. The quantum wells exhibit 2D-anisotropy of both g and which can be explained in terms of the Bychkov–Rashba field. The latter increases with increasing Ge content in the well indicating that the increasing spin-orbit coupling is more important than interface properties. The narrow ESR permits selective spin manipulation already for x > 0.02. Large, regular arrays of Ge quantum dots (about 10⁹) were grown on prepatterned substrates. Strain in the Si capping layer lowers the conduction band relative to that of Ge causing confinement. The g-shift observed implies the possibility of g-tuning by confinement. The line width shows substantial inhomogeneous broadening whereas the longitudinal spin lifetime is hardly changed with respect to 2D structures.     

Low Temperature Behavior of a Two-Dimensional Quantum Antiferromagnet

We analyze the two-dimensional antiferromagnet with a quantum disordered ground state and a gap to bosonic excitations with finite spin. The zero temperature (T = 0) quantum phase transition is studied, and the finite temperature effects are also considered in the low temperature approximation of the Renormalization Group Method. The violation of the universality expected for d = 2 system has been shown to be logarithmic for T = 0 and T 0. We showed that chemical potential and the ground state energy depends on the interaction parameters at T = 0 and T 0. but this dependence is logarithmic.     

Filtering Spin with Tunnel-Coupled Hole Quantum Wires

Creating spin-polarized currents in nonmagnetic semiconductors is one of the key prerequisites for realizing spintronics devices. We have shown previously that the k-linear Rashba spin splitting present in two-dimensional (2D) electron systems can be utilized in a momentum-selective tunneling geometry to design a spin filter without using magnetic fields or ferromagnetic contacts. Motivated by the fact that spin–orbit effects are typically much stronger in 2D hole systems, we consider quantum wires formed by additional confinement of the lowest (heavy-hole) 2D valence subband. Its k ³-type Rashba term gives rise to a k-linear spin splitting for holes in the quantum wire. Implementation of the spin-filter design is then analogous to the electron case but, in the hole system, requires less momentum selectivity and should therefore be easier to realize.     

Excitation of Nonlinear Spin Oscillations in an Antiferromagnet under the Action of Pulsed Terahertz Pumping

Technical Physics Letters - We present a model of excitation of nonlinear spin oscillations in an antiferromagnet under the action of terahertz pulses of an electromagnetic field. It is shown that,...     

Evidence of Spin Frustration in a Vanadium Diselenide Monolayer Magnet

Monolayer VSe₂, featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition‐metal dichalcogenides (2D‐TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X‐ray and angle‐resolved photoemission, and X‐ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T‐phase and vanadium 3d¹ electronic configuration in monolayer VSe₂ grown on graphite by molecular‐beam epitaxy. Element‐specific X‐ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe₂ as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long‐range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic‐scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D‐TMDs in the search for exotic low‐dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.     

Quantum Monte Carlo Study of Inhomogeneous Bond Dilution in the Two-Dimensional Heisenberg Antiferromagnet

The problem whether a two-dimensional Heisenberg antiferromagnet on a square lattice can be driven through a quantum phase transition by either bond or site dilution has attracted a lot of recent interest. Such a diluted system is of direct relevance for antiferromagnetic layered cuprates compounds, doped with nonmagnetic impurities. Both experimental results and numerical analysis now give evidence that such systems with homogeneous site and bond dilution are driven through a classical percolation transition instead of a quantum phase transition. In this paper we show that inhomogeneous bond dilution introduces the different scenario of a percolative quantum phase transition. The ground state of the bond-diluted system can be a spin liquid characterized by an infinite percolating network with vanishing antiferromagnetic order parameter. This quantum disordered phase appears as an intermediate regime between the geometrically disordered phase and the antiferromagnetic ordered phase. Here we investigate this phenomenon using Stochastic Series Expansion Quantum Monte Carlo simulations.PACS numbers: 75.10.Jm, 75.10.Nr, 75.40.Cx, 64.60.Ak     

Tunable Spin Current Through a Vibrating Molecular Quantum Dot with Spin Bias and Spin Flip Scattering

By applying the Lang-Firsov canonical transformation and the Keldysh nonequilibrium Green’s function approach, the effect of the spin-flip scattering on the spin current through a vibrating molecular quantum dot with spin bias is theoretically investigated. We can obtain the spin current from the output terminal, and find that the sign of the spin current can be changed by adjusting the spin-flip strength, and a pure spin current can be generated via the charge bias and the spin bias. In the presence of the electron-phonon interaction, the positions of the current peaks are shifted and the spin current is remarkably suppressed, which leads to the Franck-Condon blockade. Furthermore, it is found that the competition between the EPI and the spin-flip scattering jointly determines the character of the spin current. These results offer us a way to manipulate the spin current in the spin current setup. The proposed device should be realizable with use of the present technology at low temperature.     

Spin Wave Resonance in the [(Co0.88Fe0.12)/Cu]N Synthetic Antiferromagnet

Technical Physics Letters - The [(Co0.88Fe0.12)/Cu]N synthetic antiferromagnet has been investigated by the spin wave resonance method over the entire range of angles of an external dc magnetic...     

Quantum Dot in the Pseudo-gap Sate with Spin–Orbit Interaction

We study the linear conductance in quantum dot with spin–orbit interaction coupled to Fermi liquid leads with a power-low density of states. The conductance at zero temperature is calculated as a function of the power exponent from the density of state ρ(ω)∼|ω−E _F | ^r at the Fermi energy E _F and the different energy rates. The phase shift of the conduction electrons is also r-dependent. The model can be used in the study of the quantum phase transition.     

Quantum Monte Carlo Study of Entanglement in Quantum Spin Systems

We perform an extensive quantum Monte Carlo investigation of entanglement properties in quantum spin systems close to or at a quantum critical point. Making use of the Stochastic Series Expansion method, we can systematically estimate the bipartite entanglement of the ground-state wavefunction in a large class of anisotropic spin models on unfrustrated lattices and in a uniform magnetic field. The behavior of the entanglement estimators as a function of the field shows remarkable universal features independent of the lattice dimensionality, marking both the occurrence of a field-induced quantum phase transition and of an exactly factorized state.PACS numbers: 03.67.Mn, 75.10.Jm, 73.43.Nq, 05.30.-d     

High-field ESR Studies of the Quantum Spin Dimer System Ba3Cr2O8

Results of systematic high-frequency electron spin resonance (ESR) studies of Ba₃Cr₂O₈, a weakly coupled $S=\frac{1}{2}$ dimer system, in magnetic fields up to 25 T are reported. Two pairs of ESR gapped modes corresponding to transitions from a spin-singlet ground state to the first excited triplet states with gaps, Δ _AB =563 GHz and Δ _CD =399 GHz, are revealed below H _c1=12.5 T. The detection of the ground-state excitations by means of ESR clearly indicates the presence of a non-secular term allowing these transitions. A complex structure of the microwave absorption spectrum in magnetic fields above H _c1 is observed, those peculiarities are discussed.     

Field Induced Exotic Phenomena of the S=1/2 Three-Leg Quantum Spin Nanotube

The magnetization process of the S=1/2 three-leg spin tube with an asymmetric exchange interaction in each triangle unit is investigated by the numerical exact diagonalization and the density matrix renormalization group calculation. It is found that the 1/3 magnetization plateau appears due to two different mechanisms depending on the asymmetry. The phase diagrams of the 1/3 plateau is presented. In addition some cusp-like anomalies are found in the magnetization curve.     

Observation of Quantum Anomalous Hall Effect and Exchange Interaction in Topological Insulator/Antiferromagnet Heterostructure

Integration of a quantum anomalous Hall insulator with a magnetically ordered material provides an additional degree of freedom through which the resulting exotic quantum states can be controlled. Here, an experimental observation is reported of the quantum anomalous Hall effect in a magnetically-doped topological insulator grown on the antiferromagnetic insulator Cr₂O₃. The exchange coupling between the two materials is investigated using field-cooling-dependent magnetometry and polarized neutron reflectometry. Both techniques reveal strong interfacial interaction between the antiferromagnetic order of the Cr₂O₃ and the magnetic topological insulator, manifested as an exchange bias when the sample is field-cooled under an out-of-plane magnetic field, and an exchange spring-like magnetic depth profile when the system is magnetized within the film plane. These results identify antiferromagnetic insulators as suitable candidates for the manipulation of magnetic and topological order in topological insulator films.     

膨胀石墨CuCl2-EGICs的微观结构TEM研究

采用透射电镜研究了以膨胀石墨为主体材料合成的ＣｕＣｌ_２－ＥＧＩＣｓ微观结构,包括垂直和平行石墨碳原子层的层间结构、层面结构．根据Ｘ射线衍射参数计算获得２、３、４阶ＣｕＣｌ_２－ＥＧＩＣｓ的层间距Ｉ_ｃ值,与理论计算值近似．选区电子衍射获得面内结构参数．发现ＥＧＩＣｓ衍射斑点是由石墨碳原子层单斑点和氯化物层多斑点簇组两套相迭而成．ＥＧＩＣｓ层面内碳原子层原子排布保持了石墨六角网格状的特点;氯化钢分子相对碳原子层分布有三种堆垛方式．倒易点分析认为有（２ｘ２）Ｒ（３０°）、（７^1/2ｘ７^1/2）Ｒ（０°）、（３^1/2ｘ３^1/2）Ｒ（０°）三种超晶格结构．二阶、三阶ＣｕＣｌ_２－ＧＩＣ中氯化铜点阵与碳原子点阵之间存在３０°的偏转角,而在一阶ＣｕＣｌ_２－ＧＩＣ中偏转角等于零度．根据高分辨电镜（ＨＲＥＭ）、选区电子衍射（ＳＡＤ）、能谱微区成分、光电子能潜（ＸＰＳ－ＥＳＣＡ）和俄歇电子能谱（ＸＡＥＳ）等结果,探讨和分析了ＣｕＣｌ_２－ＥＧＩＣｓ微观结构．     

Photoelastic and acousto-optical properties of Cs2HgCl4 crystals

We use a Mach-Zehdner interferometric technique to study the piezo-optical properties of Cs2HgCl4 crystals at room temperature. All piezo-optical (pi(mn)) and photoelastic (p(in)) tensor constants are obtained. A substantial photoelastic effect and low ultrasonic velocities in these crystals determine a relatively high figure of merit M2 for isotropic diffraction (for a certain geometry of acousto-optical interactions, M2 approximately 110 x 10(-15) s3/kg). The new material may be considered, therefore, a candidate for applications in acousto-optical devices. The dependence of the acoustic walk-off angle on the direction of sound propagation is calculated for the principal crystallographic planes.     

杂化钙钛矿(HOC2H4NH3)2CuCl4的制备与表征

采用低温溶液法合成了新型层状有序的含有羟基的有机/无机杂化钙钛矿材料(HOC₂H₄NH₃)₂CuCl₄, 采用元素分析、红外光谱、紫外-可见光吸收光谱、X射线衍射和X射线吸收精细结构等手段对其结构与性能进行了表征。结果表明:该材料通过无机框架诱导有机组分有序排列, 形成了规则的层状结构, 有序性高。该杂化钙钛矿材料的分解温度为212℃, 电阻率为2.86×10⁶ Ω·cm, 比不含羟基杂化钙钛矿的电阻率低两个数量级。紫外-可见光吸收光谱显示285 nm左右有一归因于电子从Cl(3p)价带顶跃迁到Cu(4s)导带底而产生的吸收峰。X射线吸收精细结构谱图表明: 二维层状杂化钙钛矿晶体中的Cu²⁺与6个Cl^-形成八面体配位, Cu-Cl键长为0.191 nm, 层间距为1.099 nm。