Artificial 'spin ice' in a geometrically frustrated lattice of nanoscale ferromagnetic islands

Frustration, defined as a competition between interactions such that not all of them can be satisfied, is important in systems ranging from neural networks to structural glasses. Geometrical frustration, which arises from the topology of a well-ordered structure rather than from disorder, has recently become a topic of considerable interest. In particular, geometrical frustration among spins in magnetic materials can lead to exotic low-temperature states, including 'spin ice', in which the local moments mimic the frustration of hydrogen ion positions in frozen water. Here we report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the low-temperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized.     

尖晶石结构Li1.33Mn1.67O4的磁学性质

采用固相烧结法制备尖晶石结构Li_1.33Mn_1.67O₄样品. 不同磁场下的直流磁化率曲线、 磁滞回线和交流磁化率均表明该样品在低温呈团簇自旋玻璃态. 通过计算材料的几何失措因子, 可知掺杂非磁性Li离子破坏了失措的Kagome点阵, 从而诱导铁磁与反铁磁相互作用的竞争, 使体系呈自旋玻璃态.      

Emergent excitations in a geometrically frustrated magnet

Frustrated systems are ubiquitous, and they are interesting because their behaviour is difficult to predict; frustration can lead to macroscopic degeneracies and qualitatively new states of matter. Magnetic systems offer good examples in the form of spin lattices, where all interactions between spins cannot be simultaneously satisfied. Here we report how unusual composite spin degrees of freedom can emerge from frustrated magnetic interactions in the cubic spinel ZnCr(2)O(4). Upon cooling, groups of six spins self-organize into weakly interacting antiferromagnetic loops, whose directors -- the unique direction along which the spins are aligned, parallel or antiparallel -- govern all low-temperature dynamics. The experimental evidence comes from a measurement of the magnetic form factor by inelastic neutron scattering; the data show that neutrons scatter from hexagonal spin clusters rather than individual spins. The hexagon directors are, to a first approximation, decoupled from each other, and hence their reorientations embody the long-sought local zero energy modes for the pyrochlore lattice.     

Geometric frustration in compositionally modulated ferroelectrics

Geometric frustration is a broad phenomenon that results from an intrinsic incompatibility between some fundamental interactions and the underlying lattice geometry. Geometric frustration gives rise to new fundamental phenomena and is known to yield intriguing effects such as the formation of exotic states like spin ice, spin liquids and spin glasses. It has also led to interesting findings of fractional charge quantization and magnetic monopoles. Mechanisms related to geometric frustration have been proposed to understand the origins of relaxor and multiferroic behaviour, colossal magnetocapacitive coupling, and unusual and novel mechanisms of high-transition-temperature superconductivity. Although geometric frustration has been particularly well studied in magnetic systems in the past 20 years or so, its manifestation in the important class formed by ferroelectric materials (which are compounds with electric rather than magnetic dipoles) is basically unknown. Here we show, using a technique based on first principles, that compositionally graded ferroelectrics possess the characteristic 'fingerprints' associated with geometric frustration. These systems have a highly degenerate energy surface and display critical phenomena. They further reveal exotic orderings with novel stripe phases involving complex spatial organization. These stripes display spiral states, topological defects and curvature. Compositionally graded ferroelectrics can thus be considered the 'missing link' that brings ferroelectrics into the broad category of materials able to exhibit geometric frustration. Our ab initio calculations allow deep microscopic insight into this novel geometrically frustrated system.     

稀土Dy~(3+)掺杂TiO_2的制备及光催化性能研究

采用溶胶-凝胶法合成了不同Dy3+掺杂量、不同焙烧温度的Ti O2光催化剂.并对所制得的光催化剂进行了SEM、XRD和FT-IR的表征和光催化活性研究.结果表明:所得粉体均为锐钛矿相纳米Ti O2,稀土掺杂能细化晶粒.以酸性品红为降解物的光催化实验表明:500℃煅烧,当Dy3+掺杂量为0.20%时其光催化活性最好,加入1 m L 30%H2O2,无灯源照射,常温常压无须通氧气条件下,催化时间6 h,降解率达到81.19%.     

基于密度泛函理论和含时密度泛函理论下富勒烯B24N24光谱性质研究

本文在B3LYP/6-31g(d)基组水平上, 利用密度泛函理论(DFT)优化了B24N24团簇的几何和电子结构. 研究结果表明, 优化所得B24N24团簇的几何结构对称性分别为S4、C2、S8和O;在基态稳定结构基础上, 得出其输运性质, 既非p型输运材料, 亦非n型输运材料, 即不具有输运性质;在优化好的基态结构基础上, 又研究了它的红外和拉曼分子振动谱, 四种团簇的红外-拉曼振动谱的振动强度排序均为O>S8>S4>C2;随后采用含时密度泛函理论(TDDFT),同样在B3LYP/6-31g(d)基组水平上, 计算了这四种物质在CH2Cl2溶剂中的UV-Vis吸收谱特性, 研究表明跃迁能级数量越多,吸收谱振动强度越强, 并且这些分子的紫外-可见吸收谱的振动强度排序为S4>C2>S8>O.     

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Materials in which magnetic and electric order coexist--termed 'multiferroics' or 'magnetoelectrics'--have recently become the focus of much research. In particular, the simultaneous occurrence of ferromagnetism and ferroelectricity, combined with an intimate coupling of magnetization and polarization via magnetocapacitive effects, holds promise for new generations of electronic devices. Here we present measurements on a simple cubic spinel compound with unusual, and potentially useful, magnetic and electric properties: it shows ferromagnetic order coexisting with relaxor ferroelectricity (a ferroelectric cluster state with a smeared-out phase transition), both having sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature, the magnetocapacitive coupling (characterized by a variation of the dielectric constant in an external magnetic field) reaches colossal values, approaching 500 per cent. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments but here is found to impede long-range order of the structural degrees of freedom that drive the formation of the ferroelectric state.     

Y3Al5O12基质中Dy3+的浓度猝灭机制

采用溶胶-凝胶/燃烧合成法制备不同掺杂浓度的Dy:Y3Al5O12(YAG)发光粉体。分析基质晶体结构、Dy3+掺量对Dy3+光致发光性能的影响,并探讨Dy3+在Y3Al5O12基质中的自身浓度猝灭机制。根据激发光谱,Dy:YAG的主激发峰位置在353 nm,对应Dy3+的6H15/2→6P7/2跃迁。在Dy:YAG晶体结构中,Dy3+取代Y3+的位置具有D2对称性,故Dy:YAG的蓝光发射强度要高于黄光发射强度,且Dy3+最佳摩尔分数为0.02;Dy3+的4F9/2→6H15/2、6H13/2跃迁发射的浓度猝灭机制均为相邻中心的电偶极-电偶极相互作用引起的交叉弛豫(4F9/2+6H15/2→6H9/2+6F3/2)所造成的。     

Evolution of Magnetic and Transport Properties in Pyrochlore Iridates A_2Ir_2O_7(A=Y,Eu,Bi)

We study the structure, magnetization, electrical transport properties and heat capacity of the pyrochlore iridates A_2Ir_2O_7(A=Y, Eu, Bi). It is found that magnetic frustration and long-range magnetic order coexist in Y_2Ir_2O_7 and Eu_2Ir_2O_7, while Bi_2Ir_2O_7 shows magnetic frustration or short-range magnetic order without long-range magnetic order. The former two compounds show insulating behavior. However, the latter one shows metallic behavior. The variations of the radius of A-site ion affect the conduction bandwidth, correlation and Ir-Ir exchange energy, which in turn determine transport properties and magnetic properties. We also find that the antiferromagnetic ordering of pyrochlore iridates is weakly frustrated. The all-in/all-out type of anisotropy based on spin-orbit coupling plays a key role in releasing the frustration.     

Unusual magnetic order in the pseudogap region of the superconductor HgBa2CuO4+delta

The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-T(c)) superconductivity, characterized by anomalous physical properties. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa(2)Cu(3)O(6+delta) (YBCO) resulted in contradictory claims of no and weak magnetic order, and the interpretation of muon spin relaxation measurements on YBCO and of circularly polarized photoemission experiments on Bi(2)Sr(2)CaCu(2)O(8+delta)(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa(2)CuO(4+delta) (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature rather than a phase transition temperature. Instead, they are consistent with a variant of previously proposed charge-current-loop order that involves apical oxygen orbitals, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point.     

新型红色长余辉发光材料Gd2O2S:EU3+,Si4+,Ti4+的合成及性能表征

采用高温固相反应法首次合成了新型红色长余辉发光材料Gd2O2S:Eu3+,Si4+,Ti4+.用X射线粉末衍射(XRD)、扫描电镜(SEM)、分光光度计等对合成产物进行了分析与表征.结果表明:Gd2O2S:Eu3+,Si4+,Ti4+的晶体结构与Gd2O2S相同,为六方晶系.颗粒的形貌为类球形.Gd2O2S:Eu3+,Si4+,Ti4+的激发光谱呈250～400 nm宽带状,激发光谱主峰位于365 nm;发射光谱为线状光谱,归属于Eu3+的5DJ(J=0,1)→7FJ(K=0,1,2,4)跃迁.最强的发射峰为627 nm和617 nm,均属于5D0→7F2跃迁,且627 nm的发射峰明显远强于617nm,显示出纯正的红色发光;并且Si4+和Ti4+离子的共掺杂可显著延长样品Cd2O2S:Eu3+的余辉时间.     

Pressure-induced crystallization of a spin liquid

Liquids are expected to crystallize at low temperature. The only exception is helium, which can remain liquid at 0 K, owing to quantum fluctuations. Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie-Weiss temperature theta(CW) (ref. 3). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry. This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below theta(CW) (refs 5-7). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb(2)Ti(2)O(7), under conditions of high pressure (approximately 9 GPa) and low temperature (approximately 1 K). This compound is a three-dimensional magnet with theta(CW) = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb(2)Ti(2)O(7) remains in a spin liquid state down to at least 70 mK (ref. 8). But we find that, under high pressure, the spins start to order or 'crystallize' below 2.1 K, with antiferromagnetic order coexisting with liquid-like fluctuations. These results indicate that a spin liquid/solid mixture can be induced by pressure in geometrically frustrated systems.     

Bose-Einstein condensation of the triplet states in the magnetic insulator TlCuCl3

Bose-Einstein condensation denotes the formation of a collective quantum ground state of identical particles with integer spin or intrinsic angular momentum. In magnetic insulators, the magnetic properties are due to the unpaired shell electrons that have half-integer spin. However, in some such compounds (KCuCl3 and TlCuCl3), two Cu2+ ions are antiferromagnetically coupled to form a dimer in a crystalline network: the dimer ground state is a spin singlet (total spin zero), separated by an energy gap from the excited triplet state (total spin one). In these dimer compounds, Bose-Einstein condensation becomes theoretically possible. At a critical external magnetic field, the energy of one of the Zeeman split triplet components (a type of boson) intersects the ground-state singlet, resulting in long-range magnetic order; this transition represents a quantum critical point at which Bose-Einstein condensation occurs. Here we report an experimental investigation of the excitation spectrum in such a field-induced magnetically ordered state, using inelastic neutron scattering measurements of TlCuCl3 single crystals. We verify unambiguously the theoretically predicted gapless Goldstone mode characteristic of the Bose-Einstein condensation of the triplet states.     

（Ba0.2Sr0.8）1-1.5xBixZn0.04T i0.98O3陶瓷结构及介电性能的研究

采用传统固相反应方法,制备了（Ba0.2Sr0.8）1-1.5xBixZn0.04Ti0.98O3（x=0.00、0.01、0.02、0.03）陶瓷材料,用X射线衍射、扫描电子显微镜和变温介电谱方法,对它们的晶格结构、微观形貌和复介电常数进行了测量和分析.结果表明：1）Zn2＋和Bi3＋进入到Ba0.2Sr0.8TiO3晶格中并与之形成ABO3钙钛矿型固溶体;2）随Bi2O3掺入,陶瓷的室温晶系结构由立方相转变为四方相,同时出现在约120K的弥散相变转化为弛豫相变;3）Bi2O3掺杂对晶粒生长有明显的抑制作用,但晶粒之间的连接性增强,陶瓷的致密度增加;4）当T=300K,f=1Hz,当Bi3＋的掺杂量为0.01mol时,陶瓷样品室温介电常最大（ε＇=1529.95）,此时室温介电损耗值为最大（tanδ=3.366×10-2）.上述结果为该类陶瓷掺杂改性研究,以及弛豫相变机制探索提供参考.     

Eu^2＋,Dy^3＋惨杂Sr2MgSi2O7长余辉发光材料的研究

采用高温固相法制备了Sr2MgSi2O7（SMS）、Sr2MgSi2O7：Eu^2＋（SMS—E）和Sr2MgSi2O7：Eu^2＋,Dy^3＋（SMS—ED）粉末刑用XRD、荧光光谱仪、单光子计数器、热释光剂量计等手段对样品进行结构和发光性能分析．结果表明：所得样品的晶相结构基本一致,在发射光谱中,SMS-ED与SMS-E相比,没有明显的新发射光谱,且发光强度较弱,但余辉的亮度和余辉的时间却有了很大的提高．     

Antiferromagnetic order as the competing ground state in electron-doped Nd1.85Ce0.15CuO4

Superconductivity in the high-transition-temperature (high-T(c)) copper oxides competes with other possible ground states. The physical explanation for superconductivity can be constrained by determining the nature of the closest competing ground state, and establishing if that state is universal among the high-T(c) materials. Antiferromagnetism has been theoretically predicted to be the competing ground state. A competing ground state is revealed when superconductivity is destroyed by the application of a magnetic field, and antiferromagnetism has been observed in hole-doped materials under the influence of modest fields. None of the previous experiments have revealed the quantum phase transition from the superconducting state to the antiferromagnetic state, because they failed to reach the upper critical field B(c2). Here we report the results of transport and neutron-scattering experiments on electron-doped Nd1.85Ce0.15CuO4 (refs 13, 14), where B(c2) can be reached. The applied field reveals a static, commensurate, anomalously conducting long-range ordered antiferromagnetic state, in which the induced moment scales approximately linearly with the field strength until it saturates at B(c2). This and previous experiments on the hole-doped materials therefore establishes antiferromagnetic order as a competing ground state in the high-T(c) copper oxide materials, irrespective of electron or hole doping.     

Ordering and manipulation of the magnetic moments in large-scale superconducting pi-loop arrays

The phase of the macroscopic electron-pair wavefunction in a superconductor can vary only by multiples of 2pi when going around a closed contour. This results in quantization of magnetic flux, one of the most striking demonstrations of quantum phase coherence in superconductors. By using superconductors with unconventional pairing symmetry, or by incorporating pi-Josephson junctions, a phase shift of pi can be introduced in such loops. Under appropriate conditions, this phase shift results in doubly degenerate time-reversed ground states, which are characterized by the spontaneous generation of half quanta of magnetic flux, with magnitude 1/2 Phi(0)(Phi(0) = h/2e = 2.07 x 10(-15) Wb) (ref. 7). Until now, it has only been possible to generate individual half flux quanta. Here we report the realization of large-scale coupled pi-loop arrays based on YBa2Cu3O7-Au-Nb Josephson contacts. Scanning SQUID (superconducting quantum interference device) microscopy has been used to study the ordering of half flux quanta in these structures. The possibility of manipulating the polarities of individual half flux quanta is also demonstrated. These pi-loop arrays are of interest as model systems for studying magnetic phenomena--including frustration effects--in Ising antiferromagnets. Furthermore, studies of coupled pi-loops can be useful for designing quantum computers based on flux-qubits with viable quantum error correction capabilities.     

有限深抛物量子阱中的类氢杂质态

运用三参数变分法,研究了在垂直于生长方向的磁场作用下抛物量子阱中类氢杂质态的束缚能和1s→2p-态跃迁能.结果表明,束缚能和跃迁能随阱宽变化有一极大值;束缚能随磁场单调增加,而跃迁能随磁场的变化出现了极小值.     

Mgn(n=2～7)团簇的几何结构研究

采用密度泛函DFT中的B3LYP方法,选择sdd和6-311+g基组,对Mgn(n=2～7)小团簇的各种可能结构进行优化,得到了它们的基态平衡结构并计算出其原子化能.研究表明:随着团簇尺寸的增大,单个原子的平均原子化能逐渐增大.同时分析了团簇的最高占据轨道与最低空轨道之间形成的能级间隙.计算出了电子亲和能和电离势.最后分析了费米能级、电子亲和能和电离势的变化规律及其原因.     

钛精粉氧化过程中物相转变

采用热重分析和恒温实验法研究了攀枝花钛精粉氧化过程中相的存在形式及其转变规律.X射线衍射结果和微观形貌分析表明:温度低于500℃,钛精粉未发生氧化反应;600℃时产生Fe2Ti3O9和TiO2相,其和原相Fe2O3相衍射峰强度均随温度的升高而增强;900℃时,Fe2Ti3O9相完全转变为Fe2TiO5相,且随温度的升高,TiO2相和Fe2O3相衍射峰强度逐渐减弱;在整个氧化过程中存在四个化学反应,其与烧结作用相互关联,使得氧化产物表面形成较多孔隙,变得凹凸不平且疏松.