We examine the entanglement of the ground state of a 16-site spin-1/2 pyrochlore cluster in the quantum spin ice regime via various calculations of ${\cal I}$-concurrence. Exact ground state solutions to a quantum spin Hamiltonian with four nearest-neighbour exchange parameters were obtained using exact diagonalization. We present results for the ground state ${\cal I}$-concurrence in a region within the parameter space of the model where the ground state is a singlet. We discuss variations in the ${\cal I}$-concurrence in the context of the composition of the ground state and we demonstrate how a lattice distortion disentangles the state.
Nd-based pyrochlore oxides of the form Nd$_{2}B_{2}$O$_{7}$ have garnered a significant amount of interest owing to the moment fragmentation physics observed in Nd$_{2}$Zr$_{2}$O$_{7}$ and speculated in Nd$_{2}$Hf$_{2}$O$_{7}$. Notably this phenomenon is not ubiquitous in this family, as it is absent in Nd$_{2}$Sn$_{2}$O$_{7}$, which features a smaller ionic radius on the $B$-site. Here, we explore the necessary conditions for moment fragmentation in the Nd pyrochlore family through a detailed study of the mixed $B$-site pyrochlore Nd$_{2}$GaSbO$_{7}$. The $B$-site of this system is characteri…
RKKY interactions and anomalous Hall effect in metallic rare-earth pyrochlores
受Pr 2 Ir 2 O 7实验的启发，我们考虑金属烧绿石系统A2 B2 O 7，其中A-位点被稀土局域矩占据，B-位点托管5d过渡金属离子与巡回强自旋轨道耦合电子。假设A位上存在非Kramers双峰，我们推导出B位巡游电子介导的RKKY相互作用，并发现扩展的非Heisenberg相互作用。通过对RKKY相互作用的简化模型的分析，我们发现了一个局域磁矩相，它具有类伊辛磁偶极和类XY磁四极螺旋有序共存的特点。这种状态打破了时间反转和晶格对称性，并重建了B位电子能带结构，产生了具有固有异常霍尔效应和不可检测的小磁化的外尔金属相。我们讨论了我们的结果对Pr 2 Ir 2 O 7的影响。
We investigate the ground-state properties of the nearest-neighbor $S=1$ pyrochlore Heisenberg antiferromagnet using two complementary numerical methods, density-matrix renormalization group (DMRG) and pseudofermion functional renormalization group (PFFRG). Within DMRG, we are able to reliably study clusters with up to 48 spins by keeping 20 000 SU(2) states. The investigated 32-site and 48-site clusters both show indications of a robust $C_3$ rotation symmetry breaking of the ground-state spin correlations and the 48-site cluster additionally features inversion symmetry breaking. Our PFFRG an…
We investigate the effect of partial replacement of extended 5d Ir4+ sites by localized 3d Cr3+ moments on the magnetocaloric properties of Y2Ir2O7 (YIO) pyrochlore iridates. We find that Y2Ir2-xCrxO7 (YICO) undergoes cluster glass transition, possibly due to RKKY like interaction between localized Cr3+ moments occupying random sites in the pyrochlore network, mediated by 5d Ir conduction electrons. The coexistence of ferromagnetic and antiferromagnetic clusters give rise to the conventional and inverse magnetocaloric effect (MCE). We observe significant enhancement of conventional as well as …
The lanthanide pyrochlore oxides Ln$_2$B$_2$O$_7$ are one of the most intensely studied classes of materials within condensed matter physics, firmly centered as one of the pillars of frustrated magnetism. The extensive chemical diversity of the pyrochlores, coupled with their innate geometric frustration, enables realization of a wide array of exotic and complex magnetic ground states. Thus, the discovery of new pyrochlore compositions has been a persistent theme that continues to drive the field in exciting directions. The recent focus on the mixed B-site pyrochlores offers a unique route tow…
Motivated by recent experiments and computational results on pyrochlore iridates, we compare single-particle properties of Y2Ir2O7 obtained from single-site dynamical mean-field calculations with results within the TRILEX approximation, where the latter takes non-local correlations into account. Our calculations are all based on ab-initio calculations within density-functional theory, and take spin-orbit coupling into account. In order to make the treatment within TRILEX feasible, we first define a single-band jeff = 1/2 model, by comparing its spectral features within DMFT to a three-band mod…
We theoretically investigate spin-lattice coupling (SLC) effects on the in-field ordering properties of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. Here, we use the two possible simplified models describing the effect of local lattice distortions on the spin ordering via the SLC, the bond-phonon and site-phonon models. It is found by means of Monte Carlo simulations that in both models, the $\frac{1}{2}$ plateau shows up in the magnetization curve being relatively robust against the breathing bond-alternation, although magnetic long-range orders (LRO's) are reali…
Breathing Pyrochlore Lattice Realized in A-Site Ordered Spinel Oxides LiGaCr4O8 and LiInCr4O8
在两种A位有序尖晶石氧化物LiGaCr 4 O 8和LiInCr 4 O 8中发现了一种独特的阻挫晶格。由于Li+和Ga 3 +/In 3+离子在A位处的大尺寸失配，由携带自旋3/2的Cr 3+离子组成的烧绿石晶格变成小四面体和大四面体的交替阵列，即，“呼吸”烧绿石晶格。我们引入了一个参数，呼吸因子Bf，它量化了烧绿石晶格中的挫折程度：Bf被定义为J '/J，其中J'和J是最近邻的磁相互作用在大，小四面体，分别。具有Bf ~ 0.6的LiGaCr 4 O 8显示出与常规Cr尖晶石氧化物如ZnCr 2 O 4类似的磁化率。与此相反，LiInCr 4 O 8与一个小的Bf ~ 0.1的磁化率表现出自旋带隙的行为，表明接近一个奇特的单重基态。磁长程序发生在13.8和15.9 K的LiGaCr 4 O 8和LiInCr 4 O 8，分别，在这两种情况下可能是由于耦合到结构扭曲。
Influence of controlled disorder on the dipolar spin ice state of Ho-based pyrochlores
Oak Ridge National Laboratory（橡树岭国家实验室）；University of California Santa Barbara（加州大学圣塔芭芭拉分校）；Perry College of Mathematics, Computing, and Sciences, University of West Georgia（佐治亚西部大学珀里数学、计算与科学学院）；Japan Atomic Energy Agency（日本原子能机构）；Brigham Young University（杨百翰大学）
Electron interactions, spin-orbit coupling, intersite correlations in pyrochlore iridates
采用密度泛函和动力学平均场方法研究了烧绿石铱酸盐Lu $_2$ Ir $_2$ O $_7$，Y $_2$ Ir $_2$ O $_7$和Eu $_2$ Ir $_2$ O $_7$ .单站点和集群动力学平均场计算进行和自旋轨道耦合。发现了顺磁金属相、具有倾斜Weyl锥的反铁磁金属相和反铁磁绝缘相，磁相呈现全进/全出磁序，与前人的研究结果一致。对于电子三维材料来说，不寻常的是，单点动力学平均场近似不能再现定性的材料趋势，特别是预测Y $_2$ Ir $_2$ O $_7$和Eu $_2$ Ir $_2$ O $_7$的顺磁相性质几乎相同，尽管在实验中Y化合物具有比Eu化合物高得多的电阻。这种定性的失败是由于这些材料的物理学中的重要性，站点间的磁相关性。
Cluster approach study of intersite electron correlations in pyrochlore and checkerboard lattices
为了处理几何阻挫烧绿石晶格和棋盘格晶格中电子关联的影响，采用了一个扩展的单轨道哈伯德模型，该模型考虑了最近邻跃迁$\sim t$和库仑排斥$\sim V$.在本研究中，我们假定了无限的在位排斥，即$U\to\infty$，从而完全禁止了双占位。一个变分Gutzwiller型的方法扩展到检查由于短程$V-$相互作用的相关性和集群近似的开发，以评估系统的变分基态能量。在一个特殊的情况下获得的分析四分之一带填充适当的LiV $_2$ O $_4$，由此产生的简单的表达式描述的基态能量在该制度的中间和强耦合$V$。像在Brinkman-Rice理论的基础上的标准Gutzwiller方法的Hubbard模型，平均值的动能被证明是强烈减少的耦合$V$接近一个临界值$V_{c}$。这一发现可能有助于解释在LiV $_2$ O $_4$中观察到的重费米子行为。
Tetrahedrally ferromagnetic correlations and a glassy-freezing anomaly in the breathing pyrochlore magnet $\mathrm{AgInCr_4S_8}$ with partial $A$-site disorder
Tokyo University of Agriculture and Technology（东京农工大学）；The Institute for Solid State Physics, The University of Tokyo（东京大学固体物理研究所）；Kyoto University（京都大学）
Magnetic and Structural Properties of A-Site Ordered Chromium Spinel Sulfides: Alternating Antiferromagnetic and Ferromagnetic Interactions in the Breathing Pyrochlore Lattice
Quadrupolar Singlet Ground State of Praseodymium in a Modulated Pyrochlore
用中子散射和EXAFS研究了Pr $_{2-x}$ Bi $_x$ Ru $_2$ O $_7$的结构和磁性. Pr的基态近似为二重态，第一激发态为单重态。该整体晶场能级方案类似于本文也报道的金属Pr $_2$ Ir $_2$ O $_7$。虽然B位（Ru）是完全有序的，但A位（Pr/Bi）的情况并非如此。Pr-O2键长的分布表明Pr环境即使对于$x=0$也是不均匀的。表面上由于Bi $^{3+}$上的6s孤对，Bi环境高度无序。相应地，我们发现非Kramers双重基态简并，否则预计Pr的烧绿石结构被解除，从而产生一个四极单基态与空间变化的能隙。对于$x=0$，低于T $_N$，Ru亚晶格反铁磁有序，传播矢量\textbf{k} =（0，0，0），对于Y $_2$ Ru $_2$ O $_7$。没有秩序与Pr子晶格观察到下降到100 mK。Pr $_{2-x}$ Bi $_x$ Ru $_2$ O $_7$的低能磁响应具有与Pr准二重态基态的不均匀分裂相关的宽谱磁激发。对于$x=0$（$x=0.97$），光谱与温度相关（无关）。看来与Bi合金化相关的无序增强了不均匀Pr晶场能级分裂，使得位间相互作用对于$x=0.97$变得无关。A位的结构复杂性可以反映在B位钌在Néel相中的滞后均匀磁化中。
The question of structural disorder and its effects on magnetism is relevant to a number of spin liquid candidate materials. Although commonly thought of as a route to spin glass behavior, here we describe a system in which the structural disorder results in long-range antiferromagnetic order due to local symmetry breaking. Nd$_2$ScNbO$_7$ is shown to have a dispersionless gapped excitation observed in other neodymium pyrochlores below T$_N$ = 0.37 K through polarized and inelastic neutron scattering. However the dispersing spin waves are not observed. This excited mode is shown to occur in on…
The Spectral and Magnetic Properties of Er$^{3+}$ and Yb$^{3+}$ Ions in Y$_{2}$Ti$_{2}$O$_{7}$ Crystals with a Pyrochlore Structure
用电子自旋共振（ESR）和选择性激光光谱研究了掺Er $^{3+}$和Yb $^{3+}$离子的立方Y $_{2}$ Ti $_{2}$ O $_{7}$单晶。ESR谱显示稀土离子取代钇离子的信号与当地三角对称的网站。确定了$g$张量分量。光学研究结果表明，形成了几种类型的杂质心，并找到了这些杂质心的基态和激发态多重态的次能级，在探测到的各种光学中心中，区分出了在ESR谱形成中占主导地位的中心。使用交换电荷模型的实验数据的分析已经使得有可能确定的晶体场的参数集的Er $^{3+}$和Yb $^{3+}$离子取代Y $^{3+}$离子在烧绿石Y $_{2}$ Ti $_{2}$ O $_{7}$的规则晶体学网站。
Polynomial Expansion Monte Carlo Study of Frustrated Itinerant Electron Systems: Application to a Spin-ice type Kondo Lattice Model on a Pyrochlore Lattice
We address the ground-state properties of the long-standing and much-studied three-dimensional quantum spin liquid candidate, the $S=\frac 1 2$ pyrochlore Heisenberg antiferromagnet. By using $SU(2)$ density-matrix renormalization group (DMRG), we are able to access cluster sizes of up to 128 spins. Our most striking finding is a robust spontaneous inversion symmetry breaking, reflected in an energy density difference between the two sublattices of tetrahedra, familiar as a starting point of earlier perturbative treatments. We also determine the ground-state energy, $E_0/N_\text{sites} = -0.49…
We study a spin-ice Kondo lattice model on a breathing pyrochlore lattice with classical localized spins. The highly efficient kernel polynomial expansion method, together with a classical Monte Carlo method, is employed in order to study the magnetic phase diagram at four representative values of the number density of itinerant electrons. We tune the breathing mode by varying the hopping ratio -- the ratio of hopping parameters for itinerant electrons along inequivalent paths. Several interesting magnetic phases are stabilized in the phase diagram parameterized by the hopping ratio, Kondo cou…
Unified Description of Spin-Lattice Coupling and Thermodynamics in the Pyrochlore Heisenberg Antiferromagnet
Institute for Solid State Physics, The University of Tokyo（东京大学固体物理研究所）；Department of Physics, The University of Tokyo（东京大学物理系）；Department of Chemistry, Graduate School of Science, Kyoto University（京都大学理学研究科化学系）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（国立材料科学研究所材料纳米结构研究中心）；Department of Engineering Science, University of Electro-Communications（电气通信大学工学部）
Almost pure $J_{\mathrm{eff}} = 1/2$ Mott state of In$_2$Ir$_2$O$_7$ in the limit of reduced inter-site hopping
铱酸烧绿石In $_2$ Ir $_2$ O $_7$是一种强$J_{\mathrm{eff}} = 1/2$莫特绝缘体，具有受抑磁性。尽管三角晶场很大，但与其他烧绿石铱酸盐A $_2$ Ir $_2$ O $_7$（A：三价阳离子）相比，观察到$J_{\mathrm{eff}} = 1/2$带中$J_{\mathrm{eff}} = 3/2$组分的少量混合物和$J_{\mathrm{eff}} = 3/2$带的少量分裂。我们认为，减少之间的$J_{\mathrm{eff}} = 1/2$和$J_{\mathrm{eff}} = 3/2$流形的站点跳跃起着主导作用，在不同的行为中的In $_2$ Ir $_2$ O $_7$相比，其他A $_2$ Ir $_2$ O $_7$。在自旋-轨道纠缠$J_{\mathrm{eff}} = 1/2$莫特绝缘体的局域物理中，不应忽略格点间跳跃的影响。
The pyrochlore ruthenate In$_2$Ru$_2$O$_7$ displays a subtle competition between spin-orbital entanglement and molecular orbital formation. At room temperature, a spin-orbit-entangled singlet state was identified. With decreasing temperature, In$_2$Ru$_2$O$_7$ undergoes multiple structural transitions and eventually forms a nonmagnetic ground state with semi-isolated Ru$_2$O units on the pyrochlore lattice. The dominant hopping through the Ru-O-Ru linkage leads to molecular orbital formation within the Ru$_2$O units. This molecular orbital formation is unique in that it involves the O$^{2-}$ a…
Universal Dynamic Magnetism in Yb-Pyrochlores with Disparate Ground States
Yb_2B_2O_7（B = Sn，Ti，Ge）烧绿石型镱磁体的结构可用S_eff = 1/2的量子自旋来描述，该量子自旋由角共享四面体网络构成，并通过各向异性交换相互作用。在结构上，只有非磁性B位阳离子，因此主要是晶格参数，在整个系列中发生变化。尽管如此，一系列的磁性行为被观察到：在Yb 2 Ti 2 O 7和Yb 2Sn 2 O 7的低温磁性具有铁磁性的字符，而Yb 2Ge 2 O 7在低温下显示反铁磁有序的Neel状态。虽然静态性质的镱焦散石是不同的，非弹性中子散射测量揭示了一个共同的特点，他们的异国情调的自旋动力学。所有三种镱高温超导体都显示出无隙连续的自旋激发，类似于低Q下的过阻尼铁磁自旋波。此外，该系列的比热也遵循一个共同的形式，即在T_C或T_N处先有一个宽的高温异常，然后有一个急剧的低温异常。我们报告的新型自旋动力学仅与宽比热异常密切相关，在急剧异常中保持不变。这一结果表明，与锐异常有关的镱辉石岩的主要序参量是“隐藏的”，而不是简单的磁偶极子序。
Effects of Crystal Structure and the On-Site Coulomb Interactions on the Electronic and Magnetic Structure of Pyrochlores $A_2$Mo$_2$O$_7$ (A= Y, Gd, and Nd)
受最近实验研究的启发，我们研究了钼高温超导体$A_2$ Mo $_2$ O $_7$（$A$ = Y，Nd，Gd）的磁结构及其对电子性质的影响.后者与12个Mo（$t_{2g}$）带的行为密切相关，这些带位于费米能级附近，并且与光谱的其余部分很好地分离。我们使用平均场Hartree-Fock方法，它结合了这些带的电子结构的精细细节，从局域密度近似，自旋轨道相互作用和Mo（4d）电子之间的原位库仑相互作用的传统计算中提取，以最一般的旋转不变形式处理。库仑排斥U在这个问题中起着非常重要的作用，半经验值U $\sim$ 1.5-2.5 eV同时解释了金属-绝缘体（M-I）相变、铁磁（FM）-自旋玻璃（SG）相变以及反常霍尔效应（AHE）的增强. M-I转变主要由$U$控制。由于双交换机制，金属侧的磁结构几乎是共线FM。进入绝缘状态的转变伴随着自旋和轨道磁矩的大倾斜。在绝缘状态下的交换相互作用的符号由Mo-Mo距离控制。较小的距离有利于反铁磁耦合，这预示着SG行为的挫折烧绿石晶格。大的AHE预计在近共线FM状态，M-I转变点附近，并与未淬灭的轨道磁化在Mo网站。我们还预测了在相同的FM化合物大的磁光效应。
The A-site ordered chromate spinels LiGa1-xInxCr4O8 host a network of size-alternating spin-3/2 Cr3+ tetrahedra known as a 'breathing' pyrochlore lattice. For the x=0.05 composition, the complex magneto-structural ordering observed in the parent x=0 material is replaced by a single transition at Tf=11 K, ascribed to the collinear nematic order caused by strong spin-lattice coupling. We present here an inelastic neutron scattering study of the spin dynamics in this composition. Above Tf , the dynamical scattering function S(Q,E) is ungapped and quasi-elastic, similar to undoped LiGaCr4O8. Below…
Multi-scale Quantum Criticality driven by Kondo-lattice Coupling in Pyrochlore Systems
烧绿石体系（$A_{2}B_{2}O_{7}$）具有$A$位稀土局域矩和$B$位$5d$传导电子，为发现奇异量子多体基态提供了极好的材料平台。值得注意的例子包括局域磁矩的U（1）量子自旋液体和传导电子的半金属非费米液体。在这里，我们研究紧急的量子相位和它们的跃迁驱动的近藤晶格耦合之间的高度纠缠的量子基态。使用重整化群方法，它表明，弱Kondo晶格耦合是无关紧要的，导致一个分馏的半金属相去耦的本地时刻和导电电子。当近藤晶格耦合增加时，该相不稳定，形成对称性破缺态。特别重要的是近藤晶格耦合和长程库仑相互作用的相反影响。前者倾向于打破粒子-空穴对称性，而后者倾向于恢复对称性，这种特征竞争导致了可能的多次相变，首先是从一个分数化的半金属相到一个分数化的费米表面态和粒子-空穴口袋，然后是第二次转变到一个分数化的铁磁态。多尺度量子临界行为出现在非零温度和外部磁场附近的量子相变。我们讨论了这些结果对Pr $_2$ Ir $_2$ O $_7$实验的影响。
