We report on a combined measurement of high-resolution x-ray diffraction on powder and Raman scattering on single crystalline NiS2-xSex samples that exhibit the insulator-metal transition with Se doping. Via x-rays, an abrupt change in the bond length between Ni and S (Se) ions was observed at the transition temperature, in sharp contrast to the almost constant bond length between chalcogen ions. Raman scattering, a complementary technique with the unique sensitivity to the vibrations of chalcogen bonds, revealed no anomalies in the phonon spectrum, consistent with the x-ray diffraction results. This indicates the important role of the interaction between Ni and S (Se) in the insulator-metal transition. The potential implication of this interpretation is discussed in terms of current theoretical models.
We performed ultrasonic measurements on (Ta$_{0.952}$V$_{0.048}$)$_2$NiSe$_5$ to investigate the contribution of acoustic phonons at the zone center to structural and possible excitonic transitions. The transverse elastic constant $C_{55}$ with irreducible representation $B_\mathrm{2g}$ exhibits softening over 90\% above a transition temperature of $T_\mathrm{s} = 280$ K, indicating the soft mode of corresponding TA phonons. Our analysis based on the quadrupole response indicates that the electron-phonon interaction contributes to the excitonic transition in addition to the hybridization betwe…
Electronic and magnetic properties of NiS_{2-x}Se_x: a comparative study
用密度泛函理论研究了黄铁矿系统NiS_{2-x} Se_x在压力下的金属-绝缘体转变和d电子离域问题.我们测试了几个近似，包括广义梯度近似（GGA），GGA+U的方法，和混合泛函。此外，我们应用GW近似和执行Hartree-Fock计算。硫族二聚体在电子结构中的重要作用在GGA中进行了讨论，GGA充分描述了晶格在金属-绝缘体转变中的作用。此外，还测定了磁相图。然而，电子性质的描述不充分-绝缘基态的NiS_2不能得到-和磁序略有高估。如果在GGA+U中考虑相关性，则发现绝缘体，但掺杂样品的非磁性基态不可接近。混合Fock交换与局部近似GGA正确地再现绝缘基态NiS_2，和金属基态掺杂和压力。绝缘体是-与早期的建议相反-莫特类型，即。e. Ni 3d {eg}态之间的能隙打开。由于Fock项，磁序被严重高估。所用的GW计算不能修正NiS_2的金属特性，也不能将能隙低估一个数量级。
The excitonic insulator Ta$_2$NiSe$_5$ experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps = 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of 0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Ni-valence band…
We investigate the non-equilibrium electronic structure and characteristic time scales in a candidate excitonic insulator, Ta$_2$NiSe$_5$, using time- and angle-resolved photoemission spectroscopy with a temporal resolution of 50 fs. Following a strong photoexcitation, the band gap closes transiently within 100 fs, i.e., on a time scale faster than the typical lattice vibrational period. Furthermore, we find that the characteristic time associated with the rise of the photoemission intensity above the Fermi energy decreases with increasing excitation strength, while the relaxation time of the …
Substrate tuning of the structural and electronic transition in thin flakes of the excitonic insulator candidate Ta$_2$NiSe$_5$
Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；Institute for Functional Matter and Quantum Technologies, University of Stuttgart（斯图加特大学功能物质与量子技术研究所）；Department of Physics, University of Tokyo（东京大学物理系）
Ta$_2$NiSe$_5$ became one of the most investigated candidate materials for hosting an excitonic insulator ground state. Many studies describe the corresponding phase transition as a condensation of excitons breaking a continuous symmetry. This view got challenged recently pointing out the importance of the loss of two mirror symmetries at a structural phase transition that occurs together with the semiconductor-excitonic insulator transition. For such a scenario an unstable optical zone-center phonon at low energy is proposed to drive the transition. Here we report on the experimental observat…
Excitonic insulator (EI) was proposed in 60's as a distinct insulating state originating from pure electronic interaction, but its material realization has been elusive with extremely few material candidates and with only limited evidence such as anomalies in transport properties, band dispersions, or optical transitions. We investigate the real-space electronic states of the low temperature phase in Ta$_2$NiSe$_5$ with an atomic resolution to clearly identify the quasiparticle energy gap together with the strong electron-hole band renormalization using scanning tunneling microscopy (STM) and …
Utilizing the time-dependent density-matrix renormalization group technique, we numerically prove photoinduced pairing states in the extended Falicov-Kimball model (EFKM) at half filling, both with and without internal SU(2) symmetry. In the time-dependent photoemission spectra an extra band appears above the Fermi energy after pulse irradiation, indicating an insulator-to-metal transition. Even in the absence of the SU(2) structure, the pair correlations are enhanced during the pump, and afterwards decrease over time. This implies the possible metallization of Ta$_2$NiSe$_5$, a strong candida…
Motivated by the existence of superconductivity in pyrite-structure CuS$_2$, we explore the possibility of ionic-liquid-gating-induced superconductivity in the proximal antiferromagnetic Mott insulator NiS$_2$. A clear gating-induced transition from a two-dimensional insulating state to a three-dimensional metallic state is observed at positive gate bias on single crystal surfaces. No evidence for superconductivity is observed down to the lowest measured temperature of 0.45 K, however. Based on transport, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, atomic force micr…
Excitonic insulator (EI) is an intriguing insulating phase of matter, where electrons and holes are bonded into pairs, so called excitons, and form a phase-coherent state via Bose-Einstein Condensation (BEC). Its theoretical concept has been proposed several decades ago, but the followed research is very limited, due to the rare occurrence of EI in natural materials and the lack of manipulating method of excitonic condensation. In this paper, we report the realization of a doping-controlled EI-to-semi-metal transition in Ta$_2$NiSe$_5$ using $in$-$situ$ potassium deposition. Combining with ang…
The quasi-one-dimensional spin ladder compounds, BaFe$_2$S$_3$ and BaFe$_2$Se$_3$, are investigated by infrared spectroscopy and density functional theory (DFT) calculations. We observe strong anisotropic electronic properties and an optical gap in the leg direction that is gradually filled above the antiferromagnetic (afm) ordering temperature, turning the systems into a metallic phase. Combining the optical data with the DFT calculations we associate the optical gap feature with the $p$-$d$ transition that appears only in the afm ordered state. Hence, the insulating ground state along the le…
Non-collinear Altermagnetic Phases in the Mott Insulator NiS$_2$
Leibniz Institute for Solid State and Materials Research（莱布尼茨固体与材料研究所）；Technische Universitat Dresden（德累斯顿工业大学）；Laboratoire Léon Brillouin, CEA, CNRS, Université Paris-Saclay（塞奈-萨克雷大学、法国原子能与替代能源委员会、法国国家科学研究中心勒昂布里渊实验室）；Université de Sherbrooke（舍布鲁克大学）；Donostia International Physics Center（圣塞巴斯蒂安国际物理中心）；Regroupement Québécois sur les Matériaux de Pointe (RQMP)（魁北克尖端材料研究联合体）
The Mott transition in honeycomb compounds with significant spin-orbit coupling is explored. At finite temperatures we identify a novel semimetallic phase located between a topological insulator and a topological Mott insulator. This semimetal is characterized by having no charge gap, no quasiparticles and gapped spin excitations with non-trivial topological properties. While charge conduction is incoherent involving mean-free paths violating the Mott-Ioffe-Regel limit, spin excitations can be transported ballistically along the edges giving rise to a quantized spin conductance. Such bad topol…
Topological spin liquid phase in a low-dimensional organic molecular compound
我们报告发现了一个相关的绝缘体的体积间隙在三分之二填充的几何挫折哈伯德模型，描述了低能量物理的Mo $_3$ S $_7$（dmit）$_3$。这与半填充时预期的莫特绝缘体非常不同。我们发现，即使在非常弱的电子-电子相互作用（$U$）下也存在的绝缘相，与Halfenic相有电化学联系，并且与Mo $_3$ S $_7$（dmit）$_3$的实验结果一致
Magnetotransport Properties of Iron-Based Ladder Compounds BaFe2S3 and BaFe2Se3 under High Pressure
Graduate School of Advanced Science and Engineering, Hiroshima University（广岛大学高级科学与工程学院）；International Institute for Sustainability with Knotted Chiral Meta Matter (WPI-SKCM2), Hiroshima University（广岛大学螺旋手性超材料可持续性国际研究所）；Department of Physics, Graduate School of Science, Tohoku University（东北大学理学院物理系）
Metal-insulator transition and topological properties of pyrochlore iridates
结合密度泛函理论（DFT）和嵌入式动力学平均场论（DMFT）方法，我们研究了$R_2$ Ir $_2$ O $_7$（$R$ =Y，Eu，Sm，Nd，Pr，Bi）的金属-绝缘体转变以及绝缘化合物的拓扑性质.使用电荷自洽DFT+DMFT方法评估的准确的自由能表明，金属-绝缘体转变发生的A-阳离子半径之间的Nd和Pr，与实验一致。全进全出的磁性相在Nd化合物中是稳定的，但在Pr化合物中不是稳定的，它产生了小的Ir $^{4+}$磁矩$\approx 0.5μ_B$，并打开了相当大的关联能隙。我们证明，在这种最先进的理论方法中，绝缘体烧绿石铱酸盐在拓扑上是微不足道的。
The antiferromagnetic (AF) compound MnBi$_{2}$Te$_{4}$ is suggested to be the first realization of an antiferromagnetic (AF) topological insulator. Here we report on inelastic neutron scattering studies of the magnetic interactions in MnBi$_{2}$Te$_{4}$ that possess ferromagnetic (FM) triangular layers with AF interlayer coupling. The spin waves display a large spin gap and pairwise exchange interactions within the triangular layer are frustrated due to large next-nearest neighbor AF exchange. The degree of frustration suggests proximity to a variety of magnetic phases, potentially including s…
Metal-insulator transition (MIT) compounds are materials that may exhibit insulating or metallic behavior, depending on the physical conditions, and are of immense fundamental interest owing to their potential applications in emerging microelectronics. There is a dearth of thermally-driven MIT materials, however, which makes delineating these compounds from those that are exclusively insulating or metallic challenging. Here we report a material database comprising temperature-controlled MITs (and metals and insulators with similar chemical composition and stoichiometries to the MIT compounds) …
Observation of an unconventional metal-insulator transition in overdoped CuO_2 compounds
用非弹性光散射方法研究了$_2$ $_2$ $_2$ O $_{8+δ}$在较宽掺杂范围内正常态的电子动力学.电子弛豫的各向异性很强，不能单独用单粒子性质来解释。结果强烈表明存在一个非常规的量子临界金属-绝缘体过渡的“热”（波节）准粒子成为绝缘，而“冷”（节）准粒子保持金属。一种现象学的发展，它允许一个定量的拉曼结果的理解，并提供了一个场景，链接单和多粒子的属性。
We propose the parent compound of the newly discovered superconducting nickelate Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ as a self-doped Mott insulator, in which the low-density Nd-$5d$ conduction electrons couple to localized Ni-3$% d_{x^{2}-y^{2}}$ electrons to form Kondo spin singlets at low temperatures. This proposal is motivated with our analyses of the reported resistivity and Hall coefficient data in the normal state, showing logarithmic temperature dependence at low temperatures. In the strong Kondo coupling limit, we derive a generalized $t$-$J$ model with both Kondo singlets and nickel holons m…
The physical properties of rare-earth (RE) dodecaborides, characterized by a cage-glass crystal structure with loosely bound RE ions, are reviewed. These compounds are strongly correlated electron systems with simultaneously active charge, spin, orbital, and lattice degrees of freedom, which explains the complexity of all $R\mathrm{B}_{12}$ compounds including antiferromagnetic (TbB$_{12}$-TmB$_{12}$) and nonmagnetic (LuB$_{12}$) metals, on one side, and the so-called Kondo insulator compound YbB$_{12}$ and Yb-based Yb$_{x}R_{1-x}$B$_{12}$ solid solutions, on the other. The development of the …
Evidence of Mott Insulator with Thermally Induced Melting Behavior in Kagome Compound Nb3Cl8
International Center for Quantum Materials, School of Physics, Peking University（北京大学物理学院量子材料国际中心）；Centre for Quantum Physics, Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE), School of Physics, Beijing Institute of Technology（北京理工大学物理学院先进光电量子结构测量教育部重点实验室量子物理中心）；Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所凝聚态物理国家实验室）；Research Center for Electronic and Optical Materials, National Institute of Material Sciences（国立材料科学研究所电子与光学材料研究中心）；Research Center for Materials Nanoarchitectonics, National Institute of Material Sciences（国立材料科学研究所材料纳米构造研究中心）；School of Materials Science and Engineering, Peking University（北京大学材料科学与工程学院）；State Key Laboratory for Mesoscopic Physics, Frontiers Science Centre for Nano-optoelectronics, School of Physics, Peking University（北京大学物理学院介观物理国家重点实验室纳米光电子前沿科学中心）；Collaborative Innovation Center of Quantum Matter（量子物质协同创新中心）
NiS$_2$ has been widely regarded as a model system to study the bandwidth-controlled Mott transition, as enabled by isovalent Se chemical substitution on the S sites. Motivated by advances in electrolyte gating, we theoretically investigate the filling-controlled Mott transition induced by gating, which has the advantage of avoiding dopant disorder and stoichiometric changes. We use combined Density Functional Theory (DFT) and Dynamical Mean Field Theory (DMFT) to study such a filling-controlled transition and compare it with the case of bandwidth control. We draw a temperature-filling phase d…
