Pressure induced insulator to metal transition in quantum spin liquid candidate NaYbS_(2)

Pressure induced insulator to metal transition followed by the appearance of superconductivity has been observed recently in inorganic quantum spin liquid candidate NaYbSe_(2).In this paper,we study the properties of isostructural compound NaYbS_(2)under pressure.It is found that the resistance of Na YbS_(2)single crystal exhibits an insulating state below 82.9 GPa,but with a drop of more than six orders of magnitude at room temperature.Then a minimum of resistance is observed at about 100.1 GPa and it moves to lower temperature with further compression.Finally,a metallic state in the whole temperature range is observed at about 130.3 GPa accompanied by a non-Fermi liquid behavior below 100 K.The insulator to metal transition,non-monotonic resistance feature and non-Fermi liquid behavior of NaYbS_(2)under pressure are similar to those of NaYbSe_(2),suggesting that these phenomena might be the universal properties in NaLnCh_(2)(Ln=rare earth,Ch=O,S,Se)system.

Gapped Spin-1/2 Spinon Excitations in a New Kagome Quantum Spin Liquid Compound Cu_3Zn(OH)_6FBr

We report a new kagome quantum spin liquid candidate CuaZn（OH）6FBr, which does not experience any phase transition down to 50inK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature （-200 K）. A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from 19F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fraetionalization in the fractional quantum Hall state.

Some experimental schemes to identify quantum spin liquids

Despite the apparent ubiquity and variety of quantum spin liquids in theory,experimental confirmation of spin liquids remains to be a huge challenge.Motivated by the recent surge of evidences for spin liquids in a series of candidate materials,we highlight the experimental schemes,involving the thermal Hall transport and spectrum measurements,that can result in smoking-gun signatures of spin liquids beyond the usual ones.For clarity,we investigate the square lattice spin liquids and theoretically predict the possible phenomena that may emerge in the corresponding spin liquids candidates.The mechanisms for these signatures can be traced back to either the intrinsic characters of spin liquids or the external field-driven behaviors.Our conclusion does not depend on the geometry of lattices and can broadly apply to other relevant spin liquids.

Quantum Spin Liquid Phase in the Shastry–Sutherland Model Detected by an Improved Level Spectroscopic Method

We study the spin-1/2 two-dimensional Shastry–Sutherland spin model by exact diagonalization of clusters with periodic boundary conditions, developing an improved level spectroscopic technique using energy gaps between states with different quantum numbers. The crossing points of some of the relative(composite) gaps have much weaker finite-size drifts than the normally used gaps defined only with respect to the ground state, thus allowing precise determination of quantum critical points even with small clusters. Our results support the picture of a spin liquid phase intervening between the well-known plaquette-singlet and antiferromagnetic ground states, with phase boundaries in almost perfect agreement with a recent density matrix renormalization group study, where much larger cylindrical lattices were used [J. Yang et al., Phys. Rev. B 105, L060409(2022)]. The method of using composite low-energy gaps to reduce scaling corrections has potentially broad applications in numerical studies of quantum critical phenomena.

Crystal growth and magnetic properties of quantum spin liquid candidate KErTe_(2)

Recently rare-earth chalcogenides have been revealed as a family of quantum spin liquid(QSL)candidates hosting a large number of members.In this paper we report the crystal growth and magnetic measurements of KErTe_(2),which is the first member of telluride in the family.Compared to its cousins of oxides,sulfides and selenides,KErTe_(2) retains the high symmetry of R3m and Er3+ions still sit on a perfect triangular lattice.The separation between adjacent magnetic layers is expectedly increased,which further enhances the two dimensionality of the spin system.Specific heat and magnetic susceptibility measurements on KErTe_(2) single crystals reveal no structural and magnetic transition down to 1.8 K.Most interestingly,the absorption spectrum shows that the charge gap of KErTe_(2) is roughly 0.93±0.35 eV,which is the smallest among all the reported members in the family.This immediately invokes the interest towards metallization even superconductivity using the compound.

Recent progress on magnetic-field studies on quantum-spin-liquid candidates

Quantum spin liquids（QSLs） represent a novel state of matter in which quantum fluctuations prevent the conventional magnetic order from being established, and the spins remain disordered even at zero temperature. There have been many theoretical developments proposing various QSL states. On the other hand, experimental movement was relatively slow largely due to limitations on the candidate materials and difficulties in the measurements. In recent years, the experimental progress has been accelerated. In this topical review, we give a brief summary of experiments on the QSL candidates under magnetic fields. We arrange our discussions by two categories： i） Geometrically-frustrated systems, including triangularlattice compounds YbMgGaO4 and YbZnGaO4, κ-（BEDT-TTF）2 Cu2（CN）3, and EtMe3 Sb[Pd（dmit）2]2, and the kagom′e system ZnCu3（OH）6 Cl2; ii） the Kitaev material α-RuCl3. Among these, we will pay special attention to α-RuCl3, which has been intensively studied by ours and other groups recently. We will present evidence that both supports and rejects the QSL ground state for these materials, based on which we give several perspectives to stimulate further research activities.

三角晶格自旋液体候选材料NaYbSe_(2)在高压下的超导转变

量子自旋液体是一种由于自旋阻挫直到零温都不能形成磁有序的新奇量子态,并且和高温超导密切相关,因此,能否通过压力或化学掺杂等方式在量子自旋液体材料中调控出超导态甚至高温超导是一个重要的物理问题.二维三角晶格稀土硫属化合物NaYbCh_(2)(Ch=O,S,Se)在比热、核磁共振、中子散射等实验中未出现长程磁有序,被认为可能具有量子自旋液体基态.本文研究了NaYbCh_(2)(Ch=Se,S,O)在压力下的电输运行为.对于NaYbSe_(2),当压力加到26.9 GPa时出现超导转变,表现出零电阻行为,其超导转变温度(Tc)约为5.6 K,并且直到45 GPa都保持基本不变,得出了其超导转变温度对压力的相图;对于NaYbS2,压力使其室温电阻从10 GPa下10^(11)Ω量级降低到67 GPa的10 Ω量级,然而其电阻随温度行为没有出现金属性,也没有发生超导转变;而对于NaYbO2,其从常压到60 GPa高压一直保持完全绝缘态,没有可观测的电阻.

Effective model for rare-earth Kitaev materials and its classical Monte Carlo simulation

Recently,the family of rare-earth chalcohalides were proposed as candidate compounds to realize the Kitaev spin liquid(KSL)[Chin.Phys.Lett.38047502(2021)].In the present work,we firstly propose an effective spin Hamiltonian consistent with the symmetry group of the crystal structure.Then we apply classical Monte Carlo simulations to preliminarily study the model and establish a phase diagram.When approaching to the low temperature limit,several magnetic long range orders are observed,including the stripe,the zigzag,the antiferromagnetic(AFM),the ferromagnetic(FM),the incommensurate spiral(IS),the multi-Q,and the 120°ones.We further calculate the thermodynamic properties of the system,such as the temperature dependence of the magnetic susceptibility and the heat capacity.The ordering transition temperatures reflected in the two quantities agree with each other.For most interaction regions,the system is magnetically more susceptible in the ab-plane than in the c-direction.The stripe phase is special,where the susceptibility is fairly isotropic in the whole temperature region.These features provide useful information to understand the magnetic properties of related materials.

强关联材料霍尔热导率实验测量综述

在材料中输入热流并在垂直于热流的方向上施加磁场时,热载流子将可能被磁场偏转,获得横向速度,从而导致材料在横向出现一个温度梯度。这种效应被称为热霍尔效应(THE)。与电霍尔效应类似,热霍尔效应被预言将在一些拥有非平庸贝利曲率的材料中出现,因此它可以揭示材料的拓扑性质。然而,热霍尔效应并不像电霍尔一样,只局限于载流子带电的体系;相反,任何种类的准粒子都可以导热。因此,热霍尔效应也可以用来探索强关联电子体系材料(尤其是绝缘体)的奇异性质。因此,热霍尔效应更具有普适性,并日益成为探测电中性激发,如声子和磁振子的强有力手段。不仅如此,有如手性声子这样超越一般非平庸贝利曲率图像的因素仍可导致热霍尔效应;探查其中的热霍尔效应将为理解材料中复杂的微观机理指明方向。但是,热信号比电信号要微弱得多。尤其是测量热霍尔效应,往往要在较大背景噪音中提取微弱的有效信号,这使霍尔热导的测量极具挑战性。但是得益于科研工作者大量的努力,该领域在近几年发展迅速,得到了许多十分有趣的结果。在本文中,我们将简要总结现有的一些令人兴奋的在霍尔热导率测量方面的成果,指出尚未解决的问题,并提出未来可能的方向。

kagome晶格中无序和自旋交换作用对量子霍尔电导率的影响

数值计算了kagome晶格中无序和海森堡交换相互作用下的量子霍尔效应,其中无序包括非磁性杂质和磁性杂质,相互作用包括近邻的铁磁和反铁磁海森堡交换相互作用.计算结果表明,非磁性和磁性无序增强将导致量子化的霍尔电导平台从高朗道能级到低朗道能级逐步被破坏.除了破坏霍尔电导平台,磁性杂质还可以解除自旋简并,使系统发生从非常规量子霍尔效应到常规量子霍尔效应的转变.对于自旋交换作用影响下的霍尔电导,铁磁相互作用同样会使系统发生从非常规量子霍尔效应到常规量子霍尔效应的转变,而反铁磁相互作用却对系统的量子霍尔电导没有影响.这暗示着在S=1/2的反铁磁kagome体系中形成的量子自旋液体仍然保有零能隙的特征.

Thermodynamics of spin-1/2 Kagomé Heisenberg antiferromagnet:algebraic paramagnetic liquid and finite-temperature phase diagram

Quantum fluctuations from frustration can trigger quantum spin liquids(QSLs) at zero temperature.However, it is unclear how thermal fluctuations affect a QSL. We employ state-of-the-art tensor network-based methods to explore the ground state and thermodynamic properties of the spin-1=2 kagomé Heisenberg antiferromagnet(KHA). Its ground state is shown to be consistent with a gapless QSL by observing the absence of zero-magnetization plateau as well as the algebraic behaviors of susceptibility and specific heat at low temperatures, respectively. We show that there exists an algebraic paramagnetic liquid(APL) that possesses both the paramagnetic properties and the algebraic behaviors inherited from the QSL. The APL is induced under the interplay between quantum fluctuations from geometrical frustration and thermal fluctuations. By studying the temperature-dependent behaviors of specific heat and magnetic susceptibility, a finite-temperature phase diagram in a magnetic field is suggested, where various phases are identified. This present study gains useful insight into the thermodynamic properties of the spin-1/2 KHA with or without a magnetic field and is helpful for relevant experimental studies.

量子自旋液体的实验研究进展

量子自旋液体是指由于其中存在的强量子涨落导致自旋即使在零温极限下也不形成磁有序的一种新的自旋量子态.区别于传统的磁有序材料,它的基态没有确定的序参量来表示,并且不伴随任何自发的对称性破缺,超越了朗道相变理论所能描述的物相范畴,代表了一种新奇的量子物态,具有非常高的理论研究价值.这一全新的物态被认为与非常规超导机制之间有着十分紧密的关系.同时在未来的量子计算方面有着非常诱人的应用前景,因此一直以来备受关注.虽然量子自旋液体理论经过近半个世纪的积淀有了长足的发展,但是由于候选材料稀少,实验测量条件苛刻等多种因素制约,导致实验方面的进展相对缓慢.近年来各项实验技术的进步和成熟为量子自旋液体候选材料的测量表征提供了有利条件,加快了实验工作的推进速度.本文将从实验的角度介绍(1)几何阻挫量子自旋液体候选材料,包括三角晶格化合物YbMgGaO4和YbZnGaO4、κ-(BEDT-TTF)2Cu2(CN)3、EtMe3Sb[Pd(dmit)2]2和kagomé格子化合物ZnCu3(OH)6Cl2;(2) Kitaev量子自旋液体候选材料铱氧化物(Na2IrO3与α-,β-,γ-Li2IrO3)和α-RuCl3.文章将着重介绍近年来在量子自旋液体实验方面的进展,之后做一个简单的总结,最后对量子自旋液体的未来发展做一个展望.

量子自旋液体候选材料的缪子自旋弛豫研究

量子自旋液体是一种新奇的磁性物态。由于极强的量子涨落,直至零温都不会出现长程序。量子自旋液体的基态不能用序参量描述,并且缺少对称性破缺,因此该物态的实现打破朗道理论的范式。对于量子自旋液体的研究有助于理解高温超导的机理,并且可以被应用在量子计算和量子信息中。目前,尽管理论上有了长足的发展,但仍旧没有任何一个材料被证实为量子自旋液体。因此,探测和确认一个真正的量子自旋液体材料是当前的研究重点。缪子自旋弛豫是一个对磁场极为敏感的实验技术,被广泛应用于量子自旋液体候选材料的研究中。该技术可以观测基态中是否存在磁有序,测量系统中的涨落频率,这两点都是表征量子自旋液体的重要性质。本文简要介绍了量子自旋液体态和缪子自旋弛豫技术,回顾了近期在不同体系的量子自旋液体候选材料中的实验结果,特别是缪子自旋弛豫的成果。这些体系包括一维反铁磁海森堡链(苯甲酸铜),三角格子(YbMgGaO4,NaYbO2和TbInO3),笼目格[ZnCu3(OH)6Cl2和Tm3Sb3Zn2O14],蜂窝状格子(Na2IrO3和α-RuCl3),以及烧绿石结构(Tb2Ti2O7,Pr2Ir2O7和Ce2Zr2O7)。

强关联电子系统中的热输运测量

当考虑电子间的库伦排斥相互作用,以及电荷、自旋和轨道之间的相互耦合时,诸多超越了近自由电子框架的新奇量子态涌现而出,如非常规超导态和量子自旋液体等。对这些新奇物态的认知不仅会拓展现有的知识框架,也有望引发新一轮的量子科技革命。因此,对强关联物理的研究是当下凝聚态物理领域的前沿课题。铜基高温超导体的母体是一种莫特绝缘体,在传统的能带论之下被预言为金属态。然而电子间的强关联行为使得它表现出绝缘体的性质。由于莫特绝缘体中库伦相互作用致使能隙打开并冻结其中的电荷自由度,所以在该体系中难以开展电输运性质的测量研究。作为一种对于元激发(不仅包括电子,还包括磁振子、自旋子等)敏感的探针,热输运测量在强关联电子系统的研究中发挥着重要的作用。本文回顾了近些年在非常规超导、重费米子系统和量子自旋液体研究中一些有趣的纵向热输运性质的研究成果,并与我们近期发表的运用横向热导率测量热霍尔现象的综述文章相互补充。

有限温度张量重正化群方法及其在阻挫量子磁性研究中的应用

阻挫量子磁体中的新奇物态与效应是凝聚态物理研究的重要前沿方向,因其与高温超导、拓扑量子计算等的密切联系,近年来吸引了人们浓厚的研究兴趣。实验上,阻挫自旋液体候选材料的新进展层出不穷,人们系统地研究了若干三角晶格、笼目晶格和六角Kitaev阻挫磁体等材料,发现其在一定条件下展现出自旋液体态的特征,但澄清其中的量子物态是充满挑战的量子多体问题。作者最近的工作指出,可以从有限温度张量重正化群多体计算入手,开展热力学性质的精确计算与分析,确定阻挫磁体的微观自旋模型,做出进一步理论预言并开展实验验证,从而建立量子磁性系统的多体计算精确研究方案。有限温度张量重正化群方法是计算大尺寸二维阻挫量子自旋模型有限温度性质的有力工具,在本文中作者首先介绍新近发展的系列张量重正化群方法,包括线性和指数张量重正化群等。随后,作者讨论有限温度张量方法在三角晶格量子伊辛磁体TmMgGaO_(4)和六角晶格Kitaev磁体α-RuCl_(3)的微观自旋模型中的具体应用:通过高精度和全面的多体计算,揭示出其中存在演生U(1)对称性与拓扑相变,以及高场量子自旋液体态等新颖的结论,这些理论预言也陆续被实验所证实。通过上述实例,作者展示了有限温度张量重正化群计算方法在自旋液体候选材料研究中的应用价值,并期待这些方法能在强关联量子物质研究中发挥重要作用。

Phase Diagram of an <i>S</i>= 1/2 <i>J</i>₁-<i>J</i>₂Anisotropic Heisenberg Antiferromagnet on a Triangular Lattice

We study the ground state of an S=1/2 anisotropic a (≡Jz/Jxy) Heisenberg antiferromagnet with nearest (J1) and next-nearest (J2) neighbor exchange interactions on a triangular lattice using the exact diagonalization method. We obtain the energy, squared sublattice magnetizations, and their Binder ratios on finite lattices with N&le;36 sites. We estimate the threshold J(t) 2 (a)?between the three-sublattice Néel state and the spin liquid (SL) state, and J(s) 2 (a)? between the stripe state and the SL state. The SL state exists over a wide range in the α-J2 plane. For α>1 , the xy component of the magnetization is destroyed by quantum fluctuations, and the classical distorted 120&deg;structure is replaced by the collinear state.

Electron-nuclear hyperfine coupling in quantum kagome antiferromagnets from first-principles calculation and a reflection of the defect effect

The discovery of ideal spin-1/2 kagome antiferromagnets Herbertsmithite and Zn-doped Barlowite represents a breakthrough in the quest for quantum spin liquids(QSLs),and nuclear magnetic resonance(NMR)spectroscopy plays a prominent role in revealing the quantum paramagnetism in these compounds.However,interpretation of NMR data that is often masked by defects can be controversial.Here,we show that the most significant interaction strength for NMR,i.e.the hyperfine coupling(HFC)strength,can be reasonably reproduced by first-principles calculations for these proposed QSLs.Applying this method to a supercell containing Cu-Zn defects enables us to map out the variation and distribution of HFC at different nuclear sites.This predictive power is expected to bridge the missing link in the analysis of the low-temperature NMR data.

Exact solution to a class of generalized Kitaev spin-1/2 models in arbitrary dimensions

We construct a class of exactly solvable generalized Kitaev spin-1/2 models in arbitrary dimensions, which is beyond the category of quantum compass models. The Jordan-Wigner transformation is employed to prove the exact solvability. An exactly solvable quantum spin-1/2 model can be mapped to a gas of free Majorana fermions coupled to static Z2 gauge fields. We classify these exactly solvable models according to their parent models. Any model belonging to this class can be generated by one of the parent models. For illustration, a two dimensional(2D) tetragon-octagon model and a three dimensional(3D) xy bond model are studied.

无能隙量子自旋液体和正方晶格自旋1/2阻挫反铁磁模型的相图

自高温超导发现以来,正方晶格上由自旋为1/2构成的包括近邻和次近邻海森堡相互作用的阻挫反铁磁模型,即J1±J2模型,因其基态被认为可以作为铜基超导的母态,在过去30多年中得到了广泛关注和深入研究.该模型包含的相互作用形式简单,如今也成为用来研究正方晶格上阻挫磁性的基本理论模型.但其基态性质仍然是一个悬而未决的问题.本文作者利用最近发展的二维张量网络态算法,对该模型进行了迄今为止精度最高的计算.详细的数值结果表明这个体系的基态包含了一个无能隙的量子自旋液体和价键固体态.他们进一步计算了相变的临界指数,提出了一个低能有效理论来描述这个量子自旋液体,并探讨了这个量子自旋液体与解禁闭量子临界性的关系.该工作通过仔细比较二维张量网络态和人们常用的密度矩阵重整化群两种算法的结果,提供了在正方晶格阻挫磁性体系的研究上第一个超越密度矩阵重整化方法的例子.他们也解释了以前研究结果矛盾之处的来源,为用二维张量网络态解决量子多体问题提供了一个范本.

Strongly correlated new state Fermi systems as a of matter

The aim of this review paper is to expose a new state of matter exhibited by strongly correlated Fermi systems represented by various heavy-fermion （HF） metals, two-dimensional liquids like 3He, compounds with quantum spin liquids, quasicrystals, and systems with one-dimensional quantum spin liquid. We name these various systems HF compounds, since they exhibit the behavior typical of HF metals. In HF compounds at zero temperature the unique phase transition, dubbed throughout as the fermion condensation quantum phase transition （FCQPT） can occur; this FCQPT creates flat bands which in turn lead to the specific state, known as the fermion condensate. Unlimited increase of the effective mass of quasiparticles signifies FCQPT; these quasiparticles determine the thermodynamic, transport and relaxation properties of HF compounds. Our discussion of numerous salient experimen- tal data within the framework of FCQPT resolves the mystery of the new state of matter. Thus, FCQPT and the fermion condensation can be considered as the universal reason for the non-Fermi liquid behavior observed in various HF compounds. We show analytically and using arguments based completely on the experimental grounds that these systems exhibit universal scaling behavior of their thermodynamic, transport and relaxation properties. Therefore, the quantum physics of different HF compounds is universal, and emerges regardless of the microscopic structure of the compounds. This uniform behavior allows us to view it as the main characteristic of a new state of matter exhibited by HF compounds.