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Tunable deconfined quantum criticality and interplay of different valence-bond solid phases

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Chinese Physics B 2020年第5期29卷 52-65页

作者：赵博文 Jun Takahashi anders w.sandvik Department of Physics Boston University590 Commonwealth AvenueBostonMassachusetts 02215USA Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190China

we use quantum Monte Carlo simulations to study an S = 1/2 spin model with competing multi-spin interactions. we find a quantum phase transition between a columnar valence-bond solid(cVBS) and a Néel antiferromagnet(AFM), as in the scenario of deconfined quantum-critical points, as well as a transition between the AFM and a staggered valence-bond solid(sVBS). By continuously varying a parameter, the sVBS–AFM and AFM–cVBS boundaries merge into a direct sVBS–cVBS transition. Unlike previous models with putative deconfined AFM–cVBS transitions, e.g., the standard J–Q model,in our extended J–Q model with competing cVBS and sVBS inducing terms the transition can be tuned from continuous to first-order. we find the expected emergent U(1) symmetry of the microscopically Z4 symmetric cVBS order parameter when the transition is continuous. In contrast, when the transition changes to first-order, the clock-like Z4 fluctuations are absent and there is no emergent higher symmetry. we argue that the confined spinons in the sVBS phase are fracton-like.we also present results for an SU(3) symmetric model with a similar phase diagram. The new family of models can serve as a useful tool for further investigating open questions related to deconfined quantum criticality and its associated emergent symmetries.

关键词： quantum phase transitions deconfined quantum criticality quantum spin systems quantum Monte Carlo simulations

Typicality at quantum-critical points

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Chinese Physics B 2018年第8期27卷 517-528页

作者：刘录 anders w.sandvik 郭文安 Department of Physics Beijing Normal University Department of Physics Boston University Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of Sciences

we discuss the concept of typicality of quantum states at quantum-critical points, using projector Monte Carlo simu- lations of an S ---- 1/2 bilayer Heisenberg antiferromagnet as an illustration. with the projection （imaginary） time t scaled as t= aLz, L being the system length and z the dynamic critical exponent （which takes the value z = 1 in the bilayer model studied here）, a critical point can be identified which asymptotically flows to the correct location and universality class with increasing L, independently of the prefactor a and the initial state. Varying the proportionality factor a and the initial state only changes the cross-over behavior into the asymptotic large-L behavior. In some cases, choosing an optimal factor a may also lead to the vanishing of the leading finite-size corrections. The observation of typicality can be used to speed up simulations of quantum criticality, not only within the Monte Carlo approach but also with other numerical methods where imaginary-time evolution is employed, e.g., tensor network states, as it is not necessary to evolve fully to the ground state but only for sufficiently long times to reach the typicality regime.

关键词： typicality quantum criticality bilayer Heisenberg antiferromagnet model imaginary projectingtime, projecting quantum Monte Carlo simulation

Consistent Scaling Exponents at the Deconfined Quantum-Critical Point

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Chinese Physics Letters 2020年第5期37卷 93-97页

作者： anders w.sandvik 赵博文 Department of Physics Boston UniversityBostonMassachusetts 02215USA Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190

we report a quantum Monte Carlo study of the phase transition between antiferromagnetic and valence-bond solid ground states in the square-lattice S=1/2 J-Q model.The critical correlation function of the Q terms gives a scaling dimension corresponding to the value v=0.455±0.002 of the correlation-length exponent.This value agrees with previous(less precise)results from conventional methods,e.g.,finite-size scaling of the nearcritical order parameters.we also study the Q-derivatives of the Binder cumulants of the order parameters for L2 lattices with L up to 448.The slope grows as L1/v with a value of v consistent with the scaling dimension of the Q term.There are no indications of runaway flow to a first-order phase transition.The mutually consistent estimates of v provide compelling support for a continuous deconfined quantum-critical point.

关键词： transition. confined scaling

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具有2个长度尺度的量子临界现象

北京师范大学学报（自然科学版） 2016年第2期52卷 141-142页

作者：邵慧郭文安 anders w.sandvik 北京计算科学研究中心北京100084 波士顿大学物理学系波士顿马塞诸塞州美国02215 北京师范大学物理学系北京100875

相变和临界现象是一种普遍存在的物理现象,它既发生于人们的日常生活中（例如冰融化成水）,又是产生新奇物态的普遍机制（例如高温超导体的形成）.通常的相变由热涨落驱动,也称经典相变.量子相变则发生在绝对零温,在外参数变化时由量子... 详细信息

相变和临界现象是一种普遍存在的物理现象,它既发生于人们的日常生活中（例如冰融化成水）,又是产生新奇物态的普遍机制（例如高温超导体的形成）.通常的相变由热涨落驱动,也称经典相变.量子相变则发生在绝对零温,在外参数变化时由量子涨落驱动.建立在对称性破缺,序参量涨落和重整化群基础上的朗道-金兹堡-威尔逊理论（Landau-Ginzburg-wilson,LGw）成功解释了经典相变.

关键词：量子相变临界现象长度高温超导体对称性破缺物理现象量子涨落参数变化

Emergent O(4) symmetry at the phase transition from plaquette-singlet to antiferromagnetic order in quasi-two-dimensional quantum magnets

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Chinese Physics B 2021年第6期30卷 570-582页

作者：孙光宇马女森赵博文 anders w.sandvik 孟子杨 Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China School of Physics Key Laboratory of Micro-Nano Measurement-Manipulation and PhysicsBeihang UniversityBeijing 100191China Department of Physics Boston University590 Commonwealth AvenueBostonMassachusetts 02215USA Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongHong KongChina

Recent experiments [Guo et al., Phys. Rev. Lett. 124 206602(2020)] on thermodynamic properties of the frustrated layered quantum magnet SrCu_(2)(BO_(3))_(2)-the Shastry–Sutherland material-have provided strong evidence for a lowtemperature phase transition between plaquette-singlet and antiferromagnetic order as a function of pressure. Further motivated by the recently discovered unusual first-order quantum phase transition with an apparent emergent O(4) symmetry of the antiferromagnetic and plaquette-singlet order parameters in a two-dimensional "checkerboard J-Q" quantum spin model[Zhao et al., Nat. Phys. 15 678(2019)], we here study the same model in the presence of weak inter-layer couplings. Our focus is on the evolution of the emergent symmetry as the system crosses over from two to three dimensions and the phase transition extends from strictly zero temperature in two dimensions up to finite temperature as expected in SrCu_(2)(BO_(3))_(2).Using quantum Monte Carlo simulations, we map out the phase boundaries of the plaquette-singlet and antiferromagnetic phases, with particular focus on the triple point where these two ordered phases meet the paramagnetic phase for given strength of the inter-layer coupling. All transitions are first-order in the neighborhood of the triple point. we show that the emergent O(4) symmetry of the coexistence state breaks down clearly when the interlayer coupling becomes sufficiently large, but for a weak coupling, of the magnitude expected experimentally, the enlarged symmetry can still be observed at the triple point up to significant length scales. Thus, it is likely that the plaquette-singlet to antiferromagnetic transition in SrCu_(2)(BO_(3))_(2) exhibits remnants of emergent O(4) symmetry, which should be observable due to additional weakly gapped Goldstone modes.

关键词： quantum phase transitions quantum spin systems emergent symmetry quantum Monte Carlo simulations

Quantum Phases of SrCu2(BO3)2 from High-Pressure Thermodynamics

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Bulletin of the Chinese Academy of Sciences 2020年第2期34卷 104-105页

作者： SUN Liling anders sandvik Institute of Physics CAS Boston University

In recent years,theoretical proposals for exotic states in quantum magnets are developing very rapidly,but many of these intriguing quantum phases and transitions have been difficult to realize experimentally.One class of such states is the valence bond solid,in which spins entangle locally and form symmetry-breaking singlet patterns.Signatures of a state with four-spin singlets were recently detected in the two-dimensional(2D)quantum magnet SrCu2(BO3)2 under high pressure.

关键词： quantum singlet symmetry

EQ2594焊带电渣堆焊方法获得超级双相不锈钢耐蚀合金堆焊层的性能评估

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金属世界 2015年第6期 26-30页

作者： Johan L?thman anders waller? Ola Runnerstam 张怀征山特维克材料科技焊接产品事业部山特维克材料科技研发部山特维克材料科技

超级双相不锈钢（SDSS）凭借卓越的耐蚀性适用于各类严苛的应用环境中,如氯化海水以及温度低于300℃的酸性含氯介质中。其典型的应用包括石油和天然气、海水淡化及其他海洋工程相关领域、脱硫装置以及造纸和化工行业中需要含氯漂白的环... 详细信息

超级双相不锈钢（SDSS）凭借卓越的耐蚀性适用于各类严苛的应用环境中,如氯化海水以及温度低于300℃的酸性含氯介质中。其典型的应用包括石油和天然气、海水淡化及其他海洋工程相关领域、脱硫装置以及造纸和化工行业中需要含氯漂白的环境。在本次研究中,sandvik 25.10.4.L超级双相不锈钢焊带通过使用非合金焊剂进行电渣堆焊（ESw）获得符合EQ 2594成分的堆焊层。结果显示,堆焊层熔敷金属的临界点蚀温度（CPT）为45℃,耐点蚀当量（PRE）值≥41,基本上不存在有害析出相。熔敷金属的化学成分符合标准,铁素体数为50-60或者体积含量30%左右。力学性能方面,已通过侧弯实验。在零下46℃环境下冲击强度约为125 J,抗拉强度达到830 MPa,延伸率在30%左右。

关键词：堆焊层双相不锈钢 EQ2594 电渣堆焊耐蚀合金熔敷金属侧弯实验脱硫装置力学性能冲击强度

EQ2594焊带在超级双相不锈钢耐蚀合金堆焊中的应用

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金属加工（热加工） 2015年第18期 28-30页

作者： Johan L?thman anders waller? Ola Runnerstam 张怀征山特维克材料科技焊接产品事业部山特维克材料科技研发部山特维克材料科技(上海)

与奥氏体不锈钢和镍基材料相比,超级双相不锈钢(SDSS)具有一系列独特特性,使SDSS适合用于各类严苛的应用环境中。本文对sandvik 25.10.4.L超级双相钢焊带通过使用非合金焊剂进行电渣堆焊(ESw)获得符合EQ 2594成分的堆焊层。结果显示,堆... 详细信息

与奥氏体不锈钢和镍基材料相比,超级双相不锈钢(SDSS)具有一系列独特特性,使SDSS适合用于各类严苛的应用环境中。本文对sandvik 25.10.4.L超级双相钢焊带通过使用非合金焊剂进行电渣堆焊(ESw)获得符合EQ 2594成分的堆焊层。结果显示,堆焊层熔敷金属的化学成分以及综合力学性能都达到了使用要求。

关键词：堆焊层双相不锈钢 EQ2594 耐蚀合金电渣堆焊熔敷金属奥氏体不锈钢双相钢铁素体含量带极堆焊