自旋阻挫重费米子体系中的量子相变

近藤效应和RKKY交换相互作用的竞争决定了多数重费米子化合物的基态性质。通过压力、磁场等非热力学参量调控,该类材料能够在绝对零温附近实现费米液体和磁有序相之间的连续转变,提供了研究量子相变的理想平台。另一方面,在绝缘的量子磁体中,自旋阻挫引起的量子涨落抑制低温下长程磁有序的发生,导致自旋液体相等新奇物态的产生。在近藤晶格中引入自旋阻挫将给重费米子材料提供一个新的调控维度,深刻改变该类材料的量子临界相图,是重费米子材料领域的一个新颖研究方向。文章首先介绍阻挫重费米子体系的研究背景,然后针对CePdAl的物性展开讨论,探讨阻挫对重费米子材料量子临界物性的影响以及量子临界相的普适性。     

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TmMgGaO4 和六角晶格Kitaev 磁体α-RuCl3 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。     

强关联材料霍尔热导率实验测量综

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。      

Strongly correlated Fermi systems as a new state 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 experimental 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.     

Thermal Entanglement and Correlated Coherence in Two Coupled Double Quantum Dots Systems

In this work, the thermal quantum correlations in two coupled double semiconductor charge qubits are investigated. This is carried out by deriving analytical expressions for both the thermal concurrence and the correlated coherence. The effects of the tunneling parameters, the Coulomb interaction, and the temperature on the thermal entanglement and on the correlated coherence are studied in detail. It is found that the Coulomb potential plays an important role in the thermal entanglement and in the correlated coherence of the system. The results also indicate that the Coulomb potential can be used for significant enhancement of the thermal entanglement and quantum coherence. One interesting aspect is that the correlated coherence capture all the thermal entanglement at low temperatures, that is, the local coherences are totally transferred to the thermal entanglement. Finally, the role played by thermal entanglement and the correlated coherence responsible for quantum correlations are focused on. It is shown that in all cases, the correlated coherence is more robust than the thermal entanglement so that quantum algorithms based only on correlated coherence may be more robust than those based on entanglement.     

Magnetic quantum criticality in quasi‐one‐dimensional Heisenberg antiferromagnet

We analyze exciting recent measurements [Phys. Rev. Lett. 114 (2015) 037202] of the magnetization, differential susceptibility and specific heat on one dimensional Heisenberg antiferromagnet Cu(C₄H₄N₂)(NO₃)₂ (CuPzN) subjected to strong magnetic fields. Using the mapping between magnons (bosons) in CuPzN and fermions, we demonstrate that magnetic field tunes the insulator towards quantum critical point related to so‐called fermion condensation quantum phase transition (FCQPT) at which the resulting fermion effective mass diverges kinematically. We show that the FCQPT concept permits to reveal the scaling behavior of thermodynamic characteristics, describe the experimental results quantitatively, and derive for the first time the (temperature—magnetic field) phase diagram, that contains Landau‐Fermi‐liquid, crossover and non‐Fermi liquid parts, thus resembling that of heavy‐fermion compounds.     

Electronic properties of CuO 2 -planes: A DMFT study

We study one-particle spectra and the electronic band-structure of a CuO ₂ -plane within the three-band Hubbard model. The Dynamical Mean-Field Theory (DMFT) is used to solve the many particle problem. The calculations show that the optical gap is given by excitations from the lower Hubbard band into the so called Zhang-Rice singlet band. The optical gap turns out to be considerably smaller than the bare charge transfer energy () for a typical set of parameters, which is in agreement with experiment. We also investigate the dependence of the shape of the Fermi surface on the different hopping parameters t CuO and t OO. A value t OO / t CuO >0 leads to a Fermi surface surrounding the M point. Received 21 September 1998 and Received in final form 8 June 1999