What are the ground states of an interacting, low-density electron system? In the absence of disorder, it has long been expected that as the electron density is lowered, the exchange energy gained by aligning the electron spins should exceed the enhancement in the kinetic (Fermi) energy, leading to a (Bloch) ferromagnetic transition. At even lower densities, another transition to a (Wigner) solid, an ordered array of electrons, should occur. Experimental access to these regimes, however, has been limited because of the absence of a material platform that supports an electron system with very high-quality (low disorder) and low density simultaneously. Here we explore the ground states of interacting electrons in an exceptionally-clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well. The large electron effective mass in this system allows us to reach very large values of the interaction parameter $r_s$, defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases, qualitatively consistent with the above scenario: a paramagnetic phase at large densities, a spontaneous transition to a ferromagnetic state when $r_s$ surpasses 35, and then a phase with strongly non-linear current-voltage characteristics, suggestive of a pinned Wigner solid, when $r_s$ exceeds $\simeq 38$. However, our sample makes a transition to an insulating state at $r_s\simeq 27$, preceding the onset of the spontaneous ferromagnetism, implying that, besides interaction, the role of disorder must also be taken into account in understanding the different phases of a realistic dilute electron system.
We perform ab-initio calculations on Bi$_\mathrm{Se}$ antisite defects in the surface of Bi$_2$Se$_3$, finding strong low-energy defect resonances with a spontaneous ferromagnetism, fixed to an out-of-plane orientation due to an exceptional large magnetic anisotropy energy. For antisite defects in the surface layer, we find semi-itinerant ferromagnetism and strong hybridization with the Dirac surface state, generating a finite energy gap. For deeper lying defects, such hybridization is largely absent, the magnetic moments becomes more localized, and no energy gap is present.
Recent experiments have pointed to a chiral p-wave-like superconductivity in epitaxial Bi/Ni bilayers that are spontaneously time-reversal symmetry breaking (TRSB), making it a promising platform for exploring physics useful for topologically protected quantum computing. Quite intriguingly, evidence has emerged that in non-epitaxial Bi/Ni bilayers, superconductivity arises due to the formation of NiBi3, which has been reported to host coexisting ferromagnetic and superconducting orders at the surface. We perform high resolution surface magneto-optic Kerr effect (SMOKE) measurements using a Sag…
Spontaneous nonreciprocal transport in a gate-tunable ferromagnetic Rashba 2-dimensional electron gas
Laboratoire Albert Fert, CNRS, Thales, Université Paris-Saclay（阿尔贝·费尔特实验室，法国国家科学研究中心，泰雷兹集团，巴黎萨克雷大学）；Helmholtz-Zentrum Berlin（柏林赫尔姆霍兹中心）；Institut für Physik, Martin-Luther-Universität, Halle-Wittenberg（马丁·路德哈勒-维滕贝格大学物理研究所）；Plateforme Mesures Physiques à Basses Températures, Sorbonne Université（索邦大学低温测量平台）；Saha Institute of Nuclear Physics（萨哈核物理研究所）；Max Planck Institute of Microstructure Physics（马克斯·普朗克微结构物理研究所）
Coupling an antisymmetric tensor field to the electromagnetic field in a dyonic Reissner-Nordström-AdS black hole background, we build a holographic model for the paramagnetism/ferromagnetism phase transition. In the case of zero magnetic field, the time reversal symmetry is broken spontaneously and spontaneous magnetization happens in low temperature. The critical exponents are in agreement with the ones from mean field theory. In the case of nonzero magnetic field, the model realizes the hysteresis loop of single magnetic domain and the magnetic susceptibility satisfies the Curie-Weiss law.
In 1929 Felix Bloch suggested that the paramagnetic Fermi sea of electrons should make a spontaneous transition to a fully-magnetized state at very low densities, because the exchange energy gained by aligning the spins exceeds the enhancement in the kinetic energy. We report here the observation of an abrupt, interaction-driven transition to full magnetization, highly reminiscent of Bloch ferromagnetism that has eluded experiments for the last ninety years. Our platform is the exotic two-dimensional Fermi sea of composite fermions at half-filling of the lowest Landau level. Via quantitative m…
Spin-flip excitations in a quantum Hall electron system at fixed filling factor nu=2 are modelled and studied under conditions of a strong Coulomb interaction when the `Landau level mixing' is a dominant factor determining the excitation energy. The `one-exciton' approach used for the purely electronic excitations in question allows us to describe the Stoner transition from the unpolarized/paramgnet state to the polarized/ferromagnet one. The theoretical results are compared with the available experimental data.
The existence of the Nagaoka ferromagnetism is examined in the context of the one-dimensional $U=\infty$ Hubbard model. We construct the exact quantum partition function to describe the physics of such a regime. Our calculation reveals that, while the ground state in an open chain is always spin-degenerate, in a finite size closed chain with at least one vacancy, the ground state can only be ferromagnetic when the number of electrons is less or equal to three. Our results shed more light on a very recent experimental verification of Nagaoka ferromagnetism in a quantum dot set up.
Weak ferromagnetism of quasi-one-dimensional S=1/2 antiferromagnet BaCu$_2$Ge$_2$O$_7$
用磁化强度测量方法研究了准一维S = 1/2反铁磁体BaCu $_2$ Ge $_2$ O $_7$的弱铁磁性.自发磁化沿b轴出现。链内最近邻自旋之间的局部对称性允许Dzyaloshinskiii-Moriya相互作用的存在，并且每个自旋的弱铁磁矩的唯一可能的空间配置决定了链间相互作用的符号。弱的$a$轴磁场可以使磁化方向改变为$a$轴方向，这表明自旋链形成弱耦合的弱铁磁链系统。
We have studied the magnetic properties of pressure-induced ferromagnet Ca2RuO4 to reveal the uniqueness of the 4d-electron ferromagnetism in the quasi-two-dimensional conductor. The magnetic parameters have been estimated from the paramagnetic susceptibility and the magnetisation process under pressure up to 2 GPa. The parameters can well be interpreted on the basis of the self-consistent renormalization theory of spin fluctuation for 3D-itinerant ferromagnet. Nevertheless, the metallic Ca2RuO4 shows quite strong anisotropy not only in the conductivity but also in the magnetisation process. S…
We present an extension of the cell-construction method for the flat-band ferromagnetism. In a rather general setting, we construct Hubbard models with highly degenerate single-electron ground states and obtain a formal representation of these single-electron ground states. By our version of the cell-construction method, various types of flat-band Hubbard models, including the one on line graphs, can be designed and shown to have the unique ferromagnetic ground states when the electron number is equal to the degeneracy of the single-electron ground states.
The (111) surface of SnTe hosts one isotropic $\barΓ$-centered and three degenerate anisotropic $\bar{M}$- centered Dirac surface states. We predict that a nematic phase with spontaneously broken $C_3$ symmetry will occur in the presence of an external magnetic field when the $N=0$ $\bar{M}$ Landau levels are $1/3$ or $2/3$ filled. The nematic state phase boundary is controlled by a competition between intravalley Coulomb interactions that favor a valley-polarized state, and weaker intervalley scattering processes that increase in relative strength with magnetic field. An in-plane Zeeman field…
We construct a model-independent framework describing stabilities of ferromagnetism in strongly correlated electron systems; Our description relies on the operator theoretic correlation inequalities. Within the new framework, we reinterpret the Marshall-Lieb-Mattis theorem and Lieb\rq{}s theorem; in addition, from the new perspective, we prove that Lieb\rq{}s theorem still holds true even if the electron-phonon and electron-photon interactions are taken into account. We also examine the Nagaoka-Thouless theorem and its stabilities. These examples verify the effectiveness of our new viewpoint.
We study the electronic structure of the Pd-terminated surface of the non-magnetic delafossite oxide metal PdCoO$_2$. Combining angle-resolved photoemission spectroscopy and density-functional theory, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability towards itinerant surface ferromagnetism. Our results reveal how this leads to a rich multi-band surface electronic structure, and provide spectroscopic evidence for an intriguing sample-dependent coupling of the surface electrons to a bosonic mode which we attribute to electron-magnon …
Pressure-induced ferromagnetism with strong Ising-type anisotropy in YbCu$_2$Si$_2$
我们报告直流磁性测量YbCu $_2$ Si $_2$在10 GPa以上的压力使用微型陶瓷砧室。YbCu $_2$ Si $_2$在8 GPa压力下发生了从非磁性相到磁性相的压力诱导相变。我们发现一个自发的直流磁化在压力诱导阶段高于9.4 GPa。测定了2.0K时铁磁转变温度Tc和自发磁矩m_0的压力依赖关系。宏观测量的m_0值不到穆斯堡尔实验的一半。这种差异可以归因于铁磁相和顺磁相之间的空间相分离。这种分离表明，压力引起的相边界之间的顺磁和铁磁状态是一阶的。此外，我们还研究了压致铁磁态的磁各向异性。压力对磁化强度的影响沿磁易$c$ -轴的磁场比沿硬$a$ -轴的磁场大得多。压力诱导相具有很强的Ising型单轴各向异性，与针对YbCu $_2$ Si $_2$提出的两种晶体电场（CEF）模型一致。
We construct a multi-band Hubbard model on the lattice obtained by "decorating" a closely packed $d$-dimensional lattice $\mathcal{M}$ (such as the triangular lattice) where $d\ge2$. We take the limits in which the Coulomb interaction and the band gap become infinitely large. Then there remains only a single band with finite energy, on which electrons are supported. Let the electron number be $N_\mathrm{e}=|\mathcal{M}|-N_\mathrm{h}$, where $|\mathcal{M}|$ corresponds to the electron number which makes the lowest (finite energy) band half-filled, and $N_\mathrm{h}$ is the number of "holes". It…
To investigate ferromagnetic semiconductors and insulators, such as the famous EuO, EuS, or CrBr$_3$, we propose a hybridized Kondo-lattice model, where, in addition to the conduction electrons, localized moments (e.g., the $4f$-electrons) are modeled as a strongly correlated band system. The quasi-empty conduction band is weakly filled due to the hybridization term. This activates the intraatomic exchange coupling between conduction and localized electrons. Temperature-dependent phase diagrams and quasiparticle densities of states are presented for various coupling and hybridization strengths…
Ferromagnetism in the one-dimensional Hubbard model with orbital degeneracy: From low to high electron density
用密度矩阵重整化群方法研究了双简并原子轨道的一维哈伯德模型的铁磁性，得到了基态相图。结果表明，当相互作用足够强时，铁磁性从低到高（0< n < 1.75）电子密度是稳定的。准长程序的三重态超导电性与铁磁有序共存的强Hund耦合区，其中轨道间相互作用U '-J是吸引的。在四分之一填充（n=1），绝缘铁磁态出现伴随着轨道准长程序。对于低密度（n<1），由于电子的虚跳跃引起的铁磁交换相互作用，铁磁性发生，与四分之一填充系统相同。这来自于强耦合极限中电荷和自旋-轨道自由度的分离。这种铁磁性对于能带结构的变化是脆弱的。对于高密度（n>1），铁磁相的相图类似于在无限维中获得的相图。在这种情况下，双交换机制是可操作的，以稳定的铁磁秩序和这种长程序是强大的对带色散的变化。在1.68<n<1.75的密度区出现部分极化态，在半填充区（n=2）的正下方出现相分离。
