Magneto-optical Kerr effect microscopy is used to show that monolayer chromium triiodide is an Ising ferromagnet with out-of-plane spin orientation. The question of what happens to the properties of a material when it is thinned down to atomic-scale thickness has for a long time been a largely hypothetical one. In the past decade, new experimental methods have made it possible to isolate and measure a range of two-dimensional structures, enabling many theoretical predictions to be tested. But it has been a particular challenge to observe intrinsic magnetic effects, which could shed light on the longstanding fundamental question of whether intrinsic long-range magnetic order can robustly exist in two dimensions. In this issue of Nature, two groups address this challenge and report ferromagnetism in atomically thin crystals. Xiang Zhang and colleagues measured atomic layers of Cr2Ge2Te6 and observed ferromagnetic ordering with a transition temperature that, unusually, can be controlled using small magnetic fields. Xiaodong Xu and colleagues measured atomic layers of CrI3 and observed ferromagnetic ordering that, remarkably, was suppressed in double layers of CrI3, but restored in triple layers. The two studies demonstrate a platform with which to test fundamental properties of purely two-dimensional magnets. Since the discovery of graphene1, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin–valley coupling2, Ising superconductors3,4,5 that can be tuned into a quantum metal6, possible Mott insulators with tunable charge-density waves7, and topological semimetals with edge transport8,9. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered10,11,12,13,14; such a crystal would be useful in many technologies from sensing to data storage15. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin–Wagner theorem16. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals17,18,19. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect20, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics12, and van der Waals engineering to produce interface phenomena15.

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Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.

Weyl and Dirac semimetals in three-dimensional solids

Discovery of a Weyl Fermion Semimetal and Topological Fermi Arcs

A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs). Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.

https://science.sciencemag.org/content/sci/early/2015/07/15/science.aaa9297.full.pdf

Discovery of a Weyl fermion semimetal and topological Fermi arcs

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

We observed quantum oscillations in thermoelectric and magnetic torque signals in noncentrosymmetric superconductor ${\mathrm{PbTaSe}}_{2}$. One oscillatory frequency stems from the orbits formed by magnetic breakdown, while others are from two-dimensional-like Fermi surfaces near the topological nodal rings. Our comprehensive understanding of the Fermi surface topology of ${\mathrm{PbTaSe}}_{2}$, including nailing down the Fermi level and detecting the Berry phases near the nodal rings, is crucial for searching plausible topological superconductivity in its bulk and surface states.

Quantum oscillations in the noncentrosymmetric superconductor and topological nodal-line semimetal PbTaSe 2

Transport and magnetic studies of ${\mathrm{PbTaSe}}_{2}$ under pressure suggest the existence of two superconducting phases with the low temperature phase boundary at $\ensuremath{\sim}0.25$ GPa that is defined by a very sharp, first order, phase transition. The first order phase transition line can be followed via pressure dependent resistivity measurements, and is found to be near 0.12 GPa near room temperature. Transmission electron microscopy and x-ray diffraction at elevated temperatures confirm that this first order phase transition is structural and occurs at ambient pressure near $\ensuremath{\sim}425$ K. The new, high temperature/high pressure phase has a similar crystal structure and slightly lower unit cell volume relative to the ambient pressure, room temperature structure. Based on first-principles calculations this structure is suggested to be obtained by shifting the Pb atoms from the $1a$ to $1e$ Wyckoff position without changing the positions of Ta and Se atoms. ${\mathrm{PbTaSe}}_{2}$ has an exceptionally pressure sensitive, structural phase transition with $\mathrm{\ensuremath{\Delta}}{T}_{s}/\mathrm{\ensuremath{\Delta}}P\ensuremath{\approx}\ensuremath{-}1400$ K/GPa near room temperature, and $\ensuremath{\approx}\ensuremath{-}1700$ K/GPa near 4 K. This first order transition causes a $\ensuremath{\sim}1$ K ($\ensuremath{\sim}25%$) steplike decrease in ${T}_{c}$ as pressure is increased through 0.25 GPa.

Highly responsive ground state of PbTaSe2 : Structural phase transition and evolution of superconductivity under pressure

Discovery of Weyl semimetal NbAs

The metallic tip-induced superconductivity in normal Weyl semimetal offers a promising platform to study topological superconductivity, which is currently a research focus in condensed matter physics. Here we experimentally uncover that unconventional superconductivity can be induced by hard point contact (PC) method of ferromagnetic tips in TaAs single crystals. The magneto-transport measurements of the ferromagnetic tip-induced superconducting (FTISC) states exhibit the quantum oscillations, which reveal that the superconductivity is induced in the topologically nontrivial Fermi surface of the Weyl semimetal, and show compatibility of ferromagnetism and induced superconductivity. We further measure the point contact spectra (PCS) of tunneling transport for FTISC states which are potentially of nontrivial topology. Considering that the magnetic Weyl semimetal with novel superconductivity is hard to realize in experiment, our results show a new route to investigate the unconventional superconductivity by combining the topological semimetal with ferromagnetism through hard PC method.

Zhujun Yuan

Papers

Quantum oscillations in the noncentrosymmetric superconductor and topological nodal-line semimetal PbTaSe 2

Highly responsive ground state of PbTaSe2 : Structural phase transition and evolution of superconductivity under pressure

Discovery of Weyl semimetal NbAs

Ferromagnetic tip induced unconventional superconductivity in Weyl semimetal

Evidence for a Bose Metal in a Two-Dimensional Crystalline Superconductor