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Time-reversal symmetry breaking Weyl state in YbMnBi2
TL;DR: In this paper, the authors used angle-resolved photoemission to detect a time-reversal symmetry breaking Weyl state in two materials with Fermi velocities superior to that of graphene and presented experimental evidence of realization of such a state in one of them, YbMnBi2.
Abstract: Detection of Dirac, Majorana and Weyl fermions in real materials may significantly strengthen the bridge between high-energy and condensed-matter physics. While the presence of Dirac fermions is well established in graphene and topological insulators, Majorana particles have been reported recently and evidence for Weyl fermions in non-centrosymmetric crystals has been found only a couple of months ago, the 'magnetic' Weyl fermions are still elusive despite numerous theoretical predictions and intense experimental search. In order to detect a time-reversal symmetry breaking Weyl state we designed two materials with Fermi velocities superior to that of graphene and present here the experimental evidence of realization of such a state in one of them, YbMnBi2. Using angle-resolved photoemission, we directly observe Weyl points and a hallmark of this exotic state - the arc of the surface states which connects these points. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties.
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Journal Article•
TL;DR: High-resolution spectroscopic imaging techniques show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states, providing strong evidence for the formation of a topological phase and edge-bound Majorana fermions in atomic chains.
Abstract: A possible sighting of Majorana states Nearly 80 years ago, the Italian physicist Ettore Majorana proposed the existence of an unusual type of particle that is its own antiparticle, the so-called Majorana fermion. The search for a free Majorana fermion has so far been unsuccessful, but bound Majorana-like collective excitations may exist in certain exotic superconductors. Nadj-Perge et al. created such a topological superconductor by depositing iron atoms onto the surface of superconducting lead, forming atomic chains (see the Perspective by Lee). They then used a scanning tunneling microscope to observe enhanced conductance at the ends of these chains at zero energy, where theory predicts Majorana states should appear. Science, this issue p. 602; see also p. 547 Scanning tunneling microscopy is used to observe signatures of Majorana states at the ends of iron atom chains. [Also see Perspective by Lee] Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.
877 citations
TL;DR: A bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions, will pave a route to the engineering of magnetically functionalized Dirac materials.
Abstract: For the innovation of spintronic technologies, Dirac materials, in which low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems because of the fascinating magnetotransport associated with extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility of more than 10,000 cm(2)/V s, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials.
190 citations
Cites background from "Time-reversal symmetry breaking Wey..."
...Single crystals of EuMnBi2, SrMnBi2, and EuZnBi2 were grown by a Bi self-flux method....
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...We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions....
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...Submitted 17 August 2015 Accepted 24 November 2015 Published 29 January 2016 10.1126/sciadv.1501117 Citation: H. Masuda, H. Sakai, M. Tokunaga, Y. Yamasaki, A. Miyake, J. Shiogai, S. Nakamura, S. Awaji, A. Tsukazaki, H. Nakao, Y. Murakami, T.-h. Arima, Y. Tokura, S. Ishiwata, Quantum Hall effect in a bulk antiferromagnet EuMnBi2 with magnetically confined two-dimensional Dirac fermions....
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...For SrMnBi2, on the other hand, the rzz value at 0 T is comparable to that for EuMnBi2 but shows aminimalmagnetoresistance effect up to 9 T. Themagnetic order in both the Eu andMn sublattices is thus essential for enhancing rzz....
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...Detailed magnetic properties for EuMnBi2....
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TL;DR: In this article, the authors present a review of the topological semimetals (TSMs) from a chemistry perspective, and describe key features of TSMs, embedded in their electronic structure, and how they can be achieved.
Abstract: Initiated by the discovery of topological insulators, topologically non-trivial materials have attracted immense interest in the physics community in the past decade. One of the latest additions to the field, the material class of topological semimetals (TSMs), has grown at an extremely fast rate. While the prototype TSM, graphene, has been known for a while, the first 3D analogues of graphene have only been discovered recently. This Review, written from a chemistry perspective, intends to make the growing field of TSMs accessible to the wider community of materials scientists and scholars from related disciplines. To this end, we describe key features of TSMs, embedded in their electronic structure, and how they can be achieved based on chemical principles. We introduce the different classes of TSMs and review their salient representatives. Finally, selected properties and potential applications of these materials are discussed.
176 citations
TL;DR: The determination of Berry phases of multiple Fermi pockets of Weyl semimetal TaP through high field quantum transport measurements is reported and the TaP single crystal has the signatures of a Weyl state, including light effective quasiparticle masses, ultrahigh carrier mobility, as well as negative longitudinal magnetoresistance.
Abstract: The recent breakthrough in the discovery of Weyl fermions in monopnictide semimetals provides opportunities to explore the exotic properties of relativistic fermions in condensed matter. The chiral anomaly-induced negative magnetoresistance and π Berry phase are two fundamental transport properties associated with the topological characteristics of Weyl semimetals. Since monopnictide semimetals are multiple-band systems, resolving clear Berry phase for each Fermi pocket remains a challenge. Here we report the determination of Berry phases of multiple Fermi pockets of Weyl semimetal TaP through high field quantum transport measurements. We show our TaP single crystal has the signatures of a Weyl state, including light effective quasiparticle masses, ultrahigh carrier mobility, as well as negative longitudinal magnetoresistance. Furthermore, we have generalized the Lifshitz-Kosevich formula for multiple-band Shubnikov-de Haas (SdH) oscillations and extracted the Berry phases of π for multiple Fermi pockets in TaP through the direct fits of the modified LK formula to the SdH oscillations. In high fields, we also probed signatures of Zeeman splitting, from which the Lande g-factor is extracted.
140 citations
TL;DR: In this article, the relativistic fermion behavior of the magnetic semimetal Sr1-yMn1-zSb2 (y,z < 0.1) has been investigated.
Abstract: Weyl (WSMs) evolve from Dirac semimetals in the presence of broken time-reversal symmetry (TRS) or space-inversion symmetry. The WSM phases in TaAs-class materials and photonic crystals are due to the loss of space-inversion symmetry. For TRS-breaking WSMs, despite numerous theoretical and experimental efforts, few examples have been reported. In this Article, we report a new type of magnetic semimetal Sr1-yMn1-zSb2 (y,z<0.1) with nearly massless relativistic fermion behaviour (m*=0.04-0.05m0, where m0 is the free electron mass). This material exhibits a ferromagnetic order for 304K < T < 565K, but a canted antiferromagnetic order with a ferromagnetic component for T < 304K. The combination of relativistic fermion behaviour and ferromagnetism in Sr1-yMn1-zSb2 offers a rare opportunity to investigate the interplay between relativistic fermions and spontaneous TRS breaking.
102 citations
References
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TL;DR: This study reports an experimental study of a condensed-matter system (graphene, a single atomic layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation and reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions.
Abstract: Quantum electrodynamics (resulting from the merger of quantum mechanics and relativity theory) has provided a clear understanding of phenomena ranging from particle physics to cosmology and from astrophysics to quantum chemistry. The ideas underlying quantum electrodynamics also influence the theory of condensed matter, but quantum relativistic effects are usually minute in the known experimental systems that can be described accurately by the non-relativistic Schrodinger equation. Here we report an experimental study of a condensed-matter system (graphene, a single atomic layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation. The charge carriers in graphene mimic relativistic particles with zero rest mass and have an effective 'speed of light' c* approximately 10(6) m s(-1). Our study reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions. In particular we have observed the following: first, graphene's conductivity never falls below a minimum value corresponding to the quantum unit of conductance, even when concentrations of charge carriers tend to zero; second, the integer quantum Hall effect in graphene is anomalous in that it occurs at half-integer filling factors; and third, the cyclotron mass m(c) of massless carriers in graphene is described by E = m(c)c*2. This two-dimensional system is not only interesting in itself but also allows access to the subtle and rich physics of quantum electrodynamics in a bench-top experiment.
18,958 citations
"Time-reversal symmetry breaking Wey..." refers background in this paper
...is well established in graphene and topological insulators [1, 2], Majorana particles have been...
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TL;DR: In this paper, the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topologically insulators have been observed.
Abstract: Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface. These states are possible due to the combination of spin-orbit interactions and time-reversal symmetry. The two-dimensional (2D) topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. A three-dimensional (3D) topological insulator supports novel spin-polarized 2D Dirac fermions on its surface. In this Colloquium the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topological insulators have been observed. Transport experiments on $\mathrm{Hg}\mathrm{Te}∕\mathrm{Cd}\mathrm{Te}$ quantum wells are described that demonstrate the existence of the edge states predicted for the quantum spin Hall insulator. Experiments on ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$, ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, and ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ are then discussed that establish these materials as 3D topological insulators and directly probe the topology of their surface states. Exotic states are described that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana fermions and may provide a new venue for realizing proposals for topological quantum computation. Prospects for observing these exotic states are also discussed, as well as other potential device applications of topological insulators.
15,562 citations
TL;DR: In this paper, the topological semimetal, a three-dimensional phase of a magnetic solid, is described and it may be realized in a class of pyrochlore iridates based on calculations using the LDA+U$ method.
Abstract: We investigate novel phases that emerge from the interplay of electron correlations and strong spin-orbit interactions. We focus on describing the topological semimetal, a three-dimensional phase of a magnetic solid, and argue that it may be realized in a class of pyrochlore iridates (such as ${\mathrm{Y}}_{2}$Ir${}_{2}$O${}_{7}$) based on calculations using the $\text{LDA}+U$ method. This state is a three-dimensional analog of graphene with linearly dispersing excitations and provides a condensed-matter realization of Weyl fermions that obeys a two-component Dirac equation. It also exhibits remarkable topological properties manifested by surface states in the form of Fermi arcs, which are impossible to realize in purely two-dimensional band structures. For intermediate correlation strengths, we find this to be the ground state of the pyrochlore iridates, coexisting with noncollinear magnetic order. A narrow window of magnetic ``axion'' insulator may also be present. An applied magnetic field is found to induce a metallic ground state.
3,865 citations
"Time-reversal symmetry breaking Wey..." refers background in this paper
...numerous theoretical predictions [12-17] and intense experimental search....
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...These requirements can be realized in noncentrosymmetric and magnetic materials with strong spin-orbit interaction and many compounds have already been nominated to host this exotic state [5, 6, 12-17]....
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TL;DR: Electrical measurements on indium antimonide nanowires contacted with one normal (gold) and one superconducting (niobium titanium nitride) electrode support the hypothesis of Majorana fermions in Nanowires coupled to superconductors.
Abstract: Majorana fermions are particles identical to their own antiparticles. They have been theoretically predicted to exist in topological superconductors. Here, we report electrical measurements on indium antimonide nanowires contacted with one normal (gold) and one superconducting (niobium titanium nitride) electrode. Gate voltages vary electron density and define a tunnel barrier between normal and superconducting contacts. In the presence of magnetic fields on the order of 100 millitesla, we observe bound, midgap states at zero bias voltage. These bound states remain fixed to zero bias, even when magnetic fields and gate voltages are changed over considerable ranges. Our observations support the hypothesis of Majorana fermions in nanowires coupled to superconductors.
3,273 citations
"Time-reversal symmetry breaking Wey..." refers background in this paper
...reported recently [3, 4] and evidence for Weyl fermions in non-centrosymmetric crystals has been found only a couple of months ago [5-11], the ag etic Weyl fer io s are still elusive despite...
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TL;DR: In this article, it was shown that the incompleteness of the previous theories lying in their disagreement with relativity or, alternatetically, with the general transformation theory of quantum mechanics leads to an explanation of all duplexity phenomena.
Abstract: The new quantum mechanics, when applied to the problem of the structure of the atom with point-charge electrons, does not give results in agreement with experiment. The discrepancies consist of “duplexity ” phenomena, the observed number of stationary states for an electron in an atom being twice the number given by the theory. To meet the difficulty, Goudsmit and Uhlenbeck have introduced the idea of an electron with a spin angular momentum of half a quantum and a magnetic moment of one Bohr magneton. This model for the electron has been fitted into the new mechanics by Pauli,* and Darwin,† working with an equivalent theory, has shown that it gives results in agreement with experiment for hydrogen-like spectra to the first order of accuracy. The question remains as to why Nature should have chosen this particular model for the electron instead of being satisfied with the point-charge. One would like to find some incompleteness in the previous methods of applying quantum mechanics to the point-charge electron such that, when removed, the whole of the duplexity phenomena follow without arbitrary assumptions. In the present paper it is shown that this is the case, the incompleteness of the previous theories lying in their disagreement with relativity, or, alternatetively, with the general transformation theory of quantum mechanics. It appears that the simplest Hamiltonian for a point-charge electron satisfying the requirements of both relativity and the general transformation theory leads to an explanation of all duplexity phenomena without further assumption. All the same there is a great deal of truth in the spinning electron model, at least as a first approximation. The most important failure of the model seems to be that the magnitude of the resultant orbital angular momentum of an electron moving in an orbit in a central field of force is not a constant, as the model leads one to expect.
3,034 citations