We simultaneously determined the physical structure and optical transition energies of individual single-walled carbon nanotubes by combining electron diffraction with Rayleigh scattering spectroscopy. These results test fundamental features of the excited electronic states of carbon nanotubes. We directly verified the systematic changes in transition energies of semiconducting nanotubes as a function of their chirality and observed predicted energy splittings of optical transitions in metallic nanotubes.
The recent discovered antiferromagnetic topological insulators in Mn-Bi-Te family with intrinsic magnetic ordering have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states with half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resolution angle-resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi2Te4 exists between the bulk band gap. Such unexpected surface state remains unchanged across the bulk Né el temperature, and is even robust against severe surface degradation, indicating additional topological protection. Through symmetry analysis and ab-initio calculations we consider different types of surface reconstruction of the magnetic moments as possible origins giving rise to such linear dispersion. Our results reveal that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases such
Diverse parallel stitched 2D heterostructures, including metal-semiconductor, semiconductor-semiconductor, and insulator-semiconductor, are synthesized directly through selective "sowing" of aromatic molecules as the seeds in the chemical vapor deposition (CVD) method. The methodology enables the large-scale fabrication of lateral heterostructures, which offers tremendous potential for its application in integrated circuits.
Electronic conduction in conjugated polymers is of emerging technological interest for high-performance optoelectronic and thermoelectric devices. A completely new aspect and understanding of the conduction mechanism on conducting polymers is introduced, allowing the applicability of materials to be optimized. The charge-transport mechanism is explained by direct experimental evidence with a very well supported theoretical model.
Low dimensional quantum magnets are interesting because of the emerging collective behavior arising from strong quantum fluctuations. The one-dimensional (1D) S = 1/2 Heisenberg antiferromagnet is a paradigmatic example, whose low-energy excitations, known as spinons, carry fractional spin S = 1/2. These fractional modes can be reconfined by the application of a staggered magnetic field. Even though considerable progress has been made in the theoretical understanding of such magnets, experimental realizations of this low-dimensional physics are relatively rare. This is particularly true for rare-earth-based magnets because of the large effective spin anisotropy induced by the combination of strong spin–orbit coupling and crystal field splitting. Here, we demonstrate that the rare-earth perovskite YbAlO3 provides a realization of a quantum spin S = 1/2 chain material exhibiting both quantum critical Tomonaga–Luttinger liquid behavior and spinon confinement–deconfinement transitions in different regions of magnetic field–temperature phase diagram.
The low energy spin excitation spectrum of the breathing pyrochlore Ba 3 Yb 2 Zn 5 O 11 has been investigated with inelastic neutron scattering. Several nearly resolution limited modes with no observable dispersion are observed at 250 mK while, at elevated temperatures, transitions between excited levels become visible. To gain deeper insight, a theoretical model of isolated Yb 3+ tetrahedra parametrized by four anisotropic exchange constants is constructed. The model reproduces the inelastic neutron scattering data, specific heat, and magnetic susceptibility with high fidelity. The fitted exchange parameters reveal a Heisenberg antiferromagnet with a very large Dzyaloshinskii-Moriya interaction. Using this model, we predict the appearance of an unusual octupolar paramagnet at low temperatures and speculate on the development of inter-tetrahedron correlations.Frustrated or competing interactions have been repeatedly found to be at the root of many unusual phenomena in condensed matter physics [1][2][3][4][5]. By destabilizing conventional long-range order down to low temperature, frustration in magnetic systems can lead to many exotic phases; from unconventional multipolar [6,7] and valence bond solid orders [1,4] to disordered phases such as classical and quantum spin liquids [1,4]. Significant attention has been devoted to understanding geometric frustration where it is the connectivity of the lattice that hinders the formation of order. Recently, however, magnets frustrated not by geometry but by competing interactions have become prominent for the novel behaviors that they host. Such competing interactions might be additional isotropic exchange acting beyond nearest neighbors [8-10], biquadratic or other multipolar interactions [11]. One possibility attracting ever increasing interest is that competing strongly anisotropic interactions may stabilize a wide range of unusual phenomena.An exciting research direction in the latter context concerns itself with so-called "quantum spin ice" [12]. This quantum spin liquid can be stabilized by perturbing classical spin ice with additional anisotropic transverse exchange interactions that induce quantum fluctuations. Particularly interesting is the potential realization of such physics in the rare-earth pyrochlores R 2 M 2 O 7 [13][14][15], where R is a trivalent 4 f rare-earth ion, and M is a non-magnetic tetravalent transition metal ion, such as M=Ti, Sn or Zr. These materials can be described in terms of pseudo spin-1/2 degrees of freedom interacting via anisotropic exchanges [12,15], where the effective spin-1/2 maps the states of the crystal-electric field ground doublet of the rare-earth ion. These materials display a wealth of interesting phenomena, from the possibility of quantum [16][17][18] ion is part of a large and small tetrahedron in the breathing pyrochlore lattice.liquids [22,23]. In many of these compounds, the physics is very delicate, showing strong sample to sample variations [24] or sensitivity to very small amounts of disorder [25,26]. Consequen...
It is generally believed that fractional quantum excitations such as spinons in one-dimensional (1D) spin chains only proliferate and govern magnetism in systems with small and isotropic atomic magnetic moments, such as spin−1/2 Cu 2+ . In contrast, large and anisotropic orbital-dominated moments, such as those produced by strong spin-orbit coupling in the rare earths, are considered to be classical, becoming static as T → 0 since the conventional Heisenberg-Dirac exchange interaction [1, 2] cannot reverse their directions. We present here the results of neutron scattering measurements on Yb 2 Pt 2 Pb that completely negate this common wisdom. A diffuse continuum of magnetic excitations is observed in Yb 2 Pt 2 Pb, direct evidence that the elementary excitations carry a fractional spin quantum number, S = 1/2. The excitations disperse in only one direction, showing that the Yb moments form spin chains that are embedded in, but effectively decoupled from the three-dimensional conduction electron bands in metallic Yb 2 Pt 2 Pb. The spectrum of magnetic excitations strongly resembles the spinon continuum found in S = 1/2 Heisenberg spin chains, and indeed comparison to the 1D XXZ Hamiltonian indicates only a moderate exchange anisotropy, ∆ = J zz /J xx ∼ 3. Here we show how the orbital physics of 4f -electron exchange interactions can reconcile this moderately-anisotropic quantum Hamiltonian with the extreme anisotropy of the putatively classical Yb (J = 7/2) magnetic moments with respect to magnetic fields. We find that the unexpected quantum behavior emerges at low energies from the competition of interactions that act on much higher energy scales, i.e. the strong on-site Coulomb and spin-orbit interactions, as well as the crystal fields, and the inter-site hopping. Our findings thus provide a unique and a hitherto unforeseen manifestation of emergence [3] of quantum physics in the system of semi-classical electronic orbitals.The unusual properties of Yb 2 Pt 2 Pb derive in part from its crystal structure (Fig. 1A,B), where the Yb 3+ ions form ladders along the c−axis, with rungs on the orthogonal bonds of the ShastrySutherland Lattice (SSL) [5] in the ab-planes. Equally important is the strong spin-orbit coupling, which combines spin and orbital degrees of freedom into a large, J = 7/2 Yb moment.The absence of a Kondo effect indicates minimal coupling of Yb to the conduction electrons of this excellent metal [6, 7]. A point-charge model (Supplementary Information) indicates that the crystal electric field (CEF) lifts the eightfold degeneracy of the Yb 3+ moments, producing a Kramers doublet ground state that is a nearly pure state of the total angular momentum, J , |J, m J = |7/2, ±7/2 . The estimated anisotropy of the Landé g-factor is in good agreement with that of the measured magnetization, g /g ⊥ = 7.5(4) [6][7][8], implying strong Ising anisotropy in Yb 2 Pt 2 Pb, which confines the individual Yb moments to two orthogonal sublattices in the The reality is, in fact, completely different.The neutron scatt...
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