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Symmetry and topology are central to understanding quantum Hall ferromagnets (QHFMs), two-dimensional electronic phases with spontaneously broken spin or pseudospin symmetry whose wavefunctions also have topological properties.sup.1,2. Domain walls between distinct broken-symmetry QHFM phases are predicted to host gapless one-dimensional modes--that is, quantum channels that emerge because of a topological change in the underlying electronic wavefunctions at such interfaces. Although various QHFMs have been identified in different materials.sup.3-8, interacting electronic modes at these domain walls have not been probed. Here we use a scanning tunnelling microscope to directly visualize the spontaneous formation of boundary modes at domain walls between QHFM phases with different valley polarization (that is, the occupation of equal-energy but quantum mechanically distinct valleys in the electronic structure) on the surface of bismuth. Spectroscopy shows that these modes occur within a topological energy gap, which closes and reopens as the valley polarization switches across the domain wall. By changing the valley flavour and the number of modes at the domain wall, we can realize different regimes in which the valley-polarized channels are either metallic or develop a spectroscopic gap. This behaviour is a consequence of Coulomb interactions constrained by the valley flavour, which determines whether electrons in the topological modes can backscatter, making these channels a unique class of interacting one-dimensional quantum wires. QHFM domain walls can be realized in different classes of two-dimensional materials, providing the opportunity to explore a rich phase space of interactions in these quantum wires. Scanning tunnelling microscopy shows that 1D channels between quantum Hall states with different valley polarization are metallic or insulating depending on constraints imposed on electron-electron interaction by valley flavour., Author(s): Mallika T. Randeria [sup.1] , Kartiek Agarwal [sup.2] , Benjamin E. Feldman [sup.1] [sup.5] [sup.6] , Hao Ding [sup.1] , Huiwen Ji [sup.3] , R. J. Cava [sup.3] , [...]

Breaking down barriers Topological states have become the subject of much attention from condensed-matter physicists, as evidence accumulates to show that these states can be found on the surface of certain materials -- in particular, bulk compounds called topological insulators. As a product of their topological nature, topological surface states are predicted to have the remarkable property of being robust against imperfections. This can allow, for example, the conduction of electronic currents without dissipation. Ali Yazdani and his team now report a tantalizing finding from scanning tunnelling microscope measurements -- that topological surface states on antimony can be transmitted with high probability through naturally occurring barriers that stop other conventional surface states of common metals. The authors suggest that their findings indicate that topological surface states could be exploited in novel applications of nanoscale electronic devices. Topological surface states are a class of electronic states that might be of interest in quantum computing or spintronic applications. They are predicted to be robust against imperfections, but so far there has been no evidence that these states do transmit through naturally occurring surface defects. Here, scanning tunnelling microscopy has been used to show that topological surface states of antimony can be transmitted through naturally occurring barriers that block non-topological surface states of common metals. Topological surface states are a class of novel electronic states that are of potential interest in quantum computing or spintronic applications.sup.1,2,3,4,5,6,7. Unlike conventional two-dimensional electron states, these surface states are expected to be immune to localization and to overcome barriers caused by material imperfection.sup.8,9,10,11,12,13,14. Previous experiments have demonstrated that topological surface states do not backscatter between equal and opposite momentum states, owing to their chiral spin texture.sup.15,16,17,18. However, so far there is no evidence that these states in fact transmit through naturally occurring surface defects. Here we use a scanning tunnelling microscope to measure the transmission and reflection probabilities of topological surface states of antimony through naturally occurring crystalline steps separating atomic terraces. In contrast to non-topological surface states of common metals (copper, silver and gold).sup.19,20,21,22,23, which are either reflected or absorbed by atomic steps, we show that topological surface states of antimony penetrate such barriers with high probability. This demonstration of the extended nature of antimony's topological surface states suggests that such states may be useful for high current transmission even in the presence of atomic-scale irregularities--an electronic feature sought to efficiently interconnect nanoscale devices., Author(s): Jungpil Seo [sup.1] , Pedram Roushan [sup.1] , Haim Beidenkopf [sup.1] , Y. S. Hor [sup.2] , R. J. Cava [sup.2] , Ali Yazdani [sup.1] Author Affiliations: (1) Joseph [...]

Author(s): Cheng Gong [1]; Lin Li [2]; Zhenglu Li [3, 4]; Huiwen Ji [5]; Alex Stern [2]; Yang Xia [1]; Ting Cao [3, 4]; Wei Bao [1]; Chenzhe Wang [1]; [...]

Author(s): Yayu Wang [1]; Nyrissa S. Rogado [2]; R. J. Cava [2, 3]; N. P. Ong (corresponding author) [1, 3] In an electric field, the flow of electrons in a [...]

Author(s): T. Valla (corresponding author) [1]; P. D. Johnson [1]; Z. Yusof [2]; B. Wells [2]; Q. Li [3]; S. M. Loureiro [4]; R. J. Cava [4]; M. Mikami [5]; [...]

Author(s): J. Snyder [1]; J. S. Slusky [2]; R. J. Cava [2]; P. Schiffer (corresponding author) [1] The large degeneracy of states resulting from the geometrical frustration of competing interactions [...]

Author(s): D. C. Larbalestier (corresponding author) [1, 2]; L. D. Cooley [1]; M. O. Rikel [1]; A. A. Polyanskii [1]; J. Jiang [1]; S. Patnaik [1]; X. Y. Cai [1]; [...]

Magnetoresistance is the change in a material's electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors, in magnetic memory, and in hard drives at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity. [ABSTRACT FROM AUTHOR]

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide NaxCo2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding-that spin-entropy dominates the enhancement of thermopower in transition-metal oxides-for the search for better Peltier materials. [ABSTRACT FROM AUTHOR]

The microscopic origin of superconductivity in the high-transition-temperature high-T[SUBc]) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting T[SUBc] on the degree of electronic band filling. The recent report of superconductivity[SUP1] near 4 K in the layered sodium cobalt oxyhydrate, Na[SUB0.35]CoO[SUB2]·1.3H[SUB2]O, is of interest owing to both its triangular cobalt-oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting T[SUBc] of this compound displays the same kind of behaviour on chemical doping that is observed in the high-T[SUBc] copper oxides. Specifically, the optimal superconducting T[SUBc] occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, how-ever, suggesting that Na[SUBx]CoO[SUB2]·1.3H2[SUBO], with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity. [ABSTRACT FROM AUTHOR]

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide NaxCo2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding-that spin-entropy dominates the enhancement of thermopower in transition-metal oxides-for the search for better Peltier materials. [ABSTRACT FROM AUTHOR]

Magnetoresistance is the change in a material's electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors, in magnetic memory, and in hard drives at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity. [ABSTRACT FROM AUTHOR]