Quantum spin Hall materials hold the promise of revolutionary devices with dissipationless spin currents but have required cryogenic temperatures owing to small energy gaps. Here we show theoretically that a room-temperature regime with a large energy gap may be achievable within a paradigm that exploits the atomic spin-orbit coupling. The concept is based on a substrate-supported monolayer of a high-atomic number element and is experimentally realized as a bismuth honeycomb lattice on top of the insulating silicon carbide substrate SiC(0001). Using scanning tunneling spectroscopy, we detect a gap of ~0.8 electron volt and conductive edge states consistent with theory. Our combined theoretical and experimental results demonstrate a concept for a quantum spin Hall wide-gap scenario, where the chemical potential resides in the global system gap, ensuring robust edge conductance.
The conducting interface of LaAlO3/SrTiO3 heterostructures has been studied by hard X-ray photoelectron spectroscopy. From the Ti 2p signal and its angle-dependence we derive that the thickness of the electron gas is much smaller than the probing depth of 4 nm and that the carrier densities vary with increasing number of LaAlO3 overlayers. Our results point to an electronic reconstruction in the LaAlO3 overlayer as the driving mechanism for the conducting interface and corroborate the recent interpretation of the superconducting ground state as being of the BerezinskiiKosterlitz-Thouless type.
Atom assemblies on surfaces represent the ultimate lower size limit for electronic circuits, and their conduction properties are governed by quantum phenomena. A fundamental prediction for a line of atoms confining the electrons to one dimension is the Tomonaga-Luttinger liquid 1. Yet, astonishingly, this has not been observed in surface systems so far. Here we scrutinize self-organized chains of single-atom width by scanning tunnelling spectroscopy and photoemission. The lowenergy spectra univocally show power-law behaviour. Even more, the density of states obeys universal scaling with energy and temperature. This demonstrates paradigmatic Tomonaga-Luttinger liquid properties 2,3 encountered at the atomic scale, with bearing for the conductivity of wires and junctions. Local control enables us to study modified interactions due to defects or bridging atoms not previously possible. Arrays of single atoms on surfaces provide an environment for a rich variety of quantum phenomena, especially regarding the electron states responsible for conduction. Their properties can be probed locally with scanning tunnelling microscopy (STM). Key examples include the superposition of electron waves in quantum corrals, leading to new coherent states 4. A challenge remains the exotic correlated state predicted to occur when the electrons are squeezed into one dimension, as in a linear chain of atoms. Quantum theory describes this regime as a Tomonaga-Luttinger liquid 1 (TLL) with collective excitations of spin and charge. It reveals itself in characteristic power-law behaviour of the excitation spectra 2,3 , with markedly depressed density of states at the chemical potential (where conduction takes place). This state is highly fragile and collapses on slight coupling to the second dimension. Experimental indications of TLL low-energy spectra are scarce, consisting of one-dimensional (1D) crystals 5-7 , carbon nanotubes 8,9 and GaAs channels 10,11 (compiled in Supplementary Table S1). Surprisingly, and contrary to expectations, this phenomenon has not been found so far in atom chains at surfaces, although such behaviour will dramatically affect atomic leads and junctions 12. Creation of a TLL state in such chains would be highly intriguing because this promises local atom manipulations that tune the interactions. Concerning approaches to build suitable chains, artificial atom placement by an STM tip 13 leads to short arrays, which do not suffice for an extended 1D regime. Hence, our approach is to use self-organized chains of large extent formed by noble-metal atoms on the semiconducting Ge(001) surface 14,15. Here Au-induced chains show metallic tunnelling conductivity at room temperature 15. Self-organized formation of Au atom wires along the Ge(001) dimer rows leads to c(8 × 2) long-range order covering the
Claessen, R.; Sing, M.; Schwingenschlogl, U.; Blaha, P.; Dressel, M.; Jacobsen, Claus Schelde Published in: Physical Review LettersLink to article, DOI: 10.1103/PhysRevLett.88.096402 Publication date: 2002 Document VersionPublisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Claessen, R., Sing, M., Schwingenschlogl, U., Blaha, P., Dressel, M., & Jacobsen, C. S. (2002). Spectroscopic signatures of spin-charge separation in the quasi-one-dimensional organic conductor TTF-TCNQ. Physical Review Letters, 88(9), 096402. DOI: 10.1103/PhysRevLett.88.096402 VOLUME 88, NUMBER 9 The electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant discrepancies to band theory. We demonstrate that the measured dispersions can be consistently mapped onto the one-dimensional Hubbard model at finite doping. This interpretation is further supported by a remarkable transfer of spectral weight as a function of temperature. The ARPES data thus show spectroscopic signatures of spin-charge separation on an energy scale of the conduction bandwidth.
PACS. 71.10.Pm -Fermions in reduced dimensions. PACS. 71.27.+a -Strongly correlated electron systems.Abstract. -The momentum and energy dependence of the weight distribution in the vicinity of the one-electron spectral-function singular branch lines of the 1D Hubbard model is studied for all values of the electronic density and on-site repulsion U . To achieve this goal we use the recently introduced pseudofermion dynamical theory. Our predictions agree quantitatively for the whole momentum and energy bandwidth with the peak dispersions observed by angleresolved photoelectron spectroscopy in the quasi-1D organic conductor TTF-TCNQ.The finite-energy spectral dispersions recently observed in quasi-one-dimensional (1D) metals by angle-resolved photoelectron spectroscopy (ARPES) reveal significant discrepancies from the conventional band-structure description [1,2]. The study of the microscopic mechanisms behind these unusual finite-energy spectral properties remains until now an interesting open problem. There is some evidence that the correlation effects described by the 1D Hubbard model might contain such finite-energy mechanisms [1,2]. However, for finite values of the on-site repulsion U very little is known about its finite-energy spectral properties, in contrast to simpler models [3]. Bosonization [4] and conformal-field theory [5] do not apply at finite energy. For U → ∞ the method of Ref.[6] provides valuable qualitative information, yet a quantitative description of the finite-energy spectral properties of quasi-1D metals requires the solution of the problem for finite values of U . The method of Ref. [7] refers to features of the insulator phase. For U ≈ 4t, where t is the transfer integral, there are numerical results for the one-electron spectral function [8] which, unfortunately, provide very little information about the microscopic mechanisms behind the finite-energy spectral properties. Recent preliminary results obtained by use of the finite-energy holon and spinon description introduced in Refs. [9-11] predict separate one-electron charge and spin spectral branch lines [1]. For the electron-removal spectral function these lines show quantitative agreement with the peak dispersions observed by ARPES in the quasi-1D organic conductor TTF-TCNQ [1]. However,
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