Hasil untuk "cond-mat"

Liujun Zou, A. Vishwanath, N. Yuan et al.

The recent discovery of insulating behavior and superconductivity in twisted graphene bilayers[1, 2] has attracted a great deal of attention among the contributors to the cond-mat archive, as over 60 entries have been posted in the period March-October 2018 (see also a previous entry in this journal club[3]). There are many reasons for this impact: new broken symmetry phases in a two dimensional system, unconventional superconductivity, and novel features in the electronic bands, among others. The focus of this commentary is on interesting contributions to the latter issue, the unconventional electronic bands of twisted graphene layers. As it is argued below, these bands show features not encountered before in condensed matter physics. The articles listed above have clarified significantly the mysteries of these bands, although a number of problems remain open. Space considerations prevent a detailed analysis of the many insights into other topics, such as superconductivity itself. Many interesting papers have also been posted in the cond-mat repository recently. The electronic structure of twisted bilayers was considered shortly after research on graphene began[4]. This seminal paper provides the minimal model for this structure. The basic idea follows the approach which leads from atomistic to tight binding and to continuum

A. Srivastava, Anuradha Pillai, D. Gupta

Guadalupe María Milián Ávila

E. Yoshikawa, H. Komine, S. Goto et al.

J. Gabelli, G. Fève, T. Kontos et al.

S. Nigg, Rosa López, M. Büttiker

D. Ast

H. Förster, S. Pilgram, M. Büttiker

D. Agterberg, M. Sigrist

M. Büttiker

M. Moskalets, M. Büttiker

Márkus

S. Sachdev, M. Vojta

It was argued many years ago that translational symmetry breaking due to the appearance of spin-Peierls ordering (or bond-charge stripe order) is a fundamental property of the quantum paramagnetic states of a large class of square lattice antiferromagnets. Recently, such states were shown to be a convenient point of departure for studying translational symmetry breaking in doped antiferromagnets: these results are briefly reviewed here with an emphasis on experimental implications. In the presence of stronger frustration, it was also argued that the insulating antiferromagnet can undergo a transition to a deconfined state with no lattice symmetry breaking. This transition is described by a fully-frustrated Ising model in a transverse field: details of this earlier derivation of the Ising model are provided here--this is motivated by the reappearance of the same Ising model in a recent study of the competition between antiferromagnetism and d-wave superconductivity by Senthil and Fisher (cond-mat/9910224).

M. Putti, V. Braccini, E. Galleani et al.

S. Drechsler, Jiří Málek, Johannes Richter et al.

The microscopic origin of the recently observed helical structure in LiCu2O2 [1] is considered. It is shown that the frustrated antiferromagnetic double chain scenario adopted in Ref.1 is unrealistic. It should be replaced by a frustrated single-chain (CuO2) scenario proposed in Ref. 2 with ferromagnetic nearest neighbor and antiferromagnetic next nearest neighbor exchange integrals \. [1] T. Masuda et al. Phys. Rev. Lett. 92 (2004), 177201. [2] A. Gippius et al. Phys. Rev. B 70 (2004), R01426; cond-mat/0312576.

M. Davis, C. Gardiner, R. Ballagh

We extend earlier models of the growth of a Bose-Einstein condensate c.w. [Gardiner et al., Phys. Rev. Lett. 79, 1793 (1997); e-print cond-mat/9801027; Phys. Rev. Lett. 81, 5266 (1998)] to include the full dynamical effects of the thermal cloud by numerically solving a modified quantum Boltzmann equation. We determine the regime in which the assumptions of the simple model of Gardiner et al. [Phys. Rev. Lett. 81, 5266 (1998)] are a reasonable approximation, and compare our results with those that were earlier compared with experimental data. We find good agreement with our earlier modeling, except at higher condensate fractions, for which a significant speedup is found. We also investigate the effect of the final temperature on condensate growth, and find that this has a surprisingly small effect. The particular discrepancy between theory and experiment found in our earlier model remains, since the speedup found in these computations does not occur in the parameter regime specified in the experiment.

B. Ruzicka, L. Zulian, G. Ruocco

Alessio Cardillo, S. Scellato, V. Latora

Yasuhiro Hatsugai

Recently, by using quantized Berry phases, a prescription for a local characterization of gapped topological insulators has been given (Hatsugai 2006 Preprint cond-mat/0603230). It requires that the ground state is gapped and is invariant under some anti-unitary operation. A spin liquid which is realized as a unique ground state of the Heisenberg spin system with frustrations is a typical target system, since pairwise exchange couplings are always time-reversal invariants even with frustrations. As for a generic Heisenberg model with a finite excitation gap, we locally modify the Hamiltonian by a continuous SU(2) twist only at a specific link and define the Berry connection by the derivative. Then the Berry phase evaluated by the entire many-spin wavefunction is used to define the local topological order parameter at the link. We numerically apply this scheme for several spin liquids and show its physical validity. For example, it implies that the Haldane phase of the S = 1 chains is characterized by uniform π quantized Berry phases.