Latest Papers in Condensed Matter Physics

Mesoscale And Nanoscale Physics

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Pairing in twisted double-bilayer graphene and related moiré superlattice systems (1906.03258v1)

Mathias S. Scheurer, Rhine Samajdar, Subir Sachdev

2019-06-07

We present a systematic classification and analysis of possible pairing instabilities in graphene-based moir'e superlattices. Motivated by recent experiments on twisted double-bilayer graphene showing signs of triplet superconductivity, we analyze both singlet and triplet pairing separately, and describe how these two channels behave close to the limit where the system is invariant under separate spin rotations in the two valleys, realizing an SU(2) SU(2) symmetry. Further, we discuss the conditions under which singlet and triplet can mix via two nearly degenerate transitions, and how the different pairing states behave when an external magnetic field is applied. We find that an approximate SU(2) SU(2) symmetry can generically account for the linear increase of the critical temperature with small magnetic fields, and we map out the possible forms of the phase diagram as a function of temperature and magnetic field. We examine which of the pairing states can arise in mean-field theory and the type of pairing favored in the presence of strong ferromagnetic fluctuations, which are expected to be present in twisted double-bilayer graphene. Finally, we also detail the differences in the classification when the additional microscopic or emergent symmetries relevant for twisted bilayer graphene and ABC trilayer graphene on hexagonal boron nitride are taken into account. Our study illustrates that graphene superlattices provide a rich platform for exotic superconducting states, and could allow for the admixture of singlet and triplet pairing even in the absence of spin-orbit coupling.

Moiré-pattern fluctuations and electron-phason coupling in twisted bilayer graphene (1905.10850v2)

Héctor Ochoa

2019-05-26

In twisted bilayer graphene, long-wavelength lattice fluctuations on the scale of the Moir'e period are dominated by phason modes, i.e., acoustic branches of the incommensurate lattice resulting from coherent superpositions of optical phonons. In the limit of small twist angles, these modes describe the sliding motion of domain walls (also known as stacking solitons) separating regions of partial commensuration. The soliton network is a soft elastic manifold, whose reduced rigidity arises from the competition between intralayer (elastic) and interlayer (adhesion) forces governing lattice relaxation. Shear deformations of the beating pattern dominate the electron-phason coupling to the leading order in , the ratio between interlayer and intralayer hopping parameters. This coupling lifts the layer degeneracy of the Dirac cones at the corners of the Moir'e Brillouin zone, which could explain the observed 4-fold (instead of 8-fold) Landau level degeneracy. Electron-phason scattering gives rise to a linear-in-temperature contribution to the resistivity that increases with decreasing twist angle due to the reduced rigidity of the soliton network. This contribution, however, seems to be insufficient to explain the huge enhancement of the resistivity of the normal state close to the magic angle.

Reconstructing non-equilibrium regimes of quantum many-body systems from the analytical structure of perturbative expansions (1903.11646v3)

Corentin Bertrand, Serge Florens, Olivier Parcollet, Xavier Waintal

2019-03-27

We propose a systematic approach to the non-equilibrium dynamics of strongly interacting many-body quantum systems, building upon the standard perturbative expansion in the Coulomb interaction. High order series are derived from the Keldysh version of determinantal diagrammatic Quantum Monte Carlo, and the reconstruction beyond the weak coupling regime of physical quantities is obtained by considering them as analytic functions of a complex-valued interaction . Our advances rely on two crucial ingredients: i) a conformal change of variable, based on the approximate location of the singularities of these functions in the complex -plane; ii) a Bayesian inference technique, that takes into account additional known non-perturbative relations, in order to control the amplification of noise occurring at large . This general methodology is applied to the strongly correlated Anderson quantum impurity model, and is thoroughly tested both in- and out-of-equilibrium. In the situation of a finite voltage bias, our method is able to extend previous studies, by bridging with the regime of unitary conductance, and by dealing with energy offsets from particle-hole symmetry. We also confirm the existence of a voltage splitting of the impurity density of states, and find that it is tied to a non-trivial behavior of the non-equilibrium distribution function. Beyond impurity problems, our approach could be directly applied to Hubbard-like models, as well as other types of expansions.

Tuning the exchange bias on a single atom from 1 mT to 10 T (1906.03213v1)

Kai Yang, William Paul, Fabian D. Natterer, Jose L. Lado, Yujeong Bae, Philip Willke, Taeyoung Choi, Alejandro Ferrón, Joaquín Fernández-Rossier, Andreas J. Heinrich, Christopher P. Lutz

2019-06-07

Shrinking spintronic devices to the nanoscale ultimately requires localized control of individual atomic magnetic moments. At these length scales, the exchange interaction plays important roles, such as in the stabilization of spin-quantization axes, the production of spin frustration, and creation of magnetic ordering. Here, we demonstrate the precise control of the exchange bias experienced by a single atom on a surface, covering an energy range of four orders of magnitude. The exchange interaction is continuously tunable from milli-eV to micro-eV by adjusting the separation between a spin-1/2 atom on a surface and the magnetic tip of a scanning tunneling microscope (STM). We seamlessly combine inelastic electron tunneling spectroscopy (IETS) and electron spin resonance (ESR) to map out the different energy scales. This control of exchange bias over a wide span of energies provides versatile control of spin states, with applications ranging from precise tuning of quantum state properties, to strong exchange bias for local spin doping. In addition we show that a time-varying exchange interaction generates a localized AC magnetic field that resonantly drives the surface spin. The static and dynamic control of the exchange interaction at the atomic-scale provides a new tool to tune the quantum states of coupled-spin systems.

Topological photocurrent responses from chiral surface Fermi arcs (1906.03207v1)

Guoqing Chang, Jiaxin Yin, Titus Neupert, Daniel S. Sanchez, Ilya Belopolski, Songtian S. Zhang, Tyler A. Cochran, Ming-Chien Hsu, Shin-Ming Huang, Biao Lian, Su-Yang Xu, Hsin Lin, M. Zahid Hasan

2019-06-07

The nonlinear optical responses from topological semimetals are crucial in both understanding the fundamental properties of quantum materials and designing next-generation light-sensors or solar-cells. However, previous work was focusing on the optical effects from bulk states only, disregarding topological surface responses. Here we propose a new (hitherto unknown) surface-only topological photocurrent response from chiral Fermi arcs. Using the ideal topological chiral semimetal RhSi as a representative, we quantitatively compute the topologically robust photocurrents from Fermi arcs on different surfaces. By rigorous crystal symmetry analysis, we demonstrate that Fermi arc photocurrents can be perpendicular to the bulk injection currents regardless of the choice of materials' surface. We then generalize this finding to all cubic chiral space groups and predict material candidates. Our theory reveals a powerful notion where common crystalline-symmetry can be used to induce universal topological responses as well as making it possible to completely disentangle bulk and surface topological responses in many conducting material families.

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