The renormalization of the band structure at zero temperature due to electron-phonon coupling is explored in diamond, BN, LiF and MgO crystals. We implement a dynamical scheme to compute the frequency-dependent self-energy and the resulting quasiparticle electronic structure. Our calculations reveal the presence of a satellite band below the Fermi level of LiF and MgO. We show that the renormalization factor (Z), which is neglected in the adiabatic approximation, can reduce the zero-point renormalization (ZPR) by as much as 40%. Anharmonic effects in the renormalized eigenvalues at finite atomic displacements are explored with the frozen-phonon method. We use a non-perturbative expression for the ZPR, going beyond the Allen-Heine-Cardona theory. Our results indicate that high-order electron-phonon coupling terms contribute significantly to the zero-point renormalization for certain materials.PACS numbers: 63.20.kd, 63.20.dk, 71.15.Mb, 71.20.Nr The electron-phonon coupling is at the heart of numerous phenomena such as optical absorption 1,2 , thermoelectric transport 3 , and superconductivity 4-7 . It is also a crucial ingredient in basic electronic structure calculations, giving renormalized quasiparticle energies and lifetimes. This renormalization causes the temperature dependence of the band gap of semiconductors 8 , and accounts for the zero-point renormalization (ZPR), while the lifetime broadenings are observed through the electron mobility 9,10 and in photo-absorption/emission experiments 11 . Obtaining the quasiparticle structure from first principles has been a challenge, addressed for the first time for bulk silicon by King-Smith et al.12 , in 1989, using density functional theory (DFT), with a mixed frozen-phonon supercell and linear response approach. These authors pointed the inadequate convergence of their results with respect to phonon wavevector sampling, due to the limited available computing capabilities. Fifteen year passed, before Capaz et al. computed it for carbon nanotubes 13 using DFT with frozen phonons. At variance with the frozen-phonon approach, the theory of Allen, Heine and Cardona (AHC) [14][15][16] casts the renormalization and the broadening in terms of the first-order derivatives of the effective potential with respect to atomic positions. Used initially with empirical potentials, tight-binding or semi-empirical pseudopotentials 14-20 , AHC was then implemented with the density functional perturbation theory (DFPT) 21-24 , providing an efficient way to compute the phonon band structure and the electron-phonon coupling altogether. This powerful technique allowed A. Marini to compute, from first principles, temperature-dependent optical properties 25 . While DFPT has been widely applied to predict structural and thermodynamical properties of solids 26 , few * gabriel.antonius@gmail.com studies have used it to compute the phonon-induced renormalization of the band structure. The scarcity of experimental data is at least partly responsible for this imbalance. Whereas the phonon s...

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Classical chaotic systems exhibit exponentially diverging trajectories due to small differences in their initial state. The analogous diagnostic in quantum many-body systems is an exponential growth of out-of-time-ordered correlation functions (OTOCs). These quantities can be computed for various models, but their experimental study requires the ability to evolve quantum states backward in time, similar to the canonical Loschmidt echo measurement. In some simple systems, backward time evolution can be achieved by reversing the sign of the Hamiltonian; however in most interacting many-body systems, this is not a viable option. Here we propose a new family of protocols for OTOC measurement that do not require backward time evolution. Instead, they rely on ordinary time-ordered measurements performed in the thermofield double (TFD) state, an entangled state formed between two identical copies of the system. We show that, remarkably, in this situation the Lyapunov chaos exponent λL can be extracted from the measurement of an ordinary two-point correlation function. As an unexpected bonus, we find that our proposed method yields the so-called "regularized" OTOC -a quantity that is believed to most directly indicate quantum chaos. According to recent theoretical work, the TFD state can be prepared as the ground state of two weakly coupled identical systems and is therefore amenable to experimental study. We illustrate the utility of these protocols on the example of the maximally chaotic Sachdev-Ye-Kitaev model and support our findings by extensive numerical simulations.

Strain-induced Landau levels in wafer-scale graphene at room temperature are studied using momentum-resolved spectroscopy.

Recent work has shown that coupling two identical Sachdev-Ye-Kitaev (SYK) models can realize a phase of matter that is holographically dual to an eternal traversable wormhole. This phase supports revival oscillations between two quantum chaotic systems that can be interpreted as information traversing the wormhole. Here we generalize these ideas to a pair of coupled SYK models with complex fermions that respect a global U(1) charge symmetry. Such models show richer behavior than conventional SYK models with Majorana fermions and may be easier to realize experimentally. We consider two different couplings, namely, tunneling and charge-conserving two-body interactions, and obtain the corresponding phase diagram using a combination of numerical and analytical techniques. At low temperature, we find a charge-neutral gapped phase that supports revival oscillations, with a ground state close to the thermofield double, which we argue is dual to a traversable wormhole. We also find two different gapless non-Fermi liquid phases with tunable charge density which we interpret as dual to a "large" and "small" charged black hole. The gapped and gapless phases are separated by a first-order phase transition of the Hawking-Page type. Finally, we discuss an SU(2)-symmetric limit of our model that is closely related to proposed realizations of SYK physics with spinful fermions in graphene, and explain its relevance for future experiments on this system.

Several condensed-matter platforms have been proposed recently to realize the Sachdev-Ye-Kitaev (SYK) model in their low-energy limit. In these proposed realizations, the characteristic SYK behavior is expected to occur under certain assumptions about the underlying physical system that (i) render all bilinear terms small compared to four-fermion interactions and (ii) ensure that the coupling constants are approximately all-to-all and independent random variables. In this work we explore, both analytically and numerically, the family of models that arises when we relax these assumptions in ways motivated by real physical systems. By relaxing (i) and allowing large bilinear terms, we obtain a novel, exactly-solvable cousin of the SYK model. It exhibits two distinct phases separated by a quantum phase transition characterized by a power-law, ∼ |ω| −1/3 scaling of the low-energy spectral density, despite being a non-interacting model. By relaxing (ii), we obtain close relatives of the SYK model which exhibit interesting behaviors, including a chaotic non-Fermi liquid phase with continuously varying fermion scaling dimension, and a phase transition to a disordered Fermi liquid as a function of interaction range and disorder length scale. arXiv:1803.07197v3 [cond-mat.str-el]

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We describe a simple setup generating pure valley currents -valley transport without charge transport -in strained graphene nanoribbons with zigzag edges. The crucial ingredient is a uniaxial strain pattern which couples to the low-energy Dirac electrons as a uniform pseudomagnetic field. Remarkably, the resulting pseudo-Landau levels are not flat but disperse linearly from the Dirac points, with an opposite slope in the two valleys. We show how this is a natural consequence of an inhomogeneous Fermi velocity arising in an exactly-solvable low-energy theory describing the system. The velocity of the valley currents can be controlled by tuning the magnitude of strain and by applying bias voltages across the ribbon. Furthermore, applying an electric field along the ribbon leads to pumping of charge carriers between the two valleys, realizing a valley analog of the chiral anomaly in one spatial dimension. These effects produce unique signatures that can be observed experimentally by performing ordinary charge transport measurements. K K' arXiv:1909.01442v1 [cond-mat.mes-hall]