Current control of systems with a Peierls distortion by magnetic field

We study the tunneling phenomenon of a ladder system with a Peierls distortion in a magnetic flux, and the response of electrons the insulator is investigated when the tunneling current flows on one-dimensional gapped chains along the external electric field. Without the magnetic field, the ladder system is insulated by the charge density wave order. Then, by the increase of the magnetic field, it becomes metallic with the disappearance of the distortion of the lattice. Finally, the gap appears, and it becomes a insulator. At the metallic state, the topological transition also occurs. To show this phenomenon, we consider the distortion by the phonon in the ladder model, and calculate the distortion gap and the transition probability by using both Landau-Zener formula and the instanton method. The transition to the metallic states will be applied to the current control by the magnetic field.Comment: 8 pages, 5 figure

Semiclassical Lattice effects on interband tunneling of a two-state system

Previously, we have shown that the transition probability of the Landau-Zener problem in periodic lattice systems becomes large by taking into account the nonlinearity of the energy spectra, compared with the probability by the conventional Landau-Zener formula. The enhancement comes from the nonlinearity peculiar to the periodic lattice system, and this effect from the lattice on transition action cannot be neglected in the transition process. In the present paper, we first give a brief review of the previous work, and construct the transfer matrix of the Landau-Zener problem by the semiclassical description for lattice systems. Next, we study a ladder lattice system and show that the transition action obtains a phase due to the nonlinearity. Then, we consider the double-passage problem of the ladder system within the semiclassical description. We find the oscillation of the probability by the transition phase by the lattice effect. This phase comes from the semiclassical analysis unlike the Stokes phase, and we show that the oscillation is mainly contributed by the transition phase by the lattice effect, when the hybridization of the ladder is strong.Comment: 10 pages, 9 figure

Thermoelectricity by Perfectly Conducting Channels in Quantum Spin Hall Systems

Thermoelectric transport of two-dimensional quantum spin Hall systems are theoretically studied in narrow ribbon geometry. We find that at high temperature electrons in the bulk states dominate. By lowering temperature, the "perfectly conducting" edge channels becomes dominant, and a bulk-to-edge crossover occurs. Correspondingly, by lowering temperature, the figure of merit first decreases and then will increase again due to edge-state-dominated thermoelectric transport.Comment: 5 pages, 3 figures, minor change

Interfacial Fermi Loops from Interfacial Symmetries

We propose a concept of interfacial symmetries such as interfacial particle-hole symmetry and interfacial time-reversal symmetry, which appear in interfaces between two regions related to each other by particle-hole or time-reversal transformations. These symmetries result in novel dispersion of interface states. In particular for the interfacial particle-hole symmetry the gap closes along a loop ("Fermi loop") at the interface. We numerically demonstrate this for the Fu-Kane-Mele tight-binding model. We show that the Fermi loop originates from a sign change of a Pfaffian of a product between the Hamiltonian and a constant matrix.Comment: 5 pages, 2 figure

Landau-Zener tunneling problem for Bloch states

We study the Landau-Zener tunneling problem for particles bound in periodic lattice insulators. To this end, we construct the path integral based on the Bloch and Wannier functions in the presence with an external force, and the transition amplitude is calculated for the Su-Schrieffer-Heeger model. We find that the tunneling probability in bulk periodic systems becomes drastically larger than that by the Landau-Zener formula. This enhancement is prominent for small values of the external field or small hopping integral compared with the gap, and comes from the difference between the Dirac and the periodic dispersions. In addition, when the lattice effect is strong, another analytical formula of the tunneling probability is given with a different behavior from the Landau-Zener formula. Finally, we discuss the observation scheme for the lattice effect.Comment: 6 pages, 3 figure

Completely flat bands and fully localized states on surfaces of anisotropic diamond-lattice models

We discuss flat-band surface states on the (111) surface in the tight-binding model with nearest-neighbor hopping on the diamond lattice, in analogy to the flat-band edge states in graphene with a zigzag edge. The bulk band is gapless, and the gap closes along a loop in the Brillouin zone. The verge of the flat-band surface states is identical with this gap-closing loop projected onto the surface plane. When anisotropies in the hopping integrals increase, the bulk gap-closing points move and the distribution of the flat-band states expands in the Brillouin zone. Then when the anisotropy is sufficiently large, the surface flat bands cover the whole Brillouin zone. Because of the completely flat bands, we can construct surface-state wavefunctions which are localized in all the three directions.Comment: 9 pages, 9 figure

The ALP miracle revisited

We revisit the ALP miracle scenario where the inflaton and dark matter are unified by a single axion-like particle (ALP). We first extend our previous analysis on the inflaton dynamics to identify the whole viable parameter space consistent with the CMB observation. Then, we evaluate the relic density of the ALP dark matter by incorporating uncertainties of the model-dependent couplings to the weak gauge bosons as well as the dissipation effect. The preferred ranges of the ALP mass and coupling to photons are found to be $0.01\lesssim m_\phi \lesssim 1$\,eV and $g_{\phi \gamma \gamma} = {\cal O}(10^{-11})$\,GeV$^{-1}$, which slightly depend on these uncertainties. Interestingly, the preferred regions are within reach of future solar axion helioscope experiments, IAXO and TASTE, and laser-based stimComment: 26 pages, 8 figures; matched with the published versio

Axion domain wall baryogenesis

We propose a new scenario of baryogenesis, in which annihilation of axion domain walls generates a sizable baryon asymmetry. Successful baryogenesis is possible for a wide range of the axion mass and decay constant, $m \simeq 10^8 -10^{13}$ GeV and $f \simeq 10^{13} - 10^{16}$ GeV. Baryonic isocurvature perturbations are significantly suppressed in our model, in contrast to various spontaneous baryogenesis scenarios in the slow-roll regime. In particular, the axion domain wall baryogenesis is consistent with high-scale inflation which generates a large tensor-to-scalar ratio within the reach of future CMB B-mode experiments. We also discuss the gravitational waves produced by the domain wall annihilation and its implications for the future gravitational wave experiments.Comment: 25 pages, 3 figures; v2: accepted for publication in JCA

Domain Wall Formation from Level Crossing in the Axiverse

We point out that domain wall formation is a more common phenomenon in the Axiverse than previously thought. Level crossing could take place if there is a mixing between axions, and if some of the axions acquire a non-zero mass through non-perturbative effects as the corresponding gauge interactions become strong. The axion potential changes significantly during the level crossing, which affects the axion dynamics in various ways. We find that, if there is a mild hierarchy in the decay constants, the axion starts to run along the valley of the potential, passing through many crests and troughs, until it gets trapped in one of the minima; the {\it axion roulette}. The axion dynamics exhibits a chaotic behavior during the oscillations, and which minimum the axion is finally stabilized is highly sensitive to the initial misalignment angle. Therefore, the axion roulette is considered to be accompanied by domain wall formation. The cosmological domain wall problem can be avoided by introducing a small bias between the vacua. We discuss cosmological implications of the domain wall annihilation for baryogenesis and future gravitational wave experiments.Comment: 5 pages, 3 figure

Level Crossing between QCD Axion and Axion-Like Particle

We study a level crossing between the QCD axion and an axion-like particle, focusing on the recently found phenomenon, the axion roulette, where the axion-like particle runs along the potential, passing through many crests and troughs, until it gets trapped in one of the potential minima. We perform detailed numerical calculations to determine the parameter space where the axion roulette takes place, and as a result domain walls are likely formed. The domain wall network without cosmic strings is practically stable, and it is nothing but a cosmological disaster. In a certain case, one can make domain walls unstable and decay quickly by introducing an energy bias without spoiling the Peccei-Quinn solution to the strong CP problem.Comment: 20 pages, 5 figure