Engineering topics

Aoki, Hideo

Publications and source records attributed to Aoki, Hideo.

9 records

DOE OSTI2020

Optical imprinting of superlattices in two-dimensional materials

We propose a novel optical method to imprint superlattice structures in two-dimensional electronic systems. By changing the shape of the optical field, we synthesize various lattice structures with different spatial symmetry, periodicity, and strength. We find that the wide optical tunability allows one to tune different properties of the effective band structure, including Chern number, energy bandwidths, and band gaps. We consider the high-frequency regime where the electronic system can remain in the quasi-equilibrium phase for an extended amount of time. The spatiotemporal reconfigurablity of our approach opens up new possibilities to control light-matter interaction to generate novel electronic states and optoelectronic devices.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Higgs Mode in the d -Wave Superconductor Bi 2 Sr 2 CaCu 2 O 8 + x Driven by an Intense Terahertz Pulse

We investigated the terahertz (THz)-pulse driven nonlinear response in the d-wave cuprate superconductor Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) using a THz pump near-infrared probe scheme in the time domain. We have observed an oscillatory behavior of the optical reflectivity that follows the THz electric field squared and is strongly enhanced below T c . The corresponding third-order nonlinear effect exhibits both A 1g and B 1g symmetry components, which are decomposed from polarization-resolved measurements. Comparison with a BCS calculation of the nonlinear susceptibility indicates that the A 1g component is associated with the Higgs mode of the d-wave order parameter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

First-principles design of a half-filled flat band of the kagome lattice in two-dimensional metal-organic frameworks

Here, we design from first principles a type of two-dimensional metal-organic framework (MOF) using phenalenyl-based ligands to exhibit a half-filled flat band of the kagome lattice, which is one of a family of lattices that show Lieb-Mielke-Tasaki’s flat-band ferromagnetism. Among various MOFs, we find that trans-Au-THTAP (THTAP=trihydroxytriaminophenalenyl) has such an ideal band structure, where the Fermi energy is adjusted right at the flat band due to unpaired electrons of radical phenalenyl. The spin-orbit coupling opens a band gap giving a nonzero Chern number to the nearly flat band, as confirmed by the presence of the edge states in first-principles calculations and by fitting to the tight-binding model. This is a novel and realistic example of a system in which a nearly flat band is both ferromagnetic and topologically nontrivial.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

Plateau structure in the Faraday rotation in the graphene quantum Hall system and the frequency-driven two-parameter scaling

Flow diagram of (σ{sub xx},σ{sub xy}) in finite-frequency (ω) regime is numerically studied for the graphene quantum Hall effect (QHE). In a small-ω regime, the ac flow diagram exhibits a behavior qualitatively similar to the dc flow diagram, where the dynamical length scale acts as a cutoff in the ac region, and the flows are between σ{sub xy} = ±(1/2)e{sup 2}/h reflecting the graphene QHE. The dynamical flow is compared with the temperature-driven flow.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

Chiral symmetry and fermion doubling in the zero-mode Landau levels of massless Dirac fermions with disorder

The effect of disorder on the Landau levels of massless Dirac fermions is examined for the cases with and without the fermion doubling. To tune the doubling a tight-binding model having a complex transfer integral is adopted to shift the energies of two Dirac cones, which is theoretically proposed earlier and realizable in cold atoms in an optical lattice. In the absence of the fermion doubling, the n = 0 Landau level is shown to exhibit an anomalous sharpness even if the disorder is uncorrelated in space (i.e., large K-K' scattering). This anomaly occurs when the disorder respects the chiral symmetry of the Dirac cone.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2011

Robustness of the Edge States in Graphene Quantum Hall System: Does the Chiral Symmetry Degraded by t' Matter?

While usual edge states in the quantum Hall effect reside between adjacent Landau levels, QHE in graphene has a peculiar edge mode at E = 0 that resides right within the n = 0 Landau level as protected by the chiral symmetry. In real graphene, small but finite second-neighbor hopping exists, which degrades the chiral symmetry in the bipartite lattice. Here we have found that the edge mode is again embedded in the n = 0 bulk Landau level despite the broken chiral symmetry due to the second-neighbor hopping, where the charge is accumulated along the zigzag edge. Hence the behavior of the zero mode has turned out to be distinguished from the case of zero magnetic field, which is related to the stability of topological compensation. This charge density accumulated along the zigzag edge can be measured with an STM imaging in magnetic fields.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Entanglemet entropy of the bond order phase in graphene in magnetic fields

The quantum Hall effect (QHE) in graphene has a peculiar edge mode at E = 0 that resides right within the n = 0 Landau level as protected by the chiral symmetry, in contrast that the usual edge states in the quantum Hall effect (QHE) reside between adjacent Landau levels. We investigate edge states of graphene with the Kekule bond order, which respect the chiral symmetry using the entanglement entropy.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Dynamical Band Flipping in Fermionic Lattice Systems: An ac-Field-Driven Change of the Interaction from Repulsive to Attractive

We show theoretically that the sudden application of an appropriate ac field to correlated lattice fermions flips the band structure and effectively switches the interaction from repulsive to attractive. The nonadiabatically driven system is characterized by a negative temperature with a population inversion. We numerically demonstrate the converted interaction in an ac-driven Hubbard model with the nonequilibrium dynamical mean-field theory solved by the continuous-time quantum Monte Carlo method. Based on this, we propose an efficient ramp-up protocol for ac fields that can suppress heating, which leads to an effectively attractive Hubbard model with a temperature below the superconducting transition temperature of the equilibrium system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2011

Collective modes in multiband superfluids and superconductors: Multiple dynamical classes

One important way to characterize the states having a gauge symmetry spontaneously broken over multibands is to look at their collective excitation modes. We find that a three-band system has multiple Leggett modes with significantly different masses, which can be classified into different dynamical classes according to whether multiple interband Josephson currents add or cancel. This provides a way to dynamically characterize multiband superconductivity while the pairing symmetry is a static property.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗