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Reversible control of the topological invariants from nontrivial to trivial states has fundamental implications for quantum information processors and spintronics, by realizing of an on/off switch for robust and dissipationless spin-current. Although mechanical strain has typically advantageous for such control of topological invariants, it is often accompanied by in-plane fractures and is not suited for high-speed, time-dependent operations. Here, we use ultrafast optical and THz spectroscopy to investigate topological phase transitions by light-driven strain in Bi$_2$Se$_3$, a material that requires substantial strain for $\mathrm{Z}_2$ switching. We show that Bi$_2$Se$_3$ experiences ultrafast switching from being a topological insulator with spin-momentum-locked surfaces, to hybridized states and normal insulating phases at ambient conditions. Light-induced strong out-of-plane strain can suppress the surface-bulk coupling, enabling differentiation of surface and bulk conductance at room temperature, far above the Debye temperature. We illustrate various time-dependent sequences of transient hybridization, as well as the switching operation of topological invariants by adjusting the photoexcitation intensity. The abrupt alterations in both surface and bulk transport near the transition point allow for coherent conductance modulation at hyper-sound frequencies. Our findings regarding light-triggered ultrafast switching of topological invariants pave the way for high-speed topological switching and its associated applications., Comment: 14 pages, 4 figure, and Supplementary Material

A Kagome lattice, formed by triangles of two different directions, is known to have many emergent quantum phenomena. Under the breathing anisotropy of bond strengths, this lattice can become a higher-order topological insulator (HOTI), which hosts topologically protected corner states. Experimental realizations of HOTI on breathing Kagome lattices have been reported for various artificial systems, but not for simple natural materials with an electronic breathing Kagome lattice. Here we prove that a breathing Kagome lattice and HOTI are hidden inside the electronic structure of hexagonal transition metal dichalcogenides (h-TMD). Due to the trigonal prismatic symmetry, $sp^2$-like hybrid d-orbitals create an electronic Kagome lattice with anisotropic inter-site and on-site hopping interactions. We demonstrate that HOTI h-TMD triangular nanoflakes host topologically protected corner states, which could be quantum-mechanically entangled with triple degeneracy. Because h-TMDS are easily synthesizable and stable at ambient conditions, our findings open new avenue for quantum physics based on simple condensed matter systems., Comment: 21 pages, 4 figures, Supplementary information

Strain engineering of perovskite quantum dots (pQDs) enables widely-tunable photonic device applications. However, manipulation at the single-emitter level has never been attempted. Here, we present a tip-induced control approach combined with tip-enhanced photoluminescence (TEPL) spectroscopy to engineer strain, bandgap, and emission quantum yield of a single pQD. Single CsPbBr$_{x}$I$_{3-x}$ pQDs are clearly resolved through hyperspectral TEPL imaging with $\sim$10 nm spatial resolution. The plasmonic tip then directly applies pressure to a single pQD to facilitate a bandgap shift up to $\sim$62 meV with Purcell-enhanced PL quantum yield as high as $\sim$10$^5$ for the strain-induced pQD. Furthermore, by systematically modulating the tip-induced compressive strain of a single pQD, we achieve dynamical bandgap engineering in a reversible manner. In addition, we facilitate the quantum dot coupling for a pQD ensemble with $\sim$0.8 GPa tip pressure at the nanoscale. Our approach presents a new strategy to tune the nano-opto-electro-mechanical properties of pQDs at the single-crystal level.

Friction-driven static electrification is familiar and fundamental in daily life, industry, and technology, but its basics have long been unknown and have continually perplexed scientists from ancient Greece to the modern high-tech era. Despite its simple manifestation, triboelectric charging is believed to be very complex because of the unresolvable interfacial interaction between two rubbing materials. Here, we for the first time reveal a simple physics of triboelectric charging and triboelectric series based on friction-originated thermoelectric charging effects at the interface, characterized by the material density (${\rho}$), specific heat (c), thermal conductivity (k), and Seebeck coefficient (S) of each material. We demonstrate that energy dissipational heat at the interface induces temperature variations in the materials and thus develops electrostatic potentials that will initiate thermoelectric charging across the interface. We find that the trends and quantities of triboelectric charging for various polymers, metals, semiconductors, and even lightning clouds are simply governed by the triboelectric factor ${\xi}=S/\sqrt{{\rho}ck}$. The triboelectric figure-of-merit is expressed with the triboelectric power K=${\xi}\sqrt{t/{\pi}}$, of which the difference can be maximized up to 1.2 V/W cm$^{-2}$ at the friction time t = 1 s. Our findings will bring significant opportunities for microscopic understanding and management of triboelectricity or static electrification., Comment: 34 pages, 9 figures, 2 tables, supplementary materials (15 pages)

The atomic variations of electronic wavefunctions at the surface and electron scattering near a defect have been detected unprecedentedly by tracing thermoelectric voltages given a temperature bias [Cho et al., Nature Mater. 12, 913 (2013)]. Because thermoelectricity, or Seebeck effect, is associated with heat-induced electron diffusion, how the thermoelectric signal is related to the atomic-scale wavefunctions and what the role of the temperature is at such a length scale remain very unclear. Here we show that coherent electron and heat transport through a point-like contact produces an atomic Seebeck effect, which is described by mesoscopic Seebeck coefficient multiplied with an effective temperature drop at the interface. The mesoscopic Seebeck coefficient is approximately proportional to the logarithmic energy derivative of local density of states at the Fermi energy. We deduced that the effective temperature drop at the tip-sample junction could vary at a sub-angstrom scale depending on atom-to-atom interaction at the interface. A computer-based simulation method of thermoelectric images is proposed, and a point defect in graphene was identified by comparing experiment and the simulation of thermoelectric imaging., Comment: Main text (16 pages including 4 figures and 34 references) plus Supplementary Information (10 pages including 6 figures)

Heat is a familiar form of energy transported from a hot side to a colder side of an object, but not a notion associated with microscopic measurements of electronic properties. A temperature difference within a material causes charge carriers, electrons or holes, to diffuse along the temperature gradient inducing a thermoelectric voltage. Here we show that local thermoelectric measurements can yield high sensitivity imaging of structural disorder on the atomic and nanometre scales. The thermopower measurement acts to amplify the variations in the local density of states at the Fermi-level, giving high differential contrast in thermoelectric signals. Using this imaging technique, we uncovered point defects in the first layer of epitaxial graphene, which generate soliton-like domain wall line patterns separating regions of the different interlayer stacking of the second graphene layer., Comment: Main Text (19 pages including 5 figures) + Supplementary Information (6 pages including 10 figures, simplified version)

Fullerene coalescence experimentally found in fullerene-embedded single-wall nanotubes under electron-beam irradiation or heat treatment is simulated by minimizing the classical action for many atom systems. The dynamical trajectory for forming a (5,5) C$_{120}$ nanocapsule from two C$_{60}$ fullerene molecules consists of thermal motions around potential basins and ten successive Stone-Wales-type bond rotations after the initial cage-opening process for which energy cost is about 8 eV. Dynamical paths for forming large-diameter nanocapsules with (10,0), (6,6), and (12,0) chiral indexes have more bond rotations than 25 with the transition barriers in a range of 10--12 eV., Comment: 4 pages, 2 figures, 1 supplementary movie at http://dielc.kaist.ac.kr/yonghyun/coal.mpeg. To be published in Physical Review Letters

We investigate the energy spectra of clean incommensurate double-walled carbon nanotubes, and find that the overall spectral properties are described by the so-called critical statistics of Anderson metal-insulator transition. In the energy spectra, there exist three different regimes characterized by Wigner-Dyson, Poisson, and semi-Poisson distributions. This feature implies that the electron transport in incommensurate multi-walled nanotubes can be either diffusive, ballistic, or intermediate between them, depending on the position of the Fermi energy., Comment: final version to appear in Phys. Rev. Lett