Ab initio study of spin-phonon dynamics and band topology through the real-time time-dependent density functional theory

Department of PhysicsAmong diverse hierarchical theoretical methods in the field of condensed matter, Kohn and Sham???s prescription of density functional theory (named the Kohn-Sham equation) is particularly advantageous because of its practicality and simplicity. This equation allows the investigation of the electronic structure of materials on a first-principle basis without requiring inputs derived from prior knowledge. This thesis presents the integration of the Kohn-Sham density-functional equation into a time-evolution package to construct a real-time and time-dependent density functional calculation method. Using this computational tool, the real-time dynamics of electrons in materials were investigated. Numerical analysis of physical phenomena was performed that cannot readily be assessed using simple model-based theories or static calculations. In this thesis, three separate studies are presented. The first is regarding the correlation between the spin state of conduction valley and specific phonon mode in monolayer MoS2. The second study is the revelation of Berry curvature and band topology in time-propagating Bloch states under static E-field. The last study is on the real-time dynamics of ultrafast charge transfer. The focus is on exhibiting the unique advantages of the real-time propagation calculation method. It is proposed that the real-time propagation package can be further developed as a general tool to be applied to the calculations of material responses upon an infinitesimal probing perturbation or a strong external driver.ope

Conversion of multilayer graphene into continuous ultrathin sp 3-bonded carbon films on metal surfaces

The conversion of multilayer graphenes into sp 3-bonded carbon films on metal surfaces (through hydrogenation or fluorination of the outer surface of the top graphene layer) is indicated through first-principles computations. The main driving force for this conversion is the hybridization between sp 3 orbitals and metal surface d z 2 orbitals. The induced electronic gap states and spin moments in the carbon layers are confined in a region within 0.5â.nm of the metal surface. Whether the conversion occurs depend on the fraction of hydrogenated (fluorinated) C atoms at the outer surface and on the number of stacked graphene layers. In the analysis of the Eliashberg spectral functions for the sp 3 carbon films on a metal surface that is diamagnetic, the strong covalent metal-sp 3 carbon bonds induce soft phonon modes that predominantly contribute to large electron-phonon couplings, suggesting the possibility of phonon-mediated superconductivity. Our computational results suggest a route to experimental realization of large-area ultrathin sp 3-bonded carbon films on metal surfaces.open3

Optoelectronic manifestation of the orbital angular momentum driven by chiral hopping in helical Se chains

Chiral materials have garnered significant attention in the field of condensed matter physics. Nevertheless, the magnetic moment induced by the chiral spatial motion of electrons in helical materials, such as elemental Te and Se, remains inadequately understood. In this work, we investigate the development of quantum angular momentum enforced by chirality using static and time-dependent density functional theory calculations for an elemental Se chain. Our findings reveal the emergence of an unconventional orbital texture driven by the chiral geometry, giving rise to a non-vanishing current-induced orbital moment. By incorporating spin-orbit coupling, we demonstrate that a current-induced spin accumulation arises in the chiral chain, which fundamentally differs from the conventional Edelstein effect. Furthermore, we demonstrate the optoelectronic detection of the orbital angular momentum in the chiral Se chain, providing a conceptually novel alternative to the interband Berry curvature, which is ill-defined in low dimensions.Comment: 24 pages, 4 figure

Prediction of ferroelectricity-driven Berry curvature enabling charge- and spin-controllable photocurrent in tin telluride monolayers

In symmetry-broken crystalline solids, pole structures of Berry curvature (BC) can emerge, and they have been utilized as a versatile tool for controlling transport properties. For example, the monopole component of the BC is induced by the time-reversal symmetry breaking, and the BC dipole arises from a lack of inversion symmetry, leading to the anomalous Hall and nonlinear Hall effects, respectively. Based on first-principles calculations, we show that the ferroelectricity in a tin telluride monolayer produces a unique BC distribution, which offers charge- and spin-controllable photocurrents. Even with the sizable band gap, the ferroelectrically driven BC dipole is comparable to those of small-gap topological materials. By manipulating the photon handedness and the ferroelectric polarization, charge and spin circular photogalvanic currents are generated in a controllable manner. The ferroelectricity in group-IV monochalcogenide monolayers can be a useful tool to control the BC dipole and the nonlinear optoelectronic responses

A time-based Chern number in periodically-driven systems in the adiabatic limit

To define the topology of driven systems, recent works have proposed synthetic dimensions as a way to uncover the underlying parameter space of topological invariants. Using time as a synthetic dimension, together with a momentum dimension, gives access to a synthetic 2D Chern number. It is, however, still unclear how the synthetic 2D Chern number is related to the Chern number that is defined from a parametric variable that evolves with time. Here we show that in periodically driven systems in the adiabatic limit, the synthetic 2D Chern number is a multiple of the Chern number defined from the parametric variable. The synthetic 2D Chern number can thus be engineered via how the parametric variable evolves in its own space. We justify our claims by investigating Thouless pumping in two 1D tight-binding models, a three-site chain model and a two-1D-sliding-chains model. The present findings could be extended to higher dimensions and other periodically driven configurations.Comment: 6 pages, 4 figure

The ferroelectric photo ground state of SrTiO3: Cavity materials engineering

Optical cavities confine light on a small region in space, which can result in a strong coupling of light with materials inside the cavity. This gives rise to new states where quantum fluctuations of light and matter can alter the properties of the material altogether. Here we demonstrate, based on first-principles calculations, that such light-matter coupling induces a change of the collective phase from quantum paraelectric to ferroelectric in the SrTiO3 ground state, which has thus far only been achieved in outof-equilibrium strongly excited conditions [X. Li et al., Science 364, 1079-1082 (2019) and T. F. Nova, A. S. Disa, M. Fechner, A. Cavalleri, Science 364, 1075-1079 (2019)]. This is a light-matter hybrid ground state which can only exist because of the coupling to the vacuum fluctuations of light, a photo ground state. The phase transition is accompanied by changes in the crystal structure, showing that fundamental ground state properties of materials can be controlled via strong light-matter coupling. Such a control of quantum states enables the tailoring of materials properties or even the design of novel materials purely by exposing them to confined light.We are grateful for the illuminating discussions with Dmitri Basov, Atac Imamoglu, Jerome Faist, Jean-Marc Triscone, Peter Littlewood, Andrew Millis, Michael Ruggenthaler, Michael A. Sentef, and Eugene Demler. We acknowledge financial support from the European Research Council (Grant ERC2015AdG694097) , Grupos Consolidados (Grant IT124919) , the Japan Society for the Promotion of Science KAKENHI program (Grant JP20K14382) , and the Cluster of Excellence "CUI: Advanced Imag-ing of Matter" of the Deutsche Forschungsgemeinschaft (Grant EXC 2056 Project 390715994) . The Flatiron Institute is a division of the Simons Foundation. S.L. and D.S. acknowledge support from the Alexander von Humboldt Foundation

Mechanisms for Long-Lived, Photo-Induced Superconductivity

Advances in the control of intense infrared light have led to the striking discovery of metastable superconductivity in $\mathrm{K}_3\mathrm{C}_{60}$ at 100K, lasting more than 10 nanoseconds. Inspired by these experiments, we discuss possible mechanisms for long-lived, photo-induced superconductivity above $T_{c}$. We analyze a minimal model of optically-driven Raman phonons coupled to inter-band electronic transitions. Using this model, we develop a possible microscopic mechanism for photo-controlling the pairing interaction by displacively shifting the Raman mode. Leveraging this mechanism, we explore two pictures of long-lived, light-induced superconductivity far above $T_c$. We first investigate long-lived, photo-induced superconductivity arising from the metastable trapping of a displaced phonon coordinate. We then propose an alternate route to long-lived superconductivity. Within this paradigm, the slow equilibration of quasi-particles enables a long-lived, non-thermal superconducting gap. We conclude by discussing implications of both scenarios to experiments that can be used to discriminate between them. Our work provides falsifiable, mechanistic explanations for the nanosecond scale photo-induced superconductivity found in $\mathrm{K}_3\mathrm{C}_{60}$, while also offering a theoretical basis for exploring long-lived, non-equilibrium superconductivity in other quantum materials.Comment: 7 pages Main Text, 9 pages Supplementary Material, 4 figure

Dynamical amplification of electric polarization through nonlinear phononics in 2D SnTe

Ultrafast optical control of ferroelectricity using intense terahertz fields has attracted significant interest. Here we show that the nonlinear interactions between two optical phonons in SnTe, a two-dimensional in-plane ferroelectric material, enables a dynamical amplification of the electric polarization within subpicoseconds time domain. Our first-principles time-dependent simulations show that the infrared-active out-of-plane phonon mode, pumped to nonlinear regimes, spontaneously generates in-plane motions, leading to rectified oscillations in the in-plane electric polarization. We suggest that this dynamical control of ferroelectric material, by nonlinear phonon excitation, can be utilized to achieve ultrafast control of the photovoltaic or other nonlinear optical responses

Dissimilar anisotropy of electron versus hole bulk transport in anatase TiO2: Implications for photocatalysis

Recent studies on crystal facet manipulation of anatase TiO2 in photocatalysis have revealed that reduction and oxidation reactions preferably occur on (100)/(101) and (001) facets, respectively; however, a fundamental understanding of their origin is lacking. Here, as a result of first-principles calculations, we suggest that a dissimilar trend in the anisotropy of electron vs hole bulk transport in anatase TiO2 can be a dominant underlying mechanism for the difference in photochemical activity. Photoexcited electrons and holes are driven to different facets, i.e., electrons on (100)/(101) and holes on (001), leading to the observed preference for either reduction or oxidation. This trend of electrons vs holes found in pure TiO2 applies even for cases where a variety of dopants or defects is introduced.clos