Session S08: Transport, Optical, and Thermodynamic Phenomena

Landau-Zener tunneling problem for particles in periodic lattice

The Landau-Zener (LZ) problem plays an important role for the tunneling in nanoscale systems. Conventionally, the LZ formula is widely used for the calculation of the tunneling probability, and this formula is obtained by a linear approximation in the vicinity of the band edges for periodic lattice systems. Therefore, when we take into account the nonlinear effect by the periodic lattice, it is not sufficient for the calculation of the tunneling probability by the LZ formula. In this presentation, we report the LZ tunneling problem for particles bound in the periodic lattice [1]. 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 probability is calculated for the Su-Schrieffer-Heeger model with a constant gap. Then, we find that the tunneling probability becomes drastically larger than that by the LZ formula. This enhancement is prominent for small values of the external field or small hopping integral, and comes from the difference between the Dirac and the periodic dispersions. [1] Ryuji Takahashi and Naoyuki Sugimoto, Phys. Rev. B 95, 224302(2017).   

The Berry curvature dipole and dc photocurent in Weyl semimetal material

Noncentrosymmetric metals are anticipated to exhibit a dc photocurrent in the nonlinear optical response caused by the Berry curvature dipole in momentum space. Weyl semimetals (WSMs) are expected to be excellent candidates for observing these nonlinear effects because they carry a large Berry curvature concentrated in small regions, i.e., near the Weyl points. We have implemented the semiclassical Berry curvature dipole formalism into an ab initio scheme and investigated the second-order nonlinear response for two representative groups of materials: the TaAs-family type-I WSMs and MoTe2-family type-II WSMs. Both types of WSMs exhibited a Berry curvature dipole, in which type-II Weyl points are usually superior to the type-I because of the strong tilt. Corresponding nonlinear susceptibilities in several materials promise a nonlinear Hall effect in the dc field limit, which is within the experimentally detectable range.   

Shift current driven by ultrashort light pulses in insulating and metallic inversion asymmetric systems: Time-dependent non-equilibrium Green's function approach

Using the formalism of time dependent non-equilibrium Green's functions (TD-NEGF) we investigate the transport properties of Rice-Mele model when irradiated with ultrashort light pulse in the absence of any bias voltage. The model consists of a chain in which there are two kinds of atoms leading to different onsite energies along with hopping potential that changes from one atom to other. We then attach leads made out of a simple 1D chain and illuminate the Rice-Mele chain with an ultrashort (i.e. femtosecond duration) light pulse whose central frequency is equal to the energy gap of Rice-Mele model. While the time-dependent current oscillates following the pulse, there is an average current and a total charge is injected into the leads. Such a shift current exists even when Rice-Mele chain is short enough that evanescent wave functions from the leads fill its gap and make the whole device metallic because the whole process can be viewed as non-adiabatic charge pumping where the key requirement is broken left-right spatial symmetry. Finally, we quantify the velocity of propagation of shift current generated in the center of clear or disordered Rice-Mele chain and compare with recent experiments [Ref : M. Nakamura et al., Nature Commun. 8, 281(2017)].   

Lattice dynamics and anharmonicity: comparing first principles calculations and measurements

The interplay of experiment and theory is critical to building basic insights and fundamental principles into physical phenomena, and yet comparison of measurement and calculations is not always straightforward in practice. Here we discuss the relationship of measurement probes (Raman, neutrons and x-rays) and first principles calculations of lattice vibrations and anharmonicity as they apply to a variety of semiconducting materials. In particular, we will discuss our recent efforts in benchmarking phonon frequencies, density of states and lifetimes in MoS₂, CuCl, Tl₃VSe₄, MoCl₃, hBN and other materials.   

Diffusive real-time dynamics of a particle with Berry curvatures

Motivated by the recent developments of time-resolved experiments, we study theoretically the influence of Berry phase on the real-time dynamics focusing on the diffusive dynamics, i.e., the time-dependence of the distribution function. We found that the dynamics at the early stage is deeply influenced by the Berry curvatures in real-space (B), momentum-space (Ω), and also the crossed space between these two (C). For example, it is found that Ω induces the rotation of the wave packet and causes the time-dependence of the mean square displacement of the particle to be linear in time t at the initial stage; it is qualitatively different from the t³ dependence in the absence of the Berry curvatures. It is also found that Ω and C modifies the characteristic time scale of the thermal equilibration of momentum distribution. Moreover, the dynamics under various combinations of B, Ω and C shows singular behaviors such as the critical slowing down or speeding up of the momentum equilibration and the reversals of the direction of rotations. These results offer yet another method to disentangle Berry curvatures in terms of time-resolved experiments. Possible experimental observations for excitons and trions in transition metal dichalcogenides are also discussed.   

Optical investigation on oxygen defect states in SrTiO3 compounds

We investigated oxygen defect states in SrTiO₃ (STO) thin films by using the photoluminescence spectroscopy and spectroscopic ellipsometry. The oxygen-deficient STO thin films were thermally treated in various oxygen environments systematically. The visible emission and absorption increased simultaneously with the increase of the oxygen vacancy concentration in the STO thin films. On the basis of the absolute values of the optical constants, we discussed the origins of the visible absorption features in STO, in relation to the defect formation and related electronic reconstruction. We also investigated the STO single crystals in the similar experimental procedure, and compared the results with the case of the thin films.
  

CH3NH3PbX3 perovskites: Excellent microwave absorption

Given the remarkable progress in physical and structural performances of organic-inorganic lead halide perovskites as a superstar in the photovoltaic field, the study of some intrinsic properties are still missing. Herein, we report the microwave absorption performance in CH₃NH₃PbX₃ (X = I, Br and Cl) perovskite single crystals in terms of complex permittivity and permeability. We find that the CH₃NH₃PbI₃, CH₃NH₃PbBr₃ and CH₃NH₃PbCl₃ perovskites possess excellent microwave absorbability, and the optimum reflect losses reach -55.2, -54.7 and -46.4 dB at 16.8, 15.5 and 13.5 GHz with a matching thickness of 1.62, 1.76 and 1.95 mm, respectively. We explore the absorption mechanism. The microwave absorption properties are ascribed to the combination of dielectric and magnetic loss, but mainly for dielectric loss. In addition, the stability of microwave absorbability is also investigated, significantly dependent on the decomposition of corresponding perovskites. These results highlight the importance to the discovery of microwave absorbability in organic-inorganic lead halide perovskites. More importantly, our study provides perovskite absorbers as a new variable to consider in the quest for future microwave devices.   

Field Modulation Imaging of Polarization Domain Dynamics in Single-Crystalline Organic Ferroelectric Thin Films at Various Temperatures

Ferroelectrics have switchable spontaneous electric polarization, the feature of which is useful in nonvolatile memories or ferroelectric field-effect transistors. Among a variety of ferroelectric materials, proton-transfer-type organic ferroelectrics [1] are promising as key materials in flexible printed electronics due to the processability at ambient conditions. However, the studies of these materials are limited, because the ferroelectric domain dynamics as well as film fabrication methods have been unknown.
Here report a new noncontact method to visualize the ferroelectric domain structures over a wide area of ferroelectric films using CMOS image sensors. We call the technique as “ferroelectrics field modulation imaging (FFMI)”, in which the polarization domains can be visualized by detecting the change of optical absorption induced by applied ac electric fields. We demonstrate that the polarization domains are visualized in single-crystal thin films of [Hdppz][Hca], a proton-transfer-type organic ferroelectrics [1], at various temperatures. We show that the domain-wall motion is more suppressed in thin films by possible large potential barriers than in bulk crystals, based on the Arrhenius analyses of the measurements.
[1] S. Horiuchi et al. JACS 135, 4492 (2013).   

Quantization of the heat polarization in twisted spacetime

We study the heat polarization in the quantum Hall system in 2+1-dimensional twisted spacetime. In analogy with the quantum Hall effect, the quantized thermal Hall effect can be explained by defining the heat polarization and the heat magnetization, the theory of which is based on the spacetime geometry. In this study, we consider a diffeomorphism of spacetime by twisting the boundary condition in both spatial and temporal directions. We will show that the diffeomorphism in the quantum Hall system induces heat transfer between boundaries, which gives rise to the heat polarization.   

Optical phonons in layer crystal of As2S3: effect of weak interlayer interaction.

As₂S₃ is known as layer crystal with structure which is very similar to the metal chalcogenides as MoS₂ and some other. In such crystalline structure, the molecular unit is indefinitely extended in two dimensions. Center-zone optical phonons in 2D layer crystal As₂S₃ have been investigated in Raman scattering in temperature range of 4K-300K for both polarizations in the layer plane. Analysis of the symmetry of the crystal and 2D symmetry of the individual layer reveals that two symmetries predict different selection rules for optical phonons. Because crystal unit cell contains two layers, there are two times more (60) normal mode for the crystal compare to the layer (30). In the vanishing interaction between the layers, 60 modes collapse to 30 degenerated doublets. Weak interlayer interaction lifts these degeneracies with results of closely spaced doublets. Our experimental data show strong polarization dependence of Raman bands in ac plane at 134.4, 152.3, 185.5, 290.0, 309.4 and triplet 351.8, 354.1, 356.9 cm^-1. The important evidence for the dominance of layer symmetry, along with very weak interaction between the layers, provides understanding of structural motives of As₂S₃ and may predict optical/electronic properties of similar 2D materials.   

Ab initio Thermochemistry of the Si2N2(NH) analog of Si2N2O

The potential of designing silicon-based oxide/nitride with new properties through nanostructure and chemical tuning continues to captivate researchers. In this context, while oxynitrides of Si, Ge and Si/Ge are being actively explored, N-rich alternatives to the ubiquitous Si₃N₄ end member are far less known or studied. Recently, the imide-based Si₂N₂(NH) was synthesized and found to adopt a crystalline structure analogous to that of Si₂N₂O. Here we present a DFT study of the thermochemistry of Si-N-H compounds including Si₃N₄, Si₂N₂(NH) and Si(NH)₂, focusing on their relative stability and structural polymorphism. Using density functional theory (LDA) in the static lattice approximation, and at P=0, we find that the tetragonal and monoclinic phases of Si₂N₂O are ~0.6 and ~1.4 eV above the body centered orthorhombic ground state structure, quite similar to their Si₂N₂(NH) counterparts (~0.8 and ~1.8 eV, respectively). We will elucide the analogies in the polymorphism in these oxide/imide systems in terms of their p-T phase diagrams, based on ab initio quasiharmonic phonon-derived DFT thermochemistry.   

Two Kinds of Local Temperature in Boltzmann Theory of Inhomogeneous Vibrational Heat Transport

Heat in insulators, carried by phonons, is described by the Peierls Boltzmann equation (PBE). In nanoscale situations, the heat current is partly diffusive (from phonons with mean free paths (mfp) less than sample size L), and partly ballistic (mfp > L). We find that, when the temperature gradient varies spatially, the local temperature T(r) appearing in the PBE differs from the temperature associated with the total non-equilibrium energy. This latter temperature is probably measured by most external probes. Our relaxation time approximation (RTA) solutions of the PBE quantify the difference, which is typically 5—20%. The difference depends on how much dispersion there is in phonon scattering rates 1/τ_Q, and disappears if 1/τ_Q=constant. The temperature difference does not seem to be an artifact of the RTA.   

Spin-momentum locked interaction between guided photons and surface electrons in topological insulator Bi2Se3

The propagation of electrons and photons can respectively have the spin-momentum locking effect (SML) which correlates the spin with the linear momentum. A direct connection between the electron and photon spin occurs in topological insulators (TIs) with lifted spin degeneracy, which results in circular photogalvanic effect. Here, we study the photogalvanic effects in exfoliated Bi₂Se₃ by measuring the dependence of photoresponse on crystal orientation. In contrast to the linear photogalvanic effect that shows 3-fold rotation symmetry on crystal orientation, an isotropic photoresponse is observed for the circular photogalvanic effect, indicates the corresponding optical transitions indeed involve surface bands. Furthermore, we demonstrate an optoelectronic device that integrates a TI with a photonic waveguide. Interaction between the photons in the waveguide, which carries optical spin, and the surface electrons in a Bi₂Se₃ generates a directional photocurrent. Because of SML, the device works in a non-reciprocal way such that changing the light propagation direction reverses the optical spin and thus the direction of the photocurrent in TI.   

Ultralow thermal conductivity of single crystalline BaTiS3

BaTiS3 (BTS) is a new perovskite-like material that has promising optical, electronic and thermal properties, with potential applications in electronics and thermoelectrics. We report measurements of thermal conductivity of BTS single crystal from 80 K – 300 K using time-domain thermoreflectance. The room temperature thermal conductivity of BTS is found to be 0.5 W m-1 K-1, which is less than 10% of that of commonly known BaTiO3. To understand the origin of such low thermal conductivity we perform ab initio calculations using the temperature dependent effective potential (TDEP) method [1, 2] to study phonon transport at finite temperature. The strong temperature dependence of phonon modes and low group velocities lead to ultralow values of lattice thermal conductivity, supporting the experimental observation.
[1] O. Hellman, I. A. Abrikosov, S. I. Simak. Physical Review B 84, 180301(2011).
[2] O. Hellman, I. A. Abrikosov. Physical Review B 88, 144301(2013).