Optimal thermoelectric figure of merit of Si/Ge core-shell nanowires

We investigate the thermoelectric energy conversion efficiency of Si and Ge nanowires, and in particular, that of Si/Ge core-shell nanowires. We show how the presence of a thin Ge shell on a Si core nanowire increases the overall figure of merit. We find the optimal thickness of the Ge shell to provide the largest figure of merit for the devices. We also consider Ge core/Si shell nanowires, and show that an optimal thickness of the Si shell does not exist, since the figure of merit is a monotonically decreasing function of the radius of the nanowire. Finally, we verify the empirical law relating the electron energy gap to the optimal working temperature that maximizes the efficiency of the device.

Strain-induced band gap engineering in layered TiS3

By combining ab initio calculations and experiments, we demonstrate how the band gap of the transition metal trichalcogenide TiS3 can be modified by inducing tensile or compressive strain. In addition, using our calculations, we predicted that the material would exhibit a transition from a direct to an indirect band gap upon application of a compressive strain in the direction of easy electrical transport. The ability to control the band gap and its nature could have a significant impact on the use of TiS3 for optical applications. We go on to verify our prediction via optical absorption experiments that demonstrate a band gap increase of up to 9% （from 0.99 to 1.08 eV） upon application of tensile stress along the easy transport direction.

High-harmonic generation from spin-polarised defects in solids

The generation of high-order harmonics in gases enabled to probe the attosecond electron dynamics in atoms and molecules with unprecedented resolution.Extending these techniques to solids,which were originally developed for atomic and molecular gases,requires a fundamental understanding of the physics that has been partially addressed theoretically.Here,we employ timedependent density-functional theory to investigate how the electron dynamics resulting in high-harmonic emission in monolayer hexagonal boron nitride is affected by the presence of vacancies.We show how these realistic spin-polarised defects modify the harmonic emission and demonstrate that important differences exist between harmonics from a pristine solid and a defected solid.In particular,we found that the different spin channels are affected differently by the presence of the spin-polarised point defect.Moreover,the localisation of the wavefunction,the geometry of the defect,and the electron–electron interaction are all crucial ingredients to describe high-harmonic generation in defected solids.

LJItrasensitive H2S gas sensors based on p-type WS2 hybrid materials

Owing to their higher intrinsic electrical conductivity and chemical stability with respect to their oxide counterparts, nanostructured metal sulfides are expected to revive materials for resistive chemical sensor applications. Herein, we explore the gas sensing behavior of WS2 nanowire-nanoflake hybrid materials and demonstrate their excellent sensitivity （0.043 ppm-1） as well as high selectivity towards H2S relative to CO, NH~, H2, and NO （with corresponding sensitivities of 0.002, 0.0074, 0.0002, and 0.0046 pprn-1, respectively）. Gas response measurements, complemented with the results of X-ray photoelectron spectroscopy analysis and first-principles calculations based on density functional theory, suggest that the intrinsic electronic properties of pristine WS2 alone are not sufficient to explain the observed high sensitivity towards H2S. A major role in this behavior is also played by O doping in the S sites of the WS2 lattice. The results of the present study open up new avenues for the use of transition metal disulfide nanomaterials as effective alternatives to metal oxides in future applications for industrial process control, security, and health and environmental safety.

Effect of spin-orbit coupling on the high harmonics from the topological Dirac semimetal Na3Bi

In this work,we performed extensive first-principles simulations of high-harmonic generation in the topological Diract semimetal Na_(3)Bi using a first-principles time-dependent density functional theory framework,focusing on the effect of spin-orbit coupling(SOC)on the harmonic response.We also derived an analytical model describing the microscopic mechanism of strong-field dynamics in presence of spin-orbit coupling,starting from a locally U(1)×SU(2)gauge-invariant Hamiltonian.Our results reveal that SOC:(i)affects the strong-field excitation of carriers to the conduction bands by modifying the bandstructure of Na_(3)Bi,(ii)makes each spin channel reacts differently to the driven laser by modifying the electron velocity(iii)changes the emission timing of the emitted harmonics.Moreover,we show that the SOC affects the harmonic emission by directly coupling the charge current to the spin currents,paving the way to the high-harmonic spectroscopy of spin currents in solids.

Common microscopic origin of the phase transitions in Ta_(2)NiS_(5) and the excitonic insulator candidate Ta_(2)NiSe_(5)

The structural phase transition in Ta-NiSes has been envisioned as driven by the formation of an excitonic insulating phase.However,the role of structural and electronic instabilities on crystal symmetry breaking has yet to be disentangled.Meanwhile,the phase transition in its complementary material Ta_(2)NiS_(5)does not show any experimental hints of an excitonic insulating phase.We present a microscopic investigation of the electronic and phononic effects involved in the structural phase transition in Ta_(2)NiSe_(5)and Ta-Niss using extensive first-principles calculations.In both materials the crystal symmetries are broken by phonon instabilities,which in tum lead to changes in the electronic bandstructure also observed in the experiment.A total energy landscape analysis shows no tendency towards a purely electronic instability and we find that a sizeable lattice distortion is needed to open a bandgap.We conclude that an excitonic instability is not needed to explain the phase transition in both Ta_(2)NiSe_(5)and Ta_(2)NiS_(5).

Orbital magneto-optical response of periodic insulators from first principles

Magneto-optical response,i.e.optical response in the presence of a magnetic field,is commonly used for characterization of materials and in optical communications.However,quantum mechanical description of electric and magnetic fields in crystals is not straightforward as the position operator is ill defined.We present a reformulation of the density matrix perturbation theory for time-dependent electromagnetic fields under periodic boundary conditions,which allows us to treat the orbital magneto-optical response of solids at the ab initio level.The efficiency of the computational scheme proposed is comparable to standard linearresponse calculations of absorption spectra and the results of tests for molecules and solids agree with the available experimental data.A clear signature of the valley Zeeman effect is revealed in the continuum magneto-optical spectrum of a single layer of hexagonal boron nitride.The present formalism opens the path towards the study of magneto-optical effects in strongly driven low-dimensional systems.

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.

Phonon-mediated unconventional superconductivity in rhombohedral stacked multilayer graphene

Understanding the origin of superconductivity in correlated two-dimensional materials is a key step in leveraging material engineering techniques for next-generation nanoscale devices.While it is widely accepted that phonons fluctuations only mediate conventional(s-wave)superconductivity,the common phenomenology of superconductivity in Bernal bilayer and rhombohedral trilayer graphene,as well as in a large family of graphene-based moirésystems,suggests a common superconducting mechanism across these platforms.In particular,in all these platforms some superconducting regions violate the Pauli limit,indicating unconventional superconductivity,naively ruling out conventional phonon-mediated pairing as the underlying mechanism.Here we combine first principles simulations with effective low-energy theories to investigate the superconducting mechanism and pairing symmetry in rhombohedral stacked graphene multilayers.

Enhanced high harmonic efficiency through phonon-assisted photodoping effect

High-harmonic generation(HHG)has emerged as a central technique in attosecond science and strong-field physics,providing a tool for investigating ultrafast dynamics.However,the microscopic mechanism of HHG in solids is still under debate,and it is unclear how it is modified in the ubiquitous presence of phonons.