A configuration space approach is developed to uncover generic stability peculiarities for the lowest energy neutral and charged exciton complexes (biexciton and trion) in quasi-one-dimensional semiconductors. Trions are shown to be more stable than biexcitons in strongly confined structures with small reduced electron-hole masses. Biexcitons are more stable in less confined structures with large reduced electron-hole masses. In semiconducting carbon nanotubes, in particular, the trion binding energy is shown to be greater than that of the biexciton by a factor of $\sim 1.4$, decreasing with diameter, thus revealing the general physical principles that underlie recent experimental observations.

We present measurements of the $D^{\prime \prime }$ Raman mode in graphene and carbon nanotubes at different laser excitation energies. The Raman mode around 1050–1150 ${\text{cm}}^{-1}$ originates from a double-resonant scattering process of longitudinal acoustic (LA) phonons with defects. We investigate its dependence on laser excitation energy, on the number of graphene layers, and on the carbon nanotube diameter. We assign this Raman mode to so-called inner processes with resonant phonons mainly from the $\Gamma \text{−}K$ high-symmetry direction. The asymmetry of the $D^{\prime \prime }$ mode is explained by additional contributions from phonons next to the $\Gamma \text{−}K$ line. Our results demonstrate the importance of inner contributions in the double-resonance scattering process and add a fast method to investigate acoustic phonons in graphene and carbon nanotubes by optical spectroscopy.

We have investigated the variation in the magnetization of highly ordered pyrolytic graphite (HOPG) after neutron irradiation, which introduces defects in the bulk sample and consequently gives rise to a large magnetic signal. We observe strong paramagnetism in HOPG, increasing with the neutron fluence. The induced paramagnetism can be well correlated with structural defects by comparison with density-functional theory calculations. In addition to the in-plane vacancies, the transplanar defects also contribute to the magnetization. The lack of any magnetic order between the local moments is possibly due to the absence of hydrogen/nitrogen chemisorption, or the magnetic order cannot be established at all in the bulk form.

The strong coupling between the crystal structure and magnetic state (ferromagnetic or helical antiferromagnetic) of ${\mathrm{FeMnP}}_{0.75}{\mathrm{Si}}_{0.25}$ is investigated using density functional theory in combination with atomistic spin dynamics. We find many competing energy minima for drastically different ferromagnetic and noncollinear magnetic configurations. We also find that the appearance of a helical spin-spiral magnetic structure at finite temperature is strongly related to one of the crystal structures reported for this material. Shorter Fe-Fe distances are found to lead to a destabilized ferromagnetic coupling, while out-of-plane Mn-Mn exchange interactions become negative with the shortening of the interatomic distances along the $c$ axis, implying an antiferromagnetic coupling for the nearest-neighbor Mn-Mn interactions. The impact of the local dynamical correlations is also discussed.

A series of $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right){\mathrm{OHFeSe}}_{1-x}{\mathrm{S}}_x$ $(0\le x\le 1)$ samples were successfully synthesized via hydrothermal reaction method and the phase diagram is established. Magnetic susceptibility suggests that an antiferromagnetism arising from $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OH}$ layers coexists with superconductivity, and the antiferromagnetic transition temperature nearly remains constant for various S doping levels. In addition, the lattice parameters of the both $a$ and $c$ axes decrease and the superconducting transition temperature $T_c$ is gradually suppressed with the substitution of S for Se, and eventually superconductivity vanishes at $x=0.90$. The decrease of $T_c$ could be attributed to the effect of chemical pressure induced by the smaller ionic size of S relative to that of Se, being consistent with the effect of hydrostatic pressure on $\left({\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\right)\mathrm{OHFeSe}$. But the detailed investigation on the relationships between $T_{\mathrm{c}}$ and the crystallographic facts suggests a very different dependence of $T_{\mathrm{c}}$ on anion height from the Fe2 layer or $Ch-\mathrm{Fe}2-Ch$ angle from those in FeAs-based superconductors.

The recently described phenomenon of resonant acoustic spin pumping is due to resonant coupling between an incident elastic wave and spin waves in a ferromagnetic medium. A classical one-dimensional discrete model of a ferromagnet with two forms of magnetoelastic coupling is treated to shed light on the conditions for resonance between phonons and magnons. Nonlinear phonon-magnon interactions in the case of a coupling restricted to diagonal terms in the components of the spin degrees of freedom are analyzed within the framework of the multiple timescale perturbation theory. In that case, one-phonon-two-magnon resonances are the dominant mechanism for pumping. The effect of coupling on the dispersion relations depends on the square of the amplitude of the phonon and magnon excitations. A straightforward analysis of a linear phonon-magnon interaction in the case of a magnetoelastic coupling restricted to off-diagonal terms in the components of the spins shows a one-phonon to one-magnon resonance as the pumping mechanism. The resonant dispersion relations are independent of the amplitude of the waves. In both cases, when an elastic wave with a fixed frequency is used to stimulate magnons, application of an external magnetic field can be used to approach resonant conditions. Both resonance conditions exhibit the same type of dependency on the strength of an applied magnetic field.

We present a way of constructing continuous matrix product states (cMPS) for coupled fields. The cMPS is a variational ansatz for the ground state of quantum field theories in one dimension. Our proposed scheme is based on a physical interpretation in which the cMPS class can be produced by means of a dissipative dynamic of a system interacting with a bath. We study the case of coupled bosonic fields. We test the method with previous DMRG results in coupled Lieb-Liniger models. Besides, we discuss a novel application for characterizing the Luttinger liquid theory emerging in the low-energy regime of these theories. Finally, we propose a circuit QED architecture as a quantum simulator for coupled fields.

Optical and Hall effect measurements have been performed on single crystals of ${\mathrm{Pb}}_{0.77}{\mathrm{Sn}}_{0.23}\mathrm{Se}$, a IV-VI mixed chalcogenide. The temperature-dependent (10–300 K) reflectance was measured over 40–7000 ${\mathrm{cm}}^{-1}$ (5–870 meV) with an extension to 15 500 ${\mathrm{cm}}^{-1}$ (1.92 eV) at room temperature. The reflectance was fitted to the Drude-Lorentz model using a single Drude component and several Lorentz oscillators. The optical properties at the measured temperatures were estimated via Kramers-Kronig analysis as well as by a Drude-Lorentz fit. The carriers were p type with the carrier density determined by Hall measurements. A signature of valence intraband transition is found in the low-energy optical spectra. It is found that the valence-conduction band transition energy as well as the free-carrier effective mass reach minimum values at 100 K, suggesting temperature-driven band inversion in the material. Density functional theory calculations for the electronic band structure are also presented.

${\mathrm{Pb}}_{0.77}{\mathrm{Sn}}_{0.23}\mathrm{Se}$ is a promising thermoelectric alloy that exhibits a temperature dependent band inversion below 300 K. Recent work has shown that this band inversion also coincides with a trivial to nontrivial topological phase transition. To understand how the properties critical to thermoelectric efficiency are affected by the band inversion, we measured the broadband optical response of ${\mathrm{Pb}}_{0.77}{\mathrm{Sn}}_{0.23}\mathrm{Se}$ as a function of temperature. We find clear optical signatures suggesting the band inversion occurs at $160\pm 15$ K, and use the extended Drude model to accurately determine a $T^{3/2}$ dependence of the bulk carrier lifetime, associated with electron-acoustic phonon scattering. Due to the high bulk carrier doping level, no discriminating signatures of the topological surface states are found, although their presence cannot be excluded from our data.

We study the relaxation of an electron spin qubit in a Si quantum dot due to electrical noise. In particular, we clarify how the presence of conduction-band valleys influences spin relaxation. In single-valley semiconductor quantum dots, spin relaxation is through the mixing of spin and envelope orbital states via spin-orbit interaction. In Si, the relaxation could also be through the mixing of spin and valley states. We find that the additional spin relaxation channel, via spin-valley mixing and electrical noise, is indeed important for an electron spin in a Si quantum dot. By considering both spin-valley and intravalley spin-orbit mixings and Johnson noise in a Si device, we find that the spin relaxation rate peaks at the hot spot, where the Zeeman splitting matches the valley splitting. Furthermore, because of a weaker field dependence, the spin relaxation rate due to Johnson noise could dominate over phonon noise at low magnetic fields, which fits well with recent experiments.

We investigate the electronic and vibrational spectra of ${\mathrm{SrTiO}}_3$, as well as the coupling between them, using first-principles calculations. We compute electron-phonon scattering rates for the three lowest-energy conduction bands and use Boltzmann transport theory to calculate the room-temperature mobility of ${\mathrm{SrTiO}}_3$. The results agree with experiment and highlight the strong impact of longitudinal optical phonon scattering. Our analysis provides important insights into the key factors that determine room-temperature mobility, such as the number of conduction bands and the nature and frequencies of longitudinal phonons. Such insights provide routes to engineering materials with enhanced mobilities.

Excitation of electron-hole pairs in the vicinity of the Dirac cone by the Coulomb interaction gives rise to an asymmetric Breit-Wigner-Fano line shape in the phonon Raman spectra in graphene. This asymmetric line shape appears due to the interference effect between the phonon spectra and the electron-hole pair excitation spectra. The calculated Breit-Wigner-Fano asymmetric factor $1/q_{\mathrm{BWF}}$ as a function of the Fermi energy shows a V-shaped curve with a minimum value at the charge neutrality point and gives good agreement with the experimental results.

Understanding the crystal structure and electronic states of the organic semiconductor is of fundamental importance for developing the materials for the organic electronics. However, the theoretical treatment of organic semiconductors remains challenging, as the semilocal density functional theory fails to describe the dispersion forces accurately. We use van der Waals inclusive density functionals to study the zinc phthalocyanine polymorphs. It is found that the structure and energetics are well described with the van der Waals density functional, and as a result, the electronic band structure is nicely reproduced. Furthermore, we reveal that the distance between the molecules and the molecule tilt angle are important factors that determine the electronic band dispersion.

We investigate proximity-induced superconductivity in micrometer-long bismuth nanowires connected to superconducting electrodes with a high critical field. At low temperature we measure a supercurrent that persists in magnetic fields as high as the critical field of the electrodes (above 11 T). The critical current is also strongly modulated by the magnetic field. In certain samples we find regular, rapid SQUID-like periodic oscillations occurring up to high fields. Other samples exhibit less periodic but full modulations of the critical current on Tesla field scales, with field-caused extinctions of the supercurrent. These findings indicate the existence of low dimensionality, phase coherent, interfering conducting regions through the samples, with a subtle interplay between orbital and spin contributions. We relate these surprising results to the electronic properties of the surface states of bismuth, strong Rashba spin-orbit coupling, large effective $g$ factors, and their effect on the induced pair correlations. In particular, we emphasize the possible contribution of topological edge states of specific facets of the nanowires.

Layered materials, such as graphite/graphene, boron nitride, transition metal dichalcogenides, represent materials in which reduced size, dimensionality, and symmetry play critical roles in their physical properties. Here, we report on a comprehensive investigation of the phonon properties in the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ two-dimensional (2D) crystals, with the combination of Raman spectroscopy, first-principles calculations, and group theory analysis. Low frequency $(<30\mathrm{c}{\mathrm{m}}^{-1})$ interlayer vibrational modes are revealed in few-quintuple-layer (QL) $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\text{/}\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ 2D crystals, which are absent in the bulk crystal as a result of different symmetries. The experimentally observed interlayer shear and breathing mode frequencies both show blueshifts, with decreasing thickness in few-QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (down to 2QL) and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ (down to 1QL), in agreement with first-principles calculations and a linear chain model, from which the interlayer coupling force constants can be estimated. Besides, an intense ultralow $(<12\mathrm{c}{\mathrm{m}}^{-1})$ frequency peak is observed in 2–4QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, which is tentatively attributed to a substrate-induced interface mode supported by a linear chain model analysis. The high frequency Raman peaks exhibit frequency shifts and broadening from 3D to 2D as a result of the phonon confinement effect. Our studies shed light on a general understanding of the influence of dimensionality and crystal symmetry on the phonon properties in layered materials.

Few-layer graphene is a layered carbon material with covalent bonding in the layers and weak van der Waals interactions between the layers. The interlayer energy is more than two orders of magnitude smaller than the intralayer one, which hinders the description of the static and dynamic properties within electron band structure models. We overcome this difficulty by introducing two sets of matrix elements—one set for the covalent bonds in the graphene layers and another one for the van der Waals interactions between adjacent graphene layers in a tight-binding model of the band structure. Both sets of matrix elements are derived from an ab initio study on carbon dimers. The matrix elements are applied in the calculation of the phonon dispersion of graphite and few-layer graphene with AB and ABC layer stacking. The results for few-layer graphene with AB stacking agree well with the available experimental data, which justifies the application of the matrix elements to other layered carbon structures with van der Waals interactions such as few-layer graphene nanoribbons, multiwall carbon nanotubes, and carbon onions.

We study the structural, ferroelectric, and magnetic properties of the potentially multiferroic Aurivillius phase material ${\mathrm{Bi}}_5{\mathrm{FeTi}}_3{\mathrm{O}}_{15}$ using first-principles electronic structure calculations. Calculations are performed both with PBE and PBEsol exchange correlation functionals. We conclude that PBE systematically overestimates the lattice constants and the magnitude of the ferroelectric distortion, whereas PBEsol leads to good agreement with available experimental data. We then assess a potential site preference by comparing ten different distributions of the ${\mathrm{Fe}}^{3+}\mathrm{cation}$ on the perovskite $B$ sites. We find a slight preference for the “inner” site, consistent with recent experimental observations. We obtain a large value of $\sim 55\mu {\mathrm{C}/\mathrm{cm}}^2$ for the spontaneous electric polarization, which is rather independent of the specific Fe distribution. Finally, we calculate the strength of the magnetic coupling constants and find strong antiferromagnetic coupling between ${\mathrm{Fe}}^{3+}\mathrm{cations}$ in nearest-neighbor positions, whereas the coupling between further neighbors is rather weak. This poses the question whether magnetic long-range order can occur in this system in spite of the low concentration of magnetic ions.

We report the frequency dependence of the spin current emission (spin pumping) in a hybrid ferrimagnetic insulator/normal metal system as a function of the insulating layer thickness. The system is based on an yttrium iron garnet (YIG) film [0.2, 1, and $3\mu \mathrm{m}$] grown by liquid-phase epitaxy coupled with a spin current detector of platinum[6 nm]. A strong YIG thickness dependence of the efficiency of the spin pumping has been observed. The highest conversion factor $\Delta V/P_{\mathrm{abs}}$ has been demonstrated for the thinner YIG (1.79 and 0.55 mV/mW at 2.5 and 10 GHz, respectively), which is of interest for research heading towards YIG-based devices. Furthermore, we demonstrate the threshold frequency dependence of the three-magnon splitting process, which is shown to cease to exist for the thinner YIG of $0.2\mu \mathrm{m}$.