Structures, electronic properties, and phonon dispersions of monolayer $MX$${}_2$ ($M=$ Mo, W; $X=$ S, Se) under various types of mechanical strains are investigated with density functional calculations. By analyzing the orbitals near band gaps, we demonstrate that the $X$-$M$-$X$ bond angle, together with the $X$-$X$ distances, plays one of the most important roles in determining band gaps, as it is directly related to the coupling of $d$ orbitals of the $M$ atoms and $p$ orbitals of the $X$ atoms. It is found that the band gaps of monolayer $MX$${}_2$ are more sensitive to biaxial strains. This notion can be attributed to the fact that, under biaxial strains, $MX$${}_2$ tends to be stretched more significantly along the out-of-plane direction, resulting in greater changes in bond lengths/angles than uniaxial strains. While most theoretical reports suggested systematic reduction of band gaps under mechanical strains, we found that the direct band gaps can be robustly widened by applying compressive biaxial strains. Our results are of immediate importance for the recent experimental advances in applying compressive strains, which was not applicable before. On the other hand, the anisotropy of $X$-$M$-$X$ bond angles induced by uniaxial strains leads to splitting in the phonon dispersion, and is directly relevant to recent Raman spectroscopy results. As most transition-metal dichalcogenides share the same structural characters, we expect that the notions derived here are applicable to other layered transition-metal oxides/chalcogenides and open up new venues to engineer their properties.

Entanglement between individual spins can be detected by using thermodynamics quantities as entanglement witnesses. This applies to collective spins also, provided that their internal degrees of freedom are frozen, as in the limit of weakly coupled nanomagnets. Here, we extend such approach to the detection of entanglement between subsystems of a spin cluster, beyond such weak-coupling limit. The resulting inequalities are violated in spin clusters with different geometries, thus allowing the detection of zero- and finite-temperature entanglement. Under relevant and experimentally verifiable conditions, all the required expectation values can be traced back to correlation functions of individual spins, which are now made selectively available by four-dimensional inelastic neutron scattering.

A first-principles description of the spin Nernst effect, denoting the occurrence of a transverse spin current due to a temperature gradient, is presented. The approach, based on an extension to the Kubo-Středa equation for spin transport, supplies in particular the formal basis for investigations of diluted as well as concentrated alloys. Results for corresponding applications to the alloy system Au${}_x$Cu${}_{1-x}$ give the intrinsic and extrinsic contributions to the relevant transport coefficients. Using scaling laws allows in addition splitting the extrinsic contribution into its skew scattering and side-jump parts.

We present a detailed theoretical analysis of biexciton state generation in InAs-GaAs quantum dots by strong, chirped laser pulses. Specifically, we derive an accurate analytical expression, which not only provides a clear physical picture of the process, but also allows identifications of laser parameter regimes where efficient biexciton generation should be possible, even at temperatures up to 80 K. The results are confirmed by numerical simulations, in very good agreement with the model proposed. A clear choice of parameters is proposed, which might pave the way towards the optimal design of high-fidelity sources of entangled photon pairs based on individual quantum dots.

Subwavelength light concentration by means of plasmonic nanoantennas is known to significantly enhance the nonlinear response. In nonlinear schemes involving multiple frequencies, however, it remains challenging to design nanoantennas that respond resonantly to more than one or, eventually, to all interacting frequencies. Considering plasmonic nanowire antennas, we hereby demonstrate the potential to engineer their resonances at more than one frequency involved in the nonlinear process by carefully tailoring the antenna terminations. Although we consider here the degenerate nonlinear process of second-harmonic generation, our approach can easily be extended to other nonlinear processes.

The real-time electronic dynamics on material surfaces is critically important to a variety of applications. However, their simulations have remained challenging for conventional methods such as the time-dependent density-functional theory (TDDFT) for isolated and periodic systems. By extending the applicability of TDDFT to systems with open boundaries, we achieve accurate atomistic simulations of real-time electronic response to local perturbations on material surfaces. Two prototypical scenarios are exemplified: the relaxation of an excess electron on a graphene surface and the electron transfer across the molecule-graphene interface. Both the transient and long-time asymptotic dynamics are validated, which accentuates the fundamental importance and usefulness of an open-system TDDFT approach. The simulations also provide insights into the characteristic features of temporal electron evolution and dissipation on surfaces of bulk materials.

Motivated by recent experiments on ultracold fermions we study the effect of the spin-orbit interaction in an exactly solvable one-dimensional model with strong local attractive correlations between the fermions. We show that the asymptotes of correlation functions, calculated in the framework of the conformal field theory and finite size corrections of the Bethe ansatz exact solution, are strongly affected by the spin-orbit coupling. In the mixed phase, the correlation functions consist of terms that are the product of a power law of the distance and an oscillating function of the distance. We obtain the critical exponents for superfluidity and density waves. The leading exponents decrease as a function of spin-orbit coupling. In particular, the exponent for superfluidity is the smallest one signaling an instability to a phase where a weak interchain coupling can produce superfluidity of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type. The spatial oscillation of the order parameter is also modified by the spin-orbit parameter, i.e., its modulation is not just given by the spin imbalance.

Electron irradiation is investigated as a way to dope the topological insulator Bi${}_2$Te${}_3$. For this, $p$-type Bi${}_2$Te${}_3$ single crystals have been irradiated with 2.5 MeV electrons at room temperature and electrical measurements have been performed in situ as well as ex situ in magnetic fields up to 14 T. The defects created by irradiation act as electron donors, allowing the compensation of the initial hole-type conductivity of the material as well as the conversion of the conductivity from $p$ to $n$ type. The changes in carrier concentration are investigated using the Hall effect and Shubnikov–de Haas (SdH) oscillations, clearly observable in the $p$-type samples before irradiation, but also after the irradiation-induced conversion of the conductivity to $n$ type. The SdH patterns observed for the magnetic field along the trigonal axis can be entirely explained assuming the contributions of only one valence and one conduction band, respectively, and Zeeman splitting of the orbital levels.

We modeled the electron and hole states in $\mathrm{Si}/{\mathrm{SiO}}_2$ quantum wells within a basis of standing waves using the 30-band $\mathbf{k}·\mathbf{p}$ theory. The hard-wall confinement potential is assumed, and the influence of the peculiar band structure of bulk silicon on the quantum-well sub-bands is explored. Numerous spurious solutions in the conduction-band and valence-band energy spectra are found and are identified to be of two types: (1) spurious states which have large contributions of the bulk solutions with large wave vectors (the high-$k$ spurious solutions) and (2) states which originate mainly from the spurious valley outside the Brillouin zone (the extravalley spurious solutions). An algorithm to remove all those nonphysical solutions from the electron and hole energy spectra is proposed. Furthermore, slow and oscillatory convergence of the hole energy levels with the number of basis functions is found and is explained by the peculiar band mixing and the confinement in the considered quantum well. We discovered that assuming the hard-wall potential leads to numerical instability of the hole states computation. Nonetheless, allowing the envelope functions to exponentially decay in a barrier of finite height is found to improve the accuracy of the computed hole states.

Ab initio molecular dynamics simulations based on density functional theory were performed to generate amorphous models of the phase change compound In${}_3$SbTe${}_2$ by quenching from the melt. In-Sb and In-Te are the most abundant bonds with only a minor fraction of Sb-Te bonds. The bonding geometry in the amorphous phase is, however, strongly dependent on the density in the range 6.448–5.75 g/cm${}^3$ that we investigated. While at high density the bonding geometry of In atoms is mostly octahedral-like as in the cubic crystalline phase of the ternary compound In${}_3$SbTe${}_2$, at low density we observed a sizable fraction of tetrahedral-like geometries similar to those present in the crystalline phase of the two binary compounds InTe and InSb that the ternary system can be thought to be made of. We show that the different ratio between octahedral-like and tetrahedral-like bonding geometries has fingerprints in the optical and vibrational spectra.

We report ab initio calculations of the stability, lattice dynamics, and electronic and thermoelectric properties of cubic La${}_{3-y}$$X_4$ ($X$ = S, Se, Te) materials in view of analyzing their potential for thermoelectric applications. The lanthanum motions are strongly coupled to the tellurium motions in the telluride, whereas the motions of both types of atoms are decoupled in the sulfides. Nevertheless, this has no impact on their thermal properties because experimentally all compounds have low thermal conductivity. We believe that this is due to umklapp scattering of the acoustical modes, notably by the low-energy optical modes at about 7–8 meV found in all three chalcogenides, as in cage compounds such as skutterudites or clathrates, even though there are no cages in the cubic Th${}_3$P${}_4$ structure. We find that the energy band gap increases from the telluride to the sulfide, in good agreement with experiments. However, due to their similar band structure, we find that all three compounds have almost identical thermoelectric properties. Our results agree qualitatively with experiments, especially in the case of the telluride for which a great amount of data exists. All our results indicate that the sulfides have a strong potential for thermoelectricity and could replace tellurides if the charge-carrier concentration is optimized. Finally, we predict also a larger maximum $ZT$ for the $p$-type doped materials than for the $n$-type doped ones, even though compounds with $p$ doping have still to be synthesized. Thus our results indicate the possibility to make high-temperature performing thermogenerators based only on La${}_3$$X_4$ compounds.

We present macroscopic and neutron diffraction data on multiferroic lightly Co-doped Ni${}_3$V${}_2$O${}_8$. Doping Co into the parent compound suppresses the sequence of four magnetic phase transitions, and only two magnetically ordered phases—the paraelectric high-temperature incommensurate (HTI) phase and the ferroelectric low-temperature incommensurate (LTI) phase—can be observed. Interestingly, the LTI multiferroic phase with a spiral (cycloidal) magnetic structure is stabilized down to at least 1.8 K, which could be revealed by measurements of the electric polarization and confirmed by neutron diffraction on single-crystal samples. The extracted magnetic moments of the LTI phase contain, besides the main exchange, also fine components of the cycloid allowed by symmetry, which result in a small amplitude variation of the magnetic moments along the cycloid propagation due to the site-dependent symmetry properties of the mixed representations. In the HTI phase, a finite imaginary part of the spine magnetic moment could be deduced yielding a spin cycloid instead of a purely sinusoidal structure with an opposite spin chirality for different spine spin chains. The magnetic ordering of the cross-tie sites in both phases is different in comparison to the respective ones in the pure Ni compound. A wider temperature stability range of the HTI phase has been observed in comparison to Ni${}_3$V${}_2$O${}_8$, which can be explained by an additional single-ion easy-axis anisotropy due to Co doping. The larger incommensurability of the Co-doped compounds yields a larger ratio between the competing next-nearest-neighbor and nearest-neighbor interaction, which is $J_2/J_1=0.43$ (0.47) for a doping level of 7% (10%) Co compared to 0.39 in the parent compound.

The hexaferrites are known to exhibit a wide range of magnetic structures, some of which are connected to important technological applications and display magnetoelectric properties. We present data on the low magnetic field structures stabilized in a Y-type hexaferrite as observed by resonant soft x-ray diffraction. The helical spin block arrangement that is present in zero applied magnetic field becomes fanlike as a field is applied in plane. The propagation vectors associated with each fan structure are studied as a function of magnetic field, and a new magnetic phase is reported. Mean field calculations indicate this phase should stabilize close to the boundary of the previously reported phases.

Changes of the electronic and magnetic structure near the martensitic phase transition of Ni-Mn-In Heusler alloys doped with Co are investigated by experiment and theory. The nonstoichiometric Ni${}_{48}$Co${}_5$Mn${}_{35}$In${}_{12}$ epitaxial film undergoes a transition from a weakly magnetic martensitic phase below $T_m=350$ K to a ferromagnetic austenitic phase above $T_m$. Element-specific magnetic moments and the unoccupied density of states function is investigated using x-ray magnetic circular dichroism. We find an antiparallel alignment of Mn and Ni/Co magnetic moments in both phases. The electronic structure is calculated using the SPR-KKR Green's function approach considering experimental results on the atomic disorder in the sample. The good agreement of experimental and theoretically simulated x-ray absorption spectra in both phases confirms that a band Jahn-Teller like lifting of a degenerate $d$ state is the origin of the energetic preference of the martensitic phase at low temperatures.

Nuclear magnetic resonance (NMR), neutron diffraction (ND), x-ray diffraction, magnetic susceptibility $\chi$, and specific-heat measurements on the frustrated $A$-site spinel CoAl${}_2$O${}_4$ compound reveal a collinear antiferromagnetic ordering below $T_{\mathrm{N}}$ = 9.8(2) K. A high-quality powder sample characterized by x-ray diffraction that indicates a relatively low Co-Al inversion parameter $x=0.057\left(20\right)$ in (Co${}_{1-x}$Al${}_x$)[Al${}_{2-x}$Co${}_x$]O${}_4$, shows a broad maximum around 15 K in $\chi \left(T\right)$ and a sharp peak at $T_{\mathrm{N}}$ in heat capacity $C_{\mathrm{p}}$. The average-ordered magnetic moment of Co${}^{2+}$ ($S=\frac{3}{2}$) ions at the $A$ site is estimated to be 2.4(1) ${\mu }_{\mathrm{B}}$ from NMR and 1.9(5) ${\mu }_{\mathrm{B}}$ from ND which are smaller than the expected value of 3 ${\mu }_{\mathrm{B}}$ for $S=\frac{3}{2}$ and $g$ = 2. Antiferromagnetic spin fluctuations and correlations in the paramagnetic state are revealed from the $\chi$, NMR, and ND measurements, which are due to spin frustration and site inversion effects in the system. The ND data also show short-range dynamic magnetic ordering that persists to a temperature that is almost twice $T_{\mathrm{N}}$.

We investigate the temperature dependence of the lower critical field $H_{c1}\left(T\right)$ of a high-quality FeSe single crystal under static magnetic fields $H$ parallel to the $c$ axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding $H_{c1}\left(T\right)$ was deduced by taking into account demagnetization factors. A pronounced change in the $H_{c1}$(T) curvature is observed, which is attributed to anisotopic s-wave or multiband superconductivity. The London penetration depth ${\lambda }_{ab}\left(T\right)$ calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean $s$-wave superconductor. Using either a two-band model with $s$-wave-like gaps of magnitudes ${\Delta }_1=0.41\pm 0.1$ meV and ${\Delta }_2=3.33\pm 0.25$ meV or a single anisotropic $s$-wave order parameter, the temperature dependence of the lower critical field $H_{c1}\left(T\right)$ can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless.

In this work, we generalize the theory of localized surface plasmons to the case of high-$T_{\mathrm{c}}$ cuprate superconductors, spatially confined in the form of small spherical particles. At variance from ordinary metals, cuprate superconductors are characterized by a low-energy bulk excitation known as the Josephson plasma wave (JPW), arising from interlayer tunneling of the condensate along the $c$ axis. The effect of the JPW is revealed in a characteristic spectrum of surface excitations, which we call Josephson surface plasmons. Our results, which apply to any material with a strongly anisotropic electromagnetic response, are worked out in detail for the case of multilayered superconductors supporting both low-frequency (acoustic) and transverse-optical JPW. Spatial confinement of the Josephson plasma waves may represent a new degree of freedom to engineer their frequencies and to explore the link between interlayer tunneling and high-$T_{\mathrm{c}}$ superconductivity.

In studying Majorana fermions (MFs) in a spin ladder model, we numerically show that their qubit state can be read out by measuring fusion excitations in quenched inhomogeneous spin ladders. We construct an exactly solvable $T$-junction spin ladder model that can be used to implement MF braid operations. With braiding simulated numerically as nonequilibrium quench processes, we verify that the MFs in our spin ladder model obey non-Abelian braiding statistics. Our scheme provides a promising platform to study exotic properties of MFs and a broad range of applications in topological quantum computation.

Structure-property relationships are established in the nonlinear optical (NLO) material, zinc tris(thiourea)sulfate (ZTS), via an experimental charge-density study, x-ray constrained wave-function refinement, and quantum-mechanical calculations. The molecular charge-transfer characteristics of ZTS, that are important for NLO activity, are topologically analyzed via a multipolar refinement of high-resolution x-ray diffraction data, which is supported by neutron diffraction measurements. The extent to which each chemical bond is ionic or covalent in nature is categorized by Laplacian-based bonding classifiers of the electron density; these include bond ellipticities, energy densities, and the local source function. Correspondingly, the NLO origins of ZTS are judged to best resemble those of organic NLO materials. The molecular dipole moment, ${\mathit{\mu }}_{\mathbf{i}}$, and (hyper)polarizability coefficients, ${\mathit{\alpha }}_{\mathbf{ij}}$ and ${\mathit{\beta }}_{\mathbf{ijk}}$, are calculated from the experimental diffraction data using the x-ray constrained wave-function method. Complementary gas-phase ab initio quantum-mechanical calculations of ${\mathit{\mu }}_{\mathbf{i}}$, ${\mathit{\alpha }}_{\mathbf{ij}}$, and ${\mathit{\beta }}_{\mathbf{ijk}}$ offer a supporting comparison. When taken alone, the experimental charge-density analysis does not fare well in deriving ${\mathit{\mu }}_{\mathbf{i}}$, ${\mathit{\alpha }}_{\mathbf{ij}}$, or ${\mathit{\beta }}_{\mathbf{ijk}}$, which is not entirely surprising given that the associated calculations are only generally valid for organic molecules. However, by refining the x-ray data within the constrained wave-function method, the evaluations of ${\mathit{\mu }}_{\mathbf{i}}$, ${\mathit{\alpha }}_{\mathbf{ij}}$, and ${\mathit{\beta }}_{\mathbf{ijk}}$ are shown to agree very well with those from ab initio calculations and show remarkable normalization to experimental refractive index measurements. The small differences observed between ab initio and x-ray constrained wave-function refinement results can be related directly to gas- versus solid-state phase differences. ${\mathit{\mu }}_{\mathbf{i}}$ is found to be 28.3 Debye (gas phase) and 29.7 Debye (solid state) while ${\mathit{\beta }}_{\mathbf{ijk}}$ coefficients are not only significant but are also markedly three dimensional in form. Accordingly, substantial octupolar as well as dipolar NLO contributions in ZTS are indicated, which challenges the traditional focus on dipolar NLO molecules. This evaluation of NLO properties and their relation to the molecular structure offers several ways by which ZTS may be more widely functionalized as a NLO material. More generally, this case study on ZTS demonstrates how experimental and computational techniques can be combined to understand NLO structure-property relationships, an important tool for the quantum-tailored molecular design of next-generation metalorganic NLO materials.