The potential of quarternary wurtzite ${\mathrm{TM}}_{x/2}{\mathrm{M}}_{x/2}{\mathrm{Al}}_{1-x}\mathrm{N}$ ($\mathrm{TM}=\text{Ti}$, Zr, Hf; $\mathrm{M}=\text{Mg}$, Ca, Zn) alloys for piezoelectric applications is investigated using first-principles calculations. All considered alloys show increased piezoelectric response compared to pure AlN, and competing with the best ternary system proven to date: ScAlN. ${(\text{Zr},\text{Hf})}_{x/2}{(\text{Mg},\text{Ca})}_{x/2}{\mathrm{Al}}_{1-x}\mathrm{N}$ alloys are particularly promising. Calculations reveal positive mixing enthalpies indicative for phase separating systems; their values are smaller compared to related nitride alloys, which still can be grown as metastable thin films. The wurtzite phase of the alloys is lowest in energy at least up to $x=0.5$ and for ${\mathrm{Ti}}_{x/2}{\mathrm{Zn}}_{x/2}{\mathrm{Al}}_{1-x}\mathrm{N}$ in the full composition range. Moreover, calculations reveal that wurtzite ${\mathrm{TM}}_{0.5}{\mathrm{Zn}}_{0.5}\mathrm{N}$ ($\mathrm{TM}=\text{Ti}$, Zr, Hf) are piezoelectric alloys with $d_{33,f}=19.95$, 29.89, and 24.65 pC/N respectively, up to six times that of AlN. Finally, we discuss the physical origin behind the increased piezoelectric response and show that the energy difference between tetrahedrally coordinated zinc-blende (B3) and the layered hexagonal $\left(B_k\right)$ phases of the ${\mathrm{TM}}_{0.5}{\mathrm{M}}_{0.5}\mathrm{N}$ alloy can be used as a descriptor in a high-throughput search for complex wurtzite alloys with high piezoelectric response.

The elastic energy functional of a system of discrete dislocation lines is well known from dislocation theory. In this paper, we demonstrate how the discrete functional can be used to systematically derive approximations which express the elastic energy in terms of dislocation densitylike variables which average over the discrete dislocation configurations and represent the dislocation system on scales above the spacing of the individual dislocation lines. We study the simple case of two-dimensional systems of straight dislocation lines before we proceed to derive energy functionals for systems of three-dimensionally curved dislocation lines pertaining to a single as well as to multiple slip systems. We then illustrate several applications of the theory including Debye screening of dislocations in two and three dimensions, and the derivation of back stress and friction stress terms entering the stress balance from the free-energy functionals.

We have performed high-pressure magnetotransport and x-ray diffraction measurements on ferromagnetic $\mathrm{LaC}{\mathrm{o}}_5$, confirming the theoretically predicted electronic topological transition driving the magnetoelastic collapse seen in the related compound $\mathrm{YC}{\mathrm{o}}_5$. Our x-ray diffraction results show an anisotropic lattice collapse of the $c$ axis near 10 GPa that is also commensurate with a change in the majority charge carriers evident from high-pressure Hall effect measurements. The coupling of the electronic, magnetic, and lattice degrees of freedom is further substantiated by the evolution of the anomalous Hall effect, which couples to the magnetization of the ordered state of $\mathrm{LaC}{\mathrm{o}}_5$.

The differential conductance of NbN/GdN/TiN superconductor/ferromagnetic insulator/normal-metal junctions, with a thick NbN layer shows a large zero-field voltage offset interpreted as a spin-filtered Zeeman splitting of the NbN density of states by an effective exchange field $\left(H_0\right)$ from the GdN. The splitting increases linearly, with applied field $\left(H_{\mathrm{ext}}\right)$ enabling the relative sign of $H_0$ and $H_{\mathrm{ext}}$ to be determined. We show that the short NbN coherence length concentrates $H_0$ at the NbN/GdN interface and eliminates any averaging over the GdN domain structure leading to a large zero-field splitting.

We study magnetic properties of ${\mathrm{NdFe}}_{11}\mathrm{Ti}X$, where $X=\mathrm{B}$, C, N, O, and F, by using first-principles calculations based on density functional theory. Its parent compound ${\mathrm{NdFe}}_{11}\mathrm{Ti}$ has the ${\mathrm{ThMn}}_{12}$ structure, which has the symmetry of space group $I4/mmm$, No. 139. The magnetization increases by doping B, C, N, O, and F at the $2b$ site of the ${\mathrm{ThMn}}_{12}$ structure. The amount of the increase is larger for $X=\mathrm{N}$, O, F than for $X=\mathrm{B}$, C. On the other hand, the crystal field parameter $\langle r^2\rangle A_2^0$, which controls the axial magnetic anisotropy of the Nd $4f$ magnetic moment, depends differently on the dopant. With increase of the atomic number from $X=\mathrm{B}, \langle r^2\rangle A_2^0$ increases, takes a maximum value for $X=\mathrm{N}$, and then turns to decrease. This suggests that in ${\mathrm{NdFe}}_{11}\mathrm{Ti}X$, nitrogen is the most appropriate dopant among B, C, N, O, and F for permanent magnets in terms of magnetization and anisotropy. The above calculated properties are explained based on the detailed analysis of the electronic structures of ${\mathrm{NdFe}}_{11}\mathrm{Ti}X$.

We discuss a mechanism for the realization of supersolids in lattices in the absence of intersite interactions that surprisingly works as well at unit filling. This mechanism, that we study for the case of the sawtooth lattice, is based on the existence of frustrated and unfrustrated plaquettes. For sufficiently large interactions and frustration the particles gather preferentially at unfrustrated plaquettes breaking spontaneously translational invariance, resulting in a supersolid. We show that for the sawtooth lattice the supersolid exists for a large region of parameters for densities above half filling. Our results open a feasible path for realizing supersolids in existing ultracold atomic gases in optical lattices without the need for long-range interactions.

We introduce and analyze a mean-field model for polariton condensates which includes a velocity dependence of the effective polariton mass due to the photon and exciton components. The effective mass depends on the in-plane wave vector $\mathbf{k}$, which at the inflection point of the lower polariton energy branch becomes infinite, and above this becomes negative. The polariton condensate modes of this mean-field theory are now sensitive to mass variations and, for certain points of the energy dispersion, the polariton condensate mode represents fractional quantum mechanics. The impact of the generalized kinetic-energy term is elucidated by numerical studies in two dimensions showing significant differences for large velocities. Analytical expressions for plane-wave solutions as well as a linear waves analysis show the significance of this model.

The response function of graphene is calculated in the presence of a constant current across the sample. For small drift velocities and finite chemical potential, analytic expressions are obtained and consequences on the plasmonic excitations are discussed. For general drift velocities and zero chemical potential, numerical results are presented and a plasmon gain region is identified that is related to interband transitions.

Excitonic fine structure splitting in quantum dots is closely related to the lateral shape of the wave functions. We have studied theoretically the fine structure splitting in InAs quantum dots with a type-II confinement imposed by a GaAsSb capping layer. We show that very small values of the fine structure splitting comparable with the natural linewidth of the excitonic transitions are achievable for realistic quantum dots despite the structural elongation and the piezoelectric field. For example, varying the capping layer thickness allows for a fine tuning of the splitting energy. The effect is explained by a strong sensitivity of the hole wave function to the quantum dot structure and a mutual compensation of the electron and hole anisotropies. The oscillator strength of the excitonic transitions in the studied quantum dots is comparable to those with a type-I confinement which makes the dots attractive for quantum communication technology as emitters of polarization-entangled photon pairs.

The interplay of interactions and disorder is studied using the Anderson-Hubbard model within the typical medium dynamical cluster approximation. Treating the interacting, nonlocal cluster self-energy $\left[{\mathrm{\Sigma }}_c\left[\tilde{\mathcal{G}}\right](i,j\ne i)\right]$ up to second order in the perturbation expansion of interactions, $U^2$, with a systematic incorporation of nonlocal spatial correlations and diagonal disorder, we explore the initial effects of electron interactions ($U$) in three dimensions. We find that the critical disorder strength ($W_c^U$), required to localize all states, increases with increasing $U$; implying that the metallic phase is stabilized by interactions. Using our results, we predict a soft pseudogap at the intermediate $W$ close to $W_c^U$ and demonstrate that the mobility edge (${\omega }_{\epsilon }$) is preserved as long as the chemical potential, $\mu$, is at or beyond the mobility edge energy.

Despite the recent progress on two-dimensional multilayer materials (2DMMs) with weak interlayer interactions, the investigation of 2DMMs with strong interlayer interactions is far from sufficient. Here, we report on first-principles calculations that clarify the structural evolution and optoelectronic properties of such a 2DMM, multilayer silicene. With our global optimization algorithm, we discover the existence of rich dynamically stable multilayer silicene phases, whose stability is closely related to the extent of $s{p}^3$ hybridization that can be evaluated by average bonds and effective bond angles. Stable Si(111) surface structures are obtained when the silicene thickness gets up to four, showing the critical thickness for a structural evolution. We also find that multilayer silicene with $\pi$-bonded surfaces presents outstanding optoelectronic properties for solar cells and optical fiber communications due to the incorporation of $s{p}^2$-type bonds in the $s{p}^3$-type bond dominated system. This study helps to complete the picture of the structure and related property evolution of 2DMMs with strong interlayer interactions.

Ferromagnetism in half-metallic two-dimensional materials can lead to unique spintronics applications. Here we report first-principles calculations that predict monolayer graphyne nanoribbons (GyNRs), an alternative to graphene, doped randomly with $3d$-series transition metal atoms at medium-to-high concentrations (2%–5%) can be ferromagnetic (FM). Furthermore, Mn- and Co-doped GyNRs are half-metallic with 100% spin polarization at the Fermi level and can act as perfect spin filters. The high spin polarization of the current is preserved up to large bias voltages. This study provides a basis for the fabrication of GyNRs with ferromagnetism and spin-polarized electron transport properties.

Ultrasound velocity measurements were performed on a single crystal of spin-frustrated ferrite spinel ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$ from 300 K down to 2 K. In this cubic crystal, all the symmetrically independent elastic moduli exhibit softening with a characteristic minimum with decreasing temperature below $\sim 100$ K. This elastic anomaly suggests a coupling between dynamical lattice deformations and molecular-spin excitations. In contrast, the elastic anomalies, normally driven by the magnetostructural phase transition and its precursor, are absent in ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$, suggesting that the spin-lattice coupling cannot play a role in relieving frustration within this compound. The present study infers that, for ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$, the dynamical molecular-spin state evolves at low temperatures without undergoing precursor spin-lattice fluctuations and spin-lattice ordering. It is expected that ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$ provides the unique dynamical spin-lattice liquidlike system, where not only the spin molecules but also the cubic lattice fluctuate spatially and temporally.

Optical solitons are an ideal platform for the implementation of communication lines, since they can be packed extremely close one to another without risking partial loss of the encoded information due to their interaction. On the other hand, soliton-soliton interactions are needed to implement computations and achieve all-optical information processing. Here, we study how bright dissipative polariton solitons interact and exploit their interaction to implement and and or gates with state of the art technology. Moreover, we show that the soliton-soliton interaction can be used to determine the sign of ${\alpha }_2$, the parameter describing the interaction between polaritons with opposite spin.

We report calculations of phonon dispersions of iron (Fe) at its $\gamma -\delta$ phase transition using a self-consistent ab initio lattice dynamical method in conjunction with an effective magnetic force approach via the antiferromagnetic approximation. Our results show that anharmonic phonon-phonon interactions play a crucial role in stabilizing the $\delta$-Fe phase in the open bcc lattice. In contrast, the lattice dynamics of the close-packed fcc $\gamma$-Fe phase are dominated by magnetic interactions. Simultaneous considerations of the lattice anharmonic and magnetic interactions produced temperature-dependent phonon dispersions for $\delta$-Fe and $\gamma$-Fe phases in excellent agreement with recent experimental measurements. The present results highlight the key role of lattice anharmonicity in determining the structural stability of iron at high temperatures, which has significant implications for other high-temperature paramagnetic metals like Ce and Pu.

We study the “flux-noise” spectrum of random-bond quantum Heisenberg spin systems using a real-space renormalization group (RSRG) procedure that accounts for both the renormalization of the system Hamiltonian and of a generic probe that measures the noise. For spin chains, we find that the dynamical structure factor $S_q\left(f\right)$, at finite wave vector $q$, exhibits a power-law behavior both at high and low frequencies $f$, with exponents that are connected to one another and to an anomalous dynamical exponent through relations that differ at $T=0$ and $T=\infty$. The low-frequency power-law behavior of the structure factor is inherited by any generic probe with a finite bandwidth and is of the form $1/f^{\alpha }$ with $0.5<\alpha <1$. An analytical calculation of the structure factor, assuming a limiting distribution of the RG flow parameters (spin size, length, bond strength) confirms numerical findings. More generally, we demonstrate that this form of the structure factor, at high temperatures, is a manifestation of anomalous diffusion which directly follows from a generalized spin-diffusion propagator. We also argue that $1/f$-noise is intimately connected to many-body-localization at finite temperatures. In two dimensions, the RG procedure is less reliable; however, it becomes convergent for quasi-one-dimensional geometries where we find that one-dimensional $1/f^{\alpha }$ behavior is recovered at low frequencies; the latter configurations are likely representative of paramagnetic spin networks that produce $1/f^{\alpha }$ noise in SQUIDs.

Recently, several groups have reported observations of collective modes of the charge order present in underdoped cuprates. Motivated by these experiments, we study theoretically the oscillations of the order parameters, both in the case of pure charge order and for charge order coexisting with superconductivity. Using a hot-spot approximation we find in the coexistence regime two Higgs modes arising from hybridization of the amplitude oscillations of the different order parameters. One of them has a minimum frequency that is within the single-particle energy gap and which is a nonmonotonic function of temperature. The other—high-frequency—mode is smoothly connected to the Higgs mode in the single-order-parameter region, but quickly becomes overdamped in the case of coexistence. We explore an unusual low-energy damping channel for the collective modes, which relies on the band reconstruction caused by the coexistence of the two orders. For completeness, we also consider the damping of the collective modes originating from the nodal quasiparticles. At the end we discuss some experimental consequences of our results.

Using angle-resolved photoemission spectroscopy, we studied the electronic structures of ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ and ${\mathrm{RbFe}}_2{\mathrm{As}}_2$. Contrary to other iron-based superconductors where the band structures are usually depicted in the two-Fe Brillouin zone (BZ), we found that the distribution of electronic spectral weight in ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ and ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ follows the one-Fe BZ, and that the emerging band structure is qualitatively consistent with theoretical band calculations of the one-Fe BZ except for some shadow band effect. Our data suggest that the interlayer separation is an important tuning factor for the physics of FeAs layers, the increase of which can reduce the coupling between Fe and As and lead to the emergence of the electronic structure in accord with the one-Fe symmetry of the Fe square lattice. Our finding puts strong constraints on the theoretical models constructed on the basis of the one-Fe BZ.