We determine strain-temperature phase diagrams that map the stability of ferroelectric ordering and domains in nanothin (1.6-20 nm) films of PbTiO${}_3$ using molecular dynamics based on a first-principles model Hamiltonian. The ferroelectric domain structure is shown to depend sensitively on the epitaxial strain and the nature of the electrodes that screen the surface charges of a ferroelectric. We show that a broad peak in the dielectric response as a function of temperature marks a diffuse transition to a flux closure stripe-domain structure in the ultrathin films with incomplete screening of charges at their interface with the electrode. We demonstrate that the strain-phonon coupling is fundamental to the stability and topology of such a domain structure, which can be engineered at the nanoscale with a suitable choice of electrodes and substrate, and used in ultrahigh-density memory chips.

We present a theoretical study of a two-level quantum-dot–acoustic-cavity system operated as a phonon laser. In a semiclassical treatment, a set of phonon laser equations is derived and compared to a full quantum mechanical model developed earlier [J. Kabuss, A. Carmele, T. Brandes, and A. Knorr, Phys. Rev. Lett. 109, 054301 (2012)]. Benchmarked with the full quantum theory, the analytic results, gained from the semiclassical treatment give reliable information on the basic operational limits of the quantum-dot phonon laser.

We propose an effect whereby an electric current along the interface between a ferromagnetic and normal metal leads to injection of pure spin current into the normal metal, if the magnetization direction in the ferromagnet varies spatially along the direction of current. For the specific example of a ferromagnetic spin spiral, we compute the inverse spin Hall effect voltage this spin current gives rise to when injected into a Pt layer. Furthermore, we show that this pure spin current leads to modification of the parameters that govern spin transfer and current-driven domain-wall motion, which can be used to tune the latter in layered magnetic systems. This effect in principle enables in situ control over the location, magnitude, and polarization of spin-current injection in devices.

Recently, Duda et al. [J. C. Duda, P. E. Hopkins, Y. Shen, and M. C. Gupta, Phys. Rev. Lett. 110, 015902 (2013)] reported that the fullerene derivative [6,6]-phenyl-C${}_{61}$-butyric acid methyl ester (PCBM) has the lowest thermal conductivity Λ ever observed in a fully dense solid, Λ ≈ 0.03 W m${}^{-1}$ K${}^{-1}$. We have investigated a variety of phases and microstructures of PCBM and the closely related compound [6,6]-phenyl-C${}_{61}$-butyric acid n-butyl ester (PCBNB) and find that the thermal conductivities of PCBM and PCBNB films are mostly limited to the range 0.05 < Λ < 0.06 W m${}^{-1}$ K${}^{-1}$ with a few samples having slightly higher Λ. The conductivities we observe are ≈70$\%$ larger than reported by Duda et al. but are still “ultralow” in the sense that the thermal conductivity is a factor of ≈3 below the conductivity predicted by the minimum thermal conductivity model using an estimate of the thermally excited modes per molecule.

The graphene allotropes $\alpha$-, $\beta$-, and $\gamma$-graphyne derive from graphene by insertion of acetylenic groups. The three graphynes are the only members of the graphyne family with the same hexagonal symmetry as graphene itself, which has as a consequence similarity with their electronic and vibrational properties. Here, we study the electronic band structure, phonon dispersion, and Raman spectra of these graphynes within an ab initio-based nonorthogonal tight-binding model. In particular, the predicted Raman spectra exhibit a few intense resonant Raman lines, which can be used for identification of the three graphynes by their Raman spectra for future applications in nanoelectronics.

We study the consequences of nonequilibrium heating and inverse proximity effect in normal metal–insulator–superconductor–insulator–normal metal (NISIN) junctions with a simple quasi-one-dimensional model. We especially focus on observables and parameter regions that are of interest in the design of SINIS coolers with quasiparticle traps. We present numerical results calculated by solving the Usadel equation and also present analytical approximations in two limiting cases: a short junction with a non-negligible resistance in both ends and a long junction with a transparent contact at one end.

We study the electronic structure of finite armchair graphene nanoribbons using density-functional theory and the Hubbard model, concentrating on the states localized at the zigzag termini. We show that the energy gaps between end-localized states are sensitive to doping, and that in doped systems, the gap between the end-localized states decreases exponentially as a function of the ribbon length. Doping also quenches the antiferromagnetic coupling between the end-localized states leading to a spin-split gap in neutral ribbons. By comparing $dI/dV$ maps calculated using the many-body Hubbard model, its mean-field approximation and density-functional theory, we show that the use of a single-particle description is justified for graphene $\pi$ states in case spin properties are not the main interest. Furthermore, we study the effect of structural defects in the ribbons on their electronic structure. Defects at one ribbon terminus do not significantly modify the electronic states localized at the intact end. This provides further evidence for the interpretation of a multipeak structure in a recent scanning tunneling spectroscopy (STS) experiment resulting from inelastic tunneling processes [van der Lit et al., Nat. Commun. 4, 2023 (2013)]. Finally, we show that the hydrogen termination at the flake edges leaves identifiable fingerprints on the positive bias side of STS measurements, thus possibly aiding the experimental identification of graphene structures.

The deformation potentials $a_{cz}-D_1$, $a_{ct}-D_2$, $D_3$, $D_4$, and $D_5$ are determined for random AlGaN and InGaN alloys using electronic band structure calculations based on the density functional theory. A sublinear composition dependence is obtained for $a_{cz}-D_1$ and $D_3$ in AlGaN, and $D_3$ in InGaN, whereas superlinear behavior on composition is found for $a_{ct}-D_2$, $D_4$, and $D_5$ in AlGaN, and $a_{ct}-D_2$ and $D_5$ in InGaN. The optical polarization properties of nitride quantum wells are very well described by the k·p method when the obtained deformation potentials are included. In $m$-plane AlGaN/AlN and InGaN/GaN quantum wells, the difference between the interband transition energies for light polarized parallel and orthogonal to the crystalline $c$ axis compares more favorably to experimental data, than when deformation potentials previously reported in literature are used.

We report the discovery of a diluted magnetic semiconductor, Li(Zn,Mn)P, in which charge and spin are introduced independently via lithium off-stoichiometry and the isovalent substitution of Mn${}^{2+}$ for Zn${}^{2+}$, respectively. Isostructural to (Ga,Mn)As, Li(Zn,Mn)P was found to be a $p$-type ferromagnetic semiconductor with excess lithium providing charge doping. First-principles calculations indicate that excess Li is favored to partially occupy the Zn site, leading to hole doping. Ferromagnetism with Curie temperature up to 34 K is achieved while the system still shows semiconducting transport behavior.

Using large-scale determinant quantum Monte Carlo simulations in combination with the stochastic analytical continuation, we study two-particle dynamical correlation functions in the anisotropic square lattice of weakly coupled one-dimensional (1D) Hubbard chains at half filling and in the presence of weak frustration. The evolution of the static spin structure factor upon increasing the interchain coupling is suggestive of the transition from the power-law decay of spin-spin correlations in the 1D limit to long-range antiferromagnetic order in the quasi-1D regime and at $T=0$. In the numerically accessible regime of interchain couplings, the charge sector remains gapped. The low-energy momentum dependence of the spin excitations is well described by the linear spin-wave theory with the largest intensity located around the antiferromagnetic wave vector. This magnon mode corresponds to a bound state of two spinons. At higher energies the spinons deconfine and we observe signatures of the two-spinon continuum which progressively fade away as a function of interchain hopping.

Monte Carlo simulations shed light on the stress state of Si nanocrystals embedded in amorphous silica, unraveling and explaining its nature and origins. This is achieved by generating detailed stress maps and by calculating the stress profile as a function of size and distance between the nanocrystals. For normal oxide matrix densities, the average stress in the nanocrystal core is found to be compressive, reaching values of 3–4 GPa, in excellent agreement with experimental measurements. It drastically declines at the interface, despite the existence of several highly strained geometries. Tensile conditions prevail in nanocrystals embedded in densified silica matrices. The nanocomposites are shown to be stable, at close interdot distances, against segregation and phase separation.

The understanding and control of the dynamics of hybrid modes consisting of strongly coupled surface plasmon polaritons and molecular excitations of dye molecules is of great timely interest, as it allows one to tailor interactions between optical signals as needed for active all-optical devices. Here we utilize dye molecules with an especially large Stokes shift to demonstrate the absence of mutual scatterings among the strongly coupled hybrid modes. We employ a quantum mechanical three-level model and show that the hybrid modes decay via dephasing and internal relaxation of the molecules to a fluorescing state of the dye, which can be used as a measure for the decay. Our results provide essential information about the dynamics of the strongly coupled modes.

Molecular junctions and similar devices described by an energy dependent transmission coefficient can have a high linear response thermoelectric figure of merit. Since such devices are inherently nonlinear, the full thermodynamic efficiency valid for any temperature and chemical potential difference across the leads is calculated. The general features in the energy dependence of the transmission function that lead to high efficiency and also high power output are determined. It is shown that the device with the highest efficiency does not necessarily lead to large power output. To illustrate this, we use a model called the $t$-stub model representing tunneling through an energy level connected to another energy level. Within this model both high efficiency and high power output are achievable. Futhermore, by connecting many nanodevices it is shown to be possible to scale up the power output without compromising efficiency in an (exactly solvable) $n$-channel model even with tunneling between the devices.

One-third of a monolayer of Bi alloyed into the Ag(111) surface yields a pair of Rashba spin-split free-electron-like surface states. The splitting in momentum space is the largest of all surface alloys investigated. Using first-principles calculations, we have determined the spin splitting in this system as a function of atomic corrugation of the top atomic layer and the strengths of the atomic spin-orbit coupling in Bi and Ag. The calculated splitting is proportional to the strengths of the atomic spin-orbit coupling, but it peaks at a certain surface corrugation parameter. These findings indicate that the observed giant spin splitting is caused by a near-optimal surface corrugation and a large atomic spin-orbit coupling in Bi. The results offer a useful guide for searching for two-dimensional systems with large surface spin effects.

We investigate the dynamics and outcome of a quantum phase transition from an antiferromagnetic to a phase-separated ground state in a spin-1 Bose-Einstein condensate of ultracold atoms. We explicitly demonstrate double universality in the dynamics within experiments with various quench times. Furthermore, we show that spin domains created in the nonequilibrium transition constitute a set of mutually incoherent quasicondensates. The quasicondensates appear to be positioned in a semiregular fashion, which is a result of the conservation of local magnetization during the postselection dynamics.

We study the Josephson effect in a triplet superconductor-ferromagnet-singlet superconductor junction. We show that the interaction of tunneling Cooper pairs with the interface magnetization can permit a Josephson current at the lowest order of a tunneling Hamiltonian perturbation theory. Two conditions must be satisfied for this to occur: The magnetization of the ferromagnet has a component parallel to the $\mathbf{d}$ vector of the triplet superconductor, and the gaps of the superconductors have the same parity with respect to the interface momentum. The resulting charge current displays an unconventional dependence on the orientation of the magnetic moment and the phase difference. This is accompanied by a phase-dependent spin current in the triplet superconductor, while a phase-independent spin current is always present. The tunneling perturbation theory predictions are confirmed using a numerical Green's function method. An analytical treatment of a one-dimensional junction demonstrates that our conclusions are robust far away from the tunneling regime and reveals signatures of the unconventional Josephson effect in the critical currents.

UMo${}_{0.18}$ can absorb hydrogen at elevated H${}_2$ pressures. The product is a compact but brittle hydride UH${}_3$Mo${}_{0.18}$, which is to some extent analogous to β-UH${}_3$, but it is almost amorphous (the grain size smaller than 1.2 nm). It is ferromagnetic, and its Curie temperature (200 K) is higher, despite the disorder, than that of β-UH${}_3$ (≈170 K). We also registered an increase in U magnetic moments. The randomness, together with the large anisotropy inherent to U systems, leads to a very high coercivity, reaching 3.7 T at low temperatures. As amorphization normally tends to suppress magnetic ordering of U compounds, the hydride represents a different class of materials, amorphous U-based ferromagnets with relatively very high Curie temperatures.

We develop in full detail the formalism of tangent states to the manifold of matrix product states, and show how they naturally appear in studying time evolution, excitations, and spectral functions. We focus on the case of systems with translation invariance in the thermodynamic limit, where momentum is a well-defined quantum number. We present some illustrative results and discuss analogous constructions for other variational classes. We also discuss generalizations and extensions beyond the tangent space, and give a general outlook towards post-matrix product methods.

We report a soft x-ray nanodiffraction study of antiferromagnetic domains in the strongly correlated bilayer manganite La${}_{0.96}$Sr${}_{2.04}$Mn${}_2$O${}_7$. We find that the antiferromagnetic domains form a unique domain pattern with each domain having an intrinsic memory of its spin direction. This can be explained by the presence of crystallographic or magnetic imperfections locked in during the crystal growth process which pin the antiferromagnetic domains. The antiferromagnetic domain pattern shows two distinct types of domain. One of the domain types was observed to contain a periodic ripple in the manganese spin direction with a period of approximately 4 $\mu$$\mathrm{m}$. We propose that the loss of inversion symmetry within a bilayer is responsible for this ripple structure through a Dzyaloshinskii-Moriya-type interaction.