It is widely known that the interfacial Dzyaloshinskii-Moriya interaction (DMI) may stabilize Néel walls rather than Bloch walls in magnetic thin films. When the DMI is weak, it results in a “tilted” Bloch wall. However, for most applications, domain walls are in nanowires rather than thin films. Here we present a semianalytic two-parameter calculation for the static domain wall in a nanowire of finite width and thickness, with DMI. The DMI strength that is needed to force a Néel wall is smaller in nanowires than in films due to demagnetizing energy. Even nanowires that are hundreds of nanometers wide may have different domain wall solutions than thin films and so their finite size must be considered. The impact of this result on current experiments is briefly discussed. We extend the model to show that applying a weak magnetic field allows the domain wall type to be tuned.

We consider the spin-orbit-induced spin Hall effect and spin swapping in diffusive superconductors. By employing the nonequilibrium Keldysh Green's function technique in the quasiclassical approximation, we derive coupled transport equations for the spectral spin and particle distributions and for the energy density in the elastic scattering regime. We compute four contributions to the spin Hall conductivity, namely, skew scattering, side jump, anomalous velocity, and the Yafet contribution. The reduced density of states in the superconductor causes a renormalization of the spin Hall angle. We demonstrate that all four of these contributions to the spin Hall conductivity are renormalized in the same way in the superconducting state. In its simplest manifestation, spin swapping transforms a primary spin current into a secondary spin current with swapped current and polarization directions. We find that the spin-swapping coefficient is not explicitly but only implicitly affected by the superconducting gap through the renormalized diffusion coefficients. We discuss experimental consequences for measurements of the (inverse) spin Hall effect and spin swapping in four-terminal geometries. In our geometry, below the superconducting transition temperature, the spin-swapping signal is increased an order of magnitude while changes in the (inverse) spin Hall signal are moderate.

The perovskite ${\mathrm{Ba}}_8{\mathrm{CoNb}}_6{\mathrm{O}}_{24}$ comprises equilateral effective spin-$\frac{1}{2} {\mathrm{Co}}^{2+}$ triangular layers separated by six nonmagnetic layers. Susceptibility, specific heat, and neutron scattering measurements combined with high-temperature series expansions and spin-wave calculations confirm that ${\mathrm{Ba}}_8{\mathrm{CoNb}}_6{\mathrm{O}}_{24}$ is basically a two-dimensional magnet with no detectable spin anisotropy and no long-range magnetic ordering down to 0.06 K. In other words, ${\mathrm{Ba}}_8{\mathrm{CoNb}}_6{\mathrm{O}}_{24}$ is very close to be a realization of the paradigmatic spin-$\frac{1}{2}$ triangular Heisenberg model, which is not expected to exhibit symmetry breaking at finite temperatures according to the Mermin and Wagner theorem.

We use neutron diffraction and muon spin relaxation to study the effect of in-plane uniaxial pressure on the antiferromagnetic (AF) orthorhombic phase in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and its Co- and Ni-substituted members near optimal superconductivity. In the low-temperature AF ordered state, uniaxial pressure necessary to detwin the orthorhombic crystals also increases the magnetic ordered moment, reaching an 11% increase under 40 MPa for ${\mathrm{BaFe}}_{1.9}{\mathrm{Co}}_{0.1}{\mathrm{As}}_2$, and a 15% increase for ${\mathrm{BaFe}}_{1.915}{\mathrm{Ni}}_{0.085}{\mathrm{As}}_2$. We also observe an increase of the AF ordering temperature ($T_N$) of about 0.25 K/MPa in all compounds, consistent with density functional theory calculations that reveal better Fermi surface nesting for itinerant electrons under uniaxial pressure. The doping dependence of the magnetic ordered moment is captured by combining dynamical mean field theory with density functional theory, suggesting that the pressure-induced moment increase near optimal superconductivity is closely related to quantum fluctuations and the nearby electronic nematic phase.

Element boron tends to form an icosahedral motif involving 26 electrons, leading to intriguing bonding conditions which complicate understating the structural variations under high pressure. Here we used density function theory (DFT) to examine the mechanical response of β- and recent discovered τ-boron to shear along the most plausible slip system. We found that the failure mechanism of $\beta -{\mathrm{B}}_{106}$ is fracturing a ${\mathrm{B}}_{28}$ triply fused icosahedral cluster without destroying a regular ${\mathrm{B}}_{12}$ icosahedron, while the failure of $\tau -{\mathrm{B}}_{106}$ arises from the disintegration of a ${\mathrm{B}}_{28}$ cluster and one nearby icosahedron. The failure of $\beta -{\mathrm{B}}_{106}$ leads to a ${\mathrm{B}}_{12}$-embedded amorphous structure which transforms to the second amorphous phase with a fully deconstructed icosahedra at 81 GPa. The second amorphous phase retains the deconstructed icosahedra at ambient conditions which is different from the normal amorphous boron containing regular icosahedra which are bonded randomly to each other. The second amorphous phase is more stable than $\beta -{\mathrm{B}}_{106}$ above 90 GPa, which explains the previous experiments on pressure-induced amorphization. In addition, forming the second highest density amorphous phase likely causes the brittle failure of β-B and related materials.

We introduce a framework of utilizing origami folding to redistribute the inclusions of a phononic structure to achieve significant phononic band-gap adaptation. Cylindrical inclusions are attached to the vertices of a Miura-Ori sheet, whose 1 degree-of-freedom rigid folding can enable fundamental reconfigurations in the underlying periodic architecture via switching between different Bravais lattice types. Such a reconfiguration can drastically change the wave propagation behavior in terms of band gap and provide a scalable and practical means for broadband wave tailoring.

The dynamics of a single magnetic skyrmion in an atomic spin system under the influence of a scanning tunneling microscope is investigated by computer simulations solving the Landau-Lifshitz-Gilbert equation. Two possible scenarios are described: manipulation with aid of a spin-polarized tunneling current and by an electric field created by the scanning tunneling microscope. The dynamics during the creation and annihilation process is studied and the possibility to move single skyrmions is showed.

Recently, the iridate double perovskite ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$ has attracted considerable attention due to the report of unexpected magnetism in this ${\mathrm{Ir}}^{5+}$ $\left(5d^4\right)$ material, in which according to the $J_{\mathrm{eff}}$ model, a nonmagnetic ground state is expected. However, in recent works on polycrystalline samples of the series ${\mathrm{Ba}}_{2-x}{\mathrm{Sr}}_x{\mathrm{YIrO}}_6$ no indication of magnetic transitions have been found. We present a structural, magnetic, and thermodynamic characterization of ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$ single crystals, with emphasis on the temperature and magnetic field dependence of the specific heat. As determined by x-ray diffraction, the ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$ single crystals have a cubic structure, with space group $Fm\overline{3}m$. In agreement with the expected nonmagnetic ground state of ${\mathrm{Ir}}^{5+}$ $({5d}^4)$ in ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$, no magnetic transition is observed down to 430 mK. Moreover, our results suggest that the low-temperature anomaly observed in the specific heat is not related to the onset of long-range magnetic order. Instead, it is identified as a Schottky anomaly caused by paramagnetic impurities present in the sample, of the order of $n\sim 0.5\left(2\right)\%$. These impurities lead to non-negligible spin correlations, which nonetheless, are not associated with long-range magnetic ordering.

We calculate the Josephson current density $j\left(\varphi \right)$ for a Weyl superconductor–normal-metal–superconductor junction for which the outer terminals are superconducting Weyl metals and the normal layer is a Weyl (semi)metal. We describe the Weyl (semi)metal using a simple model with two Weyl points. The model has broken time-reversal symmetry, but inversion symmetry is present. We calculate the Josephson current for both zero and finite temperature for the two pairing mechanisms inside the superconductors that have been proposed in the literature, zero-momentum BCS-like pairing and finite-momentum FFLO-like pairing, and assuming the short-junction limit. For both pairing types we find that the current is proportional to the normal-state junction conductivity, with a proportionality coefficient that shows quantitative differences between the two pairing mechanisms. The current for the BCS-like pairing is found to be independent of the chemical potential, whereas the current for the FFLO-like pairing is not.

The problem of a quantum Ising degree of freedom coupled to a gapless bosonic mode appears naturally in many one-dimensional systems, yet surprisingly little is known how such a coupling affects the Ising quantum critical point. We investigate the fate of the critical point in a regime, where the weak coupling renormalization group (RG) indicates a flow toward strong coupling. Using a renormalization group analysis and numerical density matrix renormalization group (DMRG) calculations we show that, depending on the ratio of velocities of the gapless bosonic mode and the Ising critical fluctuations, the transition may remain continuous or become fluctuation-driven first order. The two regimes are separated by a tricritical point of a novel type.

We measure temperature-dependent one-photon and two-photon induced photoluminescence from (${\mathrm{CH}}_3{\mathrm{NH}}_3$)${\mathrm{PbBr}}_3$ single crystals cleaved in ultrahigh vacuum. An approach is presented to extract absorption spectra from a comparison of both measurements. Cleaved crystals exhibit broad photoluminescence spectra. We identify the direct optical band gap of 2.31 eV. Below 200 K, the band gap increases with temperature, and it decreases at elevated temperature, as described by the Bose-Einstein model. An excitonic transition is found 22 meV below the band gap at temperatures $<200$ K. Defect emission occurs at photon energies $<2.16$ eV. In addition, we observe a transition at 2.25 eV (2.22 eV) in the orthorhombic (tetragonal and cubic) phase. Below 200 K, the associated exciton binding energy is also 22 meV, and the transition redshifts at higher temperature. The binding energy of the exciton related to the direct band gap, in contrast, decreases in the cubic phase. High-energy emission from free carriers is observed with higher intensity than reported in earlier studies. It disappears after exposing the crystals to air.

The ability in experiments to control the relative twist angle between successive layers in two-dimensional (2D) materials offers an approach to manipulating their electronic properties; we refer to this approach as “twistronics.” A major challenge to theory is that, for arbitrary twist angles, the resulting structure involves incommensurate (aperiodic) 2D lattices. Here, we present a general method for the calculation of the electronic density of states of aperiodic 2D layered materials, using parameter-free Hamiltonians derived from ab initio density-functional theory. We use graphene, a semimetal, and ${\mathrm{MoS}}_2$, a representative of the transition-metal dichalcogenide family of 2D semiconductors, to illustrate the application of our method, which enables fast and efficient simulation of multilayered stacks in the presence of local disorder and external fields. We comment on the interesting features of their density of states as a function of twist angle and local configuration and on how these features can be experimentally observed.

The single-particle Green's function (GF) of mesoscopic structures plays a central role in mesoscopic quantum transport. The recursive GF technique is a standard tool to compute this quantity numerically, but it lacks physical transparency and is limited to relatively small systems. Here we present a numerically efficient and physically transparent GF formalism for a general layered structure. In contrast to the recursive GF that directly calculates the GF through the Dyson equations, our approach converts the calculation of the GF to the generation and subsequent propagation of a scattering wave function emanating from a local excitation. This viewpoint not only allows us to reproduce existing results in a concise and physically intuitive manner, but also provides analytical expressions of the GF in terms of a generalized scattering matrix. This identifies the contributions from each individual scattering channel to the GF and hence allows this information to be extracted quantitatively from dual-probe STM experiments. The simplicity and physical transparency of the formalism further allows us to treat the multiple reflection analytically and derive an analytical rule to construct the GF of a general layered system. This could significantly reduce the computational time and enable quantum transport calculations for large samples. We apply this formalism to perform both analytical analysis and numerical simulation for the two-dimensional conductance map of a realistic graphene $p-n$ junction. The results demonstrate the possibility of observing the spatially resolved interference pattern caused by negative refraction and further reveal a few interesting features, such as the distance-independent conductance and its quadratic dependence on the carrier concentration, as opposed to the linear dependence in uniform graphene.

We introduce a new model for quasi-one-dimensional materials, motivated by intriguing but not yet well-understood experiments that have shown two-dimensional polymer films to be promising materials for thermoelectric devices. We consider a two-dimensional material consisting of many one-dimensional systems, each treated as a Luttinger liquid, with weak (incoherent) coupling between them. This approximation of strong interactions within each one-dimensional chain and weak coupling between them is the “quasiatomic limit.” We find integral expressions for the (interchain) transport coefficients, including the electrical and thermal conductivities and the thermopower, and we extract their power law dependencies on temperature. Luttinger liquid physics is manifested in a violation of the Wiedemann-Franz law; the Lorenz number is larger than the Fermi liquid value by a factor between ${\gamma }^2$ and ${\gamma }^4$, where $\gamma \ge 1$ is a measure of the electron-electron interaction strength in the system.

Previous theoretical and experimental work has put forward 50-period semiconductor superlattices as fast, true random number generators at room temperature. Their randomness stems from feedback between nonlinear electronic dynamics and stochastic processes that are intrinsic to quantum transitions. This paper theoretically demonstrates that shorter superlattices with higher potential barriers contain fully chaotic dynamics over several intervals of the applied bias voltage compared to the 50-period device which presented a much weaker chaotic behavior. The chaos arises from deterministic dynamics, hence it persists even in the absence of additional stochastic processes. Moreover, the frequency of the chaotic current oscillations is higher for shorter superlattices. These features should allow for faster and more robust generation of true random numbers.

We theoretically study the anisotropic magnetotransport in Dirac-Weyl magnetic junctions where a doped ferromagnetic Weyl semimetal is sandwiched between doped Dirac semimetals. We calculate the conductance using the Landauer formula and find that the system exhibits extraordinarily large anisotropic magnetoresistance (AMR). The AMR depends on the ratio of the Fermi energy to the strength of the exchange interaction. The origin of the AMR is the shift of the Fermi surface in the Weyl semimetal, and the mechanism is completely different from the conventional AMR originating from the spin dependent scattering and the spin-orbit interaction.

A two-dimensional (2D) percolation transition in Fe/W(110) ultrathin magnetic films occurs when islands in the second atomic layer percolate and resolve a frustrated magnetic state to produce long-range in-plane ferromagnetic order. Novel measurements of percolation using the magnetic susceptibility $\chi \left(\theta \right)$ as the films are deposited at a constant temperature, allow the long-range percolation transition to be observed as a sharp peak consistent with a critical phase transition. The measurements are used to trace the paramagnetic-to-ferromagnetic phase boundary between the $T=0$ percolation magnetic transition and the thermal Curie magnetic transition of the undiluted film. A quantitative comparison to critical scaling theory is made by fitting the functional form of the phase boundary. The fitted parameters are then used in theoretical expressions for $\chi \left(T\right)$ in the critical region of the paramagnetic state to provide an excellent, independent representation of the experimental measurements.

Organic molecules with a permanent electric dipole moment have been widely used as a template for further growth of molecular layers in device structures. Key properties of the resulting organic films such as energy level alignment (ELA), work function, and injection/collection barrier are linked to the magnitude and direction of the dipole moment at the interface. Using angle-resolved photoemission spectroscopy (ARPES), we have systematically investigated the coverage-dependent work function and spectral line shapes of occupied molecular energy states (MESs) of chloroaluminium-phthalocyanine (ClAlPc) grown on Ag(111). We demonstrate that the dipole orientation of the first ClAlPc layer can be controlled by adjusting the deposition rate and postannealing conditions, and we find that the ELA at the interface differs by $\sim 0.4\mathrm{eV}$ between the Cl up and down configurations of the adsorbed ClAlPc molecules. These observations are rationalized by density functional theory (DFT) calculations based on a realistic model of the ClAlPc/Ag(111) interface, which reveal that the different orientations of the ClAlPc dipole layer lead to different charge-transfer channels between the adsorbed ClAlPc and Ag(111) substrate. Our findings provide a useful framework toward method development for ELA tuning.

A robust molecular-dynamics simulation method for calculating dislocation core energies has been developed. This method has unique advantages: It does not require artificial boundary conditions, is applicable for mixed dislocations, and can yield converged results regardless of the atomistic system size. Utilizing a high-fidelity bond order potential, we have applied this method in aluminium to calculate the dislocation core energy as a function of the angle $\beta$ between the dislocation line and the Burgers vector. These calculations show that, for the face-centered-cubic aluminium explored, the dislocation core energy follows the same functional dependence on $\beta$ as the dislocation elastic energy: $E_c=A{sin}^2\beta +B{cos}^2\beta$, and this dependence is independent of temperature between 100 and 300 K. By further analyzing the energetics of an extended dislocation core, we elucidate the relationship between the core energy and the core radius of a perfect versus an extended dislocation. With our methodology, the dislocation core energy can accurately be accounted for in models of dislocation-mediated plasticity.