Spherical objects, such as clusters, nanoparticles, or aerosol particles, are sputtered when exposed to energetic irradiation. We use Monte Carlo (MC) and molecular dynamics (MD) computer simulation to study this process, with 20 keV Ar impact on a-Si clusters as a prototypical example. The sputter yield is quantified as being influenced by oblique incidence and target curvature. Cluster radii $R$ are scaled to the energy deposition depth, $a$. For large $R$ $\left(R/a>5\right)$ sphere sputtering follows closely the sputtering of planar targets, if the variation of the incidence angle on the sphere surface is taken into account. For smaller radii, the yield increases due to the influence of curvature. For radii $R/a\lesssim 1$ pronounced forward sputtering leads to a maximum in the sputter yield. For smaller $R$, sputter emission becomes isotropic, but decreases in magnitude since not all the projectile energy is deposited in the sphere. However, for all spheres studied $\left(R\ge 0.05a\right)$ the average sputter yield is larger than for infinitely large spheres $\left(R\to \infty \right)$. A simple model based on linear collision cascade theory and assuming that the energy deposition profile is independent of the sphere size predicts sputtering for large spheres well, but fails for small spheres where it strongly underestimates sputtering. The MC data for the smaller spheres are supplemented by MD calculations, which indicate a significant additional contribution caused by spike sputtering.

We analyze a simple exchange-based two-qubit gate for singlet-triplet qubits in gate-defined semiconductor quantum dots that can be implemented in a single exchange pulse. Excitations from the logical subspace are suppressed by a magnetic field gradient that causes spin-flip transitions to be non-energy-conserving. We show that the use of adiabatic pulses greatly reduces leakage processes comapred to square pulses. We also characterize the effect of charge noise on the entanglement fidelity of the gate both analytically and in simulations; demonstrating high entanglement fidelities for physically realistic experimental parameters. Specifically we find that it is possible to achieve fidelities and gate times that are comparable to single-qubit states using realistic magnetic field gradients.

A Cooper-pair or electron-hole splitter is a device capable of spatially separating entangled fermionic quasiparticles into mesoscopic solid-state systems such as quantum dots or quantum wires. We theoretically study such a splitter based on a pair of helical Luttinger liquids, which arise naturally at the edges of a quantum spin Hall insulator. Equipping each helical liquid with a beam splitter, current-current cross correlations can be used to construct a Bell inequality whose violation would indicate nonlocal orbital entanglement of the injected electrons and/or holes. Due to the Luttinger-liquid correlations, however, the entanglement is suppressed depending on ambient temperature and voltage bias.

We report neutron diffraction experiments performed in the tetragonal antiferromagnetic heavy fermion system ${\mathrm{CeRhIn}}_{5-x}$${\mathrm{Sn}}_x$ in its $(x,T)$ phase diagram up to the vicinity of the critical concentration $x_c$ $\approx 0.40$, where long range magnetic order is suppressed. The propagation vector of the magnetic structure is found to be ${\mathbf{k}}_{\mathbf{IC}}=(1/2,1/2,k_l)$ with $k_l$ increasing from $k_l=0.298$ to $k_l=0.410$ when $x$ increases from $x=0$ to $x=0.26$. Surprisingly, for $x=0.30$, the order has changed drastically and a commensurate antiferromagnetism with ${\mathbf{k}}_{\mathbf{C}}=(1/2,1/2,0)$ is found. This concentration is located in the proximity of the quantum critical point where superconductivity is expected.

We present a detailed characterization of the recently discovered $\text{i-}R\text{-Cd}$ $(R=\mathrm{Y},\text{Gd-Tm})$ binary quasicrystals by means of x-ray diffraction, temperature-dependent dc and ac magnetization, temperature-dependent resistance, and temperature-dependent specific heat measurements. Structurally, the broadening of x-ray diffraction peaks found for $\text{i-}R\text{-Cd}$ is dominated by frozen-in phason strain, which is essentially independent of $R$. i-Y-Cd is weakly diamagnetic and manifests a temperature-independent susceptibility. i-Gd-Cd can be characterized as a spin glass below 4.6 K via dc magnetization cusp, a third order nonlinear magnetic susceptibility peak, a frequency-dependent freezing temperature, and a broad maximum in the specific heat. $\text{i-}R\text{-Cd}$ $(R=\text{Ho-Tm})$ is similar to i-Gd-Cd in terms of features observed in thermodynamic measurements. i-Tb-Cd and i-Dy-Cd do not show a clear cusp in their zero-field-cooled dc magnetization data, but instead show a more rounded, broad local maximum. The resistivity for $\text{i-}R\text{-Cd}$ is of order $300\mu \Omega$ cm and weakly temperature dependent. The characteristic freezing temperatures for $\text{i-}R\text{-Cd}$ $(R=\text{Gd-Tm})$ deviate from the de Gennes scaling, in a manner consistent with crystal electric field splitting induced local moment anisotropy.

Analysis of neutron diffraction, dc magnetization, ac magnetic susceptibility, heat capacity, and electrical resistivity for DyRuAsO in an applied magnetic field are presented at temperatures near and below those at which the structural distortion ($T_S=25$ K) and subsequent magnetic ordering ($T_N=10.5$ K) take place. Powder neutron diffraction is used to determine the antiferromagnetic order of Dy moments of magnitude 7.6(1)${\mu }_B$ in the absence of a magnetic field, and demonstrate the reorientation of the moments into a ferromagnetic configuration upon application of a magnetic field. Dy magnetism is identified as the driving force for the structural distortion. The magnetic structure of analogous TbRuAsO is also reported. Competition between the two magnetically ordered states in DyRuAsO is found to produce unusual physical properties in applied magnetic fields at low temperature. An additional phase transition near $T^{*}=3$ K is observed in heat capacity and other properties in fields $\gtrsim 3$ T. Magnetic fields of this magnitude also induce spin-glass-like behavior including thermal and magnetic hysteresis, divergence of zero-field-cooled and field-cooled magnetization, frequency dependent anomalies in ac magnetic susceptibility, and slow relaxation of the magnetization. This is remarkable since DyRuAsO is a stoichiometric material with no disorder detected by neutron diffraction, and suggests analogies with spin-ice compounds and related materials with strong geometric frustration.

Dynamics of the Kitaev chain model under the effect of parametric pumping is studied. Two contributions to dynamical characteristics are considered: from the extended eigenstates and from the edge bound state (zero Majorana modes). It is shown that in the dynamical regime the frequencies of Rabi oscillations for zero Majorana modes are much larger than those related to gapped extended states. In the steady-state regime, the Rabi oscillations are blurred due to relaxation processes, and only oscillations of the characteristics of the model with the pumping frequency exist, producing absorption of the pumping power by extended states. Experimental realizations of the considered effect are discussed.

The superconducting state of the 17 K Si-doped Nb-oxynitride superconductor, namely, $({\mathrm{Nb}}_{0.87}$${\mathrm{Si}}_{0.09}{\square }_{0.04})$$({\mathrm{N}}_{0.87}$${\mathrm{O}}_{0.13})$, has been investigated using muon spin rotation and relaxation. Our results show that there is no evidence of time-reversal symmetry breaking and that the temperature dependence of the magnetic penetration depth is consistent with an isotropic singlet “$s$-wave” ground state. The magnetic penetration depth has been determined to be 218(5) nm and the superconducting gap was found to be 3.4(1) meV, giving a BCS ratio of 4.6. The superelectron density and effective mass were found to be $8.7\times {10}^{27}$ ${\mathrm{m}}^{-3}$ and $15m_e$, respectively. Also, using the Uemura plot, it is suggested that the mechanisms for superconductivity in this material may not be entirely conventional and could be similar to the cuprates, organics, and heavy fermion systems.

We study superconductivity in isolated superconducting nanocubes and nanosquares of size $L$ in the limit of negligible disorder $\delta /{\Delta }_0\ll 1$ and $k_{F}L\gg 1$ for which mean-field theory and semiclassical techniques are applicable, with $k_F$ the Fermi wave vector, $\delta$ the mean level spacing, and ${\Delta }_0$ the bulk gap. By using periodic orbit theory and number theory we find explicit analytical expressions for the size dependence of the superconducting order parameter. Our formalism takes into account contributions from both the spectral density and the interaction matrix elements in a basis of one-body eigenstates. The leading size dependence of the energy gap in three dimensions seems to be universal as it agrees with the result for chaotic grains. In the region of parameters corresponding to conventional metallic superconductors, and for sizes $L\gtrsim 10$ nm, the contribution to the superconducting gap from the matrix elements is substantial $(\sim$$20\%)$. Deviations from the bulk limit are still clearly observed even for comparatively large grains $L\sim 50$ nm. These analytical results are in excellent agreement with the numerical solution of the mean-field gap equation.

Indium substitution turns the topological crystalline insulator (TCI) ${\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5}\mathrm{Te}$ into a possible topological superconductor. To investigate the effect of the indium concentration on the crystal structure and superconducting properties of $({\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5})$${}_{1-x}$${\mathrm{In}}_x\mathrm{Te}$, we have grown high-quality single crystals using a modified floating-zone method and have performed systematic studies for indium content in the range $0\le x\le 0.35$. We find that the single crystals retain the rocksalt structure up to the solubility limit of indium $\left(x\sim 0.30\right)$. Experimental dependencies of the superconducting transition temperature $\left(T_c\right)$ and the upper critical magnetic field $\left(H_{c2}\right)$ on the indium content $x$ have been measured. The maximum $T_c$ is determined to be 4.7 K at $x=0.30$, with ${\mu }_0{H}_{c2}(T=0)\approx 5$ T.

The microscopic origin of high conductivity at room temperatures in lithium superionic conductors has remained a fundamental unsolved problem, although the recent discovery of ${\mathrm{Li}}_{10}$${\mathrm{GeP}}_2$${\mathrm{S}}_{12}$ was a great step toward the application of solid electrolytes. We achieve long-time (2-ns) tight-binding molecular dynamics simulations of ${\mathrm{Li}}_{4-x}$${\mathrm{Ge}}_{1-x}$${\mathrm{P}}_x$${\mathrm{S}}_4$ and observe the diffusion process where lithium atoms collectively hop into neighboring lithium sites by kicking the lithium atoms occupying these sites out. Furthermore, it is found that excess lithium atoms or doped lithium vacancies trigger a new diffusion process and drastically reduce the activation energy. We discuss the dynamic properties of lithium atoms in these materials, such as the diffusion constant, the activation energy, and the diffusion path.

For magnons, the Dzyaloshinskii-Moriya interaction accounts for spin-orbit interaction and causes a nontrivial topology that allows for topological magnon insulators. In this theoretical investigation we present the bulk-boundary correspondence for magnonic kagome lattices by studying the edge magnons calculated by a Green function renormalization technique. Our analysis explains the sign of the transverse thermal conductivity of the magnon Hall effect in terms of topological edge modes and their propagation direction. The hybridization of topologically trivial with nontrivial edge modes enlarges the period in reciprocal space of the latter, which is explained by the topology of the involved modes.

We construct a low-energy effective field theory of fermions interacting via short-range interactions in a simple two-band model of a Weyl semimetal on the cubic lattice and investigate possible broken-symmetry ground states through a one-loop renormalization group (RG) analysis. Using the symmetries of the noninteracting Hamiltonian to constrain the form of the interaction term leads to four independent coupling constants. We investigate the stability of RG flows towards strong coupling and find a single stable trajectory. In order to explore possible broken-symmetry ground states, we calculate susceptibilities in the particle-hole and particle-particle channels along this trajectory and find that the leading instability is towards a fully gapped spin-density wave (SDW) ground state. The sliding mode of this SDW couples to the external electromagnetic fields in the same way as the Peccei-Quinn axion field of particle physics. We also study the maximally symmetric version of our model with a single independent coupling constant. Possible ground states in this case are either gapless ferromagnetic states where the spin waves couple to the Weyl fermions like the spatial components of a (possibly chiral) gauge field, or a fully gapped spin-singlet Fulde-Ferrell-Larkin-Ovchinnikov superconducting state.

Native defect levels in ternary compound ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ single crystals were studied by low-temperature photoluminescence (PL) and photoconductivity (PC) measurements. From the PL measurements, a broad emission band centered at 1.64 eV was observed at low temperatures. The peak can be decomposed using two standard Gaussian functions to reveal two emission bands at 1.55 and 1.66 eV. The PL peak at 1.55 eV is attributed to donor-acceptor pair recombination between a sulphur vacancy $\left({\mathrm{V}}_{\mathrm{S}}\right)$ deep donor $(E_d=0.57\text{eV})$ and an antisite defect $\left({\mathrm{S}}_{\mathrm{I}}\right)$ shallow acceptor $(E_a=20\text{meV})$. The 1.66-eV emission band is attributed to self-activated luminescence involving a defect complex and is described using a configuration coordinate model. Within this framework, the 1.66-eV emission band is associated with a S vacancy donor bound to a Tl vacancy acceptor that forms a ${\mathrm{V}}_{\mathrm{S}}\text{−}{\mathrm{V}}_{\mathrm{Tl}}$ Schottky pair. The photoconductivity spectra show the presence of a deep donor level located at 0.46 eV below the conduction-band edge, in good agreement with that measured by PL spectroscopy.

We study the low-energy physics of topological insulator (TI) nanoribbons proximity-coupled to $s$-wave superconductors (SCs) by explicitly incorporating the proximity effects that emerge at the TI-SC interface. We construct a low-energy effective theory that incorporates the proximity effect through an interface contribution containing both normal and anomalous terms and an energy-renormalization matrix. We show that the strength of the proximity-induced gap is determined by the transparency of the interface and the amplitude of the low-energy TI states at the interface. Consequently, the induced gap is strongly band-dependent and collapses for bands containing states with low amplitude at the interface. We find that states with energies within the bulk TI gap have surface-type character and, in the presence of proximity-induced or applied bias potentials, have most of their weight near either the top or the bottom surface of the nanoribbon. As a result, single-interface TI-SC structures are susceptible to experiencing a collapse of the induced gap whenever the chemical potential is far enough from the value corresponding to the bulk TI Dirac point and crosses weakly coupled bands. We also find that changing the chemical potential in single-interface structures using gate potentials may be ineffective, as it does not result in a significant increase of the induced gap. On the other hand, we find that symmetric structures, such as a TI nanowire sandwiched between two superconductors, are capable of realizing the full potential of TI-based structures to harbor robust topological superconducting phases.

We study the phenomenon of adiabatic quantum charge pumping in systems supporting fractionally charged fermionic bound states in two different setups. The first quantum pump setup consists of a charge density modulated quantum wire, and the second one is based on a semiconducting nanowire with Rashba spin-orbit interaction, in the presence of a spatially oscillating magnetic field. In both these quantum pumps transport is investigated in an N-X-N geometry, with the system of interest (X) connected to two normal-metal leads (N), and the two pumping parameters are the strengths of the effective wire-lead barriers. Pumped charge is calculated within the scattering matrix formalism. We show that quantum pumping in both setups provides a unique signature of the presence of the fractional-fermion bound states, in terms of the asymptotically quantized pumped charge. Furthermore, we investigate shot noise arising due to quantum pumping, verifying that the quantized pumped charge corresponds to minimal shot noise.

We present the magnetoresistance (MR) of highly doped monolayer graphene layers grown by chemical vapor deposition on 6H-SiC. The magnetotransport studies are performed on a large temperature range, from $T=1.7$ K up to room temperature. The MR exhibits a maximum in the temperature range 120–240 K. The maximum is observed at intermediate magnetic fields ($B=2–6$ T), in between the weak localization and the Shubnikov-de Haas regimes. It results from the competition of two mechanisms. First, the low-field magnetoresistance increases continuously with $T$ and has a purely classical origin. This positive MR is induced by thermal averaging and finds its physical origin in the energy dependence of the mobility around the Fermi energy. Second, the high-field negative MR originates from the electron-electron interaction (EEI). The transition from the diffusive to the ballistic regime is observed. The amplitude of the EEI correction points towards the coexistence of both long- and short-range disorder in these samples.