We study spin injection and spin extraction signals in lateral spin-valve structures consisting of ferromagnetic ${\mathrm{Co}}_2\mathrm{FeSi}$ contacts on $n$-type GaAs transport channels. The dependence of the spin-valve signals on the bias current is found to strongly depend on the degree of ordering in the ${\mathrm{Co}}_2\mathrm{FeSi}$ lattice. For the fully ordered $L{2}_1$ phase, the signal is equal for injection and extraction and independent of the bias current. In contrast, a strong dependence of the relative signal strengths (spin injection versus extraction) on the bias current is observed for the partially disordered $B2$ phase. We explain the strongly different behavior by the crucial influence of the respective electronic band structure on the spin generation processes.

We study the magnetic field response of Majorana-Kramers pairs of one-dimensional time-reversal (TR)-invariant topological superconductors (class DIII) with or without a coexisting chirality symmetry. In addition to explaining the anomalous magnetic field response of all the DIII class topological superconducting systems proposed in the literature, we provide a realistic route to engineer a “true” TR-invariant topological superconductor, i.e., one whose pair of Majorana bound states at each end is split by an applied Zeeman field in arbitrary direction. We also prove that quite generally the splitting of the Majorana bound states in a time-reversal-invariant topological superconductor by time-reversal breaking fields is highly anisotropic in spin space.

We propose a theoretical concept of switching between direct and indirect band gap character in silicon quantum dots (SiQDs) by the use of surface potential induced by the ligands or environment in which SiQDs are immersed—both cases are studied. Theoretical simulations show that the density of states of confined electrons in both real and $k$ space can be dramatically altered by engineering the local electrostatic field. Especially interesting is modification of the lowest excited states, which appear in the $\Gamma$ valley for electronegative field that “pulls” electrons towards the SiQD surface. Opposite sign of the field does not have such effect at all. Hence we conclude a general trend of promotion of directlike radiative transitions by electronegative capping/environment. The rates are enhanced by more than two orders of magnitude compared to “normal” SiQDs, which can be as high as the values characteristic for direct band gap semiconductors. This model is in agreement with observed experimental properties of SiQDs with covalently bonded electronegative ligands.

The renormalization of electronic eigenenergies due to electron-phonon interactions (temperature dependence and zero-point motion effect) is important in many materials. We address it in the adiabatic harmonic approximation, based on first principles (e.g., density-functional theory), from different points of view: directly from atomic position fluctuations or, alternatively, from Janak's theorem generalized to the case where the Helmholtz free energy, including the vibrational entropy, is used. We prove their equivalence, based on the usual form of Janak's theorem and on the dynamical equation. We then also place the Allen-Heine-Cardona (AHC) theory of the renormalization in a first-principles context. The AHC theory relies on the rigid-ion approximation, and naturally leads to a self-energy (Fan) contribution and a Debye-Waller contribution. Such a splitting can also be done for the complete harmonic adiabatic expression, in which the rigid-ion approximation is not required. A numerical study within the density-functional perturbation theory framework allows us to compare the AHC theory with frozen-phonon calculations, with or without the rigid-ion approximation. For the two different numerical approaches without non-rigid-ion terms, the agreement is better than 7 $\mu \mathrm{eV}$ in the case of diamond, which represent an agreement to five significant digits. The magnitude of the non-rigid-ion terms in this case is also presented, distinguishing specific phonon modes contributions to different electronic eigenenergies.

The thermal response of nanoscale cylindrical inclusions of amorphous $\mathrm{Si}{\mathrm{O}}_2$ embedded in crystalline quartz $\left(c-\mathrm{Si}{\mathrm{O}}_2\right)$ is investigated by means of small-angle x-ray scattering. The inclusions are generated by swift heavy-ion irradiation that leads to the formation of amorphous ion tracks along the ion trajectories. During in situ annealing between room temperature and 620 °C, we observe an irreversible expansion of the track cylinders followed by a reversible contraction. The reversible “elastic” response of the tracks can be modeled using the temperature-dependent elastic properties of bulk amorphous and crystalline $\mathrm{Si}{\mathrm{O}}_2$ for the inclusions and the matrix, respectively. A high initial interface pressure of the nanocylinders is apparent from the calculations.

Inspired by the Nagaoka ferromagnetism, we propose an itinerant model to study the transition between the Mott singlet state and a ferromagnetic state by emulating a doping process in finite lattices. In the Nagaoka ferromagnetism, the total spin of the system takes the maximum value when an electron is removed from the half-filled system. To incorporate a procedure of the electron removal, our model contains extra sites as a reservoir of electrons, and the chemical potential of the reservoir controls the distribution of electrons. As a function of the chemical potential, the system exhibits ground-state phase transitions among various values of the total spin, including a saturated ferromagnetic state due to the Nagaoka mechanism at finite hole density. We discuss the nature of the ferromagnetism by measuring various physical quantities, such as the distribution of electrons, the spin correlation functions, the magnetization process in the magnetic field, and also the entanglement entropy.

We study symmetries and defects of a wide class of two-dimensional Abelian topological phases characterized by Lie algebras. We formulate the symmetry group of all Abelian topological field theories. The symmetries relabel quasiparticles (or anyons) but leave exchange and braiding statistics unchanged. Within the class of $ADE$ phases, in particular, these anyonic symmetries have a natural origin from the Lie algebra. We classify one-dimensional gapped phases along the interface between identical topological states according to symmetries. This classification also applies to gapped edges of a wide range of fractional quantum spin Hall (QSH) states. We show that the edge states of the $ADE$ QSH systems can be gapped even in the presence of time-reversal and charge-conservation symmetries. We distinguish topological point defects according to anyonic symmetries and bound quasiparticles. Although in an Abelian system, they surprisingly exhibit non-Abelian fractional Majorana-like characteristics from their fusion behavior.

The ground-state properties of one-dimensional ${}^3\mathrm{He}$ are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well interpolated by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation functions. The density dependence of the Luttinger parameter is explicitly found, and interestingly it shows a nonmonotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, a “Bose gas.” a “super-Tonks-Girardeau” regime, and a “quasicrystal.” The obtained results are applicable to unpolarized, partially, or fully polarized ${}^3\mathrm{He}$.

We study strongly correlated helical liquids with and without Rashba coupling using quantum Monte Carlo simulations of the Kane-Mele model with a Hubbard interaction at the edge. Independent of the Rashba coupling, we find that interactions enhance spin correlations and suppress the spectral weight at the Fermi level. For sufficiently strong interactions, a gap can be observed in the single-particle spectral function. However, based on a finite-size scaling analysis and theoretical arguments, we argue that this gap is closed by order parameter fluctuations in the Luttinger liquid phase even at zero temperature, and filled in by thermally induced kinks in the order parameter in the Mott phase at finite temperatures. While the bosonization suggests an umklapp-driven Mott transition only in the presence of Rashba coupling and hence a significant impact of the latter, our numerical results are almost unaffected by Rashba coupling even at low temperatures.

A different approach to calculate the $I\text{−}V$ characteristics of $p^{+}(\mathrm{Ga},\mathrm{Mn})\mathrm{As}\text{/}{n}^{+}\mathrm{GaAs}$ spin injectors is presented. The vanishing of the spin-extraction transmission coefficients at the spin-split valence-band edges leads us to predict a dip or plateau in the $I\text{−}V$ characteristics of this kind of diodes. We show that this minimum, or the inflection point, shifts with the exchange energy. Within this approach and using Kane-like densities of states with a detached impurity band, and exchange energies reported in the literature, excellent agreement is found with observed low-bias features of $I\text{−}V$ characteristics.

We report on the carrier dynamics in InGaN/GaN dot-in-nanowire quantum dots, revealed by a systematic mapping between the optical properties and structural parameters of the quantum dots. Such a study is made possible by using quantum dots with precisely controlled locations and sizes. We show that the carrier dynamics is governed by two competing mechanisms: (1) Excitons are protected from surface recombination by a potential barrier formed due to strain relaxation at the sidewall surface. (2) Excitons can overcome the potential barrier by tunneling and thermal activation. This carrier dynamics model successfully explains the following surprising experimental findings on individual quantum dots. First, there exist strong statistical correlations among multiple optical properties of many individual quantum dots, despite variations of these properties resulting from inevitable structural variations among the quantum dots. Second, the antibunching property of the quantum dot emission exhibits an abnormal ladle-shaped dependence on the decay time and temperature. Our results can guide the way toward nitride-based high-temperature single-photon emitters and nanophotonic devices.

We calculate the inelastic scattering rates and the hot-electron inelastic mean free paths for both monolayer and bilayer graphene on a polar substrate. We study the quasiparticle self-energy by taking into account both electron-electron and electron–surface-optical-phonon interactions. In this calculation the leading-order dynamic screening approximation $({\mathrm{G}}_0\mathrm{W}$ approximation) is used to obtain the quasiparticle self-energy by treating electrons and phonons on an equal footing. We find that the strong coupling between the surface-optical phonon and the plasmon leads to an additional decay channel for the quasiparticle through the emission of the coupled mode, and gives rise to an abrupt increase in the scattering rate, which is absent in the uncoupled system. In monolayer graphene a single jump in the scattering rate occurs, arising from the emission of the low-energy branch of the coupled plasmon-phonon modes. In bilayer graphene the emission of both low- and high-energy branches of the coupled modes contributes to the scattering rate and gives rise to two abrupt changes in the scattering rate. The jumps in the scattering rate can potentially be used in hot-electron devices such as switching devices and oscillators.

Graphene's exceptional mechanical properties, including its highest-known stiffness (1 TPa) and strength (100 GPa), have been exploited for various structural applications. However, graphene is also known to be quite brittle, with experimentally measured tensile fracture strains that do not exceed a few percent. In this work, we introduce the notion of graphene kirigami, where concepts that have been used almost exclusively for macroscale structures are applied to dramatically enhance the stretchability of both zigzag and armchair graphene. Specifically, we show using classical molecular-dynamics simulations that the yield and fracture strains of graphene can be enhanced by about a factor of 3 using kirigami as compared to standard monolayer graphene. Finally, we demonstrate that this enhanced ductility in graphene may open up interesting opportunities in coupling to graphene's electronic behavior.

The existing controversies about the space group symmetries of $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_x\mathrm{T}{\mathrm{i}}_{1-x}\right){\mathrm{O}}_3$ (PZT) above and below the antiferrodistortive (AFD) phase-transition temperature $\left(T_{\mathrm{AFD}}\right)$ in the ${\mathrm{Zr}}^{4+}$-rich $\left(0.620\lesssim x\lesssim 0.940\right)$ compositions are addressed using the results of dielectric, synchrotron x-ray powder diffraction (SXRPD) and neutron powder diffraction (NPD) studies. These compositions undergo an AFD phase transition above room temperature due to tilting of oxygen octahedra leading to a superlattice phase of PZT. We have substituted 6% ${\mathrm{Sr}}^{2+}$ at a ${\mathrm{Pb}}^{2+}$ site to enhance the tilt angle and thereby the intensity of the superlattice peaks. The real and imaginary parts of the complex dielectric permittivity have been used to locate the paraelectric to ferroelectric and ferroelectric to AFD phase transitions. Rietveld analysis of SXRPD and NPD profiles unambiguously reject the rhombohedral phases in the $R3c$ and $R3m$ space groups below and above $T_{\mathrm{AFD}}$, respectively, with or without a coexisting monoclinic phase in the Cm space group and confirm that the true symmetries are monoclinic in the Cc and Cm space groups below and above $T_{\mathrm{AFD}}$, respectively. Based on these and previous findings a phase diagram of PSZT for $0.40\le x\le 0.90$ showing stability fields of monoclinic Cc and monoclinic Cm, tetragonal $P4mm$, and cubic $Pm\overline{3}m$ phases has also been presented.

The structure and lattice dynamics of rock-salt thermoelectric materials SnTe and PbTe are investigated with single-crystal and powder neutron diffraction, inelastic neutron scattering (INS), and first-principles simulations. Our first-principles calculations of the radial distribution function in both SnTe and PbTe show a clear asymmetry in the first nearest-neighbor (1NN) peak, which increases with temperature, in agreement with recent experimental reports. We show that this peak asymmetry for the 1NN Sn-Te or Pb-Te bond results from large-amplitude anharmonic vibrations (phonons). No atomic off centering is found in our simulations. In addition, the atomic mean-square displacements derived from our diffraction data reveal stiffer bonding at the anion site, in good agreement with the partial phonon densities of states from INS and first-principles calculations. These results provide clear evidence for large-amplitude anharmonic phonons associated with the resonant bonding leading to the ferroelectric instability.

We report measurements of the Nernst effect and of the magnetoresistance of granular aluminum films near the metal-to-insulator transition. These films show sharp transitions as a function of temperature and magnetic field. At low temperatures the Nernst signal displays a sharp peak at a field where more than 90% of the normal-state resistance has been restored, suggesting a transition involving entropy transport after superconducting coherence has been destroyed. At temperatures higher than the critical temperature the fluctuation paraconductivity scales with the Nernst signal, in agreement with a description in terms of fluctuations of the order parameter.

The search for exotic materials with a linear Dirac-like dispersion in the electronic structure is one of the most challenging tasks of the condensed matter community and materials science. Revealing the nature of the interplay between such a Dirac-like and superconducting states is a crucial issue for the study of fundamental physics. Here we report the experimental observations of a large linear magnetoresistance (MR) in the Ca-intercalated graphite superconductor ${\mathrm{CaC}}_6$. A large nonsaturating MR with a magnitude as high as 244% is observed at low temperature under a magnetic field of 9 T. The magnetic field $\left(B\right)$ dependence of MR shows a linear behavior above 3 T at low temperature, which deviates from the classical $B^2$ behavior, pointing to the existence of an intrinsic linear Dirac-like state. The presence of such a low-energy Dirac-like dispersion in energy band structure is confirmed qualitatively by performing first-principles calculations. These findings may pave an avenue for potential applications in magnetoelectronic sensors and for further studying the interplay between the linear Dirac-like and superconducting states in exotic materials.

The cuprate superconductor ${\mathrm{Ca}}_x{\mathrm{La}}_{1-x}{\mathrm{Ba}}_{1.75-x}{\mathrm{La}}_{0.25+x}{\mathrm{Cu}}_3{\mathrm{O}}_y$ has been proposed as a model system which allows systematic variation in the optimum transition temperature, $T_{c,\max }$, with doping and ion size. However, we show that the $T_{c,\max }$ values are anomalous in this system. We determine the doping accurately and show that the phase curve, as a function of doping, is anomalously truncated so that values of $T_{c,\max }$ appear to decrease with decreasing $x$, concealing the generic increase expected as ion size increases. We also thereby resolve its reported anomalous pseudogap behavior from NMR and anomalous superconducting gap from angle-resolved photoemission spectroscopy.

We study the properties of vortex solutions and magnetic response of two-component $U\left(1\right)\times U\left(1\right)\times {\mathbb{Z}}_2$ superconductors, with phase separation driven by intercomponent density-density interaction. Such a theory can be viewed arising from the breakdown of $SU\left(2\right)$ symmetry by a biquadratic interaction between the components of the field. Depending on the symmetry-breaking term, there are two ground-state phases: one where both components of the doublet are equal (the miscible phase) and one where only one component assumes a nonzero vacuum expectation value (the immiscible state). In the latter phase, the spectrum of topological excitations contains both domain walls and vortices. We show the existence of another kind of excitation that has properties of both topological excitations at the same time. This kind of excitation combines vorticity together with a circular domain wall, interpolating between inequivalent broken states, that shows up as a ring of localized magnetic flux. Asymptotically, this resembles a vortex carrying multiple flux quanta, but because the magnetic field is localized at a given distance from the center this looks like a pipe. The isolated multiquanta pipelike vortices can be either stable or metastable, even if the system is not type 1. We discuss the response of such a system to an externally applied magnetic field. Also we discuss magnetic response of $\mathit{SU}$(2) superconductor.

We investigated order parameter textures of the rotating superfluid ${}^3\mathrm{He}$-A phase in a single narrow cylinder with a diameter of about 10 times the dipole coherence length by the cw-NMR method. It is theoretically proposed that in such a narrow cylinder, a few special textures will appear due to the confinement in a cylindrical geometry. We observed three types of NMR spectra in the A phase. The NMR spectra of the textures were identified by a comparison with the spin waves excited in the NMR potential using their numerically calculated resonance frequencies and relative intensities. We have established a method to selectively generate each one of the textures by controlling the conditions when the A phase was formed, such as the applied magnetic field, rotation angular velocity, and temperature.