In the presented work, Sc${}_{3.1}$In, a weak itinerant ferromagnet with no magnetic constituents, is doped with Er${}^{3+}$ local moment ions, to form (Sc${}_{1-x}$Er${}_x$)${}_{3.1}$In. As $x$ increases, the Weiss-like temperature $\theta$ stays positive and nearly triples up to $x=0.10$. Moreover, Er doping of as little as $x=0.02$ induces a cluster-glass state, which persists up to $x=0.10$, as evidenced by dc and ac susceptibility measurements, and confirmed by the Vogel-Fulcher analysis.

Recent observation of zero bias conductance peaks in semiconductor wire/superconductor heterostructures has generated great interest, and there is a hot debate on whether the observation is associated with Majorana fermions (MFs). Here we study the local and crossed Andreev reflections of two normal leads attached to the two ends of a superconductor-semiconductor wire. We show that the MFs induced crossed Andreev reflections have significant effects on the shot noise of the device and strongly enhance the current-current correlations between the two normal leads. The measurements of shot noise and current-current correlations can be used to identify MFs.

We describe structural, magnetic, and transport properties of the Cd${}_{1-}$${}_x$${}_{-y}$Mn${}_x$Cr${}_y$Te ferromagnetic semiconductor alloy. Bulk Cd${}_{1-x-y}$Mn${}_x$Cr${}_y$Te crystals containing up to 10$\%$ of Cr ($y$ $=$ 0.10) were grown by the Bridgman method and were found to be strongly $p$ type and ferromagnetic, with the highest Curie temperature of 362 K observed for $y$ $=$ 0.10. Magnetotransport measurements reveal the presence of the anomalous Hall effect up to and above room temperature, providing evidence that the observed ferromagnetism is characteristic of bulk Cd${}_{1-x-y}$Mn${}_x$Cr${}_y$Te. Our experiment data suggest that Cr ions play a crucial role in establishing ferromagnetism in this quaternary alloy.

Energy stabilization of the superatom molecular orbitals (SAMOs) in fullerenes is investigated with the goal of involving their nearly free-electron bands in practical charge transport applications. Combining low-temperature scanning tunneling microscopy-based spectroscopic methods and density functional theory calculations on an endohedral metallofullerene La@C${}_{82}$, we confirm that the $s-$SAMO of C${}_{82}$ fullerene is stabilized by as much as 2 eV with respect to that of C${}_{60}$ by endohedral doping with the La atom. On the copper metal substrate, the $s-$SAMO energy is further lowered to just 1 eV above the Fermi level, making the applications of $s-$SAMO state in transport more plausible. We conclude that in an endohedral metallofullerene, the $s-$SAMO state is stabilized through the hybridization with the $s$-symmetry valence state of the metal atom and the stabilization energy correlates with the ionization potential of the free atom.

An atomic force microscope is used to locally manipulate a single layer graphene sheet. Transport measurements in this region as well as in the unmanipulated part reveal different charge carrier densities, while mobilities stay in the order of ${10}^4$ cm${}^2$(V$$s)${}^{-1}$. With a global backgate, the system is tuned from a unipolar $n-n^{\prime }$ or $p-p^{\prime }$ junction with different densities to a bipolar $p-n$ junction. Magnetotransport across this junction verifies its nature, showing the expected quantized resistance values as well as the switching with the polarity of the magnetic field. Measurements at higher magnetic fields show a suppression of the mixing of edge states with different polarity at the $p-n$ junction.

Topological superconductors in one spatial dimension exhibiting a single Majorana bound state at each end are distinguished from trivial gapped systems by a ${\mathbb{Z}}_2$ topological invariant. Originally, this invariant was calculated by Kitaev in terms of the Pfaffian of the Majorana representation of the Hamiltonian: The sign of this Pfaffian divides the set of all gapped quadratic forms of Majorana fermions into two inequivalent classes. In the more familiar Bogoliubov de Gennes mean-field description of superconductivity, an emergent particle-hole symmetry gives rise to a quantized Zak-Berry phase, the value of which is also a topological invariant. In this work, we explicitly show the equivalence of these two formulations by relating both of them to the phase winding of the transformation matrix that brings the Majorana representation matrix of the Hamiltonian into its Jordan normal form.

Density functional theory is used to systematically study the electronic properties of doped MoS${}_2$ monolayers, where the dopants are incorporated both via S/Mo substitution or as adsorbates. Among the possible substitutional dopants at the Mo site, Nb is identified as suitable $p$-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS${}_2$ conduction band, making the system $n$ type. Finally, the adsorption of charged molecules is considered, mimicking an ionic liquid environment. We find that molecules adsorption can lead to both $n$- and $p$-type conductivity, depending on the charge polarity of the adsorbed species.

Complexes of substitutional copper (Cu${}_s$) with hydrogen in silicon are investigated by standard deep-level transient spectroscopy (DLTS) and high-resolution Laplace-transform DLTS. Hydrogen is introduced into the near-surface layer of copper-doped crystals during wet chemical etching and moved deeper into the sample by annealing Schottky contacts under reverse bias at 350–380 K. Two novel centers are observed to form in the hydrogenated region. Each of them possesses two deep levels which are only slightly different from the Cu${}_s$ levels. Analysis of the depth profiles allows us to identify the observed centers as complexes of Cu${}_s$ with one and two hydrogen atoms and reveals the formation of an electrically inactive complex with three hydrogen atoms. The identification is confirmed by the numerical modeling of the copper-hydrogen interaction which enables a quantitative description of the experimentally measured concentrations of different complexes. The variations of the Cu${}_s$ donor and acceptor levels due to successive hydrogenation are found to be similar to those for isoelectronic Ag and Au impurities.

Motivated by the hot issue of the pairing symmetry of superconducting (SC) graphene, we theoretically study the electronic Raman spectra on a two-dimensional honeycomb lattice with two possible SC pairing symmetries, the $d_{x^2-y^2}+i{d}_{xy}$ wave ($d+i{d}^{\prime }$ wave) and extended $s$ wave, at half filling and different doping levels. When it is not doped, the Raman spectrum for $d+i{d}^{\prime }$-wave pairing consists of a low-energy kink and a high-energy Raman peak, while the spectrum for the extended-$s$-wave pairing is similar to that in the normal state. On the other hand, when it is doped, besides the high-energy Raman peak, a low-energy pair breaking peak arises in both of the two pairings, which is forbidden in the otherwise normal state graphene at the same doping level. For the Raman spectrum of $d+i{d}^{\prime }$-wave pairing, there is an energy cutoff at low energy, below which no Raman absorption is allowed, indicating a full gap. While for the extended-$s$-wave pairing, with the increasing of the doping level, the low energy Raman behavior changes from a linear line to the appearance of cutoff energy, indicating that the gap changes from nodal to full. Therefore, we propose that the distinct features of Raman spectra can be used to differentiate the two pairings of SC graphene.

We analytically compute the RKKY range function as induced by two-dimensional (2D) hole gases. The bulk valence-band includes heavy-hole (HH) and light-hole (LH) states and their dynamics is described by the Luttinger Hamiltonian which we adopt as our framework. We show that even for situations where only the lowest HH-like subband is occupied the resulting form of the RKKY function can be very different as compared to the one of a 2D electron gas. The associated spin susceptibility tensor has entries along the quantum-well directions and perpendicular to it. Our formulas for the spin susceptibility tensor reveal the crucial influence of HH-LH mixing which gives rise to large anisotropies both among the in-plane components as well as among the in-plane components and the component perpendicular to the quantum well.

The discovery of a high-mobility two-dimensional electron gas (2DEG) in wurtzite ZnO/Zn(Mg)O heterostructures is promising for applications due to the high mobility of the carriers. In this paper, we study the formation and properties of the 2DEG at ZnO/Zn(Mg)O interfaces using first-principles calculations based on hybrid density functional theory. The 2DEG arises from the polarization discontinuity at the interface between the two materials. The uncompensated bound charge at the interface gives rise to an electric field in the bulk of ZnO which confines free carriers close to the interface. We find that the type of the confined carriers is determined by the interface termination, while the amount of charge and the confinement width could be controlled by the Mg doping and the device dimensions.

Kohn anomalies in three-dimensional metallic crystals are dips in the phonon dispersion that are caused by abrupt changes in the screening of the ion cores by the surrounding electron gas. These anomalies are also present at the high-symmetry points $\Gamma$ and $K$ in the phonon dispersion of two-dimensional graphene, where the phonon wave vector connects two points on the Fermi surface. The linear slope around the kinks in the highest optical branch is proportional to the electron-phonon coupling. Here, we present a combined theoretical and experimental study of the influence of the dielectric substrate on the vibrational properties of graphene. We show that screening by the dielectric substrate reduces the electron-phonon coupling at the high-symmetry point $K$ and leads to an upshift of the Raman 2D line. This results in the observation of a Kohn anomaly that can be tuned by screening. The exact position of the 2D line can thus be taken also as a signature for changes in the (electron-phonon limited) conductivity of graphene.

X-ray magnetic circular dichroism experiments have been performed up to ±17 T to investigate the magnetic configuration and magnetization reversal of an original zero-magnetization ferromagnet Sm${}_{1-x}$Gd${}_x$Al${}_2$ ($x$ $=$ 0.028), both as a single epitaxial layer and as a pinning layer in an exchange-coupled system. The Sm${}_{0.972}$Gd${}_{0.028}$Al${}_2$ single layer appears to exhibit an extraordinary large coercivity that exceeds 20 T below its magnetic compensation ($T_{\mathrm{comp}}$). Despite such huge magnetic stability in the single layer, interface exchange coupling in the bilayer drives the formation of domains in Sm${}_{0.972}$Gd${}_{0.028}$Al${}_2$ and their reversal upon field, both below and at magnetic compensation. Increasing the cooling field yields the increase in exchange-favored domains in Sm${}_{0.972}$Gd${}_{0.028}$Al${}_2$, surprisingly also at $T_{\mathrm{comp}}$, whereas, Zeeman energy does not favor this specific orientation neither at compensation nor during the cooling process. We propose a possible scenario for those domains’ formation and highlight the way the external magnetic field may tune the magnetic configuration in such a zero-magnetization ferromagnet. The cooling field also consequently influences the SmAl${}_2$ magnetization reversal which is biased by pinned magnetic components in Sm${}_{0.972}$Gd${}_{0.028}$Al${}_2$; the bias field is satisfactorily explained in considering different pinned contributions with opposite magnetic orientations.

The quantitative understanding of converse magnetoelectric effects, i.e., the variation of the magnetization as a function of an applied electric field, in extrinsic multiferroic hybrids is a key prerequisite for the development of future spintronic devices. We present a detailed study of the strain-mediated converse magnetoelectric effect in ferrimagnetic Fe${}_3$O${}_4$ thin films on ferroelectric BaTiO${}_3$ substrates at room temperature. The experimental results are in excellent agreement with numerical simulation based on a two-region model. This demonstrates that the electric field induced changes of the magnetic state in the Fe${}_3$O${}_4$ thin film can be well described by the presence of two different ferroelastic domains in the BaTiO${}_3$ substrate, resulting in two differently strained regions in the Fe${}_3$O${}_4$ film with different magnetic properties. The two-region model allows to predict the converse magnetoelectric effects in multiferroic hybrid structures consisting of ferromagnetic thin films on ferroelastic substrates.

The effect of Ir substitution for Os in CeOs${}_2$Al${}_{10}$, with an unusually high Néel temperature of $T^{*}\sim 28.5$ K, has been studied by high-resolution neutron diffraction and magnetization measurements. A small amount of Ir ($\sim$$8\%$) results in a pronounced change of the magnetic structure of the Ce sublattice. The induced magnetic ground state is controlled by single ion anisotropy and implies an antiferromagnetic arrangement of the Ce moments along the $a$ axis, as expected from the anisotropy of the paramagnetic susceptibility. The value of the ordered moments, 0.92(1)${\mu }_B$, is substantially bigger than in the undoped compound, whereas the transition temperature is reduced down to 21 K. A comparison of the observed phenomena with the recently studied CeRu${}_{1.9}$Rh${}_{0.1}$Al${}_{10}$ system, exhibiting similar behavior [Kondo et al., J. Phys. Soc. Jpn. 82, 054709 (2013)], strongly suggests the electron doping as the main origin of the ground state changes. This provides additional ways for exploring the anomalous magnetic properties of the Ce(Ru/Os)${}_2$Al${}_{10}$ compounds.

A topological excitation called skyrmion has been observed experimentally in chiral magnets without spatial inversion symmetry. The dynamics of a skyrmion is equivalent to an electron moving in a strong magnetic field. As a skyrmion involves large number of spins, it is not clear whether there exist observable quantum effects. In this work, we study the quantum dynamics of a skyrmion in a pinning potential. Without a pinning potential, the skyrmion performs cyclotron motion due to the strong emergent magnetic field originating from the Berry phase of spins, and all skyrmions occupy the lowest Landau level. Their wave functions are strongly localized in a region with radius less than $1\AA$ when no driving force is applied. Thus in most circumstances, the quantum effects of a skyrmion are weak. In the presence of a pinning potential, the lowest Landau level for skyrmions is split into quantized levels, characterized by the orbital moments. The transition between quantized levels can be observed experimentally by microwave absorption measurements in low temperature region. The quantum effects are more prominent for a skyrmion with a small size, which can be realized in magnets with a large Dzyaloshinskii-Moriya interaction.

We investigate to what extent the magnetic properties of deposited nanostructures can be influenced by selecting as a support different surfaces of the same substrate material. Fully relativistic ab initio calculations were performed for Co monolayers and adatoms on Pd(100), Pd(111), and Pd(110) surfaces. Changing the crystallographic orientation of the surface has a moderate effect on the spin magnetic moment, a larger effect on the orbital magnetic moment, but sometimes a dramatic effect on the magnetocrystalline anisotropy energy and on the magnetic dipole term $T_{\alpha }$. The dependence of $T_{\alpha }$ on the magnetization direction $\alpha$ can lead to a strong apparent anisotropy of the spin magnetic moment as deduced from the x-ray magnetic circular dichroism sum rules. For systems in which the spin-orbit coupling is not very strong, the $T_{\alpha }$ term can be understood as arising from the differences between components of the spin magnetic moment associated with different magnetic quantum numbers $m$.

We consider nonchiral symmetry-protected topological phases of matter in two spatial dimensions protected by a discrete symmetry such as ${\mathbb{Z}}_K$ or ${\mathbb{Z}}_K\times {\mathbb{Z}}_K$ symmetry. We argue that modular invariance/noninvariance of the partition function of the one-dimensional edge theory can be used to diagnose whether, by adding a suitable potential, the edge theory can be gapped or not without breaking the symmetry. By taking bosonic phases described by Chern-Simons $K$-matrix theories and fermionic phases relevant to topological superconductors as an example, we demonstrate explicitly that when the modular invariance is achieved, we can construct an interaction potential that is consistent with the symmetry and can completely gap out the edge state.