The magnetotransport phase diagram of half-doped manganites ${\text{Ln}}_{0.5}{\text{A}}_{0.5}{\text{MnO}}_3$ ($\text{Ln}={\text{La}}^{3+}$, ${\text{Nd}}^{3+}$, etc., and $\text{A}={\text{Sr}}^{2+}$, ${\text{Ca}}^{2+}$, etc.) is primarily dictated by the bare conduction bandwith ($W_0$), which itself is controlled by the Mn-O-Mn bond angle, and the carrier concentration. In thin films, epitaxial strain (ε) provides an additional tool to tune $W_0$ by selecting orbital ordering at fixed carrier concentration. Here, we will show that compressive or tensile epitaxial strain on half-doped manganites can have a tremendous and distinct effect on ${\text{La}}_{0.5}{\text{Sr}}_{0.5}{\text{MnO}}_3$ (LSMO5) and ${\text{La}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$ (LCMO5), having broad or narrow $W_0$, respectively. It is found that in LSMO5, large compressive strain triggers a change from a ferromagnetic and metallic ground state to an insulating and antiferromagnetic state whereas a tensile strain produces an antiferromagnetic but metallic state. In contrast, under strain, LCMO5 remains an antiferromagnetic insulator irrespectively of the strain state. These results illustrate that orbital ordering largely depends on the interplay between $W_0$ and ε and provide a guideline towards responsive manganite layers.

Classical percolation theory ignores the possibility of quantum tunneling, which is important for transport on the nanoscale. Here we show that the electrical conductance of a two-dimensional percolating-tunneling system exhibits an exponential dependence on surface coverage, which is in contrast to the classically expected power law, and that there is no evidence for an intrinsic threshold for tunneling conduction. We then show that the percolating-tunneling system is identical to that of an equivalent system of point defects, hence creating a link between continuum percolation and the theory of hopping conduction in semiconductors.

We find a giant inverse proximity effect that arises in small low-dimensional superconductor-normal-superconductor (S/N/S) junctions, where a proximity coherence length is larger than the coherence length of a source superconductor, due to a huge backaction exerted by normal conductors on the proximity superconducting regions. Inverse proximity develops even in case where dimensions of the normal parts are smaller than the proximity coherence length and significantly suppresses superconducting characteristics of the entire system, including critical current, well below those for the usual S/N/S structures.

We show that obliquely incident, transversely magnetic-polarized plane waves can be totally transmitted (with zero reflection) through epsilon-near-zero (ENZ) bilayers characterized by balanced loss and gain with parity-time ($\mathcal{PT}$) symmetry. This tunneling phenomenon is mediated by the excitation of a surface wave localized at the interface separating the loss and gain regions. We determine the parameter configurations for which the phenomenon may occur and, in particular, the relationship between the incidence direction and the electrical thickness. We show that, below a critical threshold of gain and loss, there always exists a tunneling angle which, for moderately thick (wavelength-sized) structures, approaches a critical value dictated by the surface-wave phase-matching condition. We also investigate the unidirectional character of the tunneling phenomenon, as well as the possible onset of spontaneous symmetry breaking, typical of $\mathcal{PT}$-symmetric systems. Our results constitute an interesting example of a $\mathcal{PT}$-symmetry-induced tunneling phenomenon, and may open up intriguing venues in the applications of ENZ materials featuring loss and gain.

Traps are localized, deep lying energetic states and are generally considered detrimental for organic semiconducting devices. In this work, we investigate their influence on the magnetic field sensitive current in organic devices, an effect often called organic magnetoresistance. Polymer thin films were doped with materials well known to influence trap characteristics in such devices. We analyzed the observed trends in the intrinsic magnetic field effects in terms of a trapped trion model, which incorporates the spin dependent formation of excitations at these trap sites. The results provide strong evidence that the intrinsic traps in the polymer material are indeed at the origin of these magnetic field effects. This shows that traps have beneficial properties and that their control may provide an appealing route for the engineering of efficient organic magnetic field sensors.

We analyze the electronic properties of adatom-doped graphene in the low-impurity-concentration regime. We focus on the Anderson localized regime and calculate the localization length $\xi$ as a function of the electron doping and an external magnetic field. The impurity states hybridize with carbon's $p_z$ states and form a partially filled band close to the Dirac point. Near the impurity band center, the chiral symmetry of the system's effective Hamiltonian is partially preserved, which leads to a large enhancement of $\xi$. The sensitivity of transport properties, namely, Mott's variable range hopping scale $T_0$, to an external magnetic field perpendicular to the graphene sheet leads to a colossal magnetoresistance effect, as observed in recent experiments.

We have investigated the intrasubband spin-density excitation (SDE) in an asymmetrically doped GaAs-AlGaAs single quantum well with balanced Rashba and Dresselhaus spin-orbit interaction strengths by inelastic light scattering. For this unique symmetry, the combined spin-orbit field is either parallel or antiparallel to the $\left[1\overline{1}0\right]$ in-plane direction of the quantum well for all wave vectors of the two-dimensional reciprocal space. In backscattering geometry, the SDE is formed by spin-flip intrasubband transitions of the spin-split subband. Via the splitting of the intrasubband SDE, we have directly detected the spin splitting of the conduction band due to the anisotropic spin-orbit field. As expected, the splitting is nonzero if a wave vector is transferred perpendicular to the direction of the spin-orbit field and close to zero for a parallel wave-vector transfer. The extracted values for the spin-orbit strength and for the wavelength of a persistent spin helix compare well with results of previous experiments of direct spatial mapping of the spin helix.

We reveal the giant magnetoresistance induced by the spin-polarized current in the ferromagnet (F${}_1$)/ topological insulator (TI)/ferromagnet (F${}_2$) junction, where two ferromagnets are deposited on the diffusive surface of the TI. We can increase and reduce the value of the giant magnetoresistance by tuning the spin-polarized current, which is controlled by the magnetization configurations. The property is intuitively understood by the nonequilibrium spin-polarized current, which plays the role of an effective electrochemical potential on the surface of the TI.

We calculate the effect of spin waves on the properties of finite-size spin chains with a chiral spin ground state observed on biatomic Fe chains deposited on iridium(001). The system is described with a Heisenberg model supplemented with a Dzyaloshinskii-Moriya coupling and a uniaxial single ion anisotropy that presents a chiral spin ground state. Spin waves are studied using the Holstein-Primakoff boson representation of spin operators. Both the renormalized ground state and the elementary excitations are found by means of Bogoliubov transformation, as a function of the two variables that can be controlled experimentally, the applied magnetic field and the chain length. Three main results are found. First, because of the noncollinear nature of the classical ground state, there is a significant zero-point reduction of the ground-state magnetization of the spin spiral. Second, there is a critical external field from which the ground state changes from chiral spin ground state to collinear ferromagnetic order. The character of the two lowest-energy spin waves changes from edge modes to confined bulk modes over this critical field. Third, in the spin-spiral state, the spin-wave spectrum exhibits oscillatory behavior as function of the chain length with the same period of the spin helix.

A twisted magnetic structure in Ni ultrathin films attached to antiferromagnetic FeMn is revealed by a combination of the depth-resolved x-ray magnetic circular dichroism (XMCD) and the polarized neutron reflectivity (PNR) techniques. The depth-resolved XMCD at remanent magnetization shows that the perpendicular magnetization component in the Ni film decreases around the interface to FeMn when the film exhibits perpendicular magnetization, whereas the in-plane component is kept constant through the whole film in the case of in-plane magnetization. Moreover, the PNR data shows that when a weak in-plane magnetic field is applied to a film, which exhibits perpendicular magnetization at the remanent state, an in-plane magnetization component is induced in Ni around the interface to FeMn. These results are reasonably interpreted by assuming that the magnetic moment in the Ni layer is twisted from the perpendicular to the in-plane directions towards the interface to FeMn. Such a magnetic structure is supposed to be induced by a magnetic anisotropy interaction at the interface between ferromagnetic Ni and antiferromagnetic FeMn, which makes the Ni magnetic moment around the interface rotate towards the in-plane direction.

Nodal superconductors without inversion symmetry exhibit nontrivial topological properties, manifested by topologically protected flat-band edge states. Here we study the effects of edge roughness and strong edge disorder on the flat-band states using large-scale numerical simulations. We show that the bulk-edge correspondence remains valid for rough edges and demonstrate that midgap states generically appear at the boundary of nodal noncentrosymmetric superconductors, for almost all edge orientations. Moderately strong nonmagnetic disorder shifts some of the edge states away from zero energy, but does not change their total number. Strong spin-independent edge disorder, on the other hand, leads to the appearance of new weakly disordered midgap states in the layers adjacent to the disordered edge, i.e., at the interface between the bulk topological superconductor and the one-dimensional Anderson insulator formed by the strongly disordered edge layers. Furthermore, we show that magnetic impurities, which lift the time-reversal symmetry protection of the flat-band states, lead to a rapid decrease of the number of edge states with increasing disorder strength.

A Monte Carlo simulation method is used to study the energetics and configuration of binary alloys when grain boundary states are included as potential equilibrium features. For certain sets of alloy properties, a nanostructured grain assembly is found to be the most energetically favorable state, and is stabilized by grain boundary segregation of solute. The conditions for stability against grain coarsening and the “grain boundary energy” requirement are clarified, with emphasis on the closed system conditions that prevail in nanostructured alloys. Two thermodynamic parameters, the grain boundary area potential and the grain boundary formation energy, are quantitatively disentangled and shown to differently reflect grain stability and the energy state of interfaces. These discussions provide insights on how alloying can be used to actively manipulate nanocrystalline grain sizes.

We have determined the field dependence of collective magnetic excitations in iron oxide nanoparticles of cubic shape with 8.42(2) nm edge length and a narrow log normal size distribution of 8.2(2)% using time-of-flight neutron spectroscopy. The energy dependence of the uniform precession modes was investigated up to 5 T applied field and yields a Landé factor $g=2.05\left(2\right)$ as expected for maghemite ($\gamma$-Fe${}_2$O${}_3$) nanoparticles. A large effective anisotropy field of $B_{A,\mathrm{eff}}=0.45\left(16\right)$ T was determined, in excellent agreement with macroscopic measurements. This anisotropy is attributed to enhanced shape anisotropy in these monodisperse cubic nanoparticles. The combination of our results with macroscopic magnetization information provides a consistent view of the energy scales of superparamagnetic relaxation and collective magnetic excitations in magnetic nanoparticles.

The electronic structure of a quantum spin liquid compound, ${\text{EtMe}}_3\text{Sb}[{\text{Pd}\left(\text{dmit}\right)}_2{]}_2$, has been studied with angle-resolved photoemission spectroscopy, together with two other ${\text{Pd}\left(\text{dmit}\right)}_2$ salts in the valence bond solid or antiferromagnetic state. ${\text{EtMe}}_3\text{Sb}[{\text{Pd}\left(\text{dmit}\right)}_2{]}_2$, being a Mott insulator, has its lower Hubbard band identified, with a soft energy gap at the chemical potential. In addition, the spectral features exhibit anomalously broad linewidth and negligible dispersion, which fit well to the calculated energy levels of an isolated $[{\text{Pd}\left(\text{dmit}\right)}_2{]}_2$ dimer. We suggest that these electronic characteristics might be the manifestation of the spin-charge separation in a two-dimensional quantum spin liquid.

We present a comprehensive electron transport model to analyze thermoelectric properties of both $n$- and $p$-type bulk ${\text{Mg}}_2$${\text{Si}}_{1\text{−}x}$${\text{Sn}}_x$ ($0\le x\le 1$) solid solutions. A temperature-dependent multiparabolic bands model is used to describe the band structures of the alloys, and the transport properties are calculated using the linearized Boltzmann transport equations under the relaxation time approximation. A variety of experimental data from literature are fitted very well by this model and analyzed for further material optimization. Our analysis shows that the compositions of $x$ = 0.6 to 0.7 exhibit the highest thermoelectric figure of merit zT among $n$-type ${\text{Mg}}_2$${\text{Si}}_{1\text{−}x}$${\text{Sn}}_x$ in the midtemperature range 600 to 900 K due to both the high power factors achieved by the convergence of the two conduction bands and low electronic thermal conductivities. For the $p$-type materials, we find that the bipolar electronic thermal conductivity is a major factor limiting the figure of merit. Low Sn content ($x$ < 0.4) alloys show a larger figure of merit among the $p$-type materials due mainly to their lower bipolar thermal conductivities with larger band gaps. Finally, we propose that hot carrier energy filtering can be very useful for these alloys as it can simultaneously reduce the electronic thermal conductivity and enhance the power factor. A zT greater than 3 is possible for $n$-type ${\text{Mg}}_2$${\text{Si}}_{0.4}$${\text{Sn}}_{0.6}$ ($x$ = 0.6) at 700 K, if electrons with energies lower than 0.4 eV are effectively prevented from participating in transport.

We review and expand on previous work that treats relaxation physics of low-lying excited states in ideal, single-electron, silicon quantum dots in the context of quantum computing. These states are of three types: orbital, valley, and spin. The relaxation times depend sensitively on system parameters such as the dot size and the external magnetic field. Generally, however, orbital relaxation times are short in strained silicon (${10}^{-7}$ to ${10}^{-12}$ s), spin-relaxation times are long (${10}^{-6}$ to $\gg$1 s), while valley relaxation times are expected to lie in between. The focus is on relaxation due to emission or absorption of phonons, but for spin relaxation we also consider competing mechanisms such as charge noise. Where appropriate, comparison is made to reference systems such as quantum dots in III-V materials and silicon donor states. The phonon-bottleneck effect is shown to be rather small in the regime of interest. We compare the theoretical predictions to some recent spin relaxation experiments and comment on the possible effects of nonideal dots.

We investigate the surface electronic structure and thermodynamic stability of the ${\text{SrTiO}}_3$ (111) slabs using density functional theory. We observe that, for Ti-terminated slabs it is indeed possible to create a two-dimensional electron gas (2DEG). However, the carrier density of the 2DEG displays a strong thickness dependence due to the competition between electronic reconstruction and polar distortions. As expected, having a surface oxygen atom at the Ti termination can stabilize the system, eliminating any electronic reconstruction, thereby making the system insulating. An analysis of the surface thermodynamic stability suggests that the Ti terminated (111) surface should be experimentally realizable. This surface may be useful for exploring the behavior of electrons in oxide (111) interfaces and may have implications for modern device applications.

We find a systematic reappearance of massive Dirac features at the edges of consecutive minibands formed at magnetic fields $B_{p/q}=\frac{p}{q}{\varphi }_0/S$ providing rational magnetic flux through a unit cell of the moiré superlattice created by a hexagonal substrate for electrons in graphene. The Dirac-type features in the minibands at $B=B_{p/q}$ determine a hierarchy of gaps in the surrounding fractal spectrum and show that these minibands have topological insulator properties. Using the additional $q$-fold degeneracy of magnetic minibands at $B_{p/q}$, we trace the hierarchy of the gaps to their manifestation in the form of incompressible states upon variation of the carrier density and magnetic field.

Utilizing the anisotropic thermal expansion of $a$-plane sapphire, we study the role of anisotropic in-plane strain on ZnO quantum well structures. X-ray data reveal a deformation of the hexagonal unit cell. The symmetry reduction shows up in the optical spectra by distinct linear-polarization features, both for the neutral and charged exciton transition. We elaborate the excitonic fine structure behind this observation, accounting for spin-orbit coupling and electron-hole exchange in the presence of anisotropic strain. The consequences for optical spin control are discussed.