We investigate the presence of percolating states in disordered two-dimensional topological insulators. In particular, we uncover a close connection between these states and the so-called topological Anderson insulator, which is a topologically nontrivial phase induced by the presence of disorder. The decay of this phase could previously be connected to a delocalization of bulk states with increasing disorder strength. We identify this delocalization to be the result of a percolation transition of states that circumnavigate the hills of the bulk disorder potential.

We investigate the depth dependence of the canted magnetization in a bcc Fe ultrathin film grown on a vicinal Ag substrate. This study is performed for different Fe thicknesses, in the vicinity of the spin reorientation transition, by soft x-ray resonant magnetic reflectivity (SXRMR). Above 5.5 monolayers, the high spatial resolution of SXRMR allows us to describe a gradient of the canted magnetization. The tilt angle is larger near the Ag interface and decreases towards the surface. This noncollinearity is ascribed to a different balance of various anisotropies across the layer thickness. Below that value, a large increase of the tilt angle is observed and ascribed to the spin reorientation transition. Unlike the previous situation, the tilt angle is found to be homogeneous throughout the film. This can be related either to the out-of-plane spin reorientation associated to a reduced number of monolayers or to the $q$-space limitation of the SXRMR technique at the Fe $L_3$ edge.

We report magnetic and superconducting properties of the modified spin-valve system ${\mathrm{CoO}}_x/\text{Fe1}/\mathrm{Cu}/\text{Fe2}/\mathrm{Cu}/\mathrm{Pb}$. Introduction of a Cu interlayer between Fe2 and Pb layers prevents material interdiffusion process, increases the Fe2/Pb interface transparency, stabilizes and enhances properties of the system. This allowed us to perform a comprehensive study of such heterostructures and to present theoretical description of the superconducting spin-valve effect and of the manifestation of the long-range triplet component of the superconducting condensate.

Static strain aging, a phenomenon caused by diffusion of solute atoms to dislocations, is an important contributor to the strength of substitutional alloys. Accurate modeling of this complex process requires both atomic spatial resolution and diffusional time scales, which is very challenging to achieve with commonly used atomistic computational methods. In this paper, we use the recently developed “diffusive molecular dynamics” (DMD) method that is capable of describing the kinetics of the solute segregation process at the atomic level while operating on diffusive time scales in a computationally efficient way. We study static strain aging in the Al-Mg system and calculate the depinning shear stress between edge and screw dislocations and their solute atmospheres formed for various waiting times with different solute content and for a range of temperatures. A simple phenomenological model is also proposed that describes the observed behavior of the critical shear stress as a function of segregation level.

The exact mechanism responsible for the ferroelectricity in hexagonal manganite has been the subject of intense debate. Whether ferroelectricity and ferroelastic order appear at the same temperature and the role of covalency in the ferroelectric order are still discussed. High-temperature phase transitions are here investigated through the prism of lattice-dynamics evolution with temperature. Comparison is made between the $\mathrm{YMn}{\mathrm{O}}_3$ and $\mathrm{YbMn}{\mathrm{O}}_3$ behavior using polarized Raman spectroscopy. While $\mathrm{YMn}{\mathrm{O}}_3$ shows two phase transitions (isosymetric at about 900 K and ferroelastic at 1200 K) $\mathrm{YbMn}{\mathrm{O}}_3$ presents no lattice instability up to 1350 K. Phonons involved in the lattice instability and the zone tripling transition are identified. Moreover, peculiar hardening on heating of some $\mathrm{YbMn}{\mathrm{O}}_3$ phonons is revealed and discussed, which further highlights the dynamical difference between these two hexagonal manganites.

Thanks to a multiple-scattering theory algorithm, we present a way to focus energy at the deep subwavelength scale, from the far field, inside a cubic disordered bubble cloud by using broadband time reversal (TR). We show that the analytical calculation of an effective wavenumber performing the independant scattering approximation (ISA) matches the numerical results for the focal extension. Subwavelength focusings of $\lambda /100$ are reported for simulations with perfect bubbles (no loss). A more realistic case, with viscous and thermal losses, allows us to obtain a $\lambda /14$ focal spot, with a low volume fraction of scatterers $\left(\mathrm{\Phi }={10}^{-2}\right)$. Bubbly materials could open a new perspective for acoustic actuation in the microfluidic context.

Refined infrared magnetotransmission experiments have been performed in magnetic fields $B$ up to 35 T on a series of multilayer epitaxial graphene samples. Following the main optical transition involving the $n=0$ Landau level (LL), we observe a new absorption transition increasing in intensity with magnetic fields $B\ge 26$ T. Our analysis shows that this is a signature of the breaking of the SU(4) symmetry of the $n=0$ LL. Using a quantitative model, we show that the only symmetry-breaking scheme consistent with our experiments is a charge density wave (CDW).

Accurate models of carrier transport are essential for describing the electronic properties of semiconductor materials. To the best of our knowledge, the current models following the framework of the Boltzmann transport equation (BTE) either rely heavily on experimental data (i.e., semiempirical), or utilize simplifying assumptions, such as the constant relaxation time approximation (BTE-cRTA). While these models offer valuable physical insights and accurate calculations of transport properties in some cases, they often lack sufficient accuracy—particularly in capturing the correct trends with temperature and carrier concentration. We present here a transport model for calculating low-field electrical drift mobility and Seebeck coefficient of $n$-type semiconductors, by explicitly considering relevant physical phenomena (i.e., elastic and inelastic scattering mechanisms). We first rewrite expressions for the rates of elastic scattering mechanisms, in terms of ab initio properties, such as the band structure, density of states, and polar optical phonon frequency. We then solve the linear BTE to obtain the perturbation to the electron distribution—resulting from the dominant scattering mechanisms—and use this to calculate the overall mobility and Seebeck coefficient. Therefore, we have developed an ab initio model for calculating mobility and Seebeck coefficient using the Boltzmann transport (aMoBT) equation. Using aMoBT, we accurately calculate electrical transport properties of the compound $n$-type semiconductors, GaAs and InN, over various ranges of temperature and carrier concentration. aMoBT is fully predictive and provides high accuracy when compared to experimental measurements on both GaAs and InN, and vastly outperforms both semiempirical models and the BTE-cRTA. Therefore, we assert that this approach represents a first step towards a fully ab initio carrier transport model that is valid in all compound semiconductors.

We have studied the Pr- and La-doping effects on the magnetic anisotropy in the antiferro-magnetic (AFM) phase of ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$. The crystalline electric field (CEF) splitting in ${\mathrm{PrRu}}_2{\mathrm{Al}}_{10}$ was found to be as large as $\sim 800$ K with a singlet ground state. In ${\mathrm{Ce}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$, the CEF level scheme of the Pr ion is not changed with $x$. The AFM moment ($m_{\mathrm{AF}}$) is rotated from $c$ to $b$ axis in both systems at $x_c^{\mathrm{sr}}\sim 0.03$ and $\sim 0.07$ for Ln=Pr and La, respectively. As the ionic radius of La and Pr is larger and smaller than that of Ce, respectively, these results indicate that the chemical pressure effect is not associated with the rotation of $m_{\mathrm{AF}}$, but is caused by the suppression of the $c-f$ hybridization originating from the decrease of $4f$ electrons of Ce ions by Ce-site substitution. Since a small amount of Pr or La doping changes easily the magnetization easy axis of all the moments on Ce sites, the origin of the magnetic anisotropy is not the local single ion effect but the bandlike effect through the anisotropic $c-f$ hybridization. The magnetic phase diagrams of ${\mathrm{Ce}}_{1-x}{\mathrm{Ln}}_x{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ indicate that above $x_c^{\mathrm{sr}}$, the AFM order with $m_{\mathrm{AF}}\parallel b$ continues to exist up to $x_c$, which is $\sim 0.4$ and $\sim 0.6$ in Ln=Pr and Ln=La, respectively. This indicates that even in the sample with an AFM transition temperature ($T_0$) near $x_c$, the anisotropic $c-f$ hybridization dominates the AFM order. A large positive transverse magnetoresistance is seen below $T_0$, but a very small one above $T_0$. Together with the results of Hall resistivity and the observation of Shubnikov–de Haas oscillation, we propose that there exist large Fermi surfaces above $T_0$ and small ones below $T_0$. A gap is opened by the AFM order on almost the area of the large Fermi surface, and small Fermi surfaces are constructed below $T_0$, although we do not know the mechanism, which might be specific to the AFM order in Kondo semiconductors. The largest suppression of the magnetic scattering below $T_0$ is observed for the current $I\parallel a$ and the smallest one for $I\parallel b$. This anisotropy may be associated with the anisotropic $c-f$ hybridization, which may contribute to the anisotropic magnetic scattering of the conduction electron below $T_0$.

We propose and study the charge transport through single and double quantum point contacts setup between helical Majorana modes and an interacting helical Luttinger liquid. We show that the differential conductance decreases for stronger repulsive interactions and that the point contacts become insulating above a critical interaction strength. For a single-point contact, the differential conductance as a function of bias voltage shows a series of peaks due to Andreev reflection of electrons in the Majorana modes. In the case of two point contacts, interference phenomena make the structure of the individual resonance peaks less universal and show modulations with different separation distance between the contacts. For small separation distance, the overall features remain similar to the case of a single-point contact.

This study combines surface-sensitive photoemission experiments with density functional theory to give a microscopic description of H-adsorption-induced modifications of the $\mathrm{ZnO}\left(10\overline{1}0\right)$ surface electronic structure. We find a complex adsorption behavior caused by a strong coverage dependence of the H adsorption energies: Initially, O-H bond formation is energetically favorable and H acting as an electron donor leads to the formation of a charge accumulation layer and to surface metallization. The increase of the number of O-H bonds leads to a reversal in adsorption energies such that Zn-H bonds become favored at sites close to existing O-H bonds, which results in a gradual extenuation of the metallization. The corresponding surface potential changes are localized within a few nanometers both laterally and normal to the surface. This localized character is experimentally corroborated by using subsurface bound excitons at the $\mathrm{ZnO}\left(10\overline{1}0\right)$ surface as a local probe. The pronounced and comparably localized effect of small amounts of hydrogen at this surface strongly suggests metallic character of ZnO surfaces under technologically relevant conditions and may, thus, be of high importance for energy level alignment at ZnO-based junctions in general.

A mean-field theory formalism is employed to analyze the nonlocal plasmon dispersion relation of monolayer graphene which is Coulomb coupled to a thick conductor. We calculate numerically the undamped plasmon excitation spectrum for arbitrary wave number. For gapped graphene, both the low-frequency (acoustic) and high-frequency (surface) plasmons may lie within an undamped opening in the particle-hole region. Furthermore, we find undamped plasmon excitations in a region of frequency-wave vector space which has no counterpart for free-standing gapped graphene.

The Majorana phase boundaries in planar 2D hybrid (semiconductor-superconductor) nanowires are modified by orbital effects due to off plane magnetic components. We show that Majorana zero modes survive sizable vertical field tiltings, uncovering a remarkable phase diagram. Analytical expressions of the phase boundaries are given for the strong orbital limit. These phase boundaries can be fulfilled with attainable setups, such as an InAs nanowire of $150\mathrm{nm}$ in transverse width.

We study the effect of interactions on the properties of a model 2D topological Kondo insulator phase. Loosely motivated by recent proposals where graphene is hybridized with impurity bands from heavy adatoms with partially filled $d$ shells, we introduce a model Hamiltonian which we believe captures the essential physics of the different competing phases. We show that there are generically three possible phases with different combinations of Kondo screening and magnetic order. Perhaps the most dramatic example of many-body physics in symmetry-protected topological phases is the existence of the exotic edge states. We demonstrate that our mean-field model contains a region with a time-reversal-invariant bulk phase but where time-reversal symmetry is spontaneously broken at the edge. Such a phase would not be possible in a noninteracting model. We also comment on the stability of this phase beyond mean-field theory.

We present an analytical nonlocal effective medium approximation to describe the optical nonlocal effects in metallic nanorod metamaterials based on Mie scattering theory. It is shown that the developed nonlocal effective medium theory can predict a coexistence state of two modes around the epsilon-near-zero region, where strong optical nonlocal effects lead to the behavior of both positive refraction and negative refraction in the nanorod metamaterials. Outside of the coexistence region, only one mode can be excited and its behavior can be well described using the local effective medium theory.

The interaction of atomic hydrogen and ammonia with as-grown N-polar InN surfaces is investigated using in situ photoelectron spectroscopy. Changes in the surface electronic properties, including the band alignment and work function, as well as the chemical bonding states of the substrate and adsorbates are characterized. Ammonia molecules are dissociating at the InN surface, resulting in adsorption of hydrogen species. Consequently, the considerable changes of the chemical and electronic properties of the InN surface during ammonia interaction are almost identical to those found for adsorption of atomic hydrogen. In both cases, hydrogen atoms preferentially bond to surface nitrogen atoms, resulting in the disappearance of the nitrogen dangling-bond-related occupied surface state close to the valence band edge at $\sim 1.6$ eV binding energy and the formation of new occupied electron states at the conduction band edge. Furthermore, a decrease in work function during adsorption from 4.7 to 3.7–3.8 eV, as well as an increase in the surface downward band bending by 0.3 eV, confirm that hydrogen is acting as electron donor at InN surfaces and therefore has to be considered as one main reason for the surface electron accumulation observed at N-polar InN samples exposed to ambient conditions, for example as the dissociation product of molecules. The measured formation and occupation of electronic states above the conduction band minimum occur in conjunction with the observed increase in surface electron concentration and underline the relationship between the energy position of occupied electron states and surface band alignment for InN as a small-band-gap semiconductor.

We have theoretically analyzed coherent nuclear-spin dynamics induced by electron transport through a quantum-dot spin valve. The hyperfine interaction between electron and nuclear spins in a quantum dot allows for the transfer of angular momentum from spin-polarized electrons injected from ferromagnetic or half-metal leads to the nuclear spin system under a finite voltage bias. Accounting for a local nuclear-spin dephasing process prevents the system from becoming stuck in collective dark states, allowing a large nuclear polarization to be built up in the long-time limit. After reaching a steady state, reversing the voltage bias induces a transient current response as the nuclear polarization is reversed. Long-range nuclear-spin coherence leads to a strong enhancement of spin-flip transition rates (by an amount proportional to the number of nuclear spins) and is revealed by an intense current burst, analogous to superradiant light emission. The crossover to a regime with incoherent spin flips occurs on a relatively long-time scale, on the order of the single-nuclear-spin dephasing time, which can be much longer than the time scale for the superradiant current burst. This conclusion is confirmed through a general master equation. For the two limiting regimes (coherent/incoherent spin flips), the general master equation recovers our simpler treatment based on rate equations, but is also applicable at intermediate dephasing. Throughout this work, we assume uniform hyperfine couplings, which yield the strongest coherent enhancement. We propose realistic strategies, based on isotopic modulation and wave-function engineering in core-shell nanowires, to realize this analytically solvable “box-model” of hyperfine couplings.

We improvise an approach to carry out first-principles simulations of graphene-based vertical field-effect tunneling transistors that consist of a graphene$|h-\mathrm{BN}|$graphene multilayer structure. Within the density functional theory framework, we exploit the effective screening medium (ESM) method to properly treat boundary conditions for electrostatic potentials and investigate the effect of gate voltage. The distribution of free carriers and the band structure of both top and bottom graphene layers are calculated self-consistently. The dielectric properties of $h\text{-BN}$ thin films sandwiched between graphene layers are computed layer-by-layer following the theory of microscopic permittivity. We find that the permittivities of BN layers are very close to that of crystalline $h\text{-BN}$. The effect of interface with graphene on the dielectric properties of $h\text{-BN}$ is weak according to an analysis on the interface charge redistribution.

Voltage bistability is demonstrated at low temperature in a biased unipolar GaAs/AlGaAs superlattice, using a design based on tunneling from a charge reservoir into a Wannier-Stark ladder and competition between nearest and next-nearest neighbor coherent injection channels. The static conduction characteristics are accounted for by a density matrix approach of coherent injection in an inhomogeneous three-level system with resistive load. Electrostatic retroaction on the energy levels is shown to provide an adequate nonlinear feedback mechanism for bistability, as well as for other instability regimes including sustained Bloch oscillation above a retroaction threshold.

$\text{Ca}{\left(\text{OH}\right)}_2$ crystals, well known as portlandite, are grown in layered form, and we found that they can be exfoliated on different substrates. We performed first principles calculations to investigate the structural, electronic, vibrational, and mechanical properties of bulk, bilayer, and monolayer structures of this material. Different from other lamellar structures such as graphite and transition-metal dichalcogenides, intralayer bonding in $\text{Ca}{\left(\text{OH}\right)}_2$ is mainly ionic, while the interlayer interaction remains a weak dispersion-type force. Unlike well-known transition-metal dichalcogenides that exhibit an indirect-to-direct band gap crossover when going from bulk to a single layer, $\text{Ca}{\left(\text{OH}\right)}_2$ is a direct band gap semiconductor independent of the number layers. The in-plane Young's modulus and the in-plane shear modulus of monolayer $\text{Ca}{\left(\text{OH}\right)}_2$ are predicted to be quite low while the in-plane Poisson ratio is larger in comparison to those in the monolayer of ionic crystal BN. We measured the Raman spectrum of bulk $\text{Ca}{\left(\text{OH}\right)}_2$ and identified the high-frequency OH stretching mode $A_{1g}$ at $3620{\mathrm{cm}}^{-1}$. In this study, bilayer and monolayer portlandite [$\text{Ca}{\left(\text{OH}\right)}_2$] are predicted to be stable and their characteristics are analyzed in detail. Our results can guide further research on ultrathin hydroxites.