We investigate the sensitive interplay between magnetic, electronic, and structural properties in the ferrimagnetic oxide ${\text{NiFe}}_2{\text{O}}_4$. Emphasis is placed on the impact of reduced dimensionality in the crossover from bulk-like to ultrathin films. We observed an enhanced saturation magnetization $M_S$ for ultrathin ${\text{NiFe}}_2{\text{O}}_4$ films on ${\text{Nb-SrTiO}}_3$ (001) substrates that co-occurs with a reduced out-of-plane lattice constant under compressive in-plane epitaxial strain. We found a bulk-like cationic coordination of the inverse spinel lattice independent of the ${\text{NiFe}}_2{\text{O}}_4$ film thickness, thus ruling out a cationic inversion that nominally could account for an enhanced $M_S$. Moreover, our study instead uncovers a reduction of the unit cell volume, i.e., an auxetic behavior in ultrathin ${\text{NiFe}}_2{\text{O}}_4$ films.

Layered thin films of $\mathrm{Pb}\left(\mathrm{Z}{{\mathrm{r}}_1}_{-x}\mathrm{T}{\mathrm{i}}_x\right){\mathrm{O}}_3$ (PZT) and $\mathrm{L}{\mathrm{a}}_x\mathrm{S}{{\mathrm{r}}_1}_{-x}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) are well-known multiferroic systems that show promise for numerous applications including data storage devices and spintronics. In this work, structure-property relationships are explored in novel $\mathrm{PZT}/\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ (CFO)/LSMO heterostructures with optimized ferroic properties. High quality, epitaxial PZT/CFO/LSMO heterostructures with the thickness of the CFO layer varying from 0 to 50 nm were grown on $\mathrm{SrTi}{\mathrm{O}}_3$ (100) substrates using an optimized pulsed laser deposition technique. An ultrathin (10–20 nm) CFO layer was found to simultaneously improve the ferromagnetic and ferroelectric (FE) characteristics of the system through distinct mechanisms. The increase in magnetization and magnetic coercivity in the CFO-containing samples was associated with a tetragonal distortion of the CFO lattice under epitaxial strain, while perpendicular anisotropy generated by the distortion stabilized an out-of-plane orientation of the easy axis of magnetization in the thinnest CFO layers. Trapped charge at the CFO/PZT interface in PZT/CFO/LSMO induced an internal built-in field in the heterostructures, resulting in the accumulation of higher switched charges during voltage cycling and enhanced polarization in the samples over PZT/LSMO. An increase in electric coercivity was also observed in the CFO-containing heterostructures, and is discussed in terms of a dielectric/FE layered capacitor model. Above a critical thickness, ∼50 nm, the presence of a CFO layer has a negative effect on both magnetization and polarization in PZT/CFO/LSMO as compared to PZT/LSMO.

Energies of arbitrary small- and large-angle noncollinear excited magnetic configurations are computed using a highly accurate constrained density functional theory approach. Numerical convergence and accuracy are controlled by the choice of Lagrange multipliers ${\lambda }_I$ entering the constraining conditions. The penalty part $E_p$ of the constrained energy functional at its minimum is shown to be inversely proportional to ${\lambda }_I$, enabling a simple, robust, and accurate iterative procedure to be followed to find a convergent solution. The method is implemented as a part of ab initio vasp package, and applied to the investigation of noncollinear B2-like and $\langle 001\rangle$ double-layer antiferromagnetic configurations of bcc iron, ${\mathrm{Fe}}_2$ dimer, and amorphous iron. Forces acting on atoms depend on the orientations of magnetic moments, and the proposed approach enables constrained self-consistent noncollinear magnetic and structural relaxation of large atomic systems to be carried out.

We show that optically pumped semiconductors can exhibit superconductivity. We illustrate this phenomenon in the case of a two-band semiconductor tunnel-coupled to broad-band reservoirs and driven by a continuous wave laser. More realistically, we also show that superconductivity can be induced in a two-band semiconductor interacting with a broad-spectrum light source. We furthermore discuss the case of a three-band model in which the middle band replaces the broad-band reservoirs as the source of dissipation. In all three cases, we derive the simple conditions on the band structure, electron-electron interaction, and hybridization to the reservoirs that enable superconductivity. We compute the finite superconducting pairing and argue that the mechanism can be induced through both attractive and repulsive interactions and is robust to high temperatures.

A chain of magnetic atoms with noncollinear spin configuration on a superconductor is a promising new system that can host Majorana fermions (MFs). In this study, we clarify that in the presence of MFs, an odd-frequency Cooper pair is generated at the edge of the chain. Furthermore, it is revealed that this feature is robust against the distance between magnetic atoms as far as this distance is shorter than the coherence length of the superconductor. We also elucidate the close relationship between the pair amplitude of the odd-frequency pair and the direction of the MF spin. If Rashba-type spin-orbit coupling is included, MFs can be realized even in a collinear alignment of magnetic atoms, i.e., in a ferromagnetic or an antiferromagnetic chain on a superconductor. Then, the odd-frequency pairing is generated at the edge, similar to the noncollinear case. Based on our results, it can be concluded that the detection of the zero-energy peak of the local density of states by scanning tunneling microscopy at the edge of the magnetic chain is strong evidence for the generation of odd-frequency pairing.

The superconducting and normal-state properties of ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2 \left(0.1\le x\le 0.9\right)$ have been studied via electrical resistivity, magnetic susceptibility, and specific heat measurements. By using suitable synthesis conditions, Sm exhibits considerable solubility into the ${\mathrm{CeOBiS}}_2$-type ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ lattice. In addition to a considerable enhancement of the superconducting volume fraction, it is found that the superconducting transition temperature $T_c$ is dramatically enhanced with increasing Sm concentration to 5.4 K at $x=0.8$. These results suggest that $T_c$ for ${\mathrm{SmO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ could be as high as $\sim 6.2$ K, and comparably high $T_c$ values might also be obtained in the compounds $Ln{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2 \left(Ln=\text{Eu}-\text{Tm}\right)$ if they can be synthesized.

We report a study of the local magnetism in thin films of ${\mathrm{SrRuO}}_3$ grown on (111) and (001) oriented ${\mathrm{SrTiO}}_3$ substrates using x-ray magnetic circular dichroism spectroscopy (XMCD) at the Ru-$L_{2,3}$ edges. The application of the sum rules to the XMCD data gives an almost quenched orbital moment and a spin moment close to the value expected for the low spin state $S=1$. Full-multiplet cluster calculations indicate that the low spin state is quite stable and suggest that the occurrence of a transition to the high spin state $S=2$ in strained thin films of ${\mathrm{SrRuO}}_3$ is unlikely as it would be too expensive in energy.

We report on a theoretical study of the electronic, optical, and magneto-optical properties of digital ferromagnetic heterostructures based on Mn $\delta$-doped GaAs. We consider different structures corresponding to Mn contents within the range 12%–50% and we study how the system changes as a function of the doping concentration. Our first-principles approach includes the spin-orbit interaction in a fully relativistic pseudopotential scheme and the local-field effect in the description of the optical absorption. We show that Mn $\delta$-doped GaAs shares many properties with the uniformly doped $\mathrm{Ga}{}_{1-x}\mathrm{Mn}{}_x\mathrm{As}$ system, i.e., half-metallicity, similar absorption spectra, and moderate Kerr rotation angles in the visible spectral region.

We address the properties of excitons in monolayer ${\mathrm{MoS}}_2$ from a theoretical point of view, showing that low-energy excitonic states occur both at the Brillouin-zone center and at the Brillouin-zone corners, that binding energies at the Brillouin-zone center deviate strongly from the ${(n-1/2)}^{-2}$ pattern of the two-dimensional hydrogenic model, and that the valley-degenerate exciton doublet at the Brillouin-zone center splits at finite momentum into an upper mode with nonanalytic linear dispersion and a lower mode with quadratic dispersion. Although monolayer ${\mathrm{MoS}}_2$ is a direct-gap semiconductor when classified by its quasiparticle band structure, it may well be an indirect gap material when classified by its excitation spectra.

We settle a general expression for the Hamiltonian of the electron-zone-center optical phonon deformation potential (DP) interaction in the case of nonpolar core-shell cylindrical nanowires (NWs). On the basis of a long-range phenomenological continuum model for the optical modes and by taking into account the bulk phonon dispersions, we study the size dependence and strain-induced shift on the electron-phonon coupling strengths for Ge-Si and Si-Ge NWs. We derive analytically the DP electron-phonon Hamiltonian and report some numerical results for the frequency core modes and vibrational amplitudes. Our approach allows for the unambiguous identification of the strain and confinement effects on the optical phonons at the $\Gamma$ point. We explore the dependence of mode frequencies, phonon amplitudes, and hole-DP scattering rate on the spatial symmetry and the structural parameters of these core-shell structures, which constitute a basic tool for the characterization and device applications of these novel nanosystems.

Terahertz magnetospectroscopy experiments on high quality CdTe/CdMgTe quantum wells were carried out at low temperatures and high magnetic fields. Samples of two different geometries were considered: a large-area grid-gated sample and a split-gate quantum point contact (QPC). The spectra show features originating from a cyclotron resonance transition and magnetoplasmon excitations. Depending on the sample geometry, plasmons characterized by an effective dielectric function of a mixed gated/ungated (in grid-gated samples) or ungated type (in the QPC) were excited. In each case, the resulting plasmon dispersion relation was determined and we show that it can be precisely described within a theory based on a local approximation of a high-frequency magnetoconductivity tensor of a two-dimensional electron gas by taking into account a polaron effect and plasmon-longitudinal optical (LO) phonon interaction.

We derive the local density of states from itinerant and boundary states around transport barriers and edges in graphene and show that the itinerant states lead to mesoscale undulations that could be used to probe their scattering properties in equilibrium without the need for lateral transport measurements. This finding will facilitate vetting of extended structural defects, such as grain boundaries or line defects as transport barriers for switchable graphene resonant tunneling transistors. We also show that barriers could exhibit double minima and that the charge density away from highly reflective barriers and edges scales as $x^{-2}$.

Measurements and calculations have shown significant disagreement regarding the sign and temperature variations of the thermal expansion coefficient (TEC) of graphene $\alpha \left(T\right)$. Here we report dedicated Raman scattering experiments conducted for graphene monolayers deposited on silicon nitride substrates and over a broad temperature range extending over 150–800 K. The relation between those measurements for the $G$ band and the graphene TEC, which involves correcting the measured signal from the mismatch contribution of the substrate, is analyzed based on different theoretical candidates for $\alpha \left(T\right)$. Contrary to calculations in the quasiharmonic approximation, a many-body potential reparametrized for graphene correctly reproduces experimental data, suggesting that the TEC is more likely to be positive above room temperature.

We predict that ${\mathrm{CaFeAs}}_2$, a newly discovered iron-based high-temperature $\left(T_c\right)$ superconductor, is a staggered intercalation compound that integrates topological quantum spin Hall (QSH) and superconductivity (SC). ${\mathrm{CaFeAs}}_2$ has a structure with staggered CaAs and FeAs layers. While the FeAs layers are known to be responsible for high $T_c$ superconductivity, we show that with spin orbital coupling each CaAs layer is a $Z_2$ topologically nontrivial two-dimensional QSH insulator and the bulk is a three-dimensional weak topological insulator. In the superconducting state, the edge states in the CaAs layer are natural one-dimensional topological superconductors. The staggered intercalation of QSH and SC provides us a unique opportunity to realize and explore physics, such as Majorana modes and Majorana fermion chains.

We consider a quantum dot coupled to a topological superconductor and two normal leads and study the transport properties of the system. Using a Keldysh path-integral approach, we study current fluctuations (shot noise) within the low-energy effective theory. We argue that the combination of the tunneling conductance and the shot noise through a quantum dot allows one to distinguish between the topological (Majorana) and nontopological (e.g., Kondo) origin of the zero-bias conduction peak. Specifically, we show that, while the tunneling conductance might exhibit a zero-bias anomaly due to Majorana or Kondo physics, the shot noise is qualitatively different in the presence of Majorana zero modes.

We study quantum evolution of the mutual information of a quantum dot connected to left and right leads initially maintained at chemical potentials ${\mu }_L$ and ${\mu }_R$, respectively, within the noninteracting resonant-level model. The full nonequilirbium mixed state density matrix of the whole system is written down exactly, and the mutual information of the dot with respect to the leads is computed. A strong and direct correlation is found between the Landauer current and the mutual information at all times, the steady-state values in particular displaying a quadratic relationship at high temperatures. Strikingly, it is found that one can obtain a maximal mutual information by simply applying a sufficiently large “source-drain” voltage $V_{\mathrm{SD}}$ even at high temperatures.

We present a detailed analysis and understanding of Kelvin probe force microscopy (KPFM) data for a system of point charges in a vacuum-dielectric tip-sample system. Explicit formulae describing the KPFM signal $\Delta V$ are derived for the two KPFM operation modes, namely amplitude modulation and frequency modulation (FM). The formulae allow for a physical interpretation of the resulting KPFM signal, reveal contributing parameters, and especially disclose an additive behavior. We numerically evaluate these equations and show exemplary KPFM slice data for a single point charge. The theoretical analysis is complemented by two-dimensional FM-KPFM maps obtained experimentally on 2,5-dihydrobenzoic acid on calcite $(10.4)$. The molecules assemble in two coexisting phases understood as protonated and deprotonated molecules. The two-dimensional maps reveal a convex shape of the KPFM signal $\Delta V$ across the islands and a clear difference between the two phases is revealed. Furthermore, we apply the theoretically suggested difference strategy to extract the molecular component from the measured total KPFM signal.

Many-body localization provides a generic mechanism of ergodicity breaking in quantum systems. In contrast to conventional ergodic systems, many-body-localized (MBL) systems are characterized by extensively many local integrals of motion (LIOM), which underlie the absence of transport and thermalization in these systems. Here we report a physically motivated construction of local integrals of motion in the MBL phase. We show that any local operator (e.g., a local particle number or a spin-flip operator), evolved with the system's Hamiltonian and averaged over time, becomes a LIOM in the MBL phase. Such operators have a clear physical meaning, describing the response of the MBL system to a local perturbation. In particular, when a local operator represents a density of some globally conserved quantity, the corresponding LIOM describes how this conserved quantity propagates through the MBL phase. Being uniquely defined and experimentally measurable, these LIOMs provide a natural tool for characterizing the properties of the MBL phase, in both experiments and numerical simulations. We demonstrate the latter by numerically constructing an extensive set of LIOMs in the MBL phase of a disordered spin-chain model. We show that the resulting LIOMs are quasilocal and use their decay to extract the localization length and establish the location of the transition between the MBL and ergodic phases.