The effects of normal compressive strain and in-plane strain on the structures of phosphorene have been investigated by using first-principles calculations. It is quite intriguing to find that a structural transformation from pristine Z-phosphorene to a new A-phosphorene occurs under the normal compressive strain of $\varepsilon =48\%$ or the anisotropic biaxial in-plane strain of ${\varepsilon }_{\mathrm{x}}=-16\%$ and ${\varepsilon }_{\mathrm{y}}=54\%$. In the extreme case where the pucker structure is flattened into a plane, the phosphorene structure is quite unstable at finite temperatures, transforming into another new H-phosphorene phase. The anisotropic structure of A-phosphorene gives rise to its direction-dependent mechanical properties whereas H-phosphorene exhibits isotropic mechanical properties. Both A-phosphorene and H-phosphorene are semiconductors with indirect band gaps of about 0.42 and 1.94 eV that use the Perdew-Burke-Ernzerhof exchange-correlation functional, respectively. The electronic properties of the two new phases are found to be sensitive to the magnitude and direction of the applied strains, which offer an effective method to modulate them in future device engineering.

The structural description of even the most basic monatomic amorphous materials is under considerable debate. In this work, an intuitive computational technique has been developed to construct three-dimensional statistical density maps to directly visualize local atomic structure of amorphous germanium (a-Ge), enabling the interpretation of recent state-of-the-art experiments and simulations. The continuous random network (CRN) model is compared to our experimental model refined through a Reverse Monte Carlo routine. In this refinement a-Ge has two dominant structures: a fourfold coordinated tetrahedron and a buckled threefold coordinated local structure similar to silicene and germanene. These structures account for 95.7% of the total atoms in a 5:2 ratio respectively. Our method shows well defined structural ordering in the second shell of a-Ge. This novel visualization tool enables the interpretation of complex disordered materials and reveals the bimodal structures of a-Ge.

The motions of micrometer-sized solid deuterium particles in liquid ${}^4\mathrm{He}$, at temperatures between approximately 1.2 and 3 K, are visualized in the proximity of an oscillating cylinder of rectangular cross section (3 mm high and 10 mm wide). The cylinder is oscillating vertically, perpendicularly to its cross-section width, at frequencies between 0.05 and 1.25 Hz, and amplitudes of 5 and 10 mm, resulting in Reynolds numbers $Re$ up to ${10}^5$. The aim of the reported experiments is to investigate systematically the macroscopic vortical structures shed at the cylinder sharp edges, by tracking the deuterium particles. We find that large-scale, millimeter-sized vortices are generated in the surrounding fluid by the oscillating cylinder, both in viscous He I and superfluid He II. An estimate of the strength of the shed vortical structures reveals that, for $Re>{10}^4$, the corresponding magnitudes are approximately equal in He I and He II if, in He II, the kinematic viscosity is suitably defined. For $Re<{10}^4$, the strength of the large-scale vortices is smaller in He II than in He I. Although the outcome is partly affected by the larger scatter of the He I data and possibly also by the much larger heat conductivity of superfluid ${}^4\mathrm{He}$, we argue that the fundamental physical reason for observing this difference is that, at these Reynolds numbers, the experimentally probed length scales in He II are smaller than the average distance between quantized vortices—the quantum length scale of the flow. The result strongly suggests that, similarly to thermal counterflow, both viscous and quantum features can be observed in mechanically driven flows of He II, depending on the length scales at which the quantum flow is probed.

Spin-dependent electronic transport through multiferroic $\mathrm{Co}/{\mathrm{PbTiO}}_3/\mathrm{Co}$ tunnel junctions is studied theoretically in view of the recent observation of an inverse TMR in $\mathrm{Co}/{\mathrm{PbTiO}}_3/{\mathrm{LaSrMnO}}_3$ heterostructures. Conductances calculated within the Landauer-Büttiker formalism yield a four-conductance state characterized by sizable positive tunnel magnetoresistances (TMR) and tunnel electroresistances (TER). The conductances depend crucially on the details of the electronic structure at the interfaces. In particular, the spin polarization of the tunneling electronic states is affected by the hybridization of orbitals and the associated charge transfer at both interfaces. Digital doping of the ${\mathrm{PbTiO}}_3$ barrier with Zr impurities at the ${\mathrm{TiO}}_2/{\mathrm{Co}}_2$ interface removes the excessive metalization of the barrier and significantly enhances the TMR but is not sufficient to switch the TMR's sign. Our results indicate that the origin of the TMR inversion might be attributed to the magnetoelectrically active ${\mathrm{LaSrMnO}}_3/{\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ interface.

We theoretically investigate the electronic properties of the interface between quantum spin Hall (QSH) and quantum anomalous Hall (QAH) insulators. A robust chiral gapless state, which substantially differs from edge states of QSH or QAH insulators, is predicted at the QSH/QAH interface using an effective Hamiltonian model. We systematically reveal distinctive properties of interface states between QSH and single-valley QAH, multivalley high-Chern-number QAH and valley-polarized QAH insulators based on tight-binding models using the interface Green's function method. As an example, first-principles calculations are conducted for the interface states between fully and semihydrogenated bismuth (111) thin films, verifying the existence of interface states in realistic material systems. Due to the physically protected junction structure, the interface state is expected to be more stable and insensitive than topological boundary states against edge defects and chemical decoration. Hence our results of the interface states provide a promising route towards enhancing the performance and stability of low-dissipation electronics in real environment.

The anisotropic optical properties of multiferroic ${\mathrm{BiFeO}}_3$ thin films have been determined with Mueller matrix ellipsometry at room temperature. The full dielectric tensors of tetragonal-like and rhombohedral-like ${\mathrm{BiFeO}}_3$ phases epitaxially grown on ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ single crystal substrates, respectively, within the spectral range of 0.6 and 6.5 eV are reported. Strain-driven anisotropy changes and transition shifts are observed as well as evidence of sub-band gap many-particle excitations are found. The transition shifts, mostly to higher energies for the highly strained tetragonal-like ${\mathrm{BiFeO}}_3$ phase on ${\mathrm{LaAlO}}_3$, are indicative of band structure differences. Additionally, optical modeling, confirmed by piezoelectric force microscopy studies, revealed that the average polarization direction of bivariant ${\mathrm{BiFeO}}_3$ on ${\mathrm{LaAlO}}_3$ is not parallel to the crystallographic [001] direction but tilted by about $7^{\circ }$. Spectral weight analyses reveal phase-dependent differences, underlining that theoretical calculations of optical spectra need further improvement to appropriately account for electronic and excitonic correlations to fully understand multiferroic ${\mathrm{BiFeO}}_3$.

Injecting spin-polarized carriers into semiconductor lasers provides important opportunities to extend what is known about spintronic devices, as well as to overcome many limitations of conventional (spin-unpolarized) lasers. By developing a microscopic model of spin-dependent optical gain derived from an accurate electronic structure in a quantum-well-based laser, we study how its operation properties can be modified by spin-polarized carriers, carrier density, and resonant cavity design. We reveal that by applying a uniaxial strain, it is possible to attain a large birefringence. While such birefringence is viewed as detrimental in conventional lasers, it could enable fast polarization oscillations of the emitted light in spin lasers, which can be exploited for optical communication and high-performance interconnects. The resulting oscillation frequency ($>200$ GHz) would significantly exceed the frequency range possible in conventional lasers.

Conductive atomic force microscopy (CAFM) is employed to investigate the current injection from a nanometric contact (a Pt coated tip) to the surface of ${\text{MoS}}_2$ thin films. The analysis of local current-voltage characteristics on a large array of tip positions provides high spatial resolution information on the lateral homogeneity of the $\mathrm{tip}/{\text{MoS}}_2$ Schottky barrier ${\mathrm{\Phi }}_B$ and ideality factor $n$, and on the local resistivity ${\rho }_{\text{loc}}$ of the ${\text{MoS}}_2$ region under the tip. Here, ${\mathrm{\Phi }}_B=300\pm 24\text{meV}, n=1.60\pm 0.23$, and ${\rho }_{\text{loc}}=2.99\pm 0.68\mathrm{\Omega }\text{cm}$ are calculated from the distributions of locally measured values. A linear correlation is found between the ${\rho }_{\text{loc}}$ and ${\mathrm{\Phi }}_B$ values at each tip position, indicating a similar origin of the ${\rho }_{\text{loc}}$ and ${\mathrm{\Phi }}_B$ inhomogeneities. These findings are compared with recent literature results on the role of sulfur vacancy clusters on the ${\text{MoS}}_2$ surface as preferential paths for current injection from metal contacts. Furthermore, their implications on the behavior of ${\text{MoS}}_2$ based transistors are discussed.

We study the exchange interaction and the subsequent collective behavior of magnetic impurities embedded in a disordered two-dimensional helical metal. The exchange coupling follows a statistical distribution whose moments are calculated to the lowest order in ${\left(p_F\ell \right)}^{-1}$, where $p_F$ is the Fermi momentum of itinerant electrons and $\ell$ is the mean free path. We find that (i) the first moment of the distribution decays exponentially, and (ii) the variance of the interaction is long range, however, it becomes independent of the orientation of the localized magnetic moments due to the locking between spin and momentum of the electrons that mediate the interaction. As a consequence, long-range magnetic order tends to be suppressed, and a spin glass phase emerges. The formalism is applied to the surface states of a three-dimensional topological insulator. The lack of a net magnetic moment in the glassy phase and the full randomization of spin polarization at distances larger than $\ell$ excludes a spectral gap for surface states. Hence, nonmagnetic disorder may explain the dispersion in results for photoemission experiments in magnetically doped topological insulators.

Over the past few years, bulk pyrochlore iridates of the form $A_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ (where $A$ is a rare earth element, Ir is iridium, and O is oxygen) have been studied as model systems for investigating the interplay of electronic correlations and strong spin-orbit coupling, particularly with the aim of finding correlation-driven topological phases. In this work, we use cellular dynamical mean field theory (CDMFT) to study effects of electronic correlations beyond Hartree-Fock theory in thin films of pyrochlore irradiates grown along the $\left[111\right]$ direction. We focus on the bilayer and trilayer systems, and compute the phase diagrams of these systems as a function of electron-electron interaction strength, which is modeled by an onsite Hubbard interaction. By evaluating the $Z_2$ invariant and Chern number using formulas based on the single-particle Green's function and the quasiparticle effective Hamiltonian, we show that onsite correlations can drive an interaction-induced topological phase transition, turning a time-reversal-invariant topological insulator and a nearly flat band metal to a correlated Chern insulator (CI) in bilayer and trilayer systems, respectively. By comparing with the Hartree-Fock results, the CDMFT results show that quantum fluctuations enhance the robustness of the interaction-driven CI phase in the thin films. Furthermore, our numerical analysis of the quasiparticle spectrum reveals that the topological phases we find in our many-body calculations are adiabatically connected to those in the single-particle picture.

The In/Si(111) system reveals an anisotropy in the electrical conductivity and is a prototype system for atomic wires on surfaces. We use this system to study and tune the interwire interaction by adsorption of oxygen. Through rotational square four-tip transport measurements, both the parallel $\left({\sigma }_{\left|\right|}\right)$ and perpendicular $\left({\sigma }_{\perp }\right)$ components are measured separately. The analysis of the I(V) curves reveals that ${\sigma }_{\perp }$ is also affected by adsorption of oxygen, showing clearly an effective interwire coupling, in agreement with density-functional-theory-based calculations of the transmittance. In addition to these surface-state mediated transport channels, we confirm the existence of conducting parasitic space-charge layer channels and address the importance of substrate steps by performing the transport measurements of In phases grown on Si(111) mesa structures.

Two-dimensional (2D) topological insulators (TIs) hold promise for applications in spintronics based on the fact that the propagation direction of an edge electronic state of a 2D TI is locked to its spin orientation. Here, using first-principles calculations, we predict a family of robust 2D TIs in monolayer square transition-metal dichalcogenides $M{X}_2(M=\mathrm{Mo},\mathrm{W};X=\mathrm{S},\mathrm{Se},\mathrm{Te})$, which show sizeable intrinsic nontrivial band gaps ranged from 24 to 187 meV, thus ensuring the quantum spin Hall (QSH) effect at room temperature. Different from the most known 2D TIs with comparable band gaps, these sizeable energy gaps arise from the strong spin-orbit interaction related to $d$ electrons of the Mo/W atoms. A pair of topologically protected helical edge states emerges at the edge of these systems with a Dirac-type dispersion within the bulk band gap. The topologically nontrivial natures are confirmed by the nontrivial ${\mathrm{Z}}_2$-type topological invariant. More interestingly, with applied strain, a topological quantum phase transition between a QSH phase and a trivial insulating/metallic phase can be realized, and the corresponding topological phase diagram is well established.

In this work we aim to resolve the counterintuitive diffusion behavior of Fe and N atoms in an iron mononitride (FeN) thin film. It was observed that in spite of their larger size, Fe atoms tend to diffuse more rapidly than smaller N atoms. This only happens in the N-rich region of the Fe-N phase diagram; in N-poor regions, the N diffusion coefficient is orders of magnitude larger than that of Fe. Detailed self-diffusion measurements performed in FeN thin films reveal that the diffusion mechanism of Fe and N is different: Fe atoms diffuse through a complex process which, in addition to volume diffusion, is predominantly controlled by fast grain boundary diffusion. On the other hand, N atoms diffuse through a classical volume diffusion process. Observed results are explained in terms of stronger Fe-N (than Fe-Fe) bonds generally predicted theoretically for mononitride compositions of transition metals.

The hyperfine coupling between the spin of a charge carrier and the nuclear spin bath is a predominant channel for the carrier spin relaxation in many organic semiconductors. We theoretically investigate the hyperfine-induced spin relaxation of a carrier performing a random walk on a $d$-dimensional regular lattice, in a transport regime typical for organic semiconductors. We show that in $d=1$ and 2, the time dependence of the space-integrated spin polarization $P\left(t\right)$ is dominated by a superexponential decay, crossing over to a stretched-exponential tail at long times. The faster decay is attributed to multiple self-intersections (returns) of the random-walk trajectories, which occur more often in lower dimensions. We also show, analytically and numerically, that the returns lead to sensitivity of $P\left(t\right)$ to external electric and magnetic fields, and this sensitivity strongly depends on dimensionality of the system $(d=1$ versus $d=3)$. Furthermore, we investigate in detail the coordinate dependence of the time-integrated spin polarization $\sigma \left(\mathbf{r}\right)$, which can be probed in the spin-transport experiments with spin-polarized electrodes. We demonstrate that, while $\sigma \left(\mathbf{r}\right)$ is essentially exponential, the effect of multiple self-intersections can be identified in transport measurements from the strong dependence of the spin-decay length on the external magnetic and electric fields.

Fe-27 at.% Pt was aged at 1123 K and quenched to room temperature (RT) to set up a state of thermal equilibrium. The local atomic arrangement was studied by diffuse x-ray scattering above (at 427 K) and below (at RT) the Curie temperature as well as at RT under a saturating magnetic field. The separated short-range order scattering remained unchanged for all three states, with maxima at 100 positions. Effective pair interaction parameters determined by the inverse Monte Carlo method gave an order-disorder transition temperature of about 1088 K, close to direct experimental findings. The species-dependent static atomic displacements for the first two shells show large differences, with a strong increase in magnitude from the state at 427 K over RT to the state under saturating magnetic field. This outcome is in agreement with an increase in atomic volume of Fe with increasing local magnetic moment. Electronic-structure calculations closely reproduce the values for the static atomic displacements in the ferromagnetic state, and predict their dependence on the atomic configuration. They also reveal a strong dependence of the magnetic exchange interactions in Fe-Pt on the atomic configuration state and lattice parameter. In particular, the increase of the Curie temperature in a random state relative to that in the ordered one is demonstrated to be related to the corresponding change of the magnetic exchange interactions due to the different local atomic chemical environment. There exists a similar strong concentration dependence of the chemical interactions as in the case of magnetic exchange interactions. Theoretical effective interactions for Fe-27 at.% Pt alloy are in good agreement with experimental results, and they also reproduce well the ${\mathrm{L}1}_2$-A1 transition temperature.

${\mathrm{BiMnO}}_3$ (BMO), ferromagnetic (FM) below $T_c\simeq 100\mathrm{K}$, was believed to also be ferroelectric (FE) due to a noncentrosymmetric $C2$ structure, until diffraction data indicated that its space group is the centrosymmetric $C2/c$. Here we present infrared phonon spectra of BMO, taken on a mosaic of single crystals, which are consistent with $C2/c$ at any $T>10\mathrm{K}$, as well as room-temperature Raman data which strongly support this conclusion. We also find that the infrared intensity of several phonons increases steadily for $T\to 0$, causing the relative permittivity of BMO to vary from 18.5 at 300 K to 45 at 10 K. At variance with FE materials of the displacive type, no appreciable softening has been found in the infrared phonons. Both their frequencies and intensities, moreover, appear insensitive to the FM transition at $T_c$.

We propose a method to compute the transmission through correlated heterostructures by combining density functional and many-body dynamical mean field theories. The heart of this combination consists in porting the many-body self-energy from an all electron basis into a pseudopotential localized atomic basis set. Using this combination we study the effects of local electronic interactions and finite temperatures on the transmission across the ${\text{Cu}}_4\text{Co}{\text{Cu}}_4$ metallic heterostructure. It is shown that as the electronic correlations are taken into account via a local but dynamic self-energy, the total transmission at the Fermi level gets reduced (predominantly in the minority-spin channel), whereby the spin polarization of the transmission increases. The latter is due to a more significant $d$-electron contribution, as compared to the noncorrelated case in which the transport is dominated by $s$ and $p$ electrons.

We study the thermodynamics of the diffusive SFS trilayer composed of thin superconductor (S) and ferromagnet (F) layers. On the basis the self-consistent solutions of nonlinear Usadel equations in the F and S layers we obtain the Ginzburg–Landau expansion and compute the condensation free energy and entropy of the 0 (even) and $\pi$ (odd) order parameter configurations. The first order $0\text{–}\pi$ transition as a function of temperature $T$ occurs, which is responsible for a jump of the averaged magnetic field penetration depth $\lambda \left(T\right)$ recently observed in experiments [N. Pompeo et al., Phys. Rev. B 90, 064510 (2014)]. The generalized Ginzburg-Landau functional was proposed to describe the SFS trilayer for arbitrary phase difference between the superconducting order parameters in the S layers. The temperature dependence of the SFS Josephson junction critical current demonstrates the strong anharmonicity of the corresponding current-phase relation in the vicinity of the $0\text{–}\pi$ transition. In an rf superconducting quantum interference device (SQUID), coexistence of stable and metastable 0 and $\pi$ states provides integer and half-integer fluxoid configurations.