The structural transformation of multiferroic ${\mathrm{EuTiO}}_3$ has been intensively investigated by synchrotron x-ray diffraction at pressures up to 50.3 GPa and temperatures from 50 to 500 K. A pressure-induced antiferrodistortive phase transition from cubic $Pm\overline{3}m$ to tetragonal $I4/mcm$ space group has been observed, identical to the one that has been previously explored at ambient pressure and low temperatures. Several compression/decompression cycles at different temperatures have been carried out to accurately map the transition, and as a result a $P\text{−}T$ phase diagram for ${\mathrm{EuTiO}}_3$ has been constructed. The observed phase transition exhibits a positive $d{P}_c/dT$ ($P_c$ being the critical pressure of the transition) and has many similarities with isostructural ${\mathrm{SrTiO}}_3$, although the absence of magnetoelectric interactions in the latter accounts for the different phase boundaries between the two materials.

Modeling of nanostructure evolution in solids requires comprehensive data on the properties of defects such as the vacancy and foreign atoms. Since most processes occur at elevated temperatures, not only the energetics of defects in the ground state, but also their temperature-dependent free energies must be known. The first-principles calculation of contributions of phonon and electron excitations to free formation, binding, and migration energies of defects is illustrated in the case of bcc Fe. First of all, the ground-state properties of the vacancy, the foreign atoms Cu, Y, Ti, Cr, Mn, Ni, V, Mo, Si, Al, Co, O, and the O-vacancy pair are determined under constant volume (CV) as well as zero-pressure (ZP) conditions, and relations between the results of both kinds of calculations are discussed. Second, the phonon contribution to defect free energies is calculated within the harmonic approximation using the equilibrium atomic positions determined in the ground state under CV and ZP conditions. In most cases, the ZP-based free formation energy decreases monotonously with temperature, whereas for CV-based data both an increase and a decrease were found. The application of a quasiharmonic correction to the ZP-based data does not modify this picture significantly. However, the corrected data are valid under zero-pressure conditions at higher temperatures than in the framework of the purely harmonic approach. The difference between CV- and ZP-based data is mainly due to the volume change of the supercell since the relative arrangement of atoms in the environment of the defects is nearly identical in the two cases. A simple transformation similar to the quasiharmonic approach is found between the CV- and ZP-based frequencies. Therefore, it is not necessary to calculate these quantities and the corresponding defect free energies separately. In contrast to ground-state energetics, the CV- and ZP-based defect free energies do not become equal with increasing supercell size. Third, it was found that the contribution of electron excitations to the defect free energy can lead to an additional deviation of the total free energy from the ground-state value or can compensate the deviation caused by the phonon contribution. Finally, self-diffusion via the vacancy mechanism is investigated. The ratio of the respective CV- and ZP-based results for the vacancy diffusivity is nearly equal to the reciprocal of that for the equilibrium concentration. This behavior leads to almost identical CV- and ZP-based values for the self-diffusion coefficient. Obviously, this agreement is accidental. The consideration of the temperature dependence of the magnetization yields self-diffusion data in very good agreement with experiments.

Single spin carrying defects are key elements in quantum information and nanosensing technology with the nitrogen-vacancy (NV) center in diamond being the outstanding example. In a combined electron paramagnetic resonance and first-principles modeling study we verify the existence of such NV centers in $4H-\mathrm{SiC}$ in the form of silicon vacancy-nitrogen pairs (${\mathrm{V}}_{\mathrm{Si}}{\mathrm{N}}_{\mathrm{C}}$) and explore their basic magneto-optical properties. Similar to the NV center in diamond, the ${}^3{A}_2$ triplet ground state (with a zero-field splitting of $D=429\times {10}^{-4}{\mathrm{cm}}^{-1}$) of the singly negative charge state in axial coordination can be optically spin polarized at room temperature. Given the mature technology of $4H-\mathrm{SiC}$ the NV center in this material is expected to play an important role in future spintronics applications.

We present $\beta$-detected NMR measurements of the spin-lattice relaxation of ${}^8\mathrm{Li}^{+}$ implanted into an epitaxial heterostructure based on a 100 nm thick film of ferromagnetic (FM) EuO as a function of temperature through its FM transition. In the FM state, the spin-lattice relaxation rate follows the same temperature dependence, determined by magnon scattering mechanisms, observed in the bulk by ${}^{153}\mathrm{Eu}$ NMR, but above 40 K, the signal is wiped out. We also find that ${}^8\mathrm{Li}^{+}$ stopped in material adjacent to the magnetic layer exhibits spin relaxation related to the critical slowing of the Eu spins. A particularly strong relaxation in the Au overlayer suggests an unusual strong nonlocal coupling mechanism to ${}^8\mathrm{Li}$ in the metal.

The magnetism of FeRh (001) films strongly depends on film thickness and surface terminations. While the magnetic ground state of bulk FeRh is $G$-type antiferromagnetism, the Rh-terminated films exhibit ferromagnetism with strong perpendicular magnetocrystalline anisotropy whose energy $+2.1$ meV/□ is two orders of magnitude greater than bulk $3d$ conventional magnetic metals (□ is the area of a two-dimensional unit cell). While the Goodenough-Kanamori-Anderson rule on the superexchange interaction is crucial in determining the magnetic ground phases of FeRh bulk and thin films, the magnetic phases are the results of interplay and competition between three mechanisms—the superexchange interaction, the Zener-type direct interaction, and energy gain by Rh magnetization.

The intracluster exchange interactions within the “butterfly” $\left[{\mathrm{Fe}}_3\mathrm{Ln}\right({\mu }_3-\mathrm{O}{)}_2\left({\mathrm{CCl}}_3\mathrm{COO}{)}_8\right({\mathrm{H}}_2\mathrm{O}\left)\right(\mathrm{THF}{)}_3]$ molecules, where Ln(III) represents a lanthanide cation, have been determined by a combination of x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometry (VSM) along with an interaction model. We have studied the compounds with $\mathrm{Ln}=\mathrm{Tb}$ and Ho, both non-Kramers lanthanides and with high uniaxial anisotropy, and $\mathrm{Ln}=\mathrm{Lu}$(III) and Y(III) as pseudolanthanides, which supply nonmagnetic Ln reference cases. At low temperature, the three Fe atoms can be considered as a self-unit with total spin $S_{{\mathrm{Fe}}_3}=5/2$. Using the element selectivity of the XMCD magnetometry, measured at the Ln $L_{2,3}$ edges, together with the VSM measurements, the local magnetization of the Ln ion and the ${\mathrm{Fe}}_3$ subcluster, as a function of the field and low temperature ($T\approx 2.5\mathrm{K}$), has been determined separately. These results are described quantitatively in the framework of a theoretical model based on an effective spin Hamiltonian, which considers the competing effects of intracluster interactions and the external applied magnetic field. The $\mathrm{Ln}-{\mathrm{Fe}}_3$ exchange interaction within the $\left\{{\mathrm{Fe}}_3{\mathrm{LnO}}_2\right\}$ cluster has been determined to be antiferromagnetic, in both Tb and Ho compounds, with ${\mathcal{J}}_{\mathrm{FeTb}}/{\mathrm{k}}_{\mathrm{B}}=-0.13\left(1\right)\mathrm{K}$ and ${\mathcal{J}}_{\mathrm{FeHo}}/{\mathrm{k}}_{\mathrm{B}}=-0.18\left(1\right)\mathrm{K}$, respectively. In both cases, a field-induced reorientation of the ${\mathrm{Fe}}_3$ and Ln spins from antiparallel to parallel orientation takes place at a threshold field ${\mu }_{0}H=1.1$ and 2 T, for the $\left\{{\mathrm{Fe}}_3{\mathrm{TbO}}_2\right\}$ and $\left\{{\mathrm{Fe}}_3{\mathrm{HoO}}_2\right\}$ compounds, respectively. By comparison with other compounds of the series with uniaxial anisotropy, it is concluded that the polarizability of the ${\mathrm{Fe}}_3$ subcluster magnetic moment decreases in the trend $\left\{{\mathrm{Fe}}_3{\mathrm{YO}}_2\right\}\to \left\{{\mathrm{Fe}}_3{\mathrm{TbO}}_2\right\}\to \left\{{\mathrm{Fe}}_3{\mathrm{HoO}}_2\right\}\to \left\{{\mathrm{Fe}}_3{\mathrm{DyO}}_2\right\}$, because of the increasing opposition of the exchange antiferromagnetic field caused by the Ln ion. In the $\mathrm{Ln}=\mathrm{Tb}$, Ho, and Dy, the magnetization of the whole molecule is dominated by the anisotropy of the Ln ion. The intracluster ${\mathrm{Fe}}_3-\mathrm{Ln}$ exchange interactions are very weak compared to the Ln ligand field and Fe-Fe exchange interactions.

We study the renormalization of the gap of an $s$-wave superconductor in the presence of two magnetic impurities. For weakly bound Shiba states, we analytically calculate the part of the gap renormalization that is sensitive to the relative orientation of the two impurity spins. For impurities with a strong exchange coupling to the conduction electrons, we solve the gap equation self-consistently by numerics and find that the subgap Shiba state turns into a supragap Andreev state when the local gap parameter changes sign under the impurities.

We describe a kinetic model of recombination of nonequilibrium quasiparticles generated by single photon absorption in superconducting current-carrying nanowires. The model is developed to interpret two-photon detection experiments in which a single photon does not possess sufficient energy for breaking superconductivity at a fixed low bias current. We show that quasiparticle self-recombination in relaxing hotspots dominates diffusion expansion effects and explains the observed strong bias current, wavelength, and temperature dependencies of hotspot relaxation in tungsten silicide superconducting nanowire single-photon detectors.

We study the time evolution of the entanglement negativity after a local quantum quench in (1 + 1)-dimensional conformal field theories (CFTs), which we introduce by suddenly joining two initially decoupled CFTs at their end points. We calculate the negativity evolution for both adjacent intervals and disjoint intervals explicitly. For two adjacent intervals, the entanglement negativity grows logarithmically in time right after the quench. After developing a plateau-like feature, the entanglement negativity drops to the ground-state value. For the case of two spatially separated intervals, a light-cone behavior is observed in the negativity evolution; in addition, a long-range entanglement, which is independent of the distance between two intervals, can be created. Our results agree with the heuristic picture that quasiparticles, which carry entanglement, are emitted from the joining point and propagate freely through the system. Our analytical results are confirmed by numerical calculations based on a critical harmonic chain.

The effect of boundary disorder on electronic systems is particularly interesting for topological phases with surface and edge states. Using exact diagonalization, it has been demonstrated that the surface states of a three-dimensional (3D) topological insulator survive strong surface disorder, and simply get pushed to a clean part of the bulk. Here we explore a method which analytically eliminates the clean bulk and reduces a $D$-dimensional problem to a Hamiltonian-diagonalization problem within the $(D-1)$-dimensional disordered boundary. This dramatic reduction in complexity allows the analysis of significantly bigger systems than is possible with exact diagonalization. We use our method to analyze a 2D topological spin-Hall insulator with nonmagnetic and magnetic edge impurities, and we calculate the disorder-induced redistribution of probability density (or local density of states) in the insulating bulk, as well as the transport effects of edge impurities. The analysis reveals how the edge recovers from disorder scattering as the disorder strength increases.

The electrical creation and detection of spin accumulation in ferromagnet/semiconductor Schottky contacts that exhibit highly nonlinear and rectifying electrical transport is evaluated. If the spin accumulation in the semiconductor is small, the expression for the spin voltage is identical to that of linear transport. However, if the spin accumulation is comparable to the characteristic energy scale that governs the degree of nonlinearity, the spin detection sensitivity and the spin voltage are notably reduced. Moreover, the nonlinearity enhances the backflow of spins into the ferromagnet and its detrimental effect on the injected spin current, and the contact resistance required to avoid backflow is larger than for linear transport. It is also shown that by virtue of the nonlinearity, a nonmagnetic metal contact can be used to electrically detect spin accumulation in a semiconductor.

We report a study of disorder effects on epitaxial graphene in the vicinity of the Dirac point by magnetotransport. Hall effect measurements show that the carrier density increases quadratically with temperature, in good agreement with theoretical predictions which take into account intrinsic thermal excitation combined with electron-hole puddles induced by charged impurities. We deduce disorder strengths in the range $10.2–31.2\mathrm{meV}$, depending on the sample treatment. We investigate the scattering mechanisms and estimate the impurity density to be $3.0–9.1\times {10}^{10}{\mathrm{cm}}^{-2}$ for our samples. A scattering asymmetry for electrons and holes is observed and is consistent with theoretical calculations for graphene on SiC substrates. We also show that the minimum conductivity increases with increasing disorder strength, in good agreement with quantum-mechanical numerical calculations.

Bernal-stacked few-layer graphene has been investigated by analyzing its Landau-level spectra through quantum capacitance measurements. We find that surface relaxation, which is insignificant in trilayer graphene, starts to manifest in Bernal-stacked tetralayer graphene. In trilayer graphene, the interlayer interaction parameters are generally similar to those of graphite. However, in tetralayer graphene, the hopping parameters of the two bulk layers are quite different from those of the two outer layers. This represents direct evidence of the surface relaxation phenomenon. Traditionally, the van der Waals interaction between the carbon layers is thought to be insignificant. However, we suggest that the interlayer interaction is an important factor in explaining the observed results, and the symmetry-breaking effects in graphene sublattice are not negligible.

We study transport properties for a quantum dot coupled to normal leads with a pseudogap density of states at zero temperature, using the second-order perturbation theory based on the Keldysh formalism. We clarify that the hybridization function $\mathrm{\Gamma }\left(\omega \right)\propto {\left|\omega \right|}^r(0\le r<1)$ induces the cusp or dip structure in the density of states in the dot when finite bias voltage is applied to the interacting quantum dot system. It is found that the current-voltage characteristics and differential conductance are drastically changed at $r=1/2$.

We present a scheme to generate entangled photons using nitrogen vacancy (NV) centers in diamond. We show how the long-lived nuclear spin in diamond can mediate entanglement between multiple photons, thereby increasing the length of the entangled photon string. With the proposed scheme one could generate both $n$-photon Greenberger-Horne-Zeilinger and cluster states. We present an experimental scheme realizing the same and estimate the rate of entanglement generation both in the presence and absence of a cavity.

Many $\mathrm{Ti}{\mathrm{O}}_2$ applications (e.g., in heterogeneous catalysis) involve contact with ambient atmosphere and/or water. The resulting hydroxylation can significantly alter its surface properties. While the behavior of a single, isolated OH species on the model metal oxide surface of rutile $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ is relatively well understood, much less is known regarding highly hydroxylated surfaces and/or whether $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ could be fully hydroxylated under ultrahigh vacuum conditions. Here we report the in situ formation of a well-ordered, fully hydroxylated $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)-(1\times 1)$ surface using an enhanced photochemical approach, the key parts of which are predosing of water and multistep dissociative adsorption and subsequent photolysis of the carboxylic (trimethyl acetic) acid. Combining scanning tunneling microscopy, ultraviolet photoelectron spectroscopy, and density functional theory results, we show that the attained “super OH” surface is also fully reduced, as a result of the photochemical charging of electron traps associated with the OH groups.

Using a variational Monte Carlo method, we investigate the nematic state in iron-based superconductors based on the three-band Hubbard model. Our results demonstrate that the nematic state, formed by introducing an anisotropic hopping order into the projected wave function, can arise in the underdoped regime by an off-site Coulomb interaction $V$ induced $d$-wave Pomeranchuk instability. An analysis of $V$ in momentum space indicates that the interaction between electrons around $(\pi ,0)$ and $(0,\pi )$ points is crucial for the formation of the nematic state. The resulting anisotropic kinetic energy and spin correlation, as well as unequal occupation of $d_{xz}$ and $d_{yz}$ orbitals, are all suppressed upon electron doping, which are consistent with the doping dependence of intrinsic anisotropy revealed by optical spectrum measurement and angle-resolved photoemission spectroscopy.

After Anderson's prediction of disorder-induced insulating behavior, extensive work found no singularities in the density of states of localized systems. However, Johri and Bhatt [Phys. Rev. Lett. 109, 076402 (2012) and Phys. Rev. B 86, 125140 (2012)] recently uncovered the existence of a nonanalyticity in the density of states near the band edge of noninteracting systems with bounded disorder, in an energy range outside that captured by previous work. Moreover, this feature marks the boundary of an energy range in which the localization is sharply suppressed. Given strong current interest in the effect of interactions on disordered systems, we explore here the effect of a Hubbard $U$ interaction on this behavior. We find that in ensembles of small systems a cusp in the density of states persists and continues to be associated with a sharp suppression of the localization. We explore the origins of this behavior and discuss its connection with many-body localization.