Magnetotransport studies in assemblies of chemically synthesized FeCo nanoparticles have been performed between 1.8 and 300 K. The samples display tunnel magnetoresistance (TMR), the amplitude of which ranges between 3 and 11% at low temperature, and could persist up to 0.5% at room temperature. A quantitative analysis of the resistance-temperature and current-voltage characteristics evidences the presence of cotunneling effect inside the samples, which becomes the dominant transport mechanism below 40 K. The presence of cotunneling at low temperature is clearly correlated with an increase of TMR amplitude. Interestingly, in some samples, the TMR amplitude increases with voltage, a phenomenon which is shown to be correlated with an increase of the number of junctions involved in the tunneling process. Extent of cotunneling and TMR amplitude was found to be higher in case of samples prepared by drop casting compared to those prepared by dielectrophoresis. In the former case, the assemblies are more ramified than in the latter case, indicating the importance of controlling the structural properties, and especially the mean number of neighbors of an assembly, in order to observe the cotunneling enhancement of the TMR amplitude. Apart from TMR, two other types of magnetoresistance occurring at larger magnetic field were measured. One of them could reach up to 16% in certain cases. The cotunneling enhancement of TMR amplitude in chemically synthesized nanoparticles could be used to improve response of cheap magnetic sensors elaborated using on-chip deposition of colloidal solution of magnetic nanoparticles.

Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the $4d^3$ and $5d^3$ double pervoskite antiferromagnets ${\mathrm{La}}_2{\mathrm{LiRuO}}_6$ and ${\mathrm{La}}_2{\mathrm{LiOsO}}_6$. These reveal the development of an inelastic spin gap in ${\mathrm{La}}_2{\mathrm{LiRuO}}_6$ and ${\mathrm{La}}_2{\mathrm{LiOsO}}_6$ of $\sim 1.8\left(8\right)$ and 6(1) meV, below their respective ordering temperatures $T_N,\sim 23.8$ and 30 K. The bandwidths of the spin excitations are shown to be $\sim 5.7\left(9\right)\text{and}12\left(1\right)\mathrm{meV}$, respectively, at low temperatures. Spin gaps are surprising in such magnets as the $t_{2g}$ levels of ${\mathrm{Ru}}^{5+}$ or ${\mathrm{Os}}^{5+}$ are expected to be half-filled, resulting in an anticipated orbital singlet for both materials. We compare these results in monoclinic double perovskites ${\mathrm{La}}_2{\mathrm{LiRuO}}_6$ and ${\mathrm{La}}_2{\mathrm{LiOsO}}_6$ with those in cubic ${\mathrm{Ba}}_2{\mathrm{YRuO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{YOsO}}_6$ as well as with those in other monoclinic ${\mathrm{La}}_2{\mathrm{NaRuO}}_6,{\mathrm{La}}_2{\mathrm{NaOsO}}_6$, and ${\mathrm{Sr}}_2{\mathrm{ScOsO}}_6$ and model the inelastic magnetic scattering with linear spin-wave theory using minimal anisotropic exchange interactions. We discuss the possible role of the distortion of the face-centered-cubic double perovskite structure on the spin gap formation and geometric frustration in these materials and show that $T_N$ scales with the top of the spin-wave band in all members of these families that display long-range order.

We use the density matrix renormalization group method to compute the frequency and momentum-resolved spin-spin correlation functions of a dimerized spin-1/2 chain under a magnetic field at finite temperature. The spectral features strongly depend on the regime of the magnetic field. For increasing magnetic fields, the transitions from a gapped spin liquid phase to a Tomonaga-Luttinger liquid, and then to a totally polarized phase, can be identified in the spectra. Compared to the zero-temperature case, the finite-temperature excitations give rise to additional spectral features that we compute numerically and identify analytically as transitions from thermally excited states. We compute quantitatively the broadening of the dispersion of a single spin-flip excitation due to the temperature and find a strong asymmetric broadening. We discuss the consequences of these findings for neutron experiments on dimerized one-dimensional quantum chains.

We have measured the growth of the spin glass correlation length through the aging effect. Measurements were made on bulk ${\mathrm{Cu}}_{0.95}{\mathrm{Mn}}_{0.05}$ and a ${\mathrm{Cu}}_{0.88}{\mathrm{Mn}}_{0.12}$ thin film multilayer with CuMn layer thicknesses of 4.5 nm separated by 60-nm Cu layers. As the glass temperature $T_g$ is approached ($0.9T_g<T<0.96T_g$) in the bulk sample, we find that the waiting time effect (as measured by the time associated with the inflection point of the decay) as a function of increasing temperature, shifts to shorter timescales. For $T>0.96T_g$, there is no waiting time effect on the magnetization decay. In the temperature region $0.96T_g\text{–}1.00T_g$, all decays collapse onto a single decay curve indicating an end of aging even for long waiting times ($t_w=10000s$). For the thin film, all effects due to the waiting time disappear at around $0.89T_f$, where $T_f$ is the freezing temperature marking the onset of irreversibility. These results are interpreted in terms of the spin glass correlation length saturating at a constant value after reaching a characteristic length scale, either the size of the crystallites in the bulk, or the thickness of the 4.5-nm film.

Superconducting states with broken time-reversal symmetry are rarely found in nature. Here we predict that it is inevitable that the time-reversal symmetry is broken spontaneously in a superconducting Josephson junction formed by two superconductors with different pairing symmetries dubbed as $\pi /2$-Josephson junction. While the leading conventional Josephson coupling vanishes in such an $\pi /2$-Josephson junction, the second-order coupling from tunneling always generates chiral superconductivity orders with broken time-reversal symmetry. Josephson frequency in the $\pi /2$ junction is doubled, namely $\omega =4eV/h$. The result can not only provide a way to engineer topologically trivial or nontrivial time-reversal breaking superconducting states, but also be used to determine the pairing symmetry of unconventional superconductors.

We study how nonhelical spin textures at the boundary between a topological insulator (TI) and a superconductor (SC) affect the proximity-induced superconductivity of the TI interface state. We consider TIs coupled to both spin-singlet and spin-triplet SCs, and show that for the spin-triplet parent SCs, the resulting order parameter induced onto the interface state sensitively depends on the symmetries which are broken at the TI-SC boundary. For chiral spin-triplet parent SCs, we find that nodal proximity-induced superconductivity emerges when there is broken twofold rotational symmetry which forces the spins of the nonhelical topological states to tilt away from the interface plane. We furthermore show that the Andreev conductance of lateral heterostructures joining TI-vacuum and TI-SC interfaces yields experimental signatures of the reduced symmetries of the interface states.

The Meyer-Neldel behavior of conductivity of low-doped manganite ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_{\mathit{x}}{\mathrm{MnO}}_3$ single crystals has been investigated. The evolution of the isokinetic temperature of conductivity, modified by Ca-doping, hydrostatic pressure, and current bias has been determined. In addition, the evolution of isokinetic temperature with ageing has also been studied. The Meyer-Neldel behavior of the manganite system stems from the multiexcitation entropy mechanism. The isokinetic temperature turned out to be a sensitive parameter characterizing changes in the transport properties of mixed valence manganites, which in the presence of a detailed theoretical model of the excitations coupling in manganites could become a powerful tool for the characterization and investigation of transport properties of manganites.

We report on the single-crystal growth and anisotropic physical properties of ${\mathrm{CeAgAs}}_2$. The compound crystallizes as an ordered variant of the ${\mathrm{HfCuSi}}_2$-type crystal structure and adopts the orthorhombic space group $Pmca$ (No. 57) with two symmetry-inequivalent cerium atomic positions in the unit cell. The orthorhombic crystal structure of the single crystal was confirmed from the powder x-ray diffraction and from electron diffraction patterns obtained from the transmission electron microscope. The anisotropic physical properties have been investigated on a good-quality single crystal by measuring the magnetic susceptibility, isothermal magnetization, electrical transport, and heat capacity. The magnetic susceptibility and magnetization measurements revealed that this compound orders antiferromagnetically with two closely spaced magnetic transitions at $T_{\mathrm{N}1}=6$ K and $T_{\mathrm{N}2}=4.9$ K. Magnetization studies have revealed a large magnetocrystalline anisotropy due to the crystalline electric field with an easy axis of magnetization along the [010] direction. The magnetic susceptibility measured along the [001] direction exhibited a broad hump in the temperature range 50 to 250 K, while typical Curie-Weiss behavior was observed along the other two orthogonal directions. The electrical resistivity and the heat capacity measurements revealed that ${\mathrm{CeAgAs}}_2$ is a Kondo-lattice system with a magnetic ground state.

A procedure for removing explicit orbital dependence from meta-generalized-gradient approximation (meta-GGA) exchange-correlation functionals by converting them into Laplacian-dependent functionals recently was developed by us and shown to be successful in molecules. It uses an approximate kinetic energy density functional parametrized to Kohn-Sham results (not experimental data) on a small training set. Here we present extensive validation calculations on periodic solids that demonstrate that the same deorbitalization with the same parametrization also is successful for those extended systems. Because of the number of stringent constraints used in its construction and its recent prominence, our focus is on the SCAN meta-GGA. Coded in vasp, the deorbitalized version, SCAN-L, can be as much as a factor of three faster than original SCAN, a potentially significant gain for large-scale ab initio molecular dynamics.

The semiclassical dynamics of Bloch electrons in a crystal under slowly varying deformation is developed in the geometric language of a lattice bundle. Berry curvatures and gradients of energy are introduced in terms of lattice covariant derivatives, with the corresponding connections given by the gradient and rate of strain. A number of physical effects are discussed: an effective post-Newtonian gravity at band bottom, polarization induced by spatial gradient of strain, orbital magnetization induced by strain rate, and the electron energy-stress tensor.

The ${\mathrm{QED}}_3$-Gross-Neveu model is a $(2+1)$-dimensional $\mathrm{U}\left(1\right)$ gauge theory involving Dirac fermions and a critical real scalar field. This theory has recently been argued to represent a dual description of the deconfined quantum critical point between Néel and valence bond solid orders in frustrated quantum magnets. We study the critical behavior of the ${\mathrm{QED}}_3$-Gross-Neveu model by means of an $\epsilon$ expansion around the upper critical space-time dimension of $D_{\mathrm{c}}^{+}=4$ up to the three-loop order. Estimates for critical exponents in $2+1$ dimensions are obtained by evaluating the different Padé approximants of their series expansion in $\epsilon$. We find that these estimates, within the spread of the Padé approximants, satisfy a nontrivial scaling relation, which follows from the emergent $\mathrm{SO}\left(5\right)$ symmetry implied by the duality conjecture. We also construct explicit evidence for the equivalence between the ${\mathrm{QED}}_3$-Gross-Neveu model and a corresponding critical four-fermion gauge theory that was previously studied within the $1/N$ expansion in space-time dimensions $2<D<4$.

Topological nodal line semimetals, hosting one-dimensional Fermi lines with symmetry protection, have become a hot topic in topological quantum matter. Due to the breaking of time reversal symmetry in a magnetic system, nodal lines require protection by additional symmetries. Here, we report the discovery of antiferromagnetic type-I and type-II nodal lines coexisting in the monolayer ${\mathrm{CrAs}}_2$ based on a systematic first-principles calculation. Remarkably, the type-I nodal line in ${\mathrm{CrAs}}_2$ is filling enforced by nonsymmorphic analog symmetry and robust against spin-orbital coupling. The type-II nodal lines are protected by the mirror symmetry in the absence of spin-orbital coupling. The antiferromagnetic monolayer ${\mathrm{CrAs}}_2$ proposed here may provide a platform for the correlation between magnetism and exotic topological phases.

We consider spin-resolved cyclotron trajectories in a magnetic focusing device with quantum point source and drain contacts defined within the InSb two-dimensional electron gas and their mapping by the scanning gate microscopy. Besides the perpendicular component of the external magnetic field which bends the electron trajectories, we consider an in-plane component which introduces the spin dependence of the cyclotron radius. We demonstrate that the focusing conductance peaks become spin split by the in-plane magnetic field component on the order of a few tesla and that the spin-resolved trajectories can be traced separately with the conductance mapping.

We study the addition energy spectra of phosphorene quantum dots, focusing on the role of dot size, edge passivation, number of layers, and dielectric constant of the substrate where the dots are deposited. We show that for sufficiently low dielectric constants (${\varepsilon }_{\text{sub}}<4$), Coulomb blockade can be observed in dot sizes larger than 10 nm, for both passivated and unpassivated edges. For higher dielectric constants (up to ${\varepsilon }_{\text{sub}}=30$), Coulomb blockade demands smaller dot sizes, but this depends on whether the edges are passivated or not. This dramatic role played by the substrate is expected to impact the development of applications based on phosphorene quantum dots.

We present a detailed study on the $4f$ ground-state symmetry of the pressure-induced superconductor ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$ probed by soft x-ray absorption and hard x-ray photoemission spectroscopy. The revised Ce $4f$ ground states are determined as $|{\mathrm{\Gamma }}_7\rangle =\sqrt{0.45}|J_z=\pm \frac{5}{2}\rangle -\sqrt{0.55}|\mp \frac{3}{2}\rangle$ with $\mathrm{\Sigma }$-type in-plane rotational symmetry. This gives an in-plane magnetic moment consistent with the antiferromagnetic moment as reported in neutron measurements. Since the in-plane symmetry is the same as that for the superconductor ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$, we propose that the charge distribution along the $c$ axis plays an essential role in driving the system into a superconducting phase.

The relatively unexplored family of pnictide-based antiperovskites has been shown to harbor prospects for topological phases. Using the example of ${\mathrm{Ca}}_3\mathrm{BiP}$, we demonstrate a cascade of phases as the initial cubic symmetry is broken successively. Initially a small-gap $Z_2$ topological insulator, spin-orbit coupling leads to band re-ordering resulting in a topological semimetal phase. Compressive uniaxial (001) strain leads back to a small-gap $Z_2$ topological insulator, with the expected gapless boundary modes. Tensile (001) strain leaves the system with a pair of Dirac points along $(0,0,\pm k_{\circ })$ pinning the Fermi level, producing an unusual double meniscus of connected Fermi arcs on the (100) and (010) surfaces. Finally, breaking time-reversal symmetry by an applied Zeeman field produces a new phase with a pair of multi-Weyl nodes (massive or massless depending on direction, conventionally called semi-Dirac in two-dimensional systems) simultaneous with a sister pair of Dirac modes along each $\pm k_z$ axis, combining to pin the Fermi level in the close vicinity of these varied single-particle excitations.

We present a detailed discussion of the electronic band structure and excitonic dispersion of hexagonal boron nitride (hBN) in the single layer configuration and in three bulk polymorphs (usual ${\mathrm{AA}}^{\prime }$ stacking, Bernal AB, and rhombohedral ABC). We focus on the changes in the electronic band structure and the exciton dispersion induced by the atomic configuration and the electron-hole interaction. Calculations are carried out at the level of ab initio many-body perturbation theory (GW and Bethe Salpeter equation) and of a purposely developed tight-binding model. We confirm the change from direct to indirect electronic gap when going from single layer to bulk systems and we give a detailed account of its origin by comparing the effect of different stacking sequences. We emphasize that the inclusion of the electron-hole interaction is crucial for the correct description of the momentum-dependent dispersion of the excitations. As a result the electron-hole dispersion is flatter than the one obtained from the band structure. In the AB stacking this effect is particularly important as the lowest-lying exciton is predicted to be direct despite the indirect electronic band gap.

We have found a photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the ${\mathrm{BL}}_C$ band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free-electron concentration of about ${10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$). The electron- and hole-capture coefficients for this defect-related band are estimated as ${10}^{-9}$ and ${10}^{-10}\mathrm{c}{\mathrm{m}}^3/\mathrm{s}$, respectively. The ${\mathrm{BL}}_C$ band is observed only in GaN samples with relatively high concentration of carbon impurity, where the yellow luminescence (the YL1 band) with a maximum at 2.2 eV is the dominant defect-related PL. Both the YL1 and ${\mathrm{BL}}_C$ bands likely originate from the ${\mathrm{C}}_{\mathrm{N}}$ defect, namely, from electron transitions via the $-/0$ and $0/+$ thermodynamic transition levels of the ${\mathrm{C}}_{\mathrm{N}}$. The ${\mathrm{BL}}_C$ band appears only at high excitation intensities in $n$-type GaN samples codoped with Si and C, and it can be found in a wide range of excitation intensities in semi-insulating (presumably $p$-type) GaN samples doped with C. The properties and behavior of the YL1 and ${\mathrm{BL}}_C$ bands can be explained using phenomenological models and first-principles calculations.

Based on density functional simulations combined with the Landauer transport theory, the mechanical strain impacts on the chemical bonds of phosphorene and their effects on the electronic properties are studied. Moreover, the effect of the tensile strain along the zigzag direction on the charge transport properties of a two-terminal phosphorene device is evaluated. Enhancement of the intraplanar interactions, in particular between the next-nearest neighbors in strained phosphorene, is found to be essential in the band-structure evolution. The charge transport analyzing shows that phosphorene has a strong piezoconductance sensitivity, which makes this material highly desirable for high-pressure nanoelectromechanical applications. The piezoconductance gauge factor increases by strain from 46 in 5% tension to 220 in 12% tension, which is comparable to state-of-the-art silicon strain sensors. The transmission pathways monitor the current flowing in terms of the chemical bonds and hopping, however, the transport mostly arises from the charge transferring through the chemical bonds. The strong anisotropy in the transport properties along zigzag and armchair directions is observed.