We report on the electronic structure of a clean, well ordered ${\mathrm{CePd}}_7$ surface alloy that was grown in a unique $(\sqrt{5}\times \sqrt{5})R26.6^{\circ }$ structure, as evidenced by low-energy electron diffraction and Auger electron spectroscopy. Resonant photoemission measurements at the $4d\to 4f$ absorption edge evidenced strong Ce 4$f$ derived states having two components near the Fermi level. This observation can be explained by considering very different hybridizations between Ce 4$f$ and the Pd 4$d$ electrons in the surface and deeper layers. In the former, the hybridization is weak and leads to a spin-dependent screening with low Kondo temperature, while in the latter the hybridization recovers the value of bulk alloy and thus leads to strong charge fluctuations.

We measure the optical-absorption spectra and optical conductivities of excitons and trions in monolayers of metal dichalcogenide MoS${}_2$ and compare the results with theoretical models. Our results show that the Wannier-Mott model for excitons with modifications to account for small exciton radii and large exciton relative wave function spread in momentum space, phase space blocking due to Pauli exclusion in doped materials, and wave-vector-dependent dielectric constant gives results that agree well with experiments. The measured exciton optical-absorption spectra are used to obtain experimental estimates for the exciton radii that fall in the $7–10\text{Å}$ range and agree well with theory. The measured trion optical-absorption spectra are used to obtain values for the trion radii that also agree well with theory. The measured values of the exciton and trion radii correspond to binding energies that are in good agreement with values obtained from first-principles calculations.

Identifying the effects of surface-bulk coupling is a key challenge in exploiting the topological nature of the surface states in many available three-dimensional topological “metals.” Here we combine an effective-model calculation and an ab initio slab calculation to study the effects of the lowest order surface-bulk interaction: hybridization. In the effective-model study, we discretize an established low-energy effective four-band model and introduce hybridization between surface bands and bulk bands in the spirit of the Fano model. We find that hybridization enhances the energy gap between bulk and Dirac surface states and preserves the latter's spin texture qualitatively, albeit with a reduced spin-polarization magnitude. Our ab initio study finds the energy gap between the bulk and the surface states to grow upon an increase in the slab thickness, very much in qualitative agreement with the effective-model study. Comparing the results of our two approaches, we deduce that the experimentally observed low magnitude of the spin polarization can be attributed to a hybridization-type surface-bulk interaction. We discuss evidence for such hybridization in existing ARPES data.

Composite multiferroics are materials exhibiting the interplay of ferroelectricity, magnetism, and strong electron correlations. Typical example—magnetic nano grains embedded in a ferroelectric matrix. Coupling of ferroelectric and ferromagnetic degrees of freedom in these materials is due to the influence of ferroelectric matrix on the exchange coupling constant via screening of the intragrain and intergrain Coulomb interaction. Cooling typical magnetic materials the ordered state appears at lower temperatures than the disordered state. We show that in composite multiferroics the ordered magnetic phase may appear at higher temperatures than the magnetically disordered phase. In nonmagnetic materials such a behavior is known as inverse phase transition.

The magnetic structures of the CeScSi-type GdMgSn and GdMgPb compounds have been studied by both neutron powder diffraction and Mössbauer spectroscopy (${}^{119}Sn$ and ${}^{155}Gd$). The neutron diffraction results show that the two compounds adopt incommensurate antiferromagnetic structures at 5.4 K with propagation vectors $\vec{k}=[0.910,0.077,0]$ for GdMgSn and $\vec{k}=[0.892,0,0]$ for GdMgPb. The magnetic moments lie in the basal plane, which is confirmed by both ${}^{119}Sn$ and ${}^{155}Gd$ Mössbauer spectroscopy. Mössbauer spectroscopy refinements and simulations reveal that the magnetic structure of GdMgSn is cycloidal at low temperature and undergoes a transition to a modulated magnetic structure above $T\sim 40$ K. A similar magnetic transition is inferred for GdMgPb. The magnetic structures of GdMgSn and GdMgPb are compared with those of other CeScSi-type compounds.

Neutron-diffraction experiments on the (Mn,Fe)${}_2$(P,Si)-type compounds have shown a site preference of Si atoms in the hexagonal structure. The degree of ordering of Si depends on the Si/P ratio, while it is independent of the Mn/Fe ratio. The ferromagnetic-paramagnetic magnetoelastic transition is closely related to the size of the magnetic moment on the 3$f$ site. A preferred occupation of Si atoms on the 2$c$ site stabilizes and decreases the magnetic moment on the 3$f$ and 3$g$ site, respectively, which is supported by our first-principles density functional theory calculations. This effect, together with the contribution from the Si substitution-induced changes in the interatomic distances, leads to a phase transition that is tunable in temperature and degree of first order in ${\mathrm{Mn}}_{1.25}{\mathrm{Fe}}_{0.70}{\mathrm{P}}_{1-x}{\mathrm{Si}}_x$ compounds. These results provide us with further insight into the relationship between the magnetoelastic phase transition and the local atomic coordination.

Current-driven spin torques in metallic spin valves composed of antiferromagnets are theoretically studied using the nonequilibrium Green's function method implemented on a tight-binding model. We focus our attention on G-type and L-type antiferromagnets in both clean and disordered regimes. In such structures, spin torques can either rotate the magnetic order parameter coherently (coherent torque) or compete with the internal antiferromagnetic exchange (exchange torque). We show that, depending on the symmetry of the spin valve, the coherent and exchange torques can either be in the plane, $\propto \mathbf{n}\times (\mathbf{q}\times \mathbf{n})$ or out of the plane $\propto \mathbf{n}\times \mathbf{q}$, where $\mathbf{q}$ and $\mathbf{n}$ are the directions of the order parameter of the polarizer and the free antiferromagnetic layers, respectively. Although disorder conserves the symmetry of the torques, it strongly reduces the torque magnitude, pointing out the need for momentum conservation to ensure strong spin torque in antiferromagnetic spin valves.

We re-investigate the magnetically frustrated, diamond-lattice-antiferromagnet spinels FeAl${}_2$O${}_4$ and MnAl${}_2$O${}_4$ using magnetization measurements and diffuse scattering of polarized neutrons. In FeAl${}_2$O${}_4$, macroscopic measurements evidence a “cusp” in zero field-cooled susceptibility around 13 K. Dynamic magnetic susceptibility and memory effect experiments provide results that do not conform with a canonical spin-glass scenario in this material. Through polarized neutron-scattering studies, absence of long-range magnetic order down to 4 K is confirmed in FeAl${}_2$O${}_4$. By modeling the powder averaged differential magnetic neutron-scattering cross section, we estimate that the spin-spin correlations in this compound extend up to the third nearest-neighbor shell. The estimated value of the Landé $g$ factor points towards orbital contributions from Fe${}^{2+}$. This is also supported by a Curie-Weiss analysis of the magnetic susceptibility. MnAl${}_2$O${}_4$, on the contrary, undergoes a magnetic phase transition into a long-range ordered state below $\approx$40 K, which is confirmed by macroscopic measurements and polarized neutron diffraction. However, the polarized neutron studies reveal the existence of prominent spin fluctuations co-existing with long-range antiferromagnetic order. The magnetic diffuse intensity suggests a similar short-range order as in FeAl${}_2$O${}_4$. Results of the present work support the importance of spin-spin correlations in understanding magnetic response of frustrated magnets like $A$-site spinels which have predominant short-range spin correlations reminiscent of the “spin-liquid” state.

We report neutron scattering measurements on low-energy ($\hslash \omega \sim 5$ meV) magnetic excitations from a series of ${\mathrm{Fe}}_{1+y-z}$(Ni/Cu)${}_z$${\mathrm{Te}}_{1-x}$${\mathrm{Se}}_x$ samples which belong to the “11” Fe-chalcogenide family. Our results suggest a strong correlation between the magnetic excitations near (0.5,0.5,0) and the superconducting properties of the system. The low-energy magnetic excitations are found to gradually move away from (0.5,0.5,0) to incommensurate positions when superconductivity is suppressed, either by heating or chemical doping, confirming previous observations.

The ideal strength of monolayer materials possessing semimetallic, semiconducting, and insulating ground states is computed using density functional theory. Here we show that, as in graphene, a soft mode occurs at the K point in BN, graphane, and MoS${}_2$, while not in silicene. The transition is first order in all cases except graphene. In BN and graphane the soft mode corresponds to a Kekulé-like distortion similar to that of graphene, while MoS${}_2$ has a distinct distortion. The phase transitions for BN, graphane, and MoS${}_2$ are not associated with the opening of a band gap, which indicates that Fermi surface nesting is not the driving force. We perform an energy decomposition that demonstrates why the soft modes at the K point are unique and how strain drives the phonon instability.

We report a low-temperature spectroscopic characterization of the ground-state hyperfine transitions in ${}^{151}\mathrm{Eu}{}^{3+}{\mathrm{:Y}}_2{\mathrm{SiO}}_5$ via coherent Raman scattering. Inhomogeneous linewidths of 21 and 38 kHz were measured for the $I_z=\pm \frac{1}{2}$ $\leftrightarrow$ $\pm \frac{3}{2}$ and the $\pm \frac{3}{2}$ $\leftrightarrow$ $\pm \frac{5}{2}$ ground-state transitions, respectively. Spin-echo decays were recorded for the $I_z=\pm \frac{1}{2}$ $\leftrightarrow$ $\pm \frac{3}{2}$ transition at zero field and for a small static magnetic field. In the latter case, analysis by a simple model gave an amplitude of 284 Hz and a correlation time of 3.5 ms for the transition frequency fluctuations. Application of a dynamical decoupling sequence resulted in coherence lifetimes up to 474 ms, an 18-fold increase compared to the spin intrinsic phase memory time of 26 ms.

In this work, we studied a two-dimensional ferromagnetic system using Monte Carlo simulations. Our model includes exchange and dipolar interactions, a cubic anisotropy term, and uniaxial out-of-plane and in-plane ones. According to the set of parameters chosen, the model including uniaxial out-of-plane anisotropy has a ground state which consists of a canted state with stripes of opposite out-of-plane magnetization. When the cubic anisotropy is introduced, zigzag patterns appear in the stripes at fields close to the remanence. An analysis of the anisotropy terms of the model shows that this configuration is related to specific values of the ratio between the cubic and the effective uniaxial anisotropy. The mechanism behind this effect is related to particular features of the anisotropy's energy landscape since a global minima transition as a function of the applied field is required in the anisotropy terms. This mechanism for zigzag formation could be present in monocrystal ferromagnetic thin films in a given range of thicknesses.

The triangular based antiferromagnet $\alpha$-${\mathrm{NaFeO}}_2$ has been studied by magnetization, dielectric, and neutron diffraction measurements as a function of temperature and magnetic field. The appropriate $(H-T)$ phase diagram was constructed revealing a complex behavior due to a competition between several magnetic phases. In zero field, the system undergoes a sequence of magnetostructural transitions; initially from paramagnetic $R\overline{3}m{1}^{\prime }$ phase to the incommensurate spin density wave (ICM1) at $T_{\mathrm{N}1}=11$ K with the nonpolar (3+1) magnetic superspace group $C2/m{1}^{\prime }(0,\beta ,\frac{1}{2})s0s$, then, below $T_{\mathrm{N}2}=7.5$ K, the ICM1 phase coexists with the polar cycloidal ordering (ICM2) possessing the $Cm{1}^{\prime }(0,\beta ,\frac{1}{2})0s$ superspace symmetry and finally the commensurate collinear ordering (CM) with the nonpolar magnetic space group $P_a{2}_1/m$ develops below $T_{\mathrm{N}3}=5.5$ K as the ground state of the system. A small amount of ICM2 coexists with the ICM1 and CM phases resulting in a nonzero measured polarization below $T_{\mathrm{N}2}$. Magnetic field destabilizes the collinear ground state and promotes the polar ICM2 phase resulting in a drastic increase of the polarization. The symmetry of the zero field cycloidal structure allows the two orthogonal components ${\mathit{p}}_1\propto {\mathit{r}}_{ij}\times ({\mathit{S}}_i\times {\mathit{S}}_j)$ and ${\mathit{p}}_2\propto {\mathit{S}}_i\times {\mathit{S}}_j$ to contribute to the macroscopic polarization through the inverse DM effect. The applied magnetic field reduces the symmetry of the ICM2 phase down to the triclinic $P1(\alpha ,\beta ,\gamma )0$, resulting in admixture of another cycloidal and helical components both generating magnetic field switchable polarization ${\mathit{p}}_3$ perpendicular to ${\mathit{p}}_1$ and ${\mathit{p}}_2$.

The feasibility of thermally driven spin injectors built from half-metallic Heusler alloys inserted between Al leads was investigated by means of ab initio calculations of the thermodynamic stability and electronic transport. We have focused on two main issues and found that (i) the interface between Al and the closely lattice-matched Heusler alloys of type Co${}_2$Ti$Z$ ($Z$ = Si or Ge) is stable under various growth conditions; and (ii) the conventional and spin-dependent Seebeck coefficients in such heterojunctions exhibit a strong dependence on both the spacer and the atomic composition of the Al/Heusler interface. The latter quantity gives a measure of the spin accumulation and varies between $+8$ and $-3$ $\mu$V/K near 300 K, depending on whether a Ti-Ge or a Co-Co plane makes the contact between Al and Co${}_2$TiGe in the trilayer. Our results show that it is in principle possible to tailor the spin-caloric effects by a targeted growth control of the samples.

Polarized small-angle neutron scattering (PSANS) experimental results obtained on arrays of ferromagnetic $\mathrm{Co}$ nanowires ($\varphi \approx 13$ nm) embedded in self-organized alumina (${\mathrm{Al}}_2{\mathrm{O}}_3$) porous matrices are reported. The triangular array of aligned nanowires is investigated as a function of the external magnetic field with a view to determine experimentally the real space magnetization $\vec{M}\left(\vec{r}\right)$ distribution inside the material during the magnetic hysteresis cycle. The observation of field-dependent SANS intensities allows us to characterize the influence of magnetostatic fields. The PSANS experimental data are compared to magnetostatic simulations. These results evidence that PSANS is a technique able to address real-space magnetization distributions in nanostructured magnetic systems. We show that beyond structural information (shape of the objects, two-dimensional organization) already accessible with nonpolarized SANS, using polarized neutrons as the incident beam provides information on the magnetic form factor and stray fields ${\mu }_0{H}_{\mathrm{d}}$ distribution in between nanowires.

We investigate transport properties of a double quantum dot based a Cooper pair splitter, where the superconducting lead consists of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. The proposed device can be used to explore the symmetry of the superconducting order parameter in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ by testing the presence of gapless chiral edge states, which are predicted to exist if the bulk superconductor is described by a chiral $p$-wave state. The odd orbital symmetry of the bulk order parameter ensures that we can realize a regime where the electrons tunneling into the double-dot system come from the chiral edge states and thereby leave their signature in the conductance. The proposed Cooper pair splitter has the potential to probe the chirality of the order parameters in topological superconductors.

The main purpose of this work is to propose a theoretical description of the surface plasmon polariton stimulated electron emission process at metal surfaces in which the primary electron ejection from the conduction band is treated quantum mechanically in order to go beyond the approximate approaches used up to now to represent this first step. Our theoretical results are well supported by experimental energy spectra obtained for some tens of femtosecond laser pulses impinging on a gold grating target at various intensities in the GW/cm${}^2$ range. The present model which allows us to discuss various features of the surface plasmon photoemission process such as the role of the surface plasmon phase is rather simple and could be applied with slight modifications to study the stimulated photoemission of metallic nanoparticles which has been a subject of growing interest during the past few years.

Weyl metal is regarded as a platform toward interacting topological states of matter, where its topological structure gives rise to anomalous transport phenomena, referred to as chiral magnetic effect and “negative” magnetoresistivity, the origin of which is chiral anomaly. Recently, the negative magnetoresistivity has been observed with the signature of weak antilocalization at $x=3–4\%$ in ${\mathrm{Bi}}_{1-x}$${\mathrm{Sb}}_x$, where a magnetic field is applied in parallel with an electric field ($\mathit{E}\parallel \mathit{B}$). Based on the Boltzmann equation approach, we find the negative magnetoresistivity in the presence of weak antilocalization. An essential ingredient is to introduce the topological structure of chiral anomaly into the Boltzmann equation approach, resorting to semiclassical equations of motion with Berry curvature.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of ${\mathrm{BaNi}}_2{P}_2$ that shows a superconducting transition at $T_c$ $\sim$ 2.5 K. We observed hole and electron Fermi surfaces (FSs) around the Brillouin zone center and corner, respectively, and the shapes of the hole FSs dramatically changed with photon energy, indicating strong three dimensionality. The observed FSs are consistent with band-structure calculations and de Haas-van Alphen measurements. The mass enhancement factors estimated in the normal state were $m^{*}$/$m_b$ $\le$ 2, indicating weak electron correlation compared to typical iron-pnictide superconductors. An electronlike Fermi surface around the $Z$ point was observed in contrast with BaNi${}_2$As${}_2$ and may be related to the higher $T_c$ of ${\mathrm{BaNi}}_2{P}_2$.

Through systematic examination of symmetrically nonequivalent configurations, first-principles calculations have identified a new ground state of ${\mathrm{Cu}}_2\mathrm{Se}$, which is constructed by repeating sextuple layers of Se-Cu-Cu-Cu-Cu-Se. The layered nature is in accord with electron and x-ray diffraction studies at and below room temperature and also is consistent with transport properties. Magnetoresistance measurements at liquid helium temperatures exhibit cusp-shaped field dependence at low fields and evolve into quasilinear field dependence at intermediate and high fields. These results reveal the existence of weak antilocalization effect, which has been analyzed using a modified Hikami, Larkin, and Nagaoka model, including a quantum interference term and a classical quadratic contribution. Fitting parameters suggest a quantum coherence length $L$ of 175 nm at 1.8 K. With increasing temperature, the classical parabolic behavior becomes more dominant, and $L$ decreases as a power law of $T^{-0.83}$.