We report on the unusual magnetic properties in a double layer Ruddlesden-Popper phase tetragonal $\mathrm{S}{\mathrm{r}}_3\mathrm{MnTi}{\mathrm{O}}_7$ by the measurements of dc magnetization, heat capacity, and neutron diffraction data. Despite of a high Curie-Weiss temperature of 427 K, no long-range magnetic order was observed down to 3.5 K, below which the magnetic susceptibility $\chi \left(T\right)$ and thermodynamic results suggest the presence of a glassy phase in magnetism. By the analysis of the nonlinear component of dc susceptibility and the corresponding scaling behavior, it is shown that the nature of the magnetic state below 3.5 K is not a true conventional spin glass type. Moreover, the low temperature magnetic susceptibility and specific heat behavior were found to be strikingly similar to that of lightly doped spin liquid systems. On the basis of the above results, we predict the coexistence of correlated (spin liquid) and uncorrelated spin (spin glass) states in $\mathrm{S}{\mathrm{r}}_3\mathrm{MnTi}{\mathrm{O}}_7$.

In copper- and iron-based unconventional superconductors, the Bogoliubov quasiparticles interact with a spin resonance at momentum $(\pi ,\pi )$. This interaction is revealed by specific signatures in the quasiparticle spectroscopies, like kinks in photoemission and dips in tunneling. We study these signatures, as they appear inside and around a vortex core in the local density of states (LDOS), a property accessible experimentally by scanning tunneling spectroscopy. Our model retains the whole nonlocal structure of the self-energy in space and time and is therefore not amenable to a Hamiltonian treatment using Bogoliubov–de Gennes equations. The interaction with the spin resonance does not suppress the zero-bias peak at the vortex center, although it reduces its spectral weight; neither does it smear out the vortex LDOS, but rather it adds structure to it. Some of the signatures we find may have been already measured in FeSe, but remained unnoticed. We compare the LDOS as a function of both energy and position with and without coupling to the spin resonance and observe, in particular, that the quasiparticle interference patterns around the vortex are strongly damped by the coupling. We study in detail the transfer of spectral weight induced both locally and globally by the interaction and also by the formation of the vortex. Finally, we introduce a new way of imaging the quasiparticles in real space, which combines locality and momentum-space sensitivity. This approach allows one to access quasiparticle properties that are not contained in the LDOS.

We present a class of Rashba systems in hexagonal semiconducting compounds, where an electrical control over spin-orbital texture is provided by their bulk ferroelectricity. Our first-principles calculations reveal a number of such materials with large Rashba coefficients. We, furthermore, show that strain can drive a topological phase transition in such materials, resulting in a ferroelectric topological insulating state. Our findings can open avenues for interplay between Rashba effect, ferroelectricity, and topological phenomena.

Using the atomic density function theory (ADFT), we examine the point defect absorption at [110] symmetrical tilt grain boundaries in body-centered cubic iron. It is found that the sink strength strongly depends on misorientation angle. We also show that the ADFT is able to reproduce reasonably well the elastic properties and the point defect formation volume in $\alpha$-iron.

Using first-principles mean-field alloy theory, we calculate the vacancy formation energies of the face-centered-cubic Pd-Ag alloys as a function of chemical composition. The effect of Fermi surface topological transition on the composition dependence of the vacancy formation energies is detectable and is consistent with what has previously been shown for the bulk properties of ${\mathrm{Pd}}_{1-x}{\mathrm{Ag}}_x$.

We present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be $3{\mu }_B$, 866 K and $0.9{\mu }_B$, 358 K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered $Y$-type structure while CMCA has $L{2}_1$ disordered structure. The antisite disorder adds a somewhat unique property to the second compound, which arises due to the probabilistic mutual exchange of Al positions with Cr/Mn and such an effect is possibly expected due to comparable electronegativities of Al and Cr/Mn. Ab initio simulation predicted a unique transition from half metallic ferromagnet to metallic antiferromagnet beyond a critical excess concentration of Al in the alloy.

Two-dimensional magnetostatic cloaks, even when perfectly designed to mitigate the magnetic field disturbance of a scatterer, may be still detectable with Aharonov-Bohm (AB) measurements, and therefore may affect quantum interactions and experiments with elongated objects. We explore a multilayered cylindrical cloak whose permeability profile is tailored to nullify the magnetic-flux perturbation of the system, neutralizing its effect on AB measurements, and simultaneously optimally suppress the overall scattering. In this way, our improved magnetostatic cloak combines substantial mitigation of the magnetostatic scattering response with zero detectability by AB experiments.

The Brillouin zone spin filtering mechanism of enhanced tunneling magnetoresistance (TMR) is described for magnetic tunnel junctions (MTJs) and studied for an example of the MTJ with hcp Co electrodes and hexagonal BN $(h$-BN) spacer. Our calculations based on the local density approximation of density-functional theory (LDA-DFT) for $\text{Co}\left(0001\right)/h\text{-BN}/\text{Co}\left(0001\right)$ MTJ predict high TMR in this device due to Brillouin zone filtering mechanism. Owning to the specific complex band structure of the $h$-BN the spin-dependent tunneling conductance of the system is ultrasensitive to small variations of the Fermi energy position inside the BN band gap. Doping of the BN and, consequentially, changing the Fermi energy position could lead to variation of the TMR by several orders of magnitude. We show also that taking into account correlation effects on beyond DFT level is required to accurately describe position of the Fermi level and thus transport properties of the system. Our study suggests that new MTJ based on hcp Co-Pt or Co-Pd disordered alloy electrodes and $p$-doped hexagonal BN spacer is a promising candidate for the spin-transfer torque magnetoresistive random-access memory.

The layered honeycomb magnet $\alpha -{\mathrm{RuCl}}_3$ has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled $j_{\mathrm{eff}}=\frac{1}{2}{\mathrm{Ru}}^{3+}$ magnetic moments. Here, we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently assumed trigonal three-layer stacking periodicity. We report electronic band-structure calculations for the monoclinic structure, which find support for the applicability of the $j_{\mathrm{eff}}=\frac{1}{2}$ picture once spin-orbit coupling and electron correlations are included. Of the three nearest-neighbor Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the $b$ axis nonequivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the previously reported spin gap in powder inelastic neutron scattering measurements, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction, show a single magnetic transition upon cooling below $T_{\mathrm{N}}\approx 13$ K. The analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

The electronic structure of the new diluted magnetic semiconductor ${\text{Ba}}_{1-x}{\text{K}}_x{\text{(Zn}}_{1-y}{\text{Mn}}_y$)${}_2{\text{As}}_2$ ($x=0.30$, $y=0.15$) in single crystal form has been investigated by angle-resolved photoemission spectroscopy (ARPES). Measurements with soft x rays clarify the host valence-band electronic structure primarily composed of the As $4p$ states. Two hole pockets around the $\mathrm{\Gamma }$ point, a hole corrugated cylinder surrounding the $\mathrm{\Gamma }$ and $Z$ points, and an electron pocket around the $Z$ point are observed, and explain the metallic transport of ${\text{Ba}}_{1-x}{\text{K}}_x{\text{(Zn}}_{1-y}{\text{Mn}}_y$)${}_2{\text{As}}_2$. This is contrasted with ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{As}$ (GaMnAs), where it is located above the As $4p$ valence-band maximum (VBM) and no Fermi surfaces have been clearly identified. Resonance soft x-ray ARPES measurements reveal a nondispersive (Kondo-resonance-like) Mn $3d$ impurity band near the Fermi level, as in the case of GaMnAs. However, the impurity band is located well below the VBM, unlike the impurity band in GaMnAs, which is located around and above the VBM. We conclude that, while the strong hybridization between the Mn $3d$ and the As $4p$ orbitals plays an important role in creating the impurity band and inducing high temperature ferromagnetism in both systems, the metallic transport may predominantly occur in the host valence band in ${\text{Ba}}_{1-x}{\text{K}}_x{\text{(Zn}}_{1-y}{\text{Mn}}_y$)${}_2{\text{As}}_2$ and in the impurity band in GaMnAs.

Ordered phases such as charge- and spin-density wave state accompany either full or partial gapping of Fermi surface (FS) leading a metal-insulator or metal-metal transition (MMT). However, there are examples of MMT without any signatures of symmetry breaking. One example is ${\mathrm{Na}}_2{\mathrm{Ti}}_2{\mathrm{Sb}}_2\mathrm{O}$, where a partial gapping of FS is observed but a density wave ordering has not been found. Here we propose a microscopic mechanism of such a MMT, which occurs due to a momentum-dependent spin-orbit-coupled molecular orbital polarization. Since a molecular $d$ orbital polarization is present due to a small spin-orbit coupling of Ti, there is no spontaneous symmetry breaking involved. However, a sharp increase of polarization happens above a critical electron interaction which gaps out the $d$ orbtial FS and reduces the density of states significantly, while the rest of FS associated with Sb $p$ orbtials is almost intact across MMT. Experimental implications to test our proposal and applications to other systems are also discussed.

In this paper we theoretically investigate transverse magnetic focusing in two-dimensional electron and hole gases with strong spin-orbit interactions. We present a general result for spin-orbit interactions with singular winding numbers in the adiabatic limit. We then present results for systems with two spin-orbit interactions of different winding number, using the concrete and experimentally relevant case of an applied in-plane magnetic field in hole systems with a Rashba type spin-orbit interaction. We predict that the application of a large in-plane field will have a strong effect on the magnetic focusing spectrum.

We reveal a new type of supercritical behavior in gapped graphene with two oppositely charged impurities by studying the two-dimensional Dirac equation for quasiparticles with the Coulomb potential regularized at small distances accounting the lattice effects. By utilizing the variational Galerkin-Kantorovich method, we show that for supercritical electric dipole the wave function of the electron bound state changes its localization from the negatively charged impurity to the positively charged one as the distance between the impurities changes. Such a migration of the wave function corresponds to the electron and hole spontaneously created from the vacuum in bound states screening the positively and negatively charged impurities of the supercritical electric dipole, respectively. We generalize our results to a particle-hole asymmetric case, where the charges of impurities differ in signs and absolute values, and demonstrate that the necessary energetic condition for the supercriticality of novel type to occur is that the energy levels of single positively and negatively charged impurities traverse together the energy distance separating the upper and lower continua. The robustness of the supercriticality of novel type is confirmed by the study of an exactly solvable one-dimensional problem of the Dirac equation with the square well and barrier potential modeling an electric-dipole potential.

The bridge between quantum vacuum photon modes and properties of patterned surfaces is currently being established on solid theoretical grounds. Based on these foundations, the manipulation of quantum vacuum photon modes in a nanostructured cavity is theoretically shown to be able to change the Lifshitz–van der Waals forces from attractive to repulsive regime. Since this concept relies on surface nanopatterning instead of chemical composition changes, it drastically relaxes the usual conditions for achieving repulsive Lifshitz–van der Waals forces. As a case study, the potential interaction energy between a nanopatterned polyethylene slab and a flat polyethylene slab with water as the intervening medium is calculated. Extremely small corrugation heights ($<10$ nm) are shown to be able to change the Lifshitz–van der Waals force from attractive to repulsive, the interaction strength being controlled by the corrugation height. This new approach could lead to various applications in surface science.

In this paper we present a detailed analysis of the structural, electronic, and optical properties of an $m$-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.

Recent low-temperature heat capacity $\left(C_P\right)$ measurements on polycrystalline samples of the pyrochlore antiferromagnet ${\mathrm{Tb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+\delta }$ have shown a strong sensitivity to the precise Tb concentration $x$, with a large anomaly exhibited for $x\sim 0.005$ at $T_C\sim 0.5$ K and no such anomaly and corresponding phase transition for $x\le 0$. We have grown single-crystal samples of ${\mathrm{Tb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+\delta }$, with approximate composition $x=-0.001,+0.0042$, and $+0.0147$, where the $x=0.0042$ single crystal exhibits a large $C_P$ anomaly at $T_C=0.45$ K, but neither the $x=-0.001$ nor the $x=+0.0147$ single crystals display any such anomaly. We present new time-of-flight neutron scattering measurements on the $x=-0.001$ and the $x=+0.0147$ samples which show strong $\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)$ quasi-Bragg peaks at low temperatures characteristic of short-range antiferromagnetic spin ice (AFSI) order at zero magnetic field but only under field-cooled conditions, as was previously observed in our $x=0.0042$ single crystal. Furthermore, the frozen AFSI state displays a gapped spin excitation spectrum around $\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)$, with a gap of $\sim 0.1$ meV, again similar to previous observations on the $x=0.0042$ single crystal. These results show that the strong $\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)$ quasi-Bragg peaks and gapped AFSI state at low temperatures under field-cooled conditions are robust features of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, and are not correlated with the presence or absence of the $C_P$ anomaly and phase transition at low temperatures. Further, these results show that the ordered state giving rise to the $C_P$ anomaly is confined to $0\le x\le 0.01$ for ${\mathrm{Tb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+\delta }$, and is not obviously connected with conventional order of magnetic dipole degrees of freedom.

We study the Hall and Nernst effects across the antiferromagnetic transition that reconstructs the quasi-two-dimensional Fermi surface of the metallic local moment antiferromagnet ${\text{PdCrO}}_2$. We show that nonmonotonic temperature dependence in the Hall effect and a sign change in the Nernst effect above the ordering temperature cannot be understood within a simple single-band transport model. The inclusion of coherent scattering by critical antiferromagnetic fluctuations can qualitatively account for these features in the transport coefficients. We discuss the implications of this for the pseudogap phase of the cuprate superconductors, which have a similar Fermi surface and where the same transport signatures are observed.

The interaction between two topological objects, an x-ray beam carrying orbital angular momentum (OAM) and a magnetic vortex, is studied theoretically. The resonant x-ray scattering intensity is calculated as a function of the relative position of the magnetic and x-ray vortices. For a homogeneous system, the charge scattering is zero. For magnetic scattering, the intensity profile strongly depends on the relative topological indices of the x-ray and magnetic singularities. A strong enhancement in the intensity profile is observed for equal winding factors. Additionally, the profile displays edge effects, which depend on the scattering conditions, the radial dependence of the magnetic vortex, and the Laguerre-Gaussian mode of the OAM x-ray beam. The potential of resonant OAM x-ray scattering from magnetic vortices opens the door to study the dynamics and switching of magnetic vortices.

We present density functional theory (DFT) $+$ quasiparticle self-energy $\left(G_0{W}_0\right)+$ Bethe-Salpeter calculations of the real and imaginary parts of the long-wavelength dielectric function of LiF between ambient pressure and $P=5$ Mbars. While the optical absorption spectrum is predicted to show dramatic pressure-dependent features above the optical gap, the index of refraction well below the gap is shown to exhibit the same trends as that seen in both DFT calculations and experiment: a roughly linear increase with density. This increase does not result from a decrease in the band gap, but rather follows from the increase in oscillator strength which counteracts a smaller increase in band gap with $P$. Our calculations also suggest that the index of refraction (for visible and near-UV light) of the higher-$T{B}_2$ phase should be quite close to that of the $B_1$ (ambient crystalline) phase. These findings may be of interest to researchers who use LiF as a window material in dynamic compression experiments.