First-principles alloy theory predicts that at room temperature the paramagnetic NiCoFeCrAl${}_x$ high entropy alloys adopt the face centered cubic (fcc) structure for $x\lesssim 0.60$ and the body centered cubic (bcc) structure for $x\gtrsim 1.23$, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions.

We have investigated the electron pair emission from a Cu(111) surface excited with a primary electron beam. We use the diffraction of electron pairs to identify the contribution of the Shockley surface state to the intensity. Exploring energy and momentum conservation laws makes it possible to select this state and to investigate the electron-electron interaction within this two-dimensional electronic system. We also find for this system the emergence of a zone of reduced intensity, which is the manifestation of the exchange-correlation hole. Furthermore, the region of reduced intensity displays an intensity pattern that reflects the symmetry of the surface.

We here revisit the electrical properties of LuFe${}_2$O${}_4$, compound candidate for exhibiting multiferroicity. Measurements of dc electrical resistivity as a function of temperature, electric-field polarization measurements at low temperatures with and without magnetic field, and complex impedance as a function of both frequency and temperature were carried out in a LuFe${}_2$O${}_4$ single crystal, perpendicular and parallel to the hexagonal $c$ axis, and in several ceramic polycrystalline samples. Resistivity measurements reveal that this material is a highly anisotropic semiconductor, being about two orders of magnitude more resistive along the $c$ axis. The temperature dependence of the resistivity indicates a change in the conduction mechanism at $T_{\mathrm{CO}}$ ≈ 320 K from thermal activation above $T_{\mathrm{CO}}$ to variable range hopping below $T_{\mathrm{CO}}$. The resistivity values at room temperature are relatively small and are below 5000 Ω cm for all samples but we carried out polarization measurements at sufficiently low temperatures, showing that electric-field polarization curves are a straight line as expected for a paraelectric or antiferroelectric material. Furthermore, no differences are found in the polarization curves when a magnetic field is applied either parallel or perpendicular to the electric field. The analysis of the complex impedance data corroborates that the claimed colossal dielectric constant is a spurious effect mainly derived from the capacitance of the electrical contacts. Therefore, our data unequivocally evidence that LuFe${}_2$O${}_4$ is not ferroelectric.

We study the decoherence of a spin in a quantum dot due to its hyperfine coupling to a randomly fluctuating bath of nuclear spins. The system is modeled by the central spin model with the spin bath initially being at infinite temperature. We calculate the spectrum and time evolution of the coherence factor using a Monte Carlo sampling of the exact eigenstates obtained via the algebraic Bethe ansatz. The exactness of the obtained eigenstates allows us to study the nonperturbative regime of weak magnetic fields in a full quantum mechanical treatment. In particular, we find a large nondecaying fraction in the zero-field limit. The crossover from strong to weak fields is similar to the decoherence starting from a pure initial bath state treated previously. We compare our results to a simple semiclassical picture [Merkulov et al., Phys. Rev. B 65, 205309 (2002)] and find surprisingly good agreement. Finally, we discuss the effect of weakly coupled spins and show that they will eventually lead to complete decoherence.

We propose an effective lattice Hamiltonian for monolayer MoS${}_2$ in order to describe the low-energy band structure and investigate the effect of perpendicular electric and magnetic fields on its electronic structure. We derive a tight-binding model based on the hybridization of the $d$ orbitals of molybdenum and $p$ orbitals of sulfur atoms and then introduce a modified two-band continuum model of monolayer MoS${}_2$ by exploiting the quasidegenerate partitioning method. Our theory proves that the low-energy excitations of the system are no longer massive Dirac fermions. It reveals a difference between electron and hole masses and provides trigonal warping effects. Furthermore, we predict a valley-degeneracy-breaking effect in the Landau levels. In addition, we also show that applying a gate voltage perpendicular to the monolayer modifies the electronic structure, including the band gap and effective masses.

We study the electronic transport properties of graphene with covalently bonded hydrogen impurities. Our measurements reveal low-energy resonant scattering processes within the transport for each charge carrier type. The observed resonances exhibit a strong energy dependence and are accompanied by a sharp increase in the scattering cross section. The ability to observe the scattering resonances was found to depend on the amount of disorder introduced into the graphene through the bonding of hydrogen. The results are shown to be in agreement with a theory regarding low-energy resonant scattering off a short-range impurity in graphene that takes into account both intravalley and intervalley scattering. Theory dictates that the observed resonances are the result of the formation of quasibound states of the Dirac fermions in graphene due to a divergence in one or more of the scattering lengths for the short-range hydrogen impurity potential. We anticipate our experimental results to have implications in graphene valley physics as well as graphene chemical modification, scattering, and localization theories.

A comparative study of the structure and magnetism of Fe/CoO(111) and Fe/CoO(001) epitaxial bilayers was performed to investigate the role of uncompensated spins in the exchange bias (EB) phenomenon. Low-energy electron diffraction, x-ray photoelectron spectroscopy, conversion electron Mössbauer spectroscopy (CEMS), and the magneto-optic Kerr effect were used to characterize the structural and magnetic properties of the bilayers. Magnetically compensated and uncompensated CoO films were prepared using molecular beam epitaxy through the evaporation of single Co atomic layers and their subsequent oxidation (layer-by-layer technique) on MgO crystals with (001) and (111) orientations. Two-monolayer-thick ${}^{57}$Fe probes located on top of the oxide films and covered with ${}^{56}$Fe allowed for an analysis of the interfacial chemical and magnetic structure using CEMS. For both structures, submonolayer oxidation of the iron detected at the Fe/CoO interface was found to be accompanied by the formation of a mixed FeCo region. The Fe layers showed fourfold magnetocrystalline anisotropy when grown on CoO(001) and weak uniaxial anisotropy when grown on CoO(111). Although the structural quality and composition of the two structures were comparable, they exhibited distinct EB properties. A hysteresis loop shift as high as 354 Oe at 80 K was obtained for the Fe/CoO(111) bilayer, compared to only 37 Oe for the magnetically compensated Fe/CoO(001).

Although the corresponding carbon-metal interactions can be very different, a similar nonlinear growth behavior of graphene has been observed for different metal substrates. To understand this interesting experimental observation, a multiscale “standing-on-the-front” kinetic Monte Carlo study is performed. An extraordinary robust geometry effect is identified, which solely determines the growth kinetics and makes the details of carbon-metal interaction not relevant at all. Based on such a geometry-determined mechanism, the epitaxial growth behavior of graphene can be easily predicted in many cases. As an example, the orientation-sensitive growth kinetics of graphene on an Ir(111) surface has been studied. Our results demonstrate that the lattice mismatch pattern at the atomic level plays an important role in macroscopic epitaxial growth.

We report a numerical implementation of the Landau-Lifshitz-Baryakhtar theory that dictates that the micromagnetic relaxation term obeys the symmetry of the magnetic crystal, i.e., replacing the single intrinsic damping constant with a tensor of corresponding symmetry. The effect of anisotropic relaxation is studied in a thin saturated ferromagnetic disk and an ellipse with and without uniaxial magnetocrystalline anisotropy. We investigate the angular dependence of the linewidth of magnonic resonances with respect to the given structure of the relaxation tensor. The simulations suggest that the anisotropy of the magnonic linewidth is determined by two factors: the projection of the relaxation tensor onto the plane of precession and the ellipticity of the latter.

By using a combination of x-ray spectroscopic and diffraction measurements at high pressures together with density functional theory (DFT) calculations, we show that the increase in Curie temperature $T_{\mathrm{C}}$, induced by Si substitutions in Gd${}_5$(Si${}_x$Ge${}_{1-x}$)${}_4$ giant magnetocaloric materials is predominately electronically driven as opposed to lattice driven. Whereas, a lattice contraction with applied pressure increases the strength of exchange (magnetic) interactions between Gd spins, leading to a modest increase in $T_{\mathrm{C}}$ at a rate of 1.2 K/Å${}^3$, much larger enhancements in $T_{\mathrm{C}}$ are obtained with Si doping for the same volume reduction (13.5 K/Å${}^3$), indicating that volume (lattice) effects are secondary. Similarly, an orthorhombic [$O$(II)] to monoclinic ($M$) structural phase transition is observed to take place with applied pressure in the paramagnetic state of a Gd${}_5$(Si${}_{0.125}$Ge${}_{0.875}$)${}_4$ sample at room temperature at a much smaller volume than needed to drive the same structural transition with Si doping, indicating that, even in the absence of magnetic order, electronic effects with Si doping dominate the energetics of structural transformations over lattice (volume) effects. DFT calculations show that the electronic mechanism behind this effect is a stronger Si 3$p$-Gd 5$d$ than Ge 4$p$-Gd 5$d$ hybridization, a critical ingredient in mediating indirect exchange interactions between localized Gd 4$f$ spins. The results highlight the strong sensitivity of the magnetic ordering temperature to the nature of p-d hybridization, opening opportunities for tailoring the magnetocaloric properties of these compounds by substituting other $p$ and rare-earth elements at the Si/Ge and Gd sites, respectively.

The ambient temperature equation of state of ruby in the corundum phase ($\alpha$-Al${}_2$O${}_3$) has been measured up to 165 GPa in a diamond anvil cell, using a soft pressure transmitting medium. No clear sign of phase transformation or amorphization has been observed in this range, which could affect its luminescence signal. The equation of state of $\alpha$-Al${}_2$O${}_3$ has also been calculated within density functional theory, with two different approximations of the exchange-correlation energy (local density and generalized gradient). With suitable correction, these equations of state are predictive within $\Delta P/P=2.5\%$.

Nanoscale magnets might form the building blocks of next generation memories. To explore their functionality, magnetic sensing at the nanoscale is key. We present a multifunctional combination of a nanometer-sized superconducting quantum interference device (nanoSQUID) and a Ni nanotube attached to an ultrasoft cantilever as a magnetic tip. By scanning the Nb nanoSQUID with respect to the Ni tube, we map out and analyze their magnetic coupling, demonstrate the imaging of an Abrikosov vortex trapped in the SQUID structure — which is important in ruling out spurious magnetic signals — and reveal the high potential of the nanoSQUID as an ultrasensitive displacement detector. Our results open a new avenue for fundamental studies of nanoscale magnetism and superconductivity.

In organic light-emitting diodes with small area the current may be dominated by a finite number, $N$, of sites in which electron-hole ($e$-h) recombination occurs. As a result, averaging over the hyperfine magnetic fields, ${\mathit{b}}_h$, that are generated in these sites by the environment nuclei is incomplete. This creates a random (mesoscopic) current component, $\delta I\left(\mathit{B}\right)$, at field $\mathit{B}$ having relative magnitude $\sim$$N^{-1/2}$. To quantify the statistical properties of $\delta I\left(\mathit{B}\right)$ we calculate the correlator $K(\mathit{B},\Delta \mathit{B})=\langle \delta I(\mathit{B}-\frac{\Delta \mathit{B}}{2})\delta I(\mathit{B}+\frac{\Delta \mathit{B}}{2})\rangle$ for parallel, $\Delta \mathit{B}\parallel \mathit{B}$, and perpendicular, $\Delta \mathit{B}\perp \mathit{B}$ orientations of $\Delta \mathit{B}$. We demonstrate that mesoscopic fluctuations develop at fields $\left|\mathit{B}\right|\gg |{\mathit{b}}_h|$, where the average magnetoresistance is near saturation. These fluctuations originate from the slow beating between the singlet ($S$) and triplet ($T_0$) states of the recombining $e$-h spin-pair partners. We identify the most relevant processes responsible for the current fluctuations as being due to anomalously slow beatings that develop in sparse $e$-h polaron pairs at sites for which the ${\mathit{b}}_h$ projections on the external field direction almost coincide.

Surface plasma resonance (SPR) excitation in a Co-based two-dimensional magnetoplasmonic crystal was found to strongly enhance the second-harmonic generation (SHG) efficiency. Large changes in the phase shift between magnetic and nonmagnetic SHG contributions were observed in the transversal Kerr effect as a function of the incidence angle. The activation of a nonlocal quadrupole mechanism of the magnetic SHG due to the SPR excitation was found to lead to an unusual phase behavior in the vicinity of the resonance.

The adsorption of aromatic molecules on metal surfaces leads to a complex reorganization of the molecular and metal wave functions. Various processes such as charge transfer, hybridization between molecular and metallic states, and the formation of dispersing bands within the interface have been demonstrated for organometallic interface systems. For the model molecule 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), we compare highly ordered monolayers on Ag(110) and Cu(100), which allows us to identify changes of the interfacial electronic structure when altering the coupling strength with the substrate by means of angle-resolved photoelectron spectroscopy. The stronger coupling to the Ag(110) substrate goes along with a shorter photohole lifetime and a stronger hybridization of the NTCDA lowest unoccupied molecular orbital with metal states. Supported by ab initio calculations, we show that the observed band dispersion is greatly enhanced due to the interaction with Ag(110) while the laterally denser adsorption geometry of NTCDA on Cu(100) entails a larger intermolecular wave-function overlap, and the presence of the substrate results in no further bandwidth enhancement.

Transport properties of massive Dirac particles are investigated through an oscillating barrier. The Floquet quasienergies related to the time-dependent potential appear both in transmission and reflection as sidebands around the incoming electron's energy. We take all relevant sidebands into account and present time averaged transmission and reflection probabilities in a wide energy range. Most qualitative features of scattering on a static barrier—like Klein paradox—are still visible, but the transmission probability in the evanescent regime observably increases due to the oscillation of the potential. The strongly inelastic scattering process is shown to lead to multiple Fano-type resonances and temporal trapping of the particles inside the oscillating potential. We also present a detailed study of the time evolution of the wave packets generated in the scattering process. Our results can be relevant for graphene with an induced energy gap.

The chemical potential of a graphene layer can be locally interrupted by electrostatic gating or chemical doping. When properly designed, electrically small local interruption can possess quasistatic resonance and be strongly excited by electromagnetic field. This excitation generates a mixed-type semilocalized plasmon wave, e.g., it launches TM mode within the interruption domain and TE modes into the surrounding layer. Since the resonance is quasistatic, it is essentially independent of the interruption size and it exists also for sizes much smaller than the corresponding wavelengths of the aforementioned modes. Furthermore, the interruption's polarizability can be defined and calculated. Unlike the conventional polarizability, which is defined directly via a particle's dipole response, here it is defined via the induced currents in the interruption and in its surrounding. We verify our results by comparing them to full-wave numerical simulations. The results have potential applications in many one-atom-thick metamaterials and devices.

We report magnetic, transport, and neutron diffraction measurements as well as a doping study of the V-phase compound Zr${}_4$Fe${}_4$Si${}_7$. This compound exhibits collinear antiferromagnetic order below $T_N=98\pm 1$ K with a staggered moment of 0.57(3)${\mu }_B/$Fe as $T\to$ 0. The magnetic order can be quenched with Co substitution to the Fe site, but even then a 1.5${\mu }_B/$Fe paramagnetic moment remains. The resistivity and heat capacity of Zr${}_4$Fe${}_4$Si${}_7$ are Fermi-liquid-like below 16 and 7 K, respectively, and reveal correlations on the scale of those observed in superconducting Fe pnictides and chalcogenides. Electronic structure calculations overestimate the ordered moment, suggesting the importance of dynamical effects. The existence of magnetic order, electronic correlations, and spin fluctuations make Zr${}_4$Fe${}_4$Si${}_7$ distinct from the majority of Fe-Si compounds, fostering comparison instead with the parent compounds of Fe-based superconductors.

A thin film of ferromagnetically ordered material proximate to the surface of a three-dimensional topological insulator explicitly breaks the time-reversal symmetry of the surface states. For an out-of-plane ferromagnetic order parameter on the surface, the parity is also broken since the Dirac fermions become massive. This leads, in turn, to the generation of a topological Chern-Simons term by quantum fluctuations. On the other hand, for an in-plane magnetization the surface states remain gapless for the noninteracting Dirac fermions. In this work we study the possibility of spontaneous breaking of parity due to a dynamical gap generation on the surface in the presence of a local, Hubbard-like interaction of strength $g$ between the Dirac fermions. A gap and a Chern-Simons term are generated for $g$ larger than some critical value $g_c$, provided the number of Dirac fermions $N$ is odd. For an even number of Dirac fermions the masses are generated in pairs having opposite signs, and no Chern-Simons term is generated. We discuss our results in the context of recent experiments in EuS/Bi${}_2$Se${}_3$ heterostructures.