We consider a single impurity with spin $S$ embedded in a three-dimensional antiferromagnetic system which is close to the quantum critical point (QCP) separating magnetically ordered and disordered phases. Approaching the QCP from the disordered phase, we study the spatial distribution of the spin density and staggered magnetization induced by the impurity. Using two methods (self-consistent Born approximation and renormalization group), we found a power-law decay of the spin density $\propto 1/r^3$, and of the staggered magnetization $\propto 1/r$ with relevant logarithmic corrections. We demonstrate that the local spin at the impurity's site, $r=0$, approaches to zero at the QCP. We show that in the semiclassical limit of large $S$ the problem is equivalent to the exactly solvable independent boson model. Our results demonstrate existence of spin-charge separation in three-dimensional systems at the QCP.

This work describes detection of the laser preparation and subsequent coherent manipulation of the quantum states of orbital levels of donors in doped Si, by measuring the voltage drop across an irradiated Si sample. This electrical signal, which arises from thermal ionization of excited orbital states, and which is detected on a millisecond time scale by a voltmeter, leads to much more sensitive detection than can be had using optical methods, but has not before been quantitatively described from first principles. We present here a unified theory which relates the voltage drop across the sample to the wave function of the excited donors, and compare its predictions to experiments in which pairs of picosecond pulses from the Dutch free-electron laser FELIX are used to resonantly and coherently excite P donors in Si. Although the voltage drop varies on a millisecond time scale we are able to measure Ramsey oscillation of the excitation on a picosecond time scale, thus confirming that the donor wave function, and not just its excited state population, is crucial in determining the electrical signal. We are also able to extract the recombination rate coefficient to the ground state of the donor as well as the photoionization cross section of the excited state and phonon induced thermal ionization rate from the excited state. These quantities, which were previously of limited interest, are here shown to be important in the description of electrical detection, which, in our unoptimized configuration, is sensitive enough to enable us to detect the excitation of $\sim {10}^7$ donors.

Utilizing the notion of metamaterials, in recent years the concept of a circuit and lumped circuit elements have been extended to the optical domains, providing the paradigm of optical metatronics, i.e., metamaterial-inspired optical nanocircuitry, as a powerful tool for design and study of more complex systems at the nanoscale. In this paper we present a design for a new metatronic element, namely, a metatronic transistor that functions as an amplifier. As shown by our analytical and numerical paper here, this metatronic transistor provides gain as well as isolation between the input and output ports of such two-port device. The cascadability and fan-out aspects of this element are also explored.

The structural properties of several compositions of Ge-Sb-Te liquids are studied from a combination of neutron diffraction and density functional-based molecular dynamics. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, neighbor distributions, and compare our results with experimental findings. Most noticeable is the excellent agreement found in the reproduction of the structure in real and reciprocal spaces, resulting from the incorporation of dispersion forces in the simulation. The vibrational density of states is also measured from inelastic neutron scattering for different compositions and compared to the simulated counterpart, which exhibits an excellent agreement at low frequency.

Although glasses have been largely used for a wide variety of technological applications, much of their underlying structure and dynamical properties remain puzzling. We present a detailed investigation of the intermediate-range structure of a series of alkaline borate glasses carried out by performing neutron diffraction measurements. We propose that the first sharp diffraction peak of glasses arises from the periodicity of the boundaries of voids in a random network and explain its compositional and pressure dependence. In this framework, the planar section of a void is an $n$-membered ring of all-side vertex sharing basic structural units. Furthermore, we suggest that thermally activated relaxations responsible for the ultrasonic absorption in borate glasses stem from transverse motions of atoms bridging structural units confining voids.

${\mathrm{SrRu}}_2{\mathrm{O}}_6$ has unique magnetic properties. It is characterized by a very high Néel temperature, despite its quasi-two-dimensional structure, and has a magnetic moment more than twice reduced compared to the formal ionic count. First-principles calculations show that only an ideal Néel ordering in the Ru plane is possible, with no other metastable magnetic solutions, and, highly unusually, yield dielectric gaps for both antiferromagnetic and nonmagnetic states. We demonstrate that this strange behavior is the result of the formation of very specific electronic objects, recently suggested for a geometrically similar ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ compound, whereby each electron is well localized on a particular ${\mathrm{Ru}}_6$ hexagon, and completely delocalized over the corresponding six Ru sites, thus making the compound both strongly localized and highly itinerant.

The magnetic, structural, and vibrational properties of $\mathrm{YM}{\mathrm{n}}_2{\mathrm{O}}_5$ multiferroic have been studied by means of neutron, x-ray powder diffraction, and Raman spectroscopy at pressures up to 6 and $30\mathrm{GPa}$, respectively. Application of high pressure, $P>1\mathrm{GPa}$, leads to a gradual suppression of the commensurate and incommensurate antiferromagnetic (AFM) phases with a propagation vector $q=(1/2,0,q_z\sim 1/4)$ and appearance of the commensurate AFM phase with $q=(1/2,0,1/2)$. This observation is sharply contrasting to general trend towards stabilization of commensurate AFM phase with $q=(1/2,0,1/4)$ found in other $R\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_5$ compounds upon lattice compression. At $P\sim 16\mathrm{GPa}$ a structural phase transformation accompanied by anomalies in lattice compression and pressure behavior of vibrational modes was observed. The comparative analysis of high-pressure and $R$-cation radius variation effects clarified a role of particular magnetic interactions in the formation of the magnetic states of $R\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_5$ compounds.

The magnetic and structural properties of single-crystal ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{V}}_2{\mathrm{O}}_4$ samples have been investigated by performing specific heat, susceptibility, neutron diffraction, and x-ray diffraction measurements. As the orbital-active ${\mathrm{Fe}}^{2+}$ ions with larger ionic size are gradually substituted by the orbital-inactive ${\mathrm{Co}}^{2+}$ ions with smaller ionic size, the system approaches the itinerant electron limit with decreasing V-V distance. Then, various factors such as the Jahn-Teller distortion and the spin-orbital coupling of the ${\mathrm{Fe}}^{2+}$ ions on the A sites and the orbital ordering and electronic itinerancy of the ${\mathrm{V}}^{3+}$ ions on the B sites compete with each other to produce a complex magnetic and structural phase diagram. This phase diagram is compared to those of ${\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{V}}_2{\mathrm{O}}_4$ and ${\mathrm{Mn}}_{1-x}{\mathrm{Co}}_x{\mathrm{V}}_2{\mathrm{O}}_4$ to emphasize several distinct features.

We find the admittance of a topological Josephson junction $Y(\omega ,{\phi }_0,T)$ as a function of frequency $\omega$, the static phase bias ${\phi }_0$ applied to the superconducting leads, and temperature $T$. The dissipative part of $Y$ allows for spectroscopy of the subgap states in the junction. The resonant frequencies ${\omega }_{\mathcal{M},n}\left({\phi }_0\right)$ for transitions involving the Majorana ($\mathcal{M}$) doublet exhibit characteristic kinks in the ${\phi }_0$ dependence at ${\phi }_0=\pi$. The kinks, associated with decoupled Majorana states, remain sharp and the corresponding spectroscopic lines are bright at any temperature, as long as the leads are superconducting. The developed theory may help with regard to extracting quantitative information about Majorana states from microwave spectroscopy.

Results for supercell-size convergence of formation energies and charge transition levels of vacancy complexes $V_n(1\le n\le 11)$ in crystalline Si are reported for the ab initio density functional theory (DFT) with generalized gradient approximation (GGA) pseudopotentials. When extrapolated to the dilute limit, the formation energy of an uncharged vacancy becomes $3.74\pm 0.1\mathrm{eV}$, and the binding energy of an uncharged divacancy becomes $1.9\pm 0.2\mathrm{eV}$. Stable $V_n$ clusters are built on the basis of sixfold rings $(n\le 6)$ and of octahedral voids $(n\ge 7)$. If the well-known underestimate of the band gap by the DFT and the accuracy of extrapolations are taken into account, the extrapolated levels are in good agreement with experiment. We discuss the implications for simulation of vacancy clustering during thermal quenching, for interpretation of deep-level spectroscopy and electron paramagnetic resonance in irradiated Si, and for cell and Brillouin-zone sampling choice when DFT-related methods beyond local-density approximation or GGA are used.

We use density functional theory to explain the preferred structure of partially reduced ${\mathrm{CeO}}_2\left(111\right)$. Low-energy ordered structures are formed when the vacancies are isolated (maximized intervacancy separation) and the size of the ${\mathrm{Ce}}^{3+}$ ions is minimized. Both conditions help minimize disruptions to the lattice around the vacancy. The stability of the ordered structures suggests that isolated vacancies are adequate for modeling more complex (e.g., catalytic) systems. Oxygen diffusion barriers are predicted to be low enough that O diffusion between vacancies is thermodynamically controlled at room temperature. The O-diffusion-reaction energies and barriers are decreased when one Ce $f$ electron hops from a nearest-neighbor Ce cation to a next-nearest-neighbor Ce cation, with a barrier that has been estimated to be slightly less than the barrier to O diffusion in the absence of polaron hopping. This indicates that polaron hopping plays a key role in facilitating the overall O diffusion process, and depending on the relative magnitudes of the polaron hopping and O diffusion barriers, polaron hopping may be the kinetically limiting process.

We have studied the spin correlations with $\mathbf{k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ propagation vector, which appear below 0.4  K in ${\text{Tb}}_2{\text{Ti}}_2{\text{O}}_7$ spin liquid by combining powder neutron diffraction and specific heat on ${\mathrm{Tb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+y}$ samples with $x=0$, 0.01, and $-0.01$. The $\mathbf{k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ order clearly appears on all neutron patterns by subtracting a pattern at 1.2(1) K. Refining the subtracted patterns at 0.07 K yields two possible spin structures, with spin-ice-like and monopolelike correlations. Mesoscopic correlations involve Tb moments of 1 to $2{\mu }_{\mathrm{B}}$ ordered on a length scale of about 20 Å. In addition, long-range order involving a small spin component of 0.1 to $0.2{\mu }_{\mathrm{B}}$ is detected for the $x=0$ and 0.01 samples showing a peak in the specific heat. Comparison with previous single-crystal data suggests that the $(\frac{1}{2}, \frac{1}{2}, \frac{1}{2})$ order settles in through nanometric spin textures with dominant spin-ice character and correlated orientations, analogous to nanomagnetic twins.

The electronic, magnetic, and transport properties of Fe-intercalated $2H\text{−}{\mathrm{TaS}}_2$ have been investigated by means of the Korringa-Kohn-Rostoker (KKR) method. The nonstoichiometry and disorder in the system have been accounted for using the coherent potential approximation (CPA) alloy theory. A pronounced influence of disorder on the spin magnetic moment has been found for the ferromagnetically ordered material. The same applies for the spin-orbit-induced orbital magnetic moment and magnetocrystalline anisotropy energy. The temperature dependence of the resistivity of disordered $2H{\mathrm{Fe}}_{0.28}{\mathrm{TaS}}_2$ investigated on the basis of the Kubo-Středa formalism in combination with the alloy analogy model has been found in very satisfying agreement with experimental data. This also holds for the temperature-dependent anomalous Hall resistivity ${\rho }_{\mathrm{xy}}\left(T\right)$. The role of thermally induced lattice vibrations and spin fluctuations for the transport properties is discussed in detail.

We investigate the direct correspondence between Co band ferromagnetism and structural parameters in the pnictide oxides $R\mathrm{CoPO}$ for different rare-earth ions ($R=\mathrm{La},\mathrm{Pr},\mathrm{Nd},\mathrm{Sm}$) by means of muon-spin spectroscopy and ab initio calculations, complementing our results published previously [G. Prando et al., Common effect of chemical and external pressures on the magnetic properties of $R\mathrm{CoPO}$ ($R=\mathrm{La},\mathrm{Pr}$), Phys. Rev. B 87, 064401 (2013)]. We find that both the transition temperature to the ferromagnetic phase $T_{\mathrm{C}}$ and the volume of the crystallographic unit cell $V$ are conveniently tuned by the $R$ ionic radius and/or external pressure. We report a linear correlation between $T_{\mathrm{C}}$ and $V$ and our ab initio calculations unambiguously demonstrate a full equivalence of chemical and external pressures. As such, we show that $R$ ions influence the ferromagnetic phase only via the induced structural shrinkage without involving any active role from the electronic $f$ degrees of freedom, which are only giving a sizable magnetic contribution at much lower temperatures.

The rigid double-torus torsional oscillator (TO) is constructed to reduce any elastic effects inherent to complicated TO structures, allowing explicit probing for a genuine supersolid signature. We investigated the frequency- and temperature-dependent response of the rigid double-torus TO containing solid ${}^4\mathrm{He}$ with 0.6-ppb ${}^3\mathrm{He}$ and 300-ppb ${}^3\mathrm{He}$. We did not find evidence to support the frequency-independent contribution proposed to be a property of supersolid helium. The frequency-dependent contribution which comes from the simple elastic effect of solid helium coupled to the TO is essentially responsible for the entire response. The magnitude of the period drop is linearly proportional to $f^2$, indicating that the responses observed in this TO are mostly caused by the overshoot of “soft” solid helium against the wall of the torus. Dissipation of the rigid TO is vastly suppressed compared to that of nonrigid TOs.

The chemical potential of a superconductor is of critical importance since, at equilibrium, it is the energy where electrons pair and form the superconducting condensate. However, in nonequilibrium measurements, there may be a difference between the chemical potential of the quasiparticles and that of the pairs. Here we report a systematic time- and angle-resolved photoemission study of the pump-induced change in the chemical potential of an optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) sample in both its normal and superconducting states. The change in chemical potential can be understood by separately considering the change in the valence band energy relative to the vacuum and the change in chemical potential relative to the valence band energy. We attribute the former effect to a changing potential barrier at the sample surface and the latter effect to the conservation of charge in an asymmetrical density of states. The results indicate that the pair and quasiparticle chemical potentials follow each other even on picosecond timescales.

Several materials with energy bands crossing giving rise to relativisticlike electronic excitations have recently been discovered beyond the seminal example of graphene. These relativistic phases exhibit remarkable properties, among which is a finite minimal conductivity. While the density of propagating wave states vanishes at the band crossing, the conductivity may remain finite due to evanescent modes in a confined geometry. Studying the generalizations of graphene to three-band models we find that this property is intimately related to the existence of topological Berry phases. We show that the topological robustness of the latter originates from geometrical properties of the underlying lattice, which are encoded into duality properties of the Hamiltonian.

A giant field enhancement factor, defined as the ratio between the intensity of the resonant and the incident fields, is achievable in a photonic crystal (PhC) slab realized in a low contrast dielectric medium. The key point is the careful control of some parameters, first among all the slab thicknesses, which allows for stabilizing the coupling resonant mechanism. These modes are closely correlated with symmetry-protected bound states in continuum. Our study proves, in frequency as in time domain, that such modes can be excited with a normal incident beam and that the giant resonant enhanced field starts to grow and is established after more than ${10}^5\text{−}\mathrm{cycle}$ time. Up to six orders of magnitude of field enhancement distributed along all the slab is achievable, making it easier to use in many applications than localized enhancement.

In a pressure-temperature $\left(P\text{−}T\right)$ diagram for synthesizing $\mathrm{IrT}{\mathrm{e}}_2$ compounds, the well-studied trigonal $\left(H\right)$ phase with the $\mathrm{Cd}{\mathrm{I}}_2$-type structure is stable at low pressures. The superconducting cubic $\left(C\right)$ phase can be synthesized under higher temperatures and pressures. A rhombohedral phase with the crystal structure similar to the $C$ phase can be made at ambient pressure; but the phase contains a high concentration of Ir deficiency. In this paper we report that a rarely studied monoclinic $\left(M\right)$ phase can be stabilized in narrow ranges of pressure and temperature in this $P\text{−}T$ diagram. The peculiar crystal structure of the $M\text{−}\mathrm{IrT}{\mathrm{e}}_2$ eliminates the tendency to form Ir-Ir dimers found in the $H$ phase. The $M$ phase has been fully characterized by structural determination and measurements of electrical resistivity, thermoelectric power, DC magnetization, and specific heat. These physical properties have been compared with those in the $H$ and $C$ phases of $\mathrm{I}{\mathrm{r}}_{1-x}\mathrm{T}{\mathrm{e}}_2$. Moreover, magnetic and transport properties and specific heat of the $M\text{−}\mathrm{IrT}{\mathrm{e}}_2$ can be fully justified by calculations with the density-functional theory presented in this paper.