The growth of one and two atomic layers of iron on a W(110) substrate was followed by low-energy electron microscopy. The near-surface structural properties of the perfectly flat pseudomorphic films were studied by quantitative low-energy electron diffraction analysis from areas of uniform thickness as well as by the density functional theory. A strong relaxation of the outermost atomic layers was found in Fe mono- and bilayers on W(110). By calculating the phonon dispersion relations and phonon density of states, the stability of the pseudomorphic iron bilayer on a tungsten substrate has been addressed. To complete the physical picture, an iron trilayer has also been analyzed in order to identify the source of instability for its pseudomorphic phase. Our results show that the surface instability originates from the softening of the in-plane surface modes along the $\left[1\overline{1}0\right]$ direction, although the soft modes were not observed. The enhanced magnetic moments calculated within the density functional theory are in good agreement with experimental findings reported for these systems.

The emerging field of quantum thermodynamics is contributing important results and insights into archetypal many-body problems, including quantum phase transitions. Still, the question whether out-of-equilibrium quantities, such as fluctuations of work, exhibit critical scaling after a sudden quench in a closed system has remained elusive. Here, we take a novel approach to the problem by studying a quench across an impurity quantum critical point. By performing density matrix renormalization group computations on the two-impurity Kondo model, we are able to establish that the irreversible work produced in a quench exhibits finite-size scaling at quantum criticality. This scaling faithfully predicts the equilibrium critical exponents for the crossover length and the order parameter of the model, and, moreover, implies an exponent for the rescaled irreversible work. By connecting the irreversible work to the two-impurity spin correlation function, our findings can be tested experimentally.

We study the stability of three-dimensional incompressible Weyl semimetals in the presence of random quenched charge impurities. Combining numerical analysis and scaling theory, we show that, in the presence of sufficiently weak randomness, (i) the Weyl semimetal remains stable, while (ii) the double-Weyl semimetal gives rise to compressible diffusive metal where the mean density of states at zero energy is finite. At stronger disorder, the Weyl semimetal undergoes a quantum phase transition and enter into a metallic phase. The mean density of states at zero energy serves as the order parameter and displays single-parameter scaling across such a disorder driven quantum phase transition. We numerically determine various exponents at the critical point, which appear to be insensitive to the number of Weyl pairs. We also extract the extent of the quantum critical regime in disordered Weyl semimetals and the phase diagram of dirty double-Weyl semimetals at finite energies.

We report phonon dispersion curves obtained at a $\mathrm{Mo}{\mathrm{S}}_2\left(0001\right)$ surface in the friction process with a load and shear. An atomic force microscope tip used to apply stresses generated lattice strain on an oscillating $\mathrm{Mo}{\mathrm{S}}_2\left(0001\right)$ surface, which dissipated via acoustic phonons. The dissipation energy of the phonons strongly depended on the size of the lattice strain. The motion of the acoustic phonons consisted of a longitudinal mode and a transverse mode, but the occurrence of their phonon modes depended on the crystallographic direction, which reflects the atomic arrangement of the $\mathrm{Mo}{\mathrm{S}}_2\left(0001\right)$ surface. Thus, we can control the energy dissipation and friction by using the phonon dissipation curves in the friction process with a load and shear.

With x-ray absorption spectroscopy and polarized neutron reflectometry we studied how the magnetic proximity effect at the interface between the cuprate high-$T_C$ superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO) and the ferromagnet ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ (LCMO) is related to the electronic and magnetic properties of the LCMO layers. In particular, we explored how the magnitude of the ferromagnetic Cu moment on the YBCO side depends on the strength of the antiferromagnetic (AF) exchange coupling with the Mn moment on the LCMO side. We found that the Cu moment remains sizable if the AF coupling with the Mn moments is strongly reduced or even entirely suppressed. The ferromagnetic order of the Cu moments thus seems to be intrinsic to the interfacial ${\mathrm{CuO}}_2$ planes and related to a weakly ferromagnetic intraplanar exchange interaction. The latter is discussed in terms of the partial occupation of the Cu $3d_{3z^2-r^2}$ orbitals, which occurs in the context of the so-called orbital reconstruction of the interfacial Cu ions.

Gauge theories, while describing fundamental interactions in nature, also emerge in a wide variety of physical systems. Abelian gauge fields have been predicted and observed in a number of novel quantum many-body systems, topological insulators, ultracold atoms, and many others. However, the non-Abelian gauge field, while playing the most fundamental role in particle physics, up to now has remained a purely theoretical construction in many-body physics. In this paper, we report an observation of a non-Abelian gauge field in a spin-orbit coupled quantum system. The gauge field manifests itself in quantum magnetic oscillations of a hole doped two-dimensional (2D) GaAs heterostructure. Transport measurements were performed in tilted magnetic fields, where the effect of the emergent non-Abelian gauge field was controlled by the components of the magnetic field in the 2D plane.

The atomic and electronic structures of ${60}^{\circ }$ glide perfect and ${30}^{\circ }/{90}^{\circ }$ glide partial dislocations in CdTe are studied using combined semi-empirical and density functional theory calculations. The calculations predict that the dislocation cores tend to undergo significant reconstructions along the dislocation lines from the singly-periodic (SP) structures, yielding either doubly-periodic (DP) ordering by forming a dimer or quadruply-periodic (QP) ordering by alternating a dimer and a missing dimer. Charge modulation along the dislocation line, accompanied by the QP reconstruction for the Cd-/Te-core ${60}^{\circ }$ perfect and ${30}^{\circ }$ partials or the DP reconstruction for the Cd-core ${90}^{\circ }$ partial, results in semiconducting character, as opposed to the metallic character of the SP dislocation cores. Dislocation-induced defect states for the ${60}^{\circ }$ Cd-/Te-core are located relatively close to the band edges, whereas the defect states lie in the middle of the band gap for the ${30}^{\circ }$ Cd-/Te-core partial dislocations. In addition to the intracore charge modulation within each QP core, the possibility of intercore charge transfer between two different dislocation cores when they are paired together in the same system is discussed. The analysis of the electronic structures reveals the potential role of the dislocations on charge transport in CdTe, particularly in terms of charge trapping and recombination.

We present a combined theoretical and experimental study of alloys of the high performance piezoelectric PZT (${\mathrm{PbZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$) with BZnT (${\mathrm{BiZn}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$) and BZnZr (${\mathrm{BiZn}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_3$), focusing on atomic displacements, ferroelectric polarization, and elastic stability. From theory we find that the 75-25 PZT-BZnT alloy has substantially larger cation displacements, and hence ferroelectric polarization than the PZT base material, on the tetragonal side of the phase diagram. We also find a possible morphotrophic phase boundary in this system by comparing displacement patterns and optimized $c/a$ ratios. Elastic stability calculations find the structures to be essentially stable. Experiments indicate the feasibility of sample synthesis within this alloy system, although measurements do not find significant polarization, probably due to a large coercive field.

Co-Pd multilayers obtained by Pd capping of pre-deposited Co nanoparticles on amorphous alumina are systematically studied by means of high-resolution transmission electron microscopy, x-ray diffraction, extended x-ray absorption fine structure, SQUID-based magnetometry, and x-ray magnetic circular dichroism. The films are formed by CoPd alloyed nanoparticles self-organized across the layers, with the interspace between the nanoparticles filled by the non-alloyed Pd metal. The nanoparticles show atomic arrangements compatible with short-range chemical order of $\mathrm{L}{1}_0$ strucure type. The collective magnetic behavior is that of ferromagnetically coupled particles with perpendicular magnetic anisotropy, irrespective of the amount of deposited Pd. For increasing temperature three magnetic phases are identified: hard ferromagnetic with strong coercive field, soft-ferromagnetic as in an amorphous asperomagnet, and superparamagnetic. Increasing the amount of Pd in the system leads to both magnetic hardness increment and higher transition temperatures. Magnetic total moments of 1.77(4) ${\mu }_B$ and 0.45(4) ${\mu }_B$ are found at Co and Pd sites, respectively, where the orbital moment of Co, 0.40(2) ${\mu }_B$, is high, while that of Pd is negligible. The effective magnetic anisotropy is the largest in the capping metal series (Pd, Pt, W, Cu, Ag, Au), which is attributed to the interparticle interaction between de nanoparticles, in addition to the intraparticle anisotropy arising from hybridization between the $3d–4d$ bands associated to the Co and Pd chemical arrangement in a $\mathrm{L}{1}_0$ structure type.

We analyze the possible interaction-induced superconducting instabilities in noncentrosymmetric systems based on symmetries of the normal state. It is proven that pure electron-phonon coupling will always lead to a fully gapped superconductor that does not break time-reversal symmetry and is topologically trivial. We show that topologically nontrivial behavior can be induced by magnetic doping without gapping out the resulting Kramers pair of Majorana edge modes. In the case of superconductivity arising from the particle-hole fluctuations associated with a competing instability, the properties of the condensate crucially depend on the time-reversal behavior of the order parameter of the competing instability. When the order parameter preserves time-reversal symmetry, we obtain exactly the same properties as in the case of phonons. If it is odd under time reversal, the Cooper channel of the interaction will be fully repulsive leading to sign changes of the gap and making spontaneous time-reversal-symmetry breaking possible. To discuss topological properties, we focus on fully gapped time-reversal-symmetric superconductors and derive constraints on possible pairing states that yield necessary conditions for the emergence of topologically nontrivial superconductivity. These conditions might serve as a tool in the search for topological superconductors. We also discuss implications for oxide heterostructures and single-layer FeSe.

We determine the surface reconstruction of $\mathrm{SrTi}{\mathrm{O}}_3$ used to achieve superconducting FeSe films in experiments, which is different from the $1\times 1\mathrm{Ti}{\mathrm{O}}_2$-terminated $\mathrm{SrTi}{\mathrm{O}}_3$ assumed by most previous theoretical studies. In particular, we identify the existence of a double $\mathrm{Ti}{\mathrm{O}}_2$ layer at the FeSe/$\mathrm{SrTi}{\mathrm{O}}_3$ interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient $\mathrm{SrTi}{\mathrm{O}}_3$ surface to FeSe when the double $\mathrm{Ti}{\mathrm{O}}_2$ layer is present. The double layer helps to remove the hole pocket in the FeSe at the Γ point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this superconductor.

The role of oxygen doping in CdTe is addressed by first-principles calculations. Formation energies, charge transition levels, and quasiparticle defect states are calculated within the $\text{DFT}+GW$ formalism. The formation of a new defect is identified, the $\left({\text{O}}_{\text{Te}}{\text{-Te}}_{\text{Cd}}\right)$ complex. This complex is energetically favored over both isovalent $\left({\mathrm{O}}_{\text{Te}}\right)$ and interstitial oxygen $\left({\mathrm{O}}_{\text{i}}\right)$, in the Te-rich limit. We find that the incorporation of oxygen passivates the harmful deep energy levels associated with $\left({\mathrm{Te}}_{\text{Cd}}\right)$, suggesting an improvement in the efficiency of CdTe based solar cells. Substitutional $\left({\text{O}}_{\text{Cd}}\right)$ is only stable in the neutral charge state and undergoes a Jahn–Teller distortion. We also investigate the diffusion profiles of interstitial oxygen and find a low-energy diffusion barrier of only 0.14 eV between two structurally distinct interstitial sites.

The conductivity and dielectric permittivity spectra of polycrystalline manganites $\mathrm{P}{\mathrm{r}}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ with $x=0.3$, 0.4, and 0.5 have been measured in a broad frequency range ($5–3000\mathrm{c}{\mathrm{m}}^{-1}$) down to low temperatures. From the dispersion analysis the existence of a Drude component can be established that is associated with small polarons coherently moving within a band several meV wide. The temperature-dependent conductivity shows an activated behavior with an activation energy of approximately 165 meV above 170 K and 4.8 meV below 120 K. The change in the activation energy is assigned to the onset of magnetic order at 120 K. In all three samples an absorption band is discovered at $40–60\mathrm{c}{\mathrm{m}}^{-1}$ that is associated with the transition between Stark-split $\mathrm{P}{\mathrm{r}}^{3+}$ electron states which gain optical activity due to coupling to acoustical phonons.

We investigated the ground state of Ho atoms adsorbed on the Pt(111) surface, for which conflicting results exist. The density functional theory (DFT) calculations yielded the Ho ground state as $|J_z=\pm 8\rangle$. Interpretation of x-ray absorption spectroscopy and x-ray magnetic circular dichroism spectra and the magnetization curves indicated the ground state as $|J_z=\pm 6\rangle$. Superposition model is employed to predict the crystal-field (CF) parameters based on the structural data for the system Ho/Pt(111) obtained from the DFT modeling. Simultaneous diagonalization of the free-ion $\left(H_{\mathrm{FI}}\right)$ and the trigonal CF Hamiltonian $\left(H_{\mathrm{CF}}\right)$ within the whole configuration $4f^{10}$ of $\mathrm{H}{\mathrm{o}}^{3+}$ ion was performed. The role of the trigonal CF terms, neglected in the pure uniaxial CF model used previously for interpretation of experimental spectra, is found significant, whereas the sixth-rank CF terms may be neglected in agreement with the DFT predictions. The results provide substantial support for the experimental designation of the $|J_z=\pm 6\rangle$ ground state, albeit with subtle difference due to admixture of other $|J_z\rangle$ states, but run against the DFT-based designation of the $|J_z=\pm 8\rangle$ ground state. A subtle splitting of the ground energy level with the state (predominantly), $|J_z=\pm 6\rangle$ is predicted. This paper provides better insight into the single-ion magnetic behavior of the Ho/Pt(111) system by helping to resolve the controversy concerning the Ho ground state. Experimental techniques with greater resolution powers are suggested for direct confirmation of this splitting and $C_{3v}$ symmetry experienced by the Ho atom.

Time-resolved scanning Kerr microscopy data are presented that illustrate the refraction and total reflection of spin waves in a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ film at a film-thickness step. In our experiments spin waves are excited with microwaves on coplanar wave guides. By changing the orientation of an externally applied magnetic field, we can manipulate the group velocity of the wave and hence control the angle of incidence on the step. An interface mode bound at the step is observed, when the magnetic field vector is perpendicular to the step. We point out the general conditions under which such interface modes can be expected in media with strongly anisotropic dispersion.

We present detailed studies of critical behavior in the strongly site-disordered dilute ferromagnetic semiconductor (Ga,Mn)As. (Ga,Mn)As has a low saturation magnetization and relatively strong magnetocrystalline anisotropy. This combination of properties inhibits domain formation, thus removing a principal experimental difficulty in determining the critical coefficients $\beta$ and $\gamma$. We find that there are still a large number of problems to overcome in terms of measurement procedures and methods of analysis. In particular, the combined effects of disorder and inhomogeneity limit the accessible critical region. However, we find that accurate and reproducible values of the critical exponents $\beta$ and $\gamma$ can be obtained from Kouvel-Fisher plots of remanent magnetization and magnetic susceptibility for our (Ga,Mn)As samples. The values of $\beta$ and $\gamma$ obtained are consistent with those of the three-dimensional Heisenberg class, despite the very strong disorder present in this system, and they are inconsistent with mean field behavior. Log-log plots of $M\left(H\right)$ data for our samples are consistent with the three-dimensional Heisenberg value of the critical exponent $\delta$, but accurate values of $\delta$ could not be obtained for our samples from these plots. We also find that accurate values of the critical exponent $\alpha$ could not be obtained by fitting to the measured temperature derivative of resistivity for our samples. We find that modified Arrott plots and scaling plots are not a practical way to determine the universality class or critical exponents, though they are found to be in better agreement with three-dimensional Heisenberg values than mean field values. Below the critical temperature range, we find that the magnetization shows power-law behavior down to a reduced temperature of $t\sim 0.5$, with a critical exponent $\beta \sim 0.4$, a value appreciably lower than the mean field value of $\beta =0.5$. At lower temperatures, Bloch 3/2 law behavior is observed due to magnons.

The ferrimagnetic spinel ${\mathrm{Mn}}_3{\mathrm{O}}_4$ exhibits large and anisotropic changes in electronic and structural properties in response to an applied magnetic field. These changes are thought to result from the field-dependent tuning—via strong spin-lattice coupling—between two nearly degenerate magnetostructural phases. Recent variable-magnetic-field studies of ${\mathrm{Mn}}_3{\mathrm{O}}_4$ have been performed on melt-grown crystals, which can exhibit twin domains due to a Jahn-Teller structural transition below the melting temperature. Because of the near degeneracy of the magnetostructural phases, however, strain associated with the twin domains likely affects the magnetic responses of ${\mathrm{Mn}}_3{\mathrm{O}}_4$. In this report, we present a variable-magnetic-field Raman scattering study of untwinned ${\mathrm{Mn}}_3{\mathrm{O}}_4$ crystals grown out of a flux below the Jahn-Teller structural transition. We measure distinct $\mathbf{q}=0$ magnetic and vibrational excitation spectra for each isolated magnetostructural phase of untwinned ${\mathrm{Mn}}_3{\mathrm{O}}_4$ crystals and determine the symmetries of the observed excitations. We determine how the magnetostructural phase mixture changes in response to magnetic fields applied in the magnetic easy plane. Last, by comparing results on flux- and melt-grown ${\mathrm{Mn}}_3{\mathrm{O}}_4$ crystals, we show that the intrinsic mixture of the two magnetostructural phases is indeed strongly influenced by the presence of twin domains.

There is now strong theoretical evidence that a wide range of frustrated magnets should support quantum spin-nematic order in an applied magnetic field. Nonetheless, the fact that spin-nematic order does not break time-reversal symmetry makes it very difficult to detect in experiment. In this article, we continue the theme begun in Phys. Rev. B 88, 184430 (2013), of exploring how spin-nematic order reveals itself in the spectrum of spin excitations. Building on an earlier analysis of inelastic neutron scattering [Phys. Rev. B 91, 174402 (2015)], we show how the NMR $1/T_1$ relaxation rate could be used to identify a spin-nematic state in an applied magnetic field. We emphasize the characteristic universal features of $1/T_1$ using a symmetry-based description of the spin-nematic order parameter and its fluctuations. Turning to the specific case of spin-1/2 frustrated ferromagnets, we show that the signal from competing spin-wave excitations can be suppressed through a judicious choice of nuclear site and field direction. As a worked example, we show how ${}^{31}\mathrm{P}$ NMR in the square lattice frustrated ferromagnet $\mathrm{BaCdVO}({\mathrm{PO}}_4{)}_2$ is sensitive to spin-nematic order.