We compute the zero-temperature conductivity in the two-dimensional quantum $\mathrm{O}\left(N\right)$ model using a nonperturbative functional renormalization-group approach. At the quantum critical point we find a universal conductivity ${\sigma }^{*}/{\sigma }_Q^{}$ (with ${\sigma }_Q^{}=q^2/h$ the quantum of conductance and $q$ the charge) in reasonable quantitative agreement with quantum Monte Carlo simulations and conformal bootstrap results. In the ordered phase the conductivity tensor is defined, when $N\ge 3$, by two independent elements, ${\sigma }_{\mathrm{A}}\left(\omega \right)$ and ${\sigma }_{\mathrm{B}}\left(\omega \right)$, respectively associated with $\mathrm{SO}\left(N\right)$ rotations which do and do not change the direction of the order parameter. Whereas ${\sigma }_{\mathrm{A}}(\omega \to 0)$ corresponds to the response of a superfluid (or perfect inductance), the numerical solution of the flow equations shows that ${lim}_{\omega \to 0}{\sigma }_{\mathrm{B}}\left(\omega \right)/{\sigma }_Q^{}={\sigma }_{\mathrm{B}}^{*}/{\sigma }_Q^{}$ is a superuniversal (i.e., $N$-independent) constant. These numerical results, as well as the known exact value ${\sigma }_{\mathrm{B}}^{*}/{\sigma }_Q^{}=\pi /8$ in the large-$N$ limit, allow us to conjecture that ${\sigma }_{\mathrm{B}}^{*}/{\sigma }_Q^{}=\pi /8$ holds for all values of $N$, a result that can be understood as a consequence of gauge invariance and asymptotic freedom of the Goldstone bosons in the low-energy limit.

The mechanisms for strong electron-phonon coupling predicted for hydrogen-rich alloys with high superconducting critical temperature ($T_c$) are examined within the Migdal-Eliashberg theory. Analysis of the functional derivative of $T_c$ with respect to the electron-phonon spectral function shows that at low pressures, when the alloys often adopt layered structures, bending vibrations have the most dominant effect. At very high pressures, the H-H interactions in two- and three-dimensional extended structures are weakened, resulting in mixed bent (libration) and stretch vibrations, and the electron-phonon coupling process is distributed over a broad frequency range leading to very high $T_c$.

Oxygen $K$-edge x-ray absorption spectra of liquid water are computed based on configurations from advanced ab initio molecular dynamics simulations, as well as an electron excitation theory from the GW method. One the one hand, the molecular structures of liquid water are accurately predicted by including both van der Waals interactions and a hybrid functional (PBE0). On the other hand, the dynamic screening effects on electron excitation are approximately described by the recently developed enhanced static Coulomb-hole and screened-exchange approximation of W. Kang and M. S. Hybertsen [Phys. Rev. B 82, 195108 (2010)]. The resulting spectra of liquid water are in better quantitative agreement with the experimental spectra due to the softened hydrogen bonds and the slightly broadened spectra originating from the better screening model.

Epitaxial $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ bilayers are expected to be highly efficient spin injectors into Si owing to the spin filter effect of $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$. To exploit the full potential of this system, understanding the microscopic origin of magnetically dead layers at the $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ interface is necessary. In this paper, we study the cation distribution, electronic structures, and the magnetic properties of $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4\left(111\right)$ layers with various thicknesses (thickness $d=1.4$, 2.3, 4, and 11 nm) in the epitaxial $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4\left(111\right)/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\left(111\right)/\mathrm{Si}\left(111\right)$ structures using soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) combined with cluster-model calculation. The magnetization of $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ measured by XMCD gradually decreases with decreasing thickness $d$, and finally, a magnetically dead layer is clearly detected at $d=1.4\mathrm{nm}$. The magnetically dead layer has frustration of magnetic interactions, which is revealed from comparison between the magnetizations at 300 and 6 K. From analysis using configuration-interaction cluster-model calculation, the decrease of $d$ leads to a decrease in the inverse-to-normal spinel structure ratio and also a decrease in the average valence of Fe at the octahedral sites. These results strongly indicate that the magnetically dead layer at the $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ interface originates from various complex networks of superexchange interactions through the change in the cation distribution and electronic structure. Furthermore, from comparison of the magnetic properties between $d=1.4$ and 2.3 nm, it is found that the ferrimagnetic order of the magnetically dead layer at the $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ interface is partially restored by increasing the thickness from $d=1.4$ to 2.3 nm.

An efficient way for control of the spin wave propagation in a magnetic medium is the use of periodic patterns known as magnonic crystals (MCs). However, the fabrication of MCs especially bicomponents, with periodicity in nanoscale, is a challenging task due to the requirement for sharp interfaces. An alternative method to circumvent this problem is to use homogeneous ferromagnetic film with a modified periodically surrounding. In this work we demonstrate that the magnonic band structure is formed in thin Py film due to dynamical magnetostatic coupling with the array of Ni stripes. We show that the band gap width can be systematically tuned by changing separation between film and stripes. We show also the effect of nonreciprocity, which is seen at the band gap edge which is shifted from the Brillouin zone boundary and also in nonreciprocal interaction of propagating spin waves in Py film with the standing waves in Ni stripes. Our findings open a possibility for further investigation and exploitation of the nonreciprocity and band structure in magnonic devices.

A magnetic field free current-induced deterministic switching is demonstrated in a perpendicularly magnetized all-metallic Pt/Co/Pt thin film system with a small tilt in anisotropy axis. We realized this in devices where the ultrathin Co layer was grown using an oblique angle sputter deposition technique that had resulted in a small tilt of magnetic anisotropy from the film normal. By performing out-of-plane magnetization hysteresis measurements under bias magnetic field applied along various in-plane directions the tilt angle was estimated to be around $3.3^{\circ }$($\pm 0.3^{\circ }$). A deterministic current-induced magnetization switching could be achieved when the in-plane current was applied perpendicular to the anisotropy tilt axis, but the switching was stochastic when the current was applied in the direction of the tilt (in the tilt plane). By preparing Pt/Co/Pt stacks with unequal top and bottom Pt thickness, sufficient spin-orbit torque (SOT) could be applied to switch the magnetization of the Co layer at current densities as low as $1.5\times {10}^7$ A/${\mathrm{cm}}^2$. The switching phase diagram (SPD) constructed by plotting the critical current density versus applied in-plane magnetic field ($H_x^{\mathrm{IB}}$) confirms spin Hall effect based SOT mechanism to be responsible for the magnetization switching. The asymmetry observed in the SPD (about $H_x^{\mathrm{IB}}=0$) is in agreement with the macrospin simulations and it suggests that the tilt in the magnetic anisotropy from the film normal makes the switching deterministic even without an in-plane magnetic field bias.

We study how the shape of the spin-wave resonance lines in rf voltage induced FMR can be used to extract the spin-wave density of states and the Gilbert damping within the precessing layer in nanoscale magnetic tunnel junctions that possess perpendicular magnetic anisotropy. We work with a field applied along the easy axis to preserve the cylindrical symmetry of the uniaxial perpendicularly magnetized systems. We first describe the experimental setup to study the susceptibility contributions of the spin waves in the field-frequency space. We then identify experimentally the maximum device size above which the spin waves confined in the free layer can no longer be studied in isolation as the linewidths of their discrete responses make them overlap into a continuous density of states. The rf voltage induced signal is the sum of two voltages that have comparable magnitudes: a first voltage that originates from the linear transverse susceptibility and rectification by magnetoresistance and a second voltage that arises from the nonlinear longitudinal susceptibility and the resultant time-averaged change of the exact micromagnetic configuration of the precessing layer. The transverse and longitudinal susceptibility signals have different dc bias dependencies such that they can be separated by measuring how the device rectifies the rf voltage at different dc bias voltages. The transverse and longitudinal susceptibility signals have different line shapes; their joint studies in both fixed field-variable frequency, or fixed frequency-variable field configurations can yield the Gilbert damping of the free layer of the device with a degree of confidence that compares well with standard ferromagnetic resonance. Our method is illustrated on FeCoB-based free layers in which the individual spin waves can be sufficiently resolved only for disk diameters below 200 nm. The resonance line shapes on devices with 90-nm diameters are consistent with a Gilbert damping of $0.011$. A single value of the damping factor accounts for the line shape of all the spin waves that can be characterized. This damping of 0.011 exceeds the value of 0.008 measured on the unpatterned films, which indicates that device-level measurements are needed for a correct evaluation of dissipation.

Quantum spin liquids (QSLs) are highly entangled states of quantum magnets which lie beyond the Landau paradigm of classifying phases of matter via broken symmetries. A physical route to arriving at QSLs is via frustration-induced quantum melting of ordered states such as valence bond crystals or magnetic orders. Here we show, using extensive exact diagonalization (ED) and density-matrix renormalization group (DMRG) studies of concrete $SU\left(2\right)$ invariant spin models on honeycomb, triangular, and square lattices, that chiral spin liquids (CSLs) emerge as descendants of triple-$Q$ spin crystals with tetrahedral magnetic order and a large scalar spin chirality. Such ordered-to-CSL melting transitions may yield lattice realizations of effective Chern-Simons-Higgs field theories. Our work provides a distinct unifying perspective on the emergence of CSLs and suggests that materials with certain noncoplanar magnetic orders might provide a good starting point to search for CSLs.

The absence of both spin freezing and of a static Jahn-Teller effect have led to the proposition that ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ is a quantum spin-orbital liquid. However, theoretical understanding of the microscopic origin of this behavior has been hampered by a lack of consensus on the lattice structure. Cu ions have been proposed to realize either a triangular lattice, a short-range ordered honeycomb lattice, or a disordered lattice with stripelike correlations. Here we analyze the stability of idealized versions of these arrangements using density functional theory. We find stripe order of Cu ions to be energetically favored, hinting towards stripelike local Cu-Cu arrangements, while long-range order is presumably hindered due to disorder effects. Furthermore, we find evidence of significant interlayer interactions between Cu-Sb dumbbells, which affects the out-of-plane arrangement. Analysis of the relaxed crystal structures, electronic properties, and tight-binding parameters provides clues as to the nature of the Jahn-Teller distortions.

The asymmetry between electron and hole doping in high critical-temperature superconducting (HTS) cuprates is key information for the understanding of Cooper pair formation mechanisms. Despite intensive studies on different cuprates, a comprehensive description of related magnetic and charge excitations is still fragmentary. In the present work, artificial cuprates were used to cover the entire phase diagram within the same HTS family. In particular, Cu $L_3$-edge resonant inelastic x-ray scattering (RIXS) measurements were performed on artificial $n$- and $p$-type infinite layer (IL) epitaxial films. Beside several similarities, RIXS spectra show noticeable differences in the evolution, with doping level, of magnetic and charge intensity and damping. Compatible trends can be found in spectra measured on bulk cuprates, as well as in theoretical calculations of the spin dynamical structure factor $S(\mathbf{q},\omega )$. The findings give a deeper insight into the evolution of collective excitations across the cuprate phase diagram, and on underlying general features, only connected to the doping type. Moreover, they pave the way to the exploration of general properties of HTS physics over a broad range of conditions, by means of artificial compounds not constrained by the thermodynamic limitations governing the chemical stability of bulk materials.

It has recently been reported that Bi-doped ${\mathrm{LiNbO}}_3$ exhibits more excellent photorefractive properties than the traditional Fe doping. Bi-induced structural and physical properties remain unverified by either experiment or theory, however. Thus, here the basic characteristics of Bi-doped ${\mathrm{LiNbO}}_3$, such as the preferable Bi doping site, local lattice distortion, and the effect of Bi doping on the electronic structure and optical properties, are investigated by density functional theory with a hybrid functional. In particular, we focus on the effect of a Bi lone electron pair on the structural distortion and polaronic behavior of ${\mathrm{LiNbO}}_3$. The calculated results show that Bi substitutional Li in its +4 charge state (${{\mathrm{Bi}}_{\mathrm{Li}}}^{4+}$) and Bi substitutional Nb in its neutral state (${{\mathrm{Bi}}_{\mathrm{Nb}}}^0$) are energetically preferable in the majority of ${\mathrm{LiNbO}}_3$ samples. The incorporation of Bi could form a small bound electron polaron in ${\mathrm{LiNbO}}_3$. The strongly polarized localization of the Bi $6s^2$ lone electron pair around the Bi center dominantly contributes to the large local lattice relaxation and the huge energy gain of ${{\mathrm{Bi}}_{\mathrm{Li}}}^{2+}$ that result in the negative $U$ effect. A new ${{\mathrm{Bi}}_{\mathrm{Li}}}^{4+/2+}$ photorefractive center that is 2.2 eV deeper than the intrinsic ${{\mathrm{Nb}}_{\mathrm{Li}}}^{4+/2+}$ photorefractive center is introduced by Bi doping.

Based on first-principles calculations, we find that LiZnBi, a metallic hexagonal $ABC$ compound, has a pair of three-dimensional Dirac nodes and exhibits nontrivial topological properties under proper strain configurations. The nontrivial topological nature of the strained LiZnBi is directly demonstrated by calculating its ${\mathbb{Z}}_2$ index and the surface states. The low-energy states are shown to be sensitive to strain configurations, and we propose that these nontrivial topological properties can be observed under compressive in-plane strain in experiments. The finding of the Dirac semimetal phase in LiZnBi may intrigue further research on the topological properties of hexagonal $ABC$ materials and promote new practical applications.

We study the dynamics of a pulse-gated semiconductor double-quantum-dot qubit. In our experiments, the qubit coherence times are relatively long, but the visibility of the quantum oscillations is low. We show that these observations are consistent with a theory that incorporates decoherence arising from charge noise that gives rise to detuning fluctuations of the double dot. Because effects from charge noise are largest near the singlet-triplet avoided level crossing, the visibility of the oscillations is low when the singlet-triplet avoided level crossing occurs in the vicinity of the charge degeneracy point crossed during the manipulation, but there is only modest dephasing at the large detuning value at which the quantum phase accumulates. This theory agrees well with experimental data and predicts that the visibility can be increased greatly by appropriate tuning of the interdot tunneling rate.

Two-electron states of a double quantum dot (DQD) under irradiation of coherent boson (photon and phonon) fields are studied by measuring spin-flip tunneling current in the Pauli spin blockade regime. This measurement scheme allows us to investigate Rabi splitting and associated boson dressed states particularly in the deep dispersive regime where the detuning $\delta \equiv \hslash \omega -E_{\mathrm{AB}}$ between the boson energy $\hslash \omega$ and energy spacing $E_{\mathrm{AB}}$ of the two-level system is significantly large ($\delta \sim \hslash \omega$), where the permanent dipole moment in the DQD plays a significant role in the hybridization.

We study manifestations of spin-charge separation (SCS) in transport through a tunnel-coupled interacting single-channel quantum ring. We focus on the high-temperature case (temperature $T$ larger than the level spacing $\mathrm{\Delta }$) and discuss both the classical (flux-independent) and interference contributions to the tunneling conductance of the ring in the presence of magnetic flux. We demonstrate that the SCS effects, which arise solely from the electron-electron interaction, lead to the appearance of a peculiar fine structure of the electron spectrum in the ring. Specifically, each level splits into a series of sublevels, with their spacing governed by the interaction strength. In the high-$T$ limit, the envelope of the series contains of the order of $T/\mathrm{\Delta }$ sublevels. At the same time, SCS suppresses the tunneling width of the sublevels by a factor of $\mathrm{\Delta }/T$. As a consequence, the classical transmission through the ring remains unchanged compared to the noninteracting case: the suppression of tunneling is compensated by the increase of the number of tunneling channels. On the other hand, the flux-dependent contribution to the conductance depends on the interaction-induced dephasing rate which is known to be parametrically increased by SCS in an infinite system. We show, however, that SCS is not effective for dephasing in the limit of weak tunneling. Moreover, generically, in the almost closed ring, the dephasing rate does not depend on the interaction strength and is determined by the tunneling coupling to the leads. In certain special symmetric cases, dephasing is further suppressed. Similarly to the spinless case, the high-$T$ conductance shows, as a function of magnetic flux, a sequence of interaction-induced sharp negative peaks on top of the classical contribution.

We report results of first-principles study of the adsorption of atomic oxygen and magnesium on the Fe(001) surface. Two adsorption scenarios were considered. In the first process, the Mg atoms were adsorbed on the Fe(001) surface with preadsorbed O atoms, whereas in the second scenario metallic Mg preadsorbed on the Fe(001) surface was exposed to oxygen. For both O and Mg atoms, fourfold hollow sites were found as the energetically most favorable adsorption sites on the clean Fe(001) surface. The result of both adsorption scenarios was the formation of a MgO adlayer on the Fe(001) surface with a sharp MgO/Fe interface. In particular, the deposition of Mg atoms on O/Fe(001) showed that magnesium can pull out O adatoms from the Fe surface. Structural, electronic, and magnetic properties were analyzed as a function of O and Mg coverages. The calculated electronic structure and magnetic moments showed that the full MgO monolayer affects the properties of the Fe surface much weaker than an incomplete MgO adsorbate layer.

Plasmonic lenses, even of simple design, may have intricate spectral behavior. The spectral response of a plasmonic lens to a local, broadband excitation has rarely been studied despite its central importance in future applications. Here we use the unique combination of scanning tunneling microscopy (STM) and angle-resolved optical spectroscopy to probe the spectral response of a plasmonic lens. Such a lens consists of a series of concentric circular slits etched in a thick gold film. Spectrally broad, circular surface plasmon polariton (SPP) waves are electrically launched from the STM tip at the plasmonic lens center, and these waves scatter at the slits into a narrow, out-of-plane, light beam. We show that the angular distribution of the emitted light results from the interplay of the size of the plasmonic lens and the spectral width of the SPP nanosource. We then propose simple design rules for optimized light beaming with the smallest possible footprint. The spectral distribution of the emitted light depends not only on the SPP nanosource, but on the local density of electromagnetic states (EM-LDOS) at the nanosource position, which in turn depends on the cavity modes of the plasmonic microstructure. The key parameters for tailoring the spectral response of the plasmonic lens are the period of the slits forming the lens, the number of slits, and the lens inner diameter.

Double Dirac fermions have recently been identified as possible quasiparticles hosted by three-dimensional crystals with particular nonsymmorphic point-group symmetries. Applying a combined approach of $ab$ initio methods and dynamical mean-field theory, we investigate how interactions and double Dirac band topology conspire to form the electronic quantum state of ${\mathrm{Bi}}_2{\mathrm{CuO}}_4$. We derive a downfolded eight-band model of the pristine material at low energies around the Fermi level. By tuning the model parameters from the free band structure to the realistic strongly correlated regime, we find a persistence of the double Dirac dispersion until its constituting time-reversal symmetry is broken due to the onset of magnetic ordering at the Mott transition. Our calculations suggest that the double Dirac fermions in ${\mathrm{Bi}}_2{\mathrm{CuO}}_4$ can be restored by experimentally accessible hydrostatic pressures. In light of the growing attention to the topological quantum chemistry approach, our results on ${\mathrm{Bi}}_2{\mathrm{CuO}}_4$ show how many-body effects must be included beyond the static mean-field level for reliable predictions on new materials.