One of the best known causes of dissipation in ac-driven quantum systems stems from photon absorption causing transitions between levels. Dissipation can also be caused by the retarded response to the time-dependent excitation, and in general gives insight into the system's relaxation times and mechanisms. Here we address the dissipation in a mesoscopic normal wire with superconducting contacts, that sustains a dissipationless supercurrent at zero frequency and that may therefore naively be expected to remain dissipationless at a frequency lower than the superconducting gap. We probe the high-frequency linear response of such a normal metal/superconductor (NS) ring to a time-dependent flux by coupling it to a highly sensitive multimode microwave resonator. Far from being the simple, dissipationless derivative of the supercurrent-versus-phase relation, the ring's ac susceptibility also displays a dissipative component whose phase dependence is a signature of the dynamical processes occurring within the Andreev spectrum. We show how dissipation is driven by the competition between two mechanisms. The first is the relaxation of the Andreev level distribution function, while the second corresponds to microwave-induced transitions within the spectrum. Depending on the relative strength of those contributions, dissipation can be maximal at $\pi$, a phase at which the proximity-induced minigap closes, or can be maximal near $\pi /2$, a phase at which the dc supercurrent is maximal. We also find that the dissipative response paradoxically increases at low temperature and can even exceed the normal-state conductance. The results are successfully confronted with theoretical predictions of the Kubo linear response and time-dependent Usadel equations, derived from the Bogoliubov–de Gennes Hamiltonian describing the SNS junction. These experiments thus demonstrate the power of the ac susceptibility measurement of individual hybrid mesoscopic systems in probing in a controlled way the quantum dynamics of Andreev bound states. By spanning different physical regimes, our experiments provide unique access to inelastic scattering and spectroscopy of an isolated quantum coherent system, and reveal the associated relaxation times. This technique should be a tool of choice to investigate topological superconductivity and detect the topological protection of edge states.

We present a study of the charge order of 214 stripe ordered superconducting cuprates $\left({\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}\right){\mathrm{Sr}}_x{\mathrm{CuO}}_4$ and ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ for doping levels $0.11\le \mathit{p}\le 0.14$ by means of resonant x-ray scattering. Up to 6 T, we find no field dependence on either the integrated intensity or the correlation length of the charge modulations, providing evidence for strong stability of charge order under applied fields. The magnetic field data support a strong pinning scenario induced by the low-temperature tetragonal distortion and static disorder, and they highlight the role of the symmetry of the lattice on the stabilization of electronic periodicities.

In this study, realization and applications of anisotropic zero-refractive-index materials are proposed by exposing the unit cells of photonic crystals that exhibit Dirac-like cone dispersion to rotational symmetry reduction. Accidental degeneracy of two Bloch modes in the Brillouin zone center of two-dimensional $C_2$-symmetric photonic crystals gives rise to the semi-Dirac cone dispersion. The proposed $C_2$-symmetric photonic crystals behave as epsilon-and-mu-near-zero materials (${\varepsilon }^{\mathrm{eff}}\approx 0, {\mu }^{\mathrm{eff}}\approx 0$) along one propagation direction, but behave as epsilon-near-zero material (${\varepsilon }^{\mathrm{eff}}\approx 0, {\mu }^{\mathrm{eff}}\ne 0$) for the perpendicular direction at semi-Dirac frequency. By extracting the effective medium parameters of the proposed $C_4$- and $C_2$-symmetric periodic media that exhibit Dirac-like and semi-Dirac cone dispersions, intrinsic differences between isotropic and anisotropic materials are investigated. Furthermore, advantages of utilizing semi-Dirac cone materials instead of Dirac-like cone materials in photonic applications are demonstrated in both frequency and time domains. By using anisotropic transmission behavior of the semi-Dirac materials, photonic application concepts such as beam deflectors, beam splitters, and light focusing are proposed. Furthermore, to the best of our knowledge, semi-Dirac cone dispersion is also experimentally demonstrated for the first time by including negative, zero, and positive refraction states of the given material.

Strain in epitaxial ${\mathrm{HoNiO}}_3$ is found to be capable of influencing the charge and the spin of Ni ions and the orbital hybridization of Ni-O and further manipulating charge ordering and orbital ordering by our first-principles calculation. The tensile strain can encourage Jahn-Teller distortion instead of breathing distortion, which leads to the phase transition from the charge-ordering state to the orbital-ordering state. Furthermore, we calculated the potential energy of the two modes, which reveals that the biaxial strain tunes the competition between the breathing and Jahn-Teller distortions.

We show that approximate structural relaxation with a neural network enables orders of magnitude faster global optimization with an evolutionary algorithm in a density functional theory framework. The increased speed facilitates reliable identification of global minimum energy structures, as exemplified by our finding of a hollow ${\mathrm{Pt}}_{13}$ nanoparticle on an MgO support. We highlight the importance of knowing the correct structure when studying the catalytic reactivity of the different particle shapes. The computational speedup further enables screening of hundreds of different pathways in the search for optimum kinetic transitions between low-energy conformers and hence pushes the limits of the insight into thermal ensembles that can be obtained from theory.

We demonstrate symmetric wave propagations in asymmetric nonlinear systems. By solving the nonlinear Schördinger equation, we first analytically prove the existence of symmetric transmission in asymmetric systems with a single nonlinear delta-function interface. We then point out that a finite width of the nonlinear interface region is necessary to produce nonreciprocity in asymmetric systems. However, a geometrical resonant condition for breaking nonreciprocal propagation is then identified theoretically and verified numerically. With such a resonant condition, the nonlinear interface region of finite width behaves like a single nonlinear delta-barrier so that wave propagations in the forward and backward directions are identical under arbitrary incident wave intensity. As such, reciprocity reemerges periodically in the asymmetric nonlinear system when changing the width of interface region. Finally, similar resonant conditions of discrete nonlinear Schördinger equation are discussed. Therefore, we have identified instances of reciprocity that breaking spatial symmetry in nonlinear interface systems is not sufficient to produce nonreciprocal wave propagation.

Electronic structure and optical properties of triangular phosphorene quantum dots have been investigated theoretically. Based on systematic configuration interaction calculations, the ground and excited states of the interacting many-electron system together with its optical absorption spectrum are obtained. For the nanodot with 60 phosphorus atoms in various dielectric environments, it is found that the spin gap of the correlated system surprisingly overlaps its optical gap over a large range of the effective dielectric constant. The overlapping of the spin and optical gaps can be attributed to the fact that the extra correlation energy in the spin singlet almost compensates the exchange energy in the spin triplet in the presence of strong long-range electron-electron interactions. Moreover, both the spin and optical gaps are shown to be greatly suppressed as the screening effect becomes strong. When the dielectric constant decreases below 2.65, it is seen that the spin gap becomes negative and the quantum dot undergoes a phase transition from nonmagnetic to ferromagnetic. Our results are compared with the previous experimental and theoretical works.

We present a methodology to calculate the formation energy of a charged defect at a surface, an interface, or a two-dimensional material in the presence of a macroscopic electric field. We demonstrate that the proposed formalism corrects for electrostatic artifacts in standard repeated-slab calculations and allows us to extract reliably the formation energy in the isolated defect limit independently of vacuum thickness, slab thickness, or lateral supercell size. The formalism does not enter the self-consistency loop of a density functional theory (DFT) calculation, but requires as input only the electrostatic potential of the converged calculation. Thus, employing the proposed scheme does not require any changes in the DFT code, but only a postprocessing module that we provide for various codes.

The interband $\pi$ and $\pi +\sigma$ plasmons in pristine graphene and the Dirac plasmon in doped graphene are not applicable, since they are broad or weak, and weakly couple to an external longitudinal or electromagnetic probe. Therefore, the ab initio density functional theory is used to demonstrate that the chemical doping of the graphene by the alkali or alkaline-earth atoms dramatically changes the poor graphene excitation spectrum in the ultraviolet frequency range (4–10 eV). Four prominent modes are detected. Two of them are the intralayer plasmons with square-root dispersion, characteristic of the two-dimensional modes. The remaining two are the interlayer plasmons, very strong in the long-wavelength limit but damped for larger wave vectors. The optical absorption calculations show that the interlayer plasmons are both optically active, which makes these materials suitable for small-organic-molecule sensing. This is particularly intriguing because the optically active two-dimensional plasmons have not been detected in other materials.

Substitutionally boron-doped multiwalled carbon nanotubes (B-CNTs) with lengths mainly less than $0.5\mu \mathrm{m}$ and diameters 10–30 nm have been obtained by arc-discharge evaporation of the graphite anode containing boron material. The broad peak has been observed in the midinfrared conductivity spectra of the thin film comprising B-CNTs. The peak was suggested to be associated with a phenomenon known as localized plasmon resonance. Theoretical analysis has been done to confirm the possibility of this phenomenon to occur in the B-CNTs.

In positive muon spin rotation and relaxation spectroscopy it is becoming customary to take advantage of density functional theory (DFT) based computational methods to aid the experimental data analysis. DFT-aided muon site determination is especially useful for measurements performed in magnetic materials, where large contact hyperfine interactions may arise. Here we present a systematic analysis of the accuracy of the ab initio estimation of muon's hyperfine contact field on elemental transition metals, performing state-of-the-art spin-polarized plane-wave DFT and using the projector-augmented pseudopotential approach, which allows one to include the core state effects due to the spin ordering. We further validate this method in not-so-simple, noncentrosymmetric metallic compounds, presently of topical interest for their spiral magnetic structure giving rise to skyrmion phases, such as MnSi and MnGe. The calculated hyperfine fields agree with experimental values in all cases, provided the spontaneous spin magnetization of the metal is well reproduced within the approach. To overcome the known limits of the conventional mean-field approximation of DFT on itinerant magnets, we adopt the so-called reduced Stoner theory [L. Ortenzi et al., Phys. Rev. B 86, 064437 (2012)]. We establish the accuracy of the estimated muon contact field in metallic compounds with DFT and our results show improved agreement with experiments compared to those of earlier publications.

Strongly interacting phonons and orbital excitations are observed in the same energy range for ${\mathrm{CuSb}}_2{\mathrm{O}}_6$, unlocking a so-far unexplored type of electron-phonon interaction. An orbital wave at $\sim 550{\mathrm{cm}}^{-1}$ softens on warming and strongly interferes with a phonon at $\sim 500{\mathrm{cm}}^{-1}$, giving rise to a merged excitation of mixed character. An electronic continuum grows on warming to the orbital ordering temperature $T_{OO}=400$ K, generating an important phonon decay channel. This direct and simultaneous observation of orbital and vibrational excitations reveals details of their combined dynamics. In addition, phonon frequency anomalies due to magnetic correlations are observed below $\sim 150$ K, much above the three-dimensional magnetic ordering temperature $T_N^{3D}=8.5$ K, confirming one-dimensional magnetic correlations along Cu-O-O-Cu linear chains in the paramagnetic state.

We show theoretically and numerically that an antiferromagnetic domain wall can be moved by a rotating magnetic field in the presence of Dzyaloshinskii-Moriya interaction (DMI). Two motion modes are found: rigid domain wall motion at low frequency (corresponding to the perfect frequency synchronization) and the oscillating motion at high frequency. In the full synchronized region, the steady velocity of the domain wall is universal, in the sense that it depends only on the frequency of the rotating field and the ratio between DMI strength and exchange constant. The domain wall velocity is independent of the Gilbert damping and the rotating field strength. Moreover, a rotating field in megahertz is sufficient to move the antiferromagnetic domain wall.

The structural, electric, and magnetic properties of bulk hexagonal $\mathrm{LuFe}{\mathrm{O}}_3$ are investigated. Single phase hexagonal $\mathrm{LuFe}{\mathrm{O}}_3$ has been successfully stabilized in the bulk form without any doping by sol-gel method. The hexagonal crystal structure with $P{6}_{3}cm$ space group has been confirmed by x-ray-diffraction, neutron-diffraction, and Raman spectroscopy study at room temperature. Neutron diffraction confirms the hexagonal phase of $\mathrm{LuFe}{\mathrm{O}}_3$ persists down to 6 K. Further, the x-ray photoelectron spectroscopy established the 3+ oxidation state of Fe ions. The temperature-dependent magnetic dc susceptibility, specific heat, and neutron-diffraction studies confirm an antiferromagnetic ordering below the Néel temperature $\left(T_{\mathrm{N}}\right)\sim 130\mathrm{K}$. Analysis of magnetic neutron-diffraction patterns reveals an in-plane ($ab$-plane) ${120}^{\circ }$ antiferromagnetic structure, characterized by a propagation vector $k=\left(000\right)$ with an ordered moment of $2.84{\mu }_{\mathrm{B}}/\mathrm{F}{\mathrm{e}}^{3+}$ at 6 K. The ${120}^{\circ }$ antifferomagnetic ordering is further confirmed by spin-orbit coupling density functional theory calculations. The on-site coulomb interaction ($U$) and Hund's parameter $\left(J_H\right)$ on Fe atoms reproduced the neutron-diffraction ${\mathrm{\Gamma }}_1$ spin pattern among the Fe atoms. $P\text{−}E$ loop measurements at room temperature confirm an intrinsic ferroelectricity of the sample with remnant polarization $P_{\mathrm{r}}\sim 0.18\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$. A clear anomaly in the dielectric data is observed at $\sim T_{\mathrm{N}}$ revealing the presence of magnetoelectric coupling. A change in the lattice constants at $T_{\mathrm{N}}$ has also been found, indicating the presence of a strong magnetoelastic coupling. Thus a coupling between lattice, electric, and magnetic degrees of freedom is established in bulk hexagonal $\mathrm{LuFe}{\mathrm{O}}_3$.

We classify and characterize three-dimensional $\mathrm{U}\left(1\right)$ quantum spin liquids [deconfined $\mathrm{U}\left(1\right)$ gauge theories] with global symmetries. These spin liquids have an emergent gapless photon and emergent electric/magnetic excitations (which we assume are gapped). We first discuss in great detail the case with time-reversal and $\text{SO}\left(3\right)$ spin rotational symmetries. We find there are 15 distinct such quantum spin liquids based on the properties of bulk excitations. We show how to interpret them as gauged symmetry-protected topological states (SPTs). Some of these states possess fractional response to an external $\mathrm{SO}\left(3\right)$ gauge field, due to which we dub them “fractional topological paramagnets.” We identify 11 other anomalous states that can be grouped into three anomaly classes. The classification is further refined by weakly coupling these quantum spin liquids to bosonic symmetry protected topological (SPT) phases with the same symmetry. This refinement does not modify the bulk excitation structure but modifies universal surface properties. Taking this refinement into account, we find there are 168 distinct such $\mathrm{U}\left(1\right)$ quantum spin liquids. After this warm-up, we provide a general framework to classify symmetry enriched $\mathrm{U}\left(1\right)$ quantum spin liquids for a large class of symmetries. As a more complex example, we discuss $\mathrm{U}\left(1\right)$ quantum spin liquids with time-reversal and $Z_2$ symmetries in detail. Based on the properties of the bulk excitations, we find there are 38 distinct such spin liquids that are anomaly-free. There are also 37 anomalous $\mathrm{U}\left(1\right)$ quantum spin liquids with this symmetry. Finally, we briefly discuss the classification of $\mathrm{U}\left(1\right)$ quantum spin liquids enriched by some other symmetries.

We study the topological properties and transport in the Bernevig-Hughes-Zhang model undergoing a slow quench between different topological regimes. Due to the closing of the band gap during the quench, the system ends up in an excited state. We prove that for quenches that preserve the time-reversal symmetry, the ${\mathbb{Z}}_2$ invariant remains equal to the one evaluated in the initial state. On the other hand, the bulk spin Hall conductivity does change, and its time average approaches that of the ground state of the final Hamiltonian. The deviations from the ground-state spin Hall conductivity as a function of the quench time follow the Kibble-Zurek scaling. We also consider the breaking of the time-reversal symmetry, which restores the correspondence between the bulk invariant and the transport properties after the quench.

Density functional approximations to the exchange-correlation energy are designed to be exact for an electron gas of uniform density parameter $r_s$ and relative spin polarization $\zeta$, requiring a parametrization of the correlation energy per electron ${\epsilon }_c(r_s,\zeta )$. We consider three widely used parametrizations [J. P. Perdew and A. Zunger, Phys. Rev. B 23, 5048 (1981) or PZ81, S. H. Vosko, L. Wilk, and M. Nusair, Can. J. Phys. 58, 1200 (1980) or VWN80, and J. P. Perdew and Y. Wang, Phys. Rev. B 45, 13244 (1992) or PW92] that interpolate the quantum Monte Carlo (QMC) correlation energies of Ceperley-Alder [Phys. Rev. Lett. 45, 566 (1980)], while extrapolating them to known high-$(r_s\to 0)$ and low- $(r_s\to \infty )$ density limits. For the physically important range $0.5\le r_s\le 20$, they agree closely with one another, with differences of 0.01 eV (0.5%) or less between the latter two. The density parameter interpolation (DPI), designed to predict these energies by interpolation between the known high- and low-density limits, with almost no other input (and none for $\zeta =0$), is also reasonably close, both in its original version and with corrections for $\zeta \ne 0$. Moreover, the DPI and PW92 at $r_s=0.5$ are very close to the high-density expansion. The larger discrepancies with the QMC of Spink et al. [Phys. Rev. B 88, 085121 (2013)], of order 0.1 eV (5%) at $r_s=0.5$, are thus surprising, suggesting that the constraint-based PW92 and VWN80 parametrizations are more accurate than the QMC for $r_s<2$. For $r_s>2$, however, the QMC of Spink et al. confirms the dependence upon relative spin polarization predicted by the parametrizations.

Complex ordered phases involving spin and charge degrees of freedom in condensed matter, such as layered cuprates and nickelates, are exciting but not well understood solid-state phenomena. The rich underlying physics of the overdoped high-temperature superconductor $\mathrm{L}{\mathrm{a}}_{7/4}\mathrm{S}{\mathrm{r}}_{1/4}\mathrm{Cu}{\mathrm{O}}_4$ and colossal dielectric constant insulator $\mathrm{L}{\mathrm{a}}_{5/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Ni}{\mathrm{O}}_4$ is studied from first principles within density functional (perturbation) theory, including an effective Hubbard potential $U$ for the exchange and correlation of $d$ orbitals. Charge density wave (CDW) and spin density wave (SDW) orders are found in both materials, where the stripes are commensurate with the lattice. The SDWs are accompanied by complex antiferromagnetic spin arrangements along the stripes. The first series of conduction bands related to the pseudogap observed in the cuprate are found to be directly related to CDW order, while the colossal dielectric constant in the nickelate is demonstrated to be a result of vibronic coupling with CDW order. Differences between the two oxides are related to how the stripes fill with carriers.

We propose theoretically a method that allows to measure all the components of the quantum geometric tensor (the metric tensor and the Berry curvature) in a photonic system. The method is based on standard optical measurements. It applies to two-band systems, which can be mapped to a pseudospin, and to four-band systems, which can be described by two entangled pseudospins. We apply this method to several specific cases. We consider a 2D planar cavity with two polarization eigenmodes, where the pseudospin measurement can be performed via polarization-resolved photoluminescence. We also consider the $s$ band of a staggered honeycomb lattice with polarization-degenerate modes (scalar photons), where the sublattice pseudospin can be measured by performing spatially resolved interferometric measurements. We finally consider the $s$ band of a honeycomb lattice with polarized (spinor) photons as an example of a four-band model. We simulate realistic experimental situations in all cases. We find the photon eigenstates by solving the Schrödinger equation including pumping and finite lifetime, and then simulate the measurements to finally extract realistic mappings of the k-dependent tensor components.