We report ${}^{139}\mathrm{La}$ and ${}^{63}\mathrm{Cu}$ NMR investigation of the successive charge order, spin order, and superconducting transitions in superoxygenated ${\mathrm{La}}_2{\mathrm{CuO}}_{4+y}$ single crystal with stage-4 excess oxygen order at $T_{\text{stage}}\simeq 290$ K. We show that the stage-4 order induces tilting of ${\mathrm{CuO}}_6$ octahedra below $T_{\text{stage}}$, which in turn causes ${}^{139}\mathrm{La}$ NMR line broadening. The structural distortion continues to develop far below $T_{\text{stage}}$, and completes at $T_{\text{charge}}\simeq 60$ K, where charge order sets in. This sequence is reminiscent of the the charge-order transition in Nd codoped ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$ that sets in once the low-temperature tetragonal phase is established. We also show that the paramagnetic ${}^{63}\mathrm{Cu}$ NMR signals are progressively wiped out below $T_{\text{charge}}$ due to enhanced low-frequency spin fluctuations in charge-ordered domains, but the residual ${}^{63}\mathrm{Cu}$ NMR signals continue to exhibit the characteristics expected for optimally doped superconducting ${\mathrm{CuO}}_2$ planes. This indicates that charge order in ${\mathrm{La}}_2{\mathrm{CuO}}_{4+y}$ does not take place uniformly in space. In addition, unlike the typical second-order magnetic phase transitions, low-frequency Cu spin fluctuations as probed by ${}^{139}\mathrm{La}$ nuclear spin-lattice relaxation rate do not exhibit critical divergence at $T_{\text{spin}}$($\simeq T_c$) $=42$ K. These findings, including the spatially inhomogeneous nature of the charge-ordered state, are qualitatively similar to the case of ${\mathrm{La}}_{1.885}{\mathrm{Sr}}_{0.115}{\mathrm{CuO}}_4$ [Imai et al., Phys. Rev. B 96, 224508 (2017) and Arsenault et al., Phys. Rev. B 97, 064511 (2018)], but both charge and spin order take place more sharply in the present case.

We present a first-principles band calculation for the quasi-one-dimensional (Q1D) organic superconductor ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{ClO}}_4$. An effective tight-binding model with the TMTSF molecule to be regarded as the site is derived from a calculation based on maximally localized Wannier orbitals. We apply a two-particle self-consistent (TPSC) analysis by using a four-site Hubbard model, which is composed of the tight-binding model and an onsite (intramolecular) repulsive interaction, which serves as a variable parameter. We assume that the pairing mechanism is mediated by the spin fluctuation, and the sign of the superconducting gap changes between the inner and outer Fermi surfaces, which correspond to a $d$-wave gap function in a simplified Q1D model. With the parameters we adopt, the critical temperature for superconductivity estimated by the TPSC approach is approximately 1 K, which is consistent with experiment.

We establish the topology of the spin-singlet superconducting states in the bare hyperhoneycomb lattice, and we derive analytically the full phase diagram using only symmetry and topology in combination with simple energy arguments. The phase diagram is dominated by two states preserving time-reversal symmetry. We find a line-nodal state dominating at low doping levels that is topologically nontrivial and exhibits surface Majorana flatbands, which we show perfectly match the bulk-boundary correspondence using the Berry phase approach. At higher doping levels, we find a fully gapped state with trivial topology. By analytically calculating the topological invariant of the nodal lines, we derive the critical point between the line-nodal and fully gapped states as a function of both pairing parameters and doping. We find that the line-nodal state is favored not only at lower doping levels but also if symmetry-allowed deformations of the lattice are present. Adding simple energy arguments, we establish that a fully gapped state with broken time-reversal symmetry likely appears covering the actual phase transition. We find this fully gapped state to be topologically trivial, while we find an additional point-nodal state at very low doing levels that also break time-reversal symmetry and has nontrivial topology with associated Fermi surface arcs. We eventually address the robustness of the phase diagram to generalized models also including adiabatic spin-orbit coupling, and we show how all but the point-nodal state are reasonably stable.

For $D$-dimensional weakly interacting topological insulators in certain symmetry classes, the topological invariant can be calculated from a $D$- or $(D+1)$-dimensional integration over a certain curvature function that is expressed in terms of single-particle Green's functions. Based on the divergence of curvature function at the topological phase transition, we demonstrate how a renormalization group approach circumvents these integrations and reduces the necessary calculation to that for the Green's function alone, rendering a numerically efficient tool to identify topological phase transitions in a large parameter space. The method further unveils a number of statistical aspects related to the quantum criticality in weakly interacting topological insulators, including correlation function, critical exponents, and scaling laws, that can be used to characterize the topological phase transitions driven by either interacting or noninteracting parameters. We use 1D class BDI and 2D class A Dirac models with electron-electron and electron-phonon interactions to demonstrate these principles and find that interactions may change the critical exponents of the topological insulators.

This paper experimentally demonstrates the effects of inharmonic photonic transition in tailored aperiodic space-time refractive index modulated media. Such effects introduce a pure frequency mixing based on the simultaneous and distinct shifts in the spatial and temporal frequencies. The medium is characterized with a periodic temporal modulation and a tailored aperiodic spatially modulated permittivity and permeability, yielding aperiodic, large and tunable photonic band gaps. Since the medium is time periodic, an infinite number of space-time mixing products are generated with a distance equal to the temporal frequency of the pump wave. However, thanks to the tailored spatial aperiodicity of the medium and associated photonic band gaps, transition to unwanted space-time mixing products is prohibited. Interesting features include tunability of the operation frequencies of the mixer via space-time modulation parameters, high isolation, linear response, and possibility of conversion gain due to the transfer of energy and momentum of the space-time modulation to the input wave. We derive the analytical solution for such mixer with aperiodic space-modulated permittivity and permeability and periodic time modulation, and then provide the synthesis procedure which takes into account the effects of space-time modulation inhomogeneity. Finally, to see the effect of the tailoring of space modulation, we compare the experimental results of the aperiodic space-time modulated pure mixer with those of the conventional periodic uniform space-time modulated medium.

Topological nodal rings can be classified into three types according to the slopes in their energy dispersion. Type-I nodal rings consist of type-I points, type-II rings consist of type-II points, and type-III rings consist of anisotropic points. Taking carbon networks as an example, we show that all three types can exist and transform from one to another under experimentally realizable strain. Within a given type, a transition between a topological metal phase and a semiconductor phase also takes place, and each type of nodal rings shrinks into the corresponding type of semi-Dirac points, including novel type-II and -III semi-Dirac points. These topological features are expected to exhibit diverse electron-hole pocket patterns and Landau levels, giving rise to unusual transport properties.

We have studied the structural, electronic, and magnetic properties of spinel ${\mathrm{Co}}_3{\mathrm{O}}_4$(111) surfaces and their interfaces with ZnO(0001) using density functional theory within the generalized gradient approximation with the on-site Coulomb repulsion term. Two possible forms of spinel surface, containing ${\mathrm{Co}}^{2+}$ or ${\mathrm{Co}}^{3+}$ ions and terminated with either cobalt or oxygen ions, were considered, as well as their interface with zinc oxide. Our calculations demonstrate that ${\mathrm{Co}}^{3+}$ ions attain nonzero magnetic moments at the surface and interface, in contrast to the bulk, where they are not magnetic, leading to the ferromagnetic ordering. Since heavily Co doped ZnO samples can contain a ${\mathrm{Co}}_3{\mathrm{O}}_4$ secondary phase, such magnetic ordering at the interface might explain the origin of the magnetism in such diluted magnetic semiconductors.

The generalized tight-binding model is developed to investigate the rich and unique electronic properties of $AB$-bt (bottom-top) bilayer silicene under uniform perpendicular electric and magnetic fields. The first pair of conduction and valence bands, with an observable energy gap, displays unusual energy dispersions. Each group of conduction/valence Landau levels (LLs) is further classified into four subgroups, i.e., the sublattice- and spin-dominated LL subgroups. The magnetic-field-dependent LL energy spectra exhibit irregular behavior corresponding to the critical points of the band structure. Moreover, the electric field can induce many LL anticrossings. The main features of the LLs are uncovered with many van Hove singularities in the density-of-states and nonuniform delta-function-like peaks in the magnetoabsorption spectra. The feature-rich magnetic quantization directly reflects the geometric symmetries, intralayer and interlayer atomic interactions, spin-orbital couplings, and field effects. The results of this work can be applied to novel designs of Si-based nanoelectronics and nanodevices with enhanced mobilities.

The nonlinear magneto-optic responses are investigated for gapped graphene and doped graphene in a perpendicular magnetic field. The electronic states are described by Landau levels, and the electron dynamics in an optical field is obtained by solving the density matrix in the equation of motion. In the linear dispersion approximation around the Dirac points, both linear conductivity and third-order nonlinear conductivities are numerically evaluated for infrared frequencies. The nonlinear phenomena, including third harmonic generation, Kerr effects, and two-photon absorption, and four-wave mixing, are studied. All optical conductivities show strong dependence on the magnetic field. At weak magnetic fields, our results for doped graphene agree with those in the literature. We also present the spectra of the conductivities of gapped graphene. At strong magnetic fields, the third-order conductivities show peaks with varying the magnetic field and the photon energy. These peaks are induced by the resonant transitions between different Landau levels. The resonant channels, the positions, and the divergences of peaks are analyzed. The conductivities can be greatly modified, up to orders of magnitude. The dependence of the conductivities on the gap parameter and the chemical potential is studied.

Zirconium tungstate undergoes a sequence of phase transitions from cubic ($\alpha -{\mathrm{ZrW}}_2{\mathrm{O}}_8$) to orthorhombic ($\gamma -{\mathrm{ZrW}}_2{\mathrm{O}}_8$) to amorphous ($a-{\mathrm{ZrW}}_2{\mathrm{O}}_8$) upon increasing pressure at room temperature. The amorphous phase is known to undergo anomalous endothermic recrystallization into a high-temperature $\beta -{\mathrm{ZrW}}_2{\mathrm{O}}_8$ phase above ${600}^{\circ }\mathrm{C}$ at ambient pressure (and back to $\alpha -{\mathrm{ZrW}}_2{\mathrm{O}}_8$ when brought to room temperature). The endothermic recrystallization of $a-{\mathrm{ZrW}}_2{\mathrm{O}}_8$ is preceded by an irreversible exothermic structural relaxation. New W-O bonds are formed upon amorphization, continuing a tendency of increasing W coordination number in going from $\alpha$ to $\gamma -{\mathrm{ZrW}}_2{\mathrm{O}}_8$. In fact, contrarily to $\alpha -{\mathrm{ZrW}}_2{\mathrm{O}}_8$, in which one-quarter of the oxygen atoms are bonded only to one W (terminal oxygens), previous works found no evidence of single-bonded oxygen atoms in $a-{\mathrm{ZrW}}_2{\mathrm{O}}_8$. It thus could be argued that the irreversible character of the structural relaxation of $a-{\mathrm{ZrW}}_2{\mathrm{O}}_8$ is due to W-O bond breaking upon annealing of the amorphous phase. To test this hypothesis, x-ray diffraction, ${}^{17}\mathrm{O}$ magic-angle spinning NMR, Raman, and far-infrared analyses were performed on samples of amorphous zirconium tungstate previously annealed to increasingly higher temperatures, looking for any evidence of features that could be assigned to the presence of terminal oxygen atoms. No evidence of single-bonded oxygen was found before the onset of recrystallization. Furthermore, the kinetics of the structural relaxation of $a-{\mathrm{ZrW}}_2{\mathrm{O}}_8$ is consistent with a continuous spectrum of activation energy, spanning all the range from 1 to $2.5\mathrm{eV}$. These findings suggest that the structural relaxation of amorphous zirconium tungstate, however irreversible, is not accompanied by W-O bond breaking, but most probably characterized by a succession of (mostly) irreversible local atomic rearrangements.

The phase diagram of a cubic chiral magnet MnSi with multiple Dzyaloshinskii-Moriya (DM) vectors as a function of temperature $T$ and dc magnetic field $H_{\mathrm{dc}}$ was investigated using intensity mapping of the odd-harmonic responses of ac magnetization $(M_{1\omega }$ and $M_{3\omega })$, and the responses at phase boundaries were evaluated according to a prescription [J. Phys. Soc. Jpn. 84, 104707 (2015)]. By evaluating $M_{3\omega }/M_{1\omega }$ appearing at phase boundaries, the robustness of noncollinear spin texture in both the helimagnetic (HM) and the skyrmion lattice (SkL) phases of MnSi was discussed. The robustness of vortices-type solitonic texture SkL in MnSi is smaller than those of both the single DM HM and chiral soliton lattice phases of a monoaxial chiral magnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$, and furthermore the robustness of the multiple DM HM phase in MnSi is smaller than that of its SkL. Through magnetic diagnostics over the wide $T-H_{\mathrm{dc}}$ range, we found a new paramagnetic (PM) region with ac magnetic hysteresis, where spin fluctuations have been observed via electrical magnetochiral effect. The anomalies observed in the previous ultrasonic attenuation measurement correspond to the peak positions of out-of-phase $M_{1\omega }$. The appearance of a new PM region occurs at a characteristic magnetic field, above which indeed the SkL phase appears. It has us suppose that the new PM region could be a phase with spin fluctuation like the skyrmion gas phase.

Weyl nodes have been experimentally realized in photonic, electronic, and phononic crystals. However, magnonic Weyl nodes are yet to be seen experimentally. In this paper, we propose Weyl magnon nodes in noncoplanar stacked frustrated kagome antiferromagnets, naturally available in various real materials. Most crucially, the Weyl nodes in the current system occur at the lowest excitation and possess a topological thermal Hall effect, therefore they are experimentally accessible at low temperatures due to the population effect of bosonic quasiparticles. In stark contrast to other magnetic systems, the current Weyl nodes do not rely on time-reversal symmetry breaking by the magnetic order. Rather, they result from explicit macroscopically broken time reversal symmetry by the scalar spin chirality of noncoplanar spin textures and can be generalized to chiral spin liquid states. Moreover, the scalar spin chirality gives a real space Berry curvature which is not available in previously studied magnetic Weyl systems. We show the existence of magnon arc surface states connecting projected Weyl magnon nodes on the surface Brillouin zone. We also uncover the first realization of triply-degenerate nodal magnon point in the noncollinear regime with zero scalar spin chirality.

We study the motion of an electron bubble in the zero-temperature limit where neither phonons nor rotons provide a significant contribution to the drag exerted on an ion moving within the superfluid. By using the Gross-Clark model, in which a Gross-Pitaevskii equation for the superfluid wave function is coupled to a Schrödinger equation for the electron wave function, we study how vortex nucleation affects the measured drift velocity of the ion. We use parameters that give realistic values of the ratio of the radius of the bubble with respect to the healing length in superfluid ${}^4\mathrm{He}$ at a pressure of one bar. By performing fully three-dimensional spatiotemporal simulations of the superfluid coupled to an electron, that is modeled within an adiabatic approximation and moving under the influence of an applied electric field, we are able to recover the key dynamics of the ion-vortex interactions that arise and the subsequent ion-vortex complexes that can form. Using the numerically computed drift velocity of the ion as a function of the applied electric field, we determine the vortex nucleation limited mobility of the ion to recover values in good agreement with measured data.

Three models of the ${\mathrm{C}}_{60}$ molecular crystal are studied using molecular dynamics simulations to resolve the roles played by intermolecular and intramolecular degrees of freedom (DOF) in its structural, mechanical, and thermal properties at temperatures between 35 and 400 K. In the full DOF model, all DOF are active. In the rigid body model, the intramolecular DOF are frozen, such that only center of mass (COM) translations and molecular rotations/librations are active. In the point mass model, the molecule is replaced by a point mass, such that only COM translations are active. The zero-pressure lattice constants and bulk moduli predicted from the three models fall within ranges of 0.15 and 20%. The thermal conductivity of the point mass model is the largest across the temperature range, showing a crystal-like temperature dependence (i.e., it decreases with increasing temperature) due to the presence of phonon modes associated with the COM translations. The rigid body model thermal conductivity is the smallest and follows two distinct regimes. It is crystal-like at low temperatures and becomes temperature invariant at high temperatures. The latter is typical of the behavior of an amorphous material. By calculating the rotational diffusion coefficient, the transition between the two regimes is found to occur at the temperature where the molecules begin to rotate freely. Above this temperature, phonons related to COM translations are scattered by the rotational DOF. The full DOF model thermal conductivity is larger than that of the rigid body model, indicating that intramolecular DOF contribute to thermal transport.

Raman spectroscopy experiments on single crystals of ${\mathrm{CuSb}}_2{\mathrm{O}}_6$ and ${\mathrm{CoSb}}_2{\mathrm{O}}_6$ quasi-one-dimensional antiferromagnets with trirutile crystal structure were performed, with a focus on the first material. The observed Raman-active phonon modes and previously reported infrared-active modes were identified with the aid of ab initio lattice dynamics calculations. The structural transition between monoclinic $\beta -{\mathrm{CuSb}}_2{\mathrm{O}}_6$ and tetragonal $\alpha -{\mathrm{CuSb}}_2{\mathrm{O}}_6$ phases at $T_s=400$ K is manifested in our spectra by a “repulsion” of two accidentally quasidegenerate symmetric modes below $T_s$, caused by a phonon mixing effect that is only operative in the monoclinic $\beta -{\mathrm{CuSb}}_2{\mathrm{O}}_6$ phase due to symmetry restrictions. Also, two specific phonons, associated with ${\mathrm{CuO}}_6$ octahedra rotation and with a Jahn-Teller elongation mode, soften and broaden appreciably as $T\to T_s$. A crossover from a displacive to an order-disorder transition at $T_s$ is inferred.

Using time-dependent Ginzburg-Landau theory we find oscillations of critical current density $j_c$ as a function of the length $L$ of the bridge formed from a two-band superconductor. We explain this effect by the appearance of the phase solitons in the bridge at $j<j_c$, whose number changes with a change of $L$. In the case of sufficiently strong interband coupling, oscillations of $j_c$ disappear.

The nature of the ground state of the spin $S=1/2$ Heisenberg antiferromagnet on the kagome lattice with breathing anisotropy (i.e., with different superexchange couplings $J_{▵}$ and $J_{▿}$ within elementary up- and down-pointing triangles) is investigated within the framework of Gutzwiller projected fermionic wave functions and Monte Carlo methods. We analyze the stability of the $\text{U}\left(1\right)$ Dirac spin liquid with respect to the presence of fermionic pairing that leads to a gapped ${\mathbb{Z}}_2$ spin liquid. For several values of the ratio $J_{▿}/J_{▵}$, the size scaling of the energy gain due to the pairing fields and the variational parameters are reported. Our results show that the energy gain of the gapped spin liquid with respect to the gapless state either vanishes for large enough system size or scales to zero in the thermodynamic limit. Similarly, the optimized pairing amplitudes (responsible for opening the spin gap) are shown to vanish in the thermodynamic limit. Our outcome is corroborated by the application of one and two Lanczos steps to the gapless and gapped wave functions, for which no energy gain of the gapped state is detected when improving the quality of the variational states. Finally, we discuss the competition with the “simplex” ${\mathbb{Z}}_2$ resonating-valence-bond spin liquid, valence-bond crystal, and nematic states in the strongly anisotropic regime, i.e., $J_{▿}\ll J_{▵}$.

The Edelstein effect occurring in systems with broken inversion symmetry generates a spin polarization when an electric field is applied, which is most advantageous in spintronics applications. Unfortunately, it became apparent that this kind of magnetoelectric effect is very small in semiconductors. We here demonstrate that correlation effects can strongly enhance the magnetoelectric effect. Particularly, we observe a strong enhancement of the Edelstein effect in $f$-electron systems close to the coherence temperature, where the $f$ electrons change their character from localized to itinerant. We furthermore show that this enhancement can be explained by a coupling between the conduction electrons and the still localized $f$ electrons.

Topological crystalline insulator is a recently discovered topological phase of matter. It possesses multiple Dirac surface states, which are protected by the crystal symmetry. This is in contrast to the time-reversal symmetry that is operative in the well-known topological insulators. In the presence of a Zeeman field and/or strain, the multiple Dirac surface states are gapped. The high-Chern-number quantum anomalous Hall (QAH) state is predicted to emerge if the chemical potential resides in all the Zeeman gaps. Here, we use molecular-beam epitaxy to grow 12 double-layer (DL) pure and Cr-doped SnTe (111) thin film on heat-treated $\mathrm{SrTi}{\mathrm{O}}_3$ (111) substrate using a quintuple layer of insulating ${\left(\mathrm{B}{\mathrm{i}}_{0.2}\mathrm{S}{\mathrm{b}}_{0.8}\right)}_2\mathrm{T}{\mathrm{e}}_3$ topological insulator as a buffer film. The Hall traces of Cr-doped SnTe film at low temperatures display square hysteresis loops indicating long-range ferromagnetic order with perpendicular anisotropy. The Curie temperature of the $12\mathrm{DL}\mathrm{S}{\mathrm{n}}_{0.9}\mathrm{C}{\mathrm{r}}_{0.1}\mathrm{Te}$ film is ∼110 K. Due to the chemical potential crossing the bulk valence bands, the anomalous Hall resistance of $12\mathrm{DL}\mathrm{S}{\mathrm{n}}_{0.9}\mathrm{C}{\mathrm{r}}_{0.1}\mathrm{Te}$ film is substantially lower than the predicted quantized value $\left(\sim 1/4h/e^2\right)$. It is possible that with systematic tuning the chemical potential via chemical doping and electrical gating, the high-Chern-number QAH state can be realized in the Cr-doped SnTe (111) thin film.