We discuss an even-odd effect for an impurity with $N$-fold-degenerate internal states immersed in a two-dimensional superfluid–Mott-insulator quantum critical bath which is described by a spin-$S$ XY Bose-Kondo impurity model with $N=2S+1$. Using a dimensional- and momentum-cutoff regularized renormalization group and unbiased large-scale Monte Carlo numerical simulations, we establish the phase diagram for the $S=1$ impurity with all relevant terms included. We show that a threefold-degenerate $S=1$ impurity coupled to a critical bath has a nondegenerate ground state with a quantized charge, in qualitative contrast to the spin-1/2 case where the impurity is partially screened. We then argue that all impurities with odd-$2S$ degeneracy share the same universal physics as the spin-1/2 case and all impurities with even-$2S$ degeneracy are the same as the spin-1 case. We validate our conjecture with unbiased Monte Carlo simulations up to $S=2$. A physical consequence of this even-odd effect is that two spin-$1/2$ impurities in the critical bath form a long-range entangled state at a low temperature, which can be realized in ultracold atoms in an optical lattice.

Using elastic neutron scattering on single crystals of ${\mathrm{MoTe}}_2$ and ${\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{Te}}_2$ ($x\lesssim 0.01$), the temperature dependence of the recently discovered $T_d^{*}$ phase, present between the low-temperature orthorhombic $T_d$ phase and high-temperature monoclinic $1T^{\prime }$ phase, is explored. The $T_d^{*}$ phase appears only on warming from $T_d$ and is observed in the hysteresis region prior to the $1T^{\prime }$ transition. This phase consists of four layers in its unit cell, and is constructed by an “AABB” sequence of layer stacking operations rather than the “AB” and “AA” sequences of the $1T^{\prime }$ and $T_d$ phases, respectively. Though the $T_d^{*}$ phase emerges without disorder on warming from $T_d$, on cooling from $1T^{\prime }$ diffuse scattering is observed that suggests a frustrated tendency toward the “AABB” stacking.

We investigate the ground-state and finite-temperature properties of the mixed-spin $(s,S)$ Kitaev model. When one of the spins is a half integer and the other is an integer, we introduce two kinds of local symmetries, which results in a macroscopic degeneracy in each energy level. Applying the exact diagonalization to several clusters with $(s,S)=(1/2,1)$, we confirm the presence of this large degeneracy in the ground states, in contrast to conventional Kitaev models. By means of the thermal pure quantum state technique, we calculate the specific heat, entropy, and spin-spin correlations in the system. We find that in the mixed-spin Kitaev model with $(s,S)=(1/2,1)$, at least, the double-peak structure appears in the specific heat and the plateau in the entropy at intermediate temperatures, indicating the existence of spin fractionalization. Deducing the entropy in the mixed-spin system with $s,S\le 2$ systematically, we clarify that the smaller spin $s$ is responsible for the thermodynamic properties at higher temperatures.

The origin of multiferroicity in ${\mathrm{NdCrTiO}}_5$ has been examined. A first-order phase transition, due to breaking of the inversion symmetry, near the low-temperature antiferromagnetic ordering ($T_N\sim 21$ K) where spontaneous electric polarization also appears, is confirmed. From synchrotron x-ray powder diffraction measurements, the structure at low temperatures is found to be noncentrosymmetric (space group $Pba2$), justifying the polar behavior of this compound below $T_N$. Temperature-dependent Raman measurements reveal that the lattice is strongly coupled with the spins, pointing to the spin lattice correlations in a noncentrosymmetric space group being responsible for invoking the ferroelectricity below 21 K. Through time-of-flight neutron powder diffraction experiments, it is confirmed that the magnetic sublattices, both Nd and Cr, simultaneously order at $T_N$, and at temperatures below 15 K the Cr moments are found to be saturated while the Nd moments continue to grow until 6 K.

Single-crystal x-ray-diffraction experiments and electrical resistivity measurements were performed on intermetallic compounds of formula $R{\mathrm{NiC}}_2$ [rare earth ($R$): Y, Dy, Ho, and Er]. In the series of diffraction studies, we found three kinds of satellite reflections that had different wave vectors [incommensurate ${\mathbf{q}}_{1ic}\text{:}(0.5,0.5+\eta ,0)$ and commensurate ${\mathbf{q}}_{1c}\text{:}(0.5,0.5,0)$ and ${\mathbf{q}}_{2c}\text{:}(0.5,0.5,0.5)]$ at temperatures where their resistivities show anomalies, such as a hump, a sharp increase, or a dent. Satellite reflections with nearly identical wave vectors have also been reported in $R{\mathrm{NiC}}_2$ composed of earlier lanthanide atoms (${\mathrm{GdNiC}}_2$ and ${\mathrm{TbNiC}}_2$). These results indicate the formation of a charge density wave (CDW) also in $R{\mathrm{NiC}}_2$ studied in this paper. We found strong competition between the ${\mathbf{q}}_{1ic}$ and the ${\mathbf{q}}_{2c}$ CDWs in ${\mathrm{DyNiC}}_2$ and ${\mathrm{YNiC}}_2$, and that only the ${\mathbf{q}}_{2c}$ CDW existed in ${\mathrm{HoNiC}}_2$ and ${\mathrm{ErNiC}}_2$. Based on the results obtained in the present paper and those of ${\mathrm{SmNiC}}_2,{\mathrm{GdNiC}}_2$, and ${\mathrm{TbNiC}}_2$ reported before, we discuss the CDW transitions in the $R{\mathrm{NiC}}_2$ family.

In spintronics applications, ultrafast spin dynamics have to be controlled at femtosecond timescales via femtosecond laser radiation. At such ultrafast timescales, the effect of the Gilbert damping factor $\alpha$ on ultrafast demagnetization time ${\tau }_M$ should be considered. In previous explorations for the relationship between these two parameters, it was found that the theoretical calculations based on the local spin-flip scattering model do not agree with the experimental results. Here, we find that in $\mathrm{F}{\mathrm{e}}_{81}\mathrm{G}{\mathrm{a}}_{19}$(FeGa)/$\mathrm{I}{\mathrm{r}}_{20}\mathrm{M}{\mathrm{n}}_{80}$(IrMn) bilayers, the unconventional IrMn thickness dependence of $\alpha$ results from the competition between spin currents pumped from the ferromagnetic (FM) FeGa layer to the antiferromagnetic (AFM) IrMn layer and those pumped from the AFM layer to the FM layer. More importantly, we establish a proportional relationship between the change of the ultrafast demagnetization rate and the enhancement of Gilbert damping induced by the spin currents via interfacial spin chemical potential ${\mu }_s$. Our work builds a bridge to connect the ultrafast demagnetization time and Gilbert damping in ultrafast photoinduced spin-current-dominated systems, which not only explains the disagreement between experimental and theoretical results in the relation of ${\tau }_M$ with $\alpha$ but provides further insight into ultrafast spin dynamics as well.

We have performed X-band electron spin resonance (ESR) measurements on a single crystal of the metallic chiral helimagnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ from 3.5 to 180 K and for the external magnetic fields $H_{\mathrm{ext}}$, up to 4 kOe, perpendicular to the $c$ axis (the helical axis of ${\mathrm{CrNb}}_3{\mathrm{S}}_6$). This field-crystalline configuration is expected to provide the chiral soliton lattice (CSL) state in this system. The main resonance line can be fit with a Dysonian function above $T_{\mathrm{c}}=127\mathrm{K}$, but additional features in the spectra were observed below 105 K. Specifically, spiked anomalies superposed on the main signals were observed for magnetic fields between $H_{\mathrm{c}1}$ and $H_{\mathrm{c}2}$ that are the appearing and disappearing fields of the spiked anomalies, respectively. The resulting magnetic field vs temperature phase diagram possesses three regions, which are interpreted as different dynamical responses in the CSL phase. In addition, the values of $H_{\mathrm{c}2}$ are close to those reported by the $d^{2}M/d{H}^2$ curve [Tsuruta et al., Phys. Rev. B 93, 104402 (2016)]. Furthermore, the field range between $H_{\mathrm{c}1}$ and $H_{\mathrm{c}2}$, where the spiked anomalies exist, depends on the field direction and shifts to higher fields when turning to the $c$ axis, thereby providing additional evidence that these spiked anomalies must be related to the chiral soliton dynamics.

The electrical conductivity induced near the superconducting transition by thermal fluctuations was measured in different granular aluminum films. The seemingly anomalous behavior at high reduced temperatures and magnetic fields is explained by taking into account a total-energy cutoff in the superconducting fluctuation spectrum in both the direct (Aslamazov-Larkin) and the indirect (anomalous Maki-Thompson) contributions to the fluctuation effects. The analysis allowed a reliable determination of the coherence length amplitudes, which resulted to be much larger ($20–48$ nm) than the grains size ($5–10$ nm). This suggests that the grains are strongly Josephson-coupled, while the $T_c$ value is still as high as twice the bulk value. These results could contribute to identifying the mechanisms enhancing $T_c$ in these materials.

It was recently argued that SU(3) chains in the $p$-box symmetric irreducible representation (irrep) exhibit a “Haldane gap” when $p$ is a multiple of 3 and are otherwise gapless [Nucl. Phys. B 924, 508 (2017)]. We extend this argument to the self-conjugate irreps of SU(3) with $p$ columns of length 2 and $p$ columns of length 1 in the Young tableau ($p=1$ corresponding to the adjoint irrep), arguing that they are always gapped but have spontaneously broken parity symmetry for $p$ odd but not even.

We investigate the many-body effects of a magnetic adatom in ferromagnetic graphene by using the numerical renormalization group method. The nontrivial band dispersion of ferromagnetic graphene gives rise to interesting Kondo physics different from that in conventional ferromagnetic materials. For a half-filled impurity in undoped graphene, the presence of ferromagnetism can bring forth Kondo correlations, yielding two kink structures in the local spectral function near the Fermi energy. When the spin splitting of local occupations is compensated by an external magnetic field, the two Kondo kinks merge into a full Kondo resonance characterizing the fully screened ground state. Strikingly, we find the resulting Kondo temperature monotonically increases with the spin polarization of Dirac electrons, which violates the commonsense belief that ferromagnetic bands are usually detrimental to Kondo correlations. Doped ferromagnetic graphene can behave as half-metals, where its density of states at the Fermi energy linearly vanishes for one spin direction but remains finite for the opposite direction. In this regime, we demonstrate an abnormal Kondo resonance that occurs in the first spin direction but is completely absent in the other one.

Using density functional theory with van der Waals (vdW) corrections, we study the collapse of free-standing single-walled carbon nanotubes (also called “dogbone” nanotubes). Their thermodynamic stability is strongly influenced by the initial stacking sequence, with lateral shear allowing registry change with turbostratic stacking predominant. The electronic structure of collapsed zigzag and armchair carbon nanotubes is investigated, demonstrating sensitivity to the lattice registry. The opening of small (meV) band gaps is shown for both armchair and zigzag collapsed nanotubes, arising from quantum confinement and charge transfer between the bilayer graphenelike central region and nanotubelike edges. Different scaling rules for the band gaps of collapsed carbon nanotubes are obtained as a function of their widths taking stacking and chirality into account. We reconcile a complete understanding of electronic properties in these deformed tubes with literature theoretical and experimental results, suggesting collapsed nanotubes can be promising candidates as conductive nanoribbons in electronic and spintronic device applications.

We study a quantum dot coupled to two semiconducting reservoirs, when the dot level and the electrochemical potential are both close to a band edge in the reservoirs. This is modeled with an exactly solvable Hamiltonian without interactions (the Fano-Anderson model). The model is known to show an abrupt transition as the dot-reservoir coupling is increased into the strong-coupling regime for a broad class of band structures. This transition involves an infinite-lifetime bound state appearing in the band gap. We find a signature of this transition in the continuum states of the model, visible as a discontinuous behavior of the dot's transmission function. This can result in the steady-state dc electric and thermoelectric responses having a very strong dependence on coupling close to critical coupling. We give examples where the conductances and the thermoelectric power factor exhibit huge peaks at critical coupling, while the thermoelectric figure of merit $ZT$ grows as the coupling approaches critical coupling, with a small dip at critical coupling. The critical coupling is thus a sweet spot for such thermoelectric devices, as the power output is maximal at this point without a significant change of efficiency.

The recent discovery of the antiferromagnetic (AFM) topological insulator (TI) ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has triggered great research efforts on exploring novel magnetic topological physics. Based on first-principles calculations, we find that the manipulation of magnetic orientation and order not only significantly affects material symmetries and orbital hybridizations, but also results in variant magnetic topological phases in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. We thus predict a series of topological quantum phase transitions that are magnetically controllable in the material, including phase transitions from an AFM TI to an AFM mirror topological crystalline insulator, from a type-II to type-I topological Weyl semimetal, and from an axion insulator to a Chern insulator. The findings open opportunities for future research and applications of magnetic topological materials.

We find an intervalley wave collective mode in two- and three-dimensional Dirac semimetals in the presence of a valley population imbalance. The dispersion relation of this mode is gapless, proportional to the square of the wave vector at small frequencies, and inversely proportional to the electron-electron exchange interaction energy. The valley wave serves as an energy gain source for the external field, that generates the intervalley transitions. The spin-wave analog is discussed for the case of a semimetal with nonequilibrium spin orientation.

Controlling spin-orbit interaction and its effect on superconductivity has been a long-standing problem in two-dimensional inversion-symmetry-broken superconductors. An open challenge is to understand the role of various energy scales in shaping the complex phase diagram in these systems. From a combined experimental and theoretical study of resistance fluctuations and its higher-order statistics, we propose a phase diagram for the superconducting phase in the magnetic-field–spin-orbit interaction energy plane for the quasi-two-dimensional electron gas at the interface of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. The relative variance of resistance fluctuations increases by few orders of magnitude below the spin-orbit field $B_{\text{SO}}$ and a non-Gaussian component to the fluctuations arises for fields below the upper critical field $B_{c2}$. Theoretical calculations show that the non-Gaussian noise predominantly arises due to percolative nature of the superconducting transition. We quantify the strength and the relative importance of the spin-orbit interaction energy, Zeeman energy, and the pairing potential. Our work highlights the important role played by the interplay between these energy scales in framing the fascinating phases seen in two-dimensional inversion-symmetry-broken superconductors.

We study the energy spectrum of a two-dimensional electron in the presence of both a perpendicular magnetic field and a potential. In the limit where the potential is small compared to the Landau level spacing, we show that the broadening of Landau levels is simply expressed in terms of the structure factor of the potential. For potentials that are either periodic or random, we recover known results. Interestingly, for potentials with a dense Fourier spectrum made of Bragg peaks (as found, e.g., in quasicrystals), we find an algebraic broadening with the magnetic field characterized by the hyperuniformity exponent of the potential. Furthermore, if the potential is self-similar such that its structure factor has a discrete scale invariance, the broadening displays log-periodic oscillations together with an algebraic envelope.

Control of structural coupling at complex-oxide interfaces is a powerful platform for creating ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy, in conjunction with scanning transmission electron microscopy, to probe the depth-dependent and single-unit-cell resolved electronic structure of an isovalent manganite superlattice $\left[\mathrm{E}{\mathrm{u}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3\right]\times 15$ wherein the electronic-structural properties are intentionally modulated with depth via engineered oxygen octahedra rotations/tilts and A-site displacements. Our unit-cell resolved measurements reveal significant transformations in the local chemical and electronic valence-band states, which are consistent with the layer-resolved first-principles theoretical calculations, thus opening the door for future depth-resolved studies of a wide variety of heteroengineered material systems.

Energy measurements of electrons emitted from a semiconductor can reveal internal physical processes hitherto elusive. Signatures of hot-electron processes in heterostructures have been observed from cesiated, light-emitting, and p-i-n diodes. In p-i-n devices with AlGaN barriers, a high-energy peak was measured and ascribed to a trap-assisted Auger recombination process. Temperature dependent measurements of light-emitting diodes with AlGaN electron blocking layers also show such hot carriers when electrons thermally reach these barriers, identifying carrier escape as the mechanism of thermal droop and demonstrating the efficacy of such barriers to partially mitigate thermal droop.

The experimental discovery and theoretical analysis of an epitaxial (2 × 2) honeycomb $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_3$ monolayer on a Au(111) surface is reported. The oxide monolayer is grown by Nb deposition and subsequent annealing in an oxidizing atmosphere. Scanning tunneling microscopy (STM) images show that the films form a well-ordered honeycomb lattice, and low energy electron diffraction patterns confirm that the films adopt a (2 × 2) periodicity with respect to the Au(111) substrate. Density functional theory (DFT) modeling shows that the Nb atoms are located in Au(111) threefold hollow sites and the O atoms are located in on-top positions. DFT also demonstrates the existence of a strong interfacial interaction characterized by a large electron transfer towards the Au substrate, an increase of the Nb oxidation state, and substantial film rumpling. High-resolution STM images, supported by simulations, are able to discriminate between Nb atoms adsorbed in fcc or hcp hollow sites on the Au(111) substrate.