We demonstrate that a finite-size chiral soliton lattice formed in a chiral helimagnet with fixed boundary conditions exhibits magnetization jumps in a response to the magnetic field applied perpendicular to the chiral axis. The imposed boundary conditions lead to confinement of topological charges and quantized spatial periods of the soliton lattice. Building an envelope of the ground-state energies belonging to different topological sectors, we find the magnetization jumps related with the level crossing. After numerically establishing the quantization condition, we also develop a field-theoretical model to support the numerical results.

We report on a superconductor LiFeO${}_2$Fe${}_2$Se${}_2$ with $T_{\mathrm{c}}\sim 43$ K, synthesized via the hydrothermal method. The anti-PbO-type spacer layer of LiFeO${}_2$ has been successfully intercalated between the anti-PbO-type FeSe layer, which results in a high-$T_{\mathrm{c}}$ superconductivity in this iron selenide material. In this material, the LiFeO${}_2$ layer is electrically neutral and does not act as an electron donor, which is remarkably different from other heavily electron-doped intercalated FeSe-derived superconductors. Based on our results and previous reports on FeSe-based superconductors, we figure out a $\mathsf{V}$-shaped anion height from the Fe layer dependence of $T_{\mathrm{c}}$ in the FeSe-derived superconductors, which is in contrast to the inverse $\mathsf{V}$-shaped one for the FeAs-based superconductors. Our studies expand the categories of the iron-based superconductors and open a door in the quest for potential superconductors with various spacer layers, which is helpful for the study of the underlying physics in iron-based high-$T_{\mathrm{c}}$ superconductors.

Freestanding silicene is predicted to display comparable electronic properties as graphene. However, the yet synthesized silicenelike structures have been only realized on different substrates which turned out to exhibit versatile crystallographic structures that are very different from the theoretically predicted buckled phase of freestanding silicene. This calls for a different approach where silicene is stabilized using very weakly interacting surfaces. We propose here a route by using graphene bilayer as a scaffold. The confinement between the flat graphene layers results in a planar clustering of Si atoms with small buckling, which is energetically unfavorable in vacuum. Buckled hexagonal arrangement of Si atoms similar to freestanding silicene is observed for large clusters, which, in contrast to Si atoms on metallic surfaces, is only very weakly van der Waals coupled to the graphene layers. These clusters are found to be stable well above room temperature. Our findings, which are supported by density-functional tight-binding calculations, show that intercalating bilayer graphene with Si is a favorable route to realize silicene.

The angular dependence of magnetization was measured in an optimally Co-doped BaFe${}_2$As${}_2$ single crystal, which is irradiated by 2.6 GeV uranium to introduce columnar defects (CDs). At low fields ($\mathit{H}$), we observe the irreversible magnetization $M_{\mathrm{irr}}$ insensitive to the angle $\theta$ around $\mathit{H}\parallel \mathrm{CDs}$. The field dependence of the angular range $2{\theta }_L$ is proportional to $1/H$, quite consistent with the expectation of the lock-in transition. This manifests the first observation of the fingerprint of the Bose glass phase in iron-based superconductors.

We study the orbital-selective superconducting pairing in a five-orbital $t$-$J_1$-$J_2$ model for iron pnictides. Depending on the orbital selectivity of electron correlations and the orbital characters along the Fermi surface, the superconducting gap in an $A_{1g}$ pairing state may exhibit anisotropy. This anisotropy varies with the degree of $J_1$-$J_2$ magnetic frustration. In the superconducting state, the frequency dependence of the dynamical spin susceptibility at the antiferromagnetic wave vector $(\pi ,0)$ shows a resonance, whose width is enhanced by the orbital selectivity of the superconducting gap. When the degree of the orbital selectivity is sufficiently strong, the resonance peak may be split in two. We discuss the implications of our results on the recent angle-resolved photoemission and neutron-scattering measurements in several superconducting iron pnictides.

We study analytically the evolution of superconductivity in clean quasi-two-dimensional multiband superconductors as the film thickness enters the nanoscale region by mean-field and semiclassical techniques. Tunneling into the substrate and finite lateral size effects, which are important in experiments, are also considered in our model. As a result, it is possible to investigate the interplay between quantum coherence effects, such as shape resonances and shell effects, with the potential to enhance superconductivity, and the multiband structure and the coupling to the substrate that tend to suppress it. The case of magnesium diboride, which is the conventional superconductor with the highest critical temperature, is discussed in detail. Once the effect of the substrate is considered, we still observe quantum size effects such as the oscillation of the critical temperature with the thickness but without a significant enhancement of superconductivity. In thin films with a sufficiently longer superconducting coherence length, it is, however, possible to increase the critical temperature above the bulk limit by tuning the film thickness or lateral size.

A computation of the cuprate phase diagram is presented. Adiabatic deformability back to the density function band structure plus symmetry constraints lead to a Fermi liquid theory with five interaction parameters. Two of these are forced to zero by experiment. The remaining three are fit to the moment of the antiferromagnetic state at half-filling, the superconducting gap at optimal doping, and the maximum pseudogap. The latter is identified as orbital antiferromagnetism. Solution of the Hartree-Fock equations gives, in quantitative agreement with experiment, (1) quantum phase transitions at 5% and 16% $p$-type doping, (2) insulation below 5%, (3) a $d$-wave pseudogap quasiparticle spectrum, (4) pseudogap and superconducting gap values as a function of doping, (5) superconducting $T_c$ versus doping, (6) London penetration depth versus doping, and (7) spin wave velocity. The fit points to superexchange mediated by the bonding O atom in the Cu-O plane as the causative agent of all three ordering phenomena.

We show how all-angle negative refraction can occur in a uniaxial antiferromagnet close to the magnon resonance frequency. This behavior is based on the fact that, in such cases, the antiferromagnet acts as an indefinite permeability medium, i.e., not all its permeability tensor components are of the same sign. If an external magnetic field is applied, the angle of refraction becomes tunable, and can be made to change sign. We illustrate these effects using the example of MnF${}_2$ at low temperature, and show that in this system negative refraction should occur with a large figure of merit.

Majorana fermions often coexist with other low-energy fermionic degrees of freedom. In such a situation, topological quantum computation requires the use of fermionic zero modes of a many-body system. We classify all such modes for interacting fermions and show how to select the mode that maximizes the decoherence time. We find that in a typical interacting system the maximal decoherence time is within one order of magnitude from the decoherence time of a qubit based on the local part of the fermion parity operator.

We propose a scheme for high-order sideband generation (HSG) by utilizing strong coupling between a single quantum dot (QD) and a photonic crystal cavity beyond the weak-excitation approximation. Here the hybrid system is coherently driven by an external bichromatic field consisting of a continuous-wave (cw) control field and a cw probe field. We take account of nonlinear terms in the Heisenberg-Langevin equations, i.e., the nonlinear nature of the QD, and give an effective method to deal with such a problem. It is shown, due to the presence of nonlinear terms, that the HSG with large intensities can be realized with experimentally achievable system parameters. This investigation may provide further insight into the understanding of solid-state cavity quantum electrodynamics system and find applications in optical comb and communication in a photonic crystal platform. In addition, the present theory can also be applied to other similar systems, such as a single quantum emitter positioned close to a microtoroidal resonator with the whispering-gallery-mode fields propagating inside the resonator.

We present here an x-ray absorption study of the $R$Fe${}_2$O${}_4$ series (LuFe${}_2$O${}_4$, YFe${}_2$O${}_4$, YbFe${}_2$O${}_4$, and LuCoFeO${}_4$) at the Fe $K$-edge. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectra were measured at temperatures ranging from 100 K to 390 K crossing the charge ordering (CO) transition temperatures on both isotropic and oriented powder samples. Unpolarized and polarized x-ray absorption spectra with the x-ray polarization parallel and perpendicular to the hexagonal $c$ axis were obtained separately. The XANES spectra show almost no dependence with either polarization or temperature. This indicates that, in contrast to its average crystallographic structure, the local electronic and geometrical state of the Fe atom is barely anisotropic, and it remains the same above and below the proposed CO. Moreover, the linear combination of two spectra corresponding to the pure ionic states Fe${}^{2+}$ and Fe${}^{3+}$ does not fit to the experimental XANES of either LuFe${}_2$O${}_4$, YFe${}_2$O${}_4$, or YbFe${}_2$O${}_4$, discarding total ionic segregation in favor of an intermediate mixed valence state. The maximum charge disproportionation compatible with the XANES spectra is 0.5 ± 0.1 electrons in the whole temperature range. The polarized EXAFS spectra have allowed us to analyze the local structure separating the different contributions mixed up in the polycrystal. Best-fit results show that the five oxygen atoms of the first coordination shell are at nearly the same distance but with a large Debye-Waller factor. Neither the interatomic distances nor the Debye-Waller factors of the first oxygen coordination shell change with temperature, indicating that the dynamical structural distortion of the high-temperature symmetric hexagonal phase freezes upon cooling down. Thus, the local structure instability of the mixed valence is in the origin of the structural transitions.

We discuss, by means of mode-matching analysis for the Dirac equation, how splittings of the Landau-level (LL) degeneracies associated with spin, valley, and layer degrees of freedom affect the ballistic conductance of graphene bilayer. The results show that for wide samples ($W\gg L$) the Landauer-Büttiker conductance reaches the maximum $G\simeq s{e}^2/\left(\pi h\right)\times W/L$ at the resonance via each LL, with the prefactor varying from $s=8$ if all three degeneracies are preserved, to $s=1$ if all the degeneracies are split. In the absence of bias between the layers, the degeneracies associated with spin and layer degrees of freedom may be split by manipulating the doping and magnetic field; the conductance at the zeroth LL is twice as large, while the conductance at any other LL equals to the corresponding conductance of graphene monolayer. The presence of bias potential allows one also to split the valley degeneracy. Our results show that the charge transfer at each LL has pseudodiffusive character, with the second and third cumulant quantified by $\mathcal{F}=1/3$ and $\mathcal{R}=1/15$ (respectively). In case the electrochemical potential is allowed to slowly fluctuate in a finite vicinity of LL, the resulting charge-transfer characteristics are still quantum limited, with $\mathcal{F}\simeq 0.7$ and $\mathcal{R}\simeq 0.5$ in the limit of large fluctuations. Analogously, the above values of $\mathcal{F}$ and $\mathcal{R}$ are predicted to be approached in the limit of high source-drain voltage difference applied. The possible effects of indirect interlayer hopping integrals are also briefly discussed.

We experimentally investigate Andreev bound states (ABSs) in a carbon nanotube quantum dot (QD) connected to a superconducting Nb lead (S). A weakly coupled normal metal contact acts as a tunnel probe that measures the energy dispersion of the ABSs. Moreover, we study the response of the ABS to nonlocal transport processes, namely, Cooper pair splitting and elastic co-tunnelling, which are enabled by a second QD fabricated on the same nanotube on the opposite side of S. We find an appreciable nonlocal conductance with a rich structure, including a sign reversal at the ground-state transition from the ABS singlet to a degenerate magnetic doublet. We describe our device by a simple rate equation model that captures the key features of our observations and demonstrates that the sign of the nonlocal conductance is a measure for the charge distribution of the ABS, given by the respective Bogoliubov-de Gennes amplitudes $u$ and $v$.

Electronic localization in narrow graphene constrictions is theoretically studied, and it is found that long-lived ($\sim$1 ns) quasibound states (QBSs) can exist in a class of ultrashort graphene quantum point contacts (QPCs). These QBSs are shown to originate from the dispersionless edge states that are characteristic of the electronic structure of generically terminated graphene, in which pseudo-time-reversal symmetry is broken. The QBSs can be regarded as interface states confined between two graphene samples, and their properties can be modified by changing the sizes of the QPC and the interface geometry. In the presence of bearded sites, these QBSs can be converted into bound states. Experimental consequences and potential applications are discussed.

Multiferroic tunnel junctions (MFTJs) are usually composed of two ferromagnetic electrodes and an ultrathin layer of ferroelectric barrier, with many physical properties sensitive to the interfacial details. In Fe/BaTiO${}_3$ heterostructures, two types of interfacial atomic configurations were observed successively in previously reported experiments. In this work, we consider two types of Fe/BaTiO${}_3$ MFTJs which differ only in the interfacial structure via ab initio studies. Various physical properties are systematically investigated to reveal the interface effect. Due to the existence of the additional FeO monolayer, an unstable chemical bond forms within the interface, which leads to the vanishing critical thickness for ferroelectricity and enhanced magnetoelectric coupling of the MFTJs with this interface.

We characterize the phase diagram of anisotropic Heisenberg spin glasses, finding both the spin and the chiral glass transition. We remark on the presence of strong finite-size effects in the chiral sector. On the spin glass sector, we find that the universality class is that of Ising spin glasses. Our data are compatible with a unique phase transition for the chiral and spin glass sector. We focus on keeping finite-size effects under control, and we stress that they are important to understand experiments. Thanks to large GPU clusters we have been able to thermalize cubic lattices with up to ${64}^3$ spins, over a vast range of temperatures (hence, of relaxation times).

We present a detailed investigation of the magnetic properties in SiC single crystals bombarded with neon ions. Through careful measuring of the magnetization of virgin and irradiated SiC, we decompose the magnetization of SiC into paramagnetic, superparamagnetic, and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. We qualitatively explain the magnetic properties as a result of the intrinsic clustering tendency of defects.

Recently discovered magnetoelectricity in the rare-earth aluminum borates RAl${}_3$(BO${}_3$)${}_4$ has attracted attention due to the large values of magnetoinduced electric polarization. We have observed for the first time the magnetoelectric polarization in TbAl${}_3$(BO${}_3$)${}_4$ exhibiting anomalous temperature dependence: an electric polarization induced by in-plane magnetic field ($P\left|\right|a,H\perp c$ axis) which is small at low temperatures ($4\mathrm{K}$), remarkably increases by almost an order of magnitude at high temperatures ($150–300\mathrm{K}$). The observed nonmonotonic temperature behavior of the field-induced polarization, including a change of sign at $\sim 65–70\mathrm{K}$, is attributed to the competition of the ground and excited crystal-field states of Tb${}^{3+}$ ions. Quantum theory analysis, involving the combination of analytical and numerical methods, has enabled us to quantitatively describe the observed magnetic and magnetoelectric properties of TbAl${}_3$(BO${}_3$)${}_4$.

Recent discoveries in neutron scattering experiments for Kapellasite and Herbertsmithite as well as theoretical calculations of possible spin liquid phases have revived interest in magnetic phenomena on the kagome lattice. We study the quantum phase diagram of the $S=1/2$ Heisenberg kagome model as a function of nearest neighbor coupling $J_1$ and second neighbor coupling $J_2$. Employing the pseudofermion functional renormalization group (PFFRG), we find four types of magnetic quantum order ($\mathbf{q}=0$ order, cuboc order,ferromagnetic order, and $\sqrt{3}\times \sqrt{3}$ order) as well as extended magnetically disordered regions by which we specify the possible parameter regime for Kapellasite. In the disordered regime $\frac{J_2}{J_1}\ll 1$, the flatness of the magnetic susceptibility at the zone boundary which is observed for Herbertsmithite can be reconciled with the presence of small $J_2>0$ coupling. In particular, we analyze the dimer susceptibilities related to different valence-bond crystal (VBC) patterns, which are strongly inhomogeneous, indicating the rejection of VBC order in the RG flow.

In superconducting copper oxides, some Cu-O bond-stretching phonons around 70 meV show anomalous giant softening and broadening of electronic origin, and electronic dispersions have large renormalization kinks near the same energy. These observations suggest that phonon broadening originates from quasiparticle excitations across the Fermi surface and the electronic dispersion kinks originate from coupling to anomalous phonons. We measured the phonon anomaly in underdoped ($x=0.05$) and overdoped ($x=0.20$ and 0.25) La${}_{2-x}$Sr${}_x$CuO${}_4$ by inelastic neutron and x-ray scattering with high resolution. Combining these and previously published data, we found that doping dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e., the two phenomena are not connected. We show that these results provide indirect evidence that the phonon anomaly originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity.