It is known that peculiar plasmons whose frequencies are purely imaginary exist in the interior of a two-dimensional electronic system described by the Drude model. We show that when an external magnetic field is applied to the system, these bulk plasmons are still nonoscillating and are isolated from the magnetoplasmons by the energy gap of the cyclotron frequency. These are mainly in a transverse magnetic mode and can combine with a transverse electronic mode locally at an edge of the system to form edge magnetoplasmons. With this observation, we reveal the intrinsic long lifetime of edge magnetoplasmons for the first time.

Employing density functional theory–based methods, we investigate monolayer and bilayer structures of hexagonal ${\mathrm{SnS}}_2$, which is a recently synthesized monolayer metal dichalcogenide. Comparison of the $1H$ and $1T$ phases of monolayer ${\mathrm{SnS}}_2$ confirms the ground state to be the $1T$ phase. In its bilayer structure we examine different stacking configurations of the two layers. It is found that the interlayer coupling in bilayer ${\mathrm{SnS}}_2$ is weaker than that of typical transition-metal dichalcogenides so that alternative stacking orders have similar structural parameters and they are separated with low energy barriers. A possible signature of the stacking order in the ${\mathrm{SnS}}_2$ bilayer has been sought in the calculated absorbance and reflectivity spectra. We also study the effects of the external electric field, charging, and loading pressure on the characteristic properties of bilayer ${\mathrm{SnS}}_2$. It is found that (i) the electric field increases the coupling between the layers at its preferred stacking order, so the barrier height increases, (ii) the bang gap value can be tuned by the external E field and under sufficient E field, the bilayer ${\mathrm{SnS}}_2$ can become a semimetal, (iii) the most favorable stacking order can be switched by charging, and (iv) a loading pressure exceeding 3 GPa changes the stacking order. The E-field tunable band gap and easily tunable stacking sequence of ${\mathrm{SnS}}_2$ layers make this 2D crystal structure a good candidate for field effect transistor and nanoscale lubricant applications.

The scaling behavior of Model C describing the dynamical behavior of the $n$-component nonconserved order parameter coupled statically to a scalar conserved density is considered in $d$-dimensional space. Conditions for the realization of different types of scaling regimes in the $(n,d)$ plane are studied within the field-theoretical renormalization group approach. Borders separating these regions are calculated on the base of high-order RG functions using $\epsilon$ expansions as well as by fixed dimension $d$ approach with resummation.

Magnetization dynamics in an artificial square spin-ice lattice made of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ with magnetic field applied in the lattice plane is investigated by broadband ferromagnetic resonance spectroscopy. The experimentally observed dispersion shows a rich spectrum of modes corresponding to different magnetization states. These magnetization states are determined by exchange and dipolar interaction between individual islands, as is confirmed by a semianalytical model. In the low field regime below 400 Oe a hysteretic behavior in the mode spectrum is found. Micromagnetic simulations reveal that the origin of the observed spectra is due to the initialization of different magnetization states of individual nanomagnets. Our results indicate that it might be possible to determine the spin-ice state by resonance experiments and are a first step towards the understanding of artificial geometrically frustrated magnetic systems in the high-frequency regime.

Quantum Monte Carlo simulations at zero temperature of a ${}^3\mathrm{He}$ monolayer adsorbed on graphite, either clean or preplated with ${}^4\mathrm{He}$, unexpectedly point to a gas-liquid phase transition at a very low areal density of the order of $0.01{\AA }^{-2}$. This result stems from an essentially unbiased calculation of the ground-state energy for an infinite, defect-free substrate, which interacts with He atoms via a realistic potential, whereas the interaction between two He atoms includes two- and three-body terms. The sensitivity of the gas-liquid coexistence region on the model Hamiltonian employed is discussed.

A comparative study of field-induced domain switching and lattice strain was carried out by in situ electric-field-dependent high-energy synchrotron x-ray diffraction on a morphotropic phase boundary (MPB) and a near-MPB rhombohedral/pseudomonoclinic composition of a high-performance piezoelectric alloy $(1-x)\mathrm{PbTi}{\mathrm{O}}_3-\left(x\right)\mathrm{BiSc}{\mathrm{O}}_3$. It is demonstrated that the MPB composition showing large $d_{33}\sim 425pC/N$ exhibits significantly reduced propensity of field-induced domain switching as compared to the non-MPB rhombohedral composition $(d_{33}\sim 260pC/N)$. These experimental observations contradict the basic premise of the martensitic-theory-based explanation which emphasizes on enhanced domain wall motion as the primary factor for the anomalous piezoelectric response in MPB piezoelectrics. Our results favor field-induced structural transformation to be the primary mechanism contributing to the large piezoresponse of the critical MPB composition of this system.

The low temperature physics of parahydrogen ($p-{\mathrm{H}}_2$) confined in cylindrical channels of diameter of the order of 1 nm is studied theoretically by quantum Monte Carlo simulations. On varying the attractive strength of the wall of the cylindrical pore, as well as its diameter, the equilibrium phase evolves from a single quasi-one-dimensional (1D) channel along the axis, to a concentric cylindrical shell. It is found that the quasi-1D system retains a strong propensity to crystallization, even though on weakly attractive substrates quantum fluctuations reduce somewhat such a tendency compared to the purely 1D system. No evidence of a topologically protected superfluid phase (in the Luttinger sense) is observed. Implications on the possible existence of a bulk superfluid phase of parahydrogen are discussed.

We report detailed study of the transport properties of ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ ($x$ = 0 and 0.14) single crystals grown by vapor transport method. 14% S doping is found to significantly suppress the structural transition from $T_s\sim$ 86 K in FeSe to $\sim 49$ K, although the superconducting transition temperature $T_c$ is only slightly affected. A pronounced linear magnetoresistance (MR) is observed in both FeSe and ${\mathrm{FeSe}}_{0.86}{\mathrm{S}}_{0.14}$ single crystals, which is found to be triggered by the structural transition. The linear MR and related discussion indicate the possible existence of Dirac-cone-like state, which may come from the band shift induced by ferro-orbital order. The mobility of the Dirac-cone-like band is found to decrease after S doping. Besides, the invalid Kohler's scaling of MR is found for temperature below $T_s$ in both crystals, however, the reestablishment of the Kohler's scaling at temperatures below 30 K is observed in FeSe, but not in ${\mathrm{FeSe}}_{0.86}{\mathrm{S}}_{0.14}$. All these observations above support that the orbital ordering causes the band reconstruction in FeSe, and also that the orbital ordering in FeSe is suppressed by the chemical pressure from S doping.

We theoretically investigate k-space structures of $d_{x^2-y^2}\text{-wave}$ superconducting (SC) and spin-density-wave (SDW) orders in their coexistent phase induced by a paramagnetic pair-breaking (PPB) effect in relation to the high-field and low-temperature SC phase in ${\mathrm{CeCoIn}}_5$. It is shown that, in k space, the SDW order develops near the gap nodes where the SC order is suppressed by PPB, and the nesting condition for the SDW ordering is satisfied. By comparing the results in the $d_{x^2-y^2}\text{-wave}$ SC model and those in an artificial model with no sign change of the gap function in k space with each other, it is shown that the $d_{x^2-y^2}\text{-wave}$ SC and SDW orders are enhanced altogether in k space due to the sign change of the $d_{x^2-y^2}\text{-wave}$ gap function there, and that this mutual enhancement largely stabilizes the coexistence of these orders in real space. It is also discussed that the field dependence of a SDW moment can be affected by the k-space structure of these orders, which is dependent on the curvature of the Fermi surface.

We present a mainly analytical study of the entanglement spectrum of Bernal-stacked graphene bilayers in the presence of trigonal warping in the energy spectrum. Upon tracing out one layer, the entanglement spectrum shows qualitative geometric differences to the energy spectrum of a graphene monolayer. However, topological quantities such as Berry-phase-type contributions to Chern numbers agree. The latter analysis involves not only the eigenvalues of the entanglement Hamiltonian but also its eigenvectors. We also discuss the entanglement spectra resulting from tracing out other sublattices. As a technical basis of our analysis, we provide closed analytical expressions for the full eigensystem of bilayer graphene in the entire Brillouin zone with a trigonally warped spectrum.

${\mathbb{Z}}_d$ parafermions are exotic non-Abelian quasiparticles generalizing Majorana fermions, which correspond to the case $d=2$. In contrast to Majorana fermions, braiding of parafermions with $d>2$ allows one to perform an entangling gate. This has spurred interest in parafermions, and a variety of condensed matter systems have been proposed as potential hosts for them. In this work, we study the computational power of braiding parafermions more systematically. We make no assumptions on the underlying physical model but derive all our results from the algebraical relations that define parafermions. We find a family of $2d$ representations of the braid group that are compatible with these relations. The braiding operators derived this way reproduce those derived previously from physical grounds as special cases. We show that if a $d$-level qudit is encoded in the fusion space of four parafermions, braiding of these four parafermions allows one to generate the entire single-qudit Clifford group (up to phases), for any $d$. If $d$ is odd, then we show that in fact the entire many-qudit Clifford group can be generated.

It is shown theoretically that a one-dimensional crystal with time-reversal and particle-hole symmetries is characterized by a topological invariant that predicts the existence or otherwise of edge states. This is confirmed experimentally through the construction and simulation of a photonic crystal analog in the microwave regime. It is shown that the edge mode couples to modes external to the photonic crystal via a Fano resonance.

We study momentum-space entanglement in quantum spin-half ladders consisting of two coupled critical $XXZ$ spin-half chains, using field theoretical methods developed by Lundgren et al. [Phys. Rev. B 88, 245137 (2013)] and exact diagonalization. When the system is gapped, we analytically find that the momentum-space entanglement Hamiltonian is described by a chiral conformal field theory with a central charge of 2. This is in contrast to entanglement Hamiltonians of various real-space partitions of gapped-spin ladders that have a central charge of 1. When the system is gapless, the entanglement Hamiltonian consists of one gapless mode that is linear in subsystem momentum and one mode with a flat dispersion relation. In the gapless region, we extend the work of Lundgren et al. to include the effect of a certain irrelevant term on the entanglement spectrum. In both the gapless and gapped phases, the momentum-space entanglement entropy obeys a volume law. In the gapless region, we find a subleading constant term in the entanglement entropy which contains information about the underlying field theory of the system. The analytical predictions for the entanglement spectrum are consistent with results from exact diagonalization.

We report the transition from robust ferromagnetism to a spin-glass state in nanoparticulate $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ through solid solution with $\mathrm{BaTi}{\mathrm{O}}_3$. The field- and temperature-dependent magnetization and the frequency-dependent ac magnetic susceptibility measurements strongly indicate the existence of a spin-glass state in the system, which is further confirmed from memory effect measurements. The breaking of long-range ordering into short-range magnetic domains is further investigated using density-functional calculations. We show that Ti ions remain magnetically inactive due to insufficient electron leakage from $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ to the otherwise unoccupied $\mathrm{Ti}\text{−}d$ states. This results in the absence of a Mn-Ti-Mn spin exchange interaction and hence the breaking of the long-range ordering. Total-energy calculations suggest that the segregation of nonmagnetic Ti ions leads to the formation of short-range ferromagnetic Mn domains.

We theoretically study the differential conductance of a one-dimensional normal-superconductor-normal-superconductor (N-SNS) junction with a phase bias applied between the two superconductors. We consider specifically a junction formed by a spin-orbit coupled semiconducting nanowire with regions of the nanowire having superconducting pairing induced by a bulk $s$-wave superconductor. When the nanowire is tuned into a topologically nontrivial phase by a Zeeman field, it hosts zero-energy Majorana modes at its ends as well as at the interface between the two superconductors. The phase-dependent splitting of the Majorana modes gives rise to features in the differential conductance that offer a clear distinction between the topologically trivial and nontrivial phases. We calculate the transport properties of the junction numerically and also present a simple analytical model that captures the main properties of the predicted tunneling spectroscopy.

Hole-doped high-temperature cuprate superconductors below optimum doping have electronlike Fermi surfaces occupying a small fraction of the Brillouin zone. There is strong evidence that this is linked to charge density wave (CDW) order, which reconstructs the large holelike Fermi surfaces predicted by band structure calculations. Recent experiments have revealed the structure of the two CDW components in the benchmark bilayer material ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\text{−}x}$ in high field where quantum oscillation (QO) measurements are performed. We have combined these results with a tight-binding description of the bands in a single bilayer to give a minimal model revealing the essential physics of the situation. Here we show that this approach, combined with the effects of spin-orbit interactions and the pseudogap, gives a good qualitative description of the multiple frequencies seen in the QO observations in this material. Magnetic breakdown through weak CDW splitting of the bands will lead to a field dependence of the QO spectrum and to the observed fourfold symmetry of the results in tilted fields.

Using an exact numerical solution and semiclassical analysis, we investigate quantum oscillations (QOs) in a model of a bilayer system with an anisotropic (elliptical) electron pocket in each plane. Key features of QO experiments in the high temperature superconducting cuprate YBCO can be reproduced by such a model, in particular the pattern of oscillation frequencies (which reflect “magnetic breakdown” between the two pockets) and the polar and azimuthal angular dependence of the oscillation amplitudes. However, the requisite magnetic breakdown is possible only under the assumption that the horizontal mirror plane symmetry is spontaneously broken and that the bilayer tunneling $t_{\perp }$ is substantially renormalized from its ‘bare’ value. Under the assumption that $t_{\perp }=\tilde{Z}{t}_{\perp }^{\left(0\right)}$, where $\tilde{Z}$ is a measure of the quasiparticle weight, this suggests that $\tilde{Z}\lesssim 1/20$. Detailed comparisons with new ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.58}$ QO data, taken over a very broad range of magnetic field, confirm specific predictions made by the breakdown scenario.

Recent discoveries of supposedly pure $\alpha$-tetragonal boron require to revisit its structure. The system is also interesting with respect to a new type of geometrical frustration in elemental crystals, which was found in $\beta$-rhombohedral boron. Based on density functional theory calculations, the present study has resolved the structural and thermodynamic characteristics of pure $\alpha$-tetragonal boron. Different from $\beta$-rhombohedral boron, the conditions for stable covalent bonding (a band gap and completely filled valence bands) are almost fulfilled at a composition ${\mathrm{B}}_{52}$ with two $4c$ interstitial sites occupied. This indicates that the ground state of pure $\alpha$-tetragonal boron is stoichiometric. However, the covalent condition is not perfectly fulfilled because nonbonding in-gap states exist that cannot be eliminated. The half occupation of the $4c$ sites yields a macroscopic amount of residual entropy, which is as large as that of $\beta$-rhombohedral boron. Therefore $\alpha$-tetragonal boron can be classified as an elemental crystal with geometrical frustration. Deviations from stoichiometry can occur only at finite temperatures. Thermodynamic considerations show that deviations $\delta$ from the stoichiometric composition (${\mathrm{B}}_{52+\delta }$) are small and positive. For the reported high-pressure syntheses conditions $\delta$ is predicted to be about 0.1 to 0.2. An important difference between pure and C- or N-containing $\alpha$-tetragonal boron is found in the occupation of interstitial sites: the pure form prefers to occupy the $4c$ sites, whereas in C- or N-containing forms, a mixture of $2a$, $8h$, and $8i$ sites are occupied. The present article provides relations of site occupation, $\delta$ values, and lattice parameters, which enable us to identify pure $\alpha$-tetragonal boron and distinguish the pure form from other ones.

We present single-crystal growth and magnetic property studies of tellurium-bridged copper spin-$\frac{1}{2}$ system ${\mathrm{Ba}}_2{\mathrm{CuTeO}}_6$. The spin-exchange interaction among copper spins via $\mathrm{Cu}\text{−}\mathrm{O}\text{−}\mathrm{Te}\text{−}\mathrm{O}\text{−}\mathrm{Cu}$ super-superexchange route leads to a novel two-leg spin ladder system. Spin susceptibility $\chi \left(T\right)$ data indicate that the triclinic ${\mathrm{Ba}}_2{\mathrm{CuTeO}}_6$ undergoes a stepwise crossover for exchange couplings revealed by a broad maximum of $\chi \left(T\right)$ near $T_{max}\sim 75$ K and an anisotropic cusp in $\frac{d\chi }{dT}\left(T\right)$ at $T_N\sim 15$ K to signify a three-dimensional (3D) antiferromagnetic long-range ordering (LRO). The 3D LRO has been suggested from the anisotropic behavior of $\chi \left(T\right)$ with strong $c$-axis spin anisotropy and the signature of spin-flop transition from the isothermal magnetization below ${\mathit{T}}_N$. Analysis of magnetic heat capacity ($C_m$) at $T_N\sim 15$ K indicates that most of the spin entropy $(\sim 92\%)$ has already been released above $T_N$, which supports the picture of consecutive spin entropy reduction upon cooling with Te-bridged two-leg spin ladder system with strong intraladder and interladder couplings. Theoretical $\mathrm{DFT}+U$ calculations have been performed to search for the ground-state magnetic configuration and also to evaluate exchange-coupling constants that support the magnetic model deduced from the combined spin susceptibility and crystal structure symmetry analysis.