The paper analyzes the transverse forces (the Magnus and the Lorentz forces) on vortices in superfluids put into periodic potentials at $T=0$. The case of weak potential and the tight-binding limit described by the Bose-Hubbard model were addressed. The analysis was based on the balance of true momentum and quasimomentum. A special attention was paid to the superfluid close to the superfluid-insulator transition. In this area of the phase diagram the theory predicts the particle-hole symmetry line where the Magnus force changes sign with respect to that expected from the sign of velocity circulation. Our analysis has shown that the magnitude of the Magnus force is a continuous function at crossing the particle-hole symmetry line. This challenges the theory connecting the magnitude of the Magnus force with topological Chern numbers and predicting a jump at crossing this line. Disagreement is explained by the role of intrinsic pinning and guided vortex motion ignored in the topological approach. It is one more evidence that in general topological arguments are not sufficient for derivation of equations of vortex motion.

We discuss heat transport in thermally-biased long Josephson tunnel junctions in the presence of an in-plane magnetic field. In full analogy with the Josephson critical current, the phase-dependent component of the heat current through the junction displays coherent diffraction. Thermal transport is analyzed as a function of both the length and the damping of the junction, highlighting deviations from the standard “Fraunhofer” pattern characteristic of short junctions. The heat current diffraction patterns show features strongly related to the formation and penetration of Josephson vortices, i.e., solitons. We show that a dynamical treatment of the system is crucial for the realistic description of the Josephson junction, and it leads to peculiar results. In fact, hysteretic behaviors in the diffraction patterns when the field is swept up and down are observed, corresponding to the trapping of vortices in the junction.

We investigated the superconducting order parameter of the filled skutterudite ${\mathrm{LaPt}}_4{\mathrm{Ge}}_{12}$, with a transition temperature of $T_c=8.3$ K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting $s$-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in the context of previous studies on the substitution series ${\mathrm{Pr}}_{1-x}{\mathrm{La}}_x{\mathrm{Pt}}_4{\mathrm{Ge}}_{12}$. They suggest compatible order parameters for the two end compounds ${\mathrm{LaPt}}_4{\mathrm{Ge}}_{12}$ and ${\mathrm{PrPt}}_4{\mathrm{Ge}}_{12}$. This is not consistent with a single $s$-wave gap in ${\mathrm{LaPt}}_4{\mathrm{Ge}}_{12}$ considering previous reports of unconventional and/or multiband superconductivity in ${\mathrm{PrPt}}_4{\mathrm{Ge}}_{12}$.

We demonstrate substantial enhancement in the energy product of MnBi-based magnets by forming robust ferromagnetic exchange coupling between a MnBi layer and a thin CoFe layer in a unique perpendicular coupling configuration, which provides increased resistance to magnetization reversal. The measured nominal energy product of $172\mathrm{kJ}/{\mathrm{m}}^3$ at room temperature is the largest value experimentally attained for permanent magnets free of expensive raw materials. Our finding shows that exchange-coupled MnBi/CoFe magnets are a viable option for pursuing rare-earth-free magnets with energy products approaching those containing rare-earth elements.

We study the collective behavior of Majorana modes in the vortex state of chiral $p$-wave superconductors. Away from the isolated vortex limit, the zero-energy Majorana states communicate with each other on a vortex lattice, and form a coherent band structure with a nontrivial topological character. We reveal that the topological nature of Majorana bands changes sensitively via quantum phase transitions in two-dimensional (2D) systems, by sweeping magnetic field or Fermi energy. Through the idea of dimensional reduction, we show the existence of a generic superconducting Weyl phase in a low magnetic field region of quasi-2D chiral superconductors.

The diffusion dynamics of hydrogen in bulk and nanocrystalline palladium has been examined using quasielastic neutron scattering (QENS). With respect to bulk ${\mathrm{PdH}}_{0.73}$, two relaxation processes were found. For both processes, the variation of the relaxation times with momentum transfer was well reproduced by a model of jump diffusion between adjacent octahedral sites. Upon cooling the fast relaxation fraction decreases. The result suggests that the slow relaxation corresponds to jumps between the ground states and the fast one between excited states. In nanocrystalline ${\mathrm{PdH}}_{0.47}$ with a size of 8 nm, we found a fast diffusion process with a smaller activation energy in addition to the one observed in the bulk sample. This process could be due to the motion of hydrogen atoms in the subsurface region where the potential energy surface is substantially modified by surface strain/distortion effects.

The complex magnetism and transport properties of tetragonal ${\mathrm{Ce}}_3{\mathrm{Cu}}_4{\mathrm{As}}_4{\mathrm{O}}_2$ were examined through neutron scattering and physical property measurements on polycrystalline samples. The lamellar structure consists of alternating layers of ${\mathrm{CeCu}}_4{\mathrm{As}}_4$ with a single square Ce lattice and ${\mathrm{Ce}}_2{\mathrm{O}}_2$ bilayers. Peaks in the specific heat at the Néel temperature $T_N=24$ K, $T_2=16$ K, and $T_3=1.9$ K indicate three magnetic phase transitions or distinct crossover phenomena. For $T<T_N$ neutron diffraction indicates the development of ferromagnetic $ab$ sheets for both Ce sites, with alternating polarization along $\mathbf{c}$, and a wave vector ${\mathbf{k}}_1={\mathbf{c}}^{*}$. For $T<T_2$, quasi-two-dimensional low-energy spin fluctuations with ${\mathbf{k}}_2=\frac{1}{2}{\mathbf{a}}^{*}$ and polarized perpendicular to ${\mathbf{k}}_2$ are suppressed. The data are consistent with quasi-two-dimensional antiferromagnetic order in the ${\mathrm{CeCu}}_4{\mathrm{As}}_4$ planes polarized along the ${\mathbf{k}}_2$ wave vector. $T_3$ marks a spin-flop transition where the ${\mathbf{k}}_1$ staggered magnetization switches to in-plane polarization. There are significant transport anomalies associated with the transitions, in particular a substantial reduction in resistivity for $T<T_N$. At $T=100$ mK the magnetic correlation length exceeds $75\AA$ and the ${\mathbf{k}}_1$ modulated staggered moment is $0.85{\mu }_B$, which matches the $0.8{\mu }_B$ saturation magnetization achieved for ${\mu }_{0}H=7$ T at $T=2$ K. We trace the unusual sequence of magnetic transitions to competing interactions and anisotropies in the alternating quasi-two-dimensional magnetic layers.

We present a variable variance Preisach model that fully accounts for the different magnetization processes of a multilayer structure with perpendicular magnetic anisotropy by adjusting the evolution of the interaction variance as the magnetization changes. We successfully compare in a quantitative manner the results obtained with this model to experimental hysteresis loops of several ${[\mathrm{CoFeB}/\mathrm{Pd}]}_n$ multilayers. The effect of the number of repetitions and the thicknesses of the CoFeB and Pd layers on the magnetization reversal of the multilayer structure is studied, and it is found that many of the observed phenomena can be attributed to an increase of the magnetostatic interactions and subsequent decrease of the size of the magnetic domains. Increasing the CoFeB thickness leads to the disappearance of the perpendicular anisotropy, and such a minimum thickness of the Pd layer is necessary to achieve an out-of-plane magnetization.

We show how to construct fully symmetric states without topological order on a honeycomb lattice for $S=\frac{1}{2}$ spins using the language of projected entangled pair states. An explicit example is given for the virtual bond dimension $D=4$. Four distinct classes differing by lattice quantum numbers are found by applying the systematic classification scheme introduced by two of the authors [S. Jiang and Y. Ran, Phys. Rev. B 92, 104414 (2015)]. Lack of topological degeneracy or other conventional forms of symmetry breaking in the proposed wave functions are checked by numerical calculations of the entanglement entropy and various correlation functions. Exponential decay of all correlation functions measured are strongly indicative of the energy gap for the putative parent Hamiltonian of the state. Our work provides the first explicit realization of a featureless quantum state for spin-$\frac{1}{2}$ particles on a honeycomb lattice.

The symmetry of the pairing state in iron pnictide superconductor ${\mathrm{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ is still controversial. At optimal doping ($x\approx 0.4$), it is very likely $s$ wave, but for $x=1$ there are experimental and theoretical arguments for both $s$ wave and $d$ wave. Depending on the choice for $x=1$, intermediate $s+is$ and $s+id$ states have been proposed for intermediate doping $0.4<x<1$. In both states, the time-reversal symmetry is broken and a spontaneous magnetization is allowed. In this work we study a spontaneous magnetization induced by a nonmagnetic defect in the $s+is$ and $s+id$ states by using a perturbation theory and numerical calculations for the Ginzburg-Landau free energy functional. We show that the angular dependence of the magnetization is distinct in these two states due to the difference in symmetry properties of the order parameters. Our results indicate a possible way to distinguish between the $s+is$ and $s+id$ pairing symmetries in multiband superconductors.

A highly anisotropic superconducting gap is found in single crystals of FeSe by studying the London penetration depth $\mathrm{\Delta }\lambda$ measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with introduced pointlike disorder, indicating the absence of symmetry-imposed nodes. Surprisingly, the superconducting transition temperature $T_c$ increases by 0.4 K from $T_{c0}\approx 8.8$ K while the structural transition temperature $T_s$ decreases by 0.9 K from $T_{s0}\approx 91.2$ K after electron irradiation. We discuss several explanations for the $T_c$ enhancement and propose that local strengthening of the pair interaction by irradiation-induced Frenkel defects most likely explains the phenomenon.

Density functional theory provides the most widespread framework for the realistic description of the electronic structure of solids, but the description of strongly correlated systems has remained so far elusive. We consider a particular limit of electrons and ions in which a one-band description becomes exact all the way from the weakly correlated metallic regime to the strongly correlated Mott-Hubbard regime. We provide a necessary condition a density functional should fulfill to describe Mott-Hubbard behavior in this one-band limit and show that it is not satisfied by standard and widely used local, semilocal, and hybrid functionals. We illustrate the condition in the case of few-atom systems and provide an analytic approximation to the exact exchange-correlation potential based on a variational wave function which shows explicitly the correct behavior, combining in a neat way lattice and continuum methods.

We report measurements of the magnetic susceptibility of twinned single crystals of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ from just above their superconducting transition temperatures to 300 K with magnetic fields of up to 5 T applied parallel and perpendicular to the ${\mathrm{CuO}}_2$ planes at seven values of $x$. Appropriate analysis allows the relatively small, but still important, Curie terms to be separated from other contributions to the susceptibility. Our data support a picture in which the Curie terms arise from oxygen disorder in the Cu-O chains. This agrees with published work on polycrystalline samples where the sample cooling rate was varied, but here we show that the Curie plots flatten out above 200 K. We identify small effects of charge density wave (CDW) instabilities in the temperature $\left(T\right)$ derivative of the in-plane susceptibility $d{\chi }_{ab}\left(T\right)/dT$ and discuss their $x$ dependence. For $x=0.67$ we make a detailed comparison with published high energy x-ray diffraction data using a minimal model involving Fermi arcs, thereby obtaining values for the CDW energy gap and the Helmholtz free energy in a coherence volume. At 80 and 100 K the latter is comparable with, or smaller than, $k_{B}T$, respectively, highlighting the probable importance of thermal fluctuations. We note that the effect of the Lorentz force on charge carriers in the Fermi arcs could provide a simple mechanism for enhancing the CDWs in high magnetic fields, as suggested by recent experiments.

Subnanosecond dynamics of optically excited electrons bound to excited states of neutral magnesium donor centers in silicon has been investigated. Lifetimes of nonequilibrium electrons have been derived from the decay of the differential transmission at photon energies matching the intracenter and the impurity-to-conduction band transitions. In contrast to hydrogenlike shallow donors in silicon, significantly longer lifetimes have been observed. This indicates weaker two-phonon and off-resonant interactions dominate the relaxation processes in contrast to the single-intervalley-phonon-assisted impurity-phonon interactions in the case of shallow donors in silicon.

Time-reversal invariant topological superconductors are characterized by the presence of Majorana Kramers pairs localized at defects. One of the transport signatures of Majorana Kramers pairs is the quantized differential conductance of $4e^2/h$ when such a one-dimensional superconductor is coupled to a normal-metal lead. The resonant Andreev reflection, responsible for this phenomenon, can be understood as the boundary condition change for lead electrons at low energies. In this paper, we study the stability of the Andreev reflection fixed point with respect to electron-electron interactions in the Luttinger liquid. We first calculate the phase diagram for the Luttinger liquid-Majorana Kramers pair junction and show that its low-energy properties are determined by Andreev reflection scattering processes in the spin-triplet channel, i.e., the corresponding Andreev boundary conditions are similar to that in a spin-triplet superconductor–normal lead junction. We also study here a quantum dot coupled to a normal lead and a Majorana Kramers pair and investigate the effect of local repulsive interactions leading to an interplay between Kondo and Majorana correlations. Using a combination of renormalization group analysis and slave-boson mean-field theory, we show that the system flows to a new fixed point which is controlled by the Majorana interaction rather than the Kondo coupling. This Majorana fixed point is characterized by correlations between the localized spin and the fermion parity of each spin sector of the topological superconductor. We investigate the stability of the Majorana phase with respect to Gaussian fluctuations.

Recently, new materials exhibiting exotic band structures characterized by Dirac cones, nontrivial flat bands, and band crossing points have been proposed on the basis of effective two-dimensional lattice Hamiltonians. Here, we show using atomistic tight-binding calculations that these theoretical predictions could be experimentally realized in the conduction band of superlattices nanolithographed in III-V and II-VI semiconductor ultrathin films. The lithographed patterns consist of periodic lattices of etched cylindrical holes that form potential barriers for the electrons in the quantum well. In the case of honeycomb lattices, the conduction minibands of the resulting artificial graphene host several Dirac cones and nontrivial flat bands. Similar features, but organized in different ways, in energy or in $k$-space are found in kagome, distorted honeycomb, and Lieb superlattices. Dirac cones extending over tens of meV could be obtained in superlattices with reasonable sizes of the lithographic patterns, for instance in InAs/AlSb heterostructures. Bilayer artificial graphene could be also realized by lithography of a double quantum-well heterostructure. These new materials should be interesting for the experimental exploration of Dirac-based quantum systems, for both fundamental and applied physics.

We studied nitric oxide (NO) molecules on Cu(110) and Cu(001) surfaces with low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). NO monomers on the surfaces are characterized by STM images reflecting $2{\pi }^{*}$ resonance states located at the Fermi level. NO is bonded vertically to the twofold short-bridge site on Cu(110) and to the fourfold hollow site on Cu(001). When NO molecules form dimers on the surfaces, the valence orbitals are modified due to the covalent bonding. We measured inelastic electron tunneling spectroscopy (IETS) for both NO monomers and dimers on the two surfaces, and detected characteristic structures assigned to frustrated rotation and translation modes by density functional theory simulations. Considering symmetries of valence orbitals and vibrational modes, we explain the intensity of the observed IETS signals in a qualitative manner.

We investigate the spin-stripe mechanism responsible for the peculiar nanometer modulation of the incommensurate magnetic order that emerges between the vector-chiral and the spin-density-wave phase in the frustrated zigzag spin-1/2 chain compound $\beta -{\mathrm{TeVO}}_4$. A combination of magnetic-torque, neutron-diffraction, and spherical-neutron-polarimetry measurements is employed to determine the complex magnetic structures of all three ordered phases. Based on these results, we develop a simple phenomenological model, which exposes the exchange anisotropy as the key ingredient for the spin-stripe formation in frustrated spin systems.