The electronic structure of the giant vortex states in a mesoscopic superconducting disk is studied in a dirty limit using the Usadel approach. The local density of states profiles are shown to be strongly affected by the effect of quasiparticle (QP) tunneling between the states localized in the vortex core and the ones bound to the sample edge. Decreasing temperature leads to a crossover between the edge-dominated and core-dominated regimes in the magnetic field dependence of the tunneling conductance. This crossover is discussed in the context of the efficiency of quasiparticle cooling by the magnetic-field-induced QP traps in various mesoscopic superconducting devices.

Recent development in exact classification of a superconducting gap has elucidated various unconventional gap structures, which have not been predicted by the classification of order parameter based on the point group. One of the important previous results is that all symmetry-protected line nodes are characterized by nontrivial topological numbers. Another intriguing discovery is the gap structures depending on the angular momentum $j_z$ of normal Bloch states on threefold and sixfold rotational-symmetric lines in the Brillouin zone. Stimulated by these findings, we classify irreducible representations of the Bogoliubov-de Gennes Hamiltonian at each $\mathit{k}$ point on a high-symmetry $n$-fold $(n=2$, 3, 4, and 6) axis for centrosymmetric and paramagnetic superconductors, by using the combination of group theory and $K$ theory. This leads to the classification of all crystal symmetry-protected nodes (including $j_z$-dependent nodes) on the axis that crosses a normal-state Fermi surface. As a result, it is shown that the classification by group theory completely corresponds with the topological classification. Based on the obtained results, we discuss superconducting gap structures in SrPtAs, ${\mathrm{CeCoIn}}_5,{\mathrm{UPt}}_3$, and UCoGe.

Magnetic skyrmions in bulk crystals are linelike topologically protected spin textures. They allow for the propagation of magnons along the skyrmion line but are localized inside the skyrmion line. Analogous to the vortex line, these propagating modes are the Kelvin modes of a skyrmion line. In crystals without an inversion center, it is known that the magnon dispersion in the ferromagnetic state is asymmetric in the wave vector. It is natural to expect that the dispersion of the Kelvin modes is also asymmetric with respect to the wave vector. We study the Kelvin modes of a skyrmion line in the ferromagnetic background. In contrast, we find that the lowest Kelvin mode is symmetric in the wave vector in the low-energy region despite the inversion symmetry breaking. Other Kelvin modes below the magnon continuum are asymmetric, and most of them have a positive group velocity. Our results suggest that a skyrmion line can function as a one-way waveguide for magnons.

The nature and mechanism of superconductivity in the extremely electron-doped FeSe-based superconductors remains an outstanding problem. In these systems, the hole-like band has moved below the Fermi energy, and various spin-fluctuation theories involving pairing between states near the electron Fermi surface and states of this incipient band have been proposed. Here, using a nonperturbative dynamic cluster quantum Monte Carlo calculation for a bilayer Hubbard model, we show how spin-fluctuation scattering involving intermediate virtual states of the incipient band leads to an effective pairing interaction for the electrons on the remaining Fermi surface. The key feature of this interaction is that it is retarded due to the incipient nature of the hole band and the dynamics of the spin-fluctuation interaction. This allows the pairs to avoid the instantaneous repulsive Coulomb interaction and leads to a frequency dependence of the gap which should be observable in tunneling experiments. Our work provides a different perspective for the pairing mechanism in systems with an incipient band that can be tested in future experiments.

Ultrafast dynamics are generally investigated using stroboscopic pump-probe measurements, which characterize the sample properties for a single, specific time delay. These measurements are then repeated for a series of discrete time delays to reconstruct the overall time trace of the process. As a consequence, this approach is limited to the investigation of fully reversible phenomena. We recently introduced an off-axis zone plate based x-ray streaking technique, which overcomes this limitation by sampling the relaxation dynamics with a single femtosecond x-ray pulse streaked over a picosecond long time window. In this article we show that the x-ray absorption cross section can be employed as the contrast mechanism in this novel technique. We show that changes of the absorption cross section on the percent level can be resolved with this method. To this end we measure the ultrafast magnetization dynamics in CoDy alloy films. Investigating different chemical compositions and infrared pump fluences, we demonstrate the routine applicability of this technique. Probing in transmission the average magnetization dynamics of the entire film, our experimental findings indicate that the demagnetization time is independent of the specific infrared laser pump fluence. These results pave the way for the investigation of irreversible phenomena in a wide variety of scientific areas.

We utilize near-infrared femtosecond pulses to investigate coherent phonon oscillations of ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. The coherent $A_g$ phonon mode of the lowest frequency changes abruptly not only its amplitude but also the oscillation phase as the spin order develops. In addition, the phonon mode shows a redshift entering the magnetically ordered state, which indicates a spin-phonon coupling in the system. Density functional theory calculations reveal that the $A_g$ oscillations result in octahedral tilting distortions, which are exactly in sync with the lattice deformation driven by the magnetic ordering. We suggest that the structural distortions by the spin-phonon coupling can induce the unusual oscillation phase shift between impulsive and displacive type oscillations.

We theoretically investigate spin transport at the interface between a ferromagnetic insulator (FI) and a superconductor (SC). Considering a simple FI-SC interface model, we derive formulas for the spin current and spin-current noise induced by microwave irradiation (spin pumping) or the temperature gradient (the spin Seebeck effect). We show how the superconducting coherence factor affects the temperature dependence of the spin current. We also calculate the spin-current noise in thermal equilibrium and in nonequilibrium states induced by the spin pumping and compare them quantitatively for an yttrium iron garnet–NbN interface.

We report a ferroelectric order at $\sim 98$ K for ${\mathrm{NiFe}}_2{\mathrm{O}}_4$, which carries an inverse spinel structure with a centrosymmetric $Fd\overline{3}m$ structure at room temperature. The value of spontaneous electric polarization is considerably high as $\sim 0.29 \mu \mathrm{C}/{\mathrm{cm}}^2$ for 5 kV/cm poling field. The electric polarization decreases considerably ($\sim 17\%$) around the liquid-nitrogen temperature upon application of 50 kOe field, proposing a significant magnetoelectric coupling. The synchrotron diffraction studies confirm a structural transition at $\sim 98$ K to a noncentrosymmetric structure of $P{4}_{1}22$ space group. The occurrence of polar order is associated with an ordered occupancy of Ni and Fe atoms at the octahedral sites of the $P{4}_{1}22$ structure, instead of random occupancies at the octahedral site of the inverse spinel structure. The results propose that ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ is a member in the type-II multiferroics.

We discuss the type of pairing in the hexagonal pnictide superconductor SrPtAs, taking into account its multiband structure. The topological chiral $d$-wave state with time-reversal symmetry breaking has been anticipated from the spontaneous magnetization observed by the muon-spin-relaxation experiment. We point out in this paper that the recent experimental reports on the nuclear-spin-lattice relaxation rate $T_1^{-1}$ and superfluid density $n_s\left(T\right)$, which seemingly support the conventional $s$-wave pairing, are also consistent with the chiral $d$-wave state. The compatibility of the gap and multiband structures is crucial in this argument. We propose that the measurement of the bulk quasiparticle density of states would be useful for the distinction between two pairing states.

We report the ac magnetic susceptibility, electrical resistance, and x-ray diffraction measurements of ${\mathrm{PtH}}_x\left(x\simeq 1\right)$ in diamond anvil cells, that reveal the superconducting transition. At 30 GPa, when ${\mathrm{PtH}}_x$ is in a ${P6}_3/mmc$ structure, ${\mathrm{PtH}}_x$ exhibits the superconducting transition at 7 K. Remarkably, the observed $T_c$ is higher than that of powdered Pt by more than three orders of magnitude. It is suggested that the noble metals in the periodic table of elements possibly form a region where hydrides have higher $T_c$ than elements.

Near a quantum-critical point in a metal a strong fermion-fermion interaction, mediated by a soft boson, destroys fermionic coherence and also gives rise to an attraction in one or more pairing channels. The two tendencies compete with each other, and in a class of large $N$ models, where the tendency to incoherence is parametrically stronger, one would naively expect an incoherent (non-Fermi liquid) normal state behavior to persist down to $T=0$. However, this is not the case for quantum-critical systems, described by Eliashberg theory. In such systems, the part of the fermionic self-energy $\mathrm{\Sigma }\left({\omega }_m\right)$, relevant for spin-singlet pairing, is large for a generic Matsubara frequency ${\omega }_m=\pi T(2m+1)$ but vanishes for fermions with ${\omega }_m=\pm \pi T$, while the pairing interaction between fermions with these two frequencies remains strong. It has been shown [Y. Wang et al. Phys. Rev. Lett. 117, 157001 (2016)] that due to this peculiarity, the onset temperature for the pairing $T_p$ is finite even at large $N$, when the scaling analysis predicts a non-Fermi liquid normal state. We consider the system behavior below $T_p$ and contrast the conventional case, when ${\omega }_m=\pm \pi T$ are not special, and the case when the pairing is induced by fermions with ${\omega }_m=\pm \pi T$. We obtain the solution of the nonlinear gap equations in Matsubara frequencies and then convert to real frequency axis and obtain the spectral function $A(k,\omega )$ and the density of states $N\left(\omega \right)$. In a conventional BCS-type superconductor, $A(k,\omega )$ and $N\left(\omega \right)$ are peaked at the gap value $\mathrm{\Delta }\left(T\right)$, and the peak position shifts to a smaller $\omega$ as temperature increases towards $T_p$, i.e., the gap “closes in.” We show that, when the pairing is induced by fermions with ${\omega }_m=\pm \pi T$, the situation is qualitatively different from the standard BCS result. Namely, the peak in $N\left(\omega \right)$ remains at a finite frequency even at $T=T_p-0$, the gap just “fills in” near this $T$. The spectral function $A(k,\omega )$ either shows almost the same “gap filling” behavior as the density of states, or its peak position shifts to zero frequency already at a finite $\mathrm{\Delta }$ (“emergent Fermi arc” behavior), depending on the position of $\mathbf{k}$ on the Fermi surface. As an example, we compare our results with the data for the cuprates and argue that “gap filling” behavior holds in the antinodal region, while the “emergent Fermi arc” behavior holds in the nodal region.

We investigate the ionic Hubbard model (IHM) at half-filling in the limit of strong correlations and large ionic potential. The low-energy effective Hamiltonian in this limit, obtained by a similarity transformation, is a modified $t-J$ model with effective second-neighbor hopping terms. We explore the possibilities of $d$-wave pairing and extended $s$-wave pairing superconducting (SC) phases on a two-dimensional square lattice at zero temperature within a Gutzwiller projected renormalized mean-field theory. In the sector of solutions that forbid spin-ordering, the system shows a finite nonzero $d$-wave as well as extended $s$-wave pairing amplitude for $\mathrm{\Delta }\sim U\gg t$. The width of the superconducting phase in $U-\mathrm{\Delta }$ regime shrinks with increase in $U$ and $\mathrm{\Delta }$, though the extended $s$-wave pairing phase is higher in energy than the $d$-wave pairing superconducting phase. But in a spin-resolved renormalized mean-field calculation, which allows for an antiferromagnetic (AF) order along with the $d$-wave or extended $s$-wave pairing, the SC phase is no longer viable and the system shows a direct transition from an AF ordered phase to a paramagnetic band insulator. Except for a thin sliver of a half-metallic AF phase close to the AF transition point, most of the AF ordered phase is a Mott insulator. We benchmarked the AF Mott insulator to band insulator transition within the Gutzwiller projected renormalized mean-field theory against the dynamical mean-field theory solved using continuous time quantum Monte Carlo. Our work suggests that the ground-state phase diagram of the IHM at half-filling in the limit of extreme correlations does not have any SC phase. The SC phase seen in the paramagnetic sector is a metastable phase, being higher in energy than the AF Mott insulator phase.

We propose a type of topological states of matter exhibiting topologically nontrivial edge states (ESs) within gapless bulk states (GBSs) protected by both spin rotational and reflection symmetries. A model presenting such states is simply comprised of a one-dimensional reflection symmetric superlattice in the presence of spin-orbit (SO) coupling containing an odd number of sublattices per unit cell. We show that the system has a rich phase diagram including a topological metal (TM) phase where nontrivial ESs coexist with nontrivial GBSs at Fermi level. Topologically distinct phases can be reached through a subband gap closing-reopening transition depending on the relative strength of inter- and intraunit cell SO couplings. Moreover, the topological class of the system is AI with an integer topological invariant called $\mathbb{Z}$ index. The stability of TM states is also analyzed against Zeeman magnetic fields and on-site potentials resulting in that the spin rotational symmetry around the lattice direction is a key requirement for the appearance of such states. Also, possible experimental realizations are discussed.

We discuss the relations between restricted Boltzmann machine (RBM) states and the matrix product states (MPS) for the ground states of 1D translational invariant stabilizer codes. A generic translational invariant and finitely connected RBM state can be expressed as an MPS, and the matrices of the resulting MPS are of rank 1. We dub such an MPS as an RBM-MPS. This provides a necessary condition for exactly realizing a quantum state as an RBM state, if the quantum state can be written as an MPS. For generic 1D stabilizer codes having a nondegenerate ground state with periodic boundary condition, we obtain an expression for the lower bound of their MPS bond dimension, and an upper bound for the rank of their MPS matrices. In terms of RBM, we provide an algorithm to derive the RBM for the cocycle Hamiltonians whose MPS matrices are proved to be of rank 1. Moreover, the RBM-MPS produced by our algorithm has the minimal bond dimension. A family of examples is provided to explain the algorithm. We finally conjecture that these features hold true for all the 1D stabilizer codes having a nondegenerate ground state with periodic boundary condition, as long as their MPS matrices are of rank 1.

Molecular dynamics (MD) simulation has emerged as a powerful predictive tool to study mechanical and thermal properties of materials, without the requirement for any fitting parameter inputs such as phonon relaxation time. However, the lack of suitable interatomic potentials for many complex materials greatly prohibits effective use of MD simulations to investigate properties of bulk materials and nanostructures. In this paper, we use the method of fitting to an ab initio energy surface to develop interatomic potential parameters for the complex binary material antimony telluride, which has important applications in thermoelectric energy generation. Density-functional theory is used to calculate the ground-state electronic structure of the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ crystal, following which the total energies of a series of artificially distorted lattice configurations are calculated to create the energy surface. A Morse potential functional form is fitted to the energy surface and experimental data, and the parameters are used to calculate the bulk crystal properties and phonon spectra using lattice dynamics. Our parameters are able to reproduce the lattice structure, elastic constants, and acoustic phonon dispersion in good agreement with experimental data. MD simulations are performed using the fitted potential to calculate the thermal conductivity of bulk ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ using the Green-Kubo method. The predicted thermal conductivity shows a $1/T$ variation in both in-plane and cross-plane directions with the results in the range of experimental measurements. Frequency domain normal mode analysis is used to calculate the modal phonon relaxation times and the accumulation of thermal conductivity with respect to phonon mean free paths. The results show that phonons with mean free paths between 3 and 100 nm contribute to 80% of the total cross-plane thermal conductivity.

We study the time-dependent dynamical phase of fractional quasiparticles in quantum Hall states using Ramsey interferometry. A Ramsey two-pulse voltage modulation generates an interference term in the current noise of a quantum point contact oscillating with the dynamical phase. We show that the interference pattern probes the Green's function of the fractional quasiparticles in the time domain and reveals the fractional charge. We address both the case of a pointlike tunnel junction and the case of momentum-resolved tunneling which also provides information on the speed of those quasiparticles propagating along the edge. For the $5/2$ fractional quantum Hall case, the Ramsey signal can reveal the charge $e/4$ and differentiate between Pfaffian and anti-Pfaffian states.

We propose a lattice density-functional theory for ab initio quantum chemistry or physics as a route to an efficient approach that approximates the full configuration interaction energy and orbital occupations for molecules with strongly correlated electrons. We build on lattice density-functional theory for the Hubbard model by deriving Kohn-Sham equations for a reduced then full quantum chemistry Hamiltonian and demonstrate the method on the potential energy curves for the challenging problem of modeling elongating bonds in a linear chain of six hydrogen atoms. Here the accuracy of the Bethe-ansatz local-density approximation is tested for this quantum chemistry system, and we find that, despite this approximate functional being designed for the Hubbard model, the shapes of the potential curves generally agree with the full configuration interaction results. Although there is a discrepancy for very stretched bonds, it is lower than when using standard density-functional theory with the local-density approximation.

We propose a simple strategy to find possible linear double Weyl semimetals (WSMs) among nonmagnetic materials. This strategy can reproduce several known robust WSM candidates, covering orthorhombic, tetragonal, and cubic lattice systems. This kind of WSM hosts Weyl points (WPs) carrying double chiral charges when spin degeneracy is considered while spin-orbit coupling (SOC) is switched off, and dispersions along all three directions are linear, distinct from the parabolic double WPs proposed in $\mathrm{HgC}{\mathrm{r}}_2\mathrm{S}{\mathrm{e}}_4$. When SOC is included, these double-charge WPs split into two single spin-1/2 WPs with equal chirality. Following this strategy and combining with ab initio structural prediction techniques, we propose a candidate for such kind of WSMs, rooted in a high-pressure phase of $\mathrm{Ti}{\mathrm{S}}_2$ with space group $P\text{−}62m$. In this system, all the WPs are pinned at the same energy due to crystal symmetry. In addition, the minimum spacing between opposite-chirality WPs is $\sim 0.23\AA {}^{-1}$, which makes it a promising robust WSM candidate.

We develop a generalization of the Kohn-Sham density-functional theory (KS-DFT)+Hubbard $U$ ($\mathrm{DFT}+U$) method to the excited-state regime. This has the form of Hubbard $U$ corrected linear-response time-dependent DFT, or “$\mathrm{TDDFT}+U$.” Combined with calculated linear-response Hubbard $U$ parameters, it may provide a computationally light, first-principles method for the simulation of tightly bound excitons on transition-metal ions. Our presented implementation combines linear-scaling $\mathrm{DFT}+U$ and linear-scaling TDDFT, but the approach is broadly applicable. In detailed benchmark tests on two Ni-centered diamagnetic coordination complexes with variable $U$ values, it is shown that the Hubbard $U$ correction to an approximate adiabatic semilocal exchange-correlation interaction kernel lowers the excitation energies of transitions exclusively within the targeted localized subspace by increasing the exciton binding of the corresponding electron-hole pairs. This partially counteracts the Hubbard $U$ correction to the exchange-correlation potential in KS-DFT, which increases excitation energies into, out of, and within the targeted localized subspace by modifying the underlying KS-DFT eigenspectrum. This compensating effect is most pronounced for optically dark transitions between localized orbitals of the same angular momentum, for which experimental observation may be challenging, and theoretical approaches are at their most necessary. Interestingly, we find that first-principles $\mathrm{TDDFT}+U$ seems to offer a remarkably good agreement with experiment for a perfectly closed-shell complex on which approximate TDDFT underperforms, but only when $\mathrm{TDDFT}+U$ is applied to the DFT eigenspectrum and not to the $\mathrm{DFT}+U$ one. In tests on an open-shell, noncentrosymmetric, high-spin cobalt coordination complex, we find that first-principles $\mathrm{TDDFT}+U$ again compensates for the $\mathrm{DFT}+U$ blueshift in $3d\to 3d$ transitions, but that using the DFT eigenspectrum is not viable due to the emergence of a singlet instability. Overall, our results point to shortcomings in the contemporary $\mathrm{DFT}+U$ corrective potential, either in its functional form, or when applied to transition-metal orbitals but not to ligand ones, or both.