Thin magnetic films with Dzyaloshinskii-Moriya interactions are known to host skyrmion crystals, which typically have a hexagonal lattice structure. We investigate skyrmion-lattice configurations in synthetic antiferromagnets, i.e., a bilayer of thin magnetic films that is coupled antiferromagnetically. By means of Monte Carlo simulations, we find that by tuning the interlayer coupling the skyrmion lattice structure can be tuned from square to hexagonal. We give a simple interpretation for the existence of this transition based on the fact that for synthetic antiferromagnetic coupling the skyrmions in different layers repel each other and form each others' dual lattice. Our findings may be useful to experimentally switch between two lattice configurations to, for example, modify spin-wave propagation.

In the intermediate mixed state (IMS) of type-II/1 superconductors, vortex lattice (VL) and Meissner state domains coexist due to a partially attractive vortex interaction. Using a neutron-based multiscale approach combined with magnetization measurements, we study the continuous decomposition of a homogeneous VL into increasingly dense domains in the IMS in bulk niobium samples of varying purity. We find a universal temperature dependence of the vortex spacing, closely related to the London penetration depth and independent of the external magnetic field. The rearrangement of vortices occurs even in the presence of a flux freezing transition, i.e., pronounced pinning, indicating a breakdown of pinning at the onset of the vortex attraction.

We consider a superconducting film exchange-coupled to a close-by chiral magnetic layer and study how magnetic skyrmions can induce the formation of Majorana bound states (MBS) in the superconductor. Inspired by a proposal by Yang et al. [Phys. Rev. B 93, 224505 (2016)], which suggested MBS in skyrmions of even winding number, we explore whether such skyrmions could result from a merger of ordinary skyrmions. We conclude that the formation of higher-winding skyrmions is not realistic in chiral magnets. Subsequently, we present a possibility to obtain MBS from realistic skyrmions of winding number one, if a skyrmion-vortex pair is formed instead of a bare skyrmion. Specifically, we show that MBS are supported in a pair of a circular skyrmion and a vortex which both have a winding number of one. We back up our analytical prediction with results from numerical diagonalization and obtain the spatial profile of the MBS. In light of recent experimental progress on the manipulation of skyrmions, such systems are promising candidates to achieve direct spatial control of MBS.

We present a theory of an optically induced valley polarization in an interacting, time-reversal-symmetric Weyl semimetal placed under strong magnetic fields. Because the application of a magnetic field reduces the symmetry of the crystal, the optical absorption intensity differs at Weyl nodes that were equivalent by symmetry at zero field. At strong magnetic field, the difference in the absorption intensity reaches 100% for a sizable frequency interval of the incident light. This complete valley polarization originates from interband transitions involving the chiral Landau level, and can be controlled by changing the directions of the magnetic field and the light propagation. We identify the splitting of $0\to 1$ or $-1\to 0$ inter-Landau-level transitions as an observable signature of the complete valley polarization, and discuss its manifestation in the TaAs family of materials.

By extending our victory implementation of the parquet approach to include nonlocal Coulomb interactions, we study the extended Hubbard model on the two-dimensional square lattice with a particular focus on the competition of the nonlocal charge and spin fluctuations. Surprisingly, we find that their competition, as the mechanism driving the phase transition towards the charge density wave, dominates only in a very narrow parameter regime in the immediate vicinity of the phase transition. Due to the special geometry and the Fermi surface topology of the square lattice, antiferromagnetic fluctuations dominate even for sizable next-nearest-neighbor interactions. Our conclusions are based on the consistent observations in both the single- and two-particle quantities, including the self-energy, the single-particle spectral function, the two-particle susceptibility, the density-density vertex function, and the optical conductivity. Our work unbiasedly establishes the connection of these quantities to the charge fluctuations, and the way of interpretation can be readily applied to any many-body method with access to the two-particle vertex.

The lattice construction of Euclidean path integrals has been a successful approach of deriving 2+1D field theory dualities with a $\mathrm{U}\left(1\right)$ gauge field. In this work, we generalize this lattice construction to dualities with non-Abelian gauge fields. We construct the Euclidean space-time lattice path integral for a theory with strongly interacting $\mathrm{SO}\left(3\right)$ vector bosons and Majorana fermions coupled to an $\mathrm{SO}\left(3\right)$ gauge field and derive an exact duality between this theory and the theory of a free Majorana fermion on the space-time lattice. We argue that this lattice duality implies the desired infrared duality between the field theory with an $\mathrm{SO}\left(3\right)$ vector critical boson coupled to an $\mathrm{SO}{\left(3\right)}_1$ Chern-Simons gauge theory, and a free massless Majorana fermion in 2+1D. We also generalize the lattice construction of dualities to models with $\mathrm{O}\left(3\right)$ gauge fields.

For an optical absorber, its efficiency and bandwidth determine ultimately its light-collection performance. A general way to realize the high-efficiency absorption and manipulate the absorption bandwidth is certainly important and highly desired. In this paper, based on the coupled-mode theory, a general strategy for tailoring the bandwidth of an absorber (to broaden or narrow it) is presented. A graphene-based absorber integrated with a lossless resonator is designed, and it is shown that by increasing the layer number of graphene and adjusting structure parameters, the broadband absorber at critical coupling could be realized. Meanwhile, by adjusting the location of graphene in the structure, an ultra-narrow-band absorber could also be realized. Present work of controlling both efficiency and bandwidth of an absorber would be particularly favorable for applications in light harvesting, light emitting devices, and optical modulators.

The densities of phonon states of thin Sn films on InSb substrates are determined during different stages of the $\alpha \text{−}\mathrm{Sn}$ to $\beta \text{−}\mathrm{Sn}$ phase transition using nuclear inelastic x-ray scattering. The vibrational entropy and internal energy per atom as a function of temperature are obtained by numerical integration of the phonon density of states. The free energy as a function of temperature for the nanoscale samples is compared to the free energy obtained from ab initio calculations of bulk tin in the $\alpha \text{−}\mathrm{Sn}$ and $\beta \text{−}\mathrm{Sn}$ phase. In thin films this phase transition is governed by the interplay between the vibrational behavior of the film (the phase transition is driven by the vibrational entropy) and the stabilizing influence of the substrate (which depends on the film thickness). This brings a deeper understanding of the role of lattice vibrations in the phase transition of nanoscale Sn.

We study the spin dynamics and spin noise in a double quantum dot, taking into account the interplay between hopping, exchange interaction, and the hyperfine interaction. At short timescales the spin relaxation is governed by the spin dephasing in the random nuclear fields. At long timescales the spin polarization obeys universal power law $1/t$ independent of the relation between all the parameters of the system. This effect is caused by the competition between the spin blockade effect and the hyperfine interaction. The spin noise spectrum of the system universally diverges as $ln(1/\omega )$ at low frequencies.

In this work we propose a way to unveil the type of environmental noise in strongly driven superconducting flux qubits through the analysis of the Landau-Zener-Stückelberg (LZS) interferometry. We study both the two-level and multilevel dynamics of the flux qubit driven by a $\mathrm{dc}+\mathrm{ac}$ magnetic field. We find that the LZS interference patterns exhibit well-defined multiphoton resonances whose shape strongly depends on the timescale and the type of coupling to a quantum bath. For the case of transverse system-bath coupling, the $n$-photon resonances are narrow and nearly symmetric with respect to the dc magnetic field for almost all timescales, while in the case of longitudinal coupling they exhibit a change from a wide symmetric to an antisymmetric shape for times of the order of the relaxation time. We find this dynamic behavior relevant for the interpretation of several LZS interferometry experiments in which the stationary regime is not completely reached.

Monoclinic $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ has a large band gap of 4.8 eV, and can therefore be used as a contact material that is transparent to visible and UV light. However, indirect free-carrier absorption processes, mediated by either phonons or charged impurities, will set a fundamental limit on transparency. We use first-principles calculations to accurately assess the absorption cross section and to elucidate the microscopic origins of these processes. Phonon-assisted absorption is dominated by the emission of phonons, and is therefore always possible. This indirect absorption is inversely proportional to the cube of the wavelength. The presence of charged impurities, whether intentional or unintentional, leads to additional absorption, but for realistic concentrations, phonon-assisted absorption remains the largest contribution. Direct free-carrier absorption also leads to below-gap absorption, with distinct peaks where optical transitions match energy differences to higher conduction bands. In contrast, indirect absorption uniformly reduces transparency for all sub-band-gap wavelengths.

The dynamic coupling between spin and charge currents in nonmagnetic conductors is considered. As a consequence of this coupling, the spin dynamics is directly reflected in the electrical impedance of the sample, with a relevant frequency scale defined by spin relaxation and spin diffusion. This allows the observation of the electron spin resonance by purely electrical measurements.

The discovery of topological quantum materials represents a striking innovation in modern condensed matter physics with remarkable fundamental and technological implications. Their classification has been recently extended to topological Weyl semimetals, i.e., solid-state systems which exhibit the elusive Weyl fermions as low-energy excitations. Here we show that the Nernst effect can be exploited as a sensitive probe for determining key parameters of the Weyl physics, applying it to the noncollinear antiferromagnet ${\mathrm{Mn}}_3\mathrm{Ge}$. This compound exhibits anomalous thermoelectric transport due to enhanced Berry curvature from Weyl points located extremely close to the Fermi level. We establish from our data a direct measure of the Berry curvature at the Fermi level and, using a minimal model of a Weyl semimetal, extract the Weyl point energy and their distance in momentum space.

We present momentum-resolved spectral functions and band gaps from bare and self-consistent second-order perturbation theory for insulating periodic solids. We establish that, for systems with large gap sizes, both bare and self-consistent perturbation theory yield reasonable gaps. However, smaller gap sizes require a self-consistent adjustment of the propagator. In contrast to results obtained within a quasiparticle formalism used on top of bare second-order perturbation theory, no unphysical behavior of the band gap is observed. Our implementation of a fully self-consistent, $\mathrm{\Phi }$-derivable, and thermodynamically consistent finite-temperature diagrammatic perturbation theory forms a framework on which embedding theories such as the dynamical mean-field theory and self-energy embedding theories can be implemented.

Exciton-polaritons (polaritons herein) offer a unique nonlinear platform for studies of collective macroscopic quantum phenomena in a solid-state system. Shaping of polariton flow and polariton confinement via potential landscapes created by nonresonant optical pumping has gained considerable attention due to the flexibility and control enabled by optically induced potentials. Recently, large density-dependent energy shifts (blueshifts) exhibited by optically trapped polaritons at low densities, below the bosonic condensation threshold, were interpreted as an evidence of strong polariton-polariton interactions [Y. Sun et al., Nat. Phys. 13, 870 (2017)]. In this work, we further investigate the origins of these blueshifts in optically induced circular traps and present evidence of significant blueshifts of the polariton energy due to reshaping of the optically induced potential with laser pump power. Our work demonstrates the strong influence of the effective potential formed by an optically injected excitonic reservoir on the energy blueshifts observed below and up to the polariton condensation threshold and suggests that the observed blueshifts arise due to interaction of polaritons with the excitonic reservoir, rather than due to polariton-polariton interaction.

We study the intrinsic anomalous Hall effect (AHE) in both two-dimensional and bulk ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ (FGT) van der Waals crystals, where long-range ferromagnetic order has been observed in FGT with thickness down to monolayer. We demonstrate that the FGT with number of layers ($n$) equal to 2 exhibits the strongest AHE, and the trend of the calculated anomalous Hall conductivity (AHC) per layer with $n$ is roughly consistent with the experimental observations. Such layer dependence of AHE is in sharp contrast to the similar values of magnetic moments for FGT with different thickness. The distribution of Berry curvature and configurations of Fermi lines in the reciprocal space indicate that the interlayer hybridization in bilayer FGT induces great modification of the energy dispersions around the Fermi level, leading to much more occurrence of avoided crossings of Fermi lines near the Brillouin zone boundaries. Despite the much lower value of the AHC in the pristine monolayer FGT, it can be significantly enhanced by electron doping. Our study thus provides an in-depth understanding of the evolution of the AHE and electronic structure of FGT from bulk to monolayer.

Despite many years of efforts, attempts to reach the quantum regime of topological surface states (TSS) on an electrically tunable topological insulator (TI) platform have so far failed on binary TI compounds such as $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ due to high density of interfacial defects. Here, utilizing an optimal buffer layer on a gatable substrate, we demonstrate the first electrically tunable quantum Hall effects (QHE) on TSS of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. On the $n$ side, well-defined QHE shows up, but it diminishes near the charge neutrality point (CNP) and completely disappears on the $p$ side. Furthermore, around the CNP the system transitions from a metallic to a highly resistive state as the magnetic field is increased, whose temperature dependence indicates presence of an insulating ground state at high magnetic fields.

A number of praseodymium-containing cobalt perovskites undergo temperature-dependent metal-insulator transitions that involve a transfer of electrons between cobalt and praseodymium cations. We have used high-resolution inelastic x-ray scattering to directly measure the changes in the acoustic phonon dispersion through this transition in one such single crystal, ${\left(\mathrm{P}{\mathrm{r}}_{0.85}{\mathrm{Y}}_{0.15}\right)}_{0.7}\mathrm{C}{\mathrm{a}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3-\delta }$, in an effort to understand the role of the lattice in the transition. Transverse and acoustic modes were measured in the pseudocubic (200), (220), and (222) zones both above and below the transition temperature. The resultant dispersion curves reveal that the transition is associated with a significant hardening of the pseudocubic $C_{11}$ elastic constant, whereas $C_{12}$ and $C_{44}$ do not significantly change. Analysis of the temperature-dependent damping of the longitudinal acoustic mode in the (200) zone reveals a peak in the damping at temperatures just above the transition temperature. Significant elastic diffuse scattering is also observed above the transition temperature, likely resulting from clusters in which the electron transfer is beginning to occur as a precursor to the phase transition. The presence of these clusters likely limits the phonon lifetimes, thus leading to the observed damping.

Single-crystal diffuse scattering is generally interpreted using correlation parameters that describe probabilities for certain configurations on a local scale. In this paper we present an interpretation of diffuse maxima using a disordered superspace approach. In $(D+d)$-dimensional superspace two modulation functions are disordered along the superspace axis ${\mathit{a}}_{s,i}$ for $i=1,\cdots ,D$, while the periodicity along the internal dimensions is maintained. This simple approach allows the generation of substitutionally disordered model structures that show diffuse maxima of any width at any position in reciprocal space. The extinction rules that are introduced by superspace symmetry are also fulfilled by the diffuse maxima from structures generated using the disordered superspace approach. In this paper we demonstrate the disordered superspace approach using a simple two-dimensional binary substitutionally disordered system. The extension of the approach to $(3+d)$-dimensional superspace is trivial. The treatment of displacement and magnetic disorder as well as size-effect-like distortions in a similar manner is possible.