Icosahedral quasicrystals spontaneously form from the melt in simulations of Al-Cu-Fe alloys. We model the interatomic interactions using oscillating pair potentials tuned to the specific alloy system based on a database of density functional theory (DFT)-derived energies and forces. Favored interatomic separations align with the geometry of icosahedral motifs that overlap to create face-centered icosahedral order on a hierarchy of length scales. Molecular dynamics simulations, supplemented with Monte Carlo steps to swap chemical species, efficiently sample the configuration space of our models, which reach up to 9846 atoms. Exchanging temperatures of independent trajectories (replica exchange) allows us to achieve thermal equilibrium at low temperatures. By optimizing structure and composition we create structures whose DFT energies reach to within $\sim 2$ meV/atom of the energies of competing crystal phases. Free energies obtained by adding contributions due to harmonic and anharmonic vibrations, chemical substitution disorder, phasons, and electronic excitations, show that the quasicrystal becomes stable against competing phases at temperatures above 600 K. The average structure can be described succinctly as a cut through atomic surfaces in six-dimensional space that reveal specific patterns of preferred chemical occupancy. Atomic surface regions of mixed chemical occupation demonstrate the proliferation of phason fluctuations, which can be observed in real space through the formation, dissolution and reformation of large-scale icosahedral motifs—a picture that is hidden from diffraction refinements due to averaging over the disorder and consequent loss of information concerning occupancy correlations.

Diffusion Monte Carlo using continuous real-space wave functions is one of the most accurate scalable many-body methods for solid-state systems. However, to date, spin-orbit interactions have not been incorporated into large-scale calculations at a first-principles level, only having been applied to small systems. In this technique, we use explicitly correlated first-principles diffusion Monte Carlo calculations to derive an effective spin-orbit model Hamiltonian. The simplified model Hamiltonian is then solved to obtain the energetics of the system. To demonstrate this method, benchmark studies are performed in main-group atoms and monolayer tungsten disulfide, where high accuracy is obtained.

We study the thermodynamics and critical behavior of neutron ${}^{3}P_2$ superfluids in the inner cores of neutron stars. ${}^{3}P_2$ superfluids offer a rich phase diagram including uniaxial/biaxial nematic phases, the ferromagnetic phase, and the cyclic phase. Using the Bogoliubov–de Gennes equation as in superfluid Fermi liquid theory, we show that a strong (weak) magnetic field drives the first-order (second-order) transition from the dihedral-two biaxial nematic phase to the dihedral-four biaxial nematic phase at low (high) temperatures and their phase boundaries are divided by the critical end point (CEP). We demonstrate that the set of critical exponents at the CEP satisfies the Rushbrooke, Griffiths, and Widom equalities, indicating a different universality class. At the CEP, the ${}^{3}P_2$ superfluid exhibits critical behavior with nontrivial critical exponents, indicating an alternative universality class. Furthermore, we find that the Ginzburg-Landau (GL) equation up to the eighth-order expansion satisfies three equalities and properly captures the physics of the CEP. This implies that the GL theory can provide a tractable way for understanding critical phenomena which may be realized in the dense core of realistic magnetars.

We study quantized vortices in ${}^{3}P_2$ superfluids using a microscopic theory for the first time. The theory is based on the Eilenberger equation to determine the order parameters and the Bogoliubov-de Gennes (BdG) equation to obtain the eigenenergies and the core magnetization. Within axisymmetric vortex configurations, we find several stable and metastable vortex configurations which depend on the strength of a magnetic field, similar to a $v$ vortex and $o$ vortex in ${}^3\mathrm{He}$ superfluids. We demonstrate that the $o$ vortex is the most stable axisymmetric vortex in the presence of a strong magnetic field, and we find two zero-energy Majorana fermion-bound states in the $o$-vortex core. We show that the profiles of the core magnetization calculated using the BdG equation are drastically different from those calculated using only the order parameter profiles known before.

Ultrafast (130-fs) x-ray diffraction at the Linac Coherent Light Source has been applied to observe shock melting, which is driven by a rapid (120-ps) laser pulse impinging on a thin (few micrometers) bilayer of aluminum/zirconium. At a pressure of 100 GPa in the aluminum (130 GPa in the zirconium), there is rapid melting of both metals and the recrystallization of zirconium into the bcc $\beta$ phase. We observe the solidification of the melt starting a few hundred picoseconds following the shock melting, out to 50 ns when the zirconium is fully crystallized into the bcc $\beta$ phase at a residual temperature of approximately 2000 K. The pressure is obtained directly from the early time x-ray data, whereas the additional information from the x-ray line width and intensity at longer times inform a model of crystal nucleation and growth.

We perform a systematic ab initio study of the electronic structure of Sr(V,Mo,$\mathrm{Mn}){\mathrm{O}}_3$ perovskites, using the parameter-free $GW+\text{EDMFT}$ method. This approach self-consistently calculates effective interaction parameters, taking into account screening effects due to nonlocal charge fluctuations. Comparing the results of a three-band $\left(t_{2g}\right)$ description to those of a five-band $\left(t_{2g}+e_g\right)$ model, it is shown that the $e_g$ states have little effect on the low-energy properties and the plasmonic features for the first two compounds but play a more active role in ${\mathrm{SrMnO}}_3$. In the case of ${\mathrm{SrMnO}}_3$, paramagnetic $GW+\text{EDMFT}$ yields a metallic low-temperature solution on the verge of a Mott transition, while antiferromagnetic $GW+\text{EDMFT}$ produces an insulating solution with the correct gap size. We discuss the possible implications of this result for the nature of the insulating state above the Néel temperature, and the reliability of the $GW+\text{EDMFT}$ scheme.

We explore the low-energy dynamics of the four siblings of Lorentz symmetry enriched $\mathrm{SU}\left(2\right)$ Yang-Mills theories with a theta term at $\theta =\pi$ in $(3+1)\mathrm{d}$. Due to a mixed anomaly between time reversal symmetry and the one-form center symmetry, the low-energy dynamics cannot be symmetric trivially gapped. We focus on two possible scenarios: (1) time reversal symmetry is spontaneously broken by the two confining vacua and (2) a deconfined, gapless, and time reversal symmetric $\mathrm{U}\left(1\right)$ Maxwell gauge theory [e.g., $\mathrm{U}\left(1\right)$ spin liquid in condensed matter]. In the first scenario, we find that the antiunitary time reversal symmetry in the bulk induces a ${\mathbb{Z}}_2$ unitary symmetry on the domain wall between the two vacua. We discuss how the Lorentz symmetry and the unitary ${\mathbb{Z}}_2$ symmetry enrich the domain-wall topological field theory. In the second scenario, we relate the symmetry enrichments of the $\mathrm{SU}\left(2\right)$ Yang-Mills to that of the $\mathrm{U}\left(1\right)$ Maxwell gauge theory. This further opens up the possibility that $\mathrm{SU}\left(2\right)$ QCD with large and odd flavors of fermions could be a direct second-order phase transition between two phases of $\mathrm{U}\left(1\right)$ gauge theories as well as between a $\mathrm{U}\left(1\right)$ gauge theory and a trivial vacuum (e.g., a trivial paramagnet), where the gauge group is enhanced to be non-Abelian at and only at the transition. We characterize these transitions and name them as gauge enhanced quantum critical points.

We study trace estimators for equilibrium thermodynamic observables that rely on the idea of typicality and derivatives thereof such as the finite-temperature Lanczos method (FTLM). As numerical examples quantum spin systems are studied. Our initial aim was to identify pathological examples or circumstances, such as strong frustration or unusual densities of states, where these methods could fail. It turned out that all investigated systems allow such approximations. Only at temperatures of the order of the lowest energy gap is the convergence somewhat slower in the number of random vectors over which observables are averaged.

We formulate a Bardeen-Cooper-Schriffer (BCS) theory of quasiparticles in a degenerate Fermi gas strongly coupled to photons in a optical cavity. The elementary photonic excitations of the system are cavity polaritons, which consist of a cavity photon and an excitation of an atom within the Fermi sea. The excitation of the atom out of the Fermi sea leaves behind a hole, which together results in a loosely bound Cooper pair, allowing for the system to be written by a BCS wave function. As the density of the excitations is increased, the excited atom and hole become more strongly bound, crossing over into the molecular regime. This thus realizes an alternative BCS to BEC crossover scenario, where the participating species are quasiparticle excitations in a Fermi sea consisting of excited atoms and holes.

Magnetic frustration in metals is scarce and hard to pinpoint, but exciting due to the possibility of the emergence of fascinating novel phases. The cubic intermetallic compound ${\mathrm{HoInCu}}_4$ with all holmium atoms on an fcc lattice exhibits partial magnetic frustration, yielding a ground state where half of the Ho moments remain without long-range order, as evidenced by our neutron scattering experiments. The substitution of In with Cd results in ${\mathrm{HoCdCu}}_4$ in a full breakdown of magnetic frustration. Consequently we found a fully ordered magnetic structure in our neutron diffraction experiments. These findings are in agreement with the local energy scales and crystal electric field excitations, which we determined from specific heat and inelastic neutron scattering data. The electronic density of states for the itinerant bands acts as a tuning parameter for the ratio between nearest-neighbor and next-nearest-neighbor interactions and thus for magnetic frustration.

Study of the adsorption of a transition metal on the surface of a layered material and the possible subsequent intercalation into that layered material is of fundamental interest and potential technological importance. In the present work, we choose the transition metal Cu as the adsorbate or intercalant and $2H-\mathrm{Mo}{\mathrm{S}}_2$ as the layered material. Energetics are calculated characterizing four of the most basic surface and interfacial phenomena: adsorption, intercalation, diffusion, and adhesion. Using first-principles density functional theory (DFT), we find that intercalating a Cu atom into the van der Waals (vdW) gap below the $\mathrm{Mo}{\mathrm{S}}_2$ (0001) surface is 0.665 eV more favorable than adsorbing the Cu atom on top of the surface, i.e., intercalation of single Cu atoms is strongly favored thermodynamically. Also, we find that the system with adsorbed Cu is magnetic, while the system becomes nonmagnetic after the Cu atom is intercalated into the vdW gap. We obtain the diffusion barriers of the Cu atom on the surface and in the vdW gap to be 0.23 and 0.32 eV, respectively. We also obtain an adhesion energy of $0.874\mathrm{J}/{\mathrm{m}}^2$ for a Cu (111) slab bonding with a $2H-\mathrm{Mo}{\mathrm{S}}_2$ (0001) slab. The DFT value of adhesion energy, as well as the gap width at the interface between the Cu and $\mathrm{Mo}{\mathrm{S}}_2$, depends strongly on the choice of the functional. From our analysis on bulk properties of both $\mathrm{Mo}{\mathrm{S}}_2$ and Cu, we suggest that our vdW-DF2-B86R results listed above are reliable for applications, e.g., interpretation of experimental results and physical modeling of this adsorption system.

The magneto-optical response of Fe and Ni during ultrafast demagnetization is studied experimentally and theoretically. We have performed pump-probe experiments in the transverse magneto-optical Kerr effect (T-MOKE) geometry using photon energies that cover the $M$ absorption edges of Fe and Ni between 40 and 72 eV. The magnetic asymmetry was obtained by forming the difference of reflected intensities obtained for two opposite orientations of the sample magnetization. Density functional theory (DFT) was used to calculate the magneto-optical response of different magnetic configurations, representing different types of excitations: long wavelength magnons, short wavelength magnons, and Stoner excitations. In the case of Fe, we find that the calculated asymmetry is strongly dependent on the specific type of magnetic excitation. Our modeling also reveals that during remagnetization Fe is, to a reasonable approximation, described by magnons, even though small nonlinear contributions could indicate some degree of Stoner excitations as well. In contrast, we find that the calculated asymmetry in Ni is rather insensitive to the type of magnetic excitations. However, there is a weak nonlinearity in the relation between asymmetry and the off-diagonal component of the dielectric tensor, which does not originate from the modifications of the electronic structure. Our experimental and theoretical results thus emphasize the need to consider a coupling between asymmetry and magnetization that may be more complex than a simple linear relationship. This insight is crucial for the microscopic interpretation of ultrafast magnetization experiments.

We propose a mechanism of torque generation by injection of an orbital current, which we call orbital torque. In a magnetic bilayer consisting of a nonmagnet (NM) and a ferromagnet (FM), we consider a situation where the spin-orbit coupling (SOC) is present only in the FM. Although the SOC is absent in the NM, the orbital Hall effect can arise in the NM. When the resulting orbital Hall current is injected to the FM, the SOC of the FM converts the orbital angular momentum into spin, which exerts torque to the magnetization of the FM. Remarkably, even for small SOC strength comparable to that of $3d$ FMs, the orbital torque can be comparable to the spin torque induced by the spin Hall effect of the NM with strong SOC. This provides a way to experimentally probe the OHE and opens a venue to achieving spin-torque devices based on light elements that exhibit gigantic orbital response. Experimental implications are discussed.

Topological defects appear in symmetry breaking phase transitions and are ubiquitous throughout Nature. As an ideal testbed for their study, defect configurations in nematic liquid crystals (NLCs) could be exploited in a rich variety of technological applications. Here we report on robust theoretical and experimental investigations in which an external electric field is used to switch between predetermined stable chargeless disclination patterns in a nematic cell, where the cell is sufficiently thick that the disclinations start and terminate at the same surface. The different defect configurations are stabilized by a master substrate that enforces a lattice of surface defects exhibiting zero total topological charge value. Theoretically, we model disclination configurations using a Landau-de Gennes phenomenological model. Experimentally, we enable diverse defect patterns by implementing an in-house-developed atomic force measurement scribing method, where NLC configurations are monitored via polarized optical microscopy. We show numerically and experimentally that an “alphabet” of up to 18 unique line defect configurations can be stabilized in a 4 × 4 lattice of alternating $s=\pm 1$ surface defects, which can be “rewired” multistably using appropriate field manipulation. Our proof-of-concept mechanism may lead to a variety of applications, such as multistable optical displays and rewirable nanowires. Our studies also are of interest from a fundamental perspective. We demonstrate that a chargeless line could simultaneously exhibit defect-antidefect properties. Consequently, a pair of such antiparallel disclinations exhibits an attractive interaction. For a sufficiently closely spaced pair of substrate-pinned defects, this interaction could trigger rewiring, or annihilation if defects are depinned.

We report the existence of flat bands in a $p$-wave superconducting Kitaev ladder. We identify two sets of parameters for which the Kitaev ladder sustains flat bands. These flat bands are accompanied by highly localized eigenstates known as compact localized states. Invoking a Bogoliubov transformation, the Kitaev ladder can be mapped into an interlinked cross-stitch lattice. The mapping helps to reveal the compactness of the eigenstates, each of which covers only two unit cells of the interlinked cross-stitch lattice. The Kitaev Hamiltonian undergoes a topological-to-trivial phase transition when certain parameters are fine tuned. Correlation matrix techniques allow us to compute entanglement entropy of the many-body eigenstates. The study of entanglement entropy affords fresh insight into the topological phase transitions in the model. Sharp features in entanglement entropy when bands cross indicate a deep underlying relationship between entanglement entropy and dispersion.

Multi-Weyl semimetals feature band crossings with the dispersion that is, in general, linear in only one direction, and as a consequence their band structure is characterized by the monopole charge $n$ which can be greater than one. We show that a single screw dislocation defect oriented in the direction connecting the nodal points, which acts as an effective pseudomagnetic flux tube, can serve as a direct probe of the monopole charge $n\ge 1$ characterizing the bulk band structure of a multi-Weyl semimetal. To this end, as a proof of principle, we propose a rather simple mesoscopic setup in which the monopole charge leaves a direct imprint on the conductance measured in the plane perpendicular to the dislocation. In particular, the ratio of the positions of the neighboring maxima in the conductance as a function of the gate voltage can serve to deduce the monopole charge, while the value of the effective pseudomagnetic flux can be extracted from the position of a conductance maximum. We expect that these findings will prompt further studies on the role of multiple dislocations, as well as other topological lattice defects, such as grain boundaries and disclinations, in topological nodal materials.

We address the coexistence of massless and massive topological edge states at the interface between two materials with different topological phases. We modify the well-known Bernevig-Hughes-Zhang model to introduce a smooth function describing the band inversion and the band bending due to electrostatic effects between the bulk of the quantum well and the vacuum. Within this minimal model we identify distinct parameter sets that can lead to the coexistence of the two types of edge states and that determine their number and characteristics. We propose several experimental setups that could demonstrate their presence in two-dimensional topological systems, as well as ways to regulate or tune the contribution of the massive edge states to the conductance of associated electronic devices. Our results suggest that such states may also be present in novel two-dimensional van der Waals topological materials.

Long-range interactions allow far-distance quantum correlations to build up very fast. Nevertheless, numerical simulations demonstrated a dramatic slowdown of entanglement entropy growth after a sudden quench. In this work, we unveil the general mechanism underlying this counterintuitive phenomenon for $d$-dimensional quantum spin systems with slowly decaying interactions. We demonstrate that the semiclassical rate of collective spin squeezing governs the dynamics of entanglement, leading to a universal logarithmic growth in the absence of semiclassical chaos. In fact, the standard quasiparticle contribution is shown to get suppressed as the interaction range is sufficiently increased. All our analytical results agree with numerical computations for quantum Ising chains with long-range couplings. Our findings thus identify a qualitative change in the entanglement production induced by long-range interactions, and they are experimentally relevant for accessing entanglement in highly controllable platforms, including trapped ions, atomic condensates, and cavity-QED systems.

It is demonstrated that the Lindemann's criterion of melting can be formulated for two-dimensional classical solids using statistical mechanics arguments. With this formulation the expressions for the melting temperature are equivalent in three and two dimensions. Moreover, in two dimensions the Lindemann's melting criterion essentially coincides with the Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young melting condition of dislocation unbinding.

We study experimentally nanoscale Josephson junctions and Josephson spin valves containing strongly ferromagnetic Ni interlayers. We observe that in contrast to junctions, spin valves with the same geometry exhibit anomalous $I_c\left(H\right)$ patterns with two peaks separated by a dip. We develop several techniques for in situ characterization of micromagnetic states in our nanodevices, including magnetoresistance, absolute Josephson fluxometry, and first-order-reversal-curves analysis. They reveal a clear correlation of the dip in supercurrent with the antiparallel state of a spin valve and the peaks with two noncollinear magnetic states, thus providing evidence for generation of spin-triplet superconductivity. A quantitative analysis, based on micromagnetic simulations, brings us to the conclusion that the triplet current in our Ni-based spin valves is approximately three times larger than the conventional spin-singlet supercurrent.