We report spatially resolved small-angle neutron-scattering measurements of the conical and skyrmion states of a bulk single crystal of nickel-substituted ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, with a nominal concentration of Ni of 14%. We observe a significant spatial dependence of the structure of these magnetic states, characterized by increased disorder and misalignment with respect to the applied field as we approach the edge of the sample. Remarkably, the edge skyrmion state is also characterized by an extended stability towards lower temperatures. Surprisingly, in the same region of the sample, the metastable skyrmion state did not show simple decay. Instead, only a fraction of the scattered intensity appeared to decay, and the intensity therefore did not approach zero during our measurements. We suggest that the increased local disorder and the coexistence of conical and skyrmion states, induced by demagnetization effects at the edge of the sample, are responsible for the increased stability of this skyrmion state. We also infer that the unclear metastable behavior of the skyrmion lattice at the edge of the sample is due to the local geometry of the sample, which induces coexistence of different skyrmion states whose lifetimes are superimposed and difficult to separate in the data.

The functional renormalization group (FRG) provides a flexible tool to study correlations in low-dimensional electronic systems. In this paper, we present an FRG approach to the steady state of quantum wires out of thermal equilibrium. Our method is correct up to second order in the two-particle interaction and accounts for inelastic scattering. We combine semianalytic solutions of the flow equations with MPI parallelization techniques, which allows us to treat systems of up to 60 lattice sites. The equilibrium limit is well understood and serves as a benchmark. We compute effective distribution functions, the local density of states, and the steady-state current and demonstrate that all of these quantities depend strongly on the choice of the cutoff employed within the FRG. Nonequilibrium is plagued by the lack of physical arguments in favor of a certain cutoff as well as by the appearance of secular higher-order terms which are only partly included in our approach. This demonstrates the inadequacy of a straightforward second-order FRG scheme to study interacting quantum wires out of equilibrium in the absence of a natural cutoff choice.

We compute the Floquet Hamiltonian $H_F$ for weakly interacting fermions subjected to a continuous periodic drive using a Floquet perturbation theory (FPT) with the interaction amplitude being the perturbation parameter. This allows us to address the dynamics of the system at intermediate drive frequencies $\hslash {\omega }_D\ge V_0\ll {\mathcal{J}}_0$, where ${\mathcal{J}}_0$ is the amplitude of the kinetic term, ${\omega }_D$ is the drive frequency, and $V_0$ is the typical interaction strength between the fermions. We compute, for random initial states, the fidelity $F$ between wave functions after a drive cycle obtained using $H_F$ and that obtained using exact diagonalization (ED). We find that FPT yields a substantially larger value of $F$ compared to its Magnus counterpart for $V_0\le \hslash {\omega }_D$ and $V_0\ll {\mathcal{J}}_0$. We use the $H_F$ obtained to study the nature of the steady state of an weakly interacting fermion chain; we find a wide range of ${\omega }_D$ which leads to subthermal or superthermal steady states for finite chains. The driven fermionic chain displays perfect dynamical localization for $V_0=0$; we address the fate of this dynamical localization in the steady state of a finite interacting chain and show that there is a crossover between localized and delocalized steady states. We discuss the implication of our results for thermodynamically large chains and chart out experiments which can test our theory.

The optoelectronic properties of confined water form one of the most active research areas in the past few years. Here we present the multiscale methodology to discern the out-of-plane electronic and dipolar dielectric constants (${\varepsilon }_{\perp }^{\text{el}}$ and ${\varepsilon }_{\perp }^{\text{dip}}$) of strongly confined water. We reveal that ${\varepsilon }_{\perp }^{\text{el}}$ and ${\varepsilon }_{\perp }^{\text{dip}}$ become comparable for water confined in angstrom-scale channels (with a height of less than $15\AA$) within graphene (GE) and hexagonal boron nitride (hBN) bilayers. Channel height ($h$) associated with a minimum in both ${\varepsilon }_{\perp }^{\text{el}}$ and ${\varepsilon }_{\perp }^{\text{dip}}$ is linked to the formation of the ordered structure of ice for $h\approx \left(7\text{–}7.5\right)\AA$. The recently measured total dielectric constant ${\varepsilon }_{\perp }^T$ of nanoconfined water [L. Fumagalli et al., Science 360, 1339 (2018)] is corroborated by our results. Furthermore, we evaluate the contribution from the encapsulating membranes to the dielectric properties, as a function of the interlayer spacing, i.e., the height of the confining channel for water. Finally, we conduct analysis of the optical properties of both confined water and GE membranes, and show that the electron energy loss function of confined water strongly differs from that of bulk water.

The structural stability and electronic states of GaSe monolayer with trigonal-antiprismatic (AP) structure, which is a recently discovered polymorph, were studied by first-principles calculations. The AP-phase GaSe monolayer was found stable, and the differences in energy and lattice constant were small when compared to those calculated for a GaSe monolayer with conventional trigonal-prismatic (P) structure which was found to be the ground state. Moreover, it was revealed that the relative stability of P-phase and AP-phase GaSe monolayers reverses under tensile strain. These calculation results provide insight into the formation mechanism of AP-phase GaSe monolayers in epitaxially grown GaSe thin films.

Dispersion of the absolute second-order susceptibility of both $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{W}{\mathrm{S}}_2$ is assessed on a wide spectral excitation range (710–1300 nm) by using second-harmonic scattering spectroscopy (SHS). SHS is an accurate ensemble measurement here applied on well-dispersed suspensions of monodisperse liquid-exfoliated nanosheets showing a high monolayer content. The as-derived, high susceptibility values shed light on the discrepancies between available literature values while evidencing resonances associated with the main excitonic transitions.

Recent experiments demonstrate the induction of long-range order in correlated electron systems via external perturbations, which calls for a better understanding of nonthermal ordered states and nonequilibrium symmetry breaking. Here, we reveal a mechanism based on entropy cooling and entropy trapping in a strongly correlated multiorbital system. We consider a two-orbital Hubbard model with Hund coupling and crystal-field splitting and show that in the vicinity of a spin-state transition, crystal-field quenches can induce an excitonic condensation at initial temperatures above the highest ordering temperature in equilibrium. We furthermore identify a dynamical phase transition in the evolution of the order parameter and show that such quenches can result in long-lived nonthermal excitonic condensates which have no analog in the equilibrium phase diagram.

We take advantage of recent improvements in the grand canonical hybrid Monte Carlo algorithm, to perform a precision study of the single-particle gap in the hexagonal Hubbard model, with on-site electron-electron interactions. After carefully controlled analyses of the Trotter error, the thermodynamic limit, and finite-size scaling with inverse temperature, we find a critical coupling of $U_c/\kappa =3.834\left(14\right)$ and the critical exponent $z\nu =1.185\left(43\right)$. Under the assumption that this corresponds to the expected antiferromagnetic Mott transition, we are also able to provide a preliminary estimate $\beta =1.095\left(37\right)$ for the critical exponent of the order parameter. We consider our findings in view of the $\text{SU}\left(2\right)$ Gross-Neveu, or chiral Heisenberg, universality class. We also discuss the computational scaling of the hybrid Monte Carlo algorithm, and possible extensions of our work to carbon nanotubes, fullerenes, and topological insulators.

We implement the Bogoliubov-de Gennes (BdG) equation in real-space using the screened Korringa-Kohn-Rostoker (KKR) method. This allows us to solve, self-consistently, the superconducting state for 3D crystals including substitutional impurities with a full normal-state DFT band structure. We apply the theoretical framework to bulk Nb with impurities. Without impurities, Nb has an anisotropic gap structure with two distinct peaks around the Fermi level. In the presence of nonmagnetic impurities, those peaks are broadened due to the scattering between the two bulk superconducting gaps, however the peaks remain separated. As a second example of self-consistent real-space solutions of the BdG equations, we examine superconducting clusters embedded within a nonsuperconducting bulk metallic host. This allows us to estimate the coherence length of the superconductor and we show that, within our framework, the coherence length of the superconductor is related to the inverse of the gap size, just as in bulk BCS theory.

We investigate fractional quantum Hall states for model interactions restricted to a repulsive hard core. When the hard core excludes relative angular momentum $m=1$ between spinless electrons the ground state at Landau level filling factor $\nu =\frac{1}{3}$ is known to be exactly given by the Laughlin wave function. When we exclude relative angular momentum 3 only, Wójs, Quinn, and Yi have suggested the appearance of a liquid state with non-Laughlin correlations. We study this special hard-core interaction at filling factor $\frac{1}{3}$ on the sphere, torus, and cylinder geometry. An analysis of the charged and neutral gaps on the sphere geometry points to a gapless state. On the torus geometry the projected static structure factor has a two-peak feature pointing to one-dimensional density ordering. To clarify the nature of the ground state we perform extended density matrix renormalization group studies on the cylinder geometry for up to 30 particles. The pair correlation function allows us to conclude that the ground state is a two-particle bubble phase.

We show the existence of virtual polarization states during the interaction of modes in metasurface stacks. In support of our findings we experimentally realize a metasurface stack, consisting of an isotropic layer of nanopatches and an anisotropic layer of nanowires. Utilizing an analogy to the interaction of electrons at junctions in mesoscopic electron transport via Feynman paths, we present a semi-analytic description of the modal interaction inside this stack. We then derive a series of all possible reflection paths light can take inside the metasurface stack.

We experimentally investigate the valley-Hall effect for interfacial edge states, highlighting the importance of the modal patterns between geometrically distinct regions within a structured elastic plate. These experiments, for vibration, are at a scale where detailed measurements are taken throughout the system and not just at the input/output ports; this exposes the coupling between geometrically distinct modes that underlie the differences between wave transport around gentle and sharp bends.

The melting curve of vanadium at high pressure and temperature (P-T) is of great interest to our understanding of $d$-orbital transition metals with simple crystal structures at extreme P-T conditions. Here we have investigated the melting curve and crystal structures of polycrystalline vanadium at high P-T using synchrotron x-ray diffraction (XRD) in laser-heated diamond anvil cells (LH DACs) up to ∼100 GPa and ∼4400 K, a two-stage light-gas gun with in situ shock temperature measurements up to ∼256 GPa and ∼6200 K, and ab initio molecular dynamics (AIMD) with density functional theory computations up to ∼200 GPa. The occurrence of the diffuse scattering signals in high P-T XRD patterns is used as the primary criterion to determine the melting curve of body-centered cubic (bcc) vanadium up to ∼100 GPa in LH DACs. Analysis of thermal radiation spectra of shocked vanadium using a quasispectral pyrometer constrains the melting curve up to ∼246 GPa and ∼5830 K, which is consistent with our static results using the Simon equation. The present static and dynamic experiments on the melting curve of vanadium are consistent with our AIMD simulations with the two-phase melting modeling, and are overall consistent with other theoretical simulations using the Z method. The results reconcile the recently reported theoretical discrepancy, and refute a higher melting curve report given by self-consistent ab initio lattice dynamics calculations. The consistencies among our studies indicate that one does not have to invoke superheating as a hypothesis to describe the solid-liquid equilibrium boundary of vanadium as an explanation for static vs dynamic experimental results. Our static and dynamic results with in situ diagnostics of melting and two-phase AIMD simulation have implications for studying melting curves of other $d$-orbital transition metals and their alloys at extreme P-T conditions.

The ability to measure temperature in shock wave experiments has been a long-standing scientific challenge complicated by the short timescales involved ($<1$ microsecond) and the presence of nonthermal or external light sources that pollute measured radiances using optical pyrometry methods. In the current work, we present the first experimental data on cerium metal designed to measure the temperature on-Hugoniot from the low-pressure $\alpha$ phase well into the high-pressure liquid phase. Radiance data obtained in this work were used to determine the Hugoniot curves for solid ($\alpha$-Ce) and liquid cerium by measuring the longitudinal stress and temperature simultaneously in the shocked state. These data were used to estimate the temperature for incipient shock melting and complete melting, and to further constrain a multiphase equation of state developed to describe the dynamic response of cerium at high pressures.

Considerable attention has been paid recently to $\mathrm{Fe}{\mathrm{Te}}_2{\mathrm{O}}_5\mathrm{Cl}$ due to reduced dimensionality and frustration in the magnetic subsystem, succession of phase transitions, and multiferroicity. The efforts to grow its selenite sibling resulted in the mixed halide compound $\mathrm{Fe}\left({\mathrm{Te}}_{1.5}{\mathrm{Se}}_{0.5}\right){\mathrm{O}}_5\mathrm{Cl}$, which was found crystallizing in a different structural type and possessing properties drastically different from those of a parent system. Its magnetic subsystem features weakly coupled ${\mathrm{Fe}}^{3+}-{\mathrm{Fe}}^{3+}$ dimers showing the regime of short-range correlations at $T_{\mathrm{M}}\sim 70\mathrm{K}$ and long-range order at $T_{\mathrm{N}}=22\mathrm{K}$. In a magnetically ordered state, sizable spin-orbital interactions lead to a small canting of ${\mathrm{Fe}}^{3+}$ moments. Magnetic dipole-dipole interactions contribute significantly to the experimentally observed orientation of magnetization easy axis in the $ac$ plane. The first principles calculations of leading exchange interactions were found in agreement with measurements of thermodynamic properties and Raman spectroscopy.

Ferromagnetic channels subject to spin injection at one extremum sustain long-range coherent textures that carry spin currents known as dissipative exchange flows (DEFs). In the weak injection regime, spin currents carried by DEFs decay algebraically and extend through the length of the channel, a regime known as spin superfluidity. Similar to fluids, these structures are prone to phase slips that manifest as vortex-antivortex pairs. Here, we numerically study vortex shedding from DEFs excited in a magnetic nanowire with a physical obstacle. Using micromagnetic simulations, we find regimes of laminar flow and vortex shedding as a function of obstacle position tunable by the spin injection sign and magnitude. Vortex-antivortex pairs translate forward (VF regime) or backward (VB regime) with respect to the detector's extremum, resulting in well-defined spectral features. Qualitatively similar results are obtained when temperature, anisotropy, and weak nonlocal dipole fields are included in the simulations. These results provide clear features associated with DEFs that may be detected experimentally in devices with nominally identical boundary conditions. Furthermore, our results suggest that obstacles can be considered as DEF control gates, opening an avenue to manipulate DEFs via physical defects.

We investigate the Heisenberg model on a decorated square (Fisher) lattice in the presence of first-neighbor $J_1$, second-neighbor $J_2$, and third-neighbor $J_3$ exchange couplings, with antiferromagnetic $J_1$. The classical ground-state phase diagram obtained within a Luttinger-Tisza framework is spanned by two antiferromagnetically ordered phases, and an infinitely degenerate antiferromagnetic chain phase. Employing classical Monte Carlo simulations we show that thermal fluctuations fail to lift the degeneracy of the antiferromagnetic chain phase. Interestingly, the spin-wave spectrum of the Néel state displays three Dirac nodal loops out of which two are symmetry protected while for the antiferromagnetic chain phase we find symmetry-protected Dirac lines. Furthermore, we investigate the spin $S=\frac{1}{2}$ limit employing a bond operator formalism which captures the singlet-triplet dynamics, and find a rich ground-state phase diagram host to a variety of valence bond solid orders in addition to antiferromagnetically ordered phases.

Inelastic neutron scattering is used to investigate the temperature dependence of spin correlations in the 3-dimensional XY antiferromagnet $\mathrm{Ni}{\left({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x\right)}_2·4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2, x=0.14\left(1\right)$, tuned close to the chemical-composition-induced soft-mode transition. The local dynamic structure factor shows $\hslash \omega /T$ scaling behavior characteristic of a quantum-critical point. The deviation of the measured critical exponent from spin wave theoretical expectations is attributed to disorder. Another effect of disorder is local excitations above the magnon band. Their energy, structure factor, and temperature dependence are well explained by simple strong-bond dimers associated with Br-impurity sites.