The growth of the spin glass correlation length has been measured as a function of the waiting time $t_{\mathrm{w}}$ on a single crystal of CuMn (6 at. %), reaching values $\xi \sim 150$ nm, larger than any other glassy correlation length measured to date. We find an aging rate $dln{t}_{\mathrm{w}}/dln\xi$ larger than found in previous measurements, which evinces a dynamic slowing down as $\xi$ grows. Our measured aging rate is compared with simulation results by the Janus Collaboration. After critical effects are taken into account, we find excellent agreement with the Janus data.

Polycrystalline samples of layered pnictogen (Pn) diselenide $\mathrm{Nd}{\mathrm{O}}_{0.8}{\mathrm{F}}_{0.2}\mathrm{S}{{\mathrm{b}}_1}_{-x}\mathrm{B}{\mathrm{i}}_x\mathrm{S}{\mathrm{e}}_2\left(x=0\mathrm{to}0.8\right)$ were successfully synthesized by solid-state reactions. Electrical resistivity in the synthesized samples systematically decreases with an increase in Bi content $x$, accompanied by an insulator to metal-like transition at $x=0.5-0.6$. Crystal structure analysis using synchrotron x-ray diffraction suggests that this transition correlates with anomalous change in $c$-axis length and/or distortion in Pn−Se conducting plane. Emergence of pressure-induced superconductivity was investigated using a diamond anvil cell with boron-doped diamond electrodes, employing $x=0.3$ and 0.7 as the representative samples. For the Sb-rich one $\left(x=0.3\right)$, we observed a signature of superconducting transition with ${T_c}^{\mathrm{onset}}=5.15\pm 0.05\mathrm{K}$ at 50 GPa. The ${T_c}^{\mathrm{onset}}$ of $x=0.3$ increased with increasing pressure and reached 7.65 ± 0.05 K at 70.8 GPa, followed by the gradual decrease in $T_c$ up to 90 GPa. For the Bi-rich one $\left(x=0.7\right)$, a superconducting transition with ${T_c}^{\mathrm{onset}}=5.5\pm 0.6\mathrm{K}$ was induced at 43.5 GPa, which is almost comparable to that of $x=0.3$; besides, upper critical field $\left(H_{c2}\right)$ is evaluated to be $\sim 10$ T for $x=0.7$, which is higher than that of $x=0.3(H_{c2}=6.7$ T at 50 GPa).

We systematically extend Bogoliubov theory beyond the mean-field approximation of the Bose-Hubbard model in the superfluid phase. Our approach is based on the time-dependent variational principle applied to the family of all Gaussian states (i.e., Gaussian TDVP). First, we find the best ground-state approximation within our variational class using imaginary time evolution in 1D, 2D, and 3D. We benchmark our results by comparing to Bogoliubov theory and DMRG in 1D. Second, we compute the approximate one- and two-particle excitation spectrum as eigenvalues of the linearized projected equations of motion (linearized TDVP). We find the gapless Goldstone mode, a continuum of two-particle excitations and a doublon mode. We discuss the relation of the gap between Goldstone mode and two-particle continuum to the excitation energy of the Higgs mode. Third, we compute linear response functions for perturbations describing density variation and lattice modulation and discuss their relations to experiment. Our methods can be applied to any perturbations that are linear or quadratic in creation/annihilation operators. Finally, we provide a comprehensive overview how our results are related to well-known methods, such as traditional Bogoliubov theory and random phase approximation.

We investigate charge relaxation in disordered and quasiperiodic quantum wires of spinless fermions $(t-V$ model) at different inhomogeneity strength $W$ in the localized and nearly localized regime. Our observable is the time-dependent density correlation function, $\mathrm{\Phi }(x,t)$, at infinite temperature. We find that disordered and quasiperiodic models behave qualitatively similar: Although even at longest observation times the width $\mathrm{\Delta }x\left(t\right)$ of $\mathrm{\Phi }(x,t)$ does not exceed significantly the noninteracting localization length, ${\xi }_0$, strong finite-size effects are encountered. Our findings appear difficult to reconcile with the rare-region physics (Griffiths effects) that often is invoked as an explanation for the slow dynamics observed by us and earlier computational studies. Motivated by our numerical data we discuss a scenario in which the MBL-phase splits into two subphases: in ${\mathrm{MBL}}_{\text{A}}\mathrm{\Delta }x\left(t\right)$ diverges slower than any power, while it converges towards a finite value in ${\mathrm{MBL}}_{\text{B}}$. Within the scenario the transition between ${\mathrm{MBL}}_{\text{A}}$ and the ergodic phase is characterized by a length scale, $\xi$, that exhibits an essential singularity $ln\xi \sim 1/|W-W_{{\text{c}}_1}|$. Relations to earlier numerics and proposals of two-phase scenarios will be discussed.

The spin-orbital polarization of superconducting excitations in momentum space is shown to provide distinctive marks of unconventional pairing in the presence of inversion symmetry breaking. Taking the prototypical example of an electronic system with atomic spin-orbit and orbital-Rashba couplings, we provide a general description of the spin-orbital textures and their most striking changeover moving from the normal to the superconducting state. We find that the variation of the spin texture is strongly imprinted by the combination of the misalignment of spin-triplet $\mathbf{d}$ vector with the inversion asymmetry $\mathbf{g}$-vector coupling and the occurrence of superconducting nodal excitations. Remarkably, the multiorbital character of the superconducting state allows us to unveil a unique type of topological transition for the spin winding around the nodal points. This finding indicates the fundamental topological relation between chiral and spin winding in nodal superconductors. By analogy between spin- and orbital-triplet pairing we point out how orbital polarization patterns can also be employed to assess the character of the superconducting state.

We present a semianalytic and asymptotically exact solution to the problem of phonon-induced decoherence in a quantum dot-microcavity system. Particular emphasis is placed on the linear polarization and optical absorption, but the approach presented herein may be straightforwardly adapted to address any elements of the exciton-cavity density matrix. At its core, the approach combines Trotter's decomposition theorem with the linked cluster expansion. The effects of the exciton-cavity and exciton-phonon couplings are taken into account on equal footing, thereby providing access to regimes of comparable polaron and polariton timescales. We show that the optical decoherence is realized by real phonon-assisted transitions between different polariton states of the quantum-dot–cavity system, and that the polariton line broadening is well-described by Fermi's golden rule in the polariton frame. We also provide purely analytic approximations which accurately describe the system dynamics in the limit of longer polariton timescales.

An effective $\mathbf{k}·\mathbf{p}$ Hamiltonian is applied to spin-orbit-split states in the surface alloys ${\mathrm{PbAg}}_2$ and ${\mathrm{BiAg}}_2$ on Ag(111) with the aim to infer the microscopic mechanism behind the momentum dependence of the spin polarization beyond the standard Rashba model. The Hamiltonian is derived from the ab initio wave functions at $\overline{\mathrm{\Gamma }}$ without introducing any adjustable parameters so as to accurately reproduce the spin of the surface-alloy states, in particular the sign reversal of the spin polarization within a nondegenerate band. We establish the origin of the sign reversal and describe the spin-orbit splitting around $\overline{\mathrm{\Gamma }}$ as a combination of the linear and cubic Rashba effects. Furthermore, we predict the behavior of the ${\mathrm{PbAg}}_2$ and ${\mathrm{BiAg}}_2$ states under the influence of an in-plane and out-of-plane magnetization for a surface alloy in contact with a magnetic layer. Apart from the surface-alloy states, we identify a substrate-related surface state originating from the same branch of the complex band structure as the surface state on a clean Ag(111).

We have studied temperature-driven effects on photoemission spectra of hollandite-type ${\mathrm{Ba}}_x{\mathrm{Ti}}_8{\mathrm{O}}_{16+\delta }$ across its metal-insulator transition (MIT) at 220 K by using surface-sensitive photon energies from 27 to 70 eV. The surface Ti $3d$ spectral weight was suppressed and shifted towards higher binding energy from the bulk if the surface was obtained by cleavage above the MIT temperature. The Ti $3d$ spectral weight was recovered once the sample was cooled across the MIT, indicating a transfer of Ti $3d$ electrons from the bulk to the surface across the MIT. The recovered Ti $3d$ spectral shape is consistent with the bulk-sensitive result and indicates strong localization character of the Ti $3d$ electron even in the metallic phase. Resonant photoemission spectroscopy with absorption from the Ti $3p$ core level exhibits resonances of the O $2p$-Ti $3d$ and O $2p$-Ti $4s,p$ hybridized states. The result suggests that the O $2p$-Ti $4s,p$ hybridization plays a role in the interplay between the bulk and the surface in ${\mathrm{Ba}}_x{\mathrm{Ti}}_8{\mathrm{O}}_{16+\delta }$.

We performed electron spin-resonance (ESR), magnetic, and optical absorption studies of a model system used to investigate the metal-insulator transition in a deformable lattice. The ESR and magnetic studies revealed that the small polarons have no correlation to the conductivity below or above the mobility edge. The conducting transition is marked by the appearance of an asymmetric ESR signal simultaneously with a significant suppression of the small bipolaron optical absorption band indicating that the extended states result from an abrupt dissociation of the small bipolarons. Immediately above the mobility edge competing phases of extended states, small polarons, and small bipolarons coexist. We propose a phase diagram for the charge and spin dynamics of an interacting gas of small (bi)polarons across the discontinuous conducting transition that has implications for understanding the long-standing conundrum of Mooij correlations.

We employ density functional theory (DFT) to analyze the dispersion of the electronic state that exists at the commensurate interface between a monolayer of 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) and the Ag(111) surface. First, we present and verify a hydrogen-termination approach which allows a meaningful DFT description of the interface state with relatively thin silver slabs. Complemented with a projection technique which maps the interface electronic structure onto the original Ag(111) Shockley state, the DFT calculations enable us to analyze the evolution of the dispersion of the NTCDA/Ag(111) interface state when changing of the molecular coverage. Our calculations yield a difference between the interface state energy and the Shockley state energy that scales linearly with coverage. Furthermore, they predict a pronounced anisotropy of the dispersion of the interface state at long wavelengths which also depends linearly on the molecular coverage. The dispersion anisotropy is fully confirmed by our Fourier transform (FT) scanning tunneling spectroscopy (STS) experiments performed on a relaxed phase NTCDA/Ag(111) monolayer. Using feature detection STS (FD-STS), we moreover measure a band gap in the interface state band structure at the Brillouin zone boundary which indicates Bragg scattering of the interface state electrons in the periodic potential of the molecular layer. We thus observe an influence of the molecular layer on the interface state both at long (DFT, STS) and short wavelengths (STS).

Magnons in antiferromagnetic insulators couple strongly to conduction electrons in adjacent metals. We show that this interfacial tie can lead to superconductivity in a trilayer consisting of a metal sandwiched between two antiferromagnetic insulators. The strength of the induced electron-electron coupling, and consequently the critical temperature, strongly increases with increasing interface exchange coupling. For a fixed electron-electron coupling strength, the critical temperature is proportional to the magnon gap, a natural energy scale. However, the magnon gap also enters in the coupling strength, so that in the weak coupling limit, an enhancement of the magnon gap reduces the critical temperature. We estimate the critical temperature in $\mathrm{Mn}{\text{F}}_2$-Au-$\mathrm{Mn}{\text{F}}_2$ to be on the order of $1\mathrm{K}$. Umklapp scattering at metal-antiferromagnet interfaces leads to $d$-wave pairing, in contrast to $p$-wave superconductivity mediated by magnons in ferromagnets.

The ability to entangle distant quantum nodes is essential for the construction of quantum networks and for quantum information processing. For solid-state quantum emitters used as qubits, it can be achieved by photon interference. When the emitter is subject to spectral diffusion, this process can become highly inefficient, impeding the achievement of scalable quantum technologies. We study two-photon interference in the context of a Hong-Ou-Mandel (HOM)-type experiment for two separate quantum emitters, with different detunings with respect to a specific target frequency. We evaluate the second order coherences that characterize photon indistinguishability between the two emitters. We find that the two-photon interference operation that is inefficient in the absence of a control protocol, when the two detunings are different and spectral overlap is lessened, can be highly improved by a periodic sequence of $\pi$ pulses at a set target frequency. Photon indistinguishability in solid state and other quantum emitters subject to spectral diffusion can thus be enhanced by the proposed pulse sequence and similar external control protocols despite the fluctuations in the environment.

In the multiferroic hexagonal manganite ${\mathrm{HoMnO}}_3$, inelastic neutron scattering and synchrotron-based THz spectroscopy have been used to investigate the spin waves associated with the Mn order together with Ho crystal-field excitations. While the Mn order sets in first below 80 K, a spin reorientation occurs below 37 K, a rare feature in the rare-earth manganites. We show that several Ho crystal-field excitations are present in the same energy range as the magnons, and that they are all affected by the spin reorientation. Moreover, several anomalous features are observed in the excitations at low temperature. Our analysis and calculations for the Mn spin waves and Ho crystal-field excitations suggest Mn-Ho coupling mechanisms as well as electric charge redistribution affecting the dynamics.

The ${\mathrm{Eu}}^{+2}$ spins-7/2 in ${\mathrm{EuNi}}_2{\mathrm{As}}_2$ with the body-centered tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ structure order antiferromagnetically below the Néel temperature $T_{\mathrm{N}}=15$ K into a helical antiferromagnetic (AFM) structure with the helix axis aligned along the tetragonal $c$ axis and the Eu ordered moments aligned ferromagnetically within the $ab$ plane as previously reported from neutron diffraction measurements [T. Jin et al., Phys. Rev. B 99, 014425 (2019)]. Here we study the crystallographic, magnetic, thermal, and electronic transport properties of Bi-flux-grown single crystals using single-crystal x-ray diffraction, anisotropic magnetic susceptibility $\chi$, isothermal magnetization $M$, heat capacity $C_{{}_{\mathrm{p}}}$, and electrical resistivity $\rho$ measurements versus applied magnetic field $H$ and temperature $T$. Vacancies are found on the Ni sites corresponding to the composition ${\mathrm{EuNi}}_{1.95\left(1\right)}{\mathrm{As}}_2$. A good fit of the $\rho \left(T\right)$ data by the Bloch-Grüneisen theory for metals was obtained. The ${\chi }_{ab}\left(T\right)$ data below $T_{\mathrm{N}}$ are fitted well by molecular field theory (MFT), and the helix turn angle $kd$ and the Eu-Eu Heisenberg exchange constants are extracted from the fit parameters. The $kd$ value is in good agreement with the neutron-diffraction result. The magnetic contribution to the zero-field heat capacity below $T_{\mathrm{N}}$ is also fitted by MFT. The isothermal in-plane magnetization $M_{ab}$ exhibits two metamagnetic transitions versus $H$, whereas $M_c(T=2\mathrm{K})$ is nearly linear up to $H=14$ T, both behaviors being consistent with MFT. The $M_c(H,T),\rho (H_c,T)$, and $C_{\mathrm{p}}(H_c,T)$ data yielded a $H_c\text{−}T$ phase diagram separating the AFM and paramagnetic phases in good agreement with MFT. Anisotropic $\chi \left(T\right)$ literature data for the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type helical antiferromagnet ${\mathrm{EuRh}}_2{\mathrm{As}}_2$ were also fitted well by MFT. A comparison is made between the crystallographic and magnetic properties of ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type $\mathrm{Eu}{M}_{2}P{n}_2$ compounds with $M=\mathrm{Fe}$, Co, Ni, Cu, or Rh, and $Pn=\mathrm{P}$ or As, where only ferromagnetic and $c$-axis helical AFM structures are found.

Exploring new strategies to perform magnon logic is a key requirement for the further development of magnon-based spintronics. In this paper, we realize a three-terminal magnon transport device to study the possibility of manipulating magnonic spin information transfer in a magnetic insulator via localized magnetic fields and heat generation. The device comprises two parallel Pt wires as well as a Cu center wire that are deposited on the ferrimagnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. While the Pt wires act as spin current injector and detector, the Cu wire is used to create local magnetostatic fields and additional heat, which impact both the magnetic configuration and the magnons within the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ below. We show that these factors can create a nonlocal signal that shows similar features as compared to an electrically induced magnon flow. Furthermore, a modulation of the spin transport signal between the Pt wires is observed, which can be partly explained by thermally excited spin currents of different polarization. Our results indicate a potential way towards the manipulation of nonlocal magnon signals, which could be useful for magnon logic.

We have performed transverse- and zero-field muon spin rotation/relaxation experiments, as well as magnetometry measurements, on samples of ${\mathrm{Fe}}_{1\text{−}x}{\mathrm{V}}_x\mathrm{Se}$ and their $\text{Li}+{\mathrm{NH}}_3$ intercalates ${\mathrm{Li}}_{0.6}{\left({\mathrm{NH}}_2\right)}_{0.2}{\left({\mathrm{NH}}_3\right)}_{0.8}{\mathrm{Fe}}_{1\text{−}x}{\mathrm{V}}_x\mathrm{Se}$. We examine the low vanadium substitution regime: $x=0.005$, 0.01, and 0.02. The intercalation reaction significantly increases the critical temperature $\left(T_{\mathrm{c}}\right)$ and the superfluid stiffness for all $x$. The nonintercalated samples all exhibit $T_{\mathrm{c}}\approx 8.5$ K while the intercalated samples all show an enhanced $T_{\mathrm{c}}>40$ K. Vanadium substitution has a negligible effect on $T_{\mathrm{c}}$, but seems to suppress the superfluid stiffness for the nonintercalated samples and weakly enhance it for the intercalated materials. The optimal substitution level for the intercalated samples is found to be $x=0.01$, with $T_{\mathrm{c}}\approx 41\mathrm{K}$ and ${\lambda }_{ab}\left(0\right)\approx 0.18\mu \mathrm{m}$. The nonintercalated samples can be modeled with either a single $d$-wave superconducting gap or with an anisotropic gap function based on recent quasiparticle imaging experiments, whereas the intercalates display multigap nodal behavior which can be fitted using $s+d$- or $d+d$-wave models. Magnetism, likely from iron impurities, appears after the intercalation reaction and coexists and competes with the superconductivity. However, it appears that the superconductivity is remarkably robust to the impurity phase, providing an avenue to stably improve the superconducting properties of transition metal substituted FeSe.

The rate of energy transfer from ion projectiles onto the electrons of a solid target is hard to determine experimentally in the velocity regime between the adiabatic limit and the Bragg peak. First-principles simulations have lately offered relevant new insights and quantitative information for prototypical homogeneous materials. Here, we study the influence of structural anisotropy on electronic stopping power with time-dependent density functional theory simulations of a hydrogen projectile in graphite. The projectile traveled at a range of angles and impact parameters for velocities between 0.1 and 1.4 a.u., and the electronic stopping power was calculated for each simulation. After validation with average experimental data, the anisotropic crystal structure was found to have a strong influence on the stopping power, with a difference between simulations parallel and perpendicular to the graphite plane of up to 25%, more anisotropic than expected based on previous work. The velocity dependence at low velocity displays clear linear behavior in general, except for projectiles traveling perpendicular to graphitic layers, for which a thresholdlike behavior is obtained. For projectiles traveling along graphitic planes, metallic behavior is observed with a change of slope when the projectile velocity reaches the Fermi velocity of the electrons.

The phase diagram and the order parameters of the exactly solvable quantum one-dimensional model are analyzed. The model in its spin representation is the dimerized $XY$ spin chain in the presence of uniform and staggered transverse fields. In the fermionic representation this model is the dimerized noninteracting Kitaev chain with a modulated chemical potential. The model has a rich phase diagram which contains phases with local and nonlocal (string) orders. We have calculated within the same systematic framework the local order parameters (spontaneous magnetization) and the nonlocal string order parameters, along with the topological winding numbers for all domains of the phase diagram. The topologically nontrivial phase is shown to have a peculiar oscillating string order with the wave number $q=\pi /2$, awaiting for its experimental confirmation.

We explore an opportunity to induce and control tetragonal distortion in materials. The idea involves formation of a binary alloy from parent compounds having body-centered and face-centered symmetries. The concept is illustrated in the case of ${\mathrm{FeNi}}_{1\text{−}x}{\mathrm{Co}}_x$ magnetic alloy formed by substitutional doping of the ${L1}_0$ FeNi magnet with Co. Using electronic structure calculations we demonstrate that the tetragonal strain in this system can be controlled by concentration and it reaches maximum for $x=0.5$. This finding is then applied to create a large magnetocrystalline anisotropy (MAE) in ${\mathrm{FeNi}}_{1\text{−}x}{\mathrm{Co}}_x$ system by considering an interplay of the tetragonal distortion with electronic concentration and chemical anisotropy. In particular, we identify an ordered ${\mathrm{FeNi}}_{0.5}{\mathrm{Co}}_{0.5}$ system with MAE larger by a factor 4.5 from the ${L1}_0$ FeNi magnet.

Perpendicularly magnetized thin films with a strong Dzyaloshinskii-Moriya interaction (DMI) exhibit chiral spin structures such as Néel domain walls and skyrmions. These structures are promising candidates for next-generation magnetic memory devices. Determining the magnitude of the DMI accurately is key to engineering materials for such applications. Existing approaches are based on quantities extracted either from magnetization dynamics, which present experimental and theoretical challenges, or from measurements of quasistatic domain spacing, which so far have been analyzed using incomplete models or prohibitively slow micromagnetic simulations. Here, we use a recently developed analytical model of stripe domain widths in perpendicularly magnetized multilayers to extract the DMI from domain images combined with magnetometry data. Our approach is tested on micromagnetically simulated domain patterns, where we achieve a 1% agreement of the extracted DMI with the DMI used to run the simulation. We then apply our method to determine the thickness-dependent DMI in two experimental materials, one with (${[\mathrm{Pt}(2.5–7.5\mathrm{nm})/{\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}\left(0.8\mathrm{nm}\right)/\mathrm{MgO}\left(1.5\mathrm{nm}\right)]}_{13}$) and one without (${[\mathrm{Pt}(2.5–7.5\mathrm{nm})/\mathrm{Co}\left(0.8\mathrm{nm}\right)/\mathrm{Pt}\left(1.5\mathrm{nm}\right)]}_{13}$) inversion symmetry breaking. We discuss the means to obtain realistic error bars with our method. Our results demonstrate that analytical domain spacing analysis is a powerful tool to extract the DMI from technologically relevant multilayer materials.