Low-energy electron diffraction (LEED) is a widely used technique in surface-science laboratories. Yet, it is rarely used to its full potential. The quantitative information about the surface structure, contained in the modulation of the intensities of the diffracted beams as a function of incident electron energy, LEED $I\left(V\right)$, is underutilized. To acquire these data, only minor adjustments would be required in most experimental setups, but existing analysis software is cumbersome to use and often computationally inefficient. The ViPErLEED (Vienna package for Erlangen LEED) project lowers these barriers, introducing a combined solution for user-friendly data acquisition, extraction, and computational analysis. These parts are discussed in three separate publications. Here, the focus is on the computational part of ViPErLEED, which performs highly automated LEED-$I\left(V\right)$ calculations and structural optimizations. Minimal user input is required, and the functionality is significantly enhanced compared to existing solutions. Computation is performed by embedding the existing Erlangen tensor-LEED package (TensErLEED). ViPErLEED manages additional parallelization, monitors convergence, and processes all input and output. This makes LEED $I\left(V\right)$ more accessible to new users while minimizing the potential for errors and the manual labor. Added functionalities include intelligent structure-dependent defaults for most calculation parameters, automatic detection of bulk and surface symmetries and their relationship, automated search procedures that preserve the symmetry and speed up convergence, adjustments to the TensErLEED code to handle larger systems than before, as well as parallelization and optimization. Modern file formats are used as input and output, and there is a direct interface to the atomic simulation environment (ASE) package. The software is implemented primarily in Python (version $\ge 3.7$) and provided as an open-source package (GNU GPLv3 or any later version). A structure determination of the $\alpha {\text{-Fe}}_2{\text{O}}_3\left(1\overline{1}02\right)-(1\times 1)$ surface is presented as an example for the application of the software.

Topological insulators are quantum materials involving time-reversal protected surface states making them appealing candidates for the design of the next generation of highly efficient spintronic devices. The very recent observation of large transient spin-charge conversion and subsequent powerful THz emission from $\mathrm{Co}\text{/}{\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ bilayers clearly demonstrates such potentiality and feasibility for the near future. Among the exotic properties appearing in and at the surface of such quantum materials, spin-momentum locking and Rashba-Edelstein effects remain as key ingredients to effectively convert the spin degree of freedom into a charge or a voltage signal. In this work, we extend our analyses to the quantification of orbital momentum-locking and related orbital charge conversion effects in ${\mathrm{Bi}}_{.85}{\mathrm{Sb}}_{.15}$ via orbital Rashba-Edelstein effects. In that sense, we will provide some clear theoretical and numerical insights implemented by multiorbital and slab tight-binding methods to clarify our recent experimental results obtained by THz spectroscopy in the time domain.

Polytypes of tetralayer graphene (TLG: Bernal, rhombohedral, and mixed stacking) are crystalline structures with different symmetries. Among those, mixed-stacking tetralayers lack inversion symmetry, which allows for intrinsic spontaneous out-of-plane electrical polarization, inverted in the mirror-image pair, ABCB and ABAC stackings. Here, we compare the intrinsic polarization of such TLGs with the symmetry-breaking effect of a substrate, which can also generate out-of-plane electric dipole moments with different sizes in all four polytypes, including ABCB and ABAC twins. We analyze their temperature and magnetic field dependence, in view of understanding the origin of the recently measured Kelvin probe force microscopy maps of tetralayer flakes [S. S. Atri et al.Adv. Phys. Res. 3, 2300095 (2023)], and notice that the intrinsic contribution could be singled out based on magnetic field dependence of polarization measured at low temperatures.

Light-matter interfaces have now entered a new stage marked by the ability to engineer quantum correlated states under driven-dissipative conditions. To propel this new generation of experiments, we are confronted with the need to model nonunitary many-body dynamics in strongly coupled regimes by transcending traditional approaches in quantum optics. In this work, we contribute to this program by adapting a functional-integral technique, conventionally employed in high-energy physics, in order to obtain nonequilibrium dynamics for interacting light-matter systems. Our approach is grounded in constructing “two-particle irreducible” (2PI) effective actions, which provide a nonperturbative and conserving framework for describing quantum evolution at a polynomial cost in time. We apply our method to complement the analysis of spin-glass formation in the context of frustrated multimode cavity quantum electrodynamics, initiated in our accompanying work [Hosseinabadi et al., Phys. Rev. Res. 6, 043313 (2024)]. Finally, we outline the capability of the technique to describe other near-term platforms in many-body quantum optics, and its potential to make predictions for this new class of experiments.

In this computational study, we explore a viable route to access the Kitaev-quantum spin liquid (QSL) state in a recently synthesized monolayer of a so-called spin-orbit assisted Mott insulator ${\mathrm{OsCl}}_3$. In addition to other magnetic ground states in different regions, a substantial region of small $J_{\text{H}}$/$U$ in our Hubbard $U$–Hund's $J_{\text{H}}$ quantum phase diagram, obtained by combining second-order perturbation and pseudo-fermion functional renormalization group calculations, hosts Kitaev-QSL phase. Only the first neighbor Kitaev interaction of a smaller magnitude, nearly independent of the choice of $U$, is obtained in this region. Experimental tuning of the single parameter $J_{\text{H}}$ to access the QSL state can purportedly be achieved with epitaxial strain in advanced crystal growth techniques. Furthermore, negligibly small farther neighbor interactions appear as a distinct feature of monolayer ${\mathrm{OsCl}}_3$, suggesting this material to be a better candidate for the exploration of the possible Kitaev-QSL state than previously proposed materials.

We present a combined experimental and theoretical study on the quasi-one-dimensional organic conductor (${\mathrm{TMTTF})}_2{\mathrm{PF}}_6$, and elucidate the variation of its physical properties under pressure. We fully resolve the crystal structure by single crystal x-ray diffraction measurements using a diamond anvil cell up to 8 GPa, and based on the structural data, we perform first-principles density-functional theory calculations and derive the ab initio extended Hubbard-type Hamiltonians. Furthermore, we compare the behavior of the resistivity measured up to 3 GPa using a BeCu clamp-type cell and the ground state properties of the obtained model numerically calculated by the many-variable variational Monte Carlo method. Our main findings are as follows: (i) The crystal was rapidly compressed up to about 3 GPa where the volume drops to 80% and gradually varies down to 70% at 8 GPa. The transfer integrals increase following such behavior whereas the screened Coulomb interactions decrease, resulting in a drastic reduction of correlation effect. (ii) The degree of dimerization in the intrachain transfer integrals, as the result of the decrease in structural dimerization together with the change in the intermolecular configuration, almost disappears above 4 GPa; the interchain transfer integrals also show characteristic variations under pressure. (iii) The results of identifying the characteristic temperatures in the resistivity and the charge and spin orderings in the calculations show an overall agreement: The charge ordering sensitively becomes unstable above 1 GPa, while the spin ordering survives up to higher pressures. These results shed light on the similarities and differences between applying external pressure and substituting the chemical species (chemical pressure).

Levitated nanoparticles are a promising platform for sensing applications and for macroscopic quantum experiments. While the nanoparticles' motional temperatures can be reduced to near absolute zero, their uncontrolled internal degrees of freedom remain much hotter, inevitably leading to the emission of heat radiation. The decoherence and motional heating caused by this thermal emission process is still poorly understood beyond the case of the center-of-mass motion of point particles. Here, we present the master equation describing the impact of heat radiation on the motional quantum state of arbitrarily sized and shaped dielectric rigid rotors. It predicts the localization of spatio-orientational superpositions only based on the bulk material properties and the particle geometry. A counterintuitive and experimentally relevant implication of the presented theory is that orientational superpositions of optically isotropic bodies are not protected by their symmetry, even in the small-particle limit.

We investigate analytically the performance of many-body energy functionals, derived by Klein, Luttinger, and Ward, at different levels of diagrammatic approximations, ranging from second Born, to GW, to the so-called T matrix, for the calculation of total energies and potential energy surfaces. We benchmark our theoretical results on the extended two-site Hubbard model, which is analytically solvable and for which several exact properties can be calculated. Despite its simplicity, this model displays the physics of strongly correlated electrons: it is prototypical of the ${\mathrm{H}}_2$ dissociation, a notoriously difficult problem to solve accurately for the majority of mean-field-based approaches. We show that both functionals exhibit good to excellent variational properties, particularly in the case of the Luttinger-Ward one, which is in close agreement with fully self-consistent calculations, and we elucidate the relation between the accuracy of the results and the different input one-body Green's functions. Provided that these are wisely chosen, we show how the Luttinger-Ward functional can be used as a computationally inexpensive alternative to fully self-consistent many-body calculations, without sacrificing the precision of the results obtained. Furthermore, in virtue of this accuracy, we argue that this functional can also be used to rank different many-body approximations at different regimes of electronic correlation, once again bypassing the need for self-consistency.

Particles in a crowded environment exhibit slow anomalous diffusion, and their efficient manipulation is important in controlling transport phenomena in complex materials. Skyrmions and half-skyrmions, spatially localized quasiparticles observed in magnetic systems and liquid crystals, also exhibit diffusive motion. They exhibit normal diffusion in dilute conditions. However, the cooperative dynamics and diffusion of skyrmions and half-skyrmions in their condensed liquid, hexatic, and crystalline phases are elusive. Here we show in the half-skyrmion condensed phases that the fusion and fission of half-skyrmions, not their diffusion, are responsible for the primary structural relaxation. The fusion and fission occur due to the nonconserved nature of the quasiparticle number density. The diffusion, which contributes to the secondary structural relaxation, is suppressed by cages formed by surrounding half-skyrmions, whereas enhanced by Mermin-Wagner fluctuation characteristic to two-dimensional systems, leading to subdiffusive motion. Large displacement of half-skyrmions is locally excited by fusion-fission and bond breaking between adjacent half-skyrmions via heterogeneous elastic fields. Furthermore, the motion of half-skyrmions couples with transverse and longitudinal sound wave excitation differently, where the transverse sound wave is more attenuated than the longitudinal one due to the coupling between transverse sound wave and half-skyrmion deformation. We also discuss the relationship between half-skyrmion diffusion in our system and skyrmion diffusion in magnetic systems. Our result provides a qualitative difference in dynamical properties between half-skyrmion gaseous and condensed phases, suggesting the efficient manipulation of high-density half-skyrmions and skyrmions.

Quasiparticle poisoning errors in Majorana-based qubits are not suppressed by the underlying topological properties, which undermines the usefulness of this proposed platform. This work tackles the errors originating from intrinsically excited quasiparticles by developing an erasure conversion scheme based on local quasiparticle detection. To model such measurements, we begin by constructing the quasiparticle position operator for the Kitaev chain. A measurement probe coupling to this operator is shown to allow projective measurements in the Wannier quasiparticle basis. Detection of quasiparticles in a region of width $d$ adjacent to each Majorana zero-energy mode allows implementation of an error-detecting Majorana stabilizer code ${\mathcal{C}}_d$ based on microscopic fermionic (nontopological) physical degrees of freedom. The implementation of ${\mathcal{C}}_d$ converts a large fraction of Pauli errors to erasure errors, thus achieving “erasure conversion” in Majorana qubits. We show that the fraction of Pauli errors escaping conversion to erasure errors is exponentially small in $d$, a result tied to the exponential localization of Wannier functions which we prove rigorously. The suppression in Pauli error rate comes at the cost of the erasure rate increasing sublinearly with $d$, but this can be readily compensated for by a suitable outer code, with the net effect being a higher threshold rate of quasiparticle poisoning. The framework developed here serves as a basis for understanding how realistic measurements, such as conductance measurements, could be utilized for achieving fault tolerance in these systems.

The combination of structural chirality and magnetism leads to the formation of spin chirality through noncoplanar magnetic structures, resulting in unusual electronic transport properties. The spin chirality generates nonzero Berry curvature in real space, acting as an emergent magnetic field and contributing to the unconventional anomalous Hall effect, known as the geometrical or topological Hall effect (THE). This study unveils the remarkable occurrence of THE in a chiral antiferromagnetic (AFM) semiconductor $\mathrm{Eu}{\mathrm{Ir}}_2{\mathrm{P}}_2$ in the hopping regime. It exhibits a complex incommensurately spiral AFM ground state due to its chiral crystalline structure, providing fertile ground for the emergence of topologically nontrivial spin textures such as skyrmions. A substantial THE is observed under finite magnetic fields, making $\mathrm{Eu}{\mathrm{Ir}}_2{\mathrm{P}}_2$ an exceptional case within the ultralow-conductivity hopping regime for investigating the interplay between topologically nontrivial magnetic structures and hopping carriers. Owing to its semiconducting nature, we have formulated a theoretical model based on Mott's variable range-hopping mechanism, effectively elucidating the temperature and magnetic field-dependent behavior of THE. $\mathrm{Eu}{\mathrm{Ir}}_2{\mathrm{P}}_2$ thus serves as an ideal candidate for comprehending transport properties in the hopping regime and offers a unique opportunity for the implementation of AFM semiconductor-based spintronic devices.

Magnetic kagome metals have attracted tremendous research interests recently, because they represent an ideal playground for exploring the fascinating interplay between their intrinsically inherited topologically nontrivial electron band structures, magnetism and electronic correlation effects, and the resultant novel electronic/magnetic states and emergent excitations. In this work, we report a comprehensive single-crystal neutron diffraction investigation of the ground-state magnetic structures of the recently discovered V-based topological kagome metals $R{\mathrm{V}}_6{\mathrm{Sn}}_6$ (R = Tb, Dy, Ho, Er). Furthermore, the sample synthesis details and our systematic studies of crystal structure, low-temperature magnetic and thermodynamic properties of these compounds via various in-house characterization techniques are also reported. Our single-crystal neutron diffraction measurements confirm that the long-range magnetic order forms below 4.3 K for $R=\mathrm{Tb}$, 3.0 K for $R=\mathrm{Dy}$, 2.4 K for $R=\mathrm{Ho}$, and 0.6 K for $R=\mathrm{Er}$, respectively. The ground-state magnetic structures of the studied compounds are comprehensively determined via the magnetic crystallography approaches. It can be revealed that $R{\mathrm{V}}_6{\mathrm{Sn}}_6$ ($R$ = Tb, Dy, Ho) have a collinear ferromagnetic order in the ground state, with the ordered magnetic moment aligned along the $c$ axis for $R$ = Tb, Ho, while approximately $20{}^{\circ }$ tilted off from the $c$ axis for $R$ = Dy. In contrast, ${\mathrm{ErV}}_6{\mathrm{Sn}}_6$ shows an A-type antiferromagnetic structure with a magnetic propagation vector k = (0, 0, 0.5), and with the ordered magnetic moment aligned in the $ab$ plane. The ordered magnetic moments are determined as 9.4(2) ${\mu }_B$, 6.6(2) ${\mu }_B$, 6.4(2) ${\mu }_B$, and 6.1(2) ${\mu }_B$ for $R$ = Tb, Dy, Ho, and Er, respectively. A comparison of the low-temperature magnetic structures for both the extensively investigated topological kagome metal series of $R{\mathrm{V}}_6{\mathrm{Sn}}_6$ and $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ is given in detail. This allows to gain new insights into the complex magnetic interactions, diverse single-ion magnetic anisotropies and spin dynamics in these compounds. The reported ground-state magnetic structures in $R{\mathrm{V}}_6{\mathrm{Sn}}_6$ ($R$ = Tb, Dy, Ho, Er) can pave the way for further explorations of the possible interplay between magnetism and topologically nontrivial electron band structures in the magnetically ordered phase regime.

Metamaterials engineered to host topological states of matter in controllable quantum systems hold promise for the advancement of quantum simulations and quantum computing technologies. In this context, the Su-Schrieffer-Heeger (SSH) model has gained prominence due to its simplicity and practical applications. Here we present the implementation of a gate-tunable, five-unit-cell resonator-based SSH chain on a one-dimensional lattice of superconducting resonators. We achieve electrostatic control over the inductive intracell coupling using semiconductor nanowire junctions, which enables the spectroscopic observation of a transition from a trivial to a topological phase in the engineered metamaterial. In contrast to prior work, our approach offers precise and independent in situ tuning of the coupling parameters, which is critical to directly approximate ideal tight-binding Hamiltonians via the control over local variables. Our results supplement efforts towards gate-controlled superconducting electronics and large controllable resonator-based lattices to enable quantum simulations.

Taking advantage of the anomalous Hall effect, we electrically probe low-frequency magnetization fluctuations at room temperature in a thin ferromagnetic $\mathrm{Pt}/\mathrm{Co}/{\mathrm{AlO}}_x$ layer stack with perpendicular magnetic anisotropy. We observe a strong enhancement of the Hall voltage fluctuations within the hysteretic region of the magnetization loop. Analyzing both the temporal evolution of the anomalous Hall voltage and its frequency-dependent noise power density, we identify two types of magnetic noise: abrupt changes in the magnetic domain configuration, evident as Barkhausen-like steps in the Hall voltage time trace, yield a noise power density spectrum scaling with frequency as $1/f^{\beta }$ with $\beta \approx 2.0$. In contrast, quasistationary magnetization configurations are connected with a magnetic noise power density with an exponent $\beta \approx 1.0$. The observation of Barkhausen steps and relaxation effects shows that the magnetic system is in a nonstationary state in the hysteresis region, such that the fluctuation-dissipation theorem cannot be expected to hold. However, the time-dependent change in the Hall voltage for constant magnetic field strength resembles the integrated noise power.

In quantum mechanics, supersymmetry (SUSY) posits an equivalence between two elementary degrees of freedom, bosons, and fermions defined by local rules. Here we apply it to find connections between bosonic and fermionic lattice models in the realm of condensed-matter physics and uncover a novel fivefold way topology it demands in these systems. At the single-particle level, our connections pair a bosonic and fermionic lattice model, either describing the hopping of number-conserving particles or local couplings between fermion parity-conserving particles. The pair are isospectral except for zero modes, such as flat bands, quadratic band touchings, and nexus points, whose existence is undergirded by the Witten index of the SUSY theory. We develop a unifying framework to formulate these SUSY connections in terms of general lattice graph correspondences. Notably, in this framework, the supercharge operator that generates SUSY is Hermitian and can itself be interpreted as a hopping Hamiltonian on a bipartite lattice, a feature that enables the discovery of materials or model lattices hosting the SUSY partners. To illustrate the power of SUSY, we present 16 use cases of SUSY, that span topics including frustrated magnets, Kitaev spin liquids, and topological superconductors, the majority of which turn out to provide insights into the discovery and design of flat bands and topological materials.

A generalization of the graphene honeycomb model to the case where each site in the honeycomb lattice contains a $n$-fold degenerate set of eigenstates of the $C_3$ symmetry has been recently proposed to describe several systems, including triangulene crystals and photonic lattices. These generalized honeycomb models are defined by $(n_a,n_b)$, the number of $C_3$ eigenstates in the $a$ and $b$ sites of the unit cell, resulting in $n_a+n_b$ bands. Thus, the (1,1) case gives the coventional honeycomb model that describes the two low-energy bands in graphene. Generalizations, such as (2,1), (2,2), and (3,3) display several nontrivial features, such as coexisting graphenelike Dirac cones with flat bands, both at zero and finite energy, as well as robust degeneracy points where a flat band and a parabolic band meet at the $\mathrm{\Gamma }$ point. Here we explore the edge states of this class of crystals, using as reference triangulene crystals, and we find several types of edge states absent in the conventional (1,1) honeycomb case, associated to the nontrivial features of the two-dimensional bands of the high-fold case. First, we find dispersive edge states associated to the finite-energy flat bands, that occur both at the armchair and zigzag termination. Second, in the case of noncentrosymmetric triangulene crystals that lead to a $S=1$ Dirac band, we have a bonding-antibonding pair of dispersive edge states, localized in the same edge so that their energy splitting is reduced as their localization increases, opposite to the conventional behavior of pairs of states localized in opposite edges. Third, for the (3,3) case, that hosts a gap separating a pair of flat conduction and valence bands, we find nondispersive edge states with $E=0$ in all edge terminations.